Lanthanide(III) nitrate complexes with a tetramethyl 4-(methyl)benzoate pendant-armed hexaazamacrocyclic ligand

The [2+2] barium(II) templated cyclocondensation of 2,6-diformylpyridine and ethylendiamine followed by the N-alkylation in acetonitrile of the secondary amines led to the corresponding saturated tetramethyl 4-(methyl)benzoate pendant-armed hexaazamacrocyclic ligand, L1. The assignment of the proton and carbon NMR signals was based on standard COSY, DEPT-135 and HMQC measurements. The structure of L1 could be confirmed by X-ray diffraction showing a parallel arrangement of the ligand molecules leading to hollow tubular structures of molecular dimensions. The coordination capability of L1 towards a series of lanthanide(III) ions by direct reaction between the hydrated nitrate metal salt and the macrocyclic ligand in a 1:1 molar ratio in acetonitrile has been studied. The complexes were characterized by elemental analysis, IR spectroscopy and LSI mass spectrometry. The crystal structure determined by X-ray diffraction of [L1H4][Eu(NO3)5(H2O)](NO3)2·1.5CH3CN·0.25H2O has been characterized as an ionic mixed complex. The cation corresponds to the protonation of the four –NH groups on the macrocyclic ligand, and a well-separated nitrate Eu(III) complex affords the electric neutrality of the structure. One can also consider the presence of hydrogen bonds between another two nitrate anions and the protonated amine groups together with some π–π interactions between these nitrates and the pyridine rings.