Structural characterization of cobalt(III), nickel(II), copper(II) and iron(III) complexes of tetraazamacrocycles with N-carboxymethyl arms

Abstract

The single crystal structures of complexes [CoL1]Br0.5·[NO3]0.5·5H2O 1, [Ni(HL1)]Br·4H2O 2, Ca[CuL3]NO3· CH3CN·2H2O 3 and [CoL3]·2H2O 4 were determined, where H2L1 is 3,11-bis(carboxymethyl)-7-methyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene and H3L3 is 3,7,11-tris(carboxymethyl)-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene. In all these complexes the metal centre is encapsulated by the macrocycle in a distorted octahedral environment, the four nitrogen atoms of the tetraaza ring defining the equatorial planes. In the complexes of H2L1 the axial positions are occupied by the two carboxymethyl groups which are adjacent to the pyridine ring, while in the complexes of H3L3 one of these positions is occupied by one of the carboxymethyl arms adjacent to the pyridine ring and the other by the group opposite to the pyridine moiety. The remaining carboxymethyl arm adjacent to the pyridine ring is further away from the metal centre and plays an important role in the crystal structures of both complexes. Crystals of 3 display a three-dimensional polymeric structure derived from the Ca–O bonds. In the lattice the Ca2+ ions are surrounded by eight oxygen atoms, with two calcium ions bridged by two oxygen atoms of carboxylate groups bonded to the copper centres. This large spherical molecule has a centrosymmetric structure with the centre of the core, Ca2O2, localised on a crystallographic inversion centre. Additionally, the complexes of H3L3 (3 and 4) have interesting superstructures based on several CO· · ·HO hydrogen interactions between the carboxylate pendant arms and water molecules. The Fe3+ complexes of H2L2 (3,11-bis(carboxymethyl)-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene) and of H3L3 were characterised by EPR and Mossbauer spectroscopy. Both techniques revealed that the complexes of the bis(carboxymethyl) derivative appear as an equilibrium of high- and low-spin state species, the relative amount of the latter increasing with decrease of temperature, and that the complex of the tris(carboxymethyl) derivative appears practically as a high-spin state species. Molecular mechanics studies have provided data to explain the different behaviour of the iron(III) complexes of both ligands.