Engineering short, strong hydrogen bonds in urea di-carboxylic acid complexes

A series of seven 2 : 1 molecular complexes of urea (U) and methyl ureas with di-carboxylic acids (A) are reported, along with the results of their study by variable temperature diffraction. These all contain short, strong O-H center dot center dot center dot O hydrogen bonds and a recurring acid center dot center dot center dot amide heterodimer forming U-A-U synthons. Despite differences in the degree of saturation of the linking C-C groups of the di-carboxylic acids and the single or double methyl substitution of one of the N atoms of the urea, the packing arrangements are remarkably similar in five of the complexes; the exceptions being N-methylurea oxalic acid and N,N-dimethylurea fumaric acid. The five similar molecular complexes all show contraction of one unit cell parameter on increasing temperature due to rearrangements of the weaker interactions which hold together the U-A-U units. The strength of the short, strong O-H center dot center dot center dot O hydrogen bond is shown to be linked both to the length of the connecting bridge between the carboxylic acid groups of the acid, and to the Delta pK(a) values between the two components.