Tetranuclear homo- and heteroalumoxanes containing reactive functional groups: Syntheses and X-ray crystal structures of [{[LA1(Me)](μ-O)(MH 2)}2]

Article abstract of DOI:10.1002/anie.200460395

A core with corners: The first alumoxane containing an {Al ₄O₂} core as well as a gallium congener with an {Al ₂Ga₂O₂} core are prepared by the reaction of an organoaluminum monohydroxide with MH₃·NMe₃ (see formula; M = Al, Ga; Ar = 2,6-iPr₂C₆H₃). These compounds contain reactive hydride groups on the central M₂O ₂ rings and methyl groups on the terminal aluminum atoms.

Full text of DOI:10.1002/anie.200460395

Communications
Al and Ga Compounds
Tetranuclear Homo- and Heteroalumoxanes
Containing Reactive Functional Groups:
Syntheses and X-ray Crystal Structures of
[{[LAl(Me)](m-O)(MH2)}2]**
Sanjay Singh, S. Shravan Kumar,
Vadapalli Chandrasekhar, Hans-Jürgen Ahn,
Marianna Biadene, HerbertW. Roesky,*
Narayan S. Hosmane, Mathias Noltemeyer, and
Hans-Georg Schmidt
There is considerable interest in organoaluminum compounds
À
containing Al O bonds. This is primarily due to the discovery
of methylalumoxane (MAO) as an extremely potent cocata-
[*] Dipl.-Chem. S. Singh, Dipl.-Chem. S. S. Kumar,
Dipl.-Chem. H.-J. Ahn, Dipl.-Chem. M. Biadene,
Prof. Dr. H. W. Roesky, Dr. M. Noltemeyer, H.-G. Schmidt
Institut für Anorganische Chemie
Georg-August-Universität Göttingen
Tammannstrasse 4, 37077 Göttingen (Germany)
Fax: (+49)551-393-373
E-mail: hroesky@gwdg.de
Prof. Dr. V. Chandrasekhar
Department of Chemistry
Indian Institute of Technology Kanpur
Kanpur-208016 (India)
Prof. Dr. N. S. Hosmane
Department of Chemistry and Biochemistry
Northern Illinois University, Dekalb, IL 60115-2862 (USA)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Göttinger Akademie der Wissenschaften. S.S. thanks the
Graduiertenkolleg 782 for a fellowship. V.C. thanks the Alexander
von Humboldt foundation for the Friedrich Wilhelm Bessel
Research Award. We thank the National Science Foundation for
supporting this work and N.S.H acknowledges the senior award of
the Alexander von Humboldt foundation. M=Al, Ga, L=2,6-
iPr2C6H3NC(CH3)CHC(CH3)NC6H3-2,6-iPr2 =HC{(CMe)(2,6-
iPr2C6H3N)}2.
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DOI: 10.1002/anie.200460395
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Angewandte
Chemie
lyst in the polymerization of ethylene and propylene by
Group 4metallocenes. [1] There have been several attempts to
prepare discrete molecular organoalumoxanes with well
defined structures because the active site structure in MAO
is not known. These efforts focussed on the controlled
hydrolysis of aluminumtrialkyls or -triaryls where the sub-
stituent groups on aluminum are sterically encumbered.[2]
Although several interesting compounds have been isolated
by the above approach and structurally characterized, none of
these was as effective as MAO as a cocatalyst. The second
problem of using controlled hydrolysis as a synthetic tool is
the lack of predictability of the reaction in being able to assess
the nature of the product. The task of assembling compounds
Scheme 1. Preparation of the tetranuclear alumoxane 2 (M=Al) and
the gallium congener 3 (M=Ga); Ar=2,6-iPr2C6H3.
metric stretch of the AlH2 fragment. The corresponding
stretching modes for 3 appear at 1901 and 1929 cmÀ1. The
1H NMR resonance signals for 2 and 3 are broad at room
temperature. However, a variable temperature 1H NMR
spectroscopy study revealed that the room-temperature
spectra of 2 and 3 are simplified by a dynamic process and
upon cooling the broad signals become sharp and eventually
resolve into different resonances for the two halves of the
molecules as expected based on the solid-state structures for 2
and 3. For 2 (À608C) and 3 (À708C) two sharp singlets were
observed for the methyl groups which are located on the
terminal aluminum atoms. This observation suggests that the
local environment around the two aluminum centers is
slightly different at least at lower temperatures. This feature
is also reflected in the hydride resonances of the central
Al2O2H4 and Ga2O2H4 units. Although the exact nature of the
dynamic process involved has not been clearly established it is
À À
with Ga O Ga bonds is similarly fraught with synthetic
challenges although some trinuclear[3] and a tetranuclear[4]
derivatives are known. To assemble soluble lipophilic organo-
alumoxanes in a predictable and rational manner a change in
the synthetic approach is required. Thus, instead of taking the
À
À
À À
Al C or Al H bonds as the starting point for making Al O
Al bonds it would be more convenient to use organoalumi-
À
num hydroxides containing Al O linkages as the precursors.
The recent isolation of a discrete aluminum dihydroxide
[LAl(OH)2],[5] prompted us to look for a suitable synthon that
would allow a rational and facile assembly of structural units
of alumoxanes as well as heteroalumoxanes. A second goal
was to retain reactive groups on the metal centers in such
compounds. This arrangement would allow a stepwise syn-
thesis of cage structures. Herein, we describe the synthesis
and structural characterization of [{[LAl(Me)](m-O)(AlH2)}2]
(2) and [{[LAl(Me)](m-O)(GaH2)}2] (3; L = HC{(CMe)(2,6-
iPr2C6H3N)}2). The synthesis of 2 and 3 were possible by
preparing an unprecedented terminal aluminum mono-
hydroxide [LAlMe(OH)] (1).[6] Compounds 2 and 3 are the
first examples of a tetranuclear alumoxane[7] and a gallium
congener, respectively. Apart from being examples of simple
building blocks (Al2O2 and Ga2O2) these compounds also
contain a pair of reactive MH2 (M = Al, Ga) groups in the
À
possible that this may be due to the restricted rotation of Al
À
O Al(Ga) bond at lower temperatures.
Single crystals of 2 and 3 suitable for X-ray structural
analysis were obtained from their n-hexane solutions
(Figure 1 and Figure 2). Both the compounds crystallize
with a molecule of n-hexane, compound 2 in the monoclinic
space group P21/c and 3 in P21/m.[11]
The X-ray crystal structure of 2 reveals that it is a dimer of
the [LAl(Me)OAl(H)2] unit. The structure contains two
terminal LAlMe units that are linked to
a central
À
central core and Al Me groups as the terminal end groups,
which should allow their rapid elaboration.
{(H)2AlO}2 core. The terminal aluminum centers are part of
the six-membered C3N2Al rings. Each aluminum center of the
central {(H)2AlO}2 four-membered ring contains two reactive
hydride groups. These are located above and below the plane
of the Al2O2 ring. The two methyl groups on the terminal Al
atoms are cis with respect to each other. The central four-
membered ring is nearly planar. The terminal six-membered
rings are displaced in an approximately perpendicular manner
with respect to the central {(H)2AlO}2 ring. The metric
The reaction of [LAl(OH)Me] with a stoichiometric
amount of MH3·NMe3 (M = Al, Ga) in toluene at 08C results
in a vigorous evolution of hydrogen and the formation of 2
and 3 in good yields (Scheme 1). Thus, this synthetic method
represents a viable and rational route for the preparation of
novel compounds. While compound 2 is the first example of
an alumoxane with an Al4O2 moiety which also carries
reactive hydride groups on the aluminum centers, compound
3 is the first example of a Al-O-Ga unit bearing reactive
hydride groups.
Compounds 2 and 3 have been unambiguously charac-
terized by means of spectroscopic, spectrometric, and crys-
tallographic techniques. Both 2 and 3 are colorless crystalline
solids and are thermally quite stable. They decompose with
melting at 260 and 2348C, respectively. The EI mass spectrum
of 2 revealed that the most intense peak appears at m/z 1007
and corresponds to the loss of one hydrogen atom from the
molecular ion. Similarly a peak at m/z 1079 in 3 is due to
[M+ÀCH3]. The IR spectrum of 2 shows a sharp doublet (1833
and 1850 cmÀ1) corresponding to the symmetric and antisym-
À
parameters observed in 2 are not unusual. Thus, the Al O
bonds in the Al2O2 ring (1.824(14)–1.842(14), av 1.833 Š) are
À
longer than the terminal Al O bonds (1.748(14) Š and
1.772(13) Š, av 1.760 Š) and are similar to those found in
aluminum alkoxides for example [{(tBu)2(Me)COAlH2}2]
[8]
[9]
À
À
(Al O 1.841 Š) and [{tBuOAlH2}2] (Al O 1.815 Š). The
average O-Al-O and Al-O-Al angles in the Al2O2 ring are
86.10 and 93.438, respectively, while the average exocyclic Al-
O-Al angles are 122.02 and 144.088.
The molecular structure of 3 is analogous to that of 2.
Thus, compound 3 also exists as a dimer and contains reactive
hydride groups on the central gallium atoms of the planar
Ga2O2 ring. The terminal C3N2Al rings are arranged in an
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those observed in [{tBuOGaH2}2] (101.48).[9] The slightly
wider angle in the latter is perhaps due to the bulky tBu group
that elongates the O···O diagonal in the four-membered ring.
The average exocyclic Al-O-Ga angles in 3 are 121.44 and
À
143.288. The average terminal Al O bond in 3 (1.744 Š) is
À
similar to that found in 2. The Ga O bond within the Ga2O2
ring (1.917(2)–1.939(2) Š, av 1.928 Š) are similar to those
À
found in gallium alkoxides for example [{tBuOGaH2}2] (Ga
O
1.908 Š)[9] and in [{Me2NCH2CH2OGaH2}2] (Ga O
À
1.911 Š).[10]
In conclusion we have demonstrated a new synthetic
strategy for the preparation of an alumoxane and a corre-
sponding gallium derivative. This procedure utilizes the
À
reaction of an Al OH motif and is a change in the standard
À
À
synthetic protocols which rely on the use of Al C or Al H
motifs. The resulting compounds [{[LAl(Me)](m-O)(AlH2)}2]
and[{[LAl(Me)](m-O)(GaH2)}2] are novel tetranuclear build-
ing blocks. The reactive hydride and methyl groups present on
the central and terminal metal atoms in these compounds
should allow a further elaboration of these tetranuclear
structures.
À
Figure 1. Molecular structure of 2. The hydrogen atoms of the C H
bonds and hexane molecule are omitted for clarity. Selected inter-
atomic distances [Š] and bond angles [8]: Al(1)-O(1) 1.748(14), Al(2)-
O(1) 1.838(14), Al(2)-O(2) 1.828(14), Al(4)-O(2) 1.824(14), Al(4)-O(1)
1.842(14), Al(2)···Al(4) 2.679, Al(3)-O(2) 1.772(13), Al(1)-N(1)
1.913(16), Al(1)-C(1) 1.965(2); Al(1)-O(1)-Al(2) 145.44(8), Al(1)-O(1)-
Al(4) 121.13(8), O(1)-Al(2)-O(2) 86.10(6), Al(2)-O(2)-Al(4) 94.36(6),
Al(3)-O(2)-Al(2) 142.73(8), Al(3)-O(2)-Al(4) 122.91(8), N(1)-Al(1)-N(2)
96.51(7), N(1)-Al(1)-O(1) 113.52(7).
Experimental Section
All manipulations were performed under a dry and oxygen-free
atmosphere (N2 or Ar) using Schlenk line and glove box techniques.
2: [LAl(Me)OH] (0.95 g, 2.00 mmol) dissolved in toluene
(20 mL) was added dropwise at 08C to a stirred (1.0m) solution of
AlH3·NMe3 (2.10 mL, 2.10 mmol) in toluene (15 mL). The solution
was allowed to warm to room temperature and stirred for 15 h. After
removal of all the volatiles the residue was extracted with n-hexane
(40 mL). Partial removal of the solvent and storage of the remaining
solution at room temperature for 2 days afforded colorless crystals of
2. Yield (0.75 g, 74% with respect to 1). M.p. 258–2608C (decomp);
elemental analysis (%) calcd for C60H92Al4N4O2 (1008.65): C 71.40, H
9.19, N 5.55; found: C 71.75, H 9.55, N 5.14; EI-MS: m/z (%): 1007
(92) [M+ÀH], 993 (72) [M+ÀMe], 979 (60) [M+ÀAlÀ2H], 965 (100)
[M+ÀAlÀMeÀ3H], 951 (20) [M+À2AlÀ3H]; IR (Nujol): n˜ = 1850,
1833, 1552, 1527, 1318, 1260, 1177, 1101, 1055, 1023, 936, 874, 803, 726,
724, 656, 634 cmÀ1, 1H NMR (500 MHz, C7D8, À608C): d = 7.26–6.89
(m; Ar), 4.80 (s, 1H; g-CH), 4.79 (s, 1H; g-CH), 3.93 (br, 4H; AlH2),
3.54(sept, 3JH,H = 6.3 Hz, 2H; CHMe2), 3.40 (sept, 3JH,H = 6.5 Hz, 2H;
3
3
CHMe2), 3.27 (sept, JH,H = 6.5 Hz, 2H; CHMe2), 2.97 (sept, JH,H
=
6.5 Hz, 2H; CHMe2), 1.70–1.66 (m, 12H; CHMe2), 1.46 (s, 6H; CMe),
1.39 (s, 6H; CMe), 1.34(d, 3JH,H = 6.3 Hz, 6H; CHMe2), 1.20 (m, 12H;
CHMe2), 1.14(d, 3JH,H = 6.2 Hz, 6H; CHMe2), 1.04(d, 3JH,H = 6.4Hz,
6H; CHMe2), 0.92 (d, 3JH,H = 6.4Hz, 6H; CH Me2), 0.07 (s, 3H;
AlMe), À0.81 ppm (s, 3H; AlMe); 1H NMR (500 MHz, C7D8, 1008C):
d = 7.16–6.96 (m; Ar), 5.02 (s, 2H; g-CH), 3.65 (br, 4H; AlH2), 3.40
(sept, 3JH,H = 6.7 Hz, 4H; CHMe2), 3.14(br, 4H; C HMe2), 1.60 (s,
12H; CMe), 1.38–1.31 (br, 24H; CHMe2 and CMe), 1.10 (m, 24H;
CHMe2), À0.56 ppm (s, 6H; AlMe).
À
Figure 2. Molecular structure of 3. The hydrogen atoms of the C H
bonds and hexane molecule are omitted for clarity. Selected inter-
atomic distances [Š] and bond angles [8]: Al(1)-O(1) 1.733(2), Ga(1)-
O(1) 1.933(2), Ga(1)-O(2) 1.924(2), Ga(2)-O(2) 1.917(2), Ga(2)-O(1)
1.939(2), Ga(1)-H(1) 1.516, Ga(1)···Ga(2) 2.849(7), Al(2)-O(2)
1.755(2), Al(1)-N(1) 1.920(19), Al(1)-C(1) 1.967(4); Al(1)-O(1)-Ga(1)
145.00(14), Al(1)-O(1)-Ga(2) 120.25(13), O(1)-Ga(1)-O(2) 84.73(9),
Ga(1)-O(2)-Ga(2) 95.78(9), Ga(1)-O(2)-Al(2) 141.57(13), Ga(2)-O(2)-
Al(2) 122.65(12), N(1)-Al(1)-N(2) 96.19(12), N(1)-Al(1)-O(1)
114.44(8).
3: The preparation of 3 was carried out by using a similar
procedure as that for 2. The quantities of the reactants used were
[LAl(Me)OH] (1.43 g, 3.00 mmol), GaH3·NMe3 (0.40 g, 3.00 mmol).
Yield (1.30 g, 79% with respect to 1). M.p. 2348C (decomp);
elemental analysis (%) calcd for C60H92Al2Ga2N4O2 (1094.81): C
65.82, H 8.47, N 5.12; found: C 65.67, H 8.33, N 5.29; EI-MS: m/z (%):
1094(24) [ M+], 1079 (100) [M+ÀMe], 1052 (16) [M+ÀAlÀMe], 1022
(20) [M+ÀGaÀ2H], 1007 (20) [M+ÀMeÀGaÀ2H]; IR (Nujol): n˜ =
1929, 1901, 1585, 1551, 1521, 1315, 1293, 1256, 1183, 1176, 1107, 1098,
938, 874, 797, 770, 755, 737, 709, 659, 642, 616, 531, 507 cmÀ1; 1H NMR
(500 MHz, C7D8, À708C): d = 7.22–6.88 (m, Ar), 5.20 (s, 2H; GaH2),
5.05 (s, 2H; GaH2), 4.70 (s, 1H; g-CH), 4.63 (s, 1H; g-CH), 3.77 (m,
approximately perpendicular manner with respect to the
central Ga2O2 ring. Also, the methyl groups on the terminal
aluminum centers are cis with respect to each other. The
average O-Ga-O and Ga-O-Ga angles within the Ga2O2 ring
are 84.73 and 95.278, respectively. This may be compared with
4942
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Chemie
2H; CHMe2), 3.45 (m, 2H; CHMe2), 3.32 (m, 2H; CHMe2), 2.96 (m,
2H; CHMe2), 1.68 (d, 3JH,H = 17.3 Hz, 12H; CHMe2), 1.49 (d, 3JH,H
tal size = 0.2 ” 0.1 ” 0.2 mm3, 1calcd = 1.093 MgmÀ3, 2.22 ꢀ 2q ꢀ
=
58.938, T= 103(2) K, l = 1.54178 Š, m = 0.973 mmÀ1, F(000) =
2392, 61271 reflections collected, 9543 were independent and
were used in the structure refinement of 786, R1 = 0.0403 (I >
2s(I)), wR2 = 0.1162 (all data), min./max. residual electron
density 0.411/À0.310 eŠÀ3. Crystallographic data for compound
3·C6H14 (C66H106Al2Ga2N4O2): Mr = 1180.95, monoclinic, P21/m,
a = 9.4431(13), b = 17.889(2), c = 20.491(5) Š, b = 103.1178(14),
V= 3371.1(10) Š3, Z = 2, crystal size = 0.3 ” 0.2 ” 0.2 mm3,
5.8 Hz, 12H; CHMe2), 1.40 (s, 12H; CMe), 1.14(dd, 3JH,H = 17.3 Hz,
18H; CHMe2), 0.92 (d, 3JH,H = 6.0 Hz, 6H; CHMe2), 0.04(s, 3H;
AlMe), À0.95 ppm (s, 3H; AlMe); 1H NMR (500 MHz, C7D8, 708C):
d = 7.17–6.96 (m; Ar), 4.91 {s, 6H (2H, g-CH and 4H, GaH2)}, 3.48
(br, 4H; CHMe2), 3.15 (br, 4H; CHMe2), 1.56 (s, 12H; CMe), 1.43 (br,
3
12H; CHMe2), 1.32 (br, 12H; CHMe2), 1.17 (d, JH,H = 6.5 Hz, 12H;
CHMe2), 1.10 (d, 3JH,H = 8.7 Hz, 12H; CHMe2), À0.65 ppm (s, 6H;
AlMe).
1calcd = 1.163 MgmÀ3
0.71073 Š, m = 0.868 mmÀ1
,
3.06 ꢀ 2q ꢀ 49.608, T= 133(2) K, l =
F(000) = 1268, 20878 reflections
,
collected, 5960 were independent and were used in the structure
Received: April 21, 2004
refinement of 388, R1 = 0.0362 (I > 2s(I)), wR2 = 0.09995 (all
À3
data), min./max. residual electron density 0.719/À0.536 e Š
.
Keywords: aluminum · alumoxanes · gallium · hydrides ·
.
CCDC-236234( 3) and CCDC-236235 (2) contain the supple-
mentary crystallographic data. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+ 44)1223-336-033 or deposit@
ccdc.cam.ac.uk). The structures were solved by direct methods
using SHELXS-97 and refined against F2 on all data by full-
matrix least squares; G. M. Sheldrick programs for crystal
structure refinement, Universitꢀt Göttingen, Göttingen (Ger-
many), 1997. Hydrogen atoms were included at geometrically
calculated positions and refined using a riding model.
hydroxides
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Angew. Chem. Int. Ed. 2004, 43, 4940 –4943
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