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)(2)Re(COPh)] (1) with CF3SO3Me afforded, besides the expected carbene complex [Cp(CO)(2)Re=C(OMe)(Ph)] (2) and the alkyl-acyl derivative [Cp(CO)(2)Re(Me)(COPh)] (3), a third structural isomer [Cp(CO)(2)Re{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 sigma-donor interaction, while in solution an equilibrium between sigma-bound (eta(1)) and pi-bound (eta(2)) forms occurs, as judged by IR data and H-1 and C-13 variable temperature NMR spectra (pi/sigma ratio 2.87 at 183 K and 1.16 at 263 K in CD2Cl2, DeltaH(o) = -4.5 U mol(-1) for the sigmadouble left right arrowpi reaction, E-a 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.