Cycloaddition of CO2 to epoxides using di-nuclear transition metal complexes as catalysts

Article abstract of DOI:10.1039/c5nj03198b

Studies on the reaction and conversion of CO₂ to valuable products have made much progress in recent years, and the search for efficient catalysts is also expanding. Cycloaddition of CO₂ to epoxides was carried out selectively using di-nuclear Cu^II, Co^II and Ni^II complexes (C1, C2 and C3, respectively) as catalysts. The complexes were synthesized in good yield and characterized by various physical and spectroscopic methods. In all complexes the ligand L acted as a bidentate NO donor favouring distorted octahedral, tetrahedral or square planar geometry for C1, C2 and C3, respectively. Complex C2 in the presence of n-Bu₄NI as a cocatalyst showed the highest activity among the reported complexes in the cycloaddition reaction.

Full text of DOI:10.1039/c5nj03198b

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PAPER (or FOCUS or PERSPECTIVE)
Cycloaddition of CO2 to epoxides using di-
†
nuclear transition metal complexes as catalysts
Cite this: DOI: 10.1039/c3nj00000x
Mohmmad Y. Wani,a* Santosh Kumar,a Claudia T. Arranja,a Carlos M. F.
*
Dias,a and Abilio J.F.N Sobrala
Received 00th XXXXX 2013,
Accepted 00th XXXXX 2013
Studies on the reaction and conversion of CO2 to valuable products have made much progress
in recent years, and the search for efficient catalysts is also expanding. Cycloaddition of CO2
DOI: 10.1039/c3nj00000x
to epoxides was carried out selectively using di-nuclear CuII, CoII and NiII complexes (C1
and C3 respectively) as catalysts. The complexes were synthesized in good yield and
characterized by various spectroscopic methods. In all complexes the ligand acted as a
bidentate NO donor favouring distorted octahedral, tetrahedral or square planar geometry for
C1 C2 and C3 respectively. Complex C2 in presence of n-Bu4NI as cocatalyst showed the
highest activity among the reported complexes in the cycloaddition reaction.
, C2
www.rsc.org/njc
L
,
1. Introduction
The cycloaddition between epoxides and carbon dioxide was
usually carried out in conventional noxious organic solvents by
The use of carbon dioxide as a safe and cheap C1 building
block for the synthesis of organic chemicals can contribute to a
more sustainable chemical industry.1,2,3 CO2 is
2,7
various catalyst systems;
most catalysts for this process
a
require the use of high reaction temperatures and/or high
pressures of carbon dioxide. However, new excellent catalysts
with high stability and reactivity have been designed which can
work at room temperature and pressure under solvent free
conditions.8,9,10 Metal oxides were the first catalysts to be used
for the cycloaddition reaction.6 Research in this field has
expanded in recent years and different heterogenous and
homogenous catalysts, zeolites, and porphyrin based catalysts
have been used for this cycloaddition.2,11 Schiff base and salen
complexes as homogeneous catalysts have been found to be
efficient catalysts for the cycloaddition reaction of CO2 to
epoxides with high turnover number (TON) and high turnover
frequency (TOF).12 A variety of metal complexes, including
Zn, Al, and Co, have also been reported as efficient catalysts
for the cycloaddition of CO2 to epoxides.13 Use of bimetallic
complexes is however scarce possibly due to emergence of
studies showing no benefit of di-nuclearity in contrast to mono
nuclearity.13 Some recent studies have come which support the
use of di-nuclear metal complexes for the cycloaddition of CO2
and epoxides.2,11,14,15 A study proves that the di-nuclear system
maintains its activity even under highly diluted conditions at
which the mononuclear system loses its efficiency.14 Flexibly
tethered di-nuclear cobalt salen and chromium salphen
complexes have also exhibited a noticeably higher activity than
their mononuclear analogues.14 Salen type and porphyrin based
catalysts have already been used, however no literature
precedents were found describing the use of oxime based
complexes as catalysts. Therefore the development of efficient
non-precious metal catalysts for CO2 conversion reactions is a
challenging goal in both synthetic and industrial organic
chemistry. Following our work on CO2 capture and
thermodynamically stable and chemically inert molecule due to
the negative adiabatic electron affinity (EA) and large
ionization potential (IP), thus making its conversion into useful
products and chemicals difficult under normal conditions.4
Methods to overcome the high energy barriers are based upon
reduction, oxidative coupling with unsaturated compounds on
low valent metal complexes and increasing the electrophilicity
of the carbonyl carbon.5 Several different catalysts ranging from
heterogenous to homogenous to ionic liquids have been
designed for the conversion of CO2 to useful products.1,2,3,4
Amongst these the catalytic cycloaddition of CO2 to epoxides,
leading to the synthesis of cyclic carbonates and polycarbonates
has received much attention.3,4 Cyclic carbonates are important
class of compounds with several applications e.g., ethylene
carbonate is an excellent solvent for many polymers and resins,
they can be used as electrolytes in lithium ion batteries, as
precursors for pharmaceutical intermediates, raw materials for
plastics, and as environmentally friendly nonprotic solvents and
degreaser.6
aDepartamento de Quimica, FCTUC, Universidade de Coimbra,
Rua Larga, 3004-535 Coimbra, Portugal.
*Corresponding authors: Tel.: +351 239854476; Fax: +351
239827703; e-mail: wani@uc.pt (M.Y. Wani); asobral@ci.uc.pt
(Abilio J.F.N. Sobral)
†Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/b000000x/
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VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C5NJ03198B
conversion16-21 here we report the synthesis and catalytic FTIR spectra of C3 complex (Fig.
1). For all of the metal
behaviour of di-nuclear CuII, CoII and NiII complexes of complexes, two new bands appear at low frequencies, which
benzene-1,4-diylbis(N-hydroxy-methanimine) (
cycloaddition of CO2 to epoxides.
L) towards the
are absent in the free ligand spectrum. These bands, in the 450
680 cm-1 range, are due to the M
–
O and M N vibrations, an
–
–
indication of coordination of the ligand to the metal centres
probably through the deprotonated -OH and -C=N groups.
2. Results and discussion
The ligand (
reaction of
hydroxylamine hydrochloride under basic conditions in water
in 1:2 molar ratio as outlined in Scheme . Complex C1 was
prepared by mixing 1:2 ratio of and CuCl2.2H2O, in THF at
room temperature. Similarly complexes C2 and C3 were
prepared by adding a solution of CoCl2.6H2O and NiCl2.6H2O
to half molar amounts of in THF (Tetrahydrofuran) at room
L) was prepared by a Schiff base condensation
benzene-1,4-dicarboxaldehyde
(
1
)
with
1
L
L
temperature, respectively. In case of complex C3, the metal salt
was dissolved in a minimum quantity of ethanol and mixed
slowly with the ligand dissolved in THF under magnetic
stirring. Complexes were obtained in high yield (70−75%) and
were purified by washing with n-hexane and recrystallized in
ethanol/THF. They were remarkably stable in air in the solid
state for a few days and soluble in DMF, DMSO, MeOH, EtOH
and DCM but insoluble in water and have melting points higher
than 250°C. The spectral data confirm that, for all three
complexes studied, the metal-to-ligand stoichiometry is 2:1.
Fig. 1. FTIR spectra of ligand (L) and complex C3 showing weakening
and shift of C=N stretching around 1620-1650 cm-1 and stretching
vibrations around 650-1000 cm-1 corresponding to THF (ATR mode).
The chemical structures of all the compounds were confirmed
1
by electronic (UV-Vis), FTIR, H NMR, 13C NMR, MS (ESI+)
and TGA/DTG/DSC studies.
2.2. Electronic Spectra
The electronic spectrum of the ligand, shows two bands in the
N OH
H
O
O
H
i)
region 285-360 nm, corresponding to the π→π* and
n→π*
HO N
transition due to the (–C=N–OH) chromophore. These bands
get red shifted by 5-10 nm and 10-40 nm respectively, in the
electronic spectra of the complexes. In addition to ligand-
originated bands, additional absorption bands due to metal-
originated d-d transitions and charge transfer were observed in
the spectra of the complexes, characteristic to their
geometries.22 In all cases, intense absorptions are observed
below 400 nm, which are assigned to metal-to-ligand charge
transfer transitions. The electronic spectrum of the Cu(II)
complex (C1) display a band in the 690-790 nm region,
ii)
N
O
M
O
N
M
2
assigned to B1g→2Eg transitions. Another intense band at 385
nm refers to charge transfer transition (L→CuII CT), indicating
Ni
OH2
Cl
H2O
Cu
its tetragonally distorted octahedral geometry.22 Co(II) complex
=
Co
M
O
H2O
(
C2) showed two absorption bands in the visible region. First
2
Cl
OH2
Cl
band at 525 nm is assigned to A1g→2Eg transitions and the
second band at 680 nm is assigned to A1g→2B1g transition,
2
C3
C1
C2
favouring a square planar geometry.22 The electronic spectrum
of Ni(II) complex (C3) display absorption bands in the regions
Scheme 1. Synthesis of ligand (
Reaction Conditions: (i) NH2OH.HCl, NaHCO3, Water (ii) CuCl2.6H2O
C1), CoCl2.6H2O (C2), THF, reflux; NiCl2.6H2O (C3), THF/ Ethanol
L) and metal complexes C1-C3.
410, and 760 nm assigned to A2g(F)→3T1g(P) and
3
3A2g(F)→3T1g(F) transitions, respectively, indicating its
tetrahedral geometry.22 Thus, on the basis of these evidences,
complexes C1 shows octahedral geometry, C2 follows square
planar geometry and C3 displays tetrahedral geometry as
(
reflux.
2.1. FTIR spectra
shown in scheme 1. (See Fig. 2).
The infrared spectrum of the free ligand presents a broad band
at 3200-3450 cm-1 corresponding to the -OH vibration. The
changes in stretching frequencies concerned with ν(OH) could
not be identified as that region is merged with ν(OH) of water
and appear as a broad band in that region in the complexes. The
strong band observed at 1620-1680 cm-1 in the spectrum of the
free ligand is assigned to the C=N vibration. The weakening
and negative shift of 20-30 cm-1 of υ (C=N) stretch in the
complexes indicates the involvement of azomethine nitrogen in
complexation. Stretching vibrations around 650-1000 cm-1
corresponding to THF molecules were also observed in the
2 | J. Name., 2012, 00, 1-3
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ARTICLE
DOI: 10.1039/C5NJ03198B
1.5 moles of CoO as final solid products (Found 20.35%; Calc.
20.25%) Fig. 3. The thermal decomposition of complexes C1
and C3 is similar to that of the decomposition of C2. Although
decomposed fragments of the ligand could not be approximated
due to continuous weight loss, the total % weight loss of the
complexes corresponds to the loss of the ligand after
considering the transfer of one oxygen atom to the metal ion
and residue corresponds to the metal oxide. In all complexes,
the experimental mass loss values agree well with the
calculated values.
Fig. 2. UV-visible absorption spectra of the L, C1, C2 and C3 metal
complexes (c = 9.4×10-3 M) in THF.
2.3. Nuclear magnetic resonance spectra
Further evidence for the formation of complexes was obtained
from the 1HNMR spectra. The complexes showed resonances at
ca. 3.0-5.0 ppm, attributed to -OH proton resonance, so
coordination of the deprotonated oxygen to the metal centre
could not be ascertained. The aromatic ring protons were
observed with the expected chemical shift and integral values in
the same region as those of free ligand. The most interesting
1
Fig. 3. TGA-DTG-DSC curve of C2
feature in the HNMR spectra of Ni(II) complex (C3) is the
existence of peaks corresponding to the THF molecules. Broad
THF peaks at 4.68 and 2.11 ppm that are significantly shifted
downfield from the signals for free THF indicates that the metal
possesses direct and strong bonding with the THF molecule in
the structure of the complex. Moreover, the 13C NMR spectrum
of the ligand taken in DMSO gave signals in good agreement
with the probable structures. The ligand showed signals at
158.9 ppm assigned to azomethine (C=N) carbon. The signals
from phenyl ring carbons were in the expected range. 13C NMR
spectra were also used for the elucidation of the coordination
mode of the ligand in complexes. Assignments of the signals
are based on the chemical shifts, intensity patterns and
coordination induced shift (CIS), ꢀδ [ꢀδ = δ (complex) - δ (free
ligand)], of the signals for carbon atom in the vicinity of the
coordinating functions. As a result of variation of electron
density on coordination, azomethine carbon signal is shifted
downfield by 5-10 ppm in their respective complexes, which
indicates coordination of nitrogen lone pair to metal. Other
phenyl ring carbons in these complexes resonate nearly at the
same region as those of free ligand. In comparison to the free
2.5. ESI-MS studies
The positive ion ESI mass spectrum of the ligand (
L) and
complexes (C1 C2 and C3) showed a number of informative
,
fragment ions of different intensities confirming their molecular
weights. The result presented here is interpreted in terms of
simple bond cleavages. The molecular ion peak was observed
as 164.1 [M+H]+ for ligand and the major fragmentation
pathway involved the cleavage of N-OH bonds and subsequent
-C=N bond cleavage giving fragment at m/z 150.1, 140.1, 115.1
and 88.5, corresponding to phenyl ring fragment. The mass
spectrum of the metal complexes (C1, C2 and C3) obeyed a
similar pattern of fragmentation with the molecular ion peaks
observed as [M+Na++H]+, [M+Na+] (Metal adduct ions) and
[M+H]+.
2.6. Cycloaddition Studies
The metal complexes (C1
the cycloaddition reaction between carbon dioxide and
epoxides to afford the five membered cyclic carbonate (Scheme
).
, C2 & C3) were used as catalysts for
2
ligand, all signals in C1
consequence of complexation with the Lewis acidic metals.
2.4. TGA/DTG/DSC studies
Thermal behaviour of the complexes (C1
studied in the temperature range 25-1200
indicates that the decomposition of the obtained complexes
proceeds in several stages. The TG-DSC thermogram of C2
exhibits an exothermic event at around 280 C (553.15 K)
, C2 and C3 are shifted downfield, a
O
C1, C2 or C3
nBu4NI
O
O
CO2
+
,
C2 and C3) was
O
R
°
C and the analysis
R
R= CH3, Ph
°
Scheme 2. Synthesis of cyclic carbonate from CO2 and epoxides.
accompanied by a mass loss of 30.50% (Calc. 30.48%),
assignable for the removal of coordinated water molecules and
chloride ions or THF molecules in C3. The pyrolysis of C2
The reaction conditions were standardized by observing the
effect of temperature, pressure and catalyst loading on the yield
and percentage conversion of epoxides (propylene oxide and
styrene oxide) into cyclic carbonates. Cycloaddition of CO2
was carried out in absence of a solvent, by charging the
respective epoxides and catalyst into a stainless steel Parr
showed that the complex is stable up to 280
continuous mass loss occurs in the range 280
Additional highly exothermic peaks at 425 C and 465
attributed to the decomposition of 1.15 moles of ligand (Found
°
C and then
950 C.
C are
–
°
°
°
35.90%; Calc. 35.82%), which continues up to 930°C, forming
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 3