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

Abstract

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 (C(1)^N^C(1)) or 2,6-bis(3-butylbenzimidazol-2-ylidene)pyridine (C(2)^N^C(2)), 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(Ru(II)/Os(II)) → 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(Ru(II)/Os(II)) → 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. [Ru(II)(C^N^C)(N^N)L](n+) is emissive with emission maxima of around 600-700 nm observed upon photoexcitation of their dπ(Ru(II)) → 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.

Luminescent Ruthenium(II) Complex Bearing Bipyridine and N-Heterocyclic Carbene-based C∧N∧C Pincer Ligand for Live-Cell Imaging of Endocytosis

“…The bond distances of Ru–C NHC and Ru–C CN are 2.054(3)–2.063(3) and 2.002(3) Å respectively. These Ru–C NHC distances are similar to those in chloride-ligated complexes [Ru II (C ∧ N ∧ C)(N ∧ N)Cl] + (N ∧ N = 2,2′-bipyridine-like diimine ligands, Ru–C NHC = 2.048(2)–2.062(4) Å)30.…”

“…With reference to our previous spectroscopic studies on the [Ru II (C ∧ N ∧ C)(bpy)L] n+ system, the lowest-energy absorption band (λ max = 464 nm, ε max = 7.1 × 10 3  dm 3 mol −1 cm −1 ) is assigned as d π (Ru II ) → π*(bpy) MLCT transition30. The assignment is further supported by time-dependent density functional theory (TD-DFT) calculation on model complex 1′ , where its metal core is the same as that in 1 except the butyl chains on the C ∧ N ∧ C are replaced by methyl groups to reduce the computational cost.…”

“…All reagents and solvents were used as received. Complex [Ru(C ∧ N ∧ C)(bpy)(OH 2 )](ClO 4 ) 2 was prepared according to literature method30. 1 H, 13 C{ 1 H}, DEPT-135, 1 H– 1 H COSY, and 1 H– 13 C HSQC NMR spectra were recorded on Bruker 400 DRX FT-NMR spectrometer.…”

“…In these years, our group have initiated a program to develop new luminescent organometallic Ru(II)- and Os(II)-bipyridine complexes2223242526272829303132. Recently, we have developed luminescent Ru(II)-bipyridine complexes bearing neutral tridentate NHC-based pincer ligands 2,6-bis(1-butylimidazol-2-ylidene)pyridine (C ∧ N ∧ C)30.…”

“…Recently, we have developed luminescent Ru(II)-bipyridine complexes bearing neutral tridentate NHC-based pincer ligands 2,6-bis(1-butylimidazol-2-ylidene)pyridine (C ∧ N ∧ C)30. We envision that Ru(II) complexes in the form of [Ru II (C ∧ N ∧ C)(bpy)L] n+ would be a valuable system for the design of luminescent molecular probes for live-cell imaging, as the C ∧ N ∧ C and bpy ligands on the [Ru II (C ∧ N ∧ C)(bpy)L] n+ are inert towards ligand substitution, therefore would in principle minimize the chance of complicating any biological pathways via ligand dissociation.…”