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Cubane-Like Li4H4 and Li3H3Li(OH) Stabilized in Molecular Adducts with Alanes.

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DOI: 10.1002/anie.200501864
A cubic Li4H4 species can be isolated in molecular form through the
coordination of three HAl{N(SiMe3)2}2 ligands. Steric factors prevent
the coordination of a fourth ligand and result in the formation of an
aggregate with nearly perfect C3 symmetry. For more information see
the Communication by M. Veith et al. on the following pages.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 5968 ? 5971
Lithium Hydride
DOI: 10.1002/anie.200501864
Cubane-Like Li4H4 and Li3H3Li(OH): Stabilized
in Molecular Adducts with Alanes**
Michael Veith,* Peter Knig, Andreas Rammo, and
Volker Huch
Lithium hydride forms molecular adducts, especially with boranes, Rx(H)3xB, but also with
alanes, Rx(H)3xAl, most of which can be described as boranate or aluminate anions with
lithium as the cation. A frequently observed
common structural element of these adducts is a
four-membered Li2H2 ring that arises from dimerization of the simple lithium boranates and
aluminates.[1?10] A particularly simple example
from the area of lithium aluminates can be
described briefly thus: [(tBu)3AlH]Li is a dimer
and has a four-membered {Li2H2} ring to which
the aluminum atoms are bonded exocyclically through the
hydridic hydrogen atom.[11] A different structural principle is
the formation of Li-H-Al units where the metal atoms are
each held together through hydrogen bonds and thus form a
ring system.[12?15] In this way [{[(Me3Si)2N]AlH3Li�OEt2}2] is
formed from an eight-membered {Li2Al2H4} ring with the
hexamethyldisilazyl groups as ligands on the aluminum atoms
and the two Et2O molecules linked to the lithium atoms.[16]
The stabilization of larger LiH units in a molecule appears to
be exceptionally difficult. To our knowledge such lithium
hydride oligomers could only be confirmed structurally for
one example: [(tBuO)16Li16]Li17H17�cyclohexane.[17] This
alkoxo-hydrido lithium compound is a complex agglomerate
of LiOtBu and LiH.
Herein we report for the first time a Li4H4 cube that is
coordinated through three bis(amino)alane units and as a
result may be isolated in molecular form. We also describe the
reaction of this molecule with an equivalent of water in which
only one of the hydridic hydrogen atoms reacts. We have used
the reaction of tert-butoxyalane[18] with hexamethyldisilazyllithium[19] for the in situ generation of lithium hydride (see
also the synthesis of [{[(Me3Si)2N]AlH3Li�OEt2}2] from
hexamethyldisilazane and lithium alanate[16]). According to
the reaction in Equation (1) a product mixture is formed in
this reaction that consists of the lithium alanates 2 and 3 as
[*] Prof. M. Veith, Dr. P. K;nig, Dr. A. Rammo, Dr. V. Huch
Anorganische Chemie
Universit?t des Saarlandes
Im Stadtwald, 66041 Saarbr@cken (Germany)
Fax: (+ 49) 681-302-3995
[**] This work was supported financially by the Fonds der Chemischen
Industrie and the DFG (SFB 277 and SP 1072 ?Semiconductor and
Metal Clusters as Building Blocks for Organized Structures?).
Angew. Chem. Int. Ed. 2005, 44, 5968 ?5971
well as 1 which is the adduct of Li4H4 with three molecules of
[(Me3Si)2N]2Al-H .
We were able to separate all the components of this
reaction mixture by fractional crystallization and to characterize them spectroscopically and by X-ray crystallography. In
toluene solution 1?3 give simple 1H, 13C, and 29Si NMR
spectra, which suggests weak associations or small degrees of
oligomerization. The IR spectrum of 1 has a relatively broad
absorption in the metal hydride region with a maximum at
1732 cm1 and an extensive shoulder at 1700 cm1, a situation
which is in agreement with the different hydride bridges
found in the solid.
A ball and stick model of 1 derived from the X-ray
structural analysis is reproduced in Figure 1.[20] The molecule
has an approximate threefold axis that runs through atoms H7
and Li2. The molecule is most simply described as a Li4H4
cube at three edges of which Al?H units of HAl{N(SiMe3)2}2
are inserted so that Li?H?Al?H loops are formed from the
original Li?H edges. In spite of their weak Lewis basicity, a
hexamethyldisilylazyl group of each the three aluminum
atoms coordinates to one of the lithium atoms Li1, Li2, and
Li3. This coordination is shown by the relatively short Li?N
Figure 1. Molecular structure of 1. The methyl groups of the trimethylsilyl units (yellow Si) have been omitted for clarity. Selected (averaged)
bond lengths [*] and angles [8]: H7?Li(1,3,4) 2.03(2), H(1,2,3)?Li(1,3,4) 1.89(2), H(4,5,6)?Li2 1.815(9), H(4,5,6)?Li(4,1,3) 2.26(2), Al(1,2,3)?H(6,4,5) 1.64(1), Al(1,2,3)?H(1,2,3) 1.59(1), Al(1,2,3)?N(1,4,5)
1.846(5), Al(1,2,3)?N(2,3,6) 1.892(6), Li(1,3,4)贩種(6,2,3) 2.191(8), Li(1,3,4)贩稨(6,4,5) 2.77(2); Li1-H1-Al1 118.2(3), Li3-H2-Al2 116.6(3),
Li4-H3-Al3 116.3(3), average angle about H7 100.6(3), about Li2
101.8(6), angle-sum about N1 359.5, N2 355.2, N3 355.7, N4 358.6,
N5 359.0, N6 355.9.
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
distances (see Figure 1). Owing to the insertion of the Al?H
units into the three edges of the Li4H4 cube the Li1?H6, Li3?
H4, and Li4?H5 distances are greatly elongated, which is
reinforced by the concomitant N coordination to these atoms
(Figure 1). The different coordination behavior of the two
hexamethyldisilylazyl groups on each Al atom is also evident
from the angle-sum around the nitrogen atoms, almost
trigonal planar environment are found for N1, N4 and N5,
whereas N2, N3, and N6 are slightly pyramidalized. The
lithium atoms Li1, Li3, and Li4 are approximately tetrahedrally coordinated by three hydride ligands and a nitrogen
base, whereas Li2 is surrounded by a trigonal-pyramidal
arrangement of hydrides. The hydrides can be divided into
three categories: H1?H3 form angular bridges (mean value:
1178) between lithium and aluminum atoms, H4?H6 are
coordinated by the metal atoms in an almost trigonal-planar
arrangement, whereas H7 is surrounded only by lithium
atoms (mean angle: 100.6(3)8) in a trigonal-pyramidal
arrangement. Thus hydrogen atom H7 is the most accessible,
at least in the kinetic sense (see below).
The other two main products of the reaction form
coordination polymers in their crystal structures (Figure 2
and Figure 3). In the case of [{Li{H(OtBu)2Al[N(SiMe3)2]}}n]
(2) one-dimensional helical structures are formed from two
crystallographically different units through almost linear Li?
H?Al hydrogen bonds. They consist formally of aluminate
units {H(OtBu)2Al[N(SiMe3)2]} that chelate the formal lithium cation through the oxygen atom of the alcohol residue
(Figure 2).[20] The lithium atoms are coordinated by oxygen
The crystal structure of 3 is remarkable (Figure 3).[20] Two
crystallographically independent strands of [{Li(H2Al[N(SiMe3)2]2)}n] run through the crystal as Al?H?Li?H?Al zigzag
chains, which surprisingly (in spite of the weak Lewis basicity
of the nitrogen atom in {N(SiMe3)2}) are reinforced by Al?N?
Figure 3. Molecular structure of 3. (only one of the two crystallographically independent but structurally very similar polymers is shown).
Selected (averaged) bond lengths [*] and angles [8]: Al1?H1 1.52(1),
Al1?H2 1.79(1), Al1?N1 1.932(5), Al1?N2 1.846(5), Li1?H2 1.62(2),
Li1?H1? 1.81(2), Li1?N1 2.135(8), N1?Si(1,2) 1.752(9), N2?Si(3,4)
1.745(1); H2-Al1-N1, Al1-N1-Li1, N1-Li1-H2, Li1-H2-Al1 101.6(2), Li1H1?-Al1? 166.8(2).
Li bridges so that four-membered Al?N?Li?H rings are
formed. An alternative description is that the lithium atom is
coordinated by two hydrogen atoms and a nitrogen atom, this
is in contrast to the isolated molecular diethyl ether adduct
[Li(OEt2)2(mH)2Al{N(SiMe3)2}2] in which the lithium atom is
chelated by only two hydride hydrogen atoms.[16] Clearly the
loss of base requires the lithium atoms to expand their
coordination sphere from 2 to 3 with the silylazyl ligands.
If Li4H4[HAl{N(SiMe3)2}2]3 (1) is treated with a small
amount of water only H7 of the seven hydride atoms
(Figure 1) reacts, in the hydrolysis product Li4H3 (OH)
[HAl{N(SiMe3)2}2]3 (4; Figure 4 and the reaction in Equation (2)) it is substituted by an OH group.
Li4 H4 紿AlfN餝iMe3 �g2 3 �� H2 O
! H2 � Li4 H3 餙H藿HAlfN餝iMe3 �g2 3 �
Figure 2. Molecular structure of 2. Selected bond lengths [*] and
angles [8]: Al1?H1 1.61(1), Al2?H2 1.61(1), Li1??H2 1.95(2), Li2?H1
1.86(2), Al1?N1 1.843(5), Al1?O1 1.769(4), Al1?O2 1.788(4), Li1?O1
1.910(7), Li1?O2 1.924(7), Al2?N2 1.848(5), Al2?O3 1.780(4), Al2?O4
1.771(4), Li2?O3 1.883(7), Li2?O4 1.890(7); O1-Li1-O2 82.2(1), O1Al1-O2 90.27(8), Al1-H1-Li2 159.3(2), O3-Li2-O4 83.7(1), O3-Al2-O4
90.32(8), Al2-H2-Li1? 164.7(2).
and hydrogen atoms in a distorted trigonal-planar arrangement, and the hexamethyldisilazyl substituents form simple
terminal ligands to the aluminum atoms. An example of an
isolated lithium alanate that is chemically very similar to 2 is
[Li(OEt2)(mOCMetBu2)2Al(H)(OCMetBu2)] in which polymerization is impeded by the bonding of the Et2O base to the
lithium atom.
In many respects the structure of 4 resembles that of 1
since the whole basket-shaped Li4H3[HAl{N(SiMe3)2}2]3 part
is essentially identical in both molecules (Figure 1 and
Figure 4;[20] 1 and 4 crystallize in different crystal lattices
and are therefore not isotopic). The LiO1 bond in 4 is about
0.15 A longer than the LiH7 bond in 1, thus, the Li4H3[HAl{N(SiMe3)2}2]3 basket part of 4 is somewhat extended
(the nonbonding Li贩稬i distances in 4 are about 0.08 A longer
than in 1) and consequently the angles at the oxygen atom O1
are also more acute than at H7. As can be seen from the
hydrolysis of 1, the three HAl{N(SiMe3)2}2 loops not only
stabilize the Li4H4 units coordinatively but also protect them
in the kinetic sense. It is only at the unprotected, free position
(H7 in 1) that substitution occurs (in careful hydrolysis).
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2005, 44, 5968 ?5971
Figure 4. Molecular structure of 4. The methyl groups of the trimethylsilyl units (yellow Si) have been omitted for clarity. Selected (averaged)
bond lengths [*] and angles [8]: O1?Li(1,2,3) 2.183(9), Al(1,2,3)?N(2,4,6) 1.896(5), Al(1,2,3)?N(1,3,6) 1.841(5), Li4?H(1,4,5) 1.85(1), Li(1,2,3)?H(2,6,3) 1.90(1), Al(1,2,3)?H(2,3,6) 1.57(1), Al(1,2,3)?H(1,4,5)
1.62(2); Li1-O1-Li2 94.5(1), Li2-O1-Li3 94.4(1), Li1-O1-Li3 94.6(1).
Experimental section
A stirred solution of (H2AlOtBu)2 (0.79 g, 3.84 mmol) in toluene
(5.0 mL) at room temperature was slowly treated dropwise with a
solution of LiN(SiMe3)2 (2.58 g; 13.4 mmol) in toluene (15.0 mL).
After 18 h stirring the reaction mixture was heated for 7 h at about
95 8C. After cooling overnight in the oil bath the partly crystalline
precipitate (amount: 0.6 g) is collected by filtration. Colorless crystals
of 1 (0.05 g, 1.8 %) and 2 (0.40 g, 15.3 %) separate from the filtrate at
room temperature. The crystals differ in their crystalline habit. After
further concentration, further crystals separate, which after recrystallization from toluene yields 0.20 g (7.3 %) of colorless crystals of 3. If
a solution of 1 is treated with wet toluene, crystals of 4 form after a
few hours. Solvent in each case [D8]toluene: 1: 1H NMR
(200.13 MHz): d = 0.38 ppm, 13C NMR (50.3 MHz): d = 5.96 ppm,
Si NMR (39.7 MHz): d = 0.01 ppm; IR: (Al-H): n? = 1732, 1700 cm1;
2: 1H NMR: d = 0.45 (SiCH), 1.31 ppm (CCH), 13C NMR: d = 6.04
(SiC), 33.7 (CH), 68.9 ppm (CC), 29Si NMR: d = 2.96 ppm; 3: 1H
NMR: 0.37 ppm, 13C NMR: d = 5.90 ppm, 29Si NMR: d = 0.25 ppm. 4
was only characterized by X-ray diffraction.
Received: May 30, 2005
Published online: August 31, 2005
Keywords: alanes � hydrides � lithium � structure determination
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[20] Crystal data: 1: C36H115Al3Li4N6Si12, Mr = 1078.12 g mol1, orthorhombic, space group Pbca, a = 1547.9(3), b = 1960.8(4), c =
4791.7(10) pm,
V = 14543(5) L 106 pm3,
Z = 8,
1calcd =
0.985 Mg m3, F(000) = 4736, Stoe-IPDS-Image-Plate-System,
T = 293 K, 1.57 q 18.298, 40 984 reflections, of which 5219
were symmetry independent (SHELXS-86,97), refinement
(SHELXL-93,97) with anisotropic temperature factors for all
non-hydrogen atoms; the hydrogen atoms on the aluminum
atoms were freely refined, whereas those on the carbon atoms
were treated as rigid groups; R1 = 0.0386 (I > 2sI), wR2 = 0.0899.
2: C28H74Al2Li2N2O4Si4, Mr = 683.09 g mol1, monoclinic, space
group P21/c, a = 1065.3(2), b = 2588.1(5), c = 1701.0(3) pm, b =
106.54(3)8, V = 4495.8(14) L 106 pm3, Z = 4, 1calcd = 1.009 Mg m3,
F(000) = 1504, Stoe-IPDS, T = 293 K, 2.01 q 24.068, 27 817
reflections, of which 7004 were symmetry independent, (Rint =
0.0450), 7004 data, no restraints, 387 parameters, solution and
refinement as for 1, R1 = 0.0369 (I > 2sI), wR2 = 0.0908. 3:
C24H75Al2Li2N4Si8, Mr = 712.44 g mol1, monoclinic, space group
P21, a = 1345.4(3), b = 952.2(2), c = 1898.7(4) pm, b = 100.60(3)8,
V = 2390.9(9) L 106 pm3, Z = 2, 1calcd = 0.990 Mg m3, F(000) =
782, Stoe-IPDS, T = 293 K, 2.04 q 23.978, 14 969 reflections,
of which 6965 were symmetry independent, (Rint = 0.0701), 6965
data, 1 restraint, 373 parameters, solution and refinement as for
1, R1 = 0.0800 (I > 2sI), wR2 = 0.2521. 4: C36H115Al3Li4N6OSi12,
monoclinic, space group I2/a, a = 2495.5(5), b = 2107.0(4), c =
2974.0(6) pm, b = 96.05(3)8, V = 15550(5) L 106 pm3, Z = 8,
1calcd = 0.977 Mg m3, F(000) = 4996, Stoe-IPDS, T = 293 K,
1.81 q 24.138, 48 651 reflections, of which 11 659 were symmetry independent, (Rint = 0.0970), 11 659 data, no restraints, 605
parameters, solution and refinement as in 1, R1 = 0.0545 (I >
2sI), wR2 = 0.1446. CCDC-275399 (1), CCDC-275400 (2),
CCDC-275401 (3), and CCDC-275402 (4) contain the detailed
crystallographic data of this publication. The data may be
obtained free of charge under erhNltlich (or may be requested from: Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ; Fax: (+ 44) 1223-336-033; oder
2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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like, adduct, stabilizer, molecular, li3h3li, li4h4, cubana, alanes
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