Reactivity of (Dicarboxamide)M^II–OH (M = Cu, Ni) Complexes — Reaction with Acetonitrile to Yield M^II Cyanomethides

Tehranchi, J.; Donoghue, P. J.; Cramer, C. J.; Tolman, W. B.
Eur. J. Inorg. Chem. 2013, 4077 (doi:10.1002/ejic.201300328).

The complexes [Bu₄N][L^MeM^II–OH] [L^Me = 2,6-dimethylphenyl-substituted pyridine(dicarboxamide); M = Cu or Ni] react with CH₃CN to yield [Bu₄N][L^MeM–CH₂CN] cyanomethide complexes, which were fully characterized, including by X-ray crystallography. These conversions contrast with the usual reactions of metal hydroxide complexes with nitriles, which typically involve attack at the nitrile carbon atom and formation of amides or carboxylic acids. Kinetic studies (M = Cu) revealed a first-order dependence on the complex and a kinetic isotope effect [k(CH₃CN)/k(CD₃CN)] of 4. Various mechanisms involving either intra- or intermolecular deprotonation steps are proposed. In addition, [Bu₄N][L^MeCu–OH] was oxidized by ferrocenium hexafluorophospate (FcPF₆) to a proposed Cu^III complex L^MeCuOH at low temperature, and comparisons of its stability and reactivity with dihydroanthracene were drawn to its previously described congener with isopropyl substituents on the phenyl rings of the supporting ligand. The cyanomethide complex [Bu₄N][L^MeCu(CH₂CN)] was also reversibly oxidized both electrochemically (E_1/2 = –0.345 V vs. Fc/Fc⁺) and chemically (FcPF₆, –25°C). The product was formulated as L^MeCu^III(CH₂CN), a new Cu^III–alkyl complex relevant to such species proposed during copper-catalyzed organic reactions.