Cooperative effects of heterodinuclear IrIII?MII complexes on catalytic H2 evolution from formic acid dehydrogenation in water

Article abstract of DOI:10.1021/acs.inorgchem.0c00812

Novel heterodinuclear Ir^III?M^II complexes (M = Co, Ni, or Cu) with two adjacent reaction sites were synthesized by using 3,5-bis(2-pyridyl)-pyrazole (Hbpp) as a structure-directing ligand and employed as catalysts for H₂ evolution through formic acid dehydrogenation in water. A cooperative effect of the hetero-metal centers was observed in the H₂ evolution in comparison with the corresponding mononuclear Ir^III and M^II complexes as the components of the Ir^III?M^II complexes. The H₂ evolution rate for the Ir^III?M^II complexes was at most 350-fold higher than that of the mononuclear Ir^III complex. The catalytic activity increased in the following order: Ir^III?Cu^II complex < Ir^III?Co^II complex < Ir^III?Ni^II complex . The Ir^III?H intermediates of the Ir^III?M^II complexes were successfully detected by ultraviolet?visible, ¹H , nuclear magnetic resonance, and ESI-TOF-MS spectra. The catalytic enhancement of H₂ evolution by the Ir^III?M^II complexes indicates that the Ir^III?H species formed in the Ir^III moiety act as reactive species and the M^II moieties act as acceleration sites by the electronic effect from the M^II center to the Ir^III center through the bridging bpp^? ligand. The Ir^III?M^II complexes may also activate H₂O at the 3d M^II centers as a proton source to facilitate H₂ evolution. In addition, the affinity of formate for the Ir^III?M^II complexes was investigated on the basis of Michaelis?Menten plots; the Ir^III?Co^II and Ir^III?Ni^II complexes exhibited affinities that were relatively higher than that of the Ir^III?Cu^II complex. The catalytic mechanism of H₂ evolution by the Ir^III?M^II complexes was revealed on the basis of spectroscopic detection of reaction intermediates, kinetic analysis, and isotope labeling experiments.

Full text of DOI:10.1021/acs.inorgchem.0c00812

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Article
Cooperative Effects of Heterodinuclear IrIII−MII Complexes on
Catalytic H2 Evolution from Formic Acid Dehydrogenation in Water
Dachao Hong,* Yoshihiro Shimoyama, Yuji Ohgomori, Ryoichi Kanega, Hiroaki Kotani,
Tomoya Ishizuka, Yoshihiro Kon, Yuichiro Himeda, and Takahiko Kojima
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ABSTRACT: Novel heterodinuclear IrIII−MII complexes (M =
Co, Ni, or Cu) with two adjacent reaction sites were synthesized by
using 3,5-bis(2-pyridyl)-pyrazole (Hbpp) as a structure-directing
ligand and employed as catalysts for H2 evolution through formic
acid dehydrogenation in water. A cooperative effect of the hetero-
metal centers was observed in the H2 evolution in comparison with
the corresponding mononuclear IrIII and MII complexes as the
components of the IrIII−MII complexes. The H2 evolution rate for
the IrIII−MII complexes was at most 350-fold higher than that of
the mononuclear IrIII complex. The catalytic activity increased in
the following order: IrIII−CuII complex < IrIII−CoII complex <
IrIII−NiII complex . The IrIII−H intermediates of the IrIII−MII
1
complexes were successfully detected by ultraviolet−visible, H
nuclear magnetic resonance, and ESI-TOF-MS spectra. The catalytic enhancement of H2 evolution by the IrIII−MII complexes
indicates that the IrIII−H species formed in the IrIII moiety act as reactive species and the MII moieties act as acceleration sites by the
electronic effect from the MII center to the IrIII center through the bridging bpp− ligand. The IrIII−MII complexes may also activate
H2O at the 3d MII centers as a proton source to facilitate H2 evolution. In addition, the affinity of formate for the IrIII−MII complexes
was investigated on the basis of Michaelis−Menten plots; the IrIII−CoII and IrIII−NiII complexes exhibited affinities that were
relatively higher than that of the IrIII−CuII complex. The catalytic mechanism of H2 evolution by the IrIII−MII complexes was
revealed on the basis of spectroscopic detection of reaction intermediates, kinetic analysis, and isotope labeling experiments.
evolution. For example, metal complexes with ligands having
uncoordinated functional groups (carboxyl, hydroxy, and
pyridyl groups) have been reported to exhibit high activity in
H2 evolution from formic acid dehydrogenation with high
turnover numbers (TONs).27,28 In all cases, the uncoordinated
functional groups in the mononuclear metal complexes can
bring protons into the vicinity of the active metal sites (proton
relay) toward efficient H2 evolution.27,28 However, the
efficiency of H2 evolution based on formic acid dehydrogen-
ation by mononuclear metal complexes is limited by the ability
of organic ligands as proton acceptors.
INTRODUCTION
■
Hydrogen (H2) is known as a clean and renewable energy
source because water is the sole product after H2 combustion
and H2 can be generated from water by solar energy.1−3
However, H2 gas requires high pressure and energy to remain a
liquid for transportation. Thus, extensive efforts have been
devoted to developing H2 storage technologies using metal
hydrides,4−6 metal−organic frameworks,7−9 or liquid chem-
icals.10−16 Among liquid chemicals, formic acid/formate has
been recognized as one of the promising candidates because of
its miscibility and stability in water and the relatively high
volumetric density of hydrogen (d = 1.22 g cm−3).16 The
utilization of formic acid as a H2 carrier requires efficient
catalysts to convert formic acid into H2 and CO2 in water (eq
1).
On the other hand, multinuclear metal complexes and/or
clusters are known to acts as active sites in enzymes to perform
various reactions.29−31 In particular, active sites of metal-
loenzymes composed of two sorts of metal ions have been
HCOOH → H2 + CO2
(1)
Received: March 17, 2020
Mononuclear metal complexes using Ir,17−20 Rh,21,22
Ru,23,24 and Fe25,26 have been reported to catalyze formic
acid dehydrogenation in water to produce H2. The fine-tuning
of reactivity of metal centers by coordinated organic ligands is
significantly effective for improving the efficiency of H2
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known to play cooperative roles in catalysis by the metal
centers in the proximity as observed in [CuFe]CcO
(cytochrome c oxidase),32 [CuZn]SOD (superoxide dismu-
tase),33 and [NiFe]hydrogenase.34,35 In these enzymes, each
metal center plays a different role to achieve the required task
for the enzyme as a reactive site as well as a Lewis-acidic site.
To promote H2 evolution from formic acid dehydrogenation, it
would be valid to use a dinuclear complex, in which one metal
center acts as a reactive site and the other acts as an
acceleration site in place of the aforementioned uncoordinated
protic functional groups. This strategy may provide a facile
method for preparing efficient catalysts for the reaction with
cooperative effects. However, such cooperativity of two
different metal centers has yet to be investigated and
understood in the H2 evolution from formic acid dehydrogen-
ation in water.
Previously, we reported a heterodinuclear Ru−Co complex,
[RuII(p-cymene)(μ-bpp)(Cl)CoII(dipc)(H2O)] [p-cymene =
4-isopropyltoluene, H2dipc = 2,6-pyridinedicarboxylic acid,
and Hbpp = 3,5-bis(2-pyridyl)-pyrazole], in which Hbpp was
used as a dinucleating and structure-directing ligand, to
conduct catalytic hydrogenation of alkenes with use of
NaBH4 as a hydride source.36 In the hydrogenation reactions
by the Ru−Co heterodinuclear complex, the Ru center is
converted to be a Ru−hydrido complex and the Co center acts
as an activator of a proton source.36 Thus, we assumed that the
same strategy would be effective for the H2 evolution from
formic acid dehydrogenation. On the basis of the synthesis
strategy of the heterodinuclear Ru−Co complex, we herein
design and synthesize a series of novel heterodinuclear IrIII−
MII complexes, [IrIII(Cp*)(μ-bpp)(Cl)MII(dipc)(H2O)] [M =
Co (1), Ni (2), or Cu (3); Cp* = 1,2,3,4,5-pentamethylcy-
clopentadienyl] as shown in Chart 1, as catalysts for H2
evolution based on formic acid dehydrogenation in water.
The [IrIII(Cp*)] moiety introduced in the heterodinuclear
IrIII−MII complexes is expected to act as a reactive site, because
mononuclear Ir complexes with a Cp* ligand have been
reported to show excellent activities in H2 evolution from
formic acid dehydrogenation; the Cp* ligand is suggested to
stabilize an Ir−hydrido species formed as an intermediate in
the reaction.17−20 The other [MII(dipc)] moiety is expected to
be an acceleration site for cooperative catalysis. To realize and
elucidate cooperative effects of hetero-metal centers in the H2
evolution, we have also synthesized the mononuclear Ir
complex, [IrIII(Cp*)(Mebpp)(Cl)] (4) (Mebpp = 1-methyl-
3,5-dipyridylpyrazole), and [MII(dipc)(H2O)n]2+ [M = Co (5),
Ni (6), or Cu (7)] as reference catalysts (Chart 1). The N-
methyl pyrazole moiety in complex 4 is adopted to distinguish
the cooperativity of the second metal center from an influence
of protons because the sp2 N of the pyrazole ring of the Hbpp
ligand may function as a proton acceptor.27,37 The H2
evolution from formic acid dehydrogenation was examined in
aqueous solutions at 25 °C. The heterodinuclear IrIII−MII
complexes exhibit cooperative effects on the H2 evolution in
terms of H2 evolution rates as compared to mononuclear Ir
complex 4.
The H2 evolution rate for 2 was >350-fold higher than that
of mononuclear Ir complex 4. The IrIII−H intermediates of the
heterodinuclear IrIII−MII complexes in the H2 evolution were
detected by ultraviolet−visible (UV−vis), 1H nuclear magnetic
resonance (NMR), and ESI-TOF-MS spectra. We have gained
insights into the cooperative effects of heterodimetal centers
on the H2 evolution from formic acid dehydrogenation based
on kinetic analysis, electrochemical, and isotope labeling
experiments.
RESULTS AND DISCUSSION
Synthesis and Characterization of Heterodinuclear
■
Metal Complexes. Heterodinuclear complexes 1−3 were
38
obtained by reacting [IrIII(Cp*)(Hbpp)(Cl)]BF4 and the
respective [MII(dipc)(H2O)n] (M = Co for 1, Ni for 2, and Cu
for 3)36,39 in MeOH. The formation of complexes 1−3 was
1
confirmed by ESI-TOF-MS measurements (Figure S1). H
NMR spectra of complexes 1−3 revealed the paramagnetic
feature derived from the CoII, NiII, and CuII center, respectively
(Figure S2). The reference mononuclear IrIII complex (4) was
Chart 1. Structures of the Catalysts Used in This Work
1
also synthesized and characterized by ESI-TOF-MS and H
NMR spectra (Figure S3). The crystals of 1−3 were obtained
by vapor diffusion of Et2O as a poor solvent to MeOH
solutions of 1−3. We have successfully determined the crystal
structures of complexes 1−3 by X-ray crystallography. ORTEP
drawings of 1−3 are displayed in Figure 1.
The crystallographic parameters and selected bond lengths
of 1−3 are listed in Tables S1 and S2. The distances between
two metal centers in 1, 2, and 3 are 4.494(7), 4.560(5), and
4.289(5) Å, respectively. On the basis of these distances,
complexes 1, 2, and 3 should have enough space to capture
small molecules such as formic acid between the two metal
centers. In both complexes 1 and 2, cobalt and nickel centers
are located outside of the mean plane of the bpp− ligand due to
hydrogen bonding between the aqua (1) or methanol (2)
ligand and the Cl ligand (O5−Cl bond length, 3.219 Å in 1
and 3.238 Å in 2). The result suggests that the two metal
centers fixed in the proximity of each other enable the
heterodinuclear complexes to provide a cooperative effect
when potential substrates are trapped in clefts between the two
metal centers. As for 3, the copper center held no sixth ligand
as compared to 1 and 2 and showed the Cu−N4 bond of 2.221
Å due to the Jahn−Teller effect is longer than the Co−N4
(2.133 Å) and Ni−N4 (2.093 Å) bonds. In addition, a τ
B
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evolution rates. Complex 2 having the Ir and Ni centers
exhibited the highest H2 evolution rate (0.17 μmol min−1)
among the heterodinuclear complexes (1 for 0.13 μmol min−1
and 3 for 0.038 μmol min−1) and mononuclear complex 4 (4.3
× 10−4 μmol min−1). In addition, complex 2 also exhibited a
H2 evolution rate that was higher than that of [IrIII(Cp*)-
(Hbpp)(Cl)]+ (0.061 μmol min−1) with a nonmethylated bpp
ligand (Figure S4), suggesting the advantage of the second
metal ions in the structures. A high TON of >4200 was also
observed for 2 in an elongated reaction time (75 h) as shown
in Figure S5, indicating the durability of the catalyst. The H2
evolution rate observed for 2 was significantly enhanced >350-
fold as compared to that observed for 4 and the mixture of 4
with 5, 6, or 7 (Figure 2b). No H2 evolution was obtained for
complexes 5−7, suggesting the IrIII moiety of 1−3 acts as the
active metal site for H2 evolution. The catalytic activity of H2
evolution by the heterodinuclear complexes decreased in the
following order: 2 > 1 > 3 > 4. Thus, the results demonstrate
that the cooperative effect of hetero-metal centers has
improved the catalytic activity in H2 evolution.
Figure 1. ORTEP drawings (50% probability thermal ellipsoids) of
dinuclear complexes 1−3. Hydrogen atoms except water and
methanol have been omitted for the sake of clarity.
Time courses of the H2 evolution for 1−3 were also
examined at various pHs in formate buffers by varying the
ratios of HCOOH and HCOONa while maintaining the sum
concentration of 0.10 M (Figure S6). The pH dependence of
the H2 evolution rates for 1−3 is displayed in Figure 2d. The
H2 evolution rates for 1 and 2 increased along with a decrease
in pH in the region from 7.1 to 3.6 and exhibited saturation
below pH 3.6. In light of the pKa value of formic acid (pKa =
3.75),20 the concentration of HCOOH should be important
for the H2 evolution rates for 1 and 2. In contrast, the H2
evolution rate for 3 exhibited a remarkable increase below pH
4 and exhibited the highest H2 evolution rate at pH 2.4 as a
lower limit. The sequential increase of H2 evolution rates in
accordance with an increase in proton concentration was also
observed in the reported mononuclear Ir complexes.20,27b
Solvent effects were also examined in the H2 evolution by 2 in
various solvents in the presence of 0.10 M formic acid as
shown in Figure S7a. A small amount of H2 evolution was
observed in protic solvents [MeOH (pKa = 15.5), EtOH (pKa
= 15.9), and iPrOH (pKa = 16.5)] in the order MeOH >
EtOH > iPrOH but not in aprotic solvents [MeCH and N,N-
dimethylacetamide (DMA)]. The dependence of H2 evolution
rates on the concentration of H2O in MeCN demonstrates that
H2O is required for improving the efficiency of H2 evolution
(Figure S7b).
value40 around the copper center in 3 was calculated to be
0.37, indicating a pseudo-square-pyramidal geometry around
the CuII center in 3.
Cooperative Catalytic H2 Evolution through Formic
Acid Dehydrogenation by the Hetero-Metal Com-
plexes. Catalytic H2 evolution from formic acid dehydrogen-
ation using complexes 1−4 as catalysts was examined in an
aqueous formate buffer solution (HCOOH/HCOONa, 0.10
M, pH 3.6) at 25 °C. The evolved gases were quantified by gas
chromatography (GC). The evolution of H2 gas and an
equimolar amount of CO2 gas was observed from the reaction
solutions with complexes 1−4, and no CO was detected by
GC. Figure 2 shows the time courses of H2 evolution by 1−4
and the mixtures of 4 with 5, 6, or 7 together with their H2
Detection of the IrIII−MII Intermediate in H2
Evolution. So far, an intermediate in formic acid dehydrogen-
ation by mononuclear IrIII complexes has been characterized as
IrIII−H species.41−43 In this work, we have also successfully
detected the Ir−HIII species at the Ir moiety of complexes 1−3
1
by UV−vis, H NMR, and ESI-TOF-MS measurements. The
spectroscopic and spectrometric results for 2 are displayed in
Figure 3. The UV−vis spectrum of 2a showed a new
absorption band at λmax = 357 nm in 0.10 M formate buffer
at pH 3.6 (Figure 3a,b). The peak cluster of the species formed
was observed by ESI-TOF-MS to confirm the formation of an
IrIII−H species (2a in Figure 3a) in the course of H2 evolution.
When deuterated formate (DCOONa) was used in place of
HCOONa, the peak cluster was shifted from 796.10 to 797.18
(Figure 3c), demonstrating the formation of the corresponding
IrIII−D species. The 1H NMR spectrum of 2 in 0.10 M formate
buffer in D2O exhibited sharp signals in the paramagnetic
region (Figure 3d).44 The peak observed at −11.19 ppm
Figure 2. Time courses of H2 evolution from the HCOOH/
HCOONa (0.10 M, pH 3.6) buffer (4.0 mL) containing a catalyst
[(a) 1−4 and (b) 4, 4 with 5, 4 with 6, and 4 with 7] (0.025 mM) at
25 °C under an Ar atmosphere. (c) H2 evolution rates of catalysts.
The inset shows an expanded presentation for the rates for 4, 4 with
5, 4 with 6, and 4 with 7. (d) pH dependence on the initial rates of H2
evolution by 1−3.
C
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exhibited a distance that was shorter than those of 1a (4.390
Å) and 2a (4.427 Å). The results can explain that the IrIII−H
1
signal of H NMR for 3a shows a larger paramagnetic shift
compared to those for 1a and 2a because the distance from the
paramagnetic MII centers intensely influences the degree of a
45,46
1
paramagnetic shift of a H NMR signal.
In addition, IrIII−
H···H2O−CoII(NiII) hydrogen bonding was observed on the
optimized structures of 1a and 2a (Figure S11). This result
suggests that the internal proton of the aqua ligand bound to
the CoII or NiII center may assist the H2 evolution. TD-DFT
calculations based on the optimized structures of IrIII−H
intermediates 1a−3a were also conducted to indicate that the
oscillator strengths were in agreement with the observed UV−
vis spectra of the intermediates, allowing us to assign the
absorption around 360 nm to the MLCT transitions in the
(bpp)IrIII−H moieties (Figure S12).
Kinetic Analysis of the Heterodinuclear Complexes
for H2 Evolution. To scrutinize the details of the reaction
mechanism and the key factor of the cooperative effect in the
heterodinuclear complexes, we conducted kinetic analyses on
the H2 evolution from formic acid dehydrogenation. The
dependence of initial H2 evolution rates on the catalyst
concentrations was investigated for 1−4 in formate buffers
(HCOOH/HCOONa in H2O) at pH 3.6 or pD 3.6 at 25 °C
as shown in Figure 4 (see also Figures S13−S15 for the time
Figure 3. (a) Schematic illustration of the formation of IrIII−hydrido
species 2a by the reaction of 2 with formate (HCOO− or DCOO−).
(b) UV−vis spectra of 2 in the water (black line) or 0.10 M formate
buffer (red lines) under an Ar atmosphere at room temperature. (c)
ESI-TOF-MS spectra of 2a in a H2O/HCOONa or D2O/DCOONa
solution diluted by MeOH or MeOD, respectively. (d) 1H NMR
spectra of 2a formed by the reaction of 2 (2.0 mM) with HCOONa
or DCOONa in D2O under an Ar atmosphere at 25 °C.
should be derived from the IrIII−H species (2a) because the
chemical shift fell within the range of those reported for IrIII−
H species.41−43 Additionally, the H NMR peak of an IrIII−H
1
species (4a) derived from 4 was also observed at −11.60 ppm
(Figure S8). Furthermore, the use of 0.10 M deuterated
formate buffer in D2O resulted in the disappearance of the
1
signal at −11.19 ppm in the H NMR spectrum (Figure 3d),
indicating that the hydrido ligand in 2a should be derived from
the formyl hydrogen in formic acid. In the same manner, the
formation of the corresponding IrIII−H species 1a and 3a was
also confirmed by UV−vis, 1H NMR, and ESI-TOF-MS
spectra as shown in Figures S9 and S10. The 1H NMR peaks of
IrIII−H species 1a and 3a were observed at −11.28 and −20.90
ppm, respectively. These results demonstrate that the IrIII−H
species should be formed by the reaction of formate with the
IrIII moiety of 1−3 to act as the reactive intermediates.
Figure 4. Concentration dependence of catalysts [(a) 1, (b) 2, (c) 3,
and (d) 4] on the initial H2 evolution rates. The initial H2 evolution
rates were obtained from the time course of H2 evolution in H2O with
HCOOH/HCOONa (0.10 M, 4.0 mL, pH 3.6), H2O with DCOOH/
DCOONa (0.10 M, 4.0 mL, pH 3.6), or D2O with HCOOH/
HCOONa (0.10 M, 4.0 mL, pD 3.6) containing a catalyst (0.025
mM) at 25 °C under an Ar atmosphere.
We have calculated optimized structures of the IrIII−H
species 1a−3a using density functional theory (DFT)
calculations to support the experimental results for the
intermediate detection. The optimized structures and selected
bond lengths for the calculated IrIII−H species are displayed in
Figure S11 and Table S3. One aqua ligand is coordinated to
the CoII and NiII centers of 1a and 2a but not to the CuII
center of 3a corresponding to the coordination environments
found in the crystal structures as shown in Figure 1. On the
basis of the calculated distance between the H atom of an IrIII−
H moiety and a MII center, the IrIII−H species of 3a (4.278 Å)
courses of H2 evolution). The catalyst concentrations of 3 and
4 were made lower (<25 μM) due to their limited solubility in
water. The initial H2 evolution rates for 1−4 increased in
proportion to the increase in the catalyst concentration. The
turnover frequencies (TOFs) calculated from the slopes in
Figure 4 are summarized in Table 1. Complex 2 exhibited the
highest rate (1.2 min−1) in comparison with those of 1 (0.96
min−1), 3 (0.43 min−1), and 4 (4.3 × 10−3 min−1). The first-
order dependence of the initial rates on the catalyst
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Table 1. Summary of Kinetic Analysis of Catalytic Formic
Acid Dehydrogenation by 1−4
Scheme 1. Proposed Mechanism of H2 Evolution from
Formic Acid Dehydrogenation by 2
a
TOFD O
TOF
TOFDCOOH
b
b
2 b
catalyst (min−1
)
(min−1
)
(min−1
)
KIEDCOOH KIED O
2
1
2
3
4
0.96
1.2
0.43
0.0043
0.73
0.54
0.30
0.31
0.28
0.18
1.3
2.2
1.3
1.0
3.1
4.2
2.4
1.6
0.0043
0.0028
a
Conditions: 10−100 μM for 1 and 2, 3−25 μM for 3 and 4, 25 °C,
the solvent being formate buffer (0.10 M and pH 3.6 in H2O or pD
3.6 in D2O) or formate-d buffer (0.10 M and pH 3.6 in H2O). TOF
b
values were calculated on the basis of the slopes in Figure 4.
concentrations indicates that one molecule of a catalyst (1−4)
is involved in the rate-determining step (RDS).
The H2 evolution was also performed using deuterium-
labeled formic acid (DCOOD) and water (D2O) to investigate
kinetic isotope effects (KIEs) on the reaction by 1−4. Table 1
shows the values of TOFs using DCOOH (TOFDCOOH) and
TOFs in D2O (TOFD O) that calculated from the slopes of
2
plots depicted in Figure 4. The corresponding KIEs using
DCOOH (KIEDCOOH) and D2O (KIED O) were calculated
2
Table 2. Isotope Distribution of Hydrogen in the Catalytic
H2 Evolution from Formic Acid Dehydrogenation by 1−3
from TOFDCOOH/TOF and TOFD O/TOF, respectively. The
2
use of DCOOH instead of HCOOH in the reaction decreased
the H2 evolution rates for complexes 1−3. Complex 1
exhibited a TOFDCOOH (0.75 min−1) that was higher than
that of 2 (0.54 min−1), although the catalytic activity of H2
evolution from HCOOH using 1 was lower than that for 2. As
shown in Table 1, the KIEDCOOH values for complexes 1−3
were determined to be >1, whereas the mononuclear Ir
complex (4) exhibited no KIE, i.e., KIEDCOOH = 1. Thus, the
KIE on the formation of IrIII−H intermediates was observed
for the heterodinuclear complexes but not for the mononuclear
Ir complex. The result suggests that the 3d MII moieties of 1−3
may influence the formation of IrIII−H intermediates.
hydrogen gas (%)
entry
catalyst
substrate
solvent
H2
HD
D2
a
1
2
3
4
5
6
1
DCOOH
DCOOH
DCOOH
HCOOH
HCOOH
HCOOH
H2O
H2O
H2O
D2O
D2O
D2O
83
84
83
2
3
0
17
16
17
24
24
24
0
0
0
a
2
a
3
b
1
74
73
76
b
2
b
3
a
Conditions: 0.10 mM 1, 0.10 mM 2, 0.025 mM 3, solvent being
formate-d buffer (0.10 M and pH 3.6 in H2O), reaction time of 2 h.
Conditions: 0.10 mM 1, 0.10 mM 2, 0.025 mM 3, solvent being
formate buffer (0.10 M and pD 3.6 in D2O), pD = pH meter reading
b
On the other hand, H2 evolution rates by 1−4 were
+0.4,48 reaction time of 5.5 h.
dramatically decreased when D2O was used in place of H2O.
The values of KIED O were calculated to be 3.1 for 1, 4.2 for 2,
2
dominant product (83−84%) with a certain amount of HD
evolution (∼17%) (entries 1−3 in Table 2) in the reaction.
However, no D2 was observed in the reaction using
DCOOH. This suggests that the Ir−D species formed by the
reaction of DOO− undergoes rapid H/D exchange with H2O
to afford the IrIII−H species in H2O. Such rapid H/D exchange
in aqueous media has been reported for mononuclear IrIII−H
complexes.49,50 The rapid exchange was also confirmed in the
HCOOH dehydrogenation by 1−3 in D2O; D2 was obtained
as the main product (73−76%), and almost no H2 was evolved
in the HCOOH/D2O system (entries 4−6 in Table 2). No
difference among complexes 1−3 was observed in the isotope
distribution in the evolved dihydrogen. Compared to the
evolved HD ratio in H2O, a slightly higher HD ratio was
observed in D2O due to the H2O content (0.1%). The HD
evolution suggests a small portion of protic IrIII−H species
exists in D2O, which is consistent with the observation of IrIII−
2.4 for 3, and 1.6 for 4. All of the observed values of KIED O for
2
complexes 1−4 exhibited larger values compared to those of
KIEDCOOH. The results indicate that the formation of Ir−HIII
intermediates by the reaction of formate with the catalysts
occurs faster than that of IrIII−H intermediates with protons.
Thus, the RDS in the catalytic system should be a reaction of
IrIII−H intermediates with oxonium ions (protons) to evolve
H2 (Scheme 1).47 The larger KIED O values for 1 and 2
2
compared to those of 3 and 4 may suggest that CoII and NiII
centers of 1 and 2 activate the coordinated H2O as a proton
source to assist the reaction of IrIII−H intermediates and
oxonium ions, resulting in higher H2 evolution rates for 1 and
2 (Figure 2). This assumption can be supported by the results
of DFT calculations for IrIII−H intermediates 1a and 2a
(Figure S11), in which the aqua ligands bound to CoII and NiII
centers are within the range of distances to form intramolecular
hydrogen bonding with the hydride ligands.
1
H intermediates by H NMR spectra measured in D2O.
The isotope distribution in the evolved dihydrogen (H2,
HD, and D2) by 1−3 was scrutinized using DCOOH in H2O
and HCOOH in D2O. The observed distribution of the
dihydrogen is summarized in Table 2. When DCOOH
dehydrogenation by 1−3 in H2O was performed, H2 was the
Difference in Reactivity between IrIII−MII Complexes.
The dependence of the initial rates of H2 evolution on the
concentration of formate buffer (0.010−0.20 M) was examined
for 1−4. The results exhibited saturation behaviors at higher
concentrations of HCOOH as shown in Figure 5, indicating a
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spectral changes of 1−3 were observed when sodium acetate
was added to the aqueous solutions of 1−3; however, no
spectral change was observed for 4. The CH313COO−-
coordinated species of 4 was also evidenced by a 13C NMR
spectrum (Figure S18). The formation of these acetate-
coordinated species was supported by DFT calculations
(Figure S19). Bridging coordination of acetate to dinuclear
complexes 1 and 2 and monodentate coordination of acetate to
the IrIII centers of complexes 3 and 4 were suggested to
support the arguments about the participation of the CoII and
NiII centers in formate binding. The results may also suggest
that the difference in the structures of formate-bound
intermediates should affect the feasibility of activating the
formate ion toward the formation of the IrIII−H intermediates.
To evaluate the electronic effect of 3d metal ions in the
dinuclear complexes, we have conducted electrochemical
measurements in formate buffer. Under the conditions,
complexes 1−4 have converted to corresponding IrIII−H
species as described above (Figure 3). Therefore, the
electrochemical events observed at the positive side of the
rest potentials contributed to the oxidation potential (Eox) of
the IrIII−H species (Figure S20). The Eox values of the IrIII−H
species were determined on the basis of Osteryoung square-
wave voltammetry (OSWV) techniques (Table 3 and Figure
S14). The Eox values for 1 and 2 were more negative than those
observed for 3 and 4, indicating the stronger reductants of 1
and 2 were consistent with the higher reactivity in H2
evolution as mentioned above. Thus, the NiII and CoII centers
of 1 and 2 influence the reactivity of the corresponding IrIII−H
species. On the other hand, the Eox value for 3 was comparable
with that of 4, although higher reactivity of H2 evolution was
observed for 3. These results suggest that the higher electron
density at the IrIII−H moieties for 1 and 2 provides the higher
hydrido character that also contributes to promoting the
efficiency of H2 evolution. This result may be supported by the
fact that bond lengths of the IrIII−Cl bonds in 1−3 decrease in
the following order: 1 [2.4183(6) Å] ∼ 2 [2.4129(6) Å] > 3
[2.3996(8) Å] (Table S2). Thus, the anionic character of the
chloro ligand should be stronger for 1 and 2 than for 3, in turn,
that of the hydrido ligands of the IrIII−H intermediates.
Mechanistic Insight into the Catalytic H2 Evolution
from Formic Acid Dehydrogenation. On the basis of the
observations including the detection of the IrIII−H inter-
mediates and kinetic analysis mentioned above, the catalytic
cycle by 2 in a formate buffer solution is proposed as shown in
Scheme 1. Complex 2 reacts with formate to form a plausible
formate-coordinated species that releases CO2 to produce the
IrIII−H intermediates through β-elimination. The NiII center is
demonstrated to improve the reactivity of the IrIII−H species.
In light of the crystal structure of 1 or 2 (Figure 1), an aqua
ligand on the CoII or NiII center is directed to the hydrido
ligand at the IrIII center, respectively, as demonstrated by DFT
calculations (Figure S11). It is reasonable to propose that the
MeOH molecule bound to a labile NiII center should be easily
substituted by a H2O molecule when 2 is dissolved in an
aqueous buffer solution. Because a CoII center of a
heterodinuclear RuII−CoII complex in our previous report36
can activate methanol as a proton source to enhance
hydrogenation reactions, a similar phenomenon may occur in
the H2 evolution by the dinuclear IrIII−MII complexes. Thus,
the CoII center in 1 and the CoII center in 2 are also expected
to contribute to the activation of the aqua ligand to enhance
H2 evolution. The aqua ligand in the close vicinity of the
Figure 5. Concentration dependence of HCOOH/HCOONa (4.0
mL, pH 3.6) on the initial H2 evolution rate for (a) 1 (50 μM, red
dots), 2 (50 μM, orange dots), 3 (25 μM, green dots), and (b) 4 (25
μM, blue dots). The initial H2 evolution rates were obtained from the
time course of H2 evolution in various concentrations of formate
buffers (0.010−0.20 M).
coordination equilibrium between catalysts and formate. To
investigate the affinity of formate for complexes 1−4, we have
made curve fits of the plots based on the Michaelis−Menten
equation (eq 2)
v = Vmax[HCOOH]/(KM + [HCOOH])
(2)
where v is the initial H2 evolution rate and Vmax is the
maximum H2 evolution rate. The KM values for 1 and 2 are
smaller than that of 3 but larger than that of 4 as shown in
Table 3. The results suggest that 1 and 2 show a greater affinity
Table 3. Comparison of Michaelis-Menten Constants (KM),
V
1−4
max Values, and Oxidation Potentials of IrIII−H Species for
a
b
catalyst
KM (M−1
)
Vmax (μmol min−1
)
Eox for IrIII−H species
1
2
3
4
0.027
0.026
0.050
0.016
0.28
0.27
0.059
0.00050
+0.18(1)
+0.18(1)
+0.28(2)
+0.27(3)
a
Conditions: 50 μM 1, 50 μM 2, 25 μM 3, 25 μM 4, 298 K, solvent
b
being formate buffer (0.010−0.20 M and pH 3.6 in H2O). The Eox
values were determined from electrochemical measurements con-
ducted in formate buffer (0.10 M and pH 3.6) containing 1, 2, or 3
(0.50 mM) at 25 °C. Eox values were determined from the average of
three different runs of Osteryoung square-wave voltammetry
(OSWV), and the potential was referenced to a Ag/AgCl (saturated
KCl solution) electrode.
for formate compared to that of 3. The greater affinity of 1 and
2 can be ascribed to the fact that the formate anion can bind to
the cleft between IrIII/CoII and IrIII/NiII centers in 1 and 2,
respectively, as a bridging ligand (Scheme 1). On the other
hand, the CuII center in 3 should be less cooperative in the
binding of formate due to the Jahn−Teller effect, resulting in
the weak affinity. This result suggests that the second basic
metal center should participate in the binding of formate to 1
and 2. The affinity of 1−3 for the formate ion may influence
the formation rates of IrIII−H spices in the H2 evolution. As
the heterodinuclear metal complexes (1−3) are uncharged, the
greatest affinity of 4 can be ascribed to the positive charge of 4.
The obtained Vmax values for 1−4 are consistent with the
results in Figure 2. The possible formate-coordinated species of
complexes 1−4 were indirectly characterized by ESI-TOF-MS
spectrometry by using the acetate ion instead of the formate
ion (Figure S16). The acetate-coordinated species of 1−4 were
also characterized by UV−vis spectroscopy (Figure S17). The
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[CuII(dipc)(H2O)2],39 and [IrIII(Cp*)(Cl)(Hbpp)]BF4 were syn-
38
hydrido ligand at the IrIII center can efficiently react with the
hydrido ligand to generate H2. In the case of 3, the oxidation
potential of the IrIII−H species (3a) is similar to that of 4a.
The CuII center of 3a can interact with a water molecule
thesized according to the reported procedure.
Synthesis of [IrIII(Cp*)(Cl)(μ-bpp)CoII(dipc)(H2O)] (1). A
solution of [CoII(dipc)(H2O)3] (28.2 mg, 0.10 mmol) in 10 mL of
MeOH was added to [(Cp*)Ir(Cl)(Hbpp)]BF4 (67.6 mg, 0.10
mmol) in 10 mL of MeOH, and the mixture turned orange. The
reaction mixture was stirred at room temperature for 17 h under air.
The resultant solution was filtered to remove the undissolved
precipitate, and the filtrate was evaporated under reduced pressure.
The residue was dissolved in a small amount of MeOH, and Et2O was
poured into the mixture to obtain a pink-orange solid. The solid was
washed with Et2O and dried in vacuo to give a pale yellow powder
weakly in aqueous solution judging from the KIED O values in
2
Table 1. The weakly trapped water molecule at the CuII center
in the vicinity of the IrIII−H may cause the higher reactivity of
3 in the H2 evolution as compared to that of 4.
CONCLUSION
■
1
We have designed and successfully synthesized novel
heterodinuclear IrIII−MII [M = Co (1), Ni (2), or Cu (3)]
complexes and used them as efficient catalysts for H2 evolution
from formic acid dehydrogenation in water at 25 °C. The
crystal structures of 1−3 were determined by X-ray
crystallography, revealing that the two metal centers (IrIII
and MII) are located close to each other to function
cooperatively in the catalysis. The comparison of H2 evolution
rates was performed among the heterodinuclear metal
complexes (1−3), a mononuclear IrIII complex (4), and the
corresponding MII complexes (5−7) that are the components
of complexes 1−3. The H2 evolution rate for 2 was 350-fold
higher than that of 4 and the mixture of the corresponding
component metal complexes. The significant enhancements of
H2 evolution rates for the heterodinuclear metal complexes
should be derived from the cooperative effect of IrIII and MII
centers. The enhancement was observed in the order 2 > 1 > 3,
indicating the introduction of a NiII center, which is less
distorted octahedral and makes the aqua ligand more acidic, is
effective for H2 evolution through the cooperative effect. The
formation of IrIII−H intermediates (1a−3a) was observed by
(77.2 mg, 0.096 mmol, 95% yield based on the Ir source). H NMR
(CD3OD): δ 102.63, 85.28, 75.73, 42.74, 38.74, 38.41, 36.09, 23.20,
19.55, 13.37, 13.00, 1.85, −0.53, −1.78. Anal. Calcd for
C30H30ClCoIrN5O5·2.5H2O: H, 3.93; C, 41.36; N, 8.04. Found: H,
3.61; C, 41.11; N, 7.98. MS (ESI-TOF, MeOH): m/z 827.11 (calcd
for [1 − OH2 − Cl− + MeO− + Na+]+ 827.09).
Synthesis of [IrIII(Cp*)(Cl)(μ-bpp)NiII(dipc)(MeOH)] (2). A 4
mL methanol solution of [NiII(dipc)(H2O)3] (28.7 mg, 0.10 mmol)
was added to [(Cp*)Ir(Cl)(Hbpp)]BF4 (67.2 mg, 0.10 mmol)
suspended in 4.0 mL of MeOH. The mixture turned yellow-green.
The reaction mixture was stirred at room temperature for 20 h under
air. The resultant solution was concentrated under reduced pressure,
and Et2O was poured into the mixture, resulting in the formation of a
needle-shaped crystalline solid. The solid was washed with Et2O and
dried in vacuo to give a yellow-green powder (72.4 mg, 0.090 mmol,
1
90% yield based on the Ir source). H NMR (CD3OD): δ 68.92,
67.06, 65.25, 49.72, 48.19, 30.03, 19.59, 19.26, 15.65, 15.07, 10.35,
9.76, −0.27. Anal. Calcd for C31H31ClIrN5NiO5·2.75H2O: C, 41.86;
H, 4.14; N, 7.87. Found: C, 41.69; H, 3.83; N, 7.79. MS (ESI-TOF,
MeOH): m/z 826.08 (calcd for [2 − MeOH − Cl− + MeO− + Na+]+
826.11).
Synthesis of [IrIII(Cp*)(Cl)(μ-bpp)CuII(dipc)] (3). A solution of
[CuII(dipc)(H2O)2] (30.7 mg, 0.10 mmol) in 3.0 mL of MeOH was
added to a solution of [(Cp*)Ir(Cl)(Hbpp)]BF4 (69.0 mg, 0.10
mmol) suspended in MeOH. The mixture turned green. The reaction
mixture was stirred at room temperature for 17 h under air. The
resultant solution was concentrated under reduced pressure, and Et2O
was poured into the mixture, resulting in the formation of a
precipitate. The solid was washed with Et2O and dried in vacuo to give
a pale-green powder (87.8 mg, 0.10 mmol, quant. based on the Ir
1
UV−vis, ESI-TOF-MS, and H NMR measurements. On the
basis of the results, a plausible mechanism was proposed as
shown in Scheme 1. Michaelis−Menten analysis has unraveled
that complexes 1 and 2 have affinities for formate that are
greater than that of 3. Plausible formate-coordinated species of
1−3, which should be a coordination equilibrium of formate,
can release CO2 to yield corresponding IrIII−H intermediates
as reactive species, followed by the reaction with protons to
evolve H2 molecules. Kinetic analysis and isotope labeling
experiments have revealed that the RDS step of the H2
evolution by 1−3 involves a reaction of the IrIII−H
intermediates and oxonium ions. The higher activity of 1
and 2 compared to the activity of 3 and 4 is attributed to the
fact that a H2O molecule coordinated to CoII and NiII centers
of 1 and 2 supplies a proton to assist the H2 evolution. The Cu
center in 3 can trap a water molecule weakly in the vicinity of
the reactive IrIII−H site, resulting in an activity that is higher
than that of 4. In addition, the lower oxidation potentials of the
IrIII−H species 1a and 2a may also contribute to the
enhancement of H2 evolution. Finally, we believe this work
will provide valuable fundamentals for the development of
heterodinuclear molecular complexes for designing efficient
homogeneous catalysts through cooperative effects of two
different metal ions located in adjacent positions.
1
source). H NMR (CD3OD): δ 11.67, 10.58, 8.88, 8.39, 7.65, −0.22.
Anal. Calcd for C30H27ClCuIrN5O4·0.5H2O: H, 3.43; C, 43.85; N,
8.52. Found: H, 3.33; C, 43.86; N, 8.40. MS (ESI-TOF, MeOH): m/z
777.08 (calcd for [3 − Cl−]+ 777.10).
Synthesis of [IrIII(Cl)(Mebpp)]BF4 (4·BF4). [IrIII(Cl)2(Cp*)]2
(121 mg, 0.15 mmol) was slowly dropped into a methanol solution
containing Mebpp (79.3 mg, 0.34 mmol) and NH4·BF4 (102 mg, 0.97
mmol), and the mixture was stirred for 3 h at 25 °C under air. The
solution was concentrated under reduced pressure to a small volume.
A yellow precipitate that formed in the solution was removed by
filtration and washed with a small amount of methanol. A yellow
powder of 4·BF4 (189.3 mg, 0.28 mmol, 92% yield) was obtained. 1H
NMR (CD3OD): δ 8.95 (d, J = 5.2 Hz, 1H, Py-a), 8.83 (d, J = 4.8 Hz,
1H, Py-a′), 8.33 (d, J = 7.6 Hz, 1H, Py-d), 8.25 (td, J = 8.0 Hz, J = 1.6
Hz, 1H, Py-c), 8.07 (td, J = 8.0 Hz, J = 1.6 Hz, 1H, Py-c′), 7.99 (d, J =
7.6 Hz, 1H, Py-d′), 7.75 (ddd, J = 7.2 Hz, J = 5.2 Hz, J = 1.2 Hz, 1H,
Py-b), 7.75 (s, 1H, Pz), 7.57 (ddd, J = 7.6 Hz, J = 4.8 Hz, J = 1.2 Hz,
1H, Py-b′), 4.48 (s, 3H, Pz-Me), 1.77 (s, 15H, Cp*-Me). Anal. Calcd
for C24H27BClF4IrN4: H, 3.97; C, 42.02; N, 8.17. Found: H, 3.86; C,
42.05; N, 8.06. MS (ESI-TOF, MeOH): m/z 599.12 (calcd for [4]+
599.15).
EXPERIMENTAL SECTION
General. Chemicals and solvents were purchased from commercial
sources and used as received unless otherwise mentioned. H NMR
X-ray Crystallography. Single crystals of 1−3 suitable for X-ray
crystallography were obtained by recrystallization with vapor diffusion
of Et2O into a methanol solution of 1, 2, and AcOEt into a methanol
solution of 3. A crystal was mounted on the goniometer using a
mounting loop. All diffraction data were collected on a Bruker
APEXII diffractometer at 120 K with a graphite-monochromated Mo
Kα radiation source (λ = 0.71073 Å) by 2θ scanning. The structure
■
1
measurements were performed on a Bruker AVANCE 400
spectrometer and a JEOL JNM-ECX 400 spectrometer. ESI-TOF-
MS spectra were measured on a JEOL JMS-T100CS spectrometer.
The Mebpp ligand,51 [CoII(dipc)(H2O)3],36 [NiII(dipc)(H2O)3],39
G
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was determined by a direct method (SIR-97)52 and expanded with a
differential Fourier technique. All non-hydrogen atoms were refined
anisotropically, and the refinement was carried out with full-matrix
least squares on F. All calculations were performed using the
Yadokari-XG crystallographic software package.53 Crystallographic
data for 1−3 are summarized in Table S1. In the structural
refinements, the contribution of the solvent molecules of crystal-
lization was subtracted from the diffraction pattern by the “Squeeze”
program.54 The crystallographic data of 1−3 have been deposited in
the Cambridge Crystallographic Data Centre (CCDC) as CCDC
entries 1971003 (1), 1971004 (2), and 1971005 (3).
AUTHOR INFORMATION
Corresponding Author
■
Dachao Hong − Interdisciplinary Research Center for Catalytic
Chemistry, National Institute of Advanced Industrial Science
and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan;
Global Zero Emission Research Center, National Institute of
Advanced Industrial Science and Technology, Tsukuba, Ibaraki
305-8569, Japan; orcid.org/0000-0003-0581-1315;
Email: hong-d@aist.go.jp
Authors
ESI-TOF-MS Measurements to Detect the Hydrido Species
Derived from 1−3. A catalyst (1.0 mg) dissolved in 2,2,2-
trifluoroethanol (TFE, 50 μL) was added to formate buffer (0.10
M, 1.0 mL) in a vial at room temperature. The reaction solution (100
μL) was diluted in MeOH (1.0 mL). A small portion of the reaction
mixture was taken by a gastight syringe and subjected to ESI-TOF-MS
measurements. The orifice voltage was adjusted to 30 V. The
deuterium labeling experiments were performed using the same
procedure with the use of DCOOH/DCOONa in D2O, and MeOD
(methanol-d1) in place of HCOOH/HCOONa in H2O and MeOH.
H2 Evolution Reaction. A typical procedure is as follows. A vial
containing a formate buffer (0.10 M, 4.0 mL) was sealed with a
rubber septum and deaerated by being purged with Ar for 20 min. A
deaerated TFE solution (50 μL) of a catalyst was injected into the
formate buffer in the vial by a syringe to start the reaction. After a
certain reaction time, the evolved gas in the headspace of the reaction
vial was sampled by a gastight syringe (100 μL) and quantified by a
Shimadzu GC-2014 gas chromatograph [Ar carrier gas, a packed
Shincarbon ST column (φ 3 mm × 4 m) at 120 °C] equipped with a
thermal conductivity detector (TCD). H2, HD, and D2 gases were
analyzed on a GL Science GC-390 gas chromatograph [Ne carrier,
Hydro Isopack (2.0 m, 4.0 mm inside diameter, GTR TEC Co., Ltd.)
at 77 K (liquid N2)] with a TCD (150 °C, 60 mA).
Electrochemical Measurements. Electrochemical measure-
ments were performed on a potentiostat/galvanostat instrument
(HZ-7000, HOKUTO DENKO). Cyclic voltammetry was carried out
in a formate buffer (0.10 M, pH 3.6) containing a catalyst (0.50 mM)
at 298 K with use of glassy carbon as the working electrode, a
platinum wire as the counter electrode, and a Ag/AgCl (saturated KCl
solution) reference electrode. Osteryoung square-wave voltammetry
(OSWV) was performed from the rest potentials of catalyst solutions
to 0.8 V.
DFT Calculations. DFT calculations were carried out for the
potential energy surfaces using the B3LYP method.55 For the Ir, Ni,
Co, and Cu atoms, we used the SDD basis sets,56 and for the H, C, N,
O, and Cl atoms, we used the 6-311G(d,p) basis sets.57 TD-DFT
calculations58 were performed on the basis of the optimized
structures. The Gaussian 09 program package59 was used for all
DFT calculations.
Yoshihiro Shimoyama − Interdisciplinary Research Center for
Catalytic Chemistry, National Institute of Advanced Industrial
Science and Technology (AIST), Tsukuba, Ibaraki 305-8565,
Japan; orcid.org/0000-0003-3728-9381
Yuji Ohgomori − Interdisciplinary Research Center for Catalytic
Chemistry, National Institute of Advanced Industrial Science
and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
Ryoichi Kanega − Global Zero Emission Research Center,
National Institute of Advanced Industrial Science and
Technology, Tsukuba, Ibaraki 305-8569, Japan; orcid.org/
0000-0003-0790-956X
Hiroaki Kotani − Department of Chemistry, Faculty of Pure and
Applied Sciences, University of Tsukuba, CREST, Japan Science
and Technology Agency (JST), Tsukuba, Ibaraki 305-8571,
Japan; orcid.org/0000-0001-7737-026X
Tomoya Ishizuka − Department of Chemistry, Faculty of Pure
and Applied Sciences, University of Tsukuba, CREST, Japan
Science and Technology Agency (JST), Tsukuba, Ibaraki 305-
8571, Japan; orcid.org/0000-0002-3897-026X
Yoshihiro Kon − Interdisciplinary Research Center for Catalytic
Chemistry, National Institute of Advanced Industrial Science
and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan;
Global Zero Emission Research Center, National Institute of
Advanced Industrial Science and Technology, Tsukuba, Ibaraki
305-8569, Japan; orcid.org/0000-0002-7217-0557
Yuichiro Himeda − Global Zero Emission Research Center,
National Institute of Advanced Industrial Science and
Technology, Tsukuba, Ibaraki 305-8569, Japan; orcid.org/
0000-0002-9869-5554
Takahiko Kojima − Department of Chemistry, Faculty of Pure
and Applied Sciences, University of Tsukuba, CREST, Japan
Science and Technology Agency (JST), Tsukuba, Ibaraki 305-
8571, Japan; orcid.org/0000-0001-9941-8375
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00812
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00812.
ACKNOWLEDGMENTS
■
This work was partly supported by Grants-in-Aid (24245011,
20K15328, 17H03027, and 15J04635) from the Japan Society
for the Promotion of Science (JSPS). D.H. gratefully
acknowledges support from JSPS by the Leading Initiative
for Excellent Young Researchers (LEADER). The authors also
acknowledge Dr. Hiroki Nagashima (AIST) for assisting with
Figures S1−S14 and Tables S1−S6 (PDF)
Accession Codes
CCDC 1971003−1971005 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
paramagnetic H NMR measurements.
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