Switchable C- and N-Bound Isomers of Transition-Metal Cyanocarbanions: Synthesis and Interconversions of Cyclopentadienyl Ruthenium Complexes of Phenylsulfonylacetonitrile Anions

Graphical Abstract

Sliding makes it easy: The synthesis, structure, and specific linkage isomerization of bistable C- and N-bound isomers of transition-metal cyanocarbanions are described. Experimental and theoretical studies revealed that both the N-to-C and C-to-N isomerizations proceed unimolecularly through the sliding of the metal atom over the CCN surface (see graphic).

Abstract

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) ruthenium 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 phenylsulfonylacetonitrile and performing ligand-exchange reactions with the resulting compounds. Structural characterization by X-ray diffraction indicates that the cyanocarbanion moiety of 3 has an α-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 CCN π-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 μ²-C,N coordination dimers [{Ru{CH(CN)SO₂Ph}(Cp)(PPh₃)}₂] (15 and 16).