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(Me2pipdt)­(CO)4] (2) both have dithiolene-type ligands (mdt2– = 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 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 Me2pipdt ligand. A survey of the Cambridge Structural Database identifies other W­(CO)4 compounds with pseudo C4v disposition of CO ligands and suggests a d4 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 CH2Cl2 reveals a reduction wave at −1.14 V (vs Fc+/Fc) with an unusual offset between the cathodic and the anodic peaks (ΔEp) of 0.130 V, which is followed by a second, reversible reduction wave at −1.36 V with ΔEp = 0.091 V. The larger ΔEp 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 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.