Connecting [NiFe]- and [FeFe]-Hydrogenases: Mixed-Valence Nickel–Iron Dithiolates with Rotated Structures

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]­BF4 ([1]­BF4, where dppe = Ph2PCH2CH2PPh2 and pdt2– = −SCH2CH2CH2S−)
with P-donor ligands (L) afforded the substituted derivatives [(dppe)­Ni­(pdt)­Fe­(CO)2L]­BF4 incorporating L = PHCy2 ([1a]­BF4), PPh­(NEt2)2 ([1b]­BF4), P­(NMe2)3 ([1c]­BF4), P­(i-Pr)3 ([1d]­BF4), and PCy3 ([1e]­BF4). The related precursor [(dcpe)­Ni­(pdt)­Fe­(CO)3]­BF4 ([2]­BF4, where dcpe = Cy2PCH2CH2PCy2) gave the more electron-rich
family of compounds [(dcpe)­Ni­(pdt)­Fe­(CO)2L]­BF4 for L = PPh2(2-pyridyl) ([2a]­BF4), PPh3 ([2b]­BF4), and PCy3 ([2c]­BF4). For bulky and strongly
basic monophosphorus ligands, the salts feature distorted coordination
geometries at iron: crystallographic analyses of [1e]­BF4 and [2c]­BF4 showed that they adopt
“rotated” FeI centers, in which PCy3 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 NiIIFeI (S = 1/2) systems
according to electron paramagnetic resonance spectroscopy, although
with attenuated 31P 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 FeI-centered d­(z2) orbital,
orthogonal to the Fe–P bond. The PCy3 complexes,
rare examples of species featuring “rotated” Fe centers,
both structurally and spectroscopically incorporate features from
homobimetallic mixed-valence diiron dithiolates. Also, when the NiS2Fe 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)2L]+/2+. The resulting unsaturated 32e– dications represent the closest approach to modeling the highly
electrophilic Ni–SIa state. In the case of L = PPh2 (2-pyridyl), chelation of this ligand accompanies the second
oxidation.