Ruthenium Azocarboxamide Half-Sandwich Complexes: Influence of the Coordination Mode on the Electronic Structure and Activity in Base-Free Transfer Hydrogenation Catalysis

Article abstract of DOI:10.1021/acs.organomet.6b00424

Azocarboxamides were used as chelating ligands in ruthenium half-sandwich complexes. The synthesis and characterization of two new complexes with an unprecedented coordination motif are presented together with an in-depth investigation of two recently published complexes. Three different coordination modes of the ligands were realized, as evident by NMR spectroscopy and single-crystal X-ray diffraction. The use of base during the synthesis leads to a coordination of a deprotonated ligand, while the introduction of additional donor atoms results in a noncoordinated amide group. The first systematic experimental (cyclic voltammetry and UV-vis-NIR and EPR spectroelectrochemistry) and theoretical (DFT) investigation of the electronic structure of metal complexes bearing this redox-active ligand class is presented, revealing redox processes with ligand contribution. The absorption spectra and electrochemistry are mainly determined by the protonation state of the ligand. While complexes 2[PF₆], 3[PF₆], and 4[PF₆] with neutral azocarboxamides show similar electronic spectra and cyclovoltammograms, the incorporation of a deprotonated monoanionic ligand in complex 1 leads to significant changes of these properties. In contrast, the catalytic activity in the base-free transfer hydrogenation reaction is mainly dependent on the coordination of the amide group, with only minor effects of the protonation state. While complexes 3[PF₆] and 4[PF₆], with an uncoordinated amide group, are inactive without the addition of base, complexes 1 and 2[PF₆], with a metal-bound amide group, show activity under base-free conditions. The impact of the position of the amide group together with the detection of metal hydride species in ¹H NMR spectroscopy suggests the operation of metal-ligand bifunctional catalysis to take place when no base is added.

Full text of DOI:10.1021/acs.organomet.6b00424

Article
pubs.acs.org/Organometallics
Ruthenium Azocarboxamide Half-Sandwich Complexes: Influence of
the Coordination Mode on the Electronic Structure and Activity in
Base-Free Transfer Hydrogenation Catalysis
†
,‡
†
‡
§
†,‡
Michael G. Sommer, Sofiya Marinova, Michael J. Krafft, Damijana Urankar, David Schweinfurth,
‡
,§
,†,‡
Martina Bubrin, Janez Kosm
̌
rlj,* and Biprajit Sarkar*
†
Institut fu
̈
r Chemie und Biochemie, Anorganische Chemie, Freie Universitat
Institut fur Anorganische Chemie, Universitat
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Vecn
S Supporting Information
̈
Berlin, Fabeckstraße 34-36, D-14195 Berlin, Germany
‡
̈
̈
Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
a pot 113, SI-1000, Ljubljana, Slovenia
§
̌
*
ABSTRACT: Azocarboxamides were used as chelating ligands in ruthenium
half-sandwich complexes. The synthesis and characterization of two new
complexes with an unprecedented coordination motif are presented together
with an in-depth investigation of two recently published complexes. Three
different coordination modes of the ligands were realized, as evident by NMR
spectroscopy and single-crystal X-ray diffraction. The use of base during the
synthesis leads to a coordination of a deprotonated ligand, while the introduction
of additional donor atoms results in a noncoordinated amide group. The first
systematic experimental (cyclic voltammetry and UV−vis−NIR and EPR
spectroelectrochemistry) and theoretical (DFT) investigation of the electronic
structure of metal complexes bearing this redox-active ligand class is presented,
revealing redox processes with ligand contribution. The absorption spectra and electrochemistry are mainly determined by the
protonation state of the ligand. While complexes 2[PF ], 3[PF ], and 4[PF ] with neutral azocarboxamides show similar
6
6
6
electronic spectra and cyclovoltammograms, the incorporation of a deprotonated monoanionic ligand in complex 1 leads to
significant changes of these properties. In contrast, the catalytic activity in the base-free transfer hydrogenation reaction is mainly
dependent on the coordination of the amide group, with only minor effects of the protonation state. While complexes 3[PF6]
and 4[PF ], with an uncoordinated amide group, are inactive without the addition of base, complexes 1 and 2[PF ], with a
6
6
metal-bound amide group, show activity under base-free conditions. The impact of the position of the amide group together with
1
the detection of metal hydride species in H NMR spectroscopy suggests the operation of metal−ligand bifunctional catalysis to
take place when no base is added.
INTRODUCTION
substrate is reduced after direct coordination to the metal
center of the catalyst.
■
6
,7
1
The reduction of unsaturated molecules containing CC or
2
In general a strong base, such as KOH or KOtBu, is required
as additional reactant for the reaction to proceed. However, a
limited number of systems are known where no addition of
CO double bonds by transition metal catalyzed transfer
hydrogenation is a field of great interest in current research.
This approach allows for mild reaction conditions without the
need of molecular hydrogen under high pressures. In addition,
the commonly used hydrogen source iPrOH is inexpensive and
nontoxic, and the moderate boiling point eases the separation
from reaction products. Ruthenium(II) complexes with a
variety of chelating ligands, e.g., phosphines, amines, or
8
base is needed. This activity without additional base is
achieved by introducing rather basic positions in the ligand, e.g.,
amides or carbanionic moieties. Base-free conditions are
advantageous due to the possibility of reducing base-labile
substrates and the lack of corrosive properties of the reaction
8h
3
mixture.
carbenes, have been widely used for this type of reaction.
Azo compounds are widely used as redox-active ligands in
It has been shown that the reactivity and selectivity of
catalysts for the reduction of ketones can be improved by the
introduction of a N−H moiety in the ligand close to the metal
9
coordination chemistry. The special case of azocarboxamides
allows for an additional coordination of the amide oxygen as
well as the (deprotonated) amide nitrogen atom (Chart 1).
These possibilities of changing the coordination motif and
therefore studying the influence of the position and protonation
4
center. In this case a metal−ligand bifunctional catalysis takes
place. In this so-called “outer-sphere” mechanism the substrate
oxygen atom is not directly coordinated to the metal center but
rather to the acidic hydrogen of the ligand.
2a,c,5
This behavior is in contrast to the “inner-sphere” mechanism,
which is observed in similar catalytic systems. Here the
Received: May 26, 2016
6
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00424
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Article
Chart 1. Azocarboxamides L1−L3, Used As Ligands in This
Work
Electronic and geometric structures of these metal complexes
are reported. The activity of these complexes as (pre)catalysts
for transfer hydrogenation, particularly under base-free
conditions, is discussed.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthesis and
characterization of complexes 1 and 2[PF ] have been
6
11
published elsewhere. Complexes 3[PF ] and 4[PF ] were
6
6
obtained by the reaction of [RuCl (cym)] (cym = p-cymene)
2
2
with L2 and L3, respectively, in the presence of NH PF in dry
4
6
acetonitrile under an inert atmosphere (Scheme 1). The
syntheses are analogous to the synthesis of 2[PF ] and require
6
only small adjustments of the workup procedure (see
Experimental Section).
state of the amide group on the catalytic activity make this
ligand class interesting for applications in transfer hydro-
genation catalysis. Despite this fact, azocarboxamide ligands
have never been used as ligands in transfer hydrogenation
catalysis. In one study, a ruthenium(II) complex with an
azocarboxamide-related semicarbazone ligand was used as a
transfer hydrogenation catalyst. However, the influence of
different coordination motifs was not investigated, and an
Scheme 1. Synthesis of New Complexes 3[PF ] and 4[PF ]
6
6
10
external base was used in this case. Furthermore, the presence
of the redox-active azo group and the proton-responsive amide
group makes the investigation of the redox properties and
electronic structures of azocarboxamide-containing metal
complexes a topic of current interest, as these might in
principle be used to generate intriguing chemical reactivity.
We present here the investigations of the aforementioned
properties of four ruthenium cymene complexes, 1−4[PF ]
6
11
(Chart 2), two new and two recently published by us, with
Chart 2. Ruthenium Complexes Discussed in This Work
1
H NMR spectroscopy of 3[PF ] and 4[PF ] shows a
6
6
significant downfield shift of the heterocycle signals compared
to the free ligands. This indicates the coordination of the
ligands via a heterocycle-nitrogen atom. Single crystals of these
complexes could be obtained by slow evaporation of the solvent
of a DCM/hexane solution of the compounds.
The X-ray crystal structure analysis of 3[PF ] confirmed the
6
ligand coordination in a N(azo)-N(heterocycle)-chelating
fashion (Figure 1). The complex shows a half-sandwich piano
stool geometry. This coordination motif is completed by a
azocarboxamide ligands L1−L3 (Chart 1). The previously
published complexes contained L1 as ligand, which does not
contain any heterocycles on the ligand backbone. In the present
work, we use two further azocarboxamide ligands that contain
additional heterocycles (L2 and L3). It is shown that the
introduction of additional donor atoms results in an
unprecedented coordination motif of azocarboxamides. The
effects of the protonation state of the amide as well as different
coordination modes on the complex properties are discussed.
Figure 1. ORTEP plot of 3[PF ] at 50% probability. Counterions and
H atoms are omitted for clarity.
6
B
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6
+
chlorido and η -cymene ligand. The amide nitrogen and oxygen
Table 1. Electrochemical Potentials vs Fc/Fc of Complexes
and Ligands in V
atoms are not coordinated in this complex. The quality of the
crystals of 4[PF ] was poor; therefore bond lengths are not
discussed. However, the coordination mode of the ligand is
clearly seen to be the same as in 3[PF ] (Figure S1). This is in
6
a
a
a
a
compd (solvent)
E1/2
Epc1
Epa1
Epc2
Epa2
L1 (DCM)
L2 (DCM)
L3 (DCM)
−1.33
−1.16
−0.98
−0.50
−0.61
−0.67
6
−2.00
−1.75
−1.16
−0.98
−0.60
contrast to complexes 1 and 2[PF ] with the heterocycle-free
6
1
1
ligand L1. These findings suggest a stronger interaction
between the heterocycle nitrogen and the ruthenium than that
between the amide oxygen and the metal center. The bond
length between the metal center and the azo nitrogen of
.013(4) Å is slightly shorter than the bonds between the
ruthenium and the heterocycle nitrogen, at 2.074(3) Å in
11
2
3
4
1
1
[PF ] (DCM)
−0.35
−0.32
0.00
6
[PF ] (DCM)
6
[PF ] (DCM)
6
(DCM)
0.81
−1.22
−1.14
2
11
(CH CN)
0.77
−0.53
−1.25
−0.95
3
a
3
[PF ]. The NN bond within the azo group is 1.280(4) Å
For irreversible processes cathodic (Epc) and anodic (Epa) peaks are
6
II
listed separately.
for 3[PF ]. These bond lengths indicate a Ru center
6
III
coordinated to a neutral azo group, as a Ru center
coordinated to a reduced azo group would result in shorter
+
Fc for 2[PF ], 3[PF ], and 4[PF ], respectively. However,
these potentials were outside the available potential window of
the solvent. As was reported earlier, coordination of the
deprotonated ligand in metal complex 1 leads to a big change in
the electrochemical behavior. In DCM, only one irreversible
reduction wave is observed (Figure 3), while in acetonitrile the
6
6
6
9a
metal−ligand bonds and a longer NN bond. The CO
11
bond of 1.201(5) Å for 3[PF ] is close to the bond in
6
11
noncoordinated azocarboxamide.
Electrochemistry. The combination of redox-active ligands
with a redox-active metal center raises the question of redox
processes of the complexes and the investigation of the origin
of these processes. To investigate the electrochemical behavior
of the free ligands and their metal complexes, cyclic
voltammetry was employed. In dichloromethane (DCM),
ligand L1 shows one irreversible reduction in the available
solvent window (Figure S3). For ligands L2 and L3 two
irreversible reductions are observed (Figures S4 and S5). Azo
groups are known to undergo a two-step reduction. The
appearance of a second reduction within the available potential
window can be explained by the attachment of the electron-
withdrawing heterocycles to the redox-active azo group.
Complexes 2[PF6], 3[PF ], and 4[PF ] show a first,
9a
12
11
6
6
reversible reduction and a second, irreversible reduction. A
chloride abstraction after the second reduction is a plausible
chemical reaction following the electrochemical process. Figure
Figure 3. Cyclic voltammogram of 1 in DCM/0.1 M Bu NPF at 295
K vs Fc/Fc .
4
6
+
2
shows the cyclic voltammogram of 3[PF ] as a representative
6
1
1
wave splits into two irreversible waves. This indicates that the
wave in DCM is a two-electron reduction. In addition, complex
1
11
shows a reversible oxidation in both solvents. The ease of
oxidation of this complex is likely one of the reasons for its
11
good cytotoxic properties, as reported previously. Redox
reactions are often responsible for cytotoxic behavior. The
difference between the oxidation and reduction potential of
1
.9 V (in CH CN) is in good agreement with the observation
3
of an MLCT band at 650 nm (Table S3). Electrochemical
potentials of all compounds are listed in Table 1.
To investigate the question of a possible redox-induced
proton transfer in complexes 2[PF ], 3[PF ], and 4[PF ], the
6
6
6
Figure 2. Cyclic voltammogram of 3[PF ] in DCM/0.1 M Bu NPF at
influence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) addi-
tion on the electrochemistry was investigated. While the cyclic
voltammograms of 2[PF ] and 4[PF ] did not show any well-
6
4
6
+
2
95 K vs Fc/Fc .
6
6
sample. The cyclic voltammogram of 4[PF ] is shown in the
defined waves after base addition, 3[PF ] showed a well-
defined change in the electrochemical behavior upon
incremental base addition (Figure S6).
6
6
Supporting Information (Figure S2). The reduction potentials
follow the same trend as in the free ligands (Table 1). The
coordination of the ligands to the metal centers leads to
anodically shifted reduction potentials, which reflects the
electron-withdrawing effect of the cationic metal centers. No
oxidative processes were observed for complexes 2[PF6],
While both redox waves of the initial compound decrease,
two reversible redox waves appear. These waves are anodically
shifted compared to the native compound. These results
indicate a deprotonation of the amide, followed by a chloride
abstraction, as a simple deprotonation should not lead to an
anodic shift of the redox potentials. The absence of chloride
and an acidic proton in the proposed compound is also in
accordance with the reversibility of the reduction, as chloride
3
[PF ], and 4[PF ]. The MLCT bands at 533, 502, and 516
6 6
nm (vide infra) for 2[PF ], 3[PF ], and 4[PF ], respectively,
6
6
6
together with the reduction potentials (Table 1) suggest
oxidation processes at approximately 2.0, 2.2, and 2.4 V vs Fc/
C
DOI: 10.1021/acs.organomet.6b00424
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Article
loss is a common chemical reaction after the electrochemical
reduction.
Table 2. Hyperfine Coupling Constants in MHz Used for
Simulation and Calculated at the B3LYP/def2-TZVP-ZORA
Level of Theory
EPR Spectroelectrochemistry and Spin Density Cal-
culations. The reversible redox processes were investigated
with EPR spectroelectrochemistry. The in situ one-electron-
A
•
•
•
99,101Ru
14N
14N
14N
1H
1H
reduced radicals 2 , 3 , and 4 show sharp, isotropic signals in
DCM at room temperature with g-values of 1.999, 1.998, and
2•
18.3
20.3
17.1
18.3
16.5
23.9
31.0
26.2
21.6
15.9
23.2
17.1
7.0
5.5
6.5
2.3
1.998, respectively (Figures 4, S7, and S8). In addition
DFT
3•
18.8
12.4
19.6
10.4
18.2
7.5
10.1
10.2
12.6
4.9
8.1
7.9
DFT
4•
11.2
9.8
DFT
ZORA) calculations of the one-electron-reduced complexes
that show only 4%, 6%, and 10% spin population on the
•
•
•
ruthenium for 2 , 3 , and 4 , respectively. The calculated spin
densities of the singly reduced free ligands (Figures S11, S12,
and S13) are similar to those of the respective metal complexes.
Note that small negative values can arise as artifacts when spin-
unrestricted calculations are employed. However, these small
negative values do not change the interpretation of the results.
The one-electron-reduced forms can therefore be described as
Figure 4. X-Band EPR spectrum of electrochemically in situ generated
•
3
in DCM/0.1 M Bu NPF6 at 295 K (bottom) together with
4
II
−•
[
Ru Cl(cym)(L )].
simulation (top).
In contrast, the in situ one-electron-oxidized compound 1+
•
was EPR-silent at room temperature. A frozen solution in DCM
11
hyperfine coupling was observed for these signals. In the case
showed a broad signal with high g-anisotropy. As UV−vis−
NIR spectroelectrochemistry points to a mixture of two
compounds after oxidation (vide infra), an unambiguous,
detailed interpretation of the EPR signal is not possible.
However, the observed signal likely points to a metal-centered
oxidation.
UV−Vis−NIR Spectroelectrochemistry and TD-DFT
Calculations. Compound 1 shows intense bands at 464 and
305 nm. TD-DFT calculations show that these transitions are
mixed MLCT and ILCT bands (Table S3 and Figure S17).
Upon oxidation, the intensity of absorption between 320 and
1400 nm increases, with new bands appearing at 340, 495, 770,
and 1000 nm. The investigation of the backward process
revealed a two-step re-reduction. The main spectral change of
the first step is that the bands at 770 and 1000 nm vanish. The
second step eventually restores the starting spectrum to 90%
•
14
of 2 only hyperfine coupling to the nitrogen atoms ( N, I = 1)
9
9,101
is resolved and Ru satellites (
Ru, I = 5/2, combined
abundance of about 30%) are observed. This spectrum without
1
1
•
simulation has already been published. The EPR signals of 3
•
1
and 4 show additional coupling to two or one proton ( H, I =
1/2), respectively. The number of proton hyperfine interactions
agrees with the spin density calculations that show significant
spin density on two of the heterocycle carbon atoms (Figures 5,
•
S9, and S10), one of which is substituted with chlorine in 4 .
Parameters used for simulation of the spectra are listed in Table
2.
These characteristics of the signals reveal metal-bound
organic radical character and therefore point to ligand-centered
reduction. This is supported by DFT (B3LYP/def2-TZVP-
(
Figures S14, S15, and S16). This indicates that the oxidation
of 1 leads to a mixture of two compounds, which both can be
reduced to 1. As the nature of these two compounds is unclear,
DFT calculations of these compounds were not performed.
The cyclic voltammogram revealed that the reductions of 1 are
irreversible, and therefore no spectroscopic investigation of
these processes was done.
Complex 2[PF ] shows an MLCT (as assigned by TD-DFT,
6
Tables S4, S5 and Figures S18, S19) band at 533 nm that is
shifted to 515 nm upon reduction (Figure 6, Table 3). This is
in agreement with the ligand-centered reduction, making the
ligand less electron accepting. A second intense band at 406 nm
shows mixed ILCT/MLCT character and is shifted to 354 nm
after the one-electron reduction. The change in the
coordination mode when going to 3[PF ] has only a minor
6
effect on the energy of the MLCT band, now lying at 502 nm.
However, the transition at higher energy is significantly shifted
to 295 nm. In addition, a shoulder at 720 nm is observed. This
transition was found to be of mixed MLCT/ILCT character by
TD-DFT. When reducing the compound, the band at 720 nm
gains intensity and is shifted to 690 nm. The most prominent
change in the spectrum is the appearance of a new band at
•
Figure 5. Spin density of 3 calculated at the B3LYP/def2-TZVP-
ZORA level of theory represented at an isovalue of 0.005. Blue color
and red color indicate positive and negative (artifacts due to spin
polarization in spin-unrestricted calculations) sign, respectively. Atoms
are labeled with their respective spin population (Lo
̈
wdin, threshold
5
%).
D
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reactivity, complexes 1−4[PF ] were tested for their ability to
6
catalyze transfer hydrogenation reactions (Table 4). The
Table 4. Comparison of Activities of Different Catalysts with
a
and without Base
b
[
Ru]
loading
time
base
yield /%
1
0.5 mol %
0.5 mol %
0.5 mol %
0.5 mol %
0.5 mol %
0.5 mol %
0.5 mol %
0.5 mol %
0.5 mol %
0.5 mol %
0.1 mol %
0.1 mol %
0.1 mol %
0.1 mol %
1 h
0.2 equiv KOH
70
25
70
10
90
0
1
2
2
3
3
4
4
1
2
1
1
2
2
1 h
[PF6]
[PF6]
[PF6]
[PF6]
[PF6]
[PF6]
1 h
0.2 equiv KOH
0.2 equiv KOH
0.2 equiv KOH
1 h
1 h
1 h
1 h
40
0
1 h
19 h
19 h
65 h
65 h
65 h
65 h
>99
70
95
95
85
85
[PF6]
0.2 equiv KOH
0.2 equiv KOH
[PF6]
[PF6]
a
Reaction conditions: substrate concentration of 0.125 mol/L in
iPrOH at 100 °C with the indicated loading of the respective catalyst
b
for the indicated time; base added where indicated. Yield determined
1
by H NMR spectroscopy; hexamethylbenzene used as standard.
reduction of benzophenone to diphenylmethanol with iPrOH
as the hydrogen source was examined for all complexes. A
solution of benzophenone and 0.5 mol % of the catalyst in
iPrOH was heated to 100 °C for 1 h in the presence and
absence of 0.17 equiv of KOH as an external base. When
adding KOH to the reaction mixture, all complexes showed
activity in the transfer hydrogenation reaction. While 1 and
2
3
4
[PF ] both resulted in a 70% yield of diphenylmethanol,
6
[PF ] has a higher activity and leads to a yield of 90% and
6
[PF ] shows a lower activity and only 40% yield. Regarding
6
Figure 6. UV−vis−NIR spectral changes during the first reduction of
the observation of identical activity of 1 and 2[PF ], which was
also found for the reduction of acetophenone in the presence of
base with two different catalyst loadings (Table 5), it is
6
2
[PF ] (top), 3[PF ] (middle), and 4[PF ] (bottom) in DCM/0.1 M
6 6 6
Bu NPF at 295 K (OTTLE spectroelectrochemistry).
4
6
noteworthy that in previous experiments 2[PF ] could be
converted to 1 by the addition of base. We therefore assume
the formation of the same active species by the reaction of 1
6
Table 3. UV−Vis−NIR Data of the Complexes Measured in
11
DCM/0.1 M Bu NPF at 295 K (OTTLE
4
6
Spectroelectrochemistry)
and 2[PF ] with an excess of base.
6
1
compd
λmax/nm (ε/M− cm−1
406 (15 400); 533 (7720)
354 (18 000); 515 (7100)
)
When no external base was used, the reactivity changed
drastically. Complexes 3[PF ] and 4[PF ], in which the amide
2
2
3
3
4
4
[PF ]
6
6
6
•
group is not coordinated to the metal center, do not show
catalytic activity at all. In contrast, when the amide group of the
ligand is coordinated to the metal center, as in 1 and 2[PF6],
the resulting complexes do not require the addition of external
base in order to be active in transfer hydrogenation catalysis. As
expected when deprotonating L1 before the coordination, the
complex shows a higher activity in the base-free transfer
hydrogenation reaction. As a general observation, the addition
of external base accelerates the reaction. Nevertheless, by
increasing the reaction time with complex 1 a quantitative yield
of the reduction product can be achieved. To compare the
maximum turnover numbers (TON) of (pre)catalysts 1 and
[PF ]
295 (11 100); 347 (sh); 502 (5800); 720 (sh)
295 (11 100); 345 (12 800); 550 (sh); 690 (1200)
290 (18 300); 516 (11 870); 780 (sh)
6
•
[PF ]
6
•
290 (18 300); 358 (19 800); 532 (8330) 810 (1260)
•
3
45 nm. The spectra of 4[PF ] and 4 show similar bands to
6
•
those of 3[PF ] and 3 with the general trend of a shift to lower
6
energy (Figure 6, Table 3). This is in agreement with L3 being
12
more electron deficient than L2. A detailed analysis of TD-
DFT data, including pictures of the contributing orbitals, is
available in the Supporting Information.
2[PF ], the reduction of benzophenone with 0.1 mol % of
6
Transfer Hydrogenation. To investigate the effects of the
different coordination modes and ligand substituents on the
metal complex was allowed to react for 65 h both with and
without the addition of KOH. Under these conditions there
E
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d
Table 5. Comparison of Catalysts for Reduction of Acetophenone and Substrate Scope for Base-Free Transfer Hydrogenation
a
b
1
c
Determined by GC-MS; hexadecane used as standard. Determined by H NMR; hexamethylbenzene used as standard. Isolated yield; substrate
d
concentration of 0.03 mol/L. Reaction conditions: substrate concentration of 0.125 mol/L in iPrOH at 100 °C with the indicated loading of the
respective catalyst for the indicated time; base added where indicated.
was no significant difference between the basic and base-free
conditions. Complex 1 showed a moderately higher TON of
hydride species that are observed in the catalytic system of the
present study (vide infra) may give rise to the same reactivity.
The fact that 1-phenylbut-1-ene was isolated as the main
product strongly indicates that CC bonds are only reduced
when conjugated to a carbonyl group by the employed catalytic
system. This points to an involvement of the carbonyl group
(possibly a 1,4-type of reaction) in the reduction of the CC
bond when fully reduced 4-phenylbutan-2-ol is formed.
9
50 than 2[PF ] (TON = 850) (Table 4). As we were
6
particularly interested in the investigation of base-free transfer
hydrogenation, the substrate scope for this reaction with
complex 1, the most active compound under base-free
conditions, was examined. It was found that 1 can efficiently
catalyze the reduction of ketones as well as aldehydes. When
using α,β-unsaturated ketone 4-phenyl-3-buten-2-one as the
substrate, a mixture of products was obtained. 1-Phenylbut-1-
ene was obtained in 65% yield along with 4-phenylbutan-2-ol in
The activity of the base-free catalytic system shows activity of
8e,g
the same order of magnitude as recently studied catalysts
under base-free conditions. However, this is only a moderate
activity compared to the original Noyori catalysts (comparable
30% yield (Table 5). The formation of 1-phenylbut-1-ene is
15
possibly the result of a dehydration of allylic alcohol formed by
transfer hydrogenation to the carbonyl group of 4-phenyl-3-
buten-2-one. This reaction has been observed in the treatment
of allylic alcohols with the hydridic reagent cyanoborohy-
turnover frequencies at 28 °C).
When moving to the N-alkylation with alcohols, a reaction
2g,16
observed under various conditions,
without an external base
the formation of imine in 60% yield was found with only traces
of the N-alkylated amine product. When adding KOtAmylate, a
base that leads to moderate yields in the reaction even without
an additional catalyst (Table 6), the alkylated amine was
1
3
dride. The carbonyl to methylene reduction in α,β-
unsaturated ketones has also been observed when using
1
4
13
silanes or cyanoborohydride as hydride source. Ruthenium
F
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Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 6. N-Alkylation of Aniline with Base and under Base-
Free Conditions
formation of the same metal hydride compound for both metal
complexes (Figure S24).
a
NMR studies revealed that 2[PF ] converts to 1 in solution.
6
1
13
Upon dissolution of 2[PF ] in CD OD at 23 °C, H, C, and
6
3
15
11
N NMR resonances characteristic of 1 slowly started to
appear (Figures S29 and S30). Within 29 h, 45% conversion
was noted, as judged by integration. In CD Cl solution, on the
2
2
b
other hand, complexes 1 and 2[PF ] were found to be stable
[
Ru]
loading
time
base
yield /% a; b
6
for several days with no detectable interconversion or
decomposition.
These findings suggest the formation of the same catalytic
1
1
2 mol %
1 mol %
4 h
KOtAmylate
30; 70
18 h
traces; 70
20; 35
4
h
KOtAmylate
species when using 1 and 2[PF ]. As 1 shows higher activity,
a
6
Reaction conditions: 10 equiv of benzyl alcohol, 1 equiv of aniline,
indicated loading of 1 and 0.6 equiv of base is indicated, neat reaction
at 120 °C for the indicated time. Yield determined by GC-MS;
hexadecane used as standard.
we propose the partial conversion of 2[PF ] to 1 followed by
6
b
formation of the catalytically active species.
In combination with the different activity in the presence and
absence of base, NMR experiments, i.e., the observation of a
hydride signal only for 1 and 2[PF ] under base-free
6
observed in 30% yield along with 70% of the imine; this is only
a small improvement in the activity of the base alone (Table 6).
This observation shows that complex 1 is effective for the
oxidation of benzyl alcohol and condensation with aniline, but
not for the reduction of the imine.
conditions, point to different reaction mechanisms for base-
free and base-assisted catalysis. In the case that no additional
base is present in the reaction mixture a metal-bound amide
group is crucial for the complexes to show catalytic activity. In
our opinion, the need for a metal-coordinated amide group
under base-free conditions suggests the incorporation of this
group in the catalytic cycle. Following the recently revised
outer-sphere mechanism for the transfer hydrogenation
reaction catalyzed by ruthenium arene complexes with N−H
Metal hydride species are intermediates in most proposed
catalytic cycles for transfer hydrogenation reactions. Therefore,
we addressed the question of available ruthenium hydrides in
the catalytic systems at hand. A solution of the (pre)catalyst in
iPrOH was heated to 100 °C both with and without the
addition of KOH. After 20 min the solvent was evaporated and
5d,e
ligands
we think a mechanism as shown in Scheme 2 is
1
likely. Through initial loss of a chloride anion the coordinatively
unsaturated intermediate a is formed. After stepwise uptake of a
proton and subsequently a hydride from iPrOH a ruthenium
hydride intermediate with a protonated azocarboxamide ligand
(structure c) is formed. Hydride transfer to preorganized (via
H-bonding from N−H) substrate and proton transfer from
iPrOH (path A) or the ligand (path B) leads to the formation
of the product. Tertiary amides are known to bond rather
a H NMR spectrum in dry CD Cl was recorded. When KOH
2
2
was present in the reaction mixture, no hydride signal was
detected. The same holds true for the experiments with 3[PF6]
and 4[PF ] without KOH, a system that was found to be
6
inactive in the transfer hydrogenation catalysis. However, when
1
heating 1 and 2[PF ] in iPrOH, the H NMR spectra both
6
showed a signal at −10.22 ppm, a typical chemical shift for
metal hydrides. In combination with identical signals in the
region between 4.7 and 5.7 ppm this is indicative of the
17
weakly to metal centers. This may lead to a rearrangement of
Scheme 2. Proposed Outer-Sphere Mechanism for the Base-Free Transfer Hydrogenation with 1, Based on the Revised
5d,e
Mechanism
for Noyori-Type Catalysts with N−H Ligands
G
DOI: 10.1021/acs.organomet.6b00424
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Organometallics
Article
the protonated ligand to oxygen coordination in compound e.
As in e the N−H bond is not in proximity to the hydride, this
compound is thought to be catalytically inactive (resting state).
This fact may lead to lower concentrations of catalytically active
c and can be an explanation for the only moderate activity of
this catalytic system. However, we cannot rule out the
possibility of e being the starting point of a different reaction
pathway. Theoretical calculations can be employed to further
investigate mechanistic details. However, this is beyond the
scope of this work and will be a topic for future research.
EXPERIMENTAL SECTION
■
Commercially available chemicals were used without further
purification. All solvents were dried with appropriate drying agents,
distilled, and degassed by standard techniques prior to use. H NMR
spectra were recorded with Bruker AV 250, Jeol ECS 400, and JEOL
ECZ 400R spectrometers at 25 or 20 °C. Some H, C, and
NMR spectra were recorded with a Bruker Avance III 500 MHz
instrument at 23 °C. Chemical shifts are reported in ppm relative to
tetramethylsilane or with reference to the residual solvent peaks.
chemical shifts are referenced to external ammonia. GC-MS analysis
was performed on an Agilent 7820A GC system combined with
Agilent 5977E GC/MSD (column: Agilent HP-5 ms UI, 30 m × 250
μm, 0.25 μm, p/n 190915-433UI). Data for crystal structure analysis
were collected on a Bruker Kappa Apex II Duo at 100 K and refined
1
1
13
15
N
1
5
N
CONCLUSION
■
In conclusion, we presented the comparison of two previously
1
1
18
published and two new half-sandwich ruthenium complexes
with azocarboxamide ligands. The newly presented complexes
using the SHELX program package. CCDC 969056 and 969057
contain the cif files of this work.
Cyclic voltammograms were recorded with the M 273 A
potentiostat and the M 175 function generator of Princeton Applied
Research by working in anhydrous and degassed solvents with 0.1 M
NBu PF (dried, >99.0%, electrochemical grade, Fluka) as electrolyte.
3
[PF ] and 4[PF ] show an unprecedented coordination motif
6 6
of the azocarboxamides with an uncoordinated amido group. In
11
contrast, the previously published complexes show coordina-
tion of the amide group via either the oxygen atom or the
deprotonated nitrogen atom. In total three different coordina-
tion modes of this type of ligands are compared. All complexes
were investigated electrochemically and spectroscopically. In
addition, their ability to catalyze the transfer hydrogenation
reaction, especially under base-free conditions, was tested. This
is the first systematic investigation of this redox-active ligand
class.
4
6
A three-electrode setup was used with a glassy carbon working
electrode, a coiled platinum wire as counter electrode, and a coiled
silver wire as a pseudoreference electrode. The ferrocene/ferrocenium
couple was used as internal reference.
UV−vis−NIR spectra were recorded with a TIDAS diode array
spectrometer from J&M. Measurements were carried out in an
optically transparent thin-layer electrochemical (OTTLE) cell (CaF2
windows) with a platinum-mesh working electrode, a platinum-mesh
counter electrode, and a silver-foil pseudoreference electrode in
anhydrous and degassed solvents and 0.1 M Bu NPF .
The redox behavior and electronic spectra are mainly
determined by the protonation state of the ligand. Complexes
4
6
EPR spectra were recorded with a Bruker EMX spectrometer. The
one-electron-reduced forms were generated in situ in anhydrous and
degassed solvents and 0.1 M Bu NPF with a two-electrode (platinum
bearing a neutral azocarboxamide ligand, with (2[PF ]) or
6
without (3[PF ] and 4[PF ]) the coordination of the amide
6
6
4
6
group, show similar absorption spectra, and electrochemical
reductions are observed at similar potentials. In contrast,
complex 1 with coordination of the deprotonated amide of the
monoanionic azocarboxamide ligand shows an absorption band
at lower energy, and all redox potentials are shifted cathodically.
As a result, in addition to the reductions an electrochemical
oxidation is observed for 1.
wire) configuration. Spectra were simulated with the Easyspin toolbox.
Elemental analysis was recorded on a Vario Micro Cube
(Elementaranalysensysteme GmbH).
1
1
19
20
The syntheses of ligands L1, L2, and L3 and complexes
1
1
11
[
RuCl(cym)(L1 )] (1) and [RuCl(cym)(L1)]PF (2[PF ]) have
‑
H
6
6
been presented elsewhere.
Synthesis of [RuCl(cym)(L2)]PF6 (3[PF ]). Under an inert
6
atmosphere of argon [RuCl (cym)]2 (70.0 mg, 0.114 mmol) and
2
When comparing the activity in the base-free transfer
hydrogenation reaction, a different dependence on the
NH PF (38.2 mg, 0.234 mmol) were dissolved in dry, degassed
4
6
acetonitrile (5 mL) and stirred for 1.5 h at room temperature. The
formed precipitate was filtered off, and the filtrate was added to a
solution of L2 (51.8 mg; 0.229 mmol) in dry, degassed acetonitrile
(2.5 mL). The resulting mixture was stirred at room temperature
under an inert atmosphere of argon for 18 h. The solvent was
evaporated under reduced pressure, and the resulting solid was
dissolved in DCM (approximately 4 mL). Upon addition of hexane
coordination mode is observed. Complexes 3[PF ] and
6
4
[PF ] with an uncoordinated amide group are completely
6
inactive under base-free conditions. However, both complexes
and 2[PF ] show activity for base-free transfer hydro-
1
6
genation. Air- and chromatography-stable (pre)catalyst 1,
containing a deprotonated amide-N bound to the metal center,
shows the highest activity under base-free conditions. Catalyst
loadings of 0.1 mol % were sufficient for yields up to 95% under
base-free conditions. These results show that without the
addition of external base the catalytic activity is determined by
whether or not the amide group is coordinated to the metal
center. Therefore, the incorporation of the amide group in the
catalytic process is strongly suggested by the results of this
study. In combination with the detection of a metal hydride this
points toward a ligand−metal bifunctional catalysis. It has also
been shown that the amide group can switch between O- and
N-coordination under suitable conditions. The knowledge
about the role of the amide group can be used for the design of
new catalysts.
(approximately 2 mL), the product was collected in a good yield as a
dark red solid (105.8 mg, 0.165 mmol, 72%). Single crystals of the
product, suitable for X-ray diffraction, were obtained from a saturated
solution in DCM/hexane (2:1 vol) by slow evaporation of the solvent
at room temperature and ambient pressure.
Anal. Calcd for C H ClF N OPRu: C 41.16; H 3.77; N 8.73.
2
2
24
6
4
1
Found: C 40.39; H 3.63; N 8.64. H NMR (250 MHz, acetone-d ): δ
10.22 (s, 1H, amide), 9.80 (d, J
6
3
= 4.8 Hz, 1H, pyridyl), 8.98 (d,
H−H
3
J
H−H = 7.9 Hz, 1H, pyridyl), 8.68−8.55 (m, 1H, pyridyl), 8.17−8.05
(m, 1H, pyridyl), 7.88−7.78 (m, 2H, Ph), 7.57−7.45 (m, 2H, Ph),
.38−7.26 (m, 1H, Ph), 6.70 (m, 2H, cym-Ph), 6.48−6.39 (m, 2H,
7
3
cym-Ph), 2.86 (sept, J
= 6.9 Hz, 1H, CH(CH ) ), 2.32 (s, 3H,
H−H
3 2
CH ), 1.18−1.06 (m, 6H, CH(CH ) ).
3
3 2
Synthesis of [RuCl(cym)(L3)]PF6 (4[PF ]). Under an inert
6
atmosphere of argon [RuCl (cym)]2 (50.0 mg, 0.081 mmol) and
2
We presented here a new catalyst system based on ruthenium
arene complexes with azocarboxamide ligands for base-free
transfer hydrogenation catalysis. As these ligand classes are both
proton and electron responsive, they might find use in various
other types of bifunctional catalysis.
NH PF (27.3 mg, 0.167 mmol) were dissolved in dry, degassed
4
6
acetonitrile (4 mL) and stirred for 1.5 h at room temperature. The
formed precipitate was filtered off, and the filtrate was added to a
suspension of L3 (51.8 mg; 0.229 mmol) in dry, degassed acetonitrile
(4 mL). The resulting mixture was stirred at room temperature under
H
DOI: 10.1021/acs.organomet.6b00424
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Organometallics
Article
an inert atmosphere of argon for 20 h. The solvent was evaporated
under reduced pressure, and the resulting solid was dissolved in DCM
N-Alkylation Reaction. Metal complex 1, aniline (1 equiv), and
benzyl alcohol (10 equiv) were placed in a Schlenk tube under an inert
atmosphere of nitrogen. Hexadecane (0.5 equiv) and, when indicated,
KOtAmylate (1.7 M in toluene, 0.6 equiv) were added, and the
mixture was heated to 120 °C for 18 h in the closed reaction vessel. An
aliquot (ca. 0.1 mL) was taken and passed through a plug of silica with
EtOAC (ca. 2.5 mL). The resulting sample was submitted to GC-MS
analysis. The yield was obtained by comparison of the product peak
area to the standard peak area. The area ratio was corrected with a
correction factor determined by measurement of samples with known
molar ratio. As a control experiment the reaction was performed
without catalyst under otherwise the same conditions.
(
15 mL). After addition of hexane (20 mL) and refrigeration at 0 °C
for 3 h the product was collected by filtration as a microcrystalline dark
red powder in good yield (69.0 mg, 0.102 mmol, 61%). Single crystals
for X-ray diffraction were obtained from a saturated solution in DCM/
hexane (2:1 vol) by slow evaporation of the solvent at room
temperature and ambient pressure.
Anal. Calcd for C H Cl F N OPRu: C 37.24; H 3.27; N 10.34.
2
1
22
2
6
5
1
Found: C 37.24; H 3.44; N 10.35. H NMR (250 MHz, acetone-d ): δ
6
3
1
9
0.46 (s, 1H, amide), 9.30−9.23 (m, 1H, pyridazinyl), 8.61 (d, J
=
H−H
.0 Hz, 1H, pyridazinyl), 7.89−7.82 (m, 2H, Ph), 7.58−7.48 (m, 2H,
Metal Hydride Formation. The respective metal complex (5 mg)
was dissolved in dry, degassed iPrOH (5 mL) under an inert
atmosphere of nitrogen. The resulting solution was heated to 100 °C
in a closed flask. After 20 min the solvent was evaporated under
Ph), 7.39−7.29 (m, 1H, Ph), 6.87−6.78 (m, 2H, cym-Ph), 6.62−6.55
3
(
3
m, 2H, cym-Ph), 2.88 (sept, J
= 6.9 Hz, 1H, CH(CH ) ), 2.32 (s,
H−H 3 2
3
3
H, CH ), 1.23 (d, J
= 6.9 Hz, 3H, CH(CH ) ), 1.17 (d, J
=
H−H
3
H−H
3
2
6
.9 Hz, 3H, CH(CH ) ).
3
2
reduced pressure at ambient temperature. The resulting solid was
Transfer Hydrogenation Catalysis: Benzophenone. The metal
1
dissolved in dry, degassed CD Cl2 (0.8 mL) and the H NMR
complex and, where indicated, 0.17 equiv (with respect to substrate) of
KOH were placed in a Schlenk flask together with benzophenone. The
flask was evacuated and flushed with nitrogen. Dry iPrOH was added
2
spectrum was recorded under a nitrogen atmosphere.
DFT Calculations. DFT calculations were done with the ORCA
2
2
3
.0.0 program package using the BP86 and B3LYP functional for the
(
substrate concentration 0.125 mol/L), and the closed flask was
23
geometry optimization and single-point calculations, respectively.
heated to 100 °C. After the indicated time of reaction the solvent was
evaporated under reduced pressure, hexamethylbenzene (0.1 equiv
with respect to substrate) was added, and the yield was measured by
The restricted and unrestricted DFT method were employed for
closed- and open-shell molecules, respectively. All calculations were
2
4
run with empirical van der Waals correction (D3). Convergence
criteria were set to default for the geometry optimizations (OPT) and
tight for SCF calculations (TIGHTSCF). Relativistic effects were
NMR in CDCl . The amount of diphenylmethanol was calculated by
3
the integration of the methine signal at 5.80 ppm (1H) and
comparison to the standard signal at 2.24 ppm (18H). As a control
the remaining amount of benzophenone was calculated by the integral
of the benzophenone signal at 7.80 ppm (4H).
Acetophenone. The metal complex and, where indicated, 0.17
equiv (with respect to substrate) KOH were placed in a Schlenk flask.
The flask was evacuated and flushed with nitrogen. Dry iPrOH
25
included with the zeroth-order relativistic approximation (ZORA).
26
Triple-ζ-valence basis sets (TZVPP-ZORA) were employed for all
atoms. Calculations were performed using the resolution of the
27
identity approximation with matching auxiliary basis sets. Low-lying
excitation energies were calculated with time-dependent DFT (TD-
DFT). Solvent effects were taken into account with the conductor-like
(
substrate concentration 0.125 mol/L), acetophenone (1 equiv), and
hexadecane (1 equiv) were added, and the closed flask was heated to
00 °C for the indicated time. An aliquot (ca. 0.1 mL) was taken and
28
screening model (COSMO) for all calculations. Spin densities were
29
̈
calculated according to the Lowdin population analysis. Molecular
1
orbitals and spin densities were visualized with the Molekel 5.4.0.8
passed through a plug of silica with EtOAC (ca. 2.5 mL). The resulting
sample was submitted to GC-MS analysis. The yield was obtained by
comparison of the product peak area to the standard peak area. The
area ratio was corrected with a correction factor determined by
measurement of samples with known molar ratio.
30
program.
ASSOCIATED CONTENT
Supporting Information
■
*
S
4-Bromobenzyldehyde. The metal complex was placed in a
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00424.
Schlenk flask together with 4-bromobenzaldehyde. The flask was
evacuated and flushed with nitrogen. Dry iPrOH was added (substrate
concentration 0.125 mol/L), and the closed flask was heated to
Crystallographic details, cyclic voltammograms, spectroe-
lectro-EPR and spin density calculations, spectroelectro-
UV−vis−NIR and TD-DFT calculations, and NMR
spectra (PDF)
Crystallographic data (CIF)
Cartesian coordinates for all calculated structures (XYZ)
1
00 °C. After 19 h the solvent was evaporated under reduced pressure,
hexamethylbenzene (0.1 equiv with respect to substrate) was added,
and the yield was measured by NMR in CDCl . The amount of
3
product was calculated by the integration of the methylene signal at
4
(
.67 ppm (2H) and comparison to the standard signal at 2.24 ppm
18H).
,6-Dimethylbenzyldehyde. The metal complex was placed in a
2
Schlenk flask together with 2,6-dimethylbenzaldehyde. The flask was
purged with nitrogen. Dry iPrOH was added (substrate concentration
AUTHOR INFORMATION
Corresponding Authors
E-mail: Janez.Kosmrlj@fkkt.uni-lj.si.
E-mail: Biprajit.Sarkar@fu-berlin.de.
■
*
*
0
.125 mol/L), and the closed flask was heated to 100 °C. After 19 h
the solvent was evaporated under reduced pressure, hexamethylben-
zene (0.1 equiv with respect to substrate) was added, and the yield was
measured by NMR in CDCl . The amount of product was calculated
3
Notes
by the integration of the methylene signal at 4.74 ppm (2H) and
comparison to the standard signal at 2.24 ppm (18H).
The authors declare no competing financial interest.
4
-Phenylbut-3-en-2-one. The metal complex 1 (2 mol %) and 4-
phenylbut-3-en-2-one (68.5 mg, 0.47 mmol) were placed in a Schlenk
flask. The flask was evacuated and flushed with nitrogen. iPrOH (15
mL) was added, and the closed flask was heated to 100 °C for 24 h.
The solvent was evaporated under reduced pressure, and the crude
product was purified by silica column chromatography. 1-Phenylbut-1-
ene (42 mg, 0.32 mmol, 68%) was collected with pentane as eluent,
and 4-phenylbutan-2-ol (20 mg, 0.13 mmol, 28%) was obtained with
ACKNOWLEDGMENTS
■
The Fond der Chemischen Industrie (FCI) is kindly acknowl-
edged for financial support of this work (Chemiefonds-
Stipendium for M.G.S. and D.S.). The high-performance
computing facilities of the ZEDAT at FU Berlin are
acknowledged for the access to Soroban. We are grateful to
Prof. Dr. W. Kaim for access to the instrumental facilities at the
University of Stuttgart. Financial support from the Ministry of
1
DCM/EtOAc (100/15) as eluent. H NMR spectra of the products
21
were in accordance with literature.
I
DOI: 10.1021/acs.organomet.6b00424
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Education, Science and Sport, Republic of Slovenia, the
Slovenian Research Agency (Grant P1-0230), is acknowledged.
(14) (a) Li, J.; Peng, J.; Bai, Y.; Chen, L.; Lai, G. Phosphorus, Sulfur
Silicon Relat. Elem. 2011, 186, 1621−1625. (b) Xing, P.; Zang, W.;
Huang, Z.; Zhan, Y.; Zhu, C.; Jiang, B. Synlett 2012, 23, 2269−2273.
(
15) Matsumura, K.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am.
Chem. Soc. 1997, 119, 8738−8739.
16) For selected examples see: (a) Agrawal, S.; Lenormand, M.;
REFERENCES
■
(
1) For selected examples see: (a) Zassinovich, G.; Mestroni, G.;
(
Gladiali, S. Chem. Rev. 1992, 92, 1051−1069. (b) Brunner, H.; Leitner,
W. Angew. Chem., Int. Ed. Engl. 1988, 27, 1180−1181. (c) Xue, D.;
Chen, Y. C.; Cui, X.; Wang, Q. W.; Zhu, J.; Deng, J. G. J. Org. Chem.
Martin-Matute, B. Org. Lett. 2012, 14, 1456−1459. (b) Bartoszewicz,
A.; Marcos, R.; Sahoo, S.; Inge, A. K.; Zou, X.; Martin-Matute, B.
Chem. - Eur. J. 2012, 18, 14510−14519. (c) Blank, B.; Michlik, S.;
Kempe, R. Chem. - Eur. J. 2009, 15, 3790−3799. (d) Hamid, M. H.;
Allen, C. L.; Lamb, G. W.; Maxwell, A. C.; Maytum, H. C.; Watson, A.
J.; Williams, J. M. J. Am. Chem. Soc. 2009, 131, 1766−1774.
2
005, 70, 3584−3591. (d) Ding, B.; Zhang, Z.; Liu, Y.; Sugiya, M.;
Imamoto, T.; Zhang, W. Org. Lett. 2013, 15, 3690−3693.
2) For selected examples see: (a) Noyori, R.; Yamakawa, M.;
Hashiguchi, S. J. Org. Chem. 2001, 66, 7931−7944. (b) Samec, J. S. M.;
Backvall, J. E.; Andersson, P. G.; Brandt, P. Chem. Soc. Rev. 2006, 35,
37−248. (c) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem.
(
(e) Kawahara, R.; Fujita, K.; Yamaguchi, R. J. Am. Chem. Soc. 2010,
̈
1
32, 15108−15111. (f) Prades, A.; Corberan, R.; Poyatos, M.; Peris, E.
2
Chem. - Eur. J. 2008, 14, 11474−11479. (g) Weickmann, D.; Frey, W.;
Plietker, B. Chem. - Eur. J. 2013, 19, 2741−2748.
(
2
006, 4, 393−406. (d) Baratta, W.; Benedetti, F.; Del Zotto, A.;
Fanfoni, L.; Felluga, F.; Magnolia, S.; Putignano, E.; Rigo, P.
Organometallics 2010, 29, 3563−3570. (e) Clapham, S. E.; Hadzovic,
A.; Morris, R. H. Coord. Chem. Rev. 2004, 248, 2201−2237.
17) Niklas, N.; Alsfasser, R. Dalton. Trans. 2006, 3188−3199.
18) Sheldrick, G. M. SHELXL-97, Program for Refinement of Crystal
(
Structures; University of Go
19) Cappoen, D.; Majce, V.; Uythethofken, C.; Urankar, D.; Mathys,
V.; Kocevar, M.; Verschaeve, L.; Polanc, S.; Huygen, K.; Kosmrlj, J.
Eur. J. Med. Chem. 2014, 74, 85−94.
20) Kosmrlj, J.; Kocevar, M.; Polanc, S. J. Chem. Soc., Perkin Trans. 1
998, 3917−3920.
21) (a) Rodríguez, D.; Per
̈ ̈
ttingen: Gottingen, Germany, 1997.
(
f) Ghoochany, L. T.; Farsadpour, S.; Sun, Y.; Thiel, W. R. Eur. J.
(
Inorg. Chem. 2011, 2011, 3431−3437. (g) Gnanamgari, D.; Sauer, E. L.
O.; Schley, N. D.; Butler, C.; Incarvito, C. D.; Crabtree, R. H.
Organometallics 2009, 28, 321−325.
̌
̌
(
̌
̌
(
(
3) Noyori, R. Angew. Chem., Int. Ed. 2002, 41, 2008−2022.
1
4) Takehara, J.; Hashiguchi, S.; Fujii, A.; Inoue, S.; Ikariya, T.;
(
́
ez Sestelo, J.; Sarandeses, L. A. J. Org.
Noyori, R. Chem. Commun. 1996, 233−234.
Chem. 2004, 69, 8136−8139. (b) Shi, L.; Tu, Y. Q.; Wang, M.; Zhang,
F. M.; Fan, C. A.; Zhao, Y. M.; Xia, W. J. J. Am. Chem. Soc. 2005, 127,
(
5) (a) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000,
1
2
5
1
6
22, 1466−1478. (b) Eisenstein, O.; Crabtree, R. H. New J. Chem.
013, 37, 21−27. (c) Hasanayn, F.; Morris, R. H. Inorg. Chem. 2012,
1, 10808−10818. (d) Dub, P. A.; Ikariya, T. J. Am. Chem. Soc. 2013,
35, 2604−2619. (e) Dub, P. A.; Gordon, J. C. Dalton Trans. 2016, 45,
756−6781.
1
(
7
0836−10837.
22) Neese, F. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2, 73−
8.
(
23) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Becke,
A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (c) Lee,
C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys.
(
6) Baratta, W.; Chelucci, G.; Gladiali, S.; Siega, K.; Toniutti, M.;
Zanette, M.; Zangrando, E.; Rigo, P. Angew. Chem. 2005, 117, 6370−
375.
7) (a) Baratta, W.; Herdtweck, E.; Siega, K.; Toniutti, M.; Rigo, P.
Organometallics 2005, 24, 1660−1669. (b) Baratta, W.; Schutz, J.;
Herdtweck, E.; Herrmann, W. A.; Rigo, P. J. Organomet. Chem. 2005,
90, 5570−5575. (c) Baratta, W.; Ballico, M.; Esposito, G.; Rigo, P.
Chem. - Eur. J. 2008, 14, 5588−5595.
8) For selected examples see: (a) Dong, Z. R.; Li, Y. Y.; Chen, J. S.;
1
(
2
988, 37, 785−789.
24) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys.
010, 132, 154104.
6
(
̈
(
25) van Wu
̈
llen, C. J. Chem. Phys. 1998, 109, 392−399.
(26) Pantazis, D. A.; Chen, X. Y.; Landis, C. R.; Neese, F. J. Chem.
6
Theory Comput. 2008, 4, 908−919.
27) (a) Neese, F. J. Comput. Chem. 2003, 24, 1740−1747.
b) Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Chem. Phys.
009, 356, 98−109. (c) Vahtras, O.; Almlof, J.; Feyereisen, M. W.
Chem. Phys. Lett. 1993, 213, 514−518. (d) Whitten, J. L. J. Chem. Phys.
973, 58, 4496−4501.
28) (a) Klamt, A.; Schu
993, 799−805. (b) Sinnecker, S.; Rajendran, A.; Klamt, A.;
(
(
(
Li, B. Z.; Xing, Y.; Gao, J. X. Org. Lett. 2005, 7, 1043−1045. (b) Elliott,
A. G.; Green, A. G.; Diaconescu, P. L. Dalton Trans. 2012, 41, 7852−
2
̈
7
̃
854. (c) Esteruelas, M. A.; García-Yebra, C.; Onate, E. Organo-
1
metallics 2008, 27, 3029−3036. (d) He, L. P.; Chen, T.; Xue, D. X.;
(
̈ ̈
urmann, G. J. Chem. Soc., Perkin Trans. 2
Eddaoudi, M.; Huang, K. W. J. Organomet. Chem. 2012, 700, 202−206.
1
(
e) Kumar, M.; DePasquale, J.; White, N. J.; Zeller, M.; Papish, E. T.
Diedenhofen, M.; Neese, F. J. Phys. Chem. A 2006, 110, 2235−2245.
Organometallics 2013, 32, 2135−2144. (f) Ruff, A.; Kirby, C.; Chan, B.
C.; O’Connor, A. R. Organometallics 2016, 35, 327−335. (g) Taghi-
zadeh Ghoochany, L.; Kerner, C.; Farsadpour, S.; Menges, F.; Sun, Y.;
Niedner-Schatteburg, G.; Thiel, W. R. Eur. J. Inorg. Chem. 2013, 2013,
(
29) Lo
̈
wdin, P. O. J. Chem. Phys. 1950, 18, 365−375.
(30) Portmann, S. Molekel, version 5.4.0.8; CSCS/UNI: Geneva,
Switzerland, 2009.
4
305−4317. (h) Carrion
B. R.; Rodríguez, A. M. Organometallics 2009, 28, 3822−3833.
i) Castarlenas, R.; Esteruelas, M. A.; Onate, E. Organometallics 2008,
, R.; Peris, E. Organometallics 2008, 27,
́
́ ́
, M. C.; Sepulveda, F.; Jalon, F. A.; Manzano,
(
2
1
(
̃
7, 3240−3247. (j) Corberan
954−1958.
9) For selected reviews and examples see: (a) Kaim, W. Coord.
́
Chem. Rev. 2001, 219−221, 463−488. (b) Das, A.; Scherer, T. M.;
Mobin, S. M.; Kaim, W.; Lahiri, G. K. Chem. - Eur. J. 2012, 18, 11007−
1
4
(
1018. (c) Samanta, S.; Ghosh, P.; Goswami, S. Dalton Trans. 2012,
1, 2213−2226.
10) Anitha, P.; Manikandan, R.; Endo, A.; Hashimoto, T.;
Viswanathamurthi, P. Spectrochim. Acta, Part A 2012, 99, 174−180.
11) Sommer, M. G.; Kureljak, P.; Urankar, D.; Schweinfurth, D.;
Stojanovic, N.; Bubrin, M.; Gazvoda, M.; Osmak, M.; Sarkar, B.;
Kosmrlj, J. Chem. - Eur. J. 2014, 20, 17296−17299.
12) Kosmrlj, J.; Kocevar, M.; Polanc, S. Synlett 2009, 2009, 2217−
235.
13) Srikrishna, A.; Viswajanani, R.; Sattigeri, J. A.; Yelamaggad, C. V.
Tetrahedron Lett. 1995, 36, 2347−2350.
(
́
̌
(
̌
̌
2
(
J
DOI: 10.1021/acs.organomet.6b00424
Organometallics XXXX, XXX, XXX−XXX