New imine-phosphine palladium complexes catalyze copolymerization of CO-ethylene and CO-norbornylene and provide well-characterized stepwise insertion intermediates of various unsaturated substrates

Article abstract of DOI:10.1021/om990222+

New imine-phosphine palladium complexes [Pd(P-N)(CH₃)(CH₃CN)](BF₄) (P-N = o-(diphenylphosphino)-N-benzaldimine derivatives) are synthesized and are found to be active for copolymerization of CO-ethylene and CO-norbornylene

Full text of DOI:10.1021/om990222+

2574
Organometallics 1999, 18, 2574-2576
New Im in e-P h osp h in e P a lla d iu m Com p lexes Ca ta lyze
Cop olym er iza tion of CO-Eth ylen e a n d
CO-Nor bor n ylen e a n d P r ovid e Well-Ch a r a cter ized
Step w ise In ser tion In ter m ed ia tes of Va r iou s
Un sa tu r a ted Su bstr a tes
K. Rajender Reddy, Chi-Li Chen, Yi-Hung Liu, Shie-Ming Peng,
J wu-Ting Chen, and Shiuh-Tzung Liu*
Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, Republic of China
Received March 30, 1999
Summary: New imine-phosphine palladium complexes
[Pd(P-N)(CH3)(CH3CN)](BF4) (P-N ) o-(diphenylphos-
phino)-N-benzaldimine derivatives) are synthesized and
are found to be active for copolymerization of CO-
ethylene and CO-norbornylene, respectively. Stepwise
addition of carbon monoxide and various olefins into the
new complexes is investigated.
and strained alkenes have been isolated.10 The avail-
ability of â-hydrogen in simple alkenes which leads to
decomposition is generally believed to be responsible for
difficulty in isolating inserted intermediates. However,
the success of ethylene11 and propene12 insertion into
the Pd-acyl bond demonstrates that the steric require-
ment is not the sole factor for the stability of inserted
products but the electronic factors governed by the
auxiliary ligands also play an important role. We report
in this communication that new imine-phosphine pal-
ladium complexes exhibit catalytic activity in copolym-
erization of ethylene-carbon monoxide (E-CO) and
norbornylene-CO. Still, the controlled reactions of such
imine-phosphine Pd(II) complexes uniquely provide the
well-characterized intermediates of sequential insertion
of CO with ethylene as well as functionalized olefins and
alkynes.
Using the new imine-phosphine complex [Pd(P-N)-
(CH3)(CH3CN)](BF4) (P-N ) o-(diphenylphosphino)-N-
benzaldimine (1a ))13 as catalyst, copolymerization of
E-CO can be carried out under mild conditions. In one
of the typical reactions, 1a (22.0 mg, 0.03 mmol) with
CO and ethylene (40 psi for each in a 200 mL autoclave)
in 75 mL of CH2Cl2 at 75-80 °C produces 0.5 g of
polyketone after 48 h.14 Complex 1a also affords the
copolymerization of norbornylene-CO, but the fluori-
Insertion of unsaturated molecules into a metal-
carbon bond is a fundamental step in palladium(II)-
catalyzed copolymerization of carbon monoxide and
olefins.1,2 Among the studied systems, catalysts are
confined to either bidentate phosphine3 and imine4 or
monodentate phosphine.5 Although the soft and hard
donor natures of phosphorus and nitrogen are known
to bind Pd in a unique way, the imine-phosphine
ligands, in which imine acts as a weak π-acceptor and
phosphine as a good σ-donor, have not been well
explored. The importance of these ligands in Pd-based
catalysis, however, is evident with some recent observa-
tion in copolymerization,6 ethylene oligomerization,7 and
cross-coupling reactions.8
From a mechanistic viewpoint, although the basic
mechanism for chain propagation of CO/olefin copolym-
erization at the cis site of Pd(II) species has been
established,9 only the inserted intermediates with bulky
(1) (a) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
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Herrmann, W. A. Applied Homogeneous Catalysis with Organometallic
Compounds; VCH: Weinheim, Germany, 1996; Vol. 1.
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Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663.
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(b) Abu-Surrah, A. S.; Wursche, R.; Rieger, B.; Eckert, G.; Pechhold,
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Soc. 1995, 117, 4455. (d) J iang, Z.; Adams, S. E.; Sen, A. Macromol-
ecules 1994, 27, 2694. (e) Drent, E.; van Broekhoven, J . A. M.; Doyle,
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J . Am. Chem. Soc. 1992, 114, 5894. (b) Sen, A.; J iang, Z. Macromol-
ecules 1993, 26, 911.
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Acta 1996, 79, 1387. (b) Sperrle, M.; Consiglio, G. Chem. Ber. 1997,
130, 1557 and references therein.
(7) van den Beuken, E. K.; Smeets, W. J . J .; Spek, A. L.; Feringa,
B. L. J . Chem. Soc., Chem. Commun. 1998, 223.
(8) (a) Shirakawa, E.; Yoshida, H.; Takaya, H. Tetrahedron Lett.
1997, 38, 3759. (b) Shirakawa, E.; Yoshida, H.; Kurahashi, T.; Nakao,
Y.; Hiyama, T. J . Am. Chem. Soc. 1998, 120, 2975.
(9) (a) Mecking, S.; J ohnson, L. K.; Wang, L. Brookhart, M. J . Am.
Chem. Soc. 1998, 120, 888. (b) Margl, P.; Ziegler, T. Organometallics
1996, 15, 5519. (c) Svensson, M.; Matsubara, T.; Morokuma, K.
Organometallics 1996, 15, 5568. (d) Rix, F. C.; Brookhart, M.; White,
P. S. J . Am. Chem. Soc. 1996, 118, 4746. (e) Brumbaugh, J . S.; Whittle,
R. R.; Parvez, M.; Sen, A. Organometallics 1990, 9, 1735. (f) Zuideveld,
M. A.; Kamer, P. C. J .; van Leeuwen, P. W. N. M.; Klusener, P. A. A.;
Stil, H. A.; Roobeek, C. F. J . Am. Chem. Soc. 1998, 120, 7977.
(10) (a) Carfagna, C.; Formica, M.; Gatti, G.; Musco, A.; Pierleoni,
A. J . Chem. Soc., Chem. Commun. 1998, 1113. (b) Markies, B. A.;
Kruis, D.; Rietveld, M. H. P.; Verkerk, K. A. N.; Boersma, J .; Kooijman,
H.; Lakin, M. T.; Spek, A. L.; van Koten, G. J . Am. Chem. Soc. 1995,
117, 5263. (c) van Asselt, R.; Gielens, E. E. C. G.; Rulke, R. E.; Vrieze,
K.; Elsevier: C. J . J . Am. Chem. Soc. 1994, 116, 977. (d) Ozawa, F.;
Hayashi, T.; Koide, H.; Yamamoto, A. J . Chem. Soc., Chem. Commun.
1991, 1469.
(11) (a) Rix, F. C.; Brookhart, M. J . Am. Chem. Soc. 1995, 117, 1137.
(b) Green, M. J .; Britovsek, G. J . P.; Cavell, K. J .; Gerhards, F.; Yates,
B. F.; Frankcombe, K.; Skelton, B. W.; White, A. H. J . Chem. Soc.,
Dalton Trans. 1998, 1137.
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Hiyama, T.; Matsubara, T.; Koga, N. J . Am. Chem. Soc. 1997, 119,
12779.
(13) Spectroscopic data for complex 1a : 1H NMR (300 MHz, CDCl3)
3
δ 0.57 (d, J P-H ) 1.6 Hz, 3 H, PdCH3), 1.81 (s, 3 H, Pd-NCCH3),
7.09-7.57 (m, 16 H), 8.03 (m, 1 H), 8.23 (d, J ) 7 Hz, 2 H), 9.26 (s, 1
H); 13C NMR δ -2.1 (Pd-CH3), 1.81 (NCCH3); 31P NMR δ 38.6.
10.1021/om990222+ CCC: $18.00 © 1999 American Chemical Society
Publication on Web 06/15/1999

Communications
Organometallics, Vol. 18, No. 14, 1999 2575
Sch em e 1. Step w ise CO/Eth ylen e In ser tion in to th e P d -C Bon d a
a
Numbering of aromatic carbons in ORTEP plot of cation 5 omitted for clarity. Selected bond distances (Å): Pd-C7 ) 2.003(9),
C5-O2 ) 1.20(1), C2-O1 ) 1.18(1).
Sch em e 2. Resu lts of In ser tion Stu d ies w ith Va r iou s Un sa tu r a ted Ca r bon -Ca r bon Su bstr a tesa
a
Selected bond distances (Å): Pd-O1 ) 2.088(8), Pd-C3 ) 2.00(1), C4-O1 ) 1.21(2) for 6; Pd-O1 ) 2.104(4), Pd-C4 )
2.000(6), C2-O1 ) 1.283(9) for 7; Pd-O1 ) 2.080(4), Pd-C6 ) 1.986(5), C2-O1 ) 1.238(7), C3-C6 ) 1.352(8) for 8; Pd-O2 )
2.148(3), Pd-C7 ) 2.058(4), C5-O2 ) 1.245(5) for 9.
nated-benzaldimine-phosphine derivative 1b (Ar )
p-FC6H4-) provides a better yield. The resulting mate-
rial is a white solid and is soluble in most organic
solvents. The molecular weight was determined by GPC
analysis (Mn ) 2500, Mw/Mn ) 1.26).15
At 25 °C, consecutive bubbling of CO and ethylene
into the dichloromethane solution of 1a uniquely re-
sulted in the formation of 2-5 (Scheme 1).16 Intermedi-
ates 3 and 5, which have been isolated in the solid state,
are stable. In contrast, acyl complexes 2 and 4 decom-
pose slowly in solution as well as in the solid state.
Appearance of a single 31P signal for 2-5 at 18.4, 36.7,
19.9, and 36.8 ppm, respectively, suggests the quantita-
tive formation of only one product in each step. The
infrared spectrum of compound 5 shows two CdO
stretching bands at 1712 and 1629 cm-1 corresponding
to free and coordinated carbonyl groups, respectively.
(14) CO/ethylene copolymer: IR(KBr) νCO 1691 cm-1
(CDCl3 + CF3COOH) 36.2, 213.8.
;
13C NMR δ-
(15) In a 100 mL autoclave was placed 1b (30 mg), norbornylene
(350 mg), CH2Cl2 (20 mL), CH3CN (50 mL), and CO (60 psi). When
the mixture was heated at 50-60 °C for 24 h, the polymer was
precipitated, which was washed with aqueous hydrochloric acid and
hexane and dried (150 mg): 1H NMR (CDCl3) δ 3.30-1.90 (br, 4 H),
1.85-0.85 (br, 6 H); 13C NMR δ 211.2-209 (br), 57.9-49.0 (Br), 40.8-
36.4 (br), 30.9-27.0 (br); IR(KBr) νCO 1720 cm-1
.

2576 Organometallics, Vol. 18, No. 14, 1999
Communications
Coordination of the CdO moiety to the palladium is
well-documented via the formation of a five-membered
chelate.10 The possibility of six-membered chelation in
compound 4 is ruled out on the basis of the 13C NMR
spectrum, which shows two peaks at 223.1 and 206.6
ppm corresponding to metal-bound acyl and free car-
bonyl groups, respectively. No shift in the latter peak
is observed upon ethylene insertion leading to the
formation of 5. The chelated carbonyl carbon in 5,
however, is shifted downfield and appears at 231.3 ppm.
The X-ray single-crystal structure of 5 reveals its
composition in detail. The coordinating alkyl group is
cis to the phosphine, which is in agreement with the
crystal structure data available for the Pd-imine-
phosphine complex7 and is considered to be the ther-
modynamically favored form.17 However, the other
isomer in which alkyl group is trans to phosphine,
reported by Shirakawa and co-workers for the Pd
complex,8a is not observed in our studies. In viewing the
retention of stereochemistry through all insertion steps
in 1a , the new imine-phosphine ligands appear to
stabilize the alkyl and acyl species in a selective
manner. Complex 5 is stable at room temperature even
in air without any decomposition for several days,
indicating that the â-elimination process is suppressed.
In fact, the similar intermediates for the copolymeriza-
tion of norbornylene and carbon monoxide can be
isolated.
To understand the relative migratory insertion rates
for the present system, 0.013 mmol each of 1a and 3
are dissolved in 0.5 mL of CDCl3 (with 5% CD3CN)
under a CO atmosphere. The relative intensity of CO-
inserted products (2 and 4) in the 31P NMR spectrum
suggests that the first insertion is 1.6 times faster than
the second insertion. Similarly, under an ethylene
atmosphere the relative consumption (disappearance)
of intensity of 31P signals corresponding to 2 and 4
shows that the first and second ethylene insertion rates
are basically the same. The absence of any CO insertion
for 3 in CDCl3 suggests that releasing the chelation via
the dissociation of the coordinated carbonyl moiety
should be a prerequisite for CO insertion. Further
evidence is that addition of the limited amount of CH3-
CN facilitates CO insertion. Presumably acetonitrile,
even a stronger ligand than CO, significantly stabilizes
the CO-insertion intermediates. This also explains the
slightly faster carbonylation in 1a , where there is no
such chelation.
Besides ethylene, other substituted olefins and alkynes
undergo similar insertion reactions with such new
imine-phosphine complexes, which are summarized in
Scheme 2.18 Vinyl acetate is found to undergo alkyl
migration with 1a and acyl migration with 2 to yield
the insertion products 6 and 7, respectively. In both
cases, the insertion takes place at the terminal carbon
of the olefin. Insertion of ethylphenylacetylene into the
metal-acyl bond proceeds regioselectively at room tem-
perature in dichloromethane solution to produce 8.
Although the insertion of the alkyne into a Pd-acyl
bond is well-documented,19 the intermolecular insertion
of substituted acetylene into a Pd-acyl bond with
exclusive regioselectivity is unprecedented. Even a
consecutive insertion of ethylene, carbon monoxide, and
methyl acrylate is accomplished under mild conditions
to give 9 as the exclusive product. When the reactions
are monitored by 31P NMR, all are shown to proceed
cleanly to yield a single product without the formation
of the other possible stereoisomeric metal complexes in
each instance, revealing again the unique features of
these imine-phosphine complexes. All products in
Scheme 2 have been characterized spectroscopically and
further confirmed by X-ray single-crystal analyses.
In conclusion, the present results show that by using
imine-phosphine ligands it is possible to stabilize the
CO- and alkene- or alkyne-inserted products, which may
thus serve as a catalyst for copolymerization with
functionalized olefins and/or alkynes. The imine-phos-
phine complex also demonstrates its versatility toward
different olefins in the insertion reaction. Subsequent
insertion of various olefinic substrates into the M-C
bond and their synthetic application are currently under
investigation.
Ack n ow led gm en t. We thank the National Science
Council (Grant No. NSC88-2113-M002-32) for financial
support.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental procedures and complete descriptions of the X-ray
crystallographic structure determinations of 5-9, including
figures showing ORTEP plots and tables of crystal data, atomic
coordinates, isotropic and anisotropic thermal parameters, and
bond distances and angles. This material is available free of
charge via the Internet at http://pubs.acs.org.
(16) Spectroscopic data for complex 2: IR (KBr) νCO 1707 cm-1 1H
;
NMR (500 MHz, -50 °C, CDCl3) δ 1.76 (s, 3 H, Pd-NCCH3), 1.99 (s,
3 H, PdCOCH3), 7.20-7.69 (m, 16 H), 7.97 (m, 1 H), 8.28 (m, 2 H),
9.11 (s, 1 H); 13C NMR δ 2.0 (NCCH3), 37.3 (COCH3), 226.9 (PdCOCH3);
31P NMR δ 18.4. Complex 3: IR (KBr) νCO 1636 cm-1 (coordinated);
1H NMR (200 MHz, CDCl3) δ 1.86 (t, J ) 7 Hz, 2 H, Pd-CH2), 2.18 (s,
3 H, COCH3), 3.10 (t, J ) 7 Hz, 2 H), 7.23-7.69 (m, 17 H), 8.29 (m, 2
H), 9.15 (s, 1 H); 13C NMR δ 21.6 (PdCH2), 27.5 (COCH3), 50.4 (PdCH2),
232.9 (COCH3); 31P NMR δ 36.7. Complex 4: 1H NMR (300 MHz,
CDCl3) δ 1.77 (s, 3 H, PdNCCH3), 1.94 (s, 3 H, COCH3), 2.26 (br, 2 H,
PdCOCH2), 2.72 (br, 2 H), 7.19-7.69 (m, 16 H), 7.89 (1 H), 8.26 (2 H),
9.07 (s, 1 H); 13C NMR δ 29.3 (COCH3), 37.4 (COCH2), 44.5 (PdCOCH2),
206.6 (COCH3), 223.1 (PdCO); 31P NMR δ 19.9. Complex 5: IR (KBr)
OM990222+
(18) Selective spectral data for 6-9 are as follows. Complex 6: IR
(KBr) νCO 1612 cm-1 1H NMR δ 1.04 (s, 3 H, -CH3), 1.59 (m, 2 H,
;
-CH2-), 2.0 (s, 3H, COCH3), 5.27 (m, 1 H, PdCH-); 31P NMR δ 34.7.
νCO 1712 (free), 1629 (coordinated) cm-1
;
1H NMR (200 MHz, CDCl3)
Complex 7: IR (KBr) νCO 1713, 1610 cm-1 1H NMR δ 1.92 (s, 3 H),
;
δ 1.89 (m, 5 H, COCH3 + PdCOCH2), 2.32 (t, J ) 7 Hz, 2 H), 2.77 (t,
J ) 7 Hz, 2 H), 3.03 (t, J ) 7 Hz, 2 H), 7.18-7.95 (m, 17 H), 8.30 (m,
2 H), 9.20 (s, 1 H); 13C NMR δ 20.7 (PdCH2), 29.1 (COCH3), 33.7
(COCH2), 36.7 (COCH3), 48.9 (PdCH2CH3), 206.3 (COCH3), 231.3
(CH2COCH2); 31P NMR δ 36.8.
2.12 (s, 3 H), 2.78 (m, 2 H), 5.12 (m, 1 H, PdCH-); 31P NMR δ 35.67.
Complex 8: IR (KBr) νCO 1607 cm-1; 1H NMR δ 0.77 (t, J ) 7 Hz, 3 H,
-CH3), 1.80 (q, J ) 7 Hz, 2 H, -CH2-), 2.08 (s, 3 H, COCH3); 31P
NMR δ 38.9. Complex 9: IR (KBr) νCO 1723, 1696, 1623 cm-1; 1H NMR
δ 1.85 (s, 3 H, COCH3), 2.22 (m, 2 H, -CH2CHPd), 2.87 (m, 4 H,
-CH2CH2-), 3.5 (m, 1 H, PdCH-); 31P NMR δ 37.7.
(17) de Graaf, W.; Harder, S.; Boersma, J .; van Koten, G.; Kanters,
J . A. J . Organomet. Chem. 1988, 358, 545.
(19) Samsel, E. G.; Norton, J . R. J . Am. Chem. Soc. 1984, 106, 5505.