Reduction and Oxidation of the Arachno Clusters Fe2(CO)6(μ-S)2PtL2: Characterization of Distinct One- and Two-Electron Transfer Sites in Heteropolynuclear Compounds

Abstract

The reduction and oxidation properties of the arachno-heteropolynuclear metal clusters Fe2(CO)6(μ3-S)2PtL2, where L2 = bipyridine (1), phenanthroline (2), diphenylphosphinoethane (3), diphenylphosphinoferrocene (4), or cyclooctadiene (5), have been examined by electrochemistry, IR spectroscopy, and ESR spectroscopy. An electron-transfer series involving up to five electrons is observed. The common characteristics are a fully reversible one-electron oxidation process to a monocation (in the range 0.13−0.38 V vs ferrocene) and a chemically reversible but electrochemically quasi-reversible two-electron reduction process (range −1.9 to −2.4 V). Complexes 1 and 2 also display a fully reversible one-electron reduction at a potential less negative (ca. −1.8 V) than the subsequent two-electron reduction. The LUMOs of 1 and 2 are predicted by EHMO calculations to be primarily π*-diimine based. Spectral data from the monoanions 1- and 2- are consistent with this orbital assignment: very small shifts of the metal−carbonyl IR frequencies (ca. −13 cm-1) from those of the neutral complexes are observed, and the 195Pt hyperfine splitting in ESR spectra is very small compared to that expected for a metal-localized radical. The two-electron reductions forming, e.g., 13- or 32-, appear to involve the Fe2S2 moiety and most likely are concomitant with breaking and making of an Fe−Fe bond. Carbonyl IR energies are lowered in excess of 100 cm-1 in the two-electron reduction. The oxidation processes appear to be localized on the Fe2(CO)6S2 fragment, with IR shifts of 46−76 cm-1 for 1+−5+ and small 195Pt hyperfine splittings. X-ray crystal structures are reported for 2 and 5.