Synthesis, Electrochemical Properties And Catalytic Hydroxylation Application Of [Fe-Fe]-Hydrogenase Model Complexes

A serious of novel biomimic hydrogenase complexes, diiron-hexacarbonyl-butanedithiol complex（μ-dmedt）[Fe（CO）3]2（I, dmedt = 2, 3-butanedithiol） and N-heterocyclic carbene（NHC） substituted（μ-dmedt）[Fe-Fe]-NHC（II,（μ-dmedt）[Fe（CO）2]2[IMe（CH₂）2IMe], IMe = 1-methylimidazol-2-ylidene; III, {（μ-dmedt）[Fe（CO）5]}2[IMe（CH₂）2IMe]; IV,（μ-dmedt）[Fe2（CO）5]IMes, IMes = 1, 3-bis（2, 4, 6-trimethylphenyl）imidazol-2-ylidene; V,（μ-dmedt）[Fe2（CO）5]IMe, IMe = 1, 3-dimethylimidazol-2-ylidene; VI,（μ-dmedt）[Fe2（CO）5]IPr, IPr = 1, 3- bis（2, 6-diisopropylphenyl） imidazol-2-ylidene） were synthesized and characterized by solution IR spectra, NMR spectra, elemental analysis and X-ray crystallography. The electrochemical properties of complexes I-VI and different proton sources（HOAc or H₂O） were investigated by cyclic voltammetry in coordinating solvent CH3 CN to evaluate the effects of different NHC ligands on the redox properties of the iron atoms of the series complexes. The catalytic reactivity for direct hydroxylation of benzene to phenol by [Fe-Fe]-NHC complexes was investigated. In addition, the optimized structures of I, II and IV, and the Gibbs free energy of their oxidation products were obtained by density functional theory calculation（DFT）.The symmetrically substituted cisoid basal/basal coordination complex II displays the lowest reduction potential own to its most δ-donating of NHC and the orientation of the NHC donor carbon as a result of the constraints of the bridging bidentate ligands. The kind of configuration of II is the first obtained in the field of [Fe-Fe]-NHC. And IPr as a ligand is first used to coordinate with [2Fe2S] active center. All the new complexes are catalysts for proton reduction to hydrogen. The redox potential is very close to the electron cloud density around Fe-Fe bond. All of the six complexes are relatively stable to air, water and weak acid.A new catalytic application for the [Fe-Fe]-NHC model complexes in the directly catalytic hydroxylation of benzene to phenol was also studied. Under the optimized experimental conditions（II, 0.01mmol; benzene, 0.1m L; CH3 CN, 2.0m L; H₂O2, 6.0mmol; 60°C, 3h）, the maximal phenol yield is 26.7%.The DFT calculation results indicated that DFT optimized structures of complexes I, II and IV are agree with the experimental results by X-ray crystallography. The Gibbs free energy of Fe-based oxygenated intermediate, FeII-（μ-O）-FeII, is lower than S-based oxygenated intermediate, [2Fe2S（S=O）]. So the former is thermodynamically favored and more stable than the latter.