Iodide-Promoted Dehydrogenation of Formic Acid on a Rhodium Complex

Article abstract of DOI:10.1002/ejic.201501061

Efficient and simple catalyst systems for dehydrogenation of formic acid (FA) to produce hydrogen are always desirable. In this work, the catalytic dehydrogenation of FA to H₂ and CO₂ by using the readily available [RhCp?Cl₂]₂ was found to be accelerated simply by the addition of halide anions, iodide being the most effective. At 60 C, with [RhCp?Cl₂]₂ in azeotropic FA and triethylamine (TEA), the initial turnover frequency of dehydrogenation in the presence of I^- (4375 h^-1) is seven times as high as that of the reaction without additive (625 h^-1). Preliminary mechanistic studies suggest that the dehydrogenation is turnover-limited by the hydride-formation step, which could be facilitated by the presence of I^-.

Full text of DOI:10.1002/ejic.201501061

DOI: 10.1002/ejic.201501061
Full Paper
Formic Acid Dehydrogenation
Iodide-Promoted Dehydrogenation of Formic Acid on a
Rhodium Complex
Zhijun Wang,[
a,b,c]
Sheng-Mei Lu, Jianjun Wu, Can Li,* and Jianliang Xiao*
[b]
[a]
[b]
[a]
Abstract: Efficient and simple catalyst systems for dehydro- azeotropic FA and triethylamine (TEA), the initial turnover fre-
–
–1
genation of formic acid (FA) to produce hydrogen are always quency of dehydrogenation in the presence of I (4375 h ) is
desirable. In this work, the catalytic dehydrogenation of FA to seven times as high as that of the reaction without additive
–1
H and CO by using the readily available [RhCp*Cl ] was found (625 h ). Preliminary mechanistic studies suggest that the de-
2
2
2 2
to be accelerated simply by the addition of halide anions, hydrogenation is turnover-limited by the hydride-formation
–
iodide being the most effective. At 60 °C, with [RhCp*Cl ] in step, which could be facilitated by the presence of I .
2
2
Introduction
the mechanistic details of the effect of the iodide ion on these
reactions remain unclear, its softness, trans-effect, and size may
play a role. Here we have observed that halide anions can accel-
erate the FA dehydrogenation catalyzed by [RhCp*Cl ] in the
Hydrogen generation from formic acid (FA) has attracted much
[
1]
attention in recent years. This reaction could provide a tech-
nology for supplying hydrogen to, for example, small mobile
or stationary applications. Accordingly, studies on the catalytic
dehydrogenation of FA have been carried out by a number of
research groups, some reporting excellent turnover frequencies
2
2
FA–triethylamine (TEA) azeotrope. The most effective halide ion
–
is I , and it enhances the reaction rate by seven times with an
–1
increase in the initial TOF from 625 to 4375 h at 60 °C. A
preliminary mechanistic study suggests that the dehydrogen-
ation is turnover-limited by hydride formation and the iodide
anion facilitates this step while prolonging the catalyst lifetime.
[
2–4]
(
TOFs).
In the area of homogeneous catalysis, the highest
–
1
[4i]
TOF of 487,500 h was demonstrated by Li et al., who used
a [Cp*Ir(L)Cl]Cl catalyst (Cp* = pentamethylcyclopentadienide;
L = 2,2′-bi-2-imidazoline). As with this catalyst developed by Li
et al., most of the reported catalysts comprise relatively compli-
cated or expensive bidentate/tridentate ligands attached to the
Results and Discussion
metal center. Although they can be highly active in the de- Formic Acid Dehydrogenation
hydrogenation reaction, considering the practicality of the reac-
tion in real life, developing efficient catalyst systems without
complicated or delicate ligands would be preferable.
The precatalyst [RhCp*Cl2]2 has been shown to catalyze the
transfer hydrogenation of N-heterocycles with the FA–TEA (FT)
azeotrope in the presence of halide ions and particularly iod-
Halide ions, one of the most common and simple additives
and ancillary ligands for transition metal complexes used in ca-
talysis, have frequently been found to exert some unusual influ-
[
6]
ide. Hydrogen results from the decomposition of FA; thus the
–
combination of [RhCp*Cl ] and X is capable of FA dehydro-
2
2
[
5]
genation. At the starting point, we therefore examined the de-
hydrogenation of FA catalyzed by [RhCp*Cl ] in the absence or
ence on catalytic reactions. In particular, the significant pro-
2
2
moting effect of the iodide anion in hydrogenation and trans-
fer-hydrogenation reactions has been recognized.[5b,6] Although
presence of halide anions at 60 °C in the FT azeotrope, where
the FA/TEA mol ratio is 2.5. The amount of evolved gas was
determined by the water displacement method, and gas chro-
matography confirmed the products to be H and CO (in a 1:1
[
a] Liverpool Centre for Materials & Catalysis,
Department of Chemistry, University of Liverpool,
Liverpool L69 7ZD, UK
2
2
ratio) with a negligible level of CO (Figure S1). As shown in
Table 1 and Figure 1, both the TOF and TON are increased upon
the addition of potassium halides, KI enabling the fastest reac-
tion. In the presence of KI, the initial TOF increased most signifi-
E-mail: J.Xiao@liverpool.ac.uk
http://pcwww.liv.ac.uk/~jxiao/
[
b] State Key Laboratory of Catalysis,
Dalian Institute of Chemical Physics,
Chinese Academy of Sciences,
cantly to 4375 h– from 625 h without any halide additive.
1
–1
4
57 Zhongshan Road, Dalian 116023, China
Comparing the TOF values obtained with KI, NaI, and LiI, we
E-mail: canli@dicp.ac.cn
http://www.canli.dicp.ac.cn/
concluded that this enhancement arises from the halide anions
rather than the alkali cations. In addition, the presence of I–
[
c] Graduate University of Chinese Academy of Sciences,
Beijing 100049, China
makes this catalyst more productive, increasing the TON from
1
51 to 706 (at 40 % FA conversion, Table 1, entries 1 and 5)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201501061.
after 30 min.
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Table 1. Dehydrogenation of FA in the presence of different alkali halide
salts.
The dependence of the rate on catalyst concentration was
[
a]
explored by using 0.1–5.0 μmol of [RhCp*Cl ] in combination
2
2
with 500 μmol of KI in 1.5 mL of FT azeotrope at 60 °C (Fig-
ure S3). The double logarithmic plot of the initial rate against
the catalyst concentration shows a linear dependence on [Rh]
(Figure 2a). As shown, the reaction proceeds with an order of
Entry
Additive
Gas
TON
(30 min)
Conversion
TOFmax
–1 [c]
[
b]
[b]
[
mL]
[h
]
1
2
3
4
5
6
7
–
66
62
151
142
307
296
706
699
669
9 %
8 %
625
875
KF
KCl
KBr
KI
NaI
LiI
134
129
308
305
292
17 %
17 %
40 %
40 %
38 %
1625
2009
4375
4375
4250
0
.90 with respect to the catalyst concentration, [Rh], which is
close to 1.0. This is consistent with the involvement of a mono-
nuclear rhodium species in the catalytic cycle (also see below),
and no other polymeric compounds are likely to be the active
catalyst. Beyond the [Rh] concentration of 1.07 mM, the initial
[
2 2
a] General reaction conditions: 5.0 μmol of [RhCp*Cl ] , 1.0 mmol of alkali
TOF declines gradually (Figure S3). This may result from catalyst
aggregation at a higher concentration (vide infra).
halide, 1.5 mL of FT azeotrope, 60 °C. Approximately 16 mmol of FA is present
in 1.5 mL of FT azeotrope. [b] The amount of gas was recorded and the TON
was calculated after 30 min reaction. [c] TOFmax was calculated on the basis
of the volume of evolved gas from 3 to 6 min after the beginning of the
reaction, to rule out the influence of temperature fluctuation due to the
injection of FT solution and to avoid the initial induction period (Figure 1).
Each reaction was repeated at least twice with an error less than 5 %. No
reaction was observed without catalyst.
Figure 1. Time course of gas evolution during the dehydrogenation of FA in
the presence of various inorganic salts (5.0 μmol of [RhCp*Cl ] , 1.0 mmol of
2 2
inorganic salts (6 mol-%), 1.5 mL of FT azeotrope, 60 °C).
Other rhodium complexes, as well as those based on iridium
and ruthenium, were also investigated as catalyst precursors in
–
combination with I for FA dehydrogenation (see Table S1). All
the complexes tested showed very low activity, except for
–
1
[
Ru(p-cymene)Cl ] , which gave a moderate TOF of 1500 h .
2 2
2
Figure 2. (a) Plot of ln (initial rate of H evolution) vs. ln [Rh] ([Rh] is the
–
This value is six times that observed for the reaction without I
Table S1, entries 2–3), which indicates that the promoting ef-
concentration of rhodium monomer). Conditions: 0.1–5.0 μmol of
[RhCp*Cl ] , 500 μmol of KI, 1.5 mL of FT azeotrope, 60 °C; (b) Plot of ln (initial
(
2
2
–
2 2
rate of H2 evolution) vs. ln [KI] (0.8 μmol of [RhCp*Cl ] , 40–500 μmol of KI,
fect of I is not limited to [RhCp*Cl ] .
2
2
1
.5 mL of FT azeotrope, 60 °C).
The dependence of the rate on the concentration of KI was
Kinetics of Formic Acid Dehydrogenation
investigated by using 0.8 μmol of [RhCp*Cl ] with 40–500 μmol
2
2
To gain an understanding of the dehydrogenation reaction, the of KI in 1.5 mL of FT azeotrope (Figure S4). The reaction rate
dependence of the rate on temperature, catalyst concentration, shows a fractional order of 0.68 with respect to iodide concen-
amount of KI, and FA/TEA ratio were investigated. The reaction tration, up to [KI] ≈ 0.1
M (nKI = 160 μmol) (Figure 2b). This
rate increases with temperature, and the temperature depend- result indicates that the iodide ion is likely to be involved in
ence of the initial rates follows the Arrhenius equation (Fig- the rate-determining step (RDS) and in the equilibria prior to it.
ure S2). The apparent activation energy (E ) for FA dehydro-
With different FA/TEA ratios, the time course of gas evolution
a
–1
genation is estimated to be about 64 kJ mol , which is near was recorded as shown in Figure S5. The FT mixtures were pre-
the typical range of noble-metal-catalyzed hydrogen genera- pared with molar ratios of FA/TEA in the range 2.25:1 to 3:1.
[
4g,4h,4i]
tion from FA in aqueous solution.
Figure 3 shows the variation of the initial TOF and TON with
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the mol fraction of FA in the starting solution. The final TONs with the same amount of KCl has only an insignificant effect on
increase approximately linearly with the increase in the amount the dehydrogenation catalyzed by [Cp*RhI ] (Figure S8).
2
2
of FA (blue line). In contrast, the initial TOFs show a reverse
dependence on the mol fraction of FA: more FA gives rise to
–
lower TOFs. Since [HCOO ] is reversely proportional to [HCOOH]
in the FA/TEA mixture, the reverse dependence suggests that
–
the dehydrogenation rate depends on [HCOO ]. In line with
this, no gas bubble was observed in neat FA, and no Rh–H
species was detected in neat FA, either (Figure S6). These obser-
vations appear to indicate that the dehydrogenation is rate-
limited by the step of hydride formation, where coordination of
the formate is necessary.
Figure 4. FA dehydrogenation with [RhCp*Cl
2 2 2 2
] and [RhCp*I ] (5.0 μmol of
[7]
[
RhCp*X , 1 mmol of KI when added, 1.5 mL of FT azeotrope, 60 °C).
2 2
]
We also monitored the FA dehydrogenation with [RhCp*Cl ]
2
1
2
–
at room temperature in the absence or presence of I by H
NMR spectroscopy over a period of 72 h (Figure S9; for details
see the Experimental Section). Comparing Figures S9a and S9b
shows that the FA (indicated by the formate proton signal) was
consumed to a greater extent after 72 h in the presence of I–
(54 % vs. 18 % FA conversion), which is consistent with the re-
sults from Figure 1. Figure 5 shows the hydride part (δ = –5 to
Figure 3. TON and TOF values observed in FT solutions with various FA/TEA
2 2
ratios. Conditions: 0.8 μmol of [RhCp*Cl ] , 320 μmol of KI, 1.5 mL of FT
mixture solution with varying FA/TEA ratio in the range 2.25:1 to 3:1, 60 °C.
Each data is the average value of two runs, and the TON was measured by
using the final gas amount as shown in Figure S5, from 20 to 50 min. The
TOF was calculated on the basis of the volume of evolved gas from 3 to 6 min
after the beginning of the reaction, to rule out the influence of temperature
fluctuation due to the injection of FT solution and to avoid the initial induc-
tion period.
In addition to TEA, other amines were also tested for the
reaction (Figure S7). Tertiary amines show superior reactivity
compared with primary and secondary amines. This may be
because the latter two types of amines are better hydrogen-
bond donors, reducing the concentration of free iodide ions
needed for higher dehydrogenation rates (vide supra).
The Structure of Active Catalysts
–
Next, efforts were made to investigate the role of I in this reac-
tion. Firstly, a contrast experiment was conducted by using
[
RhCp*Cl ] and [RhCp*I ] as catalyst precursors. As shown in
2 2 2 2
Figure 4, without KI, the FA dehydrogenation rates for both
rhodium complexes are low and essentially identical. When KI
is added, both reactions are accelerated considerably and to a
similar extent. These findings rule out the possibility that the
iodide effect arises from the iodide-ligated rhodium dimer,
[
RhCp*I ] , and suggest that the same active catalyst is gener-
2 2
–
ated in the presence of I regardless of which rhodium precur-
sor is used. The necessity for a large excess of coordinating I–
also indicates that the active catalyst is less likely to be other
Figure 5. 1H NMR spectra of the reaction mixture of [RhCp*Cl
azeotrope in CDCl (hydride part): (a) without KI; (b) in the presence of KI
250 μmol). General conditions: 5.0 μmol of [RhCp*Cl , 0.1 mL of FT azeo-
2 2
] with FT
3
(
2 2
]
III
forms of dimeric Cp*Rh –I species. It is noted that replacing KI
trope, room temperature. Timing started after the injection of FT solution.
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+
–
22 ppm) of Figure S9 (enlarged). Without KI (Figure 5a), a Rh– can be ascribed to [Rh Cp* Cl(H)(OCHO)] (d, Scheme 1, vide
2
2
H hydride triplet at δ = –8.87 ppm along with a new chemical infra), a rhodium dimer complex containing a bridging hydride
1
[9]
shift at δ = 1.90 ppm [Cp* proton, Figure S9e] appeared in- and a bridging formato ligand ( H NMR: δ = 7.39 ppm). When
stantly (less than 5 min) upon mixing the rhodium precursor KI was added, the newly generated mass peak at m/z =
+
with the FT azeotrope in CDCl , indicating quick formation of 648.9554 supports the formation of [Rh Cp* I(H)(OCHO)] (D,
3
2
2
a binuclear rhodium species with a bridging hydride.[8] Other Scheme 1),[10] an analogue of d, while that at m/z = 730.8639
+
[11]
rhodium species bearing bridging hydride ligands started to may correspond to [Rh Cp* (H)I ] (E, Scheme 1).
Each of
2
2
2
appear later (δ = –11.71 and –20.39 ppm). In the presence of these species is expected to display a triplet hydride signal in
KI (Figure 5b), the NMR spectra show that similar rhodium spe-
cies are formed quickly in addition to a new peak at δ =
–
8.85 ppm, which overlaps with a slightly smaller peak centered
at δ = –8.87 ppm [Figure S9g]. The latter peak presumably arises
from the same species as that in the absence of the iodide.
However, the peak at δ = –11.72 ppm disappeared after 5 h
while that at δ = –8.85 ppm disappeared after 24 h, accompa-
nied with the appearance of new, unknown rhodium species
with bridging hydride. It is also worth noting that there is no
visible doublet hydride resonance during the catalytic reaction,
again indicating that the dehydrogenation is likely to be turn-
over-limited by the step of hydride formation. Since gas evolu-
tion was observed over the entire period of 72 h in both cases,
the disappearance of the triplets mentioned is consistent with
the corresponding binuclear rhodium species not being the ac-
tive catalyst.
Mass spectrometry was then employed in an attempt to
identify these rhodium species. Figure 6 shows the mass frag-
ment of [RhCp*Cl ] reacting with the FT azeotrope in methanol
Figure 6. Mass spectrum of the reaction mixture of [RhCp*Cl
2 2
] with FT azeo-
2
2
trope in methanol (1.6 μmol of [RhCp*Cl , 0.2 mL of FT azeotrope, 1.0 mL
2 2
]
within 5 min of the beginning of the reaction (also see Fig-
of methanol, room temperature). Top: without KI; bottom: with 120 μmol of
ure S10). In the absence of KI, the mass peak at m/z = 557.0228
KI.
2 2
Scheme 1. Plausible reaction mechanism for the dehydrogenation of FA with [RhCp*Cl ]
in the absence (1) or presence (2) of I–.
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1
its H NMR spectrum and thus appears to correspond well to
those revealed in Figure 5.
Proposed Mechanism
Taken together, a plausible reaction mechanism is tentatively
suggested for the dehydrogenation of FA catalyzed by
[
[
RhCp*Cl2]2 (Scheme 1). With or without KI, the dimeric
RhCp*Cl2]2 firstly dissociates under the reaction conditions,
turning into the catalytically active monomeric rhodium species
–
[12]
a in the absence of I and C in its presence.
The latter is in
fast equilibrium with formato A, the iodo analogue of a. Subse-
quently, rhodium hydrides b and B are formed from a and A
by decarboxylation; their protonation releases H and regener-
2
Figure 7. Longer-time reaction with intermittent addition of FA (5.0 μmol of
ates the active catalysts. During the catalytic turnover, the spec-
tator dimeric species d, D, and E, containing bridging hydride,
are formed upon dimerization of the monomers in the catalytic
cycle. As the reaction progresses, other unknown rhodium spe-
[
2 2
RhCp*Cl ] , 1.0 mmol of KI, 1.5 mL of FT azeotrope, 60 °C). The amount of
FA added was roughly based on the amount of evolved gas (0.12, 0.12, 0.12,
0.12, 0.12, 0.15, and 0.15 mL).
1
[13]
cies are formed, as shown in the H NMR spectra in Figure 5.
Conclusions
These species are not considered as reactive species. However,
they may be in fast equilibrium with the catalytically active
The dehydrogenation of FA catalyzed by a simple, readily avail-
able rhodium catalyst is reported. The activity of the
[
9a]
monomers; otherwise, the catalyst would deactivate quickly.
As indicated above, the overall dehydrogenation is likely to be
[
RhCp*Cl ] catalyst is enhanced by the addition of halide ions.
2 2
[
14,15]
The catalytic activity increases with the atomic number of hal-
controlled by the step of hydride formation in turnover.
–
ide ions, and with I , the activity is increased to sevenfold that
Thus, the promoting effect of iodide can be ascribed to the fact
without added halide. Not only does the iodide ion increase
the reaction rate, but it also prolongs the catalyst lifetime. Ki-
netic studies and NMR spectroscopic monitoring suggest that
the active catalyst is mononuclear and is in equilibrium with
binuclear hydrides outside of the catalytic cycle. The catalytic
turnover appears to be controlled by the rate of hydride forma-
tion, which is facilitated by iodide anions coordinated to the
that decarboxylation from A is easier than that from a, which
III
in turn may be because iodide is a better ligand to Rh than
[
17]
chloride. Since the decarboxylation could proceed via a tran-
sition state involving dissociation of the formate to form an
[
4o,15,16]
ion pair,
a better coordinating iodide anion would be
expected to stabilize the transition state that contains a posi-
III
tively charged Rh better. However, an excess amount of iodide
III
Rh center.
will shift the equilibrium between C and A to favor C, thereby
giving rise to the saturation kinetics observed (Figure 2).
Experimental Section
1
General: The H NMR spectra were recorded with a Bruker Avance
Stability of the Catalyst
400 NMR spectrometer. GC analysis results were obtained from
Techcomp GC7890II (equipped with a TCD or FID detector and
methanizer). HRMS data were recorded with a Finnigan MAT 95
system. The FT azeotrope was distilled prior to use. [RhCp*I ] was
The stability of this catalyst system for FA dehydrogenation was
also investigated. During a single FA decomposition reaction
2
2
with 5.0 μmol of [RhCp*Cl ] and 1.0 mmol of KI in 1.5 mL of
[18]
2
2
prepared according to the literature.
FT azeotrope at 60 °C, the catalyst is observed to lose activity
eventually (at conversion values less than 50 %, Figure 1), and
gas evolution could not be fully recovered with the addition of
either FA or FT azeotrope (Figure S11). This could be caused by
the irreversible formation of some deactivated rhodium hydride
Procedure of FA Dehydrogenation: [RhCp*Cl2]2 and KI were
placed in a thick-walled reaction vessel fitted with a side tube and
a rubber septum. The vessel was then preheated to a given temper-
ature by means of an oil bath under ambient atmosphere. FT azeo-
trope (1.5 mL) was injected through the septum; 5 seconds later,
species as a result of the consumption of FA (Figure S12). Never- stirring and timing were started. The side arm of the vessel was
theless, the addition of KI prolongs the lifetime (TON) of the connected to a gas collection apparatus (standard water displace-
catalytic system; in its absence, the catalyst has low activity ment apparatus, with a graduated cylinder to determine volume),
and the volume of the evolved gas was recorded. The products of
right from the beginning (Figure 4). One way to keep this reac-
tion going is to add an additional amount of FA into the system
at certain time intervals. This is shown in Figure 7, where more
FA decomposition were confirmed to be H and CO in a ratio of
2
2
1:1 with no observable evolution of CO.
than 1000 mL of H were released upon seven consecutive ad- Calculation of TONs and TOFs
2
ditions of FA, although the dehydrogenation rate became pro-
gressively slower. Thus, the TON was enhanced from 706 (Ta- corrected by taking into account the expansion of gas due to the
The TON was calculated by Equation (1). The total gas volume was
ble 1) to over 2540.
injection of FT azeotrope (1.5 mL) into the preheated vessel and
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494 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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also the contributions from the solvent and water vapor (an average
value of 1.5 mL gas), which were measured in an experiment with-
out the catalyst. At room temperature, Vm,20 °C = 24 L mol (see the
S. C. E. Tsang, Angew. Chem. Int. Ed. 2012, 51, 11275–11278; Angew.
Chem. 2012, 124, 11437; e) Q.-Y. Bi, X.-L. Du, Y.-M. Liu, Y. Cao, H.-Y. He, K.-
N. Fan, J. Am. Chem. Soc. 2012, 134, 8926–8933; f) Z.-L. Wang, J.-M. Yan,
Y. Ping, H.-L. Wang, W.-T. Zheng, Q. Jiang, Angew. Chem. Int. Ed. 2013, 52,
–
1
Supporting Information).
4
406–4409; Angew. Chem. 2013, 125, 4502; g) S. Zhang, O. Metin, D. Su,
TON = [Vtotal/(2 × Vm,20 °C)]/ncat (1)
S. Sun, Angew. Chem. Int. Ed. 2013, 52, 3681–3684; Angew. Chem. 2013,
1
2
25, 3769; h) M. Yadav, T. Akita, N. Tsumori, Q. Xu, J. Mater. Chem. 2012,
2, 12582–12586; i) Q.-L. Zhu, N. Tsumoriab, Q. Xu, Chem. Sci. 2014, 5,
where n denotes number of mol, and ncat = 2 ndimer
.
–
1
195–199; j) C. Feng, Y. Hao, L. Zhang, N. Shang, S. Gao, Z. Wang, C. Wang,
RSC Adv. 2015, 5, 39878–39883; k) Y.-L. Qin, Y.-C. Liu, F. Liang, L.-M. Wang,
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C. W. Yoon, T. Asefa, J. Mater. Chem. A 2014, 2, 20444–20449.
4] For selected examples of homogeneous FA dehydrogenation, see: a) C.
Fellay, P. J. Dyson, G. Laurenczy, Angew. Chem. Int. Ed. 2008, 47, 3966–
The calculation of TOF is time-related (h ) and based on the TON
value. For example: using 0.8 μmol of [RhCp*Cl ] and 320 μmol of
2
2
KI in 1.5 mL of FT (2.25:1) solution, 42 mL of gas was released at
0 °C in 2 min, thus:
6
[
–
1
TOF = [42/(2 × 24) × 1000]/(1.6) × 30 = 16406 h .
3
968; Angew. Chem. 2008, 120, 4030; b) B. Loges, A. Boddien, H. Junge,
NMR Spectroscopic Study of FA Dehydrogenation: At room tem-
M. Beller, Angew. Chem. Int. Ed. 2008, 47, 3962–3965; Angew. Chem. 2008,
120, 4026; c) A. Boddien, F. Gärtner, C. Federsel, P. Sponholz, D. Mellmann,
R. Jackstell, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2011, 50, 6411–
6414; Angew. Chem. 2011, 123, 6535; d) M. Czaun, A. Goeppert, J. Ko-
thandaraman, R. B. May, R. Haiges, G. K. S. Prakash, G. A. Olah, ACS Catal.
perature, FA dehydrogenation catalyzed by [RhCp*Cl ] with or
2
2
–
1
without I was tracked up to 72 h by H NMR spectroscopy (Fig-
ure S8). For reaction without KI, [RhCp*Cl2]2 (5.0 μmol) was dis-
solved in CDCl (1.0 mL) first. Then, FT azeotrope (0.1 mL) was intro-
3
2014, 4, 311–320; e) G. A. Filonenko, R. V. Putten, E. N. Schulpen, M. E. J.
duced, and gas bubbles were observed upon shaking immediately.
The NMR tube was sealed and inserted into the spectrometer im-
Hensen, E. A. Pidko, ChemCatChem 2014, 6, 1526–1530; f) R. Tanaka, M.
Yamashita, L. W. Chung, K. Morokuma, K. Nozaki, Organometallics 2011,
1
mediately to obtain the first H NMR spectrum. The NMR spectra at
3
0, 6742–6750; g) Y. Manaka, W. H. Wang, Y. Suna, H. Kambayashi, J. T.
other times (from 5 min to 72 h) were collected subsequently. For
reaction with KI, [RhCp*Cl2]2 (5.0 μmol) and KI (250 μmol) were
mixed in CDCl3 (1.0 mL) first. When FT azeotrope (0.1 mL) was
added, gas bubbles were observed upon shaking immediately. The
same procedure as that described above was followed.
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(
[RhCp*Cl2] or [RhCp*Cl2]2 + KI) was dissolved in methanol first.
2
Then, the FT azeotrope was injected into the vial. After shaking the
vial, the MS analysis was performed. All of the spectra were re-
corded within 5 min after addition of the FT azeotrope (Figure S9).
Acknowledgments
We thank the Dalian Institute of Chemical Physics (DICP) – Uni-
versity of Liverpool – British Petroleum (BP) joint PhD program
for a studentship and British Petroleum (BP) for financial sup-
port (Z. W.).
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Cp*
2
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[
[
[
10] Species
Rh Cp*
11] Species E is one of the most possible configurations for [Rh Cp* HI ] ;
D is one of the most possible configurations based on
+
[
2
2
IH(OCHO)] ; please see the Supporting Information.
+
2
2
2
please see the Supporting Information.
12] Species C is one of the most possible speculated configurations, but
[
without direct support from experimental data.
5
2
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[14] The possibility that the protonation step could be the RDS cannot be
ruled out, however, as the equilibrium between the monomer and dimer
Eur. J. Inorg. Chem. 2016, 490–496
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could render the concentration of the monomeric Rh–H too low to be
detected by 1H NMR spectroscopy.
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[
[
15] In line with this view, we have recently shown that hydride formation is
more energy-costly than hydride transfer in a kinetic, NMR spectro-
scopic, and DFT study of an iridium-catalyzed transfer hydrogenation of
imines with formic acid: H.-Y. T. Chen, C. Wang, X. Wu, X. Jiang, C. R. A.
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[17] Although KBr and KCl also promote this reaction, their effect is much
less significant than that of KI (Figure 1), probably because the iodide is
softer and a stronger nucleophile in protic solvents and hence is ex-
pected to bind to the iridium more easily (see ref.[ ).
5]
[18] W. D. Jones, F. J. Feher, Inorg. Chem. 1984, 23, 2376–2388.
Received: September 16, 2015
Published Online: January 12, 2016
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim