Mono(amidinate) yttrium alkyl complexes: The effect of ligand variation on ethene polymerization catalysis

Article abstract of DOI:10.1002/zaac.200600145

Yttrium dialkyl complexes were prepared for three different 2,6-diisopropylphenyl-substituted benzamidinate ligands. The electron-donating ability of the parent ligand (L1) was decreased by a pentafluorophenyl backbone substituent (L2), and enhanced by addition of a pendant Lewis base (L3). The corresponding cationic monoalkyl species were generated and compared in catalytic ethene polymerization.

Full text of DOI:10.1002/zaac.200600145

DOI: 10.1002/zaac.200600145
Mono(amidinate) Yttrium Alkyl Complexes: The Effect of Ligand Variation on
Ethene Polymerization Catalysis1)
Sergio Bambirra, Edwin Otten, Daan van Leusen, Auke Meetsma, and Bart Hessen
Groningen/The Netherlands, Center for Catalytic Olefin Polymerization, Stratingh Institute for Chemistry and Chemical Engineering,
University of Groningen
Received May 5th, 2006.
Dedicated to Professor Glen Deacon on the Occasion of his 70th Birthday
Abstract. Yttrium dialkyl complexes were prepared for three diffe-
rent 2,6-diisopropylphenyl-substituted benzamidinate ligands. The
electron-donating ability of the parent ligand (L1) was decreased
by a pentafluorophenyl backbone substituent (L2), and enhanced
by addition of a pendant Lewis base (L3). The corresponding cat-
ionic monoalkyl species were generated and compared in catalytic
ethene polymerization.
Chart
Keywords: Yttrium; Catalysis; Polymerization
The chemistry of cationic rare earth metal (Ln) alkyl species is a
field that was only recently been explored [1Ϫ3]. Cationic species
of the type [LLnR]ϩ with a variety of monoanionic ancillary li-
gands have been shown to be effective catalysts for the polymeriz-
ation of ethene [4Ϫ6], the hydroamination/cyclization of aminoalk-
enes [8, 9] and the linear dimerization of alkynes [10]. We recently
reported cationic rare earth metal alkyl species with a sterically de-
manding amidinate ligand [PhC(NAr)2]Ϫ (L1) (Ar ϭ 2,6-iPr2C6H3)
[11, 12]. It was observed for Ln ϭ Y that upon generation of the
cations [L1YR2(THF)x] (x ϭ 1, 2) [11], with either one or two do-
nor THF molecules coordinated to the metal, the one with x ϭ 1
effects living polymerization of ethene, whereas the species with
x ϭ 2 required the presence of an aluminum alkyl species (TiBAO)
to show activity. It was proposed that the additional Lewis acid is
required to capture of one of the THF molecules, freeing a coordi-
nation site to capture the ethene substrate.
Results and Discussion
To follow up on these observations we have prepared two new ami-
dinate ligands: one contains an electron withdrawing pentafluoro-
phenyl group on the amidinate backbone [C6F5C(NAr)2] (Ar ϭ
2,6-iPr2C6H3) (L2), with the intention to make the metal center
more electrophilic. The other, [PhC(N(CH2)2NMe2)(NAr)] (L3),
carries a pendant dimethylamine functionality on one of the nitro-
gen atoms, anticipated to coordinate to the metal center to yield a
trihapto-bound ligand.
The complexes were generated following the well-established alkane
elimination route [13, 14], in which the homoleptic trialkyl complex
[Y(CH2SiMe3)3(THF)2] [15] was reacted with L2H or L3H. Crys-
tallization from pentane afforded the dialkyl complexes 2 and 3,
respectively, in good yields (scheme 1). As was the case with ligand
L1, complex 2 can also be obtained with an additional coordinated
molecule of THF (2·THF), depending on the chosen conditions.
At room temperature, both complexes 2 and 3 feature simple NMR
spectra (C6D6, C7D8), suggesting an averaged C2v (2) or Cs (3) sym-
metry. The H-NMR NMe2 resonance in 3 broadens significantly
* Prof. Dr. B. Hessen,
Stratingh Institute for Chemistry and Chemical Engineering
University of Groningen
Nijenborgh 4
9747 AG Groningen (The Netherlands)
E-mail: B.Hessen@rug.nl
1
at Ϫ70 °C but does not reach decoalescence at Ϫ90 °C. This sug-
gests that the pendant amine is coordinated to the metal, but also
that the complex is highly fluxional. In both complexes the NMR
resonances of the YCH2 group show characteristic coupling to the
1
89Y (I ϭ /2) nucleus.
1) Paper presented at the XVIIIth Tage der Seltenen Erde (Terrae
Rarae 2005) at Bonn-Röttgen/Germany, November 30th Ϫ Decem-
ber 2nd, 2005 (www.Terrae-rarae.de).
Complexes 2 and 3 were characterized by single crystal X-ray dif-
fraction, and their molecular structures are shown in Figs. 1 and
2, respectively. Complex 2 is isostructural to 1 [11]. The amidinate
ligand is N,NЈ-dihapto-bound to yttrium with slightly different
Supporting information for this article is available on the
WWW under http://www.wiley-vch.de/home/zaac or from the
author
˚
˚
YϪN bond distances of 2.3989(17) A (YϪN1) and 2.3675(16) A
1950
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2006, 632, 1950Ϫ1952

Mono(amidinate) Yttrium Alkyl Complexes
Fig. 2 Molecular structure of 3, 50 % probability ellipsoids.
˚
Selected interatomic distances/A and angles/deg: Y1ϪO1 2.374(3), Y1ϪN11
Scheme 1
2.620(3), Y1ϪN12 2.340(3), Y1ϪN13 2.464(3), YϪC124 2.431(4), YϪC128
2.435(4), N12ϪC15 1.328(5), N13ϪC15 1.339(5), C124ϪYϪC128
116.62(13), N12ϪYϪN13 55.78(11).
As reported for other amidinate dialkyl systems [12], the corre-
sponding cationic monoalkyl species can readily be generated by
reaction of 2 or 3 with [HNMe2Ph][B(C6F5)4]. In C6D5Br as the
solvent at Ϫ20 °C, the cation [L3Y(CH2SiMe3)(THF)]ϩ shows
1H-NMR resonances consistent with an asymmetric structure with
a coordinated NMe2-group.
In Table 1, a series of ethene polymerization experiments with the
various species is listed. As described before [11], compound 1
yields a living ethene polymerization catalyst at 50 °C, whereas
1·THF is inactive under the same conditions. This one can only be
activated in the presence of an excess of aluminoxane Lewis acid
(TiBAO). In contrast, the fluorinated analogue 2 yields a cation
that is inactive at 50 °C. At higher temperatures it is active, but
gradually loses the living nature of the polymerization, yielding at
100 °C polyethene with a relatively low Mw and a polydispersity of
2. Interestingly, the cation derived from 2 can be active at 50 °C,
provided an additional equivalent of B(C6F5)3 Lewis acid is added.
The polyethene produced then has again a narrow polydispersity.
In the presence of TiBAO, 2·THF has an activity slightly superior
to that of the non-fluorinated analogue. In great contrast, the sys-
Fig. 1 Molecular structure of 2, 50 % probability ellipsoids.
Table 1 Catalytic ethene polymerization with 2 and 3 in conjunc-
tion with [HNMe2Ph][B(C6F5)4] activatora)
˚
Selected interatomic distances/A and angles/deg: YϪO 2.3390(17), YϪN1
2.3989(17), YϪN2 2.3675(16), YϪC32 2.385(2), YϪC36 2.379(2), N1ϪC13
1.338(3), N2ϪC13 1.326(3), C32ϪYϪC36 115.16(8), N1ϪYϪN2 56.68(6).
Dialkyl
Temp PE yield/g Activityc 10Ϫ3 Mw Mw/Mn
1
50
50
7.6
Ϫn.p.
18.9
Ϫn.p.
5.85
1.83
460
Ϫ
1146
Ϫ
350
102
280
1252
Ϫ
854
Ϫ
688
Ϫ
126
29
698
213
Ϫ
1.2
Ϫ
1.9
Ϫ
1.7
2.0
1.3
2.48
Ϫ
1·THF
1·THF /TiBAOb) 50
(YϪN2). The main difference with 1 is that 2 shows a somewhat
˚
˚
2
2
2
50
80
100
50
shorter YϪO(THF) bond distance: 2.3390(17) A vs. 2.361(2) A for
1, consistent with an increased electron deficiency of the metal
center in 2. The molecular structure of 3 shows that the amino-
amidinate is indeed tridentately bound to the yttrium atom, adopt-
ing an approximately facial [16, 17] coordination mode. The nitro-
gen atom N12 is non-planar: the sum of the angles around N12 is
344.7°. The nitrogen atom of the Me2N group (N11) is located
trans to the oxygen atom of the THF ligand (N11ϪYϪO ϭ
2/B(C6F5)3
4.60
2·THF /TiBAOb) 50
20.66
Ϫn.p.
Ϫn.p.
3/B(C6F5)3
3/TiBAO
50
50
Ϫ
Ϫ
Ϫ
a) Conditions: toluene solvent (150 ml), 50 °C, 10 µmol Y dialkyl complex,
10 µmol activator, 5 bar ethene pressure, 20 min run time. b) Toluene solvent
(200 ml), 100 µmol Al. c) kg(PE) mol(Y)Ϫ1 hϪ1 barϪ1
.
˚
170.73(2)°) with a YϪN11 distance of 2.620(3) A.
Z. Anorg. Allg. Chem. 2006, 1950Ϫ1952
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1951

S. Bambirra, E. Otten, D. van Leusen, A. Meetsma, B. Hessen
tem 3, with the pendant Lewis base on the ligand, is inactive under
any of the applied conditions.
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These results may suggest that the actual active species in this
family of catalysts is the base-free amidinate yttrium alkyl cation.
The presence of an additional (but labile) Lewis base like THF can
stabilize the catalyst and suppress chain transfer (leading to living
catalyst behavior for 1 at moderate temperatures). When the Lewis
base is too strongly associated, the catalyst activity is quenched.
We are presently investigating whether this hypothesis is correct
or not.
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Supporting Material Available. Text giving full experimental and
characterization data of the various compounds disclosed in this
paper may be obtained at http://hessen.fmns.rug.nl/Supportinginfor-
mation/ZAAC-SupportMaterial-Bambirra.pdf. Crystallographic
data for the structures of 2 and 3 have been deposited with the
Cambridge Crystallographic Data Centre, CCDC no. 601755 &
601756. Copies of the data can be obtained free of charge on appli-
cation to The Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (Fax: ϩ44Ϫ1223/336033; e-mail for inquiry:
fileserv@ccdc.cam.ac.uk).
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Acknowledgment. This investigation was financially supported by
ExxonMobil Chemicals and the National Research School Combi-
nation Catalysis (NRSC-C).
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1952
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 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2006, 1950Ϫ1952