2174 Organometallics, Vol. 28, No. 7, 2009
Dorcier et al.
One of the advantages of surface organometallic chemistry
(SOMC) is the ability to create stable grafted complexes
presenting coordinative unsaturation, in particular due to the
impossibility of bimolecular deactivation.21 Catalytic systems
derived from the SOMC approach also exhibit a very high
percentage of active sites (“single sites”). One of the easiest
ways to obtain such complexes covalently bound to silica is
the reaction of M-C bonds of MRn alkyl complexes with one
surface silanol to form a SiO-M bond. In particular, with this
approach, well-defined isolated (2,3-dimethyl-1,4-[(2′,6′)-diiso-
propylphenyl]-N,N′-diazadiene)(CH2SiMe3)Fe(II) centers grafted
onto Aerosil silica were sythesized,22 and active Zr ethylene
polymerization catalysts were obtained from well-defined
zirconium complexes grafted onto alumina, silica-alumina, or
activated silica supports. The activity observed for the Zr grafted
complexes was attributed to the presence of some “cation-like”
surface species,23 which could be described as “floating
cations”24,25 (e.g., [Cp*Zr(Me)2 · NEt2Ph]+) on a silica correctly
activated by B(C6F5)3.
Boennemann has shown that hydroxyl groups activated by
the gaseous Lewis acid BF3 on the surface of the support serve
as effective anchoring sites for organometallic complexes of
nickel cyclooctadienyl. The resulting species described as
{SiO2}O-(BF3)-Ni-(η1,η2-C8H13) are active for the oligo-
merization of olefins.26 We wish to report here a new method,
which consists of grafting first the molecular complex [(R-
diimine)Ni(CH2SiMe3)2] (1) (R-diimine ) (2,6-iPr2C6H4)-
NdCMe-CMedN(2,6-iPr2C6H4)),27 to obtain the well-defined
[t SiO-Ni(R-diimine)(CH2SiMe3)] supported onto Aerosil SiO2-
(700) by the SOMC method. The resulting surface organometallic
complex can then be activated by BF3 to form an active catalyst
for the polymerization of ethylene, which may be a cationic
R-diimine nickel species obtained by coordination of the strong
Lewis acid on the oxygen bound to Ni.
Figure 1. Infrared spectra of the grafting reaction of [(R-
diimine)Ni(CH2SiMe3)2] (1) onto SiO2-(700) by impregnation: (a)
silica partially dehydroxylated at 700 °C for 15 h; (b) the same
sample, after reaction with 1 at 25 °C.
respectively, to the ν(sp2 C-H) and ν(CdN) vibration. Moreover, a
broadband also appeared at 3695 cm-1 (ν0). This latter band,
observed in the case of several supported complexes, is
attributed to the interaction of residual silanols with the ligands
coordinated to the metal.29 The irreversible disappearance of
free silanols and the appearance of ν(CH), ν(CdN), and δ(CH) bands
is in agreement with a chemical grafting of 1 and the formation
of a (t SiO-Ni) bond.
The reaction of [(R-diimine)Ni(CH2SiMe3)2] (1) and SiO2-(700)
(0.7 OH/nm2, i.e., 0.22 ( 0.03 mmol OH/g of SiO2-(700)) was also
carried out in a benzene solution at 25 °C on a larger scale (ca.
1 g). After three washing cycles with benzene and drying under
vacuum at 25 °C, a blue solid was obtained. The Ni loading was
0.76 wt %, according to elemental analysis (0.13 mmol Ni/g),
which corresponds to a partial consumption of the surface silanols
(ca. 60% of the 0.22 mmol OH/g of SiO2-(700)). This is consistent
with what has already been observed when monitoring this reaction
by in situ IR spectroscopy (vide supra). During the grafting reaction,
tetramethylsilane (SiMe4) was evolved and quantified by GC
analysis: 1.0 ( 0.1 SiMe4/grafted Ni. This is consistent with the
exchange of only one neosilyl group of the complex 1 by one siloxy
group of SiO2-(700). Moreover, according to the elemental analysis
of the sample, the resulting solid contained, respectively, 5.15 wt
% C and 0.42 wt % N. This corresponds to 33 ( 3 C/Ni (expected
32 for 2) and 2.3 ( 0.35 N/Ni (expected 2 for 2). All of these
results are thus in full agreement with the formation of the surface
species [(t SiO)Ni(R-diimine)(CH2SiMe3)]SiO2-(700) (2) (Scheme 1).
According to a molecular modeling approach using the Tripos force
field of the Sybyl program package, the maximum loading for this
surface fragment due to its size is 0.17 mmol per g of SiO2-(700)
support. This means that ca. 75% of this theoretical maximum
surface coverage would be reached for this complex with the
consumption of ca. 60% of the support surface silanols.
Results and Discussion
During the reaction of a disk of SiO2-(700) (ca. 30 mg) with a
benzene solution of [(R-diimine)Ni(CH2SiMe3)2] (1), the solid
turned blue. The sample was further washed twice with toluene
and pentane, to remove the excess of complex, and then dried
under vacuum (10-5 torr, 2 h). It can be observed by IR that
the band attributed to isolated silanol groups, ν(O-H) at 3747
cm-1, disappeared almost completely (Figure 1). Concomitantly,
other vibration bands appeared in the 3000-2700 cm-1 (ν2 to
ν4) and 1500-1300 cm-1 (δ6 to δ10) regions, which are assigned,
respectively, to ν(CH) and δ(CH) vibrations of the hydrocarbyl
groups of the neosilyl and diimine ligands,28 while the bands
at 3070 cm-1 (ν1) and 1586 cm-1 (ν5) are only characteristic of
the diimine ligand of the nickel-supported complex and assigned,
1
(21) Lefebvre, F.; Thivolle-Cazat, J.; Dufaud, V.; Niccolai, G. P.; Basset,
J. M. Appl. Catal., A 1999, 182, 1–8.
The H MAS NMR spectrum of 2 displays broads signals,
which were assigned by comparison with the spectrum of the
molecular complex in solution (Figure S1 in the Supporting
Information) to the aromatic protons (C3-H and C4-H, δ at
ca. 7 ppm), the isopropylic methyne protons (C5-H, δ at ca.
3.5 ppm), the remaining surface silanols (t SiO-H, δ at ca.
1.8 ppm), the protons of the methyl groups of isopropyls and
of the methylene of neosilyls (C6H3 and C9H2, δ at ca. 0.3-1.5
ppm), and the protons of the methyl groups in R position of
imines and of the neosilyls (C8H3 and C10H3, δ at ca. -0.5 to
(22) Roukoss, C.; Fiddy, S.; de Mallmann, A.; Rendon, N.; Basset, J. M.;
Kuntz, E.; Coperet, C. Dalton Trans. 2007, 5546–5548.
(23) Jezequel, M.; Dufaud, V.; Ruiz-Garcia, M. J.; Carrillo-Hermosilla,
F.; Neugebauer, U.; Niccolai, G. P.; Lefebvre, F.; Bayard, F.; Corker, J.;
Fiddy, S.; Evans, J.; Broyer, J. P.; Malinge, J.; Basset, J. M. J. Am. Chem.
Soc. 2001, 123, 3520–3540.
(24) Walzer, J.; Flexer, J. U.S. Patent 5,643,847, 1997.
(25) Millot, N.; Soignier, S.; Santini, C. C.; Baudouin, A.; Basset, J. M.
J. Am. Chem. Soc. 2006, 128, 9361–9370.
(26) Bonnemann, H.; Jentsch, J. D. Appl. Organomet. Chem. 1993, 7,
553–565.
(27) Schleis, T.; Spaniol, T. P.; Okuda, J.; Heinemann, J.; Mulhaupt,
R. J. Organomet. Chem. 1998, 569, 159–167.
(28) Silverstein, R. M.; Bassler, G. C.; Girolami, G. S. Spectroscopic
Identification of Organic Compounds; Wiley: New York, 1991.
(29) Nedez, C.; Theolier, A.; Lefebvre, F.; Choplin, A.; Basset, J. M.;
Joly, J. F. J. Am. Chem. Soc. 1993, 115, 722–729.