Synthesis,structure and bonding of 2-aminopyridinium heptamolybdate trihydrate

Summary 2-aminopyridinium heptamolybdate trihydrate crystallizes in the monoclinic system with space group P2₁/n and Z=4 (R=0.030). The unit cell dimensions area=14.8161(4) Å,b=17.5073(4) Å,c=20.8492(6) Å, =107.503(2)°, V=5157.7(2) ų. The Mo₇O₂₄]^6– anions in the 2-aminopyridinium, ammonium⁽⁴⁾, guanidium⁽²⁾, propyl- and isopropyl-ammonium⁽¹⁾ molybdates, while similar, show slightly differences in several bond lengths and angles. The distinguishing features of 2-aminopyridinium heptamolybdate trihydrate structure is its extensive hydrogen bonding. The planar cations and the water molecules are positioned so as to be able to form hydrogen bonds with either molybdate oxygen atoms or water oxygen atoms. Four different types of hydrogen bonds have been found-: N-H... O (mono- and bifurcated), N-H... Ow (monofurcated), Ow-Hw... O (mono- and bi-furcated) and Ow-Hw...OW (monofurcated). The closest approach distances associated with 27 of these potential hydrogen bonds vary from 2.67 to 3.24 Å^(7,8)). The proposed strong hydrogen bonding interactions appear to stabilize the structure and explain the way of three water molecules are lost upon heating. Some of these hydrogen bonds can play an important role in the possible photochromism of this compound.