Redox-Controlled Interconversion between Trigonal Prismatic and Octahedral Geometries in a Monodithiolene Tetracarbonyl Complex of Tungsten

Abstract

The tetracarbonyl compounds [W­(mdt)­(CO)₄] (<b>1</b>) and [W­(Me₂pipdt)­(CO)₄] (<b>2</b>) both have dithiolene-type ligands (mdt^2– = 1,2-dimethyl-1,2-dithiolate; Me₂pipdt = 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 <b>1</b> and a dithioketone ligand, hence W(0) oxidation state, for <b>2</b>. Density functional theory (DFT) calculations on <b>1</b> 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 <b>2</b> 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 Me₂pipdt ligand. A survey of the Cambridge Structural Database identifies other W­(CO)₄ compounds with pseudo <i>C</i>_4<i>v</i> disposition of CO ligands and suggests a d⁴ electron count to be a probable common denominator. Reduction of <b>1</b> induces a geometry change to octahedral because the singly occupied molecular orbital (SOMO) is at lower energy in this geometry. The cyclic voltammogram of <b>1</b> in CH₂Cl₂ reveals a reduction wave at −1.14 V (vs Fc⁺/Fc) with an unusual offset between the cathodic and the anodic peaks (Δ<i>E</i>_p) of 0.130 V, which is followed by a second, reversible reduction wave at −1.36 V with Δ<i>E</i>_p = 0.091 V. The larger Δ<i>E</i>_p observed for the first reduction is evidence of the trigonal prism-to-octahedron geometry change attending this process. Tungsten L₁-edge X-ray absorption (XAS) data indicate a higher metal oxidation state in <b>1</b> than <b>2</b>. Electron paramagnetic resonance data for [<b>1</b>]⁻ and [<b>2</b>]⁻ are <i>both</i> diagnostic of dithiolene ligand-based sulfur radical, indicating that one-electron reduction of <b>1</b> <i>involves two-electron reduction of tungsten and one-electron oxidation of dithiolene ligand</i>