Troponate/Aminotroponate Ruthenium-Arene Complexes: Synthesis, Structure, and Ligand-Tuned Mechanistic Pathway for Direct C-H Bond Arylation with Aryl Chlorides in Water

Article abstract of DOI:10.1021/acs.inorgchem.6b01028

A series of water-soluble troponate/aminotroponate ruthenium(II)-arene complexes were synthesized, where O,O and N,O chelating troponate/aminotroponate ligands stabilized the piano-stool mononuclear ruthenium-arene complexes. Structural identities for two of the representating complexes were also established by single-crystal X-ray diffraction studies. These newly synthesized troponate/aminotroponate ruthenium-arene complexes enable efficient C-H bond arylation of arylpyridine in water. The unique structure-activity relationship in these complexes is the key to achieve efficient direct C-H bond arylation of arylpyridine. Moreover, the steric bulkiness of the carboxylate additives systematically directs the selectivity toward mono- versus diarylation of arylpyridines. Detailed mechanistic studies were performed using mass-spectral studies including identification of several key cyclometalated intermediates. These studies provided strong support for an initial cycloruthenation driven by carbonate-assisted deprotonation of 2-phenylpyridine, where the relative strength of η⁶-arene and the troponate/aminotroponate ligand drives the formation of cyclometalated 2-phenylpyridine Ru-arene species, [(η⁶-arene)Ru(κ²-C,N-phenylpyridine) (OH₂)]⁺ by elimination of troponate/aminotroponate ligands and retaining η⁶-arene, while cyclometalated 2-phenylpyridine Ru-troponate/aminotroponate species [(κ ²-troponate/aminotroponate)Ru(κ²-C,N-phenylpyridine)(OH₂)₂] was generated by decoordination of η⁶-arene ring during initial C-H bond activation of 2-phenylpyridine. Along with the experimental mass-spectral evidence, density functional theory calculation also supports the formation of such species for these complexes. Subsequently, these cycloruthenated products activate aryl chloride by facile oxidative addition to generate C-H arylated products.

Full text of DOI:10.1021/acs.inorgchem.6b01028

Article
pubs.acs.org/IC
Troponate/Aminotroponate Ruthenium−Arene Complexes:
Synthesis, Structure, and Ligand-Tuned Mechanistic Pathway for
Direct C−H Bond Arylation with Aryl Chlorides in Water
Ambikesh D. Dwivedi,† Chinky Binnani,† Deepika Tyagi,† Kuber S. Rawat,† Pei-Zhou Li,‡ Yanli Zhao,‡
Shaikh M. Mobin,† Biswarup Pathak,†,§ and Sanjay K. Singh*,†,§
†Discipline of Chemistry, School of Basic Sciences and §Centre for Material Science and Engineering, Indian Institute of Technology
Indore, Indore 453552, Madhya Pradesh, India
‡Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link 637371, Singapore
S
* Supporting Information
ABSTRACT: A series of water-soluble troponate/aminotroponate ruthenium-
(II)−arene complexes were synthesized, where O,O and N,O chelating troponate/
aminotroponate ligands stabilized the piano-stool mononuclear ruthenium−arene
complexes. Structural identities for two of the representating complexes were
also established by single-crystal X-ray diffraction studies. These newly synthesized
troponate/aminotroponate ruthenium−arene complexes enable efficient C−H
bond arylation of arylpyridine in water. The unique structure−activity relationship
in these complexes is the key to achieve efficient direct C−H bond arylation of
arylpyridine. Moreover, the steric bulkiness of the carboxylate additives system-
atically directs the selectivity toward mono- versus diarylation of arylpyridines.
Detailed mechanistic studies were performed using mass-spectral studies including
identification of several key cyclometalated intermediates. These studies provided
strong support for an initial cycloruthenation driven by carbonate-assisted depro-
tonation of 2-phenylpyridine, where the relative strength of η6-arene and the
troponate/aminotroponate ligand drives the formation of cyclometalated 2-phenylpyridine Ru−arene species, [(η6-arene)Ru(κ2-C,N-
phenylpyridine) (OH2)]+ by elimination of troponate/aminotroponate ligands and retaining η6-arene, while cyclometalated
2
2-phenylpyridine Ru−troponate/aminotroponate species [(κ -troponate/aminotroponate)Ru(κ2-C,N-phenylpyridine)(OH2)2] was
generated by decoordination of η6-arene ring during initial C−H bond activation of 2-phenylpyridine. Along with the experimental
mass-spectral evidence, density functional theory calculation also supports the formation of such species for these complexes. Sub-
sequently, these cycloruthenated products activate aryl chloride by facile oxidative addition to generate C−H arylated products.
hydroxy group analogous to β-diketone, and (iii) negative charge
accumulation predominantly on nitrogen atoms of aminotropone
ligands. Moreover, the planar large seven-membered homoar-
omatic rings of tropolone or aminotropones substantially con-
tribute in the stability of these complexes.
In recent years, remarkable progress has been accomplished in
C−H bond activation as a direct route for C−C bond formation
reactions.1a,15 Several other metals such as Pd,16 Rh,17 and Ir18
based complexes were also explored for C−H bond activation,
but ruthenium complexes, being relatively less expensive, non-
toxic, and stable in air and water, gained advantages over others.12
Among the most efficient ruthenium catalysts investigated so far
for C−H bond activation or functionalization, ruthenium−arene
complexes such as [{(η6-arene)RuCl2}2] or its carboxylate com-
plexes represent the highly active class of catalysts.19 However,
other ruthenium complexes such as those containing or involving
INTRODUCTION
■
Chemistry of ruthenium(II)−arene-based complexes has been
established as a promising field of organometallic chemistry
because of the structural versatility and unique tunable structure−
activity behavior shown by these complexes. Therefore, these
complexes are being used as important candidates as catalysts in a
wide range of organic transformations, biological applications
such as active water-soluble antitumor agents, and so on.1−3
Careful observation of the scientific progress in the field of
arene−ruthenium complexes revealed that the most active class of
these complexes contains nitrogen or oxygen donor ligands.1,2
In this context, chemistry of tropolone and 2-aminotropones
has been extensively explored in recent past for several appli-
cations, such as anticancer agent4 and as active catalyst for
various important reactions such as intramolecular hydroamina-
tion,5−7 ethylene polymerization,8 olefin polymerization,9 and
so on10−14 (shown in Scheme 1). Tropolone-based complexes
have several unique features such as (i) the formation of strained
five-membered chelating ring with the metal center, (ii) have enolic
Received: April 25, 2016
© XXXX American Chemical Society
A
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Scheme 1. Tropolone and Aminotropone-based Metal Complexes
phosphine ligands and others like RuCl2(PPh3)3, [RuCl2(cod)]2,
and [(η6-arene)RuCl2(1,2-diphenylvinyl)phosphine)] as well as
[{(η6-arene)RuCl2}2] with excess of PPh3 were also explored, but
those are expected to follow a different reaction pathway.20 Most
of these reactions were performed in organic solvents, such as
N-methyl-2-pyrrolidone (NMP) or toluene, but recently huge
efforts have been devoted to develop efficient catalytic systems for
C−H bond activation in more environmentally tolerant solvents
such as diethylcarbonate or water.21
Analogous to carboxylate ligands, ruthenium−arene complexes
containing several other O,O or N,O chelating ligands for C−H
bond activation were also reported in water, where diarylation is
favored with N,O chelating ligands, but phosphine-containing
complexes showed poor catalytic activity.22 Similarly, β-ketonates-
based ruthenium−arene complexes have also been employed for
C−H bond arylation in NMP, where sterically bulky diketonate
favors monoarylation.23 Unlike weakly coordinating carboxylate
ligands, these strongly chelating O,O or O,N ligands displayed
high affinity toward metal center, and therefore we anticipated that
presumably ruthenium−arene complexes containing such stron-
gly coordinating chelating ligands might follow a different catalytic
reaction pathway or involve different catalytically active species
during C−H bond activation, in contrast to the well-established
carboxylate/carbonate-driven cycloruthenated species, [(η6-arene)-
Ru(κ2-C,N-phenylpyridine) (sol)]+ observed with ruthenium−arene
complexes.
arylation of 2-phenylpyridine with 4-chloroanisole in water.
Effect of several carboxylate additives and structural motifs of
troponate/aminotroponate ruthenium−arene complexes on
catalytic activity and selectivity toward C−H bond mono vs
diarylation were explored. Main focus of the present report is to
extensively investigate, identify, and characterize catalytically
active species and/or reaction intermediates of the C−H bond
arylation as catalyzed by newly synthesized troponate/amino-
troponate ruthenium−arene complexes using mass-spectral and
1H NMR studies. On the basis of these studies, along with the
support from density functional theory (DFT) calculation, the
catalytic reaction pathway was proposed.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Troponate-/Amino-
troponate Ruthenium(II)−Arene Complexes. Troponate/
aminotroponate ruthenium(II)−arene complexes were synthesized
by reacting tropolone (L1), 2-(isopropylamino)tropone (L2), or
2-(cyclohexylamino)tropone (L3) ligands with ruthenium−arene
precursors. Aminotropone-based ligands L2 and L3 were pre-
pared by direct nucleophilic displacement of the tosyl group
of the 2-(tosyloxy)tropone by using excess of corresponding
amine (isopropylamine or cyclohexylamine) applying an earlier
reported method.6 The synthesized ligands were characterized
by several spectro-analytical techniques, which confirmed the
proposed structures (see the Experimental Section). Structural
characterization of the ligand L3 was also elucidated by single-
crystal X-ray diffraction of its suitable crystals obtained by slow
evaporation of hexane−ethyl acetate (98:2, v/v) at room temper-
ature. ORTEP diagram of the ligand L3 is shown in Figure 1.
Crystallographic data and bond parameters for L3 are listed
in Tables S1 and S2. The C−O and C−N bond lengths
(1.2478(18) and 1.3317(19) Å, respectively) are of intermediate
value for single and double bonds, which is similar to reported
aminotropone ligand.8
Herein, we report the synthesis and characterization of eight novel
ruthenium−arene complexes, [Ru]-1−[Ru]-8, containing tropo-
nate (L1) and substituted aminotroponate (isopropylaminotropo-
nate (L2) and cyclohexylaminotroponate (L3)) ligands. Structures
of the ligand L3 and two of the representative ruthenium−arene
complexes [(η6-p-cymene)Ru(κ2-O,O-troponate)Cl] ([Ru]-1) and
[(η6-p-cymene)Ru(κ2-O,N-isopropylaminotroponate)Cl] ([Ru]-3)
were authenticated by single-crystal X-ray diffraction studies.
Further, these complexes were employed for ortho C−H bond
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ligand. However, the carbonyl carbons, in particular, of the
tropolone ligand in complexes [Ru]-1 and [Ru]-5 resonated in
more deshielded region at 184.29−184.87 ppm compared to that
for free tropolone ligand (171.84 ppm), suggesting the coor-
dination of the ligand to the metal center. The conjugative and
electron-donating abilities of the troponate/aminotroponate ligands
decreases electron density that troponate/aminotroponate ring
might have contributed in the observed downfield shift in the
1H and 13C NMR spectra.24 In agreement with the NMR results,
Fourier transform infrared (FTIR) spectra of the complexes [Ru]-1
and [Ru]-5 show a typical band at 1588.21 and 1588.38 cm−1,
respectively, for the υ(CO), while it appeared at 1609 cm−1 in
the free tropolone ligand, suggesting the lengthening of CO
bonds upon coordination of tropolone to metal center.25,26 Cyclic
voltammograms of the representative complexes [Ru]-1 and
[Ru]-5 were obtained in CH2Cl2 containing 0.1 M (n-Bu4N)PF6
as supporting electrolyte at a sweep rate of 50 mV/s at room
temperature (Figures S1 and S2). On the one hand, in the anodic
window (0 to +2.0 mV) of cyclic voltammogram for complexes
[Ru]-1 and [Ru]-5, a well-defined quasi-reversible oxidation
reduction wave was observed at 0.939 and 1.023 mV, respec-
tively,27 which may be attributed to RuII/III redox couple.
Analogously for the aminotroponate ruthenium−arene complex
[Ru]-2, the RuII/III redox couple appeared at 1.173 mV. On the
other hand, tropolone-based reduction waves appeared in the
cathodic potential window (0 to −2.0 mV) of cyclic voltammo-
gram of these complexes (Table S4). Methyl substitution at arene
ligand in complex [Ru]-1 resulted in a shift of RuII/III redox
couple toward less positive side, due to the increase in electron
density on the metal center, which destabilizes the ground-state
energy and therefore leads to a lowering of the metal-based
oxidation potentials.28
Figure 1. Single-crystal X-ray structure of 2-(cyclohexylamino)tropone
(L3). Selected bond lengths (Å): C13−O1 = 1.2478(18), C7−N1 =
1.3317(19), C1−N1 = 1.4548(19), bond angles (deg): C7−C13−O1 =
116.37(13), C13−C7−N1 = 111.27(12) and torsion angles (deg):
O1−C13−C7−N1 = −4.5(2), C8−C7−C13−O1 = 174.1(2), C12−
C13−C7−N1 = 174.6(1).
Tropolone (L1) or aminotropone (L2 and L3) ligands were
reacted with ruthenium−arene precursors, [{(η6-arene)RuCl2}2]
(η6-arene = η6-C10H14 and η6-C6H6) in methanol using a base (or
without base) to afford water-soluble mononuclear piano-stool
troponate/aminotroponate ruthenium(II)−arene complexes in
good yield (Scheme 2). The synthesized complexes have the
general formula of [(η6-arene)Ru(κ2-L)Cl] (L = L1, η6-arene =
η6-C10H14 ([Ru]-1) and η6-C6H6 ([Ru]-5); L = L2, η6-arene =
η6-C10H14 ([Ru]-3) and η6-C6H6 ([Ru]-7); L = L3, η6-arene =
η6-C10H14 ([Ru]-4) and η6-C6H6 ([Ru]-8) and [{(η6-arene)Ru-
(κ2-L1) (PPh3)]+ (η6-arene = η6-C10H14 ([Ru]-2) and η6-C6H6
([Ru]-6)). The spectro-analytical analyses of the synthesized
complexes corroborated well with the proposed structures (see
the Experimental Section).
1
In the H NMR spectra of the mononuclear complex [Ru]-
1−[Ru]-8, arene protons displayed downfield shift as compared
to that in the respective precursor ruthenium−arene dimer. More-
over, ring carbon of the coordinated tropolone in complexes
[Ru]-1 and [Ru]-5 also resonated in slightly deshielded region
(126.34−137.80 ppm) compared to that of the free tropolone
X-ray suitable crystals of complexes [Ru]-1 and [Ru]-3 were
obtained by slow evaporation method using dichloromethane
and diethyl ether as solvents, and their molecular structures
were confirmed by single-crystal X-ray diffraction analysis.
Scheme 2. Synthesis of Ruthenium(II)−Arene Complexes Containing Tropolone-based Ligands
C
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Table 1. Crystal Data and Structure Refinement Details for
[Ru]-1 and [Ru]-3
structure
empirical formula
Fw
complex [Ru]-1
C17H19ClO2Ru
391.84
complex [Ru]-3
C20H26ClNORu
432.94
T (K)
293(2)
293(2)
λ (Å)
0.71073
0.71073
cryst system
space group
monoclinic
P21/n
monoclinic
P21/c
cryst size, mm (l × k × h) 0.12 × 0.10 × 0.09
0.33 × 0.27 × 0.21
14.7074(4)
16.6899(3)
7.7339(2)
90.00
a, Å
10.6918(5)
13.3978(6)
11.4591(5)
90.00
b, Å
c, Å
Figure 2. Single-crystal X-ray structure of troponate ruthenium(II)
α, deg
p-cymene complex ([Ru]-1). Selected bond lengths (Å): Ru1−Cavg
=
β, deg
99.654(4)
90.00
102.751(2)
90.00
2.167, Ru1−Cct = 1.645, Ru1−Cl1 = 2.4094(8), Ru1−O1 =
2.0789(18), Ru1−O2 = 2.079(2), C1−O1 = 1.284(3), C7−O2 =
1.289(3), bond angles (deg): O1−Ru1−O2 = 76.63(7), O1−Ru1−
Cl1 = 84.48(6), O2−Ru1−Cl1 = 84.47(6) and torsion angle (deg):
O1−C1−C7−O2 = −1.5(4).
γ, deg
V, Å3
1618.23(12)
4
1851.58(8)
4
Z
ρcalcd, g cm−3
μ, mm−1
F(000)
θ range, deg
completeness to θmax
1.608
1.553
1.135
0.997
792
888
3.24−29.84
83.2
3.535−24.994
99.8
no. of data collected/
unique data
7476/3856
14 307/3250
[R(int) = 0.0240]
193/0
[R(int) = 0.0522]
222/0
params/restraints
goodness of fit on F2
1.043
1.100
final R indices [I > 2σ(I)] R1 = 0.0318 wR2 =
R1 = 0.0297 wR2 =
0.0728
0.0688
R indices (all data)
R1 = 0.0479 wR2 =
0.0786
R1 = 0.0350 wR2 =
0.0782
(1.289 Å) are in complex [Ru]-1, suggesting a delocalization of
the CO bond of the tropolone ligand upon coordination.26
The bite angle (O1−Ru1−O2) of the tropolone ligand in com-
plex [Ru]-1 is 76.63°. The Cl1−Ru1−O1 and Cl1−Ru1−O2
bond angles are 84.48° and 84.47°, respectively, for complex
[Ru]-1. The Ru1−O1 and Ru1−N1 bond lengths in complex
[Ru]-3 are 2.0463 and 2.115 Å, respectively. The C−O (1.277 Å)
bond length appears slightly shorter in complex [Ru]-3 than in
the Ru−tropolone complex [Ru]-1. Moreover, the bite angle of
the aminotroponate ligand in complex [Ru]-3 is 75.86°, which
deviates only slightly from the bite angle of the parent complex
[Ru]-1 with troponate ligand. Replacing one of the oxygen atoms
in ligand L1 with nitrogen (in ligand L3) resulted in no
significant distortion in the planarity of the tropolone backbone
(torsion angle O1−C1−C7−N1 = 0.5(4)). For complex [Ru]-3,
the Cl1−Ru1−O1 and Cl1−Ru1−N1 bond angles are 85.93°
and 84.68°. Bond angle between the legs and the centroid of the
η6-arene ring (Ct) is 129.63°−131.17° and 126.70°−136.67° in
complexes [Ru]-1 and [Ru]-3, respectively.2,26 The observed
values are comparable to those reported for other analogous
complexes.26,29
Arene−Ru(II) Catalyzed C−H Bond Arylation in Water.
At an outset of our investigations for C−H bond arylation,
2-phenylpyridine (0.5 mmol) was treated with 4-chloroanisole
(1.25 mmol) in water in the presence of 5 mol % troponate
Ru−p-cymene catalyst, [Ru]-1, and 3 equiv of K2CO3 at 100 °C,
without using any carboxylate additive. Results inferred that [Ru]-1
catalyst is reactive in water and facilitates C−H bond arylation
even with aryl chlorides with 86% conversion of 2-phenylpyridine.
The monoarylated to diarylated product selectivity was found to be
Figure 3. Single-crystal X-ray structure of isopropylaminotroponate−
ruthenium(II) p-cymene complex ([Ru]-3). Selected bond lengths (Å):
Ru1−Cavg = 2.188, Ru1−Cct = 1.676, Ru1−Cl1 = 2.4196(8), Ru1−O1 =
2.0463(19), Ru1−N1 = 2.115(3), C1−O1 = 1.277(4), C7−N1 =
1.319(4), bond angles (deg): C1−Ru1−N1 = 84.68(7), O1−Ru1−Cl1 =
85.93(6), O1−Ru1−N1 = 75.86(9) and torsion angle (deg): O1−C1−
C7−N1 = 0.5(4).
The ORTEP view along with the selected bond parameters of
the complexes [Ru]-1 and [Ru]-3 are shown in Figures 2 and 3,
respectively. Crystallographic data and bond parameters of these
complexes are listed in Table 1 and Table S3, respectively.
Both the complexes adopted the piano-stool geometry, where the
η6-coordinated arene ring is present as top disc of stool, and two
O atoms of the ligand L1 or N and O atoms of the ligand L3
along with one chloride ligand are as three legs of the stool.
The complex [Ru]-1 crystallized in monoclinic crystal system
with P21/n space group. The arene ring (in η6-p-cymene) appears
almost planar for both the complexes [Ru]-1 and [Ru]-3, where
the displacements of the arene ring centroid from the Ru(II)
center are 1.645 and 1.676 Å, respectively, for complexes [Ru]-1
and [Ru]-3. The Ru−Ccentroid and Ru−C bond distances are also
comparable with other similar complexes.2,26 The angle between
the legs and the centroid of the η6-arene ring (Ct) are in the
range of 129.63°−131.17°. Both the Ru−oxygen bond distances,
2.0789 Å (Ru1−O1) and 2.079 Å (Ru1−O2), are comparable
for the coordinated tropolone in complex [Ru]-1.27 Moreover,
the comparable bond lengths of C1−O1 (1.284 Å) and C7−O2
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activation reaction pathway, as carboxylate can bind with metal
center (such as acetate) or remains in the solution (such as
pivalate) and therefore may be involved in intra/intermolecular
deprotonation of 2-phenylpyrdine.31,32 To further investigate
this phenomenon, we performed mass-spectral analysis of
the reaction mixture obtained after 3 h of reaction of [Ru]-1
catalyst (0.025 mmol) with carboxylate (0.05 mmol) in the pres-
ence of K2CO3 (1.5 mmol) at 100 °C in water under Ar atmo-
sphere (Figure 5). Mass-spectral results showed that, along with
the peaks of the [Ru]-1 catalyst at m/z 357.33 ([M-Cl]+), acetate-
coordinated species [(η6-arene)Ru(κ2-troponate)(κ1-acetate)] +
Na+ at m/z 439.28 was also observed with relative abundance of
37%. In contrast to the high abundance of acetate-coordinated
species, abundance for analogous pivalate coordinated species,
[(η6-arene)Ru(κ2-troponate)(κ1-pivalate)] + CH3CN + 2Na+ m/z
547.07 was observed to be very low (12%). These results are
consistent with earlier reports that acidity of pivalate increases in
water, which destabilizes the Ru−pivalate bond and therefore
results in the increment of free pivalate in the reaction mixture.21
Moreover, increase in the steric bulkiness of these carboxylates,
from acetate to pivalate, presumably also favors easy decoordina-
tion of bulky carboxylates. Interestingly, reaction with carboxylates
does not lead the de-coordination of troponate ligand in [Ru]-1
catalyst, suggesting the stability of troponate Ru−arene species.
Further, to investigate and establish structure−activity
relationship, [Ru]-1 to [Ru]-7, structural analogues of [Ru]-1
catalyst, were synthesized using different η6-p-cymene/benzene
and tropolone/aminotropone ligands; catalytic efficiency of these
complexes was evaluated for the direct C−H bond arylation of
2-phenylpyridine under the optimized reaction condition (Table 2).
Results implied that Ru−p-cymene complexes [Ru]-1−[Ru]-4
(Table 2, entries 1−4) displayed high conversions of 2-phenyl-
pyridine (75% to 95%); in contrast, Ru−benzene complexes
(Table 2, entries 5−7) exhibited relatively low conversions (48% to
90%). Moreover, Ru−p-cymene complexes favor mono- to diary-
lated (m/d) products (m/d 61:39 [Ru]-1 and 73:27 [Ru]-2),
more in comparison to Ru−benzene complexes, m/d 85:15,
[Ru]-5 and 80:20, [Ru]-6 (Table 2, entries 1, 2, 5, and 6). In
contrast to troponate Ru−p-cymene complex [Ru]-1, a remark-
able improvement in mono- to diarylated product ratio was
observed with aminotroponate Ru−p-cymene complex, [Ru]-3
(m/d 82:18) and [Ru]-4 (m/d 83:17; Figure 6). However, the
substitution at the N atom of the aminotroponate Ru−arene
complexes has no distinct effect on mono- to diarylated ratio
(m/d, N-isopropyl = 82:18, [Ru]-3 and N-cyclohexyl = 83:17),
[Ru]-4 (Figure 6).
Table 2. Screening of Troponate-/Aminotroponate Ru−Arene
a
Catalysts for C−H Bond Arylation Reaction in Water
conv
(%)
sel %
b
entry
1
catalysts
(m/d)
TON
17.2
[(η6-p-cymene)Ru(κ2-O,O-troponate)Cl]
([Ru]-1)
86
95
75
81
58
90
48
61/39
2
3
4
5
6
7
[(η6-p-cymene)Ru(κ2-O,O-troponate)
PPh3]Cl ([Ru]-2)
73/27
82/18
83/17
85/15
80/20
85/15
19.0
15.0
16.2
11.6
18.0
9.6
[(η6-p-cymene)Ru(κ2-N,O-
isopropylaminotroponate)Cl] ([Ru]-3)
[(η6-p-cymene)Ru(κ2-N,O-
cyclohexylaminotroponate)Cl] ([Ru]-4)
[(η6-benzene)Ru(κ2-O,O-troponate)Cl]
([Ru]-5)
[(η6-benzene)Ru(κ2-O,O-troponate)PPh3]
Cl ([Ru]-6)
[(η6-benzene)Ru(κ2-N,O-
isopropylaminotroponate)Cl] ([Ru]-7)
a
Reaction conditions: 2-phenylpyridine (0.5 mmol), 4-chloroanisole
(1.25 mmol), [Ru] catalyst (5 mol %), K2CO3 (1.5 equiv), water (5 mL),
b
1
16 h. Obtained from H NMR.
61:39 (results are shown in Table 2). Notably, the [Ru]-1
catalyst exhibited high conversion even in the absence of carbox-
ylate additives. Literature reports evidenced that carboxylates have
a significant role as deprotonating agent during the C−H bond
activation reactions catalyzed by transition-metal complexes.1a,30
Therefore, as an attempt to investigate the role of carboxylate
additives on the C−H bond arylation (conversion and selectivities
for mono- and diarylated products), [Ru]-1 catalyst was treated
with 2-phenylpyridine and 4-chloroanisole under analogous con-
ditions as described above but with added potassium salts of
carboxylates, starting from acetate to propionate, isobutyrate, and
pivalate. Using acetate and its methyl-substituted analogues, we
could be able to establish a relation between the steric bulkiness
of additives and the selectivity of C−H bond arylated products
(mono and diarylated). As depicted in Figure 4, with an increase
Further to elucidate the reaction mechanism, extensive mass
spectrometric investigations were performed for the ruthenium-
catalyzed C−H bond activation under the optimized reaction
condition. As previously reported, ruthenium complexes are
found to be very efficient to form cyclometalated intermediates
by C−H activation at ortho position of the phenyl ring.33
Stoichiometric reactions of 2-phenylpyridine and [Ru]-1 catalyst
were performed in water in the presence of K2CO3 (3 equiv)
base at 100 °C in catalyst-to-substrate molar ratio of 1:1. After 3 h,
reaction mixture was subjected to mass-spectral analysis, where
several cycloruthenated 2-phenylpyridine species were identified.
To our surprise, along with the well-established key intermediate
cyclometalated species {(η6-arene)Ru(κ2-C,N-phenylpyridine)}+
[Ru]-A at m/z 390.01, another cyclometalated species with tro-
ponate ligand at m/z 379.13 corresponding to {(κ2-troponate)-
Ru(κ2-C,N-phenylpyridine)} ([Ru]-B) was also observed, where
tropolone ligand was retained instead of p-cymene ring.
Figure 4. Effect of additives on the selectivity toward mono- vs di-
C−H arylated products from the reaction of 2-phenylpyridine with
4-chloroanisole catalyzed by [Ru]-1 catalyst.
in the bulkiness of additives, the selectivity shifts toward diarylated
product in the order of pivalate > isobutyrate > propionate >
acetate. The selectivity for monoarylated product was 82% with
acetate, which was further decreased to 38% and 28%, respectively,
with potassium propionate and potassium isobutyrate. This trend
persists even to bulkier potassium pivalate, where only 1% selec-
tivity for monoarylated product (99% diarylation) was observed.
The observed behavior of carboxylate additives on the
selectivity for C−H bond arylation is interesting and suggests
the intriguing involvement of carboxylates in C−H bond
E
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Figure 5. Mass-spectral analysis of the reaction of [Ru]-1 catalyst with acetate and pivalate to identify (a) (η6-arene)Ru(κ2-troponate)(κ1-acetate)] +
Na+ and (b) [(η6-arene)Ru(κ2-troponate)(κ1-pivalate)] + CH3CN + 2Na+ species.
there is more tendency of arene (p-cymene) cleavage to generate
the [Ru]-B species. Mass spectra of the reaction mixture obtained
from the reaction of [Ru]-1 catalyst with 2-phenylpyridine at
substrate/catalyst ratios of 1:1, 2:1, and 5:1 revealed that the
relative percentages of the formation of [Ru]-A and [Ru]-B are
60:40, 42:58, and 16:84, respectively (Figure S3). To further
investigate the relevance of this newly identified troponate Ru−
cyclometalated species [Ru]-B, reaction mixture was analyzed by
mass spectrometry with respect to reaction time. Notably, the
mass-spectral analysis revealed a systematic enhancement in the
abundance of the [Ru]-B species with time, whereas that of
[Ru]-A species decreases (Figure 7). To further authenticate if
analogous species were also generated with the troponate/
aminotroponate Ru(II) complexes, we performed the mass-spectral
analysis of the reaction of [Ru]-3 catalyst with 2-phenylpyridine
in substrate-to-catalyst ratio of 2:1 under analogous reaction con-
Figure 6. Effect of different Ru(II) catalysts on the selectivity of mono-
vs di- C−H arylated products of 2-phenylpyridine.
ditions. Analogous to that observed with [Ru]-1 catalyst, replacing
troponate ligand with N-isopropyl−aminotroponate ligand in
[Ru]-3 also revealed the formation of {(κ2-aminotroponate)Ru-
(κ2-C,N-phenylpyridine)} ([Ru]-B3) cyclometalated species as
Analogous reactions were conducted but with high substrate-
to-catalyst (S/C) ratio of 2:1 or larger (5:1), which inferred that
major component (90%) in comparison to [Ru]-A species (10%).
Figure 7. Time-dependent study for the formation of (a) species {(η6-arene)Ru(κ2-C,N-phenylpyridine)}+, (b) species {(κ2-troponate)Ru(κ2-C,N-
□
phenylpyridine)}, in substrate-to-catalyst ratio 2:1. ( ) Vacant sites.
F
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Therefore, high catalytic efficiency and the detection of
troponate/aminotroponate Ru−cyclometalated species [Ru]-
B1−[Ru]-B3 also suggested the key role of the newly identified
species in C−H bond arylation reaction. Notably, reaction of
[Ru]-1 with 4-chloroanisole in S/C ratio of 5:1 in the presence
of K2CO3 in water at 100 °C revealed that the [Ru]-1 is
inactive in the presence of aryl chloride. Contrarily, cyclo-
ruthenation of 2-phenylpyridine occurred readily with [Ru]-1
catalyst to generate catalytically active cycloruthenated species
[Ru]-A and [Ru]-B (Figure 7). This further supports that
initial C−H bond activation takes place by deprotonation and
not by oxidative addition of C−H bond.34 Literature also
revealed that proton abstraction by a carbonate base is
energetically favorable process (−13.7 kcal/mol) over hydride
abstraction pathway (+28.2 kcal/mol), and during this process
base to metal synergy plays a crucial role.35 The initial
carbonate-induced deprotonation of the ortho C−H bond
of the phenyl group of 2-phenylpyrdine by [Ru]-1 (as also
observed for other complexes [Ru]-2−[Ru]-7), led to the
formation of species [Ru]-A, originated by the release of tro-
polone ligand during the deprotonation process, and [Ru]-B,
by elimination of η6-arene ring.31 Reaction of species [Ru]-A
with 2.5 equiv of 4-chloroanisole in water at 100 °C resulted in
no reaction in the absence of a base (K2CO3) after 8 h, whereas
with 3 equiv of K2CO3, both mono- and diarylated products in
78:22 ratio were observed with 14% conversion. Unfortunately,
all our attempts to isolate species [Ru]-B failed, presumably,
due to the high reactivity of this species. Interestingly, per-
forming the same reaction in methanol formed only [Ru]-A
species, even after prolonged reaction time, suggesting the crucial
role of water in the C−H bond-activation reactions.
Stability of the complexes [Ru]-1 and [Ru]-2 in water was
monitored by mass-spectral studies over a period of 0 s to
one week. Complexes showed high stability in water at room
temperature or heating at 100 °C with no sign of decomposition
in the absence of base (Figures S5 and S6). These complexes
were also found to be stable in water even in the presence of base
K2CO3 under stirring at room temperature or at 100 °C for 3 d
(Figure S7). However, in the presence of K2CO3 at 100 °C,
release of chloro ligand from complex [Ru]-1 was observed to be
a facile process to form the solvated species [(η6-p-cymene)Ru-
(κ2-troponate)(CH3OH)]+ (m/z 387.1; mass spectra were
obtained by diluting the reaction mixture in methanol). Inter-
estingly, complex [Ru]-2 also showed the release of PPh3 in the
presence of base at 100 °C, but this process was a little sluggish,
as prolonged heating for 3 d was required to completely release
PPh3 (Figure S8). Moreover, no sign of the release of arene
ligand was observed during mass-spectral studies of the com-
plexes [Ru]-1 and [Ru]-2. Prolonged heating at 100 °C with
base for one week resulted in the decomposition of these com-
plexes.
[(κ2-troponate)Ru(κ2-phenylpyridine)(sol)2] [Ru]-B1, respec-
tively. This effect is further expected to be responsible for
the formation of analogous species [Ru]-B3, also for amino-
troponate complexes [Ru]-3, [(κ2-isopropylaminotroponate)-
Ru(κ2-phenylpyridine)(sol)2] (Figure S4).
Support for our experimental results comes from DFT calcu-
lations by comparing the relative energies for the formation of
species [Ru]-A, [(η6-arene)Ru(κ2-C,N-phenylpyridine)(sol)]+,
and [Ru]-B, [(κ2-troponate/aminotroponate)Ru(κ2-phenyl-
pyridine)(sol)2] from complexes [Ru]-1, [Ru]-2, and [Ru]-3.
The reaction free energy profile, Figure 8, shows an uphill
Figure 8. Reaction free energy profile for the formation of
cycloruthenated species, [Ru]-A and [Ru]-B, from the reaction of
phenylpyridine with troponate/aminotroponate Ru−arene complexes
([Ru]-1−[Ru]-3).
reaction pathway for the formation of both species [Ru]-A and
[Ru]-B from troponate/aminotroponate Ru−arene complexes.
Though the formation of [Ru]-B is highly favored over [Ru]-A
for all the ruthenium complexes, [Ru]-1 complex showed only
marginally higher selectivity for species [Ru]-B over [Ru]-A,
which is in close agreement with our mass-spectral experimental
findings. Moreover, binding energy (EB) calculations (Table S5)
also showed that p-cymene is a better leaving group than
tropolone/aminotropolone, and thus the formation of species
[Ru]-B is favored over species [Ru]-A.
On the basis of the mass-spectral identification and well-
supported by DFT calculation of new cycloruthenated Ru(II)
species, [Ru]-B formed as a major component by the release
of η6-arene group during the initial deprotonation step of
2-phenylpyrdine, along with the well-established species [Ru]-A
(minor). Role of species [Ru]-A is well-established in C−H
bond activation reaction catalyzed by Ru−arene complexes, but
our experimental evidence revealed that the formation of species
[Ru]-B is favored over species [Ru]-A for the troponate/amino-
troponate ruthenium−arene complexes. Since all the troponate/
aminotroponate ruthenium−arene complexes displayed high cata-
lytic activity for C−H bond arylation reaction, species [Ru]-B
play a key role in the oxidative addition step of C−H bond acti-
vation reaction. This observation is also in good agreement with
the previous finding by Ackermann et al., that RuCl3(H2O)n
is able to catalyze the C−H arylation in the absence of arene
ligands.36 Hence, a reaction pathway is proposed (shown in
Scheme 3) for the C−H bond arylation of 2-phenylpyridine
catalyzed by troponate /aminotroponate ruthenium−arene
complexes. Notably, troponate/aminotroponate ruthenium−
arene complexes are inactive toward direct reaction with aryl
chloride, whereas corresponding C−H bond activation of
Therefore, we anticipated that under catalytic reaction condi-
tions, in the presence of base and 2-phenylpyridine at 100 °C,
release of arene ligand became a more favorable process to gener-
ate the catalytically active arene-free ruthenium species [Ru]-B.
Further, the chelation and strong conjugation effect over seven
carbon atoms in the planar ring of tropolone favors the strong
interaction of tropolone to ruthenium metal center. As coordination
of 2-phenylpyridine earlier to the C−H deprotonation step is
crucial, the relative competitive coordination strength of η6-arene
and troponate ligands drive the de-coordination of either troponate
ligand or η6-arene, followed by deprotonation step to form
[(η6-arene)Ru(κ2-C,N-phenylpyridine)(sol)]+ [Ru]-A and
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well-established arene−ruthenium cycloruthenated species [Ru]-A.
It is worth mentioning here that arene-free ruthenium species,
for instance, RuCl3(H2O)n, were also reported to be active for
catalytic C−H arylation reactions.36 These cycloruthenated
species were readily generated by carbonate-assisted deproto-
nation of 2-phenylpyridine by ruthenium complexes. Con-
sistent with the above results, reaction free energies, calculated
using DFT calculations, also favored the formation of species
[Ru]-B over [Ru]-A. In contrast to the parent ruthenium cata-
lysts, which were inactive with 4-chloroanisole, these cyclo-
ruthenated species were found to be active toward oxidative
addition of 4-chloroanisole to generate C−H arylated products.
On the basis of the experimental evidence, a reaction pathway
was proposed showing the key importance of the species
[Ru]-B in C−H bond arylation of 2-phenylpyridine. We believe
observations marked in the present study using troponate/
aminotroponate ruthenium(II)−arene complexes will contribute
in enhancing the mechanistic understanding of such catalytic
systems in C−H activation reactions and help in development of
new and highly active catalytic system. Further investigations in
this direction are underway.
Scheme 3. Proposed Reaction Pathway for C−H Bond
Arylation of 2-Phenylpyrdine Catalyzed by Troponate-
Ruthenium(II)−Arene Complex [Ru]-1
EXPERIMENTAL SECTION
■
General Procedure for the Synthesis of Tropolone-based
Ligands. Ligands L2 and L3 were synthesized using a literature-
reported method with some modifications.6 2-Tosyloxytropone (276 mg,
1.0 mmol), corresponding amine (1.26 mmol), and triethylamine (0.153 g,
1.51 mmol) were dissolved in ethanol (25 mL). The mixture was refluxed
for 12 h and then cooled to room temperature. All volatiles were removed
under reduced pressure, and the oily residue was taken up in 2 N NaOH
(15 mL). The aqueous phase was extracted with CH2Cl2 (4 × 10 mL).
The organic phases were washed with brine (20 mL) and dried with
Na2SO4. The crude product was purified by column chromatography using
silica gel.
2-phenylpyridine was faster. Given the experimental results for
C−H bond arylation of 2-phenylpyridine by several troponate/
aminotroponate Ru−arene complexes, it has been established that
(i) these complexes are highly efficient catalyst for C−H bond
arylation, (ii) increase in steric bulkiness enhances the conversion
as well as selectivity toward diarylated product, and (iii) replacing
troponate with aminotroponate ligands in ruthenium−arene com-
plexes remarkably enhances the catalytic efficiency and selectivity
toward diarylated product.
Synthesis of 2-(Isopropylamino)cyclohepta-2,4,6-trienone. Li-
gand L2 was prepared by following the above general procedure
using 2-tosyloxytropone (0.276 g, 1.0 mmol), isopropylamine (0.074 g,
1.26 mmol), and triethylamine (0.153 g, 1.51 mmol). Yield: 0.110 g,
(67.4%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.18−7.04
(m, 3H), 6.56 (t, J = 8.0 Hz, 1H), 6.47 (d, J = 8.0 Hz, 1H), 3.78−3.70
1
(m, 1H), 1.24 (d, J = 4.0 Hz, 6H). H NMR (400 MHz, deuterated
dimethyl sulfoxide (DMSO-d6)): δ (ppm) = 7.29−7.23 (m, 2H),
6.93 (d, J = 8.0 Hz, 1H), 6.69−6.61 (m, 2H), 3.91−3.83 (m, 1H), 1.23
(d, J = 4.0 MHz, 6H). 13C NMR (100 MHz, CDCl3): δ (ppm) =
176.57, 154.67, 137.18, 136.29, 128.24, 121.83, 108.89, 43.81, 22.02.
High-resolution mass spectrometry (HRMS) (electrospray ionizatin
(ESI)) m/z calculated for 2-(isopropylamino)cyclohepta-2,4,6-trien-
one: 164.1070 [M + H]+, found 164.3049 [M + H]+.
CONCLUSION
■
We successfully synthesized a series of half-sandwiched
ruthenium(II) arene complexes with O,O and O,N donor
troponate/aminotroponate ligands, and structures of two
representative complexes were established by single-crystal
X-ray diffraction studies. These synthesized complexes exhibited
higher catalytic activity for direct ortho C−H bond arylation of
2-phenylpyridine with 4-chloroanisole at 100 °C in water, without
the use of any carboxylate additive, with enhanced selectivity for
monoarylated product over diarylated product. In comparison to
troponate ruthenium(II)−arene complex [Ru]-1, the selectivity
toward monoarylated products is more prominent with amino-
troponate ruthenium(II)−arene complexes [Ru]-3 and [Ru]-4.
Notably, these complexes are also catalytically active in the pres-
ence of carboxylate additive, where selectivity toward diarylated
product is favored over monoarylated product with the more
bulky carboxylates: pivalate > isobutyrate > propionate > acetate.
Extensive mass-spectral studies were performed to identify
active key intermediates of the catalyst and to evaluate their
role in the C−H bond activation pathway. Our studies showed
the formation of a new cycloruthenated species, [Ru]-B, by
the release of arene ring from the catalyst, is favored over the
Synthesis of 2-(Cyclohexylamino)cyclohepta-2,4,6-trienone. Li-
gand L3 was prepared by following the above general procedure using
2-tosyloxytropone (0.276 g, 1.0 mmol), cyclohexylamine (0.125 g,
1.26 mmol), and triethylamine (0.153 g, 1.51 mmol). Yield: 0.140 g
(69%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.17−7.05 (m, 3H),
6.60−6.50 (m, 2H), 3.46−3.41 (m, 1H), 2.02−1.96 (m, 2H), 1.77−
1.74 (m, 2H) 1.62−1.59 (m, 1H), 1.37−1.30 (m, 4H), 1.27−1.21
1
(m, 1H). H NMR (400 MHz, DMSO-d6): δ (ppm) = 7.28−7.23
(m, 2H), 7.93 (d, J = 12.0 Hz, 1H), 7.74 (d, J = 4.0 Hz, 1H), 6.64
(t, J = 4.0 Hz, 1H), 3.60−3.54 (m, 1H), 1.92−1.89 (m, 2H), 1.70−
1.67 (m, 2H) 1.60−1.57 (m, 1H), 1.43−1.29 (m, 4H), 1.23−1.18
(m, 1H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 176.49, 154.61,
137.12, 136.28, 128.03, 121.78, 108.94, 51.04, 32.07, 25.57, 24.65.
HRMS (ESI) m/z calculated for 2-(cyclohexylamino)cyclohepta-2,4,6-
trienone: 204.1389 [M + H]+, found 204.3550 [M + H]+. CCDC
deposition number of the ligand L3 is 1431337.
Procedure for the Synthesis of Ruthenium(II)−Arene Complexes
[Ru]-1−[Ru]-8 Containing Tropolone and Aminotropone Ligands.
Synthesis of [(η6-p-Cymene)Ru(κ2-O,O-tropolone)Cl]. Tropolone
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(L1) (0.050 g, 0.41 mmol) and KOtBu (0.046 g, 0.41 mmol) were
suspended in methanol (25 mL). The suspension was stirred at room
temperature for 3 h then added to [{(η6-C10H14)RuCl2}2] (0.122 g,
0.20 mmol) in suspension. The suspension was stirred at room temper-
ature for 24 h. Solution was evaporated to dryness, and the residue was
dissolved in dichloromethane and precipitated by pouring in excess of
diethyl ether. Red crystalline solid was obtained. Yield: 0.121 g (77.3%).
FTIR (KBr, cm−1): 1221, 1337 υ(C−C), 1508 υ(CC), 1588.21
υ(C = O). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.21 (m, 4H), 6.78
(m, 1H), 5.55 (d, 2H, J = 5.2 Hz), 5.33 (d, 2H, J = 4.8 Hz), 2.89
(sept, 1H), 2.34 (s, 3H), 1.33 (d, 6H, J = 6.8 Hz). 13C NMR (100 MHz,
CDCl3): δ (ppm) = 184.87, 137.80, 127.37, 126.34, 100.48, 96.55,
80.43, 78.89, 31.36, 22.65, 18.89. MS (ESI) m/z calculated for [(η6-
C10H14)Ru(L1)]+ (L1 = tropolone), 357.0 [M − Cl]+, found 357.1
[M − Cl]+. Anal. Calcd: C, 52.04; H, 4.84; O, 8.16. Found: C, 52.08; H,
4.83; O, 8.11%. CCDC deposition number of the complex [Ru]-1 is
1431336.
dichloromethane followed by precipitation with excess of diethyl ether.
It can be also prepared by following the procedure for the synthesis of
complex [Ru]-1. Red microcrystalline solid was obtained. Yield: 0.110 g,
(82%). FTIR (KBr, cm−1): 1139, 1360 υ(C−C), 1508 υ(CC),
1588.38 υ(CO). 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.23−7.21
(m, 4H), 6.84−6.80 (m, 1H), 5.69 (s, 6H). 13C NMR (100 MHz,
CDCl3): δ (ppm) = 184.29, 137.55, 127.07, 126.41, 80.93. MS (ESI)
m/z calculated for [(η6-C6H6)Ru(L1)]+ (L1 = tropolone), 301.0
[M − Cl]+, found 301.0 [M − Cl]+. Anal. Calcd: C, 46.56; H, 3.30;
O, 9.55. Found: C, 46.58; H, 3.17; O, 9.52%.
Synthesis of [(η6-Benzene)Ru(κ2-O,O-tropolone)PPh3]Cl. Complex
[Ru]-6 was prepared by following the procedure for the synthesis of
complex [Ru]-2, using [(η6-C6H6)RuCl2(PPh3)] (0.102 g, 0.20 mmol)
and tropolone (L1) (0.026 g, 0.21 mmol) in methanol (25 mL).
Red microcrystalline solid was obtained. Yield: 0.102 g, (85%).
1H NMR (400 MHz, CDCl3): δ (ppm) = 7.75−7.72 (m, 5H), 5.43−
5.38 (m, 10H), 7.21−7.21 (m, 3H), 5.39 (m, 1H), 5.39 (m, 1H),
5.39 (s, 6H). MS (ESI) m/z calculated for [(η6-C6H6)Ru(L1)PPh3]+
(L1 = tropolone), 563.1 [M+], found 563.1 [M+]. Anal. Calcd: C,
62.26; H, 4.38; O, 5.35. Found: C, 62.19; H, 4.13; O, 5.61%.
Synthesis of [(η6-Benzene)Ru(κ2-O,N-2-(cyclohexylamino)-
cyclohepta-2,4,6-trienone)Cl]. Complex [Ru]-7 was prepared by
following the procedure for the synthesis of complex [Ru]-3, using
[{(η6-C6H6)RuCl2}2] (0.225 g, 0.45 mmol). Red microcrystalline solid
Synthesis of [(η6-p-Cymene)Ru(κ2-O,O-tropolone)PPh3]Cl. [(η6-
C10H14)RuCl2(PPh3)] (0.114 g, 0.20 mmol) and tropolone (L1)
(0.026 g, 0.21 mmol) were dissolved in methanol (25 mL), and the
solution was refluxed for 12 h. Solution was evaporated to dryness, and
the residue was dissolved in dichloromethane and precipitated by diethyl
ether. It can be also synthesized by using an alternative method, for
which [(η6-p-cymene)RuCl(κ2-O,O-tropolone)] (0.0.078 g, 0.20 mmol)
and PPh3 (0.079 g, 0.30 mmol) were suspended in methanol (30 mL),
and then the suspension was stirred for 24 h at room temperature.
Red microcrystalline solid was obtained. Yield: 0.101 g (77%). 1H NMR
(400 MHz, CDCl3): δ (ppm) = 7.93−7.84 (m, 5H), 7.53−7.44
(m, 10H), 7.43−7.38 (m, 4H), 7.09(t, J = 8.0 Hz, 1H), 5.26 (b, 2H),
5.06 (b, 2H), 2.92−2.90 (m, 1H), 1.94 (s, 3H), 1.17 (d, J = 4.0 Hz, 6H).
MS (ESI) m/z calculated for [(η6-C10H14)Ru(L1)PPh3]+ (L1 = tropolone),
619.1 [M+], found 619.1 [M+]. Anal. Calcd: C, 64.26; H, 5.24; O, 4.89.
Found: C, 64.19; H, 5.28; O, 4.91%.
1
was obtained. Yield: 0.121 g (80%). H NMR (400 MHz, CDCl3):
δ (ppm) = 6.83−6.60 (m, 4H), 6.20 (t, J = 8.8 Hz, 1H), 5.61 (s, 6H),
4.41−4.35 (m, 1H), 1.38 (d, J = 6.4 Hz, 6H). MS (ESI) m/z calculated
for [(η6-C6H6)Ru(L2)]+ (L2 = 2-(isopropylamino)cyclohepta-2,4,6-
trienone), 342.0 [M − Cl]+, found 342.0 [M − Cl]+. Anal. Calcd: C,
50.86; H, 5.07; O, 4.29; N, 3.71. Found: C, 50.66; H, 4.87; O, 4.50; N,
3.93%.
Synthesis of [(η6-Benzene)Ru(κ2-O,N-2-(cyclohexylamino)-
cyclohepta-2,4,6-trienone)Cl]. Complex [Ru]-8 was prepared by
following the procedure for the synthesis of complex [Ru]-7, using
2-(cyclohexylamino)cyclohepta-2,4,6-trienone (L3; 0.203 g, 1.00 mmol).
Red microcrystalline solid was obtained. Yield: 0.201 g (53%). MS (ESI)
m/z calculated for [(η6-C6H6)Ru(L3)]+ (L3 = 2-(cyclohexylamino)-
cyclohepta-2,4,6-trienone), 382.1 [M − Cl]+, found 382.4 [M − Cl]+.
Anal. Calcd: C, 54.74; H, 5.32; O, 3.84; N, 3.36. Found: C, 53.13; H,
5.01; O, 2.90; N, 3.10%.
Synthesis of [(η6-p-Cymene)Ru(κ2-O,N-2-(isopropylamino)-
cyclohepta-2,4,6-trienone)Cl]. 2-(Isopropylamino)cyclohepta-2,4,6-
trienone (L2; 0.163 g, 1.00 mmol) and triethylamine (0.505 g,
5.00 mmol) were suspended in methanol (30 mL), and the suspension
was refluxed for 3 h. [{(η6-C10H14)RuCl2}2] (0.275 g, 0.45 mmol) was
then added to the above solution, and the solution was refluxed for
another 12 h. Solution was filtered and evaporated to dryness, and the
residue was dissolved in dichloromethane followed by the precipitation
with diethyl ether. Brown crystalline solid was obtained. Yield: 0.320 g
(82%). 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 7.79 (d, J = 8.0 Hz,
1H), 7.40 (d, J = 8.0 Hz, 1H), 6.79 (t, J = 8.0 Hz, 1H), 6.30 (d, J = 8.0 Hz,
1H), 6.10 (t, J = 8.0 Hz, 1H), 5.80 (d, J = 4.0 Hz, 2H), 5.68 (d, J = 4 Hz,
2H), 4.27−4.20 (m, 1H), 2.86−2.78 (sept, 1H), 2.09 (s, 3H), 1.50 (d, J =
4.0 Hz, 3H), 1.40 (d, J = 4.0 Hz, 3H), 1.19 (d, J = 4.0 Hz, 6H). MS (ESI)
m/z calculated for [(η6-C10H14)Ru(L2)]+ (L2 = 2-(isopropylamino)-
cyclohepta-2,4,6-trienone), 398.1 [M − Cl]+, found 398.1 [M − Cl]+.
Anal. Calcd: C, 55.48; H, 6.05; O, 3.70; N, 3.24. Found: C, 55.76; H,
6.15; O, 4.12; N, 3.33%. CCDC deposition number of the complex
[Ru]-3 is 1441581.
General Procedure for Catalytic C−H Bond Arylation of
Heteroarenes with Aryl Halides. All the reactions were performed
under Ar atmosphere. C−H bond arylation reaction of heteroarene
was performed in a two-necked round-bottom flask. Flask was charged
with ruthenium catalyst (5 mol %, 0.025 mmol) and K2CO3 (0.207 g,
1.50 mmol) with distilled water (5 mL). Solution was stirred for
15 min and then added to heteroarene (0.5 mmol) and aryl halide
(1.25 mmol). The reaction mixture was degassed with Ar along with
the balloon at the top of the fitted condenser. The reaction was
continued to stir for 16 h at 100 °C. After completion of reaction time,
reaction mixture was cooled to room temperature and extracted with
ethyl acetate (3 × 10 mL), and the organic layer was dried over Na2SO4.
Then extract was washed with 10 mL of saturated brine solution to
remove moisture, and the solvent volume was reduced under pressure.
The conversion and selectivity of synthesized monoarylated and biary-
Synthesis of [(η6-p-Cymene)Ru(κ2-O,N-2-(cyclohexylamino)-
cyclohepta-2,4,6-trienone)Cl]. Complex [Ru]-4 was prepared by
following the procedure for the synthesis of complex [Ru]-3, using
2-(cyclohexylamino)cyclohepta-2,4,6-trienone (L3) (0.203 g, 1.00 mmol).
Red microcrystalline solid was obtained. Yield: 0.339 g (79%).
1H NMR (400 MHz, DMSO-d6): δ (ppm) = 6.84−6.68 (m, 3H), 6.31
(d, J = 8.0 Hz, 1H), 6.11 (t, J = 8.0 Hz, 1H), 5.80 (d, J = 6.0 Hz, 2H),
5.76 (d, J = 6.0 Hz, 2H), 3.76−3.66 (m, 1H), 2.81 (sept, 1H), 2.09
(s, 3H), 1.93−1.90 (m, 2H), 1.83−1.77 (m, 3H), 1.68−1.64 (m, 1H),
1.41−1.28 (m, 4H), 1.17 (d, J = 4.0 Hz, 6H). MS (ESI) m/z calculated
for [(η6-C10H14)Ru(L3)]+ (L3 = 2-(cyclohexylamino)cyclohepta-2,4,6-
trienone), 438.1 [M − Cl]+, found 438.1 [M − Cl]+. Anal. Calcd: C,
58.40; H, 6.39; O, 3.38; N, 2.96. Found: C, 57.90; H, 5.99; O, 4.01; N,
3.13%.
1
lated products were determined by H NMR. Products were purified
and collected from the crude reaction mixture using column chro-
matography on silica gel with ethyl acetate/n-hexane as eluents in
1:99 (v/v) ratio.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.6b01028.
■
S
Materials and instrumentation, tabulated crystal data and
refinement details for ligand L3, tabulated bond lengths
and bond angles for ligand L3, selected bond lengths and
bond angles for complexes [Ru]-1 and [Ru]-3, tabulated
cyclic voltammetric data of ruthenium complexes, cyclic
Synthesis of [(η6-Benzene)Ru(κ2-O,O-tropolone)Cl]. [{(η6-
Benzene)RuCl2}2] (0.100 g, 0.20 mmol) and tropolone (L1; 0.050 g,
0.41 mmol) were suspended in methanol (25 mL). The suspension was
refluxed for 12 h and filtered, and then precipitate was dissolved in
I
DOI: 10.1021/acs.inorgchem.6b01028
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
voltammograms, analytical data for catalytic C−H bond
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X-ray crystallographic information (CIF)
X-ray crystallographic information (CIF)
X-ray crystallographic information (CIF)
22, 3533−3545.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: sksingh@iiti.ac.in.
(10) Schutte, M.; Roodt, A.; Visser, H. G. Inorg. Chem. 2012, 51,
11996−12006.
■
(11) Yoshida, J.; Kuwahara, K.; Yuge, H. J. Organomet. Chem. 2014,
756, 19−26.
Notes
(12) Mori, A.; Mori, R.; Takemoto, M.; Yamamoto, S.; Kuribayashi,
D.; Uno, K.; Kubo, K.; Ujiie, S. J. Mater. Chem. 2005, 15, 3005−3014.
The authors declare no competing financial interest.
(13) Dehnen, S.; Burgstein, M. R.; Roesky, P. W. J. Chem. Soc., Dalton
̈
ACKNOWLEDGMENTS
Trans. 1998, 2425−2429.
■
(14) Becica, J.; Jackson, A. B.; Dougherty, W. G.; Kassel, W. S.; West,
N. M. Dalton Trans. 2014, 43, 8738−8748.
Authors thank IIT Indore, CSIR, New Delhi, and SERB (DST),
New Delhi, for financial support. SIC, IIT Indore, and NTU,
Singapore, are acknowledged for instrumental facilities. A.D.D.
thanks CSIR New Delhi research scheme (01(2722)/13/EMR-II)
for JRF grant. C.B. thanks MHRD for the fellowship. D.T. thanks
UGC New Delhi for SRF grant.
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