A new chiral N-heterocyclic carbene silver(i) cylinder: Synthesis, crystal structure and catalytic properties

Article abstract of DOI:10.1039/c000793e

A new chiral cyclic triimidazoline salt and its chiral N-heterocyclic carbene trisilver(i) cylinder-like cage was prepared via self-assembly, with size-selective catalytic performance for the cyanosilylation of several Schiff-base compounds.

Full text of DOI:10.1039/c000793e

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A new chiral N-heterocyclic carbene silver(I) cylinder: synthesis,
crystal structure and catalytic propertiesw
Dehui Wang,a Bingguang Zhang,*b Cheng He,a Pengyan Wua and Chunying Duan*a
Received 12th January 2010, Accepted 4th May 2010
First published as an Advance Article on the web 20th May 2010
DOI: 10.1039/c000793e
A new chiral cyclic triimidazoline salt and its chiral N-hetero-
cyclic carbene trisilver(I) cylinder-like cage was prepared via
self-assembly, with size-selective catalytic performance for the
cyanosilylation of several Schiff-base compounds.
triimidazoline salt (CNHC)(BF4)3 (Scheme 1).z The bowl-
shaped chiral CNHC was synthesized from the chiral reactant
R,R-1,2-diamino hexane. 1H NMR spectrum of CNHC exhibited
only one set of signals corresponding to imidazoline protons at
about 9.15 ppm,12 indicating the presence of only one configu-
ration of the three imidazoline groups. API-MS spectrum
showed intense peaks at 227.4, 384.4 and 855.4, corresponding
to the [CNHC]3+, [(CNHC)(BF4)]2+ and [(CNHC)(BF4)2]+
species, respectively. X-ray structure analysis had unequivo-
cally confirmed the existence of a tricationic imidazolidinium.
The macrocycle triimidazoline salt crystallized in a chiral P63
The coordination paradigm pioneered by Lehn and Sauvage,1
and as developed in the groups of Stang, Fujita, Raymond and
others for closed systems, has allowed the synthesis of an
enormous number and variety of discrete, isolable structures
featuring well-defined nanoscale cavities.2 These capsules are
invariably comprised of many metal–ligand components,
which often self-assemble rapidly and in high yield into a
single gigantic species—sometimes even protein-size. Like
natural enzyme pockets,3 such nanovessels provide a very
specific environment in terms of size, shape, and chemical
properties around a bound guest, which not only dictates the
selectivity of guest binding, but can also be exploited to
control the structure or the chemical reactivity of encapsulated
species.4 Accordingly, the incorporation of chiral organo-
metallic precursors as building blocks for the creation of optical
active metallorganic polyhedrons will lead to stereochemical
selective catalytic performance within.5
ꢀ
space group. One-third of the imidazoline cation, one BF4
anion, as well as one-third of a water molecule and one third of
an acetonitrile molecule were found in an asymmetry unit. The
CNHC cation had a crystallographic C3 axis across the centre
of the cyclic cation. The three cationic imidazoline units were
symmetrically related and consequently all of these optical
active carbon atoms were present in a R-configuration. An
imidazoline N–C bond length of 1.31 A on average and
N–C–N angle of 113.5(4)1 were consistent with those in related
compounds.13 The upper-rim of the CNHC cation was com-
posed by three active imidazoline protons, whereas the lower-
rim of the cage was composed by three cyclohexanes. The
diameters of the upper and bottom rims were 0.74 nm and
0.62 nm, respectively.
On the other hand, the field of metal complexes of N-hetero-
cyclic carbenes (NHC) has grown explosively in the last two
decades and numerous studies have been reported owing to
their applications in homogeneous and asymmetric catalyses.6
For example, Grubbs’ second generation catalyst is well
known to display an incredible application in olefin metathesis,
which is a powerful tool in organic synthesis and polymer
chemistry.7 Au(I), Rh(I), Ir(I), Cu(I), Pd(II), Ni(II) complexes of
NHC as high-efficiency catalysts have also been reported.6b,8
Silver NHC complexes have played an important role in the
development of metal-carbene systems.9 But few reports have
explored the applications of silver NHCs in asymmetric
catalysis.10
Further reaction of the chiral macrocycle triimidazoline
salt with 3 equivalents of Ag2O in DMSO solution afforded
the complex Ag3(CNHC)2(BF4)3 in a good yield. 1H NMR
spectrum of the complex revealed the absence of the imidazolidine
protons in the CNHC compound during the reaction. Single
crystal structural analysis revealed the silver N-heterocyclic
carbene cylinder consisted of a trisilver(I) chiral cationic cage.
The cylinder also crystallized in a chiral P63 space group. An
asymmetry unit was comprised of one-third of the cationic
ꢀ
cage, one BF4 anion, one lattice water molecule and two-
third of lattice dichloromethane molecules. The two CNHC
N-heterocyclic carbene ligands with the same geometrical
As a continuation of our research work on the imidazoline-
based robust crypt and metal-tunable nanocages,11 we report
here the synthesis, crystal structure and catalytic properties of
a novel chiral N-heterocyclic carbene silver(I) cage Ag–CNHC
assembled from a new positively charged chiral macrocycle
a State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Dalian, 116012, China. E-mail: cyduan@dlut.edu.cn
b Key Laboratory of Catalysis and Materials Science of the State
Ethic Affairs Commission & Ministry of Education, South-Central
University for Nationalities, Wuhan 430074, China
w Electronic supplementary information (ESI) available: Crystal data
in CIF files, experimental details and additional spectroscopic data.
CCDC 761389 and 761390. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c000793e
Scheme 1 Synthetic procedure of the chiral macrocycle triimidazoline
salt CHNC and its silver cage Ag–CNHC.
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Fig. 2 ESI-MS and CD spectra of the Ag3(CNHC)2(BF4)3 complex
in CH2Cl2 solution.
Fig. 1 Molecular structure of the chiral Ag3(CNHC)2 cation showing
the cylinder geometry. The anions, solvent molecules and H atoms are
omitted for clarity. Selected bond lengths (A) and angles (1):
Ag(1)–C(1) 2.078(6), Ag(1)–C(16) 2.118(7), N(1)–C(1) 1.392(6),
N(2)–C(1) 1.396(6), N(3)–C(16) 1.399(7), N(4)–C(16) 1.395(7) and
C(1)–Ag(1)–C(16) 176.40(2).
at m/z = 1855.59 was assignable to the molecular ion
{[Ag3(CNHC)2](BF4)2}+ (Fig. S2).w Circular dichroism (CD)
studies of the silver cage in CH2Cl2 solution showed bands at
229 and 270 nm with positive Cotton effects and one band at
242 nm with one negative Cotton effect, indicating the homo-
chirality of the carbene moieties in the [Ag3(CNHC)2]3+
cation even in solution. These results suggested the possible
application of the silver complex in homogeneous enantio-
selective catalysis.
conformation were joined together by three silver atoms to
consolidate the cage, and the [Ag3(CNHC)2]3+ cation posi-
tioned at a crystallographic C3 axis with the centres of the two
cyclic tricarbene ligands both positioned on the same axis.
Each silver atom coordinated to two carbene donors from two
ligands with the Ag–C distances ranging from 2.078(6) to
2.118(7) A, quite close to those in several related complexes.14
The C–Ag–C bond angle of 176.0(2)1 suggested the three
atoms were approximately positioned in a line and the cage
thus was described as a cylinder. The N–C bond length of the
carbene group was 1.39 A on average, which was obviously
longer than those in its imidazoline form. Since the two chiral
N-heterocyclic carbene moieties coordinated to one silver ion
had the same chirality, and these optical active carbon atoms
were R-configurations, accordingly, all the optical carbon
atoms in the cylinder were present in a homochiral fashion
(Fig. 1).
To further investigate the enantioselective catalysis proper-
ties of the chiral cage, cyanosilylation of imines, which is one
of the most efficient and general methods for producing
optically active a-amino nitriles derivatives, including a-amino
alcohols and direct precursors of a-amino acids derivatives,
was carried out.16 A variety of chirally modified catalysts have
been utilized for the asymmetric transformation, and some of
them have achieved high enantioselectivity. However, the
development of a suitable chiral catalyst for this reaction still
remain a great challenge.17 As shown in Table 1, the loading of
only 2% mol ratio of Ag3(CNHC)2(BF4)3 (0.01 mmol) lead to
the almost complete conversion of N-benzylidenebenzenamine
and N,N-dimethyl-4-((phenylimino)methyl)benzenamine at
room temperature, while the blank experiments had average
yields lower than 15%. The control reactions by using the
CNHC (0.01 mol) as the catalyst under the same experimental
conditions gave yields lower than 50%. The significant enhance-
ment of the yield prompting by the silver cage compared with
the CNHC demonstrated that the silver center had additional
catalytic driving forces. Interestingly, only traces of the product
were observed when the bulky Schiff-base substrates with
larger sizes were used. The lower catalytic activity in entries
4 and 5 is likely due to that the size of the opening within the
silver cylinder is too small for N-benzylidenenaphthalen-1-
amine 1-naphthaldehyde or N-(naphthalen-1-ylmethylene)-
benzenamine to pass through and access the catalytic sites.
The size-selective performance suggested that the reaction
occurred within cavities of the silver cylinder.
The three silver atoms are positioned in the three vertexes of
an equilateral triangle with a Agꢂ ꢂ ꢂAg separation of 8.26 A.
Whereas the two ligands located above or beneath of the
triangle with the benzene rings in each ligand were almost
parallel to the triangle plane. The cylinder had a diameter of
about 1 nm (9.5 A) and a height calculated from the two
cyclohexanes linked by one silver atom of about 13.4 A. The
inner volume of the cylinder was estimated to be about 450 A3,
which was largeꢀenough for encapsulation of small substrates.
One of the BF4 ions was encapsulated in the center of the
cylinder through the weak Fꢂ ꢂ ꢂAg interactions (the separation
of 3.49 A). Consequently, these coordinatively unsaturated
silver atoms were well positioned within the inner surface of
the cylinder, which could interact with guest molecules that
enter the cavity of the cylinder. These silver ions might serve as
potential weak Lewis acids, thus the trisilver cylinder had the
potential to function as active homogeneous catalyst for acid
promoted reactions.15
It should also be noted that the products of phenylamino
acetonitrile derivatives had an enantioselectivity ee lower than
30%, which is not larger than those in the case of the CNHC
to prompt the relative reactions. It seems that the enantio-
selectivity of the cyano-silylation reaction was mainly con-
trolled by the chirality of the CNHC backbone. The absence of
additional enantioselectivity of the cylinder-like silver complex
was possibly attributed to that the linear coordination mode of
ESI-MS spectra of the silver cage exhibited two intense
peaks at m/z = 884.24 and 560.51 with the isotopic distribu-
tion patterns separated by 0.50 ꢃ 0.01 and 0.33 ꢃ 0.01 Da,
demonstrating the presence of positive charged species
{[Ag3(CNHC)2]BF4}2+ and [Ag3(CNHC)2]3+, respectively in
solution (Fig. 2). While in its MALDI-Tof spectrum, the peak
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Table
catalyzed by Ag–CNHC
1
Cyanosilylation of imines derivatives with Me3SiCN
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Ar1CHQNAr2
Yield(%)a
ee (%)
1
2
3
4
5
PhCHQNPh
98
99
10
28
27
p-(CH3)N–PhCHQNPh
p-CH3–PhNQCHPh
1-Np–NHQCHPh
1-Np–CHQNPh
83
o5
o5
a
Reactions conditions: To a mixture of Me3SiCN (1.2 mmol) and
imines derivatives (0.5 mmol) was added Ag–CNHC (0.01 mmol) and
the resulting mixture was stirred at rt for seven days. The conversions
1
were determined by H NMR, based on starting materials.
the silver centers did not provide enough steric hindrance to
constrain the spatial of products.
In summary, we have developed a chiral N-heterocyclic
carbene silver(I) cage Ag–CNHC assembled from a new positively
charged chiral macrocycle triimidazoline salt (CNHC). The
catalytic properties based on cyanosilylation of imines reac-
tions demonstrated the size-selectivity catalytic performance,
suggesting the possible applications of this kind of carbene
silver(I) cage in chiral homogeneous catalyses. Work is currently
in progress on further investigation of the silver and other
metal-CNHC complexes, and on the choosing of suitable
auxiliary coordination ligands to improve the efficiency and
enantioselectivity of several important catalytic reactions.
This work was supported by the National Natural Science
Foundation of China.
Notes and references
z Crystal data of CNHCꢂ3BF4ꢂCH3CNꢂH2O: C47H62N7OB3F12
,
Mr
= 1001.47, Hexagonal, space group P63, colorless block,
a = 17.009(1), c = 11.895(1) A, V = 2980.5(2) A3, Z = 2, Dc =
1.116 g cmꢀ3, m(Mo-Ka) = 0.093 mmꢀ1, T = 298(2) K. 3428 unique
reflections [Rint = 0.0666]. Final R1 [with I > 2s(I)] = 0.0784, wR2
(all data)
= 0.2399. CCDC number 761390. Crystal data of
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Ag3(CNHC)2ꢂ3BF4ꢂ2CH2Cl2ꢂ3H2O: C92H118Ag3B3Cl4 F12N12O3,
Mr = 2165.82, Hexagonal, space group P63, colorless block, a =
14.539(1), c = 27.762(1), V = 5082.3(3), Z = 2, Dc = 1.415 g cmꢀ3
,
m(Mo-Ka) = 0.751 mmꢀ1, T = 298(2) K. 5889 unique reflections
[Rint = 0.0702]. Final R1 [with I > 2s(I)] = 0.0759, wR2 (all data) =
0.2141, Flack parameter 0.32(7). CCDC number 761389. The struc-
tures were solved by direct methods and refined on F2 using full matrix
least-squares methods using SHELXTL version 5.1. Anisotropic
thermal parameters were refined for non-hydrogen atoms within the
main backbone of the molecules. Except the solvent molecules,
hydrogen atoms were localized in their calculated positions and refined
using a riding model. For the Ag complex, the adjacent atom distances
in the cyclohexane rings, benzene rings and heterocyclic carbene rings
were fixed to be the same, respectively. Several lattice molecules were
refined disordered.
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ꢁc
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4730 | Chem. Commun., 2010, 46, 4728–4730