Crystal structures and supra­molecular features of 9,9-dimethyl-3,7-di­aza­bicyclo­[3.3.1]nonane-2,4,6,8-tetra­one, 3,7-di­aza­spiro­[bi­cyclo­[3.3.1]nonane-9,1′-cyclo­penta­ne]-2,4,6,8-tetra­one and 9-methyl-9-phenyl-3,7-di­aza­bicyclo­[3.3.1]nonane-2,4,6,8-tetra­one di­methyl­formamide monosolvate

Compounds (I), C(9)H(10)N(2)O(4), (II), C(11)H(12)N(2)O(4), and (III), C(14)H(12)N(2)O(4)·C(3)H(7)NO represent 9,9-disubstituted-3,7-di­aza­bicyclo­[3.3.1]nonane-2,4,6,8-tetra­one deriv­atives with very similar mol­ecular geometries for the bicyclic framework: the dihedral angle between the planes of the imide groups is 74.87 (6), 73.86 (3) and 74.83 (6)° in (I)–(III), respectively. The dimethyl derivative (I) is positioned on a crystallographic twofold axis and its overall geometry deviates only slightly from idealized C (2v) symmetry. The spiro-cyclo­pentane derivative (II) and the phen­yl/methyl analog (III) retain only inter­nal C (s) symmetry, which in the case of (II) coincides with crystallographic mirror symmetry. The cyclo­pentane moiety in (II) adopts an envelope conformation, with the spiro C atom deviating from the mean plane of the rest of the ring by 0.548 (2) Å. In compound (III), an N—H⋯O hydrogen bond is formed with the di­methyl­formamide solvent mol­ecule. In the crystal, both (I) and (II) form similar zigzag hydrogen-bonded ribbons through double inter­molecular N—H⋯O hydrogen bonds. However, whereas in (I) the ribbons are formed by two trans-arranged O=C—N—H amide fragments, the amide fragments are cis-positioned in (II). The formation of ribbons in (III) is apparently disrupted by participation of one of its N—H groups in hydrogen bonding with the solvent mol­ecule. As a result, the mol­ecules of (III) form zigzag chains rather than the ribbons through inter­molecular N—H⋯O hydrogen bonds. The crystal of (I) was a pseudo-merohedral twin.