Chromium(III) and chromium(IV) bis(trimethylsilyl)amido complexes as ethylene polymerisation catalysts

Article abstract of DOI:10.1039/b412584c

Oxidation of Cr[N(SiMe₃)₂]₂(THF) ₂ with iodine and dicumyl peroxide results in tetrahedral Cr(IV) Cr[N(SiMe₃)₂]₂I₂ and trigonal planar Cr(III) Cr[N(SiMe₃)

Full text of DOI:10.1039/b412584c

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C O M M U N I C A T I O N
Chromium(III) and chromium(IV) bis(trimethylsilyl)amido complexes
as ethylene polymerisation catalysts†
Katherine H. D. Ballem,a Vidhath Shetty,a Nola Etkin‡,*a Brian O. Patrickb and
Kevin M. Smith*a
a
Department of Chemistry, University of Prince Edward Island, 550 University Avenue,
Charlottetown, PE, Canada C1A 4P3. E-mail: kmsmith@upei.ca; Fax: +902 566 0632;
Tel:+ 902 566 0375
Department of Chemistry, University of British Columbia, Vancouver, BC, Canada V6T 1Z1
b
Received 16th August 2004, Accepted 24th September 2004
First published as an Advance Article on the web 8th October 2004
the Cr(II) bis(amido) 1 reacted readily with I2 in Et2O to give
Oxidation of Cr[N(SiMe3)2]2(THF)2 with iodine and dicumyl
peroxide results in tetrahedral Cr(IV) Cr[N(SiMe3)2]2I2 and
trigonal planar Cr(III) Cr[N(SiMe3)2](OCMe2Ph)2, respectively;
both complexes have been characterised by single-crystal X-ray
diffraction, and both are active for ethylene polymerisation with
alkylaluminium co-catalysts.
Cr[N(SiMe3)2]2I2 3 in 80% yield as thick black plates, as shown
in Scheme 1. The Cr(IV) bis(iodo) complex exhibits a broad
singlet in its 1H NMR spectrum (C6D6, 300.1 MHz, 6.38 ppm,
mꢀ = 100 Hz) for the equivalent N(SiMe3)2 methyl groups,
distinct from the corresponding signals for 1 (30.5 ppm, mꢀ =
800 Hz) and 2 (25.9 ppm, mꢀ = 1040 Hz). The recently reported
Cr(IV) Cr[N(CH2CH2NSiMe3)3]X complexes also display SiMe3
signals in the region 5.88 to 3.28 ppm with mꢀ values of between
150 and 250 Hz.12
While examples of homoleptic tetrahedral Cr(IV) d2 CrX4
compounds (X = OR, NR2, R) have been known for many
years,1–4 there has been recent interest in mixed-ligand Cr(IV)
complexes. Initially motivated by the polymerisation activity
of silica-supported Cr(IV) organometallic complexes,5 recent
computational studies have predicted that cationic Cr(IV)
bis(amido) alkyl species should be highly active olefin poly-
merisation catalysts.6 A very recent computational study
has also proposed that cationic Cr(IV) metallacycles are
important intermediates for the Cr-catalysed trimerisation of
ethylene to 1-hexene.7 We would like to report the synthesis,
X-ray crystal structures, and olefin polymerisation activities
of Cr[N(SiMe3)2]2I2 and Cr[N(SiMe3)2](OCMe2Ph)2.
Scheme 1 Synthesis of 3 and 4. Reagents and conditions: i, I2, Et2O;
ii, dicumyl peroxide, pentane.
The bis(trimethylsilyl)amido ligand played a pre-eminent
role in the initial exploration of metal amido compounds.8
Since this pioneering work it has been almost entirely super-
seded, in part due to advances in ligand design and synthesis
in contemporary transition metal amido chemistry, and in
part due to the notorious propensity of the N(SiMe3)2 group
to decompose via N–Si and C–H bond cleavage reactions.9
Despite its perceived obsolescence as an ancillary ligand, we
found the bis(trimethylsilyl)amido group to be attractive for
the investigation of mixed-ligand Cr(IV) compounds for several
reasons. Both Cr[N(SiMe3)2]2(THF)2 1 and Cr[N(SiMe3)2]3 2
are well-known, beautifully crystalline compounds that are
readily prepared in multigram quantities from commercially
available starting materials.10 The equivalence and relatively un-
hindered rotation of the methyl groups are expected to simplify
the 1H NMR spectra of paramagnetic Cr[N(SiMe3)2]nXm
species. Well-defined, stable Cr(II) and Cr(III) complexes with
multidentate amido ligands containing NSiMe2R structural
units have been previously prepared.11 Most significantly,
Filippou and co-workers have demonstrated that the Me3Si-
substituted triamido amine ligand can stabilize a range of
Cr[N(CH2CH2NSiMe3)3]X Cr(IV) complexes.12
Compound 3 was also characterized by X-ray diffraction
(Fig. 1).§ The complex adopts a distorted tetrahedral structure,
and possesses a crystallographic C2 axis bisecting the N–Cr–N
and I–Cr–I angles.
Fig. 1 The molecular structure of 3. Selected bond lengths (Å) and
angles (°); Cr1–N1 1.8355(14), Cr1–I1 2.5610(3), N1–Si1 1.8036(15),
N1–Si2 1.7918(15), N1–Cr1–N1* 116.90(9), N1–Cr1–I1 116.44(4),
N1–Cr1–I1* 103.42, I1–Cr1–I1* 99.322(14).
In 1998, Cummins and co-workers prepared Cr(NRArF)2I2
by reaction of Cr(NRArF)3 with iodine (R = C(CD3)2CH3,
ArF = 2,5-C6H3FMe).13 Our initial attempt to extend this
reaction to the N(SiMe3)2 system using the analogous Cr(III)
tris(amido) complex was unsuccessful: no new chromium
Homoleptic Cr(OR)4 complexes are useful synthetic pre-
cursors to CrR4 species, as an alternative to the oxidation
of Cr(III) [CrR4]− “ate” complexes obtained by alkylating
CrCl3.4 Inspired by the synthesis of Cr(OCMe3)4 by treatment
of Cr(C6H6)2 with di-tert-butylperoxide,1 we attempted to
prepare a mixed ligand Cr(IV) bis(amido) complex by reacting
1 with PhMe2COOCMe2Ph. As illustrated in Scheme 1, the
1
species were detected by H NMR after treatment of 2 with
I2 in Et2O at room temperature for several days. However,
† Electronic supplementary information (ESI) available: Experimental
details. See http://www.rsc.org/suppdata/dt/b4/b412584c/
‡ Correspondence regarding polymerisation studies (netkin@upei.ca)
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 3 4 3 1 – 3 4 3 3
3 4 3 1

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Table 1 Results of ethylene polymerisation runs using pre-catalysts 1–4a
Run
Pre-catalyst/ mmol
Activatorb/ mmol (equiv)
Time/min
Yield PEc/g
Activity/g mmol−1 h−1 bar−1
1
2
3
4
5
6
7
8
9
3 (0.017)
3 (0.017)
3 (0.018)
3 (0.016)
3 (0.017)
3 (0.018)
2 (0.018)
4 (0.019)
1 (0.017)
MAO (3.0/180)
DEAC (0.52/31)
MAO (3.2/180)
DEAC (0.54/34)
MAO (3.0/180)
DEAC (0.54/30)
DEAC (0.54/30)
DEAC (0.54/28)
DEAC (0.54/32)
5
5
0.068
0.010
0.384
0.401
0.508
0.754
0.027
0.683
0.703
48
7
43
50
30
42
1
30
30
60
60
60
60
60
36
41
a General conditions: Schlenk tests carried out at 1 bar ethylene in toluene (40 mL) at ambient temperature, reaction quenched at indicated time
with HCl in methanol, solid PE washed with methanol and water (50 mL each) and dried in a 50 °C vacuum oven. b MAO = methylaluminoxane,
DEAC = diethylaluminium chloride. c Solid polyethylene.
reaction unexpectedly gave a Cr(III) mono(amido) complex,
Cr[N(SiMe3)2](OCMe2Ph)2 4 in 25% yield as blue crystals. The
1H NMR spectrum of 4 (C6D6, 300.1 MHz) displays broad
peaks at 53.6 ppm (mꢀ = 1360 Hz) and 21.2 ppm (mꢀ = 1090 Hz),
assigned to the SiMe3 and CMe2 groups of 4, respectively, based
on their relative integration. The molecular structure of 4 was
determined by X-ray diffraction and is shown in Fig. 2.§ Like
3, 4 also possesses a C2 axis bisecting the O–Cr–O angle. While
trigonal planar CrX3 tris(amido) complexes are known,8 we
are aware of only one previously reported example of a Cr(III)
trigonal planar complex bearing alkoxide ligands.14
polymerisation.6 Oxidation of Cr(II) and Cr(III) amido com-
pounds10,11 provides access to Cr(IV) mixed-ligand amido com-
plexes amenable to further synthetic elaboration.12,13 Further
studies of the properties of the polyethylene generated by
DEAC-activated 1, 3, and 4, and the synthesis and reactivity
of well-defined Cr(IV) complexes derived from 3 are currently
under investigation.
This work was supported by UPEI and the Natural Sciences
and Engineering Research Council of Canada (NSERC).
Notes and references
§ Crystal data for C12H36CrI2N2Si4 3: M = 626.59, monoclinic, space
group = C2/c, a = 16.9513(9), b = 8.7479(3), c = 18.2601(9) Å, b =
111.51(1)°, V = 111.510(10) Å3, T = 173 K, Z = 4, l(Mo-Ka) = 30.93 cm−1
,
20227 reflections measured, 3015 unique, 102 variable parameters
(Rint = 0.029), final residuals R1 = 0.019 [for 2904 reflections with
I > 2r(I)], wR2 = 0.051 [all data]. Crystal data for C24H40CrNO2Si2
4: M = 482.75, monoclinic, space group = C2/c, a = 18.480(3),
b = 9.380(1), c = 17.280(3) Å, b = 116.135(5)°, V = 2689.0(7) Å3,
T = 173 K, Z = 4, l(Mo-Ka) = 5.33 cm−1, 38321 reflections measured,
3175 unique, 142 variable parameters (Rint = 0.023), final residuals
R1 = 0.026 [for 2911 reflections with I > 2r(I)], wR2 = 0.073 [all data].
CCDC reference numbers 247651 and 247652. See http://www.rsc.org/
suppdata/dt/b4/b412584c/ for crystallographic data in CIF or other
electronic format.
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D a l t o n T r a n s . , 2 0 0 4 , 3 4 3 1 – 3 4 3 3

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D a l t o n T r a n s . , 2 0 0 4 , 3 4 3 1 – 3 4 3 3
3 4 3 3