Nickel phenyl complexes with chela ting K2-P,O ligands as catalysts for the oligomerization of ethylene into linear α-olefins

Article abstract of DOI:10.1039/a709204k

Starting from [Ni(COD)₂] and the phosphorus ylide Ph₃P(o-C₆H₄O), the complexes [NiPh{Ph₂P(o-C₆H₄O)}-(PR₃)] [PR₃ = PMe₃ (2a), PMe₂Ph (2b), PMePh₂ (2c), PCy₃ (2d), PPh₃ (2e), PTol₃ (2f), P(p-C₆H₄OMe)₃ (2g), P(OMe)₃ (2h), P(p-C₆H₄Cl)₃ (2i), P(p-C₆H₄F)₃ (2j), P(p-C₆H₄CF₃)₃ (2k)] were synthesized in the presence of the corresponding phosphine. The bis-chelate complex cis-[Ni{Ph₂P(o-C₆H₄O)}₂] (3) was formed as a minor by-product during these reactions, but was the only isolable compound when the reactions were conducted at temperatures above 60°C. Oxidative addition of a P-Ph bond to the Ni⁰ centre was also used to synthesize [NiPh{Ph₂PCH...C(...O)Ph}(PMe₃)] (1a) from the α-ketophosphorus ylide Ph₃P=CHC(=O)Ph and PMe₃. Reaction of [Ph₃P(o-C₆H₄NH₂)]Br with [Ni(COD)₂] and PTol₃ yielded the expected compound [NiPh{Ph₂P(o-C₆H₄NH)}(PTol₃)] (6) via deprotonation of the NH₂ function by an excess of PTol₃. Experiments to study the potential of these nickel complexes as catalysts for ethylene oligomerization into linear α-olefins (> 95%) showed widely varying activities [500-180000 mol C₂H₄ (mol catalyst h)^-1] and mass distributions of the α-olefins. In contrast to the nickel phosphino enolate complexes of the type [NiPh{Ph₂PCH...C(...O)Ph}(PR₃)] [PR₃ = PMe₃ (1a), PCy₃ (1b), PPh₃ (1c)], the corresponding nickel phosphino phenolates [NiPh{Ph₂P(o-C₆H₄O)}(PR₃)] (2) generally showed a marked tendency to oligomerize ethylene into α-olefins of higher molecular weight: C₄₌ to C₃₀₌ for 1 versus C₄₌ to C₉₀₌ for 2. The complex [NiPh{Ph₂P(o-C₆H₄NH)}(PTol₃)] (6), on the other hand, showed no activity for ethylene oligomerization.

Full text of DOI:10.1039/a709204k

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Nickel phenyl complexes with chelating j2-P,O ligands as catalysts
for the oligomerization of ethylene into linear a-oleÐns¤
Jorg Pietsch,a,b Pierre Braunstein*,a and Yves Chauvin*,b
a L aboratoire de Chimie de Coordination (UMR CNRS 7513), Universite L ouis Pasteur,
4 rue Blaise Pascal, F-67070 Strasbourg cedex, France
b Institut Francais du Petrole, BP 311, F-92506 Rueil-Malmaison, France
Starting from [Ni(COD) ] and the phosphorus ylide Ph P(o-C H O), the complexes [Nw iPhMPh P(o-C H Oz )N-
2
3
6
4
2
6 4
(PR )] [PR \ PMe (2a), PMe Ph (2b), PMePh (2c), PCy (2d), PPh (2e), PTol (2f), P(p-C H OMe) (2g),
3
3
3
2
2
3
3
3
6
4
3
P(OMe) (2h), P(p-C H Cl) (2i), P(p-C H F) (2j), P(p-C H CF ) (2k)] were synthesized in the presence
3 3
3
6
4
3
6
4
3
6 4
of the corresponding phosphine. The bis-chelate complex cis-[Nw iMPh P(o-C H Oz )N ] (3) was formed
2
6
4
2
as a minor by-product during these reactions, but was the only isolable compound when the reactions were
conducted at temperatures above 60 ¡C. Oxidative addition of a PwPh bond to the Ni0 centre was also used to
synthesize [Nw iPhMPh PCH}C(}Oz )PhN(PMe )] (1a) from the a-ketophosphorus ylide Ph PxCHC(xO)Ph
2
3
3
and PMe . Reaction of [Ph P(o-C H NH )]Br with [Ni(COD) ] and PTol yielded the expected compound
3
3
6
4
2
2
3
[Nw iPhMPh P(o-C H Nz H)N(PTol )] (6) via deprotonation of the NH function by an excess of PTol .
2
6
4
3
2
3
Experiments to study the potential of these nickel complexes as catalysts for ethylene oligomerization into linear
a-oleÐns ([95%) showed widely varying activities [500È180 000 mol C H (mol catalyst h)~1] and mass
2
4
distributions of the a-oleÐns. In contrast to the nickel phosphino enolate complexes of the type
[Nw iPhMPh PCH}C(}Oz )PhN(PR )] [PR \ PMe (1a), PCy (1b), PPh (1c)], the corresponding nickel
2
3
2
3
3
3
3
3
phosphino phenolates [Nw iPhMPh P(o-C H Oz )N(PR )] (2) generally showed a marked tendency to oligomerize
6
4
ethylene into a-oleÐns of higher molecular weight: C to C
for 1 versus C to C
for 2. The complex
4/
30/
4/
90/
[Nw iPhMPh P(o-C H Nz H)N(PTol )] (6), on the other hand, showed no activity for ethylene oligomerization.
2
6
4
3
Complexes phenylnickel a ligands chelatants j2-P,O pour lÏoligomerisation catalytique de lÏethylene en a-oleÐnes
lineaires. Les complexes [Nw iPhMPh P(o-C H Oz )N(PR )] [PR \ PMe (2a), PMe Ph (2b), PMePh (2c), PCy
2
6
4
3
3
3
2
2
3
(2d), PPh (2e), PTol (2f), P(p-C H OMe) (2g), P(OMe) (2h), P(p-C H Cl) (2i), P(p-C H F) (2j), P(p-
3
3
6
4
3
3
6
4
3
6 4 3
C H CF ) (2k)] ont ete obtenus par reaction de [Ni(COD) ] avec les ylures de phosphore Ph P(o-C H O), en
3 3
6
4
2
3
6 4
presence des phosphines correspondantes. Le complexe bis-chelate cis-[Nw iMPh P(o-C H Oz )N ] (3) est un produit
2
6
4
2
secondaire de ces reactions, mais il devient le seul compose isolable lorsque les reactions sont e†ectuees a des
temperatures superieures a 60 ¡C. LÏaddition oxydante dÏune liaison PwPh au centre Ni0 a egalement permis de
preparer [Nw iPhMPh PCH}C(}Oz )PhN(PMe )] (1a) au depart de lÏylure de phosphore a-cetonique
2
3
Ph PxCHC(xO)Ph et de PMe . La reaction entre [Ph P(o-C H NH )]Br, [Ni(COD) ] et PTol conduit au
3
3
3
6
4
2
2
3
2
produit attendu [Nw iPhMPh P(o-C H Nz H)N(PTol )] (6) suite a la deprotonation de la fonction NH par lÏexces de
2
6
4
3
PTol . Les tests menes pour evaluer le potentiel de ces complexes du nickel pour lÏoligomerisation catalytique
3
de lÏethylene en a-oleÐnes lineaires ([95%) montrent des activites [500È180000 mol C H (mol catalyseur h~1)]
2
4
et des distributions en masse des a-oleÐnes variables. Au contraire des complexes phosphino enolates du nickel
du type [Nw iPhMPh PCH}C(}Oz )PhN(PR )] [PR \ PMe (1a), PCy (1b), PPh (1c)], les complexes phosphino
2
3
3
3
3
3
phenolates correspondants [Nw iPhMPh P(o-C H Oz )N(PR )] (2) montrent en general une forte tendance a
2
6
4
3
oligomeriser lÏethylene en a-oleÐnes de poids moleculaires plus eleves: de C a C
pour 1 contre C a C
4/
30/
4/
90/
pour 2. Le complexe [Nw iPhMPh P(o-C H Nz H)N(PTol )] (6), par contre, nÏa montre aucune activite catalytique
2
6
4
3
en oligomerisation de lÏethylene.
The oligomerization of ethylene into linear a-oleÐns by the
Shell Higher OleÐns Process (SHOP) is of academic as well as
industrial interest and has been investigated for a number of
years.2 Research has been particularly focused on tuning the
activity and selectivity (a-oleÐn distribution) of catalysts
numerous ways, using P,N-,1 P,O-,3 As,O-,4 O,O-,5 S,S-,6 or
N,N-7 chelating ligands, but no systematic variation of the
phosphine ligands PR appears to have been reported until
3
now. Presumably this is related to earlier statements that the
PR ligand does not inÑuence the activity and selectivity
3
derived
from
precursor
complexes
such
as
of the catalyst, but is only necessary to stabilize the
[Nw iPhMPh PCH}C(}Oz )PhN(PPh )] (1c), which are strongly
nickel complex.2 Nevertheless, such an inÑuence has
been recognized.3i,3k,3l,8 In this work we set out to estab-
lish a qualitative relationship between the donor properties
of some phosphine ligands and their inÑuence on both the
activity and the selectivity of the new oligomerization
catalysts [Nw iPhMPh P(o-C H Oz )N(PR )] [PR \ PMe (2a),
2
3
dependent on the nature of the donor atoms, the electronic
and steric e†ects of the substituents, the ring size of the chelate
and the nature of the two-electron donor ligand. For this
reason, the environment of the nickel centre was modiÐed in
2
6
4
3
3
3
PMe Ph (2b), PMePh (2c), PCy (2d), PPh (2e), PTol (2f),
* E-mail: braunst=chimie.u-strasbg.fr
¤ Complexes with Functional Phosphines. Previous papers, see ref. 1.
2
2
3
3
6
3
P(p-C H OMe) (2g), P(OMe) (2h), P(p-C H Cl) (2i), P(p-
6
4
3
3
4
3
New J. Chem., 1998, Pages 467È472
467

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C H F) (2j), P(p-C H CF ) (2k)]. Furthermore, the inÑu-
spectra showed the characteristic fragment ions generated by
successive loss of the phenyl group coordinated to the nickel
centre and the monodentate phosphine ligand, leaving behind
a fragment with the nickel atom coordinated by the chelating
phosphino phenolate.
6
4
3
6
4
3 3
ence of the chelating phosphino phenolate ligand on the cata-
lytic properties of the nickel complexes was evidenced by
comparison of 2a, 2d and 2e with the related phosphino
enolate
complexes
[Nw iPhMPh PCH}C(}Oz )PhN(PR )]
2 3
[PR \ PMe (1a), PCy (1b), PPh (1c)].
Whereas
[Nw iPhMPh PCH}C(}Nz Ph)PhNPR ] [PR \ PMe
3
formation
of
the
complexes
(4a),
PMe Ph (4b), PMePh (4c), Ph PxCHC(xNPh)Ph-N (4d)],1b
3
3
3
3
2
3
3
Results and Discussion
Synthesis of the complexes
2
2
3
and [Nw iPhMPh PN}C(}Nz Ph)PhNPR ] [PR \ PMe (5a),
2
3
3
3
PMe Ph (5b), PMePh (5c), Ph PxNC(xNPh)Ph-N (5d)]1a
2
2
3
from [Ni(COD) ], the ylide Ph PxCHC(xNPh)Ph or
Complexes 1 and 2 were synthesized under conventional con-
ditions by oxidative addition of the phosphorus ylides
Ph PxCHC(xO)Ph [eq. (1)] and Ph P(o-C H O) [eq. (2)]
2
3
Ph PxNC(xNPh)Ph, respectively, and tertiary phosphine
has been found to be strongly dependent on the steric demand
3
3
3
6
4
of the phosphine ligand, the new P,N-chelate compound
respectively, to [Ni(COD) ] in the presence of the correspond-
2
[Nw iPhMPh P(o-C H Nz H)N(PTol )] (6) was easily obtained [eq.
ing phosphine (see Experimental). This reaction has been orig-
2
6
4
3
(3)]. This reÑects the much lower bulkiness of the NH group
in the latter chelate compared with that of the NwPh group
in the former complexes.
inally developed by Keim and coworkers to prepare
square-planar complexes containing both a NiwC r bond
and a bidentate, three-electron donor P,O-chelating ligand.9
The products were generally isolated as yellow powders or, in
some cases, as yellow-orange crystals, except for compounds
2i–2k, which were generated in situ and directly used for the
catalytic experiments. All complexes are soluble in solvents
such as benzene or toluene, but they are sparingly or not
soluble in nonpolar hydrocarbons such as pentane.
Ph2
P
Ph2
P
Ph
Ph
N
Ni
Ni
Ph
N
Ph
N
PR3
PR3
Ph
Ph
4a PR3 = PMe3
4b PR3 = PMe2Ph
4c PR3 = PMePh2
5a PR3 = PMe3
5b PR3 = PMe2Ph
5c PR3 = PMePh2
Ph2
4d PR3
=
5d PR3 = Ph3P=NC(=NPh)Ph-N
P
Ph
Ph
O
Ph3P=CHC(=NPh)Ph-N
– 2 COD
toluene
(1)
Ni(COD)2 + Ph3P CHC
Ni
+
PR3
Ph
O
PR3
Again, the oxidative addition of a PwPh bond to a Ni0 centre
was used for the synthesis of 6, but an important modiÐcation
had to be introduced owing to the inaccessibility of the phos-
phorus ylide Ph P(o-C H NH).11 One equivalent of
1a PR3 = PMe3
1b PR3 = PCy3
1c PR3 = PPh3
3
6 4
[Ni(COD) ] was reacted with one equivalent of the phos-
2
phonium salt [Ph P(o-C H NH )]Br in the presence of two
3
6
4
2
equivalents of PTol in order to enable deprotonation of the
3
coordinated NH group, as recently observed in related reac-
2
tions.12
The yields ranged from 30È80%, but owing to similar solu-
bility properties co-precipitation of starting materials or by-
products such as the orange cis-[Nw iMPh P(o-C H Oz )N ] (3)
The product was characterized by its 31P NMR spectrum,
which showed the usual AB pattern that is typical for square-
planar complexes of this type. The catalytic experiments were
carried out without further puriÐcation of the product.
2
6
4
2
was observed in many cases. In the case of PR \ PPh , we
3
3
veriÐed that complex 3 was the only isolable product when
the reactions were conducted at temperatures above 60 ¡C,
obviously owing to an instability of the NiwPh complexes
under these conditions. In order to facilitate comparison of
spectroscopic data and physical properties, this bis-chelate
complex was independently prepared from NiCl É 6H O and
Catalytic experiments
Toluene solutions of complexes 1 and 2 were reacted with eth-
ylene (100 lmol catalyst, 60È95 ¡C, 6 MPa ethylene, 90È300
min)3b to form linear a-oleÐns as oligomerization products.
Turnover numbers (TON), averaged over the reaction time,
and molecular weight distributions with the Schultz-Flory a
value determined from the GC traces when applicable, are
given in Table 1. Depending on the nature of the chelate
2
2
two equivalents of Ph P(o-C H OH) in the presence of
2
6 4
NEt .10
3
The isolated compounds were generally characterized by
1H and 31PM1HN NMR spectroscopy and elemental analyses
and sometimes by mass spectroscopy. The spectroscopic data
are in agreement with previously reported characteristics for
ligand as well as on the auxiliary ligand PR , di†erent behav-
3
compounds of the type [Nw iPh(POz )(PR )].1b,3b,9b Their
iour of the organometallic compounds was observed. Com-
3
31PM1HN NMR spectra exhibited typical AB patterns with
plexes 1, which all contain the phosphino enolate chelating
coupling constants in the range 245È290 Hz while the PMe
derivatives 1a and 2a showed coupling constants of about 300
ligand, all gave linear a-oleÐns in the range of C
([95% linear a-oleÐns) with the maximum of the product dis-
to C
3
4/ 30/
Hz. Surprisingly, the complexes 2bÈ2d and 2h only gave rise to
broad signals at ambient temperature. However, upon cooling
of a toluene solution of 2d to [40 ¡C, two characteristic
doublets appeared in the spectrum, indicating a dynamic equi-
librium between the complex and a coordinatively unsatu-
rated species and free phosphine in solution. The 31PM1HN
chemical shifts for both the chelating and the monodentate
phosphines were found in the expected range. The mass
tribution lying between C and C
. Their catalytic activ-
8/
10/
ity showed large di†erences as could be expected from
literature data. 1a was scarcely active in catalytic oleÐn oligo-
merization, which is in agreement with earlier observations
that PEt is a catalyst poison for oligomerization catalysts
3
such as 1c.3k,13 Insertion of ethylene into the NiwPh bond
was veriÐed by performing the reaction with 200 lmol of the
nickel complex 1a and identifying the styrene found in the
468
New J. Chem., 1998, Pages 467È472

View Article Online
Table 1 Reaction conditions and results for ethylene oligomerization reactions with complexes 1È3, 6 and 7
Molecular weight
distribution
C H
Schultz-
Flory
2
4
n
/
V
ml
/
T
¡C
/
T
/
Pressure/
MPa
t
/
TON
mol (mol h)~1
a/
converted/ Linear
non S.F.b
complex
toluene
startvend
max
¡C
reaction
average
Complex
lmol
min
g
a-oleÐns
max
a
c
SF
0.5
1a
1b
1c
213
13
26
40
40
40
75È85
65È90
60È77
85 6.0È6.1
[200 6.1È6.5
145 6.0È6.1
60
12
15
È
180 000
82 000
È
13
15
C
C
C
ÈC
d,e
4/ 24/
ÈC
C
broad
4/ 30/
ÈC
40/
e
4/ 30/
(C
ÈC
)e
10/ 20/
2a
2b
2c
2d
2e
147
172
78
95
26
40
40
40
40
40
86È88
82È88
87È97
86È90
71È89
88 6.0È6.1
89 6.0È6.5
103 6.2È6.3
100 6.1È6.5
93 6.0È6.2
135
85
70
110
165
500
1600
17 000
7500
5
11
44
37
36
C
C
C
C
C
ÈC
n.d
4/ 24/
ÈC
e
0.42
0.45
4/ 24/
ÈC
n.d.
4/ 90/
ÈC
4/ 90/
ÈC
18 000
broad
4/ 80/
(C ÈC
)
)
4/ 80/
2f
29
40
77È86
90 6.0È6.1
160
13 000
29
C
ÈC
broad
4/ 80/
(C ÈC
4/ 80/
2g
2h
2i
2j
2k
111
98
60f
120f
70f
40
40
40
40
20
77È90
84È86
77È87
75È89
85È87
91 6.1È6.4
98 6.0È6.1
99 6.0È6.3
94 5.9È6.0
88 5.9È6.3
90
60
65
40
205
7300
6900
22 000
12 500
900
34
19
41
28
6
C
C
C
C
C
ÈC
C
C
C
4/ 100/
ÈC
26/
40/
30/
4/ 100/
ÈC
4/ 100/
ÈC
0.87
È
4/ 80/
ÈC
C
ÈC
4/ 90/
22/ 48/
3 ] 2 AlEt
7 ] 2 AlEt
30
147
20
20
55È70
68È73
70 4.3È5.6
150 5.0È7.5
200
20
È
7300
È
10
È
n.a.g
È
n.d.
3
3
3 ] 2 AlEt ] PPh
60
93
20
20
80È82
70È76
82 5.9È6.4
76 6.2È6.4
180
250
1000
È
4
C
ÈC
h
d
È
È
È
È
3
3
3
4/ 26/
ÈC
7 ] 2 AlEt ] PPh
traces
C
3
4/ 10/
3 ] 2 AlEt ] PMe
59
98
20
20
78È85
77È81
85 6.0È6.3
81 5.5È6.0
180
120
È
È
traces
traces
C
ÈC
4/ 6/
ÈC
d
3
3
3
7 ] 2 AlEt ] PMe
C
d
d
0.45
3
4/ 10/
6
150f
40
78È83
83 5.3È5.5
65
È
traces
C
ÈC
4/ 10/
a Turnover number averaged over t
; TON \ mol C H converted (mol catalyst h)~1. b Non-Schultz-Flory distribution, perhaps due to
reaction
2 4
difficulties with the analysis of the solid, liquid and gas phase. c a \ X /X
. d Trace amounts of oleÐns. e Trace amounts of styrene. f Complex
n
n~2
generated in situ; estimated amount. g Products were not analysed by GC. h Contained some PE or C ÈC oleÐns; solid not analysed by GC.
30 90
liquid phase together with traces of low molecular weight a-
oleÐns, thus indicating an extremely low catalytic activity. The
well-known complex 1c was reported to exhibit activities
ranging from about 60002a,9a to 100 0002c mol C H (mol
with two equivalents of AlEt in toluene only led to an inac-
tive system that did not oligomerize or polymerize ethylene
(100 lmol catalyst, 60È95 ¡C, 6 MPa ethylene, 90È300 min).3b
This contrasts with the behaviour of the active catalytic
3
2
4
catalyst h)~1 consistent with our own Ðndings (Table 1), while
1b showed a much higher activity of about 180 000 mol C H
species generated in situ from cis-[Nw iMPh PCH}C(}Oz )PhN]
2
(7) and two equivalents of AlEt , which polymerized ethylene
3
2
4
(mol catalyst h)~1 (Table 1). Whereas for 1c the exothermic
reaction could still be controlled by cooling and decreasing
the amount of catalyst, this was no longer possible for 1b.
With the latter compound the temperature rose to more than
200 ¡C within less than 30 s after the reaction started, thus
leading to almost immediate decomposition of the active
species.
in a rapid exothermic reaction (cf. ref. 3j, 3k) at temperatures
around 65 ¡C and 0.6 MPa. It is noteworthy that in the pres-
ence of one equivalent of PPh , both 3 and 7 led to oligomeri-
3
zation catalysts upon reaction with two equivalents of AlEt ,
3
although of very low activity. However, the related complexes
1c and 2e showed a much higher activity for ethylene oligo-
merization than the AlEt -activated bis-chelate complexes
3
In contrast to 1, the phosphino phenolate complexes 2 gen-
erally showed a marked tendency to oligomerize ethylene into
since only traces of linear a-oleÐns from C
to C
were
4/
26/
formed by the latter catalysts under similar reaction condi-
tions.
a-oleÐns of higher molecular weight, from C
to C
90/
oligo-
and
10/
to produce only minor amounts of the C
mers. Only 2a and 2b gave a-oleÐns of low molecular weight
to C
To explain the di†erent properties of complexes 1 and 2 as
catalyst precursors in ethylene oligomerization, two important
factors that inÑuence the chain length of the oleÐnic products
have to be considered. First, the chelate part of the nickel
complexes plays an important role in the modiÐcation of the
mass distribution of the oligomers, but an inÑuence of the
4/
10/
in the range C to C
tribution was usually found between C
. The maximum of the product dis-
4/
24/
and C
(2dÈ2i),
20/
40/
but some exceptions were observed: a signiÐcant shift to lower
molecular weights was found for 2a and 2b (both C to C
)
4/
6/
and 2c (C to C
). A less distinct but unexpected shift was
auxiliary ligand PR has also to be considered. A slight
8/
12/
3
observed for 2j (C
to C
). The maximum of the product
decrease of the electron-donor ability of the P,O chelate was
10/
20/
distribution was shifted to C
pounds 2cÈ2j were highly active catalysts for the oligomeriza-
tion reaction with turnover rates of 7000È20 000 mol C H
(mol catalyst h)~1, complexes 2a, 2b and 2k showed much
lower but still noticeable activities of 500È1500 mol C H
(mol catalyst h)~1. The exothermic oligomerization reactions
were easily controlled by cooling and adjustment of the cata-
lyst concentration. All catalysts proved to be highly selective
and produced less than 2% nonlinear or internal oleÐns.
Reaction of the bis-chelate cis-[Nw iMPh P(o-C H Oz )N ] (3)
ÈC
for 2k. Whereas com-
recently found to favor b-elimination and thus formation of
lower molecular weight a-oleÐns.3b Thus the clear tendency of
complexes 2cÈ2k to form oligomers of higher molecular
weight than those observed with 1b and 1c should be related
to a higher electron-donor capacity of the phosphino phenol-
ate ligand in 2 compared with the phosphino enolate ligand in
1. On the other hand, coordination of a strong phosphine
ligand trans to the phosphorus donor of the phosphino
enolate chelate limits the molecular weight of the products,
suggesting that a catalyst without any donor ligand would
40/ 50/
2
4
2
4
2
6
4
2
New J. Chem., 1998, Pages 467È472
469

View Article Online
give the highest molecular weights.3j The corresponding three-
coordinate, 14-electron species is of course unstable and a
complex with this stoichiometry was found to be a dimer of
the two four-coordinate species, with the enolate oxygen
bridging the two nickel centres.3j The results of 2a and 2b are
consistent with this view since their better donor phosphine
ligand leads to lower molecular weight oligomers. However,
according to ref. 3k and 13 and the behaviour of 1a, complex
2a was not expected to be an active catalyst and 2b should
show a much smaller activity, if any. These observations
presence of PPh , the latter exclusively produced linear a-
oleÐns, which convincingly supports the described model.
Thus, besides the important electronic and steric inÑuence of
3
the chelating ligand,2h7 the mass distribution also depends on
the nature of the auxiliary ligand PR and can be tuned by the
3
introduction of di†erent phosphines. As seen above, this inÑu-
ence clearly depends on the ability of the phosphine to coordi-
nate more or less strongly to the metal atom: a-oleÐns of
higher molecular weight were produced by the complexes
2dÈ2k that contain the weakly bound phosphines PPh ,
3
PTol , etc., whereas complexes 2a and 2b with the more
3
undoubtedly reÑect the often denied inÑuence of the PR
ligand on the catalytic activity and the mass distribution of
3
strongly coordinating ligands PMe and PMe Ph gave oligo-
3
2
the a-oleÐns.2,3d This inÑuence could be explained in terms of
mers of lower molecular weight. Compound 2c with the
an equilibrium in solution between coordinated and free PR
that is essential for deciding whether b-elimination or chain
PMePh ligand appeared to behave in an intermediate
manner as it gave oligomers in the range of C to C
3
2
, but
4/
80/
growth occurs. Such an equilibrium was Ðrst mentioned by
Keim13 and is conÐrmed by the 31P NMR data we obtained
for some complexes: 2b and 2c each showed two broad signals
at room temperature in contrast to the usual, well-resolved
AB-type pattern for this type of compound. Especially the
temperature-dependent spectrum of 2d, which shows a broad
singlet at room temperature and two sharp doublets typical
for an AB-system at [40 ¡C, leads us to consider the
occurrence of an equilibrium between coordinated and free
with the maximum of the mass distribution still lying at lower
molecular masses than for 2dÈ2k. The notable results with 2j,
which indicated a distinct shift to lower molecular weight a-
oleÐns compared to its homologues, lack deÐnitive explana-
tion but appear contradictory since in this case an
electron-withdrawing e†ect of the ligand appears to promote
chain transfer of the a-oleÐns. This has already been recog-
nized earlier for other compounds that are active in ethylene
oligomerization.14 That a similar e†ect is not observed with
2k is not clearly understood but note that this catalyst was
prepared in situ.
PR in solution under the conditions of the oligomerization
3
reaction. Of course, this does not preclude the possibility of
pentacoordination when the oleÐn approaches the metal
centre. A chain-growth mechanism involving retention of both
Concerning the obvious di†erence between the activities of
the PMe complexes 1a and 2a, the reason should be sought
3
the P,O chelate and the PR ligand would imply changes of
in the di†erent electronic e†ect of the two chelate ligands. The
3
coordination numbers between four and Ðve and does not
more electron-donating phosphino phenolate generates an
appear consistent with the lability observed for PR in active
catalyst complexes.
electron-rich metal centre, which disfavours PMe -
coordination and thus leads to low, but noticeable activity
3
3
As shown in Scheme 1, dissociation of the phosphine ligand
[500 mol C H (mol catalyst h)~1]. In 1a on the other hand,
2
4
PR generates a free coordination site on the nickel centre
the more electron-deÐcient nickel centre generated by coordi-
nation of the less electron-donating phosphino enolate leads
3
and enables coordination of an ethylene molecule, which in a
second step undergoes insertion into a NiwPh, NiwH or
Niwalkyl bond and thus leads to chain growth. Chain growth
terminates when the phosphine recoordinates to the nickel
centre since ethylene can no longer easily enter the coordi-
nation sphere of the metal. From this 16-electron nickel
complex, the b-elimination step can procede via an 18-electron
pentacoordinated intermediate and is able to compete with
the otherwise much faster chain-growth reaction and conse-
quently leads to formation of the a-oleÐn products. In the
to a stronger NiwPMe bond and consequently this complex
3
showed practically no catalytic activity for the oligomeriza-
tion of ethylene. For compounds 2aÈ2k only two rough trends
could be observed: Ðrst, the more strongly coordinating phos-
phines in 2a and 2b gave rise to low activities, whereas the
weaker donor phosphines generated more active systems
owing to their lower tendency to coordinate to the nickel
centre. Second, sterically hindered phosphines like PCy in 2d
3
also gave rise to highly active species because of easy disso-
absence of a ligand like PR that helps terminate chain
growth, catalytic systems would lead to the formation of poly-
ciation of the metalÈphosphorus bond (cf. the 31PM1HN NMR
data of 2d). The relatively low catalytic activity of 2k was
attributed to inhibition by impurities that were present in the
material obtained from complex synthesis.
3
ethylene, as indeed observed for the AlEt -activated bis-
3
chelate complex cis-[Nw iMPh PCH}C(}Oz )PhN ] (7).3k In the
2
2
In agreement with recent observations made with
the P,N-chelate compounds of the type
4
and 5,1
Ph
P
P
Ph
via PR3 dissociation
[Nw iPhMPh P(o-C H Nz H)N(PTol )] (6) is not an active catalyst
Ni
+
C2H4
Ni
2
6
4
3
or pentacoordinated
intermediate
for the conversion of ethylene to higher a-oleÐns under the
O
O
PR3
PR3
chosen conditions.3b Only traces of C
to C
linear a-
4/
10/
oleÐns were found at the end of the reaction. This could be
due to the instability of the active species under these condi-
tions, as was already proposed for complexes 4 and 5.
styrene via β-elimination
P
H
Ni
α-olefin CH2=CH–R via β-elimination
O
Experimental
Reagents and physical measurements
PR3
R
P
– PR3
Ni
All operations were performed in Schlenk-type Ñasks under
high purity argon, using vacuum line techniques. The solvents
were puriÐed and dried under argon by conventional
methods. The 1H NMR spectra were recorded at 200 MHz on
a Bruker AC 200 F, the 31PM1HN NMR spectra at 81 MHz on
a Bruker CXP 200 and at 122 MHz on a Bruker AC300
instrument. The spectra were recorded at room temperature,
except when indicated explicitly. 1H and 31P shifts are given
relative to internal TMS and external H PO , respectively. A
O
PR3
P
H
+ PR3
– C2H4 + C2H4
– PR3
Ni
Ni
O
+ C2H4
H
R
P
P
P
+ C2H4
+ C2H4 + C2H4
chain growth
Ni
Ni
O
O
O
3 4
positive sign denotes a shift downÐeld from that of the refer-
Scheme 1
470
New J. Chem., 1998, Pages 467È472

View Article Online
ence. Coupling constants are given in Hz. The electron impact
mass spectra (El, 70 eV) were recorded on a Fisons ZAB-HF
spectrometer. Reactions with ethylene were performed in a
130 ml double-walled stainless steel autoclave, Ðtted with a
manometer, a septum inlet and a magnetic stirrer. The pro-
ducts were analyzed by gas phase chromatography with a
Hewlett Packard 5890 Series II instrument on a PONA
column (methylsilicone, diameter: 0.22 mm, length: 50 m, tem-
work-up, as described for 2a, yielded 2b as a bright yellow
powder with some 3 as an impurity. Yield: 0.30 g (31%). Anal.
calcd for C
H
OP Ni (551.23): C, 69.73; H, 5.49; O, 2.90; P,
32 30
2
11.2. Found: C, 68.34; H, 5.50; O, 4.10; P, 10.2. 1H NMR
(C D ): 7.7È6.4 (24H, aromatic H), 1.06 (br, 6H, PMe Ph);
6
6
2
31PM1HN NMR (C D ): AB spin system d 21.5 (br, Ph P),
6
2
6
A
2
d [ 7.4 (br, PMe Ph), 2J not resolved; MS (EI): m/z 550
B
AB
(M`), 473 (M` [ Ph), 412 (M` [ PMe Ph), 335 (M` [ Ph
2
perature program from 35È270 ¡C) for the C ÈC
fraction
[ PMe Ph).
4/ 30/
2
and on a capillary Supelco Polywax SPB1 column (diameter:
0.53 mm, length: 30 m, ““on column injectionÏÏ, 3 min at 5 ¡C
then temperature program 10 ¡C min~1 up to 380 ¡C) for the
[Nw iPh{Ph P(o-C H Oz )}(PMePh )] (2c). A cold solution of
2
6
4
2
0.58 g (2.1 mmol) [Ni(COD) ] in 40 ml toluene was added
2
C
to C
ÈC
fraction. The highest mass detected corresponded
slowly to a suspension of 390 ll (2.1 mmol) PMePh and
0.74 g (2.1 mmol) Ph P(o-C H O) in 20 ml toluene at 0 ¡C.
4/ 100/
2
. The phosphines P(C H X) (X \ Cl, F, CF , OMe)
100/
6
4
3
3
3
6 4
were purchased from Strem and used without further puriÐ-
cation. The other phosphines were purchased from Aldrich
and puriÐed either by recrystallization or by degassing. High
purity ethylene was purchased from Air Liquide and used as
received.
The mixture became immediately orange and formed a clear
orange solution within 2 h. After stirring at room temperature
for another 14 h, the solution was heated to 50 ¡C for 2 h and
subsequently the solvent was removed in vacuo. Usual work-
up, as described for 2a, yielded 2c as a bright yellow powder,
still containing some 3 and other unidentiÐed impurities due
to similar solubility properties. Yield: 0.58 g (43%). 1H NMR
(C D ): 8.0È6.1 (29H, aromatic H), 1.14 (br, 3H, PMePh );
Synthesis
6
6
2
[Ni(COD) ],15 [Nw iPhMPh PCH}C(}Oz )PhN(PR )] [PR \
31PM1HN NMR (C D ): AB spin system d 22.1 (d, Ph P), d
3.0 (d, PMePh ), 2J \ 245; MS (EI): m/z 612 (M`).
AB
2
2
3
3
6
6
A
2
B
PCy (1b),1b PPh (1c)9a], cis-[Nw iMPh P(o-C H Oz )N ] (3),10 cis-
3
3
2
6
4
2
2
[Nw iMPh PCH}C(}Oz )PhN ] (7),16 Ph PxCHC(xO)Ph,17
2
2
3
3
2
Ph P(o-C H O)11 and [Ph P(o-C H NH )]Br11 were synthe-
[Nw iPh{Ph P(o-C H Oz )}(PCy )] (2d). A cold solution of
3
6
4
6
4
2
6
4
3
sized according to the published methods.
0.53 g (1.9 mmol) [Ni(COD) ] in 30 ml toluene was added
2
slowly to a suspension of 0.53 g (1.9 mmol) PCy and 0.67 g
(1.9 mmol) Ph P(o-C H O) in 20 ml toluene at 0 ¡C. The
3
[Nw iPh{Ph PCH}C(}Oz )Ph}(PMe )] (1a). A cold solution
2
3
3
6 4
of 0.39 g (1.4 mmol) [Ni(COD) ] in 30 ml toluene was added
mixture acquired immediately an intense yellow tint and
formed a clear orange solution within 1 h. After stirring at
room temperature for another 15 h, the solution was heated to
50 ¡C for 2 h and subsequently the solvent was removed in
vacuo. The residue was taken up in 30 ml toluene, the solution
Ðltered and 60 ml pentane was added. At [ 18 ¡C orange
crystals were formed within a few days. These were isolated,
washed with 2 ] 5 ml pentane and dried in vacuo. Yield: 0.88
2
slowly to a suspension of 145 ll (1.4 mmol) PMe and 0.53 g
3
(1.4 mmol) Ph PxCHC(xO)Ph in 20 ml toluene at 0 ¡C. The
3
mixture acquired immediately an intense yellow tint and a
clear solution was formed within 1 h. After stirring at room
temperature for 16 h, the resulting orange solution was heated
to 50 ¡C for 2 h and subsequently the solvent was removed in
vacuo. The residue was taken up in 5 ml toluene, the solution
was Ðltered and 100 ml pentane was added. The orange crys-
tals that were formed at [18 ¡C overnight were isolated,
washed with 2 ] 5 ml pentane and dried in vacuo. Yield: 0.50
g (68%). Anal. calcd for C
7.56; O, 2.31; P, 8.93. Found: C, 72.75; H, 7.74; O, 2.18; P,
H
OP Ni (693.52): C, 72.74; H,
42 52
2
9.38. 1H NMR (C D ): 7.6È6.4 (19H, aromatic H), 2.1È0.9
6
6
g (71%). Anal. calcd for C
5.87; O, 3.11; P, 12.02. Found: C, 67.53; H, 5.90; O, 3.20; P,
H
OP Ni (515.20): C, 67.61; H,
(33H, PCy ); 31PM1HN NMR (C H , 20 ¡C): 19.1 (s); 31PM1HN
29 30
2
3
6 6
NMR (toluene-d , [40 ¡C): AB spin system d 23.4 (d, Ph P),
d 20.5 (d, PCy ), 2J \ 269.5.
AB
8
A
2
12.1. 1H NMR (C D ): 8.3È6.8 (20H, aromatic H), 5.27 (s, 1H,
6
6
B
3
PCH), 0.82 (d, 2J \ 8.8, 9H, PMe ); 31PM1HN NMR (C D ):
PH
3
6 6
AB spin system d 15.8 (d, Ph P), d [ 16.7 (d, PMe ),
[Nw iPh{Ph P(o-C H Oz )}(PPh )] (2e). A cold solution of
A
2
B
3
2
6
4
3
2J \ 298.1.
0.58 g (2.1 mmol) [Ni(COD) ] in 40 ml toluene was added
AB
2
slowly to a suspension of 0.55 g (2.1 mmol) PPh and 0.74 g
(2.1 mmol) Ph P(o-C H O) in 20 ml toluene at 0 ¡C. The
3
[Nw iPh{Ph P(o-C H Oz )}(PMe )] (2a). As described for 1a,
2
6
4
3
3
6 4
0.46 g (1.7 mmol) [Ni(COD) ] in 30 ml toluene was reacted
mixture became dark red immediately and formed a clear red
solution within 1 h. After stirring at room temperature for
another 15 h, the resulting yellow-orange solution was heated
to 50 ¡C for 2 h and subsequently the solvent was removed in
vacuo. Usual work-up, as described for 2a, yielded 2e as a
bright yellow powder, still containing some of the starting
2
with a suspension of 176 ll (1.7 mmol) PMe and 0.60 g (1.7
mmol) Ph P(o-C H O) in 20 ml toluene. The residue obtained
3
3
6 4
after evaporation of the solvent was taken up in 10 ml toluene,
the solution was Ðltered and 100 ml pentane were added. The
bright yellow precipitate was isolated by Ðltration, washed
with 2 ] 5 ml pentane and dried in vacuo. A second crop of
the product was obtained from the Ðltrate at [18 ¡C. Both
fractions still contain some of the bis-chelate complex cis-
[Nw iMPh P(o-C H Oz )N ] (3) due to similar solubility properties.
PPh and 3 due to similar solubility properties. Yield: 0.45 g
3
(33%). Anal. calcd for C
H
OP Ni (675.37): C, 74.69; H,
42 34
2
5.07; O, 2.37; P, 9.17. Found: C, 73.88; H, 5.08; O, 3.09; P,
9.06. 1H NMR (C D ): 7.9È5.8 (aromatic H); 31PM1HN NMR
2
6
4
2
6
6
Yield: 0.38 g (47%). Anal. calcd for C
H
OP Ni (489.16): C,
(C D ): AB spin system d 23.7 (d, Ph P), d 18.7 (d, PPh ),
27 28
2
6
6
A
2
B
3
66.3; H, 5.77; O, 3.27; P, 12.7. Found: C, 65.1; H, 5.65; O,
2J \ 286.8; MS (EI): m/z 674 (M`), 597 (M` [ Ph), 412
(M` [ PPh ), 335 (M` [ Ph [ PPh ).
AB
3.09; P, 11.9. 1H NMR (C D ): 7.6È6.4 (19H, aromatic H),
6
6
3
3
0.77 (d, 2J \ 9.3, 9H, PMe ); 31PM1HN NMR (C D ): AB
PH
3
6 6
spin system d 20.8 (d, Ph P), d [ 16.6 (d, PMe ), 2J
\
[Nw iPh{Ph P(o-C H Oz )}(PTol )] (2f). As described for 2e,
A
2
B
3
AB
2
6
4
3
302.4; MS (EI): m/z 488 (M`), 411 (M` [ Ph), 412 (M`
[ PMe ), 335 (M` [ Ph [ PMe ).
0.53 g (1.9 mmol) [Ni(COD) ] in 30 ml toluene was reacted
2
with a suspension of 0.58 g (1.9 mmol) PTol and 0.67 g (1.9
mmol) Ph P(o-C H O) in 20 ml toluene. Usual work-up, as
described for 2a, yielded 2f as a bright yellow powder. Yield:
1.15 g (84%). Anal. calcd for C
H, 5.6. Found: C, 74.9; H, 5.4. 1H NMR (C D ): 7.9È6.2 (31H,
aromatic H), (s, 9H, PC H Me); 31PM1HN NMR (C D ): AB
3
3
3
3
6 4
[Nw iPh{Ph P(o-C H Oz )}(PMe Ph)] (2b). As described for
2
6
4
2
1a, 0.43 g (1.6 mmol) [Ni(COD) ] in 30 ml toluene was
H
OP Ni (717.45): C, 75.3;
2
45 40
2
reacted with a suspension of 227 ll (1.6 mmol) PMe Ph and
0.56 g (1.6 mmol) Ph P(o-C H O) in 20 ml toluene. Usual
2
6
6
3
6
4
6
4
6 6
New J. Chem., 1998, Pages 467È472
471

View Article Online
spin system d 25.0 (d, Ph P), d 18.5 (d, PTol ), 2J \ 287.6;
canula to the autoclave and the same procedure as described
above was followed.
A
2
B
3
AB
MS (EI): m/z 716 (M`), 639 (M` [ Ph), 412 (M` [ PTol ),
3
335 (M` [ Ph [ PTol ).
3
Reactions of AlEt -activated 3 and 7 with ethylene in the
3
[Nw iPh{Ph P(o-C H Oz )}{P(p-C H OMe) } ]
(2g).
As
presence of PPh or PEt . Solutions of 60È90 lmol of the
2
6
4
6
4
3
3
3
described for 2e, 0.38 g (1.4 mmol) [Ni(COD) ] in 30 ml
nickel bis-chelate complexes and one equivalent of PPh or
2
3
toluene was reacted with a suspension of 0.45 g (1.4 mmol)
PEt in 20 ml toluene were reacted with two equivalents of
AlEt as described above and the usual procedure for the
catalysis experiments was followed.
3
3
P(p-C H OMe) and 0.49 g (1.4 mmol) Ph P(o-C H O) in 20
6
4
3
3
6 4
ml toluene. Usual work-up, as described for 2a, yielded 2g as a
yellow-ocre powder. Due to similar solubility properties, the
product still contained a small amount of 3 and some of the
starting phosphine P(p-C H OMe) . Yield: 0.64 g (57%).
Acknowledgements
6
4
3
31PM1HN NMR (C D ): AB spin system d 22.3 (d, Ph P), d
14.7 [d, P(p-C H OMe) ], 2J \ 289.7.
AB
Financial support from the Centre National de la Recherche
ScientiÐque and the Institut Francais du Petrole (IFP) is
gratefully acknowledged. We are grateful to Drs. D. Com-
mereuc, H. Olivier and L. Saussine (IFP) for numerous dis-
cussions. J. P. thanks the Alexander von Humboldt Stiftung
for the award of a Feodor Lynen fellowship and the CNRS for
a temporary position.
6
6
A
2
B
6
4
3
[Nw iPh{Ph P(o-C H Oz )}{P(OMe) } ] (2h). A cold solution
2
6
4
3
of 0.47 g (1.7 mmol) [Ni(COD) ] in 30 ml toluene was added
2
slowly to a suspension of 200 ll (1.7 mmol) P(OMe) and 0.60
g (1.7 mmol) Ph P(o-C H O) in 20 ml toluene at 0 ¡C. The
3
3
6 4
mixture acquired an intense yellow tint within 1 h, but no
clear solution was formed. After stirring at room temperature
for another 15 h, the resulting orange cloudy solution was
heated to 50 ¡C for 2 h. The now clear solution was evapo-
rated, whereupon a yellow solid precipitated soon. The
residue was taken up in 10 ml toluene and the suspension was
Ðltered. The product was washed with 2 ] 5 ml pentane and
dried in vacuo, while 50 ml pentane were added to the solu-
tion. Thus, a second crop of 2h was obtained, which was also
washed with 2 ] 5 ml pentane and dried in vacuo. Due to
similar solubility properties, the product still contained a
References
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small amount of 3 and residual P(OMe) . Yield: 0.51 g (53%).
3
31PM1HN NMR (C D ): AB spin system d 24.0 (br, Ph P), d
6
6
A
2
B
18.1 [br, P(OMe) ], 2J not resolved.
3
AB
[Nw iPh{Ph P(o-C H Oz )}(PR )] [PR = P(p-C H Cl) (2i),
2
3
6
4
3
3
6
4
3
P(p-C H F) (2j), P(p-C H CF ) (2k)]. As described for 2e,
6
4
6
4
3 3
about 1.5 mmol [Ni(COD) ] in 30 ml toluene was reacted
2
with suspensions of one equivalent of the corresponding phos-
phine PR and one equivalent of Ph P(o-C H O) in 20 ml
3
3
6 4
toluene. The dark red solids thus obtained were used for the
catalytic experiments without further puriÐcation and charac-
terization.
[Nw iPh{Ph P(o-C H Nz H)}(PTol )] (6). As described for 2e,
2
6
4
3
0.45 g (1.6 mmol) [Ni(COD) ] in 30 ml toluene was reacted
2
with a suspension of 0.97 g (3.2 mmol) PTol and 0.70 g (1.6
mmol) [Ph P(o-C H NH )]Br in 20 ml toluene. At the end of
the reaction, the solid [HPTol ]Br was removed by Ðltration
and the clear red solution was evaporated. The dark red solid
5 W. Keim, B. Ho†mann, R. Lodewick, M. Peuckert and G.
Schmitt, J. Mol. Cat., 1979, 6, 796.
3
3
6
4
2
6 K. J. Cavell and A. F. Masters, Aust. J. Chem., 1986, 39, 1129.
7 (a) V. M. Mohring and G. Fink, Angew. Chem., Int. Ed. Engl.,
1985, 24, 1001; (b) W. Keim, R. Appel, A. Storeck, C. Kruger and
R. Goddard, Angew. Chem., Int. Ed. Engl., 1981, 20, 116.
8 W. Keim and R. Nucker, Chem. Ing. T ech., 1994, 66, 950.
9 (a) W. Keim, F. H. Kowaldt, R. Goddard and C. Kruger, Angew.
Chem., Int. Ed. Engl., 1987, 17, 466; (b) W. Keim, A. Behr, B.
Gruber, B. Ho†mann, F. H. Kowaldt, U. Kurschner, B. Limbacker
and F. P. Sistig, Organometallics, 1986, 5, 2356.
3
thus obtained was used for the catalytic experiments without
further puriÐcation. 31PM1HN NMR (C D ): AB spin system
6
AB
6
d 28.5 (d, Ph P), d 21.9 (d, PTol ), 2J \ 276.4.
A
2
B
3
Reactions
10 T. B. Rauchfuss, Inorg. Chem., 1977, 16, 2966.
Reactions of 1, 2 and 6 with ethylene. The nickel complexes
(20È200 lmol) were dissolved in 40 ml toluene, transferred via
canula to the autoclave and stirred under 0.5 MPa ethylene
for 16 h. Then temperature and pressure were increased to the
standard conditions 80È90 ¡C and 6 MPa. After 1È3 h, the
autoclave was cooled to ambient temperature, the pressure
released slowly and the products analyzed by gas phase chro-
matography.
11 M. K. Cooper, J. M. Downes, P. A. Duckworth and E. R. T.
Tiekink, Aust. J. Chem., 1992, 45, 595.
12 J. Pietsch, L. Dahlenburg, A. Wolski, H. Berke and I. L. Ere-
menko, J. Organomet. Chem., 1995, 495, 113.
13 W. Keim, Chem. Ing. T ech., 1984, 56, 850.
14 S. Mercier, PhD Thesis, Universite de Paris VI, 1993.
15 R. A. Schunn, S. D. Ittel and M. A. Cushing, Inorg. Synth., 1990,
28, 94.
16 P. Braunstein, D. Matt, D. Nobel, F. Balegroune, S.-E. Bouaoud,
D. Grandjean and J. Fischer, J. Chem. Soc., Dalton T rans., 1988,
353.
Reactions of AlEt -activated 3 and 7 with ethylene. Typi-
3
cally, a solution of 30È150 lmol of the nickel bis-chelate com-
17 T. M. G. Carneiro, J. Dupont, M. Luke and D. Matt, Quim. Nova,
1988, 11, 215.
plexes in 20 ml toluene was cooled to 0 ¡C and two
equivalents of AlEt were added. These mixtures were stirred
3
for 20 min while warming to room temperature, then the
Received in Montpellier, France, 10th March 1997;
Paper 7/09204K
resulting yellow/brown clear solutions were transferred via
472
New J. Chem., 1998, Pages 467È472