X-ray Diffraction and 1H NMR in Solution: Structural Determination of Lanthanide Complexes of a Py2N6Ac4 Ligand

Complexes between the Py2N6Ac4 (H4L) ligand containing four carboxylate pendant arms and trivalent lanthanide ions have been synthesized, and structural studies have been made both in the solid state and aqueous solution. The crystal structures of the La, Ce, Sm, Tb, Dy, Ho, Er, Tm, and Lu complexes, with chemical formulas [LaH2L](NO3)·3H2O (1), [Ce4L2](NO3)4·30H2O (2), [SmHL]·EtOH·3H2O (5), [TbHL]·EtOH·3H2O (8), [DyHL]·2EtOH·2H2O (9), [HoHL]·3H2O (10), [ErHL]·EtOH·3H2O (11) [TmHL]·EtOH·3H2O (12), and [LuHL]·3H2O (14), have been determined by single-crystal X-ray crystallography. In the solid state, the complexes of the lighter lanthanide ions La3+−Dy3+ show a 10-coordinated geometry close to a distorted bicapped antiprism, where the carboxylate pendants are situated alternatively above and below the best plane that contains the nitrogen donor atoms. The complexes of the heavier ions, Ho3+−Lu3+, have a 9-coordinated geometry close to distorted tricapped trigonal prism, with one of the pendant carboxylate groups uncoordinated. The ligand is in a “twist−fold” conformation, where the twisting of the pyridine units is accompanied by an overall folding of the major ring of the macrocycle so that the pyridine nitrogen atoms and the metal are far from linear. The aqueous solution structures of the complexes were thoroughly characterized, the diamagnetic ones (La3+ and Lu3+) by their COSY NMR spectra, and the paramagnetic complexes using a linear least-squares fitting of the 1H LIS (lanthanide-induced shift) and LIR (lanthanide-induced relaxation) data with rhombic magnetic susceptibility tensors. The solution structures obtained for the La3+−Dy3+ complexes (10-coordinate) and for the Tm3+−Lu3+ complexes (9-coordinate) are in very good agreement with the corresponding crystal structures. However, the 10-coordinate structure is still exclusive in solution for the Ho3+ complex and predominant for the Er3+ complex