Hydrolysis of B2pin2 over Pd/C Catalyst: High Efficiency, Mechanism, and in situ Tandem Reaction

Article abstract of DOI:10.1002/ejic.202001132

A facile and effective synthesis of H₂ or D₂ from Pd/C catalyzed hydrolysis of B₂pin₂ has first been developed. Among them, B₂pin₂ is frequently used for borylation reaction, and has rarely been used for hydrogen evolution. The kinetic isotope effects (KIEs) and tandem reaction for diphenylacetylene and norbornene hydrogenation have confirmed both two H atoms of H₂ gas are provided from H₂O. This is contrary to other boron compounds hydrolysis (including NH₃BH₃, NaBH₄), which generates H₂ with only one H atom provided by water and the other one by boron compounds. Note that the hydrolysis of B₂pin₂ in D₂O also provides an easy and useful synthesis of D₂.

Full text of DOI:10.1002/ejic.202001132

Communications
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Hydrolysis of B2pin2 over Pd/C Catalyst: High Efficiency,
Mechanism, and in situ Tandem Reaction
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Ning Li+,[a] Jialu Shen+,[a] and Xiang Liu*[a]
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by Song’s group (Scheme 1a).[27] In 2016, Prabhu’s group further
A facile and effective synthesis of H2 or D2 from Pd/C catalyzed
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confirmed that clean hydrogen gas is produced from water by
Pd(OAc)2-catalyzed hydrolysis of B2pin2 (Scheme 1b).[28,29] In
2019, with a combined ethanol/B2pin2 system as the hydrogen
source, Jin and Liu’s group also reported CuCl/Ligand-catalyzed
semihydro-genation of alkynes to synthesize Z-alkenes with
excellent stereoselectivity (Scheme 1c).[30] As a part of our
current studies on the development of new hydrogen storage
materials and new catalytic system for H2 evolution,[31] herein,
we first report Pd/C as an efficient and reusable catalyst for H2
evolution from the hydrolysis of B2pin2 (Scheme 1d). To our
knowledge, this is the first systematic study of hydrogen
evolution from the hydrolysis of B2pin2, although Prabhu’s
group and Nielsen and Skrydstrup’s group mentioned H2
evolution from the hydrolysis of B2pin2.[25,28] Among them, Pd/C
is a very cheap and commercial source of palladium,[32] it shows
efficient catalytic activity in the H2 evolution even without
ligands. We found Pd/C, as a heterogeneous catalyst, has been
successfully recycled 5 times without any activity loss for H2
evolution by the simple filtration. In addition, kinetic isotope
effects (KIEs) and tandem reaction for diphenylacetylene and
norbornene hydrogenation have also been investigated for the
mechanism study of the B2pin2 hydrolysis.
hydrolysis of B2pin2 has first been developed. Among them,
B2pin2 is frequently used for borylation reaction, and has rarely
been used for hydrogen evolution. The kinetic isotope effects
(KIEs) and tandem reaction for diphenylacetylene and norbor-
nene hydrogenation have confirmed both two H atoms of H2
gas are provided from H2O. This is contrary to other boron
compounds hydrolysis (including NH3BH3, NaBH4), which gen-
erates H2 with only one H atom provided by water and the
other one by boron compounds. Note that the hydrolysis of
B2pin2 in D2O also provides an easy and useful synthesis of D2.
Introduction
Over the past two decades, it has become an significant
mainstream in the world to explore clean and renewable
energy, due to the ever-increasing of the depletion of non-
renewable fossil resources (including natural gas, oil and coal)
and the crazy development of global economy.[1–4] Among all
kinds of renewable energy resources, hydrogen is regarded as
one of the most potential alternative source with high chemical
energy (142 MJ/kg) due to its clean, abundant, efficient and
sustainable characteristics.[5–7] However, the safe production,
storage and transport of H2 is still a challenging task. As H2 is in
the form of gas at normal atmospheric pressure and needs low
temperature to be liquefied H2 (70 g/L at 20 K) or high pressure
to be converted to compressed H2 (40 g/L at 700 bar), which
limits the development of hydrogen energy technology .[8,9]
Recently, catalytic hydrolysis of hydrogen storage materials
(such as borohydrides, organic compounds and metal hydrides)
has attracted more and more attention, because it is a safe,
facile and efficient method for producing, transporting and
storing hydrogen.[10–17] Typically, B2pin2 (bis(pinacolato)diboron)
is often applied as a current borylation source,[18–22] and it has
rarely been used for reduction reactions.[23–26] For example, in
2016, Pd(OAc)2-catalyzed transfer hydrogenation of N-heteroar-
omatics via a combined B2pin2/H2O system, had been reported
[a] N. Li,+ J. Shen,+ Prof. Dr. X. Liu
College of Materials and Chemical Engineering,
Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion
Materials,
Analysis and Testing Center, China Three Gorges University,
Yichang, Hubei 443002, China
E-mail: xiang.liu@ctgu.edu.cn
[+] These authors are contributed equally to this work and should be considered
as co-first authors.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202001132
Scheme 1. B2Pin2-Assisted Transfer Hydrogenation Reactions
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Results and Discussion
slope of the logarithmic plot of the H2 evolution rate and the
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Pd/C concentration is 0.80, indicating that the generation rate
of H2 is first-order in Pd/C concentration. At the same time, The
Figure 3 exhibits that the plot of H2 generation vs. concen-
tration of B2pin2 is 0.70, showing the activation of B2pin2 is also
first-order. Then the time dependence of H2 generation at
various temperatures (from 298 K to 313 K) has been inves-
tigated in the Figure 4a. The activation energy is 25.65 KJ/mol
for Pd/C based on the Arrhenius equation (Figure 4b).
First, the size of Pd/C is 3.29 nm, confirmed by transmission
electron microscopy (TEM) in the Figure 1. The B2pin2 hydrolysis
catalyzed by various amount of Pd/C (from 0.2 mol% to
1.0 mol%) have been conducted at 30 C (Eq. 1). The plots of
the hydrogen volume vs. time for various amounts of Pd/C has
°
been shown in Figure 2a. The Figure 2b represents that the
Our previous work has demonstrated that both of the
hydrogen atoms in H2 comes from water for the metal-catalyzed
B2(OH)4 hydrolysis, and kinetic isotope effect (KIE) has confirmed
that the OÀ H bond cleavage of H2O is the rate-determining
step.[31] Here, the KIE using D2O instead of H2O for Pd/C
catalyzed H2 evolution upon B2pin2 hydrolysis has also been
investigated. Indeed, the KIE value found for the present
reaction (Figure 5) including the B2pin2/D2O system, kH/kD =
3.26, is very high and represents a primary KIE, confirming that
the OÀ H bond cleavage of H2O is the rate-determining step.
The durability of Pd/C catalyst has been investigated due to
its importance for practical applications (Figure 6). Pd/C has
been collected and reused by filtration, washing and drying
after the reaction. The H2 evolution has been re-initiated by
adding the re-obtained Pd/C into another fresh solution of
B2pin2 and H2O. The result exhibited that Pd/C catalyst has been
reused 5 times with a slightly decrease in the catalytic activity.
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Figure 1. (a) The TEM of Pd/C; (b). Distribution diagram of Pd/C.
Figure 2. (a) Time plots of the catalytic evolution of H2 by various amount of
Pd/C; (b) Plots of the rates of H2 generation vs. the amount of the Pd/C, both
on natural logarithmic scale.
Figure 5. Hydrogen evolution from B2pin2 with H2O and D2O.
Figure 3. (a) Plots of the volume of hydrogen generated vs. time for the
evolution of H2 catalyzed by 4 mol% Pd/C at various amount of B2pin2; (b)
and the plot of hydrogen generation rate versus the concentration of B2pin2
both in natural logarithmic scale.
Figure 4. (a) Plots of volume of H2 vs. time for the 0.4 mol% of Pd/C
catalyzed hydrolysis of B2pin2 at different temperatures; (b) Arrhenius plots
obtained from the kinetic data.
Figure 6. The B2pin2 hydrolysis in the presence of 0.4% Pd/C during the
reusability test.
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For further studying H2 evolution upon the hydrolysis of
50% deuteriated (Eq. 2 and Figure S6). The uneven and
excessive distribution is probably attributed to the facile
migration of H or D atoms between the homobenzylic and
benzylic positions.[33,34] For the hydrogenation of norbornene,
the desired product 4b is provided in 99% yield (Eq. 3 and
Figure S7), exactly verifying that both two D atoms of D2 gas
are provided from D2O. This is contrary to other boron
compounds hydrolysis (including NH3BH3, NaBH4), which gen-
erates H2 with only one H atom provided by water and the
other one by boron compounds.[35]
A mechanistic proposal of Pd/C catalyzed B2pin2 hydrolysis
based on our experimental results follows (Figure 8): originally,
intermediate A has been formed by oxidative addition of B2pin2
and Pd/C. Then intermediate B is obtained from A after the
attack of water molecules. Subsequently, B further isomerized
into intermediate C smoothly. Next, the generation of a
dihydride Pd(H)2 species from C, via releasing a molecule of
(Bpin)2O, leads to the formation of H2.[36]
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B2pin2 catalyzed by Pd/C, the tandem reaction for the hydro-
genation of 1,1-diphenylethylene and norbornene with H2,
in situ generated from B2pin2 hydrolysis, has been investigated
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in a sealed two-chamber system at 30 C for 12 h (Figure S1),
The result exhibits that all the hydrogenations afford 99% yield
of the desired products, verifying the evolution of H2 (Figure S2
and Figure S3). Moreover, the reaction product of (Bpin)2O was
also confirmed by NMR spectrum (Figure S4) and mass spec-
trum (Figure S5). While the D2 was produced from the
hydrolysis of B2pin2 with D2O. This D2 has been used for the
in situ hydrogenations of 1,1-diphenylethylene and norbornene,
respectively, catalyzed by Pd/C in the right side (Figure 7). For
the hydrogenation of 1,1-diphenylethylene (1a) with D2, the
benzylic position is 81% deuteriated whereas methyl group is
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Conclusion
In summary, a novel and efficient Pd/C-catalyzed hydrogen
production from the hydrolysis of B2pin2 has been developed
for the first time. Typically, B2pin2 is often applied as a current
borylation source, and it has rarely been used for reduction
reactions. Among them, Pd/C is a very cheap and commercial
source of palladium, it shows efficient catalytic activity in the H2
evolution even without ligands. We found Pd/C, as a heteroge-
neous catalyst, has been successfully recycled 5 times without
any activity loss for H2 evolution by simple filtration. Moreover,
kinetic isotope effects (KIEs) and tandem reaction have
confirmed both two H atoms of H2 gas are provided from H2O.
This is contrary to other boron compounds hydrolysis (including
NH3BH3, NaBH4), which generates H2 with only one H atom
provided by water and the other one by boron compounds.
Note that the hydrolysis of B2pin2 in D2O also provides an easy
and useful synthesis of D2. In addition, this system has
enormous potential to be tapped in large-scale industrial
production and utilization.
Figure 7. Sealed two-chamber system for the hydrogenation of diphenyl-
acetylene and norbornene with D2
Acknowledgements
Financial support from the National Natural Science Foundation
of China (21805166), the 111 Project (D20015), the Engineering
Research Center of Eco-environment in Three Gorges Reservoir
Region, Ministry of Education, China Three Gorges University
(KF2019-05) and the outstanding young and middle-aged science
and technology innovation teams, Ministry of Education, Hubei
province, China (T2020004) is gratefully acknowledged.
Conflict of Interest
The authors declare no conflict of interest.
Figure 8. Proposed mechanism for the hydrolysis of B2Pin2 reaction
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Manuscript received: December 16, 2020
Revised manuscript received: December 30, 2020
Accepted manuscript online: January 5, 2021
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Hydrolysis of B2pin2 over Pd/C
Catalyst: High Efficiency,
Mechanism, and in situ Tandem
Reaction
The new H2 evolution upon B2pin2 hy-
drolysis is efficiently catalyzed by Pd/
C. The kinetic isotope effects and
tandem reaction have verified both
two H atoms of H2 gas are provided
from H2O. Note that the hydrolysis of
B2pin2 in D2O also provides an easy
and useful synthesis of D2. In addition,
this system has enormous potential to
be tapped in large-scale industrial
production and utilization.
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