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

Abstract

As biomimetic models for the active site of Fe-only hydrogenases,six new N-substituted diiron azadithiolates (ADT) were prepared.Treatment of CH 2Cl 2 solutions of primary amines RNH 2 with paraformaldehyde followed by an excess of SOCl 2 gave N, N-bis(chloromethyl)amines RN(CH 2Cl) 2 ( 1,R = CH 2CO 2Et; 2,C 6H 4C(O)Me- p; 3,C 6H 4CO 2Me- p; 4,C 6H 4SCN- p) in 30–90% yields.Further treatment of the chloromethylated amines 1– 4 with ( μ-LiS) 2Fe 2(CO) 6 in THF resulted in formation of the corresponding N-substituted ADT-type models [( μ-SCH 2) 2NR]Fe 2(CO) 6 ( 5,R = CH 2CO 2Et; 6,C 6H 4C(O)Me- p; 7,C 6H 4CO 2Me- p; 8,C 6H 4SCN- p) in 24–75% yields.Also prepared were the N-substituted models [( μ-SCH 2) 2NC(O)CH 2C 10H 7- α]Fe 2(CO) 6 ( 9) and 1,4-[Fe 2(CO) 6( μ- SCH 2) 2NC(O)] 2C 6H 4 ( 10) by reaction of CH 2Cl 2 solutions of [( μ-SCH 2) 2NH]Fe 2(CO) 6 with α-C 10H 7CH 2COCl and 1,4-C 6H 4(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 CF 3CO 2H or weak acid HOAc under electrochemical conditions. In addition, two mechanisms ECCE and EECC are preliminarily suggested for such two electrocatalytic H 2 production processes, respectively.