Topological control in the hydrogen bond-directed self-assembly of ortho-, meta-, and para-phenylene-substituted dioxamic acid diethyl esters

Abstract

The structures of the series of N,N′-1,n-phenylenebis(oxamic acid ethyl ester) molecules with n = 2 (H₂Et₂opba, 1), 3 (H₂Et₂mpba, 2), and 4 (H₂Et₂ppba, 3) have been determined by single-crystal X-ray diffraction (XRD) methods. Density functional (DF) calculations have been performed on the simplest model system N-phenyloxamic acid methyl ester (HMepma). Compounds 1–3 have either folded (H₂Et₂opba), bent (H₂Et₂mpba), or linear (H₂Et₂ppba) almost planar (periplanar) molecular configurations with the two oxalamide moieties being slightly tilted up and down, respectively, with respect to the benzene ring. The energy calculations as a function of the torsion angle (ϕ) around the N(amide)–C(benzene) bond for HMepma reveal that the minimum energy syn and anti periplanar conformations of the carboxamide functions are more stable than the corresponding syn and anti planar ones (ϕ = 0 and 180°) by 0.18 and 0.13 kcal mol⁻¹, respectively. The calculated ϕ values for the syn and anti periplanar minimized conformers of HMepma are 16.0 and 200.0°, respectively, in reasonable agreement with the experimental values for 1–3 [ϕ = 39.0(4) and 225.0(3) (H₂Et₂opba), 32.6(5) (H₂Et₂mpba), and 34.7(2)° (H₂Et₂ppba)]. This situation likely minimizes the forced repulsive interactions between the amide hydrogen and the nearest benzene hydrogen atoms while it maximizes the attractive interactions between the carbonyl amide oxygen and the nearest benzene hydrogen atoms, which are then implicated in a relatively weak, intramolecular C–H(benzene)⋯OC(amide) hydrogen bond [d(H⋯O) = 2.45(2)–2.57(2) Å]. A supramolecular aggregation of molecules into either a duplex (H₂Et₂opba) or a brick-wall sheet (H₂Et₂ppba) occurs for 1 and 3, respectively, through moderately strong, intermolecular N–H(amide)⋯OC(amide) hydrogen bonds [d′(H⋯O) = 2.17(2)–2.37(2) Å]. By contrast, moderately weak, intermolecular N–H(amide)⋯OC(ester) hydrogen bonds between the H₂Et₂mpba molecules are involved in 2 to give a meso-helical chain with a unique hydrogen-bonded oxalamide acid ester dimeric unit. The energy calculations as a function of the intermolecular N–H(amide)⋯OC(ester) hydrogen bond distance (d′) for the {HMepma}₂ dimer show an energy minimum at 2.37 Å, in excellent agreement with the experimental value of 2 [d′(H⋯O) = 2.42(4) Å]. The calculated value of the hydrogen bond energy for {HMepma}₂ (E_HB = 4.83 kcal mol⁻¹) is consistent with a partially covalent nature of the interaction between the amide hydrogen and the carbonyl ester oxygen atoms, as confirmed by the existence of a significant electron density delocalization within the resulting four-center H₂O₂ diamond core.