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-1κ C )(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 4 c ; 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*).

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