Characterisation and substitution kinetics of different cobalt(III) tripod complexes

[摘要] English: The synthesis and reactions of Co(III) complexes with tripod-type ligands such as l-leucine-N,N-diacetic acid (lda) and l-phenylalanine-N,N-diacetic acid (pda) have a widespread interest, mainly because of the fact that these complexes can be used as biological model complexes and because lda as well as pda labilises usually inert metal centres. Uehara and co-workers (1971:1552) were the first to prepare Co(III)-lda andpda complexes. Unfortunately metal complexes of cobalt(III) containing lda and pda as possible multidentate or tripod ligand are rarely mentioned in the literature and little information on their structure and chemistry is available. No kinetic studies on cobalt(III)-lda and-pda complexes have been published. The question regarding the identity of the different Co(III)-L (L = lda, pda) species in solution at different pH levels has been accounted for in this study (see Scheme 1). Scheme in PDF full text. l-Leucine-N,N-diacetic acid (lda) and l-phenylalanine-N,N-diacetic acid (pda) were synthesised according to the procedure developed by Bocarsly et al. (1990:4898). The synthesis of lda and pda were confirmed by means of IR and 1H NMR spectrometry. Both [Co(lda)(μ-OH)]2 2- and [Co(pda)(μ-OH)]2 2- were prepared similar to the method described by Uehara and co-workers (1971:1552). The synthesis of [Co(lda)(μ-OH)]2 2- and [Co(pda)(μ-OH)]2 2- were confirmed by means of IR, UV/VIS and 1H NMR spectrometry. The IR stretching frequencies obtained for above-mentioned complexes are indicative of COO- groups coordinated to a metal centre such as Co(III). The 1H NMR spectra for both [Co(lda)(µ-OH)]2 2- and [Co(pda)(μ-OH)]2 2- also indicated that all the protons of lda and pda are non-equivalent. [Co(lda)(μ-OH)]2 2- and [Co(pda)(μ-OH)]2 2- undergo bridge-cleavage upon acidification with H+ ions to form [Co(lda)(H2O)2] and [Co(pda)(H2O)2], respectively. Further acidification of [Co(lda)(H2O)2] and [Co(pda)(H2O)2] leads to the stepwise dissociation of lda and pda, respectively. The formation of an ion association species between [Co(L)(H2O)2] (L = lda, pda) and H+ ions upon addition of acid is postulated. This ion associated species dissociates in the rate determining step to form the tridentate L (L = lda, pda) complex, [Co(µ3-L)(H2O)3]+. The values of k1 were determined as 0.128(8) s-1 at 25.9 µC for lda complex and as 0.115(7) s-1 at 25.7 µC for pda complex. Another acid-base equilibrium is observed when the pH of both [Co(lda)(H2O)2] and [Co(pda)(H2O)2] solutions are increased. It was concluded that the newly formed species are not the dimer, but rather [Co(lda)(H2O)(OH)]- and [Co(pda)(H2O)(OH)]- which reverts back to the dimer at pH 6 - 7 after several days. The acid dissociation constants of [Co(lda)(H2O)2] and [Co(pda)(H2O)2] were spectrophotometrically determined as 6.11(2) and 6.74(1), respectively. The substitution reactions between [Co(L)(H2O)2]/[Co(L)(H2O)(OH)]- (L = lda, pda) 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 of [Co(lda)(H2O)2] and [Co(pda)(H2O)2] (k1 = 2.43(3) x 10-3 M-1 s-1 at 24.5 °C) and (k1 = 2.3(4) x 10-2 M-1 s-1 at 25.0 °C), respectively, at low pH is about 125 times faster than the rate of substitution of the second aqua ligand (k3 = 1.52(6) x 10-4 M-1 s-1 at 25.0 °C) and (k3 = 1.36(2) x 10-4 M-1 s-1 at 25.0 °C), respectively. The [Co(lda)(H2O)(OH)]- and [Co(pda)(H2O)(OH)]-complexes reacts about 70 times faster at 25.0 °C with NCS-, respectively, than the [Co(lda)(H2O)2] and [Co(pda)(H2O)2] complexes with NCS- (k2 = 1.34(9) M-1 s-1 vs. 2.43(3) x 10-2 M-1 s-1 for k1 at 24.5 °C) and (k2 = 1.44(8) M-1 s-1 vs. 2.3(4) x 10-2 M-1 s-1 for k1 at 25.0 °C), respectively. This clearly indicates that the hydroxo ligand labilises the cis-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. The Co(III)-lda and-pda complexes that were isolated and characterised can successfully be used as biological model complexes in future studies. These complexes could for example be used to simulate the bonding of metal ion to functional groups of wool fibre or might have uses as models in pharmacology.

[发布机构] University of the Free State