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 [{C 2 Me 4 (C 5 H 4 ) 2 }Mo(OH)(OH 2 )][OTs] ( 3 ) and the related hydroxo-bridged dimer [{C 2 Me 4 (C 5 H 4 ) 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 {C 2 Me 4 (C 5 H 4 ) 2 }MoH 2 ( 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 {C 2 Me 4 (C 5 H 4 ) 2 }Mo(CO)H ( 4 ) complex was prepared. On the basis of the ν(C⋮O) stretching frequencies, the ansa ligand C 2 Me 4 (C 5 H 4 ) 2 was found to be electron-withdrawing relative to two η 5 -C 5 H 5 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 Cp 2 Mo(OH)(OH 2 ) + and Cp‘ 2 Mo(OH)(OH 2 ) + catalysts. In all cases, the Cp 2 Mo(OH)(OH 2 ) + 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 Cp 2 Mo(OH)(OH 2 ) + complex, these two opposing trends converge, and optimal activity is observed.