Thermodynamic and structural aspects of manganese(II) complexes with polyaminopolycarboxylic ligands based upon 1,4,7,10-tetraazacyclododecane (cyclen). Crystal structure of dimeric [MnL]2·2CH3OH containing the new ligand 1,4,7,10-tetraazacyclododecane-1,4-diacetate

Abstract

The formation of manganese(II) complexes with polyaminopolycarboxylic ligands based upon 1,4,7,10-tetraazacyclododecane (cyclen) has been studied in aqueous solution by means of potentiometric and microcalorimetric techniques affording log K, ΔH° and TΔS° values for the complexation reactions. The ML complexes present high stability constants, due to both favourable enthalpic and entropic contributions; with the unique exception of DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), the entropic term is prevalent. The new ligand 1,4-DO2A (1,4,7,10-tetraazacyclododecane-1,4-diacetic acid) forms a more stable complex than the 1,7-DO2A isomer. The thermodynamic results are discussed in terms of ligand characteristics: number and type of donor atoms, overall charge, substituent location. The crystal structure of [MnL]₂·2CH₃OH (H₂L = 1,4-DO2A), obtained by single crystal X-ray analysis, displays a centrosymmetric dimeric [Mn₂L₂] unit in which each manganese atom is seven-coordinated by four nitrogen atoms of a cyclen macrocycle, one oxygen atom of each of the acetate groups linked to this macrocyclic unit, and one oxygen atom pertaining to an acetate group of the other cyclen molecule in the same dimeric unit and bridging the two metal centers. The resulting binuclear complex contains a doubly bridged bimetallic core with Mn ⋯ Mn distance of 3.537(1) Å.