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

Article abstract of DOI:10.1039/a905993h

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.

Full text of DOI:10.1039/a905993h

The insertion of sulfur dioxide into palladium–methyl bonds: the synthesis and
X-ray crystal structure of an unusual [(dppp)PdOS(Me)O]2[BArA4]2 dimer
Derek P. Gates, Peter S. White and Maurice Brookhart*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3290, USA. E-mail:
brook@net.chem.unc.edu
Received (in Bloomington, IN, USA) 26th July 1999, Accepted 22nd October 1999
The migratory insertion of sulfur dioxide into the palla-
dium(II)–methyl bond of [(dppp)Pd(Me)(OEt2)]BArA4 [ArA =
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.
crystals. Indeed, pale yellow crystals suitable for X-ray
diffraction were isolated after recrystallisation of the crude
product from dichloromethane–hexanes at 230 °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 phospho-
rus 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.
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(NC-
Me)]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 spectro-
scopic detection of the copolymerisation of ethylene and SO2.
The cationic 1,3-bis(diphenylphosphino)propane (dppp) pal-
ladium(ii) complex 1 can be prepared by the low temperature
Fig. 1 Molecular structure of 4 (BArA4 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 (d 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.
reaction of (dppp)PdMe2 6 with H(OEt2)2[BArA4] [ArA =
C6H3(CF3)2-3,5].6 We have previously studied the reactions of
carbon monoxide with 1 at 270 °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 d 15.7
indicating equivalence of the phosphorus nuclei. In addition,
two minor products (ca. 20%) with singlet resonances at d 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 d 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
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
2
10
[(dppp)Pd(Me)(h -C2H4)]BArA4 which was characterised by
31P [d = 17.5 (d, J 56 Hz), 21.5 (d, J 56 Hz)] and H NMR
1
spectroscopy [d 5.18 (bound C2H4), 0.34 (dd, Pd–CH3, J 7, 4
Hz)]. Upon warming the reaction mixture to 223 K for ca. 30
Chem. Commun., 2000, 47–48
This journal is © The Royal Society of Chemistry 2000
47

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
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.
We thank NSF for support of this work. D. P. G gratefully
acknowledges NSERC of Canada for a Postdoctoral Fellow-
ship. Thanks to S. Svejda for providing a sample of CDCl2F and
to S. Shultz for useful discussions.
bond of the methyl ethylene complex observed to form 5. This
2
suggests that formation of
4
from [(dppp)Pd(Me)(h -
C2H4)]BArA4 (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 SO2 via 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)].†
Notes and references
† 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 278 °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 278 °C and layering pentanes (1.5 mL) over the
solution. Overnight at 230 °C light yellow crystals formed. Yield 28 mg
(30%). 1H NMR (CD2Cl2, 193 K); major isomer (78%): d 7.72 (s, 8H,
BArA4: o-H), 7.53 (s, 4H, BArA4: 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: d 0.42 (s, CH3) (8%), 0.25 (s, CH3) (14%); 31P (CD2Cl2, 193 K) d
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%.
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 d 22.0 and 25.5
(dppp)Pd(Me)S(O)2Me 7: 1H NMR (CD2Cl2, 293 K) d 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, JPH 6 Hz, 3H, CH3); 31P NMR (CD2Cl2, 293 K); d 18.9 (JPP 48
Hz), 3.1 (JPP 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 mmol) was added at 278 °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)
d 4.1–3.6 [br, 2H, SO2CH2], 3.3 [br, 1H, CH(Bu)SO2], 2.5–1.7 (br, a-CH2),
1.7–1.3 (br, 4H, CH2CH2), 0.9 (t, 3H, CH3); GPC (THF, vs. PS standards):
Mn = 61000, PDI 5.8. Identical conditions were applied for the reactions in
the absence of 1 or in the presence of galvinoxyl (10 mg).
1
(JPP 50 Hz) and d 18.8 and 17.4 (JPP 20 Hz). In the H NMR
spectrum, in addition to 4, two new products were observed
which exhibited a methyl singlet at d 1.6 and a methyl doublet
of doublets at d 0.0 (JHP 7, 4 Hz). The signal at d 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 d 22.1 (d, J 50 Hz) and d 25.6 (d, J 50 Hz) can be
assigned to 2. The low field methyl resonance at d 1.6 [31P
resonances d 18.8 (d, J 20 Hz), d 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.
‡ Crystal data for C65H44BCl4F24O2P2PdS 4: M = 1666.04, triclinic, space
¯
group P1, a = 13.0395(6), b = 17.1606(8), c = 17.9784(8) Å, a =
72.309(1), b = 78.658(1), g = 68.341(1)°, V = 3545.8(3) Å3, T = 173 K,
Z = 2, m(Mo-Ka) = 0.59 mm21, 2q @ 56°, 42774 reflections measured,
17105 unique (Rint = 0.031), 10812 observed [Inet > 2.5s(Inet)]. Rf = 0.079
(observed data), Rw = 0.104 (unique data). The data were collected using
a Bruker SMART diffractometer using the w scan mode, and solved using
direct methods and refined by full matrix least squares on Fo. CCDC
129/236.
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; M. Brookhart, F. C. Rix, J. M. DeSimone, J. C. Barborak,
J. Am. Chem. Soc., 1992, 114, 5894; F. C. Rix, M. Brookhart and P. S.
White, J. Am. Chem. Soc., 1996, 118, 4746.
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 (Mn = 61 000: 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 3 1026 mol), the amount of hex-1-ene consumed (1.6 3
1024 mol) and the estimated Mn 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
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)2CH2CHNCHCH3. 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 mol21 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 copoly-
merisation of SO2 with olefins. Temperatures as low as 2100 °C can be
used. C. P. Tsonis, in Polymeric Materials Encyclopedia, ed. J. C.
Salamone, CRC Press, New York, 1996, vol. 9, p. 6866.
Communication a905993h
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Chem. Commun., 2000, 47–48