Synergistic oxygen atom transfer by ruthenium complexes with non-redox metal ions

Article abstract of DOI:10.1039/c6dt01077f

Non-redox metal ions can affect the reactivity of active redox metal ions in versatile biological and heterogeneous oxidation processes; however, the intrinsic roles of these non-redox ions still remain elusive. This work demonstrates the first example of

Full text of DOI:10.1039/c6dt01077f

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Synergistic oxygen atom transfer by ruthenium
complexes with non-redox metal ions†
Cite this: DOI: 10.1039/c6dt01077f
Zhanao Lv, Wenrui Zheng, Zhuqi Chen,* Zhiming Tang, Wanling Mo and
Guochuan Yin*
Non-redox metal ions can affect the reactivity of active redox metal ions in versatile biological and
heterogeneous oxidation processes; however, the intrinsic roles of these non-redox ions still remain
elusive. This work demonstrates the first example of the use of non-redox metal ions as Lewis acids to
sharply improve the catalytic oxygen atom transfer efficiency of a ruthenium complex bearing the classic
2
,2’-bipyridine ligand. In the absence of Lewis acid, the oxidation of ruthenium(II) complex by PhI(OAc)
2
generates the Ru(IV)vO species, which is very sluggish for olefin epoxidation. When Ru(bpy) Cl was
2
2
tested as a catalyst alone, only 21.2% of cyclooctene was converted, and the yield of 1,2-epoxycyclo-
octane was only 6.7%. As evidenced by electronic absorption spectra and EPR studies, both the oxidation
2
of Ru(II) by PhI(OAc) and the reduction of Ru(IV)vO by olefin are kinetically slow. However, adding non-
redox metal ions such as Al(III) can sharply improve the oxygen transfer efficiency of the catalyst to 100%
conversion with 89.9% yield of epoxide under identical conditions. Through various spectroscopic
characterizations, an adduct of Ru(IV)vO with Al(III), Ru(IV)vO/Al(III), was proposed to serve as the active
species for epoxidation, which in turn generated a Ru(III)–O–Ru(III) dimer as the reduced form. In particu-
lar, both the oxygen transfer from Ru(IV)vO/Al(III) to olefin and the oxidation of Ru(III)–O–Ru(III) back to
the active Ru(IV)vO/Al(III) species in the catalytic cycle can be remarkably accelerated by adding a non-
redox metal, such as Al(III). These results have important implications for the role played by non-redox
metal ions in catalytic oxidation at redox metal centers as well as for the understanding of the redox
mechanism of ruthenium catalysts in the oxygen atom transfer reaction.
Received 19th March 2016,
Accepted 7th June 2016
DOI: 10.1039/c6dt01077f
www.rsc.org/dalton
recently, and the oxidation mechanisms of RuvO species have
been thoroughly investigated. Attempts have been made to
Introduction
Ruthenium based compounds have attracted continuous atten- optimize the performance of ruthenium catalysts through
tion because they are versatile synthetic tools for selective oxi- chemical
dation processes in both homogeneous and heterogeneous ligands.
modification
of
their
attached
organic
6
,18,29,41–44
However, chemical modification of organic
1
–11
conversions, including asymmetric epoxidation of olefins,
ligands still has major drawbacks, such as high cost, un-
C–C predictable stability, and time-consuming procedures;
and water examples also exist where unexpected steric hindrance in the
In most cases, the active site of the Ru center is first coordination sphere strongly influences the reactivity of a
1
2–16
17–19
oxidation of alcohols,
oxidative esterification,
2
0–23
24–26
coupling,
oxidation of aryl sulfides,
2
7–32
oxidation.
recognized as a RuvO group in which ruthenium is in a RuvO system.
2
4,43
3
,5,33–40
formal high oxidation state (Ru(IV) or Ru(VI)).
Due to the
Alternatively, non-redox metal ions serving as Lewis acids
rich oxidative properties of the RuvO species, a large number offer another efficient strategy for regulating the reactivity of
of examples related to ruthenium catalysts have emerged redox metal ions through bridging or ligation with the metal-
oxo or metal-hydroxo moiety in a variety of homogeneous
chemical transformations. The role of these non-redox metal
ions has attracted much attention in both the biological and
Key laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong
University of Science and Technology), Ministry of Education, Key laboratory of
chemical communities because understanding how these non-
redox metal ions participate in oxidation events and affect the
Material Chemistry and Service Failure, School of Chemistry and Chemical
Engineering, Huazhong University of Science and Technology, Wuhan 430074,
PR China. E-mail: zqchen@hust.edu.cn, gyin@hust.edu.cn
reactivity of redox-active metal ions is highly important for
†
Electronic supplementary information (ESI) available: Crystallographic data in
45–48
understanding the mechanism of metalloenzymes,
such
CIF and GC-MS graph, NMR and HSQC spectra, UV-Vis absorption spectra or
other reaction information. CCDC 1411258. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c6dt01077f
as the essential roles of Ca(II) in Photosystem II in water oxi-
4
9
dation. On the other hand, many non-redox metal ions are
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frequently employed as additives to improve the stability aspects of the oxidation, which can also be useful in hetero-
and/or to manipulate the activity of transition metal catalysts; geneous catalysis as well as biological metalloenzymes.
5
0,51
2+
however, their pivotal role is still elusive.
Ruthenium imine complexes, such as [Ru(bpy)
n
]
(bpy =
To date, the available examples of adding Lewis acids to 2,2′-bipyridine), are among the most well-known ruthenium
manipulate the stoichiometric oxidation properties of tran- complexes and have been investigated extensively as analogues
5
2–56
57–61
62,63
sition metals include manganese,
iron,
cobalt
to natural chromophores for utilization in water oxidation as
6
4
and osmium complexes. Collins found that non-redox metal well as key materials and building blocks for various photo-
ions can accelerate triphenylphosphine oxygenation by their nic/electronic devices.
2
9,32,67–70
Thus, the fundamental knowl-
(
TAML)Mn(V)(O) analogues but do not improve olefin epoxi- edge of the redox properties of bpy based RuvO species is of
5
2
dation. Borovik showed that Ca(II) has a marked influence on paramount importance to understand and optimize its redox
the rate of dioxygen activation by a monomeric Mn(II) complex, performance. However, the examples of catalytic oxidation by
and a heterobimetallic complex containing Mn(III)–(μ-OH)– [Ru(bpy)
2
+
n
]
are surprisingly limited; to the best of our knowl-
Ca(II) cores was confirmed by its single crystal X-ray struc- edge, no example of an efficient oxygen atom transfer reaction
5
6
2+
ture. Goldberg observed that Zn(II) may interact with the catalyzed by [Ru(bpy)n] has been reported. Here, we present a
Mn(V)uO group in a (corrolazine)Mn(V)(O) complex and accel- non-redox metal ion-promoted oxygen atom transfer reaction
erate its rate in both hydrogen abstraction and electron trans- with ruthenium imine complexes for the first time. The
5
5,65,66
fer reactions.
Sc(III) to (N4Py)Fe(IV)(O) complex can accelerate electron-trans- cyclooctene to a yield of 89.9% with the cis-Ru(bpy)
fer reactivity because of a large positive shift in the one-elec- lyst, whereas a yield of only 6.7% can be achieved with the
tron reduction potential by binding the Sc(III) cation. Also, ruthenium(II) complex alone. A comprehensive mechanism
the mechanism of sulfoxidation by Fe(IV)vO was changed explaining this Lewis acid-accelerated-epoxidation was further
from a direct oxygen atom transfer pathway to an electron investigated and may shed some light on the factors that favor
transfer pathway. Lau also demonstrated that adding FeCl3 the oxidation reactivity of ruthenium catalysts; it also illus-
as a Lewis acid can accelerate catalytic alkane oxidation by gen- trates a novel strategy to improve the catalytic reactivity of a
erating a [Os(VIII)(N)(O)Cl
Nam and Fukuzumi revealed that binding addition of Al(III) dramatically accelerated the epoxidation of
2
2
Cl cata-
5
9
5
8
−
64
4
] /Lewis acid adduct.
Borovik variety of redox metal complexes.
further accomplished the synthesis and characterization of a
Co(III)–(μ-OH)–Ca(II) complex and revealed the changes in the
redox properties of Co(II) complexes correlated with the coordi-
Experimental section
6
3
nation of Ca(II).
The above findings clearly demonstrate that non-redox The ruthenium(II) complex cis-Ru(bpy)
2
Cl
Iodosobenzene diacetate (PhI-
high valent transition metal ions, although in most examples, (OAc)2), sodium trifluoromethanesulfonate (NaOTf), mag-
only accelerated electron transfer rates were observed. In par- nesium trifluoromethanesulfonate (Mg(OTf) ), and scandium
ticular, most of these reported examples are based on stoichio- trifluoromethanesulfonate (Sc(OTf) came from Aladdin.
metric oxidations, while the examples of Lewis-acid- Other trifluoromethanesulfonates, including Ca(OTf)2,
accelerated catalytic oxidation, which are more analogous to Al(OTf) , Y(OTf) and Ba(OTf) , were purchased from Shanghai
biological metalloenzymes and chemical oxidations by redox DiBai Chemical Company. Olefins and epoxides were pur-
2
was synthesized
4
4,71
metal ions as Lewis acids can modulate the redox behaviors of according to the literature.
2
3
)
3
3
2
1
8
18
oxide catalysts, remain limited. Recently, we found that the chased from either Aldrich or Alfa Aesar. H2 O (97%
O
presence of non-redox metal ions such as Al(III) can accelerate atom) came from Aladdin. In all cases, solvents were de-
the catalysis of sulfide oxidation by a manganese complex hydrated via 4 Å MS (activated at 400 °C for 2 h in a muffle
5
3
having a cross-bridged cyclam ligand. Moreover, we observed furnace) prior to use. Di-2-pyridyl ketone (dpk), 2,2′-bipyri-
the first example in which non-redox metal ions greatly midine (bpm) and 2,2′-bipyridine (bpy) were purchased from
promote the catalytic epoxidation of olefin by dissociating the Sinopharm Chemical Reagents. 2,3-Bis(2-pyridyl) pyrazine
sluggish Mn(III)–(μ-O) –Mn(IV) species and generating the (bpp) was from Aldrich and 1,10-phenanthroline-5,6-dione
2
n+
54
M vO/Lewis acid adduct as the active species. Inspired by (ptd) was synthesized following the procedure in our previous
the above findings, we wished to discern whether the addition publication. The structures of these ligands have been illus-
7
2
of non-redox metal ions could also manipulate the physical trated in Scheme 1.
1
and chemical properties of a Ru catalyst by interacting with
H NMR data of ligands: bpy (DMSO-d6): δ 7.47 (t, 2H), 7.96
): δ 7.56 (m, 2H),
mizing the performance of the ruthenium catalyst through 8.41 (m, 2H), 9.06 (m, 2H); bpm (CDCl ): δ 7.38 (t, 2H), 8.95 (d,
the RuvO species. In contrast to the extensive studies on opti- (t, 2H), 8.40 (d, 2H), 8.70 (d, 2H); ptd (CDCl
3
3
3
chemical modification of its organic ligands, introducing non- 4H); dpk (CDCl ): δ 7.46 (m, 2H), 7.89 (m, 2H), 8.09 (m, 2H),
redox metal ions as additives demands no extra synthetic pro- 8.75 (m, 2H); bpp (DMSO-d6): δ 7.28 (m, 2H), 7.85 (m, 4H),
cedures and can be easily performed. More importantly, 8.21 (m, 2H), 8.76 (m, 2H).
understanding how the electronic and catalytic properties of
UV-Vis spectra were collected on an Analytik Jena Specord
these complexes in various states are modulated by non-redox 205 spectrophotometer. GC-MS analysis was performed on an
metal ions will provide more information on the mechanistic Agilent7890A/5975C system. EPR experiments were conducted
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carried out in parallel. The reactions were performed at least
in triplicate, and the average data are used in the discussion.
General procedure for Lewis acid promoted catalytic
epoxidation by ruthenium(II) complex in the presence of water
A mixture solution of 0.8 mL of acetone and 0.2 mL of CH
containing 1 mM cis-Ru(bpy) Cl , 2 mM 2,2′-bipyridine and
mM PhI(OAc)2 was stirred at room temperature for 5 h.
Then, 0.1 mmol olefin, 0.002 mmol Lewis acid, 0.2 mmol
PhI(OAc) and a certain amount of water were added to this
solution. The reaction solution was stirred in a water bath at
08 K for 10 h. The yield of epoxide and the conversion of
olefin were quantitatively analyzed by HPLC using the internal
standard method, and the yield of benzaldehyde was quantitat-
ively analyzed by GC using the internal standard method.
Control experiments with the ruthenium(II) complex or Lewis
2 2
Cl
2
2
5
2
3
Scheme 1 Molecular structures of the ligands studied in this study.
at 130 K on a Bruker A200 instrument, with a center field of acid alone as the catalyst were carried out in parallel. Reactions
352.488 G, frequency of 9.395 GHz, power of 19.44 mW, were performed at least in triplicate, and the average data are
3
modulation amplitude of 2.00 G and receiver gain of 1.00 × used in the discussion.
3
1
0 . X-Ray photoelectron emission spectra were obtained on a
Lewis acid promoted catalytic epoxidation by ruthenium(II)
complex in the presence of O-water
Kratos AXIS ULTRA DLD-600 W system, and all photoelectron
1
8
1
peaks were referenced to the carbon C 1s peak at 284.4 eV. H
1
3
and C NMR spectra were recorded on a Bruker AV-400 instru- The mixture solution of 0.8 mL of acetone and 0.2 mL of
ment using TMS as an internal reference; extra PhI and CH Cl containing 1 mM cis-Ru(bpy) Cl , 2 mM 2,2′-bipyridine
and 5 mM PhI(OAc) was stirred at room temperature for 5 h.
Then 0.1 mmol olefin, 0.002 mmol Lewis acid, 0.2 mmol
2
2
2
2
oxidant were removed by washing with cyclohexane.
2
1
8
18
2 2
PhI(OAc) and 0.1 mL H O (97% O enrichment as received)
General procedure for Lewis acid promoted catalytic
epoxidation by ruthenium(II) complexes
were added to this solution. The reaction mixture was stirred
in a water bath at 308 K for 10 h, and the product analysis was
performed by GC-MS using the same procedure as that used in
2 2
A mixture solution of 0.8 mL of acetone and 0.2 mL of CH Cl
containing 1 mM ruthenium(II) complex, 2 mM 2,2′-bipyridine
1
8
normal epoxidation. The O enrichments are calculated based
and 5 mM PhI(OAc) was stirred at room temperature for 5 h.
2
16
18
on the peak abundances of O- and O-epoxide in the GC-MS
graphs.
Then, 0.1 mmol olefin, 0.002 mmol Lewis acid and 0.2 mmol
PhI(OAc) were added to this solution. The reaction solution
2
was stirred in a water bath at 308 K for 10 h. The yield of
epoxide and the conversion of olefin were quantitatively ana-
lyzed by GC using the internal standard method. Control
experiments with the ruthenium(II) complex or Lewis acid
alone as the catalyst were carried out in parallel. The reactions
were performed at least in triplicate, and the average data are
used in the discussion.
Crystal structure analysis
cis-Ru(bpy)
2
Cl
2
(19.4 mg, 40 μmol), Sc(OTf)
3
(39.3 mg,
80 μmol) and PhI(OAc)
2
(64.4 mg, 200 μmol) were dissolved in
acetonitrile (20 mL) followed by stirring at r.t. in air for 1 h.
The solution color changed rapidly from purple to yellow.
III
2 3 3 2 3
Single crystals of Ru (bpy) (CF SO )Cl ·CH CN suitable for
X-ray crystallography were grown by slow vapor diffusion of
diethyl ether into the above reaction mixture at 5 °C. A red
prism-shaped crystal with dimensions of 0.25 × 0.17 ×
General procedure for Lewis acid promoted catalytic
epoxidation of cis- and trans-stilbene by ruthenium(II)
complexes
0.14 mm was selected for X-ray structure analysis. Intensity
data for this compound were collected on a Bruker D8 Quest
A mixture solution of 0.8 mL of acetone and 0.2 mL of CH
2
Cl
2
diffractometer at 173(2) K using graphite-monochromated Mo
containing 1 mM ruthenium(II) complex, 2 mM 2,2′-bipyridine Kα radiation (λ = 0.71073 Å) with a ω-scan method. The centro-
and 5 mM PhI(OAc) was stirred at room temperature for 5 h. symmetric monoclinic space group P2 /n was determined by
2
1
Then, 0.1 mmol olefin, 0.002 mmol Lewis acid and 0.2 mmol systematic absences and statistical tests and verified by sub-
PhI(OAc) were added to this solution. The reaction solution sequent refinement. The structure was solved by direct
2
was stirred in a water bath at 298 K for 12 h. The yield of methods and refined by full-matrix least-squares methods on
2
epoxide and the conversion of olefin were quantitatively ana- F . One target molecule was co-crystallized with 1 equiv. of
lyzed by HPLC using the internal standard method, and the acetonitrile. The crystal data and structure refinement results
III
8 2 2 3 3 2
yield of benzaldehyde was quantitatively analyzed by GC using of Ru (C10H N ) (CF SO )Cl are given in Tables S5–S8,† and
the internal standard method. Control experiments with the the structure of the six-coordinate complex is shown in
ruthenium(II) complex or Lewis acid alone as catalyst were Fig. S7.†
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II
Given that the Ru complex in this work is a potential
photosensitizer, the epoxidation of 1,2-cyclooctene was investi-
gated in the dark; the results have been provided in Table S10
Results and discussion
Synergistic effect in epoxidation
Initial assays were performed with cyclooctene as a test sub- in the ESI.† The synergetic effect between Ru(bpy)
2 2
Cl and
strate, using dehydrated acetone/CH Cl (4 : 1, v/v) as the Lewis acid (Al(OTf) ) was still found to occur in the dark. For
2
2
3
solvent and PhI(OAc)
compiled in Table 1, together with Al(OTf)
the purpose of comparison. cis-Ru(bpy) Cl was initially used sion was 19.6% and the yield was 7.6% for the epoxide
2
as the oxidant. The results obtained are example, a conversion of 11.5% and a yield of 4.2% were
3
as an additive, for achieved by adding Al(OTf) , while for Ru(bpy) Cl , the conver-
3
2
2
2
2
as the catalyst; hereafter, it will be referred to as Ru(bpy)
2
Cl
2
.
(Table S10†). Without any irradiation, epoxidation with Ru
Cl and Al(OTf) under identical conditions still showed
As listed in Table 1, when Ru(bpy) Cl was tested as the (bpy)
2
2
2
2
3
catalyst alone, only 21.2% of cyclooctene was converted and efficient activity, with 99.8% conversion of the substrate and
the yield of 1,2-epoxycyclooctane was only 6.7%, indicating 90.2% yield of the epoxide (see Table S10 in the ESI†). Thus,
II
that Ru(bpy)
2
Cl
2
is very sluggish in catalyzing cyclooctene these data clearly exclude the influence of the Ru complex as
epoxidation. When 2 equiv. of Al(OTf) were added to the reac- a potential photosensitizer.
3
tion solution, 100% conversion with 89.9% yield of epoxide
This synergistic effect has also been observed by adding
could be achieved under the identical conditions. In a control other non-redox metal ions to the reaction mixture. The
experiment, Al(OTf) alone as the catalyst provided only 15.2% addition of either Sc(III) or Y(III) provided 100% conversion,
3
conversion and 3.7% yield, which was close to the blank test and the yields of epoxide were 72.5% and 73.1%, respectively.
as listed in entry 1 (12.4% conversion and 1.4% yield). Appar- Non-redox metal ions with a positive charge of 2+, such as
ently, Lewis acid alone has almost no catalytic activity for Mg(II), Ca(II), Ba(II) and Zn(II), also showed similar promotional
olefin epoxidation in our case. In complementary experiments, effects. For example, adding 2 equiv. of Mg(II) or Ca(II) resulted
3
Ru(OTf) was used as the precursor to synthesize the ruthe- in 100% conversion with epoxide yields of 77.0% and 72.6%,
nium(II) catalyst in situ; this demonstrated very sluggish respectively, whereas Mg(II) or Ca(II) alone provided only 19.3%
activity, providing only 12.9% of conversion with a 10.1% yield or 11.0% conversion with 2.5% or 4.4% yield in control experi-
of epoxide, revealing that the promotional effect cannot be ments, respectively. Meanwhile, adding 6 equiv. of NaOTf only
−
attributed to the presence of OTf anion. Meanwhile, a provided 21.9% conversion with 5.0% yield of epoxide, which
3
mixture of Al(OTf) and bpy ligand as catalyst also showed very was identical to the results of the control experiments without
sluggish activity, in which only 18.0% conversion and 7.0% any additives (21.2% conversion with 6.7% yield). Because the
yield were achieved; this excluded the possibility that the concentration of OTf− under these conditions is identical to
3
synergistic effect originates from the potentially generated that of 2 equiv. of Al(OTf) , the above results reveal that the
complex of Al(III) with the ligand. Therefore, this great improve- promotional effect cannot be attributed to the presence of
2
+
−
ment in the catalytic activity of the [Ru(bpy)2] complex by OTf anion. It is worth noting that in the control experiments,
adding non-redox metal ions Al(III) strongly supports that the all of these non-redox metal ions alone as catalysts demon-
synergistic effect occurs between the ruthenium catalyst and strate very poor catalytic activity for epoxidation (Table 2).
the Lewis acid in epoxidation. The addition of extra ligand
2,2′-bpy) can slightly improve the yield of epoxide by prevent- effect of Lewis acid on Ru(II) complex-mediated epoxidation
ing the existence of ligand-free ruthenium species, which can (Fig. 1). In the presence of Ru(II) catalyst or Lewis acid alone,
The catalytic kinetics further confirms the promotional
(
lead to ring opening of epoxides with Lewis acid (Table S3†).
the catalytic epoxidation of cyclooctene is apparently sluggish,
Table 2 Catalytic oxidation of cyclooctene to 1,2-epoxycyclooctane by
ruthenium(II) complexes in the presence of non-redox metal ions as
Lewis acids
3
Table 1 Influence of Al(OTf) concentration on the catalytic oxidation
of cyclooctene
Entry Catalyst (1 mM) Lewis acid
Conv. (%) Yield (%) Additives
Conversion (%)
Yield (%)
1
2
3
4
5
6
7
8
9
—
Ru(bpy)
—
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
bpy
—
—
12.4
21.2
15.2
1.4
6.7
3.7
—
NaOTf
Mg(OTf)
Ca(OTf)
2
Ba(OTf)
Zn(OTf)
Sc(OTf)
Al(OTf)
Y(OTf)
3
21.2
6.7
a
2
Cl
2
21.9 (20.5)
100 (19.3)
100 (11.0)
55.9 (12.9)
100 (11.9)
100 (5.0)
100 (13.1)
100 (12.5)
5.0 (2.2)
77.0 (2.5)
72.6 (4.4)
46.9 (1.7)
68.4 (1.1)
72.5 (2.1)
89.9 (3.0)
73.1 (3.7)
Al(OTf)
Al(OTf)
Al(OTf)
Al(OTf)
Al(OTf)
—
3
3
3
3
3
(2 mM)
2
2
Cl
2
Cl
2
Cl
2
Cl
2
2
2
2
2
(0.5 mM) 30.3
9.9
(1 mM)
(2 mM)
(4 mM)
61.5
99.9
99.3
12.9
18.0
34.7
89.9
70.4
10.1
7.0
2
2
3
a
(OTf)
3
3
Al(OTf)
3
(2 mM)
Conditions: acetone/CH
2
Cl
2
(4 : 1, v/v) 1 mL, cyclooctene 0.1 M, ruthe- Conditions: acetone/CH
0.2 M, 308 K, Ru(bpy) Cl 1 mM, 2,2′-bipyridine 2 mM, Lewis acid 2 mM, PhI(OAc)
was in situ synthesized by Ru(OTf) and bpy 0.2 M, 308 K, 10 h. The data in parentheses represent control experi-
2 2
Cl (4 : 1, v/v) 1 mL, cyclooctene 0.1 M, cis-
nium(II) catalyst 1 mM, 2,2′-bipyridine 2 mM, PhI(OAc)
2
2
2
2
a
1
0 h. Ru(bpy)
2
(OTf)
3
3
a
ligand.
ments with Lewis acids only. NaOTf 6 mM.
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substrates, Ru(II) catalyst or Lewis acid alone showed sluggish
activity for epoxidation. For example, in Table 3, when cyclo-
hexene was used as the substrate, Ru(II) catalyst alone demon-
strated sluggish activity in epoxidation, giving only 15.2%
conversion with 3.8% yield of epoxide. Under identical con-
ditions, the addition of 2 equiv. of Al(III) generated high oxi-
dation activity, with 100% conversion and 82.2% yield of
epoxide. Similar promotion effects by adding non-redox metal
ions can also be observed in the cases of norbornene, styrene
and terminal linear olefins such as 1-hexene and 1-dodecene.
For example, by adding 2 equiv. of Al(III), the conversion of
1-hexene was increased to 94.6% with 74.6% yield of epoxide,
while the conversion of 1-dodecene was increased to 100%
with 67.3% yield of epoxide. 36.7% yield of benzaldehyde
product was detected by GC when styrene was used as the sub-
strate. To further verify the stability, styrene oxide and 2,3-
epoxynorbornane were tested as substrates under identical
conditions, while 73.2% 2,3-epoxynorbornane and 82.6%
styrene oxide were converted, respectively. Therefore, lower
Fig. 1 Kinetics of Lewis acid-accelerated epoxidation by the ruthenium(II)
complex. Conditions: solvent: acetone 0.8 mL CH
tene 0.1 M, cis-Ru(bpy) Cl 1 mM, 2,2’-bipyridine 2 mM, Al(OTf)
PhI(OAc) 0.2 M, 308 K.
2
Cl
2
0.2 mL, cyclooc-
2
2
3
2 mM,
2
whereas the combination of Ru(II) complex and Lewis acid epoxide yield or selectivity for styrene and norbornene were
leads to a remarkable promotion effect. These results are observed in our system. Meanwhile, either Ru(II) catalyst or
highly consistent with the data in Table 1.
Lewis acid alone provided limited substance conversion and
This synergistic oxygen atom transfer by adding non-redox epoxide yield, which again demonstrates the synergistic effect
metal ions was also observed in the epoxidation of other cyclo- between Ru(II) catalyst and Lewis acid for the oxygen atom
olefins and terminal linear olefins. In all of the investigated transfer reaction.
Table 3 Al(OTf)
3
promoted olefin epoxidations by Ru(bpy)
Substrate Product
2 2
Cl catalyst
Entry
1
Catalyst
Conversion (%)
Yield (%)
—
12.4
21.2
15.2
100
1.4
6.7
3.7
Ru(II)
3
+
Sc
3
3
3
+
+
+
Ru(II) + Sc
89.9
a
2
—
11.2
15.2
5.1
3.5
3.8
2.8
Ru(II)
3
+
Sc
Ru(II) + Sc
100
82.2
b
3
—
8.4
44.3
16.1
98.9
3.0
15.4
3.3
Ru(II)
3
+
Sc
Ru(II) + Sc
33.6
c
4
—
16.1
21.6
15.3
100
0.6
2.4
2.7
Ru(II)
3
+
Sc
3
3
3
+
+
+
Ru(II) + Sc
30.0
d
5
—
5.8
18.8
7.5
1.6
2.0
2.3
Ru(II)
3
+
Sc
Ru(II) + Sc
94.6
74.6
d
6
—
5.8
24.1
13.4
100
0.9
8.0
6.5
Ru(II)
3
+
Sc
Ru(II) + Sc
67.3
Conditions: acetone/CH
2
Cl
2
(4 : 1, v/v) 1 mL, olefin 0.1 M, cis-Ru(bpy)
2
Cl
2
1 mM, 2,2′-bipyridine 2 mM, Lewis acid 2 mM, PhI(OAc)
2
0.2 M, 308 K,
a
b
c
d
10 h. 293 K, 6 h. 293 K, 12 h. 293 K, 12 h, 36.7% yield of benzaldehyde was detected by GC. Olefin 0.05 M, Ru(II) complex 0.5 mM, 2,2′-bipyr-
idine 1 mM, Lewis acid 1 mM, PhI(OAc) 0.1 M, 308 K, 2 h.
2
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Reaction mechanism
absorption bands (Fig. 2a), an intense band centered at
3
80 nm and another broad band from 450 nm to 800 nm with
The reaction data presented above clearly reveal that a syner-
gistic effect exists between Ru(bpy) Cl and non-redox metal
2 2
λ
max at 560 nm. The lowest energy bands in the visible region
for these Ru(II) complexes are generally associated with metal-
to-ligand charge transfer (MLCT) states resulting from the pro-
motion of an electron from a metal d-orbital to the π*-orbital
of the bpy ligand (d → π*). The electronic transitions centered
at 380 nm have previously been assigned to both LMCT (π →
ions as Lewis acids in the catalysis of olefin epoxidation,
which is the first example where non-redox metal ions as
Lewis acids accelerate direct oxygen atom transfer catalyzed by
ruthenium catalysts. Based on our previous studies and results
from other labs, the promotional effects of those non-redox
metal ions were generally attributed to their linkages to the
7
5
e *) and MLCT (d → π*). For the Ru–Cl complexes, the MLCT
g
2
+
n+
bands are red shifted with regard to the analogous Ru(bpy)
3
redox metal ions through the M vO functional
5
2–57,59,63,64,73
species (λmax at 450 nm) because of the relative stabilization of
group.
In literature reports, the active site of the
4
3,75
the dπ(Ru) levels provoked by the chloro ligand.
Upon addition of 5 equiv. of PhI(OAc) as oxidant, the deep
purple color of Ru(bpy) Cl gradually changed to yellow, and
Ru center for epoxidation is commonly recognized as Ru(IV)v
3
,5,32–36,74
2
O,
which prompted us to explore the origin of the
2
2
increase in catalytic reactivity as follows.
we named this unknown species compound 1 (Fig. 2(a)). The
above oxidation process was much slower compared to the
case of adding Lewis acid (vide infra, Fig. 5). The concomitant
change in the absorption spectra was identical: the molar
extinction coefficient of the absorption peak at 380 nm
Epoxidation pathway in the absence of Lewis acid. The elec-
tronic absorption data for the complex and the kinetic
changes of the spectra during stoichiometric oxidation are
illustrated in Fig. 2. Due to the interfering UV absorption from
acetone in the mixed solution, only signals beyond 320 nm
3
−1
−1
3
−1
decreased from ε = 6.3 × 10 M cm to ε = 4.2 × 10 M
2 2
were shown herein. Ru(bpy) Cl alone gives two district
−
1
cm , whereas the distinct band at 560 nm decreased from ε =
3
−1
−1
−1
−1
6
.2 × 10 M cm to ε = 1.2 × 103 M cm . The absorption
spectrum of compound 1 is highly consistent with previous
7
6–79
results for the (bpy)
2
Ru(IV)vO species,
which gives the
first evidence of our proposed reaction pathway: Ru(II) is oxi-
dized to the Ru(IV)vO species using PhI(OAc) as a stoichio-
2
1
1,25,80
metric oxidant.
Upon addition of 20 equiv. of
cyclooctene as the substrate, this yellow solution of high valent
species, (bpy) Ru(IV)vO, gradually changes back to deep
2
purple. This reduced species is named compound 2, which
showed an identical electronic absorption spectrum to that of
Ru(bpy) Cl (Fig. 2(b)). Moreover, along with the reduction of
2
2
high valent ruthenium species, the epoxide product can be
found by GC-MS analysis. Therefore, the above evidence clearly
demonstrates that in the absence of Lewis acid, (bpy) Ru(IV)v
2
O was generated as the key intermediate and was responsible
for the direct oxygen transfer to the substance, after which the
Ru(IV)vO species was reduced back to Ru(II) to complete the
catalytic cycle. Meanwhile, a weak but nonetheless detectable
shoulder peak appeared at around 680 nm and then decayed
during the process of oxygen atom transfer from the high
valent ruthenium species to cyclooctene (inset in Fig. 2b). This
shoulder peak may be attributed to a small amount of Ru(III)–
O–Ru(III) dimer originating from the comproportionation of
Ru(IV)vO and Ru(II) (vide infra). Eventually, this shoulder peak
disappeared and an identical absorption spectrum of
Ru(bpy) Cl was regenerated upon addition of extra cyclooc-
2
2
tene, implying that the catalytic cycle occurs between the Ru(II)
and Ru(IV)vO species.
The above reaction pathway was further confirmed by the
1
Fig. 2 (a) UV-Vis absorption spectral changes of Ru(bpy)
upon addition of 5 equivalents of PhI(OAc) at 308 K in acetone/CH
1 : 1, v/v). (b) UV-Vis absorption spectral changes of (a) upon addition of
2
Cl
2
(0.3 mM) change in the NMR spectra (Fig. 3). Initially, the H NMR spec-
2
2
Cl
2
2 2
trum of Ru(bpy) Cl
was identical to the reported value of cis-
Ru(bpy) –Cl ]. Eight resonances appear from 16 protons of
the two bipyridyl rings chelated to Ru(II), and the assignments
of the above resonances are listed in Fig. S1.† Upon the
(
81
[
2
2
20 equivalents of cyclooctene (red line: initial absorption spectrum, blue
line: eventual absorption spectrum, inset: unexpected peak at 680 nm
that appears then decays during the process of reduction by
cyclooctene).
addition of 5 equiv. of PhI(OAc) , 16 resonances with the same
2
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Together with the electronic absorption data mentioned
above, compound 2 can be confirmed as Ru(bpy) Cl . Very
2
2
small peaks from high valent ruthenium species were found in
1
the H NMR spectrum of compound 2, possibly due to the
presence of a small amount of PhI(OAc)2 to oxidize the
Ru(bpy) Cl (Fig. 3, compound 2, blue line). It is thus inferred
2 2
that the epoxidation pathway in the absence of Lewis acid can
be described as follows: Ru(IV)vO (compound 1), generated
from the oxidation of Ru(bpy)
the olefin and is reduced back to Ru(bpy)
2
Cl
2
, transfers an oxygen atom to
Cl (compound 2) to
2
2
complete the catalytic cycle. The above reaction pathway is
clear but not efficient, as evidenced by the epoxidation data in
Table 3.
1
Fig. 3
Ru(bpy)
H
Cl
NMR spectra of ruthenium complexes. Compound 1:
upon addition of 5 equivalents of PhI(OAc) (extra PhI and
Epoxidation pathway in the presence of Lewis acid. The elec-
tronic absorption data for stoichiometric oxidation with the
addition of Lewis acid are illustrated in Fig. 4. In the control
experiments, neither Al(OTf) nor PhI(OAc) gave any absorp-
2
2
2
oxidant were removed by washing and recrystallization); compound 2:
compound 1 upon addition of 20 equiv. of cyclooctene.
3
2
tion bands beyond 350 nm. Upon addition of PhI(OAc)
2
, the
2
+
MLCT (d → π*) absorption of Ru(II)(bpy)
2
at 560 nm vanished
integral area indicated that 16 protons of the two bipyridyl immediately in the presence of Al(OTf) , which indicates a
3
rings became non-equivalent after the oxidation. This un- much faster oxidation process compared with the case without
symmetrical high valent ruthenium complex (compound 1) is Al(OTf) (Fig. 5). This absorption kinetics is highly consistent
3
proposed to have coordination as a Ru(IV)vO species, perhaps with the epoxidation kinetics of the Ru(II) catalyst shown in
cis-[Ru(IV)vO(bpy) Cl ], which is possible due to the following Fig. 1, indicating that the oxidation from Ru(II) to Ru(IV) was
2 2
reasons: (1) EPR studies demonstrate that the high valent enormously accelerated by Lewis acid. Herein, we named this
ruthenium species exist in the form of Ru(IV) rather than Ru(VI) high valent ruthenium complex compound 3. Surprisingly,
(see EPR section). (2) A six-coordinate Ru(IV)-oxo complex when 20 equiv. of cyclooctene was added as the substrate in
should be in the S = 1 spin state and thus should be the presence of Lewis acid, oxygen atom transfer from com-
8
2–86
paramagnetic.
The well-resolved spectra shown in Fig. 3 pound 3 to the olefin led to another unexpected ruthenium
and 8 disclosed that the Ru(IV) complexes should be seven- species with a distinct absorption band centered at 650 nm
coordinate in the S = 0 spin state with a coordination structure (Fig. 2b), named compound 4. As shown in Fig. 4c, compound
3
1,82,84,87,88
as cis-[Ru(IV)vO(bpy) Cl ].
(3) The 16 non-equi- 4 and compound 2 gave distinguished electronic absorption
2
2
valent protons of the two bipyridyl rings indicated that the bands, suggesting that an alternative reaction pathway occurs
coordination of Ru(IV) species was in the cis form. In the case when catalytic epoxidation is conducted in the presence or
of trans-[(bpy) Cl Ru(IV)vO], it will give only 8 non-equivalent absence of Lewis acid.
2
2
protons due to axial symmetry. (4) When we attempted to grow
Under the oxidation conditions described above, this ruthe-
a crystal of the high valent Ru complex that is believed to be nium species can be regenerated for several cycles, which
responsible for the epoxidation, an unexpected single crystal further evidences its role in the catalytic cycle. Initially, com-
of Ru(III) complex, cis-[Ru(bpy) Cl ](OTf), was collected and pound 3 was prepared by the oxidation of Ru(bpy) Cl . When
2 2
2 2
analyzed (see Fig. S7 and Table S4†). This Ru(III) complex an excess amount of cyclooctene was added to the solution at
showed no epoxidation reactivity towards the olefin, and was T = 0 min (Fig. 5, red line), it initiated a slow growth of the
proposed to be generated from the reduction of instable high- characteristic absorption signal at 650 nm, demonstrating the
valent Ru species. cis-[Ru(bpy)
chloro ligands coordinated to the central ion, whereas OTf
2 2
Cl ](OTf) was found to have two formation of the reduced ruthenium species, compound 4.
−
Then, adding PhI(OAc)2 triggered a much faster oxidation
anion acts as a counterion in the outer coordination sphere of from compound 4 to compound 3 to complete the catalytic
Ru(III). From the single crystal structure of this reduced Ru(III) cycle, and this oxidation process was manifested by the
complex, one may conclude that the chloro ligand is more immediate decay of the band at 650 nm. Once again, with the
likely to be coordinated to the high-valent Ru(IV) ions than the addition of extra cyclooctene, PhI(OAc) was consumed while
2
−
OTf ligand. This single crystal structure also reveals that the the reduced ruthenium species was regenerated, as observed
promotional effect cannot be attributed to the presence of by the recovery of its absorption at 650 nm. This regeneration
−
OTf anion.
can be repeated several times without any loss of the active
When extra cyclooctene was added as the substrate to the catalyst (see the intensity of the band at 650 nm after each
high valent ruthenium species (compound 1), the reduced cycle), which further confirms that this unknown species was
ruthenium species (compound 2) was obtained and gave an involved as the key intermediate in the catalytic cycle in the
1
identical H NMR spectrum to Ru(bpy)
2
Cl
2
; the epoxide presence of Lewis acid. As can be seen, the oxidation of com-
product can be found by GC-MS analysis at the same time. pound 3 to compound 4 is significantly faster than the epoxi-
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Fig. 5 Kinetic trace at 560 nm corresponding to a reaction mixture of
Ru(bpy)
cyclooctene at 308 K and a trace at 650 nm corresponding to a reaction
mixture containing Ru(bpy) Cl (0.3 mM), 5 equivalents of PhI(OAc)
equivalents of Al(OTf) and 30 equivalents of cyclooctene at 308 K.
2 2 2
Cl (0.3 mM), 5 equivalents of PhI(OAc) , and 30 equivalents of
2
2
2
,
2
3
disappearance of this peak indicated the oxidation from Ru(II)
2
+
(
(
bpy)
bpy)
2
to Ru(IV)vO, followed by reduction back to Ru(II)
upon addition of extra cyclooctene. As can be seen,
2
+
2
the catalytic reaction rate was obviously accelerated with the
addition of Lewis acid, and more importantly, the reaction
pathways are totally different in the absence and presence of
Lewis acid. Thus, collecting the valence and coordination
information of compound 4, with the absorption peak at
650 nm, is essential to understand the mechanism of Lewis
acid-accelerated-epoxidation.
A Ru-bpy based complex that possesses a similar absorption
band centered at 672 nm was first prepared and well character-
ized as an oxo-bridged Ru(III) dimer, [(bpy) ClRu(III)–(μ-O)–
2
2
+
89
Ru(III)Cl(bpy)
2
] , by Meyer and co-workers.
[(bpy)
2 2
(H O)
4
+
Ru(III)–(μ-O)–Ru(III)(H
2
2
O)(bpy) ]
also showed a very similar
absorption peak at 660 nm. DFT calculation indicated that the
band around 650 to 670 nm can be assigned mainly to the
III
III
III
III
transition from dπ(Ru –O–Ru ) to dπ*(Ru –O–Ru ); thus,
ruthenium dimers with other states show distinguished elec-
Fig. 4 (a) UV-Vis absorption spectra of compound
3
(0.3 mM
and 2 equi-
). (b) UV-Vis absorption spectra of compound such as [(bpy)
(0.3 mM compound 3 upon addition of 20 equiv. of cyclooctene.)
c) UV-Vis absorption spectra of compound 2 and compound 4. Con-
ditions: Ru(bpy) Cl in acetone/CH Cl (1 : 1, v/v) at 308 K.
9
0,91
tronic absorption bands.
Alternatively, Ru(III)/Ru(IV) dimers
Ru(bpy)
2 2 2
Cl upon addition of 5 equivalents of PhI(OAc)
2
+
valents of Al(OTf)
3
2
ClRu(III)–(μ-O)–Ru(IV)Cl(bpy)
2
8
]
showed a dis-
and no signal
1,89
4
(
tinct absorption band centered at 470 nm,
could be observed in the region of 600 to 700 nm from either
2
2
2
2
9
2
Ru(III)–(μ-O)–Ru(IV) or Ru(IV)–(μ-O)–Ru(IV) with bpy ligand.
The above results strongly suggest that our unexpected species,
compound 4, which can be generated from the oxygen transfer
dation step, indicating that the rate-determining step (rds) of reaction between high valent Ru(IV) species and olefin in the
this catalysis involves the interaction of the double bond of the presence of Lewis acid, is an oxo-bridged Ru(III) dimer.
alkene with Ru(IV)vO groups and eventual transfer of the
To further verify the oxidation states of ruthenium in com-
oxygen atom to the olefin. Similarly, a kinetic trace at 560 nm pound 4, X-ray photoelectron data of the different ruthenium
corresponding to the reaction in the absence of Lewis acid was complexes were collected, and the results are illustrated in
also recorded under identical conditions. According to our pre- Fig. 6. Peaks occurring in the region of 272 to 292 eV were
vious discussion for the case without Lewis acid, the absorp- attributed to ruthenium (Ru 3d3/2 and Ru 3d5/2) and carbon
2
+
tion at 560 nm represented the Ru(II)(bpy)2 species, and the (C 1s) electron transitions. Herein, all binding energies were
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Fig. 6 X-ray photoelectron emission spectra of Ru(bpy)
2
Cl
2
(A);
[
Ru(bpy)
2 2 3 3
Cl ](CF SO ) (B); compound 4 (C). The C 1s reference peak is
defined as 284.4 eV in all cases.
1
Fig. 7 H NMR spectra of the ruthenium complexes. Compound 3:
Ru(bpy) Cl upon addition of 2 equivalents of Al(OTf) and 5 equivalents
of PhI(OAc) (extra PhI and oxidant were removed by washing and
recrystallization); compound 4: compound 3 upon addition of 20 equiva-
2
2
3
2
measured relative to an arbitrarily assigned value of 284.4 eV,
which is due to the C 1s peak from the bpy ligand, because the lents of cyclooctene.
absolute binding energies varied somewhat from sample to
8
9
sample due to surface charging effects. cis-Ru(II)(bpy) Cl
2
2
was first measured as the standard and showed a binding
energy of 280.0 eV for the Ru 3d5/2 peak. This result is in (compound 3) can be assigned as an adduct of Ru(IV)vO/Al(III)
excellent agreement with the value obtained in the literature species, possibly cis-[Ru(IV)vO(bpy) Cl ]/Al(III), based on the
2
2
8
9
(279.9 eV for cis-Ru(II)(bpy)
2 2
Cl ). The single crystal of Ru(III) following reasons: (1) EPR study demonstrates that the high
complex [Ru(III)(bpy) Cl ](OTf) gave a binding energy of valent ruthenium species exists in the form of Ru(IV) rather
2
2
2
82.1 eV, which is also in good agreement with previously than higher states such as Ru(V) or Ru(VI) (see EPR section). (2)
1
found values for Ru(III) (281.9 eV for [Ru(III)(bpy)
2
Cl
2
]Cl). In the The H NMR signal of compound 3 is similar to, but not the
case of compound 4, a broadening ruthenium peak was same as that of compound 1, Ru(IV)vO. The differences in the
detected with a value of 280.9 eV, also consistent with Ru chemical shifts of the protons between compound 3 and com-
3d5/2, that is, Ru(II) < Ru(III)–O–Ru(III) < Ru(III). Our results also pound 1 may be induced by the linkages of the non-redox
follow this rule, which further supports the hypothesis pre- metal ion to the Ru(IV)vO functional group (vide infra). Sur-
viously supported by the electronic absorption results that prisingly, with the addition of an extra amount of cyclooctene,
compound 4 can be recognized as an oxo-bridged Ru(III). Also, only 8 resonances appear for compound 4. As can be seen, the
the lack of splitting of the Ru 3d3/2 and Ru 3d5/2 peaks for com- chemical shifts of these 8 resonances are clearly different from
pound 4 indicates that the two ruthenium centers are equi- those of cis-Ru(bpy) Cl , which is consistent with the results of
2
2
valent, or nearly equivalent.
the electronic spectra shown in Fig. 4c and further demon-
The above XPS data, together with evidence from the elec- strates that adding Lewis acid has changed the epoxidation
tronic absorption data, suggest that compound 4 is a Ru(III)/ pathway of the ruthenium catalyst. There are two speculations
Ru(III) dimer with bpy ligand. In recent years, related spectral to explain these 8 resonances in compound 4, which has been
characterization and redox properties of such ruthenium previously proposed to be a Ru(III)/Ru(III) dimer. The first
dimers with bpy analogues as ligands, well-known as the speculation arises from the possibility that one bpy ligand is
“blue-dimer”, have received intense scientific scrutiny because dissociated from the metal ion during the redox process, and
they are recognized as the key precursors to release molecular only one bipyridine is coordinated to each Ru(III) ion. There-
2
7,32,93–96
oxygen in water oxidation.
However, oxo-bridged fore, the remaining two bpy ligands are equivalent in chemical
Ru(III) dimers were seldom reported as an intermediate in cata- environment and thus give 8 resonances from 16 protons.
lyzed epoxidation, even though some well-designed dinuclear However, the above speculation on the dissociation of the
ruthenium complexes containing organic bridging ligands complex is not favored because the consumption and regener-
3
,5,6
have been prepared as redox catalysts in olefin epoxidation.
ation of compound 4 can be easily repeated without any
The epoxidation reaction in the presence of Lewis acid was decrease in the content of active ruthenium species (see
also monitored by NMR spectroscopy, and the results are Fig. 4). Furthermore, resonances from the protons in disso-
shown in Fig. 7. Upon addition of 2 equiv. of Lewis acid and 5 ciated bpy ligand should be observed in the NMR spectra as
equiv. of PhI(OAc) , 16 resonances with the same integral area well as the ligated bpy, which is obviously not the case in
2
were observed for compound 3, demonstrating that 16 protons Fig. 7. The other explanation is that 8 resonances appear from
of the two bipyridyl rings became non-equivalent after the oxi- 4 equivalent bpy ligands due to a higher level symmetry from
dation. This unsymmetrical high valent ruthenium complex the geometric point of view; based on this, compound 4 can
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be speculated as a di-oxo bridged Ru(III) dimer, [(bpy)
2
Ru(III)– these resonances, those in the region of 8.88 to 8.65 ppm show
2
+
1
(
μ-O) –Ru(III)(bpy) ] . On the basis of the well-investigated H the most obvious shifts with the addition of Al(III). These res-
2
2
NMR information of bpy ligand and cis-Ru(bpy)
assignments of resonances originating from compound 4 re tions, as shown in Fig. S3.† Compared with compound 1, all
listed in Fig. S2 and Table S3.† the resonances were located at a higher field in compound 3
2 2
Cl , detailed onances are assigned to the four protons at the 4,4′,5,5′ posi-
Another aspect that is worthy of discussion is how the (8.85 to 8.62 ppm). The protons at the 4′ and 4 positions in
Lewis acid influences the electronic factor of high valent compound 1 individually show a doublet structure with J = 9
Ru(IV)vO species, which plays a pivotal role in oxygen atom Hz and overlap each other, appearing as a triplet structure
transfer. As mentioned above, in the absence of Lewis acid, (8.72 to 8.66 ppm). Alternatively, in compound 3, these two
compound 1 was generated from the chemical oxidation of doublet structures with J = 8 Hz (8.70 to 8.63 ppm) become
Ru(bpy) Cl and was recognized as Ru(IV)vO based on various separated and appear as a hyperfine structure of four peaks.
2
2
photonic and electronic characterizations. Meanwhile, with The above data clearly demonstrate that the linkage of Al(III) to
the addition of Lewis acid, the generated compound 3 also the Ru(IV)vO center leads to a weaker deshielding effect on
showed 16 resonances with the same integral area, indicating the protons in the bpy ligand. We believe that the changes in
1
6 non-equivalent protons in two bpy ligands. However, the the chemical shifts can be attributed to a perturbation of the
chemical shifts of the protons in compound 3 were different magnetic anisotropy rather than an influence on the electron
from those of compound 1. To assign each proton accurately density through a conjugative or inductive effect, based on the
and to further explore the interaction between the Lewis acid following speculations. Considering electronic factors, the
1
3
1
13
and the Ru(IV)vO species, the C NMR and H– C HSQC linkage of Al(III) to the Ru(IV)vO center should not greatly
spectra of compound 3 were measured and are shown in influence the electron density of the protons at the 2,7 posi-
1
3
Fig. S3.† We initialized the assignment according to the
C
tions in bipyridine because they are separated by three or
into the
NMR results: the electron density of the carbons in the pyri- more bonds. Alternatively, introducing Al(OTf)
3
dine ring follows the order of ortho- > para- > meta-, leading to Ru(IV)vO center may significantly disturb the geometric factor
a stronger deshielding effect in the ortho-position and showing in the complex due to the steric hindrance of the triflate
a chemical shift to a lower field. The above trend is consistent moiety; this causes certain changes in the magnetic anisotropy
in both the free and coordinated ligands. Thus, assignment of and thus, in turn, influences the chemical shifts of the
1
3
the C resonances in compound 3 followed straightforwardly protons. The protons at the 4,5 positions, the chemical shifts
1
3
81
from the C NMR spectra of bpy ligand and Ru(bpy)
2
Cl
2
,
of which are located at a lower field than those of the other
and these results are shown in Fig. S3.† Together with the protons in the para-positions of bipyridine (protons at the 2,7
1
13
H– C HSQC spectrum, the resonances of each proton in positions) due to the shielding effect from both pyridine rings,
compound 3 were carefully assigned and are summarized in are more sensitive to changes in the geometric environment in
Table S3.† the ruthenium complex. Therefore, these protons provide the
1
1
On the basis of the above assignments and analysis, the H most obvious changes in the H NMR spectra with the
NMR spectra of compound 1 and compound 3, which are addition of Lewis acid. In particular, compound 1 together
respectively recognized as the key intermediates for the oxygen with 6 equiv. of NaOTf as additive was also measured and
atom transfer in the different reaction pathways, are compared listed for the purpose of comparison. In this case, no concomi-
1
in Fig. 8. Some peaks in the black line in Fig. 8 located at 7.1 tant change in the H NMR spectrum can be observed, clearly
1
to 7.8 ppm are due to unreacted PhI(OAc) and PhI. Among all supporting that the Al(OTf) -induced shifting in the H NMR
2
3
−
spectrum was not due to the presence of OTf , which is in
good agreement with our catalytic epoxidation data (Table 2).
EPR is uniquely sensitive to paramagnetic intermediates.
Mononuclear Ru(II) and Ru(IV) are EPR silent, while the mono-
9
7
nuclear Ru(III) center gives characteristic EPR signals. Fig. 9
shows the EPR spectra of Ru(II)(bpy) Cl that underwent oxi-
2
2
2
dation by PhI(OAc) . Initially, the signal from the mononuclear
Ru(II) center was almost silent. However, some weak but detect-
able peaks corresponding to Ru(III) can be observed under the
experimental conditions. This is due to the fact that Ru(II)
(
2 2 3
bpy) Cl was synthesized using RuCl as a precursor through
reduction, during which a trace amount of Ru(III) residue may
still remain in the recrystallized Ru(II) catalyst and may easily
be observed due to the EPR silence of the Ru(II) center. Upon
addition of excess of PhI(OAc) as oxidant and freezing within
2
2 2
a certain time after mixing, Ru(II)(bpy) Cl undergoes oxi-
1
dation, during which distinct signals with gxx = 2.60, gyy = 2.40
and gzz = 1.66 can be observed, indicating the formation of
Fig. 8 H NMR spectra of high valent ruthenium complexes with
different Lewis acids.
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species is more isotropic in comparison with Ru(III) and has
9
8
very characteristic signals and hyperfine splittings.
instance, the experimental EPR spectra of cis-Ru(V)(O)(OH)
bpy) showed a distinct signal in the region of 3200 to
300 G. However, the abovementioned signals from the Ru(V)
For
(
3
2
9
9
center were not detected during the oxidation process in this
case, which implies that no further oxidation such as Ru(IV) →
Ru(V) or Ru(IV) → Ru(VI) is occurring. In the case of oxidation
with the addition of Lewis acid, the abovementioned for-
mation and disappearance of the Ru(III) species was much
faster, as shown in Fig. 9b, because the oxidation from Ru(II)
to Ru(IV)vO was significantly accelerated by the addition of
Lewis acid. This phenomenon is also in good agreement with
the acceleration demonstrated in the electronic absorption
spectra (Fig. 4a and 5). Furthermore, compound 4 exhibits no
EPR signal, and this EPR silence is in agreement with the rela-
tively large antiferromagnetic spin–spin coupling of the
1
00
Ru(III)/Ru(III) dimer.
Generally, epoxidation of cis-stilbene can provide further
mechanistic information because the ratio of cis and trans-
epoxide products from cis-stilbene is highly dependent on the
reaction pathway as well as the coordination environments of
the redox metal ions. At least two pathways have been reported
1
01,102
with distinctly different transition states:
(1) the radical
Fig. 9 (a) EPR spectra of the ruthenium complex with oxidant and path, in which direct attack of the substrate by the metal-oxo
Al(OTf)
g = 2.60. Conditions: cis-Ru(bpy)
equivalents of Al(OTf) , 20 equiv. of cyclooctene, 130 K.
3
as additive. (b) Change in the intensity of ruthenium complex at
species generates an olefinic CvC π bond-broken radical, fol-
lowed by a collapse or rotation–collapse process, thus provid-
2
Cl 1 mM, 5 equivalents of PhI(OAc) ,
2
2
2
3
1
03,104
ing a mixture of cis and trans epoxides;
(2) concerted
oxygen atom transfer, in which 2 + 2 cycloaddition of the
n+
M vO moiety to olefin generates a metallaoxetane intermedi-
paramagnetic Ru(III) species. The formation of this Ru(III) ate, followed by its subsequent breakdown to form an epoxide
1
05,106
species is more likely due to the comproportionation of fresh while retaining the steric structure of the olefin.
Comple-
generated Ru(IV) and the remaining Ru(II) species. Then, the mentarily, in some Ru(IV)vO based catalytic epoxidation reac-
characteristic signal from the Ru(III) species decays along with tions, a radical path has also been suggested that leads to the
5
further oxidation, and the residual EPR signal exhibits an sterically-retained product. Herein, Ru(II) catalyst alone pro-
intensity of less than 5% compared with the highest value. The vided 3.4% cis-epoxide and 0.3% trans-epoxide when cis-stil-
3
+
above phenomenon clearly demonstrates the formation and bene was used as the substrate. Adding Al can sharply
disappearance of the Ru(III) species and furthermore suggests improve the yields of both cis- and trans-epoxide. For instance,
3
+
the formation of the Ru(IV)vO moiety in compound 1. The in the presence of 2 equiv. of Al , Ru(II) catalyst gave 39.1%
ruthenium species at higher valences (Ru(V) or Ru(VI)) were not yield of cis-epoxide and 7.0% yield of trans-epoxide (Table 4).
favored in our proposed mechanism because the Ru(V)vO Meanwhile, no cis/trans isomerization takes place for cis-stil-
Table 4 Influence of the Al(OTf)
3
concentration on the catalytic oxidation of stilbene by the ruthenium(II) complexes
Yield (%)
3
+
Substrate
Ru(II) : Al
0 : 0
Conversion (%)
cis-Epoxide
trans-Epoxide
Benzaldehyde
cis-Stilbene
12.4
15.5
17.7
74.7
8.5
12.9
9.8
0.9
3.4
0.8
39.1
0
0
0
0.1
0.3
0.5
7.0
0.2
2.0
1.0
36.2
6.4
5.8
5.7
15.6
4.8
8.2
1
0
1
: 0
: 2
: 2
trans-Stilbene
0 : 0
1
0
1
: 0
: 2
: 2
5.9
17.6
85.5
0
Conditions: acetone/CH
Cl
2 2
(4 : 1, v/v) 1 mL, olefin 0.1 M, cis-Ru(bpy)
Cl
2 2
1 mM, 2,2′-bipyridine 2 mM, PhI(OAc)
2
0.2 M, 298 K, 12 h.
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bene, which is proved by the fact that no trans-stilbene product
was found by GC-MS analysis. The above data point towards a
mechanism of concerted oxygen atom transfer from the active
species, Ru(IV)vO or Ru(IV)vO/Al(III), to the double bond of
olefin. Although the ratios of cis and trans-epoxide products
reported herein are difficult to compare directly with other
complexes from the literature because the catalysts and oxi-
dants used are different, the results of catalytic epoxidation in
this paper are consistent with a general trend that Ru(IV)vO
3
,5,43
based species provide a certain steric selectivity.
In the
Scheme 2 Reaction pathways of epoxidation.
case of trans-stilbene, Ru(bpy) Cl combined with 2 equiv. of
2
2
Al(III) as catalyst provides 36.2% yield of trans-epoxide with
7.6% yield of benzaldehyde, while Ru(bpy) Cl alone gave
1
2
2
only 2.0% yield of trans-epoxide with 8.2% yield of benz- atom to olefin and leads to the formation of a Ru(III)/Ru(III)
aldehyde. Notably, there was no cis-stilbene epoxide formation dimer (compound 4) which can be easily oxidized back to com-
from trans-stilbene.
pound 3 in the presence of oxidant. As evidenced by the elec-
Another key piece of evidence that the active Ru(IV)vO tronic absorption spectra and EPR studies (Fig. 5 and 9b),
species is responsible for the oxygen atom transfer reaction both the oxygen transfer from Ru(IV)vO/Al(III) to olefin and
1
8
comes from O isotope labelling experiments (Fig. 10). A the oxidation of Ru(III)–O–Ru(III) to Ru(IV)vO/Al(III) are remark-
metal oxo species can exchange its oxo functional group with ably accelerated by adding Al(III) compared to the case without
1
8
n+ 18
18
O-water to form M v O and subsequently incorporate
O
Al(III). The improved catalytic epoxidation efficiency with the
In the epoxi- addition of Lewis acid (as shown in Table 1) was also attribu-
dation of cis-stilbene catalyzed by Ru(II) complex alone, only ted to this acceleration effect (Scheme 2).
1
8
107
into olefin, forming O-epoxide products.
1
8
4
.6% cis-epoxide with a 36.0% abundance of O and 12.4%
To further explore the origin of this Ru(III)–O–Ru(III) dimer,
benzaldehyde with a 31.9% abundance of O were produced Ru(II)(bpy) Cl was mixed with one equivalent of high valent
when 0.1 mL H2 O was added to 1.0 mL of reaction solution. Ru(IV)vO/Al(III) (compound 3), and its UV-Vis spectrum
In the presence of 2 equiv. of Al(OTf)
, the yield of cis-epoxide immediately changed to that of compound 4 (as shown in
1
8
2
2
1
8
3
1
8
sharply increased to 28.7% with 35.1% O abundance, and Fig. S9†), supporting the formation of the Ru(III)–O–Ru(III)
the yield of benzaldehyde also increased to 36.4% with 33.3% dimer. This result demonstrated the comproportionation of
1
8
18
O abundance. The above O isotope labelling results further Ru(IV)vO/Al(III); Ru(II) would generate the Ru(III)–O–Ru(III)
bolster the support for the abovementioned mechanism where species. Theoretically, the oxygen atom transfer from Ru(IV)v
Ru(IV)vO is responsible for the oxygen atom transfer reaction.
O/Al(III) to olefin would generate the Ru(II) species, and the
Taken together, in the absence of Lewis acid, the oxidation Ru(II) species would be further oxidized to Ru(IV)vO/Al(III) in
of Ru(bpy) Cl
generates the Ru(IV)vO species (compound 1), the presence of Lewis acid to complete the catalytic cycle.
which transfers the oxygen atom to olefin and then is reduced However, as demonstrated in Fig. 5, the oxidation of low valent
back to cis-Ru(bpy) Cl
(compound 2) to complete the catalytic ruthenium complex to Ru(IV)vO/Al(III) is significantly faster
cycle; this is a sluggish process. With the addition of Al(OTf)
than the epoxidation step, indicating that the rate-determining
as Lewis acid, the oxidation of Ru(bpy) Cl generates an step (rds) of this process involves the interaction of the double
2
2
2
2
3
2
2
adduct of Ru(IV)vO and Al(III), proposed as Ru(IV)vO/Al(III) bond of the alkene with Ru(IV)vO groups and the eventual
(
compound 3). This adduct can efficiently transfer the oxygen transfer of the oxygen atom to the olefin. Thus, Ru(II) species
cannot be directly observed in the Lewis acid-accelerated
reactions.
Apparently, our results have illustrated a novel strategy to
explore the catalytic reactivity of redox metal complexes
without chemical modification of their organic ligands under
oxidation conditions. To further verify this strategy, ruthenium
complexes with different ligands are employed as catalysts for
the epoxidation of cyclooctene (Table 5). All these ruthenium
complexes alone, as well as Ru(bpy)
2 2
Cl catalyst, are very slug-
gish in olefin epoxidation. For example, Ru(ptd) Cl (ptd =
2
2
1
2
,10-phenanthroline-5,6-dione) alone as the catalyst only gave
8.1% conversion with 10.5% yield in the epoxidation of
cyclooctene. Remarkably, the addition of Lewis acid sharply
promoted conversion of up to 100% with a 66.4% yield of
epoxide. This acceleration has been observed in each case,
which further supports the effectiveness of our strategy. That
18
Fig. 10 Isotopic labelling experiments using H
dation of cis-stilbene.
2
O for the catalytic oxi-
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Table 5 Catalytic oxidation of cyclooctene by Ru(II) complexes bearing Graduates’ Innovation Fund of Huazhong University of Science
various ligands
and Technology (No. 0118650009). The GC-MS and XPS analy-
sis was performed in the Analytical and Testing Center of
Huazhong University of Science and Technology.
Ligand
Al(OTf)
3
(mM)
Conversion (%)
Yield (%)
a
bpy
0
2
0
2
0
2
0
2
0
2
21.2
99.9
32.2
100
63.8
100
28.1
100
26.2
80.7
6.7
89.9
19.3
70.1
30.8
80.2
10.5
66.4
10.3
49.6
bpm
dpk
ptd
Notes and references
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