Synthetic models of [FeFe]-hydrogenase: case studies utilizing azadithiolate and nitrosyl coligands
Hydrogenases are enzymes that catalyze the reversible interconversion of protons, electrons, and dihydrogen (2H+ + 2e− <-> 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 CH2C6H5, BArF4- = tetrakis(bis(3,5-trifluoromethyl)phenyl)borate, and dppv = cis-1,2-bis(diphenylphosphino)ethylene). Solutions of [Fe2(adtR)(CO)3(PMe3)(dppv)]+ react with high pressures of H2 to give the hydride [(mu-H)Fe2(adt)(CO)3(PMe3)(dppv)]BArF4. The oxadithiolate [Fe2(odt)(CO)3(PMe3)(dppv)]+ (odt = 2-oxa-1,3-dithiolate) and propanedithiolate [Fe2(pdt)(CO)3(PMe3)(dppv)]+ are unreactive toward H2, thus implicating a role for the amine. According to the proposed mechanism, H2 binds to the mixed-valence cation to give the dihydrogen adduct whereupon the H2 undergoes intramolecular heterolysis utilizing the amine. Use of D2 in place of H2 gave the deuteride as well as the hydride, implicating the transient formation of free protons that exchange with water. 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)(PMe3)]+ ([2]+ for R = H and [2’]+ for R = CH2C6H5) indicate that addition of H2 followed by deprotonation are (i) endothermic for the mixed valence (FeIIFeI) state and (ii) exothermic for the diferrous (FeIIFeII) state. The diferrous state is shown to be unstable with respect to coordination of the amine to Fe, a derivative of which was characterized crystallographically. The redox and acid-base properties for the mixed valence models differ strongly for those containing the amine cofactor versus those derived from propanedithiolate. Protonation of [2’]+ induces disproportionation to a 1:1 mixture of the ammonium-Fe(I)Fe(I) and the dication [2’]2+ (Fe(II)Fe(II)). This effect is consistent with substantial enhancement of the basicity of the amine in the Fe(I)Fe(I) state vs the Fe(II)Fe(I) state. The Fe(I)Fe(I) ammonium compounds are rapid and efficient H-atom donors toward the nitroxyl compound TEMPO. The atom transfer is proposed to proceed via the hydride, as indicated by the reaction of [HFe2[(SCH2)2NH](CO)2(dppv)2]+ with TEMPO. Collectively, the results suggest that proton-coupled electron-transfer pathways should be considered for H2 activation by the [FeFe]-hydrogenases. Chapter Four probes the impact of substitution of nitrosyl ligands on diiron(I) dithiolato carbonyls. Treatment of Fe2(S2CnH2n)(CO)5-x(PMe3)x or Fe2(S2CnH2n)(CO)5-y-z(PMe3)y (dppv)z with NOBF4 afforded the nitrosyl complexes [Fe2(S2CnH2n)(CO)4-x(PMe3)x(NO)]BF4 and [Fe2(S2CnH2n)(CO)4-y-z(PMe3)y (dppv)z(NO)]BF4, respectively (x = 1-2; y = 1 and z = 1, or y = 0 and z =2). These nitrosyl complexes can also be synthesized by the sequential oxidation of an FeIFeI precursor with one equiv FcBF4 followed by treatment with one equiv of NO. Whereas the monophosphine nitrosyl derivative is largely undistorted, the bisphosphine nitrosyl derivatives are distorted such that the CO ligand on the Fe(CO)(PMe3)(NO)+ fragment is semibridging. Two isomers of [Fe2(S2C3H6)(CO)3(PMe3)2(NO)]BF4 were characterized spectroscopically and crystallographically. Each isomer features electron-rich [Fe(CO)2PMe3] and electrophilic [Fe(CO)(PMe3)(NO)]+ fragments. These species are in equilibrium with an unobserved isomer that reversibly binds CO (deltaH = -35 kJ/mol, deltaS = -139 J/mol•K) to give the symmetrical adduct [Fe2(S2C3H6)(mu-NO)(CO)4(PMe3)2]BF4. In contrast to Fe2(S2C3H6)(CO)4(PMe3)2, the tricarbonyl nitrosyl complexes readily undergo CO-substitution to give the trisphosphine derivatives. Whereas [Fe2(S2C2H4)(CO)3(PMe3)2(NO)]BF4 undergoes substitution with PMe3 via an adduct containing a bridging nitrosyl, the chelate complex [Fe2(S2C2H4)(CO)3(dppv)(NO)]BF4 instead undergoes substitutiton via the formation of a double-adduct with PMe3. The intermediate [Fe2(S2C2H4)(CO)3(dppv)(PMe3)2(NO)]BF4 was crystallized and displays partial iron-dithiolate bond cleavage as well as FeIIFe0 mixed valency. Substitution of the nitrosyl complexes with cyanide was also explored, and in this way we synthesized Fe2(S2C2H4)(CN)(CO)3(PMe3)(NO), Fe2(S2C2H4)(CN)(CO)(dppv)(PMe3)(NO), and [Et4N][Fe2(S2C2H4)(CN)2(CO)(dppv)(NO)]. In these cases, migration of the nitrosyl to the cyanide-bound Fe was observed. 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 Fe2(S2C3H6)(CO)4(PMe3)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(S2C3H6)(t-X)(CO)4(PMe3)2]+ and [Fe2(S2C3H6)(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.
hydrogenase, diiron, azadithiolate, nitrosyl, hydrogen activation
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