Toward functional models of the nickel sites in [FeNi] and [FeNiSe] hydrogenases: Syntheses, structures, and reactivities of nickel(II) complexes containing [NiN3S2] and [NiN3Se2] chromophores

Article abstract of DOI:10.1021/ja00110a013

The reaction of [Ni(terpy)Cl₂] with ～2 equiv of 2,4,6-(Me)₃C₆H₂Se^- in 3:1 acetonitrile/ethanol affords [Ni(terpy)(2,4,6-(Me)₃C₆H₂Se)₂] (7), while [Ni(DAPA)Cl₂] (DAPA = 2,6-bis[1-(phenylimino)ethyl]pyridine) reacts with ～2 equiv of PhS^- and PhSe^- in neat ethanol or acetonitrile to yield [Ni(DAPA)(SPh)₂] (8) and [Ni(DAPA)-(SePh)₂] (9), respectively. All three complexes contain the distorted trigonal bipyramidal (TBP) NiN₃E₂ (E = S, Se) chromophore. Previous X-ray absorption spectroscopic data have indicated a distorted TBP NiN₃S₂ coordination for the nickel site of the hydrogenase (H₂ase) from Thiocapsa roseopersicina. Complex 7 crystallizes in the monoclinic space group P2₁/n with a = 13.170(6) ?, b = 16.091(5) ?, c = 15.111(8) ?, β = 114.42(2)°, V = 2916(2) A°³, and Z = 4. The structure of 7 was refined to R = 4.78% on the basis of 2730 reflections (I > 4σ(I)). Complex 8·CH₃-CN crystallizes in the monoclinic space group P2₁/c with a = 23.012(7) ?, b = 17.814(5) ?, c = 15.698(4) ?, β = 108.52(2)°, V = 6099(5) ?³, and Z = 8. The structure of 8·CH₃CN was refined to R = 6.46% on the basis of 6133 reflections (I > 4σ(I)). Complex 9·CH₃CN also crystallizes in the monoclinic space group P2₁/c with a = 23.209(2) ?, b = 17.960(1) ?, c = 15.749(1) ?, β = 108.482(6)°, V = 6225 ?³, and Z = 8. The structure of 9·CH₃CN was refined to 3.90% on the basis of 5808 reflections (I > 4σ(I)). Reduction of the terpy analogue 7 with aqueous dithionite gives rise to the corresponding Ni(I) complex which binds CO (reversibly) and H^-. The EPR parameters of the CO and hydride adducts resemble the Ni-CO and Ni-C signal of the H₂ases. Much like the other terpy analogues reported previously by this group, oxidation of 7 affords unstable Ni(III) products in low yields. The two DAPA analogues (8 and 9), on the other hand, are readily oxidized and reduced by biologically relevant oxidants and reductants, and the transformation Ni(III) ← Ni(II) → Ni(I) is reversible. The Ni(III) species (10 and 13) derived from 8 and 9 via oxidation with [Fe(CN)₆]^3- are comparatively stable and do not bind CO (or H^-). The single electron in both 10 and 13 resides in the d_z² orbital. Upon reduction with aqueous dithionite, 8 and 9 produce the corresponding Ni(I) species 11 and 14 with the single electron in the d_x²_-y² orbital. These Ni(I) complexes are quite stable at low temperatures but slowly lose thiolates/selenolates at room temperature to give [Ni(DAPA)(solv)₂]⁺. Both 11 and 14 bind CO reversibly. The affinity of the Ni(I) (but not the Ni(III)) model complexes toward CO strongly suggests the presence of Ni(I) in the C form of the H₂ases since the enzymes bind CO only in the Ni-C form. Reaction of NaBH₄ with 8 and 9 results in the hydride adducts 19 and 20. These hydride adducts are stable under basic conditions. The absence of any detectable proton hyperfine coupling indicates that the H^- ligand is located at the basal plane of the Ni(I) center. The EPR parameters of the CO and hydride adducts are quite similar to those of the Ni-CO and Ni-C signals of the H₂ases. Under basic conditions, both 8 and 9 react with dihydrogen at ambient temperature and pressure to afford the hydride adducts 19 and 20 in significant yields. This reaction is quite remarkable since the model complexes mimic the reductive activation step of the biological nickel site in such a reaction to ultimately produce Ni-C-like signals. Taken together, the present results strongly suggest a Ni(I)-H^- formalism for the nickel site in the C form of the H₂ases. In addition, enhancement of the intensities of the EPR signals of the hydride adducts in the presence of a base indicates heterolytic cleavage of H₂ (coordinated or not) at the Ni(I) site of the model complexes and probably also at the enzyme active sites.