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X-ray evidence for the relationship between pyridyl side chain basicity and the Z/E preferences of 5-halogen substituted (pyridin-2-yl)aminomethane-1,1-diphosphonic acids; implications for metal ions coordination in solution.
Autorzy
Rok wydania
2012
Czasopismo
Strony
167-185
DOI
10.3998/ark.5550190.0013.412
Kolekcja
Język
Angielski
Typ publikacji
Artykuł
The crystal structures of three salts (1-a−3-a), products of the reactions between 5-chloro (1), 5-bromo (2) and 5-iodo (3) substituted (pyridin-2-yl)aminomethane-1,1-diphosphonic acids and 4-aminopyridine were determined by single crystal X-ray diffraction and discussed with respect to molecular geometry and solid state organization. In all 1-a−3-a, bisphosphonate dianion adopts the opposite (E) conformation with respect to the C2−N1 bond, compared with the parent zwitterion. This provides further evidence that intermolecular N−H···O hydrogen bonds involving both exocyclic and pyridinium N atoms as proton donors and an O atom of the phosphonic/phosphonate group as acceptor play a significant role in stabilizing the Zconformation of this particular subclass of acids. The solution behavior and complex-formation abilities of [(5-iodopyridin-2-yl)amino]methane-1,1-diphosphonic acid (3) were also studied. Compound 3 exists in solution as the Z/E mixture. However, the barrier to rotation around the C2−N1 bond in 3 is lower,compared with 1 and 2. This is important for the complex-formation processes in the Zn(II), Mg(II) and Ca(II) solutions with 3.
Słowa kluczowe
Bisphosphonates, X-ray structures, complex-formation, equilibria, NMR, potentiometry, ESI-MS
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