Probing the effects of ligand structure on activity and selectivity of Cr(iii) complexes for ethylene oligomerisation and polymerisation

Article abstract of DOI:10.1039/b716078j

A series of distorted octahedral Cr(iii) complexes containing tridentate S-, S/O- or N-donor ligands comprised of three distinct architectures: facultative {S(CH₂CH₂SC₁₀H₂₁) ₂ (L¹) and O(CH₂CH₂SC ₁₀H₂₁)₂ (L²)}, tripodal {MeC(CH ₂SⁿC₄H₉)₃ (L ³), MeC(CH₂SC₁₀H₂₁)₃ (L⁴)} and macrocyclic {(C₁₀H₂₁)[9]aneN ₃ (L⁵), (C₁₀H₂₁)₃[9] aneN₃ (L⁶), with [9]aneN₃ = 1,4,7-triazacyclononane} are reported and characterised spectroscopically. Activation of [CrCl₃(L)] with MMAO produces very active ethylene trimerisation, oligomerisation and polymerisation catalysts, with significant dependence of the product distribution upon the ligand type present. The properties of the parent [CrCl₃(L)] complexes are probed by cyclic voltammetry, UV-visible, EPR, EXAFS and XANES measurements, and the effects upon activation with Me₃Al investigated similarly. Treatment with excess Me₃Al leads to substitution of Cl ligands by Me groups, generation of an EPR silent Cr species (consistent with a change in the oxidation state of the Cr to either Cr(ii) or Cr(iv)) and substantial dissociation of the neutral S and S/O-donor ligands. The Royal Society of Chemistry.

Full text of DOI:10.1039/b716078j

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Probing the effects of ligand structure on activity and selectivity of Cr(III)
complexes for ethylene oligomerisation and polymerisation
Jerome O. Moulin,a John Evans,*a David S. McGuinness,c Gillian Reid,*a Adam J. Rucklidge,b
Robert P. Toozeb and Moniek Trompa
Received 17th October 2007, Accepted 30th November 2007
First published as an Advance Article on the web 3rd January 2008
DOI: 10.1039/b716078j
A series of distorted octahedral Cr(III) complexes containing tridentate S-, S/O- or N-donor ligands
comprised of three distinct architectures: facultative {S(CH2CH2SC10H21)2 (L1) and
O(CH2CH2SC10H21)2 (L2)}, tripodal {MeC(CH2SnC4H9)3 (L3), MeC(CH2SC10H21)3 (L4)} and
macrocyclic {(C10H21)[9]aneN3 (L5), (C10H21)3[9]aneN3 (L6), with [9]aneN3 = 1,4,7-triazacyclononane}
are reported and characterised spectroscopically. Activation of [CrCl3(L)] with MMAO produces very
active ethylene trimerisation, oligomerisation and polymerisation catalysts, with significant dependence
of the product distribution upon the ligand type present. The properties of the parent [CrCl3(L)]
complexes are probed by cyclic voltammetry, UV-visible, EPR, EXAFS and XANES measurements,
and the effects upon activation with Me3Al investigated similarly. Treatment with excess Me3Al leads to
substitution of Cl ligands by Me groups, generation of an EPR silent Cr species (consistent with a
change in the oxidation state of the Cr to either Cr(II) or Cr(IV)) and substantial dissociation of the
neutral S and S/O-donor ligands.
stabilities.14 The application of soft thioether-containing ligands in
homogeneous catalysis has recently been reported by McGuinness
Introduction
and co-workers11 using mixed donor ligands, HN(CH2CH2SR)2
(R = Me, Et, nBu, ndecyl) which, when reacted with [CrCl3(THF)3]
and activated with methylaluminoxane (MAO), give rise to highly
active and selective catalysts for the trimerisation of ethylene to
1-hexene. [CrCl3(L)] (L = HN(CH2CH2SC10H21)2) is the most
effective pre-catalyst. From a mechanistic point of view, it is
believed that a change in the oxidation state of the metal centre
occurs upon activation with MAO. Used as a cocatalyst, MAO
is normally thought to implicate a cationic active metal centre21
through the Cr(II)/Cr(IV) couple.22 Even if this route seems more
likely than the cycle involving neutral species (Cr(I)/Cr(III)),
evidence to support it is still very limited.
Here we describe the synthesis and investigation of a series
of Cr(III) complexes of the type [CrCl3(L)], where L is a neutral
tridentate ligand, for ethylene trimerisation, oligomerisation and
polymerisation reactions. This work focuses on the effect of the
ligand donor set and architecture on the activity and selectivity.
The ligands, L, were chosen to cover a wide area of catalytic
activity; from the geometry—open-chain, tripodal, macrocycle—
to the type of donor atoms, hard donor amine (N) and ether
(O) atoms and soft donor thioether (S) atoms. Using long
chain terminal alkyl groups improves the solubility of the Cr
complexes in organic solvents for the catalytic studies, and can
lead to increased activity.11 The series of Cr(III) complexes has
been studied using IR, UV-visible and EPR spectroscopy. Partial
structural data were obtained via Cr K-edge X-ray Absorption
Fine Structure (XAFS) measurements. The activated catalysts
were then employed for ethylene trimerisation, oligomerisations
or polymerisation using MMAO (modified methylaluminoxane)
as cocatalyst. The structures of the complexes derived from
[CrCl3(L)] (L = L1–L4) in toluene and activated with an excess of
the molecular alkylating agent Me3Al have also been investigated
In recent years there has been a substantial acceleration in research
activity concerned with homogeneous transition metal catalysts
for ethylene oligomerisation and polymerisation.1–3 Chromium
catalysts play an important role in both of these processes.4,5
The chromium-based heterogeneous Phillips catalyst for ethylene
polymerisation6 produces polyethylene with broad molecular
weight distributions (C6–C20 range) as a unique property. Presently,
a-olefins are produced by the Alfen7 or Shell Higher Olefin8
Processes. Cr-based catalysts also play a central role in the selective
ethylene trimerisation and tetramerisation reactions,9–14 and Cr(III)
complexes coordinated to a range of tridentate ligands bearing a
combination of S, N and P donor atoms have also shown great
potential in ethylene polymerization.15–17
In contrast to the wide range of Cr(III) complexes with hard N
and O donor ligands, far fewer examples of complexes with soft
donor ligands, such as thioethers, are known,18,19 and there are
few reported examples of Cr(III) thioether complexes as catalysts
for ethylene trimerisation, oligomerisation or polymerisation. On
the other hand, Cr(III) complexes coordinated to harder N-based
acyclic and cyclic ligands do show good activities for the combined
oligomerisation and polymerisation.4–6
The trimerisation of ethylene to 1-hexene is of wide interest
due to the importance of this co-monomer in the production of
linear low-density polyethylene (LLDPE).20 While some ethylene
trimerisation catalysts are based on Ti and Ta, Cr-based sys-
tems generally display higher activities, selectivities, and thermal
aSchool of Chemistry, University of Southampton, Southampton, UK SO17
1BJ. E-mail: J.Evans@soton.ac.uk, G.Reid@soton.ac.uk
bSASOL Technology UK, Purdie Building, University of St. Andrews, St.
Andrews, Fife, UK
cSchool of Chemistry, University of Tasmania, Private Bag 75, Hobart, 7001,
Australia
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4
4
4
using Cr K-edge EXAFS and X-ray Absorption Near-Edge Struc-
ture (XANES) measurements, UV-visible and EPR spectroscopy.
Me3Al was specifically employed in preference to MMAO for the
spectroscopic studies to permit accurate stoichiometric ratios to
be used, avoiding the difficulties associated with the ill-defined
MMAO reagent.
(F) ← A2g (t2); T1g (P) ← A2g (t3)27 (the latter is usually
obscured due to more intense LMCT transitions). Although the
complexes reported here have symmetry lower than Oh, the absence
of clear splitting of the d–d transitions means that analysis of
the electronic spectra based on approximate Oh symmetry is
reasonable. Two low energy d–d transitions are observed for each
of the Cr(III) complexes. The complexes [CrCl3(L1)], [CrCl3(L3)]
and [CrCl3(L4)] exhibit Dq ∼1 440 cm−1 (Table 1), similar to
those reported in the literature for related species,28–30 e.g. Dq =
1 433 cm−1 for [CrCl3{[9]aneS3}], 1 420 cm−1 for [CrCl3{[18]aneS6}]
and 1 470 cm−1 for [CrCl3{MeC(CH2SCH3)3}]. The geometry of
the ligand (open-chain vs. tripodal) does not affect the ligand-
field splitting. The aza-macrocycle complexes [CrCl3(L5)] and
[CrCl3(L6)] give a higher value with Dq ∼ 1 680 cm−1, relatively
low compared to species such as [Cr{H2NCH2CH2NH2}3]3+ (Dq =
2 180 cm−1), [Cr{[9]aneN3}2]3+ (2 270 cm−1) and [Cr{[18]aneN6}]3+
(2 146 cm−1),31 reflecting the presence of N6 donor sets in these
species compared to N3Cl3 donor sets in the new complexes.
We also probed the redox properties of [CrCl3(L)] (L = L1–
L6) by cyclic voltammetry. Electrochemical data were recorded at
room temperature under an N2 atmosphere and with a scan rate
of 100 mV s−1 over a range of 1.5 to −1.5 V. A 0.2 M solution of
Results and discussion
The ligands are comprised of three distinct geometries: fac-
ultative {S(CH2CH2SC10H21)2 (L1), O(CH2CH2SC10H21)2 (L2)},
tripodal (MeC(CH2SnC4H9)3 (L3) MeC(CH2SC10H21)3 (L4)) and
macrocyclic {(C10H21)[9]aneN3 (L5), (C10H21)3[9]aneN3 (L6)} and
their preparations and spectroscopic properties are described in
the Experimental section. The methods used were based upon
modifications of literature preparations for similar species.23–26
The distorted octahedral complexes [CrCl3(L)] (L = L1–L6) were
prepared by reaction of [CrCl3(THF)3] with one mol. equiv. of L
in anhydrous CH2Cl2 and isolated in good yields as purple or
green coloured solids (L = L1–L5) or oily compounds (L = L6) by
removal of the solvent in vacuo and washing with hexane. These
neutral compounds are readily soluble in both chlorinated solvents
and organic solvents such as toluene. The complexes [CrCl3(L)] (L
= L3–L6) are also relatively sensitive to moisture, hence were stored
and handled in a dry N2-purged glove-box.
[Bun N]BF4 in anhydrous CH2Cl2 was employed as base electrolyte
with a double platinum working and counter electrode and a
standard calomel reference electrode (Table 2).
The Cr(III) thioether complexes show irreversible reductions
(Table 2), possibly due to a structural rearrangement which is slow
in comparison to the electron transfer process, and/or irreversible
ligand loss upon reduction. No reductive activity was observed for
the N-donor ligand complexes.
4
The IR spectroscopic data for the complexes [CrCl3(L)] (L = L1–
L6) (Table 1) provide evidence for coordinated L. Peaks observed
in the region 200–400 cm−1 may be tentatively assigned to t(Cr–
Cl), hence give some structural information on the complexes (mer
vs. fac isomers). The steric constraints of the tripodal ligands L3
and L4 are such that monomeric complexes must be fac isomers.
This is borne out by the two bands observed in the 200–400 cm−1
region assigned to the m(Cr–Cl) stretching modes. Similarly the
macrocyclic geometry of the tridentate ligands L5 and L6 is
such that they coordinate facially in octahedral complexes. The
IR spectrum for [CrCl3(L5)] also exhibits a peak at 3189 cm−1,
characteristic of the N–H stretching mode. The IR spectrum of
[CrCl3(L1)] shows a broad peak at 342 cm−1 and earlier work
on crown thioether chemistry and the implications of the ligand
design suggest the complex adopts a facial geometry. On the other
hand, spectroscopic studies indicate the mer isomer is formed for
[CrCl3(L2)], which reveals three m(Cr–Cl) as expected (C2v or C8).
The octahedral Cr(III) d3 system is expected to exhibit three
quartet excited states and hence show three spin-allowed d–d
Table 2 Electrochemical data and Dq values for [CrCl3(L)] (L = L1–L6)
in 0.2 M [Bun4N]BF4 in CH2Cl2, scan rate = 100 mV s−1, EPC = reduction
potential
Complex
E
PC/Va
Dq/cm−1
[CrCl3(L1)]
[CrCl3(L2)]
[CrCl3(L3)]
[CrCl3(L4)]
[CrCl3(L5)]
[CrCl3(L6)]
−1.28
−1.25
−1.35
−1.27
1 430
1 396
1 440
1 439
1 684
1 680
b
—
—
b
a Voltages are quoted vs. [Fe(g -C5H5)2]/[Fe(g -C5H5)2]+. b No reduction
observed.
5
5
4
4
transitions in the UV-visible spectrum: T2g
←
4A2g (t1); T1g
Table 1 Selected electronic and IR spectroscopica data for [CrCl3(L)] (L = L1–L6)
e1,
Complex
t1/cm−1
t2/cm−1
Dq/cm−1
e2/dm3 mol−1 cm−1
m(Cr–Cl)/cm−1
[CrCl3(L1)]
[CrCl3(L2)]
[CrCl3(L3)]
[CrCl3(L4)]
[CrCl3(L5)]
[CrCl3(L6)]
14 300
13 960
14 400
14 390
16 840
16 800
19 330
21 050
20 320
20 280
20 620
21 790
1 430
1 396
1 440
1 439
1 684
1 680
200, 280
250, 400
185, 220
—
—
104, 179
342 (br)
389, 364, 347
357, 334
360, 333
327, 307
315, 308
a UV-visible spectra recorded in toluene solution. IR spectra recorded as Nujol mulls.
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XAFS spectra of parent Cr(III) complexes
tridentate ligand containing donor atoms of the same type (in our
case, L = S or N; M = Cr; X = Cl), there are three environments:
X–M–X, L–M–L and X–M–L, and the multiple scattering effects
in the EXAFS measurements are negligible as they tend to ‘cancel’
one another. However, in fac-MX3L3 there is only one type of
X–M–L multiple scattering unit and in this case the multiple
We have carried out Cr K-edge XAFS measurements on the parent
Cr(III) complexes to probe d(Cr–S), d(Cr–N), d(Cr–O) and d(Cr–
Cl) in the first coordination sphere and to allow comparisons with
the EXAFS data on the activated complexes (below).
Spectra were recorded from solid pellets of the complex diluted
with BN and also for 5.0 mM solutions in toluene. Cr K-edge
XAFS measurements on the solution formed after the in situ
reaction between the complex and Me3Al were also undertaken.
The EXAFS data for [CrCl3(L1)] (Fig. 1) show a good fit
for 2.7 Cl and 2.8 S atoms, with the bond lengths consistent
scattering provides a large contribution to the EXAFS (L1, L3–6
)
(Fig. 1). The partial structural data for the complexes [CrCl3(L)]
(L = L1–L6) obtained via Cr K-edge EXAFS are summarised in
Table 3.
with crystal structures involving other Cr(III) thioether complexes,
Catalysis
1
˚
˚
˚
e.g. [CrCl3(L )] d(Cr–Cl) = 2.32(6) A, d(Cr–S) = 2.45(5) A,
The complexes [CrCl3(L)] (L = L1–L6) (10 lmol for L1, L5
and L6 in 100 mL toluene and 30 lmol for L2, L3 and L4 in
80 mL of methylcyclohexane) were studied for their catalytic
activities towards ethylene oligomerisation and polymerisation
using MMAO as activator. The results at 40 bar of ethylene and
with a MMAO/Cr ratio of 300 : 1 are summarised in Table 4.
fac-[CrCl3{[18]aneS6}] d(Cr–S) = 2.459(3), 2.442(5), 2.440(5) A,
28
˚
d(Cr–Cl) = 2.305(5), 2.279(5), 2.291(5) A. The Debye–Waller
terms are relatively low, which suggests that both shells are integral
to the fit. The model proposed gives the best results when multiple
scattering is included, supporting the facial configuration deduced
from the IR spectra (vide supra). For mer-MX3L3, where L is a
Fig. 1 The Cr K-edge k3-weighted EXAFS, Fourier transform and sine transform for a solid pellet of [CrCl3(L1)]/BN.
Table 3 Cr K-edge k3-weighted EXAFS data for [CrCl3(L)] (L = L1–L6) measured as powders diluted with BN
2
2c
2
2
C.N.a,b Cr–Cl
d(Cr–Cl)/A
2r /A
C.N. Cr–Ed
d(Cr–E)/A
Re (%)
Eff/eV
˚
˚
˚
˚
2r /A
Complex
[CrCl3{L1}]
2.7(4)
2.7(4)
2.32(6)
2.25(3)
0.011(1)
0.015(5)
S: 2.8(5)
S: 2.0(2)
O: 0.8(2)
S: 2.5(4)
S: 2.5(2)
N: 2.5(5)
N: 3.0(1)
S: 2.45(5)
S: 2.37(7)
O: 2.10(3)
S: 2.43(3)
S: 2.38(4)
N: 2.01(8)
N: 2.00(6)
0.010(2)
S: 0.008(0)
O: 0.007(7)
0.010(2)
0.001(7)
0.007(6)
0.004(9)
23.2
27.0
−6.3(3)
−2.8(8)
[CrCl3{L2}]
[CrCl3{L3}]
[CrCl3{L4}]
[CrCl3{L5}]
[CrCl3{L6}]
2.5(3)
2.5(5)
2.7(2)
2.5(4)
2.26(2)
2.25(3)
2.33(9)
2.32(6)
0.011(8)
0.003(8)
0.007(6)
0.011(8)
25.2
25.9
26.0
25.2
−5.7(9)
−1.1(7)
−3.4(8)
4.8(1)
ꢀ
ꢀ
a C.N. = coordination number. b Standard deviations in parentheses. c Debye–Waller factor. d E = S, O or N. e Defined as [ (vT − vE)k3dk/ (vEk3dk)] ×
100 (where T is theoretical and E is experimental). f Ef, the difference calculated between the calculated Fermi level energy and the theoretically known
values for the element.
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Table 4 Summary of the results of ethylene trimerisation, oligomerisation and polymerisation using [CrCl3(L)] (L = L1–L6) activated with MMAO
a
Complex
T/◦C
C6 (wt%)
71.8
1-C6 (wt%)
99.4
C8–C22 (wt%)
27.1
Productivityc/g g(Cr)−1
PE/G
PE (wt%)
[CrCl3(L1)]
[CrCl3(L2)]
[CrCl3(L3)]
[CrCl3(L4)]
[CrCl3(L5)]
[CrCl3(L6)]
80
80
80
80
90
90
3165
2295
31089
8827
5804
3168
0.05
3.58
48.5
13.77
0.76
0.53
1.0
—
—
—
8.4
10.6
b
b
b
—
—
—
—
—
—
—
—
—
b
b
b
b
b
b
15.7
14.9
92.8
89.6
63.8
63.2
a Selectivity for 1-hexene as a percentage of total C6 fraction. b Any liquid phase was absorbed by the polymer and not analyzed further. c Note that the
productivity reported is not necessarily a true reflection of the stability of the catalyst as they were stopped after 30 minutes, calculation of the productivity
based on a wt/wt basis (polymer/chromium).
Table 5 Selected UV-visible spectroscopic data for a 5 mM toluene
solution of [CrCl3(L)] (L = L1–L6) before and after treatment with 5 mol.
equivs. of Me3Al (m1 corresponds to the lowest energy transition in each
case)
The [CrCl3(L)] complexes exhibit moderate to high activities
for ethylene oligomerisation and polymerisation. The products
obtained using [CrCl3(L5)] and [CrCl3(L6)] are comprised of
oligomers with a distribution that closely resembles Schulz–Flory
rules.32–34 The weight of C8–C22 fractions was used to calculate
the Schulz–Flory distribution coefficient b by fitting the following
simplified Schulz–Flory equation:
Complex
t1a/cm−1
t1b/cm−1
[CrCl3(L1)]
[CrCl3(L2)]
[CrCl3(L3)]
[CrCl3(L4)]
[CrCl3(L5)]
[CrCl3(L6)]
14 300
13 960
14 400
14 390
16 840
16 800
16 015
16 795
16 990
16 865
17 450
17 375
vq = b/(1 + b)q where vq = mole fraction of C2q+2 olefin.
Calculation of the b factor for [CrCl3(L5)] and [CrCl3(L6)] gave
values of 0.68 and 0.70 respectively, which are comparable with
literature values for related compounds.35
a Before treatment. b After treatment with 5 mol. equivs. of Me3Al.
Complexes containing N–H groups generally show relatively
high catalytic activity compared with other analogues. However,
this does not apply for [CrCl3(L5)], leading to the suggestion
that the stereochemistry of the complex might also significantly
influence the catalytic activity and selectivity.
Surprisingly, the thioether complex [CrCl3(L1)] gave an active
and selective catalyst for the trimerisation of ethylene to 1-hexene,
although the catalyst was active only for a few minutes (probably
due to the loss of the thioether ligand, vide infra). This is the
first example of a fac-CrCl3 complex bearing a simple trithioether
ligand showing such activity. The catalyst based upon [CrCl3(L2)],
containing the mixed S/O-donor ligand gave no evidence for
C6 formation. Complexes [CrCl3(L3)] and [CrCl3(L4)] showed
significant polymerisation activity, with the butyl derivative giving
higher productivity under the same catalytic conditions.
Fig. 2 UV-visible spectra of a 5 mM toluene solution of [CrCl3(L3)] before
(red) and after (black) treatment with five mol. equivs. of Me3Al.
To investigate the first stage of activation of these complexes, we
have followed the reaction with Me3Al by UV-visible spectroscopy.
To a freshly prepared 5 mM toluene solution of [CrCl3(L)] was
added five mol. equivs. of Me3Al and UV-visible spectra were
recorded after ∼one minute. Excess Me3Al was used to allow for
possible multiple substitution of the Cl atoms by Me groups. Fig. 2
shows typical UV-visible spectra for a complex in solution under
these conditions.
Addition of Me3Al causes an immediate change in the colour
of the solution from purple to yellow/green. The UV-visible
spectrum shows a change from two bands to one, the latter at
different energy (Table 5). Either a blue shift is occurring as the
chlorine ligands are substituted by methyl groups in Cr(III) species
and the peak corresponding to m2 is obscured by the charge transfer
bands, and/or substitution of the chlorine atoms by methyl groups
is accompanied by a change in oxidation state at the chromium.
The latter would lend support to a Cr(II)/(IV) redox cycle.
EPR studies
The reactions between [CrCl3(L)] (L = L1–L6) and Me3Al have also
been studied by EPR spectroscopy. Q- and K-band EPR spectra
were recorded for the complexes in a frozen toluene–CH2Cl2 glass
at 115 K. A broad isotropic signal characteristic for paramagnetic
d3 systems was observed for all the chromium(III) complexes, with
g values of ∼1.99, as expected for Cr(III).
Addition of three mol. equivs. of Me3Al to a CH2Cl2–toluene
solution of the parent Cr(III) complex and quenching to 77 K
leads to significant diminution of the strong isotropic d3 signal,
giving an EPR silent species in each case (Fig. 3), consistent with a
change in the oxidation state from Cr(III) to either Cr(II) or Cr(IV).
Addition of excess (30 mol. equivs.) Me3Al led to complete loss of
the EPR resonance in all cases.
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These results are consistent with substitution of all of the Cl atoms
by Me groups upon addition of Me3Al. It is also clear that the
neutral S- and S/O-donor ligands substantially dissociate after
addition of Me3Al, leaving on average one S atom coordinated to
Cr in ∼one third of the molecules.
We have also used XANES spectroscopy to investigate the
absorption edge features for [CrCl3(L)] (L = L1–L4) before and
after addition of excess (30 equiv.) Me3Al (Fig. 5 displays the
XANES for [CrCl3(L1)] and [CrCl3(L3)]). It is clear from the
spectra that significant changes in edge position and shape occur.
Moreover, the energy shift of the absorption edge upon addition of
AlMe3 is negative for compounds containing L1 and L2, whereas
the energy shift is positive for compounds containing L3 and L4. In
our previous work we have demonstrated that significant changes
within the XANES region can result from changes in oxidation
state, coordination number, ligand donor type and geometry.36
Hence, it is not possible at this stage to make definitive assignments
on the basis of these spectra alone. Further experimental work and
theoretical modelling are required to determine how the individual
factors influence the XANES in these types of compounds.
Fig. 3 EPR spectra (Q-band) of [CrCl3(L6)] in toluene–CH2Cl2 before
(blue) and after (red) treatment with three mol. equivs. of Me3Al (115 K),
g = 1.98.
XAFS studies on Cr complexes with added Me3Al
The Cr K-edge EXAFS data for [CrCl3(L)] (L = L1–L4) after
treatment with 30 mol. equivs. of Me3Al (excess) in toluene are
presented in Fig. 4 and Table 6. The analysis for the activated
systems shows a first coordination shell of on average three C
Conclusions
˚
atoms at ∼2.1 A. A second shell was modelled satisfactorily by
A range of new Cr(III) complexes with various neutral tridentate
N-, S- and O-donor ligands has been investigated using IR,
˚
˚
∼0.3 S atoms, giving d(Cr–C) = 2.07 A and d(Cr–S) = 2.44 A.
Table 6 Cr K-edge k3-weighted EXAFS structural data for 5 mM toluene solutions of [CrCl3(L)] (L = L1–L4) treated with an excess of Me3Al
2
2
2
2
˚
˚
˚
˚
Complex
C.N. C
d(Cr–C)/A
2r /A
C.N. S
d(Cr–S)/A
2r /A
R (%)
FI
Ef/eV
[CrCl3(L1)]
[CrCl3(L2)]
[CrCl3(L3)]
[CrCl3(L4)]
3.0(2)
2.6(4)
2.8(5)
2.9(7)
2.07(7)
2.11(6)
2.09(3)
2.06(4)
0.011(4)
0.015(1)
0.011(9)
0.013(1)
0.3(3)
0.5(1)
0.3(7)
0.2(3)
2.44(5)
2.47(2)
2.44(3)
2.40(7)
0.007(1)
0.009(4)
0.003(4)
0.006(1)
37.6
39.8
36.2
36.7
11.0
10.4
11.2
12.4
−3.1(8)
−2.0(9)
−8.2(1)
−4.6(5)
Fig. 4 The Cr K-edge k3-weighted EXAFS, Fourier transform and sine transform for a 5 mM toluene solution of [CrCl3(L1)] treated with an excess
(30 mol. equivs.) of Me3Al at room temperature.
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Fig. 5 Normalized X-ray absorption data for [CrCl3(L)]; 5 mM in toluene solution (solid black line) and with excess Me3Al (30 mol. equivs.) (dashed
line). (a) L = L1; (b) L = L3.
1
and 13C{ H} NMR spectra were recorded on a Bruker AV300
UV-visible and EPR spectroscopy along with cyclic voltammetry
and XAFS studies. Treatment of these compounds with MMAO
led to catalytically active species; the catalytic activity towards
ethylene trimerisation, oligomerisation and polymerisation shows
significant dependence upon the ligand donor set. Of the trithio-
ether ligand complexes only [CrCl3(L1)] with the facultative S–S–
S donor set is active towards selective trimerisation of ethylene.
Substitution of the central S-donor by oxygen suppresses this
activity whilst allowing some polymerisation. The activated solu-
tions of the complexes containing the tripod S3 donor ligands (L3
and L4) produced polymer with MMAO co-catalyst. In contrast,
the complexes containing the hard N3-donor ligands (L = L5
or L6) produced a Schulz–Flory distribution of oligomers and each
exhibited an electronic transition at ∼17 400 cm−1 after treatment
with Me3Al (cf. ∼16 600 cm−1 for the S-based ligand complexes),
suggesting a subtly different ligand environment is present in these
systems with retention of some N-donor ligand coordination.
Further experiments to investigate this are underway.
The UV-visible and EPR measurements suggest that a change
in the oxidation state of the Cr centre occurs in all cases upon
addition of excess Me3Al, plausibly to EPR silent Cr(II). Investiga-
tion of the partial structure of the active species has been achieved
using XAFS spectroscopy. Treatment of the Cr(III) complexes with
excess Me3Al leads to substitution of the Cl ligands by Me groups
and substantial dissociation of the ligands L1–L4 from the metal
centre. However, the nature of structural changes responsible for
the difference in catalytic activity between L1 and L2–L4 remains
unclear at present. One limitation of the EXAFS experiments
described here is the relatively long time-scale for data acquisition
(presently ∼1–3 h) due to the need to use dilute solutions and the
low energy of the Cr edge being investigated. Experiments to eluci-
date these structural differences and to trap the early intermediates
in the catalysis are being developed in our laboratories.
spectrometer and were referenced internally from the residual
protio solvent (1H) or the signals of the solvent (13C). Mass
spectra were on electrospray mode (positive or negative) using
a VG Biotech Platform. Microanalyses were obtained from the
University of Strathclyde microanalytical service and from the
London Metropolitan University elemental analysis service. Infra-
red spectra were recorded as Nujol mull between CsI plates,
using a Perkin Elmer 983 IR spectrometer over the range 4000–
180 cm−1. UV-visible spectra were recorded on a Perkin Elmer
Lambda 19 spectrometer in toluene or CH2Cl2 solutions or by
diffuse reflectance. EPR spectra were obtained from the EPSRC
National EPR Service, University of Manchester. K-band (∼24.0
GHz) and Q-band (∼34.00 GHz) spectra were recorded using
an ESP 300 spectrometer. Cyclic voltammetry measurements
were performed using an Eco Chemie PGSTAT20. Platinum
beads were used as both working and auxiliary electrodes and
a calomel reference electrode was used with 0.2 M tetrabuty-
lammonium tetrafluoroborate [Bun N]BF4 as base electrolyte. All
4
experiments were performed in degassed CH2Cl2 and standardised
against the ferrocene/ferrocenium couple. The Cr K-edge XAFS
measurements were performed at station BM26A (DUBBLE)
at the European Synchrotron Radiation Facility in Grenoble,
France.37 Data were collected in transmission mode from samples
diluted with boron nitride and in fluorescence mode (using a 9
element germanium detector from Ortec, with XPRESS signal
processing38) from 5 mM toluene solutions. For the Me3Al-
activated species the solutions were prepared in a Schlenk tube
under rigorously anhydrous conditions. The reaction mixture was
stirred for ∼30 s after addition of Me3Al and then transferred
to the dry, N2-purged XAFS cell for data collection. The spectra
were calibrated using a Cr foil, using the 1st maximum of the
1st derivative at 5989.0 eV. Background subtraction utilized the
program PAXAS39 and EXAFS analysis employed the spherical
wave code EXCURV98.40 The ligands L1–L6 were prepared using
modified literature procedures.23–26
Experimental
All procedures involving air sensitive materials were performed
under an inert atmosphere of N2 and using standard Schlenk
techniques. Solvents were dried using standard techniques and
distilled under nitrogen prior to use or stored in Young’s am-
poules over dry molecular sieves under nitrogen. Unless stated,
commercial reagents were used as received from Aldrich. 1H
Procedure for catalysis
The reactor was prepared by filling/venting with ethylene (5 ×
10 bar). Toluene (100 mL) or methylcyclohe◦xane (80 mL) was
◦
added and the reactor heated to 80 C or 90 C. To activate the
1182 | Dalton Trans., 2008, 1177–1185
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complexes [CrCl3(L)], MMAO (1.9 M in heptane) was added to the
complexes previously dissolved in toluene. The solution was mixed
and immediately added to the reactor. Ethylene (40 bar) was added
and fed as required to maintain a constant pressure. The reactor
was allowed to run until ethylene uptake ceased. The reactor was
then cooled with ice to ∼10 ◦C and the ethylene slowly vented.
Nonane (1000 lL) was added followed by MeOH and HCl (10%)
to destroy the MMAO. The reactor was opened and the polymer
collected, dried, weighed and when possible a liquid fraction was
collected for GC analysis.
cooled, filtered and the filtrate concentrated in vacuo and was
washed with a saturated NaHCO3 solution. Diethyl ether (50 mL)
was added and the aqueous layer was extracted. The organic
layer was dried (MgSO4), filtered and the solvent was removed
in vacuo to afford a clear orange oil which was distilled (80 ◦C;
0.5 mm Hg). Yield: 4.1 g, 51%. 1H NMR (CDCl3): d 0.89 (t, 9H,
CH3C3H6S–), 1.06 (s, 3H, CH3C), 1.35 (m, 6H, CH3CH2C2H4S),
1.55 (m, 6H, C2H5CH2CH2S–), 2.51 (t, 6H, C3H7CH2S), 2.63 (s,
1
6H, C4H9SCH2). 13C{ H} NMR (CDCl3): d 13.66 (CH3C3H6S–),
21.95 (CH3CH2C2H4S–), 23.80 (CH3C), 32.03 (C2H5CH2CH2S–),
33.82 (C3H7CH2S), 38.88 (CH3C), 41.84 (C4H9SCH2). Calculated
for C17H36S3: C, 60.65; H, 10.78; found C, 60.56; H, 10.81%.
HRMS analysis (ESI): m/z calculated [M + H]+: 336.1981; found:
336.1979.
Preparations
S(CH2CH2SC10H21)2 (L1). Bis(2-mercaptoethyl)sulfide (7.16 g,
46.0 mmol) was dissolved in EtOH (200 mL) under N2, and
sodium (2.14 g, 93.0 mmol) was added in small pieces. When
the sodium had dissolved the mixture was heated to reflux and
1-iododecane (25.0 g, 93.0 mmol) added dropwise. The EtOH was
removed in vacuo and water (35 mL) was added. The mixture
was extracted with Et2O (3 × 35 mL) and the organic extracts
were dried (MgSO4), filtered, and the solvent was removed in
vacuo to afford the ligand as a white solid. Yield: 16.28 g,
80%. Mp: 54.5–55.5 ◦C. 1H NMR (CDCl3): d 0.88 (t, 6H,
CH3C9H18S–), 1.26 (s, 28H, CH3C7H14C2H4S–), 1.58–2.54 (m, 8H,
MeC(CH2SC10H21)3 (L4). MeC(CH2Br)3 (4.0 g, 13.0 mmol)
was dissolved in EtOH (200 mL) and NaOH (2.1 g, 52.0 mmol)
was added to the mixture. Decanethiol (7.5 g, 43.0 mmol) was
added dropwise over a period of 30 min and the resulting mixture
was refluxed under N2 for 24 h. The resulting mixture was cooled,
filtered and the filtrate was washed with saturated NaHCO3 solu-
tion (30 mL). The aqueous layer was then extracted with diethyl
ether (3 × 50 mL). The organic layer was dried (MgSO4), filtered
and the solvent was removed in vacuo to afford an orange oil.
Yield: 2.75 g, 36%. 1H NMR (CDCl3): d 0.90 (t, 9H, CH3C9H18S–),
1.08 (s, 3H, CH3C), 1.29 (m, 42H, CH3C7H14C2H4S–), 1.62 (m,
6H, C8H17CH2CH2S–), 2.52 (t, 6H, C9H19CH2S–), 2.60 (s, 6H,
1
C8H17CH2CH2S–), 2.74 (m, 8H, C10H21SC2H4S–). 13C{ H} NMR
(CDCl3): d 14.25 (CH3C9H18S–), 22.81 (CH3CH2C8H16S–), 28.88,
29.04, 29.38, 29.45, 29.67, 29.70, 29.88 (C2H5C7H14CH2S–), 32.0,
32.48 (C9H19CH2SCH2CH2S). Calculated for C24H50S3: C, 66.36;
H, 11.52; S, 22.12; found C, 66.25; H 11.60; S, 22.19%.
1
C10H21SCH2). 13C{ H} NMR (CDCl3): d 14.07 (CH3C9H18S–),
22.66 (CH3CH2C8H16S–), 23.80 (CH3C), 29.00, 29.26, 29.30,
29.54, 29.96 (C3H7C5H10C2H4S–), 30.83 (C8H17CH2CH2S–), 31.89
(C2H5CH2C7H14S–), 34.06 (C9H19CH2S–), 40.75 (CH3C), 41.84
(C10H21SCH2). HRMS analysis (ESI): m/z calculated [M + H]+:
588.4796; found: 588.4789.
O(CH2CH2SC10H21)2 (L2). To a solution of bis(2-chloroethyl)-
ether (3.42 g, 24.0 mmol) in EtOH (250 mL) was added NaOH
(2.868 g, 1.25 mol). 1-Decanethiol (8.35 g, 48.0 mmol) was
added dropwise and the mixture refluxed for 4 h under N2.
The solvent was then removed in vacuo and water (35 mL) was
added to the remaining oil. The mixture was extracted with
diethyl ether (3 × 35 mL). The organic extracts were dried
(MgSO4), filtered and the solvent was removed in vacuo to
afford a colourless oil that crystallised upon standing at room
temperature. Yield: 8.1 g, 81%. 1H NMR (CDCl3): d 0.80 (t,
6H, CH3C9H18S–), 1.17 (m, 28H, CH3C7H14C2H4S), 1.56–2.46
(m, 8H, C8H9CH2CH2S–), 2.63 (t, 4H, C10H21SCH2CH2O–), 3.55
1-Decyl-1,4,7-triazacyclononane (C10H21)[9]aneN3 (L5). De-
canoyl chloride (2.01 g, 10 mmol) was added dropwise at
room temperature to
a stirred solution containing 1,4,7-
triazacyclononane-1,4-dicarboxylic acid di-tert-butyl ester (3.15 g,
9.60 mmol) and triethylamine (2.7 mL) in distilled CH2Cl2
(100 mL). The solution was left to stir overnight under N2 and
then washed with water (3 × 30 mL); the organic layer was dried
(MgSO4), filtered and the CH2Cl2 was removed. The intermediate
1,4,7-tri(nonylcarbonyl)-1,4,7-triazacyclononane was obtained as
a pale yellow oil. Yield: 1.96 g, 69%. 1H NMR (CDCl3): d 0.94 (t,
3H, CH3C8H16CON–), 1.25–1.30 (m, 12H, CH3C6H12CH2CON–),
1.32–1.65 (m, 20H, C(CH3)3 and C7H15CH2CON–), 2.22–2.39
(m, 4H, [9]aneN3 ring), 3.10–3.35 (m, 4H, [9]aneN3 ring), 3.43–
1
(t, 4H, C10H21SCH2CH2O–). 13C{ H} NMR (CDCl3): d 14.48
(CH3C9H18S–), 23.06 (CH3CH2C8H16S–), 29.28, 29.65, 29.70,
29.95, 30.22, 31.93, 32.29, 33.10 (C2H5C8H16SCH2CH2O–), 71.16
(C10H21SCH2CH2O–). Calculated for C24H50OS2: C, 68.83; H,
12.03; found C, 68.78; H, 12.00%.
1
3.6 (m, 4H, [9]aneN3 ring). 13C{ H} NMR (CDCl3): d 14.07
MeC(CH2SnC4H9)3 (L3). Ammonia (100 mL) was condensed
onto dry, degassed THF (100 mL) in a 1 L flask equipped with
a condenser and using a Me2CO/CO2 slush bath (−78 ◦C). A
small piece of solid Na was added to the mixture, leading to a
dark blue solution. (nC4H9)2S2 (12.82 g, 72.0 mmol) was added
slowly to produce a creamy white mixture. The remaining Na
(3.31 g, 0.144 mol) was added slowly in small pieces and the NH3
was then allowed to evaporate. The mixture was concentrated in
vacuo to remove the THF. A mixture of ethanol and MeC(CH2Br)3
(7.34 g, 24.0 mmol) was added dropwise to the Na+S(nC4H9)−
first diluted with 150 mL of ethanol. The resulting mixture was
refluxed for 70 h under nitrogen. The grey reaction mixture was
(CH3C8H16CON–), 22.63, 24.22, 25.35, 28.85, 29.25, 29.47,
31.85, 33.40, 35.28, 48.18, 49.67, 52.94 ([9]aneN3 ring and
=
CH3C8H16CON–), 155.36 (C O BOC), 173.13 (C9H19CON–).
Electrospray MS (CH2Cl2): m/z = 484 ((C9H19CO)-diBOC-
[9]aneN3 + H)+.
The amide (C9H19CO)-diBOC-[9]aneN3 (3.30 g, 6.83 mmol) was
dissolved in dry THF (50 mL); a solution of BH3THF (48 mL)
was added, and the mixture was refluxed overnight under N2. After
cooling to room temperature, the excess BH3THF was quenched
by slow addition of methanol, and the colourless solution was
evaporated to leave a colourless oil. This was dissolved in 1-
butanol (50 mL), water (50 mL), and concentrated HCl (80 mL)
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and refluxed overnight. After the mixture was cooled, sodium
hydroxide was added until pH > 12, and the product was
extracted with CH2Cl2 (6 × 50 mL). The combined organic
extracts were dried (MgSO4), filtered and evaporated in vacuo to
afford a purple solid which was washed with a minimum amount
of hexane and dried in vacuo. Yield: 1.02 g, 67%. UV-visible (DR):
517, 699 nm. Calculated for C24H50Cl3CrS3: C, 48.59; H, 8.50; S,
16.22; found C, 48.55; H, 8.52; S, 16.22%.
1
give the desired product as an orange oil. Yield: 0.9 g, 49%. H
[CrCl3(L2)]. Method as above to give a green solid. Yield: 71%.
UV-visible (DR): 716, 475 nm. Calculated for C24H50Cl3CrOS2: C,
49.95; H, 8.73; found: C, 49.87; H, 8.77%.
NMR (CDCl3): d 0.95 (t, 3H, CH3C9H18N–), 1.12–1.65 (m, 16H,
CH3C8H16CH2N–), 2.38–2.55 (m, 2H, C9H19CH2N–), 2.56–2.75
1
(m, 4H, [9]aneN3 ring), 2.78–3.04 (m, 8H, [9]aneN3 ring). 13C{ H}
[CrCl3(L3)]. Method as above to give a purple solid. Yield:
75%. UV-visible (DR): 694, 492 nm. Calculated for C17H36Cl3CrS3:
C, 41.25; H, 7.33; found: C, 41.17; H, 7.35%.
NMR (CDCl3): d 14.08 (CH3C9H18N), 22.66, 27.23, 27.31, 29.28–
29.56 (C3H7C4H8C3H6N–), 31.87 (CH3C8H16CH2N–), 43.15,
44.72 ([9]aneN3 ring), 49.78 (C9H19CH2N–), 56.55 ([9]aneN3 ring).
Electrospray MS (CH2Cl2): m/z = 270 ((C10H21)[9]aneN3 + H)+.
Calculated for C16H35N3: C, 71.31; H, 13.09; N, 15.59; found C,
71.24; H, 12.90; N, 15.50%.
[CrCl3(L4)]. Method as above to give a dark purple solid.
Yield: 76%. UV-visible (DR): 695, 493 nm. Calculated for
C35H72Cl3CrS3: C, 56.24; H, 9.71; found: C, 56.36; H, 9.76%.
1,4,7-Tris(decyl)-1,4,7-triazacyclononane
(C10H21)3[9]aneN3
(L6). Decanoyl chloride (4.87 g, 25.0 mmol) was added
dropwise at room temperature to a stirred solution containing
1,4,7-triazacyclononane (1.0 g, 7.75 mmol) and NEt3 (6.5 mL,
46.6 mmol) in distilled CH2Cl2 (100 mL). The solution was left to
stir overnight under N2 and then washed with water (3 × 30 mL);
the organic layer was dried (MgSO4), filtered and the CH2Cl2
was removed. The intermediate 1,4,7-tris(nonylcarbonyl)-1,4,7-
triazacyclononane was obtained as a pale yellow solid. Yield:
[CrCl3(L5)]. Method as above to give a dark purple solid.
Yield: 70%. UV-visible (DR): 594, 485 nm. Calculated for
C16H35Cl3CrN3: C, 44.92; H, 8.25; N, 9.82; found: C, 44.82; H,
8.27; N, 9.76%.
[CrCl3(L6)]. Method as above to give a brown oil. Yield: 48%.
UV-visible (DR): 595, 459 nm. Calculated for C36H75Cl3CrN3: C,
61.04; H, 10.67; N, 5.93; found: C, 61.13; H, 10.75; N, 5.88%.
1
3.47 g, 76%. H NMR (CDCl3): d 0.8 (t, 9H, CH3C8H16CON–),
1.19 (m, 36H, CH3C6H12C2H4CON–), 1.53 (m, 6H,
C7H15CH2CH2CON–), 2.19 (m, 6H, C7H15CH2CH2CON–),
3.32 (s, 6H, [9]aneN3 ring), 3.61 (s, 6H, [9]aneN3 ring).
Acknowledgements
We thank the EPSRC and SASOL Technology UK for financial
support, the ESRF for access to the synchrotron radiation facility
via projects CH-2125 and CH-2209 and Dr Wim Bras and
Mr Sergey Nikitenko for assistance in using Beamline BM26A
(DUBBLE). We also thank Dr Eric McInnes and Dr Joanna
Wolowska for access to the EPSRC EPR service and for helpful
discussions.
1
13C{ H} NMR (CDCl3): d 14.01 (CH3C8H16CON–), 22.58
(CH3CH2C7H14CON–), 25.20 (C7H15CH2CH2CON–), 29.44–
29.46 (C3H7C4H8C2H4CON–), 31.79 (C2H5CH2C6H12CON–),
33.16 (C8H17CH2CON–), 49.16, 51.05 ([9]aneN3 ring),
=
174.35 (C O). Electrospray MS (CH2Cl2): m/z
=
592.5
+
−1
=
((C9H19CO)3[9]aneN3 + H) . IR: m(C O) = 1637 cm .
The triamide ((C9H19CO)3[9]aneN3) (3.30 g, 5.58 mmol) was
dissolved in dry THF (50 mL); a solution of BH3THF (39 mL)
was added, and the mixture was refluxed overnight under a
dinitrogen atmosphere. After being allowed to cool at room
temperature, the excess BH3THF was quenched by slow addition
of methanol, and the colourless solution was evaporated to leave
a colourless oil. This was dissolved in 1-butanol (50 mL), water
(50 mL), and concentrated HCl (80 mL) and refluxed overnight.
After the mixture was cooled, NaOH was added until pH >
12, and the product was extracted with CH2Cl2 (6 × 50 mL).
The combined organic extracts were dried (MgSO4), filtered and
the solvent evaporated in vacuo to give the desired product as
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