A palladium(ii) triangle as building blocks of microporous molecular materials: Structures and catalytic performance

Article abstract of DOI:10.1039/b916916d

A porous molecular crystalline solid based on amide-containing Pd(ii) triangles was created for size-selective heterogeneous catalysis of the Knoevenagel condensation reaction.

Full text of DOI:10.1039/b916916d

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A palladium(II) triangle as building blocks of microporous molecular
materials: structures and catalytic performancew
Yang Liu, Rong Zhang, Cheng He,* Dongbin Dang and Chunying Duan*
Received (in Cambridge, UK) 17th August 2009, Accepted 3rd November 2009
First published as an Advance Article on the web 26th November 2009
DOI: 10.1039/b916916d
A porous molecular crystalline solid based on amide-containing
Pd(II) triangles was created for size-selective heterogeneous
catalysis of the Knoevenagel condensation reaction.
for selective sorption, and has the potential to serve as active
sites for base-type catalysis of special organic reactions,9 such
as the Knoevenagel condensation reaction.10
Ligand L, 3,305,50-tetra[N-(4-pyridyl)methylcarboxylamide]
diphenylmethane was synthesized from 3,30,5,5-tetra-carboxy-
diphenylmethane tetrachloride with the reaction of 4-amino-
methyl pyridine. Crystals of 1 suitable for single crystal
structural analyses were obtained by layering methanol onto
a dimethyl sulfoxide (DMSO) solution containing ligand L
and Pd(NO3)2ꢀ2H2O in a 1 : 1 stoichiometry.z Compound 1
[Pd3(C41H36N8O4)3(NO3)6ꢀ34CH3OHꢀ8H2O] crystallized in a
space group P-43m. A cationic triangle was comprised of three
palladium(II) ions and three ligands. Each palladium(II) ion
was coordinated by four pyridine sites from two of the ligands
in a planar configuration and each ligand bridged two
palladium(II) ions alternatively. The Pdꢀ ꢀ ꢀPd separation within
one triangle was 1.27 nm, and the diameter of the cycle was
about 1.29 nm. The oxygen atoms of the amide groups
extended outwards and the N–H bonds toward the inside of
the cycle to form weak hydrogen bonds with the disordered
solvent molecules, the Nꢀ ꢀ ꢀO distance was 3.59 A and the
N–Hꢀ ꢀ ꢀO angle was 1521, respectively (Fig. 1).
Hybrid porous materials resulting from the reaction between
organic and inorganic species have attracted considerable
attention, because of their profound range of applications
such as in separation, gas storage and catalysis.1 The ability
to incorporate guest-accessible functional sites into the cavities of
these materials makes them excellent candidates as heterogeneous
catalysts.2 On the other hand, the coordination paradigm
pioneered by Lehn and Sauvage, and as developed in the groups
of Stang, Fujita, Raymond and others for closed systems, has
allowed for the synthesis of an enormous number and variety of
discrete, isolable structures featuring well-defined nanoscale
cavities.3,4 Like natural enzyme pockets,5 these structures are able
to bind substrates and fix them into orientations.6 They could
be used as synthetic hosts for mediating chemical reactions,
exhibiting high shape and size selectivity.
In this regard, the use of metal–organic macrocycles as
molecular building blocks for the creation of extent porous
materials has implications for a whole generation of hetero-
geneous catalysts. These materials would be expected to
typically display the combined advantages of homogeneous
and heterogeneous catalysis,7 such as high activity and
selectivity on one hand, and easy separation, and efficient
recycling that results in an overall lower cost compared to
others.8 The ready tunability of such an approach leads to
unique and useful heterogeneous catalysis, since the rigid and
shape-persistent metal–organic macrocycles are versatile
building blocks for supramolecular design and possibly
suited for the hierarchical assembly of porous materials with
utilizable functional groups.
Interestingly, in the crystal structure of compound 1, four
triangles positioned at four of the eight vertexes of a cubic cell,
with each pair of triangles positioned at two vertical diagonals
of the square face of to form a large tetrahedron cavity
(Fig. 2). The cavities thus exhibited a psuedospherical void
with the internal diameter extended to ca. 2.2–2.3 nm, and the
volume of a van der Waals sphere of the structure that would
just fit inside the void was ca. 2.8 nm3. These triangles
themselves acted as windows of the cavities, allowing the guest
molecules to ingress or egress. In the meantime, each triangle
Through carefully incorporating amide groups as
guest-accessible functional organic sites within the well-
defined metal–organic macrocyclic system (Scheme 1), herein
we employed a new porous material based on Pd(II)-based
triangles to demonstrate the possibility of producing size-
selective heterogeneous catalytic properties. The ligand was a
four-connector containing four pyridyl groups as efficient
coordination sites. The amide group possesses two types of
hydrogen bonds that could act as attractive interaction sites
State Key Laboratory of Fine Chemicals, Dalian University of
Technology Dalian, 116012, China. E-mail: cyduan@dlut.edu.cn
w Electronic supplementary information (ESI) available: Experimental
details, additional spectroscopic data and crystal data file in CIF
format. CCDC 718744 and 752539. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b916916d
Fig. 1 Molecular structure of the palladium(II) triangle. The metal,
oxygen, nitrogen and carbon atoms are drawn in green, red, blue
and grey, respectively. Pd–N bond distances were from 2.029(7) to
2.069(8) A.
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Table
derivatives with those substrates, catalyzed by crystals of compound 1
1
Knoevenagel condensation reaction of salicylaldehyde
Substrates
Conversion (%)
80
Fig. 2 Molecular packing of the palladium(II) triangle in compound 1
showing the large apertures and voids, the solvent molecules are
omitted for clarity.
1
2
3
4
5
6
7
8
9
10
16
20
13
35
38
19
15
also exhibited six windows with the size of 0.62 ꢂ 0.72 nm2 in
the side walls. Each triangle was also further connected to six
neighbors via two-fold C–Hꢀ ꢀ ꢀO hydrogen bonds between the
oxygen atoms of the amide groups in one triangle and
the carbon atoms in the phenyl ring of the other one, with
the Cꢀ ꢀ ꢀO distance of 3.36 A and N–Hꢀ ꢀ ꢀO angle of 1451,
respectively. Several methanol molecules and water molecules
as well as nitrate anions were found to fill in the 3D body, such
a large volume of the cavity should allow two or more
molecules to inhabit, and provided a platform to carry out
catalytic reaction within the cavity.
Crystalline powder of as-synthesized compound 1 was
insoluble in water or common organic solvents, and was only
soluble in DMSO. The as-synthesized crystals of compound 1
possibly lost some of the solvent molecules and part of its
porosity in ambient conditions. The profile of N2 sorption of
compound 1 at 77 K (supporting materials S7w) showed no
adsorption in the lower relative pressure region, and an abrupt
rise at P/P0 E 0.9 which may be due to the adsorption
between the particles of the compound,11 suggesting that the
porous structural property was partly lost after the removal of
solvent molecules at typical sample preparation conditions.
However, when the desolvated amorphous powder was
immersed into methanol or exposed to methanol vapor,12 it
changed immediately to the original crystalline phase, which
was detected by the XRPD patterns. Because of the low
solubility and the results of experiments on vapor exposure,
the possibility of recrystallization could be ruled out.13
Consequently, the origin porosity of the crystals could be well
recovered under the conditions where the solvent molecules or
suitable guest molecules were involved. These types of flexible
frameworks provide porous structures that are much more
useful for molecular recognition or selective guest inclusion
than these robust porous structures.14 The weight loss is about
30% at 250 1C in the TGA curve, corresponding to the loss of
the adsorbed solvents of the re-sorption crystalline powder
(ca. 37 methanol molecules per triangle, similar to the number
of solvent molecules in the original porous structure).
*Reactions were performed with 2 mmol substrates in 10 mL of
benzene using 0.1 g (0.08 mmol, 4 mol%) of the as-synthesized
crystalline solid of compound 1 for 3 h. The conversions were
1
determined by H NMR, based on starting materials.
as substrates could be comparable to the base catalysts
reported.17 Dynamics studies of the conversion–time curve
revealed that the reaction was second grade and the rate
constant was 8.6 ꢃ 0.6 Mꢄ1
h
ꢄ1. Without compound 1, the
reaction of malononitrile and salicylaldehyde in the same
benzene solution did not fully proceed, supporting the possible
base-catalytic activity of compound 1. The control experiment
of using the N-benzylbenzamide as a catalyst in homogenous
THF solution just resulted in 30% conversion, demonstrating
the enhanced catalytic activity of compound 1 under the
heterogeneous catalytic conditions. It is also postulated that
the presence of the porosity of the crystalline powder
conditions where the solvent was involved is one of the most
important factor to enhance the heterogeneous catalytic
activity of the amide groups.
Knoevenagel condensations that require the formation of
the anion of the active methylene containing compound under
a weak base-catalyzed mode15 were performed to characterize
the possible catalytic behaviour of compound 1. As shown in
Table 1, the as-synthesized crystals of 1 could promote the
reaction of salicylaldehyde derivatives and active methylene
compounds to generate coumarin derivatives.16 The yield
conversion of ca. 80% using salicylaldehyde and malononitrile
At the same experimental conditions, ethyl cyanoacetate
reacted with salicylaldehyde negligibly, and the conversions of
ethylacetoacetate and diethyl malonate were about 20% and
13%, respectively. When the size of the salicylaldehyde
derivatives increased, the relative conversions of these
Knoevenagel condensations about malononitrile decreased
obviously (Table 1). Although it could not be proven beyond
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a shadow of a doubt that the base-catalyzed reactions were
displayed within the cavities of the porous material, the
size-selective catalytic properties and the efficiency of the
catalytic reaction all supported this hypothesis.
Pd(NO3)22H2O. Yield: 20%. 1H NMR (400 MHz, DMSO-d6,ppm):
d = 3.96 (s, 2H), 4.47 (s, 8H), 7.59 (d, 8H), 7.95 (s, 4H), 8.34 (s, 2H),
9.12 (d, 8H), 9.32 (m, 4H). Anal. Calcd. for Pd3(C41H36N8O4)3(NO3)6ꢀ
34CH3OHꢀ8H2O: C, 46.69; H, 6.49; N, 10.40%. Found: C, 45.90; H,
6.84; N, 10.01%. Crystallography: Intensity data of compound 1 were
collected on a Siemens SMART-CCD diffractometer with graphite-
monochromated Mo-Ka (l = 0.71073 A) using the SMART and
IR spectra of the crystalline powder obtained by immersing
as-synthesized compound 1 in benzene containing malono-
nitrile exhibited a peak at about 2218 cmꢄ1, attributed to the
CRN stretching vibrations. The red-shift of the vibration
compared to the free malononitrile (2278 cmꢄ1),18 suggested
the potential interaction between the amide group and the CN
group. 1H NMR (d6-DMSO) of the desolvated powder
obtained by immersed as-synthesized compound 1 in benzene
containing malononitrile exhibited the small but significant
down-field shift of the methylene hydrogen atoms on
malononitrile, also demonstrating the amide groups could
form hydrogen bonds with the malononitrile groups. Detailed
NMR analyses exhibited the possible 1 : 3 stoichiometric
host–guest complexation behaviour of the triangles and the
malononitrile or salicylaldehyde molecules. The NMR of
as-synthesized compound 1 immersed with other active
methylene compounds and salicylaldehyde derivatives only
showed little sorption with the stoichiometry of host–guest
complexation lower than 1 : 0.5. Thus the size selective catalytic
behaviour of compound 1 should be attributed to the possible
selective sorption of cavities in the crystalline solid. ESI-MS
spectra of compound 1 in DMSO solution exhibited three
SAINT programs. Crystal data for 1: C157H260N30O72Pd3; Mr =
3
ꢀ
4039.15, cubic; P43m; a = 30.749(3) A, V = 29073(6) A ; Z = 4;
Dc = 1.251 g cmꢄ3; T = 180(2) K. Rint = 0.1226. The final refinement
gave R1 = 0.0898, wR2 = 0.1921 and Goof = 0.870 for 1926 observed
reflections with I > 2s(I).
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This work is supported by the National Natural Science
Foundation of China (20801008 and 20871025), Postdoctoral
Science Foundation of China (20080431135) and the Start-up
Fund of the University of Dalian Technology.
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Notes and references
z Preparation of compound 1: Crystals of compound 1 suitable for
X-ray structural analyses were obtained by layering methanol (12 mL)
on a DMSO solution 9 mL containing 0.015 g L and 0.056 g
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This journal is The Royal Society of Chemistry 2010
748 | Chem. Commun., 2010, 46, 746–748