| [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-nLn]. 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. In this thesis, the effect of ligand exchange on the structure and property of the diiron dithiolate complexes were studied.A series of unsymetrically diphosphine substituted [2Fe2S] model complexes, [(μ-pdt){Fe(CO)2L1}{Fe(CO)2L2}] [pdt = propane-1,3-dithiolato; L1 = PMe3, L2 = PMe2Ph, 4; L1 = PMe3, L2 = PPh3, 5; L1 = PMe3, L2 = PCy3,6; L1 = PMe3, L2 = P(OEt)3, 7; L1 = PMe2Ph, L2 = PPh3, 8; L1 = PMe2Ph, L2 = P(OEt)3, 9; L1 = P(OEt)3, L2 = PPh3,10; L1= P(OEt)3, L2 = PCy3, 11] and [(μ-edt){Fe(CO)2(PMe3)}{Fe(CO)2(PPh3)}] (edt = ethane-1,2-dithiolato, 12) were successfully synthesized by stepwise phosphine ligand replacement. X-ray single crystal diffraction reveals that bulky phosphine ligand prefers the apical site, while the smaller one coordinates at the basal site. When two phosphine ligands, i.e., PPh3 and P(OEt)3 in complex 10, are both bulky ligands, an apical/apical coordination conformation is preferred. Electrochemistry results indicated that different phosphine ligands led to ca. 200 mV difference for the redox potentials of the unsymetrically diphosphine substituted complexes.Mono- and disubstituted complexes, [(μ-pdt)Fe2(CO)5(THP)] (13) and [(μ-pdt){Fe(CO)2(THP)}2] (14), were synthesized for the purpose of introduction of a water soluble phosphine ligand, tris(hydroxymethyl)phosphine (THP), to the [2Fe2S] model complex. Disubstituted complex 14 possesses good water solubility in water and bad stability in air atmosphere. Complex 13 has poor solubility in pure water while it can be dissolved in CH3CN/H2O mixed solution. The catalytic activity for proton reduction of complex 13 in CH3CN/H2O (1:1, v/v) is higher than that in pure CH3CN in the presence of HOAc. Indefinitely extended sandwich FeS/OH/FeS packing mode is found in the packing diagram of complex 13. Wavily assembled two dimensional networks are constructed by intra- and intermolecular O-H…O hydrogen bonds. Right- and left-handed O-H…O-H helical chains are alternately arranged in the extended network.Phosphine ligands featuring an internal base of pyridine ring were introduced to the [2Fe2S] model by synthesis of complexes [(μ-pdt)Fe2(CO)5L] (L = Ph2PCH2Py, 15; Ph2PPy, 16). Potential shifts of 360 and 490 mV to positive direction were found for the first reductions of complexes 15 and 16, respectively, in the presence of HOTf. It is proposed that the anodic shifts are caused by the protonation of the pyridyl-N atoms in Ph2PCH2Py and Ph2PPy. Protonated species [(μ-pdt)Fe2(CO)5L][OTf] (L = [Ph2PCH2PyH]+, 19; [Ph2PPyH]+, 20) were isolated and characterized by X-ray crystal diffraction. Complexes [(μ-pdt){Fe(CO)2(PMe3)}{Fe(CO)2L}] (L = Ph2CH2Py, 17; Ph2PPy, 18) were synthesized for the enhancement on the protophilicity of the Fe-Fe bonds while remaining the pyridine ring as an internal base in the same molecular. Low temperature NMR (-55℃) techniques and in situ IR (-10℃) spectroscopy were used to trace the protonation process of complex 18 in the presence of strong acid (HOTf). The results showed that complex 18 was protonated first on the Fe-Fe bond. The doubly protonated species [18HyH]2+ was formed and detected at low temperature while it would decomposed at room temperature.In order to tune the reduction potential of the [2Fe2S] complex, the rigid and conjugated bridge containing the electron withdrawing group (C=O) was introduced to the [2Fe2S] model. Complexes [μ-SC6H4-2-(CO)S-μ]Fe2(CO)6 (21) and [μ-2-SC5H3N-3-(CO)S-μ]Fe2(CO)6 (22)were prepared by the reactions of Fe2(CO)9 and (?) or (?) in THF at room temperature, respectively. X-ray crystal diffraction confirms the plane structure of the bridge in complexes 21 and 22. The results of electrochemistry indicate that the rigid and conjugated bridge indeed results in the positive shift of the reduction potentials. The first reduction event of complex 22 appears at -1.18 V (vs. Fc/Fc+), which is 100 mV positive than that of complex 21.All synthesized diiron dithiolate complexes were characterized by IR, NMR, MS and elemental analysis. Structures of complexes 4-6, 8-10, 12, and 15-22 were determined by X-ray single crystal diffraction.
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