Porous coordination polymers of diverse topologies based on a twisted tetrapyridylbiaryl: Application as nucleophilic catalysts for acetylation of phenols

Article abstract of DOI:10.1002/chem.201404552

Porous coordination polymers (CPs) with partially uncoordinated pyridyl rings based on rationally designed polypyridyl linkers are appealing from the point of view of their application as nucleophilic catalysts. A D_2d^--symmetric tetradentate organic linker L, that is, 2,2 ',6,6'-tetramethoxy-3,3',5,5'-tetrakis(4-pyridyl)biphenyl, was designed and synthesized for metal-assisted self-assembly aimed at porous CPs. Depending on the nature of the metal ion and the counter anion, the ligand L is found to function as a 3- or 4-connecting building block leading to porous CPs of diverse topologies. The reaction of L with Zn(NO₃)₂ and Cd(NO₃)₂ yields porous 2D CPs of "fes" topology, in which the tetrapyridyl linker L serves as a 3-connecting unit with its free pyridyl rings well exposed into the pores. The functional utility of these porous CPs containing uncoordinated pyridyl rings is demonstrated by employing them as efficient heterogeneous nucleophilic catalysts for acetylation of a number of phenols with varying electronic properties and reactivities.

Full text of DOI:10.1002/chem.201404552

DOI: 10.1002/chem.201404552
Full Paper
&
Coordination Polymers
Porous Coordination Polymers of Diverse Topologies Based on
a Twisted Tetrapyridylbiaryl: Application as Nucleophilic Catalysts
for Acetylation of Phenols
[a]
[b]
[a]
Saona Seth, Paloth Venugopalan, and Jarugu Narasimha Moorthy*
Abstract: Porous coordination polymers (CPs) with partially
uncoordinated pyridyl rings based on rationally designed
polypyridyl linkers are appealing from the point of view of
their application as nucleophilic catalysts. A D -symmetric
topologies. The reaction of L with Zn(NO ) and Cd(NO )
3 2 3 2
yields porous 2D CPs of “fes” topology, in which the
tetrapyridyl linker L serves as a 3-connecting unit with its
free pyridyl rings well exposed into the pores. The functional
utility of these porous CPs containing uncoordinated
pyridyl rings is demonstrated by employing them as efficient
heterogeneous nucleophilic catalysts for acetylation of a
number of phenols with varying electronic properties and
reactivities.
2
d
tetradentate organic linker L, that is, 2,2’,6,6’-tetramethoxy-
,3’,5,5’-tetrakis(4-pyridyl)biphenyl, was designed and syn-
3
thesized for metal-assisted self-assembly aimed at porous
CPs. Depending on the nature of the metal ion and the
counter anion, the ligand L is found to function as a 3- or 4-
connecting building block leading to porous CPs of diverse
Introduction
introduce functionality, which is not achieved by de novo
approaches involving organic linker design and metal ion
[10]
Porous coordination polymers (CPs) are of tremendous
choice. It occurred to us that incomplete metal–ligand coor-
dination of a polydentate ligand that leads to porous CPs
could be an exciting alternative, provided that the uncoordi-
nated ligand in the CPs serves as a chemical functionality. We
envisaged that porous CPs with partially uncoordinated pyridyl
rings—the functionalities for nucleophilic catalysis—based on
metal-assisted self-assembly of rationally designed polypyridyl
linkers should be appealing for application as nucleophilic cat-
[
1]
contemporary interest in view of their functional utility in
a myriad of applications that include gas storage and separa-
[
2]
[3]
[4]
[5]
tion, sensing, magnetic properties, proton conductivity,
[
3b,6]
bioimaging, and drug delivery.
In the realm of synthetic or-
ganic chemistry, they offer unique advantages as microreactors
with well-defined cavity sizes and shapes to promote organic
transformations and also regulate product, regio-, and stereo-
[
7]
[11]
selectivities. CPs are, therefore, invaluable as heterogeneous
alysts; the literature reveals a few instances of inadvertent
[
8]
and recyclable catalysts with environmentally beneficial attri-
butes. They are more advantageous than inorganic zeolites in
terms of ease with which they are synthesized, functional
diversity, and control of the chemical environment and pore
characteristics through judicious choice of organic linkers.
For functional applications, the chemical functionality in CPs,
in particular in metal–organic frameworks (MOFs) that consti-
tute a subclass of CPs, is generally incorporated in the design
of the organic building blocks/ligands or through choice of the
synthesis and applications of porous CPs with partially uncoor-
[12]
dinated pyridyl rings based on polypyridyl ligands. We re-
ported some time ago that the metal-assisted self-assembly of
D -symmetric tetradentate 3,3’,5,5’-tetrakis(4-pyridyl)bimesityl
2
d
leads to 1) porous CPs of the topologies of diamond, PtS, and
[13a]
corundum that are extended,
and 2) discrete face-centered
M6L12 metal–organic cubes in which only two of the four pyrid-
[13b]
yl rings are utilized in metal–ligand coordination.
This intri-
guing observation spurred us to delve more into ligands of
this type in quest of porous CPs with partially uncoordinated
pyridyl groups for application as nucleophilic catalysts. We de-
signed the ligand L, that is, 2,2’,6,6’-tetramethoxy-3,3’,5,5’-tetra-
kis(4-pyridyl)biphenyl shown in Scheme 1, with the objective
to set it up for coordination polymerization with different
metal ions to afford porous CPs of diverse topologies and to
examine the accessibility of those that contain uncoordinated
pyridyl rings. Our rationale for the design of the ligand L was
the following: first, considerable torsional flexibility for the pyr-
idyl rings in L should facilitate formation of CPs that are struc-
turally well defined; second, creation of pores within the CPs
generated by metal-assisted self-assembly should be assured
[
9]
metal ions. In recent years, postsynthetic modification of
MOFs has been demonstrated as a very effective protocol to
[
a] S. Seth, Prof. Dr. J. N. Moorthy
Department of Chemistry, Indian Institute of Technology
Kanpur 208016 (India)
Fax: (+91)512-2597436
E-mail: moorthy@iitk.ac.in
[b] Prof. Dr. P. Venugopalan
Department of Chemistry
Panjab University, Sector 14, Chandigarh 160014 (India)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404552.
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Scheme 1. Synthetic route for the tetradentate tetrapyridyl ligand L. I-DiR=2,6-dimethoxyiodobenzene, NIS=N-iodosuccinimide, TFA=trifluoroacetic acid.
based on the known guest inclusion propensity of aromatics
oxaborolan-2-yl)pyridine, to yield the target tetradentate tetra-
pyridyl ligand L, that is, 2,2’,6,6’-tetramethoxy-3,3’,5,5’-tetra-
kis(4-pyridyl)biphenyl, in 72% yield of isolated product.
[
14]
that are 1,3-diarylated;
third, the methoxy substituents at
ortho positions of the biaryl should increase the steric bulk, in
addition to ensuring the twisted D2d symmetry, to preclude
[
15]
self-interpenetration of the derived CPs. Herein, we report
access to a series of porous CPs of different topologies based
on the reaction of L with different transition-metal ions at
room temperature (RT). It is shown that one of the pyridyl
rings of L remains uncoordinated in at least three CPs such
that the ligand L serves essentially as an unconventional 3-con-
necting linker leading to porous 2D networks of the topolo-
gies of fes and hcb. The functional utility of these porous CPs
containing uncoordinated pyridyl rings is demonstrated by
employing them as efficient heterogeneous catalysts for
acetylation of phenols in a recyclable manner.
Synthesis of coordination polymers with tetrapyridylbiaryl L
and X-ray crystal structure determinations
The synthesis of CPs with tetrapyridyl ligand L was attempted
with a number of transition-metal ions at RT. A series of CPs
was obtained over a period of 2–3 weeks when the solutions
of the transition-metal ions in MeOH/MeCN were layered over
solutions of the tetrapyridyl ligand L in DMF. The crystals of
CPs that developed slowly were isolated and characterized by
single-crystal XRD analyses. Details of all CPs characterized are
given in Table 1 along with solvent combinations employed
and codes assigned; also noted are contents of the asymmetric
unit and the network topology observed in each case (see
below).
Results and Discussion
Synthesis of tetradentate tetra-
pyridylbiaryl L
Table 1. The CPs obtained upon reaction of the ligand L with various transition-metal salts.
The target 4-connecting ligand L
was synthesized starting from
Metal salt
Solvent
Asymmetric unit of CP
Code
Net topology
CoCl
CdCl
CuI
2
·6H
·6H
2
O
O
DMF/MeOH
DMF/MeOH
DMF/CH
DMF/CH CN
DMF/CH
DMF/CH CN
DMF/CH
DMF/CH
[CoLCl
[CdLCl
2
(DMF)1.5(CH
3
OH)1.5(H
2
O).5
]
CP-1
CP-2
CP-3
CP-4
CP-5
CP-6
CP-7
CP-8
PtS
PtS
PtS
dia
dia
fes
fes
hcb
1,3-dimethoxybenzene,
which
2
2
2
(DMF)(CH OH) (H O)]
3
2 2
was converted to 2,6-dimethox-
yiodobenzene by ortho lithiation
followed by quenching with
3
CN
[Cu
[CuLCl(DMF)4]
[AgL(NO )(DMF)
[Zn(L)(NO ) (DMF) (CH CN) ]
2 2 2
LI (DMF) ]
CuCl
3
AgNO
Zn(NO ) ·6H O
3
3
CN
3
2
]
[
16]
3
2
2
3
3
2
2
3
2
iodine.
Ullmann coupling of
Cd(NO
CuBr
3
)
2
3
CN
CN
[Cd(L)(NO
3
)
2
(DMF)
2
(CH
3
CN)
2
]
1,3-dimethoxybenzene with the
3
[CuLBr(DMF)(CH
3
CN)]
iodinated derivative under modi-
fied conditions led to 2,2’,6,6’-
tetramethoxybiphenyl in a near
[
16]
quantitative yield (Scheme 1). Iodination of the latter with
our modified conditions by employing N-iodosuccinimide
yielded the corresponding tetraiodo derivative, that is, 3,3’,5,5’-
Based on the structural analyses, it was noted quickly that at
least three sets of CPs of the tetradentate ligand L formed
with the following pairs of metal salts are isostructural: CoCl2
and CdCl , CuCl and AgNO , and Zn(NO ) and Cd(NO ) . The
[
17]
tetraiodo-2,2’,6,6’-tetramethoxybiphenyl.
This compound
2
3
3 2
3 2
was subjected to Suzuki coupling with commercially available
pyridine boronate ester, namely, 4-(3,3,4,4-tetramethyl-1,3,2-di-
CP formed with CuI was found to be very different from that
formed with CuCl, but is related topologically to those formed
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with CoCl and CdCl . In the following the structural aspects of
ence essentially manifests in extended geometry of the PtS
network. As in the case of CP-1 and CP-2, the structure is
doubly interpenetrated in this instance as well, with a solvent-
accessible volume of 37.5%.
2
2
the CPs and their topologies are discussed.
Treatment of the ligand L with CoCl2 and CdCl2 led to
isostructural CPs, that is, CP-1 and CP-2, as revealed by X-ray
crystallography. The crystals of both were found to belong to
the monoclinic crystal system with the space group C2/c. In
the crystals, the ligand L and the metal ions are connected by
coordination bonds leading to a three-dimensional CP that is
bi-porous (Figure 1c). The coordination geometry of the metal
The reaction of the ligand L with CuCl and AgNO , as
3
mentioned earlier, led to isostructural CPs, CP-4 and CP-5, with
the space group C2/c. In the crystal structures, one observes
that the ligand L is 4-connecting as that of the metal ion, for
which the coordination geometry is tetrahedral (Figure 1d).
Thus, the iterative and infinite
linking of 4-connecting organic
linkers of D2d symmetry and
metal ions of T symmetry leads
d
to CPs with extended diamond-
[13a,19]
oid topology
(Figure 1e).
The three-dimensional porous
cationic network in each case is,
however, found to be doubly in-
terpenetrated as those of CP-1,
CP-2, and CP-3 (Figure 1 f). Nota-
bly, the counter anions in both
cases were found to be located
within the voids. The network
structures are still porous with
solvent-accessible volumes of
21.7% (CP-4) and 16.9% (CP-5),
as revealed by Platon analyses.
The reaction of ligand L with
Zn(NO ) and Cd(NO ) led to iso-
2
+
Figure 1. a) Coordination geometry of Co and the connectivity of ligand L in CP-1. b) The 4-connecting planar
Cu node coordinated by the ligand L in CP-3. c) Twofold interpenetrated PtS network of CP-1 shown by
3
2
3 2
2 2
I
reducing the ligand L to a pair of 3-connecting units and the metal node as 4-connecting center. d) The basic
metal–organic adamantanoid repeating unit of CP-4. e) The 3D open framework structure of CP-4 down the c
structural CPs, that is, CP-6 and
CP-7, respectively, with the
axis. f) Twofold interpenetration of the diamondoid network of CP-4 shown by reducing the ligand to a pair of
space group P2 /c. Analysis of
À
1
3
-connecting units and the metal ion as 4-connecting node. Hydrogen atoms, Cl ions, and solvent molecules
the ligand and metal ion con-
nectivities in the crystal struc-
have been omitted for clarity.
tures shows that one of the pyr-
À
ion is found to be octahedral with the Cl ions occupying axial
positions and the pyridyl rings of the ligand L occupying equa-
torial positions. Overall, the metal ion serves as a 4-connecting
square-planar node, whereas the ligand L functions as a pair of
idyl rings of the ligand L remains uncoordinated. Thus, the tet-
radentate ligand L turns out to be a 3-connecting one with un-
usual pyramidal disposition for metal–ligand coordination. The
crystal packing analyses of these CPs correspond to the topol-
[20]
3
-connecting units (Figure 1a). The topology of the resultant
ogy of the fes network (Figure 2d). Thus, the crystals are
truly bi-porous with solvent-accessible volumes of 37.1 and
38.2%, respectively. Remarkably, one observes that the uncoor-
dinated pyridyl rings are disposed into the cavities in which
the guest molecules reside (Figure 2d and e). Some differences
in the coordination geometries of the metal ions in both Zn-
and Cd-CPs are noteworthy. In the former, the metal ion is of
[
13a,18]
CPs is akin to that of PtS
in the realm of inorganic struc-
tures, but extended. The open networks are doubly interpene-
trated in both cases, as shown in Figure 1c for CP-1. Platon
analyses show that the solvent-accessible volumes are 32.5
and 34.8% for CP-1 and CP-2, respectively.
Brown tetragonal crystals (space group: P4 2 2) of CP-3
were obtained by slow diffusion of a solution of CuI in acetoni-
3
1
À
distorted octahedral geometry with one of the NO3 anions
trile into a solution of L in DMF. The crystal structure analyses
undergoing chelation and the other linked in a monodentate
I
2+
show that the Cu ions exist as dimers with the two metal ions
fashion (Figure 2a). In contrast, in CP-7, the Cd ion is hepta-
coordinated with both nitrate anions chelating the metal ion
(see Figure 2b).
À
bridged by two I ions (Figure 1b). The dimer, in principle,
serves as a 4-connecting bimetallic node of D2h symmetry.
Thus, linking of the D -symmetric ligand L and D -symmetric
An altogether different CP was isolated when the tetrapyrid-
yl ligand L was treated with CuBr. The crystal structure analyses
of the resultant product CP-8 showed that the ligand employs
only three pyridyl rings for coordination polymerization with
2d
2h
node leads to a 3D CP that is porous. The topology of the
latter can be readily likened to those of CP-1 and CP-2, which
are of PtS type. The difference between the topologies of CP-
I
1
/CP-2 and CP-3 is marginal in that the metal ions in the latter
Cu ions, which are tetrahedrally connected by three pyridyl
are of D4h symmetry, whereas they are D2h in CP-3. This differ-
rings and the bromide counter anion. In essence, the metal ion
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Diverse topologies of CPs and
influence of counter anions
From the structural studies of
CPs obtained upon reaction of
the unique twisted D -symmet-
2d
ric tetrapyridyl ligand L with dif-
ferent metal ions, it is evident
that the metal-assisted self-as-
sembly of L leads to open frame-
work structures of distinct topol-
ogies; indeed, the latter can be
predicted a priori. Given that the
D -symmetric allene-like ligand
2
d
L can be regarded as an extend-
ed tetrahedron, its self-assembly
with the Td metal ion should
lead to an extended diamond
network. In line with this expect-
ation, the self-assembly of L with
metal salts such as CuCl and
AgNO , which exhibit T coordi-
3
d
nation geometry, led to dia-
mondoid structures, (see Figur-
es 1d–f). In a similar manner,
linking of the ligand L with
metal ions of square-planar ge-
ometry should be expected to
deliver polymeric structures of
PtS topology. Indeed, the ligand
L with metal salts such as CdCl2
and CoCl2 that display square-
Figure 2. The 3-connecting T disposition of the metal ion and 3-connecting ligand L in a) CP-6 and b) CP-7; ob-
serve the differences in the coordination modes of the nitrate anions. c) The 3-connecting pyramidal disposition
of the metal ion and connectivity of L in CP-8. d) The 2D corrugated layer of CP-6. e) Packing of the corrugated
2
D fes nets in CP-6 by reducing the ligand L into a 3-connecting pseudo-trigonal unit and the metal ions into 3-
connecting nodes. Notice that the free pyridyl groups (nitrogen atoms are shown in dark color) project out of the
D layers. f) Crystal packing of CP-8 showing the channels down the a axis. g) Corrugated hcb nets of CP-8 by
2
reducing the ligand L into a 3-connecting pseudo-trigonal unit and the metal ions into 3-connecting nodes. Note
that the uncoordinated pyridyl groups (in dark color) protrude into pores of the neighboring layer in each case.
Hydrogen atoms and solvent molecules have been omitted for clarity.
planar connectivity with the
À
in this instance acts as a 3-connecting node as does the linker
axial positions occupied by Cl ions lead to CPs of PtS topolo-
gy (Figure 1c). The case of the CP formed with CuI is different.
In this instance, one observes a dicopper moiety bridged by
L (Figure 2c). Topologically, the 2D CP corresponds to the
[
21]
hcb type. The 2D nets are found to be corrugated with the
pores being filled by the humps of the offset layers below,
which leads to an overall low porosity that is about 18.1%
À
two I ions (Figure 1b). It turns out that the coordination ge-
ometry for the metal ion with respect to the linkage of the
ligand L is distorted square planar. Thus, the combination of
D -symmetric L with the dicopper node leads to a metal–or-
(Figure 2 f and g).
All the CPs of the tetrapyridyl ligand L, that is, CP-1 to CP-8,
2
d
are characterized by considerable void volumes in the crystal
lattice, as revealed by the crystal structures. The Platon analy-
ses show that the solvent-accessible volumes range from 17 to
ganic CP the structure of which is topologically equivalent to
PtS. Incidentally, all of these CPs, that is, CP-1–CP-5, are 3D
open framework structures that are doubly interpenetrated.
Despite this interpenetration, the solvent-accessible volumes in
these CPs range from 16.9 to 38.2%. Remarkably, the ligand L
employs only three of its pyridyl rings for its self-assembly
when reacted with Zn(NO ) , Cd(NO ) , and CuBr leading to 2D
38%; it should be noted that we have used the term “porous”
loosely to refer to the presence of solvent-accessible volumes
in the crystals of all CPs. The bulk purity for each of the CPs
was established by comparison of the powder X-ray diffraction
3
2
3 2
(PXRD) pattern recorded for as-synthesized material with that
polymers. In the CPs formed with Zn(NO ) and Cd(NO ) , the
3 2 3 2
simulated for the structure determined by single-crystal X-ray
analyses. The PXRD patterns were found to match the simulat-
ed profiles of all CPs satisfactorily with the exception of those
of CP-1 and CP-8 (see the Supporting Information). For the
latter, the crystals were found to collapse and become pow-
dery upon removal from the mother liquor. Of all the CPs, only
CP-2 and CP-3 were found to be stable to desolvation under
vacuum (see the Supporting Information).
metal ions are bound by counter anions such that only three
sites with T disposition are available for coordination with pyr-
idyl moieties. Consequently, the CPs derived therefrom corre-
spond to fes topology, in which the organic linker and metal
ions are both 3-connecting with pseudo-trigonal and T disposi-
tions for linkage (Figure 2e). However, in the CP formed with
À
CuBr, the Br counterion that caps the otherwise tetrahedral
I
Cu ion renders the metal ion essentially 3-connecting with
trigonal disposition for linkage with the ligand L, which is
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pseudo-trigonal (Figure 2c). Thus, one obtains an hcb network
Figure 2g). In the 2D polymers, the 2D layers are found to be
Table 2. Screening of the reaction conditions for catalytic acetylation of
p-bromophenol, a representative case, in the presence of CP-6 and
CP-7.
(
[
a]
stacked one over the other in the crystal lattice with overall
solvent-accessible volumes ranging between 18.1 and 38.2%.
The limited, yet meaningful set of CPs formed with the
twisted ligand L amply demonstrate the fact that the latter
provides access to open frameworks of diverse topologies de-
pending on 1) whether the ligand L serves as a 3- or 4-con-
necting linker and 2) the nature of the metal salt employed. It
emerges compellingly that in the CPs formed with cuprous
salts, the counter anion crucially determines the network top-
Entry
Medium
CP-6
Conv.
CP-7
Conv.
t
t
[
h]
[%]
[h]
[%]
1
2
3
4
neat Ac
dichloromethane
acetonitrile
tetrahydrofuran
2
O
2
100
100
80
6
40
72
48
100
100
76
[
b]
24
72
72
[b]
À
[
b]
ology. Although I facilitates formation of a dicopper unit that
82
100
displays square-planar connectivity leading to the network of
[
a] The reactions were run at RT on 20 mg (0.12 mmol) of the phenol in
À
PtS topology, Cl remains uncoordinated to the metal ion
the presence of 2 mg (10 wt%) of the catalyst in 0.5 mL solvent. [b] Four
equivalents of acetic anhydride were employed as the acetylating agent.
thereby permitting T connectivity leading to a cationic dia-
d
À
mondoid network. In contrast, Br caps up one site of the tet-
I
rahedral Cu reducing it to a 3-connecting species, which seem-
ingly influences the ligand L to become 3-connecting, so that
actions were run under identical conditions in CH Cl ; the reac-
2
2
the network observed is
a
(6,3) honeycomb. Although
tion with CP-7 was found to be perceptibly slower and the
complete disappearance of the starting p-bromophenol, a rep-
resentative case, was found to occur only after about 40 h as
monitored by TLC and NMR analyses (see Table 2). Figure 3
shows the relative percent conversions of p-bromophenol
without any added catalyst and in the presence of CP-6 and
CP-7. Thus, acetylation of a variety of phenols of varying elec-
tronic attributes and reactivities was explored by employing
CP-6 as a heterogeneous catalyst.
a number of instances of counter anions influencing the net-
work topologies are known, formation of highly disparate net-
works of the kind found herein, that is, PtS, dia, and hcb, for
the same ligand and metal ion based on variation in the coor-
dination modes and flexibility of the polypyridyl ligand to
[
12b,22]
adopt 3- or 4-connecting geometry is rare.
Along the
same lines, one observes that the variation of the counter
À
À
anion from Cl in CdCl to NO3 in Cd(NO ) manifests in two
2
3 2
distinct CPs with PtS and fes topologies. Here again, the coun-
ter anion seemingly dictates the ligand L to adopt 3- or 4-
connecting geometries in fes and PtS structures, respectively.
Application of CP-6 and CP-7 for heterogeneous
nucleophilic catalysis: acetylation of phenols
It was our objective at the outset to examine if the ligand L
leads to porous CPs with partially uncoordinated pyridyl rings,
which can be exploited for nucleophilic catalysis. We were,
indeed, delighted by the observation of uncoordinated pyridyl
rings that are disposed well into the void regions occupied by
solvent molecules in the isostructural CP-6 and CP-7 to explore
heterogeneous nucleophilic catalysis. The free pyridyl rings
could be considered as immobilized and explored for hetero-
geneous nucleophilic catalysis to permit recyclability and facile
workup. A prototype example of catalysis by pyridines is the
acetylation of phenols. Thus, the utility of CP-6 and CP-7 was
investigated for acetylation of p-bromophenol as a representa-
tive case under heterogeneous conditions at RT. The results of
screening for acetylation of p-bromophenol using acetic
anhydride by employing CP-6 and CP-7 as heterogeneous and
recyclable catalysts in various solvents are collected in Table 2.
In a typical experiment, 10 wt% of the solid CP-6/7 was em-
ployed with respect to the phenol reactant. Although the reac-
tion was found to be complete in neat acetic anhydride in 2 h
with CP-6, that in CH Cl containing 4 equivalents of Ac O was
Figure 3. Comparative study of rates of acetylation of p-bromophenol with-
out any added catalyst and with CP-6 and CP-7 as added heterogeneous
catalysts.
In Table 3 are collected the results of acetylation of diverse
phenols with Ac O (4 equiv) in CH Cl at RT in the presence of
2
2
2
10 wt% of CP-6. The simple workup involved dilution of the
reaction mixture with ethyl acetate followed by filtration of the
solid catalyst. The filtrate was washed with brine followed by
Na CO solution, and the solvent was stripped off. As can be
2
2
2
found to go to completion in 24 h in the presence of the same
catalyst. Notably, the Cd-CP, that is, CP-7, was found to be mar-
ginally inferior to that of the Zn-CP, namely, CP-6, when the re-
2
3
seen from the results in Table 3, the acetylation of phenols
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geneous catalysts was further explored for their recyclability.
The fact that the catalytic activity was not lost was established
by recycling the catalyst at least three times for the case of
acetylation of p-bromophenol. Although the yields were quan-
titative, the reaction occurred over a slightly longer period
during the third recycling. The recyclability in conjunction with
the PXRD analyses of the recycled catalysts (see Figure 4) sug-
gest that the crystallinity of the CPs is conserved throughout
Table 3. Acetylation of phenols to the corresponding acetates with CP-6
as catalyst.
[
a]
Entry Substrate
Catalyst
t
Product
Yield
[%]
[
b]
[mol%] [h]
1
2
1.73
24
10
99
93
1.12
1.12
1.28
3
4
10
11
92
97
5
6
1.54
1.44
36
48
96
98
7
8
9
1.22
1.09
2.06
60
36
72
97
96
<10
[
a] All the reactions were run at RT on 0.2 g (0.97–1.78 mmol) of the
phenol by employing 4 equivalents of acetic anhydride in the presence of
.02 g (10 wt%) CP-6 in 1 mL of CH Cl as solvent. [b] Yield of isolated
product.
0
2
2
Figure 4. PXRD profiles of CP-6: simulated based on single-crystal X-ray
structure, for as-synthesized sample and after one, two, and three catalytic
cycles of acetylation of p-bromophenol.
occurs in a facile manner at RT to afford the corresponding
aryl acetates in quantitative yields. The substrates containing
electron-withdrawing groups were found to be acetylated
faster than those containing electron-donating groups. The
fact that the acetylation proceeds within the pores of the CP is
evidenced from the result observed with sterically hindered
and bulky 2,4-di-tert-butylphenol. Acetylation of this substrate
was found to be too slow in the presence as well as in the ab-
sence of the added CP-6 catalyst (conversion ca. <10% after
and that the reactions necessarily occur within the pores of
CP-6. Evidently, the layered structures of both CP-6 and CP-7
support transport of phenol reactants, acetic anhydride re-
agent, and the acetylated phenol in and out of the pores
[23]
within the crystals, of course with size and shape selectivity.
Why is it that CP-6 is marginally better than CP-7 as a nucle-
ophilic catalyst for acetylation of phenols despite the fact that
both are isostructural? To answer this, let us first consider the
mechanism of acetylation of phenols with acetic anhydride in
the presence of pyridine. The latter acts as a nucleophilic cata-
lyst and activates the electrophilic acetic anhydride by forming
acetylpyridinium acetate. This intermediate is subsequently
attacked by the phenol as a nucleophile to produce phenyl
acetate. It has also been shown unambiguously in the early
investigations involving the deuterium isotope effect and
kinetic studies that intermolecular hydrogen bonding plays an
72 h). However, addition of a small amount of pyridine was
found to drive the reaction to completion. This clearly suggests
that the open pyridine catalytic sites within the CPs are not
readily accessed by this particular substrate. Notably, the acety-
lation of phenols with neat Ac O without added pyridine
2
under homogeneous conditions does proceed, but very slowly.
For example, the acetylation of p-bromophenol in neat Ac O
2
was found to be incomplete even after 48 h at RT. In contrast,
in the presence of CP-6, the reactions went to completion in
a short period of time. Of course, it is needless to mention that
the reactions occur readily with added pyridine as a homogene-
ous catalyst. The advantage thus offered by the CPs as hetero-
[24]
important role in the acetylation of phenols.
Accordingly,
hydrogen-bonded phenols are better nucleophiles, and the
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stronger the hydrogen bonding is, the more nucleophilic are
the phenols. Thus, when one compares hydrogen bonding in
p-cresol and in p-chlorophenol, the stronger hydrogen bond-
ing in the latter renders it more nucleophilic. Indeed, acetyla-
tion of p-chlorophenol has been shown to occur 2–4 times
faster than that of p-cresol under identical conditions. Similar
considerations should apply in the present instance involving
acetylation of phenols catalyzed by CP-6 and CP-7. In both,
there are strongly hydrogen-bond-accepting sites in the form
of nitrate anions. Thus, the phenols are likely to react as hydro-
gen-bonded nucleophiles under the employed conditions of
catalysis with CP-6 or CP-7 in CH Cl as the solvent. The obser-
vation that phenols substituted with electron-withdrawing
groups undergo acetylation faster with CP-6 can now be readi-
ly understood based on more nucleophilicity resulting from
stronger hydrogen bonding. In CP-6, one of the nitrate anions
is coordinated to the metal ion in a chelating fashion, whereas
the other is coordinated in a monodentate fashion. In contrast,
in CP-7, both nitrate anions are coordinated to the metal ion
in a chelating fashion. Thus, for a given phenol, the hydrogen
bonding should be expected to be stronger with CP-6 than
with CP-7 due to the presence of the monodentate nitrate
anion. We are thus tempted to ascribe the marginally higher
reactivity observed with CP-6 over CP-7 to the relatively stron-
ger hydrogen bonding of the phenols with the former. In sup-
port of this, we find that the shortest distance between one of
the oxygen atoms of the monocoordinated nitrate anion and
the free pyridine nitrogen atom in CP-6 is 3.57 ꢁ, which is less
than that in CP-7; in the latter, this distance is 4.51 ꢁ. Thus,
stronger hydrogen bonding with oxygen atoms of the non-
chelated nitrate anion in conjunction with better proximity to
the free pyridine nitrogen that activates the electrophile
should promote the reaction comparatively better in CP-6
than in CP-7.
which metalloporphyrins serve as pillars.[27] To the best of our
knowledge, the results described herein constitute the first
examples of direct nucleophilic catalysis of acetylation of
phenols. The results of catalysis clearly demonstrate the fact
that porous CPs with partially uncoordinated N-heterocyclic
polydentate ligands can be exciting metal–organic materials
for catalytic applications.
Conclusion
We have designed and synthesized a twisted tetratopic D -
2d
2
2
symmetric ligand L, that is, 3,3’,5,5’-tetrakis(4-pyridyl)-2,2’,6,6’-
tetramethoxybiphenyl, which is endowed with considerable
torsional freedom for coordination polymerization. It is shown
that the tetrapyridyl ligand L undergoes metal-assisted self-as-
sembly to afford CPs of diverse topologies. The limited, yet
meaningful set of CPs amply suggest the fact that the ligand L
lends itself to diverse structures by exhibiting flexibility as a 3-
and 4-connecting molecular building block depending on the
nature of the metal ion and its associated counter anion. In
the CPs formed with CdCl , CoCl , CuI, CuCl, and AgNO , the
2
2
3
ligand L functions as a 4-connecting unit leading to predicta-
ble 3D open framework structures of PtS and diamondoid top-
ologies. In contrast, the ligand L serves as a 3-connecting
building block in its self-assembly with Zn(NO ) , Cd(NO ) , and
3
2
3 2
CuBr leading to 2D porous CPs of fes and hcb topologies. The
observation of free pyridyl rings being disposed into the void
regions in the CPs of L formed with Zn(NO ) and Cd(NO )
3 2
3
2
was exploited to demonstrate their functional utility as hetero-
geneous nucleophilic catalysts. A variety of phenols are shown
to be acetylated at RT in the presence of 10 wt% of CP-6 and
4
equivalents of Ac O. The fact that the catalysis truly occurs
2
within the voids of the CPs has been demonstrated based on
the reactivity of a sterically hindered phenol, which was found
to not undergo acetylation over a period of three days. The
crystallinity of the CPs as well as their catalytic efficacy has
been found to be unaffected even after three recycling
experiments.
Application of CPs and MOFs as catalysts in organic synthe-
[
8]
sis is an ongoing quest. The literature reveals only limited in-
stances, wherein one of the pyridyl rings in the CPs of poly-
[
12a,c]
pyridyl ligands remains uncoordinated.
Not surprisingly,
such CPs have been explored as catalysts. In a seminal paper
by Kim and co-workers on the application of the CPs formed
with pyridyl-functionalized d-tartaric acid in the presence of
Zn(NO ) , it was demonstrated that the asymmetric transesteri-
The chemical functionality in CPs for functional applications
is introduced in different ways, both prior to and post synthe-
sis. Partial coordination polymerization of a polydentate ligand
that leads to porous CPs containing uncoordinated center(s)
constitutes an alternative approach, provided that the uncoor-
dinated center serves as a chemical functionality. It is shown
that the uncoordinated pyridyl ring in the CPs of a tetratopic
tetrapyridyl can be exploited as a nucleophilic catalyst for
heterogeneous catalytic applications of CPs. We are currently
exploring ways of introducing chemical functionality through
this approach.
3
2
fication was promoted by the uncoordinated pyridyl groups lo-
[
25]
cated in the cavities of the MOF. Indeed, they also demon-
strated size-selectivity of substrates in the transesterification re-
action. Chen et al. reported a luminescent MOF formed with
III
pyridine-3,5-dicarboxylate and Eu salt in which the pyridyl
groups were found to be uncoordinated and exposed into the
channels of the framework. They showed that these open
[
26]
basic sites could be used in sensing of metal ions. Recently,
Huh et al. have shown that the Zn–bisSalen MOF (bisSalen=
N,N’,N’’,N’’’-tetrakis[3-tert-butyl-5-(4-pyridinyl)salicylidene]-1,2,
Experimental Section
4,5-benzenetetraamine), which contains free pyridyl groups
Synthesis of 4-connecting 2,2’,6,6’-tetramethoxy-3,3’,5,5’-
tetrakis(4-pyridyl)biphenyl (L)
disposed inward of the 1D channel of the MOF, could be
employed as a heterogeneous catalyst in transesterification
reactions.[12c] Hupp and co-workers have reported an
impressive example of acetyl transfer reaction in a MOF in
A mixture of toluene (15 mL), EtOH (10 mL), and water (5 mL) was
placed in a two-necked round-bottom flask and degassed by bub-
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bling N2 for 15 min. Subsequently, the flask was charged with
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.57 mmol), 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine
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4.21 g, 20.57 mmol), Na CO (2.18 g, 20.57 mmol), and Pd(PPh3)4
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4
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H NMR (500 MHz, CDCl , 258C, TMS): d=3.38 (s, 12H), 7.45 (s, 2H;
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[
Typical procedure for acetylation of phenols using CP-6 as
catalyst
Acetic anhydride (4 equiv) was added to a mixture of phenol
(
0.20 g) and CP-6 (20 mg, 10 wt%) catalyst in CH Cl (1.0 mL), and
2 2
placed in a capped vial. The reaction mixture was monitored by
TLC analysis. After completion of the reaction, the MOF catalyst
was isolated by filtration and washed thoroughly with ethyl ace-
tate. The filtrate was thoroughly washed with aqueous Na CO so-
2
3
lution to remove the unreacted anhydride and acetic acid. The or-
ganic layer was dried over anhydrous Na SO and evaporated to
2
4
obtain the pure products.
CCDC-1013777 (CP-1), 1013778 (CP-2), 1013779 (CP-3), 1013780
CP-4), 1013781 (CP-5), 659282 (CP-6), 1013783 (CP-7), and
013784 (CP-8) contain the supplementary crystallographic data
(
1
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·
coordination
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Received: July 24, 2014
Published online on && &&, 0000
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FULL PAPER
A miss by chance is a hit: Metal-assist-
ed self-assembly of a polypyridyl ligand
may lead to porous coordination poly-
mers (CPs) in which one of the pyridyl
groups misses coordination with the
metal ions (see figure). Such CPs can be
exploited as heterogeneous nucleophilic
catalysts for organic reactions. A variety
of phenols are acetylated at room
temperature with twisted tetrapyridyl-
biaryl-based CPs containing free pyridyl
groups as catalysts.
&
Coordination Polymers
S. Seth, P. Venugopalan, J. N. Moorthy*
& – &&
&
Porous Coordination Polymers of
Diverse Topologies Based on
a Twisted Tetrapyridylbiaryl:
Application as Nucleophilic Catalysts
for Acetylation of Phenols
&
&
Chem. Eur. J. 2014, 20, 1 – 10
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!