Synthesis, structure and catalytic activity of bis(phenoxyiminato)iron(III) complexes in coupling reaction of CO2 and epoxides

Article abstract of DOI:10.1016/j.ica.2015.11.023

Here we described the synthesis and characterization of a series of new bis(phenoxyiminato)Fe(III)-chloro complexes using a variety of techniques, including: elemental analysis, IR-, MS(EI), UV-Vis-spectroscopy, and X-ray diffraction. A solid-state structure of bis(N-salicylidene-3-phenylpropylamine)iron(III)chloride reveals a distorted square-pyramid coordination with crystallographic C₂-symmetry wherein chloride occupies the axial position. Isolated complexes were also evaluated as catalysts for a coupling reaction of CO₂ and various epoxides, including propylene, 1-hexene, cyclohexene and styrene oxides to form corresponding cyclic carbonates. Accordingly, the ligand structure and the solvent used influenced the catalytic activity and marked enhancement in the activity was observed using DMF as a solvent. Under optimized reaction conditions (145 °C and 10 bar of CO₂) bis(N-salicylidene-2-phenylethylamine)iron(III)chloride gave cyclopropylene carbonate with considerable high TON value (1029) within 3 h.

Full text of DOI:10.1016/j.ica.2015.11.023

Inorganica Chimica Acta 442 (2016) 81–85
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structure and catalytic activity of bis(phenoxyiminato)iron
(III) complexes in coupling reaction of CO2 and epoxides
⇑
Feda’a Al-Qaisi, Nevil Genjang, Martin Nieger, Timo Repo
Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
Here we described the synthesis and characterization of a series of new bis(phenoxyiminato)Fe(III)-chloro
complexes using a variety of techniques, including: elemental analysis, IR-, MS(EI), UV–Vis-spectroscopy,
and X-ray diffraction. A solid-state structure of bis(N-salicylidene-3-phenylpropylamine)iron(III)chloride
reveals a distorted square-pyramid coordination with crystallographic C2-symmetry wherein chloride
occupies the axial position. Isolated complexes were also evaluated as catalysts for a coupling reaction
of CO2 and various epoxides, including propylene, 1-hexene, cyclohexene and styrene oxides to form
corresponding cyclic carbonates. Accordingly, the ligand structure and the solvent used influenced the
catalytic activity and marked enhancement in the activity was observed using DMF as a solvent. Under
optimized reaction conditions (145 °C and 10 bar of CO2) bis(N-salicylidene-2-phenylethylamine)iron
(III)chloride gave cyclopropylene carbonate with considerable high TON value (1029) within 3 h.
Ó 2015 Published by Elsevier B.V.
Received 23 September 2015
Received in revised form 18 November 2015
Accepted 25 November 2015
Available online 30 November 2015
Keywords:
CO2 activation
CO2 insertion
Cyclic carbonate synthesis
Iron catalysis
Epoxide coupling
1. Introduction
Although iron is environmentally benign and an attractive alter-
native for other transition metals, reports on iron-based catalysts in
Schiff base ligands derived from an aldehyde and a primary
amine, and their complexes with transition metals, have been
widely studied as catalyst precursors in several organic transfor-
mations. Chelating salicyliminato-type ligands are of particular
interest as they can enhance stability of the complexes and there-
fore often present in catalytically active species [1]. For example,
Schiff-base complexes of iron(III) have excellent catalytic perfor-
mances in a wide range of reactions [2], such as the storage and
transport of dioxygen [3], oxidation of alkanes [4], phosphate ester
hydrolysis [5], and DNA synthesis [6].
The use of carbon dioxide as a C1 source for the synthesis of
organic chemicals is a pathway to a sustainable chemical industry.
Currently the major industrial conversions of CO2 are limited to the
synthesis of urea, salicylic acid, inorganic carbonates, ethylene/
propylene carbonates, and as an additive to CO in methanol syn-
thesis [7]. Of these, cyclic carbonates are important as intermedi-
ates in polycarbonate synthesis [8], electrolytes in lithium ion
batteries, and as polar solvents. There are already several systems
based on alkali metal salts [9], ammonium salts [10], phosphines
[11], ionic liquids systems [12], and metal complexes [13], includ-
ing salen complexes of aluminum [14], cobalt [15], chromium [16],
and zinc [17] to catalyze the coupling of CO2 and epoxides to cyclic
carbonates.
cyclic carbonate synthesis are rare. Reported examples of divalent
iron complexes are [trans-N,N-bis(quinolin-2-ylmethyl)cyclohex-
ane-1,2-diamine]Fe(II)chloride [18] and [N,N-bis-2-pyridinyl-
methylene-cyclohexane-1,2-diamine]Fe(II)chloride [19] which,
along with Bu4NBr, catalyze propylene carbonate synthesis. Inter-
estingly, comparison of reactivity between iron(II) and iron(III)
complexes carrying the same tetradentate Schiff-base ligand shows
higher activity for iron(III) [20]. The second reported example of
iron(III) complexes are bimetallic iron catalysts utilized for polycar-
bonate synthesis. Remarkably, these catalysts produce cyclic car-
bonates even at RT and 1 atm [21] or 2 atm [22] CO2 pressure,
although with cost of TOF values. Moreover, efficient iron(III) amine
triphenolate complexes are reported for synthesis of either the cyc-
lic or the polymeric carbonates from cyclohexene oxide [23] with
stereochemical control in the formation of cyclic carbonates
derived from internal epoxides [24]. In this respect we report here
on the synthesis and characterization of bis(phenoxyiminato)Fe
(III)-chloro complexes and their catalytic activity in a coupling reac-
tion of CO2 and epoxides using Bu4PBr as a cocatalyst.
2. Materials and methods
2.1. Materials
⇑
Epoxides, anhydrous FeCl3, tetrabutylphosphonium bromide
(TBPB), and other chemicals were purchased from Sigma Aldrich
Corresponding author.
E-mail address: timo.repo@helsinki.fi (T. Repo).
http://dx.doi.org/10.1016/j.ica.2015.11.023
0020-1693/Ó 2015 Published by Elsevier B.V.

82
F. Al-Qaisi et al. / Inorganica Chimica Acta 442 (2016) 81–85
and used without further purification. Carbon dioxide was pur-
chased from AGA with 99.9% purity.
2b: Yield: 95%. Color: oily yellow. UV–Vis (EtOH): kmax = 325,
389 nm. IR:
= 1612 cmÀ1 (m, C@N). 1H NMR (CDCl3) (ppm): CH
m
(Ph): 6.8–7.52(m); CH3(CH3C@N): 2.48(s); CH2(CH2Ph): 3.1(t);
CH2(NCH2): 3.88(t).
2.2. Analysis
2.5. Synthesis of complexes
Elemental analyses were performed using an EA 1110
CHNS-OCE instrument. IR and UV–Vis spectra were collected with
a Perkin Elmer Spectrometer and a Hewlett Packard 8453 spec-
trophotometer, respectively. Thermal gravimetric analysis (TGA)
was carried out using Mettler TGA850. EI-mass spectrometer were
run with JEOL JMS-SX 102 mass spectrometer (ion voltage 70 eV).
1H NMR spectra were obtained using a 300 MHz Bruker instru-
ment. The spectra were collected at 25 °C, and chemical shifts were
reported in ppm relative to TMS as an external standard.
Bis(N-salicylideneaniline)iron(III)chloride (3a), bis(N-salicyli-
denebenzylamine)iron(III) chloride (3b), bis(N-salicylidene-3-phenyl-
propylamine)iron(III)chloride (3c), bis(N-salicylidene-p-chloroaniline)
iron(III)chloride (3d), bis(N-salicylidenecyclohexylamine)iron(III)
chloride (3e).
Ligand (1a–1e) (1.6 mmol) was added slowly to a solution of
FeCl3 (0.77 mmol) in ethanol (15.0 ml). Upon addition, a dark solu-
tion was formed. The solution was heated at 60 °C for 30 min, and
then stirred at room temperature for 2 h. The solvent was removed
in vacuum and the precipitate was thrice washed with petroleum
ether (5 ml), then again dried under vacuum.
2.3. Crystal structure determination of 3c
The single-crystal X-ray diffraction study was carried out on a
Bruker-Nonius KappaCCD diffractometer at 123(2) K (using Mo
Ka radiation (k = 0.71073 Å). Direct Methods (SHELXS-97) [25] were
3a: Yield: (55%). Color: deep red. UV–Vis (EtOH): kmax (lg
= 486 nm (3.45). IR:
= 1634 cmÀ1 (m, C@N). Anal. Calc. for
26H20ClFeN2O2: C, 64.55; H, 4.17; N, 5.79. Found: C, 64.63; H,
e)
m
C
used for structure solution and refinement was carried out using
SHELXL-2014 (full-matrix least-squares on F2) [25]. Hydrogen atoms
were localized by difference electron-density determination and
refined using a riding model. A semi-empirical absorption correc-
tion was applied.
4.12; N, 5.78. MS (EI, +)m/z: 91(100, FeCl); 197(95, Fe-L); 448(100,
L-Fe-L); 483(10, M+).
3b: Yield: (73%). Color: deep red. UV–Vis (EtOH): kmax (lg
e)
= 456 nm(3.45). IR:
m
= 1601 cmÀ1 (m, C@N). Anal. Calc. for
C
28H24ClFeN2O2Á3H2O: C, 59.43; H, 5.35; N, 4.95. Found: C, 58.97;
H, 5.07; N, 5.31. MS (EI, +)m/z: 18(100, H2O); 91(100, FeCl); 211
2.4. Synthesis of ligand precursors
(100, L); 266(10, Fe-L); 476(20, L-Fe-L); 510(15, [MÀ1]+).
3c: Yield: (40%). Color: deep red. UV–Vis (EtOH): kmax (lge)
Synthesis of N-salicylideneaniline (1a), N-salicylideneben-
= 420 nm (3.45). IR:
m
= 1618 cmÀ1 (m, C@N). Anal. calc. for
zylamine
(1b),
N-salicylidene-3-phenylpropylamine
(1c),
C32H32ClFeN2O20.5H2O: C, 67.11; H, 5.60; N, 4.87. Found: C, 67.29;
N-salicylidene-p-chlorophenylamine (1d), and N-salicylidenecy-
clohexylamine (1e).
H, 5.58; N, 4.90. MS (EI, +)m/z: 18(100, H2O); 91(100, FeCl); 239(40,
L); 329(100, FeCl-L); 532(100, L-Fe-L); 566(50, [MÀ1]+); 567(50, M).
A solution of salicylaldehyde (0.012 mol) in methanol (5 ml)
was added to a solution of the desired amine (0.012 mol) in metha-
nol (5 ml) with continuous stirring. The mixture was further stirred
at RT for 1 h and a precipitation formed. After stirring, the product
was filtered, washed with petroleum ether and vacuum dried.
1a: Yield: 98%. Color: yellow. UV–Vis (EtOH): kmax = 336,
3d: Yield: (88 %). Color: deep red. UV–Vis (EtOH): kmax (lge)
= 340 nm (3.45). IR:
m
= 1637 cmÀ1 (m, C@N). Anal. calc. for
C26H18Cl3FeN2O2: C, 56.51; H, 3.28; N, 5.07. Found: C, 56.55; H,
3.27; N, 5.08. MS (EI, +)m/z: 320(10, Fe-L); 516(40, C26H18ClFeN2O2).
3e: Yield: (80 %). Color: reddish brown. UV–Vis (EtOH): kmax
(lge) = 456 nm (3.45). IR: m
= 1603 cmÀ1 (m, C@N). Anal. Calc. for
427 nm. IR:
m
= 1614 cmÀ1 (m, C@N) 1H NMR (CDCl3) (ppm): CH
C
26H32ClFeN2O2Á2H2O: C, 58.71; H, 6.82; N, 5.21. Found: C, 58.3;
(HC@N): 8.7(s); CH(Ph): 7–7.52(m).
H, 6.35; N, 4.81. MS (EI, +)m/z: 18(100, H2O); 91(50, FeCl); 203
1b: Yield: 65%. Color: oily yellow. UV–Vis (EtOH): kmax = 316,
(100, L); 495(5, M+).
404 nm. IR:
m
= 1628 cmÀ1 (m, C@N). 1H NMR (CDCl3) (ppm): CH
Bis(N-methylsalicylidenebenzylamine)iron(III)chloride (4a) and
bis(N-methylsalicylidene-2-phenylethylamine)iron(III)chloride
(4b).
Ligand (2a–2b) (1.6 mmol) was added slowly to a solution of
FeCl3 (0.77 mmol) in ethanol (15.0 ml). Upon addition, a dark solu-
tion was formed. The solution was heated at 60 °C for 30 min, and
then stirred at room temperature for 2 h. The solvent was removed
under vacuum, precipitate was thrice washed petroleum ether
(5 ml), and then vacuum dried.
(HC@N): 8.5(s); CH(Ph): 7–7.52(m); CH2: 4.8(s).
1c: Yield: 98%. Color: yellow. UV–Vis (EtOH): kmax = 315,
399 nm. IR:
m
= 1630 cmÀ1 (m, C@N). 1H NMR (CDCl3) (ppm):
CH(HC@N): 8.7(s); CH(Ph): 7–7.52(m). CH2(PhCH2A): 2.62(t);
CH2(PhCH2CH2A): 1.98(m); CH2(NCH2A): 3.71(t).
1d: Yield: 93%. Color: yellow. UV–Vis (EtOH): kmax = 317,
340 nm. IR:
m
= 1608 cmÀ1 (m, C@N). 1H NMR (CDCl3) (ppm): CH
(HC@N): 8.7(s); CH(Ph): 7–7.52(m).
1e: Yield: 98%. Color: yellow. UV–Vis (EtOH): kmax = 314,
4a: Yield: (62 %). Color: reddish brown. UV–Vis (EtOH): kmax
398 nm. IR:
m
= 1627 cmÀ1 (m, C@N). 1H NMR (CDCl3) (ppm): CH
(lge) = 489 nm (3.45). IR: m
= 1599 cmÀ1 (m, C@N). Anal. Calc. for
(HC@N): 8.48(s); CH(Ph): 6.8–7.5(m). CH2(cyclohexyl): 1.2–2(m);
CH(cyclohexyl): 3.25(m).
C
30H28ClFeN2O2Á2.5H2O: C, 61.6; H, 5.69; N, 4.79. Found: C,
61.65; H, 5.62; N, 4.81. MS (EI, +)m/z: 18(100, H2O); 91(100, FeCl);
Synthesis of N-methylsalicylidenebenzylamine (2a), and N-
methylsalicylidene-2-phenylethylamine (2b)
255(20, L).
4b: Yield: (71%). Color: bright red. UV–Vis (EtOH): kmax (lg
= 420 nm (3.45). IR:
e)
A solution of methylsalicylaldehyde (0.012 mol) in methanol
(5 ml) was added to a solution of the desired amine (0.012 mol)
in methanol (5 ml) with continuous stirring. The mixture was stir-
red at room temperature for 1 h, during which, a precipitate was
formed. After an additional 2 h stirring, the product was filtered,
washed with petroleum ether and vacuum dried.
m
= 1599 cmÀ1 (m, C@N). Anal. Calc. for
C
32H32ClFeN2O2Á2H2O: C, 63.64; H, 6.01; N, 4.64. Found: C, 63.68;
H, 6.06; N, 4.67. MS (EI, +)m/z: 18(100, H2O); 91(100, FeCl); 239
(90, L); 329(30, FeCl-L); 567(10, M+).
2.6. General procedure of coupling reaction
2a: Yield: 99%. Color: pale yellow. UV–Vis (EtOH): kmax = 323,
389 nm. IR:
m
= 1607 cmÀ1 (m, C@N). 1H NMR (CDCl3) (ppm): CH
The coupling of propylene oxide (PO) and CO2 was investigated
as a model reaction. All coupling reactions were performed in a
(Ph): 6.8–7.52(m); CH3(CH3C@N): 2.48(s); CH2(NCH2Ph): 4.97(s).

F. Al-Qaisi et al. / Inorganica Chimica Acta 442 (2016) 81–85
83
100 ml stainless steel autoclave equipped with a magnetic stirring
bar. The autoclave reactor was charged with iron catalyst, cocata-
lyst and propylene oxide. The reactor was pressurized with CO2
to 10 bar and then heated to a preselected temperature. After a
required time, the reaction was stopped by cooling the reactor in
ice water. After slow venting a sample was taken from the reaction
solution and dissolved in a proper amount of CDCl3 for 1H NMR
measurements for yield determination.
The X-ray structure consists of two phenoxyaldimine ligands
and a chloride coordinated to the iron(III) center giving a distorted
square-pyramid geometry with crystallographic C2-symmetry. The
Fe atom lies 0.54 Å above the ligand plane. The FeAN bond lengths
of 2.132(4) Å are comparable with the corresponding values in
similar types of complexes, such as [Fe2O(bipy)4Cl2]2+ [average
2.16 Å] [26] and [Fe2O(phen)4(phCOO)2]2+ [2.14(5) and 2.17(5) Å]
[27]. Two oxygen atoms of hydroxyl-groups coordinate to Fe(III)
ion with the FeAO distances of 1.873(3) Å.
3. Results and discussion
3.1. Ligands and complexes synthesis
3.2. Catalytic activity
Ligand precursors, phenoxyaldimines (1a–1e) and phenoxyke-
timines (2a–2b), were prepared in high to moderate yields by
the condensation reaction of salicylaldehyde or o-hydroxyace-
tophenone with stoichiometric amounts of the desired amine in
the presence of a catalytic amount of formic acid (Scheme 1). The
condensation reactions were followed by IR spectroscopy, wherein
disappearance of the carbonyl band and the appearance of a new
band between 1618 and 1642 cmÀ1 is an indication of imine for-
mation. The isolated ligand precursors were characterized by 1H
NMR, IR-, and UV–Vis spectroscopy (see Supporting information).
The iron complexes 3a–e and 4a–b were synthesized in high
yields by a treatment of anhydrous FeCl3 with two equivalents of
the corresponding bidentate ligand precursors in methanol. Iso-
lated complexes are air-stable and were studied with IR, UV–vis,
elemental analysis, and EI-MS analysis. In IR spectra, a slight shift
The Fe(III) complexes were studied also as catalysts in synthesis
of cyclic carbonates from carbon dioxide and epoxides. The cou-
pling reactions were carried out using 0.01 mmol of a Fe(III)com-
plex mixed with 0.04 mmol of a cocatalyst, including Bu4NBr,
Bu4PBr, and 4-dimethylaminopyridine (DMAP). In addition, several
simple amines such as, imidazole, morpholine, and piperidine were
also investigated as cocatalysts, but failed to generate any notable
activity under applied conditions. In an earlier study on synthesis
of cyclic carbonates from the corresponding epoxides, it was
noticed that Bu4NBr is a more efficient cocatalyst than Bu4NI or
Bu4NCl [28] (see below).
Complexes 3a, 3d, 4a, and 4b showed the highest catalytic
activity among the iron complexes tested when Bu4PBr was used
as a cocatalyst (Table S1 in Supplementary). As shown in Table 2
the four most active complexes with the most active cocatalyst
Bu4PBr give almost identical conversion values. The complex 4b
with Bu4PBr was chosen as a catalyst for further studies using
the coupling of propylene oxide and CO2 as the model reaction.
The coupling reactions are clearly influenced by the nature of
the solvent employed (Table S2 in Supplementary) and the highest
activities were obtained in DMF, 1,4-dioxane, and toluene (Table 3).
Of the series of solvents, DMF is the most polar and most likely able
to support polar transition states during epoxide ring-opening and
ring-closing steps after CO2 insertion to the FeAO bond. In fact,
DMF alone can catalyze the coupling reaction of epoxides with
of the imine bonds (m(C@N)) was observed indicating complexa-
tion. For example, imine bands in complexes 3a and 3d are
observed at 1634 and 1637 cmÀ1 respectively, whereas in the cor-
responding ligand precursors 1a and 1d, the band appears at 1614
and 1608 cmÀ1 (Fig. 3S Supporting information). In addition, the
complexes were characterized by thermal gravimetric analysis
(TGA) (Fig. 4S and Table 3S).
Crystals suitable for X-ray analysis were obtained for 3c from
saturated methanol solution (Fig. 1). Selected bond lengths and
bond angles in 3c are presented in Table 1.
R
R
O
N
N
O
O
N
FeCl3
RNH2
Fe
Cl
(3a-e)
Cl
OH
OH
MeOH
R
(1a-e)
R:
3c
3a
3b
3d
3e
R
R
O
N
N
O
O
N
FeCl3
RNH2
Fe
Cl
OH
OH
(4a and b)
MeOH
R
(2a and b)
R:
4a
4b
Scheme 1. Synthesis of phenoxyaldimines (1a–e) and phenoxyketimines (2a and b) via
a
Schiff base condensation reaction starting from salicylaldehyde or o-
hydroxyacetophenone and amine. Fe(III) complexes 3a–e and 4a–b were prepared by treating the corresponding ligand precursors with Fe(III)Cl3.

84
F. Al-Qaisi et al. / Inorganica Chimica Acta 442 (2016) 81–85
Table 3
Conversions (%, measured by 1H NMR) of coupling reactions between carbon dioxide
and different epoxides using catalyst 4b in the presence of Bu4PBr.
Solvent
Time
(h)
Epoxide
Propylene
oxide
1-Hexene
oxide
Styrene
oxide
Cyclohexene
oxide
DMF
7
4
2
78
–
69
64
62
40
70
66
46
22
16
0
1,4-Dioxane
Toluene
7
4
2
89
–
52
66
60
31
83
53
28
13
7
0
7
4
2
89
–
49
62
46
22
67
62
52
17
6
7
Reaction conditions: 145 °C, 10 bar CO2; catalyst = 0.02 mmol, Bu4PBr = 0.08 mmol,
0.5 ml solvent, and 16 mmol epoxide.
Table 4
Conversions (%, measured by 1H NMR) of coupling reactions between carbon dioxide
and propylene oxide using 4b in the presence of Bu4PBr in DMF (0.5 ml).
4b (mmol)
Bu4PBr (mmol)
0.04 (TON)
Fig. 1. Crystal structure for 3c where two phenoxyaldimine ligands occupy the
plane, the imine nitrogens are trans to each other, and ClÀ takes the apical position
(displacement parameters are drawn at 50% probability level).
0.08 (TON)
0.1 (TON)
–
0.01
0.02
26 (108)
28 (465)
56 (465)
30 (51)
49 (813)
73 (606)
31 (51)
62 (1029)
89 (739)
Reaction conditions: 3 h, 145 °C, 10 bar CO2, and 16.6 mmol propylene oxide.
Bold font in the table highlights the best conversions.
Table 1
Selected bond lengths (Å) and angles (°) in 3c.
Bond lengths (Å)
Fe(1)AO(1)#1
Fe(1)AO(1)
Angles (°)
CO2 under harsh conditions (110 °C and 50 bar of CO2 for 20 h)
[29].
To optimize the ratio between the catalyst and cocatalyst,
experiments with different amounts of complex 4b and Bu4PBr
were performed using DMF as a solvent (Table 4). The highest
1.873(3)
1.873(3)
2.132(4)
O(1)#1AFe(1)AO(1)
O(1)#1AFe(1)AN(8)
128.2(2)
86.75(13)
Fe(1)AN(8)
O(1)AFe(1)AN(8)
O(1)#1AFe(1)AN(8)#1
87.18(13)
87.18(13)
conversion (89%) was obtained for
a solution containing
Fe(1)AN(8)#1
Fe(1)ACl(1)
2.132(4)
2.235(2)
O(1)AFe(1)AN(8)#1
N(8)AFe(1)AN(8)#1
86.75(13)
166.06(19)
0.02 mmol of 4b and 0.1 mmol of Bu4PBr, while the highest
TON value (1029) was obtained 0.01:0.1 complex to nucleophile
ratio.
O(1)#1AFe(1)ACl(1)
O(1)AFe(1)ACl(1)
N(8)AFe(1)ACl(1)
N(8)#1AFe(1)ACl(1)
115.90(11)
115.90(11)
96.97(9)
96.97(9)
3.3. Reaction mechanism
Symmetry codes: (#1) Àx + 1, y, Àz + 1/2.
The mechanism of the cycloaddition of CO2 to epoxides, in
the presence of the transition metal catalysts and quaternary
ammonium salts as cocatalysts, has been addressed in the liter-
ature [30]. A plausible mechanism involving two nucleophiles
for cyclic-carbonate synthesis was proposed (Scheme 2). At first,
epoxide replaces a coordinated solvent (1) (here DMF) and coor-
dinates to the metal in the axial position trans to the chloride,
thus generating a six-coordinated intermediate. Subsequently,
nucleophilic attack by Br anion to the coordinated and activated
epoxide (2), initiates the ring opening. Next, the formed alkox-
ide species (3) acts in turn as a nucleophile that attacks CO2
to form a metal carbonate species (4). Then an intramolecular
ring-closure forms cyclic carbonate.
Therefore, the anion of the cocatalyst has a marked role in the
coupling reaction: strong nucleophilic character is preferred for
the ring opening of epoxide (step 2) while during the ring-closing
(step 4) the anion should act as a proper leaving group. It seems
that bromide anion presents the best combination of nucleophilic-
ity and leaving group character. Both tetrabutylphosphonium bro-
mide and tetrabutylammonium bromide are effective BrÀ sources
the former being a slightly more effective with the studied Fe(III)
complexes.
Table 2
Conversions (%, measured by 1H NMR) of coupling reactions between carbon dioxide
and epoxides using 3a, 3d, 4a, and 4b in the presence of DMAP, Bu4PBr, and Bu4NBr in
0.5 ml DCM.
Catalyst
Epoxide
Cocatalyst
DMAP
Bu4NBr
Bu4PBr
–
PO
HO
2
–
11
7
14
5
3a
3d
4a
4b
PO
HO
47
41
67
16
75
52
PO
HO
72
46
74
40
81
57
PO
HO
79
43
72
29
82
47
PO
HO
79
44
75
33
83
59
Reaction conditions: 24 h, 120 °C, 12.5 bar CO2; catalyst = 0.01 mmol, Bu4NBr =
0.04 mmol, and 16.6 mmol epoxide. PO = propylene oxide, HO = 1-hexene oxide.

F. Al-Qaisi et al. / Inorganica Chimica Acta 442 (2016) 81–85
85
(d) I. Manirul, P. Sumantra, R. Singha, M. Paramita, J. Inorg. Organomet. Polym
Mater. 23 (3) (2013) 560;
O
(e) P. Comba, Y. Lee, W. Nam, A. Waleska, Chem. Commun. 50 (2014) 412.
[5] S. Albedyhl, M. Pouchot, C. Belle, B. Krebs, J. Pierre, E. Aman, S. Torelli, Eur. J.
Inorg. Chem. (2001) 1457.
[6] (a) P. Nordlund, H. Eklund, J. Mol. Biol. 232 (1993) 123;
(b) Y. Jiaojiao, S. Wanling, Zhongguo Yiyao Daobao 41 (2015) 33.
[7] T. Sakakura, J.C. Choi, H. Yasuda, Chem. Rev. 107 (2007) 2365.
[8] (a) J.H. Clements, Ind. Eng. Chem. Res. 42 (2003) 663;
(b) A.G. Shaikh, S. Sivaram, Chem. Rev. 96 (1996) 95.
O
DMF
Fe
O
R
R
O
O
Cl
(1)
Br
R
R
[9] (a) K. Roshan, A. Kathalikkattil, J. Tharun, D. Kim, Y. Won, D. Park, Dalton Trans.
43 (2014) 2023;
O
Br
O
(b) B. Barkakaty, K. Morino, A. Sudo, T. Endo, Green Chem. 12 (2010) 42;
(c) P. Ramidi, P. Munshi, Y. Gartia, S. Pulla, A. Biris, A. Paul, A. Ghosh, Chem.
Phys. Lett. 512 (2011) 273;
Bu4P
Bu4P
O
Fe
Cl
(d) N. Kihare, N. Hara, T. Endo, J. Org. Chem. 58 (1993) 6198.
[10] (a) H. Ju, M. Manju, K. Kim, S. Park, D. Park, J. Ind. Eng. Chem. 14 (2008) 157;
(b) J. Wang, K. Dong, W. Cheng, J. Sun, S. Zhang, Catal. Sci. Technol. 2 (2012)
1480.
Fe
Cl
(4)
(2)
Br
[11] (a) H. Hu, J. Martin, L. Sun, RSC Adv. 2 (2012) 3810;
(b) D. Wei-Li, J. Bi, L. Sheng-Lian, L. Xu-Biao, T. Xin-Man, A. Chak-Tong, Appl.
Catal. A 470 (2014) 183.
[12] (a) P. Agrigento, S.M. Al-Amsyar, B. Sorée, M. Taherimehr, M. Gruttadauria, C.
Aprile, P. Pescarmona, Catal. Sci. Technol. 4 (2014) 1598;
(b) J. Wang, W. Cheng, J. Sun, T. Shi, X. Zhang, S. Zhang, RSC Adv. 4 (2014) 2360;
(c) Y. Zhao, C. Yao, G. Chen, Q. Yuan, Green Chem. 15 (2013) 446;
(d) L. Han, H. Choi, D. Kim, S. Park, B. Liu, D. Park, J. Mol. Catal. A: Chem. 338
(2011) 58;
R
Bu4P
O
CO2
Fe
Cl
(3)
(e) G. Fiorani, W. Guo, A. Kleij, Green Chem. 17 (2015) 1375.
[13] (a) C. Martín, G. Fiorani, A. Kleij, ACS Catal. 5 (2015) 1353;
(b) K. Nakano, S. Hashimoto, M. Nakamura, T. Kamada, K. Nozaki, Angew.
Chem. Int. Ed. 50 (2011) 4868.
Scheme 2. Proposed mechanism for coupling reaction.
[14] (a) J. Rintjema, W. Guo, E. Martin, E. Escudero, A. Kleij, Chem. Eur. J. 21 (2015)
10754;
4. Conclusions
(b) M. North, R. Pasquale, Angew. Chem. Int. Ed. 48 (2009) 2946.
[15] (a) R.L. Paddock, S.T. Nguyen, Chem. Commun. (2004) 1622;
(b) T. Chang, L. Jin, H. Jing, ChemCatChem 1 (2009) 379;
(c) C. Li, P. Anastas (Guest Eds.), Chem. Soc. Rev. 41 (2012) 1462.;
(d) A. Sibaouih, P. Ryan, M. Leskela, B. Rieger, T. Repo, Appl. Catal. A 365 (2009)
194.
[16] (a) R.L. Paddock, S.T. Nguyen, J. Am. Chem. Soc. 123 (2001) 11498;
(b) D.J. Darensbourg, R.M. Mackiewicz, J. Am. Chem. Soc. 127 (2005) 14026;
S. Iksi, A. Aghmiz, R. Rivas, M. Dolores Gonzalez, L. Cuesta-Aluja, J. Castilla, A.
Orejon, F. Guemmout, A. Masdeu-Bulto, J. Mol. Catal. A: Chem. 143 (2014) 383;
(d) L. Cuesta-Aluja, M. Djoufak, A. Aghmiz, R. Rivas, L. Christ, A. Masdeu-Bultó,
J. Mol. Catal. A: Chem. 161 (2014) 381.
A series of new Fe(III)-based transition-metal complexes bear-
ing Schiff-base ligands has been synthesized and characterized in
this study. The complexes are easy to synthesize, air-stable, and
with Bu4PBr were found to be active catalysts for the cycloaddition
of CO2 to epoxides affording cyclic carbonates under moderate
temperature and pressure. A significant enhancement of reactivity
was observed upon addition of a polar solvent to the reaction med-
ium. The highest reactivity was achieved by using DMF as a solvent
(TOF = 343 hÀ1).
[17] (a) Y. Shen, W. Duan, M. Shi, J. Org. Chem. 68 (2003) 1559;
(b) G.A. Luinstra, G.R. Haas, F. Molnar, V. Bernhart, R. Eberhardt, B. Rieger,
Chem. Eur. J. 11 (2005) 6298;
Acknowledgment
A. Decortes, M. Martinez Belmonte, J. Benet- Buchholz, A.W. Kleij, Chem.
Commun. 46 (2010) 4580.
[18] J. Dengler, M. Lehenmeier, S. Klaus, C. Anderson, E. Herdtweck, B. Rieger, Eur. J.
Inorg. Chem. (2011) 336.
We are grateful for the financial support of the Magnus Ehrn-
rooth Foundation.
[19] X. Sheng, L. Qiao, Y. Qin, X. Wang, F. Wang, Polyhedron 74 (2014) 129.
[20] M.A. Fuchs, T.A. Zevaco, E. Ember, O. Walter, I. Held, E. Dinjus, M. Doring,
Dalton Trans. 42 (2013) 5322.
Appendix A. Supplementary material
[21] A. Buchard, M. Kember, K. Sandeman, C. Williams, Chem. Commun. 47 (2011)
212.
[22] C. Whiteoak, E. Martin, M. Belmonte, J. Benet-Buchholz, A. Kleij, Adv. Synth.
Catal. 354 (2012) 469.
[23] M. Taherimehr, S. Al-Amsyar, C. Whiteoak, A. Kleij, P. Pescarmona, Green
Chem. 15 (2013) 3083.
[24] C. Whiteoak, E. Martin, E. Adan, A. Kleij, Adv. Synth. Catal. 355 (2013) 2233.
[25] G.M. Sheldrick, Acta Crystallogr. A64 (2008) 112–122.
[26] D. Xiang, X. Tan, S. Zhang, Y. Han, K. Yu, W. Tang, Polyhedron 17 (1998) 2095.
[27] J. Li, Y. Zou, J. Wu, M. Xu, Z. You, Polyhedron 14 (1995) 3519.
[28] M. Fuchs, T. Zevaco, E. Ember, O. Walter, I. Held, E. Dinjus, M. Doring, Dalton
Trans. 42 (2013) 5322.
CCDC 1417046 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif. Supplementary data associated with this arti-
cle can be found, in the online version, at http://dx.doi.org/10.
1016/j.ica.2015.11.023.
References
[29] L. Jiang, R. Hua, Synth. Commun. 36 (2006) 3141.
[30] (a) T. Sakakura, K. Kohno, Chem. Commun. (2009) 1312;
(b) M. North, R. Pasquale, C. Young, Green Chem. 12 (2010) 1514;
(c) Y. Yang, Y. Hayashi, Y. Fujii, T. Nagano, Y. Kita, T. Ohshima, J. Okuda, K.
Mashima, Catal. Sci. Technol. 2 (2012) 509;
[1] (a) R. Clarke, T. Storr, Dalton Trans. 43 (2014) 9380;
(b) B. Karimi, A. Zamani, J. Clark, Organometallics 24 (2005) 4695;
(c) G. Wu, S. Wei, W. Ren, X. Lu, T. Xu, D. Darensbourg, J. Am. Chem. Soc. 133
(2011) 15191.
[2] K. Gupta, A. SutarK, Coord. Chem. Rev. 252 (2008) 1420.
[3] C. Trapti, G. Bhadra, PCT Int. Appl. (2014). WO 2014100059 A2 20140626.
[4] (a) A. Rosenzweig, C. Frederick, S. Lippard, P. Norlund, Nature 366 (1993) 537;
(b) M. Costas, K. Chen, L. Que, Coord. Chem. Rev. 517 (2000) 200;
(c) Y. Raffard-Pons, N. Moll, F. Banse, K. Miki, M. Nierlich, J.-J. Girerd, Eur. J.
Inorg. Chem. (2002) 1941;
(d) J. Meléndez, M. North, P. Villuendas, C. Young, Dalton Trans. 40 (2011)
3885;
(e) J. Song, Z. Zhang, S. Hu, T. Wu, T. Jiang, B. Han, Green Chem. 11 (2009) 1031;
(f) J. Comerford, I. Ingram, M. North, X. Wu, Green Chem. 17 (2015) 1966.