Binuclear lanthanide complexes of 2,3-dihydroxy-1,4-naphthoquinone

Abstract

Binuclear octa-coordinated lanthanide(III) complexes of 2,3-dihydroxy-1,4-naphthoquinone ( isnphth) possessing [M 2L 3(H 2O) 4] composition are synthesized and characterized by infrared spectroscopy, 1H and 13C NMR spectroscopy and thermal studies. The thermal decomposition revealed two stage patterns, 1st step corresponding to loss of four coordinated water molecules while the bonded isnphth molecules are decomposed during 2nd step. During the 2nd stage, the structure of these complexes is reorganized forming two distinct monomeric species implying that the ligand bridges as well as intermolecular associations are essential for thermal stability. Energy of activation ( E a) for 2nd stage exhibits linear increase with decrease in the size of lanthanide ion. 1H and 13C NMR spectrum of isnphth and its lanthanide chelate signifies strong hydroxyl oxygen and quinone binding. IR absorption spectral data indicate bonding through both hydroxyl oxygen with quinone carbonyls acting as bridge between lanthanide ions and there exists intensive network of intermolecular hydrogen bonding involving coordinated water and quinone carbonyls. The absorption spectrum of Nd(III) complex suggests a weak covalent character in metal–ligand bonding as revealed from nephelauxetic ratio ( β = 0.973), covalent factor ( b 1/2 = 0.0641) and Sinha's factor ( δ = 1.705).