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 Fe(2)(xdt)(CO)(2)(PMe(3))(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 C(2)-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 (31)P{(1)H} NMR measurements indicate that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate Fe(2)(Hpdt)(CO)(2)(PMe(3))(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 PMe(3) ligands are basal.