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dc.contributor.advisorPurcelI, W.
dc.contributor.advisorBasson, S. S.
dc.contributor.authorVisser, Hendrik Gideon
dc.date.accessioned2017-03-22T09:38:52Z
dc.date.available2017-03-22T09:38:52Z
dc.date.issued2000-11
dc.identifier.urihttp://hdl.handle.net/11660/5959
dc.description.abstractEnglish: 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.en_ZA
dc.description.abstractAfrikaans: Die sintese en reaksies van Co(III)- en Cr(III)-komplekse met nitrilotriasynsuur (nta) as tetradentate ligand lok wye belangstelling, veralomdat hierdie komplekse as biologiese modelkomplekse gebruik kan word en omdat die nta ligand metaalsentra wat gewoonlik inert is, labiiiseer. Mori et al (1958:940) en Uehara et al. (1967:2317) het aanvanklik verskillende Co(III)-nta en Cr(III)-nta komplekse respektiewelik berei. Sedertdien is hierdie komplekse in verskeie kinetiese en sintetiese studies as uitgangstof gebruik (Visser et al. 1997:2581; Visser et al. 1994:1051 en Thacker & Higginson, 1975:704). Tot en met tyde van hierdie studie was die struktuur van hierdie komplekse nog nie opgeklaar met sekerheid nie en die suiwerheid van die bereide komplekse was onder verdenking. Die vraag rondom die identiteit van die verskillende Co(III)-nta spesies in oplossing (sien Skema 1) by verskillende pH's is grootliks in hierdie studie beantwoord. Skema 1 Komplekse en reaksies van Co(III)-nta. (Sien PDF) Die struktuur van die kompleks wat eerste deur Mori et al (1958:940) berei en karakteriseer was, is uiteindelik met behulp van X-straal kristallografie bepaal as [Co(nta)(Jl-OH)]l-. Kristalle van Cs2[Co(nta)(Jl-OH)].4H20 kristalliseer in die ortorombiese ruimtegroep, 141/a(R1::;0.0322). Die Co-N bindingsafstand is bereken as 1.922(6) A. Hierdie studie het bevind dat die hidrokso brue van Co(nta)(Jl-OH)]l- gesplyt word met byvoeging van H+ ione om [Co(nta)(H20)2] te vorm. Die pKavan hierdie reaksie is as 3.09(3) bepaal. Verdere byvoeging van suur lei tot die stapsgewyse dissosiasie van die nta ligand. Dit word gepostuleer dat 'n ioongeassosieerde spesie tydens die reaksie tussen [Co(nta)(H20)2] en H+ ione met byvoeging van suur vorm. Hierdie ioongeassosieerde spesie dissosieer in die tempobepalende stap om 'n tridentate nta kompleks, [Co(Tl3-nta)(H20bt, te vorm. Die waarde van k1 by 25.9 °C is as 0.13(1) S-1 bepaal. Nog 'n suur-basis ewewig is tydens die die pH-verhoging van 'n [Co(nta)(H20)2] oplossing waargeneem. Die gevolgtrekking is dat die nuutgevormde spesies nie die dimeer is nie, maar eerder [Co(nta)(H20)(OH)r wat terugkeer na die dimeer by pH 6 - 7 na verloop van 'n paar dae. Hierdie tweede pKais as 6.52(2) bepaal. Die substitusiereaksies tussen [Co(nta)(H20)2] en NCS- ione is ook ondersoek. NCSione substitueer die akwaligande stapsgewys by pH ::; 2.0. Die substitusie van die eerste akwa ligand (k1 = 2.4(1) x 10-2M-1S-1by 24.7 °C) is omtrent 'n faktor 120 keer vinniger as die tempo van die tweede akwa-substitusie (kJ ::; 1.~8(6) x 10-4M-1S·1by 24.7°C). Die [Co(nta)(H20)OHr kompleks reageer omtrent 70 keer vinniger by 24.7 °C met NCS-as wat [Co(nta)(H20)2]met NCS· reageer (k2= 1.68(5) M-1S·1vs. 2.4(1) x 10-2 M-1S·1vir k1by 24.7 °C). Hierdie resultate toon duidelik dat die hidrokso ligand die cisakwa binding sodanig labiiiseer dat 'n toename in tempo waargeneem word. Verder toon dit aan dat die hidroksiedligande nie deur NCS· ione by hoër pH gesubstitueer word nie aangesien slegs een reaksie by hierdie pH waargeneem word. Die hidrokso brue van Co(nta)(Jl-OH)]22.word ook by hoër pH's deur verskillende ligande soos en, dmap of py gesplyt. As gevolg hiervan is verskeie [Co(nta)(LL')] en [Co(nta)(L)2](LL' = verskillende N,N and N,O donors en L::; dmap, py) komplekse berei. Die X-straal kristalstruktuurbepaling van [Co(nta)(N,N-Et2en)] is 'n resultaat van een van die bogenoemde reaksies. Kristalle van [Co(nta)(N,N-Ehen)] kristalliseer in die ororombiese ruimtegroep, Pbcm (R1 = 0.0309). Die Co-N bindingsafstand is bepaal as 1.950(4) A. Die brugsplytingsreaksies van u-hidrokso gebrugde Co(III)-komplekse is nog nie voorheen ondersoek nie. Die substitusiereaksies tussen [Co(nta)(1l-0H)]22- en verskillende ligande soos dmap, py, en and N,N-Ehen is by pH 9 - 11.5 ondersoek. Dit word gepostuleer dat [Co(nta)(1l-0H)]l- 'n ewewig met 'n mono-u-hidrokso gebrugde spesie in waterige, basiesie oplossings vorm en dat beide spesies met die inkomende ligande reageer. Hierdie reaksies lei tot die vorming van ioongeassosieërde spesies wat in die tempobepalende stap dissosieer om die produkte te vorm. Die bestaan van die gevormde mono-u-hldrokso gebrugde kompleks is ook bevestig aangesien die ewewigskonstante, PKoH, as 3.3 bepaal is vir al die reaksies wat bestudeer is. Die resultate toon ook dat die rnono-u-hidrokso spesie meer labiel ten opsigte van substitusie as die dimeer self is. Laasgenoemde word geïllustreer deur die feit dat k1 < k2 is vir al die reaksies wat ondersoek is. Die waardes van k1wissel tussen 8.7(7) X 10-5S-1en 3.3(7) x 10-3S-1en k2 tussen 6.8(2) x 10-4S-1en 5.7(2) x 10-2S-1. Die sintese en karakterisering van Cs2[Co(nta)(C03)].H20 is ook onderneem. Hierdie kompleks kristalliseer in die monokliniese ruimtegroep P21/c (R1 = 0.0249) en kan as 'n alternatiewe uitgangstof in die sintese van verskillende Co(III)-nta komplekse gebruik word. Die Co-N bindingsafstand is as 1.920(2) A bereken. Die onsekerheid aangaande die identiteit van Cr(III)-nta komplekse wat aanvanklik deur Uehara et al. (1967:2317) berei is, is uit die weg geruim met die X-straal kristalstruktuurbepaling van Cs2[Cr(nta)(1l-0H)].4H20. CS2[Cr(nta)(1l-0H)].4H20 kristalliseer in twee verskillende ruimtegroepe, tetragonaal 141/a (R1 = 0.0354) en monoklinies P21/c (R1 = 0.0354). Die Cr-N bindingsafstande is respektiewelik as 2.048(9) en 2.061 (3) A bepaal. Die vervorming van die glisinato ringe van gek06rdineerde nta neem af in die volgorde G > R vir al die komplekse wat ondersoek is. Die R ringe is amper ten volle planêr vir al die gevalle, terwyl die G ringe almal nie-planêr is.af
dc.language.isoenen_ZA
dc.publisherUniversity of the Free Stateen_ZA
dc.subjectCobalt(III)en_ZA
dc.subjectChromium(III)en_ZA
dc.subjectNitrilotriacetic aciden_ZA
dc.subjectSubstitution and ring cleaving reactionsen_ZA
dc.subjectSingle crystal x-ray crystallographyen_ZA
dc.subjectChromium groupen_ZA
dc.subjectChemical kineticsen_ZA
dc.subjectSubstitution reactionsen_ZA
dc.subjectThesis (Ph.D. (Chemistry))--University of the Free State, 2000en_ZA
dc.titleCharacterisation and substitution kinetics of hromium(III)- and obalt(III)nitrilotriacetato complexesen_ZA
dc.typeThesisen_ZA
dc.rights.holderUniversity of the Free Stateen_ZA


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