Structural, Spectral, and Electrochemical Properties of Nickel(II), Copper(II), and Zinc(II) Complexes Containing 12-Membered Pyridine- and Pyridol-Based Tetra-aza Macrocycles

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The structural, electronic, and electrochemical properties of a series of novel 12-membered pyridine- and pyridol-based tetra-aza transition-metal (Ni, Cu, Zn) complexes {[M(II)(L1)Cl](ClO4), [M(II)(L2)Cl](ClO4), and [M(II)(L3)Cl](ClO4)} are described (L1 (Pyclen) = 1,4,7,10-tetra-aza-2,6-pyridinophane; L2 = 3,6,9,15-tetraazabicyclo[9.3.1]penta-deca-1(15),11,13-trien-13-ol; L3 = 3,6,9,15-tetra-azabicyclo[9.3.1]penta-deca- 1(15),11,13-trien-12-ol). The subtle variations in the chemical properties of these complexes were investigated using X-ray crystallography, UV-vis and NMR spectroscopy, and cyclic voltammetry. In the solid-state, the Ni(II) complexes adopt a unique bimetallic and cis-octahedral (μ-Cl)2 coordination sphere, and the electronic studies provide further evidence for the existence of a six-coordinate Ni(II) species in solution. The pyridol-based Cu(II) and Zn(II) complexes contain five-coordinate (N4Cl) geometries in the solid-state, in which the four N-donor atoms are not coplanar. Hydroxylation of the pyridine ring was found to increase the amount of π electronic charge density residing throughout the aromatic system of the ligand backbone, increase the strength of the M-Cl and M-N (pyridine) basal x- and y-plane interactions, and decrease the axial M-N bonding interaction. The electrochemical studies demonstrate that (i) the Lewis-acidity of the metal center systematically decreases across the series {[Cu(II)(L3)Cl](ClO4) > [Cu(II)(L1)Cl](ClO4) > [Cu(II)(L2)Cl](ClO4)}, and (ii) the aromatic backbones allow access to both Cu(I) and Cu(III) species in solution. Overall, the experimental findings are consistent with the idea that p-hydroxylation enhances the Lewis-basicity of pyridine-based macrocycle and decreases the Lewis-acidity of the metal-ion, while m-hydroxylation decreases the electron-donating ability of the backbone and increases the metal-ion Lewis-acidity.