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)(1η5-pentamethylcyclopentadienyl)(μ-2,3,9,10-tetramethyl-1,4,8,11-tetrathiaundeca-2,9-diene-1,11-diido-1κ2S,S′′′:2κ4S,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')FeCpR(CO)]+ (CpR = Cp or Cp*).

Keywords

• crystal structure
• NiFe hydrogenase
• enzyme model
• bioinorganic
• sulfur ligand