Protonation and electrochemical properties of a bisphosphide diiron hexacarbonyl complex bearing amino groups on the phosphide bridge

Abstract

A bisphosphide-bridged diiron hexacarbonyl complex 3 with NEt₂ groups on the phosphide bridge was synthesized to examine a new proton relay system from the NEt₂ group to the bridging hydride between the two iron centers. As a precursor of the bridging moiety, peri-Et₂NP–PNEt₂-bridged naphthylene 5 was synthesized by the reaction of 1,8-dilithionaphthylene with two equivalents of Cl₂PNEt₂ followed by reductive P–P bond formation by magnesium. The reaction of the diphosphine ligand 5 with Fe₂(CO)₉ gave the diiron hexacarbonyl complex 3, in which the P–P bond of the ligand was cleaved to form the bisphosphide-bridge. The molecular structure of 3 indicated that the trigonal plane of the NEt₂ group was forced to face the Fe–Fe bond to avoid steric congestion with the naphthylene group linking the two phosphide groups. The NEt₂ group could be protonated by p-toluenesulfonic acid. Density functional theory (DFT) calculations confirmed that the proton of the N(H)Et₂ group adopted a position close to the bridging hydride. The DFT results for the ferrocene analogue 1, in which the 1,8-naphthylene group of 3 was replaced with the 1,1′-ferrocenylene group, also revealed that the most stable orientation of the protonated NHEt₂ group was that in the protonated 3. As a result, electrochemical proton reduction reactions using complexes 1 and 3 proceeded with similar catalytic efficiencies. Unfortunately, the catalytic efficiencies (CEs) of these complexes were much lower than those of the complexes with a proton relay system of the terminal hydrogen, indicating that the reactive properties of the bridging hydride in the present proton relay system cannot exceed those of the terminal hydride.