Characterisation and substitution kinetics of chromium(III) - N-(carbamoylmethyl)-iminodiacetato complexes

[摘要] English: N-Carbamoylmethyl-iminodiacetic acid (H2ada) is widely used in biologicalbuffers of pH 6.0 - 7.4 and is also a well known chelating agent that acts as a tetradentate ligand under most circumstances. The first known structure determination with coordinated ada2- was reported by Sivak and co-workers (1995:1057) during a study in which they characterised [VO(O2)(η4-ada]-. Only one complex, [Ni(η3-ada)(bipy)(H20]·4H20, has been reported in the literature in which ada2- is coordinated as a tridentate ligand (Bugella-Altamirano et al.,2000:727).The synthesis of Cs[Cr(η3-ada)2]-2H20 was confirmed by means of IR, UV/VISspectroscopy and X-ray crystallography. The Cs[Cr(η3-ada)2]-2H20 complex is the first metal(lll)-ada complex and only the second metal-ada complex with ada2- coordinated in tridentate mode.The IR vibrational frequencies obtained for Cs[(η3-ada)2]-2H20 are indicative of COO- groups coordinated to the Cr(llll) metal centre and of uncoordinated glycinamido groups. The crystal structure determination confirmed the IR data by clearly showing that all the COO- groups are coordinated to the Cr(lll) centre and that both amido groups remained uncoordinated. The intermediates and the final products of the kinetic studies perfonned in this study, were also successfully isolated and characterised by means of IR, UV/VIS spectrums and X-ray crystallography. (see Scheme 1).Scheme in PDF full text.The interest in the characterization of rnetal-ada complexes have increased in the last few years, but kinetic studies on metal-ada complexes are yet to be published. The first UV/VIS and kinetic investigation on a metal-ada complex was performed in this study.The UV/VIS investigation revealed that the protonation of the [Cr(η3-ada)2]- complex leads to the breaking of a Cr-O(carboxylate) bond and the subsequent coordination of an aqua ligand to the Cr(lll) centre, forming the [Cr(η3-ada)(η2-Hada)(H20)] intermediate. This first reaction of the stepwise dissociation of the two ada2- ligands from (Cr(η3-ada)2]- was investigated (k1=4.34(4)x10-1 M-1s-1 at 19.6°C).The substitution reaction between [Cr(η3-ada)(η2-Hada)(H20)] and NCS- was alsostudied. At pH = 0.8 NCS- ions substituted the aqua ligand (2.2 x 10-1 M-1 s-1 at25°C) before the ada2- ligands dissociated from the Cr(lll)-ada complex over a period of three days.The above mentioned reactions that were kinetically investigated in this study all have relatively fast reaction rates. This is indicative of the labilisation of the Cr(lll) complex by the electron rich ada2- ligands, in which the electron density is donated to the normally inert Cr(lll) centre making it react more like the labile Cr(ll) species.The various Cr(lll)-ada 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