Barbiturates with hydrogen-bonded layer and framework structures

Abstract

Four N–H⋯OC bonded structures for 5,5-substituted derivatives of barbituric acid are reported, which are distinct from the eight fundamental types that have been previously described. The N–H⋯OC-bonded layer structures L-4, L-5 and L-6 are present in crystal polymorphs of 5-(2-bromoallyl)-5-isopropyl barbituric acid (L-4: 1, noctal), 5,5-dichlorobarbituric acid (L-6: 2a, L-5: 2b) and 5,5-dibromobarbituric acid (L-5: 3a). The monoclinic polymorph (3b) of the dibromo compound contains the framework F-2. In each case, the essential building block consists of two molecules which are doubly N–H⋯OC-bonded via a central R²₂(8) ring. The L-4 structure is closely related to the common 1D ladder topology C-4. It serves also as a building unit for the L-5 sandwich structure. The essential rings of L-4 and L-6 have the same graph-set descriptor R⁶₆(28) but are topologically distinct from one another. In the F-2 framework, ten molecules are linked together to give an L-shaped R¹⁰₁₀(48) ring. The IR spectra of the crystal forms 1, 2a–b and 3a–b are discussed and a previously established classification scheme correlating the H-bonding of barbiturates with their IR characteristics is extended.