General transfer hydrogenation by activating ammonia-borane over cobalt nanoparticles

Article abstract of DOI:10.1039/c5ra19869k

Cobalt nanoparticles containing both Co²⁺ and Co⁰ species supported on carbon nitride can function as heterogeneous nanocatalysts for a general transfer hydrogenation reaction in aqueous ammonia-borane solution at room temperature. The conversions of nitroarenes, olefins, imines, aldehydes, ketones and cyanobenzene are high with superior selectivity under mild conditions. This noble-metal-free catalyst is also cheap and reusable.

Full text of DOI:10.1039/c5ra19869k

RSC Advances
View Article Online
View Journal | View Issue
PAPER
General transfer hydrogenation by activating
ammonia-borane over cobalt nanoparticles†
Cite this: RSC Adv., 2015, 5, 102736
*
Tian-Jian Zhao,‡ Ya-Nan Zhang,‡ Kai-Xue Wang, Juan Su, Xiao Wei and Xin-Hao Li
Cobalt nanoparticles containing both Co2+ and Co0 species supported on carbon nitride can function as
heterogeneous nanocatalysts for a general transfer hydrogenation reaction in aqueous ammonia-borane
solution at room temperature. The conversions of nitroarenes, olefins, imines, aldehydes, ketones and
cyanobenzene are high with superior selectivity under mild conditions. This noble-metal-free catalyst is
also cheap and reusable.
Received 25th September 2015
Accepted 24th November 2015
DOI: 10.1039/c5ra19869k
www.rsc.org/advances
Ammonia-Borane (AB), a widely investigated hydrogen-rich reusability for most reactions under mild conditions.7,8 All
compound for hydrogen storage, shows great potential as current results promise a great room to develop efficient
a solid hydrogen source for hydrogenation reactions.1,2 The high heterogeneous catalysts for transfer hydrogenation of unsatu-
stability under ambient conditions in combination with its high rated organic compounds using AB,3–6 although a cheap
hydrogen content, low toxicity and high solubility in water transition-metal nanoparticle-based catalyst has not been re-
makes AB an ideal solid hydrogen source for green organic ported for AB-based transfer hydrogenation reaction as far as we
synthesis. Transfer hydrogenation of unsaturated compounds know. Obviously, cheap transition metal-based catalysts with
by using a hydrogen-containing molecule has attracted much reusability are preferred for practical use.7
attention for organic synthesis. The pioneering work using AB
Here, we report a proof-of-concept study that cobalt nano-
for hydrogenation of various unsaturated carbon and nitrogen particles with Co2+ species on the surface can be applied as
bonds promises the availability of such a strategy.3–5 Transfer efficient heterogeneous catalyst for highly effective and selective
hydrogenation of polarized C]C from AB could also proceed transfer hydrogenation reactions by AB under mild conditions
even without the presence of catalyst.3 However, organic or without sacricial additives. Our catalytic system is general for
organometallic complexes are usually utilized as homogenous transfer hydrogenation of a series of unsaturated organic
catalysts to activate AB for most transfer hydrogenation reac- compounds, including olens, nitroarenes, ketones, imines
tions due to the relatively high activation barrier for non- and aromatic aldehydes at room temperature.
activated substrates.4,5 Recent breakthroughs in applying
An in situ reduction protocol was applied to fabricate cobalt
a unique P(III) 4 P(V) redox cycle in 1,3,2-diazaphospholene for nanoparticle-carbon nitride dyad (Co/CN), whereby meso-
catalytic hydrogenation of N]N bond with AB rather suggested porous carbon nitride (CN) was used as the catalyst support.
the critical effect of redox cycle in certain organocatalyst for The formation of Co nanoparticles can be directly conrmed by
activating AB and substrates.4 Redox pairs in inorganic solids, the powder X-ray diffraction (XRD) pattern of the sample
such as Fe2+/Fe3+, Co2+/Co0 and IÀ/IO3Àpairs, might also be (Fig. S1†). Weak and broad peaks of metallic Co maybe could be
available to activate small molecules, including AB here, for distinguished in the sample of Co/CN. The presence of Co
organic synthesis.
components could not be distinguished by low-magnication
Indeed, various supported noble metal nanoparticles, transmission electron microscopy (TEM) observation due to
including single-component nanoparticles or alloy nano- the similar contrast between Co species and CN components
particles have been investigated as efficient catalysts for transfer (Fig. S3a†). Nevertheless, the well preservation of the porous
hydrogenation reactions using AB or various derivatives.6 The structure was conrmed by TEM image (Fig. 1a) aer loading Co
reusability of these noble metal heterogeneous catalysts could nanoparticles. Both the specic surface area and the size
obviously benet industrial applications. Moreover, powerful distribution (Fig. S2 and Table S1†) of the Co/CN sample
catalysts should also offer excellent activity, selectivity and decreased slightly aer deposition of Co species. HRTEM
images (Fig. 1b and S3†) revealed the formation of poly-
crystalline Co nanoparticles with tiny crystal domains sized
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University,
from several to ten nanometers, matching well with the trend
reected by weak and broad XRD peaks. Clearly, the (111) plane
of metallic cobalt with a spacing of 0.204 nm was the dominant
plane in our sample, matching well with the XRD results. The
Shanghai 200240, P. R. China. E-mail: xinhaoli@sjtu.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra19869k
‡ These authors contributed equally.
102736 | RSC Adv., 2015, 5, 102736–102740
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Table 1 Hydrogenation of nitrobenzene to aniline using ABa
Entry
Catalyst
C [%]
S [%]
1b
2c
3
4
5
—
Co/CN
CN
Co
Co/C
Co/N-LC
Co/CN
Co/CN
Co*/CN
Co(OH)2/CN
—
—
—
11
41
6
>99
—
30
—
—
—
—
68
97
71
>99
—
70
—
6
7
8d
9e
10e
a
Reaction condition: 0.5 mmol nitrobenzene, 10 mL of water, 2 mmol
AB, 20 mg of catalyst, 298 K, 5 min; conversion (C) and selectivity (S)
b
were demonstrated via GC (byproduct: nitrosobenzene). Without
c
d
catalyst. Without NH3BH3. Hydrogen gas (1 bar) was used as the
e
only hydrogen source, 12 h. 1 h.
a conversion of 11% and a selectivity of 68% to aniline and 32%
to nitrosobenzene (Entry 4, Table 1). Co nanoparticles sup-
ported on carbon black (C, surface area: 62.5 m2 gÀ1) or nitrogen
doped layered carbon (N-LC, surface area: 190 m2 gÀ1
) could
11
offer moderate conversions and selectivity (Entry 5 and 6,
Table 1). Though both the conversions and selectivity need to be
optimized, all these results conrmed the possibility of using
transition metal nanoparticles for triggering the transfer
hydrogenation reaction using AB. Surprisingly, the conversion
of nitrobenzene selectivity to aniline over Co/CN (weight
percentage of Co: 30%) both reached nearly 100% (Entry 7,
Table 1), whilst bare CN offered no conversion (Entry 3, Table 1).
These results suggested that the interaction between Co NPs
and CN indeed promotes the nal activity of Co/CN. The elec-
tron transfer from CN to metallic Co nanoparticles at their
interface was ambiguously conrmed by the signicant
decrease in the photoluminance intensity aer loading Co NPs
(Fig. S4†), whilst the band structure (Fig. S5†) and also chemical
structure (Fig. S6†) of CN were not disturbed obviously. The
rectied contact between CN and metal signicantly facilitated
the catalytic activity via Mott–Schottky effect.12,13
Fig. 1 TEM images (a) of Co/CN with mesoporous structure and
corresponding elemental mapping images (c) HRTEM image (b) and
XPS Co 2p spectrum (d) of Co species suggested a proposed structure
(e).
elemental mapping images (Fig. 1c) unambiguously demon-
strated the homogeneous distribution of Co species on the
surface of CN support. More importantly, amorphous domains
(marked with an arrow in Fig. 1b) were also observed on the
outer surface and at the intersections of Co nanocrystals,
proposed to be cobalt oxides or hydroxides as demonstrated by
the X-ray photoelectron spectroscopy (XPS) results (Fig. 1d).9 All
these results suggested a hybrid structure of Co nanoparticles
with metallic cores and amorphous shells as depicted in Fig. 1e.
It should be noted that the amorphous layer mainly formed
aer the exposure of the sample to air and could be reduced
back to metallic Co in situ by AB in the catalytic reaction. More
importantly, such an amorphous layer could further prevent
embedded Co nanoparticles from oxidation. This explains the
good stability of Co nanoparticles in Co/CN during the XPS and
XRD measurements in air. As a result, the Co/CN catalyst could
be easily stored and used under ambient conditions.
More importantly, the unique coupled structure of Co0/Co2+-
species in Co/CN was found to be the critical factor for ensuring
the high activity and selectivity of our catalyst. The Co*/CN
catalyst, obtained by using strong reductant (NaBH4), could
only offer moderate conversion and poor selectivity under xed
conditions (Entry 9, Table 1). Another control sample Co(OH)2/
CN without Co0 species (Fig. S7†) was totally inert to the same
reaction (Entry 10, Table 1).
Aromatic amines are key intermediates in the synthesis of
ne chemicals, biologically active compounds and more.10 The
efficient conversion of nitrobenzene to aniline under mild
conditions via relatively sustainable methods is thus of great
interests. We thus rstly tested the hydrogenation of nitroben-
zene at room temperature by using aqueous solution of AB and
Co nanocatalysts. Blank tests without catalyst (Entry 1, Table 1)
or AB (Entry 2, Table 1) were conducted without obvious
conversions of the substrate. Bare Co nanoparticles could give
The catalytic activity of Co/CN catalysts also depended on the
loadings of Co species on the surface of CN (Table S2†). As the
optimum catalyst for the hydrogenation reaction here, 30%
Co/CN was used for all following catalytic reactions.
Nitroarenes substituted by both electron-repelling and
electron-attracting groups could be transformed to corresponding
amines with high conversions and excellent selectivity (Table 2).
Both orientation effect (Entries 1–3 & 6–8, Table 2) and space
steric hindrance (Entries 4 and 5, Table 2) of the substituted
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 102736–102740 | 102737

View Article Online
RSC Advances
Paper
Table 2 Hydrogenation of nitroarenes using Co/CN and ABa
Table 3 Hydrogenation of olefins using Co/CN and ABa
Entry
1
Substrate
t [min]
C [%]
S [%] Entry
Substrate
t [min]
C [%]
S [%]
1
2
25
90
>99
30
>99
>99
20
25
20
20
64
46
26
65
>99
>99
>99
>99
2
3
20
30
>99
>99
>99
>99
3
4
4
5
15
30
>99
>99
>99
>99
5
6
5
>99
>99
6
7
30
15
>99
>99
>99
>99
7
30
—
—
a
Standard reaction conditions as described in Table 1.
8
30
15
30
15
>99
>99
98
>99
>99
>99
>99
Table 3), appears to be necessary to facilitate transfer hydroge-
nation for full conversion to target product within 5 min, as
demonstrated by the high conversion of cis-stilbene whilst
9
10
11
Table 4 Hydrogenation of aldehydes and ketones using Co/CN and
ABa
98
Entry
Substrate
t [min]
C [%]
>99
S [%]
>99
12
30
>99
>99
1
2
15
5
a
Standard reaction conditions as described in Table 1.
>99
>99
3
4
5
6
5
10
30
10
>99
>99
80
>99
>99
>99
>99
groups didn't disturb the hydrogenation rate of corresponding
substrates. No dehalogenation was detected for these haloge-
nated substrates (Entries 6–10, Table 2). The Co/CN catalyst also
showed its advantages over noble metal based catalysts on
resistance to the poison effect of N or S atoms. Co/CN catalyst
could remain active in the presence of sulfur, reducing 4-
nitrothioanisole to 4-aminothioanisole (Entry 11, Table 2). 6-
Nitroquinoline was also reduced in excellent yields (Entry 12,
Table 2) in the same catalytic system.
>99
We further tested the possibility of hydrogenation of olens
(Table 3) over Co/CN using AB as the hydrogen source. Cyclo-
hexene was reduced to cyclohexane (yield: 90%) without other
by-product (Entry 1, Table 3). Styrene and substituents (Entry 2–5,
Table 3) could also be generally reduced to the corresponding
alkanes with good conversions (26–65%) and excellent selec-
tivity (>99%). For internal olens a cis geometry (Entry 6,
7
10
15
64
44
>99
>99
8
a
Standard reaction conditions as described in Table 1.
102738 | RSC Adv., 2015, 5, 102736–102740
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
trans-stilbene was totally unreactive. The signicant difference
between the conversions of cis- and trans-stilbene may be due to
steric constraints or to the orientation of the interaction
involved between the olens and the surface of the catalyst.14
We also investigated the hydrogenation of aromatic alde-
hydes and ketones in our catalytic system. Benzaldehyde and
substituents (Entries 1–4, Table 4) were completely reduced to
alcohols within short time. 2-Furaldehyde was found to reach
a yield of 99% for furfuryl alcohol (Entry 6, Table 4). Note that
such hydrogenation reactions were usually conducted at high
temperature in high-pressure hydrogen gas.7 Moreover, both
non-activated (Entry 7, Table 4) and activated ketones (Entry 8,
Table 4) were reduced to alcohols with high selectivity.
Benzimidazole with C]N bond and cyanobenzene with
C^N bond could be transferred to secondary amine (yield:
99%) and benzylamine (yield: 82%, byproduct: 18% phenyl-
methanimine) respectively under standard conditions (Scheme
S1†). All these results revealed the generality of our catalytic
system for hydrogenation of unsaturated compounds. The
transfer hydrogenation reactions could be scaled up facilely
with constant yields for typical substrates (Table S3†), prom-
ising the great potential of Co/CN as sustainable catalysts for
practical use.
In order to demonstrate the reaction path of AB and various
substrate over Co/CN, a series of control reactions were con-
ducted. Taking the transfer hydrogenation of nitrobenzene as
an example, complete conversion of substrate could be achieved
within 2 minutes under standard conditions with a conversion
of AB of 25% (Fig. 2a and S8†). When substrate was not
involved, 10 mL of H2 forms in our catalytic system via the
decomposition of AB with a conversion of AB to be 14%. No
obvious conversion of nitrobenzene (Entry 8, Table 1) could be
observed over Co/CN when hydrogen gas (1 bar) was used for
the hydrogen reaction at room temperature for 12 h, excluding
the possible role of as-formed hydrogen gas as reductant here.
The fact that possible intermediates between AB and substrate
were not detected in the 11BNMR spectra of byproducts from AB
(Fig. 2b) further demonstrated a direct hydrogen transfer from
AB to substrate on the surface of Co/CN. AB was nally split into
H3BO3, BO2À and other borate species in water solution,15 no
Scheme 1 Proposed reaction mechanism for transfer hydrogenation
reactions from AB over Co/CN.
matter whether organic substrates were involved. Combining
these results with the Co2+/Co0 directed hydrogenation process
of various substrates, we propose a direct hydrogen transfer
path (Scheme 1) from AB to substrate over Co/CN. Co2+/Co0
pairs in Co/CN were believed to be active centers in our catalytic
system, although more detailed investigations of the reaction
mechanism and the possibility of formation of CoHx or Co-
amidoborane intermediates are still in progress.16
Heterogeneous catalysts are preferred due to their stability
and reusability. The catalytic transfer hydrogenation of nitro-
benzene under ambient conditions was studied as a model
reaction to investigate the stability and reusability of the Co/CN
catalyst. Co/CN catalyst was reused for 9 times without any
signicant loss of catalytic activity (Fig. S9†). Aer multiple use,
a slight decrease in the crystallinity of Co saturated substrates
was observed, whilst the decrease in the amount of metallic Co
nanoparticles didn't disturb the nal activity of Co/CN at all
(Fig. S10 and 11†).
Moreover, the excellent chemoselectivity of the Co catalysts
is highly impressive here. In contrast, noble metal nanoparticle
based catalyst could also trigger the reduction of these
substrates but offer worse selectivity.6 For example, halogenated
compounds shown here could be hydrogenated with high
selectivity (>99%) over Co/CN (Entry 10 of Table 2, Entry 5 of
Table 3 and Entry 4–5 of Table 4). As shown in Table S4,†
complete or partial dehalogenation occurred in all these reac-
tions when tested over Pd/CN catalysts. Note that such a deha-
logenation reaction was general for noble metal-based
nanocatalysts.
Conclusions
In summary, noble metal-free Co/CN catalysts were successfully
applied as heterogeneous catalyst for the highly effective and
Fig. 2 Conversions of AB (b) over Co/CN with (red) or without (blue)
the presence of substrate and the 11BNMR spectra of corresponding
decomposition products of AB. Standard reaction conditions were
applied except a shorter reaction time (2 minutes). The 11BNMR spectra
of pure AB (black) was also tested and an external reference
BF3$(C2H5)2O was set as 0 ppm.
selective transfer hydrogenation of
a series of organic
compounds in aqueous AB solution. The transfer hydrogena-
tion of nitroarenes, olens, imines, aldehydes, ketones and
cyanobenzene generally proceeded under mild conditions
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 102736–102740 | 102739

View Article Online
RSC Advances
Paper
¨
(ambient atmosphere and room temperature) and afforded the
desired products in good to excellent yields in water. To the best
of our knowledge, these results constitute the rst example of
using non-noble metal heterogeneous catalyst to extract the
hydrogen of AB for organic synthesis at room temperature.
Further extending the application of rst-row transition metal
based nanocatalysts for facilitating synthetically useful trans-
formations is promising and will be tried in the near future.
5 (a) R. V. Jagadeesh, G. Wienhofer, F. A. Westerhaus,
A. E. Surkus, M. M. Pohl, H. Junge, K. Junge and M. Beller,
Chem. Commun., 2011, 47, 10972; (b) D. Wu, Y. Li,
R. Ganguly and R. Kinjo, Chem. Commun., 2014, 50, 12378
and references therein.
6 For example see: (a) C. A. Jaska and I. Manners, J. Am. Chem.
Soc., 2004, 126, 2698; (b) M. E. Sloan, A. Staubitz, K. Lee and
I. Manners, Eur. J. Org. Chem., 2011, 672; (c) H. Goksu,
¨
¨
S. F. Ho, O. Metin, K. Korkmaz, A. M. Garcia,
¨
M. S. Gultekin and S. Sun, ACS Catal., 2014, 4, 1777.
Acknowledgements
7 (a) V. Molinari, C. Giordano, M. Antonietti and D. Esposito, J.
Am. Chem. Soc., 2014, 136, 1758; (b) M. Shalom, V. Molinari,
D. Esposito, G. Clavel, D. Ressnig, C. Giordano and
M. Antonietti, Adv. Mater., 2014, 26, 1272; (c) L. B. Lv,
T. N. Ye, L. H. Gong, K. X. Wang, J. Su, X. H. Li and
J. S. Chen, Chem. Mater., 2015, 27, 544.
8 (a) L. H. Gong, Y. Y. Cai, X. H. Li, Y. N. Zhang, J. Su and
J. S. Chen, Green Chem., 2014, 16, 3746; (b) Y. N. Zhang,
X. H. Li, Y. Y. Cai, L. H. Gong, K. X. Wang and J. S. Chen,
RSC Adv., 2014, 4, 60873; (c) X. H. Li, Y. Y. Cai, L. H. Gong,
W. Fu, K. X. Wang, H. L. Bao, X. Wei and J. S. Chen,
Chem.–Eur. J., 2014, 20, 16732.
9 K. Artyushkova, S. Pylypenko, T. S. Olson, J. E. Fulghum and
P. Atanassov, Langmuir, 2008, 24, 9082.
10 N. Ono, The Nitro Group in Organic Synthesis, Wiley-VCH,
2001.
11 X. H. Li, S. Kurasch, U. Kaiser and M. Antonietti, Angew.
Chem., Int. Ed., 2012, 51, 9689.
12 (a) X. H. Li, J. S. Zhang, X. F. Chen, A. Fischer, A. Thomas,
M. Antonietti and X. C. Wang, Chem. Mater., 2011, 23,
4344; (b) Y. Wang, X. C. Wang and M. Antonietti, Angew.
Chem., Int. Ed., 2012, 51, 68; (c) X. H. Li, X. C. Wang and
M. Antonietti, Chem. Sci., 2012, 3, 2170; (d) X. H. Li,
M. Baar, S. Blechert and M. Antonietti, Sci. Rep., 2013, 3,
1743; (e) X. H. Li and M. Antonietti, Chem. Soc. Rev., 2013,
42, 6593; (f) Y. Y. Cai, X. H. Li, Y. N. Zhang, X. Wei,
K. X. Wang and J. S. Chen, Angew. Chem., Int. Ed., 2013, 52,
11822.
13 (a) J. Yang, H. Liu, W. N. Martens and R. L. J. Frost, J. Phys.
Chem. C, 2010, 114, 111; (b) N. Kumar, E. A. Payzant,
K. Jothimurugesanc and J. J. Spivey, Phys. Chem. Chem.
Phys., 2011, 13, 14735.
14 G. R. Geier and T. Sasaki, Tetrahedron, 1999, 55, 1859.
15 M. Chandra and Q. Xu, J. Power Sources, 2006, 156, 190.
16 W. Xu, G. Wu, W. Yao, H. Fan, J. Wu and P. Chen, Chem.–Eur.
J., 2012, 18, 13885 and references therein.
This work was nancially supported by National Basic Research
Program of China (2013CB934102, 2011CB808703), the
National Natural Science Foundation of China (21331004,
21301116), SJTU-UM joint grant and Eastern Scholar Program.
We thank Prof. Mark Barteau of University of Michigan for
fruitful discussion and proofreading.
Notes and references
1 (a) G. Alcaraz and S. Sabo-Etienne, Angew. Chem., Int. Ed.,
2010, 49, 7170; (b) C. W. Hamilton, R. T. Baker, A. Staubitz
and I. Manners, Chem. Soc. Rev., 2009, 38, 279; (c)
J. M. Yan, X. B. Zhang, S. Han, H. Shioyama and Q. Xu,
Angew. Chem., Int. Ed., 2008, 47, 2287; (d) J. M. Yan,
X. B. Zhang, T. Akita, M. Haruta and Q. Xu, J. Am. Chem.
Soc., 2010, 132, 5326; (e) L. T. Guo, Y. Y. Cai, J. M. Ge,
Y. N. Zhang, L. H. Gong, X. H. Li, K. X. Wang, Q. Z. Ren,
J. Su and J. S. Chen, ACS Catal., 2015, 5, 388.
2 (a) A. D. Sutton, A. K. Burrell, D. A. Dixon, E. B. Garner,
J. C. Gordon, T. Nakagawa, K. C. Ott, J. P. Robinson and
M. Vasiliu, Science, 2011, 331, 1426; (b) A. Staubitz,
A. P. M. Robertson and I. Manners, Chem. Rev., 2010, 110,
4079.
3 (a) J. S. M. Samec, J. E. Backvall, P. G. Andersson and
P. Brandt, Chem. Soc. Rev., 2006, 35, 237; (b) C. Ruchardt,
¨
M. Gerst and J. Ebenhoch, Angew. Chem., Int. Ed., 1997, 36,
1406; (c) X. Yang, L. Zhao, T. Fox, Z. X. Wang and H. Berke,
Angew. Chem., Int. Ed., 2010, 49, 2058; (d) X. Yang, T. Fox
and H. Berke, Chem. Commun., 2011, 47, 2053; (e) X. Yang,
T. Fox and H. Berke, Tetrahedron, 2011, 67, 7121; (f)
X. H. Yang, T. Fox and H. Berke, Org. Biomol. Chem., 2012,
10, 852; (g) E. M. Leitao, N. E. Stubbs, A. P. M. Robertson,
H. Helten, R. J. Cox, G. C. Lloyd-Jones and I. Manners, J.
Am. Chem. Soc., 2012, 134, 16805; (h) H. C. Johnson,
E. M. Leitao, G. R. Wjittell, I. Manners, G. C. Lloyd-Jones
and A. S. Weller, J. Am. Chem. Soc., 2014, 136, 9078.
4 C. C. Chong, H. Hirao and R. Kinjo, Angew. Chem., Int. Ed.,
2014, 53, 3342.
102740 | RSC Adv., 2015, 5, 102736–102740
This journal is © The Royal Society of Chemistry 2015