Structural studies of lanthanide complexes with tetradentate nitrogen ligands

Abstract

Complexes have been synthesised with bis(2-pyridine carboxaldehyde) ethylenediimine ( 1) and bis(2-pyridine carboxaldehyde)propylene-1,3-diimine ( 2) with all of the available lanthanide trinitrates. Crystal structures were obtained for all but one complex with 1 and for all but one complex with 2. Four distinct structural types were established for 1 but only two for 2, although in all cases the structures contained one ligand bound to the metal in a tetradentate fashion. With 1, the four different structures of the lanthanide(III) nitrate complexes included 11-coordinate [Ln( 1)(NO 3) 3(H 2O)] for Ln = La; 10 coordinate [Ln( 1)(NO 3) 3(H 2O)] with one monodentate and two bidentate nitrates for Ln = Ce, then 10-coordinate [Ln( 1)(NO 3) 3] for Ln = Pr–Yb with three bidentate nitrates; and 9-coordinate [Ln( 1)(NO 3) 3] with one monodentate and two bidentate nitrates for Ln = Lu. On the other hand for 2 only two distinct types of structure are obtained, the first type with Ln = La–Pr and the second type for Ln = Sm–Lu, although all are 10-coordinate with stoichiometry [Ln( 2)(NO 3) 3]. The difference between the two types is in the disposition of the ligand relative to the nitrates. With the larger lanthanides La–Pr the ligand is found on one side of the coordination sphere with the three nitrate anions on the other. In these structures, the ligand is folded such that the angle between the two pyridine rings approaches 90°, while with the smaller lanthanides Sm–Lu, two nitrates are found on one side of the ligand and one nitrate on the other and the ligand is in an extended conformation such that the two pyridine rings are close to being coplanar. In both series of structures, the Ln–N and Ln–O bond lengths were consistent with the lanthanide contraction though there are significant variations between ostensibly equivalent bonds which are indicative of intramolecular hydrogen bonding and steric crowding in the complexes.