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

The complex cation in the title compound, (carbonyl-1κC)(1η5-penta-methyl-cyclo-penta-dien-yl)(μ-2,3,9,10-tetra-methyl-1,4,8,11-tetra-thia-undeca-2,9-diene-1,11-diido-1κ2S,S''':2κ4S,S',S'',S''')ironnickel(Fe-Ni) hexa-fluoro-phosphate, [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- (penta-methyl-cyclo-penta-dien-yl). The ratio of hexa-fluoro-phosphate anion per complex cation is 1:1. The structure at 150 K has ortho-rhom-bic (Pbcn) symmetry. The atoms of the complex cation are located on general positions (multiplicity = 8), whereas there are two independent hexa-fluoro-phosphate 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 inter-esting 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 cyclo-penta-dienyl. 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 mol-ecules 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 inter-planar fold angles that are similar between the two: 39.56 (5), 41.99 (5) (independent mol-ecules 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*).