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.