Protonation of diaza cyclic complexones: 1H NMR, calorimetric and molecular mechanics studies

Abstract

A detailed study of the sequences of protonation of three normal- and medium-ring cyclic diamino-carboxylate ligands (PIPDA, DACHDA and DACODA) has been carried out by ¹H NMR spectroscopy. Theoretical molecular mechanics calculations were carried out to predict the most stable conformations of their monoprotonated forms (HL^–) and to estimate relevant bond lengths and contributions to overall energy.

Determinations of the enthalpy changes were made by microcalorimetry, to confirm and relate the conclusions of these studies to the thermodynamic functions of the reactions of protonation.

The results suggest that a strong NH ⋯ N hydrogen bond (bond length 2.02 Å) is formed in monoprotonated 15-diazacycloctane-N,N′- diacetate (DACODA), the ring of which adopts a boat conformation, whereas monoprotonated 1,4-diazacycloeptane-N,N′-diacetate (DACHDA) adopts a semi-chair, and piprazine-N,N′-diacetate (PIPDA) adopts a trans-chair conformation for which NH ⋯ N hydrogen bonds are not possible. The doubly protonated species (H₂L) of these ligands seems to have both protons involved in intramolecular NH ⋯₂OC^– bonds tothe acetate groups.

Hydrogen bonding and differences in ring inductive effects may explain the large discrepancies in the values of the protonation constants of the three ligands.