Influence Of Ligands On The Oxidation Property And H₂ Activation Capability Of [FeFe]-Hydrogenase Models

[FeFe]-hydrogenases ([FeFe]-H2ases) are a kind of metalloenzymes that can reversibly catalyze the redox of H2/H+ at remarkable reaction rates. The reaction undergoes via a pathway with very low energy barriers. The structural and functional mimicking of the [FeFe]-H2ase active site has attracted extensive attention in recent decades. There are many reports on the biomimic of proton reduction to H2 by [FeFe]-H2ase models, while less attention has been paid in modeling of their H2 oxidation function. This thesis focuses on hydrogen activation by [FeFe]-H2ase models. A series of [FeFe]-H2ase active site models were designed and synthesized. The influence of different diphosphine ligands and the S-to-S bridging units on the oxidation property and hydrogen activation capability of these 2Fe2S complexes was studied by using electrochemical and in-situ IR spectroscopic techniques.First, we prepared and characterized two 2Fe2S complexes, [(?-edt){Fe(CO)3}{Fe(CO)(?2-P2N2)}] (1, edt=ethane-1,2-dithiolate, P2N2= 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) and [(?-edt){Fe(CO)3}{Fe(CO)(?2-P2N)}] (2, P2N=N,N-bis((diphenylphosphino)methyl)-propanamine), which have the same S-to-S bridge, an ethylene bridge, but different diphosphine ligands. Complex 1 contains a pendant diamine diphosphine ligand (P2N2), while 2 has a uniamine diphosphine ligand (P2N). X-ray crystallographic studies revealed that the noticeable difference between these two complexes in their crystal structures is that the two P atoms of diphosphine ligand in 1 are in a basal-basal configuration due to the bulky volume of P2N2 ligand, while the two P atoms of diphosphine ligand in 2 feature a basal-apical configuration. Cyclic voltammetric studies showed that the oxidation property of these 2Fe2S complexes, especially the second oxidation event, is significantly influenced by the chelating diphosphine ligand. The Fe?Fe?/Fe?Fe? and Fe?Fe?/Fe?Fe? oxidation peaks of 1 are very close and not well separated, while the two oxidation peaks of 2 are completely separated by 0.9 V (?Ep=?Epox2-Epoxl?). The in situ IR spectroscopy further demonstrated the difference of these two complexes in their oxidation property. Complex 1 successively took place two-step single-electron oxidation in the presence of a mild oxidant (Fc+), while under the same conditions, the oxidation of 2 stopped at the singel-electron oxidation stage, which could be reduced to fully recover the initial complex. The in-situ IR spectroscopic studies indicate that2 can catalyze H2 oxidation under mild conditions, although in low turnover numbers, while 1 is decomposed under the same conditions.To explore the effect of S-to-S linker in [FeFe]-H2ase models on H2 activation, we prepared and characterized other two 2Fe2S complexes, [(?-bdtMe){Fe(CO)3}{Fe(CO)(?2-P2N)}] (3, bdtMe= 4-methylbenzene-1,2-dithiolate) and [(?-adtBn){Fe(CO)3}{Fe(CO)(?2-P2N)}] (4, adtBn=N-benzyl-2-azapropane-1,3-dithiolate). These two complexes together with 2 form a series of 2Fe2S complexes, which have the same P2N ligand but different S-to-S bridges. X-ray crystallographic studies showed that the framework structures of 3 and 4 are very similar with that of 2 although they have different S-to-S linkers. Cyclic voltammetric studies revealed that the potentials for the first oxidation event of 2,3, and 4 are almost identical, while the potentials of their second oxidation events are quite different. Therefore the ?Ep? values of these complexes are different; it is in an order of ?(2)>?Ep(3)>?Ep(4). The in-situ IR spectra showed that the single-electron oxidized species of 2 has good stability in solution and it can activate H2 in the presence of excess of oxidant, while the single-electron oxidized species of 3 and 4 are not stable in solution and decomposed to mononuclear iron complexes at room temperature, and therefore cannot activate H2 under mild conditions.