Mixed-valence nickel-iron dithiolate models of the [NiFe]-hydrogenase active site

A series of mixed-valence nickel-iron dithiolates is described. Oxidation of (diphosphine)Ni(dithiolate)Fe(CO)(3) complexes 1, 2, and 3 with ferrocenium salts affords the corresponding tricarbonyl cations (dppe)Ni(pdt)Fe(CO)(3)](+) (1](+)), (dppe)Ni(edt)Fe(CO)(3)](+) (2](+)) and (dcpe)Ni(pdt)Fe(CO)(3)](+) (3](+)), respectively, where dppe = Ph(2)PCH(2)CH(2)PPh(2), dcpe = Cy(2)PCH(2)CH(2)PCy(2), (Cy = cyclohexyl), pdtH(2) = HSCH(2)CH(2)CH(2)SH, and edtH(2) = HSCH(2)CH(2)SH. The cation 2](+) proved unstable, but the propanedithiolates are robust. IR and EPR spectroscopic measurements indicate that these species exist as C(s)-symmetric species. Crystallographic characterization of 3]BF(4) shows that Ni is square planar. Interaction of 1]BF(4) with P-donor ligands (L) afforded a series of substituted derivatives of type (dppe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = P(OPh)(3) (4a]BF(4)), P(p-C(6)H(4)Cl)(3) (4b]BF(4)), PPh(2)(2-py) (4c]BF(4)), PPh(2)(OEt) (4d]BF(4)), PPh(3) (4e]BF(4)), PPh(2)(o-C(6)H(4)OMe) (4f]BF(4)), PPh(2)(o-C(6)H(4)OCH(2)OMe) (4g]BF(4)), P(p-tol)(3) (4h]BF(4)), P(p-C(6)H(4)OMe)(3) (4i]BF(4)), and PMePh(2) (4j]BF(4)). EPR analysis indicates that ethanedithiolate 2](+) exists as a single species at 110 K, whereas the propanedithiolate cations exist as a mixture of two conformers, which are proposed to be related through a flip of the chelate ring. M?ssbauer spectra of 1 and oxidized S = 1/2 4e]BF(4) are both consistent with a low-spin Fe(I) state. The hyperfine coupling tensor of 4e]BF(4) has a small isotropic component and significant anisotropy. DFT calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the SOMOs in complexes of the present type are localized in an Fe(I)-centered d(z(2)) orbital. The DFT calculations allow an assignment of oxidation states of the metals and rationalization of the conformers detected by EPR spectroscopy. Treatment of 1](+) with CN(-) and compact basic phosphines results in complex reactions. With dppe, 1](+) undergoes quasi-disproportionation to give 1 and the diamagnetic complex (dppe)Ni(pdt)Fe(CO)(2)(dppe)](2+) (5](2+)), which features square-planar Ni linked to an octahedral Fe center.