Synthesis, characterisation and catalytic behaviour of a novel class of chromium(III) and vanadium(III) complexes containing bi- and tri-dentate imidazole chelating ligands: A comparative study

Article abstract of DOI:10.1039/B207248C

The syntheses and characterisation of tri- and bi-dentate coordinated chromium(III) and vanadium(III) complexes of the general composition [MCL₃(NDN)], [MCL₃(NN)]₂ {M = Cr, (NDN) = (mim)₃COCH₃ 2a [mim = 1-methylimidazolyl], (mim)₂CHCH₂PPh₂ 2b, (mim)₂CHCH₂C(O)But 2c, (tBupim)₃P 2d [tBupim = 1-isopropyl-4-tert-butylimidazolyl], (NN) = (mim)₂CH₂ 3e, (mim)₂CH₂PPh₂ 3f, (mim)₂CO 3g, (Bzmim)₂CO 3h [Bzmim = 1-methybenzimidazolyl], (tBupim)₂CO 3i, (mim)₂C=NPh 3j; M = V: (NDN) = (mim)₃COCH₃ 5a; (NN) = (mim)₂CO 5g}, [CrCl₃(mim)₃] 4k, and [CrCl₃(MeTAM)] 6 (MeTAM = 1-methyltriacetylmethane) is described. Crystallisation of 3g from CH₃CN/Et₂O gave the mononuclear complex [CrCl₃{(mim)₂CO} (CH₃CN)] 3g′. The molecular structure of 3g′ shows the chromium atom is quasi-octahedral, six-coordinate, the three coordinated chlorine atoms disposed met in the coordination sphere. The electronic spectra of the chromium complexes exhibit d-d transitions typical of a pseudo-octahedral coordinated d³ ion, falling into the region ν₁ ⁴A_2g → ⁴T_2g 600-700 nm and ν₂ ⁴A_2g → ⁴T_1g(F) 430-470 nm, and 10Dq values between 14 400 and 16 700 cm^-1. In the presence of MMAO the complexes give active catalyst systems for the conversion of ethylene into 1-alkenes or polymers, with activities and selectivities depending on the electronic and steric factors of the ligand system and the metal centre, respectively.

Full text of DOI:10.1039/B207248C

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Synthesis, characterisation and catalytic behaviour of a novel class
of chromium(III) and vanadium(III) complexes containing bi- and
tri-dentate imidazole chelating ligands: a comparative study
Thomas Rüther,*†a Kingsley J. Cavell,*b Nathalie C. Braussaud,a Brian. W. Skeltonc and
Allan H. Whitec
a School of Chemistry, University of Tasmania, GPO Box 252-75, Hobart, Tasmania 7001,
Australia
b Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff, UK CF1 3TB
c Department of Chemistry, University of Western Australia, Crawley, WA 6009, Australia
Received 24th July 2002, Accepted 7th October 2002
First published as an Advance Article on the web 14th November 2002
The syntheses and characterisation of tri- and bi-dentate coordinated chromium() and vanadium() complexes
of the general composition [MCl3(N∧D∧N)], [MCl3(N∧N)]2 {M = Cr, (N∧D∧N) = (mim)3COCH3 2a [mim =
1-methylimidazolyl], (mim)2CHCH2PPh2 2b, (mim)2CHCH2C(O)But 2c, (tBupim)3P 2d [tBupim = 1-isopropyl-4-
tert-butylimidazolyl], (N∧N) = (mim)2CH2 3e, (mim)2CH2PPh2 3f, (mim)2CO 3g, (Bzmim)2CO 3h [Bzmim =
1-methybenzimidazolyl], (tBupim) CO 3i, (mim) C᎐NPh 3j; M = V: (N∧D∧N) = (mim) COCH 5a; (N∧N) =
᎐
2
2
3
3
(mim)2CO 5g}, [CrCl3(mim)3] 4k, and [CrCl3(MeTAM)] 6 (MeTAM = 1-methyltriacetylmethane) is described.
Crystallisation of 3g from CH3CN/Et2O gave the mononuclear complex [CrCl3{(mim)2CO}(CH3CN)] 3gЈ. The
molecular structure of 3gЈ shows the chromium atom is quasi-octahedral, six-coordinate, the three coordinated
chlorine atoms disposed mer in the coordination sphere. The electronic spectra of the chromium complexes exhibit
4
d–d transitions typical of a pseudo-octahedral coordinated d3 ion, falling into the region ν1 A2g
and ν2 A2g
4T2g 600–700 nm
4T1g(F) 430–470 nm, and 10Dq values between 14400 and 16700 cmϪ1. In the presence of MMAO the
4
complexes give active catalyst systems for the conversion of ethylene into 1-alkenes or polymers, with activities and
selectivities depending on the electronic and steric factors of the ligand system and the metal centre, respectively.
More recently, these studies have also furnished a number of
Introduction
nitrogen chelated chromium olefin polymerisation23,24 and
trimerisation25,26 catalysts that can be regarded as homo-
geneous non-cyclopentadiene model systems for industrial
heterogeneous processes27 (Phillips catalyst). In a previous note
we have reported the synthesis and catalytic behaviour of six
chromium() complexes bearing bi- and tri-dentate ligands
based on a bis(imidazole) framework containing additional
functional groups which differ in their electronic properties and
chemical hardness (HSAB concept).28 With the aim of gaining
a better understanding of how variations of the ligand system
within a class of catalysts influences the electronic properties of
the coordinated metal centre or impacts on their catalytic per-
formance, we have extended our previous studies to other
imidazole chelates, two imidazole chelated vanadium() com-
plexes, and two chromium() complexes bearing the podand
ligands 1-methyltrisacetylmethane (MeTAM) and trisamino-
triethylamine (tren) . In this paper we describe in greater
detail the synthesis and characterization of this new class of
chromium() chloro complexes, including their magnetic prop-
erties, electronic spectra, and electrochemical investigation (the
ligands used in this work are shown in Fig. 1). We also report a
rare example of a structurally characterized chromium()
complex where the imidazole ligand is an integral part of the
coordination environment.
Numerous middle and late transition metal complexes bearing
chelating imidazole ligands are known and they play a major
role as models to mimic the active sites of metallo enzymes.1
Applications outside this area, such as catalytic hydro-
amination, hydrosilylation and Heck coupling reactions have
recently been reported by Field and coworkers and our group
for imidazole chelated rhodium2 and palladium3 complexes,
respectively. By contrast and despite the fact that chromium()
complexes with unidentate and chelating N-heterocyclic lig-
ands (e.g. pyridine, phenanthroline, pyrazolylborate) are well
established,4–10 very few examples with non-histidine type
imidazole ligands11–15 are known.16 Sanchez and Losada 14 have
reported chromium() halogen complexes of the type
[CrL6]Cl3, [CrXL5]X2 (X = Cl, Br), cis- and trans-[CrCl2L4]Cl,
and trans-[CrCl2(L)4]Cl with unidentate imidazoles (L =
imidazole, 1-methylimidazole or 2-methylimidazole). Structur-
ally characterised complexes bearing chelating benzimidazole
ligands have been described recently by Castillo-Blum and
coworkers.15
Apart from the lack of a well established chemistry for
chromium imidazole complexes, our interest in such combin-
ations has been stimulated by recent developments in homo-
geneous catalysis. Most notably these led to a great variety of
new N-ligated transition metal complexes capable of catalysing
the oligo- and poly-merisation of olefins,17,18 and it has been
demonstrated that steric and electronic variations in the ligand
system govern their activity and product selectivity.19–22
Results and discussion
Synthesis of the complexes
† Current address: Centre for Green Chemistry, Monash University,
PO Box 23, Melbourne, VIC 3800, Australia. E-mail:
thomas.ruether@sci.monash.edu.au
The ligands have been synthesized according to published
procedures2,29–31 or by methods developed in our laboratories.32
4684
J. Chem. Soc., Dalton Trans., 2002, 4684–4693
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b207248c

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Fig. 1 Ligands used in this work.
The complexes have been prepared by adding a solution
to drastically diminished C and N microanalytical values. In
of the respective ligand to a solution of CrCl3(THF)3. Using
CH2Cl2 or THF as a solvent, this method of preparing
CrCl3(mim)n complexes in high yields (ca. 90%) is generally
applicable for the majority of the complexes presented in this
work (Scheme 1). The reactions proceeded smoothly at ambient
temperature and in most cases the green or grey-green products
precipitated from the reaction medium. We note however that
extensive trituration with CH2Cl2 is essential to obtain pure
products. The, compared to the other ligands, relatively bulky
ligand (tBupim)3P, did not react under these conditions; even
on refluxing the reaction mixture in CHCl3 for one day in the
presence of a trace amount of zinc dust, the purple colour per-
sisted and most of the ligand was recovered. Complex 2d has
been prepared by addition of a methanol solution of the ligand
to a mixture of solid CrCl3 and a trace amount of zinc dust.
The reaction mixture gradually developed a green colour and
was heated to 60 ЊC for 1–2 h prior to work up. Different
reactivity of related ligand systems towards transition metal
THF adducts is not uncommon. In order to demonstrate the
advantage of using chelating ligands to support catalytically
active metal centres we have also prepared the previously
unknown [CrCl3(1-methylimidazole)3] 4k, containing three
unidentate imidazole units.
this regard we have undertaken controlled studies on an analyt-
ically pure sample of complex 2a. Although the green colour of
the complex did not change upon exposure to air, the analytical
results demonstrate the problem regarding water absorption:
water free C14H18N6OCl3Cr (2a) requires C, 37.81; H, 4.09; N:
18.90; exposure to air for a few hours forming C14H18N6-
OCl3Crؒ2.5H2O—requires: C, 34.32; H, 4.74; N, 17.16; found:
C, 34.89; H, 5.36; N, 17.26; exposure to air for 1 day, forming
C14H18N6OCl3Crؒ6H2O—requires: C, 30.41; H, 5.48; N, 15.21;
found: C, 30.54; H, 4.95; N, 14.81%.
Crystals suitable for X-ray diffraction separated from an
acetonitrile solution of 3g upon slow diffusion of diethyl ether
at Ϫ20 ЊC. Instead of the anticipated dinuclear [CrCl3{(mim)2-
CO}]2 3g, the crystal structure (see below) reveals a mono-
nuclear complex [CrCl3(mim)2CO}(CH3CN)]ؒCH3CN 3gЈ, in
which one solvent molecule occupies the sixth coordination
site and one solvent molecule is occluded in the crystal. 3gЈ
represents the first example of a structurally characterized
chromium() complex bearing a bis(imidazole) chelating
ligand. Together with the complexes [CrCl2{tris(2-benzimid-
azolylmethyl)amine}]Clؒ2H2O and [Cr2(2-guanidinobenzimid-
azole)(µ-OH)2](ClO4)4ؒ5H2O reported by Castillo-Blum and
coworkers,15 these are the only structurally characterized
examples for chromium() in which the imidazole heterocycle
is not a sub-unit of a more complex ligand environment.
Although five-coordinate chromium complexes bearing
nitrogen ligands have been described,8,24,33 we favour a dinuclear
structure6,7 for complexes 3 on the basis of their MS spectra,
showing dinuclear pentachloro molecular ions, their electronic
The complexes 2, 3 and 4 have been characterized by IR, MS,
UV-vis and microanalysis. FAB and ESI mass spectrometry
proved to be a particularly useful analytical method in that it
allowed the quick and sufficient determination of the chemical
composition. Satisfactory microanalyses were often difficult to
obtain due to the moisture sensitivity of the complexes, leading
J. Chem. Soc., Dalton Trans., 2002, 4684–4693
4685

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Scheme 1 Syntheses of the complexes.
spectra, which are distinct from those of five-coordinate
chromium() complexes,34 and their magnetic properties (see
below).7 However, from the crystal structure of 3gЈ it follows,
that the dimeric complexes 3 are cleaved when dissolved in
coordinating solvents such as CH3CN.
Molecular structure of 3gЈ
The results of the low-temperature single crystal X-ray struc-
ture determination (Fig. 2) are consistent with the above formu-
lation of 3gЈ, in terms of stoichiometry and connectivity,
as [Cl3CrL(NCCH3)]ؒCH3CN. One formula unit, devoid of
crystallographic symmetry, comprises the asymmetric unit of
the structure. The chromium atom is quasi-octahedral, six-
coordinate, the three coordinated chlorine atoms disposed mer
in the coordination sphere. Their Cr–Cl distances exhibit only
minor differences, with the difference between Cr–Cl(1,2) trans
to different atoms, similar to that between Cr–Cl(2,3) trans to
each other, although Cr–Cl(1), trans to an imidazole-nitrogen is
the longest. The difference in the Cr–imidazole-N distances,
trans to Cl and MeCN, is more substantial. Their mean is simi-
lar to that found in hydridotris(1-pyrazolyl)borate Cr() com-
plexes9,10 and to that for the Cr–benzimidazole-N distances
(2.047(4)–2.062(4) Å) in the cis-dichloro{tris(2-benzimidazolyl-
Fig.
2
Projection of complex 3gЈ. 50% probability amplitude
displacement ellipsoids are shown for the non-hydrogen atoms,
hydrogen atoms having arbitrary radii of 0.1 Å.
methyl)amine}chromium() cation;15 in the latter Cr–N trans
to Cr–Cl is 2.047(4), and trans to itself 2.050(3), 2.062(4) Å.
Although the Cr–central N of the tripod is appreciably longer
(2.153(4) Å), Cr–Cl trans to the two different types of nitrogen
therein are relatively insensitive (2.318(1), 2.291(1) Å) to that
difference.
4686
J. Chem. Soc., Dalton Trans., 2002, 4684–4693

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Table 1 The chromium environment. r (Å) is the chromium–ligand atom distance; other entries in the matrix are the angles (Њ) subtended at the
chromium by the relevant atoms at the head of the row and column
Atom
r
Cl(2)
Cl(3)
N(11)
N(21)
N(101)
Cl(1)
Cl(2)
Cl(3)
N(11)
N(21)
N(101)
2.3290(3)
2.3199(3)
2.3090(3)
2.063(1)
2.0394(9)
2.070(1)
92.04(1)
91.60(1)
176.23(2)
177.79(4)
87.75(3)
88.65(3)
95.12(3)
88.11(3)
90.60(3)
87.07(4)
89.59(3)
89.64(3)
91.35(3)
88.21(4)
174.85(4)
The dihedral angle between the C3N2 planes of the bidentate ligand is 24.63(5)Њ, chromium atom deviations being 0.126(2), 0.303(2) Å. The
C3N2 dihedral angles to the central C2CO plane are 21.30(5), 24.63(5)Њ. O(0) ؒ ؒ ؒ H(16a, 26a) are 2.53(2), 2.62(1) Å. H(15) ؒ ؒ ؒ N(101) is 2.62(2),
H(25) ؒ ؒ ؒ Cl(1,2) 2.89(1), 2.93(1) Å.
Table 2 Magnetic moments and electronic spectral data of 2a–c, 3e–h,j; 4k and 6
Complex
µeff/µB
ν1/nm
ν2/nm
Shoulder/nm
725
10Dq/cmϪ1
ε1; ε2/ MϪ1 cmϪ1
2a
2b
2c
3e
3f
3.77
4.2
3.61
3.57
n. d.
—
3.68
—
—
3.63
3.5
n. d.
n. d.
n. d.
623
601
650
642
693
666
645
621, 544
615
n. d.
648
637
613
678
457
431
453
445
468
451
∼425 (sh)
—
16051
16638
15384
15576
14423
15000
15503
—
16250
n. d.
15432
15701
16313
14737
83; 143
95; 78
37; 84
37; 63
174; 198
—
67
—
—
n. d.
108
732
718
719, 753
3fa
3g
3ge
3gb
3gЈ
3h
3j
—
n. d.
obsc.
obsc.
447
720
700, 710
65
23; 40
54; 210
4k
6
440
a Diffuse reflectance spectrum at 19 K. b Diffuse reflectance spectrum at r.t.
In the present complex, the pair of imidazole C3N2 rings are
appreciably inclined to each other, being folded about the cen-
tral CO pivot (Table 1). This may be a consequence of some
incompatibility between the ‘bite’ angle of a planar ligand viz-
à-viz the pair of sites available for its accommodation. A further
relevant determinant may be accommodation of steric inter-
actions between H(n5) and the adjacent equatorial ligands
(H(15) ؒ ؒ ؒ N(101) 2.62(2); H(25) ؒ ؒ ؒ Cl(1) 2.89(1) Å) and/or
those between the carbonyl pivot and the methyl substituents
(H(16a, 26a) ؒ ؒ ؒ O(0) 2.53(2), 2.62(1) Å); some asymmetry is
evident in the deviations of the chromium atom from the two
C3N2 planes.
bands of the ligand systems, which are centred at approximately
300 nm. A low energy shift of the CT bands is observed for
those ligands containing additional chromophoric groups in
4
the backbone, so that the A2(F)
4T1(F) transition in 3g
appears as a shoulder, or is obscured in 3h,j. In addition
shoulders at the low energy tail of the first transition (720–
730 nm), observed in most cases, may be attributed to spin-
forbidden transitions and to LMCT bands.12,13 Since the
molecular structure of 3gЈ suggests that the complexes contain-
ing bidentate ligands are mononuclear in acetonitrile solution
with one solvent molecule occupying the sixth coordination
site, we have recorded the diffuse reflectance spectra for
complexes 3f,g. They exhibit bands characteristic of a pseudo-
octahedral chromium() chromophore, supporting the pro-
posed dimeric structure of 3e–j with both bridging and terminal
chlorines. Similar observations have been made for [CrCl3-
Magnetic properties
We have reviewed the magnetic properties of imidazole chro-
mium complexes reported in our previous note,28 and collected
magnetic susceptibility data for powdered samples of com-
plexes 2a–c,3e,g,gЈ,h in the temperature range 4–300 K on a
SQUID magnetometer, allowing more accurate determination
of µeff values than in the previous measurements (Evans bal-
ance). The complexes have µeff values typically between 3.5 and
4.2 µB (Table 2), which are in agreement with three unpaired
electrons. Above T ∼ 25 K essential temperature-independent
behaviour (2a,c; 3g,gЈ,h) or very little (2b: µeff 3.85–4.2; 3e:
µeff 3.25–3.57) temperature-dependence is observed, and thus
none of the complexes shows antiferromagnetic coupling.
(bipy)].7 However, the visible spectrum of 3g at 19 K shows
4
splitting of the A2g
4T2g transition, indicating lowering of
the symmetry from Oh, a common feature of a number of other
chromium() complexes.5,34 The observation of the lowest
energy spin-allowed transition for the chromium() centre
allows us to estimate the splitting parameter 10Dq to lie
between 14 737 and 16 640 cmϪ1
.
However, the positions of transitions found in this series of
chromium() complexes, all based on a bis(1-methylimidazole)
framework, merit some further comment. Among the imidazole
complexes, the additional donor moiety in complexes 2 causes
an approximately 500 cmϪ1 greater splitting parameter relative
to 3 which bear bidentate ligands. Values for ν1 and ν2 are simi-
lar to those observed for recently reported chromium() chloro
complexes bearing tridentate benzimidazole chelating ligands.15
Changes in the individual Dq values of complexes containing
the tripod ligands are in agreement with the nature of the third
donor function, linked to the bis(imidazole) framework
(N∧N∧N; N∧O∧N; N∧P∧N). Thus the highest Dq
value is observed for N∧P∧N system reflecting the σ-donor/
Electronic spectra
UV-vis spectra have been recorded for 1–5 × 10Ϫ3 solutions of
complexes 2a–c,e;3e–g,h,j in CH3CN or MeOH. The results are
summarised in Table 2. The majority of the complexes exhibit
d–d transitions typically observed5,34 for pseudo-octahedral
4
chromium in the oxidation state ϩ3, with ν1 A2g
4T2g 600–
4
700 nm, ν2 A2g
4T1g(F) 430–470 nm and the third absorp-
4
tion, ν3 A2g
4T1(P) being obscured by charge transfer (CT)
J. Chem. Soc., Dalton Trans., 2002, 4684–4693
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Table 3 Peak potentials Ep observed in the cyclic voltammograms of
complexes 2a–c, 4k and 3e–h,ja
Complex
Epred/V
Epox1/V
Epox2/V
2a
Ϫ1.8
Ϫ1.5
Ϫ0.67
Ϫ0.91
Ϫ0.63
Ϫ0.72
Ϫ0.72
—
Ϫ0.94
—
Ϫ0.53
0.69
0.51
0.74
0.75
0.74
0.72
0.56e
0.85
0.89
2bb
2c
Ϫ1.71
Ϫ1.42
Ϫ1.41
Ϫ1.33
Ϫ1.06
Ϫ1.58
Ϫ1.61
3ec
3fd
3g
3h
3jc
4k
a Conditions: scan rate ν = 500 mV sϪ1; c = 5 mM; supporting electrolyte
ox3
red2
ox3
0.1 M Bu4NPF6. b Ep = 1.37 V. c Ep
= 1.66 V. d Ep = 1.39 V.
e Quasi-reversible potential given as E1/2
.
Fig. 3 Cyclic voltammograms of 2a recorded in CH3CN (0.1 M
Bu4NPF6) at various scan rates using a 3 mm diameter glassy carbon
electrode.
π-acceptor properties of the phosphine ligand. The low Dq
value in 2c can be attributed to the strong electron withdrawing
and hard nature of the oxygen ligand. Interestingly, in the series
of bidentate coordinated complexes, reduced Dq values are
observed for those ligands containing electron-accepting
groups in the bridge. Generally the positions of d–d bands seen
in the present complexes are very similar to those reported for
the related chromium() chloro 2, 2Ј-bipyridine complexes
[CrCl3(bipy)]n and [CrCl3(bipy)L] (L = CH3CN, py, pyO,
OPPh3).7 In the latter a low energy shift of the 4A2g
4T2g band
upon replacing the third pyridine donor with oxygen donors
(pyO, OPPh3) has also been observed. The electronic transitions
in the tripod imidazole complexes 2 are similar to those
reported for the tris(pyrazol-1-yl)borate complex [CrCl3(Tp)]-
[AsPh4],9 but shifted to lower energy compared to [CrCl3(Tp)]-
[HPMe3]10 and the related complexes [CrCl3(Tp)(L)], bearing
additional donors L = THF,10 py, Hpz.9 Given the relative ease
with which the bis(1-methylimidazol-1-yl) framework can be
functionalised, these ligands represent a valuable opportunity
for the tuning of ligand field parameters and offer interesting
applications in coordination chemistry.
Fig. 4 Cyclic voltammogram of (mim)2CO 1g recorded in CH3CN
(0.1 M Bu4NPF6) at a scan rate of ν = 500 mV sϪ1 using a 3 mm diameter
glassy carbon electrode.
Electrochemistry
Cyclic voltammograms (CV) for the complexes 2a–c;3e–g,h,j;4k
and the ligands (mim)3COCH3 and (mim)2CO have been
recorded in CH3CN containing 0.1 M tetrabutylammonium
hexafluorophosphate as supporting electrolyte at a glassy
carbon working electrode at various scan rates (50–2000 mV
ox2
irreversible oxidation waves (Epox1 = Ϫ0.59 to Ϫ0.94 and Ep
=
0.70 to 0.89 V), apart from 3h, which displays a quasi-reversible
oxidation at 0.56 V (∆Epox = 0.34 V). When the voltammograms
of complexes 2 and 3 are scanned in the positive direction, the
s
Ϫ1). All redox potentials are referenced in volts vs. the ferro-
ox2
second oxidation process Ep is not detected or of very low
cenium/ferrocene couple (Fcϩ/Fc). The results are collected in
Table 3 and representative cyclic voltammograms for the com-
plexes and ligands are shown in Fig. 3 and Fig. 4, respectively.
Both ligands undergo a reversible reduction at E1/2 = Ϫ1.76
[(mim)3COCH3] and Ϫ1.73 V [(mim)2CO] where E1/2 is calcu-
lated as the average of the reduction and oxidation peak poten-
tials in this process. These values can be compared to those
found for substituted 2,6-bis[1-(3,5-dimethoxybenzyl)benz-
imidazole-2-yl]pyridine ligands35 (Ϫ1.84 to Ϫ1.98 V) under
similar conditions.
ox1
intensity. In the bidentate coordinated complexes 3, Ep is
either absent (3g,j) or only observed at scan rates ≥100 mV
(3e,f,h). A third irreversible oxidation is observed in 2b (1.37 V)
and 3f (1.39 V), which may be assigned to an oxidation of the
phosphine group in these ligands.
Synthesis of [VCl3{(mim)3COCH3}] 5a, [VCl3{(mim)2CO}-
(THF)] 5g and [CrCl3(MeTAM)] 6 (see Scheme 2)
Several groups have reported non-cyclopentadiene vanad-
ium() complexes and their different catalytic behaviour com-
pared to other systems.36 In addition, the role of the number of
valence electrons in transition metal pre-catalysts used in the
olefin polymerisation has been pointed out by Britovsek and
co-workers.17 With a view to contrasting the properties of
d3 (mim)nCr systems with their d2 vanadium counterparts, we
have attempted to synthesise related complexes containing the
bidentate (mim)2CO and tridentate (mim)3COCH3 ligand. The
reaction of VCl3(THF)3 with these ligands in CH2Cl2 pro-
ceeds sluggishly resulting in green–brown reaction mixtures.
Replacing CH2Cl2 with THF leads to the precipitation of
green solids. The isolated complexes are very air sensitive and
decompose on standing at room temperature under nitrogen.
The cyclic voltammograms of the complexes containing tri-
podal ligands and three unidentate imidazole units respectively
exhibit complex irreversible ligand based reduction processes
red
with peak potentials Ep Ϫ1.5 to Ϫ1.84 V. The relative ease
with which reduction occurs in 2b is due to the π-acceptor
properties of the PPh2 group. The irreversible reductions in
complexes 3 containing bidentate ligands are observed at less
negative potentials (Ϫ1.06 to Ϫ1.58 V). Thus, as a result of
coordination to chromium() the potentials are less negative
than in the free ligands. At scan rates ≥500 mV a second
irreversible reduction is found in complexes 3e,j (Ϫ1.74,
Ϫ1.77 V). After switching the potential, products resulting
from the irreversible reductions give rise to one or two
4688
J. Chem. Soc., Dalton Trans., 2002, 4684–4693

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conversion of ethylene. Among the bidentate coordinated com-
plexes lacking bulky substituents the benzimidazole derivative
3h (Table 4, entry 10) exhibits the highest activities. The low
activity observed for the precursor 4k (entry 13) bearing three
monodentate 1-methylimidazole ligands clearly demonstrates
the advantage of chelating ligands in stabilizing intermediates
occurring in catalytic cycles.
The test runs with complexes 2d and 3i bearing ligands con-
taining bulky substituents provide interesting insight into how
steric bulk placed in proximity to the metal centre affects selec-
tivities in this type of reaction. Theoretical studies on the
recently developed Ni(), Pd(), Fe() and Co() catalysts con-
taining sterically demanding nitrogen ligands have revealed,
that subtle placing of bulky substituents in the ligand system
allows the blocking of axial coordination sites, thus suppressing
β-hydride transfer as a chain termination mechanism.20,21
Hence it has been shown experimentally that, by varying the
steric properties of the ligand system, each class of catalysts is
capable of producing a great variety of products, ranging from
linear and highly branched polymers to short chain oligo-
mers.19,38 Our results obtained for the pre-catalysts 2d and 3i
(entries 5 and 11) are in agreement with these studies. Although
the bulky tBu substituents in the ligand system of 3i are situated
close to the metal center as shown in the proposed structure
in Fig. 5, their position does not allow reaching across the
Scheme 2 Syntheses of the complexes 5a,g and 6.
They analyse to the general compositions [VCl3{(mim)2CO}-
(THF)] (5g) and, less satisfactorily, [VCl3{(mim)3COCH3}] (5a).
For both products the presence of the ligand system is
confirmed by MS and IR (5g) (ν(C᎐O) 1655 cmϪ1), the former
showing ions at m/z = 407 and 311 ᎐for the fragments [(mim)3-
COCH3VCl2]ϩ and [(mim)2COVCl2]ϩ, respectively. High air
sensitivity, instability and difficulties in characterisation of this
type of complexes has also been reported by other groups.36
Since in industrially utilised heterogeneous chromium poly-
olefin catalysts (Phillips catalyst)26,27 the metal centre resides in
a hard oxygen donor environment and closely related homo-
geneous model systems are scarce,37 we have chosen 1-methy-
triacetylmethane (MeTAM) as a hard, albeit neutral tripodal
oxygen ligand. Addition of one equivalent of MeTAM
dissolved in CH2Cl2 to CrCl3(THF) in THF resulted in an
immediate colour change to green. The product [CrCl3-
(MeTAM)] (6) was isolated as a green moisture sensitive solid
upon repeated precipitation from CH2Cl2/hexanes at Ϫ20 ЊC
and characterized by IR, MS, UV-vis and microanalysis. The
IR spectrum of 6 (KBr; ν(CO) = 1572 cmϪ1) shows a low
frequency shift of approximately 140 cmϪ1of the carbonyl band,
relative to the free ligand, confirming its coordination. The
electronic spectrum of 6 exhibits two transitions at ν1 = 678 nm
and ν2 = 440 nm, characteristic of a pseudo-octahedral chrom-
ium() chromophore. The ligand field splitting parameter
10Dq (14737 cmϪ1) is considerably smaller than in the tripod
imidazole complexes 2.
Fig. 5 Steric situation at the metal centre in complexes 2d and 3i.
coordination plane to block axial sites. As a consequence,
1-alkenes ranging from C4 to C30 rather than polyethylene
(12 wt%) are produced. We note however that the maximum is
found at C14–C16, whereas pre-catalysts lacking bulky substitu-
ents give 1-alkenes with a maximum at C8. The activity and
selectivity is the highest observed among the series of
complexes tested. By contrast, complex 2d in which the metal
centre is embraced by the three tBu groups of the tripod ligand
(Fig. 5) gives mainly polyethylene. The relatively encumbered
steric situation may account for the reduced activity compared
to 3i and 2a. On the other hand the considerable polymer
build-up hampered effective stirring, thus preventing obtaining
higher activities.
Complex 2a, as a representative example in the series of
imidazole based catalyst precursors, has been tested under
various conditions (Table 5). Upon reducing the pressure the
activity of 2a/MMAO decreases with decreasing monomer
concentration. Surprisingly, but not unusually, when toluene is
replaced with chlorobenzene as a solvent, polyethylene is pro-
duced with similar activity and only trace amounts of low
molecular weight hydrocarbons have been detected in the GC
samples.
Catalysis
In our previous note we have reported that exposure of toluene
solutions of 2,3/modified methylaluminoxane (MMAO) to 40
bar ethylene resulted in the formation of linear 1-alkenes in the
range of C4–C30 (maximum at C8) with selectivities of up to
79%.28 Catalytic test experiments for the conversion of ethylene
have now been extended to the novel complexes 2d, 3i,j, 4k, 5
and 6. The catalytic results are summarised in Tables 4–6 and
the data reported earlier have been included for convenient
comparison (Table 4).
Generally, all of the new imidazole coordinated complexes
give, upon activation with MMAO, active catalysts for the
J. Chem. Soc., Dalton Trans., 2002, 4684–4693
4689

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Table 4 Ethylene oligomerisation and polymerisation in toluenea
Entry Catalyst/mmol MMAO/equiv.b Activityb/g mmolϪ1 hϪ1 barϪ1 1-Alkenes Isoalkenes
cAlkanes Isoalkanes Polymer (wt%)
1
2
3
4
5
6
7
8
9
10
11
12
13
2a/0.11
2b/0.1
2b/0.12
2c/0.1
2d/0.093
3e/0.09
3f/0.06
3f/0.04
3g/0.06
3h/0.047
3i/0.053
3j/0.11
4k/0.1
400
400
182
400
400
400
170
408
414
400
400
400
400
5.2
2.7
2.3
2.3
2.7
1.4
1.4
2.3
1.5
3.6
10.8
0.8
0.5
79
65
77
9
12
21
12
20
—
4
—
2
3.4
9
3.3
15
70
1.7
2.1
2.6
5.5
6.5
10
11
13
65
d
50
70
60
57
68
81
26
—
16
15
20
17
15
6.5
23
—
25
15
18
24
16
11
29
—
9
—
2
2
1
1.5
22
—
12
2
2
a 50 ml toluene; p(CH2᎐᎐CH2) 40 bar, T = 100 ЊC, 1 h. b Per [Cr]. c Determined by GC–MS. d 1-Alkenes are the predominant products of the liquid
phase.
Table 5 Catalytic testing of 2a under varying conditionsa
Catalyst/
mmol
MMAO/
equiv.
Act.b/g mmolϪ1
Ϫ1 barϪ1
Polymer
(wt%)
Entry
Conditions
h
1-Alkenes
Isoalkenes
Alkanes
Isoalkanes
1
2
3
4
0.11
0.12
0.11
0.11
400
400
400
200
C6H5Clc
14 bar CH2᎐᎐CH2
6 bar CH2᎐᎐CH2
40 bar CH2᎐᎐CH2
4.9
5.8
3.2
1.3
—
62
—
18
n.d.
13
—
20
—
—
100
1.5
0.2
6.6
67
20
—
a 50 ml toluene (except entry 1), T = 100 ЊC, 1 h. b Per [Cr]. c p(CH2᎐᎐CH2) = 40 bar.
Table 6 Catalytic testing of 3–6 in toluenea
Entry
Catalyst/mmol
T /ЊC
Act.b/g mmolϪ1 hϪ1 barϪ1
1-Alkenes
Isoalkenes
Alkanes
Polymer (wt%)
1
2
3
5a/0.11
5a/0.12
5g/0.1
5g/0.09
6/0.01
7/0.11
100
1–40
30–45
1–40
100
80
0.5
1.3
1.1
2.2
6.6
7.7
—
—
—
100
64
100
93
8
13.5
c
c
c
—
—
n.d.
—
—
n.d.
6
11.5
4
n.d.
88e
80
5d
6
8.5
a 400 equivalents MMAO; 50 ml toluene, p(CH2᎐᎐CH2) = 40 bar, 1 h. b Per [Cr]. c C4 hydrocarbons only. d 400 equivalents MAO instead of MMAO
used as activator. e Odd numbered 1-alkenes were also detected.
The vanadium complexes 5a,g, although not carrying bulky
ligands, give predominantly polymer (Table 6) whereas their
chromium counterparts under similar conditions produce
oligomers. They also appear to be less stable, e.g. considerably
higher activities have been obtained at low temperatures
(entries 2 and 4) and a significant exothermic reaction has been
observed on pressurising solutions of 5a,g/MMAO with 40 bar
ethylene. Quite different behaviour of vanadium complexes
compared to other early transition metals supported by the
same ligand system, and deactivation of vanadium precatalysts
due to reduction during the catalytic cycle, has been reported
recently by other groups.36
Generally, from these studies it is clear that tripod ligands are
better suited to stabilise and protect the metal centre during
the catalytic cycle than related bidentate ligands. The results
from the catalytic testing of the imidazole chelated complexes
show that, in agreement with the metal centres residing in a
bis(imidazole) framework, these complexes display similar
activities and selectivities, with the observed differences arising
from variations in the additional substituents and donor centres
respectively.
Conclusion
Complex 6, containing a tripodal oxygen ligand as a neutral
mimic for the hard oxygen environment in heterogeneous
supporting materials, shows somewhat higher activity than its
imidazole chelated counterparts (Table 6, entry 5).
A number of novel chromium() complexes bearing bi- and tri-
dentate imidazole chelate ligands as an integral part of the
coordination environment have been prepared and character-
ised, adding to the very limited number of such complexes
known to date. The complex [CrCl3{mim)2CO}(CH3CN)]
represents a rare example of a structurally characterized
chromium() complex where the imidazole ligand is integral
to the coordination environment. A particular feature of this
series of complexes is, that the electronic properties of the
central metal may be tuned by introducing different substitu-
ents and functional groups or additional donor groups into the
bis(imidazole) ligand framework. The chromium complexes
have been probed in the catalytic conversion of ethylene
into higher molecular weight products. Upon activation with
MMAO, 1-alkenes or polyethylene are produced with activities
To further probe how electronic and binding properties of
the supporting ligand system in chromium() complexes
impacts on their performance as catalysts in the conversion of
olefins we have chosen the reported39 complex [CrCl2(tren)]Cl
(7) containing the tetradentate amino ligand triaminotriethyl-
amine. Catalytic testing of 7 under the conditions used for the
imidazole systems gave improved selectivities and enhanced
activity (Table 6, entry 6). The better performance of 7 may be
partly attributed to the unique geometric properties of the tren
ligand, which have been shown to greatly affect the kinetics of
substitution reactions in chromium complexes.39,40
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J. Chem. Soc., Dalton Trans., 2002, 4684–4693

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and selectivities largely depending on the steric and electronic
nature of the ligand system. Two isostructural, albeit less well
characterised, vanadium() complexes containing bi- and tri-
dentate ligands, respectively, generated mainly polymer under
similar conditions.
4k. Found: C, 35.69; H 4.70; N, 20.68. C12H18Cl3CrN6
requires C, 35.61; H; 4.49; N, 20.77%; m/z 403 (Mϩ, 30%), 368
(MϪCl, 100%), 333 (MϪ2Cl, 8%), 286 (L2CrCl2, 89%), 251
(L2CrCl, 56%) (FAB MS).
[CrCl3{(tBupim)3P}] 2d. The ligand 1d (0.122 g, 0.23 mmol),
CrCl3 (0.037 g, 0.23 mmol) and a trace of Zn dust were mixed
as solids and 8 ml methanol added. Upon stirring and heating
to 45 ЊC the colour of the slightly turbid reaction mixture
changed to green–turquoise. After 4 h and cooling to room
temperature the reaction mixture was filtered through Celite
and the solvent removed under reduced pressure. Washing of
the crude product with hexanes (2 × 5 ml) and drying under
vacuum gave [CrCl3{(tBupim)3P}] 2d as a mint green solid
(0.142 g, 90%). Found: C 50.34, H 8.58, N 11.72. C30H51-
Cl3CrN6Pؒ2H2O requires C, 49.96; H, 7.7; N, 11.66%; m/z 630
(LCrClϩO, 18%); 327 (L, 100%) (FAB MS).
Experimental
Unless otherwise stated, all manipulations were carried out
using standard Schlenk techniques or in a nitrogen glovebox
(Innovative Technology Inc.). All solvents for use in an inert
atmosphere were purified by standard procedures and distilled
under nitrogen immediately prior to use. The metal chlorides
[CrCl3(THF)3],41 [VCl3(THF)3],42 and the ligands2,29–32 were
prepared according to literature procedures, except for
MeTAM, which was a loan from Dr S. B. Wild (ANU/
Canberra). For the preparation of [CrCl3(tren)] (7) a literature
procedure39 was followed, except that [CrCl3(THF)3] was used
as a starting material and CH2Cl2 as a solvent. Anhydrous
CrCl3, 1-methylimidazole and triaminotriethylamine (tren) were
purchased from Aldrich and used as received. Modified methyl-
aluminoxane (MMAO) solution in heptane was purchased
from AKZO. Ethylene was purchased from BOC gases. Tetra-
butylammonium hexafluorophosphate (Bu4NPF6) (Aldrich)
was recrystallised from ethanol prior to use.
[CrCl3{(tBupim)2CO}]2 3i. (tBupim)2CO 1i (0.095 g, 0.26
mmol) in tetrahydrofuran (7 ml) was added to a stirred solu-
tion of [CrCl3(THF)] (0.099 g, 0.26 mmol) in tetrahydrofuran
(10 ml). The solution turned brown and some solid material
formed after a few hours. After stirring overnight the olive
brown reaction mixture was filtered through Celite and the
solvent removed under reduced pressure. Washing of the crude
product with Et2O (2 × 5 ml) and drying under vacuum gave
[CrCl3{(tBupim)2CO}]2 3i as a green solid (0.118 g, 88%).
Found: C, 48.65; H, 7.84; N, 9.54. C42H68Cl6Cr2N8O2 requires
C, 48.79; H, 6.64; N, 10.84%; m/z 874 (L2Cr2Cl2 ϪO), 1%) 359
(LH, 100%) (ESI MS).
Catalytic test experiments were carried out in a 300 ml stain-
less steel autoclave (Parr) fitted with a glass liner and magnetic
stir bar.
Solution phase voltammetric experiments in CH3CN using
Bu4NPF6 (0.1 M) as a supporting electrolyte were carried out in
a standard three-electrode arrangement with a glassy carbon
(GC) disk working electrode, a platinum wire as the counter
electrode and an Ag/Agϩ (CH3CN, 10 mM AgNO3) double
junction reference electrode. A BAS100B (Bioanalytical Sys-
tems) electrochemical workstation was used to conduct the
voltammetric experiments. Magnetic susceptibility measure-
ments were carried out in the temperature range 4–300 K on a
Quantum Design MPMS SQUID magnetometer and electronic
spectra were recorded using either a Cary 17 or 5 spectrometer.
IR spectra were recorded on a Bruker IFS-66 FTIR spectro-
meter. Gas chromatography was performed on a Hewlett-
Packard 5890 series II gas chromatograph fitted with a flame
ionisation detector. Elemental analysis (Carlo Erba EA 1108
elemental analyser), MS (LSIMS: Kratos Concept ISQ MS,
10 kV Cs ions, m-nitrobenzyl alcohol (mnba) matrix; ESI:
Finnigan LCQ, direct infusion 3 µl minϪ1, needle voltage 4.5 kV,
capillary voltage 20 V, sheath gas 30 psi), and GC–MS
were carried out by the Central Science Laboratory (CSL),
University of Tasmania. Extreme water sensitivity leads to poor
microanalytical data for some of the complexes.
[VCl3{(mim)3COCH3}] 5a. (mim)3COCH3 1a (0.091 g, 0.32
mmol) in tetrahydrofuran (7 ml) was added to a stirred solution
of [VCl3(THF)3] (0.119 g, 0.32 mmol) in tetrahydrofuran (3 ml).
A pale purple precipitate forms immediately. Upon stirring
overnight the reaction mixture turns pale green. The product
was allowed to settle and the almost colorless tetrahydrofuran
phase decanted. Washing with tetrahydrofuran (2 × 5 ml), Et2O
(2 × 5 ml) and drying under vacuum gave [VCl3{(mim-
)3COCH3}] 5a as a green solid. (0.14 g, 99%). Found: C, 34.36;
H, 3.71; N, 17.20. C14H18Cl3N6OV requires C, 37.87; H, 4.10;
N, 18.95%; m/z 408 (MϪCl, 9%); 388 (LVClϩO, 42%); 322
(LV–CH3 Ϫ3Cl, 12%); 255 (LϪOCH3) (FAB MS).
[VCl3{(mim)2CO}(THF)] 5g. A suspension of (mim)2CO 1c
(0.085 g, (0.44 mmol) in tetrahydrofuran (15 ml) was added to a
stirred solution of [VCl3(THF)3] (0.166 g, 0.44 mmol) in tetra-
hydrofuran 5 ml. A bright green precipitate begins to form after
a few minutes. After 2 h the product was allowed to settle and
the almost colourless tetrahydrofuran phase decanted. Washing
with tetrahydrofuran (2 × 5 ml), Et2O (2 × 5 ml) and drying
under vacuum gave [VCl3{(mim)2CO}(THF)] 5g as a green
solid (0.166 g, 90%). Found: C, 38.32; H, 3.98; N, 12.90.
General synthetic procedure for chromium imidazole complexes
[CrCl3{(Bzmim)2CO}]2 3h, [CrCl3{(mim)2CNPh}]2 3j and
[CrCl3(mim)3] 4k
C13H18Cl3N4O2V requires C, 37.20; H, 4.33; N, 13.35%; νmax
/
cmϪ1 (C᎐O) 1654 (KBr); m/z 428 (LVClϩmnba, 20%); 311
᎐
1.03 equivalents of the respective ligand dissolved in CH2Cl2
were added to a CH2Cl2 solution of (THF)3CrCl3. The products
started to precipitate within minutes. After stirring for 12 h the
solvent was separated and the crude product stirred again in
CH2Cl2 overnight. Removal of the CH2Cl2 followed by washing
with CH2Cl2, three portions of hexanes and drying in vacuo
gave the respective complexes as green solids (ca. 90%).
(LVCl2, 14%); 292 (LVClϩO, 100%); 276 (LVCl, 23%); 191
(LH, 15%) (FAB MS).
[CrCl3(MeTAM)] 6. MeTAM 1 (0.099 g, 0.63 mmol) in
CH2Cl2 (3 ml) was added to a stirred solution/suspension of
[CrCl3(THF)3] (0.235 g, 0.63 mmol) in tetrahydrofuran (4 ml).
The solution gradually changes colour from purple to green.
After 24 h the almost clear green reaction mixture was filtered
through Celite and the solvents removed under reduced
pressure. The remaining dark green sticky solid was stirred in
hexanes for 24 h after which time the hexane phase was
decanted and the green solid dried under vacuum. The crude
[CrCl3(MeTAM)] 6 was further purified by repeated precipit-
ation from CH2Cl2/hexanes at Ϫ20 ЊC, followed by washing
with hexanes and drying under vacuum (0.119–0.139 g,
3h. Found: C, 45.56; H, 3.67; N, 11.30. C34H28Cl6Cr2N8O2
requires C, 45.51; H, 3.15; N, 12.49%; νmax/cmϪ1 (C᎐O) 1680
᎐
(KBr); m/z 465 (LCrCl3 ϩH2O, 100%), 429 (LCrCl2 ϩOH,
100%), 343 (LCrH, 8%) (FAB MS).
3j. Found: C, 42.84; H 4.77; N, 14.31. C30H30Cl6Cr2N10
requires C, 42.84; H; 3.58; N, 16.53%; νmax/cmϪ1 (C᎐NPh) 1622
᎐
(KBr); m/z 810 (MϪCl, 5%), 387 (LCrCl2, 100%), 352 (LCrCl,
65%) (FAB MS).
J. Chem. Soc., Dalton Trans., 2002, 4684–4693
4691

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J. Chem. Soc., Dalton Trans., 1998, 2703–2706; (g) C. Place,
J.-L. Zimmermann, E. Mulliez, G. Guillot, C. Bois and J.-C.
Chottard, Inorg. Chem., 1998, 37, 4030–4039.
60–70%). Found: C, 31.31; H, 4.07. C8H12Cl3CrO3 requires C,
30.55; H 3.85%; νmax/cmϪ1 (C᎐O) 1571 cmϪ1 (KBr); m/z 278
᎐
[MϪCl]ϩ (100%) (FAB MS).
2 (a) S. Elgafi, L. D. Field, B. A. Messerle, P. Turner and T. W.
Hambley, J. Organomet. Chem., 1999, 588, 69–77; (b) S. Burling,
L. D. Field and B. A. Messerle, Organometallics, 2000, 19, 87–90;
(c) S. Elgafi, L. D. Field, B. A. Messerle, T. W. Hambley and
P. Turner, J. Chem. Soc., Dalton Trans., 1997, 2341–2345.
3 (a) M. C. Done, T. Rüther, K. J. Cavell, M. Kilner, E. J. Peacock,
N. Braussaud, B. W. Skelton and A. H. White, J. Organomet. Chem.,
2000, 607, 78–92; (b) T. Rüther, M. C. Done, K. J. Cavell, E. J.
Peacock, B. W. Skelton and A. H. White, Organometallics, 2001, 20,
5522–5531.
4 (a) D. K. Geiger, Coord. Chem. Rev., 1998, 172, 163–165;
(b) R. Colton, Coord. Chem. Rev., 1984, 58, 249–250; (c) L. F.
Larkoworthy, K. B. Nolan and P. O’Brien, in Comprehensive
Coordination Chemistry, eds. G. Wilkinson, R. D. Gillard and
J. A. Clevery, Pergamon, Oxford, 1987, vol. 3, ch. 35, pp. 815–823;
(d ) F. H. Burstall and R. S. Nyholm, J. Chem. Soc., 1952, 3570;
(e) M. Hughes and D. Macero, Inorg. Chem., 1976, 15, 2040–2044;
( f ) T. Fujihara, T. Schönherr and S. Kaizaki, Inorg. Chim. Acta.,
1996, 106, 135–141; (g) O. Toshiyuki, K. Mashima, S.-I. Kawamura
and K. Kitaura, Bull. Chem. Soc. Jpn., 2000, 73, 1735–1748.
5 C. S. Garner and D. A. House, Transition Met. Chem., 1970, 6,
59–150.
Structure determination of 3gЈ
A full sphere of low-temperature CCD area-detector diffract-
ometer data was measured (Bruker AXS instrument, ω-scans,
2θmax = 75Њ; monochromatic Mo-Kα radiation, λ = 0.71073 Å;
T ca. 153 K) yielding 37346 reflections, merging to 9433
independent (Rint = 0.033) after ‘empirical’/multiscan absorp-
tion correction (proprietary software), 6992 with F > 4σ(F)
being considered ‘observed’ and used in the full matrix least
squares refinement, refining anisotropic displacement par-
ameter forms for the non-hydrogen atoms and (x,y,z,Uiso) for
the hydrogen. Conventional residuals on |F| at convergence
were R = 0.030, Rw = 0.035 (weights: (σ2(F) ϩ 0.0004F 2)Ϫ1).
Neutral atom complex scattering factors were employed within
the Xtal 3.7 program system.43 Pertinent results are given below
and in Table 1 and Fig. 2.
Crystal data: C11H13Cl3CrN5OؒCH3CN, M = 430.7, mono-
clinic, space group P21/n (C25h, no. 14, variant), a = 6.9757(2),
b = 14.6946(5), c = 17.5803(6) Å, β = 94.419(1)Њ, V = 1797 Å3. Dc
(Z = 4) = 1.592 g cmϪ3. µMo = 11.0 cmϪ1; specimen: 0.5 × 0.2 ×
6 D. H. Brown and R. T. Richardson, J. Inorg. Nucl. Chem., 1973, 35,
755–759.
7 D. A. Edwards and S. C. Jennison, Transition Met. Chem., 1981, 6,
235–238.
8 J. A. Broomhead, J. Evans, W. D. Grumley and M. Sterns, J. Chem.
Soc., Dalton Trans., 1977, 173–176.
0.1 mm; ‘T min, max = 0.72, 0.83. |∆ρmax| = 0.55(7) e ÅϪ3
.
CCDC reference number 181231.
See http://www.rsc.org/suppdata/dt/b2/b207248c/ for crystal-
lographic data in CIF or other electronic format.
9 M. J. Abrams, R. Faggiani and C. J. L. Lock, Inorg. Chim. Acta.,
1985, 106, 69–74.
10 C.-H. Li, J.-D. Chen., L.-S. Liou and J. -C. Wang, Inorg. Chim.
Acta., 1998, 269, 302–309.
General procedure for the catalysis
11 (a) J. Chatlas, S. Kaizaki, E. Kita, P. Kita, N. Sakagami and
R. Van Eldik, J. Chem. Soc., Dalton Trans., 1999, 91–95; (b) L. K.
Mishra and J. P. Singh, Indian J. Chem., Sect. A, 1982, 21, 1135–
1136.
MMAO (1.82 M in heptane) was added slowly with stirring to a
suspension of the selected complex in 5 ml toluene upon which
the complexes dissolved. A colour change to olive or brown was
observed. A 300 ml autoclave (dried at 100 ЊC, cooled to RT
under vacuum and back-filled with nitrogen) was charged with
50 ml toluene and the catalyst solution added via a syringe. The
reactor was immersed into a pre-heated oil-bath and pressur-
ized with ethylene. The reactions were terminated by cooling
the reactor in an ice salt-bath, and venting of excess ethylene
followed by injection of n-nonane (internal standard), slow
addition of a few ml of MeOH and dilute HCl. The solid
material was collected on a frit, washed with MeOH, dilute
HCl, MeOH and dried in a high vacuum at 60 ЊC. The filtrate
was dried over Na2SO4 and analysed by GC and GC/MS.
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Acknowledgements
The Australian Research Council (SPIRT Grant) and Orica
Australia are gratefully acknowledged for financial support
and for providing post-doctoral salaries for T. R. We thank
Dr S. B. Wild (ANU/Canberra) for a generous loan of
MeTAM. We also thank the staff from the Central Science
Laboratory, University of Tasmania, for their assistance with
elemental analyses, and in particular Dr Noel Davis for
numerous MS measurements. We are grateful to Professor
A. M. Bond (Monash University/Melbourne) for his helpful
discussions and for providing the equipment to conduct electro-
chemical studies. Dr B. Moubaraki (Monash University/
Melbourne) is thanked for magnetic moment measurements.
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