Synthesis and structures of aluminium monohydride and chalcogenides bearing a bidentate [N,O] ligand

Article abstract of DOI:10.1039/b411617h

The aluminium monohydride (3-tBu-5-Me-2-(O)C₆H ₂CH₂-N-2,6-iPr₂C₆H ₃)AlH(NMe₃) (2) was prepared by treatment of the bidentate salicylaldimine [3-tBu-5-Me-2-(OH)C₆H

Full text of DOI:10.1039/b411617h

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F U L L P A P E R
Synthesis and structures of aluminium monohydride and
chalcogenides bearing a bidentate [N,O] ligand
Ying Peng, Haijun Hao, Vojtech Jancik, Herbert W. Roesky,* Regine Herbst-Irmer and
Jörg Magull
Institut für Anorganische Chemie der Universität Göttingen, Tammannstrasse 4,
D-37077 Göttingen, Germany. E-mail: hroesky@gwdg.de
Received 29th July 2004, Accepted 10th September 2004
First published as an Advance Article on the web 28th September 2004
The aluminium monohydride (3-tBu-5-Me-2-(O)C6H2CH2-N-2,6-iPr2C6H3)AlH(NMe3) (2) was prepared by
treatment of the bidentate salicylaldimine [3-tBu-5-Me-2-(OH)C6H2CHN-2,6-iPr2C6H3] (1) with a small excess
of AlH3·NMe3 in high yield. Compound 2 reacted with sulfur and selenium respectively to afford the dimeric
aluminium chalcogenide [(3-tBu-5-Me-2-(O)C6H2CH2-NH-2,6-iPr2C6H3)Al(l-E)]2 [E = S (3), E = Se (4)]. During the
formation of 2 hydrogen migration from the aluminium centre to the ligand backbone occurred. A possible reaction
mechanism for 3 and 4 is discussed and the molecular structures of compounds 2–4 were determined by X-ray
structural analyses.
common organic solvents, such as toluene, benzene, hexane
Introduction
and pentane. The broad IR band at 1837 cm−1 can be assigned
Much attention is paid to the salicylaldiminato ligand family
due to its easy accessibility and diversity as chelates with or
without pendant arms. Moreover, salicylaldiminato ligands
have played an important role in a range of olefin polymeriza-
to the Al–H stretching frequency.7 The absorptions for the
CN double bond and O–H group are absent in the IR of
2. Compound 2 crystallizes with one molecule of toluene.
The structure of 2 shows the distorted tetrahedral aluminium
tion catalyst systems1 and development of transition and main
centre (Fig. 1). Selected bond lengths and angles are listed in
group metal coordination chemistry. This type of ligand was
Table 1. The terminal Al–H bond length (1.479(2) Å) is simi-
introduced to aluminium chemistry mainly to prepare the alkyl
lar to that in [ArN(CH2)3NAr]AlHNMe3 (Ar = 2,6-iPr2C6H3)
aluminium complexes2 and their cationic derivatives by reaction
(1.52 Å).4a The Al–Nligand bond distance (1.799(1) Å) is much
with AlR2X (R = alkyl group, X = alkyl group or Cl), which can
shorter than those in [3,5-tBu2-2(O)C6H2CHNR]AlMe2
be used as ethylene polymerization catalysts.2d To the best of our
(R = 2,6-Me2C6H3 or 2,6-iPr2C6H3) (1.972(3), 1.972(3) Å) bear-
knowledge, there have been no reports on aluminium hydride
species stabilized by Schiff base [N,O] chelate ligands.
Heavier Group 13 element chalcogenides have been widely
ing the unchanged bidentate salicylaldiminato ligands.2a The
Al–NNMe3 bond length (2.000(1) Å) is comparable to those in
[ArN(CH2)3NAr]AlRNMe3 (Ar = 2,6-iPr2C6H3, R = H, F)
studied due to their important applications in chemical
(2.024(2), 2.000(2) Å).4a The Al–O distance (1.741(1) Å) is a
vapor deposition (CVD) and catalysis,3 and organoaluminium
little shorter than those in [3,5-tBu2-2(O)C6H2CHNR]AlMe2
hydrides of low aggregation have proved to be effective
reagents for preparing compounds with elemental chalcogens
(R = 2,6-Me2C6H3 or 2,6-iPr2C6H3) (1.755(3), 1.773(3) Å).2a
The N(1)–C(24) bond length (1.478(2) Å) shows a typical C–N
or organochalcogenides.4 As an extension of this type of
single bond character compared to the corresponding retained
reaction, we explore a bidentate salicylaldimine [3-tBu-5-Me-
CN double bond in {3,5-tBu2-2(O)C6H2CHNR}AlMe2
2-(OH)C6H2CHN-2,6-iPr2C6H3] (1)5 and its reaction with
(R = 2,6-Me2C6H3 or 2,6-iPr2C6H3) (1.285(5), 1.300(5) Å).2a
AlH3·NMe3 to afford an aluminium monohydride (3-tBu-5-Me-
2-(O)C6H2CH2-N-2,6-iPr2C6H3)AlH(NMe3) (2) by elimination
of H2 together with hydrogen migration from the metal to the
ligand. In this paper we also describe its chalcogenide derivatives
[(3-tBu-5-Me-2-(O)C6H2CH2-NH-2,6-iPr2C6H3)Al(l-E)]2 [E = S
(3), Se (4)] from the reaction of 2 with sulfur and selenium, and
the possible reaction mechanism is discussed as well.
Results and discussion
Reaction of 1 with a small excess of AlH3·NMe3 in toluene
6
at 0 °C or alternatively under refluxing conditions in toluene
afforded the aluminium monohydride (3-tBu-5-Me-2-
(O)C6H2CH2-N-2,6-iPr2C6H3)AlH(NMe3) (2) in good yield
under elimination of H2 (Scheme 1). Furthermore, a hydrogen
migration from the aluminium centre to the parent ligand
backbone occurred at the CHN double bond to give the
CH2N single bond. A migration reaction of a methyl group was
not observed when AlMe3 was used instead, and the CHN
double bond was retained within the bidentate salicylaldiminato
ligand.2a–c We assume that this is due to the higher reactivity of
AlH3·NMe3 compared to that of AlMe3. Moreover we believe
that both the 2,6-iPr2C6H3 group on the aldimine nitrogen
and the bulky ortho tBu group on the phenoxide ring of 1
prevent the dimerization of 2. Compound 2 is well soluble in
Scheme 1 Synthesis of 2, 3 (E = S) and 4 (E = Se), Ar = 2,6-iPr2C6H3.
3 5 4 8
D a l t o n T r a n s . , 2 0 0 4 , 3 5 4 8 – 3 5 5 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Table 1 Selected bond distances (Å) and bond angles (°) for com-
pounds 2, 3 and 4
2·C7H8
Al(1)–O(1)
Al(1)–N(2)
Al(1)–H(1)
1.741(1)
2.000(1)
1.479(2)
Al(1)–N(1)
N(1)–C(24)
1.799(1)
1.478(2)
O(1)–Al(1)–N(1)
O(1)–Al(1)–N(2)
N(1)–C(24)–C(14)
H(1)–Al(1)–N(1)
102.32(6)
95.45(5)
115.63(1)
118.9(6)
N(1)–Al(1)–N(2)
H(1)–Al(1)–O(1)
H(1)–Al(1)–N(2)
117.07(6)
121.2(6)
100.3(6)
3·2C7H8
S(1)–Al(1)
O(1)–Al(1)
N(1)–C(24)
2.185(2)
1.750(1)
1.510(2)
S(1)–Al(1A)
Al(1)–N(1)
2.232(2)
2.005(1)
Al(1)–S(1)–Al(1A)
O(1)–Al(1)–S(1)
O(1)–Al(1)–S(1A)
S(1)–Al(1)–S(1A)
77.53(3)
119.92(4)
117.74(4)
102.47(3)
O(1)–Al(1)–N(1)
N(1)–Al(1)–S(1)
N(1)–Al(1)–S(1A)
C(14)–C(24)–N(1)
96.99(5)
118.01(5)
100.86(5)
111.15(1)
Fig. 1 The molecular structure of 2. the solvent molecule and the
hydrogen atoms are omitted for clarity except for the Al–H hydrogen
atom.
Treatment of 2 with elemental sulfur and selenium in toluene
yielded the dimeric aluminium chalcogenides [(3-tBu-5-Me-
2-(O)C6H2CH2-NH-2,6-iPr2C6H3)Al(l-E)]2 [E = S (3), Se (4)]
bearing the hydrogenated bidentate salicylaldiminato ligand
(Scheme 1). Compounds 3 and 4 are air and moisture sensi-
tive and soluble in toluene, while sparingly soluble in benzene.
They were characterized by IR and NMR spectroscopy and
EI mass spectrometry, as well as elemental analyses. No bands
for the CN stretches were found in the IR spectrum. We
assume that the formation of 3 and 4 might proceed through
a reactive intermediate [(3-tBu-5-Me-2-(O)C6H2CH2-N-2,6-
iPr2C6H3)Al(EH)(NMe3)] (E = S, Se) (Scheme 1). The conver-
sion of Al–H to Al–EH was observed in compounds stabilized
by the bulky b-diketiminato ligand.8 The intermediate then
dimerizedto[(3-tBu-5-Me-2-(O)C6H2CH2-N-2,6-iPr2C6H3)Al(l-
EH)]2 with elimination of NMe3. Under heating the unstable
[(3-tBu-5-Me-2-(O)C6H2CH2-N-2,6-iPr2C6H3)Al(l-EH)]2 was
converted to the stable products 3 and 4 with hydrogen migra-
tion from l-EH to nitrogen. The absorption bands at 3175 and
3220 cm−1 assigned to the NH stretching frequency in the IR
spectrum of 3 and 4 respectively support the proposed hydrogen
migration. The proton of the NH group was also found in the
4·2C7H8
Se(1)–Al(1)
Al(1)–O(1)
N(1)–C(12)
2.314(1)
1.749(2)
1.511(3)
Se(1)–Al(1A)
Al(1)–N(1)
2.366(1)
2.009(2)
O(1)–Al(1)–Se(1)
N(1)–Al(1)–Se(1)
N(1)–Al(1)–Se(1A)
Se(1)–Al(1)–Se(1A)
118.45(6)
118.48(7)
102.25(7)
104.50(3)
O(1)–Al(1)–N(1)
O(1)–Al(1)–Se(1A)
Al(1)–Se(1)–Al(1A)
C(2)–C(12)–N(1)
96.43(8)
116.08(6)
75.50(3)
110.7(2)
1
molecular structures by X-ray analyses and H NMR spectro-
scopy. A comparable migration phenomena was observed in
previous examples.4a
Compounds 3 and 4 crystallize with two molecules of
toluene respectively. The molecular structures of 3 and 4 show
that both of them have a dimeric structure featuring a fused
planar four-membered ring with a central Al2E2 core (Figs. 2
and 3). Selected bond lengths and angles are shown in Table 1.
The Al–N bond length (2.005(1) Å for 3; 2.009(2) Å for 4) is
much longer than that in 2 (1.799(1) Å), however similar to those
in {3,5-tBu2-2(O)C6H2CHNR}AlMe2 (R = 2,6-Me2C6H3 or
2,6-iPr2C6H3) (1.972(3), 1.972(3) Å) bearing the unchanged
bidentate salicylaldiminato ligand.2a The Al–O distance
(1.750(1) Å for 3; 1.749(2) Å for 4) is comparable to that in 2
(1.741(1) Å). The N–C bond length (1.510(2) Å for 3; 1.511(3) Å
for 4) shows single bond character like that of 2. The Al–E dis-
tance (2.185(2), 2.232(2) Å for 3; 2.314(1), 2.366(1) Å for 4) is
analogous to those of similar Al2E2 species.4 The E(1)–Al–E(1A)
angle (102.47(3)° E = S 3; 104.50 (3)° E = Se 4) is in the range
of those reported.4b
Fig. 2 The molecular structure of 3. Solvent molecules and the
hydrogen atoms are omitted for clarity except for the N–H hydrogen
atoms.
3 and 4. A possible mechanism for the latter reaction is
discussed.
Experimental
General
Concluding remarks
This work provides an aluminium monohydride employing a
bidentate salicylaldiminato ligand. During the formation of
the aluminium monohydride, a hydrogen migration occurred
together with the elimination of H2. Reactions of aluminium
hydride with chalcogens (S, Se) result in the dimeric products
All manipulations were carried out using Schlenk line tech-
niques or in a glove box under a purified nitrogen atmosphere.
Solvents were dried according to standard methods and freshly
distilled prior to use. Elemental sulfur, selenium were purchased
D a l t o n T r a n s . , 2 0 0 4 , 3 5 4 8 – 3 5 5 1
3 5 4 9

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Table 2 Crystallographic data for compounds 2, 3 and 4
Compound
2·C7H8
3·2C7H8
4·2C7H8
Empirical formula
M
C34H51AlN2O
530.75
C62H84Al2N2O2S2
1007.39
C62H84Al2N2O2Se2
1101.19
k/Å
T/K
1.54178
100(2)
1.54178
100(2)
0.71073
133(2)
Crystal system
Space group
Monoclinic
P21/c
Triclinic
PL
Triclinic
PL
a/Å
b/Å
c/Å
a/°
9.100(1)
9.632(1)
36.703(1)
90
96.65(1)
90
3195(1)
4
1.103
0.746
12.149(1)
12.345(1)
12.554(1)
108.94(1)
118.62(1)
97.75(1)
1465(1)
1
1.142
1.431
10.048(4)
12.4855(5)
13.1473(6)
77.061(3)
68.039(3)
89.284(3)
1486.23(11)
1
b/°
c/°
V/Å3
Z
qc/Mg m−3
1.230
1.315
l/mm−1
F(000)
1160
13836
4447 (Rint = 0.0327)
4447 / 0 / 61
1.045
544
11561
4107 (Rint = 0.0299)
4107 / 249 / 394
1.047
580
50725
5105 (Rint = 0.0437)
5105 / 0 / 292
1.020
No. of reflns collected
No. of indep reflns
Data/restaints/paramers
GOF on F2
R1, wR2 (I > 2r(I))
R1,wR2 (all data)
Largest diff peak,hole/e Å−3
0.0338, 0.0845
0.0413, 0.0886
0.169,−0.243
0.0324, 0.0859
0.0369, 0.0891
0.200,−0.232
0.0345, 0.0878
0.0367,0.0891
0.920,−0.633
crude product was recrystallized from toluene, and the filtrate
was kept at −20 °C to obtain colorless crystals (3.10 g, 79%).
Mp 109–112 °C; IR (Nujol): ν = 1837 cm−1 (Al–H stretch); 1H

NMR (300.13 MHz, C6D6,): d 1.02 (d, 3H, CHMe2), 1.19 (d, 3H,
CHMe2), 1.43 (d, 6H, CHMe2), 1.72 (s, 9H, CMe3), 1.79 (s, 9H,
NMe3), 2.26 (s, 3H, Me), 3.72 (sept, 2H, CHMe2), 3.86 (d, 1H,
CH2N), 4.69 (d, 1H, CH2N), 6.65 (d, 1H, OAr–H), 6.99–7.18 (m,
4H, NAr–H, OAr–H); EI -MS (70 eV): m/z (%) only CH frag-
ments; elemental analysis (%) calcd. for C27H43AlN2O (438.62):
C, 73.93; H, 9.88; N, 6.39; Found: C, 74.29; H 9.51; N, 5.53.
[(3-tBu-5-Me-2-(O)C6H2CH2-NH-2,6-iPr2C6H3)Al(l-S)]2
(3). Toluene (20 mL) was added to a mixture of 2 (0.558 g,
1.28 mmol) and S (0.042 g, 1.28 mmol), and the resulting solu-
tion was warmed to 60 °C and stirred for 15 h. After cooling
to room temperature the yellow-green solution was filtered and
the filtrate was concentrated to ca. 8 mL. Yellow crystals were
obtained after storing the resulting solution at −20 °C for 5 days
−1

(0.72 g, 62%). Mp 121 °C decomp.; IR (Nujol): ν = 3175 cm
Fig. 3 The molecular structure of 4. Solvent molecules and the
hydrogen atoms including the N–H hydrogen atoms are omitted for
clarity.
(N–H stretch); 1H NMR (300.13 MHz, C6D6,): d 0.88, 0.92, 1.09,
1.14 (d, 24H, CHMe2), 1.52 (s, 2H, NH), 1.45, 1.71 (s, 18H,
CMe3), 2.10, 2.18 (s, 6H, Me), 3.15, 3.25, 3.41, 3.57 (sept, 4H,
CHMe2), 4.58, 4.64 (d, 4H, CH2N), 5.80 (d, 1H, OAr–H), 6.01
(d, 1H, OAr–H), 6.40 (d, 1H, OAr–H), 6.51 (d, 1H, OAr–H),
7.01–7.15 (m, 6H, NAr–H); EI-MS (70 eV): m/z (%) 822 (12)
[M+ − H], 162 (100) [iPr2C6H4]; elemental analysis (%) calcd. for
C48H68Al2N2O2S2 (823.16): C 70.04, H 8.33, N 3.40. Found C
70.75, H 8.46, N 3.50.
6
from Aldrich and used as received. AlH3·NMe3 was prepared
as described in the literature.
NMR spectra were recorded on Bruker AM 200, AM 300
spectrometers. Chemical shifts are reported in ppm with
1
reference to SiMe4 (external) for H nuclei. Mass spectra were
obtained on Finnigan MAT system 8230 or Varian MAT CH5
mass spectrometers by EI-MS methods. Melting points of all
new compounds were measured on a Büchi melting point B-540
apparatus in sealed capillaries and are uncorrected. IR spectra
were recorded on a Bio-Rad Digilab FTS-7 spectrometer (only
the characteristic absorptions are reported) and the samples
were prepared as Nujol mulls between KBr plates. Elemental
analyses were performed by the Analytisches Labor des Instituts
für Anorganische Chemie der Universität Göttingen.
[(3-tBu-5-Me-2-(O)C6H2CH2-NH-2,6-iPr2C6H3)Al(l-Se)]2
(4). Toluene (30 mL) was added to a mixture of 2 (0.768 g,
1.75 mmol) and Se (0.28 g, 3.54 mmol). The solution was
refluxed for 2 h. The unreacted Se was removed by filtration.
The filtrate was concentrated to ca. 8 mL and kept at 0 °C to
obtain yellow crystals (1.06 g, 60%). Mp 118 °C decomp.; IR
−1
1

(Nujol): ν = 3220 cm (N–H stretch); H NMR (300.13 MHz,
C6D6,): d 0.87, 0.94, 1.14, 1.44 (d, 24H, CHMe2), 1.50, 1.56 (s,
18H, CMe3), 1.88 (s, 2H, NH), 2.10, 2.15 (s, 6H, Me), 2.64, 3.25,
3.43, 3.57 (sept, 4H, CHMe2), 4.35, 4.39 (d, 4H, CH2N), 6.05 (d,
1H, OAr–H), 6.21 (d, 1H, OAr–H), 6.33 (d, 1H, OAr–H), 6.47
(d, 1H, OAr–H), 7.01–7.15 (m, 6H, NAr–H); EI-MS (70 eV):
m/z (%) 916 (4) [M+ − H], 162 (100) [iPr2C6H4]; elemental
analysis (%) calcd. for C62H84Al2N2O2Se2 (1101.23): C, 67.62; H,
7.69; N, 2.54. Found: C, 67.25; H, 7.81; N, 2.72.
Preparations
[3-tBu-5-Me-2-(O)C6H2CH2-N-2,6-iPr2C6H3]AlH(NMe3)
(2). A solution of AlH3·NMe3 (17.2 mL of a 0.8 M solution
in toluene, 13.76 mmol) was added at 0 °C to a solution of 1
(3.14 g, 8.95 mmol) in toluene (40 mL). The reaction mixture
was slowly warmed to room temperature and stirred for an
additional 15 h. The volatiles were removed in vacuo. The
3 5 5 0
D a l t o n T r a n s . , 2 0 0 4 , 3 5 4 8 – 3 5 5 1

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V. C. Gibson, A. J. P. White and D. J. Williams, J. Am Chem. Soc.,
2003, 125, 8450–8451.
X-Ray structural determination and refinement for 2, 3, 4
Data for compounds 2 and 3 were collected on a Bruker three-
circle diffractometer equipped with a SMART 6000 CCD
detector and for 4 on a Stoe IPDS II two-circle diffractometer.
All structures were solved by direct methods (SHELXS-97)9
and refined with all data by full-matrix least-squares methods
on F2.10 The non-hydrogen atoms were refined anisotropically,
except those of the highly disordered toluene molecules in 4
which were refined isotropically. The hydrogen atoms were
included at geometrically idealized positions and refined with
the riding model, except for the Al–H and N–H hydrogen atoms
which were located by difference Fourier synthesis and refined
isotropically. The toluene molecules in 2 were disordered over
two positions and refined with distance restraints and restraints
for the anisotropic displacement parameters.
2 (a) P. A. Cameron, V. C. Gibson, C. Redshaw, J. A. Segal, G. A.
Solan, A. J. P. White and D. J. Williams, J. Chem. Soc., Dalton Trans.,
2001, 1472–1476; (b) D. Pappalardo, C. Tedesco and C. Pellecchia,
Eur. J. Inorg. Chem., 2002, 621–628; (c) M. S. Hill, A. R. Hutchison,
T. S. Keizer, S. Parkin, M. A. VanAelstyn and D. A. Atwood,
J. Organomet. Chem., 2001, 628, 71–75; (d) P. A. Cameron, V. C.
Gibson, C. Redshaw, J. A. Segal, M. D. Bruce, A. J. P. White
and D. J. Williams, Chem. Commun., 1999, 1883–1884; (e) P. A.
Cameron, V. C. Gibson, C. Redshaw, J. A. Segal, A. J. P. White
and D. J. Williams, J. Chem. Soc., Dalton Trans., 2002, 415–422.
3 (a) Reviews: M. B. Power, A. R. Barron, D. Hnyk, H. E. Robertson
and D. W. H. Rankin, Adv. Mater. Optics Electron., 1995, 5,
177–185; (b) J. L. Atwood, in Coordination Chemistry of Aluminum,
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D. Vidovic and J. Magull, Eur. J. Inorg. Chem., 2003, 3113–3119
and references therein; (b) V. Jancik, M. M. Moya Cabrera, H. W.
Roesky, R. Herbst-Irmer, D. Neculai, A. M. Neculai, M. Noltemeyer
and H.-G. Schmidt, Eur. J. Inorg. Chem., 2004, 3508–3512.
5 (a) M. Pickel, T. Casper, A. Rahm, C. Dambouwy and P. Chen,
Helv. Chim. Acta, 2002, 12, 4337–4352; (b) D. Zhang, G.-X. Jin,
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6 R. A. Kovar and J. O. Callaway, Inorg. Synth., 1977, 17, 36–42.
7 S. Cucinella, A. Mazzei and W. Marconi, Inorg. Chim. Acta Rev.,
1970, 4, 51–71.
CCDC reference numbers 245956 (2), 245957 (3) and
245958 (4).
See http://www.rsc.org/suppdata/dt/b4/b411617h/ for crystal-
lographic data in CIF or other electronic format.
See Table 2 for crystallograpic details.
Acknowledgements
This work was supported by the Deutsche Forschungsgemein-
schaft, the Göttinger Akademie der Wissenschaften and the
Fonds der Chemischen Industrie.
8 (a) C. Cui, H. W. Roesky, H. Hao, H.-G. Schmidt and
M. Noltemeyer, Angew. Chem., 2000, 112, 1885–1887; C. Cui,
H. W. Roesky, H. Hao, H.-G. Schmidt and M. Noltemeyer,
Angew. Chem. Int. Ed., 2000, 39, 1815–1817; (b) V. Jancik, Y. Peng,
H. W. Roesky, J. Li, D. Neculai, A. M. Neculai and R. Herbst-
Irmer, J. Am. Chem. Soc., 2003, 125, 1452–1453.
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