Electron diffraction and crystal engineering of Pigment Red 5, C30H31ClN4O7S

Abstract

Pigment Red 5 (P.R.5) is an industrial organic hydrazone pigment with non-optimal application properties. In its application media – paints and printing inks – the pigment is not dissolved, but finely dispersed; hence, the crystal structure is maintained and has a strong effect on the properties. Hitherto, the crystal structure of P.R.5 has not been known, because the pronounced insolubility in water and in common organic solvents hampers the growth of single crystals for X-ray analysis. Since structure determination from powder data failed as well, the structure was determined by 3-dimensional electron diffraction (3D-ED). P.R.5 crystallises in the triclinic space group P (Z = 2), with layers of largely planar molecules. The structure was later confirmed by single-crystal X-ray analysis of a tiny crystal. The structure contains a void of about 36 ų between the molecules—a feature unprecedented in any known commercial organic pigment, as such voids deteriorate the packing efficiency, the lattice energy, and the application properties. To address these drawbacks, crystal engineering strategies were applied. First, lattice-energy minimisations guided the selection of potential derivatives and solid solutions. Several promising solid solutions were then synthesised and analysed by X-ray powder diffraction and single crystal analysis, and, indeed, according to the lattice-energy minimisations, the void could be filled.