Multicomponent self-sorting of a Pd7 molecular boat and its use in catalytic Knoevenagel condensation

Article abstract of DOI:10.1039/c2cc37377g

Unique three-component self-assembly of a cis-blocked 90° Pd(ii) acceptor with a mixture of tri- and tetra-imidazole donors led to the self-sorting of a Pd₇ molecular boat with an internal nanocavity, which catalyses the Knoevenagel condensation of a series of aromatic aldehydes with 1,3-dimethylbarbituric acid and Meldrum's acid in aqueous media.

Full text of DOI:10.1039/c2cc37377g

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Multicomponent self-sorting of a Pd molecular boat and its use in
7
catalytic Knoevenagel condensation
Dipak Samanta and Partha Sarathi Mukherjee*
A unique three–component self-assembly of a cis-blocked 90° Pd(II) acceptor with a mixture of
tri- and tetra-imidazole donors led to the self-sorting of a Pd molecular boat, which was found to
7
catalyse Knoevenagel condensations in aqueous medium.

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Multicomponent self-sorting of a Pd molecular boat and its use in
7
catalytic Knoevenagel condensation
*
Dipak Samanta and Partha Sarathi Mukherjee
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Unique three–component self-assembly of a cis-blocked 90°
Pd(II) acceptor with a mixture of tri- and tetra-imidazole
prediction of the final structure from the mixture of ligands
having multiple donor sites.
Herein, we report an example of multicomponent selfꢀsorted Pd7
nanocage [{(tmen)Pd} (timb) (tim) ](NO ) (H O) (1), [tmen =
donors led to the self-sorting of a Pd molecular boat with
7
7
2
2
3 14
2
20
internal nanocavity, which catalyses the Knoevenagel
condensation of a series of aromatic aldehydes with 1,3-
dimethylbarbituric acid and Meldrum’s acid in aqueous
media.
55
N,N,N’,N’ꢀtetramethylethylenediamine, timb
tim
imidazolyl)benzene] composed of both triꢀ and tetraꢀtopic
linkers (Scheme 1). The cavity of this cage was utilized as
=
1,3,5ꢀtris(1ꢀ
1
0
imidazolyl)benzene,7
=
1,2,4,5ꢀtetrakis(1ꢀ
8
nanoreactor for catalytic Knoevenagel condensations of a series
Construction of supramolecular nanocages containing porosity/ 60 of aromatic aldehydes with 1,3ꢀdimethylbarbituric acid (e) and
channels is highly appealing for their potential applications in
catalysis, gas adsorption, sensing, separation etc. Recent past has
Meldrum’s acid (f) in aqueous medium.
1
1
2
2
3
3
4
5
0
5
0
5
0
witnessed that architectures having predetermined structures and
functions can be obtained by engineering of complementary
2
building units via selfꢀassembly. In this context, metal–ligand
3
coordination driven selfꢀassembly has been proven to be a
potential protocol for the construction of nanoscopic metallacages
due to predictable directionality and high bondꢀenthalpy. Pd(II)/
Pt(II) in combination with polypyridyl donors have been widely
4
used to construct several discrete architectures. Twoꢀcomponent
selfꢀassembly has been widely used and easy to control. Recently,
Scheme 1. Threeꢀcomponent selfꢀassembly of a Pd cage (1) from cisꢀ
blocked Pd(II) 90° acceptor (M), triꢀimidazole (timb) and tetraꢀimidazole
7
5
multiꢀcomponent selfꢀassembly has come up as an effective and
alternative approach to accomplish complex architectures using 65 (tim) donors.
more than two components in a single step. Though the selective
formation of a single discrete product from multicomponents is
In all the previous multicomponent coordinationꢀdriven
assembly, a ditopic donor was assembled with metal acceptor in
combination with a triꢀ/ or tetraꢀtopic donor. To take
entropically unfavorable, a few strategies have been formulated to
sweep over the entropic penalty using driving forces such as
steric hindrance, hostꢀguest interaction, etc.(higher enthalpic
multicomponent directional selfꢀassembly to next level of
6
contribution). Imidazole is better donor compared to pyridyl
o
7
7
8
8
0
5
0
5
complexity, we performed selfꢀassembly of a 90 Pd(II) acceptor
donor and more importantly, imidazole can have at least two
different conformations (in plane) due to free rotation of CꢀN
bond that connects them to the backbone of the ligand. This
makes polyimidazole linkers to gain special attention. Until now,
a very few 3D architectures are known that are obtained from a
mixture of ditopic and triꢀ or tetratopic donors with metal
acceptors (multicomponent approach), with or without employing
with a mixture of tritopic (timb) and tetratopic (tim) donors. One
9
can anticipate trifacial barrel and semi cylindrical molecular
10
barrel from the combination of the acceptor with individual
donor separately and a molecular boat composed of all the three
components (Scheme 2). In the recent past, we have established
the formation of semiꢀcylindrical barrel via twoꢀcomponent selfꢀ
10
assembly of Pd(II) acceptor with triimidazole donor (timb).
5
ꢀ6
templates. To the best of our knowledge, a selfꢀassembled
multicomponent discrete architecture composed of both triꢀ and
tetraꢀtopic donors is yet to be reported due to difficulty in
However, exclusive selfꢀsorting of a Pd7 molecular boat,
o
incorporating both triꢀ and tetraꢀimidazole donors with 90
acceptor, was rather surprising due to absence of such kind of
report in literature.
_
_______________________________________________
Department of Inorganic and Physical Chemistry, Indian Institute of
Science, Bangalore-560 012, India. Fax: 91-80-2360-1552; Tel; 91-80-
A yellow aqueous solution of cisꢀ(tmen)Pd(NO ) was added into
3
2
4
5
5
0
solid mixture of triꢀ and tetraꢀtopic donors (timb and tim) in 7:2:2
ratio and allowed to stir for 24 h at room temperature. An
immediate sharp visual color change from yellow to colorless and
slow consumption of suspended solid donor indicated the
progress of the reaction. Pure complex 1 as offꢀwhite precipitate
was isolated by triturating the concentrated solution with acetone.
2
293-3352 ; E-mail: psm@ipc.iisc.ernet.in
†
Electronic Supplementary Information (ESI) available: Synthesis,
spectroscopic characterization of the cage 1, the product of the catalytic
Knoevenagel condensations and crystallographic analysis for 1. For
cryatallographic data in CIF, ESI see DOI: 10.1039/b000000x/
This journal is © The Royal Society of Chemistry [year]
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method was 1.0 nm. Three different imidazole rings in timb are in
different magnetic environments in complex 1. Likewise, one half
of tim is different from the other half due to different connectivity
which is reflected in the NMR spectrum (Fig.1). More
interestingly, two tim and two timb units are converged inward to
45
ꢀ
adopt a boat like architecture. Notably, two NO3 are located in
the internal cavity of complex 1 and the persisting counterꢀanions
placed outside the cage.
50
Scheme 2. Possible molecular architectures from the individual linkers
(tim or timb) with the acceptor (M) and threeꢀcomponent assembled cage
55
(1).
5
0
5
0
1
Though H NMR analysis was quite complicated with four sets of
each four protons of tim and three sets of four protons of timb
(
Fig. 2 Molecular structure of complex 1. Colour codes: light green = Pd;
blue = N, grey = C. For clarity, all hydrogen atoms, methyl groups,
counter anions and solvent molecules are removed.
ꢀ
10
2
Fig. 1), identical diffusion coefficients (2.6 × 10 m /s) for all
the proton signals in diffusionꢀordered (DOSY) NMR
spectroscopic analysis (ESI†), ruled out the formation of mixture
of products. Similarly, expected downfield shifts of the tim as
well as timb were noticed in the proton signals (Fig. 1), resulting
from loss of electron density of the donors upon coordination to
Pd(II). Several salient peaks at m/z 1159.7, 852.9, 546.5 and
343.5 correspond to [1 – 3NO ] , [1 – 4NO ] , [1 – 6NO ]
and [1 – 9NO ] , respectively in the ESIꢀMS spectrum further
support the formation of [7 + 2 + 2] selfꢀassembled product.
Other possible combination of Pd acceptor, triꢀ and
tetraimidazole donors in 5:2:1 ratio also yielded 1 along with
Pd (timb) cage.
1
1
2
6
6
7
7
8
8
0
5
0
5
0
5
In biological systems, the reactivity of one molecule can be tuned
by sequestering it from the bulk to achieve high catalytic activity,
selectivity of enzyme to the substrate, and storage or transport of
11
proteins. The interior cavity of the cage (Fig. 2) is surrounded
by the aromatic walls of the triꢀ and tetraꢀimidazole donors. Such
cavity provides a suitable environment to encapsulate large planar
aromatic guest such as 9ꢀanthracenealdehyde (a) through πꢀπ
ꢀ
3+
ꢀ 4+
3
ꢀ 6+
3
3
ꢀ
9+
3
10
stacking interaction with the walls of the cage. When an excess
of a was suspended in an aqueous solution of 1, the colorless
solution turned yellow in 1 h upon stirring at room temperature.
Though NMR spectrum of the encapsulated complex was very
complicated, UVꢀVis spectroscopic analysis (ESI†) and sharp
color change indicated encapsulation of a within the hydrophobic
cavity of 1. This prompted us to explore complex 1 as a catalyst
for Knoevenagel condensation of a number of aromatic aldehydes
with e (dimethylbarbituric acid) and f (Meldrum’s acid) at
ambient condition especially in green aqueous medium (Scheme
6
4
3
).
1
Fig. 1 H NMR spectrum of complex 1 recorded in D
2
O (colour codes:
blue circles = timb, red squares = tim, green triangles = tmen).
25
30
35
40
Finally, singleꢀcrystal Xꢀray analysis (ESI†) unambiguously
established the formation of a Pd7 molecular boat (Fig. 2)
comprising of both tim and timb linkers. Single crystals suitable
for Xꢀray diffraction were obtained by slow vapor diffusion of
acetone into aqueous solution of the complex 1 at room
temperature for two weeks. Though there are severe disorders in
nitrate anions and solvent molecules due to poor diffraction of the
crystal, low temperature Xꢀray diffraction data disclosed the
Scheme 3. Knoevenagel condensation of aromatic aldehydes using
molecular cage 1 as a catalyst in aqueous medium.
90
Water is generally not considered as favorable solvent for
Knoevenagel condensation as water is a byproduct of such
reaction and may drive the backward reaction. Therefore,
performing dehydration reactions like Knoevenagel condensation
in environmental friendly aqueous medium is a challenging
formation of a Pd molecular boat without any doubt. Complex 1,
7
that crystallized in triclinic system with space group Pꢀ1 has three
different Pd(II) centers. Pd1ꢀ Pd4 are linked to one timbꢀN and 95 approach. Aldehyde a was treated with one equivalent of e in
one timꢀN. Pd5 and Pd6 are coordinated to two timꢀN, and Pd7 is
connected to two timbꢀN to attain square planar geometry with
PdꢀN average bond distances in the range of 2.03 – 2.04 Å. The
average diagonal distance between the orthogonal Pdꢀmetal
presence of 10 mol% of cage 1 in 1 mL of water and the mixture
was allowed to stir. After 72 h, the condensation product (2) was
extracted with CDCl3 and NMR spectroscopy revealed that
compound 2 was formed in ~35 % yield (with respect to
00 aldehyde). The product was characterized by elemental analyses,
ESIꢀMS as well as NMR (ESI†). To conclude the catalytic
activity of 1, we carried out the same reaction in absence of cage
1
centers is 17.05 Å. BET surface area was very low of the order of
2
0
.49 m /g with pore diameter calculated using HarvathꢀKawazoe
2
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in water, and found that the reaction proceeded poorly with very
crystal Xꢀray analysis showed that the cage 1 has a large internal
cavity. Furthermore, the preferential affinity of the cavity towards
aromatic molecules through πꢀπ stacking has been successfully
employed to catalyze the Knoevenagel condensation of a series of
aromatic aldehydes with 1,3ꢀdimethylbarbituric acid /Meldrum’s
acid in green aqueous medium. Construction of water soluble
other polyhedral cages via multiꢀcomponent selfꢀassembly
including their hostꢀguest chemistry and further exploitation in
low yield (~5 %). Even in organic solvents (CHCl , acetone,
3
dichloromethane), only ~3ꢀ4 % of aldehyde transformed to
product after 72 h. We believe that, at first, the aromatic aldehyde
is encapsulated within the hydrophobic cavity of the cage (1) via
πꢀπ stacking interaction between the aromatic walls of the cage
and aromatic backbone of aldehyde to form hostꢀguest complex.
After the reaction, due to the hydrophobicity of the pocket, the
loss of water takes place easily to generate dehydrated product
which is too bulky for encapsulation and readily comes out of the
cage.
Similarly, the condensation of 1ꢀpyrene aldehyde (b) with e,
efficiently elevated to ~33 % within 3 h whereas the reaction is
scarcely occurred without cage (~2 %). We have also performed
the same reaction with individual components [cisꢀ
7
7
8
0
5
0
5
0
5
0
5
0
12
catalytic reactions have potential to explore.
1
1
2
2
3
The authors are thankful to the CSIR, New Delhi, India for
financial support. We are thankful to the Johnson Matthey Ltd.
U.K. for loan of PdCl . D. S. is grateful to CSIR, India for SPMꢀ
fellowship. Authors also acknowledge financial assistance
received from STCꢀIISc.
‡ This article is part of the ChemComm ‘Emerging Investigators 2013’
2
(
tmen)Pd(NO ) , tim or timb] and found that reaction progressed
3 2
only up to ~3 % which clearly suggested that the hydrophobic
cavity is crucial for the reaction. Relatively smaller aldehyde, 2ꢀ
naphthaldehyde (c), in general, reacts with active methylene
compound even in aqueous media due to higher reactivity.
However, when we performed the reaction in presence of the
cage, the yield of the condensation product (4 and 5) significantly
increased (Table 1), which demonstrates that the cavity of the
cage plays a vital role in promoting the reaction. In contrast,
benzaldehyde (d) undergoes condensation with e at almost equal
rate corresponding to the reaction without cage (Table 1).
Initially, it was thought that due to higher reactivity of e, cage has
not influenced the yield. To verify this, we replaced e with f,
where f is lesser reactive than e owing to higher pKa and found
that in this case also, cage doesn’t regulate the yield. We also
checked the reaction of aliphatic aldehyde like isobutyraldehyde
with e for 10 h in presence of 1, which showed no rate
enhancement. Such observation demonstrates that the cavity of
the cage 1 is quite selective in promoting the rate of Knoevenagel
condensation of polycyclic aldehydes, which are very less
reactive at ambient condition in aqueous medium without any
catalyst.
themed issue.
Notes and references
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In conclusion, we have demonstrated social selfꢀsorting of an
1
1
unprecedented Pd molecular cage (1) via multiꢀcomponent selfꢀ
7
6
0
assembly of triꢀ and tetraꢀimidazole donors without employing
any template. Such interesting social selfꢀsorting was possible
due to having required directions of the donor nitrogens of the
building units. This approach may have wide application in
generating further complex architectures of varied shapes and
sizes using multiple donors with higher denticity. Along with the
structural confirmation through ESIꢀMS and NMR studies, single
1
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6
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