π‑Bonded Dithiolene Complexes: Synthesis, Molecular Structures, Electrochemical Behavior, and Density Functional Theory Calculations

The synthesis and X-ray molecular structure of the first
metal-stabilized o-dithiobenzoquinone [Cp*Ir-o-(η4-C6H4S2)] (2) are described. The presence of the metal stabilizes
this elusive intermediate by π coordination and increases the
nucleophilic character of the sulfur atoms. Indeed, the π-bonded
dithiolene complex 2 was found to react with the organometallic
solvated species [Cp*M­(acetone)3]­[OTf]2 (M =
Rh, Ir) to give a unique class of binuclear dithiolene compounds [Cp*Ir­(C6H4S2)­MCp*]­[OTf]2 [M = Rh
(3), Ir (4)] in which the elusive dithiolene
η-C6H4S2 acts as a bridging
ligand toward the two Cp*M moieties. The electrochemical behavior
of all complexes was investigated and provided us with valuable information
about their redox properties. Density functional theory (DFT) calculations
on the π-bonded dithiobenzoquinone ligand and related bimetallic
systems show that the presence of Cp*M at the arene system of the
dithiolene ligand increases the stability compared to the known monomeric
species [Cp*Ir-o-(C6H4S2-κ2-S,S)] and enables these complexes Cp*Ir­(C6H4S2)­MCp*]­[OTf]2 (3 and 4) to act as electron reservoirs. Time-dependent DFT calculations
also predict the qualitative trends in the experimental UV–vis
spectra and indicate that the strongest transitions arise from ligand–metal
charge transfer involving primarily the HOMO–1 and LUMO. All
of these compounds were fully characterized and identified by single-crystal
X-ray crystallography. These results illustrate the first examples
describing the coordination chemistry of the elusive o-dithiobenzoquinone to yield bimetallic complexes with an o-benzodithiolene ligand. These compounds might have important
applications in the area of molecular materials.