A Pd(ii) catecholato complex bearing 5,5′-divinyl-2,2′-bipyridine: synthesis, characterization, and electrochemical disproportionation in solutions and electropolymerized films

Abstract

A new Pd(II) catecholato (Cat) complex bearing 5,5′-divinyl-2,2′-bipyridine as an ancillary ligand (complex 2) was synthesized, and its electrochemical behavior in solutions and electropolymerized films was characterized. A comparison of 2 with an analog containing ^tBu groups (complex 1) revealed that the introduced vinyl groups positively shifted the reduction potential of the bipyridine (Bpy) moiety, in line with the lowered lowest unoccupied molecular orbital, thus redshifting the ligand-to-ligand charge transfer (LL'CT) transition absorption from the Cat to Bpy ligands across the Pd(II) center. Complex 2 exhibits a reversible redox wave for the semiquinone (SQ)/Cat couple in DMF/ⁿBu₄NPF₆. On the other hand, the addition of Cl⁻ substantially changed the voltametric properties, indicating disproportionation between the electrogenerated SQ complexes to the regenerated 2 and ligand dissociation products. In contrast to previous reports on the disproportionation of chemically isolated SQ complexes, we found electrochemical features of the repeated anodic oxidation of regenerated 2 followed by disproportionation. The electropolymerized films of 2 on indium tin oxide electrodes exhibited a widely separated pair of anodic and cathodic peaks, similar to those of monomer 2 in solution. The disproportionation of the constituent complexes in the metallopolymers was spectroelectrochemically monitored by the disappearance of the characteristic LL'CT absorption, indicative of ligand dissociation. These findings provide new insights into the fabrication of redox-active metallopolymer films and their applications, which exploit their electrochemical reactivity and bistability in molecular devices.