Facile synthesis of monomeric alumatranes

Article abstract of DOI:10.1021/ja0626786

Alumatranes, tricyclic neutral molecules featuring a transannular N → Al bond, can act as Lewis acids that activate substrates in the axial coordination site. Treatment of tris(2-hydroxy-3,5-dimethylbenzyl)-amine with AlMe₃ afforded dimeric (AIL)₂ 1 [wherein L = tris(2-oxy-3,5-dimethylbenzyl)amine]. X-ray diffraction analysis revealed bridging between AIL monomers by two Al-O bonds. Reactions of 1 with substrates containing O or N donors generated the alumatranes THF-AIL 2, PhCHO-AIL 3, H₂NCH₂CH₂-NH₂-AIL 4, and [PhO-AIL]^- 5, in which the apical added ligand on the five-coordinate aluminum center causes variation in the transannular bond distance. Water coordinates with 1 a -20 0C to form the alumatrane H₂O-AIL 6 that undergoes partial hydrolysis at room temperature to produce 7, which X-ray crystallography showed to be composed of four AIL fragments linked by an (H ₂O)₂(HO)₂Al(OH)₂Al(OH) ₂-(H₂O)₂ framework in which the O ₄AlO₂AlO₄ moiety is of local D_2h symmetry. According to X-ray analysis, 7 can crystallize in at least two polymorphic modifications: triclinic 7a and monoclinic 7b. The reaction of 3 with water also generated 6 and 7, depending on the reaction temperature. Dimeric 1 was found to promote the reaction of benzaldehyde with trimethylsilyl cyanide at room temperature to provide 2-trimethylsilyoxyphenylacetonitrile in 95% yield.

Full text of DOI:10.1021/ja0626786

Published on Web 09/29/2006
Facile Synthesis of Monomeric Alumatranes
Weiping Su, Youngjo Kim,† Arkady Ellern, Ilia A. Guzei, and John G. Verkade*
Contribution from the Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011
Received April 17, 2006; E-mail: jverkade@iastate.edu
Abstract: Alumatranes, tricyclic neutral molecules featuring a transannular N f Al bond, can act as Lewis
acids that activate substrates in the axial coordination site. Treatment of tris(2-hydroxy-3,5-dimethylbenzyl)-
amine with AlMe3 afforded dimeric (AlL)2 1 [wherein L ) tris(2-oxy-3,5-dimethylbenzyl)amine]. X-ray
diffraction analysis revealed bridging between AlL monomers by two Al-O bonds. Reactions of 1 with
substrates containing O or N donors generated the alumatranes THF-AlL 2, PhCHO-AlL 3, H2NCH2CH2-
NH2-AlL 4, and [PhO-AlL]- 5, in which the apical added ligand on the five-coordinate aluminum center
causes variation in the transannular bond distance. Water coordinates with 1 at -20 °C to form the
alumatrane H2O-AlL 6 that undergoes partial hydrolysis at room temperature to produce 7, which X-ray
crystallography showed to be composed of four AlL fragments linked by an (H2O)2(HO)2Al(OH)2Al(OH)2-
(H2O)2 framework in which the O4AlO2AlO4 moiety is of local D2h symmetry. According to X-ray analysis,
7 can crystallize in at least two polymorphic modifications: triclinic 7a and monoclinic 7b. The reaction of
3 with water also generated 6 and 7, depending on the reaction temperature. Dimeric 1 was found to
promote the reaction of benzaldehyde with trimethylsilyl cyanide at room temperature to provide
2-trimethylsilyoxyphenylacetonitrile in 95% yield.
still relatively rare.10 It is extensively documented that aluminum
Introduction
compounds can serve as powerful Lewis acid catalysts for
organic transformations, and numerous studies of the mecha-
nisms of such reactions implicate trigonal bipyramidal (TBP)
alumatranes as active intermediates.1,12 We therefore envisioned
Polydentate ligands can influence the reactivity of a metal
center in a complex, not only by exerting steric and electronic
effects, but also by imposing a specific coordination geometry
on the metal ion. In the present context, Nelson et al. recently
showed that an Al(III) complex with a trigonal monopyramidal
coordination geometry is an active Lewis acid catalyst for a
ketene aldehyde cycloaddition reaction, while its tetrahedral
analogue is inactive in this process.1 Tripodal tetradentate ligands
define a large family of trigonal bipyramidal metal complexes
commonly referred to as atranes.2,3 These appealingly sym-
metrical compounds possess a pseudo threefold symmetric
environment around the metal ion generated by the tripodal
ligand and the exocyclic axial group,3 a flexible transannular
bond between the metal ion and an axial ligand atom in the
tripodal ligand,2 and the possibility of 3d orbital involvement
for substrate binding at the exocyclic apical position,3 provided
that site is vacant or possesses a ligand that is relatively easily
displaced. Thus, complexes of this type have been utilized for
activation of small molecules4-6 and for generating novel
structural motifs,3,5,7,8 such as ligand-metal multiple bonds.7
Although contributions from several laboratories including
ours have led to the development of atrane chemistry involving
a wide variety of metals,2-7,9-11 examples of alumatranes are
(5) (a) Macbeth, C. E.; Golombek, A. P.; Young, V. G., Jr.; Yang, C.; Kuczera,
K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938. (b) Macbeth,
C. E.; Gupta, R.; Mitchell-Koch, K. R.; Young, V. G., Jr.; Lushington, G.
H.; Thompson, W. H.; Hendrich, M. P.; Borovik, A. S. J. Am. Chem. Soc.
2004, 126, 2556. (c) Larsen, P. L.; Gupta, R.; Powell, D. R.; Borovik, A.
S. J. Am. Chem. Soc. 2004, 126, 6552.
(6) Hu, X.; Castro-Rodriguez, I.; Meyer, K. J. Am. Soc. Chem. 2004, 126,
13464.
(7) (a) Cummings, C. C.; Schrock, R. R.; Davis, W. M. Inorg. Chem. 1994,
33, 1448. (b) Johnson-Carr, J. A.; Zanetti, N. C.; Schrock, R. R.; Hopkins,
M. D. J. Am. Chem. Soc. 1996, 118, 11305. (c) Hu, X.; Meyer, K. J. Am.
Chem. Soc. 2004, 126, 16322.
(8) For examples of threefold symmetric metal complexes supported by
tridentate ligands, which are similar to atranes and possess metal-ligand
multiple bonds, see: (a) Jenkins, D. M.; Betley, T. A.; Peters, J. C. J. Am.
Chem. Soc. 2002, 124, 11238. (b) Brown, D. S.; Betley, T. A.; Peters, J.
C. J. Am. Chem. Soc. 2003, 125, 322. (c) Betley, T. A.; Peters, J. C. J. Am.
Chem. Soc. 2004, 126, 6252.
(9) (a) Timosheva, N. V.; Chandrasekaran, A.; Day, R. O.; Holmes, R. R.
Organometallics 2000, 19, 5614. (b) Chandrasekaran, A.; Day, R. O.;
Holmes, R. R. J. Am. Chem. Soc. 2000, 122, 1066. (c) Timosheva, N. V.;
Chandrasekaran, A.; Day, R. O.; Holmes, R. R. Organometallics 2001,
20, 2331. (d) Timosheva, N. V.; Chandrasekaran, A.; Day, R. O.; Holmes,
R. R. J. Am. Chem. Soc. 2002, 124, 7035. (e) Chandrasekaran, A.; Day, R.
O.; Holmes, R. R. Inorg. Chem. 2000, 39, 5683. (f) Timosheva, N. V.;
Chandrasekaran, A.; Holmes, R. R. Inorg. Chem. 2004, 43, 7403.
(10) (a) Kim, Y.; Verkade, J. G. Inorg. Chem. 2003, 42, 4804. (b) Pinkas, J.;
Verkade, J. G. Inorg. Chem. 1993, 32, 2711. (c) Pinkas, J.; Gaul, B.;
Verkade, J. G. J. Am. Soc. Chem. 1993, 115, 3925. (d) Waldner, K. F.;
Laine, R. M.; Dhumrongvaraporn, S.; Tayaniphan, S.; Narayanan, R. Chem.
Mater. 1996, 8, 2850.
(11) (a) Filippou, A. C.; Schneider, S.; Schnakenburg, G. Inorg. Chem. 2003,
42, 6974. (b) Groysman, S.; Goldberg, I.; Goldschmidt, Z.; Kol, M. Inorg.
Chem. 2005, 44, 5073. (c) Lehtonen, A.; Sillanpa¨a¨, R. Organometallics
2005, 24, 2795. (d) Bull, S. D.; Davidson, M. G.; Johnson, A.; Robinson,
D. E. J. E.; Mahon, M. F. Chem. Commun. 2003, 14, 1750. (e) Kol, M.;
Shamis, M.; Goldberg, I.; Goldschmidt, Z.; Alfi, S.; Hayat-Salant, E. Inorg.
Chem. Commun. 2001, 4, 177. (f) Hwang, J.; Govindaswamy, K.; Koch,
S. A. Chem. Commun. 1998, 1667. (g) Kim, Y.; Verkade, J. G. Organo-
metallics 2002, 21, 2395.
† Permanent address: Department of Chemistry and Basic Science
Research Institute, Chungbuk National University, Cheongju, Chungbuk
361-763, Korea.
(1) Nelson, S. G.; Kim, B.-K.; Peelen, T. J. J. Am. Chem. Soc. 2000, 122,
9318.
(2) (a) Verkade, J. G. Acc. Chem. Res. 1993, 26, 896. (b) Verkade, J. G. Coord.
Chem. ReV. 1994, 137, 233.
(3) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9.
(4) (a) Yandulov, D. V.; Schrock, R. R. J. Am. Chem. Soc. 2002, 124, 6252.
(b) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76. (c) Yandulov,
D. V.; Schrock, R. R. Inorg. Chem. 2005, 44, 1103.
9
10.1021/ja0626786 CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 13727-13735
13727

A R T I C L E S
Su et al.
Scheme 1. Transformations Stemming from 1a
a The cation for 5 is [HP(i-PrNCH2CH2)3N]+.
that a labile molecule serving as an exocyclic axial ligand on
an alumatrane could potentially function as a substrate in a
Lewis acid catalyzed reaction. Recently, we reported for the
first time the molecular structure of a monomeric alumatrane,
namely Me2HNAlL, wherein L ) tris(2-oxy-3,5-dimethylbenzyl)-
amine.10a
acetylacetone with the aim of affording a higher-coordinate
aluminum complex containing an acetylacetone moiety as well
as ligand L (Scheme 1). We also demonstrate that 1 efficiently
catalyzes the reaction of benzaldehyde with trimethylsilyl
cyanide under mild conditions to give 2-trimethylsilyoxyphe-
nylacetonitrile.
Earlier work on atranes focused on examples containing three
five-membered chelating rings.2,3 Recently, however, Holmes
et al. synthesized tris-(2-hydroxy-3,5-dimethylbenzyl)amine and
its bulkier analogue tris-(2-hydroxy-3-tert-butyl-5-methylben-
zyl)amine to create a series of silatranes containing six-
membered rings.9 The flexibility of the six-membered rings in
these compounds allowed these investigators to observe the
effect of the exocyclic apical substituent on the transannular
bond length.
Herein, we report the synthesis of the dimeric 1 and the
cleavage of 1 into alumatranes 2-6 with ligands containing O
or N donors, the reaction of 3 and 6 with water to give 7, the
reaction of 3 to give 6, and a failed attempt to cleave 1 with
Experimental Section
General. All reactions were carried out under argon with the strict
exclusion of moisture using Schlenk techniques, unless otherwise stated.
Toluene, pentane, and THF were distilled from sodium/benzophenone
under nitrogen, and CH2Cl2 was dried by distillation from CaH2. All
deuterated solvents and ethylenediamine were distilled from CaH2 and
stored over activated 3 Å molecular sieves under argon. Benzaldehyde
was purchased from Aldrich and distilled under reduced pressure prior
to use. Tris(2-hydroxy-3,5-dimethylbenzyl)amine was prepared accord-
ing to a published procedure.9b Other chemicals were obtained from
Aldrich and used as received. H NMR spectra and 13C NMR spectra
1
were recorded on a Varian Germini-300 spectrometer at 300 and 75.5
MHz, respectively. Electrospray MS analysis (ESI-MS) in toluene was
recorded on a Finnigan AQA apparatus. Elemental analyses were carried
out by Desert Analytics or Instrument Services of this department.
Single-crystal X-ray diffraction data were collected under N2 flow at
-100 °C on a Bruker 1000 CCD diffractometer.
(12) (a) Ooi, T.; Uraguchi, D.; Kagoshima, N.; Maruoka, K. J. Am. Chem. Soc.
1998, 120, 5327. (b) Arai, T.; Sasai, H.; Yamaguchi, K.; Shibasaki, M. J.
Am. Chem. Soc. 1998, 120, 441. (c) Murakata, M.; Jono, T.; Mizuno, Y.;
Hoshino, O. J. Am. Chem. Soc. 1997, 119, 11713. (d) Simonsen, K. B.;
Svenstrup, N.; Roberson, M.; Jørgensen, K. A. Chem.-Eur. J. 2000, 6,
123.
Synthesis of (AlL)2 [L ) Tris(2-oxy-3,5-dimethylbenzyl)amine]
1. To a suspension of 1.677 g of tris(2-hydroxy-3,5-dimethylbenzyl)-
9
13728 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Facile Synthesis of Monomeric Alumatranes
A R T I C L E S
amine (4.000 mmol) in 20 mL of toluene was slowly added 2 mL of
a 2 M toluene solution of trimethylaluminum (4.000 mmol) via syringe.
Stirring this reaction mixture for 2 min at room temperature afforded
a yellow solution that was stirred for two more hours to generate a
white precipitate. The solid was filtered off, washed with toluene (2 ×
5 mL) and pentane (2 × 8 mL), and dried under vacuum to afford
1.507 g of 1 (yield, 85%) as a white solid. Single crystals suitable for
X-ray diffraction were obtained from a toluene/pentane (v/v ) 1:12)
solution of 1 at room temperature. Anal. Calcd for 1, C54H60Al2N2O6:
C, 73.12; H, 6.82; N, 3.16. Found: C, 73.49; H, 6.62; N, 3.06. ESI-
MS (m/z): [M + H]+ ) 887. Because of very broad overlapped peaks
from 2.3 to 5.9 ppm (assigned to the methylene groups), clear 1H NMR
spectra could not be obtained even at 50 °C. Poor solubility of 1 in
toluene-d8 and benzene-d6 prevented us from obtaining 13C NMR spectra
for 1.
6) solution of [HP(C3H7NCH2CH2)3N][5] at -20 °C. 1H NMR
(CDCl3): δ 7.09 (m, 5H, OPh); 6.78 (s, 3H, Ar); 6.54 (s, 3H, Ar);
5.38 (d, JPH ) 495.1 Hz, 1H, PH); 4.25 (b, 3H, ArCH2N); 3.47-3.34
(m, 3H, NCHMe2); 3.08-3.06 (m, 6H, CH2NCHMe2); 2.76-2.70 (m,
9H, N(CH2)3 and ArCH2N); 2.16 (s, 9H, ArCH3); 2.07 (s, 9H, ArCH3);
1.02 (d, J ) 6.7 Hz, 18H, NCH(CH3)2). 13C NMR (CDCL3): δ 164.8
(Ar); 155.9 (Ar); 130.4 (Ar); 129.3 (Ar); 128.5 (Ar); 128.3 (Ar); 127.0
(Ar); 126.9 (Ar); 125.5 (Ar); 124.1 (Ar); 122.1 (Ar); 121.3 (Ar); 114.1
(Ar); 59.4 (ArCH2N); 46.9 (d, JPC ) 16.0 Hz, NCHMe2); 46.5 (d, JPC
) 7.5 Hz, CH2NCHMe2); 32.9 (d, JPC ) 6.1 Hz, N(CH2)3); 21.3 (CH-
(CH3)2); 20.7 (ArCH3); 17.1 (ArCH3). Anal. Calcd for [HP(C3H7NCH2-
CH2)3N]+5, C48H69AlN5O4P: C, 68.79; H, 8.30; N, 8.36. Found: C,
68.76; H, 8.10; N, 8.01.
Synthesis of H2O-AlL, 6. Method A. A -20 °C solution of water
(18 mg, 1 mmol) in 70 mL of CH2Cl2 was added to a flask containing
443 mg (0.500 mmol) of 1. The reaction mixture was stirred at -20
°C for 5 h to generate a colorless solution. This solution was
concentrated to 5 mL, and then 30 mL of pentane was added. The
resulting solution was stored at -20 °C for a few days to afford 194
mg of 6 (yield, 42%) as a colorless crystalline solid. Because 6 is
unstable in solution at room temperature, its 1H NMR spectrum showed
several impurity peaks that are ascribable to decomposition. The peaks
assigned to 6 are 6.72 (s, 3H, Ar); 6.42 (s, 5H, Ar and H2O); 4.19 (b,
3H, ArCH2N); 2.26, (s, 9H, ArCH3); 2.10 (s, 9H, ArCH3). Attempts to
obtain satisfactory elemental analyses failed.
Synthesis of THF-AlL, 2. Compound 1 (443 mg, 0.500 mmol)
was dissolved in 15 mL of THF, giving a colorless solution. The solvent
was evaporated from this solution under reduced pressure to afford
515 mg of 2 (yield, 99%) as a white solid. Single crystals suitable for
X-ray diffraction were obtained from a THF/pentane (v/v ) 1:12)
solution of 2 at -20 °C. 1H NMR (CDCl3): 6.86 (s, 3H, Ar); 6.60 (s,
3H, Ar); 4.56 (b, 4H, CH2CH2O); 4.25 (d, J ) 10.2 Hz, 3H, ArCH2N);
2.83 (d, J ) 10.2, 3H, ArCH2N); 2.19-2.18 (m, 22H, ArCH3 and CH2-
CH2O). 13C NMR (CDCl3): δ 154.5 (Ar); 131.2 (Ar); 127.2 (Ar); 127.1
(Ar); 126.0 (Ar); 120.8 (Ar); 71.6 (CH2O); 58.9 (ArCH2N); 25.9 (CH2-
CH2O); 20.7 (ArCH3); 17.3 (ArCH3). Anal. Calcd for 2, C31H38-
AlNO4: C, 72.21; H, 7.43; N, 2.72. Found: C, 72.21; H, 7.44; N, 2.75.
Synthesis of PhCHO-AlL, 3. To a suspension of 443 mg of 1
(0.500 mmol) in 20 mL of toluene was added 0.200 mL of benzaldehyde
(1.97 mmol). The reaction mixture was stirred at room temperature
for 2 h to generate a yellowish solution that was concentrated under
reduced pressure to 3 mL, followed by addition of 35 mL of pentane.
This solution was stored at -20 °C for a few days to yield 368 mg of
Method B. To a flask containing 275 mg (0.500 mmol) of 3 was
added a mixture of 9 mg (0.5 mmol) of water in 70 mL of CH2Cl2 at
-20 °C. This mixture was stirred at -20 °C for 5 h, giving rise to a
colorless solution. Removal of the solvent under reduced pressure
afforded a white residue that was dissolved in 5 mL of cold (-20 °C)
toluene followed by addition of 40 mL of -20 °C pentane. After several
days at -20 °C, 81 mg of 6 (yield, 35%) was obtained as colorless
crystals.
1
3 (yield, 67%) as yellow crystals. H NMR (C6D6): δ 10.06 (s, 1H,
Synthesis of 7 from 1 (Method A). A -20 °C solution of water
(36 mg, 2.0 mmol) in 70 mL of CH2Cl2 was charged to a flask
containing 443 mg (0.500 mmol) of 1 at -20 °C. The reaction mixture
was allowed to warm slowly to room temperature while being stirred
over 6 h to generate a colorless solution. Slow evaporation of the solvent
under an argon flow afforded 153 mg of 7 (yield, 46%) as colorless
PhCHO); 7.61 (d, J ) 7.0 Hz, 2H, ArCHO); 7.0 (s, 3H, Ar); 6.84-
6.79 (m, 3H, ArCHO); 6.51 (s, 3H, Ar); 4.39 (b, 3H, ArCH2N); 2.75
(b, 3H, ArCH2N); 2.34 (s, 9H, ArCH3); 2.23 (s, 9H, ArCH3). 13C
NMR: 198.5 (PhCHO); 155.1 (Ar); 136.3 (Ar); 134.6 (Ar); 131.7 (Ar);
131.5 (Ar); 128.1 (Ar); 127.7 (Ar); 127.1 (Ar); 125.7 (Ar); 121.0 (Ar);
58.9 (ArCH2N); 20.6 (ArCH3); 16.8 (ArCH3). Anal. Calcd for 3, C34H36-
AlNO4: C, 74.30; H, 6.60; N, 2.55. Found: C, 73.98; H, 6.84; N, 2.62.
Synthesis of H2NCH2CH2NH2-AlL, 4. To a suspension of 443 mg
of 1 (0.500 mmol) in 20 mL of toluene was added 0.53 mL of
ethylenediamine (7.9 mmol). The reaction mixture was stirred at room
temperature for 3 h to generate a colorless solution. The volatiles were
evaporated under vacuum to afford 490 mg of the desired product 4
(yield, 98%) as a white solid. Single crystals suitable for X-ray
diffraction were obtained from a toluene/pentane (v/v ) 1:10) solution
of 4 at -20 °C. 1H NMR (C6D6): δ 6.97 (s, 2H, Ar); 6.90 (s, 1H, Ar);
6.47 (s, 2H, Ar); 6.43 (s, 1H, Ar); 4.26 (d, J ) 14.2 Hz, 3H, ArCH2N);
3.02 (b, 2H, AlNH2CH2); 2.83 (m, 2H, AlNH2CH2CH2NH2); 2.46 (d,
J ) 14.0 Hz, 3H, ArCH2N); 2.30 (s, 9H, ArCH3); 2.23 (s, 6H, ArCH3);
2.19 (s, 3H, ArCH3), 2.09-2.05 (m, 2H, NH2). 13C NMR (C6D6): δ
155.1 (Ar); 131.2 (Ar); 129.2 (Ar); 126.6 (Ar); 125.5(Ar); 121.1 (Ar);
58.9 (ArCH2N); 43.6 (NH2CH2); 41.8 (NH2CH2); 20.6 (ArCH3); 17.0
(ArCH3). Anal. Calcd for 4, C29H38AlN3O3: C, 69.16; H, 7.61; N, 8.34.
Found: C, 69.31; H, 7.72; N, 7.99.
Synthesis of PhO-AlL-, 5. To a suspension of 443 mg (0.500
mmol) of 1 in 20 mL of CH2Cl2 was added the solution obtained by
treatment of 94 mg (1.0 mmol) of phenol with 300 mg (1 mmol) of
tri-isopropylproazaphosphatrane [P(i-PrNCH2CH2)3N] in 20 mL of
toluene. The reaction mixture was stirred at room temperature for 3 h
to generate a colorless solution. The volatiles were evaporated under
reduced pressure to afford 831 mg of [HP(i-PrNCH2CH2)3N][5] (yield,
99%) as a white solid. Single crystals suitable for X-ray diffraction
were obtained from a toluene/methylene chloride/pentane (v/v/v ) 1:1:
1
crystals. H NMR (CDCl3): δ 9.09 (b, 2H, AlOH2); 6.91 (s, 1H, Ar);
6.62-6.53 (m, 3H, Ar); 6.35 (s, 1H, Ar); 5.96 (s, 1H, Ar); 4.35 (d, J
) 13.8 Hz, 1H, ArCH2N); 4.16-4.04 (m, 2H, ArCH2N); 3.07 (s, 1H,
OH); 2.81-2.67 (m, 3H, ArCH2N); 2.32 (s, 3H, ArCH3); 2.25 (s, 3H,
ArCH3); 2.08 (s, 3H, ArCH3); 1.96 (s, 3H, ArCH3); 1.71 (s, 3H, ArCH3);
1.67 (s, 3H, ArCH3). 13C NMR (CDCl3): δ 153.9 (Ar); 153.4 (Ar);
152.7 (Ar); 132.6 (Ar); 131.6 (Ar); 130.3 (Ar); 127.7 (Ar); 127.4 (Ar);
127.2 (Ar); 125.6 (Ar); 125.1 (Ar); 121.9 (Ar); 121.8 (Ar); 118.2 (Ar);
59.4 (ArCH2N); 58.7 (ArCH2N); 20.8 (ArCH3); 20.6 (ArCH3); 20.3
(ArCH3); 17.8 (ArCH3); 16.9 (ArCH3); 15.7 (ArCH3). Anal. Calcd for
7, C108H134Al6N4O22: C, 64.79; H, 6.75; N, 2.80. Found: C, 64.62; H,
6.87; N, 2.72.
Method B. A solution of 6 (100 mg, 0.217 mmol) in CH2Cl2 was
exposed to air for a few days via a needle through the septum of the
flask to afford 27 mg of colorless crystals of 7 in 37% yield. For the
1H NMR spectrum, see Method A.
Method C. A toluene solution of 3 (200 mg, 0.364 mmol) was
exposed to air for a few days via a needle through the septum of the
flask to generate 52 mg of colorless crystals of 7 in a 43% yield. For
1
the H NMR spectrum of the product, see Method A.
X-ray Structure Determination. Crystals suitable for X-ray analysis
were selected from underneath a solvent layer and were covered with
premixed epoxy glue to prevent decomposition, as almost all of the
substances investigated were extremely unstable in the atmosphere. The
sample was immediately mounted under a stream of cold nitrogen and
centered in the X-ray beam using a video camera.
9
J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13729

A R T I C L E S
Su et al.
Crystal evaluation and data collection were performed on a Bruker
CCD-1000 diffractometer with Mo KR (λ ) 0.71073 Å, graphite
monochromator) radiation with a detector to crystal distance of 5.03
cm. As almost all of the aforementioned samples were twinned, three
series of ω scans at different starting angles were obtained to analyze
the reflection profiles and to estimate the exposure time for data
collection. Each series consisted of 30 frames collected at intervals of
0.3° in a 10° range about ω, with an exposure time of 30 s per frame.
Members of this class of compounds diffracted very weakly and were
limited to low-resolution angles (usually, above 0.9 Å). No preliminary
indexing was performed. The data were obtained using the full sphere
routine by harvesting four sets of frames with 0.3° scans in ω with an
exposure time 30-120 s per frame. The dataset was integrated with
SMART software14 and analyzed with RLATT software14 to separate
the reflections belonging to one crystal for further calculations. Those
datasets were corrected for Lorentz and polarization effects. The
absorption correction was based on fitting a function to the empirical
transmission surface as sampled by multiple equivalent measurements13
using SADABS software.14
The positions of some core non-hydrogen atoms for all the structures
were found by direct methods. The remaining atoms were located in
an alternating series of least-squares cycles and difference Fourier maps
using SHELXTL14 software. All non-hydrogen atoms were refined in
a full-matrix anisotropic approximation. Typically, all other hydrogen
atoms were placed in the structure factor calculation at idealized
positions and were allowed to ride on the neighboring atoms with
relative isotropic displacement coefficients. In some cases, hydrogen
atoms belonging to water ligands were found objectively on Fourier
difference maps, and after that their parameters were constrained.
Atomic coordinates, bond lengths and angles, and anisotropic parameters
have been deposited at the Cambridge Crystallographic Data Center
(CCDC 297572-297580).
Structure of 1. A colorless plate (0.20 mm × 0.20 mm × 0.08
mm), C54H60Al2N2O6 × C7H8, M ) 979.13, at 298 K is monoclinic,
space group C2/c, a ) 30.924(15), b ) 11.069(5), c ) 17.077(8) Å, â
) 97.377(3)°, Z ) 4, V ) 5247(4) Å3, R1 ) 0.072, wR2 ) 0.206, GOF
) 1.058.
Structure of 2. A colorless plate (0.20 mm × 0.20 mm × 0.08
mm), C31H38AlNO4, M ) 515.60, at 298 K is monoclinic, space group
P21/c, a ) 9.3865(16), b ) 17.609(3), c ) 16.824(3) Å, â )
116.153(13)°, Z ) 4, V ) 2757.8(8) Å3, R1 ) 0.046, wR2 ) 0.124,
GOF ) 1.024.
Structure of 3. A colorless block (0.40 mm × 0.30 mm × 0.30
mm), C34H36AlNO4, M ) 549.62, at 183 K is rhombohedral, space
group R3h, a ) b ) c ) 11.7638(15) Å, R ) â ) γ ) 116.153(13)°,
Z ) 2, V ) 1508.8(3) Å3, R1 ) 0.053, wR2 ) 0.139, GOF ) 1.056.
Structure of 4. A colorless plate (0.28 mm × 0.22 mm × 0.10
mm), C35H44AlN3O3′, M ) 575.66, at 173 K is triclinic, space group
P1h, a ) 10.631(3), b ) 12.816(3), c ) 13.411(4) Å, R ) 71.575(4)°,
â ) 71.114(4)°, γ ) 81.389(5)°, Z ) 2, V ) 1637.9(8)A3, R1 ) 0.060,
wR2 ) 0.162, GOF ) 1.032.
Structure of 5. A colorless plate (0.30 mm × 0.20 mm × 0.10
mm), (C15H34AlNO4)+(C33H35PN4)- × CH2Cl2, M ) 922.96, at 183 K
is monoclinic, space group P21/n, a ) 13.024(6), b ) 18.322(8), c )
20.516(10) Å, â ) 96.352(9)°, Z ) 4, V ) 4866(4) Å3, R1 ) 0.073,
wR2 ) 0.180, GOF ) 1.064.
P1h, a ) 18.433(7), b ) 19.237(7), c ) 22.352(9) Å, R ) 104.229(7)°,
â ) 106.203(7)°, γ ) 104.718(7)°, Z ) 2, V ) 6920(5) Å3, R1 )
0.106, wR2 ) 0.295, GOF ) 1.167.
Structure of 7b. A colorless block (0.40 mm × 0.20 mm × 0.18
mm), C108H134Al6N4O22, M ) 2002.07, at 203 K is monoclinic, space
group C2/c, a ) 37.345(11), b ) 18.611(5), c ) 26.234(7) Å, â )
134.62(1)°, Z ) 4, V ) 12978(6) Å3, R1 ) 0.071, wR2 ) 0.218, GOF
) 1.059.
Structure of 7c. A colorless block (0.20 mm × 0.20 mm × 0.15
mm), C108H134Al6N4O22‚C6H14‚6(CH2Cl2), M ) 2597.80, at 173 K is
orthorhombic, space group Aba2, a ) 27.854(10), b ) 25.160(8), c
)18.562(8) Å, Z ) 4, V ) 13009(8) Å3, R1 ) 0.069, wR2 ) 0.182,
GOF ) 1.041.
The Reaction of 3 with Trimethylsilyl Cyanide in Toluene. To a
solution of 275 mg (0.500 mmol) of compound 3 in 10 mL of toluene
was added 64 mg (0.65 mmol) of trimethylsilyl cyanide. After being
stirred at room temperature for 12 h, all volatiles were removed under
reduced pressure at 40 °C. The reaction mixture was diluted with 50
mL of hexanes and filtered, and the filtrate was concentrated under
reduced pressure. The residue obtained was purified by column
chromatography on silica gel (eluent: 5% ethyl acetate in hexanes) to
give 101 mg of R-trimethylsilyoxyphenylacetonitrile (yield, 98%). The
1H NMR and 13C NMR spectra are in accord with those reported in
the literature.15d
Trimethylsilyl Cyanation of Benzaldehyde Catalyzed by 1. To
each of the following suspensions of 5 mol % (44.4 mg, 0.500 mmol),
10 mol % (88.8 mg, 0.100 mmol), and 25 mol % (222 mg, 0.250 mmol)
of 1 in 10 mL of toluene was added 106 mg (1.00 mmol) of
benzaldehyde, whereupon a bright yellow solution formed. The reaction
mixtures were stirred for 30 min, and then 129 mg (1.3 mmol) of
trimethylsilyl cyanide was added, causing the solution to become pale
yellow immediately. The reaction mixtures were stirred at room
temperature for 12 h, during which time a white solid precipitated in
the presence of the highest concentration of catalyst and a turbid solution
was obtained when the two lower concentrations of catalyst were
present. To remove excess trimethylsilyl cyanide, all volatiles were
removed under reduced pressure at 40 °C. Then 50 mL of hexanes
was added, and the precipitated catalyst was filtered off. The filtrate
was concentrated under reduced pressure, and the residue was purified
by column chromatography on silica gel (eluent: 5% ethyl acetate in
hexanes) to produce 191 mg (93% yield, 5 mol % catalyst), 201 mg
(98% yield, 10 mol % catalyst), and 205 mg (99% yield, 25 mol %
catalyst) of R-trimethylsilyoxyphenylacetonitrile for which the 1H NMR
and 13C NMR spectra were in accord with those reported in the
literature.15d The reactants were also added in reverse order. Thus, to
a suspension of 10 mol % (88.8 mg, 0.100 mmol) of compound 1 in
10 mL of toluene was added 129 mg (1.30 mmol) of trimethylsilyl
cyanide, giving a brownish solution. The reaction mixture was stirred
for 30 min, after which 106 mg (1.00 mmol) of benzaldehyde was
added, causing the solution to became pale yellow. The reaction mixture
was stirred at room temperature for 12 h, during which time a turbid
solution was obtained. Workup followed the same procedure as above,
and after column chromatography on silica gel, 189 mg (92% yield)
of R-trimethylsilyoxyphenylacetonitrile was obtained.
Results and Discussion
Structure of 6. A colorless block (0.20 mm × 0.20 mm × 0.15
mm), 2(C27H32AlNO4)‚C7H8, M ) 1015.17, at 173 K is triclinic, space
group P1h, a ) 13.350(5), b ) 14.565(6), c ) 15.156(6) Å, R )
73.818(6)°, â ) 68.657(6)°, γ ) 79.629(6)°, Z ) 2, V ) 2627.4(17)
Å3, R1 ) 0.058, wR2 ) 0.140, GOF ) 1.040.
In earlier work, we synthesized the alumatrane Me2HN-AlL
via the reaction of tris(2-hydroxy-3,5-dimethylbenzyl)amine with
Al2(NMe2)6.10a The dimethylamine liberated in this reaction
formed a robust adduct with the AlL moiety, and attempts to
synthesize other alumatranes by displacing dimethylamine from
Structure of 7a. A colorless plate (0.10 mm × 0.10 mm × 0.03
mm), C108H134Al6N4O22, M ) 2002.07, at 293 K is triclinic, space group
(15) (a) Yang, W.-B.; Fang, J.-M. J. Org. Chem. 1998, 63, 1356. (b) Whitesell,
J. K.; Apodaca, R. Tetrahedron Lett. 1996, 37, 2525. (c) Sasai, H.; Arai,
S.; Shibasaki, M. J. Org. Chem. 1994, 59, 2661. (d) Costa, D. J.; Boutin,
N. E.; Riess, J. G. Tetrahedron 1974, 30, 3793.
(13) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
(14) SHELXTL program library and NT version, version 5.1; Bruker Analytical
X-ray Systems: Madison, WI, 2003.
9
13730 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Facile Synthesis of Monomeric Alumatranes
A R T I C L E S
Å),18 which suggests a substantial degree of single bond
character. This conjecture is supported by our observation in
the molecular structures reported in the present work that, in
the absence of overriding effects, the aluminum atom is
displaced from the plane of the equatorial oxygens toward Nax
(see below). It should be noted, however, that transannular
distances in atranes can also be influenced in the solid state by
dipole forces, and differences in these distances may not be
entirely due to substituent effects.19 The average obtuse Oeq-
Al-Nax angle of 93.26(5)° indicates that the Al atom is slightly
displaced toward Nax from the plane defined by three equatorial
oxygen atoms, as also occurs in Me2HN-AlL.10b
The positive ion electrospray MS spectrum of a solution of
1 in toluene displayed a peak at m/z ) 887, corresponding to
its dimeric structure. The absence of additional peaks in the
ESI MS spectrum suggests that other species in toluene solution
are absent or are present in undetectably low concentration. In
previous literature reports,10b,c,20 evidence was presented for
various degrees of association of alumatranes: dimeric in the
gas phase; monomeric, hexameric, and octameric in solution;
and tetrameric in the solid phase. The aforementioned ESI MS
results given for 1 point to the persistence of a dimeric structure
for solid 1 in solution.
Figure 1. ORTEP of the molecular structure of 1 at the 50% probability
level with atomic labeling. Hydrogen atoms were omitted for clarity.
Me2HN-AlL with other ligands such as benzaldehyde and
phenol were unsuccessful.16 To achieve the goal of synthesizing
a coordinately unsaturated alumatrane that would readily be
ligated by a substrate molecule and hence activate it, we sought
to use trimethylaluminum as the metal source for the reaction
with tris(2-hydroxy-3,5-dimethylbenzyl)amine in toluene, in
which unreactive methane would be generated. Because only
dimeric 1 was formed in this reaction, we turned our attention
to the possibility of cleaving dimeric 1.
Cleavage of Dimeric 1 to Monomeric Alumatranes and
Their Structural Characterization. In view of the distortion
of the trigonal bipyramidal geometry adopted by the aluminum
atoms in 1, we hypothesized that there might be sufficient strain
in this dimer to permit its symmetrical cleavage with selected
ligands. Dissolving 1 in THF at room temperature allowed
adduct 2 to be formed in virtually quantitative isolated yield.
Adduct 2 is also easily synthesized in quantitative yield by
combining 1 with 2 equiv of THF in toluene. The expected
molecular structure of 2 was confirmed by X-ray diffraction
analysis (Figure 2). Here the Al atom adopts a nearly ideal
trigonal bipyramidal (TBP) structure with a THF molecule at
the apical position. The sum of the Oeq-Al-Oeq angles is
359.63(3)°, and the Nax-Al-Oax angle [179.15(8)°] is quite
linear. The average of the three Oeq-Al bonds in 2 [1.7535(17)
Å] is significantly shorter than the Oax-Al bond [1.9613(16)
Å]. The transannular N-Al bond length in 2 [2.0685(18) Å] is
within 3 times the estimated standard deviation (esd) of that in
1 [2.083(3) Å]. The average acute Oeq-Al-Oax bond angle
[87.98(7)°] and the average obtuse Oeq-Al-Nax [92.02(7)°]
indicates slight displacement of the aluminum atom in 2 from
the equatorial plane toward Nax, as is also found in 1 and in
Me2HN-AlL.10c
Synthesis and Structural Characterization of 1. This
compound was synthesized in 85% yield by combining tri-
methylaluminum with tris(2-hydroxy-3,5-dimethylbenzyl)amine
in equimolar quantities in toluene at room temperature. As
shown in Figure 1, the molecular structure determined by X-ray
means is composed of two alumatrane units (related by a C2
axis through the molecular center) linked by two Al-O bonds.
To our knowledge, the dimeric structure of 1 in the solid state
represents the only structurally characterized example of an
alumatrane dimer thus far reported. The aluminum atoms in this
structure adopt a distorted TBP geometry with a Nax-Al-Oax
angle of 141.41(13)°.
Two Al-Oeq bonds [1.733(3) and 1.755(3) Å] are signifi-
cantly shorter than those between the Al-O bonds involving
the bridging oxygen atoms [ave ) 1.886(3) Å]. The shorter
aryloxide Al-O distances, which are also observed in Me2HN-
AlL10a and in other alumatranes (see below), suggest a degree
of π-bonding between the oxygen lone pairs and aluminum-
centered σ* orbitals.17 The transannular interaction distance Al-
Nax in 1 [2.083(3) Å] is the same as the N-Al bond length in
Me2HN-AlL [2.083(3) Å].10a This distance is essentially equal
to the sum of the covalent radii of the Al and N atoms (2.05
In the 1H and 13C NMR spectra of 2, equivalency of the three
identical arms of the ligand was observed, indicating average
(18) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles
of Structure and Reactivity, 4th ed.; Harper-Collins: New York, 1993; p
292.
(19) Ka´rpa´ti, T.; Veszpre´mi, T.; Thirupathi, N.; Liu, X.; Wang, Z.; Ellern, A.;
Nyula´szi, L.; Verkade, J. G. J. Am. Chem. Soc. 2006, 128, 1500-1512.
(20) (a) Hein, F.; Albert, P. W. Z. Anorg. Allg. Chem. 1952, 269, 67. (b)
Mehrotra, R. C.; Mehrotra, R. K. J. Indian Chem. Soc. 1962, 39, 677. (c)
Lacey, M. J.; McDonald, C. G. Aust. J. Chem. 1967, 29, 1119. (d)
Shkklover, V. E.; Struchkov, Yu. T.; Voronkov, M. G.; Ovchinnikova, Z.
A.; Baryshok, V. P. Dokl. Akad. Nauk SSSR (Engl. Transl.) 1984, 277,
723. (e) Voronkov, M. G.; Baryshok, V. P. J. Organomet. Chem. 1982,
239, 199. (f) Paz-sandoval, M. A.; Fernandez-Vincent, C.; Uribe, G.;
Contreras, R.; Klaebe, A. Polyhedron 1988, 7, 679. (g) Healy, M. D.;
Barron, A. R. J. Am. Chem. Soc. 1989, 111, 398. (h) Mehrotra, R. C.; Rai,
A. K. Polyhedron 1991, 10, 1967. (i) Narayanan, R.; Laine, R. M. Appl.
Organomet. Chem. 1997, 11, 919. (j) Opornsawad, Y.; Ksapabutr, B.;
Wongkasemjit, S.; Laine, R. M. Eur. Polym. J. 2001, 37, 1877.
(16) Attempts to remove the dimethylamine ligand in Me2HNAlL by protonation
with 1 equiv of F3CCOOH or F3CSO3H failed, resulting in decomposition.
Benzaldehyde and phenol also did not displace the dimethylamine ligand
from Me2HNAlL.
(17) (a) Power, M. B.; Bott, S. G.; Clark, D. L.; Atwood, J. L.; Barron, A. R.
Organometallics 1990, 9, 3086. (b) Healy, M. D.; Wielda, D. A.; Barron,
A. R. Organometallics 1988, 7, 2543. (c) Healy, M. D.; Ziller, J. W.; Barron,
A. R. J. Am. Chem. Soc. 1990, 112, 2949.
9
J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13731

A R T I C L E S
Su et al.
The 1H NMR and 13C NMR spectra of 3, like those of 2, are
consistent with threefold symmetry in solution. It is interesting
to note that the 13C NMR spectrum of 3 exhibits a downfield
shift of 8 ppm for the carbonyl carbon compared with that of
uncoordinated benzaldehyde, consistent with the presence of a
more electrophilic carbonyl carbon in 3 as the result of a shift
of lone pair density from the carbonyl oxygen to aluminum.
The activation of benzaldehyde in compound 3 was demon-
strated by its reaction with an equivalent of trimethylsilyl
cyanide (eq 1). Compound 1 serves in such a catalytic capacity
for the addition of trimethylsilyl cyanide to benzaldehyde (eq
2), a transformation that Lewis acids readily facilitate.15 Here,
benzaldehyde apparently symmetrically cleaves dimeric 1,
generating 3 in situ as an intermediate. Only one literature report
was found stating that this reaction proceeds in high yield in
the absence of a catalyst.24 In our hands, only a 15% yield of
product was obtained under such conditions. However, in the
presence of 5, 10, or 25 mol % loading of 1, we obtained
excellent isolated yields (93-100%) of R-trimethylsilyoxyphe-
nylacetonitrile. Because this compound is easily hydrolyzed in
the presence of moisture, the HCN thus generated could function
as a catalyst unless strong precautions are taken. On the other
hand, adventitious water could also be expected to destroy
catalyst 1 by producing catalytically inactive 6 and/or 7. In our
reaction of 1 equiv of 1 with excess benzaldehyde for 30 min
prior to addition of the trimethylsilyl cyanide, 2 equiv of
intermediate 3 would be expected to form from the 5, 10, or 25
mol % loading of dimeric 1 via its cleavage into 10, 20, and 50
mol % of monomeric 1, respectively. By reversing the order of
addition of the reagents (see Experimental Section), a 92%
isolated yield of product was obtained. In that experiment, the
brownish solution obtained on initial addition of the trimeth-
ylsilyl cyanide suggests it ligates to the aluminum center in the
monomeric cleavage product of dimeric 1. Subsequent addition
of benzaldehyde could then displace the trimethylsilyl cyanide
Lewis base to form 3, as was suggested by the replacement of
the brownish color of the solution of the trimethylsilyl cyanide
adduct by the yellow color of the benzaldehyde adduct 3.
Nucleophilic attack of the liberated trimethylsilyl cyanide on
the carbonyl carbon would then follow.
Figure 2. ORTEP of the molecular structure of 2 at the 50% probability
level with atomic labeling. Hydrogen atoms were omitted for clarity.
Figure 3. ORTEP of the molecular structure of 3 at the 50% probability
level with atomic labeling. Hydrogen atoms were omitted for clarity.
pseudo C3V molecular symmetry in solution owing to a rapid
twisting molecular motion around the molecular axis on the
NMR time scale, in contrast to the mirror symmetry of the
molecular crystal structure.
Treatment of a suspension of 1 in toluene with 4 equiv of
benzaldehyde gave a solution of the benzaldehyde adduct 3 that
was isolated as yellow crystals. We were prompted to synthesize
3 by the realization that many reactions catalyzed by aluminum
complexes involve carbonyl-bearing substrates whose carbonyl
group can be activated by oxygen coordination to the
metal.1,12,21-23 The molecular structure of 3 shown in Figure 3,
like that of 2, possesses a near-ideal TBP coordination geometry
featuring a linear Oax-Al-Nax linkage [180.00(18)°] and an
aluminum center somewhat displaced toward Nax as indicated
by the Oeq-Al-Nax angle [92.95(6)°]. Although the Al-Oap
bond length [1.9815(15) Å] in 3 is only slightly longer than
that in 2 [1.9613(16) Å], the transannular bond distance in 3
[2.0983(15) Å] is distinctly longer than that in 2 [2.0685(18)]
Å. The differences in these distances are consistent with the
expected poorer donor character of the benzaldehyde oxygen
compared with the oxygen in THF.
The reaction of 1 with ethylenediamine (en) in toluene yielded
alumatrane 4 in which the en is coordinated to the aluminum
center by only one nitrogen. The molecular structure of 4,
established by X-ray diffraction analysis, is shown in Figure 4.
The transannular bond in 4 [2.1079(19) Å] is slightly longer
than those in 2 [2.0685(18) Å] and 3 [2.0983(15) Å], and it is
(21) (a) Lewis Acids in Organic Synthesis; Yamamoto, H., Ed.; Wiley-VCH:
Weinheim, 2000; Vols. 1 and 2. (b) Lewis Acid Reagents: A Practical
Approach; Yamamoto, H., Ed.; Oxford University Press: Oxford, 1999.
(22) Saito, S.; Nagahara, T.; Shiozawa, M.; Nakadai, M.; Yamamoto, H. J. Am.
Chem. Soc. 2003, 125, 6200.
(23) Ooi, T.; Takahashi, M.; Maruoka, K. J. Am. Chem. Soc. 1996, 118, 11307.
(24) Manju, K.; Trehan, S. J. Chem. Soc., Perkin Trans. 1 1995, 19, 2383.
9
13732 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Facile Synthesis of Monomeric Alumatranes
A R T I C L E S
Figure 4. ORTEP of the molecular structure of 4 at 50% probability level
with atomic labeling. Hydrogen atoms were omitted for clarity.
Figure 5. ORTEP of the molecular structure of 5 at the 50% probability
level with atomic labeling. Hydrogen atoms were omitted for clarity.
also slightly longer than the Al-NH2CH2CH2NH2 bond in 4
[2.0146(19) Å]. Neither intermolecular nor intramolecular
hydrogen bonds were observed in the structure.
(2)°] and the obtuse Oeq-Al-Oax angles [94.2(2)°]. The
phenolate anion in 5 is expected to function as a stronger
electron donor than the neutral ligands present in 2-4 and in
Me2HN-AlL.10a This supposition is consistent with the obser-
vation of a distinctly longer transannular bond in 5 [2.233(6)
Å] compared with that in 1 [2.083(3) Å], 2 [2.0685(18) Å], 3
[2.0983(15) Å], 4 [2.1079(19) Å], and Me2HN-AlL [2.083(3)
Å].10a
The reaction of 1 with en was carried out in an attempt to
synthesize a six-coordinate complex possessing a bidentate
ethylenediamine ligand, or a five-coordinate en complex in
which the tripodal ligand did not coordinate its central nitrogen
to the aluminum (or did so only weakly). The preservation of
the characteristic atrane structure is indicative of the stereo-
electronic stability of its tris-chelated structure involving three
six-membered rings. The 1H NMR spectrum of compound 4 in
C6D6 shows the presence of two chemically different phenyl
rings in a 2:1 ratio which may be associated with sterically
hindered rotation of the en ligand around the Al-N axis and/
or intramolecular hydrogen bonding between the hydrogens of
the free -NH2 group of the en ligand with an oxygen atom of
the aryloxide.
Unlike the reaction of 1 with en, its reaction with acetylac-
etone produced tris-acetylacetonatato aluminum(III) along with
free tris(2-hydroxy-3,5-dimethylbenzyl)amine. In view of the
comparatively great strength of the Al-O bond, it is reasonable
to suppose that the reaction was driven by formation of six
Al-O bonds at the cost of breaking only three Al-O bonds
and one Al-N bond.
Combination of a solution of [HP(i-PrNCH2CH2N)3N][OPh]
(made from an equimolar mixture of P(i-PrNCH2CH2N)3N and
phenol) with 1 afforded [HP(i-PrNCH2CH2N)3N][5]. This
compound allowed us the opportunity to gauge the influence
of an anionic exocyclic ligand on the alumatrane structure in
view of the fact that the exocyclic ligands in 2-4 and in Me2-
HN-AlL10a are charge neutral. Of particular interest here was
the effect of the phenolate ligand on the length of the
transannular bond in anionic 5 whose molecular structure we
obtained by X-ray analysis. As depicted in Figure 5, the metal
adopts a TBP coordination geometry with an average Al-Oeq
bond length [1.752(6) Å] close to that in 2-4. Coordination of
the anionic phenolate to aluminum, however, gives rise to a
short Al-Oax bond [1.768(6) Å] in 5 that is significantly shorter
by about 0.2 Å than that in 2. Moreover, the aluminum position
is above the plane defined by the three equatorial O atoms, in
contrast to its location in 1-4 and in Me2HN-AlL,10a where it
is below this plane in each case. The metal location in 5 is
reflected by the average of the acute Oeq-Al-N angles [85.9-
Synthesis of 6 and 7. The reaction of 1 with water at -20
°C yielded the water-coordinated monomeric alumatrane 6 in
42% yield, while the reaction of 1 with water at room
temperature led to its partial hydrolysis giving 7 which consists
of four alumatrane units symmetrically connected into a ring
by a tetra-aqua dimeric aluminum hydroxide molecule (see
later). These observations suggested that 6 is unstable at room
temperature, probably undergoing hydrolysis to generate 7.
Further experiments showed that indeed 6 reacted with atmo-
spheric moisture at room temperature to give 7 in 37% yield.
The sensitivity to atmospheric moisture of the benzaldehyde-
coordinated alumatrane 3 at room temperature and at -20 °C
gave rise to the formation of 7 in 43% yield and 6 in 35% yield.
Both 6 and 7 were structurally characterized by single-crystal
X-ray diffraction experiments. It should be mentioned that it
was possible to crystallize two different polymorphs of 7
(triclinic 7a and monoclinic 7b) as well as solvate 7c. As
depicted in Figure 6, two molecules of 6 are linked by an
unsymmetrical hydrogen bond between a water ligand of one
alumatrane unit and an aryloxide oxygen atom in the neighbor-
ing alumatrane unit. Although the two alumatranes in this dimer
are inequivalent because of the hydrogen bond, all the corre-
sponding bond distances and angles are very similar. The
aluminum atoms in a dimeric unit of 6 each possess a TBP
coordination geometry with an average Oeq-Al bond distance
[1.755(3) Å] close to that in 1-5 and Me2HN-AlL.10a The
average of the Oax-Al bonds in the two units of dimeric 6
[1.933(4) Å] is slightly shorter than that in 2 [1.9613(16) Å],
and the average of the transannular bond distances in 6 [2.063(4)
Å] is shorter than those in 1-5 and in Me2HN-AlL.10a The
transannular distance in 6 is, however, within 3 times the esd
of that in 2.
As depicted in Figure 7, the molecular structure of 7a can
be viewed as a tetramer consisting of four alumatrane units
9
J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13733

A R T I C L E S
Su et al.
centers in these units are located in the plane defined by their
three equatorial oxygen atoms as indicated by the sum of the
Oeq-Al-Oeq angles of 360.0(8)°. The average Al-Oeq bond
length of [1.777(5) Å] is within the range (1.745-1.759 Å) of
those observed in 1-6. The four alumatranyl Al atoms in 7 are
each bound to an anionic hydroxyl ligand. The average
transannular bond distance in 7 [2.073(6) Å] is significantly
shorter than that in compound 5 [2.233(6) Å] which also bears
an anionic ligand. The marked difference in the transannular
bond distance in these two compounds can be attributed to the
fact that the hydroxyl in 7 is a bridging ligand, while the ligand
in 5 is monodentate and bears a full negative charge. As an
expected consequence, the average Al-Oax bond distance in 7
[1.854(5) Å] is longer than that in 5 [1.768(6) Å].
Although the instability of 6 at room temperature precluded
1
obtaining definitive H NMR spectra, such a spectrum of 7
shows that there are three inequivalent arms of the tripodal
ligand in the four alumatranyl units owing to sterically restricted
rotation of these units around their pseudo threefold axis.
Interestingly, the hydroxyl proton (3.07 ppm) and ligated water
proton (9.09 ppm) resonances appear at separate chemical shifts
at room temperature.
Figure 6. ORTEP of the molecular structure of 6 showing the dimer formed
by hydrogen bonding. All hydrogen atoms were omitted except those
participating in dimerization.
The hydrolysis of aluminum alkoxides is a reaction that has
been widely used for the synthesis of porous or nanosized
materials.26 For a metal alkoxide or halide, this process is
proposed to be initiated by nucleophilic attack of water at the
metal center, followed by formation of HOR or HX, and metal
hydroxide, of which the latter then undergoes polycondensation
to provide the corresponding metal oxide.27 Water attack at the
metal center is expected to involve the temporary formation of
a water-coordinated complex. Because departure of the alkoxide
or halide is apparently too fast to stop hydrolysis at the stage
of a water-coordinated complex, no such complex has yet been
reported for a neutral water-coordinated aluminum alkoxide. In
this regard, however, Kawashima and co-workers recently
reported a water-coordinated silicon atrane that is representative
of an intermediate in the hydrolysis of alkoxysilanes.28 Our
observations that 1 and 3 are easily converted to 6 and that 6 is
converted to 7 under mild conditions suggest that 6 is a model
of the first intermediate in the hydrolysis of an aluminum
alkoxide. Our observation that 6 is easily transformed to 7 is
consistent with the notion that 7 is representative of a subsequent
condensed intermediate that could well be formed in such
hydrolyses.
Figure 7. Computer drawing of the molecular structure of 7a. Ellipsoids
for the carbon atoms and hydrogen atoms were not displayed for clarity.
The full ORTEP diagram can be found in the Supporting Information.
bridged by the dimeric aluminum hydroxide tetra-aqua fragment
(H2O)2(HO)2Al(OH)2Al(H2O)2(OH)2 in which both aluminum
atoms are octahedrally coordinated. In this framework, the pairs
of terminal OH and H2O groups on each aluminum are mutually
trans and cis, respectively, with the terminal hydroxyls each
shared with an AlL fragment. The two six-coordinate aluminum
centers are linked by two hydroxyl bridges. The seven Al-µ2-
OH bond distances in 7a fall in the range of 1.852-1.891 Å,
distances that are comparable to those for such linkages reported
in the literature.25 The four Al-OH2 bond lengths in 7a range
from 1.900(5) to 1.907(5) Å, of which the upper end of the
range is within 3 times the esd of the Al-OH2 bond length in
6 [(1.933(4) Å) and close to Al(III)-OH2 bond distances
reported in the literature (1.916 Å).25 The four alumatranyl units
in 7 are related by a C2 symmetry axis, and each of these units
features a TBP aluminum coordination geometry. The metal
Conclusions
As summarized in Scheme 1, dimeric 1 is a versatile starting
material for the synthesis of monomeric alumatranes with a
(26) (a) Kim, H. J.; Lee, H. C.; Rhee, C. H.; Chung, S. H.; Lee, H. C.; Lee, K.
H.; Lee, J. S. J. Am. Chem. Soc. 2003, 125, 13354. (b) Cabrera, S.; Haskouri,
J. E.; Alamo, J.; Beltra´n, A.; Beltra´n, D.; Mendioroz, S.; Marcos, M. D.;
Amoros, P. AdV. Mater. 1999, 11, 379. (c) Pu, L.; Bao, X.; Zou, J.; Feng,
D. Angew. Chem., Int. Ed. 2001, 40, 1490. (d) Zou, J.; Pu, L.; Bao, X.;
Feng, D. Appl. Phys. Lett. 2002, 80, 1079. (e) Lee, H. C.; Kim, H. J.;
Chung, S. H.; Lee, K. H.; Lee, H. C.; Lee, J. S. J. Am. Chem. Soc. 2003,
125, 2882. (f) Hant, S. M.; Attard, G. S.; Riddle, R.; Ryan, K. M. Chem.
Mater. 2005, 17, 1434. (g) Li, Y.; Zhang, W.; Zhang, L.; Yang, Q.; Wei,
Z.; Feng, Z.; Li, C. J. Phys. Chem. B 2004, 108, 9739. (h) Yue, Y.; Cedeon,
A.; Bonardet, J. L.; Melosh, N.; D’Esinose, J. B.; Fraissard, J. Chem.
Commun. 1999, 1967.
(27) (a) Cerveau, G.; Corriu, R. J. P.; Framery, E. Chem. Mater. 2001, 13, 3373.
(b) Misra, C. Industrial Alumina Chemicals; ACS Monograph 184;
American Chemical Society: Washington, DC, 1986; p 133.
(28) Kobayashi, J.; Kawaguchi, K.; Kawashima, T. J. Am. Chem. Soc. 2004,
126, 16318.
(25) Casey, W. H.; Olmstead, M. M.; Phillips, B. L. Inorg. Chem. 2005, 44,
4888.
9
13734 J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006

Facile Synthesis of Monomeric Alumatranes
A R T I C L E S
variety of substituents, including a novel water-coordinated
alumatrane. Comparison of the transannular bond lengths in the
alumatrane monomers 2-7 and Me2HN-AlL10b allowed in-
sights into the influence of the axial ligands on the transannular
bond length, although these distances lie in a somewhat narrow
range (2.063-2.233 Å). It has been shown by 13C NMR
spectroscopy that coordination of the aluminum center in 3 to
the oxygen of a carbonyl group appears to reduce the electron
density on the carbonyl carbon, thereby enhancing the electro-
philicity of this atom. Moreover, the fact that 3 is an isolable
compound is indicative of considerable Lewis acidity of the
aluminum center. We believe these findings are consistent with
promising prospects for the utility of alumatranes such as 1 as
Lewis acidic catalysts, and investigations along these lines are
underway. On the basis of the literature and our observations
of the facile conversion of 1 and 3 to 6, and of 6 to 7, we can
suggest that 6 models the initial intermediate in the hydrolysis
of an aluminum alkoxide and that 7 models a condensed
intermediate formed later in such hydrolyses.
Acknowledgment. We sincerely thank Dr. Junji Kobayashi
for fruitful discussions. We thank the National Science Founda-
tion for grant support.
Supporting Information Available: Crystallographic data
(CIF), chromatograms, ORTEP, and experimental results. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA0626786
9
J. AM. CHEM. SOC. VOL. 128, NO. 42, 2006 13735