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A benzophenone-diphenylamine co-crystal nucleates from a submerged eutectic-is this a possible mechanism of grinding induced synthesis of such materials?The early discovery of co-crystals, or molecular complexes as they are also referred to in the literature, resulted from the measurement of two component phase diagrams 1 and associated crystallisation from melt phases. 2 Subsequently crystallisation from solution has also been used as a method for preparing co-crystals, particularly when suitable samples are required for X-ray analysis and structure determination. 3,4 These methods have been complemented, in recent years, by the use of co-grinding of potential co-crystal formers either as dry solids or in the presence of a solvent drop. 5 This grinding technique has been popularised on two counts-first that it requires little or no solvent and hence is seen as environmentally friendly 6,7 and second because it is possible to use this as a screening aid to generate novel co-crystalline phases not apparently found in conventional crystallisation from solution. 5 Despite its evident utility little work has been devoted to understanding the mechanism of this grinding process, an issue bound to limit its ultimate commercial efficacy. For example, Kuroda et al. 8 produced co-crystals of racemic-bis-b-naphthol and benzoquinone by solid state grinding and postulated that the shearing and molecular diffusion processes occurring during grinding generated a different adduct structure to that recovered from solution. In the work of Rastogi et al. with picric acid complexes, 9,10 on the other hand, vapour diffusion was suggested as a mass transfer mechanism during solid state grinding. Shan et al. 11 explained solvent drop grinding on the basis of additional degrees of freedom, enhancement of molecular collisions and formation of tiny co-crystal seeds. In the wider context of grinding induced solid state covalent synthesis, Rothenberg et al. 12 have presented evidence suggesting the formation of a liquid phase in the binary phase diagram as essential to facilitate intermolecular contacts, mass transfer and chemical reaction. Recently Kaupp proposed a three step solid-state mechanism, derived from atomic force microscopy studies, which involves long-range anisotropic molecular migration. 13 In the work presented here we have studied one particular cocrystal forming system-benzophenone (BZP) and diphenylamine (DPA) in which we found that a co-crystal can form under solidstate grinding (mortar and pestle) at ambient temperatures. The binary phase diagram for this system was first reported in 1933 by Warner and Lee, 14 later revisited by Rastogi et al. 15 and is reproduced in Fig. 1. We found that co-grinding of the two components in solid form created the stable polymorphic structure first identified by Warner and Lee 14 as having a melting point of 40.2 uC and a crystal structure identical to that first determined in 1972 16 based on an H-bonded carbonyl-amine dimer. In order to explore in more detail how this reaction procee...

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