Synthesis and crystal structures of three amidinatoaluminum compounds and their catalytic behavior in the Tishchenko reaction

Article abstract of DOI:10.1016/j.poly.2015.01.045

Three amidinatoaluminum compounds, [{MeC(NCy)₂}AlMe(μ-OMe)]₂ (1), [{(PhN)MeC(NCy)}AlMe(μ-OMe)]₂ (2) and [{(PhN)MeC(N^tBu)}AlMe(μ-OMe)]₂ (3), were prepared by the insertion reaction of oxygen with the corresponding aluminum amidinate in high yield and characterized by ¹H and ¹³C NMR spectra and single crystal X-ray diffraction analysis. 1-3 were used as pre-catalysts to catalyze the Tishchenko reaction and both 2 and 3 exhibited good to excellent catalytic activity under mild conditions.

Full text of DOI:10.1016/j.poly.2015.01.045

Polyhedron 90 (2015) 118–122
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and crystal structures of three amidinatoaluminum
compounds and their catalytic behavior in the Tishchenko reaction
Shaofeng Zhang a, Hongfei Han a, Zhiqiang Guo b, Hongbo Tong b, Xuehong Wei a,
⇑
a The School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
b Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three amidinatoaluminum compounds, [{MeC(NCy)2}AlMe(
l
-OMe)]2 (1), [{(PhN)MeC(NCy)}AlMe(l-
OMe)]2 (2) and [{(PhN)MeC(NtBu)}AlMe(
l-OMe)]2 (3), were prepared by the insertion reaction of oxygen
Received 18 November 2014
Accepted 31 January 2015
Available online 17 February 2015
with the corresponding aluminum amidinate in high yield and characterized by 1H and 13C NMR spectra
and single crystal X-ray diffraction analysis. 1–3 were used as pre-catalysts to catalyze the Tishchenko
reaction and both 2 and 3 exhibited good to excellent catalytic activity under mild conditions.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Amidinatoaluminum compound
Oxygen
Catalysis
Tishchenko reaction
Insertion reaction
1. Introduction
Usually, aluminum(III) displays a high oxophilicity ability, sev-
eral examples of the interaction of aluminum complexes with oxy-
Many natural products and synthetic pharmaceuticals contain
amidine functional groups, which often play an essential role in
their biological activities [1]. Amidinates [R1NC(R3)NR2]ꢀ (Lꢀ; R1,
R2 = H, alkyl, cycloalkyl, aryl or trimethylsilyl; R3 = H, alkyl, or aryl)
have been long studied as chelating ligands for main group, transi-
tion metal and lanthanide elements [2] due to their versatile steric
and electronic properties [3], which can be easily tuned by changing
the various R1, R2 and R3 groups [4]. Large numbers of metal amidi-
nate complexes have been synthesized and applied in many areas,
including catalytic reactions, organic transformations and atomic
layer deposition [5]. More specifically, aluminum amidinates are
of interest as potential reagents in organic synthesis [6], as catalysts
for olefin polymerization [7], and as precursors for thin film depo-
sition [8], and have attracted increasing attention in recent years.
Three major synthetic routes for N,N-disubstituted aluminum ami-
dinates are (i) insertion of carbodiimides into Al–C bonds, (ii) con-
densation reactions of amidines with aluminum reagents and (iii)
displacement reactions of amidino lithium compounds with alu-
minum halide [9]. For instance, Rowley et al. synthesized aluminum
amidinates ([CH3C(Ni-Pr)2]2AlCH3, [CH3C(Ni-Pr)2]3Al) containing
two and three acetamidinate units via insertion of 2 or 3 equiva-
lents of diisopropylcarbodiimide into trimethylaluminum [10].
gen, as well as details of the reaction mechanism have been
reported [11]. However, the insertion reaction of oxygen into alu-
minum amidinate alkyls has not been reported to date. To the best
of our knowledge, the sole example, [{HC(NDipp)2}AlMe(l-OMe)]2,
was synthesized and well characterized by Richards’ group [7b],
what is more, it was obtained unexpectedly by using an
undried solvent. It is noteworthy that it was unsuccessfully synthe-
sised by the addition of stoichiometric quantities of water or oxy-
gen to anhydrous reactions. Here we report the synthesis and
structures of three amidinatoaluminum compounds containing
the l-OCH3 moiety, and their use as pre-catalysts for the dimeriza-
tion of aldehydes to the corresponding carboxylic esters was also
investigated.
2. Experimental
2.1. General remarks
All manipulations were performed under a nitrogen atmo-
sphere with rigorous exclusion of oxygen and water using standard
Schlenk and cannula techniques. All solvents were freshly distilled
and stored over a potassium mirror or activated molecular sieves
prior to use. The deuterated solvent C6D6 was dried over activated
molecular sieves (4 Å) and vacuum transferred before use.
CyN@C@NCy (Cy = cyclohexyl, Alfa Aesar) and AlMe3 (2.0 M solu-
tion in hexane; Alfa Aesar) were obtained commercially and used
⇑
Corresponding author. Tel.: +86 351 7018390; fax: +86 351 7011688.
E-mail address: xhwei@sxu.edu.cn (X. Wei).
http://dx.doi.org/10.1016/j.poly.2015.01.045
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

S. Zhang et al. / Polyhedron 90 (2015) 118–122
119
as received. PhN@C@NCy and PhN@C@Nt-Bu were prepared
Table 1
Selected bond lengths (Å) and angles (°) for compound 1.
according to the literature procedures [12]. Glassware was oven-
dried at 105 °C overnight. 1H NMR (300 MHz) and 13C NMR
(75 MHz) spectra of the compounds were recorded on a Bruker
DRX 300 instrument and referenced internally to the residual sol-
vent resonances (chemical shift data in d). All NMR spectra (1H and
13C) were measured in C6D6 at 298 K. Elemental analyses were per-
formed on a Vario EL-III instrument. Crystallographic measure-
Al1–O1
Al1–N1
Al1–C15
N1–C13
1.9190(11)
2.0524(14)
1.9769(18)
1.318(2)
Al1–O10
Al1–N2
Al1–Al10
1.8270(11)
1.9280(14)
2.9418(9)
1.339(2)
N2–C13
N1–C1
O1–C16
1.460(2)
1.4194(18)
76.53(5)
N2–C7
1.460(2)
O1–Al10
1.8270(11)
160.97(6)
99.48(6)
121.20(8)
98.36(7)
100.01(7)
143.04(11)
103.47(5)
124.48(10)
123.28(14)
124.42(15)
O1–Al1–O10
O10–Al1–N1
O10–Al1–N2
O1–Al1–C15
N1–Al1–N2
C1–N1–Al1
C13–N1–Al1
C16–O1–Al1
C1–N1–C13
N1–C13–C14
O1–Al1–N1
O1–Al1–N2
N2–Al1–C15
O10–Al1–C15
N1–Al1–C15
C7–N2–Al1
Al1–O10–Al10
C160–O10–Al1
C7–N2–C13
N2–C13–C14
97.98(5)
ments were performed with Mo Ka radiation (k = 0.71073 Å) on a
117.51(6)
121.06(7)
66.53(6)
146.10(12)
88.66(10)
129.20(10)
122.39(14)
124.83(15)
Bruker Smart Apex CCD diffractometer at 296(2) K. Crystals were
coated in oil and then directly mounted on the diffractometer
under a stream of cold nitrogen gas. Details of the modeling of dis-
order in the crystals can be found in their CIF files.
2.2. Synthesis of 1, [{MeC(NCy)2}AlMe(l-OMe)]2
0
Symmetry transformations used to generate equivalent atoms: ꢀx, 1 ꢀ y, 1ꢀz.
To a stirred solution of N,N0-dicyclohexylcarbodiimide (0.62 g,
3.0 mmol) in hexane (20 mL) was added trimethylaluminium
(1.50 mL of a 2.0 M solution in hexane, 3.0 mmol) dropwise at
room temperature. After being stirred for 6 h, stoichiometric quan-
tities of dry oxygen were introduced slowly into the solution via a
syringe and the reaction was carried out for 30 min at ꢀ78 °C. The
2
1
1
2
3
o
2
1
2
1
1
1
2
2
2
Scheme 1. Synthetic routes to complexes 1–3.
Fig. 2. Molecular structure of complex 2. Thermal ellipsoids are shown at the 30%
0
probability level. Hydrogen atoms are omitted for clarity. (Symmetry codes:
ꢀx + 1, ꢀy, ꢀz).
resulting mixture was then warmed to room temperature and stir-
red overnight. The white precipitate that formed was collected by
filtration and extracted into tetrahydrofuran. The filtrate was con-
centrated to ca. 15 mL and stored at ꢀ10 °C for several days, yield-
ing colorless crystals of 1. Yield: 0.71 g, 80.4%. Anal. Calc. for
C32H62Al2N4O2: C, 65.92; H, 10.64; N, 9.03. Found: C, 65.77; H,
10.48; N, 9.24%. 1H NMR (C6D6, 25 °C) d (ppm): ꢀ0.19 (s, 6H,
AlCH3), 1.22–1.36 (m, 8H, C6H11), 1.55 (s, 6H, (CyN)2CCH3), 1.59–
1.72 (m, 16H, C6H11), 1.86–1.97 (m, 16H, C6H11), 3.10 (m, 4H,
C6H11), 3.68 (s, 6H, OCH3). 13C NMR (C6D6, 25 °C) d (ppm): ꢀ9.54
(OCH3), 10.31 (AlCH3), 25.26, 26.25 (C6H11NCCH3), 35.09, 35.43,
51.46, 54.55 (C6H11), 170.95 (NCN).
2.3. Synthesis of 2, [{(PhN)MeC(NCy)}AlMe(l-OMe)]2
Compound 2 was isolated using the same procedures as
described for compound 1, except N-phenyl-N0-cyclohexylcarbodi-
imide was employed. Yield: 0.65 g, 74.9%. Anal. Calc. for C32H52Al2
N4O2: C, 66.78; H, 9.12; N, 9.48. Found: C, 66.59; H, 9.04; N, 9.63%.
Fig. 1. Molecular structure of complex 1. Thermal ellipsoids are shown at the 30%
0
probability level. Hydrogen atoms are omitted for clarity. (Symmetry codes: ꢀx,
1 ꢀ y, 1 ꢀ z).

120
S. Zhang et al. / Polyhedron 90 (2015) 118–122
Fig. 3. Molecular structure of complex 3. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms are omitted for clarity. (Symmetry codes: 0 1 ꢀ x, 1 ꢀ y,
1 ꢀ z).
Table 2
1H NMR (C6D6, 25 °C) d (ppm): ꢀ0.26 (s, 6H, AlCH3), 1.27–1.35 (m,
Selected bond lengths (Å) and angles (°) for compound 2.
4H, C6H11), 1.61 (s, 6H, C6H11NCCH3), 1.56–1.67 (m, 8H, C6H11),
Al1–O1
Al1–N1
Al1–C15
N1–C7
N1–C1
O1–C16
O1–Al1–O10
O10–Al1–N1
O10–Al1–N2
O1–Al1–C15
N1–Al1–C15
C1–N1–Al1
C7–N1–Al1
C160–O10–Al1
C1–N1–C7
N1–C7–C14
1.808(2)
1.917(3)
1.972(4)
1.339(4)
1.439(4)
1.434(4)
76.75(10)
120.41(12)
99.00(10)
98.36(7)
118.52(14)
139.5(2)
94.83(18)
123.8(2)
124.4(3)
123.6(3)
Al1–O10
Al1–N2
Al1–Al10
N2–C7
N2–C8
O1–Al10
O1–Al1–N1
O1–Al1–N2
N1–Al1–N2
O10–Al1–C15
N2–Al1–C15
C8–N2–Al1
C16–O1–Al1
Al1–O1–Al10
C7–N2–C8
N2–C7–C14
1.913(2)
2.068(3)
2.9181(18)
1.315(4)
1.446(4)
1.83–1.91 (m, 8H, C6H11), 3.17 (m, 2H, C6H11), 3.76 (s, 6H, OCH3),
7.11 (d, 6H, C6H5), 7.28 (t, 4H, C6H5). 13C NMR (C6D6, 25 °C) d
(ppm): -8.94 (OCH3), 8.28 (AlCH3), 22.26, 23.15 (C6H11NCCH3),
35.75, 38.42 (C6H11), 128.54, 131.21 (C6H5), 160.72 (NCN).
1.808(2)
97.04(10)
157.62(11)
65.80(10)
121.06(7)
99.93(14)
147.3(2)
130.95(19)
103.25(10)
122.9(3)
2.4. Synthesis of 3, [{(PhN)MeC(Nt-Bu)}AlMe(
l-OMe)]2
Compound 3 was isolated using the same procedures as
described for compound 1, except N-phenyl-N0-tert-butylcarbodi-
imide was employed. Yield: 0.67 g, 85.1%. Anal. Calc. for
C
28H46Al2N4O2: C, 64.31; H, 8.91; N, 10.43. Found: C, 64.25; H,
8.83; N, 10.52%. 1H NMR (C6D6, 25 °C) d (ppm): ꢀ0.23 (s, 6H,
AlCH3), 1.40 (s, 18H, (C(CH3)3) 1.68 (s, 6H, C6H5NCCH3), 3.71 (s,
6H, OCH3), 7.37 (d, 6H, C6H5), 7.54 (t, 4H, C6H5). 13C NMR (C6D6,
126.8(3)
0
Symmetry transformations used to generate equivalent atoms: ꢀx + 1, ꢀy, ꢀz.
25 °C)
d
(ppm): ꢀ7.12 (OCH3), 11.68 (AlCH3), 32.16, 32.93
(C(CH3)3), 47.86 (C(CH3)3), 127.85, 129.27 (C6H5), 163.29 (NCN).
Table 3
2.5. General procedure employed for the Tishchenko reaction
Selected bond lengths (Å) and angles (°) for compound 3.
Al1–O1
Al1–N1
Al1–C14
N1–C5
N1–C1
O1–C13
O1–Al1–O10
O10–Al1–N1
O10–Al1–N2
O1–Al1–C14
N1–Al1–N2
C1–N1–Al1
C5–N1–Al1
C13–O1–Al1
C1–N1–C5
N1–C5–C2
1.9169(12)
2.0656(15)
1.9653(19)
1.315(2)
1.481(2)
1.419(2)
77.07(6)
100.08(6)
95.72(6)
120.91(7)
66.16(6)
142.99(11)
89.22(10)
123.50(11)
125.70(14)
109.54(14)
Al1–O10
Al1–N2
Al1–Al10
N2–C5
N2–C7
O1–Al10
O1–Al1–N1
O1–Al1–N2
N2–Al1–C14
O10–Al1–C14
N1–Al1–C14
C7–N2–Al1
Al1–O10–Al10
C130–O10–Al1
C7–N2–C5
N1–C5–C6
1.8092(12)
1.9128(14)
2.9155(10)
1.348(2)
The pre-catalyst, amidinatoaluminum compound (0.8 mmol)
was placed in a dry Schlenk flask under a nitrogen atmosphere,
and freshly distilled aldehyde (80 mmol) was introduced. The
mixture was immediately stirred at room temperature for 30 min
to produce the corresponding ester. The reaction was quenched
with 0.5 ml of water and the product was distilled to collect the
corresponding ester. The yields reported are the isolated yield.
1.413(2)
1.8092(12)
158.28(6)
117.54(6)
121.43(8)
100.14(7)
99.68(7)
139.90(11)
102.93(6)
132.65(11)
125.13(14)
129.01(16)
3. Results and discussion
3.1. Synthesis and characterization
All of the amidinatoaluminum compounds 1, 2 and 3 were syn-
thesized by a one-pot reaction route. As shown in Scheme 1, the
carbodiimide and trimethylaluminum were mixed in hexane at
room temperature. After stirring for an appropriate time, dry oxy-
gen was slowly introduced into the mixture at ꢀ78 °C. Complexes
1–3 were prepared in good yield and each of 1, 2 and 3 was easily
purified by recrystallization from a mixed solution of hexane and
THF, and characterized by 1H and 13C NMR spectra in C6D6 at
0
Symmetry transformations used to generate equivalent atoms: 1 ꢀ x, 1 ꢀ y, 1 ꢀ z.
ambient temperature, and single crystal X-ray diffraction data.
Attempts to add stoichiometric quantities of water to the reaction
system were unsuccessful. The reaction of aluminum amidinates
with water gave a viscous substance and we believed that the

S. Zhang et al. / Polyhedron 90 (2015) 118–122
121
Table 4
The Tishchenko reaction of aldehydes using complexes 1–3 catalysts.
O
catalyst
2 RCHO
R
O
R
Entry
Catalyst
mol%
Substrate
Temperature (°C)
Time (h)
Yielda (%)
1
2
3
4
5
6
7
8
1
2
3
3
3
3
3
3
3
3
3
3
1
1
1
0.75
0.5
0.2
1
1
1
isobutyraldehyde
isobutyraldehyde
isobutyraldehyde
isobutyraldehyde
isobutyraldehyde
isobutyraldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
propionaldehyde
acetaldehyde
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
80
80
80
80
r.t.
r.t.
0.5
0.5
0.5
0.5
0.5
0.5
2
4
6
8
0.5
16
47
>99
>99
97
89
69
51
81
96
97
9
10
11
12
1
1
1
>99
95
a
Isolated yield.
aluminum amidinates were hydrolyzed to aluminum hydroxide. It
is noteworthy to mention that compounds 1, 2 and 3 are slightly
air and moisture stable, meaning they can be exposed to the atmo-
sphere for several minutes.
of guanidinatozinc complexes [20] exhibiting excellent catalytic
activities for the dimerization of aromatic aldehydes to the corre-
sponding esters under mild conditions. To expand the family of
the Tishchenko reaction catalysts, the amidinatoaluminum alkox-
ides 1, 2 and 3 were used as pre-catalysts to catalyze the dimeriza-
tion of alkyl aldehydes.
3.2. Molecular structures of compounds 1–3
Isobutyraldehyde was selected as a model substrate, and the
amidinatoaluminum compounds 1–3 were screened as potential
catalysts (1.0 mol%) for the Tishchenko reaction; the results are
summarized in Table 4. The dimerization of aldehydes was carried
out at room temperature for 30 min under an atmosphere of N2.
Delightedly, isobutyl isobutyrate was obtained in excellent yields
(>99%) in the presence of complexes 2 or 3, whilst the activity of
complex 1 was much lower (Table 4, entries 1–3). This indicates
the highly efficient catalytic behavior is due to an electronic effect
caused by the phenyl group. Complex 3 at 0.75, 0.5 or 0.2 mol%
loading gave a 97%, 89% or 69% yield, respectively, of isobutyl
isobutyrate under the same conditions (Table 4, entries 4–6).
In view of the above results, we proceeded to explore the sub-
strate scope of the reaction. The dimerization of benzaldehyde
was carried out at 80 °C and compound 3 exhibited high activity
after different times (2, 4, 6, 8 h, Table 4, entries 7–10). In addition,
good to excellent yields were obtained when complex 3 was used
to promote propionaldehyde or acetaldehyde to the corresponding
ester. Because of the poor solubility of the catalyst in acetaldehyde,
the reaction time was longer than for propionaldehyde (Table 4,
entries 11 and 12).
The ORTEP drawing of the molecular structure of compound 1 is
shown in Fig. 1. Selected bond lengths and angles are given in
Table 1. Crystalline 1 is a centrosymmetric dimer with a central,
virtually square Al1O1Al10O10 core, which is close to that of
[{HC(NDipp)2}AlMe(l-OMe)]2 [7b]. Each aluminum atom is five-
coordinated, in a distorted trigonal bipyramidal environment in
which the nitrogen atom N1 and oxygen atom O1 occupy axial
positions. The N1–C13 and N2–C13 bond lengths of 1.318(2) and
1.339(2) Å are slightly longer than those of 1.311(4) and
1.320(4) Å in [{HC(NDipp)2}AlMe(l-OMe)]2, respectively, while
the Al–N bond lengths of 1.9280(14) and 2.0524(14) Å are slightly
shorter than those of 1.973(3) and 2.071(3) Å in [{HC(NDipp)2}-
AlMe(
2.9418(9) and 1.9190(11) Å are similar to those (2.919(2) and
1.830(3) Å) in [{HC(NDipp)2}AlMe( -OMe)]2, respectively.
l
-OMe)]2, respectively. The Al–Al0 and Al1–O1 distances of
l
As illustrated in Figs. 2 and 3, respectively, the molecular struc-
tures of crystalline 2 and 3 are extremely similar to 1, and selected
bond distances and angles are summarized in Tables 2 and 3,
respectively.
3.3. Complexes 1, 2 and 3 as catalysts for the Tishchenko reaction
4. Conclusions
The highly efficient conversion of aldehydes to their corre-
sponding symmetric esters, better known as the Tishchenko reac-
tion (or the Claisen–Tishchenko reaction) has been known for
more than a hundred years [13]. Since then, the Tishchenko reac-
tion has been used as an efficient method for the preparation of
dimeric esters in industry [14], and it is inherently environmental-
ly benign since it utilizes catalytic conditions and is 100% atom
economic. A number of transition metal, main group and non-met-
al compounds are reported to catalyze the reaction. For example,
Hill and co-workers investigated the activity of heavier group 2
(Ca, Sr, Ba) metal amides [15], and a homogeneous magnesium
thiolate system was reported in detail [16]. A magnesium guani-
dinate compound [17], Th(IV) compounds [18], Ni(0)/NHC [19]
and selenide ions [16c] have been used as effective catalysts in
the Tishchenko reaction. Very recently, our group reported a series
In summary, three amidinatoaluminum compounds supported
by different amidinate ligands were synthesized and characterized
by satisfactory C, H and N microanalysis, 1H and 13C{1H} spectra in
C6D6 at ambient temperature and single crystal X-ray structural
data. The complexes 2 and 3 exhibited excellent catalytic activities
for the solvent-free Tishchenko reaction under mild conditions.
Acknowledgments
Financial support from the National Natural Science Foundation
of China (Nos. 21272142 and 21101101) and the Natural Science
Foundation of Shanxi Province (Nos. 2011021011-1, 2008011021,
2012021007-1) are gratefully acknowledged.

122
S. Zhang et al. / Polyhedron 90 (2015) 118–122
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(g) M.P. Coles, Dalton Trans. (2006) 985.
CCDC 1032299-1032301 contain the supplementary crystallo-
graphic data for 1, 2 and 3, respectively. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated
with this article can be found in the online version or by direct
contacting the authors.
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