Through-Space Stabilization of Carbon-Centered Aryl Dicyanomethyl Radicals

Stable organic radicals have diverse applications in medicine, biology, materials, and quantum sciences. Understanding the factors that affect radical stability is essential for exploiting radicals for such applications. Aryl dicyanomethyl radicals are an intriguing class of carbon-centered radicals due to their notable air and thermal stabilities, but these species are known to dimerize to form sigma or pi dimers at low temperatures. However, whether such carbon-centered radicals could potentially be stabilized by exploiting through-space interactions remains unknown. To evaluate whether through-space stabilization is possible, aryl dicyanomethyl radicals with nitro groups in the ortho positions were synthesized. Control radicals lacking ortho groups, or with only sterically bulky ortho groups, exist as dimers in the solid state and in cooled solutions, while the ortho dinitro-substituted species is observed as the free radical even at low temperature and is stable for hours in solution and for months in the solid state. Density functional theory calculations reveal a stereoelectronic interaction between the lone pair of the oxygen on the nitro group and the empty p-orbital of the radical carbon that twists the dicyanomethyl group out of the plane with the aryl ring, an interaction that only occurs with very strong donating groups. Overall, this work demonstrates a new approach for obtaining free aryl dicyanomethyl radicals by exploiting through-space interactions with the radical center.