Switchable C- and N-bound isomers of transition-metal cyanocarbanions: Synthesis and interconversions of cyclopentadienyl ruthenium complexes of phenylsulfonylacetonitrile anions

The synthesis, structure, and dynamic behavior of bistable C- and N-bound isomers of transition-metal cyanocarbanions are described. A series of C-bound cyclopentadienyl (Cp) rutheniurn phosphane complexes, [Ru{CH(CN)SO2Ph}(Cp)L1L2] (3), and their N-bound isomers, [Ru+ (Cp)(NCCH-SO2Ph)L1L2] (4), were prepared by treating [RuCl(Cp)(PR3)(2)] with the sodium salt of phenylsulfonyl-acetonitrile and performing ligand-exchange reactions with the resulting compounds. Structural characterization by X-ray diffraction indicates that the cyanocarbanion moiety of 3 has an alpha-metalated structure, whereas that of 4 has a zwitterionic, end-on structure. Heating these complexes in aprotic solvents gives rise to irreversible linkage isomerization between C- and N-bound isomers, in which the relative thermal stabilities vary greatly depending on the steric and electronic nature of the ligands. Mechanistic studies of N-to-C isomerization revealed that the reaction proceeds irreversibly in a unimolecular manner without the formation of coordinatively unsaturated species. A metal-sliding process, which occurs over the -C-C N pi-conjugated surface of the cyanocarbanion moiety, was suggested by results from kinetic studies and density functional theory (DFT) calculations. C-to-N isomerizations proceed by the above-mentioned intramolecular process, with a temperature-dependent contribution from the formation and cleavage of pi(2)-C,N coordination dimers [{Ru(CH(CN)SO2Ph)(Cp)(PPh3)}2] (15 and 16).