Evidence for a trajectory of hydrolytic reactions brought about by [L3Zn-OH] species

A kinetic study has been performed on the hydrolytic cleavage of various tr iorganophosphates PO(OR)(2)(OR') by three pyrazolylborate-zinc hydroxide co mplexes [Tp'Zn-OH] to form [Tp'Zn-OPO(OR)(2)] and HOR'. The nature of the r eactions (first order both in the zinc complex and the phosphate) and the s trongly negative activation entropies (-54 to -126Jmol(-1) K-1) indicate an intimate association of the Zn-OH and P=O functions in the rate-determinin g step. Some ester cleavages by [Tp'Zn-OH] show the same kinetic pattern an d 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 catalyz ed hydrolyses. A trajectory for reactions passing through this intermediate has been constructed with the Burgi-Dunitz structure correlation method, b ased on the geometries of SO[Tp'Zn(X)(Y)] species with truly five-coordinat e zinc centers. Its first step is the approach of the substrate's oxygen at om to the tetrahedral L3Zn-OH species along the axis of a trigonal bipyrami d. In the resulting four-center intermediate with five-coordinate zinc a Be rry pseudorotation describes the synchronous formation of the Zn-O-(substra te) and breaking of the Zn-O-(OH) bonds. The concluding step in the coordin ation 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 d ioxide hydrolysis, is in accord with theoretical models of carbonic anhydra se action.