Selective photocatalytic C-C bond cleavage under ambient conditions with earth abundant vanadium complexes

Article abstract of DOI:10.1039/c5sc02923f

Selective C-C bond cleavage under ambient conditions is a challenging chemical transformation that can be a valuable tool for organic syntheses and macromolecular disassembly. Herein, we show that base metal vanadium photocatalysts can harvest visible light to effect the chemoselective C-C bond cleavage of lignin model compounds under ambient conditions. Lignin, a major aromatic constituent of non-food biomass, is an inexpensive, accessible source of fine chemical feedstocks such as phenols and aryl ethers. However, existing lignin degradation technologies are harsh and indiscriminately degrade valuable functional groups to produce intractable mixtures. The selective, photocatalytic depolymerization of lignin remains underexplored. In the course of our studies on lignin model compounds, we have uncovered a new C-C activation reaction that takes place under exceptionally mild conditions with high conversions. We present our fundamental studies on representative lignin model compounds, with the aim of expanding and generalizing the substrate scope in the future. Visible light is employed in the presence of earth-abundant vanadium oxo catalysts under ambient conditions. Selective C-C bond cleavage leads to valuable and functionally rich fine chemicals such as substituted aryl aldehydes and formates. Isotope labeling experiments, product analyses, and intermediate radical trapping, together with density functional theory studies, suggest a unique pathway that involves a photogenerated T₁ state during the C-C bond cleavage reactions. Our study demonstrates a sustainable approach to harvest sunlight for an unusual, selective bond activation, which can potentially be applied in organic transformations and biomass valorization.

Full text of DOI:10.1039/c5sc02923f

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DOI: 10.1039/C5SC02923F
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Selective Photocatalytic C-C Bond Cleavage under Ambient
Conditions with Earth Abundant Vanadium Complexes
Received 00th January 20xx,
Accepted 00th January 20xx
Sarifuddin Gazi,a,b Wilson Kwok Hung Ng,a Rakesh Ganguly,a Adhitya Mangala Putra Moeljadi,a
Hajime Hirao,*a and Han Sen Soo*a,b
DOI: 10.1039/x0xx00000x
Selective C-C bond cleavage under ambient conditions is a challenging chemical transformation that can be a valuable tool
for organic syntheses and macromolecular disassembly. Herein, we show that base metal vanadium photocatalysts can
harvest visible light to effect the chemoselective C-C bond cleavage of lignin model compounds under ambient conditions.
Lignin, a major aromatic constituent of non-food biomass, is an inexpensive, accessible source of fine chemical feedstocks
such as phenols and aryl ethers. However, existing lignin degradation technologies are harsh and indiscriminately degrade
valuable functional groups to produce intractable mixtures. The selective, photocatalytic depolymerization of lignin
remains underexplored. In the course of our studies on lignin model compounds, we have uncovered a new C-C activation
reaction that takes place under exceptionally mild conditions with high conversions. We present our fundamental studies
on representative lignin model compounds, with the aim of expanding and generalizing the substrate scope in the future.
Visible light is employed in the presence of earth-abundant vanadium oxo catalysts under ambient conditions. Selective C-
C bond cleavage leads to valuable and functionally rich fine chemicals such as substituted aryl aldehydes and formates.
Isotope labeling experiments, product analyses, and intermediate radical trapping, together with density functional theory
studies, suggest a unique pathway that involves a photogenerated T1 state during the C-C bond cleavage reactions. Our
study demonstrates a sustainable approach to harvest sunlight for an unusual, selective bond activation, which can
potentially be applied in organic transformations and biomass valorization.
www.rsc.org/
disassembly and valorization of macromolecules such as non-
food biomass.
The increasingly severe effects of global climate change and
Introduction
Photoredox catalysis has recently emerged as a versatile
methodology to harvest visible light as a sustainable source of
energy to facilitate chemical conversions in organic reactions.1-
6 Pioneering studies have been reported in the construction of
complex organic molecules through C-C bond formation,1-6 but
the application of photocatalytic processes in selective C-C
bond activation reactions has been largely neglected. Related
to this context, our team has been developing (photo)catalysts
for bioinspired oxidation reactions,7,8 proton reduction, and
degradation of pollutants,9 as part of a common theme to
achieve a sustainable, integrated artificial photosynthetic
system.10-13 Intrigued by the dearth of research on photoredox
catalysis in selective C-C bond cleavage reactions, we
endeavored to explore the development of base-metal
photocatalyzed reactions to be potentially applied in the
environmental pollution due to fossil fuel combustion have led
to growing interest in the development of non-food biomass
as feedstocks for fuels and chemicals.14,15 In particular, the
conversion of biomass into fine chemicals and pharmaceutical
precursors is attractive because of the inherent chemical
complexity of biopolymers. One of the most under-utilized
renewable resources is non-food plant biomass, the major
constituents of which are biomacromolecules such as
cellulose, hemicelluloses, and lignin.1 Lignin (Fig. 1), which
constitutes up to 30% by weight in biomass, is especially
appealing as a source of aromatic small molecules such as
phenols and aryl ethers.16 However, unlike cellulose and
hemicelluloses that have already been utilized in biorefineries
for biofuel production,17 lignin is often regarded as waste since
it is notoriously resistant to oxidative, hydrolytic,
photochemical, and even biological degradation.15,16
Consequently, lignin is typically burned for energy production
despite the presence of rich chemical functionalities that
would otherwise require energy-intensive multi-step chemical
a.Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, 21 Nanyang Link, Nanyang Technological University,
Singapore 637371. E-mail: hirao@ntu.edu.sg; hansen@ntu.edu.sg
b.Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE), 1
Create Way, Singapore 138602.
†Electronic Supplementary Information (ESI) available: Detailed synthetic
procedures and characterization, photoluminescence and TCSPC, TEM, as well as
DFT calculations are included. See DOI: 10.1039/x0xx00000x
syntheses.18
The
chemoselective
photochemical
depolymerization of lignin remains an underexplored field.19
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photosensitizers and sacrificial amine-formate reductants,19
View Article Online
DOI: 10.1039/C5SC02923F
which can be more sustainable if earth-abundant
(photo)catalysts and reagents like air are utilized instead.34,35
Fig. 1 Representative fragment of biomass lignin and one of the model compounds
used in our studies.
Fig. 2 Previously reported strategies for selective bond cleavage of lignin model
compounds include initial oxidation, followed by thermal or noble-metal driven
photocatalytic conditions.19,23,25 Our current procedure involves mild, photocatalytic C-
C cleavage with earth-abundant vanadium oxo catalysts.
The exclusive utilization of lignin as a biofuel has been
considered a waste of its potential.14-16,18,20 Biomass lignin (Fig.
1) is a complex polymer that can realistically be harnessed as
the largest source of inexpensive, easily accessible precursors
for the production of numerous aromatic building blocks
through judicious disasssembly.18 Current technology for the
valorization of lignin involves harsh solvothermal or pyrolytic
processes in the presence of excess reagents like acids or
H2.15,16,21,22 This causes the unselective decomposition of lignin
into many products with low yields, or even mineralization and
charring. Patently, a cost-effective, environmentally benign
protocol for lignin depolymerization through chemoselective
bond cleavage will be critical for the sustainable production of
chemical feedstocks from biomass lignin.19,23-25
In the course of our studies on the disassembly of lignin model
compounds, we have uncovered a photocatalytic earth-
abundant vanadium oxo complex that can effect selective C-C
bond cleavage of specific bonds in some aliphatic alcohols.
Herein, we describe the fundamental studies on the reactivity
and mechanistic understanding of the degradation of lignin
model compounds, with the intention of future expansion and
generalization of the substrate scope. A representative lignin
model compound (11, Fig. 1) with a -O-4 linkage is employed
as the substrate,25-36 since this moiety constitutes over 50% of
the inter-monomer functionalities in lignin, and has been
Recently, the selective bond cleavage of lignin model
compounds and even lignin in some cases, by molecular
catalysts, has been reported in a few seminal papers. This
includes the use of molecular catalysts based on Ru,26,27 Ni,24
and V28-33 under thermal conditions. For instance, Bergman
and Ellman adopted a tandem dehydrogenative-reductive
ether cleavage approach with Ru complexes,26 whereas
Hartwig reported efficient Ni complexes for the reductive
hydrogenation of aryl ether linkages commonly found in lignin.
On the other hand, Toste et al. and Hanson and Silks
independently demonstrated the promise of earth-abundant V
complexes in catalyzing selective C-O and C-C bonds in lignin
model compounds with high yields at moderately elevated
temperatures.28-33 Stahl and Westwood also separately
described novel two-step thermal depolymerization of lignin
itself (Fig. 2), through the initial oxidation of benzylic alcohols,
followed by C-O bond cleavage via acid-promoted or reductive
pathways.23,25 The first successful degradation of lignin and
lignin model compounds under ambient conditions was
reported by Stephenson and coworkers, involving a tandem
TEMPO-oxidation and photocatalytic C-O bond cleavage using
visible light irradiation (Fig. 2).19 Their protocol is notable for
the high yields and preservation of valuable alcohol, aldehyde,
and ether functional groups in the products. However, the
second step includes the use of expensive Ir-based
proposed
as
a
vulnerable
link
for
chemical
cleavage.15,16,18,21,22,25,37 We show that in the presence of our
vanadium catalyst, upon irradiation with visible light (> 420
nm) under AM1.5 solar intensity and ambient temperature
and pressure, a solution of the model compound
6 is
converted into an aryl aldehyde and an aryl formate (Fig. 2).
Mechanistic studies with isotope labeling, product analyses,
and density functional theory (DFT) calculations suggest a
distinct, unique, and functional-group tolerant C-C bond
activation pathway for lignin substrates under exceptionally
mild reaction conditions.
Results and discussion
Thermal and photocatalytic lignin model degradation by
monomeric vanadium(V) oxo complexes.
The detailed synthetic procedures of the vanadium(V) oxo
complexes
Supplementary Information (ESI†). Complex
compound synthesized from a hydrazone benzohydroxamate
ligand , which is redox non-innocent. The ligand was
2
and
5
are described in the Electronic
2
is a new
1
1
synthesized in two steps from commercial reagents by
adapting a previously published procedure.38 In the presence
2 | Chem. Sci., 2015, 00, 1-3
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thoroughly. Complex
experiments to represent the V(V) oxo catalyst reported
previously by Toste and coworkers.30,36
5
has been prepared VifeowrArticcloe nOtnrlionel
DOI: 10.1039/C5SC02923F
The single crystal X-ray diffraction structures of complexes
and are depicted in Fig. 3. Complex displays an octahedral
geometry in the solid state supported by ligand , with a
2
5
2
1
characteristic V=O bond distance of 1.59 Å. An equatorial
methoxide (O5) and an axial methanol ligand (O4) are bound,
with the V-O bond for the methanol ligand (2.34 Å) being
dramatically elongated compared to the methoxide (1.76 Å).
Other salient parameters of
(Tables S1 – S5). Complex
2
5
are summarized in the ESI†
crystallizes in two different
polymorphs from methanol (MeOH), depending on the
temperature. At low temperatures (–20 ºC), the monomeric
polymorph (Fig. 3b) was isolated, featuring a square pyramidal
V center possessing an axial oxo ligand. On the other hand, the
dimeric, centrosymmetric polymorph (Fig. S1 in the ESI†)
crystallized at room temperature, with an octahedral V
Fig. 3 Oak Ridge Thermal Ellipsoid Plots (ORTEPs) from single crystal X-ray diffraction
experiments of (a) complex 2, and (b) complex 5. (c) UV-vis absorption spectra of 1
(red), 2 (red), 4 (blue), and 5 (green) in CH3CN (0.10 mM). (d) CV of 1.0 mM 1 (black line
with Fc+/Fc as internal standard), and 2 (red) using 0.10 M n-Bu4NPF6 as electrolyte in
geometry and the alkoxide group of ligand
bridging donor. In solution with acetonitrile (CH3CN) as the
solvent, both and are believed to be monomeric or in
4 behaving as the
CH3CN at a scan rate of 100 mV s-1
.
2
5
equilibrium, with no perceptible effects on the catalytic
activity.
of stoichiometric equivalents of V(V) oxytripropoxide and
1,
complex was prepared in high yields and characterized
2
Table 1 Thermal reactivity of complexes 2 and 5.
Products[a]
Substrate
Entry
Amount
(mol %)
Reaction Conversion
Vanadium
catalyst
Temperature
(°C)
time (h)
(%)
OH
O
O
O
HO
O
1
5
10
~25
24
18
O
O
O
OH
O
O
12 (17%)
11%
O
11
11
O
HO
10
24
100
2
~80
5
O
12 (85%)
80%
2
2
24
36
3
4
11
11
~25
~80
10
10
0
O
O
O
25
OH
32 (25%)
O
O
[a] The yields are derived from the 1H NMR spectra with 1,1,2,2-tetrachloroethane as an internal standard. All the reactions were carried out in CH3CN in the dark
under ambient air.
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With complexes
2
and
5
in hand, we investigated their after oxidation of the benzylic alcohol under similar reaction
View Article Online
DOI: 10.1039/C5SC02923F
reactivity with lignin model compound 11 under thermal conditions over 36 h (entry 4). The product selectivity is
conditions (Table 1). As expected,
towards lignin degradation through selective C-O bond brown while
cleavage similar to the prior report by the Toste group. At charge transfer (LMCT) transitions occurring in these d0 V(V)
room temperature, showed slow reactivity with 11 (18 % oxo complexes. Accordingly, the UV-visible (UV-vis) spectrum
conversion, entry 1), but gave > 95% conversion with good of exhibits an absorption band in the near-UV region with
selectivity under reflux conditions (entry 2). The products arise max at 396 nm (Fig. 3c) tailing to 500 nm, which can be
from C-O bond cleavage and give predominantly attributed to LMCT based on DFT calculations (vide infra).
unsaturated ketones. In contrast, catalytic amounts of
5
showed catalytic reactivity patently different under thermal conditions. However,
2 has a
5
has a red color, alluding to the ligand-to-metal
5
2
,-
2
resulted in only about 25% conversion of 11 to the ketone (32),
Table 2 Optimization of photocatalytic reaction of lignin model compound 11.
Vanadium
Catalyst
Amount
(mol %)
Irradiation
time (h)
Conversion
(%)
Substrate
Identifiable
products
Entry
O
OH
O
O
11
O
1
VO(acac)2
10
24
O
OH
O
O
6 (11%)
11
11
VO(OPr)3
2
3
10
10
24
24
7
6 (7%)
O
O
HO
11
>50
5
O
O
O
20%
12 (30%)
O
O
O
O
O
6 (18%)
9 (15%)
4a
5
2
2
5
5
0
11
11
24
24
6a (7%),
~41
6 (39%),
9 (34%)
7 (9%)
HCOOH (10%)
7 (6%)
6 (70%), 6a (24%),
6
11
10
24
100
2
9 (61%)
HCOOH (32%)
7b
8c
11
11
10
10
24
16
~3
6 (3%)
2
2
100
6 (50%),6a (49%),7 (30%)
9 (54%) HCOOH (45%)
6a (3%), 7 (10%)
9
11
20
24
~35
6 (32%),
2
9 (25%) HCOOH (10%)
The yields are calculated based on 1H NMR spectra with 1,1,2,2-tetrachloroethane as an internal standard. Unless otherwise mentioned, the reactions were carried out
in CH3CN under aerobic conditions with visible light irradiation (λ > 420 nm, AM1.5 solar irradiation). During irradiation, the temperature was maintained below 30 ºC
a
c
by water circulation through a transparent glass jacket. Reaction performed in the dark. b Reaction performed under an Ar atmosphere. Reaction carried out with
AM1.5 solar irradiation without visible light filter.
4 | Chem. Sci., 2015, 00, 1-3
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The UV region consists of more intense absorptions typical of Prompted by the absorption and emission properties of these
View Article Online
salicylaldimine intra-ligand charge transfers. Similarly,
displays a near-UV absorption band with a blue-shifted
5

also complexes, we probed their photocataDlyOtiIc: 10b.1e0h3a9v/Cio5rSCi0n29t2h3eF
max at degradation of 11. Remarkably, when a solution of 11 and 5
was irradiated under ambient conditions by a solar
are illustrated in Fig. 3c for simulator with a visible light filter (> 420 nm) for 24 h, >40% of
comparison. Both ligands evidently do not exhibit absorption 11 were chemoselectively converted (Table 2, entry 5), in
above 380 nm, lending credence to the assignment of the contrast to the lower thermal activity of . The products have
visible absorption bands as arising from LMCT transitions. The been identified as 4-ethoxy-3-methoxybenzaldehyde ( ) and 2-
UV-vis spectra of VO(acac)2 and VO(OPr)3 are shown in Fig. S2a methoxyphenylformate ( ), arising from selective aliphatic C-C
on the same scale, and both complexes absorb less light in the bond cleavage reactions under ambient conditions in the
visible region relative to
consistent with the low preserve of enzymatic processes.37,39 These products were
photodegradation products yields (Table 2, entries 1 and 2). identified by NMR spectroscopy, high-resolution mass
Notably, also has a photoluminescence max at 480 nm with spectrometry (HRMS), isotope labeling studies (vide infra), and
330 nm that tails into the visible region (Fig. 3c). The UV-vis mol% of
spectra of ligands and
2
1
4
2
6
9
2
,
2

lifetimes of 9.9 ns (59%), 2.0 ns (29%), and < 110 ps (12%) as also confirmed by independent syntheses. The product
determined by time-correlated single photon counting (TCSPC, selectivity and reaction conditions are distinct from all previous
Fig. S2 in the ESI†). Additional photophysical studies are reports about degradation of lignin model compounds. A
underway to probe this intriguing phenomenon, which has not catalyst concentration of 10 mol%
been commonly observed among molecular vanadium > 95% consumption of 11 and high yields of
systems, and will be reported in a separate manuscript. (61%, entry 6). However, further increase in the concentration
of led to lower conversion and product yields, possibly due
2
gave the best results with
6
(70%) and
9
2
to reduced light transmission through the sample (entry 9).
Under the optimized conditions for visible light
photodegradation of 11
,
formic acid, 4-ethoxy-3-
methoxybenzoic acid (6a), and guaiacol were also observed by
1H NMR spectroscopy or isolated by preparatory thin layer
chromatography (TLC), as illustrated in Fig. 4. The clean and
controlled transformation of 11 into chemical products that
retain valuable functional groups occurs under exceptionally
mild conditions, with the major energy inputs being light and
the water circulation needed to maintain temperatures below
30 ºC. A large scale photolysis of 11 (0.300 g, 0.86 mmol) was
carried out with 10 mol% of
2. Due to the poor light
transmission properties of the reaction solution, the photolysis
experiment was conducted with a rudimentary homemade
flow reactor (Fig. S71) with a residence time of around 3 min.
After 24 h of irradiation under visible light, the products
6
(67%) and
over silica.
9 (56%) were isolated by column chromatography
To elucidate the nature of photocatalysis by
control experiments were conducted to ascertain the role of
reagents and reaction conditions. In the absence of , lignin
2, a series of
2
model compound 11 did not react when irradiated with visible
light (> 420 nm) for 24 h at ambient temperature, since 11
does not absorb visible light. Likewise, no reaction was
observed when 11 was stirred in the dark with
(entry 4). Under an Ar atmosphere, surprisingly remained
photocatalytically active with an approximately 3% yield of
whereas was not observed in the NMR spectra (entry 7).
Compound could have formed from a stoichiometric
reaction, whereas can still be produced in the absence of O2
since
can act as a stoichiometric VV oxidant, as supported by
our DFT calculations (vide infra). When the reaction was
conducted with AM1.5 solar irradiation without the visible
light filter, the rate accelerated resulting in complete
consumption of 11 within 16 h (entry 8). However, the yields
2 for 24 h in air
2
6
9
6
9
Fig.
4 Product distribution from photocatalytic degradation of lignin model 11.
2
Compounds 6a, 7, and formic acid are secondary products from 6 and 9. Labeling
experiments with 13C isotopes indicate that the label is incorporated into aryl formate
9-13C1 and formic acid. Labeling experiments with deuterium provide evidence that no
isotope scrambling occurs prior to or after C-C bond cleavage, suggesting that benzylic
alcohol oxidation is not part of the mechanism. Moreover, the deuterium label on
formic acid suggests that formic acid is a hydrolysis secondary product from the aryl
formate.
of
6 and 9 decreased vis-à-vis the optimized conditions (entry
6), with increased amounts of 6a and guaiacol. Compound 6a
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is believed to have arisen by aerial oxidation of
Chemical Science
, whereas over the photochemical reaction.36 When VtiheweArticcolelOonrleinde
6
guaiacol and the formic acid could be derived from
by complexes VIVO(acac)2 and VVO(OPr)3DOwI: e10re.103u9/sCe5dSC0u2n9d2e3Fr
hydrolysis, as supported by isotope labeling studies below. The photocatalytic conditions, only trace amounts of were
9
6
quantum efficiency of the photocatalytic degradation of 11 identified by 1H NMR spectroscopy (entries 1 and 2). These
was measured with the use of ferrioxalate as the chemical experiments support the critical role of LMCT for visible light
actinometer, and was found to be 1.3% at 436 nm under absorption, and redox non-innocent ligands in mediating the
visible light irradiation with a 420 nm filter, and 1.5% at 334 photocatalysis, as verified by DFT calculations (vide infra).
nm using AM1.5 irradiation without a 420 nm filter (in the Moreover, an unprecedented, general pathway for the
ESI†).
Inspired by these findings, we examined the photocatalytic alcohol groups may be accessible for other V(V) oxo catalysts
activity of 10 mol% of catalyst under visible irradiation and that absorb visible light.
selective photodriven C-C bond cleavage adjacent to aliphatic
5
observed 50% conversion of 11 under similar reaction
conditions. Two products are identical with those obtained
Mechanistic understanding from substrate screening and isotope-
labeling.
under thermal conditions, namely 12 and guaiacol, while
6
and
9
are different compared to the thermal experiments and
To ascertain the identity of the products and obtain additional
mechanistic insights, we carried out isotope labeling studies.
similar to the results with photocatalyst
2 (entry 3). This
experiment suggests that two simultaneous catalytic pathways
may operate for , with the thermal reaction predominant
5
Fig. 5 NMR (300 MHz, CD3CN) spectra of lignin model compounds before and after photocatalytic reactions. (a) Compound 11; (b) 11 after photolysis; (c) 11-13C2 after
photoreaction; (d) 11-D1 after photoreaction (inset: 1H-decoupled 2H NMR spectrum of 6-D); and (e) 11-D2 after photoreaction (inset: 1H-decoupled 2H NMR spectrum after partial
conversion. The peak at 4.27 ppm is from unreacted 11-D2). The unlabeled products 6 (red square), 9 (red triangle), and formic acid (red hexagon) are indicated at their distinctive
chemical shifts. The 13C labeled 9-13C1 (blue triangle) and 13C labeled formic acid (blue hexagon) have been split due to the 1H coupling with 13C. The deuterated 6-D (green square),
9-D (grey triangle), and formic acid (grey hexagon) have been detected by 2H NMR spectroscopy.
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1
Three different isotopically labeled lignin model compounds The H (Fig. 5c) and 13C NMR spectra of both products (Fig. S3
View Article Online
DOI: 10.1039/C5SC02923F
11-13C2
, 11-D1, and 11-D2 were synthesized and used as in ESI†) confirm the assignment of the H NMR chemical shifts
1
substrates in NMR tube experiments with CD3CN as the and the regioselective aliphatic C-C bond cleavage sites. When
1
solvent under the optimal conditions from Table 2 entry 6. 11 is deuterated at the benzylic position (11-D1), the H NMR
1
2
NMR experiments for H, H, and 13C were conducted before spectrum of the product mixture no longer displays the
2
and after irradiation. With the 13C labeled 11-13C2 as substrate, aldehyde proton signal at 9.8 ppm (Fig. 5d), while the H NMR
13C-labeled formic acid (blue hexagons) and 2- spectrum (inset) clearly shows the corresponding deuterium
methoxyphenylformate (9-13C1, blue triangles) were observed label (green square). The product 6-D was isolated by
1
by H NMR spectroscopy, with the formate protons split into preparatory TLC and characterized by 1H and 2H NMR
doublets due to coupling with 13C (Fig. 5c).
spectroscopy as well as by HRMS, suggesting that the benzylic
hydrogen is not scrambled or exchanged during the
photocatalytic C-C bond cleavage reactions. When another
deuterated lignin model 11-D2 was subjected to photocatalytic
degradation, we found that the reaction rate was slower and it
generated deuterated 2-methoxyphenylformate (9-D, grey
triangle, Fig. 5e) and almost exclusively deuterated formic acid
(grey hexagon, inset), further confirming the absence of
1
isotope scrambling. The absence of formic acid in the H NMR
spectrum with 11-D2 as the substrate also suggests that the
formic acid produced during this photocatalytic reaction
originated from the 2-methoxyphenylformate fragment, likely
via hydrolysis (Fig. 5e and inset). This proposal is confirmed by
the identification of guaiacol by NMR spectroscopy and liquid
chromatography-mass spectrometry (LC-MS), although
isolation by preparatory TLC proved challenging possibly due
to decomposition of guaiacol.
These isotope labeling studies confirm that
2 is a photocatalyst
for the unique and selective C-C bond cleavage of 11, ruling
out the benzylic hydrogen abstraction and oxidation pathways
proposed in prior reports.23,25-27,30,36 The product distribution
of the isotope labeling experiments is summarized in Fig. 4. A
systematic series of various lignin model compounds have
been investigated under the same photocatalytic reaction
conditions to explore the generality and selectivity of the C-C
cleavage reactions (Fig. 6). Model compound 13 was degraded
to yield
6
and
9
, albeit with lower yields than for 11. Similarly,
and
compound 15 was photocatalytically converted into
6
9
cleanly (Fig. 6), with a dramatically lower yield. These results
suggest that the primary alcohol in 11 is important for
facilitating the chemical reactivity, although our DFT
calculations indicate that the lower yields from degradation of
13 and 15 over 24 h may have kinetic origins (vide infra). The
results from substrates 13 and 15, as well as the DFT studies
also indicate that a reaction path involving the tandem
oxidation of the primary alcohol to an aldehyde, followed by a
retro-aldol reaction is probably not operational.23,27,32,33
A set of mono-aryl aliphatic alcohol substrates was also
subjected to the same photocatalytic conditions, and the
product distribution gave additional insights into the catalytic
mechanism. After photolysis, 17 was oxidized to
6 and formic
Fig. 6 Lignin model compounds and aliphatic alcohols subjected to the photocatalytic
conditions. Substrates 13 and 15 demonstrate that the benzylic alcohol is sufficient for
C-C bond cleavage activity. Substrates 17, 20, and 23 show that electron-rich and
electron-deficient aryl rings react, although benzylic alcohol oxidation instead of C-C
bond cleavage dominates with electron-deficient aryl rings. Substrates 28 and 31
illustrate the necessity of aliphatic alcohol groups and the incompatibility with phenolic
functions respectively. Substrates 35 and 36 demonstrate the potential versatility of
this photocatalytic protocol for C-C bond cleavage reactivity under ambient conditions.
acid as well as 6a, but the ketone 18 was also observed as a
minor product (Fig. 6), with no aldehyde derived from primary
alcohol oxidation. Interestingly, the ratio of the oxidized
ketones (21 and 24) to the corresponding aldehyde originating
from C-C bond cleavage (benzaldehyde and 4-
nitrobenzaldehyde, respectively) was found to rise when
increasingly electron-deficient aryl groups were introduced at
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the benzylic alcohol (20 and 23 in Fig. 6). This phenomenon supporting ligand
Chemical Science
to the VV center in
2 is required to trigger
1
View Article Online
suggests a vital role of the aryl ring in dictating the selectivity the intramolecular radical oxidative proDcOesI:s1e0s.1.0S3i9n/Cce5SCo0rg29a2n3iFc
of our unusual photocatalytic reactions. On the other hand, radicals are expected after the C-C cleavage reaction, the
protection of the hydroxyl group of the benzylic alcohol as a stable radical (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO)
methoxy ether provided 25, and the reaction yielded a more has been employed as a trap to capture intermediates during
complex mixture including aldehydes and formic acid upon the degradation of the model compounds. TEMPO (5
photodegradation (Fig. S4 in the ESI†).
When 27 (Fig. 6) was photolyzed with the same protocol,
equivalents) was introduced during the photocatalytic reaction
and of stoichiometric amounts of 11 with under an Ar
9
2
formic acid were observed in poor yields at low conversion. atmosphere to maximize yields of the intermediate. The
Furthermore, irradiation of the ether-protected model 28 and aldehyde was still observed but an additional new product
the phenolic substrate 31, in the presence of , resulted in no 34) derived from the radical coupling between the other
6
2
(
degradation (Fig. 6). Alcohol functionalities appear to be fragment of 11 and TEMPO was detected by LCMS (Fig. S6 in
essential to direct the selective C-C bond cleavage, but binding the ESI†). The compound 34 was isolated by preparatory TLC
of the aryl ring to the vanadium center as a phenolate shuts and confirmed with 1H, 13C, and DEPT NMR spectroscopy,
down the catalytic activity. To probe whether unactivated NOESY as well as HRMS. The proton-decoupled 13C NMR
alcohols can undergo this remarkable photocatalyzed C-C spectrum confirms the number of distinct carbons in the
cleavage, both glycerol (a by-product of biodiesel molecule, of which five carbons are CH (DEPT 90, Fig. S63 in
production)40 and 1-butanol were used as substrates. the ESI†), four carbons are CH2, and five carbons are CH3 (DEPT
Gratifyingly, formic acid was observed as one of the products 135, Fig. S62 in the ESI†). The NOESY data indicated
for both substrates as anticipated (Fig. 6), although the correlations between the TEMPO methyl protons with the
identification of the remaining products will require additional aliphatic CH, CH2, and aromatic CH protons, consistent with
studies. The photocatalytic cleavage of C-C bonds in alcohols our structural formulation of compound 34 (Fig. S64 in the
shows promise for further generalization. Electrochemical ESI†). The isolation of 34 and
6 supports the intermediacy of a
experiments were then initiated to probe the feasibility and controlled radical cleavage pathway.
thermodynamics of this photochemical reaction.
In addition, transmission electron microscopy (TEM)
measurements have been conducted before and after the
photocatalytic reactions to explore the operation of
heterogeneous catalysis. The TEM images of the reaction
mixture before irradiation showed small amounts of
amorphous residue, whereas sporadic amounts of some
irregular particles appeared after irradiation (Fig. S7 in the
ESI†). These amorphous materials could have arisen due to
precipitation of the catalyst or substrate after solvent
evaporation. Dynamic light scattering (DLS) experiments
conducted before photodegradation showed the presence of
small amounts of particles with sizes between 30 to 60 nm,
Electrochemical measurements and intermediate trapping.
Cyclic voltammetry (CV) measurements conducted on
photocatalyst
2 shows a reversible wave at –0.14 V relative to
ferrocene (Fc, Fig. 3d). This can be attributed to the VV/VIV
redox reaction. By assuming a LMCT band edge of ~550 nm
(Fig. 3c), which corresponds to an optical band gap of at least
2.25 eV, the absorbed photon energy far exceeds the dark T1
excited state that mediates the C-C bond cleavage, as
supported by DFT calculations below.
The CV of 11 is illustrated in Fig. S5a (in the ESI†), with two
irreversible oxidation waves observed at +0.94 and +1.40 V.
Alkoxy and alkyl groups are known to be electron-donating
moieties for aromatic rings; the oxidation wave at +0.94 V
likely corresponds to oxidation of the tri-substituted aryl ring,
whereas the wave at +1.40 V corresponds to oxidation of the
other di-alkoxy aryl ring. However, these redox potentials arise
from the outer-sphere single electron transfer processes only.
On the other hand, when 11 binds to VV, the C-C bond adjacent
to the alkoxide becomes activated and will require
substantially lower potentials for cleavage, which has been
substantiated by our DFT calculations. To further support the
hypothesis of a LMCT-initiated photocatalytic reaction, we
attempted to synthesize the one-electron reduced congener of
whereas larger aggregates between 200 nm to 1
m were
detected after 24 h irradiation. To rule out the possibility of
heterogeneous catalysis, an experiment was performed under
the conditions of Table 2 entry 6, and the reaction mixture was
centrifuged at the end of 24 h. After a fresh batch of 11 was
added to the supernatant that was decanted and the solution
was irradiated over 24 h, the product mixture contained
6
(40%), (34%), and formic acid (21%). On the other hand,
9
when 11 was added to the little residue that was collected
from centrifugation, no reaction was observed. Moreover,
experiments have been performed with suspended V2O5, and
no photodegradation of 11 was observed after irradiation for
24 h under the same ambient conditions as the experiments
with catalyst 2. We believe that the small amounts of particles
2
, the vanadium(IV) oxo 3. However, the product obtained by
are unlikely to explain the remarkable selectivity of our C-C
bond cleavage reactions, especially since 11 displays no
reactivity when irradiated with V2O5 particles under the typical
catalytic conditions. Based on the experimental studies, a
cogent mechanistic pathway involving LMCT in 2, followed by
selective C-C cleavage of the bound aliphatic alcohol function
is proposed. Theoretical studies have been pursued to gain a
one-electron reduction using cobaltocene was poorly soluble
and could not be fully characterized. Nonetheless, the EPR
spectrum (Fig. S5b in the ESI†) confirms the presence of a
paramagnetic VIV species. With
3 as the photocatalyst,
irradiation of 11 with > 420 nm light led to no degradation of
11 in the absence of air, confirming that LMCT from the
8 | Chem. Sci., 2015, 00, 1-3
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deeper understanding of the exceptionally mild, unique, and The lowest-energy (singlet) exited state S1 excitation is of a
View Article Online
potentially general photocatalytic reaction.
π(L)
excitation energy of 536 nm (53.4 kcal mol-1), which falls within
the range of wavelengths for light absorption by observed by
UV-vis spectroscopy (Fig. 3c). The visible absorption band with
max at 396 nm should be due to a higher-energy excitation,
most probably to the S3 state that lies 401 nm above S0 and is
also of a π(L) dπ*(M) LMCT type (Fig. 7a).
The TD-DFT calculation also suggested that the triplet T1 state
is less stable than S0 by 49.6 kcal mol-1 and is thus slightly
lower in energy than the S1 state. Intersystem crossing from S1
to T1 may be facile since the two excited states should be in
thermal equilibrium at room temperature. As illustrated in Fig.
→
dπ*(M) LMCT type (Fig. 7a, 7b, DaOnId: 107.1c0)39a/nCd5SCh0a2s92a3nF
TD-DFT calculation of 2.
2
The electronic structure of
2 was examined by a time-
a
dependent DFT (TD-DFT) calculation using the B3LYP functional
and the 6-31G* basis set.41,42 Gaussian 09 was used for the
calculations.43 The TD-DFT calculation was performed at the
→
ground-state (S0) geometry of
2 without the labile methanol
ligand. At the valence edge, the occupied molecular orbitals
are composed mostly of π orbitals of the ligand, whereas the
unoccupied molecular orbitals always involve large
contributions from the d orbitals at the vanadium center (Fig.
S10 in the ESI†). As such, the six low-lying excited states are all
associated with LMCT (Table S16 in the ESI†).
7d, the spin density of
diazo and oxygen donors of the ligand
2
in the T1 state is distributed in the
and the V-based dxy
1
orbital; therefore, the T1 state is also viewed as an LMCT-type
excited state. However, we note that these TD-DFT
calculations correspond to the allowed vertical transitions into
the vibronically excited states according to the Franck-Condon
principle. Subsequently, intramolecular relaxation, internal
conversion, and intersystem crossing should take place on
ultrafast time-scales, which may lead to dark states from which
the C-C bond cleavage occurs. Therefore, we sought a feasible
reaction mechanism for this unusual reaction from
a
geometry-optimized, dark T1 excited state using DFT
calculations.
Exploration of reaction mechanisms using DFT calculations.
Having established that
2 has photo-accessible S1 and T1
excited states, we proceeded to explore viable reaction
pathways for the selective C-C bond cleavage reaction of the
V-bound 11. Our isotope labelling experiments suggest that C-
C bond cleavage takes place without activation of any C-H
bonds. Therefore, we sought a pathway that included direct C-
C cleavage via a single electron transfer (SET) process as
frequently observed in natural lignin peroxidase
enzymes.34,35,37,39 The substrate we used for calculations is
similar to 11, but the ethoxy group on the phenyl group on the
left-hand side and the methoxy group on the phenyl group on
the right-hand side in Fig. 1 were replaced with methoxy and
hydrogen, respectively.
It was found that the C-C cleavage in the S0 state is a very
unfavourable process (Fig. S11). This corroborates the slow
thermal reactivity of
product being the ketone 32 instead of
2
even at 80 ºC, with the major observed
and (Table 1, entry
6
9
Fig. 7(a) Absorption spectrum obtained from a TD-DFT/B3LYP/6-31G* calculation. To
compare the experimental and theoretical spectra, the theoretical absorption curve
was scaled such that the absorbance at 396 nm became 0.2. Oscillator strengths for TD-
DFT data are shown in an arbitrary unit. The spatial distributions of HOMO (b) and
LUMO (c) indicate that the S0–S1 transition is characterized as a LMCT. ∆E(HOMO-LUMO) (=
3.21 eV) is much larger than the S0–S1 excitation energy obtained by TDDFT (2.31 eV).
The spin density distribution for the T1 state (d) indicates that the spin is localized at
the vanadium center.
4). It is therefore more probable that the reaction is triggered
by photoexcitation. In the reactant complex (RC) for the
reaction involving
11 is bound to the vanadium of
2
and a model of 11, the benzylic oxygen of
. A TD-DFT calculation
2
determined the S0-S1 excitation energy for this species at the
ground-state geometry as 48.9 kcal mol-1, which is similar to
the excitation energy calculated for 2
(53.4 kcal mol-1).
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View Article Online
DOI: 10.1039/C5SC02923F
Fig. 8(a) Energy profile (kcal mol-1) for the first half of the reaction under aerobic conditions, determined at the B3LYP(SCRF)/6-311+G(d,p)//B3LYP/6-31G* level with zero-point
energy corrections. The S1 state was obtained by broken-symmetry (BS) calculations. BS calculations did not allow us to determine the S1 state for RC (*), because the calculation
converged back to the stable S0 state. Nonetheless, as shown by the TDDFT calculations for 2, it should lie close in energy to the T1 state. A light-green arrow indicates a
photoexcitation. (b) The second half of the reaction. The S1 state could not be determined for the first few species by BS calculations, but it should be almost as stable as T1.
10 | Chem. Sci., 2015, 00, 1-3
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DOI: 10.1039/C5SC02923F
Although TD-DFT appears to describe the photoexcitation of
2
step, photoexcitation of the VV peroxide may occur to
reasonably well, geometry optimization of various generate the T1 or S1 excited state to complete the reaction.
electronically excited species with TD-DFT is computationally
too demanding. We have seen above that T1, which can be
treated by ordinary DFT calculations, has an LMCT state. In
addition, many of the intermediates and transition states on
the S1 excited-state pathway can be described using broken-
symmetry (BS) DFT calculations. Therefore, we decided to use
(BS-)DFT calculations to explore the subsequent
photochemical reactions of 2. The methanol ligand of 2 was
removed. Geometry optimization and frequency calculations
were then performed at the B3LYP/6-31G* level in the gas
phase, and the energies were refined by performing single-
point calculations with the 6-311+G(d,p) basis set, while
including the effect of acetonitrile solvent with an IEFPCM-
SCRF model.44
Geometry relaxation of T1 led to a somewhat smaller S0-T1
energy gap (20.2 kcal/mol) for RC (Fig. 8a). It should be noted
that B3LYP may possibly underestimate the energies of
excited-states.45-47 Intriguingly, DFT calculations identified low-
energy-barrier, exothermic pathways on the excited S1 and T1
Fig. 9 Illustration of LMCT-driven homolytic C–C bond cleavage.
The transient peroxide in the excited state undergoes
homolytic O-O bond cleavage and subsequent homolytic C-C
bond cleavage in a virtually barrierless fashion, to produce
9
energy surfaces (Fig. 8a). This suggests that the photoexcited
2
and formaldehyde (or a hydroxymethyl radical). Importantly,
although the vanadium is temporarily reduced to VIV in the
midst of the reaction under aerobic conditions (Fig. 8), it is
oxidized back to the VV state, thereby permitting repeated
usage of the vanadium complex for the catalytic reactions.
In the absence of air, the photocatalytic reactions can still
possesses an exceptionally mild, thermally accessible pathway
for aliphatic C–C bond cleavage by inner-sphere SET from 11 to
the ligand-centered cation radical, reminiscent of enzymes like
lignin peroxidases and cytochromes P450.34,35,37,39,48,49 The
facile LMCT-driven homolytic C–C bond cleavage in our
reaction is due to an efficient SET to the ligand’s
 orbital that
proceed, since
2
is nominally a moderate VV oxidant. Although
is lower in energy than the vanadium’s d orbital (Fig. 9).
Although the oxidation of 11 ostensibly requires at least +0.94
V by electrochemical measurements (Fig. S5a), binding of the
benzylic alkoxide to VV activates the adjacent C-C bond (red
O2 is not present in the system, after the first C-C bond
cleavage (RC to Int1 in Fig. 8a), the substrate radical can be
trapped by another VV oxo to produce a VIV alkoxide complex;
the barrier height for this process is only 3.2 kcal mol-1 (Fig.
bond in Fig. 9). Furthermore, photoexcitation of
2 eventually
10). This process will lead to consumption of
thus decreasing the amount of that can be used for
homolytic C-C bond cleavage of the substrate to produce Int1
The resultant VIV alkoxide will then exchange ligands with
another molecule of
in the system to form a VV oxo alkoxide
2 in the system,
results in an energetic, dark T1 excited state after non-radiative
relaxation processes. The additional impetus from the T1
excited state together with formation of the C=O double bond
in the aldehyde, more than compensates for cleavage of the C-
C single bond to produce Int 1 (Fig. 8a). In other words, the
dark T1 excited state, which may be accessible only via
photoexcitation and relaxation, combined with the Lewis
acidic V center, allowed us to target proximal C-C bonds that
are otherwise not electrochemically oxidizable. Our additional
DFT calculations also suggested that when the substrate 11 is
bound to the vanadium center via the primary alcohol, the C-C
bond cleavage is not as favourable as the substrate bound via
the benzylic alcohol (Fig. S12); this is presumably due to the
lower stability of the resultant substrate radical. The resulting
substrate radical is trapped by O2 (see Fig. S13 in the ESI†)
2
.
2
complex (Int3A). We found two possible pathways for photo-
excitation-driven reactions of this VV complex.
In one of the two mechanisms, homolytic C–C bond cleavage
on the excited state energy surface is accompanied by
hydrogen abstraction from the hydroxyl group of glycol via
TS2A, with an energy barrier of 8.9 kcal mol-1. Consequently,
formate
9
and a VIII complex are generated (Fig. 10, top right).
Thus, this pathway leads to further depletion of VV in the
system. The other pathway proceeds with an energy barrier of
9.0 kcal mol-1 in the excited state, and a transiently formed VIV
species is oxidized back to VV, thus allowing the catalytically
useful VV species to be regenerated for subsequent use. Both
under aerobic conditions, while the aldehyde
6 dissociates.
Subsequently, heterolytic cleavage of the peroxide O-O and
aliphatic C-C bonds, coupled with proton transfer occurs on
the S0 surface via TS3 (Fig. 8b). This concerted process has a
thermally accessible barrier of 19.7 kcal mol-1, generating
of these processes lead to the formation of 9, but their energy
barriers are somewhat higher than the barrier for LMCT-driven
C-C cleavage of 11 (Fig. 8a). Therefore, after the formation of
the VIV complex, a next round of the C-C cleavage of 11 should
occur more favourably than these formate generation
formaldehyde and
9 (Fig. 8b), the latter of which has been
isolated experimentally. Alternatively, in analogy to the first
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processes. A set of C-C cleavage and formation of the VIV coworkers.30,36 The activation barrier was calculVaietwedArtiaclse O3n6lin.1e
DOI: 10.1039/C5SC02923F
species Int2A requires consumption of two molecules of
ultimately resulting in depletion of the available
system. These arguments explain why the reaction is not benzylic hydrogen to the V oxo has an activation barrier of
efficient and negligible amounts of
are formed under 20.2 kcal mol-1, which is much higher than the barrier for
2
,
kcal mol-1 on the S0 surface (Fig. S14). Even on the
2
in the photoexcited T1 energy surface, H-abstraction from the
9
anaerobic conditions (Table 2). Our DFT studies of the aerobic excited-state C-C bond cleavage (Fig. 8a). An alternative H-
and anaerobic reactions therefore suggest that the high abstraction reaction from the C-H bond adjacent to the
conversion efficiency of the aerobic reaction considered here hydroxymethyl group also has a high energy barrier of 29.2
originates from the LCMT-type photoexcitation of the kcal mol-1 (Fig. S15) and is endothermic. These calculations are
vanadium complex as well as the efficient regeneration of VV consistent with the fact that the thermal catalytic reactions of
complexes in the presence of O2. Attempts were also made to
2 exhibit low yields, and oxidation of the benzylic alcohol of 11
understand the selectivity of C-C bond cleavage over C-H is slow even at 80 ºC.
oxidation. Calculations were performed for the H-abstraction
pathway of the ground state VV oxo proposed by Toste and
Fig. 10 DFT calculated energy profile (kcal mol-1) for the reaction under anaerobic conditions, determined at the B3LYP(SCRF)/6-311+G(d,p)//B3LYP/6-31G* level with zero-point
energy corrections. Note that the step from RC to Int1 is the same as that in the aerobic reaction (Fig. 8a).
12 | Chem. Sci., 2015, 00, 1-3
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Since
2
can be easily photoexcited, outer sphere mechanisms reactive and versatile aldehyde groups. VieTwhAertoiclreeOtniclinael
DOI: 10.1039/C5SC02923F
might exist whereby the substrate undergoes a C-C bond calculations at the DFT level confirm that
2 has unusual, facile
cleavage without its direct coordination to vanadium. Two mechanistic pathways on the T1 energy surface generated only
outer sphere mechanisms in the T1 state were examined (Fig. via photoexcitation, and exhibits greater selectivity for C-C
S16); however, these reactions were not as favourable as the bond cleavage over HAT by the VV oxo. Our fundamental study
reaction presented in Fig. 8. Hence, the reaction of the demonstrates a unique and eco-friendly approach to harvest
vanadium complex considered here seems to be distinct from solar energy to perform C-C activation reactions, which can
the cases of other photosensitizers that rely on an outer potentially be employed in late-stage transformations of
sphere electron transfer mechanism.19,34,35 We also explored if complex organic molecules or disassembly and valorization of
an equatorial oxo arrangement is possible for the catalyst
2 or (bio)macromolecules in the future.
RC in Fig. 8a. However, in the former case, the axial isomer
was more stable, and the equatorial isomer is possible only
when vanadium has a methanol ligand. The equatorial isomer
could not be located on the potential energy surface for RC
without a methanol ligand. These additional calculations are
Acknowledgements
This work was supported by a NTU start-up grant (M4081012,
M4080551), the Nanyang Assistant Professorship (M4081154,
M4080755), and the Singapore-Berkeley Research Initiative for
Sustainable Energy (SinBeRISE) CREATE Programme. This
research programme is funded by the National Research
Foundation (NRF), Prime Minister’s Office, Singapore under its
Campus for Research Excellence and Technological Enterprise
(CREATE). W.K.H.N. and H.H. thank the High Performance
Computing Centre of Nanyang Technological University for
computer resources. The authors thank Dr. Subas K. Muduli for
assistance with TEM measurements. We are also grateful to
Dr. Xiangyang Wu and Prof. Edwin Yeow for help with TCSPC
experiments.
presented in Fig. S17.In the presence of substrates 13 and 15
,
the yields of products are diminished over the course of 24 h
irradiation, but the absence of the primary alcohol in both
substrates does not completely shut down the activity. As
shown in Fig. S20, the highest energy transition state over the
two steps in Fig. 8a and 8b is lowest for 11 (6.1 kcal mol–1)
compared to 13 (7.3 kcal mol–1) and 15 (8.6 kcal mol–1), lending
support to a kinetic origin for the lower product yields from 13
and 15. For phenolic substrates like 31, the photocatalytic
reactivity is completely absent. The greater acidity of a
phenolic group allows the phenoxide to bind strongly to VV
oxo. However, efficient LMCT-driven C-C cleavage, as depicted
in Fig. 9, is not possible in this case. Indeed, the aliphatic C-C
bond cleavage along the T1 energy surface has a high energy
barrier (Fig. S18), suggesting that for such substrates, even
photoexcitation will not provide a viable route for aliphatic C-C
cleavage. This result highlights the need to pre-process natural
lignin19,23,25,34 to passivate all phenolic groups, before a
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We have prepared a photocatalyst consisting of an earth-
abundant VV oxo complex supported by a redox non-innocent
salicylaldimine ligand 1. A catalytic amount of 2 can effect the
chemoselective, aliphatic C-C bond cleavage of representative
lignin model compounds under ambient conditions with visible
light (> 420 nm) irradiation at moderate to high yields. The
product analysis is supported by isotope labeling studies,
which confirm that the lignin model compounds cleave
regioselectively into two primary degradation products. The
resulting aryl aldehyde and aryl formates are important
building blocks in organic synthesis, since they possess highly
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Chemical Science
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ARTICLE
Chemical Science
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