Commensurate and incommensurate hydrogen bonds. An exercise in crystal engineering.
Ureas characteristically form one-dimensional hydrogen-bonded alpha-networks with a repeat distance of about 4.60 A. Oxamides form similar alpha-networks with a longer 5.05 A 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 alpha-networks with incommensurate distances were forced into the same crystal. The two all-urea crystals and the two all-oxamide crystals contained the expected alpha-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 beta-networks with alternating parallel urea and oxamide subnetworks. The repeat distances averaged 4.87 A, a value close to the value expected for oxamides, but shorter than any previously observed examples. In the fourth mixed crystal, the urea alpha-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.
Nguyen
T L
TL
Department of Chemistry, State University of New York, Stony Brook, NY 11794, USA.
Fowler
F W
FW
Lauher
J W
JW
D003460:Crystallization; D005998:Glycine / Q000737:chemistry; D006860:Hydrogen Bonding; D008958:Models, Molecular; D010072:Oxamic Acid / Q000031:analogs & derivatives* / Q000737:chemistry; D011725:Pyridines / Q000737:chemistry; D014508:Urea / Q000737:chemistry*
D011725:Pyridines; D014508:Urea; D010072:Oxamic Acid; C008728:oxamide; D005998:Glycine