Synthesis and characterization of mixed ligand cyclopalladates with 2-phenylpyridine and substituted phenanthrolines: Investigation into the hydroxylation reaction of 2-phenylpyridine

Abstract

Mixed ligand cyclopalladated complexes with 2-phenylpyridine (ppy) and 2,9-dimethyl-1,10-phenanthroline (ncp), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bcp), 4,7-diphenyl-1,10-phenanthroline (bphen) were synthesized and characterized. The crystal structures of [Pd(ppy)(ncp)Cl] (1), [Pd(ppy)(ncp)Η2O]PF6·0.5(CH3CH2)O (3) and {[Pd(ppy)(ncp)]2(μ-Cl)}PF6 (4) were determined by single-crystal diffraction methods. In all cases, the results showed a pseudo-pentacoordinated palladium environment due to the anisobidentate binding of the 2,9-dimethyl-1,10-phenanthroline. The structures of the complexes in acetone solution were studied using NMR spectroscopic techniques, mass spectrometry and conductivity measurements. The results revealed the retention of their solid-state structure. The complexes (1), (4) and [Pd(ppy)(phen)]PF6, (6) were examined for their activity in the hydroxylation reaction of 2-phenylpyridine using NMP (N-methyl-2-pyrrolidone) and molecular oxygen as oxidant. The complex [Pd(ppyO)(phen)]PF6 (8), was isolated from the hydroxylation reaction mixture and could potentially imply that the reaction mechanism includes the formation of high-valent palladium species with Pd-OH bond(s).