Organometallic catalysis in aqueous solution.: The hydrolytic activity of a water-soluble ansa-molybdocene catalyst

The synthesis, characterization, and reactivity of the new water-soluble ansa-molybdocene catalyst [{C2Me4(C5H4)(2)}Mo(OH)(OH2)][OTs] (3) and the related hydroxo-bridged dimer [{C2Me4(C5H4)(2)}Mo-(mu-OH)](2)[OTs](2) (5) are described. The effect of the ethylene bridge on the metallocene structure was evaluated by comparing the crystal structures of {C2Me4(C5H4)(2)}MoH2 (2) and 5 to those of the non-ansa analogues. The ethylene bridge changed the bite angles of the metallocene fragment by only a few degrees in both ansa structures. To probe the electronic consequences of the tetramethylethylene bridge, the {C2Me4(C5H4)(2)}Mo(CO)H (4) complex was prepared. On the basis of the nu(C O) stretching frequencies, the ansa ligand C2Me4(C5H4)2 was found to be electron-withdrawing relative to two eta(5)-C5H5 ligands. The reactivity of 3 in nitrile hydration, phosphate ester hydrolysis, and carboxylic acid ester hydrolysis was explored, and the rate constants for these transformations were compared to rate constants obtained using the Cp2Mo(OH)(OH2)(+) and Cp'Mo-2(OH)(OH2)(+) catalysts. In all cases, the Cp-2-Mo(OH)(OH2)(+) Catalyst, having intermediate electron density, had the largest rate constants. The reactivity trends for the three catalysts are explained by the relative electrophilicities of the Mo centers. If electron-donating cyclopentadienyl ligands are employed, the reactivity of the bound substrate is decreased relative to Cp and the rate is decreased. Conversely, if electron-withdrawing Cp cyclopentadienyl ligands are employed, the reactivity of the bound hydroxo nucleophile is decreased and the rate is decreased. In the case of the Cp2Mo(OH)(OH2)(+) complex, these two opposing trends converge, and optimal activity is observed.