Iron complexes with polydentate phosphines as unusual catalysts for alcohol oxidation

The iron(II) compounds [Fe(triphos)(CH3CN)3](OTf)2 (triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane; OTf = CF3SO3) (1), [Fe(dppm)2(CH3CN)2](OTf)2 (dppm = bis(diphenylphosphino)methane) (2), [Fe(dppe)2(CH3CN)2](OTf)2 (dppe = 1,2-bis(diphenylphosphino)ethane) (3), [Fe(dppp)(CH3CN)4](OTf)2 (dppp = 1,3-bis(diphenylphosphino)propane) (4) and [Fe(PSP))(CH3CN)3](OTf)2 (PSP = bis(2-(diphenylphosphino)ethyl)sulfide) (5) were synthesized and characterized by NMR and (2, 3 and 4) also by X-ray crystallography. Such complexes catalyzed the selective oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones. The catalytic reactions were performed in acetonitrile in mild experimental conditions (r.t. or 50°C) using tert-butylhydroperoxide (TBHP) as oxidizing agent. By following the reaction of complex 4 with the oxidant by UV-visible spectroscopy, it was possible to evidence formation of the corresponding iron-peroxide intermediate. Comparison of ESI-MS spectra acquired on a solution of 1 or 4 before and after TBHP addition suggests ligand oxidation to iron-phosphine oxide complexes.