Oxovanadium(iv and v) and copper(ii) complexes of N-salicyl-glycylglycine and N-salicyl-glycylglycylglycine

Abstract

By reaction of salicylaldehyde and GlyGly or GlyGlyGly, followed by reduction with NaBH₄, N-salicyl-glycylglycine 1 and N-salicyl-glycylglycylglycine 2, the reduced Schiff bases H₂sal-RGG and H₂sal-RGGG, containing the GlyGly or GlyGlyGly moities, are prepared and characterised. Their acid–base properties and complexation with V^IVO²⁺ and Cu^II are studied by pH-potentiometry, visible absorption and EPR spectrometries, and the protonation and complex formation constants are determined. Vanadium and copper complexes are also prepared and characterised. Differences in the metal binding abilities of 1 and 2 are found. Amide deprotonation is proved but its extent depends on the system, being particularly important in the M–H₂sal-RGG systems with the formation of the (O-phenolate, N-amine, N-amide, COO⁻) (6+5+5)-membered joined chelate system. Depending on the system, at physiological pH this binding mode and/or 2 × (O-phenolate, N-amine) are dominant. The aminophenolate chelates formed with sal-RGG and sal-RGGG behave as anchoring donor sites and the ML and ML₂ complexes, involving O-phenolate, N-amine chelation, form in higher relative concentration than in the corresponding M–GlyGly or M–GlyGlyGly systems. Moreover, while for the V^IVO–H₂sal-RGG system the N-amide deprotonation/co-ordination is strongly promoted, it is not so favoured in the Cu^II–H₂sal-RGG and Cu^II–H₂sal-RGGG systems. The increased stability induced by the additional O-phenolate donor is particularly relevant for sal-RGG and hydrolysis of the MLH₋₁ complexes starts several pH units higher than in the GlyGly systems. In contrast to V^IV, the V^V complexes formed with sal-RGGG were found to be more stable than those of sal-RGG.