Initial Structure Modification of Tetrahedral to Planar Nickel(II) in a Nickel−Iron−Sulfur Cluster Related to the C-Cluster of Carbon Monoxide Dehydrogenase

A method has been devised that creates a planar NiII site from a tetrahedral site in a NiFe3S4
cubane-type cluster. Reaction of [(Ph3P)NiFe3S4(LS3)]2- (2) with 1,2-bis(dimethylphosphino)ethane affords
[(dmpe)NiFe3S4(LS3)]2- (3), isolated in ca. 45% yield as (Et4N)2[3a]·2.5MeCN and (Et4N)2[3b]·0.25MeCN,
both of which occur in triclinic space group P1̄. Each crystalline form contains two crystallographically
inequivalent clusters with the same overall structure but slightly different dimensions. The cluster is bound
by three thiolate terminal ligands to semirigid cavitand ligand LS3. The NiFe3S4 core contains three tetrahedral
sites, one Fe(μ3-S)3(SR) and two Fe(μ3-S)2(μ2-S)(SR) with normal metric features, and one distorted square
planar Ni(μ3-S)2P2 site in a Ni(μ3-S)2Fe face with mean bond lengths Ni−P = 2.147(9) Å and Ni−S =
2.29(2) Å. The opposite Fe2(μ3-S)(μ2-S) face places the μ2-S atom at nonbonding and variable distances
(2.60−2.90 Å) above the nickel atom. Binding of the strong-field ligand dmpe results in a planar NiII site
and deconstruction of the full cubane geometry. The structure approximates that established crystallographically in the C-cluster of C. hydrogenoformans carbon monoxide dehydrogenase whose NiFe4S4 core
contains a planar NiS4 site and three tetrahedral FeS4 sites in a fragment that is bridged by sulfide atoms
to an exo iron atom. Mössbauer studies of polycrystalline samples containing both clusters 3a and 3b
reveal the presence of at least two cluster types. The spectroscopically best defined cluster accounts for
ca. 54% of total iron and exhibits hyperfine interactions quite similar to those reported for the S = 5/2 state
of the protein-bound cubane-type cluster [ZnFe3S4]1+, whose Mössbauer spectrum revealed the presence
of a high-spin Fe2+ site and a delocalized Fe2.5+Fe2.5+ pair. Development of reactions leading to a planar
nickel and a sulfide-bridged iron atom is requisite to attainment of a synthetic analogue of this complex
protein-bound cluster. This work demonstrates a tetrahedral (2) → planar (3) NiII stereochemical conversion
can be effected by binding of ligands that generate a sufficiently strong in-plane ligand field (dmpe = 1,2-bis(dimethylphosphino)ethane, LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzene(3−)).