Assembled Multinuclear Ruthenium(II)-NNNN Complexes: Synthesis, Catalytic Properties, and DFT Calculations

Article abstract of DOI:10.1021/acs.organomet.9b00669

Using a coordinatively unsaturated 16-electron mononuclear ruthenium(II)-pyrazolyl-imidazolyl-pyridine complex [Ru(II)-NNN] as the building block and oligopyridines as the polydentate ligands, pincer-type tri- A nd hexanuclear ruthenium(II) complexes [Ru(II)-NNNN]_n were efficiently assembled. These complexes were characterized by elemental analyses, NMR, IR, and MALDI-TOF mass spectroscopies. In refluxing 2-propanol, the multinuclear ruthenium(II)-NNNN complexes exhibited exceptionally high catalytic activity for the transfer hydrogenation of ketones at very low concentrations and reached turnover frequencies (TOFs) up to 7.1 × 10⁶ h^-1, featuring a remarkable cooperative effect from the multiple Ru(II)-NNNN functionalities. DFT calculations have revealed the origin of the high catalytic activities of these Ru(II)-NNNN complexes.

Full text of DOI:10.1021/acs.organomet.9b00669

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Assembled Multinuclear Ruthenium(II)−NNNN Complexes:
Synthesis, Catalytic Properties, and DFT Calculations
Tingting Liu,†,‡,§ Kaikai Wu,† Liandi Wang,† Hongjun Fan,*,†,∥ Yong-Gui Zhou,*,†
and Zhengkun Yu*,†,⊥
†Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning 116023, PR China
‡University of Chinese Academy of Sciences, Beijing 100049, PR China
§Institute of Chemistry, Henan Academy of Sciences, Zhengzhou 450002, PR China
∥State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457
Zhongshan Road, Dalian 116023, PR China
⊥State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354
Fenglin Road, Shanghai 200032, PR China
S
* Supporting Information
ABSTRACT: Using a coordinatively unsaturated 16-electron mononuclear
ruthenium(II)-pyrazolyl-imidazolyl-pyridine complex [Ru(II)−NNN] as the
building block and oligopyridines as the polydentate ligands, pincer-type tri-
and hexanuclear ruthenium(II) complexes [Ru(II)−NNNN]n were efficiently
assembled. These complexes were characterized by elemental analyses, NMR, IR,
and MALDI-TOF mass spectroscopies. In refluxing 2-propanol, the multinuclear
ruthenium(II)−NNNN complexes exhibited exceptionally high catalytic activity
for the transfer hydrogenation of ketones at very low concentrations and reached
turnover frequencies (TOFs) up to 7.1 × 106 h−1, featuring a remarkable
cooperative effect from the multiple Ru(II)−NNNN functionalities. DFT
calculations have revealed the origin of the high catalytic activities of these
Ru(II)−NNNN complexes.
and co-workers developed a diruthenium Ru(II)−NNNN
complex incorporating a naphthyridine−diimine ligand as an
efficient catalyst for the acceptorless dehydrogenation of
alcohols,12b and they also synthesized a Ru(II)−CNNO
complex catalyst bearing naphthyridine-functionalized N-
heterocyclic carbene (NHC) ligands for the dehydrogenative
coupling reactions between alcohols and amines, exhibiting
both metal−metal and metal−ligand cooperation.12c Although
the preparation methods and stoichiometric reactivity of
multinuclear complexes have been extensively investigated,
less attention has been paid to their catalytic activities.13
Catalytic transfer hydrogenation (TH) reaction has been
considered as a concise method for the reduction of ketones to
the corresponding alcohols.14 Mononuclear ruthenium(II)
complexes have usually been applied as the catalysts for such
TH reactions,15−18 but multinuclear ruthenium(II) complexes
have rarely been studied in this area. During the ongoing
investigation of pincer-type Ru(II)−NNN complex catalysts
for TH reaction,19 we established several highly active pincer-
type diruthenium(II) complex catalysts,20 among which
complex A20c was assembled by means of coordinatively
unsaturated 16-electron mononuclear ruthenium(II)-pyrazolyl-
INTRODUCTION
■
In homogeneous catalysis and organic synthesis, construction
of an efficient catalyst system has been a challenging task.1
Multinuclear complex catalysts have recently been paid much
attention because they are usually expected to exhibit different
reactivity in comparison to that of the corresponding
mononuclear complexes due to the possible electronic
interactions between the metal centers and cooperative
activation of the substrates.2 In this regard, noncovalent
interactions,3 such as van der Waals, π−π stacking, metal−
ligand interactions, dipole−dipole interaction, hydrogen
bonding, and so on, have been used for the construction of
multinuclear structures inspired by many functional biological
systems, which results in fascinating catalytic,4 electro-
chemical,5 photophysical,6 magnetic,7 and host−guest8 proper-
ties. Coordination-driven assembly evolved into a useful
methodology to access multinuclear structures due to its
highly directional and predictable feature and specific
stoichiometry.9 Stang et al. established the general principle
for the construction of macrocycles using ditopic subunits.10
Anderson and co-workers used oligopyridines to assemble with
small template molecules for the direct synthesis of macro-
cyclic zinc-porphyrin nanorings.11 To date, many multinuclear
complexes have been constructed by assembly of monometallic
building blocks and a polydentate ligand,12 among which Bera
Received: October 9, 2019
© XXXX American Chemical Society
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Figure 1. Our previous work on dinuclear Ru(II)−NNNN complex catalysts.19e,20c,d
imidazolyl-pyridine complex 1a and 4,4′-(CH2)3-linked bipyr-
idine and complex B20d was constructed by using a π-linker-
supported bis(pyrazolyl-imidazolyl-pyridine) ligand (Figure 1).
As compared with corresponding mononuclear pincer-type
Ru(II)−NNN complexes 1a and 1b,19e diruthenium(II)
complexes A and B exhibit much higher catalytic activities
for the TH reactions of ketones, demonstrating a remarkable
cooperative effect from the diruthenium metal centers.
Complex A is bestowed with the most flexible molecular
structure in which the two metal centers may cooperatively
interact in the confined microenvironment and thus enhance
the catalytic activity of the complex. The high catalytic activity
of complex B is presumably attributed to its good stability and
the possible cooperativity of the two coexisting metal centers
coordinated by the coplanar NNN-heteroaryl-NNN ligand.
Due to the easy manipulation and high efficiency related to
the assembly strategy for the construction of a multinuclear
complex,9−13 we envisioned that coordinatively unsaturated
16-electron pincer-type Ru(II)−NNN complex 1a,19e as a
mononuclear complex building block, might be utilized to
assemble multinuclear Ru(II)−NNNN complexes in the
presence of polydentate nitrogen-containing ligands. Herein,
we disclose the synthesis, characterization, and catalytic
properties of the tri- and hexanuclear ruthenium(II)−NNNN
complexes assembled by using a 16-electron mononuclear
complex building block and polydentate oligopyridine ligands
(Scheme 1). DFT calculations were also conducted to
rationalize the exceptionally high catalytic activities of these
Ru(II)−NNNN complexes at very low concentrations.
Scheme 1. Strategy To Assemble Multinuclear Ru(II)−
NNNN Complexes
RESULTS AND DISCUSSION
■
Synthesis of Multinuclear Ru(II)−NNNN Pincer Com-
plexes. Oligopyridines 1,3,5-tri(pyridin-4-yl)benzene (2a)21a
and 4,4′-(5′-(4-(pyridin-4-yl)phenyl)-[1,1′:3′,1′′-terphenyl]-
4,4’’-diyl)dipyridine (2b)21b were prepared and applied to
the synthesis of trinuclear Ru(II)−NNNN pincer complexes 3
and 4, respectively, from their reactions with the mononuclear
Ru(II)−NNN complex 1a by means of the assembly strategy
as shown in Scheme 1. At ambient temperature, treatment of
complex 1a and oligopyridine 2a in a 3:1 molar ratio in the
mixed solvent of CH2Cl2/MeOH (v/v, 5/1) afforded complex
3 in 94% yield (Scheme 2). In a similar manner, the reaction of
ligand 2b with 1a efficiently formed complex 4 (91%). To
evaluate the steric and electronic impacts of the oligopyridine
ligands on the catalytic activity of the resultant multimetallic
complexes, hexanuclear Ru(II)−NNNN pincer complexes 6
and 7 were also prepared (91−92%) from the reactions of
complex 1a and oligopyridine ligands 5a and 5b21c,d in a 6:1
molar ratio (Scheme 3). In ligands 2 and 5, the linkers
(spacers) varied from terphenyl to ethynyl-bridged terphenyl.
In order to make a comparison, a relevant mononuclear
Ru(II)−NNNN pincer complex, that is, complex 9, was also
synthesized (92%) from the 1:1 molar ratio reaction of
complex 1a with 4-phenylpyridine (8):
B
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Scheme 2. Synthesis of Trinuclear [Ru(II)−NNNN]3
Complexes
Scheme 3. Synthesis of Hexanuclear [Ru(II)−NNNN]6
Complexes
and the resonance signals at 93.9 and 87.0 ppm in the 13C{1H}
NMR spectrum reveal existence of the ethynyl moiety,
suggesting the presence of ligand 5b in complex 7. The 31P
NMR signals are shown at 33.5, 33.5, 33.5, 33.0, and 33.5 ppm
for complexes 3, 4, 6, 7, and 9 in the 31P{1H} NMR spectra,
respectively, implicating a similar coordination pattern around
the Ru(II) metal centers in these Ru(II)−NNNN complexes.
In comparison to the 31P{1H} NMR spectrum of complex 1a
(δ(31P) = 33.8 ppm),19e it is clear that the 31P resonance
signals of complexes 3, 4, 6, 7, and 9 are shifted 0.3−0.8 ppm
upfield, suggesting that the coordinatively unsaturated metal
center in Ru(II)−NNN complex 1a is more electronically
positive than those in the coordinatively saturated Ru(II)−
NNNN complexes. The compositions of complexes 3, 4, 6,
and 7 were also analyzed by MALDI-TOF mass spectrometry.
The molecular ion peaks [3 + H]+, [4 + H]+, [6 + H]+, and [7
+ H]+ were not observed in the mass spectra, while those peaks
corresponding to the fragments [1a + H]+, [2 + H]+, and [5 +
H]+ were detected, respectively. It is noteworthy that the
fragment of mononuclear building block complex 1a, that is,
[1a + H]+, was found in the mass spectra of all complexes 3, 4,
6, and 7, coalesced by the protonated ligand fragments, which
has suggested the possible compositions of the complexes.
Theoretical stimulation of the mass spectra is consistent with
the experimental results (see the Supporting Information for
details).
Characterization of the Complexes. Complexes 3, 4, 6,
7, and 9 were fully characterized by NMR and FT-IR
spectroscopies, elemental analysis, and MALDI-TOF mass
spectrometry. The NMR analyses are consistent with their
1
compositions. The H NMR spectrum of complex 3 reveals
two doublets at 8.71 and 8.00 ppm for the 4-pyridyl moiety of
ligand 2a and a singlet at 8.27 ppm for the 1,3,5-phenyl core,
respectively. The resonance signal of the three pyrazolyl-CH
protons appears at 6.38 ppm as a singlet, suggesting occurrence
of Ru−N coordination between ligand 2a and the Ru(II)−
NNN building block, that is, mononuclear Ru(II)−NNN
complex 1a, which led to complex 3. Similarly, the proton
resonance signals of the 4-pyridyl moiety appear at 8.69 and
7.81 ppm as two doublets, and that of the 1,3,5-triphenyl-
substituted benzene core is shown at 8.09 ppm as a singlet,
revealing the existence of ligand 2b in trinuclear complex 4.
The 1H NMR spectrum of complex 6 exhibits two doublets at
8.42 and 7.57 ppm, respectively, corresponding to the proton
resonances of the 4-pyridyl moieties of ligand 5a, and the
resonance signal of the six pyrazolyl-CH protons appears at
6.37 ppm as a singlet. These spectral features have suggested
the occurrence of Ru−N coordination between mononuclear
complex 1a and ligand 5a in complex 6. Two doublets appear
at 8.57 and 7.20 ppm for the resonance signals of the 4-pyridyl
Comparison of the Catalytic Activities of Complexes
3, 4, 6, 7, and 9. Next, the TH reactions of selected ketones
1
moieties of ligand 5b in the H NMR spectrum of complex 7,
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were performed to compare the catalytic activities of
complexes 3, 4, 6, 7, and 9 (Table 1). Under the typical
conditions for a TH reaction,19 acetophenone was reduced to
1-phenylethanol in refluxing 2-propanol. With a loading of
0.025 mol % Ru for a complex catalyst, the TH reaction
proceeded smoothly, furnishing the target alcohol product in
96−98% yields over a period of 1−2 min by using complexes
3, 4, and 6 (Table 1, entry 1), exhibiting a positive cooperative
effect from the multiple Ru(II)−NNNN functionalities. It is
noteworthy that mononuclear Ru(II)−NNN complex 1a could
only be applied as the catalyst for the same reactions at a ≥
0.05 mol % Ru loading.19e Complex 7 showed a much lower
catalytic activity than complexes 3, 4, 6, and 9, rendering
formation of 1-phenylethanol in 90% yields over a period of 30
min, which is presumably attributed to the introduction of an
ethynyl linker (spacer) into the ligand that disfavors the
electron transfer in a multimetallic complex molecule.22 Under
the same conditions, bimetallic complex B can be applied as
the catalyst for the TH reaction of acetophenone at a
minimum Ru loading of 0.05 mol %, achieving 98% yield
and a TOF value of 1.5 × 105 h−1 within 20 min.20d The
present results have revealed that complexes 3, 4, and 6 are
much more catalytically active than bimetallic complex B at a
lower ruthenium loading. Complexes 3, 4, and 6 could all
efficiently catalyze the TH reaction of 2′-chloroacetophenone
to completion within 1/6 min. Complex 3 achieved the highest
TOF value 7.1 × 106 h−1 by using a catalyst loading as low as
0.008 mol % Ru; both complexes 4 and 6 could be used as the
catalysts with a loading as low as 0.0125 mol % Ru (Table 1,
entry 2). However, a minimum loading of 0.025 mol % Ru was
required for complexes 7 and 9. Both 3′- and 4′-
chloroacetophenones efficiently underwent the TH reactions
in the presence of complex catalysts 3, 4, and 6, respectively,
whereas complex 9 required a longer time for completing the
reactions at the same ruthenium loadings (Table 1, entries 3
and 4), revealing the lower catalytic activity of complex 9. It
seems that the ethynyl linker in complex 7 remarkably
diminished its catalytic activity for the same reactions.
Complex 3 catalyzed the TH reaction of sterically hindered
2′-methylacetophenone to reach 98% yield within 5 min, and
complex 6 promoted the same reaction to give the product in
97% yield over a period of 30 min, while complexes 4, 7, and 9
catalyzed the same reaction to form the target product in 45−
62% yields within 30 min (Table 1, entry 5). Overall, complex
3 exhibited the highest catalytic activity in most of the TH
reactions of the selected acetophenones as shown in Table 1.
Oligopyridine ligand 2a may bestow assembled pincer-type
triruthenium(II)−NNNN complex 3 with the most compatible
steric and electronic impacts on the ruthenium metal centers,
which thus enhances its catalytic activity for the transfer
hydrogenation of ketones. In comparison to those known
dinuclear pincer-type Ru(II)−NNNN complexes,20 tri- and
hexanuclear complexes 3, 4, and 6 could exhibit much higher
catalytic activity in the TH reactions of acetophenones.
Table 1. Comparison of the Catalytic Activities of
Complexes 3, 4, 6, 7, and 9
a
The reaction kinetics was studied by the TH reaction of 4′-
chloroacetophenone to understand the catalytic activity
differences between complexes 3, 4, 6, 7, and 9 (Figure 2).
It is clear that complexes 3, 4, and 6 exhibited much higher
catalytic activities than those of complexes 7 and 9 with the
catalytic activity order 3 ≥ 4 > 6 > 9 > 7 under the stated
conditions. The curves for the calculated turnover frequencies
against the concentrations of 4′-chloroacetophenone for the
five complex catalysts fall on top of each other, which
a
Conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); 0.025 mol
% Ru (ketone/iPrOK/Ru = 4000:20:1); 0.1 MPa N2, 82 °C.
b
c
Determined by GC analysis. Turnover frequency (moles of ketone
converted per mole of Ru per hour) at 50% conversion of the ketone
d
e
substrate. Using 0.008 mol % Ru. Using 0.0125 mol % Ru.
implicates that catalyst deactivation is not a factor in this TH
reaction, and the catalytic activity differences were caused by
D
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transfer hydrogenation reaction of 3′-bromoacetophenone
catalyzed by a monometallic Ru(II)−CNN complex catalyst
bearing an NH functionality, achieving a TOF value 3.8 × 106
h−1.23a Our complexes 3 and 6 are unambiguously among the
few known most active transition metal complex catalysts for
the transfer hydrogenation of ketones to date. Fluoro- and
trifluoromethyl-substituted acetophenones were also efficiently
reduced to the corresponding alcohols. In the case of using
complex 3 as the catalyst, the reactivity order of the ketone
substrates is 2′-CF3 > 4′-CF3 > 3′-CF3, while the reactivity
order was altered to 4′-CF3 > 2′-CF3 > 3′-CF3 by using
complexes 4 and 6 as the catalysts (Table 2, entries 14−16),
suggesting that the coordinating arms of the ligand may affect
the interaction of the substrate with the catalytically active
metal center. Both sterically hindered benzophenone and 2-
acetylnaphthalene, as well as aliphatic cyclic and acyclic
ketones, could also be reduced to the corresponding alcohols
by variation of the reaction time and catalyst loadings (Table 2,
entries 17−20).
Tentative Preparation of the Ruthenium(II)−NNNN
Hydride Complexes. To probe into the reaction mechanism,
the preparation of catalytically active species was attempted by
reacting complex 3 with a base in refluxing 2-propanol under
the TH reaction conditions. The Ru(II) hydride complexes
generated from the corresponding Ru(II)−Cl complex catalyst
precursors are usually considered as the catalytically active
species for the transfer hydrogenation of ketones.17b,18−20
Thus, complex 3 was reacted with tBuOK or iPrOK in 2-
propanol under a nitrogen atmosphere. Unfortunately, the
expected triruthenium(II) hydride complex was not success-
fully isolated, and no RuH species was detected in the reaction
mixture by proton NMR spectral analysis. Complex 3 was also
reacted with NaBH4 at 0 °C under a nitrogen atmosphere or
treated with HCO2H/Et3N or atmospheric hydrogen in
isopropanol at room temperature to 50 °C; no RuH species
was detected in the reaction mixtures by proton NMR spectral
analysis. These results have suggested that the multinuclear
Ru(II)−NNNN hydride complexes are unstable under the
stated conditions as compared with those mononuclear
Ru(II)−NNN hydride complexes generated from the mono-
nuclear Cl−Ru(II)−NNN complexes.19,20,24
Figure 2. Representative reaction kinetics profiles.
the inherent catalyst rates of the complex catalysts23c (Figure
3).
Figure 3. Correlation of TOFs and concentrations of the ketone.
Theoretical Calculations. DFT (M06−2x25 functional,
def2-tzvp for ruthenium and cc-pVDZ for other atoms, and
Gaussian16)26 calculations were conducted to study the
structures of the multinuclear Ru(II)−NNNN complexes and
the possible catalytically active species in the TH reactions of
ketones, and we rationalize the enhanced catalytic activity of
these complexes (see the Supporting Information for details).
Coordination and Optimized Structures. First, the
coordination structure of complex 3 was investigated by using
its mononuclear analog, that is, the 1:1 molar ratio adduct of
mononuclear Ru(II)−NNN complex 1a and ligand 2a (1a·2a,
Scheme 4), in which ligand 2a only contributes one pyridyl
moiety to coordinate with the metal center of complex 1a
(Scheme 3). It was found that in the most stable conformation
of the resultant mononuclear Ru(II)−NNNN complex 10, the
PPh3 ligand is positioned trans to ligand 2a, which is exactly
the same as that shown in Scheme 2. The conformer with the
Cl atom trans to ligand 2a, that is, complex 10a, is less stable
by 14.1 kcal/mol than the former, and the conformer of
complex 10b with the NNN ligand of 1a trans to ligand 2a is
less stable by 12.5 kcal/mol. A similar structure was found for
complex 9, the 1:1 molar ratio adduct of complex 1a and
Transfer Hydrogenation of Ketones Catalyzed by
Complexes 3, 4, and 6. Then, the TH reactions of various
ketones were explored by using complexes 3, 4, and 6 as the
catalysts. With a 0.025 mol % Ru loading, complexes 3, 4, and
6 efficiently catalyzed the TH reaction of acetophenone in
refluxing 2-propanol, forming the target alcohol product in
96−98% yield within 1−2 min (Table 2, entry 1). However,
propiophenone required a higher catalyst loading, i.e., 0.1 mol
% Ru, to complete the reaction (Table 2, entry 2). For the
reduction of methyl-substituted acetophenones, a higher
catalyst loading (0.05−0.1 mol % Ru) was usually necessary
for complex 4 (Table 2, entries 3−5). A smaller amount of
complex catalyst 3 but a higher loading of complex 6 was
provided for the TH reaction of 3′-methoxyacetophenone
(Table 2, entry 6), reaching TOFs ranging from 4.2 × 105 to
5.2 × 106 h−1. The chloro and bromo substituents favored the
TH reactions, and the catalyst loading could be further
reduced to 0.0125 or 0.008 mol % Ru (Table 2, entries 7−11).
In the cases of 2′-chloroacetophenone and 4′-bromoacetophe-
none, complex 3 exhibited a very high catalytic activity with a
0.008 mol % Ru loading, reaching TOF values 7.1 × 106 h−1
and 5.8 × 106 h−1, respectively. Baratta et al. reported the
E
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a
Table 2. Transfer Hydrogenation of Ketones Catalyzed by Complexes 3, 4, and 6
a
b
Conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); 0.025 mol % Ru (ketone/iPrOK/Ru = 4000:20:1); 0.1 MPa N2, 82 °C. Determined by
c
d
GC analysis. Turnover frequency (moles of ketone converted per mole of Ru per hour) at 50% conversion of the ketone substrate. 0.1 mol % Ru.
e
f
g
0.05 mol % Ru. 0.0125 mol % Ru. 0.008 mol % Ru.
ligand 4-phenylpyridine (8) (eq 1). In trinuclear Ru(II)−
NNNN complex 3, ligand 2a is situated at the molecular core,
and the three PPh3 ligands are placed at the peripheral
positions of the Ru-NNN building block planes. It has been
well-known that transition metal catalyzed organic trans-
formations often occur via a ligand dissociation−substrate
coordination mechanism. In our case, the PPh3 ligand is
initially dissociated to offer a coordinatively unsaturated site at
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Scheme 4. Coordination Structures of Adduct 1a·2a
the metal center for coordination−activation of the ketone
substrate in the catalytic cycle. The calculations have revealed
that dissociation of the PPh3 ligand does not obviously change
the structure of the Ru metal center in complex 10. Thus, in
the subsequent calculations the PPh3 ligand was omitted in the
theoretical models. With this simplification, the structures of
corresponding monohydride complex 3′ from complex 3
(Figure 4), which is supposed to be the catalytically active
species for the TH reaction, and complex 3″ in which the Ru−
H metal center of complex 3′ is coordinated by one molecule
of the ketone substrate (simplified as acetone) are optimized as
shown in Figures 5 and 6. Interestingly, it was found that
Figure 5. Optimized structure of complex 3″.
ligand 2a is positioned trans to the hydride in complex 3″, and
the NNN ligand of complex 1a is situated trans to the
coordinated ketone substrate (Figure 5). Similar structural
features have also been found in the corresponding hydride
species of complexes 9 and 10, that is, Ru(II)−H complexes 9′
and 10′.
Binding Energies. To understand the enhanced catalytic
activity of the multinuclear Ru(II)−NNNN complex catalysts,
their coordination and transfer hydrogenation behaviors were
Figure 4. Optimized structure of complex 3′.
G
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complex 3′ was considered. Such an interaction may also be
contributed to the increase of the binding energy of the
Ru(II)−H metal center and the ketone substrate. By means of
IQA methods27 utilizing the program ADF 201828 (M06−2x
functional, tzp basis sets), the interaction energy between the
ketone molecule coordinated to the Ru(II)−H metal center
and the two Ru−Cl metal centers was calculated to be −0.75
kcal/mol, which results completely from the electrostatic
interaction. This value is quite close to the binding energy
difference between Ru(II)−H complexes 10′ and 3′ (−0.73
kcal/mol), implicating that the enhanced binding capability of
complex 3′ arises from the weak electrostatic interaction
between the ketone substrate and the Ru(II)−Cl fragments.
The interaction between the coordinated ketone substrate and
the two nearest CH3 groups of the NNN ligand was also
calculated with energy of 0.03 kcal/mol which could be
ignored.
Proposed Mechanism. Thus, a plausible explanation is
proposed to rationalize the enhanced catalytic activity of the
multinuclear Ru(II)−NNNN complex catalysts. It is the weak
electrostatic interaction between the coordinated ketone
substrate and the metal centers of the Ru−Cl fragments, that
is, the electrostatic attraction between the negatively charged
oxygen of the ketone and the positively charged metal centers,
which enhances the total binding strength of the Ru(II)−H
metal center to the ketone substrate. As the spacer (linker)
length in the oligopyridine ligand is extended, the electrostatic
interaction between the coordinated ketone substrate and the
Ru(II)−Cl fragments is then diminished, resulting in the
observed decrease in the catalytic activity from 3 to 4 and from
6 to 7, respectively, as shown in Tables 1 and 2 as well as
Figures 2 and 3.
Figure 6. Optimized structure of the TH reaction transition state
(bond lengths in Å).
modeled by means of Ru(II)−H complexes 9′, 10′, and 3′ as
the model complex catalysts. The energies of the ketone
substrate binding to the Ru(II)−H metal center of these three
hydride complexes are 17.21, 17.53, and 18.26 kcal/mol,
respectively. Upon binding of the ketone substrate, the energy
barriers for the subsequent hydride transfer from the Ru(II)−
H metal center to the ketone carbonyl are calculated to be
3.50, 3.72, and 3.90 kcal/mol (Figure 6). The energies for the
PPh3 ligand to bind the Ru(II)−H metal centers in 9′, 10′, and
3′ are 36.41, 36.85, and 36.49 kcal/mol, respectively, which are
much greater than those for the ketone substrate to bind the
same Ru(II)−H metal center. On the basis of the DFT
calculations, it can be concluded that both dissociation of the
PPh3 ligand from the metal center and coordination of the
ketone substrate to the metal center are crucial for the TH
reaction to efficiently occur. The enhanced catalytic activity for
complex catalyst 3′ mostly arises from the stronger capability
of its Ru(II) metal center(s) to bind the ketone substrate,
while the energy barrier for the transfer hydrogenation step is
not sensitive to the environmental change around the metal
centers in complexes 9′, 10′, and 3′.
CONCLUSIONS
■
In summary, multinuclear ruthenium(II)−NNNN complexes
have been constructed by assembly of a coordinatively
unsaturated 16-electron mononuclear pincer-type Ru(II)−
NNN complex and oligopyridine ligands. 1,3,5-Tri(pyridin-4-
yl)-benzene-based triruthenium(II)−NNNN pincer complex
has exhibited exceptionally high catalytic activity for the
transfer hydrogenation of ketones at a very low loading. The
enhanced catalytic activities of these multinuclear Ru(II)−
NNNN complexes for the transfer hydrogenation of ketones
have been rationalized by DFT calculations. The present
assembly strategy provides a concise route to highly active
transition metal complex catalysts.
The energy difference for binding the ketone substrate
between complexes 9′ and 3′ is ca. 1.1 kcal/mol. By means of
Arrhenius equation with the assumption that the pre-
exponential factor does not change, it is predicted that the
TH reaction rate in the case of using complex 3′ as the model
catalyst is about 6.4 times faster than that obtained by using
the mononuclear Ru(II)−H complex 9′ as the catalyst at room
temperature, which is within the experimental rate difference in
our work as shown in Table 1.
To rationalize the stronger binding capability of multi-
nuclear Ru(II)−NNNN complex 3′ for the ketone substrate,
the structures and natural charges of the Ru(II)−H metal
centers in complexes 9′, 10′, and 3′ were explored by variation
of the chemical environment around the metal centers. It was
found that all the Ru(II)−N, Ru(II)−O, and Ru(II)−H bond
lengths are very close in these Ru−H complexes with a
maximum deviation of less than 0.005 Å (see the Supporting
Information for details). Ru(II)−hydride complexes 10′ and 3′
are almost structurally bestowed with the same Ru(II)−H
metal center. This is also true for the natural charges on the
Ru(II)−H metal centers with a maximum deviation of 0.005 e.
These results have suggested that although the multinuclear
Ru(II)−NNNN complex molecules are much bigger than the
mononuclear one, the structural and electronic properties of
the Ru(II) metal centers are actually very analogous. Next, the
weak interaction of the ketone substrate coordinated to the
Ru(II)−H metal center with the Ru(II)−Cl moieties in
EXPERIMENTAL SECTION
■
1
General Considerations. H and 13C{1H} NMR spectra were
recorded on a Bruker DRX-400 spectrometer and all chemical shift
values refer to δ of TMS = 0.00 ppm, CDCl3 (δ(1H), 7.26 ppm;
δ(13C), 77.16 ppm), and DMSO-d6 (δ(1H), 2.50 ppm; δ(13C), 39.52
ppm). Elemental analysis was achieved by the Analysis Center, Dalian
University of Technology and Dalian Institute of Chemical Physics,
Chinese Academy of Sciences. Matrix-assisted laser desorption/
ionization time of flight mass spectra were recorded on a MALDI-
TOF/TOF 5800 spectrometer. All the chemical reagents were
purchased from commercial sources and used as received unless
otherwise indicated.
Typical Procedures for the Synthesis of Complexes 3, 4, 6,
7, and 9. Synthesis of Complex 3. Under a nitrogen atmosphere, a
mixture of complex 1a (68.6 mg, 0.1 mmol) and ligand 2a (10.3 mg,
0.03 mmol) in 3 mL of CH2Cl2/CH3OH (v/v, 5/1) was stirred at 25
°C for 5 h. All the volatiles were removed under reduced pressure, and
H
DOI: 10.1021/acs.organomet.9b00669
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
the resultant residue was subjected to purification by recrystallization
in CH2Cl2/n-hexane (v/v, 1/3) at ambient temperature, affording
complex 3 as a red solid (74 mg, 94%).
Synthesis of complex 6. Under a nitrogen atmosphere, a mixture
of complex 1a (68.6 mg, 0.1 mmol) and ligand 5a (16.7 mg, 0.017
mmol) in 3 mL of CH2Cl2/CH3OH (v/v, 5/1) was stirred at 25 °C
for 5 h. All the volatiles were removed under reduced pressure and the
resultant residue was subjected to purification by recrystallization in
CH2Cl2/n-hexane (v/v, 1/3) at ambient temperature, affording
complex 6 as a red solid (78 mg, 92%).
Synthesis of complex 9. Under a nitrogen atmosphere, complex 1a
(68.6 mg, 0.1 mmol) and ligand 8 (15.5 mg, 0.1 mmol) in 3 mL of
CH2Cl2/CH3OH (v/v, 5/1) was stirred at 25 °C for 5 h. All the
volatiles were removed under reduced pressure, and the resultant
residue was subjected to purification by recrystallization in CH2Cl2/n-
hexane (v/v, 1/3) at ambient temperature to afford complex 9 as a
dark red solid (80 mg, 92%).
[C35H29ClN5PRu + H]+: 688.0971. Found: 687.9938. MALDI-TOF
MS (m/z) Calcd for [C39H27N3 + H]+: 538.2278. Found: 538.1482.
1
Complex 6. 78 mg, 92%, red solid, mp > 300 °C dec. H NMR
(400 MHz, DMSO-d6, 23 °C) δ 8.42 and 8.08 (d each, J = 5.1 and 8.1
Hz, 12:6 H), 7.55−7.64 (m, 6:6:12 H), 7.43 (m, 12:12 H), 7.31 (d, J
= 8.2 Hz, 6 H), 7.16−7.24 (m, 54 H), 7.04−7.11 (m, 36:6:6 H), 6.98
(t, J = 7.4 Hz, 6 H), 6.37 (s, 6 H), 2.69 (s, 18 H), 2.53 (s, 18 H).
13C{1H} NMR (100 MHz, DMSO-d6, 23 °C) δ 160.1, 156.7, 155.3,
151.5, 150.1, 147.0, 146.1, 139.7, 135.8, 131.5, 131.4 and 128.8 (Cq
each), 132.9 (d, J = 16.2 Hz, o-C of PPh3), 131.8 (d, J = 74.7 Hz, i-C
PPh3), 129.2 (s, p-C of PPh3), 127.6 (d, J = 14.4 Hz, m-C of PPh3),
144.5, 128.7, 127.5, 124.9, 120.5, 120.4, 119.5, 118.7, 117.1, 116.1,
112.4, 107.9, 14.4, 14.1. 31P{1H} NMR (162 MHz, DMSO-d6, 23 °C)
δ 33.5. IR (KBr pellets, cm−1): ν 3435, 3050, 2954, 2918, 2855, 2584,
2347, 1911, 1813, 1775, 1606, 1573, 1550, 1499, 1478, 1459, 1435,
1409, 1354, 1325, 1280, 1219, 1186, 1157, 1142, 1093, 1048, 1033,
1001, 982, 930, 844, 820, 789, 746, 619, 577, 528, 514, 499, 465, 436.
Anal. Calcd for C282H222Cl6N36P6Ru6: C, 66.15; H, 4.37; N, 9.85.
Found: C, 66.07; H, 4.34; N, 9.82. MALDI-TOF MS (m/z) Calcd for
[C35H29ClN5PRu + H]+: 688.0971. Found: 688.0164. MALDI-TOF
MS (m/z) Calcd for [C72H48N6 + H]+: 997.4013. Found: 997.2916.
Typical Procedure for the Transfer Hydrogenation Reaction
of Ketones. The catalyst solution was prepared by dissolving
complex 3 (9.4 mg, 0.004 mmol) in 2-propanol (60 mL). Under a
nitrogen atmosphere, a mixture of a ketone (2.0 mmol), 7.5 mL of the
catalyst solution (0.0005 mmol), and 2-propanol (12.1 mL) was
stirred at 82 °C for 10 min. Then, 0.2 mL of iPrOK solution (0.01
mmol, 0.05 M) in 2-propanol was introduced to initiate the reaction.
At the stated time, 0.1 mL of the reaction mixture was sampled and
immediately diluted with 0.5 mL of 2-propanol precooled at 0 °C for
GC analysis. After the reaction was complete, the reaction mixture
was condensed under reduced pressure and subjected to purification
by flash silica gel column chromatography to afford the corresponding
alcohol product which was identified by comparison with the
authentic sample through NMR and GC analyses.
1
Complex 7. 80 mg, 91%, red solid, mp > 300 °C dec. H NMR
(400 MHz, DMSO-d6, 23 °C) δ 8.57 (d, J = 5.7 Hz, 12 H), 8.16 and
7.70 (d and m, J = 8.2 Hz,6:12 H), 7.38−7.51 (m, 24 H), 7.19−7.24
(m, 54:6:12 H), 7.02−7.12 (m, 36:6:12 H), 6.39 (s, 6 H), 2.70 (s, 18
H), 2.54 (s, 18 H). 13C{1H} NMR (100 MHz, DMSO-d6, 23 °C) δ
157.0, 153.7, 151.6, 151.4, 145.0, 144.9, 140.6, 139.4, 131.5 (d, J =
62.2 Hz, i-C of PPh3), 130.4, 130.0 and 125.7 (Cq each), 149.9,
136.1, 132.8 (d, J = 16.2 Hz, o-C of PPh3), 129.4, 127.7 (d, J = 14.6
Hz, m-C of PPh3), 125.3, 121.5, 118.8, 117.7, 116.8, 112.6, 121.1,
117.3, 108.7, 93.3 and 87.0 (CC, Cq each), 14.4, 14.1. 31P{1H}
NMR (162 MHz, DMSO-d6, 23 °C) δ 33.0. IR (KBr pellets, cm−1): ν
3441, 2957, 2919, 2584, 2217, 1955, 1774, 1604, 1550, 1501, 1478,
1435, 1409, 1354, 1280, 1234, 1214, 1185, 1157, 1142, 1048, 1032,
1018, 1000, 985, 913, 824, 789, 772, 655, 640, 620, 578, 514, 499,
464, 436. Anal. Calcd for C294H222Cl6N36P6Ru6: C, 67.08; H, 4.25; N,
9.58. Found: C, 67.15; H, 4.22; N, 9.49. MALDI-TOF MS (m/z)
Calcd for [C35H29ClN5PRu + H]+: 688.0971. Found: 687.9626.
MALDI-TOF MS (m/z) Calcd for [C84H48N6 + H]+: 1141.4012.
Found: 1141.2973.
1
Complex 3. 74 mg, 94%, red solid, mp > 300 °C dec. H NMR
(400 MHz, DMSO-d6, 23 °C) δ 8.71 (d, J = 5.9 Hz, 6 H), 8.27 (s, 3
H), 8.08 (d, J = 8.0 Hz, 3 H), 8.00 (d, J = 6.0 Hz, 6 H), 7.62 (t, J = 8.1
Hz, 3 H), 7.56 (d, J = 7.7 Hz, 3 H), 7.43 and 7.31 (d each, J = 8.0 and
8.1 Hz, 3:3 H), 7.19−7.24 (m, 27 H), 7.05−7.11 (m, 3:18 H,), 6.98
(t, J = 7.5 Hz, 3 H), 6.38 (s, 3 H), 2.69 (s, 9 H), 2.52 (s, 9 H), 2.52.
13C{1H} NMR (100 MHz, DMSO-d6, 23 °C) δ 156.9, 154.4, 151.5,
150.1, 146.3, 145.5, 144.7, 139.2, 131.6 (d, J = 62.0 Hz, i-C of PPh3)
and 129.0 (Cq each), 150.2, 136.0, 132.9 (d, J = 15.7 Hz, o-C of
PPh3), 129.3, 127.7 (d, J = 13.9 Hz, m-C of PPh3), 126.2, 121.8,
121.1, 120.5, 117.9, 117.4, 116.5, 112.6, 108.4, 14.4, 14.2. 31P{1H}
NMR (162 MHz, DMSO-d6, 23 °C) δ 33.5. IR (KBr pellets, cm−1): ν
3430, 3051, 2955, 2920, 2853, 2568, 2373, 2344, 1813, 1774, 1735,
1719, 1551, 1500, 1478, 1408, 1355, 1280, 1218, 1186, 1157, 1117,
1070, 1048, 1033, 1000, 983, 913, 892, 843, 818, 720, 669, 658, 620,
573, 528, 502, 468, 436. Anal. Calcd for C126H102Cl3N18P3Ru3: C,
63.83; H, 4.34; N, 10.63. Found: C, 63.73; H, 4.38; N, 10.52.
MALDI-TOF MS (m/z) Calcd for [C35H29ClN5PRu + H]+:
688.0971. Found: 688.0292. MALDI-TOF MS (m/z) Calcd for
[C21H15N3 + H]+: 310.1339. Found: 310.1230.
1
Complex 9. 80 mg, 92%, red solid, mp > 300 °C dec. H NMR
(400 MHz, DMSO-d6, 23 °C) δ 8.64 (dd, J = 4.5, 1.7 Hz, 2 H), 8.07
(d, J = 8.1 Hz, 1 H), 7.81 (dd, J = 9.5, 2.6 Hz, 2 H), 7.71 (dd, J = 4.5,
1.7 Hz, 2 H), 7.62 (t, J = 8.0 Hz, 1 H), 7.57−7.45 (m, 4 H), 7.43 (d, J
= 8.0 Hz, 1 H), 7.31 (d, J = 7.5 Hz, 1 H), 7.25−7.18 (m, 9 H), 7.08
(dtd, J = 10.0, 8.3, 1.5 Hz, 7 H), 6.93−6.99 (m, 1 H), 6.37 (s, 1 H),
2.69 (s, 3 H), 2.52 (s, 3 H). 13C{1H} NMR (100 MHz, DMSO-d6, 23
°C) δ 160.1, 156.7, 155.3, 151.7, 151.5, 147.0, 146.1, 144.5 and 150.2
(Cq each), 137.1, 135.8, 132.9 (d, J = 16.4 Hz, o-C of PPh3), 131.8
(d, J = 61.6 Hz, i-C of PPh3), 129.2, 129.2, 129.1, 127.6 (d, J = 14.4
Hz, m-C of PPh3), 126.8, 121.2, 120.5, 119.5, 118.7, 117.1, 116.1,
112.4, 107.9, 14.4, 14.1. 31P{1H} NMR (162 MHz, DMSO-d6, 23 °C)
33.5. IR (KBr pellets, cm−1): ν 3439, 3052, 3004, 2958, 2921, 2869,
1607, 1574, 1550, 1500, 1481, 1459, 1436, 1410, 1396, 1354, 1326,
1280, 1219, 1186, 1157, 1141, 1094, 1070, 1048, 1034, 1011, 1002,
982, 841, 834, 788, 767, 698, 659, 623, 578, 562, 527, 515, 500, 462,
447, 437,427. Anal. Calcd for C46H38ClN6PRu: C, 65.59; H, 4.55; N,
9.98. Found: C, 65.47; H, 4.60; N, 9.89.
1
Complex 4. 77 mg, 91%, red solid, mp > 300 °C dec. H NMR
(400 MHz, DMSO-d6, 23 °C) δ 8.69 (d, J = 5.5 Hz, 6 H), 8.06−8.10
(m, 3:3:6 H), 7.98 (d, J = 8.1 Hz, 6 H), 7.81 (d, J = 5.4 Hz, 6 H), 7.62
(t, J = 8.0 Hz, 3 H), 7.55 (d, J = 7.6 Hz, 3 H), 7.44 and 7.31 (d each, J
= 8.0 and 8.1 Hz, 3:3 H), 7.19−7.24 (m, 27 H), 7.04−7.11 (m, 3:18
H), 6.97 (t, J = 7.4 Hz, 3 H), 6.37 (s, 3 H), 2.69 (s, 9 H), 2.52 (s, 9
H). 13C{1H} NMR (100 MHz, DMSO-d6, 23 °C) δ 158.8, 156.9,
154.4, 151.5, 151.4, 146.4, 145.5, 144.7, 141.0, 140.8, 131.6 (d, J =
62.0 Hz, i-C of PPh3) and 129.0 (Cq each), 150.3, 136.5, 136.0, 132.9
(d, J = 16.4 Hz, o-C of PPh3), 129.4, 128.1, 127.7 (d, J = 14.6 Hz, m-C
of PPh3), 127.4, 124.8, 121.1, 120.5, 117.9, 117.4, 116.5, 112.6, 108.4,
14.4, 14.2. 31P{1H} NMR (162 MHz, DMSO-d6, 23 °C) δ 33.5. IR
(KBr pellets, cm−1): ν 3050, 2960, 2918, 2854, 2755, 2677, 1956,
1815, 1720, 1626, 1573, 1551, 1500, 1478, 1435, 1409, 1395, 1356,
1301, 1270, 1229, 1186, 1157, 1141, 1094, 1069, 1049, 1021, 1005,
983, 930, 889, 845, 791, 659, 619, 590, 515, 500, 462, 436, 410. Anal.
Calcd for C144H114Cl3N18P3Ru3: C, 66.54; H, 4.42; N, 9.70. Found: C,
66.41; H, 4.47; N, 9.61. MALDI-TOF MS (m/z) Calcd for
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00669.
■
S
Experimental materials and procedures, analytical data
and NMR spectra of compounds and complexes
MALDI-TOF MS spectra of complexes, theoretical
methods and results (PDF)
Cartesian coordinates (XYZ)
I
DOI: 10.1021/acs.organomet.9b00669
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Yan, L.-K.; Su, Z.-M. Two-State Reactivity Mechanism of Benzene
C−C Activation by Trinuclear Titanium Hydride. J. Am. Chem. Soc.
2016, 138, 11069−11072.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: zkyu@dicp.ac.cn (Z.K.Y.).
*E-mail: fanhj@dicp.ac.cn (H.J.F.).
*E-mail: ygzhou@dicp.ac.cn (Y.G.Z.).
■
(5) (a) Gu, X.; Li, X.; Wu, S.; Shi, J.; Jiang, G.; Jiang, G.; Tian, S. A
Sensitive Hydrazine Hydrate Sensor Based on A Mercaptomethyl-
terminated Trinuclear Ni(II)-Complex Modified Gold Electrode. RSC
Adv. 2016, 6, 8070−8078. (b) Mognon, L.; Mandal, S.; Castillo, C. E.;
Fortage, J.; Molton, F.; Aromí, G.; Benet-Buchhlolz, J.; Collomb, M.-
N.; Llobet, A. Synthesis, Structure, Spectroscopy and Reactivity of
New Heterotrinuclear Water Oxidation Catalysts. Chem. Sci. 2016, 7,
3304−3312. (c) Liu, R.; von Malotki, C.; Arnold, L.; Koshino, N.;
ORCID
Tingting Liu: 0000-0002-0156-3054
Kaikai Wu: 0000-0003-1950-5343
Liandi Wang: 0000-0003-4996-3687
Hongjun Fan: 0000-0003-3406-6932
Yong-Gui Zhou: 0000-0002-3321-5521
Zhengkun Yu: 0000-0002-9908-0017
Higashimura, H.; Baumgarten, M.; Mullen, K. Triangular Trinuclear
̈
Metal-N4 Complexes with High Electrocatalytic Activity for Oxygen
Reduction. J. Am. Chem. Soc. 2011, 133, 10372−10375.
(6) (a) Yamazaki, Y.; Umemoto, A.; Ishitani, O. Photochemical
Hydrogenation of π-Conjugated Bridging Ligands in Photofunctional
Multinuclear Complexes. Inorg. Chem. 2016, 55, 11110−11124.
(b) Sahara, G.; Abe, R.; Higashi, M.; Morikawa, T.; Maeda, K.; Ueda,
Y.; Ishitani, O. Photoelectrochemical CO2 Reduction Using a
Ru(II)−Re(I) Multinuclear Metal Complex on A P-type Semi-
conducting NiO Electrode. Chem. Commun. 2015, 51, 10722−10725.
(7) (a) Andruh, M. Heterotrimetallic Complexes in Molecular
Magnetism. Chem. Commun. 2018, 54, 3559−3577. (b) Dul, M.-C.;
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21672209 and 21871253) and the strategic pilot
science and technology project of the Chinese Academy of
Sciences (XDB17010200) for support of this research.
̈
Pardo, E.; Lescouezec, R.; Journaux, Y.; Ferrando-Soria, J.; Ruiz-
García, R.; Cano, J.; Julve, M.; Lloret, F.; Cangussu, D.; Pereira, C. L.
́
́
M.; Stumpf, H. O.; Pasan, J.; Ruiz-Perez, C. Supramolecular
Coordination Chemistry of Aromatic Polyoxalamide Ligands: A
Metallosupramolecular Approach Toward Functional Magnetic
Materials. Coord. Chem. Rev. 2010, 254, 2281−2296.
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DOI: 10.1021/acs.organomet.9b00669
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DOI: 10.1021/acs.organomet.9b00669
Organometallics XXXX, XXX, XXX−XXX