The insertion of sulfur dioxide into palladium–methyl bonds: the synthesis and X-ray crystal structure of an unusual [(dppp)PdOS(Me)O]₂[BAr′₄]₂ dimer

Abstract

The migratory insertion of sulfur dioxide into the palladium(II)–methyl bond of [(dppp)Pd(Me)(OEt₂)]BAr′₄ [Ar′ = C₆H₃(CF₃)₂-3,5] to yield a unique dimeric eight-membered palladacycle was followed by NMR spectroscopy, and the product characterised by X-ray crystallography.

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Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyse the copolymerisation of olefins with carbon monoxide to form polyketones.¹ Sulfur dioxide is an attractive monomer for catalytic copolymerisations with olefins since SO₂, like CO, is known to undergo facile insertion reactions into a variety of transition metal–alkyl bonds.^2,3 Indeed, Drent recently patented alternating copolymerisation of ethylene with SO₂ using various palladium(II) complexes.⁴ In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF₄ was an effective catalyst for the copolymerisation of SO₂ with ethylene, propylene and cyclopentene.⁵ Here, we report our preliminary investigations of the insertion reactions of SO₂ into Pd(II)–methyl bonds and the attempted spectroscopic detection of the copolymerisation of ethylene and SO₂.

The cationic 1,3-bis(diphenylphosphino)propane (dppp) palladium(II) complex 1 can be prepared by the low temperature reaction of (dppp)PdMe₂6 with H(OEt₂)₂[BAr′₄] [Ar′ = C₆H₃(CF₃)₂-3,5].⁶ We have previously studied the reactions of carbon monoxide with 1 at −70 °C, and have found that insertion proceeds via initial displacement of diethyl ether with CO followed by migratory insertion into the Pd–Me bond.⁷ Immediately after treating a solution of 1 in CD₂Cl₂ with an excess of SO₂ (ca. 25 equiv.) at 193 K, the ¹H NMR spectrum shows the presence of free diethyl ether. Analysis of the reaction mixture by ³¹P NMR spectroscopy (193 K) shows the formation of a major product (ca. 80%) which exhibits a singlet at δ 15.7 indicating equivalence of the phosphorus nuclei. In addition, two minor products (ca. 20%) with singlet resonances at δ 15.4 and 14.5 are also observed. The expected products, 2 and 3, both of which would possess inequivalent phosphorus atoms, are not observed under these conditions. In addition, the ¹H NMR spectrum exhibits three new singlets⁵ at δ 0.52 (78%), 0.42 (8%) and 0.25 (14%) which are assigned to –CH₃ groups, no signals were observed downfield at ca. 2–3 where resonances for an –S(O)₂CH₃ group would be expected.⁸ A preparative scale reaction† was carried out in an attempt to isolate single crystals. Indeed, pale yellow crystals suitable for X-ray diffraction were isolated after recrystallisation of the crude product from dichloromethane–hexanes at −30 °C.‡

The molecular structure of the isolated product 4 is shown in Fig. 1, and confirms that insertion of SO₂ into the Pd–Me bond had taken place, however dimerisation resulted in the formation of an eight-membered cyclic compound in which the phosphorus atoms are equivalent, consistent with the ³¹P NMR observations. In particular, the eight-membered ring consists of a plane of palladium and oxygen atoms, with the sulfur atoms located above and below the plane. The six-membered rings formed by the dppp ligand are in the chair conformation.

Fig. 1 Molecular structure of 4 (BAr′₄ anion omitted for clarity) with thermal ellipsoids shown at the 50% probability level. Selected bond lengths (Å) and angles (°): Pd(1)–P(1) 2.2354(20), Pd(1)–P(2) 2.2393(19), Pd(1)–O(1) 2.078(5), Pd(1)–O(2) 2.088(5), O(1)–S(1) 1.494(5), O(2)–S(1) 1.507(5), S(1)–C(3) 1.786(10); P(1)–Pd(1)–P(2) 90.12(7), P(1)–Pd(1)–O(2) 89.00(15), P(2)–Pd(1)–O(1) 90.47(15), O(1)–Pd(1)–O(2) 90.92(19), O(1)–S(1)–O(2) 109.5(3), O(1)–S(1)–C(3) 98.4(5), Pd(1)–O(1)–S(1) 128.0(3).

A variable temperature NMR experiment was carried out on 4 in CD₂Cl₂ solution. As the temperature is raised from 193 K the three singlet ³¹P resonances (δ 15.7, 15.4, 14.5) begin to broaden at ca. 260 K and coalesce to a singlet above ca. 300 K. This process is reversible. Similarly, the three methyl signals also coalesce to a singlet but at a somewhat lower temperature (ca. 280 K) owing to the smaller spread in resonance frequencies. Since the inversion barrier at tetrahedral sulfur in 4 is expected to be high, the most likely source of the isomerism observed here⁹ comes from ring inversion of the chair conformation of the dppp ligand. We have also studied the behaviour of 1 towards mixtures of ethylene and SO₂ in an attempt to spectroscopically observe the insertion of SO₂ and ethylene in an alternating fashion. Thus, treatment of a solution of 1 in CD₂Cl₂ with a mixture of ethylene (10 equiv.) and SO₂ (10 equiv.) results in quantitative displacement of ether with ethylene at 193 K forming [(dppp)Pd(Me)(η²-C₂H₄)]BAr′ ₄¹⁰ which was characterised by ³¹P [δ = 17.5 (d, J 56 Hz), −1.5 (d, J 56 Hz)] and ¹H NMR spectroscopy [δ 5.18 (bound C₂H₄), 0.34 (dd, Pd–CH₃, J 7, 4 Hz)]. Upon warming the reaction mixture to 223 K for ca. 30 min complex 4 was formed in nearly quantitative yield. No evidence for insertion of ethylene into the Pd–Me bond was detected, nor was any direct insertion of SO₂ into the Pd–Me bond of the methyl ethylene complex observed to form 5. This suggests that formation of 4 from [(dppp)Pd(Me)(η²-C₂H₄)]BAr′ ₄ (or 1) involves displacement of ethylene (or diethyl ether for 1) to yield 2 which then undergoes a rapid migratory insertion reaction to form 3 which then dimerises yielding 4. This is in contrast to observations by Jones for analogous L₂PtMe₂ complexes which readily insert SO₂via a five coordinate intermediate.⁸ We have also carried out the analogous reaction of Pd complex 6 with SO₂ to form 7 presumably also via a five coordinate intermediate [eqn. (1)].†

In an attempt to obtain conclusive evidence for a migratory insertion mechanism, a low-temperature NMR experiment was carried out using CDCl₂F as solvent. Thus, SO₂ (10 equiv.) was added to a solution of ether complex 1 in CDCl₂F at 153 K. Upon warming to 173 K several new products could be detected in addition to 1. The ³¹P NMR spectrum exhibited resonances assigned to 4, and also two sets of doublets at δ 22.0 and −5.5 (J_PP 50 Hz) and δ 18.8 and 17.4 (J_PP 20 Hz). In the ¹H NMR spectrum, in addition to 4, two new products were observed which exhibited a methyl singlet at δ 1.6 and a methyl doublet of doublets at δ 0.0 (J_HP 7, 4 Hz). The signal at δ 0.0 is clearly due to a Pd–CH₃ group of a (dppp)Pd(CH₃)⁺ moiety and, since this species is not observed in the absence of SO₂, we assign this resonance to complex 2, [(dppp)Pd(CH₃)(SO₂)]⁺. The ³¹P resonances at δ 22.1 (d, J 50 Hz) and δ −5.6 (d, J 50 Hz) can be assigned to 2. The low field methyl resonance at δ 1.6 [³¹P resonances δ 18.8 (d, J 20 Hz), δ 17.4 (d, J 20 Hz)] is assigned to monomer 3, a precursor to dimer 4, formed via a migratory insertion reaction of 2. The ligand (L) occupying the fourth coordination site in 3 is likely to be either SO₂ or Et₂O. As the reaction mixture was warmed in 5 °C increments, the signals for 2, 3 and 4 increased in intensity and those for 1 decreased. At 213 K, only isomers of 4 could be detected. We speculate that in 3 the SO₂Me is bound through oxygen, rather than sulfur, based on the molecular structure of the final product 4.

A copolymerisation was attempted using hex-1-ene (10% v/v) in CH₂Cl₂ purged with SO₂ in the presence of a catalytic quantity of 1 (14 mg).¹¹† After ca. 14 h, the reaction was quenched with methanol. Solvent removal in vacuo, yielded a small amount of a polymeric material (23 mg). The polymer was precipitated with methanol and was characterised by gel permeation chromatography (M_n = 61000: PDI = 5.8), and the ¹H and ¹³C NMR data are identical with those reported for the hex-1-ene/SO₂ alternating copolymer prepared using a free-radical initiator.¹² Comparing the amount of 1 used as initiator (9 × 10⁻⁶ mol), the amount of hex-1-ene consumed (1.6 × 10⁻⁴ mol) and the estimated M_n of the polymer produced (61000) suggests that if chain growth occurred at Pd only a very small fraction of the Pd centers were active (<5%). Results from repeated polymerisations proved quite erratic with varying amounts of polymer produced both in the presence and absence of 1. Reactions conducted in the presence of the radical scavenger galvinoxyl failed to produce polymer.¹³ These results, coupled with the failure to spectroscopically observe ethylene insertion in these systems and the ready formation of the stable dimer 4, suggest that the copolymerisation of hex-1-ene and SO₂ under conditions reported above does not occur by a coordination–insertion process initiated by 1. A radical chain growth mechanism appears very likely, although we cannot rule out copolymerization in the presence of 1 which is initiated by traces of a palladium complex of unknown structure present in 1 and which can be deactivated by galvinoxyl. The copolymerizations reported by Sen and coworkers⁵ were carried out at higher pressures (600 psi), so it is uncertain how these results relate to those experiments.

Acknowledgements

We thank NSF for support of this work. D. P. G gratefully acknowledges NSERC of Canada for a Postdoctoral Fellowship. Thanks to S. Svejda for providing a sample of CDCl₂F and to S. Shultz for useful discussions.

References

1. See, for example: ; A. Sen, Adv. Polym. Sci., 1986, 73/74, 125; E. Drent, J. A. M. van Broekhoven and M. J. Doyle, J. Organomet. Chem., 1991, 417, 235; F. C. Rix, M. Brookhart and P. S. White, J. Am. Chem. Soc., 1996, 118, 4746.
2. For general reviews, see: ; G. J. Kubas, Acc. Chem. Res., 1994, 27, 183; A. Wojcicki, Adv. Organomet. Chem., 1974, 12, 31; W. Kitching and C. W. Fong, Organomet. Chem. Rev. A, 1970, 5, 281.
3. The reaction of SO₂ with ethylene in the presence of catalytic amounts of PdCl₂ has been reported to produce EtS(O)₂CH₂CH[double bond, length half m-dash]CHCH₃. See: ; H. S. Klein, Chem. Commun., 1968, 377.
4. E. Drent, US Pat. 4 808 697, 1989..
5. L. M. Wojcinski, M. T. Boyer and A. Sen, Inorg. Chim. Acta, 1998, 270, 8.
6. M. Brookhart, B. Grant and A. F. Volpe, Organometallics, 1992, 11, 3920.
7. J. Ledford, C. S. Shultz and M. Brookhart, unpublished work..
8. M. S. Morton, R. J. Lachicotte, D. A. Vicic and W. D. Jones, Organometallics, 1999, 18, 227.
9. Quantitative analysis of the spectra were not performed but the barrier to isomer interconversions must lie in the range 14–15 kcal mol⁻¹ based on ³¹P NMR linewidths at 263 K..
10. J. Ledford and M. Brookhart, unpublished work..
11. Hex-1-ene was chosen as comonomer because the ethylene/SO₂ copolymers are poorly soluble and difficult to characterise..
12. K. J. Ivin, M. Navratil and N. A. Walker, J. Polym. Sci. Part A-1, 1972, 10, 701.
13. Polysulfones are generally prepared by radical initiated copolymerisation of SO₂ with olefins. Temperatures as low as −100 °C can be used. ; C. P. Tsonis, in Polymeric Materials Encyclopedia, ed. J. C. Salamone, CRC Press, New York, 1996, vol. 9, p. 6866..

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Footnotes

† Preparation of 4: under an atmosphere of argon, SO₂(g) (150 mL, 6 mmol) was slowly purged through a cloudy white stirred suspension of 1 (102 mg, 0.07 mmol) in CH₂Cl₂ (7 mL) at −78 °C. After stirring for 10 min a clear yellow solution was observed and the solvent was removed in vacuo yielding a yellow solid. The crude product was recrystallized by dissolution in 3 mL of CH₂Cl₂ at −78 °C and layering pentanes (1.5 mL) over the solution. Overnight at −30 °C light yellow crystals formed. Yield 28 mg (30%). ¹H NMR (CD₂Cl₂, 193 K); major isomer (78%): δ 7.72 (s, 8H, BAr′₄: o-H), 7.53 (s, 4H, BAr′₄: p-H), 7.5–7.2 (m, 20H, C₆H₅), 2.49 (br, 2H, PCH₂), 2.34 (br, 2H, PCH₂), 2.07 (m, 2H, CH₂), 0.52 (s, CH₃), minor isomers: δ 0.42 (s, CH₃) (8%), 0.25 (s, CH₃) (14%); ³¹P (CD₂Cl₂, 193 K) δ 15.7 (major isomer, ca. 80%), 15.4, 14.5 (minor isomers, ca. 20%). Anal. Calc.: C, 49.32; H, 2.83. Found: C, 49.37; H, 2.81%.

(dppp)Pd(Me)S(O)₂Me 7: ¹H NMR (CD₂Cl₂, 293 K) δ 7.6–7.2 (m, 20H, C₆H₅), 2.4 (m, 4H, PCH₂), 2.21 (s, 3H, SO₂CH₃), 2.1 (m, 2H, CH₂), 0.52 (pseudo t, J_PH 6 Hz, 3H, CH₃); ³¹P NMR (CD₂Cl₂, 293 K); δ 18.9 (J_PP 48 Hz), 3.1 (J_PP 48 Hz).

Polymerisation studies: a 10% solution of freshly distilled hex-1-ene (5 mL) in CH₂Cl₂ (45 mL) was purged with SO₂ for 7 min at room temperature. A solution of 1 (14 mg, 9 μmol) was added at −78 °C, and the reaction was stirred at room temperature for 14 h. Methanol (ca. 10 mL) was added to quench the reaction, the solution concentrated in vacuo, and methanol added to precipitate the polymer. Yield 23 mg. ¹H NMR (CDCl₃) δ 4.1–3.6 [br, 2H, SO₂CH₂], 3.3 [br, 1H, CH(Bu)SO₂], 2.5–1.7 (br, α-CH₂), 1.7–1.3 (br, 4H, CH₂CH₂), 0.9 (t, 3H, CH₃); GPC (THF, vs. PS standards): M_n = 61000, PDI 5.8. Identical conditions were applied for the reactions in the absence of 1 or in the presence of galvinoxyl (10 mg).

‡ Crystal data for C₆₅H₄₄BCl₄F₂₄O₂P ₂PdS 4: M = 1666.04, triclinic, space group P1̄, a = 13.0395(6), b = 17.1606(8), c = 17.9784(8) Å, α = 72.309(1), β = 78.658(1), γ = 68.341(1)°, V = 3545.8(3) ų, T = 173 K, Z = 2, μ(Mo-Kα) = 0.59 mm⁻¹, 2θ ⩽ 56°, 42774 reflections measured, 17105 unique (R_int = 0.031), 10812 observed [I_net > 2.5ς(I_net)]. R_f = 0.079 (observed data), R_w = 0.104 (unique data). The data were collected using a Bruker SMART diffractometer using the ω scan mode, and solved using direct methods and refined by full matrix least squares on F_o. CCDC 129/236.

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