Syntheses and structures of an alumole and its dianion

Article abstract of DOI:10.1002/anie.201304143

Base free: An alumole was synthesized and treatment with lithium afforded the lithium salt of the alumole dianion. The structures of these two molecules were then investigated. The Ci￡C bond lengths of the AlC ₄ ring in the dianion are nearly equal. DFT calculations revealed that the 3p(Al)-π* conjugation lowers the LUMO level of the alumole and that coordination of two lithium cations to the alumole dianion results in a planar AlC₄ ring.

Full text of DOI:10.1002/anie.201304143

Angewandte
Chemie
DOI: 10.1002/anie.201304143
Organoaluminum Compounds Very Important Paper
Syntheses and Structures of an “Alumole” and Its Dianion**
Tomohiro Agou, Tatsuya Wasano, Peng Jin, Shigeru Nagase, and Norihiro Tokitoh*
Dedicated to Professor Renji Okazaki on the occasion of his 76th birthday
Heteroles of electron-deficient group 13 elements are
expected to have low-lying LUMOs owing to the orbital
interactions between the empty p orbital of group 13 elements
and the p* orbitals. For instance, boroles (boracyclopenta-
dienes) exhibit various fascinating properties and reactivity,[1]
including extremely high electrophilicity of the boron center;
this electrophilicity has been exemplified by the generation of
Scheme 1. Syntheses of alumole 1 and the lithium salt [Li+(thf)]2[12ꢀ
(Mes*=2,4,6-(tBu)3C6H2).
]
the corresponding 5p-radical anions[2] and 6p-dianions.[3]
Such intriguing properties of boroles invoke the question of
whether the aluminum analogues of boroles, that is, alumoles,
would also possess peculiar characteristics derived from the
conjugation involving the empty 3p(Al) orbital.[4,5] To date,
only a few examples of alumole/Lewis base complexes have
been structurally characterized.[6,7] The coordination of Lewis
bases to the aluminum center may substantially affect the
electronic structures, therefore the synthesis of Lewis base
free alumoles has been desired to provide a basis for the
elucidation of the intrinsic nature of alumoles.
X-ray crystallographic analysis of 1 revealed its molecular
structure.[13] In the unit cell two crystallographically inde-
pendent molecules were found; the structure for one of the
two molecules is shown in Figure 1a. The AlC4 ring is
completely planar as shown by the sum of the internal bond
angles of 5608 with the tri-coordinated aluminum atom
Herein, we report the synthesis of stable alumole 1, which
is not coordinated to a Lewis base. Reduction of alumole
1 afforded the lithium salt of the alumole dianion [Li+(thf)]2-
[12ꢀ]. Structures of these alumole derivatives have been
elucidated.
On the basis of the successful application of 2,4,6-
(tBu)3C6H2 (Mes*) group in the syntheses of stable galloles[8]
and dibenzoboroles,[9] this bulky group was chosen as a sub-
stituent on the aluminum atom to protect the alumole from
Lewis base attack. Treatment of Mes*AlCl2 (2)[10] with 1,4-
dilithiobutadiene (3)[11] in toluene afforded alumole 1 as
a colorless solid (48%),[12] which is stable under an inert
atmosphere (Scheme 1). Alumole 1 is readily hydrolyzed to
give 4,5-diethyl-3,5-octadiene and Mes*H on exposure to air
and moisture. Addition of THF or Et2O to a C6D6 solution of
1 did not affect its 1H NMR spectrum, thus suggesting that the
aluminum center of 1 maintains a tri-coordinated structure in
solution even in the presence of such coordinative solvents.
Figure 1. Molecular structures and selected bond lengths (ꢀ) of a) 1
and b) [Li+(thf)]2[12ꢀ] (thermal ellipsoids are set at 50% probability
level). Hydrogen atoms are omitted for clarity.
showing planar geometry (S(ffAl) = 3608). The butadiene
moiety of the AlC4 ring exhibits apparent bond alternation.
ꢀ
The Al C bond lengths in the AlC4 ring are comparable to
those of alkenylaluminum compounds (e.g., (E)-
[*] Dr. T. Agou, T. Wasano, Prof. Dr. N. Tokitoh
Institute for Chemical Research, Kyoto University
Gokasho, Uji, Kyoto 611-0011 (Japan)
ꢀ
=
ꢀ
ꢀ
[(Me3Si)2HC]2Al CH CH SiMe3:
Al C(alkene) =
1.951(5) ꢀ).[14]
E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp
Dr. P. Jin, Prof. Dr. S. Nagase
Fukui Institute for Fundamental Chemistry, Kyoto University
Kyoto 606-8103 (Japan)
Treatment of 1 with an excess amount of lithium in THF
afforded the lithium salt of the alumole dianion,
[Li+(thf)]2[12ꢀ], as air- and moisture-sensitive orange crystals
in 41% yield (Scheme 1).[15] The choice of reductant is crucial;
treatment of 1 with Na, K, or KC8 gave a complicated mixture
and there was no evidence for the generation of the
corresponding metal salts.
[**] This work was partially supported by JSPS KAKENHI Grant (Nos.
22350017, 24550048, 24655028, and 24109013) and by the
“Molecular Systems Research” project of RIKEN Advanced Science
Institute. T.A. thanks to the Kyoto Technoscience Center for the
financial support.
X-ray crystallographic analysis of [Li+(thf)]2[12ꢀ] revealed
a C2 symmetric structure with a C2 axis passing through the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304143.
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Al(1) CMes* bond and the center of the AlC4 ring.[13] The two
imaginary vibrational frequency. Alternatively, the pyramidal
form (S(ffAl) = 3328), having Cs symmetry, was the energy
minimum. Even in the case of structural optimization of Li-
free dianion 12ꢀ, not the planar form but the pyramidal form
(S(ffAl) = 3328) was obtained as the equilibrium structure,
thus indicating that the bulky substituent would not enforce
the planar geometry of the alumole dianion. The pyramidal
structures of Li-free alumole dianions 12ꢀ and 42ꢀ suggest that
the negative charges are localized on the aluminum atoms.[16]
On the other hand, the energy-minimum structure of the
ꢀ
lithium cations are located above and below the planar AlC4
5
ꢀ
ring and bound to the AlC4 ring in a h fashion. The C C bond
lengths in the AlC4 ring are nearly equal, and the Al C bond
lengths in the AlC4 ring are slightly shortened compared with
those in 1.
ꢀ
In the 13C NMR spectrum of [Li+(thf)]2[12ꢀ] in C6D6, the
AlC4-ring carbon atom resonances (d(Ca) = 102.6 ppm,
d(Cb) = 112.6 ppm) were shifted upfield to those of
1 (d(Ca) = 144.0 ppm, d(Cb) = 156.4 ppm). The 7Li NMR
chemical shift of [Li+(thf)]2[12ꢀ] (d = ꢀ6.0 ppm) is in accord-
ance with the calculated value (d = ꢀ5.5 ppm), thus indicating
that the contact ion-pair structure of [Li+(thf)]2[12ꢀ] is
retained in solution.
lithium salt of 42ꢀ exhibited a planar AlC4 ring without C C
ꢀ
bond length alternation, as in the case of the experimentally
observed structure of [Li+(thf)]2[12ꢀ]. The distances between
the lithium cations and the AlC4 ring atoms in Li+ [42ꢀ] are
2
To elucidate the electronic structure of 1, the molecular
orbitals of parent alumole 4, butadiene 5, and dimethylalane 6
were computed (Figure 2). The structural parameters of the
comparable to those observed in the crystal structure of
[Li+(thf)]2[12ꢀ]. It can be concluded that coordination of two
lithium cations on the alumole dianion with inverse sandwich
geometry makes the alumole dianion planar.
The small negative nucleus-independent chemical shift
(NICS(0)) values of the Li-free dianion 42ꢀ (planar form: d =
ꢀ5.10 ppm, pyramidal form: d = ꢀ3.10 ppm) suggest that the
AlC4 ring in 42ꢀ is nearly nonaromatic. In contrast, lithium salt
Li+ [42ꢀ] has a considerably negative NICS value (d =
2
ꢀ15.48 ppm) at the center of the AlC4 ring. Almost the
same NICS value (d = ꢀ15.01 ppm) was calculated at the
center of the AlC4 ring in [Li+(thf)]2[12ꢀ]. Therefore, the
coordination of the lithium cations should significantly
influence not only the geometry but also the electronic
states of the alumole dianions, thus indicating a closo-cluster
description for the bonding of the lithium salt
[Li+(thf)]2[12ꢀ].[17] This bonding picture is of interest from
the viewpoint of spherical aromaticity.
Figure 2. Frontier orbitals of parent alumole 4 and the corresponding
orbital energies.
In summary, we have synthesized a Lewis base free
alumole and its dianion. The AlC4 ring of the alumole exhibits
bond alternation. Reduction of the alumole with lithium
afforded the lithium salt of the alumole dianion. DFT
calculations revealed that the 3p(Al)-p* conjugation effec-
tively lowers the LUMO energy level of the alumole.
Coordination of two lithium cations to the alumole dianion
is a key factor to keep the planar AlC4 ring structure. Further
investigation of the bonding situation and properties of the
lithium salt of the alumole dianion is currently underway.
AlC4 ring in 4 are in good agreement with those of 1. The
frontier orbitals of 4 can be explained by the interactions
between the p/p* orbitals of 5 and the 3p(Al) orbital of 6. The
HOMO of 4 is seemingly similar to that of 5. The p* orbital of
5 and the 3p(Al) orbital of 6 can be favorably overlapped,
thus making the LUMO level of 4 lower than those of 5 and 6.
Such a lowered LUMO level should explain the high electron
acceptability of 1.
Structures of the dianions of parent alumole 4 were
optimized (Figure 3). The planar form of Li-free dianion 42ꢀ,
having C2v symmetry, was the transition state with one
Experimental Section
General experimental: Solvents were purified by the Ultimate
Solvent System, Glass Contour Company (n-hexane and toluene)[18]
and by distillation from a potassium mirror (THF and C6D6). NMR
spectra were measured on a Bruker Avance-600 or a JEOL AL-300
spectrometer. Chemical shifts (d) are reported in ppm and are
referenced against solvent signals (1H, 13C) or external standards
(Al(NO3)3/D2O (27Al) or LiBr/D2O (7Li)). Mass spectra were
recorded on a Bruker micrOTOF mass spectrometer equipped with
an AMR DART-SVP ion source using He as an ionization gas.
Melting points were determined on a Yanaco micro melting point
apparatus and uncorrected. Elemental analyses were performed at
the microanalytical Laboratory of the Institute for Chemical
Research, Kyoto University.
Synthesis of alumole 1: Mes*AlCl2 (2) (0.46 g, 1.3 mmol) was
added to a toluene solution (5 mL) of 1,4-dilithiobutadiene 3 (0.40 g,
1.3 mmol) at room temperature. The solution was stirred for 4 h. The
Figure 3. Optimized geometries and the selected bond lengths (ꢀ) of
the dianions of parent alumole 4.
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solvent was removed under reduced pressure, and the residue was
dissolved in n-hexane and filtered. The filtrate was concentrated and
stored at ꢀ308C to afford 1 as colorless crystals (0.28 g, 0.64 mmol,
48%). m.p. 113–1158C. 1H NMR (600 MHz, C6D6): d = 1.09 (t, J =
7.5 Hz, 6H, a-CH2CH3), 1.12 (t, J = 7.5 Hz, 6H, b-CH2CH3), 1.30 (s,
9H, p-C(CH3)3), 1.55 (s, 18H, o-C(CH3)3), 2.46 (q, J = 7.5 Hz, 4H, b-
CH2CH3), 2.48 (q, J = 7.5 Hz, 4H, a-CH2CH3), 7.48 ppm (s, 2H,
ArH); 13C NMR (151 MHz, C6D6): d = 15.16 (b-CH2CH3), 17.54 (a-
CH2CH3), 21.99 (b-CH2CH3), 24.81 (a-CH2CH3), 31.47 (o-C(CH3)3),
31.47 (p-C(CH3)3), 34.83 (p-C(CH3)3), 38.22 (o-C(CH3)3), 121.12 (m-
calculated for parent alumole 4; these indexes suggest that the
antiaromaticity of 4 is much lower compared to that of the
parent borole (HBC4H4). Similarly, the small positive NICS(0)
value of 1 (+ 2.78 ppm) indicates the quite low antiaromaticity
of 1; a) P. von Raguꢁ Schleyer, P. K. Freeman, H. Jiao, B.
Goldfuss, Angew. Chem. 1995, 107, 332; Angew. Chem. Int. Ed.
Engl. 1995, 34, 337; b) P. v. R. Schleyer, C. Maerker, A.
Dransfeld, H. Jiao, N. J. R. van Eikema Hommes, J. Am.
Chem. Soc. 1996, 118, 6317; c) M. K. CyraÇsky, T. M. Krygowsky,
A. R. Katritzky, P. v. R. Schleyer, J. Org. Chem. 2002, 67, 1333;
d) H. Fallah-Bagher-Shaidaei, C. S. Wannere, C. Corminboeuf,
R. Puchta, P. v. R. Schleyer, Org. Lett. 2006, 8, 863; e) D. B.
Chesnut, L. J. Bartolotti, Chem. Phys. Lett. 2000, 316-331, 175;
f) J. Poater, X. Fradera, M. Duran, M. Solꢂ, Chem. Eur. J. 2003, 9,
400.
ꢀ =
Ar(C)), 131.68 (ipso-Ar(C)), 144.03 (Al C C), 150.66 (p-Ar(C)),
ꢀ =
156.42 (Al C C), 158.81 ppm (o-Ar(C)); No signal was observed in
the 27Al NMR spectrum even after a long-time measurement for a few
days; HRMS (DART-TOF, positive-mode) m/z calcd for C30H50Al
([M + H]+): 437.3722, found: 437.3691. Elemental analysis calcd (%)
for C30H49Al (1): C, 82.51; H, 11.31; found: C, 82.25; H, 11.57.
Synthesis of [Li+(thf)]2[12ꢀ]: Lithium (0.015 g, 2.1 mmol) was
added to a THF solution (1.5 mL) of alumole 1 (0.16 g, 0.37 mmol) at
room temperature, and the mixture was stirred for 12 h. Excess
lithium was removed by filtration, and the filtrate was concentrated
and stored at ꢀ308C to afford [Li+(thf)]2[12ꢀ] as orange crystals
(0.089 g, 0.15 mmol, 41%). m.p. 1498C (dec.). 1H NMR (600 MHz,
C6D6): d = 1.17–1.20 (m, 8H, OCH2CH2), 1.33 (t, J = 7.5 Hz, 6H, b-
CH2CH3), 1.41 (t, J = 7.5 Hz, 6H, a-CH2CH3), 1.51 (s, 9H, p-
C(CH3)3), 2.11 (s, 18H, o-C(CH3)3), 2.85 (q, J = 7.5 Hz, 4H, b-
CH2CH3), 2.89 (q, J = 7.5 Hz, 4H, a-CH2CH3), 3.30–3.32 (m, 8H,
OCH2CH2), 7.76 ppm (s, 2H, ArH); 13C NMR (151 MHz, C6D6): d =
19.84 (b-CH2CH3), 21.43 (b-CH2CH3), 21.99 (a-CH2CH3), 25.26
(OCH2CH2), 26.27 (a-CH2CH3), 31.88 (p-C(CH3)3), 34.86 (o-
C(CH3)3), 35.13 (p-C(CH3)3), 38.65 (o-C(CH3)3), 68.63 (OCH2CH2),
[5] Recently, syntheses and properties of dibenzogallole derivatives
have been reported: T. Matsumoto, K. Tanaka, Y. Chujo, J. Am.
Chem. Soc. 2013, 135, 4211.
[6] a) H. Hoberg, R. Krause-Gçing, J. Organomet. Chem. 1977, 127,
C29; b) C. Krꢃger, J. C. Sekutowski, H. Hoberg, R. Krause-
Gçing, J. Organomet. Chem. 1977, 141, 141.
[7] Intermediary formation of alumoles: a) E. Negishi, D. Y. Kon-
dakov, D. Choueiry, K. Kasai, T. Takahashi, J. Am. Chem. Soc.
1996, 118, 9577; b) Z. Xi, P. Li, Angew. Chem. 2000, 112, 3057;
Angew. Chem. Int. Ed. 2000, 39, 2950; c) C. Zhao, P. Li, Z. Xi,
Chem. Eur. J. 2002, 8, 4292; d) J. J. Eisch, W. C. Kaska, J. Am.
Chem. Soc. 1962, 84, 1501; e) J. J. Eisch, W. C. Kaska, J. Am.
Chem. Soc. 1966, 88, 2976; f) H. Hoberg, W. Richter, J. Organo-
met. Chem. 1980, 195, 347.
[8] A. H. Cowley, F. P. Gabbaꢄ, A. Decken, Angew. Chem. 1994, 106,
1429; Angew. Chem. Int. Ed. Engl. 1994, 33, 1370.
[9] A. Wakamiya, K. Mishima, K. Ekawa, S. Yamaguchi, Chem.
Commun. 2008, 579.
[10] R. J. Wehmschulte, P. P. Power, Inorg. Chem. 1996, 35, 3262.
[11] L. Liu, W.-X. Zhang, Q. Luo, H. Li, Z. Xi, Organometallics 2010,
29, 278.
[12] The UV/Vis spectrum of 1 in n-hexane has an absorption at
lmax = 318 nm (e = 2.1 ꢅ 103), which is at a much shorter wave-
length than those of the previously reported borole derivatives.
The hypsochromic shift may be related with the much lower
antiaromaticity of 1 compared to those of the borole derivatives.
A relationship between the UV/Vis absorption properties and
the antiaromaticity of pentaarylborole derivatives has been
noted: H. Braunschweig, I. Fernꢆndez, G. Frenking, T. Kupfer,
Angew. Chem. 2008, 120, 1977; Angew. Chem. Int. Ed. 2008, 47,
1951.
ꢀ =
ꢀ =
102.62 (Al C C), 112.56 (Al C C), 119.23 (m-Ar(C)), 142.02 (ipso-
Ar(C)), 147.93 (p-Ar(C)), 159.72 ppm (o-Ar(C)); 7Li NMR
(117 MHz, C6D6): d = ꢀ5.96 ppm; 7Li NMR (117 MHz, THF): d =
ꢀ5.84 ppm; 27Al NMR (156 MHz, C6D6): d = 198 ppm (s, broad,
Dw1/2 = ca. 7000 Hz); MS (DART-TOF, positive-mode) m/z 437 ([M-
[Li(thf)]2]+). Elemental analysis calcd (%) for [Li+(thf)]2[12ꢀ]: C,
76.73; H, 11.02; found: C, 75.96; H, 11.20 (Because [Li+(thf)]2[12ꢀ] is
highly air and moisture sensitive, the elemental analysis was unsat-
isfactory.)
Computational details: Geometry optimization and frequency
calculations were carried out using the B3PW91 functional with 6-
31G(d) (1, 12ꢀ, and [Li+(thf)]2[12ꢀ]) or 6-311 + G(2df) (4, 5, 6, 42ꢀ, and
Li+2[42ꢀ]) basis sets. The optimized geometries of
1
and
[Li+(thf)]2[12ꢀ] agree well with those found in the crystal structures.
Single point energies and NICS values were calculated at the
B3PW91/6-311 + G(2df) level using the optimized geometries. The
Gaussian 09 program package was used for all the calculations.[19]
[13] Crystallographic data for 1: triclinic, space group P-1, a =
11.3385(7), b = 14.0940(13), c = 19.1071(14) ꢀ, a = 70.677(4),
b = 88.960(3), g = 80.284(3)8, V= 2837.7(4) ꢀ3, Z = 4, R1 (I >
2s(I)) = 0.0538, wR2 (all data) = 0.1512; Crystallographic data
for [Li+(thf)]2[12ꢀ]: tetragonal, space group P43212, a = b =
10.4548(3), c = 34.1514(10) ꢀ, V= 3732.9(2) ꢀ3, Z = 4, R1 (I >
2s (I)) = 0.0375, wR2 (all data) = 0.1015. CCDC 915171 (1) and
915172 ([Li+(thf)]2[12ꢀ]) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Received: May 14, 2013
Revised: May 29, 2013
Published online: August 8, 2013
Keywords: aluminum · density functional calculations ·
.
heterocycles · lithium · structure elucidation
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[4] Various aromaticity indexes, including nucleus-independent
chemical shifts (NICS), harmonic oscillator model of aromaticity
(HOMA), and aromatic stabilization energies, have been
[14] W. Uhl, E. Er, A. Hepp, J. Kçsters, J. Grunenberg, Organo-
metallics 2008, 27, 3346.
[15] Reduction of a pentaphenylalumole/Et2O complex with lithium
and subsequent treatment with NiBr2 afforded a unique triple-
decker dinuclear nickel complex that does not contain alumi-
num: H. Hoberg, R. Krause-Gçing, C. Krꢃger, J. C. Sekutowski,
Angew. Chem. 1977, 89, 179; Angew. Chem. Int. Ed. Engl. 1977,
16, 183.
[16] The equilibrium structures of 42ꢀ and 12ꢀ are comparable to
those of phospholes and heavier group 14 heterole anions. In the
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cases of these heteroles, the pyramidal forms and the planar
forms correspond to the energy minima and the first-order
saddle points on the potential surfaces, respectively: a) W. P.
Freeman, T. D. Tilley, F. P. Arnold, A. L. Rheingold, P. K.
Gantzel, Angew. Chem. 1995, 107, 2029; Angew. Chem. Int.
Ed. Engl. 1995, 34, 1887; b) W. P. Freeman, T. D. Tilley, L. M.
Liable-Sands, A. L. Rheingold, J. Am. Chem. Soc. 1996, 118,
10457; c) L. Nyulꢆszi, Chem. Rev. 2001, 101, 1229.
[18] A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen,
F. J. Timmers, Organometallics 1996, 15, 1518.
[17] A closo-7-vetex AlLi2C4 cluster description as shown below has
been proposed for the bonding situation of [Li+(thf)]2[12ꢀ] by
a reviewer. The cluster core consists of 16 skeletal electrons
(3 electrons from each of the CEt units, 2 electrons from the
AlMes* unit, and two negative charges), which agrees with the
pentagonal bipyramidal structure as observed in the crystal and
optimized structures of [Li+(thf)]2[12ꢀ].
[19] Gaussian 09 (Revision C.01), M. J. Frisch, et al., Gaussian, Inc.,
Wallingford CT, 2010. For full reference, see Supporting
Information.
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