Acceleration of solid-state β–α isomerization of the cobaloxime complex in mixed crystal containing the product

Abstract

It has been reported¹ that when β-cyanoethyl cobaloximes with various base ligands (cobaloxime = 2,3-butanedionedioxi-matocobalt(III)) are exposed to visible light in the solid state, β–α isomerization of the cyanoethyl group occurs¹. During a detailed study of one such reaction, the β–α isomerization of (β-cyanoethyl)(3-methylpyridine)cobaloxime, we found that some batches of synthesized material were isomerized abnormally quickly. Using crystallography, we show here that the anomalous crystals are mixtures of the original material and the reaction product with an α-cyanoethyl group. To our knowledge, this is the first example of an ‘autocatalytic’ solid-state reaction and of the possible artificial control of a reaction achieved by modifying crystals, although the use of mixed crystal has been reported for engineering the solid-state reaction of benzylbenzylidenecyclo-propenones². The explanation seems to be that there is more room in the mixed crystals for the movement of the atoms of the cyanoethyl group involved in the β–α isomerization, which is consistent with the conclusions of our earlier studies of crystalline-state racemization^3–5.