Connecting [NiFe]- and
[FeFe]-Hydrogenases: Mixed-Valence
Nickel–Iron Dithiolates with Rotated Structures
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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)<sub>3</sub>]BF<sub>4</sub> ([<b>1</b>]BF<sub>4</sub>, where dppe = Ph<sub>2</sub>PCH<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub> and pdt<sup>2–</sup> = −SCH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>S−)
with P-donor ligands (L) afforded the substituted derivatives [(dppe)Ni(pdt)Fe(CO)<sub>2</sub>L]BF<sub>4</sub> incorporating L = PHCy<sub>2</sub> ([<b>1a</b>]BF<sub>4</sub>), PPh(NEt<sub>2</sub>)<sub>2</sub> ([<b>1b</b>]BF<sub>4</sub>), P(NMe<sub>2</sub>)<sub>3</sub> ([<b>1c</b>]BF<sub>4</sub>), P(<i>i</i>-Pr)<sub>3</sub> ([<b>1d</b>]BF<sub>4</sub>), and PCy<sub>3</sub> ([<b>1e</b>]BF<sub>4</sub>). The related precursor [(dcpe)Ni(pdt)Fe(CO)<sub>3</sub>]BF<sub>4</sub> ([<b>2</b>]BF<sub>4</sub>, where dcpe = Cy<sub>2</sub>PCH<sub>2</sub>CH<sub>2</sub>PCy<sub>2</sub>) gave the more electron-rich
family of compounds [(dcpe)Ni(pdt)Fe(CO)<sub>2</sub>L]BF<sub>4</sub> for L = PPh<sub>2</sub>(2-pyridyl) ([<b>2a</b>]BF<sub>4</sub>), PPh<sub>3</sub> ([<b>2b</b>]BF<sub>4</sub>), and PCy<sub>3</sub> ([<b>2c</b>]BF<sub>4</sub>). For bulky and strongly
basic monophosphorus ligands, the salts feature distorted coordination
geometries at iron: crystallographic analyses of [<b>1e</b>]BF<sub>4</sub> and [<b>2c</b>]BF<sub>4</sub> showed that they adopt
“rotated” Fe<sup>I</sup> centers, in which PCy<sub>3</sub> 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<sup>II</sup>Fe<sup>I</sup> (<i>S</i> = <sup>1</sup>/<sub>2</sub>) systems
according to electron paramagnetic resonance spectroscopy, although
with attenuated <sup>31</sup>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 [<b>1e</b>]<sup>+</sup> is mostly localized
in a Fe<sup>I</sup>-centered d(<i>z</i><sup>2</sup>) orbital,
orthogonal to the Fe–P bond. The PCy<sub>3</sub> 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<sub>2</sub>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)<sub>2</sub>L]<sup>+/2+</sup>. The resulting unsaturated 32e<sup>–</sup> dications represent the closest approach to modeling the highly
electrophilic Ni–SI<sub>a</sub> state. In the case of L = PPh<sub>2</sub> (2-pyridyl), chelation of this ligand accompanies the second
oxidation