Synergistic Cu2 Catalysts for Formic Acid Dehydrogenation

Article abstract of DOI:10.1021/jacs.9b03532

Hexanuclear copper hydride complexes, [Cu₆(μ₃-H)₂(meso-L⁴)₃(RNC)₄](PF₆)₄ (R = ^tBu (6a), Cy (6b)), were prepared by using a new linear tetraphosphine, meso-Ph₂PCH₂P(Ph)(CH₂)₄P(Ph)CH₂PPh₂ (meso-L⁴), and were converted into active catalysts of [Cu₂(μ-O₂CH)(meso-L⁴)(RNC)₂]⁺ under the reaction conditions of formic acid dehydrogenation, where unsymmetric dinuclear copper sites supported by the tetradentate phosphine and isocyanide ligands were essential to demonstrate effective catalytic activity.

Full text of DOI:10.1021/jacs.9b03532

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Communication
2
Synergistic Cu Catalysts for Formic Acid Dehydrogenation
Takayuki Nakajima, Yoshia Kamiryo, Masayo Kishimoto, Kaho
Imai, Kanako Nakamae, Yasuyuki Ura, and Tomoaki Tanase
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03532 • Publication Date (Web): 14 May 2019
Downloaded from http://pubs.acs.org on May 14, 2019
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Synergistic Cu2 Catalysts for Formic Acid Dehydrogenation
9
Takayuki Nakajima*, Yoshia Kamiryo, Masayo Kishimoto, Kaho Imai, Kanako
Nakamae, Yasuyuki Ura, Tomoaki Tanase*
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Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoya-
nishi-machi, Nara 630-8506, Japan
Supporting Information Placeholder
the Cu2(–H) unit.9 In contrast, similar syntheses using those with
ABSTRACT: Hexanuclear copper hydride complexes, [Cu6(μ3–
H)2(meso–L4)3(RNC)4](PF6)4 (R = tBu (6a), Cy (6b)), were
different central methylene chains and configurations at the inner P
atoms (n = 2: meso– and rac–L2 (dpmppe), 3: meso–L3 (dpmppp))
afforded tri- and tetranuclear copper hydride complexes 2–5, which
were entirely inert against CO2.10 These results strongly suggested
that the structures and reactivity of multinuclear copper hydride
complexes could be tuned by varying the multidentate phosphine
ligands.
prepared by using
a new linear tetraphosphine, meso–
Ph2PCH2P(Ph)(CH2)4P(Ph)CH2PPh2 (meso–L4), and were
converted into active catalysts of [Cu2(O2CH)(meso–
L4)(RNC)2]+ under the reaction conditions of FA dehydrogenation,
where unsymmetric dinuclear copper sites supported by the
tetradentate phosphine and isocyanide ligands were essential to
demonstrate effective catalytic activity.
BF4
Ph
*
*
Ph
P
Ph2P
P
n P
Ph
PPh2
P
Ph
Ph2P
Cu
L1
n = 1: dpmppm (
)
Cu
Ph2P
P
PPh2
L2
2: dpmppe (
3: dpmppp (
4: dpmppb (
)
)
)
H
Hydrogen gas is one of the best promising energy carriers, if it
is green, since it can be storable and used on demand to
complement intermittent energy gaps with an aim of constructing
sustainable energy systems with minimizing environmental
impacts. From a view point of storing and transporting H2 gas
efficiently as non-hazardous material, formic acid (FA) has been
L3
P
PPh2
L4
Ph
Ph
1
L1
: n = 1 (meso- )
Ph2
P
Ph2
P
BF4
(PF6)2
PhP
Ph
Cu
PPh2
Cu
Ph
Ph
P
PPh2
H
considered as
a viable hydrogen carrier, and hence, its
Cu
P
P
H
H
H
decomposition into H2 and CO2 has been extensively studied as
well as hydrogenation of CO2 into FA.1 While noble metal
homogeneous catalysts based on Ru, Rh, and Ir have widely been
investigated,1 non-noble base metallic systems except for Fe and
Ni1 are yet to be studied, and in particular, selective decomposition
of FA by Cu based systems is still largely unexplored; only very
poor catalytic activity was reported with Cu(OAc)2 and Et3N
combination2 although gas-phase FA decomposition was recently
reported.3 For the past a couple of decades, copper hydride species
have attracted considerable interest due to their application for the
chemoselective reductions of unsaturated organic compounds
including CO2.4,5 The reactivity and selectivity are manipulated by
the nature of the copper hydride intermediates, which have
originally been supported by monophosphine ligands.6 In recent
years, however, with an aim of establishing synergistic reactivity
arising from multinuclear Cu–H species, several groups have
reported syntheses of CuH complexes supported by di- and
tridentate phosphine ligands, such as 1,2-bis(diphenylphos-
phino)benzene (dppbz), bis(diphenylphosphino)methane (dppm),
bis(diphenylphosphino)amine (dppa), and 1,1,1-tris(diphenyl-
phosphinomethyl)ethane, as well as N-heterocyclic carbenes.7,8
n
n
Cu
Cu
Cu
Cu
Ph
P
Ph2P
P
P
Ph2P
PPh2
Ph
Ph
Ph2P
PPh
2
L2
L3
: n = 2 (meso-
)
)
L1
n = 1 (meso-
)
3
: n = 3 (meso-
Ph2
P
Ph2
P
Ph
Ph
(PF6)2
(PF6)2
Ph
P
Ph
P
Ph2P
L
Cu
P
P
Cu
Cu
PPh2
Cu
Cu
H
H
L
H
H
Cu
L
L
Ph2P
P
P
Cu
PPh2
Cu
Ph
Ph
P
P
P
Ph2
P
Ph
Ph2
Ph
L2
L2
n = 2 (rac-
n = 2 (meso-
)
)
t
t
5
L = BuNC ( ), CyNC, XylNC
4
L = BuNC ( ), CyNC
Figure 1. Structures of tetradentate phosphine ligands L1–4 and
copper hydride complexes 1–5.
In the present study, we have utilized a new tetradentate
phosphine ligand, meso–L4 (n = 4, dpmppb) (Figure 1) to elucidate
its effects on the structures and reactivity of copper hydride
complexes, and synthesized hexanuclear Cu–H complexes that
catalyze a facile decomposition of FA into H2 and CO2. After
optimizing catalytic conditions, an unsymmetric dinuclear copper
motif bridged by the tetraphosphine was proposed as an active
species.
Recently we have synthesized di- and tetranuclear copper
hydride complexes [Cu2(H)(mesoL1)2]+ (1) and [Cu4(4H)-
(H)2(mesoL1)2]+ supported by a linear tetraphosphine ligand (n
= 1: L1 = dpmppm) (Figure 1), that showed facile reactivity toward
CO2 to give formate-bridged complexes via insertion of CO2 into
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Reaction of meso–L4 with CuCl and RNC in the presence of
1
2
3
4
5
6
7
NaBH4 and NH4PF6 afforded colorless crystals of [Cu6(μ3–
Scheme 1. Reactions of 6a with CO2 or HCO2H.
t
H)2(meso–L4)3(RNC)4](PF6)4 (R = Bu (6a), Cy (6b)) (Figure 2).
The crystal structures of 6a,b (Figures 2b, S1, and S2) were
essentially similar to consist of two trinuclear copper units,
{Cu3(μ3–H)(meso–L4)(RNC)2}2+, connected by one meso–L4 with
crystallographically imposed inversion center. Each Cu3 unit is
bridged by a 3–hydride to result in distorted triangular cores,
unlike the isosceles triangles of 2 and 3 with meso–L2,3 (Figure 1).
CO2 (1 atm), r.t.
HBF4
L4
[Cu2(O2CH)(meso- )L2]+
6a
HCO2H
quant.
N2 (1 atm), r.t.
HCO2H
L = tBuNC, CH3CN
7a
H2
8
9
(a)
Ph
(PF6)4
Several efforts were made to synthesize dinuclear copper
Ph2
P
Ph
P
2 L
complexes supported by tetradentate phosphine ligands Ln similar
to 7a. Reaction of meso–L3 with [Cu(CH3CN)4]PF6 in CH3CN
afforded [Cu2(meso–L3)(CH3CN)3](PF6)2 (8) (Figure S3), which
possesses a cradle-type unsymmetric dinuclear scaffold with
Cu∙∙∙Cu = 3.3830(7) Å. Although 8 itself showed poor activity
P
PPh2
Cu
L
Ph
Ph
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Cu
Cu
H
P
P
P
H
P
Cu
Ph
Cu
Ph
L
Cu
P
P
P
Ph2P
Ph2
Ph2
L
Ph
t
t
6a
6b
L = BuNC ( ), CyNC (
)
(conv. 9%) for FA decomposition, addition of BuNC (2 eq.) to 8
increased the catalytic activity drastically (conv. 100%) (Table S8),
demonstrating that tBuNC is indispensable to active species which
are likely formulated as [Cu2(O2CH)(mesoLn)(tBuNC)2]PF6 (n
= 3, 4).
(b)
Table 1. FA Dehydrogenation with Various Phosphine Ligandsa
[Cu(CH3CN)4]PF6
Phosphine ligands
tBuNC (1.0 eq. vs Cu)
NEt3 (0.4 eq. vs FA)
HCO2H
H2 + CO2
70 oC, 3 h, CD3CN
entry Ligandb
Conv.
(%)
7
TON
TOF
Figure 2. (a) Schematic structure of 6a and 6b. (b) Perspective
view for the complex cation of 6a.
(Cu mol-1) (h-1·Cu mol-1)
1
2
3
4
5
6
7
8
9
0.5 meso–L1
5.3×10
0
1.8×10
0
0.5 rac–L1
0.5 meso–L2
0.5 rac–L2
0.5 meso–L3
0.5 rac–L3
0.5 meso–L4
2 PPh3
0
Complex 6a reacted gradually with CO2 (1 atm) at r.t. in CD3CN,
and after 30 h, the hydride peak at –0.22 ppm for 6a in H NMR
3.7×102
4.7×102
6.4×102
2.1×102
7.2×102
6.8×10
4.7×102
1.7×102
3.0×10
1.1×102
1.2×102
1.6×102
2.1×102
7.0×10
2.4×102
2.3×10
1.8×102
5.5×10
1.0×10
3.8×10
1
49
62
85
28
95
9
63
22
4
spectra diminished and a characteristic signal for a formate
complex 7a appeared at 8.85 ppm (Scheme 1). When the solution
containing 7a was stored under N2 for 30 h, 6a was regenerated
(Figure S16). Treatment of 7a with HBF4 afforded FA
quantitatively (Scheme 1). In light of the stoichiometric reactions,
those of 6a with FA were investigated to explore catalytic activity
for FA decomposition. Reaction of 6a with ca. 50 eq. of FA in
CD3CN for 20 h afforded 7a quantitatively with formation of H2
and CO2, which were confirmed by GC and 1H/13C NMR
techniques. After 60 h, all FA was consumed and 6a was recovered
in ~50% yield as a mixture with 7a (Figure S17). The ESI–MS
spectra of the reaction mixture showed a mono-cation peak at m/z
843.098 corresponding to {Cu2(O2CH)(meso–L4)}+. Complex 7a
is thus assumed an unsymmetric dinuclear copper formate complex
of [Cu2(O2CH)(meso–L4)L2]PF6 (L = tBuNC or CH3CN)
(Scheme 1), which is consistent with the 1H{31P} NMR spectra of
the reaction mixture (Figure S17(c)). The bridging mode of the
formate, O,O’ or O, was not determined. It should be noted that
O’Hair et al. recently reported gas-phase catalytic decomposition
of FA, in which bimetallic [MM’(–H)(–dppm)]+ (M, M’ = Cu,
Ag, Au) were estimated as active species on the basis of MS
experiments and DFT calculations.3
1 dppmc
10
11
12
1 dppec
1 dpppc
1 dppbc
15
a
To a NMR tube were added [Cu(CH3CN)4]PF6 (1.3 mg, 3.4 mol),
phosphine (6.8 mol of P), CD3CN (0.45 mL), tBuNC (0.30 mg 3.6 mol),
MeNO2 for internal standard (5.6 mg, 92 mmol), NEt3 (97 mg, 0.96 mmol),
and HCO2H (114 mg, 2.5 mmol, 7.5×102 eq. vs Cu). The mixture was
heated at 70 oC for 3 h. The conversion (%), TON (Cu mol-1), and TOF (h-1
1
Cu mol-1) were calculated by using internal standard method of H NMR
measurement. bThe number before the abbreviation indicates equivalent
amounts for Cu to adjust Cu:P ratio to 1:2. c Ph2P(CH2)nPPh2 (n = 1: dppm,
2: dppe, 3: dppp, 4: dppb).
To evaluate the effect of phosphine ligands on the FA
dehydrogenation, several phosphines were also examined (Table 1).
The phosphines were mixed in an appropriate ratio with
t
[Cu(CH3CN)4]PF6 in the presence of BuNC with a hope of
Since 6a showed the catalytic activity for FA dehydrogenation,
those of related copper hydride complexes such as 1–6,
[Cu6H6(PPh3)6], [Cu8H6(dppm)5](PF6)2, and [Cu3H(O2CH)-
(dppm)3]PF6 were investigated at 45 C in CD3CN for 3 h, and
surprisingly, only 6 showed remarkable activity (Table S6). Addi-
tion of NEt3 (40 mol% vs FA) enhanced catalytic activity, and
increasing temperature to 70 oC led to the best result with 6 (conv.
100%), while 4 and 5 also showed moderate activities (conv. 48
and 66%) (Table S7).
generating the dinuclear active species like [Cu2(O2CH)-
(mesoL3,4)(tBuNC)2]PF6 in situ. Among tetradentate phosphines
Ln (n = 1–4), catalytic activities are higher in an order of meso–L4
> meso–L3 > rac–L2 > meso–L2 > rac–L3 >> meso–L1 >> rac–L1.
Monophosphine such as PPh3 was not effective and among
bidentate phosphines, dppm showed a moderate activity (conv.
63%) compared with dppe, dppp, and dppb, which may indicate
preferential formation of dinuclear copper complex in the case of
8
o
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dppm.11 Among other isocyanides, CyNC gave an excellent effect
comparable to that of BuNC (Tables S11,S12). These results
H2 generation step from C3 + HCO2H to B3 + H2 is exothermic with
Go 17.6 kcal/mol probably with low energy barriers.
t
1
2
3
4
5
6
7
8
strongly suggested that unsymmetric dinuclear copper species
supported by the tetraphosphine and isocyanides are essential for
the effective catalysts. The present catalytic system could be
carried out in a large scale with a usual flask (FA/Cu = 5 x 103), the
activity of which was monitored by volume of generated H2 and
CO2 gases,12 and the best performance with 8 in the presence of
tBuNC (2 eq.) and NEt3 (0.4 eq vs. FA) at 70 ºC for 8 h reached
TON = 1.5 x 103 per Cu without obvious decomposition of the
active species generated in situ (Figure S23), which is in accord
with the results of recycling experiments with 6a (see SI).
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18
19
Scheme 2. Possible reaction mechanism for FA
dehydrogenation.
CO2
H
O
H
C
O
2+
2+
2+
H
O
O
L
L
L
L
L
L
Cu
Cu
P
Cu
Cu
Cu
Cu
20
21
22
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60
P
PhP
P
P
PhP
PhP
Ph2
PPh2
Ph2
Ph2
PPh2
PPh2
P
P
n
Ph
n
n
Ph
Ph
Cn
Figure 3. DFT optimized structures of A3, B3, C3, and GS1, and
Bn
An
simplified Gibbs free-energy profiles. Numerical values in
L = tBuNC
parentheses are difference in Go relative to A3 in kcal mol-1.
298
Estimated transition states, TS1 and TS2, for decarboxylation
HCO2H
H2
of GS1 to give C3.
Based on the results of catalytic reactions, a possible reaction
mechanism is proposed in Scheme 2. The formate complex [Cu2(–
O2CH–O,O’)(Ln)(tBuNC)2]+ (An, n = 2–4) might be generated
under the reaction conditions and structural change of the formate
ligand is likely to occur from O,O’–bridging mode to O–
bridging to form [Cu2(–O2CH–O)(Ln)(tBuNC)2]+ (Bn) due to
coordination of bulky isocyanide ligand (tBuCN), which is
supported by DFT calculations (vide infra). Since any hydride
species were not detected in the catalytic reaction conditions,
decarboxylation of Bn to give a –hydride complex, [Cu2(–
H)(Ln)(tBuNC)2]+ (Cn), is estimated as rate-determining step,
and Cn readily reacts with FA, generating Bn and H2, to complete
the catalytic cycle. The assumption is consistent with a small
primary deuterium kinetic isotope effect (KIE) of 2.98(2) derived
from the catalytic decomposition of DCOOH (see SI, Figure
S25).13
As to the rate-determining step, we have tried to find a transition
state (TS) from B3 to C3, and at first, obtained a high energy ground
state (GS1) that involves a loosely trapped formate in O,H–
fashion before C–H cleavage reaction (Figures 3 and S26d). DFT
calculations were performed to find a transition state between GS1
and C3, but reliable structures were not obtained presumably due to
shallow potentials. Nevertheless, on the basis of GS1, two
transition states, TS1 and TS2, could be envisaged for C–H bond
cleavage of the formate (Figure 3). A hydride abstraction proceeds
on one Cu side with anchoring O atom to another Cu center in TS1,
and this type of mechanism was proposed by O’Hair et al. for the
gas-phase FA dehydrogenation.3a Alternatively,
a hydride
abstraction is promoted on the both Cu centers with H bridging
fashion in TS2, which is reminiscent of the transition state
estimated for insertion of CO2 into Cu2(H) unit of 1.8 The present
DFT calculations do not determine the exact reaction mechanism,
but demonstrate that synergistic effects of the unsymmetric Cu2
unit play important role for the hydride abstraction; stabilization of
the hydride species as the Cu2(H) form might be a driving force
of -hydride elimination.
In order to confirm the proposed mechanism, theoretical
calculations with DFT methods were carried out on a model system
with meso–L3, including [Cu2(–O2CH–O,O’)(meso–L3)-
(tBuNC)2]+ (A3), [Cu2(–O2CH–O)(meso–L3)(tBuNC)2]+ (B3),
and [Cu2(–H)(meso–L3)(tBuNC)2]+ (C3) (Figures 3 and S26). Due
to repulsive interaction with bulky tBuNC ligands, the formate–O
bridged structure B3 (Cu∙∙∙Cu = 3.503 Å) is appreciably stable by
Go 3.3 kcal/mol in comparison with the formate–O,O’ bridged
structure A3 (Cu∙∙∙Cu = 3.909 Å), and is thus estimated to be
involved in the catalytic cycle. The H structure of C3 is also
identified with appropriate metric parameters (Cu∙∙∙Cu = 2.773 Å),
which are comparable to those of 18 (Figures S26 and S27). These
results revealed that the 2,2–bridging mode of meso–L3, entirely
close to that of 8, can accommodate the unsymmetric dicopper
centers flexibly to promote the decomposition. The difference of
Gibbs free energy between B3 and C3 + CO2 is +8.7 kcal/mol,
indicating an endothermic process for the decarboxylation, and the
In conclusion,
a
new linear tetraphosphine, meso–
Ph2PCH2P(Ph)(CH2)4P(Ph)CH2PPh2 (meso–L4) was synthesized
and utilized to create hexanuclear copper hydride complexes, [Cu6-
(μ3–H)2(meso–L4)3(RNC)4](PF6)4 (R = tBu, Cy), that were found to
catalyze a facile decomposition of FA into H2 and CO2. After
optimizing catalytic conditions, an unsymmetric dinuclear copper
motif bridged by meso–L3,4, [Cu2(meso–L3,4)(tBuNC)2]2+, was
proposed as an active species to promote the FA decomposition by
virtue of synergistic effects of the Cu2 scaffold. The present system
is the first viable catalytic dehydrogenation of FA by using cheap
base metallic Cu complexes.
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Jordan, A. J.; Lalic, G.; Sadighi, J. P. Coinage Metal Hydrides: Synthesis,
Characterization, and Reactivity. Chem. Rev. 2016, 116. 8318.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b#####.
Experimental procedure, NMR and ESI mass spectra, Tables for
catalytic FA decomposition, DFT optimized structures (PDF).
Crystallographic data for 6a,b and 8.
1
2
3
4
5
6
7
8
(5) (a) Motokura, K.; Kashiwame, D.; Takahashi, N.; Miyaji, A.; Baba,
T. Highly Active and Selective Catalysis of Copper Diphosphine
Complexes for the Transformation of Carbon Dioxide into Silyl Formate.
Chem. Eur. J. 2013, 19, 10030. (b) Zhang, L.; Cheng, J.; Hou, Z. Highly
efficient catalytic hydrosilylation of carbon dioxide by an N-heterocyclic
carbene copper catalyst. Chem. Commun. 2013, 49, 4782. (c) Shintani, R.;
Nozaki, K. Copper-Catalyzed Hydroboration of Carbon Dioxide.
Organometallics 2013, 32, 2459. (d) Zall, C. M.; Linehan, J. C.; Appel, A.
M. Triphosphine-Ligated Copper Hydrides for CO2 Hydrogenation:
Structure, Reactivity, and Thermodynamic Studies. J. Am. Chem. Soc. 2016,
138, 9968. (e) Jordan, A. J.; Wyss, C. M.; Bacsa, J.; Sadighi, J. P. Synthesis
and Reactivity of New Copper(I) Hydride Dimers. Organometallics 2006,
35, 613. (f) Zall, C. M.; Linehan, J. C.; Appel, A. M. A Molecular Copper
Catalyst for Hydrogenation of CO2 to Formate. ACS Catal. 2015, 5, 5301.
(g) Watari, R.; Kayaki, Y.; Hirano, S.; Matsumoto, N.; Ikariya, T.
Hydrogenation of Carbon Dioxide to Formate Catalyzed by a Copper/1,8-
Diazabicyclo[5.4.0]undec-7-ene System. Watari, R.; Kayaki, Y.; Hirano,
S.; Matsumoto, N.; Ikariya, T.; Adv. Synth. Catal. 2015, 357, 1369.
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AUTHOR INFORMATION
Corresponding Authors
*t.nakajima@cc.nara-wu.ac.jp
*tanase@cc.nara-wu.ac.jp
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ORCID
Takayuki Nakajima: 0000-0003-3884-9205
Kanako Nakamae: 0000-0002-4256-9305
Yasuyuki Ura: 0000-0003-0484-1299
Tomoaki Tanase: 0000-0003-1838-0112
Synthesis
and
Molecular
Geometry
of
Hexameric
Triphenylphosphinocopper(1) Hydride and the Crystal Structure of
H6Cu6(PPh3)6 HCONMe. Inorg. Chem. 1972, 11, 1818. (b) Stevens, R. C.;
McLean, M. R.; Bau, R. Neutron Diffraction Structure Analysis of a
Hexanuclear Copper Hydride Complex, H6Cu6[P(p-tolyl)3]6: An
Unexpected Finding. J. Am. Chem. Soc. 1989, 111, 3472. (c) Goeden, G.
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Copper Polyhydrides. J. Am. Chem. Soc. 1985, 107, 7774. (e) Albert, C. F.;
Healy, P. C.; Kildea, J. D.; Raston, C. L.; Skelton, B. W.; White, A. H.
Lewis-Base Adducts of Group 11 Metal(1) Compounds. 49. Structural
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by a Grant-in-Aid for Scientific
Research (no. 18H03914) and on Priority Area 2107 (no.
22108521, 24108727) and 2802 (no. 17H05374, 19H04582)
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan. The computations were performed using
Research Center for Computational Science, Okazaki, Japan.
The authors thank Prof. Kohtaro Osakada of Tokyo Institute of
Technology for help in MS spectral measurements.
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and
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