Commensurate and Incommensurate Hydrogen Bonds. An Exercise in Crystal Engineering

Ureas characteristically form one-dimensional hydrogen-bonded α-networks with a repeat distance of about 4.60 Å. Oxamides form similar α-networks with a longer 5.05 Å repeat distance. The urea of glycine and the oxamide of glycine were each cocrystallized with a series of four bipyridines, including two urea derivatives and two oxamide derivatives. This series of eight cocrystals was studied by X-ray diffraction in order to see what would happen when molecules that would normally form α-networks with incommensurate distances were forced into the same crystal. The two all-urea crystals and the two all-oxamide crystals contained the expected α-networks with repeat distances in accordance with normal urea or oxamide values. Four of the crystals were mixed, containing both oxamide and urea molecules. Three consisted of two-dimensional β-networks with alternating parallel urea and oxamide subnetworks. The repeat distances averaged 4.87 Å, a value close to the value expected for oxamides, but shorter than any previously observed examples. In the fourth mixed crystal, the urea α-network formed with a normal urea repeat distance, but the oxamide network did not form, the oxamide adopting an unusual molecular conformation that maximizes intramolecular hydrogen bonds instead.