Cationic rhenium(I) complexes bearing a π-accepting pyridoannulated N-heterocyclic carbene ligand: Synthesis, photophysical, electrochemical and theoretical investigation

Abstract

A novel family of photoactive cationic tris-carbonyl rhenium complexes of general formula fac-[Re(CO)3(pyipy)(L)]PF6, where pyipy is the 2-(pyridyl)-imidazo[1,5-a]pyridyl-3-ylidene ligand and L = pyridine (3·PF6) or PPh3 (4·PF6), is herein presented. Compounds 3·PF6 and 4·PF6 are straightforwardly synthetized from the corresponding chloro-derivative fac-[ReCl(CO)3(pyipy)] via halogen abstraction with silver(I) salt, followed by coordination of the neutral monodentate L ligand. The target complexes were characterized by 1H and 13C NMR as well as FT-IR spectroscopy and high-resolution mass spectrometry (HR-MS). In addition, X-ray diffractometric analysis was carried out by solving the single-crystal structure of compound 3·PF6. For both complexes, dilute samples in CH3CN solution at room temperature display a structured photoluminescence profile in the red region that is ascribed to a long-lived excited state (τ = 19.3–30.0 μs in degassed condition) with mainly triplet ligand-centered (3LC) character. This assignment is further corroborated by the minor hypsochromic shift of the emission profile observed for samples in 2-MeTHF at 77 K glassy matrix, in agreement with previous studies on the corresponding neutral counterparts. Electrochemical characterization by cyclic voltammetry (CV) shows two irreversible processes, one in the positive and one in the negative bias, which are associated to the metal-NHC moiety and the chelating NHC ligand, respectively. Finally, both electronic and optical features are further studied by means of computational investigation at density functional theory (DFT) and time-dependent DFT (TD-DFT) level of theory, which supports the attribution of the electronic transitions involved.