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The nucleobase adenine plays a pivotal role in the chemistry of life, but is also becoming increasingly interesting as a building block in the synthesis of functional solid materials.Although commercially available as a solid, adenine's solid-state chemistry has so far been neglected. In this comprehensive study it is shown that adenine is most often marketed as a mixture of two polymorphs, one previously known, and a new polymorph.Both polymorphs exhibit layered structures with different hydrogen-bonding patterns within layers. The crystal structure of the new polymorph was elucidated using synchrotron powder X-ray diffraction. Polymorph occurrence conditions, interconversion and the difference in their thermodynamic stability were established theoretically and experimentally revealing the polymorph with Z = 2 (known) as stable relative to the polymorph with Z = 1 (new). The adenine layers in both polymorphs are connected by weak interaction likely resulting in stacking faults which are manifested in anisotropic line broadening of their powder diffraction patterns. Analysis of a few commercial samples of adenine revealed them all to be a polymorph mixture, which could be inconvenient in experiments where properties of the solid material could be relevant.

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The Schiff base derived from salicylaldehyde and 2‐amino‐3‐hydroxypyridine affords a diversity of solid forms, two polymorphic pairs of the enol‐imino (D1 a and D1 b) and keto‐amino (D2 a and D2 b) desmotropes. The isolated phases, identified by IR spectroscopy, X‐ray crystallography, and 13C cross‐polarization/magnetic angle spinning (CP/MAS) NMR spectroscopy, display essentially planar molecular conformations characterized by strong intramolecular hydrogen bonds of the OH⋅⋅⋅N (D1) or NH⋅⋅⋅O (D2) type. A change in the position of the proton within this O⋅⋅⋅H⋅⋅⋅N system is accompanied by substantially different molecular conformations and, subsequently, by divergent supramolecular architectures. The appearance and interconversion conditions for each of the four phases have been established on the basis of a number of solution and solvent‐free experiments, and evaluated against the results of computational studies. Solid phases readily convert into the most stable form (D1 a) upon exposure to methanol vapor, heating, or by mechanical treatment, and these transformations are accompanied by a change in the color of the sample. The course of thermally induced transformations has been monitored in detail by means of temperature‐resolved powder X‐ray diffraction and infrared spectroscopy. Upon dissolution, all forms equilibrate immediately, as confirmed by NMR and UV/Vis spectroscopy in several solvents, with the equilibrium shifted far towards the enol tautomer. This study reveals the significance of peripheral groups in the stabilization of metastable tautomers in the solid state.

This work reports the experimentally studied mechanochemical formation of rhodacycles by ball milling pyridine-and quinoline-derived substrates and [Cp*RhCl 2 ] 2 in the presence of NaOAc. Ex-situ analysis of the mechanochemical reactions using powder X-ray diffraction (PXRD), solidstate UV-vis spectroscopy and ATR-FTIR spectroscopy revealed the formation of unexpected cocrystals between the substrates and the rhodium dimer prior to the CÀ H activation step. This sequence of events differs from the generally accepted steps in solution in which cleavage of [Cp*RhCl 2 ] 2 is initiated by acetate ions. Additionally, the mechanochemical approach enabled the synthesis of the six-membered rhodacycle [Cp*Rh(2-benzilpyridine)Cl], a metal complex repeatedly reported as inaccessible in solution. Altogether, the results of this investigation clarify some of the fundamental aspects of mechanochemical cyclometallations.

This work presents a successful application of a recently reported supramolecular strategy for stabilization of metastable tautomers in cocrystals to monocomponent, non-heterocyclic, tautomeric solids. Quantum-chemical computations and solution studies show that the investigated Schiff base molecule, derived from 3-methoxysalicylaldehyde and 2-amino-3-hydroxypyridine (ap), is far more stable as the enol tautomer. In the solid state, however, in all three obtained polymorphic forms it exists solely as the keto tautomer, in each case stabilized by an unexpected hydrogen-bonding pattern. Computations have shown that hydrogen bonding of the investigated Schiff base with suitable molecules shifts the tautomeric equilibrium to the less stable keto form. The extremes to which supramolecular stabilization can lead are demonstrated by the two polymorphs of molecular complexes of the Schiff base with ap. The molecules of both constituents of molecular complexes are present as metastable tautomers (keto anion and protonated pyridine, respectively), which stabilize each other through a very strong hydrogen bond. All the obtained solid forms proved stable in various solid-state and solvent-mediated methods used to establish their relative thermodynamic stabilities and possible interconversion conditions.