Kinetics and mechanism of the bicarbonate dehydration of the half-sandwich zinc(II) complexes [TpPh]ZnX ([TpPh] = hydrotris(3-phenylpyrazolyl)borate; X− = OH−, N3−, NCS−)

Abstract

The preparation and characterization of the half-sandwich zinc complexes [TpPh]ZnX ([TpPh] = hydrotris(3-phenylpyrazolyl)borate; X− = OH− (1), N3− (2), NCS− (3)) have been described. The bicarbonate dehydration kinetic measurements are performed by the stopped-flow techniques at pH < 7.9. The apparent dehydration rate constant kobsd varies linearly with the total Zn(II) concentration, and the catalytic activity of the model complexes decreases in the order 1>2>3. The catalytic activity decreases with increasing pH, which indicates that the aqua model 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 kobsd values increase with increasing reaction temperature, and the apparent activation energies of the model complexes with small inorganic ions are remarkably higher, this being the origin of inhibition. The large negative entropy of activation also indicates an associative mode of activation in the rate-determining step. The inhibition ability of NCS− is strong than that of N3−, which can be rationalized by the decrease in effective atomic charges of the Zn(II) ions as revealed by the theoretical calculations.