A kinetic study has been performed on the hydrolytic cleavage of various triorganophosphates PO-(OR) 2 (OR') by three pyrazolylborate ± zinc hydroxide complexes [Tp'Zn À OH] to form [Tp'Zn À OPO(OR) 2 ] and HOR'. The nature of the reactions (first order both in the zinc complex and the phosphate) and the strongly negative activation entropies (À 54 to À 126 J mol) indicate an intimate association of the ZnÀOH and PO functions in the rate-determining step. Some ester cleavages by [Tp'Zn À OH] show the same kinetic pattern and similar activation parameters. The observations are in accord with a four-center arrangement (ZnOPO or ZnOCO) in the activated complex, that is, the hybrid mechanism discussed for zinc enzyme as well as zinc complex catalyzed hydrolyses. A trajectory for reactions passing through this intermediate has been constructed with the Bürgi ± Dunitz structure correlation method, based on the geometries of 30 [Tp'Zn(X)(Y)] species with truly five-coordinate zinc centers. Its first step is the approach of the substrates oxygen atom to the tetrahedral L 3 ZnÀOH species along the axis of a trigonal bipyramid. In the resulting four-center intermediate with five-coordinate zinc a Berry pseudorotation describes the synchronous formation of the Zn À O-(substrate) and breaking of the ZnÀO-(OH) bonds. The concluding step in the coordination sphere of zinc is the expulsion of the former OH oxygen, now part of the substrate, again along the axis of a trigonal bipyramid. This mechanism, which is applicable to phosphate as well as to ester, amide, and carbon dioxide hydrolysis, is in accord with theoretical models of carbonic anhydrase action.
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