DOTA complexes with divalent zinc, cadmium and mercury: X-ray and solid-state NMR studies and solution isomerism

DOTA is widely regarded as a "prototype chelator" in the coordination chemistry of macrocyclic ligands. Its macrocyclic cavity can adapt to the size and coordination requirements of various metal ions. However, its complexes with divalent metal ions have been less explored than those with trivalent ions. Here, we studied the complexes of d¹⁰ metal ions, Zn(II), Cd(II) and Hg(II), which are chemically similar but have different ionic radii and prefer different coordination numbers (CNs). Solid-state structures of the [M(dota)]^2- anions show different coordination modes going from an octahedron and CN 6 for Zn(II) to a twisted-square antiprismatic (TSA) arrangement with CN (6 + 2) for Cd(II) and CN (6 + 1)/(6 + 2) for Hg(II). The coordination spheres are distorted from the ideal arrangements, and they consist of four amine groups (forming an N₄ plane) and 2-4 oxygen atoms of the carboxylate pendant arms. The oxygen atoms of the pendant arms are also bound to Ba(II) or Ca(II) counter-ions in various metal-ion bridging modes. The ¹³C and ¹⁵N solid-state NMR data correspond well to the structures determined by X-ray diffraction. The fluxionality of the coordination sphere was investigated by variable-temperature ¹³C NMR spectroscopy in solution. The measurements pointed to a higher rigidity of the macrocycle chelate rings compared to those formed by the pendant arms. The pendants are highly fluxional due to the easy change of their coordination modes as was observed in the solid state. Overall, the results confirm the size-dependent coordination behaviour of the internal cavity of DOTA-like macrocycles, which differs significantly from those observed in complexes of trivalent metal ions, e.g. trivalent lanthanides.