“…The only exception is to now the zwitterion of the parent acid 1-a (Scheme 1), which despite almost equal populations of the E and Z in solution crystallizes as the E isomer [11]. Herein we present the solid state and solution behavior of three new members of this family, namely, (3,5-dibromo-6-methylpyridin-2-yl)aminomethane-1,1-diphosphonic acid (2), (3-chloro-5-trifluoromethylpyridin-2-yl)aminomethane-1,1-diphosphonic acid (3) and (pyrimidin-2-yl)aminomethane-1,1-diphosphonic acid (4). The latter compound may be considered as the derivative of 1-a, in which atom C-3 is replaced by a nitrogen atom (Scheme 2).…”