Cis -Oxoruthenium complexes supported by chiral tetradentate amine (N4) ligands for hydrocarbon oxidations

Article abstract of DOI:10.1039/c7sc05224c

We report the first examples of ruthenium complexes cis-[(N₄)Ru^IIICl₂]⁺ and cis-[(N₄)Ru^II(OH₂)₂]²⁺ supported by chiral tetradentate amine ligands (N₄), together with a high-valent cis-dioxo complex cis-[(N₄)Ru^VI(O)₂]²⁺ supported by the chiral N₄ ligand mcp (mcp = N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine). The X-ray crystal structures of cis-[(mcp)Ru^IIICl₂](ClO₄) (1a), cis-[(Me₂mcp)Ru^IIICl₂]ClO₄ (2a) and cis-[(pdp)Ru^IIICl₂](ClO₄) (3a) (Me₂mcp = N,N′-dimethyl-N,N′-bis((6-methylpyridin-2-yl)methyl)cyclohexane-1,2-diamine, pdp = 1,1′-bis(pyridin-2-ylmethyl)-2,2′-bipyrrolidine)) show that the ligands coordinate to the ruthenium centre in a cis-α configuration. In aqueous solutions, proton-coupled electron-transfer redox couples were observed for cis-[(mcp)Ru^III(O₂CCF₃)₂]ClO₄ (1b) and cis-[(pdp)Ru^III(O₃SCF₃)₂]CF₃SO₃ (3c′). Electrochemical analyses showed that the chemically/electrochemically generated cis-[(mcp)Ru^VI(O)₂]²⁺ and cis-[(pdp)Ru^VI(O)₂]²⁺ complexes are strong oxidants with E° = 1.11-1.13 V vs. SCE (at pH 1) and strong H-atom abstractors with D_O-H = 90.1-90.8 kcal mol^-1. The reaction of 1b or its (R,R)-mcp counterpart with excess (NH₄)₂[Ce^IV(NO₃)₆] (CAN) in aqueous medium afforded cis-[(mcp)Ru^VI(O)₂](ClO₄)₂ (1e) or cis-[((R,R)-mcp)Ru^VI(O)₂](ClO₄)₂ (1e?), respectively, a strong oxidant with E(Ru^VI/V) = 0.78 V (vs. Ag/AgNO₃) in acetonitrile solution. Complex 1e oxidized various hydrocarbons, including cyclohexane, in acetonitrile at room temperature, affording alcohols and/or ketones in up to 66% yield. Stoichiometric oxidations of alkenes by 1e or 1e? in ^tBuOH/H₂O (5:1 v/v) afforded diols and aldehydes in combined yields of up to 98%, with moderate enantioselectivity obtained for the reaction using 1e?. The cis-[(pdp)Ru^II(OH₂)₂]²⁺ (3c)-catalysed oxidation of saturated C-H bonds, including those of ethane and propane, with CAN as terminal oxidant was also demonstrated.

Full text of DOI:10.1039/c7sc05224c

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cis-Oxoruthenium complexes supported by chiral
tetradentate amine (N4) ligands for hydrocarbon
oxidations†
Cite this: DOI: 10.1039/c7sc05224c
ab
ac
Chun-Wai Tse,
Yungen Liu,
Toby Wai-Shan Chow,a Chaoqun Ma,c
Wing-Ping Yip,a Xiao-Yong Chang,
and Chi-Ming Che
Kam-Hung Low,a Jie-Sheng Huang
a
a
abc
*
We report the first examples of ruthenium complexes cis-[(N4)RuIIICl2]+ and cis-[(N4)RuII(OH2)2]2+
supported by chiral tetradentate amine ligands (N4), together with a high-valent cis-dioxo complex cis-
[(N4)RuVI(O)2]2+ supported by the chiral N4 ligand mcp (mcp ¼ N,N0-dimethyl-N,N0-bis(pyridin-2-
ylmethyl)cyclohexane-1,2-diamine). The X-ray crystal structures of cis-[(mcp)RuIIICl2](ClO4) (1a), cis-
[(Me2mcp)RuIIICl2]ClO4 (2a) and cis-[(pdp)RuIIICl2](ClO4) (3a) (Me2mcp ¼ N,N0-dimethyl-N,N0-bis((6-
methylpyridin-2-yl)methyl)cyclohexane-1,2-diamine,
pdp
¼
1,10-bis(pyridin-2-ylmethyl)-2,20-
bipyrrolidine)) show that the ligands coordinate to the ruthenium centre in a cis-a configuration. In
aqueous solutions, proton-coupled electron-transfer redox couples were observed for cis-[(mcp)
RuIII(O2CCF3)2]ClO4 (1b) and cis-[(pdp)RuIII(O3SCF3)2]CF3SO3 (3c0). Electrochemical analyses showed that
the chemically/electrochemically generated cis-[(mcp)RuVI(O)2]2+ and cis-[(pdp)RuVI(O)2]2+ complexes
are strong oxidants with Eꢀ ¼ 1.11–1.13 V vs. SCE (at pH 1) and strong H-atom abstractors with DO–H
¼
90.1–90.8 kcal molꢁ1. The reaction of 1b or its (R,R)-mcp counterpart with excess (NH4)2[CeIV(NO3)6]
(CAN) in aqueous medium afforded cis-[(mcp)RuVI(O)2](ClO4)2 (1e) or cis-[((R,R)-mcp)RuVI(O)2](ClO4)2
(1e*), respectively, a strong oxidant with E(RuVI/V) ¼ 0.78 V (vs. Ag/AgNO3) in acetonitrile solution.
Complex 1e oxidized various hydrocarbons, including cyclohexane, in acetonitrile at room temperature,
affording alcohols and/or ketones in up to 66% yield. Stoichiometric oxidations of alkenes by 1e or 1e* in
tBuOH/H2O (5 : 1 v/v) afforded diols and aldehydes in combined yields of up to 98%, with moderate
enantioselectivity obtained for the reaction using 1e*. The cis-[(pdp)RuII(OH2)2]2+ (3c)-catalysed
oxidation of saturated C–H bonds, including those of ethane and propane, with CAN as terminal oxidant
was also demonstrated.
Received 9th December 2017
Accepted 1st February 2018
DOI: 10.1039/c7sc05224c
rsc.li/chemical-science
efficient and selective C–H and C]C functionalizations,
wherein the widely employed N4 ligands include mcp, pdp and
Introduction
The selective oxidations of hydrocarbons1 including alkanes
and alkenes, and oxidation of alcohols,2 catalysed by metal
complexes under mild conditions are important reactions in
chemical synthesis. Iron and manganese complexes bearing
tetradentate pyridylmethyl amine or quinolylamine N4
ligands1b,h,i,k–o,r constitute one of the platforms for performing
bqcn ligands and their derivatives (examples depicted in
Fig. 1).3–6 These acyclic chiral tetradentate amine (N4) ligands, in
most scenarios, coordinate to metal ions in a cis-a conguration
to form octahedral metal complexes (Fig. 2), leaving a pair of cis
sites for oxidant activation or substrate binding. Stereoretentive
C–H hydroxylation,3a,b,f,4a,b enantioselective epoxidation5c–f,6a,c–e
and asymmetric cis-dihydroxylation (AD) of alkenes5g,6b have
been achieved under limiting substrate conditions. One type of
proposed active metal–oxo intermediates in these oxidation
reactions catalysed by metal chiral N4 complexes is the corre-
sponding high-valent cis-dioxo complexes, i.e., cis-M(O)2 species
supported by chiral N4 ligands.6b In the iron-catalysed systems,
cis-[(N4)FeV(O)(OR)]2+ (R ¼ H or acyl) active intermediates,5h,7,8,9
and a cis-[(N4)FeV(O)2]+ active intermediate in alkene cis-dihy-
droxylation,10 have been proposed; isolation of these active
species and elucidation of the reaction mechanisms in these
iron systems have oen been difficult because of the
aState Key Laboratory of Synthetic Chemistry, Department of Chemistry, The University
of Hong Kong, Pokfulam Road, Hong Kong, China. E-mail: cmche@hku.hk
bHKU Shenzhen Institute of Research and Innovation, Shenzhen, Guangdong 518053,
China
cDepartment of Chemistry, Southern University of Science of Technology, Shenzhen,
Guangdong 518055, China
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization, Scheme S1, Tables S1–S6, Fig. S1–S20. CCDC 1589975 (1a),
CCDC 1589976 (2a), CCDC 1589977 (3a), CCDC 1589978 (5d), CCDC 1589979 (6d).
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c7sc05224c
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cis-[(6,60-Cl2bpy)2RuVI(O)2]2+ are known to react with simple satu-
rated alkanes (e.g., cyclohexane) stoichiometrically.15,16 The
t
related catalytic oxygenation of cyclohexane with BuOOH could
be performed with [(Me3tacn)RuIIICl3]19 and cis-[(Cl2bpy)2-
RuII(OH2)2]2+ as catalysts.16b Du Bois and co-workers recently
demonstrated selective C–H functionalization catalysed by
[(Me3tacn)RuIIICl3] with ceric ammonium nitrate (CAN) as
a terminal oxidant to give tertiary C–H hydroxylation products.20
Using bis(bipyridine)Ru catalysts, the selective functionalization
of amine derivatives was attainable with various oxidants and acid
additives.21 These studies highlight the amendable oxidation
capabilities of the cis-dioxoruthenium(VI) moiety and the under-
developed potential of ruthenium catalysts in C–H oxidation.
Thus far, studies on highly oxidizing cis-dioxoruthenium(VI)
complexes have focused on tridentate Me3tacn and simple
bidentate aromatic diimine ligands.15–18 The Me3tacn ligand is
not exible for structure modication.21 Ruthenium complexes
with aromatic diamine ligands may undergo cis–trans isomeri-
zation22 and ligand loss in a high oxidation state.17 These
difficulties can potentially be resolved by utilizing the above-
mentioned chiral N4 ligands: the rst coordination sphere is
highly tuneable by ligand modication, as revealed by recent
works from White,3d Costas,3b,4c,d and their co-workers; the
higher rigidity and denticity can provide better conformational
stability under catalytic conditions.
In this work, we aim to (i) isolate/generate cis-dioxor-
uthenium(VI) complexes bearing chiral tetradentate amine (N4)
ligands, (ii) study the redox potentials and hydrocarbon oxida-
tion reactions of these cis-[(N4)RuVI(O)2]2+ complexes, and (iii)
gain insight into the activity of chiral Ru(N4) complex in
asymmetric oxidation reactions. Until now, studies on ruthe-
nium complexes supported by chiral N4 ligands (mcp, pdp,
bqcn and their derivatives) have been limited,23–25 including
a report involving some data of [RuII(mcp)Cl2]-catalysed oxida-
tion of thioanisole with H2O2,25a another report involving
the synthesis and crystallographic characterization of
[RuII((R,R)-pdp)(NCMe)2]2+,25b and density functional calcula-
Fig. 1 Structures and abbreviations of chiral N4 ligands used in this
study. Ligands were prepared as either the (R,R)-enantiomer or
a racemic mixture.
extraordinary reactivity of high-valent iron–oxo complexes. A
proposed cis-[(N4)MnV(O)2]+ intermediate, cis-[((S,S)-bqcn)
MnV(O)2]+,6b in manganese-catalysed enantioselective cis-dihy-
droxylation of alkenes was also found to be insufficiently stable
for isolation. While several cis-[(N4)ReV(O)2]+ complexes and
a cis-[(N4)ReVI(O)2]2+ complex have been isolated and structur-
ally characterized in our recent work,11 the former were not
reactive towards organic substrates, and concerning hydro-
carbon oxidation reactivity, the latter only reacted with weak
C–H bonds (bond dissociation energy: ꢂ76 kcal molꢁ1) of 1,4-
cyclohexadiene, 9-10-dihydroanthracene and xanthene at 80 ꢀC
to give dehydrogenation or ketone products.
To search for isolable cis-dioxo metal complexes that are sup-
ported by the abovementioned chiral N4 ligands and are reactive
towards hydrocarbon oxidation, including the cis-dihydroxylation
of alkenes and the oxidation of strong C–H bonds at room
temperature, we directed our efforts to ruthenium systems. High-
valent Ru–oxo complexes are generally more stable than their iron
counterparts due to their lower redox potentials as well as
substitutional inertness of auxiliary ligands.12 cis-Dioxor-
uthenium(VI) complexes can have a delicate balance between
stability and reactivity that allows them to be isolated/character-
ized12,13 or even studied in reactions with organic substrates in
tions on a hypothetical monooxo ruthenium(IV) species cis-
[(bqcn)RuIV(O)(NCMe)]2+
.
No examples of the corresponding
26
a
stoichiometric manner.14–16 Several cationic cis-dioxor-
uthenium(VI) complexes, including cis-[(Tet-Me6)RuVI(O)2]2+ (Tet-
Me6 ¼ N,N,N0,N0-3,6-hexamethyl-3,6-diazooctane-1,8-diamine),14a
cis-[(Me3tacn)RuVI(O)2(O2CCF3)]+ (Me3tacn ¼ 1,4,7-trimethyl-tri-
azacyclononane),15a cis-[(6,60-Cl2bpy)2RuVI(O)2]2+ (6,60-Cl2bpy ¼
6,60-dichloro-2,20-bipyridine),16a cis-[(bpy)2RuVI(O)2]2+ (bpy ¼ 2,20-
bipyridine)17 and cis-[(dmp)2RuVI(O)2]2+ (dmp ¼ 2,9-dimethyl-1,10-
phenanthroline),18 have been isolated and/or spectroscopically
characterized. Among them, cis-[(Me3tacn)RuVI(O)2(O2CCF3)]+ and
cis-dioxo ruthenium chiral N4 complexes have been re-
ported.23–26 Herein, we describe the syntheses, characterization,
and electrochemical and reactivity studies of a series of chiral
Ru(N4) complexes, including a highly reactive chiral cis-[(N4)
RuVI(O)2]2+ complex that can perform dihydroxylation of
alkenes and oxidation of strong C–H bonds of alkanes
(including cyclohexane) and oxidation of alcohols at room
temperature. The studies on cis-[(N4)RuVI(O)2]2+ complexes
provide insight into the reactivity and electrochemical proper-
ties of the analogous highly oxidizing cis-[(N4)M(O)2]n+ (n ¼ 1
or 2; M ¼ Fe or Mn) species.6b,10
Results
Synthesis and characterization
In this work, a series of ruthenium complexes bearing six chiral
tetradentate amine N4 ligands (Fig. 1) and different auxiliary
ligands were prepared (Schemes 1 and 2). The reaction of
Fig. 2 Different wrapping modes of acyclic tetradentate N4 ligands in
an octahedral environment.
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K2[RuIIICl5(OH2)] with the mcp, Me2mcp, pdp or Me2pdp ligand
in ethanol under reuxing conditions (Scheme 1) gave the cor-
responding cis-[(N4)RuIIICl2]+ complex (1a, 2a, 3a or 4a) in 32–
97% yield.27 The reaction of 1a with Zn/Hg in distilled water at
80 ꢀC for 30 min, followed by subsequent treatment of the
solution with AgOTf and 0.2 M CF3CO2H, afforded cis-[(mcp)
RuIII(O2CCF3)2]ClO4 (1b) in 20% yield (Scheme 1).28 To prepare
ruthenium complexes containing the bqcn and Me2bqcn
ligands, an alternative synthetic method was developed. Treat-
ment of bqcn or Me2bqcn with a slight excess (1.2 equiv.) of
[RuII(OH2)6](OTs)2 under Ar in THF furnished the OTsꢁ salt of
cis-[(N4)RuII(OH2)2]2+ (5c or 6c) in good yield (up to 71%). A
similar treatment using pdp or Me2pdp gave the OTsꢁ salt of 3c
or 4c. Recrystallization of 5c$OTs or 6c$OTs in acetonitrile in
the presence of LiClO4 produced cis-[(bqcn)RuII(NCMe)2](ClO4)2
(5d) and cis-[(Me2bqcn)RuII(NCMe)2](ClO4)2 (6d), respectively
(Scheme 2).
Fig.
3
ORTEP drawings of the complex cations of cis-[(bqcn)
RuII(NCMe)2](ClO4)2 (5d). (a, left) cis-a isomer (a-5d). (b, right) cis-
b isomer (b-5d). Hydrogen atoms are omitted for clarity. Thermal
ellipsoids are drawn at the 30% probability level.
symmetry (see Experimental section); no interconversion to the
cis-b conformer was observed by standing the solution at room
temperature for days. In contrast, the H NMR spectrum of 5d
The structures of 1a, 2a, 3a, 5d and 6d (as ClO4ꢁ salts) were
established by X-ray crystallography. All these complexes, except
5d, adopt a cis-a conguration (Fig. 3 and S1–S5, ESI†), where
the two terminal pyridyl/quinolyl groups are positioned trans to
each other. For 5d, its crystal structure showed that the cis-a and
cis-b isomers are present in a 1 : 1 ratio in the unit cell.29 The
two isomers could not be separated by repeated recrystalliza-
tions. Fig. 3a depicts the structure of cis-a-5d; its two methyl
groups on the cyclohexane-1,2-diamine nitrogen atoms are
oriented anti to each other (C40 and C47). In the cis-b isomer
(Fig. 3b), the corresponding two methyl groups (C10 and C17)
show the opposite (syn) orientation.
1
comprises a mixture of signals from the cis-a and cis-b isomers.
The UV-Vis absorption spectra of the cis-[(N4)RuIIICl2]+
complexes (1a, 2a, 3a and 4a) in acetonitrile solution are char-
acterized by pp(Cl) / Ru(III) LMCT transition band at l 400–
450 nm (3 ¼ 900–2000 dm3 molꢁ1 cmꢁ1, Fig. S6, ESI†).27 In
aqueous solutions, the cis-[(N4)RuII(OH2)2]2+ complexes (3c, 4c,
5c and 6c) show intense absorption bands at 361–477 nm (3 ¼
4700–6900 dm3 molꢁ1 cmꢁ1, Fig. S7, ESI†) assignable to dp (Ru)
/ pp* (pyridyl or quinolyl) MLCT transitions.
The cis-dichlororuthenium(III) complexes display different
cyclic voltammetric behaviours in acetonitrile solutions (Fig. S9,
ESI†). Complexes 1a and 3a display a reversible couple at E1/2
The 1H NMR spectra of 3c, 4c (in CD3CN) and 6d show
signals indicative of a cis-a conguration (Fig. 2) with C2
¼
ca. 0 V vs. SCE. This is assigned to the RuIII/II couple: cis-[(N4)
RuIIICl2]+ + eꢁ / cis-[(N4)RuIICl2]0. The RuIV/III couple was not
observed at potentials up to 1.6 V vs. SCE. For 2a, where the N4
ligand possesses a methyl substituent on the pyridyl moiety, the
RuIII/II couple is irreversible. The irreversible reduction of cis-
[(N4)RuIIICl2]+ occurs at Epc ¼ ꢁ0.01 V; upon the reverse scan, an
oxidation wave appears at Epa ¼ 0.69 V, which is attributed to
the oxidation of cis-[(Me2mcp)RuIICl(NCMe)]+ to cis-[(Me2mcp)
RuIIICl(NCMe)]2+ aer a ligand exchange reaction of [(Me2mcp)
RuIICl2]0 with the solvent.30,31 The cyclic voltammograms of 5d
and 6d in MeCN display reversible oxidation couples at E1/2
¼
1.35 V and 1.36 V vs. SCE, respectively (Fig. S11, ESI†). The
electrochemical reaction is assigned to: cis-[(N4)RuIII(NCMe)2]3+
+ eꢁ / cis-[(N4)RuII(NCMe)2]2+.
Scheme 1 Preparation of 1a–4a and 1b.
Aqueous electrochemistry of cis-[(mcp)RuIII(O2CCF3)2]ClO4
(1b) and cis-[(pdp)RuIII(O3SCF3)2]CF3SO3 (3c0)
The cyclic voltammogram of cis-[(mcp)RuIII(O2CCF3)2]ClO4 (1b)
at pH 1 displays three reversible/quasi-reversible couples (i), (ii)
and (iii) at E1/2 ¼ 0.37, 0.92 and 1.11 V vs. SCE, respectively
(Fig. 4a). Using rotating-disk electrode voltammetry, the
coulombic stoichiometries of the redox couples were deter-
mined to be 1.0, 1.9 and 1.1 for couples (i), (ii) and (iii),
respectively (Fig. 4b). With reference to previous work,27 these
couples could be assigned to RuIII/II, RuV/III and RuVI/V redox
processes, and the electrochemical reactions (1)–(3) are
Scheme 2 Preparation of 3c–6c, 5d and 6d.
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relative to the RuIII/II couple is attributed to the rate-
determining deprotonation of [RuIII(OH)] or [RuIII(OH2)] prior
to the oxidation of RuIII to RuIV.35 Couple III is assigned as
a RuVI/IV couple (eqn (8)).36 The natures of couples I, II and III
were examined by rotating-disk electrode voltammetry (Fig. 5b),
showing that the limiting current/number of electrons involved
in couples I and (II and III) has a ratio of 1 to 2.7.37
The complex cis-[(pdp)RuII(OH2)2](OTs)2 (3c$OTs) similarly
shows a reversible couple at E1/2 ¼ 0.36 V and an irreversible
oxidation wave at Epa ¼ 1.14 V at pH 1 (Fig. S14a, ESI†).38 At pH
1, cis-[(bqcn)RuII(OH2)2](OTs)2 (5c$OTs) shows a reversible RuIII/
II couple at E1/2 ¼ 0.45 V and a shoulder oxidation wave at Epa
¼
1.15 V (Fig. S14b, ESI†), while cis-[(Me2bqcn)RuII(OH2)2](OTs)2
(6c$OTs) shows a reversible RuIII/II couple at E1/2 ¼ 0.49 V and
a shoulder oxidation wave at Epa ¼ 1.08 V (Fig. S14c, ESI†). The
s-donating ability of the N4 ligands follows the order of mcp z
pdp > bqcn > Me2bqcn, as revealed by the E1/2 values of the RuIII/
II
couples (Table 1).39 However, varying the structure of the N4
ligand has a minor effect on the redox potentials of the elec-
trochemically generated cis-dioxoruthenium(VI) complexes
(DEpa ꢂ 70 mV).
Fig. 4 (a, upper) Cyclic voltammogram of cis-[(mcp)RuIII(O2CCF3)2]
ClO4 (1b) at pH 1. (b, lower) Rotating-disk electrode voltammogram of
cis-[(mcp)RuIII(O2CCF3)2]ClO4 (1b) at pH 1. Working electrode: edge-
plane pyrolytic graphite for CV; glassy carbon for RDEV. Rest poten-
tials: ca. 0.55 V.
Variable-pH cyclic voltammetry of 3c0 was conducted in
Britton–Robinson buffer.40–42 Selected voltammograms at pH ¼
2.56, 5.02 and 6.37 are displayed in Fig. S15 (ESI†). Above pH
1.98, couple III splits into two one-electron couples (RuV/IV and
RuVI/V); the former, which merges with couple II to form a new
couple IV, can be assigned as a RuV/III couple (eqn (9)). The latter
one is designated as couple V (eqn (10)). The Pourbaix diagram
depicted in Scheme 3. At pH 5, the E1/2 of couples (i) and (iii) shi
to 0.25 and 0.98 V, respectively, and couple (ii) splits into two
reversible one-electron couples (iv) and v at 0.65 and 0.77 V,
respectively (Fig. S12, ESI†). Couples (iv) and (v) are assigned to
RuIV/III and RuV/IV couples (eqn (4) and (5) in Scheme 3). The
cathodic shi in the E1/2 of couple (iii) with an increasing pH is in
accordance with other dioxoruthenium(VI) complexes.14a,32,33,34
The electrochemical properties of cis-[(pdp)RuIII(O3SCF3)2]-
CF3SO3 (3c0, Scheme S1, ESI†) in 0.1 M CF3SO3H at pH 1 are
reminiscent of that of 1b. As depicted in Fig. 5a, 3c0 shows
a reversible couple I at E1/2 ¼ 0.36 V and a quasi-reversible
couple III at E1/2 ¼ 1.13 V (Epa ¼ 1.19 V) vs. SCE. Notably, at
the foot of couple III, there is a less dened couple II at E1/2
¼
0.95 V. Couple I (DEp ꢂ 60 mV; ipa/ipc ꢂ 1) is attributed to
a RuIII/II couple (eqn (6) in Scheme 4). Couple II is assigned as
a RuIV/III couple (eqn (7)). Its much smaller current measured
Fig. 5 Cyclic voltammogram (a, upper) at 0.1 V sꢁ1 and rotating-disk-
electrode voltammogram (b, lower) at 100 rpm of 3c0 in 0.1 M
Scheme 3 Proposed redox couples for cis-[(mcp)RuIII(O2CCF3)2]ClO4 CF3SO3H (pH 1). Working electrode: edge-plane pyrolytic graphite for
(1b) in different pH buffer solutions. The cis-sign is omitted for clarity. CV; glassy carbon for RDEV.
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Fig. 6 Pourbaix diagram of 3c0. Data points at pH 1 were extracted
from the cyclic voltammogram in 0.1 M CF3SO3H (i.e., Fig. 5a), while
data points at pH $ 2 were extracted from variable-pH measurements
in Britton–Robinson buffer.
Scheme
4
Proposed redox couples for cis-[(pdp)RuIII(O3SCF3)2]
CF3SO3 (3c0) in different pH buffer solutions. The cis-sign is omitted for
clarity.
Bordwell.43,44 The DO–H value is calculated to be 90.8 kcal molꢁ1
for cis-[(pdp)RuVI(O)2]2+ (Scheme 5) and 90.1 kcal molꢁ1 for cis-
[(mcp)RuVI(O)2]2+.
from pH 1 to 7.96 is shown in Fig. 6. For couple I (RuIII/II), there
are two straight-line fragments with slopes of ꢁ56 and ꢁ122 mV
per pH unit at 1 < pH < 6.37 and 6.37 < pH < 7.24, respectively,
corresponding to the electrochemical reactions described in
eqn (6) and (11). The breakpoint (pH ¼ 6.4) of the plot for couple
I is logically the pKa value of cis-[(pdp)RuIII(OH)(OH2)]2+, which
is comparable to that of cis-[(Tet-Me6)RuIII(OH)(OH2)]2+ (pKa ¼
6.5).14a For couple IV (RuV/III), three linear segments with slopes
of ꢁ57, ꢁ85 and ꢁ52 mV per pH unit are found at 1.98 < pH <
5.72, 5.72 < pH < 6.37 and 6.37 < pH < 7.96, respectively. The
corresponding electrochemical reactions are described in eqn
(9), (12) and (13). For couple V (RuVI/V), its potential shis
cathodically with a slope of ꢁ51 mV pHꢁ1 at 1.98 < pH < 5.02.
This is in line with its one-proton one-electron nature (equation
(10)). At 5.02 < pH < 7.96, it becomes insensitive to pH, sug-
gesting a one-electron process that does not involve proton loss
(equation (14)). From this observation, together with the
breakpoint of the plot of couple IV, the pKa value of cis-[(pdp)
RuV(O)(OH)]2+ is estimated to be 5.6.
DO–H ¼ 23.06Eꢀ + 1.37pKa + C45
(15)
Isolation or generation of cis-dioxoruthenium(VI) complexes
via chemical oxidation
Treatment of cis-[(mcp)RuIII(O2CCF3)2]+ (1b) with excess CAN in
aqueous solution gave cis-[(mcp)RuVI(O)2]2+ (1e), which was
isolated as a pale green perchlorate salt in 66% yield (Scheme 6,
see Experimental section for details). The UV-visible absorption
spectrum of
a freshly prepared solution of cis-[(mcp)
RuVI(O)2](ClO4)2 (Fig. 7) in acetonitrile shows a prominent
absorption peak at lmax ¼ 260 nm (3 ¼ 8700 dm3 molꢁ1 cmꢁ1),
a broad shoulder band at 340 nm (3 ¼ 2210 dm3 molꢁ1 cmꢁ1
)
and a weak absorption band at 700 nm (3 ¼ 80 dm3
molꢁ1 cmꢁ1). The high-resolution ESI mass spectrum of 1e
shows a prominent ion species centred at m/z ¼ 229.0631 that
matches the formulation and the isotope distribution pattern of
[(mcp)Ru(O)2]2+ (Fig. S16, ESI†). The IR spectrum of 1e shows
two peaks at 845 and 868 cmꢁ1, which are assigned to the
With the above electrochemical information in hand, the
bond dissociation energy (DO–H) for cis-[(pdp)RuV(O)(O–H)]2+ to
form cis-[(pdp)RuVI(O)2]2+ can be obtained from eqn (15), based
on the thermochemical method developed by Mayer and
Table 1 Redox potentials of ruthenium N4 complexes in aqueous
solution at pH 1 (ref. 39)
E1/2 of RuIII/II
RuVI/V or RuVI/IV
Complex
couple (V vs. SCE)
oxidation (V vs. SCE)
1b (N4 ¼ mcp)
3c0 (N4 ¼ pdp)
5c$OTs (N4 ¼ bqcn)
6c$OTs
0.37
0.36
0.45
0.49
E1/2 ¼ 1.11
E1/2 ¼ 1.13
Epa ¼ 1.15
Epa ¼ 1.08
(N4 ¼ Me2bqcn)
Scheme 5 Thermochemical cycle of cis-[(pdp)RuVI(O)2]2+
.
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Fig. 8 (Upper) Simulated ESI-MS pattern of {[(pdp)RuVI(O)2]ClO4}+.
(Lower) Experimental ESI-MS signals for
a reaction mixture of
3c$CF3SO3 and 6 equiv. of CeIV(ClO4)4, [Ru] ¼ 1 ꢃ 10ꢁ4 M.
3c$CF3SO3 with a CeIV oxidant were performed in water and
monitored by high-resolution ESI-MS. Under dilute conditions
([Ru] ¼ 1 ꢃ 10ꢁ4 M), treatment of 3c$CF3SO3 with 4 equiv. of
CeIV(ClO4)4 generated predominant ruthenium signals at m/z ¼
220.05 and 228.56. These signals are attributed to [(pdp)
RuIV(O)]2+ and [(pdp)RuV(O)(OH)]2+ species (Fig. S17, ESI†).
When a slight excess of CeIV(ClO4)4 (6 equiv.; 150% for a 4eꢁ
oxidation process) was employed, a new signal that corresponds
to {[(pdp)RuVI(O)2]ClO4}+ was observed at m/z ¼ 555.05 (Fig. 8).
Notably, its signal intensity dropped signicantly aer ca. 1 min
Scheme 6 Preparation of 1e and generation of cis-[(pdp)RuVI(O)2]2+
.
symmetric and asymmetric stretches of the cis-dioxor-
uthenium(VI) moiety.14a,15a,16a Complex 1e is diamagnetic, as
1
ꢀ
revealed by its H NMR signals. Notably, 1e is stable at ꢁ15 C
under argon for a few hours but decomposes in aqueous tert-
butanol or acetonitrile within 30 min to give a dark brown
solution, the ESI-MS analysis of this solution showed peaks
centred at m/z ¼ 460.1, which corresponds to [(mcp)Ru(OH)2]+
in aqueous tert-butanol, and m/z ¼ 254.1, which corresponds to
[(mcp)Ru(NCMe)2]2+ in acetonitrile. In aqueous solution at pH
1, cis-[(mcp)RuVI(O)2](ClO4)2 (1e) shows an identical cyclic vol-
tammogram as that as 1b. The cyclic voltammogram of 1e in
Table
2 Stoichiometric oxidation of alkenes by cis-[((R,R)-mcp)
RuVI(O)2](ClO4)2 (1e*) in aqueous tert-butanola
Entry
Alkene subtrate
Product(s)
% Yieldb (% ee)
70c
acetonitrile shows a reversible one-electron couple at E1/2
¼
0.78 V vs. Ag/AgNO3 (0.1 M in MeCN), attributed to a RuVI/V
couple: cis-[(mcp)RuVI(O)2]2+ + eꢁ / [(mcp)RuV(O)2]+. Based on
this redox potential, 1e is a stronger oxidant than cis-[(Tet-Me6)
RuVI(O)2](ClO4)2 (E1/2 ¼ 0.53 V vs. Ag/AgNO3).14a
1
2
cis-Diol
syn-Diol
anti-Diol
PhCHO
syn-Diol
anti-Diol
PhCHO
syn-Diol
anti-Diol
PhCHO
27
20 (24)
28 (35)
45c
21 (30)
25 (36)
52c
19 (28)
26 (33)
53c
Similar oxidation of cis-[(pdp)RuIII(O3SCF3)2]+ (3c0) or in situ
generated cis-[(pdp)RuII(OH2)2]2+ by CAN did not furnish
isolable cis-[(pdp)RuVI(O)2](ClO4)2 (Scheme 6). Upon addition of
the CAN solution into an ice-cooled solution of 3c0, a dark
3
4
ꢁ
brown solution resulted, and subsequent addition of ClO4 or
ꢁ
PF6 did not induce solid formation. Small-scale reactions of
42 (27)
50c
5
6
7
39 (33)
51c
43 (28)
48c
45 (34)
47c
8
a
Reaction conditions: 1e* (0.3 mmol), substrate (30 mmol), tBuOH/H2O
(5 : 1 v/v, 12 mL), under argon, room temperature, and 30 min.
Fig. 7 UV-Vis absorption spectrum of cis-[(mcp)RuVI(O)2](ClO4)2 (1e)
in acetonitrile.
b
Isolated yield, calculated as mmol of product per mmol of 1e.
c
Determined by GC.
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(Fig. S18, ESI†). A cis-dioxo-Ru(V) species was also detected at m/ in 85% and 5% yields, respectively, whereas use of cis-[(Tet-Me6)
z ¼ 456.10 (Fig. S19a, ESI†), with its signal intensity remaining RuVI(O)2](ClO4)2 gave 22% cis-cyclooctane-1,2-diol and 60% 1,8-
constant for at least 3 min (Fig. S19b†). At a higher concentra- octanedialdehyde.47 Apart from the organic products, a green
tion of 3c$CF3SO3 ([Ru] ¼ 1 ꢃ 10ꢁ3 M), a complicated spectrum ruthenium compound was isolated at the end of the reaction of
dominated by noise signals was obtained with just 4 equiv. of 1e with cyclooctene. ESI-MS analysis revealed a prominent ion
CeIV(ClO4)4. Most likely, the decomposition of Ru(pdp) peak at m/z ¼ 460.1; its m/z ratio and isotopic distribution
complexes under oxidizing condition is signicantly fast with pattern are consistent with a [(mcp)RuIII(OH)2]+ formulation.
[Ru] $ 1 mM. This may account for the difficult isolation of cis-
Using chiral (R,R)-mcp as a ligand, the chiral cis-dioxor-
[(pdp)RuVI(O)2](ClO4)2
experiment.
in
the
large-scale
preparative uthenium(VI) complex, cis-[((R,R)-mcp)RuVI(O)2](ClO4)2 (1e*),
was prepared. Several stoichiometric alkene oxidation reactions
were performed by reacting 1e* (0.3 mmol) with excess alkene
substrate (30 mmol, 100 equiv.) in a degassed (5 : 1 v/v) tert-
Stoichiometric oxidation of hydrocarbons by cis-[(mcp)
butanol/H2O mixture (12 mL) under argon at room temperature
for 30 min (Table 2). Aryl alkenes were oxidized to their corre-
sponding diols (39–48% yields) with ee values ranging from 24
to 36%, accompanied by the formation of C]C bond cleavage
products in considerable amounts (45–53%). In the reaction of
1e* with styrene, for instance, a 42% yield of styrene glycol (27%
ee) and 50% yield of benzaldehyde were obtained (entry 5, Table
2). Similarly, trans-b-(trimethylsilyl)styrene reacted with 1e* to
afford a 19% yield of syn-diol (28% ee) and 26% yield of anti-diol
(33% ee) along with a 53% yield of benzaldehyde (entry 4, Table
2). There is no major difference in the reactions of 1e* with
trans-b-methylstyrene and with cis-b-methylstyrene, which
afforded the enantio-enriched syn-diol in 20% yield (24% ee)
and 21% yield (30% ee), anti-diol in 28% yield (35% ee) and 25%
yield (36% ee), benzaldehyde in 45% and 52% yields, respec-
tively (entries 2 and 3, Table 2). The effects of para-substituents
on the enantioselectivity of p-substituted styrenes in the reac-
tion with 1e* were examined (entries 5–8, Table 2); the para-
substituents CH3, Cl and Br had no signicant effect on either
the yields (39–45%) or ee (28–34%) of the diol products.
The stoichiometric oxidations of alcohols and alkanes by 1e
were studied. When 1e was treated with benzyl alcohol (100
equiv.) in acetonitrile at room temperature for 30 min, benzal-
dehyde was formed in 90% yield (Table 3, entry 1). Similarly,
other primary alcohols such as 1-heptanol and 1-octanol were
oxidized by 1e to give a mixture of aldehyde and carboxylic acid
(entries 2 and 3, Table 3). Under these conditions, cyclooctene
reacted with 1e to afford cyclooctene oxide and 1,8-octane-
dialdehyde in 30% and 58% yields, respectively (entry 4, Table
RuVI(O)2](ClO4)2 (1e)
The results of the electrochemical studies suggest that cis-
[(mcp)RuVI(O)2](ClO4)2 (1e) is a strong oxidant (Eꢀ ¼ 1.11 V vs.
SCE at pH 1). In aqueous tert-butanol, freshly prepared 1e could
stoichiometrically oxidize cyclooctene to give a mixture of cis-
cyclooctane-1,2-diol (27%) and 1,8-octanedialdehyde (70%)
(Table 2, entry 1).46 Compared with our previous works, [(Me3-
tacn)RuVI(O)2(O2CCF3)]ClO4 oxidized cyclooctene stoichiomet-
rically to give cis-cyclooctane-1,2-diol and 1,8-octanedialdehyde
Table 3 Stoichiometric organic oxidations by cis-[(mcp)RuVI(O)2](ClO4)2
(1e) in acetonitrilea
Entry
Substrate
Product(s)
PhCHO
Yieldb (%)
1
2
90c
60
22
55
26
3
58c
4
5
30
26c
55c
62c
3). Complex 1e could also oxidize saturated C–H bonds. For
instance, ethylbenzene (BDEC–H ¼ 85.4 kcal molꢁ1
)
was
48
6
7
8
58
66
Table 4 Oxidation of cis-1,2-dimethylcyclohexane with CAN cata-
lysed by 1b and cis-[(N4)RuII(OH2)2]2+ complexesa
Reaction time Conversion Product yield (%)
9d
58
60
Entry Catalyst (min)
(%)
based on conversion
1
2
3
4
5
1b
15
15
30
15
30
60
80
8
73
6
62
64
55
61
61
10d
3c$OTs
4c$OTs
5c$OTs
6c$OTs
a
Reaction conditions: 1e (0.3 mmol), substrate (30 mmol), MeCN (12
b
mL), under argon, room temperature, and 30 min. Isolated yield,
c
a
calculated as mmol of product per mmol of 1e. Determined by GC.
Reaction conditions: substrate (0.25 mmol), catalyst (2 mol%), CAN
d
1e* instead of 1e was used.
(0.75 mmol), tBuOH/H2O (1 : 1 v/v, 4 mL), and room temperature.
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oxidized by 1e in acetonitrile to give acetophenone (55% yield) for tertiary C–H hydroxylation. Analysis of the crude reaction
and 1-phenylethanol (26% yield) (entry 5, Table 3). Notably, mixture by 1H NMR spectroscopy revealed the C7 : C3 selectivity
cyclohexane (BDEC–H ¼ 99.5 kcal molꢁ1
) was oxidized to give to be a ratio of >10 : 1. Aer purication, a C7-hydroxylated
48
cyclohexanone in 62% yield (entry 6, Table 3). Similar to the product (P6) was obtained in 80% yield (entry 7, Table 5).
reported cis-dioxoruthenium(VI) complexes,14a,16a when ada- Reactions of S7 and S8 similarly occurred at the 3ꢀ C–H bond
mantane was employed as a substrate, C–H oxidation occurred which were remote from the electron-withdrawing ester/amide
primarily at the 3ꢀ carbon; 1-adamantanol was formed as the
sole product in 58% yield (entry 7, Table 3). The oxidation of cis-
4-methylcyclohexyl benzoate afforded the tertiary alcohol in
moderate yield (66%) with complete retention of the congu-
Table 5 Oxidation of tertiary and benzylic C–H bonds with CAN
catalysed by 3c$OTsa
ration; no epimerized product was observed (entry 8, Table 3).
Reaction of 1e* with the two racemic substrates in entries 9 and
10 (Table 3) predominantly gave oxygenated products at the
tertiary C–H bonds; however, chiral HPLC analysis of the
tertiary alcohol product revealed no kinetic resolution effect (ee
<2%). These organic transformations were accompanied by the
reduction of cis-dioxoruthenium(VI) to cis-[(mcp)RuII(NCMe)2]
(ClO4)2 (1d), which was isolated and characterized (ESI†).
Catalytic oxidation of alkanes with CAN mediated by cis-[(pdp)
RuII(OH2)2]2+ (3c)
The catalytic activities of the cis-[(mcp)RuIII(O2CCF3)2]+ (1b) and
cis-diaquoruthenium(II) complexes (3c–6c) towards the hydrox-
ylation of C(sp3)–H bonds were examined using CAN as
a terminal oxidant. cis-1,2-Dimethylcyclohexane (S1) was chosen
as an initial test substrate (Table 4). The reaction of S1 with
2 mol% 3c$OTs and 3 equiv. of CAN for 15 min at room
temperature in aqueous tert-butanol afforded a tertiary alcohol
product (P1) in 64% yield based on 80% conversion (entry 2,
Table 4).49 The stereogenic centres are retained in the alcohol
product, indicating that the hydroxylation reaction does not
involve long-lived carbon-based radicals that can epimerize.
Among the screened ruthenium catalysts, 3c$OTs showed the
highest catalytic activity. When 4c$OTs or 6c$OTs was employed
as the catalyst, particularly, the substrate conversion was <10%
(entries 3 and 5, Table 4).50 Therefore, subsequent studies
focused on the use of 3c$OTs as a catalyst. In a control experi-
ment, in which the ruthenium catalyst was replaced by
[RuII(OH2)6](OTs)2, S1 remained intact for a 30 min reaction
Reaction Conversion Products (yield in %
(Table 5, entry 2). Subsequent addition of 3c$OTs to this reac-
tion mixture followed by stirring for 15 min afforded P1 in 64%
yield based on 61% conversion.
Entry Substrate time
(%)
based on conversion)
1
S1
S1
S2
S3
S4
15 min
30 min
45 min
1 h
80
<1
51
52
74
P1 (64)
—
2b
3c
4d
5c
Oxidation of methylcyclohexane (S2) gave a tertiary alcohol
product (P2) with high selectivity (96%) based on 52% conver-
sion (Table 5, entry 3). Similarly, S3 was oxidized to P3 with good
selectivity (entry 4, Table 5). For the oxidation of adamantane
(S4), apart from ordinary oxygenation products, such as Ad-1-ol
(P4a, 47% yield) and “Ad-2-ol + Ad-2one” (P4b, 3% yield),
adamantan-1,3-diol (P4c) was also formed in 32% yield (entry 5,
Table 5). Most likely, the initial hydroxylation of S4 gives P4a;
the latter, being more soluble, was efficiently further hydroxyl-
ated to yield P4c.51 The normalized 3ꢀ/2ꢀ selectivity is as high as
79 : 1, showing the strong preference of the active oxidant to (0.75 mmol), tBuOH/H2O (1 : 1 v/v, 4 mL), and room temperature.
attack 3ꢀ over 2ꢀ C–H bonds. Following this preference, the
oxidation of racemic S5 produced P5 in 48% isolated yield (entry
6, Table 5).52 Compound S6 has two possible sites (C3 and C7)
P2 (96)
P3 (84)
P4a (47), P4b (3),
P4c (32)
P5 (83)
P6 (80)
P7 (85)
P8 (89)
P9 (91)
P10 (89)
1.5 h
6d
7
8
S5
S6
S7
S8
S9
S10
1 h
1.5 h
1 h
1 h
15 min
40 min
59
40
60
65
28
84
9
10
11c,e
a
Reaction conditions: substrate (0.25 mmol), catalyst (2 mol%), CAN
b
[RuII(OH2)6](OTs)2 was used as the catalyst. c tBuOH/H2O (3 : 1 v/v, 4
d
mL) was used as the solvent. CF3CH2OH/H2O (3 : 1 v/v, 4 mL) was
used as the solvent. e 1.5 mmol CAN was used.
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Table 7 Oxidation of secondary and primary C–H bonds with CAN
catalysed by 3c$OTsa
groups, in 51% and 58% isolated yields, respectively (entries 8
and 9, Table 5). In entries 10 and 11 (Table 5), the benzylic C–H
bonds in ethylbenzene and tetralin were oxidized to give ace-
topheone (P9) and tetralone (P10), respectively. No aromatic
ring degradation products were found. Thus, the possible
involvement of RuO4 was unlikely as it is known to degrade
aromatic rings.53 A kinetic isotope effect (KIE) of kH/kD ¼ 5.2 was
found in the competitive oxidation of an equimolar mixture of
ethylbenzene and d10-ethylbenzene, indicative of C–H bond
cleavage in the rate-determining step (RDS) or in a product-
determining step following the RDS.54
Reaction
time (h)
Conversion
(%)
Products (yield in %
based on conversion)
For more complex substrates, the catalyst loading was Entry
increased to 5 mol% to furnish oxidation products in isolated
Substrate
1b
S16
S17
S18
S19
1.5
1.5
3
40
P16 (95)
yields ranging from 37% to 76% (55–93% based on conversion,
Table 6). In general, sterically unhindered tertiary C–H bonds
were preferred over unactivated methylene centres (entries 1
and 4, Table 6). For the oxidation of S12 that contains both
tertiary and benzylic C–H bonds, only the ketone product P12
was formed (entry 2, Table 6). Notably, the reaction of S13 gave
desaturation product P13 (entry 4, Table 6), presumably via an
alcohol intermediate.55
d
2b,c
—
P17 (TON ¼ 9)
P18 (TON ¼ 8)
P19 (TON ¼ 3)
3e
—
—
4e
3
a
Reaction conditions: substrate (0.25 mmol), catalyst (2 mol%), CAN
(0.75 mmol), tBuOH/H2O (1 : 1 v/v, 4 mL), and room temperature.
b tBuOH/H2O (3 : 1 v/v, 4 mL) was used as the solvent. 1.5 mmol CAN
c
d
was used. Conversion was not determined because of the high
e
volatility of the substrate. Gaseous substrate used in excess (100 psi).
Interestingly, this catalytic protocol was also found capable
of oxidizing strong secondary and primary C–H bonds of light
Lastly, oxidation of ethane (BDEC–H ¼ 100.5 kcal molꢁ1
afforded acetic acid with a TON ¼ 3 (entry 4, Table 7).56
)
48
alkanes (Table 7). The oxidation of cyclooctane (BDEC–H
¼
95.7 kcal molꢁ1
)
for 1.5 h gave cyclooctanone in 95% yield
48
based on 40% conversion (entry 1, Table 7). Similarly, cyclo-
hexane (BDEC–H ¼ 99.5 kcal molꢁ1
)
was oxidized to cyclohex-
48
General remarks/discussion
General properties of the ruthenium N4 complexes
anone with a turnover number (TON) of 9 (entry 2, Table 7). The
oxidation of propane (BDEC–H ¼ 98.1 kcal molꢁ1
)
afforded
48
acetone with a TON ¼ 8 for a 3 h reaction (entry 3, Table 7). Two series of ruthenium complexes, cis-[(N4)RuIIICl2]+ (1a–4a)
and cis-[(N4)RuII(OH2)2]2+ (3c–6c), were prepared. Owing to the
lability of aqua ligands, [RuII(OH2)6](OTs)2 is an efficient
precursor for the synthesis of cis-[(N4)RuII(OH2)2]2+ complexes
Table 6 Oxidation of pharmaceutical ingredients and natural product
(3c–6c). The cis-diaquoruthenium(II) complexes were isolated as
derivatives with CAN catalysed by 3c$OTsa
ditosylate (OTsꢁ) salts and are air sensitive. In aqueous solu-
tions under aerobic conditions, they are susceptible to oxida-
tion, as determined by the depletion of the characteristic MLCT
transition band at 360–480 nm. Accompanying the UV-Vis
spectral changes, the predominant species observed in ESI-MS
analysis changed from [(N4)RuII(OTs)]+ to [(N4)RuIII(OH)2]+.
Complexes without ortho-methyl substituents on the pyridyl/
quinolyl moieties (3c and 5c) are less prone to aerobic oxida-
tion; the process requires hours to complete. In contrast,
complexes 4c and 6c are readily oxidized to Ru(III) species within
1 h.
The structural analyses of the cis-[(N4)RuIIICl2]+ complexes by
X-ray crystallography show that the cis-a conguration is the
predominantly preferred geometry. 1H NMR spectroscopy of the
cis-[(N4)RuII(OH2)2]2+ complexes in CD3CN or the bis(acetoni-
Reaction
time
Conversion
(%)
Products (yield in %
based on conversion)
trile)ruthenium(II) complex revealed that the coordination
geometry depends on the ligand structure. In particular, the
bqcn ligand coordinates to the ruthenium centre in an unse-
lective manner affording a mixture of cis-a and cis-b isomers,
which do not interconvert in acetonitrile solution. In the X-ray
crystal structures of cis-a-5d and cis-b-5d, the N-methyl groups
have different orientations (anti or syn). Thus, interconversion
between the two isomers requires (i) breakage of the Ru–
Entry
Substrate
1
2
3
4
S11
S12
S13
S14
40 min
40 min
20 min
13 h
50
82
68
73
P11 (84)
P12 (93)
P13 (55)
P14 (78)
a
Reaction conditions: substrate (0.2 mmol), catalyst (5 mol%), CAN (1.2
mmol), tBuOH/H2O (1 : 1 v/v, 4 mL), and room temperature.
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Table 8 Hydrogen-atom affinity of selected metal–oxo complexes
N(quinolyl) bond, (ii) breakage of the Ru–N(amine) bond, and
(iii) epimerization of the N-methyl group followed by migration
of the acetonitrile ligand. These are expected to have large
kinetic barriers, therefore, interconversion between the two
isomeric forms is slow, and the ligand topology is likely deter-
mined at the synthetic stage of 5c$OTs. Similar arguments have
been addressed by Nam, Shin and co-workers; they found that
cis-a- or cis-b-[(bqcn)FeII(NCMe)2]2+ could be independently
obtained with different synthetic methods and that these
isomers do not interconvert in solution at room temperature.57
Complex
DO–H
Reference
cis-[(bpy)2RuVI(O)2]2+
cis-[(pdp)RuVI(O)2]2+
93.5a
90.8
90.1
90
17
This work
This work
59, 60
65
62
61
63
62
66, 67
68
cis-[(mcp)RuVI(O)2]2+
[(TSMP)FeIV(O)]
cis-[(Me3tacn)RuVI(O)2(O2CCF3)]2+
[(N4Py)RuIV(O)(OH2)]2+
[(bpy)2(py)RuIV(O)]2+
87.5a
84.8
84
trans-[(N2O2)RuVI(O)2]2+
[(TPA)RuIV(O)(OH2)]2+
[(Me2EBC)MnIV(O)(OH)]+
82.8
82.7
84.3
80
79
76.3
MnVIIO4
ꢁ
Electrochemistry/reduction potentials
[(phen)2MnIV(m-O)2MnIII(phen)2]3+
69
64
trans-[(14-TMC)RuVI(O)2]2+
Aqueous electrochemical measurements (at pH 1) of cis-[(mcp)
RuIII(O2CCF3)2]ClO4 (1b) and cis-[(pdp)RuIII(O3SCF3)2]-CF3SO3
(3c0) revealed the strong oxidizing powers of their correspond-
ing cis-dioxoruthenium(VI) species. The highly anodic redox
potentials (Eꢀ ¼ 1.11–1.13 V vs. SCE) are comparable to those of
cis-[(6,60-Cl2bpy)2RuVI(O)2]2+ (1.17 V)58 and electrochemically
a
Calculated from the reported electrochemical data.
species, if it exists, would be much more reactive than the cis-
dioxoruthenium(VI) counterpart. The highly anodic/oxidizing
reduction potential of the cis-dioxoiron(V) species may not be
favourable for alkene dihydroxylation, as side reactions (e.g.,
C]C cleavage) may become dominant. In comparison, the FeV/
generated cis-[(TPA)RuVI(O)2]2+ (1.1 V, TPA
idylmethyl)amine).42
¼
tris(2-pyr-
The aqueous electrochemical data allow the determination
of the hydrogen-atom affinity of the cis-dioxoruthenium(VI)
complexes. The DO–H values are calculated to be 90.8 kcal molꢁ1
for cis-[(pdp)RuVI(O)2]2+ and 90.1 kcal molꢁ1 for cis-[(mcp)
III
couple of cis-[(L-N4Me2)FeV(O)2]+ (L-N4Me2 ¼ N,N0-dimethyl-
2,11-diaza[3.3](2,6)pyridinophane), an intermediate proposed
to be involved in alkene dihydroxylation,10 was computed to
occur at 1.34 V vs. SCE at pH 1.71
RuVI(O)2]2+. Referring to Table 8, these values are comparable to
those of cis-[(bpy)2RuVI(O)2]2+ (93.5 kcal molꢁ1
)
and [(TSMP)
17
FeIV(O)] (90 kcal molꢁ1, H2TSMP ¼ meso-tetrakis(sulfonatome-
Reactivity of cis-dioxoruthenium(VI)
sityl)porphyrin),59,60 but are considerably larger than those of
several
(mono)oxoruthenium(IV)
complexes
(82.7– The results presented in this work show the strong oxidizing
84.8 kcal molꢁ1),61,62 trans-dioxoruthenium(VI) complexes (76.3– power of cis-dioxoruthenium(VI) complexes containing chiral N4
82.8 kcal molꢁ1),63,64 another cis-dioxoruthenium(VI) complex ligands by electrochemical analysis and their reactivity with
supported by the Me3tacn ligand (87.5 kcal molꢁ1
)
65 and several hydrocarbons.
complexes are known,14a,15a,16a,17 chiral ones have not been re-
Although
several
cis-dioxoruthenium(VI)
Mn–oxo complexes (79–84.3 kcal molꢁ1).66–69
Another piece of interesting information can be extracted ported in the literature to the best of our knowledge. In this
from the pH-dependent cyclic voltammogram of 3c0, where the work, we isolated and spectroscopically characterized the chiral
RuV/III couple was observed over the pH range of 1.98 to 7.96. At complex cis-[((R,R)-mcp)RuVI(O)2](ClO4)2 (1e*). Complex 1e*
pH 4.1, for example, the potential of the redox couple cis-[(pdp) could effect the stoichiometric oxidations of alcohols, alkanes
RuV(O)(OH)]2+ + 2eꢁ + 2H+ / cis-[(pdp)RuIII(OH)(OH2)]2+ occurs and alkenes, as was found for other cis-dioxoruthenium(VI)
at 0.76 V vs. SCE. This can provide a basis to estimate the redox complexes. In the reaction of 1e* with alkenes, considerable
potential of the putative cis-[(pdp)FeV(O)(OH)]2+ or cis-[(pdp) amounts of dihydroxylation products were obtained with
FeV(O)2]+ species, which are perceived to be strong oxidants but moderate enantioselectivities (ꢂ30% ee, Table 2), albeit with
have not been reported in the literature. We previously reported the predominant products being C]C bond cleavage ones,
a density functional theory (DFT) study of trans-dioxo complexes such as carbonyl compounds. In addition, a mixture of syn- and
of iron, ruthenium and osmium, trans-[(NH3)2(NMeH2)2- anti-diols was obtained, which possibly indicates the non-
MVI(O)2]2+ (M ¼ Fe, Ru, Os), where the reduction potentials of concerted nature of the dihydroxylation reaction.72,73 The
the corresponding FeVI/V and RuVI/V couples were estimated to reactivity/selectivity in the Ru((R,R)-mcp)-mediated asymmetric
be 1.3 V and 0.56 V vs. NHE, respectively.70 This theoretical study cis-dihydroxylation (AD) reaction of alkenes is in great contrast
implies that a O]Fe]O complex would be ꢂ0.7 V more to some of the known, highly efficient chiral Fe(N4) or Mn(N4)
oxidizing than the corresponding O]Ru]O complex with the catalysts. For instance, cis-[((R,R)-Me2bqcn)FeII(OTf)2] and cis-
same ligand system. If the same relationship can be applied to [((S,S)-bqcn)MnIICl2] gave cis-diols in up to 95% yields and
Fe/Ru(N4) complexes in a cis-conguration, the potential of the 99.8% ee via proposed cis-[((R,R)-Me2bqcn)FeIII(OOH)]2+ and cis-
redox couple cis-[(pdp)FeV(O)(OH)]2+ + 2eꢁ + 2H+ / cis-[(pdp) [((S,S)-bqcn)MnV(O)2]+ intermediates, respectively.5g,6b
FeIII(OH)(OH2)]2+ (or cis-[(pdp)FeV(O)2]+ + 2eꢁ + 2H+ / cis-[(pdp)
Based on the stoichiometric reaction of cis-[(mcp)RuVI(O)2]2+
FeIII(OH)2]+, depending on the pKa value) would occur at (1e) with alkenes, a related catalytic reaction was developed
approximately 1.4–1.5 V vs. SCE at pH 4.1, which is equivalent to using NaIO4 as a terminal oxidant. cis-[(mcp)RuIII(O2CCF3)2]
1.6–1.7 V vs. SCE at pH 1. This suggests that a cis-dioxoiron(V) ClO4 (1b) turned out to be an efficient catalyst for the oxidative
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scission of aryl alkenes to carbonyl compounds (Table S5, ESI†, S1 to P1 in Table 5, S11 to P11 in Table 6), which is a funda-
6 examples). At a catalyst loading of 1 mol%, aryl C]C bonds mentally dening feature in C–H functionalization rendering
are cleaved to aldehydes or ketones in high conversions (83– this method of synthetic value. When the substrate contains
100%) and high yields (89–100%).74 Over-oxidation of aldehydes multiple tertiary C–H bonds (S8–S10, Table 5), hydroxylation
to carboxylic acids was not observed by controlling the stoi- preferentially occurs at the most electron-rich site, as was also
chiometry of NaIO4 (10% excess). The timespan of the reaction observed in other Fe/Mn-catalysed C–H hydroxylation systems
(1 h) is comparable to that (30 min) reported by Bera and co- (e.g., cis-[(pdp)FeII(NCMe)2]2+/H2O2/AcOH).3a,b,4b This similar
workers using an abnormal-NHC–Ru(II) catalyst.75
reactivity pattern implies the common electrophilic nature of
Using cis-[(mcp)RuIII(O2CCF3)2]ClO4 (1b) as a catalyst and cis-dioxoruthenium(VI) and the active oxidant in Fe(pdp)-
H2O2 as a terminal oxidant, we also developed a catalytic catalysed reactions. In literature, the identity of the latter was
protocol for the oxidation of alcohols (Table S6, ESI†, 14 investigated by multiple research groups which has led to
examples). Alcoholic substrates were effectively oxidized to different formulations.5d,7,8,9 Talsi, Bryliakov and co-workers
carbonyl compounds or carboxylic acids in yields up to 98% (see identied an S ¼ 1/2 species by EPR and assigned it to [(pdp)
the ESI† for a more detailed description). ESI-MS analysis of FeV(O)(OAc)]2+
.
Based on computational results, Wang, Que,
5d
a mixture of 1b and H2O2 did not reveal formation of 1e or other Shaik and co-workers suggested a cyclic Fe(III) peracetate
high-valent ruthenium–oxo complexes. The active intermediate complex that undergoes O–O bond cleavage to a transient
could be hydroperoxo- or peroxo-Ru(III) species, which has yet to oxoiron(IV)–AcOc species which performs efficient C–H
be claried.
Reports on the oxidation of alkanes catalysed by ligand-
hydroxylations.7
We also demonstrated the strong oxidizing power of this
supported ruthenium complexes are sparse in the literature.76 catalytic system in the reaction with propane and ethane (Table
In 2010, Du Bois and co-workers developed a RuCl3/pyridine/ 7). Although the turnover numbers are not impressive, identi-
KBrO3 protocol for the hydroxylation of various substituted cation of appreciable amounts of the various oxidation prod-
alkane substrates;77 In 2012, they improved the yield and ucts is signicant, as light alkanes oen exhibit resistance to
allowed a lower catalyst loading by employing [(Me3tacn) oxidation. To the best of our knowledge, this represents a rare
RuIIICl3] as a catalyst in combination with AgClO4 as an additive example of ruthenium-catalysed/mediated oxidation of light
and CAN as a terminal oxidant.20 Using desorption electrospray alkanes (<C4), except Drago's reported work on the cis-[(dmp)
ionization mass spectrometry (DESI-MS), cis-[(Me3tacn) RuII(S)2]2+ (S ¼ MeCN or H2O)-catalysed hydroxylation of
RuVI(O)2(OH)]+ was identied to be
hydroxylating agent, but the possible involvement of Ru(V) and/
a
plausible reactive methane with H2O2.80
Some issues remain to be resolved/explored that are worth
or Ru(IV) species could not be discounted.78 In this work, stoi- being addressed. First, the stability/robustness of the highly
chiometric reactions between 1e and several alkane substrates oxidizing cis-dioxoruthenium(VI) species is of concern. In CAN-
(Table 3) provided direct evidence that cis-dioxoruthenium(VI) driven catalytic oxidation of alkanes, the turnover number
preferentially oxidizes the tertiary C–H bonds in hydrocarbons. based on 3c is typically less than 30. Post-reaction analysis of
The same selectivity was observed in catalytic experiments. the mixture revealed that the catalyst had degraded/
Aqueous electrochemical and ESI-MS experiments showed that decomposed almost completely. A likely deactivation pathway
cis-[(pdp)RuVI(O)2]2+ is accessible via the successive oxidative of the catalyst is the oxidation of the ligand by the strongly
deprotonation of a low-valent precursor, such as 3c or 3c0. The oxidizing Ru–oxo intermediate. Indeed, it was noted that
DO–H values of cis-[(Me3tacn)RuVI(O)2(O2CCF3)]2+ and cis-[(pdp) complexes 4c and 6c showed much poorer activities than 3c and
RuVI(O)2]2+ are calculated to be 87.5 and 90.8 kcal molꢁ1
respectively (vide supra). We anticipate that the Ru(pdp) molecular oxidation of the ortho-Me group by the Ru–oxo
complex, with an additional driving force of 3.3 kcal molꢁ1 moiety.50 Although our recent work on the Fe(N4)-catalysed AD
,
5c (Tables 4 and S4, ESI†), presumably due to the intra-
,
would be as reactive as the Ru(Me3tacn) complex in alkane reaction showed that installation of an ortho-Me group could
oxidation reactions. Additionally, the use of a chiral N4 sup- substantially improve the catalyst activity (particularly the
porting ligand might incorporate chirality into the oxygenated enantioselectivity),5g this strategy cannot be directly trans-
products.3c,4d A catalytic system for the oxidation of alkanes by planted to the ruthenium chemistry. For the Fe((R,R)-Me2bqcn)-
cis-[(pdp)RuII(OH2)2]2+ (3c) with CAN is herein reported. catalysed AD reaction, the active intermediate was proposed to
Compared to the [(Me3tacn)RuIIICl3]/AgClO4/CAN system, our be [((R,R)-Me2bqcn)FeIII(OOH)]2+ rather than dioxoiron(V).5g
system avoids the use of a Ag+ salt as a chloride scavenger, and From ESI-MS experiments, it was also demonstrated that the
no pretreatment of the catalyst is required.79 In general, 2– decomposition of Ru(pdp) complexes under oxidizing condition
5 mol% catalyst (3c) and 3–6 equiv. of CAN afforded 3ꢀ C–H is considerably fast when [Ru] $ 1 mM. Thus, a delicate balance
hydroxylated products in isolated yields of ca. 50% (Tables 4 between the oxidizing power and stability of the active inter-
and 5), which are comparable to those in other Ru-catalysed mediate is yet to be achieved for efficient ruthenium-catalysed
C–H hydroxylation protocols (e.g., [(Me3tacn)RuIIICl3]/AgClO4/ C–H oxidation. Moreover, either stoichiometrically or catalyti-
CAN and cis-[(tBu2bpy)2RuIICl2]/H5IO6/CF3SO3H).20,21 In C–H cally, the studied chiral ruthenium complexes (1e*, 3c–6c*) did
oxidation with substrates containing a mixture of tertiary and not show noticeable enantioselectivity in reactions with racemic
secondary C–H bonds, the reaction occurs preferentially at the tertiary alkane substrates (e.g., entries 9, 10, Table 3; entry 6,
tertiary position and is highly stereoretentive (e.g., oxidation of Table 5). This suggests, without any directing group,3c there is
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not sufficient chiral differentiation between the two isomeric
forms by kinetic resolution at the chiral ruthenium centre.
K. P. Bryliakov and E. P. Talsi, Coord. Chem. Rev., 2014,
276, 73; (l) W. N. Oloo and L. Que Jr, Acc. Chem. Res., 2015,
´
48, 2612; (m) O. Cusso, X. Ribas and M. Costas, Chem.
Commun., 2015, 51, 14285; (n) G. B. Shul'pin, Catalysts,
Conclusions
´
2016, 6, 50; (o) G. Olivo, O. Cusso and M. Costas, Chem.–
In this work, we reported the preparation and electrochemistry
of several ruthenium complexes bearing tetradentate N4 ligands
including cis-[(mcp)RuIII(O2CCF3)2]ClO4 (1b) and cis-[(pdp)
RuIII(O3SCF3)2]CF3SO3 (3c0). Complex cis-[(mcp)RuVI(O)2](ClO4)2
(1e) was obtained from CAN oxidation of 1b in aqueous solu-
tion. Complex 1e is a powerful oxidant with E(RuVI/V) ¼ 0.78 V
(vs. Ag/AgNO3) in acetonitrile or Eꢀ ¼ 1.11 V (vs. SCE) at pH 1. In
aqueous tert-butanol, [((R,R)-mcp)RuVI(O)2](ClO4)2 (1e*) under-
went stoichiometric alkene cis-dihydroxylation to afford cis-diol
in 24% ee for trans-b-methylstyrene oxidation. With high
hydrogen-atom affinities (DO–H ¼ 90.1–90.8 kcal molꢁ1), 1e and
chemically generated cis-[(pdp)RuVI(O)2]2+ are active oxidants
for C–H oxidation. cis-[(pdp)RuII(OH2)2]2+ (3c), in combination
with CAN as a terminal oxidant, catalysed the oxidation of
unactivated C–H bonds including those of some pharmaceu-
tical ingredients and natural product derivatives. This work
demonstrates that efficient oxidation catalysts can be con-
structed based on the cis-dioxoruthenium(VI) moiety on a N4
ligand platform. The diversity and exibility of chiral N4 ligand
design will direct subsequent efforts to improve the reaction
selectivities.4d Further studies are also directed to gain a better
understanding of the reaction mechanism in hydrocarbon
oxidations and to explore other catalytic activities of chiral
Ru(N4) complexes.
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Conflicts of interest
There are no conicts to declare.
W.
Nam,
Chem.
Commun.,
2007,
4623;
(b)
R. V. Ottenbacher, D. G. Samsonenko, E. P. Talsi and
K. P. Bryliakov, Org. Lett., 2012, 14, 4310; (c) M. Milan,
G. Carboni, M. Salamone, M. Costas and M. Bietti, ACS
Catal., 2017, 7, 5903; (d) M. Milan, M. Bietti and M. Costas,
ACS Cent. Sci., 2017, 3, 196.
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A. K. Tipton, K. Chen, D.-H. Jo and L. Que Jr, J. Am. Chem.
Soc., 2001, 123, 6722; (b) K. Suzuki, P. D. Oldenburg and
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Acknowledgements
This work was supported by Hong Kong Research Grants
Council General Research Fund (17303815, 17301817) and Basic
Research Program-Shenzhen Fund (JCYJ20170412140251576,
JCYJ20150629151046879). We thank the X-Ray Crystallography
Laboratory in the Department of Chemistry at The University of
Hong Kong for instrumental support (Bruker D8 Venture).
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C.-X. Miao, S. Wang, X. Hu, C. Xia, F. E. Kuhn and W. Sun,
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a similar protocol was
unsuccessful. Treatment of 2a with Zn/Hg at 80 ꢀC in
water, followed by metathesis with AgOTf afforded a light
green
solution.
However,
the
corresponding
triuoroacetato complex 2b was not isolated upon addition
of NaClO4.
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C
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a
constant of
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51 In a control experiment where the substrate (adamantane)
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<1%).
`
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Chem. Sci.
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