Crystal structure of a 2:1 piroxicam-gentisic acid co-crystal featuring neutral and zwitterionic piroxicam mol-ecules

A new 2:1 co-crystal of piroxicam and gentisic acid [systematic name: 4-hy-droxy-1,1-dioxo-N-(pyridin-2-yl)-2H-1λ⁶,2-benzo-thia-zine-3-carboxamide-2-(4-oxido-1,1-dioxo-2H-1λ⁶,2-benzo-thia-zine-3-amido)-pyridin-1-ium-2,5-di-hydroxy-benzoic acid, 2C₁₅H₁₃N₃O₄S·C₇H₆O₄] has been synthesized using a microfluidic platform and initially identified using Raman spectroscopy. In the co-crystal, one piroxicam mol-ecule is in its neutral form and an intra-molecular O-H⋯O hydrogen bond is observed. The other piroxicam mol-ecule is zwitterionic (proton transfer from the OH group to the pyridine N atom) and two intra-molecular N-H⋯O hydrogen bonds occur. The gentisic acid mol-ecule shows whole-mol-ecule disorder over two sets of sites in a 0.809 (2):0.191 (2) ratio. In the crystal, extensive hydrogen bonding between the components forms layers propagating in the ab plane.