title: Redox-controlled interconversion between trigonal prismatic and octahedral geometries in a monodithiolene tetracarbonyl complex of tungsten.
abstract: The tetracarbonyl compounds [W(mdt)(CO)(4)] (1) and [W(Me(2)pipdt)(CO)(4)] (2) both have dithiolene-type ligands (mdt(2-) = 1,2-dimethyl-1,2-dithiolate; Me(2)pipdt = 1,4-dimethylpiperazine-2,3-dithione) but different geometries, trigonal prismatic (TP) and octahedral, respectively. Structural data suggest an ene-1,2-dithiolate ligand description, hence a divalent tungsten ion, for 1 and a dithioketone ligand, hence W(0) oxidation state, for 2. Density functional theory (DFT) calculations on 1 show the highest occupied molecular orbital (HOMO) to be a strong W-dithiolene π bonding interaction and the lowest unoccupied molecular orbital (LUMO) its antibonding counterpart. The TP geometry is preferred because symmetry allowed mixing of these orbitals via a configuration interaction (CI) stabilizes this geometry over an octahedron. The TP geometry for 2 is disfavored because W-dithiolene π overlap is attenuated because of a lowering of the sulfur content and a raising of the energy of this ligand π orbital by the conjugated piperazine nitrogen atoms in the Me(2)pipdt ligand. A survey of the Cambridge Structural Database identifies other W(CO)(4) compounds with pseudo C(4v) disposition of CO ligands and suggests a d(4) electron count to be a probable common denominator. Reduction of 1 induces a geometry change to octahedral because the singly occupied molecular orbital (SOMO) is at lower energy in this geometry. The cyclic voltammogram of 1 in CH(2)Cl(2) reveals a reduction wave at -1.14 V (vs Fc(+)/Fc) with an unusual offset between the cathodic and the anodic peaks (ΔE(p)) of 0.130 V, which is followed by a second, reversible reduction wave at -1.36 V with ΔE(p) = 0.091 V. The larger ΔE(p) observed for the first reduction is evidence of the trigonal prism-to-octahedron geometry change attending this process. Tungsten L(1)-edge X-ray absorption (XAS) data indicate a higher metal oxidation state in 1 than 2. Electron paramagnetic resonance data for [1](-) and [2](-) are both diagnostic of dithiolene ligand-based sulfur radical, indicating that one-electron reduction of 1 involves two-electron reduction of tungsten and one-electron oxidation of dithiolene ligand.
authors.affiliation: Department of Chemistry, Tulane University, 6400 Freret Street, New Orleans, Louisiana 70118-5698, USA.
authors.forename: Yong
authors.initials: Y
authors.lastname: Yan
authors.forename: Perumalreddy
authors.initials: P
authors.lastname: Chandrasekaran
authors.forename: Joel T
authors.initials: JT
authors.lastname: Mague
authors.forename: Serena
authors.initials: S
authors.lastname: DeBeer
authors.forename: Stephen
authors.initials: S
authors.lastname: Sproules
authors.forename: James P
authors.initials: JP
authors.lastname: Donahue
mesh_terms: D056831:Coordination Complexes / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D011789:Quantum Theory; D014414:Tungsten / Q000737:chemistry*
chemical_list: D056831:Coordination Complexes; D014414:Tungsten