Solid state and solution behaviour of N-(2-pyridyl)- and N-(4-methyl-2-pyridyl)aminomethane-1,1-diphosphonic acids

The crystal structures of N-(2-pyridyl)aminomethane-1,1-diphosphonic acid (1) and N-(4-methyl-2-pyridyl)aminomethane-1,1-diphosphonic acid (2) are determined and discussed with respect to their geometry and solid state organization. Compounds 1 and 2 crystallize as the opposite E and Z isomers. However, in solution both appear as a mixture of the Z and E forms. From the temperature dependence of NMR spectra, the Z/E ratio is assigned to be nearly equal in 1 and 0.69:0.31, respectively, in 2. The role of a substituent position on the pyridyl ring for the Z/E ratio in solution and for solid state conformations of N-(2-pyridyl)aminomethane-1,1-diphosphonic acids is discussed as well.