Structural characterization of [κ2-(t-Bu)2PCH2C(O)C6 H5]PdMe(η2-C2H4)+ BAr′4-: A model for the catalyst resting state for ethylene polymerization

Article abstract of DOI:10.1021/om020951g

The Pd methyl-ethylene complex (P,O)Pd-Me(η²-C₂H₄)⁺ BAr′₄- ((P,O) = phenacyldi-tert-butylphosphine; Ar′ = 3,5-(CF₃)₂C₆H₃) has been isolated and structural characterized by X-ray crystallography. The ethylene ligand is oriented perpendicular to the square plane and is cis to the methyl ligand. This compound closely models the resting state(s) for ethylene oligomerization /polymerization by such complexes.

Full text of DOI:10.1021/om020951g

Organometallics 2003, 22, 621-623
621
Str u ctu r a l Ch a r a cter iza tion of
-
[K2-(t-Bu )2P CH2C(O)C6H5]P d Me(η2-C2H4)+BAr ′4 : A Mod el
for th e Ca ta lyst Restin g Sta te for Eth ylen e
P olym er iza tion
J on M. Malinoski, Peter S. White, and Maurice Brookhart*
Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
Received November 18, 2002
Summary: The Pd methyl-ethylene complex (P,O)Pd-
Me(η2-C2H4)+BAr′4- ((P,O) ) phenacyldi-tert-butylphos-
phine; Ar′ ) 3,5-(CF3)2C6H3) has been isolated and
structurally characterized by X-ray crystallography. The
ethylene ligand is oriented perpendicular to the square
plane and is cis to the methyl ligand. This compound
closely models the resting state(s) for ethylene oligomer-
ization/ polymerization by such complexes.
The development of single-site, homogeneous transi-
tion-metal catalysts for olefin polymerization has led to
a greater understanding of the nature of the catalytic
species and the mechanisms of monomer enchainment
in these systems. Much of the work in this area has
focused on early-metal d0 catalysts, in particular met-
allocene complexes.1-5 Recently, however, considerable
efforts have been directed at developing cationic and
neutral late-transition-metal catalysts for olefin po-
lymerization.6-9 The discovery that Ni(II) and Pd(II)
complexes bearing bulky R-diimine ligands are highly
active catalysts has led to a number of experimental and
theoretical mechanistic investigations.10-16 Central to
these studies is the generation of a cationic catalyst
precursor which, upon addition of an olefin such as
ethylene, rapidly forms an alkyl-olefin species which
models the resting state for catalytic olefin polymeri-
zation (eq 1). Metal alkyl-olefin complexes are typically
thermally unstable, as migratory insertion of ethylene
into the metal-alkyl bond is facile. These species are
normally observed in situ via NMR experiments at low
temperatures. Warming of these samples allows obser-
vation of migratory insertion rates as well as chain
propagation and isomerization processes.
We recently reported a series of Ni(II) and Pd(II)
complexes bearing the bulky P,O chelate phenacyldi-
tert-butylphosphine that are active at elevated temper-
ature and ethylene pressure for the production of linear
polyethylene in the case of the Ni(II) catalyst and
ethylene oligomers in the case of the Pd(II) com-
plexes.17,18 Complexes used for initiation of polymeri-
zation are shown in Figure 1. We report here the
synthesis, isolation, and structural characterization of
the Pd methyl-ethylene complex (P,O)PdMe(η2-C2H4)+-
-
BAr′4 ((P,O) ) phenacyldi-tert-butylphosphine; Ar′ )
(1) Waymouth, R. M.; Coates, G. W. Science 1995, 267, 217.
(2) Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143.
(3) Coates, G. W. Chem. Rev. 2000, 100, 1223.
3,5-(CF3)C6H3), a model for the catalyst resting state
in the (P,O)PdII system.
(4) Tian, J .; Hustad, P. D.; Coates, G. W. J . Am. Chem. Soc. 2001,
123, 5134.
(5) Matsui, S.; Mitani, M.; Saito, J .; Tohi, Y.; Makio, H.; Matsukawa,
N.; Takagi, Y.; Tsuru, K.; Nitaburu, M.; Nakano, T.; Tanaka, H.;
Kashiwa, N.; Fujita, T. J . Am. Chem. Soc. 2001, 123, 6847.
(6) Ittel, S. D.; J ohnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100,
1169.
(7) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 429.
(8) Keim, W.; Kowalt, F. H.; Goddard, R.; Kru¨ger, C. Angew. Chem.,
Int. Ed. Engl. 1978, 17, 466.
(9) Wang, C.; Friedrich. S.; Younkin, T. R.; Li, R. T.; Grubbs, R. H.;
Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3419.
(10) J ohnson, L. K.; Killian, C. M.; Brookhart, M. J . Am. Chem. Soc.
1995, 117, 6414.
(11) J ohnson, L. K.; Mecking, S.; Brookhart, M. J . Am. Chem. Soc.
1996, 118, 267.
The previously reported ether adduct (P,O)PdMe-
(OEt2)+BAr′4 (1) was prepared from the neutral com-
-
plex (P,O)PdMeCl via chloride abstraction by NaBAr′4
in diethyl ether solvent. Complex 1 is a powdery white
solid that is stable for several days at ambient temper-
ature under an inert atmosphere. Treatment of a
solution of 1 in methylene chloride at -30 °C with a
large excess of ethylene followed by precipitation into
stirring pentane at -78 °C yields the white solid (P,O)-
PdMe(η2-C2H4)+BAr′4 (2) (eq 2). Complex 2 is stable
-
up to 0 °C in the solid state and can be stored for several
days under argon at -35 °C. Above 0 °C the white
powder quickly decomposes to an uncharacterizable
(12) Mecking, S.; J ohnson, L. K.; Wang, L.; Brookhart, M. J . Am.
Chem. Soc. 1998, 120, 888.
(13) Svejda, S. A.; J ohnson, L. K.; Brookhart, M. J . Am. Chem. Soc.
1999, 121, 10634.
(14) Tempel, D. J .; J ohnson, L. K.; Huff, R. L.; White, P. S.;
Brookhart, M. J . Am. Chem. Soc. 2000, 122, 6686.
(15) Michalak, A.; Ziegler, T. Organometallics 2000, 19, 1850.
(16) Michalak, A.; Ziegler, T. J . Am. Chem. Soc. 2002, 124, 7519.
(17) Liu, W.; Malinoski, J . M.; Brookhart, M. Organometallics 2002,
21, 2836.
(18) Wang, L.; Brookhart, M.; J ohnson, L. K.; Ittel, S. D.; Wang, Y.;
Kunitsky, K.; Kreutzer, K. A.; Guan, Z.; Liu, W.; Malinoski, J . M. WO
0259165, 2002.
10.1021/om020951g CCC: $25.00 © 2003 American Chemical Society
Publication on Web 01/21/2003

622 Organometallics, Vol. 22, No. 4, 2003
Communications
F igu r e 1. (P,O)NiII and (P,O)PdII precatalysts for ethylene
polymerization.
-
F igu r e 3. Thermal ellipsoid plot of 2. The BAr′4 coun-
terion is omitted for clarity. Relevant bond distances (Å)
and angles (deg): Pd(1)-C(1) ) 2.029(8), Pd(1)-C(2) )
2.297(7), Pd(1)-C(3) ) 2.274(8), Pd(1)-P(1) ) 2.2782(18),
Pd(1)-O(4) ) 2.188(5), C(2)-C(3) ) 1.346(12), Pd-(C(2)-
C(3) centroid) ) 2.18, C(5)-C(6) ) 1.518(10), C(5)-O(4) )
1.245(8); C(1)-Pd(1)-C(2) ) 88.9(3), C(1)-Pd(1)-C(3) )
93.7(3), P(1)-Pd(1)-O(4) ) 82.83(18), C(6)-P(1)-C(13) )
102.6(4), C(6)-P(1)-C(17) ) 104.6(3).
η2 fashion and is oriented nearly perpendicular to the
square plane of the molecule (81.4(7)° relative to the
plane defined by C(1)-Pd(1)-P(1)). Although there are
no other reported crystal structures of Pd methyl-
ethylene complexes, the near-orthogonal orientation of
the bound ethylene is consistent with other examples
of d8 metal-ethylene complexes.19-21 The carbon-
carbon bond length for the bound ethylene is very
similar to that in free ethylene, indicating little back-
bonding from Pd into the antibonding π* orbital.22,23 The
distance from Pd to the ethylene C(2)-C(3) centroid is
2.18 Å. As expected on electronic grounds, the strong
donor methyl ligand lies trans to the carbonyl oxygen
and ethylene is trans to the strong donor phosphine
ligand. The phenyl ring of the P,O ligand is slightly
twisted from coplanarity with C(6)-C(5)-O(4) (torsion
angle C(6)-C(5)-C(7)-C(8) ) 14.7(14)°), while the
tetrahedral geometry of phosphorus results in projection
F igu r e 2. 1H NMR spectrum of (P,O)PdMe(η2-C2H4)+-
-
BAr′4 (400 MHz, CD2Cl2, -20 °C).
(19) Reported crystal structures of square-planar Pt(II) methyl-
ethylene complexes: (a) Fusto, M.; Giordano, F.; Orabona, I.; Ruffo,
F.; Panunzi, A. Organometallics 1997, 16, 5981. (b) Reinartz, S.; White,
P. S.; Brookhart, M.; Templeton, J . L. Organometallics 2000, 19, 3854.
(20) Other examples of d8 Pd(II) and Pt(II) ethylene crystal struc-
tures: (a) Dempsey, J . N.; Baenziger, N. C. J . Am. Chem. Soc. 1955,
77, 4984. (b) Love, R. A.; Koetzle, T. F.; Williams, G. J . B.; Andrews,
L. C.; Bau, R. Inorg. Chem. 1975, 14, 2653. (c) Hodgson, M.; Parker,
D.; Taylor, R. J .; Gerguson, G. Chem. Commun. 1987, 1309.
(21) Other examples of transition-metal methyl-ethylene crystal
structures: (a) Churchill, M. R.; Wasserman, H. J . Inorg. Chem. 1981,
20, 4119. (b) Miao, F. M.; Prout, K. Acta Crystallogr., Sect. B 1982,
38, 945. (c) Umezawa-Vizzini, K.; Lee, T. R. J . Organomet. Chem. 1999,
579, 122. (d) Lundquist, E. G.; Folting, K.; Huffman, J . C.; Caulton,
K. G. Organometallics 1990, 9, 2254.
dark oil. In CD2Cl2 solution 2 begins to decompose at
-50 °C; however, its stability can be greatly enhanced
by the addition of several equivalents of ethylene. Figure
1
2 shows a H NMR spectrum of 2 in CD2Cl2 at -20 °C
with 3 equiv of added ethylene. The Pd-methyl reso-
nance is visible at δ 0.95 (3J HP ) 2 Hz), while the Pd-
(η2-C2H4) resonance appears at δ 5.57 as a broad singlet,
indicating rapid exchange of bound and free ethylene
on the NMR time scale. Even at -95 °C separate
resonances for free and bound ethylene were not ob-
served. The 31P{1H} spectrum of 2 at -40 °C shows one
sharp singlet at δ 66.3 ppm, which is indicative of a
single Pd species.
(22) The CdC bond length in free ethylene has been measured to
be 1.339 Å: Tables of Interatomic Distances and Configuration in
Molecules and Ions; Sutton, L. E., Ed.; The Chemical Society: London,
1958.
(23) (a) Mingos, D. M. P. Bonding of Unsaturated Organic Molecules
to Transition Metals. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: New York,
1982; Vol. 3, pp 1-88. (b) Segal, J . A.; J ohnson, B. F. G. J . Chem. Soc.,
Dalton Trans. 1975, 677. (c) Koemm, U.; Kreiter, C. G. J . Organomet.
Chem. 1982, 240, 27. (d) Albright, T. A.; Hoffman, R.; Thibeault, J . C.;
Thorn, D. L. J . Am. Chem. Soc. 1979, 101, 3801.
X-ray-quality crystals of 2 were grown from diethyl
ether/pentane at -35 °C under argon. An ORTEP
diagram of complex 2 is shown in Figure 3. The coor-
dination geometry around Pd is square planar, with the
methyl and ethylene ligands in a cis configuration
(C(1)-Pd(1)-(C(2)-C(3) centroid) ) 91.3(3)°). The eth-
ylene unit binds trans to the phosphorus ligand in an
(24) A five-coordinate Pd(II) methyl complex bearing an η2-maleic
anhydride ligand has been reported: Albano, V. G.; Castellari, C.;
Cucciolito, M. E.; Panunzi, A.; Vitagliano, A. Organometallics 1990,
9, 1269.

Communications
Organometallics, Vol. 22, No. 4, 2003 623
of the bulky tert-butyl groups toward the axial sites
above and below the square plane. To the best of our
knowledge, this is the first example of a structurally
characterized Pd(II) alkyl-ethylene complex, the cata-
lyst resting state for ethylene polymerization.
for 2 matches that previously measured using 1 as a
precursor for the active systems.
Ack n ow led gm en t. We thank DuPont and the Na-
tional Science Foundation (Grant No. CHE-0107810) for
financial support of this work.
The kinetics of insertion of ethylene into the Pd-Me
1
bond in 2 were measured by H NMR spectroscopy. At
0 °C the observed rate constant was kobs ) 5.2 × 10-5
s-1, corresponding to a free energy barrier to insertion
of ∆Gq ) 21.3 kcal/mol. Subsequent insertions resulted
in polyethylene oligomers that were identical with those
produced by complex 1. The insertion barrier measured
Su p p or tin g In for m a tion Ava ila ble: Text detailing the
synthesis and characterization of 2 as well as crystal structure
data. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM020951G