Solid-State N–H···O/O–H···N Tautomerism in Resonance-Assisted 1‑(Arylazo)-2-Naphthols and Its Through-Space π* ← π Perturbation in TCNQ Cocrystals. A Variable-Temperature X‑ray Crystal Study

Aryl-substituted 1-arylhydrazo-naphthalen-2-ones and 1-arylazo-naphthalen-2-ols display ···HN–NC–CO··· ⇄ ···NN–CC–OH··· ketohydrazone–azoenol prototropic tautomerism in the solid state by forming a strong intramolecular resonance-assisted H-bond (RAHB) that can change from pure N–H···O to pure O–H···N through dynamically disordered N–H···O ⇄ O–H···N bonds according to the electronic properties of the substituents. The compounds of the series aryl = 4-F-phenyl, 4-CH₃-phenyl, 4-Cl-phenyl, and 1-naphthalenyl and their cocrystals with TCNQ have been studied by X-ray crystallography at temperatures varying from 100 to 295 K. The packing is extensively discussed in terms of charge-transfer (CT) or electron donor–acceptor (EDA) interactions, showing that the herringbone packing of pure azonaphthols and the columnar one of their TCNQ cocrystals are respectively determined by specific σ* ← π and π* ← π interactions. It is shown that cocrystallization with TCNQ induces two main effects: (i) in TCNQ itself, a significant change from quinoid to aromatic geometry, from which a CT of some 0.40 e can be estimated; (ii) in azonaphthols, a shift of the N–H···O ⇄ N···H–O equilibrium toward the pure N–H···O tautomer, accompanied by a decrease of π-delocalization inside the conjugated ···HN–NC–CO··· resonant fragment. These findings suggest that through-space effects caused by TCNQ cocrystallization on the RAHB equilibrium are formally equivalent to through-bond effects caused by the aryl substituents.