An Amphiphilic (salen)Co Complex – Utilizing Hydrophobic Interactions to Enhance the Efficiency of a Cooperative Catalyst

Article abstract of DOI:10.1002/adsc.202100494

An amphiphilic (salen)Co(III) complex is presented that accelerates the hydrolytic kinetic resolution (HKR) of epoxides almost 10 times faster than catalysts from commercially available sources. This was achieved by introducing hydrophobic chains that increase the rate of reaction in one of two ways – by enhancing cooperativity under homogeneous conditions, and increasing the interfacial area under biphasic reaction conditions. While numerous strategies have been employed to increase the efficiency of cooperative catalysts, the utilization of hydrophobic interactions is scarce. With the recent upsurge in green chemistry methods that conduct reactions ‘on water’ and at the oil-water interface, the introduction of hydrophobic interactions has potential to become a general strategy for enhancing the catalytic efficiency of cooperative catalytic systems. (Figure presented.).

Full text of DOI:10.1002/adsc.202100494

Title: An Amphiphilic (salen)Co Complex – Utilizing Hydrophobic
Interactions to Enhance the Efficiency of a Cooperative Catalyst
Authors: Pablo Solís-Muñana, Joanne Salam, Chloe Z.-J. Ren, Bronte
Carr, Andrew E. Whitten, Gregory G. Warr, and Jack L. Y.
Chen
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202100494
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10.1002/adsc.202100494
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.202100494
An Amphiphilic (salen)Co Complex – Utilizing Hydrophobic
Interactions to Enhance the Efficiency of a Cooperative
Catalyst
Pablo Solís-Muñana,a,b Joanne Salam,a,b Chloe Z.-J. Rena,b, Bronte Carra, Andrew E. Whitten,c
Gregory G. Warrd and Jack L.-Y. Chena,b,e,
*
a
Centre for Biomedical and Chemical Sciences, School of Science
Auckland University of Technology, 34 St Paul St, Auckland 1010, New Zealand
E-mail: jack.chen@aut.ac.nz
b
c
d
e
The MacDiarmid Institute for Advanced Materials and Nanotechnology
Wellington 6011, New Zealand
Australian Nuclear Science and Technology Organisation (ANSTO)
New Illawarra Rd, Lucas Heights, NSW 2234, Australia
School of Chemistry, The University of Sydney,
Sydney, New South Wales 2006, Australia
Department of Biotechnology, Chemistry and Pharmaceutical Sciences
Università degli Studi di Siena, Via Aldo Moro, 53100 Siena, Italy
Received: 22nd April 2021
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.20######.
Abstract. An amphiphilic (salen)Co(III) complex is
presented that accelerates the hydrolytic kinetic
resolution (HKR) of epoxides almost 10 times faster than
catalysts from commercially available sources. This was
achieved by introducing hydrophobic chains that
increase the rate of reaction in one of two ways – by
enhancing cooperativity under homogeneous conditions,
and increasing the interfacial area under biphasic
reaction conditions. While numerous strategies have
been employed to increase the efficiency of cooperative
catalysts, the utilization of hydrophobic interactions is
scarce. With the recent upsurge in green chemistry
methods that conduct reactions ‘on water’ and at the oil-
water interface, the introduction of hydrophobic
interactions has potential to become a general strategy
for enhancing the catalytic efficiency of cooperative
catalytic systems.
Keywords: Cooperative catalysis; self-assembly;
preorganization; hydrolytic kinetic resolution;
amphiphiles
Figure 1. Schematic representations of (a) previous
strategies used to enhance the activity of cooperative
catalysts and (b) utilization of hydrophobic interactions to
enhance cooperativity in a self-assembled catalyst.
Nature commonly exploits the pre-organization of
functional groups to create highly effective catalytic
systems.[1] This allows proximal functional groups to
work cooperatively, achieving rate accelerations much
greater than the sum of each of the groups acting on
their own.[2] This emergent property has drawn the
attention of chemists, and numerous cooperative
catalytic systems have been reported over the last
decades.[3] One example that stands out for its utility in
both academic and industrial settings is the hydrolytic
kinetic resolution (HKR) of terminal epoxides
using(salen)Co complexes, first reported by Jacobsen
and co-workers.[4] This reaction is an economically
viable method for obtaining optically pure terminal
epoxides and 1,2-diols from racemic mixtures of
terminal epoxides.[5] The cooperative system requires
two Co(III) ions in the rate-determining-step, one
acting as a Lewis acid to increase the electrophilicity
of the epoxide whilst the other stabilizes the
nucleophilic hydroxide anion.[6] The reaction can be
conducted under neat conditions using only water as a
1
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10.1002/adsc.202100494
Advanced Synthesis & Catalysis
reagent, or with addition of a water miscible solvent to
generate a homogeneous system.
The catalytic activity of (salen)Co complex 4 was
first investigated in the HKR of epichlorohydrin under
solvent-free conditions (Figure 2). Water acts as a
reagent in this reaction and was added to the epoxide
substrate and complex 4, forming a biphasic mixture.
The mixture was stirred vigorously at room
temperature and aliquots were taken at specific time
intervals and analyzed by chiral GC to determine the
reaction conversion.[4a] Under these neat conditions,
complete chiral resolution (>99% e.e.) of
epichlorohydrin was achieved within 70 min using
0.10 mol% of complex 4, whereas 37% e.e. was
Cooperative catalysts require the participation of
two or more functional groups in the rate-determining
step, and thus benefit from pre-organization that brings
the required functionality into proximity. This can be
achieved by direct covalent linkage of the requisite
groups onto a molecular scaffold[7] or by using more
advanced strategies such as the incorporation of
catalytic units into oligomers,[8] dendrimers,[9] the
surfaces of particles and polymers[10] or by hydrogen
bonded self-assembly.[11] All these strategies have
been successfully utilized to generate catalytic systems
with higher efficiencies than their monomeric
counterparts. Yet, one strategy seldom considered for
the pre-organization of cooperative catalysts is the use
of hydrophobic interactions. The lack of examples in
the literature is likely due to the fact that the majority
of organic reactions are performed in organic solvents,
where hydrophobic interactions are often disregarded.
However, the recent drive for green synthetic methods
can be expected to give greater emphasis on the
utilization of hydrophobic interactions, with an
evident shift towards performing reactions in benign
solvents such as water,[12] on-water reactions,[13] and
an increasing interest in reactions that occur at the
water-oil interface.[14]
We recently performed conceptual studies to
demonstrate that self-assembly driven by hydrophobic
interactions is an effective strategy to improve the
efficiency of cooperative catalysts in aqueous and
semi-aqueous systems.[15,16] In addition to being more
synthetically accessible than catalysts attached to
molecular scaffolds, dendrimers or nanoparticles, the
modularity of the system allowed facile optimization
of the catalyst system when two different catalytically
active building blocks were used.[17] The weak and
reversible nature of hydrophobic interactions also
allowed design of systems where control of the self-
assembly process can lead to stimuli-responsive and
switchable catalytic properties.[15a]
Scheme 1. Jacobsen’s (salen)Co complex 1 and modified
(salen)Co complexes 2-4 with incorporation of sidechains
with differing lengths.
While noteworthy as proof-of-concept studies, the
above examples did not catalyze a practically useful
reaction. To demonstrate that introduction of
hydrophobic interactions can be a general method for
increasing the efficiency of cooperative catalysts, we
embarked on demonstrating this effect on the
(salen)Co complexes used in the hydrolytic kinetic
resolution (HKR) of terminal epoxides (Scheme 1).[4]
We were drawn to this reaction as it is a widely
employed process with a well-established cooperative
mechanism.[6]
Amphiphilic analogues of (salen)Co complex (1)
were made by replacing the two tert-butyl groups with
fatty acid esters (Scheme 2). Synthesis of the salen
ligands was achieved in five steps starting from 2-tert-
butylhydroquinone, using a synthetic route adapted
from one previously used to access an oligomeric
(salen)Co complex.[8a] By changing the fatty acid
employed in the fourth step of this sequence, three
amphiphilic analogues (2-4) were made containing
different degrees of hydrophobicity.
Scheme 2. Synthesis of amphiphilic (salen)Co(III)
complexes 2-4. i) DMAP (0.15 equiv.), imidazole (2 equiv.),
pivaloyl chloride (1.2 equiv.), CH2Cl2, quantitative; ii) 2,6-
lutidine (2 equiv.), SnCl4, (0.5 equiv.), paraformaldehyde (9
equiv.), toluene, 89%; iii) KOH (10 equiv.), EtOH/H2O
(1:1), 76%; iv) carboxylic acid (1 equiv.), DMAP (0.2
equiv.), EDC (1.2 equiv.), DMF/CH2Cl2 (1:5), n = 6, 82%;
n = 10, 52%; n = 16, 88%; v) (1R,2R)-(-)-1,2-
diaminocyclohexane (0.5 equiv.), K2CO3 (2 equiv.),
H2O/THF (1:4), n = 6, 43%; n = 10, 65%; n = 16, 67%; vi)
Co(OAc)2·4H2O (1 equiv.) MeOH/toluene (1:1), n = 6,
50%; n = 10, 36%; n = 16, 95%; vii) p-PTSA.H2O (1 equiv.)
MeOH/toluene/CH2Cl2 (10:3:10), n = 6, 62%; n = 10, 68%;
n = 16, 64%.
2
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10.1002/adsc.202100494
Advanced Synthesis & Catalysis
Figure 3. a) Initial rates of reaction for the HKR of
epichlorohydrin at variable catalyst loadings under
homogenous conditions. Red: complex 4. Blue: complex 1;
b) Initial rates of hydrolytic kinetic resolution of
epichlorohydrin in THF vs number of carbons in the salen
sidechain, catalyst loading 0.1 mol%. Blue = catalyst 1;
green = complex 2; yellow = complex 3; red = complex 4.
Reaction conditions for both a and b: epichlorohydrin (1
equiv., 1 volume), THF (1 volume), H2O (0.6 equiv.), rt, 400
rpm.
Figure 2. Hydrolytic kinetic resolution of epichlorohydrin
utilizing complexes 1 to 4 under solvent free conditions.
[Catalyst] = 0.10 mol%, epichlorohydrin (1 equiv.), H2O
(0.6 equiv.).
observed with complex 1 under the same conditions
(Figure 2). Analysis of the initial rates revealed an
almost ten-fold rate difference between complex 1 (kobs
= 0.86 h-1) and complex 4 (kobs = 8.2 h-1), with the kobs
determined from plots of -ln([epoxide]/[epoxide]0)
versus time (see SI). A positive correlation between
the reaction rate and the length of the added sidechain
was also observed (2.4 h-1 for complex 2 and 6.5 h-1 for
complex 3).
constant K2, describing the equilibrium between the
dimeric and monomeric forms of the (salen)Co(III)
complex.
2Co(III)monomeric
Co(III)dimeric
(1)
(2)
(3)
(4)
K2 = [Co(III)dimeric]/[Co(III)monomeric]2
rate = k[Co(III)dimeric]
To provide insight into the origin of this rate
acceleration, we measured the kinetic profile of
complexes 4 and 1 under different reactions conditions.
The experiments were first conducted under
homogenous conditions, to allow direct comparison to
prior studies demonstrating the cooperativity of these
catalysts in the HKR (Figure 3a).[6,7b,11a,18,19] This
required addition of THF to the reaction mixture to
form a fully miscible reaction mixture. The plot of the
kobs of the reaction against [catalyst]2 confirmed the
second-order dependence of the rate on catalyst
rate = kK2[Co(III)monomeric]2
[Co(III)total] = 2[Co(III)dimeric] + [Co(III)monomeric] (5)
If Co(III)dimeric << [Co(III)monomeric]
Then rate = kK2 ([Co(III)total]-2[Co(III)dimeric])2
≈ kK2[Co(III)total]2
(6)
concentration, confirming that
a
cooperative
mechanism is operative with complex 4. The higher
activity of complex 4 can be seen by the much steeper
curve compared to complex 1 and the origin of this
increase can be rationalized in a similar way as to a
treatment by Hong.[11a] Firstly, we can assume that
formation of the active catalyst results from
dimerization of monomeric Co(III) complexes
(equation 1). The rate of the reaction is proportional to
the dimeric complex (equation 3) and can be expressed
in terms of the concentration of the monomer
(equation 4). A second assumption is that the
concentration of the active dimeric species is much
lower than the concentration of the monomeric catalyst
(and hence the observation of second order
dependence typical for non-covalently-linked,
cooperative systems).[20] This allows simplification of
the expression to be directly proportional to the total
Co(III) complex concentration (equation 6). This
equation shows that there is a direct relationship
between the rate of the reaction and the equilibrium
If hydrophobic interactions were in fact an
important factor in the formation of this cooperative
catalyst, changes in the length of the hydrophobic
chains should result in changes in the catalytic activity.
To investigate this hypothesis, we examined the initial
rates of (salen)Co complexes 2 and 3. A positive linear
relationship was found between the initial rates of the
HKR reaction, and the number of carbon atoms present
in the hydrocarbon sidechain of the (salen)Co complex
(Figure 3b). This trend also rules out changes in
activity due to electronic effects, which have been
previously observed in other analogues.[11a] Small
angle neutron scattering (SANS) experiments also
excluded the formation of micellar aggregates with
amphiphilic complex 4 (see SI, section 5),[21] which is
in line with the observed second-order rate dependence
on catalyst concentration.
To estimate the relative difference in magnitude
between the dimerization constants of complexes 4
and 1, the corresponding Ni(II) analogues 5 and 6 were
3
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10.1002/adsc.202100494
Advanced Synthesis & Catalysis
1
synthesized (Table 1).[22] The H NMR spectra of
(salen)Ni(II) complexes 5 and 6 were measured at
different concentrations in a mixture of THF-d8 and
D2O to simulate the reaction conditions (see SI).
Changes in the chemical shift of the aromatic protons
were used to estimate their association constants.[23]
HKR substrate were examined using dynamic light
scattering (DLS). The size of the water in oil (styrene
oxide) emulsions in the absence of catalyst was
observed to be 2-3 µm. The same mixture in the
presence of complex 1 (0.02 mol%) showed droplet
sizes of ~1 µm, suggesting that complex 1 has some
effect in stabilizing the interface, resulting in a greater
number of droplets of smaller size. The introduction of
complex 4 resulted in a more pronounced effect,
decreasing the size of the emulsions to the range of
400-600 nm. These results suggest that under neat
conditions, complex 4 increases the rate of the reaction
by stabilization of nanoemulsions, thereby increasing
the interfacial area between the reagent water and the
epoxide substrate.
1
Non-linear least squares fitting of the H NMR data
into the monomer-dimer (MD) model estimated a
dimerization constant of 1.43 ± 0.75 for complex 6 and
0.36 ± 0.75 for complex 5. Alternatively, fitting into
the equal K (EK) model estimated an association
constant of 2.82 ± 2.19 for complex 6 and 0.68 ± 0.57
for complex 5. Both models estimated the
dimerization/association constant of functionalized
complex 6 to be approximately 4 times that of complex
5, which is consistent with the 5 times difference in
rate observed between the two complexes (see Figure
3a).
Figure 4. a) Initial rates of the HKR of epichlorohydrin (h-
1) vs [catalyst] with variable catalyst loadings under neat
(biphasic)
conditions.
Experimental
conditions:
epichlorohydrin (1 equiv.), H2O (0.6 equiv.), rt, stirring at
400 rpm, red = complex 4, blue = complex 1; b) Diameter
of water droplets measured by dynamic light scattering.
Experimental conditions: styrene oxide = 375 µL, milli-Q
water = 36 µL, [catalyst] = 0.02 mol%. Grey line = no
catalyst, blue line = complex 1, red line = complex 4. The
samples were stirred for 2 minutes and left to rest for 10
minutes.
Table 1. Dimerization and association constants obtained
by fitting chemical shifts in the H NMR of compounds 5
and 6 into the monomer-dimer (MD) model and the equal K
(EK) isodesmic model.
1
While (salen)Co(III) complex 4 yielded faster rates
than control complex 1 in the kinetic studies under
homogeneous conditions, this rate enhancement (~5
times) was significantly lower than what was observed
under neat, biphasic conditions (~10 times). We thus
proceeded to examine the kinetics of the reaction
under the biphasic conditions in which the HKR
reaction is commonly utilized, and the initial rates of
reaction were measured for a range of loadings of
complexes 1 and 4 (Figure 4a). It became apparent that
under these biphasic conditions, complex 4 became
increasingly more effective than 1 as the concentration
is increased (e.g. at 0.06 mol% catalyst loading,
complex 4 is 5 times more effective than complex 1,
whereas at 0.10 mol%, complex 4 is 9.5 times more
effective than complex 1). Note that the ratio of
reaction rates between complexes 4 and 1 was constant
at different catalyst concentrations when performed
under homogenous conditions.
It is worth noting that for most previously described
strategies for enhancing cooperative catalysis
(covalent linkage, immobilization onto nanoparticles),
the difference in catalytic activity between the newly
designed catalyst and monomeric complex 1 can be
expected to decrease at higher concentrations, as
described by Kleij.[7b] This is because the advantage
gained by preorganization is negated at higher
concentrations due to the second order dependence of
rate on catalyst concentration. In these examples, the
kinetics can be described by a two-term equation
involving both intramolecular and intermolecular
components (equation 7).[7c]
Rate = kintra [catalyst] + kinter [catalyst]2
(7)
Taking for example two (salen)Co(III) complexes
joined covalently onto a molecular scaffold – the
intramolecular component refers to catalysis by Co
sites on the same molecule, while the intermolecular
component refers to the reaction catalyzed by Co sites
As we were working in biphasic conditions, we
rationalized that a possible reason for an increasingly
more effective complex 4 at higher concentrations may
be due to more efficient emulsion stabilization.
Reaction mixtures containing styrene oxide as the
4
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10.1002/adsc.202100494
Advanced Synthesis & Catalysis
on different molecules. At low concentrations, the kintra
term is dominant due to preorganization of the Co sites
within the same molecule which enhances
cooperativity.
However,
at
high
catalyst
concentrations, the kintra term will not always
compensate for a decrease in the kinter term due to a
halving in the total number of discrete catalysts
because of preorganization. Thus, when comparing to
complex 1 which has solely a kinter term, the difference
in rate between the two complexes will decrease at
higher concentrations. In contrast to these previous
examples, our current system does not rely on pre-
formed dimers, and association between monomers is
only expected to increase at higher catalyst
concentrations and lead to enhanced activity.
To demonstrate that our catalyst is practically useful,
the reactivity of (salen)Co(III) complex 4 with a range
of different substrates was examined under neat
conditions (Table 2). Reaction of terminal epoxides
containing an aliphatic chain proceeded quickly using
complex 4, with the HKR of 1,2-epoxyhexane
complete (>99% e.e.) after 1 hour (entry 2), while in
the same time period, 66% e.e. was obtained using
complex 1 (entry 1). 2-Vinyloxirane required a higher
equivalence of water to obtain high enantioselectivity,
and complete chiral resolution was achieved in 5 hours
with complex 4 (entry 10). Epoxides containing ether
side chains were also good substrates, with the HKR
of butyl glycidyl ether complete within 25 min (entry
8) while the HKR of benzyl glycidyl ether was
complete within 80 min using 0.1 mol% of complex 4
(entry 12). Within in the same time period, catalysis
with complex 1 arrived to 67% and 78% e.e.
respectively (entries 7 and 11). Styrene oxide, known
to be a slow reacting substrate,[4a] was conducted using
0.5 mol% of catalyst. Complete resolution was
achieved using complex 4 within three hours (entry 4),
while 73% e.e. was reached with complex 1 (entry 3).
HKR of the sterically demanding 3,3-dimethyl-1,2-
epoxybutane required 4 days to reach completion
(entry 14). The control conditions reach 49% e.e. after
this time (entry 13), which clearly demonstrates the
Table 2. Substrate scope for the hydrolytic kinetic
resolution of terminal epoxides using complex 4. The
reactions were conducted in solvent free conditions. the
enantiomeric excess was determined by either chiral GC or
chiral HPLC and the yield of the product is inferred from
the ee (see SI).
a
because any added surfactants will dilute the
concentration of (salen)Co complexes at the interface,
preventing the required cooperativity for effective
catalysis. In the current example, by incorporating
amphiphilicity into the catalyst itself, significant rate
enhancement was observed under industrially relevant,
neat conditions, without the need for added solvent.
We believe that this concept will become increasing
valuable for the design of green chemical methods that
accelerate reactions occurring ‘on-water’ and at the
oil-water interface. The reversible nature of these
hydrophobic interactions also provides modularity and
the potential to readily mix and match different
monomeric catalysts for the development of novel
cooperative systems.
advantage to be had with a more active catalyst. Unlike
8e, 11a]
many previously reported examples,[7b,
this
catalyst is efficient under solvent free conditions and
does not require the addition of an organic solvent to
homogenize the reaction mixture.
In conclusion, this work demonstrates that
hydrophobic interactions can be used to enhance the
efficiency of a practically useful cooperative catalyst.
The results from kinetic studies reveal that under
homogenous conditions, this rate acceleration is due to
an increase in the association constant, leading to Experimental Section
enhanced cooperativity. Under neat biphasic
A specific volume of a solution of complex 4 in chloroform
conditions, the amphiphilic nature of the catalyst acts
to increase the interfacial area between the two
immiscible reactants, resulting in an almost 10 times
increase in reaction rate over catalysts from
commercially available sources. It is worth noting that
Jacobs has previously shown that the addition of
external surfactants does not result in rate
enhancement in the HKR reaction.[24] This is likely
was introduced to a vial and the chloroform evaporated.
Milli-Q water (0.6 eq.) was added followed by racemic
epichlorohydrin (1.0 eq., 1 volume). For experiments under
homogenous conditions, THF (2 volumes) was also added.
The reaction mixtures were stirred at 400 rpm and aliquots
of the reaction mixture were taken at specific time intervals
to monitor the reaction progress. The aliquots were filtered
through a plug of silica and eluted with Et2O before chiral
GC or HPLC analysis.
5
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10.1002/adsc.202100494
Advanced Synthesis & Catalysis
Acknowledgements
[10] a. X. Zhu, K. Venkatasubbaiah, M. Weck, C. W. Jones,
ChemCatChem 2010, 2, 1252-1259; b. T. Belser, E. N.
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M. Rossbach, K. Leopold, R. Weberskirch, Angew.
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[11] a. J. Park, K. Lang, K. A. Abboud, S. Hong, Chem. Eur.
J. 2011, 17, 2236-2245; b. J. Park, K. Lang, K. A.
Abboud, S. Hong, J. Am. Chem. Soc. 2008, 130,
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[12] T. Kitanosono, K. Masuda, P. Xu, S. Kobayashi, Chem.
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[13] S. Narayan, J. Muldoon, M. G. Finn, V. V. Fokin, H.
C. Kolb, K. B. Sharpless, Angew. Chem. Int. Ed. 2005,
44, 3275-3279; Angew. Chem. 2005, 117, 3339-3343.
[14] a. M. F. Ruiz-Lopez, J. S. Francisco, M. T. C. Martins-
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[15] a. C. Z. J. Ren, P. Solís Muñana, J. Dupont, S. S. Zhou,
J. L. Y. Chen, Angew. Chem. Int. Ed. 2019, 58, 15254-
15258; Angew. Chem. 2019, 131, 15398-15402; b. P.
Solís Muñana, G. Ragazzon, J. Dupont, C. Z. J. Ren,
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[16] It is important to note that the mechanism of rate
acceleration in these examples is distinctly different to
conventional micellar catalysis. In the above systems,
rate enhancement is due to an increase in cooperativity
between catalytic units, whereas micellar catalysis
relies on an increase in reactant concentration in the
hydrophobic core of the micellar structures.
This work was supported by a Catalyst: Seeding Grant (CSG-
AUT1701), administered by the Royal Society of New Zealand and
funded by the Ministry of Business, Innovation and Employment;
and by a grant from the Australian Centre for Neutron Scattering,
ANSTO (Neutron proposal: 8756). Funding was also received
from the MacDiarmid Institute for Advanced Materials and
Nanotechnology (Wellington, NZ).
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[19] In these prior examples, either the product of the
reaction (the 1,2-diol), or a solvent was added to keep
the reaction mixture homogeneous. In our studies,
tetrahydrofuran was used.
[20] If dimeric or larger assemblies was predominant in the
system, then rate
=
k[Co(III)dimeric]
≈
(1/2)k[Co(III)total]. In that case, first order
dependence on [catalyst] would be observed.
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6
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10.1002/adsc.202100494
Advanced Synthesis & Catalysis
COMMUNICATION
An Amphiphilic (salen)Co Complex – Utilizing
Hydrophobic Interactions to Enhance the
Efficiency of a Cooperative Catalyst
Adv. Synth. Catal. Year, Volume, Page – Page
Pablo Solís-Muñana, Joanne Salam, Chloe Z.-J.
Ren, Bronte Carr, Andrew E. Whitten, Gregory G.
Warr and Jack L.-Y. Chen
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