Synthesis, Electrochemistry And Protonation Of Phosphine Substituted [2Fe2S] Complexes

The iron-only hydrogenase can catalyze the reversible reduction of proton to hydrogen atextraordinarily high rates. The active site, wherein the catalytic chemistry takes place, adoptsa bi-octahedral butterfly geometry, highly resembling the well-known organometalliccomplex [Fe₂(μ-SR)₂(CO)_6-xL_x]. Because of the simple structure and high efficiency,biomimetic models related to the active site of the Fe-only hydrogenase are of particularinterest to bioinorganic chemists. The major challenge is to understand the enzymaticcatalytic mechanism and to explore for the synthetic catalysts that play with hydrogenase-likefunction. A series of phosphine-substituted diiron dithiolate complexes were synthesized inthis thesis as the structural and functional models of the Fe-only hydrogenase active site. Thestructures, electrochemistry and protonation of these model complexes were studied.In order to investigate the function of the bridged-N in the active site of Fe-onlyhydrogenase, azadithiolate diiron model complex [{(μ-SCH₂)₂N(4-NO₂C₆H₄)}Fe₂(CO)₆] (1)and its mono- and di-phosphine substituted complex [{(μ-SCH₂)₂N(4-NO₂C₆H₄)}-Fe₂(CO)₅(PMe₃)] (2) and [{(μ-SCH₂)₂N(4-NO₂C₆H₄)}{Fe(CO)₂L}₂](3, L=PMe₃; 4, PMe₂Ph)were synthesized. The reaction of complex 3 with triflic acid gave two protonated products,[3(FeHFe)]⁺[PF₆]^- and [3(SH)]⁺(OTf)^-. The obtained complexes were characterized by IR,NMR, MS methods. The molecular structures of all diiron complexes except for[3(SH)]⁺(OTf)^- were determined by single crystal X-ray difffation. The variable temperature³¹P NMR spectra of complexes 2 and 3 were studied to get some information about theexistence of the conformation isomers of 2 and 3 in solution. The electrocatalytic property of{(μ-SCH₂)₂NC₆H₅}[Fe(CO)₂PMe₃]₂ (6) was investigated for the reduction of protons fromHOAc to molecular hydrogen. The azadithiolate (ADT) bridged diiron complex 6 exhibitedhigher electrocatalytic activity for reduction of protons from HOAc than its analogouspropanedithiolate (PDT) bridged diiron complex under the same measuring condition.For the purposes of (1) connecting a photosensitizer to the biomimetic model complexthrough the carboxy group and (2) immobilising the carboxy functionalized diiron complex tosome semiconductors or to an electrode, we try to introduce a carboxyl-functionalizedphosphine ligand to the diiron complex. The reaction of [{(μ-SCH₂)₂NC₃H₇}{Fe₂(CO)₆}] (7)with Ph₂PCH₂COOH afforded a decarboxylate product [(μ-SCH₂)₂NC₃H₇]-[Fe(CO)₃][Fe(CO)₂(Ph₂PCH₃)] (10), but the reaction of model complex 7 with Ph₂PCH₂CH₂COOH gave the expected target complex [{(μ-SCH₂)₂NC₃H₇}-{Fe(CO)₃}{Fe(CO)₂(Ph₂PCH₂CH₂COOH)}] (11). In order to understand this decarboxylationreaction, the reaction of (μ-PDT)Fe₂(CO)₆ and complex [{(μ-SCH₂)₂NH(CH₂C₆H₄-2-Br)}-Fe₂(CO)₆]⁺ClO₄^- (9) with Ph₂PCH₂COOH were carried out under the same condition whichgave expected products [(μ-PDT){Fe(CO)₃}{Fe(CO)₂(Ph₂PCH₂COOH)}] (12) and[{(μ-SCH₂)₂NH(CH₂C₆H₄-2-Br)}Fe₂(CO)₅Ph₂PCH₂COOH]⁺ClO₄^- (13), respectively. Theseexperimental results indicate that bridging-N atom plays an important role as a proton transferrelay in this decarboxylate reaction. A plausible decarboxylation reaction mechanism isproposed. On the basis of experimental facts, these results provide experimental evidence forchemists to better understanding the important function of the bridging-N atom as an internalbase in the Fe-only hydrogenase active site and the mechanism of enzymetic H₂ generation.