Site-isolation effects in a dendritic nickel catalyst for the oligomerization of ethylene

Article abstract of DOI:10.1021/ja046901f

Dendrimers, specifically suited to construct site-isolated groups due to their well-defined hyperbranched structure, have been used as a ligand design element for the construction of nickel catalysts for ethylene oligomerization. The dendritic P,O ligand indeed suppresses the formation of inactive bis-(P,O)Ni complexes in toluene, as is evident from NMR studies, and, as a consequence, outperforms the parent ligand in catalysis in this solvent. The dendritic effect observed in methanol is more subtle because both the dendritic ligand 1 and the parent 2 form bis(P,O)nickel complexes in solution according to NMR spectroscopy. Unlike the parent complex 8, the dendritic bis(P,O)Ni complex 7 derived from dendrimer ligand 1 is able to dissociate to a mono-ligated species under catalytic conditions, that is, 40 bar ethylene and 80 °C, which can enter the catalytic cycle. Indeed, dendritic ligand 1 gives much more active nickel catalysts for the oligomerization in methanol than does 2.

Full text of DOI:10.1021/ja046901f

Published on Web 10/20/2004
Site-Isolation Effects in a Dendritic Nickel Catalyst for the
Oligomerization of Ethylene
Christian Mu¨ller, Lily J. Ackerman, Joost N. H. Reek,* Paul C. J. Kamer, and
Piet W. N. M. van Leeuwen*
Contribution from the Van’t Hoff Institute for Molecular Sciences, Nieuwe Achtergracht 166,
1018 WV Amsterdam, The Netherlands
Received May 26, 2004; E-mail: reek@science.uva.nl
Abstract: Dendrimers, specifically suited to construct site-isolated groups due to their well-defined
hyperbranched structure, have been used as a ligand design element for the construction of nickel catalysts
for ethylene oligomerization. The dendritic P,O ligand indeed suppresses the formation of inactive bis-
(P,O)Ni complexes in toluene, as is evident from NMR studies, and, as a consequence, outperforms the
parent ligand in catalysis in this solvent. The dendritic effect observed in methanol is more subtle because
both the dendritic ligand 1 and the parent 2 form bis(P,O)nickel complexes in solution according to NMR
spectroscopy. Unlike the parent complex 8, the dendritic bis(P,O)Ni complex 7 derived from dendrimer
ligand 1 is able to dissociate to a mono-ligated species under catalytic conditions, that is, 40 bar ethylene
and 80 °C, which can enter the catalytic cycle. Indeed, dendritic ligand 1 gives much more active nickel
catalysts for the oligomerization in methanol than does 2.
Introduction
examples of dendritic effects on catalyst properties have been
reported in the literature.3,6 Here, we report on a new nickel
Careful positioning of the functional groups in a polymeric
protein framework optimizes biological functions of natural
systems. This concept of optimization by site-isolation can also
be applied to artificial systems and has been utilized for various
applications including catalysis.1 Dendrimers2 are specifically
suited to construct site-isolated functional groups due to their
well-defined hyperbranched structure,3 and site-isolation effects
have been explored by encapsulation of various functional
groups including photoactive4 and catalytic functions.5,6 The
distinct environment around the metal center created by a
dendritic scaffold can in principle lead to properties that differ
significantly from the parent compounds. So far, only a few
catalyst system for the oligomerization of ethylene7 in which
the catalyst-embedding by a dendrimer has been used as a design
element. This important industrial reaction (Shell Higher Olefins
Process) is catalyzed by nickel complexes bearing bidentate P,O
ligands, such as o-diphenylphosphinophenols.8 However, the
formation of bis(P,O)nickel complexes can, especially in polar
solvents, retard the reaction, as it is a pathway for catalyst
deactivation.9 We anticipated that the embedding of such a
(P,O)Ni catalyst in a dendritic framework, such as 1, would
reduce dimerization and therefore enhance the productivity of
the catalyst (Figure 1).
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10.1021/ja046901f CCC: $27.50 © 2004 American Chemical Society

Dendritic Nickel Catalyst
A R T I C L E S
Figure 1. Core-functionalized dendritic P,O ligand 1 and its smaller parent
ligand 2 used in this study to investigate site-isolation effects in nickel-
catalyzed oligomerization reactions.
Figure 2. Yield of insoluble high molecular weight oligomer (0) and
soluble low molecular weight oligomer (9) using catalysts based on 1 and
2 under various conditions (entries 1-6, Table 1). Entry: 1,2, toluene; 3-5,
methanol; 6, water; 5,6, in the presence of PPh3 (1 equiv).
Scheme 1. Synthesis of Dendritic Ligand 1 and Parent Ligand 2
(R ) H)
Table 1. Ethylene Oligomerization Using Nickel Catalysts Based
on Dendritic Ligand 1 and Parent Ligand 2a
b
yield of
TOFave
1
entry
catalyst system (solvent)
oligomers [g]
[h-
]
1
2
3
4
5
6
2/Ni (toluene)
1/Ni (toluene)
2/Ni (methanol)
1/Ni (methanol)
1/Ni, PPh3 (methanol)
1/Ni, PPh3 (H2O)
2.52
5.39
tracesc
2.27
2.34
2.45
3600
7700
n.d.
3242
3342
3500
Scheme 2. Dendritic Mono-ligated 5 Only Partly (17%)
Transforms to Bis-ligated Complex 7 in Toluene after the Addition
of 1 equiv of Ligand 1, While the Formation of 8 Is Quantitative
under Similar Conditions
a Conditions: T ) 80 °C, 50 bar ethylene, 25 mL of solvent, reaction
time 30 min. b Average turn over frequency (mol of ethene‚mol-1 of Ni‚h-1).
c Reaction time 4 h.
added to the NMR solution of 6, an orange precipitate
immediately formed, and no 31P signals could be detected.
Isolation and characterization of the precipitate confirmed the
formation of bis(P,O)nickel complex (8) (FAB-MS: m/z )
613.10 ([M + H]+ calcd for C36H29O2P2Ni: 613.10)). In contrast
to the experiments with parent ligand 2, addition of a second
equivalent of 1 to the solution 5 did not result in the formation
of precipitate, and only a small amount (17%) of the bis(P,O)-
nickel complex 7 is present in solution (mixture of cis/trans
isomers, 31P NMR ∼34-38 ppm). The identity of complexes 7
and 8 was confirmed by independent syntheses.
Result and Discussion
The dendrimer-substituted o-diphenylphosphino phenol 1 was
synthesized by coupling lithiated dendritic wedges (4)6h to EVE-
protected diethyl arylphosphonite (3) (EVE ) ethyl vinyl ether)
and subsequent removal of the EVE protecting group with
pyridinium p-toluenesulfonate (PPTS, Scheme 1). The dendritic
ligand was completely characterized with 1H NMR, 31P NMR,
elemental analysis, and mass spectrometry. In a similar manner,
the parent compound o-diphenylphosphinophenol 2 was pre-
pared, and the spectroscopic data are identical to those reported
in the literature.10
31P NMR experiments were carried out to probe the ability
of dendritic ligand 1 to prevent the formation of bis(P,O)nickel
complexes. When 1 or 2 was reacted with an equimolar amount
of Ni(COD)2 in toluene-d8, mono-ligated species (5 and 6,
respectively; Scheme 2) were formed as shown by 31P chemical
shifts (∼11-16 ppm).11 When a second equivalent of 2 was
These results demonstrate the ability of dendritic ligand 1 to
suppress the formation of inactive bis(P,O)nickel complexes in
toluene. Thus, 1 was anticipated to form a more productive
catalyst than parent ligand 2 in the oligomerization of ethylene.
Catalysis experiments performed with 1 and 2 in toluene (Figure
2, Table 1, entries 1 and 2) indeed show that dendritic catalyst
1/Ni(COD)2 produced higher yields of oligomers than the parent
2/Ni(COD)2 (5.39 g oligomer, TOFave ) 7700 h-1 versus 2.52
g, TOFave ) 3600 h-1). Products of both catalytic reactions of
ethylene were mainly insoluble higher oligomers (>C30), and
only traces of low molecular weight (soluble) oligomers were
detected by GC.12
For nickel complexes based on o-diphenylphosphinophenol
2, it is known that the formation of bis(P,O)nickel complexes
is favored in polar solvents.9a When 1 or 2 was reacted with an
equimolar amount of Ni(COD)2 in THF, already small amounts
(10) (a) Herd, O.; Hessler, A.; Hingst, M.; Machnitzki, P.; Tepper, M.; Stelzer,
O. Catal. Today 1998, 42, 413-420. (b) Sua´rez, A.; Me´ndez-Rojas, M.
A.; Pizzano, A. Organometallics 2002, 21, 4611-4621.
(11) Heinicke, J.; Ko¨hler, M.; Peulecke, N.; He, M.; Kindermann, M. K.; Keim,
W.; Fink, G. Chem.-Eur. J. 2003, 9, 6093-6107 and references therein.
(12) (a) Singleton, D. M. (Shell Oil Corp.), U.S. 4,472,522; Chem. Abstr. 1985,
102, 46405. (b) Singleton, D. M. (Shell Oil Corp.), U.S. 4,472,525; Chem.
Abstr. 1985, 102, 85118.
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A R T I C L E S
Mu¨ller et al.
Scheme 3. Dendritic Complex 7 Is in Equilibrium with
Mono-ligated 5 under Catalytic Conditions in THF/MeOH, While
Formation of 8 Is Irreversible
Figure 3. Mol % and wt % of oligomer fractions versus the number of
carbon atoms (run 5). Inset: Schulz-Flory distribution of oligomers (run
5).16
(Table 1, entry 4) gave an active catalyst when applied in
methanol (2.27 g of oligomer, TOFave ) 3242 h-1). Under these
conditions, we also observed a product distribution that differs
from the reaction applied in toluene; a significant amount (30
wt %) of low molecular weight oligomers (mainly C6-C30) were
formed together with insoluble higher molecular weight oligo-
mers (70 wt %, >C30). Interestingly, less than 0.05% isomer-
ization to internal alkenes was observed, and the high selectivity
to R-olefins is of importance for commercial processes. Related
(P,O)Ni systems also show decreased molecular weights when
the catalysis is carried out in polar solvents.13 The overall
productivity of 1/Ni(COD)2 in methanol is only one-half as high
as that observed in toluene. We ascribe this effect to the presence
of larger quantities of catalytically inactive bis(P,O)nickel
complexes in methanol than in toluene, although we cannot
completely exclude the role of methanol which can compete
with ethylene for a coordination site on the metal complex. It
should be mentioned here that aggregation of dendrimer 1 in
more polar solvents could provide an alternative explanation
for the lower activity observed in MeOH. However, 1H-DOSY
NMR experiments showed that dendrimer 1 has a similar relative
diffusion (0.2 as compared to tetramethoxysilane as internal
standard) in toluene and MeOH-d4/THF-d8 (6:4 mixture) at
concentrations relevant to catalysis (up to 1.2 mM), indicating
that the dendritic ligand essentially remains monomeric with a
conformation that is comparable in these solvent systems.
It is known for similar catalyst systems that triphenylphos-
phine enhances â-H elimination relative to the insertion reaction
and therefore favors the formation of low molecular weight
oligomers.14 We found that PPh3 has a similar effect on the
dendritic catalyst system 1/Ni(COD)2 in methanol as mainly
low molecular weight oligomers were formed (80 wt %). The
productivity of the catalyst remained the same. A detailed
analysis of the obtained oligomers by GC revealed a Schulz-
Flory distribution with R ) 0.61 (Figure 3).15
of the bis(P,O)nickel complexes (7 and 8, respectively; 10-
20%) were observed by 31P NMR in addition to mono-ligated
species 5 and 6. Interestingly, upon addition of MeOH to the
NMR solution, 7 and 8 were the only remaining species
observed, although only equimolar quantities of ligand and Ni-
(COD)2 are present. Under these conditions, the dendritic ligand
was unable to suppress formation of the bis(P,O)Ni complex 7,
and there was no difference in the coordination chemistry
behavior of 1 and 2 with Ni(COD)2 in THF/MeOH as observed
by 31P NMR. However, identical NMR experiments in THF/
MeOH carried out under conditions more relevant to catalysis
(40 bar ethylene and 80 °C) revealed a crucial difference in the
behavior of 1 and 2 (Scheme 3). For the parent ligand 2, only
the 31P resonance for the bis(P,O)nickel complex 8 was observed
under these conditions, and no oligomers are detected in the
NMR solution by GC-MS. In contrast, for dendritic ligand 1,
only the 31P resonance for the bis(P,O)nickel complex 7 was
observed under 40 bar ethylene at room temperature, but when
the complex was heated to 80 °C, a resonance at 16 ppm
indicative of mono-ligated species 5 appeared (20%). When the
complex was cooled to room temperature, the resonance at 16
ppm disappeared, and again only 7 was observed, indicating
that the ligation mode of the dendritic catalyst is dependent on
the conditions and fully reversible. Oligomers were formed in
the NMR solution as detected by GC-MS, supporting the
formation of active mono-ligated species under these conditions.
Pressurizing the NMR tube at room temperature did not result
in the formation of oligomers, showing that elevated tempera-
tures are required to form the active mono-ligated species.
Importantly, these experiments reveal that dendritic ligand 1
forms a bis(P,O)nickel complex 7 that is in equilibrium with
an active, mono-ligated species 5 under catalytic conditions,
whereas the parent ligand remains a bis-ligated species under
these conditions.
We also studied the dendritic effect in H2O, and we observed
that the dendritic catalyst generated from ligand 1 and Ni(COD)2
in the presence of 1 equiv of PPh3 performed in H2O just as
In agreement with our NMR results, the dendritic effect is
more pronounced when the oligomerization catalysis is per-
formed in methanol. Almost no catalytic activity was observed
for the parent complex (2/Ni) in methanol, and only a trace
amount of product was obtained (Table 1, entry 3). Similar
results were reported by Heinicke et al. who found no catalytic
activity of comparable species in ethanol as a solvent due to
the formation of catalytically inactive bis(P,O)nickel complexes.9a
In contrast, the dendrimer-substituted ligand 1 and Ni(COD)2
(13) Starzewski, A. O. In Late Transition Metal Polymerization Catalysis; Rieger,
B., Saunders Baugh, L., Kacker, S., Striegler, S., Eds.; Wiley-VCH:
Weinheim, 2003; Chapter 1 and references therein.
(14) (a) Pietsch, J.; Braunstein, J. P.; Chauvin, Y. New J. Chem. 1998, 467-
472. (b) Heinicke, J.; He, M.; Dal, A.; Klein, H.-F.; Hetche, O.; Keim,
W.; Flo¨rke, U.; Haupt, J. Eur. J. Inorg. Chem. 2000, 431-440.
(15) (a) Schulz, G. V. Z. Phys. Chem., Abt. B 1935, 30, 379-398. (b) Flory, P.
J. J. Am. Chem. Soc. 1940, 62, 1561-1565.
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Dendritic Nickel Catalyst
A R T I C L E S
well as in methanol (Table 1, entry 6). A similar productivity
was observed in H2O as compared to methanol (2.45 g of
oligomer, versus 2.34 g,), and mainly low molecular weight
oligomers (C6-C30) were formed (73 wt %). It should be pointed
out that the THF solution containing the dendritic nickel-
precatalyst is soluble in methanol, whereas in water the addition
of this solution leads to the formation of an emulsion.
the core-functionalized dendritic catalyst is far more active than
its parent complex due to the site-isolation. We believe that the
increased propensity of the dendrimer ligand to form mono-
ligated species reported here might be applied to other transition-
metal catalysts for which the formation of bis-ligated or
bimetallic complexes plays a role in catalyst deactivation.17
Acknowledgment. We thank Dr. Eric J. M. de Boer and
Harry van der Heijden (Shell International Chemicals, Amster-
dam) for helpful discussions and GC-analyses and Ir. J. M. J.
Paulusse for assistance with the DOSY. We thank CW-NWO
for financial support of C.M. and the USA-NSF for an
International Postdoctoral Fellowship for L.J.A.
Conclusions
We have successfully applied the dendrimer scaffold in
catalyst design and have shown that the dendrimers-substituted
o-diphenylphosphinophenol 1 suppresses the formation of bis-
(P,O)Ni complexes in toluene solution. Consequently, dendrimer
ligand 1 outperforms the parent ligand 2 for the oligomerization
of ethylene in toluene. The dendritic effect observed in methanol
is more subtle because both 1 and 2 form bis(P,O)nickel
complexes in solution, but bis(P,O)Ni complex 7 derived from
dendrimer ligand 1 is able to dissociate to a mono-ligated species
under reaction conditions, resulting in a dendritic catalyst that
is far more active. This is one of the rare examples in which
Supporting Information Available: Experimental details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA046901F
(16) The C6, C8, and C28 slightly deviate from the linear plot, which is probably
due to the volatility and insolubility of these fractions.
(17) Ulman, M.; Grubbs, R. H. J. Org. Chem. 1999, 64, 7202-7207.
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