Synthesis, Characterization And Electrochemical Studies Of Hydrogenase Active Site Models

Hydrogenases are enzymes that control formation and consumption of protonsand electrons by catalyzing the reversible proton reduction, and they play a centralrole in the natural hydrogen and energy metabolism. In recent years, it has receivedconsiderable attention due to its attractive application prospect in new energy. Thestudies in this field are in two aspects: on the one hand, it involves the structure,properties and catalytic mechanism of the natural hydrogenases that could strengthenour understanding in the structure and function of hydrogenases, on the other hand, itinvolves the study of biomimetic synthetic chemistry and the electrochemicalproperties of synthetic models, and this shows practical significance in hydrogenproduction in order to deal with the most important problems in this world, energycrisis and environment pollution. In this paper we synthesized a series of [FeFe] and[NiFe] hydrogenase models, and studied their electrochemical properties andelectrocatalytic behaviors. In addition, some electrochemistry of hydrogenase activesite models synthesized by our group have also been studied systematically. The mainresults are described as follows:1. Five new all-CO [2Fe₂Se] models [（μ-SeCH_2）2NR]Fe₂（CO）₆containing differencefunctional groups in the bridge head and two monosubstituted models[（μ-SeCH_2）2NC₆H₄OH-p]Fe₂（CO）5L （L=P（OEt）3; CNC₆H11） were synthesized andcharacterized. The structure of model2was determined by single crystal X-raydiffraction techaniques, and the electrochemistry of these models were studied inMeCN or DMF solution. It is suggested that the Fe-Fe bond length of2is slightlylonger than that of its S analog, and so more close to the corresponding bondlength of the reduced form of [FeFe] hydrogenases. All of these models cancatalyze hydrogen production in HOTs or HOAc acid. The all-CO [2Fe₂Se]Models can electrocatalyze HOTs reduction to hydrogen at the first reductionpotential（-1.5^-1.6V）. The first reduction potential of monosubstituted modelsshifted negatively by ca.200mV, and they can electrocatalyze HOAc to hydrogen at about-2.2^-2.3V, which is a more negative electrocatalytic potential thantheir parent model.2. Twelve models related to [FeFe] hydrogenase active sites synthesized by ourgroup were studied by cyclic voltammetry（CV） and controlled potentialelectrolysis（CPE）. The electrochemical redox processes were analysized, and theelectrocatalytic behaviors and mechanisms were investigated in the present ofdifferent acids. It is shown that the diiron thiadiselenolate parent model exhibitsdifferent electrocatalytic potential in the presence of different acids such as HOTs,CF₃COOH and HOAc, which means that different mechanisms occurred.Phosphine or nitrogen heterocyclic carbene monosubstituted diironthiadiselenolate and diiron oxadiselenolate type models exhibit betterelectrocatalytic activities when compared to their parent models, but it costs abigger overpotential. The electrocatalytic activity of diiron propaneditellurolatecomplex is in the same level comparing with the Se analog, and is better than theS analog in the presence of HOTs in MeCN. However, the electrocatalyticacitivities of [2Fe₂E] clusters are found to decrease on going from S to Se to Te inthe prense of HOAc in MeCN solution.3. Five trinuclear [NiFe₂] models [（Ph2P（CH₂）n）₂NR]Ni（μ3-Se）₂Fe₂（CO）₆（n=0,1）containing different nitrogen heterocyclic diphosphine ligands and fourmononuclear Ni models （dipp）Ni（SCH₂CH（COOEt）NHBoc）₂were synthesizedand characterized firstly. The structure of the [NiFe₂] models were determinedby single crystal X-ray diffraction techaniques, and the electrochemistry of themwere studied in MeCN or CH₂Cl2solution. It is suggested that the [NiFe₂] modelscan electrocatalytic HOTs to hydrogen at ca.-1.5V（vs Fc/Fc+）, and the active siteis located at the metal Ni center based on the spectroelectrochemical（SEC） studyof the reduction process by in situ IR techniques. The mononuclear Ni Modelsexhibit poor electrocatalytic activity in the present of HOAc in MeCN. In addition,the electrochemical properties of three mononuclear Ni complexes [N2S2]NiX2（X=Cl, Br, I） and one trinuclear [NiFe₂] model [N2S2]Ni（μ3-S）₂Fe₂（CO）₆related tothe active site of [NiFe] hydrogenases were investigated. It is suggested that thetrinuclear [NiFe₂] model can catalyze HOAc to hydrogen in MeCN，and this is the fisrt case associated with the electrocatalytic investigation of [NiFe]-hydrogenasemodels in the presence of HOAc.4. The electrochemical properties of two [Fe]-hydrogenase models were investigatedpreliminarily. It is suggested that they displayed an irreversible reduction at ca.-2.0V and an irreversible oxidation at ca.0.4V in MeCN. Both of them cancatalyze CF₃COOH or HOAc to hydrogen at a potential that is close to theelectrocatalytic potential of [FeFe]-hydrogenase models.