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

Ruthenium(II) and osmium(II) complexes [M((CNC)-N-boolean AND-C-boolean AND)((NN)-N-boolean AND)L](n+) (L = Cl-, n = 1; L = CH3CN, t-BuNC, n = 2) containing a neutral tridentate N-heterocyclic carbene (NHC)-based pincer ligand, either 2,6-bis(1-butylimidazol-2-ylidene)pyridine ((CNC1)-N-1 boolean AND-C-boolean AND) or.2,6-bis(3-butylbenzimidazol-2-ylidene)pyridine ((CNC2)-N-2 boolean AND-C-boolean AND), and a neutral 2,2'-bipyridine-type aromatic diimine have been prepared. Investigations into the effects of varying M (Ru and Os), (CNC)-N-boolean AND-C-boolean AND, (NN)-N-boolean AND, and L on the structural, electrochemical, absorption, and emission characteristics associated with [M((CNC)-N-boolean AND-C-boolean AND)((NN)-N-boolean AND)L](n+) are presented. Interestingly, spectroscopic findings and time-dependent density functional theory (TD-DFT) calculations in this work support a d(pi)(Ru-II/Os-II) -> pi*((NN)-N-boolean AND) metal-to-ligand charge transfer (MLCT) assignment for the lowest-energy transition in [M((CNC)-N-boolean AND-C-boolean AND)((NN)-N-boolean AND)L](n+) and not a d(pi)(Ru-II/Os-II) -> pi*((CNC)-N-boolean AND-C-boolean AND) 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(pi)(Ru/Os) -> pi*(tpy) MLCT transitions. [Ru-II(CNAC)(NAN)Lr is emissive with emission maxima of around 600-700 nm observed upon photoexcitation of their d(pi)(Ru-II/Os-II) -> pi*((NN)-N-boolean AND) MLCT bands. The electronic structures for [Ru((CNC)-N-boolean AND-C-boolean AND)((NN)-N-boolean AND)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)-N-boolean AND-C-boolean AND)((NN)-N-boolean AND)Cl](+) are NAN-based.