Solid state structure and solution thermodynamics of three-centered hydrogen bonds (O⋯H⋯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 N¹,N²-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 [δH6(DMSO-d₆)-δH6(CDCl₃)] and Δδ(NH)/ΔT measurements, in DMSO-d₆as 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-d₆solution because of the unfavorable steric effects of the o-benzoyl group. The estimated ΔH° and ΔS° values for the hydrogen bonding disruption by DMSO-d₆of 28.3(0.1) kJ·mol^-1and 69.1(0.4) J·mol^-1·K^-1for 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⋯A (A = O, π) and dipolar C=O⋯A (A = CO, π) 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.