Synthesis, structure and electrochemical properties of N-substituted diiron azadithiolates as active site models of Fe-only hydrogenases

As biomimetic models for the active site of Fe-only hydrogenases,six new N-substituted diiron azadithiolates (ADT) were prepared.Treatment of CH2Cl2 solutions of primary amines RNH2 with paraformaldehyde followed by an excess of SOCl2 gave N,N-bis(chloromethyl)amines RN(CH2Cl)2 (1,R = CH2CO2Et;2,C6H4C(O)Me-p;3,C6H4CO2Me-p;4,C6H4SCN-p) in 30–90% yields.Further treatment of the chloromethylated amines 1–4 with (μ-LiS)2Fe2(CO)6 in THF resulted in formation of the corresponding N-substituted ADT-type models [(μ-SCH2)2NR]Fe2(CO)6 (5,R = CH2CO2Et;6,C6H4C(O)Me-p;7,C6H4CO2Me-p;8,C6H4SCN-p) in 24–75% yields.Also prepared were the N-substituted models [(μ-SCH2)2NC(O)CH2C10H7-α]Fe2(CO)6 (9) and 1,4-[Fe2(CO)6(μ- SCH2)2NC(O)]2C6H4 (10) by reaction of CH2Cl2 solutions of [(μ-SCH2)2NH]Fe2(CO)6 with α-C10H7CH2COCl and 1,4-C6H4(COCl)2 in 81% and 28% yields, respectively. All the new compounds 1–10 were characterized by elemental analysis and spectroscopy, as well as for 5–7 and 9 by X-ray crystallography. The crystallographic studies indicated that the functionality of 5 attached to the bridged N atom lies in an equatorial position, whereas those of functionalities of 6, 7, and 9 are located in an axial position. This is presumably due to different electronic and steric effects between the N-substituted aliphatic and aromatic functionalities. More interestingly, model 7 has been found to be a catalyst for proton reduction in the presence of either strong acid CF3CO2H or weak acid HOAc under electrochemical conditions. In addition, two mechanisms ECCE and EECC are preliminarily suggested for such two electrocatalytic H2 production processes, respectively.