Terminal vs Bridging Hydrides of Diiron Dithiolates: Protonation of Fe2(dithiolate)(CO)2(PMe3)4

This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe2(xdt)­(CO)2(PMe3)4, where xdt is edt (ethanedithiolate, 1), pdt (propanedithiolate, 2), and adt (2-aza-1,3-propanedithiolate, 3), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds 1–3 oxidize near −950 mV vs Fc+/0. Crystallographic analyses confirm that 1 and 2 adopt C2-symmetric structures (Fe–Fe = 2.616 and 2.625 Å, respectively). Low-temperature protonation of 1 afforded exclusively [μ-H1]+, establishing the non-intermediacy of the terminal hydride ([t-H1]+). At higher temperatures, protonation afforded mainly [t-H1]+. The temperature dependence of the ratio [t-H1]+/[μ-H1]+ indicates that the barriers for the two protonation pathways differ by ∼4 kcal/mol. Low-temperature 31P­{1H} NMR measurements indicate that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate [Fe2(Hpdt)­(CO)2(PMe3)4]+ ([S-H2]+). This intermediate converts to [t-H2]+ and [μ-H2]+ by first-order and second-order processes, respectively. DFT calculations support transient protonation at sulfur and the proposal that the S-protonated species (e.g., [S-H2]+) rearranges to the terminal hydride intramolecularly via a low-energy pathway. Protonation of 3 affords exclusively terminal hydrides, regardless of the acid or conditions, to give [t-H3]+, which isomerizes to [t-H3′]+, wherein all PMe3 ligands are basal.