Spectroscopic and calorimetric studies of the complexing ability of some polyamido polymers containing amino-acid residues

Abstract

Complexes formed by two polymers L¹(derived from glycine) and L²(derived from β-alanine) and their corresponding non-macromolecular models with copper(II) have been studied in aqueous solution at different pH by e.s.r. and Fourier-transform i.r. (F.t.i.r.) spectroscopy and by calorimetry at 25 °C. The e.s.r. and F.t.i.r. data reveal that L¹ forms a single complex in which co-ordination occurs through one amino nitrogen and one carboxylic group. Each copper(II) ion is co-ordinated to one repeat unit in the polymer. These findings are consistent with the –ΔH^⊖ value of 30 kJ mol^–1 which is independent of pH, and is very close to that measured for the similar complex with the non-macromolecular model. For L² the e.s.r. spectra show the presence of two complexes, one, involving only oxygen donor atoms, formed at low pH and the other, involving one co-ordinated nitrogen atom also, has similar parameters to the complex formed by L¹. The calorimetric data for the first complex, [Cu(HL)]²⁺(L refers to the repeating monoanionic unit of the polymer), reveal that –ΔH^⊖ is independent of pH but only an approximate value can be obtained because of the small amount of this species formed. The ΔH^⊖ value for the second complex, [CuL]⁺, is very close to that of the simple [Cu(alaO)]⁺[alaO =β-alaninate(1–)] complex. F.t.i.r. spectra seem to exclude the participation of the amido carbonyl oxygen to the co-ordination. The dependence of ΔS^⊖ on the degree of complex formation is attributed to the statistical entropy contribution.