A ketone complex by alkylation of an acyl anion. Synthesis, crystal structure and spectroscopic characterization of [Cp(CO)2Re{OC(Me)Ph}]

Reaction of Li[Cp(CO)2Re(COPh)] (1) with CF3SO3Me afforded, besides the expected carbene complex [Cp(CO)2ReC(OMe)(Ph)] (2) and the alkyl–acyl derivative [Cp(CO)2Re(Me)(COPh)] (3), a third structural isomer [Cp(CO)2Re{OC(Me)Ph}] (4), which contains an acetophenone molecule coordinated to the metal center. The X-ray analysis showed that in solid acetophenone is bound to ‘CpRe(CO)2’ exclusively through an oxygen σ-donor interaction, while in solution an equilibrium between σ-bound (η1) and π-bound (η2) forms occurs, as judged by IR data and 1H and 13C variable temperature NMR spectra (π/σ ratio 2.87 at 183 K and 1.16 at 263 K in CD2Cl2, ΔH°=−4.5 kJ mol−1 for the σ⇔π reaction, Ea 58(1) kJ mol−1). In solvents different from Et2O (n-hexane, THF, acetone) and with alkylating agent different from CF3SO3Me (MeI, Me3OBF4) the formation of 4 was negligible. It has been demonstrated that 4 does not originate by acetophenone reductive elimination from 3. No evidence of the involvement of radicals has been obtained.