Connecting [NiFe]- and [FeFe]-hydrogenases: mixed-valence nickel-iron dithiolates with rotated structures.

Abstract

New mixed-valence iron-nickel dithiolates are described that exhibit structures similar to those of mixed-valence diiron dithiolates. The interaction of tricarbonyl salt [(dppe)Ni(pdt)Fe(CO)(3)]BF(4) ([1]BF(4), where dppe = Ph(2)PCH(2)CH(2)PPh(2) and pdt(2-) = -SCH(2)CH(2)CH(2)S-) with P-donor ligands (L) afforded the substituted derivatives [(dppe)Ni(pdt)Fe(CO)(2)L]BF(4) incorporating L = PHCy(2) ([1a]BF(4)), PPh(NEt(2))(2) ([1b]BF(4)), P(NMe(2))(3) ([1c]BF(4)), P(i-Pr)(3) ([1d]BF(4)), and PCy(3) ([1e]BF(4)). The related precursor [(dcpe)Ni(pdt)Fe(CO)(3)]BF(4) ([2]BF(4), where dcpe = Cy(2)PCH(2)CH(2)PCy(2)) gave the more electron-rich family of compounds [(dcpe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = PPh(2)(2-pyridyl) ([2a]BF(4)), PPh(3) ([2b]BF(4)), and PCy(3) ([2c]BF(4)). For bulky and strongly basic monophosphorus ligands, the salts feature distorted coordination geometries at iron: crystallographic analyses of [1e]BF(4) and [2c]BF(4) showed that they adopt "rotated" Fe(I) centers, in which PCy(3) occupies a basal site and one CO ligand partially bridges the Ni and Fe centers. Like the undistorted mixed-valence derivatives, members of the new class of complexes are described as Ni(II)Fe(I) (S = ½) systems according to electron paramagnetic resonance spectroscopy, although with attenuated (31)P hyperfine interactions. Density functional theory calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the spin for [1e](+) is mostly localized in a Fe(I)-centered d(z(2)) orbital, orthogonal to the Fe-P bond. The PCy(3) complexes, rare examples of species featuring "rotated" Fe centers, both structurally and spectroscopically incorporate features from homobimetallic mixed-valence diiron dithiolates. Also, when the NiS(2)Fe core of the [NiFe]-hydrogenase active site is reproduced, the "hybrid models" incorporate key features of the two major classes of hydrogenase. Furthermore, cyclic voltammetry experiments suggest that the highly basic phosphine ligands enable a second oxidation corresponding to the couple [(dxpe)Ni(pdt)Fe(CO)(2)L](+/2+). The resulting unsaturated 32e(-) dications represent the closest approach to modeling the highly electrophilic Ni-SI(a) state. In the case of L = PPh(2) (2-pyridyl), chelation of this ligand accompanies the second oxidation.