Luminescent mononuclear mixed ligand complexes of copper(i) with 5-phenyl-2,2′-bipyridine and triphenylphosphine

Abstract

Reaction of 5-phenyl-2,2′-bipyridine (L) with a mixture of CuI or [Cu(CH₃CN)₄]BF₄ and PPh₃ leads to mononuclear heteroleptic complexes [CuL(PPh₃)I] (1) and [CuL(PPh₃)₂]BF₄ (2). According to X-ray diffraction, L crystallizes in the monoclinic space group P2₁/n, exhibiting a disorder over four orientations. Complexes 1 and 2 crystallize in the monoclinic space groups P2₁/c and P2₁, respectively. 1 comprises a discrete neutral molecule, while 2 has an ionic structure containing [CuL(PPh₃)₂]⁺ and BF₄^–. Both structures reveal that each tetracoordinated copper(I) atom is linked to two nitrogen atoms of L, one iodide and one PPh₃ in the structure of 1, or two PPh₃ in the structure of 2 with the formation of a distorted tetrahedral coordination core. The structure of 2 is additionally stabilized by a weak intramolecular π⋯π stacking interaction formed between two adjacent phenyl rings of two PPh₃ ligands. Hirshfeld surface analysis showed that the structures of both complexes are mainly characterized by H⋯H and C⋯H contacts as well as by I⋯H in the structure of 1 and F⋯H in the structure of 2. The 2D fingerprint plots of two different molecules in the structure of L showed that both molecules exhibit contacts for π⋯π stacking interactions. The factors important for the stability of 1 and 2 were further quantitatively and qualitatively characterized by the charge and energy decomposition method ETS-NOCV. According to diffuse reflectance spectroscopy in the solid state, free L exhibits bands exclusively in the UV region, while the spectra of 1 and 2 also contain bands in the visible range up to about 500 and 600 nm. All three compounds were found to be emissive in the solid state. DFT calculations have shown that, while emission of L is due to the ligand-centered π → π* transition, luminescence of 1 and 2 was assigned to a (M + X)LCT and MLCT excited states, respectively.