| dc.description.abstract | Based on the studies on the lanthanide complexes of pyridine?1?oxide and its methyl?substituted derivatives, it has been concluded that the methyl substitution at the 4?position of the pyridine ring does not bring down the ligand?to?metal ratio (L:M), the L:M ratio for pyridine?1?oxide and 4?methylpyridine?1?oxide complexes being 8:1. The aim of the present investigation was to ascertain whether the above conclusion holds good in the complexes of lanthanides with substituted pyridine?1?oxides with substituents other than the methyl group at the 4?position of the aromatic ring.
The stoichiometries and the probable coordination numbers of the new complexes prepared and characterised in the present investigation are presented in Table?1.
The present studies on the complexes of lanthanide perchlorates with 4?substituted and 2,4?disubstituted pyridine?1?oxides reveal that the L:M ratio is affected by substituents other than the methyl group at the 4?position of the aromatic ring. For example, while 4?methylpyridine?1?oxide complexes exhibit an L:M ratio of 8:1, the corresponding complexes of 4?nitropyridine?1?oxide, 4?cyanopyridine?1?oxide and 4?dimethylaminopyridine?1?oxide show lower L:M ratio. Similarly, L:M ratio in 4?nitro?2?picoline?1?oxide, 4?chloro?2?picoline?1?oxide, and 4?dimethylamino?2?picoline?1?oxide complexes of lanthanides is lower than that in the corresponding complexes of 2,4?lutidine?1?oxide. This means that substituents like –NO?, –CN and –Me? at the 4?position of the aromatic ring can produce steric hindrance at the coordination site.
It is well known that lanthanides usually exhibit high coordination numbers. In complexes with low L:M ratio, the lanthanides generally tend to attain high coordination numbers by dimerisation or polymerisation and/or by allowing even weakly coordinating anions to coordinate to the metal. Of the complexes with low L:M ratio prepared in the present study, 4?nitropyridine?1?oxide and 4?nitro?2?picoline?1?oxide complexes have been assigned bridged structures in accordance with their analytical and infrared data, suggesting thereby that they are dimers in the solid state. Complexes of 2,4?lutidine?1?oxide and 4?dimethylaminopyridine?1?oxide with lanthanides have also been assigned dimeric formulae on the basis of their analytical results. In 4?cyanopyridine?1?oxide and 4?nitro?2?picoline?1?oxide complexes of lanthanides, high coordination number is attained by perchlorate coordination, which is indicated by their infrared and conductivity data.
It is surprising that the complexes of 4?chloro?2?picoline?1?oxide with Er and Yb have an L:M ratio of 5:1. In addition, it is found that the perchlorates are all ionic in these complexes, and also the absence of water molecules in the complexes excludes the possibility of high coordination number around the metal ion.
The observed values of ??NO for the present complexes are smaller than those observed for the complexes of transition metals with aromatic amine oxides, indicating thereby a weaker interaction of the present set of ligands with the lanthanide ions. This could be attributed to the fact that the Ln–ligand bond in the lanthanide complexes is poorly covalent and that the f?orbitals do not participate in bonding, unlike the d?orbitals of transition metals.
The last part of the present work includes the study of the nature of coordination of a few bidentate N?oxide?derived ligands containing C=O or oxime groups in addition to the N?O group. As only cationic and anionic complexes of lanthanides with picolinic acid?1?oxide are known, preparation and characterisation of neutral complexes of lanthanides with picolinic acid?1?oxide was included in the present study.
The complexes of picolinamide?1?oxide and 4?chloropyridine?2?aldoxime?N?oxide with lanthanides were studied in view of the fact that picolinamide?1?oxide is a neutral bidentate ligand unlike the picolinic acid?1?oxide and 4?chloropyridine?2?aldoxime?N?oxide, neutral bidentate ligands with the oxime group instead of C=O group.
The physical studies on the neutral complexes of picolinic acid?1?oxide, presently prepared, show that the L:M ratio is 3:1 which is lower than that in the corresponding complexes of lanthanides reported by West and coworkers. The studies on the complexes of picolinamide?1?oxide and 4?chloropyridine?2?aldoxime?N?oxide reveal that they possess an L:M ratio of 4:1, meaning also that the binding of the oxime nitrogen to the metal in the 4?chloropyridine?2?aldoxime?1?oxide complexes is quite interesting. | |
