Synthesis, structural characterization, catalytic properties, and theoretical study of compounds containing an Al-O-M (M = Ti, Hf) core

Article abstract of DOI:10.1021/ic060538r

Two single oxygen-bridged heterobimetallic oxides of Al(III) with group 4 metals (Ti, Hf) have been prepared. The reaction of LAlMeOH (1) [L = CH(N(Ar)(CMe))₂, Ar = 2,6-iPr₂C₆H₃] with dimethylmetallocenes of Ti

Full text of DOI:10.1021/ic060538r

Inorg. Chem. 2007, 46, 1056−1061
Synthesis, Structural Characterization, Catalytic Properties, and
Theoretical Study of Compounds Containing an Al
Core
−O−M (M ) Ti, Hf)
Prabhuodeyara M. Gurubasavaraj, Swadhin K. Mandal, Herbert W. Roesky,* Rainer B. Oswald,
Aritra Pal, and Mathias Noltemeyer
Institut fu¨r Anorganische Chemie der Georg-August-UniVersita¨t Go¨ttingen, Tammannstrasse 4,
37077 Go¨ttingen, Germany
Received March 30, 2006
Two single oxygen-bridged heterobimetallic oxides of Al(III) with group 4 metals (Ti, Hf) have been prepared. The
reaction of LAlMeOH (1) [L CH(N(Ar)(CMe))2, Ar 2,6-iPr2C6H3] with dimethylmetallocenes of Ti and Hf in
toluene (80 C) and ether (room temperature), respectively, resulted in the formation of LAl(Me)( -O)M(Me)Cp2 [M
Ti (2), Hf (3)] in moderate to good yield. Compounds 2 and 3 were characterized by elemental analysis, IR,
)
)
°
µ
)
NMR (1H and 13C), EI-MS, and single-crystal X-ray structural analysis. Furthermore, compound 2 showed good
catalytic activity in ethylene and styrene homopolymerization, while compound 3 is less active in ethylene
polymerization. The styrene polymerization yields atactic polystyrene.
Introduction
metallic cocatalysts is a very important subject which can
provide high catalytic activity with a low cocatalyst to
catalyst precursor ratio, and this allows unprecedented control
over the polymer microstructure generating new polymers
with improved properties. Well-defined single-site metal-
locene catalysts are slowly replacing the conventional
heterogeneous Ziegler-Natta catalysts.
Recent work from our laboratory described the preparation
of heterobimetallic oxides of aluminum with zirconium which
showed high catalytic activity in the olefin polymerization
reaction.4 Instead of routine long-chain and three-dimensional
cage compounds, we isolated unprecedented aluminum
dihydroxide-containing terminal OH groups LAl(OH)2 [L )
CH(N(Ar)(CMe))2, Ar ) 2,6-iPr2C6H3] by treatment of LAlI2
with KOH containing 10-15% H2O and KH in liquid
ammonia.5 Then, we reported an improved route to LAl-
(OH)2 using a strong nucleophilic reagent N-heterocyclic
carbene as a HCl acceptor for the reaction of LAlCl2 and
stoichiometric amounts of water.6 In the course of the
synthesis of LAlMe(OH) from LAlMeCl, a stepwise process
The synthesis and characterization of heterobimetallic
oxides, which are used as polyfunctional catalysts and
precursors for the preparation of bi- and polymetallic
heterogeneous catalysts, have been the topic of various
academic and industrial studies1 since the discovery of
catalytic olefin polymerization by Ziegler and Natta. These
heterogeneous catalysts have complicated structural features
and are insoluble in solvents advantageous for polymerization
reactions. Investigations by Sinn and Kaminsky2 reveal that
soluble metallocene catalysts in combination with methyl-
alumoxane (MAO) achieve extremely high activities in the
polymerization of olefins leading to new developments in
this field. These investigations are accompanied by an
increased understanding of the factors that are important for
stabilizing polymerization-active metal centers and control-
ling their activity and selectivity.3 The design and synthesis
of new transition metal precursors and main group organo-
* To whom correspondence should be addressed. E-mail: hroesky@
gwdg.de.
(1) (a) Cope´ret, C.; Chabanas, M.; Saint-Arroman, R. P.; Basset, J.-M.
Angew. Chem. 2003, 115, 164-191.; Angew. Chem., Int. Ed. 2003,
42, 156-181. (b) Roesky, H. W.; Haiduc, I.; Hosmane, N. S. Chem.
ReV. 2003, 103, 2579-2595.
(2) Andresen, A.; Cordes, H.-G.; Herwig, J.; Kaminsky, W.; Merck, A.;
Mottweiler, R.; Pein, J.; Sinn, H.; Vollmer, H.-J. Angew. Chem. 1976,
88, 689-690; Angew. Chem., Int. Ed. 1976, 15, 630-632.
(3) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391-1434.
(4) Bai, G.; Singh, S.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G.
J. Am. Chem. Soc. 2005, 127, 3449-3455.
(5) Bai, G.; Peng, Y.; Roesky, H. W.; Li, J.; Schmidt, H.-G.; Noltemeyer,
M. Angew. Chem. 2003, 115, 1164-1167; Angew. Chem., Int. Ed.
2003, 42, 1132-1135.
(6) Jancik, V.; Pineda, L. W.; Pinkas, J.; Roesky, H. W.; Neculai, D.;
Neculai, A. M.; Herbst-Irmer, R. Angew. Chem. 2004, 116, 2194-
2197; Angew. Chem., Int. Ed. 2004, 43, 2142-2145.
1056 Inorganic Chemistry, Vol. 46, No. 4, 2007
10.1021/ic060538r CCC: $37.00
© 2007 American Chemical Society
Published on Web 01/20/2007

Compounds Containing an Al-O-M (M ) Ti, Hf) Core
Table 1. Crystal Data and Structure Refinement for Compounds 2 and
3
was followed with 1 equiv of water. Recently, we became
interested in the preparation of heterodinuclear complexes
of aluminum with early transition metals and the use of such
systems as catalysts for the polymerization of olefins.4
Herein, we report the reaction of LAlMeOH (1) [L ) CH-
(N(Ar)(CMe))2, Ar ) 2,6-iPr2C6H3] with methylated met-
allocene derivatives of group 4 metals (Ti, Hf) leading to
the isolation and characterization of discrete new µ-oxo
heterobimetallic systems. Compounds 2 and 3 have been
tested as catalysts for the polymerization of olefins. Com-
pound 2 showed high catalytic activity with a low MAO to
catalyst ratio in ethylene and styrene polymerization, whereas
compound 3 showed low activity in the polymerization of
ethylene.
2
3
formula
fw
C41H57AlN2OTi
668.77
C41H57AlHfN2O
799.36
color
yellow
100(2)
triclinic
P1h
9.572(2)
10.422(2)
20.060(3)
90.13(2)
90.55(2)
114.14(2)
1886.0(6)
2
colorless
133(2)
triclinic
temp (K)
cryst syst
space group
a (Å)
b (Å)
c (Å)
P1h
9.921(2)
10.276(2)
19.616(3)
88.28(2)
87.17(2)
68.47(2)
1857.9(6)
2
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
F
calcd (g cm-3
)
1.216
2.464
720
1.429
2.864
820
µ (mm-1
F(000)
)
Experimental Section
θ range (deg)
4.41-58.99
-10 e h e 10
-11 e k e 11
-22 e l e 20
2.08-24.81
-11 e h e 11
-12 e k e 12
-23 e l e 23
6373/1/431
full-matrix
least-squares on F2
1.028
0.0178,
0.0369
0.0221,
0.0375
index ranges
General Comments. All experimental manipulations were
carried out under an atmosphere of dry nitrogen using standard
Schlenk techniques. The samples for spectral measurements were
prepared in a glovebox. The solvents were purified according to
conventional procedures and were freshly distilled prior to use.
Titanocene and hafnocene dichlorides were purchased from Aldrich
and Fluka, respectively, and were used as received. Cp2TiMe2 and
Cp2HfMe2 were prepared by the procedure which was published
elsewhere.7 NMR spectra were recorded on a Bruker Avance 500
instrument, and the chemical shifts are reported with reference to
tetramethylsilane (TMS). IR spectra were recorded on a Bio-Rad
Digilab FTS-7 spectrometer. Mass spectra were obtained on a
Finnigan MAT 8230 spectrometer by the EI technique. Melting
points were obtained in sealed capillaries on a Bu¨chi B 540
instrument. Elemental analyses were performed at the Analytical
Laboratory of the Institute of Inorganic Chemistry at Go¨ttingen,
Germany.
Preparation of LAl(Me)(µ-O)Ti(Me)Cp2 (2). A solution of
freshly prepared Cp2TiMe2 (0.21 g, 1.01 mmol) in toluene (20 mL)
was added via cannula to a solution of LAlMeOH (1) [L ) CH-
(N(Ar)(CMe))2, Ar ) 2,6-iPr2C6H3] (0.48 g, 1.01 mmol) in toluene
(20 mL) at ambient temperature. (NOTE: Care must be taken
because Cp2TiMe2 is photosensitive.) The reaction mixture was
heated to 80 °C for 18 h under stirring. The yellow precipitate
formed was filtered off, washed with n-hexane, and dried in
vacuum. Yield: 0.41 g (61%). mp (dec): 250 °C. 1H NMR (500.13
data/restraints/params 5056/704/531
refinement method
full-matrix
least-squares on F2
GOF on F2
R1, wR2
(I > 2σ(I))
R1, wR2
1.195
0.0603,
0.1205
0.0760,
0.1265
(all data)
largest diff
0.301/-0.368
0.370/-0.412
peak, hole (e Å-3
)
was slowly warmed to ambient temperature and stirred for 18 h.
The precipitate was filtered, washed with n-hexane, and dried in
vacuum. Yield: 0.54 g (67.4%). mp (dec): 391 °C. 1H NMR
(500.13 MHz, C6D6, 25 °C, TMS): δ 7.13-7.24 (m, 6H, m-, p-Ar-
H), 5.40 (s, 10H, C5H5), 4.80 (s, 1H, γ-CH), 3.30 (sept, 4H, 3JH-H
3
) 6.8 Hz, CH(CH3)2), 1.76 (s, 6H, CH3), 1.61 (d, 12H, JH-H
)
3
6.8 Hz, CH(CH3)2), 1.42 (d, 12H, JH-H ) 6.8 Hz, CH(CH3)2),
0.08 (s, 3H, Hf-CH3), -0.27 (s, 3H, Al-CH3). 13C NMR (125.75
MHz, C6D6, 25 °C, TMS): δ 168.3 (CN), 149.5, 146.6, 144.7,
132.4, 135.5, 137.9 (i-, o-, m-, p-Ar), 116.3 (C5H5), 102.0 (γ-CH),
52.9 (Hf-CH3), 32.5 (Al-CH3). EI-MS (70 eV) [m/e (%)]: 785 (100)
[M+ - Me], 770 (8) [M+ - 2Me], 202 (26) [DippNCMe]+. Anal.
Calcd for C41H57AlHfN2O (Mr ) 799.36): C, 61.60; H, 7.19; N,
3.50. Found: C, 59.08; H, 6.85; N, 3.32.
X-ray Structure Determination of 2 and 3. Data for the
structure of 2 were collected on a Bruker three-circle diffractometer
equipped with a SMART 6000 CCD detector. The data for the
structure of 3 were collected on a STOE IPDS II diffractometer.
Intensity measurements were performed on a rapidly cooled crystal.
The structures were solved by direct methods (SHELXS-97)8 and
refined with all data by full-matrix least-squares on F2.9 The
hydrogen atoms on the C-H bonds were placed in idealized
positions and refined isotropically with a riding model, whereas
the non-hydrogen atoms were refined anisotropically. Crystals of
2 were pseudomerohedrally twinned. The fractional contribution
of the minor domain refined to 0.061(1). The Cp groups and the
Me group attached to the titanium were disordered. They were
refined with distance restraints and restraints for the anisotropic
displacement parameters. Crystal data are shown in Table 1.
MHz, C6D6, 25 °C, TMS): δ 7.13-7.24 (m, 6H, m-, p-Ar-H),
3
5.30 (s, 10H, C5H5), 4.90 (s, 1H, γ-CH), 3.10 (sept, 4H, JH-H
)
3
6.8 Hz, CH(CH3)2), 1.68 (s, 6H, CH3), 1.40 (d, 12H, JH-H ) 6.8
Hz, CH(CH3)2), 1.31 (d, 12H, 3JH-H ) 6.8 Hz, CH(CH3)2), -0.18
(s, 3H, Ti-CH3), -0.91 (s, 3H, Al-CH3). 13C NMR (125.75 MHz,
C6D6, 25 °C, TMS): δ 165.4(CN), 145.3, 144.8, 142.8, 128.6,
125.9, 125.2 (i-, o-, m-, p-Ar), 111.3 (C5H5), 97.0 (γ-CH), 27.9
(Ti-CH3), 26.5 (Al-CH3). EI-MS (70 eV) [m/e (%)]: 653 (100)
[M+ - Me], 638 (48) [M+ - 2Me], 202 (26) [DippNCMe]+. Anal.
Calcd for C41H57AlN2OTi (Mr ) 668.75): C, 73.64; H, 8.59; N,
4.19. Found: C, 72.28; H, 8.47; N, 4.17.
Preparation of LAl(Me)(µ-O)Hf(Me)Cp2 (3). Freshly sublimed
Cp2HfMe2 (0.34 g, 1 mmol), dissolved in ether (20 mL), was
transferred using a cannula to a flask charged with 1 (0.48 g, 1
mmol) in diethyl ether (30 mL) at -30 °C. The reaction mixture
(8) Sheldrick, G. M. SHELXS-97, Program for Structure Solution. Acta
Crystallogr., Sect. A 1990, 46, 467-473.
(9) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(7) Samuel, E.; Rausch, M. D. J. Am. Chem. Soc. 1973, 95, 6263-6267.
(b) Alt, H. G.; DiSanzo, F. D.; Rausch, M. D.; Uden, P. C. J.
Organomet. Chem. 1976, 107, 257-263.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1057

Gurubasavaraj et al.
Scheme 1
Polymerization of Ethylene and Styrene. On a high-vacuum
line (10-5 Torr), polymerizations were carried out in a 200 mL
autoclave (Bu¨chi). In a typical experiment, 100 mL of dry toluene
(from Na/K) was vacuum transferred into the polymerization flask
and saturated with 1.0 atm of rigorously purified ethylene (for
ethylene homopolymerization) or with argon in the presence of 10
mL of dry styrene (from CaH2) (for styrene homopolymerization).
The catalyst (see Tables 3 and 4) was placed in the Schlenk flask,
and the appropriate MAO (1.6 M in toluene) was added. The
mixture was stirred for 20 min at room temperature to activate the
catalyst. The catalyst solution was then quickly injected into the
rapidly stirred autoclave using a gastight syringe. After a measured
time interval, the polymerization was quenched by the addition of
5 mL of methanol, and the reaction mixture was then poured into
800 mL of methanol. The polymer was allowed to fully precipitate
overnight, and then it was collected by filtration, washed with fresh
methanol, and dried.
Computational Details. The calculations were performed at a
well-established DFT level of theory using the B3LYP-functional10-11
as implemented in the Gaussian program package12 and the basis
sets LANL2DZ13 for Ti and 6-31G14-15 with additional double-
diffuse functions for the remaining atoms. In the first step, the
compound was fully optimized to its equilibrium structure. The
analysis of the resulting electronic wavefunction for this structure
was then used to obtain the shape of the molecular orbitals and to
analyze the bonding situation by means of a NBO analysis.16-18
in ether in the range from -30 °C to ambient temperature
results in the formation of the µ-O-bridged heterodinuclear
compound [LAlMe(µ-O)Hf(Me)Cp2] (3, Scheme 1) in good
yield (67%).
Compounds 2 and 3 are not soluble in toluene, hexane,
and ether, but they are soluble in hot toluene and are
characterized by analytical, spectroscopic, and single-crystal
X-ray diffraction studies. The IR spectra of 2 and 3 show
no OH absorptions in the range from 3000 to 3600 cm-1,
confirming the completion of the reaction by deprotonation.
Compound 2 is a yellow crystalline solid that melts at 250
°C, while 3 is a colorless crystalline solid that melts at 391
°C. Decomposition was observed at the melting points of 2
and 3. Unlike Cp2TiMe2, complex 2 is thermally stable and
not photosensitive. Compound 2 can be stored for a period
of time at room temperature in the absence of air and
moisture. The mass spectral data for both 2 and 3 are in
agreement with the assigned structures. Neither of them
exhibits a molecular ion. Compound 2 shows the base peak
at m/e 638 corresponding to [M - 2Me]+. The next most-
intense peak for compound 2 is observed at m/e 653 which
can be assigned to [M - Me]+. The base peak for compound
3 is observed at m/e 785 representing [M - Me]+. The next
most-intense peak at m/e 770 shows the loss of another
methyl group corresponding to [M - 2Me]+. Both com-
pounds 2 and 3 exhibit ions at m/e 202 which can be assigned
to [DippNCMe]+.19 The 1H NMR spectrum of 2 exhibits two
resonances (-0.91 and -0.18 ppm) which can be attributed
to the Me protons of the AlMe and TiMe groups, respec-
tively, whereas the respective AlMe and HfMe groups in
compound 3 resonate at -0.27 and 0.08 ppm. The charac-
teristic Cp protons for 2 and 3 appear as singlets (5.3 and
5.4 ppm). In addition, a set of resonances assignable to the
isopropyl and methyl protons associated with the â-diketimi-
nato ligand is found in the range between 1.76 and 1.01 ppm,
and the absence of the OH proton resonance features both 2
and 3. The 27Al NMR is silent because of the quadruple
moment of aluminum.
Results and Discussions
Using the advantage of the oxophilicity of group 4 metals
and the Bro¨nsted acidic character of the proton of the Al-
(O-H) moiety, we isolated compounds 2 and 3 by treatment
of equivalent amounts of LAlMeOH (1) [L ) CH(N(Ar)-
(CMe))2, Ar ) 2,6-iPr2C6H3] and Cp2MMe2 (M ) Ti, Hf).7
Reaction of 1 with Cp2TiMe2 at 80 °C led to the inter-
molecular elimination of methane and the formation of the
µ-O-bridged heterodinuclear complex [LAlMe(µ-O)Ti(Me)-
Cp2] (2, Scheme 1) in moderate yield (61%). Similarly,
treatment of 1 with a stoichiometric amount of Cp2HfMe2
(10) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-78.
(11) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989,
157, 200-206.
(12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision C.02; Gaussian, Inc., Wallingford CT, 2004.
(13) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270-283.
(14) Petersson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081-
6090.
Molecular Structure Description of 2 and 3. The yellow
single crystals of 2 and the colorless single crystals of 3 were
obtained from cooling their hot toluene solutions and were
unambiguously analyzed by X-ray diffraction studies (Figures
1 and 2). The crystallographic data for the structural analysis
(15) Petersson, G. A.; Bennett, A.; Tensfeldt, T. G.; Al-Laham, M. A.;
Shirley, W. A.; Mantzaris, J. J. Chem. Phys. 1988, 89, 2193-2218.
(16) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211-7218.
(17) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736-1740.
(18) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899-
926.
(19) Prust, J.; Most, K.; Mu¨ller, I.; Alexopoulos, E.; Stasch, A.; Uso´n, I.;
Roesky, H. W. Z. Anorg. Allg. Chem. 2001, 627, 2032-2037.
1058 Inorganic Chemistry, Vol. 46, No. 4, 2007

Compounds Containing an Al-O-M (M ) Ti, Hf) Core
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Compounds 2 and 3a
compound 2
Ti(1)-O(1)
Al(1)-N(1)
Al(1)-N(2)
Xcp1-Ti(1)
1.808(3)
1.926(3)
1.919(3)
2.134
Ti(1)-C(29)
Al(1)-O(1)
Al(1)-C(28)
Xcp2-Ti(1)
2.239(9)
1.715(3)
1.958(4)
2.081
N(2)-Al(1)-N(1)
O(1)-Al(1)-N(1)
N(2)-Al(1)-C(28)
Al(1)-O(1)-Ti(1)
Xcp1-Ti(1)-C(29)
O(1)-Ti(1)-C(29)
Xcp1-Ti(1)-O(1)
95.7(1)
O(1)-Al(1)-N(2)
O(1)-Al(1)-C(28)
N(1)-Al(1)-C(28)
Xcp1-Ti(1)-Xcp2
Xcp2-Ti(1)-C(29)
Xcp2-Ti(1)-O(1)
113.9(2)
115.2(2)
108.2(2)
130.6
99.7
110.4
111.0(2)
110.9(2)
151.7(2)
100.6
95.6(5)
111.8
compound 3
Hf(1)-O(1)
Al(1)-N(1)
Al(1)-N(2)
Xcp1-Hf
1.919(2)
1.932(2)
1.913(2)
2.249
Hf(1)-C(7)
Al(1)-O(1)
Al(1)-C(6)
Xcp2-Hf
2.281(2)
1.71(2)
1.965(2)
2.224
Figure 1. Molecular structure of 2 in the crystal (50% probability
ellipsoids); hydrogen atoms are omitted for clarity.
N(2)-Al(1)-N(1)
O(1)-Al(1)-N(1)
N(2)-Al(1)-C(6)
Al(1)-O(1)-Hf(1)
Xcp1-Hf-C(7)
95.1(1)
111.6(1)
111.7(1)
158.4(1)
102.3
O(1)-Al(1)-N(2)
O(1)-Al(1)-C(6)
N(1)-Al(1)-C(6)
Xcp1-Hf-Xcp2
114.0(1)
113.3(1)
109.8
129.8
103.3
Xcp2-Hf-C(7)
O(1)-Hf-C(7)
99.5(1)
110.0
Xcp2-Hf-O(1)
107.3
Xcp1-Hf-O(1)
a XCp ) centroid of the Cp ring.
(1)°) angle in 3, and the corresponding Al-(µ-O)-Zr bond
angle in LMeAl-(µ-O)-ZrRCp2 (L ) CH(N(Ar)(CMe))2,
Ar ) 2,6-iPr2C6H3, R ) Me, Cl) (158.2(1)°).4 This can
probably be attributed to the increasing atomic radii from
Ti to Zr causing gradual opening of the Al-O-M (MdTi
or Zr) bond angle and to the bulkiness of the ligands
surrounding the metal centers. However, the Al-(µ-O)-M
(M ) Ti, Hf) angles in 2 and 3 are significantly less opened
than those of homobimetallic M-(µ-O)-M (M ) Zr, Hf)
in (Cp2ZrMe)2(µ-O) (174.1(3)°)22 and (Cp2HfMe)2(µ-O)
(173.9(3)°).23 The Al-Me bond length in compound 2
(1.958(4) Å) is similar to that of LAlMeOH and LMeAl(µ-
O)ZrRCp2 (L ) CH (N(Ar)(CMe))2, Ar ) 2,6-iPr2C6H3, R
) Me, Cl) (1.961(3) Å).4
The Ti(1)-O(1) bond distance (1.808(3) Å) in compound
2 is significantly shorter when compared to those in [Cp2-
Ti(CF3CdC(H)CF3)]2O (av Ti-O ) 1.856(6)Å)24 and the
alkoxide bridged cluster (Ti4Zr2O4(OBu)n(OMc)10 (OMc )
methacrylate; n ) 2, 4, 6; av Ti-O ) 2.041(5) Å).25 The
Ti(1)-C(29) bond length (2.239(9) Å) is slightly longer when
compared to those (av 2.175(5)) in Cp2TiMe2.26
Figure 2. Molecular structure of 3 in the crystal (50% probability
ellipsoids); hydrogen atoms are omitted for clarity.
of compounds 2 and 3 are given in Table 1. The important
bond parameters are listed in Table 2.
Compounds 2 and 3 crystallize in the triclinic space group
P1h. Both compounds show the aluminum atom bonded
through an oxygen atom to titanium and hafnium, respec-
tively, and contain a bent Al-O-M (M ) Ti, Hf) core. The
aluminum atom exhibits a highly distorted tetrahedral
geometry with two nitrogen atoms of the â-diketiminato
ligand, a methyl group, and one (µ-O) unit. The titanium
and hafnium exhibit tetrahedral geometry and their coordina-
tion spheres are completed by two Cp ligands and one methyl
group around each metal atom. The Me groups on Al and
Ti in 2 and 3 are bent out of the Al-O-M (M ) Ti, Hf)
plane in a cis configuration.
The Al-(µ-O) bond length (1.715(3) Å) in 2 is in good
agreement with LMeAl-(µ-O)-ZrRCp2 (L ) CH(N(Ar)-
(CMe))2, Ar ) 2,6-iPr2C6H3, R ) Me, Cl) (av 1.721 Å)4
but is longer than those found in compounds [{(Me3-
Si)2HC}2Al]2(µ-O) (1.687(4) Å)20 and [HC{(CMe)-
(NMe)}2AlCl]2(µ-O) (1.677(6) Å).21 The Al-(µ-O)-Ti angle
(151.7(2)°) in 2 is smaller than the Al-(µ-O)-Hf (158.4-
The Ti-XCp (XCp ) centroid of the Cp ring) distances in
2 are almost identical (av 2.108 Å) and are similar to those
in dimethyltitanocene (av Ti-XCp ) 2.078 Å).26 The XCp1
-
Ti-XCp2 (XCp ) centroid of the Cp ring) bond angle (130.6°)
(20) Uhl, W.; Koch, M.; Hiller, W.; Heckel, M. Angew. Chem. 1995, 107,
1122-1124; Angew. Chem., Int. Ed. 1995, 34, 117-119.
(21) Kuhn, N.; Fuchs, S.; Niquet, E.; Richter, M.; Steimann, M. Z. Anorg.
Allg. Chem. 2002, 628, 717-718.
(22) Hunter, W. E.; Hrncir, D. C.; Vann Bynum, R.; Penttila, R. A.;
Atwood, J. L. Organometallics 1983, 2, 750-755.
(23) Fronczek, F. R.; Baker, E. C.; Sharp, P. R.; Raymond, K. N.; Alt, H.
G.; Rausch, M. D. Inorg. Chem. 1976, 15, 2284-2289.
(24) Rausch, M. D.; Sikora, D. J.; Hrncir, D. C.; Hunter, W. E.; Atwood,
J. L. Inorg. Chem. 1980, 19, 3817-3821.
(25) Moraru, B.; Kickelbick, G.; Schubert, U. Eur. J. Inorg. Chem. 2001,
1295-1301.
(26) Thewalt, U.; Wo¨hrle, T. J. Organomet. Chem. 1994, 464, C17-C19.
Inorganic Chemistry, Vol. 46, No. 4, 2007 1059

Gurubasavaraj et al.
Table 3. Ethylene Polymerization Data for Compounds 2 and 3 as
Table 4. Styrene Polymerization Data for Compound 2 as Catalysta
Catalystsa
b
MAO/
t
PS
(g)
activity
Mw/
Mn
Tg
(°C)
MAO/
catalyst
t
PE
(g)
activity
Mw/
Mn
catalyst catalyst (min)
(×104)
Mw
catalyst
(min)
(×106)
Mw
2
2
2
500
800
1500
120
120
120
0.35
0.8
1.7
0.78
1.8
3.8
83.5
2
2
2
2
2
2
3
3
3
50
100
200
300
400
500
300
400
500
45
30
30
17
15
15
30
30
30
1.16
3.5
5.0
4.0
4.5
0.08
0.36
0.51
0.73
0.92
1.03
0.02
0.04
0.06
152817
6.01
12989 7.46 76.5
81.7
a Polymerization conditions: for 2, 22.5 µmol, 100 mL of toluene at
25 °C, and 10 mL styrene. Activity ) g PE/mol cat h. b DSC.
97909
121996
106020
4.74
4.57
2.86
5.0
0.17
0.43
0.61
a Polymerization conditions: for 2 and 3, 19.5 µmol, 100 mL of toluene
at 25 °C, and 1 atm of ethylene pressure. Activity ) g PE/mol cat h.
Figure 3. Plot of the activity toward the MAO/2 ratios for ethylene
polymerization.
Figure 4. Computed frontier orbitals for the complex 2: (a) LUMO, (b)
HOMO, and (c) HOMO-1.
in compound 2 is comparable to that in Cp2TiMe2 (XCp1
-
Ti-XCp2 ) 134.5°).26
Table 3 clearly demonstrate that compound 2 acts as
moderately active catalyst even at low MAO/2 ratios; a
similar result was previously observed for the corresponding
Zr analogue of 2.4
Styrene Polymerization Studies. The catalytic property
of complex 2 for the polymerization of styrene was prelimi-
narily investigated. This complex shows living catalyst
activity at ambient temperature in toluene when activated
with MAO. All polymeric materials were isolated as white
powders, and Table 4 summarizes the activity values of
catalyst 2.
The DSC measurements of the polymers show that the
characteristic glass-transition temperatures (Tg) are in the
range from 76 to 83 °C which is within the typical Tg range
for the atactic polymers.28 Melting points (Tm) for the
polymers were not observed. The GPC for measured
polyethylene samples exhibit monomodal characteristics. The
polydispersities show narrow distribution ranging from 2 to
6, which is typical for single-site catalysts.
Results of Computational Studies. To understand the
catalytic process, we have carried out ab initio calculations
for compound 2. The frontier orbitals of complex 2 are shown
in Figure 4. As can be directly seen from the orbital pictures,
the HOMO and LUMO orbitals are located on different areas
The Al-(µ-O) (1.71(2) Å) and Al-Me (1.965(2) Å) bond
lengths in 3 are in the same range as those observed in 2.
The Hf(1)-O(1) (1.919(2) Å) and Hf(1)-C(7) bond lengths
(2.281(2) Å) in 3 are shorter when compared to Hf-O (av
1.943 Å) and Hf-C (av 2.350 Å) in the homobimetallic
compound (Cp2RHf-(µ-O)-HfRCp2) (R) Me,23 Cl27). The
Hf-XCp (X ) centroid of the Cp ring) distance in 3 (av
2.237 Å) is comparable to those of the homobimetallic (Cp2-
HfMe)2(µ-O) Hf-XCp (av 2.210 Å).23 The XCp1-Hf-XCp2
(X ) centroid of the Cp ring) bond angle (129.8°) in
compound 3 is close to that in [(Cp2HfMe)2(µ-O)] (128.5°).23
Ethylene Polymerization Studies. The methylalumoxane
(MAO)-activated compound of 2 exhibits high catalytic
activity for the polymerization of ethylene, whereas the
methylalumoxane (MAO)-activated compound of 3 shows
low activity for the ethylene polymerization. All polymeric
materials were isolated as white powders. Table 3 sum-
marizes the polymerization results of catalysts 2 and 3. Under
comparable polymerization conditions, the MAO/2 catalyst
system shows almost similar activity to that of MAO/
LMeAl-(µ-O)-Zr(Me)Cp2 (L ) CH(N(Ar)(CMe))2, Ar )
2,6-iPr2C6H3).4 Figure 3 exhibits the plot of activity for
different ratios of MAO/2 revealing a gradual increase in
the activity with the MAO/2 ratios. The data presented in
(27) Pa´rka´nyi, L.; Sharma, S.; Cervantes-Lee, F.; Pannell, K. H. Z.
Kristallogr. 1993, 208, 335-337.
(28) Guo, N.; Li, L.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 6542-
6543.
1060 Inorganic Chemistry, Vol. 46, No. 4, 2007

Compounds Containing an Al-O-M (M ) Ti, Hf) Core
Table 5. Selected Calculated and X-ray Bond Distances (Å) and Bond
Angles (deg)
changes of the electronic density on the Ti atom during the
catalytic step.
bond
calcd X-ray
1.749 1.715 O(1)-Al(1)-N(2)
1.840 1.808 O(1)-Al(1)-C(28) 115.36 115.20
angles
calcd
X-ray
In summary, we report the preparation, structural charac-
terization, and catalytic property of two kinetically stable
heterobimetallic complexes of Al(III) with Ti and Hf
respectively binding through an oxygen bridge. Compound
2 exhibits high catalytic activity in ethylene and styrene
homopolymerization. Computational study shows that the Ti
cation center after activation by MAO is stabilized by the
donor acceptor interaction between the Ti(1)-O(1) bonding
orbital with an unoccupied antibonding Al(1)-C(28) orbital.
Presently, we are studying the use of these compounds for
copolymerization reactions.
Al(1)-O(1)
Ti(1)-O(1)
114.02 113.90
Al(1)-C(28) 1.974 1.958 Al(1)-O(1)-Ti(1)
Ti(1)-C(29) 2.181 2.239 O(1)-Ti(1)-C(29)
153.81 151.70
95.59 95.60
of the molecule, and therefore, the metal centers Ti and Al
show a completely different chemical behavior. As shown
in Table 5, the resulting structure compares very well with
the data obtained by X-ray crystallography, thus making it
possible to obtain further insight into the molecular orbitals
and bonds through analysis of the electronic structure.
The electronic stabilization of the metal cation formed
during the activation by MAO is one of the key steps in the
polymerization process. To address the factor which governs
the stability of the cation, we studied the bonding situation
in complex 2. The NBO analysis16-18 shows that the bond
formed between the Ti atom and the oxygen (p-rich orbital
on oxygen and pure d-orbital on titanium) obtains a
significant stabilization of 26 kcal/mol through a donor
acceptor interaction between the Ti(1)-O(1) bonding orbital
with an unoccupied antibonding Al(1)-C(28) orbital. The
combination of this stabilization with the lowest-unoccupied
molecular orbital prevents bond breaking resulting from
Acknowledgment. This work is dedicated to Professor
Karl Christe on the occasion of his 70th birthday. This work
is supported by the Go¨ttinger Akademie der Wissenschaften,
Deutsche Forschungsgemeinschaft and the Fonds der Chem-
ischen Industrie. S.K.M. thanks the Alexander von Humboldt
Stiftung for a research fellowship. We thank the referees for
their valuable suggestions.
Supporting Information Available: X-ray data (CIF) of 2 and
3. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC060538R
Inorganic Chemistry, Vol. 46, No. 4, 2007 1061