Thermodynamic and kinetic studies on the Cu2+ coordination chemistry of a novel binucleating pyridinophane ligand

Abstract

The synthesis and coordination chemistry of the novel pyridine-functionalized ligand 2,6,9,12,16-pentaaza[17]-(2,6)pyridinophane (L¹) is described. The compound behaves as a hexaprotic base in aqueous solution. NMR studies indicate a protonation pattern in which the sp² pyridine nitrogen (N(py)) does not undergo a net protonation although it is involved in formation of hydrogen bonds. L¹ forms mono and binuclear hydroxylated complexes. The crystal structure of [CuL¹](ClO₄)₂ shows a square planar coordination for Cu²⁺ with the pyridine and the three central nitrogens of the chain forming the vertices of the square. The benzylic nitrogens could be occupying the axial positions of a strongly axially asymmetrically elongated octahedron. The kinetic data show that decomposition of the [Cu(HL¹)]³⁺, [Cu(L¹)]²⁺ and [Cu₂(L¹)(OH)]³⁺ complexes takes place in two steps the slower one being ascribed to the dissociation of the Cu–N(py) bond.