Preparation of monomeric [LAl(NH2)2] - A main-group metal diamide containing two terminal NH2 groups

Article abstract of DOI:10.1002/anie.200353541

The reactions of [LA/Cl₂] (L = HC[C(Me)N(Ar)]₂, Ar = 2,6-iPr₂C₆H₃) with NH₃ or H ₂O in the presence of a 1,3-di-tert-butylimidazol-2-ylidene as acceptor for the HCl lead to unique species of composition [LAl(NH ₂)₂] (depicted) or [LAl(OH)₂], respectively. The by-product 1,3-di-tert-butylimidazolium chloride can be recovered by filtration and recycled to the corresponding carbene.

Full text of DOI:10.1002/anie.200353541

Communications
Main-Group Metal Complexes
Preparation of Monomeric [LAl(NH2)2]—A
Main-Group Metal Diamide Containing Two
Terminal NH2 Groups**
Vojtech Jancik, Leslie W. Pineda, Jiri Pinkas,
Herbert W. Roesky,* Dante Neculai, Ana M. Neculai,
and Regine Herbst-Irmer
Dedicated to Professor Dietmar Seyferth
on the occasion of his 75th birthday
Aluminum amides supported by organic ligands are impor-
tant molecular precursors for the preparation of aluminum
À
À
nitrides, such as Al N based semiconductors and Al N
ceramics.[1] Amides with the general formula [R2Al(NH2)], in
which R corresponds to an organic substituent, are rare. The
presence of a reactive NH2 group, which can be involved in
further substitution reactions, may lead to mixed-metal
À
À
imides that have the Al N(H) M skeleton (M = metal
atom). However the known amides show a strong tendency
to oligomerize and form unstable trimers [{R2Al(m-NH2)}3]
(R = Me, tBu)[2a,b] or dimers [{R2Al(m-NH2)}2] (R =
Me3Si)[2b–d] because of the Lewis acidity of the aluminum
center and the presence of the electron lone pair at the
nitrogen atom of the NH2 group. The steric bulk of the
substituents is the major factor that determines the degree of
association. The most recent example of such an aluminum
amide [{[(Me3Si)2Al(m-NH2)2]3}Al] was published in 1988 by
Janik et al., in which the central aluminum cation is octahe-
drally coordinated to three [(Me3Si)2Al(m-NH2)2]À ions.[2c] The
only known aluminum amide that contains a terminal NH2
group
[AlCl3(NH2iPr)][{Al(NH3)(NH2)[Al(NHiPr)-
(NiPr)Cl]2}2] was prepared in low yield in 1997 by Chang
et al. by using AlCl3, iPrNHLi, and an excess of iPrNH2.[3]
However, the mechanism of the formation of the product
remains unclear. The adducts of composition R3Al·NH3 are
thermodynamically unstable and thus decompose at elevated
temperatures to the corresponding amides under elimination
of alkanes.[2a,4] So far there are no reports known on the
preparation of aluminum amides by treating KNH2 or NaNH2
with the corresponding aluminum halides. Previous experi-
[*] Dipl.-Chem. V. Jancik, Dipl.-Chem. L. W. Pineda,
Prof. Dr. H. W. Roesky, Dr. D. Neculai, Dr. A. M. Neculai,
Dr. R. Herbst-Irmer
Institut für Anorganische Chemie
Georg-August Universität Göttingen
Tammannstrasse 4, 37077 Göttingen (Germany)
Fax: (+49)551-39-3373
E-mail: hroesky@gwdg.de
Prof. Dr. J. Pinkas
Department of Inorganic Chemistry
Masaryk University, 61137 Brno (Czech Republic)
[**] L=HC[C(Me)N(Ar)]2, Ar=2,6-iPr2C6H3. This work was supported
by the Deutsche Forschungsgemeinschaft, the VW Foundation and
the Göttinger Akademie der Wissenschaften. V. Jancik thanks the
Fonds der Chemischen Industrie for a predoctoral fellowship.
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DOI: 10.1002/anie.200353541
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Angewandte
Chemie
ments indicate that such reactions result mostly in the
decomposition of the starting material to yield insoluble
white powders under elimination of the free ligand.[5]
Recently, we showed that an N-heterocyclic carbene can
be used as an HCl acceptor for the preparation of a GeII
hydroxide derivative when water is used as a source of OH
ions.[6] Consequently we tried the same procedure for the
preparation of aluminum amides and hydroxides by adding
NH3 and H2O, respectively, to the corresponding aluminum
chloride derivative. It should be mentioned that previous
such as hexane, toluene, or THF, which allows an easy
separation of the product from the reaction mixture by
filtration. In addition the imidazolium chloride can be easily
recycled to the free carbene by using a strong base such as
KOtBu or NaH.[8]
Surprisingly, compound 3 is monomeric in the solid state
and what is even more striking, the NH2 groups are not
involved in any kind of hydrogen bonding as shown by X-ray
structural analysis and IR spectroscopy. Moreover, 3 is
thermally stable and can be maintained at 708C for 2 h
without any significant decomposition, which is also reflected
by its high melting point (1668C). Based on the assumption
that 3 is exposed to air, the rate of decomposition is
significantly slower than that of 4 (see below). Furthermore
we carried out some additional investigations on 4 to learn
more about its stability. Compound 4 is unstable and
decomposes upon heating to temperatures exceeding 708C
or rapidly after contact with air as indicated by temperature
dependent 1HNMR studies (Figure 1).
À
attempts to prepare compounds with terminal Al OHand
À
Al NH2 groups in the presence of NMe3, NEt3, or pyridine as
an HCl acceptor were not successful.[5] The addition of dry
ammonia to
a toluene solution of [LAlCl2] (1; (L =
HC[C(Me)N(Ar)]2, Ar= 2,6-iPr2C6H3)[7] and two equivalents
of 1,3-di-tert-butylimidazol-2-ylidene (2)[8] over a period of
10 min at À208C resulted in the formation of a slurry of
insoluble 1,3-di-tert-butylimidazolium chloride. Subsequent
filtration followed by extraction of the remaining solid with
toluene gave an oily residue after removal of
the volatiles in vacuo. Treatment of the residue
with cold pentane afforded [LAl(NH2)2] (3) as
a white powder in 70% yield. Furthermore,
when water is used instead of ammonia and
benzene as a solvent a white microcrystalline
powder of [LAl(OH)2] (4) can be obtained
within 10 min in a yield of 65%. Previously 4
was prepared in an NH3(l)/toluene two-phase
system,[9] after 7 h in a yield of 48%. This new
method clearly demonstrates the advantages
Figure 1. 1H NMR spectroscopic kinetic study of the thermal decomposition of 4 to the free
ligand (LH). Resonances between d=1.5 and 1.7 ppm are assigned to the a-methyl groups (a 4;
d LH), and the doublets belong to the diastereotopic methyl groups of the iPr moieties (b+c 4;
e+f LH). The only soluble organic product of this decomposition is the free ligand, which was
identified by comparison with an original sample. The spectrum taken at 308C represents pure 4
in a sealed tube, whereas, the following spectrum was taken after three days. Further spectra
show thermally initiated decomposition of 4, which is slow below 608C, but accelerates signifi-
cantly at 708C. Finally the last spectrum was recorded after 15 min maintained at 708C and con-
firms the thermal lability of 4. Similar degradation of 4 was observed after its exposure to air in
both the solid and solution state.
for the preparation of aluminum amides and
hydroxides. Scheme 1 summarizes the reactions
for preparing 3 and 4.
So far we have not given an answer to the
question: Why does the addition of the N-
heterocyclic carbene lead to the desired prod-
uct? Clearly, because of the high reactivity of 3
and 4 towards protons it appears that the amine
is not suitable as a HCl acceptor. On the one
hand, there is always an equilibrium between
the protonated amine and the free base and
thus the protons cause side reactions. On the other hand, in
the presence of the N-heterocyclic carbene there is no such an
The 1HNMR spectrum of 3 shows the typical pattern for
the ligand (L) and a broad singlet at À0.55 ppm assigned to
the NH2 moieties with 15N satellites (1JNH = 64 Hz). The NH2
groups resonate in the 15N NMR at À378 ppm, whereas the
remaining two nitrogen atoms have a resonance at À205 ppm.
The IR spectrum shows two weak sharp absorptions for nasym
at 3468 and nsym at 3396 cmÀ1, which also confirm the absence
of hydrogen bonds in the crystal lattice. Finally because of the
high thermal stability of 3, the EIMS spectrum shows the
molecular ion at m/z 476 (16%) while the most intense peak
at m/z 444 (100%) was assigned to the [M+À2NH2] fragment.
Single crystals of 3 suitable for X-ray structural analysis
were obtained by slow crystallization of its saturated pentane
solution at À328C. Compound 3 crystallizes in the monoclinic
space group P2(1)/c.[11] Figure 2 shows the molecular struc-
ture and numbering Scheme of 3.
À
equilibrium of free protons due to the favored covalent C H
bond formation.[10] Moreover, the resulting imidazolium
chloride is very sparingly soluble in hydrocarbon solvents
The AlN4 core has a deformed tetrahedral geometry with
the smallest and biggest N-Al-N angle of 95.78 and 117.28,
Scheme 1. Syntheses of 3 and 4.
Angew. Chem. Int. Ed. 2004, 43, 2142 –2145
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Communications
Experimental Section
All manipulations were performed under a dry and oxygen-free
atmosphere (N2 or Ar) by using Schlenk-line and glovebox techni-
ques.
Synthesis of 3: Dry gaseous NH3 was added in excess to a solution
of (2.410 g, 4.675 mmol) [LAlCl2] and 1.686 g (9.349 mmol) of 1,3-di-
tert-butylimidazol-2-ylidene in toluene (60 mL) cooled to À208C.
Immediately after the addition of NH3, a precipitate of 1,3-di-tert-
butylimidazolium chloride was formed. After 10 min the gas stream
of NH3 was disconnected, the cooling bath was removed, and the
suspension was stirred for additional 10 min. The excess ammonia was
released through a mineral oil bubbler attached to the flask. The
precipitate was removed by filtration, washed twice with toluene
(10 mL), and all the volatiles were removed in vacuo. The oily residue
was treated twice with cold pentane (5 mL) and after filtration and
drying in vacuo, 3 was obtained as a white microcrystalline powder.
Yield 1.67 g (75%). 1HNMR (500 MHz, C 6D6, 258C, TMS): d =
À0.52 (bs, 1J(N,H) = 64 Hz, 4H, NH2), 1.17 (d, 3J(H,H) = 6.9 Hz,
12H, CH(CH3)2), 1.37 (d, 3J(H,H) = 6.9 Hz, 12H, CH(CH3)2), 1.58 (s,
6H, CH3), 3.47 (sept, 3J(H,H) = 6.9 Hz, 4H, CH(CH3)2), 4.88 (s, 1H,
g-CH), 7.05–7.20 ppm (m, 6H, m-, p- Ar-H); 13C NMR (125.8 MHz,
C6D6, 258C, TMS): d = 23.4, 24.8 (CH(CH3)2), 25.4 (CH(CH3)2), 28.5
(CH3), 96.5 (g-CH), 124.3, 127.0, 141.2, 144.5 (i-, o-, m-, p- Ar),
Figure 2. Thermal ellipsoids plot of 3 showing the atoms at the 50%
probability level. H atoms, except NH, are omitted for clarity. Selected
bond lengths [Š] and angles [8]: Al(1)-N(1) 1.921(2), Al(1)-N(2)
1.903(2), Al(1)-N(3) 1.790(2), Al(1)-N(4) 1.788(2), N(3)-H(1) 0.88(2),
N(3)-H(2) 0.85(2), N(4)-H(3) 0.87(2), N(4)-H(4) 0.86(2); N(1)-Al(1)-
N(2) 95.7(1), N(1)-Al(1)-N(3) 107.2(1), N(1)-Al(1)-N(4) 117.2(1),
N(3)-Al(1)-N(4) 112.2(1), H(1)-N(3)-H(2) 106(2), H(3)-N(4)-H(4)
109(2), H(1)-N(3)-Al(1) 122(1), H(2)-N(3)-Al(1) 127(1), H(3)-N(4)-
Al(1) 123(1), H(4)-N(4)-Al(1) 125 (1).
169.2 ppm (C N); 15N NMR (50.7 MHz, C6D6, 258C, MeNO2): d =
=
À378 (NH2); À205 ppm (C N); 27Al NMR (78.2 MHz, C6D6, 258C,
=
[Al(OH2)6]3+): d = 102 ppm (n1/2 = 4031 Hz); IR (KBr pellet): n˜ =
3468 vw, 3396 vw (NH) cmÀ1; EI-MS (70 eV): m/z (%): 476 (16)
[M+], 459 (20) [M+ÀNH3], 444 (100) [M+À2NH2]; elemental analysis
(%) calcd for C29H45AlN4 (476.7): C 73.1, H9.5; found: C 72.9, H9.4.
Synthesis of 4: H2O (180 mL, 9.989 mmol) was added quickly to a
solution of [LAlCl2] (2.560 g, 4.966 mmol) and 1,3-di-tert-butylimida-
zol-2-ylidene (1.790 g, 9.932 mmol) in benzene (60 mL) cooled to
108C. Immediately after the addition of water, a slurry of the 1,3-di-
tert-butylimidazolium chloride was formed. The suspension was
vigorously stirred for an additional 10 min and filtered. The precip-
itate was washed twice with benzene (5 mL) and all the volatiles were
removed in vacuo. The solid residue was treated twice with cold
pentane (5 mL) and after filtration and drying in vacuo, 4 was
obtained as a white powder. Yield 1.55 g (65%). 1HNMR (200 MHz,
respectively. The N(1)-Al-N(2) angle (95.78) within the six-
membered ring is in the normal range, whereas the large
N(3)-Al-N(4) angle of 112.28 (compare with 86.9–106.18 in
the dimeric and trimeric cyclic species)[2,3] might be due to the
monomeric nature of 3 and thus missing the ring strain
characteristics of the cyclic congeners. There are also
À
significant differences of the Al N bond lengths within the
À
À
molecule. The Al N(1) and Al N(2) bond lengths (1.921,
À
À
1.903 Š) are in the normal range, while the Al N(3) and Al
N(4) bond lengths (1.790, 1.788 Š) represent the shortest
bonds for NH2 groups with Al so far known (compare with
3
C6D6, 258C, TMS): d = 0.22 (s, 2H, OH), 1.16 (d, J(H,H) = 6.8 Hz,
1.873–2.034 Š).[2,3] A similar shortening of the Al (NH2)terminal
À
12H, CH(CH3)2), 1.42 (d, 3J(H,H) = 6.8 Hz, 12H, CH(CH3)2), 1.58 (s,
6H, CH3), 3.47 (sept, 3J(H,H) = 6.8 Hz, 4H, CH(CH3)2), 4.92 (s, 1H,
g-CH), 7.14–7.16 ppm (m, 6H, m-, p- Ar-H); 13C NMR (125.8 MHz,
C6D6, 258C, TMS): d = 23.1, 24.8 (CH(CH3)2), 25.2 (CH(CH3)2), 28.3
(CH3), 96.5 (g-CH), 124.5, 127.4, 139.9, 144.8 (i-, o-, m-, p- Ar),
bond length was observed in [AlCl3(NH2iPr)][{Al(NH3)-
(NH2)[Al(NHiPr)(NiPr)Cl]2}2] and was assigned to the ionic
resonance effects of the Al N bond.[3,12] The hydrogen atoms
À
170.3 ppm (C N); IR (KBr pellet): n˜ = 3458 wbr (OH) cmÀ1. EI-MS
of the NH2 groups were localized in the difference electron-
=
(70 eV): m/z (%): 478 (38) [M+], 460 (10) [M+ÀH2O], 445 (100)
[M+ÀH2OÀCH3]; elemental analysis (%) calcd for C29H43AlN2O2
(480.7): C 72.5, H9.4; found: C 72.4, H9.5.
À
density map and the N Hbond lengths (0.85, 0.86, 0.87, and
0.88 Š) are in the range of known compounds (0.75–
1.10 Š).[2,3] The nitrogen atoms of the NH2 groups have
almost planar environments (the sum of the surrounding
angles 3568—N(3) and 3578—N(4)), which indicates a low-
ering of the inversion barrier at the nitrogen center due to the
electropositive aluminum atom.[13] A similar phenomenon
Received: December 16, 2003 [Z53541]
Keywords: aluminum · amides · carbenes · hydroxides
.
*
was observed for [Cp2 TiNH2] and [{[DippN(SiMe3)]-
Ge(NH2)NH}3]
(Cp* = C5Me5,
Dipp = diisopropyl-
phenyl).[14,15]
[1] a) D. A. Neumayer, J. G. Ekerdt, Chem. Mater. 1996, 8, 9 – 25,
and references therein; b) A. C. Jones, P. O'Brien in CVD of
Compound Semiconductors, VCH, Weinheim, 1997; c) Chemis-
try of Aluminum, Gallium, Indium and Thallium (Ed.: A. J.
Downs), Blackie, Glasgow, 1993; d) D. C. Boyd, R. T. Haasch,
P. R. Mantell, R. K. Schulze, J. F. Evans, W. L. Gladfelter, Chem.
Mater. 1989, 1, 119 – 124; e) A. Ochi, H. K. Bowen, W. E. Rhine,
Mater. Res. Soc. Symp. Proc. 1988, 121, 663 – 666; f) F. N. Tebbe,
U.S. Pat. 4696968, 1987; g) L. V. Interrante, L. E. Carpenter, C.
Whitmarsh, W. Lee, G. A. Slack, Mater. Res. Soc. Symp. Proc.
1986, 73, 359 – 366; h) L. V. Interrante, W. Lee, M. McConnel, N.
In summary, we have presented a novel method for
preparing aluminum amides and hydroxides from the corre-
sponding chlorides. The main by-product—1,3-di-tert-butyli-
midazolium chloride—can be easily separated from the
product by filtration and recycled to the free base.
[LAl(NH2)2] is to the best of our knowledge the first
monomeric main-group metal diamide that contains two
terminal NH2 groups. Further research will be focused on the
preparation of the heavier Group 13 analogues.
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Chemie
Lewis, E. Hall, J. Electrochem. Soc. 1989, 136, 472 – 478; i) A.
Rabineau in Compound Semiconductors, Vol. 1 (Eds.: R. K.
Willardson, H. L. Goerring), Reinhold Publishing Corp., New
York, 1962, chap. 19, pp. 174–176.
[2] a) L. V. Interrante, G. A. Siegel, M. Garbauskas, C. Hejna, G. A.
Slack, Inorg. Chem. 1989, 28, 252 – 257; b) F. C. Sauls, C. L.
Czekaj, L. V. Interrante, Inorg. Chem. 1990, 29, 4688 – 4692;
c) J. F. Janik, E. N. Duesler, R. T. Paine, Inorg. Chem. 1988, 27,
4335 – 4338; d) J. F. Janik, E. N. Duesler, R. T. Paine, Inorg.
Chem. 1987, 26, 4341 – 4345.
[3] C.-C. Chang, M.-D. Li, M. Y. Chiang, S.-M. Peng, Y. Wang, G.-H.
Lee, Inorg. Chem. 1997, 36, 1955 – 1960.
[4] J. Müller, U. Ruschewitz, O. Indris, H. Hartwig, W. Stahl, J. Am.
Chem. Soc. 1999, 121, 4647 – 4652.
[5] V. Jancik, J. Prust, H. W. Roesky, unpublished results.
[6] L. W. Pineda, V. Jancik, H. W. Roesky, D. Neculai, A. M.
Neculai, Angew. Chem.; Angew. Chem. Int. Ed. 2004, in press.
[7] M. Stender, B. E. Eichler, N. J. Hardman, P. P. Power, J. Prust, M.
Noltemeyer, H. W. Roesky, Inorg. Chem. 2001, 40, 2794 – 2799.
[8] A. J. Arduengo III, H. Bock, H. Chen, M. Denk, D. A. Dixon,
J. C. Green, W. A. Herrmann, N. L. Jones, M. Wagner, R. West,
J. Am. Chem. Soc. 1994, 116, 6641 – 6649.
[9] G. Bai, Y. Peng, H. W. Roesky, J. Li, H.-G. Schmidt, M.
Noltemeyer, Angew. Chem. 2003, 115, 1168 – 1172; Angew.
Chem. Int. Ed. 2003, 42, 1132 – 1135.
[10] D. Bourissou, O. Guerret, F. P. Gabbaï, G. Bertrand, Chem. Rev.
2000, 100, 39 – 91.
[11] Crystal data for 3: C29H45AlN4, Mr= 476.67, monoclinic, space
group P2(1)/c, a = 16.995(2), b = 13.057(2), c = 13.565(2) Š, b =
109.26(2)8, V= 2841.7(7) Š3, Z = 4, 1calcd = 1.114 MgmÀ3
,
F(000) = 1040,
l = 1.54178 Š,
T= 100(2) K,
m(CuKa) =
0.781 mmÀ1. Data for the structure were collected on a Bruker
three-circle diffractometer equipped with a SMART 6000 CCD
detector. Intensity measurements were performed on a rapidly
cooled crystal (dimensions 0.30 ” 0.10 ” 0.10 mm3) in the range
8.7 ꢀ 2q ꢀ 116.28. Of the 17547 measured reflections, 3915 were
independent. The structure was solved by direct methods
(SHELXS-97)[16] and refined with all data by full-matrix least-
2
À
squares on F . The hydrogen atoms of C Hbonds were placed in
idealized positions, whereas the hydrogen atoms from the NH2
moieties were localized from the difference electron-density
map and refined isotropically. The final refinements converged
at R1 = 0.0390 for I > 2s(I), wR2 = 0.1033 for all data. The final
difference Fourier synthesis gave a min/max residual electron
density À0.281/ + 0.227 eŠÀ3. CCDC-226258 (3) contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB21EZ, UK; fax:
(+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
[12] a) D. B. Collum, Acc. Chem. Res. 1993, 26, 227 – 234, and
references therein; b) M. F. Lappert, P. P. Power, A. R. Sanger,
R. C. Srivastava, Metal and Metalloid Amides, Ellis-Horwood,
Chichester, 1980, and references therein.
[13] A. Greenberg, C. M. Breneman, J. F. Liebman, The Amide
Linkage: Structural Significance in Chemistry, Biochemistry, and
Materials Science, Wiley-Interscience, New York, 1999, and
references therein.
[14] E. Brady, J. R. Telford, G. Mitchell, W. Lukens, Acta Crystallogr.
Sect. C 1995, 51, 558 – 560.
[15] K. Wraage, H.-G. Schmidt, M. Noltemeyer, H. W. Roesky, Eur. J.
Inorg. Chem. 1999, 863 – 867.
[16] G. M. Sheldrick, SHELXS-97, Program for Crystal Structure
Refinement, University of Gꢀttingen, Gꢀttingen (Germany),
1997.
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