Synthesis and characterization of carboxy-functionalized diiron model complexes of [FeFe]-hydrogenases: Decarboxylation of Ph2PCH2COOH promoted by a diiron azadithiolate complex

Two carboxy-functionalized diiron complexes [{(μ-SCH2)2X}{Fe(CO)3}{Fe(CO)2L}] (X = NC3H7, L = Ph2PCH2CH2COOH, 4; X = CH2, L = Ph2PCH2COOH, 5) were prepared, as biomimetic models of the [FeFe] hydrogenase active site, from the CO-replacement of [{(μ-SCH2)2NC3H7}Fe2(CO)6] (1) and (μ-pdt)Fe2(CO)6 (2) by phosphine ligands in CH3CN at 40 °C, respectively. In contrast, the reaction of 1 with Ph2PCH2COOH under the same condition afforded complex [{(μ-SCH2)2NC3H7}{Fe(CO)3}{Fe(CO)2(Ph2PCH3)}] (3) with a decarboxylated phosphine ligand. The molecular structures of complexes 3–5 were determined by X-ray crystallographic analyses, which show that they have similar frameworks with the phosphine ligand on the apical position. The interesting C–H···S contacts between the methylene hydrogen atoms of the PhCH2COOH ligand and the μ-S atoms of the pdt-bridge are found in the crystal of 5. According to the experimental evidence, a plausible mechanism, via sequential phosphine coordination, N-protonation, and decarboxylation steps, is proposed for the formation of 3 and for explanation of the contrastive reactivities of the adt- (2-aza-1,3-propanedithiolato) and the pdt- (1,3-propanedithiolato) bridged diiron complexes toward decarboxylation of the Ph2PCH2COOH ligand.