Trinuclear Mn2+/Zn2+based microporous coordination polymers as efficient catalysts foripso-hydroxylation of boronic acids

Article abstract of DOI:10.1039/d0dt00794c

Two microporous coordination polymers based on hourglass trinuclear building units, [Mn₃(bpdc)₃(bpy)]·2DMF and [Zn₃(bpdc)₃(bpy)]·2DMF·4H₂O (bpdc = 4,4′-biphenyl dicarboxylic acid, bpy = 4,4′-bipyridine), have been synthesized under solvothermal conditions employing DMF as the solvent. Each structure consists of two crystallographically distinct M²⁺(M1 and M2) centers that are connectedviacarboxylate bridges from six bpdc ligands, generating a trinuclear metal cluster, [M₃(bpdc)₃(bpy)]. Cluster representation of the structure resulted in an interpenetrated net of rarehextopological type. Catalytic activities of the CPs have been assessed for the oxidative hydroxylation of phenylboronic acids (PBAs) using aqueous hydrogen peroxide (H₂O₂). Various substituted aryl/hetero-arylboronic acids RB(OH)₂[R = phenyl, 2,4-difluorophenyl, 4-aminophenyl, 2-thiopheneetc.] underwentipso-hydroxylation smoothly at room temperature to generate the corresponding phenols in excellent yields. The main advantages of this protocol are the aqueous medium reaction, heterogeneous catalytic system, and short reaction time with excellent yield.

Full text of DOI:10.1039/d0dt00794c

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Trinuclear Mn2+/Zn2+ based microporous coordination polymers as
efficient catalysts for ipso-hydroxylation of boronic acids
Sanchay J. Bora,*a Rima Paul,a Anurag Dutta,b Shyam Goswami,a Ankur K. Guha c and
Ashim J. Thakur b
aDepartment of Chemistry, Pandu College, Guwahati-781012, Assam, India
bDepartment of Chemistry, Tezpur University, Napaam, Tezpur-784028, Assam, India
cDepartment of Chemistry, Cotton University, Guwahati-781001, Assam, India
Tel: +91-361-2570450, E-mail: sanchay.bora@gmail.com
Abstract
Two microporous coordination polymers based on hourglass trinuclear building units,
[Mn3(bpdc)3(bpy)].2DMF and [Zn3(bpdc)3(bpy)].2DMF.4H2O (bpdc = 4,4-biphenyl
dicarboxylic acid, bpy = 4,4-bipyridine) have been synthesized under solvothermal
conditions employing DMF as the solvent. Each structure consists of two
crystallographically distinct M2+ (M1 & M2) centers that are connected via carboxylate
bridges from six bpdc ligands, generating a trinuclear metal cluster, [M3(bpdc)3(bpy)].
Cluster representation of the structure resulted in the interpenetrated net of the rare hex
topological type. Catalytic activities of the CPs have been assessed for oxidative
hydroxylation of phenylboronic acids (PBA) with the aid of aqueous hydrogen peroxide
(H2O2). Various substituted aryl/hetero-arylboronic acids RB(OH)2 [R = phenyl, 2,4-
difluorophenyl,
4-aminophenyl,
2-thiophene
etc.] underwent ipso-hydroxylation
smoothly at room temperature to generate corresponding phenols in excellent yield. The
main advantages of this protocol are the aqueous medium reaction, heterogeneous
catalytic system, and short reaction time with excellent yield.
Keywords: Trinuclear metal cluster, topological analysis, ipso-hydroxylation,
phenylboronic acid, heterogeneous catalysis, density functional theory
CCDC Deposition Number 1987377-1987378 contains the supplementary crystallographic data for 1 & 2.
These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ,UK; fax: +44 1223 336
033; or e-mail: deposit@ccdc.cam.ac.uk.

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Introduction
In addition to the widespread application in the field of gas storage and separation,1-3
Metal-Organic Frameworks (MOFs) or porous coordination polymers (PCPs) have
publicized themselves to be worthy catalysts in a number of organic transformations.4-11
They belong to a fascinating class of porous crystalline materials built from metal
ions/nodes and poly-functional organic linkers.12-15 The uniform pore sizes/environments
and high surface area possessed by these materials provide an edge over traditional
microporous and mesoporous materials, including zeolite, mesoporous silica and
activated carbon. Although, a rapid progress has been witnessed in the construction of
functional MOFs, it is also a great challenge to rationally prepare and control the
structures and composition of the target molecules with desired properties.16-19 The most
effective and facile method of MOF construction involves intelligent selection of metal
ions and multifunctional organic linkers. Dicarboxylate ligands spanning network nodes
to link metal centers have proven to be good candidates for the construction of new
MOFs.20-23 Unlike other neutral linker molecules, these anionic dicarboxylate linkers
generate infinite coordination networks in which it is not necessary to accommodate other
counter ions to achieve electro neutrality. A linear dicarboxylate linker 4,4-
biphenyldicarboxylate has been used extensively in the synthesis of new porous
framework materials.24-27
Phenols are considered as versatile precursors to a large assemblage of polymers,
drugs, herbicides and anti-oxidants in simple and polyphenolic forms.28-30 The general
procedures for its synthesis involve nucleophilic substitution of activated aryl halides or
Cu-catalyzed conversion of diazoarenes31 and Pd-catalyzed conversion of aryl halides32
to the desired products. It is to be noted that the involvement of fairly inactive starting
materials in the concerned existing methodologies, leads to routes that are either
economically not viable for large-scale synthesis or require harsher conditions such as
high temperature, high pressure, handling hazardous chemicals etc. Adding to this list is
the requirement of additives, solvent issues, lower yields and longer reaction time. As the
quest for an efficient alternative methodology was on, arylboronic acids emerged as an
excellent starting material for the synthesis of phenols. The magnanimity of the
2

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importance of boronic acid moieties is due to their higher availability, stability and
greater functional group diversity.33-35 Concerning ourselves to the situation in hand,
reports say that ipso-hydroxylation of aryl boronic acids could be carried out by various
heterogeneous as well as homogeneous catalytic systems such as biosilica-H2O2,36 I2-
H2O2,37 Al2O3-H2O2,38 PEG-H2O2,39 NH2OH,40 KOH-TBHP,41 CuSO4-Phenantholine,42
H3BO4-H2O2,43 Amberlite IR-120 resin,44 supported silver nano particle,45 organic
hypervalent iodine (III),46 H2O2-natural base47 etc. As the homogeneous system suffers
from the issue of recyclability, we avert our attention towards the heterogeneous system.
Moreover, from earlier reports of homogeneous systems, it is also found that the amount
of H2O2 required was quite high. H2O2 is a potent organic functional group transformer
and its presence in excess might lead to undesired side products. Further, the reactions
involving minimal reagents are universally accepted from the green chemistry point of
view, as these reactions result in high atom efficiency, high atom economy, low E-factor
and high mass intensity. Therefore, it is highly desirable to develop synthetic
methodologies that practices minimum amount of reagents/catalysts. In our effort, we
tried to design a heterogeneous catalytic system, which would function efficiently in the
required reaction, all the while consuming minimal amount of H2O2. To the best of our
knowledge, ipso-hydroxylation has never been reported by using Mn(II)/Zn(II) as the
metallic core in the heterogeneous system. The novelty of this work lies in the catalysts
as well as efficacy of the catalytic system.
Herein, we report the synthesis of two microporous CPs based on
[M3(bpdc)3(bpy)] hourglass SBUs [M = Mn (1), Zn (2)] by solvothermal process. We
demonstrate the crystal structures as well as the topological exploration of the
isostructural crystalline solids. The remarkable catalytic activity of the MOFs towards the
conversion of PBAs into phenols in aqueous medium has also been presented. Also, we
have proposed a plausible mechanism for the catalytic reaction pathway, which has been
further established by using density functional theory (DFT).
Experimental
Materials and Methods
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Materials used in this work were obtained from commercial sources and used without
further purification. Zinc nitrate hexahydrate was purchased from Merck (India) whereas
manganese nitrate tetrahydrate, 4,4′-bipyridine and biphenyl-4,4'-dicarboxylic acid were
purchased from Sigma Aldrich, (USA). FT-IR spectra were recorded on a Perkin-Elmer
RX1 spectrophotometer in the mid-IR region (4000 to 450 cm-1) for KBr pellets.
Thermogravimetric (TG) data were obtained using a Mettler Toledo TGA/DSC1 STARe
system in the 40-800 C range. Thermal decompositions of all the complexes were
studied under N2 atmosphere at a heating rate of 10 °C min-1. Magnetic susceptibilities
were measured at 300K on a Sherwood Mark 1 Magnetic Susceptibility Balance by
Evans Method. The specific surface area and porosity were measured in a Backman
Coulter Surface Area and Pore Size Analyzer (SA-3100). For this, the sample was first
degassed for 2 h at 200 °C and then BET analysis was performed.
Synthesis of [Mn3(bpdc)3(bpy)]·2DMF (1)
.
A mixture of manganese nitrate tetrahydrate Mn(NO3)2 4H2O (0.251g, 1mmol), bpdc
(Na2C14H8O4) (0.242g, 1mmol) and bpy (C10H8N2) (0.156g, 1mmol) in 10 ml DMF was
placed in a stainless steel autoclave, which was sealed and placed in a programmable
oven. The mixture was heated at 120 °C at 5 °C/min for 24hrs followed by further
cooling at room temperature. The resulting colorless crystals were filtered, washed with
water and then dried in a vacuum desiccator over fused CaCl2. Yield: 0.27 g (67%). eff =
6.17 BM. IR spectral data (KBr disc, cm-1): 3407(w), 3045(w), 2849(w), 1606(s),
1536(sh), 1380(s), 1289(s), 1068(w), 1005(w), 813(w), 772(m), 696(w), 628(w) [s,
strong; m, medium; w, weak; br, broad; sh, shoulder].
Synthesis of [Zn3(bpdc)3(bpy)]·2H2O·4H2O (2)
.
A mixture of zinc nitrate hexahydrate Zn(NO3)2 6H2O (0.297g, 1mmol), bpdc
(Na2C14H8O4) (0.242g, 1mmol) and bpy (C10H8N2) (0.156g, 1mmol) in DMF was placed
in a stainless steel autoclave, which was sealed and placed in a programmable oven. The
mixture was heated at 120 °C at 5 °C/min for 24 hrs followed by further cooling at room
temperature. The resulting colorless crystals were filtered, washed with water and then
dried in a vacuum desiccator over fused CaCl2. Yield: 0.31 g (72%). IR spectral data
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(KBr disc, cm-1): 3416(w), 3073(w), 2890(w), 1606(s), 1541(w), 1410(w), 1541(m),
1282(s), 1074(w), 1006(w), 825(w), 776(s), 681(s), 640(m).
X-ray Crystallographic Procedures
Suitable crystals of compound 1 & 2 were obtained by solvothermal process. In each
case, the chosen crystal was mounted on a glass fibre for intensity data collection at room
temperature using graphite monochromatized Mo-Kα radiation on a Bruker SMART
CCD Diffractometer.48 The unit cell measurement, data collection (φ and ω scan),
integration, scaling and absorption corrections for the crystals were done using Bruker
Apex II software.49 Both the structures were solved using direct method followed by full
matrix least square refinements against F2 (all data HKLF 4 format) using SHELXTL.50 A
multi-scan absorption correction, based on equivalent reflections, was applied to the data.
Anisotropic refinement was used for all non-hydrogen atoms. Hydrogen atoms were
placed geometrically and held in the riding model. The disordered carboxylate O-atoms
in both the structures were modelled with PART and SIMU commands in SHELXL. The
contributions of the most disordered solvent molecules were removed from the
diffraction data of 2 using the SQUEEZE routine of PLATON software,51 and then final
refinements were carried out. The structural diagrams were drawn using Diamond.52
Powder X-ray diffraction patterns in the 3−60º 2-range were recorded on a
Philips X’Pert PRO instrument using Cu-Kα radiation (1.5418 Å) at a scan rate of 0.5 sec
(0.5º 2per step at 40 KV/30 mA. The calculated diffraction patterns assuming
Bragg-Brentano geometry were obtained from results of single crystal structure analyses
using the computer program PowderCell.53
Topological Analysis
Topological analysis was performed with the ToposPro program package and the TTD
collection of periodic network topologies.54 The RCSR three-letter codes55 were used to
designate the network topologies. Those nets that are absent in the RCSR are designated
with the TOPOS NDn nomenclature,56 where N is a sequence of coordination numbers of
all non-equivalent nodes of the net, D is the periodicity of the net (D = M, C, L, T for 0-,
1-, 2-, 3-periodic nets), and n is the ordinal number of the net in the set of all non-
isomorphic nets with the given ND sequence. The standard description of a MOF
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includes simplification procedure, i.e., representation of the network in terms of a graph-
theory approach. Metal atoms remain intact during simplification, but ligands are
represented by its center of mass, keeping the connectivity of the ligand with its
neighbors. Thus, this description characterize the way through which ligands and metal
centers are connected.
Catalysis
In a 50 mL round-bottomed flask, a mixture of PBA (1 mmol), H2O2 (30% aq, 0.2 mL),
Mn/Zn-MOF (5 mg) and 2 mL of water was added and stirred at room temperature in
aerobic condition. The reaction was monitored by TLC. After completion, the reaction
mixture was diluted with 20 mL of water and extracted with (3  20) mL of diethylether
and the organic layer was washed with brine, dried over Na2SO4 and evaporated in a
rotary evaporator under reduced pressure. The crude was purified by column
chromatography (hexane/ethyl acetate, 9:1) on mesh silica (100–200) to get the desired
product. The products were confirmed by 1H NMR and 13C NMR spectroscopy.
Computational Details
To investigate the energetic of the proposed reaction mechanism (see text below), we
have carried out theoretical calculations using density functional theory. All the structures
were fully optimized without any symmetry constraints at M06-2X/6-311++G** level of
theory.57 For the sake of computational time, we have considered only the monomer
complex. Harmonic vibrational frequency calculations were performed at the same level
of theory to understand the nature in the potential energy surface. All intermediates and
products are found to be at their local minima with all real values of the hessian matrix
while the transition state was found to have one imaginary value of hessian matrix. All
these calculations were performed using Gaussian 16 suite of program.58
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Table 1 Crystal and structure refinement data for [Mn3(bpdc)3(bpy)]·2DMF (1) &
[Zn3(bpdc)3(bpy)]·2DMF·4H2O (2)
Details
[Mn3(bpdc)3(bpy)]·2DMF (1)
[Zn3(bpdc)3(bpy)]·2DMF·4H2O (2)
Empirical formula
Formula weight
C58H46N4O14Mn3
1187.81
C58H54N4O18Zn3
1291.10
Temperature/ K
293(2)
293(2)
Wavelength
Crystal system, space group
a/Å
0.71073Å
0.71073Å
Orthorhombic, Pbcn
14.5387(4)
25.0165(8)
18.1249(7)
6592.2(4)
4, 1.228
1.141
2496
Orthorhombic, Pbcn
14.5419(9)
25.0959(17)
18.1368(12)
6618.9(7)
4, 1.192
0.622
2436
b/Å
c/Å
Volume/Å3
Z, Calculated density/ gcm-3
Absorption coefficient/ mm-1
F(000)
Crystal size/ mm
range for data collection/º
Index ranges
0.32 × 0.24 × 0.12
1.6 to 25.2
-17 ≤h ≤10, -29 ≤k ≤30, -21 ≤l ≤20 -20 ≤h ≤18, -34 ≤k ≤34, -25≤l ≤25
0.18 × 0.15 × 0.12
2.2 to 30.1
Reflections collected/ unique
Refinement method
Data / restraints / parameters
Goodness-of-fit* on F2
28622 / 5957 [R(int) = 0.042]
Full-matrix least-squares on F2
5957/36/387
90716/ 9564 [R(int) = 0.048]
Full-matrix least-squares on F2
9564 / 24 / 387
1.084
0.058 / 0.1708
1.050
Final R indices† [I>2(I)] R1/ 0.083 / 0.246
wR2
R indices† (all data) R1/ wR2
0.105 / 0.273
0.000
0.0836 / 0.184
0.038
1.54 and -1.47
Shift / esd max
Largest diff. peak and hole /e.Å-3 2.002 and -0.906
†wR2 = { [ w(Fo2–Fc2)2 ] /  [ w(Fo2)2 ] }1/2; R1 =  | |Fo| – |Fc| | /  |Fo *GooF = S = { [w(Fo2- Fc2)2] / (n-p)2}1/2
Results and Discussion
Colorless
block-shaped
crystals
of
[Mn3(bpdc)3(bpy)].2DMF
(1)
and
[Zn3(bpdc)3(bpy)].2DMF.4H2O (2) were obtained via solvothermal reaction of the
respective metal nitrates with bpdc and bpy using DMF as a solvent. In an earlier study,59
a species of similar composition as that of 2 was obtained in the form of colorless crystals
by reacting a pre-formed compound Zn(bpdc)(H2O)2·H2O with bpy in DMF. In the
present study, we have succeeded in isolating pure samples of
[Zn3(bpdc)3(bpy)].2DMF.4H2O (2) in good yield by following a simple synthetic method
as described in the experimental section. Both of these solid species 1 and 2 are sparingly
soluble in water and common organic solvents such as alcohol, acetone etc. They are
non-hygroscopic and compositionally stable in air at room temperature (up to 40 °C) for
an extended period of time.
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Structural description
Single crystal X-ray diffraction analyses reveal that the compounds 1 and 2 are analogous
because these two crystalline solids differ only on the basis of the metal ions present and
may be termed as isostructural. Both these solids crystallize in the orthorhombic system
with space group Pbcn. The asymmetric unit of each structure consists of two
crystallographically distinct M2+ (M1 & M2) centers that are connected via carboxylate
bridges from six bpdc ligands, generating a trinuclear (hourglass type) metal cluster,
[M3(bpdc)3(bpy)] (Figure 1). The octahedrally coordinated M2 center is linked to two
tetrahedral M1 centers through six bridging bpdc ligands. Each of these two M1···M2
couples (i.e., M1···M2···M1) is connected via two 2- and one 3- mode of carboxylate
bridges. The remaining two apical positions of the tetrahedral M1 centers are occupied by
the N-atoms of two bpy molecules. In these structures, the M2 ion lies at the special
position (0,y,¼), while the rest of the atoms lie at general positions. The selected bond
parameters for compounds 1 and 2 are listed in Table S1. The three M–O (carboxylate)
distances for the terminal M1 centers lie in the range 1.9341(3)-1.9599(1) Å, whereas for
the central M2 atom they are between 2.0348(4) and 2.1449(1) Å. From the observed
M1−O and M1−N distances (~2 Å) and L−M1−L angles (ranging from ~95° to ~121° for 1
& 2), it is evident that that the tetrahedral ligand arrangement around M1 is considerably
distorted. The bridging carboxylate oxygens viz., O(4), O(5) and O(6) connected to M2
center are disordered over two positions and was modelled accordingly using SHELXL
(Experimental Section). The adjacent metal nodes within the cluster are separated from
each other at a distance of 3.5378(2) Å and 3.5367(3) Å for the compounds 1 and 2
respectively. Further, these trinuclear building units are connected through the bpy
ligands in 1 with an intermetallic separation of the 11.1836(7) Å extending in the
direction of crystallographic c-axis. The corresponding distance observed in compound 2
is 11.17751(6) Å.
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Figure 1. (a) Molecular structure of [Mn3(bpdc)3(bpy)]·2DMF (1) presented in wireframe. Mn atoms are
shown in pink, O atoms in red, N atoms in blue and C atoms in green. (b) ORTEP diagram of the
asymmetric unit with 50% probability ellipsoids. Only non-hydrogen atoms of the asymmetric SBU are
labelled.
The crystal structure analysis of [Zn3(bpdc)3(bpy)]·2DMF·4H2O (2) has been
carried out and solved successfully confirming Pbcn to be the most likely space group for
the compound, whereas the previously reported structure59 was determined by assuming
P21/n as the space symmetry. Moreover, the number of uncoordinated solvent molecules
obtained for both the compounds were also different. The standard representation
(ToposPro Analysis) of the structure resulted in the underlying net of 4,4,4,6T43
topological type (Figure 2). The observed double interpenetrated net of hex topological
type is quite rare among the MOFs. During the cluster simplification procedure more
complex building blocks of a structure can be identified. In the structure under
examination, complex trinuclear zinc cluster was recognized.
Figure 2. (a) A standard representation of the Zn-MOCP (2). The 4- and 6-coordinated zinc atoms and 4-
coordinated biphenyl 4,4-dicarboxylates are considered as the nodes of the underlying net. (b) & (c)
Depiction of the double interpenetrated hex underlying net formed via the 8-connected metal clusters by
means of the 4,4'-biphenylene moieties (yellow) and 4,4'-bipyridine ligands (green).
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TGA, PXRD and Physisorption Studies
The observed peaks in the experimental powder patterns of 1 and 2 are also seen in the
powder pattern calculated on the basis of structural results obtained from single-crystal
X-ray data. A comparison of experimental PXRD patterns of 1 and 2 along with the
calculated pattern of 1 is shown in Figure 3. The close similarity between observed and
calculated PXRD patterns suggests the purity of crystalline bulk sample and also the
authenticity of the structure described by us. Powder XRD data with hkl indices for the
prominent lines have been determined by comparing the two patterns. Furthermore,
nearly super-imposable PXRD patterns of both the complexes confirm their structural
analogy.
Figure 3. Comparison of PXRD patterns obtained for 1 and 2 along with the calculated (indexed) pattern.
The structural analogy between 1 and 2 was evaluated by using a method
developed by Fábián and Kálmán. According to this, for highly similar structures, the
unit cell similarity index60 (Π) value is expected to be equal to zero. In the present study,
this value is estimated to be 0.00163835, which corroborates the close structural
similarity obtained from crystal structure analyses. The other two related parameters61
viz., mean elongation value (ϵ) and the so-called asphericity index (A) of the compounds
were found to be 0.00134934 and 0.454187 respectively. Further, the product ϵA, also
known as lattice distortion index approaches the value of 0.000612854. The above figures
clearly indicate the isostructurality of the two crystalline solids.
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Thermal decompositions of 1 and 2 were studied up to a temperature of 800 °C
under N2 atmosphere and the resulting patterns are presented in Figure 4. Both the
compounds release the constituent organic matters in an unidentified manner leaving
behind Mn3O4 and ZnO as the final products of 1 and 2 respectively at ~550 °C.
However, at temperature 90-160 °C, the Zn-CP shows a combined weight loss of 11.5%
indicating the expulsion of four water molecules along with two molecules of DMF
(calculated weight loss of 11.32%) from the solid sample. While the gaseous products
have not been experimentally identified by us, the observed PXRD patterns (SI) confirm
the identity of the residue. Nevertheless, the TG results clearly suggest that the polymeric
species 1 and 2 can be thermolysed to produce their respective metal oxides as pure
products at relatively low temperature.
Figure 4. Temperature dependent weight loss curves of 1 and 2 under flowing N2 gas.
The surface area and porosity of Zn-CP (2) were measured using Brunauer–
Emmett–Teller (BET) equation following the Barrett–Joyner–Halanda (BJH) method.62
From the N2 adsorption–desorption isotherm of 2 in Figure S2, the BET surface area of
the particles is 11.0227 m2g−1 as calculated by linear part of the BET plot. The total pore
volume at P/Po = 0.985 is 0.0194 cm3g−1. The BET isotherm corresponds to type IV,
which is the characteristic of microporous adsorbents.63
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Catalytic activities
Catalytic properties of as-synthesized CPs were assessed for oxygenated ipso-
hydroxylation of a number of substituted PBAs using H2O2 in aqueous medium (Scheme
1). The reaction between PBA (1 mmol) and 30% aqueous H2O2 (2 mL) was carried out
in the presence of Zn-CP (15 mg), which was completed within a short period of 0-5
minute with an isolated yield of 95%. A blank reaction was performed with 30% aqueous
H2O2 and without the catalyst. This reaction was continued for 12 hrs. The result was
only 45% conversion of PBA to phenol. Further, hydroxylation of the PBA in the
presence of the catalyst, only, did not yield phenol. Hence, it is obvious that both H2O2
and Zn-CP are needed for the effective conversion of PBA to phenol. In addition to this, a
series of reactions were performed in order to optimize the reaction conditions for
effective hydroxylation of PBAs. At first, the volume of H2O2 was gradually lowered
from 2 mL to 0.2 mL, keeping the Zn-CP amount fixed at 15 mg (Entries 4-6, Table 2).
Noticeably, it was observed that merely 0.2 mL of the oxidant was enough to get the best
conversion in the presence of 15 mg of the catalyst. Another set of experiments was also
performed to optimize the amount of catalyst required.
Scheme 1. Ipso-hydroxylation of aryl/hetero-arylboronic acids
We began with minimizing the amount of the catalyst with 0.2 mL of the oxidant (30%
aqueous H2O2) and it was found that 5 mg of the catalyst was effective enough for the
desired conversion (Entry 8, Table 2). The same procedure was followed to derive an
optimized reaction scheme using Mn-CP as the catalyst for the conversion of PBA to
phenol. Thus, the optimum yielding reaction condition was to use 1 mmol of PBA, 0.2
mL of 30% aqueous H2O2, and 5 mg of the catalyst in 2 mL of water (Entry 8, Table 2).
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Table 2 Optimization reaction conditiona for catalyst and H2O2
Entry M-CP H2O2 Water Time
Yield(%)b Yield(%)b
(mg)
(ml)
(ml)
(M=Zn)
(M=Mn)
1
2
3
4
5
6
7
8
15
-
2
2
-
1
0.5
0.2
0.2
0.2
-
-
-
2
2
2
2
2
0-5min
12h
12h
0-5min
0-5min
0-5min
0-5min
0-5min
95
45
0
95
95
95
95
95
93
45
0
93
93
93
93
93
15
15
15
15
10
5
Standard Conditions: aPBA (1 mmol), M-MOF, H2O2, H2O (2 mL), bisolated yield (%)
These encouraging findings, prompted us to explore the applicability of the
protocol with wide array of sterically hindered and substituted aryl/hetero-arylboronic
acids. Phenols with electron withdrawing and electron donating groups at the ortho-,
meta-, and para- positions, such as -CN, -CHO, -OMe, -Me, -Et, and -Cl were obtained
with moderate to excellent yields (Table 3). Interestingly, we didn’t notice any
significant contribution of the electronic factor over the yield. However, the time required
for the ipso-hydroxylation of sterically hindered arylboronic acids and also the ones
containing electron withdrawing groups, was found to be comparatively higher.
A plausible mechanistic pathway for Zn-CP catalyzed ipso-hydroxylation of
arylboronic acids is depicted in Scheme 2. It was further established by DFT
calculations (Figure S4). The first step is the addition of H2O2 to A to generate the
intermediate B. The oxygen atom of H2O2 molecule attacks the Zn-center of the catalyst
and thereby facilitating the cleavage of Zn-O(carboxylate) bond. This step is found to be
exergonic by 6.4 kcal/mol. The next step is the addition of Ph-B(OH)2 to B to generate
another intermediate C. In this step, addition of Ph-B(OH)2 cleaves one of the Zn-O bond
of the active site, thereby resulting in the detachment of the -Ph-COOH group, which
obviously remains attached to the CP. The following steps involve the nucleophilic attack
of the peroxide ion on boronic acid. This step is found to be endergonic by 8.2 kcal/mol.
The intermediate C then releases Ph-OB(OH)2 and forms the intermediate D. This step is
again exergonic by 2.3 kcal/mol. Subsequent release of H2O regenerates the active site of
the catalyst. Intermediate C undergoes aryl group migration from boron to oxygen (1,2-
aryl shift) generating Ph-OB(OH)2, which on hydrolysis produces the major product,
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phenol. We believe that the reaction pathway for the Mn-CP catalysed ipso-hydroxylation
of PBAs will follow the same mechanism as done by Zn-CP.
Table 3. Zn/Mn-CP catalysed oxidative hydroxylation of aryl/hetero-arylboronic acids.
(I) 0-5min
Zn-CP: 95%
Mn-CP: 93%
(II) 20min
Zn-CP: 92%
Mn-CP: 92%
(III) 10min
Zn-CP: 92%
Mn-CP: 90%
(IV) 8min
Zn-CP: 93%
Mn-CP: 92%
(V) 15min
Zn-CP: 85%
Mn-CP: 88%
(VI) 20min
Zn-CP: 91%
Mn-CP: 90%
(VII) 0-5min
Zn-CP: 95%
Mn-CP: 90%
(VIII) 30min
Zn-CP: 94%
Mn-CP: 89%
(IX) 0-5min
Zn-CP: 92%
Mn-CP: 90%
(X) 30min
Zn-CP: 87%
Mn-CP: 83%
(XI) 0-5min
Zn-CP: 93%
Mn-CP: 93%
(XII) 10min
Zn-CP: 90%
Mn-CP: 88%
(XIII) 25min
Zn-CP: 91%
Mn-CP: 90%
(XIV) 20min
Zn-CP: 85%
Mn-CP: 89%
(XV) 20min
Zn-CP: 93%
Mn-CP: 90%
aStandard Conditions: Aryl/hetero-arylboronic acid (1 mmol), M-MOF (5 mg), H2O2 (0.20 mL), isolated
yield (%), all the compounds were characterized by 1H and 13C NMR spectroscopy.
The catalysts were tested for their reusability using the model reaction and it was
found that their catalytic activities were well maintained up to the 6th cycle. Since the
amount of catalyst required is very low, testing the recyclability beyond this point was
not carried out due to difficulties in the efficient isolation of the catalyst. Further, at low
substrate concentration (5 mmol PBA), leaching of bpdc and bpy ligands were not
detected in the reaction mixture even up to 6th cycle. However, during scaling up the
reaction (with 10 mmol PBA), traces of bpy were detected in the 4th-6th cycles of the
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catalyst recyclability test (Table 4). The presence of bpy ligand in the reaction mixture
was confirmed under UV light and also by using TLC. As it is evident from the reaction
mechanism that bpy is hardly playing any role in the catalytic process, the trace amount
of leaching might have caused a slight lowering of the yield of phenol, but the overall
performance was fair enough to establish that the CPs were highly stable during the
course of the reaction. It should also be noted that bpdc did not appear in the reaction
mixture at any stage of the reaction or during the recyclability test, even up to the 6th
cycle.
Moreover, the heterogeneous nature of the catalyst was determined by inductively
coupled plasma mass spectrometry (ICP-MS) analysis and Atomic absorption
spectroscopy (AAS) analysis.
Scheme 2. Plausible mechanism for Zn-CP (2) catalysed ipso-hydroxylation of PBA.
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Table 4. Reusability of the catalysts 1 and 2 in the model reactiona
Entry
Run
Time
(min)
Yield
cbpy detected
during the use of
Zn-CP
Yield
cbpy detected during the
(Zn-CP)b
(Mn-CP)b
use of Mn-CP
1
2
3
4
5
6
1st
2nd
3rd
4th
5th
6th
1
2
2
5
5
6
95
95
95
93
93
90
None
93
93
92
90
88
85
None
None
None
Trace
Trace
Trace
None
None
None
Trace
Trace
Standard Conditions: aPBA (5 mmol), M-MOF (25 mg), H2O2 (1 mL), H2O (10 mL), bisolated yield (%).
cReaction conditions: PBA (10 mmol), M-MOF (50 mg), H2O2 (2 mL), H2O (10 mL)
In both the analyses, the filtrates, collected after the completion of Zn-CP and
Mn-CP catalyzed reaction, were subjected to the respective metal ion detection protocol,
i.e. to check if Zn and Mn have leached out of the MOF and has catalyzed the reaction in
a homogeneous way. However, the metal ions were not detected in the respective filtrates
and thus it can be ascertained that the metal ions did not leach out during the reaction and
catalyzed the reaction heterogeneously. We would also like to mention that the hot
filtration test to determine the heterogeneous nature of the catalyst was not performed
because Hydrogen peroxide is used as an oxidant and it can also carry forward the
reaction, to a certain extent.
Conclusions
Solvothermal
synthesis
of
two
microporous
CPs
containing
linear
biphenylenedicarboxylic acids as ligands have been described. Both the compounds
adopt [M(bpdc)3(bpy)] secondary building units (SBUs) leading to networks of
interpenetrated net with rare hex topology. Catalytic activities of as-prepared complexes
were tested for ipso-hydroxylation of PBAs using hydrogen peroxide. These room
temperature reactions give isolated an yield of the desired product up to 95%. The
reusability of Zn-catalyst was witnessed up to 6 runs. To the best of our knowledge, ipso-
hydroxylation has never been reported by using Zn/Mn as the metallic core in
heterogeneous system. The novelty of this work lies in the catalyst as well as efficacy of
the catalytic system.
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Acknowledgements
Financial support from Department of Science and Technology, India (FASTTRACK
Grant No. SB/FT/CS-047/2013) is gratefully acknowledged. The authors thank
Department of Instrumentation and USIC, Gauhati University, Guwahati for providing
with the X-ray diffraction data. The authors also thank Dr. Nithi Phukan and Dr.
Achintya Jana for their help during refinement of the X-ray crystallographic data.
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Table of Contents for
Trinuclear Mn2+/Zn2+ based microporous coordination polymers as
efficient catalysts for ipso-hydroxylation of boronic acids
Sanchay J. Bora,*a Rima Paul,a Anurag Dutta,b Shyam Goswami,a Ankur K. Guha c and Ashim J. Thakur b
Two efficient catalysts based on trinuclear Mn(II)/Zn(II) clusters were
synthesized and used for ipso-hydroxylation of phenylboronic acids into the
corresponding phenols with excellent yield.
Graphical Abstract
19