Synthesis And Characterization Of Heterocycle Containing Diiron Azadithiolate Complexes As The Active Models Of Hydrogenase

Fe-only hydrogenase can catalyze the reduction of protons to molecular hydrogen and the oxidation of hydrogen to protons at extraordinarily high rates. Owing to the butterfly geometry and high efficiency, the chemical mimic and the mechanism of hydrogen production of the hydrogenases active site are of particular interest to researchers in this field. Decreasing the electron density of the Fe atoms of the [2Fe2S] core can lead to lower the reduction potentials, which is the key to produce dihydrogen. Herein, a series of diiron disulfide complexes, through introducing five- or six- membered heterocyclic rings, were synthesized as the structural and functional models of the Fe-only hydrogenase active site. We expected that the protonation after introduction of an electron-rich five-membered ring or the direct attachment of the electron-poor six-membered heterocycle on the Fe-S cluster might have some favorable effects on its reduction potential.Furan, thiophen and pyridine were introduced as a N-subsititutents for the synthesis of a series of complexes [{(μ-SCH₂)₂NCH₂(2-C₄H₃O)}Fe₂(CO)₆](4), [{(μ-SCH₂)₂NCH₂(2-C₄H₃S)} Fe₂(CO)₆] (5), [{(μ-SCH₂)₂NCH₂(5-Br-2-C₄H₃O)} Fe₂(CO)₆] (8), [{(μ-SCH₂)₂NCH₂ (2-C₅H₄N)}Fe₂(CO)₆] (25), with the basic structure of azadithiolate (ADT). The electrochemical characteristic of 4 and 8 showed that their reductive potentials were lowered to reach -1.13 V (for furan derivative) and -1.09 V (for thiophene derivative) in the presence of HClO₄. In addition, the introduction of bromo to the thiophene ring on complex 8 made it better catalytic capability. Under the strong acid, the protonation, occurred on the N-bridged atom for 4, 5 and 8, but on the pyridine N for 25, which was the protonation product 25H.The ligand exchanges of complex 4 and 25 were further studied. Ligand exchanges of carbonyls with the electron-donation ligands can strengthen the electron density of diiron atoms, therefore are easier to capture a proton. In this study, complexes 17-20 with mono phosphorous ligands, 21, 22 with diphosphorous ligands, and moreover, complexes 23 and 26 with bis 1, 3-dimethylimidazol-2-ylidene carbene ligands were synthesized. The reduction potential of complex 22 was observed at -2.3 V, due to the increased electron density of Fe-Fe bond by the electron-donation ligand PMe₃. The protonation occurred between diiron core in the presence of F₃CCO₂H. Furthermore, the reduction potential of 23 and 26 were at around -2.5 V, attributed to the stronger electron-donation carbene ligands. In addition, the IR results of the disubstituted carbene complex 23 and 26 were optimistic, owing to the lower wave number of carbonyl peak at 1963-1886cm^-1, which shows that electron-donating ability is stronger and capturing proton is easier. The negative hydrogen signals were also detected at -27 and -55 ppm by ¹H NMR.In addition, a new sort of NBS reaction was found unexpectedly in the synthesizing of complex 8. When we tried to perform the bromination of complex 5 by using NBS, we did not obtain the expected bromo-substituted product, but the regeneration of the starting material [(μ-S₂)Fe₂(CO)₆] (1) was observed. A possible mechanism was proposed after the auxiliary experiments with different reaction conditions, reactants, as well as the characterization of structures.All complexes were characterized by MS, NMR and IR spectra and the structures of 4, 5, 8, 17, 19, 21-23, 25, 25H and 26 were determined by X-ray crystal diffraction.