953398-00-0Relevant articles and documents
Organometallic catalysis in aqueous solution. The hydrolytic activity of a water-soluble ansa-molybdocene catalyst
Ahmed, Takiya J.,Zakharov, Lev N.,Tyler, David R.
, p. 5179 - 5187 (2007)
The synthesis, characterization, and reactivity of the new water-soluble ansa-molybdocene catalyst [{C2Me4(C5H 4)2}Mo(OH)(OH2)][OTs] (3) and the related hydroxo-bridged dimer [(C2Me4(C5H 4)2)Mo(M-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 (C2Me 4(C5H4)2)MoH2 (2) and 5 to those of the nonansa 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 2Me4(C5H4)2)Mo(CO)H (4) complex was prepared. On the basis of the V(C≡O) stretching frequencies, the ansa ligand C2Me4(C5H4) 2 was found to be electron-withdrawing relative to two η5C5H5 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)(OH 2)+ and Cp′2Mo(OH)(OH2) + catalysts. In all cases, the Cp2Mo(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 electrondonating 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.
