Definition of the Labile Capping Bond Effect in Lanthanide Complexes

Two macrocyclic ligands containing a cyclen unit, a methyl group, a picolinate arm, and two acetate pendant arms attached to two nitrogen atoms of the macrocycle either in trans (1,7-H₃Medo2 ampa = 2,2′-(7-((6-carboxypyridin-2-yl)methyl)-10-methyl-1,4,7,10-tetraazacyclododecane-1,4-diyl)diacetic acid) or in cis (1,4-H₃Medo2 ampa) positions are reported. These ligands provide eight-coordination to the Ln³⁺ions, leaving a coordination position available for a water molecule that occupies a capping position in the twisted square antiprismatic polyhedron (1,4-H₃Medo2 ampa) or one of the positions of the square antiprism (1,7-H₃Medo2 ampa). The charge neutral [Gd(1,7-Medo2 ampa)] complex presents an unprecedentedly low water-exchange rate (k_ex²⁹⁸=8.8×10³s⁻¹), whereas water exchange in [Gd(1,4-Medo2 ampa)] is three orders of magnitude faster (k_ex²⁹⁸=6.6×10⁶s⁻¹). These results showcase the labile capping bond phenomenon: A ligand occupying a capping position is hindered by the environment and thus is intrinsically labile.