Protonation and Electrochemical Properties of Bisphosphide Diiron Hexacarbonyl Complex Bearing Amino Groups on the Bridge

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 a 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 diironhexacarbonyl 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 the bridging hydride. The DFT results on the ferrocene analog 1, in which the 1,8-naphthylene group of 3 was replaced with a 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 using complexes 1 and 3 proceeded with similar catalytic efficiency. Unfortunately, the catalytic efficiencies (CE) of these complexes were much lower than those of complexes with a proton relay system to the terminal hydrogen, indicating the reactive property of the bridging hydribe in the present proton relay system cannot exceed that of the terminal hydride.