Unusual Reactivity and Photophysical Properties of Platinum(II) Pincer Complexes Containing 6,6'-Diphenyl-2,2'-bipyridine Ligands

A series of novel 6,6'-diphenyl-2,2'-bipyridine ligands modified with the substituents at phenyl and bipyridine moieties have been prepared and used in the synthesis of pincer platinum chloride complexes 1-3, [Pt(NNC-R)Cl], where NNC-R - dimethyl 6,6'-diphenyl-[2,2'-bipyridine]-5,5'-dicarboxylate (R = H, (1)), dimethyl 6,6'-bis(4-fluorophenyl)-[2,2'-bipyridine]-5,5'-dicarboxylate (R = F (2)), dimethyl 6,6'-bis(4-methoxyphenyl)-[2,2'-bipyridine]-5,5'-dicarboxylate (R = OMe (3)). Reactions of the obtained compounds with triphenylphosphine proceed through unusual pathway, which gives the [Pt(NNC-R)(PPh3)(2)Cl] products (1a-3a) containing two phosphine ligands, with the chloride retained in coordination sphere and diphenyl-bipyridine fragment coordinated in eta(1)-mode through metalation of a pyridine ring. All complexes have been characterized with mass-spectrometry and NMR spectroscopy, solid-state structure of 1-3 and 1a, 3a was revealed by using XRD crystallography. The complexes 1-3 are phosphorescent in dichloromethane solution and in solid state. Their photophysical characteristics were determined and analyzed by DFT calculations, which gave assignment of emissive excited state character with the major contribution from the intraligand charge transfer ((ILCT)-I-3, Ph -> bipyridine) and ligand centered ((LC)-L-3, bipyridine fragment) transitions.

Automated summary

A series of novel 6,6'-diphenyl-2,2'-bipyridine ligands modified with substituents were synthesized and used in the synthesis of pincer platinum chloride complexes. The obtained compounds reacted with triphenylphosphine in an unusual pathway to give products with two phosphine ligands. These complexes exhibited phosphorescence in solution and solid state, with their photophysical characteristics explained by DFT calculations.