Copper based coordination polymers based on metalloligands: Utilization as heterogeneous oxidation catalysts

Article abstract of DOI:10.1039/c8dt03836h

This work presents the synthesis and characterization of two Cu(ii)-based coordination polymers prepared by utilizing two different Co(iii)-based metalloligands offering appended arylcarboxylic acid groups. Both coordination polymers are three-dimensional in nature and present pores and channels filled with water molecules. Both coordination polymers function as heterogeneous catalysts for the epoxidation of various olefins using O₂ while employing isobutyraldehyde as the coreductor and for peroxide-mediated oxidation of assorted benzyl alcohols. The catalytic results illustrate efficient oxidation reactions, whereas the hot-fltration test and leaching experiments indicate the true heterogeneous nature of the catalysis.

Full text of DOI:10.1039/c8dt03836h

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Copper based coordination polymers based on
metalloligands: utilization as heterogeneous
oxidation catalysts†
Cite this: Dalton Trans., 2018, 47,
16985
Gulshan Kumar, Firasat Hussain
and Rajeev Gupta
*
This work presents the synthesis and characterization of two Cu(II)-based coordination polymers prepared
by utilizing two different Co(III)-based metalloligands offering appended arylcarboxylic acid groups. Both
coordination polymers are three-dimensional in nature and present pores and channels filled with water
molecules. Both coordination polymers function as heterogeneous catalysts for the epoxidation of
various olefins using O2 while employing isobutyraldehyde as the coreductor and for peroxide-mediated
oxidation of assorted benzyl alcohols. The catalytic results illustrate efficient oxidation reactions, whereas
the hot-fltration test and leaching experiments indicate the true heterogeneous nature of the catalysis.
Received 22nd September 2018,
Accepted 2nd November 2018
DOI: 10.1039/c8dt03836h
rsc.li/dalton
robust yet catalytically active CPs.14 In this context, CPs based
on metalloligands have emerged as suitable alternatives to
Introduction
In recent times, coordination polymers (CPs) have emerged as metal–organic frameworks (MOFs) based on organic linkers.15
a novel class of porous materials due to their fascinating archi- Our group has developed a variety of metalloligand-based CPs
tectures1 and noteworthy applications in sorption,2 separ- functioning as noteworthy heterogeneous catalysts for various
ation,3 catalysis,4 sensing,5 conduction,6 and ion exchange and organic transformations.15,16 Such transformations vary from
transport.7 Such interest is largely due to their high surface Lewis-acid catalyzed reactions16a,b to oxidation reactions15b to
area, porosity, and tunable architectures that often regulate challenging coupling reactions.16c
their applications.1,8 For designing new CPs, many factors play
Oxidation reactions of olefins to epoxides and alcohols to
crucial roles in controlling both the dimensionality and topo- aldehydes are fundamental reactions to produce numerous
logy of the resultant material;9 however, the framework struc- fine chemicals.17 While many homogenous catalysts including
ture of a CP primarily depends on the coordination prefer- metal salts have been developed,18 their efficiency, stability
ences of a metal and the type of ligand as a linker.10 In this and recyclability are the major challenges.19 Therefore, signifi-
context, the use of carboxylate based ligands has been quite cant efforts have been devoted to develop heterogeneous cata-
successful in the construction of assorted CPs.11 Such a note- lysts to overcome such drawbacks.20 In this context, CPs
worthy feature is not only due to the ability of a carboxylate containing catalytically active transition metals provide better
group to coordinate a variety of metal ions, but also due to its options owing to their high surface area, porosity, stable
capability to support assorted coordination geometries.11 architectures, and particularly recyclability.21 Recently, a few
Another notable feature that carboxylate-based ligands impart copper-based CPs have been utilized as heterogeneous cata-
in the resultant CPs is their enhanced thermal stability that lysts for olefin-epoxidation and benzyl alcohol oxidation reac-
has greatly assisted in heterogeneous catalysis.12
tions, thus adequately justifying the role of Cu(II) ions in
In pursuit of robust heterogeneous catalysts, the structural promoting oxidation reactions.22 In this work, we present the
and thermal stability of CPs and the non-interpenetrable synthesis and characterization of two Cu(II)-based CPs, 1-Cu
nature of the resultant network play important roles.13 These and 2-Cu, and their role as heterogeneous catalysts for the
points warrant the development of next-generation stable and oxidation of a variety of olefins and assorted benzyl alcohols.
Department of Chemistry, University of Delhi, Delhi – 110 007, India.
Experimental section
Materials and reagents
E-mail: rgupta@chemistry.du.ac.in; Tel: +91-11-27666646
†Electronic supplementary information (ESI) available: Figures for FTIR, UV/VIS
and EPR spectra, TGA, DSC, PXRD, optical images, kinetic plots and a table for
All commercially available chemicals and reagents were used
without further purification unless otherwise stated. Standard
X-ray data collection and solution part. CCDC 1869008. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c8dt03836h
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methods were used to purify organic solvents. Metalloligands 1.171.33.49b.25 The structure was solved by direct methods
Et4N[Co(L4COOH)2] (1) and Et4N[Co(L3COOH)2] (2) were syn- using SIR-200426 and refined by using the SHELXL program27a
thesized according to our earlier report.23
in Olex227b in the space group I4/m in a tetragonal lattice
system. All non-hydrogen atoms were refined anisotropically.
All hydrogen atoms were located from the Fourier map.
Syntheses
[{(1)Cu2.5(µ4–H2O)0.625(H2O)2.5}·(H2O)8]n (1-Cu). Metalloligand However, in the solid-state structure, most of the lattice water
1 (100 mg, 0.100 mmol) was dissolved in 10 ml MeOH and a molecules were highly disordered, so those oxygen atoms were
5 ml MeOH solution of Cu(OAc)2·H2O (50 mg, 0.25 mmol) was modelled and refined with partial occupancy. The refinement
added to it under magnetic stirring. The mixing of two solu- also showed large solvent accessible voids of total potential
tions caused an immediate precipitation; however, the reaction solvent area volume of 2140 Å3 with the total volume per unit
mixture was further stirred for 2 h to ensure complete precipi- cell of 6261 Å3. For 1-Cu, about 34% (2140 Å3) of the unit cell
tation. The precipitate was filtered and dissolved in 2 ml water amounts to solvent accessible voids which can host the
and layered with tert-butanol. A pale green coloured crystalline remaining water molecules. The solvent accessible voids were
material resulted after a period of 6–7 d, which was filtered located at two positions 0(x) 0(y) 0.5(z) of 1070 Å3 and 0.5(x)
and dried under vacuum. Yield: 208 mg (83%). 0.5(y) 0(z) of 1070 Å3, respectively. As there was a large amount
C42H64.25CoCu2.5N6O23.12 (1223.13): calcd C, 41.24; H, 3.71; N, of diffused electron density (all less than 1.0 e Å3, however) the
6.78; found C, 41.59; H, 4.04; N, 6.79. FTIR spectrum (Zn–Se model was SQUEEZED to remove any diffused solvent by using
ATR, selected peaks): (ν/cm−1): 3401 (OH), 1573, 1565 (CvO).
the programme PLATON SQUEEZE.28 The latter improved R =
[(2)Cu2(µ2-OH)(H2O)6·{Cu(H2O)6}·2H2O]n (2-Cu). This com- 0.0688 for 2954 (observed) and R = 0.0756 for 29 481 reflections
pound was synthesized by layering an aqueous solution (3 ml) with 196 parameters and no restraints. The fact that the
of Cu(OAc)2·H2O (50 mg, 0.25 mmol) over a solution of metal- residual electron density was less than 1 eÅ3 signifies that this
loligand 2 (100 mg, 0.100 mmol) dissolved in DMSO (5 ml) large amount of diffused electron density may be due to a
with an intermediate layer of tert-butanol. A green coloured large number of water molecules present only in the voids.
crystalline product resulted at the interface of two solutions The final model being submitted here is the SQUEEZED one.
within 4–5 d that was filtered, washed with diethyl ether and The present structure solution is the best model with the avail-
dried under vacuum. Yield: 215 mg (88%). C42H71CoCu3N6O27 able data for poorly diffracting crystals of 1-Cu. Details of the
(1321.45): calcd C, 38.18; H, 3.89; N, 6.36; found C, 38.24; crystallographic data collection and structure solution para-
H, 3.75; N, 6.39. FTIR spectrum (Zn–Se ATR, selected peaks): meters are provided in Table S1 (ESI†). The network topology
(ν/cm−1): 3387 (OH), 1593, 1556 (CvO).
of 1-Cu was calculated by ToposPro 5.1.0.7 using the cluster
simplification method.29
Physical measurements
General procedure for the catalytic reaction
Elemental analysis data were obtained with an Elementar
Analysen Systeme GmbH Vario EL-III instrument. FTIR spectra
were recorded with a PerkinElmer Spectrum-Two spectrometer
having Zn–Se ATR. NMR spectral measurements were carried
out with a Jeol 400 MHz spectrometer. PerkinElmer Clarus 580
and Shimadzu QP 2010 instruments were respectively used for
gas chromatography (GC) and GC-MS studies having an
RTX-5SIL-MS column. Thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) were performed with
DTG 60 Shimadzu and TA-DSC Q200 instruments, respectively,
under a nitrogen atmosphere. X-ray powder diffraction studies
were performed either with an X’Pert Pro from PANalytical or a
Bruker AXS D8 Discover instrument (Cu-Kα radiation, λ =
1.54184 Å). The samples were ground and subjected to the
range of θ = 5–50° with a slow scan rate at room temperature.
The experimental X-ray powder diffraction patterns both for
1-Cu and 2-Cu were fitted to Le Bail refinement using TOPAS3
software with lattice parameters generated from the single
crystal X-ray diffraction data.24
In a typical epoxidation reaction, an olefin was treated with
isobutyraldehyde in 1 : 3 ratio under an O2 atmosphere in the
presence of 2 mol% of a catalyst (1-Cu or 2-Cu). Thin layer
chromatography (TLC) and/or gas chromatography (GC) tech-
niques were used to ascertain the progress of the reaction.
After completion, the reaction was quenched by the addition
of 2 ml of ethyl acetate which resulted in complete separation
of the catalyst which was filtered off and dried under vacuum.
The ethyl acetate layer was washed with water (×2 times) and
dried over anhyd. Na2SO4. The solvent was evaporated to
isolate the organic product(s). Using flash column chromato-
graphy on silica gel with 5% EtOAc/hexanes as the eluent, the
crude organic product(s) were purified and analyzed by the
1
GC/GC-MS techniques in all cases and H NMR spectroscopy
in a few selected cases.
Results and discussion
Crystallography
Synthesis and characterization of 1-Cu and 2-Cu
Single crystal X-ray diffraction data collection for 1-Cu was Green-coloured crystalline 1-Cu and 2-Cu were respectively syn-
done on an Oxford Xcalibur CCD diffractometer. The crystals thesized by the reaction of metalloligands 1 and 2 with
used were coated with paratone oil. The data collection, cell Cu(OAc)2. Both CPs show νO–H stretches at 3380–3400 cm−1
refinement and reduction were done by CrysAlisPro, ver. due to the presence of coordinated and lattice water molecules
16986 | Dalton Trans., 2018, 47, 16985–16994
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(Fig. S1 and S2, ESI†).15c,d,30 Amidic νcvo and arylcarboxylate (Fig. S7B and S8B, ESI†). Crystals of 2-Cu diffracted poorly and
νcoo stretches were noted as strong bands between therefore its crystal structure could not be obtained. However,
1557–1573 cm−1 15c,d,30
For both CPs, TGA confirmed the pres- its XRPD pattern closely matched with that of our earlier Ni-
.
ence of ligated and lattice water molecules by displaying based CP using the identical metalloligand (Fig. S8, ESI†).16c
weight loss between 30 and 160 °C (Fig. S3 and S4, ESI†). For Such a comparison allowed us to tentatively suggest its compo-
1-Cu, the observed weight change of 18.04% (calcd 18.51%) sition as follows: [(2)Cu2(µ2-OH)(H2O)6·{Cu(H2O)6}], where
corresponds to three coordinated and eight lattice water mole- [Cu(H2O)6]2+ balances the overall 2− charge on the polymer.16c
cules. A similar thermal change was noted for 2-Cu for the loss
Crystal structure of 1-Cu
of one bridging hydroxide and six coordinated and eight
lattice water molecules (Obs. 20.41%; calcd 20.37%). DSC 1-Cu was structurally characterized and the crystal structure is
studies further strengthen the TGA observations by displaying shown in Fig. 1. 1-Cu crystallized in a tetragonal cell with I4/m
exothermic features for the loss of ligated and lattice water space group. The asymmetric unit consisted of 1/4 of a metal-
molecules. Diffuse-reflectance absorption spectra for both CPs loligand, 1/2 Cu(II) ion, 1/4 of a bridging water molecule, one
display broad features at ca. 450 and ca. 715 nm (Fig. S5 and coordinated water molecule, and two lattice water molecules
S6, ESI†). The latter spectral band is assigned to the d–d tran- (Fig. 1a). Every metalloligand provides one negative charge
sition based on the Cu(II) ion.31 The broadness of the peak at whereas all four deprotonated carboxylate groups contribute
ca. 715 nm is also due to the presence of the d–d transition of four negative charges, therefore 2.5 Cu(II) ions balance the
Co(III) ions of a metalloligand.23,32
5− charge with the following network composition:
X-ray powder diffraction (XRPD) patterns confirmed the [{(1)Cu2.5(µ4–H2O)0.625(H2O)2.5}]n. This formula was established
crystalline homogeneity and single crystalline phase of the by a combination of electron diffraction density, TGA, and
bulk products (Fig. S7A and S8A, ESI†). In both cases, experi- elemental analyses.33
mental XRPD patterns were fitted to Le Bail refinement24 with
In a metalloligand, a Co3+ ion is coordinated by four Namide
lattice parameters generated from the single crystal X-ray diffr- groups in a distorted basal plane whereas two Npyridine atoms
action data and such an exercise provided good match occupy the axial positions maintaining a compressed octahedral
Fig. 1 (a) Asymmetric unit of 1-Cu, hydrogen atoms and lattice water molecules are omitted for clarity. Selected bond distances (Å): Co1–N(1),
1.958(4); Co1–N(2), 1.850(5); Cu1–O(5), 2.355(3); Cu1–O(10), 2.372(7); Cu1–O(1), 2.509. Selected bond angles (°): N(1)–Co1–N(2), 81.63(10); O(5)–
Cu1–O(10), 82.88(19); (b) stick representation of a selected part of the crystal structure of 1-Cu; colour codes: brown, cobalt; yellow, copper; grey,
carbon; red, oxygen. (c) Coordination environment around the Cu(II) ions and their bonding to Ocarboxylate groups from different metalloligands. (d)
3D network structure displaying the presence of pores in 1-Cu. (e) A section of 1-Cu showing the presence of lattice water molecules in the channels
throughout the network; (f) topological view of the network where yellow and green colours respectively represent Cu2+-based SBUs and Co3+
based metalloligands.
-
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geometry (Fig. 1b).16,34 Such a structural feature has been noted
The presence of secondary Cu(II) ions in two CPs suggested
in our earlier metalloligands and metalloligand-based CPs.16,34 their possible redox involvement. For such a purpose, both 1-
The Co–Namide bond distance (1.944) was noted to be longer than Cu and 2-Cu were suspended in a MeOH solution of KI.
the Co–Npyridine distance (1.833), as observed earlier.16,34
Notably, both 1-Cu and 2-Cu were able to oxidize the iodide
A metalloligand offers four arylcarboxylate fragments that ion to molecular iodine which further reacted with the free
coordinate to a group of four Cu(II) ions in different directions, iodide ion to form the triiodide ion. The formation of the triio-
thus generating a 3D network (Fig. 1b–d). Such four Cu(II) ions dide ion was confirmed by the observation of signature peaks
are bridged by a μ4-aqua group. Any Cu(II) ion displays a at ca. 450, 360 and 290 nm in the UV-visible spectrum
unique eight-coordinated geometry where six sites come from recorded in MeOH (Fig. S12, ESI†). The generation of the triio-
Ocarboxylate from different metalloligands, one from a μ4-bridg- dide ion infers that the secondary Cu(II) ions in the CPs were
ing water molecule and one from a terminal water molecule reduced to the Cu(I) state. Therefore, the EPR spectrum of 1-
(Fig. 1c). Therefore, the resulting secondary building units Cu was recorded before and after the addition of KI to gain
(SBUs) have the following composition: [Cu4(μ4-H2O)(RCOO)8]. insight into the mechanism. The solid-state powder EPR spec-
Such SBUs functioned as 8-connected nodes that were further trum of 1-Cu displayed tetragonal features with g⊥ = 2.08 and
connected to metalloligands, acting as four-connected g∥ = 2.34 (Fig. S13, ESI†).31 Such spectral features justify the
nodes.15b,16c A combination of SBUs and metalloligands gener- solid-state crystal structure. Interestingly, these spectral fea-
ates a 3D network with a flu topology and point symbol tures disappeared after the reaction with the iodide ion, thus
{412·612·84}{46}2 (Fig. 1f).35 1-Cu exhibited pores of 11.59 × supporting the reduction of the Cu(II) state to the Cu(I) state.
16.78 Å2 dimensions throughout the 3D network (Fig. 1e). As a reference, Co3+-based metalloligands do not react with the
Such pores were connected to form channels occupying lattice iodide ion. Importantly, the in situ generated Cu(I) state is re-
water molecules. We believe that such a structural feature oxidized to the Cu(II) form in open air, most probably due to
should benefit substrate accessibility during the hetero- the molecular oxygen. The facile redox conversion between the
geneous catalysis.
Cu2+–Cu+ states suggests the potential role of the present CPs
in redox-based catalytic reactions. Therefore, epoxidation of
olefins and oxidation of benzyl alcohols were investigated.
Exchange studies
The crystal structure of 1-Cu exhibited the presence of co-
ordinated water molecules. In order to evaluate their possible
Catalytic applications of 1-Cu and 2-Cu
displacement and/or exchange by a potential substrate or a The crystal structure of 1-Cu exhibited the presence of pores
reagent, we carried out a few exchange experiments.36 For the and channels whereas exchange and I2 uptake studies not only
solvent exchange experiment, 1-Cu was heated up to 80 °C inferred the possible replacement of the metal-bound water
under vacuum for 6 h in order to remove both coordinated molecules but also inclusion abilities. Such features together
and lattice water molecules. The desolvated 1-Cu was allowed with facile Cu2+–Cu+ conversion created potential catalytic
to equilibrate in
a
sealed environment containing D2O opportunities.16,22
vapours. This exercise resulted in exchange of both co-
ordinated and lattice H2O molecules by D2O which was
Epoxidation reactions
evident from the observation of νO–D stretches at 2500 cm−1 Oxidation of olefins is an industrially and commercially impor-
(Fig. S9, ESI†). A similar experiment involving MeOH vapours tant reaction to obtain various fine chemicals.17 In this
resulted in the exchange of water molecules with MeOH. For context, epoxidation is a significant reaction in organic chem-
such a sample, the νc–o (potentially ligated MeOH) stretch was istry as an epoxide can lead to a diverse range of ring-opened
noticed at 1020 cm−1 in the FTIR spectrum (Fig. S10, ESI†).
products.37 Because of its low cost and environmentally
friendly nature, epoxidation of olefins using molecular oxygen
as an oxidant and an aldehyde as a co-reactant is an attractive
Iodine uptake study
To evaluate the possible inclusion abilities of the present CPs, option.38 Both 1-Cu and 2-Cu were tested for the epoxidation
vapour uptake of molecular iodine was investigated using 1-Cu reactions. Initially, several control experiments were carried
as a representative case. For such an experiment, 20 mg out to optimize the reaction conditions to obtain the desirable
sample of desolvated 1-Cu was allowed to equilibrate in a product in high yield while minimizing the formation of unde-
sealed tube containing molecular iodine. The original green sirable by-products (Table 1). Several oxidizing agents such as
colour of 1-Cu was changed to dark brown suggesting the H2O2, TBHP (in water), TBHP (in decane), PhIO, N-methyl-mor-
inclusion of molecular iodine within the pores of 1-Cu pholine-N-oxide, meta-chloroperbenzoic acid, NaOCl and O2/
(Fig. S11, ESI†). The net weight gain was 1.9 mg corresponding isobutyraldehyde were screened with styrene as the model sub-
to 9.5% weight change. The inclusion of molecular iodine was strate and 1-Cu as a representative catalyst in order to find out
found to be reversible and application of either vacuum at the best oxidising agent (entries 1–7). Notably, only molecular
ambient temperature or suspending 1-Cu in any solvent was O2 with isobutyraldehyde resulted in high epoxidation with a
found to release I2 in a time-dependent manner. Such a fact small amount of benzaldehyde as the by-product. Various sol-
suggests the stable nature of CPs in accommodating and vents were screened to evaluate their possible effect on the
releasing molecular iodine from their porous structure.
epoxidation reaction (entries 8–12). Importantly, with most of
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Table 1 Optimization of reaction conditions for the epoxidation of styrenea
Yieldb (%)
Entry
Oxidising agent
Catalyst
Solvent/additive
T (°C)
E
A
1
2
3
4
5
6
7
8
H2O2
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu (0.5 mol%)
1-Cu (1.0 mol%)
1-Cu (1.5 mol%)
1-Cu (2.0 mol%)
1-Cu
1-Cu
1-Cu
1-Cu
1-Cu
CuCl
CuCl2
—
—
—
—
—
—
—
MeOH
EtOH
CH3CN
CH2Cl2
DMF
Pyridine
tert-BuOH
NaOAc
Morpholine
—
—
—
—
—
—
—
—
—
—
—
—
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
30
40
50
60
70
50
50
50
7
2
19
5
7
3
t-BuOOH
61
15
12
8
PhIO
N-Methyl-morpholine-N-oxide
m-Chloroperbenzoic acid
NaOCl
0
0
Isobutyraldehyde + O2
PhIO
90
15
52
43
21
17
9
60
86
87
19
42
68
87
55
72
90
78
63
0
10
32
27
19
8
11
3
28
14
13
4
9
t-BuOOH
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
Isobutyraldehyde + O2
7
10
13
19
17
10
22
37
0
0
0
0
0
Cu(OAc)2
a Conditions: catalyst: 2 mol%; time: 6 h. b Yield was calculated using gas chromatography.
the solvents, the amount of benzaldehyde as the by-product was the results are presented in Tables 2–4. In all cases, an olefin
quite high. More importantly, solvent-free reaction conditions was allowed to react with isobutyraldehyde in a 1 : 3 ratio in
provided the best results with a minimum amount of benz- the dioxygen environment with only 2 mol% of a catalyst (1-Cu
aldehyde (entry 7). We also attempted to understand the role of a and 2-Cu) at 50 °C under solvent-free conditions. Under these
few additives in the epoxidation reaction.23,39 The use of NaOAc, conditions, a smooth reaction took place producing a high
pyridine, t-BuOH and morpholine as the additives did not yield of the respective epoxide along with a small amount of
improve the epoxidation of styrene (entries 13–16). A variation in the corresponding aldehyde (Table 2). The presence of an
catalyst loading suggests that increasing the amount of 1-Cu electron-donating and/or electron-withdrawing substituent at
increased the epoxidation yield while the best results were the para-position of styrene did not result in a significant
obtained with 2 mol% catalyst load (entries 17–20; Fig. S14, difference in the epoxidation and both CPs were equally
ESI†). Epoxidation reactions were also attempted at different effective; producing 79–90% of styrene-epoxide and 10–21% of
temperatures. Quantitative conversion (epoxide: 90%; benz- the corresponding aldehyde (entries 1–6). Interestingly, when
aldehyde: 10%) was observed at 50 °C whereas both lower and sterically hindered substrates were used, the product yield
higher temperatures resulted in either a lower product yield or decreased due to the limited accessibility of a substrate
higher yield of benzaldehyde (entries 21–25). The use of a few through the pores and channels of the CPs (entries 7–9). In
metal salts (CuCl, CuCl2, and Cu(OAc)2) as the catalysts did not fact, an interesting trend was noticed between the substrate
result in any epoxidation of styrene (entries 26–28). Such a fact size and epoxidation yield: 1-vinylnaphthalene (7.00 × 8.81 Å2;
clearly establishes the critical role played by the copper-based 62%), 2-vinylnapthalene (7.00 × 10.37 Å2; 67%) and 9-vinylan-
CPs in promoting the epoxidation reactions.
thracene (8.42 × 10.34 Å2; 53%).40 Such a trend supports the
These control and optimization experiments allowed the effect of substrate size on the epoxidation reaction.15d
epoxidation reaction of assorted substrates to be explored and Although epoxidation of both cis-stilbene and trans-stilbene
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Table 2 Epoxidation of substituted styrenes and assorted olefins using
catalysts 1-Cu and 2-Cu under solvent-free conditionsa
Table 4 Epoxidation of linear olefins using catalysts 1-Cu and 2-Cu
under solvent-free conditionsa
Yieldb (%)
Yieldb (%)
1-Cu
E
2-Cu
E
1-Cu
E
2-Cu
E
Entry
R(Ar)–
A
A
1
2
3
4
78
62
67
73
22
31
23
7
74
67
73
68
26
28
27
10
Entry
R(Ar)-
A
A
1
2
3
4
5
6
7
8
–H
–OMe
–Me
–Cl
–NO2
–C(CH3)3
1-Vinylnaphthalene
2-Vinylnaphthalene
9-Vinylanthracene
cis-Stilbene
trans-Stilbene
90
79
83
82
85
83
62
67
53
47
43
10
21
17
18
15
17
7
88
82
82
86
87
85
65
64
49
51
46
12
18
18
14
13
15
9
9
7
6
8
a Reaction conditions: Catalyst 2 mol%; time: 6 h; temperature: 50 °C.
b Yield was calculated using gas chromatography.
5
3
5
8
9
10
11
to excellent yields of the desirable epoxide (E) were noted
whereas the amount of keto product (K-1) was limited (entries
1–5). In the case of cyclohexene, the respective dione (K-2) was
also obtained as the by-product in addition to the keto product
(K-1) (entry 2). Importantly, 1-methylcyclohexene showed high
epoxide formation among the series, possibly due to the +I
effect of the methyl group.41
Both CPs, 1-Cu and 2-Cu, were also effective in promoting
the epoxidation of long-chain terminal alkenes (entries 1–4,
Table 4). Both CPs were successfully able to oxidize 1-hexene
(74–78%), 1-heptene (62–67%), 1-nonene (67–73%), and
1-decene (68–73%) effortlessly. Notably, oxidative product con-
version (epoxide + ketone) of various terminal alkenes was
nearly quantitative in most cases.15d
a Reaction conditions: Catalyst 2 mol%; time: 6 h; temperature: 50 °C.
b Yield was calculated using gas chromatography.
Table 3 Epoxidation of cyclic olefins using catalysts 1-Cu and 2-Cu
under solvent-free conditionsa
Yieldb (%)
Benzyl alcohol oxidation reactions
1-Cu
2-Cu
After successful epoxidation reactions, the catalytic activities of
both 1-Cu and 2-Cu were further extended to the oxidation of
various benzyl alcohols.42 At first, control experiments were
conducted taking benzyl alcohol as a model substrate along
with 1-Cu as a representative catalyst to optimize the reaction
conditions (Table 5). These experiments allowed us to select
tert-butyl hydroperoxide (TBHP) as the best oxidant (entries
1–3). It was also noted that the substrate oxidation depended
upon the amount of TBHP being used in the catalytic reaction
(Fig S15, ESI†). Investigation of catalyst loading illustrates the
best results with 2 mol% load whereas a decreased amount of
catalyst hampered the oxidation (entries 4–7). For the oxi-
dation of benzyl alcohol, solvent-free reaction conditions
showed the best results when compared with those in different
solvents (H2O, MeOH, CH3CN, EtOAc, EtOH and DMF; entries
8–13). Additional experiments suggested the requirement of a
mild elevated temperature of 50 °C for the best oxidation
Entry
1
Substrate
E
K1/K2
E
K1/K2
89
11
83
17
2
3
4
5
81
79
80
91
19c
21
20
9
79
81
85
87
21c
19
15
13
a Reaction conditions: Catalyst 2 mol%; time: 6 h; temperature: 50 °C.
b Yield was calculated using gas chromatography. c In the case of cyclo-
hexene, the yields of both K-1 and K-2 products are shown together.
was comparatively poor (entries 10 and 11), benzaldehyde was (entries 14–17), whereas the use of Cu(OAc)2, CuCl and CuCl2
produced only in a trace amount.15d
as the catalysts produced negligible benzaldehyde (entries
Both CPs, 1-Cu and 2-Cu, were further used for the epoxi- 18–20). Such a fact necessitates the role of a well-defined cata-
dation of assorted cyclic olefins (Table 3).15d In all cases, good lyst in promoting the oxidation reaction.
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Table 5 Optimization of reaction conditions for the oxidation of benzyl
alcohol
better for smaller substrates when compared to the bulkier
substrates (entries 7–9). Notably, a similar observation was
noted for the epoxidation reactions of bulkier substrates.
Collectively, these results point towards the substrate accessi-
bility as a limiting factor.43
Leaching experiment
Entry
Catalyst/oxidising agents/solvents
T (°C)
Yield (%)
It is important to substantiate that the secondary Cu(II) ions
are not leached out from the present CPs during the course of
the catalytic reaction; therefore, a hot-filtration leaching test
was executed.15d,44 For the oxidation of benzyl alcohol using
TBHP, 1-Cu was selected as a representative catalyst to perform
this test (Fig. 2).42 During the course of catalytic reaction
(black dots), 1-Cu was filtered off after 3 h and the reaction
was further allowed to proceed (red dots). As can be seen from
Fig. 2, removal of 1-Cu resulted in nearly termination of oxi-
dation as was monitored till 10 h. Importantly, re-addition of
1-Cu to the same reaction mixture at 5 h immediately caused
significant oxidation of benzyl alcohol to benzaldehyde (green
triangles). This experiment unambiguously infers that the cata-
lytically active species are not leached out from the present
CPs and they act as true heterogeneous catalysts.
1
2
3
4
5
6
7
8
1-Cu + H2O2
50
50
50
50
50
50
50
50
50
50
50
50
50
30
40
50
70
50
50
50
5
12
97
22
53
82
97
78
78
71
23
54
24
62
83
97
95
2
1-Cu + PhIO
1-Cu + t-BuOOH
1-Cu + t-BuOOH (0.5 mol% Cat.)
1-Cu + t-BuOOH (1.0 mol% Cat.)
1-Cu + t-BuOOH (1.5 mol% Cat.)
1-Cu + t-BuOOH (2.0 mol% Cat.)
1-Cu + t-BuOOH, H2O
1-Cu + t-BuOOH, MeOH
1-Cu + t-BuOOH, CH3CN
1-Cu + t-BuOOH, EtOAc
1-Cu + t-BuOOH, EtOH
1-Cu + t-t-BuOOH, DMF
1-Cu + t-BuOOH
9
10
11
12
13
14
15
16
17
18
19
20
1-Cu + t-BuOOH
1-Cu + t-BuOOH
1-Cu + t-BuOOH
Cu(OAc)2 + t-BuOOH
CuCl + t-BuOOH
4
5
CuCl2 + t-BuOOH
Further support came from the inductively coupled plasma
mass spectrometry (ICP-MS) measurements of hot filtrates
from the catalytic reactions.22a–c The copper species leached
To expand the scope of catalysis, oxidation of various sub- into the solution was noted to be only 1.1 and 1.7% during the
stituted benzyl alcohols and related substrates was performed
epoxidation and benzyl alcohol oxidation reactions, respect-
using both CPs (Table 6). These alcohols were oxidized to their ively, using 1-Cu as the representative catalyst. On the other
corresponding aldehydes conveniently within 6 h. Both CPs hand, negligible leaching of copper (0.2%) was noted when 1-
were equally effective and resulted in high conversion. For
Cu was suspended in DMF for 48 h. These experiments indi-
example, benzyl alcohol and assorted para-substituted benzyl cate the stable nature of the present CPs during the hetero-
alcohols produced the corresponding aldehydes in nearly geneous oxidation reactions.
similar yields (entries 1–6). However, oxidation was slightly
Table 6 Oxidation of benzyl alcohol, substituted benzyl alcohols and
other bulkier alcohols using catalysts 1-Cu and 2-Cu under solvent-free
conditions
Yielda (%)
Entry
R−
1-Cu
2-Cu
1
2
3
4
5
6
7
8
9
–H
–OMe
–2-NO2
3-NO2
4-NO2
–Cl
Naphthalen-1-ylmethanol
Naphthalen-2-ylmethanol
Anthracen-9-ylmethanol
97
95
93
95
96
98
88
84
76
98
96
95
93
96
95
86
87
81
Fig. 2 (a) Oxidation of benzyl alcohol with tert-BuOOH in the presence
of 1-Cu as a catalyst. (b) Catalyst 1-Cu was filtered off at 3 h leading to
nearly termination of the catalytic reaction. (c) Catalyst 1-Cu was re-
added at 5 h causing commencement of the reaction. In all cases,
%-conversion was measured using a gas chromatograph.
Reaction conditions: Catalyst 2 mol%; time: 6 h; temperature: 50 °C.
a Yield was calculated using gas chromatography.
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thesized by using two unique cobalt-based metalloligands
decorated with appended arylcarboxylate groups. CP 1-Cu was
structurally characterised and it showed a porous 3D architec-
ture due to the coordination of secondary Cu(II) ions to the
appended carboxylate groups from metalloligands. Both CPs,
1-Cu and 2-Cu, acted as heterogeneous catalysts for the epoxi-
dation of various olefins using O2 and employing isobutyralde-
hyde as the co-reductor and for the peroxide-mediated oxi-
dation of assorted benzyl alcohols under solvent-free con-
ditions. Reusability and leaching experiments supported the
stable and true heterogeneous nature of catalysis, thus enhan-
cing the significance of the present copper-based coordination
polymers in oxidation catalysis.
Fig. 3 Reusability of 1-Cu as a catalyst for five consecutive cycles for
the oxidation of benzyl alcohol using 1-Cu as a catalyst.
Conflicts of interest
Recyclability experiment
There are no conflicts to declare.
Taking advantage of the heterogeneous nature of catalysis,
both CPs can be easily recovered from the reaction mixture by
simple filtration. Such a fact provided the opportunity to check
their recyclability. Taking 1-Cu as a representative catalyst,
recyclability was checked for five consecutive cycles with no
apparent loss of catalytic activity for the oxidation of benzyl
alcohol using TBHP (Fig. 3). In addition, convenient separ-
ation of CPs from the reaction mixture allowed characteriz-
ation of them and comparison of them with pristine samples.
The FTIR spectrum (Fig. S16, ESI†) and powder XRD pattern
(Fig. S17, ESI†) of the recovered 1-Cu were found to closely
match those of the pristine sample. Such a fact confirms that
both the structural integrity and crystallinity of a CP are
retained during the catalytic reaction.
Acknowledgements
RG acknowledges the financial support from the Science and
Engineering Research Board (EMR/2016/000888), New Delhi
and the University of Delhi. The authors thank CIF-USIC at
this university for the instrumental facilities including X-ray
data collection and AIRF-JNU, New Delhi for EPR and GC-MS
studies.
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