posted on 2018-08-16, 12:35authored byYu Zhang, Jeffrey L. Petersen, Carsten Milsmann
The complexes Zr(HCNN)2 and Zr(MeCNN)2 were prepared via reaction
of the ligand precursors
2-phenyl-6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine,
H2HCNN, and 2-(3,5-dimethyl-phenyl)-6-(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine, H2MeCNN, 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 (3IL/3LMCT). 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(MeCNN)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(MeCNN)2 readily promotes the photochemical homocoupling
of benzyl bromide in the presence of a sacrificial benzimidazolium
hydride reductant, Zr(HCNN)2 undergoes an intramolecular
photochemical reaction with formation of a new carbon–carbon
bond between the phenyl units of the two HCNN ligands.