Indenyl Rhodium Complexes with Arene Ligands: Synthesis and Application for Reductive Amination

Article abstract of DOI:10.1021/acs.organomet.8b00311

An efficient protocol for synthesis of indenyl rhodium complexes with arene ligands has been developed. The hexafluoroantimonate salts [(σ⁵-indenyl)Rh(arene)](SbF₆)₂ (arene = benzene (2a), o-xylene (2b), mesitylene (2c), d

Full text of DOI:10.1021/acs.organomet.8b00311

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Indenyl Rhodium Complexes with Arene Ligands: Synthesis and
Application for Reductive Amination
Vladimir B. Kharitonov,† Maria Makarova,‡ Mikhail A. Arsenov,† Yulia V. Nelyubina,† Olga Chusova,§
Alexander S. Peregudov,† Semen S. Zlotskii,∥ Denis Chusov,† and Dmitry A. Loginov*,†
†A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova str. 28, 119991 Moscow,
Russian Federation
‡Dmitry Mendeleev University of Chemical Technology of Russia, Miusskaya sq. 9, Moscow 125047, Russian Federation
§Faculty of Science, RUDN University, 6 Miklukho-Maklaya St., Moscow 117198, Russian Federation
∥Ufa Oil Institute, Kosmonavtov Str., 1, Ufa 450062, Russian Federation
S
* Supporting Information
ABSTRACT: An efficient protocol for synthesis of indenyl
rhodium complexes with arene ligands has been developed.
The hexafluoroantimonate salts [(η5-indenyl)Rh(arene)]-
(SbF6)2 (arene = benzene (2a), o-xylene (2b), mesitylene
(2c), durene (2d), hexamethylbenzene (2e), and [2.2]-
paracyclophane (2g)) were obtained by iodide abstraction
from [(η5-indenyl)RhI2]n (1) with AgSbF6 in the presence of
benzene and its derivatives. The procedure is also suitable for
the synthesis of the dirhodium arene complex [(μ-η:η′-1,3-dimesitylpropane){Rh(η5-indenyl)}2](SbF6)4 (3) starting from 1,3-
dimesitylpropane. The structures of [2e](SbF6)2, [2g](SbF6)2, and [3](SbF6)4 were determined by X-ray diffraction. The last
species has a sterically unfavorable conformation, in which the bridgehead carbon atoms of the indenyl ligand are arranged close
to the propane linker between two mesitylene moieties. Experimental and DFT calculation data revealed that the benzene
ligand in 2a is more labile than that in the related cyclopentadienyl complexes [(C5R5)Rh(C6H6)]2+. Complex 2c effectively
catalyzes the reductive amination reaction between aldehydes and primary (or secondary) amines in the presence of carbon
monoxide, giving the corresponding secondary and tertiary amines in very high yields (80−99%). This protocol is the most
active in water.
effective synthetic methods to prepare indenyl complexes is a
very important challenge of organometallic chemistry.
At the same time, the strongly bonded halide anions can
decrease the activity of the catalyst by blocking of vacant
coordination sites at the metal atom. In general, the use of
silver salts as halogen scavengers is necessary for the generation
of catalytically active species.6 In contrast, more weakly bonded
neutral ligands, e.g. carbonyl, acetonitrile, olefins, and arenes,
do not have this disadvantage due to their easy thermal
lability.7 To the best of our knowledge, indenyl rhodium
complexes with arenes are known only for the 1,2,3,4,7-
pentamethylindenyl ligand.8 In particular, we have found
earlier that the benzene complex [(η5-C9H2Me5)Rh(C6H6)]2+
shows moderate catalytic activity in the oxidative coupling of
benzoic acid with diphenylacetylene, giving 1,2,3,4-tetraphe-
nylnaphthalene.9 Herein we report the synthesis of parent
arene rhodium complexes with an unsubstituted indenyl ligand
and their application in catalytic reductive amination of
aldehydes.
INTRODUCTION
■
The catalytic applications of indenyl metal complexes are well-
known.1 For example, Gimeno and co-workers2 have shown
that the ruthenium derivative (η5-indenyl)Ru(cod)Cl effec-
tively catalyzes the cycloaddition reactions and hydration of
alkynes, with yields and selectivity being considerably higher
than those in the reactions catalyzed by cyclopentadienyl
congeners.
The enhanced catalytic activity of indenyl complexes in
comparison to cyclopentadienyl analogs is attributed to easy
slippage of the indenyl ligand from an η5 to an η3 coordination
mode (the so-called indenyl effect).3 The structural flexibility
of the indenyl ligand facilitates the ligand substitution and
catalytic reactions. In addition, the indenyl moiety can play the
role of a sterical bulky ligand in the chiral reactions. For
example, the heptamethylindenyl rhodium complex [(η5-
C9Me7)RhCl2]2 proved to be an effective catalyst for the
diastereoselective coupling of O-substituted arylhydroxamates
and cyclopropenes.4 Though it has been a very long time since
the indenyl effect was discovered by Hart-Davis and Mawby in
1969,5 the indenyl complexes of transition metals are still
poorly studied in catalysis, which can be explained by their
limited commercial availability. Therefore, the development of
Received: May 14, 2018
© XXXX American Chemical Society
A
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Very recently, we have found that the iodide [(η5-
indenyl)RhI2]n (1) has high catalytic activity in the reductive
amination of aldehydes or ketones in the presence of carbon
monoxide as a reducing agent.10 The classical two-step
synthesis of amines includes Schiff base formation and its
subsequent hydrogenation. The method where CO is used as a
reducing agent instead of H2 gas or hydride reagents has a
number of advantages in comparison with the classical method.
In the first place, it provides high selectivity due to the absence
of an external hydrogen source, which makes hydrogenation of
functional groups present in the substrates impossible.11−13
Second, carbon monoxide, the reducing agent in the
aforementioned method, is widely available and inexpensive
in comparison to hydride reducing agents such as NaBH3CN.
Moreover, the method is highly atom economical and the only
stochiometric byproduct is carbon dioxide. CO-mediated
reductive addition has been applied to the synthesis of a
number of practically important compounds, including chiral
ligands14 and drugs.15,16
nucleophilic anions, e.g. AgOAc and AgOTf, instead of
AgSbF6. The use of AgBF4 or AgPF6 leads to the desired
products, but in lower yields and in impure form. We propose
that these products are contaminated by solvate complexes
which may be formed as a result of arene replacement with the
assistance of the counterion.
Further investigation showed that this procedure is also
suitable for the synthesis of the dinuclear complex [(μ-η:η′-1,3-
dimesitylpropane){Rh(η5-indenyl)}2](SbF6)4 ([3](SbF6)4), in
which two (η5-indenyl)Rh fragments are coordinated to
different aromatic rings (Scheme 2). An attempt to prepare
Scheme 2. Synthesis of the Dinuclear Arene Complex
[3](SbF6)4
However, in terms of green chemistry atom economy has
been the only advantage of this reductive amination reaction
until now. Moreover, from an ecological point of view it had a
serious disadvantagelow yields of the desired amines if the
experiment is conducted in ecologically friendly solvents.
Surprisingly, with the use of the new indenyl rhodium
complexes as catalysts for the reductive amination, various
solvents were tested and water was found to be the best solvent
for the reaction among them. Using this approach, we were
able to prepare both aliphatic and aromatic amines.
RESULTS AND DISCUSSION
■
Synthesis and Reactivity. Recently, we have shown that
iodide complex 1 is a useful synthon of the (η5-indenyl)Rh
fragment, as illustrated by the preparation of the cyclo-
pentadienyl derivatives and rhodacarboranes.10,17 In the
present study, we used 1 for the synthesis of arene complexes.
Thus, iodide abstraction from 1 by AgSbF6 in the presence of
benzene or its derivatives in nitromethane gives the dicationic
mononuclear arene complexes [(η5-indenyl)Rh(arene)]-
(SbF6)2 ([2a−e](SbF6)2, Scheme 1). Apparently, the reaction
the mononuclear complex [(η5 -indenyl)Rh(1,3-
dimesitylpropane)](SbF6)2 ([2f](SbF6)2) by using a 4-fold
excess of 1,3-dimesitylpropane led to formation of an
inseparable mixture of [2f](SbF6)2 and [3](SbF6)4 in a 2:1
ratio. At the same time, the reaction of an excess of 1 with
[2.2]paracyclophane exclusively leads to the mononuclear
complex [(η5-indenyl)Rh([2.2]paracyclophane)](SbF6)2
([2g](SbF6)2), in which one of the aromatic rings remains
uncoordinated (Scheme 3).
Scheme 1. Synthesis of the Mononuclear Arene Complexes
[2a−e](SbF6)2
Scheme 3. Reaction of 1 with [2.2]Paracyclophane
The arene complexes [2a−g](SbF6)2 and [3](SbF6)4 are
indefinitely stable in the solid state, but they readily undergo
solvolysis by coordinating solvents (such as acetone,
acetonitrile, and dimethyl sulfoxide) with elimination of free
arene, indicating substitutional lability of the arene ligand.
Higher lability of the arene can have a good effect on the
catalytic activity. The arene exchange reaction, in which one
arene ligand is replaced by another arene, is well-known for
rhodium complexes.20 In particular, examples of such reactions
have been described for cyclopentadienyl, carborane, and
proceeds via intermediate formation of the nitromethane
18
solvate [(η5-indenyl)Rh(MeNO2)3](SbF6)2 which is an
analogue of the well-known cyclopentadienyl derivatives
[(η5-C5R5)Rh(MeNO2)3]2+.19 However, the step-by-step
procedure with preliminary generation of the solvate (due to
its low stability) did not give the desired arene complexes. It is
worth noting that the nature of the counterion significantly
affects the outcome of the reaction. For example, arene
complexes were not obtained with the use of silver salts with
B
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triple-decker derivatives. To estimate the lability of arene
ligand in the indenyl complexes, we carried out the
comparative 1H NMR study of arene exchange in the benzene
derivatives [2a](SbF6)2, [2a](BF4)2, and related rhodium
complexes (Table 1). The reaction of the complex with an
excess of mesitylene in nitromethane-d3 was chosen as a model.
replacement of the arene ligand.21 The latter is in accordance
with the difficulty in isolating cations 2a−e as salts with the
−
BF4 anion in pure form (vide supra).
Recently, we have shown that the thermal arene exchange in
the (indenyl)iron complexes [(η5-indenyl)Fe(arene)]+ pro-
ceeds by an associative mechanism via arene slippage from η6
to η3 coordination mode rather than indenyl slippage.21e We
consider that the rhodium complexes undergo arene exchange
in the same manner. Otherwise, methyl groups in the indenyl
ligand should suppress the indenyl slippage and thereby
decrease the reaction rate. However, the methylated complex
[(C9H2Me5)Rh(C6H6)](BF4)2 proved to be more reactive
than the parent analog [2a](BF4)2 (Table 1, entry 9 vs entry
3).
Table 1. Comparative Study of Arene Exchange in
a
[2a](SbF6)2, [2a](BF4)2, and Related Rhodium Complexes
To elucidate the influence of the ligand at the rhodium atom
on the Rh−C6H6 bonding, we performed energy decom-
position analysis (EDA)22 for 2a and the related rhodium
complexes (Table 2). According to the EDA method, the
interaction energy between the bonding fragments ΔEint can be
divided into three main components:
b
entry
complex
time, h conversion,
%
1
2
3
4
5
6
7
8
[2a](SbF6)2
[2a](SbF6)2
[2a](BF4)2
[2a](BF4)2
5
24
5
24
5
17
74
32
92
8
[Cp*Rh(C6H6)](SbF6)2
[Cp*Rh(C6H6)](SbF6)2
[Cp*Rh(C6H6)](BF4)2
[CpRh(C6H6)](BF4)2
[(C9H2Me5)Rh(C6H6)](BF4)2
[CpCo(μ-1,3-C3B2Me5)Rh(C6H6)](BF4)2
[(7,8-C2B9H11)Rh(C6H6)]BF4
ΔEint = ΔEelstat + ΔEPauli + ΔEorb
48
5
5
61
53
9
ΔEelstat is the electrostatic interaction energy between the
fragments with a frozen electron density distribution, ΔEPauli
presents the repulsive four-electron interactions between
occupied orbitals (Pauli repulsion), and ΔEorb refers to the
stabilizing orbital interactions. The ratio ΔEelstat/ΔEorb
indicates the electrostatic/covalent character of the bond.
The bond dissociation energy De is defined as
c
9
5
5
62
<1
10
11
12
5
5
44
49
d
[(9-SMe2-7,8-C2B9H10)Rh(C6H6)](BF4)2
a
Reagents and conditions: rhodium complex (0.016 mmol),
mesitylene (0.14 mmol), nitromethane-d3 (0.5 mL), room temper-
b
c
1
D = −(ΔEint + ΔEprep
)
e
ature. Determined by H NMR spectroscopy. C9H2Me5 denotes
d
1,2,3,4,7-pentamethylindenyl. Significant decomposition of the
where ΔEprep (the fragment preparation energy) is the energy
that is necessary to promote the fragments from their
equilibrium geometry and electronic ground state to the
geometry and electronic state that they have in the optimized
structure.
carborane derivative was observed.
It was found that the benzene ligand in the indenyl
complexes is more labile than in the cyclopentadienyl analogs
(Table 1, entry 3 vs entry 8 as well as entry 9 vs entry 7). In
general, the methylated derivatives have a higher rate of the
arene exchange in comparison with the parent analogs (entry 7
vs entry 8 as well as entry 9 vs entry 3) that can be explained
by the highest dissociation energies of the Rh−C6H6 bond for
the parent compounds (vide infra). The carborane and triple-
decker derivatives showed lower reactivity in the arene
exchange reaction in comparison to the cyclopentadienyl and
indenyl complexes (entries 10−12 vs entries 7 and 9). In
general, the salts with a tetrafluoroborate anion are
considerably more reactive than those with a hexafluoroantim-
onate anion (entry 3 vs entry 1 as well as entry 7 vs entry 5),
suggesting the crucial participation of counterions in
We found that in agreement with the highest reactivity of
the indenyl complexes in the arene exchange reaction they
have weaker Rh−C6H6 bond than cyclopentadienyl analogs
(Table 2, entries 1 and 2 vs entries 3 and 4). The loosening of
the bond is mainly caused by the decrease in orbital
interactions and the increase in Pauli repulsion. The
introduction of methyl groups in the indenyl ring leads to
the following decrease of ΔEorb and thereby ΔEint (entry 2 vs
entry 1). The decrease in ΔEorb upon methylation can be
explained by the decrease in benzene-to-rhodium donation.
The ΔEint values correlate well with the dissociation energies,
since the preparation energy is almost the same in all cases.
Surprisingly, EDA data for complexes with boron-heterocyclic
Table 2. Results of EDA (Energy Values in kcal mol−1) of Cations [(L)Rh(C6H6)]n+ Using [(L)Rh]n+ and C6H6 as Interacting
Fragments at the BP86/TZP Level
a
a
entry
L
ΔEint
ΔEPauli
ΔEelstat
ΔEorb
ΔEprep
De
1
2
3
4
5
6
7
indenyl (2a)
C9H2Me5
Cp
−81.27
−62.59
−102.53
−68.81
−62.19
−54.46
−69.77
118.46
115.12
113.72
108.71
115.45
117.95
111.24
−83.03 (41.6)
−80.28 (45.2)
−81.48 (37.7)
−76.95 (43.3)
−78.85 (44.4)
−81.50 (47.3)
−77.77 (43)
−116.70 (58.4)
−97.42 (54.8)
−134.76 (62.3)
−100.57 (56.7)
−98.79 (55.6)
−90.90 (52.7)
−103.24 (57.0)
2.97
2.80
3.24
2.59
3.83
4.69
6.30
78.30
59.79
99.29
66.22
58.36
49.77
63.47
Cp*
CpCo(μ-1,3-C3B2Me5)
7,8-C2B9H11
9-SMe2-7,8-C2B9H10
a
The values in parentheses give the percentage contribution to the total attractive interactions.
C
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and carborane ligands (entries 5−7) did not agree with the
experimental data. For example, the rhodacarborane [(7,8-
C2B9H11)Rh(C6H6)]+ with the weakest Rh−C6H6 bond had
lower reactivity in the arene exchange in comparison with the
indenyl and cyclopentadienyl complexes, which can be
explained by kinetic factors due to better positive charge
delocalization in polyhedral compounds.20d
X-ray Diffraction. The structures of [2e](SbF6)2, [2g]-
(SbF6)2, and [3](SbF6)4 were determined by X-ray diffraction
(Figures 1−3). In [2e](SbF6)2, the cations pack into infinite
Figure 2. Crystal structure of complex [2g](SbF6)2 with 50% thermal
ellipsoids. Counterions and all hydrogen atoms are omitted for clarity.
Selected interatomic distances (Å): Rh1−C1 2.233(9), Rh1−C2
2.169(9), Rh1−C3 2.167(9), Rh1−C4 2.145(9), Rh1−C5 2.221(9),
Rh1−C10 2.411(10), Rh1−C11 2.209(9), Rh1−C12 2.227(9), Rh1−
C13 2.359(11), Rh1−C14 2.220(9), Rh1−C15 2.213(9), C1−C2
1.434(14), C1−C5 1.460(14), C2−C3 1.412(14), C3−C4 1.503(13),
C4−C5 1.410(14), C10−C11 1.405(13), C10−C15 1.426(13),
C11−C12 1.443(14), C12−C13 1.437(12), C13−C14 1.394(15),
C14−C15 1.407(15), C20−C21 1.409(15), C20−C25 1.407(14),
C21−C22 1.365(15), C22−C23 1.394(15), C23−C24 1.448(15),
C24−C25 1.359(16).
Figure 1. Crystal structure of complex [2e](SbF6)2 with 50% thermal
ellipsoids. Counterions and all hydrogen atoms are omitted for clarity.
Selected interatomic distances (Å): Rh1−C1 2.230(7), Rh1−C2
2.131(8), Rh1−C3 2.047(9), Rh1−C4 2.082(8), Rh1−C5 2.207(8),
Rh1−C10 2.285(7), Rh1−C11 2.317(7), Rh1−C12 2.333(8), Rh1−
C13 2.314(7), Rh1−C14 2.284(7), Rh1−C15 2.238(7), C1−C2
1.408(12), C1−C5 1.506(11), C2−C3 1.407(11), C3−C4 1.418(11),
C4−C5 1.406(13).
columns bound by the anions into the 3D framework by a
number of weak C−H···F contacts. The latter are supported by
F···π contacts with indenyl rings, the smallest C···F distance
being 2.930 Å. In [3](SbF6)4, only cation−anion interactions
contribute to the crystal formation: most of them are C−H···F
contacts with a F···π interaction involving the indenyl rings
(C···F 3.121 Å).
The indenyl ligand in these complexes is almost planar; the
hinge angles23,24 are 0.1, 3.2, and 3.5°, respectively. At the
same time, the slip parameter,21e,24,25 which is used to describe
the slip-fold distortion in the indenyl complexes, has a
considerably higher value for the hexamethylbenzene deriva-
tive [2e](SbF6)2 (0.13 Å) in comparison with other complexes
(0.07 for [2g](SbF6)2 and [3](SbF6)4), which can be expected
from the higher steric hindrance of hexamethylbenzene.
The Rh···C6Me6 distance in [2e](SbF6)2 (1.78 Å) is also
longer than the same distance in the cyclopentadienyl analog
[CpRh(C6Me6)](BF4)2 (1.689 Å),19b suggesting weaker
bonding of hexamethylbenzene with the [Rh(η5-indenyl)]2+
fragment than with [RhCp]2+. In contrast, the Rh···indenyl and
Rh···Cp distances in these complexes are very close (1.76 Å).
Both benzene rings of the paracyclophane ligand in cation
2g deviate from planarity toward a boat conformation, similar
to the case for the free ligand26 and its known complexes.27
The C−C bonds of the coordinated six-membered ring
(average 1.42 Å) are slightly longer than those of the
uncoordinated ring (average 1.40 Å) due to the loosening of
π bonds upon coordination. In contrast to hexamethylbenzene
Figure 3. Crystal structure of complex [3](SbF6)4 with 50% thermal
ellipsoids. Counterions and all hydrogen atoms are omitted for clarity.
Selected interatomic distances (Å): Rh1−C1 2.232(9), Rh1−C2
2.158(9), Rh1−C3 2.158(9), Rh1−C4 2.176(8), Rh1−C5 2.232(9),
Rh1−C10 2.278(8), Rh1−C11 2.250(9), Rh1−C12 2.246(10), Rh1−
C13 2.226(9), Rh1−C14 2.286(8), Rh1−C15 2.254(8), C1−C2
1.427(13), C1−C5 1.446(13), C2−C3 1.418(14), C3−C4 1.431(14),
C4−C5 1.450(13).
D
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derivatives, the Rh···paracyclophane distance in [2g](SbF6)2
(1.78 Å) is somewhat shorter than the corresponding distances
in the related complexes [Cp*Rh([2.2]paracyclophane)]-
(BF4)2 (1.805 Å)27c and [(η-7,8-C2B9H11)Rh([2.2]-
paracyclophane)]BF4 (1.817 Å).20c
Table 3. Screening of Rhodium Complexes in Reductive
Amination Reactions
Recently, we have found that (indenyl)rhodacarboranes
adopt the sterically unfavorable eclipsed cisoid conformation,
in which the bridgehead carbon atoms of the indenyl ligand are
located close to the carborane cage carbon atoms.17 This
phenomenon was explained by the trans effect and the
symmetry of molecular orbitals of indenyl and carborane
ligands. Surprisingly, the arene dinuclear complex [3](SbF6)4
also has a sterically unfavorable conformation, in which the
bridgehead carbon atoms of the indenyl ligand are arranged
close to the propane linker between two mesitylene moieties.
As a result of the trans effect, the Rh1−C10, Rh1−C14, and
Rh1−C15 bonds (average 2.27 Å) are considerably longer
than the Rh1−C11, Rh1−C12, and Rh1−C13 bonds (average
2.24 Å). Nevertheless, we consider that the sterically
unfavorable conformation in this case is due to crystal-packing
effects rather than thermodynamic factors. In particular, this
correlates well with the 2D ROESY NMR study of [3](SbF6)4
(see Figure S8 in the Supporting Information), which showed
the absence of spin interactions between the protons of the
indenyl ligand and the propane linker.
entry
complex
yield, %
1
2
3
4
5
6
7
8
1
34
84
81
57
80
83
68
79
[2c](SbF6)2
[2d](SbF6)2
[2e](SbF6)2
[2g](SbF6)2
[3](SbF6)4
[2a](BF4)2
[(C9H2Me5)Rh(C6H6)](BF4)2
benzene complex [2e](SbF6)2 led to only a 57% yield of the
desired amine (entry 4). This can be explained by the strong
binding of the hexamethylbenzene ligand to the rhodium atom,
which inhibits the removal of the arene ligand. Therefore, we
can suggest that catalytic efficiency is limited by the stage of
removal of the arene ligand. At the same time, a change of the
counterion to BF4− did not have a significant effect on the yield
(entries 7 and 8 vs entries 2−6). The benzene complex
[2a](BF4)2 furnished the desired amine in 68% yield. The
methylated derivative [(C9H2Me5)Rh(C6H6)](BF4)2 showed
somewhat higher activity in comparison to the parent complex
(entry 8 vs entry 7), which is in accordance with the lower
dissociation energy of the Rh−C6H6 bond in the former (vide
supra).
The complex [2c](SbF6)2 was used for the substrate scope
screening. We found that aromatic and aliphatic aldehydes as
well as aromatic and aliphatic amines (both primary and
secondary) are suitable for the reaction. The methodology
takes advantage of the unique deoxygenative potential of
carbon monoxide, which enables full compatibility with a range
of functional groups prone to reduction (e.g., N-benzyl and
halocyclopropanes) (Figure 4).
Application to Direct Reductive Amination. Since
reductive amination is one of the most convenient and versatile
methods of C−N bond formation, we have investigated the
catalytic activity of several rhodium complexes in the reductive
amination reaction using carbon monoxide as a reducing agent
(Scheme 4).28 Such reductive amination does not need an
Scheme 4. Reductive Amination Using Carbon Monoxide as
a Reducing Agent
external hydrogen source and has become one of the most
selective approaches for the amine synthesis (even more
selective than sodium cyanoborohydride).11
On the basis of previous studies by our group12,30 and that
of Denmark,31 we hypothesize that the cycle starts with the
formation of hydroxide anion and ammonium cation (Figure
5). An intermolecular attack of hydroxide on the precatalytic
species leads to intermediate A, and intramolecular attack on
the carbonyl group leads to species B. Subsequent elimination
of carbon dioxide yields hydride C, which reacts with an
iminium cation/Schiff base, leading to the formation of the
product. We believe that the key step of both processes
involves changing the configuration of the complex from η5 to
η3.
Attempts of C−H Activation. Finally, we tested the
catalytic activity of indenyl arene complexes for the C−H
activation of various aromatic substrates with directing group
(such as −NHAc, −C(O)NHOBoc, and −C(O)OH).
Unfortunately, it was found that even the most labile benzene
derivative, [2a](SbF6)2, exhibited very low reactivity (Scheme
5). For example, the reaction of acetanilide with 1-phenyl-1-
propyne in dichloromethane at a 5.0 mol % loading of
[2a](SbF6)2 gave the corresponding indole derivative in 34%
yield, which is comparable with that for the classical catalyst
However, usually this protocol does not work in water.12,15
Even the water-gas shift reaction could not provide such a
transformation in pure water.29 Recently, we have reported the
first example of such a transformation in water, which is
important since it is a highly accessible, nontoxic, and
nonflammable solvent.10 Herein, we report that the new
rhodium arene complexes are the most active species in such
reactions.
As a model reaction we chose the reaction between p-
tolylaldehyde and p-anisidine (Table 3). Since 1 mol % of the
iodide complex 1 was shown to give full conversion in such a
transformation,10 we decided to decrease the catalyst loading
to 0.4 mol %. Complex 1 gave only a 34% yield of the desired
product. Screening of various indenyl rhodium complexes with
arene ligands showed higher activity, in general (entries 2−8 vs
entry 1). The mesitylene complex [2c](SbF6)2 led to an 84%
yield, and the use of the durene and [2.2]paracyclophane
complexes [2d](SbF6)2 and [2g](SbF6)2 respectively led to
80% and 81% yields. The dimeric complex [3](SbF6)4 also
exhibited relatively high catalytic activity and furnished the
product in 83% yield. However, the use of the hexamethyl-
32
[Cp*RhCl2]2 but is lower than that for the functionalized
E
DOI: 10.1021/acs.organomet.8b00311
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Article
Scheme 5. Attempted C−H Activation Reactions
Figure 4. Products synthesized by reductive amination using carbon
monoxide as the reducing agent and [2c](SbF6)2 as the catalyst.
Conditions: 120 °C, 30 bar of CO. Yields of amines are given for 1.0
mol % catalyst loading with the exception of 4-methoxy-N-(4-
methylbenzyl)aniline, for which 0.4 mol % of the catalyst was
employed.
similar pattern was observed earlier for (indenyl)iridium and
(cyclohexadienyl)rhodium complexes.24c,35
CONCLUSIONS
■
The reaction of the iodide complex [(η5-indenyl)RhI2]n (1)
with arenes in the presence of AgSbF6 has been shown to be an
effective method for the synthesis of mono- and binuclear
indenyl rhodium complexes with arene ligands. The outcome
of the reaction strongly depends on the nature of the
counterion. For example, strongly nucleophilic anions, e.g.
acetate and triflate anions, did not afford arene complexes,
while the use of a non-nucleophilic hexafluoroantimonate
anion allowed us to prepare the desired complexes in pure
form. In accordance with the weaker Rh−C6H6 bonding in
[(η5-indenyl)Rh(C6H6)]2+ (2a) in comparison to that in
cyclopentadienyl analogs, the cation 2a readily undergoes an
arene exchange reaction. Indenyl arene complexes proved to be
the most active catalysts in the reaction of reductive amination
with carbon monoxide in water. The methodology enables full
compatibility with a range of functional groups prone to
reduction (e.g., N-benzyl, PMB, halocyclopropanes).
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out under an
argon atmosphere in anhydrous solvents, which were purified and
dried using standard procedures. Isolation of all products was carried
Figure 5. Plausible reaction mechanism. Counterions for the rhodium
species are omitted for clarity.
1
out in air. H and 13C{1H} NMR spectra were recorded on a Bruker
Avance 500 spectrometer operating at 500.13 and 125.8 MHz,
respectively. Chemical shifts are reported in ppm using the residual
signals of the solvents as internal standards. The signals of the carbon
atoms of the indenyl ligand in the 13C NMR spectra were assigned by
analogy with the (indenyl)iron complexes according to the conven-
tional numbering scheme.21e 2D ROESY NMR experiments were
performed on a Bruker Avance 600 spectrometer at 298 K. Complex
110 was prepared by the literature procedure. All other reagents were
purchased from Acros or Aldrich and used as received. Column
chromatography was carried out using Macherey−Nagel silica gel 60
(0.04−0.063 mm particle size).
indenyl derivative [(η5-indenylE)RhI2]2 (indenylE is 1-
(COOEt)-2-Me-3-benzylindenyl).1c The use of acetone or
1,2-dichloroethane as a solvent as well as the addition of
KOAc, CsOAc, or KI did not improve the yield of the product.
Similarly, the low catalytic activity of [2a](SbF6)2 was
observed for the couplings of O-Boc-phenylhydroxamic acid
with norbornene33 or benzoic acid with diphenylacetylene,34
which gave only a trace amount of the corresponding
dihydroisoquinolin-1-one or naphthalene, respectively. At the
same time, the use of an equimolar quantity of [2a](SbF6)2 in
the last reaction increased the yield of naphthalene up to
almost quantitative. Therefore, we can conclude that the low
catalytic activity in C−H activation reactions is probably
connected with the low stability of the catalytic species. A
[(η5-indenyl)Rh(arene)](SbF6)2 ([2a−e,g](SbF6)2). MeNO2 (1
mL) was added to a mixture of 1 (57 mg, 0.121 mmol), AgSbF6 (83
mg, 0.241 mmol), and arene (0.5 mL of benzene, o-xylene, or
mesitylene; 30 mg of durene, hexamethylbenzene, or [2.2]-
paracyclophane). The reaction mixture was vigorously stirred for 1
h, and the precipitate of AgI was centrifuged off. Then an excess of
ether was added. The precipitate that formed was washed with
F
DOI: 10.1021/acs.organomet.8b00311
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
CH2Cl2 (2 × 2 mL), reprecipitated from nitromethane by ether, and
dried in vacuo. Compounds [2a−g](SbF6)2 were obtained as an pale
yellow solids.
C6H2Me3), 18.0 (s, C6H2Me3). Anal. Calcd for C39H42F24Rh2Sb4: C,
28.22; H, 2.56. Found: C, 28.39; H, 2.54.
General Procedure for Catalytic Reductive Amination.
[2c](SbF6)2 (1.0 mol %), p-anisidine (150 mol %), and p-
tolualdehyde (100 mol %) were charged into a glass vial in a 10
mL stainless steel autoclave. A 0.2 mL portion of water was added,
and the autoclave was sealed, flushed three times with 10 bar of CO,
and then charged with 30 bar of CO. The reactor was placed into an
oil bath preheated to 120 °C. After 22 h of heating, the reactor was
cooled to room temperature and depressurized. The reaction mixture
was transferred into a flask, and the autoclave was washed with
dichloromethane (2 × 1 mL); the product was extracted with
dichloromethane (3 × 1 mL), the combined organic layers were
filtered through a silica gel pad, and the solvent was removed on a
rotary evaporator. The residue was purified by column chromatog-
raphy.
Computational Details. The geometries have been optimized
without constraints at the gradient corrected DFT level of theory
using the exchange functional of Becke36 and the correlation
functional of Perdew37 (BP86). An all-electron triple-ζ basis set
augmented by one polarization function TZP was used. The bonding
interactions were studied by means of Morokuma−Ziegler energy
decomposition analysis.38 The calculations were carried out using the
ADF 2010.01 program package.39
[2a](SbF6)2: arene = benzene, yield 49 mg (53%). 1H NMR
(CD3NO2): δ 8.13 (m, 2H, C9H7), 8.02 (m, 2H, C9H7), 7.41 (s, 6H,
C6H6), 7.40 (m, 2H, C9H7), 6.64 (m, 1H, C9H7). 13C NMR
(CD3NO2): δ 142.8 (s, C4/7), 127.6 (s, C5/6), 111.2 (d, 1JRh−C = 6.3
Hz, C8/9), 108.9 (d, 1JRh−C = 5.0 Hz, C6H6), 95.0 (d, 1JRh−C = 7.5 Hz,
1
C2), 92.3 (d, JRh−C = 7.5 Hz, C1/3). Anal. Calcd for
C15H13F12RhSb2: C, 23.47; H, 1.71. Found: C, 23.86; H, 1.81.
[2b](SbF6)2: arene = o-xylene, yield 68 mg (71%). 1H NMR
(CD3NO2): δ 8.28 (m, 2H, C9H7), 7.80 (m, 2H, C9H7), 7.43 (m, 2H,
C6H4Me2), 7.31 (m, 2H, C6H4Me2), 7.23 (m, 2H, C9H7), 6.61 (m,
1H, C9H7), 1.96 (s, 6H, C6H4Me2). 13C NMR (CD3NO2): δ 141.9 (s,
1
C4/7), 126.1 (s, C5/6), 124.5 (d, JRh−C = 3.8 Hz, C6H4Me2), 110.1
1
1
(d, JRh−C = 6.3 Hz, C8/9), 109.2 (d, JRh−C = 5.0 Hz, C6H4Me2),
1
1
104.9 (d, JRh−C = 5.0 Hz, C6H4Me2), 95.2 (d, JRh−C = 7.5 Hz, C2),
91.7 (d, JRh−C = 7.5 Hz, C1/3), 17.7 (s, C6H4Me2). Anal. Calcd for
1
C17H17F12RhSb2: C, 25.66; H, 2.15. Found: C, 25.71; H, 2.20.
[2c](SbF6)2: arene = mesitylene, yield 64 mg (66%). 1H NMR
(CD3NO2): δ 8.19 (m, 2H, C9H7), 7.89 (m, 2H, C9H7), 7.15 (m, 2H,
C9H7), 7.02 (s, 3H, C6H3Me3), 6.59 (m, 1H, C9H7), 2.46 (s, 9H,
C6H3Me3). 13C NMR (CD3NO2): δ 140.5 (s, C4/7), 125.2 (s, C5/6),
124.7 (d, 1JRh−C = 5.0 Hz, C6H3Me3), 108.8 (d, 1JRh−C = 6.3 Hz, C8/
1
1
9), 104.8 (d, JRh−C = 5.0 Hz, C6H3Me3), 94.5 (d, JRh−C = 7.5 Hz,
C2), 91.0 (d, 1JRh−C = 7.5 Hz, C1/3), 19.5 (s, C6H3Me3). Anal. Calcd
for C18H19F12RhSb2: C, 26.69; H, 2.37. Found: C, 26.84; H, 2.48.
[2d](SbF6)2: arene = durene, yield 56 mg (56%). 1H NMR
(CD3NO2): δ 8.26 (m, 2H, C9H7), 7.76 (m, 2H, C9H7), 7.27 (s, 2H,
C6H2Me4), 7.18 (m, 2H, C9H7), 6.43 (m, 1H, C9H7), 2.30 (s, 12H,
C6H2Me4). 13C NMR (CD3NO2): δ 138.6 (s, C4/7), 123.2 (s, C5/6),
119.4 (d, 1JRh−C = 4.0 Hz, C6H2Me4), 106.7 (d, 1JRh−C = 6.0 Hz, C8/
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00311.
■
S
Crystallographic data and NMR spectra (PDF)
Cartesian coordinates for optimized structures (XYZ)
1
1
9), 106.3 (d, JRh−C = 6.0 Hz, C6H2Me4), 94.4 (d, JRh−C = 8.0 Hz,
C2), 89.1 (d, 1JRh−C = 8.0 Hz, C1/3), 16.1 (s, C6H2Me4). Anal. Calcd
for C19H21F12RhSb2: C, 27.70; H, 2.57. Found: C, 28.08; H, 2.40.
Accession Codes
CCDC 1838580−1838582 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
[2e](SbF6)2: arene = hexamethylbenzene, yield 89 mg (87%). H
NMR (CD3NO2): δ 8.23 (m, 2H, C9H7), 7.72 (m, 2H, C9H7), 6.86
(m, 2H, C9H7), 6.36 (m, 1H, C9H7), 2.24 (s, 18H, C6Me6). 13C NMR
(CD3NO2): δ 140.9 (s, C4/7), 124.9 (s, C5/6), 120.4 (d, 1JRh−C = 5.0
1
1
Hz, C6Me6), 108.6 (d, JRh−C = 6.3 Hz, C8/9), 97.3 (d, JRh−C = 7.5
1
Hz, C2), 92.3 (d, JRh−C = 7.5 Hz, C1/3), 18.4 (s, C6Me6). Anal.
Calcd for C21H25F12RhSb2: C, 29.61; H, 2.96. Found: C, 29.48; H,
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.A.L.: dloginov@ineos.ac.ru.
ORCID
Yulia V. Nelyubina: 0000-0002-9121-0040
Denis Chusov: 0000-0001-6770-5484
Dmitry A. Loginov: 0000-0002-7512-5057
2.97.
■
1
[2g](SbF6)2: arene = [2.2]paracyclophane, yield 74 mg (69%). H
NMR (CD3NO2): δ 8.00 (m, 2H, C9H7), 7.86 (m, 2H, C9H7), 7.13
(m, 2H, C9H7), 6.91 (s, 4H, C6H4), 6.31 (s, 4H, C6H4), 6.26 (m, 1H,
C9H7), 3.37 (m, 8H, CH2). 13C NMR (CD3NO2): δ 143.7 (d, 1JRh−C
= 2.5 Hz, C6H4), 140.9 (s, C6H4), 140.2 (s, C4/7), 135.6 (s, C6H4),
1
1
126.4 (s, C5/6), 108.1 (d, JRh−C = 6.3 Hz, C8/9), 99.7 (d, JRh−C
=
6.3 Hz, C6H4), 91.6 (d, 1JRh−C = 7.5 Hz, C2), 90.5 (d, 1JRh−C = 7.5 Hz,
C1/3), 35.4 (s, CH2), 33.9 (s, CH2). Anal. Calcd for
C25H23F12RhSb2: C, 33.41; H, 2.69. Found: C, 33.73; H, 2.77.
[(μ-η:η′-1,3-dimesitylpropane){Rh(η5-indenyl)}2](SbF6)4 ([3]-
(SbF6)4). MeNO2 (1 mL) was added to a mixture of 1 (85 mg,
0.180 mmol), AgSbF6 (124 mg, 0.360 mmol), and 1,3-dimesitylpro-
pane (25 mg, 0.090 mmol). The reaction mixture was vigorously
stirred for 1 h, and the precipitate of AgI was centrifuged off. Then an
excess of ether was added. The precipitate that formed was washed
with CH2Cl2 (2 × 2 mL), reprecipitated from nitromethane by ether,
and dried in vacuo. Compound [3](SbF6)4 was obtained as an orange
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Russian Foundation for Basic
Research (grant no. 16-33-60140 mol_a_dk, 18-03-00839 A)
and the Council of the President of the Russian Federation
(Grant for Young Scientists No. MK-520.2017.3). The
publication has been prepared with the support of the
RUDN “5-100”. The contribution of the Center of Molecular
Composition Studies of INEOS RAS is gratefully acknowl-
edged.
1
solid. Yield: 119 mg (80%). H NMR (CD3NO2): δ 8.27 (m, 4H,
C9H7), 7.84 (m, 4H, C9H7), 7.17 (s, 4H, C6H2Me3), 7.10 (m, 4H,
C9H7), 6.54 (m, 2H, C9H7), 2.53 (s, 6H, C6H2Me3), 2.51 (m, 4H,
CH2), 2.39 (s, 12H, C6H2Me3), 1.7 (m, 2H, CH2). 13C NMR
(CD3NO2): δ 140.5 (s, C4/7), 124.7 (s, C5/6), 123.3 (d, 1JRh−C = 3.8
Hz, C6H2Me3), 122.4 (d, 1JRh−C = 5.0 Hz, C6H2Me3), 118.3 (d, 1JRh−C
REFERENCES
■
1
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1JRh−C = 5.0 Hz, C6H2Me3), 95.0 (d, JRh−C = 7.5 Hz, C2), 91.1 (d,
1JRh−C = 7.5 Hz, C1/3), 27.8 (s, CH2), 24.4 (s, CH2), 19.7 (s,
G
DOI: 10.1021/acs.organomet.8b00311
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