New routes to synthetically useful, sterically encumbered arylaluminum halides and hydride halides

Article abstract of DOI:10.1021/ic9515416

The synthesis and structural characterization of the compounds Mes*AlCl₂(THF) (1), Mes*AlCl₂ (2), Mes*Al(H)Cl(THF) (3a), Mes*Al(H)Cl (4a), and (Mes*AlH2): (5) (Mes* = 2,4,6-t-Bu₃C₆H₂^-) are described as well as those for two compounds 3b and 4b that are analogs of 3a and 4a but have H:Cl ratios that are less than 1:1. All compounds were characterized by ¹H, ¹³C NMR, and IR spectroscopy, and 1, 2, 3a, and 4b were additionally characterized by X-ray crystallography. Compound 1 is best synthesized by the reaction of [(THF)₂LiH₃AlMes*]₂ (6) with 6 equiv of Me₃SiCl. A more conventional route involving the addition of (THF)₂LiMes* to 2 equiv of AlCl₃ in toluene usually affords a mixture of 1 and AlCl₃·THF. Recrystallization of 1 from n-hexane results in a species that has less than 1 equiv of THF per Mes* AlCl₂ The THF free complex 2 may be obtained in quantitative yield by heating 1 for 20 min at 90 °C under reduced pressure. Compound 3a may be obtained by treating a 1:1 mixture of Mes*Li(THF)₂ and LiAlH₄ with 2 equiv of Me₃₀SiCl or by the addition of slightly less than 4 equiv of Me₃SiCl to 6. The THF can be removed from 3a by sublimation to give 4a. The related compounds 3b and 4b, which display an aluminum-bound H:Cl ratio that is deficient in H, can be obtained by reactions with slightly more than 2 equiv of Me₃SiCl. Crystal data at 130 K with Cu Kα (λ = 1.541 78 A?) radiation: 1, C₂₂H₃₇AlCl₂O, a = 11.889(3) A?, b = 9.992(3) A? , c = 19.704(5) A?, orthorhombic, space group Pca2₁, Z = 4, R = 0.068 for 1556 (I > 2σ(I)) data; 2, C₁₈H₂₉AlCl₂, a = 12.147(5) A?, b = 18.042(6) A?, c = 17.771(7) A?, β= 95.77(3)°, monoclinic, space group P2₁/n, Z = 8, R = 0.032 for 4610 (I > 2σa(I)} data; 3a, C₂₂H₃₈AlClO, a = 16.887(7) A?, b = 16.333(6) A?, c = 8.739(3) A?, β= 101.41(3)°, monoclinic, space group P2₁/c, Z = 4, R = 0.073 for 2752 (I > 2σ(I)) data; 4b, C₁₈H_29.64AlCl_1.36, a = 12.077(3) A?, b = 17.920(3) A?, c = 17.634(5) A? ; β = 95.21(2) A?, monoclinic, space group P2₁/n, Z = 8, R = 0.070 for 4261 (I > 2σ(I)) data.

Full text of DOI:10.1021/ic9515416

3262
Inorg. Chem. 1996, 35, 3262-3267
New Routes to Synthetically Useful, Sterically Encumbered Arylaluminum Halides and
Hydride Halides
Rudolf J. Wehmschulte and Philip P. Power*
Department of Chemistry, University of California, Davis, California 95616
ReceiVed December 1, 1995X
The synthesis and structural characterization of the compounds Mes*AlCl2(THF) (1), Mes*AlCl2 (2), Mes*Al-
(H)Cl(THF) (3a), Mes*Al(H)Cl (4a), and (Mes*AlH2)2 (5) (Mes* ) 2,4,6-t-Bu3C6H2-) are described as well as
those for two compounds 3b and 4b that are analogs of 3a and 4a but have H:Cl ratios that are less than 1:1. All
1
compounds were characterized by H, 13C NMR, and IR spectroscopy, and 1, 2, 3a, and 4b were additionally
characterized by X-ray crystallography. Compound 1 is best synthesized by the reaction of [(THF)2LiH3AlMes*]2
(6) with 6 equiv of Me3SiCl. A more conventional route involving the addition of (THF)2LiMes* to 2 equiv of
AlCl3 in toluene usually affords a mixture of 1 and AlCl3‚THF. Recrystallization of 1 from n-hexane results in
a species that has less than 1 equiv of THF per Mes*AlCl2. The THF free complex 2 may be obtained in quantitative
yield by heating 1 for 20 min at 90 °C under reduced pressure. Compound 3a may be obtained by treating a 1:1
mixture of Mes*Li(THF)2 and LiAlH4 with 2 equiv of Me3SiCl or by the addition of slightly less than 4 equiv
of Me3SiCl to 6. The THF can be removed from 3a by sublimation to give 4a. The related compounds 3b and
4b, which display an aluminum-bound H:Cl ratio that is deficient in H, can be obtained by reactions with slightly
more than 2 equiv of Me3SiCl. Crystal data at 130 K with Cu KR (λ ) 1.541 78 Å) radiation: 1, C22H37AlCl2O,
a ) 11.889(3) Å, b ) 9.992(3) Å , c ) 19.704(5) Å, orthorhombic, space group Pca21, Z ) 4, R ) 0.068 for
1556 (I > 2σ(I)) data; 2, C18H29AlCl2, a ) 12.147(5) Å, b ) 18.042(6) Å, c ) 17.771(7) Å, â ) 95.77(3)°,
monoclinic, space group P21/n, Z ) 8, R ) 0.032 for 4610 (I > 2σ(I)) data; 3a, C22H38AlClO, a ) 16.887(7) Å,
b ) 16.333(6) Å, c ) 8.739(3) Å, â ) 101.41(3)°, monoclinic, space group P21/c, Z ) 4, R ) 0.073 for 2752
(I > 2σ(I)) data; 4b, C18H29.64AlCl1.36, a ) 12.077(3) Å, b ) 17.920(3) Å, c ) 17.634(5) Å ; â ) 95.21(2) Å,
monoclinic, space group P21/n, Z ) 8, R ) 0.070 for 4261 (I > 2σ(I)) data.
Introduction
and relatively expensive Mes*Br) reagent was required to obtain
a good (65%) yield of the product.3 Subsequent experience has
shown that the use of less than this amount of LiMes* afforded
mixtures of products from which it proved very difficult to
obtain a pure Mes*AlX2 sample due to contamination with
aluminum halide and/or aluminate complexes. Given the key
role of the Mes*AlX2 halides as starting materials, it was a
priority to develop reliable and efficient routes to such species.
In addition, it was desired to examine the tendency of Mes*AlX2
compounds to form complexes with Et2O or THF, the two most
commonly used ether donor solvents.
In this paper, the synthesis of Mes*AlCl2(THF) (1) in good
yield by two separate routes and its X-ray crystal structure are
described. It is also shown that 1 readily desolvates under mild
conditions to give the hitherto uncharacterized species Mes*AlCl2
(2). The previously unreported hydride halide compounds
Mes*Al(H)Cl(THF), 3a, and its desolvated analogue, 4a, as well
as the related species 3b and 4b, involving ratios of aluminum-
bound H:Cl that are less than 1:1, are now reported.
Sterically hindered, neutral organohalide derivatives of the
heavier main group III elements Al, Ga, and In are key starting
materials for a variety of compounds with unusual bonding and
structures.1,2 Among the most crowded, well-characterized
organohalides are the recently reported Mes* (Mes* ) 2,4,6-
t-Bu3C6H2) derivatives of formula Mes*MX2 (M/X ) Al/Br,3
Ga/Cl,3,4 Ga/Br,4 In/Cl,3,4 In/Br4) or Mes*2MX (M/X ) Ga/
Cl,5 In/Cl,6 In/Br7 ). A notable feature of the dihalides is that
they are monomeric in the solid state whereas all previously
known uncomplexed organodihalide derivatives were associated
through halide bridging. In addition, it was noted3 that neither
Mes*AlBr2, Mes*GaCl2, nor Mes*InCl2 formed stable com-
plexes with Et2O even though they were synthesized in an Et2O
solvent mixture. The synthesis of all the Mes*-substituted metal
halide derivatives has involved the familiar lithium halide salt
elimination reaction. For the Ga and In derivatives, moderate
to good yields of the product were obtained by stoichiometric
reaction of LiMes* (generated from Mes*Br and Li(n-Bu)) with
the metal halide.3,4 For the only currently known, well-
characterized aluminum derivative Mes*AlBr2, it was reported
that more than 2 equiv of LiMes* (generated from Li(n-Bu)
Experimental Section
General Procedures. All reactions were performed under N2 by
using either modified Schlenk techniques or a Vacuum Atmospheres
HE43-2 drybox. Solvents were freshly distilled from sodium-
potassium alloy and degassed twice before use. Mes*Br8 and [(THF)2-
X Abstract published in AdVance ACS Abstracts, May 1, 1996.
(1) Uhl, W. Angew. Chem., Int. Ed. Engl. 1993, 32, 1386.
(2) Brothers, P. J.; Power, P. P. AdV. Organomet. Chem. 1996, 39, 1-69.
(3) Petrie, M. A.; Power, P. P.; Dias, H. V. R.; Ruhlandt-Senge, K.;
Waggoner, K.M.; Wehmschulte, R. J. Organometallics 1993, 12, 1086.
(4) Schultz, S.; Pusch, S.; Pohl, E.; Dielkus, S.; Herbst-Irmer, R.; Meller,
A.; Roesky, H. W. Inorg. Chem. 1993, 32, 3343.
9
LiH3AlMes*]2 were prepared by literature procedures. AlCl3 and
Me3SiCl were purchased from Aldrich and used as received. Infrared
spectra were recorded in the range 4000-200 cm-1 as a Nujol mull
between CsI plates using a Perkin-Elmer PE-1430 spectrometer, and
NMR spectra were recorded on a General Electric GE-300 spectrometer.
(5) Meller, A.; Pusch, S.; Pohl, E.; Ha¨ming, L.; Herbst-Irmer, R. Chem.
Ber. 1993, 126, 2255.
(6) Cowley, A. H.; Isom, H. S.; Decken, A. Organometallics 1995, 14,
2589.
(7) Rahbarnoohi, H.; Heeg, M. J.; Oliver, J. P. Organometallics 1994,
13, 2123.
(8) Pearson, D. E.; Frazer, M. G.; Frazer, V. S.; Washburn, L. C. Synthesis
1976, 621.
(9) Wehmschulte, R. J.; Ellison, J. J.; Ruhlandt-Senge, K.; Power, P. P.
Inorg. Chem. 1994, 33, 6300.
S0020-1669(95)01541-2 CCC: $12.00 © 1996 American Chemical Society

Arylaluminum Halides and Hydride Halides
Inorganic Chemistry, Vol. 35, No. 11, 1996 3263
Mes*Li(THF)2. Mes*Li(THF)2 was synthesized by a slightly
modified literature procedure.10 A solution of 12.93 g (39.7 mmol) of
Mes*Br in 20 mL of THF/80 mL n-pentane was treated at ca. -78 °C
with 30 mL (48.0 mmol) of a 1.6 M n-BuLi solution in hexanes and
stirred for 4 h at this temperature. The colorless precipitate was washed
twice with n-pentane (2 × 50 mL) at -78 °C, collected on a sintered-
large (>1 mm) colorless crystals (parallelepipeds) formed. Yield:
58.2%. Mp: turns opaque at 100 °C, melts at 146-150 °C. 1H NMR
(C6D6): 7.37 (s, m-H, 2H), 1.45 (s, o-CH3, 18H), 1.27 ppm (s, p-CH3,
9H). 13C{1H} NMR: 158.6 (o-C), 152.7 (p-C), 121.2 (m-C), 37.5 (o-
C(CH3)3), 35.0 (p-C(CH3)3), 32.9 (o-CH3), 31.3 ppm (p-CH3). 27Al
NMR (C6D6, 80 °C): 130 ppm (s, broad, ν1/2 ) ca. 4700 Hz).
1
glass frit, and dried under reduced pressure. The H NMR spectrum
Mes*Al(H)Cl‚THF (3a). Method A. A ca. -78 °C slurry of
Mes*Li(THF)2 in 1:1 n-pentane/Et2O (80 mL) (generated in situ from
6.08 g (18.7 mmol) of Mes*Br and 12.2 mL of 1.6 M n-BuLi in hexane)
was transferred via a cannula to a slurry of LiAlH4 (0.64 g, 17.0 mmol)
in Et2O (30 mL) at -78 °C, the mixture was warmed to room
temperature and stirred for a further 16 h. After cooling to -78 °C,
the colorless, cloudy (LiH) solution was treated dropwise with Me3-
SiCl (4.5 mL, 35 mmol, 3.80 g). The mixture was then warmed to
room temperature and stirred for an additional 1 h. The clear, colorless
supernatant was decanted, and the volatile materials were removed
under reduced pressure. The colorless solid was then extracted with
n-hexane (80 mL), the extract was filtered, and the filtrate was
concentrated to ca. 50 mL and cooled in a -20 °C freezer for 24 h to
yield 1.4 g of 3a as large colorless crystals. Yield: 21.6%. The crystal
used for the structure determination was taken from this batch. 1H
NMR (C6D6): 7.47 (s, m-H, 2H), 4.8 (s, broad, ν1/2 ) ca. 60 Hz, Al-
H, 1H), 3.43 (m, OCH2, 4H), 1.60 (s, ï-CH3, 18H), 1.36 (s, p-CH3,
9H), 1.04 ppm (m, CH2). 13C{1H} NMR (C6D6): 160.2 (o-C), 149.8
(p-C), 121.0 (m-C), 70.6 (OCH2), 39.0 (o-C(CH3)3), 34.8 (p-C(CH3)3),
33.5 (o-CH3), 31.6 (p-CH3), 24.8 ppm (CH2). IR: νAl-H 1880 (sh),
1832 (st), 1785 (m), 1757 cm-1 (m). Concentration of the mother liquor
to ca. 30 mL and crystallization at -20 °C afforded 2.04 g of a ca. 1:1
mixture (by NMR) of 3a and Mes*AlH2.11
shows the presence of two THF molecules per Mes* unit. Yield: 68%.
1H NMR (C6D6): 7.59 (s, m-H, 2H), 3.38 (m, OCH2, 8H), 1.64 (s,
o-CH3, 18H), 1.57 (s, p-CH3, 9H), 1.24 ppm (m, CH2, 8H).
Mes*AlCl2‚THF (1). Method A. A slurry of 0.48 g (0.55 mmol)
of [(THF)2LiH3AlMes*]2 in THF/hexane (30 mL of a 1:2 mixture) was
treated with 0.42 mL (3.3 mmol, 0.36 g) of Me3SiCl at -78 °C. The
resulting clear colorless solution was warmed to room temperature and
stirred overnight. After removal of volatile materials, the colorless solid
was extracted with n-hexane (40 mL). Concentration to ca. 20 mL
followed by crystallization at -20 °C overnight gave 0.28 g of small
colorless crystals of 1. Yield: 61%. Mp: 99-100 °C. 1H NMR
(C6D6): 7.45 (s, m-H, 2H), 3.58 (m, OCH2, 4H), 1.59 (s, o-CH3, 18H),
1.33 (s, p-CH3, 9H), 1.02 ppm (m, CH2, 4H). 13C{1H} NMR (C6D6):
161.1 (o -C), 150.4 (p-C), 121.3 (m-C), 72.6 (OCH2), 39.3 (o-C(CH3)3),
34.7 (p-C(CH3)3), 33.5 (o-CH3), 31.5 (p-CH3), 24.7 ppm (CH2). 27Al
NMR: 108 ppm (s, broad, ∆ν1/2 ) ca. 6300 Hz).
Method B. Mes*Li(THF)2 (3.00 g, 7.5 mmol) was added to a slurry
of AlCl3 (2.00 g, 15.0 mmol) in PhMe (40 mL) at -78 °C via a solids-
addition funnel in several portions (ca. 30 min). The colorless slurry
was kept at ca. -78 °C for an additional 1 h, slowly warmed to room
temperature, and stirred for a further 16 h. The volatile materials were
removed from the slightly cloudy solution under reduced pressure, after
which the remaining colorless sticky solid was warmed to ca. 40 °C
for 1 h under reduced pressure. The colorless solid was then extracted
twice with 80 and 40 mL of n-hexane, respectively. The combined
hexane extracts were concentrated to ca. 50 mL and slowly cooled to
-20 °C for 24 h to yield 1 as large (>1 mm) colorless plates of
sufficient quality for X-ray diffraction studies. The sample also contains
Method B. A slurry of [(THF)2LiH3AlMes*]2 (3.8 mmol) in PhMe
(50 mL) was treated dropwise with Me3SiCl (1.93 mL, 1.65 g, 15.2
mmol) at -78 °C. The mixture was warmed to room temperature and
stirred overnight (16 h). After removal of the volatile materials under
reduced pressure the colorless, slightly sticky solid was extracted with
n-hexane (60 mL). The supernatant was decanted, concentrated to 20
mL, and cooled to -20 °C for 4 days. As no crystals had formed,
THF (1 mL) was added and the solution was concentrated to ca. 5 mL
and cooled in -20 °C freezer. After 2 days, large, colorless crystals
of 3b (Mes*Al(H)0.64Cl1.36‚THF) had formed. Yield: 1.78 g. Mp: 77-
95 °C. 1H NMR (C6D6): 7.48, 7.46 (2s m-H, 2H), 4.8 (s, broad, ν1/2
) ca. 60 Hz, Al-H, 0.66H), 3.43 (m, OCH2, 4H), 1.60 (s, o-CH3,
18H), 1.36 (s, p-CH3, 9H), 1.04 ppm (m, CH2, 4H). IR: νAl-H 1885
(sh), 1836 (st), 1810 (sh), 1770 cm -1 (w). The reaction of 2.07 g (2.4
mmol) of [(THF)2LiH3AlMes*]2 with 3.4 equiv (1.05 mL, 8.2 mmol,
0.90 g) of Me3SiCl affords Mes*Al(H)0.9Cl1.1‚THF in 49% yield. 27Al
NMR: 121 ppm (s, br, ∆ν1/2 ) ca. 8300 Hz). IR: νAl-H ) 1832 cm-1
(st).
1
ca. 0.3 equiv of AlCl3‚THF (by H, 27Al NMR). This contaminant
can be removed by sublimation to afford 2 (vide infra), and pure 1
may be reconstituted by adding THF. Yield of the mixture of 1 and
AlCl3‚THF: 2.29 g, 73.4% (or 56% crude Mes*AlCl2‚THF). Mp:
softens at 60 °C, melts at 85-90 °C. 1H NMR (C6D6): 7.45 (s, m-H,
2H), 3.63 (m, OCH2, 5.3H), 1.58 (s, o-CH3, 18H), 1.32 (s, p-CH3),
1.00 ppm (m, CH2, 5.3H). 13C{1H} NMR (C6D6): 160.9 (o-C), 150.6
(p-C), 121.3 (m-C), 72.7 (OCH2), 39.1 (o-C(CH3)3), 34.7 (p-C(CH3)3),
33.5 (o-CH3), 31.5 (p-CH3), 24.6 ppm (CH2). 27Al NMR (C6D6, νo )
78.340 477 MHz): 87.4 ppm (s, ν1/2 ) ca. 430 Hz, AlCl3‚THF).
Concentration of the supernatant liquid to ca. 4-5 mL, followed by
cooling in a -20 °C freezer overnight, gave 0.18 g of a mixture of 1
and 2 in the form of large (>1.5 mm) colorless plates with the average
composition (by 1H NMR) of Mes*AlCl2‚0.35THF. The crystal used
for the structure determination of 2 was taken from this batch.
Removal of Excess AlCl3‚THF. Method A. A mixture of 1 equiv
of Mes*AlCl2‚THF and ca. 0.4 equiv of AlCl3‚THF (1.83 g containing
ca. 1.5 mmol of AlCl3‚THF) in n-pentane (50 mL) was reacted with
0.59 g (1.5 mmol) of Mes*Li(THF)2 at -78 °C. The resultant mixture
was warmed to room temperature and stirred overnight. The colorless
precipitate (LiCl) was removed, and the colorless solution was
concentrated to ca. 30 mL to yield 0.59 g of colorless pyramid-shaped
(ca. 1 mm) crystals after 20 h at room temperature. Cooling of the
mother liquor to -20 °C for 4 days gave another 0.68 g of 1. Yield:
Mes*Al(H)Cl (4a). A 0.23 g (0.6 mmol) sample of Mes*Al(H)-
Cl‚THF (3a) was sublimed at ca. 110-120 °C under reduced pressure,
and 0.13 g of colorless microcrystalline Mes*Al(H)Cl (4a) was
recovered. 1H NMR (C6D6): 7.38 (s, m-H, 2H), 5.25 (s, broad, ν1/2
≈
40 Hz, Al-H, 1H), 1.44 (s, o-CH3, 18H), and 1.30 (s, p-CH3, 9H).
13C{1H} NMR (C6D6): 158.5 (o,C), 121.0 (m-C), 38.3 (o-C(CH3)3),
34.9 (p-C(CH3)3), 32.9 (o-CH3), 31.4 ppm (p-CH3). IR: νAl-H 1882
(st), 1813 cm-1 (w). Mp: softens at 85 °C, melts from 100-110 °C.
Mes*AlH0.64Cl1.36 (4b). A 0.92 g (2.4 mmol) sample of Mes*Al(H)0.64
-
Cl1.36‚THF (3b) was sublimed at ca. 110-120 °C under reduced
pressure. The colorless microcrystalline sublimate (0.63 g) was
crystallized from n-hexane by cooling at ca. -20 °C for 3 days.
Yield: 0.28 g. 1H NMR (C6D6): 7.38 (s, m-H, 2H), 5.25 (s, broad,
ν1/2 ) ca. 60 Hz, Al-H, 0.6H), 1.44 (s, o-CH3), 1.30 ppm (s, p-CH3).
13C{1H } NMR (C6D6): 158.4 (o-C), 151.8 (p-C), 121.0 (m-C), 38.2
(o-C(CH3)3), 34.9 (p-C(CH3)3), 32.9 (o-CH3), 31.4 ppm (p-CH3). IR:
νAl-H 1877 (st), 1849 (sh), 1805 cm-1 (w). Mp: 110-121 °C.
X-ray Crystallographic Studies. The crystals were removed from
the Schlenk tube under a stream of N2 and immediately covered with
a layer of hydrocarbon oil. A suitable crystal was selected, attached
to a glass fiber, and immediately placed in the low-temperature nitrogen
1
59%. AlCl3‚THF content: <5% according to H and 27Al NMR.
Method B. A crystalline mixture of Mes*AlCl2‚THF and AlCl3‚
THF (approximate ratio 1:0.4; 1.09 g) was heated for 3 h to 80 °C
under reduced pressure (0.02 mmHg). A colorless microcrystalline
solid (AlCl3‚THF) sublimed. The remaining off-white powder was pure
Mes*AlCl2 (2). Yield: 0.64 g, 85%.
Mes*AlCl2 (2). Compound 1 (0.31 g, 0.75 mmol) was heated under
reduced pressure (ca. 0.02 mmHg) to 90 °C for 20 min. The colorless
solid, which is essentially pure Mes*AlCl2, was dissolved in n-hexane
(30 mL), the solution was filtered, and the filtrate was concentrated to
ca. 10 mL and cooled in a -20 °C freezer. After 3 days, 0.15 g of
(10) Weidenbruch, M.; Kramer, K. J. Organomet. Chem. 1985, 291, 159.
(11) Wehmschulte, R. J.; Power, P. P. Inorg. Chem. 1994, 33, 5611.

3264 Inorganic Chemistry, Vol. 35, No. 11, 1996
Wehmschulte and Power
Table 1. Crystallographic Dataa for 1, 2, 3a, and 4b
1
2
3a
4b
formula
fw
color and habit
cryst system
space group
a, Å
C22H37AlCl2O
415.40
colorless plate
orthorhombic
Pca21
11.889(3)
9.992(3)
19.704(5)
C18H29AlCl2
343.29
colorless plate
monoclinic
P21/n
12.147(5)
18.042(6)
17.771(7)
95.77(3)
C22H38AlClO
380.95
colorless plate
monoclinic
P21/c
16.887(7)
16.333(6)
8.739(3)
101.41(3)
2363(2)
C18H29.64AlCl1.36
321.34
colorless prism shaped fragment
monoclinic
P21/n
12.077(3)
17.920(3)
17.634(5)
95.21(2)
b, Å
c, Å
â, deg
V, Å3
2341(1)
3875(2)
3801(2)
Z
4
8
4
8
cryst dimens, mm
0.61 × 0.60 × 0.10
0.58 × 0.46 × 0.40
0.56 × 0.55 × 0.22
0.90 × 0.52 × 0.28
d
calc, g cm-3
1.179
1.177
1.071
1.123
µ, mm-1
2.906
1556 (I > 2σ(I))
0.068, 0.174
3.373
4610 (I > 2σ(I))
0.032, 0.079
1.823
2752 (I > 2σ(I))
0.073, 0.189
2.602
4261 (I > 2σ(I))
0.070, 0.187
no. of obsd rflcns
R, wR2
aData were collected at 130 K with a Syntex P21 diffractometer (1, 3a, 4b) or a Siemens P4/RA diffractometer (2) using Cu KR (λ ) 1.541 78
Å) radiation.
stream as described in ref 12. All data were collected near 130 K with
either a Syntex P21 (graphite monochromator) in the cases of 1, 3a,
and 4b or a Siemens P4/RA diffractometer (nickel foil monochromator)
in the case of 2 using Cu KR (λ ) 1.541 78 Å) radiation. Both
diffractometers were equipped with a locally modified Enraf-Nonius
universal low-temperature device for low-temperature work.
A different approach using the Grignard reagent Mes*MgBr
and the addition of 1 equiv of AlBr3 does not lead to Mes*AlBr2
but to the isolation of Mes*2Mg16 in ca. 50% yield. Mes*GaCl2
can be conveniently synthesized by the reaction of Mes*Li-
(THF)2 with 2 equiv of GaCl3 in n-pentane.15
It seems that in the 1:1 reactions with the MX3 (M ) Al,
Ga) salt the LiCl, LiBr, and MgBr2 byproducts form insoluble
or relatively inert complexes such as LiMCl4, LiMBr4, and
MgBrMBr4 with unreacted MX3. In contrast to its gallium and
indium congeners, Mes*2AlX (X ) Cl, Br) has not yet been
reported. Bearing the probable formation of aluminate salts in
mind, it was decided to react Mes*Li, which was isolated as
Mes*Li(THF)2, with 2 equiv of AlCl3 in toluene. This resulted
in a milky solution. Removal of the volatile materials and
extraction with n-hexane, followed by crystallization at -20
°C, gives large (ca. 1-2 mm) colorless crystals, which are
Mes*AlCl2‚THF (1) according to the X-ray structure determi-
nation. NMR spectroscopic examinations nevertheless reveal
a THF:Mes* ratio that is greater than 1:1, and the 27Al NMR
spectrum displays a narrow line in the 80-90 ppm range, con-
Crystallographic programs used for the structure solutions were those
of the SHELXTL Version 5.03 (1994) program package installed on
an IBM-compatible 486 computer. Scattering factors were obtained
from ref 13. An absorption correction was applied by using the method
described in ref 14. Some details of the data collection and refinement
are given in Table 1. Coordinates for selected atoms are given in Table
2, and selected bond distances and angles are given in Table 3. Further
details are provided in the Supporting Information. The crystal
structures were solved by direct methods, and they were refined by
full-matrix least-squares procedures. All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms attached to the carbons were
included in the refinement at calculated positions using a riding model
with C-H ) 0.96 Å and UH ) 1.2UC and 1.5UC for methyl hydrogens.
Hydrogen atoms bound to aluminum were located on Fourier difference
maps and, with the exception of those in 4b, were allowed to refine
freely with fixed isotropic parameters.
.
sistent with the presence of some AlCl3 THF contamination of
Discussion
the product in the crystals.17 The excess (cocrystallized) AlCl3‚
THF can be removed either (a) by reaction of this mixture with
the appropriate amount of Mes*Li(THF)2 in n-pentane or (b)
by sublimation at 80 °C under reduced pressure, which leaves
pure 2 as a powder. Pure 1 can then be obtained by the addition
of THF.
The main objectives of this work were the development of a
more practical route to a Mes*AlX2 (X ) halide) starting
material and the study of some of its properties. The work may
be summarized by Scheme 1, which presents an overall view
of the relationships among the compounds under discussion.
The original preparation of Mes*AlBr2 required the use of
2.5 equiv of the expensive Mes*Br precursor.3 Attempts to react
Mes*Li (generated by treatment of Mes*Br with n-BuLi in a
THF/n-pentane 1:4 mixture at ca. -78 °C, followed by two
washings with n-pentane) with 1 equiv of AlBr3 in either
n-hexane or toluene led to impure oils from which Mes*AlBr2
could eventually be isolated (after several crystallizations and
sublimation) in yields that were often less than 20%.15 Interest-
ingly, the reaction of Mes*Li with 1 equiv of GaCl3 under the
same conditions led to a mixture of Mes*2GaCl and Mes*GaCl2.15
The solvent-free Mes*AlCl2 (2) can be isolated in the form
of large (ca. 2 mm) colorless plates either by repeated
crystallization of 1 from n-hexane or by heating of 1 to 90 °C
under reduced pressure.
The alternative (and higher yielding) method for the synthesis
of 1 arose from parallel work involving the synthesis of
(Mes*AlH2)2 (5).11 This molecule was originally synthesized
by the reaction of MeI with [(THF)2LiH3AlMes*]2 as described
by eq 1. It was later found that Me3SiCl could be used instead
[(THF)2LiH3AlMes*]2 + 2MeI f
(12) This method is described by: Hope, H. A Practicum in Synthesis and
Characterization. In Experimental Organometallic Chemistry; Wayda,
A. L., Darensbourg, M. Y., Eds.; ACS Symposium Series 357;
American Chemical Society: Washington, DC, 1987; Chapter 10.
(13) International Tables for Crystallography; D. Reidel Publishing Co.:
Dordrecht, The Netherlands, 1993; Vol. C.
6
2LiI + 2MeH + (H2AlMes*)2 (1)
5
(14) Parkin, S. R.; Moezzi, B.; Hope, H. J. Appl. Crystallogr. 1995, 28,
53.
(15) Brothers, P. J.; Wehmschulte, R. J.; Power, P. P. Unpublished results,
1993, 1994.
(16) (a) Wehmschulte, R. J.; Power, P. P. Organometallics 1995, 14, 3264.
(b) Wehmschulte, R. J.; Power, P. P. Unpublished results, 1995.
(17) No¨th, H.; Rurla¨nder, R.; Wolfgardt, P. Z. Naturforsch. 1982, 87B,
29.

Arylaluminum Halides and Hydride Halides
Inorganic Chemistry, Vol. 35, No. 11, 1996 3265
Table 2. Atom Coordinates (×104) for Important Atoms in 1, 2,
3a, and 4b
Table 3. Selected Bond Distances (Å) and Angles (deg) for
Compounds 1, 2, 3a, and 4b
Compound 1
Compound 1
Al(1)
Cl(1)
Cl(2)
O(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
8360(2)
8857(2)
7024(2)
9630(4)
8485(6)
9102(6)
9719(6)
9713(6)
8981(6)
8325(6)
-1614(2)
523(1)
557(1)
Al(1)-Cl(1)
Al(1)-Cl(2)
2.173(3)
2.131(3)
Al(1)-C(1)
Al(1)-O(1)
1.975(7)
1.902(5)
479(2)
-1769(2)
-2099(5)
-2585(7)
-3809(7)
-4069(7)
-3216(7)
-2142(7)
-1854(7)
-194(1)
-10(2)
1392(4)
1454(4)
2043(4)
2595(4)
2580(4)
2005(4)
Cl(1)-Al(1)-Cl(2)
C(1)-Al(1)-Cl(1)
C(1)-Al(1)-Cl(2)
107.1(1)
115.2(2)
126.5(2)
C(1)-Al(1)-O(1)
Cl(1)-Al(1)-O(1)
Cl(2)-Al(1)-O(1)
107.1(3)
92.7(2)
102.0(2)
Compound 2
Al(1)-Cl(1)
Al(1)-Cl(2)
Al(1)-C(1)
2.120(1)
2.129(1)
1.942(2)
Al(2)-Cl(3)
Al(2)-Cl(4)
Al(2)-C(19)
2.123(1)
2.123(1)
1.936(2)
Compound 2
Cl(1)-Al(1)-Cl(2) 109.6(1) Cl(3)-Al(2)-Cl(4)
111.4(1)
Al(1)
Cl(1)
Cl(2)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
Al(2)
Cl(3)
Cl(4)
C(19)
C(20)
C(21)
C(22)
C(23)
C(24)
7577(1)
7334(1)
9268(1)
6597(2)
6492(2)
5932(2)
5468(2)
5547(2)
6090(2)
2657(1)
1878(1)
4362(1)
1779(2)
1238(2)
566(2)
1718(1)
872(1)
1374(1)
547(1)
Cl(1)-Al(1)-C(1)
Cl(2)-Al(1)-C(1)
133.1(1) Cl(3)-Al(2)-C(19) 119.6(1)
117.4(1) Cl(4)-Al(2)-C(19) 128.8(1)
2051(1)
2203(1)
1945(1)
2371(1)
3055(1)
3295(1)
2892(1)
5657(1)
6532(1)
5576(1)
5017(1)
4373(1)
3964(1)
4162(1)
4796(1)
5222(1)
1488(1)
2019(1)
2760(1)
3256(1)
3058(1)
2321(1)
1803(1)
1653(1)
1001(1)
1492(1)
2231(1)
1921(1)
2350(1)
3085(1)
3380(1)
2978(1)
Compound 3a
Al(1)-Cl(1)
Al(1)-C(1)
2.174(2)
1.983(5)
Al(1)-H(1)
Al(1)-O(1)
1.73(5)
1.908(3)
Cl(1)-Al(1)-C(1)
Cl(1)-Al(1)-O(1)
Cl(1)-Al(1)-H(1)
128.2(2)
103.6(1)
114(2)
C(1)-Al(1)-O(1)
C(1)-Al(1)-H(1)
O(1)-Al(1)-H(1)
103.2(2)
105(2)
97(2)
Compound 4b
Al(1)-Cl(1)
Al(1)-Cl(2)
Al(1)-H(1)
Al(1)-C(1)
2.186(2)
2.070(10)
1.61(6)
Al(2)-Cl(3)
Al(2)-Cl(4)
Al(2)-H(2)
Al(2)-C(19)
2.169(2)
2.130(2)
1.45(12)
1.947(4)
1.944(4)
377(2)
892(2)
1591(2)
Cl(1)-Al(1)-C(1) 119.88(14) Cl(3)-Al(2)-C(19) 117.78(14)
Cl(1)-Al(1)-H(1) 103(3)
Cl(1)-Al(1)-Cl(2) 126.4(3)
Cl(2)-Al(1)-C(1) 113.5(3)
C(1)-Al(1)-H(1) 137(3)
Cl(3)-Al(2)-Cl(4) 108.41(9)
Cl(3)-Al(2)-H(2) 132.(5)
Cl(4)-Al(2)-C(19) 133.8(2)
C(19)-Al(2)-H(2) 109(5)
Compound 3a
Al(1)
Cl(1)
H(1)
O(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
8749(1)
9581(1)
8856(31)
9183(2)
7576(3)
7082(3)
6395(3)
6158(3)
6583(3)
7261(3)
9666(1)
8897(1)
9606(32)
10740(2)
9830(3)
9532(3)
9983(3)
10690(3)
10909(3)
10491(3)
2485(2)
1523(2)
4491(62)
2407(4)
1684(5)
2698(5)
2894(5)
2083(5)
926(5)
has less than one aluminum hydrogen per chlorine, i.e. a species
such as 3b. The synthesis of 3a may also be accomplished in
[(THF)2LiH3AlMes*]2 + 4Me3SiCl f
2LiCl + 4Me3SiH + 2Mes*Al(H)Cl(THF) (2)
3a
691(5)
[(THF)2LiH3AlMes*]2 + 6Me3SiCl f
Compound 4b
Al(1)
Cl(1)
Cl(2)
H(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
Al(2)
Cl(3)
Cl(4)
H(2)
C(19)
C(20)
C(21)
C(22)
C(23)
C(24)
2674(1)
4449(1)
1845(8)
2482(58)
1742(3)
1567(3)
926(4)
673(1)
429(1)
1574(5)
1300(37)
62(2)
1553(1)
1562(1)
1031(5)
903(36)
2147(3)
2900(2)
3338(2)
3063(2)
2321(2)
1859(2)
3654(1)
3522(1)
4492(1)
4047(73)
3009(2)
3231(2)
2723(2)
1995(2)
1787(2)
2275(2)
2LiCl + 6Me3SiH + 2Mes*AlCl2(THF) (3)
1
reduced yield by the one-pot reaction of Mes*Li(THF)2 with
LiAlH4, followed by treatment with Me3SiCl. The route to (H2-
AlMes*)2 described by eq 4 was investigated in order to avoid
272(2)
-172(2)
-837(2)
-1031(2)
-608(2)
1662(1)
1951(1)
808(1)
1053(67)
2173(2)
2867(2)
3258(2)
3004(2)
2322(2)
1906(2)
437(4)
605(4)
2Mes*Li(THF)2 + 2LiAlH4 f
1228(3)
7316(1)
5565(1)
7557(2)
7915(94)
8300(3)
8814(3)
9411(4)
9547(4)
9064(3)
8453(3)
[(THF)2LiH3AlMes*]2 + 2LiH 4Me SiCl8
3
6
(Mes*AlH2)2 + 4LiCl + 4Me3SiH + 4THF (4)
5
the necessity of isolating [(THF)2LiH3AlMes*]2, 6. Apparently,
(Mes*AlH2)2, 5, reacts with Me3SiCl at a rate competitive with
that of LiH, thereby giving some 3 as product. Attempts to
synthesize 3a by exchange reactions involving 1 and 5 led to
mixtures of products which according to H NMR consisted
mainly of Mes*AlHnCl2-n‚THF species.
The THF from 3 can be easily removed by sublimation at
110-120 °C under reduced pressure without affecting the H:Cl
ratio. It is notable, however, that the synthesis of Mes*Al(H)-
Cl.THF (H:Cl ratio 1:1) according to eq 2 can be further
complicated by an equilibrium involving (Mes*AlH2)2, Mes*Al-
(H)Cl‚THF, and Mes*AlCl2‚THF. It should be noted that
Mes*Al(H)nCl2-n‚THF (n >1) is less soluble than (Mes*AlH2)2
1
of MeI to afford elimination of Me3SiH instead of methane.
The addition of further Me3SiCl leads to the substitution of
further hydrogens on the aluminum by chlorine. Thus, the
addition to 6 of 4 instead of 2 equiv of Me3SiCl gives 3a in the
presence of THF whereas 6 equiv of Me3SiCl affords compound
1 in which all the hydrogens are replaced by chlorine (eqs 2
and 3). Care has to be exercized in adding exactly the correct
number of equiv of Me3SiCl in the case of eq 2 since the
addition of more than 4 equiv will result in a species which

3266 Inorganic Chemistry, Vol. 35, No. 11, 1996
Wehmschulte and Power
Structures. The structures of compounds 511 and 69 have
been described elsewhere. The structure of 1, which is
illustrated in Figure 1, bears a resemblance to the structure of
the less sterically encumbered MesAlCl2(THF).3 There are
important differences in structural detail, however, that may be
attributed to the greater size of the Mes* group in comparison
to Mes. The Al-C and Al-O distances in 1 are ca. 0.05 Å
longer than the corresponding lengths in MesAlCl2(THF). The
Cl-Al-Cl angle is ca. 3° narrower in 1 and the C-Al-Cl
angles in 1 can be up to 11° wider than the same feature in
MesAlCl2(THF). The higher steric crowding in 1 is also
manifested in the distortion of the C(1)-C(6) phenyl ring. The
C(1)-C(4) vector deviates by 30.5° from the Al-C(1) vector.
Furthermore, the ring itself is also distorted toward a boat
conformation so that the planes defined by C(1)-C(2)-C(6)
and C(3)-C(4)-C(5) are folded toward the plane C(2)-C(3)-
C(5)-C(6) by angles of 13.2 and 4.8°, respectively. Removal
of THF from 1 under mild conditions results in the three-
coordinate species 2. The lowering of the aluminum coordina-
tion number from 4 to 3 results in slightly shorter metal-carbon
and metal-halide bonds as well as wider interligand angles.
The crystals of 2 are not isomorphous with their Ga or In
analogues, nor are they isomorphous with the Al species
Mes*AlBr2 although all the compounds bear a close structural
resemblance. They are, however, isomorphous with 4b (vide
infra). The two crystallographically independent molecules in
2 have almost identical structures; the only significant differ-
ences relate to slight changes in the orientation of the t-Bu
groups. There are also close approaches (2.1 Å) of two H atoms
from ortho t-Bu groups to the Al center which are similar to
those observed in Mes*AlBr2. The positioning of the aromatic
ring plane approximately perpendicular to the metal coordination
plane thus appears to take place for two reasons: to minimize
steric congestion and to permit agostic interactions between the
t-Bu hydrogens and the metal.
Figure 1. Thermal ellipsoidal plot (30%) of 1. H atoms are omitted
for clarity.
Figure 2. Thermal ellipsoidal plot (30%) of one of the independent
molecules of 2. H atoms are omitted for clarity.
in hexane or pentane solutions which contain small amounts
(ca. 5-10 equiv) of THF. Thus, a synthesis following the exact
stoichiometry given by eq 2 can lead to Mes*Al(H)n Cl2-n‚THF
with n <1 in the isolated crystals. The employment of less
than 4 equiv of Me3SiCl per [(THF)2LiH3AlMes*]2 increases
the H:Cl ratio to 0.9:1.1.
The structure of 3a resembles that of 1 except the two
formulas differ by the substitution of a hydrogen instead of a
chlorine at the aluminum; however, the crystals of 1 and 3a
are not isomorphous. Nonetheless, the Al-Cl and Al-C bond
lengths are very similar in the two compounds. The Al-H
Scheme 1. Summary of Relationships among Compounds 1-6

Arylaluminum Halides and Hydride Halides
Inorganic Chemistry, Vol. 35, No. 11, 1996 3267
observed in the X-ray crystal structure where the two crystal-
lographically independent molecules have H-Cl compositions
of H0.85Cl1.15 and H0.43Cl1.57. The positions of the partially
occupied H(1)/Cl(2) and H(2)/Cl(4) are exchanged with respect
to each other. The hydrogen atoms H(1) and H(2) deviate
slightly from the averaged Cl2AlC plane, and the Al-H
distances are 1.45(12) and 1.61(6) Å in good agreement with
11
the values in (Mes*AlH2)2 and Mes2AlH.6 There is a 90°
torsion angle between the aluminum coordination plane and the
plane of the aromatic ring. In addition, there are short contacts
(ca. 2.1 Å) between hydrogens from the ortho t-Bu groups and
the Al centers. It is interesting to note that the structure of 4b
closely resembles those of 2 and Mes*AlBr2 and is in sharp
contrast to the dimeric arrangement of (Mes*AlH2)2. A dimeric
structure of the type Mes*(Cl)Al(µ-H)2Al(Cl)Mes* might have
been expected for 4 especially in view of the relative strength
of the Al-H-Al bridge in (Mes*AlH2)2 (6) which is apparently
not cleaved by nucleophiles like THF. It is also notable that 2
and 4b are essentially isomorphous with very similar cell
constants. In effect, the structure of 4b may be most simply
described as consisting of Mes*Al(H)Cl contaminated with some
Mes*AlCl2.
Figure 3. Thermal ellipsoidal plot (30%) of 3a. H atoms are omitted
for clarity.
Conclusion
Reaction of [(THF)2LiH3AlMes*]2 with an appropriate num-
ber of equivalents of Me3SiCl affords various arylaluminum
halides in good yield. Although ether complexes of Mes*AlX2
(X ) Cl, Br) have not been isolated, it is possible to crystallize
the THF-complexed species Mes*AlCl2(THF) and Mes*Al(H)-
Cl(THF). These compounds are the first Lewis-base complexes
of Mes*-substituted Al derivatives. However, even the THF
can be removed by mild heating under reduced pressure. The
relatively weak coordinating ability of the Al center with regard
to Et2O or THF is due to the large size of the Mes* substituent
and the presence of agostic interactions involving the metal and
C-H bonds from the ortho t-Bu groups.
Figure 4. Thermal ellipsoidal plot (30%) of one of the independent
molecules of 4b. H atoms are omitted for clarity.
distance is 1.73(5) Å which is ca. 0.2 Å longer than the terminal
Al-H bond in (Mes*AlH2)211 or Mes*2AlH.6 The high standard
deviations, however, makes such comparisons of limited value.
The Al-O distance 1.908(3) Å is essentially identical to the
value observed in 1.
Acknowledgment. We are grateful to the National Science
Foundation for financial support.
Desolvation of 3b by heating under reduced pressure invari-
ably afforded species of formula Mes*Al(H)0.64(Cl)1.36 (4b) upon
recrystallization. The amount of Cl always exceeded that of
the aluminum hydrogen in several preparations. The ratio of
Al-H to Mes* can be determined approximately by integration
of the 1H NMR signals. Different ratios of H:Cl are also
Supporting Information Available: Full tables of data collection
parameters, atom coordinates, bond distances and angles, anisotropic
thermal parameters, and hydrogen coordinates (35 pages). Ordering
information is given on any current masthead page.
IC9515416