Structural And Functional Mimic Of The Active Site Of[FeFe]-Hydrogenases

[FeFe]-hydrogenase ([FeFe]-H2ase) is so far the most efficient catalyst for water reduction, which has a catalytic rate of6000～9000mol H2per second. Chemists have extensively studied its structural and functional mimics and catalytic mechanism since the crystal structure of [FeFe]-H2ase was determined. Chemists have got some features of the hydrogenases existing in nature by studying specially designed model complexes. The intermediates of one electron oxidation state of [FeFe]-H2ases models obtained in lab show almost the same configuration as the Hox state of hydrogenases. These models have similar electron distribution and magnetic property as the Hox. Unfortunately, although the models possess similar structure of [FeFe]-H2ases, they have catalytic properties different from hydrogenases. Some Co and Ni complexes show good photochemical and electrochemical catalytic H2-evolving properties, giving examples for mimicking the function of hydrogenases by small molecules, though no breakthrough is found in functional mimic of [FeFe]-H2ases.Tetra-and hexanuclear iron-sulfur complexes2,2’and3were prepared and structurally characterized. These mini dendritic iron complexes display redox feature quite different from that of most reported polynucleariron complexes. Complexes2and3display, respectively, two and three consecutive two-electron, metal-center-based reductionevents in the range of-1.33to-1.81V, with relatively narrow potential spans for four-and six-electron transformations. SEC-IR spectra give experimental evidence for the formation of a μ-CO bridge in the two-electron-reduced [Fe2S2(CO)6] unit of2and3. The good reversibility of the reduction processes implies the stabilization of the reduced species of2and3by the conjugated and rigid bridge.A specially degnined bridge was introduced into the model complex6of [FeFe]-hydrogenase, The "cap-like" bridge resulted in an asymmetric coordination environment of6. The repulsive force from the bridge made the iron center oxidized more easily than its analogue6b. When complex6was oxidized, the ketone carbonyl of the bridge reacted with the water in solvent, and formed a gem-diol sepcies. The oxygen of the ketone coordinates to the open site on the iron center resulting from rotation, thereby forming a covalent bond with the iron atom to afford a relative stable oxidized product7. This complex shows a similar structure with the oxidized state (Hoxair) of the [FeFe]-H2ase active site, and provides a new Fe"Fe" model for the mimic of [FeFe]-H2ases.A cobalt complex9proves to be an excellent catalyst for electrochemical production of hydrogen both in neutral water and sea water, which displays an extremely low overpotential (80mV) while maintaining high activity and good durability. It displays an activity of1470mol H2(mol cat)-1h-1(cm2Hg)-1over20h CPE (controlled potential electrolysis) experiment at an applied potential of-1.0V in phosphate buffer at pH7, without apparent deactivation. The other merit of this catalyst is its self-assembling property from simple salts of earth-abundant elements, which is reminiscent of self-formation of many metalloenzymes in nature. The discovery of this unusual molecular cobalt catalyst shows promise in using earth-abundant metal-based catalysts as surrogates of precious metal-based ones such as platinum to create highly efficient and economical catalyst systems for water reduction to a sustainable fuel-hydrogen.