The geometrical basis of crystal chemistry. Part 6

Abstract

A preliminary X-ray study of the yellow wool dye Coomassie Yellow 7G is reported. The unusual diffraction effects from dry crystals and the very large unit cell of the wet crystals suggest a framework structure. Although the structures of the crystals are far too complex to analyse directly, some suggestions are made as to the possible nature of the framework. A protein-like azo compound It has been known for some time that certain complex dyes have unusual properties, for example, the formation of highly thixotropic aqueous pastes or very viscous aqueous solutions or gels. An X-ray study of the compound terials can be obtained. Three characteristic developments are shown in Fig. l(a)-(c). This compound can also be obtained in a relatively insoluble form (< 1% in boiling water) but it has not been found possible to obtain crystals of this form large enough for single-crystal work. The X-ray powder photograph of the insoluble form is that of a normal CH~ I CH 3 SO 3 CO \ \ N-ICH.CO. H-@ /-~ ~ / ~-NH.CO.CH ~T// I SOa CHa CO I CH3 Na~ (marketed as the wool dye Coomassie Yellow 7G) shows that certain of its crystalline forms exhibit quite novel X-ray diffraction effects, indicating that the crystals are of a type not hitherto encountered. The observations recorded here are of a very preliminary character; no systematic study has been made of either the physical chemistry of the compound or the X-ray diffrac-tion effects. The results of this very incomplete study are presented in this form because no detailed work is contemplated in the immediate future and also because the structures of these crystals are probably examples of complex framework structures of the kind discussed in Part 4 and later in the present paper. The compound is prepared by tetrazotizing o-toli-dine-6:6'-disulphonic acid, coupling with two molecules of acetoacetanilide, and then precipitating the sodium salt of the dye by the addition of brine. The product, a yellow crystalline powder, is normally quite soluble in water (> 10% in boiling water), and if it is dissolved in hot water and the solution cooled a very viscous solution is obtained which sometimes does not crystallize over a period of days or even weeks. However, by controlled salting-out or the addition of ethyl alcohol beautifully crystalline ma-microcrystalline powder (Fig. 2(a)), whereas those of the soluble forms of Fig. 1 (a)-(c), after grinding lightly, show only a small number of very diffuse rings, resembling a photograph of a glass or liquid rather than a crystalline powder (Fig. 2(b)). Although they are apparently all quite normal crystals, readily obtainable with dimensions of the order of a millimetre or so, they give none of the sharp X-ray diffractions usually regarded as characteristic of crystals. In an ion of the kind shown above the possibilities of tautomerism and stereo-isomerism are considerable, and polymorphism and different degrees of hydration are also possible. It is therefore not surprising that by varying the conditions of crystallization a number of different products can be obtained, and it is not possible to say anything about their intramolecular configurations. Chemical analyses have been made of various batches of crystals obtained by salting-out, filtering, and drying at room temperature. These analyses indicated considerable and variable degrees of hydration (up to 20 H20 for 1 formula-weight of dye), but since the crystals were still losing weight at the time of analysis the initial water contents are not known.