Spectroscopic properties, catalytic activities and mechanism studies of [(TpPh)Co(X)(CH3OH)m]·nCH3OH: bicarbonate dehydration in the presence of inhibitors Original Research Article

Two inhibitor-containing ‘half-sandwich’ cobalt(II) complexes [(TpPh)Co(X)(CH3OH)m]·nCH3OH ((TpPh)=hydrotris(3-phenylpyrazolyl)borate; 1: X−=N3−, 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 N3− 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 HCO3− are performed by the stopped-flow techniques at pH<7.9. The apparent dehydration rate constant kobs 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 HCO3−. The kobs 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 N3−, which can be rationalized by the decreases in the CoN(N3−/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.