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 = Ph2PCH2CH2PPh2, dcpe = Cy2PCH2CH2PCy2, (Cy = cyclohexyl), pdtH2 = HSCH2CH2CH2SH, and edtH2 = HSCH2CH2SH. The
cation [2]+ proved unstable, but the propanedithiolates
are robust. IR and EPR spectroscopic measurements indicate that these
species exist as Cs-symmetric
species. Crystallographic characterization of [3]­BF4 shows that Ni is square planar. Interaction of [1]­BF4 with P-donor ligands (L) afforded a series of substituted
derivatives of type [(dppe)­Ni­(pdt)­Fe­(CO)2L]­BF4 for L = P­(OPh)3 ([4a]­BF4), P­(p-C6H4Cl)3 ([4b]­BF4), PPh2(2-py) ([4c]­BF4), PPh2(OEt) ([4d]­BF4),
PPh3 ([4e]­BF4), PPh2(o-C6H4OMe) ([4f]­BF4), PPh2(o-C6H4OCH2OMe) ([4g]­BF4), P­(p-tol)3 ([4h]­BF4), P­(p-C6H4OMe)3 ([4i]­BF4), and PMePh2 ([4j]­BF4). 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.
Mössbauer spectra of 1 and oxidized S = 1/2 [4e]­BF4 are both consistent with a
low-spin Fe­(I) state. The hyperfine coupling tensor of [4e]­BF4 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(z2) 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.