Predicting molecular isomerism of symmetrical and unsymmetrical N,N′-diphenyl formamidines in the solid-state: crystal structure, Hirshfeld surface analysis, pairwise interaction energy, and ΔHfusion and ΔSfusion correlations

Abstract

Eight N,N′-diphenylformamidines with the general formula [N-(Ar),N′-(Ar′)] were synthesized and characterized using spectroscopic and analytical techniques. Four were symmetrical (Ar = Ar′) while the other four were unsymmetrical (Ar ≠ Ar′). Five of the compounds were characterized using single crystal X-ray diffraction. Solid-state structure analysis showed two molecular isomers, E_syn and E_anti. Sterics and electronics play a role in the particular isomer and conformation and directly influence the intermolecular interactions in their crystal structures. All E_anti isomers were found to form dimeric molecular units with R²₂(8) graph set descriptors, while E_syn isomers form one dimensional supramolecular chains. In addition, C–H⋯π, π⋯π and Cl⋯Cl intermolecular interactions are found to support the classic N–H⋯N hydrogen bonds in E_syn isomers. Molecular pairwise interaction energy calculations show that the electrostatic energy (E_ele) contributes more than dispersion energy (E_dis) for E_anti isomers, with the exception of symmetrical formamidines with bulky substituents on the aryl rings. E_syn isomers have a relatively higher contribution of E_dis than E_ele, with the exception of a case in which both E_syn and E_anti isomers exist in the same crystal lattice. Interestingly, an inversely proportional relationship exists between the E_dis : E_ele ratios and the experimental thermodynamic terms ΔH_fusion and ΔS_fusion. Based on this, we see that by experimentally determining the ΔH_fusion and ΔS_fusion of N,N′-diphenylformamidines, one can ultimately establish molecular isomerism without the need for crystal structure determination.