Yttrium alkyl complexes with a sterically demanding benzamidinate ligand: Synthesis, structure and catalytic ethene polymerisation

Article abstract of DOI:10.1039/b208502j

The benzamidinate yttrium dialkyl complexes [PhC-(NAr)₂]Y(CH₂SiMe₃)₂(THF) _n (Ar = 2,6-diisopropylphenyl; n = 1, 2) were prepared; when activated with [PhNMe₂H][B(C₆F₅)

Full text of DOI:10.1039/b208502j

Yttrium alkyl complexes with a sterically demanding benzamidinate
ligand: synthesis, structure and catalytic ethene polymerisation†
Sergio Bambirra, Daan van Leusen, Auke Meetsma, Bart Hessen* and Jan H. Teuben
Centre for Catalytic Olefin Polymerisation, Stratingh Institute of Chemistry and Chemical Engineering,
University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands. E-mail: hessen@chem.rug.nl
Received (in Cambridge, UK) 2nd September 2002, Accepted 7th January 2003
First published as an Advance Article on the web 17th January 2003
The benzamidinate yttrium dialkyl complexes [PhC-
(NAr)2]Y(CH2SiMe3)2(THF)n (Ar = 2,6-diisopropylphenyl;
the higher coordination number of the metal in 4. The Y–O(2)
distance in 4, 2.451(1) Å, is elongated by 0.090 Å relative to the
Y–O(1) distance in 3. This Y–THF interaction in 4 is likely to
be relatively weak, leading to facile loss of a THF molecule.
Similar behaviour was recently reported for the pair of yttrium
compounds Y(CH2SiMe3)3(THF)n (n = 2, 3), where one of the
Y–O distances in the tris-THF adduct is very long (2.500(1) Å)
and in which this THF molecule is readily lost.9
n
=
1, 2) were prepared; when activated with
[PhNMe2H][B(C6F5)4], the species with n = 1 polymerises
ethene to polyethene with a narrow polydispersity.
In contrast to the extensively developed families of cationic
group 4 metal alkyl olefin polymerisation catalysts,1 very few
examples exist to date of well-defined cationic olefin polymer-
isation catalysts for the group 3 and lanthanide metals.2,3 In the
past we reported attempts to prepare yttrium dialkyl complexes,
with amidinate and amidinate-amine ancillary ligands, that
could be converted to active cationic monoalkyl species.4 These
efforts were not successful due to the occurrence of ligand
redistribution, yielding bis(amidinate) compounds, when alkyl
groups other than the highly sterically demanding CH(SiMe3)2
were employed. Here we report the use of a sterically
demanding amidinate ancillary ligand, N,NA-bis(2,6-diisopro-
pylphenyl)benzamidinate, in a successful synthesis of an
amidinate yttrium bis(trimethylsilylmethyl) complex. This
compound can be activated with the Brønsted acid activator
[PhNMe2H][B(C6F5)4] to give an active ethene polymerisation
catalyst, yielding polyethene with a narrow polydispersity.
The sterically demanding amidine ArNNCPh-NHAr (1, Ar =
2,6-Pri2C6H3) was prepared from benzoic acid and the corre-
sponding aniline with the use of silylated polyphosphoric acid,
as reported for other N,NA-diaryl benzamidinates.5 Alter-
natively, it can be obtained in stepwise fashion through the
corresponding imidoyl chloride, as reported by Boeré et al. for
ArNNC(p-tolyl)-NHAr.6 The latter ligand has been used
recently by Tolman et al. to prepare (amidinate)yttrium
bisamide derivatives.7
In C6D6 solution, the NMR spectra of 310 and 4 are
practically indistinguishable (apart from the shift and intensity
of the THF resonances), which may indicate that one of the THF
molecules in 4 is bound reversibly. For 3, the ambient
temperature NMR spectrum suggests C2v symmetry (equivalent
Y-alkyl and N-aryl groups), indicating that rearrangement
around the Y-centre is fast on the NMR timescale.
Reaction of
3
with the Brønsted acid activator
[PhNMe2H][B(C6F5)4] in THF-d8 results in a clean reaction to
As seen by NMR spectroscopy, the amidine 1 reacts cleanly
in benzene-d6 with Y(CH2SiMe3)3(THF)2 (2)8 to give SiMe4
and an amidinate yttrium dialkyl species. Performing the
reaction in pentane, followed by evaporation of the solvent and
by removal of residual THF from the material (by stirring with
additional pentane that was subsequently removed in vacuo),
allowed crystallisation of the dialkyl complex as its mono-THF
adduct, [PhC(NAr)2]Y(CH2SiMe3)2(THF) (3, Scheme 1). Re-
markably, work-up of this reaction mixture by simply concen-
trating and cooling the pentane solution formed initially cleanly
yielded crystals of the corresponding bis-THF adduct,
[PhC(NAr)2]Y(CH2SiMe3)2(THF)2 (4). Both 3 and 4 were
characterised by single crystal X-ray diffraction, and their
structures are shown side by side in Fig. 1.
Scheme 1 Reagents and conditions: i, pentane, 20 °C, 3 h, evaporation to
dryness, pentane extraction; ii, pentane, 20 °C, 3 h, concentration and
cooling to 230 °C.
Both complexes have an N,NA-dihapto-amidinate ligand and
in both cases the THF molecule(s) occupy a position roughly in
the plane defined by the yttrium atom and the two ligand
nitrogen atoms. The alkyl groups are located above and below
that plane, oriented in such a way as to minimise interaction
with the ligand 2,6-Pri2C6H3 groups. The CH2–Y–CH2 angle in
4 is 140.56(6)°, and relaxes to 119.48(19)° in 3. As expected,
the distances between yttrium and the ligating atoms are larger
for 4 than for 3 (generally by about 0.03–0.05 Å) in response to
Fig. 1 Molecular structures of 3 (left) and 4 (right) (hydrogen atoms omitted
for clarity, unlabeled atoms are carbon). Selected interatomic distances (Å)
and angles (°) for 3: Y–N(1) 2.339(3), Y–N(2) 2.369(2), Y–C(32) 2.374(4),
Y–C(36) 2.384(4), Y–O 2.361(2), N(1)–Y–N(2) 57.27(9), C(32)–Y–C(36)
119.5(2), O–Y–C(32) 94.1(1), O–Y–C(36) 91.4(1), Y–C(32)–Si(1)
129.1(2), Y–C(36)–Si(2) 136.1(3); for 4: Y–N(1) 2.395(1), Y–N(2)
2.397(1), Y–C(40) 2.427(2), Y–C(44) 2.433(2), Y–O(1) 2.427(1), Y–O(2)
2.451(1), N(1)–Y–N(2) 55.70(4), C(40)–Y–C(44) 140.56(6), O(1)–Y–O(2)
138.80(4), Y(1)–C(40)–Si(1) 137.32(9), Y–C(44)–Si(2) 135.72(9).‡
† Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b2/b208502j/
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give SiMe4, PhNMe2 and an ionic yttrium alkyl species
formulated as {[PhC(NAr)2]Y(CH2SiMe3)(THF-d8)n}-
abstraction appears to be unfavourable, and that can produce
polyethene with a narrow polydispersity. There are several
examples of cationic transition-metal catalysts that can effect
living linear insertion polymerisation of ethene,14,15 but such
behaviour at a relatively high reaction temperature of 50 °C is
quite unusual.
[B(C6F5)4] (5). In the 13C NMR spectrum,11 the YCH2
resonance in 5 is shifted downfield and the JYC is increased
somewhat relative to the neutral dialkyl 3, which is similar to
the behaviour observed previously for cationic YCH2SiMe3
species.3 In a more weakly coordinating polar solvent, bromo-
benzene-d5, the resulting cationic species was found to be too
unstable for characterisation.
This investigation was supported by ExxonMobil Chemical
Company.
When activated by [PhNMe2H][B(C6F5)4] in toluene in the
presence of ethene, 3 readily polymerises the monomer to yield
polyethene. In Table 1, the data are given for ethene
polymerisation runs at 50 °C with varying run times, from 5 min
to 30 min. It was observed that the Mw of the product increases
from 430.103 over 5 min to 1211.103 over 30 min, with a
remarkably low polydispersity Mw/Mn of around 1.2. Simple
calculations indicate that about 1.1(1) polymer chain per
yttrium is produced in all the runs. This suggests that the
polymerisation of ethene by 3/[PhNMe2H][B(C6F5)4] has living
character, i.e. that chain transfer via b-H elimination is
unfavourable. The exact study of this phenomenon in this
system is complicated by experimental difficulties, such as
inhomogeneity of the reaction mixture by precipitation of the
polymer during the run. This may be associated with the
observation that, although the catalyst productivity per unit of
time decreases with increasing run time (from 1037 kg(PE)
mol21 h21 bar21 over 5 min to 400 kg(PE) mol21 h21 bar21
over 30 min), the low polydispersity of the polymer is
retained.
In contrast with the results for 3, the combination of the bis-
THF adduct 4 with [PhNMe2H][B(C6F5)4] in the presence of
ethene did not result in catalytic polymerisation. Apparently the
bonding of the second molecule of THF to the cationic metal
species is sufficiently strong to suppress catalytic activity,
suggesting that the (amidinate)Y(alkyl)(THF)-cation is respon-
sible for olefin capture and subsequent insertion. In the presence
of an excess of partially hydrolysed tris(isobutyl)aluminium
(TIBAO) scavenger12 (Al to Y ratio 10+1), the combination
4/[PhNMe2H][B(C6F5)4] is effective as ethene polymerisation
catalyst (Table 1, bottom two entries). The Mw of the polyethene
is lower than in the absence of TIBAO, and its polydispersity is
around 2. The polymer Mw does increase with run time to some
extent, and it can be calculated that in both runs approximately
5 polymer chains per yttrium are produced. This behaviour may
be similar to that observed in the polymerisation of ethene by
some group 3 and group 4 metal metallocene catalysts in the
presence of main group metal alkyls, where a polymer chain
may be transferred from the group 3 metal to the main group
metal and vice versa, and where chain termination by b-H
transfer is insignificant.13
Notes and references
‡ Crystallographic data: for 3: C43H69N2OSi2Y, M = 775.11, triclinic,
¯
space group P1, a = 12.537(1), b = 13.340(1), c = 13.925(1) Å, a =
101.964(9), b = 98.18(1), g = 93.65(1)°, U = 2244.6(3) Å3, T = 130 K,
Z = 2, Dc = 1.147 g cm23, m = 13.8 cm21, Enraf-Nonius CAD4-F
diffractometer, l(Mo-Ka) = 0.71073 Å, 9732 unique reflections, final
residuals wR(F2) = 0.1390, R(F) = 0.0547 for 7479 reflections with Fo 4
4s(Fo) and 702 parameters. For 4: C47H77N2O2Si2Y, M = 847.22, triclinic,
¯
space group P1, a = 10.8885(6), b = 11.7658(6), c = 20.830(1) Å, a =
80.246(1), b = 85.581(1), g = 69.055(1)°, U = 2455.8(2) Å3, T = 90 K,
Z = 2, Dc = 1.146 g cm23, m = 12.71 cm21, Bruker SMART APEX CCD
diffractometer, l(Mo-Ka) = 0.71073 Å, 11035 unique reflections, final
residuals wR(F2) = 0.0782, R(F) = 0.0306 for 9712 reflections with Fo 4
4s(Fo) and 795 parameters. CCDC reference numbers 192682 and 192683.
See http://www.rsc.org/suppdata/cc/b2/b208502j/ for crystallographic files
in CIF format.
1 H. H. Brintzinger, D. Fischer, R. Mülhaupt, B. Rieger and R. M.
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10 Selected NMR data for 3: 1H NMR (300 MHz, C6D6): d 3.69 (m, 4 H a-
THF), 1.14 (m, 4 H b-THF), 20.11 (d, 2JYH = 3.0 Hz, 4 H, CH2SiMe3).
13C NMR (75.4 MHz, C6D6): d 70.7 (t, 1JCH = 147.6 Hz, a-THF), 39.5
In conclusion, the use of a sterically encumbered benzamidi-
nate ancillary ligand has enabled the synthesis of a mono-
(amidinate) yttrium dialkyl that, with the Brønsted acid
activator [PhNMe2H][B(C6F5)4], can be converted into a
cationic monoalkyl species. The latter provides an active ethene
polymerisation catalyst system in which chain transfer via b-H
1
1
(dt, JYC = 40.3 Hz, 1JCH = 100.1 Hz, YCH2SiMe3), 24.9 (t, JCH
=
124.4 Hz, b-THF).
1
11 Selected NMR data for 5: H NMR (500 MHz, THF-d8): d 20.15 (d,
2JYH = 3.0 Hz, 2 H, CH2SiMe3). 13C{1H} NMR (125.7 MHz, THF-d8):
d 42.3 (d, 1JYC = 42.7 Hz, YCH2SiMe3).
12 Prepared according to: J. F. van Baar, P. A. Schut, A. D. Horton, T.
Dall’occo and G. M. M. van Kessel, World Pat., WO 00/35974,
2000.
Table 1 Catalytic ethene polymerisation with 3 and 4 in conjunction with
[PhNMe2H][B(C6F5)4] activatora
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Int. Ed. Engl., 1996, 35, 1854; E. G. Samsel, US Pat., 5210338,
1993.
Dialkyl
Time/min PE yield/g Activityc 1023 Mw
Mw/Mn
3a
5
10
20
30
10
5
4.2
6.4
7.6
10.0
0.0
11.1
18.9
1.04
0.79
0.46
0.40
0.0
430
644
854
1269
—
1.2
1.2
1.2
1.1
—
14 M. Brookhart, J. M. DeSimone, B. E. Grant and M. J. Tanner,
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15 For a recent review on living olefin insertion polymerisation, see: G. W.
Coates, P. D. Hustad and S. Reinartz, Angew. Chem., Int. Ed., 2002, 41,
2236.
3a
3a
3a
4a
4/TIBAOa,b
2.67
1.15
361
689
2.1
1.9
4/TIBAOa,b
20
a Conditions: toluene solvent (150 ml), 50 °C, 10 mmol Y dialkyl complex,
10 mmol activator, 5 bar ethene pressure. b Toluene solvent (200 ml), 100
mmol Al. c 103 kg(PE) mol(Y)21 h21 bar21
.
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