Homoleptic tetraazaphenanthrene-based copper(I) complexes: Synthesis, spectroscopic characterization, crystal structures and computational studies

Three new Cu(I) complexes containing bidentate N^N donor ligands with the general formula [Cu(N^N)₂][PF₆] (N^N = 2,3-diphenyl-6,7-di-p-tolyl-1,4,5,8-tetraazaphenanthrene (^1L), 2,3-diphenyl-6,7-di(2-thienyl)-1,4,5,8-tetraazaphenanthrene (^2L), and 2,3-diphenyl-6,7-di-p-fluorophenyl-1,4,5,8-tetraazaphenanthrene (^3L), were prepared by the reaction of [Cu(CH₃CN)₄][PF₆] with two equivalents of the N^N ligand. Single-crystal X-ray diffraction analysis confirmed that in each complex the metal displays a distorted tetrahedral geometry surrounded by the four N atoms of the two sterically hindered substituted tetraazaphenanthrene (TAP) ligands. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT), calculations were used to study the ground state properties and interpret the absorption spectra for these Cu(I) complexes. The calculations show that the lowest-energy excitations of all complexes are dominated by dπ(Cu) → π^∗(L), metal-to-ligand charge transfer, (MLCT) excitations. Electronic difference density maps (EDDMs) were calculated, indicating the change of electron density in the singlet excited states. The degree of filling of the coordination sphere (G parameter) by the ligands was calculated taking into account the ligand–ligand overlap, and compared to the related bis(2,9-disubstituted phenanthroline) Cu(I) complexes.