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(H ₂O)](CF₃-SO₃)₃·3H ₂O, crystallizing in space group P2₁, have been studied by single-crystal X-ray diffraction. The Ln coordination is a O ₄N₄ 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(H ₂O)₉](CF₃-SO₃)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.