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(C^N^C)­(N^N)­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 (C1^N^C1) or 2,6-bis­(3-butylbenzimidazol-2-ylidene)­pyridine (C2^N^C2), and a neutral 2,2′-bipyridine-type aromatic diimine have been prepared. Investigations into the effects of varying M (Ru and Os), C^N^C, N^N, and L on the structural, electrochemical, absorption, and emission characteristics associated with [M­(C^N^C)­(N^N)­L]n+ are presented. Interestingly, spectroscopic findings and time-dependent density functional theory (TD-DFT) calculations in this work support a dπ(RuII/OsII) → π*­(N^N) metal-to-ligand charge transfer (MLCT) assignment for the lowest-energy transition in [M­(C^N^C)­(N^N)­L]n+ and not a dπ(RuII/OsII) → π*­(C^N^C) 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 dπ(Ru/Os) → π*­(tpy) MLCT transitions. [RuII(C^N^C)­(N^N)­L]n+ is emissive with emission maxima of around 600–700 nm observed upon photoexcitation of their dπ(RuII) → π*­(N^N) MLCT bands. The electronic structures for [Ru­(C^N^C)­(N^N)­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(C^N^C)­(N^N)­Cl]+ are N^N-based.