Spectroscopic properties, catalytic activities and mechanism studies of [(Tp Ph)Co(X)(CH 3OH) m]· nCH 3OH: bicarbonate dehydration in the presence of inhibitors

Abstract

Two inhibitor-containing ‘half-sandwich’ cobalt(II) complexes [(Tp Ph)Co(X)(CH 3OH) m ]· nCH 3OH ((Tp Ph)=hydrotris(3-phenylpyrazolyl)borate; 1: X −=N 3 −, m=1, n=2; 2: X −=NCS −, m=0, n=0) have been synthesized and used as the catalysts in the bicarbonate dehydration reaction. The structures of 1 and 2 were determined by X-ray diffraction analysis, which shows that N 3 − and NCS − coordinate to the Co(II) ions of 1 and 2, respectively, with the CoN bond lengths of 1.992(6) Å and 1.901(3) Å. The coordination geometries of the Co(II) complexes in solution are five-coordinated trigonal bipyramid as revealed by the spectroscopic measurements. The dehydration kinetic measurements of HCO 3 − are performed by the stopped-flow techniques at pH<7.9. The apparent dehydration rate constant k obs varies linearly with Co(II) complex and H + concentrations, respectively, and the catalytic activity of 2 is lower than that of 1. The aqua Co(II) complex must be the reactive catalytic species in the catalyzed dehydration reaction and the rate-determining step is the substitution of the labile water molecule by HCO 3 −. The k obs values increase with increasing reaction temperature, and the large negative entropy of activation also indicates the associative activation mode. The inhibition ability of NCS − is stronger than that of N 3 −, which can be rationalized by the decreases in the CoN(N 3 −/NCS −) bond lengths and effective atomic charges of the Co(II) ions based on the X-ray crystallographic data and theoretical calculations in this work.