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Unsaturated Vicinal Frustrated Lewis Pair Formation by Electrocyclic Ring Closure and Their Reaction with Nitric Oxide Thomas Ö zgün,† Guo-Qiang Chen,† Constantin G. Daniliuc,† Alison C. McQuilken,‡ Timothy H. Warren,‡ Robert Knitsch,§ Hellmut Eckert,§,⊥ Gerald Kehr,† and Gerhard Erker*,† †

Organisch-Chemisches Institut, Westfälische Wilhelms-Universität, Corrensstrasse 40, 48149 Münster, Germany Department of Chemistry, Georgetown University, Box 571227, Washington, D.C. 20057-1227, United States § Institut für Physikalische Chemie, Westfälische Wilhelms-Universität, Corrensstrasse 30, 48149 Münster, Germany ‡

S Supporting Information *

ABSTRACT: The Lewis acidic β-styryl-B(C6F5)2 reagent 6a undergoes a clean 1,1-carboboration reaction with 1-(PMes2)-2cyclohexenyl acetylene 9 at 60 °C to give the vicinal P/Bsubstituted conjugated triene product 10a. At 80 °C this undergoes a stereoselective thermally induced disrotatory electrocyclic ring closure to give the cyclohexadiene-derived P/B system 11. Subsequent TEMPO oxidation gave the substituted phenylene-bridged P/B product 12. Both 11 and 12 are active phosphane/borane frustrated Lewis pairs (FLPs). The FLP 11 reacts in a typical way with phenylacetylene to give the phosphonium/alkynylborate product 13. Compound 12 cleaves dihydrogen at near ambient conditions to give the respective phosphonium/hydridoborate zwitterion 14. Both the FLPs 11 and 12 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 19 and 21, respectively (both characterized by X-ray diffraction and by EPR spectroscopy). The FLPs 11 and 12 are thermally robust. At elevated temperatures (11: 75 °C, 12: 100 °C) they undergo a coupling reaction with dimethyl acetylenedicarboxylate with carbon−carbon bond activation at a P-mesityl substituent.

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INTRODUCTION Intramolecular phosphane/borane systems have played an important role in the development of frustrated Lewis pair (FLP) chemistry1−3 and especially the vicinal C2-bridged systems.4,5 The ethylene-bridged P/B FLP 1 has been one of the most active nonmetallic dihydrogen activating systems.6,7 Autrey et al. determined the rate law of the H2-splitting reaction of this system as v = 0.70 ± 0.03 [M−1 s−1][1][H2].8 In contrast to the saturated system 1 (see Scheme 1) a series of unsaturated C(sp2)−C(sp2) bridged phosphane/borane compounds 2 were unreactive toward dihydrogen.9 Some showed a very limited FLP activity, namely, the cooperative P/B addition to the carbon atom of, for example, n-butylisocyanide.10 We had investigated whether partially or fully unsaturated

compounds of the types 3 and 4 featuring one or two exomethylene groups at the bridge would be hydrogen activators or not. We prepared a couple of derivatives of 3 and found these systems to cleave dihydrogen at mild conditions. Compound 3b was found to serve as an active metal-free hydrogenation catalyst of a few enamine samples.11 We prepared two examples of the compound type 412 by a sequence of 1,1-carboboration13−16 followed by thermally induced 6π-electrocyclization.17 The 1,1-carboboration products 7 were inert toward H2. Both the compounds 4a and 4b (see Scheme 2) cleaved dihydrogen under mild conditions. From a competition experiment we learned that 4a was ca. 6 times slower in heterolytic dihydrogen cleavage as compared to the saturated parent compound 1, but it turned out that 4a was thermally more robust than the sensitive compound 1.12 Only at ca. 100 °C did the FLP 4a (R: Ph) undergo a slow deactivation reaction by the often observed intramolecular nucleophilic aromatic substitution route.1,13,18 Oxidation of 4 by treatment with two molar equivalents of TEMPO19 gave the aromatic phenylene-bridged P/B FLP systems 8. Even these unsaturated vicinal FLPs cleaved dihydrogen, although compound 8a was ca. 38 times less reactive than 4a. The compounds 8 are thermally very robust; 8a could be kept for

Scheme 1

Received: August 3, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acs.organomet.6b00627 Organometallics XXXX, XXX, XXX−XXX

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The X-ray crystal structure analysis showed the presence of the newly formed conjugated hexatriene system. The central carbon atoms (C1, C2) have the PMes2 and B(C6F5)2 functional groups attached (see Figure 1). There is a marked

Scheme 2

several days at 160 °C without any measurable decomposition.12 One of the unique features of our reaction sequence is the formation of the active P/B FLP at the conjugated cyclohexadiene framework by thermolysis of the respective substituted hexatriene isomers that were obtained by 1,1carboboration. We assumed that this was a Woodward− Hoffmann allowed concerted disrotatory electrocyclic ringclosure reaction,17 but it had to be shown to have the correct stereochemistry. Therefore, we prepared a suitable stereochemically labeled example and studied its thermally induced cyclohexadiene P/B FLP formation. The resulting P/B FLP and its aromatic follow-up product were then used for the preparation of some new persistent P/B FLPNO• nitroxide radicals by their cooperative reaction with nitric oxide (NO).20−22 The results of this study will be described in this account.

Figure 1. View of the molecular structure of the 1,1-carboboration product 10a (thermal ellipsoids are shown at the 30% probability level). Selected bond lengths (Å) and angles (deg): B1−P1 2.116(4), P1−C1 1.809(3), C2−B1 1.622(5), C1−C2 1.354(4), C2−C3 1.454(4), C3−C4 1.332(5), C2−C1−P1 97.0(2), C1−C2−B1 107.8(3), C2−C1−C5 129.3(3), C1−C2−C3−C4 172.6(3), C2− C1−C5−C6 −56.2(5).

interaction between the P1/B1 Lewis base/Lewis acid centers. Carbon atom C2 has the trans-β-styryl substituent attached, and carbon atom C1 bears the cyclohexenyl substituent. The C1 to C4 unit is close to coplanar (θ C1−C2−C3−C4 172.6(3)°), whereas the cyclohexenyl π-system is markedly rotated out of the conjugated diene plane (θ C2−C1−C5−C6 −56.2(5)°). In solution (CD2Cl2) compound 10a shows a 31P NMR resonance at δ 8.8 and a 11B NMR signal at δ −2.0. It shows a pair of 1H NMR signals of the β-styryl substituent at δ 7.14 and 6.40 of one H relative intensity each with a vicinal 3JHH coupling constant of 16.3 Hz, which confirms the transstereochemistry of this group. The olefinic 1H NMR feature of the distal cyclohexenyl group occurs at δ 5.87 (s, 1H relative intensity). Due to the presence of conformational chirality, the low-temperature 19F NMR spectrum shows a total of 10 separate signals of the B(C6F5)2 moiety (for details see the Supporting Information). The thermally induced electrocyclic ring-closure reaction of 10a was achieved by keeping the P/B-substituted conjugated hexatriene system for 3 d at 80 °C in toluene solution. The isomeric ring-closed cyclohexadiene P/B product 11 was isolated as a white solid in 88% yield. The compound was characterized by C,H-elemental analysis, by spectroscopy, by some chemical reactions (see below), and by X-ray diffraction (suitable single crystals were obtained from pentane/dichloromethane at −35 °C by the diffusion method). The X-ray crystal structure analysis revealed formation of the cyclohexadiene core of compound 11 by means of a disrotatory electrocyclic ringclosure reaction. This core contains a conjugated diene unit with the B(C6F5)2 and PMes2 substituents attached at the central sp2-hybridized carbon atoms (C2, C1; see Figure 2). The newly formed distal C4−C5 carbon−carbon σ-bond bears the pair of carbon substituents in a vicinal cis-arrangement.

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RESULTS AND DISCUSSION Formation of the Annulated P/B FLPs. The suitably stereochemically labeled substrate 10a was readily prepared by the 1,1-carboboration reaction of the Mes2P-phosphanylsubstituted conjugated enyne 9 with trans-β-bis(pentafluorophenyl)boryl styrene (6a)23 (see Scheme 3). The reaction Scheme 3

required 3 d at 60 °C to go to completion, and we isolated compound 10a from the workup procedure as a white, amorphous solid in 90% yield. Crystallization from dichloromethane at −35 °C gave single crystals of compound 10a that were suited for its characterization by X-ray diffraction. B


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persistent nitroxide radical.19−21 Therefore, we treated the cyclohexadiene-based FLP 11 with two molar equivalents of TEMPO (2 d, 60 °C in benzene). Workup furnished the respective substituted phenylene-based FLP 12, which we isolated in 80% yield. The X-ray crystal structure analysis confirmed the presence of the aromatic phenylene core that was cyclohexene annulated through carbon atoms C5 and C10 (see Figure 3). The phenylene core has the phenyl substituent bonded at carbon atom C4 and the B(C6F5)2/PMes2 pair at C2 and C1. There is a B1−P1 interaction.

Figure 2. Projection of the molecular structure of the cyclohexadienederived FLP 11 (thermal ellipsoids are shown at the 50% probability level). Selected bond lengths (Å) and angles (deg): B1−P1 2.204(3), C1−P1 1.818(3), B1−C2 1.608(4), C1−C2 1.479(3), C1−C10 1.345(3), C2−C3 1.331(4), C4−C5 1.554(3), C2−C1−P1 98.6(1), C1−C2−B1 105.2(2), C2−C3−C4 120.9(2), C2−C1−P1−B1 −7.1(1), C1−C2−C3−C4 −9.7(3).

Carbon atom C4 has the phenyl substituent attached, and carbon atom C5 is part of the annulated six-membered ring. The carbon atom C10 is trigonal planar (being part of the C1C10 carbon−carbon double bond). The C5−C10 sixmembered ring attains a distorted chairlike conformation. We note that there is a marked B1−P1 Lewis acid/Lewis base interaction, despite the fact that compound 11 (in contrast to its precursor 10a) was shown to undergo a variety of typical frustrated Lewis pair reactions (see below). In solution the FLP shows heteroatom NMR signals at δ 10.4 (31P) and δ 6.4 (11B). Compound 11 shows three broad o, p, m-19F NMR resonances at 299 K, which indicated conformational equilibration on the NMR time scale potentially by reversible opening of the P−B bond. Lowering the temperature rapidly leads to decoalescence of each signal to a 1:1 intensity set, as it would be expected for the structure of 11, as observed in the solid state (see above). Further lowering of the monitoring temperature leads to splitting of the pair of p-19F NMR C6F5 resonances into two pairs in a ca. 4:1 ratio. At the same time we observe the occurrence of four o-C6F5 signals of the minor component at 213 K and a partially decoalesced set of o-C6F5 19F NMR resonances of the major component (the spectra are depicted in the Supporting Information). It appears that frozen rotation around the B−C6F5 vectors at low temperature has resulted in the occurrence of a pair of diastereomeric conformational isomers of compound 11. We observed a similar coalescence behavior of the 1H NMR spectra of 11: at low temperature six equal-intensity methyl singlets of the pair of mesityl groups at phosphorus were observed (see the Supporting Information for details). At 213 K the olefinic 1 H NMR signal of compound 11 was located at δ 5.88 (singlet, 1H) and the pair of methine hydrogens at the newly formed (Ph)CH−CH− single bond occurs at δ 4.14 (−CHPh−) and 2.34 (m) with a 3JHH coupling constant of 10.3 Hz. Cyclohexadienes can be oxidized to the respective aromatic systems by 2-fold hydrogen atom abstraction with a suitable

Figure 3. Molecular structure of the FLP 12 (thermal ellipsoids are shown at the 30% probability level). Selected bond lengths (Å) and angles (deg): P1−B1 2.169(6), P1−C1 1.804(5), C2−B1 1.631(7), C1−C2 1.385(7), C1−C10 1.406(7), C2−C3 1.391(7), C4−C5 1.416(7), C2−C1−P1 98.0(3), C1−C2−B1 108.6(4), C2−C3−C4 120.3(5), B1−P1−C1−C2 0.7(3), C1−C2−C3−C4 3.2(7).

The 13C NMR spectrum of the FLP 12 shows six resonances of the newly formed pentasubstituted central phenylene group and the CH resonance at δ 7.31 in the 1H NMR. We located the 11B NMR resonance at δ 2.0 and the 31P NMR signal at δ 4.1. Compound 12 shows dynamic temperature-dependent NMR spectra. At low temperature (193 K) we have monitored decoalesced 1H and 19F NMR spectra, indicating a frozen chiral rotameric conformation. Consequently, we observed a total of nine 19F NMR resonances at that temperature (two m-C6F5 signals overlapping) of the B(C6F5)2 group and a total of six separate methyl 1H NMR signals of the pair of mesityl groups at phosphorus (see the Supporting Information for further details). Figure 4 summarizes the 11B and 31P solid-state NMR spectra of compounds 10a, 11, and 12. Both the 11B isotropic chemical shifts and the nuclear electric quadrupolar coupling constants increase in the order 10a (δ 1.58, 1.86 MHz), 12 (δ 4.7, 1.92 MHz), 11 (δ 8.8, 2.28 MHz), indicating a successive diminution of the covalent interactions between the Lewis centers. These differences are also reflected in the B···P distances in accordance with previous DFT calculations conducted on a large set of intramolecular borane-phosphane FLPs24 and also correlate well with the differences in reactivities of these compounds (see below). Similar trends are observed in the proton-decoupled 31 P MAS NMR spectra, whose asymmetric line shapes arise from direct and indirect spin− C


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Figure 4. Experimental and simulated 11B MAS NMR (left) and 31P{1H} CPMAS NMR spectra (right) of compounds 10a, 11, 12, and 14, measured at a magnetic field strength of 7.05 T. Gray lines shown in the figure indicate the simulated subspectra of individual components. The 31P resonances of 10a−12 consist of two components caused by direct and indirect spin−spin coupling to 11B and 10B in the corresponding isotopologues [+: impurities].

spin couplings to the quadrupolar isotopes 11B and 10B as discussed in ref 24. Both the 11B and 31P spectra of compound 11 are consistent with the formation of one dominant stereoisomer. Some FLP Reactions of Compounds 11 and 12. Many frustrated Lewis pairs react with terminal acetylenes by a deprotonation route25 and so does the cyclohexadiene-derived FLP 11. Treatment of 11 with phenylacetylene at 80 °C in toluene solution (16 h) gave the phosphonium/alkynylborate product 13 (isolated in 39% yield, see Scheme 4). It showed a Scheme 4

Figure 5. View of the molecular structure of the phosphonium/ alkynylborate product 13 (thermal ellipsoids are shown at the 50% probability level). Selected bond lengths (Å) and angles (deg): P1−C1 1.804(3), B1−C2 1.664(4), C1−C2 1.516(4), C1−C10 1.366(4), C2−C3 1.336(4), C61−C62 1.205(4), C2−C1−P1 117.1(2), C62− C61−B1 170.9(3), C61−C62−C71 176.3(3), P1−C1−C2−B1 −29.4(4).

typical 31P NMR resonance doublet at δ −29.3 with 1JPH = 496 Hz (11B NMR resonance at δ −17.7).26 The 1H NMR spectra at 253 K showed six separate methyl singlets and four aromatic methine proton resonances of the pair of mesityl substituents at phosphorus, which indicated the presence of the chirality centers at the core plus hindered rotation around the P−C (mesityl) vectors under these conditions (for further details see the Supporting Information). Compound 13 was characterized by X-ray diffraction (see Figure 5). It shows the bicyclic core structure with a cisarrangement of the vicinal pair of hydrogens at carbon atoms C4 and C5. In the product 13 the B1···P1 separation is large, at 3.354 Å. Both the phosphorus and the boron atoms show pseudotetrahedral coordination geometries (∑P1CCC 341.3°, ∑B1CCC 332.8°). The boron atom B1 has the phenylacetylide residue attached to it, and the phosphorus atom P1 is found protonated. We had previously found that both the ring-closed compounds 4a and 8 (see Scheme 2) are active FLPs that were able to heterolytically cleave dihydrogen under mild

conditions. However, the phenylene-bridged FLP 8 was found to be 38 times less active than the cyclohexadiene-derived system 4a.12 In order to characterize the hydrogen splitting power of our new systems, we, therefore, reacted the new even more bulky annulated phenylene-containing system FLP 12 with dihydrogen (see Scheme 5). This system proved to be an active hydrogen-cleaving reagent. We stirred the pale yellow Scheme 5

D


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(∑B1CCC 335.9°, ∑P1CCC 347.8°). Each of these heteroatoms has a hydrogen atom attached. Both the B1−H and the P1−H vectors are oriented toward each other with a B1−H···H−P1 separation of 1.838 Å. We had shown that both the FLPs 4a and 8a reacted with dimethyl acetylenedicarboxylate under forcing conditions by addition involving a carbon−carbon bond activation step.12 Our new FLP 11 reacts in a similar way with this acetylenic ester reagent. Heating of the mixture of 11 with one molar equivalent of dimethyl acetylenedicarboxylate for 6 d at 75 °C in toluene solution gave the product 17, which we isolated as a yellow solid in 50% yield. We assume a reaction pathway as shown in Scheme 6 that is initiated by ester coordination to the

solution of the FLP 12 in CH2Cl2 for 24 h at room temperature (rt) in a hydrogen atmosphere (1.5 bar). Workup involving removal of the dichloromethane solvent in vacuo and washing with pentane gave the phosphonium/hydridoborate product 14 as a white solid in 88% yield. In CD2Cl2 solution (299 K) the compound showed a typical 1H NMR phosphonium [P]+H doublet at δ 9.48 with 1JPH = 507 Hz (31P NMR δ −28.5), and we observed the broad partially relaxed 1:1:1:1 hydridoborate 1 H NMR quartet at δ 3.90. The corresponding 11B NMR signal was found at δ −20.8. It showed a 1JBH coupling constant of 83 Hz. Compound 14 showed six separate 1H NMR mesityl methyl singlets and four 1H NMR methine signals of the PMes2 moiety. Due to the PHMes2 conformational chirality, we find the 19 F NMR signals of a pair of diastereotopic C6F 5 substituents at the tetracoordinate boron atom. The solid-state 11B MAS NMR spectrum of compound 14 (see Figure 4) indicates that hydrogenation diminishes the 11B electric field gradient significantly, resulting in a substantially reduced nuclear electric quadrupolar coupling constant (about 0.5 MHz). Hydrogenation also results in low-frequency shifts of both the 11B and 31P resonances. The 31P NMR line shape indicates further that spin−spin coupling to boron is significantly weaker than in the free FLP. We have also reacted FLP 12 with deuterium under similar conditions. This experiment gave a ca. 54:46 mixture of the starting material 12 with [P]D+/[B]D− product 14-D2. The reduced compound 14-D2 showed a 1:1:1 intensity triplet in the 31P NMR spectra at δ −28.6 with 1JPD = 79 Hz. In the 2H NMR spectrum we observed the respective [P]2H resonance as a corresponding doublet at δ 9.46 and a broad [B]2H NMR signal at δ 3.88 (for further details and the depicted spectra see the Supporting Information). Compound 14 was also characterized by an X-ray crystal structure analysis (see Figure 6). It shows the central aromatic

Scheme 6

boron Lewis acid functionality of 11. This apparently activates the acetylenic ester to enter in an electrophilic aromatic substitution reaction with one of the mesityl groups at phosphorus to generate the intermediate 16. This may then be stabilized by methyl group transfer induced by nucleophilic phosphane attack at the central carbon atom of the allenic ester enolate by making use of the distal orthogonal α,β-unsaturated ester moiety (remotely related to a Baylis−Hillman reaction),27 which would eventually give the observed product 17 (see Scheme 6). The X-ray crystal structure analysis of compound 17 shows the newly formed seven-membered annulated heterocycle. It has the quaternary carbon atom C12 attached to it, which contains an ester group from the acetylenedicarboxylate reagent and the methyl substituent that had been transferred from the mesityl substituent. The FLP-derived framework features the vicinal cis-arrangement of the substituents at carbon atoms C4/ C5, which is so typical for this system. The −B(C6F5)2 Lewis acid at the ring carbon atom C2 now has the oxygen atom (O3) of the ester enolate function attached to it (see Figure 7). In the 1H NMR spectrum at 299 K in CD2Cl2 solution we observe a pair of OCH3 signals, five methyl singlets of the CH3 substituents at the aryl substituents at phosphorus, and the singlet of the transferred methyl group. The 19F NMR spectrum shows four o-, two p-, and four m-C6F5 signals. In both spectra we have observed a set of broad signals of an unidentified minor compound (ca. 30%). This might be the

Figure 6. Molecular structure of the dihydrogen splitting product 14 (thermal ellipsoids are shown at the 30% probability level).

ring with C−C bond lengths in a narrow range between 1.378(6) and 1.419(6) Å. The C5/C10 annulated cyclohexene moiety features a half-chair conformation. The phenyl substituent at carbon atom C4 is rotated markedly out of the central aromatic plane [θ C3−C4−C51−C52 111.7(5)°] The B/P centered heteroatom substituents are found attached at carbon atoms C2 and C1, respectively [C2−B1 1.640(7) Å, C1−P1 1.809(4) Å]. Both the boron and the phosphorus atoms show pseudotetrahedral coordination geometries E


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Figure 8. Molecular structure of compound 18 (thermal ellipsoids are shown at the 50% probability level). Selected bond lengths (Å) and angles (deg): C1−P1 1.827(2), B1−C2 1.654(4), C1−C2 1.423(3), C1−C10 1.422(3), C2−C3 1.401(3), C11−C14 1.354(3), C12−C22 1.523(3), C12−C27 1.534(3), C2−C1−P1 113.1(2), C11−P1−C1 103.9(1), C12−C11−P1 111.9(2), C2−B1−O3−C14 −8.3(3), P1− C11−C14−O3 0.5(3).

Figure 7. Molecular structure of the carbon−carbon bond activation product 17 (thermal ellipsoids are shown at the 50% probability level). Selected bond lengths (Å) and angles (deg): P1−C1 1.812(2), B1−C2 1.660(3), C1−C2 1.521(3), C1−C10 1.352(3), C2−C3 1.341(3), C11−C14 1.348(3), C12−C22 1.523(3), C12−C27 1.538(3), C2− C1−P1 111.7(2), C11−P1−C1 103.0(2), C12−C11−P1 111.3(2), C2−B1−O3−C14 −5.5(3), P1−C11−C14−O3 1.9(3).

persistent FLPNO• nitroxide radicals.20−22 They had chemical and spectroscopic features similar to the ubiquitous organic nitroxide radicals such as TEMPO29 or PINO.30 We had employed a FLPNO• example even as controlling reagent in a nitroxide-mediated styrene polymerization (NMP) reaction.20 The new cyclohexadiene-derived P/B FLP 11 reacts readily with NO. We exposed a solution of 11 in dichloromethane at rt to a NO atmosphere (1.5 bar). The pale yellow solution turned dark turquoise within 15 min. Workup gave the FLPNO• product 19 as a pale turquoise solid in 80% yield (see Scheme 8). We characterized the compound by C,H,N elemental

epimer of 17 or a conformational isomer. Consequently, we observed a sharp 31P NMR resonance at δ 15.9, which we attribute to the major compound 17 along with a broad signal at δ 16.5 of the minor component. We observed a single 11B NMR resonance at δ 0.8. The aromatic phenylene-bridged FLP 12 is markedly less reactive than 11, but it is thermally robust and allows reactions at higher temperatures. It undergoes the analogous reaction with dimethyl acetylenedicarboxylate; only here this reaction needs much more forcing reaction conditions. Heating of the respective reaction mixture for 6 d at 100 °C gave the C−C activation product 18, which we isolated in 62% yield (see Scheme 7). In this case we observed a single set of NMR signals

Scheme 8

Scheme 7

(e.g., 1H NMR: six methyl singlets, two OCH3 singlets, and a total of 10 well-separated 19F NMR resonances; 31P NMR: δ 15.1; 11B NMR: δ 1.1; for further details see the Supporting Information). Compound 18 was also characterized by X-ray diffraction (see Figure 8). Reactions of the Annulated FLPs 11 and 12 with Nitric Oxide; Formation of the Persistent FLPNO• Nitroxide Radicals. We had shown that a variety of vicinal P/B FLPs react cooperatively by joint P/B addition to a single carbon atom of an isonitrile10 or carbon monoxide28 to form the respective P/B-containing heterocycles. A number of vicinal P/ B FLPs added analogously to NO to give the respective

analysis, by EPR spectroscopy, and by X-ray diffraction. The Xray crystal structure analysis (see Figure 9) showed that both the phosphorus Lewis base and the boron Lewis acid had added to the NO nitrogen atom. The N−O bond length in nitric oxide is 1.15 Å,31 and the N1−O1 bond length in the persistent radical 19 is longer (see Table 1), but is still within a range that indicates substantial delocalization of the unpaired electron F


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obtained A(14N) hyperfine coupling is markedly smaller than that of, for example, the persistent TEMPO radical [A(14N) = 43.5 MHz in toluene],32 which indicates that 19 is a more oxygen-centered radical. Persistent radicals undergo H atom abstractions with suitable substrates. The P/B FLPNO• 19 is a rather reactive radical. It reacted with cyclohexadiene at rt within 1 h to give benzene and the diamagnetic P/B FLPNOH product 20. In solution compound 20 shows the 1H NMR resonances of a pair of diastereotopic mesityl substituents at phosphorus (31P NMR: δ 28.7) and the 19F NMR signals of a pair of diastereotopic C6F5 rings at boron (11B: δ −6.6). The X-ray crystal structure analysis of 20 (the structure is depicted in the Supporting Information) shows a markedly elongated N1−O1 bond (see Table 1) relative to its paramagnetic precursor 19. This elongation is caused by occupation of the delocalized odd electron in the course of the H atom abstraction reaction. The B1−N1 bond in 20 is slightly shortened, as is the P1−N1 linkage. This may indicate a participation of the phosphinimine resonance structure for the description of the bonding features of this P/B FLPNOH compound. The phenylene P/B FLP 12 reacts equally facilely with nitric oxide. The reaction took ca. 15 min under our typical conditions, and we isolated the persistent FLPNO• radical 21 as a pale green solid from the dark green reaction mixture in 84% yield. The X-ray crystal structure analysis (see Figure 11 and Table 1) shows an even shorter N1−O1 bond and rather long P1−N1 and B1−N1 linkages. Again, the nitrogen atom is planar tricoordinate. The EPR spectrum of the P/B FLPNO• radical shows a signal at giso = 2.0052. It also shows a relatively small A(14N) hyperfine coupling (23.3 MHz) as compared to, for example, TEMPO, rendering 21 a more oxygen-centered radical (see Figure 12). Consequently, the nitroxide radical 21 is a reactive

Figure 9. View of the molecular structure of the persistent FLPNO• radical 19 (thermal ellipsoids are shown at the 30% probability level). Selected bond lengths (Å) and angles (deg): P1−N1 1.690(3), P1−C1 1.804(3), O1−N1 1.340(4), N1−B1 1.578(5), B1−C2 1.624(5), C1− C10 1.345(5), C1−C2 1.491(5), C2−C3 1.337(5), C3−C4 1.506(5), C4−C5 1.538(5), C5−C10 1.519(5), N1−P1−C1 93.8(2), O1−N1− B1 123.4(3), O1−N1−P1 117.2(2), B1−N1−P1 119.4(2), N1−B1− C2 100.3(3), C10−C1−C2 123.2(3), C10−C1−P1 125.7(3), C2− C1−P1 110.3(2), P1−C1−C2−B1 −4.1(4).

Table 1. Selected Structural Parameters of the Persistent FLPNO• Nitroxide Radicals 19 and 21 and Their Diamagnetic FLPNOH Derivatives 20 and 22a

O1−N1 P1−N1 B1−N1 P1−C1 B1−C2 N1−P1−C1 O1−N1−B1 N1−B1−C2 ∑N1POB a

FLPNO• 19

FLPNOH 20

FLPNO• 21

FLPNOH 22

1.340(4) 1.690(3) 1.578(5) 1.804(3) 1.624(5) 93.8(2) 123.4(3) 100.3(2) 360.0

1.434(2) 1.636(2) 1.546(3) 1.804(2) 1.622(4) 93.4(1) 121.9(2) 99.4(2) 359.8

1.294(3) 1.728(3) 1.597(5) 1.800(3) 1.618(5) 92.8(1) 125.3(3) 97.7(3) 360.0

1.423(4) 1.646(3) 1.539(5) 1.812(4) 1.622(5) 93.1(2) 120.7(3) 98.7(3) 359.5

Bond lengths in Å and angles in deg.

between nitrogen and oxygen. Both the P1−N1 and B1−N1 bonds are within an element−nitrogen single-bond range. The X-band EPR spectrum of compound 19 is shown in Figure 10. Simulation gave the characteristic parameters. The

Figure 11. Projection of the molecular structure of phenylene-bridged persistent P/B FLPNO• radical 21 (thermal ellipsoids are shown at the 30% probability level). Selected bond lengths (Å) and angles (deg): P1−N1 1.728(3), P1−C1 1.800(3), B1−N1 1.597(5), B1−C2 1.618(5), O1−N1 1.294(3), C1−C2 1.400(4), C1−C10 1.409(4), C2−C3 1.392(4), C3−C4 1.380(5), C4−C5 1.417(5), N1−P1−C1 92.8(1), O1−N1−B1 125.3(3), O1−N1−P1 122.8(2), B1−N1−P1 111.9(2), N1−B1−C2 97.7(3), C2−C1−C10 123.9(3), C10−C1−P1 126.3(2), C2−C1−P1 109.7(2), P1−C1−C2−B1 4.5(4).

Figure 10. X-band EPR spectrum (bottom) and simulation (top) of the persistent FLPNO• nitroxide radical 19 (rt, fluorobenzene, giso = 2.0053, A(14N) = 22.2 MHz, A(31P) = 49.6 MHz, A(11B) = 9.5 MHz). G


Article

Organometallics

conjugated enynes. Subsequent thermally induced concerted disrotatory electrocyclic ring closure gave the corresponding cyclohexadiene-derived unsaturated P/B system, which could even conveniently be oxidized using TEMPO to yield the respective phenylene-bridged P/B compounds. These new cyclized products are active P/B FLPs. They underwent the typical cooperative P/B addition reaction to the nitrogen atom of NO to give the persistent P/B FLPNO• radicals in high yield. The FLPs 4, 8, 11, and 12 may be less active FLP systems compared to their saturated parent P/B FLP 1, but they more than offset this by their remarkable thermal robustness. The FLP 1 can hardly be employed in reactions that require temperatures above ca. 50 °C due to the onset of unwanted decomposition. In contrast, in this study we cleanly carried out the C−C activation reaction of compound 11 with dimethyl acetylenedicarboxylate, which required heating at 75 °C for almost 1 week. The phenylene derivative (12) underwent the analogous reaction at 100 °C (6 d duration) without any significant signs of decomposition. It seems that our new reaction sequence of 1,1-carboboration followed by electrocyclic ring closure (and subsequent oxidation) provides us with a convenient entry to new P/B FLP reagents that enlarge the spectrum of FLP reactions and offers utilizing a larger temperature window for the development and performance of frustrated Lewis pair reactions. This may become useful for extending the scope of frustrated Lewis pair reactions with organic substrates.

Figure 12. X-band EPR spectrum (bottom) and simulation (top) of compound 21 (rt, fluorobenzene, giso = 2.0052, A(14N) = 23.3 MHz, A(31P) = 48.9 MHz, A(11B) = 10.0 MHz).

hydrogen atom abstractor. It reacts with 1,4-cyclohexadiene under mild conditions (rt, in benzene solution) to give the diamagnetic P/B FLPNOH product 22. It was isolated from the reaction mixture as a colorless solid in 88% yield (see Scheme 9). Scheme 9

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EXPERIMENTAL SECTION

General Procedures. All syntheses involving air- and moisturesensitive compounds were carried out using standard Schlenk-type glassware (or in a glovebox) under an atmosphere of argon. Solvents were dried and stored under an argon atmosphere. NMR spectra were recorded on an Agilent DD2-500 MHz (1H: 500 MHz, 13C: 126 MHz, 19 F: 470 MHz, 11B: 160 MHz, 31P: 202 MHz) and on an Agilent DD2600 MHz (1H: 600 MHz, 13C: 151 MHz, 19F: 564 MHz, 11B: 192 MHz, 31P: 243 MHz). 1H NMR and 13C NMR: chemical shifts are given relative to TMS and referenced to the solvent signal. 19F NMR: chemical shifts are given relative to CFCl3 (δ = 0, external reference). 11 B NMR: chemical shifts are given relative to BF3·Et2O (δ = 0, external reference). 31P NMR: chemical shifts are given relative to H3PO4 (85% in D2O) (δ = 0, external reference). NMR assignments were supported by additional 2D NMR experiments. Elemental analyses were performed on an Elementar Vario El III. IR spectra were recorded on a Varian 3100 FT-IR (Excalibur Series). Melting points and decomposition points were obtained with a DSC 2010 (TA Instruments). HRMS was recorded on a GTC Waters Micromass (Manchester, UK). X-ray diffraction: For compounds 10a, 10b, 12, 14, 19, 21, and 22 data sets were collected with a Nonius Kappa CCD diffractometer. Programs used: data collection, COLLECT;33 data reduction, Denzo-SMN;34 absorption correction, Denzo;35 structure solution, SHELXS-97;36 structure refinement, SHELXL-97;37 and graphics, XP (BrukerAXS, 2000). For compounds 18 and 20 data sets were collected with a Kappa CCD APEXII Bruker diffractometer. For compounds 11, 17, and 20′ data sets were collected with a D8 Venture Dual Source 100 CMOS diffractometer. Programs used: data collection, APEX2 V2014.5-0 (Bruker AXS Inc., 2014); cell refinement, SAINT V8.34A (Bruker AXS Inc., 2013); data reduction, SAINT V8.34A (Bruker AXS Inc., 2013); absorption correction, SADABS V2014/2 (Bruker AXS Inc., 2014); structure solution, SHELXT-2014 (Sheldrick, 2014); structure refinement, SHELXL2014 (Sheldrick, 2014); and graphics, XP (Bruker AXS Inc., 2014). RValues are given for observed reflections, and wR2 values are given for all reflections. Exceptions and special features: For compound 11 one pentane molecule, for compound 14 one dichloromethane molecule, for compounds 19 and 20′ two dichloromethane molecules, and for

The X-ray crystal structure of 22 shows the typically elongated N1−O1 bond relative to its precursor 21 (see Table 1) and shortened P1−N1 and B1−N1 bond lengths (the structure is depicted in the Supporting Information). The 1H NMR spectrum of compound 22 shows the −OH resonance at δ 4.76. It shows a single set of signals of the pair of symmetry-equivalent mesityl groups at phosphorus and three 19 F NMR signals of the −B(C6F5)2 group. We find a 31P NMR signal at δ 33.7 and a sharp 11B NMR resonance at δ −5.4. Compound 22 shows four separate 1H NMR multiplets of the tetramethylene unit of the annulated cyclohexene ring and the typical 1H NMR set of signals of the distal phenyl substituent at the central phenylene unit.

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CONCLUSIONS The saturated ethylene-bridged phosphane/borane 1 is a very reactive FLP.7 Autrey et al. determined the rate law of the heterolytic cleavage of dihydrogen at this system as v = k[1][H2] with a second-order rate constant (at rt) of 0.70 ± 0.03 [M−1 s−1].8 It was tempting to use the 1,1-carboboration reaction of dimesitylphosphinoacetylenes with a variety of reactive alkyl- or aryl-B(C6F5)2 boranes as a convenient entry to the related unsaturated C2-bridged P/B FLP systems. It was disappointing to learn that the respective compounds 2 were very unreactive and did not undergo the majority of typical FLP reactions at all.9 This and a recent related study12 have now lifted this serious restriction from the 1,1-carboboration route to active FLP systems. Our new reaction sequence makes the P/B-substituted cyclohexatrienes (7, 10a) conveniently available by means of the 1,1-carboboration reactions of the alkenylB(C6F5)2 reagents with the respective Mes2P-substituted H


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Organometallics

(d, 4JPH = 3.5 Hz, 1H, m-Mesa), 6.81 (s, 1H, m′-Mesa), 6.79 (m, 1H, m-Mesb), 6.40 (d, 3JHH = 16.3 Hz, 1H, CHPh), 6.38 (d, 4JPH = 2.5 Hz, 1H, m′-Mesb), 5.87 (s, 1H, 2-CH), 2.57 (s, 3H, o-CH3Mes,b), 2.45 (m, 3H, o-CH3Mes,a), 2.30, 2.20 (each m, each 1H, 3-CH2)t, 2.23 (s, 3H, p-CH3Mes,a), 2.20 (s, 3H, o′-CH3Mes,a), 2.10 (s, 3H, p-CH3Mes,b), 1.77, 1.70 (each m, each 1H, 5-CH2)t, 1.65, 1.52 (each m, each 1H, 4CH2)t, 1.61 (s, 3H, o′-CH3Mes,b), 1.58, 1.43 (each m, each 1H, 6-CH2)t [t tentatively assigned]. 13C{1H} NMR (151 MHz, dichloromethaned2, 213 K): δ 166.3 (d, 2JPC = 29.6 Hz, BC), 143.1 (d, 2JPC = 17.8 Hz, o-Mesa), 143.0 (o′-Mesa), 142.5 (d, 2JPC = 3.4 Hz, o-Mesb), 141.3 (d, 4JPC = 2.4 Hz, p-Mesa), 139.9 (p-Mesb), 139.8 (d, 2JPC = 10.2 Hz, o′-Mesb), 138.7 (d, 1JPC = 50.6 Hz, PC), 136.3 (CHPh), 136.1 (iPh), 133.2 (d, 2JPC = 3.3 Hz, 1-C), 130.2 (d, 3JPC = 8.3 Hz, m-Mesb), 130.0 (d, 3JPC = 8.3 Hz, 2-CH), 129.6 (d, 3JPC = 5.9 Hz, m′-Mesa), 129.3 (d, 3JPC = 9.2 Hz, m-Mesa), 129.2 (d, 3JPC = 9.2 Hz, m′-Mesb), 128.3 (m-Ph), 128.1 (p-Ph), 126.6 (o-Ph), 125.4 (d, 1JPC = 25.3 Hz, iMesa), 124.6 (d, 1JPC = 39.9 Hz, i-Mesb), 124.4 (d, 3JPC = 46.8 Hz,  CH), 27.4 (5-CH2)t, 25.3 (3-CH2)t, 25.0 (br d, 3JPC = 13.5 Hz, oCH3Mes,b), 23.7 (dm, J = 8.7 Hz, o-CH3Mes,a), 22.4 (6-CH2)t, 22.3 (d, 3 JPC = 2.2 Hz, o′-CH3Mes,a), 22.0 (dd, J = 9.5 Hz, J = 3.9 Hz, o′CH3Mes,b), 21.5 (4-CH2)t, 20.7 (p-CH3Mes,a), 20.1 (p-CH3Mes,b) [C6F5 not listed; t tentatively assigned]. 31P{1H} NMR (243 MHz, dichloromethane-d2, 213 K): δ 8.8 (partial relaxed 1:1:1:1 q, JPB ≈ 25 Hz). 11B{1H} NMR (192 MHz, dichloromethane-d2, 213 K): δ −2.0 (ν1/2 ≈ 1900 Hz). 19F NMR (564 MHz, dichloromethane-d2, 213 K): δ −125.4 (m, o), −131.7 (m, o′), −158.0 (t, 3JFF = 21.3 Hz, p), −163.8 (m, m′), −164.6 (m, m) (each 1F, C6F5) [Δ19Fmp = 5.8, 6.6], −128.4 (o), −130.7 (o′), −159.4 (m, p), −165.6 (m, m′), −165.9 (m, m) (each 1F, C6F5) [Δ19Fmp = 6.2, 6.5]. Synthesis of Compound 10b. A solution of borane 6b (1.54 g, 3.60 mmol, 1.0 equiv) in toluene (5 mL) was added dropwise to a solution of phosphane 9 (1.35 g, 3.60 mmol, 1.0 equiv) in toluene (5 mL). The reaction mixture was stirred at 60 °C for 3 days, and then all volatiles were removed in vacuo. The resulting sticky red residue was dissolved in pentane (5 mL), which was subsequently removed in vacuo. The obtained solid was washed with pentane (5 × 3 mL) and dried to finally give compound 10b as an off-white solid (1.26 g, 1.6 mmol, 43%). Crystals suitable for the X-ray crystal structure analysis were obtained from a solution of 10b in pentane at −35 °C. Dec: 212 °C. Anal. Calcd for C44H42BF10P: C: 65.85; H: 5.27. Found: C: 65.79; H: 5.21. IR (KBr): ν̃ [cm−1] = 5340 (w), 4382 (w), 4310 (w), 3901 (w), 3819 (w), 3747 (w), 3673 (w), 3628 (w), 3156 (w), 3026 (s), 2954 (s), 2863 (s), 2739 (w), 2632 (w), 2568 (w), 2400 (w), 2360 (w), 2220 (w), 2093 (w), 1828 (w), 1772 (w), 1735 (w), 1699 (w), 1645 (s), 1605 (m), 1515 (m), 1445 (m), 1381 (m), 1177 (w), 1114 (s), 1032 (m), 966 (m), 925 (m), 854 (s), 805 (m), 772 (m), 743 (s), 695 (s), 668 (s), 630 (s), 609 (s), 574 (m), 554 (m), 533 (m), 498 (m), 456 (m), 417 (m). 1H NMR (500 MHz, dichloromethane-d2, 213 K): δ 6.95 (d, 4JPH = 3.0 Hz, 1H, m-Mesa), 6.78 (s, 1H, m′-Mesa), 6.76 (s, 1H, m′-Mesb), 6.34 (s, 1H, m-Mesb), 6.15 (d, 3JHH = 16.0 Hz, 1H, CH), 5.72 (s, 1H, 2-CH), 5.62 (d, 3JHH = 16.0 Hz, 1H, CHtBu), 2.53 (s, 3H, o′-CH3Mes,b), 2.44 (br, 3H, o-CH3Mes,a), 2.21 (s, 3H, pCH3Mes,a), 2.18, 2.13 (each m, each 1H, 3-CH2)t, 2.16 (s, 3H, o′CH3Mes,a), 2.09 (s, 3H, p-CH3Mes,b), 1.71, 1.67 (each br m, each 1H, 5CH2)t, 1.59, 1.45 (each m, each 1H, 4-CH2), 1.56 (s, 3H, o-CH3Mes,b), 1.53, 1.40 (each m, each 1H, 6-CH2), 0.75 (s, 9H, CH3tBu) [t tentatively assigned]. 13C{1H} NMR (126 MHz, dichloromethane-d2, 213 K): δ 167.9 (br d, 2JPC = 30 Hz, BC), 151.9 (CHtBu), 143.1 (d, 2JPC = 17.2 Hz, o-Mesa), 143.0 (o′-Mesa), 142.5 (d, 2JPC = 3.4 Hz, o′-Mesb), 141.1 (d, 4JPC = 1.7 Hz, p-Mesa), 139.7 (d, 2JPC = 13.2 Hz, oMesb), 139.7 (d, 4JPC = 2.1 Hz, p-Mesb), 135.5 (d, 1JPC = 51.1 Hz, PC), 133.0 (d, 2JPC = 3.5 Hz, 1-C), 130.1 (d, 3JPC = 8.2 Hz, m′Mesb), 129.5 (d, 3JPC = 6.9 Hz, 2-CH), 129.5 (d, 3JPC = 6.9 Hz, m′Mesa), 129.3 (d, 3JPC = 9.1 Hz, m-Mesa), 129.1 (d, 3JPC = 9.1 Hz, mMesb), 125.6 (d, 1JPC = 25.0 Hz, i-Mesa), 124.8 (d, 1JPC = 39.6 Hz, iMesb), 120.2 (d, 3JPC = 45.5 Hz, CH), 33.3 (CtBu), 28.0 (CH3tBu), 27.4 (5-CH2)t, 25.2 (3-CH2)t, 24.9 (m, o′-CH3Mes,b), 23.6 (m, oCH3Mes,a), 22.4 (6-CH2)t, 22.3 (d, 3JPC = 2.3 Hz, o′-CH3Mes,a), 22.0 (dd, J = 9.3 Hz, J = 3.4 Hz, o-CH3Mes,b), 21.5 (4-CH2)t, 20.7 (pCH3Mes,a), 20.0 (p-CH3Mes,b) [C6F5 not listed; t tentatively assigned].

compound 21 one phenyl group and one part of the six-membered ring C5 to C10 are disordered over two positions. Several restraints (SADI, SAME, ISOR, and SIMU) were used in order to improve refinement stability. For compounds 10a and 12 one badly disordered pentane molecule and for compounds 17 and 18 one badly disordered half-pentane molecule were found in the asymmetric unit and could not be satisfactorily refined. The program SQUEEZE38 was therefore used to remove mathematically the effect of the solvent. The quoted formula and derived parameters are not included in the squeezed solvent molecules. Compound 10b presents one tBu group and one six-membered ring at the C1 atom disordered over two positions. Several restraints (SADI, SAME, ISOR, and SIMU) were used in order to improve refinement stability. Moreover, one badly disordered pentane molecule was found in the asymmetrical unit and could not be satisfactorily refined. The program SQUEEZE was therefore used to remove mathematically the effect of the solvent. The quoted formula and derived parameters are not included in the squeezed solvent molecule. Materials: Vinylboranes 6a and 6b were prepared according to the literature.39 Synthesis of Compound 9.13 n-Butyllithium (10.0 mL, 16.0 mmol, 1.0 equiv) was added dropwise to a solution of 1ethynylcyclohexene (1.9 mL, 1.7 g, 16.0 mmol, 1.0 equiv) in tetrahydrofuran (80 mL) at −78 °C. The reaction mixture was warmed to rt and then stirred for 10 min. A solution of dimesitylchlorophosphane (4.9 g, 16.0 mmol, 1.0 equiv) in tetrahydrofuran (30 mL) was added dropwise to the dark brown suspension at −78 °C, and the reaction mixture was stirred for 10 min at that temperature and then for another 2 h at ambient temperature. All volatiles were removed in vacuo, and the sticky residue was suspended in pentane (100 mL). The resulting suspension was filtered via cannula (Whatman glass fiber filter), and all volatiles were removed from the filtrate in vacuo. The crude product was purified via column chromatography (silica: CH2Cl2/CyH = 15:85; Rf 0.70) to give compound 9 as a colorless, crystalline solid (5.1 g, 13.6 mmol, 85%). Mp: 80 °C. Anal. Calcd for C26H31P: C: 83.39; H: 8.34. Found: C: 83.20; H: 8.43. IR (KBr): ν̃ [cm−1] = 3063 (w), 2958 (w), 2855 (w), 2727 (w), 2661 (w), 2466 (w), 2300 (w), 2133 (m), 1908 (w), 1881 (w), 1720 (w), 1601 (m), 1553 (w), 1465 (m), 1433 (s), 1407 (m), 1372 (m), 1346 (w), 1074 (w), 1028 (m), 996 (w), 917 (m), 845 (s), 787 (m), 712 (w), 689 (m), 616 (m), 600 (m), 558 (s), 460 (w), 431 (w). 1H NMR (500 MHz, dichloromethane-d2, 299 K): δ 6.81 (dm, 4 JPH = 3.2 Hz, 4H, m-Mes), 6.08 (m, 1H, 2-CH), 2.37 (s, 12H, o-Mes), 2.24 (s, 6H, p-Mes), 2.11 (m, 2H, 6-CH2), 2.09 (m, 2H, 3-CH2), 1.62 (m, 2H, 5-CH2), 1.57 (m, 2H, 4-CH2). 13C{1H} NMR (126 MHz, dichloromethane-d2, 299 K): δ 142.2 (d, 2JPC = 15.6 Hz, o-Mes), 138.7 (p-Mes), 135.6 (d, 4JPC = 2.6 Hz, 2-CH), 130.5 (d, 1JPC = 12.6 Hz, iMes), 130.2 (d, 3JPC = 3.6 Hz, m-Mes), 121.7 (d, 3JPC = 1.5 Hz, 1C), 109.2 (d, 2JPC = 8.8 Hz, C), 84.3 (d, 1JPC = 3.4 Hz, PC), 28.8 (d, 4JPC = 1.6 Hz, 6-CH2), 26.1 (3-CH2), 23.0 (d, 3JPC = 14.4 Hz, o-CH3Mes), 22.6 (5-CH2), 21.8 (4-CH2), 21.0 (p-CH3Mes). 31P{1H} NMR (202 MHz, dichloromethane-d2, 299 K): δ −56.5 (ν1/2 ≈ 3 Hz). Synthesis of Compound 10a. A solution of phosphane 9 (417.6 mg, 1.1 mmol, 1.0 equiv) in toluene (6 mL) was added to a solution of borane 6a (500.0 mg, 1.1 mmol, 1.0 equiv) in toluene (4 mL). The reaction mixture was stirred at 60 °C for 3 d, and then all volatiles were removed in vacuo. The resulting sticky residue was dissolved in pentane (3 mL), which was subsequently removed in vacuo. The obtained solid was washed with pentane (5 × 3 mL) to give compound 10a as a white powdery solid (822.0 mg, 1.0 mmol, 90%). Crystals suitable for the X-ray crystal structure analysis were obtained from a dichloromethane solution of compound 10a at −35 °C. Dec: 200 °C. Anal. Calcd for C46H38BF10P: C: 67.17; H: 4.66. Found: C: 66.78; H: 4.60. IR (KBr): ν̃ [cm−1] = 3696 (w), 3023 (m), 2929 (s), 2856 (m), 2347 (w), 1947 (w), 1723 (w), 1642 (s), 1604 (s), 1553 (w), 1515 (s), 1455 (m), 1380 (m), 1341 (w), 1287 (s), 1269 (m), 1248 (m), 1152 (w), 1091 (s), 1034 (m), 1024 (w), 996 (s), 852 (s), 805 (m), 773 (m), 741 (s), 696 (s), 643 (m), 629 (m), 607 (m), 575 (m), 553 (m), 499 (m), 453 (w), 418 (m). 1H NMR (600 MHz, dichloromethane-d2, 213 K): δ 7.28 (m, 2H, o-Ph), 7.23 (m, 2H, mPh), 7.18 (m, 1H, p-Ph), 7.14 (br d, 3JHH = 16.3 Hz, 1H, CH), 6.97 I


Article

Organometallics P{1H} NMR (202 MHz, dichloromethane-d2, 213 K): δ 10.9 (partial relaxed 1:1:1:1 q JPB ≈ 26 Hz). 31P{1H} NMR (202 MHz, dichloromethane-d2, 299 K): δ 12.8 (ν1/2 ≈ 37 Hz). 11B{1H} NMR (160 MHz, dichloromethane-d2, 213 K): δ −3.0 (ν1/2 ≈ 2400 Hz). 19F NMR (470 MHz, dichloromethane-d2, 213 K): δ −125.9 (m, o), −131.2 (m, o′), −158.7 (br t, 3JFF = 21.3 Hz, p), 164.7 (m, m), −164.9 (m, m′) (each 1F, C6F5) [Δ19Fm,p = 6.0, 6.2]; −128.8 (m, o), −130.7 (m, o′), −159.8 (br t, 3JFF = 17.0 Hz, p), −165.9 (m, m′), −166.1 (m, m) (each 1F, C6F5) [Δ19Fm,p = 6.1, 6.3]. Synthesis of Compound 11. A solution of compound 10a (515.2 mg, 626.3 μmol, 1.0 equiv) in toluene (5 mL) was heated at 80 °C for 3 d in a Schlenk flask. The volatiles were removed in vacuo, and the sticky residue was suspended in pentane (3 mL). The sticky solid was collected and washed with pentane (3 × 2 mL) to give compound 11 as a white solid (454.2 mg, 552.2 μmol, 88%). Crystals suitable for Xray crystal structure analysis were obtained by slow diffusion of pentane into a solution of compound 11 in dichloromethane at −35 °C. Mp: 130 °C. Anal. Calcd for C46H38BF10P: C: 67.17; H: 4.66. Found: C: 66.79; H: 5.02. IR (KBr): ν̃ [cm−1] = 3696 (w), 3027 (m), 2959 (m), 2934 (s), 2856 (m), 2797 (w), 2295 (w), 1642 (m), 1602 (s), 1556 (w), 1515 (s), 1460 (s), 1384 (m), 1283 (m), 1246 (m), 1204 (w), 1094 (s), 1040 (w), 1011 (w), 968 (s), 907 (w), 853 (s), 834 (w), 769 (m), 739 (s), 702 (s), 675 (m), 640 (m), 597 (w), 555 (s), 534 (w), 510 (w), 482 (w), 427 (m). A solution of the white solid in dichloromethane-d2 showed at 213 K a mixture of two isomers, 11 and 11′ [ratio ca. 77:23 (31P)]. Major component (11): 1H NMR (500 MHz, dichloromethane-d2, 213 K): δ 7.29 (m, 2H, m-Ph), 7.22 (m, 2H, o-Ph), 7.19 (m, 1H, p-Ph), 7.00 (d, 4JPH = 3.2 Hz, 1H, mMesa), 6.79 (m, 1H, m-Mesb), 6.78 (m, 1H, m′-Mesa), 6.51 (m, 1H, m′-Mesb), 5.88 (br, 1H, 3-CH), 4.14 (br d, J = 10.3 Hz, 1H, 4-CH), 2.62 (s, 3H, o-CH3Mes,b), 2.45 (s, 3H, o-CH3Mes,a), 2.34 (m, 1H, 5-CH), 2.26 (s, 3H, p-CH3Mes,a), 2.15 (s, 3H, p-CH3Mes,b), 2.09 (2H), 1.47 (1H), 1.07 (1H), 1.29 (1H), 0.63 (1H), 1.17 (1H), 0.81 (1H) (each m, CH2), 1.96 (s, 3H, o′-CH3Mes,b), 1.83 (s, 3H, o′-CH3Mes,a). 13C{1H} NMR (126 MHz, dichloromethane-d2, 213 K): δ 153.1 (br dm, J = 3.8 Hz, 10-C)t, 145.7 (br, 2-C)t, 143.8 (d, 2JPC = 19.1 Hz, o-Mesa), 142.6 (i-Ph)t, 142.6 (d, 2JPC = 6.5 Hz, o-Mesb), 142.0 (d, 2JPC = 6.5 Hz, o′Mesa), 141.7 (d, 2JPC = 14.9 Hz, o′-Mesb), 141.0 (d, 4JPC = 3.1 Hz, pMesa), 140.7 (d, 4JPC = 2.1 Hz, p-Mesb), 131.2 (d, 3JPC = 7.2 Hz, m′Mesa), 130.3 (d, 3JPC = 11.2 Hz, m-Mesa), 130.1 (d, 3JPC = 7.8 Hz, mMesb), 128.6 (o-Ph), 128.3 (d, 3JPC = 9.9 Hz, m′-Mesb), 127.8 (m-Ph), 126.0 (d, 1JPC = 53.5 Hz, 1-C), 125.8 (p-Ph), 124.7 (br d, J = 42.7 Hz 3-CH), 123.8 (d, 1JPC = 41.9 Hz, i-Mesb), 121.7 (d, 1JPC = 25.6 Hz, iMesa), 47.9 (d, 3JPC = 10.9 Hz, 5-CH), 45.6 (4-CH), 33.5 (dm, J = 6.8 Hz), 27.8, 27.6, 25.8 (CH2), 25.4 (d, 3JPC = 10.5 Hz, o-CH3Mes,a), 24.3 (d, 3JPC = 3.3 Hz, o′-CH3Mes,a), 22.9 (m, o-CH3Mes,b), 20.7 (m, o′CH3Mes,b), 20.6 (d, J = 1.4 Hz, p-CH3Mes,a), 20.3 (d, J = 1.1 Hz, pCH3Mes,b) [C6F5 not listed, t tentatively assigned]. 31P{1H} NMR (202 MHz, dichloromethane-d2, 213 K): δ 10.4 (ν1/2 ≈ 40 Hz). 11B{1H} NMR (160 MHz, dichloromethane-d2, 213 K): δ 3.6 (broad). 11B{1H} NMR (160 MHz, dichloromethane-d2, 299 K): δ 6.4 (ν1/2 ≈ 550 Hz). 19 F NMR (470 MHz, dichloromethane-d2, 213 K): δ −124.5, −126.0, −129.3, −130.1 (each br, each 1F, o-C6F5), −158.4 (br t, 3JFF = 19.7 Hz), −159.5 (br t, 3JFF = 20.8 Hz) (each 1F, p-C6F5), −164.6, −165.6 (each m, each 2F, m-C6F5). Minor component (11′): 1H NMR (500 MHz, dichloromethane-d2, 213 K): δ 7.20 (m, 2H, m-Ph), 7.16 (m, 2H, o-Ph), 7.14 (m, 1H, p-Ph), 7.01 (br, 1H, m-Mesa), 6.94 (br, 1H, m-Mesb), 6.80 (br, 1H, m′-Mesa), 6.47 (br, 1H, m′-Mesb), 5.92 (br, 1H, 3-CH), 3.47 (br m, 1H, 4-CH), 2.78 (m, 1H, 5-CH), 2.76 (s, 3H, o-CH3Mes,b), 2.27 (s, 3H, p-CH3Mes,a), 2.21 (s, 3H, o-CH3Mes,a), 2.18 (s, 3H, p-CH3Mes,b), 2.14 (1H), 1.61 (1H), 1.48 (1H), 1.46 (1H), 1.27 (1H), 1.14 (1H), 1.05 (1H), 0.94 (1H) (each m, CH2), 1.98 (s, 3H, o′-CH3Mes,b), 1.76 (s, 3H, o′-CH3Mes,a). 13C{1H} NMR (126 MHz, dichloromethane-d2, 213 K): δ 149.8 (br d, J = 0.4.2 Hz, 10-C), 144.2 (d, 2JPC = 20.0 Hz, o-Mesa), 142.8 (m, o-Mesb), 142.6 (i-Ph)t, 142.2 (m, o′-Mesb), 142.0 (m, p-Mesa), 141.9 (d, 2JPC = 6.5 Hz, o′-Mesa), 141.1 (m, p-Mesb), 131.7 (d, 3JPC = 8.2 Hz, m′-Mesa), 130.2 (d, 3JPC = 10.1 Hz, m-Mesa), 129.9 (d, 3JPC = 8.2 Hz, m-Mesb), 128.7 (o-Ph), 128.4 (d, 1JPC = 53.2 Hz, 1-C), 128.0 (m, m′-Mesb), 128.0 (m-Ph), 126.2 (br, 3-CH), 125.8 (p-Ph), 123.6 (d, 1JPC = 41.6 Hz, i-Mesb),

120.3 (d, 1JPC = 26.6 Hz, i-Mesa), 45.4 (br, 4-CH), 39.8 (d, 3JPC = 9.6 Hz, 5-CH), 31.7 (d, 3JPC = 8.2 Hz), 28.5, 25.2, 23.3 (CH2), 24.2 (d, 3 JPC = 3.3 Hz, o′-CH3Mes,a), 24.2 (d, 3JPC = 9.0 Hz, o-CH3Mes,a), 21.4 (m, o-CH3Mes,b), 20.7 (m, o′-CH3Mes,b), 20.6 (m, p-CH3Mes,a), 20.4 (pCH3Mes,b), n.o. 2-C [C6F5 not listed, t tentatively assigned]. 31P{1H} NMR (202 MHz, dichloromethane-d2, 213 K): δ 12.7 (partial relaxed br 1:1:1:1 q, 1JPB ∼ 15 Hz). 11B{1H} NMR (160 MHz, dichloromethane-d2, 213 K): δ 3.6 (broad). 11B{1H} NMR (160 MHz, dichloromethane-d2, 299 K): δ 6.4 (ν1/2 ≈ 550 Hz). 19F NMR (470 MHz, dichloromethane-d2, 213 K): δ −121.7, −127.4, −129.9, −130.1 (each m, each 1F, o-C6F5), −158.1 (br t, 3JFF = 19.8 Hz), −159.6 (t, 3 JFF = 20.2 Hz) (each 1F, p-C6F5), −164.5 (2F), −165.4 (1F), 165.5 (1F) (each m, m-C6F5). Synthesis of Compound 12. (2,2,6,6-Tetramethylpiperidin-1yl)oxyl (TEMPO) (152.0 mg, 972.6 μmol, 2.0 equiv) and compound 11 (400.0 mg, 486.3 μmol, 1.00 equiv) were dissolved in benzene (5 mL). The reaction mixture was stirred at 60 °C for 2 d in a Schlenk flask. The volatiles were removed in vacuo, and the sticky red residue was suspended in pentane (3 mL). The suspension was stored at −35 °C for 1 d for a complete precipitation. Washing the crude off-white product with cold pentane (5 × 2 mL) gave compound 12 as a white, fluffy solid (319.2 mg, 0.4 mmol, 80%). Crystals suitable for the X-ray crystal structure analysis were obtained by slow diffusion of pentane into a solution of compound 12 in dichloromethane at −35 °C. Mp: 136 °C. Anal. Calcd for C46H36BF10P: C: 67.33; H: 4.42. Found: C: 67.65; H: 5.17. IR (KBr): ν̃ [cm−1] = 3688 (m), 3016 (w), 3028 (w), 2935 (m), 2860 (m), 2773 (w), 2739 (w), 2473 (w), 2399 (w), 2348 (w), 2314 (w), 1962 (w), 1886 (w), 1812 (w), 1735 (w), 1642 (s), 1603 (s), 1559 (m), 1515 (s), 1464 (m), 1381 (m), 1324 (w), 1305 (m), 1270 (m), 1260 (m), 1201 (w), 1178 (m), 1160 (m), 1092 (s), 1030 (m), 962 (s), 894 (m), 851 (s), 771 (s), 704 (s), 678 (m), 637 (m), 574 (w), 554 (m), 509 (w), 473 (w), 440 (m), 408 (w). 1H NMR (500 MHz, dichloromethane-d2, 299 K): δ 7.42 (3H), 7.34 (3H) (each m, Ph, 3-CH), 6.81 (d, 4JPH = 3.2 Hz, 4H, m-Mes), 2.55 (2H), 2.37 (2H), 1.60 (4H) (each br m, CH2), 2.25 (s, 6H, p-CH3Mes), 2.09 (s, 12H, o-CH3Mes). 13C{1H} NMR (126 MHz, dichloromethaned2, 299 K): δ 156.6 (br d, 2JPC = 30.2 Hz, 2-C), 148.2, (d, J = 2.8 Hz) 138.9 (d, J = 0.9 Hz), 135.9 (d, J = 8.5 Hz) (4,5,10-C)t, 148.0 (dm, 1 JFC ≈ 240 Hz, C6F5), 142.7 (br d, 2JPC = 8.9 Hz, o-Mes), 142.5 (d, J = 1.9 Hz, i-Ph), 141.2 (br d, 4JPC = 2.9 Hz p-Mes), 140.2 (dm, 1JFC ≈ 250 Hz, C6F5), 137.0 (dm, 1JFC ≈ 250 Hz, C6F5), 134.8 (d, 1JPC = 53.5 Hz, 1-C)t, 131.5 (dm, J = 46.2 Hz, 3-CH), 130.9 (br d, 3JPC = 8.9 Hz, m-Mes), 129.5, 128.4, 127.3 (p) (Ph), 127.4 (br, i-C6F5), 124.5 (br dm, 1JPC = 35.1 Hz, i-Mes), 28.7 (d, J = 1.0 Hz), 27.9 (d, J = 4.1 Hz), 23.5, 22.4 (CH2), 22.8 (br dm, 3JPC = 5.4 Hz, o-CH3Mes), 20.8 (d, J = 1.4 Hz, p-CH3Mes) [t tentatively assigned]. 31P{1H} NMR (202 MHz, dichloromethane-d2, 299 K): δ 5.8 (ν1/2 ≈ 36 Hz). 11B{1H} NMR (160 MHz, dichloromethane-d2, 299 K): δ 5.7 (ν1/2 ≈ 500 Hz). 19F NMR (470 MHz, dichloromethane-d2, 299 K): δ −128.1 (br, 2F, oC6F5), −159.1 (t, 1F, 3JFF = 20.2 Hz, p-C6F5), −165.5 (m, 2F, m-C6F5) [Δ19Fmp = 6.4]. 1H NMR (500 MHz, dichloromethane-d2, 193 K): δ 7.39 (2H), 7.31 (4H) (each m, Ph, 3-CH), 6.94 (d, 4JPH = 2.3 Hz, 1H, m-Mesa), 6.85 (s, 1H, m′-Mesa), 6.66 (s, 1H, m-Mesb), 6.52 (d, 4JPH = 2.0 Hz, 1H, m′-Mesb), 2.55 (2H), 2.43 (1H), 1.97 (1H), 1.61 (1H), 1.52 (2H), 1.38 (1H) (each br m, CH2), 2.25 (s, 3H, p-CH3Mes,a), 2.09 (s, 3H, p-CH3Mes,b), 2.06 (s, 3H, o′-CH3Mes,b), 2.03 (s, 3H, o-CH3Mes,b), 1.96 (s, 3H, o-CH3Mes,a), 1.83 (s, 3H, o′-CH3Mes,a). 13C{1H} NMR (126 MHz, dichloromethane-d2, 193 K): δ 155.0 (br d, 2JPC = 30.6 Hz, 2-C), 146.8, 137.7, 134.4 (d, J = 8.4 Hz)(4,5,10-C)t, 143.3 (d, 2JPC = 18.3 Hz, o-Mesa), 142.1 (o′-Mesa), 141.2 (m, i-Ph), 141.2 (p-Mesa), 141.1 (d, 2JPC = 16.0 Hz, o′-Mesb), 140.5 (o-Mesb), 140.2 (m, p-Mesb), 133.7 (d, 1JPC = 54.2 Hz, 1-C)t, 131.4 (br d, 3JPC = 7.0 Hz, m′-Mesa), 130.6 (dd, J = 45.0 Hz, J = 12.0 Hz, 3-CH), 130.0 (br d, 3JPC = 10.7 Hz, m-Mesa), 129.7 (d, 3JPC = 7.4 Hz, m-Mesb), 128.8, 128.4, 127.8, 127.5, 126.6 (p) (each br, Ph), 128.1 (d, 3JPC = 9.4 Hz, m′-Mesb), 125.5 (d, 1JPC = 40.3 Hz, i-Mesb), 120.0 (d, 1JPC = 28.5 Hz, i-Mesa), 28.2, 27.2 (d, J = 3.7 Hz), 22.6, 21.5 (CH2), 24.7 (d, 3JPC = 3.6 Hz, o′CH3Mes,a), 22.9 (dd, J = 10.1 Hz, J = 4.0 Hz, o-CH3Mes,a), 21.0 (m, oCH3Mes,b), 20.7 (m, o′-CH3Mes,b), 20.5 (p-CH3Mes,a), 20.0 (p-CH3Mes,b) [C6F5 not listed; t tentatively assigned]. 31P{1H} NMR (202 MHz,

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Article

Organometallics dichloromethane-d2, 193 K): δ 4.1 (ν1/2 ≈ 40 Hz). 11B{1H} NMR (160 MHz, dichloromethane-d2, 193 K): δ 2.0 (broad). 19F NMR (470 MHz, dichloromethane-d2, 193 K): δ −126.2 (m, o), −130.7 (m, o′), −158.7 (br t, 3JFF = 21.4 Hz, p), −164.3 (m, m), −164.5 (m, m′) (each 1F, C6F5) [Δ19Fmp = 5.6, 5.8], −127.6 (m, 1F, o), −129.1 (m, 1F, o′), −157.8 (br t, 1F, 3JFF = 21.6 Hz, p), −165.5 (m, 2F, m,m′) (C6F5) [Δ19Fmp = 7.7]. Synthesis of Compound 13. Phenylacetylene (12.4 mg, 122 μmol, 1.0 equiv) and compound 11 (100 mg, 122 μmol, 1.0 equiv) were dissolved in toluene (4 mL) and then heated for 16 h at 80 °C. All volatiles were removed in vacuo, and the yellow oil was suspended in pentane (2 mL). The collected solid was washed with pentane (3 × 2 mL). After drying in vacuo compound 13 was obtained as a white solid (43.8 mg, 47.4 μmol; 39%). Crystals suitable for the X-ray crystal structure analysis were obtained from a dichloromethane solution of compound 13 at −35 °C. Mp: 182 °C. Anal. Calcd for C54H44BF10P: C: 70.14; H: 4.80. Found: C: 68.60; H: 4.56. IR (KBr): ν̃ [cm−1] = 3060 (w), 3027 (w), 2931 (w), 2361 (w) 1945 (w), 1747 (w), 1639 (w), 1603 (w), 1547 (w), 1511 (m), 1452 (s), 1382 (w), 1296 (w), 1269 (m), 1248 (m), 1154 (w), 1083 (s), 1033 (w), 966 (s), 910 (w), 854 (w), 784 (w), 757 (m), 738 (w), 696 (m), 645 (m), 605 (w), 558 (w), 490 (w), 421 (w). 1H NMR (500 MHz, dichloromethane-d2, 253 K): δ 9.70 (d, 1JPH = 495.9 Hz, PH), 7.31 (m, 2H, o-Ph), 7.30 (m, 2H, m-Ph), 7.24 (m, 2H, m-Ph), 7.21 (m, 1H, p-Ph), 7.20 (m, 1H, p-Ph), 7.15 (m, 2H, o-Ph), 6.91 (br d, 3JPH = 4.1 Hz, 1H, m-Mesa), 6.84 (br d, 3JPH = 4.4 Hz, 1H, m-Mesb), 6.82 (br d, 3JPH = 3.8 Hz, 1H, m′-Mesa), 6.77 (br d, 3JPH = 4.6 Hz, 1H, m′-Mesb), 5.90 (br, 1H, 3CH), 3.82 (dm, 3JHH = 8.8 Hz, 1H, 4-CH), 2.66 (s, 3H, o-CH3Mes,b), 2.65 (s, 3H, o-CH3Mes,a), 2.52/2.04, 1.72/1.24, 1.63/1.17, 1.47/0.95 (each m, each 1H, CH2), 2.34 (m, 1H, 5-CH), 2.26 (s, 3H, pCH3Mes,b), 2.23 (s, 3H, p-CH3Mes,a), 1.89 (s, 3H, o′-CH3Mes,b), 1.77 (s, 3H, o′-CH3Mes,a). 13C{1H} NMR (126 MHz, dichloromethane-d2, 253 K): δ 168.4 (d, 2JPC = 7.7 Hz, 10-C)t, 144.9 (d, 2JPC = 9.2 Hz, o-Mesb), 144.8 (d, 4JPC = 2.8 Hz, p-Mesa), 144.2 (d, 4JPC = 2.8 Hz, p-Mesb), 143.4 (d, 2JPC = 11.0 Hz, o-Mesa), 142.8 (d, 2JPC = 9.4 Hz, o′-Mesa), 142.4 (i-Ph), 142.2 (d, 2JPC = 10.9 Hz, o′-Mesb), 134.6 (br dm, J = 14.2 Hz, 3-CH), 132.3 (d, 3JPC = 11.3 Hz, m′-Mesa), 131.6 (d, 3JPC = 11.5 Hz, m-Mesb), 131.2 (o-Ph), 130.6 (d, 3JPC = 10.8 Hz, m′-Mesb), 130.4 (d, 3JPC = 10.7 Hz, m-Mesa), 129.0 (o-Ph), 128.2 (m-Ph), 128.1 (m-Ph), 126.9 (i-Ph), 126.5 (p-Ph), 126.2 (p-Ph), 118.9 (d, 1JPC = 77.8 Hz, i-Mesa), 115.5 (d, 1JPC = 83.4 Hz, i-Mesb), 113.1 (d, 1JPC = 73.1 Hz, 1-C)t, 110.3 (br 1:1:1:1 q, 1JCB ∼ 80 Hz, BC), 98.7 (br, PhC), 51.3 (d, 3JPC = 12.4 Hz, 5-CH), 43.0 (4-CH), 35.0 (d, 3JPC = 9.8 Hz), 31.7, 29.2, 26.3 (CH2), 23.8 (d, 3JPC = 10.7 Hz, o-CH3Mes,a), 22.3 (d, 3JPC = 5.8 Hz, o′-CH3Mes,a), 21.5 (d, 3JPC = 4.4 Hz, o-CH3Mes,b), 21.1 (p-CH3Mes,a), 20.9 (p-CH3Mes,b), 20.4 (d, 3JPC = 10.4 Hz, o′CH3Mes,b), n.o. (2-C) [C6F5 not listed; t tentatively assigned]. 31P NMR (202 MHz, dichloromethane-d2, 253 K): δ −29.3 (d, 1JPH = 496.6 Hz). 31P{1H} NMR (202 MHz, dichloromethane-d2, 253 K): δ −29.3 (ν1/2 ≈ 15 Hz). 11B{1H} NMR (160 MHz, dichloromethane-d2, 253 K): δ −17.7 (ν 1/2 ≈ 50 Hz). 19 F NMR (470 MHz, dichloromethane-d2, 253 K): δ −129.2 (br, 2F, o), −162.5 (t, 3JFF = 20.8 Hz, 1F, p), −166.5 (m, 2F, m) (C6F5) [Δ19Fmp = 3.2], −129.6 (m, 2F, o), −161.0 (t, 3JFF = 20.7 Hz, 1F, p), −165.9 (m, 2F, m) (C6F5) [Δ19Fmp = 4.9]. Synthesis of Compound 14. A yellow solution of compound 12 (200.0 mg, 243.7 μmol, 1.0 equiv) in CH2Cl2 (2 mL) was degassed at −78 °C and then exposed to a dihydrogen atmosphere (1.5 bar) at room temperature. The solution was stirred for 24 h at ambient temperature, before the volatiles were removed in vacuo and the offwhite residue was washed with pentane (3 × 2 mL). After drying in vacuo, compound 14 was obtained as a white solid (175.7 mg, 213.6 μmol, 88%). Crystals suitable for the X-ray crystal structure analysis were obtained from a solution of compound 14 in dichloromethane at −35 °C. Dec: 167 °C. Anal. Calcd for C46H38BF10P: C: 67.17; H: 4.66. Found: C: 67.36; H: 5.28. IR (KBr): ν̃ [cm−1] = 3418 (w), 3316 (w), 3055 (w), 3028 (w), 2941 (w), 2854 (w), 2443 (w), 2360 (w), 1939 (w), 1775 (w), 1639 (m), 1604 (m), 1559 (w), 1509 (s), 1461 (s), 1380 (m), 1332 (w), 1293 (w), 1271 (m), 1206 (w), 1176 (w), 1134 (w), 1083 (s), 1030 (m), 967 (s), 905 (m), 857 (m), 768 (m), 741

(w), 698 (m), 678 (w), 647 (m), 617 (w), 575 (w), 555 (m), 512 (w), 488 (w), 429 (m). 1H NMR (600 MHz, dichloromethane-d2, 299 K): δ 9.48 (d, 1JPH = 507.3 Hz, 1H, PH), 7.38 (m, 2H, m-Ph), 7.31 (m, 1H, p-Ph), 7.24 (m, 2H, o-Ph), 7.22 (d, 4JPH = 5.5 Hz, 1H, 3-CH), 7.07 (d, 4JPH = 4.4 Hz, 1H, m-Mesa), 7.00 (d, 4JPH = 4.6 Hz, 1H, mMesb), 6.96 (d, 1H, 4JPH = 3.5 Hz, m′-Mesa), 6.81 (d, 4JPH = 4.4 Hz, 1H, m′-Mesb), 3.90 (partial relaxed 1:1:1:1 q, 1JBH ≈ 85 Hz, 1H, BH), 2.67/2.20 (each m, each 1H, 9-CH2)t, 2.54 (m, 2H, 6-CH2)t, 2.34 (s, 3H, p-CH3Mes,a), 2.32 (s, 3H, p-CH3Mes,b), 2.31 (s, 3H, o-CH3Mes,a), 2.23 (s, 3H, o-CH3Mes,b), 2.14 (s, 3H, o′-CH3Mes,b), 1.91 (s, 3H, o′CH3Mes,a), 1.69 (1H), 1.50 (2H), 1.39 (1H) (each m, 7,8-CH2)t [t tentatively assigned]. 13C{1H} NMR (151 MHz, dichloromethane-d2, 299 K): δ 146.5 (d, 4JPC = 3.9 Hz, 4-C), 145.2 (d, 4JPC = 2.8 Hz, pMesb), 144.9 (d, 4JPC = 3.0 Hz, p-Mesa), 144.4 (d, 2JPC = 10.1 Hz, o′Mesb), 143.6 (d, 2JPC = 11.8 Hz, o-Mesa), 143.4 (d, 2JPC = 8.4 Hz, o′Mesa), 142.7 (d, 2JPC = 9.9 Hz, o-Mesb), 142.0 (d, J = 0.9 Hz, i-Ph), 141.6 (d, 2JPC = 14.2 Hz, 10-C)t, 137.4 (br d, 3JPC = 16.8 Hz, 3-CH), 134.6 (d, 3JPC = 12.4 Hz, 5-C)t, 132.6 (d, 3JPC = 11.0 Hz, m′-Mesa), 132.1 (d, 3JPC = 11.5 Hz, m′-Mesb), 131.6 (d, 3JPC = 10.6 Hz, m-Mesa), 130.8 (d, 3JPC = 10.5 Hz, m-Mesb), 129.4 (o-Ph), 128.4 (m-Ph), 127.3 (p-Ph), 120.5 (d, 1JPC = 83.1 Hz, 1-C), 117.1 (d, 1JPC = 76.3 Hz, iMesa), 116.2 (d, 1JPC = 80.9 Hz, i-Mesb), 32.1 (d, 3JPC = 7.7 Hz, 9CH2)t, 28.1 (d, 4JPC = 1.6 Hz, 6-CH2)t, 22.5 (7,8-CH2)t, 22.4 (d, 3JPC = 5.2 Hz, o′-CH3Mes,a), 21.4 (d, J = 1.4 Hz, p-CH3Mes,a), 21.3 (d, 3JPC = 11.5 Hz, o-CH3Mes,a), 21.2 (p-CH3Mes,b), 21.1 (br d, 3JPC = 6.1 Hz, o′CH3Mes,b), 21.0 (d, 3JPC = 11.9 Hz, o-CH3Mes,b), n.o. (2-C) [C6F5 not listed; t tentatively assigned]. 31P NMR (243 MHz, dichloromethaned2, 299 K): δ −28.5 (br d, 1JPH ≈ 508 Hz). 31P{1H} NMR (243 MHz, dichloromethane-d2, 299 K): δ −28.5 (m). 11B NMR (192 MHz, dichloromethane-d2, 299 K): δ −20.8 (d, 1JBH ≈ 83 Hz). 11B{1H} NMR (192 MHz, dichloromethane-d2, 299 K): δ −20.8 (ν1/2 ≈ 55 Hz). 19F NMR (564 MHz, dichloromethane-d2, 299 K): δ −130.9 (m, 2F, o), −163.2 (t, 3JFF = 20.1 Hz, 1F, p), −166.9 (m, 2F, m) (C6F5) [Δ19Fmp = 3.7], −133.0 (m, 2F, o), −162.7 (t, 3JFF = 20.1 Hz, 1F, p), −166.3 (m, 2F, m) (C6F5) [Δ19Fmp = 3.6]. Generation of Compound 14-D2. A yellow solution of compound 12 (20.0 mg, 24.4 μmol, 1.0 equiv) in CD2Cl2 (0.5 mL) was degassed at −78 °C using a J-Young NMR tube. Then the solution was exposed to a deuterium gas atmosphere (1.5 bar). After shaking the tube for 16 h at room temperature, the reaction mixture was characterized by NMR experiments. A mixture of compounds 12 and 14-D2 (12:14-D2 ≈ 54:46 (31P{1H}) was observed). The NMR data of compounds 14-D2 and 12 are consistent with those listed for isolated compounds 14 and 12 (see above). Compound 14-D2: 2H NMR (92 MHz, dichloromethane, 299 K): δ 9.46 (d, 1JPD = 77.1 Hz, 1D, PD), 3.88 (br, 1D, BD). 31P NMR (243 MHz, dichloromethane-d2, 299 K): δ −28.6 (br 1:1:1 t, 1JPD ≈ 79 Hz). 31P{1H} NMR (243 MHz, dichloromethane-d2, 299 K): δ −28.6 (br 1:1:1 t, 1JPD ≈ 79 Hz). 11B NMR (192 MHz, dichloromethane-d2, 299 K): δ −21.0 (ν1/2 ≈ 70 Hz). 11B{1H} NMR (192 MHz, dichloromethane-d2, 299 K): δ −21.0 (ν1/2 ≈ 70 Hz). 19F NMR (564 MHz, dichloromethane-d2, 299 K): δ −131.0, −133.0 (each m, each 2F, o), −162.7, −163.3 (each t, 3JFF = 20.1 Hz, each 1F, p), −166.3, −166.9 (each m, each 2F, m) (C6F5). Synthesis of Compound 17. Dimethyl acetylenedicarboxylate (17.9 μL, 20.7 mg, 145.9 μmol, 1.0 equiv) and the P/B-FLP 11 (120.0 mg, 145.9 μmol, 1.0 equiv) were dissolved in toluene (3 mL). The orange reaction mixture was stirred at 75 °C for 6 d in a Schlenk flask. The volatiles were removed in vacuo, and the sticky residue was suspended in pentane (5 mL). The suspension was filtered via cannula (Whatman glass fiber filter). The solid was washed with pentane (5 × 3 mL) and dried in vacuo to give compound 17 as a pale yellow solid (70.8 mg, 73.4 μmol, 50%). Crystals suitable for X-ray crystal structure analysis were obtained from a dichloromethane solution of compound 17 at room temperature. Mp: 259 °C. Anal. Calcd for C52H44BF10O4P: C: 64.74; H: 4.60. Found: C: 64.99; H: 4.73. IR (KBr): ν̃ [cm−1] = 3448 (w), 3193 (w), 3121 (w), 3604 (w), 3029 (m), 2991 (m), 2942 (m), 2862 (m), 2788 (w), 2735 (w), 2688 (w), 2390 (w), 2087 (w), 1946 (w), 1872 (w), 1792 (w), 1736 (s), 1683 (w), 1643 (m), 1605 (m), 1558 (w), 1515 (m),1451 (s), 1041 (w), 1377 (m), 1341 (m), 1276 (m), 1248 (m), 1206 (m), 1180 (w), 1148 (w), 1088 (s), 1030 K


Article

Organometallics

CH2)t, 1.58 (s, 3H, CH3), 1.51 (m, 2H, 8-CH2)t [t tentatively assigned]. 13C{1H} NMR (126 MHz, dichloromethane-d2, 299 K): δ 174.6 (d, 3JPC = 8.6 Hz, OC), 162.6 (d, 2JPC = 16.3 Hz, O2C), 154.9 (d, 2JPC = 22.4 Hz, 2-CAr), 146.0 (d, 2JPC = 9.8 Hz, o-Mes)t, 145.7 (d, 4JPC = 2.6 Hz, 4-CAr), 145.4 (d, 2JPC = 12.9 Hz, o′-Mes)t, 144.3 (d, 4JPC = 3.0 Hz, p-Mes), 143.9 (dd, J = 3.4 Hz, J = 1.1 Hz, 4C)t, 142.1 (d, J = 1.2 Hz, i-Ph), 142.0 (d, 2JPC = 9.9 Hz, 6-CAr), 139.8 (dd, J = 18.5 Hz, J = 9.8 Hz, 3-CH), 135.8 (d, 1JPC = 80.8 Hz, 1-C), 134.8 (d, 2JPC = 11.3 Hz, 10-C)t, 134.6 (d, 3JPC = 11.5 Hz, 5-C)t, 133.2 (d, 3JPC = 10.2 Hz, 5-CHAr), 131.7 (d, 3JPC = 6.2 Hz, m′-Mes), 131.6 (d, 3JPC = 5.1 Hz, m-Mes), 129.4 (o-Ph), 128.4 (m-Ph), 127.2 (p-Ph), 124.6 (d, 1JPC = 84.2 Hz, 1-CAr), 123.5 (d, 3JPC = 10.0 Hz, 3-CHAr), 116.3 (d, 1JPC = 82.8 Hz, i-Mes), 77.8 (d, 1JPC = 115.4 Hz, PC), 55.7 (d, 2JPC = 15.2 Hz, CMe), 53.7 (d, 4JPC = 1.2 Hz, OMe), 52.4 (OMeCO), 29.4 (d, 3JPC = 7.1 Hz, 9-CH2)t, 28.7 (d, 4JPC = 1.6 Hz, 6CH2)t, 28.2 (CH3), 24.2 (dd, J = 5.8 Hz, J = 3.3 Hz o′-Mes)t, 23.9 (dd, J = 7.0 Hz, J = 1.5 Hz, o-Mes)t, 23.0 (8-CH2)t, 22.3 (7-CH2)t, 21.7 (d, J = 1.3 Hz, 4-CH3Ar), 20.9 (d, 3JPC = 3.4 Hz, 6-CH3Ar), 20.7 (p-Mes), n.o. (2-C) [C6F5 not listed; t tentatively assigned]. 31P{1H} NMR (202 MHz, dichloromethane-d2, 213 K): δ 15.1 (ν1/2 ≈ 5 Hz). 11B{1H} NMR (160 MHz, dichloromethane-d2, 299 K): δ 1.1 (ν1/2 ≈ 350 Hz). 19 F NMR (470 MHz, dichloromethane-d2, 299 K): δ −128.5 (m, o), −136.8 (m, o′), −160.0 (t, 3JFF = 20.3 Hz, p), −164.9 (m, m′), −165.5 (m, m) (each 1F, C6F5) [Δ19Fmp = 4.9, 5.5], −128.8 (m, o), −132.6 (m, o′), −162.8 (t, 3JFF = 20.4 Hz, p), −166.2 (m, m′), −168.6 (m, m) (each 1F, C6F5) [Δ19Fmp = 3.4, 5.8]. Synthesis of Compound 19. A solution of compound 11 (200.0 mg, 243.1 μmol, 1.0 equiv) in CH2Cl2 (1 mL) was degassed at −78 °C and exposed to a nitric oxide atmosphere (1.5 bar). The solution was stirred for 15 min, all volatiles were removed in vacuo, and the residue was suspended in pentane (3 mL). The solid was collected and washed with pentane (3 × 2 mL). After drying in vacuo compound 19 was obtained as a pale turquoise solid (166.4 mg, 195.2 μmol, 80%). Crystals suitable for the X-ray crystal structure analysis were obtained by slow diffusion of pentane into a solution of compound 19 in dichloromethane at −35 °C. Dec: 168 °C. Anal. Calcd for C46H38BF10PNO: C: 64.80; H: 4.49; N: 1.64. Found: C: 64.63; H: 4.49; N: 1.69. IR (KBr): ν̃ [cm−1] = 3515 (w), 3063 (w), 3032 (w), 2937 (m), 2862 (w), 2811 (w), 2349 (w), 2297 (w), 1739 (w), 1644 (s), 1604 (m), 1577 (w), 1553 (w), 1515 (s), 1453 (s), 1382 (m), 1318 (w), 1280 (m), 1249 (w), 1183 (w), 1149 (w), 1098 (s), 1033 (w), 974 (s), 926 (w), 853 (m), 817 (w), 794 (w), 772 (w), 743 (w), 703 (s), 646 (s), 599 (w), 567 (m), 510 (w), 468 (w). Synthesis of Compound 20. 1,4-Cyclohexadiene (131.6 mg, 1.64 mmol, 10.0 equiv) was added to a turquoise solution of compound 19 (140.0 mg, 164.2 μmol, 1.0 equiv) in benzene (2 mL) at room temperature. The solution became colorless upon stirring for 1 h. The volatiles were removed in vacuo, and the off-white residue was recrystallized by the layering method with CH2Cl2 (1 mL) and pentane (7 mL). The crystalline solid was ground and washed with pentane (3 × 1 mL). After drying in vacuo, compound 20 was obtained as a white solid (88.4 mg, 103.6 μmol, 63%). Crystals suitable for the X-ray crystal structure analysis were obtained by slow evaporation from a dichloromethane solution of compound 20. Dec: 214 °C. Anal. Calcd for C46H39BF10PNO: C: 64.73; H: 4.61; N: 1.64. Found: C: 64.71; H: 4.50; N: 1.58. IR (KBr): ν̃ [cm−1] = 3515 (m), 3061 (w), 3033 (w), 2936 (m), 2859 (w), 2816 (w), 2733 (w), 2340 (w), 1946 (w), 1640 (m), 1603 (m), 1590 (w), 1554 (w), 1512 (s), 1456 (s), 1406 (w), 1385 (w), 1340 (w), 1276 (m), 1246 (w), 1207 (w), 1162 (w), 1139 (w), 1084 (s), 1040 (m), 967 (s), 927 (w), 892 (w), 852 (m), 803 (w), 779 (w), 754 (w), 741 (w), 703 (m), 689 (m), 645 (s), 599 (w), 572 (m), 510 (w), 473 (w), 452 (w). 1H NMR (500 MHz, dichloromethane-d2, 299 K): δ 7.33 (m, 2H, m-Ph), 7.29 (m, 2H, oPh), 7.23 (m, 1H, p-Ph), 7.00 (d, 4JPH = 4.3 Hz, 2H, m-Mesa), 6.92 (d, 4 JPH = 4.6 Hz, 2H, m-Mesb), 6.33 (br, 1H, 3-CH), 4.63 (t, J = 9.1 Hz, 1H, OH), 3.89 (br dd, 3JHH = 9.5 Hz, J = 2.0 Hz, 1H, 4-CH), 2.60/ 2.13 (each m, each 1H, 9-CH2), 2.37 (s, 6H, o-CH3Mes,a), 2.36 (s, 3H, p-CH3Mes,a), 2.33 (m, 1H, 5-CH), 2.29 (s, 3H, p-CH3Mes,b), 2.20 (s, 6H, o-CH3Mes,b), 1.56/1.13 (each m, each 1H, 6-CH2), 1.53/1.14 (each m, each 1H, 7-CH2), 1.27/0.46 (each m, each 1H, 8-CH2). 13C{1H}

(m), 972 (s), 926 (w), 859 (m), 801 (w), 783 (m), 741 (m), 691 (s), 658 (m), 642 (s), 616 (w), 654 (m), 525 (w), 460 (w), 435 (w). A solution of the yellow solid in dichloromethane-d2 showed at 299 K a mixture of two compounds. The major component was assigned as compound 17, and the minor one has not been identified yet [ratio ca. 64:36 (31P)] [Ar = 4,6-dimethylphenylene]. Major component 17: 1H NMR (600 MHz, dichloromethane-d2, 299 K): δ 7.23 (m, 2H, m-Ph), 7.17 (m, 1H, p-Ph), 7.11 (m, 1H, 3-CHAr), 7.00 (m, 2H, o-Ph), 6.97 (br d, 4JPH = 5.5 Hz, 1H, 5-CHAr), 6.75 (br d, 4JPH = 2.7 Hz, 1H, mMes), 6.57 (br d, 4JPH = 3.9 Hz, 1H, m′-Mes), 5.86 (m, 1H, 3-CH), 3.95 (dm, 3JHH = 10.8 Hz, 1H, 4-CH), 3.68 (s, 3H, OMeCO), 3.49 (s, 3H, OMe), 2.72 (s, 3H, o′-CH3Mes), 2.55/2.06 (each m, each 1H, 9CH2), 2.35 (s, 3H, 4-CH3Ar), 2.35 (m, 1H, 5-CH), 2.25 (s, 3H, oCH3Mes), 2.20 (s, 3H, p-CH3Mes), 2.07 (s, 3H, 6-CH3Ar), 1.91 (s, 3H, CH3), 1.53, 1.16 (each m, each 1H, 7-CH2), 1.33/1.02 (each m, each 1H, 6-CH2), 1.30/0.94 (each m, each 1H, 8-CH2). 13C{1H} NMR (151 MHz, dichloromethane-d2, 299 K): δ 174.7 (d, 3JPC = 6.9 Hz, OC), 165.8 (d, 2JPC = 18.3 Hz, O2C), 154.6 (d, 2JPC = 21.8 Hz, 2CAr), 148.4 (d, 2JPC = 6.7 Hz, 10-C), 145.9 (d, 2JPC = 10.0 Hz, o-Mes), 145.7 (d, 4JPC = 2.7 Hz, 4-CAr), 145.2 (d, 2JPC = 12.7 Hz, o′-Mes), 144.1 (d, 4JPC = 2.9 Hz, p-Mes), 142.8 (i-Ph), 141.8 (d, 2JPC = 10.3 Hz, 6-CAr), 140.3 (br, 2-C), 138.6 (br, 3-CH), 133.0 (d, 3JPC = 10.3 Hz, 5CHAr), 132.1 (d, 3JPC = 12.3 Hz, m′-Mes), 131.9 (d, 3JPC = 11.0 Hz, mMes), 129.1 (o-Ph), 128.5 (m-Ph), 126.5 (p-Ph), 124.6 (d, 1JPC = 81.6 Hz, 1-C), 123.9 (d, 1JPC = 86.5 Hz, 1-CAr), 123.6 (d, 3JPC = 10.2 Hz, 3CHAr), 117.0 (d, 1JPC = 82.0 Hz, i-Mes), 71.8 (d, 1JPC = 116.9 Hz, PC), 56.7 (d, 2JPC = 15.3 Hz, CMe), 53.7 (OMe), 52.4 (OMeCO), 48.4 (d, 3JPC = 13.0 Hz, 5-CH), 43.8 (d, 4JPC = 2.1 Hz, 4-CH), 33.5 (d, 3 JPC = 9.4 Hz, 9-CH2), 29.7 (CH3), 29.5 (m, 6-CH2), 28.6 (d, J = 1.5 Hz, 8-CH2), 26.6 (7-CH2), 24.9 (m, o′-CH3Mes), 23.8 (d, 3JPC = 6.8 Hz, o-CH3Mes), 21.7 (d, J = 1.3 Hz, 4-CH3Ar), 20.9 (d, 3JPC = 3.7 Hz, 6CH3Ar), 20.7 (p-CH3Mes), [C6F5 not listed]. 31P{1H} NMR (243 MHz, dichloromethane-d2, 299 K): δ 15.9 (ν1/2 ≈ 4 Hz). 11B{1H} NMR (192 MHz, dichloromethane-d2, 299 K): δ 0.8 (ν1/2 ≈ 330 Hz). 19F NMR (564 MHz, dichloromethane-d2, 299 K): δ −127.2 (m, o), −137.6 (m, o′), −160.9 (t, 3JFF = 20.2 Hz, p), −165.4 (m, m′), −166.1 (m, m) (each 1F, C6F5) [Δ19Fmp = 4.5, 5.1], −130.4 (m, o), −133.2 (m, o′), −162.6 (t, 3JFF = 20.4 Hz, p), −166.3 (m, m′), −168.2 (m, m) (each 1F, C6F5) [Δ19Fmp = 3.7, 5.5]. Minor component: 31P{1H} NMR (243 MHz, dichloromethane-d2, 299 K): δ 16.5 (ν1/2 ≈ 30 Hz). 11 1 B{ H} NMR (192 MHz, dichloromethane-d2, 299 K): δ 0.8 (ν1/2 ≈ 330 Hz). 19F NMR (564 MHz, dichloromethane-d2, 299 K): δ −127.0, −129.7, −132.3, −136.9 (each br, each 1F, o), −160.5, −162.9 (each br, each 1F, p), not resolved (4F, m) (C6F5). Synthesis of Compound 18. Dimethyl acetylenedicarboxylate (18.0 μL, 20.8 mg, 146.2 μmol, 1.0 equiv) and the P/B-FLP 12 (120.0 mg, 146.2 μmol, 1.0 equiv) were dissolved in toluene (4 mL). The reaction mixture was heated at 100 °C for 6 d in a Schlenk flask. The volatiles were removed in vacuo, and the sticky residue was suspended in pentane (5 mL). The suspension was filtered via cannula (Whatman glass fiber filter). The solid was washed with pentane (5 × 3 mL) and dried in vacuo to give compound 18 as a pale orange solid (87.7 mg, 91.1 μmol, 62%). Crystals suitable for the X-ray crystal structure analysis were obtained from a dichloromethane solution of compound 18 at −35 °C. Mp: 268 °C. Anal. Calcd for C52H42BF10O4P: C: 64.88; H: 4.40. Found: C: 64.91; H: 4.59. IR (KBr): ν̃ [cm−1] = 3473 (w), 3027 (w), 2938 (m), 2864 (w), 2668 (w), 2540 (w), 2293 (w), 2092 (w), 1744 (s), 1613 (s), 1561 (w), 1515 (s), 1450 (s), 1377 (w), 1338 (m), 1244 (m), 1024 (w), 1150 (w), 1088 (s), 1030 (w), 974 (s), 926 (w), 875 (w), 854 (m), 814 (w), 778 (m), 764 (w), 747 (m), 710 (w), 690 (m), 662 (w), 641 (w), 604 (w), 563 (w), 521 (w), 459 (w), 444 (w), 419 (w). [Ar = 4,6-dimethylphenylene]. 1H NMR (500 MHz, dichloromethane-d2, 299 K): δ 7.34 (m, 2H, m-Ph), 7.27 (m, 1H, pPh), 7.16 (m, 2H, o-Ph), 7.13 (dd, J = 8.1 Hz, J = 5.3 Hz, 1H, 3-CH), 7.04 (m, 1H, 3-CHAr), 7.03 (m, 1H, 5-CHAr), 6.84 (dm, 4JPH = 3.6 Hz, 1H, m-Mes), 6.42 (dm, 4JPH = 4.6 Hz, 1H, m′-Mes), 3.66 (s, 3H. OMeCO), 3.38 (s, 3H, OMe), 2.74/2.28 (each m, each 1H, 9-CH2)t, 2.55/2.50 (each m, each 1H, 6-CH2)t, 2.39 (d, J = 0.8 Hz, 3H, 4CH3Ar), 2.34 (s, 3H, o-CH3Mes)t, 2.20 (s, 3H, p-CH3Mes), 2.09 (s, 3H, 6-CH3Ar), 1.90 (s, 3H, o′-CH3Mes)t, 1.59/1.43 (each m, each 1H, 7L


Article

Organometallics NMR (126 MHz, dichloromethane-d2, 299 K): δ 159.5 (d, 2JPC = 12.8 Hz, 10-C), 144.0 (d, 4JPC = 3.0 Hz, p-Mesb), 143.8 (d, 2JPC = 11.4 Hz, o-Mesb), 143.0 (d, 4JPC = 2.9 Hz, p-Mesa), 142.9 (i-Ph), 142.0 (br, 2C), 141.6 (d, 2JPC = 10.4 Hz, o-Mesa), 132.7 (d, 3JPC = 11.7 Hz, mMesb), 132.3 (d, 3JPC = 11.7 Hz, m-Mesa), 129.3 (o-Ph), 129.2 (br m, 3-CH), 128.6 (m-Ph), 126.6 (p-Ph), 124.9 (d, 1JPC = 86.3 Hz, i-Mesa), 122.5 (d, 1JPC = 90.0 Hz, i-Mesb), 120.4 (d, 1JPC = 99.3 Hz, 1-C), 50.5 (d, 3JPC = 13.3 Hz, 5-CH), 44.1 (4-CH), 35.6 (d, 3JPC = 7.3 Hz, 9CH2), 30.2 (br d, 4JPC = 1.9 Hz, 8-CH2), 30.0 (br, 6-CH2), 26.9 (7CH2), 24.2 (d, 3JPC = 4.2 Hz, o-CH3Mes,a), 23.8 (d, 3JPC = 5.4 Hz, oCH3Mes,b), 21.2 (d, J = 1.9 Hz, p-CH3Mes,b), 21.2 (d, J = 1.3 Hz, pCH3Mes,a) [C6F5 not listed]. 31P{1H} NMR (202 MHz, dichloromethane-d2, 299 K): δ 28.7 (ν1/2 ≈ 25 Hz). 11B{1H} NMR (160 MHz, dichloromethane-d2, 299 K): δ −6.6 (ν1/2 ≈ 130 Hz). 19F NMR (470 MHz, dichloromethane-d2, 299 K): δ −128.7 (m, 2F, o), −161.5 (t, 3 JFF = 20.3 Hz, 1F, p), −165.9 (m, 2F, m) (C6F5) [Δ19Fmp = 4.4], −136.8 (m, 2F, o), −162.6 (t, 3JFF = 20.3 Hz, 1F, p), −165.3 (m, 2F, m) (C6F5) [Δ19Fmp = 2.7]. Synthesis of Compound 21. Analogously to the synthesis of the P/B FLPNO• nitroxide radical 19 a solution of compound 12 (100.0 mg, 121.9 μmol, 1.0 equiv) in CH2Cl2 (1 mL) was degassed at −78 °C and exposed to a nitric oxide atmosphere (1.5 bar). After stirring for 15 min and workup P/B FLPNO• nitric oxide radical 21 was obtained as a pale green solid (87.4 mg, 102.8 μmol, 84%). Crystals suitable for the X-ray crystal structure analysis were obtained by slow diffusion of pentane into a solution of compound 21 in dichloromethane at −35 °C. Dec: 193 °C. Anal. Calcd for C46H36BF10PNO: C: 64.96; H: 4.27; N: 1.65. Found: C: 65.34; H: 4.28; N: 1.31. IR (KBr): ν̃ [cm−1] = 3518 (w), 3455 (w), 3031 (w), 2940 (m), 2864 (w), 2741 (w), 2624 (w), 2386 (w), 2348 (w), 2302 (w), 1732 (w), 1642 (m), 1604 (m), 1554 (w), 1515 (s), 1459 (s), 1382 (m), 1322 (w), 1283 (m), 1253 (m), 1205 (w), 1178 (w), 1157 (w), 1103 (s), 1029 (w), 974 (s), 926 (w), 881 (w), 850 (m), 836 (m), 809 (w), 772 (m), 748 (w), 702 (m), 659 (m), 643 (m), 609 (w), 571 (w), 515 (w), 478 (w), 455 (w), 429 (w). Synthesis of Compound 22. 1,4-Cyclohexadiene (75.4 mg, 94.6 μmol, 10.0 equiv) was added to a green solution of compound 21 (80.0 mg, 94.1 μmol, 1.0 equiv) in benzene (2 mL) at room temperature. The solution became colorless upon stirring for 1 h. The volatiles were removed in vacuo, and the off-white residue was washed with pentane (3 × 1 mL). After drying in vacuo, compound 22 was obtained as a white solid (70.8 mg, 83.1 μmol, 88%). Crystals suitable for the X-ray crystal structure analysis were obtained by slow evaporation of a solution of compound 22 in dichloromethane at room temperature. Dec: 220 °C. Anal. Calcd for C46H37BF10PNO: C: 64.88; H: 4.38; N: 1.64. Found: C: 65.00; H: 4.44; N: 1.50. IR (KBr): ν̃ [cm−1] = 3494 (w), 2934 (w), 2864 (w), 1642 (m), 1604 (m), 1556 (w), 1515 (s), 1456 (s), 1381 (w), 1342 (w), 1227 (m), 1252 (m), 1212(w), 1159 (w), 1090 (s), 1041 (w), 971 (s), 933 (w), 853 (m), 822 (w), 775 (m), 741 (w), 703 (m), 659 (m), 642 (m), 576 (w), 507 (w), 482 (w), 440 (w). 1H NMR (500 MHz, dichloromethane-d2, 299 K): δ 7.82 (br, 1H, 3-CH), 7.42 (m, 2H, m-Ph), 7.35 (m, 1H, p-Ph), 7.33 (m, 2H, o-Ph), 6.97 (d, 4JPH = 4.4 Hz, 4H, m-Mes), 4.76 (quint, J = 5.0 Hz, 1H, OH), 2.68 (m, 2H, 9-CH2), 2.56 (m, 2H, 6-CH2), 2.33 (s, 6H, p-CH3Mes), 2.13 (s, 12H, o-CH3Mes), 1.67 (m, 2H, 8-CH2), 1.57 (m, 2H, 7-CH2). 13C{1H} NMR (126 MHz, dichloromethane-d2, 299 K): δ 156.5 (br, 2-C), 148.0 (dm, 1JFC ≈ 240 Hz, C6F5), 138.2 (dm, 146.5 (d, 4JPC = 3.1 Hz, 4-CH), 143.6 (d, 4JPC = 2.9 Hz, p-Mes), 143.2 (br d, 2JPC = 10.8 Hz, o-Mes), 142.3 (d, J = 0.9 Hz, i-Ph), 138.2 (d, 2JPC = 16.6 Hz, 10-C), 138.2 (dm, 1JFC ≈ 240 Hz, C6F5), 137.2 (dm, 1JFC ≈ 250 Hz, C6F5), 134.7 (d, 3JPC = 12.2 Hz, 5-C), 133.3 (m, 3-CH), 132.7 (d, 3JPC = 11.6 Hz, m-Mes), 129.5 (o-Ph), 128.4 (m-Ph), 128.3 (d, 1JPC = 100.5 Hz, 1-C), 127.5 (br, i-C6F5), 127.4 (p-Ph), 122.2 (d, 1JPC = 87.4 Hz, i-Mes), 30.3 (d, 3JPC = 5.6 Hz, 9-CH2), 29.0 (d, 4JPC = 1.8 Hz, 6-CH2), 23.5 (br d, 3JPC = 3.6 Hz, o-CH3Mes), 23.0 (8-CH2), 22.9 (7CH2), 21.2 (d, J = 1.5 Hz, p-CH3Mes). 31P{1H} NMR (202 MHz, dichloromethane-d2, 299 K): δ 33.7 (ν1/2 ∼ 40 Hz). 11B{1H} NMR (160 MHz, dichloromethane-d2, 299 K): δ −5.4 (ν1/2 ≈ 150 Hz). 19F NMR (470 MHz, dichloromethane-d2, 299 K): δ −131.7 (br, 2F, o-

C6F5), −161.2 (t, 3JFF = 20.3 Hz, 1F, p-C6F5), −165.4 (m, 2F, m-C6F5) [Δ19Fmp = 4.2]. Solid-State NMR Data. Solid-state NMR measurements were carried out on a Bruker Avance III 300 spectrometer (magnetic field strength of 7.05 T). Magic angle spinning was used in 4 mm NMR double and triple resonance probes. For 11B{1H} MAS NMR experiments a short duration pulse of 0.5−1 μs was used, corresponding approximately to a 30° flip angle to achieve uniform excitation. A spinning speed of 12.0 kHz and relaxation delays of 20− 80 s were used. Chemical shifts are reported relative to an external sample of BF3·Et2O. 31P{1H} CPMAS NMR spectra were obtained at 12.5 kHz spinning rate using proton π/2-pulses between 5 and 7 μs and a contact time of 3 to 5 ms with a ramp (90−100% of the maximum power on the 1H channel). The relaxation delay was 5−15 s. Spectra were externally referenced to 85% H3PO4. TPPM-15 proton decoupling was conducted during acquisition of 11B and 31P spectra with pulses of about 5.5−8 μs length, corresponding to 10/12 πpulses. Spectral deconvolution was done with DMFIT software (version 2011).40 EPR Data. The EPR measurements were performed in quartz tubes. Solution EPR spectra were recorded on a JEOL JES-FA200 continuous wave spectrometer equipped with an X-band Gunn oscillator bridge and a cylindrical mode cavity. For all samples, a modulation frequency of 100 kHz was employed. All spectra were obtained from freshly prepared fluorobenzene solutions (0.005 M) at 298 K. Spectral simulation was performed using the program QCMP 136 by Prof. Dr. Frank Neese from the Quantum Chemistry Program Exchange as used by Neese et al.41 The fittings were performed by the “chi by eye” approach. Collinear g and A tensors were used. Coupling to 10B (I = 3; 19.9% abundant) was neglected in all simulations.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00627. Details on the experimental procedures, characterization of all new compounds, solid-state NMR data, and EPR data (PDF) Crystal structure data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ⊥

Instituto de Fı ́sica, Sao Carlos, Universidade de Sao Paulo, CP 369, 13560-970 Sao Carlos, SP, Brazil. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Deutsche Forschungsgemeinschaft and the European Research Council is gratefully acknowledged. T.H.W. thanks the American Chemical Society Petroleum Research Fund (51971-ND3) and the Georgetown Environment Initiative for financial support.

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REFERENCES

(1) (a) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124−1126. Review: (b) Kehr, G.; Schwendemann, S.; Erker, G. Top. Curr. Chem. 2012, 332, 45−84. (2) (a) Wang, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. J. Am. Chem. Soc. 2014, 136, 3293−3303. (b) Chen, G.-Q.; Kehr, G.; Daniliuc, C. G.; Wibbeling, B.; Erker, G. Chem. - Eur. J. 2015, 21, 12449−12455. M


Article

Organometallics

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