Making crystals from crystals: three solvent-free routes to the hydrogen bonded co-crystal between 1,1′-di-pyridyl-ferrocene and anthranilic acid

The organometallic complex [Fe(η5-C5H4–C5H4N)2] has been reacted with the anthranilic acid (C6H4)NH2COOH to generate the hydrogen bonded supramolecular macrocycle {[Fe(η5-C5H4–C5H4N)2] [(C6H4)NH2COOH]}2. The product has been fully characterized by means of X-ray powder diffraction, 13C and 15N solid-state NMR and single crystal X-ray diffraction. It has been shown that the same product can be obtained, quantitatively, by three different processes, namely kneading, i.e. solid-state mixing in the presence of a catalytic amount of MeOH, wet compression, i.e. pressure without mixing in the presence of MeOH, and vapour digestion, i.e. a mixture of the solid reactants is left in an atmosphere of MeOH vapours. The product can also be obtained via thermally induced reaction of a mixture of the two solid reactants, while no reaction is observed by dry mixing and dry compression. These experiments provide new insights into the factors controlling the process suggesting that the reaction requires the intermediacy of a catalytic amount of solvent or melting of one reactant to take place.