Thermogravimetric and differential scanning calorimetric investigations of manganese–glycine interactions

Thermogravimetric (t.g.) and differential scanning calorimetric (d.s.c.) data have been used to study metal–amino acid interactions in adducts of general formula MnCl2 · ngly (gly = glycine, n = 0.7, 2.0, 4.0 and 5.0). All the prepared adducts exhibit only a one step mass loss associated with the release of glycine molecules, except for the 0.7gly adduct, which exhibits two glycine mass loss steps. From d.s.c. data, the enthalpy values associated with the glycine mass loss can be calculated: MnCl2 · 0.7gly = 409 and 399 kJ mol−1, MnCl2 · 2.0gly = 216 kJ mol−1, MnCl2 · 4.0gly = 326 kJ mol−1 and MnCl2 · 5.0gly = 423 kJ mol−1, respectively. The enthalpy associated with the ligand loss, plotted as function of the number of ligands for the n = 2.0, 4.0 and 5.0 adducts, gave a linear correlation, fitting the equation: ΔH (ligand loss)/kJ mol−1 = 67 × (number of ligands, n) + 76. A similar result was achieved when the enthalpy associated with the ligand loss was plotted as a function of the νa(COO−) bands associated with the coordination through the carboxylate group, 1571, 1575 and 1577 cm−1, respectively, for the n = 2.0, 4.0 and 5.0 adducts, giving the equation ΔH (ligand loss) /kJ mol−1 = 33.5 × νa(COO−) /cm−1 − 52418.5. This simple equation provides evidence for the enthalpy associated with the ligand loss being very closely related to the electronic density associated with the metal–amino acid bonds.