Doctoral Degrees (Chemistry)

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Open Access
The analysis of natural and sulfited commercial quebracho (Schinopsis lorentzii) and Acacia (Acacia mearnsii) proathocyanidin extracts with electrospray ionisation mass spectrometry
(University of the Free State, 2013) Jordaan, Maryam Amra; Van der Westhuizen, J. H.
Quebracho (Schinopsis lorentzii and Schinopsis balansae) heartwood and black wattle (Acacia mearnsii) bark extracts are important renewable industrial sources of proanthocyanidins (PACs). These extracts are used industrially in leather tanning and adhesive manufacturing. These applications are derived from their chemical properties. The poly hydroxy groups of PACs complex with proteins via hydrogen bonds and thus transforms raw skin into leather. The phloroglucinol or resorcinol type A-rings are nucleophilic and polymerise with aldehydes to form natural adhesives. The ortho hydroxy group on the B-ring form insoluble complexes with heavy metals and can be used in water purification applications. The extracts are often treated with sodium hydrogen sulphate (sulfitation) to enhance their industrial usefulness. From a literature search and discussions with role players in the black wattle and quebracho PAC extract manufacturing industry, it became evident that knowledge on the composition of commercial PACs extracts and chemical changes that takes place during sulfitation is unsatisfactory. These PAC extracts are complex due to variable hydroxylation patterns of the constituent flavan-3-ol aromatic rings, different configurations of the C-2, C-3 and C-4 stereogenic centres, different degrees of polymerisation, and the existence of angular oligomers. Gel or paper chromatography fractionations of the complex extracts are hampered by poor resolution due to their hydrophilic polyphenolic nature and efforts to isolate pure compounds have been restricted to the isolation of mainly monomers and a few dimers and trimers. PACs of the commercially important quebracho (Schinopsis lorentzii and Schinopsis balansae) and black wattle (Acacia mearnsii) extracts have a strong and stable interflavanyl bond. This stability is important from an industrial point of view as it leads to durable leather and adhesive products. It is attributed to the absence of 5-OH groups in the aromatic moieties of the extender fisetinidol and robinetinidol flavan-3-ols units. However, from an analytical point of view it is not advantageous. The high temperatures thus required to hydrolyse the interflavanyl bonds with weak acids; leads to decomposition of the intermediate monomers that renders conventional thiolysis and phloroglucinolysis based analytical methods unreliable. In this thesis we used electrospray mass spectrometry (ESI-MS) to investigate the composition of PACs in black wattle extract and the changes that takes place in the chemical composition of quebracho PACs during sulfitation. We furthermore use all the information available from literature on the phytochemistry of flavan-3-ols and PACs and the syntheses of flavan-3-ol oligomers to guide us in our ESI-MS interpretations. Previous research in our group established that quebracho PACs always consist of a catechin starter unit to which one, two or more fisetinidol extender units are attached. The first and second extender units are always attached to the relatively reactive phloroglucinol A-ring of the catechin starter unit to form predominantly dimers and angular trimers. Further extender units are attached to the relatively less reactive resorcinol A-rings of already incorporated fisetinidol extender units. This explains the relatively short degree of polymerisation of quebracho PAC extracts and their popularity as a tanning agent. Large PACs will not penetrate the spaces between skin proteins and cannot act as a tanning agent. In this thesis we established that black wattle PACs have, in addition to catechin starter units, also gallocatechin starter units and, in addition to fisetinidol extender units, also robinetinidol extender units. Acacia PACs are thus more complex combinations of catechin, gallocatechin, fisetinidol and robinetinidol monomers. This contrasts with quebracho PACs that only contain catechin and fisetinidol monomers. The higher degree of hydroxylation of gallocatechin and robinetinidol explains the higher water solubility of black wattle PACs and the less frequent need for sulfitation. We also established that during sulfitation of quebracho PACs, a sulfonic acid moiety is introduced in both the C-2 and C-4 position of the pyran heterocyclic C-ring. In the case of C-2 sulfitation, the heterocyclic ring is opened. This enhances the reactivity of the A-ring towards the reaction with formaldehyde (adhesive formation) and increases water solubility due to removal of rigidity and introduction of a polar sulfonic acid group. In the case of C-4 sulfitation, the interflavanyl bond is broken. Polarity and water solubility is thus not only increased via an additional sulfonic acid moiety, but due to the presence of shorter oligomers and a smaller average chain length. We also developed a chromatographic method to estimate the degree of sulfitation of quebracho PAC extract. We believe that we have made a valuable contribution towards a better understanding of the composition of black wattle and sulfited quebracho PAC extracts and have identified a number of misconceptions.
Open Access
Analysis of zirconium containing materials using multiple digestion and spectrometric techniques
(University of the Free State, 2014-10) Lotter, Steven James; Purcell, W.; Nel, J. T.
English: The preparation of pure zirconium metal for nuclear applications is difficult due to the non-reactivity of zirconium minerals, such as zircon. The ability to accurately analyse zirconium-containing materials across the whole beneficiation chain is of crucial importance to the zirconium industry as a whole. The development of such an analytical technique is problematic, however, as the very properties which make these materials desirable also make quantification of their components extremely difficult. Certified reference materials for the fluoride-containing Necsa zirconium process products were not available. Therefore in-house reference materials were created by crystallisation of several (cation)xZrF4+x compounds. Potassium catena di-μ-fluoridotetrafluoridozirconate( IV), cesium hexafluoridozirconate(IV) and tetraethyl ammonium catena di-μ-fluorido-bis-(trifluoridozirconate(IV)) monohydrate were prepared and characterised by X-ray crystallography and qualitative XRD. Coordination numbers for the zirconium atoms in each of these crystals were found to be 8, 6 and 7 respectively. Bridging fluorine bond lengths were determined to be approximately 2.06 and 1.97 Å for the potassium and tetraethyl ammonium complexes while terminal bond lengths were found to be 2.17 (potassium), 2.007 (cesium) and 2.15 (tetraethyl ammonium) Å. ICP-OES lower limits of detection for zirconium in the 3.25% nitric acid matrix were found to be 1.6 ppb with lower limits of quantification being ten times this value. ICPOES zirconium recoveries for these crystals were 101(1) and 100(2)% for the potassium and cesium crystals respectively. Dissolution of various commercial and Necsa process samples was problematic and thus several digestion methods were investigated. Sulphuric acid, ammonium bifluoride and hydrofluoric acid were all investigated along with microwave assistance. A microwave-assisted acid digestion method was developed capable of complete dissolution of all zirconium compounds with ICP-OES analytical recoveries of 102.0(9), 100(2) and 101(3)% for 99.98% zirconium metal foil, ZrC and ZrH2 respectively. In order to circumvent the dissolution step a solid state GD-OES method was developed wherein sample powders were pressed into disks with a binder material, either copper or graphite. Initially instrument response across different samples was inconsistent but after optimisation of several instrument parameters, such as applied voltage and pre-burn time, a calibration curve with a R2 value of 0.9805 was achieved using multiple sample materials. This was achieved using the radio frequency glow discharge source operating at 900 V applied voltage and 14 W applied power with a 5-minute pre-burn period. Results for Necsa process products were largely in line with those achieved by the ICP-OES method.
Open Access
Analysis of zirconium containing materials using multiple digestion and spectrometric techniques
(University of the Free State, 2014-10) Lotter, Steven James; Purcell, W.; Nel, J. T.
English: The preparation of pure zirconium metal for nuclear applications is difficult due to the non-reactivity of zirconium minerals, such as zircon. The ability to accurately analyse zirconium-containing materials across the whole beneficiation chain is of crucial importance to the zirconium industry as a whole. The development of such an analytical technique is problematic, however, as the very properties which make these materials desirable also make quantification of their components extremely difficult. Certified reference materials for the fluoride-containing Necsa zirconium process products were not available. Therefore in-house reference materials were created by crystallisation of several (cation)xZrF4+x compounds. Potassium catena di-μ-fluorido-tetrafluoridozirconate(IV), cesium hexafluoridozirconate(IV) and tetraethyl ammonium catena di-μ-fluorido-bis-(trifluoridozirconate(IV)) monohydrate were prepared and characterised by X-ray crystallography and qualitative XRD. Coordination numbers for the zirconium atoms in each of these crystals were found to be 8, 6 and 7 respectively. Bridging fluorine bond lengths were determined to be approximately 2.06 and 1.97 Å for the potassium and tetraethyl ammonium complexes while terminal bond lengths were found to be 2.17 (potassium), 2.007 (cesium) and 2.15 (tetraethyl ammonium) Å. ICP-OES lower limits of detection for zirconium in the 3.25% nitric acid matrix were found to be 1.6 ppb with lower limits of quantification being ten times this value. ICP-OES zirconium recoveries for these crystals were 101(1) and 100(2)% for the potassium and cesium crystals respectively. Dissolution of various commercial and Necsa process samples was problematic and thus several digestion methods were investigated. Sulphuric acid, ammonium bifluoride and hydrofluoric acid were all investigated along with microwave assistance. A microwave-assisted acid digestion method was developed capable of complete dissolution of all zirconium compounds with ICP-OES analytical recoveries of 102.0(9), 100(2) and 101(3)% for 99.98% zirconium metal foil, ZrC and ZrH2 respectively. In order to circumvent the dissolution step a solid state GD-OES method was developed wherein sample powders were pressed into disks with a binder material, either copper or graphite. Initially instrument response across different samples was inconsistent but after optimisation of several instrument parameters, such as applied voltage and pre-burn time, a calibration curve with a R 2 value of 0.9805 was achieved using multiple sample materials. This was achieved using the radio frequency glow discharge source operating at 900 V applied voltage and 14 W applied power with a 5-minute pre-burn period. Results for Necsa process products were largely in line with those achieved by the ICP-OES method.
Open Access
Biflavonoied diastereo-isomere en hul biogenetiese voorlopers uit Berchemia SPP.: pogings tot sintese van analoe
(University of the Free State, 1976-02) Volsteedt, Francois du Rouan; Roux, D. G.
Afrikaans: Hierdie studie behels die struktuurondersoek, omskakelingsreaksies en pogings tot sintese van sommige van die mono- en biflavonoïede uit die kernhout van Berchernia zeyheri met 'n aanvullende studie op die kernhoutstowwe van die taksonomies notiverwante spesie; B discolor. Die verbindings uit B. zeyheri (rooi ivoor) kan in twee groepe geklassifiseer word, nl. mono- en biflavonoïede, almal op dieselfde 4' ,5,7- trihidroksi- (of sy ekwivalent) fenoliese patroon gegrond. Die eerste groep sluit die volgende in: maesopsin {2-(4-hidroksibensiel)-2,4,6-trihidroksibenso[b]furan-3(2H)-oon}; 7-metielmaesopsin (nnut); α,2’,4,4’,6’-pentahidroksicis- en trans-chalkoon; 2,3-trans-aromadendrin (3,4’,5,7-tetrahidroksiflavanoon); kaempferol (3,4’,5,7-tetrahidroksiflavoon); naringenin (4’,5,7-trihidroksiflavanoon) en 4,4’,6-trihidroksiauroon. Uit die beskikbare eksperimentele gegewens kon geen aanduiding van die struktuur van die rooi pigment wat verantwoordelik is vir die besondere voorkoms van die kernhout, verkry word nie. Die relatiewe hoë opbrengs van maesopsin het toegelaat dat verskeie nuwe omskakelings op die tetra-O-metieleter uitgevoer kan word met die doel om soortgelyke reaksies op daardie biflavonoïede, met een of meer maesopsineenhede waarvan genoegsaam verkry kon word, te herhaal. Reduksie met LiA1H4 het die ooreenstemmende 3-ol gelewer; brominering die 7-broomderivaat; en fotolise die 2’-hidroksi-α-metoksi-cis-chalkoonanaloog sowel as 1-(2-hidroksi-4,6-dimetoksifeniel)-2,2-dimetoksi-3-(4-metoksifeniel)-propan-1-oon. 2-(α-Asetoksi-4-metoksibensiel)-2,4,6-trimetoksi- en 2-(4-metoksibensoïel)-2-asetoksi-4,6-dimetoksibesno[b]furan-3(2H)-oon is vir die eerste keer verkry deur oksidasie van sintetiese 2’-hidroksi-α-metoksi-trans-chalkoon met Pb(0Ac)4 in asynsuur. Die struktuur van die cis- en trans-α-hidroksiechalkone is deur sintese en fotolitiese omskakeling bevestig. Die struktuur van die “konvensionele” chalkoon is deur sintese van 2’-hidroksi-4,4’,6’-trimetoksi-trans-chalkoon bevestig. Oksidasie van laasgenoemde met Pb(OAc)4 en Tl(NO3)3 lewer onderskeidelik tri-0-metielauroon en –isoflavoon. Al die biflavonoïede is nuwe natuurprodukte en sluit die volgende opties onaktiewe verbindings in (slegs triviale name genoem): zeyherin en isozeyherin, beide enantiomere pare bestaande uit twee I-2,II-7-gekoppelde bensokumaranonieleenheid verbind en neo- en isoneoberchenin wat slegs verskil t.o.v. die I-3,II-5-interflavonoïedbinding van voorafgaande twee. Al vier is enantiomere pare. ‘n Verdere biflavonoïed, rhamnin, saamgestel uit ‘n naringenieleenheid en ‘n terminale α-hidroksichalkooneenheid (I-3,II-3’-gekoppel), lewer vir die eerste keer direkte bewys (met die moontlikheid van ringopening in ag geneem) van inkorporasie van ‘n α-hidroksichalkoon as biogenetiese voorloper. Die bestaan van die verbinding dui aan dat ringsluiting van die terminale eenheid by berchenin en isoberchenin na intermolekulêre koppeling plaasvind. Twee verdure biflavonoïede, laktoon A en B genoem, identies aan die zeyherins behalwe dat die terminale eenheid (maesopsin) ’n bensielsuuromskakeling onder die toestande van metilering ondergaan het en derhalwe nie as natuurprodukte beskou word nie, is as enantiomorfe mengsels verkry. Hul dien egter as ‘n indirekte bewys vir α-hidroksichalkone as biogenetiese voorlopers, en dat siklisering by die zeyherins soos in die geval van berchenins moontlik eers na intermolekulêre koppeling geskied. Geeneen van die pare biflavonoïede ondergaan onderlinge omskakeling by ca. 150° (ontbinding) nie, sodat hul nie rotasie isomere verteenwoordig nie, maar wel diastereoisomere. Bibensokumaranonielmetaan, moontlik ‘n rotameriese mengsel volgens duplisering van seine in die KMR-spektrum van die 0-metieleter, bestaan uit twee maesopsineenhede, unike I-5,II-7 oor ‘n CH2-brug verbind. ‘n Verbinding verwant aan laasgenoemde, n. 2,7-bi(4-hidroksibensiel)-2,4,6-trihidroksibenso[b]furan-3(2H)-oon is ook geïsoleer. Al die biflavonoïede is uniek in die opsig dat hul 5-ledige heterosikliese ring(e) of ‘n α-hidroksichalkoon as eenhede bevat. As gevolg van ‘n gebrek aan material is slegs enkele omskakelings op sommige van die biflavonoïede uitgevoer. Reduksie van 0-metielzeyherin met KBH4 skakel slegs die karbonielgroep van die terminale eenheid na die alcohol om. Dieselfde resultaat is op die berchenins verkry. Fotolise van 0-metielzeyherin lei tot omskakeling van die terminale eenheid na ‘n ketalstruktuur soortgelyk as die verkry uit die fotolise van 0-metielmaesopsin. Deur die uitskakeling van suurtoestande en hitte is die 0-metieleters van maesopsin, zeyherin, isozeyherin, berchenin en isoberchenin na voltooiing van die ondersoek as opties aktiewe verbindings geïsoleer na metilering met diasometaan. In ‘n poging tot sintese van ‘n biflavonoïed is gevind dat wanneer ‘n gesubstitueerde fenol i.p.v. ‘n flavonoïedeenheid gebruik word onder toestande wat fenolkoppeling bevorder [alkalise K3Fe(CN)6], die reaksie van die verwagte afwyk deurdat koppeling in die β- i.p.v. die α-posisie van die chalkoon plaasvind. In die spesifieke geval koppel 2’,4-dihidroksi-4’,6’-dimetoksi-trans-chalkoon in die β-posisie met 3,5-dimetoksifenol in teenwoordigheid van K3Fe(CN)6 om vier diastereoisomere 2-[α-(4-hidroksi-2,6-dimetoksifeniel)-4-hidroksibensiel]-4,6-dimetoksiebenso[b]furan-3(2H)-one en 4’-hidroksie-4,6-dimetoksiauroon in goeie opbrengste te lewer. Die volgende verbindings is uit die kernhout van die verwante B. discolour geïsoloeer: maesopsin, alfitonien {2-(3,4-dihidroksibensiel)-2,4,6-trihidroksibenso[b]furan-3(2H)-oon}; α,2’,4,4’,6’-pentahidroksi- en α-2’,3,4,4’,6’-heksahidroksitrans-chalkoon (beide nuwe natuurprodukte); (-)-epi- en (+)-katesjien; kaempferol (3,4’,5,7-tetrahidroksiflavoon), kwersitien (3,3’,4’,5,7-pentahidroksiflavoon), 3,4’,5-trihidroksidihidrostilbeen, asook twee verbindings waarvan nog geen struktuurtoeseggings moontlik was nie. Fotolise van die heksametoksi-trans-chalkoon, derivaat van bogenoemde α-heksahidroksichalkoon, het die cis-isomeer gelewer. Slegs ‘n ketalanaloog is na die foto-oksidatiewe omskakeling van penta-0-metielalfitonien geïsoleer, in teenstelling met die addisionele cis-α-metoksichalkoon verkry by die fotolise van tetra-0-metielmaesopsin. Skrille kontras bestaan dus tussen die inhoudstowwe van die twee spesies. Benewens ‘n aanvullende katekoloksigeneringspatroon is die opvallendste egter die afwesigheid van die verwagte biflavonoïede by B. discolour op grond van taksonomiese verwantskap. Alhoewel twee α-hidroksichalkone in die kernhout voorkom dui die afwesigheid van biflavonoïede in teenstelling met die geval van B. zeyheri moontlik op ‘n verskil in die redokspotensiaal van die twee spesies se ensiemsisteme en/of dat ‘n α-hidroksichalkoon slegs tesame met ‘n “konvensionele” chalkoon, soos in die geval van rooi ivoor, as biogenetiese voorlopers vir biflavonoïede optree. Die aanwesigheid van α-hidroksichalkone en verwante hemiketale in hoë konsentrasie dui op ‘n chemotaksonomiese verwantskap tussen twee spesies.
Open Access
Bulky metal complexes as model nanoscale catalysts
(University of the Free State, 2012-11) Young, Cyril; Roodt, Andreas; Bezuidenhout, Ben
English: The lifestyle of modern society has created a massive demand for various chemicals such as fuels, chlorine-free refrigerants, high-strength polymers, stain-resistant fibres, cancer treatment drugs and thousands of other products. The demand for these compounds can only be met through the use of catalysts. Heterogeneous catalysis has become a fundamental part of the industrial scale production of these chemicals. Although heterogeneous catalysis is better suited for these processes than its homogeneous counterpart, some of the systems are plagued by poor distribution of the active metal species throughout the support. The aim of this study was to investigate the feasibility of synthesising robust, planar, bridging ligands that could act as spacers between active metal species in the deposition of active catalysts onto heterogeneous supports. By choosing different building blocks, for the simple Schiff base reaction, the distance and proximity between active metal species could theoretically be controlled for a desired application. 1,10-Phenanthroline and diamide type ligands (Figure 1) were identified as possible candidates for this application Figure 1: Different ligand systems identified as possible dispersion spacers. (A) represents the diamide type ligands and (B) the 1,10-phenanthroline ligands. The aim of this study was pursued by the identification and synthesis of building blocks such as 5,6-diamino-1,10-phenanthroline and 1,10-phenanthroline-5,6-dione which could act as bridging ligands and could be used to construct larger bridging systems. The bridging ligands and building blocks were coordinated to square planar metal centres such as platinum and palladium. This would enhance the possibility of creating a single layer network on the surface of the support. The ligands and complexes were characterised using solid state techniques and single crystal X-Ray Diffractometry to investigate the planarity of these species and the coordination mode to some of the diamide type complexes that have not found many applications in this field. The Heck coupling was identified as a standard reaction which could be utilised to test the catalytic properties of the palladium species. The catalytic activity of a range of diamide and 1,10-phenanthroline type ligands was evaluated after the optimisation of the Heck coupling. It was found that reducing the electron density on the five and six position of the phenanthroline ring drastically enhances the catalytic capabilities of these compounds. The diamide type complexes and larger bridging ligands showed less promising results.
Open Access
Characterisation and substitution kinetics of hromium(III)- and obalt(III)nitrilotriacetato complexes
(University of the Free State, 2000-11) Visser, Hendrik Gideon; PurcelI, W.; Basson, S. S.
English: The synthesis and reactions of Co(llI) and Cr(lIl) complexes with nitrilotriacetic acid (nta) as tetradentate ligand have widespread interest, mainly because of the fact that these complexes can be usedas biological model complexes and because nta labilises usually inert metal centres. Mori et al (1958:940) and Uehara et al. (1967:2317) were the first to prepare different Co(III)-nta and Cr(III)-nta complexes respectively. Since then these complexes have been used in several kinetic and synthetic studies (Visser et al. 1997:2581; Visser et al. 1994:1051 and Thacker & Higginson, 1975:704). However the identity and purity of these complexes were questionable and had not been solved up to the time of this study. The question regarding the identity of the different Co(III)-nta species in solution at different pH levels have largely been accounted for in this study (refer to Scheme 1). Scheme 1 Complexes and reactions of Co(III)-nta. (Refer to PDF attached) The identity of the complex first prepared by Mori et al (1958:940) was finally characterised with X-ray crystallography as being [Co(nta)(1l-0H)]22-. Crystals of CS2[Co(nta)(J.l-OH)].4H20 crystallises in the orthorombic space group 141/a (R1= 0.0322). The Co-N bonding distance was determined as 1.922(6) A. [Co(nta)(J.l-OH)]}- undergo bridge-cleavage upon acidification with H+ ions to form [Co(nta)(H20)2]. The pKa of this reaction was determined as 3.09(3). Further acidification of [Co(nta)(H20)2] leads to the stepwise dissociation of nta. The formation of an ion associated species between [Co(nta)(H20)2] and H+ions upon addition of acid is postulated. This ion associated species dissociates in the rate determining step to form the tridentate nta complex, [Co(,,3-nta)(H20hr. The value of k1 at 25.9 °C was determined as 0.13(1) S-1. Another acid-base equilibrium is observed when the pH of a [Co(nta)(H20)2]solution is increased. It was concluded that the newly formed species is not the dimer, but rather [Co(nta)(H20)(OH)]" which reverts back to the dimer at pH 6 - 7 after several days. This second pKawas determined as 6.52(2). The substitution reactions between [Co(nta)(H20)2] and NCS- ions have been investigated, At pH = 2.00 NCS-ions substitute the aqua ligands in a stepwise fashion. The substitution of the first aqua ligand (k1= 2.4(1) x 10-2M-1 S-1at 24.7 °C) is about 120 orders of magnitude faster than the rate of substitution of the second aqua ligand (~ = 1.98(6) X 10-4M-1S-1at 24.7 °C). The [Co(nta)(H20)OHr complex reacts about 70 times faster at 24.7 "C with NCS- than [Co(nta)(H20)2] with NCS- (k2 = 1.68(5) M-1S-1vs. 2.4(1) x 10-2 M-1 S-1for k1 at 24.7 °C). This clearly indicates that the hydroxo ligand labinses the eis-aqua bond so that an increase in rate is observed. Hydroxide is not substituted by NCS- ions at higher pH so that only one reaction is observed spectrophotometrically. [Co(nta)(wOH)]i- undergo bridge cleavage at higher pH upon addition of various ligands like en, dmap or py. As a result of this several [Co(nta)(LL')] and [Co(nta)(L)2] (LL' = various N,N and N,O donors and L = dmap, py) complexes have been synthesised. The X-ray crystallographical structure determination of [Co(nta)(N,N-Et2en)] is a result of one of the synthetic studies. Crystals of [Co(nta)(N,N-Et2en)] crystallises in the orthorombic space group Pbcm (R1 = 0.0309). The Co-N bonding distance was determined as 1.950(4) A. The bridge cleavage reactions of u-hydroxo bridged Co(III)-complexes have not been studied to our knowledge. The substitution reactions between [Co(nta)(1l-0H)]l- and various ligands like dmap, py, en and N,N-Et2en have been investigated at pH 9 - 11.5. It is suggested that [Co(nta)(1l-0H)]/- equilibrates rapidly in aqueous basic solutions with a mono-u-hydroxo bridged species and that both these species react with the incoming ligand to form ion associated species (rapid) which dissociates in the rate determining step to the products. The existence of the formed mono-u-hydroxo bridged complex was confirmed by the fact that the value for the equilibrium constant, pKoH, was determined as 3.3 for all the reactions studied. This mono-u-hydroxo species is more labile towards substitution than the dimer itself as is illustrated by the fact that k1 < k2 for all the reactions studied. The values of k1 varied between 8.7(7) X 10-5 S-1 and 3.3(7) x 10-3 S-1 and those of k2 between 6.8(2) x 10-4S-1 and 5.7(2) x 10-2S-1. The synthesis and characterisation of Cs2[Co(nta)(C03)].H20 was also undertaken. This complex crystallises in the monoclinic space group P21/c (R1 = 0.0249) and can be used as an alternative to [Co(nta)(1l-0H)]l- for the synthesis of different Co(III)-nta complexes. The Co-N bonding distance was calculated as 1.920(2) A. The uncertainty surrounding the identity of the Cr(III)-nta complexes first prepared by Uehara et al. (1967:2317) have been erased with the X-ray crystal structure determination of Cs2[Cr(nta)(j.l-OH)].4H20. CS2[Cr(nta)(1l-0H)].4H20 crystallise in two different space groups, tetragonal 141/a (R1 = 0.0354) and monoclinic P21/c (R1 = 0.0354). The Cr-N bonding distances were 2.048(9) and 2.061 (3) A respectively. The strain experienced by the glycinato rings of coordinated nta decreases in the order G > R for all the complexes studied. The R rings in all the complexes are almost perfectly planar in all cases, while the G rings are non-planar.
Open Access
Chemical kinetics, electrochemistry and structural aspects of ferrocene-containing b-diketonato complexes of rhodium(I) and iridium(I)
(University of the Free State, 1999) Conradie, Jeanet
English: Synthetic routes to prepare new Rh(I)-β-diketonate complexes [Rh(FcCOCHCOR)(CO)2] and [Rh(FcCOCHCOR)(CO)(PPh3)] with Fc = ferrocenyl and R = Fc, C6H5, CH3 and CF3 have been developed and optimized. Optimized synthetic routes to iridium(I) complexes, [Ir(R'COCHCOR)(cod)], with R' = Fc and R = C6H5, CH3 and CF3, or with R' = CF3 and R = CH2CH3, CH(CH3)2 and C(CH3)3 have also been developed. 1H and 31P NMR studies indicated that for complexes of the type [Rh(β-diketonato)(CO)(PPh3)] with an unsymmetrical β-diketonato ligand, at least two main isomers exist in solution. The structure of one isomer of [Rh(fctfa)(CO)(PPh3)], as well as crystal structures of FcCOCH2COCF3, [Rh(fctfa)(CO)2] and [Rh(fctfa)(CO)(PPh3)(CH3)(I)] were solved. The chemical kinetics of the oxidative addition of iodomethane to [Rh(FcCOCHCOR)(CO)(PPh3)] has been studied utilizing IR, UV/visible, 1H NMR and 31P NMR techniques. The NMR studies revealed that the rate of oxidative addition of iodomethane to the different [Rh(FcCOCHCOR)(CO)(PPh3)] isomers was the same. A complete general reaction sequence for the oxidative addition of iodomethane to all [Rh(bidentate ligand)(CO)(PPh3)] complexes is: First set of reactionsSecond set of reactionsThird set of reactions{[Rh(III)-alkyl1] [Rh(III)-acyl1] }K2=k2/k-2Rh(I)+ CH3I k1k-1k3k-3k4k-4[Rh(III)-alkyl2][Rh(III)-acyl2] 1H and 31P NMR studies further showed that all rhodium-containing complexes in the above mentioned reaction scheme, are actually composed of at least two main isomers, that is Rh(I)A the rate of substitution becomes faster when the group electronegativity of the R groups increases. This tendency is, as expected, exactly the opposite to what was observed during oxidative addition. A general reaction mechanism for both Rh and Ir complexes was presented. An additional study on the rate of the β-diketonato substitution with 1,10-phenanthroline in complexes of the type [Ir(CF3COCHCOR)(cod)] with R = CH3, CH2CH3, CH(CH3)2 and C(CH3)3 showed that the size of R does not hamper the rate of substitution. All substitution reactions were independent of a solvent step. The cyclic voltammetry study of all the ferrocene-containing β-diketonato complexes of rhodium(I) and iridium(I) synthesized, exhibited a single electrochemically reversible redox couple corresponding to the formal reduction potential of the ferrocenyl group of the β-diketonato ligand coordinated to the rhodium or iridium complexes, as well as an electrochemically irreversible anodic oxidation peak which corresponds to the oxidation of the metal = Rh or Ir. The 31P NMR study on different six-membered chelate complexes, [Rh(L,L'-BID)(CO)(PPh3)], and related Rh(III) complexes, indicated a general decrease in coupling constants 1J(31P-103Rh) as the Rh-P bond length, determined by X-ray crystallography, increases according to the relationship d(Rh-P) = -0.0014(1) x 1J(31P-103Rh) + 2.49(2). The Rh-P bond lengths, d(Rh-P), varied between 2.23 Å and 2.36 Å. The electron density on the Rh(I) and Ir(I) metal centres was manipulated over a wide range by changing the R group on the coordinated ligand (FcCOCHCOR)- from the highly electron donating Fc group (χFc = 1.87) to C6H5, (χC6H5 = 2.21) to CH3(χCH3 = 2.34) to the strongly electron withdrawing CF3 group (χCF3 = 3.01). The effect of the different R groups on the β-diketonato ligand (FcCOCHCOR)- coordinated to the rhodium(I) and iridium(I) complexes was not only observed in kinetic rate constants, but also in formal reduction potentials of the ferrocenyl group, the oxidation potential of Rh(I) or Ir(I), pKa-values, IR stretching frequencies, and crystallographic bond lengths.
Open Access
Comparison of the influence of CeYAG and MCM-41 as nanofillers on the properties of polycarbonate and poly( methyl methacrylate)
(University of the Free State (Qwaqwa Campus), 2016-01) Sibeko, Motshabi Alinah; Luyt, A. S.
This study reports on the morphology of poly( methyl methacrylate) (PMMA) and polycarbonate (PC) filled with mesoporous silica (MCM-41) and cerium doped yttrium aluminum garnet (Ce:YAG) at contents in the range of 0.1 to 5 wt.%. The interactions between the polymer and fillers in these composites, and their thermomechanical, mechanical and thermal degradation properties were studied. The techniques used were small angle X-ray scattering (SAXS), transmission electron microscopy (TEM), CP-MAS-NMR spectroscopy, X-ray diffraction (XRD), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), impact testing and luminescence spectroscopy. The samples containing more than 0.5 wt.% of filler were Jess transparent than those containing smaller amounts, due to the presence of agglomerates. MCM- 41 particles were well dispersed at low loadings, but formed agglomerates at higher loadings, while the Ce:Y AG particles were not too well dispersed in both polymers at low loadings. Mixing with PMMA and PC did not alter the pore dimensions in the MCM-41 structure, and it maintained its hexagonal structure, even though the polymer chains partially penetrated the pores during composite preparation. Both polymers have carbonyl groups that had hydrogen bond interactions with the silanol group (Si-OH) on the surfaces of the MCM-41 particles. In the case of Ce: Y AG the interaction was through electron donor-acceptor interaction between the carbonyl oxygen lone pair in the polymers and the yttrium cation (Y3+). PMMA, however, showed a stronger interaction than PC. The addition of MCM-41 and Ce:Y AG increased the storage and loss modulus of PMMA and PC above the glass transition temperature. In the presence of MCM- 4 I the increase in modulus was due to the interaction of the polymer chains with the porous filler which restricted the mobility of the polymer chains and increased the stiffness of the composites. The impact strength of the polymer increased with the addition of MCM-41 and Ce: Y AG, but the concentration corresponding to the maximum increase depended on the type of filler. The combination of blue LEDs with the PMMA/Ce:YAG composites loaded with 5 wt.% and PC/Ce:YAG composites loaded with 2 wt.% gave off light in the white region, making them suitable for applications in white light emitting diodes.
Open Access
Computational and characterization studies of homogeneous and heterogeneous tris(beta-diketonato) complexes with catalytic applications
(University of the Free State, 2017-06) Gostynski, Roxanne; Conradie, J.; Erasmus, E.
English: Nine [Mn(β-diketonato)3] [β-diketonato = dipivaloymethanato (dpm), 1; acetylacetonato (acac), 2; benzoylacetonato (ba), 3; dibenzoylmethanato (dbm), 4; trifluoroacetylacetonato (tfaa), 5; thenoyltrifluoroacetonato (tfth), 6; trifluorofuroylacetonato (tffu), 7; trifluorobenzoylacetonato (tfba), 8 and hexaflouroacetylacetonato, 9] complexes were synthesized by adapted methods from literature. An attempt was made to graft [Mn(β-diketonato)3] complexes 2-9 onto two dimensional (2-D) Si-wafer supports (S5-S12) via a silane linker. The X-ray photoelectron spectroscopy (XPS) Mn:F atomic ratio results of fluorine containing [Mn(β-diketonato)3] complexes grafted onto amino-functionalized Si-wafer (S9-S12) gave an indicaton that the [Mn(β-diketonato)3] complexes decomposed during the grafting process. Studying Mn2O3 on the hydroxylated Si-wafer surface by the use of XPS, it was confirmed that the Mn 2p photoelectron lines observed for S5-S12 consist mostly of Mn2O3 that has no interaction with the silane linker. Eight [Mn(β-diketonato)3] complexes 1-8 was wet impregnated onto various three dimensional (3-D) solid supports (S12- S28) to form [Mn(β-diketonato)3] model catalysts. Additionally a selection of [M(acac)3] complexes ([Co(acac)3], 12; [Rh(acac)3], 13, and [Ir(acac)3], 14) were grafted onto two dimensional (2-D) Si-wafer supports (S29-S31). While [M(acac)3] complexes ([Cr(acac)3], 10; [Fe(acac)3], 11; [Co(acac)3], 12; [Rh(acac)3], 13, and [Ir(acac)3], 14) were wet impregnated onto various three dimensional (3-D) solid supports (forming S32-S36). The model 2-D and 3-D catalysts were studied and characterized by the means of XPS, TGA and computational chemistry calculations. Selected model 2-D (S5, S7, S8, S12 and S29-S31) and Mn(acac)3 immobilized onto SiO2 (3-D catalysts, S14 as is and heat treated at 100, 130, 250 and 350 °C) were tested for the catalytic self-solvating reaction between ethanol and hexamethylenediisocyanate (HDI) to form hexamethylenediurethane (HDU) to mimic the industrial production of polyurethane. The model 2-D [Mn(β-diketonato)3] (S5, S7, S8 and S12) catalysts showed that as the total group electronegativity, 3(R + R'), increase a general decrease in turnover frequency (TOF) was observed. The model 2-D [M(acac)3] catalysts (S29-S31) showed that with an increase in the metal centre’s Pauling electronegativity a general increase in TOF was observed. The catalytic test on the Mn(acac)3 immobilized onto SiO2 showed that the sample heat treated at 100°C before hand has the highest TOF, which is most probably due to the loss of only one -diketonato ligand making it more active. The chromium(0) Fischer carbene complexes (Cr-FCCs) ([Cr(CO)4(PPh3)=C(OEt)(Fu)], C1; [Cr(CO)5=C(NHCy)(Fu)], C2; [Cr(CO)5=C(NHCy)(Th)], C3 and [Cr(CO)5=C(OEt)(ThTh)], C4) study was in collaboration with the research group of Dr. M Landman at the University of Pretoria. The four Cr(0) FCCs, were obtained and characterized by electrochemistry, XPS and computational chemistry calculations in this study. Penta-carbonyl Cr-FCCs [Cr(CO)5=C(OEt)(Fu)], C5, and [Cr(CO)5=C(OEt)(Th)], C6, were also supplied and anchored onto an amino-functionalized Si-wafer to create an immobilized Cr-FCCs C7 and C8. XPS results showed that the Cr(CO)5=C(OEt)(Fu)], C5, and [Cr(CO)5=C(OEt)(Th)], C6, was successfully anchored onto an amino-functionalized Si-wafer (C7 and C8). The electrochemical study of C1-4 showed that the oxidation potential (Epa) is influenced by the varied substituents on different sites of the Cr-FCCs. The oxidation order of the Cr-FCCs is: [Cr(CO)5=C(OEt)(ThTh)], C4 > [Cr(CO)5=C(NHCy)(Th)], C3 > [Cr(CO)5=C(NHCy)(Fu)], C2 > [Cr(CO)4(PPh3)=C(OEt)(Fu)], C1. The reduction followed the same trend except for the [Cr(CO)4(PPh3)=C(OEt)(Fu)], C1, complex that is reduced at a higher potential than the aminocarbene complexes, [Cr(CO)5=C(OEt)(ThTh)], C4 > [Cr(CO)4(PPh3)=C(OEt)(Fu)], C1 > [Cr(CO)5=C(NHCy)(Th)], C3 > [Cr(CO)5=C(NHCy)(Fu)], C2. The electronic energies of the different conformations obtained by the computational study showed that certain conformations are preferred over others. The computational results were in good agreement with experimental characterization method results. Computational study of HOMO and LUMO orbitals of the Cr- FCCs confirmed that the oxidation process is metal based and the reduction is based across the carbene ligand. The correlation of the oxidation potential (Epa) of the Cr0/Cr+1 redox couple and reduction potential (Epc) of the ligand based reduction with the HOMO energy (EHOMO) and LUMO energy (ELUMO) respectively, showed that with decreasing molecular orbital energy (HOMO and LUMO) an increase to more positive Epa and Epc potentials are obtained.
Open Access
Conformational analysis of oligomeric profisetinidins
(University of the Free State, 2007-02) Potgieter, Eleonora Deborah; Ferreira, D.; Brandt, E. V.
English: The profisetinidins are an important class of condensed tannins, or proanthocyanidins. Historically, studies towards the structure and conformation of proanthocyanidins were done on their peracetate and permethyl acetate derivatives. A current upsurge in industrial and biological applications of proanthocyanidins has prompted the present efforts at detailed analysis of the conformational behaviour of the naturally occurring free phenolic oligomeric profisetinidins. Studies towards the structure and conformational analysis of a small number of free phenolic dimeric procyanidins that are 4→8 coupled and only one free phenolic dimeric profisetinidin, fisetinidol-(4α→8)-catechin, have hitherto been reported. This study centres on the use of 1H, 13C, gradient COSY, COSY 45, COSY 90W, NOESY PH and HMQC NMR experiments in different solvents and at different temperatures to assign the hydrogen and some carbon resonances of the free phenolic profisetinidins that are found in commercially important southern hemisphere trees, namely Black Wattle (Acacia mearnsii) and Quebracho (Schinopsis balansae). These results, together with data obtained from CD spectra in methanol, were then used to study the conformations of these compounds. Dimers with 2,3-trans-3,4-trans (2,4-cis) configuration, namely fisetinidol-(4α→8)-catechin, fisetinidol-(4α→6)-catechin, ent-fisetinidol-(4β→8)-catechin and ent-fisetinidol-(4β→6)- catechin all displayed sets of duplicate resonances on 1H NMR spectra, indicating the presence of rotamers on an NMR time-scale at ambient temperatures. The proton resonances of the rotamers of the 4→6 linked dimers displayed insignificant chemical shift differences due to the similar magnetic environments and linear shape of both rotamers. The proton resonances of the rotamers of the 4→8 linked dimers displayed significant chemical shift differences due to the presence of compact and extended rotamers resulting in large changes in magnetic environment due to anisotropic effects. The type of solvent, temperature as well as the relative presence of water or cadmium nitrate had a strong influence on the relative concentrations of the rotamers, the conformations of the heterocyclic C- and F-rings as well as the visibility of hydroxy groups. The F-rings all displayed A/E- conformational exchange with line shapes indicating possible skewed boat conformations in some instances. The C-ring conformations ranged from rings with A/E conformational exchange to preferred E-conformers. The dimers with 2,3-trans-3,4-cis (2,4-trans) configuration, namely fisetinidol-(4β→8)- catechin, fisetinidol-(4β→6)-catechin and ent-fisetinidol-(4α→8)-catechin displayed only one set of resonances on 1H NMR spectra at ambient temperatures. The presence of intramolecular hydrogen bonding and limited conformational exchange was confirmed by the following observations: a) Selective broadening of proton resonances in both the heterocyclic and aromatic regions of 1D NMR spectra. b) Sharpening of resonances in 1D spectra at elevated temperatures. c) The presence of abundant coupling between heterocyclic and aromatic ring protons as observed on 2D spectra. d) Coupling between 2-HC and 4-HC on 2D NMR spectra. The C-rings had preferred A-conformations, with the F-rings displaying A/E conformational exchange with line shapes indicating possible skewed boat conformations in some instances. 2D NMR experiments afforded estimations, in some cases, of the angles between the plane of the B-ring and the 2-CC→2-HC bond, the plane of the D-ring and the 4-CF→4-HF bond as well as the plane of the D-ring and the 4-CC→4-HC. The resonances of 4-CC of the 2,3-trans-3,4-trans dimers displayed significant chemical shift differences (± 41 ppm) compared to 4-CC of the 2,3-trans-3,4-cis dimers (± 31 ppm).This could serve as a possible indicator of the relative configurations of the C-rings of 2,3-trans profisetinidins dimers. CD studies of al seven abovementioned dimers, as well as four trimers from Acacia mearnsii displayed complex curves with a number of strong Cotton effects. Although some trends were observed, it was abundantly evident that the chiroptical characteristics of this class of compounds are too complex to be interpreted in terms of the empirical quadrant rule.
Open Access
The constitution of oligomeric benzofuranoids
(University of the Free State, 1999-05) Bekker, Riaan; Brandt, E. V.; Ferreira, D.
English: Berchemia zeyheri is known for its unique red heartwood, a property that was probably responsible for the first phytochemical investigation into the flavonoid content of this tree. The heartwood contains a unique series of biflavonoids with one of more benzofuranoid moieties. These are usually found in diastereomeric mixtures, the biogenetic origin and stereochemistry of which have hitherto been unknown. This investigation thus represents a renewed effort to solve some of the intricate problems associated with these compounds. The high concentration of maesopsin in the heartwood made extensive enrichment and fractionation by the use of Craig countercurrent distribution techniques and Sephadex LH- 20 gelchromatography necessary. The two diastereomers of 4',5, 7-tri-O-methylnaringenin-(3a~ 7)-2,4,4',6-tetra-Omethylmaesopsin were, for the first time, successfully isolated and separated. Reduction of these diastereomers with Na(CN)BH3 gave two enantiomeric pure fragments. The conformations of the heterocyclic rings of these fragments were established by molecular mechanics (MMX and GMMX) and semi-empirical methods (AMI). These results allowed the absolute configuration of the fragments to be deduced from CD-curves of the compounds by application of Snatzke's rule for a,~-unsaturated five-membered cyclic rings. A n.O.e. correlation observed for one of the diastereomers only, correlates the stereocenter of the maesopsin moiety, of known absolute configuration, with a specific configuration of the naringenin unit, thus defining the absolute configuration of the dimer. These results also allowed the determination of the absolute stereochemistry of two regioisomers of the above dimers, 4',5, 7-tri-O-methylnaringenin-(3a~5)-2,4,4',6-tetra-Omethylmaesopsin and its epimer. The 13CNMR spectra of these related dimers were also studied and fully elucidated by means of HMQC and HMBC experiments. The structure and stereochemistry of two novel isoflavanone-benzofuranone biflavonoids, 4',5,7-tri-Omethyldihydrogenistein-fêcc-» 7)-2,4,4',6-tetra-O-methylmaesopsin and its epirner, were similarly determined. Resolution of maesopsin, the main metabolite in the heartwood, by means of HPLC using a chiral column, for the first time gave access to the two enantiomers of this benzofuranoid. 4,4',6- Tri-O-methyl-2-deoxymaesopsin-(2~ 7)-2,4,4',6-tetra-O-methyl-maesopsin and its epimer, consist of two benzofuranoid constituent units. An X-ray crystal structure was obtained for the one diastereomer, but due to the presence of a symmetric Pbea point group, only the relative configuration could be determined. After the racemic nature of each of the diastereomers was determined, each epimer was resolved with HPLC into its constituent enantiomers. The information obtained from the CD curves and crystal structure allowed the determination of the absolute stereochemistry of each of the enantiomers. Four further epimeric biflavonoids were isolated as the hepta-O-methyl ethers. Evidence obtained from 13C NMR data suggested the presence of a y-lactone functionality in the upper benzofuranoid moiety, identifying the dimers as the epimers of 4,6-dimethoxy-3-( 4- methoxy-benzyl)benzo[b ]furan-2(3H)-one-(2~5)-2,4,4',6-tetra-O-methylmaesopsin and the (2~ 7)-coupled regio-isomer. In order to supplement the above data, an asymmetric synthesis of maesopsin was attempted. The first attempt involved the oxidation of 2-( 4-methoxybenzyl)-4,6- dimethoxybenzo[b ]furan-3(2H)-one, obtained by reduction of the corresponding aurone, with AD-mix-a, a stereoselective catalyst, or chiral oxaziridine. The former afforded the desired product in low yield but no stereo selectivity while the latter method realized a much-improved yield, but still with no selectivity. This lack of selectivity is attributed to equilibrium of the product with the a-diketone. Attempts to prevent the formation of this equilibrium product were unsuccessful. A second synthetic attempt involved benzylation of 2,4,6-trimethoxybenzo[b]furan-3-(2H)-one with (-)-sparteine as chiral auxiliary, but again resulted in high yields but no stereo selectivity.
Open Access
Coordination chemistry and solution behaviour of gold (I) complexes
(University of the Free State, 2007-11) Sam, Zolisa Agnes; Roodt, A.
English: The aim of this study was to extend the knowledge base of gold(I) coordination chemistry and investigate the substitution behaviour of these complexes with sulphurdonor ligands. The water-soluble and air-stable ligand 1,3,5-triaza-7- phosphatricyclo[3.3.1.13,7]decane (PTA) with low steric demand was employed in the synthesis of the various complexes in this study. The [Au(PTA)Cl] complex was reacted with S-donor ligands such as SCN¯, thiourea and methyl thiourea and the equilibrium constant determination was done using 31P NMR by monitoring the chemical shift change when stoichiometric amounts of the ligand are added to the [Au(PTA)Cl] solution. The equilibrium constants obtained were 0.070(6), 4.191(1) and 6.734(3) for SCN¯, thiourea and methyl thiourea as entering ligands, respectively. The X-ray crystal structure of the inclusion of the PTA ‘guest’ molecule into the ‘host’ b- cyclodextrin (b-CD) was determined. The PTA-b-CD·8H2O inclusion compound crystallises in the monoclinic space group P21 with eight solvent water molecules in an asymmetric unit and was refined to a final R value of 4.35%. The packing within the PTA-b-CD·8H2O inclusion compound is of a ‘herring-bone’ type motif. An attempt to include auranofin {[Au(PEt3)(Sgluc)]; Sgluc = thioglucose} into the cavity of the b- cyclodextrin resulted in only auranofin crystallising without being included. However, since the data collection for the auranofin compound was done at 100 K, careful observations in parameters such as selected torsion and bond angles were noted to have 1-4° changes as compared to the known auranofin structure investigated at room temperature, which clearly indicated a phase change and a new polymorph at 100 K. The pure auranofin compound investigated in this study crystallises in the monoclinic space group P21 and was refined to a final R value of 1.59%. Further study of the interactions of b-cyclodextrin with PTA and related ligands and gold(I) complexes in this study was investigated with NMR spectroscopy. This was done by the determination of the equilibrium constant when these complexes are included into the b-cyclodextrin. The equilibrium constants calculated for PTA, (PTAMe)+ and PPh3 were 8.7(1) x 102 and 23(4) and 10(6) M-1, respectively, while for the [Au(PTAMe)Cl] and [Au(dppe)2]Cl equilibrium constants of 23(5) and 6(1) M-1 were obtained. The PTA ligand clearly showed the largest ‘host-guest’ stability. The solubility of the phenyl compounds, following the inclusion, was not improved that much as compared to the PTA compounds which may be due to steric hindrance and orientation of the phenyl groups being too large to be incorporated into the b-cyclodextrin. This phenomenon is also noted in the unsuccessful incorporation of auranofin which may be due the orientation of the ethyl groups into b-cyclodextrin. In an attempt to increase the solubility of these gold(I) complexes, bidentate P-donor ligand systems with a bridging ferrocene group, functionalised by hydrophilic moieties, were synthesised. These complexes were unambiguously characterised by X-ray crystallography. The following dinuclear gold(I) crystal structures are reported, with their general crystal data reported in parenthesis: [(AuCl)2(m-dppf-CH(CH3)N(CH3)2)] (Monoclinic, P21/n, R = 4.94%) [(AuSCN)2(m-dppf-CH(CH3)N(CH3)2)] (Triclinic, P 1 , R = 5.76%) [(AuCl)2(m-dppf-CH(CH3)OAc)] (Orthorhombic, Pbca, R = 4.09%) [(AuSCN)2(m-dppf-CH(CH3)OAc)] (Triclinic, P 1 , R = 4.36%) [(AuSCN)2(m-dppf)] (Monoclinic, C2/c, R = 2.36%) The SCN¯ ligand in the thiocyanato gold(I) dppf structures coordinated to the soft Au(I) metal centre via the softer S atom. An interesting factor was the isomorphism identified for the two [(AuSCN)2(m-dppf-CH(CH3)N(CH3)2)] and [(AuSCN)2(m-dppf-CH(CH3)OAc)] structures while the structure for the [(AuSCN)2(m-dppf)] compound was interlinked by short Au…Au contacts of 2.9798(7) Å of which none were observed for the other structures. Auranofin and its derivative compounds such as PTA-auranofin, PTAMe-auranofin and triethylarsine-auranofin were utilised and tested for biological activity against cancer cell lines. Furthermore, a preliminary chemiluminescence assay was done with the compounds at three different concentrations (0.3, 3.1 and 12.5 mM for each compound) to determine their effect on the chemiluminescence of isolated blood neutrophils. The auranofin compound was included in the investigations as reference. The A2780 human ovarian cancer cell lines are the most sensitive to all derivatives while the arsine-auranofin compound showed good activity against A2780 human ovarian cancer cell lines with an IC50 of only about 0.0076 mg/mL. Generally, auranofin and arsineauranofin gave results closely related to each other with arsine-auranofin having higher toxicity to other cells whereas PTA-auranofin and PTAMe-auranofin showed more correlation to each other and had less activity. For the preliminary chemiluminescence assays it can be mentioned that for all auranofin derivatives, at low concentrations the compounds act as stimulants to the neutrophil chemiluminescence activity and at higher concentrations the compounds act as inhibitors to neutrophil activity. Complex substitution behaviour was observed for selected gold(I) dinuclear {[(AuCl)2(m- dppf-CH(CH3)OAc)]} and mononuclear systems {[Au(PX)Cl]; X = Ph3 or Ph2Fc} when the chloride is substituted with ligands such as L-cysteine and SCN¯ as studied by UVVis and 31P NMR. Furthermore, fast reaction kinetics for the chloride substitution with ligands such as SCN¯ and dimethylthiourea from the mononuclear [Au(PPh3)Cl] complex was investigated with stopped-flow techniques. The overall rate constants for the substitutions from [Au(PPh3)Cl] with SCN¯ and dimethyl thiourea representing the forward reactions were obtained as k1 = 13(1) and 2.17(1) x 103 M-1s-1 respectively. Thus, it was concluded that chloride substitution reactions on linear gold(I) systems are extremely fast reactions.
Open Access
A crystallographic and mechanistic investigation of rhenium (I) tricarbonyl complexes as model radiopharmaceuticals
(University of the Free State, 2011-11) Brink, Alice; Roodt, Andreas; Visser, Hendrik G
English: Rhenium(I) and technetium(1) tricarbonyl complexes hold significant potential as model radiopharmaceuticals which could be utilized as therapeutic and imaging agents. 99mTechnetium in particular, is one radionuclide which is used in more than 80% of radiopharmaceuticals as a diagnostic agent. The biodistribution of a potential radiopharmaceutical can in principle be manipulated by the coordination of a biologically active molecule to the radionuclide. The principle aim of this study was to gain further insight into the chemistry, coordination and kinetic behaviour of fac-[M(CO)3+ (M = Tc, Re) complexes. With this idea in mind, a range of salicylidene ligands, SaIH, were synthesised according to the Schiff-base reaction. These organic ligands were synthesized as potential bifunctional chelators between the tricarbonyl radionuclide and the biologically active directing molecule. The ligands contain various amine compounds which were coordinated to the imine nitrogen atom on the salicyclidene "backbone". The imino substituents consisted of aromatic, aliphatic and biologically active moieties with varying steric, electronic and biological properties and included amines such as m-toluidine, 3-methylbutylamine, aniline, histamine, tryptamine, tyramine etc. The ligands were characterized via NMR and IR spectroscopy. A single crystal X-ray diffraction study of the ligands was reported and revealed the various orientations of the substituents relative to the salicylidene backbone. The reported X-ray crystallographic structure determinations included the following ligands: 2-(m-tolyliminomethyl)phenol, 5- methyl-2-(m-tolyliminomethyl)phenol, 4-fluoro-2-(m-tolyliminomethyl)phenol, 2-(4-nitrophenyliminomethyl) phenol, 2-[(4-hydroxyphenyl)iminomethyl]-5-methylphenol, 2-[(2- imidazol-4-yl)ethyliminomethyl]-5-methylphenol, 2-[(2-indol-3-yl-ethyl)iminomethyl]-5- methylphenol, 2-(9-ethylcarbazol-3-yliminomethyl)-5-methylphenol, 2-[2-(4-hydroxy- phenyl)ethyliminomethyl]-5-methylphenol, 5-methyl-2-(1,2,4-triazol-3-yliminomethyl)-phenol. A further important part of this investigation was concerned with the synthesis and evaluation of the solid state characteristics of the SalH ligands coordinated to the rhenium(I) metal centre. The advantage of the fac-[M'(CO)3(S)3]+complex (M = Tc or Re; S = H20 or other coordinated solvents) is the high stability of the classic low spin d6 [M(CO)3]+ core in water and the potential of exchanging the labile solvent ligands. The SalH bidentate ligands were bonded to the fac-[M(CO)3]+ core according to the [2+1] approach, thus leaving an 'open' third position occupied by either a solvent or by a neutral monodentate ligand. The complexes were also characterized via NMR and IR spectroscopy. The solid state behaviour of these fac-[Re(Sal)(CO)3(S)] complexes were investigated by X-ray crystallography, and included the complexes: fac-[Re(Sal-mTol)(CO)3(HOCH3)], fac-[Re(Sal- Ph)(C0)3(HOCH3)],fac-[Re(Sal-pTol)(CO)3(HOCH3)],fac-[Re(Sal-CyHex)(C0)3(HOCH3)], fac-[Re(Sal-3MeBu)(CO)3(HOCH3)], fac-[Re(Sal-Ph)(CO)3(NCsHs)], fac-[Re(Sal- 3MeBu)(CO)3(NCsHs)], fac-[Re(5Me-Sal-Hist)(CO)3], fac-[Re(5Me-Sal-Trypt)(CO)3 (NCsHs)], fac-[Re(5Me-Sal-Carba)(CO)3(NCsHs)], fac-[Re(5Me-Sal-mTol)(C0)3(HOCH3)], fac-[Re(5Me-Sal-3Me2Bu)(CO)3(HOCH3)]. The coordination geometry around the Re(l) metal centre in the crystal structures was a distorted octahedron. The imino substituents crystallize in similar orientations despite various steric sizes, while the bond angles and lengths were not significantly affected by the different nitrogen-coordinated substituents. The study of the solid state coordination of the complexes was further supplemented with a theoretical DFT (density functional theory) study for specifically the 2-(mtolyliminomethyl) phenol, 5-methyl-2-(m-tolyliminomethyl)phenol, 2-(9-ethylcarbazol-3- yliminomethyl)-5-methylphenol,fac-[Re(Sal-mTol)(CO)3(S)],fac-[Re(Sal-Ph)(COh(S)],fac- [Re(5Me-Sal-Carba)(CO)3(S)] compounds. The comparison between the optimised structure and the crystal data reveal small differences, illustrating that predictions can be made in terms of the coordination of the bidentate ligands to the rhenium metal centre utilising DFT techniques. A kinetic study of substitution of the coordinated solvent ligand of fac-[Re(Sal)(CO)3(S)] complexes was done. The following complexes were evaluated: fac-[Re(SalmTol)( CO)3(HOCH3)], fac-[Re(Sal-pTol)(CO)3(HOCH3)], fac-[Re(Sal-Ph)(CO)3(HOCH3)], fac-[Re(Sal-3MeBu)(CO)3(HOCH3)], fac-[Re(Sal-CyHex)(CO)3(HOCH3)] for entering ligands 3-chloropyridine, pyridine, 4-picoline and 4-dimethylaminopyridine in methanol as solvent. The rates were quite fast and varied from seconds to minutes under the conditions studied. Only one reaction was observed under dry conditions. The Sal-Re ligand systems allowed the unique opportunity to confirm non-associative behaviour since for selected ligands, pyridine and DMAP, limiting kinetic profiles were obtained; clear evidence of either I or D intimate mechanisms. An interchange dissociative (Id) mechanism was proposed for the substitution reaction with positive activation entropy and enthalpy values. An in vitro cancer screen was conducted on selected non-coordinated ligands and fac- [Re(Sal)(CO)3(S)] complexes in a 3-ce]] line panel consisting of TKlO (renal), UACC62 (melanoma) and MCF7 (breast) cancer cells using a Sulforhodamine B (SRB) assay. No significant cell activity was found but limited cell growth inhibition was observed.
Open Access
A crystallographic investigation of multidentate ligand hafnium(IV) halido complexes
(University of the Free State, 2021-11) Viljoen, Johannes Augustinus; Roodt, Andreas; Visser, Hendrik G.
Hafnium, the chemical element with atomic number 72 in the titanium-triad on the periodic table, was first discovered by Dirk Coster and George von Hevesy in 1923 by analysing X-Ray spectra of Norwegian zircon samples (sea sand). Hafnium is considered relatively abundant and comprises about three parts per million by weight of the earth's crust. Unfortunately, hafnium only occurs naturally in zirconium ores at an approximate ratio of 1:50 and has almost identical general chemical properties to that of zirconium. Zirconium has a very desirable application in the nuclear industry as a cladding material for nuclear fuel rods due to its low affinity for thermal neutrons. However, for zirconium to be used as a cladding material in nuclear reactors, it must be virtually hafnium free since hafnium has a 600 times larger affinity for thermal neutrons than that of zirconium, turning it into a “poison” in zirconium cladding materials. However, this high affinity for thermal neutrons is exactly why pure Hf makes an excellent shielding material. Therefore, for hafnium and zirconium to be utilised in the nuclear industry, it is apparent why the effective and economically viable separation of these metals to their pure chemical state is vital. The principle aim of this study is focused on studying the solid-state and solution behaviour of hafnium(IV), with various N- and O-donating multidentate ligand systems, in order to obtain valuable new data that could potentially be used to compare to zirconium(IV) to evaluate if metal separation is feasible for industrial applications. Therefore, in this study, the solid-state and solution behaviour of novel hafnium(IV) and zirconium(IV) coordination compounds, obtained from a systematic range of N- and O- bi- and multidentate ligands, is investigated. A detailed description of the synthesis of the hafnium(IV) and zirconium(IV) complexes containing different N,O- and O,O’-bidentate ligand systems as well as N,O,O,N- and N,N’,N’,N-tetradentate ligands are reported and characterised by means of IR, UV/Vis and NMR (1H and 13C) spectroscopies. Moreover, it is clearly illustrated by this study that it is possible to synthesise hafnium(IV) and zirconium(IV) complexes under simple bench-top aerobic conditions. Furthermore, the solid-state structural characterisation by means of single crystal X-Ray Diffraction studies of ten of these synthesised complexes is described in detail. Thus, six novel hafnium(IV)-oxinato complexes containing a systematic range of substituents on the outer periphery of the ligand are firstly presented: (1) [Hf(Ox)4]·2(C7H8), (2) [Hf(5-ClOx)4]·2(C7H8), (3) [Hf(diMeOx)4]·2(DMF), (4) [Hf(diClOx)4]·3DMF, (5) [Hf(diBrOx)4]·3DMF and (6) [Hf(diIOx)4]·DMF and described. The complexes contain non- and substituted 8-hydroxyquinoline (OxH) ligands [with (5-ClOxH) = 5-chloro-8-hydroxyquinoline, (diMeOxH) = 5,7-dimethyl-8-hydroxy-quinoline, (diClOxH) = 5,7-dichloro-8-hydroxyquinoline, (diBrOxH = 5,7-dibromo-8-hydroxyquinoline, (diIOxH) = 5,7-diiodo-8-hydroxyquinoline], and are discussed and compared with regard to the intimate geometric environment around the hafnium(IV) metal centre. It is clearly illustrated by this study that tetrakis-(oxinato)hafnium(IV) complexes prefer the square-antiprismatic coordination geometry in the D2-corner clipped isomer form. It is further shown that all the hafnium(IV) -and zirconium(IV) oxine complexes are very similar with relation to bond lengths, angles and packing modes. Secondly, two new hafnium(IV)-β-diketonato complexes – (7) [Hf(dbm)4] and (8) [Hf(OH)(tfba)3]2·2DMF – are structurally evaluated and described [where (dbmH) = dibenzoylmethane and (tfbaH) = 4,4,4-trifluoro-1-phenyl-1,3-butanedione]. Focus is placed on the influence of ligands when increasing the metallocycle from five (oxine-type ligands) to six (acac-type ligands) and using acac-type ligands containing different electron-donating properties. The effect of electron-withdrawing substituents on the ligand backbone with regards to coordination mode and coordination geometry. These examples show and conclude that hafnium-β-diketonato complexes also prefer a square-antiprismatic coordination geometry in the D2-corner clipped isomer form. Moreover, all the hafnium(IV)- and zirconium(IV)-β-diketonato complexes are very similar with respect to their general geometry and corresponding bond lengths, angles and packing modes. Similar to the oxine-type complexes, this suggests that hafnium(IV) and zirconium(IV) has a certain preference for the chelation sites of the coordinating atoms, regardless of the steric or electronic properties of the ligands as a whole. Thirdly, a novel hafnium(IV) and zirconium(III) complex containing a N,O,O,N- and N,N’,N’,N-tetradentate ligand, respectively – (9) [Hf(salophen)2·2(C7H8), (10) [Zr(pbp)(H2O)2]2NO3·(MeOH) – are structurally evaluated and compared with regard to the intimate geometric environment around the metal centres [where (salophen) =N,N'-bis(salicylidene)-1,2-phenylene-diamine & (pbpH) = N,N'-(1,2-phenylene)bis-(pyridine-2-carboxamide)]. It was found that the eight coordinated hafnium(IV) complex favours a square-antiprismatic coordination polyhedron. However, the six coordinated zirconium(III) complex formed a distorted octahedron as coordination polyhedron around the metal centre, as expected for many six coordinated metal complexes. Finally, preliminary liquid-liquid extraction experiments are described herein, and it is concluded to in principle be considered a preliminary proof-of-concept for separation. The preliminary extraction investigation reported includes liquid-liquid extractions of zirconium- and hafnium-oxine complexes from methanol into dichloromethane and competition extractions, as qualitatively evaluated by (I) UV-Vis spectroscopy and (II) inductively coupled plasma optical emission spectroscopy.
Open Access
A crystallographic, computational and mechanistic study of rhodium enaminoketonado complexes
(University of the Free State, 2013-01) Venter, Gertruida Jacoba Susanna; Roodt, Andreas; Steyl, Gideon
English: This study includes the investigation of enaminoketones as ligand systems in rhodium complexes with possible future application in catalysis. In order to evaluate the influence of substituents on the phenyl ring on activity of the complex, a range of 4-(phenylamino)pent-3-en-2-onate (PhonyH) derivatives with chloride substituents on different positions on the phenyl ring were synthesized and characterized through X-ray crystallography as well as infrared and NMR spectroscopy. The compounds crystallize in a range of space groups and varying crystal systems, are stable in air over a period of several years and soluble in most solvents. The optimized structures of these compounds were calculated using DFT methods. The relative energies of the optimized structures adopt a cumulative nature ‒ the relative energy of 2,4-Cl2-PhonyH with regard to unsubstituted PhonyH is roughly equal to the sum of the relative energies of 2-Cl- PhonyH and 4-Cl-PhonyH, while the relative energy of 2,6-Cl2-PhonyH is twice the relative energy of 2-Cl-PhonyH. The distortion of the phenyl ring from the ideal planar position presented in the calculated structures corresponds to the distortion observed in the solid state. The synthesis of the uncoordinated compounds was followed by the synthesis and characterization of a range of substituted dicarbonyl-[4-(phenylamino)pent-3-en-2-onato]- rhodium(I) complexes. The complexes crystallized in varying crystal systems and space groups. The trans influence of nitrogen was confirmed through the difference in the Rh-CO bonds: the Rh-C bond trans to the nitrogen atom is longer than the Rh-C bond trans to oxygen. The impact of the chloride substituents was observed from differences in geometrical parameters and is supported by information from the calculated structures and literature. The optimized structures of these complexes were calculated using DFT methods, and their optimized energies follow the same cumulative trend as observed in the uncoordinated compounds. A range of carbonyl-[4-(phenylamino)pent-3-en-2-onato]-triphenylphosphine-rhodium(I) {[Rh(N,O-Bid)(CO)(PPh3)]} complexes were synthesized and characterized, containing both electron-withdrawing chloride atoms and electron-donating methyl groups. These complexes displayed poor solubility, but once dissolved, were stable over a period of several months. Isomorphism was observed between [Rh(2,6-Cl2-Phony)(CO)(PPh3)] and [Rh(2,6-Me2- Phony)(CO)(PPh3)]. [Rh(2,6-Cl2-Phony)(CO)(PPh3)] and [Rh(2,6-Me2-Phony)(CO)(PPh3)] were chosen to investigate the exchange of triphenylphosphine coordinated in [Rh(N,O-Bid)(CO)(PPh3)] complexes with the uncoordinated phosphine, allowing for the comparison of the electronic effect of the substituents on the phenyl rings. The method chosen for the investigation was magnetization spin transfer, an NMR technique which utilizes the magnetic properties of nuclei and determines the kinetic properties of the exchange reaction by following the rate at which magnetic equilibrium is restored. The rate of the phosphine exchange reaction in [Rh(2,6-Cl2-Phony)(CO)(PPh3)] was determined as approximately three times faster than the rate of reaction for phosphine exchange in [Rh(2,6- Me2-Phony)(CO)(PPh3)]. The decreased electron density surrounding the rhodium atom in [Rh(2,6-Cl2-Phony)(CO)(PPh3)] allows for the reversal of the reaction as indicated by the k-1 values of approximately 11 s-1 calculated from the [Rh(2,6-Cl2-Phony)(CO)(PPh3)] exchange reaction. This value is absent in the reaction of the [Rh(2,6-Me2-Phony)(CO)(PPh3)] complex. The activation parameters of the exchange reaction in [Rh(2,6-Cl2-Phony)(CO)(PPh3)] (ΔH‡ = 25(3) kJ.mol-1 and ΔS‡ = -117(9) J.K-1.mol-1) correlate well with the parameters of the exchange reaction in [Rh(2,6-Me2-Phony)(CO)(PPh3)] (ΔH‡ = 24(4) kJ.mol-1 and ΔS‡ = -124(12) J.K-1.mol- 1). In both cases the value for entropy, ΔS‡, is negative, indicating an associative mechanism. The relative contribution of TΔS‡ to ΔG‡ is approximately 60% for both complexes, whereas the enthalpy (ΔH‡) terms are correspondingly small. This indicates that the activation process is primarily controlled by entropy and involves the formation of a stable, well-ordered transition state while bond weakening is less important. The relatively constant values for ΔG‡ imply that the exchange reaction is not very sensitive to changes in temperature.
Open Access
A crystallographic, computational and mechanistic study of rhodium enaminoketonato complexes
(University of the Free State, 2013-01) Venter, Gertruida Jacoba Susanna; Roodt, Andreas; Steyl, Gideon
English: This study includes the investigation of enaminoketones as ligand systems in rhodium complexes with possible future application in catalysis. In order to evaluate the influence of substituents on the phenyl ring on activity of the complex, a range of 4-(phenylamino) pent-3-en-2-onate (PhonyH) derivatives with chloride substituents on different positions on the phenyl ring were synthesized and characterized through X-ray crystallography as well as infrared and NMR spectroscopy. The compounds crystallize in a range of space groups and varying crystal systems, are stable in air over a period of several years and soluble in most solvents. The optimized structures of these compounds were calculated using DFT methods. The relative energies of the optimized structures adopt a cumulative nature - the relative energy of 2,4-C-PhonyH with regard to unsubstituted PhonyH is roughly equal to the sum of the relative energies of 2-CIPhonyH and 4-CI-PhonyH, while the relative energy of 2,6-Cb-PhonyH is twice the relative energy of 2-CI-PhonyH. The distortion of the phenyl ring from the ideal planar position presented in the calculated structures corresponds to the distortion observed in the solid state. The synthesis of the uncoordinated compounds was followed by the synthesis and characterization of a range of substituted dicarbonyl-[ 4-(phenylamino )pent-3-en-2-onato]- rhodium(I) complexes. The complexes crystallized in varying crystal systems and space groups. The trans influence of nitrogen was confirmed through the difference in the Rh-CO bonds: the Rh-C bond trans to the nitrogen atom is longer than the Rh-C bond trans to oxygen. The impact of the chloride substituents was observed from differences in geometrical parameters and is supported by information from the calculated structures and literature. The optimized structures of these complexes were calculated using OFT methods, and their optimized energies follow the same cumulative trend as observed in the uncoordinated compounds. A range of carbonyl-[4-(phenylamino) pent-3-en-2-onato ]-triphenylphosphine-rhodium(l) {[Rh(N,O-Bid)(CO)(PPh3)]} complexes were synthesized and characterized, containing both electron-withdrawing chloride atoms and electron-donating methyl groups. These complexes displayed poor solubility, but once dissolved, were stable over a period of several months. Isomorphism was observed between [Rh(2,6-Ch-Phony)(CO)(PPh3)] and [Rh(2,6-Me2- Phony)(CO)(PPh3)] . [Rh(2,6-Ch-Phony)(CO)(PPh3)] and [Rh(2,6-Me2-Phony)(CO)(PPh3)] were chosen to investigate the exchange of triphenylphosphine coordinated in [Rh(N,O-Bid)(CO)(PPh3)] complexes with the uncoordinated phosphine, allowing for the comparison of the electronic effect of the substituents on the phenyl rings. The method chosen for the investigation was magnetization spin transfer, an NMR technique which utilizes the magnetic properties of nuclei and determines the kinetic properties of the exchange reaction by following the rate at which magnetic equilibrium is restored. The rate of the phosphine exchange reaction in [Rh(2,6-Ch-Phony)(CO)(PPh3)] was determined as approximately three times faster than the rate of reaction for phosphine exchange in [Rh(2,6-Me2-Phony)(CO)(PPh3)]. The decreased electron density surrounding the rhodium atom in [Rh(2,6-C12-Phony)(CO)(PPh3)] allows for the reversal of the reaction as indicated by the k-, values of approximately 11 s-1 calculated from the [Rh(2,6-Cl2Phony)(CO)(PPh3)] exchange reaction. This value is absent in the reaction of the [Rh(2,6-Me2Phony)(CO)(PPh3)] complex. The activation parameters of the exchange reaction in [Rh(2,6-C12-Phony)(CO)(PPh3)] (∆H‡ = 25(3) kJ.mol-l and ∆S‡ = -117(9) J.K-l.mol-l) correlate well with the parameters of the exchange reaction in [Rh(2,6-Me2-Phony)(CO)(PPh3)] (∆H‡ = 24(4) kJ.mol-l and ∆S‡ = -124(12) J.K-I.mol-1I). In both cases the value for entropy, ∆S‡, is negative, indicating an associative mechanism. The relative contribution of T∆S‡ to ∆G‡ is approximately 60% for both complexes, whereas the enthalpy (∆H‡) terms are correspondingly small. This indicates that the activation process is primarily controlled by entropy and involves the formation of a stable, well-ordered transition state while bond weakening is less important. The relatively constant values for ∆G‡ imply that the exchange reaction is not very sensitive to changes in temperature.
Open Access
Development of bionanocomposites based on PCL/PBS double crystalline blends and carbon nanotubes
(University of the Free State, 2018) Gumede, Thandi Patricia; Luyt, A. S.; Muller, A. J.
𝑬𝒏𝒈𝒍𝒊𝒔𝒉 The main purpose of this research was to use commercially available crystalline biobased polymers, namely poly(ε-caprolactone) (PCL) and poly(butylene succinate) (PBS), and introduce polycarbonate (PC)/multi-walled carbon nanotubes (MWCNTs) masterbatch into these polymers in order to provide additional functionalities, especially those associated with electronic applications. All the samples were prepared through melt- mixing in a twin-screw extruder. According to our results, the PCL/PBS blends showed a sea-island morphology with discrete droplets of the minor phase within the matrix of the major phase, indicating immiscibility. The introduction of the PC/MWCNTs masterbatch to PCL, PBS and the PCL/PBS blends showed partial miscibility, where PC-rich, PCL-rich and PBS-rich phases were formed. A number of MWCNTs diffused from the PC-rich phase to the PCL- rich and the PBS-rich phases, although the MWCNTs were mostly agglomerated in the PC-rich phases. However, the extent of partial miscibility was different for each system. The polar component surface energy, interfacial tension and isothermal crystallization results suggested that the MWCNTs would preferably diffuse into the PBS-rich phase, rather than the PCL-rich phase. Standard DSC measurements for the PCL/PBS blends, PCL/(PC/MWCNTs) and PBS/(PC/MWCNTs) nanocomposites demonstrated nucleation effects. In the PCL/PBS blends, nucleation was ascribed to (1) transference of the impurities from the PCL phase to the PBS phase, and (2) since the PBS crystallizes first, the PCL droplets may have crystallized by surface induced nucleation on the interface with the PBS crystallized matrix and nucleate at the interphase. In the case of the nanocomposites, the nucleation effect was attributed to the MWCNTs that diffused from the PC-rich to the PCL-rich and PBS-rich phases, even though the nucleating efficiency was lower than reported in the literature, which probably was due to the limited phase mixing between the PC-rich, the PCL-rich and PBS-rich phases. For the PCL/PBS/(PC/MWCNTs) nanocomposites, there was a decrease in the Tc values. This was due to the competition between two effects: (1) the partial miscibility of the PC-rich with the PCL-rich and PBS-rich phases, and (2) the nucleation effect of the MWCNTs. The decrease in the Tc values indicated that miscibility was the dominating effect. Isothermal crystallization experiments performed by DSC showed an increase in the overall crystallization rate of PCL with increases in MWCNTs contents, which was the result of their nucleating effect. However, for the PBS/(PC/MWCNTs) nanocomposites, the crystallization rate increased up to 0.5 wt% MWCNTs, while further increases in MWCNTs loading (and also in PC content) resulted in progressive decreases in crystallization rate. The results were explained through increased MWCNTs aggregation and reduced diffusion rates of PBS chains, as the masterbatch content in the blends increased. In the case of the PCL/PBS/(PC/MWCNTs) nanocomposites, the overall crystallization rates decreased as a result of the competition between the nucleating effect and miscibility. Since the PC-rich phase is partially miscible with the PCL-rich and PBS-rich phases, the PC probably immobilized the PCL and PBS chains and inhibited the rate of crystallization. The thermal conductivities and mechanical strengths of the nanocomposites were generally enhanced compared to those of the neat material. The nanocomposites prepared in this work could be used in applications where electrical conductivity, as well as low weight and tailored mechanical properties, are required. ___________________________________________________________________
Open Access
Diolefin complexes of transition metals as 'venus fly-trap' templates
(University of the Free State, 2011-06) Hill, Tania Nicola; Roodt, A.; Steyl, G.
English: The aim of this study was to gain a further insight into the bonding modes of 1,5-cyclooctadiene to various middle to late transition metal centres. The intension was to design a theoretical model of the influences of the variations of the metal centres and ligands on the “Venus fly-trap” system. Two principle geometric features were classified in the 1,5- cyclo-octadine when co-ordinated, i.e. the (bite) angle of the olefinic moieties to the metal centre, and the (jaw) angle, i.e. the dihedral angle between the two planes formed between the alkane ethylene groups. The latter mimicking the “jaws” of the Venus flytrap. A range of 1,5-cyclo-octadiene metal complexes were successfully synthesized and characterized via NMR and IR spectroscopy, the metal centers chosen were that of rhodium(I), platinum(II) and palladium(II). The ligands selected were divided into three categories namely · β-diketonato · β-enaminonato · tropolonato From each of these categories ligands were selected to vary the electron withdrawing or donating ability. Both symmetrical and non-symmetrical ligands were made use of, to see the influence of the nitrogen trans effect on the 1,5-cyclo-octadiene moiety. Finally, the tropolonato category added the influence of the smaller five membered metal chelate ring. A single crystal X-ray crystallographic study of the complexes was undertaken. The reported X-ray crystallographic structure determinations include the following complexes: [Pd(cod)(acac)]PF6 (1, Monoclinic P21/n, R = 3.59 %), [Pd(cod)(acac)]BF4 (2, Orthorhombic Pca21, R = 2.24 %), [Pd(cod)(thtfac)]PF6 (3, Monoclinic P21/n, R = 2.43 %), [Pd(cod)(thtfac)]BF4 (4, Monoclinic P21/n, R = 2.83 %), [Pd(cod)(tfacac)]PF6 (5, Triclinic P1 , R = 13.41 %), [Pd(cod)(hfacac)]PF6 (6, Monoclinic P21/c, R = 2.60 %), [Pt(cod)(acac)] (7, Orthorhombic Pca21, R = 1.75 %), [Pt(cod)(acac)]PF6 (8, Monoclinic P21/n, R = 3.02 %), [Pt(cod)(dbm)]BF4 (9, Triclinic P1), [Pt(cod)(thtfac)]BF4 (10, Monoclinic P21/n, R =2.80 %), [Pd(cod)(3Br-trop)]PF6 (11, Triclinic P1, R = 5.90 %), [Pd(cod)(3Br-trop)]BF4 (12, Triclinic P1, R = 3.40 %), [Pd(cod)(trop)]PF6 (13, Tetragonal P42bc, R = 4.08 %), [Pt(cod)(trop)]PF6 (14, Tetragonal P42/mbc, R = 3.45 %), [Pt(cod)(trop)]BF4 (14, Tetragonal P42bc, R = 2.91 %), [Pt(cod)(3Br-trop)]PF6 (16, Monoclinic C2/c, R = 4.38 %), [Pt(cod)(3Br-trop)]BF4 (17, Triclinic P1, R = 2.66 %), [Pd(cod)(NH-acac)]BF4 (18, Monoclinic P21/c, R = 2.24 %), [Pt(cod)(NH-acac)]BF4 (19, Monoclinic P21/c, R = 2.09 %), [Pt(cod)(NH-acac)]PF6 (20, Monoclinic C2/m, R = 3.88 %), [Pt(cod)(NMe-acac)]BF4 (21, Orthorhombic P212121, R = 1.50 %), [Pt(cod)(NMeacac)] PF6 (22, Monoclinic C2, R = 2.44 %), [Pt(cod)(NPh-acac)]BF4 (23, Monoclinic P21/n, R = 3.25 %), [Pt(cod)(NPh-acac)]PF6 (24, Triclinic P1, R = 3.29 %), [Rh(cod)(acac)] (25, Monoclinic Cc, R = 2.21 %), [Rh(cod)(dbm)] (26, Monoclinic Cc, R = 2.80 %), [Rh(cod)(thtfac)] (27, Monoclinic P21/n, R = 4.75 %) and [Rh(cod)(trop)] (28, Orthorhombic P212121, R = 1.39 %). The co-ordination geometry of the crystal structures was square planar, for palladium(II) and platinum(II) crystal structures resulting in a cationic charged species which was balanced with either BF4 - or PF6 - counter ions. Extensive hydrogen bonding was observed for the solid state structures with some interesting metal ring chelate interactions. The twist angle, the distortion from the square planar co-ordination geometry, for the crystal structures was found to be of a similar order. A theoretical DFT study was carried out on the group 7-11 transition metals with acetylacetone (Hacac), trifluoroacetylacetone (Htfacac), hexafluoroacetylacetone (Hhfacac), 4-aminopent-3-en-2-one (HNH-acac), 4-(methylamino)pent-3-en-2-one (HNMe-acac), 4-anilinopent-3-en-2-one (HNPh-acac), tropolone (Htrop) and tribromotropolone (H3Br-trop). The comparison of the calculated structures with the Xray crystal structures was found to be in good agreement with R-values of greater than 98 %. The molecular orbitals showed the influences of the π orbital delocalization and the bonding orbitals of the cyclo-octadiene with the various transition metal centres and provided an easy graphical method to compare both the variations of ligands and metal centres. Both the molecular orbital energies and band gap energies were presented and reflected the changes in the cyclo-octadiene complexes. A push-pull effect was observed though the use of the “core” co-ordination geometry, where the metal displays a lateral movement within the co-ordination area (Atot). The bite and jaw angles were determined for all the theoretical structures as well as the solid state crystal structures obtained. A maximum opening of c.a. 15° for the β-diketonato complexes while both the β-enaminonato and tropolonato complexes have a c.a. 14° variation in the jaw angle affected by both the metal and trans ligand manipulations.
Open Access
Direct synthesis of pterocarpans via aldol condensation
(University of the Free State, 2000-05) Van Aardt, Theunis G.; Van Rensburg, H.; Ferreira, D.
English: Pterocarpans, representing the second largest group of natural isoflavonoids, have received considerable interest on account of their medicinal properties over the last few years. These phytoalexins not only serve as antitoxins but also display antifungal, antiviral and antibacterial properties. Despite this, the study of these metabolites are restricted by their limited availability from natural sources. Furthermore, synthetic protocols allowing ready access to these compounds are restricted by the lack of availability of suitable starting materials and the potential introduction of stereoselectivity. Owing to the demand for enantiopure pterocarpans a direct stereoselective synthetic approach, based on the aldol condensation between appropriate phenylacetates and benzaldehydes, was developed. 2-Hydroxybenzaldehydes, protected as 2-0-methoxymethyl ethers, and 2- hydroxyphenylacetates, protected as TBDMS ethers, were subjected to aldol condensation employing lithium diisopropylamide, to afford the 2,3-diphenyl-3- hydroxypropanoates (40-76%, de = 22-100%). Subsequent reduction (LiAIH4), followed by Lewis acid (SnCI4, BnSH) deprotection of the 2'-O-MOM ethers, yielded the 3- benzylsulfanyl-2,3-diphenylpropanols (29-56%). Improved yields of these propanols were obtained by simply reversing the order of reactions (54-81%). B-ring formation using Mitsunobu conditions (TPP-DEAD) afforded the isoflavan silyl ethers in good yields (80-97%). The 2'-O-TBDMS derivatives were smoothly deprotected (TBAF) to yield the 2'-hydroxyisoflavans in excellent yields (96-99%). Finally, thiophilic Lewis acid (AgBF4, AgOTf or DMTSF) cyclisation produced the cis-pterocarpans in moderate to good yields (39-82%). Initial C-ring cyclisation (AgBF4) of the methyl 3-benzylsulfanyl-2,3-di(2- hydroxyphenyl)propanoates, followed by reduction (LiAIH4) and Mitsunobu (TPPDEAD) B-ring formation, afforded for the first time a trans-pterocarpan in a moderate overall yield of 12%. In order to address the issue of stereo control, we first attempted to introduce stereoselectivity during the aldol condensation. Stereoselective aldolisation employing diisopropylethylamine and chiral boron triflates, was evaluated utilizing achiral dibutylborontriflate. This system, though capable of effecting aldolisation, was ineffective to incorporate a broad range of substrates. Secondly, we converted the methyl propanoates to chiral derivatives of imidazolidin-2-one, bornane-l0,2-sultam and (lR,2S)-p-tol-N-norephedrine. Steric shielding of the enolates generated from these derivatives, prevented aldol condensation. Thirdly, using (-)-sparteine as chiral base afforded achiral products. Finally, in an effort to employ stereoselective epoxidation, attempts were made to synthesize 2-propenoates. All attempts to introduce the double bond gave disappointing yields. Although our attempts to introduce chirality failed, several alternatives still needs to be investigated in future endeavours. We have thus developed a highly efficient synthesis of cis-pterocarpans and succeeded in modifying this protocol to the novel synthesis of Irans-pterocarpanoids. Also, this synthetic protocol was modified to permit the stereoselective synthesis of 6ahydroxypterocarpans in high overall yields. The ease with which these protocols accommodate highly oxygenated substrates, featured by most natural pterocarpans, should contribute substantially to assess the chemical and physiological characteristics that may promote application of this class of phenolics as pharmaceutical or agricultural chemicals.
Open Access
Effect of metal oxide nano-particles on the properties and degradation behaviour of polycarbonate and poly (methyl methacrylate)
(University of the Free State (Qwaqwa Campus), 2012-10) Motaung, Tshwafo Elias; Luyt, A. S.
Melt compounding was used to prepare polycarbonate (PC) and poly(methyl methacrylate) (PMMA) nanocomposites with different amounts of metal-oxide fillers (silica, zirconia and titania). Zirconia and two types of titania were prepared by a sol-gel method, whereas a commercial hydrophobic silica having chemically surface bonded methyl groups was used. Titania nanoparticles were annealed at 200 and 600 °C to obtain the anatase and rutile phases, respectively. The effect of filler amount, in the range 1-5 wt.%, on the morphology, mechanical properties and thermal degradation kinetics was investigated by means of transmission electron microscopy (TEM), X-ray diffractometry (XRD), small-angle X-ray scattering (SAXS), dynamic mechanical analysis (DMA), thermogravimetric analyses (TGA), Fourier-transform infrared spectroscopy (FTIR), 13C cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy (13C{1H}CP-MAS NMR) and measures of proton spin-lattice relaxation time in the rotating frame (T1ρ(H)), in the laboratory frame (T1(H)) and cross polarization times (TCH). Results showed that the nanoparticles were well dispersed in the polymers whose structure remained amorphous, except for zirconia in a PC matrix, which showed the appearance of a local lamellar order around the nanoparticles. The silica, titania and zirconia nanopaticles increased the thermal stability of the polymers, except for the highest silica and zirconia contents in the PC system which showed a decrease. A similar trend in the activation energies of thermal degradation was observed. The presence of zirconia and silica showed a decrease in the storage and loss moduli at lower temperatures, probably due to a plasticization effect. The two types of titania nanoparticles influenced the rigidity of the polymers in different ways because of their different carbon contents, particle sizes and crystal structures. NMR results suggested that, in the presence of a metal oxide, the observations in the PMMA systems could be related to heteronuclear dipolar interactions between the carbonyl carbons and the surrounding hydrogen nuclei, and in the PC systems to intermolecular interactions involving the carbonyl groups.