Iron amino-bis(phenolate) complexes for the formation of organic carbonates from CO2 and oxiranes

Article abstract of DOI:10.1039/c6cy00477f

A series of iron(iii) compounds supported by tetradentate amino-bis(phenolate) ligands were synthesized and characterized using electronic absorption spectroscopy, magnetic moment measurement and MALDI-TOF mass spectrometry. The solid-state structures of

Full text of DOI:10.1039/c6cy00477f

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Iron amino-bis(phenolate) complexes for the formation of organic
carbonates from CO and oxiranes
2
a
b
a
a
Dalal Alhashmialameer, Julie Collins, Karen Hattenhauer, and Francesca M. Kerton*
a
Department of Chemistry, Memorial University of Newfoundland, St. John’s,
Newfoundland, Canada A1B 3X7. Eꢀmail: fkerton@mun.ca; Tel:
b
CꢀCART Xꢀray Diffraction Laboratory, Memorial University of Newfoundland, St.
John’s, Newfoundland
†
Electronic supplementary information (ESI) available. For ESI data in and other
electronic format see DOI:
Abstract
A series of iron(III) compounds supported by tetradentate aminoꢀbis(phenolate) ligands
were synthesized and characterized using electronic absorption spectroscopy, magnetic
moment measurement and MALDIꢀTOF mass spectrometry. The solidꢀstate structures of
1
and 2 were determined by Xꢀray diffraction and reveal iron(III) square pyramidal
compounds. The complexes were studied as catalysts for the reaction of carbon dioxide
and epoxides in the presence of a coꢀcatalyst, under solvent free conditions to yield cyclic
carbonates. Catalytic testing with TBAB as a coꢀcatalyst shows that 4 bearing electron
withdrawing groups in the ortho and paraꢀ positions of the phenolate ring exhibits the
highest catalytic activity. Kinetic studies using 1 revealed that the cycloaddition reaction
is affected by temperature as expected and the activation energy for propylene carbonate
ꢀ1
formation is 98.4 kJ mol .
Introduction
Utilization of carbon dioxide (CO ) in the preparation of chemicals with
2
commercial value has become important as it is a green, cheap, nonꢀtoxic and abundant
1
ꢀ8
feedstock. Highly reactive substrates such as epoxides allows for the thermodynamic
1
, 2, 9
stability of CO to be overcome.
The interest in cyclic carbonates as CO ꢀderived
2
2
molecules is driven by their wide applications as aprotic solvents (including their use to
prepare electrolyte solutions in lithium ion batteries) and as starting materials for
1
0
polycarbonates. Industrially, the production of cyclic carbonates requires demanding

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reaction conditions such as elevated CO pressures and high temperatures. Therefore,
2
numerous efforts have been devoted to the design of efficient catalysts for this
1
0
11ꢀ14
transformation under mild reaction conditions, including catalysts of aluminum,
1
5ꢀ18
17ꢀ22
23, 24
25
26
chromium,
cobalt,
zinc,
manganese and magnesium. In order to address
the potential toxicity associated with some of these metals, iron complexes have been
used as a promising class of catalyst. Moreover, compared with some catalysts, because
of iron’s high natural abundance, they are often cheap and some recent examples have
shown exceptional catalytic activity in the conversion of CO and epoxides to
2
2
7ꢀ29
carbonates.
To date, several ironꢀbased catalysts have shown excellent activity in the
2
6ꢀ33
production of both cyclic carbonates and polycarbonates.
A series of dinuclear iron
2
7
catalysts based on macrocyclic ligands was reported by the Williams group. Their
systems were able to catalyze the reaction of CO and epoxide to produce cyclic
2
carbonates or polycarbonates at only 1 atm pressure of CO in the presence of PPNCl as a
2
coꢀcatalyst. In 2011, a mononuclear iron(II)ꢀsystem based on a tetraamine ligand was
2
8
able to produce propylene carbonate without the addition of a coꢀcatalyst. Related
cycloaddition reactions catalyzed by monometallic and dimetallic iron(III) complexes
chelated with aminoꢀtriphenolate ligands have been investigated by Kleij and coꢀworkers
2
9
(
Figure 1). Later, the same iron complexes were shown to have high activity and
selectivity in supercritical carbon dioxide (scꢀCO2 both cyclic
) for the production of
carbonate and polycarbonate at 80 °C with a strong dependence on coꢀcatalyst loading.
In 2013, a new family of ionic monometallic iron(II) and (III) complexes containing
N O ligands demonstrated high activity for the conversion of CO and epoxide to cyclic
3
0
2
2
2
3
1
carbonates with 99% yield and TON up to 500 without the addition of a coꢀcatalyst.
Wang and coꢀworkers have reported very active iron(II) complexes which could catalyze
the cycloaddition of CO and epoxides to generate cyclic carbonates with nearly 100%
2
3
2
yield and TON of 1000 in 6 h (Figure 2). Recently, bimetallic iron(III) thioetherꢀ
triphenolate complexes have shown high activity towards the production of propylene
carbonate from the coupling of CO and propylene oxide under solventꢀfree conditions
2
3
3
with the highest reported TON to date, 3480, in only 6 h (Figure 3). Pescarmona and
coꢀworkers reported an effective bifunctional iron(III) pyridylaminoꢀbis(phenolate)

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complexes FeX[O NN’], (X=Cl, Br) to produce either cyclic carbonates (CHC) or
2
polycyclohexene (PCHC) carbonates under solvent free conditions at 60 °C and scCO2
34
medium within 18 h.
In the present study, we report the synthesis of new iron(III) complexes supported
by tetradentate aminoꢀbis(phenolate) ligands and their catalytic activity for the coupling
reaction of CO and various epoxides. Details of the spectroscopic and magnetic
2
properties of these complexes have been included.
Figure 1. Monomeric and dimeric iron(III) triphenolate complexes used as a catalyst used
2
9
by Whiteoak et al.
32
Figure 2. Iron(II) complexes reported by Sheng et al.
Figure 3. Bimetallic iron(III) bearing thioetherꢀtriphenolate ligands reported by
3
3
Capacchione, Rieger and coꢀworkers.

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Results and discussion
Synthesis and characterization of iron complexes
RR’Pip
A series of tetradentate aminoꢀbis(phenol) compounds H [O N ]
(Figure 4)
2
2
2
were synthesized using a method similar to literature procedures reported by Kerton and
3
5
coꢀworkers. As shown in Scheme 1, the desired iron(III) complexes were obtained via a
3
6
method reported by Kozak and coꢀworkers, which employs dropwise addition of a
methanol solution of anhydrous FeX (X = Cl or Br) to a methanolic slurry of the ligand
3
at room temperature. The resulting solution was neutralized using NEt and evaporated to
3
dryness. Extraction into an appropriate solvent, such as acetone, followed by filtration
and removal of the solvent afforded analytically pure paramagnetic complexes with the
3
7ꢀ39
formulation Fe[L]X.
The complexes were characterized using MALDIꢀTOF mass
spectrometry, elemental analysis, Xꢀray diffraction and UVꢀvis spectroscopy.
Figure 4. Proligands used with iron(III) in this study.
Scheme 1. Synthesis of iron(III) complexes.

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Crystal Structure Determination
Single crystals of 1 and 2 suitable for Xꢀray diffraction analysis were obtained by
slow evaporation and cooling of a saturated methanol or acetone solution at –20 °C. The
ORTEP diagrams of the structures are shown in Figure 5 and S1 and the crystallographic
data are collected in Table S1. Both complexes exhibit monometallic structures with the
iron centres bonded to the two phenolate oxygen atoms and two amine nitrogen atoms of
the ligand, which define the basal plane of the square pyramid. The apical sites are
occupied by chloride ions and the coordination geometry around each iron atom can be
described as a distorted square pyramid for both complexes. Selected bond lengths (Å)
and angles (°) for compounds 1 and 2 are given in Table S2. Since both of these
complexes are structurally identical except for one substituent, namely a methyl group
instead of tertꢀbutyl group on the phenolate rings, the observed bond lengths and angles
are very similar. For 1, the phenolate oxygen atoms exhibit bond distances to iron of
1
.869(12) and 1.8734(11) Å for FeꢀO(1) and FeꢀO(2). These Fe–O(1) and Fe–O(2)
distances lie within the range observed for 2 [1.8690(12) to 1.8805(12) Å]. These values
are similar to those observed in related square pyramidal geometry iron(III) complexes
3
3, 40ꢀ43
containing salen and bis(phenolate) ligands.
However, they are longer than the
corresponding Fe–O bond lengths observed in 5ꢀcoordinate iron(III) complexes
3
8, 39, 44, 45
46, 47
possessing diaminoꢀbis(phenolate) ligands
and salan complexes
in which the
iron(III) ion adopts a trigonal bipyramidal. Moreover, the Fe–O distances are shorter than
44, 48ꢀ51
the average bond length of 1.92 Å observed in octahedral iron(III) complexes,
suggesting relatively strong iron–oxygen overlap which is consistent with the lower
4
4, 48
coordination number (five rather than six).
The short Fe–O bond distance is also
supported by the high molar absorptivity of the LMCT band (UVꢀsection below). The
Fe–Cl(1) distance of 2.2466(6) Å in 1 and 2.2488(8) Å in 2 are shorter than those in the
trigonal bipyramidal complexes but similar to the Fe–Cl lengths observed in other square
3
8, 40, 42,
pyramidal iron(III) complexes possessing salen or diaminoꢀbis(phenolate) ligands.
4
3
The nitrogen donors in the ligand backbone exhibit bond lengths of 2.1864(13) and
2
.1681(13) Å for Fe–N(1) and Fe–N(2) in 1, and lengths of 2.1826(14) and 2.1902(14) Å
for Fe–N(1) and Fe– N(2) in 2. The FeꢀN bond distances in both complexes were close to
3
8, 40, 42, 43
the FeꢀN distances in the related square pyramidal complexes.
The Fe–O(1)–

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◦
◦
C(ipso) and Fe–O(2)–C(ipso) in 1 are 138.27(10) and 137.92(10) while 2 gives angles of
◦
◦
1
37.50(10) and 138.15(10) , which are identical to those observed in square pyramidal
3
8
iron(III) complexes of phenolate ligands. The distortion of the coordination around the
iron centre was determined by the trigonality parameter τ (τ =(β – α)/60), as β the largest
angle is O(2)ꢀFeꢀN(1) and α the second largest angle in the coordination sphere is O(1)ꢀ
5
2
FeꢀN(2). For both complexes, the trigonality index is close to zero.
Figure 5. Molecular structure (ORTEP) and partial numbering scheme for 1. Ellipsoids
are shown at the 50% probability level (Hꢀatoms omitted for clarity).
UV-visible spectroscopic and magnetic data
Based on previous work with iron(III) compounds supported by tetradentate
aminoꢀbis(phenolate) ligands, similar electronic absorption spectra were obtained for all
3
6, 38, 50
of the present complexes.
bands, we can assume that the compounds contain Fe in similar geometries. Complexes
–5 are intensely purpleꢀcoloured solids and their UVꢀvis spectra exhibit multiple intense
Since all of these complexes showed similar absorption
1
bands in the UV and visible regions. Electronic absorption spectra of 1ꢀ5 are shown in
Figure 6 and S2ꢀS5. The highest energy bands (<300 nm) are caused by π→π* transitions
involving the phenolate units. Strong bands in this region are also observed between 330
and 450 nm which are assigned to charge transfer transitions from the outꢀofꢀplane pπ
2
2
2
orbital (HOMO) of the phenolate oxygen to the halfꢀfilled d
/d orbital of highꢀspin
x −y z
iron(III). The lowest energy bands (visible region) between 450 and 700 nm arise from
chargeꢀtransfer transitions from the inꢀplane pπ orbital of the phenolate to the halfꢀfilled
dπ* orbital of iron(III) and account for the intense blue/purple colour of the complexes.

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4
4, 50
The halide ligands are anticipated to be labile in solution.
Changing the halide from
chloride to bromide resulted in the lowest energy band appearing at a longer wavelength
for the bromide complex compared to the chloride analogue. The lower energy of
absorption in 4 (with electron withdrawing groups) reflects the higher Lewis acidity of
the iron centre in this complex compared with 1ꢀ3 and 5.
Magnetic susceptibility data for powdered samples were measured at room
temperature using a JohnsonꢀMatthey balance. All compounds 1ꢀ3 and 5 exhibited
5
moments in the range of 4.6ꢀ5.1 ꢁ , consistent with high spin d ions. Complex 4,
B
however, exhibited a lower magnetic moment than expected (2.3 ꢁ ) which we postulate
B
is due to the presence of diamagnetic impurities (e.g. unreacted ligand), which was
confirmed by elemental analysis and MALDIꢀTOF mass spectrometry. Another
possibility for the lower than expected magnetic moment may be spinꢀspin coupling if a
bimetallic complex was formed – however, we observed no peaks for Fe ꢀspecies in the
2
mass spectra of 4.
1600
1400
1200
1000
8
6
4
2
00
00
00
00
0
2
00
400
600
800
Wavelenght(nm)
Figure 6. Electronic absorption spectrum of 1 in dichloromethane.

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Table 1. Cycloaddition reactions of propylene oxide and carbon dioxide catalyzed by
iron(III) complexes 1-5.
[a]
[i]
Entry
Catalyst
Coꢀ
[Fe]:[PO]:
Cocat]
Time
(h)
T
Pressure
(bar)
Conv. TON
TOF
[
ꢀ1 [j]
catalyst
(ºC)
(yield)
(h )
[h]
/
%
1
2
3
1
ꢀ
ꢀ
1:4000:0
0:4000:4
1:4000:4
22
22
22
100
100
100
20
20
20
0
ꢀ
ꢀ
TBAB
TBAB
33
1320
2960
60
1
74
135
(
70)
4
5
6
7
8
1
1
1
1
1
1
1
1
1
1
1
2
PPNCl
PPNCl
PPNN3
DMAP
DMAP
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
1:4000:4
1:4000:4
1:4000:4
1:4000:1
1:4000:4
1:4000:2
1:4000:10
1:4000:4
1:4000:4
1:4000:4
1:1000:4
1:4000:4
6
100
100
100
100
100
100
100
25
20
20
20
20
20
20
20
20
40
20
20
20
27
70
18
14
0
1080
2800
720
560
ꢀ
180
127
33
22
22
22
22
22
22
22
22
6
26
ꢀ
[b]
9
63
58
0
2520
2320
ꢀ
115
106
ꢀ
[
c]
1
1
1
0
[
d]
1
2
[
e]
100
100
100
100
84
25
58
30
3360
1000
580
1200
153
167
26
[
f]
1
3
[
g]
1
4
22
22
1
5
55
16
17
18
3
4
5
TBAB
TBAB
TBAB
1:4000:4
1:4000:4
1:4000:4
22
22
22
100
100
100
20
20
20
34
95
34
1360
3800
1360
62
173
62
[
a] Reaction conditions (unless otherwise stated): PO (7.0 × 10−2 mol), catalyst (1.75 × 10−5 mol, 0.025
mol%), TBAB (7.0 × 10−5 mol, 0.1 mol%), CO (20 bar); 100 °C, 22 h. [b, c] Reactions performed with 2
2
and 10 equivalents of TBAB respectively. [d] Reaction at room temperature. [e] At 40 bar CO [f] 6 h. [g]
2
.
1
Reaction conducted with 1000 equivalents of PO. [h] Determined by H NMR spectroscopy. [i] Overall
ꢀ1
turnover number (molPC molCat ). [j] Overall turnover frequency (TON/reaction time) observed.

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Table 2. Catalytic cyclization of carbon dioxide and epoxides using 1
[a]
[c]
Entry
Catalyst
Substrate
Conv
TON
TOF
[b]
ꢀ1 [d]
/
%
(h )
1
2
1
1
74
78
2960
3120
135
142
3
1
78
3120
142
4
5
1
1
52
53
2080
2120
95
96.4
6
7
1
1
31
9
1240
364
57
17
[
a] Reaction conditions: substrate (7.0 × 10−2 mol), catalyst (1.75 × 10−5 mol, 0.025 mol%), TBAB (7.0 ×
0−5 mol, 0.1 mol%), Fe: [epoxide]:[Cocatalyst] = 1:4000:4, CO (20 bar); 100 °C, 22 h. [b] Overall
1
2
ꢀ1
turnover number (molPC molCat ). [d] Overall turnover frequency (TON/reaction time) observed.
Cyclization of Propylene Oxide with Carbon Dioxide
3
2
Inspired by the promising results reported by the Wang, Capacchione and
3
3
Rieger groups, complexes 1–5 were evaluated as catalysts under similar conditions but
longer reaction times. The results are summarized in Table 1. 1 was studied most
extensively in order to get baseline results for comparison with other related catalysts.
Tetrabutylammonium bromide (TBAB) was used as a coꢀcatalyst and the influence of the
reaction temperature, CO pressure and mole ratio of Fe:PO was studied. The presence of
2
the coꢀcatalyst was necessary since no reactivity was observed when catalyst was used
alone (entry 1 vs. entry 3). Ionic and neutral coꢀcatalysts such as TBAB,

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bis(triphenylphosphine)iminium chloride (PPNCl), PPN azide (PPNN ) and 4ꢀ
3
dimethylaminopyridine (DMAP) were investigated with 1 and compared. The results
show that the combination of 1 and TBAB or PPNCl gives the highest activity and
conversion compared to other coꢀcatalysts (entries 3 and 5 vs. entries 6 and 8). It is well
known that the anionic group of the coꢀcatalyst can function as a nucleophile for the ringꢀ
53, 54
opening reaction of the epoxides.
It should be noted that TBAB and PPNCl salts
alone have been reported to yield only small amounts of cyclic carbonate indicating the
necessity of an active catalyst for carbonate formation in addition to the ionic coꢀ
2
8, 32ꢀ34, 55
catalyst.
For example, propylene carbonate was produced with a conversion of
3
3% in the absence of an Fe catalyst in the present student (entry 2). Under the applied
conditions, one equivalent of the neutral Lewis base coꢀcatalyst (DMAP) produced only
small amounts of propylene carbonate, with an increase to four equivalents inhibiting the
reaction (entry 7 vs. 8). This can be explained by the ability of DMAP to coordinate to
1
, 21
the metal centre which then competes with the incoming epoxide.
In addition, the
ratio between coꢀcatalyst and catalyst was also evaluated under the same conditions using
TBAB as the coꢀcatalyst. At higher ratios of coꢀcatalyst, the conversion of cyclic
carbonate increased from 63% to 74% which corresponds to an increase in TOF from 115
ꢀ1
ꢀ1
h to 135 h (entry 3 vs. 9). However, a drop in the conversion and the catalytic activity
was observed with further increase of TBAB loading (entry 10). Therefore, all reactions
were performed with four equivalents of TBAB.
Cycloadditions, as with many reactions, are strongly influenced by temperature.
At elevated temperature (100 °C), a conversion of 74% was reached in 22 h with a
ꢀ1
corresponding TOF value of 135 h (entry 3) confirming the thermal stability of the
catalytic system. However, conducting the reaction at room temperature afforded no
conversion of PO (entry 11). A similar trend has previously been noted for iron
3
2
complexes by Wang and coꢀworkers. Therefore, the cycloaddition reaction of PO and
CO to form PC typically requires high temperatures and this is in agreement with the
2
fact that selectivity towards formation of the cyclic carbonate product is
thermodynamically favored at high temperature. It can also be seen that the pressure of
CO has a significant influence on the conversion of PO. When the reaction is conducted
2
at 20 bar PCO2, conversion levels of 74% were achieved. At higher pressure, PCO2 = 40

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bar, it increased from 74% to 84% (entry 3 vs. 12) possibly due to the increased solubility
2
of CO in the epoxide at higher pressure. As expected, the PO conversion was timeꢀ
2
dependent as shortening reaction times from 22 h to 6 h led to decreases in the amount of
PC obtained (entry 3 and 13, respectively). In addition, the epoxide loading has a
significant influence on the reaction course with an increase in conversion being observed
with increasing amounts of PO from 1000 to 4000 (entry 14 vs. 3, respectively).
The cycloaddition reaction was also tested for complexes 2ꢀ5, in order to identify
the most active catalyst and any ligand effects (entries 15ꢀ18). As previously reported by
others, introducing electronꢀwithdrawing substituents in the ortho and paraꢀ positions of
the phenolate ring generates more reactive complexes for the use in the coupling reaction
34, 56, 57
of CO and epoxides.
Our results are in good agreement with this observation as 4
2
ꢀ1
displays the highest catalytic activity with a TOF of 173 h (entry 17). A possible
explanation is that a decrease in the donor ability of the ligand leads to increased Lewis
1, 2
acidity of the metal centre and enhances the ability of the metal to bind to the epoxide.
The substitution of the axial ligand by a bromide led to a drop of the catalytic activity
with conversions achieving only 34% (entry 18 cf. entry 3 for the corresponding chloride
complex, conversion 74%). A similar trend was also observed by Pescarmona and coꢀ
3
4
workers. and they attributed the low activity to the larger radius of bromide which
causes steric repulsion for the incoming epoxide substrate when approaching the metal
centre. Other reasons for the decrease in activity may be a difference in lability or
nucleophilicity of the halide anion. Therefore, overall in the present work activity
decreased in the order 4>1>3≥5>2. In contrast to the work reported by Pescarmona’s
group, only a small amount of polypropylene carbonate (4%) could be produced at higher
3
4
temperature and pressure conditions (70 °C and 70 bar of CO2).
To expand the scope of the catalytic system, several commercially available
epoxides with different electronic and steric properties were examined as substrates using
1
(Table 2). The reaction conditions were chosen according to the conditions presented in
Table 1. 1 was able to produce cyclic carbonate from various terminal epoxides
containing functional groups. It has recently been noted that the presence of such groups
can have a significant effect on the underlying mechanism of the reaction and functional
58
groups such as –OH in glycidol can serve a role in activating carbon dioxide. In our

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study, epichlorohydrin and glycidol reached conversions higher than those observed for
PO (Table 2, entry 1 vs. 2 and 3). Such observations have already been documented in
2
9, 32, 33, 59
earlier studies.
Reducing the electronꢀwithdrawing nature of the substituents on
the oxirane ring resulted in the production of cyclic carbonate in smaller amounts and low
catalytic activity (Table 2, entries 4 and 5). Styrene oxide (SO) exhibited lower reactivity
with conversion reaching only 31% (entry 6). This might be due to electronic effects that
have been studied computationally, which show that the alkoxide formed from ringꢀ
60
opening of SO is less nucleophilic and therefore less reactive towards carbon dioxide.
Further to this, switching the substrate to cyclohexene oxide led to very low conversions
compared to all other epoxides used and no polymer was formed (Table 2, entry 7).
29
32
These results are in agreement with the ones reported by the groups of Kleij, Wang,
3
3
Capacchione and Rieger.
Kinetic Measurements
At elevated temperatures, it is known that propylene carbonate is produced as the
1
9
dominant product in the coupling reaction of PO and CO . The production of cyclic
2
carbonates is proposed to occur via a backbiting mechanism from either a carbonate or an
1
9, 27
alkoxide chain end during the coupling process.
In an effort to better understand the
mechanistic aspects of the propylene oxide/carbon dioxide coupling process, a kinetic
study for the formation of cyclic propylene carbonate catalyzed by 1 and TBAB was
undertaken. Figure 7 shows the reaction profile obtained using in situ infrared
ꢀ1
spectroscopy. During the course of the reaction, a strong absorption band at 1806 cm
was seen to increase in intensity and can be assigned to the cyclic carbonate carbonyl
group.

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Figure 7. Threeꢀdimensional stack plots of the IR spectra using 1 at 20 bar, 100 °C and
Fe]:[PO]:[Cocat] 1:4000:4.
[
Furthermore, as discussed above, temperature has a clear influence on the
reaction; therefore, the formation of cyclic carbonate was monitored with respect to
increases in temperature (Figure 8). During the course of the reaction, the temperature
was gradually increased and maintained for approximately 25 minutes at each
temperature. No cyclic carbonate was observed at room temperature and a small amount
formed at 30 and 40 °C. As expected, increasing the temperature further resulted in
significant increases in the rate of cyclic carbonate formation. In addition, as shown in
the Arrhenius plot (Figure 9), the activation energy for the formation of the cyclic
carbonate could be calculated from the kinetic data. The activation barrier using the
ꢀ1
1
/TBAB catalytic system was determined to be 98.4 kJ mol , which is in good agreement
ꢀ1
with the values reported by the groups of Rieger (93.8 kJ mol ) and Darensbourg (100 kJ
ꢀ
1 28, 61
mol ),
for the cycloaddition of PO with CO using an iron(II) complex containing a
2
tetradentate bis(amino)ꢀbis(pyridyl) ligand and chromium(III) salen complex,
respectively. Thus implying that the reaction pathways followed by these catalytic
systems are likely very similar.

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0
0
0
0
0
0
0
0
0
0
.22
.20
.18
.16
.14
.12
.10
.08
.06
.04
6
0
80
100
120
140
160
180
200
Time (min)
Figure 8. Initial rates of reaction profile at various temperature based on the absorbance
of the ν(C=O) of the propylene carbonate (PC).
At 50 °C (y = 0.0001066 x + 0.04121,
2
2
R = 0.9838),
At 60 °C (y = 0.0004866 x + 0.01444, R = 0.9987),
At 70 °C (y =
2
2
0
.001141 x ꢀ 0.05657, R = 0.9971),
At 80 °C (y = 0.002372 x ꢀ 0.2107, R = 0.988).
-
-
-
-
-
-
-
-
-
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
0.00280
0.00285
0.00290
0.00295
0.00300
0.00305
0.00310
0.00315
1
/T
2
Figure 9. Arrhenius plot for the formation of PC. Straight line: y = ꢀ11840x + 27.25, R =
.9713.
0

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DOI: 10.1039/C6CY00477F
Conclusions
New air stable iron(III) complexes based on aminoꢀbis(phenolate) ligands were prepared
and characterized. The structures of 1 and 2 were determined and reveal iron(III) centres
in square pyramidal environments. The complexes in combination with TBAB exhibit
promising activity towards the catalytic formation of cyclic carbonates. It was found that
the presence of electron withdrawing groups in the ortho- and paraꢀpositions of the
phenolate rings increases the reactivity of catalysts. On the basis of the kinetic data at
different temperatures, the activation energy determined for cyclic carbonate formation
was close to those previously reported.
Acknowledgements
NSERC of Canada, Memorial University, RDCꢀNL and CFI are thanked for
operating and instrument grants. D.A. gives special thanks to the Saudi Arabian Cultural
Bureau in Canada and Taif University (Saudi Arabia) for financial support.
Experimental
General experimental conditions
Reagents were purchased from either SigmaꢀAldrich or Alfa Aesar and used
without further purification. Commercially available solvents were used without further
purification. Reactions for synthesizing ligands and iron complexes were performed in
6
2
air. H L1 and H L2 were prepared using a previously described procedure.
2
2
Instrumentation
1
13
1
H and C{ H} NMR spectra were recorded on a Bruker Avance 300 MHz
1
3
spectrometer at 25 °C and were referenced internally using the residual proton and C
resonances of the solvent. MALDIꢀTOF MS spectra were obtained using an Applied
Biosystems 4800 MALDI TOF/TOF Analyzer equipped with a reflectron, delayed ion
extraction and high performance nitrogen laser (200 Hz operating at 355 nm). Samples
were prepared at a concentration of 10.0 mg/mL in toluene. Matrix (anthracene) was

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mixed at a concentration of 10.0 mg/mL to promote desorption and ionization. Separate
vials were used to mix 20 ꢁL of the sample solution with 20 ꢁL of the matrix solution. 1
ꢁ
L of the sample and matrix mixture were spotted on a MALDI plate and left to dry.
TM
Images of mass spectra were prepared using mMass software (www.mmass.org). The
crystal structures were collected on a AFC8ꢀSaturn 70 single crystal Xꢀray diffractometer
from Rigaku/MSC, equipped with an Xꢀstream 2000 low temperature system (CCDC
numbers: 1452392ꢀ3). UVꢀvis spectra were recorded on an Ocean Optics USB4000+
fiber optic spectrophotometer. The room temperature magnetic measurements were
obtained using a JohnsonꢀMatthey magnetic susceptibility balance. The data were
corrected for the diamagnetism of all atoms and the balance was calibrated using
Hg[Co(NCS) ]. In addition to reactions described below that use a 100 mL pressure
4
vessel equipped for IRꢀmonitoring, cycloaddition reactions were also carried out in a 300
mL stainless steel Parr® 5500 autoclave reactor with a Parr® 4836 controller.
In situ monitoring of the cycloaddition reaction by IR spectroscopy
In situ monitoring was carried out using a modified 100 mL stainless steel reactor
vessel (Parr Instrument Company) equipped with a silicon sensor (SiComp), motorized
mechanical stirrer and a heating mantle. The silicon sensor was connected to a ReactIR
1
5 base unit (MettlerꢀToledo) through a DS silverꢀhalide FiberꢀtoꢀSentinel conduit. The
reactor vessel was cleaned and heated under vacuum at 80 °C overnight before
experiments. The appropriate amount of complex and coꢀcatalyst were weighed and then
dissolved in 4 g PO which afforded a purple solution. The mixture was stirred for about
1
0 min and the reaction solution was transferred into a 5 mL syringe with a cannula
needle attached. The syringe was injected into the vessel through a port. Then the vessel
was pressurized with 20 bar CO . Heating and stirring were started and the reaction was
2
monitored for the allotted time. After venting the reaction vessel, it was noted that the
mixture had changed colour ꢀ a brown solution had formed.

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Synthesis and characterization of ligands and catalysts
Synthesis of [H L3] A mixture of 3ꢀtertꢀbutylꢀ4ꢀhydroxyanisole (24.4 g, 0.123
2
mol), 37% w/w formaldehyde (10.0 mL, 0.123 mol) and homopiperazine (6.22 g, 0.0615
mol) in water (100 mL) was stirred and heated to reflux for 24 h. Upon cooling to room
temperature, solvents were decanted from the resulting white solid, which was
recrystallized from methanol and chloroform to afford a pure white powder (24 g,
1
2
8
5.7%). H NMR (300 MHz, 298 K, CDCl ) δ 10.72 (2H, s, OH), 6.80 (2H, d, J = 2.6,
3 HH
2
ArH), 6.40 (2H, d, J = 2.6, ArH), 3.73 (4H, s, ArCꢀCH ꢀN), 3.48 (6H, s, ArCꢀOCH ),
HH
2
3
3
2
.81 (4H, t, J = 6.1 Hz, NꢀCH CH ꢀN), 2.76 (4H, br, NꢀCH {CH }CH ꢀN), 1.88 (2H,
HH 2 2 2 2 2
3
13
1
quintet, J = 6.01 Hz, NꢀCH {CH }CH ꢀN), 1.4 (18H, s, ArCꢀC(CH ) ). C{ H} NMR
HH
2
2
2
3 3
(
300 MHz, 298 K, CDCl ) δ 151.8 (ArCꢀO), 150.6 (ArCꢀOCH ), 138.0 (ArCꢀC(CH ) ),
3 3 3 3
1
22.0 (ArCH), 112.8 (ArCH), 111.2 (ArCꢀCH ꢀN), 62.2 (ArCꢀCH ꢀN), 55.7 (ArCꢀ
2 2
OCH ), 54.5 (NꢀCH {CH }CH ꢀN), 53.0 (NꢀCH CH ꢀN), 34.9 (ArCꢀCꢀ(CH ) ), 29.4
3
2
2
2
2
2
3 3
(
ArCꢀCꢀ(CH ) ), 26.8 (NꢀCH {CH }CH ꢀN). MS (MALDIꢀTOF) m/z (% ion): 484.3
3
3
2
2
2
BuOMePip +•
(
100, H [N O
2
] ).
2
2
Synthesis of [H L4] A mixture of 2,4ꢀdichlorophenol (24.4 g, 0.123 mol), 37%
2
w/w formaldehyde (10.0 mL, 0.123 mol) and homopiperazine (6.22 g, 0.0615 mol) in
water (50 mL) was stirred and heated to reflux for 72 h. Upon cooling to room
1
temperature, solvents were decanted from the resulting yellow solid (18.5 g, 67%). H
2
NMR (300 MHz, 298 K, CDCl ) δ 11.65 (2H, s, OH), 7.24 (2H, d, J = 2.3 Hz, ArH),
3
HH
2
3
6
.84 (2H, d, J = 2.3 Hz, ArH), 3.75 (4H, s, ArCꢀCH ꢀN), 2.83 (4H, t, J = 6 Hz, Nꢀ
HH 2 HH
3
CH CH ꢀN), 2.79 (4H, s, NꢀCH {CH }CH ꢀN), 1.97 (2H, quintet, J = 6.04 Hz, Nꢀ
2
2
2
2
2
HH
13
1
CH {CH }CH ꢀN). C{ H} NMR (300 MHz, 298 K, CDCl ) δ 152.40 (ArCꢀO), 128.70
2
2
2
3
(
ArCCl), 126.51 (ArCCl), 123.43 (ArCH), 123.31 (ArCH), 121.37 (ArCꢀCH ꢀN), 61.11
2
(
ArCꢀCH ꢀN), 54.05 (NꢀCH {CH }CH ꢀN), 25.97 (NꢀCH {CH }CH ꢀN). MS (MALDIꢀ
2
2
2
2
2
2
2
ClClPip +•
TOF) m/z (% ion): 449 (100, H [N O
2
] ).
2
2
Synthesis of 1. To a methanol solution (30 mL) of recrystallized H L1 (2.52 g,
2
4
.7 mmol) was added a solution of anhydrous FeCl (0.762 g, 4.7 mmol) in methanol
3
resulting in purple solution. To this solution was added triethylamine (0.819 g, 9.4 mmol)

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and the resulting mixture was stirred for 2 h. After stirring, the solvent was removed
under vacuum. The purple product was dissolved in acetone (50 mL) and filtered through
Celite three times. Removal of solvent under vacuum yielded a black powder (2.61 g,
8
8%). Anal. calc’d for C H FeClN O .C H O: C, 66.71; H, 8.84; N, 4.09. Found: C,
35 54 2 2 3 6
6
6.7; H, 8.80; N, 4.19. MS (MALDIꢀTOF) m/z (%, ion): 664.2 (15,
BuBuPip +•
BuBuPip +•
BuBuPip +•
FeClK[N O
] ), 625.2 (85, FeCl[N O
] ), 590.3 (94.3, Fe[N O
2
] ).
2
2
2
2
2
UVꢀVis (CH Cl ) λ , nm (ε): 570 nm. ꢁ (solid, 25 °C) = 4.68 ꢁB
2
2
max
eff
Synthesis of 2. To a methanol solution (30 mL) of recrystallized H L2 (2.51 g,
2
5
.5 mmol) was added a solution of anhydrous FeCl (0.899 g, 5.5 mmol) in methanol
3
resulting in an intense purple solution. To this solution was added triethylamine (0.967 g,
1.1 mmol) and the resulting mixture was stirred for 2 h. After stirring, the solvent was
1
removed under vacuum. The purple product was dissolved in acetone (50 mL) and
filtered through Celite. Removal of solvent under vacuum yielded a black powder (2.5 g,
8
8
5
6%). Anal. calc’d for C H FeClN O : C, 64.27; H, 7.81; N, 5.17. Found: C, 64.14; H,
29 42 2 2
BuMePip +•
.10; N, 5.37. MS (MALDIꢀTOF) m/z (%, ion): 580.1 (12.2, FeClK[N O2
] ),
2
BuMePip +•
BuMePip +•
41.1 (32.4, FeCl[N O
] ), 506 (63, Fe[N O
2 2
] ). UVꢀVis (CH Cl ) λmax, nm
2
2
2
2
(
ε): 580 nm. ꢁ (solid, 25 °C) = 5.1 ꢁ
eff B
Synthesis of 3. To a methanol solution (30 mL) of recrystallized H L3 (2.52 g,
2
5
.5 mmol) was added a solution of anhydrous FeCl (0.896 g, 5.5 mmol) in methanol
3
resulting in an intense blue solution. To this solution was added triethylamine (0.965 g,
1.1 mmol) and the resulting mixture was stirred for 2 h. After stirring, the solvent was
1
removed under vacuum. The purple product was dissolved in acetone (50 mL) and
filtered through Celite. Removal of solvent under vacuum yielded a black powder (2.5 g,
7
6
9%). Anal. calc’d for C H FeClN O .C H O: C, 60.81; H, 7.66; N, 4.43. Found: C,
29 42 2 4 3 6
0.9; H, 7.41; N, 4.63. MS (MALDIꢀTOF) m/z (%, ion): 573.1 (98.6,
BuOMePip +•
BuOMePip +•
FeCl[N O
] ), 538.2 (43.1, Fe[N O
] ). UVꢀVis (CH Cl ) λmax, nm (ε):
2 2
2
2
2
2
5
80 nm. ꢁ (solid, 25 °C) = 5.1 ꢁ
eff B

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Synthesis of 4. To a methanol solution (30 mL) of recrystallized H L4 (2.52 g,
2
5
.6 mmol) was added a solution of anhydrous FeCl (0.91 g, 5.6 mmol) in methanol
3
resulting in an intense blue solution. To this solution was added triethylamine (0.977 g,
1.2 mmol) and the resulting mixture was stirred for 2 h. After stirring, the solvent was
1
removed under vacuum. The purple product was dissolved in acetone (50 mL) and
filtered through Celite. Removal of solvent under vacuum yielded a dark black powder
(
2.9 g, 82%). Anal. calc’d for C H FeCl N O (1.5CH OH)(0.5H L4): C, 44.34; H,
19 18 5 2 2 3 2
4
5
.22; N, 5.17. Found: C, 44.48; H, 4.66; N, 5.69. MS (MALDIꢀTOF) m/z (%, ion):
ClClPip +•
ClClPip +•
38.24 (90, FeCl[N O
] ), 503.98 (35, Fe[N O
] ). UVꢀVis (CH Cl ) λmax,
2 2
2
2
2
2
nm (ε): 550 nm. ꢁ (solid, 25 °C) = 2.3 ꢁB
eff
Synthesis of 5. To a methanol solution (30 mL) of recrystallized H L1 (2.52 g,
2
4
.7 mmol) was added a solution of anhydrous FeBr (1.389 g, 4.7 mmol) in methanol
3
resulting in an intense blue solution. To this solution was added triethylamine (0.819 g,
.4 mmol) and the resulting mixture was stirred for 2 h. After stirring, the solvent was
9
removed under vacuum. The purple product was dissolved in acetone (50 mL) and
filtered through Celite. Removal of solvent under vacuum yielded a dark black powder
(
2.9 g, 92%). Anal. calc’d for C H FeBrN O : C, 62.69; H, 8.12; N, 4.18. Found: C,
35 54 2 2
6
2.87; H, 7.97; N, 4.24. MS (MALDIꢀTOF) m/z (%, ion): 669.29 (45.5,
BuBuPip +•
BuBuPip +•
2
FeBr[N O
] ), 590.38 (81.8, Fe[N O
] ). UVꢀVis (CH Cl ) λmax, nm (ε):
2 2
2
2
2
5
80 nm. ꢁ (solid, 25 °C) = 4.85 ꢁB
eff
Spectroscopic data for carbonate products
3
1, 56 1
4-methyl-1,3-dioxolan-2-one (Table 2, entry 1).
H NMR (300 MHz, 298 K,
3
3
CDCl ): δ 4.7 (1H, m, CHO), 4.4 (1H, t, J = 7.1 Hz, OCH ), 3.8 (1H, t, J = 7.1 Hz,
3
HH
2
HH
3
13
1
OCH ), 1.26 (3H, d, J = 6 Hz, CH ). C { H} NMR (300 MHz, 298 K, CDCl ) δ 155
2
HH
3
3
(
CꢀCO), 73.5 (CꢀCH), 70.5 (CꢀCH ), 18.9 (CꢀCH ).
2 3
3
1, 33, 56 1
4-chloromethyl-1,3-dioxolan-2-one (Table 2, entry 2).
H NMR (300
3
MHz, 298 K, CDCl ) δ 4.94 (1H, m, CHO), 4.54 (1H, t, J = 8.9 Hz, OCH ), 4.35 (1H,
3
HH
2
3
13
1
dd, J = 9.2 Hz, OCH ), 3.7ꢀ3.9 (2H, m, CH Cl). C { H} NMR (300 MHz, 298 K,
HH
2
2
CDCl ) δ 154.5 (CꢀCO), 74.5 (CꢀCH), 67 (CꢀCH Cl), 44.1 (CꢀCH ).
3
2
2

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3
3, 56 1
4-hydroxymethyl-1,3-dioxolan-2-one (Table 2, entry 3).
H NMR (300
MHz, 298 K, CDCl ) δ 4.8 (1H, m, CHO), 4.4ꢀ4.6 (2H, m, OCH ), 3.6ꢀ4.1 (1H, m,
3
2
13
1
CH OH). C { H} NMR (300 MHz, 298 K, CDCl ) δ 155.6 (CꢀCO), 75.27 (CꢀCH),
2
3
6
6.02 (CꢀCH OH), 44.01 (CꢀCH ).
2
2
3
3, 56 1
4-allyloxymethyl-1,3-dioxolan-2-one (Table 2, entry 4).
H NMR (300 MHz,
2
98 K, CDCl ) δ 5.8 (1H, m, CH), 5ꢀ5.15 (2H, m, CH ), 4.71 (1H, s, OCH), 4.4 (1H, t,
3
2
3
3
JHH = 8.5 Hz, OCH ), 4.2 (1H, dd, J = 8.5 Hz, OCH ), 3.9 (2H, d, OCH ), 3.4ꢀ3.6
2
HH
2
2
1
3
1
(
2H, m, CH O). C { H} NMR (300 MHz, 298 K, CDCl ) δ 155.0 (CꢀCO), 117 (Cꢀ
2
3
CHCH ), 133.7 (CꢀCHCH ), 75.2 (CꢀCH), 72 (Cꢀ CH O), 66.1 (CꢀCH O), 44.0 (CꢀCH ).
2
2
2
2
2
3
1, 33 1
4-phenoxymethyl-1,3-dioxolan-2-one (Table 2, entry 5).
H NMR (300
MHz, 298 K, CDCl ) δ 7.3 (5H, m, ArH), 5.3 (1H, s, ArCH), 5 (1H, m, OCH), 4.6 (2H,
3
1
3
1
m, PhCH O), 4.2 (2H, m, OCH ). C { H} NMR (300 MHz, 298 K, CDCl ) δ 155.0 (Cꢀ
2
2
3
CO), 137 (ArꢀCO), 121.3 (ArCH ), 114.7 (ArꢀCꢀCH ), 75.7 (CꢀCH), 66.2 (CꢀCH O),
2
2
2
4
4.8 (CꢀCH2).
3
1, 33, 56 1
4-phenyl-1,3-dioxolan-2-one (Table 2, entry 6).
H NMR (300 MHz, 298
3
3
K, CDCl ) δ 7.3 (5H, m, ArH), 5.6 (1H, t, J = 8.2 Hz, ArCH), 4.7 (1H, t, J = 8.7
3
HH
HH
3
13
1
Hz, OCH), 4.2 (2H, t, J = 8.7 Hz, OCH ). C { H} NMR (300 MHz, 298 K, CDCl )
HH
2
3
δ 154.9 (CꢀCO), 136 (ArꢀCO), 128.5 (ArCH ), 126 (ArꢀCꢀCH ), 71.1 (CꢀCH), 51.1 (Cꢀ
2
2
CH2).
3
1, 33 1
cis-1,2-cyclohexene carbonate (Table 2, entry 7).
H NMR (300 MHz, 298
K, CDCl ) δ 2.89 (2H, m, OCHCH CH ), 1.63 (4H, m, OCHCH CH ), 1.20 (2H, m,
3
2
2
2
2
1
3
1
OCHCH CH ), 1.03 (2H, m, OCHCH CH ). C { H} NMR (300 MHz, 298 K, CDCl ) δ
2
2
2
2
3
1
54.9 (CꢀCO), 75.5 (OCHCH CH ), 26.6 (OCHCH CH ), 19.1 (OCHCH CH ).
2 2 2 2 2 2
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Graphical Abstract
Air-stable iron complexes display good activity for CO -epoxide cycloadditions and reactivity
2
trends across family are reported.