Preparation of Aluminum Hydrides with Chelating Anilido-Imine Ligands by Addition of an Al-H Bond to a C=N Bond

Article abstract of DOI:10.1002/zaac.201400592

Reacting o-C₆H₄F(CH = NC₆H₃iPr₂-2,6) (L) with H₃Al·NMe₃ resulted in the aluminum dihydride with coordinated NMe₃ of composition [o-C₆H₄F(CH₂NC₆H₃iPr₂-2,6)]AlH₂(NMe₃)(1). Furthermore, the aluminum monohydride [o-C₆H₄N(C₆H₃iPr₂-2,6)(CH₂NC₆H₃iPr₂-2,6)]AlH(NMe₃) (2) was prepared by treatment of the anilido-imine ligand o-C₆H₄N(C₆H₃iPr₂-2,6)(CH = NC₆H₃iPr₂-2,6) (L) with H₃Al·NMe₃ in high yield. The formations of the two complexes both involve aluminum hydride addition to the C=N double bond. Compounds 1 and 2 were characterized by ¹H NMR and IR spectroscopic, elementary analysis, and single-crystal X-ray diffraction studies.

Full text of DOI:10.1002/zaac.201400592

Journal of Inorganic and General Chemistry
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ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
DOI: 10.1002/zaac.201400592
Preparation of Aluminum Hydrides with Chelating Anilido-Imine Ligands
by Addition of an Al–H Bond to a C=N Bond
Weiwei Wang,[a] Zhi Yang,*[a] Xiaoli Ma,[a] Herbert W. Roesky,*[b] Yingxin Ju,[a] and
Pengfei Hao[a]
Keywords: Aluminum hydrides; Addition; Reduction; Anilido-imine ligands
Abstract. Reacting o-C6H4F(CH
=
NC6H3iPr2-2,6) (L) with
(L*) with H3Al·NMe3 in high yield. The formations of the two com-
H3Al·NMe3 resulted in the aluminum dihydride with coordinated plexes both involve aluminum hydride addition to the C=N double
NMe3 of composition [o-C6H4F(CH2NC6H3iPr2-2,6)]AlH2(NMe3)(1). bond. Compounds 1 and 2 were characterized by 1H NMR and IR
Furthermore, the aluminum monohydride [o-C6H4N(C6H3iPr2-2,6)
(CH2NC6H3iPr2-2,6)]AlH(NMe3) (2) was prepared by treatment of the
anilido-imine ligand o-C6H4N(C6H3iPr2-2,6)(CH = NC6H3iPr2-2,6)
spectroscopic, elementary analysis, and single-crystal X-ray diffraction
studies.
aldiminate ligand framework to stabilize the aluminum
center. To date, some alkylaluminum,[10] nickel,[11] copper,[12]
zinc,[13] yttrium,[14] boron,[15] iridium,[16] and platinum[17]
complexes bearing such ligands have been prepared and
their catalytic properties have also been investigated. To the
best of our knowledge, there have been no reports on alumi-
num hydride species with anilido-imine ligands. Given
the above situation, herein we report on the synthesis of
Introduction
Much attention was paid to the aluminum hydride family
because of their scientific importance and numerous applica-
tions. They are well-known reducing agents for unsaturated
molecules such as ketones, nitriles, alkenes, and alkines.[1] In
addition, aluminum hydrides are very effective precursors for
preparing aluminum chalcogenides[2] and alumoxanes by con-
trolled hydrolysis.[3] They can be used in the production of
amidoalane clusters with organic nitriles,[4] in the preparation
of carbaalane clusters and chlorinated derivatives using alk-
ines,[5] in the generation of alkoxide and aryloxide com-
plexes,[6] and in preparing aluminum containing heterocycles[7]
by elimination of H2.
[o-C6H4F(CH2NC6H3iPr2-2,6)]AlH2(NMe3)
(1)
and
[o-C6H4N(C6H3iPr2-2,6)(CH2NC6H3iPr2-2,6)]AlH(NMe3) (2)
by addition of aluminum hydride to L and L*, respectively.
Results and Discussion
In the past decade, bulky aryl-substituted β-diketimine has
attracted much attention due to its strong electron donation and
steric constraints that can stabilize main group metal com-
plexes,[8] although these β-diketiminate ligands are not easily
modified. The bidentate salicylaldiminate chelate ligand
possesses easily modified features and has been applied in ole-
fin polymerization chemistry.[9] Our current research interest
is to select a kind of novel bulky anilido–imine ligand that
combines the steric effect of the β-diketiminate ligand
framework and the more easily modified feature of the salicyl-
L and L* were synthesized by a literature procedure[11]
shown in Scheme 1. The imine L was obtained from the reac-
tion of 2,6-diisopropylaniline and o-fluorobenzaldehyde. The
anilido-imine L* can be prepared via a nucleophilic aromatic
displacement reaction of fluorine using LiNHAr (Ar = 2,6-
iPr2C6H3). Compounds 1 and 2 were synthesized in reasonable
yields by reaction of a small excess of H3Al·NMe3 with L and
L*, respectively, in toluene at 0 °C. The reactions of aluminum
hydride with L and L* are quite different, when compared with
that of the β-diketiminate ligand. Their C=N double bonds are
reduced to C-N single bonds. However, in the case of the β-
diketiminate ligand the C=N double bonds are not reduced.[18]
The reason for this different behavior might be due to the
conjugation of the C=N with the C=C double bond in the β-
diketiminate ligand. The additions of aluminum hydride to
C=N double bonds are accompanied with the hydrides turning
to protons. Meanwhile, the core structure of compound 2 turns
to an aluminum containing six-membered ring under elimi-
nation of H2. This is attributed to the sufficient reactivity of
the second hydride at the same Al atom to interact with the
* Prof. Dr. Z. Yang
Fax: +86-10-68914503
E-mail: zhiyang@bit.edu.cn
* Prof. Dr. H. W. Roesky
Fax: +49-551-39-33373
E-mail: hroesky@gwdg.de
[a] School of Chemical Engineering and Environment
Beijing Institute of Technology
100081 Beijing, P. R. China
[b] Institut für Anorganische Chemie
Georg-August-Universität Göttingen
Tammannstrasse 4
37077 Göttingen, Germany
Z. Anorg. Allg. Chem. 2015, 641, (3-4), 684–687
684
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Zeitschrift für anorganische und allgemeine Chemie
other proton of the ligand with elimination of H2. A similar (1.8214(15) Å for 1, 1.8103(17) Å and 1.8591(17) Å for 2)
migration reaction of a methyl group was not observed when are much shorter than the coordinated Al–N (imine)
AlMe3 was used instead,[10a] and the CH=N double bond was distance (1.963(1) Å) in [o-C6H4N(C6H3iPr2-2,6)(CH
=
[10a]
retained. We assume that this is due to the higher reactivity of NC6H3iPr2-2,6)]AlMe2,
which shows the covalent nature
the hydride compared to that of CH3. In the two reactions only of the Al–N bond in 1 and 2, respectively. The Al–NNMe3 bond
the mononuclear product was obtained when the reactions length (2.0188(17) Å in 1, 2.0274 Å in 2) is comparable to
were conducted in the ratio of 1:1 or 1:2. We think that the those in [ArN(CH2)3NAr]AlRNMe3 (Ar = 2,6-iPr2C6H3, R =
2,6-iPr2C6H3 groups on the nitrogen atoms prevent the dimer- H, F) (2.024(2), 2.000(2) Å].[20] The N(1)–C(13) bond length
ization of compounds 1 and 2, respectively. The two com- (1.47(2) Å] in 1 and N(1)–C(7) (1.481(2) Å] in 2 show a typi-
pounds are well soluble in common solvents such as toluene, cal C–N single bond character than the corresponding retained
dichloromethane, and tetrahydrofuran, and they can be stored C=N double bond in [o-C6H4N(C6H3iPr2-2,6)(CH
in the glove box for several months without any decomposi- NC6H3iPr2-2,6)]AlMe2 (1.298(2) Å].[10a]
=
tion.
Figure 1. Molecular structure of 1. Thermal ellipsoids are drawn at
the 50% level and the hydrogen atoms are omitted for clarity. Selected
bond lengths /Å and angles /deg: Al(1)–N(1) 1.8214(15), Al(1)–N(2)
2.0188(17), N(1)–C(13) 1.47(2) Å, N(1)–Al(1)–N(2) 111.78(7), C–
N(1)–C(13) 113.62(13), C–N(1)–Al(1) 125.54(11), C(13)–N(1)–Al(1)
119.85(11), N(1)–C(13)–C 114.48(14).
Scheme 1. Preparation of compounds 1 and 2.
1
In the H NMR spectra of compounds 1 and 2, the reso-
nances (δ = 2.60 for 1) and (δ = 2.33 for 2) show the existence
of the NMe3 group, respectively. The resonances of protons on
CH2-N can be found at δ = 4.20 for 1 and δ = 4.15 for 2,
which indicate the conversion of hydrides to protons. The reso-
1
nance for the hydride could not be observed in the H NMR
because of the quadrupolar broadening by the Al nucleus. The
presence of Al–H bonds in 1 and 2 is evident from their IR
spectra. The broad IR bands around 1835 cm–1 in 1 and
1854 cm–1 in 2 are assigned to the Al–H stretching frequency,
which compares well with the known value of 1860 cm–1.[19]
The absorptions for the C=N double bond and N–H group are
absent in the IR spectrum of 2.
Compounds 1 and 2 crystallize in the monoclinic system
C2/c and in the monoclinic P21/n space group, respectively.
The molecular structures of 1 and 2 are depicted in Figure 1 Figure 2. Molecular structure of 2. Thermal ellipsoids are drawn at
the 50% level and the hydrogen atoms are omitted for clarity. Selected
and Figure 2. Selected bond lengths and angles are listed in
bond lengths /Å and angles /deg: Al(1)–N(1) 1.8103(17), Al(1)–N(2)
1.8591(17), Al(1)–N(3) 2.0274(18), N(1)–Al(1)–N(2) 102.62(8),
N(1)–Al(1)–N(3) 108.23(8), N(2)–Al(1)–N(3) 109.07(7), C–N(1)–
the captions of the figures The tetrahedral aluminum center in
Figure 1 of compound 1 indicates the existence of two H atoms
and the coordinate NMe3 group. For the structure of 2, the
aluminum atom also adopts a distorted tetrahedral geometry
with one H atom, one nitrogen atom of the NMe3 group and
two nitrogen atoms of the anilido-imine ligand. The Al–H
bond lengths (1.48(2) Å and 1.52(2) Å for 1, 1.53(2) Å for 2)
are similar to that in [ArN(CH2)3NAr]AlHNMe3 (Ar = 2,6-
C(7) 115.39(15), C–N(1)–Al(1) 128.44(13), C(7)–N(1)–Al(1)
111.94(12), N(1)–C(7)-C 113.70(15).
Conclusions
We describe the synthesis, characterization, and X-ray struc-
iPr2C6H3) (1.52 Å).[20] The Al–N (amido) distances ture of two aluminum hydrides supported by the anilido-imine
Z. Anorg. Allg. Chem. 2015, 684–687
685
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least-squares on F2 using the SHELXL-97 program.[23] All non-
ligand. They are prepared by addition reactions of the Al–H
hydrogen atoms were refined anisotropically. Hydrogen atoms con-
nected to carbon atoms were included at geometrically calculated posi-
tions and refined by using a riding model (Table 1).
bond to the unsaturated C=N bond. The resulting compounds
are interesting precursors for the preparation of new aluminum
compounds.
Table 1. X-ray crystallographic data for 1 and 2.
1
2
Experimental Section
Empirical formula
Formula weight
Temperature /K
C22H34AlFN2
372.49
153(2)
C34H50AlN3
527.75
163(2)
General Procedures
All manipulations were carried out under a purified nitrogen atmo-
sphere using Schlenk techniques or inside an MBraun MB200 glove
box. All solvents were distilled from Na/benzophenone ketyl prior to
use. Commercially available chemicals were purchased from J&K
chemical or VAS and used as received. 1H NMR spectra were recorded
with a Varian Mercury Plus 400 spectrometer. The melting points were
measured in sealed glass tubes and were not corrected. Elemental
analyses were carried out by the Analytical Instrumentation Center of
the Peking University. The IR spectra were recorded using KBr pellets
with a Bio-Rad Digilab FTS-7 spectrometer from 4000 to 400 cm–1.
Wavelength /Å
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
β /°
0.71073
Monoclinic
C2/c
22.737(4)
11.294(2)
18.035(4)
90
102.949
90
4513.7(15)
8
1.096
0.71073
Monoclinic
P21/c
14.637(3)
11.112(2)
20.129(5)
90
105.962
90
3147.7(12)
4
1.114
γ /°
V /Å3
Z
Dcalcd. /Mg·m–3
Absorption coefficient /mm–1
F(000)
0.105
1616
0.090
1152
[o-C6H4F(CH2NC6H3iPr2-2,6)]AlH2(NMe3) (1)
θ range for data collection /°
Index ranges
2.92 to 29.13
–27 Յ h Յ 31
–14 Յ k Յ 15
–24 Յ l Յ 24
23531
0.0456
6047 / 0 / 250
1.003
R1 = 0.0664
wR2 = 0.1450
R1 = 0.0820
wR2 = 0.1563
0.233 and
–0.280
2.39 to 28.50
–17 Յ h Յ 19
–10 Յ k Յ 14
–27 Յ l Յ 27
26652
0.0437
7872 / 0 / 359
1.0002
R1 = 0.0676
wR2 = 0.1810
R1 = 0.0781
wR2 = 0.1897
0.238 and
–0.268
To a toluene solution (20 mL) of o-C6H4F(CH = NC6H3iPr2-2,6)
(0.283 g, 1.0 mmol) at 0 °C was added drop by drop H3Al·NMe3 in
toluene (1 m, 1.2 mL, 1.2 mmol). After the addition was complete, the
reaction mixture was allowed to warm to room temperature and stirring
was continued for 12 h. The solution was evaporated to dryness in
vacuo, and the residue was washed with cold n-hexane (10 mL) to
yield crystalline 1 (0.326 g,87%); m.p. 89–92 °C. IR (KBr (cm–1)): ν˜
Reflections collected
R (int)
Data / restraints / parameters
Goodness-of-fit on F2
Final Rint [I Ͼ 2σ(I)]
1
= 1835 cm–1 (m, Al-H). H NMR (400MHz, CDCl3, 25 °C, TMS): δ
= 7.15–6.80(m, 7 H, Ph-H), 4.20(s, 2 H, CH2N), 3.38(m, 2 H,
CHMe2), 2.60(s, 9 H, NMe3), 1.19(d, 6 H, CHMe2), 0.85(d, 6 H,
CHMe2); Elemental Analysis calcd. (%) for C22H34AlFN2: C 70.94,
H 9.20, N 7.52; Found: C 70.89, H 9.24, N 7.48.
Rint (all data)
Largest diff. peak /e·Å–3
CCDC-1030540 (1) and –1030541 (2) contain the supplementary
crystallographic 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.
[o-C6H4N(C6H3iPr2-2,6)(CH2NC6H3iPr2-2,6)]AlH(NMe3) (2)
To a toluene solution (20 mL) of o-C6H4N(C6H3iPr2-2,6)(CH =
NC6H3iPr2-2,6) (0.440 g, 1.0 mmol) at 0 °C was added drop by drop
H3Al·NMe3 in toluene (1 m, 1.3 mL, 1.3 mmol). After the addition was
complete, the reaction mixture was allowed to warm to room tempera-
ture and stirring was continued for 24 h. The solution was filtered
to remove any solid impurities and stored at room temperature for
crystallization. Product 2 was isolated as colorless crystals. An ad-
ditional crop of 2 was obtained from the mother liquor. Total yield:
0.437 g (82.9%); m.p. 140–142 °C. IR (KBr (cm–1)): ν˜ = 1854 cm–1
(m, Al-H). 1H NMR (400MHz, CDCl3, 25 °C, TMS): δ = 7.32–
6.88(m, 4 H, Ph-H), 6.47–6.18 (d, 6 H, Ph-H), 4.15(s, 2 H, CH2N),
3.76–3.20(m, 4 H, CHMe2), 2.33(s, 9 H, NMe3), 1.42–0.45(m, 24 H,
CHMe2.); Elemental Analysis calcd. (%) for C34H50AlN3: C 77.38, H
9.55, N 7.96; Found: C 77.28, H 9.52, N 8.01.
Acknowledgments
This work was supported by the Beijing Natural Science Foundation
(2132044), Beijing High School Youth Talent Project (YETP1191),
and the National Natural Science Foundation of China (21001016).
Support from the Deutsche Forschungsgemeinschaft is gratefully
acknowledged.
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