Properties of Fe(salen)2-μ-oxo Derivatives and their Effect on Catalytic Hydrophosphination

A series of Fe(salen)2(μ-O) and Fe(salophen)2(μ-O) complexes (1a to 1i) were synthesized and characterized to investigate the effect of ligand electronics and backbone structure on their physical and catalytic properties. UV-visible spectroscopy revealed that complexes bearing aromatic salophen ligands exhibited minimal variation in the ligand-to-metal charge transfer (LMCT) band, whereas electron withdrawing and donating groups on the phenoxy fragment of salen (1a to 1c) showed significant shifts, indicating tunable electronic properties. Cyclic voltammetry confirmed reversible Fe(III)/(II) redox couples for the salen complexes with formal potentials ranging from −0.95 to −1.42 V (vs. Fc/Fc+), demonstrating the sensitivity of the iron center to phenolate substituents and backbone length. Infrared spectroscopy of the Fe–O–Fe stretch correlated electronic effects with bond strength. Catalytic hydrophosphination of styrene with diphenylphosphine established 1a to 1c as the most active precatalysts, affording up to 81% yield of the tertiary phosphine product. In situ IR and NMR studies indicated rapid Fe–O–Fe bond cleavage to generate mononuclear Fe(III) species. A postulated mechanism involving an active iron phosphide intermediate (3a) and an inactive iron hydroxide by-product (4a) is presented.