Lanthanide-based coordination polymers as promising heterogeneous catalysts for ring-opening reactions

Article abstract of DOI:10.1039/c5ra26283f

This work presents the synthesis and structural characterization of Eu³⁺- and Tb³⁺-based coordination polymers starting from a Co³⁺-based metalloligand offering appended arylcarboxylic acid groups. Both coordination polymers function as reusable heterogeneous catalysts for ring-opening reactions utilizing amines, alcohols, thiols, and azides as the nucleophiles. The catalytic results illustrate an excellent control over the regioselectivity whereas a filtration test and mechanistic studies substantiate Lewis-acid catalyzed activation of the epoxide during the reaction.

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Lanthanide-based coordination polymers as the promising
heterogeneous catalysts for ring-opening reactions
Gulshan Kumar,a Girijesh Kumara† and Rajeev Guptaa*
Received 00th January 20xx,
Accepted 00th January 20xx
This work presents the synthesis and structural characterization of Eu3+– and Tb3+–based coordination polymers starting
from a Co3+–based metalloligand offering appended arylcarboxylic acid groups. Both coordination polymers function as the
reusable heterogeneous catalysts for the ring-opening reactions utilizing amines, alcohols, thiols, and azides as the
nucleophiles. The catalytic results illustrate an excellent control over the regioselectivity whereas filtration test and
mechanistic studies substantiate a Lewis – acid catalyzed activation of the epoxide during the reaction.
DOI: 10.1039/x0xx00000x
www.rsc.org/
such CPs were highly crystalline nature; robust architectures;
presence of catalytically relevant secondary metal ions; and
ubiquitous occurrence of labile water molecules on the catalytically-
Introduction
Over the past few decades, extensive research has been devoted to
develop coordination polymers (CPs) not only due to their
structural diversity and fascinating molecular topologies1 but also
for their potential applications as a new class of materials in the
field of sorption,2 separation,3 ion-exchange,4 proton conduction,5
magnetism,6 luminescence,7 and catalysis.8,9 In particular, their
emergence as the crystalline heterogeneous catalysts, having well-
defined pores and channels, has opened novel avenues towards
size- and shape-selective catalysis unprecedented from other
polymeric materials including pillared clays and zeolites.10
active sites. Several such features of our earlier crystalline CPs have
allowed their utilization as the heterogeneous catalysts in various
organic transformation reactions.16-18 Herein, we present the
synthesis and characterization of two novel Eu3+ (1-Eu) and Tb3+
based (1-Tb) coordination polymers prepared using an
arylcarboxylate appended Co3+–based metalloligand (1). Both CPs
function as the reusable heterogeneous catalysts in ring-opening
reactions utilizing assorted nucleophiles. In particular, we show
remarkable catalytic applications of 1-Eu and 1-Tb in aminolysis,
alcoholysis, thialysis, and azidolysis reactions.
The earlier synthetic protocols for the preparation of CPs were
limited to mixing a suitable ligand and a desirable metal ion under
the solvothermal conditions while leaving the structural diversity to
a fair amount of serendipity.11 The recent approaches are relying
more and more on the directional bonding concept utilizing
custom-made organic as well as metal-based precursors towards a
rational approach for the synthesis of CPs.1,12 In this context,
utilization of a well-defined metalloligand as the molecular building
blocks offers several advantages over the conventional
synthesis.1a,13 A metalloligand provides structural rigidity while
offering the secondary binding sites; while the former structural
feature induces elements of rational design,12a-c the latter aspect
ensures the viability of generating extended architectures. Our
group has introduced a variety of metalloligands offering hydrogen
bonding (H-bonding)14 as well as coordination bonding sensitive
functional groups.15-18 The latter metalloligands have provided one-
(1D), two- (2D), as well as three-dimensional (3D) CPs in a more
rational manner.16-18 A few important characteristics offered by
Experimental section
Materials and Reagents
The solvents were purified as per as the standard literature
method.19 Ligand H2L and metalloligand Et4N[Co(L)2] (1) was
synthesized according to our earlier report.18a
Synthesis of [{(1)Eu(OH2)5}.8H2O]n (1-Eu). 1-Eu was synthesized
by layering a solution of metalloligand 1 (100 mg, 0.1004 mmol) in
CH3OH (4 mL) over Eu(CF3SO3)3 (132 mg, 0.2209 mmol) dissolved in
water (2 mL) with an intermediate layer of tert-butanol. After a
period of 3–4 days, light green crystalline material resulted that was
filtered and dried under vacuum. Yield: 208 mg (83 %).
C42H50CoEuN6O25 (1250.137): calcd. C 40.36, H 4.03, N 6.72; found C
40.19, H 3.85, N 6.80. FTIR spectrum (Zn-Se ATR, selected peaks):
(ν/cm–1) = 3400 (OH), 1603, 1565 (C=O).
[{(1)Tb(OH2)5}.12H2O]n (1-Tb). 1-Tb was synthesized by layering
a CH3OH solution of 1 (100 mg, 0.1004 mmol) over an aqueous
solution (2 mL) of Tb(CF3SO3)3 (133.4 mg, 0.2209 mmol) with an
intermediate layer of tert-butanol. A pale green crystalline product
was resulted at the interface of two solutions within 2 days that
was filtered and dried under vacuum. Yield: 215 mg (86 %).
C42H58CoTbN6O29 (1328.183): calcd: C 37.96, H 4.40, N 6.32; found: C
38.11, H 4.56, N 6.23. FTIR spectrum (Zn-Se ATR, selected peaks):
(ν/cm–1) = 3400 (OH), 1593, 1556 (C=O).
a.Department of Chemistry, University of Delhi, Delhi-110007, India.
E-mail: rgupta@chemistry.du.ac.in
Phone: +91-11-27666646; Fax: +91 - 11 - 2766 6605
Web: http://people.du.ac.in/~rgupta/
†Present Address: Department of Chemistry, Panjab University – Chandigarh, India.
Electronic Supplementary Information (ESI) available: [Figures for FTIR and diffuse-
reflectance absorption spectra; TGA and DSC plots; PXRD plots; crystal structure;
1H and 13C NMR spectra of organic products; and tables for the H-bonding
distances]. CCDC No. 1439280 and 1439281. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/x0xx00000x.
This journal is © The Royal Society of Chemistry 20xx
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Physical measurements
Crystal system
Monoclinic
P21/n
Triclinic
The elemental analysis data were obtained with an Elementar Space group
Analysen Systeme GmbH Vario EL-III instrument. The NMR
P-1
a(Å)
b(Å)
c(Å)
α(˚)
20.9010(8)
9.7122(4)
25.5412(16)
90
11.4727(12)
15.695(2)
17.302(2)
63.875(13)
79.754(10)
82.500(10)
2747.9(6)
1
spectroscopic measurements were carried out with a Jeol 400 MHz
spectrometer. The FTIR spectra were recorded with a Perkin-Elmer
Spectrum-Two
spectrometer
having
Zn-Se
ATR.
Gas
chromatography (GC) and GC-MS studies were performed either
with a Perkin Elmer Clarus 580 or Shimadzu QP 2010 instrument,
with RTX-5SIL-MS column. Thermal gravimetric analysis (TGA) and
differential scanning calorimetry (DSC) were performed with DTG
60 Shimadzu and TA-DSC Q200 instruments, respectively, at 5 °C
min–1 heating rate under nitrogen. The 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 = 5–35° with a scan rate of 1° per minute at room temperature.
β(˚)
99.292(5)
90
γ(˚)
V(Å)3
Z
5116.7(4)
4
Crystallography
d [g cm–3
]
1.588
1.567
Single-crystal X-ray diffraction data for 1-Eu and 1-Tb were collected
on an Oxford XCalibur CCD diffractometer equipped with graphite
monochromatic MoKα radiation (λ = 0.71073 Å).20 For both CPs,
µ [mm–1
]
1.629
1.672
frames were collected at 173(2) K. An empirical absorption F(000)
2432
1288
correction was applied using spherical harmonics implemented in
SCALE3 ABSPACK scaling algorithm.20 The structures were solved by
R(int.)
0.0546
0.1175
the direct methods using SIR-9721 and refined by the full-matrix
Final R indicesa
[I>2σ(I)]
R1 = 0.0442
wR2 = 0.1093
R1 = 0.0537
wR2 = 0.1142
1.063
R1 = 0.0783
wR2 = 0.1372
R1 = 0.1473
wR2 = 0.1667
0.958
least-squares refinement techniques on F2 using the program
SHELXL-9722 incorporated in the WINGX 1.8.05 crystallographic
collective package.23 The hydrogen atoms were fixed at the
calculated position with isotropic thermal parameters except for
C38 and C40 for 1-Tb, whereas non-hydrogen atoms were refined
anisotropically except for O11W for 1-Eu and O6W1, O6W2, O8W1
and O8W2 for 1-Tb. The lattice water molecules O6W, O9W and
O10W in 1-Eu and O6W and O8W in 1-Tb were found to be
positionally disordered and were solved using the PART command.
For 1-Eu, their positions were refined anisotropically with the site
occupancy factors of 0.2730 (O6W), 0.7270 (O6W1), 0.2730 (O9W),
0.7270 (O9W1), 0.2730 (O10W) and 0.7270 (O14). However, for 1-
Tb their positions were refined isotropically with the site occupancy
factors of 0.6900 (O6W1), 0.3100 (O6W2), 0.7700 (O8W1) and
0.2300 (O8W2). The hydrogen atoms on the carboxylic acid groups
were added with fixed Uiso (1.2 of the parent atoms) using HFIX 83.
Furthermore, hydrogen atoms of the coordinated as well as
uncoordinated water molecules (except for O12W and O17W of 1-
Tb) could not be located from the Fourier map; however their
contributions are included in the empirical formulae. Details of the
crystallographic data collection and structural solution parameter
are provided in Table 1.
R indices
(All data)
GOF on F2
CCDC No.
1439280
1439281
a R1= Σ||Fo| – |Fc||/Σ|Fo|; wR2 = {Σ[w(Fo2 – Fc2 )2]/Σ[wFo4]}1/2
General Procedure for the Catalytic Reactions
Ring-opening reactions of the epoxides were carried out in oven-
dried glassware. In a typical reaction, 1 equiv. of an epoxide was
treated with 1.1 – 1.2 equiv. of the required nucleophile in presence
of 2-mol% catalyst. The reaction mixture was stirred under ambient
conditions for 4 h while progress of the reaction was monitored
either by thin-layer chromatography (TLC) or GC. Subsequently,
catalyst was filtered off after the addition of ethyl acetate. The
organic layer was separated, washed with water (3-4 times), dried
over anhydrous Na2SO4, followed by the removal of solvent under
the reduced pressure. The organic product(s) were analyzed by the
GC, GC-MS, and/or NMR techniques as and when required. The
characterization data for a few representative products is provided
Table
1 Crystallographic data collection and structural refinement
parameters for 1-Eu and 1-Tb.
1-Eu
1-Tb
below whereas their H and 13C NMR spectra are included in Figure
1
Empirical formula
Formula weight
T(K)
C42H50CoEuN6O25
C84H82Co2N12O58Tb2
2623.32
S16 – S31 in the Electronic Supporting Information.
1249.77
173(2)
Characterization Data for a Few Representative Products.
2-((4-Ethylphenyl)amino)cyclohexanol. 1H NMR spectrum (400
MHz, CDCl3): δ 7.00 (d, J = 8.4 Hz, 2H), 6.65 (d, J = 8.4 Hz, 2H), 3.34
173(2)
2 | J. Name., 2012, 00, 1-3
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(m, 1H), 3.11-3.05 (m. 1H), 2.8(brs, 2H), 2.56 (q, J = 7.6 Hz, 2H), Results and discussion
2.11-2.07 (m, 2H), 1.77-1.67 (m, 2H), 1.39-1.25 (m, 3H), 1.77 (t, J =
7.6, 3H), 1.06-0.95 (m, 1H). 13C NMR spectrum (100 MHz, CDCl3): δ
145.5, 134.4, 128.6, 114.6, 74.41, 60.5, 33.0, 31.5, 27.8, 25.0, 24.2,
15.9. HRMS (ESI): [M+H]+ Calcd. for [C14H21NO] 219.1623, found
220.2688.
Synthesis and characterization of CPs 1-Eu and 1-Tb
CPs 1-Eu and 1-Tb were synthesized by layering a solution of
metalloligand 1 in CH3OH over a solution of Ln(OTf)3 in H2O with an
intermediate layer of tert-butanol. Both CPs were isolated as pale
green crystalline material in high yield. FTIR spectra (Fig. S1 and S2
in ESI) of both CPs show νO–H stretches between 3350 and 3410 cm–
2-Phenyl-2-(phenylamino)ethanol. 1H NMR spectrum (400 MHz,
CDCl3): δ 7.40-7.41 (m, 4H), 7.28-7.24 (m, 1H), 7.09 (t, J = 7.64 Hz,
2H), 6.67 (t, J = 7.64 Hz, 1H), 6.56 (d, J = 8.4 Hz, 2H), 4.49 (q, J =
3.84, 1H), 3.92 (dd, J = 4.56 Hz and 11.4 Hz, 1H), 3.75-7.71 (m, 1H).
13C NMR spectrum (100 MHz, CDCl3): δ 147.2, 140.0, 129.1, 128.8,
127.6, 126.7, 117.8, 113.8, 67.3, 59.8. HRMS (ESI): [M+H]+ Calcd. for
[C14H15NO] 213.11, found 214.2248.
1
due to the presence of coordinated as well as lattice water
molecules.24 A strong band in 1560–1570 cm–1 region corresponds
to amidic νC=O stretches.24 TGA plots for CPs 1-Eu and 1-Tb support
the presence of water molecules. For 1-Eu, an observed weight loss
of 18.12% fits nicely with the calculated value of 17.56% in the
temperature range of 35–100 °C corresponding to the loss of five
coordinated and seven lattice water molecules as observed
crystallographically. Similarly, for 1-Tb, a weight loss between 30
and 100 °C corresponded to the loss of 13 water molecules (obs.
17.99%; calcd. 17.64%). For both CPs, DSC studies display a broad
exothermic feature for the loss of coordinated as well as lattice
water molecules in the region of 30–150 °C (Fig. S3 and S4, ESI).
Moreover, both TGA and DSC studies suggest thermal stability close
to 370 °C for both CPs. The diffuse-reflectance absorption spectra of
both CPs display signals at 470 and 635 nm (Fig. S5 and S6 in ESI). X-
ray powder diffraction (XRPD) patterns were used to confirm the
crystalline homogeneity and purity of the bulk products. The
experimental XRPD patterns closely match the ones simulated from
the single crystal diffraction data for both CPs (Fig. S7 and S8 in ESI),
thereby indicating that a single crystalline phase has resulted during
the bulk synthesis of the polymers.
2-Butoxy-2-phenylethanol. 1H NMR spectrum (400 MHz, CDCl3): δ
7.32 (d, J = 3.2 Hz, 4H), 7.28-7.24 (m, 1H), 3.97-3.94 (m, 1H), 3.84 (s,
2H), 2.46-2.44 (m, 2H), 2.04 (brs, 1H), 1.40-1.16 (m, 7H). 13C NMR
spectrum (100 MHz, CDCl3): δ 139.7, 128.7, 128.0, 127.6, 65.5, 52.6,
25.1, 14.7. HRMS (ESI): [M+H]+ Calcd. for [C12H18O2] 194.1307,
found 194.1873.
2-(tert-Butoxy)-2-phenylethanol. 1H NMR spectrum (400 MHz,
CDCl3): δ 7.35-7.29 (m, 4H), 7.26-7.24 (m, 1H), 4.62-4.59 (m, 1H),
3.51-3.44 (m, 2H), 1.72 (brs, 1H), 1.16 (s, 9H). 13C NMR spectrum
(100 MHz, CDCl3): δ 142.2, 128.2, 127.3, 126.3, 75.1, 74.9, 67.8,
28.8. HRMS (ESI): [M+H]+ Calcd. for [C12H18O2] 194.1307, found
194.1873.
2-(Benzylthio)-2-phenylethanol. 1H NMR spectrum (400 MHz,
CDCl3): δ 7.37-7.27 (m, 8H), 7.25-7.19 (m,2H), 3.86-3.80 (m, 3H),
3.69-3.65 (m, 1H), 3.56-3.53 (m, 1H), 1.99 (brs, 1H). 13C NMR
spectrum (100 MHz, CDCl3): δ 139.3, 137.8, 128.9, 128.7, 128.5,
128.2, 127.7, 127.1, 65.6, 51.9, 35.3. HRMS (ESI): [M+H]+ Calcd. for
[C15H16O1S1] 244.0922, found 245.0753.
Crystal structures
Both CPs, 1-Eu and 1-Tb, were crystallographically characterized
and their structures are shown in Fig. 1 and 2 whereas tables 2 and
3 present the selected bonding parameters. The coordination
polymer 1-Eu crystallizes in monoclinic cell with P21/n space group.
The asymmetric unit cell of 1-Eu is consist of one Co3+–based
metalloligand, one Eu3+ ion, five coordinated and seven lattice
water molecules. Similarly, 1-Tb is found to crystallize in triclinic cell
with P-1 space group and the asymmetric cell contains one
metalloligand, one Tb3+ ion, five coordinated and twelve
2-(Ethylthio)-2-phenylethanol. 1H NMR spectrum (400 MHz, CDCl3):
δ 7.32-7.24 (m, 5H), 4.11-4.09 (m, 1H), 3.97-3.94 (m, 1H), 3.84-3.80
(m, 2H), 2.45-2.39 (q, J = 6.1, 2H), 1.24-1.16 (t, J = 7.6, 3H). 13C NMR
spectrum (100 MHz, CDCl3): δ 139.7, 128.5, 127.9, 127.4, 65.5, 29.5,
25.0, 14.8. HRMS (ESI): [M+H]+ Calcd. for [C10H14OS] 182.0765,
found 182.1831.
2-Azido-2-phenylethanol. 1H NMR spectrum (400 MHz, CDCl3): δ
7.33-7.25 (m, 5H), 4.60 (s, 3H), 2.1 (brs, 1H). 13C NMR spectrum (100
MHz, CDCl3): δ 140.8, 128.5, 127.6, 126.9, 65.2139.3, 137.8, 128.9,
128.7, 128.5, 128.2, 127.7, 127.1, 65.6, 51.9, 35.3.
uncoordinated water molecules. In
a
metalloligand, two
deprotonated tridentate ligands are arranged meridionally around
the Co3+ ion maintaining a compressed octahedral geometry (Fig. 1a
3-Chloro-2-(phenylamino)propan-1-ol. 1H NMR spectrum (400 and 2a).18 The Co3+ ion is coordinated by four Namide atoms in a
MHz, CDCl3): δ 7.19 (t, J = 7.6 Hz, 2H), 6.75 (t, J = 7.6 Hz, 1H), 6.65 distorted equatorial basal plane whereas two Npyridine atoms occupy
(d, J = 8.3 Hz, 2H), 4.07 – 4.02 (m, 1H), 3.67-3.58 (m, 2H), 3.35 (dd, J the axial positions.18 The Eu3+ ion in 1-Eu shows a nine-coordinate
= 12.9 Hz, 1H), 3.20 (dd, J = 13.7 Hz, 1H). 13C NMR spectrum (100 geometry wherein two bidentate arylcarboxylate groups stemming
MHz, CDCl3): δ 129.4, 118.4, 114.1, 112.7, 68.8, 57.9, 47.3.
from two different metalloligands are present in trans manner
whereas the remaining five sites are occupied by the water
molecules (Fig. 1a). The Tb3+ ions in 1-Tb are octa-coordinated by
three Ocarboxylate atoms from two different metalloligands and five
water molecules (Fig. 2a). Notably, in 1-Tb, while one of the
arylcarboxylate groups coordinates in a bidentate manner, the
second one ligates in a monodentate form. In case of 1-Eu,
metalloligands coordinate to the europium ions and generate a 1D
Scheme 1. Synthetic route for the preparation of coordination polymers 1- chain. Such parallel 1D chains are further connected to each other
via an array of H-bonds involving amide, arylcarboxylate, and
arylcarboxylic acid groups as well as coordinated and lattice water
molecules (Fig. 1b). On the other hand, 1-Tb exhibits the
Eu and 1-Tb.
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coordination of two metalloligands to two Tb3+ ions resulting in the
A somewhat similar H-bonding pattern is observed in case of CP
formation of a dimeric repeat unit (Fig. 2b). Such a bonding pattern 1-Tb (Fig. 2b, 2c; and S9, ESI). The Tb3+ coordinated water molecule
has become possible due to the coordination of arylcarboxylate O1W in one chain shows H-bonding with O6 atom of the adjcent
fragments on syn positions of the Tb3+ ion while originating from chain through intervening water molecules O10W, O15W and
two different metalloligands. Subsequently, such dimeric repeat O16W (O10…O1W = 2.709 Å, O6⋯O16W = 2.914 Å). The O3amide of
units extend via the operation of assorted H-bonds comprising of one chain shows H-bonding with O2amide of adjacent chain through
amide, arylcarboxylate, and arylcarboxylic acid groups as well as the water molecule O14W in between them (O3⋯O14W = 3.014 Å
coordinated and lattice water molecules (Fig. 2c). In both polymers, and O2⋯O14W = 2.826 Å). The H-bonding is further extended by
a metalloligand provides three negative charge which is balanced by O3amide towards O8W (O3⋯O8W = 2.762 Å) which H-bonds to water
a Ln3+ ion. As a result, in both 1-Eu and 1-Tb, out of four, two molecule O10W while it further shows H-bonding with O2amide
arylcarboxylate groups remain protonated and therefore do not atom. The Oamide atom of one chain H-bonds to O6W which extends
participate in coordinating the lanthanide metal ion. However, such its H-bonding to O7 of a arylcarboxylate group of the next
arylcarboxylic acid groups participate in various H-bonding metalloligand within the same chain.
interactions and help in extending the structure to a 2D network (cf.
Fig. 1c; Tables S1 and S2, ESI).
---------------------------------------
Table 3 Selected bond lengths (Å) and bond angles (°) around the lanthanide
metal ion in CPs 1-Eu and 1-Tb. M stands for Eu and Tb in 1-Eu and 1-Tb,
respectively.
Fig. 1 and 2
---------------------------------------
------------------------------------
Both 1-Eu and 1-Tb show several H-bonding interactions
involving coordinated as well as lattice water molecules to that of
metalloligands offering Oamide, arylcarboxylate, and arylcarboxylic
Table 3: At the end
------------------------------------
It is notable that a combination of such H-bonding interactions
acid groups. In 1-Eu, one 1D chain is connected to the adjacent 1D connects individual 1D chains to generate interesting H-bonded 2D
chain via H-bonding of intervening water molecules (Fig. 1b and 1c). architectures in both CPs. The H-bonding distances illustrate
The Oamide atom O2 makes an H-bond with water molecule O8W moderate to strong nature of the hydrogen bonding in both CPs
which further connects to O11W which in turn is coordinated with (Tables S1 and S2, ESI). Such bonding features suggest a stable
O9 atom of arylcarboxylate group of another chain. The O2⋯O8W nature of H-bonded architectures in both CPs. In fact, TGA and DSC
and O8W⋯O11W separations were 2.786
Å and 2.836 Å, studies adequately display thermal stabilities of both CPs close to
respectively. Similarly, O8 atom of arylcarboxylic acid shows H- 370 °C. Therefore, both CPs highlight the importance of H-bonding
bonding with O7W which is connected with O1W coordinated to in the construction of stable 2D architectures.
the Eu3+ ion. The O8⋯O7W and O7W⋯O1W sepations were noted
to be 2.852 Å and 2.812 Å, respectively. Additional H-bonding is
provided by O1W of the Eu3+ centre of one 1D chain with O8W
water molecule (O1W⋯O8W = 2.701 Å) which further shows H-
bonding with O2amide of the adjcent chain (O8W⋯O2 = 2.786 Å).
Catalytic applications of 1-Eu and 1-Tb
Both CPs offered several structural features that suggested their
potential application in the catalysis: (i) 1D chain like structures of
two CPs suggest the uncluttered access of substrates towards the
lanthanide metals; (ii) presence of a large number of coordinated
yet labile water molecules on both Eu3+ and Tb3+ ions point towards
their facile replacement by the substrate(s); (iii) strong Lewis acidic
nature of Eu3+ and Tb3+ ions; (iv) thermal stability of both CPs, close
to ca. 370 °C, provide an option to carry out the reactions at
elevated temperature; (v) existence of H-bonded network
advocates the possibility of substrates and nucleophiles to have
favorable interaction within the CPs; and (vi) insolubility of two CPs
in common organic solvents provide an option to do the reactions
heterogeneously.
Table 2 Selected bond lengths (Å) and bond angles (°) around the Co3+ ion in
CPs 1-Eu and 1-Tb.
Bond
1-Eu
1-Tb
Co-N1
1.957(3)
1.860(3)
1.966(3)
1.955(3)
1.860(3)
1.965(4)
81.2(15)
162.7(14)
91.4(15)
98.0(15)
92.2(16)
81.6 (14)
97.7(14)
178.9(16)
99.7(15)
92.0(14)
99.2(14)
89.7(15)
81.5(14)
162.6(14)
81.1(15)
1.945(7)
1.862(6)
1.934(7)
1.953(7)
1.853(6)
1.949(7)
81.6(3)
163.4(3)
92.2(3)
97.1(3)
89.8(3)
81.7(3)
97.6(3)
178.5(3)
98.2(3)
90.0(3)
99.5(3)
92.5(3)
81.6(3)
164.2(3)
82.6(3)
Co-N2
Co-N3
Co-N4
Co-N5
Co-N6
N1-Co1-N2
N1-Co1-N3
N1-Co1-N4
N1-Co1-N5
N1-Co1-N6
N2-Co1-N3
N2-Co1-N4
N2-Co1-N5
N2-Co1-N6
N3-Co1-N4
N3-Co1-N5
N3-Co1-N6
N4-Co1-N5
N4-Co1-N6
N5-Co1-N6
Ring-opening reactions (RORs) of epoxides with various
nucleophiles provide an easy access to
a large number of
intermediates widely desired in the synthesis of pharmaceutically
and industrially important compounds.25–27 Importantly, RORs of
epoxides can be accomplished with versatile nucleophiles such as
amine, alcohol, thiol, cyanide, azide, etc. thus opening up the
possibility to access assorted products. For example, azidolysis of
epoxides produces 1,2-azidoalcohols which play crucial role as the
precursors to vicinal amino alcohols.28 Similarly, β-hydroxysulfide,
the product of thialysis reaction, is a significant synthon for the
synthesis of a number of biological and pharmacological relevant
compounds.29 In literature, RORs of epoxides with amine as the
nucleophile (amilolysis) have been extensively studied; however,
use of other nucleophiles such as alcohol, azide, and thiol has rarely
been studied.16a,28,29 Further, activation of an epoxide ring has been
4 | J. Name., 2012, 00, 1-3
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considered as a critical step during the RORs. Such an activation has
Thus, when an equimolar mixture of cyclohexene oxide and
been typically achieved using a suitable Lewis acidic metal ion aniline was stirred in presence of 2-mol% of CPs 1-Eu or 1-Tb at
although other chemicals, reagents, and even conditions have also room temperature under the solvent-free condition; a smooth
been utilized.25-27 Out of various metal salts, lanthanide salts and in reaction produced 2-(phenylamino)cyclohexanol in high yield
particular lanthanide triflate salts have been particularly (>97%; entry 1, Table 4). To evaluate the impact of electronic effect
successful.30 Lately, there have been attempts to heterogenize the on the RORs, several para-substituted anilines containing assorted
lanthanide-based catalysts considering their high Lewis acidic functional groups were employed (Entries 2–6; Table 4).
character.31 In this context, coordination polymers incorporating Satisfyingly, high product yield was observed in all cases (95%–
lanthanide metal ions are notable.16a,31 Keeping these points in 99%) irrespective of the nature of electronic substituent. Notably,
perspective, we attempted to utilize the present Eu3+ and Tb3+ the catalytic reaction did not proceed in absence of 1-Eu and 1-Tb
based coordination polymers, 1-Eu and 1-Tb, as the heterogeneous thus supporting the probable Lewis acid catalyzed activity of both
catalysts for the RORs.
CPs. Moreover, in a control experiment using Eu(OTf)3 or Tb(OTf)3
as the catalyst, only 5 – 8% conversion was noted (data not shown).
Table 4 Ring-opening reactions of cyclohexene oxide and styrene oxide with These experiments endorse that CPs 1-Eu and 1-Tb are responsible
aniline and para-substituted anilines by using CPs 1-Eu and 1-Tb.
for the enhanced product yield. In order to avoid competition from
the solvent molecules and greener reasons; all reactions were
performed under the solvent-free conditions. However, use of
solvents did not seem to improve the catalysis results (data not
shown). Styrene oxide, an unsymmetrical epoxide, was selected to
understand the regioselectivity. Thus, when styrene oxide was
treated with assorted substituted anilines in presence of 1-Eu and
1-Tb; nucleophilic attack exclusively took place at the benzylic
carbon atom of the epoxide (Entries 7–11; Table 4). The results
suggest that the epoxide ring has potentially interacted with the
lanthanide ion through the less-hindered side as noted before with
our earlier heterogeneous catalysts.17,18
We then extended the catalysis towards less common
nucleophiles. Both CPs 1-Eu and 1-Tb worked effortlessly for the
RORs of cyclohexene oxide as well as styrene oxide using a variety
of alcohols as the nucleophiles (alcoholysis). Despite the poor
nucleophilicity of alcohols, 80 to >99% product formation took
place within 4 h as also noted for aminolysis (Table 5). In fact, nearly
quantitative transformation was observed with various alcohols
both with cyclohexene oxide and styrene oxide. In case of
cyclohexene oxide, products were obtained as the single
diastereoisomer with trans stereochemistry. On the other hand,
styrene oxide again afforded a single regioisomer through the
incorporation of alcohol at the phenyl-substituted carbon atom, as
expected for a charge-controlled ring-opening process, irrespective
of the nature of the alcohol.
Yield [%]a
Entry
R-
Time(h)
1-Eu
1-Tb
1
2
3
4
5
6
7
8
9
H
C2H5
CH3O
Cl
F
NO2
H
C2H5
CH3O
Cl
4
4
4
4
4
6
4
4
4
4
4
98, 97,b 96c
97, 95,b 95c
98
99
96
98
97
98
97
98
96
95
92, 91,b 90c
94,93,b 92c
88
90
96
79
92
91
95
80
10
11
F
a
b
The yields were calculated using the gas chromatograph. Third run with
the reused catalyst. c Fifth run with the reused catalyst.
Table 5 Ring-opening reactions of cyclohexene oxide and styrene oxide with
various alcohols by using CPs 1-Eu and 1-Tb.
Our next nucleophile was azide (azidolysis) and we selected
trimethylsilyl azide for this purpose. Under the identical reaction
conditions, both cyclohexene oxide and styrene oxide provided
good to excellent conversion to the respective β-azido alcohols
(Entries 1 and 4; Table 6). As noted for aminolysis and alcoholysis
reactions; azidolysis of styrene oxide showed the regioselective
product formation wherein azide preferentially attacked from the
less hindered side of the epoxide that resulted in a single product.
Thialysis of cyclohexene oxide and styrene oxide afforded the
corresponding products in 82 to 92% yield (Entries 2,3 and 5,6;
Table 6). It may appear that thialysis reactions are slower in
comparison to aminolysis, alcoholysis, and azidolysis reactions; but
we did not increase the reaction time as well as temperature (25
°C) for a better comparison. The thialysis of cyclohexene oxide
provided the stereo-selective trans-β-hydroxysulfide as the only
product whereas styrene oxide afforded the exclusive regioselective
products (Entries 5– 6; Table 6). It is important to mention that the
use of azides (azidolysis) and thiols (thialysis) as the nucleophiles
are far less common when compared to well-explored aminolysis
and alcoholysis reactions.28,29
Yield [%]b
Entry
Epoxidea
R–OH
Product (R–)
1-
Eu
78
76
83
99
99
98
99
92
1-
Tb
80
78
84
97
98
96
97
96
1
2
3
4
5
6
7
8
C. O.
C. O.
C. O.
C. O.
S. O.
S. O.
S. O.
S. O.
(CH3)3COH
CH3CH2CH(CH3)OH
(CH3)2CHOH
(CH3)3C-
CH3CH2CH(CH3)-
(CH3)2CH-
C6H5CH2OH
C6H5CH2-
(CH3)3COH
(CH3)3C-
CH3CH2CH(CH3)OH
(CH3)2CHOH
CH3CH2CH(CH3)-
(CH3)2CH-
C6H5CH2OH
C6H5CH2-
a C.O. and S.O. stand for cyclohexene oxide and styrene oxide, respectively. b
The yields were calculated using the gas chromatograph.
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Table 6 Ring-opening reactions of cyclohexene oxide and styrene oxide with during the catalysis, a filtration test was performed using 1-Eu in
trimethylsilyl azide (TMSN3) and thiols by using CPs 1-Eu and 1-Tb.
the aminolysis reaction of styrene oxide with aniline (Fig. 3). In a
typical reaction, 1-Eu completes the ROR of styrene oxide with
aniline within 4 h. However, taking the advantage of heterogeneous
catalysis, if solid 1-Eu is filtered off after 2 h and the reaction is
allowed to continue; as shown in Fig. 3, there was practically no
catalysis. Subsequently, re-addition of recovered 1-Eu after a gap of
1.5 h immediately started the ROR. An overall profile illustrates that
the removal of 1-Eu ceases the ROR whereas its re-addition to the
reaction restarts the ROR. This simple test establishes the true
heterogeneous catalytic behavior of 1-Eu and asserts that the
lanthanide metal ions are not leached out from the CPs.
Both solid CPs, 1-Eu and 1-Tb, can easily be separated from the
heterogeneous reaction mixture by simple filtration; a feature
unique to the heterogeneous and solid immobilized catalysts.18b,c
Such a fact conveniently allows studying the recyclability of the
heterogeneous catalysts. Importantly, both CPs can be easily
recovered by filtration after the reaction and reused several times.
We tested both CPs for five consecutive times (Entries 1 and 7,
Table 4) and observed comparable catalytic activity with less than
2% drop in yield in the fifth run. The FTIR spectra of the recovered
polymers exactly match to that of pristine samples (Fig. S10 and
S11, ESI) whereas a comparison of PXRD patterns of the as-
synthesized samples to that of recovered CPs authenticate that the
crystallinity as well as the structural integrity of both polymers is
preserved during the catalysis (Fig. S12 and S13, ESI).
Yield[%]a
1-Tb
Entry
Epoxide
Nucleophiles
1-Eu
99
1
2
3
4
5
6
C. O.
C. O.
C. O.
S. O.
S. O.
S. O.
TMS-azide
Benzyl thiol
Ethane thiol
TMS-azide
99
90
89
99
86
84
89
91
99
Benzyl thiol
Ethane thiol
82
86
a The yields were calculated using the gas chromatograph.
Table
7 Ring-opening reactions of assorted epoxides with various
nucleophiles by using CPs 1-Eu and 1-Tb.
tert-
Butyl
phenol
tert-
Butyl
aniline
TMS-
Azide
Entrya
Aniline
Substrate
1-Eu /
1-Tb
1-Eu /
1-Tb
1-Eu /
1-Tb
1-Eu /
1-Tb
1
2
3
4
100
90
80
70
60
50
40
30
20
10
100/99
80/82
83/86
100/98
70/76
82/84
81/84
79/82
94/92
98/97
100/99
94/95
100/98
100/99
83/85
93/91
97/99
83/86
5
100/100
97/98
a The yields were calculated using the gas chromatograph.
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
Time (h)
To further establish the critical issue of regioselectivity;
assorted unsymmetrical epoxides were investigated (Entries 1-4;
Table 7).18b Importantly, these substrates offer an epoxide ring in
addition to intricately placed substituents. Moreover, various
nucleophiles were used to understand their role in controlling the
product regioselectivity (Table 7). Satisfyingly, in each case, a
perfect regioselectivity was noticed in the form of a single product.
Notably, in most cases, regioselectivity has not affected the product
yield and high to excellent yield was noticed. These experiments
confirm that the nucleophile has exclusively attacked on the less
hindered side of the epoxide ring and the regioselectivity is
controlled by the steric factor rather than resonance effect as a
prevailing cause in a few substrates, such as styrene oxide.18b
Fig. 3 (a) Ring-opening reaction of styrene oxide with aniline in presence of
coordination polymer 1-Eu (ꢀ); (b) catalyst 1-Eu was filtered after 2 h
leading to termination of reaction (ꢁ); (c) Catalyst 1-Eu was re-added after
1.5
h causing commencement of the reaction (ꢂ) and leading to
completeness.
Mechanistic insight
Our catalysis results primarily rely on the fact that the coordinated
water molecules present on the lanthanide metals are labile and
are easily replaceable. Such a critical step allows a substrate to
interact with the lanthanide metal ion before its activation followed
by reaction with the nucleophile. Therefore, in order to strengthen
the suggested mechanistic proposal; it became essential to evaluate
that the coordinated water molecules are indeed labile in nature
Leaching and Recyclability Experiments
To ascertain the true heterogeneous nature of catalysis and to
make sure that the lanthanide metal ions are not leached out
6 | J. Name., 2012, 00, 1-3
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4
5
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and the substrates and/or reagents are able to replace them;
solvent exchange studies were performed. For such an experiment,
powdered samples of both CPs were first heated at 150 °C under
vacuum to remove lattice as well as coordinated water molecules
followed by allowing the sample to equilibrate in
a sealed
environment of D2O vapors. Such an experiment resulted in clean
exchange of H2O by D2O and the νO-D stretches were observed at ca.
2500 cm-1 with a shift of ca. 1000 cm-1 from νO-H stretches (Fig. S14
and S15, ESI). It is important to mention that such a D2O–exchange
led to nearly complete replacement of both coordinated and lattice
water molecules, therefore, firmly establishing the labile nature of
the coordinated water molecules. We therefore suggest that a
substrate could easily replace the coordinated water molecule(s)
followed by its activation by the Lewis acidic lanthanide metal ion
before a nucleophile attacks to produce the desired product.
6
7
8
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Conclusions
This work has shown the synthesis and characterization of Eu- and
Tb-based coordination polymers that were synthesized using a Co-
based metalloligand offering appended arylcarboxylate groups.
Both coordination polymers displayed the generation of 2D
architectures due to the involvement of various intermolecular H-
bonds whereas thermal studies illustrated the stable nature of the
2D architecture. Both coordination polymers functioned as the
heterogeneous catalysts for the ring-opening reactions of assorted
epoxides utilizing several nucleophiles. The illustration of regio-
selective ring-opening reactions; solvent-free catalytic conditions;
and reusability of both catalysts depicted the importance of
lanthanide-based coordination polymers in the heterogeneous
catalysis.
9
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Engineering Research Board (SERB), New Delhi and the
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university for the instrumental facilities including X-ray data
collection and AIRF center of JNU, New Delhi for the GC–MS
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Appl. Crystallogr., 1999, 32, 837.
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Medicinal Chemistry, 4th ed. (Ed.: M. S. Wolff), Wiley-
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8 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
(b)
(a)
(c)
Eu
Free COOH
Group
Co
Eu
Free COOH
Group
Fig. 1 (a) Partial crystal structure of 1-Eu showing the coordination environment around the Eu3+ center and its bonding with the metalloligand; hydrogen
atoms and solvent molecules have been omitted for clarity. (b) A section of crystal structure of 1-Eu displaying arrangements of metalloligands and Eu3+ ions
generating 1D chains (shown in green and magenta colours). Lattice water molecules, shown as blue balls, help in connecting two 1D chains to a double
chain. (c) A view of 2D network created due to various H-bonding interactions involving free arylcarboxylic acid groups, Oamide groups, and coordinated as
well as lattice water molecules connecting various 1D chains (shown in green, magenta, pink, and red colours). See text for details.
(a)
(b)
(c)
Tb
Free COOH
Group
Co
Co
Tb
Tb
Co
Free COOH
Group
Tb
Fig. 2 (a) Partial crystal structure of 1-Tb showing the coordination environment around the Tb3+ center and its bonding with the metalloligand;
hydrogen atoms and solvent molecules have been omitted for clarity. (b) A section of crystal structure of 1-Tb exhibiting coordination of Tb3+ ions to
that of metalloligands generating 1D zig-zag chains. (c) Lattice water molecules, shown in blue balls, help in connecting 1D chains (shown in green
and magenta colours) involving various weak interactions. See text for details.
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Table 3 Selected bond lengths (Å) and bond angles (°) around the lanthanide metal ion in CPs 1-Eu and 1-Tb. M stands for Eu
and Tb in 1-Eu and 1-Tb, respectively.
Bond
M-O5
M-O6
M-O9
M-O10
M-O11
M-O1W
M-O2W
1-Eu
1-Tb
Bond
1-Eu
1-Tb
76.9(2)
-----
-----
-----
-----
-----
-----
-----
2.444(3)
2.426(3)
2.473(3)
2.498(3)
-----
2.457(3)
2.454(4)
2.477(4)
2.464(4)
2.488(3)
53.30(11)
143.30(11)
137.40(11)
-----
2.420(5)
2.431(6)
-----
2#O6-M-O5W
O9-M-O1W
87.05(14)
77.99(11)
77.81(12)
69.43(12)
96.25(12)
127.47(11)
71.43(11)
124.31(11)
104.99(12)
72.83(11)
75.93(11)
-----
O9-M-O2W
O9-M-O3W
O9-M-O4W
O9-M-O5W
-----
2.271(5)
2.395(5)
2.414(6)
2.403(5)
2.420(6)
2.409(6)
53.5(2)
-----
-----
85.7(2)
-----
-----
80.2(2)
-----
103.3(2)
76.1(2)
155.3(2)
85.5(2)
129.5(2)
78.0(2)
111.9(2)
150.57(19)
133.39(19)
O10- M-O1W
O10-M-O2W
O10-M-O3W
O10-M-O4W
O10-M-O5W
O11-M-O1W
O11- M-O2W
O11- M-O3W
O11-M-O4W
O11- M-O5W
O1W-M-O2W
O1W-M-O3W
O1W-M-O4W
O1W-M-O5W
O2W-M-O3W
O2W-M-O4W
O2W-M-O5W
O3W-M-O4W
O3W-M-O5W
O4W-M-O5W
M-O3W
M-O4W
M-O5W
-----
-----
-----
2#O5-M-O6#2
2#O5-M-O9
2#O5-M-O10
O5-M-O11
2#O6-M-O9
2#O6-M-O10
O6-M-O11
O9-M-O10
2#O5-M-O1W
2#O5-M-O2W
2#O5-M-O3W
2#O5-M-O4W
2#O5- M-O5W
2#O6-M-O1W
2#O6-M-O2W
2#O6-M-O3W
2#O6-M-O4W
144.3(2)
143.0(2)
101.8(2)
75.0(2)
75.9(19)
72.2(2)
84.5(19)
139.4(2)
71.8(2)
84.3(2)
71.7(2)
139.9(19)
74.1(19)
75.2(2)
131.7(2)
-----
-----
-----
-----
142.46(12)
150.50(12)
-----
76.24(13)
138.81(13)
138.39(13)
78.81(12)
72.88(15)
143.50(14)
139.05(13)
71.38(15)
141.55(14)
72.43(13)
52.11(10)
75.91(11)
71.26(12)
117.60(12)
120.37(12)
71.49(11)
129.12(11)
84.16(14)
73.89(12)
79.18(13)
Symmetry transformations used to generate equivalent atoms: for 1-Eu; #2 x+1/2,-y-1/2,z+1/2
10 | J. Name., 2012, 00, 1-3
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Graphical Abstract
Lanthanide-based coordination polymers as the promising heterogeneous
catalysts for ring-opening reactions
Gulshan Kumar, Girijesh Kumar and Rajeev Gupta*
Department of Chemistry, University of Delhi, Delhi – 110 007 (India)
Artwork:
Synopsis: Eu and Tb based coordination polymers function as the reusable heterogeneous
catalysts for the ring-opening reactions with amines, alcohols, thiols, and azides as the
nucleophiles.