MnIII Porphyrins: Catalytic Coupling of Epoxides with CO2 under Mild Conditions and Mechanistic Considerations

Article abstract of DOI:10.1002/cctc.201900745

A series of 5,10,15,20-tetrakis(2,3-dichlorophenyl)porphyrinate complexes of manganese(III) [Mn^III(T2,3DCPP)X] with six different axial ligands (X=NO₃ ^?, AcO^?, IO₃ ^?, Br^?, Cl^?, HO^?) were investigated as catalysts in the cycloaddition reactions of CO₂ and styrene oxide (SO), under mild conditions, i. e., atmospheric pressure and 60 °C. [Mn^IIIT(2,3DCPP)IO₃] showed the best catalytic performance, selectively producing the respective cyclic carbonate from diverse epoxides using tetrabutylammonium bromide as a nucleophile source. Mechanistic considerations were inferred from electronic spectra and spectrophotometric titrations, showing that there are a series of equilibriums involved in the formation of the catalytic active species. Stability constants for the proposed equilibrium models were determined using SQUAD software. A catalytic cycle has been proposed based on those observations.

Full text of DOI:10.1002/cctc.201900745

Accepted Article
Title: Mn(III) Porphyrins: Catalytic Coupling of Epoxides with CO₂ under
Mild Conditions and Mechanistic Considerations
Authors: Jorge Luiz Sônego Milani, Alexandre Moreira Meireles,
Werberson de Almeida Bezerra, Dayse Carvalho da Silva
Martins, Danielle Cangussu, and Rafael Pavão das Chagas
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Mn(III) Porphyrins: Catalytic Coupling of Epoxides with CO2 under
Mild Conditions and Mechanistic Considerations
Jorge Luiz Sônego Milani,*[a] Alexandre Moreira Meireles,[b] Werberson de Almeida Bezerra,[c] Dayse
Carvalho da Silva Martins,*[b] Danielle Cangussu[c] and Rafael Pavão das Chagas*[c]
Abstract:
A
series
of
5,10,15,20-tetrakis(2,3-
of manganese(III)
elements[6] or d-block metals[7] show excellent catalytic activity
and selectivity, with high TON and TOF values, in the
cycloaddition of epoxides with CO2. However, the conversion of
CO2 through efficient, selective and green catalysis, in mild
conditions, remains challenging. The synthesis of cyclic
carbonates is one of the few reactions that may employ CO2 in
mild conditions.[8] Examples of catalytic systems in mild
conditions include binary (concomitant use of a Lewis acid and a
nucleophile source to open the epoxide) and bifunctional
systems (a single catalyst species containing Lewis acid and
nucleophilic moieties).[7d, 9] Among these systems, heterocyclic
dichlorophenyl)porphyrinate
complexes
[MnIII(T2,3DCPP)X] with six different axial ligands (X = NO3 , AcO-,
-
-
IO3 , Br-, Cl-, HO-) were investigated as catalysts in the cycloaddition
reactions of CO2 and styrene oxide (SO), under mild conditions, i.e.,
atmospheric pressure and 60 °C. [MnIIIT(2,3DCPP)IO3] showed the
best catalytic performance, selectively producing the respective
cyclic carbonate from diverse epoxides using tetrabutylammonium
bromide as a nucleophile source. Mechanistic considerations were
inferred from electronic spectra and spectrophotometric titrations,
showing that there are a series of equilibriums involved in the
formation of the catalytic active species. Stability constants for the
proposed equilibrium models were determined using SQUAD
software. A catalytic cycle has been proposed based on those
observations.
macrocycle metal complexes, such as metalloporphyrins[10] and
11]
metallocorroles,[8b,
are promising catalysts due to the
possibility of altering electronic and steric characteristics by
modifications of its framework.[12] Also, in square pyramidal
metalloporphyrins the axial counterion may influence the stability
and catalytic activity and additionally can be readily replaced.[10c,
13]
Introduction
Metalloporphyrins (MP) have been studied lately for this
transformation and some outstanding results were reported.[10a,
14]
Bai et al.[15] observed that the nature of axial counter ion
Concerns about the increasing high levels of carbon dioxide in
the atmosphere and about the dependence of finite fossil carbon
resources to produce important chemicals have boosted the
interest in catalytic conversion of CO2. Carbon dioxide may be
employed as an abundant, cheap, renewable, non-toxic and
non-flammable C1 building block.[1] The thermodynamic stability
of CO2, however, restricts its use as a feedstock in industrial
attractive processes.[2] Therefore, the development of catalytic
systems to chemically convert CO2 to useful bulk commercial
products is an important topic of research.
Diverse metal- and organo-catalysts have been described to
catalytically transform CO2.[2-3] One of the most studied reactions
using CO2 is its cycloaddition with epoxides to produce cyclic
carbonates.[4] Cyclic carbonates have been used as polar aprotic
solvents in chemical process, electrolytes in lithium‐ion batteries
and intermediates to synthesize diols, polycarbonates and
polyurethanes.[5]
changes the Lewis acidity in metal center of the aluminum
porphyrin, affecting directly the conversion to cyclic carbonates.
Electronic effects in phenyl groups in cobalt porphyrins were
studied by Rieger and co-workers.[16] It was observed that
electron-withdrawing or electron-donating groups in para
position of the meso-aryl substituents of the macrocycle core
can modify significantly the conversion and the selectivity of the
system. Also, substituents in the aryl group of the meso
positions of porphyrin macrocycle are beneficial for this type of
reaction.[11] In second-generation metalloporphyrins, the
presence of halogen on the meso-aryl turns the metal ion
coordinated in the macrocycle more acidic, facilitating the
epoxide activation.[10c],[11]
In this sense, we have previously reported the synthesis and
characterization of new [MnIII(T2,3-DCPP)X] porphyrins (X = Br-,
-
-
Cl-, NO3 , AcO-, HO-, and IO3 , designed as MnP1-6,
Under high pressure, complexes either with main-group
respectively)
derived
from
5,10,15,20-tetrakis(2,3-
dichlorophenyl)porphyrin, (H2T2,3DCPP) (Figure 1) and
described their catalytic behavior in the cycloaddition of CO2 and
epoxides, at high pressure conditions.[17] It was observed that
under 5 bar of pressure, the system MnP2/tetrabutylammonium
bromide was very active converting 74% of styrene oxide in
styrene carbonate. Driven by this observation, we decided to
conduct studies on the performance of the MnP1-6 porphyrins
under milder conditions, using lower temperatures and
atmospheric pressure of CO2. Herein, it is reported this catalytic
studies in the cycloaddition reaction of CO2 with epoxides, in
mild conditions, showing that the binary system MnP1-6/TBAB is
[a]
[b]
[c]
Prof. Dr. J. L. S. Milani
Departmento de Química – Instituto de Ciências Exatas
Universidade Federal de Juiz de Fora
Juiz de Fora – MG – Brazil
E-mail: jorge.milani@ufjf.edu.br
M.Sc. A. M. Meireles, Prof. Dr. D. C. S. Martins
Departmento de Química – Instituto de Ciências Exatas
Universidade Federal de Minas Gerais
Belo Horizonte – Mg – Brazil
E-mail: daysequimica@ufmg.br
W. A. Bezerra, Prof. Dr. D. Cangussu, Prof. Dr. R. P. Das Chagas
Instituto de Química
Universidade Federal de Goiás
Goiânia – GO – Brazil
E-mail: rpchagas@ufg.br
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201900745
ChemCatChem
FULL PAPER
very efficient in atmospheric pressure of CO2 to produce cyclic
carbonates.
activity is as follow: IO3- (MnP6)  HO- (MnP5) > AcO- (MnP4) >
NO3- (MnP3) > Cl- (MnP2) > Br- (MnP1). Since Mn(III) is strongly
oxophilic in this oxidation state, the oxyanions can provide a
good stabilization of the metallic center.[19] Furthermore, all
oxygen atoms in the oxyanions are very electronegative and let
the manganese cation with a high Lewis acidity, favoring the
epoxide activation through Mn-O coordination.[20]
Table 1. Cycloaddition reactions using MnP1-6.
O
MnP1-6
TBAB
O
O
O
CO2 (1 atm)
Ph
Figure 1. Structure of the metalloporphyrin catalysts.
Ph
Entry
1
Catalyst
MnP1
MnP1
MnP1
MnP2
MnP2
MnP2
MnP3
MnP3
MnP3
MnP4
MnP4
MnP4
MnP5
MnP5
MnP5
MnP6
MnP6
MnP6
MnP6
Time (h)
6
Conversion (%)[a]
TON[b]
1,721
2,494
3,584
1,546
2,577
3,876
2,174
3,049
3,984
1,751
3,040
4,566
2,217
3,322
4,902
2,257
3,759
5,025
5,376
6,024
-
TOF[c]
287
104
75
Additionally, extensive UV-Vis spectroscopic studies have been
conducted and detailed equilibriums of the ligand exchange and
mechanism pathways have been proposed. Despite the fact that
17
25
36
15
26
39
22
30
40
18
30
46
22
33
49
23
38
50
54
60
14
2
24
48
6
3
mechanistic considerations have been done to explain the
4
258
107
81
15, 18]
coupling of CO2 and epoxides,[10c,
further efforts are
5
24
48
6
necessary to describe the intricate ligand exchange equilibriums
presumably involved in the systems catalyzed by
metalloporphyrins.
6
7
362
127
83
8
24
48
6
9
10
11
12
13
14
15
16
17
18
19
20
21
292
127
95
24
48
6
Results and Discussion
370
138
102
376
157
105
56
24
48
6
Catalytic studies on the cycloaddition reaction
Metalloporphyrins MnP1-6 were tested as catalysts, combined
with tetrabutylammonium bromide (TBAB) as a nucleophile
source, for the coupling reaction between CO2 and epoxides to
form cyclic carbonates, in a solvent-free process under 1 atm. In
a general procedure, a 10 mL glass round-bottom flask was
charged with catalyst, TBAB and the epoxide. Carbon dioxide
was purged into the flask and a CO2 balloon was connected to
maintain 1 atm of pressure.
In an initial screening, all metalloporphyrins MnP1-6 were active
to produce styrene carbonate from CO2 and styrene oxide (SO)
without diol formation in standard conditions: 0.01 mol% of
metalloporphyrin, 1 mol% of TBAB, and 60 °C (Table 1). The
conversions were evaluated in 6, 24 and 48 hours. In short
reaction times, conversions were similar, however, in longer
experiments, the differences in the activity are increasingly
significant. This shows the influence of ligand axial X on the
stabilization of the reaction intermediates, affecting the reaction
kinetics.
As previously reported, the axial ligand X plays a very important
role in such catalytic systems. Qin et al.[14d] observed that the
electronegativity of the axial ligands favored an increase in the
electrophilicity of aluminum metal center leading to larger
conversions. Anderson et al.[16] concluded that the lability of
ligand X is important for the reaction outcome when studying
Co(III)-porphyrins with electron-donating and withdrawing
groups in the porphyrin ring. In our case, the oxyanions had a
better performance on the catalytic systems, and the order of
24
48
96
120
24
MnP6
[d]
50
-
Reactions conditions: 0.0025 mmol catalyst (0.01 mol%), 0.25 mmol TBAB (1
mol%), SO (25.00 mmol), Temperature = 60 °C. [a] Conversions determined
on basis of 1H NMR analysis. [b] Turnover number (mol of carbonate
produced/mol catalyst). [c] Turnover frequency (TON h-1). [d] Blank
experiment: only TBAB (1 mol%), SO (25.00 mmol), temperature = 60 °C.
The metalloporphyrin catalyst is inactive in the absence of a
cocatalyst, as previously reported under high pressure
conditions.[17] In a blank experiment using only TBAB, i.e.,
without metalloporphyrin catalyst, conversion was 14% in 24
hours (Entry 21, Table 1). In the same conditions, all catalysts
led to better conversions and catalyst MnP6 was the most active
converting 38% of SO. Additional experiments using MnP6 were
conducted in 96 and 120 hours and a reasonable conversion of
60% was obtained (Entry 20, Table 1).
Catalyst MnP6 was chosen to verify the influence of other
parameters on the system, including temperature, time, catalyst
loading and amount of a range of nucleophile sources. Initially,
temperature studies were conducted and provided interesting
behavior. At ambient conditions (1 bar and 30 °C) only 4% of
styrene oxide was converted to carbonate in 24 hours (Entry 22,
Table 2). Increasing temperature improves the conversion
values up to 60 °C, which was the optimal temperature,
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converting 38% of SO in 24 h (Entry 25, Table 2). At higher
temperatures (70 °C, 100 °C and 120 °C) conversions were
lower (28, 24 and 25%, respectively; entries 26-28). When the
temperature is increased in atmospheric conditions (P[CO2] = 1
bar) two properties are affected: activation barrier (Ea or ΔG‡)
and CO2 solubility. In higher temperatures, CO2 solubility is
diminished, therefore, the cyclic carbonate formation in 70, 100
and 120 °C was similar due to an equilibrium between CO2
solubility and kinetic effects, both being strongly affected by
temperature changes.
conducted with 4 mol% of water at 60 °C (entries 42- 45, Table
2), did not produce any diol, showing selectivities higher than
99%.
Besides temperature, it was also observed that diol production is
time-dependent (see entries 29-32 and 42-45, Table 2). It could
be expected that epoxide and carbonate might be competing for
the Mn axial vacancy site, but not the diol. Since no diol
production is observed in initial stages of the reaction, it is
assumed that its formation is
a
result of carbonate
degradation.[21] Thus, selectivity may be affected in longer
reactions and higher temperatures due to degradation of
carbonate to form diol. On the other hand, in temperatures
below 70 °C, even in presence of water, selectivity is high
showing the robustness of our system in mild conditions.
Table 2. Cycloaddition reaction using MnP6/SO.
O
MnP6
O
base
Nitrogenated bases did not perform the reaction (entries 34 and
O
O
CO2 (1 atm)
Ph
35, Table 2), presumably because of the high coordination ability
18c, 22]
of 1-methylimidazole and DMAP.[15,
Metalloporphyrins
Ph
Conversion (%)[a]
TON[b]
TOF[c]
17
27
49
157
116
100
103
58
49
38
36
133
-
MnP1-6 are five-coordinated complexes with a square pyramidal
geometry (Figure 1), so there is a free vacant site at the metallic
center where the epoxide should coordinate to suffer the
cycloaddition. When 1-methylimidazole and DMAP are in the
system, there is a competition between the base and epoxide;
the more favorable and strong coordination between Mn(III) and
Entry
22
T (°C)
30
40
50
60
70
100
120
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
time (h)
24
4
411
647
23
24
6
24
24
12
38
28
24
25
28
47
73
87
16
0
1,188
3,759
2,793
2,392
2,481
1,389
2,370
3,650
4,348
3,190
-
25
24
26
24
nitrogen donor ligands[12a] turns the active catalyst in
a
27*
28*
29[d]
30[d]
24
hexacoordinated dormant species and no conversion is
observed (see mechanistic considerations).
24
24
48
31[d]
32[d]
*
*
96
120
24
33[e]
34[f]
35[g]
24
24
0
-
-
36[h]
37[i]*
38[j]
*
24
16
8
1,558
816
65
34
109
41
58
88
63
52
45
45
-
24
24
26
10
14
21
15
37
44
54
0
2,625
974
39[k]
40[l]
41[m]
42[n]
43[n]
24
24
1,385
2,119
1,508
3,731
4,367
5,435
-
24
24
72
44[n]
45[n]
*
96
*
120
24
46[o]
Reactions conditions: 0.0025 mmol catalyst (0.01 mol%), 0.25 mmol
tetrabutylammonium bromide, TBAB, (1 mol%), SO (25.00 mmol),
Temperature = 60 °C. [a] Conversions determined on basis of 1H NMR
analysis. [b] Turnover number (mol of carbonate produced/mol catalyst). [c]
Turnover frequency (TON h-1). [d] 0.02 mol% of catalyst. [e] 0.005 mol% of
catalyst. [f] 1-methyl-imidazole. [g] 4-dimethylaminopyridine (DMAP). [h]
Bis(triphenylphosphine)iminium chloride (PPNC). [i] Tetrabutylammonium
chloride (TBAC). [j] Tetrabutylammonium iodide (TBAI). [k] 0.25 mol% TBAB
(0.0625 mmol). [l] 0.50 mol% TBAB (0.125 mmol). [m] 2.00 mol% TBAB (0.500
Figure 2. 1H NMR spectra of entries 25, 26, 27, 28 and 42. The 1H NMR (500
MHz) were recorded in CDCl3 with tetramethylsilane (TMS) as an internal
reference at room temperature.
The activity of the tetrabutylammonium salts used as nucleophile
source cocatalysts varies as follow: TBAB > TBAI > TBAC. Two
factors are important when comparing the influence of
quaternary ammonium salts: nucleophilicity and leaving group
ability. The first is related to the nucleophilic attack to open the
epoxide ring and the second is related to the carbonate closure
by nucleophilic attack of Mn-O bond in the electrophilic carbon
attached to the halogen. The nucleophilicity and leaving group
ability order are inverse and by the results, we can speculate,
mmol). [n] 4 mol% (1 mmol of water). [o] Blank experiment without TBAB.
Diol was observed as a byproduct.
*
An important consideration in high-temperature reactions
(entries 27 and 28) is that diol is produced as a co-product (21
and 32%, respectively); however, in milder conditions (T <
70 °C) no diol is detected in NMR spectra (Figure 2), even when
reaction is conducted in presence of water. Experiments
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actually, that a balance between these properties makes TBAB
the best cocatalyst in the systems. [23]
When using PPNC and TBAC, which generate the same
nucleophilic chloride anion, the conversions were significantly
different (entries 36 and 37, Table 2), indicating that the cation
nature may affect the system.[18f] The selectivity of the reaction
was also affected by the nature of the cocatalyst. By using TBAB
or TBAI, no diol was detected (entries 25 and 38, Table 2),
however, when the cocatalyst is TBAC or PPNC (entries 36 and
37, Table 2), diol production was attested from NMR (Figure 3).
The basicity of anions is determinant for the conversion of cyclic
carbonate in diol, as previously reported. [24]
Figure 4. Cycloaddition reaction using MnP6 with different epoxides, in 24 and
120 hours, under 1 bar of CO2. Reactions conditions: 0.0025 mmol Mn catalyst
(0.02 mol%), 0.25 mmol TBAB (1 mol%), epoxide (12.50 mmol). The
conversions were determined on basis of 1H NMR analysis, TON = Turnover
number (mol of carbonate produced/mol catalyst), TOF = Turnover frequency
(TON h-1). * Diol was formed as a byproduct.
Electronic spectra and mechanistic considerations
Spectrophotometric titrations: stability constants
Metalloporphyrins (MP) contains four nitrogen atoms bound to
the metallic center in the macrocyclic plane. In many cases, MP
may exhibit axial ligands (L), above and/or below of this,
completing the coordination sphere of the metal ion. The number
of axial ligands (n) and the stability constants (Kx) can be
determined by spectrophotometric measurements, in which the
absorbance of one (or more) species is dependent of the ligand
and MP concentrations. Equations (1) and (2) represent the
formation of two main species, where X is iodate axial ligand.
Figure 3. 1H NMR spectra of entries 25, 38, 37 and 36. The 1H NMR (500
MHz) were recorded in CDCl3 with tetramethylsilane (TMS) as an internal
reference at room temperature.
Surprisingly, increasing the amount of catalyst (from 0.01 mol%
to 0.02 mol%) results in lower conversions after 24 and 48 hours.
In 96 and 120 hours, conversions were lower in experiments
with 0.01 mol% of catalyst, probably because the decomposition
of active species affects more the reaction with lower catalyst
load (compare entries 17-20 in Table 1 and entries 29-32 in
Table 2).

MPX + L
MP(L) + X
1 = K1
(equation 1)

2 = K1 × K2 (equation 2)
L
L
L
MP( )2
MP( ) +
It is important to note that in equation 1 the product may be
either the six-coordinate MPXL or the five-coordinate MPL,
which is dependent on both the coordinating ability of the axial
ligand X and the dielectric constant of the solvent, factors that
affect the dissociation of X.[25] Titration experiments were
performed by using CH2Cl2 as solvent at 25 oC in order to
ensure that the catalyst and the axial ligands were soluble and
available to interact with each other excluding the competition of
a coordinating solvent, such as the epoxide.
When the amount of the cocatalyst was changed to 0.25 and
0.50 mol% (entries 39 and 40, Table 2), it was observed a
significant decrease in the conversion as expected, since there
is a reduction of nucleophilic species to open the epoxide ring.
Unexpectedly, when increasing the cocatalyst to 2.00 mol%
(entry 41, Table 2), the conversion was also lower compared to
1 mol% of TBAB. This can be attributed to a formation of a
dormant species in higher concentrations of bromide anion.
Thereafter, MnP6 was tested in the cycloaddition of carbon
dioxide with a series of epoxides (Figure 4), under the optimized
conditions. All epoxides were converted to their corresponding
cyclic carbonates with good catalytic activity. The reaction with
epoxide glycidol (not presented) resulted in a complex mixture of
products, probably due to its susceptibility to polymerization.[9b]
Titrations of [MnIII(T2,3DCPP)IO3] (MnP6) with TBAB or DMAP
showed different behaviors. For TBAB as the titrant, UV-vis
spectra exhibited, initially, a hypsochromic shift of the Soret
band (band V) in 477 nm, followed by a bathochromic shift to
485 nm. Addition of DMAP as titrant led to a small hyperchromic
shift followed by a hypochromic-hypsochromic shift of the Soret
Band to 468 nm (Figure 5). These spectral variations suggest
the coordination of these ligands, independently, to MnP6. The
absence of clear isosbestic points in the UV-Vis spectra, at the
course of the titration, may indicate the formation of more than
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two absorbing species in solution, such as MPX, MPXL, MPL
and MPL2. Thus, the Fortran-based software “Stability Quotients
from Absorbance Data” (SQUAD), becomes the best option to
calculate the more reliable stability constants for proposed
equilibrium models from spectrophotometric data.[26]
The presence of eight chlorine atoms in the aryl groups at meso-
positions of MnP6 makes the Mn(III) ion more electrophilic,
favoring the coordination of axial ligands.[27] The higher stability
constants with bromide (from TBAB) may be explained by the
electron withdrawing effect of the chlorine atoms and by the
strong electrostatic interaction[28] between coordinated Mn(III)
and bromide ligand. This argument also may justify the
coordination of another bromide ligand, leading to an anionic
complex trans-[MnIII(T2,3DCPP)Br2]- in an equilibrium with
[MnIII(T2,3DCPP)Br]. Chatterjee and Chisholm verified the effect
The
best
fit
of
SQUAD
and
was
observed
when
were
[MnIII(T2,3DCPP)(L)]IO3
[MnIII(T2,3DCPP)L2]
considered in the equilibrium model, for both ligands used in
titrations experiments. Other tested models led to either non-
convergence or significantly inferior fits to the experimental data.
Table 3 lists the formation constants and statistical parameters
generated by SQUAD for the equilibrium model proposed. The
parameters Σ(Obsvd-Calc)2 and αdata, which assess the reliability
of the proposed model, are within acceptable limits.[26]
-
of azide (N3 ) ligand in [AlIII-porphyrins] containing chloride as
axial ligand, by IR spectroscopy, and demonstrated the
exchange of chloride anion by two azide ligands, leading to
anionic complex trans-[AlIII-porphyrin(N3)2]-.[29] It was also
described that the addition of tetrabutylammonium fluoride
(TBAF) to a solution of a [FeIII(TPP)F] led to a formation of
complex trans-[FeIII(TPP)F2]-.[30] For manganese(III) porphyrins,
Chang et al. observed the production of [MnIII(TPP)(OH)2]-
during the spectrophotometric titration of [MnIII(TPP)(ClO4)] with
tetramethylammonium hydroxide.[31]
Table 3. Parameters obtained by the SQUAD program for the
spectrophotometric titration of [MnIII(T2,3DCPP)IO3] with TBAB or DMAP.[a]
TBAB
DMAP
TBAB[d]
[b]
[c]
logβ1
logβ2
K1
6.23
3.54
5.78
10.17
6.13
9.36
1.7 x 106
8.8 x 103
1.9 x 102
0.003
3.4 x 103
4.0 x 102
8.4
4.8 x 105
2.7 x 103
1.8 x 102
0.002
K2
K1/k2
αdata
0.003
Σ(Obsvd. - Calc)²
0.02
0.05
0.008
[a] The titrations were realized in CH2Cl2 as solvent. Temperature = 25 oC. [b]
β1 = K1; [c] β2 = K1 × K2. [d] [MnIII(TPP)Cl] was used in the same conditions
employed for [MnIII(T2,3DCPP)IO3], for comparison.
DMAP exhibited lower values for K1 and K2 when compared to
TBAB. DMAP is an aminopyridine-type ligand known to be less
effective than imidazole-type ligands,[32] although K1 and K2
values obtained in this work are comparable to axial
coordination of imidazole in second-generation Mn(III)-
porphyrins.[12a] This observation could be justified by the
resonance effect promoted by the lone electron pair on the
amino substituent which makes the pyridinic nitrogen a good σ-
and π- donor in DMAP.[32]
Both ligands exhibited higher values for K1 when compared to K2,
and five- and six- coordinate species (MPXL, MPL and MPL2)
are likely to coexist in solution.[30a] The ratio K1/K2 may indicate a
preferential tendency to form MPL2 species. By comparing the
K1/K2 values for TBAB (1.9x102) and DMAP (8.4) it is possible to
infer that concentration of six-coordinated complex MPL2 is
higher in presence of DMAP. Boucher[33] mentioned the
possibility of neutral N-donor ligands to favor the formation of
six-coordinated complexes with [MnIII(TPP)Cl]. Bottomley and
Figure 5. Spectral shifts for the absorbance changes during
spectrophotometric titration of the MnP6, in CH2Cl2 solution, with (a) TBAB,
and (b) DMAP.
The output data from SQUAD also includes the molar
absorptivities of each species present in solution, in the range of
wavelengths considered (Figure S1 and S4, Supplementary
Material). For the MnP6 species was observed
a good
Kadish[30a]
,
studying the electrochemical properties of
correlation between the molar absorptivities obtained
experimentally (CH2Cl2) and those calculated by the SQUAD,
indicating excellent reliability of the equilibrium data calculated
from the proposed models.
[FeIII(TPP)X], described the coordination of two molecules of
pyridine or N,N-dimethylformamide (DMF) in axial positions of
the metal center replacing X ligands. A similar behavior was
observed for [MnIII(TPP)ClO4] in studies of Kelly and Kadish,[25b]
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10.1002/cctc.201900745
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who described that due to the very low coordinating ability of
ClO4 , coordination of pyridine, imidazole and DMSO occurs in
without the presence of SO or CO2.[18d] Thus, it is possible to
assume that the ligand exchange depicted in Scheme 1 should
be slow as it takes 24 hours for the absorption shift to a single
band at 477 nm. Such behavior shows the influence of axial
-
both axial positions of the metal center. The higher ratio K1/K2 for
TBAB could be justified due to the displacement out-of-plane of
Mn(III) ion promoted by the introduction of the first bromide
anion,[28] what may hinder the coordination of another bulky
bromide ligand in the trans position.
ligand X (X = Cl-, Br-, NO3 , AcO-, HO- or IO3 ) on the reaction
conversions, since a more labile Mn(III)-X bond would favor a
faster ligand exchange to form the more stable species, as
attested by the measured stability constants (logβ2 = 10.11).
-
-
For comparison,
a simple first-generation metalloporphyrin
[MnIII(TPP)Cl] was analyzed in the same conditions employed for
the spectrophotometric titration of [MnIII(T2,3DCPP)]IO3 with
TBAB. Similar behavior was observed for [MnIII(TPP)Cl], a
hypochromic shift followed to a bathochromic shift of the Soret
Band to 485 nm (not shown data). However, this
metalloporphyrins exhibited lower logβ1 and logβ2 (Table 3)
when compared to [MnIII(T2,3DCPP)IO3]. This observation can
be justified by the absence of electron-withdrawing atoms in the
structure of [MnIII(TPP)Cl], making the Mn(III) ion less
electrophilic in this compound.
Mechanistic studies
To obtain further mechanistic details, electronic spectra
measurements of MnP6 in CH2Cl2 in the presence of cocatalyst
and substrate were performed (Figure 6). Firstly, 5,000
equivalents of SO were added to a solution of MnP6 1.16 x 10-5
mol L-1 and the electronic spectrum remained practically
unchanged compared to MnP6 (Figure 6a and 6b), although SO
may eventually be poorly coordinated to manganese center.
When TBAB (50 equivalents) was added into the flask, the
solution color rapidly changed and a bathochromic shift was
observed in all regions of spectra (Figure 6c); at this point, we
suppose that there is an equilibrium between species depicted in
Scheme 1. Besides, these species may coexist in solution with
alkoxide complexes resulting from the epoxide ring-opening
(Figure 7). This is evidenced by a shoulder in the Soret band
(~470 nm, Figure S11, ESI) that was not observed in blank
experiments (MnP6/TBAB, Figure 6f and S11, ESI). Tiffner and
co-workers[8b] described a similar absorption to support the
presence of an axially ligated oxygen in Mn(III) corroles, without
assigning to be an alkoxide or a carboxylate.
Figure 6. UV-Vis spectra of the changes observed by the interaction between
MnP6 (1.16 x 10-5 mol L-1), TBAB (5.80 x 10-4 mol L-1, 50 eq.), CO2 (1 bar) and
SO (5.80 x 10-2 mol L-1, 5,000 eq.). (a) MnP6 in CH2Cl2; (b) MnP6 after
addition of SO at 25 °C; (c) MnP6/SO after addition of TBAB; (d)
MnP6/SO/TBAB after
1 bar pressure of CO2 and stirring and 1 h; (e)
MnP6/SO/TBAB/CO2 after 24 h; (f) MnP6 and TBAB, in CH2Cl2, after 1 h; (g)
MnP6 and TBAB, in CH2Cl2, after 24 h. The UV-Vis spectra could be seen,
individually, in ESI (Figures S5-10).
Surprisingly, when the flask was pressurized with 1 bar of CO2,
after one hour, the profile of Soret band changes and appears a
“triplet” (472, 477, 485 nm, Figure 6d), indicating an initial CO2
insertion, leading to the carbonate species coordinated in
manganese. At this point, we suggest that the species with
inserted CO2 may coexist with alkoxide species and species
depicted in Scheme 1. Within 24 hours the spectrum profile
completely changes by a hypsochromic shift, indicating that all
active catalytic species had CO2 inserted into alkoxide-metal
bond, which should be a slow step in reactions at 1 bar of
pressure. This is evidenced carrying out reactions in lower
temperatures (Entries 22-24, Table 2), when conversions were
Scheme 1. Possible equilibria involved in the interaction between
[MnIIIT(2,3DCPP)IO3] and TBAB, in CH2Cl2.
In the one-hour blank experiment (MnP6/TBAB, Figure 6f), the
Soret band was observed as a broad band with absorptions at
478 and 484 nm. After 24 hours (Figure 6g), the Soret band
appears as a single band with maxima of absorption at 477 nm.
The latter UV-Vis spectrum is rather similar to the spectra
calculated by the SQUAD for MPL specie (Figure S2, ESI),
indicating the formation of a more stable [MnIII(T2,3DCPP)Br], so
that this absorption only comes from coordination of bromide,
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very low, probably because the transition free energy to the
carbonate ring closure should be high.[34] Therefore, the
nucleophilic attack by alkoxide bond (O—Mn) in the electrophilic
carbon also might be a rate-determining step as can be inferred
from UV-Vis analysis.
species may coexist in diverse equilibriums. The lability of axial
ligands of the porphyrin complex is responsible for the reaction
outcome and for the preferential formation of a five- or hexa-
coordinated intermediate (Figure 7).
The influence of axial ligands in the reaction is related to their
coordinating ability. The catalytic results exhibited in Table 1
revealed that this is less pronounced in 6 hours (conversions
between 15 and 23%); however, in longer periods, as in 48
hours, conversions are rather different (from 36 to 52%). When
the epoxide is attacked by an external nucleophile, an alkoxide
bond is formed between the Mn(III) and epoxide and, at this
point there are at least two possibilities: a) the catalytic species
remain neutral and an axial ligand is uncoordinated (five-
coordinated), or b) the catalytic species becomes anionic with
tetrabutylammonium cation as counterion (six-coordinated). Both
pathways may coexist in the catalytic cycle, and the preference
for one or another depends on the coordinating ability and
lability of axial ligand X. Since axial ligand X influences on the
stabilization of the reaction intermediates, affecting the reaction
kinetics, the ligand exchange equilibriums are the responsible
for the differences in conversions.
As mentioned above, the ligand exchange depicted in Scheme 1
-
should be slow, even for the poor coordinating IO3 axial ligand
(see blank experiment in figure 6f and 6g). Thus, even if the
complexes MPL and MPL2 (L = Br-) have large values of K1 =
1.7 x 106 and K2 = 8.8 x 103 (see section 2.2.1), the
[MnIII(T2,3DCPP)X] should be active in the catalytic cycle, since
different conversions were observed using different axial ligands.
Based on those observations, we propose the catalytic cycle
presented in Figure 7. The reaction proceeds upon coordination
of SO in the metal center, followed by the TBAB addition. The
large excess of TBAB should be able to replace the ligand X or
the SO coordinated in the manganese center, leading to the
many different species, some are dormant however reversible
MPL2 (L = axial ligand, except SO) and active species such as
[MP(SO)]+ or MPL(SO), the UV-Vis spectra in Figure 6 (and
Figure S8, ESI) show the enlargement in Soret band, that
characterize the mixture of different metalloporphyrins
species.[18d] Subsequently, a nucleophilic attack by a bromide
anion from TBAB occur either on α or β carbon of epoxide,
preferentially in the less hindered carbon (β attack).[35] However,
α attack occur when some fragment of epoxide is electron-
withdrawing as a phenyl group in SO, but with low selectivity.[36]
The ring-opening reaction is followed by CO2 insertion and ring-
closure to form the respective cyclic carbonate, regenerating the
active species. Ema et al.[18f] have studied the binding constants
(Ka) for 1,2-epoxyhexene and the respective cyclic carbonate
and have verified that the epoxide and cyclic carbonates in Zn(II)
and Mg(II)-porphyrins have small values of Ka, measured in
CH2Cl2. These results indicate that the ligand exchange (cyclic
carbonate to epoxide) is not a rate-determining step.57 By
connecting the studies of Ema with the present studies, there
are three possible rate-determining steps in the catalytic cycle:
epoxide opening, carbon dioxide insertion and ring-closure, to
form the cyclic carbonate at 30 °C. The complete understanding
of the catalytic system is not trivial because a series of different
Figure 7. Catalytic cycle proposed for the cycloaddition of CO2 in SO using
-
-
[MnIII(T2,3DCPP)X] (X = Cl-, Br-, NO3 , AcO-, HO- or IO3 ) and TBAB. Ligand X
or L may or may not be coordinated, depending on its nature.
In order to understand the behavior of the system when
nitrogenated bases were used instead of TBAX or PPNC, we
investigate UV-Vis spectra in the reaction course using DMAP
as cocatalyst. DMAP and 1-methylimidazole have been reported
as good cocatalysts for copolymerization of CO2 and epoxides
catalysed by metalloporphyrins.[37] In contrast, in our work,
reactions performed in the presence of these cocatalysts did not
convert SO (Entry 34 and 35, Table 2). According to the
spectrophotometric titrations, the successive addition of DMAP
led to the formation of [MnIII(T2,3DCPP)(DMAP)]IO3 and
[MnIII(T2,3DCPP)(DMAP)2]IO3 (Scheme 2).
Scheme 2. Possible equilibria involved in the interaction between
[MnIII(T2,3DCPP)IO3] and different nitrogenated base (NB), in CH2Cl2.
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10.1002/cctc.201900745
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the reason for the inactivation of catalyst in the reactions with
DMAP.
Initially SO was added to a solution of MnP6 in CH2Cl2 and no
change was observed (Figure 8b). However, when DMAP was
added, an enlargement of Soret band was observed (Figure 8c).
This enlargement of the Soret band could be admitted to the
production of species depicted in Scheme 2, what is
corroborated by the spectrophotometric titrations (Figure 6b).
Similarly, in blank experiments of MnP6 in the exclusive
presence of DMAP, the UV-Vis spectrum (ESI, Figure S12),
exhibited the same enlargement of the Soret band, indicating the
coexistence of more than one specie in solution (possibly MPL
and MPL2, where L = DMAP).
When 1-methylimidazole was used as cocatalyst instead of
DMAP, similar behavior was observed (0% of conversion). Da
Silva and co-workers[12a] have studied similar Mn(III)-porphyrins
([MnIII(TPP)Cl],
[MnIII(APTPP)Cl],
[MnIII(Br9APTPP)Cl],
[MnIII(T4CMPP)Cl] and [MnIII(Br8T4CMPP)Cl]) in the presence of
imidazole as cocatalyst. Similarly, the authors have made a
spectrophotometric titration of Mn(III)-porphyrins and calculated
K1 and K2, in equilibria quite similar to Scheme 2. They observed
that MPL2 species had a larger  value than MPL species, what
should occur in our case. In general, imidazoles are strong π-
donors and, when compared to pyridine derivates, the smaller
size of these ligands favors the coordination in
metalloporphyrins.[32] This is
a strong evidence that 1-
methylimidazole coordinate in manganese center inactivating
the systems because the resulting complex is kinetically inert to
ligand exchange in this condition.
In the case of TBAB, regardless if a five- or six- coordinated
intermediate is formed in the catalytic system, the Mn-Br bond
seems to be sufficiently labile to exchange with epoxide to
initiate the catalytic cycle. While, for DMAP, the production of
six-coordinated MnP(DMAP)2 and the non-labile Mn-NDMAP bond
prevents the reaction to occur.
Further studies are necessary for a complete understanding of
the intricate mechanisms of ligand exchange involved in the
binary catalytic systems for the production of cyclic carbonates.
Conclusions
In summary, six manganese porphyrins (MnP1-6) were tested
as catalysts for the cycloaddition of CO2 and epoxides under
mild conditions. A series of parameters was evaluated in the
catalytic system and [MnIII(T2,3DCPP)IO3] (MnP6) showed the
best performance in the reaction with high selectivity, even in
presence of water. It could be used to convert diverse epoxides
in cyclic carbonates, converting up to 91% of epichlorohydrin,
using very low catalyst and cocatalyst loads (compared to
present literature) under atmospheric pressure and 60 °C. The
chlorinated aryl substituents on the meso positions of
metalloporphyrin may facilitate the initial reaction by enhancing
binding interactions among catalyst, cocatalyst and reactant.
Mechanistic considerations were inferred from studies based on
UV-Vis spectra and spectrophotometric titration. Stability
constants for species derived from MnP6 and bromide anion or
DMAP were determined using SQUAD software. The
intermediate [MnIII(T2,3DCPP)Br2]-, formed when TBAB is used
as cocatalyst, presents higher stability constants compared to
[MnIII(T2,3DCPP)(DMAP)2]+, produced when DMAP is used as
cocatalyst. The electron withdrawing chlorines drain the
electronic charge from the axial ligands and make the anionic
[MPBr2]- more stable than cationic [MP(DMAP)2]+, as can be
seen in titration data. Still, the former has labile Mn-Br bonds
which is essential for the reaction to occur, whilst the latter is
Figure 8. UV-Vis spectra of the changes observed by the interaction between
MnP6 (1.16 x 10-5 mol L-1), DMAP (5.80 x 10-4 mol L-1, 50 eq.), CO2 (1 bar)
and SO (5.80 x 10-2 mol L-1, 5,000 eq.). (a) MnP6 in CH2Cl2; (b) MnP6 after
addition of SO at 25 °C; (c) MnP6/SO after addition of DMAP.
It is presumable from our results and previous studies,[25b, 33, 38]
that in the presence of a large excess of DMAP, MnP6 is
converted to a six-coordinated species [MnP(DMAP)2]X. The
ratio K1/K2 calculated from SQUAD was 1.9x102 for TBAB and
8.4 for DMAP (Table 3), indicating a higher tendency to form
MPL2 species when using DMAP. Kelly and Kadish[25b] reported
that [MnIII(TPP)X] when dissolved in pyridine produces
[MnIII(TPP)(Py)2]+, when the axial ligand X is a poor coordinating
-
ion as ClO4 , I- or Br-. In our case, the axial ligand IO3- has a very
weak coordinating ability and DMAP is an even better ligand
than pyridine. Since DMAP is a very good σ- and π-donor,
plausibly, the bond Mn-NDMAP is strong and kinetically non-labile,
so the formation of an inert MPL2 species could be admitted as
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10.1002/cctc.201900745
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kinetically inert and no ligand substitution reactions occur after
[MnIII(T2,3DCPP)(DMAP)2]+ is formed in the catalytic system.
A series of ligand exchange equilibriums is involved in the
formation of the catalytic active species, what can be presumed
by accompanying the reaction with UV-vis spectra. The lability of
axial ligand X of Mn(III)-porphyrins and of the ligand L (from
nucleophile source cocatalyst) determine the coexistence of
different species in the process and are the responsible for the
reaction outcome. A plausible catalytic cycle has been proposed,
supposing that the mechanism may occur via a neutral and an
anionic route. This work illustrates the intricacy of the
mechanism of homogeneous metallic systems to convert CO2
and epoxides in carbonates and shed light to some mechanistic
aspects of the reaction.
treatment of the data obtained by the titrations was carried
with the use of the software Stability Quotients from
Absorbance Data (SQUAD).[26]
General procedure for cycloaddition reactions
In a general procedure, a 10 mL round flask with a
magnetic stirrer was charged with complexes MnP1-6
(0.0025 mmol, 0.0010-0.0013 g, 0.01 mol%), cocatalyst
(TBAB, 0.25 mmol, 0.0805 g, 1 mol%) and styrene oxide
(25 mmol, 1.75 mL). A balloon of carbon dioxide (1 bar)
was pressurized into this mixture and the reaction was
operated at the predetermined conditions. After the reaction
was completed, the vessel was cooled to room temperature
(25-30 °C) and a sample was prepared to 1H NMR analysis.
Conversion was calculated on the basis of 1H NMR
spectroscopy of the crude reaction mixture thus obtained, in
deuterated chloroform. Quantities were adjusted when
changing the proportions, type of cocatalyst and the
epoxide.
Experimental Section
Reagents
Analytical grade CH3OH, CH2Cl2, CHCl3, n-hexane and
N,N-dimethylformamide were obtained from Aldrich
Chemical Co. All the other reagents were of analytical
grade and were used without further purification. The CO2 Acknowledgements
gas (99.99%) was purchased and used without further
This work was financially supported by Fundação de Amparo à
purification. TBAB, TBAI, TBAC, PPNC, DMAP, SO, and
the other epoxides were used without further purification.
Porphyrins were obtained according to published
protocol.[17]
Pesquisa do Estado de Goiás (Nº 201610267001033), by
Fundação de Amparo à Pesquisa do Estado de Minas Gerais
(FAPEMIG), by Financiadora de Estudos e Projetos (Finep), and
by Conselho Nacional de Desenvolvimento Científico
e
Tecnológico (CNPq). W.A.B. and A.M.M. thank CNPq for
fellowships.
Instrumentation
UV-vis spectra were recorded in HP 8453A diode-array
spectrophotometer. Solution NMR spectra were collected at
ambient temperatures using Bruker Avance III 11.75 Tesla
spectrometer (500 MHz) at room temperature in deuterated
chloroform (CDCl3) with tetramethylsilane (TMS) as internal
reference.
Keywords: Carbon dioxide fixation; Homogeneous catalysis;
Metalloporphyrins; Mild conditions; SQUAD.
Axial Coordination studies to [MnIIIT(2,3DCPP)IO3]
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10.1002/cctc.201900745
ChemCatChem
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10.1002/cctc.201900745
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The coupling of CO2 with epoxides is
catalyzed by Mn(III) porphyrins, under
mild conditions, and a series of
equilibriums is involved in the
formation of the catalytic active
species. Electronic spectra and
spectrophotometric titrations provided
insights into the mechanism of the
reaction.
Jorge Luiz Sônego Milani,* Alexandre
Moreira Meireles, Werberson de
Almeida Bezerra, Dayse Carvalho da
Silva Martins,* Danielle Cangussu and
Rafael Pavão das Chagas*
Page No. – Page No.
Mn(III) Porphyrins: Catalytic Coupling
of Epoxides with CO2 under Mild
Conditions and Mechanistic
Considerations
This article is protected by copyright. All rights reserved.