An electron spin resonance study of the formation of hetero-metal ion hydroxycarboxylate chelates in nonaqueous solution

Abstract

Spectrophotometric and e.s.r. measurements have been used to characterise the equilibrium systems produced by addition of base to dimethylformamide solutions containing a non-transition-metal ion chloride, copper(II) chloride and hydroxycarboxylic acid. The non-transition-metal chlorides used were those of aluminium(III), antimony(III), arsenic(III), zinc(II), cadmium(II), mercury(II), and dimethyltin(IV). Polarographic measurements were made on similar systems but using the metal perchlorates. The ligands used were citric, malic, and mandelic acid. All the evidence indicates that on addition of base the aluminium(III), antimony(III), and arsenic(III) chelates are formed first and that in the remainder of the systems the copper(II) hydroxycarboxylate is formed first, except in the system involving dimethyltin(IV), where both chelates form on initial addition of base. The presence of the non-transition-metal hydroxycarboxylate brings about an unexpected complexity in the distribution of chelate species. A property common to many of the systems is that the non-transition-metal ion hydroxycarboxylate plays a role in the structure of the polymeric copper(II) chelates. The e.s.r. data has been interpreted in terms of chelates containing uncoupled copper(II) arising from formation of hetero-metal ion chelate species and chelates containing coupled copper(II) in polymeric species. The results obtained from the system containing copper(II) chloride, zinc chloride, mandelic acid, and triethylamine provide clearcut evidence for a species containing both copper(II) and zinc(II) present as their hydroxycarboxylates and where coupling between the copper(II) ions is maintained. The magnetic parameters of the various systems containing copper(II) have been evaluated by consideration of an axial spin Hamiltonian and one which includes the dipole–dipole interaction, making it possible to characterise cases where coupling occurs between two copper(II) ions in the polymeric chelates.