Nickel(II) compounds derived from tripod ligands: Efficient catalysts for the stereoselective heterodimerization of styrene and propene

Article abstract of DOI:10.1002/ejic.200200536

The tripod ligands RCH₂C(CH₂X)(CH₂Y)(CH₂Z) form tetracoordinate compounds with NiHal₂. If two of the donor functions X, Y, Z are phosphanes, square-planar coordination is observed. In contrast, donor sets X,Y,Z = P,N,N or N,N,N form tetrahedral compounds, by P,N or N,N coordination, that are active in the catalysis of olefin oligomerization with MAO as a co-catalyst. These catalyst systems are also active in the stereoselective heterodimerization of olefins. The co-dimerization of styrene and propene by these systems leads to 4-phenyl-2-pentene almost exclusively, with regioselectivities up to 98% and (E)/(Z) ratios of up to 95:5, surpassing other reported catalysts for this process. The products of these catalytic transformations have been fully characterized by GC/MS techniques as well by isolation, NMR and MS characterization of the pure compounds. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejic.200200536

FULL PAPER
Nickel(II) Compounds Derived from tripod Ligands: Efficient Catalysts for the
Stereoselective Heterodimerization of Styrene and Propene
[a]
[a]
Ralf Faissner and Gottfried Huttner*
Dedicated to Professor Günter Helmchen[‡]
Keywords: Tripod ligands / Nickel / Catalysis / Heterodimerization
The tripod ligands RCH
coordinate compounds with NiHal
tions X, Y, Z are phosphanes, square-planar coordination is
2
C(CH
2
X)(CH
2
Y)(CH
2
Z) form tetra-
phenyl-2-pentene almost exclusively, with regioselectivities
up to 98% and (E)/(Z) ratios of up to 95:5, surpassing other
reported catalysts for this process. The products of these
2
. If two of the donor func-
observed. In contrast, donor sets X,Y,Z = P,N,N or N,N,N form catalytic transformations have been fully characterized by
tetrahedral compounds, by P,N or N,N coordination, that are
active in the catalysis of olefin oligomerization with MAO as
a co-catalyst. These catalyst systems are also active in the
stereoselective heterodimerization of olefins. The co-dimeri-
zation of styrene and propene by these systems leads to 4-
GC/MS techniques as well by isolation, NMR and MS char-
acterization of the pure compounds.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
Results and Discussion
The propensity of tripod-derived nickel() chelate com-
pounds to act as precatalysts in styrene olefin co-dimeri-
zation was tested for compounds 1Ϫ5 (Scheme 1).
Transition metal catalysed heterodimerization of olefins
is a versatile method of carbonϪcarbon bond forma-
[
1,2]
tion.
The co-dimerization of styrene and ethene has been
An initial check as to whether the compounds Ϫ when
[3]
much studied since its first report in 1965. Many tran-
activated with MAO Ϫ lead to catalysts for ethene oligo-
sition metals are active in this process[ but catalysts con-
4,5]
[
11]
merization
found that none of the compounds contain-
taining nickel are the most active ones.[
2,6,7]
They could
ing two phosphane donors in the chelate cycle (5aϪ5d) was
active. However, compounds containing a P,N or an N,N
donor set in the chelate cycle (1Ϫ4) are active as oligomeriz-
ation catalysts. The ethene oligomers formed were soluble
in toluene. GC/MS analysis of the toluene solution and of
the gaseous phase revealed a broad distribution of oligo-
mers up to an oligomerization degree around n ϭ 12 with
the expected decay of the respective yields with increasing
degree of oligomerization (Scheme 2).
The oligomerization process is not stereoregular: instead
of one single GC/MS peak at each oligomerization degree
n, a pattern of overlapping peaks is observed, all of which
correspond to the same mass number.
even be developed into systems that allow the enantioselec-
tive hydrovinylation of styrene.[ Conversely, the long-
standing problem of co-dimerizing styrene with pro-
7]
[
4a,5b,8]
[7d,7e]
pene
has only been partially solved.
The major
prerequisite for an effective catalyst in this process appears
to be a hemilabile co-ligand.[
9]
Nickel() compounds containing P,N- or N,N-bonded
chelate ligands are promising candidates as precatalysts in
[
10]
this type of reaction. The recent synthesis
of
com-
ligands
1
1
(
RЈCH )(CH Z)C(CH -κ -X)(CH -κ -Y)NiHal
2
2
2
2
2
pounds
derived
from
potential
tripod
RЈCH C(CH X)(CH Y)(CH Z) (RЈ ϭ H, OMe; X, Y, Z ϭ
2
2
2
2
PR , NR ) has prompted the study of their catalytic proper-
2
2
This first test with ethene as a substrate showed that
compounds 1Ϫ4 Ϫ containing hemilabile P,N- and N,N-
bonded chelates Ϫ are candidates as potential precatalysts
of olefin co-dimerization. The next test therefore probed the
ties as co-dimerization catalysts. The results of such a study
are reported here.
activity of these compounds in the presence of MAO for
[
[
a]
‡]
[12]
Anorganisch-Chemisches Institut der Universität Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax: (internat.) ϩ 49-6221/545707
E-mail: g.huttner@indi.aci.uni-heidelberg.de
As a late contribution to his 60th birthday.
propene dimerization.
The test was limited to 1a, which
was found to promote the dimerization of propene with
more than 90% of the product being 2-methyl-1-pentene
(Scheme 3). This product was isolated by distillation and
Eur. J. Inorg. Chem. 2003, 2239Ϫ2244
DOI: 10.1002/ejic.200200536
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2239

R. Faissner, G. Huttner
FULL PAPER
Scheme 1
had been consumed (18 h, 0.1 mol % precatalyst). GC/MS
analysis of the products showed that only around 30% of
the products consisted of the dimer, while the rest were tri-
mers or tetramers with all combinations of constituting ole-
fins being present. However, no styrene dimer was ob-
served.
Scheme 2
Scheme 3
These findings show that regioselective hydrovinylation
of styrene is catalysed by the systems tested. They also dem-
onstrate that the catalyst systems are too active since they
activate the heterodimer for further CϪC bond formation.
Consequently, the codimerization of p-chlorostyrene and
ethene was analysed using 1a and MAO as precatalyst sys-
identified unequivocally by 1D- and 2D-NMR spec- tem. This substituted styrene should be a less active olefin
troscopy (see Exp. Sect.). than styrene, such that the formation of oligomers contain-
This test shows that the catalytic process of dimerization ing more than one p-chlorostyrene moiety should be sup-
is completely regioselective in this case.[ Regioselectivity pressed. It was observed that the catalyst system is still ac-
is an essential prerequisite for an effective heterodimeri- tive with these components. The reaction is, however, much
zation catalyst. The next test was to analyse the potential slower than with styrene, such that after 18 h (0.4 mol %
catalytic dimerization of styrene and ethane with 1a and 2 precatalyst) only 45% of p-chlorostyrene had reacted. The
as precatalysts. The formation of heterodimers was ob- almost exclusive product of this reaction is the heterodi-
served in each case. From the many conceivable kinds of meric compound 3-(p-chlorophenyl)-1-butene (Scheme 5).
isomeric heterodimers only 3-phenyl-1-butene was formed A very minor amount of a trimer composed of one p-
13]
(Scheme 4). The catalytic process was run until all styrene chlorostyrene and two ethene moieties is formed. Thus, the
Scheme 4
240
2
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Eur. J. Inorg. Chem. 2003, 2239Ϫ2244

Nickel(II) Compounds Derived from tripod Ligands
FULL PAPER
selectivity of the catalyst system for the formation of hetero- as the dimerization products. The (E) isomer is the pre-
dimers increases with decreasing activity of the olefinic dominant compound in each case, with (E)/(Z) ratios (A/B)
partners.
between 89:11 and 95:5 depending on the catalyst and the
All the above tests give credit to the idea that styrene and reaction conditions. The observed regioselectivity (up to
propene might be the proper choice of olefins to be co- 98%) and stereoselectivities (up to 95:5) are higher than
3
dimerized by the above catalyst systems.
those reported for the [(η -allyl)NiBr] system (regioselec-
2
[
7e]
As a precatalyst for the heterodimerization of styrene and tivity: 80%; stereoselectivity: 88:12).
3
propene the system [(η -allyl)NiBr] /PPh /AgOTf has been
The results of this analysis (Table 1) show that com-
2
3
[
7e]
described recently.
Ϫ20 °C to give the co-dimerization products in more than late cycle (Runs 1Ϫ10) are active as precatalysts, while none
0% yield based on styrene. of the compounds that contain two phosphorus donors in
A catalyst system composed of P,N or N,N chelate com- their chelate cycle (Runs 11Ϫ14) show any activity.
pounds 1Ϫ4 and MAO in toluene gives good yields of the The selectivity of the systems containing hemilabile P,N
This reaction was run in CH Cl at pounds that contain at least one nitrogen donor in the che-
2 2
9
desired dimers even at room temperature (see Scheme 6). and N,N chelates (1Ϫ4) with respect to the formation of 4-
The identity of the heterodimers was proved beyond doubt phenyl-2-pentene (A, B) depends on the catalyst/substrate
by their isolation, by distillation, and a combination of ratio (see Table 1). With 1.5 mol % of the catalyst (relative
mass spectra, 1D- and 2D-NMR experiments (see Exp. to styrene) up to 80% dimeric products are obtained. With
Sect.).
lower catalyst/styrene ratios the yield of dimeric products
The regioselectivity of this process Ϫ depending on the decreases and is only around 20% for a ratio of 0.1 mol %
reaction conditions Ϫ is up to 98%, corresponding to the (Table 1). The activities of the individual catalyst systems
almost exclusive formation of 4-phenyl-2-pentene[14] (A, B) based on 1Ϫ4 are not too different in this process (Table 1).
Scheme 5
Scheme 6
Table 1. Catalytic activity of tripod compounds 1Ϫ5 in the heterodimerization of styrene and propene
Run[a]
Total yield[b]
of dimers
Regioselectivity[c]
(E)/(Z) ratio of
3-phenyl-1-butene
Molar ratio
catalyst/styrene
Coordinating donors
of precatalyst
Precatalyst[d]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
81%
75%
56%
55%
54%
52%
29%
22%
21%
19%
Ϫ
85%
88%
92%
97%
89%
95%
98%
89%
61%
83%
Ϫ
89:11
89:11
90:10
90:10
89:11
90:10
95:5
89:11
90:10
89:11
Ϫ
1.50 mol %
1.50 mol %
0.75 mol %
0.30 mol %
0.50 mol %
0.75 mol %
0.10 mol %
0.10 mol %
0.75 mol %
0.10 mol %
0.75 mol %
0.75 mol %
0.75 mol %
0.75 mol %
P,N
P,N
P,N
N,N
P,N
N,N
N,N
P,N
P,N
P,N
P,P
Br
2
1b
1a
1a
2
Cl
Cl
Cl
2
2
2
Br
Br
2
2
1b
3
Cl
Cl
Cl
2
2
2
2
1
4
0
1
2
3
4
Br
2
1b
5a
5b
5c
5d
Cl
2
Cl
2
Cl
2
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
P,P
P,P
P,P
Br
2
[
a]
[b]
Determined by GC/MS. [c] Regioselectivity towards 3-
Reaction time: 18 h; solvent: toluene; temperature: 25 °C; pressure: 1 bar.
phenyl-1-butene (3-phenyl-1-butene/other dimers). [ Co-catalyst: MAO.
d]
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R. Faissner, G. Huttner
FULL PAPER
Scheme 7
Only 4, which bears an Et N donor instead of a pyrazole D) other trimers composed of one styrene and two propene
2
donor (1Ϫ3), appears to be somewhat less reactive and less units are also formed, while with lower ratios (0.1Ϫ0.75
selective (Table 1). The reactivity of all systems is such that mol %) 4,6-diphenyl-2-heptene (C, D) is by far the predomi-
within 18 h styrene is quantitatively consumed, even if the nant trimer (90Ϫ99%). The lowest percentage (65%) of C
molar ratio of catalyst/styrene is only 0.1 mol %.
In all experiments that lead to the catalytic formation of 1b as the precatalyst.
styrene/propene heterodimers (see Table 1), 4,6-diphenyl-2- All the precatalysts 1Ϫ4 are paramagnetic nickel() com-
is a by-product. This heterotrimer is composed pounds with a tetrahedral coordination around the metal
and D amongst these trimers was found with 1.5 mol % of
[
15]
heptene
of one propene and two styrene entities (C, D) (Scheme 7). atom. When the toluene solutions of these precatalysts are
The formation of C and D is completely regioselective. The treated with MAO, these blue to violet tetrahedrally coordi-
(E)/(Z) ratio (C/D) is between 80:20 and 85:15 throughout, nated compounds turn red, which is characteristic of such
such that the reaction is also stereoselective in this respect. diamagnetic square-planar nickel() compounds. This indi-
Both C and D were identified unequivocally by their iso- cates that the substitution of the halide function by a
lation by distillation, and a combination of MS, 1D- and methyl group occurs since the replacement of an NiϪHal
2D-NMR experiments (see Exp. Sect.).
bond by an NiϪC bond in this type of compound leads to
With ‘‘high’’ molar ratios of precatalyst/styrene (1.5 a transformation of tetrahedral into square-planar coordi-
mol %), in addition to the trimer 4,6-diphenyl-2-heptene (C, nation due to the increased ligand field.[10b]
Scheme 8
2242
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Nickel(II) Compounds Derived from tripod Ligands
FULL PAPER
All of the above observations may be rationalized by the GC/MS: Agilent 6890 (GC) interfaced to an Agilent 5973N (MS)
equipped with an HP-5MS capillary column (30 m long, 0.25 mm
mechanistic scheme shown in Scheme 8. In principle the
inner diameter) with helium as carrier gas. Yields of the products
scheme is similar to the accepted hypothesis concerning ole-
fin oligomerization by nickel catalysts.[7a,7e] Activation of
(Table 1) refer to dodecane as internal standard.
the precatalyst by MAO and its reaction with styrene leads
to a coordinatively unsaturated cationic NiϪH species,
which is able to add styrene. (The individual steps, which
Typical Procedure for the (Hetero)dimerization Reaction: Styrene
(
500Ϫ2000 mg), precatalyst 1Ϫ4 (5.0Ϫ20.0 mg; 0.1Ϫ1.5 mol %),
and dodecane (200 mg) as internal standard were dissolved in tolu-
ene (25 mL) at 25 °C. MAO (3.0 mL of a 10% solution in toluene)
result in the activated NiϪH species, are shown in
[
2,7a,7e]
Ϫ
refs.
) Coordinative reactivity results from X being was then added to the reaction mixture, and the solution immedi-
trapped by MAO and by the hemilability of the NiϪN co- ately turned from violet to red. Ethene or propene (1 bar), respec-
ordination (dashed lines in Scheme 8). The usual sequence tively, was then introduced into the catalyst solution and the result-
ant reaction mixture was stirred at 25 °C for 18 h under ethene
of insertion steps leads to 4-phenyl-2-pentene (A, B) when
or propene (1 bar), respectively. For GC/MS analysis the reaction
propene is added as the second olefin (inner circle of
mixture (0.5 mL) was quenched with a solution of half-concen-
trated HCl (20 mL) and diethyl ether (20 mL). The aqueous phase
was extracted with diethyl ether (20 mL). The combined organic
4
layers were washed with brine and dried with MgSO . For prepara-
tive isolation of the oligomerization products the same type of
workup procedure was applied in an appropriately scaled-up ver-
Scheme 8). When styrene is also added as the second olefin
the sequence shown in the outer circle of Scheme 8 leads to
the formation of 4,6-diphenyl-2-heptene (C, D) provided
that propene is the third olefinic constituent taken up by
the catalyst. The observed regioselectivity is in complete
agreement with this scheme. All routes incorporate 1,2-in- sion. After removing the solvent, the following products were iso-
sertions as crucial steps, from which the observed regiose- lated by Kugelrohr distillation:
lectivity follows as a natural consequence.
2
7
-Methyl-1-pentene: 1H NMR (CDCl
.2 Hz, 3 H, CH CH
), 2.10 (t, JH,H ϭ 7.5 Hz, 2 H, ϭCCH
) ppm. 13C NMR (CDCl
): δ ϭ 14.2 (s, CH
CH ), 23.1 (s, ϭCCH ), 40.4 (s, ϭCCH ), 110.2 (s, ϭCH
137.8 (s, ϭC ) ppm. MS (EI): m/z (%) ϭ 84 (34) [M ], 69 (100)
3
): δ ϭ 1.04 (t, 3
), 1.51Ϫ1.62 (m, 2 H, CH CH ), 1.75 (s, 3
), 4.80 (br. s, 2
CH ), 20.5
),
H,H
J ϭ
2
3
3
2
3
H, ϭCCH
H, ϭCH
s, CH
3
2
Conclusion
2
3
2
3
(
2
3
3
2
2
The tripod ligands RC(CH X)(CH Y)(CH Z) (X,Y,Z ϭ
ϩ
2
2
2
q
P,N,N or N,N,N) coordinate to NiHal in a bidentate mode,
ϩ
ϩ
2
[M Ϫ CH ], 55 (13) [M Ϫ CH CH ]. B.p. 60 °C (1013 mbar).
3
2
3
resulting in tetrahedral nickel() compounds that are active
as precatalysts (co-catalyst: MAO) in the heterodimeri- 3-Phenyl-1-butene: H NMR (CDCl
1
): δ ϭ 1.37 (d, 3JH,H ϭ 6.9 Hz,
3
3
H, CH
CH ), 5.98Ϫ6.07 (m, 1 H, ϭCH), 7.19Ϫ7.30 (m, 5 H, aromat. H)
ppm. C NMR (CDCl
3
), 3.45Ϫ3.61 (m, 1 H, PhCH), 5.01Ϫ5.09 (m, 2 H, ϭ
zation of styrene and propene. The current results indicate
that the yield of dimeric products and the selectivity of the
catalysts may be further increased by optimising the reac-
tion conditions (i.e. styrene/precatalyst ratio, reaction tem-
perature, solvent). The yield of dimeric products reached so
far is up to 80% with regioselectivities of up to 98% and
stereoselectivities of up to 95%.
Pyrazole as a nitrogen donor in tripod ligands has been (E)-4-Phenyl-2-pentene (A): H NMR (CDCl ): δ ϭ 1.46 (d,
found to promote this selectivity, while the third non-coor-
dinated donor function, serving as a dangling arm, has no CHCH
major influence on the catalytic activities. This finding
should stimulate further experiments in which different
groups at the dangling arm are introduced to either change
the solubility of the precatalysts or to fix them on a sup-
port.
2
1
3
3
): δ ϭ 21.1 (s, CH
3
), 43.7 (s, PhCH), 113.3
(
(
s, ϭCH ), 125.9Ϫ145.5 (m, aromat. C), 143.8 (s, ϭCH) ppm. MS
EI): m/z (%) ϭ 132 (25) [M ], 117 (100) [M Ϫ CH
2
ϩ
ϩ
3
], 91 (25)
ϩ
ϩ
ϩ
ϩ
[C
7
H
7
], 77 (18) [C
6
H
5
5 5 4 3
], 65 (10) [C H ], 51 (22) [C H ]. B.p.
3
0 °C (0.1 mbar).
1
3
3
3
J
H,H ϭ 7.1 Hz, 3 H, PhCHCH
3
), 1.80 (d, JH,H ϭ 6.2 Hz, 3 H, ϭ
3
3
), 3.54 (quint, JH,H ϭ 6.9 Hz, 1 H, PhCH), 5.51Ϫ5.68 (m,
3
3
1
H, CH
CHCHϭ), 7.24Ϫ7.44 (m, 5 H, aromat. H) ppm. C NMR
CDCl ): δ ϭ 18.1 (s, ϭCHCH ), 21.8 (s, PhCHCH ), 42.9 (s,
PhCH), 124.0 (s, CH CHϭ), 126.3Ϫ128.7 (m, aromat. C), 136.8 (s,
CHCHϭ) ppm. MS (EI): m/z (%) ϭ 146 (22) [M ], 131 (100) [M
3
CHϭ), 5.76 (dd, JH,H ϭ 6.8, JH,H ϭ 15.2 Hz, 1 H,
1
3
(
3
3
3
3
ϩ
ϩ
ϩ
ϩ
ϩ
Ϫ CH
3
], 116 (14) [M Ϫ 2CH
3
7 7 6 5
], 91 (31) [C H ], 77 (10) [C H ].
B.p. 35Ϫ40 °C (0.1 mbar).
1
Experimental Section
(E)-4,6-Diphenyl-2-heptene (C): H NMR (CDCl ): δ ϭ 1.39 (d,
J
3
3
3
H,H ϭ 7.0 Hz, 3 H, PhCHCH
CHCH ), 2.01Ϫ2.21 (m, 2 H, CH
7.2 Hz, PhCHCH
5.55Ϫ5.61 (m, 1 H, CH
15.0 Hz, 1 H, PhCHCHϭ), 7.25Ϫ7.47 (m, 10 H, aromat. H) ppm.
3
), 1.83 (d, JH,H ϭ 6.3 Hz, 3 H, ϭ
), 2.87 (sext, 1 H, 3
JH,H ϭ 7.6 Hz, 1 H, PhCHCHϭ),
General Remarks: All (hetero)dimerization reactions were carried
out under moisture- and oxygen-free conditions by means of stand-
ard Schlenk techniques. The Schlenk flasks were equipped with a
gas balloon. All solvents were dried by standard methods[ and
distilled under argon. Complexes 1Ϫ5 were prepared according to
3
2
H,H
J ϭ
3
3
), 3.25 (q,
CHϭ), 5.76 (dd,
3
3
3
H,H
JH,H ϭ 7.9, J ϭ
16]
1
3
C NMR (CDCl
37.8 (s, PhCHCH
MAO (10% solution in toluene), and dodecane were purchased CH CHϭ), 126.4Ϫ146.0 (m, aromat. C), 135.4 (s, PhCHCHϭ)
3
): δ ϭ 18.2 (s, ϭCHCH
3 3
), 23.0 (s, PhCHCH ),
[
10]
literature procedures. Ethene, propene, styrene, p-chlorostyrene,
3
), 45.1 (s, CH ), 47.1 (s, PhCHCHϭ), 125.6 (s,
2
3
ϩ
ϩ
from Aldrich and were used without further purification. NMR:
ppm. MS (EI): m/z (%) ϭ 250 (12) [M ], 145 (10) [M
Ϫ
1
ϩ
Bruker Avance DPX 200 at 200.12 MHz ( H); 50.323 MHz
PhCHCH
3
], 131 (100) [M
Ϫ
PhCH(CH
3
)CH
2
], 105 (35)
]. B.p. 215Ϫ220 °C
1
3
1
ϩ
ϩ
ϩ
(
C{ H}); T ϭ 303 K; chemical shifts (δ) in ppm with respect to [PhCHCH
3
], 91 (38) [C
7
H
7
], 77 (10) [C
6
H
5
1
13
CDCl
3
( H: δ ϭ 7.27 ppm; C: δ ϭ 77.0 ppm) as internal standard.
(0.1 mbar).
Eur. J. Inorg. Chem. 2003, 2239Ϫ2244
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2243

R. Faissner, G. Huttner
FULL PAPER
J. Am. Chem. Soc. 1998, 120, 459. [ M. Nandi, J. Jin, T. V.
RajanBabu, J. Am. Chem. Soc. 1999, 121, 9899. [ T. V. Rajan-
Babu, N. Nomura, J. Jin, B. Radetich, H. Park, M. Nandi,
7c]
Acknowledgments
We are indebted to the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie for financial support. One of us
7d]
[
7e]
Chem. Eur. J. 1999, 5, 1963.
J. Jin, T. V. RajanBabu, Tetra-
hedron 2000, 56, 2145. [ A. Bösmann, G. Francio
7f]
, E. Janssen,
M. Solinas, W. Leitner, P. Wasserscheid, Angew. Chem. 2001,
`
(R. F.) is indebted to the local Graduiertenkolleg ‘‘Selektivität in
der organischen und metallorganischen Synthese und Katalyse’’ for
a scholarship.
7g]
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