Wydział Chemii

Ewa Matczak-Jon

Barbara Kurzak

Wanda Sawka-Dobrowolska

2012

Polyhedron

31

176-187

10.1016/j.poly.2011.09.007

Naukowa

Angielski

Artykuł

The crystal structures of the two derivatives of aminomethane-1,1-diphosphonic acid with morpholinyl- (1) and thiomorpholinyl- (2) side chains were determined by single crystal X-ray diffraction and discussed with respect to molecular geometry and solid state organization. The protonation equilibria, solution behavior and complex-formation equilibria in solutions of 1 and 2 with the Zn(II), Mg(II) and Ca(II) ions were studied by means of NMR, pH-potentiometry and ESI-MS methods.

As the pK(NH)protonation constants of 1 and 2 are high (11.65 and 11.91, respectively) two different approaches were used to evaluate the pH-potentiometric data. The first approach disregarded the proton-dissociation from the NH⁺ group. In the second one, all the pK_a values were considered in the M(II):ligand formation equilibria. For 1, the accuracy of the determination was shown to be sufficient to calculate reliable stability constants of metal complexes with the use of both approaches. For 2, only approach neglecting the

protonation constant was shown to be correct.

The studied acids form dinuclear, [M₂L₃H_x], [M₂L₂H_x] and mononuclear MLH_x and ML₂H_x complexes with different degree of ligand protonation. Tendency to undergo some oligomerization with the increase in the metal and ligand concentration was demonstrated for the [CaLH] complex of 1 and 2. As far as 1 and 2 remain protonated, the Zn(II), Mg(II) and Ca(II) ions are coordinated exclusively through oxygen atoms of the phosphonate groups. The metal promoted proton dissociation from the NH⁺ ring atom takes place in alkaline pH.

Bisphosphonates, X-ray structures, Complex-formation equilibria, NMR, potentiometry, ESI-MS

https://doi.org/10.1016/j.poly.2011.09.007

http://www.elsevier.com