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

摘要

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 C_s-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.