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 (L-1), [LnL(1)(H2O)](CF3-SO3)(3).3H(2)O, crystallizing in space group P2(1), have been studied by single-crystal X-ray diffraction. The Ln coordination is a O4N4 square antiprism, the O-4 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 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)](CF3-SO3)(3) salts (space group P6(3)/m) and of six reported isostructural complexes of DOTA [L-2] revealed a similar trend. The implications of the resultant destabilization of the ground state structure for the water interchange process are discussed.