Structure Comparison of Early and Late Lanthanide( III ) Homodinuclear Macrocyclic Complexes with the Polyamine Polycarboxylic Ligand H 8 OHEC

The solid‐state structures of two new homodinuclear chelate complexes with the late lanthanide( III ) ions Yb and Lu, [Na 2 (Yb 2 OHEC)] . 14 . 5H 2 O ( 1 ), and [Na 2 (Lu 2 OHEC)] . 14 . 5H 2 O ( 2 ) (H 8 OHEC = 1,4,7,10,14,17,20,23‐octaazacyclohexacosane‐ 1,4,7,10,14,17,20,23‐octaacetic acid), have been determined by X‐ray crystal structure analysis. Each lanthanide( III ) ion is coordinated by eight donor atoms of the ligand and the geometry of the coordination polyhedron approaches a bicapped trigonal prism. These structures are compared with those of the homodinuclear chelate complexes with the same ligand and the mid to early lanthanide( III ) ions Gd, Eu, La and also Y. A distinctive structural change occurs across the lanthanide series. The centrosymmetric mid to early lanthanide( III ) complexes are all ninefold‐coordinated in a capped square antiprismatic arrangement with a water molecule coordinated in a prismatic position. This structure is maintained in aqueous solution, together with an asymmetric minor isomer. The late lanthanide( III ) OHEC complexes not only lack the inner‐sphere water, but the change of coordination sphere also results in a loss of symmetry of the whole complex molecule. The observed change of coordination mode and number of the lanthanide ion may offer a geometric model for the isomerization process in eight‐ and ninefold‐coordinated complex species that are isomers in a possible coordination equilibrium observed by NMR in aqueous solution. This model may also explain the intramolecular rearrangements necessary during water exchange in the inner coordination sphere of the complex [(Gd 2 OHEC)(H 2 O) 2 ] 2− through a slow dissociative mechanism. Protonation constants of the H 8 OHEC ligand and complex formation constants of this ligand with Gd III , Ca II , Cu II and Zn II have been determined by solution thermodynamic studies. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)