Diiron Carbonyl Complexes Bearing an N,C,S-Pincer Ligand: Reactivity toward Phosphines, Heterolytic Fe–Fe Cleavage, and Electrocatalytic Proton Reduction

The thiolate-bridged diiron carbonyl complex [{Fe­(μ-PyBPT-κ3N,C,S)­(CO)2}­Fe­(CO)3] (1) consists of two units, Fe­(PyBPT)­(CO)2 and Fe­(CO)3, where the N,C,S-pincer ligand PyBPT is a doubly deprotonated form of 3′-(2″-pyridyl)-1,1′-biphenyl-2-thiol. The two Fe complex units are connected through a thiolate S atom, π coordination, and an Fe–Fe bond. Diiron complex 1 reacted with 1 equiv of dimethylphenylphosphine to form the CO substitution product [{Fe­(μ-PyBPT-κ3N,C,S)­(CO)2}­Fe­(CO)2(PMe2Ph)] (3) via the phosphine adduct [{Fe­(μ-PyBPT-κ3N,C,S)­(CO)2}­Fe­(CO)3(PMe2Ph)] (2), which has a polarized Fe–Fe bond. A further reaction of 3 with PMe2Ph produced the N,C,S-pincer iron­(II) complex trans-[Fe­(PyBPT-κ3N,C,S)­(CO)­(PMe2Ph)2] (4) and the iron(0) complex trans-[Fe­(CO)3(PMe2Ph)2]. 1,2-Bis­(diphenylphosphino)­benzene (dppbz) abstracted the Fe­(CO)3 unit from 1 to give the dimeric diiron­(II,II) complex [{Fe­(μ-PyBPT-κ3N,C,S)­(CO)2}2] (7) and the iron(0) complex [Fe­(CO)3(dppbz)]. The asymmetric bridging ligand PyBPT and coordination of the phosphines induce polarization of the Fe–Fe bond, which leads to the formation of the iron­(II) and iron(0) complexes via heterolytic Fe–Fe cleavage. Electrochemical properties of 3 and 4 were investigated by cyclic voltammetry. Complex 3 showed two one-electron-reduction processes, the potentials of which are ca. 0.4 V more negative than those of 1. Electrocatalytic proton reduction by 3 was investigated, and the efficiency was comparable to that of 1.