Preparation, Structures And Electrochemical Properties Of Fe/S Complexes Containing A Carboxyl-Functionalized Phosphine Ligand Or A Rigid Bridge

[FeFe] Hydrogenases in microorgamisms can reversibly catalyze the proton reduction for hydrogen evolution. Studies on the crystal structures of the enzymes show that the active site of [FeFe] hydrogenases features a square-pyramidal butterfly coordination geometry, which is quite similar to that of the reported organometallic complexes formulated as [Fe₂(μ-SR)₂(CO)_6-nL_n].The simple structure and the high efficiency attract the great interest of synthesis researchers. They try to explore the catalytic mechanism for proton reduction and eventually find cheap and efficient catalysts for hydrogen production by mimicking the structure of the active site. The main work of this thesis is based on preparation, structures and electrochemical properties of Fe/S complexes containing a carboxyl-functionalized phosphine ligand or a rigid bridge.In order to investigate the function of the bridged-N in the active site of the Fe-Fe hydrogenase, we try to introduce a carboxyl-functionalized phosphine ligand to the diiron complex. The reaction of [{(μ-SCH₂)₂NC₃H₇}{Fe₂(CO)₆}] (1) with Ph₂PCH₂COOH afforded a decarboxylate product [(μ-SCH₂)₂NC₃H₇][Fe(CO)₃][Fe(CO)₂(Ph₂PCH₃)] (3), but the reaction of model complex 1 with Ph₂PCH₂CH₂COOH gave the expected target complex [{(μ-SCH₂)₂NC₃H₇}{Fe(CO)₃}{Fe(CO)₂(Ph₂PCH₂CH₂COOH)}] (4). In order to understand this decarboxylation reaction, the reaction of (μ-pdt)Fe₂(CO)₆ (pdt = propane-1,3-dithiolato) with Ph₂PCH₂COOH and [{(μ-SCH₂)₂NC₃H₇}{Fe₂(CO)₆}] (1) with Ph₂PCH₂COOC₂H₅ were carried out under the same condition which gave expected products [(μ-pdt){Fe(CO)₃}-{Fe(CO)₂(Ph_PCH₂COOH)}] (5) and [{(μ-SCH₂)₂NC₃H₇}{Fe(CO)₃}{Fe(CO)₂-(Ph₂PCH₂COOC₂H₅)}] (6), respectively. These experimental results indicate that the bridging-N atom plays an important role as a proton transfer relay in this decarboxylate reaction. A plausible decarboxylation reaction mechanism is proposed. These results provide experimental evidence for chemists to better understanding the important function of the bridging-N atom as an internal base in the Fe-Fe hydrogenase active site and the mechanism of enzymetic H₂ generation.In order to tune the reduction potential of the Fe/S complex, the rigid and conjugated bridge benzene-1,2,4,5-tetrathiolate was introduced to the Fe/S model. Complexes [(CO)₆Fe₂](μ-btt-μ)[Fe₂(CO)6] (7) (btt = benzene-1,2,4,5-tetrathiolate) and its phosphine ligand substituted complexes [(CO)₆Fe₂](μ-btt-μ)[Fe₂(CO)₅(PPyr₃)] (8) and [(PPyr₃)(CO)₅Fe₂](μ-btt-μ)[Fe₂(CO)₅(PPyr₃)] (9) were prepared by the reactions of Fe₂(CO)₉ and 1,2,4,5-Tetramercaptobenzene in tetrahydrofuran, respectively. X-ray crystal diffraction confirms the plane structure of the bridge in complexes 7, 8 and 9. The results of electrochemistry indicate that the reduction wave of complex 7 observed corresponds to a one-electron process which is different from the [2Fe2S] analog [(μ-bdt)₂Fe₂(CO)₆] (bdt = benzene-1,2-dithiolate). Complex 7 can catalyze the reduction of protons to hydrogen at -1.35 V vs. Fc/Fc⁺ in the presence of ClCH₂COOH.