Selective Cα Alcohol Oxidation of Lignin Substrates Featuring a β-O-4 Linkage by a Dinuclear Oxovanadium Catalyst via Two-Electron Redox Processes

Article abstract of DOI:10.1002/ejic.201900807

Developing highly efficient catalyst systems to transform lignin biomass into value-added chemical feedstocks is imperative for utilizing lignin as renewable alternatives to fossil fuels. Recently, the pre-activated strategy involving the selective oxidat

Full text of DOI:10.1002/ejic.201900807

DOI: 10.1002/ejic.201900807
Full Paper
Oxovanadium Catalysts
Selective Cα Alcohol Oxidation of Lignin Substrates Featuring a
ꢀ-O-4 Linkage by a Dinuclear Oxovanadium Catalyst via Two-
Electron Redox Processes
Yan-Ting Tsai,[a] Chih-Yao Chen,[a] Yi-Ju Hsieh,[a] and Ming-Li Tsai*[a]
Abstract: Developing highly efficient catalyst systems to trans- of complex 2 could serve as proton abstraction sites for both
form lignin biomass into value-added chemical feedstocks is Cα and Cγ alcohol of dimeric lignin substrates, respectively. In-
imperative for utilizing lignin as renewable alternatives to fossil terestingly, the dinuclear vanadium intermediate 4 demon-
fuels. Recently, the pre-activated strategy involving the selective strates the ability to uptake two electrons resulting from the
oxidation of Cα alcohol of lignin substrates containing (ꢀ-O-4 oxidation of Cα alcohol and yields two corresponding mono-
linkage mode has been demonstrated to significantly increase nuclear VIV intermediate 5. The mononuclear VIV intermediate
the depolymerization efficiency of native aspen lignin from 5 exhibits a characteristic 8-line EPR spectrum and possesses
10–20 to 60 wt.-%. In this study, we reported the synthesis of a one unpaired electron determined by the Evans method. The
dinuclear oxovanadium complex 2 that is capable of selectively established structure-reactivity relationships will be able to
oxidizing the Cα alcohol (80 – 100% selectivity) of various di- shed light on the future directions for rational design of highly
meric lignin substrates under a mild condition. Further investi- efficient catalysts for selective oxidation of lignin biomass.
gation of catalytic mechanism has revealed that two V=O motifs
Introduction
terns of linkage modes distributed in different agricultural resi-
dues and wood wastes, the ꢀ-O-4 linkage mode is the most
abundant component which accounts for 40 – 65% of total
linkages presented in lignin biomass. Recently, the C–O/C–C
bond cleavage of lignin model substrates or depolymerizations
of natural lignin have been demonstrated via redox-neutral,[5]
acid/base,[6] and oxidative[3d,5g,6e,7] catalyst systems. In particu-
lar, the selectivity toward C–O and C–C bond cleavage of lignin
model substrates containing the ꢀ-O-4 linkage mode have been
successfully catalyzed by a five-coordinate vanadium-salen
complex and a six-coordinate (HQ)2V(O)(OiPr) mononuclear oxo-
vanadium catalysts, respectively.[7a,7d,8] The facile non-oxidative
C–O bond cleavage catalyzed by the five-coordinate oxovana-
dium complex has been proposed via a ketyl radical intermedi-
ate generating from a one-electron process[7b] and also been
shown to depolymerize lignin samples isolated from Miscanthus
giganteus pretreated with organosolv processes.[9]
In addition to the photocatalytic carbon–carbon bond cleav-
age reaction facilitated by vanadium catalysts,[10] the two-step
strategy regarding the selective oxidation of Cα alcohol into
ketone followed by formic acid-induced C–O bond cleavage has
been developed. The approach has significantly improved the
depolymerization efficiency of native aspen lignin samples from
less than 10–20 wt.-% yields up to more than 60 wt.-%.[11] The
dramatic increase of depolymerization yield is attributed to the
weakening of Cꢀ-O bond strength by 15 kcal/mol resulting from
the selective oxidation of Cα alcohol to ketone as shown in
Scheme 1.[11,12] Although the catalytic oxidation of Cα alcohol
of lignin substrates containing ꢀ-O-4 linkage modes have been
presented by either microwave-assisted photocatalysis,[13] tran-
The over-exploitation of limited natural resources resulting from
exponential growth of human population is destined to the
depletion of fossil fuels and projected shortages of commodity
chemicals in the near future. To achieve sustainable energy soci-
ety, the development of technologies to harness renewable re-
sources as alternatives to fossil fuels is imperative. Ligno-
cellulose biomass from agricultural residues and wood wastes
produced annually is promising non-edible sources due to their
diverse chemical compositions consisted of versatile aliphatic
and aromatic building blocks.[1] In particular, lignin biomass, the
natural polymer consisting of methoxy-substituted phenolic
monomers, is the only abundant source of renewable aromatics
from biomass.[2] Despite there are existing processes (gasifica-
tion, pyrolysis, liquid phase reforming, acid/base catalyzed de-
polymerization) for converting lignin biomass into value-added
chemical feedstocks, harsh conditions (high temperatures/pres-
sures or using strong acid/base) have limited their energy effi-
ciency and environmental sustainability.[3]
The chemical structures of lignin biomass are comprised of
the polyphenolic compounds with versatile linkage modes
including ꢀ-O-4 (ꢀ-aryl ether), ꢀ-5 (phenylcoumaran), ꢀ-ꢀ
(resinol), and ꢀ-1 (spirodienone), etc.[4] Despite the diverse pat-
[a] Department of Chemistry, National Sun Yat-sen University,
Kaohsiung 80424, Taiwan
E-mail: mltsai@mail.nsysu.edu.tw
http://chem.nsysu.edu.tw/var/file/180/1180/img/780/ml_tsai_c.htm
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201900807.
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sition metal catalysts[3d,7m] or AcNH-TEMPO/HNO3/HCl catalyst of νC=N (free ligand: 1606 cm–1; chelated ligand 1601 cm–1), re-
system,[12c] the development of a highly efficient catalyst sys- spectively. The resulting mononuclear oxovanadium complex
tem that uses less-expensive additives under relatively mild re-
action conditions (weak acids/bases or O2 in air) will be neces-
sary for large-scale industrial applications.
[VO(OCH3)(CH3OH)SBH] (1) is best described as distorted octa-
hedral geometry. SBH-H2 is deprotonated by two coordinated
OiPr ligands of V(O)(OiPr)3 and provides tridentate [O,N,O] liga-
tion to the complex 1. The coordinated methoxide ligand of
complex 1 is resulting from a simple ligand-exchange reaction
between the third coordinated OiPr ligand of V(O)(OiPr)3 and
MeOH solvent (Scheme 2).
By adding one equivalent of H2O into the THF solution of
complex 1 at room temperature, the corresponding dinuclear
oxovanadium complex [(SBH)(O)V(μ-O)V(O)(SBH)] (2) was pro-
duced in nearly quantitative fashion (96% yield). The reaction
is presumably proceeded by protonating the coordinated
methoxide ligands from two complex 1 via H2O to produce two
CH3OH molecules and O2– serving as a bridged ligand for two
resulting V(O)(SBH) fragments. To quantitatively understand the
redox behavior of the synthesized oxovanadium complexes, the
cyclic voltammetry of complexes 1 and 2 were conducted in
Scheme 1. Differences in bond dissociation energies of Cα-Cꢀ and Cꢀ-O link-
age modes between ꢀ-O-4-alcohol and ꢀ-O-4-ketone lignin model substrates.
Here, we presented the synthesis of dinuclear oxovanadium
catalysts with tridentate [O,N,O] ligation mode.[14] The dinuclear
oxovanadium catalyst was characterized by single-crystal X-ray
diffraction, IR,
CH3CN solution with 0.1 M
[nBu4N][PF6] acting as supporting
51
electrolyte at room temperature. Complex 1 shows one reversi-
ble redox process at E1/2 = –0.477 V (vs. Fe+/Fe), which is attrib-
uted to a reversible VV/IV redox couple, with ΔEp = 86 mV and
ipa/ipc = 0.70 (red line, Figure 1). On the contrary, complex 2
V-NMR, and cyclic voltammetry. The dinuclear vanadium cat-
alyst 2 has shown excellent selectivity of oxidizing the Cα alco-
hol of dimeric lignin substrates containing ꢀ-O-4 linkage mode
into ketone. The unique selectivity of catalytic alcohol oxidation
has been attributed to the nature of dinuclear vanadium core displays one reversible redox couple at E1/2 = 0.055 V (vs.
that is capable of acting as a two-electron reservoir evidenced Fe+/Fe) with ΔEp = 83 mV, ipc/ipa = 0.74 and one irreversible
by its cyclic voltammogram exhibiting one reversible and one reduction wave at E = –0.335 V (vs. Fe+/Fe) (green line, Figure 1).
quasi-reversible redox couples. The detailed catalytic mecha-
nism has further been delineated via a combination of 1H-NMR,
2D-NMR (COSY, HSQC), electron paramagnetic resonance (EPR)
spectroscopy and Evans method.
The two redox couples of complex 2 were tentative assigned
as VV-VV/VV-VIV and VV-VIV/VIV-VIV, respectively. The significant
positive E1/2 of complex 2 as compared to that of complex 1
(–0.477 V for 1 and 0.055 V for 2) indicates that the complex 2
is much easier to accept one electron than complex 1. In addi-
tion, the irreversible behavior of the second reduction wave of
complex 2 implicates its ability to accept an additional electron
to form an electrochemically unstable VIV-VIV intermediate. The
diamagnetic nature of complexes 1 and 2 are demonstrated by
their 1H and 13C NMR spectra that display similar chemical shift
and splitting pattern as compared to free SBH-H2 ligand (Figure
S1–S6). In addition, the 51V NMR of complexes 1 and 2 were
determined as δ = –548 ppm and δ = –577 ppm, respectively,
which could serve as spectroscopic references to monitor the
possible structural change of oxovanadium complexes during
chemical transformations/catalytic reactions (Figure S7 and S8).
Note that the two redox waves of complex 2 both exhibit
Results and Discussion
Synthesis and Characterization of Mononuclear and
Dinuclear Oxovanadium Complexes
The mononuclear oxovanadium complex 1 chelated with tri-
dentate [O,N,O] salicylaldehyde benzoyl hydrazone (SBH–H2) li-
gand was successfully synthesized by dropwise addition of
V(O)(OiPr)3 into the MeOH solution of SBH-H2 tridentate ligand
at room temperature as reported in literature.[10b] The reaction
was monitored by the disappearance of characteristic IR νC=O
stretching frequency of SBH-H2 at 1676 cm–1 and the red shift
Scheme 2. Synthetic procedures of complex 1 and 2.
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anodic shifts relative to the redox wave corresponds to the spectively) between complexes 1 and 2 also manifest in the
VV/VIV of complex 1 could be attributed to the electrophilic differences in their metal-ligand bond lengths. The bond length
nature of 5 C.N. vanadium center in complex 2 as compared to of V=O bond in complex 2 is slightly shorter than that of com-
the 6 C.N. coordination environment in complex 1. Neverthe- plex 1 (V(1)–O(3) 1.5890(16) for complex 1; V(1)–O(4) 1.5756(14)
less, the detailed studies of coupling interactions between two for complex 2 ) and average V–O bond length provided by the
vanadium centers of complex 2 with different redox states SBH2– ligand in complex 2 is also ca. 0.018 Å shorter than that
(VV-VV/VV-VIV and VV-VIV/VIV-VIV) will be required in order to pro- in complex 1. The decrease of V=O and average V–OSBH bond
vide more insights on this observation.
lengths from complex 1 to complex 2 suggest that the rela-
tively electron deficient nature of vanadium center in complex
2 which is consistent with the cyclic voltammetry studies asso-
ciated VV/VIV and VVVV/VIVVV redox couples (E1/2 = –0.477 for
complex 1 and E1/2 = 0.055 for complex 2) of complexes 1 and
2, respectively.
Figure 2. ORTEP drawing and labeling scheme of 2 with thermal ellipsoid
drawn at 50% probability and selected bond lengths [Å] and angles [deg].
Hydrogen atoms are omitted for clarity. Complex 2: V(1)–N(1) 2.0893(17),
V(1)–O(1) 1.8205(11), V(1)–O(2) 1.9541(12), V(1)–O(3) 1.8029(10), V(1)–O(4)
1.5756(14), V(1)–V(1′) 2.9948(6), O(3)–V(1)–O(4) 107.32(7), O(1)–V(1)–O(4)
103.33(7), O(1)–V(1)–O(3) 104.78(5), O(2)–V(1)–O(4) 102.74(7), O(2)–V(1)–O(3)
84.82(5), O(1)–V(1)–O(2) 147.91(6), N(1)–V(1)–O(4) 99.18(8), N(1)–V(1)–O(3)
149.24(7), N(1)–V(1)–O(1) 83.47(6), N(1)–V(1)–O(2) 74.18(6), V(1)–O(3)–V(1′)
112.31(9).
Figure 1. The cyclic voltammograms of complexes 1 (red) and 2 (green) in
0.1
trode at a scan rate of 50 mV.
M
[nBu4N][PF6] supporting electrolyte with a glass carbon working elec-
The ORTEP plot and the selected bond lengths and bond
angles of complex 2 are shown in Figure 2. The dimeric struc-
ture of complex 2 consists of two five-coordinate monomeric
vanadium units bridged by an O2– ligand. The local geometry
of vanadium center could be determined by structural parame-
ter τ5 (τ5 = (ꢀ-α)/60ꢁ where α and ꢀ are the two greatest valence
angles of the coordination center and ꢀ > α). The τ5 of vana-
dium center is calculated as ca. 0.02 (α: ;O(1)–V(1)–O(2) ca.
148ꢁ; ꢀ: ;N(1)–V(1)–O(3) ca. 149ꢁ). Therefore, the local vana-
Synthesis of Dimeric Lignin Model Substrates Containing
ꢀ-O-4 Linkage Modes
dium geometry of complex 2 is best described as square py- To explore the chemical reactivity of oxovanadium complexes
ramidal (square pyramidal: τ5 = 0; trigonal bipyramidal: τ5 1 and 2, a series of dimeric lignin substrates containing ꢀ-O-4
=
1) where the V=O fragment located at an apical position. The ether bond were synthesized. The ꢀ-O-4 linkage was incorpo-
alterations of coordination numbers (C.N. = 6 and C.N. = 5 for rated by the reaction of 2-bromo-1-phenylethanone or its para-
complexes 1 and 2, respectively) and coordination environ- methoxy derivatives with phenoxide or its ortho-methoxy deriv-
ments (O5N1 and O4N1 donor sets for complexes 1 and 2, re- ative, respectively. This synthetic route will allow us to synthe-
Scheme 3. Synthetic procedures of lignin model substrates (S1–S8 and S1ox–S8ox) containing a ꢀ-O-4 linkage mode.
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size dimeric lignin substrates S1ox–S4ox which could serve as the transformation of complex 1 into complex 2 has been ob-
starting materials to yield the corresponding alcohol substrates served and characterized by 51V-NMR spectroscopy (Figure S9).
S1–S4 via the nucleophilic attack of NaBH4 at the Cα position Taking advantage of the robustness of complex 2, the catalyti-
(Scheme 3a). The oxidized dimeric lignin substrates with ali- cally oxidizing the Cα alcohol of the synthesized dimeric
phatic alcohol functional group at the Cα position S5ox–S8ox lignin substrates into ketone will be further pursued by the
could be obtained by the addition of HCOH into S1ox–S4ox in complex 2.
The effects of pyridine on the transformation of S1 into S1ox
were studied by using pyridine as an additive in the catalytic
system containing 5 mol % of complex 2 at 80 °C under air for
24 hours in CH3CN solution. As shown in Table 2, the conversion
of S1 is significantly enhanced from 5.4% to 100% as additive
pyridine increase from 10 mol % to 1 equivalent (Table 2, en-
tries 1–3), respectively. Increasing the amount of pyridine
added to 2 or 4 equivalents only slightly lowers the selectivity
of catalytic oxidation of the Cα alcohol from 100% to 80%
(Table 2, entries 4 and 5). Therefore, 1 equivalent of pyridine is
the optimal amount in order to achieve both excellent selecti-
vity and efficiency of converting S1 into S1ox. To exclude the
possibility of pyridine serving as a catalyst, the catalytic condi-
tion of one equivalent pyridine in CH3CN solvent at 80 °C under
air for 24 hours was conducted. As shown in the entry 6 of
Table 2, 0% of conversion has been observed.
the presence of K2CO3 (Scheme 3b). The addition of NaBH4 into
a THF solution of S5ox–S8ox results in the formation of the cor-
responding S5–S8 (Scheme 3c) which closely resembles to the
chemical structure of natural lignin substrates featuring a sec-
ondary benzylic alcohol and a primary aliphatic alcohol at the
Cα and Cγ positions, respectively. Note that the tested sub-
strates do not cover the common set of dimeric lignin model
substrates such as disubstituted guaiacyl analog and trisubsti-
tuted syringyl analog. From the systematic studies of catalytic
reactivity of the oxovanadium complexes 1 and 2 toward the
synthesized dimeric lignin substrates S1–S8, the electronic ef-
fects of the methoxy substituent on the aromatic ring and the
incorporation of primary aliphatic alcohol at the Cγ positions
could be elucidated.
Selective Alcohol Oxidation Catalyzed by Oxovanadium
Complexes
Table 2. Additive effects on the oxidation of S1 into S1ox
.
To investigate the catalytic oxidation of secondary benzylic al-
cohol of dimeric lignin substrates by oxovanadium complexes
1 and 2, the simplest synthesized lignin substrate containing
ꢀ-O-4 linkage S1 was chosen to screen an optimized catalytic
condition. As shown in Table 1, S1 was treated with the 10 mol
% of complex 1 and 5 mol % of complex 2, respectively, at
80 °C under air for 24 hours in different solvents. In CH3CN
or toluene solvent, both complexes 1 and 2 show very low
conversions (< 10%) of oxidizing the Cα alcohol of S1 into the
Cα ketone of S1ox (entries 1, 2, 6, 7). The efficiency of catalytic
oxidation of S1 into S1ox in DMF solvent slightly increases to
36.2% and 39.0% for complexes 1 and 2 (entries 3, 4, 8, 9),
respectively. Interestingly, using pyridine as a solvent achieves
almost 100% conversion of S1 (95% for both complex 1 and 2).
Unlike complex 2 remains intact during the catalytic reactions,
Entry
Cat.[mol %] Loading
Additive[a] Conv.%
Yield[b]
%
1
2
3
4
5
6
2
2
2
2
2
–
5%
5%
5%
5%
5%
–
10 mol %
20 mol %
1 eq
5.4
20.2
100.0
100.0
100.0
0
3,5
15.4
100.0
79.7
80.0
0
2 eq
4 eq
1 eq
[a] The amount of pyridine. [b] Conversions and yields determined by integra-
tion against an internal standard 2-(trimethylsilyl)ethanol.
The substrate scope has been examined with the synthe-
sized substrates S1–S8 under the established reaction condi-
tion (5 mol % loading of catalyst 2 with 1 equivalent pyridine
as additive in CH3CN solution at 80 °C under air for 24 hours).
The incorporation of methoxide functional group either on R1
or R2 positions has slightly lowered conversion from 100% for
S1 to ca. 90% for S2 and S3, respectively, over the course of 24
hours (Table 3, entries 2 and 3). If both R1 and R2 positions are
substituted with methoxy group, the conversion from S4 to
S4ox has decreased to 70% (Table 3, entry 4) which indicates
the incorporation of electron-donating groups on the aryl ring
of lignin substrates containing ꢀ-O-4 linkage has notably low-
ered conversion efficiency in the current catalyst system.
To further explore the reactivity and selectivity of complex 2
toward the substrates that closely resemble to natural lignin,
the catalytic oxidation of Cα alcohol of substrates containing
primary aliphatic CH2OH moiety on the Cꢀ position (S5–S8) was
investigated. The catalytic conversion efficiency of Cα alcohol
into the corresponding ketone for S5–S8 is 10–30% lower
Table 1. Solvent effects on the catalytic oxidation of S1 into S1ox
.
Entry
Catalyst[mol %]
Solvent
Conversion%
Yield[a]
%
1
2
3
4
5
6
7
8
10% 1
10% 1
10% 1
10% 1
10% 1
5% 2
5% 2
5% 2
5% 2
5% 2
CH3CN
Toluene
DMSO
DMF
pyridine
CH3CN
Toluene
DMSO
DMF
4.9
2.8
23.4
36.2
95.0
8.0
6.2
5.0
39.0
97.0
2.7
1.0
13.4
29.0
45.2
2.7
4.0
5.0
20.0
47.5
9
10
pyridine
[a] Conversions and yields determined by integration against an internal stan-
dard 2-(trimethylsilyl)ethanol.
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Table 3. Substrate scope of selective oxidation of dimeric lignin substrates.
Entry
Substrate/Product
R1
R2
R3
Con.%
Yield[a]
%
Sel.%
1
2
3
4
5
6
7
8
S1/ S1ox
S2/S2 ox
S3/S3 ox
S4/S4 ox
S5/S5ox
S6/S6 ox
S7/S7 ox
S8/S8 ox
H
OMe
H
OMe
H
OMe
H
H
H
OMe
OMe
H
H
OMe
OMe
H
H
H
H
100
93
90
70
77
65
62
61
100
87
70
66
75
58
53
53
100
94
78
94
97
89
85
87
CH2OH
CH2OH
CH2OH
CH2OH
OMe
[a] Conversions and yields determined by integration against an internal standard 2-(trimethylsilyl)ethanol.
(Table 3, entries 5–8) than that for substrates S1–S4. In addi- shown in Figure 3c, the added 1 equiv. pyridine dose not show
1
tion, the effects of different bases (including pyridine, K2CO3, the characteristic uncoordinated pyridine H-NMR signals with
NaHCO3, DBU, CsCO3, Et3N) on the selective oxidation of S5 to chemical shift δ > 8. This result implicates that the coordination
S5ox have been examined; only pyridine demonstrates a signifi-
cant conversion (77%) and selectivity (97%). All the other bases
show less than 16% of conversion (Table S1 and Figure S10).
Notably, the oxidation of primary aliphatic Cγ alcohol to alde-
hyde has not been observed in the current catalyst system
which indicates the complex 2 could selectively oxidize elec-
tron-deficient secondary benzylic alcohol over electron-rich pri-
mary aliphatic alcohol with 85 – 96% selectivity.
of pyridine to the vanadium center of complex 2 and yields the
corresponding complex 3. The observation might indicate that
the binding of pyridine and deprotonated S5 to the vanadium
center of complex 2 could be reversible. In the catalytic reaction
condition, it is still unclear that whether pyridine or S5 will bind
to vanadium center of complex 2 in the first step.
(b) Upon addition of the S5, the proposed oxovanadium-
1
substrate complex 4 is produced: the detailed H NMR assign-
ment of S5 was achieved by a combination of 1H-13C HSQC and
1
1H-1H COSY spectra (Figure S11). The H-NMR of S5 shows that
Mechanistic Studies of Catalytic Alcohol Oxidation of
Dimeric Lignin Substrates via Dinuclear Oxovanadium
Complex 2
there are six signals in the range of δ = 2.8 – δ = 5.8 with total
integration area ca. 6 which corresponds to the non-aromatic
1
H signal. The five H-NMR peaks were labelled from δ = 2.8 –
δ = 5.8 as Ha (δ = 3.76,1H), Hb ((δ = 3.48, 1H), Hc (δ = 4.42, 1H
1H), Hd (δ = 2.96,1H), He (δ = 4.93,1H), and Hf (δ = 3.68,1H).
In order to elucidate the mechanism associated with the highly
selective oxidation of Cα alcohol to the corresponding ketone
catalyzed by dinuclear oxovanadium complex 2, the transfor-
mation of S5 into S5ox via complex 2 was delineated step-by-
step consisting of (a) the coordination of 2 equivalent pyridine
to the vacant site of vanadium center of complex 2 (b) the
proposed structure of oxovanadium-substrate complex 4 result-
ing from the stoichiometric reaction among complex 2, pyrid-
ine and S5; (c) exploring the role of additive by reacting the
oxovanadium-substrate complex 4 with pyridine; (d) the spec-
troscopic characterizations of a key dinuclear oxovanadium in-
termediate 5 produced by uptaking two electrons from the
oxidation of Cα alcohol to ketone; (e) regenerating dinuclear
oxovanadium complex 2 by reacting intermediate 5 with O2 to
complete the catalytic cycle. The proposed elementary steps of
1
The H-13C HSQC spectrum shown in Figure S12a allows us to
determine the connectivity between H and C atoms which only
the directly bonded hydrogen and carbon shows a cross peak.
1
The H-13C HSQC spectrum of S5 indicates that Ha, Hb, Hc, and
He show cross peaks, suggesting that Hd and Hf are attached to
non-carbon atoms. Based on these observations and the chemi-
cal structure of S5, the Hd and Hf are assigned as the proton
signals of alcohol functional group. From the coupling patterns
1
of Hd and Hf shown in the H-1H COSY spectrum of S5 (Figure
S12b), Hf only shows one cross-peak between Hf and He where
Hd has exhibited cross-peaks to both Ha and Hb. Based on the
coupling patterns and chemical shifts, the Hf and Hd are as-
signed as proton signals of benzylic and aliphatic alcohol func-
tional connected to the Cα and Cγ, respectively. Since He shows
a cross peak to the benzylic alcohol proton Hf in the COSY
1
the catalytic cycle were studied by the utilizations of H NMR,
51V NMR, 2D-NMR including Correlation Spectroscopy (COSY),
Heteronuclear Single-Quantum Correlation (HSQC), Hetero- spectrum, He is then assigned as Cα-H signal. Similarly, Ha and
nuclear Multiple-Bond Correlation spectroscopy (HMBC), IR, CV, Hb demonstrated two cross peaks associated with aliphatic al-
and EPR spectroscopy.
cohol proton Hd, which indicates the Ha and Hb are magnetically
inequivalent and all bonded to the Cγ. Finally, Hc displays cross
peak to Ha, Hb, and He, which suggests its connectivity to
(a) The addition of one equivalent pyridine to the reaction
solution of complex 4 and S5 (1:1 ratio) at room temperature
was conducted to understand the possibility of pyridine serving Cꢀ. Based on the H-13C HSQC and H-1H COSY spectra of S5,
1
1
1
as coordinating ligand to vanadium center of complex 2. As the six H NMR signals in the range of δ = 2.8 – δ = 5.8 are
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Figure 3. (a) the 1H NMR of the substrate S5. (b) the 1H NMR spectrum of the reaction solution of S5 and complex 2 in 1:1 ratio. (c) the 1H NMR spectrum of
the resulting solution c with 1 equivalent pyridine. (d) the 1H NMR spectrum of the resulting solution d at 80 °C for 4 h (e) the 1H NMR spectrum of the
resulting solution d at 80 °C for 12 h.
assigned. The assignment has been further confirmed by the reveals the complete disappearance of Hd and Hf peaks, impli-
1H-13C HMBC spectrum of S5 shown in Figure S12c.
cating the ability of complex 2 to abstract two alcohol protons
of S5. The proton abstraction is likely proceeded by two V=O
The substrate binding mode was probed by the reaction of
S5 and complex 2 in 1:1 ratio under N2 atmosphere at room
moieties of complex
2 to form a tentative dicationic
temperature to prevent the subsequent catalytical alcohol oxid- [(SBH)(OH)V(μ-O)V(OH)(SBH)]2+ chelated with deprotonated S5
1
ation. The H-NMR spectrum of reaction solution (Figure 3a–b) substrate intermediate 4 (Scheme 4a). The proton abstraction
Scheme 4. Proposed catalytic mechanism of selective Cα alcohol oxidation of a lignin substrate containing ꢀ-O-4 Linkage.
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via two V=O moieties of complex 2 is further supported by diate 4 accommodating two electrons to yield the temporarily
the disappearance of characteristic V=O stretching frequency stable dinuclear VIV-VIV complex before the formation of mono-
(νV=O ca. 964 cm–1) and the growth of O-H stretching frequency nuclear intermediate 5 has been investigated.[17] The solution
(νO-H ca. 3365 cm–1) (Figure S13).
behavior of the dinuclear V(V) complex containing [V–O-V]
structure motif was extensively studied. Based on the investiga-
tion, the dinuclear structure of vanadium complex with V(V)-
V(V) oxidation state remains intact in solution even with the
coordination of pyridine on vanadium center. Upon two elec-
trons reduction, the dinuclear V(IV)-V(IV) complex subsequently
transforms into mononuclear V(IV) complex characterized by its
distinct 8-line EPR signal. In order to provide more convincing
evidence for the possible transformation of dinuclear nature of
complex 2 in solution, 1H-NMR (Figure 3) spectra, CV voltammo-
grams (Figure 4) were conducted. The CV voltammograms of
complex 2 and complex 2 coordinated with S5 both exhibit
two reversible redox waves, which also implicates the integrity
of dinuclear structure. However, after receiving two electrons
from the oxidation of benzyl alcohol, the dinuclear structure of
complex 2 may transform into two mononuclear vanadium (IV)
species via the coordination of pyridine/solvent as evidenced
by its distinct 8-line EPR signal which corresponds to the mono-
nuclear VIV intermediate determined from Evans method. We
further examined the possibility by conducting the cyclic vol-
tammetry of intermediate 4. As shown in Figure 4b, the second
(c) the role of pyridine on the oxidation of S5 into S5ox (rate-
determining step): By increasing temperature to 80 °C, the He
signal of intermediate 4 is decreased (in the presence of 1
equivalent of pyridine) accompanied by the observation charac-
teristic 1H NMR signal of S5ox (δ = 3.97, 2H; δ = 5.67 1H) as
demonstrated in Figure 3d-e and Scheme 4b. In order to probe
the role of pyridine additive, we carried out the catalytic oxid-
ation of lignin model substrate S5 by using 2,6-lutidine and 4-
dimethylaminopyridine as additives, respectively. The selectivity
and conversion of these reactions were summarized in Table
S2. By using 2,6-lutidine as an additive, there is 0% conversion
which clearly indicates the essence of pyridine coordination in
order to proceed the catalytic oxidation of lignin substrate S5.
Interestingly, using 4-dimehylaminopyridine (DMAP) as an addi-
tive slightly increase conversion rate (82%) but the selectivity
decreases from 97% to 18%. The rationale of this dramatical
decrease in selectivity toward benzylic alcohol oxidation is at-
tributed to the formation of unexpected dehydration product
S5de (48% yield). To further investigate the possible pathway of
the S5de formation, we have carried out a reaction using S5ox
and DMPA as substrate and additive, respectively, as well as 5%
complex 2 as catalyst. The 1H-NMR spectrum of this reaction
indicates 80% conversion from S5ox to S5de (Figure S14). These
results suggest that the suitable basicity range of unhindered
pyridine derivatives is required for selective oxidation of Cα al-
cohol without subsequently transforming S5ox into S5de. To fur-
ther examine the possibility that DMAP deprotonate S5ox to
yield the corresponding S5de, we mixed S5ox and DMAP in 1:1
ratio at 85 °C for 24 h. As shown in Figure S15, no S5de forma-
tion has been observed.
(d) the spectroscopic characterizations of key mononuclear
1
oxovanadium intermediate 5: the broad H NMR of the pyrid-
ine-deprotonated solution could be attributed to the presence
of a paramagnetic specie resulting from the oxidation of Cα
alcohol to ketone. To further investigate the nature of the para-
magnetic specie, the EPR spectroscopy, Evans method, and cy-
clic voltammetry experiments had been pursued. The EPR spec-
trum of the resulting solution (Figure 4a) measured at room
temperature had exhibited a well-resolved and characteristic 8-
line EPR signal with (g1 = 1.97, g2 = 1.98, g3 = 1.99; A1(V) = 145
G, A2(V) = 110 G, A3(V) = 40 G in the fast motion tumbling re-
gime (correlation time ca. 10–10 s) which is the well-docu-
mented mononuclear VIV EPR spectrum.[5a,15] The 8-line feature
also excludes the possibility of existence of delocalized VIV-VV
electronic state which has been shown to display a 15-line EPR
signal.[16] In combination with EPR results, the magnetic suscep-
tibility of the paramagnetic species was determined by the
Evans method (Figure S15), which corresponds to the formation
of two mononuclear VIV intermediates. Based on the above ex-
perimental evidence, the paramagnetic species generating by
accepting two electrons from the oxidation of S5 is presumably
proposed as two mononuclear oxovanadium intermediate 5
with VIV electronic state (Scheme 4d). The feasibility of interme-
Figure 4. Overlap of the experimental EPR spectrum of reaction solution (S5
and complex 4 in 1:1 ratio) added with 1 equivalent pyridine at 80 °C for
12 h (black line) and the simulated EPR spectrum with g1 = 1.97, g2 = 1.98,
g3 = 1.99; A1(V) = 145 G, A2(V) = 110 G, A3(V) = 40 G; correlation time =
10–9.65 s–1 (red line). (b) the overlapped CV spectra of complex 2 (green line)
and intermediate 4 (blue line) measured in CH3CN solution with a glass
carbon working electrode at a scan rate of 50 mV.
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Table 4. Current catalyst systems and representative mononuclear vanadium catalysts using O2 as oxidant for the selective oxidation of benzyl alcohol with a
ꢀ-O-4 linkage and a Cγ primary alcohol.
Entry
1
Substituent
Cat.
Selec. [%][a]
Conv. [%][a]
Yield
75
T/ °C
80
Ref.
R1=H, R2=H, R3=H, R4=H
2
97
71
89
85
55
52
39
27
31
77
55
65
62
82
75
79
84
85
This work
This work
This work
2
R1=H, R2=OMe, R3=H, R4=OMe
R1=H, R2=OMe, R3=H, R4=H
R1=H, R2=H, R3=H, R4=OMe
R1=H, R2=OEt, R3=OMe, R4=OMe
R1=OMe, R2=H, R3=OMe, R4=OMe
R1=H, R2=OEt, R3=OMe, R4=OMe
R1=OMe, R2=H, R3=OMe, R4=OMe
R1=OMe, R2=H, R3=OMe, R4=OMe
2
39
80
3
2
58
80
4
2
53
80
This work
[7b]
5
V(O)(OiPr)3
V(O)(OiPr)3
V(O)(acac)2
M1
45
80
6[b]
7
39
100
80
[7d]
[7b]
[8b]
[17]
31
8[b]
9[b]
23
100
100
M2
27
[7d]
[7b]
10[b]
11
R1=OMe, R2=H, R3=OMe, R4=OMe
R1=H, R2=OEt, R3=OMe, R4=OMe
M3
M4
68
68
95
86
65
59
100
80
[7b]
[7b]
[7e]
12
R1=H, R2=OEt, R3=OMe, R4=OMe
R1=H, R2=OEt, R3=OMe, R4=OMe
R1=OMe, R2=H, R3=OMe, R4=OMe
M5
M6
M6
62
28
85
66
95
65
41
27
55
80
80
80
13
14[b]
[a] Selec. and Conv. represent selectivity and conversion, respectively. [b] Reaction time = 48 h.
irreversible redox wave observed in the CV spectrum of com- of H2O, presumably resulting from the decomposition of H2O2
1
plex 2 (E1/2 ca. –0.335 mV, ipc/ipa ca. 0.1, ΔEp ca. 111 mV) had at 80 °C, during the catalytic cycle was also observed in the H
presumably changed toward quasi-reversible as indicated in the NMR at ca. δ = 2.6 (Figure S17). To confirm this peak as H2O,
1
second redox wave of intermediate 4 (E1/2 ca. –0.318 mV, ipc/ipa molecular sieves with 4 Å porous size was added. The H NMR
ca. 0.3, ΔEp ca. 95 mV). Note that the Fc/Fc+ redox couple used signal ca. δ = 2.6 disappeared after incubating the molecular
as an internal standard showed ipc/ipa ca. 0.99 and ΔEp ca. sieves in the resulting solution overnight.
75 mV. The different electrochemical behaviors between com-
plex 2 and intermediate 4 implicate that the substrate binding
to a dinuclear oxovanadium complex is a key step to facilitate
the two-electron transfer receiving from the oxidation of alco-
hol into ketone.
Based on the available experimental and spectroscopic evi-
dence, we have derived the plausible catalytic mechanism asso-
ciated with selectively oxidizing Cα alcohol of lignin dimeric
substrates to ketone via a dinuclear oxovanadium catalysts. The
catalytic reaction was presumably initiated by the coordination
(e) Regenerating the dinuclear oxovanadium complex 2 from of pyridine to the vanadium center of complex 2 followed by
the mononuclear intermediate 5: the regeneration of dinuclear the deprotonation of the secondary benzylic alcohol and the
vanadium complex 2 with VV-VV oxidation state was achieved primary aliphatic alcohol by two V=O motifs to produce the
by simply exposing the intermediate 5 with O2 or air dinuclear vanadium chelated with a deprotonated lignin sub-
(Scheme 4e). The two-electron transfer from the VIV core of in- strate (Scheme 4a–b). Note that the order of pyridine or a lignin
termediate 5 to O2 resulted in the formation hydrogen peroxide substrate binding to the complex 2 could be arbitrary. Upon
characterized by H2O2 strip qualitatively (Figure S16). The neces- coordinating to the dinuclear vanadium core, the acidity of
sity of exposing O2 to regenerate the complex 2 was verified Cα-H of a lignin substrate is likely to increase to facilitate the
by conducting the catalytic oxidation reaction under N2 atmos- deprotonation of Cα-H via uncoordinated pyridine (Scheme 4c).
phere. Only trace amount of S5ox was detected as compared to In addition, the coordination of the lignin substrate might also
almost 80% conversion under air/O2 atmosphere. The formation provide the facile avenues to accept two electrons yielding the
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corresponding dinuclear complex with VIV-VIV oxidation state, ing two-electron redox shuffling. Since the oxidation of alcohol
evidenced by that the intermediate 4 exhibits two redox waves to ketone is a classical two-electron oxidation reaction, the
in cyclic voltammetry studies, followed by the formation of the highly selective oxidation of Cα alcohol to ketone of various
cationic mononuclear intermediate 5 as evidenced from the lignin dimeric substrates may be rationalized by the two-elec-
EPR spectroscopy and Evans method (Scheme 4d). Since the tron capacity of dinuclear vanadium core, in contrast to the
transformation of alcohol into the corresponding ketone is an reported one-electron pathway involving ketyl radical interme-
intrinsic two-electron oxidation process, the highly selective al- diate and a subsequent C-O bond cleavage.
cohol oxidation (> 85% selectivity) could be attributed to the
dinuclear core of oxovanadium complex 2 that is capable of
serving as a two-electron reservoir. The oxidations of two pro-
posed mononuclear VIV intermediate 5 via O2 led to the forma-
tion of H2O2 which subsequently decomposed into H2O and O2
characterized by H2O2 strip and 1H NMR spectrum (Scheme 4e).
Note that the overall transformation of Cα alcohol oxidation
catalyzed by dinuclear vanadium complex 2 could also be re-
garded as a net H-atom transfer from the reactive C-H of the
benzylic alcohol.
We have summarized the catalytic selectivity and conversion
between complex 2 and some representative mononuclear va-
nadium catalysts in Table 4.[7b,7d,8b,18] In particular, we focus on
the alcohol oxidation of non-phenolic lignin model substrates
with a ꢀ-O-4 linkage mode and primary Cγ alcohol. Since the
alcohol oxidations of this research work is using air as oxidant,
we compared the results of benzyl alcohol oxidation of lignin
model substrates in literature that use air as oxidant. As indi-
cated in the Table 4, the selectivity of Cα alcohol oxidation via
the synthesized complex 2 (97% selectivity and 77% conver-
sion, entry 1) is enhanced as compared to the listed mono-
nuclear vanadium catalysts (28%-68% selectivity, entries 5–10)
with representative vanadium catalysts (V(O)(OiPr)3, V(O)(acac)2,
M1–M6). For the catalyst that performs slightly lower (> 10%)
selectivity and conversion (85% selectivity and 65% conversion,
entry 11), the reaction time is significantly longer than that in
this work (24 hours vs. 48 hours) and using pyridine as solvent.
As compared to the listed representative mononuclear vana-
dium catalysts, we believe that our catalyst system using dinu-
clear complex 2 as catalyst and pyridine as additive has made
significantly scientific advances in the field of selective oxid-
ation of benzyl alcohol in lignin model substrates containing a
ꢀ-O-4 linkage and a primary Cγ alcohol.
The catalytic mechanism has been proposed and divided
into several elementary steps involving (1) the coordination of
pyridine to the vanadium center of complex 2 and deprotona-
tion of secondary benzylic alcohol as well as primary aliphatic
alcohol located at the Cα and Cγ by the two V=O motifs of
dinuclear oxovanadium complex 2, respectively, characterized
1
by the H NMR and IR spectra. Therefore, the preservation of
V=O motif could be crucial for the binding of lignin substrates
to vanadium catalysts. (2) abstracting the Cα-H of deprotonated
lignin substrates chelating on dinuclear vanadium core via pyr-
idine followed by a two-electron transfer to the dinuclear vana-
dium core of intermediate 4 to form a transient stable dinuclear
vanadium with a VIV-VIV oxidation state and subsequently
cleave into two mononuclear intermediate 5 with VIV oxidation
state which has been characterized by its distinct 8-line feature
of VIV EPR spectrum and a magnetic susceptibility determined
by Evans method.[17] (3) regenerating the dinuclear oxovana-
dium complex 2 by reacting the two mononuclear VIV interme-
diate 5 with O2 or air and producing H2O2 by-product, which
decomposes into H2O and O2 at 80 °C, evidenced by a H2O2
1
test strip and H NMR spectra, respectively.
The excellent selectivity of dinuclear oxovanadium complex
2 could be attributed to the intrinsic nature of dinuclear vana-
dium core that is capable of serving as a two-electron reservoir
in order to facilitate the oxidation of alcohol into ketone (also
a well-known two-electron oxidation reaction). The mechanistic
insights regarding the relationships between the molecular/
electronic structure of the dinuclear oxovanadium catalyst and
the chemical reactivity of selective oxidation of Cα alcohol to
ketone of lignin dimeric substrates featuring a ꢀ-O-4 linkage
mode could be beneficial to the rational design of highly effi-
cient and selective molecular catalysts for harnessing lignin bio-
mass via two-step strategy.
Experimental Section
Conclusions
The detailed experimental procedures of synthesis of lignin dimeric
substrates, oxovanadium complexes, measurement of cyclic voltam-
In this study, the catalytic oxidation of Cα alcohol to ketone
of lignin dimeric substrates containing ꢀ-O-4 linkage has been
demonstrated via the catalyst system containing dinuclear oxo-
vanadium complex 2 with one equivalent of pyridine at 80 °C
under air. In contrast to the one reversible redox couple of
mononuclear oxovanadium catalyst 1 at E1/2 = –0.477 V, the
dinuclear oxovanadium complex 2 exhibits one reversible and
1
mogram, EPR were included in the supporting information. The H,
13C, 51V NMR spectra of complex 1 and 2; the HMBC spectrum of
S5; the overlapped IR spectrum of complex 2 and intermediate 4;
the 1H NMR spectrum of Evans method; the picture of H2O2 test
strip.
CCDC 1884038 (for 2) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre.
one irreversible redox couple at E1/2 = –0.055 V and E1/2
=
–0.335 V, respectively. Interestingly, the second redox wave of
substrate-chelating dinuclear vanadium intermediate 4 displays
a quasi-reversible behavior as compared to that of complex 2
(ipc/ipa ca. 0.1, ΔEp ca. 111 mV for complex 2; ipc/ipa ca. 0.3, ΔEp
Acknowledgments
ca. 95 mV for intermediate 4) implicating that the feasibility of We gratefully acknowledge financial support from the Ministry
intermediate 4 to exert facile chemical transformations involv- of Science and Technology (Taiwan). We also thank the National
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Received: July 24, 2019
Eur. J. Inorg. Chem. 2019, 4637–4646
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