Hydrogen-bonded networks constructed with 5-nitrobarbiturate

A series of nine compounds including tetra-alkylammonium 5-nitrobarbiturates and their urea and thiourea adducts have been obtained and characterized by single crystal X-ray analysis. The hydrogen-bonded networks in these compounds feature three different kinds of dimeric 5-nitrobarbiturate linkages (para-to-para, ortho-to-ortho and para-to-ortho), which are stabilized by formation of the cyclic urea dimer supramolecular synthon. In four thiourea complexes, the 5-nitrobarbiturate dimer is the dominant building unit, while in the urea complexes no such dimer is present due to the comparable hydrogen bonding capability of urea and 5-nitrobarbiturate. The main connection modes in the urea complexes are the traditional head-to-tail and shoulder-to-shoulder types, as well as a specific nitro-urea hydrogen motif. In [2(CH ₃)₄N⁺]·[2(C₄H ₂N₃O₅)^-]·(NH ₂)₂CS (3), the thiourea molecule uses its syn and anti hydrogen atoms as a clamp and strut, respectively, to connect two nearly parallel hydrogen-bonded 5-nitrobarbiturate zigzag ribbons. Comparison of the nine crystal structures substantiates the versatility of 5-nitrobarbiturate as a building block in the construction of hydrogen-bonded crystalline networks. The present study also shows that the size of the tetra-alkylammonium cation plays an important role in the generation and stabilization of inclusion complexes.