Characterisation and substitution kinetics of hromium(III)- and obalt(III)nitrilotriacetato complexes

[摘要] 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 thatthese complexes can be usedas biological model complexes and because nta labilisesusually inert metal centres. Mori et al (1958:940) and Uehara et al. (1967:2317) werethe first to prepare different Co(III)-nta and Cr(III)-nta complexes respectively. Sincethen these complexes have been used in several kinetic and synthetic studies (Visser etal. 1997:2581; Visser et al. 1994:1051 and Thacker & Higginson, 1975:704). Howeverthe identity and purity of these complexes were questionable and had not been solvedup to the time of this study.The question regarding the identity of the different Co(III)-nta species in solution atdifferent 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 finallycharacterised 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). Furtheracidification of [Co(nta)(H20)2] leads to the stepwise dissociation of nta. The formationof an ion associated species between [Co(nta)(H20)2] and H+ions upon addition of acidis postulated. This ion associated species dissociates in the rate determining step toform the tridentate nta complex, [Co(,,3-nta)(H20hr. The value of k1 at 25.9 °C wasdetermined as 0.13(1) S-1.Another acid-base equilibrium is observed when the pH of a [Co(nta)(H20)2]solution isincreased. 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 beeninvestigated, 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 120orders 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 timesfaster 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 ligandlabinses the eis-aqua bond so that an increase in rate is observed. Hydroxide is notsubstituted by NCS- ions at higher pH so that only one reaction is observedspectrophotometrically.[Co(nta)(wOH)]i- undergo bridge cleavage at higher pH upon addition of variousligands 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 beensynthesised. 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 beenstudied to our knowledge. The substitution reactions between [Co(nta)(1l-0H)]l- andvarious 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 solutionswith a mono-u-hydroxo bridged species and that both these species react with theincoming ligand to form ion associated species (rapid) which dissociates in the ratedetermining step to the products.The existence of the formed mono-u-hydroxo bridged complex was confirmed by thefact that the value for the equilibrium constant, pKoH, was determined as 3.3 for all thereactions studied. This mono-u-hydroxo species is more labile towards substitutionthan 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 k2between 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 beused as an alternative to [Co(nta)(1l-0H)]l- for the synthesis of different Co(III)-ntacomplexes. 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 byUehara et al. (1967:2317) have been erased with the X-ray crystal structuredetermination of Cs2[Cr(nta)(j.l-OH)].4H20. CS2[Cr(nta)(1l-0H)].4H20 crystallise in twodifferent 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 orderG > R for all the complexes studied. The R rings in all the complexes are almostperfectly planar in all cases, while the G rings are non-planar.

[发布机构] University of the Free State