Structures, protonation, and electrochemical properties of diiron dithiolate complexes containing pyridyl-phosphine ligands

Abstract

Diiron complexes containing pyridyl-phosphine ligands, that is, (μ-pdt)[Fe₂(CO)₅L] (pdt = S(CH₂)₃S, L = Ph₂PCH₂Py, 3a; Ph₂PPy, 3b) and (μ-pdt)[Fe(CO)₂(PMe₃)][Fe(CO)₂L] (L = Ph₂PCH₂Py, 4a; Ph₂PPy, 4b) were prepared as model complexes of the [FeFe]-hydrogenase active site. Protonation of 3a and 3b by HOTf afforded the pyridyl-nitrogen protonated products [3aH_N][OTf] and [3bH_N][OTf], respectively. The molecular structures of 3a, 3b, 4a, 4b, as well as [3aH_N][OTf] and [3bH_N][OTf] were confirmed by X-ray diffraction studies, which show that the Ph₂PCH₂Py ligand occupies the basal position both in 3a and its protonated species [3aH_N][OTf], while the Ph₂PPy ligand prefers the apical position in 3b and [3bH_N][OTf]. The double protonation process of complex 4b was monitored by in situ IR, ¹H and ³¹P NMR spectroscopy at low temperature. The spectroscopic evidence indicates that the protonation of 4b occurs first at the Fe–Fe bond and then at the pyridyl-nitrogen atom. Cyclic voltammograms reveal that protonation of 3a and 3b results in a considerable decrease in the overpotential for electrocatalytic proton reduction in the presence of HOTf, while the efficiency is not influenced by protonation. The electrocatalytic efficiency of 4a for proton reduction in the presence of HOAc in CH₃CN–H₂O (50 : 1, v/v) is 5 times higher than that in pure CH₃CN.