Oxidation of Alcohols to Carbonyl Compounds Catalyzed by Oxo-Bridged Dinuclear Cerium Complexes with Pentadentate Schiff-Base Ligands under a Dioxygen Atmosphere

Article abstract of DOI:10.1021/acscatal.8b01718

Ionic mononuclear and neutral dinuclear complexes of cerium(III) 3-L¹-3-L⁹ bearing a series of dianionic pentadentate Schiff-base ligands were synthesized, characterized, and used as catalysts for N-oxyl radical-free aerobic alcohol oxidation. Reactions of Ce(NO₃)₃·6H₂O with o-tert-butyl-substituted sterically hindered ligands NH(CH₂CH₂-Rfnet=CHC₆H₂-3-(^tBu)-5-R²-2-OH)₂ (for L¹H₂, R² = ^tBu; for L²H₂, R² = OMe; and for L³H₂, R² = H) in the presence of triethylamine afforded the corresponding anionic cerium complexes [HNEt₃][Ce(L^1-3)(NO₃)₂] (3-L¹-3-L³), whereas complexation with sterically less hindered ligands, such as NH(CH₂CH₂N=CHC₆H₂-3-R¹-5-R²-2-OH)₂ (for L⁴H₂, R¹ = OMe and R² = H; for L⁵H₂, R¹ = H and R² = ^tBu; for L⁶H₂, R¹ = H and R² = OMe; for L⁷H₂, R¹ = H and R² = H; for L⁸H₂, R¹ = H and R² = NO₂; and for L⁹H₂, R¹ = ^tBu and R² = NO₂), afforded neutral dinuclear complexes [Ce(L^4-9)(NO₃)]₂ (3-L⁴-3-L⁹). Among these newly prepared complexes, complex 3-L¹ was selected as the best catalyst for oxidizing primary and secondary alcohols under a dioxygen atmosphere without any N-oxyl radicals such as TEMPO to produce the corresponding carbonyl compounds, where the oxo-bridged dinuclear complex worked as a catalyst while maintaining its dinuclear skeleton during the catalytic cycle. In addition, an intramolecular redox process between the two cerium centers through the bridging oxygen atom played a key role in forming the ligand phenoxide radical-mediated TEMPO-free alcohol oxidation reaction.

Full text of DOI:10.1021/acscatal.8b01718

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Article
Oxidation of Alcohols to Carbonyl Compounds Catalyzed
by Oxo-bridged Dinuclear Cerium Complexes with
Pentadentate Schiff-base Ligands under Dioxygen Atmosphere
Satoru Shirase, Koichi Shinohara, Hayato Tsurugi, and Kazushi Mashima
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01718 • Publication Date (Web): 14 Jun 2018
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ACS Catalysis
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Oxidation of Alcohols to Carbonyl Compounds Catalyzed by
Oxo-bridged Dinuclear Cerium Complexes with Pentadentate
Schiff-base Ligands under Dioxygen Atmosphere
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Satoru Shirase, Koichi Shinohara, Hayato Tsurugi,* and Kazushi Mashima*
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan.
ABSTRACT: Ionic mononuclear and neutral dinuclear complexes of cerium(III) 3-L1―3-L9 bearing a series of dianionic pentaden-
tate Schiff-base ligands were synthesized and characterized, and used as catalysts for N-oxyl radical-free aerobic alcohol oxidation.
Reactions of Ce(NO3)3·6H2O with ortho-tert-butyl-substituted sterically hindered ligands NH(CH2CH2N=CHC6H2-3-(tBu)-5-R2-2-
OH)2 (L1H2: R2 = tBu; L2H2: R2 =OMe; and L3H2: R2 = H) in the presence of triethylamine afforded the corresponding anionic cerium
complexes [HNEt3][Ce(L1-3)(NO3)2] (3-L1—3-L3), whereas complexation with sterically less-hindered ligands, such as
NH(CH2CH2N=CHC6H2-3-R1-5-R2-2-OH)2 (L4H2: R1 = OMe, R2 = H; L5H2: R1 = H, R2 = tBu; L6H2: R1 = H, R2 = OMe; L7H2: R1 =
H, R2 = H; L8H2: R1 = H, R2 = NO2; and L9H2: R1 = tBu, R2 = NO2), afforded neutral dinuclear complexes [Ce(L4-9)(NO3)]2 (3-L4—
3-L9). Among these newly prepared complexes, complex 3-L1 was selected as the best catalyst for oxidizing primary and secondary
alcohols under dioxygen atmosphere without any N-oxyl radicals such as TEMPO to produce the corresponding carbonyl compounds,
where the oxo-bridged dinuclear complex worked as a catalyst while maintaining its dinuclear skeleton during the catalytic cycle. In
addition, an intramolecular redox process between the two cerium centers through the bridging oxygen atom played a key role in
forming the ligand phenoxide radical mediated TEMPO-free alcohol oxidation reaction.
Keywords：cerium catalysts, aerobic alcohol oxidation, oxo-bridged dinuclear complex, Schiff-base ligand, co-catalyst free oxidation
1. INTRODUCTION
essential for achieving the co-catalyst-free aerobic alcohol oxi-
dation reaction,12 which is resemble to the natural enzymatic
system for alcohol oxidation such as Galactose oxidase.13 Ac-
cordingly, the development of such an oxidation process by
metal catalysts without any co-catalysts is a long-standing and
challenging task.
Oxidation of primary and secondary alcohols to the correspond-
ing carbonyl compounds is a critical transformation in organic
synthesis. A variety of oxidants, such as peracids, peroxides,
and chlorinated compounds, have been utilized so far, but these
reagents require harsh conditions and coproduce more than stoi-
chiometric amounts of waste.1 A recent requirement for organic
reactions is the development of environmentally friendly syn-
thetic protocols, and a notable alternative is aerobic catalytic
oxidation in which dioxygen is an oxidant and H2O is the sole
byproduct. Various transition metal complexes, including Cu,2
Co,3 Fe,4 Ru,5 Pd,6 and V, 7 have been intensively investigated
as catalysts for oxidative transformations using dioxygen.8 Al-
most all of these transition metal complexes require co-catalysts,
however, such as stable N-oxyl radicals: Semmelhack first re-
ported that a copper complex in combination with 2,2,6,6-tetra-
methylpyperidine-N-oxyl (TEMPO) as a co-catalyst became a
catalyst for a practical aerobic oxidation of alcohols, wherein
the copper catalyst mediated a one-electron redox and TEMPO
concurrently functioned to abstract the α-hydrogen atoms of al-
cohols (Figure 1a).9 Since this pioneering work, many highly
active copper/nitroxyl radical catalyst systems have been exten-
sively developed, and high catalytic performance has been real-
ized as unique cooperative effects of both the metals and or-
ganic co-catalysts, although the use of costly additives such as
N-oxyl radicals should be reduced and eliminated (Figure 1a).2
In this context, Kitajima10 and Stack11 independently demon-
strated that salen-type copper complexes are effective as cata-
lysts for oxidizing alcohols without any co-catalysts; during the
catalytic cycle, the formation of phenoxide radicals at the re-
dox-active salen ligands plays an important role for abstracting
α-hydrogen atoms from alcohols, in which the metal and the
ligand cooperatively support the oxidation reaction (Figure 1b).
In most cases, including these examples, phenoxide radicals are
Figure 1. Transition metal catalyst systems for aerobic alcohol
oxidation (a) with a co-catalyst and (b) without any co-catalyst.
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We recently focused our attention on cerium complexes, be- tert-butyl substituents at the ortho-positions and different sub-
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cause the cerium atom adopts two stable oxidation states, +3
and +4, and its one-electron redox couple can be utilized for a
large number of redox processes.14 Actually, Ce(IV)-based
compounds, such as cerium ammonium nitrate (CAN), are
widely used as versatile oxidants in organic transformations, bi-
oinorganic reactions, and electron-transfer chemistry.15 Alt-
hough CAN is commonly used as a stoichiometric oxidant,
CAN becomes a catalyst for the aerobic oxidation of alcohols
in the presence of TEMPO.16 This unique one-electron redox
nature of the cerium atom, similar to that of the copper metal,
prompted us to search for a cerium complex capable of catalyt-
ically oxidizing alcohols to the corresponding aldehydes. We
previously reported that a neutral cerium(IV) complex 1 bearing
a pentadentate Schiff-base ligand performs as an excellent cat-
alyst in the presence of TEMPO, and isolated a neutral mono-
nuclear Ce(III) complex 2 as an intermediate species that was
prepared by reducing the Ce(IV) complex 1 with an organosili-
con reducing reagent (Figure 1a).17 Herein, we report a catalyst
system of ionic mononuclear and neutral dinuclear complexes
of cerium(III) bearing Schiff-base ligands without any co-cata-
lyst for oxidizing primary and secondary alcohols to the corre-
sponding carbonyl compounds. In addition, we identified a
unique nature of oxo-bridged dinuclear cerium complexes in
which one of two cerium centers functioned as a catalytically
active site while the other cerium center supported an intramo-
lecular redox process for forming the ligand phenoxide radical
as well as proceeding this TEMPO-free oxidation reaction.
stituents at the para-positions. These complexes were fully
characterized by their spectral data, combustion analyses, and a
crystallographic study for 3-L3.
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Scheme 1. Synthesis of Mononuclear Cerium(III) Complexes
3-L1―3-L3 with Pentadentate Schiff-base Ligands.
Figure 2 shows the crystal structure of the anionic part of 3-
L1, in which the cerium atom adopts a nine-coordinated geom-
etry by three nitrogen atoms and two oxygen atoms in a dis-
torted penta-coordinated equatorial plane of the ligand and two
ҝ2-(O,O)-NO3 anions at both apical sites. The dihedral angle
(39.3°) between the two phenoxy rings indicates a deformation
of the penta-coordinated equatorial plane due to steric repulsion
between the two tert-butyl groups of the two phenoxy rings.
The distances of Ce―O(phenoxide) (2.29—2.30 Å) are normal
2. RESULTS AND DISCUSSION
2-1. Synthesis of Ionic Mononuclear Cerium Complexes
An
ionic
mononuclear
cerium(III)
complex,
[HNEt3][Ce(L1)(NO3)2] (3-L1), was prepared in 83% yield by
treating Ce(NO3)3·6H2O with a pentadentate Schiff-base ligand,
NH(CH2CH2N=CHC6H2-3-R1-5-R2-2-OH)2 (L1H2: R1 = R2 =
tBu), in the presence of 2 equiv of NEt3 (Scheme 1). Complex
3-L1 was characterized by NMR measurements, despite its par-
amagnetic nature, and a single crystal X-ray diffraction study
(Figure 2; vide infra) with its combustion analysis. Its 1H NMR
spectrum in CD3CN at 30 °C displayed four paramagnetically-
shifted broad singlets centered at δ 30.95, 18.15, 11.22, and -
42.10, which were assignable to an imine proton, two aromatic
protons, and an amine proton of L1; two singlets at δ 4.26 and -
t
4.26 due to two magnetically non-equivalent Bu groups; and
four broad resonances at δ -16.25, -6.37, 3.26, and 4.54 due to
four dissymmetric –CH2CH2– protons of the ligand. Three
ethyl groups of the triethylammonium salt were observed as a
rather sharp triplet at δ 1.14 and a quartet at δ 2.06, attributed to
the methyl and methylene protons, respectively. The observed
chemical shift values were almost the same as those for mono-
nuclear cerium(III) complex 2 with the same Schiff-base ligand,
indicating that the coordination environment of L1 in complex
3-L1 was almost the same as that of 2. Advantages of complex
3-L1 were air stability in solid state and easy handling for cata-
lytic reactions compared with 2, which decomposed quickly
upon exposure to air. Similarly, mononuclear ionic cerium
Figure 2. Molecular structures of 3-L1 and 3-L3 with 50% prob-
ability ellipsoids. All hydrogen atoms and HNEt3 are omitted
+
for clarity. Selected bond lengths (Å) and angles (degree) for 3-
L1: Ce—N1 2.6697(18), Ce—N2 2.7411(19), Ce—N3
2.6438(17), Ce—O1 2.3022(14), Ce—O2 2.2852(13), Ce—O3
2.6620(15), Ce—O4 2.6504(15), Ce—O6 2.566(2), Ce—O7
2.6615(17); N1—Ce—N2 64.71(6), N2—Ce—N3 63.10(6),
N1—Ce—N3 123.99(5), O1—Ce—O2 106.24(5), O3—Ce—
O4 48.03(4), O6—Ce—O7 48.23(6), O1—Ce—N2 69.04(5),
complexes
[HNEt3][Ce(L2)(NO3)2]
[3-L2:
L2
=
NH(CH2CH2N=CHC6H2-3-tBu-5-OMe-2-O)2]
and
[HNEt3][Ce(L3)(NO3)2] [3-L3: L3 = NH(CH2CH2N=CHC6H3-
3-tBu-2-O)2]) were prepared in 89% and 95% yields, respec-
tively, by treating Ce(NO3)3·6H2O with the corresponding
Schiff-base ligands L2H2 and L3H2, whose phenoxy rings have
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ACS Catalysis
with those reported for Ce(III)―O(µ-phenoxide) (2.45 Å),20
O2—Ce—N3 68.83(5); those for 3-L3: Ce—N1 2.6618(17),
1
2
3
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Ce—N2 2.6860(15), Ce—N3 2.6544(13), Ce—O1 2.3006(10),
Ce—O2 2.3127(14), Ce—O3 2.6287(15), Ce—O4 2.6046(11),
Ce—O6 2.6939(11), Ce—O7 2.6469(12); N1—Ce—N2
63.30(4), N2—Ce—N3 64.91(5), N1—Ce—N3 127.83(5),
O1—Ce—O2 103.70(4), O3—Ce—O4 48.87(5), O6—Ce—
O7 47.91(3), O1—Ce—N1 69.35(4), O2—Ce—N3 69.44(4).
and the distance of Ce―O(phenoxide) (2.31 Å) is almost the
same as that of the ionic mononuclear cerium complexes 3-L1
and 3-L3. The distances of Ce―N(amine, imine) (2.65―2.67
Å) are typical noncovalent distances of Ce―N (2.67―2.70 Å).
The distances of Ce―O(nitrate) (2.65―2.70 Å) are also normal
for cerium(III) nitrate complexes (2.53—2.67 Å), and some-
what elongated compared with 2 (2.59—2.61 Å) due to the
trans effect of the oxygen atoms of the bridging phenoxy lig-
ands.
for a single bond compared with that reported for other ce-
rium(III) complexes (2.16—2.28 Å).17,18 The distances of
Ce―N(amine, imine) (2.64―2.74 Å) in 3-L1 are typical for da-
tive interaction of Ce―N (2.67―2.70 Å).17,18 The distances be-
tween the cerium atom and oxygen atoms of one of the two ni-
trate Ce―O(nitrate) (2.57―2.66 Å) are also normal for ce-
rium(III) nitrate complexes (2.53—2.67 Å), whereas those of
the other Ce―O(nitrate) (2.65―2.66 Å) were somewhat elon-
gated compared with those of 2 (2.59—2.61 Å) due to hydrogen
bonding between a hydrogen atom of [HNEt3]+ and nitrate ox-
ygens.17,18 The molecular structure of 3-L3 is essentially the
same as that of 3-L1, and some of the structural data are listed
in Figure 2.
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Scheme 2. Synthesis of Dinuclear Cerium(III) Complexes 3-L4
―3-L9 with Pentadentate Schiff-base Ligands.
2-2. Synthesis of Neutral Dinuclear Cerium Complexes
We found that sterically less-hindered pentadentate ligands af-
forded the corresponding neutral dinuclear cerium(III) com-
plexes. Reaction of Ce(NO3)3·6H2O with L4H2 (R1 = OMe; R2
= H) in the presence of NEt3 resulted in the formation of
[Ce(L4)(NO3)]2 (3-L4) in 83% yield (Scheme 2). Complex 3-
L4 was characterized by NMR measurements, and its dinuclear
structure was revealed by mass spectroscopy of 3-L4, displaying
a peak for monocation, [M - NO3]+ (m/z = 1080.1), together
with a single crystal diffraction analysis (Figure 3; vide infra).
No NMR signals assignable to the ammonium salt were de-
tected, distinguishing the structure of 3-L4 from the anionic
mononuclear structure of 3-L1―3-L3. Due to the paramagnetic
nature of 3-L4, all signals in its 1H NMR spectrum in DMSO-d6
were upfielded, downfielded, and broadened; four broad sin-
glets observed at δ 22.52, 12.80, 9.16 and 9.01 were assigned to
an imine proton and three aromatic protons of L4; and four
broad signals centered at δ 6.40, 5.26, -0.44 and -8.22 corre-
sponded to ethylene protons; and a broad singlet at δ 2.02 was
attributed to the four OMe groups, suggesting that the four
iminophenolato moieties were magnetically equivalent. Such a
symmetric NMR pattern might be due to a rapid exchange of
the cerium atom between the oxygen atoms of µ-phenoxide and
phenoxide. Similarly, other less-hindered ligand such as L5 (R1
= H; R2 = tBu), L6 (R1 = H; R2 = OMe), L7 (R1 = R2 = H), and
L8 (R1 = H; R2 = NO2) gave the corresponding neutral dinuclear
cerium complexes [Ce(L5―8)(NO3)]2 (3-L5―3-L8) (Scheme 2),
while a rather bulky but electron-deficient ligand L9 (R1 = tBu;
R2 = NO2) bearing a tert-butyl group at the ortho position also
produced a neutral dinuclear complex [Ce(L9)(NO3)]2 (3-L9).
Figure 3. Molecular structure of 3-L4·(dmf)2 with 50% proba-
bility ellipsoids. All hydrogen atoms are omitted for clarity. Se-
lected bond distances (Å) and angles (degree): Ce1—N1
2.6747(19), Ce1—N2 2.645(2), Ce1—N3 2.6701(19), Ce1—
O1 2.3083(17), Ce1—O2 2.453(2), Ce1—O2* 2.5148(18),
Ce1—O3 2.5013(17), Ce1—O4 2.646(2), Ce1—O5 2.699(2);
N1—Ce1—N2 64.62(6), N1—Ce1—N3 63.75(7), O1—Ce1—
O2 92.84(7), O1—Ce1—N2 69.33(7), O2—Ce1—N3 66.87(7),
O4—Ce1—O5 47.71(6).
2-3. Cerium-catalyzed Oxidation of Alcohols to Aldehydes
under Dioxygen Atmosphere
Complex 3-L4 was crystallized from N,N’-dimethylforma-
mide (DMF) layered by Et2O at room temperature to give yel-
low crystals of 3-L4·(dmf)2. Figure 3 shows the dinuclear struc-
ture of 3-L4, in which one of two phenoxy oxygen atoms of the
ligand coordinates to the other cerium atom to hold a dinuclear
unit. The cerium atom adopts a nine-coordinated geometry of
one ҝ2-(O,O)-NO3 anion, one ҝ1-O-DMF, three nitrogen atoms,
and three oxygen atoms of the Schiff-base ligand. The distances
of Ce―O(µ-phenoxide) (2.45―2.51 Å) are in good accordance
With a series of ionic mononuclear cerium(III) complexes 3-
L1—3-L3 and neutral dinuclear cerium(III) complexes 3-L4—
3-L9 in hand, we tested the catalytic performance of these com-
plexes for selective oxidation of 4-methylbenzyl alcohol (4a) to
4-methylbenzaldehyde (5a), and the results are summarized in
Table 1. Upon starting from the bulky substituted ligand system,
under the conditions including complex 3-L1 (5 mol% catalyst
loading), MS 4Å, DMF, 10 h, 100 °C, and O2 (atmospheric
pressure), 4a was oxidized to give 5a in quantitative yield (entry
1). The substituents at the para-position of the phenoxy group
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of complexes 3-L2, 3-L3, and 3-L9 sensitively affected the cat-
substituted benzyl alcohols 4h,i were oxidized to 5h,i in good
alytic activity: complex 3-L2 having a methoxy group at the
para-position gave 5a in 84% yield and 3-L3 with a phenyl
group was less effective (44% yield), whereas 3-L9 bearing an
electron-withdrawing group (NO2) exhibited almost no cata-
lytic activity (entries 2―4). Similar substituent effects at the
para-position on catalytic activity were observed for complexes
3-L5―3-L8 that had less bulky ligands, though their catalytic
activities were relatively lower than those of 3-L1―3-L3 (en-
tries 5―8). Acceleration of the catalytic activity by the bulky
and electron-donating substituents at the 3- and 5-positions of
the phenoxy ring strongly suggested that phenoxide radicals
might be involved in the alcohol oxidation process; this is a sim-
ilar tendency to the reported TEMPO-free copper(II) catalyst
system that the phenoxide radical is stabilized by the bulky and
electron-donating substituents at the 3- and 5-positions, where
ligand phenoxide radials played a crucial role for the oxidation
process.12 Thus, we selected 3-L1 as the best catalyst for co-
catalyst free cerium-catalyzed alcohol oxidation under atmos-
pheric pressure of O2. Finally, we checked the catalytic oxida-
tion of 4a using 3-L1 in the presence of TEMPOH as an additive,
giving 5a in 95% yield with a slightly lower catalytic activity
compared to 3-L1 without TEMPOH.
yields after 20 h. Ortho-substituted benzyl alcohols 4j―m
were oxidized to 5j―m in moderate to excellent yields and
showed similar reactivity to the para-substituted benzyl alco-
hols 4a―e despite their large steric hindrances.
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Table 2. Catalytic Oxidation of Benzyl Alcohols.a,b
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aAlcohol (0.200 mmol), 3-L1 (0.010 mmol), MS 4Å (40 mg),
0.5 mL of DMF, O2 (balloon), 1,3,5-trimethoxybenzene as an
internal standard. bNMR yields. c20 h.
Oxidation of secondary benzyl alcohols 4n—q was slow, and
the corresponding ketones were obtained after a longer reaction
time with 8 mol% catalyst (Table 3). An activated secondary
benzyl alcohols 4n and 4o were effectively oxidized to give 5n
and 5o in 81% and >99% yield, respectively, whereas second-
ary benzyl alcohols 4p and 4q were less reactive to afford 5p
and 5q in 37% and 52% yield. Our catalytic system can be ap-
plied to oxidation of aliphatic alcohols (4r—4t). Cinnamyl al-
cohol (4r) was smoothly converted to 5r in good yield of 89%.
Secondary aliphatic alcohols 4s and 4t were respectively oxi-
dized to the corresponding ketones 5s and 5t in moderate to ex-
cellent yields of >99% and 42% yield.
Table 1. Oxidation of 4-Methylbenzyl alcohol to 4-Methylben-
zylaldehyde by Cerium Complexes under Dioxygen Atmos-
phere.a,b
entry
Ce complex condition
conv. (%)
1
2
3
4
5
6
7
8
3-L1
3-L2
3-L3
3-L9
3-L5
3-L6
3-L7
3-L8
-
-
-
-
-
-
-
-
>99
84
44
10
64
81
43
17
Table 3. Catalytic Oxidation of Secondary Benzyl Alcohols
and Aliphatic Alcohols. a,b
9
3-L1
3-L1
3-L1
with 10 mol% TEMPOH
under air
95
10
11
91
under Ar
no reaction
a4a (0.200 mmol), [Ce] (0.010 mmol on metal), MS 4Å (40 mg),
0.50 mL of DMF, O2 (balloon), 1,3,5-trimethoxybenzene as an
internal standard. bNMR yields.
We next screened various substituted benzyl alcohols as sub-
strates and the results are shown in Table 2. We increased the
temperature for benzyl alcohol derivatives using the best cata-
lyst 3-L1 to improve the product yields for a series of substrates.
Functionalized benzyl alcohols were oxidized to the corre-
sponding aldehydes under our oxidation system. Benzyl alco-
hol and its derivatives bearing an electron-donating substituent
at the para-position afforded the corresponding aldehydes in
excellent yields (Table 2, 5b,c). Para-fluoro and trifluorome-
thyl-substituted benzyl alcohol derivatives 4d,e were slowly ox-
idized to form the corresponding aldehydes 5d,e in moderate
yields of 63% and 80% after 20 h. Meta-methyl and methoxy-
substituted benzyl alcohol 4f,g were converted to the corre-
sponding aldehydes 5f,g in 90% and 86% yield, whereas meta-
aAlcohol (0.200 mmol), 3-L1 (0.016 mmol), MS 4Å (40 mg),
0.50 mL of DMF, O2 (balloon), 1,3,5-trimethoxybenzene as an
internal standard. bNMR yields. c5 mol%.
2-4. Reaction of Cerium(III) Complexes with Dioxygen Giv-
ing Dinuclear Peroxo and Oxo Complexes
We conducted stoichiometric reactions to elucidate how O2
played a role on activating catalyst precursors 3-L1—3-L3. In
fact, we exposed three ionic mononuclear Ce(III) complexes 3-
L1—3-L3 to O2, and we isolated the corresponding peroxo-
bridged dinuclear Ce(IV) complexes [Ce(L1―3)(NO3)]2(µ-
η2:η2-O2) (6-L1—6-L3) (Scheme 3). Bubbling dioxygen into the
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(1.33―1.51 Å).17,21 By changing the oxidation state of the ce-
solution of 3-L2 in CH3CN resulted in a rapid color change from
yellow to purple, and the corresponding peroxo complex 6-L2
was isolated as purple microcrystalline solids in 40% yield. The
UV-Vis spectrum of 6-L2 showed characteristic absorption at
540 nm, corresponding to those found for the dinuclear ce-
rium(IV) µ-peroxo complex 6-L1.17 The same experimental
procedure was applied to the complexes 3-L1 and 3-L3 to pre-
pare the corresponding peroxo complexes 6-L1 and 6-L3 in 86%
and 44% yields, respectively, though 6-L1 was already prepared
by the reaction of a neutral cerium(III) complex 2 with O2 in
CH3CN and crystallographically characterized.17 The UV-Vis
spectrum of 6-L3 was almost the same as that of 6-L1 and 6-L2,
displaying characteristic absorptions at 331 nm and 515 nm.
rium center from +3 to +4, the distances of the Ce—O(phenox-
ide) bonds are shortened (2.17 Å, ca. 0.11－0.14 Å) compared
with the anionic Ce(III) complexes 3-L1 and 3-L3, which is con-
sistent with the two-electron reduction of O2 to form a -2:2-
peroxo structure.
We measured the velocity for the conversion of 3-L1 under
atmospheric pressure of dioxygen to 6-L1 at -40 °C in CH3CN,
and Figure 5 shows the time-dependent UV-Vis spectra for 3-
L1. The absorption around 365 nm for 3-L1 rapidly decreased
with an increase in new absorptions at 340 and 530 nm due to
6-L1, where two isosbestic points at 354 and 399 nm were de-
tected during the transition. The reaction rate constant (kobs) for
the formation of 6-L1 was 7.58 x 10-4 [s-1].
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12000
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3000
.3
10000
1
2500
0.25
8000
0.8
y = 7.58E-04x + 1.60E-01
R² = 0.984
6000
0.6
200.20
Scheme 3. O2 Activation by Cerium(III) Complexes 3-L1―3-
L3 to form Peroxo-bridged Dinuclear Ce(IV) Complexes 6-
L1―6-L3.
70
120
170
220
4000
0.4
time [s]
2000
0.2
0
300
400
500
wavelength (nm)
600
700
Figure 5. Time-dependent UV-Vis spectra upon bubbling O2 to
3-L1 solution (0.1 mM in CH3CN) at -40 °C (kobs = 7.58 x 10-4
[s-1]). The UV transition was recorded in every 20 sec. The for-
mation rate of 6-L1 from 3-L1 was calculated based on the ab-
sorption at λ = 530 nm.
Peroxo-bridged dicerium complex 6-L1 was further converted
to the oxo-bridged dicerium complex 7-L1 upon heating the
CH3CN solution of 6-L1 at 70 °C. In fact, heating the reaction
mixture of the peroxo complex 6-L1 with or without 4-
methylbenzyl alcohol produced the corresponding oxo-bridged
dinuclear cerium(IV) complex, [Ce(L1)(NO3)]2(µ-O) (7-L1)
(Scheme 4), which were characterized by spectroscopic meth-
ods and crystallographic analysis (vide infra), though the de-
tailed reaction mechanism for the conversion from 6-L1 to 7-L1
was not clear. No transformation from 6-L1 to 7-L1 was ob-
served at room temperature. In the 1H NMR spectrum of 7-L1,
a singlet resonance for the imine protons of the Schiff-base lig-
and was observed at  7.98, while two singlet signals due to the
phenoxy moieties were detected at  7.60 and 7.10, suggesting
a symmetric structure in solution. In addition, we found two
singlet signals assignable to the tert-butyl groups at the ortho-
and para-positions of the phenoxy rings. All the signals for 7-
L1 were detected in the region typical for diamagnetic com-
plexes, indicating the formation of dinuclear Ce(IV)—Ce(IV)
species. A cyclic voltammogram of oxo-bridged dinuclear
Ce(IV) complex 7-L1 showed one reversible oxidation wave at
-0.273 V(vs Fc) assignable to the redox behavior of Ce(IV) and
Ce(III).19
Figure 4. Molecular structure of complex 6-L2 with 50% prob-
ability ellipsoids. All hydrogen atoms are omitted for clarity.
Selected bond distances (Å) and angles (degree): Ce—N1
2.637(7), Ce—N2 2.620(7), Ce—N3 2.541(7), Ce—O1
2.172(6), Ce—O2 2.175(5), Ce—O3 2.655(6), Ce—O4
2.541(6), Ce—O6 2.339(3), Ce—O7 2.288(2), O6—O7
1.484(10); N1—Ce—N2 62.5(2), N2—Ce—N3 65.4(3), O1—
Ce—O2 95.5(2), O1—Ce—N1 69.5(2), O2—Ce—N3 71.4(2),
O3—Ce—O4
48.71(19),
Ce1―O6―Ce1*
138.9(4),
Ce1―O7―Ce1* 146.3(4).
The crystal structure of complex 6-L2 is shown in Figure 4.
Two cerium centers are bridged by side-on bounded peroxo lig-
and to form a dinuclear structure. Bond distances around the
cerium center are typical for the cerium(IV) complexes. The
distances of the Ce(IV)―O(peroxo) bonds (2.29―2.34 Å)
around the µ-peroxo moiety are in good accordance with those
reported for 6-L1 and other dinuclear Ce(IV) µ-peroxo com-
plexes (2.24―2.38 Å).17,21 The distance of the O―O(peroxo)
bond (1.48 Å) is almost the same to 6-L1, and is long enough to
be characterized as an O—O single bond that is typically found
in other dinuclear µ-η2:η2-peroxo complexes of cerium
Figure 6 shows the molecular structure of 7-L1, in which each
cerium center adopts a distorted 8 coordination geometry and
each cerium unit correlates with a C2 operation. The distance
of the Ce–O(µ-oxo) bond (2.08 Å) is typical for Ce(IV)–O(µ-
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oxo) (2.07―2.18 Å) bonds.20 The Ce―O―Ce angle (169°) is
Page 6 of 11
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2
3
4
5
6
7
8
more acute than that in the Ce(III) μ-oxo dinuclear complex
[CeCp*2(thf)]2(μ-O) (Cp* = η5-C5Me5) (176°),22 probably due
to the steric repulsion of the pentadentate ligand. Dinuclear
Ce(IV) μ-oxo complexes are uncommon, although bis(µ-
oxo)dinuclear complexes as well as polynuclear oxo bridged ce-
rium complexes were previously reported.21-23 The distances of
the Ce–O(phenoxide) bond (2.20―2.21 Å) are slightly elon-
gated compared with those of peroxo-bridged dinuclear Ce(IV)
complex 6-L1, probably due to the steric repulsion between the
supporting ligands. The distance of the Ce–O(nitrate) bond
(2.53 Å) is reasonable compared with that of reported Ce(IV)
complexes, such as complex 1 (2.51―2.52 Å).17
17000
15000
13000
11000
9000
5000
4500
4000
3500
y = -5.03x + 4660
R² = 0.998
0
100
200
7000
time[s]
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3000
1000
300 350 400 450 500 550 600 650 700
wavelength (nm)
Figure 7. Time-dependent UV-Vis spectra of 6-L1 to 7-L1 (0.2
mM in CH3CN) at 70 °C (kobs = 6.41 x 10-3 [s-1]). The UV tran-
sition was recorded in every 30 seconds. The formation rate of
7-L1 from 6-L1 were calculated based on the absorption at λ =
580 nm.
Scheme 4. Synthesis of Oxo-bridged Dinuclear Ce(IV) Com-
plex 7-L1.
We tested both of peroxo- and oxo-bridged dinuclear Ce(IV)
complexes 6-L1 and 7-L1 for the catalytic alcohol oxidation re-
action under the optimal conditions (Scheme 5). We found that
the complexes 6-L1 and 7-L1 showed catalytic activity for oxi-
dation of 4a, giving 5a in 88% and 85% yield, respectively, sug-
gesting that 3-L1 was readily converted to 7-L1 via 6-L1 upon
exposure to dioxygen. Thus, 7-L1 was an actual catalyst pre-
cursor of the catalytic cycle (vide infra).
Scheme 5. Catalytic Activity of Complexes 6-L1 and 7-L1: 4a
(0.200 mmol), [Ce] (0.0050 mmol), MS 4Å (40 mg), 0.5 mL of
DMF, O2 (balloon), 1,3,5-trimethoxybenzene as an internal
standard.
Figure 6. Molecular structure of complex 7-L1 with 50% prob-
ability ellipsoids. All hydrogen atoms and methyl groups of tBu
substituents are omitted for clarity. Selected bond distances (Å)
and angles (degree); Ce—N1 2.621(3), Ce—N2 2.606(3), Ce—
N3 2.600(3), Ce—O1 2.211(2), Ce—O2 2.196(2), Ce—O3
2.525(2), Ce—O4 2.531(2), Ce—O6 2.0804(4); N1—Ce—N2
62.56(9), N2—Ce—N3 65.55(9), O1—Ce—O2 93.87(8),
O1—Ce—N1 71.20(8), O2—Ce—N3 69.96(7). O3—Ce—O4
50.38(9), Ce1―O6―Ce1* 168.99(16).
2-5. Kinetic Study of the Alcohol Oxidation by Oxo-bridged
Dinuclear Ce(IV) Complex 7-L1
We investigated the catalytic reaction profile for the oxidation
of 4a by oxo-bridged dinuclear Ce(IV) complex 7-L1 to clarify
the dependence of the catalyst concentration. Under O2 at at-
mospheric pressure, the initial reaction rate constants (kobs) were
determined by monitoring the amount of 4-methylbenzaldehyde
under five different catalyst concentrations. A plot of the kobs
showed a linear dependence on the initial concentration of 7-L1
in the range of 1.9—7.4 mM (Figure 8a), clearly indicating that
the reaction was first-order for catalyst 7-L1. Thus, during the
reaction, the oxo-bridged dinuclear structure was kept intact
without any dissociation into the monomeric form. In addition,
the kobs was linearly fitted to the initial concentration of 4-
methylbenzyl alcohol in the range of 2.0―6.0 M (Figure 8b).
Thus, the reaction rate was first-order in both of [7-L1] and [al-
cohol]; a rate law obeying kobs[7-L1][alcohol].
Thermolysis of 6-L1 was monitored by UV-Vis spectra for
determining the formation rate of the corresponding oxo-
bridged complexes 7-L1 (Figure 7).17 Absorption for peroxo
complex 6-L1 gradually decreased with the appearance of new
absorptions at λmax = 331 and 486 nm. During this transition,
we found an isosbestic point at 491 nm. Formation of 7-L1 from
6-L1 was much slower than the peroxo formation from 3-L1 and
required a higher temperature. We further checked the effects
of O2 and benzyl alcohol for the thermolysis of 6-L1 to 7-L1:17
no rate accelerations were observed for the transition, suggest-
ing that interactions of O2 and benzyl alcohol with cerium were
not involved in the oxo-bridged formation.
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and while the other cerium forms (bridging-oxo)(nitro)ce-
1
2
3
4
5
6
7
8
rium(IV) to generate A’. Dissociation of the oxo-bridged dinu-
clear structure to the monomeric form during the redox process
was excluded due to the reversible wave of the complex 7-L1 in
the cyclic voltammogram.19 The next step is that the phenoxide
radical abstracts an -hydrogen atom from the benzylalkoxy
moiety bound to the cerium(III) atom to form benzyl-
alkoxycerium(III) species B with a carbon radical  to the oxy-
gen atom. Subsequent homolysis of the Ce—O(alkoxy) bond
in species B proceeds to afford benzaldehyde with one-electron
reduction of the oxo-bridged dicerium center, producing
[Ce(III)]——[Ce(III)]+ dinuclear intermediate C. Thus, the oxo-
bridged dinuclear motif plays an important role for the elec-
tronic communication between the two cerium centers in co-
catalyst free alcohol oxidation. Further reaction of zwitterionic
complex C with dioxygen generates a hydroperoxo species of
Ce(IV)―Ce(III) D by a concerted one-electron reduction of O2
and hydrogen abstraction from the phenol moiety of the ligand.
Such the phenoxide radical-assisted H-atom transfer from the
alkoxymetal species to O2 was reported for co-catalyst-free al-
cohol oxidation by copper(II) and zinc(II) complexes with
Schiff-base ligands.12 Intermediate A’ is regenerated by an ex-
change reaction of the hydroperoxy ligand in D and 4a, giving
hydrogenperoxide. In this catalytic cycle, the rate-determining
step is expected to be an H-atom abstraction by the ligand phe-
noxide radical, based on the KIE value (2.08) for the oxidation
of PhCH2OH/PhCD2OH.19 During the catalytic reaction, it is
assumed that MS 4Å functions for not only decomposing the in
situ-generated hydrogen peroxide as a heterogeneous catalyst24
but also trapping the in situ-generated HNO3 as a weak base to
prevent a reverse reaction from A to 7-L1.
3.00E-04
2.50E-04
2.00E-04
1.50E-04
1.00E-04
5.00E-05
(a)
y = 2.80E-05x + 6.58E-05
R² = 0.997
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
7-L1 [mM]
9
2.00E-02
1.60E-02
1.20E-02
8.00E-03
4.00E-03
0.00E+00
(b)
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16
y = 2.71E-03x - 3.21E-04
R² = 0.995
0.0 1.0
2.0 3.0
4.0 5.0 6.0
7.0 8.0
Initial Concentration of 4a [M]
Figure 8. Reaction rate dependence on (a) the catalyst concen-
tration and (b) the alcohol concentration.
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2-6. Reaction Mechanism
Scheme 6 shows a plausible mechanism for the co-catalyst-free
cerium-catalyzed alcohol oxidation reaction starting from the
oxo-bridged dinuclear Ce(IV) complex 7-L1, generated from 3-
L1 and O2 (vide supra). It is assumed that complex 7-L1 reacts
with 4a to form a dinuclear Ce(IV) benzylalkoxy complex A
with a release of HNO3 because of the 1st order dependence on
the concentration of 7-L1 for the catalytic reaction as well as the
stability of the oxo-bridged dinuclear skeleton in 7-L1 when
heated in the presence of 4a under Ar atmosphere. In the pres-
ence of external radical sources such as TEMPO, abstraction of
H-atom at the -position to the oxygen atom proceeds by the
radical, as our observation for Ce(IV)/TEMPO/O2-catalyzed al-
cohol oxidation.17 In the absence of TEMPO, formation of the
phenoxide radical in the ligand framework is indispensable for
the catalytic transformation, similar to the Galactose oxidase.13
Thus, in this benzylalkoxy-bound dinuclear Ce(IV) species A,
one of the two cerium centers accepts one-electron to be re-
duced to Ce(III) with a phenoxide radical at the Schiff-base lig-
3. CONCLUSION
We herein report a TEMPO-free cerium complex-catalyzed ox-
idation of primary and secondary alcohols to the corresponding
carbonyl compounds, as well as the synthesis of a series of ce-
rium catalyst precursors 3-L1 ―3-L9 bearing a pentadentate
Schiff-base ligand with electron-donating and –withdrawing
substituents on the phenoxy ring. In relation to the mechanistic
study, we successfully isolated oxo-bridged dicerium complex
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Page 8 of 11
7-L1 upon exposure of 3-L1 to dioxygen and further heating, as
Dioxygen by Cobaloxime and Nitric Oxide for Efficient
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3
4
5
6
7
8
a pre-catalyst for the alcohol oxidation, and proposed oxo-
bridged dicerium alkoxy complex A as the key intermediate. To
the best of our knowledge, this is the first example of the oxo-
bridged dinuclear cerium complex that worked as a catalyst for
the aerobic oxidation of alcohols while maintaining its dinu-
clear skeleton, in which an electronic communication through
the bridging oxygen atom is a key step. Further exploitation of
the oxo-bridged dinuclear complexes to catalyze other reactions
is ongoing in our laboratory.
TEMPO-Catalyzed Oxidation of Alcohols. Adv. Synth. Catal.
2011, 353, 1146–1152. (c) Iwahama, T.; Sakaguchi, S.;
Nishiyama, Y.; Ishii, Y. Aerobic Oxidation of Alcohols to Car-
bonyl Compounds Catalyzed by N-Hydroxyphthalimide
(NHPI) Combined with Co(acac)3. Tetrahedron Lett. 1995, 36,
6923–6926.
(4) Jiang, X.; Zhang, J.; Ma, S. Iron Catalysis for Room-Tem-
perature Aerobic Oxidation of Alcohols to Carboxylic Acids. J.
Am. Chem. Soc. 2016, 138, 8344−8347.
(5) (a) Kondo, T.; Kimura, Y.; Kanda, T.; Takagi, D.; Wada,
K.; Toshimitsu, A. Simple and Practical Aerobic Oxidation of
Alcohols Catalyzed by a (μ-Oxo)tetraruthenium Cluster. Green
Sustain. Chem. 2011, 1, 149–154. (b) Mizoguchi, H.; Uchida,
T.; Katsuki, T. Ruthenium-Catalyzed Oxidative Kinetic Reso-
lution of Unactivated and Activated Secondary Alcohols with
Air as the Hydrogen Acceptor at Room Temperature. Angew.
Chem., Int. Ed. 2014, 53, 3178–3182.
(6) (a) Blackburn, T. F.; Schwartz, J. Homogeneous Catalytic
Oxidation of Secondary Alcohols to Ketones by Molecular Ox-
ygen under Mild Conditions. J. Chem. Soc., Chem. Commun.
1977, 157. (b) Popp, B. V.; Stahl, S. S. Palladium-Catalyzed
Oxidation Reactions: Comparison of Benzoquinone and Molec-
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2007, 22, 149-189.
(7) (a) Kirihara, M.; Ochiai, Y.; Takizawa, S.; Takahata, H.;
Nemoto, H. Aerobic Oxidation of α-Hydroxycarbonyls Cata-
lysed by Trichlorooxyvanadium: Efficient Synthesis of α-Di-
carbonyl Compounds. Chem. Commun. 1999, 1387–1388. (b)
Maeda, Y.; Kakiuchi, N.; Matsumura, S.; Nishimura, T.;
Uemura, S. Oxovanadium Complex-catalyzed Oxidation of
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(8) (a) Cardona F, Parmeggiani C. In Transition Metal Catal-
ysis in Aerobic Alcohol Oxidation; RSC: UK, 2015. (b) Pérez,
P. In Advances in Organometallic Chemistry 1st ed.; Academic
Press; 2015, Vol. 63. (c) Parmeggiani, C.; Cardona, F. Transi-
tion Metal Based Catalysts in the Aerobic Oxidation of Alco-
hols. Green Chem. 2012, 14, 547–564. (d) Schultz, M. J.; Sig-
man, M. S. Recent Advances in Homogeneous Transition
Metal-catalyzed Aerobic Alcohol Oxidations. Tetrahedron
2006, 62, 8227–8241.
(9) (a) Semmelhack, M. F.; Schmid, C. R.; Cortés, D. A.;
Chou, C. S. Oxidation of Alcohols to Aldehydes with Oxygen
and Cupric Ion, Mediated by Nitrosonium Ion. J. Am. Chem.
Soc. 1984, 106, 3374-3376. (b) Hoover, J. M.; Ryland, B. L.;
Stahl, S. S. Mechanism of Copper(I)/TEMPO-Catalyzed Aero-
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2367.
(10) Kitajima, N.; Whang, K.; Moro-oka, Y.; Uchida, A.; Sa-
sada, Y. Oxidations of Primary Alcohols with a Copper(II)
Complex as a Possible Galactose Oxidase Model. J. Chem. Soc.,
Chem. Commun. 1986, 1504-1505.
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ASSOCIATED CONTENT
The Supporting Information is available free of charge on the
ACS Publications website http://pubs.acs.org.
All synthetic procedures, experimental details, spectral characteri-
zation, kinetic procedures, data and additional discussion (PDF)
Structure data for 3-L1 (CIF)
Structure data for 3-L3 (CIF)
Structure data for 3-L4 (CIF)
Structure data for 6-L2 (CIF)
Structure data for 7-L1 (CIF)
AUTHOR INFORMATION
Corresponding Author
*Email: tsurugi@chem.es.osaka-u.ac.jp.
*Email: mashima@chem.es.osaka-u.ac.jp
ORCID
Satoru Shirase: 0000-0002-6932-085X
Koichi Shinohara: 0000-0003-1622-0756
Hayato Tsurugi: 0000-0003-2492-7705
Kazushi Mashima: 0000-0002-1316-2995
Funding Sources
JSPS KAKENHI Grant Nos. JP15KT0064 (Grant-in-Aid for Sci-
entific Research on Innovative Areas (B)) and JP15H05808 (Pre-
cisely Designed Catalysis with Customized Scaffolding).
ACKNOWLEDGMENT
S. S. thanks the financial support by the JSPS Research Fellow-
ships for Young Scientists. This work was supported by JSPS
KAKENHI Grant Nos. JP15KT0064 (Grant-in-Aid for Scientific
Research on Innovative Areas (B)) to H. T. and JP15H05808 (Pre-
cisely Designed Catalysis with Customized Scaffolding) to K. M.
We thank Prof. Dr. Sonja Herres-Pawlis in RWTH Aachen Univer-
sity for a fruitful discussion.
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