Ruthenium(II) and osmium(II) complexes bearing bipyridine and the N-heterocyclic carbene-based C^N^C pincer ligand: An experimental and density functional theory study

摘要

Ruthenium(~(II)) and osmium(~(II)) complexes [M(CNC)(NN)L]~(n+) (L = Cl~-, n = 1; L = CH_3CN, t-BuNC, n = 2) containing a neutral tridentate N-heterocyclic carbene (NHC)-based pincer ligand, either 2,6-bis(1-butylimidazol-2-ylidene)pyridine (C~1NC~1) or 2,6-bis(3-butylbenzimidazol-2-ylidene)pyridine (C~2NC~2), and a neutral 2,2'-bipyridine-type aromatic diimine have been prepared. Investigations into the effects of varying M (Ru and Os), CNC, NN, and L on the structural, electrochemical, absorption, and emission characteristics associated with [M(CNC)(NN)L]~(n+) are presented. Interestingly, spectroscopic findings and time-dependent density functional theory (TD-DFT) calculations in this work support a d_π(Ru~(II)/Os~(II)) → π(NN) metal-to-ligand charge transfer (MLCT) assignment for the lowest-energy transition in [M(CNC)(NN)L]~(n+) and not a d_π(Ru ~(II)/Os~(II)) → π(CNC) MLCT assignment. This is in stark contrast to [Ru(tpy)(bpy)Cl]~+ and [Os(tpy)(bpy)Cl]~+ (tpy = 2,2':6',2'-terpyridine, bpy = 2,2'-bipyridine) for which the lowest-energy transitions are assigned as d_π(Ru/Os) → π(tpy) MLCT transitions. [Ru~(II)(CNC)(NN)L]~(n+) is emissive with emission maxima of around 600-700 nm observed upon photoexcitation of their d_π(Ru~(II)) → π(NN) MLCT bands. The electronic structures for [Ru(CNC)(NN)Cl]~0 have also been probed by spectroelectrochemistry, electron paramagnetic resonance (EPR) spectroscopy, and DFT calculations, which reveal that the lowest unoccupied molecular orbitals (LUMOs) for [Ru(CNC)(NN)Cl]~+ are NN-based.