Luminescence of five-coordinated nickel(ii) complexes with substituted-8-hydroxyquinolines and macrocyclic ligands

Abstract

A series of heteroleptic quinolinolate pentacoordinated nickel(II) complexes, [Ni(mcN₃)(R¹,R²,R³-8-hq)](PF₆), were synthesized and characterized by spectroscopic methods. Single-crystal X-ray diffraction studies for [(Me₃-mcN₃)Ni(N,O-2-CN-8-hq)][PF₆] (6a), [(Me₄-mcN₃)Ni(N,O-8-hq)][PF₆] (2b) and [(Me₄-mcN₃)Ni(N,O-5,7-I₂-8-hq)][PF₆] (5b) indicate that these complexes consist of a square-pyramidal ligand arrangement containing one chelating quinolinolate and one macrocyclic ligand (mcN₃). Variation of the substituents on quinolinolate ligands imposes obvious electronic or structural effects on the nickel atom. These chromophores absorb moderately in the visible region and emit in the yellowish-green spectral region from a quinolinolate-centered intraligand charge-transfer excited state. The emission maxima are in the range 520–548 nm, with quantum yields between 0.11 and 1.63%, in deoxygenated organic solvents at room temperature. TD-DFT calculations allow exploration of the photophysical properties of complex [(Me₄-mcN₃)Ni(N,O-8-hq)][PF₆] 2b and reveal the influence of the quinolinolate ligand on the HOMO/LUMO energies and oscillator strengths.