Transfer hydrogenation employing ethylene diamine bisborane in water and Pd- and ru-nanoparticles in ionic liquids

Article abstract of DOI:10.3390/molecules200917058

Herein we demonstrate the use of ethylenediamine bisborane (EDAB) as a suitable hydrogen source for transfer hydrogenation reactions on C-C double bonds mediated by metal nanoparticles. Moreover, EDAB also acts as a reducing agent for carbonyl functionalities in water under metal-free conditions.

Full text of DOI:10.3390/molecules200917058

Molecules 2015, 20, 17058-17069; doi:10.3390/molecules200917058
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
Transfer Hydrogenation Employing Ethylene Diamine
Bisborane in Water and Pd- and Ru-Nanoparticles in
Ionic Liquids
Sebastian Sahler, Martin Scott, Christian Gedig and Martin H. G. Prechtl *
Department of Chemistry, University of Cologne, Greinstrasse 6, 50939 Cologne, Germany;
E-Mails: ssahler@hiden.co.uk (S.S.); scott@itmc.rwth-aachen.de (M.S.);
cgedig@smail.uni-koeln.de (C.G.)
*
Author to whom correspondence should be addressed; E-Mail: Martin.Prechtl@uni-koeln.de;
Tel.: +49-221-4701-981; Fax: +49-221-4701-788.
Academic Editor: Nicola Cioffi
Received: 2 April 2015 / Accepted: 8 September 2015 / Published: 17 September 2015
Abstract: Herein we demonstrate the use of ethylenediamine bisborane (EDAB) as a
suitable hydrogen source for transfer hydrogenation reactions on C-C double bonds mediated
by metal nanoparticles. Moreover, EDAB also acts as a reducing agent for carbonyl
functionalities in water under metal-free conditions.
Keywords: ruthenium; palladium; transfer-hydrogenation; ethylene diamine bisborane;
ammonia borane, EDAB, reducing, carbonyl
1
. Introduction
Hydrogenation is one of the most important chemical transformations used in academia and industry
and has received notable attention in the past century [1–7]. It can be performed via transition
metal-catalyzed activation of molecular hydrogen and is used in large-scale applications such as
hydrocracking [8,9] and the Haber-Bosch-Process [10] or in medium- to lab-scale applications for the
synthesis of fine and special chemicals [1–3,11]. However, as the use of molecular hydrogen often
requires harsh reaction conditions [12] and the regio- and stereo-selectivity are challenging to
control [4,5,7,13], transfer hydrogenation has evolved as useful tool for specific and highly selective
hydrogenation under mild conditions [3,4,7,12,14,15]. Specifically, the use of alcohols such as ethanol,

Molecules 2015, 20
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isopropanol, and glycerol [4,7,16,17] and also compounds like formic acid [18] or Hantzsch’s ester [19]
have been used as suitable hydrogen sources in transfer hydrogenation reactions. Many of these
transformations can be performed by using homogenous catalysis with transition metal complexes based
on iron, palladium, ruthenium, or rhodium [4,7,15,17,20,21]. In contrast, the use of non-metal based
organo-catalysts has also been shown and several prolin-based catalysts are known [22,23]. A major
drawback of using alcohols or other carbon-based compounds in transfer hydrogenation lies in the
comparable low mass fraction of the stored hydrogen volume-to-mass ratio. For example, isopropanol
delivers only one equivalent (3.3 wt-% H
hydrogen-rich sources for transfer hydrogenation reactions are desirable. On the other hand, amine
boranes such as ammonia borane (AB) (NH BH ) [24], methyl amine borane [24,25] (MAB), and ethylene
2 2
) and formic acid (4.4 wt-% H ) as well. Therefore,
3
3
diamine bisborane [24,26] (EDAB) have received increasing attention as hydrogen storage materials
owing their hydrogen content. The mass fraction of stored hydrogen is comparably high and the
compounds are easily obtained [24]. MAB releases a mass fraction of 9% H
2
[25,27], EDAB a mass
fraction of 10% [26], and AB a mass fraction of up to 16% H at a maximum temperature of 200 °C [24].
2
Amine boranes have not only been used as possible hydrogen storage material. Additionally, these
non-toxic and water-soluble compounds have shown successful use as hydrogen sources for transfer
hydrogenation reactions. AB in particular has been used elaborately for the reduction of C=C [28],
C=O [29], C=N [13,30], or NO
boranes, the literature is scant and little is known about chemo-, regio-, and stereo-selectivity. Our group
has investigated the liberation of H from EDAB in ionic liquid media as a potential hydrogen storage
2
[13], thereby showing chemo-selectivity [13]. However, for other amine
2
material [26,31]. Following these studies, we focus on the use of EDAB as a possible hydrogen source
for transfer hydrogenation reactions.
Referring to the ongoing debate of implementing greener processes in chemistry [32,33], but also
regarding excellent behavior in catalytic conversions [34], the use of tailor-made ionic liquid media as
reaction mediums has revealed encouraging effects for dehydrogenation reactions with amine
boranes [26,31]. The incorporation of homogeneously dispersed metal nanoparticles as highly potent
catalysts has also been demonstrated [31,35–41]. Furthermore, the use of nanoparticles as catalysts for
the hydrolytic dehydrogenation of AB has been shown [42–45]. Additionally, EDAB undergoes
hydrolytic dehydrogenation with elevated hydrogen yields compared to the reaction in common organic
solvents if ionic liquids (ILs) are implemented as reaction medium. Subsequently, transition metal
nanoparticles have been used as catalysts in transfer hydrogenation reactions, reducing C=C [46] or
C=O [47] functionalities. However, to the best of our knowledge, the use of nanoparticles as transfer
hydrogenation catalysts in ILs as reaction media has not yet been reported. Furthermore, the use of
EDAB as a suitable hydrogen source in transfer hydrogenation reactions is also unknown.
2
. Results and Discussion
Herein we present the use of EDAB as a hydrogen source as well as the use of Pd- and Ru-NPs (NPs:
nanoparticles) as suitable transfer hydrogenation catalysts for the chemo-selective hydrogenation of
carbonyl functionalities and for the reduction of unsaturated carbon-carbon bonds (Scheme 1).

Molecules 2015, 20
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Scheme 1. EDAB as a hydrogen source for the reduction of carbonyl functionalities and
unsaturated carbon-carbon bonds.
The application of amine borane adducts as highly active bench-top reducing agents has long been
known [48,49], but has fallen out of favor, and recently it has been demonstrated that they are suitable
for safe reduction in pure water [50]. The reduction of carbonyl groups in different chemical surroundings
is possible employing EDAB at mild temperatures and water as the solvent (Table 1).
Cyclohexanone (Table 1, entry 1) is quantitatively reduced by EDAB. The reduction of other simple
carbonyl compounds like acetophenone (Table 1, entry 2), cyclohex-2-enone (Table 1, entry 3), and
benzaldehyde (Table 1, entry 4) succeeds with moderate yields. While the aromatic substrates remain
intact, the reduction of α, β-unsaturated cyclohex-2-enone yields a mixture of the unsaturated
cyclohex-2-enol (50% yield) and the saturated cyclohexanol (13% yield). Interestingly, the reduction of
cinnamic aldehyde (Table 1, entry 5) yields solely the unsaturated cinnamic alcohol. The double bond
remains unchanged, probably due to the stabilizing effect of the conjugated aromatic system. The
comparison of the aliphatic carbonyl substrates octanal (Table 1, entry 6) and octan-3-one (Table 1, entry 7)
reveals that aldehydes are by far more reactive toward EDAB than similar ketones. While octanal is
reduced in 33%–38% yield after 1 h, only 9% of octan-3-one reacts in the same time. The yield can be
notably enhanced by prolonging the reaction time to 24 h (53%). The direct comparison between octanal
and 3-octanone showed that 38% 1-octanol and 9% 3-octanol are produced. A similar behavior is
observed when benzaldehyde and acetophenone are reduced in direct comparison: 93% phenylmethanol
and 68% 1-phenylethanol are produced. These experiments indicate that the reduction of aldehydes is faster
than ketones under the given conditions.
The reduction of benzophenone yields diphenylmethanol in moderate amounts (48%–53%; Table 1,
entry 8), while benzoquinone does not react under the given conditions (Table 1, entry 9). In contrast to
the reduction of octan-2-one, the reduction of octan-2, 3-dione (Table 1, entry 10) yields moderate
octan-2, 3-diol amounts (37%–42%). The improved reactivity might be related to the superior solubility
of the dione in comparison to the single ketone. Reduction of 5-hydroxymethylfurfural (HMF) also shows
only a slow conversion into furandimethanol (Table 1, entry 11; 10%). Several other functional groups are
not reduced by EDAB: alkenes, aromatic double bonds, nitro groups, lactames, esters, ethers, and acids
remain inert, making EDAB highly chemo-selective for the reduction of aldehydes and ketones (Table 1,
entry 12–16). The main influences on the yields of the reductions appear to be the solubility and the
steric hindrance of the substrate.

Molecules 2015, 20
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Table 1. Metal-free reductions using EDAB at 80 °C in H
2
O.
a
#
Reaction
Yield (%)
1
99
2
3
68
b
50 (+13)
4
5
75–93
50–72
33–38
6
7
c
9 (53)
8
9
48–53
0
1
1
0
1
37–42
10
1
1
1
2
3
4
0
0
0
1
1
5
6
0
0
a
1
The yield was determined by H-NMR using Si
2
Me
6
as internal standard. Each reaction was performed at
b
c
least twice with a reaction time of 1 h; 13% of cyclohexanol was found; after 60 min and 24 h, respectively.
In comparison to the reduction employing ammonia borane (AB) or methyl amine borane
MAB) [50,51], reduction by EDAB is slightly slower, even at higher temperatures. The scope of
(
substrates is very similar for all three reducing agents; a plethora of different carbonyl compounds are
susceptible to reduction, leaving nearly all other functional groups intact.

Molecules 2015, 20
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Besides the reduction of carbonyl compounds with neat EDAB in water, the application of metal
nanoparticles as catalysts broadens the substrate scope considerably. Ruthenium and palladium were
chosen as catalysis. These metals in particular are known to be active in hydrogenation and dehydrogenation
reactions in ionic liquid media [31,35–37,39,41]. Additionally, ionic liquids were employed due to
their stabilizing effect on the nanoparticles as well as the ionic solvent properties, which provide a
good solubilization of the substrates. We evaluated the well-characterized nanoparticle catalysts
Ru@[BMMIM][NTf
(BCN)MIM: 1-butyronitrile-3-methylimidazolium) [35,39] for transfer hydrogenation with EDAB as
the hydrogen source. The results for the ruthenium-catalyzed reactions are listed in Table 2.
2
] (BMMIM: 1-n-butyl-2,3-dimethylimidazolium) and Pd@[(BCN)MIM][NTf ]
2
(
Table 2. Catalyzed transfer hydrogenations with EDAB and Ru@[BMMIM][NTf ] at 70 °C
2
after 18–24 h.
#
1
2
3
4
5
6
7
8
9
Substrate
cyclohexene
1,5-cyclooctadiene
benzonitrile
nitrobenzene
phenylacetylene
diphenylacetylene
styrene
[%] a
25
[%] Overall
TON b
72 (137)
133 (253)
0.0
25
60
0
55 (5)
0.0
0.0 (17)
8.1 (7.7)
22 (5)
45
17
16
27
45
12
33
41
72
45
26
26
108 (205)
50 (95)
45 (86)
127 (241)
51 (97)
squalene
0 (12)
33
1-octene
92 (176)
112 (214)
253 (482)
129 (245)
69 (131)
1
0
1
2
3
4
2-octene
41
1
1
1
1
1-octyne
21 (50)
31 (14)
20 (6)
22 (3)
4-octyne
1,3-cyclohexadiene
1,4-cyclohexadiene
59 (112)
a
b
In case of triple bonds the product of single hydrogenation is the Z-isomer, exclusively; The TON in brackets
is considered with the magic number approach, with a particle size of 2.0 nm [35,37] which corresponds to
2.6% surface atoms. Each reaction has been performed at least twice.
5
Among the tested compounds, simple alkenes and alkynes showed the highest conversion upon use
of the Ru@[BMMIM][NTf2] system. The hydrogenation of cyclohexene succeeds with mediocre TONs
(Table 2; entry 1). 1,5-cyclooctadiene can be hydrogenated with moderate TONs of 253 considering
only available surface atoms, where the single hydrogenation is strongly favoured over the full
hydrogenation (Table 2; entry 2; 55%:5%). The hydrogenation of 1-octyne shows an inverse selectivity
(
21%:50%) and also the highest activity observed overall (Table 2, entry 11; TON 482). In comparison
4
-octyne is hydrogenated, but to a smaller extent (TON 245) and less selective (Table 2, entry 12;
1%:14%), probably due to the higher steric hindrance. The alkynes phenylacetylene and
3
diphenylacetylene are slowly converted with TONs of 50 and 45, respectively, and selective towards the
corresponding alkenes are observed (Table 2, entries 5–6). In case of alkenes, the highest conversion is
observed with styrene (Table 2, entry 7; TON 241). Squalene, 1-octene, 2-octene as well as 1, 3- and
1
,4-cyclohexadiene are hydrogenated slightly slower (Table 2; entries 8–10 and 13–14). While
benzonitrile (Table 2, entry 3) remains unchanged, nitrobenzene (Table 2, entry 4) can be converted to

Molecules 2015, 20
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aniline with a TON of 205. No hydrogenation of arenes was observed for these functionalised arenes,
which stays in agreement with earlier results that employed elemental hydrogen as the reducing agent
for functionalised and unfunctionalized arenes [35–37,41].
Moreover, palladium nanoparticles in the IL [(BCN)MIM][NTf ] have been evaluated for transfer
2
hydrogenation from EDAB to organic substrates containing alkynes and alkenes (Table 3).
Table 3. Catalyzed transfer hydrogenations with EDAB and Pd@[(BCN)MIM][NTf ] at
2
7
0 °C after 18–24 h.
#
1
2
3
4
5
6
7
8
9
Substrate
[%] a
[%] Overall
TON b
122 (665)
phenylacetylene
diphenylacetylene
1-octyne
9.5 (8.1)
18
64–71
59
54–69 (1–9)
186–216 (1020–1182)
449 (2453)
400 (2186)
255 (1395)
245 (1337)
316 (1727)
199 (1090)
0.0
28 (32)
54
4-octyne
11
1-octene
45
45
2-octene
37
37
styrene
47
47
cyclohexene
benzonitrile
squalene
34
34
0
0
1
0
1
2
3
traces
42 (0.2)
43 (3)
4
n.d.
42
n.d.
1
1
1
1,3-cyclohexadiene
1,4-cyclohexadiene
1,5-cyclooctadiene
197 (1075)
229 (1253)
46
0.2
19 (104)
a
b
In case of triple bonds the product of single hydrogenation is the Z-isomer, exclusively; The TON (turnover
number) in brackets is considered with the magic number approach, with a particle size of 7.3 nm [39], which
corresponds to 18.3% surface atoms. Each reaction has been performed at least twice. n.d.: not determined.
The hydrogenation of phenylacetylene (Table 3, entry 1) proceeds with overall small yields and low
selectivity. Interestingly, diphenylacetylene is hydrogenated somewhat faster, despite the higher steric
demand (Table 3, entry 2). A strong selectivity towards the single hydrogenated Z-alkene is observed
(
69%:1%). The hydrogenation of 1- and 4-octyne (Table 3, entries 3–4) succeeds with TONs of 2453
and 2186, respectively. Selectivity differs strongly among these substrates: the fully hydrogenated
product of 1-octyne is found in higher abundance (32% of 59% overall yield) than that of 4-octyne (entry 4,
1
1% of 65% overall yield). Simple alkenes, i.e., 1-octene, 2-octene, styrene, and cyclohexene (Table 3,
entries 5–8), are hydrogenated with moderate TONs of 1090–1727. No hydrogenation of arenes is
observed. Similarly to the reaction employing the previously described Ru-system, nitriles are not
hydrogenated (Table 3, entry 9). Squalene (Table 3, entry 10) yields only product traces and the reduction
of nitrobenzene proceeds uncontrolled and was not further examined. The dienes 1,3- and 1,4-
cyclohexadiene (CHD) (Table 3, entries 11–12; CHD) can be hydrogenated with similar TONs (1075–1253)
and selectivities (1,3-CHD: 0.2% cyclohexane of 42% overall yield and 1,4-CHD: 3% cyclohexane of
4
6% overall yield). Hydrogenation of 1,5-cyclooctadiene (Table 3, entry 13) yields only minor amounts
of product (entry 13, 4% cyclooctene of 4.2% overall yield). Conclusively, the substrates accessible to
transfer-hydrogenation with Pd@[(CN)BMIM][NTf ] using EDAB are very similar to those of the
2
Ru-system previously described. In most cases, the Pd-system shows a higher activity and a better
selectivity towards the partial hydrogenated product (Table 4). In the reduction of 1,5-cyclooctadiene

Molecules 2015, 20
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and squalene, the Ru-system showed a considerably higher activity. The reason for these exemptions is
yet to be found.
Table 4. Comparison between Ru-NPs and Pd-NPs.
Ru-NP@(IL-1)
Conv. [%]
Pd-NP@(24) or @(IL-2)
Substrate
Sing. Transf.
Mult. Transf.
Conv. [%]
TON
TON
Sing. Mult.
Sing. Mult.
1
2
-
211
392
25
55
-
1108
106
0
2.70
34
0
a
5
4
3
4
5
-
-
0
-
-
0
17
8
-
0
318
147
deflagration
10
8
676
8
6
7
8
9
133
372
151
272
22
45
-
5
-
1202
1756
n.d.
69
47
-
2
-
-
traces
-
Squalene
-
Squalane
-
12
-
33
1418
45
1
0
1
-
331
745
41
21
-
1359
2494
37
28
-
1
1
50
32
2
0
31
14
2223
54
10
1
1
3
4
380
202
20
22
6
3
1092
1274
42
43
0
0
a
[
(BCN)M2Im][NTf2] was used.
3. Experimental Section
3.1. General
All commercial chemicals and solvents were purchased from Sigma-Aldrich (St. Louis, MO, USA),
Merck (Kenilworth, NJ, USA) and Acros (Waltham, MA, USA) and used as received. Ethylene diamine
bisborane (EDAB) [26,27,31], all used ionic liquids [39,52], ruthenium nanoparticles [37], and
palladium nanoparticles [39] were prepared using previously published procedures. Nuclear magnetic
resonance spectroscopy was performed at 300 K using a Bruker Avance II 300 or a Bruker Avance II
1
13
2
00 by Bruker (Billerica, MA, USA) at given frequency: AvanceII 300 ( H (300.1 MHz), C (75.5 MHz),
1
1
19
1
13
B (96.2 MHz), F (282.2 MHZ)), and AvanceII 200 ( H (200.1 MHz) C (50.0 MHz)). If not otherwise
indicated, all measurements were proton broad band-decoupled. The relative chemical shift δ is

Molecules 2015, 20
17065
1
13
11
referenced in the remaining signal of the given deuterated solvent for H and C spectra. For B spectra,
BF
-diethylether complex was added as standard in sealed glass ampules. For 19F spectra,
3
trifluoromethane (δ = 0 ppm) was used as internal standard, respectively. All chemical shifts are given
in parts per million (ppm). When possible, the multiplicity of each spectrum is assigned using the
following abbreviations: s: singlet, d: doublet, t: triplet, q: quartet quint: quintet, sext: sextet, sept: septet,
m: multiplet, br: broad signal, and mc: multiplet, centred. GC-MS was performed on an Agilent
GC-system series 6890 by Agilent using a capillary column HP5MS–0.25 μm of 30 m length and
0
.32 mm in diameter by J & W (J & W Scientific, branch of Agilent, Santa Clara, CA, USA). The used
temperature program 50–300 M had a temperature increase of 10 °C/min with a starting temperature of
0 °C, an ending temperature of 300 °C, and a run time of 5 min. Mass detection was carried out on an
5
Agilent 5973 Network Mass Selective Detector by Agilent. Ionization was achieved by electron impact
ionization (EI) with an ionization potential of 70 eV.
3.2. Experimental Protocols
3.2.1. Nanoparticle Synthesis
Ruthenium Nanoparticle Synthesis [35,37]
The synthesis of Ru-NPs was carried out according to literature-reported procedures [35,37].
Under glove box conditions, a screw neck vial was loaded with approximately 5–15 mg of
bis(2-methylallyl)(1,5-cyclooctadiene)ruthenium(II) and 0.65–1.45 mg of ionic liquid [BMMIM][NTf ] and
2
sealed. The reaction mixture was stirred 18 h at 75 °C to yield a brown to dark brown but clear suspension.
The obtained particle size averaged 2.0 nm with a size distribution of ± 0.5 nm [35,37].
Palladium Nanoparticle Synthesis [39]
The synthesis of Pd-NPs was carried out according to a literature-reported procedure [39].
A screw neck vial was loaded with approximately 3 mg of palladium(II)acetate and 600 mg of ionic
2
liquid [(BCN)MIM][NTf ] and sealed. The reaction mixture was stirred 3 h at 130 °C to yield an orange but
clear suspension. The obtained particle size averaged 7.1 nm with a size distribution of ± 2.2 nm [39].
3
.2.2. Metal-Free Transfer Hydrogenation in Water
Exemplary procedure (Table 1; entry 2): A vial was charged with 2.3 mmol acetophenone (Table 1)
and 5 mL demineralized water. After adding 0.5 mmol of EDAB (four H
sealed, placed in an aluminum heating block at 80 °C, and the mixture was stirred for the given time
Table 1). Subsequently, the aqueous suspension was cooled down and extracted three times with 5 mL
Et O. The organic layers were combined and the solvent was removed under reduced pressure. The product
yield was determined by NMR spectroscopy using a defined amount of Si Me as internal standard.
2
equivalents), the vial was
(
2
2
6
3
.2.3. Transfer Hydrogenations in Ionic Liquids with Metal Nanoparticles
Exemplary procedure (Table 2; entry 1): In a typical transfer hydrogenation experiment, a vial was
charged with the ruthenium nanocatalyst mixture of previously synthesized ruthenium nanoparticles

Molecules 2015, 20
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2
(3 mg Ru) in ionic liquid (1.28 g; Ru@BMMIM][NTf ]) [37] and 9 mmol cyclohexene. Subsequently,
1
.7 mmol EDAB was added and the vial was sealed. The reaction mixture was stirred overnight at
0 °C and then cooled to room temperature. A defined amount of CDCl as solvent and hexamethyl
Me ) as internal standard was added to the mixture and homogenized. A sample
was taken and the product yield was determined by NMR spectroscopy (Tables 2 and 3). The
7
3
disilane (HMDS; Si
2
6
Pd-catalyzed reactions used 600 mg [(CN)BMIM][NTf
2
] and 0.5 mg Pd nanoparticles prepared as
previously published [39].
4
. Conclusions
In summary, ethylenediamine bisborane (EDAB) is a suitable hydrogen source for transfer
hydrogenation reactions on C-C double bonds mediated by metal nanoparticles. Several organic model
compounds could be shown to undergo reduction in the presence of the Ru-system as well as the
Pd-system. In most cases, the latter showed considerably higher activity. The single hydrogenation of
triple bonds yields only Z-isomeric compounds, as expected in a heterogeneous catalytic reaction. C-N
triple bonds (nitriles) are not susceptible to the reduction. C-O double bonds can be reduced without the
presence of metal particles in water. C-C double bonds can be readily reduced, while aromatic double
bonds remain inert. In future investigations, the selectivity, time-resolved studies, and solvent effects
will be addressed. The reduction with this air- and moisture-stable amine borane adduct could
complement the synthetically interested chemist's toolkit.
Acknowledgments
We acknowledge the Ministerium für Innovation, Wissenschaft und Forschung NRW (MIWF-NRW)
for financial support within the Energy Research Program for the Scientist Returnee Award for M.H.G.
Prechtl (NRW-Rückkehrerprogramm). M.H.G.P. acknowledges the Ernst-Haage Foundation at the
Max-Planck-Institute for Chemical Energy Conversion for the Ernst-Haage Prize 2014 and the Deutsche
Forschungsgemeischaft DFG (Heisenberg-program). S.S., M.S., and C.G. want to thank M. Kessler,
L. Heim, and H. Konnerth for helpful discussion.
Author Contributions
S.S., M.S. and C.G. performed experiments and analyses. S.S., M.S., and M.H.G.P. wrote the manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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Sample Availability: Not applicable; no new compounds have been synthesised.
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