Novel Reactivity of Ruthenium Alkylidenes in Protic Solvents: Degenerate Alkylidene Proton Exchange

Abstract

A novel organometallic transformation is reported in which the alkylidene protons of water-soluble ruthenium alkylidenes 1 and 2 undergo nondestructive, degenerate exchange with solvent-derived deuterons in perdeuterated protic solvents such as D_2O and CD_3OD. Deuterated alkylidene complex (1-D) was isolated from a solution of alkylidene 1 in D_2O, and the new alkylidene was fully characterized by ^1H, ^2H, ^(13)C, and ^(31)P NMR spectroscopy and fast-atom bombardment mass spectroscopy (FAB-MS). The rate of alkylidene proton exchange for this transformation was found to correlate with the bulk dielectric constant of the solvent or solvent mixtures employed. The data support a mechanism for proton exchange involving the dissociation of a chloride ion from the ruthenium metal center. The rate of alkylidene H/D exchange for alkylidene 2 was faster than the rate of exchange for alkylidene 1, demonstrating that relative rates of exchange are influenced by the electron densities at the metal centers of these complexes. Several additional ruthenium alkylidenes were found to undergo analogous alkylidene H/D exchange reactions, including parent alkylidene (Cy_3P)_2Cl_2Ru═CHPh (3) in CD_2Cl_2/CD_3OD mixtures. These data suggest that this novel reactivity may be general for an entire class of ruthenium alkylidenes provided that protic species are available in solution and that the dielectric constant of the reaction medium is sufficiently high to ionize the halide ligands.

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