Synthesis and electropolymerization of N-heterocyclic carbene complexes of Pd(II) and Pt(II) from an emissive imidazolium salt with a terthiophene backbone

The synthesis of an emissive terthiophene-based imidazolium iodide: 1,3-dimethyl-4,6-di(thiophen-2-yl)-thienoimidazol-3-ium iodide (4), the corresponding N-heterocyclic carbene (NHC) preligand, and Pd and Pt–NHC complexes (5a, 5b) are reported. A related Pd–NHC complex (8) with a mono-thiophene backbone has also been synthesized. The solid-state structures were determined by single crystal X-ray diffraction studies. Compounds 4, 5a and 5b were characterized by absorption/emission spectroscopy and an emission-quench phenomenon of 5a was observed. The Pd complexes 5a and 8 were shown to catalyze Heck type C–C bond forming reactions. The compounds with terthiophene backbones (4, 5a, 5b) were electropolymerized onto indium tin oxide (ITO)-coated glass electrodes to yield thin films of conducting metallopolymers (CMPs) (P1–3). UV-vis-NIR spectroelectrochemistry data of P1–3 showed reversible color changes upon oxidation due to the generation of charge carrier species along the polythiophene backbones.