Thermal and spectral properties of lanthanide(III) complexes of 3-amino-2-hydroxy-1,4-naphthoquinone

Abstract

Octa-coordinated La(III), Ce(III), Pr(III), Nd(III), Sm(III), Gd(III) and Dy(III) complexes of 3-amino-2-hydroxy-1,4-naphthoquinone (3A2HNQ) of general formula [M(3A2HNQ)3(H2O)2] have been synthesized and characterized. The non-isothermal thermogravimetric profiles indicate the loss of one of the coordinated ligands along with one coordinated water molecule at a significantly higher energy of activation than that of the second step of decomposition with loss of two ligands and one water molecule. Such a difference in the energy of activation for the two steps can be correlated to the strong intermolecular hydrogen bonding interactions between quinone carbonyls and coordinated water molecules. These chelates exhibit slightly lowered magnetic moments probably due to their polymeric nature. IR and far-IR spectral data suggest that phenolic oxygen and amino nitrogen are the coordinating sites for 3A2HNQ, while coordinated water molecules are involved in the intermolecular hydrogen bonding interactions with free quinone carbonyls of 3A2HNQ molecules. The nephelauxetic ratio (β), covalency factor (b1/2) and Sinha's parameter (δ) evaluated from electronic spectral studies of Ce(III), Sm(III), Gd(III) and Dy(III) reveal a negligible amount of covalency in metal-ligand bonding.