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with the tritylium salt [Ph3C][B(C6F5)4][5a,19] were also which leads to cationic species by alkyl abstraction to sur-
studied. More cationic structures were later characterized face aluminum sites. We very recently revisited the grafting
and of particular interest to our study are alkoxy or phen- of the organometallic complexes Ti(CH2Ph)4 and
oxy ligands, as they best approximate a putative silica- Zr(CH2Ph)4 on hydroxy-rich silica (SiO2-200) and highly de-
grafted benzyl group 4 complex.
hydroxylated silica (SiO2-700). The resulting bipodal and
monopodal surface species [(ϵSiO)2M(CH2Ph)2] and
[ϵSiOM(CH2Ph)3] (M = Ti, Zr), respectively, were unam-
biguously characterized for the first time and presented an
unexpected polymerization activity in the absence of any
activating agent.[32] We also recently demonstrated the
highly beneficial influence of a new hybrid material H that
was developed on amorphous silica and features a rigid
phenol grafting site as a substitute for silica hydroxy groups
and, thus, affords reduced steric and electronic interactions
from the surface.[33] In this publication, the influence of the
phenol spacer will be studied on the grafting of
Zr(CH2Ph)4, and the resulting surface species will be struc-
turally compared to its silica-supported counterpart
Scheme 1. Homoleptic (A) and aryloxide (B) zwitterionic benzyl
complexes of zirconium following benzyl abstraction by B(C6F5)3.
Rothwell et al. structurally characterized mixed benzyl– [ϵSiOZr(CH2Ph)3]. We will also explore and fully charac-
aryloxide complexes [(ArO)x(MBn4–x)] (x = 1, 2; M = Ti, terize the cationic surface species formed by reaction with
Zr; ArO = 2,6-di-tert-butylphenoxide,[20] 2,6-diphenylphen- B(C6F5)3. The influence of both the new hybrid material
oxide,[21] or 2,6-diphenyl-3,5-dimethylphenoxide).[22] Subse- and the cocatalyst will be assayed in ethylene polymeriza-
quent activation reactions with B(C6F5)3 yielded zwitter- tion.
ionic complexes through benzyl abstraction (Scheme 1, B).
Similarly to what was observed for the homoleptic com-
Results and Discussion
plexes, upon abstraction of a benzyl group, the borate anion
remains in the coordination sphere of the metal through an
η6 coordination of the aromatic ring.[22]
The
formation
of
the
monopodal
species
[ϵSiOZr(CH2Ph)3] (2) was performed according to a litera-
ture procedure. The silica (Evonik Aerosil-200) used was
previously dehydroxylated under vacuum (10–5 mbar) at
700 °C (SiO2-700),[32] as was the hybrid material featuring
phenolic grafting sites, which was prepared by first reacting
silica with isobutylaluminum–diethyl ether to yield an alu-
minum isobutyl site [(ϵSiO)2Al(iBu)(Et2O)] concomitantly
with an adjacent [ϵSi(iBu)] fragment. This unique and re-
active [Al(iBu)] species was then selectively reacted with a
hydroquinone spacer to yield [(ϵSiO)2(AlOC6H4OH)-
(Et2O)] (H).[33] Zr(CH2Ph)4 was impregnated on this latter
material and on SiO2-700 by using an excess (1.3 equiv.) of
complex in benzene at room temperature for 2 h. After mul-
tiple washings with benzene followed by evacuation of the
volatiles, the materials 1-H and 1-SiO2-700 were obtained as
yellow powders.
Other good silica models for benzyl complexes include
the calix[4]arene-based complexes studied by Floriani et
al.[23] and the polysilsesquioxane complexes prepared by
Duchateau et al.[24] Cationic structures were obtained in
some cases after reaction with B(C6F5)3; however, these
models suffered from either selectivity issues during the
protonolysis steps or resulted in unwanted dimeric struc-
tures, which are the logical step in the thermodynamic stabi-
lization of the complexes in the homogeneous phase and
are indicative of the limits of molecular compounds as silica
models.
On the other hand, studies related to the use of group 4
metal complexes with benzyl ligands as heterogeneous poly-
merization catalysts are scarce and have not reached the
level of characterization obtained homogeneously. Their use
is mostly described in patent examples, and they are most
often used on activating supports.[25] Patents for the hetero-
genization of benzyl group 4 complexes by reaction with
silica or alumina surfaces have shown interesting polymeri-
zation activities when activated with organoaluminum com-
pounds as far back as 1971[26] and gained in importance
Reactivity of Zr(CH2Ph)4 with the Phenolic Functionalities
of the New Hybrid Material H
Compared to the spectrum of the phenolic material H
with patent applications related to zirconium tetrabenzyl (Figure 1, a), the diffuse reflectance infrared Fourier trans-
grafted on alumina.[27] The groups of Ballard,[28] form (DRIFT) spectrum of 1-H (Figure 1, b) displays new
Yermakov,[29] and Giesemann[30] conducted preliminary bands in the 3000–3100 cm–1 range attributed to νC–H
studies in surface organometallic chemistry by grafting the vibrations from aromatic rings. A new νC=C vibration band
group 4 homoleptic benzyl complexes on silica and alu- at 1600 cm–1 is also observed, which arises from the benzyl
mina. The silica-supported complexes displayed some sur- aromatic ring stretching, whereas the one at 1495 cm–1 over-
prising activity in polymerization, and the formation of laps with the intense νC=C band from the hydroquinone ring
active polymerization catalysts on alumina is to be expected at 1510 cm–1. The δC–H band observed at 1450 cm–1 can be
owing to its dual role as both support and activator,[31] attributed to the methylene group. These bands are similar
Eur. J. Inorg. Chem. 2014, 888–895
889
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