Electrochemical generation of stable carbocation-tetrafluoroborate salts and the cation-anion interactions in their solid state structures

The electrochemical generation of stable carbocations, among other heterocyclic products, by a unique electrochemical process involving the anodic oxidation of aryl-substituted ketene imines is described. The electrochemical oxidation undergoes an unusual multiannulation process to form these types of products by intermolecular cyclization. The X-ray crystal structures of two carbocation tetrafluoroborate salts, 4c and 4d, of which the latter is solvated by CH₂Cl₂, are presented. We have observed that one of the B-F bonds in 4c is relatively long with respect to the other three similar in length B-F bonds, while in the solvated salt (4d·CH₂Cl₂), one of the B-F bonds is particularly short relative to its congeners. In both cases, the "exceptional" B-F bonds are oriented toward the positive center of the carbocation. These phenomena are compared with other known X-ray structures of organic tetrafluoroborate salts and discussed.