Air-stable palladium N-Heterocyclic carbene based CCC-NHC pincer complexes: Synthesis, characterization, photophysical and Raman vibrational studies, and DFT studies, plus observation of an abnormal carbene pincer

A series of CCC-NHC pincer Pd(II)X complexes, where X = Cl, Br, or I, were synthesized by a metalation/ transmetalation reaction sequence and characterized by NMR and Raman spectroscopy, photophysical studies, Xray crystallography, and DFT computational studies. This is the first detailed report of the CCC-NHC pincer Pd complexes analogous to the previously reported Pt analogs. Photophysical measurements show that by varying the X ligand, the emission wavelength can be tuned. The chloride complex is a water and air stable blue emitter with a quantum yield of 46 % and all three complexes have photostabilities >90 %. A combined DFT and TD-DFT study has been carried out to investigate ground state and excited state geometries as well as absorption and emission processes of CCC-NHC Pd complexes. Theoretical results were compared with the corresponding experimental results and showed good agreement. Raman spectroscopy (computational and experimental) was used to examine Pd-X vibrations which have been rarely reported in the literature. Several by-products of the metalation/transmetalation procedure were identified. A rare mixed normal/abnormal carbene pincer Pd(II)Cl complex was isolated and crystallographically characterized.