Reactions of phosphorus/boron frustrated Lewis pairs with SO₂

Abstract

The frustrated Lewis pair tBu₃P/B(C₆F₅)₃ (1) readily adds SO₂ to yield the zwitterionic adduct tBu₃P⁺–S(O)–OB⁻(C₆F₅)₃ (3). A series of intramolecular vicinal P/B FLPs Mes₂P–(X)–B(C₆F₅)₂ [X = –CH₂–CH₂– (2a), –CHMe–CH₂– (2b), cyclo-C₆H₁₀ (5)] add SO₂ at −78 °C to yield the corresponding six-membered addition products 4a, 4b, 6. The adducts contain a chiral sulfur center. The [B]–O–(O)S–[P] addition products 3, 4b and 6 were characterized by X-ray diffraction.

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Introduction

The advent of frustrated Lewis pair (FLP) chemistry has provided a new strategy for small molecule binding and/or activation.¹ Foremost in these efforts is the ability of FLPs to heterolytically split the dihydrogen molecule.² There is an increasing scope of substrates that can be employed in FLP-mediated hydrogenations.³ In addition, this concept has been applied to effect the stoichiometric reduction of the aromatic rings of anilines to the corresponding cyclohexylamines.⁴ Small molecule binding is another aspect of FLP chemistry that has garnered much attention.⁵ FLP binding of alkenes, alkynes and unsaturated conjugated systems⁶ as well as a variety of carbonyl compounds⁷ have been explored. Perhaps most importantly, FLPs have been shown to bind to a number of greenhouse gases, such as carbon dioxide⁸ or nitrogen oxides.⁹ Sulfur dioxide is another environmentally problematic gas.¹⁰ Herein, we describe the ability of a small series of inter- and intramolecular FLPs to capture the gas sulfur dioxide.¹¹ The dynamic and structural implications arising from the stereochemistry at sulfur are probed.

Results and discussions

The intermolecular FLP 1 reacts with sulfur dioxide at room temperature in bromobenzene to give a new product 3 in 80% yield. This compound shows a ³¹P NMR resonance at δ = 67.8 ppm, typical of a phosphonium center. The corresponding ¹¹B NMR signal is seen at δ = 0.3 ppm, suggestive of four-coordinate boron. Single crystals of compound 3 suitable for the X-ray crystal structure analysis were obtained by crystallization from CH₂Cl₂. It confirmed that the FLP 1 had undergone 1,2-addition to a S=O bond of sulfur dioxide to form the zwitterion tBu₃P(S(O)O)B(C₆F₅)₃3. The lengths of the newly formed P1–S1 and O1–B1 bonds are found to be 2.2753(6) and 1.565(2) Å, respectively, while the S1–O1–B1 angle is 118.2(1)°. The corresponding S1–O1 bond length is 1.572(1) Å while the corresponding terminal S1–O2 bond distance is 1.465(1) Å. The angles about S sum to 314.58° (O2–S1–O1 110.07(8)°, O2–S1–P1 105.89(6)°, O1–S1–P1 98.54(5)°) consistent with a distorted trigonal pyramidal geometry at sulfur (Scheme 1, Fig. 1).

Scheme 1 Reactions of inter- and intramolecular FLPs with SO₂.

Fig. 1 Molecular structure of compound 3 (thermal ellipsoids are shown with 50% probability).

The intramolecular FLP 2a^2b also cleanly adds sulfur dioxide in a rapid reaction in pentane solution at −78 °C to yield the product 4a. This was assigned the structure of the cyclic adduct (C₆H₂Me₃)₂PCH₂CH₂B(C₆F₅)₂(S(O)O) (Scheme 1). The new compound shows typical heteronuclear NMR resonances at δ = 33.3 ppm (³¹P) and δ = 1.2 ppm (¹¹B), respectively. Compound 4a is chiral due to the presence of the stereogenic sulfur atom. Consequently, NMR signals for the diastereotopic pair of C₆F₅ substituents on boron and a pair of diastereotopic mesityl groups at phosphorus are observed. In addition, the hydrogen atoms of the ethylene linker are also pairwise diastereotopic.

The overall structural features of this type of compounds were revealed by the X-ray crystal structure analysis of the methyl substituted derivative 4b that was formed from 2b and SO₂ at −78 °C (for details see the ESI†). It features the rac-(R,^SS)–4b diastereoisomer as different conformers (see Fig. 2), that shows B1A–O1A 1.574(3) Å [B1B–O1B 1.560(3) Å] and P1A–S1A 2.336(1) Å [P1B–S1B 2.347(1) Å] bond lengths inside the respective six-membered heterocyclic distorted half-chair conformations.

Fig. 2 Molecular structure of the two conformers of the rac-(1R,^SS)-4b enantiomers found in the single crystal (thermal ellipsoids are shown with 30% probability; for clarity only the ipso-carbons of aryl ring at B and P are shown).

Compound 4a shows temperature dependent dynamic NMR spectra. Coalescence of the pairs of o-, p- and m-¹⁹F NMR resonances of the B(C₆F₅)₂ moiety was observed in the temperature range between 298 K and 339 K, indicating a rapid enantiomerization of the stereogenic sulfur atom on the NMR time scale (Scheme 2). From the dynamic ¹⁹F NMR spectra a Gibbs activation barrier of ΔG^≠_enant (333 K) = 15.4 ± 0.1 kcal mol⁻¹ was calculated for this process.

Scheme 2 Enantiomerization and fluxional behavior of 4a.

In addition a second distinct dynamic process of 4a was observed by monitoring the temperature dependent ³¹P NMR spectra. These spectra revealed the presence of a ca. 1 : 4 mixture of two isomers at 183 K, giving rise to resonances at δ = 38.0 ppm and δ = 26.8 ppm. Warming the sample rapidly resulted in coalescence of the signals at 243 K. In view of the results of a theoretical study of this system (vide infra) it is likely that these spectral changes arise from the occurrence of two ring conformers, featuring the S=O oxygen in the axial or equatorial position (Scheme 2). From the dynamic ³¹P NMR spectra an activation barrier of ΔG^≠_enant (223 K) = 10.0 ± 0.3 kcal mol⁻¹ was estimated for this conformational equilibration process (see the ESI†).

The results were augmented by a study of the reaction of the intramolecular FLP¹² derived from the hydroboration of cyclohexenyldimesitylphosphane with Piers' borane [HB(C₆F₅)₂].¹³ FLP 5 reacts readily with SO₂ to give the cyclic adduct 6 (Scheme 1). In this case the FLP framework itself contains a pair of chiral carbon centers. The resulting stereochemistry from the hydroboration affords the single diastereoisomer that has the –PMes₂ and –B(C₆F₅)₂ groups trans-1,2-attached (rac-trans-5; this study involves only racemic compounds).¹⁴ Introduction of the stereogenic sulfur center in 6 leads to the formation of a pair of diastereomers of 6 in a ca. 1 : 1.3 ratio [³¹P NMR (193 K): δ = 60.5 ppm and δ = 41.1 ppm]. Each diastereoisomer exhibits conformational equilibria similar to those observed for the monocyclic FLP/SO₂ adducts 4a/b (vide supra). At higher temperature (308 K) the reversible equilibration between the diastereoisomers of 6, by epimerization at sulfur, was evidenced by a ³¹P, ³¹P-NOESY experiment (see the ESI†). The X-ray crystal structure analysis of 6 confirmed that the phosphane component of the FLP 5 had added to the sulfur atom of sulfur dioxide. The diastereomers of 6 are present in a 3 : 1 ratio in the crystal (Fig. 3). The major adduct 6 shows typical bond lengths of P1–S1: 2.362(1) Å, S1–O(2A): 1.407(2) Å, S1–O1: 1.547(2) Å, O1–B1: 1.568(3) Å and bond angles C1–P1–S1: 102.80(7)°, P1–S1–O(2A): 114.82(11)°, O(2A)–S1–O1: 116.5(12)°, S1–O1–B1: 122.51(14)°, O1–B1–C6: 108.79(18)°.

Fig. 3 View of the molecular structures of the pair of diastereomers of the FLP-SO₂ adduct 6 in the crystal (top 75%, bottom: 25%) (thermal ellipsoids are shown with 30% probability; For clarity only the ipso-carbons of aryl ring at B and P are shown).

The FLP-SO₂ adducts 4a and 6 were also characterized by ³¹P and ¹¹B solid state NMR spectroscopy (see Fig. 4). The ³¹P{¹H} CP MAS NMR spectrum of 4a shows a pair of resonances at 39.9 and 28.7 ppm in an intensity ratio of about 1 : 3 probably originating from the two [B]–O–(O)S–[P] ring conformers. The unusually broad line shape suggests some fluxional character in the solid state. The line shape simulation for the ¹¹B MAS NMR spectrum of 4a reveals two resonances at about 1 ppm with quadrupolar coupling constants C_Q of 1.6 and 1.9 MHz and nearly identical asymmetry parameters η of about 0.5. The experimental C_Q and η values are much closer to those calculated for the [B]–O–(O)S–[P] isomer than to those calculated for the [B]–S(O)–O–[P] isomers 4a′-ax. and 4a′-eq. (see Table 1). Thus, the formation of the latter isomers can be ruled out for all the SO₂ adducts studied here.

Fig. 4 Left: ¹¹B{¹H} MAS NMR spectra of 4a and 6 (a) acquired at 7.1 T and 11.7 T, respectively, with their corresponding line shape simulations (b). Right: ³¹P{¹H} CP MAS spectrum of 4a acquired at 7.1 T and ³¹P{¹H} MAS NMR spectrum of 6 acquired at 9.4 T. * mark spinning sidebands, + impurities.

Table 1 Experimentally and theoretically determined ¹¹B NMR parameters for the [B]–O–(O)S–[P] and [B]–S(O)–O–[P] conformers of 4a and 6. [Chemical shifts are calculated on a B3-LYP/def2-TZVP, electric field gradients on a B97-D/def2-TZVP (mod.) level of theory (for the full AO basis set see ref. 16)]

	δ^isoCS/ppm ±0.5	CQ/MHz ±10%	η ±0.1	δ^isoCS/ppm (calc.)	CQ/MHz (calc.)	η (calc.)
a Fully geometry optimized structure of 4a-ax.b Fully geometry optimized structure of 4a-eq.c Fully geometry optimized structure of 4a′-ax.d Fully geometry optimized structure of 4a′-eq.e Geometry of 6-ax. taken from the crystal structure.f Geometry of 6-eq. taken from the crystal structure.g Fully geometry optimized structure of 6′-ax.h Fully geometry optimized structure of 6′-eq.i Assigned to an impurity.
4a	1.0	1.62	0.46	1.6^a	2.21^a	0.35^a
	1.0	1.92	0.48	2.2^b	2.15^b	0.37^b
				−4.4^c	1.16^c	0.71^c
				−5.4^d	1.00^d	0.56^d
6	2.5^i	1.80^i	0.01^i	−2.6^e	1.95^e	0.43^e
	0.9	1.88	0.50	0.4^f	2.02^f	0.44^f
				−2.8^g	1.05^g	0.74^g
				−1.5^h	1.25^h	0.57^h

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Fig. 5 shows results from ¹¹B{³¹P} Rotational Echo DOuble Resonance (REDOR)^15,16 experiments for measuring the boron-phosphorus distances in 4a and 6. In contrast to previous results on non-reacted FLPs with much shorter B⋯P distances,¹⁶ no dipolar oscillation typical of spin pairs are seen in these curves. Most likely, these oscillations are damped out due to subtle differences in the B⋯P distances for the different conformers and/or slow dynamic processes as suspected for 4a. For the distance determination, we use the fact that at short evolution times (ΔS/S₀ < 0.25) these curves are well approximated by parabolae, whose curvatures are proportional to the squared dipolar coupling constants.¹⁷ With the B⋯P internuclear distance for 6 known from the crystal structure (3.463(4) Å) we can use the REDOR curve of this compound (Fig. 5a) for calibrating analogous results obtained on 4a. We find a boron-phosphorus distance of 3.72 Å ± 0.2 Å in good agreement with the theoretically expected value (3.495 Å/3.506 Å) in the gas phase. Within the experimental error limit it also agrees well with the B⋯P internuclear distances of 3.501(2) Å and 3.537(6) Å for the two conformers determined from the crystal structure of 4b. The ³¹P{¹H} MAS NMR spectrum of a sample of 6 reveals two resonances at 56.5 and 39.1 ppm, respectively, close to the respective ³¹P NMR chemical shifts found for the pair of diastereoisomers of 6 in solution (see above). In the solid state these two species were initially formed in a ca. 1 : 5 ratio which over a period of 25 days changed to 1 : 10. In contrast to 4a both diastereomers possess nearly identical ¹¹B quadrupolar coupling parameters since only one resonance is detected in the ¹¹B MAS NMR spectrum.

Fig. 5 ¹¹B{³¹P} REDOR curves for 6 (a) and 4a (b) and corresponding parabolic approximations in the limit of short evolution times (for more details see ESI†).

The FLP + SO₂ reaction was investigated by state of the art DFT calculations [B2PLYP-D3/def2-TZVP//TPSS-D3/def2-QZVPP + COSMO-RS solvent corrections,^18–23 estimated accuracy absolute about 1 kcal mol⁻¹ for free enthalpies (for further details see the ESI†)]. As there were no X-ray data available for compound 4a several structural alternatives were tested. We investigated the observed [B]–O–(O)S–[P] conformational isomers 4a-eq. and 4a-ax. (see Table 2 and Fig. 6) as well as their hypothetical [B]–S(O)–O–[P] regioisomers (4a′-eq. and 4a′-ax.). The latter were found to be slightly less favourable than the observed 4a-eq./ax. pair (calculated for a bromobenzene solution). In addition the formation of the alternative five-membered isomer, [B]–S(O)₂–[P], was computed to be highly thermodynamically unfavourable with reaction (free) enthalpies of ca. 7–14 kcal mol⁻¹ higher than for any of the six-membered products.

Table 2 Free reaction enthalpies (ΔG in kcal mol⁻¹) for the formation of the different isomers of 4a and 6 in bromobenzene

Conformer	4a	rac-(R,R,^SR) 6
[B]–O–(O)S–[P], eq.	−7.9	−6.2
[B]–O–(O)S–[P], ax.	−7.7	−6.7
[B]–S(O)–O–[P], eq.	−3.6	−5.1
[B]–S(O)–O–[P], ax.	−7.1	−4.5
[B]–S(O)2–[P]	6.7	2.5

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Fig. 6 The DFT calculated [B]–O–(O)S–[P] isomers of compound 4a: (a) with O_exocycl. equatorial and (b) with O_exocycl. axial (color code: C grey, P orange, B purple, S yellow, O red, F green, H white).

The relative free enthalpies of all four six-membered isomers of 4a (and similarly of 6 see Table 2) fall in a rather narrow range of about 4 kcal mol⁻¹ for 4a and 2 kcal mol⁻¹ for 6. The [B]–O–(O)S–[P] isomer 6-ax. is computed to be the thermodynamically most stable product. Although this would suggest the observed regioselectivity is under thermodynamic control, it should be noted that the error of the computations infer the first two isomers of 6 (and 1,2,4 of 4a) are more or less isoexergonic.

We investigated the possible pathways of the observed facile enantiomerization process at sulfur in 4a by DFT and found that a true inversion process at the sulfur atom would be far too high in energy (∼50 kcal mol⁻¹). However, due to the calculations rapid reversible P–S bond rupture provides a viable pathway of enantiomerization at sulfur in such a system.

Conclusions

This study shows that FLPs react with remarkable ease with sulfur dioxide. The formation of the respective [B]–O–(O)S–[P] isomers has been observed by X-ray diffraction in three cases and by ¹¹B solid state NMR/DFT calculations in two selected cases although the [B]–S(O)–O–[P] isomers are thermodynamically feasible according to the DFT calculations. FLP-SO₂ adduct formation has remarkably low kinetic barriers, making these adducts readily available for further reactivity studies.

Experimental

Synthesis of compound 3

A solution of B(C₆F₅)₃ (25 mg, 0.049 mmol) and tBu₃P (10 mg, 0.049 mmol) in C₆H₅Br (1 mL) was degassed and filled with SO₂ (1 bar). The sample was sealed and allowed to react overnight to afford tBu₃PS(O)OB(C₆F₅)₃ (3) as a white powder (31 mg, 0.039 mmol, 80%). Crystals suitable for X-ray crystal structure analysis were obtained by crystallization from CH₂Cl₂. Anal. calcd (%) for C₃₀H₂₇BF₁₅O₂PS: C, 46.29; H, 3.50; found: C, 45.84; H, 3.40. ¹H NMR (400 MHz, 298 K, [d₂]-dichloromethane) δ = 1.43 (d, J_PH = 14.1 Hz, 27H, tBu). ¹¹B NMR (128 MHz) δ = 0.3 (ν_1/2 ∼ 200 Hz). ³¹P NMR (162 MHz) δ = 67.8.

X-ray crystal structure analysis of 3

Formula C₃₀H₂₇BF₁₅O₂PS, M = 778.36, colourless crystal, 0.24 × 0.22 × 0.19 mm, a = 9.4942(4), b = 29.0008(13), c = 11.5368(5) Å, β = 90.018(2)°, V = 3176.5(2) ų, ρ_calc = 1.628 g cm⁻³, μ = 0.270 mm⁻¹, empirical absorption correction (0.937 ≤ T ≤ 0.950), Z = 4, monoclinic, space group P2₁/c (No. 14), λ = 0.71073 Å, T = 150(2) K, ω and φ scans, 30506 reflections collected (±h, ±k, ±l), 8222 independent (R_int = 0.040) and 6900 observed reflections [I > 2σ(I)], 452 refined parameters, R = 0.034, wR² = 0.072, max. (min.) residual electron density 0.37 (−0.43) e Å⁻³, hydrogen atoms calculated and refined as riding atoms.

Preparation of compound 4a

Compound 2a, was generated in situ from dimesitylvinylphosphane (100 mg, 0.34 mmol, 1.0 eq.) and bis(pentafluorophenyl)-borane (117 mg, 0.34 mmol, 1.0 eq.) in n-pentane. SO₂ was introduced (−78 °C, 0.8 bar, 10 min.). After stirring the reaction mixture for 1.5 h workup gave compound 4a (198 mg, 83%) as a white solid. M.p.: 103 °C. Anal. calcd (%) for C₃₂H₂₆BF₁₀O₂PS: C, 54.41; H, 3.71; found C, 54.88; H, 3.91. ¹H NMR (500 MHz, 298 K, [d₂]-dichloromethane): δ = 7.00 (d, ⁴J_PH = 3.6 Hz, 4H, m-Mes^A), 6.99 (d, ⁴J_PH = 3.9 Hz, 4H, m-Mes^B), 3.75/2.93 (each m, each 1H, CH₂^P), 2.45 (br s, 6H, o-CH₃^MesA), 2.41 (br s, 6H, o-CH₃^MesB), 2.31 (s, 6H, p-CH₃^MesA,B), 1.88/1.23 (each m, each 1H, CH₂^B). ¹³C{¹H} NMR (126 MHz): δ = 144.9, 144.8 (each s, p-Mes^A,B), 144.1 (d, ²J_PC = 7.0 Hz, o-Mes^B), 143.4 (d, ²J_PC = 5.6 Hz, o-Mes^A), 132.2 (d, ³J_PC = 10.8 Hz, m-Mes^B), 131.6 (d, ³J_PC = 9.8 Hz, m-Mes^A), 122.2 (d, ¹J_PC = 39.3 Hz, i-Mes^A), 118.3 (d, ¹J_PC = 52.5 Hz, i-Mes^B), 23.4 (br, o-CH₃^MesA), 23.1 (br, o-CH₃^MesB), 22.7 (d, ¹J_PC = 22.9 Hz, CH₂^P), 21.3, 21.2 (each s, p-CH₃^MesA,B), 11.4 (br, CH₂^B), [C₆F₅ not listed]. ¹¹B{¹H} NMR (160 MHz): δ = 1.2 (ν_1/2 ≈ 400 Hz). ¹⁹F NMR (470 MHz): δ = −134.9 (m, 2F, o), −160.6 (m, 1F, p), −165.7 (m, 2F, m), Δδ¹⁹F_p,m = 5.1 [C₆F₅^A]; −135.5 (m, 2F, o), −160.7 (m, 1F, p), −165.7 (m, 2F, m), Δδ¹⁹F_p,m = 4.7 [C₆F₅^B]. ³¹P{¹H} NMR (202 MHz): δ = 33.3 (ν_1/2 ≈ 20 Hz).

Synthesis of compound 4b

Compound 2b, was generated in situ from (1-methylethenyl)di-mesitylphosphane (24.8 mg, 0.08 mmol, 1.0 eq.) and bis(pentafluorophenyl)borane (27.6 mg, 0.08 mmol, 1.0 eq.) in toluene (2 mL). The solution was briefly exposed to sulfur dioxide at −78 °C (5 min, 1.8 bar, this provided for minimal condensation of SO₂). The reaction mixture was stirred for 45 min. The excess SO₂ was vented and the obtained reaction mixture was covered with precooled n-pentane (5 mL) and stored at −36 °C to finally get compound 4b as colorless crystals which were suitable for X-ray crystal structure analysis. Two diastereomers of 4b were found in [d₂]-dichloromethane solution at 299 K (ratio: 84/16). M.p.: 126 °C. ¹¹B{¹H} NMR (160 MHz, CD₂Cl₂, 299 K): δ = 1.3 (ν_1/2 ≈ 450 Hz). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 299 K): δ = 51.2 (ν_1/2 ≈ 10 Hz, 16%), 39.5 (ν_1/2 ≈ 10 Hz, 84%). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 198 K): δ = 55.4 (ν_1/2 ≈ 15 Hz, 8%), 50.3 (ν_1/2 ≈ 35 Hz, 3%), 39.0 (ν_1/2 ≈ 25 Hz, 58%), 37.8 (ν_1/2 ≈ 30 Hz, 31%). Major diastereomer [84%]:¹H NMR (500 MHz, CD₂Cl₂, 299 K): δ = 7.02 (d, ⁴J_PH = 3.6 Hz, 2H, m-Mes^A), 7.00 (d, ⁴J_PH = 3.6 Hz, 2H, m-Mes^B), 4.14 (m, 1H, CH^P), 2.38 (br, 6H, o-CH₃^MesA), 2.35 (br, 6H, o-CH₃^MesB), 2.325 (br m, 3H, p-CH₃^MesA), 2.318 (br m, 3H, p-CH₃^MesB), 1.82 (dd, ³J_PH = 27.8 Hz, ³J_HH = 16.4 Hz, 1H, CH₂^B), 1.41 (dd, ³J_PH = 18.1 Hz, ³J_HH = 7.0 Hz, 3H, CH₃^P), 1.30 (m, 1H, CH₂^B). ¹³C{¹H} NMR (126 MHz): δ = 144.6 (d, ⁴J_PC = 3.1 Hz, p-Mes^A), 144.3 (d, ⁴J_PC = 3.8 Hz p-Mes^B), 144.0 (d, ²J_PC = 5.6 Hz o-Mes^A), 143.6 (br d, ²J_PC = 6.6 Hz o-Mes^B), 132.5 (br d, ³J_PC = 11.4 Hz, m-Mes^B), 131.69 (d, ³J_PC = 9.9 Hz, m-Mes^A), 120.4 (d, ¹J_PC = 49.6 Hz, i-Mes^B), 120.3 (d, ¹J_PC = 36.1 Hz, i-Mes^A) 31.1 (d, ¹J_PC = 15.0 Hz, CH^P), 24.6 (br, o-CH₃^MesA)^t, 23.3 (d, ³J_PC = 4.1 Hz, o-CH₃^MesB)^t, 22.3 (br d, ²J_PC = 7.9 Hz, CH₃^P), 21.19, 21.18 (each d, each ⁵J_PC = 1.8 Hz, p-CH₃^MesA,B), 21.1 (br, CH₂^B), [C₆F₅ not listed]. ¹⁹F NMR (470 MHz): δ = −135.4 (m, 2F, o), −160.6 (t, ¹J_FC = 20.4 Hz, 1F, p), −165.7 (m, 2F, m), Δδ¹⁹F_p,m = 5.1 [C₆F₅^A]; −135.6 (m, 2F, o), −161.1 (t, ¹J_FC = 20.4 Hz, 1F, p), −165.5 (m, 2F, m), Δδ¹⁹F_p,m = 4.4 [C₆F₅^B]. Minor diastereomer [16%]:¹H NMR (500 MHz): δ = 7.04 (d, ⁴J_PH = 3.7 Hz, 2H, m-Mes^A), 7.01 (d, ⁴J_PH = 3.6 Hz, 2H, m-Mes^B), 3.36 (m, 1H, CH^P), 2.37 (br, 6H, o-CH₃^MesA), 2.34 (br, 6H, p-CH₃^MesA,B), 2.27 (br, 6H, o-CH₃^MesB), 2.04 (m, 1H, CH₂^B), 1.68 (dd, ³J_PH = 27.5 Hz, ³J_HH = 16.4 Hz, 1H, CH₂^B), 1.406 (dd, ³J_PH = 17.9 Hz, ³J_HH = 7.1 Hz, 3H, CH₃^P). ¹³C{¹H} NMR (126 MHz): δ = 144.4 (d, ⁴J_PC = 3.3 Hz), 144.3^t (d, ⁴J_PC = 3.8 Hz) (p-Mes^A,B), 144.1 (d, ²J_PC = 6.3 Hz, o-Mes^A), 142.9 (d, ²J_PC = 6.3 Hz, o-Mes^B), 132.2 (d, ³J_PC = 10.6 Hz, m-Mes^B), 131.67 (d, ³J_PC = 10.8 Hz, m-Mes^A), 121.5 (d, ¹J_PC = 44.8 Hz, i-Mes^B), 121.1 (d, ¹J_PC = 34.5 Hz, i-Mes^A), 33.6 (d, ¹J_PC = 11.7 Hz, CH^P), 24.4 (br, o-CH₃^MesA), 23.7 (d, ³J_PC = 3.5 Hz, o-CH₃^MesB), 21.6 (d, ²J_PC = 12.6 Hz, CH₃^P), n.o. (p-CH₃^Mes), 20.2 (br, CH₂^B), [C₆F₅ not listed; ^t tentative assignment]. ¹⁹F NMR (470 MHz): δ = −134.5 (m, 2F, o), −161.0 (t, ¹J_FC = 20.4 Hz, 1F, p), −165.9 (m, 2F, m), Δδ¹⁹F_p,m = 5.1 [C₆F₅^A]; −134.7 (m, 2F, o), −159.9 (t, ¹J_FC = 20.4 Hz, 1F, p), −165.0 (m, 2F, m), Δδ¹⁹F_p,m = 4.4 [C₆F₅^B].

X-ray crystal structure analysis of 4b

Formula C₃₃H₂₈BF₁₀O₂PS, M = 720.39, colourless crystal, 0.20 × 0.13 × 0.05 mm, a = 13.2898(3), b = 13.3521(6), c = 22.5790(9) Å, α = 73.392(3), β = 73.073(3), γ = 83.328(4)°, V = 3670.7(2) ų, ρ_calc. = 1.304 g cm⁻³, μ = 1.903 mm⁻¹, empirical absorption correction (0.702 ≤ T ≤ 0.910), Z = 4, triclinic, space group P1̄ (no. 2), λ = 1.54178 Å, T = 223(2) K, ω and φ scans, 53 257 reflections collected (±h, ±k, ±l), 12 647 independent (R_int = 0.043) and 10 730 observed reflections [I > 2σ(I)], 877 refined parameters, R = 0.049, wR² = 0.145, max. (min.) residual electron density 0.35 (−0.32) e.Å⁻³, hydrogen atoms calculated and refined as riding atoms.

Synthesis of compound 6

Procedure A: Di(mesityl)cyclohexenylphosphane (120 mg, 0.34 mmol) and HB(C₆F₅)₂ (119 mg, 0.34 mmol) dissolved in toluene (2.5 mL) and stirred for 30 min at r.t. to give a yellow solution of 5. The solution was cooled to −78 °C and SO₂ gas was pressurized (2.0 bar) for 3 minutes. Then the solution was covered with n-pentane (−30 °C) to finally get compound 6 (136 mg, 52 %) as colourless crystals, which were suitable for X-ray crystal structure analysis. Procedure B: Di(mesityl)cyclohexenylphosphane (250 mg, 0.71 mmol) and HB(C₆F₅)₂ (247 mg, 0.71 mmol) dissolved in n-pentane (15 mL) and stirred for 30 min at r.t. to give a yellow solution of 5. The solution was cooled to −78 °C and SO₂ gas was pressurized (2.0 bar) for 5 minutes. The solvent was removed and the residue was dried at −78 °C. The colourless residue was dissolved in precooled CH₂Cl₂ and covered with precooled (−30 °C) n-pentane to finally get compound 6 (310 mg, 57 %) as colourless crystals. Anal. calcd (%) for C₃₆H₃₂PBSO₂F₁₀: C, 56.86; H, 4.24; found, C; 56.30; H, 4.06. The NMR resonances were not assigned to each diastereoisomer [1 : 1.3 (³¹P)] ¹H NMR (600 MHz, CD₂Cl₂, 193 K) [all resonances are broad] δ = 7.18/6.97, 7.14/6.93, 7.09/6.80, 7.05/6.87 (each br s, each 1H, m,m'-Mes)¹, 3.82, 3.47 (each br., each 1H, PCH), 2.79/2.24(p)/1.64, 2.74/2.30(p)/1.51, 2.67/2.31(p)/1.87, 2.54/2.25(p)/1.74 (each s, each 3H, CH₃^Mes)¹, 2.25/1.23, 2.07/0.62, 1.92/0.61, 1.88/1.35, 1.60/1.22, 1.58/1.77, 1.56/1.28, 1.42/0.99 (each br, each 1H, CH₂)², 2.31, 1.85 (each br, each 1H, BH)², [¹ from the gcosy NMR experiment;² from the ghsqc NMR experiment]. ¹¹B{¹H} NMR (192 MHz, CD₂Cl₂, 308 K): δ = 1.8 (ν_1/2 ≈ 500 Hz). ³¹P{¹H} NMR (243 MHz, CD₂Cl₂, 193 K): δ = 60.5 (ν_1/2 ≈ 30 Hz, [42%]), 41.1 (ν_1/2 ≈ 40 Hz, [58%]). ¹⁹F NMR (564 MHz, CD₂Cl₂, 193 K): δ = −128.6, −130.8, −131.7, −132.3, −132.47, −132.53, − 136.3, −139.0 (each br, each 1F, o-C₆F₅), −158.1, −159.3, −159.7, −160.1 (each br, each 1F, p-C₆F₅), −163.6 (1F), −163.9 (1F), −164.5 (5F), −164.9 (1F) (each br, m-C₆F₅).

X-ray crystal structure analysis of 6

Formula C₃₆H₃₂BF₁₀O₂PS, M = 760.46, colourless crystal, 0.28 × 0.15 × 0.06 mm, a = 10.5993(4), b = 11.8650(4), c = 14.4165(9) Å, α = 92.700(3), β = 95.322(2), γ = 111.030(13)°, V = 1678.66(13) ų, ρ_calc. = 1.504 g cm⁻³, μ = 2.113 mm⁻¹, empirical absorption correction (0.589 ≤ T ≤ 0.883), Z = 2, triclinic, space group P1̄ (no. 2), λ = 1.54178 Å, T = 223(2) K, ω and φ scans, 19 387 reflections collected (±h, ±k, ±l), 5692 independent (R_int = 0.048) and 4822 observed reflections [I > 2σ(I)], 476 refined parameters, R = 0.043, wR² = 0.116 max. (min.) residual electron density 0.24 (−0.29) e Å⁻³, hydrogen atoms calculated and refined as riding atoms.

Acknowledgements

Financial support from the Deutsche Forschungsgemeinschaft and the European Research Council is gratefully acknowledged. DWS gratefully acknowledges the financial support of NSERC of Canada, the award of a Canada Research Chair. TW thanks the Fonds der Chemischen Industrie for a research fellowship and acknowledges additional support by the NRW Forschungsschule Molecules and Materials.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: For additional experimental details, further structural information, additional data from the solid state NMR and theoretical studies. CCDC 895193–895195. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2sc21161k

‡ Computational chemistry.

§ Solid state NMR.

¶ X-ray crystal analyses.

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