Effect Of The Second Coordination Sphere On The Redox Reactions And C-H Bond Activation Of [FeFe]-Hydrogenase Active Site Models

[FeFe]-hydrogenase ([FeFe]-H2ase) can reversibly catalyze the oxidation of hydrogen and the reduction of protons. The high efficiency and special structure of the [FeFe]-H2ase active site have attracted a great attention. In the past decade, the structural and functional mimicking of the [FeFe]-H2ase active site and investigation of the enzymatic mechanism for dihydrogen oxidation and proton reduction is one of the hot topics in the field of bioinorganic chemistry. The remarkable efficiency of the [FeFe]-H2ase enzyme is closely related to its unique coordination sphere that provides a low-energy pathway for H-H bond cleavage and formation. In this thesis, a series of diiron dithiolate complexes containing a pendant phosphine or pendant amine bases were synthesized, aiming at exploring the influence of the second coordination sphere on the redox property of the [FeFe]-H2ase models.A diiron dithiolate complex with a pendant phosphine coordinated to one of the iron centers, [(μ-adtP)Fe2(CO)5] (adtP= (μ-SCH2)2NCH2C6H4-o-PPh2), was prepared and structurally characterized. The pendant phosphine is dissociated together with a CO ligand in the presence of excess PMe3, to afford complex [(μ,-adtP){Fe(CO)2(PMe3)}2] (1). Redox reactions of 1 and related complexes were studied in detail by in situ IR spectroscopy. A series of new FeIIFeI (7), FeIIFeII(4), and FeIFeI (5) complexes relevant to Hoxco, Hoxair, and Hred states of the [FeFe]-hydrogenase active site were characterized. Among these complexes, the molecular structures of the diferrous complex [(μ-adtP){Fe(CO)(PMe3)}{Fe(CO)3(PMe3)}] (4) with the internal amine and the pendant phosphine co-coordinated to the same iron center and the triphosphine diiron complex [(μ-adtP){Fe(CO)(PMe3)}{Fe(CO)2(PMe3)}] (5) were determined by X-ray crystallography. To make a comparison, the redox reactions of an analogous complex, [(μ-SCH2N(CH2Ph)CH2S){Fe(CO)2(PMe3)}2] (6), were also investigated by in situ IR spectroscopy in the absence or presence of extrinsic PPh3, which has no influence on the oxidation reaction of 6. The results clearly show that the pendant phosphine in the second coordination sphere makes the redox reaction of 1 different from that of its analogue 6.In response to whether [FeFe]-H2ase models could be extended to C-H activation and to figure out the effect of the second coordination sphere on the redox reaction of [FeFe]-H2ase models, we designed and prepared an diiron complex [(μ-dmpdt){Fe(CO)3}{Fe(CO) (PPh2NBn2)}] (9) coordinated with a pendant amine-containing diphosphine ligand and a reference complex [(μ-dmpdt){Fe(CO)3}{Fe(CO)(Pph2C5)}] (10), which is structurally analogous to 9 but without any pendant base in the molecule. The in situ IR and 31P NMR spectroscopic as well as cyclic voltammetric studies revealed that such a model complex could take place an iron-mediated intramolecular C-H heterolytic cleavage reaction, with proton removal by a nearby pendant amine, affording a FeII[Fe’IICHS] three-membered-ring product. The function of the pendant base as a proton shuttle was confirmed by the crystal structures of the N-protonated intermediate and the final deprotonated product in comparison with that of the similar but pendant-amine-free complex (10) that does not show evidence of C-H activation. The mechanism of the process was backed up by DFT calculations and the computational data suggest the C-H activation via a δ-C-H agostic interaction and an aid of the pendant base.To further explore the influence of the bridgehead steric bulk in the S-to-S linker on the intramolecular C-H heterolysis of the [FeFe]-H2ase model, two more diiron dithiolate complexes [(μ-pdt){Fe(CO)3}{Fe(CO)(PPh2NBn2)}] (11) and [(μ-depdt){Fe(CO)3}{Fe(CO) (PPh2NBn2)}] (12) were synthesized and these two complexes together with 9 give a series of models with different groups at the S-to-S bridgehead carbon. The oxidations of the complexes were studied by cyclic voltammetry and in situ IR spectroscopy, which showed that regardless of the bridgehead steric bulk, complexes 11 and 12 took place the intramolecular iron-mediated C(sp3)-H bond heterolytic cleavage reaction in the two-electron oxidation process, similar to that found for 9. The reaction of 11 bearing a simple pdt bridge is apparently slower than its analogues 9 and 12 featuring two methyl or two ethyl groups at the bridgehead of S-to-S linker, implicating the considerable influence of the bridgehead groups on the reaction. Crystallographic analysis shows a noticeable difference in structures of the two-electron oxidized complexes, [11’]+and [12’]+. In complex [11’]+, the single CO ligand of the rotated Fe(PPh2NBn2)(CO) unit is found below the Fe-Fe vector, while in [12’]+an unusually rotated Fe(PPh2NBn2)(CO) moiety positions one of the P donors within the bidentate ligand under the Fe…Fe vector. These varied structures show that the [FeFe]-H2ase mimics have a flexible coordination sphere while maintain a stable butterfly-shaped 2Fe2S framework. The studies on the reverse reduction reactions of [11’]+and [12’]+indicated that 11 with a pdt bridge was rapidly recovered almost quantitatively by in situ protonation and reduction of [11’]+, while the large bridgehead steric bulk apparently decreased the recovery of 12 from [12’]+...