Aminoxyl Radicals of B/P Frustrated Lewis Pairs: Refinement of the Spin-Hamiltonian Parameters by Field- and Temperature-Dependent Pulsed EPR Spectroscopy.

Q-band and X-band pulsed electron paramagnetic resonance spectroscopic methods (EPR) in the solid state were employed to refine the parameters characterizing the anisotropic interactions present in six nitroxide radicals prepared by N,N addition of NO to various borane-phosphane frustrated Lewis pairs (FLPs). The EPR spectra are characterized by the g-anisotropy as well as by nuclear hyperfine coupling between the unpaired electron and the 11B/10B, 14N and 31P nuclear magnetic moments. It was previously shown that continuous-wave spectra measured at X-band frequency (9.5 GHz) are dominated by the magnetic hyperfine coupling to 14N and 31P, whereas the g-tensor values and the 11B hyperfine coupling parameters cannot be refined with high precision from lineshape fitting. On the other hand, the X-band electron spin echo envelope modulation (ESEEM) and hyperfine sublevel correlation (HYSCORE) spectra are completely dominated by the nuclear hyperfine coupling to the 11B nuclei, allowing a selective determination of their interaction parameters. In the present work this analysis has been further validated by temperature dependent ESEEM measurements. In addition, pulsed EPR data measured in the Q-band (34 GHz) are reported, which present an entirely different situation: the g-tensor components can be measured with much higher precision, and the ESEEM and HYSCORE spectra contain information about all of the 10B, 11B, 14N and 31P hyperfine interaction parameters. Based on these new results, we report here high-accuracy and precision data of the EPR spin Hamiltonian parameters measured on six FLP-NO radical species embedded in their corresponding hydroxylamine host structures. While the ESEEM spectra at Q-band frequency turn out to be very complex (due to the multinuclear contribution to the overall signal) in the HYSCORE experiment the extension over two dimensions renders a better discrimination between the different nuclear species, and the signals arising from hyperfine coupling to 10B, 11B, 14N and 31P nuclei can be individually analyzed.

Electronic control in frustrated Lewis pair chemistry: adduct formation of intramolecular FLP systems with -P(C(6)F(5))(2) Lewis base components.

2-Propenylbis(pentafluorophenyl)phosphane adds Piers' borane [HB(C(6)F(5))(2)] with anti-Markovnikov orientation to yield the intramolecular vicinal frustrated P/B Lewis pair 10. The FLP 10 adds pyridine, acetonitrile or alkyl isocyanides to form simple boron Lewis acid adducts 11-14, all of which were characterized by X-ray diffraction. The FLP 10 reacts with trans-cinnamic aldehyde to give a boron carbonyl adduct 15 that was characterized by an X-ray crystal structure analysis. In contrast, the geminal FLP (C(6)F(5))(2)P-CHEt-B(C(6)F(5))(2) (3) undergoes 1,2-carbonyl addition reactions with benzaldehyde or trans-cinnamic aldehyde to yield the respective five-membered heterocyclic products (7, 8, both characterized by X-ray diffraction). The FLP 10 undergoes 1,2-P/B addition to p-tolylacetylene and to 2-methylbutenyne, respectively, to yield the corresponding six-membered heterocyclic products 17 and 18, respectively (both were characterized by X-ray diffraction).

Anomalous Staudinger reaction at intramolecular frustrated P-B Lewis pair frameworks.

The FLP-mesityl azide addition products 5, formed by FLP-addition to the terminal azide nitrogen atom, undergo N-N bond cleavage in an unusual variant of the Staudinger reaction upon thermolysis or photolysis to give an internally borane stabilized [P]=NH phosphinimine and a dimethylindazole derivative.

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 [Mes(2)P-C(6)H(10)-B(C(6)F(5))(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-CH(2)Ph 4b, both characterized by X-ray diffraction. Structurally related FLPs [Mes(2)P-CHR(1)-CHR(2)-B(C(6)F(5))(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 C(2)-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide.

Chemistry of a geminal frustrated Lewis pair featuring electron withdrawing C6F5 substituents at both phosphorus and boron.

Hydroboration of the electron poor phosphine (1-propenyl)P(C(6)F(5))(2) with Piers' borane [HB(C(6)F(5))(2)] gave the geminal frustrated Lewis pair (C(6)F(5))(2)P-CH(Et)-B(C(6)F(5))(2). It undergoes 1,2-addition reactions to an alkene and an alkyne and to the C=N bond of an isocyanate. With mesityl azide it undergoes a 1,3-addition reaction.