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

Abstract

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(C10 H15)(C11 H18 S4)(CO)]PF6 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∗).