Crystal structure and tautomeric state of Pigment Red 48:2 from X-ray powder diffraction and solid-state NMR

Abstract

Pigment Red 48:2 is an industrially important hydrazone pigment, used for the colouration of plastics and printing inks. The crystal structure of the commercial phase was solved and refined from X-ray powder diffraction data. The phase (P.R.48:2, α-phase) was found to be a monohydrate with a chemical composition of Ca[C18H11ClN2O6S]·H2O. Its tautomeric state was investigated by multinuclear solid-state NMR. The 15N CPMAS unambiguously revealed the compound to adopt the hydrazone tautomeric form in the solid state. In an unrestrained Rietveld refinement, the crystal structure did not significantly change. The crystal structure was confirmed by dispersion-corrected DFT optimisation, which also included the calculation of solid-state NMR chemical shifts. Heating the monohydrate to about 200 °C results in an anhydrate (P.R.48:2, β-phase), which has not been described before. Its crystal structure was determined by X-ray powder diffraction, and confirmed by DFT-D, too. Solid-state NMR spectra confirmed that the crystal structure of P.R.48:2 remains mostly unchanged upon dehydration and that also the hydrazone tautomeric form is maintained. Both crystal structures are similar and present a double-layer structure. The non-polar layer contains the phenyl and naphthalene moieties, whereas the polar/ionic layer comprehends the calcium ions, carboxylate, sulfonate, carbonyl groups and water molecules. The calcium ions have a coordination number of 8 and 7 in the monohydrate and anhydrate phases, respectively. For the corresponding Mn salt (Pigment Red 48:4), at least two phases were found, but the quality of the powder patterns did not allow determining their crystal structures.