Monomeric zinc(II) amide complexes supported by bidentate, benzannulated phenanthridine amido ligands

Abstract

The first examples of homoleptic zinc amides (1-Zn and 2-Zn) supported by chelating, benzannulated 4-amidophenanthridine ligands (L1, L2) are reported. The organometallic compounds were characterized fully both in solution (NMR, UV–Vis, electrochemistry, emission spectroscopy) and the solid-state (X-ray crystallography, elemental analysis). X-ray structural analysis reveals the Zn complexes are monomeric in the solid-state, with distorted sawhorse or distorted tetrahedral structures enforced by the coordination geometry of the deprotonated forms of the bidentate amine proligands L1 (4-(N-phenylamine)-2-tert-butylphenanthridine) and L2 (2,6-dimethyl-4-(N-phenylamine)phenanthridine). Cyclic voltammetry shows quasi-reversible oxidations on the electrochemical timescale, which density functional theory (DFT) assigns as arising from the highest occupied molecular orbital (HOMO) comprised largely of the nitrogen lone pair and occupied N-phenyl π-orbitals. Ligand substitution plays a role in the reversibility of the observed oxidation. In comparison, the lowest unoccupied molecular orbitals (LUMOs) of L1, L2, 1-Zn and 2-Zn are based wholly on the phenanthridine moiety. This engenders the lowest energy absorptions of both the proligands and zinc complexes with ligand-to-ligand charge transfer (LLCT) character, confirmed by time-dependent DFT (TD-DFT) calculations. Both L1 and 1-Zn are emissive in solution, with considerable quenching of emission intensity in the zinc complex. In comparison, L2 and 2-Zn are non-emissive.