Synthesis of [LAl(μ-S)2AlL] (L?=?HC(CMeNAr)2, Ar?=?2,6-Et2C6H3) with the insertion of sulfur into the Al–H bonds of LAlH2 and its application in catalysis

Article abstract of DOI:10.1016/j.ica.2017.05.034

Two aluminum complexes [LAl(μ-S)₂AlL] (1) and [LAl(SeH)₂] (2) were synthesized in good yield by reacting one equivalent of LAlH₂ (L?=?HC(CMeNAr)₂, Ar?=?2,6-Et₂C₆H₃) with one equivalent of S and with two equivalents of Se without any catalysts, respectively. Complexes 1 and 2 have been characterized by ¹H and ¹³C NMR, elemental analyses, and single crystal X-ray structural analysis. Furthermore, the good catalytic activity of 1 as bimetallic Lewis acid catalyst for the addition reaction of TMSCN to aldehydes and aldimine condensation reactions, respectively, were investigated.

Full text of DOI:10.1016/j.ica.2017.05.034

Accepted Manuscript
Research paper
Synthesis of [LAl(μ-S) AlL] (L = HC(CMeNAr) , Ar = 2,6−Et C H ) with the
2
2
2
6
3
insertion of sulfur into the Al−H bonds of LAlH and its application in catalysis
2
Mingdong Zhong, Ying Liu, Xin Liu, XiaoliMa, Zhi Yang
PII:
S0020-1693(16)30793-9
http://dx.doi.org/10.1016/j.ica.2017.05.034
ICA 17603
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
2 November 2016
13 May 2017
15 May 2017
Please cite this article as: M. Zhong, Y. Liu, X. Liu, XiaoliMa, Z. Yang, Synthesis of [LAl(μ-S) AlL] (L =
2
HC(CMeNAr) , Ar = 2,6−Et C H ) with the insertion of sulfur into the Al−H bonds of LAlH and its application
2
2
6
3
2
in catalysis, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.05.034
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Synthesis of [LAl(µ-S) AlL] ( L = HC(CMeNAr) , Ar
2
2
=
2,6−Et C H ) with the insertion of sulfur into the
2
6
3
Al−H bonds of LAlH and its application in catalysis
2
Mingdong Zhong, Ying Liu, Xin Liu, XiaoliMa,* Zhi Yang*
School of Chemical Engineering and Environment, Beijing Institute of
Technology, 100081 Beijing, China
Fax: (+86) 10-68911032; E-mail: maxiaoli@bit.edu.cn. zhiyang@bit.edu.cn
ABSTRACT: Two aluminum complexes [LAl(µ-S) AlL] (1) and
2
[
LAl(SeH) ] (2) were synthesized in good yield by reacting one equivalent
2
of LAlH (L = HC(CMeNAr) , Ar = 2,6-Et C H ) with one equivalent of S
2
2
2
6
3
and with two equivalents of Se without any catalysts, respectively.
1
13
Complexes 1 and 2 have been characterized by H and C NMR, elemental
analyses, and single crystal X-ray structural analysis. Furthermore, the good
catalytic activity of 1 as bimetallic Lewis acid catalyst for the addition
reaction of TMSCN to aldehydes and aldimine condensation reactions,
respectively, were investigated.
Keywords : aluminum complexes, bimetallic Lewis acid catalyst, addition
reaction, aldimine condensation

Introduction
The Lewis acid properties of aluminum compounds for the mediation
of various organic transformations have a long-standing tradition. Among
those compounds, inorganic aluminum reagents such as AlCl are primarily
3
used for their Lewis acid properties. Poor selectivity and control of reactivity
1,2
are often the major issues in using these compounds.
While
organoaluminum compounds such as alanes, AlR , and their derivatives
3
combine a Lewis acidic character with an intrinsic nucleophilicity, which
3-6
provides them with additional reactivity towards organic substrates. The
majority of organic aluminum compounds applied in organic reactions are
bearing multidentate ligands containing nitrogen and oxygen as hard donor
7
atoms. In contrast, chelating ligands incorporating soft donors, which are
8
coordinated to the aluminum center and function as catalysts are scarce.
Even though, recent reports have shown that the use of ancillary ligands with
softer donors could affect the Lewis acidity of the metal center, with
9
remarkable effects on the catalytic performance.
The advances that have been made in the chemistry of multiple
bonding between main group elements compounds over the past two
10
decades have been remarkable . Since the first unique monomeric
Dipp
11
Dipp
12
Dipp
[
L Al(SeH) ]
and
[L Al(SH)2]
compounds
(L
=
2
N(Ar)C(Me)CHC(Me)N(Ar) (Ar = 2,6-i-Pr C H ) )) were reported, more
2
6
3 2

13
and more organoaluminum chalcogenides have been synthesized.
Furthermore, organic aluminum compounds responsible for polymerization
reactions exhibited bimetallic cooperative effects that can lead to higher
14
catalytic activity than the corresponding mononuclear ones. However, bis-
aluminum compounds with chelating ligands incorporating soft donors that
act as moderate and stable Lewis acid catalysts have never been reported.
Recently, we reported on an aluminum(III) hydride as a main group catalyst
15
based on the principles of transition-metal catalysts. Considering the
Dipp
distinct reactivity of [L AlH ], it can be used for the activation of
2
element−element bonds and reduction of various polar functional and
11,12,16
unsaturated substrates.
Inspired by these results, we set out to
investigate on the bulky aluminum dihydride for the activation of nonpolar
element−element bonds to form bis-aluminum species. Herein, we report on
the synthesis and structural characterization of bis-aluminum compounds
with soft donor properties, as well as their catalytic activity in the addition
reaction of TMSCN to aldehydes and as Lewis acid catalyst in aldimine
condensation reaction.
EXPERIMENTAL SECTION
General Information: Unless otherwise noted, all manipulations were
performed using a vacuum line and standard Schlenk techniques with an
atmosphere of nitrogen or using glove box techniques (typically oxygen and

moisture were controlled at less than 1 ppm). All solvents including C D
6
6
and CDCl were refluxed over the appropriate drying agent and distilled
3
prior to use. Commercially available chemicals were purchased from J&K
15a
and used as received. LAlH2 was prepared as described in the literature.
Elemental analyses were performed by Analysis & Testing Center of Beijing
Institute of Technology. NMR spectra were recorded on a Bruker AM 400
spectrometer. Infrared spectra of 2 were recorded on a Perkin Elmer
spectrophotometer. Melting points were measured in sealed glass tubes.
Synthesis of [LAl(µ-S) AlL] (1). A solution of LAlH (0.390 g, 1.0 mmol)
2
2
in THF (10 mL) was added drop by drop to a suspension of sulfur (0.032 g,
.0 mmol) in THF (10 mL) at room temperature. After the addition was
1
complete, stirring of the reaction mixture was continued overnight. Then the
solvent was removed in vacuo. The solid was extracted with toluene (10
mL), and the extract was stored at -30 ˚C for 2d to afford 1 as pale yellow
crystals. An additional crop of 1 was obtained from the mother liquor. Total
1
yield: 0.359 g (85%), mp 175 ˚C (decomp). H NMR (400 MHz, C D , 25
6
6
3
˚
C): δ = 7.15-7.02 (m, 12 H, Ar−H), 4.89 (s, 2 H, γ−H), 3.03 (sept, J
H−H
3
=
6.8 Hz, 8 H, CH Me), 2.57 (sept, J = 6.8 Hz, 8 H, CH Me), 1.44 (s, 12
2
H−H
2
3
13
H, Me), 1.22 (t, J
= 6.8 Hz, 24 H, CH Me); C NMR (100.61 MHz,
H−H
2
C D , 25 ˚C): δ = 170.85 (CN), 140.27, 139.14, 127.06, 126.17 (C of Ar),
6
6

9
7.75 (γ−CH), 24.52 (CH Me), 22.72, (CH Me) 13.87 (Me). C H Al N S
2
2
50 68
2
4 2
(843.16): calcd. C 71.39, H 7.91, N 6.66; found C 70.69, H 7.35, N 6.33.
Synthesis of [LAl(SeH) ] (2). A solution of LAlH (0.390 g, 1.0 mmol) in
2
2
THF (10 mL) was added drop by drop to a suspension of selenium (0.156 g,
.0 mmol) in THF (10 mL) at room temperature. After the addition was
2
complete, stirring of the reaction mixture was continued overnight, and a
light brown solution formed with trace amount of unchanged selenium. After
filtration, the solvent was removed and the residue extracted with toluene
(
(
˚
10 mL). The extract was stored at -30˚C for 2 d to give yellow crystals of 2
1
0.284, yield 52%), mp 223 ˚C (decomp). H NMR (400 MHz, C D , 25
6
6
3
C): δ = 7.15-7.09 (m, 6 H, Ar−H), 4.91 (s, 1 H, γ−H), 3.05 (sept, J = 6.8
H−H
3
Hz, 4 H, CH Me), 2.58 (sept, J = 6.8 Hz,4 H, CH Me), 1.44 (s, 6 H, Me),
2
H−H
2
3
13
1
.21 (t, J
= 6.8 Hz, 12 H, CH Me), -2.82 (s, 2 H, SeH); C NMR
H−H
2
(
100.61 MHz, C D , 25˚C): δ = 170.85 (CN), 140.28, 139.06, 127.15,
6
6
1
26.28 (C of Ar), 98.15 (γ−CH), 24.92 (CH Me), 22.82 (CH Me), 13.98
2
2
(
Me). C H AlN Se (843.16): calcd. C 54.75, H 6.43, N 5.11; found C
25
35
2
2
53.86, H 6.70, N 5.74.
RESULTS AND DISCUSSION
LAlH reacts smoothly with one equivalent of element sulfur at room
2
temperature to afford [LAl(µ-S) AlL] (1) in high yield (Scheme 1). It is
2
noteworthy that 1 is formed even when two equivalents of sulfur were used.

[
LAl(µ-Se) AlL] could not be obtained by using this protocol. The direct
2
reaction of LAlH with elemental Se led to [LAl(SeH) ] (2) (Scheme 1). The
2
2
conversion of LAlH to 2 demonstrates the umpolung from the hydridic to
2
the protonic form of the hydrogen atoms. This assumption well explained
our experimental results that compound 2 was always obtained as the
product formed at room temperature, regardless of the amount of the
selenium used. Note, this transformation proceeded in the absence of any
promoter. Inoue and co-workers reported that dimeric imidazolin-2-iminato
aluminum dihydride could be used for the activation of nonpolar
17
element−element bonds without any catalyst. The crystals of 1 and 2 are
slightly soluble in common organic solvents such as toluene,
dichloromethane, and tetrahydrofuran, respectively. Furthermore, 1 is
unexpectedly thermal stable no changes were observed after heating at 80 ˚C
for 4 h, which is explained by the steric and electronic stabilizing properties
of the bulky β-diketiminato ligand. Compounds 1 and 2 were characterized
by analytical, spectroscopic, and single-crystal X-ray diffraction studies. The
1
H NMR spectrum of 2 shows a high-field singlet at δ = -2.82 ppm, which
11
was assigned to SeH. The absorption of the Se-H stretching frequencies
could not be observed in the IR spectrum. The molecular geometries of 1
and 2 were determined by X-ray crystallography. The pale yellow crystals of
1
and yellow crystals of 2 were obtained by slow cooling of their saturated

solutions in toluene, respectively. The molecular structures of 1 and 2 are
shown in Figures 1 and 2. The four-membered Al S ring is essentially
2
2
planar, and the Al−S bond lengths are similar (2.2212(7) and 2.2224(8) Å).
They are a little shorter with respect to those in the literature reported (2.237
17
and 2.245 Å). The S(1)−Al(1)−S(2) angle falls within the range of
18,19,20
comparable compounds (95.9º to 101.9º).
Unfortunately, the quality
of crystals of 2 did not allow the location of the hydrogen atoms on the
selenium atoms. The Al−Se bond lengths (2.349 and 2.376 Å) are nearly
similar , when compared with those of the dimeric compound:
21
[
{[N(SiMe )C(Ph)C(SiMe ) ]AlSe} ] (av 2.350 Å). The Al atom deviates
3
3 2
2
slightly from the NCCCN plane of the ligand.
Recently our group reported on aluminum compounds with chelating
ligands incorporating soft donors that exhibited good catalytic activity in
addition reactions of aldehydes with trimethylsilyl cyanide (TMSCN).
Herein, we used the bis-aluminum compound [LAl(µ-S) AlL] (1) as a
2
bimetallic catalyst. To investigate the catalytic properties of compound 1,
addition reactions were performed by adding TMSCN to aldehydes. As
shown in Scheme 2, the reaction of TMSCN with benzaldehyde gives an
almost quantitative yield of product within 4 h with 2 mol% loading of the
catalyst at ambient temperature in CDCl . Excellent yield was also obtained
3
when an aldehyde with electron-donating aromatic group was used (Scheme

2, product 2). Phenylacrolein and o-fluorobenzaldehyde were studied to
show the electronic effect on the activation by using catalyst loadings of 2
mol% (Scheme 2, products 3 and 4). Under the same conditions, the reaction
with aliphatic aldehyde exhibited lower yield than those with aromatic
aldehydes (Scheme 2, product 5). The analogous reactions with substrates
including heterocycles were also found to be less efficient than those with
aromatic aldehydes (Scheme 2, products 6 and 7). The steric effect of the
9
bulky groups on the Al center decrease the catalytic activity. However, the
22
bimetallic cooperative effect may induce the high activity.
Imines are highly useful precursors for the synthesis of chiral amines,
23
secondary amines, and important biological molecules. Traditional imines
are synthesized using a primary amine and a carbonyl compound that
generally involves heating to reflux for an extended period of time to shift
the equilibrium to the product side. This protocol often requires the use of
materials such as molecular sieves (MS), titanium tetrachloride, CuSO , zinc
4
24
dichloride, and alumina. However, bimetallic compounds as moderate and
stable Lewis acid catalysts used in this type of reaction have never been
reported to the best of our knowledge. We found that 1 is an effective
catalyst for aldimine condensation reaction. The reactions were carried out
in CDCl with 5 mol% loading of the catalyst at ambient temperature. The
3
results are shown in Scheme 3 and indicate that these reactions give an

almost quantitative yield of product within 2 h. It is generally considered
that using the catalyst in the reaction usually undergoes a preliminary
coordinative interaction between the catalytic metal center and the carbonyl
oxygen atom of the substrate followed by a nucleophilic attack.
CONCLUSION
In summary, we have prepared and structurally characterized two new
thermally stable aluminum chalcogenides [LAl(µ-S) AlL] and [LAl(SeH) ].
2
2
This work shows new strategies for the activation of element−element bonds
by aluminum dihydride without any additional catalyst. Bis-aluminum
compound 1 acts as a good bimetallic Lewis acid catalyst in addition
reaction of TMSCN to aldehydes as well as in the aldimine condensation
reactions.
ACKONWLEDGEMENT
This work was supported by the International Science & Technology
Cooperation Program of China (2014DFR61080) and Analysis & Testing
Center of Beijing Institute of Technology.
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Scheme 1 Synthesis of 1 and 2.
a
Scheme 2 Addition reactions of TMSCN to aldehyde catalyzed by 1
O
OTMS
Cat 1. 2 mol%
+
TMSCN
R
H
CN
H
R
CDCl . r.t.
3
1
.0 equiv
1.0 equiv
R = alkyl or aryl group
OTMS
CN
OTMS
OTMS
CN
OTMS
CN
H
CN
H
H
H
F
1
, 99%
2, 99%
3, 79%
4, 56%
OTMS
OTMS
CN
TMSO
S
O
CN
H
CN
H
H
5
, 76%
7, 54%
6
, 77%
a
Aldehyde, 1 mmol; TMSCN, 1 mmol; 1, 0.02 mmol; in CDCl , at ambient temperature.
3
1
Yield was obtained according to thin-layer chromatographic (TLC) and H NMR spectral
analysis.

a
Scheme 3 Aldimine condensation reaction catalyzed by 1
a
Amine, 1 mmol; Aldehyde, 1 mmol; 1, 0.05 mmol; in CDCl , at ambient temperature.
3
1
Yield was obtained according to thin-layer chromatographic (TLC) and H NMR spectral
analysis.

Fig. 1 Molecular structure of LAl(µ-S) AlL (1); hydrogen atoms and solvent
2
molecules are omitted for clarity; selected bond lengths [Å] and angles [˚]:
Al(1)−S(1) 2.2212(7), Al(1)−S(2) 2.2224(8), S(1)−Al(1)−S(2) 99.60(3),
Al(1)−S(1)−Al(2) 88.40(3), N(1)−Al(1)−N(2) 95.95(6)

Fig. 2 Molecular structure of LAl(SeH) (2); hydrogen atoms and solvent
2
molecules are omitted for clarity; selected bond lengths [Å] and angles [˚]:
Al(1)−Se(1) 2.349 (3), Al(1)−Se(2) 2.376 (2), Se(1)−Al(1)−Se(2) 105.6 (5),
N(1)−Al(1)−N(2) 97.75(6)

TOC
[
LAl(µ-S) AlL] (1) and [LAl(SeH) ] (2) were synthesized and fully
2
2
characterized. [LAl(µ-S) AlL] (1) as a bimetallic Lewis acid catalyst for
2
addition reaction of TMSCN to aldehydes and aldimine condensation
reaction were reported.

Two aluminum complexes [LAl(µ-S) AlL] (1) and [LAl(SeH) ] (2) were
2
2
synthesized in good yield by reacting one equivalent of LAlH (L =
2
HC(CMeNAr) , Ar = 2,6-Et C H ) with one equivalent of S and with two
2
2
6
3
equivalents of Se without any catalysts, respectively. Complexes 1 and 2
1
13
have been characterized by H and C NMR, elemental analyses, and single
crystal X-ray structural analysis. Furthermore, the good catalytic activity of
1
as bimetallic Lewis acid catalyst for the addition reaction of TMSCN to
aldehydes and aldimine condensation reactions, respectively, were
investigated.