Solid State Structure And Solution Thermodynamics Of Three-Centered Hydrogen Bonds (O Center Dot Center Dot Center Dot H Center Dot Center Dot Center Dot O) Using N-(2-Benzoyl-Phenyl) Oxalyl Derivatives As Model Compounds

Intramolecular hydrogen bond (HB) formation was analyzed in the model compounds N-(2-benzoylphenyl) acetamide, N-(2-benzoylphenyl) oxalamate and N-1, N-2-bis(2-benzoylphenyl) oxalamide. The formation of three-center hydrogen bonds in oxalyl derivatives was demonstrated in the solid state by the X-ray diffraction analysis of the geometric parameters associated with the molecular structures. The solvent effect on the chemical shift of H6[delta H6(DMSO-d(6))-delta H6(CDCl3)] and Delta d(NH)/Delta T measurements, in DMSO-d(6) as solvent, have been used to establish the energetics associated with intramolecular hydrogen bonding. Two center intramolecular HB is not allowed in N-(2-benzoylphenyl) acetamide either in the solid state or in DMSO-d(6) solution because of the unfavorable steric effects of the o-benzoyl group. The estimated Delta H-o and Delta S-o values for the hydrogen bonding disruption by DMSO-d(6) of 28.3(0.1) kJ.mol(-1) and 69.1(0.4) J.mol(-1).K-1 for oxalamide, are in agreement with intramolecular three-center hydrogen bonding in solution. In the solid, the benzoyl group contributes to develop 1-D and 2-D crystal networks, through C-H center dot center dot center dot A (A = O, p) and dipolar C= O center dot center dot center dot A (A = CO, pi) interactions, in oxalyl derivatives. To the best of our knowledge, this is the first example where three-center hydrogen bond is claimed to overcome steric constraints.