Synthesis, X-ray Structure, and Solution NMR Studies of Ln(III) Complexes with a Macrocyclic Asymmetric Compartmental Schiff Base. Preference of the Ln(III) Ions for a Crown-Like Coordination Site

The compartmental ligand H2LA, containing an N3O2 Schiff base and an O2O3 crown like coordination site, has
been prepared by reaction of 3,3‘-(3-oxapentane-1,5-diyldioxy)bis(2-hydroxybenzaldehyde) with 1,5-diamino-3-azamethylpentane. The formation of a [1+1] macrocycle was inferred by IR, NMR, and mass spectrometry.
When reacted with the rare-earth hydrate chlorides, LnCl3·nH2O (Ln = La, Ce, Pr, Nd, Eu, Tb, Dy, Ho, Er, Tm,
Yb, Lu, Y), H2LA or its precursors (template reaction) form the mononuclear complexes [Ln(H2LA)(H2O)4]Cl3·nH2O where the lanthanide ion coordinates the O2O3 crown like site. The solid-state X-ray structures of [Ln(H2LA)(H2O)4]Cl3·nH2O (Ln = Ce, Dy, Lu) have been determined. [Lu(H2LA)(H2O)4]Cl3·3H2O is monoclinic
space group P21/n (Z = 4) with a = 15.269(5) Å, b = 11.484(5) Å, c = 19.389(6) Å, β = 102.85(5)°; [Ce(H2LA)(H2O)4]Cl3·H2O and [Dy(H2LA)(H2O)4]Cl3·H2O are isomorphous, space group P21 (Z = 2), with a =
10.959(5) Å, b = 16.978(5) Å, c = 9.017(4) Å, β = 97.73(5)°, and a = 10.874(5) Å, b = 16.797(5) Å, c =
9.046(4) Å, β = 97.86(5)° for the cerium and dysprosium complexes, respectively. In the three compounds the
metal ion is coordinated in a similar manner by the five oxygens (two phenolic and three etheric) of the cyclic
ligand and the nine coordination around the central atom is reached by the oxygen atoms of four coordinated
water molecules. Three chlorine ions are present in the asymmetric unit. A detailed 1H NMR study was carried
out in CD3OD for both the diamagnetic and paramagnetic [Ln(H2LA)(H2O)4]Cl3 complexes in order to compare
their structure in solution with that found in the solid state. The quantitative analysis of the paramagnetic proton
shifts indicates that the complexes from La to Tm are isostructural, maintain in solution the same type of
coordination polyhedron found at the solid state, with the metal ion invariably coordinated in the O2O3 compartment,
and present a high degree of stereochemical nonrigidity. In the case of the Lu complex, the decreased fluxionality
due to the reduced ionic radius allows the observation of two isomeric species in the 1H NMR spectrum at low
temperature.