Interaction of long polyazaalkanes with zinc(II) and cadmium(II) ions. A thermodynamic and 13C nuclear magnetic resonance study

Abstract

The interaction of Zn²⁺ and Cd²⁺ ions with a series of terminally methylated open-chain polyazaalkanes CH₃NHCH₂CH₂NH[CH₂CH₂NH]_nCH₂CH₂NHCH₂CH₂NHCH₃(n= 1–5, L¹–L⁵) has been studied by potentiometry and direct microcalorimetry at 298 K in 0.15 mol dm^–3 NaClO₄, as well as by ¹H and ¹³C NMR techniques. All these ligands form mononuclear complexes with Zn²⁺ and Cd²⁺. The stability constants and the enthalpy of formation of the species [ML]²⁺ for both metal ions are roughly constant along the series. The ¹³C NMR spectra of the complexes [ZnL¹]²⁺ and [CdL¹]²⁺ indicate the presence of a C₂ symmetry axis. All the experimental evidence suggests five-co-ordination for the zinc complexes and six-co-ordination for the cadmium complexes. The great upfield shifts experienced by the methyl carbons at both ends of the chain suggest the involvement of the terminal nitrogens in the co-ordination to the metals. The three largest ligands form binuclear zinc and cadmium complexes, the stability of which increases with the number of nitrogen donors. Comparison with analogous polyazacycloalkanes shows that a macrocyclic effect is observed only for the zinc mononuclear complexes of the shortest ligand L¹ and for the cadmium complexes of L¹ and L². For all the other complexes, mono- or binuclear, a higher stability of the complexes of the open-chain ligands is observed. On the other hand, ¹³C NMR spectra of the ligands L³–L⁵ allow a general protonation pattern to be established based on minimization of repulsive interactions between same-sign charges.