Synthesis and crystal structures of guanidinatoaluminum complexes and catalytic study for MPV reduction

Article abstract of DOI:10.1016/j.poly.2017.01.030

Treatment of a secondary amine (piperidine or N-benzylmethylamine) with sequentially trimethylaluminum and carbodiimide CyN[dbnd]C[dbnd]NCy in a molar ratio of 1:1:1 afforded two mononuclear guanidinatoaluminum complexes, [{(C₅H₁₀N)C(NCy)₂}AlMe₂] (1) and. [{(Bz(Me)N)C(NCy)₂}AlMe₂] (2). Introducing dry oxygen slowly into the solution of 1 or 2, [{(C₅H₁₀N)C(NCy)₂}AlMe(μ-OMe)]₂ (3) and [{(Bz(Me)N)C(NCy)₂}AlMe(μ-OMe)]₂ (4) were obtained, respectively. Both of 3 and 4 were also prepared by treatment of piperidine or N-benzylmethylamine with trimethylaluminum and carbodiimide CyN[dbnd]C[dbnd]NCy in the presence of O₂. The complexes 1–4 were characterized by ¹H, ¹³C NMR spectra and single crystal X-ray diffraction analysis. 1–4 were also used to catalyze the Meerwein–Ponndorf–Verley (MPV) reaction.

Full text of DOI:10.1016/j.poly.2017.01.030

Accepted Manuscript
Synthesis and crystal structures of guanidinatoaluminum complexes and cata-
lytic study for MPV reduction
Hongfei Han, Zhiqiang Guo, Shaofeng Zhang, Yupeng Hua, Xuehong Wei
PII:
S0277-5387(17)30067-0
DOI:
http://dx.doi.org/10.1016/j.poly.2017.01.030
POLY 12434
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
10 December 2016
18 January 2017
19 January 2017
Please cite this article as: H. Han, Z. Guo, S. Zhang, Y. Hua, X. Wei, Synthesis and crystal structures of
guanidinatoaluminum complexes and catalytic study for MPV reduction, Polyhedron (2017), doi: http://dx.doi.org/
10.1016/j.poly.2017.01.030
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Synthesis and crystal structures of guanidinatoaluminum complexes and
catalytic study for MPV reduction
Hongfei Han[a,b] , Zhiqiang Guo [c] , Shaofeng Zhang[a] , Yupeng Hua [a] , Xuehong Wei[c]∗
a
The School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, P.R.
China
b Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, P. R. China
c Scientific Instrument Center, Shanxi University, Taiyuan 030006, P. R. China
Abstract: Treatment of a secondary amine (piperidine or N-benzylmethylamine) with sequentially
trimethylaluminum and carbodiimide CyN=C=NCy in a molar ratio of 1:1:1 afforded two
mononuclear guanidinatoaluminum complexes, [{(C5H10N)C(NCy)2}AlMe2] (1) and.
[{(Bz(Me)N)C(NCy)2}AlMe2] (2). Introducing dry oxygen slowly into the solution of 1 or 2,
[{(C5H10N)C(NCy)2}AlMe(µ-OMe)]2 (3) and [{(Bz(Me)N)C(NCy)2}AlMe(µ-OMe)]2 (4) were
obtained, respectively. Both of 3 and 4 were also prepared by treatment of piperidine or
N-benzylmethylamine with trimethylaluminum and carbodiimide CyN=C=NCy in the presence of
1
O2. The complexes 1-4 were characterized by H, 13C NMR spectra and single crystal X-ray
diffraction analysis. 1-4 were also used to catalyze the Meerwein-Ponndorf-Verley (MPV)
reaction.
Keywords: Guanidinato complexes; Oxygen; Crystal structure; MPV reaction.
1. Introduction
Amidinates and guanidinates are popular ligands due to their ease of synthesis and
modification by changing various groups, as well as their robust nature. These amidinato or
guanidinato ligands involving metals from across the periodic table have been synthesized
and applied in many areas, including synthesis [1], coordination chemistry[2] and material
science [3]. Group 13 amidinate and guanidinate complexes, especially highly electrophilic
aluminium species, are widely studied due to their potential applications as gaseous
precursors to technologically important materials [4] and excellent candidates for several
* Corresponding author. Tel.: +86 351 7018390, Fax: +86-351-7011688;
E-mail address: xhwei@sxu.edu.cn (X. Wei).

well-known catalytic applications [5]. Recently, our group explored a series of
amidinatoaluminum and guanidinato metal (Al and Zn) complexes used as the catalysts in
the Tishchenko reaction and Meerwein-Ponndorf-Verley reduction [6].
Although the high oxophilicity ability of aluminum(III) have been demonstrated by
several examples of the interaction of aluminum complexes with oxygen, and the reaction
mechanism about the reaction has been also detailed in the reported papers [7], the first oxo
amidinatoaluminum complex, [{HC(NDipp)2}AlMe(
until 2010 by Richards’ group [8]. Recently, our team has synthesized several
amidinatoaluminum complexes containing the -OCH3 moiety by the reaction of
µ-OMe)]2, was isolated unexpectedly
µ
aluminum amidinate alkyls with oxygen [6b]. Due to guanidinates considered as special
amidinates, the oxo guanidinatoaluminum complexes should be obtained by the similar
reaction procedure. We have been devoted to the research on reactivities of
(mono)guanidinatoaluminum complexes with oxygen, and a relevant paper have been
published [9]. We herein report the synthesis and characterization of the other four new
guanidinatoaluminum complexes, including two (mono)guanidinatoaluminum complex,
[{(C5H10N)C(NCy)2}AlMe2] (
1
) and [{(Bz(Me)N)C(NCy)2}AlMe2] (
-OMe)]2
µ-OMe)]2 (4). Our report also includes their application in
2
), and two oxo
guanidinatoaluminum complexes, [{(C5H10N)C(NCy)2}AlMe(
µ
(3)
and
[{(Bz(Me)N)C(NCy)2}AlMe(
the MPV reaction.
2. Experimental
2.1 General remarks
All manipulations were performed under a nitrogen atmosphere 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. Deuterated
solvent C6D6 or CDCl3 was dried over activated molecular sieves (4Å) and vacuum transferred
before use. CyN=C=NCy (Cy = cyclohexyl, Alfa Aesar) and AlMe3 (2.0M solution in hexane;
Alfa Aesar) were obtained commercially and used as received. Pyrrolidine, piperidine, N-
benzylmethylamine, aliphatic aldehydes and benzaldehyde were dried over MgSO4 and redistilled
before use. 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 solvent resonances

(chemical shift data in δ). All NMR spectra (1H, 13C) were measured in C6D6 or CDCl3 at 298 K.
Elemental analyses were performed on a Vario EL-III instrument.
The crystals of complexes 1-4 suitable for single crystal X-ray diffraction analysis were
recrystallized from the appropriate solvents. Crystallographic measurements were performed with
Mo Kα radiation (λ = 0.71073 Å) on a Bruker Smart Apex CCD diffractometer at 223(2) K.
Corrections were applied for Lorentz and polarization effects as well as absorption using
multiscans (SADABS) [10]. Each structure was solved by direct methods and refined on F2 by full
matrix least-squares (SHELX97) [11] using all unique data. Then the remaining non-hydrogen
atoms were obtained from the successive difference Fourier map. All non-hydrogen atoms were
refined with anisotropic displacement parameters, whereas the hydrogen atoms were constrained
to parent sites, using a riding model (SHELXTL) [12]. Details of the modeling of disorder in the
crystals can be found in their CIF files.
2.2 Synthesis of 1, [{(C5H10N)C(NCy)2}AlMe2]
To a stirred solution of piperidine (0.256 g, 3.0 mmol) in hexane (15 mL), trimethylaluminum
(1.50 mL, 3.0 mmol) was added dropwise. The mixture was heated to 60 °C for 12 h and then
cooled to 0 °C, N,N'-dicyclohexylcarbodiimide (0.678 g, 3.0 mmol) was added. The mixture was
stirred for 8 h to afford a cloudy solution, which was filtered. The filtrate was concentrated to ca.
5 mL in vacuo and stored at -10 °C, yielding colorless crystals of 1 (0.859 g, 82.4%). Anal. Calc.
1
for C20H38AlN3: C, 69.12; H, 11.02; N, 12.09. Found: C, 68.96; H, 10.91; N, 12.28%. H NMR
(C6D6, 25 °C): δ (ppm): -0.14 (s, 6H, AlCH3), 1.05-1.07 (m, 2H, C5H10N), 1.18-1.22 (m, 10H,
C6H11), 1.28-1.34 (m, 4H, C5H10N), 1.51-1.53 (m, 2H, C6H11), 1.66-1.68 (m, 4H, C6H11),
1.88-1.90 (m, 4H, C6H11), 2.81 (br, 4H, C5H10N), 2.96-3.00 (m, 2H, C6H11). 13C NMR (C6D6, 25
°C): δ (ppm): -8.33 (AlCH3), 25.55, 25.78, 35.36, 48.23 (C6H11), 24.13, 25.82, 53.45 (C5H10N),
166.81 (NCN).
2.3 Synthesis of 2, [{(Bz(Me)N)C(NCy)2}AlMe2]
To a stirred solution of N-benzylmethylamine (0.364 g, 3.0 mmol) in hexane (15 mL),
trimethylaluminum (1.50 mL, 3.0 mmol) was added dropwise. The mixture was heated to 60 °C
for 12 h and then cooled to 0 °C, N,N'-dicyclohexylcarbodiimide (0.678 g, 3.0 mmol) was added.
The mixture was stirred for 8 h to afford a cloudy solution, which was filtered. The filtrate was
concentrated to ca. 5 mL in vacuo and stored at -10 °C, yielding colorless crystals of 2 (0.948 g,

82.4%). Anal. Calc. for C23H38AlN3: C, 72.02; H, 9.99; N, 10.96. Found: C, 71.89; H, 9.90; N,
1
11.10%. H NMR (C6D6, 25 °C): δ (ppm): -0.12 (s, 6H, AlCH3), 1.02-1.12 (m, 6H, C6H11),
1.30-1.33 (m, 4H, C6H11), 1.46-1.48 (m, 2H, C6H11), 1.62-1.64 (m, 4H, C6H11), 1.83-1.84 (m, 4H,
C6H11), 2.33 (s, 3H, NCH3), 2.98-3.01 (m, 2H, C6H11), 3.96 (s, 2H, PhCH2), 7.03-7.06 (m, 3H,
C6H5), 7.11-7.15 (m, 2H, C6H5). 13C NMR (C6D6, 25 °C): δ (ppm): -8.23 (AlCH3), 25.65, 25.70,
35.68, 53.48 (C6H11), 36.14 (NCH3), 55.64 (PhCH2), 127.46, 127.97, 128.44, 137.07 (C6H5),
167.26 (NCN).
2.4 Synthesis of 3, [{(C5H10N)C(NCy)2}AlMe(µ-OMe)]2
Method 1. To a stirred solution of complex
1 (1.043 g, 3.0 mmol) in tetrahydrofuran (15
mL), dry oxygen was introduced slowly via a balloon and the reaction was carried out for
30 min at -78 °C. Then the resulting mixture was warmed to room temperature and stirred
overnight. The volatiles were removed under reduced pressure. The residue was washed
with hexane (2 × 5 mL) and dried in vacuo to give the product as a white solid, which was
recrystallized from hexane-tetrahydrofuran (1:2) solution at -10 °C to give the colorless
crystals of complex
3 (0.885 g, 81.2%).
Method 2. To a stirred solution of piperidine (0.256 g, 3.0 mmol) in hexane (15 mL),
trimethylaluminum (1.50 mL, 3.0 mmol) was added dropwise. The mixture was heated to 60 °C
for 12 h and then cooled to room temperature, N,N'-dicyclohexylcarbodiimide (0.678 g, 3.0 mmol)
was added. After being stirred for 6 h, dry oxygen was introduced slowly into the solution via a
balloon and the reaction was carried out for 30 min at -78 °C. Then the resulting mixture was
warmed to room temperature and stirred overnight. The white precipitate was collected by
filtration, then washed with hexane (2 × 5 mL) and dried in vacuo to give the product as a white
solid, which was recrystallized from hexane-tetrahydrofuran (1:2) solution at -10 °C to give the
colorless crystals of complex 3 (0.791 g, 72.5%). Anal. Calc. for C40H76Al2N6O2: C, 66.08; H,
10.54; N, 11.56. Found: C, 65.94; H, 10.45; N, 11.69%. 1H NMR (CDCl3, 25 °C): δ (ppm): -0.09
(s, 3H,AlCH3), 1.42 (m, 12H, C6H11), 1.79-1.81(m, 4H, C5H10N), 1.92-2.00 (m, 8H, C6H11),
2.11-2.15 (m, 2H, C5H10N), 3.03 (s, 4H, C5H10N), 3.21-3.28 (m, 2H, C6H11), 3.78 (s, 3H, OCH3).
13C NMR (CDCl3, 25 °C): δ (ppm): -6.39 (AlCH3), 23.75, 24.51, 25.99, 26.30, 27.26, 34.99, 51.27,
52.13, 54.20, 54.87 (C6H11), 25.43, 36.09, 49.93 (C5H10N), 57.14 (OCH3), 169.46 (NCN).

2.5 Synthesis of 4, [{((Me)BzN)C(NCy)2}AlMe(µ-OMe)]2
Method 1. To a stirred solution of complex (1.151 g, 3.0 mmol) in tetrahydrofuran (15
2
mL), dry oxygen was introduced slowly via a balloon and the reaction was carried out for
30 min at -78 °C. Then the resulting mixture was warmed to room temperature and stirred
overnight. The volatiles were removed under reduced pressure. The residue was washed
with hexane (2 × 5 mL) and dried in vacuo to give the product as a white solid, which was
recrystallized from hexane-tetrahydrofuran (1:2) solution at -10 °C to give the colorless
crystals of complex
4 (0.982 g, 81.9%).
Method 2. To a stirred solution of N-benzylmethylamine (0.364 g, 3.0 mmol) in hexane (15 mL),
trimethylaluminum (1.50 mL, 3.0 mmol) was added dropwise. The mixture was heated to 60 °C
for 12 h and then cooled to room temperature, N,N'-dicyclohexylcarbodiimide (0.678 g, 3.0 mmol)
was added. After being stirred for 6 h, dry oxygen was introduced slowly into the solution via a
balloon and the reaction was carried out for 30 min at -78 °C. Then the resulting mixture was
warmed to room temperature and stirred overnight. The white precipitate was collected by
filtration, then washed with hexane (2 × 5 mL) and dried in vacuo to give the product as a white
solid, which was recrystallized from hexane-tetrahydrofuran (1:2) solution at -10 °C to give the
colorless crystals of complex 4 (0.913 g, 76.2%). Anal. Calc. for C46H76Al2N6O2: C, 69.14; H,
9.59; N, 10.52. Found: C, 68.97; H, 9.48; N, 10.64%. 1H NMR (CDCl3, 25 °C): δ (ppm): -0.12 (s,
3H,AlCH3), 1.19-1.39 (m, 8H, C6H11), 1.65-1.72 (m, 4H, C6H11), 1.81-1.89 (m, 4H, C6H11),
1.92-1.93 (m, 2H, C6H11), 2.02-2.09 (m, 2H, C6H11), 2.53 (s, 3H, NCH3), 3.10-3.20 (m, 2H,
C6H11), 3.71 (s, 3H, OCH3), 4.10 (s, 2H, PhCH2), 7.11-7.17 (m, 2H, C6H5), 7.24-7.25 (m, 3H,
C6H5). 13C NMR (CDCl3, 25 °C): δ (ppm): -7.57 (AlCH3), 26.27, 27.26, 35.84, 51.77 (C6H11),
35.41 (NCH3), 55.49 (OCH3), 57.58 (PhCH2), 127.42, 128.93, 129.08, 140.08 (C6H5), 155.43
(NCN).
2.6 General procedure employed for the MPV reaction.
To a solution of catalyst in toluene was added aldehyde or ketone, followed by the addition of
2-propanol. The reaction mixture was then refluxed for the required reaction time under an
atmosphere of nitrogen. The reaction was then cooled to room temperature and the conversion
yield was determined by 1H NMR spectroscopic study based on the integration in the methylene
and the CHO region.

3. Results and discussion
3.1. Synthesis and characterization
As shown in Scheme 1, treatment of piperidine or N-benzylmethylamine with AlMe3 and
CyN=C=NCy in a molar ratio of 1:1:1 afforded two mononuclear guanidinatoaluminum complex,
[{(C5H10N)C(NCy)2}AlMe2] (1) and [{(Bz(Me)N)C(NCy)2}AlMe2] (2). While introducing dry
oxygen via a balloon into the solution of the mononuclear guanidinatoaluminum complex (1 or 2)
in tetrahydrofuran for 30 mintues at -78 °C, two new guanidinatoaluminum complexes containing
the µ-OCH3 moiety, [{(C5H10N)C(NCy)2}AlMe(µ-OMe)]2 (3), [{((Me)BzN)C(NCy)2}AlMe
-(µ-OMe)]2 (4) were isolated successfully, which were obtained by insertion of oxygen atom into
−C bond of mononuclear guanidinatoaluminum complex
the Al
(1 or 2), respectively. The dimeric
guanidinatoaluminum complexes (3 and 4) were also synthesized via one-pot reaction of Et2NH,
AlMe3, and carbodiimine in the presence of O2. The results of the elemental analyses for 1-4 are
well in accordance with the corresponding supposed compositions (as shown in the Experimental
section). All of 1-4 were characterized by 1H and 13C NMR spectra in C6D6 or CDCl3 at ambient
temperature, and further confirmed by X-ray diffraction analysis.
Me
Bz
N
Cy
Cy
N
N
O2
Al
Me
Me
2
(1) AlMe3
(2) CyN=C=NCy
Me
O
Cy
N
Cy
N
Me
O
Cy
N
Cy
N
Me
Al
Me
Al
(1) AlMe3
Bz
Me
Bz
(1) AlMe3
(2) CyN=C=NCy
Al
N
N
N
Al
RH
N
(2) CyN=C=NCy
(3) O2
(3) O2
N
O
N
Me
N
O
N
Me
Me
(1) AlMe3
Cy
Me
Cy
Cy
Me
Cy
(2) CyN=C=NCy
3
4
N
O2
Cy
Cy
N
N
Al
Me
Me
1
Scheme 1. Synthetic routes to complexes 1-4

3.2. Molecular structures of complexes 1-4
Crystals of were obtained from a saturated hexane solution at -10 °C. As is shown in
Fig. 1, the aluminum centre of the complex is four coordinate with one chelating
1
1
guanidinato unit and two C atoms coming from methyl groups giving it a distorted
tetrahedral geometry, and the four atoms N1, C7, N2, and Al1 are essentially planar with a
deviation of 0.0014 Å, which is similar to the structure of the molecular of
[(iPr2N)C{N(2,6-iPr2C6H3)}2]AlMe2 [13]. The Al1-N1 and Al1-N2 bond distances
(1.9291(15) and 1.9226(15) Å, respectively) are similar to those (1.9287(15) and
1.9239(16) Å, respectively) in [(iPr2N)C{N(2,6-iPr2C6H3)}2]AlMe2. In addition, the bite
angle of 69.77(6) ° for N1-Al1-N2 is somewhat larger than that (69.06(6) °) of
[(iPr2N)C{N(2,6-iPr2C6H3)}2]AlMe2. In the guanidinato moiety C7N3, C7-N3 bond length
is
1.369(2)
Å,
which
is
shorter
than
that
(1.371(2)
Å)
in
[(iPr2N)C{N(2,6-iPr2C6H3)}2]AlMe2. Virtually identical C7-N1 and C7-N2 bond distances
of 1.346 (2) and 1.344(2) Å substantiated a high degree of electron delocalization within
the guanidinate backbone.
Fig. 1. Molecular structure of complex 1. Thermal ellipsoids are shown at 30% probability level.
Hydrogen atoms are omitted for clarity.
Table 1
Selected bond lengths (Å) and angles (°) for complex 1
N(1)-C(7)
N(2)-C(7)
1.346(2)
1.344(2)
Al(1)-N(2)
Al(1)-C(19)
1.9226(15)
1.956(2)

N(3)-C(7)
1.369(2)
Al(1)-C(20)
1.959(2)
Al(1)-N(1)
1.9291(15)
109.97(14)
126.08(15)
123.93(15)
89.97(10)
90.29(10)
N(1)-C(7)-N(2)
N(1)-C(7)-N(3)
N(2)-C(7)-N(3)
C(7)-N(1)-Al(1)
C(7)-N(2)-Al(1)
N(1)-Al(1)-N(2)
N(1)-Al(1)-C(19)
N(1)-Al(1)-C(20)
N(2)-Al(1)-C(19)
N(2)-Al(1)-C(20)
69.77(6)
115.09(9)
115.30(9)
115.82(9)
115.00(9)
As illustrated in Figure 2, the molecular structure of crystalline 2 is similar to 1 and selected
bond distances and angles are summarized in Table 2.
Fig. 2. Molecular structure of complex 2. Thermal ellipsoids are shown at 30% probability level.
Hydrogen atoms are omitted for clarity.
Table 2
Selected bond lengths (Å) and angles (°) for complex 2
N(1)-C(7)
1.346(2)
Al(1)-N(2)
Al(1)-C(22)
Al(1)-C(23)
1.9310(13)
1.9858(15)
1.9943(15)
N(2)-C(7)
1.346(2)
N(3)-C(7)
1.371(2)
Al(1)-N(1)
1.9167(13)
109.93(12)
123.54(13)
N(1)-C(7)-N(2)
N(1)-C(7)-N(3)
N(1)-Al(1)-N(2)
N(1)-Al(1)-C(22)
69.90(5)
116.46(6)

N(2)-C(7)-N(3)
C(7)-N(1)-Al(1)
C(7)-N(2)-Al(1)
126.52(13)
90.35(9)
N(1)-Al(1)-C(23)
N(2)-Al(1)-C(22)
N(2)-Al(1)-C(23)
116.29(6)
116.63(6)
114.21(6)
89.74(9)
The molecular structure of complex 3 is depicted in Fig. 2. Selected bond lengths and angles are
given in Table 3. Complex 3 crystallize in the triclinic P-1 space group. Crystalline 3 is a
centrosymmetric dimer with a central, virtually rhombus Al1O1Al1'O1' core, which is close to that
of [{MeC(NCy)2}AlMe(µ-OMe)]2 [6b]. The geometry around the Al atom in the structures of
complex 3 is penta-coordinated with a distorted trigonal bipyramidal environment in which the
nitrogen atom N1 and oxygen atom O1 occupy axial positions. In molecular structure of 3, the
dihedral angle between the planes of Al1N1C7N2 and Al1O1Al1'O1' is 62.207(43) °, which is
slightly larger than that (61.322(42) °) in above reported complex. The Al-O bond lengths
(1.8464(14)-1.9136(13) Å) lie in the reasonable range for those (1.8270(11)-1.9190(11) Å) in
[{MeC(NCy)2}AlMe(µ-OMe)]2. The Al1-N1 and Al1-N3 bonds (2.0266(15) and 1.9291(16) Å,
respectively) of the guanidinate ligand are similar to those (2.0524(14) and 1.9280(14) Å,
respectively) in [{MeC(NCy)2}AlMe(µ-OMe)]2. For the guanidinato moiety C7N3, the bond
distances of N1-C7, N2-C7 and N3-C7 are 1.328(2), 1.387(2), and 1.345(2) Å, respectively, which
suggest a greater degree of electronic delocalization within the guanidinate backbone of 3. In
addition, the Al1-C18 bond distance of 1.978(2) Å is similar to that (1.9769(18) Å) in
[{MeC(NCy)2}AlMe(µ-OMe)]2.
Fig. 3. Molecular structure of complex 3. Thermal ellipsoids are shown at 30% probability
level. Hydrogen atoms are omitted for clarity. (Symmetry codes: ′ 1 – x, – y, 1 – z)

Table 3
Selected bond lengths (Å) and angles (°) for complex 3
N(1)-C(7)
1.328(2)
Al(1)-C(19)
Al(1)-O(1)
Al(1)-O(1)′
C(20)-O(1)
1.978(2)
N(2)-C(7)
1.387(2)
1.9136(13)
1.8464(14)
1.422(2)
N(3)-C(7)
1.345(2)
Al(1)-N(1)
2.0266(15)
1.9291(16)
126.65(16)
110.39(15)
122.92(16)
89.26(11)
92.98(11)
67.37(6)
Al(1)-N(3)
N(1)-C(7)-N(2)
N(1)-C(7)-N(3)
N(2)-C(7)-N(3)
C(7)-N(1)-Al(1)
C(7)-N(3)-Al(1)
N(1)-Al(1)-N(3)
N(1)-Al(1)-C(19)
N(1)-Al(1)-O(1)
N(1)-Al(1)-O(1)′
N(3)-Al(1)-C(19)
N(3)-Al(1)-O(1)
N(3)-Al(1)-O(1)′
O(1)-Al(1)-C(19)
O(1)-Al(1)-O(1)′
O(1)′-Al(1)-C(19)
C(20)-O(1)-Al(1)
94.81(6)
116.74(9)
101.03(6)
117.33(7)
98.95(7)
75.81(6)
100.71(8)
160.14(6)
125.66(8)
123.19(12)
Symmetry transformations used to generate equivalent atoms: ′ 1 – x, – y, 1 – z
As illustrated in Figure 4, the molecular structure of crystalline 4 is similar to 3, and selected
bond distances and angles are summarized in Table 4.
Fig. 4. Molecular structure of complex 4. Thermal ellipsoids are shown at 30% probability
level. Hydrogen atoms are omitted for clarity. (Symmetry codes: ′ – x, 1 – y, – z)

Table 4
Selected bond lengths (Å) and angles (°) for complex 4
N(1)-C(7)
1.341(2)
Al(1)-C(22)
Al(1)-O(1)
Al(1)-O(1)′
C(23)-O(1)
1.979(2)
N(2)-C(7)
1.326(2)
1.8330(14)
1.9223(13)
1.415(2)
N(3)-C(7)
1.384(2)
Al(1)-N(1)
1.9229(16)
2.0710(15)
111.68(16)
121.55(16)
126.77(16)
93.51(11)
87.54(10)
66.97(6)
Al(1)-N(2)
N(1)-C(7)-N(2)
N(1)-C(7)-N(3)
N(2)-C(7)-N(3)
C(7)-N(1)-Al(1)
C(7)-N(2)-Al(1)
N(1)-Al(1)-N(2)
N(1)-Al(1)-C(22)
N(1)-Al(1)-O(1)
N(1)-Al(1)-O(1)′
N(2)-Al(1)-C(22)
N(2)-Al(1)-O(1)
N(2)-Al(1)-O(1)′
O(1)-Al(1)-C(22)
O(1)-Al(1)-O(1)′
O(1)′-Al(1)-C(22)
C(23)-O(1)-Al(1)
99.19(6)
100.34(8)
96.97(6)
159.97(7)
123.06(8)
76.59(6)
99.00(8)
128.68(11)
117.87(9)
118.86(7)
Symmetry transformations used to generate equivalent atoms: ′ – x, 1 – y, – z
3.3. 1-4 as catalysts for the MPV reaction
According to the successful experiences [6c], benzaldehyde was selected as a model substrate,
and dry 2-propanol was employed as the reducing agent. As shown in Table 5, 1-4 were screened
as potential catalysts ([PhCHO]/[Al] = 10/1) for the MPV reduction of benzaldehyde. The
reduction of benzaldehyde was carried out in anhydrous toluene at 110 °C for 2 h under the
atmosphere of N2. The experimental results show that 1-4 exhibited moderate catalytic activity
(>69%) in the reduction of benzaldehyde under the above conditions. Compared with 2 and 4,
complex 1 or 3 as pre-catalyst in the reduction showed better activity, respectively (Table 5,
entries 1-4), which is attributed to the different substituents bound to N atoms in the complexes
1-4. Although both of complexes 1 and 3 have similar activities in MPV reaction, 3 is more
slightly air and moisture more stable than 1. So 3 was selected as pre-catalyst to initiate the
reduction of aldehyde or ketone. When increasing the molar ratio of 2-propanol to benzaldehyde
([PhCHO]/[iPrOH] = 1/2), the yield of phenylmethanol in the reduction using complex 3 as
catalyst improved evidently (Table 5, entry 5). In addition, in order to find the optimum reaction

time, the representative reaction was carried out in anhydrous toluene at 110 °C for different time
intervals, viz 2-8 h. The maximum yield was observed in 8 h, which was fixed as the optimum
time for other reactions (Table 5, entries 5-8). Employing 3 as a catalyst at 5 mol% ([PhCHO]/[Al]
= 10/1) loading, the yield of phenylmethanol in the MPV reduction decreased significantly (Table
5, entry 9).
Table 5 The MPV reaction of benzaldehyde using complexes 1-4 catalysts
Entry Catalyst [PhCHO]/[Al]/[iPrOH]
Time/h
Yield(%)a
2
2
2
2
2
4
6
8
8
1
2
3
4
5
6
7
8
9
10/1/15
10/1/15
10/1/15
10/1/15
10/1/20
10/1/20
10/1/20
10/1/20
10/0.5/20
78
69
79
70
87
92
96
99
88
1
2
3
4
3
3
3
3
3
aDetermined by 1H NMR spectroscopy.
With the optimized conditions in hand, we proceeded to explore the substrate scope of the
reduction with complex 3 as the model catalyst. The results are shown in Table 6. As expected, the
aromatic aldehydes with an electron-donating group on the phenyl ring gave lower yields than the
aldehyde with an electron-withdrawing group (Table 6, entries 1-2), which is probably due to the
increasing positive charge of the aldehyde group carbon atom caused by the electron-withdrawing
group on the para position. Except the aromatic aldehydes, three particular aliphatic aldehydes
were also reduced to corresponding alcohols under our catalytic system in 89-99% yields (Table 6,
entries 3-5). Due to the conjugative effect, the yield of citral and cinnamic aldehyde was
somewhat lower than that of citronellal. In addition, cyclohexanone and acetophenone were also
selected to examine the catalytic behavior of 3, respectively (Table 6, entries 6, 7). As a result,
compared to those asymmetric guanidinatoaluminum complexes as a catalyst at 10 mol% loading

for 2 h in the MPV reaction [6c], guanidinatoaluminum complex 3 were used in catalytic amounts
(5 mol%), and longer reaction time (8 h) was needed, which may be also attributed to different
substituents bound to the N atoms of guanidinato ligand. The result further demonstrated the types
of substituents bound to the N atoms greatly affect the electronegativities of nitrogen and
aluminum atoms in guanidinatoaluminum complexes. A proposed mechanism for the reduction of
aldehydes or ketones catalyzed by the complexes1-4 would be similar to the reaction catalyzed by
aluminum complexes chelated phenoxide ligands [14].
Table 6 The MPV reaction of aldehyde and ketone using complex 3 catalysta
Entry
1
Substrate
Product
Yield(%)b
89
2
3
>99
99
4
91
5
6
89
83
7
84
aReaction conditions: pre-catalyst (0.2 mmol), 2-propanol (8.0 mmol), aldehyde (4.0 mmol) in
toluene at 110 °C for 8 h;
bDetermined by 1H NMR spectroscopy.
4. Conclusions
In summary, four new guanidinatoaluminum complexes were synthesized and characterized by
satisfactory C, H and N microanalysis, 1H, 13C NMR spectra and single crystal X-ray structural

data. The guanidinatoaluminum complexes (1-4) used as pre-catalysts for the MPV reaction have
been studied and the complex 3 exhibited the best catalytic activities toward the reduction. Using
3 as a pre-catalyst at low (5 mol%) loading, a series of aldehydes and ketones are selectively
reduced to the corresponding alcohols in good yields.
Statement
I confirm that the reported work has not received prior publication and that no portion of this or
any other closely related work is under consideration for publication elsewhere in any medium,
including electronic journal, computer databases, and publicly accessible preprint Web sites.
Acknowledgments
Financial support from the National Natural Science Foundation of China (No.
20572065), Special Fund for Agro-scientific Research in the Public Interest (No.
201303106), Natural Science Foundation of Shanxi Province (No. 2011021011-1,
2014011049-26 and 201601D202091), and the Shanxi Coal Based Key Scientific and
Technological Project (No. MH2014-07) are gratefully acknowledged.
Appendix A. Supplementary data
CCDC 1515058, 1521422 and 1509440-1509441 contains the supplementary crystallographic
data for the complexes 1-4, 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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Synthesis and crystal structures of guanidinatoaluminum complexes and
catalytic study for MPV reduction
∗
Hong-Fei Han[a,b] , Zhi-Qiang Guo [c] , Shao-Feng Zhang [a] , Yu-Peng Hua [a] , Xue-Hong Wei[c]
a
The School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, P.R.
China
b Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, P. R. China
c Scientific Instrument Center, Shanxi University, Taiyuan 030006, P. R. China
* Corresponding author. Tel.: +86 351 7018390, Fax: +86-351-7011688;
E-mail address: xhwei@sxu.edu.cn (X. Wei).

Graphical Abstract - Synopsis
The synthesis of four guanidinatoaluminum complexes is described. The molecular structures of
four guanidinatoaluminuml complexes were confirmed by a single crystal X-ray diffraction study.
In addition, the guanidinatoaluminuml complexes were also used to catalyze the
Meerwein-Ponndorf-Verley (MPV) reaction of a series of aromatic and aliphatic aldehydes to the
corresponding alcohols.