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

The structures of the series of N,N'-1, n-phenylenebis(oxamic acid ethyl ester) molecules with n = 2 (H(2)Et(2)opba, 1), 3 (H(2)Et(2)mpba, 2), and 4 (H(2)Et(2)ppba, 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 (H(2)Et(2)opba), bent (H(2)Et(2)mpba), or linear (H(2)Et(2)ppba) 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 (phi) 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 (phi = 0 and 180 degrees) by 0.18 and 0.13 kcal mol(-1), respectively. The calculated phi values for the syn and anti periplanar minimized conformers of HMepma are 16.0 and 200.0 degrees, respectively, in reasonable agreement with the experimental values for 1-3 [phi = 39.0(4) and 225.0(3) (H(2)Et(2)opba), 32.6(5) (H(2)Et(2)mpba), and 34.7(2)degrees (H(2)Et(2)ppba)]. 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)center dot center dot center dot O=C(amide) hydrogen bond [d(H center dot center dot center dot O) = 2.45(2)-2.57(2) angstrom]. A supramolecular aggregation of molecules into either a duplex (H(2)Et(2)opba) or a brick-wall sheet (H(2)Et(2)ppba) occurs for 1 and 3, respectively, through moderately strong, intermolecular N-H(amide)center dot center dot center dot O=C(amide) hydrogen bonds [d'(H center dot center dot center dot O) = 2.17(2)-2.37(2) angstrom]. By contrast, moderately weak, intermolecular N-H(amide)center dot center dot center dot O=C(ester) hydrogen bonds between the H(2)Et(2)mpba 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)center dot center dot center dot O=C(ester) hydrogen bond distance (d') for the {HMepma}(2) dimer show an energy minimum at 2.37 angstrom, in excellent agreement with the experimental value of 2 [d'(H center dot center dot center dot O) = 2.42(4) angstrom]. 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.