Synthesis and characterization of (MesGaO)9 (Mes= Me3C6H2) and crystal structure of the first galloxane comparable to catalytically active aluminum compounds

Article abstract of DOI:10.1021/om970086d

The thermolysis of (MeS₂GaOH)₂·THF in toluene or 1,4-dioxane at 100 °C resulted in the formation of (MesGaO)₉ (1). A byproduct of this reaction is Mes₃Ga. Compound 1 was characterized by electron impact mass spectroscopy and single-crystal X-ray structural analysis. The structure consists of two six-membered (MesGaO)₃ rings connected by three μ₂-(MesGaO) units.

Full text of DOI:10.1021/om970086d

3074
Organometallics 1997, 16, 3074-3076
Syn th esis a n d Ch a r a cter iza tion of (MesGa O)9 (Mes )
Me3C6H2) a n d Cr ysta l Str u ctu r e of th e F ir st Ga lloxa n e
Com p a r a ble to Ca ta lytica lly Active Alu m in u m
Com p ou n d s†
J ens Storre,‡ Andreas Klemp,‡ Herbert W. Roesky,*,‡ Roland Fleischer,§ and
Dietmar Stalke§
Institut fu¨r Anorganische Chemie der Universita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany, and Institut fu¨r Anorganische Chemie der Universita¨t
Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
Received February 5, 1997X
Summary: The thermolysis of (Mes2GaOH)2‚THF in
toluene or 1,4-dioxane at 100 °C resulted in the formation
of (MesGaO)9 (1). A byproduct of this reaction is
Mes3Ga. Compound 1 was characterized by electron
impact mass spectroscopy and single-crystal X-ray struc-
tural analysis. The structure consists of two six-
membered (MesGaO)3 rings connected by three µ2-
(MesGaO) units.
information concerning the analogous gallium com-
pounds are limited to few hydroxides and oxide
hydroxides.11,13-17 Although Barron et al. have char-
acterized the nonameric galloxane (tBuGaO)9, there has
been no X-ray structural investigation on galloxanes.18
Recently, we have shown that controlled hydrolysis
of Mes3Ga in THF solution leads to the formation of
(Mes2GaOH)2‚THF.19 We now report the synthesis and
characterization of (MesGaO)9 (1) formed by thermolysis
of (Mes2GaOH)2‚THF (eq 1).
Controlled reactions of organoaluminum or -gallium
compounds with water lead to the formation of alumox-
anes or galloxanes with the general formulae (RMO)n
or (R2MOMR2)n (M ) Al, Ga).1 The alkyl-substituted
alumoxanes were studied in the 1960s as catalysts for
polymerization reactions.2-7 When Sinn and Kaminsky,
in 1980, found methylalumoxane (MAO) to be a highly
active cocatalyst for group 4 metallocenes (e.g., Cp2-
ZrMe2) in alkene polymerization,8 considerable impetus
was given to structural information on alumoxanes in
order to elucidate their role in these systems. Therefore,
manifold variations of organic substituents on alumi-
num have been introduced or the exchange of the metal
by gallium has been made to determine the character-
istics of metalloxanes.1 On this basis, Barron et al.
found that the hydrolysis of tBu3Al leads to hydroxides,
oxide hydroxides, or oxides.9-12 In contrast, structural
∆
9(Mes2GaOH)2‚THF
8 2(MesGaO)9 (1)
-9THF
-18MesH
Due to the high thermal stability of the hydroxide, no
reaction was observed in refluxing THF. Therefore, we
performed condensation reactions in toluene or 1,4-
dioxane (as coordinating solvent) at 100 °C. In both
solvents, the yield of the nonameric galloxane was about
60%. The main byproduct under these reaction condi-
1
tions was trimesitylgallium. The H NMR spectrum of
1 shows two sets of mesityl protons in the ratio of 2:1
(δ 6.64, 6.59 ppm for the ring protons, δ 2.57, 2.49 ppm
for the o-CH3 protons, and δ 2.07, 2.00 ppm for the para-
CH3 protons, respectively), indicating that the mesityl
groups in 1 are magnetically nonequivalent. The elec-
tron impact (EI) mass spectrum shows the molecular
ion (m/z 1844 1; m/z 1605, 100, 1 - Mes - MesH).
Single crystals suitable for an X-ray structural deter-
mination were obtained from THF. The molecular
structure of 1 is shown in Figure 1.
† Dedicated to Professor Roald Hoffmann on the occasion of his 60th
birthday.
* Author to whom correspondence should be addressed. Fax: +49-
(0)551393373. E-mail:hroesky@gwdg.de.
‡ Institut fu¨r Anorganische Chemie der Universita¨t Go¨ttingen.
§ Institut fu¨r Anorganische Chemie der Universita¨t Wu¨rzburg.
Fax: +49(0)9318884619. E-mail:dstalke@xiris.chemie.uni-wu¨rzburg.de.
X Abstract published in Advance ACS Abstracts, J une 1, 1997.
(1) Generally, alumoxanes and galloxanes are species containing an
oxygen bridge connecting two metal atoms M-O-M (M ) Al, Ga).
Oligomeric aluminum or gallium alkoxides bridged by alkoxy groups
M-O(R)-M and compounds containing no organic group are (by
definition) not included in this classification. See: (a) Pasynkiewicz,
S. Polyhedron 1990, 9, 429. (b) Atwood, J . L. In Coordination Chemistry
of Aluminum; Robinson, G. H., Ed.; VCH Publishers, Inc.: New York,
1993; p 219.
(12) Power, M. B.; Ziller, J . W.; Barron, A. R. Organometallics 1992,
11, 2783.
(13) Barron, A. R. Comm. Inorg. Chem. 1993, 14, 123.
(14) (a) Naiini, A. A.; Young, V.; Han, H.; Akinc, M.; Verkade, J . G.
Inorg. Chem. 1993, 32, 3781. (b) Atwood, A. D.; Cowley, A. H.; Harris,
P. R.; J ones, R. A.; Koschmieder, S. U.; Nunn, C. M. Organometallics
1993, 12, 24. (c) Power, M. B.; Cleaver, W. M.; Apblett, A. W.; Barron,
A. R. Polyhedron 1992, 11, 477.
(15) (a) Kenney, M. E.; Laubengayer, A. W. J . Am. Chem. Soc. 1954,
76, 4839. (b) Smith, G. S.; Hoard, J . L. J . Am. Chem. Soc. 1959, 81,
3907.
(2) Vandenberg, E. J . J . Polym. Sci. 1960, 47, 489.
(3) Colclough, R. O.; Gee, G.; J agger, A. H. J . Polym. Sci. 1960, 48,
273.
(4) Saegusa, T.; Fujii, Y.; Fujii, H.; Furukawa, J . Macromol. Chem.
1962, 55, 232.
(16) Schluter, R. D.; Isom, H. S.; Cowley, A. H.; Atwood, D. A.; J ones,
R. A.; Olbrich, F.; Corbelin, S.; Lagow, R. J . Organometallics 1994,
13, 4058.
(5) Ishida, S. I. J . Polym. Sci. 1962, 62, 1.
(6) Longiave, C.; Castelli, R. J . Polym. Sci. 1963, 4C, 387.
(7) Sakharovskaya, G. B. Zh. Obshch. Khim. 1969, 39, 788; Ibid. J .
Gen. Chem. USSR 1996, 39, 749.
(8) Sinn, H.; Kaminsky, W. Adv. Organomet. Chem. 1980, 18, 99.
(9) Mason, M. R.; Smith, J . M.; Bott, S. G.; Barron, A. R. J . Am.
Chem. Soc. 1993, 115, 4971.
(10) Harlan, C. J .; Mason, M. R.; Barron, A. R. Organometallics
1994, 13, 2957.
(11) Landry, C. C.; Harlan, C. J .; Bott, S. G.; Barron, A. R. Angew.
Chem. 1995, 107, 1315; Angew. Chem., Int. Ed. Engl. 1995, 34, 1201.
(17) Storre, J .; Belgardt, T.; Roesky, H. W.; Stalke, D. Angew. Chem.
1994, 106, 1365; Angew. Chem., Int. Ed. Engl. 1994, 33, 1244.
(18) Two structurally characterized compounds containing Ga-O-
Ga units are known but not comparable to our compound, see: (a)
Neumu¨ller, B.; Gahlmann, F. Angew. Chem. 1993, 105, 1770; Angew.
Chem., Int. Ed. Engl. 1993, 32, 1701. (b) Cowley, A. H.; Decken, A.;
Olazabal, C. A.; Norman, N. C. Inorg. Chem. 1994, 33, 3435.
(19) Storre, J .; Klemp, A.; Roesky, H. W.; Schmidt, H.-G.; Noltem-
eyer, M.; Fleischer, R.; Stalke, D. J . Am. Chem. Soc. 1996, 118, 1380.
S0276-7333(97)00086-1 CCC: $14.00 © 1997 American Chemical Society

Notes
Organometallics, Vol. 16, No. 13, 1997 3075
vacuo beginning at ca. 110 °C. Investigations of the
volatiles collected in cooling traps (-196 °C) gave
different ratios of mesitylene compared to THF at
various temperatures (3:1 to 6:1). Moreover, the solid
residue exhibits two characteristic OH vibrations in the
IR spectra (3671 and 3645 cm-1), clearly indicating
hydroxide groups and remaining THF molecules. There-
fore, we propose eq 2 as a competing reaction to eq 1.
-4
10
n(Mes2GaOH)2‚THF
mbar, ∆8
nMes Ga
[MesGa(OH) ‚THF]
+
(2)
3
2
n
2
3
However, intermediates of composition such as [Mes2-
Ga(OH)-MesGa(OH)2‚THF]n (4) cannot be ruled out. At
various reaction conditions (temperature and heating
time), the yields of 1 (20-70%), 2 (15-30%), and the
hydroxides (3 or 4, 15-50%) vary in the solid residue.
The EI mass spectrum of the residue shows only the
1
molecular ion of mesitylene (m/z 120). In the H NMR
spectrum, only the signals of 1, 2, and mesitylene can
be detected. We assume decomposition of the interme-
diates 3 and 4, respectively, in solution due to mesityl-
ene and Mes3Ga formation in vacuo. However, it has
been impossible up to now to establish a precise
condensation mechanism. The obvious temperature
dependance of the condensation reaction can be deduced
from the constant product distribution at a constant
temperature and the variety of product yields at differ-
ent temperatures (see above).
However, the experiments indicate that the equilibria
postulated for the methylalumoxane system1,21 are
feasable for the related gallium systems as well. The
existence of hydroxidic intermediates during condensa-
tion reactions has been established. Therefore, possible
interactions between hydroxide groups and the catalyti-
cally active species in polymerization reactions of olefins
with metallocenes should not be neglected.22 This is
indicated by the nonexistence of cocatalytic properties
of 1 in the polymerization of ethylene with Cp2ZrMe2.
Recently, Barron et al. introduced the idea of the “latent
Lewis acidity“ which might be responsible for the
cocatalytic activity of (tBuAlO)n.23 However, no crystal-
lographic data are available for any nonameric metal-
loxane for comparing the structural parameters of 1
with those of the homologous aluminum compounds.
Figu r e 1. ORTEP plot of [(MesGaO)9 + 10THF + Me3C6H3
] 1. The mesitylene molecule and the nine noncoordinated
THF molecules are omitted for clarity. Important average
bond distances (Å): Ga-C 1.990 , Ga-O ((MesGaO)3) 1.910
, Ga-O (µ2-(MesGaO)) 1.967.
For reasons of clarity, lattice solvent molecules are
omitted (nine noncoordinating THF molecules are lo-
cated between the mesityl groups and one THF molecule
is placed between four (MesGaO)9 units in the layer).
In addition, a single nonameric unit cocrystallizes with
a single mesitylene (C9H12) molecule. To avoid a decay
of the crystals due to loss of crystal solvent they were
mounted on a glass fiber at -50 °C.20 The nonameric
mesitylgalloxane 1 crystallizes in the orthorhombic
space group Pccn. The structure can be described
having two six-membered (MesGaO)3 rings connected
by three µ2-MesGaO units. A similiar structure was
found for the analogous (tBuAlO)9, which is isostructural
to (NaOtBu)9.9 All gallium atoms of this cage are
coordinated by three µ3-oxygen atoms. The average
gallium-carbon bond length (1.990 Å) is in the range
of known mesityl-substituted gallium compounds.19 The
average Ga-O bond length of the two (MesGaO)3 rings
is 1.910 Å, whereas the three Ga-O bond distances in
the µ2-(MesGaO) units are remarkably longer (1.967 Å
average). This bond lengthening is probably induced
by the electrostatic competition of a fourth oxygen atom
for each gallium atom in the bridging position between
the six-membered rings (average Ga(4)‚‚‚O(5′),
Ga(5)‚‚‚O(5), Ga(5′)‚‚‚O(4) 3.524 Å). Furthermore, it
seems noteworthy that the µ2-(MesGaO) units between
the rings are located closer to the bottom than to the
top ring system shown in Figure 1 (Ga-O 1.871 versus
1.944 Å).
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All experiments were per-
formed using standard Schlenk techniques under
a dry
nitrogen atmosphere due to the extreme sensitivity of the
reactants and products toward air and moisture.19 A Braun
MB 150-GI glovebox was used to store the compounds and to
prepare the samples for spectroscopic characterization. All
solvents were dried over sodium/benzophenone, freshly dis-
tilled, and degassed prior to use. (Mes2GaOH)2‚THF was
prepared as previously reported.19
To obtain additional information on condensation
reactions and the formation of Mes3Ga, we performed
solvent-free thermolysis of (Mes2GaOH)2‚THF at atmo-
spheric pressure and in vacuo (7.5 × 10-3 Torr) at
various temperatures. According to eq 1, the elimina-
tion of mesitylene starts at the decomposition point of
(Mes2GaOH)2‚THF (172 °C, atmospheric pressure)
whereas sublimation of Mes3Ga can be observed in
Elemental analyses were performed by the Analytisches
Labor des Instituts fu¨r Anorganische Chemie der Universita¨t
(21) Barron, A. R. Organometallics 1995, 14, 3581.
(22) This could also be of interest in the water-induced carbalumi-
nation of alkynes by zirconium containing catalysts. See for example:
Wipf, P.; Lim, S. Angew. Chem. 1993, 105, 1095; Angew. Chem., Int.
Ed. Engl. 1993, 32, 1068.
(23) Harlan, C. J .; Bott, S. G.; Barron, A. R. J . Am. Chem. Soc. 1995,
117, 6465.
(20) Kottke, T.; Stalke, D. J . Appl. Crystallogr. 1993, 26, 615.

3076 Organometallics, Vol. 16, No. 13, 1997
Notes
Ta ble 1. Cr ysta llogr a p h ic Da ta for 1
1
Go¨ttingen. NMR spectra were recorded on a Bruker AM 200
and were externally referenced to tetramethylsilane. FT-IR
spectra were measured on a Bio-Rad FTS-7 as Nujol mulls
compd
empirical formula
fw
C126H183Ga9O18
between KBr plates in the range 4000-400 cm-1
.
EI mass
2613.2
193(2)
0.7 × 0.7 × 0.6
orthorhombic
Pccn
18.78(3)
20.63(2)
33.65(3)
90
spectra were measured on
spectrometer.
a Finnigan MAT 8230 mass
temp (K)
cryst size (mm)
cryst syst
space group
a (Å)
P r ep a r a tion of (MesGa O)9 (1): Meth od a . A suspension
of (Mes2GaOH)2‚THF (2.0 g, 2.7 mmol) in toluene or 1,4-
dioxane (30 mL) was heated to 100 °C for 6 h. After removing
the volatiles under reduced pressure, the white solid was
washed with n-hexane (15 mL), filtered, and dried in vacuo.
Crystallization from THF gave 1 (0.7 g) in 61% yield. Mp >
350 °C. The filtrate contained 0.4 g of Mes3Ga. 1H NMR (200
MHz, C6D6): δ 6.64 (s, 12 H, Ar-H), 6.59 (s, 6 H, Ar-H), 2.57
(s, 36 H, 2,6-CH3), 2.49 (s, 18 H, 2,6-CH3), 2.07 (s, 18 H, 4-CH3),
2.00 (s, 9 H, 4-CH3) ppm. MS (70 eV): m/e 1844 ((MesGaO)9,
10), 1724 ((MesGaO)9 - MesH, 65), 1605 ((MesGaO)9 - MesH
- Mes, 100). IR (Nujol mull): 3015 (s), 1719 (m), 1601 (vs),
1558 (s), 1412 (s), 1293 (m), 1094 (vs), 1028 (vs), 947 (m), 848
(s), 737 (vs), 663 (vs), 589 (s), 572 (s), 551 (vs), 491 (m), 464
(s), 404 (vs). Anal. Calcd for C81H99Ga9O9 (1844.15): C, 52.76;
H, 5.41. Found: C, 52.5; H, 5.6.
P r epar ation of (MesGaO)9 (1): Meth od b. (Mes2GaOH)2‚
THF (0.8 g, 1.1 mmol) was heated at atmospheric pressure to
190 °C. At 172 °C, elimination of mesitylene and THF could
be observed. The volatiles were collected in a cold trap and
characterized by 1H NMR. After removal of the volatiles at
90 °C under reduced pressure, the white solid was washed with
n-hexane (15 mL), filtered, and dried in vacuo. Yield 0.3 g of
1 (71%). From the filtrate, 0.1 g of Mes3Ga was obtained.
Spectroscopic data and elemental analysis are consistent with
those of method a.
Con d en sa tion of (Mes2Ga OH)2‚THF in va cu o. (Mes2-
GaOH)2‚THF (0.8 g, 1.1 mmol) was heated in vacuo (7.5 × 10-3
Torr) to temperatures between 110 and 160 °C. Subliming
Mes3Ga was removed on a cold finger held at 30 °C. The
volatiles (mesitylene and THF) were collected in a cold trap
(-196 °C). The reaction time at each temperature was 8 h.
At various temperatures, different amounts of Mes3Ga, vola-
tiles, and solid residue were obtained. The total assay of each
reaction was correct. Spectroscopic data of the solid residue:
1H NMR (200 MHz, C6D6) δ 6.89-6.44 (m, Ar-H), 2.94-2.00
(m, 2,4,6-CH3 ppm. MS (70 eV): m/e 120 (MesH, 100). IR
(Nujol mull): 3671 (s, ν(OH)), 3645 (s, ν(OH)), 3509 (s, broad,
ν(OH)), 3016 (s), 1756 (m), 1719 (m), 1601 (vs), 1556 (s), 1411
(s), 1292 (m), 1094 (vs), 1029 (vs), 947 (m), 846 (vs), 835 (m),
736 (vs), 708 (vs), 687 (s), 663 (vs), 588 (s), 571 (s), 543 (vs),
492 (m), 464 (s), 403 (vs).
b (Å)
c (Å)
R (deg)
â (deg)
90
γ (deg)
90
13.04(3)
4
1.331
1.888
cell volume, V (nm3)
Z
Fc (g‚mm-3
)
µ (mm-1
F(000)
)
5448
2θ range (deg)
5-45
no. of data measd, unique
R,a wR2b (I > 2σI)
R, wR2 (all data)
20 787, 11 026 (Rint ) 0.0373)
0.0691, 0.1470
0.0973, 0.1608
1.259
goodness of fit, Sc
weight factors a, bd
no. of refined parameters
restraints
0.029, 35.790
1026
4045
largest diff peak, hole (e‚nm-3
)
462/-543
a
b
R ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) [∑w(Fo2 - Fc2)2]/[∑w(Fo2)2]1/2
.
c S ) [∑w(Fo - Fc2)2]/∑(n - p)]1/2
.
w-1 ) σ2(Fo2) + (aP)2 + bP; P
2
d
) [Fo + 2Fc2]/3.
2
4
[∑w(Fo2 - Fc2)2/∑wFo
]
1/2.All non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were geometrically
positioned and treated as riding. All five THF molecules
present suffer from severe disorder. Similarity restraints were
applied to geometrically equivalent bond lengths and angles
of the lattice THF molecules. They were refined as puckered
C5H10 five-membered rings because it was impossible to
identify the oxygen atom. Two THF molecules in general
positions were refined to three disordered positions with site
occupation factors of 0.4/0.4/0.2 and 0.45/0.35/0.2, respectively,
and two THF molecules in general positions were refined to
two disordered positions with site occupation factors 0.6/0.4
and 0.65/0.35, respectively. The single THF molecule at the
center of inversion was refined by suppressing the special
position constraints.
Ack n ow led gm en t. This work has been financially
supported by the BMBF, the Deutsche Forschungsge-
meinschaft, and the Hoechst AG.
X-r a y Str u ctu r e Deter m in a tion of 1. The crystal of 1
was mounted in an oil drop on a glass fiber at low tempera-
tures. Due to a phase transition of the crystals at -93 °C,
data were collected at -80 °C24 on a Stoe-Siemens-Huber
diffractometer with monochromated Mo KR radiation (λ )
0.710 73 Å) using a SMART-CCD area detector. The structure
was solved by direct methods using SHELXS-90.25 The
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, fractional coordinates, anisotropic displacement param-
eters, and bond lengths and angles and fully labeled figures
showing 50% thermal ellipsoids and hydrogen atom coordi-
nates of 1 (14 pages). Ordering information is given on any
current masthead page.
2
structure was refined against F with a weighting scheme of
2
w-1) σ2(Fo2) + (g1P)2 + g2P with P ) (Fo + 2Fc2)/3.26 The R
values are defined as R1 ) ∑||Fo| - |Fc||/∑|Fo| and wR2 )
OM970086D
(24) Kottke, T.; Lagow, R. J .; Stalke, D. J . Appl. Crystallogr. 1996,
29, 465.
(25) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(26) Sheldrick, G. M. SHELXL-93/ 96, programs for crystal structure
refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1996.