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.

ABSTRACT: The complex cation in the title compound, (carbonyl-1?C)(1?⁵-penta-methyl-cyclo-penta-dien-yl)(?-2,3,9,10-tetra-methyl-1,4,8,11-tetra-thia-undeca-2,9-diene-1,11-diido-1?²S,S''':2?⁴S,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*).

SUBMITTER: Harrison DJ 

PROVIDER: S-EPMC6127701 | biostudies-literature | 2018 Sep

REPOSITORIES: biostudies-literature