Novel phosphorescent tetradentate bis-cyclometalated C^C∗N^N-coordinated platinum complexes: Structure, photophysics, and a synthetic adventure

Abstract A long but adventurous journal to the synthesis of tetradentate bis-cyclometalated C^C ∗ N^N-coordinated platinum complexes is described, where C^C and N^N denote a five-membered chelation and C ∗ N denotes a six-membered chelation. Initial ligand designs resulted in the selective cycloplatination to form a tridentate coordination complex. The structures of the tridentate coordination complexes were confirmed by the X-ray crystallographic studies. The selective formation of the tridentate coordination complexes was analyzed, which led us to an ultimate design of the ligand 15 and 16 . Ligands 15 and 16 differ only in the N^N motif; a bipyridine-based N^N for 15 and a pyrazolylpyridine-based N^N for 16 , respectively. In both cases, a forced consecutive double cycloplatination would be possible to form a C^C ∗ N^N-coordinated platinum complex. As expected, the reaction of ligand 15 and 16 with K 2 PtCl 4 in acetic acid at reflux produced 17 and 18 , respectively. The coordination geometry was confirmed by the X-ray crystallography of the complex 17 . The square planar geometry and dual intermolecular π–π and Pt–Pt interactions were revealed in the molecular packing. The photophysical properties of the complexes were studied. It was found that the complex 18 emitted intensely yellow phosphorescence at room temperature in deoxygenated solution, while complex 17 emitted red emission in a frozen glass matrix. Both compounds have the lifetime of the excited states in the scale of micro seconds. The DFT calculations were carried out to elucidate the frontier orbitals of the complexes, which are used in interpreting the electronic spectra of the complexes.