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

The crystal structures of three salts ( 1-a−3-a), products of the reactions between 5-chloro ( 1), 5bromo ( 2) and 5-iodo ( 3) substituted (pyridin-2-yl)aminomethane-1,1-diphos phonic acids and 4aminopyridine 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 int ermolecular 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 Z conformation 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 t he 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) solution s with 3.