Doctoral Degrees (Chemistry)

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Browsing Doctoral Degrees (Chemistry) by Author "Basson, S. S."

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    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.