Synthesis, structures, and electrocatalytic properties of phosphine-monodentate, −chelate, and -bridge diiron 2,2-dimethylpropanedithiolate complexes related to [FeFe]-hydrogenases

Abstract

To further extend diiron subsite models of [FeFe]-hydrogenases, the various substitutions of all-carbonyl diiron complex Fe2(μ-Me2pdt)(CO)6 (A, Me2pdt = (SCH2)2CMe2) with monophosphines or small bite-angle diphosphines are studied as follows. Firstly, the monodentate complexes Fe2(μ-Me2pdt)(CO)5{κ1-P(C6H4R-p)3} [R = Me (1a) and Cl (1b)] and Fe2(μ-Me2pdt)(CO)5{κ1-Ph2PX'} [X' = NHPh (2a) and CH2PPh2 (2b)] are readily afforded through the Me3NO-assisted reactions of A with monophosphines P(C6H4R-p)3 (R = Me, Cl) and diphosphines (Ph2P)2X (X = NPh, CH2 (dppm)) in MeCN at room temperature, respectively. Secondly, the chelate complexes Fe2(μ-Me2pdt)(CO)4(κ2-(Ph2P)2X) [X = NPh (3a) and NBun (3b)] can be efficiently prepared by the UV-irradiated reactions of A with small bite-angle diphosphines (Ph2P)2X (X = NPh, NBun) in toluene. Thirdly, the bridge complexes Fe2(μ-Me2pdt)(CO)4(μ-(Ph2P)2X) [X = NPh (4a) and CH2 (4b)] are well obtained from the refluxing solutions of A and diphosphines (Ph2P)2X (X = NPh, CH2) in xylene. Rarely, the diphosphine-bridge complex 4b may be produced in low yield via the UV-irradiated solutions of A and the dppm ligand in toluene emitting at 365 nm. Eight new complexes obtained above have been well characterized by using element analysis, FT-IR, NMR (1H, 31P) spectroscopies, and particularly for 1a, 1b, 2a, 3b, 4a, 4b by X-ray crystallography. Meanwhile, the electrochemical and electrocatalytic properties of three representative complexes 2a, 3a, and 4a with pendant N-phenyl groups are investigated and compared by using cyclic voltammetry (CV) in the absence and presence of trifluoroacetic acid (TFA) as a proton source, indicating that they are all found to be active for electrocatalytic proton reduction to hydrogen (H2).