Electrochemistry of methylidyne and 1,2,3,5-dithiadiazole complexes of iron and nickel and a reinterpretation of the composition of "PhCN2S2Fe2(CO)6"

Abstract

The title compounds have been studied in CH2Cl2 by cyclic voltammetry. Nonacarbonyldi(μ3-methylidyne)triiron, [Fe3(CO)9(μ3-CX)2] (X = F, Cl, Br, H), undergo chemically reversible 1e− reduction (−0.72 to −0.88 V vs. SCE) and irreversible oxidation (+1.59 to +1.72 V); the compound for which X = F displays a second, irreversible reduction (−1.96 V). Using reduction potentials of comparable μ3-E (E = S, Se, NPh, PPh) clusters, μ3-methylidyne ligands are shown to be strongly basic, comparable to PPh and NPh and more basic than S or Se. The methylidyne clusters are both more difficult to oxidize and reduce than isomeric alkyne clusters [Fe3(CO)9(C2R2)], indicative of greater thermodynamic stability. The complexes previously formulated as [Fe2(CO)6(S2N2CC6H4X)] (X = H, CF3, OCH3) are reversibly reduced (−1.59 to −1.70 V) and irreversibly oxidized (+1.0 to 1.3 V), unlike free dithiadiazole radicals, which are reversibly oxidized around +0.8 V. This behaviour is not consistent with the ring-centered free radical claimed for the case where X = H; instead we find conclusive evidence that these are diamagnetic complexes of the 3-H-1,2,3,5-dithiadiazolines, [Fe2(CO)6(S2N{NH}CC6H4X)] (νNH = 3376−3381 cm−1;δNH = 6.55−7.1 ppm). Dicyclopentadienyl-dithiadiazoledinickel, [Ni2Cp2(S2N2CC6H5)], is both reversibly reduced (−0.79 V) and oxidized (+0.45 V); and irreversibly oxidized at +1.9 V. This is consistent with an unpaired electron, and only the nickel cluster has an ESR signal (g = 2.0410, no resolved hyperfine splitting). Extended Hückel theory locates the free electron in a SOMO restricted to Ni, C, and S atoms. Keywords: electrochemistry, organometallic, methylidyne, dithiadiazole, dithiadiazoline.