Tetradentate vs Pentadentate Coordination in Copper(II) Complexes of Pyridylbis(aminophenol) Ligands Depends on Nucleophilicity of Phenol Donors

The ligand binding preferences of a series of potentially pentadentate pyridylbis(aminophenol) ligands were explored. In addition to the previously reported ligands 2,2′-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H2L1) and 6,6′-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)bis(2,4-di-tert-butylphenol) (H2L1‑tBu), four new ligands were synthesized: 6,6′-(2-methyl-2(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)bis(2,4-dibromophenol) (H2L1‑Br), 6,6′-(2-methyl-2(pyridine-2-yl)propane-1,3diyl)bis(azanediyl)bis(methylene)bis(2-methoxyphenol) (H2L1‑MeO), 2,2′-(2-methyl-2(pyridine-2-yl)propane-1,3diyl)bis(azanediyl)bis(methylene)bis(4-nitrophenol) (H2L1‑NO2), and 2,2′-(2-phenylpropane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H2L2). These ligands, when combined with copper(II) salts and base, form either tricopper(II) species or monocopper(II) species depending on the nucleophilicity of the phenol groups in the ligands. All copper complexes were characterized by X-ray crystallography, cyclic voltammetry, and spectroscopic methods in solution. The ligands in trimeric complexes [{CuL1(CH3CN)}2Cu](ClO4)2 (1), [{CuL1Cl}2Cu] (1a), and [{CuL2(CH3CN)}2Cu](ClO4)2 (1b) and monomeric complex [CuL1‑tBu(CH3OH)] (2) coordinate in a tetradentate mode via the amine N atoms and the phenolato O atoms. The pyridyl groups in 1, 1a, and 2 do not coordinate, but instead are involved in hydrogen bonding. Monomeric complexes [CuL1‑Br] (3a), [CuL1‑NO2] (3b), and [CuL1‑MeONa(CH3OH)2]ClO4 (3c) have their ligands coordinated in a pentadentate mode via the amine N atoms, the phenolato O atoms, and the pyridyl N atom. The differences in tetradentate vs pentadentate coordination preferences of the ligands correlate to the nucleophilicity of the phenolate donor groups, and coincide with the electrochemical trends for these complexes.