Hydrogen-bond-assisted configurational diversity of benzamidines: experimental and theoretical study

Abstract

Configurational diversity and mechanisms of prototropic tautomerism of 15 N,N′-diarylbenzamidines in solution and their configuration and H-bonding motifs in the crystalline state are investigated. Variable-temperature 1D and 2D liquid-state NMR spectroscopic studies provide strong evidence of the solvent-dependent dynamic interconversion between all four isomers, E-anti, E-syn, Z-anti and Z-syn. Some of the isomers are prone to either linear or cyclic dimerization through H-bonding. A quantitative assessment of the changes in the proton-donating ability of the amine functionality depending on the electron-donating ability of para-substituents in all three aryl rings of N,N′-diarylbenzamidines is proposed based on the ¹H NMR data and Hammett's substituent constants. A different pattern of the ordered H-bonded network composed exclusively of non-planar E-syn isomers is observed in the solid state by X-ray diffraction studies. DFT calculations confirm the torsional flexibility of the molecules, which are able to closely approach each other and form various linear and cyclic dimers, for which the relative stability is determined by the interplay of NH⋯N hydrogen bonding and π–π stacking interactions. Rotational barrier calculations offer mechanistic insights into the tautomerization and conformational dynamics observed in the NMR spectra.