Dinuclear Quinonoid-Bridged d8Metal Complexes with Redox-Active Azobenzene Stoppers: Electrochemical Properties and Electrochromic Behavior

Abstract

The ligands 2,5-bis[2,6-(diisopropyl)anilino]-1,4-benzoquinone (L1) and azophenine (L2) were reacted with [(az-H)M(μ-Cl)2M(az–H)] (M = Pd, Pt, az = azobenzene) to generate the complexes [(az-H)Pd(μ-L1-2H)Pd(az-H)] (1), [(az-H)Pt(μ-L1-2H)Pt(az-H)] (2), and [(az-H)Pt(μ-L2-2H)Pt(az-H)] (3). Structural characterization of 1 and 2 revealed a distorted-square-planar environment around the metal centers, localization of double bonds within the L1-2H ligand, and binding of L1-2H to the metal centers through anionic O– and neutral imine type donors. Furthermore, the N═N double bond within az-H displayed a slight elongation in comparison to that in free az owing to back-bonding from the dπ metal orbitals to the π* orbitals of az-H. All complexes show an irreversible oxidation step and three stepwise, reversible one-electron-reduction steps in their cyclic voltammograms. The redox potentials of the complexes are seen to be strongly dependent on the nature of the bridging ligand. UV–vis–near-IR spectroelectrochemical measurements show that these complexes are strongly absorbing in the visible or the near-IR region, depending on the charged state of the metal complexes. The position and intensity of the absorption bands can be tuned by varying the bridging ligand and the metal center. Additionally, the absorption bands can be tuned by simple one-electron-transfer steps. EPR spectroelectrochemistry and DFT calculations have been used to shed light on the electronic structures of these metal complexes in their various redox states and to interpret the results obtained from the UV–vis–near-IR spectroelectrochemistry measurements. In this work, a comparison is being made among d8 metal complexes containing bridging quinones with a [O,O,O,O], [O,N,O,N], or [N,N,N,N] donor set, and the advantages of using the isoelectronic [NR] for [O] substitution on the quinonoid ligands for generating electrochromic metal complexes are discussed. In doing so, we also present complex 3, which is a rare example of a dinuclear metal complex containing the azophenine bridge.