The effect of a local proton source on the activity of a bromotricarbonyl Mn redox catalyst for CO₂ reduction has been investigated. The electrochemical behaviour of the novel complex [fac-Mn(dhbpy)(CO)₃Br] (dhbpy = 4-phenyl-6-(1,3-dihydroxybenzen-2-yl) 2,2′-bipyridine), containing two acidic OH groups in the proximity of the metal centre, under a CO₂ atmosphere showed a sustained catalysis in homogeneous solution even in the absence of Brønsted acids.

Pressing solid barbituric acid with KBr to prepare samples for IR spectroscopy leads to the formation of an ionic co-crystal, in which the co-former is a classical ionic salt; co-crystal formation is also obtained with the other alkali bromides (LiBr, NaBr, RbBr and CsBr) and with caesium iodide. The simultaneous presence of alkali and halide ions affects the dissolution properties of barbituric acid in water.

Vapour digestion of a mixture of solid [Fe(η⁵-C₅H₄–C₅H₄N)₂] and solid pimelic acid HOOC(CH₂)₅COOH in the presence of solvent vapours generates co-crystals of different stoichiometry depending on the protic or aprotic nature of the solvent. The nature of the products has been ascertained by a combined use of SSNMR and X-ray diffraction.