Synthesis of helical aluminium catalysts for cyclic carbonate formation

Article abstract of DOI:10.1039/c9dt00323a

Helical aluminium complexes [Al₂X₄(μ-nbptam)] (X = Me 1, Et 2), [Al₂X₄(μ-fbpam)] (R = Me 3, Et 4), [Al₃X₇(μ-nbptam)] (X = Me 5, Et 6) and [Al₃X₇(μ-fbpam)] (X = Me 7, E

Full text of DOI:10.1039/c9dt00323a

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Gaona, F. de la Cruz-Martínez, F. Juan, L. F. SÁNCHEZ-BARBA MERLO, C. Alonso-Moreno, A. M.
Rodríguez, A. Rodríguez Diéguez, J. A. Castro-Osma and A. Lara, Dalton Trans., 2019, DOI:
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0.1039/C9DT00323A.
Volume 45 Number
1
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January 2016 Pages 1–398
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Efficient extraction of sulfate from water using
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DOI: 10.1039/C9DT00323A
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ARTICLE
Synthesis of Helical Aluminium Catalysts for Cyclic Carbonate
Formation
a
a
a
b
Miguel A. Gaona, Felipe de la Cruz-Martínez, Juan Fernández-Baeza, Luis F. Sánchez-Barba,
Received 00th January 20xx,
Accepted 00th January 20xx
c
a
d
,c
Carlos Alonso-Moreno, Ana M. Rodríguez, Antonio Rodríguez-Diéguez, José A. Castro-Osma,*
Antonio Otero* and Agustín Lara-Sánchez*
,
a
,a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Helical aluminium complexes [Al
2 4 2 4 3 7
X (μ-nbptam)] (X = Me 1, Et 2), [Al X (μ-fbpam)] (R = Me 3, Et 4), [Al X (μ-nbptam)] (X = Me
5
, Et 6) and [Al X (μ-fbpam)] (X = Me 7, Et 8) have been prepared by treatment of scorpionate ligand precursors with two or
3 7
three equivalents of the corresponding trialkylaluminium derivative. The structures of these complexes have been
determined by spectroscopic methods and the X-ray crystal structure of complex 1 has also been established. These
complexes have been studied as catalysts for the chemical fixation of carbon dioxide into cyclic carbonates displaying good
catalytic activity. When cyclohexene oxide was used as a substrate, polyether–polycarbonate was obtained in a ratio which
is highly dependent on the cocatalyst and the catalyst to cocatalyst ratio used.
helicates when using an enantiomerically pure scorpionate
5
Introduction
The design of bio-inspired helical structures found in proteins
and nucleic acids has recently attracted much attention from
the scientific community. Significant progress has been made
in developing artificial helices by using supramolecular
ligand. These enantiomerically pure helicates structures could
be promising entities for their use in medicinal chemistry or
asymmetric catalysis. Interestingly, these scorpionate
aluminium complexes not only are very interesting building
blocks for the design of metallic helicates but also have
displayed high catalytic activity in a range of different
7
1
interactions, such as hydrogen bonds, - stacking or CH-
8
,9
processes.
2
interactions. Ligand design has shown to be the key factor for
3
the construction of helical architectures. In this context,
scorpionate ligands can be used for the design of helical
aluminium complexes that can act as building blocks for the
Ph
N
Et
Al Et
4
,5
Et
Et
construction of helical architectures since heteroscorpionate
ligands are highly versatile in terms of their coordination
S
N
Al
N
N
Me
N
Me
6
modes. In 2009 we reported a novel and straightforward
Me Me
approach to prepare a single helical dinuclear aluminium
complex able to act as a building block for the construction of a
single-stranded aluminium helicate self-assembled by CH-π
4
hydrogen bonds (Figure 1). The helical chirality induction was
based on the bimetallic bridging mode of the non-helical
backbone heteroscorpionate ligand. Later on, our group
developed new heteroscorpionate aluminium complexes as
appropriate building blocks to form supramolecular homochiral
Fig. 1 Molecular structure of the array resulting from the aggregation of five dinuclear
aluminium complexes. Hydrogen atoms are omitted for clarity.
a. Universidad de Castilla-La Mancha, Dpto. de Química Inorgánica, Orgánica y
Bioquímica, Facultad de Ciencias y Tecnologías Químicas, 13071-Ciudad Real,
Spain. E-mail: Agustin.Lara@uclm.es; Antonio.Otero@uclm.es.
Carbon dioxide (CO
2
) fixation into high value-added chemical
is
an abundant, cheap and non-toxic feedstock. However, CO
b.
Universidad Rey Juan Carlos, Departamento de Biología y Geología, Física y
products has drawn much attention in recent years since CO
2
Química Inorgánica, Móstoles, 28933 Madrid, Spain.
Universidad de Castilla-La Mancha, Dpto. de Química Inorgánica, Orgánica y
1
0
c.
2
Bioquímica, Facultad de Farmacia, 02071-Albacete, Spain. E-mail: utilisation in a sustainable manner is still a challenge due to its
JoseAntonio.Castro@uclm.es.
1
0
thermodynamic stability and kinetic inertness. Therefore,
d.
Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de
Granada. Avda de Fuentenueva s/n, 18071-Granada, Spain.
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
there is much research going on to develop highly efficient
†
11
catalysts for CO
2
transformation into useful products. One of
the most studied reaction of CO
2
is its reaction with epoxides to
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produce cyclic carbonates12 (Scheme 1) due to their potential
Journal Name
R
View Article Online
1
3
DOI: 10.1039/C9
X
DT00323A
industrial applications as sustainable polar aprotic solvents,
X
X
+
2AlX3
-2XH
N
E
Al
Al
X
1
4
electrolytes in Li-ion batteries and chemical intermediates for
N
N
1
5
Me
N
N
the production of chemical products or polymers.
Me
R
HN
Me
Me Me
E
N
N
Me
E = S, R = Naph, X = Me (1)
E = S, R = Naph, X = Et (2)
E = O, R = Flu, X = Me (3)
E = O, R = Flu, X = Et (4)
O
O
N
N
catalyst
+
CO2
O
O
R1
R2
Me
Me
R1
R2
E = S, R = Naph (L1)
E = O, R = Flu (L2)
R
AlX3
Scheme 1 Cyclic carbonates synthesis from epoxides and carbon dioxide.
X
+
3 AlX3
2XH
X
X
N
E
Al
N
Al
N
X
-
Although a variety of catalysts for this transformation has been
Me
N
N
Me
1
0-12
reported in the last years,
there is still a great interest in
Me Me
developing novel Earth abundant metal catalysts such as
E = S, R = Naph, X = Me (5)
E = S, R = Naph, X = Et (6)
E = O, R = Flu, X = Me (7)
E = O, R = Flu, X = Et (8)
1
6
aluminium supported by a variety of ligands. In this regard, we
have designed a range of helical aluminium complexes which
have shown to be efficient catalysts for cyclic carbonate
formation from terminal epoxides and carbon dioxide at room
Scheme 2 Synthesis of aluminium complexes 1-8.
8
The synthesis of compounds 1-8 was carried out by reaction of
the heteroscorpionate ligand precursors nbptamH (nbptamH =
N-naphtyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)thioacetamide)
2
temperature and one bar CO pressure (Fig. 2b).
Ph
Ph
N
AlMe3
O
Et
Al Et
Me
Al Me
Et
Et
N
S
N
Me
Me
Al
N
Al
N
(L
1
)
and fbpamH (fbpamH
=
N-fluorenyl-2,2-bis(3,5-
with two or three
N
N
dimethylpyrazol-1-yl)acetamide) (L
2
)
Me
Me
N
Me
N
N
Me
equivalents of the corresponding trialkylaluminium derivative
AlX ) in toluene for one hour at room temperature, affording
the organometallic aluminium complexes [Al (μ-nbptam)] (X
Me 1, Et 2), [Al (μ-fbpam)] (R = Me 3, Et 4), [Al (μ-
nbptam)] (X = Me 5, Et 6) and [Al (μ-fbpam)] (X = Me 7, Et 8),
Me Me
a)
Me Me
(
3
(
(b)
2 4
X
Fig. 2 Helical scorpionate aluminium complexes as catalysts for cyclic carbonate
formation at room temperature.
=
X
2 4
3 7
X
3 7
X
with elimination of the corresponding alkane (Scheme 2).
Herein, we report the synthesis and structural characterisation
of novel helical aluminium complexes supported by scorpionate
ligands containing a polycyclic aromatic hydrocarbon group.
The catalytic activity of these aluminium complexes for the
chemical fixation of carbon dioxide into cyclic carbonates from
their corresponding terminal and internal epoxides has been
studied.
Me3,3’ Me5,5’
AlEt3
2 3 3
Al(CH CH )
Et
Et
Et
N
N
O
Al
N
Al Et
Al(CH
2
CH
3
)
2
N
Me
N
Me
Me Me
8)
Al(CH
2
CH
3 3
)
(
Flu
Results and discussion
H4,4’
CH
2
2 3 2
Al(CH CH )
Syntheses and Structural Characterisation of Aluminium
Complexes.
Drawing upon previous works in which several scorpionate
ligands were used for the synthesis of helical aluminium
complexes,4 we have now focussed our efforts on the
development of aluminium complexes 1-8 with a polycyclic
aromatic hydrocarbon group in the acetamidate or
thioacetamidate moiety (Scheme 2). This will allow us to study
the influence of CH-π hydrogen bonds between the discrete
organometallic complexes on the synthesis of chiral helical
aluminium complexes and on the catalytic activity of these
complexes for the synthesis of cyclic carbonates from epoxides
and carbon dioxide.
,5
9
.0 8.5 8.0
7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0
ppm
1
Fig. 3 H NMR spectrum for complex 8 in C
6 6
D .
1H and 13C{ H} NMR spectra for complexes 1-8, which are
supported by an achiral ligand showed two singlets for the
pyrazole protons and carbons and four set of signals for the alkyl
ligands (See Figure 3). These data suggested the formation of a
helical structure for complexes 1-8 due to the coordination
1
1
3
1
mode of the scorpionate ligand. C{ H} NMR data for
complexes 1-8 confirmed that the scorpionate ligand is bridging
1
3
the two aluminium centres since the C resonances of the
carbon bridging the two pyrazole rings indicated a quaternary
2
4,5
sp carbon. In trimetallic aluminium complexes 5-8, the third
aluminium centre is coordinated through a dative bond to the
2
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sulphur or oxygen atom of the thioacetamidate or acetamidate
scorpionate ligand. NOESY-1D and g-HSQC NMR experiments
View Article Online
DOI: 10.1039/C9DT00323A
1
13
enabled the unequivocal assignment of most of the H and
C
NMR resonances (see Figure S1 in Supporting Information as a
representative example of the characterisation of the series).
The solid-state structure of complex 1 was determined by X-ray
crystallography (Figure 4) and the crystallographic data is given
in Table 3 in the Experimental Part. The ORTEP drawing
confirmed a distorted tetrahedral environment for the
Fig. 5 Supramolecular chains that extend along the a axis for compound 1. The red lines
show the CH- and - staking interactions.
aluminium
atoms
in
which
the
thioacetamidate
The union of these chains by means of - and CH- interactions
heteroscorpionate ligand is bridging the metal centres. Selected
interatomic distances and angles for complex 1 is given in Table
S2 of the ESI.
with toluene molecules of solvent generated
a
3D
supramolecular structure in which it is possible to observe the
formation of channels along the c axis occupied by toluene
molecules of solvent (Figure 6).
Fig. 4 ORTEP perspective for the M-enantiomer of complex 1.
Fig. 6 3D structure for compound 1 viewed along the c axis showing the channels
Complex 1 crystallises as a racemic mixture containing the two occupied with toluene molecules solvent.
enantiomers, M and P, in the unit cell (Figure S2 in the SI). The
distortion of the tetrahedral configured around the aluminium
centre was higher for complex 1 than for the previously
reported helical aluminium complexes containing a phenyl
Cyclic carbonates synthesis
Aluminium complexes 18 (Scheme 2) were investigated as
catalysts for the conversion of styrene oxide 9a and CO into
2
styrene carbonate 10a. The catalyst screening was carried out
using 5 mol% of complexes 18 and tetrabutylammonium
2
bromide (TBAB) as cocatalyst at 25°C and one bar of CO for 24
4
,5
ring
instead of a naphthyl group. The major steric
requirement of the naphthyl group of the thioacetamide moiety
dictated the twist angle between the two pyrazole rings.
Complex 1 showed shorter bond lengths for AlN(5) than for
AlN(pyrazole), and AlC bond lengths are within the expected
hours under solvent free conditions (Scheme 3). Conversion of
epoxide 9a into cyclic carbonate 10a was monitored at 3, 6 and
4
,5,8-9
range for other organometallic aluminium alkyl complexes.
1
2
4 hours by H-NMR spectroscopy and results are shown in
The 3D supramolecular structure reflects the presence of CH-
and π−π stacking intermolecular interactions that may have an
influence on the stability of the system. It is possible to observe
that the pyrazole rings coordinated to Al(1) and Al(3) atoms
exhibit a π−π stacking with the external phenyl ring of the
naphthyl moiety. In addition, one methyl group of these
pyrazole rings establishes a CH- interaction with the internal
phenyl ring of the naphthyl group. These interactions lead to the
formation of chains extended along the a axis as showed in
Figure 5.
Table 1.
1
8 (05 mol%)
O
Bu4NBr (05 mol%)
2550 oC, 1 bar, 324 h
O
+
CO2
O
O
R
R
9
a-j
10a-j
9,10: a, R = Ph; b, R = Me; c, R = Et; d, R = Bu; e, R = Oct; f, R = 4-ClC H ;
6 4
g, R = 4-BrC H ; h, R = CH OH; i, R = CH Cl; j, R = CH OPh
Scheme 3 Synthesis of cyclic carbonates 10a–j.
6
4
2
2
2
As can be seen from in Table 1, bimetallic complexes 14 were
more active than trimetallic complexes 58 using the same
catalyst loading (Table 1, entries 1–8). It is also apparent that
bimetallic ethyl aluminium complexes 2 and 4 displayed higher
catalytic activity than their corresponding bimetallic methyl
complexes 1 and 3 (entries 1–4). On the other hand, trimetallic
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Journal Name
methyl aluminium complexes 5 and 7 displayed a slightly higher pressure (Figure 7). Other cocatalysts ViewsAurtcichle Onlianes
catalytic activity than their trimetallic ethyl counterparts 6 and tetrabutylammonium fluoride (TBAF), DtOeIt:r1a0b.1u0t3y9l/aCm9DmT0o0n3iu23mA
8
(entries 5–8).
chloride (TBAC), tetrabutylammonium iodide (TBAI),
bis(triphenylphosphine)iminium chloride (PPNCl), 4-
Table 1 Catalyst screening for the synthesis of styrene carbonate 10a using a
combination of catalyst 1-8 and Bu
NBr.a
dimethylaminopyridine (DMAP) and N-methylimidazole (NMI)
displayed lower activities (Figure 7). The trend followed was Br
> I > Cl > F (Figure 7), similarly to the trend found previously for
4
Conversion
Conversion
(%) 6h
Conversion
(%) 24h
Entry
Complex
b
b
b
8,17
(
%) 3h
19
21
13
16
20
17
19
14
2
other scorpionate helical aluminium catalysts.
1
2
3
4
5
6
7
8
9c
1
2
3
4
5
6
7
8
2
2
2
2
-
40
46
28
33
42
34
40
30
2
78
92
65
69
77
70
63
58
6
To further investigate the catalytic performance of the catalyst
o
system at 25 C and one bar CO
2
pressure, the synthesis of ten
cyclic carbonates (10aj) from their corresponding terminal
epoxides (9aj) and carbon dioxide was investigated using 5
mol% of complex 2 and TBAB as a catalyst system.
O
O
O
O
O
O
O
O
O
O
O
O
0d
Ph
Me
Et
nBu
1
1
1
5
15
0
6
19
0
18
37
0
a
a
a
10d: Conv: 90%a
1
0a: Conv: 92%
Yield: 90%a
10b: Conv: n.d.
Yield: 59%a
10c: Conv: 83%
Yield: 70%a
1e
Yield: 74%a
2f
O
O
O
O
1
3g
0
2
5
O
O
O
O
O
aReactions carried out at 25°C and one bar of CO
for 24 hours using 5 mol% of a
2
nOct
0e: Conv: 99%
Yield: 95%a
4Cl-C6H4
10f: Conv: 52% , 99%
6 4
4Br-C H
10g: Conv: 43% , 100%
Yield: 41% , 95%b
b
1
combination of complex 18 and Bu
4
NBr in a 1:1 ratio. Determined by H-NMR.
a
a
b
a
b
1
c
d
e
a
b
a
0
.5 mol% of complex 2 was used. 1.0 mol% of complex 2 was used. 2.5 mol% of
Yield: 48% , 93%
f
g
4
complex 2 was used. No Bu NBr was added. Complex 2 was not added.
O
O
O
O
O
O
O
O
O
Amongst the complexes screened, complex 2 was the most
active catalyst for styrene carbonate formation from styrene
oxide and carbon dioxide at ambient temperature and pressure.
The effect of reducing the catalyst loading was investigated
HO
Cl
PhO
10j: Conv: 100%a
Yield: 92%a
1
0h: Conv: 88%a
Yield: 74%a
10i: Conv: 100%a
Yield: 94%a
Fig. 8 aSynthesis of cyclic carbonates 10a–j from epoxides 9a–j and carbon dioxide
o
(
Table 1, entries 9–11) and the results showed that 5 mol% of catalysed by a combination of 5 mol% of complex 2 and 5 mol% of Bu
4
NBr at 25 C and
b
o
2
one bar CO pressure for 24 hours. Reaction carried out at 50 C.
catalyst loading is required in order to achieve excellent
conversion. Control experiments (entries 12–13) indicated that
cyclic carbonate formation only occurs when a nucleophile As can be seen in Figure 8, the catalyst system shows excellent
cocatalyst is added into the reaction mixture. Therefore, the catalytic performance towards all the epoxides tested
influence of the cocatalyst for this reaction catalysed by converting a broad range of terminal epoxides into their
complex 2 was investigated (Figure 7).
corresponding cyclic carbonates in good yields at ambient
temperature and pressure. In general, good to excellent
conversions and yields were obtained for all the non-
9
4
9
2
o
1
00
functionalised and functionalised epoxides used at 25 C and
2
one bar CO pressure (Figure 8). The conversion of epoxides
containing an alkyl chain (10b-e) increased when increasing the
chain length probably due to a higher solubility of the catalyst
7
1
80
60
40
20
0
6
1
4
1
and CO
conversions and yields were achieved when aromatic epoxides
i and 9j were used as substrates due to the solidification of the
2
in the epoxides. On the other hand, moderate
9
1
1
0
reaction mixture. To overcome this issue, the reaction
o
temperature was raised to 50 C, obtaining excellent yields
TBAF TBAC TBAB TBAI PPNCl PPNBr DMAP NMI
Cocatalyst
(Figure 8).
To further expand the substrate scope of the catalyst system,
the synthesis of a range of disubstituted cyclic carbonates from
their corresponding internal epoxides, which are far less
Fig. 7 Influence of the cocatalyst on the synthesis of styrene carbonate 10a catalysed by
mol% complex 2 and 5 mol% of cocatalyst at 25°C and one bar CO for 24 hours.
5
2
1
2,17
reactive than terminal epoxides,
was studied (Scheme 4).
It was found that the combination of complex 2 and a bromide
anion as a nucleophile source (tetrabutylammonium bromide
(TBAB) or bis(triphenylphosphine)iminium bromide (PPNBr)
formed a highly efficient catalyst system for the styrene
o
carbonate formation at 25 C and one bar carbon dioxide
4
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2
(15 mol%)
Bu4NBr (15 mol%)
090°C, 10 bar, 16h
O
O
O
View Article Online
DOI: 10.1039/C9DT00323A
O
O
+
CO2
O
O
O
O
O
O
p
+
n
+
5
R1
R2
R3
1a-e
R1
R2
2a-e
R3
1
1
2 (0-1 mol%) /
Cocatalyst (0-3 mol%)
50-90 oC, 10-40 bar, 24 h
O
12f
PCHC
PCHO
+
CO2
1
1, 12: a, R1-R2=(CH2)3, R3= H; b, R1-R2=CH3, R3=H; c, R1=H R2-R3=CH3;
O
d, R1=H, R2-R3=Ph; e, R1=H, R2=CH3, R3=Ph
O
O
O
o
m
Scheme 4 Reaction of internal epoxides and carbon dioxide.
11f
PCHC-PCHO
We first investigated the synthesis of cyclopentene carbonate
2
Scheme 5 ROCOP of cis-cyclohexene oxide 11f and CO catalysed by complex 2.
1
2a from cis-cyclopentene oxide 11a at room temperature and
one bar CO pressure using 5 mol% of complex 2 and TBAB as
2
Therefore, the effect of changing the complex 2:TBAB ratio,
cocatalyst, reaction temperature, reaction pressure and
reaction time was investigated (Table 2).
catalyst system, however only 2% conversion was obtained
under these reaction conditions. Therefore, the reaction
o
temperature and pressure were increased at 50 C and 10 bar
2
CO respectively and the reactions were carried out using 5
2
Table 2 ROCOP of cyclohexene oxide 11f and CO catalysed by complex 2.
mol% of catalyst system for 16 hours. Under these reaction
conditions, 100% conversion of cis-cyclopentene oxide 11a into
cis-cyclopentene carbonate 12a was obtained. Then, the effect
of reducing the catalyst loading was investigated and we were
delighted to find that this catalyst system is still highly active
when the reactions were carried out using 1 mol% of both
complex 2 and TBAB (Figure 9). Having determined the optimal
reaction conditions for the synthesis of cis-cyclopentene
carbonate 12a, the synthesis of a range of disubstituted cyclic
carbonates was studied and as can be seen in Figure 9, excellent
yields were obtained under the aforementioned reaction
conditions except when trans-stilbene oxide 11d was used as
2
Cocat.
(mol %)
T
P
Conv. PCHC PCHO
Entry
o
(%)a
96
100
17
97
95
86
79
99
99
97
99
92
90
85
55
21
(%)a
36
0
(%)a
63
100
0
(
mol %)
( C) (bar)
1
2
3
4
5
6
7
8
9
1
1
-
TBAB (1)
TBAB (0)
TBAB (1)
TBAB (0.5)
TBAB (0.1)
TBAB (2)
TBAB (3)
TBAB (1)
TBAB (1)
TBAB (1)
TBAB (1)
TBAB (1)
PPNCl (1)
TBAC (1)
DMAP (1)
NMI (1)
50
50
50
50
50
50
50
50
50
70
90
50
50
50
50
50
10
10
10
10
10
10
10
20
40
10
10
10
10
10
10
10
0b
1
1
1
1
1
1
1
1
1
1
1
1
1
10
0
0c
0d
32
35
33
29
27
43
33
90
100
51
24
68
65
67
71
73
57
67
69
0
1
1
1
1
1
1
1
0
1
2
3
4
5
6
o
substrate for which a temperature of 90 C was used due to the
solid nature of the epoxide. Under these reaction conditions,
cis-cyclopentene carbonate 12a was isolated in 88% yield
(Figure 9). Moreover, cis- and trans-cyclic carbonates 12b and
1
2c were obtained in excellent yields with a stereoselectivity
31
0b
higher than 98% in both cases (Figure 9). Sterically hindered
disubstituted trans-cyclic carbonates 12d and 12e were
obtained in high yields with retention of the stereochemistry
a
Determined by 1H-NMR. b100% selectivity towards cyclohexene carbonate 12f
c
was obtained. 49% selectivity towards cyclohexene carbonate 12f was obtained.
(Figure 9). These results indicated that cyclic carbonate
d
7
6% selectivity towards cyclohexene carbonate 12f was obtained.
formation catalysed by complex 2 and TBAB occurs with
retention of the epoxide stereochemistry.
It should be noted that complex 2 showed excellent catalytic
activity for the conversion of cis-cyclohexene oxide into
poly(cyclohexene oxide) (PCHO) in the absence of a cocatalyst
O
O
O
O
O
O
O
(Table 2, entry 2). As excepted, TBAB catalysed the conversion
O
O
O
O
O
O
O
O
of epoxide 11f into cis-cyclohexene carbonate 12f in the
absence of complex 2 (Table 2, entry 3). Then, the effect of
changing the complex 2:TBAB ratio on the catalytic activity was
investigated (Table 2, entries 4-7). As can be seen in Table 2,
decreasing the amount of TBAB from 1 to 0.5 or 0.1 mol%
resulted in a decrease of the poly(cyclohexene carbonate)
Me
Me
Me
Me
Ph
Ph
Ph
Me
2a: Conv: 93%a 12b: Conv: 88% 12c: Conv: 91% 12d: Conv: 100% 12e: Conv: 95%
a
a
b
a
1
Yield: 88%a
Yield: 79%a
Yield: 78%a
Yield: 93%b
Yield: 88%a
cis/trans: > 99
cis/trans: 98:2
trans/cis: > 99
trans/cis: > 99
trans/cis: > 99
Fig. 9 Synthesis of cyclic carbonates 12a–e from internal epoxides 11a–e and carbon
dioxide catalysed by a combination of 1 mol% of complex 2 and 1 mol% of Bu NBr at 50
4
oC and 10 bar CO
b
o
2
pressure for 16 hours. Reaction carried out at 90 C.
(
PCHC) selectivity from 36% to 10% and 0% respectively. On the
other hand, increasing the concentration of TBAB from 1 to 2 or
mol% increased the conversion of cis-epoxide 11f into
cis-cyclohexene carbonate 12f instead of PCHC (Table 2, entries
, 6-7).
An increase of the reaction pressure to 20 or 40 bar (Table 2,
On the other hand, when cis-epoxide 11f was used as the
substrate, no cyclic carbonate was observed but a polyether-
polycarbonate (PCHC-PCHO) was obtained, indicating that the
catalyst system can initiate the ring-opening copolymerization
3
1
2
(ROCOP) of CO and cis-cyclohexene oxide 11f under these
o
o
entries 1, 8-9) or reaction temperature from 50 C to 90 C did
not have a pronounced effect on the PCHC selectivity (Table 2,
entries 1, 10-11). The effect of the cocatalyst was also
investigated (Table 2, entries 1, 12-15). The study revealed that
reaction conditions (Scheme 5).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Page 6 of 9
ARTICLE
changing the TBAB to PPNCl was beneficial and a slight Synthesis of [Al
Journal Name
3
4
Me (µ-nbptam)] (1): In a 100 cm Schlenk tube,
View Article Online
2
DOI: 10.1039/C9DT00323A
improvement of PCHC selectivity was observed (Table 2, entries nbptamH (500 mg, 1.30 mmol) was dissolved in dry toluene (30
3
3
1
3
and 13). Other cocatalysts such as TBAC, DMAP and NMI were cm ). A solution of AlMe (2M in toluene) (1.30 cm , 2.60 mmol)
also investigated, however all of them showed a poorer was added and the mixture stirred overnight. The solution was
o
selectivity towards the synthesis of PCHC compared to TBAB or concentrated and recrystallized at 26 C to give compound 1
PPNCl (Table 2, entries 1, 12-16).
as orange crystals. Yield: 0.52 g, 80%. Anal. Calcd for
C
1
26
H
33Al
2
N
5
S: C, 62.3; H, 6.7; N, 14.0. Found: C, 62.5; H, 6.7; N,
1
o
3.9. H NMR (500 MHz, C
6 6
D
, 25 C),  = 8.15-7.14 (m, 7H,
Conclusions
4
4’
3
Naph), 5.25 (s, 1H, H ), 5.22 (s, 1H, H ), 2.01 (s, 3H, Me ), 1.98
3
’
5
(s, 3H, Me ), 1.40 (s, 3H, Me5’), 1.31 (s, 3H, Me ), 0.22, 0.18 (s,
In this work, we report the development of heteroscorpionate
helical aluminium complexes 1-8 supported by scorpionate
ligands with a polycyclic aromatic hydrocarbon group in the
acetamidate or thioacetamidate moiety. These complexes have
been characterised by spectroscopic methods and the
molecular structure of complex 1 has been confirmed by X-Ray
diffraction analysis.
Complexes 1-8 were screened as catalysts for the synthesis of
styrene carbonate from styrene oxide and carbon dioxide,
showing that complex 2 in combination with a bromide
cocatalyst was the most active catalyst system. This catalytic
system is able to convert a range of terminal epoxides into their
corresponding cyclic carbonates at room temperature and one
bar of carbon dioxide pressure under solvent-free conditions in
good yields and excellent selectivities. Furthermore, the
’
13
1
6
C
1
H, AlMe
3
o
), 0.00, 0.09 (s, 6H, AlMe
3
). C-{ H} NMR (125 MHz,
a
4,4’
6 6
D , 25 C), δ = 102.2 (C ), 107.4, 107.4 (C ), 148.8, 148.0,
47.4, 146.3, 141.2 (C , C , Cipso), 12.1, 10.9 (Me3,3’), 9.9, 9.8
3
,3’
5,5’
5
,5’
(
(
Me ), 158.3 (NCS), 129.8-123.6 (Naph), 7.1, 9.1, 10.0
AlCH ).
3
Synthesis of [Al
carried out in a manner identical to that described for 1, using
nbptamH (500 mg, 1.30 mmol) and AlEt
cm , 2.60 mmol). Yield: 0.60 g, 82%. Anal. Calcd for
2 4
Et (µ-nbptam)] (2): The synthesis of 2 was
3
(1M in hexane) (2.60
3
C
1
30
H41Al
2
N
5
S: C, 64.6; H, 7.4; N, 12.6. Found: C, 64.9; H, 7.5; N,
1
o
2.5. H NMR (500 MHz, C
6 6
D
, 25 C),  = 8.15-7.14 (m, 7H,
4
4’
3
Naph), 5.28 (s, 1H, H ), 5.19 (s, 1H, H ), 2.00 (s, 3H, Me ), 1.97
3
’
5’
5
(s, 3H, Me ), 1.44 (s, 3H, Me ), 1.37 (s, 3H, Me ), 1.59-0.69 (m,
1
3
1
1
2H, AlCH
2
CH
3
), 0.65-0.00 (m, 8H, ALCH
2
CH
3
). C-{ H} NMR
o
a
4,4’
6 6
(125 MHz, C D , 25 C), δ = 101.9 (C ), 107.5, 107.3 (C ), 149.7,
4
catalyst system comprised by 2/Bu NBr catalyses the synthesis
3
,3’
5,5’
ipso
3
1
48.2, 146.4, 146.1, 141.1 (C , C , C ), 12.5 (Me ), 12.6
of disubstituted cyclic carbonates from internal epoxides with
retention of the epoxide stereochemistry. When using
cyclohexene oxide as substrate, the catalyst system produces a
polyether–polycarbonate material which is highly dependent
on the cocatalyst and the catalyst to cocatalyst ratio used. The
reported catalysts system showed to be more active than the
previously reported helical aluminium complexes since the
conversion of terminal epoxides into their corresponding cyclic
3
’
5,5’
(
1
Me ), 10.1, 10.0 (Me ), 158.9 (NCS), 129.2-124.6 (Naph),
0.2-8.6 (AlCH CH ), 0.7-0.8 (AlCH CH ).
Synthesis of [Al Me (µ-fbpam)] (3): The synthesis of 3 was
carried out in a manner identical to that described for 1, using
2
3
2
3
2
4
fbpamH (530 mg, 1.30 mmol) and AlMe
cm , 2.60 mmol). Yield: 0.59 g, 87%. Anal. Calcd for
3
(2M in toluene) (1.30
3
C
29
H35Al
2
N
5
O: C, 66.5; H, 6.7; N, 13.4. Found: C, 67.1; H, 6.7; N,
1
o
o
13.3. H NMR (500 MHz, C
6 6
D
, 25 C),  = 7.55-3.99 (m, 7H, Ar),
carbonates can be carried out at 25 C and one bar carbon
dioxide pressure and the combination of complex 2/Bu NBr can
catalyse the formation of disubstituted cyclic carbonates from
4
,4’
5
.20, 5.17 (s, 2H, H ), 3.48 (m, 2H, CH
2
), 1.97, 1.90 (s, 6H,
4
3
,3’
5,5’
Me ), 1.36, 1.27 (s, 6H, Me ), 0.14, 0.03, 0.29, 0.38 (s,
1
3
1
o
a
8
12H, AlMe
06.7, 106.4 (C ), 149.1-138.1 (C , C , C ), 12.4, 12.3
Me3,3’), 9.9, 9.7 (Me5,5’), 159.1 (NCO), 129.9-120.2 (Ar), 36.7
(CH ), 9.2, 9.1, 10.1 (AlCH ).
Synthesis of [Al Et (µ-fbpam)] (4): The synthesis of 4 was
carried out in a manner identical to that described for 3, using
3 6 6
). C-{ H} NMR (125 MHz, C D , 25 C), δ = 92.3 (C ),
internal epoxides and carbon dioxide.
4
,4’
3,3’
5,5’
ipso
1
(
Experimental Part
2
3
2
4
All manipulations were performed under nitrogen, using
standard Schlenk techniques. Solvents were pre-dried over
sodium wire (toluene, n-hexane) and distilled under nitrogen
from sodium (toluene) or sodium-potassium alloy (n-hexane).
Deuterated solvents were stored over activated 4 Å molecular
fbpamH (530 mg, 1.30 mmol) and AlEt
cm , 2.60 mmol). Yield: 0.69 g, 91%. Anal. Calcd for
3
(1M in hexane) (2.60
3
C
1
5
33
H43Al
2
N
5
O: C, 68.4; H, 7.5; N, 12.1. Found: C, 69.0; H, 7.7; N,
1
o
2.1. H NMR (500 MHz, C
.20 (s, 1H, H ), 5.15 (s, 1H, H ), 3.48 (m, 2H, CH
6 6
D
, 25 C),  = 7.56-6.90 (m, 7H, Ar),
1
13
sieves and degassed by several freeze-thaw cycles. H and
C
4
4’
2
), 1.98 (s, 3H,
NMR spectra were recorded on a Varian Inova FT-500
spectrometer and are referenced to the residual deuterated
solvent. Two-dimensional NMR spectra were acquired using
standard VARIAN-FT software and were processed using an IPC-
Sun computer. AlMe and AlEt were purchased from Aldrich.
3 3
nbptamH and fbpamH were prepared as previously reported.
3
3’
5
5’
Me ), 1.88 (s, 3H, Me ), 1.28 (s, 3H, Me ), 1.33 (s, 3H, Me ),
1
3
1
.50-0.90 (m, 12H, AlCH
2
CH
3 2 3
), 0.70-0.10 (m, 8H, AlCH CH ). C-
1
o
a
{
(
(
(
H} NMR (125 MHz, C D , 25 C), δ = 92.5 (C ), 106.8, 106.4
6 6
C4,4’), 148.7-137.4 (C , C , C ), 12.5, 12.4 (Me ), 10.0, 9.7
3,3’
5,5’
ipso
3,3’
5
,5’
Me ), 159.4 (NCO), 129.2-119.6 (Ar), 36.8 (CH
AlCH CH ), 0.7-0.8 (AlCH CH ).
Me (µ -nbptam)] (5): The synthesis of 5 was
carried out in a manner identical to that described for 1, using
2
), 9.6-6.8
1
8
o
2
3
2
3
The specific rotation []
Elmer 241 Polarimeter equipped with a Na lamp operating at
D
was measured at 22 C on a Perkin-
Synthesis of [Al
3
7
3
5
89 nm with a light path length of 10 cm.
nbptamH (500 mg, 1.30 mmol) and AlMe
cm , 4.0 mmol). Yield: 0.65 g, 89%. Anal. Calcd for C29
3
(2M in toluene) (2.0
S:
3
H
42Al
3
N
5
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Page 7 of 9
Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Journal Name
ARTICLE
1
C, 60.7; H, 7.4; N, 12.2. Found: C, 60.8; H, 7.5; N, 12.0. H NMR Crystal structure was solved by direct methods using the SIR97
View Article Online
DOI: 10.1039/C9DT00323A
2
o
21
(
500 MHz, C
D
6 6
, 25 C),  = 8.00-7.31 (m, 7H, Naph), 5.24 (s, 1H, program and refined by full-matrix least-squares on F
H ), 5.17 (s, 1H, H ), 1.98 (s, 3H, Me ), 1.89 (s, 3H, Me ), 1.39 (s, including all reflections using anisotropic displacement
H, Me ), 1.22 (s, 3H, Me ), 0.00, 0.12, 0.22, 0.29 (s, 12H, parameters by means of the WINGX crystallographic package.
4
4’
3
3’
5
5’
22
3
AlCH
o
1
3
1
3
), 0.41 (s, 9H, Al(CH
3
)
3
). C-{ H} NMR (125 MHz, C
D
6 6
, 25 Generally, anisotropic temperature factors were assigned to all
a
4,4’
C), δ = 105.4 (C ), 107.9, 107.5 (C ), 150.1, 148.7, 148.0, 146.6, atoms except for hydrogen atoms, which are riding their parent
42.1 (C3 , C , C ), 12.7 (Me ), 13.1 (Me ), 9.9, 9.7 (Me ), atoms with an isotropic temperature factor arbitrarily chosen as
,3’
5,5’
ipso
3
3’
5,5’
1
1
2
51.2 (NCS), 128.8-124.4 (Naph), 7.8, 8.8, 10.1 (AlCH
).
Synthesis of [Al
carried out in a manner identical to that described for 1, using angles are given in Table S2 (see the Supporting Information).
3
), 6.9 1.2 times that of the respective parent. Final R(F), wR(F ) and
(Al(CH
)
3 3
goodness of fit agreement factors, details on the data collection
3
7 3
Et (µ -nbptam)] (6): The synthesis of 6 was and analysis can be found in Table 3. Selected bond lengths and
nbptamH (500 mg, 1.30 mmol) and AlEt
cm , 4.0 mmol). Yield: 0.75 g, 86%. Anal. Calcd for C36
C, 64.4; H, 8.4 N, 10.4. Found: C, 65.0; H, 8.6 N, 10.2. H NMR Cambridge
3
(1M in hexane) (4.0 CCDC 1884536 contain the supplementary crystallographic data
3
H
56Al
3
N
5
S: for 1. These data can be obtained free of charge from The
1
Crystallographic
Data
Centre
via
o
(
500 MHz, C
6
D
6
, 25 C),  = 8.02-7.31 (m, 7H, Naph), 5.26 (s, 1H, www.ccdc.cam.ac.uk/data_request/cif.
4
4’
3
H ), 5.18 (s, 1H, H ), 1.98 (s, 3H, Me ), 1.93 (s, 3H, Me3’), 1.37
s, 3H, Me ), 1.27 (s, 3H, Me5’), 1.59-0.91 (m, 12H, AlCH
Table 3 Crystallographic data and structure refinement details for all compound 1
5
(
2
CH
3
),
0
6
.82-0.33 (m, 8H, AlCH
2
1
CH
3
), 1.30 (m, 9H, Al(CH
2
CH
3
o
)
3
), 0.14 (m,
Compound
Formula
M
1
1
3
H, Al(CH
a
2
CH
3 3
)
). C-{ H} NMR (125 MHz, C
6 6
D , 25 C), δ = 105.5
C H Al N S
4
0
49
2
5
4,4’
3,3’
(C ), 107.9, 107.5 (C ), 150.6, 149.2, 148.5, 146.2, 141.9 (C
,
685.86
1884536
Monoclinic
1
P2 /c
100
16.119(5)
24.785(5)
21.326(4)
90
115.151(16)
90
7712(3)
8
5
,5’ ipso
3
3’
5,5’
C
1
, C ), 13.0 (Me ), 13.4 (Me ), 10.0, 9.8 (Me ), 151.4 (NCS),
29.1-122.7 (Naph), 11.2-8.5 (AlCH CH ), 2.1-0.1 (AlCH CH ), 9.9
CH ), 1.9 (Al(CH CH ).
Synthesis of [Al Me (µ -fbpam)] (7): The synthesis of 7 was
carried out in a manner identical to that described for 3, using
CCDC
2
3
2
3
Crystal System
Space group
T [K]
(Al(CH
2
)
3 3
2
)
3 3
3
7
3
a [Å]
b [Å]
c [Å]
α [deg]
β [deg]
γ [deg]
V [Å3]
fbpamH (530 mg, 1.30 mmol) and AlMe
cm , 4.0 mmol). Yield: 0.66 g, 85%. Anal. Calcd for C32
C, 64.5; H, 7.5; N, 11.8. Found: C, 65.0; H, 7.7; N, 11.6. H NMR
3
(2M in toluene) (2.0
3
H
44Al
3 5
N O:
1
o
(500 MHz, C
6 6
D
, 25 C),  = 7.81-7.19 (m, 7H, Ar), 5.34 (s, 1H,
4
4’
3
H ), 5.30 (s, 1H, H ), 3.63 (m, 2H, CH
3
2
), 2.10 (s, 3H, Me ), 2.01 (s,
3
’
5
5’
H, Me ), 1.34 (s, 3H, Me ), 1.29 (s, 3H, Me ), 0.00, -0.12, -0.23,
Z
1
3
1
-0.39 (s, 12H, AlCH
3
), -0.37 (s, 9H, Al(CH
, 25 C), δ = 96.0 (C ), 108.7, 108.5 (C ), 150.5-141.3
, C , C ), 12.8, 12.5 (Me3,3’), 10.8, 10.7 (Me5,5’), 154.7
3 3
)
). C-{ H} NMR (125
-
3
Density [gcm ]
1.181
0.164
39723
0.0636
o
a
4,4’
MHz, C
6
5,5’
D
6
-1
μ [mm ]
3
,3’
ipso
(
(
(
C
Observed reflections
NCO), 128.2-121.2 (Ar), 37.5 (CH
Al(CH ).
2 3
), -4.1, -5.6, -9.2 (AlCH ), -6.8
R
int
3 3
)
b
c
R1 / wR2 [I>2σ(I)]
R1 b / wR2 (all data)
GoF
0.0842 / 0.2258
0.1092 / 0.2461
1.106
3 7 3
Synthesis of [Al Et (µ -fbpam)] (8): The synthesis of 8 was
carried out in a manner identical to that described for 3, using
fbpamH (530 mg, 1.30 mmol) and AlEt
4
c
3
3
(1M in hexane) (4.0 cm ,
-3
Largest diff. pk and hole [eÅ ]
0.995 and -0.554
.0 mmol). Yield: 0.76 g, 84%. Anal. Calcd for C39
H
1
58Al
3 5
N O: C,
[a] S = [Σw(F0
2
– Fc
[c] wR = [Σw(F
2
2
)
2
/ (Nobs – Nparam)]1/2 [b] R = Σ||F |–|F || / Σ|F |
2
1 0 c 0
2
2
2
2 1/2
6
7.5; H, 8.4; N, 10.1. Found: C, 67.7; H, 8.4; N, 10.0. H NMR (500
0
– F
c 0
) / ΣwF ]
2
2
2,0) + 2F 2)/3
o
4
w = 1/[σ (F
0
) + (aP) + bP] where P = (max(F
0 c
MHz, C
6
D
6
, 25 C),  = 7.67-7.12 (m, 7H, Ar), 5.33 (s, 1H, H ), 5.30
4
’
3
(s, 1H, H ), 3.62 (m, 2H, CH
2
), 2.08 (s, 3H, Me ), 1.99 (s, 3H,
General procedure for catalyst screening at 1 bar pressure
Styrene oxide 9a (1.7 mmol), aluminium complex 1-8 (85.0
µmol) and Bu NBr (85.0 µmol) were placed in a sample vial
fitted with a magnetic stirrer bar and placed in a large conical
flask. Cardice pellets were added to the conical flask which was
fitted with a rubber stopper pierced by a deflated balloon. The
reaction mixture was stirred at 25 C for 24 h and the conversion
of styrene oxide 9a into styrene carbonate 10a was determined
by H NMR spectroscopy after 3, 6 and 24 h.
General procedure for synthesis of cyclic carbonates at 1 bar
pressure
3
’
5
5’
Me ), 1.34 (s, 3H, Me ), 1.26 (s, 3H, Me ), 1.63-0.80 (m, 12H,
AlCH
Al(CH
MHz, C
2
CH
3
), 0.78-0.00 (m, 8H, AlCH
), 0.49 (brs, 6H, Al(CH CH
2
CH
3
), 1.24 (brs, 9H,
)
3
2
3
)
3
). 13C-{ H} NMR (125
1
4
2
CH
3
o
a
4,4’
6
5,5’
D
6
, 25 C), δ = 95.8 (C ), 108.8, 108.3 (C ), 151.4-139.1
3
,3’
, C , C ), 13.4, 13.3 (Me3,3’), 10.5, 10.4 (Me5,5’), 154.5
ipso
(
(
(
C
NCO), 127.6-120.7 (Ar), 37.6 (CH
AlCH CH ), 11.0 (Al(CH CH ), 2.0 (Al(CH
2
), 10.8-9.2 (AlCH
2 3
CH ), 2.4-0.0
o
2
3
2
)
3 3
2 3 3
CH ) ).
X-ray crystallographic structure determination. Colourless
crystals for 1 were obtained by diffusion of toluene/hexane.
Prismatic crystal was mounted on a glass fibre and used for data
collection on a Bruker D8 Venture with Photon detector
equipped with graphite monochromated MoKα radiation
1
An epoxide 9a–j (1.7 mmol), aluminium complex 2 (85.0 µmol)
and Bu NBr (85.0 µmol) were placed in a sample vial fitted with
4
a magnetic stirrer bar and placed in a large conical flask. Cardice
(λ=0.71073 Å). The data reduction were performed with the
1
9
20
APEX2 software and corrected for absorption using SADABS.
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Deibe, Cryst. Growth Des. 2015, 15, 4318; M. AlbVireewcAhrtti,clCe hOenlmine.
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pellets were added to the conical flask which was fitted with a
rubber stopper pierced by a deflated balloon. The reaction
mixture was stirred at 25 C for 24 h and the conversion of
Rev., 2001, 101, 3457
3
4
o
2
012, 14, 561.
epoxide 9a–j into cyclic carbonate epoxide 10a–j was
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1
determined by H NMR spectroscopy after 24 h. The remaining
sample was purified by flash chromatography using a solvent
system of first hexane, then hexane:EtOAc (9:1), then
hexane:EtOAc (3:1), then EtOAc to give the pure cyclic
carbonate product.
General procedure for synthesis of cyclic carbonates at 10 bar
pressure
5
6
J. A. Castro-Osma, C. Alonso-Moreno, M. V. Gómez, I.
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An epoxide 11a–e (1.7 mmol) aluminium complex 2 (17.0 µmol)
and Bu NBr (17.0 µmol) were placed in a stainless-steel reactor
4
with a magnetic stirrer bar. The autoclave was sealed,
o
pressurised to 5 bar with CO
2
, heated to 50-90 C and then
pressurised to 10 bar with CO
2
. The reaction mixture was
subsequently stirred at 50–90 °C for 16 h. The conversion of
epoxide to cyclic carbonate was then determined by analysis of
1
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a sample by H NMR spectroscopy. The remaining sample was
purified by flash chromatography using a solvent system of first
hexane, then hexane:EtOAc (9:1), then hexane:EtOAc (3:1),
then EtOAc to give the pure cyclic carbonate product.
General procedure for synthesis of cyclic carbonates at 10 bar
pressure
Cyclohexene oxide 11f (0.98 g, 10.0 mmol), complex 2 (0-17.0
µmol) and a cocatalyst (0-51.0 µmol) were placed in a stainless-
steel reactor with a magnetic stirrer bar. The autoclave was
7
L. Wang, C. Xu, Q. Han, X. Tang, P. Zhou, R. Zhang, G. Gao, B.
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sealed, pressurised to 5 bar with CO
temperature and then pressurised to 10-40 bar with CO
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, heated to the desired
. The
Ju and W. Liu, Chem. Eur. J., 2017, 23, 9804; O. Gidron, M.
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2
reaction mixture was subsequently stirred at 50–90 °C for 16 h.
The conversion of cyclohexene oxide into polycarbonate,
polyether-polycarbonate or cyclic carbonate was determined
1
by analysis of a sample by H NMR spectroscopy.
Conflicts of interest
There are no conflicts to declare.
8
9
J. A. Castro-Osma, A. Lara-Sánchez, M. North, A. Otero and P.
Villuendas, Catal. Sci. Technol., 2012, 2, 1021; J. A. Castro-
Osma, C. Alonso-Moreno, A. Lara-Sánchez, J. Martínez, M.
North and A. Otero, Catal. Sci. Technol., 2014, 4, 1674.
J. Martínez, M. Martínez de Sarasa Buchaca, F. de la Cruz-
Martínez, C. Alonso-Moreno, L. F. Sánchez-Barba, J.
Fernandez-Baeza, A. M. Rodríguez, A. Rodríguez-Diéguez, J. A.
Castro-Osma, A. Otero and A. Lara-Sánchez, Dalton Trans.,
Acknowledgements
We gratefully acknowledge financial support from the
Ministerio de Economía y Competitividad (MINECO), Spain
2
018, 47, 7471; J. A. Castro-Osma, A. Earlam, A. Lara-Sánchez,
(Grant Nos. CTQ2017-84131-R and CTQ2016-81797-REDC
A. Otero and M. North, ChemCatChem, 2016, 8, 2100; M.
Martínez de Sarasa Buchaca, F. de la Cruz-Martínez, J.
Martínez, C. Alonso-Moreno, J. Fernández-Baeza, J. Tejeda, E.
Niza, J. A. Castro-Osma, A. Otero, A. Lara-Sánchez, ACS
Omega, 2018, 3, 17581.
Programa Redes Consolider). Miguel A. Gaona acknowledges
the Universidad de Castilla-La Mancha (UCLM) for the PhD
Fellowship. Felipe de la Cruz-Martínez acknowledges the
Ministerio de Educación, Cultura y Deporte (MECD) for the FPU
Fellowship.
1
0 For recent reviews, see: P. Styring, A. Quadrelli and K.
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