N,N-Addition of Frustrated Lewis Pairs to Nitric Oxide: An Easy Entry to a Unique Family of Aminoxyl Radicals

The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes₂PC₆H₁₀B­(C₆F₅)₂] 1b reacts rapidly with NO to give the persistent FLP-NO aminoxyl radical 2b formed by P/B addition to the nitrogen atom of NO. This species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT calculations. The reactive oxygen-centered radical 2b undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH product 3b. FLP-NO 2b reacts with toluene at 70 °C in an HAA/radical capture sequence to give a 1:1 mixture of FLP-NOH 3b and FLP-NOCH₂Ph 4b, both characterized by X-ray diffraction. Structurally related FLPs [Mes₂PCHR¹CHR²B­(C₆F₅)₂] 1c, 1d, and 1e react analogously with NO to give the respective persistent FLP-NO radicals 2c, 2d, and 2e, respectively, which show similar HAA and O-functionalization reactions. The FLP-NOCHMePh 6b derived from 1-bromoethylbenzene undergoes NOC bond cleavage at 120 °C with an activation energy of E_a = 35(2) kcal/mol. Species 6b induces the controlled nitroxide-mediated radical polymerization (NMP) of styrene at 130 °C to give polystyrene with a polydispersity index of 1.3. The FLP-NO systems represent a new family of aminoxyl radicals that are easily available by N,N-cycloaddition of C₂-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide.