Electrochromic Platinum(II) Complexes Derived from Azobenzene and Zwitterionic Quinonoid Ligands: Electronic and Geometric Structures

The
ligands azobenzene (az) and the zwitterionic 4-(isopropylamino)-6-(isopropyliminio)-3-oxocyclohexa-1,4-dien-1-olate
(Q) were used to synthesize the mononuclear complex [(Q‑H)­Pt­(az‑H)] (1), and the dinuclear
complex [(Q‑H)­Pt­(μ-az‑2H)­Pt­(Q‑H)] (2). Structural characterization
of the complexes shows a distorted-square-planar environment around
the Pt­(II) centers and localization of the double bonds within the
Q‑H ligand on metal coordination. Furthermore, the
NN azo bond is elongated in the metal complexes in comparison
to free az, owing to π back-bonding from Pt­(II) to az. Complexes 1 and 2 display multiple reversible reduction
steps in their cyclic voltammograms. The complexes also exhibit strong
absorptions in the visible region, the position and intensity of which
can be influenced by the chromophore [(Q‑H)­Pt].
UV–vis–near-IR spectroelectrochemical studies show that
the absorption of these complexes in the visible as well as the near-IR
region can be controlled by electron transfer steps. Depending on
the charge state of the complexes, they are found to be either transparent
in the near-IR region but strongly absorbing in the visible or vice
versa, thus displaying strong electrochromic behavior. EPR spectroelectrochemical
studies together with DFT calculations and comparison with the complex
[(Q‑H)­Pd­(az‑H)] (3) are used to locate the site of electron transfer in these complexes
and to elucidate their electronic properties in the various redox
states. Complex 2 is a rare example where doubly deprotonated
azobenzene acts as a bridging ligand.