A new structural model for NiFe hydrogenases: an unsaturated analogue of a classic hydrogenase model leads to more enzyme-like Ni—Fe distance and inter­planar fold

摘要

The complex cation in the title compound, (carbonyl-1κC)(1η⁵-penta­methyl­cyclo­penta­dien­yl)(μ-2,3,9,10-tetra­methyl-1,4,8,11-tetra­thia­undeca-2,9-diene-1,11-diido-1κ² S,S′′′:2κ⁴ S,S′,S′′,S′′′)ironnickel(Fe—Ni) hexa­fluoro­phosphate, [FeNi(C10H15)(C11H18S4)(CO)]PF6 or [Ni(L′)FeCp*(CO)]PF6, is composed of the nickel complex fragment [Ni(L′)] coordinated as a metalloligand (using S¹ and S⁴) to the [FeCp*(CO)]⁺ fragment, where (L′)²⁻ is [S—C(Me)=C(Me)—S—(CH2)3—S—C(Me)=C(Me)—S]²⁻ and where Cp*⁻ is cyclo-C5(Me)5 ⁻ (penta­methyl­cyclo­penta­dien­yl). The ratio of hexa­fluoro­phosphate anion per complex cation is 1:1. The structure at 150 K has ortho­rhom­bic (Pbcn) symmetry. The atoms of the complex cation are located on general positions (multiplicity = 8), whereas there are two independent hexa­fluoro­phosphate anions, each located on a twofold axis (Wyckoff position 4c; multiplicity = 4). The structure of the new dimetallic cation [Ni(L′)FeCp*(CO)]⁺ can be described as containing a three-legged piano-stool environment for iron [Cp*Fe(CO)‘S2’] and an approximately square-planar ‘S4’ environment for Ni. The NiS2Fe diamond-shaped substructure is notably folded at the S—S hinge: the angle between the NiS2 plane and the FeS2 plane normals is 64.85 (6)°. Largely because of this fold, the nickel–iron distance is relatively short, at 2.9195 (8) Å. The structural data for the complex cation, which contains a new unsaturated ‘S₄’ ligand (two C=C double bonds), provide an inter­esting comparison with the known NiFe hydrogenase models containing a saturated ‘S₄’-ligand analogue having the same number of carbon atoms in the ligand backbone, namely with the structures of [Ni(L)FeCp(CO)]⁺ (as the PF₆ ⁻ salt, CH₂Cl₂ solvate) and [Ni(L)FeCp*(CO)]⁺ (as the PF₆ ⁻ salt), where (L)²⁻ is [S—CH₂—CH₂—S—(CH₂)₃—S—CH₂—CH₂—S]²⁻ and Cp⁻ is cyclo­penta­dienyl. The saturated analogues [Ni(L)FeCp(CO)]⁺ and [Ni(L)FeCp*(CO)]⁺ have similar Ni—Fe distances: 3.1727 (6), 3.1529 (7) Å (two independent mol­ecules in the unit cell) and 3.111 (5) Å, respectively, for the two complexes, whereas [Ni(L′)FeCp*(CO)]⁺ described here stands out with a much shorter Ni—Fe distance [2.9196 (8) Å]. Also, [Ni(L)FeCp(CO)]⁺ and [Ni(L)FeCp*(CO)]⁺ show inter­planar fold angles that are similar between the two: 39.56 (5), 41.99 (5) (independent mol­ecules in the unit cell) and 47.22 (9) °, respectively, whereas [Ni(L′)FeCp*(CO)]⁺ possesses a much more pronounced fold [64.85 (6)°]. Given that larger fold angles and shorter Ni—Fe distances are considered to be structurally closer to the enzyme, unsaturation in an ‘S₄’-ligand of the type (S—C₂—S—C₃—S—C₂—S)²⁻ seems to increase structural resemblance to the enzyme for structural models of the type [Ni(‘S₄’)FeCp^R(CO)]⁺ (Cp^R = Cp or Cp*).