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 LnOH2 distance along the Pr to Lu series by 0.06 (angstrom) (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 (angstrom) (Pr) to 3.04 (angstrom) (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.