Solution Study, Crystal Structure and Relaxivity Properties of a Gd3+ Complex with an Uncharged Macrocyclic Ligand Bearing Four Amidic Side Arms

Abstract

Equilibrium data on the interaction of DTMA [(DTMA = DOTA tetrakis(methylammide)] with Gd3+ in aqueous solution, properties of the complexes formed in the pH range 0.6–11.8, water proton relaxation rate enhancement, and the crystal structure analysis of the [Gd(DTMA)H2O]3+ complex are reported. In the crystal structure the metal ion is bound to the nitrogen atoms of the tetraazamacrocyclic moiety, to the amidic oxygen atoms, and to an oxygen atom of a water molecule. The nine donors are located at the vertices of a distorted square antiprism, which is capped by the coordinated water oxygen atom in the axial position. In solution [Gd(DTMA)]3+ is not very stable [logKML = 12.8(1)] and gives rise to the formation of [Gd(DTMA)OH]2+ [pKa = 7.9(1)] and [Gd(HDTMA)]4+ [logK(ML+H) = 3.4(1)]. The proton solvent relaxivity of aqueous complex solutions assumes a constant value in the pH range 3–8, increasing at higher and lower pH. For pH > 3 the data are in good agreement with a previous study on the same compound. For pH < 3 a new interpretation is presented, based on the formation of [Gd(HDTMA)]4+ and the release of Gd3+.