Synthesis and characterization of Co(iii) amidoamine complexes: Influence of substituents of the ligand on catalytic cyclic carbonate synthesis from epoxide and carbon dioxide

Article abstract of DOI:10.1039/c3dt51352a

A series of amidoamine ligands (1) and their cobalt(iii) complexes (2) were synthesized and characterized by various spectroscopic techniques including ¹H-NMR and X-ray crystallographic techniques. X-ray crystallography shows that one of the complexes, 2a, forms a chiral coordination polymer due to bridge formation with Li⁺ associated with the complex, although the ligand is achiral. Complex 2 was employed for catalytic synthesis of cyclic carbonates from epoxides and carbon dioxide (CO₂) in a solvent free condition. A strong influence of the substituents on the ligand 1 was revealed by the varied activity of complex 2. The presence of electron withdrawing groups such as chloro (2b) and nitro (2c) increases the Lewis acidity of the catalyst, which, in turn, enhances the catalytic activity of 2. An electron withdrawing group containing complexes (2b and 2c) showed exceptionally high catalytic activity with a turnover frequency (TOF) of 662 and 602 h^-1 respectively at 130°C and 300 psig CO₂ pressure. On the other hand, our studies indicate that a catalyst with an electron releasing group (2d) showed relatively lower activity with a TOF of 488 h^-1 under similar reaction conditions. Our results show that cobalt(iii) complexes follow the reactivity order of 2d < 2a < 2c < 2b.

Full text of DOI:10.1039/c3dt51352a

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Synthesis and characterization of Co(III) amidoamine
complexes: influence of substituents of the ligand on
catalytic cyclic carbonate synthesis from epoxide and
carbon dioxide†
Cite this: Dalton Trans., 2013, 42, 13151
Punnamchandar Ramidi,a Nikolay Gerasimchuk,b Yashraj Gartia,a
Charlette M. Feltona and Anindya Ghosh*a
A series of amidoamine ligands (1) and their cobalt(III) complexes (2) were synthesized and characterized
by various spectroscopic techniques including 1H-NMR and X-ray crystallographic techniques. X-ray
crystallography shows that one of the complexes, 2a, forms a chiral coordination polymer due to bridge
formation with Li+ associated with the complex, although the ligand is achiral. Complex 2 was employed
for catalytic synthesis of cyclic carbonates from epoxides and carbon dioxide (CO2) in a solvent free con-
dition. A strong influence of the substituents on the ligand 1 was revealed by the varied activity of
complex 2. The presence of electron withdrawing groups such as chloro (2b) and nitro (2c) increases the
Lewis acidity of the catalyst, which, in turn, enhances the catalytic activity of 2. An electron withdrawing
group containing complexes (2b and 2c) showed exceptionally high catalytic activity with a turnover fre-
quency (TOF) of 662 and 602 h−1 respectively at 130 °C and 300 psig CO2 pressure. On the other hand,
our studies indicate that a catalyst with an electron releasing group (2d) showed relatively lower activity
with a TOF of 488 h−1 under similar reaction conditions. Our results show that cobalt(III) complexes
follow the reactivity order of 2d < 2a < 2c < 2b.
Received 23rd May 2013,
Accepted 6th July 2013
DOI: 10.1039/c3dt51352a
www.rsc.org/dalton
1. Introduction
Commercial production of many useful chemicals and poly-
mers has been heavily dependent on petroleum feedstock.
Since crude oil is a non-renewable resource, its depletion is a
cause of serious concern. Many efforts have been made over
the past few decades to develop alternative and sustainable
materials as a solution to this problem.1–7 In terms of green
chemistry and sustainable development, the use of renewable
resources, such as carbon dioxide (CO2), as a starting material
for the production of industrially important organic com-
Scheme 1 Synthesis of cyclic carbonate using CO2 and epoxide.
cyclic carbonates via cycloaddition of CO2 to epoxide
(Scheme 1). Cyclic carbonates are colorless, odorless, and bio-
degradable chemicals with low toxicity.9–11 Because of these
advantages, cyclic carbonates have found use in several appli-
cations which includes as solvents and precursors for synthe-
sizing various pharmaceutically and industrially important
chemicals and polymers.12–20
In the past few decades, several homogeneous and hetero-
geneous catalytic systems such as ionic liquids,21,22 supported
metal oxides,23,24 alkali metal salts,25 quaternary ammonium
salts,26 and transition metal complexes,27–35 have been
employed in cyclic carbonate synthesis. However, attention
has been paid to the development of catalysts with a high
efficiency and that work under mild reaction conditions.
pounds has been
a topic of interest to the scientific
community.6–8 One such utilization of CO2 is the synthesis of
aDepartment of Chemistry, University of Arkansas at Little Rock, 2801 South
University Avenue, Little Rock, AR 72204, USA. E-mail: axghosh@ualr.edu;
Fax: +1 501 569 8838; Tel: +1 501 569 8827
bDepartment of Chemistry, Temple Hall 456, Missouri State University, Springfield,
Missouri 65897, USA
†Electronic supplementary information (ESI) available: Synthetic procedure and
characterization details of all ligands and cobalt complexes including X-ray crys-
tallographic information and data for 2a, 2b together with PLATON check-CIF
reports, product characterizations, and Hammett plot are given in ESI. CCDC
936734 and 936336. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt51352a
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Among them, bimetallic and one component binary catalytic formation of coordination polymers, even though the ligand is
systems were prominent in this pursuit.34–38
not chiral. Crystals of complex 2b were observed as a neutral
For the cycloaddition of CO2 to epoxide, activation of complex with an axial halide ligand. Catalytic activities of
epoxide and subsequent epoxide ring opening are vital steps. 2a–2d were studied on cycloaddition of CO2 to epoxide in the
Generally, such a cycloaddition reaction requires a Lewis presence of 4-dimethylaminopyridine (DMAP) as a co-catalyst.
acidic catalyst and a Lewis base as a co-catalyst.29,39 Lewis Based on the variation in their electron withdrawing (2b
acidity of any catalyst for such a reaction should play an and 2c) and electron donating (2a and 2d) properties, we
important role. It is evident from previous reports that the have observed that dichloro substituted Co(III) amidoamine
increase in Lewis acidity of the metal complexes enhances complex (2b) produced the high catalytic activity with a TOF of
their catalytic activity. For example, metal-based catalysts with 662 h−1 on propylene carbonate production. On the other
high oxidation numbers are more efficient in cycloaddition hand, complex 2d, which contains electron releasing methyl
reactions.21,39–41 In the literature many reactive catalysts are groups, showed low catalytic activity (TOF 488 h−1). A linear
made of Al(III) and Zn(II), which are fairly strong Lewis acids.41 relationship was obtained when TOF (h−1) of different catalysts
Previously, we also reported the influence of Lewis acids on were plotted against the Hammett parameters (σm) of various
catalytic activity. In our study we showed that when we substituents corroborating our hypothesis on reactivity and the
replaced counter cation Li+ of the Cr(V) amido macrocyclic effect of electron withdrawing and releasing groups on Lewis
complex with Na+ and K+, the catalytic activity dropped dra- acidity. In order to understand the activity of these cobalt com-
matically.28 We rationalized the observation based on the fact plexes, optimization studies were performed using propylene
that Li+ is more Lewis acidic and, hence, accounts for more oxide as the reference substrate.
activity compared to less Lewis acidic counter cations (Na+ and
K+) of the Cr(V) complex. We also studied the activity of various
alkali halides with Lewis bases for the cycloaddition of CO2 to
2. Results and discussion
2.1. Synthesis, characterization and structural studies of
epoxides and increased activity was observed with increas-
ing Lewis acidity of the alkali halides.27,28 Apart from these
cobalt complexes
findings, we recently reported the synthesis of cyclic carbon-
ates and oxazolidinones using a series of chlorostannoxane Co(III) complexes (Fig. 1) of the corresponding amidoamine
catalysts.42–44 The catalysts showed a strong dependence on ligands were synthesized by taking the ligand in tetrahydro-
its substituents. The catalyst with high Lewis acidity, as dic- furan (THF) under an inert atmosphere. Using n-butyllithium
tated by the substituents on the Sn centers of chlorostannox- as a strong base, the ligand was deprotonated and reacted with
anes, furnishes high reactivity towards both cyclic carbonate anhydrous Co(II) chloride. Finally, the desired Co(III) complex
and oxazolidinone synthesis.
was obtained under air exposure. The resulting Co complex
Focusing on the possibility of increasing the electrophilicity was found to be highly stable in air and moisture. All these
of the metal complexes by introducing electron withdrawing Co(III) complexes (2a–2d) were characterized by performing
groups, we developed a series of Co(III) metal complexes (2) of FT-IR, ESI-MS, NMR, and elemental analysis studies.
various amidoamine ligands (1) bearing various electron with-
For structure elucidation of these complexes, Co(III) com-
drawing and electron donating groups on the aromatic ring plexes were synthesized by using cobalt(II) halide as a metal
of the ligands (Fig. 1). Co(III) complexes (2) of corresponding precursor. Characterization details of complexes 2a–2d are
ligands (1) were successfully characterized by using various explained here. Elemental analysis values for complexes 2a–2d
analytical techniques. X-ray crystallographic studies of matched well with the theoretical values (see the Experimental
complex 2a reveal that it is an anionic cobalt complex with section). The strong and sharp NMR signals (Fig. 2) of the
two axial halides and a Li+ counter cation. Interestingly, the complexes indicate that all the complexes are low spin d6
complex crystallizes in a chiral space group due to the systems. The aromatic proton peaks of the complexes are
shifted to lower field compared with relevant ligands. Protons
of methyl groups (–CH3) attached to the amino acid part of the
ligand show a characteristic singlet peak in the region of δ =
1.6–1.5 ppm. On the other hand, CH3 protons attached to an
aromatic ring in 2d show a characteristic singlet peak at δ =
2.06 ppm. One extra peak was observed at δ = 2.06 ppm (Fig. 2)
for 2b and 2d. This extra peak was assigned to an acetonitrile
molecule which was used for crystallization and further con-
firmed by X-ray crystallography study. The Fourier transform
infrared spectroscopic (FT-IR) analysis of the complexes
showed the characteristic carbonyl stretching frequencies
in the range of 1580–1600 cm−1
, and NH stretching
3220–3460 cm−1. A characteristic C–Cl stretching frequency at
678 cm−1 for 2b and a NO2 stretching frequency at 1308 cm−1
Fig. 1 Structure of the catalysts (R1, R2 are different electron donating and
electron withdrawing groups on the catalyst structure). *Counter cation is Li+.
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Fig. 2 1H NMR spectra of 2a, 2b, 2c and 2d.
for 2c were observed. Complexes 2a–2d were further character-
ized by using ESI-MS (Fig. S9, ESI†). X-ray quality crystals of
complexes 2a and 2b with bromide were obtained from slow
evaporation of solvent acetonitrile in which the catalyst was
dissolved previously. X-ray crystallographic studies were per-
formed on suitable 2a and 2b crystals.
Molecular structure and numbering scheme for complex 2a
are shown in Fig. 3, while selected bond distances and valence
angles are summarized in Table 2. This compound represents
the cationic complex of Li(CoLBr2)·nCH3CN composition and
contains several disordered solvent molecules. It appears that
disordered solvent molecules are clustered around a Li+ cation
from one side of the molecule (Fig. S12, ESI†), and are electro-
statically attracted to the NH2 groups of the amidoamine
ligand (Fig. S13, ESI†). In addition to disorder, there was also
their partial occupancy in the crystal lattice, which further
complicated the structure determination. We refrained from
using the PLATON SQUEEZ program in order not to alter
experimental data which would affect chemical meaning in
the structure. Although we were able to model non-hydrogen
atoms in a disordered solvent, we were not able to locate
H-atoms of the methyl groups on the difference map. More-
over, an attempt to place hydrogen atoms geometrically led to
unstable refinement. Therefore, they were not included in final
refinement cycles. Their absence did not affect overall quality
of the structure due to the presence of 3 heavy atoms (Co, 2Br)
Fig. 3 Two orthogonal views of the 2a complex structure: A – side view, B –
top view showing the numbering scheme; an ORTEP drawing at the 50%
thermal ellipsoids probability level. Symmetry transformations: #1 x, y, z; #2 −x,
−y, z + 1/2, #3 −x, y, −z + 1/2; #4 x, −y, −z; #5 x + 1/2, y + 1/2, z; #6 −x + 1/2,
−y + 1/2, z + 1/2; #7 −x + 1/2, y + 1/2, −z + 1/2; #8 x + 1/2, −y + 1/2, −z.
in the structure. The Co(III) center in the complex is in a dis- Co–Namide distances are significantly shorter than the
torted octahedral environment comprised of two axial Br− Co–Namine distances. Two axial bromine atoms are at equal dis-
anions and 4 nitrogen atoms in the equatorial plane (Fig. 3). tance from the Co metal center with 2.428 Å. Notable devi-
The bond distances of Co to aromatic N (deprotonated amide ations in the four Co–N angles are observed, N(2)#1–Co–N(2),
nitrogens) atoms and amine N (aliphatic amine nitrogens) N(2)#1–Co(1)–N(1)#1, N(2)–Co(1)–N(1) angles with 86.2°,
atoms in 2a are 1.862 Å and 1.958 Å, respectively. The 85.11° and 85.11°, respectively, whereas N(1)–Co–N(1)#1 with a
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wider angle of 103.6°, which are found to be comparable with separation.47–49 Herein, we observed that complex 2a crystal-
the earlier reported Co(III) complexes of 1,2-bis(2-pyridinecar- lizes in a chiral space group P222(1) (#20), forming a coordi-
boxamide)benzene ligand.45,46
nation polymer. The metal center in 2a forms six coordinated
Details of the geometry of the coordination polyhedron of species with four nitrogen atoms (equatorial) and two bromide
the central atom are shown in Fig. S14 (ESI†). The presence of ions (axial), and it induces the chirality through the solvated
solvated with disordered CH3CN molecules in Li+ cations adds Li+ ions, which is intimately associated with the anionic units
some peculiarity to the crystal structure of 2a. Thus, the Li+ of the complex and forms bridges between them, resulting in
counter cation acts as a bridge between anionic [CoL(Br)2]− a chiral coordination polymer with achiral units. We are
units (Fig. 3, L stands for a ligand). Li+, being small in size, currently investigating crystals of 2a for further applications,
coordinates to relatively electron rich carbonyl groups of especially in the field of asymmetric synthesis.
deprotonated amide bonds of the ligand to form a coordi-
Fig. 4 shows the ORTEP diagram and the numbering
nation polymer. The shape and configuration of the coordi- scheme for complex 2b. Details of the data collection and
nation polymer formed became chiral, although the Co(III) refinement parameters of complex 2b are listed in Table 1,
complex is, obviously, non-chiral. The result is crystallization while selected bond lengths and valence angles are presented
of 2a in a chiral space group P222(1) (#20) (Table 1), and in Table 2. X-ray crystallographic studies demonstrate that the
because there are three heavy atoms present in the structure, Co(III) complex crystallizes in the orthorhombic system in
their anomalous dispersion allowed reliable determination of Pnma space group. The metal center is in a six coordinate octa-
the absolute configuration of the complex, which is reflected hedral environment comprised of four N atoms of the ligand
in its very low Flack parameter. Both Co(III) and Li+ atoms moiety in the equatorial plane (two amine ligands and two
occupy special positions in the unit cell.
deprotonated amide bonds) and two axial positions occupied
Such chiral coordination polymers can be obtained by with one bromide and one solvent molecule (acetonitrile). The
using chiral species and chiral additives, and by crystallization bond distances of Co to aromatic N (deprotonated amide
of achiral moieties without using any chiral additives, result- bond) atoms and amine N atoms in 2b are 1.868 and 1.969 Å,
ing in spontaneous chiral resolution, which is rare.47–52 respectively, which are similar to the previously reported Co(III)
The use of chiral coordination polymers has been of growing complexes of salen and macrocyclic ligands.27,53 The distances
scientific interest in recent years due to their potential appli- between the metal center and axial atoms are 2.369 and
cations in asymmetric catalysis, non-linear optics, and chiral 1.937 Å for Co–Br and Co–N (NCCH3), respectively. The N–Co–N
Table 1 Crystal data and structure refinement for the studied Co(III) complexes
2a
2b
Empirical formula
Formula weight
Temperature (K)
Wavelength
Crystal system
Space group
C9.45H10BrCo0.50Li0.50N3.24
297.71
120(2)
O
C16H21BrCl2CoN5O2
525.12
120(2)
0.71073 Å
Orthorhombic
Pnma, #62
a = 10.2678(6)
b = 13.4525(8)
c = 14.9665(9) Å
α = 90
0.71073 Å
Orthorhombic
P222(1), #20
a = 9.0155(12)
b = 17.429(2)
c = 15.794(2)
α = 90
Unit cell dimensions (Å; °)
β = 90
β = 90
γ = 90
2067.3(2), 4
1.687 Mg m−3
3.043
γ = 90
Volume (Å3), Z
2481.8(6) Å3, 8
1.594 Mg m−3
3.938
Density (calculated)
Absorption coefficient (mm−1
F(000)
Crystal size (mm)
Θ Range for data collection
Index ranges
)
1179
1056
0.25 × 0.13 × 0.10
2.54 to 27.16°
−11 ≤ h ≤ 11
−22 ≤ k ≤ 22
−20 ≤ l ≤ 20
14 362
0.49 × 0.27 × 0.22
2.04 to 29.13°
−14 ≤ h ≤ 13
−18 ≤ k ≤ 18
−20 ≤ l ≤ 20
26 557
Reflections collected
Independent reflections
Completeness to Θ
2756 [R(int) = 0.0664]
99.7%
2880 [R(int) = 0.0301]
99.6%
Absorption correction
T(max) and T(min)
Semi-empirical
0.7455 and 0.5770
Numerical
0.7620 and 0.5285
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
Full-matrix least-squares on F2
2880/0/173
0.989
R1 = 0.0394, wR2 = 0.0815
R1 = 0.0654, wR2 = 0.0911
0.469 and −0.548
1.091
R1 = 0.0383, wR2 = 0.0941
R1 = 0.0466, wR2 = 0.0987
0.922 and −1.106
Largest peak and hole (e Å−3
)
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Table 2 Selected bond length and valence angles in structures of Co(III)
complexes
2a
Bonds (Å)
Angles (°)
Ligand
N(1)–C(1)
C(1)–C(4)
C(4)–N(2)
N(2)–C(5)
C(4)–O(1)
O(1)–Li(1)
Metal center
Co(1)–N(2)#1
Co(1)–N(2)
Co(1)–N(1)
Co(1)–N(1)#1
Co(1)–Br(1)#1
Co(1)–Br(1)
1.514(6)
1.533(7)
1.316(6)
1.419(6)
1.253(5)
1.810(8)
N(1)–C(1)–C(4)
O(1)–C(4)–N(2)
N(2)–C(4)–C(1)
C(4)–O(1)–Li(1)
C(4)–N(2)–C(5)
C(5)#1–C(5)–N(2)
108.5(4)
126.5(4)
115.2(4)
166.6(3)
127.5(4)
113.6(3)
1.862(4)
1.862(4)
1.958(4)
1.958(4)
2.4279(5)
2.4279(5)
N(2)#1–Co(1)–N(2)
N(2)#1–Co(1)–N(1)
N(2)–Co(1)–N(1)
N(1)–Co(1)–N(1)#1
N(2)#1–Co(1)–Br(1)#1
N(2)–Co(1)–Br(1)#1
N(1)#1–Co(1)–Br(1)#1
N(1)#1–Co(1)–Br(1)
Br(1)#1–Co(1)–Br(1)
86.2(2)
171.29(18)
85.11(17)
103.6(2)
91.09(5)
91.53(5)
88.47(13)
89.31(13)
176.42(5)
Symmetry transformations used to generate equivalent atoms: #1 −x,
y, −z − 1/2
2b
Bonds (Å)
Angles (°)
Ligand
C(1)–N(1)
C(1)–C(4)
C(4)–O(1)
C(4)–N(2)
C(5)–N(2)
C(7)–Cl(1)
1.509(4)
1.551(4)
1.233(3)
1.342(3)
1.403(3)
1.735(3)
N(1)–C(1)–C(4)
O(1)–C(4)–N(2)
O(1)–C(4)–C(1)
N(2)–C(4)–C(1)
C(4)–N(2)–C(5)
C(7)#1–C(7)–Cl(1)
C(6)–C(7)–Cl(1)
109.6(2)
127.0(2)
119.8(2)
113.2(2)
127.0(2)
121.03(9)
118.26(19)
Metal center
Br(1)–Co(1)
Co(1)–N(2)
Co(1)–N(2)#1
Co(1)–N(3)
Co(1)–N(1)#1
Co(1)–N(1)
2.3690(7)
1.868(2)
1.868(2)
1.937(4)
1.969(2)
1.969(2)
N(2)–Co(1)–N(2)#1
N(2)#1–Co(1)–N(3)
N(2)#1–Co(1)–N(1)#1
N(3)–Co(1)–N(1)#1
N(2)#1–Co(1)–N(1)
N(1)#1–Co(1)–N(1)
N(2)#1–Co(1)–Br(1)
N(3)–Co(1)–Br(1)
86.58(13)
89.70(10)
85.19(10)
91.77(10)
171.63(10)
102.99(14)
90.13(7)
Fig. 4 Two orthogonal views of the 2b complex structure: A – side view, B –
top view showing the numbering scheme. Symmetry codes are: #1 x, y, z; #2 −x,
−y, z + 1/2, #3 −x, y, −z + 1/2; #4 x, −y, −z; #5 x + 1/2, y + 1/2, z; #6 −x + 1/2,
−y + 1/2, z + 1/2; #7 −x + 1/2, y + 1/2, −z + 1/2; #8 x + 1/2, −y + 1/2, −z.
bond angle between cobalt and aromatic N atoms is 86.58°
and the N–Co–N bond angle between Co and two amine N
atoms is 102.99°. Details of the geometry of the Co(III) coordi-
nation polyhedron in complex 2b can be found in Fig. S14
(ESI†). Complete details of the X-ray crystallography studies are
given in the ESI.† Unlike 2a, neither 2b forms any coordination
179.77(10)
88.37(8)
N(1)–Co(1)–Br(1)
Symmetry transformations used to generate equivalent atoms: #1 x,
−y + 1/2, z
polymer nor does it crystallize into a chiral space group. co-catalyst has shown a strong effect on the catalytic activity.
Complex 2b is a neutral compound and as a result it does not The reaction products were confirmed by the presence of the
have any Li cation, which is essential to interconnect different respective molecular ion peaks in GC/MS analysis and a broad
cobalt units by coordinating to amide oxygen atoms and infrared peak at ν 1800 cm−1. The absence of any other carbon-
solvents to yield a chiral material.
ate peaks in the IR spectra indicates high product selectivity.
Gas chromatography of the reaction mixture shows the pres-
ence of the product as the sole component besides unreacted
2.2 Optimization of reaction parameters
1
epoxide, suggesting high reaction selectivity. H- and 13C-NMR
Due to the solubility of the participating components in the
epoxide substrates, no co-solvent was used. Under the applied
reaction conditions, i.e. 130 °C, 300 psig CO2 pressure in 3 h,
little or no product was observed without the use of either cata-
lyst or co-catalyst, indicating that the reaction needs an active
catalytic system. Based on our previous reports on propylene
carbonate production,27,28 the addition of Lewis base as a
spectra were also used in certain cases to characterize the pro-
ducts (see ESI†).
Table 3 summarizes the activity of cobalt complexes (2a–2d)
on cycloaddition of CO2 with propylene oxide under solvent
free conditions. Initially, we studied the activity of cobalt com-
plexes on cycloaddition of CO2 to propylene oxide in the
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Table 3 Study of various co-catalysts on the reaction TOF for the catalyst 2/co-
catalysts system
Entry
2/co-catalyst
TOFa (h−1
)
1
2
3
2a only
2b only
2c only
38
64
52
4
5
6
7
8
9
10
11
12
13
14
15
16
2d only
DMAP only
2a/DMAP
2a/Py
2a/TEA
2a/TPA
30
14
583
301
396
237
186
263
465
113
662
607
488
2a/TBA
2a/MeIm
2a/DBU
2a/NMP
2b/DMAP
2c/DMAP
2d/DMAP
Fig. 5 Turnover frequency of the catalyst on propylene carbonate production
using the catalyst 2a/DMAP system as a function of DMAP equivalents. Reaction
conditions: catalyst (2.14 × 10−5 mol), propylene oxide (3 mL, 42.9 mmol),
130 °C, CO2 (300 psig) in 3 hours.
Reaction conditions: Catalyst (2.14 × 10−5 mol), propylene oxide (3 mL,
42.9 mmol, 2000 eq.), co-catalyst (4.3 × 10−5 mol), at 130 °C, for
3 hours, using 300 psig CO2 pressure. DMAP – 4-dimethylamino
pyridine; Py – pyridine; TEA – triethylamine; TPA – tripropylamine;
TBA
– tributylamine; MeIm – methylimidazole; DBU – 1,8-
diazabicycloundec-7-ene; NMP – N-methyl-2-pyrrolidone. a Moles of
propylene carbonate produced per mole of catalyst per hour.
absence of a co-catalyst. Significantly low TOFs were observed
for propylene carbonate production, indicating that the system
needs an active co-catalyst for better product yields. For this
study, we studied a range of Lewis bases as co-catalysts along
with complex 2 (Scheme 1). Table 3 shows the effect of various
co-catalysts on the TOF of propylene carbonate production.
From Table 3, the 2a/DMAP (entry 6, Table 3) system gave a
very high TOF of 583 h−1 for propylene carbonate formation in
comparison with other bases; a low TOF of 113 h−1 was
observed with the 2a/N-methyl pyrrolidine system. From
Table 3, diminishing activity was observed with less Lewis
basic and bulky co-catalysts.
With DMAP found to be the choice co-catalyst, a further
study on the influence of changes in its concentration indi-
cated a profound effect on the TOF of the propylene carbonate
formed. Fig. 5 reveals a change in TOF of propylene carbonate
with a change in the concentration of DMAP with respect to
the concentration of the catalyst. The TOF of propylene carbon-
ate reaches its maximum when the ratio of 2a : DMAP reaches
1 : 2. Beyond this catalyst to DMAP ratio, further increase in
the concentration of the co-catalyst resulted in a decrease in
the TOF, which may be due to excess DMAP saturating the
catalyst binding positions, thus leaving no room for the
binding of epoxide to the catalyst for further activity.
Fig. 6 Turnover frequency of the catalyst on propylene carbonate production
for the catalyst 2a–d/DMAP systems as a function of reaction temperature. Reac-
tion conditions: catalyst (2.14 × 10−5 mol), propylene oxide (3 mL, 42.9 mmol,
2000 eq.), co-catalyst (DMAP, 4.3 × 10−5 mol), CO2 pressure (300 psig) for
3 hours.
withdrawing atoms such as chloro (2b) and nitro (2c) on the
aromatic ring showed exceptionally high catalytic activity
(entries 14 and 15, Table 3). Complexes 2a and 2d showed rela-
tively lower activity, with the latter being the least active (entry
16, Table 3). This change in activity could be attributed to the
change in Lewis acidity of the complexes.
The activity of the cobalt complexes with DMAP was studied
at different temperatures for the synthesis of propylene car-
bonate. Fig. 6 shows the TOF of propylene carbonate pro-
duction at various temperatures. The product TOF increases
with an increase in temperature; at 150 °C it was found to have
completely converted to the product, indicating that complexes
2a–2d were found to be stable at high reaction temperatures.
In order to compare with other cobalt complexes, we chose
130 °C as the reaction temperature for the rest of the studies.
Once DMAP was found to be an active co-catalyst for the
propylene carbonate formation, we have also studied the
activity of other cobalt complexes. It is evident from Table 3
that all these cobalt complexes (2a–2d) were found to be active
in producing propylene carbonate with a good TOF. TOFs of
662, 607, and 488 h−1 were observed for cobalt complexes with
chlorine (2b), nitro (2c) and methyl (2d) groups in the ligand
framework, respectively. The cobalt complex with electron
13156 | Dalton Trans., 2013, 42, 13151–13160
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various aromatic and aliphatic epoxides for cyclic carbonate
production.
2.3 Effect of substituents on reactivity
We also observed an increasing TON of propylene carbonate
production over time with all four complexes (2a–2d). The
maximum product TON was observed in 5 h with complex 2b,
which is the most active in the series. Fig. 8 shows the TON
of propylene carbonate production at various time intervals.
Considering the above results and previously reported metal
catalyzed reaction mechanisms, we explained the effect of sub-
stituents on catalytic activity in the following way. The Lewis
acid with the Lewis base initiated epoxide to cyclic carbonate
reaction pathway has widely been discussed in many previous
reports.29,31 Initially, epoxide coordinated with the electrophi-
lic metal center, and ring opening occurred with the aid of a
nucleophile by forming an alkoxide intermediate. Insertion of
CO2 takes place, thus resulting in the formation of product by
intramolecular cyclic elimination. Thus, the Lewis acidity of
the Co(III) complex and nucleophilicity of the co-catalyst can
influence the reaction mechanism. The presence of electron
withdrawing groups, which increase the Lewis acidity of the
metal center, directly affects the coordination of the epoxide;
i.e., epoxide coordination is easier with a strong acidic metal
center.29 As we observed in the above results, complex 2b was
found to be highly active in propylene carbonate production
with TOFs of 662 h−1 whereas complex 2d was found to be
least active in the series with TOFs of 488 h−1. The high cataly-
tic activity of complex 2b was attributed to the presence of two
electron withdrawing chlorine groups, which make the metal
center more electrophilic, and the low catalytic activity in the
case of complex 2d is due to the presence of two electron
donating methyl groups. The catalytic activity order of cobalt
complexes was observed as 2b > 2c > 2a > 2d. The activities of
these cobalt complexes were evaluated over a period of time
and increased activity was observed with increasing reaction
Fig. 7 Turnover frequency of the catalyst on propylene carbonate production
for the catalyst 2a–d/DMAP systems as a function of carbon dioxide pressure.
Reaction conditions: catalyst (2.14
42.9 mmol, 2000 eq.), co-catalyst (DMAP, 4.3 × 10−5 mol), at 130 °C, for 3 hours.
×
10−5 mol), propylene oxide (3 mL,
Table 4 Study of the activity of the catalyst 2b on various epoxides
Entry
Epoxide
Selectivity (%)
Yield (%)
1
2
3
4
5
6
Propylene oxide
Epichlorohydrin
Epoxydodecane
Styrene oxide
Cyclohexene oxide
Cyclooctene oxide
99
99
99
94
99
90
∼99
∼99
∼92
71
96a
83a
Reaction conditions: catalyst (2.14 × 10−5 mol), substrate (21.5 mmol,
1000 eq.), co-catalyst (DMAP, 4.3 × 10−5 mol), at 130 °C, for 5 hours,
using 300 psig CO2 pressure. a Reactions were performed at 150 °C.
As the complex 2a/DMAP was found to be an active catalyst
system for the cycloaddition of carbon dioxide to epoxide, we
determined the optimum CO2 pressure for the reaction. The
effect of carbon dioxide pressure was studied at 130 °C by
employing a catalyst, a co-catalyst, and propylene oxide, while
maintaining the ratio of 1 : 2 : 2000. Fig. 7 shows the influence
of carbon dioxide pressure on the reaction of propylene oxide
and CO2. The maximum product yield was observed at
300 psig CO2 pressure. This CO2 pressure has been found to
be optimum and similar to our previous work.54 Similarly,
other cobalt complexes (complexes 2b, 2c and 2d) were also
studied at various CO2 pressures. The formation of product at
100 psig CO2 pressure indicates that all these cobalt complexes
were found to be active even at low reaction pressure. A similar
trend of reactivity has been observed in our previous findings
for CO2/epoxide reactions catalyzed by cyclic metal amido
complexes.27,28
The activity of the 2b/DMAP system on the synthesis of
cyclic carbonates using various aromatic and aliphatic epox-
ides was studied by employing the above optimized reaction
conditions (300 psig, 130 °C in 3 h). Table 4 summarizes the
yield of the various cyclic carbonates production. It is evident
Fig. 8 Turnover number of the catalyst on propylene carbonate production for
the catalyst 2a–d/DMAP systems as a function of reaction time. Reaction con-
ditions: catalyst (2.14 × 10−5 mol), propylene oxide (3 mL, 42.9 mmol, 2000 eq.),
from this table that 2b/DMAP is an efficient catalytic system on co-catalyst (DMAP, 4.03 × 10−5 mol), CO2 pressure (300 psig) at 130 °C.
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time. The high catalytic activity of complex 2b was observed in to dryness under reduced pressure, and the resulting solid was
all cases while studying the effect of CO2 pressure, temp- washed with a small amount of dichloromethane yielding the
erature, and reaction time. We have plotted TOF values of product 2 as a purple powder.
different catalysts against the Hammett parameters (σm) of
Complex 2a: Yield: 88%. Elemental analysis for
various substituents on the complexes, which is shown in C14H20BrCoN4O2: C, 40.50; H, 4.86; N, 13.49. Found: C, 40.36;
1
Fig. S23.† A linear relationship was observed between TOF and H, 5.02; N, 13.54%. ESI-MS (M − Br+): m/z 335.6 (Fig. S9†). H
σm values of the substituents of the catalysts in the Hammett NMR (CD3OD, 3.33): 8.87 (m, 2H), 7.03 (m, 2H), 2.06 (s, 3H,
plot. The linear relationship supports our hypothesis on the CH3CN), 1.611 (s, 12H). IR 1165 (aromatic amide), 1543
effect of electron withdrawing and donating properties of (aromatic), 1593 (carbonyl) and 3205, 3296 cm−1 (amine).
various substituents of the ligand and how they affect Lewis
Complex 2b: Yield: 90%. Elemental analysis for
acidity of metal complexes and, inevitably, the reactivity of the C14H18BrCl2CoN4O2: C, 34.74; H, 3.75; N, 11.57. Found: C,
complexes.
34.63; H, 3.84; N, 12.05. ESI-MS (M − Br+): m/z 403 (Fig. S9†).
1H NMR (CD3OD, 3.33): 8.979 (s, 2H), 2.06 (s, 3H, CH3CN),
1.585 (s, 12H). IR 678 (C–Cl), 1105 (aromatic amide), 1537
(Ph ring), 1596 (carbonyl) and 3232, 3320 cm−1 (amine).
Complex 2c: Yield: 72%. Elemental analysis for
C14H19BrCoN5O4: C, 36.54; H, 4.16; N, 15.22. Found: C, 36.46;
H, 4.28; N, 15.35. ESI-MS (M − Br+): m/z 380 (Fig. S9†). IR 1155
(aromatic amide), 1308 (nitro), 1537 (aromatic), 1596 (carbo-
nyl) and 3203, 3284 cm−1 (amine).
Complex 2d: Yield: 80%. Elemental analysis for
C16H24BrCoN4O2: C, 43.36; H, 5.46; N, 12.64. Found: C, 43.48;
H, 5.40; N, 12.54. ESI-MS (M − Br+): m/z 362.1 (Fig. S9†). 1H
NMR (CD3OD, 3.33): 8.639 (s, 2H), 2.304 (s, 6H), 2.06 (s, 3H,
CH3CN), 1.611 (s, 12H). IR 1234 (aromatic amide), 1521 (nitro),
1597 (carbonyl) and 3215, 3369 cm−1 (amine).
3. Experimental
3.1 General
All chemicals used in this study were purchased from Aldrich
Chemical Company, USA and Acros Chemical Company, USA
and used without further purification unless otherwise
mentioned. CO2 (99.9%) was obtained from Airgas. All high
pressure reactions were performed in a 100 mL Parr reactor
connected with a 4840 controller. Tetrahydrofuran (THF) was
dried over sodium and benzophenone. Elemental analyses
were performed by Midwest Microlab Inc., Indianapolis.
3.2 Spectroscopy
1H and 13C-NMR spectra were obtained either using
a
3.4 X-ray analysis
600 MHz Brüker instrument or a 400 MHz JEOL ECS 400
instrument equipped with a 5 mm triple resonance inverse
probe. The spectra were collected at 25 °C, and chemical shifts
were reported in ppm relative to TMS as the external standard,
unless otherwise stated. Infrared spectra were obtained
using a Thermo Scientific Nicolet 6700 FT-IR spectrometer.
Electrospray ionization mass spectra (ESI-MS) were obtained
using an Agilent 1000 series MSD VL spectrometer. Gas
Chromatography Mass spectrometric (GC/MS) analysis was
performed using an Agilent Technologies 6890N Network GC
system and equipped with an Agilent Technologies 5975 inert
XL mass selective detector.
Suitable single crystals of the cobalt(III) complexes 2a and 2b
were positioned in a MitiGen cryo-loop on the goniometer
head of the Bruker APEX 2 diffractometer equipped with a
SMART CCD area detector. Data sets were measured at 120 K.
The intensity data were collected in ω scan mode using the Mo
tube (Kα radiation; λ = 0.71073 Å) with a highly oriented graph-
ite monochromator. Intensities were integrated from 4 series
of 364 exposures, each covering 0.5° in ω at 20 seconds of
acquisition time, with the total data set being a sphere.55 The
space group determination was done with the aid of XPREP
software.56,57 Numerical absorption corrections were applied
based on crystal face indexing obtained using actual images
recorded by the video-microscope camera (Fig. S10, S11, ESI†).
Thus, absorption correction was performed using the SADABS
program that was included in the Bruker AXS software
3.3 Synthesis of amidoamine ligands (1) and their Co(III)-
complexes (2)
All the amidoamine ligands (1) considered for this study were package.58 Both structures were solved by direct methods and
synthesized according to the previously reported pro- refined by least squares on weighted F2 values for all reflec-
cedure.27,44 Ligands were characterized by 1H and 13C NMR tions using the SHELXTL program.56 The final analysis of the
studies (see ESI† for details).
variance between observed and calculated structure factors
Synthesis of the cobalt(III) complexes (2) was performed showed no apparent errors. The crystal data for compounds 2a
according to this general method. Ligand 1 (1 eq.) was dis- and 2b are presented in Table 1, while bond lengths and
solved in dry THF under N2 and n-BuLi (2.1 eq., 1.6 M in valence angles are summarized in Table 2. Figures for the
hexane) was added at 0 °C. Dry solid CoCl2 or CoBr2 (1 eq.) was crystal structures of studied complexes were drawn using
added to the solution. The solution was warmed up to room Mercury 4.1.2 and ORTEP 32 software59,60 at a 50% thermal
temperature and stirred overnight to yield a green precipitate. ellipsoids probability level. The PLATON checks of crystallo-
Air was admitted and a purple precipitate was collected. The graphic data and actual CIF files for the reported structures
mixture was filtered through silica gel, and the purple com- can be found in the ESI.† CCDC numbers: 936734 (2a) and
pound was eluted with methanol. The solution was evaporated 936336 (2b).
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3.5 Synthesis of cyclic carbonates
reaction conditions in the absence of a co-solvent, while main-
taining the selectivity towards a single product (cyclic carbon-
ate), is noteworthy. The possibility of the amidoamine ligand
system to stabilize even the high-valent metal centers, avail-
ability of detailed crystal data, and the option of altering the
activity of the catalyst by changing the substituent groups
provide us with various opportunities to further explore the
activity of these catalysts, especially in utilization of CO2 for
synthesizing value added products.
Scheme 1 illustrates the general method of synthesizing cyclic
carbonates using epoxide and CO2. A representative procedure
for the coupling reaction of epoxides and CO2 is as follows: a
100 mL stainless steel Parr high pressure autoclave was
initially charged with a catalyst, DMAP and propylene oxide in
the ratio 1 : 2 : 2000, respectively. The reaction vessel was
initially pressurized with CO2 and the system was allowed to
equilibrate for about 2 min. Then the autoclave was heated to
130 °C and the reaction mixture was allowed to stir for 3 h
under this condition. The vessel was then cooled quickly by
placing in an ice/water bath and the pressure was released Acknowledgements
slowly. The reaction mass was collected in dichloromethane
A.G. would like to thank the National Science Foundation
for complete transfer and the catalyst was separated by passing
through a small plug of silica gel (2 g, 60–100 mesh) and
eluting with dichloromethane. The solvent and the unreacted
substrate were removed at reduced pressure. The yield was cal-
culated either by taking the weight of the isolated product or
by comparing the ratio of the product to substrate peak areas
obtained by GC-MS analysis. Product yields were reported in
TON (moles of the product formed per mole of catalyst) and
TOF (moles of product formed per mole of catalyst in one
EPSCoR program (grant EPS-1003970) for financial support
and the National Science Foundation (grant CHE-1229149)
major research instrument grant to complete this work. N.G. is
grateful to Dr Nadia Marino (Syracuse University, SUNY) for
her help with the resolution of complicated solvent molecules
disorder in the crystal structure of 2a.
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In conclusion, we have synthesized a new class of cobalt com-
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