Electron spin resonance studies and crystal structures of copper(II) complexes of some 12-, 13- and 14-membered oxatriaza macrocycles

Abstract

Electronic and ESR spectroscopic studies in aqueous solution and single-crystal X-ray diffraction analyses of copper(II) complexes of the following oxatriaza macrocycles have been undertaken: 1-oxa-4,7,10-triazacyclododecane (L¹), 4,7,10-trimethyl-1-oxa-4,7,10-triazacyclododecane (L²), 1-oxa-4,7,11-triazacyclotridecane (L³), 1-oxa-4,8,12-triazacyclotetradecane (L⁴) and 4,8,12-trimethyl-1-oxa-4,8,12-triazacyclotetradecane (L⁵). The complexes for solution studies were prepared in an excess of NaClO₄, but the bulky counter ion PF₆^– was added to induce crystallization. The crystal structures of [CuL¹(Br)][PF₆], [CuL²(Cl)][PF₆], [CuL⁴H₂O)][PF₆]₂·H₂O and [CuL⁵(Cl)][PF₆] were determined. The co-ordination geometry around the copper atom in the four complexes can be described as a distorted square pyramid, but whereas the basal planes comprise the three nitrogens and the oxygen from the macrocycle in the last two complexes, in the first two complexes they comprise the three macrocyclic nitrogens and a halogen atom, bromine and chlorine respectively. The apical co-ordination in the latter is by the macrocyclic oxygen, but in [CuL⁴(H₂O)][PF₆]₂·H₂O it is the oxygen of a water molecule and in [CuL⁵(Cl)][PF₆] it is a chlorine atom. The main conclusions on the structures of these complexes from spectroscopic measurements and X-ray studies are in good agreement. However, for the complex of the 13-membered macrocycle (L³) published X-ray results point to a structure similar to those found in this work for the 14-membered macrocyclic complexes, while the spectroscopic studies now reported indicate a structure with a pronounced tetrahedral distortion similar to that observed in the 12-membered macrocyclic complexes. The replacement of hydrogen atoms by methyl groups on the nitrogen atoms does not affect the overall final geometry of these complexes. For this series of macrocyclic complexes, the smaller the ligand the more pronounced is the folding necessary to accommodate the metal at its centre. The structure of a complex of a 12-membered macrocycle having four donor atoms one of which approaches the metal from the apical position is reported for the first time.