Synthesis and ethylene trimerisation capability of new chromium(II) and chromium(III) heteroscorpionate complexes

Article abstract of DOI:10.1039/b926333k

Reaction of (Me₂pz)₂CHSiMe₂N(H)R (R = ⁱPr or Ph) or (Me₂pz)₂CHSiMe ₂NMe₂ with CrCl₃(THF)₃ or CrCl ₂(THF)₂ gave Cr{(Me₂pz)₂CHSiMe ₂NR¹R²}Cl₃ (R¹ = H, R² = ⁱPr (10) or Ph (11); R¹ = R² = Me (15)) or Cr{(Me₂pz)₂CHSiMe₂NR ¹R²}Cl₂(THF) (R¹ = H, R² = ⁱPr (12) or Ph (13); R¹ = R² = Me (16)), respectively. Compounds 10 and 11 were crystallographically characterized and the magnetic behaviour of all the new compounds was evaluated using SQUID magnetometry. Reaction of CrCl₃(THF)₃ with Li{C(Me ₂pz)₃}(THF) gave the zwitterionic complex Cr{C(Me ₂pz)₃}Cl₂(THF) (17) containing an apical carbanion. Reaction of the analogous phenol-based ligand (Me₂pz) ₂CHArOH (ArO = 2-O-3,5-C₆H₂^tBu ₂) with CrCl₃(THF)₃ gave Cr{(Me ₂pz)₂CHArOH}Cl₃ (19) whereas the corresponding reaction with CrCl₂(THF)₂ unexpectedly gave the Cr(iii) phenolate derivative Cr{(Me₂pz)₂CHArO}Cl₂(THF) (20) which could also be prepared from CrCl₃(THF)₃ and the sodiated ligand [Na{(Me₂pz)₂CHArO}(THF)]₂. Reaction of the corresponding ether (Me₂pz)₂CHArOMe with CrCl₃(THF)₃ or CrCl₂(THF)₂ gave Cr{(Me₂pz)₂CHArOMe}Cl₃ (23) and Cr{(Me ₂pz)₂CHArOMe}Cl₂(THF) (24), respectively. The catalytic performance in ethylene oligomerisation/polymerisation of all of the new Cr(ii) and Cr(iii) complexes was evaluated. Most of the complexes showed high activity, but produced a Schultz-Flory distribution of α-olefins. Compound 23 had an exceptionally low α-value of 0.37 and showed a preference for 1-hexene and 1-octene formation. While replacing a secondary amine (10-13) for a tertiary amine (15-16) resulted in loss of catalytic activity, replacing a phenol (19) for an anisole (23) group afforded a more selective and more active catalyst. Changing from MAO to DIBAL-O as cocatalyst induced a switch in selectivity to ethylene polymerisation.

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis and ethylene trimerisation capability of new chromium(II) and
chromium(III) heteroscorpionate complexes†
Alexander F. R. Kilpatrick,a Shaneesh Vadake Kulangara,b Michael G. Cushion,a Robbert Duchateau*b and
Philip Mountford*a
Received 14th December 2009, Accepted 9th February 2010
First published as an Advance Article on the web 5th March 2010
DOI: 10.1039/b926333k
i
Reaction of (Me2pz)2CHSiMe2N(H)R (R = Pr or Ph) or (Me2pz)2CHSiMe2NMe2 with CrCl3(THF)3
i
or CrCl2(THF)2 gave Cr{(Me2pz)2CHSiMe2NR1R2}Cl3 (R1 = H, R2 = Pr (10) or Ph (11); R1 = R2 =
i
Me (15)) or Cr{(Me2pz)2CHSiMe2NR1R2}Cl2(THF) (R1 = H, R2 = Pr (12) or Ph (13); R1 = R2 = Me
(16)), respectively. Compounds 10 and 11 were crystallographically characterized and the magnetic
behaviour of all the new compounds was evaluated using SQUID magnetometry. Reaction of
CrCl3(THF)3 with Li{C(Me2pz)3}(THF) gave the zwitterionic complex Cr{C(Me2pz)3}Cl2(THF) (17)
containing an apical carbanion. Reaction of the analogous phenol-based ligand (Me2pz)2CHArOH
t
(ArO = 2-O-3,5-C6H2 Bu2) with CrCl3(THF)3 gave Cr{(Me2pz)2CHArOH}Cl3 (19) whereas the
corresponding reaction with CrCl2(THF)2 unexpectedly gave the Cr(III) phenolate derivative
Cr{(Me2pz)2CHArO}Cl2(THF) (20) which could also be prepared from CrCl3(THF)3 and the sodiated
ligand [Na{(Me2pz)2CHArO}(THF)]2. Reaction of the corresponding ether (Me2pz)2CHArOMe with
CrCl3(THF)3 or CrCl2(THF)2 gave Cr{(Me2pz)2CHArOMe}Cl3 (23) and Cr{(Me2pz)2CHArOMe}Cl2-
(THF) (24), respectively. The catalytic performance in ethylene oligomerisation/polymerisation of all of
the new Cr(II) and Cr(III) complexes was evaluated. Most of the complexes showed high activity, but
produced a Schultz-Flory distribution of a-olefins. Compound 23 had an exceptionally low a-value of
0.37 and showed a preference for 1-hexene and 1-octene formation. While replacing a secondary amine
(10-13) for a tertiary amine (15-16) resulted in loss of catalytic activity, replacing a phenol (19) for an
anisole (23) group afforded a more selective and more active catalyst. Changing from MAO to
DIBAL-O as cocatalyst induced a switch in selectivity to ethylene polymerisation.
Of all the systems known to trimerise ethylene to 1-hexene,
the vast majority are based on chromium. Chromium is an
Ethylene oligomerisation and polymerisation belong to the same
element that provides both potent ethylene polymerisation
Introduction
catalysts4 and ethylene oligomerisation catalysts (non-selective5
and selective2,6,7). This makes chromium the ideal metal to
study the effect of the ancillary ligand system on the cata-
lyst’s behaviour. The majority of the reported chromium-based
ethylene trimerisation catalysts consist of tridentate heteroatom-
containing ancillary ligands. The most well-known ligand systems
are the meridionally bonded diphosphinoamines, triphosphines
and dithioetheramines, together with the facially bonded triaza-
cycloalkanes and tris(dimethylpyrazolyl)methane (Fig. 1).6 Bulky
type of C–C bond formations albeit that the mechanisms are
completely different. While the migratory insertion mechanism
for the polymerisation of a-olefins is well established,1 the redox
process (oxidative addition, ring expansion, reductive elimina-
tion) for selective oligomerisation is still not fully understood.2
Although significant progress has been made with respect to
elucidating mechanistic details and the oxidation state of the active
metal species,3 there are still many questions to be answered. For
example, it is not clear how the ancillary ligand system determines
the selectivity of the catalyst. Furthermore, finding the right
ancillary ligand that results in a selective oligomerisation catalyst
(rather than a non-selective oligomerisation or polymerisation cat-
alyst) remains a broadly empirical process relying on exploratory
synthesis and screening.
5
cyclopentadienyls and pyrroles can bind to the metal in an h -facial
fashion, also occupying 3 metal orbitals.6a-6b,8 Whilst ligands like
diphosphinoamines, triphosphines or dithioetheramines generally
exhibit a meridional arrangement in the catalyst precursors,
these ligands could also bind facially to the metal. With this
in mind, we reasoned that a facial tridentate ancillary ligand
might be one of the requisites for a selective chromium ethylene
trimerisation catalyst.9 In this regard we note Braunstein and Hor’s
recent reports of potent chromium-based trimerisation catalysts
stabilised by fac-coordinated bis(pyrazolyl)methane based ligands
(see below).
aDepartment of Chemistry, Chemistry Research Laboratory, University
of Oxford, Mansfield Road, Oxford, OX1 3TA, UK. E-mail: philip.
mountford@chem.ox.ac.uk
bLaboratory of Polymer Chemistry, Department of Chemical Engineering
and Chemistry, Eindhoven University of Technology, Den Dolech 2, STO
1.37, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands. E-mail:
R.Duchateau@tue.nl
As alluded to above, the tris(dimethylpyrazolyl)methane com-
plex Cr{H(Me2pz)3}Cl3 (1, Fig. 2) shows very good selectiv-
ity and activity for ethylene trimerisation on activation with
† CCDC reference numbers 758105 and 758106. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b926333k
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Fig. 1 Mer- and fac-coordinating ancillary ligands for chromium-based ethylene trimerisation catalysts.
Fig. 2 Examples of homo- and hetero-scorpionate complexes for polymerisation and trimerisation catalysis.
MAO.6c Tris(pyrazolyl)methanes11-13 are examples of scorpionate
ligands, the most well-known examples of which are anionic
tris(pyrazolyl)hydroborates [HB(Rnpz)3]- (Rnpz = pyrazolyl or
substituted pyrazolyl ring).14 While homoscorpionate ligands
have the same three moieties appended to the apical group
(e.g., BH, CH), a second important class of poly(pyrazolyl)
ligands are the heteroscorpionates. Here one of the pyrazolyl
groups has been replaced by a different C-, O-, S- or N-donor
moiety.15 While much of the early work with heteroscorpionate
complexes was concerned with synthetic and structural studies,
several reports have focussed more on their applications in
the areas of ring-opening polymerisation of cyclic esters16-18
(e.g. Mg{(Me2pz)2CHArO}N(SiHMe2)2 (2)18) and Ziegler–Natta
polymerization catalysis19,20 (e.g. Ti{(Me2pz)2CHArO}Me3 (3,
Ar = C6H2 Bu2),19c Zr{(Me2pz)2CHSiMe2NiPr}(CH2SiMe3)3 (4)
and Sc{(Me2pz)2CHSiMe2NiPr}(CH2SiMe3)2 (5)20).
t
Very recently, preliminary results for Cr(III) ethylene trimerisa-
tion precatalysts 7-9 (Fig. 2) featuring heteroscorpionate ligands
were disclosed by Braunstein and Hor.10 The 1-hexene selectivities
for all three types of complex 7-9 were competitive with that of
the “parent” system 1, but the N2S donor derivatives (7) were
significantly less active than their N2O (8) and N2N¢ (9) analogues.
These results, alongside the track records of other N2O (e.g., in
2 and 3) and N2N¢ (e.g., in 4-6) donor ligands, prompted us to
report our own results in this area. As described below, we have
used several different types of heteroscorpionate complexes of this
type with either Cr(II) or Cr(III) centres, secondary or tertiary
amine side arms and phenyl ether, phenol or phenolate side arms.
3654 | Dalton Trans., 2010, 39, 3653–3664
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Scheme 1 Synthesis of new Cr(II) and Cr(III) N2N’-donor heteroscorpionate complexes.
Quantum Interference Device) magnetometry in the range 5-
300 K. A plot of 1/cm vs. temperature is shown in Fig. 3. The
observed linear relationship is characteristic of a paramagnetic
solid that obeys the Curie–Weiss Law (cm = C/(T - q)). The
Weiss temperature (q) of -4.6 K is typical of a material that orders
antiferromagnetically at sufficiently low temperatures.21
Results and discussion
Synthesis of N2N¢ and N3 donor scorpionate Cr(II) and Cr(III)
complexes
Reaction of CrCl3(THF)3 with (Me2pz)2CHSiMe2N(H)R (R =
iPr or Ph)20 in THF at room temperature gave a colour change
from purple to pale green or pink. After work up and crys-
tallisation from acetonitrile the Cr(III) complexes Cr{(Me2pz)2-
i
CHSiMe2N(H)R}Cl3 (R = Pr (10) or Ph (11)) were obtained
in ca 85% yield (Scheme 1). Attempted complexation of the
sterically more demanding analogue (Me2pz)2CHSiMe2N(H)◦Ar
i
(Ar = 2,6-C6H3 Pr2) was unsuccessful, even after heating at 70 C
for 24 h. Compounds 10 and 11, like all the complexes described
herein, are paramagnetic and meaningful NMR spectra could not
be obtained. The solid state structures (see below) confirm the
presence of neutral ligands bound in a fac-coordinated manner
to CrCl3 moieties. The IR spectra show n(N–H) absorbances (ca
3220 cm-1) red-shifted from those of the free ligands, and the
elemental analyses are consistent with the expected composition.
Under the conditions of their synthesis and manipulation, neither
10 nor 11 exhibit any tendency to eliminate HCl.
The room temperature, solid state magnetic moments of 3.90
and 3.99 mB for 10 and 11 agree well with that expected for an
S = /2 spin ground state (4A, expected spin-only moment =
3
Fig. 3 Magnetic behaviour of Cr{(Me2pz)2CHSiMe2N(H)iPr}Cl3 (10) as
3.87 mB). The solid state magnetic behaviour of 10 was further
investigated by variable temperature SQUID (Superconducting
determined by variable temperature SQUID magnetometry.
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Fig. 4 Displacement ellipsoid plots (20% probability) of Cr{(Me2pz)2CHSiMe2N(H)iPr}Cl3 (10, left) and Cr{(Me2pz)2CHSiMe2N(H)Ph}Cl3 (11, right).
C-bound H atoms, dichloromethane of crystallization and minor disorder (for 10) omitted for clarity.
Table 1 Selected distances (A) and angles (◦) for Cr{(Me2pz)2-
˚
estimated (4.89 mB) by the spin-only approximation for a high spin
CHSiMe2N(H)iPr}Cl3 (10) and Cr{(Me2pz)2CHSiMe2N(H)Ph}Cl3 (11).
The values in brackets for 10 are for the other orientation of N(5) which is
positionally disordered over two sites)
S = 2 (5E) group state.
For comparison with 10, the solid state magnetic behaviour of 12
was investigated by variable temperature SQUID magnetometry.
A plot of 1/cm vs. temperature is shown in Fig. 5. Above ca. 60 K
the linear plot is again characteristic of a paramagnetic solid that
obeys the Curie–Weiss Law, but at lower temperatures there is a
deviation from this behaviour which may be due to large zero-field
splittings typical of S = 2 ground states.
Parameter
10
11
Cr(1)-N(1)
Cr(1)-N(3)
Cr(1)-N(5)
Cr(1)-Cl(1)
Cr(1)-Cl(2)
Cr(1)-Cl(3)
N(1)-Cr(1)-N(3)
N(1)-Cr(1)-Cl(2)
N(3)-Cr(1)-Cl(1)
N(5)-Cr(1)-Cl(1)
N(5)-Cr(1)-Cl(2)
N(5)-Cr(1)-Cl(3)
Cl(1)-Cr(1)-Cl(2)
2.129(2)
2.125(3)
2.163(10) [2.182(11)]
2.3023(9)
2.3005(8)
2.3292(9)
86.72(9)
174.95(7)
175.88(8)
82.44(18) [93.6(2)]
91.6(2) [83.10(19)]
173.6(2) [172.47(19)]
92.97(3)
2.129(3)
2.141(2)
2.190(3)
2.2747(9)
2.3291(9)
2.3280(8)
86.54(9)
169.78(7)
178.49(8)
90.83(8)
83.33(9)
178.00(8)
91.06(3)
The solid state structures of 10 and 11 are shown in Fig. 4, and
selected bond distances and angles are compared in Table 1. In
general the distances and angles are within the expected ranges.22,23
3
Each complex contains a k N–bound (Me2pz)2CHSiMe2N(H)R
ligand and an approximately octahedral chromium. There is
some positional disorder in the Me2Si–N(H) linkage of 10
(see Experimental) but this was easily modelled. There are no
substantial differences between the principal metric parameters
listed in Table 1 for the two compounds. The Cr–N(1,3) distances
are slightly shorter than their Cr–N(5) counterparts, possibly
reflecting the difference in hybridisation of the two types of
nitrogen donor (sp2 vs sp3). Somewhat surprisingly, the Cr–Cl(3)
(trans to N(H)R) distance is slightly longer than the average Cr(1)-
Cl(1,2) distances (trans to the shorter Cr–Npz bonds).
Fig. 5 Magnetic behaviour of Cr{(Me2pz)2CHSiMe2N(H)iPr}Cl2(THF)
(12) as determined by variable temperature SQUID magnetometry.
The compounds 10-13 have secondary amine donors (N(H)R).
The N-bound H atom could be lost under reaction conditions
in the presence of aluminium alkyl species (cf. Hor’s report
for 910b,10c). To test for this possible effect, we therefore also
prepared the new ligand (Me2pz)2CHSiMe2NMe2 (14), contain-
ing a pendant NMe2 donor in place of the N(H)R group
in (Me2pz)2CHSiMe2N(H)R. Compound 14 was obtained in
48% yield by reaction of the previously reported20 chlorosilane
(Me2pz)2CHSiMe2Cl with LiNMe2 in THF. Examination of the
crude reaction mixture showed evidence for the formation of
Cr(II) analogues of 10 and 11 were obtained from the re-
action of in situ prepared CrCl2(THF)2 with the same ligands
◦
(Scheme 1) at 70 C in THF, forming the sky blue THF adducts
i
Cr{(Me2pz)2CHSiMe2N(H)R}Cl2(THF) (R = Pr (12) or Ph (13))
in ca. 60% yield. As for the Cr(III) analogues, the IR spectra showed
the expected n(N–H) stretches. The solid state magnetic moments
of 4.92 and 5.08 mB, respectively, are in good agreement with that
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(Me2pz)2CHLi during this process, thus (Me2pz)2CHLi elimina-
tion appears to compete with LiCl elimination in this reaction,
contributing to the relatively modest yield of 14.
heteroscorpionate-based phenolates in general are somewhat
underdeveloped, especially in relation to catalysis.
Reaction of (Me2pz)2CHArOH (18) with CrCl3(THF)3
Reaction of 14 with CrCl3(THF)3 and in situ gene-
rated CrCl2(THF)2 required slightly more forcing conditions
than for the secondary amine analogues (higher temperatures
or longer reaction times). The target complexes (Scheme 1)
Cr{(Me2pz)2CHSiMe2NMe2}Cl3 (15, 87% yield) and Cr-
{(Me2pz)2CHSiMe2NMe2}Cl2(THF) (16, 67% yield) were
nonetheless satisfactorily obtained in analytically pure form. The
solid state magnetic moments of 3.79 mB and 5.03 mB were close to
in THF for 12 h
at 60 ◦C gave the phenol complex
Cr{(Me2pz)2CHArOH}Cl3 (19) as a dark green solid in 73%
yield (Scheme 2). The IR spectrum of 19 showed a n(O–H) band
(absent in the otherwise identical methyl ether complexes 23 and
24 discussed below) and the solid state magnetic moment was
consistent with Cr(III). Reaction of 18 with CrCl2(THF)2 also
gave a dark green product. However, although the elemental
analysis was consistent with that anticipated for the target phenol
complex Cr{(Me2pz)2CHArOH}Cl2(THF), the expected n(O–
H) band was absent from the IR spectrum. Furthermore, both
the solid state (SQUID) and solution (Evans method27,28) room
temperature magnetic moments (3.72 mB and 3.66 mB, respectively)
3
those expected according to the spin-only formula for S = /2 and
S = 2 ground states, respectively.
We and others have recently found that free HC(Me2pz)3 is read-
ily deprotonated at the apical C–H bond by lithium, magnesium or
zinc alkyl reagents to form zwitterionic tris(pyrazolyl)methanide
3
were considerably closer to that expected for S = /2 (Cr(III),
-
complexes of the type [Mn+{ C(Me2pz)3}X] (n = 1, X = alkyl or
3.87 mB) than S = 2 (high spin Cr(II), 4.89 mB). Thus it appears
that reaction of 18 with CrCl2(THF)2 spontaneously forms the
Cr(III) phenolate complex Cr{(Me2pz)2CHArO}Cl2(THF) (20,
Scheme 2), perhaps by H atom loss from a first-formed phenol
intermediate Cr{(Me2pz)2CHArOH}Cl2(THF). By way of verifi-
cation, we also prepared 20 from CrCl3(THF)3 and the sodiated
ligand18 [Na{(Me2pz)2CHArO}(THF)]2 (21). The samples of 20
prepared by the two routes were indistinguishable.
-
THF; n = 2, X = C(Me2pz)3) possessing “naked” sp3 carbanions
and formally cationic metals.24 Deprotonation can also occur
once the neutral HC(Me2pz)3 is bound to a transition metal, and
this can also form zwitterionic complexes that are highly active
Ziegler–Natta polymerisation catalysts.24b,24d Under the trimeri-
sation reaction conditions reported for Cr{HC(Me2pz)3}Cl3
◦
(MAO, 80 C),6c there is a possibility that deprotonation of
the apical C–H bond occurs. Therefore, in addition to the
heteroscorpionate complexes in Scheme 1, we also prepared the
zwitterionic, homoscorpionate complex Cr{C(Me2pz)3}Cl2(THF)
(17) from Li{C(Me2pz)3}(THF)24a,24b and CrCl3(THF)3 (eqn (1)).
Compound 17 was obtained as a green microcrystalline solid in
40% crystallised yield. The EI mass spectrum shows a fragment
peak centred on m/z = 418 amu (M+ - THF) with the correct
isotope distribution. The solid state magnetic moment of 3.76 mB
In order to block the spontaneous oxidation process leading to
20, and to evaluate the influence of the ligand-bound O–H group
in 19 on oligomerisation catalysis, we prepared the new ligand
(Me2pz)2CHArOMe (22) from 21 using MeI in THF (Scheme 2).
Reaction of 22 with CrCl3(THF)3 or CrCl2(THF)2 in THF at 70 ◦C
for 12 h gave dark red Cr{(Me2pz)2CHArOMe}Cl3 (23) or dark
blue Cr{(Me2pz)2CHArOMe}Cl2(THF) (24) in very good yields.
The solid state magnetic moments of 3.56 mB and 4.83 mB were
close to those expected for Cr(III) and high spin Cr(II) systems,
respectively.
3
is consistent with a Cr(III), S = /2 species as depicted in eqn (1).
Ethylene oligomerisation studies
All of the new N2N’ and N2O donor heteroscorpionate
complexes, together with the zwitterionic homoscorpionate
Cr{C(Me2pz)3}Cl2(THF) (17), were evaluated for their ethy-
lene oligomerisation capabilities. We also tested the previously
reported1 complex Cr{HC(Me2pz)3}Cl3 (1) under our experimen-
tal conditions in order to make valid comparisons. Catalytic runs
were carried out in toluene solvent with MAO activation (Al : Cr =
500 : 1) under 35 bar ethylene pressure at 60 ◦C. The most relevant
oligomerisation results are listed in Table 2.
(1)
The first observation that can be made is that all new complexes
are more active than 1 but invariably produce a Schultz-Flory
distribution of oligomers. Whereas most complexes display a
typical a-value between 0.59 and 0.67, with an a-value of 0.37
compound 23 shows a significantly higher selectivity for 1-hexene
(52.4%) and 1-octene (34.7%). The GC analysis of the oligomers
obtained demonstrates that in all cases 98+% of the oligomers
are a-olefins. In general, an initial temperature rise of 4-7 ◦C
occurred upon injection of pre-catalyst solution, which remained
constant until the catalyst deactivated. Reaction profile assessment
of selected active catalysts (10-13) showed that catalytic activity
had ceased after 30 min, and that either increasing the reactor
temperature to 80 ◦C or reducing it to 30 ◦C decreased the yield of
Synthesis of N2O donor heteroscorpionate Cr(II) and Cr(III)
complexes
Encouraged by Braunstein and Hor’s preliminary reports10a
concerning the N2O-donor heteroscorpionate Cr(III) complexes 8
(Fig. 2), we also extended our collection of Cr(III) and Cr(II) com-
plexes to include derivatives of the bulky bis(pyrazolyl)methane-
phenol (Me2pz)2CHArOH (18).18,19c As mentioned, a number of
transition metal and main group metal complexes of 18 and its
homologues have been reported,16,18,19c,25 although none are yet
known for chromium. Compared to the situation for chelating
phenolate ligands in general (which attract widespread interest
in main group, transition metal and f element chemistry),26
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Scheme 2 Synthesis of new Cr(II) and Cr(III) N2O-donor heteroscorpionate complexes.
Table 2 Ethylene oligomerisation results for 10-13, 19, 20, 22, 23 and 1a
Oligomers (mol%)
Pre-catalyst
Yield/mL
C6
C8
C10
C12
C14
C16
C18
a
PE/g
Activity (golig mmol-1 h-1)
10
11
12
13
19
20
23
24
1
18.2
6.3
30.2
37.1
36.6
36.8
39.2
42.2
52.4
37.6
91.5
27.2
26.8
25.5
26.4
25.4
25.7
34.7
25.9
0.1
19.6
16.9
16.8
17.8
15.5
15.1
11.8
16.2
2.4
9.9
9.8
10.6
11.9
8.9
8.7
1.1
10.1
2.1
6.9
4.6
6.2
4.6
5.8
5.2
0.0
4.6
1.7
4.0
3.2
2.4
2.5
3.1
3.0
0.0
3.6
1.3
2.1
1.6
2.0
0.0
2.0
0.2
0.0
2.0
0.9
0.61
0.59
0.67
0.64
0.61
0.59
0.37
0.61
0.79
0.70
0.62
0.20
0.28
0.35
0.10
1.80
0.38
0.16
1320
770
60.7
48.5
17.4
17.0
37.0
23.9
8.9
3250
2650
1280
1300
2310
1580
690
a Conditions: 100 mL of toluene, 35 bar C2H4, Al(MAO):Cr = 500; 20 mmol pre-catalyst loading, 30 min, 60 ◦C.
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Table 3 Polymerisation results for selected precatalysts using DIBAL-O
oligomers while leaving the mass of undesirable polyethylene co-
product approximately constant. The polyethylene obtained in all
cases shows an extremely broad multimodal product distribution,
typical for multiple site chromium catalysts.
as cocatalysta
Pre-catalyst
Polyethylene/g
Activity/kg mol-1 h-1
When comparing the catalytic behaviour of the various catalysts
in more detail, several observations can be made. The divalent
complexes 12 and 13 are clearly more active than their trivalent
equivalents 10 and 11, but the selectivity of all four is rather similar.
This suggests that both Cr(II) and Cr(III) complexes (10/12 and
11/13, respectively) are precursors to the same active species.
Based on the lack of selectivity the active species most likely
contains chromium in the divalent state.3
10
13
15
19
23
0.43
16.22
0.55
3.40
5.31
86
3240
110
680
1060
a Conditions: 100 mL toluene, 35 bar C2H4, Al : Cr = 500 with 10 mmol
pre-catalyst loading, 30 min, 50 ◦C.
The effect of changing from a pendant secondary amine to
pendant tertiary amine has also been evaluated. Surprisingly,
both the trivalent 15 and divalent 16 lack any form of activity.
There has been some debate about the role and importance of
the NH-functionality in mer-SNS and fac-NNX ligand systems.
Although it was initially assumed that deprotonation of the NH-
functionality of the SNS ligand occurs upon activation with
MAO, this was found not to be the case. The deprotonated
version of the ligand actually afforded an inactive complex and
pyridine-based SNS ligands lacking an NH functionality afford
active trimerisation catalysts.3g-3i Hor and coworkers demonstrated
that treating a bis(pyrazolyl)(methyleneamine)methane chromium
complex containing a secondary amine with AlMe3 resulted in
deprotonation of the amine, which did not affect the catalytic
behaviour.10b Replacing the secondary amine by a tertiary amine
significantly decreased the selectivity for 1-hexene and favoured
the production of polyethylene of the corresponding catalysts but
did not reduce the catalytic activity.10c
apically-deprotoned Cr{C(Me2pz)3}Cl2(THF) (17) was therefore
compared with that of 1 under identical conditions. Surprisingly,
17 shows no catalytic activity which strongly suggests that deproto-
nation of the apical C–H bond of 1 under trimerisation conditions
is very unlikely (assuming that the coordinated THF in 17
would be effectively scavenged by the MAO under polymerisation
conditions - cf. the discussion of 19 and 20 above).
As earlier reports showed that the type of cocatalyst can have
a profound effect on the catalytic selectivity,29 we tested several
pre-catalysts (10, 13, 15, 19, 23) with AlMe3 (TMA), AliBu3
(TIBAL), DIBAL-O and MMAO-7. TMA and TIBAL did not
afford any active catalyst. In agreement with earlier reports, when
activated with DIBAL-O all complexes tested produced exclusively
polyethylene (Table 3). Interestingly, with MMAO-7 as cocatalyst
only precatalyst 10 (20 mol Cr, Al : Cr = 50, 35 bar, 30 min,
toluene) gave an active catalyst that produced 6.3 g of PE (activity:
630 kg mol-1 h-1). The other pre-catalysts only afforded marginal
amounts of polymer, while no oligomers were formed at all.
The virtually identical yield and product distribution for 19 and
20 suggests that they are both precursors of the same active species,
in which the phenol is obviously deprotonated. Furthermore,
the THF present in 20 does not inhibit activity, presumably
due to the scavenging effects of MAO. The GC analysis of the
product mixtures formed from 19 and 20 revealed the unexpected
formation of a statistical distribution of odd numbered a-olefins
and saturated even numbered oligomers as by-products of the
expected distribution of even numbered a-olefins.5d The amount of
these odd numbered oligomers (approx. 1.3 mL, 6 mmol) is of the
order of the amount of MAO in the reaction mixture, which might
suggest the mechanism by which they are formed is not catalytic.
It is possible that methyl transfer of some kind from MAO
(or the AlMe3 in MAO) is responsible for this phenomenon.5b
Replacing the phenolic hydrogen in 19 for a methyl substituent
(23), cf. changing the pendant NHR donor in 10-13 for NMe2
in 15 and 16, has an unexpected positive effect on the catalytic
activity and selectivity. Although somewhat more polyethylene is
formed, the a-value of 0.37 for complex 23 shows an increased
selectivity for 1-hexene and 1-octene. Interestingly, the divalent 24
displays a somewhat lower activity than 23 but moreover, the a-
value is significantly higher yielding a broader product distribution
compared to 23. This strongly suggests that 23 and 24 do not lead
to the same active species.
Conclusions
Reactions of a series of N2N’ and N2O donor heteroscorpionate
ligands with secondary and tertiary amine donors, as well as
phenol and phenyl ether donors, were successfully complexed
to Cr(II) and Cr(III). Their solid state structures and magnetic
properties have been determined. The tris(pyrazolyl)methanide
analogue 17 was also prepared. The ethylene oligomerisation
capability of the 11 new complexes was compared with that of the
“parent” system Cr{HC(Me2pz)3}Cl3 (1) under identical reaction
conditions. The results show that very small changes either to the
nature of the pendant donor atom (N, O) and/or the presence of an
ionisable hydrogen (N–H, O–H) can have a large effect on activity
and/or selectivity. Although in almost all cases the new complexes
gave an increase in productivity compared to 1, disappointingly,
only one of them (23) had any particular selectivity for 1-hexene
and 1-octene. Removal of the apical C–H proton from 1 (forming
17) gave a switch from a highly selective and productive catalyst
to an inactive one.
Experimental section
General methods and instrumentation
As mentioned above, free tris(dimethylpyrazolyl)methane is
readily deprotonated at the apical C–H bond by strong Brønsted
bases. It might therefore be possible that deprotonation of
the coordinated ligand in Cr{HC(Me2pz)3}Cl3 (1) occurs under
trimerisation reaction conditions. The catalytic behaviour of the
All manipulations were carried out using standard Schlenk
line or dry-box techniques under an atmosphere of argon or
dinitrogen. Solvents were degassed by sparging with dinitrogen
and dried by passing through a column of the appropriate drying
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 3653–3664 | 3659
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agent.30 Deuterated solvents were refluxed over the appropriate
drying agent, distilled and stored under dinitrogen in Teflon
valve ampoules. NMR samples were prepared under dinitrogen
in 5 mm Wilmad 507-PP tubes fitted with J. Young Teflon
Cr{(Me2pz)2CHSiMe2N(H)Ph}Cl3 (11)
To a purple solution of CrCl3(THF)3 (0.30 g, 0.81 mmol) in
THF (20 mL) was added a solution of (Me2pz)2CHSiMe2N(H)Ph
(0.30 g, 0.85 mmol) in THF (20 mL), giving a light pink solution
with a small amount of solid. The resulting mixture was stirred
for 4 h at RT. The solution was concentrated to half its volume
and hexanes (10 mL) were added to complete crystallisation.
The pale pink solid was collected by filtration, washed with
hexanes (3 ¥ 5 mL) and dried in vacuo. Recrystallisation from
a saturated acetonitrile (30 mL) solution at -30 ◦C afforded 11
as a microcrystalline pink solid. Yield: 0.33 g (84%). IR (NaCl
plates, Nujol mull, cm-1): 3221 (m, nN–H), 3036 (w), 1557 (m),
1493 (w), 1415 (w), 1394 (w), 1045 (m), 851 (s), 797 (m), 698 (m).
EI-MS: m/z = 439 (50%), [M - HCl - Cl]+. Anal. found (calcd.
for C19H27Cl3CrN5Si): C, 44.49 (44.58); H, 5.37 (5.32); N, 13.68
(13.67)%. meff = 3.99 mB.
valves. H and 13C-{ H} NMR spectra were recorded on Varian
Mercury-VX 300 and Varian Unity Plus 500 spectrometers and
referenced internally to residual protio-solvent (1H) or solvent
(13C) resonances, and are reported relative to tetramethylsilane
(d = 0 ppm). Assignments were confirmed as necessary with
the use of DEPT-135, DEPT-90, and two dimensional 1H-1H
and 13C-1H NMR correlation experiments. Chemical shifts are
quoted in d (ppm) and coupling constants in Hz. IR spectra
were recorded on a Nicolet Magna 560 E.S.P. FTIR spectrometer.
Samples were prepared in a dry-box as Nujol mulls between NaCl
plates, and the data are quoted in wavenumbers (cm-1). Mass
spectra were recorded by the mass spectrometry service of Oxford
University’s Department of Chemistry. Elemental analyses were
carried out by the Elemental Analysis Service at the London
Metropolitan University. SQUID Magnetometry measurements
were carried out on a Quantum Design MPMSXL at 1000 Oe.
Gas chromatography of oligomerisation products was conducted
on a Varian 450-GC equipped with an autosampler and a factor
four capillary column VF-5 ms 25 M¥0.◦25 MM. A gradient oven
1
1
Cr{(Me2pz)2CHSiMe2N(H)iPr}Cl2(THF) (12)
Anhydrous CrCl2 (0.43 g, 3.49 mmol) in THF (40 mL) was heated
at 70 ◦C for 12 h, allowed to cool to RT and a solution of
(Me2pz)2CHSiMe2N(H)iPr (1.09 g, 3.67 mmol) in THF (30 mL)
was added. A colour change from pale green to sky blue occurred
and a small amount of solid formed. The resulting mixture was
stirred for 4 h at 70 ◦C. The solution was concentrated to one
third of its volume and 10 mL of hexanes were added to complete
crystallisation. The solid was collected by filtration, washed with
cold hexanes (3 ¥ 5 mL) and dried in vacuo, yielding 12 as a sky
blue solid. Yield: 1.96 g (62%). IR (NaCl plates, Nujol mull, cm-1):
3225 (m, nN–H), 3084 (w), 3122 (w), 1556 (m), 1416 (m), 1316 (m),
1285 (w), 1239 (w), 953 (w), 876 (m), 855 (m), 788 (m), 752 (w), 724
(w). Anal. found (calcd. for C20H37Cl2CrN5OSi): C, 46.69 (46.62);
H, 7.25 (7.18); N, 13.61 (13.61)%. meff = 4.92 mB.
◦
temperature program, starting from 50 C (for 2 min) to 280 C
◦
at a rate of 10 C min-1 and holding at the final temperature
for 3 min was employed. The chromatograms were obtained via
flame ionization detector (FID). As a transport gas ethanol was
used.
Starting materials
The following compounds were prepared according to (or
by analogy with) literature procedures: CrCl3(THF)3,31
(R2pz)2CHSiMe2Cl (R = Me or tBu),20 (Me2pz)2CHSiMe2N(H)R
i
i
(R = Pr, Ph or 2,6-C6H3 Pr2),20 (Me2pz)2CHArOH,18,19c [Na-
Cr{(Me2pz)2CHSiMe2N(H)Ph}Cl2(THF) (13)
{(Me2pz)2CHArO}(THF)]2,18
HC(Me2pz)3,32
Cr{HC(Me2-
pz)3}Cl3,6c and Li{C(Me2pz)3}(THF).24a,24b LiNMe2 was prepared
from HNMe2 and nBuLi in hexanes.33 Other reagents were
obtained commercially and used as received.
Anhydrous CrCl2 (0.28 g, 2.28 mmol) in THF (40 mL) was
heated at 70 ◦C for 12 h, allowed to cool to RT and a solution
of HC(Me2pz)2CHSiMe2N(H)Ph (0.85 g, 2.39 mmol) in THF
(30 mL) was added. A colour change from pale green to sky
blue occurred and a small amount of solid formed. The resulting
mixture was stirred for 4 h at 70 ◦C. The solution was concentrated
to one third of its volume and 10 mL of hexanes were added
to complete crystallisation. The solid was collected by filtration,
washed with cold hexanes (3 ¥ 5 mL) and dried in vacuo, yielding
13 as a sky blue solid. Yield: 1.35 g (61%). IR (NaCl plates, Nujol
mull, cm-1): 3202 (m, nN–H), 3085 (w), 1600 (w), 1591.4 (w), 1554
(m), 1418 (m), 1316 (m), 1234 (m), 1203 (m), 1067 (w), 903 (w),
855 (m), 865 (s), 837 (m), 796 (s), 763 (m), 697 (m), 666 (m). Anal.
found (calcd. for C23H35Cl2CrN5OSi): C, 50.29 (50.36); H, 6.56
(6.43); N, 12.86 (12.77)%. meff = 5.08 mB.
Cr{(Me2pz)2CHSiMe2N(H)iPr}Cl3 (10)
To a purple solution of CrCl3(THF)3 (1.02 g, 2.71 mmol) in
THF (30 mL) was added a solution of (Me2pz)2CHSiMe2N(H)iPr
(0.91 g, 2.86 mmol) in THF (30 mL), giving a green solution with
a small amount of solid. The resulting mixture was stirred for 4 h
at RT. The volume was concentrated by half and hexanes (10 mL)
were added to complete crystallisation. The green solid was
collected by filtration, washed with hexanes (3 ¥ 5 mL) and dried
in vacuo. Recrystallisation from a saturated acetonitrile (50 mL)
solution at -30 ◦C afforded 10 as a pale green, microcrystalline
solid. Yield: 1.11 g (86%). IR (NaCl plates, Nujol mull, cm-1):
3222 (m, nN–H), 3134 (w), 1558(m), 1417(s), 1304 (w), 1236(m),
1199(m), 1168 (s), 1125 (w), 1048 (w), 967 (w), 918 (m), 889 (m),
860 (s), 845 (s), 829 (m), 791(s), 765 (w), 687 (w). EI-MS: m/z =
440 (5%), [M - HCl]+; 405 (20%), [M–HCl - Cl]+. Anal. found
(calcd. for C16H29Cl3CrN5Si): C, 40.26 (40.21); H, 6.05 (6.11); N,
14.57 (14.66)%. meff = 3.90 mB.
(Me2pz)2CHSiMe2NMe2 (14)
To a solution of (Me2pz)2CHSiMe2Cl (2.66 g, 8.56 mmol) in THF
(40 mL) was added dropwise a solution of lithium dimethylamide
(0.46 g, 8.56 mmol) in THF (20 mL). The resulting yellow solution
was stirred for 1 h. The volatiles were removed under reduced
pressure to give an off white solid which was washed with Et2O
3660 | Dalton Trans., 2010, 39, 3653–3664
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(3 ¥ 50 mL) and dried in vacuo. Extraction into hot benzene
(3 ¥ 50 mL, 70 ◦C) followed by removal of the volatiles
under reduced pressure gave a yellow oil as the crude product.
Recrystallisation from pentane (20 mL) at -30 ◦C afforded 14 as an
analytically pure white solid. Yield: 1.26 g (48%). 1H NMR (C6D6,
299.9 MHz, 293 K): 5.91 (1 H, s, (Me2pz)2CHSiMe2NMe2),
5.67 (2 H, s, N2C3HMe2), 2.46 (6 H, s, CHSiMe2NMe2),
2.21 (6 H, s, N2C3HMe2), 1.84 (6 H, s, N2C3HMe2), 0.48 (6
(40%). IR (NaCl plates, Nujol mull, cm-1): 3127(w), 1556 (m),
1210 (w), 871 (m), 667 (m). EI-MS: m/z = 418 (20%), [M - THF]+.
Anal. found (calcd. for C20H28Cl2CrN6O): C, 48.85 (48.89); H, 5.74
(5.96); N, 16.99 (17.10)%. meff = 3.76 mB.
Cr{(Me2pz)2CHArOH}Cl3 (19)
To a purple solution of CrCl3(THF)3 (0.82 g, 2.20 mmol) in THF
(30 mL) was added a solution of (Me2pz)2CHArOH (0.95 g,
2.32 mmol) in THF (20 mL). The mixture was stirred for 12 h
at 60 ◦C, giving a dark green solution. The volatiles were removed
under reduced pressure giving a dark green solid that was washed
with hexanes (3 ¥ 10 mL) and dried in vacuo. Extraction into
toluene (3 ¥ 10 mL) and recrystallisation at -30 ◦C afforded 19 as
a dark green microcrystalline solid. Yield: 0.91 g (73%). IR (NaCl
plates, Nujol mull, cm-1): 3367 (br, w), 3191 (w), 3120 (w), 3093 (w),
1596 (w), 1563 (m), 1415 (m), 1343 (s), 1297 (m), 1200 (m), 1161
(m), 1046 (m), 1016 (m), 918 (w), 854 (m), 683 (w). EI-MS: m/z =
530 (20%), [M - HCl]+. Anal. found (calcd. for C25H36Cl3CrN4O):
C, 52.96 (53.03); H, 6.42 (6.40); N, 9.88 (9.89)%. meff = 3.39 mB.
1
H, s, (Me2pz)2CHSiMe2NMe2) ppm. 13C-{ H} NMR (C6D6,
75.4 MHz, 293 K): 146.8 (5-N2C3HMe2), 140.1 (3-N2C3HMe2),
106.6 (4-N2C3HMe2), 69.0 ((Me2pz)2CHSiMe2NMe2), 38.7
(CHSiMe2NMe2), 14.1 (N2C3HMe2), 11.0 (N2C3HMe2), -1.5
((Me2pz)2CHSiMe2NMe2) ppm. IR (NaCl plates, Nujol
mull, cm-1): 1553 (m), 1419 (m), 1357 (s), 1319 (w), 1290 (m),
1276 (m), 1175 (m), 1027 (m), 1002 (w), 785 (m), 667 (w), 661
(m). EI-MS: m/z = 261 (15%), [M - NMe2]+; 210 (30%), [M -
Me2pz]+; 102 (60%), [M - HC(Me2pz)2]+. Anal. found (calcd. for
C15H27N5Si): C, 58.97 (58.88); H, 8.91 (8.86); N, 22.92 (22.96)%.
Cr{(Me2pz)2CHSiMe2NMe2}Cl3 (15)
To a purple solution of CrCl3(THF)3 (1.17 g, 3.14 mmol) in THF
(30 mL) was added a solution of (Me2pz)2CHSiMe2NMe2 (14)
(1.01 g, 3.31 mmol) in THF (20 mL), giving a pink solution with a
small amount of solid. The resulting mixture was stirred for 4 h at
70 ◦C, concentrated to half its volume and hexanes (10 mL) added
to complete crystallisation. The solid was collected by filtration,
washed with hexanes (3 ¥ 5 mL) and dried in vacuo, affording 7
as a pale pink solid. Yield: 1.27 g (87%). IR (NaCl plates, Nujol
mull, cm-1): 3087 (w), 1560 (m), 1415 (w), 1304 (w), 1045 (m), 895
(m), 847 (m). Anal. found (calcd. for C15H27Cl3CrN5Si): C, 38.90
(38.84); H, 5.86 (5.87); N, 15.01 (15.10)%. meff = 3.79 mB.
Cr{(Me2pz)2CHArO}Cl2(THF) (20)
Method 1—from CrCl2. Anhydrous CrCl2 (0.075 g,
0.62 mmol) in THF (30 mL) was heated at 70 ◦C for 12 h,
allowed to cool to RT and a solution of (Me2pz)2CHArOH (0.36 g,
0.63 mmol) in THF (10 mL) was added. A colour change from
pale green to dark green occurred and the mixture was stirred for
12 h at 70 ◦C. The volatiles were removed under reduced pressure
to give a green solid that was washed with hexanes (3 ¥ 10 mL) and
dried in vacuo. Extraction into THF (3 ¥ 10 mL), concentration,
and cooling to -30 ◦C afforded 11 as a dark green microcrystalline
solid. Yield: 0.30 g (81%).
Cr{(Me2pz)2CHSiMe2NMe2}Cl2(THF) (16)
Method 2—from CrCl3(THF)3. To a purple solution of
CrCl3(THF)3 (0.62 g, 1.7 mmol) in THF (30 mL) was added a
solution of [Na{(Me2pz)2CHArO}(THF)]2 (0.88 g, 1.8 mmol) in
Anhydrous CrCl2 (0.22 g, 1.78 mmol) in THF (40 mL) was heated
at 70 ◦C for 12 h, allowed to cool to RT and a solution of
(Me2pz)2CHSiMe2NMe2 (5) (0.57 g, 1.87 mmol) in THF (15 mL)
was added. A colour change from pale green to sky blue occurred
and a small amount◦of solid formed. The resulting mixture was
stirred for 4 h at 70 C, concentrated to one third of its volume
and 10 mL of hexanes added to complete crystallisation. The solid
was collected by filtration, washed with cold hexanes (3 ¥ 5 mL)
and dried in vacuo, affording 16 as a turquoise solid. Yield: 0.60 g
(67%). IR (NaCl plates, Nujol mull, cm-1): 3085 (w), 1551 (w),
1317 (w), 1238 (w), 901 (m), 848 (m), 760 (w), 700 (w). Anal.
found (calcd. for C19H35Cl2CrN5OSi): C, 45.49 (45.59); H, 6.93
(7.05); N, 13.86 (13.99)%. meff = 5.03 mB.
◦
THF (20 mL). The mixture was stirred for 12 h at 60 C, giving
a dark green solution. The volatiles were removed under reduced
pressure giving a dark green solid that was washed with hexanes
(3 ¥ 10 mL) and dried in vacuo. Extraction into toluene (3 ¥ 10 mL)
and cooling to -30 ◦C afforded 20 as a dark green microcrystalline
solid. Yield; 0.47 g (47%). IR (NaCl plates, Nujol mull, cm-1):
3091 (w), 1600 (w), 1562 (m), 1342 (s), 1299 (m), 1201 (w), 1159
(w), 1120 (w), 972 (w), 918 (w), 855 (w), 842 (m), 776 (s), 753 (w),
682 (s). EI-MS: m/z = 566 (20%), [M - Cl]+; 494 (100%), [M -
Cl - THF]+. Anal. found (calcd. for C29H44Cl2CrN4O2): C, 57.79
(57.71); H, 7.30 (7.35); N, 9.33 (9.28)%. meff = 3.72 mB (solid state);
3.66 mB (Evans method,27,28 toluene-d8).
Cr{C(Me2pz)3}Cl2(THF) (17)
(Me2pz)2CHArOMe (22)
To a purple solution of CrCl3(THF)3 (0.59 g, 1.57 mmol) in THF
(20 mL) was added a solution of [Li{C(Me2pz)3}(THF)] (0.62 g,
1.65 mmol) in THF (30 mL). The mixture was stirred for 12 h at
60 ◦C, giving an orange solution and a small amount of solid. The
volatiles were removed under reduced pressure and the residues
were extracted into hot benzene (4 ¥ 20 mL, 60 ◦C). Volatiles
were again removed under reduced pressure giving a green solid.
Crystallisation from the minimum volume of toluene at -30 ◦C
yielded 17 as a dark green microcrystalline solid. Yield: 0.27 g
To a suspension of [Na{(Me2pz)2CHArO}(THF)]2 (1.68 g,
3.34 mmol) in THF (40 mL) at -20 ◦C was added MeI (0.23 mL,
3.67 mmol), dropwise. The mixture was allowed to warm to RT
and was stirred for a further 6 h, giving a clear yellow solution. The
volatiles were removed under reduced pressure and the resultant
solid was extracted into pentane (3 ¥ 15 mL). Removal of the
volatiles under reduced pressure yielded 22 as a yellow, glassy solid
which was recrystallised from pentane at -30 ◦C to afford 22 as a
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Dalton Trans., 2010, 39, 3653–3664 | 3661
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Table 4 X-ray data collection and processing parameters for Cr{(Me2pz)2CHSiMe2-N(H)iPr}Cl3·0.75(CH2Cl2) (10·0.75(CH2Cl2)) and
Cr{(Me2pz)2CHSiMe2N(H)Ph}Cl3·CH2Cl2 (11·CH2Cl2)
Parameter
10·0.75(CH2Cl2)
11·CH2Cl2
empirical formula
fw
C16H29Cl3CrN5Si·0.75(CH2Cl2)
C18H27Cl3CrN5Si·CH2Cl2
541.58
596.83
150
T/K
wavelength/A
150
˚
0.71073
Monoclinic
P 21/c
10.13760(10)
14.1035(2)
18.5130(3)
90
98.8488(6)
90
2615.40(6)
11908
5950
0.024
4
1.375
0.956
0.71073
Triclinic
¯
P1
Crystal system
Space group
˚
a/A
8.5884(2)
11.9779(4)
13.6741(5)
75.1113(14)
76.7912(14)
76.1372(17)
1298.79(7)
10110
5903
0.026
2
1.526
1.020
R1 = 0.0450
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Reflections measured
Unique reflections
Rint
Z
r (calcd)/Mg m-1
abs coeff/mm-1
R indices [I > 3s(I)]a
Rw = 0.0497
R1 = 0.0517
Rw = 0.0448
√
a R1 = RꢀFo| - |Fcꢀ/R|Fo|; Rw
=
{Rw (|Fo| - |Fc|)2/Rw|Fo|2}.
yellow microcrystalline solid. Yield: 0.99 g (70%). 1H-NMR (C6D6,
299.9 MHz, 293 K): 7.97 (1 H, s, (Me2pz)2CHArOMe), 7.57 (1 H,
Cr{(Me2pz)2CHArOMe}Cl2(THF) (24)
Anhydrous CrCl2 (0.14 g, 1.14 mmol) in THF (15 mL) was heated
at 70 ◦C for 12 h, allowed to cool to RT and a solution of
(Me2pz)2CHArOMe (0.51 g, 1.21 mmol) in THF (10 mL) was
added. A colour change from pale blue to dark blue occurred
and the mixture was stirred for a further 12 h at 70 ◦C. The
volatiles were removed under reduced pressure to give a blue solid
that was washed with hexanes (3 ¥ 10 mL) and dried in vacuo.
Extraction into THF (3 ¥ 10 mL), concentration and cooling to
-30 ◦C afforded 24 as a blue, microcrystalline solid. Yield: 0.51 g
(72%). IR (NaCl plates, Nujol mull, cm-1): 1700 (w), 1653 (m), 1497
(s), 1458 (s), 1378 (w), 1249 (s), 1120 (w), 964 (w), 922 (w), 842
(m), 737 (w), 664 (s). Anal. found (calcd. for C30H47Cl2Cr1N4O2):
C, 57.96 (58.32); H, 7.38 (7.67); N, 9.31 (9.07)%. meff = 4.83 mB
(solid state).
d, 3J = 3 Hz, 4-C6H2 Bu2), 7.51 (1 H, d, 3J = 3 Hz, 6-C6H2 Bu2),
5.70 (2 H, s, N2C3HMe2), 3.42 (3 H, s, (Me2pz)2CHArOMe), 2.16
(6 H, s, 3 or 5 N2C3HMe2), 2.01 (6 H, s, 3 or 5 N2C3HMe2), 1.44 (9
t
t
H, s, 3-C6H2(CMe3)2), 1.25 (9 H, s, 5-C6H2(CMe3)2) ppm. 13C-{ H}
1
t
NMR (C6D6, 75.4 MHz, 293 K): 153.3 (2-C6H2 Bu2), 148.2 (3 or 5-
t
N2C3HMe2), 142.2 (5-C6H2 Bu2), 141.2 (3 or 5-N2C3HMe2), 140.0
t
t
t
(3-C6H2 Bu2), 125.3 (4-C6H2 Bu2), 125.1 (6-C6H2 Bu2), 124.7 (1-
C6H2(tBu)2), 106.9 (4-N2C3HMe2), 74.7 ((Me2pz)2CHArOH), 35.6
(3-C6H2(CMe3)2), 34.4 (5-C6H2(CMe3)2), 31.7 (5-C6H2(CMe3)2),
30.1 (3-C6H2(CMe3)2), 13.6 (N2C3HMe2), 11.2 (N2C3HMe2) ppm.
IR (NaCl plates, Nujol mull, cm-1): 3118 (w), 3086 (w), 1557 (m),
1334 (s), 1309 (m), 1293 (m), 1229 (m), 1116 (m), 1158 (m), 1028
(m), 869 (m), 834 (m), 781 (m), 631 (w), 607 (w), 682 (s). EI-MS:
m/z = 391 (80%), [M - OMe]+; 327 (100%), [M - Me2pz]+. Anal.
found (calcd. for C26H38N4O): C, 73.79 (73.89); H, 9.01 (9.06); N,
13.30 (13.26)%.
Crystal structure determinations of
Cr{(Me2pz)2CHSiMe2N(H)iPr}Cl3 (10·0.75(CH2Cl2)) and
Cr{(Me2pz)2CHSiMe2N(H)Ph}Cl3 (11·CH2Cl2)†
Cr{(Me2pz)2CHArOMe}Cl3 (23)
Crystal data collection and processing parameters are given in
Table 4. Crystals were mounted on glass fibers using perfluo-
ropolyether oil and cooled rapidly in a stream of cold N2 using
an Oxford Cryosystems Cryostream unit. Diffraction data were
measured using an Enraf-Nonius KappaCCD diffractometer.
As appropriate, absorption and decay corrections were applied
to the data and equivalent reflections merged.34 The structures
were solved by direct methods (SIR9235) and further refinements
and all other crystallographic calculations were performed using
the CRYSTALS program suite.36 For 10, the atoms of the
CHSiMe2N(H) moiety were positionally disordered over two sites.
The disorder was modelled adequately and refinement proceeded
To a purple solution of CrCl3(THF)3 (0.65 g, 1.70 mmol) in THF
(10 mL) was added a solution of (Me2pz)2CHArOMe (0.77 g,
1.80 mmol) in THF (10 mL). The mixture was stirred for 12 h at
70 ◦C, to form a dark red solution. The volatiles were removed
under reduced pressure to give a solid that was washed with
hexanes (3 ¥ 10 mL) and dried in vacuo. Extraction into toluene
(20 mL), concentration and recrystallisation at -30 ◦C afforded
23 as a red microcrystalline solid. Yield: 0.87 g (86%). IR (NaCl
plates, Nujol mull, cm-1): 3300 (w), 1571 (w), 1230 (w), 1202 (w),
869 (w), 667 (m). Anal. found (calcd. for C26H38Cl3CrN4O): C,
53.73 (53.75); H, 6.65 (6.59); N, 9.61 (9.64)%. meff = 3.56 mB.
3662 | Dalton Trans., 2010, 39, 3653–3664
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The Royal Society of Chemistry 2010
©

View Online
normally subject to similarity restraints applied to the positional
and displacement parameters of the affected fragment. Residual
electron density was modelled a 0.75 fractional occupancy CH2Cl2
molecule of crystallization. For 11, Residual electron density was
modelled a full occupancy CH2Cl2 molecule of crystallization. For
both 11 the N-bound H atom was located and positionally refined.
Other H atoms (and all H atoms for 10) were placed in calculated
positions (with reference to a Fourier map as far as possible) and
refined in a riding model. A full listing of atomic coordinates,
bond lengths and angles and displacement parameters for all the
structures have been deposited at the Cambridge Crystallographic
Data Centre.
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General procedure for ethylene oligomerisation experiments
Catalytic runs were carried out in a 200 mL high-pressure
Bu¨chi reactor containing a heating/cooling jacket. A preweighed
amount of catalyst was dissolved in 6 mL of toluene in the
glove box and pre-activated with 1 mL of co-catalyst solution to
increase solubility. The activated catalyst solution was sonicated
for 1 min and immediately injected into the preheated reactor
already charged with co-catalyst and toluene (total volume
100 mL) and saturated with ethylene. Solutions were heated using a
thermostatic bath and charged with 35 bar of ethylene, the stirring
rate was increased to 1000 rpm and the ethylene pressure was
maintained throughout the run. The oligo/polymerisations were
quenched by venting the reactor (after cooling to below 0 ◦C
for oligomerisation runs) and addition of EtOH and HCl. The
resulting polymer was isolated by filtration, sonicated with an
acidified ethanol solution, rinsed, and thoroughly dried prior to
mass determination and GPC analysis. Oligomers were analysed
using GC and 1H NMR spectroscopy.
Acknowledgements
7 (a) A. Bollmann, K. Blann, J. T. Dixon, F. M. Hess, E. Killian, H.
Maumela, D. S. McGuinness, D. H. Morgan, A. Nevelling, S. Otto,
M. J. Overett, A. M. Z. Slawin, P. Wasserscheid and S. Kuhlmann,
J. Am. Chem. Soc., 2004, 126, 14712; (b) K. Blann, A. Bollmann,
J. T. Dixon, F. M. Hess, E. Killian, H. Maumela, D. H. Morgan,
A. Neveling, S. Otto and M. J. Overett, Chem. Commun., 2005, 620;
(c) K. Blann, A. Bollmann, J. T. Dixon, F. M. Hess, E. Killian, H.
Maumela, D. H. Morgan, A. Neveling, S. Otto and M. J. Overett,
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Wang, K. J. Yi and Q. Huang, J. Mol. Catal. A: Chem., 2006, 259, 161;
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We thank the Royal Society and EPSRC for support.
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The Royal Society of Chemistry 2010
©