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
journal contribution
posted on 2013-09-03, 00:00 authored by Lai-Hon Chung, Ka-Sin Cho, Jason England, Siu-Chung Chan, Karl Wieghardt, Chun-Yuen WongRuthenium(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.
