IS PI-BACK-BONDING IMPORTANT FOR SIGMA-BOUND ALDEHYDE AND KETONE COMPLEXES - SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF AROMATIC ALDEHYDE COMPLEXES OF THE [CPFE(CO)(2)](+) CATION

The synthesis and characterization of a series of organometallic complexes, [CpFe(CO)(2)-(O=C(H)Ar)]PF6 (Ar = Ph (1), p-MeC(6)H(4) (2), p-NMe(2)C(6)H(4) (3), p-OMeC(6)H(4) (4), p-CF3C6H4 (5), p-CIC6H4 (6), p-(dimethylamino)cinnamaldehyde (7); Cp = eta(5)-cyclopentadienyl) are presented. Oxidation of [CpFe(CO)(2)](2) with AgPF6 or [Cp(2)Fe]PF6 in the presence of a 5-fold excess of aromatic aldehyde led to the formation of 1-7 in good to excellent yields (especially with the latter oxidant). Compounds 1-7 were analyzed by a combination of H-1 NMR (solution) and FTIR spectroscopic (solid and solution state) methods. Single-crystal X-ray diffraction studies on 6 and [CpFe(CO)(2)(O=C(H)-p-OMeC(6)H(4))]SbF6 (8) were also performed. Binding of these aldehydes to [CpFe(CO)(2)](+) in solution and the solid state occurs exclusively through oxygen a-donor interactions. Structural evidence for pi-back-bonding in sigma-bound aldehyde complexes is presented. These solid-state effects, however, do not manifest themselves in the solution equilibrium binding constants for this class of substrates.