How does the combination of the nitro group and fluorine atoms affect the (co)crystallization behaviour of arylenediamines?

Abstract

A series of phenylenediamines containing the NO₂ group and 3 to 0 fluorine atoms and 18-crown-6 ether were used to study the dependence of the stoichiometry and supramolecular structure of co-crystals on the degree of fluorination of the aromatic co-former. 2,4-Diamino-3,5,6-trifluoro- and -3,5-difluoronitrobenzenes form co-crystals of two stoichiometries: 2 : 1 (preferably) and 1 : 1. 2,4-Diamino-5-fluoronitrobenzene and the non-fluorinated analogue yield only 1 : 1 co-crystals. DSC analysis indicates complete regeneration of the stoichiometry and structure of the co-crystals in the melting–crystallization cycle. According to X-ray diffraction data, all the 1 : 1 co-crystals are built from the 1D assemblies in which N–H⋯O_cr H-bond is the only structure-directing interaction. The 2 : 1 co-crystals have a 3D or 2D supramolecular structure due to additional interactions formed by the aromatic co-formers, i.e. N–H⋯O_nitro H-bond and p⋯π electron contacts. Analysis of the structure of diaminonitrobenzene homocrystals showed that in di- and trifluorinated diaminonitrobenzenes, F atoms contribute to the formation of p⋯π electron interactions, while in mono- and non-fluorinated co-formers, the ability of NH₂ groups to form the H-bonds both as donors and as acceptors increases. Factors controlling the crystallization behaviour were rationalized using quantum-chemical computations of the packing and co-crystallization enthalpies, the energies of N–H⋯X (X = O_cr, O_nitro, N_amino) and p⋯π electron interactions, as well as the MEP and NCI analyses.