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-κ³<i>N,C,S</i>)­(CO)₂}­Fe­(CO)₃] (<b>1</b>) 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 <b>1</b> reacted with 1 equiv of dimethylphenylphosphine to form the CO substitution product [{Fe­(μ-PyBPT-κ³<i>N,C,S</i>)­(CO)₂}­Fe­(CO)₂(PMe₂Ph)] (<b>3</b>) via the phosphine adduct [{Fe­(μ-PyBPT-κ³<i>N,C,S</i>)­(CO)₂}­Fe­(CO)₃(PMe₂Ph)] (<b>2</b>), which has a polarized Fe–Fe bond. A further reaction of <b>3</b> with PMe₂Ph produced the N,C,S-pincer iron­(II) complex <i>trans</i>-[Fe­(PyBPT-κ³<i>N,C,S</i>)­(CO)­(PMe₂Ph)₂] (<b>4</b>) and the iron(0) complex <i>trans</i>-[Fe­(CO)₃(PMe₂Ph)₂]. 1,2-Bis­(diphenylphosphino)­benzene (dppbz) abstracted the Fe­(CO)₃ unit from <b>1</b> to give the dimeric diiron­(II,II) complex [{Fe­(μ-PyBPT-κ³<i>N,C,S</i>)­(CO)₂}₂] (<b>7</b>) 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 <b>3</b> and <b>4</b> were investigated by cyclic voltammetry. Complex <b>3</b> showed two one-electron-reduction processes, the potentials of which are ca. 0.4 V more negative than those of <b>1</b>. Electrocatalytic proton reduction by <b>3</b> was investigated, and the efficiency was comparable to that of <b>1