Self-assembled tetranuclear palladium catalysts that produce high molecular weight linear polyethylene

Article abstract of DOI:10.1021/ja909473y

(Figure Presented) The phosphine-bis-arenesulfonate ligand PPh(2-SO ₃Li-4-Me-Ph)₂ (Li₂[OPO]) coordinates as a K²-P,O chelator in Li[(Li-OPO)PdMe(Cl)] (2a) and (Li-OPO)PdMe(L) (L = pyridine (2b); MeOH (2d); 4-(5-nonyl)pyridine) (py, 3)). 2a reacts with AgPF₆ to form {(Li-OPO)PdMe}_n (2c). Photolysis of 2d yields {(OPO)Pd}₂ (5) in which the [OPO]^2- ligand coordinates as a K³-O,P,O pincer. 3 self-assembles into a tetramer in which four (Li-OPO)PdMe(py) units are linked by Li-O bonds that form a central Li₄S₄O₁₂ cage. The Pd centers are equivalent but are spatially separated into two identical pairs. The Pd-Pd distance within each pair is 6.04 A. IR data (u(ArSO₃ ^-) region) suggest that the solid state structures of 2a-c are similar to that of 3. 3 reacts with the cryptand Krypt211 to form [Li(Krypt211)][(OPO)PdMe(py)] (4). 3 is in equilibrium with a monomeric (Li-OPO)PdMe(py) species (3) in solution. 2a-c and 3 produce polyethylene (PE) with high molecular weight and a broad molecular weight distribution, characteristic of multisite catalysis. Under conditions where the tetrameric structure remains substantially intact, the PE contains a substantial high molecular weight fraction, while, under conditions where fragmentation is more extensive, the PE contains a large low molecular weight fraction. These results suggest that the tetrameric assembly gives rise to the high molecular weight polymer. In contrast, the monomeric complex 4, which contains a free pendant sulfonate group that can bind to Pd, oligomerizes ethylene to a Schultz-Flory distribution of C₄-C₁₈ oligomers.

Full text of DOI:10.1021/ja909473y

Published on Web 12/17/2009
Self-Assembled Tetranuclear Palladium Catalysts That Produce High
Molecular Weight Linear Polyethylene
Zhongliang Shen and Richard F. Jordan*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
Received November 7, 2009; E-mail: rfjordan@uchicago.edu
Palladium catalysts based on ortho-phosphino-arenesulfonate
(PO-) ligands display interesting olefin polymerization proper-
ties. (PO)Pd(R)(L) catalysts (L ) labile ligand) polymerize
ethylene to linear polyethylene (PE) and copolymerize ethylene
with polar vinyl monomers to linear copolymers.1 However, these
catalysts typically exhibit low ethylene polymerization activity
(<100 g ·mmol-1 ·h-1 ·atm-1) and produce PEs with low molecular
weights (MWs).1c Polar comonomers invariably decrease activities
and MWs. Here we describe (PO)Pd(R)(L) catalysts that self-
assemble into tetrameric species that produce very high MW PE.
Scheme 1
The new ligand PPh(2-SO3Li-4-Me-Ph)2 (Li2[OPO], 1) reacts
with (COD)PdMeCl to yield {Li[(Li-OPO)PdMe(Cl)]}n (2a, Scheme
1). NMR studies show that 2a undergoes reversible solvolysis in
MeOH to form 2d. The reaction of 2a with AgPF6 in MeOH
followed by recrystallization from MeOH/Et2O yields the base-
free complex 2c. 2c dissolves in MeOH to form 2d and reacts with
pyridine (py) to form adduct 2b. NMR studies show that, in CD3OD,
2a,b,d contain one Pd-coordinated and one free sulfonate group,
which exchange rapidly above -20 °C.
2a-d are soluble only in solvents such as CH3OH and H2O that
can solvate the pendant ArSO3Li group. In order to prepare a more
soluble (Li-OPO)PdMe(L) complex, 4-(5-nonyl)pyridine (py′) was
used as the ligand L.2 The reaction of 2c with py′ in CH2Cl2 yields
{(Li-OPO)PdMe(py′)}4 (3), which was isolated by recrystallization
from CH2Cl2/Et2O. The solid state structure of 3 (Figure 1) is a
cubic assembly of four (Li-OPO)PdMe(py′) units linked by Li-O
bonds that form a central Li4S4O12 cage. The [Li-OPO]- ligand
acts as a κ2-P,O chelator to Pd. One oxygen of each Pd-bound
ArSO3- group (O(3)) and all three oxygens of each non-Pd-bound
ArSO3- group (O(4)-O(6)) coordinate to Li+ ions to form the cage.
Similar motifs were observed in sulfonate MOFs.3 The Pd atoms
are equivalent but are spatially separated into two pairs. The Pd-Pd
distance in each pair (Pd(1)-Pd(1A)) is 6.04 Å. The Pd-Me groups
lie directly under the py′ ring of the neighboring (Li-OPO)Pd-
Me(py′) unit. The IR spectra (υ(ArSO3-) region) of 2a-c and 3
are very similar, suggesting they have similar structures.
As the temperature of a CD2Cl2 solution of 3 is raised above
-10 °C, a new species (3′) grows in reversibly at the expense of
3. NMR studies show that 3′ is a smaller fragment of 3, most likely
a monomeric (Li-OPO)PdMe(py′) species. The 1H NMR spectrum
-
of 3′ at 80 °C in CDCl2CDCl2 contains one ArSO3 methyl
-
resonance (δ 2.27), consistent with fast ArSO3 exchange as
observed for 2a-d in CD3OD, and a Pd-Me resonance (δ 0.46)
in the range for monomeric (PO)PdMe(L) species. PGSE NMR
1
1
data show that the hydrodynamic volume of 3′ is /3 to /4 that of
3. The concentrations of 3 and 3′ fit the equilibrium expression for
a tetramer/monomer equilibrium, with Keq ) [3′]4/[3] ) 9.6(2) ×
102 M3 at 80 °C. The entropy value (∆S ) 34(2) eu) is consistent
with dissociation of 3 into multiple fragments.
NMR studies show that 3 remains intact in CD2Cl2 solution at
-10 °C and below. The 1H NMR spectrum of 3 in CD2Cl2 at -10
-
°C contains two sets of ArSO3 resonances, consistent with the
solid state structure and in contrast to the spectra of 2a-d in CD3OD
-
for which fast ArSO3 exchange is observed at this temperature.
1
The Pd-Me H NMR resonance of 3 (δ -0.48) is shifted >0.6
ppm upfield from the range observed for 2a-d and other (PO)P-
dMe(py) complexes (δ 0.2 - 0.5), consistent with the anisotropic
shielding from the proximal py′ ring expected from the solid state
structure. Pulsed field gradient spin-echo (PGSE) NMR data in
CD2Cl2 show that the hydrodynamic volume of 3 is 1.9 times larger
than that of dinuclear {(PO-3,5-tBu)PdMe}2 ((PO-3,5-tBu)-
)
2-P(3,5-tBu2-Ph)2-4-Me-benzenesulfonate) and 3.8 times larger than
that of mononuclear (PO-3,5-tBu)PdMe(py), consistent with a
tetrameric structure for 3 in solution.
Figure 1. Molecular structure of {(Li-OPO)PdMe(py′)}4 (3). Hydrogen
atoms (except C(1A)H3) are omitted. Only the nitrogen atoms of the N1A-C
py′ ligands are shown.
9
52 J. AM. CHEM. SOC. 2010, 132, 52–53
10.1021/ja909473y  2010 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Representative Ethylene Polymerization Resultsa
structure (as shown by X-ray diffraction) and produces high MW
PE. However, in solution (toluene), 3 and/or growing higher-alkyl
analogues break up to form mononuclear species (as shown by
NMR) that produce low MW PE, similar to conventional (PO)P-
d(R)(L) catalysts. The overall MWD is broad and reflects the degree
of cage fragmentation under the given polymerization conditions.
Similar reasoning explains the behavior of 2a-c. This proposal is
supported by the polymerization behavior of 3 in CH2Cl2 at 25 °C.
Under these conditions, 3 is initially completely soluble and 90%
intact and produces PE that contains a substantial high MW
component (entry 9). The broad MWD is again due to fragmentation
of 3 and/or growing higher-alkyl analogues. The lower temperature
disfavors fragmentation, but this effect may be counteracted by the
higher polarity of the solvent (ꢀ: CH2Cl2, 8.9; toluene, 2.4).
Surprisingly, 4 produces only ethylene oligomers (Table 1, entry
11) in good yield. The oligomers display a Schulz-Flory distribution
(ꢀ ) 0.69), consistent with single site catalysis. These results show
that a pendant free ArSO3- group strongly enhances chain transfer.
This work shows that the incorporation of the pendant ArSO3Li
group in (Li-OPO)PdMe(L) catalysts dramatically influences their
ethylene polymerization behavior. (Li-OPO)PdMe(L) complexes
self-assemble into tetranuclear species that produce very high MW
PE. In solution, the tetramers fragment into mononuclear species
that behave like normal (PO)Pd(R)(L) catalysts and produce low
MW PE. When the Li+ counterion is sequestered by a cryptand,
only ethylene oligomers are formed. The unusual production of a
high MW polymer by the tetranuclear species may result from steric
effects. The crystal structure of 3 shows that one axial face of the
Pd center is blocked by the Li4S4O12 cage, which may disfavor
associative chain transfer.5 It is also possible that cooperative effects
between the Pd centers in 3 are important.6 The free ArSO3- group
in 4 may facilitate chain transfer by displacing the olefin in
Pd(H)(olefin) intermediates generated by ꢀ-H elimination or by
stabilizing the transition state for ꢀ-H transfer to monomer by
coordinating to Pd. We are exploring multinuclear catalysts based
on more robust self-assembled cages.
e
g
P
(psi)
T
(°C)
time
(h)
yield
(g)
Mw
(103)
Tm
(°C)
entry
cat.
PDIe
1b
2b
3b
4b
5b
6b
7c
PO1
PO2
3
410
435
410
410
410
410
410
410
80
80
80
80
80
80
80
80
80
25
25
25
2
2
2
2
2
1
2
2
2.84
4.36
6.25
0.85
4.85
1.35
0.35
5.18
1.91
0.45
2.25
13.2
58.8
7.87
1110
382
2.0
2.5
2.6
28
49
31
2.0
60
29
2.2
-
131.4
132.9
131.7
135.4
134.3
134.3
129.3
138.7
136.5
136.3
-
2a
2b
2c
PO1
3
3
PO2
4
284
6.39
1000
915
8c
9d
10d
11d
24
24
24
80
80
29.0
oligof
a Solvent ) 50 mL, [Pd] ) 0.2 mM (10 µmol). b Solvent ) toluene.
c Solvent ) hexanes. d Solvent ) CH2Cl2. e GPC. f C4-C18 oligomers.
g DSC.
The reaction of 3 with the cryptand Krypt211 yields [Li-
(Krypt211)][(OPO)PdMe(py′)] (4). The [OPO]2- ligand of 4 binds
to Pd as a κ2-P,O chelator, and the pendant sulfonate group interacts
weakly with the Pd (Pd-O distance: 3.08 Å; sum of van der Waals
radii of Pd and O: 3.15 Å). 2d undergoes Pd-Me bond homolysis
in fluorescent room light in CD2Cl2 to yield {(OPO)Pd}2 (5) and
CH3D.4 The [OPO]2- ligands in 5 act as κ3-O,P,O pincers to each
-
Pd and also bridge via one SO3 oxygen.
In toluene at 80 °C (410 psi ethylene), mononuclear {2-(Ar2P)-
4-Me-benzenesulfonate}PdMe(py) catalysts (PO1: Ar ) Ph; PO2:
Ar ) 2-Et-Ph) produce linear PE with low to moderate MW and a
narrow molecular weight distribution (MWD; Table 1, entries 1,2),
characteristic of single site catalysis. 3 behaves similarly (entry 3).
Under these conditions, PO1 and PO2 are completely soluble and
3 is highly soluble. In contrast, 2a-c produce very high MW linear
PE with a broad bimodal MWD (entries 4-6 and Figure 2a),
suggestive of multisite catalysis. The insoluble 2a yields PE with
a large high MW fraction, while sparingly soluble 2b,c yield PEs
with a large low MW fraction.
To explore the influence of catalyst solubility, the polymer-
ization behaviors of PO1 and 3 were compared under homoge-
neous and heterogeneous conditions. PO1 behaves similarly in
toluene (initially homogeneous) and hexanes (heterogeneous) except
that the MW decreases slightly in hexanes (Table 1, entries 1,7).
In contrast, 3 generates low MW PE with a narrow MWD in toluene
(as noted above) but produces high MW PE with a broad MWD
and a significant high MW component in hexanes (entries 3,8, and
Figure 2b). These results show that the solubility of the catalyst
per se does not control the MW but rather suggest that the nuclearity
of the catalyst is a key factor. That is, under heterogeneous
conditions (hexanes), 3 substantially maintains its tetranuclear
Acknowledgment. This work was supported by the U.S.
Department of Energy (DE-FG-02-00ER15036). The authors thank
T. Huang, P. D. Hustad, and T. T. Wenzel (Dow) and M. Mitobe
and Y. Kawashima (Sumitomo) for help with GPC.
Supporting Information Available: Experimental procedures and
characterization of complexes (PDF, CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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