The heterogeneous thermal transformation of the orange diethyl NN′-diphenyl-2,5-diaminoterephthalate benzene complex to the red solvent-free compound. X-Ray crystal structures of the two forms

Abstract

The orange form of diethyl NN′-diphenyl-2,5-diaminoterephthalate (2) which had been reported by Liebermann has been found, when formed by recrystallization from benzene, to be a benzene complex (2)–benzene (1 : 1). The ready transformation of benzene complex (2-O) to the red benzene-free compound (2-R) has been studied. Reaction of single crystals proceeds by nucleation followed by frontal migration leading first to a metastable, partially disordered, yellow phase and a second front leading to the red product (2-R). The reaction is very readily affected by mechanical stress and also by the presence of the vapour of ethanol, acetone, or even water, in contrast to the transformation of diethyl 2,5-diaminoterephthalate (1-Y) discussed in the previous paper. The crystal structures of (2-O) and (2-R) were determined by X-ray methods. An orange crystal of the benzene complex of (2-O), immersed in benzene mother liquor in a capillary tube, was monoclinic, a= 15.355(5), b= 7.863(2), c= 22.303(7)Å, β= 100.18(3)°; the space group is P2₁/c with four molecules in the unit cell and the structure has been refined to an R factor of 0.055 on 2 858 non-zero reflections. The structure is composed of pairs of crystallographically independent, but centrosymmetric, molecules of (2) related to each other by a near two-fold rotation axis. The benzene molecules are ordered and situated in channels with periodic constrictions. Red crystals of (2-R) are triclinic, a= 10.124(5), b= 4.025(1), c= 13.623(8)Å, α= 90.46(4)°, β= 106.95(4)°, γ= 99.63(4)°; the space group was P with one molecule in the unit cell and the structure was refined to an R factor of 0.043 on 1 496 non-zero reflections. The structure is formed by translation in three dimensions of the crystallographically centrosymmetric molecule.