Structural Analysis of Nine-Coordinate Lanthanide Complexes: Steric Control of the Metal−Water Distance Across the Series

The structures of 10 isomorphous lanthanide (Ln) complexes of a chiral DOTA tetra-amide ligand (L1), [LnL1(H2O)](CF3SO3)3·3H2O, crystallizing in space group P21, have been studied by single-crystal X-ray diffraction. The Ln coordination
is a O4N4 square antiprism, the O4 base of which is capped by an aqua ligand. The sterically demanding position
of the latter results in the lengthening of the Ln−OH2 distance along the Pr to Lu series by 0.06 Å (after allowing
for the lanthanide contraction). In parallel, the distance between the bound water oxygen and the second-sphere
water oxygen is reduced from 3.17 Å (Pr) to 3.04 Å (Lu), consistent with the enhanced hydrogen bond acceptor
ability of the coordinated water oxygen across the series. A Cambridge Structural Database survey of [Ln(H2O)9](CF3SO3)3 salts (space group P63/m) and of six reported isostructural complexes of DOTA [L2] revealed a similar trend.
The implications of the resultant destabilization of the ground state structure for the water interchange process are
discussed.