title: Solid-state NMR strategies for the structural characterization of paramagnetic NO adducts of Frustrated Lewis Pairs (FLPs).
abstract: 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 ((1)H, (11)B, (19)F, (31)P) of these FLP adducts is reported. Signal quantification experiments using a standard addition method reveal that the (11)B and (31)P 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 (1)H and (19)F 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 (19)F 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 (19)F((11)B) 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.
authors.affiliation: Institut für Physikalische Chemie and Graduate School of Chemistry, WWU Münster, Corrensstrasse 30, D 48149 Münster, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Wiegand
authors.affiliation: Organisch-Chemisches Institut, WWU Münster, Corrensstrasse 40, D 48149 Münster, Germany.
authors.forename: Muhammad
authors.initials: M
authors.lastname: Sajid
authors.affiliation: Organisch-Chemisches Institut, WWU Münster, Corrensstrasse 40, D 48149 Münster, Germany.
authors.forename: Gerald
authors.initials: G
authors.lastname: Kehr
authors.affiliation: Organisch-Chemisches Institut, WWU Münster, Corrensstrasse 40, D 48149 Münster, Germany. Electronic address: erker@uni-muenster.de.
authors.forename: Gerhard
authors.initials: G
authors.lastname: Erker
authors.affiliation: Institut für Physikalische Chemie and Graduate School of Chemistry, WWU Münster, Corrensstrasse 30, D 48149 Münster, Germany. Electronic address: eckerth@uni-muenster.de.
authors.forename: Hellmut
authors.initials: H
authors.lastname: Eckert
keywords: DFT calculated Mulliken spin densities; Frustrated Lewis pairs; NMR of paramagnetic solids; NMR signal quantification