Engineering short, strong hydrogen bonds in urea di-carboxylic acid complexesElectronic supplementary information (ESI) available: Summary of the crystallographic data, details of the short, strong hydrogen bond lengths, details of the variation of hydrogen bond length with temperature, Fourier difference maps imaging the hydrogen atoms from both X-ray and neutron data, cifs CCDC 992787-992819. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ce00587b

• Main Authors: Jones, Andrew O. F, Leech, Charlotte K, McIntyre, Garry J, Wilson, Chick C, Thomas, Lynne H
• Format: Article
• Language: English
• ISSN: 1466-8033
• DOI: 10.1039/c4ce00587b

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 O hydrogen bonds and a recurring acid 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 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 Δp K a values between the two components. The persistence of the acid amide heterodimer and the effect of methyl substitution on the short strong O-H O hydrogen bond is investigated in urea and methylurea di-carboxylic acid molecular complexes. Temperature dependent structural changes are also reported utilising X-ray and neutron diffraction in tandem.