Redox-controlled interconversion between trigonal prismatic and octahedral geometries in a monodithiolene tetracarbonyl complex of tungsten

The tetracarbonyl compounds [W(mdt)(CO) ₄] (1) and [W(Me ₂pipdt)(CO) ₄] (2) both have dithiolene-type ligands (mdt ^2- = 1,2-dimethyl-1,2-dithiolate; Me ₂pipdt = 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 ₂pipdt 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 ₂Cl 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 1involves two-electron reduction of tungsten and one-electron oxidation of dithiolene ligand.