Mixed-Valence Nickel–Iron Dithiolate Models of the [NiFe]-Hydrogenase Active Site

Abstract

A series of mixed-valence iron-nickel 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, 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), 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 = ½ [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 a 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.

Models of the Ni-L and Ni-SIa States of the [NiFe]-Hydrogenase Active Site

“…A similar change is observed for the reduction of [(dppe)Ni(pdt)Fe(CO) 3 ] + , a Ni II Fe I /Ni I Fe I couple (Δ ν̃ CO ≈ 35 cm −1 for this tri carbonyl). 20 Oxidation state changes centered at iron typically shift ν̃ CO by ca. 100 cm −1 as observed for [(dppe)Pt(pdt)Fe(CO) 3 ] +/0 , a Pt II Fe I /Pt II Fe 0 couple.…”

“…The isomer shifts observed in the Ni I Fe II compounds [ 1a – 1c ] 0 are larger than the shift observed for the Ni II Fe I compound [(dppe)Ni(pdt)Fe(CO) 3 ]BF 4 ( δ = 0.04 mm s −1 ), while the quadrupole splittings are comparable (Δ E q = 0.57). 20 Mössbauer data for the [NiFe]-hydrogenases are sparse and suffer from difficulties in identifying and correcting for the dominating subspectra of the accessory FeS clusters. 37,38 Isomer shifts in the range of 0.05–0.15 mm s −1 have been assigned to the iron center of the [NiFe] center.…”

“…The first attempts to model the mixed-valence active site of the Ni-L state focused on [(CO) 2 LFe(pdt)Ni-(diphosphine)] + 20,28 . These cations are described as Ni II Fe I , which is reversed from the Ni I Fe II states assigned in Ni-L. More recently, we have characterized complexes with the configuration Ni I Ru II , wherein the Ru II center is redox-inactive.…”

Protonation of Nickel–Iron Hydrogenase Models Proceeds after Isomerization at Nickel

“…For example, 1 undergoes a tetrahedral → square-planar twist at Ni upon oxidation or protonation (Figure 1). While the structures of 1 , [ 1 ] + , and [ 1 H] + are established, 10−15 those of any intermediates are not. Such transformations may, for example, involve “pre-twisting” of the Ni coordination in 1 (Scheme 1, top).…”

New Systematic Route to Mixed-Valence Triiron Clusters Derived from Dinuclear Models of the Active Site of [Fe–Fe]-Hydrogenases

“…Site b presents the lowest isomer shift among the series, and the value is similar to that observed in [Fe 2 (CO) 6 (μ-pdt)] (see the Supporting Information) or similar complexes. 21,22 Consequently, site b is tentatively attributed to the terminal Fe3 center. The parameters of site c are close to those obtained for the iron center of [(κ 2 -dppe)(OC)Fe(μ-pdt)Mo-(CO) 4 ] (δ = 0.20 mm/s, ΔE Q = −0.95 mm/s, η = 0.89).…”