Concomitant and Conformational Polymorphism, Conformational Isomorphism, and Phase Relationships in 4-Cyanopyridine·4,4‘-biphenol Cocrystals

The 2:1 co-crystal of 4-cyanopyridine and 4,4‘-biphenol exists in at least two polymorphic forms. Single-crystal X-ray crystallographic analysis of forms I and II revealed conformational differences in the 4,4‘-biphenol molecules, but O−H···N(pyridine) supramolecular heterosynthons sustain both forms, suggesting that O−H···N(pyridine) interactions are favored over competing O−H···N(cyano) interactions. These conformational polymorphs crystallize concomitantly, and conformational isomorphism is also observed in this co-crystal. Experimental conditions that induce transformations between form I and form II are described. The structural differences between form I and II are discussed in the broader context of conformational polymorphism.