Simulated (red curves) and experimental (black curve) X-band ESEEM spectra for sample 5 obtained with <i>τ</i> = 250 ns.

<p>The simulations consider different sets of nuclear species interacting with a single unpaired electron. (a) ¹⁰B, ¹¹B, ¹⁴N, ³¹P, ¹⁹F, and ¹H nuclei; (b) ¹¹B nucleus only; (c) ¹⁰B nucleus only; (d) ¹⁴N nucleus only; and (e) ³¹P nucleus only. The peaks in the ¹H and ¹⁹F Zeeman frequencies are labeled in the figure.</p

Unsaturated Vicinal Frustrated Lewis Pair Formation by Electrocyclic Ring Closure and Their Reaction with Nitric Oxide

The Lewis acidic β-styryl-B­(C₆F₅)₂ reagent <b>6a</b> undergoes a clean 1,1-carboboration reaction with 1-(PMes₂)-2-cyclohexenyl acetylene <b>9</b> at 60 °C to give the vicinal P/B-substituted conjugated triene product <b>10a</b>. At 80 °C this undergoes a stereoselective thermally induced disrotatory electrocyclic ring closure to give the cyclohexadiene-derived P/B system <b>11</b>. Subsequent TEMPO oxidation gave the substituted phenylene-bridged P/B product <b>12</b>. Both <b>11</b> and <b>12</b> are active phosphane/borane frustrated Lewis pairs (FLPs). The FLP <b>11</b> reacts in a typical way with phenylacetylene to give the phosphonium/alkynylborate product <b>13</b>. Compound <b>12</b> cleaves dihydrogen at near ambient conditions to give the respective phosphonium/hydridoborate zwitterion <b>14</b>. Both the FLPs <b>11</b> and <b>12</b> cooperatively add P/B to the nitrogen atom of nitric oxide (NO) within minutes at room temperature to give the persistent P/B FLPNO^• radicals <b>19</b> and <b>21</b>, respectively (both characterized by X-ray diffraction and by EPR spectroscopy). The FLPs <b>11</b> and <b>12</b> are thermally robust. At elevated temperatures (<b>11</b>: 75 °C, <b>12</b>: 100 °C) they undergo a coupling reaction with dimethyl acetylenedicarboxylate with carbon–carbon bond activation at a P-mesityl substituent

Experimental (black curves) and simulated (red curves) X-band ESEEM spectra obtained with <i>τ</i> = 170 ns (a) and <i>τ</i> = 250 ns (b) for the sample 5 in the temperature range 100–300 K.

<p>The simulations where performed considering isotropic hyperfine coupling tensors for ¹H and ¹⁹F nuclei with <i>A</i>_<i>iso</i> = 1.8 MHz and 0.8 MHz respectively, and hyperfine and quadrupolar coupling interactions with ¹¹B using the interaction parameters listed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157944#pone.0157944.t002" target="_blank">Table 2</a>.</p

Experimental Q-band ESEEM spectra for the samples 1–5 (black curves).

<p>Red curves are EasySpin simulations considering “best fit” principal values of the interaction tensors obtained from the HYSCORE results (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157944#pone.0157944.t001" target="_blank">Table 1</a>) and Euler angles from DFT calculations [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157944#pone.0157944.ref008" target="_blank">8</a>]. The asterisk marks indicate frequency positions where spectrometer artifacts are present (narrow peaks).</p

Compounds studied within this work.

<p>Compounds studied within this work.</p

Simulated (red curves) and experimental (black curves) Q-band ESEEM spectra for sample 2.

<p>The simulations consider different sets of nuclear species interacting with a single unpaired electron. (a) ¹⁰B, ¹¹B, ¹⁴N and ³¹P nuclei (red curve); (b) ¹¹B nucleus only; (c) ¹⁴N nucleus only; (d) ¹⁰B nucleus only; (e) ¹¹B and ¹⁴N nuclei; (f) ¹⁰B and ¹⁴N nuclei; (g) ³¹P nucleus only; (h) ³¹P and ¹⁴N nuclei; (i) ³¹P and ¹⁰B nuclei. Spectra are internally normalized by the maximum intensity. The asterisk marks indicate frequency positions where spectrometer artifacts are present (narrow peaks). The blue dashed curve in (a) shows an optimized simulation including the interactions with all the nuclei and emphasizing the ¹⁴N contribution (see text).</p

2D-HYSCORE simulated spectra (left) compared with HYSCORE experimental spectrum for sample 3 (right).

<p>(a) parameters obtained from DFT calculations [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157944#pone.0157944.ref008" target="_blank">8</a>], (b) parameters extracted from X-band EPR analysis [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157944#pone.0157944.ref008" target="_blank">8</a>], and (c) optimized parameters. The simulations were performed at Q-band frequencies and a magnetic field of 1.23 T. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157944#pone.0157944.t001" target="_blank">Table 1</a> shows the set of EPR parameters used in the simulations. The diagonal peaks at around 34 MHz and 42 MHz are artifacts from the spectrometer.</p

2D-HYSCORE spectra recorded at a magnetic field strength of 1.23 T for the set of FLP samples 1–5.

<p>The anti-diagonal dashed lines cross the diagonal at the Larmor frequencies for the isotopes related in the plots. The diagonal peaks at around 34 MHz and 42 MHz are artifacts from the spectrometer.</p

X-band cw-EPR spectrum of samples 1–3 (black curves, from bottom to top) and simulation (red curves) using the parameters obtained from Q-band HYSCORE and EDFS analyses.

<p>X-band cw-EPR spectrum of samples 1–3 (black curves, from bottom to top) and simulation (red curves) using the parameters obtained from Q-band HYSCORE and EDFS analyses.</p

EPR interaction parameters used for the X-band ESEEM simulations for samples 2–5.

<p><i>A</i>_<i>iso</i>, <i>δ</i>_<i>A</i> and <i>η</i>_<i>A</i> are respectively the isotropic and anisotropy and asymmetry parameters of the ¹¹B hyperfine coupling tensor, according to the notation given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157944#pone.0157944.e002" target="_blank">Eq 2</a>. <i>α</i>, <i>β</i> and <i>γ</i> are the Euler angles, according to notation from Ref. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0157944#pone.0157944.ref016" target="_blank">16</a>], and <i>C</i>_<i>Q</i> and <i>η</i>_<i>Q</i> are the ¹¹B nuclear electric quadrupole coupling constant and the EFG asymmetry parameter.</p