Versatile Rh- and Ir-Based Catalysts for CO2 Hydrogenation, Formic Acid Dehydrogenation, and Transfer Hydrogenation of Quinolines

Article abstract of DOI:10.1021/acs.inorgchem.8b02164

Considering the interest in processes related to hydrogen storage such as CO₂ hydrogenation and formic acid (FA) decomposition, we have synthesized a set of Ir, Rh, or Ru complexes to be tested as versatile precatalysts in these reactions. In relation with the formation of H₂ from FA, the possible applicability of these complexes in the transfer hydrogenation (TH) of challenging substrates as quinoline derivatives using FA/formate as hydrogen donor has also been addressed. Bearing in mind the importance of secondary coordination sphere interactions, N,N′ ligands containing NH₂ groups, coordinated or not to the metal center, were used. The general formula of the new complexes are [(p-cymene)RuCl(N,N′)]X, X = Cl^-, BF₄^- and [Cp?MCl(N,N′)]Cl, M = Rh, Ir, where the N,N′ ligands are 8-aminoquinoline (HL1), 6-pyridyl-2,4-diamine-1,3,5-triazine (L2) and 5-amino-1,10-phenanthroline (L3). Some complexes are not active or catalyze only one process. However, the complexes [Cp?MCl(HL1)]Cl with M = Rh, Ir are versatile catalysts that are active in hydrogenation of quinolines, FA decomposition, and also in CO₂ hydrogenation with the iridium derivative being more active and robust. The CO₂ hydrogenation takes place in mild conditions using only 5 bar of pressure of each gas (CO₂ and H₂). The behavior of some precatalysts in D₂O and after the addition of 9 equiv of HCO₂Na (pseudocatalytic conditions) has been studied in detail and mechanisms for the FA decomposition and the hydrogenation of CO₂ have been proposed. For the Ru, Ir, or Rh complexes with ligand HL1, the amido species with the deprotonated ligand are observed. In the case of ruthenium, the formate complex is also detected. For the iridium derivative, both the amido intermediate and the hydrido species have been observed. This hydrido complex undergoes a process of umpolung D⁺a?" Ir-D. All in all, the results of this work reflect the active role of aNH₂ in the transfer of H⁺.

Full text of DOI:10.1021/acs.inorgchem.8b02164

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Versatile Rh- and Ir-Based Catalysts for CO2 Hydrogenation, Formic
Acid Dehydrogenation, and Transfer Hydrogenation of Quinolines
‡,#
Jairo Fidalgo,†‡,# Margarita Ruiz-Castaneda, Gabriel García-Herbosa,† Arancha Carbayo,†
̃
Felix A. Jalon, Ana M. Rodríguez,§ Blanca R. Manzano,*,‡ and Gustavo Espino*,†
́
́
†Departamento de Química, Facultad de Ciencias, Universidad de Burgos, Plaza Misael Ban
̃
uelos s/n, 09001, Burgos, Spain
Departamento de Química Inorganica, Organica y Bioquímica, Facultad de Químicas, IRICA, Universidad de Castilla-La Mancha,
Avda. Camilo J. Cela 10, 13071 Ciudad Real, Spain
‡
́
́
§
́
́
́
Departamento de Química Inorganica, Organica y Bioquímica, Escuela Tecnica Superior de Ingenieros Industriales, Avda. C. J.
Cela, 3, 13071 Ciudad Real, Spain
S
* Supporting Information
ABSTRACT: Considering the interest in processes related to hydrogen storage such as CO2
hydrogenation and formic acid (FA) decomposition, we have synthesized a set of Ir, Rh, or Ru
complexes to be tested as versatile precatalysts in these reactions. In relation with the formation
of H2 from FA, the possible applicability of these complexes in the transfer hydrogenation
(TH) of challenging substrates as quinoline derivatives using FA/formate as hydrogen donor
has also been addressed. Bearing in mind the importance of secondary coordination sphere
interactions, N,N′ ligands containing NH2 groups, coordinated or not to the metal center, were
used. The general formula of the new complexes are [(p-cymene)RuCl(N,N′)]X, X = Cl−,
−
BF4 and [Cp*MCl(N,N′)]Cl, M = Rh, Ir, where the N,N′ ligands are 8-aminoquinoline
(HL1), 6-pyridyl-2,4-diamine-1,3,5-triazine (L2) and 5-amino-1,10-phenanthroline (L3).
Some complexes are not active or catalyze only one process. However, the complexes
[Cp*MCl(HL1)]Cl with M = Rh, Ir are versatile catalysts that are active in hydrogenation of
quinolines, FA decomposition, and also in CO2 hydrogenation with the iridium derivative being more active and robust. The
CO2 hydrogenation takes place in mild conditions using only 5 bar of pressure of each gas (CO2 and H2). The behavior of some
precatalysts in D2O and after the addition of 9 equiv of HCO2Na (pseudocatalytic conditions) has been studied in detail and
mechanisms for the FA decomposition and the hydrogenation of CO2 have been proposed. For the Ru, Ir, or Rh complexes
with ligand HL1, the amido species with the deprotonated ligand are observed. In the case of ruthenium, the formate complex is
also detected. For the iridium derivative, both the amido intermediate and the hydrido species have been observed. This
hydrido complex undergoes a process of umpolung D+↔ Ir−D. All in all, the results of this work reflect the active role of −NH2
in the transfer of H+.
and transport of molecular hydrogen is dangerous and very
inefficient. As a consequence, the development of hydrogen
storage systems is highly desired.11,12 Formic acid (FA) is
considered a leading system for hydrogen storage,4,11,13
because it combines a moderately high H2 content (4.38 wt
%, 53 g·L−1) with a number of advantageous properties related
to its safe transportation and manipulation.14,15 Interestingly,
FA is a major product of biomass processing.13
Different complexes of Ru, Rh, Ir, and Fe, mainly with P- or
N-donor ligands including half-sandwich derivatives and others
that contain pincer ligands (usually of the PNP type), have
been reported as precatalysts in the FA dehydrogen-
ation.13,14,16−26 Catalytic hydrogenation of CO2 has been
reported most frequently for ruthenium complexes but iridium,
rhodium, iron, cobalt, nickel, copper, manganese, and
molybdenum derivatives have also been described.4,5,14,27−32
INTRODUCTION
■
Hydrogen-related catalytic processes are of current interest for
the scientific community, not only due to their numerous
practical applications,1 but also for the intriguing character-
istics of the transition metal hydrides and dihydrogen
complexes.2 In this context, one motivating and current
challenge related to sustainability is the catalytic CO2
hydrogenation3,4 to one-carbon molecules such as formic
acid4 (or formate)5 or methanol as an alternative to photo-6
and electrochemical7,8 CO2 reduction. The interest in formic
acid in relation to CO2 conversion derives from the fact that it
is the liquid product of CO2 hydrogenation that requires the
lowest consumption of H2.9 Formic acid can be used as a
feedstock in chemical reactions but it can also be employed in
fuel cells.
In addition to the above, hydrogen has been postulated as an
alternative energy source for future generations due to its high
gravimetric energy density and its environmental advantages
(water is the only byproduct).10 The manipulation, storage,
Received: July 31, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
The concept of TH of CO2 to formate was first proposed by
Peris in 2010.33 High activities have been attained in the
hydrogenation of bicarbonate with half-sandwich Rh, Ir, or Ru
with dihydroxy-phenanthroline ligands (proton-responsive
ligands) although the use of high pressures is required
(about 40 bar).34,35 In general, harsh conditions of temper-
ature and pressure are commonly required for such
processes.14,32 Although examples with low temperatures
have been reported (50−80 °C), the use of 100−200 °C is
rather common and pressures clearly higher than 25−25
(CO2/H2) bar are generally used. Systems that are active in
both transformations have been reported,36,37 and a detailed
review about noble and non-noble metal based catalysts for
both processes has been recently published.14 One again, high
pressures for the CO2 hydrogenation,35,37−41 high catalyst
loadings,42 and/or the use of organic solvents are usually
needed.40,41,43 Fukuzumi et al. described a process that takes
place at atmospheric pressure, although this requires a high
catalyst concentration and provides a low TON (100).44 As a
consequence, the development of new, facile, efficient, and
robust versatile catalysts for both processes that work in water,
as a benign solvent, and under mild conditions is highly
desirable.
The metal-catalyzed generation of H2 from FA is closely
related to the use of this acid (in a buffer with formate) as a
hydrogen donor for transfer hydrogenation (TH) reactions in
water.45−48 TH has been frequently used for the hydrogenation
of carbonyl compounds46,49 and imines,50 but the hydro-
genation of heteroaromatics is more challenging.51 The
selective hydrogenation51−53 of the pyridinic ring of quinolines
leads to 1,2,3,4-tetrahydroquinolines, which are important
synthetic intermediates for pharmaceuticals, agrochemicals,
and dyes.54,55 Examples of this hydrogenation with the FA/
formate mixture in water with Ir, Rh, and Ru complexes have
appeared in the bibliography.56−58 Xiao59 reported the use of
Ir complexes with low catalyst loadings, and interesting
mechanistic studies were also described. The handling of
molecular hydrogen to reduce these substrates is less
convenient and, with some exceptions,60 either high
pressures61−67 or high temperatures68 are required. It is
generally accepted that an ionic mechanism occurs that
involves hydride transfer to a previously protonated
form58−60 of the quinoline derivative. Moreover, a clear effect
of the pH of the medium has been observed and acidic values
(4.5−5) are needed for an optimal reduction.
Chart 1
dehydrogenation of formic acid into H2 and CO2, and (iii)
selective TH of quinolines with HCO2H/HCO2Na as the
hydrogen source in water.
We also report on the detection and study of the possible
intermediates in the catalytic processes and the behavior in
D2O of the metal hydrides by analyzing the presence or
absence of an umpolung process (i.e., reversal of polarity,
transformation of D+ into M−D). An interchange between a
hydrido and an amido iridium species derived from ligand HL1
was identified. The formation of HD or D2 with D2O as the
only deuterium source has also been considered. The results
reported here could open new ways for the design of improved
catalytic systems.
RESULTS AND DISCUSSION
■
Synthesis of Complexes. The structural formulas of the
complexes reported in this work are provided in Scheme 1.
The cationic p-cymene complex [(η6-p-cym)RuCl(HL1)]Cl, I,
was prepared by reacting the dimer [(η6-p-cym)Ru(μ-Cl)Cl]2
with 8-aminoquinoline (HL1) at room temperature. Treat-
ment of complex I with NaBF4 afforded 1BF4. The change in
the anion was performed in order to obtain a compound that
could give rise to single crystals appropriate for an X-ray
diffraction study. The neutral complex 2 was prepared adding
KOH after the reaction of the dimer and the ligand.
Pentamethylcyclopentadienyl (Cp*) cationic complexes of
stoichiometry [(η5-Cp*)M(N,N′)]Cl, 3 to 7, were prepared by
treating the appropriate dichloro-bridged dimer [(η5-Cp*)M-
(μ-Cl)Cl]2 (M = Rh, Ir) with the corresponding ligand.
Complex I has been reported by Turkmen et al. and Singh et
̈
al. independently,73,74 but it was synthesized in this work for
the sake of comparison in the catalytic tests.
Characterization by NMR Spectroscopy. The 1H NMR
and 13C{1H}NMR spectra are consistent with the structural
formulas depicted in Scheme 1. 2D NMR spectra and NOE
effects were used to assign the resonances (see Figure S1 for
the NOE effects in 2). The Ru(II) p-cymene complexes I,
1BF4, and 2 showed patterns consistent with a C1 symmetry.
The 2D NOESY spectrum registered for I exhibits exchange
cross-peaks between the pairs H2−H6, H3−H5, and H8−H9,
which evidence a dynamic process that allows interconversion
between enantiomers RRu and SRu.
The importance of secondary coordination sphere inter-
actions on using multifunctional ligands69 has been docu-
mented in different fields including H2 production and CO2
reduction.70 For example, in complexes with pincer ligands the
formation of a hydrogen bond with the ligand allows the
insertion of CO2 into an Ir−H bond in a very active system71
and cases of ligand-assisted H2 activation have been
reported.27,36 Long-range metal−ligand bifunctional catalysis
involving FA-assisted proton hoping in a dehydrogenation
process of this acid has also been described.72
We describe here a series of half-sandwich Ru, Ir, and Rh
complexes with κ2-N,N′-chelating ligands, which could
participate actively in the catalytic process. The ligands chosen
(see Chart 1) contain one or more −NH2 groups that are able
to form hydrogen bonds in all cases but would be arranged at
different distances from the metal center. Interestingly, it was
found that one Rh complex and another Ir derivative are
versatile catalysts that are active in (i) CO2 hydrogenation, (ii)
The resonances of the −NH2 groups deserve further
comment: Complexes I, 3, and 4, with 8-aminoquinoline,
display two resonances at very different chemical shifts (for
2
instance, δ 11.72 and 4.61 ppm, JHH = 10.55 Hz for I) and
these signals are attributed to inequivalent protons of this
group. This observation is consistent with an N,N′ chelate
coordination mode for HL1 and suggests that the −NH proton
at lower field is involved in a hydrogen bonding interaction,
possibly with the chloride ligand. Moreover, it was confirmed
that these protons do not exchange with one another on the
NMR time scale. In the case of 7, which contains ligand L3
where the amino group is not coordinated, a unique and broad
resonance was observed for the −NH2 group. The NH group
B
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Synthesis and Molecular Structure with Atomic
a
Numbering of the Complexes Described in This Paper
Figure 1. ORTEP of the cation in complex 1BF4. H atoms have been
omitted for clarity. Ellipsoids are shown at the 30% probability level.
Selected distances (Å): Ru1−Cl1 = 2.3984(8); Ru1−N1 = 2.100(2);
Ru1−N2 = 2.128(2); Ru1−Caverage = 2.19; Ru1−Centroid(p-cym) =
1.43. Bite angle N1−Ru1−N2 = 78.70(9)°.
similar except the Ru−NH2 distance that is shorter for 1BF4
(2.128(2) and 2.134 for I).74 Interestingly, the N−H bond
distances are longer in 1BF4 (0.97 Å) than in I (0.91−0.92
Å).74 The shortening of the Ru−NH2 distance implies a higher
electron donation from the N atom increasing the positive
charge of this atom and this causes a lengthening of the N−H
distances.
The 3D structure is held together by a set of intermolecular
interactions. The chloride ligand and the tetrafluoroborate
anion connect three or four, respectively, cations through
hydrogen bonds, including bifurcated and trifurcated bonds.
The hydrogen atoms that participate in these bonds are not
−
only those of the −NH2 group (interacting with the BF4
anion or the chloride ligand) but also H−C(sp2) and H−
C(sp3) atoms (see Table S3). A π−π interaction54 is also
established between two cations and this involves the aromatic
rings that bear the amino group (see Figure S2 and Table S4).
Catalytic Transfer Hydrogenation of Quinolines. It
was decided to study the catalytic activity of the complexes in
the transfer hydrogenation (TH) of quinoline and methyl- and
amino-substituted quinolines using HCO2H/HCO2Na as the
hydrogen source in water. Based on literature reports59 and on
our previous results on the TH of ketones with the same
hydrogen source, we chose to perform all the tests at pH 4.5.
2-Methylquinoline (IIa) was quantitatively reduced to 2-
methyl-1,2,3,4-tetrahydroquinoline, IIb, within 24 h at room
temperature using a 0.1 mol % loading of 3 or 4 (Table 1,
entries 3 and 6). The same result was obtained on using a
mixture of [Cp*IrCl2]2 and 8-aminoquinoline, HL1, (Table 1,
entry 12). However, on using only the iridium dimer,
[Cp*IrCl2]2, the yield decreased to 50% and activity was not
observed with HL1 in the absence of any metallic center
(entries 13 and 14). The performance of I, 6, and 7 was
disappointing (<1% yield after 24 h, entries 1, 10, and 11)
while the activity of 5 was only moderate (entry 9). Although
the activity of the Ru dimer, [(p-cym)RuCl2]2, was low (entry
15), it was higher than that of the preformed complex I. As a
control experiment, it was verified that the TH of IIa did not
take place in the absence of any precatalyst (entry 16).
a
Only one enantiomer is shown.
of 2 gives rise to a singlet at 8.17 ppm integrating for 1H.
Besides, the neutral nature of this compound leads to a shifting
to higher field of the resonances of the L1 ligand as compared
to I.
Solid-State Characterization of 1BF4. The molecular
and crystal structure of complex 1BF4 was solved by X-ray
diffraction. The crystallographic data and the distances and
bond angles are gathered in the Supporting Information
(Tables S1 and S2). An ORTEP of the cation is included in
Figure 1. Both enantiomers (RRu and SRu) are present in the
crystal. As I or the similar complex with benzene, previously
described,74 1BF4 has a three-legged piano-stool structure
formed by the η6-coordinated p-cymene ring, the (κ2-N,N′)
chelate HL1 and the chloride ligand. The 8-aminoquinoline
ligand is essentially planar. The value of the bite angle is
78.70(9)°, similar to I.74 The bond lengths within the cationic
Ru-centered component are in the expected range75−78 and
these include the Ru−Cp‑cymene average distance of 2.19 Å.
When compared with I the bonds involving the Ru atom are
Interestingly, the application of the aforementioned TH
conditions to quinoline (IIIa) using 4 as the precatalyst led to
the hydrogenation of the pyridinic ring along with the
introduction of a formyl group on the nitrogen atom in a
C
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. TH of 2-Methylquinoline (IIa) and Quinoline
(IIIa) Using Different Precatalysts
Table 2. Results Obtained in the TH of 8-Aminoquinoline
Using Different Precatalysts
a
a
Time
(h)
Yield
(%)
TON
b
Entry
Cat
Time (h) Yield (%) TON (TOF, h−1
)
b
Entry Substrate
Cat
(TOF, h−1
)
1
2
3
4
5
6
7
8
3
4
24
24
24
1
24
24
24
24
1
60
74
37
19
42
27
96
72
33
0
600
740
370
190 (190)
422
270
961
620
330 (330)
--
1
2
3
4
5
6
7
8
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
IIa
I
24
24
24
3
1
24
3
0
0
>99
8
--
--
990
2
3
3
3
4
4
4
5
6
7
[Cp*IrCl2]2 + HL1
4
5
6
80
20 (20)
990
130
60 (60)
400
2
>99
13
6
40
1
7
[Cp*IrCl2]2 + HL3
7
No
1
9
10
9
24
24
24
24
24
10
11
12
10
10
990
a
Reaction conditions: 0.1 mol % of catalyst (0.09 mol % in the case of
7). Substrate: 2.5 mmol. Reaction in water with HCO2H/HCO2Na
(pH = 4.5) in 3 mL of H2O at r. t. during 24 h. The amount of
hydrogenated product was lower than 5%. Calculated at 1 h.
1
>99
[Cp*IrCl2]2 +
HL1
b
13
14
15
16
17
IIa
IIa
IIa
IIa
IIIa
[Cp*IrCl2]2
HL1
[(p-cym)RuCl2]2
No
4
24
24
24
24
24
50
0
20
0
500
--
200
--
in the case of 4 (compare entry 2 vs 3 and entry 7 vs 8).
Moreover, it was verified that a precatalyst was necessary for
the process to occur, thus reflecting the formylation catalytic
activity of these complexes. The generation of formamides by
reaction with FA is a process that requires high temperatures
and these are clearly higher than those required for the
generation of acetamides, for example. In our examples,
however, the process takes place at room temperature. The
reported formylation of 8-aminoquinoline requires the use of
toluene at reflux.89 Hydrogenation of 8-aminoquinoline has
been reported but higher temperatures are required (toluene at
reflux).90
33
333
a
Reaction conditions: 0.1 mol % of catalyst (0.09 mol % in the case of
7). Substrate: 2.5 mmol. Reaction in water with HCO2H/HCO2Na
(pH = 4.5) in 3 mL of H2O at r.t. Calculated at 1 h.
b
one-pot reaction (IIIb, Table 1). The product was obtained in
moderate yield (Table 1, entry 17). The lower yield with
respect to the hydrogenation of IIa could be related to a lower
degree of protonation of IIIa (pKa(IIIa) = 4.94 and pKa(IIa) =
5.83). The reactivity of the formyl group introduced in IIIb
opens new reaction pathways for 1,2,3,4-tetrahydroquinoline
derivatives. The formylation of 1,2,3,4-tetrahydroquinolines
with different reagents has been described previously,79−83
usually through heterogeneous catalysts, but the two-step one-
pot process starting from quinoline is usually nonselective84
and/or requires high temperatures (sometimes the reaction is
also carried out under pressure).85−88 A case with selectivity
toward one of the two hydrogenated products (formylated and
nonformylated) thanks to pH changes with a silica-supported
iridium catalyst has recently been reported (80 °C).88
Catalytic tests were also performed to hydrogenate 8-
aminoquinoline (IVa) with 3−7. In these cases hydrogenation
was observed in a very small extent (<5%) under the
conditions explored. However, the product corresponding to
the formylation of the amino group was the main product
(Table 2). The hydrogenation of IVa was attempted again
using 4 as the precatalyst at a lower pH (3.2) in an effort to
improve the protonation of 8-aminoquinoline. Although
formylation was observed, the hydrogenation was not
improved. It is noteworthy that 7, a precatalyst that is
practically inactive in the hydrogenation of 2-methylquinoline
(IIa), gave the best result in this process with a yield of 96%.
When the activity of 4 or 7 is compared with that of mixtures
of [Cp*IrCl2]2 and the corresponding ligands (HL1 or HL3,
respectively), a decrease in the activity is observed, especially
The three quinolines tested in the catalytic experiments are
not soluble in the solutions in the catalytic conditions. Thus,
the processes are biphasic in nature.
Some cases have been reported of the dehydrogenation of
1,2,3,4-tetrahydroquinolines.91,92 Catalytic tests were per-
formed with 4 and 1,2,3,4-tetrahydroquinoline (0.1% mol
and 1% mol) or with 2-methyl-1,2,3,4-tetrahydroquinoline
with the same precatalyst (0.1% mol) at 80 °C in 2,2,2-
trifluoroethanol during 20 h. However, the amounts of the
dehydrogenated products, quinoline and 2-methylquinoline,
were very small (yield <1%).
Catalytic Dehydrogenation of Formic Acid. Complexes
I and 3−7 were tested as potential catalysts for the
dehydrogenation of a mixture of formic acid/sodium formate
at 100 °C and pH 4.5, using 0.04 mol % of the respective
precatalysts. The gas formed was collected with a gas buret. It
was verified that a certain amount of CO2 was present in the
gas mixture on using a buret filled with water (as verified by
bubbling the gas through CD3CN, signal at 125.5 ppm in the
13C{1H} NMR spectrum). As a consequence, the buret was
filled with aqueous NaOH solution (0.1 M). In this case, CO2
was not detected in the gas mixture collected. The amount of
H2 generated by I and 5−7 was very small. However, 3 and 4
were active in this process (see Table 3). In the initial period of
the reaction (5−10 min), 3 provided better performance than
D
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
spectroscopy allowed H2 and HD to be detected in a H2:HD
ratio of 20:80 (see Figure S6). A D NMR spectrum allowed
Table 3. Dehydrogenation of Formic Acid Catalyzed by the
Indicated Complexes
a
2
the detection of D2 but the amount of HD was not sufficiently
high to be observed in this spectrum (see Figure S7). Thus, the
three possible isotopomers of molecular hydrogen were
detected in the following sequence of increasing concentration:
H2 < HD ≪ D2.
As for 3, in the catalytic dehydrogenation experiment with
the HCO2H/HCO2Na mixture in D2O, both H2 and HD were
detected in a H2:HD ratio of 26:74 but in this case the
formation of D2 was not observed.
CO2 Hydrogenation. Complexes I and 3−6 were tested as
precatalysts in the hydrogenation of CO2 using pressures of 5
bar of H2 and 5 bar of CO2 in 0.1 M aqueous KOH at 80 °C
during 8 h. The results are gathered in Table 4. Potassium
b
c
Entry Cat time (min) V(H2) , L
n(H2), mol
TON (TOF, h−1
)
1
2
I
120
5
0
0
--
641
886
1087
3
3
3
3
3
3
3
4
4
4
4
4
4
4
5
6
7
0.290
0.400
0.490
0.640
0.690
0.855
0.950
0.200
0.360
0.685
1.160
1.250
1.345
1.400
0
1.18 × 10−2
1.63 × 10−2
2.00 × 10−2
2.61 × 10−2
2.87 × 10−2
3.56 × 10−2
3.88 × 10−2
8.16 × 10−3
1.47 × 10−2
2.80 × 10−2
4.74 × 10−2
5.20 × 10−2
5.60 × 10−2
5.72 × 10−2
0
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
10
21
50
60
120
180
5
10
21
1418
1560 (1560)
1935
2109
443
799
1522
2576
50
60
2826 (2826)
Table 4. Hydrogenation of CO2 Catalyzed by the
120
200
120
120
120
3043
3109
--
a
Complexes Indicated
HCO2K generated
Entry Precatalyst Time (h) T (°C)
(mol)
TON
0
0
0
0
--
--
1
2
3
4
5
6
7
8
I
I
8
3
8
3
8
3
8
8
80
40
80
40
80
40
80
80
0.00139
nd
0.00150
nd
0.00151
0.00142
nd
5562
--
5991
--
6021
5662
--
a
The reaction was performed using a mixture of HCO2H/HCO2Na
3
3
4
4
5
6
(0.05:0.105 mol) in 10 mL of H2O in the presence of the
corresponding catalyst (1.84 × 10−5 mol, 0.04 mol %) at initial pH
b
4.5 and 100 °C. Yield of H2 gas collected in a gas buret filled with a
0.1 M NaOH solution. Calculated at 60 min.
c
4 (cf. entries 2 vs 9 and 3 vs 10) whereas after approximately
21 min 4 provided a better TON value than 3 (entries 4 and
11) (see Figure S3).
nd
--
a
Precatalyst = 50 μM. 5 mL of a 0.1 M KOH solution in H2O. P(H2)
= 5 bar, P(CO2) = 5 bar.
The most active systems reported until now include complex
A of Chart 2 (3.1 μM, 90 °C, TON = 165 × 103 after 7 h,
formate was only generated in the case of complexes I, 3, and
4. When the temperature and the time were reduced to 40 °C
and 3 h, respectively, the potassium salt was not detected in the
case of I and 3, but the number of moles were only slightly
reduced in the case of 4, which was the most active precatalyst
(see Figures S8−S10 and Table S5 for more information about
the experimental procedure).
Chart 2
Some very active systems previously described include
iridium catalysts as [IrH3(iPr2PCH2NC5H3CH2PiPr2)] (200
°C, 50 bar, 0.010 μM, 300 × 103 TON, 150 × 103 TOF,
h−1),28 complex A of Chart 2 (80 °C, 50 bar, 2 μM, 79 × 103
TON, 53.8 × 103 TOF h−1)35 or the Ru derivative
[RuHCl(CO)(tBu2PCH2NC5H3CH2PtBu2)] (120 °C, 40 bar,
0.178 μM, 1100 × 103 TOF h−1).43 However, this last catalytic
process was performed in DMF. The two last ones were also
very active on FA decomposition. To the best of our
knowledge, the lowest total pressure reported (10 bar, 50
μM), similar to our examples, is reported with complex A and
gives rise to a TON value of 7200 but after 336 h (64 TOF
h−1). Thus, our results are outstanding because of the low
pressure used (10 bar, 5662 TON at 3 h with 4).
TOF(h−1) = 228 × 103),35 the Ru derivative [RuHCl(CO)-
(tBu2PCH2NC5H3CH2PtBu2)] (1.42 μM, 90 °C, TON = 706.5
× 103 after 3 or 4 h, TOF(h−1) = 256 × 103)43 and the Fe
complex [FeH(HCO2)(CO)(iPr2P(CH2)2NH(CH2)2PiPr2)]
(0.0001 mol % cat, 10 mol % LiBF4, 80 °C, TON = 983,642
in 9.5 h, TOF(h−1) = 196,728).18 However, although complex
A was used in aqueous solutions, the FA decomposition with
the Ru and Fe complexes was carried out in organic solvents as
DMF and dioxane, respectively.
From the different catalytic studies performed it is observed
that apparently the NH2 group of HL1 exerts a positive effect
as it has also been recently reported in other types of Ir
complexes with this ligand.22
In the case of 4, further experiments were carried out to
obtain information about the possible mechanism of the
process: (i) the initial rates of decomposition of the mixture
HCO2H/HCO2Na in catalytic experiments with H2O and
D2O were compared and an isotopic effect of 1.41 was
observed (r(H2O)/r(D2O) = 1.41; see Figures S4 and S5); (ii)
analysis of the gas evolved in a catalytic process performed in
Mechanistic Studies. Characterization of Catalytic
Intermediates. Some experiments were performed to get
information about the possible catalytic intermediates in the
FA decomposition process or to find out the role of the −NH2
1
group of the HL1 ligand. The H NMR spectra in D2O at 25
1
D2O showed the presence of H2, HD, and D2. H NMR
°C of complexes I, 3, 4, and 6 were recorded, and after that,
E
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. Aromatic region (left) and hydride region (right) of 1H NMR spectra of (a) Complex 4 in D2O (7.5 × 10−3 mmol) at 25 °C. The species
4 [Cp*IrCl(HL1)]+, coexist with a small amount of 4-C (C) [Cp*Ir(D2O)(L1)]+. (b−j) Spectra corresponding to the evolution with time of the
+
solution of ‘a’ after adding HCO2Na (6.5 × 10−2 mmol). 4-C and 4-F (F) [Cp*IrH(HL1)] coexist. Free formate is labeled as yellow ).
⧫
the evolution with time under pseudocatalytic conditions, i.e.,
in the presence of an excess of HCO2Na (1:9 molar ratio) in
D2O was monitored. At room temperature and at the
moderately alkaline pH (8.29) of these experiments we
anticipated that the FA decomposition reaction could be
slowed down and that catalytic intermediates could be
detected.
the corresponding hydroxo-amino complex, [Cp*Ir(OD)-
(HL1)]+ (labeled as 4-C2, see Scheme 2). From now on, we
will use the symbol C when referring to the two species C1/C2
in equilibrium.
In the first spectrum recorded after the addition of HCO2Na
to the solution of 4 in D2O (see spectrum (b) in Figure 2 and
S12) the signals attributed to 4 vanished and a sharp increase
in the peaks assigned to 4-C was observed along with the
appearance of resonances for a hydrido-derivative, Ir−H (4-F).
The latter assignment was made on the basis of the signal
observed at −9.78 ppm (see Figure 2). The formation of
monohydrides from the reaction of half-sandwich complexes of
late-transition metals in the presence of HCO2Na is well-
documented.93 Interestingly, during the first 48 h a fluctuation
was observed in the integration ratio between signals of 4-C
and 4-F together with a slow release of bubbles (presumably
H2 + CO2). However, after this time the evolution of gas had
stopped and the concentration of hydride 4-F had diminished
considerably. This finding is consistent with the arrest of the
catalytic decomposition of FA due to the expected increase in
pH even though the HCO2Na had not been completely
consumed.
The spectra of 4 in neat D2O displayed two sets of signals
for two products in an approximate 9:1 ratio (Figure 2(a)).
The major component is tentatively attributed to the chlorido-
precatalyst while the minor component is assigned as a new
derivative (labeled as 4-C) (see Scheme 2 for the labeling of
the intermediates). Several additional NMR experiments were
carried out to establish the identity of 4-C: (i) Na2CO3 was
added to 4 in D2O and this resulted in the quantitative
formation of 4-C (see Figure S11). (ii) DCl (1 M in D2O, 50
μL) was added to 4-C and resonances with δ values consistent
with 4 emerged (see Figure S11). All of these results prove that
the formation of 4-C from 4 involves a deprotonation process.
However, inasmuch as species 4 and 4-C can be distinguished
by NMR, we believe that the transformation of 4 into 4-C
must involve a second process, such as aquation (Ir−Cl → Ir−
OH2 exchange). Hence, we propose that the minor
intermediate 4-C could well be formulated as two species in
fast equilibrium: (i) the aqua-amido complex 4-C1 of formula
[Cp*Ir(D2O)(L1)]+ (see Scheme 2) where the −ND2 group
of HL1 has been converted into an amido group −ND and (ii)
A comparative analysis of the integrals for the hydride
resonance of 4-F and that for the signal at 8.95 ppm belonging
to the same species reveals a partial deuteration of the Ir−H
2
group and accordingly, the Ir−D group was detected by D
NMR spectroscopy (resonance at −9.71 ppm, see Figure
F
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Cycle Proposed for the Evolution of a Solution of 4 with HCO2Na in D2O
S13).The variation of the Z value (see eq 1) is represented in
Figure S14. The initial value of 0.72 decreased sharply in the
first hour and then diminished more slowly up to 5 h before a
value between 0.3−0.4 was reached up to 25 h. After 11 days
the value of Z was 0.25.
formate the activation steps are shifted toward the production
of 4-C1 from 4. Then, the cycle starts with an equilibrium
between 4-C1 and 4-C2. In step b, 4-C2 undergoes a
substitution reaction to give the formate complex, 4-E. This
step must be rate-determining in agreement with the
observation of 4-C. Complex 4-E rapidly evolves to the
hydride derivative 4-F. Indeed, the absence of signals for the
species 4-E and the observation of resonances for the hydrido
intermediate 4-F in this 1H NMR study indicates that step c is
fast and that step d is rate limiting. This fact is in accordance
with the isotopic effect observed when using D2O instead of
H2O always in the presence of HCO2H (vide supra). The
species 4-F undergoes an [Ir]−H ↔ [Ir]−D exchange by an
umpolung process (reversal of polarity, in this case D+ from
water D(δ+)-OD is transformed into Ir−D(δ−)). The
occurrence of the umpolung process is deduced both from
the reduced integral area of the Ir−H resonance and from the
formation of D2 in the catalytic process. The umpolung process
may take place, as observed in other examples93−95 by reaction
of D+ with the Ir−H group and formation of [Cp*Ir(HD)-
(HL1)]2+ (4-G) provided that step d is slower than step f. In
this part of Scheme 2, deuterium atoms, D, are shown in
brackets to reflect the possible incorporation of deuterium in
the cycle. Next, it is postulated that an internal deuterium
transfer process slowly converts 4-F into the undetected
intermediate 4-H (step d). Finally, the cycle is closed through
(Ir − H)
Z =
(Ir − H) + (Ir − D)
(1)
1
Ir−H = integral of the hydride resonance of 4-F in the H
NMR spectrum.
(Ir−H) + (Ir−D) = integral of the resonance at 8.95 ppm of
1
4-F in the H NMR spectrum.
From the results described above it can be concluded that
(a) complex 4 can promote the catalytic release of CO2 and H2
even under adverse conditions (alkaline pH, 8.29); (b) the
chloro-amino species 4 is only the precatalyst and it does not
take part in the catalytic cycle; (c) the species 4-C and 4-F are
active intermediates in this process and can be considered as
resting states of the catalytic cycle; (d) the hydrido species 4-F
undergoes partial deuteration, thus suggesting its participation
in an umpolung process (vide infra).
The mechanism proposed for the decomposition of
HCO2Na in the presence of 4 is outlined in Scheme 2. The
process consists of two consecutive activation steps and a
catalytic cycle in which only the species 4-C and 4-F have been
detected experimentally. We theorize that in the presence of
G
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the fast release of hydrogen (step e) and the regeneration of 4-
C1.
decreased and two new sets of signals were observed: the
formato complex (I-E with a coordinated formate ligand) and
the aqua-amido complex (I-C, its nature was also verified with
reactions similar to those used with 4-C) (see Figure S18).
The presence of the formato complex shows that in this case its
transformation into the hydrido derivative is not a fast process.
Evidence for the hydrido complex was not found and this is
consistent with a fast transformation of the hydrido species
into the aqua-amido derivative I-C.
In order to obtain more information to support our
proposal, the effect of the addition of HCl (HCl:4 ratio of
2) to the sample after 11 days, which contained mainly 4-C
and 6% of 4-F with Z = 0.25, was analyzed. The release of gas
was observed and after 5 h 61% of 4-F was present and the Z
value was 0.13. These facts indicate, as proposed, that the
acidic medium favors the formation of 4-F from 4-C because
the rate limiting step b is promoted by a decrease in the OD−
concentration and this allows the catalytic cycle to restart.
Moreover, this situation also favors the deuteration of the Ir−
H group thanks to the promotion of step f. Four days after the
addition of the acid the major product was 4-C and the
percentage of 4-F was 12%. At this point, the amount of
In the case of 6, with ligand L2, the D2O solution only
contained the initial compound and the addition of an excess
of HCO2Na initially gave rise to a mixture of three species,
namely, 6, the formate-intermediate (6-E) and the hydrido
species (6-F). This mixture evolved slowly to produce the
hydrido-derivative as the only species after 24 h (see Figure
S19) and this exhibited remarkable stabilityunlike the
analogous species with ligand HL1. This difference can be
explained as being the result of the position of the −NH2
group in the hydrido-intermediates. In the hydrido species 4-F
(and probably in 3-F and I-F), the coordinated −NH2 group
exhibits exacerbated acidity together with close proximity to
the hydride group, in such a way that one H+ (or D+) is easily
transferred to the hydride group to form H2 (or HD). In other
words, the ligand 8-aminoquinoline assists the metals
efficiently in the HCO2H dehydrogenation in a metal−ligand
bifunctional catalysis. The fact that 6 is not active in the
HCO2H dehydrogenation regardless of the pH implies that
protons are not transferred efficiently to the hydride group of
species 6-F even in the presence of HCO2H under catalytic
conditions. We also noted that for 6-F the intensity of the
hydride signal is not reduced with time with respect to other
signals of the compound, thus indicating that the Ir−H ↔ Ir−
D exchange does not take place.
−
formate had been markedly reduced to an HCO2 :(4-C+4-F)
ratio of 0.2. The value of Z at this point was 0.06 and this
shows a high degree of deuteration.
Considering all the information discussed above, we propose
that the catalytic hydrogenation of CO2 (P(CO2) = P(H2) = 5
bar) in the presence of 4 at basic pH must proceed by a
pathway opposite to that depicted in Scheme 2 (i.e., in an anti-
clockwise sense). Thus, we believe that 4-C1 is able to
coordinate a molecule of H2 to give intermediate 4-H, which in
turn can heterolytically activate this molecule to give 4-F. CO2
is then inserted into the Ir−H bond to produce 4-E, so that in
the next step the anion formate is eliminated to generate 4-C2,
which is in equilibrium with 4-C1. However, for the
−
dehydrogenation of the mixture HCO2H/HCO2 , which has
been studied at acidic pH, some species need to be
reformulated (see Scheme S1). In particular, we hypothesize
that the species 4-C1/4-C2 are not favored at acidic pH, so
instead we propose that the first active species in the
corresponding catalytic cycle must be the aqua-amino
intermediate 4-J ([Cp*Ir(DL1)(D2O)]2+). This species can
be transformed into intermediates 4-E, 4-F, and 4-H under the
experimental conditions to generate concurrently the gas
mixture of CO2 and H2. It is worth noting that the 4-C1/4-F
couple (the same will apply for 3 and I; see below) could be
considered as the dehydrogenated and hydrogenated forms,
respectively, of a pair of species in a Noyori-type mechanism.96
As for 3, the 1H NMR in D2O also revealed the coexistence
of 3 and 3-C (9:1 ratio, the similar nature of 3-C and 4-C was
verified with experiments as those performed with 4-C, Figure
S15) and after the addition of 9 equiv of HCO2Na the
transformation of 3 into 3-C takes place, with 3-C being the
only species after 72 h (see Figure S16). In this case neither
the formate complex nor the hydrido-derivative were detected
by NMR, which suggests that the release of both CO2 and H2
CONCLUSIONS
■
Two versatile precatalysts have been synthesized, namely,
[Cp*MCl(HL1)]Cl (HL1 = 8-aminoquinoline; M = Rh, 3; Ir,
4), and these are active in three processes that involve the
concerted transfer of hydrides and protons, i.e., CO2
hydrogenation, formic acid dehydrogenation, and transfer
hydrogenation of 2-methylquinoline under mild conditions.
The iridium complex 4 is more active than its relative with
rhodium, 3. The CO2 hydrogenation takes place at a very low
total pressure of 10 bar. The reaction of complexes 3 and 4 and
the ruthenium derivative [(p-cym)RuCl(HL1)]Cl with
HCO2Na in D2O leads to the formation of the hydrido
species where the NH2 group must be sufficiently acidic to
react with the hydride. This reaction liberates XY (X or Y = H,
D) and the species evolves to the amido derivative, which is
detected in all cases. This evolution is only slow enough for the
hydrido species to be detected in the Ir case. A pH-dependent
umpolung process takes place in the iridium hydrido complex,
with D+ being transformed into Ir−D. This process also leads
to the formation of D2 as the major component of the gas in
the catalytic formic acid dehydrogenation process. When the
reaction with HCO2Na is performed with the complex
[Cp*IrCl(L2)]Cl (L2 = 6-pyridyl-2,4-diamine,1,3,5-triazine)
the final product is the hydrido species and gas release was not
observeda finding in agreement with the lack of activity in
the FA dehydrogenation. Both of these facts may be related to
the absence of a coordinating −NH2 group that could assist in
the transfer of a proton to the Ir−H group. Thus, we can
conclude that the ligand 8-aminoquinoline assists the metals
−
are fast and that 3-C is the catalyst resting state in the HCO2
decomposition. In addition, the inability of 3 to produce D2 in
this reaction indicates that an umpolung process is not
operating (3-G is not formed). The ability of Ir-Cp*
complexes with bpy ligands to promote H/D umpolung
processes in contrast to the inability of the analogous Rh
derivatives have been reported previously.95 Apart from the
above, the cycles proposed for the FA dehydrogenation and
hydrogenation of CO2 for complex 3-A must be similar to
those depicted in Scheme 2 and Scheme S1, respectively.
The Ru complex I in D2O is in equilibrium with the aqua-
amino derivative, [(p-cym)Ru(HL1)(D2O)]2+, I-J (Figure
S17). After the addition of HCO2Na the signals for the latter
species disappeared, the intensity of the resonances of I
H
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
HCO2Na (3 mL, 14.0 mmol of HCO2H and 29.4 mmol of HCO2Na
in 2.8 mL of H2O, pH 4.5) was added and the mixture was stirred at
30 °C during 24 h. The reaction mixture was quenched with saturated
aqueous sodium bicarbonate. The aqueous layer was extracted with
ethyl acetate (3 × 10 mL) and the combined organic layers were
washed with brine (20 mL). The organic layer was collected and dried
over anhydrous sodium sulfate. Filtration, followed by evaporation of
the solvent under reduced pressure, gave the crude reaction mixture.
Procedure for the Catalytic Dehydrogenation of HCO2H. A
stock solution of HCO2H/HCO2Na was prepared by dissolving
HCO2Na (71.41 g) in 19 mL of concentrated formic acid (97%) and
diluting to 100 mL with distilled water.
A round-bottomed glass vessel fitted with a side arm was degassed
three times and placed under an N2 atmosphere. The HCO2H/
HCO2Na stock solution (10 mL, pH 4.5) was added. The vessel was
connected to a water-cooled condenser and this in turn to a gas
collection buret (i.e., the standard water displacement apparatus to
determine the volume of generated gas). The buret was charged with
0.1 M aqueous NaOH. The mixture was heated to the boiling point
with stirring and allowed to reflux until pressure equilibration (i.e.,
until water displacement stopped in the buret). The buret was
completely filled again with the NaOH solution and the precatalyst
(1.84 × 10−2 mmol) was added to the vessel under N2 flux. Volumes
of collected gas were recorded periodically. The catalytic activity was
calculated from the volume of displaced NaOH solution assuming
that CO2 is completely dissolved in this solution and that all the gas
collected is H2. The presence of H2 in the collected gas was confirmed
by recording a 1H NMR spectrum of this gas dissolved in CD3CN and
the absence of CO2 was verified by recording a 13C{1H} NMR
spectrum.
efficiently in the HCO2H dehydrogenation and the coordina-
tion of the NH2 group to the metal center imparts specific
properties to the complexes, a fact that may be used in the
future design of new catalytic species.
EXPERIMENTAL SECTION
■
General Methods and Starting Materials. The starting
materials RuCl3·xH2O, RhCl3·xH2O, and IrCl3·xH2O were purchased
from Johnson Matthey and used as received. The starting dimers
[RuCl(μ-Cl)(p-cym)]2,97 [RhCl(μ-Cl)(Cp*)]2, and [IrCl(μ-Cl)-
(Cp*)]2 (p-cym = p-cymene, Cp* = pentamethylcyclopentadien-
yl)98,99 were prepared according to literature procedures. The ligands
8-aminoquinoline (HL1), 6-pyridyl-2,4-diamine-1,3,5-triazine (L2),
and 5-aminophenanthroline (L3) were purchased from Sigma-Aldrich
and were used without further purification. Deuterated solvents
(CDCl3, CD3OD, and D2O) were obtained from Euriso-top.
Synthesis Method and Complex Characterization. All
synthetic manipulations were carried out under an atmosphere of
dry, oxygen-free nitrogen using standard Schlenk techniques. The
solvents were dried and distilled under a nitrogen atmosphere before
use. Elemental analyses were performed with a Thermo Fisher
Scientific EA Flash 2000 Elemental Microanalyzer. The analytical data
for the new compounds were obtained from crystalline samples when
possible. IR spectra were recorded on a Jasco FT/IR-4200
spectrophotometer (4000−400 cm−1 range) with a Single Reflection
ATR Measuring Attachment. HR ESI(+) mass spectra (position of
the peaks in Da) were recorded with an Agilent LC-MS system (1260
Infinity LC/6545 Q-TOF MS spectrometer) using DCM as the
sample solvent and (0.1%) aqueous HCO2H/MeOH as the mobile
phase and with an autospec spectrometer. NMR samples were
prepared under a nitrogen atmosphere by dissolving the appropriate
amount of compound in 0.5 mL of the respective oxygen-free
deuterated solvent and the spectra were recorded at 298 K on a
Varian Unity Inova-400 (399.94 MHz for 1H; 376 MHz for 19F; 100.6
Procedure to Detect the Gases in the Catalytic Dehydro-
genation of HCO2H with 4. In a 10 mL Schlenk flask, previously
purged with nitrogen, complex 4 (5.3 mg, 9.31 × 10−3 mmol) was
added to 5 mL of the HCO2H/HCO2Na stock solution. The mixture
was heated under reflux for 1 h and the evolved gas was
simultaneously bubbled through solutions of toluene-d8 and
toluene-H8 in different NMR tubes. The presence of H2 and HD in
1
MHz for 13C). Typically, H NMR spectra were acquired with 32
1
scans into 32 k data points over a spectral width of 16 ppm. H and
1
13C{1H} chemical shifts were internally referenced to TMS via the
the evolved gas was confirmed by recording a H NMR spectrum of
1
residual H and 13C signals of CDCl3 (δ = 7.26 ppm and δ = 77.16
the toluene-d8 solution (20:80 ratio). In the same way, the presence of
D2 was confirmed in the toluene-H8 solution.
ppm), whereas for the 1H NMR spectra recorded in D2O, 1,4-dioxane
(3.75 ppm) was used as internal reference according to the values
reported by Fulmer et al.100 Chemical shift values (δ) are reported in
ppm and coupling constants (J) in Hertz. 2D NMR spectra such as
1H−1H gCOSY, 1H−1H NOESY, 1H−13C gHSQC, and 1H−13C
Procedure to Detect the Gases in the Catalytic Dehydro-
genation of HCO2H with 3. This was carried out in a similar way to
that performed with 4. In this case, 4.4 mg (8.99 × 10−3 mmol) of 3
were used. The presence of H2 and HD in the evolved gas was
1
confirmed by recording a H NMR spectrum of this gas dissolved in
gHMBC were recorded using standard pulse sequences. The probe
temperature ( 1 K) was controlled by a standard unit calibrated with
methanol as a reference. All NMR data processing was carried out
using MestReNova v 10.0.2. For the molar conductimetry measure-
ments, the ΛM values are given in S·cm2·mol−1 and were obtained at
room temperature for 10−3 M solutions of the corresponding
complexes in CH3CN, using a CRISON 522 conductimeter equipped
with a CRISON 5292 platinum conductivity cell.
X-ray Crystallography. A summary of crystal data collection and
refinement parameters for 1BF4 (CCDC number 1858461) are given
in Table S1. A single crystal of 1BF4 was mounted on a glass fiber and
transferred to a Bruker X8 APEX II CCD diffractometer equipped
with a graphite monochromated Mo Kα radiation source (λ =
0.71073 Å). Data were integrated using SAINT101 and an absorption
correction was performed with the program SADABS.102 The
software package WINGX103,104 was used for space group
determination, structure solution and refinement by full-matrix
least-squares methods based on F2. All non-hydrogen atoms were
refined with anisotropic thermal parameters and hydrogen atoms were
placed using a “riding model” and included in the refinement at
calculated positions.
toluene-d8. The proportion observed was 74% HD and 26% H2.
However, the presence of HD and D2 was not observed in toluene-H8.
Procedure for the Catalytic Hydrogenation of CO2. In a dry
flask, solutions of KOH (0.1 M) and the corresponding precatalysts
(50 μM) in 5 mL of H2O were placed in a stainless steel autoclave.
The autoclave was purged with CO2 for 5 min. After 10 min of
stabilization at the desired temperature, 10 bar of H2/CO2 (1/1) gas
were introduced and the reaction was started with stirring (1500
rpm). The stirring was stopped after 3 or 8 h and the pressurized gas
was released. An aliquot of 0.5 mL was taken from the resulting
solutions, to which dioxane was added as a reference (0.0125 mol).
The formate concentration was determined by 1H NMR spectroscopy
(after a previous calibration; see SI).
Procedures for the Characterization of Catalytic Intermedi-
ates by NMR. 1. Reactivity of Precatalysts with HCO2Na. In a dry
NMR tube previously purged with nitrogen, a solution of the
corresponding precatalyst (7.5 × 10−3 mmol) in D2O (0.5 mL) was
introduced. A 1H NMR spectrum was recorded and then an excess of
HCO2Na was added (4.4 mg, 6.5 × 10−2 mmol). The evolution of the
1
reaction with time was monitored by H NMR spectroscopy.
In the case of 4, one spectrum was registered each 10 min. Dioxane
Synthesis and Characterization of the New Complexes. See
Supporting Information.
Procedure for the Catalytic Transfer Hydrogenation Experi-
ments. Substrate (2.5 mmol) and catalyst (2.5 × 10−3 mmol) were
placed in a carousel reaction tube. An aqueous solution of HCO2H/
2
(1 μL, 0.011 mmol) was added as internal reference. The D NMR
spectrum was recorded after 48 h.
2. Reactivity of Precatalysts with DCl. In a dry NMR tube
previously purged with nitrogen, a solution of the corresponding
I
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
precatalyst (7.5 × 10−3 mmol) in D2O (0.5 mL) was introduced. A 1H
NMR spectrum was recorded and then a solution of DCl was added
(1 M in D2O, 50 μL). The evolution of the reaction with time was
Alternative to Photo- and Electrochemical CO2 Reduction. Chem. Rev.
2015, 115 (23), 12936−12973.
(6) Izumi, Y. Recent Advances in the Photocatalytic Conversion of
Carbon Dioxide to Fuels with Water and/or Hydrogen Using Solar
Energy and Beyond. Coord. Chem. Rev. 2013, 257 (1), 171−186.
(7) Qiao, J.; Liu, Y.; Hong, F.; Zhang, J. A Review of Catalysts for
the Electroreduction of Carbon Dioxide to Produce Low-Carbon
Fuels. Chem. Soc. Rev. 2014, 43, 631.
1
monitored by H NMR spectroscopy.
3. Reactivity of the Precatalysts with Na2CO3. In a dry NMR tube
previously purged with nitrogen, a solution of 3, 4, or I (7.5 × 10−3
1
mmol) in D2O (0.5 mL) was introduced. A H NMR spectrum was
recorded and then Na2CO3 (1 mg, 9.5 × 10−3 mmol) was added. The
evolution of the reaction with time was monitored by 1H NMR
spectoscopy.
(8) Zhang, W.; Hu, Y.; Ma, L.; Zhu, G.; Wang, Y.; Xue, X.; Chen, R.;
Yang, S.; Jin, Z. Progress and Perspective of Electrocatalytic CO2
Reduction for Renewable Carbonaceous Fuels and Chemicals. Adv.
Sci. 2018, 5 (1), 1700275.
(9) Centi, G.; Perathoner, S. Opportunities and Prospects in the
Chemical Recycling of Carbon Dioxide to Fuels. Catal. Today 2009,
148 (3−4), 191−205.
(10) Rand, D. A. J.; Dell, R. M. Hydrogen Energy. Challenges and
Prospects; RSC Publishing: Cambridge, 2008.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02164.
Molecular and crystalline structure; catalytic experi-
ments and mechanistic studies (DOCX)
(11) Liu, C.; Li, F.; Ma, L.-P.; Cheng, H.-M. Advanced Materials for
Energy Storage. Adv. Mater. 2010, 22 (8), E28−E62.
(12) Schlapbach, L.; Zuttel, A. Hydrogen-Storage Materials for
̈
Accession Codes
Mobile Applications. Nature 2001, 414 (6861), 353−358.
(13) Johnson, T. C.; Morris, D. J.; Wills, M. Hydrogen Generation
from Formic Acid and Alcohols Using Homogeneous Catalysts.
Chem. Soc. Rev. 2010, 39 (1), 81−88.
(14) Sordakis, K.; Tang, C.; Vogt, L. K.; Junge, H.; Dyson, P. J.;
Beller, M.; Laurenczy, G. Homogeneous Catalysis for Sustainable
Hydrogen Storage in Formic Acid and Alcohols. Chem. Rev. 2018, 118
(2), 372−433.
CCDC 1858461 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(15) Onishi, N.; Xu, S.; Manaka, Y.; Suna, Y.; Wang, W. H.;
Muckerman, J. T.; Fujita, E.; Himeda, Y. CO2 Hydrogenation
Catalyzed by Iridium Complexes with a Proton-Responsive Ligand.
Inorg. Chem. 2015, 54 (11), 5114−5123.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: gespino@ubu.es (G.E.).
*E-mail: Blanca.Manzano@uclm.es (B.R.M.).
ORCID
■
(16) Iglesias, M.; Oro, L. A. Mechanistic Considerations on
Homogeneously Catalyzed Formic Acid Dehydrogenation. Eur. J.
Inorg. Chem. 2018, 2018, 2125.
Gabriel García-Herbosa: 0000-0002-2863-1272
(17) Mellone, I.; Gorgas, N.; Bertini, F.; Peruzzini, M.; Kirchner, K.;
Gonsalvi, L. Selective Formic Acid Dehydrogenation Catalyzed by Fe-
PNP Pincer Complexes Based on the 2,6-Diaminopyridine Scaffold.
Organometallics 2016, 35 (19), 3344−3349.
(18) Bielinski, E. A.; Lagaditis, P. O.; Zhang, Y.; Mercado, B. Q.;
Wurtele, C.; Bernskoetter, W. H.; Hazari, N.; Schneider, S. Lewis
̈
Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-
Supported Iron Catalyst. J. Am. Chem. Soc. 2014, 136 (29), 10234−
10237.
́
́
Felix A. Jalon: 0000-0002-6622-044X
Blanca R. Manzano: 0000-0002-4908-4503
Gustavo Espino: 0000-0001-5617-5705
Author Contributions
#These authors have contributed equally to the experimental
part of this paper.
Notes
̈
(19) Boddien, A.; Loges, B.; Gartner, F.; Torborg, C.; Fumino, K.;
The authors declare no competing financial interest.
Junge, H.; Ludwig, R.; Beller, M. Iron-Catalyzed Hydrogen
Production from Formic Acid. J. Am. Chem. Soc. 2010, 132 (26),
8924−8934.
ACKNOWLEDGMENTS
We gratefully acknowledge the financial support provided by
the Spanish Ministerio de Economia y Competitividad -
FEDER (CTQ2014-58812-C2-1-R). M. R.-C. is grateful for
the grant from University of Castilla-La Mancha (programa
propio).
■
(20) Celaje, J. J. A.; Lu, Z.; Kedzie, E. A.; Terrile, N. J.; Lo, J. N.;
Williams, T. J. A Prolific Catalyst for Dehydrogenation of Neat
Formic Acid. Nat. Commun. 2016, 7, 11308.
́
(21) Wang, W. H.; Hull, J. F.; Muckerman, J. T.; Fujita, E.; Hirose,
T.; Himeda, Y. Highly Efficient D2 generation by Dehydrogenation of
Formic Acid in D2O through H+/D+ exchange on an Iridium Catalyst:
Application to the Synthesis of Deuterated Compounds by Transfer
Deuterogenation. Chem. - Eur. J. 2012, 18 (30), 9397−9404.
REFERENCES
■
́
(1) Chaloner, P. A.; Esteruelas, M. A.; Joo, F.; Oro, L. A.
Homogeneous Hydrogenation; Kluwer Academic Publishers: Dordrecht,
1994.
(2) Recent Advances in Hydride Chemistry, Peruzzini, M.; Poli, R.,
Eds.; Elsevier: Amsterdam, 2001.
́
́
(22) Iturmendi, A.; Rubio-Perez, L.; Perez-Torrente, J. J.; Iglesias,
M.; Oro, L. A. Impact of Protic Ligands in the Ir-Catalyzed
Dehydrogenation of Formic Acid in Water. Organometallics 2018,
37, 3611.
(23) Wang, L.; Sun, H.; Zuo, Z.; Li, X.; Xu, W.; Langer, R.; Fuhr, O.;
Fenske, D. Activation of CO2, CS2, and Dehydrogenation of Formic
Acid Catalyzed by Iron(II) Hydride Complexes. Eur. J. Inorg. Chem.
2016, 2016 (33), 5205−5214.
(24) Xiao, P.; Wu, D.; Fang, W.-H.; Cui, G. Mechanistic Insights
into the Light-Driven Hydrogen Evolution Reaction from Formic
Acid Mediated by an Iridium Photocatalyst. Catal. Sci. Technol. 2017,
7 (13), 2763−2771.
(3) Torrente-Murciano, L.; Mattia, D.; Jones, M. D.; Plucinski, P. K.
Formation of Hydrocarbons via CO2 Hydrogenation − A
Thermodynamic Study. J. CO2 Util. 2014, 6, 34−39.
(4) Jiang, H. L.; Singh, S. K.; Yan, J. M.; Zhang, X. B.; Xu, Q. Liquid-
Phase Chemical Hydrogen Storage: Catalytic Hydrogen Generation
under Ambient Conditions. ChemSusChem 2010, 3 (5), 541−549.
(5) Wang, W. H.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.;
Fujita, E. CO2 Hydrogenation to Formate and Methanol as an
J
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
̈
(25) Boddien, A.; Mellmann, D.; Gartner, F.; Jackstell, R.; Junge, H.;
Dyson, P. J.; Laurenczy, G.; Ludwig, R.; Beller, M. Efficient
Dehydrogenation of Formic. Science (Washington, DC, U. S.) 2011,
333, 1733−1737.
(42) Himeda, Y.; Miyazawa, S.; Hirose, T. Interconversion between
Formic Acid and H2/CO2 Using Rhodium and Ruthenium Catalysts
for CO2 Fixation and H2 Storage. ChemSusChem 2011, 4 (4), 487−
493.
(26) Iguchi, M.; Himeda, Y.; Manaka, Y.; Kawanami, H. Develop-
ment of an Iridium-Based Catalyst for High-Pressure Evolution of
Hydrogen from Formic Acid. ChemSusChem 2016, 9 (19), 2749−
2753.
(43) Filonenko, G. A.; Van Putten, R.; Schulpen, E. N.; Hensen, E. J.
M.; Pidko, E. A. Highly Efficient Reversible Hydrogenation of Carbon
Dioxide to Formates Using a Ruthenium PNP-Pincer Catalyst.
ChemCatChem 2014, 6 (6), 1526−1530.
(44) Maenaka, Y.; Suenobu, T.; Fukuzumi, S. Catalytic Inter-
conversion between Hydrogen and Formic Acid at Ambient
Temperature and Pressure. Energy Environ. Sci. 2012, 5 (6), 7360−
7367.
̈
(27) Bertini, F.; Glatz, M.; Gorgas, N.; Stoger, B.; Peruzzini, M.;
Veiros, L. F.; Kirchner, K.; Gonsalvi, L. Carbon Dioxide Hydro-
genation Catalysed by Well-Defined Mn(i) PNP Pincer Hydride
Complexes. Chem. Sci. 2017, 8 (7), 5024−5029.
(28) Tanaka, R.; Yamashita, M.; Nozaki, K. Catalytic Hydrogenation
of Carbon Dioxide Using Ir (III) -Pincer Complexes and Its
Mechanistic Investigation. J. Am. Chem. Soc. 2009, 131, 14168−
14169.
(45) Wei, Y.; Wu, X.; Wang, C.; Xiao, J. Transfer Hydrogenation in
Aqueous Media. Catal. Today 2015, 247, 104−116.
(46) He, Y.-M.; Fan, Q.-H. Advances in Transfer Hydrogenation of
Carbonyl Compounds in Water. ChemCatChem 2015, 7 (3), 398−
400.
̈
(29) Bertini, F.; Gorgas, N.; Stoger, B.; Peruzzini, M.; Veiros, L. F.;
Kirchner, K.; Gonsalvi, L. Efficient and Mild Carbon Dioxide
Hydrogenation to Formate Catalyzed by Fe(II) Hydrido Carbonyl
Complexes Bearing 2,6-(Diaminopyridyl)Diphosphine Pincer Li-
gands. ACS Catal. 2016, 6 (5), 2889−2893.
(47) Wu, X.; Wang, C.; Xiao, J. Asymmetric Transfer Hydrogenation
in Water with Platinum Group Metal Catalysts. Platinum Met. Rev.
2010, 54 (1), 3−19.
(48) Wang, C.; Wu, X.; Xiao, J. Broader, Greener, and More
Efficient: Recent Advances in Asymmetric Transfer Hydrogenation.
Chem. - Asian J. 2008, 3 (10), 1750−1770.
(30) Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J. W.; Ben-
David, Y.; Milstein, D. Low-Pressure Hydrogenation of Carbon
Dioxide Catalyzed by an Iron Pincer Complex Exhibiting Noble Metal
Activity. Angew. Chem., Int. Ed. 2011, 50 (42), 9948−9952.
(49) Wu, X.; Xiao, J. Aqueous-Phase Asymmetric Transfer
Hydrogenation of Ketones ? A Greener Approach to Chiral Alcohols.
Chem. Commun. 2007, 2449−2466.
(31) Gluer, A.; Schneider, S. Iron Catalyzed Hydrogenation and
̈
́
́
Electrochemical Reduction of CO2: The Role of Functional Ligands.
J. Organomet. Chem. 2018, 861, 159−173.
(50) Fleury-Bregeot, N.; de la Fuente, V.; Castillon, S.; Claver, C.
Highlights of Transition Metal-Catalyzed Asymmetric Hydrogenation
of Imines. ChemCatChem 2010, 2 (11), 1346−1371.
(32) Wang, W.; Wang, S.; Ma, X.; Gong, J. Recent Advances in
Catalytic Hydrogenation of Carbon Dioxide. Chem. Soc. Rev. 2011, 40
(7), 3703−3727.
(51) Zhou, Y.-G. Asymmetric Hydrogenation of Heteroaromatic
Compounds. Acc. Chem. Res. 2007, 40 (12), 1357−1366.
(52) Wang, W. B.; Lu, S. M.; Yang, P. Y.; Han, X. W.; Zhou, Y. G.
Highly Enantioselective Iridium-Catalyzed Hydrogenation of Hetero-
aromatic Compounds, Quinolines. J. Am. Chem. Soc. 2003, 125 (35),
10536−10537.
(33) Sanz, S.; Benítez, M.; Peris, E. A New Approach to the
Reduction of Carbon Dioxide: CO2 Reduction to Formate by
Transfer Hydrogenation in IPrOH. Organometallics 2010, 29 (1),
275−277.
(34) Himeda, Y.; Onozawa-Komatsuzaki, N.; Sugihara, H.; Arakawa,
H.; Kasuga, K. Half-Sandwich Complexes with 4,7-Dihydroxy-1,10-
Phenanthroline: Water-Soluble, Highly Efficient Catalysts for Hydro-
genation of Bicarbonate Attributable to the Generation of an
Oxyanion on the Catalyst Ligand. Organometallics 2004, 23 (7),
1480−1483.
(35) Hull, J. F.; Himeda, Y.; Wang, W.-H.; Hashiguchi, B.; Periana,
R.; Szalda, D. J.; Muckerman, J. T.; Fujita, E. Reversible Hydrogen
Storage Using CO2 and a Proton-Switchable Iridium Catalyst in
Aqueous Media under Mild Temperatures and Pressures. Nat. Chem.
2012, 4 (5), 383−388.
(36) Gorgas, N.; Kirchner, K. Isoelectronic Manganese and Iron
Hydrogenation/Dehydrogenation Catalysts: Similarities and Diver-
gences. Acc. Chem. Res. 2018, 51 (6), 1558−1569.
(37) Bernskoetter, W. H.; Hazari, N. Reversible Hydrogenation of
Carbon Dioxide to Formic Acid and Methanol: Lewis Acid
Enhancement of Base Metal Catalysts. Acc. Chem. Res. 2017, 50
(4), 1049−1058.
(38) Tanaka, R.; Yamashita, M.; Chung, L. W.; Morokuma, K.;
Nozaki, K. Mechanistic Studies on the Reversible Hydrogenation of
Carbon Dioxide Catalyzed by an Ir-PNP Complex. Organometallics
2011, 30 (24), 6742−6750.
(53) Wang, D. S.; Chen, Q. A.; Lu, S. M.; Zhou, Y. G. Asymmetric
Hydrogenation of Heteroarenes and Arenes. Chem. Rev. 2012, 112
(4), 2557−2590.
(54) Janiak, C. A Critical Account on Π−π Stacking in Metal
Complexes with Aromatic Nitrogen-Containing Ligands. J. Chem. Soc.,
Dalton Trans. 2000, 3885−3896.
(55) Katritzky, A. R.; Rachwal, S.; Rachwal, B. Recent Progress in
the Synthesis of 1,2,2,4-Tetrahydroquinolines. Tetrahedron 1996, 52
(48), 15031−15070.
(56) Tan, J.; Tang, W.; Sun, Y.; Jiang, Z.; Chen, F.; Xu, L.; Fan, Q.;
Xiao, J. PH-Regulated Transfer Hydrogenation of Quinoxalines with a
Cp*Ir-Diamine Catalyst in Aqueous Media. Tetrahedron 2011, 67
(34), 6206−6213.
(57) Zhang, L.; Qiu, R.; Xue, X.; Pan, Y.; Xu, C.; Li, H.; Xu, L.
Versatile (Pentamethylcyclopentadienyl)Rhodium-2,2′-Bipyridine
(Cp*Rh-Bpy) Catalyst for Transfer Hydrogenation of N-Heterocycles
in Water. Adv. Synth. Catal. 2015, 357 (16−17), 3529−3537.
(58) Wang, C.; Li, C.; Wu, X.; Pettman, A.; Xiao, J. PH-Regulated
Asymmetric Transfer Hydrogenation of Quinolines in Water. Angew.
Chem., Int. Ed. 2009, 48 (35), 6524−6528.
(59) Talwar, D.; Li, H. Y.; Durham, E.; Xiao, J. A Simple Iridicycle
Catalyst for Efficient Transfer Hydrogenation of N-Heterocycles in
Water. Chem. - Eur. J. 2015, 21 (14), 5370−5379.
(60) Dobereiner, G. E.; Nova, A.; Schley, N. D.; Hazari, N.; Miller,
S. J.; Eisenstein, O.; Crabtree, R. H. Iridium-Catalyzed Hydrogenation
of N-Heterocyclic Compounds under Mild Conditions by an Outer-
Sphere Pathway. J. Am. Chem. Soc. 2011, 133 (19), 7547−7562.
(61) Yang, P. Y.; Zhou, Y. G. The Enantioselective Total Synthesis
of Alkaloid (−)-Galipeine. Tetrahedron: Asymmetry 2004, 15 (7),
1145−1149.
(62) Lu, S. M.; Han, X. W.; Zhou, Y. G. Asymmetric Hydrogenation
of Quinolines Catalyzed by Iridium with Chiral Ferrocenyloxazoline
Derived N,P Ligands. Adv. Synth. Catal. 2004, 346 (8), 909−912.
(39) Bertini, F.; Mellone, I.; Ienco, A.; Peruzzini, M.; Gonsalvi, L.
Iron(II) Complexes of the Linear Rac-Tetraphos-1 Ligand as Efficient
Homogeneous Catalysts for Sodium Bicarbonate Hydrogenation and
Formic Acid Dehydrogenation. ACS Catal. 2015, 5 (2), 1254−1265.
(40) Gao, Y.; Kuncheria, J. K.; Jenkins, H. A.; Puddephatt, R. J.; Yap,
G. The Interconversion of Formic Acid and Hydrogen/Carbon
Dioxide Using a Binuclear Ruthenium Complex Catalyst. J. Chem.
Soc., Dalton Trans. 2000, 2 (18), 3212−3217.
(41) Geri, J. B.; Ciatti, J. L.; Szymczak, N. K. Charge Effects Regulate
Reversible CO2 Reduction Catalysis. Chem. Commun. 2018, 54 (56),
7790−7793.
K
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(63) Reetz, M. T.; Li, X. Asymmetric Hydrogenation of Quinolines
Catalyzed by Iridium Complexes of BINOL-Derived Diphosphonites.
Chem. Commun. 2006, 52 (20), 2159−2160.
Catalyzed N-Formylation of Amines. Angew. Chem., Int. Ed. 2015,
54 (41), 12116−12120.
(80) Zhao, T.-X.; Zhai, G.-W.; Liang, J.; Li, P.; Hu, X.-B.; Wu, Y.-T.
Catalyst-Free N-Formylation of Amines Using BH NH and CO
2
(64) Lu, S. M.; Wang, Y. Q.; Han, X. W.; Zhou, Y. G. Asymmetric
Hydrogenation of Quinolines and Isoquinolines Activated by
Chloroformates. Angew. Chem., Int. Ed. 2006, 45 (14), 2260−2263.
(65) Zhou, H.; Li, Z.; Wang, Z.; Wang, T.; Xu, L.; He, Y.; Fan, Q.
H.; Pan, J.; Gu, L.; Chan, A. S. C. Hydrogenation of Quinolines Using
a Recyclable Phosphine-Free Chiral Cationic Ruthenium Catalyst:
Enhancement of Catalyst Stability and Selectivity in an Ionic Liquid.
Angew. Chem., Int. Ed. 2008, 47 (44), 8464−8467.
3
3
under Mild Conditions. Chem. Commun. 2017, 53 (57), 8046−8049.
(81) Shah, N.; Gravel, E.; Jawale, D. V.; Doris, E.; Namboothiri, I. N.
N. Carbon Nanotube-Gold Nanohybrid Catalyzed N-Formylation of
Amines by Using Aqueous Formaldehyde. ChemCatChem 2014, 6
(8), 2201−2205.
(82) Hulla, M.; Bobbink, F. D.; Das, S.; Dyson, P. J. Carbon Dioxide
Based N-Formylation of Amines Catalyzed by Fluoride and
Hydroxide Anions. ChemCatChem 2016, 8 (21), 3338−3342.
(83) Zheng, D.; Zhou, X.; Cui, B.; Han, W.; Wan, N.; Chen, Y.
Biocatalytic α-Oxidation of Cyclic Amines and N-Methylanilines for
the Synthesis of Lactams and Formamides. ChemCatChem 2017, 9
(6), 937−940.
(84) Rahman, S.; Fukamiya, N.; Okano, M.; Tagahara, K.; Lee, K.-H.
NII-Electronic Library Service. Chem. Pharm. Bull. 1997, 45 (9),
1527−1529.
(66) Li, Z. W.; Wang, T. L.; He, Y. M.; Wang, Z. J.; Fan, Q. H.; Pan,
J.; Xu, L. J. Air-Stable and Phosphine-Free Iridium Catalysts for
Highly Enantioselective Hydrogenation of Quinoline Derivatives. Org.
Lett. 2008, 10 (22), 5265−5268.
(67) Xu, L.; Lam, K. H.; Ji, J.; Wu, J.; Fan, Q.-H.; Lo, W.-H.; Chan,
A. S. C. Air-Stable Ir-(P-Phos) Complex for Highly Enantioselective
Hydrogenation of Quinolines and Their Immobilization in Poly-
(Ethylene Glycol) Dimethyl Ether (DMPEG). Chem. Commun. 2005,
1390−1392.
(68) Yamaguchi, R.; Ikeda, C.; Takahashi, Y.; Fujita, K. I.
Homogeneous Catalytic System for Reversible Dehydrogenation-
Hydrogenation Reactions of Nitrogen Heterocycles with Reversible
Interconversion of Catalytic Species. J. Am. Chem. Soc. 2009, 131
(24), 8410−8412.
(69) Crabtree, R. H. Multifunctional Ligands in Transition Metal
Catalysis. New J. Chem. 2011, 35 (1), 18−23.
(70) Rakowski DuBois, M.; DuBois, D. L. The Roles of the First and
Second Coordination Spheres in the Design of Molecular Catalysts
for H 2 Production and Oxidation. Chem. Soc. Rev. 2009, 38 (1), 62−
72.
(71) Schmeier, T. J.; Dobereiner, G. E.; Crabtree, R. H.; Hazari, N.
Secondary Coordination Sphere Interactions Facilitate the Insertion.
J. Am. Chem. Soc. 2011, 133 (24), 9274−9277.
(72) Barnard, J. H.; Wang, C.; Berry, N. G.; Xiao, J. Long-Range
Metal−ligand Bifunctional Catalysis: Cyclometallated Iridium Cata-
lysts for the Mild and Rapid Dehydrogenation of Formic Acid. Chem.
Sci. 2013, 4 (3), 1234.
(85) Chen, F.; Sahoo, B.; Kreyenschulte, C.; Lund, H.; Zeng, M.;
He, L.; Junge, K.; Beller, M. Selective Cobalt Nanoparticles for
Catalytic Transfer Hydrogenation of N-Heteroarenes. Chem. Sci.
2017, 8 (9), 6239−6246.
(86) Tao, L.; Zhang, Q.; Li, S. S.; Liu, X.; Liu, Y. M.; Cao, Y.
Heterogeneous Gold-Catalyzed Selective Reductive Transformation
of Quinolines with Formic Acid. Adv. Synth. Catal. 2015, 357 (4),
753−760.
́
(87) Vilhanova, B.; van Bokhoven, J. A.; Ranocchiari, M. Gold
Particles Supported on Amino-Functionalized Silica Catalyze Transfer
Hydrogenation of N-Heterocyclic Compounds. Adv. Synth. Catal.
2017, 359 (4), 677−686.
(88) Zhang, J. F.; Zhong, R.; Zhou, Q.; Hong, X.; Huang, S.; Cui, H.
Z.; Hou, X. F. Recyclable Silica-Supported Iridium Catalysts for
Selective Reductive Transformation of Quinolines with Formic Acid
in Water. ChemCatChem 2017, 9 (13), 2496−2505.
(89) Saari, W. S.; Halczenko, W.; Freedman, M. B.; Arison, B. H.
Synthesis and Reactions of Some Dihydro and Tetrahydro-4H-
Imidazo[5,4,l-Ij]Quinoline Derivatives. J. Heterocycl. Chem. 1982, 19,
837−840.
(73) Turkmen, H.; Kani, I.; Çetinkaya, B. Transfer Hydrogenation of
̈
(90) Shugrue, C. R.; Miller, S. J. Phosphothreonine as a Catalytic
Residue in Peptide-Mediated Asymmetric Transfer Hydrogenations of
8-Aminoquinolines. Angew. Chem., Int. Ed. 2015, 54 (38), 11173−
11176.
(91) Yamaguchi, R.; Ikeda, C.; Takahashi, Y.; Fujita, K.-I. J. Am.
Chem. Soc. 2009, 131, 8410−8412.
(92) Wu, J.; Talwar, D.; Johnston, S.; Yan, M.; Xiao, J. Acceptorless
Dehydrogenation of Nitrogen Heterocycles with a Versatile Iridium
Catalyst. Angew. Chem., Int. Ed. 2013, 52 (27), 6983−6987.
Aryl Ketones with Half-Sandwich Ru II Complexes That Contain
Chelating Diamines. Eur. J. Inorg. Chem. 2012, 2012, 4494−4499.
(74) Gupta, K.; Tyagi, D.; Dwivedi, A. D.; Mobin, S. M.; Singh, S. K.
Catalytic Transformation of Bio-Derived Furans to Valuable
Ketoacids and Diketones by Water-Soluble Ruthenium Catalysts.
Green Chem. 2015, 17 (9), 4618−4627.
́
́
(75) Carrion, M. C.; Jalon, F. A.; Manzano, B. R.; Rodríguez, A. M.;
́
Sepulveda, F.; Maestro, M. (Arene)Ruthenium(II) Complexes
Containing Substituted Bis(Pyrazolyl)Methane Ligands - Catalytic
Behaviour in Transfer Hydrogenation of Ketones. Eur. J. Inorg. Chem.
2007, 2007 (25), 3961−3973.
́
(93) Carrion, M. C.; Ruiz-Castaneda, M.; Espino, G.; Aliende, C.;
̃
́
́
Santos, L.; Rodríguez, A. M.; Manzano, B. R.; Jalon, F. A.; Lledos, A.
Selective Catalytic Deuterium Labeling of Alcohols during a Transfer
Hydrogenation Process of Ketones Using D 2 O as the Only
Deuterium Source. Theoretical and Experimental Demonstration of a
Ru−H/D + Exchange as the Key Step. ACS Catal. 2014, 4 (4), 1040−
1053.
́
́
́
(76) Carrion, M. C.; Sepulveda, F.; Jalon, F. A.; Manzano, B. R.;
Rodríguez, A. M. Base-Free Transfer Hydrogenation of Ketones Using
Arene Ruthenium(II) Complexes. Organometallics 2009, 28 (13),
3822−3833.
́
(77) Espino, G.; Caballero, A.; Manzano, B. R.; Santos, L.; Perez-
Manrique, M.; Moreno, M.; Jalon, F. A. Experimental and
(94) Miyake, H.; Kano, N.; Kawashima, T. Isolation of a Metastable
Geometrical Isomer of a Hexacoordinated Dihydrophosphate:
Elucidation of Its Enhanced Reactivity in Umpolung of a Hydrogen
Atom of Water. Inorg. Chem. 2011, 50 (18), 9083−9089.
(95) Wang, W. H.; Hull, J. F.; Muckerman, J. T.; Fujita, E.; Hirose,
T.; Himeda, Y. Highly Efficient D2 Generation by Dehydrogenation
of Formic Acid in D2O through H+/D+ Exchange on an Iridium
Catalyst: Application to the Synthesis of Deuterated Compounds by
Transfer Deuterogenation. Chem. - Eur. J. 2012, 18 (30), 9397−9404.
(96) Ohkuma, T.; Utsumi, N.; Tsutsumi, K.; Murata, K.; Sandoval,
C.; Noyori, R. The Hydrogenation/Transfer Hydrogenation Net-
work : Asymmetric Hydrogenation of Ketones with Chiral η 6
-Arene/N-Tosylethylenediamine-Ruthenium (II) Catalysts. J. Am.
Chem. Soc. 2006, 128 (27), 8724−8725.
́
Computational Evidence for the Participation of Nonclassical
Dihydrogen Species in Proton Transfer Processes on Ru−Arene
Complexes with Uncoordinated N Centers. Efficient Catalytic
Deuterium Labeling of H 2 with CD 3 OD. Organometallics 2012,
31 (8), 3087−3100.
(78) Martínez, M.; Carranza, M. P.; Massaguer, A.; Santos, L.;
Organero, J. A.; Aliende, C.; De Llorens, R.; Ng-Choi, I.; Feliu, L.;
Planas, M.; et al. Synthesis and Biological Evaluation of Ru(II) and
Pt(II) Complexes Bearing Carboxyl Groups as Potential Anticancer
Targeted Drugs. Inorg. Chem. 2017, 56 (22), 13679−13696.
(79) Chong, C. C.; Kinjo, R. Hydrophosphination of CO2 and
Subsequent Formate Transfer in the 1,3,2-Diazaphospholene-
L
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(97) Bennett, M. A.; Smith, A. K. Arene Ruthenium(II) Complexes
Formed by Dehydrogenation of Cyclohexadienes with Ruthenium-
(III) Trichloride. J. Chem. Soc., Dalton Trans. 1974, 233.
(98) Kang, J. W.; Moseley, K.; Maitlis, P. M. J. Am. Chem. Soc. 1969,
91 (13), 5970−5977.
(99) Yates, A.; Maitlis, P. M. Inorg. Synthesis 1992, 29, 228−234.
(100) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H.
E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. NMR
Chemical Shifts of Trace Impurities: Common Laboratory Solvents,
Organics, and Gases in Deuterated Solvents Relevant to the
Organometallic Chemist. Organometallics 2010, 29 (9), 2176−2179.
(101) SAINT v 8.37 and APEX3 v 2016.1.0. Bruker-AXS: Madison,
WI, USA; 2016.
(102) Krause, L.; Herbst-Irmer, R.; Sheldrick, G. M.; Stalke, D.
Comparison of Silver and Molybdenum Microfocus X-Ray Sources
for Single-Crystal Structure Determination. J. Appl. Crystallogr. 2015,
48 (1), 3−10.
(103) Farrugia, L. J. WinGX and ORTEP for Windows: An Update.
J. Appl. Crystallogr. 2012, 45 (4), 849−854.
(104) Sheldrick, G. M. SHELX-2014: Progressive Crystal Structure
̈
̈
Refinement; University of Gottingen: Gottingen, Germany., 2014.
M
DOI: 10.1021/acs.inorgchem.8b02164
Inorg. Chem. XXXX, XXX, XXX−XXX