Phosphine-substituted diiron complexes Fe2(μ-Rodt)(CO)6−n(PPh3)n (R = Ph, Me, H and n = 1, 2) featuring desymmetrized oxadithiolate bridges: structures, protonation, and electrocatalysis

Abstract

Two new series of phosphine-substituted diiron complexes Fe₂(μ-Rodt)(CO)_6−n(PPh₃)_n (n = 1 for 4–6 and n = 2 for 7–9) bearing desymmetrized oxadithiolate bridges (i.e., Rodt bridges), which can be considered as diiron subsite models of [FeFe]-hydrogenases, were prepared through the Me₃NO-induced replacements of all-CO diiron precursors Fe₂(μ-Rodt)(CO)₆ (Rodt = SCH(R)OCH₂S; R = Ph (1), Me (2), and H (3)) with one equivalent or excess PPh₃. All the as-obtained complexes have been fully characterized by elemental analysis, various spectroscopy methods, and especially for 4, 7–9 by X-ray crystallography. Further protonation and electrochemistry of 4–6 and 7–9 with Rodt bridges are studied and compared without and with CF₃CO₂H (TFA) and CH₃CO₂H (HOAc) as strong and weak acidic proton sources by in situ IR plus NMR spectroscopies and cyclic voltammetry. On the one hand, under excess TFA, the monosubstituted complexes 4–6 are partially protonated to produce a mixture of main precursors 4–6 and minor Fe-protonated species [4(μH)]⁺, [5(μH)]⁺, and [6(μH)]⁺, whereas the disubstituted counterparts 7–9 are completely protonated to form an isomeric mixture of their hydride species [7(μH)]⁺, [8(μH)]⁺, and [9(μH)]⁺ in transoid-dibasal and apical-basal fashions. On the other hand, complexes 7–9 show better electrocatalytic proton reduction activities (i.e., higher turnover numbers/TONs) under TFA or HOAc relative to counterparts 4–6, in which the TONs of 4, 5 and 7, 8 with Phodt or Meodt bridges are a little higher than those of 6 and 9 with odt bridges, respectively. These findings reveal that the redox and electrocatalytic behaviors of 4–6 and 7–9 with Rodt bridges are affected not only by phosphine coordination modes (PPh₃, mono- vs. di-substitution) but also by desymmetrized dithiolate bridges (Rodt, R = Ph, Me vs. H).