Planar dimeric six-membered spirane aluminum hydrazide: Synthesis and X-ray crystal structure of [LAlN(Me)NH]2 (L = HC{(2,6-i-Pr 2C6H3N)(CMe)}2)

Article abstract of DOI:10.1021/om049560n

The reaction between stoichiometric amounts of β-diketiminato- stabilized aluminum dihydride and methylhydrazine affords reddish brown crystals of a planar dimeric spirane aluminum hydrazide complex, [LAlN(Me)NH] ₂ (2).

Full text of DOI:10.1021/om049560n

Organometallics 2004, 23, 6327-6329
6327
Planar Dimeric Six-Membered Spirane Aluminum
Hydrazide: Synthesis and X-ray Crystal Structure of
[LAlN(Me)NH]2 (L ) HC{(2,6-i-Pr2C6H3N)(CMe)}2)
S. Shravan Kumar,† Sanjay Singh,† Fan Hongjun,‡ Herbert W. Roesky,*,†
Denis Vidovic´,† and Jo¨rg Magull†
Institut fu¨r Anorganische Chemie der Universita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany, and Theoretische Chemie, Universita¨t Siegen,
D-57068 Siegen, Germany
Received June 16, 2004
Summary: The reaction between stoichiometric amounts
of â-diketiminato-stabilized aluminum dihydride and
methylhydrazine affords reddish brown crystals of a
planar dimeric spirane aluminum hydrazide complex,
[LAlN(Me)NH]2 (2).
they are found to be potential precursor not only in the
formation of aluminum- and nitrogen-containing rings
and cages5-9 but also in the formation of aluminum
nitride by chemical vapor deposition or by the ther-
molysis of macroscopic samples.3 The hydrazine deriva-
tives have been prepared either by treatment of hydra-
zineswithtrialkylaluminumcompounds,whicheliminates
alkanes,5-7,14,16 or by reaction of hydrazines with di-
alkylaluminum hydrides8 or LiAlH4,11 which eliminates
hydrogen. Furthermore, they are prepared by reaction
of lithiated hydrazines with dialkylaluminum chlo-
rides11,12 or by hydroalumination of azobenzene with an
arylaluminum dihydride.13 These compounds usually
are dimeric, containing a four-membered Al2N2,5,6,12,14,16
a five-membered Al2N3,13,15 or a six-membered Al2N4
heterocycle,11,15,20 respectively. The Al2N2 dimers exist
as cis and trans isomers, and the Al2N4 rings can adopt
a chair and twist-boat conformation. In our exploration
of the reactivity of the Al-H bonds as well as of
aluminum hydrazide compounds we have studied the
reaction between LAlH2 (L ) HC{(2,6-i-Pr2C6H3N)-
(CMe)}2), which contains the bulky â-diketiminato
ligand, and methylhydrazine, which resulted in the
Introduction
Until now much of the aluminum structural chemis-
try was focused on the synthesis of aluminum amides1
and aluminum imides.2 They are found to be excellent
precursor for aluminum nitride.3 In combination with
CVD formed diamond film, aluminum nitride is a
promising piezoelectric material for acoustic wave (SAW)
application.3k Fetter et al.4 reported the reaction be-
tween hydrazine and trimethylamine alane, but were
unsuccessful in the synthesis of aluminum hydrazides.
Further work with hydrazine-aluminum compounds
was hampered by their shock sensitivity. Only in the
past few years has there been a gradual upsurge in the
synthesis of hydrazine derivatives of aluminum,5-21 as
* To whom correspondence should be addressed. E-mail:
hroesky@gwdg.de.
formation of
a structurally characterized planar
† Universita¨t Go¨ttingen.
‡ Universita¨t Siegen.
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Publication on Web 11/17/2004

6328 Organometallics, Vol. 23, No. 26, 2004
Notes
Scheme 1
dimeric spirane aluminum hydrazide, [LAlN(Me)NH]2
(2). Theoretical studies using the DFT-B3LYP method
have served to explain the planarity and the stability
of the six-membered Al2N4 ring in 2.
2.004(1) Å),15 [(Me3C)2AlNHNH2]2 (1.951(2) Å),20 and
[(Me3C)2AlNHNH]2[µ-Al(CMe3)]2 (1.941(7)-1.970(6) Å).20
The sum of the bond angles of N(1) and N(3), which are
bonded to the methyl groups, is 359.99°, indicating the
trigonal planar geometry. For N(2) and N(4) the sum of
the bond angles is 337.45°, showing clearly the pyra-
midal arrangement.
Results and Discussion
The reaction between stoichiometric amounts of 1 and
methylhydrazine in toluene refluxing solution gave
compound 2 with elimination of H2 (Scheme 1). During
the reaction the color of the solution changed from
colorless to reddish brown, and this color is sustained
during the course of the reaction. All appropriate
workup details for the experiment gave reddish brown
crystals of 2 as the only isolable compound. No reaction
was observed when the reaction was attempted at room
temperature. The air- and moisture-sensitive compound
2 was characterized by multinuclear NMR spectroscopy,
mass spectrometry, IR spectroscopy, and elemental
The theoretical calculations were carried out to
explain the planarity of the nitrogen centers in the
Al2N4 ring of compound 2. The DFT-B3LYP method was
employed within Gaussian 98,22 selecting 6-311g(d,p) as
basis sets unless otherwise specified. The geometry of
compound 2 was optimized (6-311g for phenyl, and
CHMe2 in L was replaced by H). The planar nitrogen
center and other structural parameters were success-
fully reproduced and the average bond length deviation
is less than 0.02 Å, thus showing the reliability of the
theoretical method. According to the classic chemical
bond theory and VSEPR model, nitrogen with its
coordination number three adopts a pyramidal geom-
etry. However, in compound 2 there are two planar
Al-N(Me)N centers. Several model molecules (AlH2-
(H2O)(NH2), AlH2(H2O)(NMe2), AlH2(H2O)N(NH2)2, AlCl2-
1
analysis. In the H NMR spectrum the NMe protons
resonate at δ 2.10 and the NH proton at δ 0.21.
Compound 2 is centrosymmetric and has two septets
corresponding to the CH proton of the isopropyl groups
(δ 3.46 and 3.25). In the EI mass spectrum the most
intense peak (m/z 976) corresponds to the dimer, which
is indicative of the stability of the Al2N4 ring, and a
small peak related to the monomer appears at m/z 488.
In the IR spectrum a weak band was observed at 3440
cm-1, which can be attributed to the N-H stretching
frequency.
The molecular structure of 2 in the solid state was
determined by X-ray crystal structural analysis. An
ORTEP plot of 2 is shown in Figure 1. Compound 2
crystallizes in the triclinic space group P1h with one-half
of the molecule and one-half of the n-hexane molecule
in the asymmetric unit. It is a spirane molecule in which
the six-membered Al2N4 ring is connected by â-diketim-
inato ligands at each of the aluminum centers. Com-
pound 2 has a planar Al2N4 ring. It contains a cen-
trosymmetric dimer with an inversion center. The
aromatic groups are dangling above and below the six-
membered ring, and they are slightly inclined toward
the vertical axis of the six-membered ring. The alumi-
num atom is tetracoordinate, connected to the four
nitrogen atoms in a distorted tetrahedral array. Two
methyl groups present on N(1) and N(3) are anti-
periplanar to each other. The hydrogen atoms on two
nitrogen atoms were freely refined on two split posi-
tions with half-occupancy. The Al-N bond lengths
(Al(1)-N(1) 1.787(2) Å, N(2)-Al(2) 1.804(2) Å) within
the planar Al2N4 ring of 2 are shorter in comparison
to those of [Me2AlN(SiMe3)N(tBu)H]2 (1.874(2) and
2.016(2) Å),11 [(Me3C)2AlN(SiMe3)NH2]2 (1.867(2) and
Figure 1. Molecular structure of 2 with 50% thermal
ellipsoid probability. Hydrogen atoms and the hexane
molecules are omitted for clarity. Selected bond distances
[Å] and bond angles [deg]: Al(1)-N(1) 1.787(2), N(1)-N(2)
1.445(2), N(2)-Al(2) 1.804(2), Al(1)-N(5) 1.939(2), N(6)-
Al(1)-N(5) 93.92(7), N(5)-Al(1)-N(1) 112.23(8), Al(1)-
N(1)-N(2) 126.66(13), Al(1)-N(1)-C(1) 124.61(15), C(1)-
N(1)-N(2) 108.72(17), N(1)-N(2)-Al(2) 123.54(13), N(1)-
Al(1)-N(4) 109.79(8).

Notes
Organometallics, Vol. 23, No. 26, 2004 6329
and refined against F2 on all data by full-matrix least squares
with SHELXL-97.25 All non-hydrogen atoms were refined
anisotropically. Neutral-atom scattering factors (including
anomalous scattering) were taken from International Tables
for X-Ray Crystallography.26
(H2O)(NHNH2)) were studied theoretically in order to
find the reason for the planarity of the nitrogen centers
in 2. It was observed that the geometry optimization
gives a planar nitrogen center. These results show that
the nitrogen center forms a triangular planar arrange-
ment when bonded to aluminum and two other atoms.
Thus the planar nitrogen center in 2 is due to the Al-N
bond character rather than the presence of the sterically
bulky ligands and other steric interactions. Moreover
we also calculated the curve when the planar nitrogen
center in AlH2(H2O)N(NH2)2 is distorted artificially
(torsion angle 180°) to a pyramidal conformation (torsion
angle 130°). The total energy change is only 0.95 kcal/
mol, which is small compared with that of a hydrogen
bond interaction. This may explain the pyramidal
geometry at the Al-NHN center in compound 2, as the
steric interaction prefers a staggered NH-NMe confor-
mation rather than an eclipsed one.
Synthesis of 2. To a solution of LAlH2 (0.50 g, 1.12 mmol)
in toluene (30 mL) was added dropwise a solution of methyl-
hydrazine (0.103 g, 2.24 mmol) in toluene (10 mL) at room
temperature. The resultant solution was stirred for 0.5 h before
refluxing it for 3 h. During the reaction evolution of H2 was
observed. After the removal of all volatiles the residue was
extracted with n-hexane (20 mL). The resultant solution was
kept for crystallization at room temperature for 24 h to give
reddish brown crystals of 2. Yield: 0.41 g, 76% with respect
1
to 1. Mp: 297-299 °C. H NMR (300 MHz, C6D6, 25 °C): δ
3
7.17-7.11 (m, 12 H, Ar), 4.91 (s, 2 H, γ-CH), 3.46 (sept, JHH
) 6.9 Hz, 4 H, CHMe2), 3.25 (sept, 3JHH ) 6.8 Hz, 4 H, CHMe2),
2.10 (s, 6 H, NMe), 1.57 (s, 12 H, CMe), 1.41 (d, 3JHH ) 6.7 Hz,
3
12 H, CHMe2), 1.35 (d, JHH ) 6.6 Hz, 12 H, CHMe2), 1.12 (d,
3
3JHH ) 6.9 Hz, 12 H, CHMe2), 1.06 (d, JHH ) 6.9 Hz, 12 H,
CHMe2), 0.21 (s, 2 H, NH). 13C NMR (125.13 MHz, C6D6, 25
°C): δ 161.48 (CN), 144.88-123.55 (Ar), 94.26 (γ-C), 45.64
(NMe), 31.9, 28.6 (CHMe2), 28.3, 25.16, 23.39, 22.99 (CHMe2),
20.74 (Me). IR (KBr, Nujol): ν˜ 3440 (w), 1622, 1552, 1261,
1096, 1020, 800, 721 cm-1. EIMS (70 eV): m/z (%): 976 (100)
[M+], 488 (12) [M+ - Al - L - N(Me)NH]. Anal. (%) Calcd for
C60H90Al2N8 (979.39): C, 73.58; H, 9.47; N, 11.44. Found: C,
73.10; H, 9.20; N, 10.98.
Experimental Section
All experimental manipulations were carried out under an
atmosphere of dry nitrogen using standard Schlenk tech-
niques. The samples for spectral measurements were prepared
in a drybox. Solvents were purified according to conventional
procedures and were freshly distilled prior to use. Compound
1
23 was prepared as described in the literature. NMR spectra
Crystallographic data for compound 2‚C6H14 (C66H104
-
were recorded on a Bruker AM 200 instrument, and the
chemical shifts are reported with reference to TMS. IR spectra
were recorded on a Bio-Rad Digilab FTS7 spectrometer.
Melting points were obtained on a HWS-SG 3000 apparatus
and are uncorrected. CHN analyses were performed at the
Analytical Laboratory of the Institute of Inorganic Chemistry,
Go¨ttingen, Germany. A suitable crystal of compound 2 was
mounted on a glass fiber and coated with paraffin oil. Diffrac-
tion data for 2 were collected on a STOE IPDS II diffractometer
at 133(2) K. The measurement was made with graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å). The
structure was solved by direct methods using SHELXS-9724
Al2N8): Mr ) 1063.53, triclinic, P1h, a ) 9.054(5) Å, b ) 13.218-
(7) Å, c ) 13.446(8) Å, R ) 87.23(5)°, â ) 80.54(4)°, γ )
3
78.99(4)°, V ) 1558.04(15) Å3, Z ) 1, Fcalcd ) 1.134 Mg m-
,
3.08° e 2θ e 49.76°, T ) 133(2) K, λ ) 0.71073 Å, µ ) 0.092
mm-1, F(000) ) 582, -10 e h e 10, -14 e k e 15, -15 e l e
15, 28 288 reflns collected, 5222 were independent and were
used in the structure refinement of 348, R1 ) 0.0495 (I >
2σ(I)), wR2 ) 0.1388 (all data), min./max. residual electron
density 0.636/-0.493 e‚Å- 3
.
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft and the Go¨ttinger
Akademie der Wissenschaften. S.S. thanks the Gra-
duiertenkolleg 782 for a fellowship.
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Supporting Information Available: Single-crystal X-ray
structural data of compound 2 (CIF format). This material is
available free of charge via the Internet at http://pubs.acs.org.
OM049560N
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