Synthesis of allyl- and aryl-iminopyrrolyl complexes of nickel

Article abstract of DOI:10.1039/b311323j

The 2-iminopyrrole ligand precursors 2-C₄H₃ NH[(R)C=N(2,4,6-C₆H₂Me₃)] (R = H, I; R = Me, II) were synthesised and react with NaH to give the corresponding sodium salts 1 and 2, respectively. Salt 2 re

Full text of DOI:10.1039/b311323j

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Synthesis of allyl- and aryl-iminopyrrolyl complexes of nickel†
Ronan M. Bellabarba,a Pedro T. Gomes*a and Sofia I. Pascub
a Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais,
1049-001 Lisboa, Portugal. E-mail: pedro.gomes@ist.utl.pt
b Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR
Received 16th September 2003, Accepted 19th September 2003
First published as an Advance Article on the web 7th October 2003
The 2-iminopyrrole ligand precursors 2-C H NH[(R)C᎐N(2,4,6-C H Me )] (R = H, I; R = Me, II) were synthesised
᎐
4
3
6
2
3
and react with NaH to give the corresponding sodium salts 1 and 2, respectively. Salt 2 reacts with [NiPhBr(PPh3)2] to
yield [NiPh(acetiminopyrrolyl)(PPh3)] 3. Ligand precursors II and 2 react, respectively, with equimolar amounts of
[NiMe2(TMEDA)] and [NiBr2(NCMe)2] to give [Ni(acetiminopyrrolyl)2] 4. Both 1 and 2 react with [Ni(η3-C3H5)-
(µ-Br)]2 to give complexes [Ni(η3-C3H5)(iminopyrrolyl)] 5 and 6. The crystal structures of ligand precursor II and of
complex 3 are reported and compared. Complex 3 was tested for catalytic activity for the oligomerisation of ethylene
and showed only moderate activity.
(Fig. 1) and to compare it with the metal complexes, and rele-
Introduction
vant bond distances and angles are given in Table 1. As can be
In recent years, many papers have been published in which
seen from Table 1, the pyrrole ring shows a longer bond dis-
bidentate chelating ligands containing an imine and a donor
tance between C4 and C5 than between any other carbons of
moiety have been used as precursors to the synthesis of
the ring, the shortest bond in the ring being between N1 and
new organometallic complexes as precatalysts for olefin poly-
C6. The angle between C3–N1–C6 is 109.37(12)Њ, whilst the
merisation.1 However, metal complexes of ligands containing
both an imine and a pyrrolyl moiety are uncommon. These
complexes may be formed using two different strategies: either
imine C2–N2 distance is 1.2846(19) Å and the angle at N2 is
118.91(12)Њ. The co-planarity of the pyrrole ring with the
acetimine group (torsion angle N1–C2–C3–N1 of 1.3Њ) and a
distance C2–C3 of 1.450(2) Å, shorter than normal values of a
typical C–C single bond, points to an extension of the pyrrole
ring π-electronic delocalisation towards its acetimino sub-
by in situ formation of the imine and concomitant complex-
ation to the metal centre via a template-type synthesis,2,3 or by
prior formation of the imine and reaction to form the desired
metal complex.4–10 The advantages of the latter strategy is that
stituent. The sterical hindrance produced by the two methyl
ancillary ligands can be also attached to the metal centre, whilst
substituents in the 2 and 6 positions of the mesityl ring
with the metal template synthesis bis(iminopyrrolyl)metal
complexes appear to be the favoured products. This type of
makes it nearly perpendicular (83.2Њ) to the acetiminopyrrole
plane defined by atoms N1, C3, C2 and N2. These features
only change slightly upon coordination, as will be discussed
below.
complex has been studied for olefin polymerisation catalysis in
several projects.6–9 Following Grubbs et al. work on neutral
nickel() catalysts for olefin polymerisation,11,12 we decided to
synthesise new iminopyrrolyl complexes of nickel in view of
their potential interest as olefin polymerisation catalysts. At the
end of this study we came across patents where similar ligand
precursors and nickel complexes are claimed.13,14
Results and discussion
Two (arylimino)pyrrole ligands were synthesised by conden-
sation of 2-(formyl)pyrrole or 2-(acetyl)pyrrole with mesityl-
aniline (Scheme 1). The synthesis of the formimines was
relatively straightforward and standard conditions could be
employed, but the synthesis of the acetimines required forcing
conditions and resulted in low yields (ca. 35%).
Fig. 1 Molecular structure of the ligand precursor II.
Treatment of the ligands I and II with sodium hydride results
in the formation of the pyrrolyl sodium salts 1 and 2, respect-
ively. These ligand precursors were generally prepared immedi-
ately before reaction with the appropriate nickel substrate, and
only characterised in the case of 2 by 1H NMR spectroscopy to
confirm the site of deprotonation.
Treatment of [NiPhBr(PPh3)2] with one equivalent of 2
results in the formation of 3 (Scheme 2). However, this reaction
1
is not clean, as a certain amount of 4 (as determined by H
NMR) and of [Ni(PPh3)4] (determined by elemental analysis)
were also formed in all cases independently of precautions
taken to exclude moisture or control the reaction by cooling.
In an attempt to synthesise [NiMe(acetiminopyrrolyl)-
(NCCH3)], the product 4 was also formed, in quantitative yield
(based on I), in the reaction of [NiMe2(TMEDA)] with one
equivalent of I in acetonitrile (Scheme 3). 3 was characterised
by 1H and 13C{1H} NMR spectroscopy, elemental analysis and
its identity was confirmed by mass spectrometry.
Scheme 1 Synthetic route to the ligand precursors I and II.
1
These ligands were characterised by H and 13C{1H} NMR
spectroscopy. The crystal structure of the acetimine ligand pre-
cursor (II) was determined in order to confirm the structure
† Dedicated to Professor J. J. R. Fraústo da Silva on the occasion of his
70th birthday.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 4 4 3 1 – 4 4 3 6
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Table 1 Selected bond distances (Å) and angles (Њ) for ligand precursor acetiminopyrrole II and the complex [NiPh(acetiminopyrrolyl)(PPh3)] 3
II
3
II
3
N2–C2
N2–C7
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
N1–C6
1.2846(19)
1.4266(19)
1.507(2)
1.450(2)
1.387(2)
1.410(2)
1.374(2)
1.3617(19)
1.302(3)
1.435(3)
1.501(3)
1.427(3)
1.387(4)
1.397(4)
1.396(4)
1.353(3)
1.386(3)
2.1478(7)
1.940(2)
1.959(2)
1.900(3)
1.398 (4)
1.392(4)
1.386(5)
1.379(5)
1.396(4)
1.399(4)
C2–N2–C7
N1–C3–C2
N2–C2–C3
C1–C2–C3
C2–C3–C4
N1–C3–C4
C3–C4–C5
C4–C5–C6
N1–C6–C5
C3–N1–C6
N1–Ni1–N2
P1–Ni1–C16
P1–Ni1–N1
N2–Ni1–C16
P1–Ni1–N2
N1–Ni1–C16
118.91(12)
122.28(13)
119.74(13)
116.46(13)
130.25(13)
107.36(13)
107.51(13)
107.15(13)
108.61(13)
109.37(12)
–
120.2(2)
114.8(2)
115.7(2)
120.7(2)
134.4(2)
110.6(2)
106.3(2)
106.4(2)
111.4(2)
105.3(2)
82.87(9)
88.10(7)
98.79(6)
91.43(9)
174.10(7)
166.76(11)
N1–C3
1.3747(18)
Ni1–P1
Ni1–N1
Ni1–N2
Ni1–C16
C16–C17
C17–C18
C18–C19
C19–C20
C20–C21
C21–C16
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Scheme 3
Scheme 2 Synthesis of 3 and side-products.
On the basis of these observations it would appear that Ni
complexes containing two iminopyrrolyl ligands, such as 4, are
favoured products.
The complex 3 was characterised by 1H, 31P{1H} and 13C{1H}
NMR spectroscopy, and peaks assigned by using selective
1H–1H decoupling techniques. Satisfactory elemental analysis
was obtained and suitable crystals for X-ray diffractometry
were grown. The structure is shown in Fig. 2 and selected bond
distances and angles are given in Table 1. The sum of all the
angles around the nickel centre is ca. 360Њ, indicating this atom
is in an essentially square-planar conformation (Fig. 3), albeit
distorted by the ligand (angle between the planes defined by N1,
Ni1, N2 and P1, Ni1, C16 is 13.4Њ). The acetiminopyrrolyl
chelating ligand occupies two cis positions and the triphenyl-
phosphine group is located trans to the mesitylimine moiety due
to steric reasons. The phenyl ligand is virtually perpendicular to
the square plane (88.5Њ) and the mesityl substituent of the imine
also retains its perpendicularity to this plane (79.4Њ). Com-
parison of the data in Table 1 also highlights some structural
differences between the free ligand and when it is coordinated.
The first feature to note is that the acetiminopyrrolyl ligand bite
angle described by N1–Ni–N2 is very acute, at 82.87(9)Њ. This
Fig. 2 Molecular structure of the complex 3.
value is obtained at expenses of decreases of 3.8 and, mainly,
7.7Њ in the angles defined by N2–C3–C2 or N1–C3–C2 in
relation to those observed in the organic ligand precursor II.
Also, the angle at the pyrrolyl nitrogen C3–N1–C6 is decreased
by almost 4Њ upon coordination, which is compensated by an
increase of 2.7 and 3.3Њ of the pyrrolyl angles at C6 and C3,
D a l t o n T r a n s . , 2 0 0 3 , 4 4 3 1 – 4 4 3 6
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complexes 5 and 6, respectively, are the syn-protons closest to
the pyrrole ring. The resonance at δ 7.01 and 7.12, respectively,
was shown to correspond to the proton occupying the
2-position on the pyrrole ring. Interestingly, in the case of 5, a
syn- and an anti-allyl proton are isochronous at around δ 2.0;
however, these resonances are partially masked by the mesityl
p-CH3 resonance. This is not the case in 6 where all four allyl
CH2 resonances are well separated. Both the allyl ligands in 5
and 6 are non-fluxional.
Experimental
All manipulations of air- and/or moisture-sensitive materials
were carried out under inert atmosphere using a dual vacuum/
nitrogen line and standard Schlenk techniques. Nitrogen gas
was supplied in cylinders by Air Liquide, and purified by
passage through 4Å molecular sieves. Solvents and solutions
were transferred using a positive pressure of nitrogen through
stainless steel cannulae and mixtures were filtered in a similar
way using modified cannulae that could be fitted with glass fibre
filter disks. Unless otherwise stated, reagents were purchased
from commercial suppliers (Aldrich, Fluka et sim.) and used
without further purification. All solvents to be used under inert
atmosphere were thoroughly deoxygenated and dehydrated
before use. They were dried and purified by refluxing over a
suitable drying agent followed by distillation under nitrogen.
The following drying agents were used: sodium/benzophenone
for toluene, benzene, thf and diethyl ether; calcium hydride for
hexanes, dichloromethane and o-dichlorobenzene. Deuterated
solvents were dried by storage over 4 Å molecular sieves and
degassed by the freeze–pump–thaw method. Mass spectra were
obtained from the IST mass spectrometry service. Nuclear
magnetic resonance spectra were recorded on a Varian Unity
Fig. 3 Alternative view of the molecular structure of complex 3.
ORTEP diagram showing the square planar geometry about the Ni()
centre.
respectively. In general, the bond lengths within the pyrrole ring
or in the acetiminic moiety appear not to be very significantly
affected. The N1–C6 distance decreases slightly, as does the
C4–C5 distance, whilst the N1–C3 and the C5–C6 distances
increase. The angle at the iminic nitrogen C2–N2–C7 remains
almost the same, and the imine double bond C2–N2 only
slightly increases upon coordination, indicating that π-back-
donation from the nickel centre to this group is not strong.
Preliminary experiments in which the complex 3 was tested
for catalytic ability in the polymerisation of olefins showed
it as inactive. However, when in the presence of a phosphine
scavenger like [Ni(COD)2] (1 equivalent), it promotes oligo-
merisation of ethylene at 25 and 50 ЊC, in toluene, to a mixture
of unsaturated hydrocarbons. The activity at 25 ЊC is estimated
to be 1.25 × 103 g oligomer (mol Ni)Ϫ1 hϪ1 barϪ1, which is lower
than the values observed for the oligomerisation of ethylene
with catalysts based on the analogue systems Ni salicyl-
aldimine/[Ni(COD)2] (5.7 × 103–8.2 × 104 g oligomer (mol Ni)Ϫ1
1
300 spectrometer, at the following frequencies: H at 299.995
MHz; 13C at 75.4296 MHz; 31P at 121.417 MHz. Spectra were
referenced internally using the residual protio solvent resonance
relative to tetramethylsilane (1H and 13C, δ = 0) or 85% H3PO4
(31P, δ = 0). All chemical shifts are quoted in δ (ppm) and coup-
ling constants are given in Hz. Multiplicities are abbreviated as
follows: singlet (s), doublet (d), triplet (t), heptet (h), multiplet
(m), broad (br). Elemental analyses were obtained from the IST
elemental analysis service. Masses are quoted in grams (g) for
quantities above 1 g and milligrams (mg) for smaller amounts.
The compounds 2-formylpyrrole, 2-acetylpyrrole,17 [NiPhBr-
(PPh3)2],18 and [NiMe2(TMEDA)]19 were synthesised according
to the literature procedure. [Ni(η3-C3H5)(µ-Br)]2 was prepared20
by oxidative addition of allyl bromide to [Ni(COD)2].21
h
Ϫ1 barϪ1 11,12 or the Ni diimine/MAO systems (5.5 × 104–6.2 ×
)
105 g oligomer (mol Ni)Ϫ1 hϪ1 barϪ1).15,16 We emphasise, how-
ever, that the value herein reported corresponds to non-
optimised oligomerisation conditions (temperature, pressure
and 3/[Ni(COD)2] ratio). Blank experiments revealed that
[Ni(COD)2] alone is inactive at both temperatures.
Reaction of two equivalents of 1 with [Ni(η3-C3H5)(µ-Br)]2
cleanly yields 5 (Scheme 4) as orange crystals, which was charac-
terised by H and 13C{1H} NMR spectroscopy and elemental
1
analysis; 1H–1H decoupling techniques and a COSY-NMR
experiment permitted assignments of the 1H NMR resonances.
Preparation of (1H-pyrrol-2-ylmethylene)(2,4,6-trimethyl-
phenyl)amine, formimine, I
2-Formylpyrrole (1.00 g, 10.6 mmol), mesitylaniline (1.58 g,
11.5 mmol), a catalytic amount of p-toluenesulphonic acid and
enough MgSO4 to remove any water from the reaction mixture
were suspended in absolute ethanol (5 ml) in a 50 ml round
bottom flask fitted with a condenser and a CaCl2 guard tube.
The mixture was heated to reflux overnight, during which time
the solution turned yellow–orange. The mixture was allowed
to cool, CH2Cl2 was added and the suspension was filtered
through Celite, and washed through with more CH2Cl2. After
removal of all volatiles, the product was recrystallised from
refluxing hexane to yield 2.01 g (90%) of a microcrystalline
yellow solid. Anal. found (calc. for C14H16N2): C 79.28 (79.21);
H 7.76 (7.60); N 13.22 (13.20)%. NMR: δH (CDCl3): 10.60 (1H,
Scheme 4 Synthesis of complexes 5 and 6.
The complex 6 (Scheme 4) was prepared in a similar fashion
by the reaction of 2 with [Ni(η3-C3H5)(µ-Br)]2, isolated as a
1
yellow microcrystalline product, and characterised by H and
br s, pyrrole NH), 7.94 (1H, s, N᎐CH), 6.89 (2H, s, phenyl H),
᎐
13C{1H} NMR spectroscopy and elemental analysis.
6.59 (1H, s, pyrrole 5-H), 6.54 (1H, s, pyrrole 3-H), 6.20 (1H, m,
pyrrole 4-H), 2.29 (3H, s, mesityl p-CH3), 2.12 (6H, s, mesityl
o-CH3); δC (CDCl ): 153.25 (N᎐CH), 148.27 (ipso-C), 133.45
1H–1H COSY, NOE and selective decoupling NMR experi-
ments were carried out and the 1H NMR resonances of 2, 3 and
6 were assigned by comparison between them. These experi-
ments showed that the peak occurring at δ 2.91 and 2.98 for
᎐
3
(pyrrole quat. C), 130.36 (mesityl quat. p-C), 129.08 (mesityl
m-CH), 128.35 (mesityl quat. o-C), 123.87 (pyrrole 5-CH),
D a l t o n T r a n s . , 2 0 0 3 , 4 4 3 1 – 4 4 3 6
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116.72 (pyrrole 4-CH or 3-CH), 110.34 (pyrrole 3-CH or
mesityl-H), 6.36 (1H, m, NiPh p-H), 6.27 (2H, m, NiPh m-H),
4-CH), 21.02 (mesityl p-CH3), 18.57 (mesityl o-CH3).
6.20 (1H, m, pyrrole 3-H), 5.96 (1H, br, pyrrole 4-H), 2.32 (6H,
s, mesityl-o-CH ), 1.98 (3H, s, N᎐CCH ), 1.66 (3H, s, mesityl-p-
᎐
3
3
1
CH3); δC (C D ): 170.49 (N᎐CCH ), 153.64 (d, J = 48 Hz,
᎐
Synthesis of [1-(1H-pyrrol-2-yl)ethylidene](2,4,6-trimethyl-
phenyl)amine, acetimine, II
6
6
3
PC
C–P), 144.08 (ipso-C), 143.23 (aryl quat. C), 138.64 (aryl CH),
136.58 (aryl CH), 135.08 (aryl CH), 134.93 (aryl CH), 133.66
(pyrrole quat. C), 133.11 (aryl quat. C), 132.52 (aryl CH),
130.98 (aryl quat. C), 130.07 (aryl CH), 125.28 (aryl CH),
121.52 (pyrrole 5-CH), 115.70 (pyrrole 4-CH or 3-CH), 111.68
(pyrrole 3-CH or 4-CH), 20.83 (mesityl p-CH3), 19.20 (mesityl
2-Acetylpyrrole (purified by vacuum sublimation at 140 ЊC,
10Ϫ2 mbar) (1.08 g, 10 mmol) and mesitylaniline (1.49 g,
11 mmol, 1.54 ml) were placed in a 100 ml round bottom flask
with a catalytic amount of p-toluenesulphonic acid under
nitrogen. A calcium chloride guard tube was fitted on top of
the flask, and it was immersed as far as possible in an oil bath.
The bath was heated to 140 ЊC overnight. After the flask was
allowed to cool, the remaining aniline was eliminated by trap-
to-trap distillation, and the iminopyrrole was vacuum-sublimed
into a water condenser. Any further impurities could be
removed by recrystallisation from the minimum amount of
boiling hexane. A second crop of crystals from the mother-
liquor can be obtained by evaporation and washing with cold
hexane. Combined isolated yield: 758 mg (35%). Anal. found
(calc. for C15H18N2): C 79.77 (79.61); H 8.16 (8.02); N 12.37
(12.38)%. NMR: δH (CDCl3): 9.80 (1H, br s, pyrrole NH), 6.83
(2H, s, mesityl CH), 6.75 (1H, s, pyrrole 5-H), 6.61 (1H, m,
o-CH ), 16.04 (N᎐CCH ); δ (C D ): 31 (s).
᎐
3
3
P
6
6
Synthesis of Ni(acetiminopyrrolyl)2, 4
Ni(TMEDA)Me2 (165 mg, 0.81 mmol) was dissolved in
acetonitrile and cooled to Ϫ45 ЊC. The iminopyrrole II (183 mg,
0.81 mmol) was dissolved in acetonitrile and added slowly and
dropwise to the nickel complex. The solution was allowed to
warm to room temperature and stirred overnight. After filtration,
the solvent was removed. During the removal of the solvent, the
colour changed from yellow to orange to red. The dry material
was
a red microcrystalline solid. Washing with hexanes,
extraction and crystallisation from diethyl ether yielded a red
crystalline solid. Yield: 148 mg (72%, relative to ligand). Anal.
found (calc. for C30H34N4Ni): C 69.37 (70.75); H 7.00 (6.73);
N 10.67 (11.00)%; MS: m/z 508 (Mϩ), 493 (Mϩ Ϫ CH3), 442
(Mϩ Ϫ C4H4N), 400 (Mϩ Ϫ C9H11), 349 [Mϩ Ϫ C11H14], 284
[Mϩ Ϫ C15H17N2]. NMR: δH (C6D6): 6.64 (2H, s, mesityl CH),
6.58 (1H, m, pyrrole 5-H), 6.01 (1H, m, pyrrole 3-H), 5.00 (1H,
br, pyrrole 4-H), 2.39 (6H, s, mesityl-o-CH3), 2.01 (3H, s,
pyrrole 3-H), 6.21 (1H, m, pyrrole 4-H), 2.24 (3H, s, N᎐CCH ),
᎐
3
1.95 (6H, s, mesityl o-CH3), 1.91 (3H, s, mesityl p-CH3);
δC (pyridine-d ): 158.12 (N᎐CCH ), 147.13 (mesityl ipso-C),
᎐
5
3
133.29 (pyrrole quat. C), 131.50 (mesityl quat. p-C), 128.97
(mesityl CH), 126.48 (mesityl quat. o-C), 122.76 (pyrrole
5-CH), 112.72 (pyrrole 4-CH or 3-CH), 109.75 (pyrrole 3-CH
or 4-CH), 20.76 (mesityl p-CH3), 18.11 (mesityl o-CH3), 16.77
N᎐CCH ), 1.38 (3H, s, mesityl-p-CH ); δC (C6D6): 170.5
᎐
3
3
(N᎐CCH ).
᎐
3
(N᎐CCH ), 143.4 (mesityl ipso-C), 142.5 (aryl quat. p-C), 135.9
᎐
3
(pyrrole CH), 135.8 (pyrrole quat. C), 133.4 (aryl quat. o-C),
129.5 (aryl m-CH), 117.0 (pyrrole CH), 111.4 (pyrrole CH), 21.0
Synthesis of the sodium salt of [1-(1H-pyrrol-2-yl)ethylidene]-
(2,4,6-trimethylphenyl)amine, 2
(mesityl p-CH ), 18.8 (mesityl o-CH ), 15.0 (N᎐CCH ).
᎐
3
3
3
NaH (55 mg, 60% dispersion in mineral oil, 1.38 mmol) was
placed in a Schlenk tube under nitrogen, washed twice with
hexanes and suspended in thf. The iminopyrrole II (271 mg,
1.2 mmol) was slowly added as a solid under a counterflow of
nitrogen, and immediate evolution of hydrogen occurred. After
the addition was completed, the suspension was stirred for
90 min and then filtered into another Schlenk tube. The yellow
solution was concentrated to about 5 ml, and excess hexane was
added. After a few seconds, the product began to precipitate
out of solution. The material was allowed to settle for 15 min,
then the supernatant was filtered off and the solid pumped to
dryness. Yield: 180 mg, 0.73 mmol, 60%. NMR: δH (pyridine-
d5): 7.62 (1H, s, pyrrole 5-H), 7.34 (1H, m, pyrrole 3-H), 6.80
Synthesis of Ni(ꢀ3-C3H5)(formiminopyrrolyl), 5
NaH (144 mg, 60% dispersion in mineral oil, 3.6 mmol) was
placed in a Schlenk tube under nitrogen, washed twice with
hexanes and suspended in thf. The iminopyrrole I (245 mg,
1.16 mmol) was slowly added as a solid under a counterflow of
nitrogen, and immediate evolution of hydrogen occurred with
concomitant generation of a bright pink colour. The suspen-
sion was stirred for 1 h then slowly added by filtration into a
thf solution of [Ni(η3-C3H5)(µ-Br)]2 (207 mg, 0.58 mmol) at
Ϫ70 ЊC. The solution rapidly turned orange and cloudy, and
was allowed to warm to room temperature. All volatiles were
removed under vacuum, and the residue was extracted with
hexanes until the extracts were colourless. The resulting solu-
tion was concentrated and cooled to Ϫ80 ЊC to yield the prod-
uct as orange crystals. Yield: 68%. If desired a second crop can
be obtained by concentrating the mother-liquor and further
cooling to Ϫ80 ЊC. Anal. found (calc. for C17H20N2Ni): C 65.87
(65.64); H 6.57 (6.48); N 8.92 (9.01)%. NMR: δH (C6D6): 7.01
(1H, m, pyrrole 4-H), 6.78 (2H, s, mesityl CH), 2.25 (3H, s, N᎐
᎐
CCH3), 2.16 (3H, s, mesityl p-CH3), 1.90 (6H, s, mesityl o-CH3).
Synthesis of [NiPh(acetiminopyrrolyl)(PPh3)], 3
The sodium salt 2 (164 mg, 0.66 mmol) was suspended in
toluene, with a few drops of thf to dissolve it. Ni(PPh3)2(Ph)Br
(485 mg, 0.66 mmol) was dissolved in toluene and cooled to
Ϫ80 ЊC. The sodium salt was added to the nickel complex, and
the resulting suspension was stirred for 3 h, during which time
the reaction mixture attained a temperature of ca. 0 ЊC. The
mixture was then pumped to dryness, and the following workup
procedure was applied. The residue was washed once with
hexanes and once with a very small quantity of diethyl ether.
The residue was then extracted into diethyl ether until it was
almost colourless. NMR experiments showed that the hexane
extract contained very little of the desired product, whilst the
diethyl ether extract contained it. Washing this residue with
hexane afforded a yellow solid, which was dried under vacuum.
Yield: 120 mg (30%). Anal. found (calc. for C39H37N2PNi): C
74.74 (75.14); H 6.18 (5.98); N 4.11 (4.49)%. NMR: δH (C6D6):
7.66 (6H, m, PPh3 m-H), 7.02 (2H, m, NiPh o-H), 6.80–6.90
(9H, m, PPh3 o,p-H), 6.78 (1H, m, pyrrole 5-H), 6.48 (2H, s,
(1H, br, pyrrole 5-H) 6.87 (1H, s, N᎐CH), 6.80 (1H, m pyrrole
᎐
3-H), 6.71 (1H, br, aryl H), 6.68 (1H, (br), mesityl H), 6.50 (1H,
m, pyrrole 4-H), 5.18–4.85 (1H, m, allyl central H), 2.91 (1H, m,
allyl syn-H), 2.19 (3H, s, mesityl o-CH3), 2.11 (3H, s, mesityl
o-CH3), 2.03–1.99 (5H, s ϩ m, mesityl p-CH3 ϩ allyl syn ϩ
anti-H), 1.78 (1H, m, allyl anti-H); δC (C D ): 162.5 (N᎐CH),
᎐
6
6
149.1 (mesityl ipso-C), 141.8 (mesityl quat. p-C), 139.9 (allyl
CH), 134.2 (pyrrole quat. C), 130.3 (mesityl quat. o-C), 130.1
(mesityl quat. o-C), 129.0 (mesityl m-CH), 128.7 (mesityl
m-CH), 118.0 (pyrrole CH), 113.4 (pyrrole CH), 111.0 (pyrrole
CH), 58.8 (allyl CH2), 50.1 (allyl CH2), 20.8 (mesityl p-CH3),
18.7 (mesityl o-CH3), 18.6 (mesityl o-CH3).
Synthesis of Ni(ꢀ3-C3H5)(acetiminopyrrolyl), 6
NaH (96 mg, 60% dispersion in mineral oil, 2.4 mmol) was
placed in a Schlenk tube under nitrogen, washed twice with
D a l t o n T r a n s . , 2 0 0 3 , 4 4 3 1 – 4 4 3 6
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Table 2 Crystallographic data for compounds II and 3
hexanes and suspended in thf. The acetiminopyrrole II (318 mg,
1.4 mmol) was slowly added as a solid under a counterflow of
nitrogen, and immediate evolution of hydrogen occurred. The
suspension was stirred for 1 h then slowly added by filtration
into a thf solution of [Ni(η3-C3H5)(µ-Br)]2 (252 mg, 0.7 mmol)
at Ϫ70 ЊC. The mixture went yellow and cloudy during this time
and was allowed to warm to room temperature. After 90 min,
the mixture was evaporated under vacuum to yield a dark
yellow oil. The residue was extracted with hexanes until the
washings were colourless, and the solution was concentrated
and cooled to Ϫ80 ЊC to yield yellow crystals of the desired
product. A second crop can be obtained by concentrating and
cooling the mother-liquor. Combined yield: 91%. Anal. found
(calc. for C18H22N2Ni): C 66.45 (66.51); H 7.00 (6.82) N 8.48
(8.62)%. NMR: δH (C6D6): 7.12 (1H, m, pyrrole 5-H), 6.90 (1H,
m), pyrrole 3-H), 6.77 (1H, br, mesityl H), 6.75 (1H, br, mesityl
H), 6.62 (1H, m, pyrrole 4-H), 5.15–5.04 (1H, m, allyl central
H), 2.98 (1H, m, allyl syn-H), 2.22 (3H, s, mesityl o-CH3), 2.18
(3H, s, mesityl o-CH3), 2.10 (1H, m, allyl anti-H), 2.03 (3H,
s, imine CH3), 1.92 (1H, m, allyl syn-H), 1.82 (1H, m, allyl
II
3
Chemical formula
Mr
C15H18N2
226.32
C39H37N2NiP
623.42
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/c
Unit cell dimensions:
a/Å
b/Å
c/Å
7.1279(2)
15.0718(4)
12.4133(3)
92.6733(12)
1332.11(6)
4
11.7076(2)
13.0259(2)
21.3494(3)
98.1760(12)
3222.73(9)
4
β/Њ
V/Å3
Z
Dc/g cmϪ3
1.128
1.285
T/K
150(2)
0.067
150(2)
0.682
µ/mmϪ1
Reflection collected
Independent reflections
5816
3049
7381
4081
Rint
R
Rw
0.01
0.0530
0.0575
0.02
0.0339
0.0370
anti-H), 1.62 (3H, s, mesityl p-CH3); δC (C D ): 170.48 (N᎐
᎐
6
6
Criterion for observation: I > 3σ(I ).
CCH3), 146.83 (ipso-C), 138.70 mesityl quat. p-C) 138.51 (allyl
CH), 134.01 (pyrrole quat. C), 129.97 (mesityl quat. o-C),
129.84 (mesityl quat. o-C), 129.14 (mesityl m-CH), 128.85
(mesityl m-CH), 115.84 (pyrrole CH), 112.12 (pyrrole CH),
110.93 (pyrrole CH), 54.86 (allyl CH2), 49.64 (allyl CH2), 20.83
(mesityl p-CH3), 18.40 (mesityl o-CH3), 18.30 (mesityl o-CH3),
Data collection and processing. All data were collected at
150(2) K on a Nonius KappaCCD, with graphite-mono-
chromated Mo-Kα radiation (λ = 0.71073 Å), as summarised in
Table 2. The images were processed with the DENZO and
SCALEPACK programs.22
16.68 (N᎐CCH ).
᎐
3
Structure solution and refinement. The crystal structures were
solved by direct methods using the programs SIR92.23 The
structures of compounds II and 3 were refined using full-
matrix least squares on all F data using the CRYSTALS24 and
CAMERON25 software packages. All non-hydrogen atoms
were refined anisotropically and hydrogen atoms were included
in calculated positions with isotropic thermal parameters. In
the case of compound II, The NH hydrogen atom H1 was
located in a difference Fourier map and its coordinates and
isotropic thermal parameter subsequently refined. All other
hydrogen atoms were positioned geometrically, the orientations
of the methyl groups H11–H13, H131–H133, H141–H143 and
H151–H153 having been determined by examination of a
difference Fourier map. A Chebychev polynomial weighting
scheme26 with the parameters 1.190, 0.871 and 0.730 was
applied to the crystal structure of compound II giving a final R
factor of 0.0530 and Rw = 0.0575 with a maximum residual
electron density of 0.54 e ÅϪ3. A similar weighting scheme was
applied to the structure of 3 using the parameters 0.250, 0.0981
Oligomerisation tests
Oligomerisation tests with ethylene were carried out in flame
dried 250 ml crown capped glass pressure bottles, sealed with
neoprene septa, and pump filled with nitrogen atmosphere. To
these reaction bottles 50 ml of toluene (dried over Na/K alloy)
were added and the solvent saturated with ethylene at a constant
pressure of 2 bar (absolute pressure). This value was kept con-
stant during the polymerisation runs. A solution of Ni(COD)2
(9.4 µmol in 1 ml of toluene), used as a co-catalyst, was added via
a glass syringe. The solutions were thermostated to 20 or 50 ЊC
and allowed to equilibrate for 15 min. After this period of time, a
toluene solution of nickel catalyst 3 (9.4 µmol in 1 ml of toluene)
was added to the reaction mixture. The oligomerisation reactions
were terminated after 2 h by quenching the mixture with 150 ml
of a 2% HCl–methanol solution, and no polymer precipitated
from the reaction solution. Aliquots of these solutions were
analysed by gas–liquid chromatography (Chrompack GC PONA
capillary column (0.53 mm × 30 m); oven temperature program:
40 ЊC/15 min, 10 ЊC minϪ1 ramp, 200 ЊC/20 min) showing the
presence of peaks corresponding to a mixture of ethylene
oligomers, the total amount being estimated by integration. The
reaction mixture was extracted with distilled water (3 × 100 ml)
and the organic phase volatiles were removed under vacuum to
yield an oily liquid colourless product. This fraction was analysed
and 0.0658. This yielded a final R factor of 0.0339 and Rw
=
.
0.0370 with a maximum residual electron density of 0.31 e ÅϪ3
Empirical absorption corrections were applied.27
CCDC reference numbers 212400 and 212401.
See http://www.rsc.org/suppdata/dt/b3/b311323j/ for crystal-
lographic data in CIF or other electronic format.
1
by H NMR spectroscopy showing resonances corresponding to
olefinic, methine, methylene and methyl protons (δ 6.47, 5.37,
4.93, 2.39–1.53, 1.24 and 0.83). Blank tests were carried out
using either Ni(COD)2 or compound 3 alone, at the same
pressure of ethylene and temperatures used for their mixtures,
showing no catalytic activity in the oligo- or polymerisation of
ethylene.
Acknowledgements
We thank the Fundação para a Ciência e Tecnologia, Portugal,
for financial support (Project POCTI/QUI/42015/2001, co-
financed by FEDER) and for a fellowship FCT/BPD/3574/2000
(to R. M. B.), and the Balliol College, Oxford, UK, for a
Dervorguilla Scholarship (to S. I. P.).
Crystal structure determination
Experimental. Crystals of compound II were grown by
cooling a hexane solution to Ϫ20 ЊC. Crystals of compounds 3
were grown from a diethyl ether solution which was slowly
cooled to Ϫ20 ЊC. In all cases, the crystals were isolated by
filtration, and a specimen crystal selected under an inert atmos-
phere, covered with polyfluoroether, and mounted on the end of
a glass fibre. Crystal data are summarised in Table 2.
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