| Hydrogenases are enzymes that catalyze the reversible interconversion of protons, electrons, and dihydrogen (2H+ + 2e- &rlhar2; H2). Because of the potential utility of H2 as an energy carrier, the detailed understanding of hydrogenases has received considerable attention and funding. In particular, hydrogenases are fascinating because they employ inexpensive first row transition metals, while operating at overpotentials and rates comparable with the industrial standard, Pt metal. Synthetic models of hydrogenase active sites are useful for understanding the chemistry occurring within the active site. Chapter One reviews general background information on hydrogenases as well as their synthetic models.;Chapter Two describes the oxidation of dihydrogen by a hydrogenase model. The studies are enabled by the finding that salts of [Fe2(adtR)(CO) 3(PMe3)(dppv)]+ are thermally stable when the anion is BArF4--, where adt = (azadithiolate, (SCH2)2NR), R = H and CH 2C6H5, BArF4 -- = tetrakis(bis(3,5-trifluoromethyl)phenyl)borate, and dppv = cis-1,2-bis(diphenylphosphino)ethylene).;Chapter Three summarizes studies on the redox behavior of synthetic models for the [FeFe]-hydrogenases, consisting of diiron dithiolato carbonyl complexes bearing the amine cofactor and its N-benzyl derivative. Of specific interest are the causes of the low reactivity of oxidized models toward H2, which contrasts with the high activity of these enzymes for H2 oxidation. The redox and acid-base properties of the model complexes [Fe2[(SCH2)2NR](CO)3(dppv)(PMe 3)]+ ([2]+ for R = H and [2']+ for R = CH2C6H 5) indicate that addition of H2 followed by deprotonation are (i) endothermic for the mixed valence (FeIIFeI) state and (ii) exothermic for the diferrous (FeIIFe II) state.;Chapter Four probes the impact of substitution of nitrosyl ligands on diiron(I) dithiolato carbonyls. The nitrosyl complexes discussed in this Chapter reduce at potentials that are ∼1 V milder than their carbonyl counterparts. Reduction results in bending of the nitrosyl ligand and the resulting radical displays strong hyperfine coupling to a nitrogen atom. DFT calculations, specifically NBO values, reinforce the electronic resemblance of the nitrosyl complexes with the corresponding mixed-valence diiron complexes. Unlike other diiron dithiolato carbonyls, these species undergo reversible reductions at mild conditions. The results show that the novel structural and chemical features associated with mixed valence diiron dithiolates can be replicated in the absence of mixed-valency by introducing electronic asymmetry.;Chapter Five explores the reactivity of the tetracarbonyl complexes Fe 2(S2C3H6)(CO)4(PMe 3)2 and Fe2(S2C3H6 )(CO)4(dppv) towards the electrophiles. Treatment of these diiron starting materials with [S2Me3]+ and [N2C6H5]+ afforded the terminally bound electrophile adducts [Fe2(S2C 3H6)(t-X)(CO)4(PMe3) 2]+ and [Fe2(S2C3H 6)(t-X)(CO)4(dppv)]+, where X = SMe and N2R, respectively. These intermediates thermally rearrange to isomers containing bridging electrophiles. The stability of the terminal-electrophile isomers is significantly greater when X = SMe than when X = N2R. (Abstract shortened by UMI.)... |
