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

The complex cation in the title compound, (carbonyl-1κC)(1η(5)-penta­methyl­cyclo­penta­dien­yl)(μ-2,3,9,10-tetra­methyl-1,4,8,11-tetra­thia­undeca-2,9-diene-1,11-diido-1κ(2) S,S′′′:2κ(4) S,S′,S′′,S′′′)ironnickel(Fe—Ni) hexa­fluoro­phosphate, [FeNi(C(10)H(15))(C(11)H(18)S(4))(CO)]PF(6) or [Ni(L′)FeCp*(CO)]PF(6), is composed of the nickel complex fragment [Ni(L′)] coordinated as a metalloligand (using S(1) and S(4)) to the [FeCp*(CO)](+) fragment, where (L′)(2−) is [S—C(Me)=C(Me)—S—(CH(2))(3)—S—C(Me)=C(Me)—S](2−) and where Cp*(−) is cyclo-C(5)(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)‘S(2)’] and an approximately square-planar ‘S(4)’ environment for Ni. The NiS(2)Fe diamond-shaped substructure is notably folded at the S—S hinge: the angle between the NiS(2) plane and the FeS(2) 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 ‘S(4)’ ligand (two C=C double bonds), provide an inter­esting comparison with the known NiFe hydrogenase models containing a saturated ‘S(4)’-ligand analogue having the same number of carbon atoms in the ligand backbone, namely with the structures of [Ni(L)FeCp(CO)](+) (as the PF(6) (−) salt, CH(2)Cl(2) solvate) and [Ni(L)FeCp*(CO)](+) (as the PF(6) (−) salt), where (L)(2−) is [S—CH(2)—CH(2)—S—(CH(2))(3)—S—CH(2)—CH(2)—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 ‘S(4)’-ligand of the type (S—C(2)—S—C(3)—S—C(2)—S)(2−) seems to increase structural resemblance to the enzyme for structural models of the type [Ni(‘S(4)’)FeCp(R)(CO)](+) (Cp(R) = Cp or Cp*).