Amine-containing tertiary phosphine-substituted diiron ethanedithioate (edt) complexes Fe2(μ-edt)(CO)6-nLn(n=1, 2): Synthesis, protonation, and electrochemical properties

As diiron subsite models of [FeFe]-hydrogenases for catalytic proton reduction to hydrogen (H-2), a new series of the phosphine-substituted diiron ethanedithiolate complexes Fe-2(mu-edt)(CO)(6-n)L-n(n= 1, 2) were prepared from the variable substitutions of all-CO precursor Fe-2(mu-edt)(CO)(6)(A) and tertiary phosphines (L1-L4) under different reaction conditions. While the Me3NO-assisted substitutions ofAand one equiv. ligands L1-L4 [L = Ph2P(CH2NHBut), Ph2P(CH2CH2NH2), Ph2P(NHBut), and Ph2P(C6H4Me-p)] produced the monosubstituted complexes Fe-2(mu-edt)(CO)(5)L (1-4) in good yields, the refluxing xylene solution ofAand two equiv. ligand L1 prepared complex Fe-2(mu-edt)(CO)(5){kappa(1)-Ph2P(CH2NHBut)} (1) in low yield. Meanwhile, the UV-irradiated toluene solution ofAand two equiv. ligand L3 resulted in the rare formation of the disubstituted complex Fe-2(mu-edt)(CO)(4){kappa(1), kappa(1)-(Ph2PNHBut)(2)} (5) in low yield, whereas the Me3NO-assisted substitution ofAand two equiv. ligand L4 afforded the disubstituted complex Fe-2(mu-edt)(CO)(4){kappa(1), kappa(1)-(Ph2PC6H4Me-p)(2)} (6) in good yield. All the model complexes1-6have been characterized by elemental analysis, FT-IR, NMR spectroscopy, and particularly for1,3,5by X-ray crystallography. Further, the protonations of complexes1-4are studied and compared with excess acetic acid (HOAc) and trifluoroacetic acid (TFA) by using FT-IR and NMR techniques. Additionally, the electrochemical and electrocatalytic properties of model complexes1-6are investigated and compared by cyclic voltammetry (CV), suggesting that they are electrocatalytically active for proton reduction to H(2)in the presence of HOAc.