Commensurate and incommensurate hydrogen bonds. An exercise in crystal engineering

Ureas characteristically form one-dimensional hydrogen-bonded alpha -networ ks with a repeat distance of about 4.60 Angstrom. Oxaniides form similar al pha -networks with a longer 5.05 Angstrom repeat distance. The urea of glyc ine and the oxamide of glycine were each cocrystallized with a series of fo ur 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 alpha -net works with incommensurate distances were forced into the same crystal. The two all-urea crystals and the two all-oxamide crystals contained the expect ed alpha -networks with repeat distances in accordance with normal urea or oxamide values. Four of the crystals were mixed, containing both oxamide an d urea molecules. Three consisted of two-dimensional beta -networks with al ternating parallel urea and oxamide subnetworks. The repeat distances avera ged 4.87 Angstrom, a value close to the value expected for oxamides, but sh orter than any previously observed examples. In the fourth mixed crystal, t he urea alpha -network formed with a normal urea repeat distance, but the o xamide network did not form, the oxamide adopting an unusual molecular conf ormation that maximizes intramolecular hydrogen bonds instead.