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. 2012 Feb 20;51(4):2338-48.
doi: 10.1021/ic202329y. Epub 2012 Feb 3.

Mixed-valence Nickel-Iron Dithiolate Models of the [NiFe]-hydrogenase Active Site

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Free PMC article

Mixed-valence Nickel-Iron Dithiolate Models of the [NiFe]-hydrogenase Active Site

David Schilter et al. Inorg Chem. .
Free PMC article

Abstract

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. Mössbauer 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.

Figures

Figure 1
Figure 1
Structural mimics of the active site of the [NiFe]-H2ases.
Figure 2
Figure 2
X-band EPR spectra (CH2Cl2/PhMe, 110 K) of [1]BF4 (exp.) and [1′]BF4 (exp.′). In each case the experimental spectrum could be simulated as sum of two components, denoted A+B (for [1]BF4), and A′+B′ (for [1′]BF4).
Figure 3
Figure 3
ORTEP of [3]BF4 with ellipsoids drawn at the 50% probability level. The H atoms, BF4- anion, and two CH2Cl2 solvate molecules are omitted for clarity. Selected distances (Å): Ni1-Fe1, 2.818; Ni1-P1, 2.191; Ni-P2, 2.188; Ni1-S1, 2.235; Ni1-S2, 2.227; Fe1-S1, 2.296; Fe1-S2, 2.288; Fe1-C30, 1.833; Fe1-C31, 1.799; Fe1-C32, 1.790. Selected calculated (BP/TZVP) distances (Ǻ) Ni1-Fe1, 2.76; Ni1-P1, 2.23; Ni1-P2, 2.22; Ni1-S1, 2.25; Ni1-S2, 2.24; Fe1-S1, 2.31; Fe1-S2, 2.34; Fe1-C30, 1.80; Fe1-C31, 1.79; Fe1-C32, 1.79.
Figure 4
Figure 4
X-band EPR spectra (CH2Cl2/PhMe, 110 K) of [4e]BF4. The experimental spectrum (exp.) could be simulated as sum of two components, denoted A+B.
Figure 5
Figure 5
X-band EPR spectra (CH2Cl2/PhMe, 110 K) of [4a]BF4 (exp.) and [4a′]BF4 (exp.′). Simulated spectra (sim. and sim.′, respectively) are also presented.
Figure 6
Figure 6
X-band EPR spectra (CH2Cl2/PhMe) of [2]BF4 collected at 110 K (110 K exp.) and room temperature (r.t. exp.). Simulated spectra are also presented.
Figure 7
Figure 7
Isocontour plots of the unpaired spin density distribution at 0.005 e-/a03 for the two conformers of [1]+. The central methylene of the pdt2- ligand can be oriented towards Ni (left, conformer ‘a’) or Fe (right, conformer ‘b’). Unpaired atomic spin densities are given for selected nuclei.
Figure 8
Figure 8
Isocontour plots of the unpaired spin density distribution at 0.005 e-/a03 for the two conformers of [4a]+. The central methylene of the pdt2- ligand can be oriented towards Ni (left, conformer ‘a’) or Fe (right, conformer ‘b’).
Figure 9
Figure 9
Mössbauer spectra of [4e]BF4 recorded at the applied fields and temperatures indicated. Spectra A, C and D were obtained on a solid sample while spectrum B was obtained on a 40 mM frozen solution. The magnetic field was parallel to the γ beam for spectra A and B, transverse to the γ beam for spectra C and D. The solid line through the data is a simulation using an S = ½ Hamiltonian with the parameters; δ = 0.18 mm/s, ΔEQ = 0.79 mm/s, η = 0.7, A/gnβn = ( + 6.2, -5.5, -28.1) KG, βefg = 45° and γefg = 90°.
Figure 10
Figure 10
ORTEP of [5](BF4)2·4CH2Cl2 with ellipsoids drawn at the 50% probability level. The H atoms, disordered BF -4 anions and four CH2Cl2 solvate molecules are omitted for clarity. Selected distances (Å): Ni1-Fe1, 2.818; Fe1-P1, 2.254; Fe1-P2, 2.262; Fe1-C27, 1.800; Fe1-C28, 1.817; Fe1-S1, 2.325; Fe1-S2, 2.334; Ni1-S1, 2.219; Ni1-S2, 2.253; Ni1-P3, 2.186; Ni1-P4, 2.180.
Scheme 1
Scheme 1
Scheme 2
Scheme 2
Scheme 3
Scheme 3
Scheme 4
Scheme 4
Scheme 5
Scheme 5
Scheme 6
Scheme 6

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