Mechanistic Insight into Synergistic Catalysis of Olefin Hydrogenation by a Hetero-Dinuclear RuII-CoII Complex with Adjacent Reaction Sites

Article abstract of DOI:10.1021/acs.inorgchem.9b02104

We have designed and synthesized a hetero-dinuclear Ru^II-Co^II complex with a dinucleating ligand inspired by hetero-dinuclear active sites of metalloenzymes. A synergistic effect between the adjacent Ru^II and Co^II sites has been confirmed in catalytic olefin hydrogenation by the complex, exhibiting a much higher turnover number than those of mononuclear Ru^II or Co^II complexes as the components. A Ru^II-hydrido species was detected by ¹H NMR and electrospray ionization (ESI)-time-of-flight (TOF)-MS measurements as an intermediate to react with olefins, and Co^II-bound methanol was suggested to act as a proton source.

Full text of DOI:10.1021/acs.inorgchem.9b02104

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Mechanistic Insight into Synergistic Catalysis of Olefin
II
II
Hydrogenation by a Hetero-Dinuclear Ru −Co Complex with
Adjacent Reaction Sites
Dachao Hong,*† Yuji Ohgomori, Yoshihiro Shimoyama, Hiroaki Kotani, Tomoya Ishizuka,‡
,
†
†
†
‡
Yoshihiro Kon, and Takahiko Kojima*,‡
†Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology
(
AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
‡
Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, CREST, Japan Science and Technology
Agency (JST), 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan
*
S Supporting Information
II
III
3+
13
bpp) } Mn Mn (μ-O)] complex and Ru−Zn hetero-
3
2
2
14
ABSTRACT: We have designed and synthesized a
polynuclear complexes have also been synthesized by using
bpp as a bridging ligand. In light of those complexes reported,
II
II
−
hetero-dinuclear Ru −Co complex with a dinucleating
ligand inspired by hetero-dinuclear active sites of
metalloenzymes. A synergistic effect between the adjacent
Hbpp is considered as a promising ligand for the synthesis of
hetero-dinuclear complexes because the deprotonated pyrazole
moiety can induce the second coordination site (Figure 1a). In
this context, stepwise introduction of two different metal
complexes into the Hbpp ligand can provide a sure and facile
method for synthesis of hetero-dinuclear metal complexes. It is
expected that a combination of different transition metal ions
such as 3d and 4d transition metals will offer a unique property
and catalytic performance.
II
II
Ru and Co sites has been confirmed in catalytic olefin
hydrogenation by the complex, exhibiting a much higher
II
II
turnover number than those of mononuclear Ru or Co
II
complexes as the components. A Ru -hydrido species was
1
detected by H NMR and electrospray ionization (ESI)-
time-of-flight (TOF)-MS measurements as an intermedi-
II
ate to react with olefins, and Co -bound methanol was
suggested to act as a proton source.
Herein, we report the synthesis of a hetero-dinuclear Ru−Co
II
II
complex, [Ru (p-cymene)(μ-bpp)(Cl)Co (dipc)(H O)] (1),
2
II
+
15
which is a fusion of [Ru (p-cymene)(Hbpp)(Cl)] (2) and
[Co (dipc)(Hbpp)(H O)] (3) (p-cymene = 4-isopropylto-
II
n nature, multinuclear metal complexes/clusters in enzymes
2
I
composed of different metal ions have been known to act as
luene, H dipc = 2,6-pyridinedicarboxylic acid) as components,
as shown in Figure 1b. The three complexes (1−3) were
employed as catalysts for hydrogenation of 1-dodecene using
2
1
,2
active sites to perform diverse reactions.
CuZn]SOD (superoxide dismutase), [NiFe]hydrogenase,
For example,
3
4
[
5
and the oxygen evolving complex in the photosystem II have
two kinds of metal ions working on different tasks at active
sites; the metal centers play cooperative roles to accomplish
the tasks required for the enzymes. Multimetallic active sites
that consist of hetero-metal centers and supporting ligands
have been reported to exhibit attractive features in
NaBH as a hydride source. Hydrogenation of olefins has also
4
been described using NaBH with metals salts that afford
4
16,17
uncertain catalytically active entities in early reports;
however, mechanistic insights have rarely been gained into
those reactions because the catalysis proceeds in a heteroge-
neous manner. In sharp contrast to the previous works, herein,
a metal-hydrido intermediate derived from 1 in 1-dodecene
6
−10
catalysis.
Among multinuclear complexes with different
1
metal ions, hetero-dinuclear metal complexes should offer
advantages to clarify synergistic effects, since only two distinct
sites participate in catalytic reactions. The use of dinucleating
ligands should be effective for preparing hetero-dinuclear metal
complexes; however, some difficulty can be raised in the
formation of a desired dinuclear complex. Simultaneous
introduction of two kinds of metal ions into a single
dinucleating ligand with two equivalent coordination sites
can produce dinuclear complexes with various patterns of
metal pairs. Thus, sequential insertion of two kinds of metal
ions into a single dinucleating ligand can be a promising
approach for preparing desired hetero-dinuclear complexes
selectively.
hydrogenation has been successfully detected by H NMR and
electrospray ionization (ESI)-time-of-flight (TOF)-MS meas-
urements. The rate-determining step (RDS) and catalytic
mechanism are discussed on the basis of kinetic analysis of 1-
docecene hydrogenation by 1. It is also revealed that the Ru
II
center of 1 acts as a reactive site forming the Ru -hydrido
II
intermediate and the Co center as an accelerating site by
activating methanol as a proton source to arise a synergistic
effect.
1
1
We first synthesized complexes 2 and 3 (Figure 1b) as the
components of 1 according to the literature (see the
1
5,18
Supporting Information).
Characterization of the com-
1
plexes was made by H NMR spectroscopy and ESI-TOF-MS
II
Recently, a dinuclear Ru complex with 3,5-bis(2-pyridyl)-
pyrazole (Hbpp) ligand has been reported as a catalyst for
Received: July 14, 2019
1
2
I I
water oxidation.
A pentanuclear [{Mn (μ-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02104
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Table 1. Catalytic Activity of the Metal Complexes for 1-
a
Dodecene Hydrogenation
b
c
d
cat.
conv.,
%
selec., %
yield, %
TON
1
2
3
2
65.8
7.7
0
3.0
0.7
53.5
68.4
0
87.0
100
35.2
5.3
0
2.6
0.7
464
70
0
e
+ 3
34
none
a
The reactions were performed at 25 °C in MeOH/EtOH (97/3 v/v,
2
.0 mL) containing a catalyst (25 μM), NaBH (33 mM), and 1-
4
dodecene (33 mM). The values were calculated from the average of
b
three runs. Conversion was calculated on the basis of 1-dodecene.
c
Selectivity and yields were determined for n-dodecane. Selectivity =
d
[
n-dodecane] × 100/[converted 1-dodecene]. TON was calculated
e
as [n-dodecane]/[Cat.]. [2] and [3] are 25 μM, respectively.
of 1-dodecene by the catalysts produced n-dodecane in over
5
0% selectivity. The byproduct was determined to be 2-
1
dodecene by H NMR analysis. The highest turnover number
TON) of 464 for 1 was observed among the catalysts.
(
Complexes 2 and 3 as the components of 1 exhibited lower
TON for 2 (70) and no reactivity for 3. The mixture of 2 and 3
20
suppressed the catalytic activity in comparison with 2. The
results clearly demonstrate that the synergistic effect between
the adjacent Ru and Co centers in 1 is operative in the
hydrogenation.
The homogeneous catalysis of 1 in the hydrogenation was
confirmed by a Hg poisoning test where 93% of the catalytic
performance of 1 was maintained even in the presence of a few
drops of Hg (Figure S5). To examine the effect of H gas
2
generated by methanolysis of NaBH , the hydrogenation
4
Figure 1. (a) Deprotonation−protonation equilibrium of Hbpp and
reaction by 1 was also examined under a H atmosphere
2
(
b) schematic descriptions of structures of 1−3. (c) An ORTEP
without NaBH ; however, no n-dodecane was observed in the
4
drawing of 1 with 50% probability thermal ellipsoids. Hydrogen atoms
except those of the aqua ligand are omitted for clarity.
reaction mixture. The results demonstrate that 1 acts as a
homogeneous catalyst and H gas, even if generated, is not
2
involved in the hydrogenation. Catalytic activity of 1 in
hydrogenation of other olefins was also examined as shown in
Table S3. The highest activity of 1 was observed in
hydrogenation of 1-dodecene.
To detect the intermediates for the hydrogenation, we
spectrometry (Figures S1 and S2). Finally, complex 1 was
II
18
21
obtained by reacting complex 2 with [Co (dipc)(H O) ] in
2
3
II
II
MeOH; the Ru −Co hetero-dinuclear complex was quickly
formed as confirmed by ESI-TOF-MS spectrometry (Figure
S3). The MS spectrum clearly demonstrates that two metal
ions are included in 1. The characterization of 1 was also made
performed the reaction of 1 with NaBH and monitored the
4
1
reaction by H NMR and ESI-TOF-MS measurements. The
1
by elemental analysis and H NMR measurements (Figure S4),
addition of two equivalents of NaBH resulted in spectral
4
the latter of which exhibited paramagnetic features.
change of complex 1, indicating formation of intermediates
1
Figure 1c depicts an ORTEP drawing of 1 determined by X-
ray crystallography. The crystallographic parameters and
selected bond lengths of 1 are given in Tables S1 and S2.
(
Figure 2a). The paramagnetic H NMR spectrum supports
II
that Co remains the valence of the cobalt center. A new peak
−
The bpp ligand in 1 functions to enforce the two metal sites
directed to the same side, i.e., the plausible reactive sites; the
Ru−Cl and Co(H O) moieties are compelled to be close each
2
other. The distance between Co and Ru centers is determined
to be 4.357(1) Å. The dihedral angles between the pyrazole
plane and the planes of two pyridine rings containing N1 and
N4 are 6.30° and 1.73°, respectively, indicating the pyrazole
and two pyridine moieties reside on the same plane. These
results confirm that the bpp− ligand enable the two metal
centers to locate adjacent positions, which make it possible to
display a synergistic effect by placing the potential vacant sites
on both metal centers close to each other.
To demonstrate and clarify a synergistic catalysis by 1, we
1
Figure 2. (a) H NMR spectra of 1 (5.0 mM) and 1 with the addition
have employed complexes 1−3 as catalysts for 1-dodecene
of NaBH (10 mM) or NaBD (10 mM) in MeOH-d . (b) ESI-TOF-
MS spectra of the hydrido species by the addition of NaBH in
4
4
4
19
hydrogenation by NaBH as a hydride source. The results of
4
4
the catalytic reactions are listed in Table 1. The hydrogenation
MeOH or NaBD in MeOH-d into 1.
4
1
B
DOI: 10.1021/acs.inorgchem.9b02104
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
1
at −19.87 ppm in the H NMR spectrum can be assigned to a
Scheme 1. Proposed Mechanism of 1-Dodecene
metal-hydrido (M−H) species. When NaBH was replaced by
Hydrogenation by 1 with NaBH in MeOH
4
4
NaBD , the peak at −19.87 ppm disappeared to support the
4
2
2
II
assignment. The hydrido species is ascribed to a Ru −H
II
II
II
complex, [(Ru −H)(μ-bpp)Co ], not a Co −H species,
II
II
[
Ru (μ-bpp)(Co −H)], because the relaxation time (T ) of
1
the peak at −19.87 ppm was determined to be 15.2 ms (Figure
II
S6) and the high-spin state of the Co center involved in the
II
Ru −H intermediate was also confirmed by ESR spectroscopy
II
in Figure S7. The Ru −H intermediate was also successfully
detected by ESI-TOF-MS measurements (Figure 2b). The
peak cluster due to the intermediate was observed at 705.11.
When NaBD was used in place of NaBH , the peak cluster
4
4
shifted from 705.11 to 706.12. The NMR and MS spectra
II
clearly indicate the formation of the Ru −H intermediate.
Dependence of the yield of n-dodecane on the concentration
of 1 was investigated. The turnover frequency (TOF) was
calculated to be 15.7 min− on the basis of the slope (Figure
1
S8). The hydrogenation was investigated using MeOH-d as a
1
23
solvent or NaBD as a reductant (Figure S9). The TOF in
4
−
1
MeOH-d was determined to be 10.0 min , and the KIE value
1
was calculated to be 1.6. On the other hand, the KIE value in
the reaction using NaBD was calculated to be 1.0, indicating
4
that the cleavage of the O−H bond of MeOH is involved in
the RDS. When MeOH (pK = 15.5, TON = 464) was
a
replaced by EtOH (pK = 15.9) and 2-propanol (pK = 16.5 in
a
a
water) as a solvent, the catalytic activity of n-dodecane
production (TON = 44 and 10, respectively) was significantly
II
decreased (Table S4). The results suggest that the Co center
of 1 activates MeOH as a proton donor through the
coordination to protonate the adjacent Ru-bound n-dodecyl
group for releasing n-dodecane as a product. Thus, the
intermediate should include MeOH as a ligand bound to the
Co center, i.e., [(Ru −H)(μ-bpp)(Co −O(H)Me)] (A in
Scheme 1).
sites, which can catalyze 1-dodecene hydrogenation to produce
n-dodecane by NaBH4 as a hydride source. Mechanistic
insights into the catalytic hydrogenation of 1-dodecene as a
probe substrate by 1 have been gained by the detection of
intermediates and kinetic analysis. In the homogeneous
II
II
II
Besides, the dependence of the initial rates of n-dodecane
production on the concentration of 1-dodecene was inves-
tigated to observe saturation behavior (Figure S10), indicating
II
hydrogenation reaction, the Ru center of 1 reacts with
II
II
NaBH4 to afford the Ru −H intermediate. The Ru −H
II
II
intermediate reacts with 1-dodecene to form a putative Ru −
alkyl complex. As a synergistic effect in 1, the Co center
that an insertion equilibrium is operative between [(Ru −
II
II
H)(μ-bpp)(Co −O(H)Me)] (A in Scheme 1) and 1-
activates MeOH as a proton source to enhance the rate-
determining protonolysis of the Ru−alkyl moiety to release n-
dodecane. The findings in this work will provide valuable
fundamentals to develop hetero-dinuclear molecular catalysts
that enhance efficient homogeneous catalysis through syner-
gistic effects.
dodecene. A linear increase of the initial rates corresponding
to the increase of NaBH concentration was also observed in
4
the hydrogenation (Figure S11). On the basis of the results, we
propose the catalytic mechanism of catalytic hydrogenation of
1
-dodecene by 1 with use of NaBH4 (Scheme 1). The
intermediate A is formed by the reaction of NaBH with 1 as
observed by H NMR and ESI-TOF-MS measurements. The
4
1
24
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02104.
reaction of A and 1-dodecene to produce a mixture of Ru-(1-
■
II
*
S
dodecyl) and Ru -(2-dodecyl) intermediates (B and C,
respectively) involves an insertion equilibrium as evidenced
by the saturation behavior observed in the kinetic study. On
II
the other hand, MeOH coordinated to the Co center of 1 is
Experimental details and analytical data; 1H NMR
spectra; ESI-MS spectra; X-ray crystallographic data
and bond lengths for 1; catalytic activity of 1 for olefin
and 1-dodecene hydrogenation; production of n-
activated to act as a proton source for the protonolysis of B
and C at the adjacent site to generate n-dodecane, realizing the
synergistic effect of 1. The protonolysis is considered to be
involved in the RDS in the 1-dodecene hydrogenation by 1, as
concluded from the observed solvent KIE. Finally, n-dodecane
is released through the RDS, and the resultant species reacts
dodecane from 1-dodecene; T determination for the
1
1
H NMR signal; ESR spectra; time courses of n-
dodecane yield; concentration dependence of 1 on the
initial rate of n-dodecane production; dependence of the
initial rate of n-dodecane production on the concen-
tration of 1 and 1-dodecene; time courses of n-dodecane
production (PDF)
with NaBH to reproduce the intermediate A in the catalytic
4
cycle. In addition, the formation of C is also supported by the
production of 2-dodecene as mentioned above.
In summary, we have designed and successfully synthesized
II
II
a hetero-dinuclear Ru −Co complex, 1, with adjacent reactive
C
DOI: 10.1021/acs.inorgchem.9b02104
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Accession Codes
(9) (a) Wang, Y.; Kostenko, A.; Yao, S.; Driess, M. Divalent Silicon-
Assisted Activation of Dihydrogen in a Bis(N-Heterocyclic Silylene)-
Xanthene Nickel(0) Complex for Efficient Catalytic Hydrogenation
of Olefins. J. Am. Chem. Soc. 2017, 139, 13499−13506. (b) Cammar-
ota, R. C.; Lu, C. C. Tuning Nickel with Lewis Acidic Group 13
Metalloligands for Catalytic Olefin Hydrogenation. J. Am. Chem. Soc.
CCDC 1877269 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
2
015, 137, 12486−12489.
(
10) (a) Coombs, J.; Perry, D.; Kwon, D.-H.; Thomas, C. M.; Ess,
D. H. Why Two Metals Are Better Than One for Heterodinuclear
Cobalt−Zirconium-Catalyzed Kumada Coupling. Organometallics
AUTHOR INFORMATION
■
2
018, 37, 4195−4203. (b) Pye, D. R.; Cheng, L.-J.; Mankad, N. P.
Corresponding Authors
E-mail: hong-d@aist.go.jp (D.H.).
E-mail: kojima@chem.tsukuba.ac.jp (T.K.).
Cu/Mn Bimetallic Catalysis Enables Carbonylative Suzuki−Miyaura
Coupling with Unactivated Alkyl Electrophiles. Chem. Sci. 2017, 8,
*
*
4
750−4755.
ORCID
(11) Ohtsu, H.; Shimazaki, Y.; Odani, A.; Yamauchi, O.; Mori, W.;
Itoh, S.; Fukuzumi, S. Synthesis and Characterization of Imidazolate-
Bridged Dinuclear Complexes as Active Site Models of Cu,Zn-SOD. J.
Am. Chem. Soc. 2000, 122, 5733−5741.
Dachao Hong: 0000-0003-0581-1315
Hiroaki Kotani: 0000-0001-7737-026X
Tomoya Ishizuka: 0000-0002-3897-026X
Takahiko Kojima: 0000-0001-9941-8375
(
12) (a) Bozoglian, F.; Romain, S.; Ertem, M. Z.; Todorova, T. K.;
Sens, C.; Mola, J.; Rodriguez, M.; Romero, I.; Benet-Buchholz, J.;
Fontrodona, X.; Cramer, C. J.; Gagliardi, L.; Llobet, A. The Ru-Hbpp
Water Oxidation Catalyst. J. Am. Chem. Soc. 2009, 131, 15176−
Notes
The authors declare no competing financial interest.
15187. (b) Mola, J.; Mas-Marza, E.; Sala, X.; Romero, I.; Rodríguez,
M.; Vinas, C.; Parella, T.; Llobet, A. Ru-Hbpp-Based Water-Oxidation
Catalysts Anchored on Conducting Solid Supports. Angew. Chem., Int.
Ed. 2008, 47, 5830−5832.
̃
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid (Nos. 24245011,
7H03027, and 15J04635) from Japan Society for the
Promotion of Science (JSPS). D.H. gratefully acknowledges
support from JSPS by Leading Initiative for Excellent Young
Researchers (LEADER).
1
(13) Romain, S.; Rich, J.; Sens, C.; Stoll, T.; Benet-Buchholz, J.;
Llobet, A.; Rodriguez, M.; Romero, I.; Clerac, R.; Mathoniere, C.;
́ ̀
Duboc, C.; Deronzier, A.; Collomb, M.-N. Multireversible Redox
Processes in Pentanuclear Bis(Triple-Helical) Manganese Complexes
Featuring an Oxo-Centered Triangular {Mn Mn (μ -O)} or
Mn Mn (μ -O)} Core Wrapped by Two {Mn (bpp) } .
II
III
5+
2
3
II
III
6+
II
−
{
2
3
2
3
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D
DOI: 10.1021/acs.inorgchem.9b02104
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Inorganic Chemistry
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(
22) The pristine NaBH shows mainly a quartet due to the coupling
4
11
with the B nucleus (I = 3/2) at − 0.18 ppm (Figure S13).
II
Additionally, a Ru −H species derived from the reaction of NaBH
4
1
with 2 also showed a signal at −5.92 ppm in the H NMR spectrum
(
Figure S14). This result is consistent with the observation that 2 also
performs the 1-dodecene hydrogenation under the same conditions.
II
Compared to the Ru −H species derived from 2, the upfield shift was
observed for the Ru−H species derived from 1. This shift is probably
II
caused by the paramagnetism of the neighboring high-spin Co
center.
(23) The production ratio of [n-dodecane]/[2-dodecene] was
decreased to 0.58 using NaBH in CH OD and remained at 0.82
4
3
when using NaBD in CH OH.
4
3
II
II
(
24) The [(Ru −H)(μ-bpp)Co ] species generated from 1 was also
observed in the presence of 1-dodecene (Figure S15). After the
II
II
production of n-dodecane ceased, the [(Ru −H)(μ-bpp)Co ] still
remained in the solution as detected by H NMR spectroscopy
1
(
Figure S16). Control experiments have been performed to
II
II
demonstrate the participation of [(Ru −H)(μ-bpp)Co ] in the 1-
II
dodecene hydrogenation: After quenching the [(Ru −H)(μ-bpp)
II
Co ] species by air, n-dodecane was not produced, indicating the
II
II
crucial role of the [(Ru −H)(μ-bpp)Co ] species in the hydro-
genation (Figure S17).
E
DOI: 10.1021/acs.inorgchem.9b02104
Inorg. Chem. XXXX, XXX, XXX−XXX