Equimolar complex formation of urea or thiourea with 2-alkoxy-benzamides: Structural factors required for the equimolar complex formation

Equimolar complex formation of either urea or thiourea with 2-ethoxy-benzamide (2-EB) and 2-methoxy-benzamide (2-MB) was investigated. Complex formation of urea and 2-MB was observed both by co-grinding method and by coprecipitation method. Molecular arrangement of the complex was determined by single crystal X-ray diffraction method as an equimolar complex. The crystal structure of the urea-2-MB complex, especially hydrogen bond networks, was quite different from that of thiourea-2-MB complex. In urea-2-MB equimolar complex, not only intermolecular hydrogen bond between urea and 2-MB but also hydrogen bond network between urea molecules played important roles to form the complex. When urea was co-ground with 2-EB, equimolar complex formation was not observed. Conformational change of guest molecules by the complexation was investigated in terms of intramolecular hydrogen bond length and the dihedral angles. Reduction of intramolecular hydrogen bond length of 2-MB and the conformational change to the flatter structure affected the equimolar complex formation.