A new diamido-amine ligand based on three-carbon atom arms : synthesis, structures and polymerisation capability of zirconium derivatives of MeN(CH2CH2CH2NSiMe3)2.

Article abstract of DOI:10.1039/b412381f

Zirconium compounds of the new diamido-amine ligand MeN(CH(2)CH(2)CH(2)NSiMe(3))(2) feature significantly different molecular structures and considerably improved olefin polymerisation characteristics in comparison with analogous compounds based on the two-carbon arm homologues.

Full text of DOI:10.1039/b412381f

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COMMUNICATION
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A new diamido-amine ligand based on three-carbon atom ‘‘arms’’:
synthesis, structures and polymerisation capability of zirconium
derivatives of MeN(CH2CH2CH2NSiMe3)2{
Thomas A. Lowes, Benjamin D. Ward, Robert A. Whannel, Stuart R. Dubberley and Philip Mountford*
Received (in Cambridge, UK) 11th August 2004, Accepted 23rd September 2004
First published as an Advance Article on the web 25th November 2004
DOI: 10.1039/b412381f
affords the protio ligand MeN(CH2CH2CH2N(H)SiMe3)2
(H2N2NC3, 1-H2) as a colourless oil in 84% yield (. 15 g isolated
quantities) after a simple pentane extraction.{ Reaction with
BuLi (2 equiv.) gives the structurally characterised5 dimer
[MeN(CH2CH2CH2N(Li)SiMe3)2]2 (1-Li2) in 91% yield. For the
purposes of making a strict comparison of the 3-carbon arm
N2NC3 ligand with 2-carbon arm analogues under otherwise
identical conditions we selected the ligand N2NC2,Me V (Chart 1)
reported by Bertrand in the context of main group chemistry.6
The synthesis and structures of zirconium complexes of N2NC3
and N2NC2,Me are summarised in Scheme 1. The reactions all
proceed in good yields. Alternatively, the dichloride 2 can be
prepared from 1-Li2 and [ZrCl4(THF)2], and the dibenzyl 3 can be
made from 2 and PhCH2MgBr (2 equiv.). The analogous reaction
of 2 with MeMgBr (2 equiv.) yields [Zr(N2NC3)Me2] 6. The
structures for 2–5 were confirmed by X-ray crystallography5 and
that of 3 is shown in Fig. 1 by way of example.{{ Unlike certain
Group 4 complexes of the two-carbon atom ligand I,2a the
300 MHz 1H NMR spectra of 6 (toluene-d8) show no evidence for
dissociation of the amino NMe nitrogen on the NMR timescale up
to 80 uC at which temperature thermal decomposition becomes
significant.
Scheme 1 clearly shows how the 3-carbon arm N2NC3 ligand
favours fac-coordination. It also illustrates that, with this ligand,
dichloride 2 remains monomeric whereas the otherwise identical
2-carbon arm ligand N2NC2,Me leads to binuclear 4. The binuclear
structure of 4 parallels that formed with the all-SiMe3 ligand I;2 the
mer-coordination found for N2NC2,Me in 5 is paralleled by the
coordination of the N-mesityl ligand II in its dialkyl zirconium
derivatives.3a Points of note in the structure of 3 include the well-
defined trigonal bipyramidal geometry at Zr with the longer ligand
arms allowing the metal to be more fully embraced. The
Zirconium compounds of the new diamido-amine ligand
MeN(CH2CH2CH2NSiMe3)2 feature significantly different
molecular structures and considerably improved olefin poly-
merisation characteristics in comparison with analogous
compounds based on the two-carbon arm homologues.
Early transition metal complexes of polydentate amide ligands1a,b
have been shown to be important in the polymerisation1c and,
more recently, hydroamination1d of olefins. Most relevant to our
present contribution are Group 4 (especially Zr) complexes of
diamido-amine ligands (Chart 1).2 Cloke2a and Horton2b first
introduced the all-SiMe3-substituted systems I (N2NC2,TMS) which
were susceptible to intramolecular activation of the amide-bound
SiMe3 groups. Schrock has reported the living polymerisation of
1-hexene with the mesityl functionalised analogue II.3a The SiMe3-
centred deactivation reactions of complexes of I and the success of
II and related3b non-SiMe3 systems have led to the view3c that
SiMe3 amide N-substituents are incompatible with cationic olefin
polymerisation catalysts. The flexible ligand II can bind with fac or
mer coordination. To force the apparently more favourable fac
diamide-donor mode, a recent focus has been on the tripod-like
ligand IV4a which is a modification of Gade’s original SiMe3-
functionalised ligand III.4b Despite the attractiveness of these latter
ligands, their multistep syntheses involve organic azide intermedi-
ates and a high temperature and pressure autoclave first step.4b
Here we report a new 3-carbon ‘‘arm’’ diamido-amine ligand that
is available in multigram quantities from commercially available
starting materials, and which exclusively affords fac coordination
and exhibits promising olefin polymerisation behaviour, even with
amide-SiMe3 substituents.
Reaction of the commercially available MeN(CH2CH2-
CH2NH2)2 with ClSiMe3 (2 equiv.) in the presence of NEt3
Chart 1
{ Electronic supplementary information (ESI) available: characterising
data for the new compounds. See http://www.rsc.org/suppdata/cc/b4/
b412381f/
Scheme 1 (i) [ZrCl2(NMe2)2(THF)2], yields 52% (for 2) and 77% (for 4);
(ii) [Zr(CH2Ph)4], yields 70% (for 3) and 45% (for 5).
*philip.mountford@chem.ox.ac.uk
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Fig. 2 GPC traces for polyethylene produced by [Zr(N2NC3)X2] (X 5 Cl
2, CH2Ph 3, Me 6), [Zr(N2NC2,Me)Cl2]2 4 and [Zr(N2NC2,TMS)Cl2]2 8.
than the 2-carbon analogue 5 under identical conditions. The
dimethyl compound 6 has a lower activity than that of the dibenzyl
3 (but similar polymer is formed, Fig. 2). This perhaps points to an
activator effect4a where, in the poorly polar toluene medium used
for the polymerisation, there may in fact be some self-trapping of
the 7+ prior to enchainment of monomer. Alternatively, 7+ could
interact more strongly with the AliBu3 scavenger present than the
cation derived from 3 does. Such factors will be the subject of
future studies.
Fig. 1 Molecular structure of [Zr(N2NC3)(CH2Ph)2] 3. Selected data:
Zr(1)–N(1) 2.039(2), Zr(1)–N(2) 2.517(2), Zr(1)–N(3) 2.053(2), Zr(1)–C(14)
˚
2.319(2), Zr(1)–C(21) 2.327(3) A.
conformations of the two C3 arms are different such that one
SiMe3 group (Si(1)) orientates towards the equatorial benzyl ligand
and the other one is orientated away. The Zr–N distances for 3 are
comparable to those reported for the compounds of the diamido-
pyridine ligands III and IV.
The H and 13C NMR data (100 uC, C6D4Cl2) for the soluble
1
portion of the solid, free-flowing polymers formed by the new
catalysts suggest they are non-branched and without detectable
vinyl end-groups. No significant levels of hydrocarbon or other
impurity were detected. The GPC data (Fig. 2) show that all three
catalyst systems 2/MAO, 3/TB and 6/TB produce rather similar
polymers containing low and high molecular weight fractions.
Under identical conditions the polymers formed for the 2-carbon
chain analogues are very broad and multimodal. Although
the polymers formed by 2/MAO and 3/TB each have a high
molecular weight fraction, the prominent low molecular weight
components have polydispersity indices (PDIs) between 1.5 (6/TB;
Mw 5 2.6 6 103) and 1.8 (2/MAO; Mw 5 890). We believe that
the larger PDIs for the more active 2/MAO and 3/TB systems
can be attributed to the non-isothermal experimental conditions
(DTmax between 20 and 50 uC were recorded). Indeed, preliminary
results show that diluting the catalyst solution (and reducing
the exotherm output) for 2/MAO (at constant Zr : Al ratio)
retains the lower molecular weight component and significantly
reduces the higher molecular weight material. Further work on
these aspects are in progress as well as extending the studies to
a-olefins.
With regard to olefin polymerisation catalysis, evidence of well-
defined alkyl cations is essential. Dimethyl [Zr(N2NC3)Me2] 6
reacts cleanly with [CPh3][B(C6F5)4] (TB, 1 : 1 ratio) in C6D5Br
to form [Zr(N2NC3)Me]+ 7+. There is no evidence for SiMe3
activation at room temperature, and all data point to 7+ being fully
solvent-separated from the anion. We note that the analogous
reaction with ligand IV gives a self-trapped binuclear m-methyl
cation with a {Zr2Me2(m-Me)} unit,4a highlighting again how the
new ligand N2NC3 helps enforce the formation of mononuclear
species.
All three compounds [Zr(N2NC3)X2] (X 5 Cl 2, CH2Ph 3, Me
6) are active for the polymerisation of ethylene (Table 1) with very
favourable polydispersities as indicated by the gel permeation
chromatography (GPC) data (Fig. 2). Selected data for
[Zr(N2NC2,Me)X2]n (X 5 Cl, n 5 2 4; X 5 CH2Ph, n 5 1 5)
and the previously reported2 [Zr(N2NC2,TMS)Cl2]2 8 under the
same conditions are presented for comparison.
On MAO activation (Al : Zr ratio 5 1500 : 1), the dichloride 2
(3-carbon arm) has an activity that is about two orders of
magnitude higher than that for the 2-carbon arm analogue 4. A
very similar position emerges for the previously reported 8. With
TB activation, the dibenzyl 3 is again considerably more active
Although the data in Table 1 and Fig. 2 show that the silylated
system N2NC3 has considerable merit and promise for future
development (with the added benefit of the inexpensive and
facile introduction of different SiR3 amide N-substituents), it is
clear from the literature that one should also have access to
N-arylated homologues. Therefore we also report here our
preliminary results that arylation of MeN(CH2CH2CH2NH2)2
with mesityl bromide using standard procedures7 affords
MeN(CH2CH2CH2N(mesityl)H)2 in ca. 50% isolated yield.{
Complexation reactions of this ligand are underway, together
with polymerisation studies of the compounds so formed. We will
report on this work in due course.
Table 1 Polymerisation activities for [Zr(N2NC3)X2] (X 5 Cl 2,
CH2Ph 3, Me 6), [Zr(N2NC2,Me)X2]n (X 5 Cl 4, CH2Ph 5) and
[Zr(N2NC2,TMS)Cl2]2 8a
Dichloride
pre-catalystc (avg. Mw)
Activityb
Dialkyl
pre-catalystd (avg. Mw)
Activityb
2
4
8
110 (1.81 6 105)
3
5
6
164 (2.48 6 105)
0.8 (not measured)
1.3 (7.52 6 105)
4.0 (1.46 6 106)
47 (1.29 6 104)
Conditions: 10 or 20 mmol precatalyst, 250 cm3 toluene; 5 bar
a
b
ethylene; run time 60 min; To 22 ¡ 3 uC. In kg(PE)/mol(Zr)/h/bar.
In conclusion, we have introduced a new, simple and readily-
available diamido-donor ligand, established its capability in areas
1500 equiv. MAO. 1 equiv. TB and 250 equiv. AliBu3.
c
d
114 | Chem. Commun., 2005, 113–115
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1 (a) L. H. Gade, Chem. Commun., 2000, 173; (b) R. Kempe,
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2601.
of current interest and have demonstrated how its chemistry and
applications may be developed in the future.
We thank the EPSRC and Leverhulme Trust for support and
Albermarle for samples of MAO.
2 (a) F. G. N. Cloke, P. B. Hitchcock and J. B. Love, J. Chem. Soc.,
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5 R. A. Whannel, B. D. Ward, T. A. Lowes, A. R. Cowley,
S. R. Dubberley and P. Mountford, unpublished results.
6 N. Emig, H. Nguyen, H. Krautscheid, R. Re´au, J.-B. Cazaux and
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Thomas A. Lowes, Benjamin D. Ward, Robert A. Whannel,
Stuart R. Dubberley and Philip Mountford*
Chemistry Research Laboratory, University of Oxford, Mansfield Road,
Oxford, UK OX1 3TA. E-mail: philip.mountford@chem.ox.ac.uk
Notes and references
{ Crystal data for [Zr{MeN(CH2CH2CH2NSiMe3)2}(CH2Ph)2] (3):
C27H47N3Si2Zr1, Mw 5 561.09, orthorhombic, Pna 21, a 5 20.9927(4),
˚
b 5 12.0115(2), c 5 12.1022(2) A, a 5 90.00, b 5 90.00, c 5 90.00u,
3
U 5 3051.6(1) A , Z 5 4, F(000) 5 1180.68, T 5 150 K, Nonius Kappa
˚
CCD, Mo-Ka radiation, 2.91 ¡ 2h ¡ 27.48u, 6677 independent reflections,
5525 reflections I . 3s(I), R 5 0.0273, Rw 5 0.0262. The structure was
solved using the CRYSTALS software suite.8 Notes on refinement: the
refined Flack parameter of 0.50(3) is indicative of an intimately twinned
structure since Friedel pairs were collected but not merged. CCDC 247514.
See http://www.rsc.org/suppdata/cc/b4/b412381f/ for crystallographic data
in .cif or other electronic format.
8 P. W. Betteridge, J. R. Carruthers, R. I. Cooper, K. Prout and
D. J. Watkin, J. Appl. Crystallogr., 2003, 36, 1487.
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Chem. Commun., 2005, 113–115 | 115