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]Br_0.5·[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 C[double bond, length half m-dash]O  · · ·  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 Mössbauer 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.

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