Photochemical C-C Bond Formation in Luminescent Zirconium Complexes with CNN Pincer Ligands

The complexes Zr((CNN)-C-H)(2) and Zr((CNN)-C-Me)(2) were prepared via reaction of the ligand precursors 2-phenyl-6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, (H2CNN)-C-H, and 2-(3,5-dimethyl-phenyl)-6- (5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, (H2CNN)-C-Me, with tetrabenzyl zirconium. Both complexes are photoluminescent upon excitation with visible light and exhibit remarkably long emission lifetimes of tens to hundreds of microseconds in solution at room temperature. The nature of the emissive state was investigated using density functional theory, which allowed the assignment as a predominantly intraligand triplet state with significant ligand-to-metal charge transfer contributions ((IL)-I-3/(LMCT)-L-3). Electrochemical studies revealed two fully reversible one-electron reductions for each complex and an additional reversible oxidative EC process for the more sterically protected complex Zr((CNN)-C-Me)(2). On the basis of the optical and electrochemical properties, the utility of the two zirconium complexes as photosensitizers for photoredox catalytic transformations was investigated. While Zr((CNN)-C-Me)(2) readily promotes the photochemical homocoupling of benzyl bromide in the presence of a sacrificial benzimidazolium hydride reductant, Zr((CNN)-C-H)(2) undergoes an intramolecular photochemical reaction with formation of a new carbon carbon bond between the phenyl units of the two (CNN)-C-H ligands.