Structural, Spectral, and Electrochemical Properties of Nickel(11), Copper(II), and Zinc(II) Complexes Containing 12-Membered Pyridine- and Pyridol-Based Tetra-aza Macrocycles

The structural, electronic, and electrochemical properties of a series of novel 12-membered pyridine- and pyridolbased tetra-aza transition-metal (Ni, Cu, Zn) complexes f[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),1 1,13-trien-13-ol; L3 = 3,6,9,15-tetra-azabicyclo[9.3.1]penta-deca- 1(15),11,13-trien12-o1). 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 (mu-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 pi 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 metalion, while m-hydroxylation decreases the electron-donating ability of the backbone and increases the metal-ion Lewis-acidity.