Synthesis, structures, and reactivities of guanidinatozinc complexes and their catalytic behavior in the Tishchenko reaction

Article abstract of DOI:10.1021/om400345f

Treatment of a secondary amine (pipH, Bz₂NH, Et₂NH) with sequentially diethylzinc and N,N′-dicyclohexylcarbodiimide in hexane has afforded in good yield the new crystalline guanidinatozinc complexes. Each has been X-ray and solution NMR spectrally characterized. Three are dimeric (mono)guandinatozinc alkyls, two are dinuclear (tris)guanidinatozinc amides, and one is a homoleptic zinc bis(guanidinate). The reactions of dinuclear (tris)guanidinatozinc amides with diethylzinc and N,N′- dicyclohexylcarbodiimide in the molar ratios of 1:2:1 led to the dimeric (mono)guandinatozinc alkyls; homoleptic zinc bis(guanidinate) with an equimolar portion of diethylzinc also yielded dimeric (mono)guandinatozinc alkyls. Each of the complexes exhibited good to excellent catalytic activity for the solvent-free Tishchenko reaction under mild conditions.

Full text of DOI:10.1021/om400345f

Article
pubs.acs.org/Organometallics
Synthesis, Structures, and Reactivities of Guanidinatozinc Complexes
and Their Catalytic Behavior in the Tishchenko Reaction
Jie Li,† Jingchao Shi,† Hongfei Han,† Zhiqiang Guo,‡ Hongbo Tong,‡ Xuehong Wei,*,†,‡ Diansheng Liu,‡
and Michael F. Lappert*,§
†School of Chemistry and Chemical Engneering, Shanxi University, Taiyuan 030006, People’s Republic of China
‡Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, People’s Republic of China
§Department of Chemistry, University of Sussex, Brighton, BN1 9QJ, U.K.
S
* Supporting Information
ABSTRACT: Treatment of a secondary amine (pipH, Bz2NH,
Et2NH) with sequentially diethylzinc and N,N′-dicyclohexyl-
carbodiimide in hexane has afforded in good yield the new
crystalline guanidinatozinc complexes. Each has been X-ray
and solution NMR spectrally characterized. Three are dimeric
(mono)guandinatozinc alkyls, two are dinuclear (tris)-
guanidinatozinc amides, and one is a homoleptic zinc
bis(guanidinate). The reactions of dinuclear (tris)guanidinato-
zinc amides with diethylzinc and N,N′-dicyclohexylcarbodiimide
in the molar ratios of 1:2:1 led to the dimeric (mono)guandinatozinc alkyls; homoleptic zinc bis(guanidinate) with an equimolar
portion of diethylzinc also yielded dimeric (mono)guandinatozinc alkyls. Each of the complexes exhibited good to excellent catalytic
activity for the solvent-free Tishchenko reaction under mild conditions.
INTRODUCTION
■
The coordination and applied chemistry of the guanidinato
([RNC(NR′2)NR]−, R, R′ = alkyl, aryl, silyl, etc.) ligands is
extensive and has received much attention; there have been
several excellent and comprehensive reviews.1 Since the
preparation of the first transition metal guanidinate by Lappert’s
group in 1970,2 complexes involving metals from across the
periodic table have been synthesized and applied in many areas,
including synthesis,1 stabilization of complexes of low oxidation
state metals,1a materials science,3 and catalysis.4 The mono-
Figure 1. General formula of the guanidinate ligand, the two most
common ligand-to-metal(s) bonding modes, and resonance structures
of the guanidinate anions.
anionic chelating guanidinato ligands are related to those of
amidinates, triazinates, and carboxylates, each of which can
delocalize the charge and hence increase its potential for electron
donation to the metal; several coordination modes have been
R = Ar, R′ = I6b). Compound 1 was obtained from Zn-
observed, but by far the two most common are as N,N′-chelating
[N(SiMe3)]2 + CyNCNCy,5a 2 from ZnMe2 + Me2NC-
or -bridging ligands (Figure 1).1a
Although the chemistry of numerous metal guanidinates has
been much investigated, zinc guanidinates have been largely
(NPri)[N(H)Pri],5a and 3 from 2 and 2ArOH.5a From ZnI2 +
K[N(Ar)C(NPri2)NAr], the product was 4.6b
neglected. Only a few papers have been published, by Coles’5 or
Amidinato analogues of 1, 2, and 3 lately reported are Zn-
Jones’6 group. In their studies, the zinc guanidinate compounds
were synthesized by (i) the insertion of a carbodiimide into a
compound containing a Zn−NRR′ bond, (ii) the deprotonation
of a guanidine by dimethylzinc, or (iii) a salt-metathesis reaction
between a zinc halide and an alkali metal guanidinate. As recently
noted,6a the chemistry of zinc guanidinates (or their related
amidinates) is poorly developed. An early (2004) paper reported
on their use as catalysts in the ring-opening polymerization of
lactide.5a Guanidinatozinc complexes related to the present study
have been 1 (R = Pri or Cy),5a 2,5a and 3 (R = Pri, R′ = OAr;5a
[N(Ar)C(But)NAr]2 and Pri2N[Zn(l)N(Ar)C(H)N(Ar)Znl],
prepared from respectively ZnBr2/K[N(Ar)C(But)NAr] and
ZnI2/K[N(Ar)C(NPri2)NAr].6b A further zinc amidinate
skeletally related to 2 and to one of our new zinc guanidinates
is MeZn{N(Pei)C(Me)N(Ar)}Zn(Me){N(Pri)C(Me)NPri},
isolated from the reaction of ZnMe2 + Zn[N(Pri)C(Me)NPri]2;
Received: April 20, 2013
Published: June 20, 2013
© 2013 American Chemical Society
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Article
was added to the above reactant mixture, which was stirred for 8 h to
afford a cloudy solution, which was filtered. The filtrate was
concentrated to ca. 25 mL in vacuo and stored at −10 °C, yielding
colorless crystals of 6 (1.06 g, 65%) (found: C, 65.20; H, 9.75; N, 12.83;
C59H106N10Zn2 requires C, 65.23; H, 9.84; N, 12.89), mp 156−158 °C.
1H NMR (C6D6): δ (ppm) 0.86−0.88 (m, 12 H, C6H11), 0.91−0.95 (m,
2 H, C5H10N), 1.19 (m, 6 H, C5H10N), 1.48−1.67 (m, 24 H, C6H11),
1.77−1.97 (m, 4 H, C5H10N), 2.18−2.35 (m, 12 H, C5H10N), 2.53−2.69
(m, 24 H, C6H11), 3.18−3.32 (m, 12 H, C6H11), 3.65 (m, 6 H, C6H11),
4.13−4.15 (m, 4 H, C5H10N). 13C NMR (C6D6): δ (ppm) 24.62, 24.89,
25.18, 25.82, 26.26, 35.66, 36.39, 36.78, 48.53, 66.08 (C6H11), 31.85,
33.10, 34.27, 34.76, 48.19, 51.33, 52.04, 53.63, 54.11, 57.52 (C5H10N),
167.49 (NCN).
[{(PhCH2)2NC(NC6H11)2}ZnEt]2 (7). Method 1. To a stirred solution
of dibenzylamine (0.38 mL, 2.0 mmol) in hexane (40 mL) was added
diethylzinc (2.00 mL of a 1.0 M solution in hexane, 2.0 mmol). The
mixture was heated to 60 °C for 12 h and cooled to room temperature,
and N,N′-dicyclohexylcarbodiimide (0.41 g, 2.0 mmol) was added. The
mixture was stirred for 8 h to afford a cloudy solution, which was filtered.
The filtrate was concentrated to ca. 20 mL in vacuo and stored at −10 °C,
yielding colorless crystals of 7 (0.76 g, 76%) (found: C, 70.10; H, 8.28;
N, 8.37; C58H82N6Zn2 requires C,70.08; H, 8.31; N, 8.45), mp 167−
169 °C. 1H NMR (C6D6): δ (ppm) 0.70−0.76 (m, 4 H, ZnCH2CH3),
0.97 (m, 6 H, ZnCH2CH3), 1.17−1.25 (m, 8 H, C6H11), 1.42−1.55 (m,
16 H, C6H11), 1.67−1.73 (m, 16 H, C6H11), 3.31 (m, 4 H, C6H11), 3.99
(s, 8 H, PhCH2), 6.91−7.07 (m, 20 H, C6H5). 13C NMR (C6D6): δ
(ppm) 3.22 (ZnCH2CH3), 19.42 (ZnCH2CH3), 19.42, 30.94, 42.47,
57.89 (C6H11), 59.36 (C6H5CH2), 127.48, 140.06, 144.58, 154.49
(C6H5) 173.43 (NCN).
the latter zinc amidinate was the product of the reaction of ZnI2
and 2 Li[N(Pri)C(Me)NPri].7a
Herein, we report (i) six new zinc guanidinates synthesized
from one-pot reactions of a secondary amine, diethylzinc, and
N,N′-dicyclohexylcarbodiimide and (ii) their use as catalysts for
the homodimerization of an aromatic aldehyde to the
corresponding ester.
EXPERIMENTAL SECTION
■
General Remarks. All manipulations were carried out under dry
nitrogen using standard Schlenk and cannula techniques. Solvents were
dried with appropriate drying agents, degassed, and stored over a
potassium mirror or activated molecular sieves prior to use. CyNC
NCy (Alfa Aesar) and ZnEt2 (1.0 M solution in hexane; Alfa Aesar) were
obtained commercially and used as received. Piperidine, (PhCH2)2NH,
and Et2NH (Aldrich) were dried over KOH 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 at 298 K.
Elemental analyses were performed on a Vario EL-III instrument.
Melting points were taken on an electrothermal apparatus and are
uncorrected.
Method 2. A stirred suspension of compound 8 (0.76 g, 1.17 mmol),
N,N′-dicyclohexylcarbodiimide (0.24 g, 1.17 mmol) in hexane (50 mL),
and diethylzinc (2.34 mL of a 1.0 M solution in hexane, 2.34 mmol) was
stirred at room temperature for 8 h to afford a cloudy mixture, which was
filtered. The filtrate was concentrated to ca. 15 mL in vacuo and stored at
−15 °C, yielding colorless crystals of 7 (1.51 g, 65%).
[{(PhCH2)2NC(NC6H11)2Zn}2(μ-{(PhCH2)2NC(NC6H11)2}2)(μ-N-
(PhCH2)2)] (8). To a stirred solution of dibenzylamine (0.38 mL,
2.0 mmol) in hexane (40 mL) was added diethylzinc (1.00 mL of a 1.0 M
solution in hexane, 1.0 mmol), and the mixture was heated to 60 °C for
12 h, then cooled to room temperature. N,N′-Dicyclohexylcarbodiimide
(0.31 g, 1.5 mmol) was added, and the mixture was stirred for 8 h to
afford a cloudy solution, which was filtered. The volatiles were removed
from the filtrate in vacuo, and the residue was extracted with diethyl
ether; the extract was filtered. The filtrate was concentrated and stored at
−10 °C, yielding colorless cubic crystals of compound 8 (0.60 g, 78%)
(found: C, 74.42; H, 8.25; N, 9.07; C95H122N10Zn2 requires C,74.34; H,
8.01; N, 9.13), mp 155−156 °C. 1H NMR (C6D6): δ (ppm) 0.89−0.91
(m, 12 H, C6H11), 1.48−1.66 (m, 24 H, C6H11), 1.83−1.87 (m, 24 H,
C6H11), 3.38−3.40 (m, 6 H, C6H11), 4.12 (s, 16 H,PhCH2), 7.08−7.11
(m, 16 H, ArH), 7.18−7.21 (m, 8 H, ArH), 7.25−7.28 (m, 16 H, ArH).
13C NMR (C6D6): δ (ppm) 26.62, 26.74, 35.36, 36.30, 52.33, 54.94,
Preparations. [{C5H10NC(NC6H11)2}ZnEt]2 (5). Method 1. To a
stirred solution of piperidine (0.60 mL, 6.0 mmol) in hexane (60 mL)
was added diethylzinc (6.00 mL of a 1.0 M solution in hexane,
6.0 mmol). The mixture was heated to 60 °C for 12 h and then cooled to
room temperature. N,N′-Dicyclohexylcarbodiimide (1.24 g, 6.0 mmol)
was added. The mixture was stirred for 8 h to afford a cloudy solution,
which was filtered. The filtrate was concentrated to ca. 20 mL in vacuo
and stored at −10 °C, yielding colorless crystals of 5 (1.85 g, 80%)
(found: C, 62.34; H, 9.71; N, 10.86; C40H74N6Zn2 requires C, 62.41;
H, 9.69; N, 10.92), mp 194 °C (dec). 1H NMR (C6D6): δ (ppm): 0.84
(q, J = 8.4, 4 H, ZnCH2CH3), 1.14 (t, J = 11.4, 6 H, ZnCH2CH3), 1.29−
1.34 (m, 8 H, C6H11), 1.38−1.46 (m, 4 H, C5H10N), 1.56−1.60 (m, 16
H, C6H11), 1.72−1.76 (m, 16 H, C6H11), 1.92−1.94 (m, 8 H, C5H10N),
3.20 (m, 8 H, C5H10N), 3.30 (m, 4 H, C6H11). 13C NMR (C6D6): δ
(ppm) −9.72 (ZnCH2CH3), 6.59 (ZnCH2CH3), 17.09, 18.12, 29.27,
46.31 (C6H11), 18.39, 18.53, 41.89 (C5H10N), 160.72 (NCN).
Method 2. A stirred suspension of compound 6 (0.84 g, 0.77 mmol),
N,N′-dicyclohexylcarbodiimide (0.16 g, 0.77 mmol) in hexane (40 mL),
and diethylzinc (1.54 mL of a 1.0 M solution in hexane, 1.54 mmol) was
stirred at room temperature for 8 h to afford a cloudy mixture, which was
filtered. The filtrate was concentrated to ca. 15 mL in vacuo and stored at
−10 °C, yielding colorless crystals of 5 (0.89 g, 75%).
[{C5H10NC(NC6H11)2Zn}2(μ-{C5H10NC(NC6H11)2}2)(μ-NC5H10)] (6).
To a stirred solution of piperidine (0.60 mL, 6.0 mmol) in hexane
(60 mL) was added diethylzinc (3.00 mL of a 1.0 M solution in hexane,
3.0 mmol), and the solution was heated to 60 °C for 12 h, then cooled to
room temperature. N,N′-Dicyclohexylcarbodiimide (0.93 g, 4.5 mmol)
57.97, 60.29 (C6H11), 67.07 (PhCH2), 128.50, 129.55, 140.59, 155.77
(C6H5), 164.86 (NCN).
[{(C2H5)2NC(NC6H11)2}ZnEt]2 (9). Method 1. To a stirred solution of
diethylamine (0.20 mL, 2.0 mmol) in hexane (40 mL) was added
diethylzinc (2.00 mL of a 1.0 M solution in hexane, 2.0 mmol), and the
mixture was heated to 60 °C for 12 h, then cooled to room temperature.
N,N′-Dicyclohexylcarbodiimide (0.41 g, 2.0 mmol) was added. The
mixture was stirred for 10 h, then filtered. The filtrate was concentrated
to ca. 20 mL in vacuo and stored at −10 °C, yielding colorless crystals of
9 (0.6g, 80%) (found: C, 61.25; H, 9.95; N, 11.32; C38H74N6Zn2
1
requires C,61.20; H, 10.00; N, 11.27), mp 148−150 °C. H NMR
(C5D5N): δ (ppm) 0.97 (q, J = 7.9, 4 H, ZnCH2CH3), 1.09 (t, J = 6.9,
6 H, ZnCH2CH3), 1.15−1.23 (m, 8 H, C6H11), 1.28−1.58 (m,
16 H, C6H11), 1.70−1.73 (m, 16 H, C6H11), 1.87−1.89 (m, 12 H,
N(CH2CH3)2), 3.15−3.17 (m, 8 H, N(CH2CH3)2), 3.22 (m, 4 H,
C6H11). 13C NMR (C5D5N): δ (ppm) 6.69 (ZnCH2CH3), 22.37
(ZnCH2CH3), 22.97 (N(CH2CH3)2), 62.88 (N(CH2CH3)2), 22.37,
34.62, 45.86, 51.92 (C6H11), 176.91 (NCN).
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Method 2. A stirred suspension of compound 10 (0.62 g, 1.0 mmol)
in hexane (40 mL) and diethylzinc (1.0 mL of a 1.0 M solution in
hexane, 1.0 mmol) was stirred at room temperature for 8 h to afford a
cloudy mixture, which was filtered. The filtrate was concentrated to ca.
15 mL in vacuo and stored at −15 °C, yielding colorless crystals of 9
(0.66 g, 89%).
Scheme 1. Synthetic Routes to Compounds 5−10
[Zn{Et2NC(NCy)2}2] (10). To a stirred solution of diethylamine
(0.2 mL, 2.0 mmol) in hexane (40 mL) was added diethylzinc (1.00 mL
of a 1.0 M solution in hexane, 1.0 mmol), and the mixture was heated
to 60 °C for 12 h and then cooled to room temperature. N,N′-
Dicyclohexylcarbodiimide (0.41 g, 2.0 mmol) was added. The mixture
was stirred for 10 h, then filtered. The filtrate was concentrated to 20 mL
in vacuo and stored at −10 °C, yielding colorless crystals of 10 (0.43 g,
69%) (found: C, 65.7; H, 10.42; N, 13.43; C34H64N6Zn requires C,
1
65.62; H, 10.37; N, 13.51), mp 128−130 °C. H NMR (C5D5N): δ
(ppm) 1.05 (t, J = 6.9, 12 H, N(CH2CH3)2), 1.25−1.29 (m, 8 H, C6H11),
1.44−1.65 (m, 16 H, C6H11), 2.12−2.16 (m, 16 H, C6H11), 3.18 (q, J =
6.9, 8 H, N(CH2CH3)2), 3.26−3.29 (m, 16 H, C6H11). 13C NMR
(C5D5N): δ (ppm) 1.89 (N(CH2CH3)2), 14.12, 27.00, 37.90, 43.78
(C6H11), 55.30 (N(CH2CH3)2), 170.39 (NCN).
Typical Procedure Employed for the Tishchenko Reaction.
The appropriate zinc complex (0.5 mmol) was placed in a dry Schlenk
flask with a stirring bar under an argon atmosphere, and freshly distilled
aldehyde (10 mmol) was introduced. The reaction mixture was heated
at 80 °C for 12 h. The reaction was monitored by TLC until complete
consumption of aldehyde. The product was isolated by column
chromatography on silica gel (hexane/ether = 1:15 as eluent).
X-ray Crystallography. Crystallographic measurements were
performed with Mo Kα radiation (λ = 0.71073 Å) on a Bruker Smart
Apex CCD diffractometer. Crystals were coated in oil and then directly
mounted on the diffractometer under a stream of cold nitrogen gas. A
total of N reflections was collected using the ω scan mode. Corrections
were applied for Lorentz and polarization effects as well as absorption
using multiscans (SADABS).8 Each structure was solved by direct
methods and refined on F2 by full matrix least-squares (SHELX97)9
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
mode (SHELXTL).10 Details of the modeling of disorder in the crystals
can be found in their CIF files.
was prepared as colorless crystals in moderate yield by changing
the molar ratios of N,N′-dicyclohexylcarbodiimide, diethylamine,
and diethylzinc in hexane to 2:2:1. Each of 5, 6, 7, 8, 9, and 10 was
easily purified by crystallization from hexane at −10 °C and was
characterized by satisfactory C, H, and N microanalysis, 1H and
13C{1H} NMR spectra in C6D6 at ambient temperature, and
single-crystal X-ray structural data.
Reactivities of 6, 8, or 10 with Diethylzinc and N,N′-
Dicyclohexylcarbodiimide or Diethylzinc. Consistent with
the chemical formula of the (mono)guanidinatozinc alkyl
complex 5 or 7, the dinuclear (tris)guanidinatozinc amide 6 or
8 was smoothly transformed to 5 or 7 by treatment of 6 or 8 with
the carbodiimide and diethylzinc in relative molar proportions of
1:1:2 in hexane. Treatment of 10 with an equivalent portion of
diethylzinc in hexane yielded 9 in 89% yield.
X-ray Single-Crystal Structures of 5−10. The molecular
structure of the fused tricyclic boat-shaped crystalline compound
5, having a 2-fold rotation axis through the midpoint of the
central slightly puckered Zn1N3Zn1′N3′ ring, is illustrated as an
ORTEP diagram in Figure 2 and is closely similar (except for the
NR2 groups) to that of 2. Selected geometrical parameters are
listed in Table 1. The two guanidinate ligands with Zn1 and Zn1′
adopt a κ1N−κ1,2-N′ bonding mode with a central Zn2N2 ring and
a tricyclic structure as the previously reported 2.5a The Zn1−N2
(2.029 Å) and Zn1−N3′ (2.068 Å) bond distances are shorter than
those in 2 (2.063, 2.101 Å), respectively, while the bond distance of
Zn1−N3 (2.314 Å) is much longer than that of 2 (2.201 Å).
The molecular structure of the crystalline compound 6 is
shown in an ORTEP representation in Figure 3. At its core is the
Zn1N4C19N6Zn2N10 six-membered ring, in which relative to
the N4C19N6 plane the atoms Zn1, Zn2, and N10 are 0.921 Å
above, 0.766 Å below, and 0.238 Å above, respectively; dihedral
angles between adjacent planes are as follows: N4C19N6/
N1C1N2Zn1, 80.39°; N4C19N6/N7C37N9Zn2, 89.22°;
N1C1N2Zn1/N7C37N9Zn2, 40.13°; N4C19N6/Zn1N10Zn2,
33.83°; N1C1N2Zn1/Zn1N10Zn2, 81.31°; N7C37N9Zn2/
Zn1N10Zn2, 69.44°. Selected bond lengths and angles are listed
in Table 2. The above ring with its pendant C19−N4 and
C19−N6 bonds is similar to the corresponding feature in 3,5a but
the two structures differ in their substituents and the zinc co-
ordination number (four in 5 but three in 3).
RESULTS AND DISCUSSION
■
Synthesis of Zinc Guanidinate Complexes 5−10. A one-
pot reaction route has been employed in the synthesis of zinc
guanidinate complexes. As shown in Scheme 1, the stoichio-
metric reaction of a secondary amine, diethylzinc, and N,N′-
dicyclohexylcarbodiimide in hexane was first investigated.
Hence, when piperidine, dibenzylamine, or diethylamine was
employed as the secondary amine, the (mono)guanidinatozinc
alkyl complex was obtained in good yield (80% for 5, 65% for 7,
and 80% for 9), respectively. It was evident that the insertion of a
molecule of carbodiimide into the Zn−N bond is easier than that
of the Zn−C bond. Interestingly, the minor dinuclear (tris)-
guanidinatozinc amide [{C5H10NC(NC6H11)2Zn}2(μ-
{C5H10NC(NC6H11)2}2)(μ-NC5H10)] (6) was formed when
piperidine was used. This spurred us to explore the reaction in
different molar ratios of starting materials. The results show that
the reaction products depend extremely on the molar ratios of
the amine, diethylzinc, and carbodiimide. Thus, the colorless
crystals of the complex 6 or 8 were obtained in moderate or good
yield from diethylzinc and successive portions of piperidine or
dibenzylamine and N,N′-dicyclohexylcarbodiimide in relative
molar ratios of 1:2:1.5, respectively. Attempts to prepare the
corresponding dinuclear (tris)guanidinatozinc amide based on
diethylamine under the same condition were unsuccessful.
However, the homoleptic, bis(guanidinato)zinc complex (10)
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Figure 3. Molecular structure of [{C5H10NC(NC6H11)2Zn}2(μ-
{C5H10NC(NC6H11)2}2)(μ-NC5H10)] (6) with thermal ellipsoids
drawn at the 25% probability level; hydrogen atoms are omitted for
clarity.
Figure 2. Molecular structure of [{C5H10NC(NC6H11)2}ZnEt]2 (5)
with thermal ellipsoids drawn at the 25% probability level; hydrogen
atoms are omitted for clarity.
Table 2. Selected Bond Lengths [Å] and Angles [deg] for 6
Zn1−N1
2.124(2)
2.008(2)
2.061(2)
1.998(2)
1.344(4)
1.338(4)
1.318(4)
110.92(10)
110.19(9)
100.72(10)
121.3(2)
89.10(17)
113.8(3)
88.86(18)
Zn1−N2
2.056(2)
2.013(2)
2.116(2)
2.020(2)
1.338(4)
1.332(4)
1.333(4)
64.69(9)
64.65(9)
120.0(3)
120.40(19)
92.21(18)
91.66(18)
114.8(3)
Zn1−N4
Zn1−N10
a
Table 1. Selected Bond Lengths [Å] and Angles [deg] for 5
Zn2−N7
Zn2−N9
Zn2−N6
Zn2−N10
Zn1−C19
Zn1−N3′
Zn1−C1
N1−C1
N1−C2
N2−C7
N3−C13
C19−Zn1−N2
N2−Zn1−N3′
N2−Zn1−N3
C19−Zn1−C1
N3′−Zn1-C1
C1−N1−C6
C6−N1−C2
C1−N2−Zn1
C1−N3−C13
C13−N3−Zn1′
C13−N3−Zn1
N2−C1−N3
N3−C1−N1
N3−C1−Zn1
N1−C2−C3
N2−Zn1−Zn1′
N3−Zn1−Zn1′
1.979(4)
2.068(2)
2.577(3)
1.384(3)
1.471(4)
1.464(3)
1.484(3)
120.25(15)
111.13(9)
62.23(8)
127.11(15)
107.36(9)
121.5(2)
113.0(2)
98.60(17)
119.3(2)
114.02(16)
126.17(17)
113.7(2)
122.4(2)
63.34(14)
111.8(2)
84.56(6)
43.19(6)
Zn1−N2
Zn1−N3
Zn1−Zn1′
N1−C6
N2−C1
N3−C1
N3−Zn1′
C19−Zn1−N3′
C19−Zn1−N3
N3′−Zn1-N3
N2−Zn1−C1
N3−Zn1−C1
C1−N1−C2
C1−N2−C7
C7−N2−Zn1
C1−N3−Zn1′
C1−N3−Zn1
Zn1′−N3−Zn1
N2−C1−N1
N2−C1−Zn1
N1−C1−Zn1
C19−Zn1−Zn1′
N3′−Zn1−Zn1′
C1−Zn1−Zn1′
2.029(2)
2.314(2)
3.0171(9)
1.459(4)
1.314(3)
1.382(3)
2.068(2)
124.74(15)
127.26(17)
93.16(9)
30.27(9)
32.24(8)
121.9(2)
125.1(2)
136.21(19)
119.24(16)
84.42(15)
86.82(9)
123.8(2)
51.13(13)
171.67(19)
147.66(17)
49.99(7)
64.20
N4−C19
N6−C19
N1−C1
N2−C1
N7−C37
N9−C37
N4−Zn1−N10
N6−Zn2−N10
Zn1−N10−Zn2
C19−N4−Zn1
C1−N1−Zn1
N2−C1−N1
C37−N9−Zn2
N2−Zn1−N1
N7−Zn2−N9
N6−C19−N4
C19−N6−Zn2
C1−N2−Zn1
C37−N7−Zn2
N7−C37−N9
a
Symmetry elements for 5: ′ −x+3/2, −y+1/2, z.
The molecular structure of the crystalline compound 7 is
shown as an ORTEP diagram in Figure 4, with selected bond
lengths and angles in Table 3. The two guanidinate ligands with
Zn and Zn′ present a κ1N−κ2-N bonding mode to form an eight-
membered-ring core structure. The transannular Zn−Zn′
distance is 2.857 Å (compared with 3.0171 Å in 5), and Zn or
Zn′ is 0.713 or 0.785 Å out of the N2C1N1 plane (compared
with 0.355 or 1.738 Å out of the N2C1N3 plane of 5).
The molecular structure of the crystalline compound 8 is
shown as an ORTEP diagram in Figure 5. Apart from the
exocyclic substituents at the carbon atoms C1, C28, and C55
(corresponding to C1, C19, and C37 in 6) and nitrogen atoms
Figure 4. Molecular structure of [{Bz NC(NC6H11)2}ZnEt]2 (Bz =
PhCH2) (7) with thermal ellipsoids drawn at the 25% probability level;
hydrogen atoms are omitted for clarity.
2
N3 and N4 (corresponding to N4 and N6 in 6), its geometry is
closely similar to that of 6. Selected bond lengths and angles are
listed in Table 4.
The molecular structure of the crystalline compound 9 is
shown in Figure 6 with selected data in Table 5. The core of the
centrosymmetric structure of 9 is the puckered eight-membered
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Table 3. Selected Bond Lengths [Å] and Angles [deg] for 7
Zn−N2′
N1−C1
N2−C(8)
N3−C23
Zn−Zn′
N2−Zn′
N2′−Zn−N1
N1−Zn−C14
C1−N1−Zn
C1−N2−C8
C8−N2−Zn′
C1−N3−C16
N1−C1−N2
N2−C1−N3
N2′−Zn−Zn′
N1−C2−C3
1.996(2)
1.320(3)
1.482(4)
1.455(4)
2.8570(12)
1.996(2)
111.74(10)
119.20(12)
112.58(17)
119.6(2)
112.98(17)
122.0(2)
118.6(2)
120.3(2)
69.86(8)
109.4(2)
Zn−N1
N1−C2
N3−C1
N3−C16
Zn−C14
N2−C1
N2′−Zn−C14
C1−N1−C2
C2−N1−Zn
C1−N2−Zn′
C1−N3−C23
C23−N3−C16
N1−C1−N3
N1−C2−C7
N1−Zn−Zn′
C14−Zn−Zn′
1.985(2)
1.464(3)
1.397(3)
1.468(4)
1.990(3)
1.357(4)
129.01(12)
124.7(2)
122.64(17)
127.33(19)
120.9(2)
116.3(2)
121.1(2)
111.2(2)
83.93(7)
116.45(11)
Figure 6. Molecular structure of [{C5H10NC(NC6H11)2}ZnEt]2 (9)
with thermal ellipsoids drawn at the 25% probability level; hydrogen
atoms are omitted for clarity.
a
Table 5. Selected Bond Lengths [Å] and Angles [deg] for 9
Zn1−C18
Zn1−N1
N1−C7
N2−C7′
N3−C7
N3−C16
C18−Zn1−N2
N2−Zn1−N1
N2−Zn1−Zn1′
C7−N1−C1
C1−N1−Zn1
C7′−N2−Zn1
C7−N3−C14
C14−N3−C16
N1−C1−C6
1.949(3)
1.964(3)
1.321(3)
1.299(3)
1.383(3)
1.435(4)
122.02(12)
108.71(10)
83.60(6)
120.5(2)
113.75(16)
116.26(17)
121.5(2)
117.6(2)
110.4(2)
Zn1−N2
Zn1−Zn1′
N1−C1
N2−C8
N3−C14
1.954(2)
2.742(2)
1.456(3)
1.452(3)
1.434(4)
C18−Zn1−N1
C18−Zn1−Zn1′
N1−Zn1−Zn1′
C7−N1−Zn1
C′−N2−C8
C8−N2−Zn1
C7−N3−C16
N1−C1−C2
129.26(10)
110.08(9)
74.34(7)
125.73(15)
123.8(2)
119.97(15)
119.9(2)
110.3(2)
a
Symmetry elements for 9: ′ −x+2, y, −z+3/2.
Figure 5. Molecular structure of [{Bz2NC(NC6H11)2Zn}2(μ-{Bz2NC-
(NC6H11)2}2)(μ-NBz2)] (Bz = PhCH2) (8) with thermal ellipsoids
drawn at the 25% probability level; hydrogen atoms are omitted for
clarity.
Table 4. Selected Bond Lengths [Å] and Angles [deg] for 8
Zn1−N1
Zn1−N3
Zn2−N5
Zn2−N4
N4−C28
N1−C1
N5−C5
N3−Zn1−N10
N4−Zn2−N10
Zn1−N10−Zn2
C28−N3−Zn1
C1−N1−Zn1
N2−C1−N1
C55−N6−Zn2
2.106(3)
2.008(3)
2.129(3)
1.995(3)
1.333(5)
1.334(5)
1.331(5)
110.85(14)
106.94(13)
102.39(17)
123.3(2)
89.1(2)
Zn1−N2
Zn1−N10
Zn2−N6
Zn2−N10
N3−28
N2−C1
N6−C55
N2−Zn1−N1
N5−Zn2−N6
N3−C28−N4
C28−N4−Zn2
C1−N2−Zn1
C55−N5−Zn2
N5−C55−N6
2.057(3)
2.007(3)
2.033(3)
2.021(4)
1.343(5)
1.328(5)
1.332(5)
65.02(12)
65.08(12)
119.6(3)
121.9(3)
91.5(2)
Figure 7. Molecular structure of [Zn{Et2NC(NCy)2}2] (10) with
thermal ellipsoids drawn at the 25% probability level; hydrogen atoms
are omitted for clarity.
114.4(3)
92.3(2)
88.1(2)
114.5(3)
Zn1N1C7N2′Zn1′C7N2 ring, each constituent atom of which is
in a distorted trigonal planar environment, which is closely
similar to that of 7.
The molecular structure of the crystalline compound 10 is shown
in Figure 7, with selected data in Table 6. In the mononuclear
complex 10, the zinc atom is in a distorted tetrahedral environment
and is the spiro junction of the two almost isostructural planar four-
membered rings ZnN1C7N2 and ZnN4C24N5. The endocyclic
angles at each of these rings decrease in the sequence N1C7N2
[112.5(2)°] > ZnN1C7 ≈ ZnN2C7 > N1ZnN2 [66.89(8)°].
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conditions (Table 7, entries 1−6). The activity of the complex 6
or 8 was much lower than that of the complex 5, 7, 9, or 10. This
is attributed to the highly crowded environment of the zinc atom
in complex 6 or 8, which decreased the attack probability of
benzaldehyde. Complex 9 at 2.5 mol % or 1 mol % loading gave a
93% or 80% yield, respectively, of benzyl benzoate under the
same conditions (Table 7, entries 7, 8). [The dimeric compound
names (5, 7, and 9) discussed here were still used, but their
solution states should be monomeric, and the 13CNMR data are
consistent with the monomeric solution structure.]
Table 6. Selected Bond Lengths [Å] and Angles [deg] for 10
Zn1−N1
2.0168(19)
2.0336(19)
1.338(3)
Zn−N2
2.0170(19)
2.011(2)
1.335(3)
1.323(3)
66.72(8)
90.30(14)
90.66(15)
113.4(2)
Zn1−N4
Zn1−N5
C7−N1
C7−N2
C24−N4
1.338(3)
C24−N5
N1−Zn1−N2
Zn1−N1−C7
Zn1−N4−C24
N1−C7−N2
66.89(8)
N4−Zn1−N5
Zn1−N2−C7
Zn1−N5−C24
N4−C24−N5
90.23(14)
89.27(15)
112.5(2)
In view of the above results, a variety of aromatic and
heteroaromatic aldehydes were examined, using 9 as a catalyst
at 5 mol % loading (Table 7, entries 9−15). Except for
2-pyridylaldehyde and furfural (entries 14 and 15), complex 9
catalyzed the conversion of the chosen aryl aldehyde to its
corresponding ester with good to excellent activities (Table 7,
entries 9−13).
The structure of 10 is similar to that of 15a except for the
substituents at the nitrogen atoms [NC6H11 and NEt2 in 10 and
NPri or NCy and N(SiMe3)2 in 15a].
Tishchenko Reaction of Aromatic Aldehydes Cata-
lyzed by Guanidinatozinc Complexes. The Tishchenko
reaction (eq 1), which has been known for more than a century, is
CONCLUSION
■
The synthesis and structures of six new crystalline guanidinato-
zinc complexes, 5−10, is described. Each was obtained from a
secondary amine (A), diethylzinc (B), and N,N′-dicyclohex-
ylcarbodiimide (C). The relative proportions of A:B:C were
1:1:1 (5, 7, and 9), 4:2:3 (for 6 and 8), or 2:1:2 (10). Treatment
of 6 or 8 with diethylzinc and N,N′-dicyclohexylcarbodiimide in a
molar ratio of 1:2:1 also afforded 5 or 7, respectively. Compound
9 was also obtained from 10 and an equivalent portion of
diethylzinc. Whereas 10 is mononuclear, the others contain two
zinc atoms. Compounds 6, 8, and 10 have two terminal
guanidinato ligands, while the corresponding bridging ligands are
found in 6 and 8 (one only) and 5, 7, and 9 (two each). The
coordination number of zinc is four, but compounds 5 and 7 are
three-coordinate. Compounds 6 and 8 are tricylic, with a central
six-membered ring; the core of 5, 7, and 9 is a boat-shaped
octacycle. All of the complexes exhibited good (6, 8) to excellent
(5, 7, 9, and 10) catalytic activities for the solvent-free
Tishchenko reaction under mild conditions.
an atom-efficient method for the synthesis of symmetric esters
from the dimerization of the corresponding aldehydes. Since
then, the Tishchenko reaction has been used as an efficient
method for the preparation of dimeric esters in industry, but
development of efficient catalysts is crucial. A number of catalysts
for the Tishchenko reaction have been reported, including metal,
metalloid, and nonmetal compounds, such as Na, Mg, Al, Ca, Ba,
Sr, B, Fe, Ru, Os, Hf, Zr, La, Y, and Sc.11 Recently, a magnesium
guanidinate compound,12 Th(IV) compounds,13 Ni(0)/NHC,14
including an organic compound thiolate,15 and selenide ions16
have been reported as effective catalysts in the Tishchenko
reaction. But, to the best of our knowledge, there have been no
reports on zinc compounds used as the catalyst of the
Tishchenko reaction.
Benzaldehyde was selected as a model substrate, and the guani-
dinatozinc complexes 5−10 were screened as potential catalysts
for the Tishchenko reaction (eq 1); the results are summarized in
Table 7. Good to excellent yields (78−98%) were obtained when
ASSOCIATED CONTENT
* Supporting Information
■
S
Table 7. Tischenko Reaction Catalyzed by Zinc Complexes
NMR data (S1), crystal data details of data collection and
refinements (S2), and crystallographic information files (CIF)
for 5−10 (CCDC 838523−838528) (S3). This material is
available free of charge via the Internet at http://pubs.acs.org.
a
5−10
entry catalyst loading (mol %)
substrate
yield (%)
1
5
6
7
8
9
10
9
9
9
9
9
9
9
9
9
5
5
5
5
5
5
2.5
1
5
5
5
5
5
5
5
benzaldehyde
95
80
92
78
98
96
93
80
98
98
96
88
75
54
35
2
benzaldehyde
AUTHOR INFORMATION
Corresponding Author
*E-mail: xhwei@sxu.edu.cn; m.f.lappert@sussex.ac.uk. Tel:
0086-15536566161. Fax: 0086-351-7011688.
■
3
benzaldehyde
4
benzaldehyde
5
benzaldehyde
6
benzaldehyde
7
benzaldehyde
Notes
8
benzaldehyde
The authors declare no competing financial interest.
9
4-Br-benzaldehyde
4-Cl-benzaldehyde
2-Cl-benzaldehyde
4-Me-benzaldehyde
4-MeO-benzaldehyde
2-pyridylaldehyde
furfuraldehyde
10
11
12
13
14
15
ACKNOWLEDGMENTS
■
We thank the NSFC (No. 20572065), SXNSFC (2008011021;
2008012013-2; 2011021011-1), and Research Project Supported
by Shanxi Scholarship Council of China for financial support.
REFERENCES
■
a
Reaction conditions: 80 °C, 12 h, solvent-free.
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