Solid-state NMR strategies for the structural characterization of paramagnetic NO adducts of Frustrated Lewis Pairs (FLPs)

By N,N addition of NO to the norbonane annulated borane-phosphane Frustrated Lewis pair (FLP) 1 a five-membered heterocyclic persistent aminoxyl radical 2 and its diamagnetic hydroxylamine reduction product 3 are prepared, and the comprehensive multinuclear solid state NMR characterization (¹H, ¹¹B, ¹⁹F, ³¹P) of these FLP adducts is reported. Signal quantification experiments using a standard addition method reveal that the ¹¹B and ³¹P NMR signals observed in 2 actually arise from molecular impurities of 3 embedded in the paramagnetic crystal. In contrast analogous quantification experiments reveal that the ¹H and ¹⁹F MAS-NMR spectra originate from spin-carrying molecules. Peak assignments are based on DFT-calculated Mulliken spin densities, which lead to the surprising result that the largest paramagnetic shift affecting a proton NMR resonance in 2 originates from intermolecular interactions. For the ¹⁹F nuclei, experiments and calculations indicate that paramagnetic shift effects are very small. In this case, assignments are based on DFT chemical shift calculations carried out on diamagnetic 3 and ¹⁹F(¹¹B) Rotational Echo Adiabatic Passage DOuble Resonance (REAPDOR) experiments. The set of experiments described here defines an efficient strategy for the structural analysis of paramagnetic FLP adducts.