C^C* Platinum(II) Complexes with Bis(pyridyl)borate Ligands: Synthesis, Crystal Structures, and Properties

The synthesis and characterization of cyclometalated N-heterocyclic carbene complexes of platinum(II) with auxiliary borate ligands are reported. Bis(pyridyl)borate ligands are compared in detail with bis(pyrazolyl)borate ligands with regard to their influence on structural, photophysical, and electrochemical properties. All complexes have been characterized by standard analytical methods, and three of them also by solid-state structure determinations. Absorption and emission measurements revealed that the phosphorescent emission of bis(pyridyl)borate complexes is weaker than that of the corresponding bis(pyrazolyl)borate complexes. The electrochemistry was investigated by voltammetry experiments, where the bis(pyrazolyl)borate complexes were oxidized at potentials significantly higher than those of the bis(pyridyl)borate complexes. The experimental results were accompanied by time-dependent density functional theory calculations (PBE0/6-311G* with dispersion corrections) to analyze the observed transitions.