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(μ-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 ν(C⋮O) stretching frequencies, the ansa ligand C2Me4(C5H4)2 was found to be electron-withdrawing relative to two η5-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‘2Mo(OH)(OH2)+ catalysts. In all cases, the Cp2Mo(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.