Stable Fe(iii) phenoxyimines as selective and robust CO2/epoxide coupling catalysts

Article abstract of DOI:10.1039/c8dt02919a

Three phenoxyimine Fe(iii)Cl complexes bearing electronically diverse -Cl, -H or -^tBu substituents in the ortho position were synthesised. X-ray crystallographic analysis of the complexes reveals mononuclear structures with pentacoordinate iron

Full text of DOI:10.1039/c8dt02919a

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Stable Fe(III) phenoxyimines as selective and robust
CO /epoxide coupling catalysts†
2
Cite this: DOI: 10.1039/c8dt02919a
Eszter Fazekas,
Jennifer A. Garden
Gary S. Nichol,
*
Michael P. Shaver
and
t
Three phenoxyimine Fe(III)Cl complexes bearing electronically diverse -Cl, -H or - Bu substituents in the
ortho position were synthesised. X-ray crystallographic analysis of the complexes reveals mononuclear
structures with pentacoordinate iron centres and trigonal bipyramidal geometries. All three complexes
2
demonstrated excellent catalytic activities towards CO /epoxide coupling to selectively form cyclic car-
bonates, with catalyst activity correlating with the electron withdrawing nature of the ortho-substituent
t
(
Cl > H > Bu) and thus the Lewis acidity of the metal centre. The chloro-substituted complex displayed
2
remarkable activity in the synthesis of propylene carbonate from propylene oxide and CO , reaching turn-
over frequencies (TOF) up to 760 h− in the presence of TBABr co-catalyst at 120 °C and 20 bars of CO
1
2
pressure. Importantly, the catalyst is also very robust, functioning with high substrate loading, under air or
in the presence of water. The substrate scope was successfully extended to other terminal epoxides
including epichlorohydrin (TOF = 900 h−1) and to the more challenging internal epoxide, cyclohexene
Received 17th July 2018,
Accepted 23rd August 2018
DOI: 10.1039/c8dt02919a
oxide (TOF = 80 h−1). These are amongst the highest TOF values reported for an iron CO
2
/epoxide
rsc.li/dalton
coupling catalyst to date.
(
(
for epoxide coordination and activation) and a nucleophile
for ring opening of the epoxide). Ligand design has enabled
Introduction
2
Catalytic transformations of carbon dioxide (CO ) are an the development of highly efficient catalyst systems, including
important strategy to valorise this waste gas and potentially mono- or bi-metallic ligand supported complexes with
reduce the environmental impact of its accumulation in the ammonium or phosphonium salt co-catalysts, or bifunctional
atmosphere. Epoxide coupling is an effective and atom catalysts which incorporate the onium salt into the ligand
9
–12
efficient method to use this inexpensive, renewable C1 build- backbone.
One popular ligand framework is the privileged
ing block. Depending on the catalyst system and the reaction salen ligand scaffold, which has supported many Lewis acidic
conditions used, polycarbonates and/or cyclic carbonates are complexes mediating a wide array of small and large molecule
1
3–15
formed; the latter are applied as high boiling solvents, ion-car- transformations (Scheme 1).
While the ubiquitous salen
/epoxide coupling,
The key to their bidentate phenoxyimine (half salen) analogues have been
riers for batteries, reagents for lignin functionalisation, fuel ligands have shown some success in CO
2
1
–6
additives and precursors to polymer synthesis.
1
6
CO /epoxide coupling is to combine high selectivity with high underexplored. Breaking the bridge between the phenoxy
2
activity, as the synthesis of both cyclic carbonates and poly-
carbonates requires the catalyst system to overcome the signifi-
1
,7,8
cant activation energy barrier of these transformations.
Some of the most active catalyst systems comprise homo-
geneous complexes with Lewis acidic metal centre
a
EaStCHEM School of Chemistry, University of Edinburgh, Edinburgh, EH9 3FJ, UK.
E-mail: j.garden@ed.ac.uk
1
13
†
Electronic supplementary information (ESI) available: H and C NMR spectra
and single crystal X-ray diffraction data. CCDC 1855808–1855810. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt02919a
Scheme 1 General structure of a bis(phenoxyimine) complex vs. a
salen complex.
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groups creates a more flexible coordination environment,
The complexes proved to be oxygen- and moisture-stable,
enabling the balance between the steric hindrance and the and single crystals suitable for X-ray diffraction were obtained
accessibility of the metal centre to be better fine-tuned under air. Crystals of complexes C1 and C3 were obtained via
(Scheme 1). Previous findings in the field suggest that the the slow evaporation of DCM, while crystals of C2 were
coordination mode of the metal centre is a key feature in obtained via the slow evaporation of toluene. Single crystal
allowing the efficient conversion of sterically challenging X-ray diffraction studies revealed that all three complexes have
1
internal epoxides to cyclic carbonates.
a mononuclear structure, with a pentacoordinate Fe(III) centre
We have had a longstanding interest in iron-mediated cata- displaying a slightly distorted trigonal bipyramidal (TBP) geo-
1
7,18
lysis thanks to its abundance, low cost, and low toxicity.
In metry. In each case, the TBP character is high, as established
particular, Fe compounds offer a user-friendly route into cat- using the factors of equatorial TBP (TBP : C1 ≥ 99.9%; C2 ≥
alysis, due to their air- and moisture-stability which enables 99.9%; C3 ≥ 99.9%) and axial TBP (TBP : C1 ≥ 99.9%; C2 =
3
+
e
a
2
9
convenient synthesis and handling. Only a few examples of 97.7%; C3 = 99.8%) proposed by Tamao and Ito (Fig. 1–3).
mono- or bi-metallic Fe(III) complexes have been reported as For all three complexes, the axial N–Fe–N bond angle deviates
catalysts for CO /epoxide couplings and in most cases high slightly from linearity (N–Fe–N, C1, 177.44(5)°; C2, 169.7(2)°;
2
catalyst loadings were required to achieve good C3, 174.92(7)°), while the sum of the equatorial O–Fe–Cl and
1
6,19–27
conversions.
Uniting these joint ligand and metal O–Fe–O bond angles is 360° (C1, 360.00°; C2, 360.01°; C3,
benefits, we targeted a series of phenoxyimine Fe(III) chloride 360.01°). The steric bulk of the ortho-substituents notably
complexes as robust and flexible catalysts for epoxide/CO
coupling.
2
influences the distortion of the TBP geometry at the Fe centre,
with the sterically demanding Bu group leading to the greatest
t
deviation from linearity for the N–Fe–N bond angle, and the
lowest TBP character. These ‘half salen’ frameworks signifi-
a
3
0
Results and discussion
cantly differ from Fe(III) salen complexes, as the phenoxy-
imine ligands are arranged such that both nitrogen and both
oxygen atoms are in mutually trans positions. This difference
in geometry arises from the lack of a bridge connecting the
Three phenoxyimine ligand precursors were targeted, bearing
ortho-substituents of H (L1, Scheme 2), Bu (L2) or Cl (L3).
t
Accordingly, all three ligands were synthesised in excellent
yields (95–99%) via the straightforward condensation of
methyl amine and mono-substituted salicylaldehydes
2
8
(
Scheme 2). Confirming the synthesis of L1, L2 and L3, no
1
aldehyde resonances were present in the H NMR spectra, and
diagnostic imine resonances were observed (CDCl solvent, L1,
3
δ = 8.33; L2, δ = 8.35; L3, δ = 8.30). Complexes C1–C3 were syn-
thesised through deprotonation of pro-ligands L1–L3 using
NaH (1.1 eq.) to form the corresponding Na-phenolates.
3
Subsequent reaction with anhydrous FeCl (0.5 eq.) in THF
solvent at room temperature gave an immediate colour change
from yellow to dark purple, which indicated the formation of
the bis-ligated Fe(III) chloride complexes C1–C3. After purifi-
Fig. 1 Molecular structure of C1 with ellipsoids set at the 50% prob-
cation via filtration to remove residual NaCl, the products were ability level. Hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å): Fe1–Cl1 2.2713(5), Fe1–O1 1.8758(9), Fe1–O1’ 1.8757
isolated in high yields (74–98%, Scheme 2) as dark purple/
brown solids. As the paramagnetically shifted H NMR spectra
of C1–C3 provided limited information, the stoichiometry
was confirmed via high resolution mass spectrometry and
elemental analysis.
1
(9), Fe1–N1 2.111(1), Fe1–N1’ 2.111(1). Selected bond angles (°): O1–Fe1–
Cl1 119.50(3), O1’–Fe1–Cl1 119.50(3), O1’–Fe1–O1 121.00(6), O1’–Fe1–
N1’ 90.29(4), O1–Fe1–N1’ 88.46(4), O1–Fe1–N1 90.28(4), O1’–Fe1–N1
88.46(4), N1’–Fe1–Cl1 91.28(3), N1–Fe1–Cl1 91.28(3), N1–Fe1–N1’
177.44(5).
Scheme 2 Synthesis of pro-ligands L1–L3 and Fe(III) phenoxyimine complexes C1–C3.
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These short C–O bonds are indicative of resonance delocalisa-
tion of the anionic charge on the phenoxide ligand in com-
plexes C1–C3. Furthermore, the C–C bond lengths of the
(O–)CvC–C(vN) scaffold within C1–C3 lie between the expected
bond lengths for C(aromatic)vC(aromatic) double bonds and
4
0
C
(aromatic)–C single bonds (1.40 and 1.52 Å, respectively), sug-
gestive of resonance delocalisation through the phenoxyimine
moiety. This resonance delocalisation is most pronounced for
t
the electron donating Bu-substituted complex C2 [C1–C6,
1.434(9); C6–C7, 1.43(1) Å], followed by the unsubstituted CH
analogue C1 [C1–C6, 1.417(2); C6–C7, 1.447(2) Å] and the
electron withdrawing Cl analogue C3 [C1–C6, 1.411(3); C6–C7,
1
.450(3) Å].
Fig. 2 Molecular structure of C2 with ellipsoids set at the 50% prob-
ability level. Hydrogen atoms and co-crystallised solvent have been
omitted for clarity. Selected bond lengths (Å): Fe1–Cl1 2.245(2), Fe1–O1
To test the catalytic activity of complexes C1–C3 towards
2
/epoxide coupling, propylene oxide (PO) was selected as a
CO
benchmark substrate, as it has previously been studied with
1
.880(5), Fe1–O2 1.881(4), Fe1–N1 2.103(6), Fe1–N2 2.108(6). Selected
bond angles (°): O1–Fe1–Cl1 116.1(2), O2–Fe1–Cl1 119.4(2), O2–Fe1–O1 other iron catalysts. Initially, complex C2 was tested under
24.6(2), N1–Fe1–Cl1 93.6(2), N1–Fe1–O1 87.5(2), N1–Fe1–O2 87.5(2),
1
6
1
solvent free conditions using 0.1 mol% catalyst loading with
.1 mol% tetrabutylammonium iodide (TBAI) as a co-catalyst
N2–Fe1–Cl1 96.8(2), N2–Fe1–O1 88.5(2), N2–Fe1–O2 86.9(2), N2–Fe1–
N1 169.7(2).
0
1
13
(
vs. PO), using 20 bar pressure of CO at 120 °C. H and
C
2
NMR studies of the crude mixture revealed that the catalysts
were selective towards the formation of cyclic propylene car-
4
1
bonate (Fig. S8†). Under these conditions, C2 successfully
achieved 99% conversion to cyclic propylene carbonate in
12 hours (Table 1, entry 2). Control reactions testing only the
Fe-complex C2 (entry 4) or only co-catalyst TBAI (entry 5)
showed that both components individually display low activity,
however, their combination displayed a synergistic effect with
a significantly higher conversion achieved (cf. entries 1 and 3).
A systematic optimisation of the reaction conditions was sub-
sequently performed. Catalyst C2 displayed tolerance towards
an increased substrate loading of 1 : 2000 ([Fe] : [PO]), reaching
a high TOF value of 400 h− (Table 1, entry 6). Doubling the
co-catalyst ratio from 1 to 2 equivalents (vs. catalyst C2) further
improved the turnover frequency of the catalyst system, from
1
Fig. 3 Molecular structure of C3 with ellipsoids set at the 50% prob-
ability level. Hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å): Fe1–Cl1 2.2517(6), Fe1–O1 1.871(2), Fe1–N1 2.092(2), 400 h− to 480 h (Table 1, entries 6 and 8, respectively).
1
−1
Fe1–O2 1.889(2), Fe1–N2 2.104(2). Selected bond angles (°): O1–Fe1–N1
Finally, tetrabutylammonium bromide (TBABr) was investi-
8
8
8.64(7), O1–Fe1–Cl1 117.12(5), O1–Fe1–O2 121.78(7), O1–Fe1–N2
9.43(7), N1–Fe1–Cl1 92.25(5), N1–Fe1–N2 174.92(7), O2–Fe1–N1 89.00
gated as a co-catalyst, which resulted in a modest improvement
in the catalytic activity (entry 12, TOF = 510 h− ). Kinetic
studies were performed and confirmed a linear, first-order
relationship between substrate conversion and time, highlight-
ing the catalyst stability under the reaction conditions tested
1
(
7), O2–Fe1–Cl1 121.11(5), O2–Fe1–N2 88.02(7), N2–Fe1–Cl1 92.81(5).
two phenoxy groups, enabling the phenoxyimine ligands to (Fig. 4). All three Fe(III) phenoxyimine complexes were sub-
minimise the steric repulsion of the substituted imine ligands. sequently screened for CO /PO coupling using the optimised
2
These complexes bear similar structural motifs to those reaction conditions (Table 1, entries 11–13).
observed with related phenoxyimine Fe(III)–chloride com-
The reactivity trends fall in line with the electron withdraw-
plexes, including those with bulky substituents on the imine ing/donating ability of the ortho-substituent, with the electron
1
6,31–37
groups.
The bond metrics are comparable to reported withdrawing Cl group (C3) giving the highest catalytic activity
examples of structurally characterised phenoxyimine Fe(III) (TOF = 760 h− , entry 13), followed by the ortho-H analogue C1
1
1
6
−1
t
complexes, with Fe–N bond lengths ranging from 2.092(2) to (TOF = 530 h , entry 11) and the electron donating Bu substi-
.111(1) Å and Fe–O bond lengths ranging from 1.8757(9) to tuent C2 (TOF = 510 h− , entry 12). Comparing C1–C3 to other
1
2
1
3
3,34,38
.889(2) Å.
Fe(III) catalysts known for this transformation is rather
The C–O bond lengths of complexes C1, C2 and C3 are complex due to the range of different conditions used, as
short [1.328(1) Å, 1.318(8) Å and 1.321(3) Å, respectively] in shown by specific catalytic comparators provided in the ESI
comparison to the related protonated ligand bearing a (Table S2†). However, the high catalytic activity of ortho-chloro-
3
9
naphthalene substituent on the imine N [C–O, 1.354(2) Å].
substituted C3 is particularly notable, as it is amongst the
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2
Table 1 Synthesis of propylene carbonate from CO and propylene oxide catalysed by complexes C1–C3
TOF (h−
1
)
Entry
Complex
t (h)
[PO]/[Fe]
Co-cat.
[Co-cat]/[Fe]
Conv. (%)
TON
1
2
3
4
C2
C2
C2
C2
—
24
12
2
24
2
2
2
2
2
2
2
2
2
2
2
2
26
24
1000
1000
1000
1000
1000
2000
1000
2000
2000
2000
2000
2000
2000
2000
2000
10 000
10 000
10 000
TBAI
TBAI
TBAI
—
TBAI
TBAI
TBAI
TBAI
1
1
1
0
2
1
2
2
2
2
2
2
2
2
2
2
2
2
99
99
56
13
15
40
91
48
48
69
53
51
76
43
65
11
63
51
990
990
560
130
—
800
910
960
960
1380
1060
1020
1520
860
1300
1100
6300
5100
41
83
280
5
a
5
—
6
7
8
9
1
1
1
1
1
1
1
1
1
C2
C2
C2
C1
C3
C1
C2
C3
C3
C3
C3
C3
C3
400
455
480
480
690
530
510
760
430
650
550
242
213
TBAI
TBAI
0
1
2
3
4
5
6
7
8
TBABr
TBABr
TBABr
TBABr
TBABr
TBABr
TBABr
TBABr
b
c
d
1
2
Conditions: 100 ml stainless steel autoclaves, 20 bar CO pressure, 120 °C, neat. Conversion was determined using H NMR spectra of crude reac-
b
a
tion mixtures. The reaction was carried out without an iron complex, 2 equivalents of TBAI were added per 1000 equivalents of epoxide. 100
c
d
equivalents H
2
O/[Fe] was added. The reaction was carried out under air. The reaction was carried out using unpurified propylene oxide (99%).
When the coupling reaction was set up under air, ortho-chloro
catalyst C3 still displayed high activities towards cyclic pro-
pylene carbonate formation, with only a minor reduction in
−
1
the TOF value observed (entry 13, TOF = 760 h ; entry 15,
TOF = 650 h− ). Furthermore, catalyst system C3 also displayed
some tolerance towards water; the addition of 100 equivalents
1
−
1
of water (vs. catalyst C3) still gave high TOF values of 430 h
entry 14). Catalyst C3 is noteworthy as it demonstrates high
catalytic activities even at low catalyst loadings of 0.01 mol%
(
−
1
(
2
1 : 10 000 [Fe] : [PO]), displaying a TOF of up to 550 h after
hours (entry 16), slowing only after 8 hours (Fig. S12†). The
robustness of C3 was further demonstrated when high conver-
sion was maintained using 10 000 equivalents of unpurified
(
wet) PO (entry 18). The water tolerance of catalyst C3 was
Fig. 4 Kinetic plot for the synthesis of propylene carbonate using C2
with 2000 equivalents of substrate.
supported by FT-IR spectroscopic studies (refer to ESI† for
further details).
The substrate scope of catalyst C3 towards CO
2
/epoxide
coupling was investigated by testing a range of epoxides,
fastest reported Fe-based catalyst systems for cyclic carbonate including cyclohexene oxide (CHO), epichlorohydrin, 1,2-epox-
formation. It seems likely that the presence of electron with- ybutane and 1,2-epoxy-3-phenoxypropane (Table 2). Catalyst C3
drawing chloro- groups increases the Lewis acidity of the demonstrated tolerance towards a broad substrate scope, suc-
Fe centre, facilitating epoxide coordination to promote nucleo- cessfully converting all five substrates tested to the corres-
4
2
philic attack and ring-opening. These findings conform ponding cyclic carbonates. For the terminal epoxides, the cata-
to previous studies of cobalt(III) amidoamine ligands, which lyst activity falls in line with the nature of the epoxide substitu-
showed a similar trend; complexes bearing electron withdraw- ent, with the presence of electron withdrawing substituents
ing chloro- or nitro- substituents gave significantly enhanced facilitating the epoxide ring opening. Accordingly, the highest
−
1
catalyst activities in comparison to the methyl substituted conversion was observed for epichlorohydrin (TOF = 900 h
analogues.
,
4
3
Table 2, entry 3), and the lowest conversion (of a terminal
−
1
In addition to their high catalytic activity, it is particularly epoxide) was achieved for butylene oxide (TOF = 500 h
,
1
noteworthy that C1, C2 and C3 are all air-stable complexes. entry 4). In agreement with previously observed trends, the
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5
2–55
Table 2 Substrate scope for CO
2
/epoxide coupling using catalyst C3
with Fe(III) salen complexes.
The flexible coordination
modes available when using phenoxyimine ligands may
present an advantage over the well-established salen
analogues. These findings suggest that phenoxyimine ligand
supported metal complexes have significant potential for a
2
broad scope of CO /epoxide coupling reactions.
Entry
Substrate
Substrate
loading
Conv.
(%)
TON
1520
160
TOF
−1
(h
)
Conclusions
1
2
2000
2000
76
8
760
80
In conclusion, three new iron(III) phenoxyimine complexes
were synthesised and tested in CO /epoxide coupling reac-
2
tions. All three earth-abundant, air-tolerant complexes display
excellent performance in the selective catalytic coupling of
CO /epoxides, when activated by the presence of a tetrabutyl-
2
ammonium bromide or iodide co-catalyst. These results high-
light that for this class of catalysts, the activity is influenced by
the presence of electron donating or electron withdrawing
ortho-substituents on the phenoxyimine ligand. The highest
catalyst activities were achieved using the electron withdrawing
3
2000
90
1800
900
4
5
2000
1500
50
69
1000
1035
500
518
−
1
chloro-substituted complex C3, with TOF values up to 900 h
.
Importantly, the complexes are very robust, tolerating the pres-
ence of both air and water, and very flexible, selectively
forming the cis isomer of cyclohexene carbonate from the steri-
cally challenging secondary epoxide. These iron catalyst pre-
cursors offer significant potential as stable, robust and selec-
tive catalysts for the valorisation of carbon dioxide.
Conditions: 100 ml stainless steel autoclaves, 20 bar CO
20 °C, 2 hours, neat. Conversion was determined using H NMR
spectra of crude reaction mixtures.
2
pressure,
1
1
Experimental
General procedure for ligand precursors L1–L3
lowest conversions were observed for the most sterically
hindered internal epoxide, cyclohexene oxide, as expected due
to the formation of a strained bicyclic carbonate product.
Through extending the reaction time to 48 hours, high conver-
sion of CHO to cyclohexene carbonate (CHC) was achieved
Equimolar amounts of a salicylaldehyde (salicylaldehyde, 3-tert-
butylsalicylaldehyde or 3-chlorosalicylaldehyde) and methyl-
amine (40% w/w solution in water) were dissolved in ethanol
(74%, Table S1,† entry 3). This is indeed surprising, as cata-
(
0.2 M) and stirred at room temperature for 16 hours. The
lysts which selectively give CHC in high yields are still rare, as
many catalyst systems yield a mixture of poly(cyclohexene
resulting yellow solutions were dried over MgSO , filtered, and
4
the solvents were subsequently evaporated in vacuo. L1 was
obtained as an orange oil, whilst L2 and L3 were further puri-
fied via recrystallisation from ethanol, to produce L2 as green
crystals and L3 as yellow crystals. Characterisation data for L1
1
,20,44,45
carbonate) (PCHC) and CHC.
Importantly, catalyst C3
exclusively formed cis-CHC at high conversions, which is quite
unusual as the formation of the trans-isomer often occurs
through the thermodynamically favourable back-biting of a
poly(cyclohexene carbonate) chain. However, resonances
corresponding to trans-CHC (3.9 ppm) were absent from the
5
6
was in agreement with reported values in the literature.
1
3
Data for L2: (1.86 g, 97%) H NMR (500 MHz, CDCl )
δ 14.11 (s, 1H, OH), 8.35 (s, 1H, HCvN), 7.31 (d, J = 7.7 Hz, 1H,
ArH), 7.10 (d, J = 7.6 Hz, 1H, ArH), 6.80 (t, J = 7.6 Hz, 1H, ArH),
1
H NMR spectra, and IR spectroscopy and mass spectrometry
13
(
EI and MALDI) studies of the product mixture confirmed the
3
.48 (d, J = 1.5 Hz, 3H, NCH
3 3
), 1.44 (s, 9H, CCH ). C NMR
1
,20
lack of polymer formation (Fig. S13–15†).
The formation of
(
126 MHz, CDCl ) δ 166.89 (CvN), 160.51 (C-OH), 137.43, 129.32,
3
cis-CHC occurs through a double inversion pathway, which can
be favoured through the addition of excess co-catalyst.
1
29.13, 118.75, 117.63 (Ar-C), 45.72 (N-CH
3
), 34.83 (CCH
3
), 29.32
2
0,46–50
+
+
(
CCH
Data for L3: (1.10 g, 99%) H NMR (500 MHz, CDCl3)
δ 14.51 (s, 1H, OH), 8.29 (s, 1H, HCvN), 7.39 (d, J = 7.9 Hz,
H, ArH), 7.14 (d, J = 7.7 Hz, 1H, ArH), 6.77 (t, J = 7.8 Hz, 1H,
3
). HRMS (EI): m/z [M] 191.1319 calculated [M] 191.1310.
1
It has previously been proposed that the metal geometry is
of key importance for the conversion of sterically congested
internal epoxides to the corresponding cyclic carbonates, with
complexes bearing a trigonal bipyramidal geometry around the
1
1
3
ArH), 3.49 (d, J = 1.5 Hz, 3H, CH3). C NMR (126 MHz, CDCl3)
δ 165.8 (CvN), 158.8 (C-OH), 132.7, 129.6, 122.2, 119.3, 118.1
1
,51
metal centre typically showing greater success.
Complexes
+
C1–C3 all display a distorted trigonal bipyramidal geometry, in
contrast to the square pyramidal geometries often observed
(
Ar-C) 44.9 (CH
3
). HRMS (EI): m/z [M] 169.0315 calculated
+
[M] 169.0294.
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General procedure for complexes C1–C3
Acknowledgements
To a solution of pro-ligand (L1–L3) in THF, 1.1 equivalents of
NaH was gradually added and the mixture was stirred at
ambient temperature for 1 hour. A solution of anhydrous
FeCl3 (0.5 eq.) in THF was added dropwise to afford a dark
coloured mixture that was stirred for a further 16 hours at
We gratefully acknowledge the University of Edinburgh for
funding.
room temperature. Solvents were evaporated in vacuo and the References
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2
3
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was removed by filtration through Celite and the filtrate was
dried in vacuo. The crude products were washed three times
with hexane to afford dark purple-brown powders. Single crys-
tals suitable for X-ray diffraction analysis were obtained via
slow evaporation of DCM or toluene.
1
+
Data for C1 (0.42 g, 81%) HRMS (EI): m/z [M] 359.0205
+
4 H. Zhang, H.-B. Liu and J.-M. Yue, Chem. Rev., 2014, 114,
83–898.
5
calculated [M] 359.0250 elemental analysis calculated for
8
C
16
H
16ClFeN
2
O
2
: C, 53.4; H, 4.5; N, 7.8. Found: C, 53.3; H,
B. Schäffner, F. Schäffner, S. P. Verevkin and A. Börner,
Chem. Rev., 2010, 110, 4554–4581.
4
.65; N, 7.6.
+
Data for C2 (2.11 g, 98%) HRMS (EI): m/z [M] 471.1523
+
6 A. Duval and L. Avérous, ACS Sustainable Chem. Eng., 2017,
, 7334–7343.
7
calculated [M] 471.1502 elemental analysis calculated for
C H FeN O : C, 61.1; H, 6.84; N, 5.9. Found: C, 60.2; H, 6.2;
5
2
4
32
2 2
H. Sun and D. Zhang, J. Phys. Chem. A, 2007, 111, 8036–
8043.
N, 6.0.
+
Data for C3 (0.49 g, 74%) HRMS (EI): m/z [M] 426.9474 cal-
culated [M] 426.9475 elemental analysis calculated for
+
8 C.-H. Guo, H.-S. Wu, X.-M. Zhang, J.-Y. Song and X. Zhang,
J. Phys. Chem. A, 2009, 113, 6710–6723.
M. Chihiro, T. Tomoya, O. Kanae and E. Tadashi, Angew.
Chem., Int. Ed., 2015, 54, 134–138.
C
16
H
14Cl
3
FeN
2 2
O : C, 44.85; H, 3.3; N, 6.5. Found: C, 44.7; H,
9
3
.35; N, 6.5.
General procedure for the synthesis of cyclic carbonates from
CO and epoxides
Reactions were carried out in 100 mL stainless steel autoclaves
1
0 C. J. Whiteoak, N. Kielland, V. Laserna, F. Castro-Gomez,
E. Martin, E. C. Escudero-Adan, C. Bo and A. W. Kleij,
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2
equipped with a magnetic stirrer, heating mantle (controlled 11 H. Vignesh Babu and K. Muralidharan, Dalton Trans., 2013,
by a thermocouple) and a pressure gauge. Complex C1, C2 or 42, 1238–1248.
C3 (10.0 μmol) and co-catalyst (TBAI or TBABr) were placed 12 J. Qin, P. Wang, Q. Li, Y. Zhang, D. Yuan and Y. Yao, Chem.
into the autoclave and purged with argon. The relevant Commun., 2014, 50, 10952–10955.
epoxide (2000 eq.) was added and the autoclave was sub- 13 P. G. Cozzi, Chem. Soc. Rev., 2004, 33, 410–421.
sequently pressurised with 20 bar of CO . The autoclave was 14 A. W. Kleij, Eur. J. Inorg. Chem., 2009, 193–205.
2
heated to 120 °C for 24 hours, then rapidly cooled in an ice 15 J. A. Castro-Osma, M. North and X. Wu, Chem. – Eur. J.,
bath for 1 hour and slowly vented. An aliquot of the crude reac-
2016, 22, 2011–2017.
tion mixture was immediately taken up in CDCl and analysed 16 F. A. Al-Qaisi, N. Genjang, M. Nieger and T. Repo, Inorg.
3
1
via H NMR spectroscopy, to determine epoxide conversion via
Chim. Acta, 2016, 442, 81–85.
integration of product resonances against starting material 17 K. Zhu, M. P. Shaver and S. P. Thomas, Chem. Sci., 2016, 7,
resonances (refer to ESI† for further details).
3031–3035.
1
1
2
2
8 K. Zhu, J. Dunne, M. P. Shaver and S. P. Thomas, ACS
Catal., 2017, 7, 2353–2356.
Crystallography
Single crystal X-ray diffraction data were collected on an
Rigaku Oxford Diffraction SuperNova diffractometer fitted
with an Atlas CCD detector with Mo-Kα radiation (λ = 0.7107 Å)
or Cu-Kα radiation (λ = 1.5418 Å). Crystals were mounted
under Paratone on MiTeGen loops. The structures were solved
by direct methods using SHELXS or SHELXT and refined by
9 K. Nakano, K. Kobayashi, T. Ohkawara, H. Imoto and
K. Nozaki, J. Am. Chem. Soc., 2013, 135, 8456–8459.
0 A. Buchard, M. R. Kember, K. G. Sandeman and
C. K. Williams, Chem. Commun., 2011, 47, 212–214.
1 C. J. Whiteoak, B. Gjoka, E. Martin, M. M. Belmonte,
E. C. Escudero-Adán, C. Zonta, G. Licini and A. W. Kleij,
Inorg. Chem., 2012, 51, 10639–10649.
2
full-matrix least-squares on F using SHELXL interfaced
5
7,58
through Olex2.
Molecular graphics for all structures were
22 M. A. Fuchs, T. A. Zevaco, E. Ember, O. Walter, I. Held,
E. Dinjus and M. Doring, Dalton Trans., 2013, 42, 5322–
generated using Diamond.
5329.
2
3 A. Buonerba, A. De Nisi, A. Grassi, S. Milione,
C. Capacchione, S. Vagin and B. Rieger, Catal. Sci. Technol.,
2015, 5, 118–123.
Conflicts of interest
There are no conflicts of interest to declare.
Dalton Trans.
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