Synthesis, structural characterization, and theoretical investigation of compounds containing an Al-O-M-O-Al (M = Ti, Zr) core

Article abstract of DOI:10.1021/ic701063v

We report a facile route to the first molecular compounds with the Al-O-M-O-Al (M = Ti, Zr) structural motif. Synthesis of L(Me)Al(μ-O) M(NMe₂)₂(μ-O)Al(Me)L [L = CH{N(Ar)(CMe)}₂, Ar = 2,6-iPr₂C₆H₃; M = Ti (7), Zr (8)] was accomplished by reacting the monometallic hydroxide precursor L(Me)Al(OH) (1) with Ti(NMe₂)₄ or Zr(NMe₂)₄ under elimination of Me₂NH in good yield. The crystal structural data confirm the trimetallic Al-O-M-O-Al core in both 7 and 8. Preliminary investigation on catalytic activity of these complexes reveals low activity of these complexes in ethylene polymerization as compared to the related oxygen-bridged metallocene-based heterobimetallic complexes L(Me)Al(μ-O)M(Me) Cp₂ (M = Ti, Zr) which could be attributed to the relatively lower stability of the supposed cationic intermediate as revealed by DFT calculations.

Full text of DOI:10.1021/ic701063v

Inorg. Chem. 2007, 46, 7594−7600
Synthesis, Structural Characterization, and Theoretical Investigation of
Compounds Containing an Al Al (M
Ti, Zr) Core†
−O
−M−O
−
)
Swadhin K. Mandal,‡ Prabhuodeyara M. Gurubasavaraj,‡ Herbert W. Roesky,*,‡ Rainer B. Oswald,§
Jo
rg Magull,‡ and Arne Ringe‡
1
Institute of Inorganic Chemistry, UniVersity of Go¨ttingen, Tammannstrasse 4, 37077 Go¨ttingen,
Germany, and Institute of Physical Chemistry, UniVersity of Go¨ttingen, Tammannstrasse 6,
37077 Go¨ttingen, Germany
Received May 31, 2007
We report a facile route to the first molecular compounds with the Al
Synthesis of L(Me)Al( -O)M(NMe2)2( -O)Al(Me)L [L CH N(Ar)(CMe)
was accomplished by reacting the monometallic hydroxide precursor L(Me)Al(OH) (1) with Ti(NMe2)4 or Zr(NMe2)4
under elimination of Me2NH in good yield. The crystal structural data confirm the trimetallic Al Al core in
−O
−M
−O
−Al (M
)
Ti, Zr) structural motif.
µ
µ
)
{
}
2, Ar
)
2,6-iPr2C6H3; M
)
Ti (7), Zr (8)]
−O−M−O−
both 7 and 8. Preliminary investigation on catalytic activity of these complexes reveals low activity of these complexes
in ethylene polymerization as compared to the related oxygen-bridged metallocene-based heterobimetallic complexes
L(Me)Al(
µ-O)M(Me)Cp2 (M ) Ti, Zr) which could be attributed to the relatively lower stability of the supposed
cationic intermediate as revealed by DFT calculations.
Introduction
metals in a heterometallic complex would allow more
pronounced chemical communication between the metals.
Additionally, it is a synthetic challenge to assemble different
metal centers of entirely different chemical properties into a
single molecule.
Heterobi- and heteropolymetallic compounds are of sig-
nificant interest ranging from advanced materials to valuable
catalysts. The compounds with different metals have often
modified the fundamental properties of the individual metal
atoms.1 This cooperative feature is difficult to achieve
otherwise.2 Moreover, the interest in metal and organome-
tallic oxides stems not only from the application of these
compounds in industry as catalysts or as cocatalysts but also
as a model for the fixation of catalysts on oxide surfaces.3
As a result, increasing attention has been focused on the
synthesis and characterization of heterobimetallic oxides,
which are used as polyfunctional catalysts and precursors
for the preparation of bi- and polymetallic heterogeneous
catalysts.4-7 Appropriate proximity between two different
We have recently reported the syntheses of a new class
of heterometallic complexes (3-6, Chart 1) through oxygen
bridging based on an Al-O-M motif, some of which are
excellent candidates for homogeneous catalysis.8-12 The
synthetic strategy takes advantage of unprecedented syntheses
of two monometallic hydroxide precursors, L(Me)Al(OH)
(1)8 [L ) CH{N(Ar)(CMe)}2, Ar ) 2,6-iPr2C6H3] and Cp*2-
(5) Carofiglio, T.; Floriani, C.; Rosi, M.; Chiesi-Villa, A.; Rizzoli, C.
Inorg. Chem. 1991, 30, 3245-3246.
(6) Rau, M. S.; Kretz, C. M.; Geoffroy, G. L.; Rheingold, A. L.; Haggerty,
B. S. Organometallics 1994, 13, 1624-1634.
* To whom correspondence should be addressed. E-mail: hroesky@
gwdg.de.
(7) Li, H.; Eddaoudi, M.; Ple´vert, J.; O’Keeffe, M.; Yaghi, O. M. J. Am.
Chem. Soc. 2000, 122, 12409-12410.
† Dedicated to Professor Roald Hoffmann on the occasion of his 70th
birthday.
(8) Bai, G.; Singh, S.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G.
J. Am. Chem. Soc. 2005, 127, 3449-3455.
‡ Institute of Inorganic Chemistry, University of Go¨ttingen.
§ Institute of Physical Chemistry, University of Go¨ttingen.
(1) Ueland, B. G.; Lau, G. C.; Cava, R. J.; O’Brien, J. R.; Schiffer, P.
Phys. ReV. Lett. 2006, 96, 027216.
(9) Chai, J.; Jancik, V.; Singh, S.; Zhu, H.; He, C.; Roesky, H. W.;
Schmidt, H.-G.; Noltemeyer, M.; Hosmane, N. S. J. Am. Chem. Soc.
2005, 127, 7521-7528.
(2) Tao, R.-J.; Li, F.-A.; Zang, S.-Q.; Cheng, Y.-X.; Wang, Q.-L.; Niu,
J.-Y.; Liao, D.-Z. J. Coord. Chem. 2006, 59, 901-909.
(3) Cope´ret, C.; Chabanas, M.; Saint-Arroman, R. P.; Basset, J.-M. Angew.
Chem., Int. Ed. 2003, 42, 156-181.
(4) Roesky, H. W.; Haiduc, I.; Hosmane, N. S. Chem. ReV. 2003, 103,
2579-2595.
(10) Gurubasavaraj, P. M.; Mandal, S. K.; Roesky, H. W.; Oswald, R. B.;
Pal, A.; Noltemeyer, M. Inorg. Chem. 2007, 46, 1056-1061.
(11) Singh, S.; Roesky, H. W. Dalton Trans. 2007, 1360-1370.
(12) Nembenna, S.; Roesky, H. W.; Mandal, S. K.; Oswald, R. B.; Pal,
A.; Herbst-Irmer, R.; Noltemeyer, M.; Schmidt, H.-G. J. Am. Chem.
Soc. 2006, 128, 13056-13057.
7594 Inorganic Chemistry, Vol. 46, No. 18, 2007
10.1021/ic701063v CCC: $37.00
© 2007 American Chemical Society
Published on Web 08/08/2007

Compounds Containing an Al-O-M-O-Al Core
Chart 1. Recently Developed Heterometallic Complexes with an Al-O-M Core
Scheme 1
(Me)Zr(OH) (2).13 The Bro¨nsted acidic character of the
proton in the Al(O-H) or Zr(O-H) moiety on reactions with
a variety of metal-alkyl groups allows building a new class
of metallocene based heterobimetallic complexes useful as
olefin polymerization catalysts8,10,13 at unusually low me-
thylalumoxane (MAO) concentration. This could be at-
tributed to the presence of a chemically grafted (Me)Al-O
backbone in the catalysts and also the bridging oxygen atom
which might play an important role in determining the Lewis
acidity of the active metal center. At present a number of
oxide-bridged heterobimetallic complexes with the Al-O-M
motif are known,8-12 where the metal center (M) varies from
transition metal to lanthanide to alkaline earth, but there were
no reports on the oxide-bridged trimetallic complex with an
Al-O-M-O-Al core except the one reported very re-
cently.12 Last year we have reported the first example of a
trimetallic complex containing the Al-O-Mg-O-Al skel-
eton (complex 6, Chart 1).12 However, to date there is no
report of trimetallic complexes with a transition metal center
fixed on two aluminum centers through oxide bridging.
Herein, we report the synthesis, characterization, and
theoretical investigation of a new class of oxide-bridged
trimetallic compounds bearing the Al-O-M-O-Al core
(M ) Ti, Zr). Preliminary investigation on the catalytic
activity of these complexes reveals low activity of these
complexes in ethylene polymerization as compared to the
related oxygen-bridged metallocene-based heterobime-
tallic complexes L(Me)Al(µ-O)M(Me)Cp2 (M ) Ti, Zr),
which could be attributed to the relatively lower stability of
the supposed cationic intermediate as revealed by DFT
calculations.
Results and Discussion
Syntheses of Oxygen-Bridged Trimetallic Complexes.
For sometime, we have been actively involved in the
synthesis of a compound bearing the Al-O-M-O-Al
trimetallic core with a catalytically active transition metal
center (M). It was initially anticipated that if instead of one
(Me)Al-O unit (for example complex 4, Chart 1) two such
units can be grafted around the active metallic center, the
catalytic activity of these complexes might be enhanced many
times at even lower cocatalyst to catalyst ratio. Unfortunately,
all attempts by varying the starting metallocene-based
precursors and reaction condition to synthesize such a
complex were unsuccessful by reacting L(Me)Al(µ-O)Zr-
(Me)Cp2, L(Me)Al(µ-O)Ti(Me)2Cp*, or L(Me)Al(µ-O)Ti-
(Me)2Cp (complex type 4, Chart 1) with another 1 equiv of
L(Me)Al(OH) (1) [L ) CH{N(Ar)(CMe)}2, Ar ) 2,6-
iPr2C6H3]. This might be attributed to the high steric
crowding around the metal center (M) imposed by bulky C5-
Me5 or C5H5 ligand hindering the approach of another
molecule of 1 to the M-Me unit. Also further reactivity of
the M-Me unit in the heterobimetallic complexes might be
responsible for this reluctance. However, synthesis of
complexes bearing the oxygen-bridged trimetallic Al-O-
(13) Gurubasavaraj, P. M.; Roesky, H. W.; Sharma, P. M. V.; Oswald, R.
B.; Dolle, V.; Pal, A. Organometallics 2007, 26, 3346-3351.
Inorganic Chemistry, Vol. 46, No. 18, 2007 7595

Mandal et al.
M-O-Al (M ) Ti, Zr) core was accomplished by reacting
the monometallic hydroxide precursor, L(Me)Al(OH) (1),
with sterically less crowded group 4 nonmetallocene precur-
sor M(NMe2)4 under elimination of Me2NH. Reaction of 2
equiv of 1 with an 1 equiv of M(NMe2)4 (M ) Zr, Ti) in
toluene leads to the intermolecular elimination of Me2NH
and the formation of the µ-O-bridged trimetallic complex,
L(Me)Al(µ-O)M(NMe2)2(µ-O)Al(Me)L [M ) Ti (7), Zr (8)]
(Scheme 1). The absence of the characteristic OH resonance
of L(Me)Al(OH) (1) in the 1H NMR spectrum of the reaction
mixture indicates the complete consumption of 1 into 7 and
8, respectively. These complexes (7, 8) were characterized
1
by H and 13C NMR spectroscopy, elemental analysis, EI
mass spectrometry, and single-crystal X-ray diffraction
studies. Both of these complexes are soluble in n-hexane,
pentane, toluene, and benzene at room temperature. The 1H
NMR spectra of 7 and 8 feature a characteristic singlet each
at ∼2.8 ppm attributed to the -NMe2 protons, and the Al-
(Me) protons resonate at ∼-0.6 ppm as another singlet. The
singlet at ∼2.8 ppm integrates twice against the singlet at
∼-0.6 ppm revealing the formation of trimetallic complexes
as formulated in Scheme 1. In addition, a set of resonances
assignable to the protons associated with the â-diketiminato
ligand (L) are found in the 1H NMR spectra of 7 and 8. The
27Al NMR is silent due to the quadrupolar nuclei of
aluminum. The 13C NMR spectra of 7 and 8 respectively
reveal a singlet (∼ -11.0 ppm) assigned to the aluminum-
bound methyl-carbon resonance and another singlet (∼44.0
ppm) that could be assigned to the four methyl carbon
resonances arising from the two dimethylamino groups
attached to the Ti or Zr center. The mass spectral data for 7
are in accord with the assigned structure. It exhibits the
molecular ion peak at m/e 1086.8, and the next peak for
compound 7 was observed at m/e 1071.8 corresponding to
[M - Me]+. However the mass spectrometry data for 8 are
quite different from those of 7 revealing no characteristic
fragment except the base peak at m/e 202, which can be
assigned to [DippNCMe]+.14 Analytically pure crystals
of 7 and 8 were grown from pentane and n-hexane so-
lution, respectively, and finally the structures of 7 and 8
were unambiguously determined by single-crystal X-ray
crystallography.
X-ray Crystal Structures of 7 and 8. The yellow single
crystals of 7 and colorless single crystals of 8 were analyzed
by X-ray diffraction studies (Figures 1and 2). Compound 7
was crystallized from pentane at -30 °C whereas complex
8 was crystallized from n-hexane at 0 °C. Table 1 lists the
crystallographic data, and the important bond parameters are
listed in Table 2. Compounds 7 and 8 crystallize in the
monoclinic space group P21/c. Both aluminum atoms are
bonded through an oxygen atom to titanium (in 7) and
zirconium (in 8), respectively, and contain a bent Al-O-M
(M ) Ti, Zr) core as revealed by the corresponding bond
angles (Table 2). The aluminum atom exhibits a distorted
tetrahedral geometry with two nitrogen atoms of the â-diketim-
inato ligand, a methyl group, and one (µ-O) unit. The
Figure 1. Molecular structure of 7 in the crystal (50% probability
ellipsoids). Hydrogen atoms are omitted for clarity.
Figure 2. Molecular structure of 8 in the crystal (50% probability
ellipsoids). Hydrogen atoms are omitted for clarity.
titanium or zirconium center also adopts a distorted tetra-
hedral geometry, and their coordination spheres are com-
pleted by two dimethylamino ligands and two (µ-O) units.
The Al-C(Me) bond length (average 1.96 Å in both 7 and
8) compares very well to the recently structurally character-
ized oxygen-bridged heterobimetallic compounds of the
general formula L(Me)Al(µ-O)M(R)Cp2 (R ) Me or Cl; M
) Ti , Zr, or Hf).8,10 The Al-(µ-O) bond length (average
1.73 Å in 7 and 1.72 Å in 8) is in good agreement with that
observed for L(Me)Al(µ-O)Ti(Me)Cp2 (1.715(3) Å) and
L(Me)Al(µ-O)Zr(R)Cp2 (average 1.72 Å; R ) Me or Cl)
but longer than those found in compounds [{(Me3Si)2
HC}2Al]2(µ-O) (1.687(4) Å)15 and [HC{(CMe)(NMe)}2AlCl]2-
(µ-O) (1.677(6) Å).16 The Ti-O bond distance in 7 (average
1.80 Å) and the Zr-O bond length in 8 (average 1.94 Å)
are in good agreement with that observed for L(Me)Al(µ-
O)Ti(Me)Cp2 (1.808(3) Å)10 and L(Me)Al(µ-O)Zr(R)Cp2
(average 1.92 Å), respectively.8 Two types of Al-O-M
bond angles are noticed in both 7 and 8. For example, one
Al-O-M bond angle is almost linear (175.58(8)° in 7 and
173.21(10)° in 8) while the other Al-O-M bond angle is
(15) Uhl, W.; Koch, M.; Hiller, W.; Heckel, M. Angew. Chem. 1995, 107,
1122-1124; Angew. Chem., Int. Ed. 1995, 34, 989-990.
(16) Kuhn, N.; Fuchs, S.; Niquet, E.; Richter, M.; Steimann, M. Z. Anorg.
Allg. Chem. 2002, 628, 717-718.
(14) 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.
7596 Inorganic Chemistry, Vol. 46, No. 18, 2007

Compounds Containing an Al-O-M-O-Al Core
Table 1. Crystallographic and Structure Refinement Data for Compounds 7 and 8
param
7
8
empirical formula
fw
C64H100Al2N6O2Ti
1087.36
C64H100Al2N6O2Zr
1130.68
T (K)
133(2)
133(2)
cryst system, space group
monoclinic, P21/c
22.6235(9)
17.1285(4)
17.1933(5)
90
103.433(3)
90
6480.2(4)
monoclinic, P21/c
22.6139(9)
17.1826(8)
17.2375(6)
90
102.419(3)
90
6541.2(5)
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
4
4
abs coeff (mm-1
)
0.204
0.239
F(000)
2360
2432
θ range for data collcn (deg)
no. of reflcns collcd/unique
obsd reflcns [I > 2σ(I)]
data/restns/params
goof
1.51-24.84
98 282/11 152 [R(int) ) 0.0826]
8232
11 152/0/732
0.966
1.50-24.90
66 342/11 255 [R(int) ) 0.1005]
8169
11 255/0/732
0.967
R1, wR2 [I > 2σ(I)]
0.0395, 0.0910
0.0620, 0.0977
0.342 and -0.353
0.0373, 0.0744
0.0647, 0.0810
0.243 and -0.321
R1, wR2 (all data)
largest diff peak/hole (e Å-3
)
in (Cp2ZrMe)2(µ-O) (174.1(3)°)17 and (Cp2HfMe)2(µ-O)
(173.9(3)°).18
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Compounds 7 and 8
Compound 7
Ethylene Polymerization and Computational Studies.
Preliminary experiments were carried out for ethylene
polymerization using compounds 7 and 8, respectively, as
precatalyst in the presence of methylalumoxane (MAO) as
cocatalyst. The results reveal two orders lower activity in
magnitude (on the order of 104 with MAO to catalyst ratio
800:1, activity ) g of PE/(mol of catal‚h)) even at relatively
higher MAO to catalyst ratio, when compared to the activity
observed in ethylene polymerization with metallocene-based
heterobimetallic complexes [L(Me)Al(µ-O)M(Me)Cp2 (M )
Ti, Zr) bearing the Al-O-M moiety, 4, Chart 1] reported
from our laboratory.8,10 This relatively lower activity in this
study might be attributed to the lower stability of the
supposed coordinatively unsaturated cationic intermediate of
7 or 8. Density functional calculations were carried out to
understand the bonding situation and the catalytic properties
of compounds 7 and 8 and compare them to the related
metallocene-based heterobimetallic complexes L(Me)Al(µ-
O)Ti(Me)Cp2 bearing the Al-O-Ti motif. As shown in
Table 3, the calculated equilibrium structure matches very
well with the data obtained by X-ray diffraction, thus giving
a solid foundation for the electronic structure calculations
to obtain further insight by investigating the molecular
orbitals and bonds.
Ti(1)-O(1)
Ti(1)-N(5)
Al(1)-O(1)
Al(1)-N(2)
Al(2)-O(2)
Al(2)-N(4)
1.798(1)
1.923(2)
1.725(1)
1.936(2)
1.734(2)
1.926(2)
Ti(1)-O(2)
Ti(1)-N(6)
Al(1)-N(1)
Al(1)-C(30)
Al(2)-N(3)
Al(2)-C(60)
1.809(1)
1.910(2)
1.916(2)
1.965(2)
1.908(2)
1.950(2)
O(1)-Ti(1)-O(2)
O(2)-Ti(1)-N(6)
O(2)-Ti(1)-N(5)
Al(1)-O(1)-Ti(1)
O(1)-Al(1)-N(1)
N(1)-Al(1)-N(2)
N(1)-Al(1)-C(30)
O(2)-Al(2)-N(3)
N(3)-Al(2)-N(4)
N(3)-Al(2)-C(60)
119.58(6)
106.35(7)
109.64(7)
166.18(9)
113.34(7)
94.97(7)
108.46(8)
108.22(7)
95.32(7)
O(1)-Ti(1)-N(6)
O(1)-Ti(1)-N(5)
N(6)-Ti(1)-N(5)
Al(2)-O(2)-Ti(1)
O(1)-Al(1)-N(2)
O(1)-Al(1)-C(30)
N(2)-Al(1)-C(30)
O(2)-Al(2)-N(4)
O(2)-Al(2)-C(60)
N(4)-Al(2)-C(60)
108.55(7)
106.61(7)
105.24(7)
175.58(8)
113.68(7)
116.96(8)
107.08(8)
107.85(7)
119.24(8)
113.49(8)
110.00(9)
Compound 8
Zr(1)-O(1)
Zr(1)-N(5)
Al(1)-O(1)
Al(1)-N(2)
Al(2)-O(2)
Al(2)-N(4)
1.941(2)
2.072(2)
1.716(2)
1.926(2)
1.723(2)
1.926(2)
Zr(1)-O(2)
Zr(1)-N(6)
Al(1)-N(1)
Al(1)-C(30)
Al(2)-N(3)
Al(2)-C(60)
1.944(2)
2.057(2)
1.913(2)
1.974(2)
1.907(2)
1.955(3)
O(1)-Zr(1)-O(2)
O(2)-Zr(1)-N(6)
O(2)-Zr(1)-N(5)
Al(1)-O(1)-Zr(1)
O(1)-Al(1)-N(1)
N(1)-Al(1)-N(2)
N(1)-Al(1)-C(30)
O(2)-Al(2)-N(3)
N(3)-Al(2)-N(4)
N(3)-Al(2)-C(60)
117.05(7)
107.07(7)
110.21(8)
166.50(10)
112.86(8)
95.20(9)
108.81(10)
107.74(8)
95.20(9)
O(1)-Zr(1)-N(6)
O(1)-Zr(1)-N(5)
N(6)-Zr(1)-N(5)
Al(2)-O(2)-Zr(1)
O(1)-Al(1)-N(2)
O(1)-Al(1)-C(30)
N(2)-Al(1)-C(30)
O(2)-Al(2)-N(4)
O(2)-Al(2)-C(60)
N(4)-Al(2)-C(60)
109.09(7)
107.87(7)
104.88(8)
173.21(10)
112.98(8)
117.65(10)
106.92(10)
107.56(9)
119.15(10)
113.83(10)
The bonding situation ascertained from the calculation
predicts that generating a cation from 7 or 8 is more difficult
as compared to that from the highly active catalyst L(Me)-
Al(µ-O)Ti(Me)Cp2 in which a cation is easily formed and
stabilized by the Cp ligands. The examination of the bonds
formed between the metal centers of 7 (Ti) or 8 (Zr) and
oxygen clearly explains this behavior. The M-O bonds for
both compounds are composed of a sd-hybrid orbital on the
110.60(11)
slightly bent (166.18(9)° in 7 and 166.50(10)° in 8). These
bond angles sharply contrast to the Al-O-M bond angle
observed in the heterobimetallic complexes, L(Me)Al(µ-O)-
Ti(Me)Cp2 (151.7(2)°)10 and L(Me)Al(µ-O)Zr(R)Cp2 (aver-
age 156.8°)8 or the recently characterized trimetallic complex
L(Me)Al(µ-O)Mg(THF)2(µ-O)Al(Me)L (average 154.9°)12
though these values compare well with that observed for the
homobimetallic angle, M-(µ-O)-M (M ) Zr, Hf), observed
(17) Hunter, W. E.; Hrncir, D. C.; Bynum, R. V.; Penttila, R. A.; Atwood,
J. L. Organometallics 1983, 2, 750-755.
(18) Fronczek, F. R.; Baker, E. C.; Sharp, P. R.; Raymond, K. N.; Alt, H.
G.; Rausch, M. D. Inorg. Chem. 1976, 15, 2284-2289.
Inorganic Chemistry, Vol. 46, No. 18, 2007 7597

Mandal et al.
Figure 3. Shape of the bonding orbital around the metal center in 7 and 8: (a) between Ti and oxygen; (b) between Zr and oxygen; (c) between Ti and
nitrogen of dimethyl amino ligand; (d) between Zr and nitrogen of dimethyl amino ligand.
Table 3. Selected Calculated and X-ray Bond Distances (Å) and Bond
Angles (deg) in 7 and 8
bond dists
calcd X-ray
bond angles
calcd
X-ray
Compound 7
Ti(1)-O(1) 1.809 1.798 O(1)-Ti(1)-N(6)
Ti(1)-O(2) 1.817 1.809 O(2)-Ti(1)-N(5)
Ti(1)-N(5) 1.928 1.923 O(1)-Ti(1)-O(2)
Ti(1)-N(6) 1.921 1.910 N(1)-Al(1)-N(2)
109.37 108.55
110.23 109.64
119.81 119.58
95.16
94.97
Compound 8
Zr(1)-O(1) 1.975 1.941 O(1)-Zr(1)-N(6) 109.87 109.09
Zr(1)-O(2) 1.984 1.944 O(2)-Zr(1)-N(5) 111.27 110.21
Zr(1)-N(5) 2.095 2.072 O(1)-Zr(1)-O(2) 118.60 117.05
Zr(1)-N(6) 2.087 2.057 N(1)-Al(1)-N(2)
95.08
95.20
metal with a nearly pure p-orbital from oxygen. The M-N
bonds are formed as a result of overlap between the sd-hybrid
orbital on the metal and the sp1.7-hybrid orbital on the
nitrogen. The electronic density of both M-O and M-N
bonds are primarily located on the O and N, respectively,
leaving the metal center Lewis acidic (Figure 3). The
situation is somewhat similar when the bonding in L(Me)-
Al(µ-O)Ti(Me)Cp2 is considered.10 Interestingly, the stabi-
lization of the M-O and M-N bonds occurs by means of a
strong synergistic donor acceptor interaction between the
bonding and antibonding orbitals among themselves stabiliz-
ing each of the bonds by about 15 kcal/mol. The formation
of a cation results in disruption of this mutual synergy
between these bonds around the Ti (Zr) center. Moreover,
the enhanced Lewis acidity of the Ti (Zr) center of 7 (8)
does not encourage the cation formation as easily as observed
for L(Me)Al(µ-O)Ti(Me)Cp2 in which the Cp ligand acts as
Figure 4. Relative energy needed to generate cationic species revealing
that generation of the corresponding cation of 7 or 8 is more difficult than
that of L(Me)Al(µ-O)Ti(Me)Cp2 (CpTi+).
electronic buffer transferring necessary electron density to
the metal center in the corresponding cation. This is also
evident from the thermodynamic data revealing that the
related energy needed to generate the corresponding cation
of 7 (8) is more than three times that required to generate a
cation from L(Me)Al(µ-O)Ti(Me)Cp2 (Figure 4).
Summary and Conclusion
In this contribution, we report the syntheses, characteriza-
tion, and theoretical investigation of two covalently linked
oxygen-bridged trimetallic complexes bearing the Al-O-
M-O-Al (M ) Ti, Zr) core. The synthetic strategy takes
advantage of the recently synthesized kinetically stable
7598 Inorganic Chemistry, Vol. 46, No. 18, 2007

Compounds Containing an Al-O-M-O-Al Core
-30 °C. The reaction mixture was slowly warmed to ambient
temperature and was stirred at 25 °C for 14 h. The solvent was
evaporated to dryness yielding a colorless solid, and then it was
dissolved in n-hexane (40 mL) and the solution passed through an
activated Celite pad. The resulting solution was concentrated to
approximately 15 mL under reduced pressure and kept at 0 °C for
several days yielding colorless crystals of analytical purity. Yield:
L(Me)Al(OH) (1) [L ) CH{N(Ar)(CMe)}2, Ar ) 2,6-
iPr2C6H3] precursor as the building block. The Bro¨nsted
acidic character of the proton in the Al(O-H) moiety allows
almost clean reaction with less sterically hindered group 4
metal precursor M(NMe2)4 (M ) Ti, Zr) forming compounds
with the trimetallic core. Preliminary investigation on the
catalytic activity reveals that these complexes exhibit low
activity in ethylene polymerization as compared to the
oxygen-bridged metallocene-based heterobimetallic com-
plexes L(Me)Al(µ-O)M(Me)Cp2 (M ) Ti, Zr), which could
be attributed to the relatively lower stability of the supposed
cationic intermediate as revealed by DFT calculations.
0.42 g (75%). Mp: 246-247 °C. 1H NMR (500 MHz, C6D6,
3
25 °C, TMS; δ): -0.58 (s, 6H, Al-CH3); 1.12 (d, 12H, JH-H
)
3
6.8 Hz, CH(CH3)2); 1.18 (d, 12H, JH-H ) 6.8 Hz, CH(CH3)2);
3
1.29 (d, 12H, 3JH-H ) 6.8 Hz, CH(CH3)2); 1.33 (d, 12H, JH-H
)
6.8 Hz, CH(CH3)2); 1.52 (s, 12H, CH3); 2.81 (s, 12H, Zr-N(CH3)2);
3.26 (sept, 4H, 3JH-H ) 6.8 Hz, CH(CH3)2); 3.56 (sept, 4H, 3JH-H
) 6.8 Hz, CH(CH3)2); 4.95 (s, 2H, γ-CH); 7.06-7.24 (m, 12H,
aryl protons). 13C NMR (125.75 MHz, C6D6, 25 °C, TMS; δ):
-11.1 (s, Al-CH3); 23.7 (s, CH3); 24.5 (s, CH(CH3)2); 26.1 (s,
CH(CH3)2); 28.2 (s, CH(CH3)2); 28.6 (s, CH(CH3)2); 43.3 (s, Zr-
N(CH3)2); 98.1 (γ-CH);124.5, 127.0, 141.6, 144.1, 144.8, (s, aryl
carbon, p, m, o, and i, respectively); 169.8 (s, (CN)). MS (EI) [m/z
(%)]: 202 (100) [DippNCCH3]+. Anal. Calcd for C64H100Al2N6O2-
Zr: C, 67.98; H, 8.91; N, 7.43. Found: C, 67.66; H, 9.00; N, 7.34.
X-ray Structure Determination of 7 and 8. Suitable crystals
of 7 and 8 were mounted on a glass fiber and data was collected
on an IPDS II Stoe image-plate diffractometer (graphite-mono-
chromated Mo KR radiation, λ ) 0.710 73 Å) at 133(2) K. The
data was integrated with X-Area. The structures were solved by
Direct Methods (SHELXS-97)19 and refined by full-matrix least-
squares methods against F2 (SHELXL-97).20 All non-hydrogen
atoms were refined with anisotropic displacement parameters. One
of the eight isopropyl groups in both 7 and 8 was found to be
disordered. The hydrogen atoms were refined isotropically on
calculated positions using a riding model. Crystallographic data are
presented in Table 1.
Computational Details. The calculations were performed at the
well-established DFT level of theory making use of the B3LYP
functional21,22 as implemented in the Gaussian program package23
employing a basis-set termed LANL2DZ24 for Ti and 6-31G25,26
for the remaining atoms. In the first step the compound was fully
optimized to its equilibrium structure. The resulting electronic wave
function for the structure was then analyzed to obtain the shape of
the molecular orbitals, and a NBO analysis27-29 was performed to
ascertain the bonding situation.
Experimental Section
General Comments. All experimental manipulations were
carried out under an atmosphere of dry argon 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. Ti(NMe2)4 (Alfa
Aesar) and Zr(NMe2)4 (Aldrich) were purchased from commercial
sources and used without further purification. L(Me)Al(OH) (1)
[L ) CH{N(Ar)(CMe)}2, Ar ) 2,6-iPr2C6H3] was prepared by
following the literature procedure.8 NMR spectra were recorded
on a Bruker Avance 500 instrument, and the chemical shifts
downfield from the reference standard tetramethylsilane (TMS) were
assigned positive values. 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.
Synthesis of L(Me)Al(µ-O)Ti(NMe2)2(µ-O)Al(Me)L (7). A
solution of L(Me)Al(OH) (0.477 g, 1.0 mmol) in toluene (20 mL)
was added dropwise by a syringe over a period of 15 min to a
solution of Ti(NMe2)4 (0.112 g, 0.50 mmol) in toluene (20 mL) at
-30 °C. The reaction mixture was slowly warmed to ambient
temperature and was stirred at 25 °C for 14 h. The solvent was
evaporated to dryness yielding a pasty yellow solid, and then it
was dissolved in pentane (30 mL) and the solution passed through
an activated Celite pad. The yellow crystals of the title compound
were grown from concentrated pentane solution at -30 °C.
Nucleation of crystal growth sometime starts on warming the
pentane solution from -30 °C to room temperature. Yield: 0.32 g
(60%). Mp: 170-171 °C. 1H NMR (500 MHz, C6D6, 25 °C, TMS;
Polymerization of Ethylene. The polymerization reactions were
carried out on a high-vacuum line (10-5 Torr) in an autoclave
(Buchi). 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. The catalyst
(10 µmol) was taken in the Schlenk flask, and appropriate MAO
(1.6 M in toluene, Witco GmbH) was added. The mixture was
3
δ): -0.53 (s, 6H, Al-CH3); 1.17 (d, 12H, JH-H ) 6.8 Hz, CH-
3
(CH3)2); 1.19 (d, 12H, JH-H ) 6.8 Hz, CH(CH3)2);1.29 (d, 12H,
3
3JH-H ) 6.8 Hz, CH(CH3)2); 1.31 (d, 12H, JH-H ) 6.8 Hz, CH-
(CH3)2); 1.52 (s, 12H, CH3); 2.84 (s, 12H, Ti-N(CH3)2); 3.26 (sept.,
4H, 3JH-H ) 6.8 Hz, CH(CH3)2); 3.63 (sept., 4H, 3JH-H ) 6.8 Hz,
CH(CH3)2); 4.98 (s, 2H, γ-CH); 7.06-7.22 (m, 12H, aryl protons).
13C NMR (125.75 MHz, C6D6, 25 °C, TMS; δ): -10.9 (br s, Al-
CH3); 23.9 (s, CH3); 24.7 (s, CH(CH3)2); 26.5 (s, CH(CH3)2); 28.1
(s, CH(CH3)2); 28.6 (s, CH(CH3)2); 46.1 (s, Ti-N(CH3)2); 98.5 (γ-
CH); 124.4, 127.0, 141.9, 144.3, 144.7 (s, aryl carbon, p, m, o, and
i, respectively); 170.2 (s, (CN)). MS (EI) [m/z (%)]: 1086.8 (4)
[M]+, 1071.8 (64) [M - Me]+, 202 (100) [DippNCCH3]+. Anal.
Calcd for C64H100Al2N6O2Ti: C, 70.69; H, 9.26; N, 7.73. Found:
C, 70.24; H, 9.25; N, 7.61.
(19) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467-473.
(20) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(21) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
(22) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989,
157, 200-206.
(23) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004 (see Supporting Information).
(24) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270-283.
(25) Petersen, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081-
6090.
(26) Petersen, G. A.; Bennett, A.; Tensfeldt, T. G.; Al-Laham, M. A.;
Shirley, W. A.; Mantzaris, J. J. Chem. Phys. 1988, 89, 2193-2218.
(27) Foster, J. P.; Weinhold, F. J. Am. Chem. Soc. 1980, 102, 7211-7218.
(28) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736-1740.
(29) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899-
926.
Synthesis of L(Me)Al(µ-O)Zr(NMe2)2(µ-O)Al(Me)L (8). A
solution of L(Me)Al(OH) (0.477 g, 1.0 mmol) in toluene (20 mL)
was added dropwise by a syringe over a period of 15 min to a
solution of Zr(NMe2)4 (0.133 g, 0.50 mmol) in toluene (20 mL) at
Inorganic Chemistry, Vol. 46, No. 18, 2007 7599

Mandal et al.
stirred for 20 min to activate the catalyst. The catalyst solution
was then quickly injected into the rapidly stirred flask using a
gastight syringe. After a measured time interval, the polymerization
was quenched by the addition of methanol (5 mL), and the reaction
mixture was then poured into methanol (800 mL). The polymer
was allowed to fully precipitate overnight and then collected by
filtration, washed with fresh methanol, and dried.
S.K.M thanks the Alexander von Humboldt Foundation for
a research fellowship.
Supporting Information Available: Complete ref 23, X-ray
data for 7 and 8 (CIF), and tables of crystallographic and structural
refinement data, atomic coordinates, bond lengths, bond angles,
anisotropic thermal parameters, and calculated atomic coordinates
of 7 and 8 (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgment. This work is supported by the Go¨t-
tinger Akademie der Wissenschaften, Deutsche Forschungs-
gemeinschaft, and the Fonds der Chemischen Industrie.
IC701063V
7600 Inorganic Chemistry, Vol. 46, No. 18, 2007