Isolation and X-ray Crystal Structure of an Electrogenerated TEMPO–N3 Charge-Transfer Complex

Recent advances in radical-based catalytic reactions have created an increasing demand for the understanding of their mechanistic underpinnings. Structural elucidation of transient reactive intermediates via diffraction techniques, though rarely possible, is one of the most decisive ways to support such mechanistic hypotheses. Here we present the isolation, structural elucidation, and theoretical analysis of an electrochemically generated and catalytically relevant charge-transfer species formed between the azidyl radical and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). The unusual bent N–N–N angle and the pancake bonding between these two fragments highlight the weak bonding interactions present in this complex. This X-ray structure validates computational predictions as well as mechanistic proposals of TEMPO-mediated radical azidation reactions.