Novel NCN-pincer carbene complexes of Ru(II), 1,3-bis(2-pyridylmethyl)benzimidazolineruthenium(II) bishexafluorophosphate (2), and Pt(II), 1,3-bis(2-pyridylmethyl)benzimidazolinechloroplatinum(II) hexafluorophosphate (3), complexes based on 1,3-bis(2-pyridylmethyl)-1H-benzimidazolium chloride (1) were synthesized and characterized by different spectroscopic methods. Complex 2 shows an absorption maximum at 386 nm, blue-shifted in comparison to Ru(bpy)₃ ²⁺ and Ru(tpy)₂ ²⁺, probably due to the strong σ-donor and weak π-acceptor properties of the electron-rich NHC ligand. Electrochemical studies show Ru(II)/Ru(III) and Pt(II)/Pt(IV) reversible couples at 0.67 and 0.58 eV, respectively, lower than those for the analogous complexes of ligands like bipyridine (bpy), terpyridine (tpy) and phenylbipyridine (pbpy). The solid state structure of 2 was solved by X-ray diffraction. Theoretical studies (B3LYP/LANL2DZ) of the complex show a HOMO (−0.38594 au) mainly centered on the ruthenium and benzimidazole, whereas the LUMO (−0.25130 au) is populated by pyridines. Therefore, it is assumed that the charge transfer from HOMO → LUMO is mixed ILCT (interligand charge transfer)/MLCT (metal to ligand charge transfer). The observed lower redox potentials of the Pt(II) complex compared to the Ru(II) complex is supported by theoretically calculated ionisation potentials and also electron affinity values. To the best of our knowledge, 2 is the first example of a six-membered metallacycle homoleptic chelate pincer NCN–Ru(II) N-heterocyclic carbene complex.

This work describes the synthesis and characterization of two new bis-terdentate Ru(II) complexes. Compound 1 is a homoleptic complex containing two CNC N-heterocyclic carbene (NHC) based ligands, whereas compound 2 bears one CNC ligand and an ancillary terpyridine ligand. The redox and photophysical properties of both compounds have been investigated and their X-ray crystal structures determined. Complex 1 displays a close-to-perfect octahedral coordination geometry and is not luminescent at room temperature while complex 2 features room temperature and 77 K luminescence despite its partially distorted geometry. The presence of the NHC moieties brings a significant amount of electronic density to the metal centre therefore lowering its oxidation potential with respect to that of analogous polypyridyl complexes.