Structural, Spectral, and Electrochemical Properties of Nickel(II), 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 pyridol-based tetra-aza transition-metal
(Ni, Cu, Zn) complexes {[MII(L1)­Cl]­(ClO4), [MII(L2)­Cl]­(ClO4), and
[MII(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 {[CuII(L3)­Cl]­(ClO4) > [CuII(L1)­Cl]­(ClO4) > [CuII(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.