Tungsten(0)–carbonyl complexes of naphthylazoimidazoles

Abstract

The reaction of W(CO) 6 with 1-alkyl-2-(naphthyl-α-azo)imidazole (α-NaiR) has synthesized [W(CO) 5(α-NaiR-N)] (α-NaiR-N refers to the monodentate imidazole-N donor ligand) at room temperature. The structure of [W(CO) 5(α-NaiMe-N)] shows a monodentate imidazole-N coordination of 1-methyl-2-(naphthyl-α-azo)imidazole (α-NaiMe). The complexes are characterized by elemental, mass and other spectroscopic data (IR, UV–Vis, NMR). On refluxing in THF at 323 K, [W(CO) 5(α-NaiR-N)] undergoes decarbonylation to give [W(CO) 4(α-NaiR-N,N′)] (α-NaiR-N,N′ refers to the imidazole-N(N), azo-N(N′) bidentate chelator). Cyclic voltammetry shows metal oxidation (W 0/W I) and ligand reductions (azo/azo –, azo –/azo ). The redox and electronic properties are explained by theoretical calculations using an optimized geometry. DFT computation of [W(CO) 5(α-NaiMe-N)] suggests that the major contribution to the HOMO/HOMO − 1 come from W d-orbitals and the orbitals of CO. The LUMOs are occupied by α-NaiMe functions. The back bonding interaction thus originates from the W(CO) n moiety to the LUMO of α-NaiR. A TD-DFT calculation has ascribed that HOMO/HOMO − 1 → LUMO is a mixture of metal-to-ligand and ligand-to-ligand charge transfer underlying the CO → azoimine contribution. The complexes show emission spectra at room temperature. [W(CO) 4(α-NaiR-N,N′)] shows a higher fluorescence quantum yield ( ϕ = 0.05–0.07) than [W(CO) 5(α-NaiR-N)] ( ϕ = 0.01–0.02).