Dehydrogenation of Formic Acid Promoted by a Trihydride-Hydroxo-Osmium(IV) Complex: Kinetics and Mechanism

Article abstract of DOI:10.1021/acscatal.8b02370

The preparation of the hydroxo-osmium(IV) complex OsH₃(OH){xant(PⁱPr₂)₂} (xant(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) and its catalytic efficiency for the dehydrogenation of formic acid to H₂ and CO₂ are reported. The mechanism of the dehydrogenation has been unambiguously stablished by combining the kinetic analysis of the catalysis, the isolation of the intermediates and the kinetic analysis of their decomposition, and density functional theory (DFT) calculations on the rate-determining step. Under catalytic conditions, the trihydride-hydroxo complex reacts with formic acid to afford OsH₃{κ¹-O-(HCO₂)}{xant(PⁱPr₂)₂}, which isomerizes into OsH₃{κ¹-H-(HCO₂)}{xant(PⁱPr₂)₂} by means of the slippage of the metal center through a formate O-C-H path. The κ¹-H-formate intermediate releases CO₂ to give the previously reported tetrahydride OsH₄{xant(PⁱPr₂)₂}, which undergoes protonation with formic acid. The resulting OsH₅ cation exists as an equilibrium mixture of the tautomers trihydride-compressed dihydride [OsH₃(H···H){xant(PⁱPr₂)₂}]⁺ and hydride-compressed dihydride-dihydrogen [OsH(H···H)(η²-H₂){xant(PⁱPr₂)₂}]⁺. The dissociation of H₂ from the latter leads to [OsH₃{xant(PⁱPr₂)₂}]⁺, which coordinates HCO₂^- to regenerate the trihydride-(κ¹-O-formate) complex and close the cycle. The release of CO₂ from the κ¹-H-formate intermediate is the rate-determining step of the catalysis.

Full text of DOI:10.1021/acscatal.8b02370

Subscriber access provided by University of South Dakota
Article
Dehydrogenation of Formic Acid Promoted by a Trihydride-
Hydroxo-Osmium (IV) Complex: Kinetics and Mechanism
Miguel A. Esteruelas, Cristina Garcia-Yebra, Jaime Martín, and Enrique Onate
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02370 • Publication Date (Web): 22 Oct 2018
Downloaded from http://pubs.acs.org on October 23, 2018
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
Dehydrogenation of Formic Acid Promoted by a
Trihydride-Hydroxo-Osmium (IV) Complex:
Kinetics and Mechanism
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Miguel A. Esteruelas,* Cristina García-Yebra, Jaime Martín, and Enrique Oñate
Departamento de Química Inorgánica, Instituto de Síntesis Química y Catálisis Homogénea
(ISQCH), Centro de Innovación en Química Avanzada (ORFEO-CINQA), Universidad de
Zaragoza-CSIC, 50009 Zaragoza, Spain
ABSTRACT
The preparation of the hydroxo-osmium(IV) complex OsH3(OH){xant(PiPr2)2} (xant(PiPr2)2 =
9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) and its catalytic efficiency for the
dehydrogenation of formic acid to H2 and CO2 are reported. The mechanism of the
dehydrogenation has been unambiguously stablished by combining the kinetic analysis of the
catalysis, the isolation of the intermediates and the kinetic analysis of their decomposition, and
DFT calculations on the rate-determining step. Under catalytic conditions the trihydride-hydroxo
complex reacts with formic acid to afford OsH3{κ1-O-(HCO2)}{xant(PiPr2)2}, which isomerizes
into OsH3{κ1-H-(HCO2)}{xant(PiPr2)2} by means of the slippage of the metal center through a
formate O-C-H path. The κ1-H-formate intermediate releases CO2 to give the previously reported
ACS Paragon Plus Environment
1

ACS Catalysis
Page 2 of 36
1
2
3
4
5
6
7
8
9
tetrahydride OsH4{xant(PiPr2)2}, which undergoes protonation with formic acid. The resulting
OsH5-cation exists as an equilibrium mixture of the tautomers trihydride-compressed dihydride
[OsH3(H···H){xant(PiPr2)2}]+ and hydride-compressed dihydride-dihydrogen [OsH(H···H)(η2-
H2){xant(PiPr2)2}]+. The dissociation of H2 from the latter leads to [OsH3{xant(PiPr2)2}]+, which
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
-
coordinates HCO2 to regenerate the trihydride-(κ1-O-formate) complex and to close the cycle.
The release of CO2 from the κ1-H-formate intermediate is the rate-determining step of the
catalysis.
INTRODUCTION
Hydrogen gas is a promising energy carrier as clean alternative to conventional fossil fuels. It
can be produced from any primary energy source, has an energy content per mass that is about
three times that of the carbon-based fuels, can be used by direct introduction into either an
internal combustion engine or to a fuel cell, and its oxidation with O2 gives water.1 However, its
energy content per volume is very low at standard temperature and pressure. To overcome this
difficulty, several methods are being investigated.2
The concept of chemical storage is an attractive strategy; in particular the use of organic
liquids since they can be transported through the liquid-fuel infrastructures. In these compounds,
hydrogen does not exist in its molecular form but is covalently bound. At the time and place of
energy demand, H2 is released via dehydrogenation. The hydrogen carrying liquid itself is not
consumed but is reloaded and used in further cycles.3 Formic acid is a reputed family member of
liquid organic hydrogen carriers. It is stable under ambient conditions, produced in large-scale,
and biodegradable. Furthermore, it has low flammability and toxicity, its gravimetric and
volumetric H2 capacity of 4.4 wt. % and 53.4 g/L are reasonable,4 and its recyclability by means
ACS Paragon Plus Environment
2

Page 3 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
CO2 hydrogenation undergoes a constant improvement,5 reaching complex RuHCl(CO)(PNP)
(PNP = 2,6-bis(di-ter-buthylphosphinomethyl)pyridine) a TOF of 1100000 h-1 for the reduction
to formates in 2014.5a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Both homogeneous and heterogeneous transition metal systems have proven to have the ability
of promoting the formic acid (FA) dehydrogenation to H2 and CO2.6 The majority of the
homogeneous catalysts are complexes of Fe,7 Ru,8 and Ir.9 In addition, a few precursors of Mo,10
Rh,11 Ni,12 Cu,13 and Al14 have been also reported. Further research needs to be focused on the
mechanism of the whole catalytic process. Most of them are only postulated and experimental
support is necessary. The catalysis has two stages, CO2 formation and H2 formation. The rate-
limiting step can be any of them. The main divergences are found in the former, which may
happen by hydride abstraction, β-hydride elimination, or by an outer-sphere mechanism.15
Although the latter is currently questioned.16
The use of Os in homogeneous catalysis has received significantly less attention than the use
of its group congeners Fe and Ru. With the exception of Sharpless dihydroxylation and processes
akin,17 it has been centered in some particular reactions of organic synthesis.18 However, in the
last years, it is being revealed as a promising alternative for processes related to the hydrogen
economy.19 Of special relevance are its polyhydride derivatives,20 which have shown to have the
ability of carrying out the dehydrogenation of amineboranes21 and liquid organic hydrogen
carriers such as alcohols and cyclic amines.22 Other ligand with good performance is the hydroxo
group. For instance, complex [Os(OH)(η6-p-cymene)IPr]OTf (IPr
=
1,3-bis(2,6-
diisopropylphenyl)imidazolylidene; OTf = CF3SO3) has shown great efficiency in the hydrogen
transfer from 2-propanol to aldehydes,23 the α-alkylation of arylacetonitriles and methyl
ketones,24 and the hydration of nitriles to amides.25
ACS Paragon Plus Environment
3

ACS Catalysis
Page 4 of 36
1
2
3
4
5
6
7
8
9
Hydroxo complexes of the platinum group metals are very scarce and their chemistry is
underdeveloped.26 Hydride-hydroxo derivatives are particularly challenging since formally result
from the oxidative addition of water, which is generally disfavored from a thermodynamic point
of view and thus the limited known examples display reversibility.27 Compounds of Os of this
class are Os(II)-species28 and, although the oxidation state +4 is usual for this element when
bears small size ligands, the hydride-hydroxo-Os(IV) complexes are unknown until now.
Pincer ligands show a marked ability to stabilize uncommon complexes due to the disposition
of their donor atoms.29 Ether-diphosphines have particular interest due to their flexibility, which
grant the ability of changing between κ3-mer-, κ3-fac, and κ2 coordination modes. This allows
them to adapt to the requirements of the participating intermediates of the catalytic cycles.30 In
the search for efficient catalysts for processes related to hydrogen economy and also for reactions
of interest in organic synthesis, some years ago we initiated a research program on POP
complexes of groups 819c,30c,30i,31 and 9,30h,32 which has yielded us to discover notable catalysts
for dehydrogenation of ammonia borane,30h monoalcoholysis of disilanes,32b synthesis of imines
from alcohols and amines with liberation of H2,19c hydrogen transfer from 2-propanol to ketones,
α-alkylation of phenylacetonitrile and acetophenone with alcohols,31b regio- and stereoseletive
head-to-head (Z)-dimerization of terminal alkynes,30c,31b borylation of arenes,32c and decyanative
borylation of arylnitriles.32d Now, we have discovered that the ether-diphosphine 9,9-dimethyl-
4,5-bis(diisopropylphosphino)xanthene (xant(PiPr2)2) stabilizes a hydride-hydroxo-Os(IV)
complex, which efficiently promotes the FA dehydrogenation. This paper reports the preparation
and characterization of this novel catalyst precursor, its activity in the dehydrogenation process,
and the kinetics and mechanism of the same one.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
RESULTS AND DISCUSSION
ACS Paragon Plus Environment
4

Page 5 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
Preparation and Characterization of the Catalyst Precursor. The known hydroxo
compounds of osmium include the nitrido complexes Os(N)Tp(OH)2 (Tp = hydridotris(1-
pyrazolyl)borate),33 [Os(N)(CH2SiMe3)(OH)]-, and [Os(N)(CH2SiMe3)(OH)2]-,34 the previously
[Os(OH)(η6-p-cymene)IPr]+,23
the
hydride-hydroxo
derivatives
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
mentioned
cation
OsH(OH)(PMe3)4,28a OsH(OH)(CO)(PR3)2 (PR3 = PiPr3,28b PtBu2Me28c), OsH(OH)(NH2CMe2
CMe2NH2)(PPh3)2,28d and [OsH(OH)(C≡CPh)(IPr)(PiPr3)]OTf,28e and a few dimers.35 They were
prepared by one of these methods: hydrolysis of Os-C28a,28c or Os-N28d bonds and replacement of
chloride by hydroxo.23,28a,28c,34 The first method is not suitable in our case by the lack of an
appropriate starting compound. So, we tried the substitution of chloride by hydroxo employing
the previously reported30c trihydride OsH3Cl{xant(PiPr2)2} (1) as a precursor. However, all
attempts were unsuccessful, reaching a partial substitution in the best of cases. Then, we decided
to replace the chloride ligand of 1 by a better leaving group as trifluoromethanesulfonate
(Scheme 1).
Scheme 1. Preparation of complex 3
Treatment of a toluene solution of 1 with 2.0 equiv of Me3SiOTf, at room temperature, for 10
min affords the synthetic intermediate OsH3(OTf){xant(PiPr2)2} (2), which was isolated as a
19
yellow solid in 90 % yield. The substitution reaction is supported by the F NMR spectrum, in
toluene-d8, which shows a singlet at -77.7 ppm due to the coordinated OTf¯ anion. Like observed
for its chloride precursor, the 1H NMR spectrum of 2 in toluende-d8 shows a resonance at -13.82
ppm for the inequivalent hydride ligands, which indicates the operation of two thermally
ACS Paragon Plus Environment
5

ACS Catalysis
Page 6 of 36
1
2
3
4
5
6
7
8
9
activated site exchange processes within the OsH3 unit. Between 233 and 223 K, decoalescence
occurs and two signals at -12.12 and -17.02 ppm in a 2:1 intensity ratio are observed. Although
at lower temperatures than 223 K the resonance at -12.12 ppm displays broad, the expected
second decoalescence is not reached even at 183 K. According to the trihydride character of the
compound, 400 MHz T1(min) values of 89 ± 4 and 69 ± 3 ms were found at 223 K for the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
31
hydride resonances. The P{1H} NMR spectrum shows a singlet at 54.6 ppm between 298 and
183 K.
Trifluromethanesulfonate is easily displaced by a hydroxo group, in contrast to the chloride
ligand of 1. Dropwise addition of a water KOH solution to an acetone solution of 2 at room
temperature leads to the hydroxo OsH3(OH){xant(PiPr2)2} (3), which was isolated as a yellow
solid in 92 % yield. The presence of the hydroxo ligand in this novel osmium(IV) species is
1
supported by the IR and H NMR spectrum. The IR contains the characteristic ν(OH) band at
1
3672 cm-1, whereas the H NMR spectrum in toluene-d8 shows a broad resonance at 2.87 ppm
corresponding to the OH-hydrogen atom. Like the 1H NMR spectrum of 2, the spectrum contains
a signal (δ1H, -12.25) for the hydrides, indicating that are also involved in two thermally activated
site exchange processes. At about 273 K, a first decoalescence occurs to give two resonances at -
12.46 and -13.08 ppm in a 2:1 intensity ratio. Between 203 and 193 K, the lower field signal
splits into an AB system. The JAB coupling constant decreases from 151.9 to 146.8 Hz as the
temperature does from 193 to 173 K. These unusually high value for two hydrides disposed cis
and their dependence with the temperature can be rationalized in terms of quantum-mechanical
exchange coupling between the involved hydrides.20 The 31P{1H} NMR spectrum shows a
singlet at 49.9 ppm.
ACS Paragon Plus Environment
6

Page 7 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
FA Dehydrogenation Promoted by 3: Kinetic Study. Trihydride-hydroxo complex 3 promotes
the FA dehydrogenation to H2 and CO2.36 It works well with diluted solutions and also
dehydrogenates neat formic acid, but rapidly decomposes under the latter conditions. The
reactions were performed in toluene, under atmospheric pressure, between 298 to 318 K. The
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
partial volume of hydrogen formed (VH ) was determined from the total volume (VT) of gas
2
generated according to eq 1, where Vm(H ) and Vm(CO ) are the partial molar volumes of H2 and
2
2
CO2, respectively. The total volume was measured by displacing vaseline oil from a gas burette.
VH = [VT/(Vm(H ) + Vm(CO ))]Vm(H )
(1)
2
2
2
2
The kinetics of the catalysis was studied to gain insight into the process. Initial rates (Table 1)
were determined from the gas evolution experiments (Figure 1) by using eq 2, where P is the
atmospheric pressure (atm), R is the molar gas constant, T is the temperature (K) and Vsol is the
total volume of the reaction solution.
[
]
d H2 /dt = (dVH /dt)P/푅TVsol
(2)
2
Figure 1. VH Generated from the catalytic dehydrogenation of H2CO2 (0.53 M) promoted by 3
2
in toluene at 298 K. 102[3]: 0.61 M (blue circle), 1.23 (orange square), 1.86 (grey triangle), 2.48
(yellow diamond), 3.06 (green cross).
ACS Paragon Plus Environment
7

ACS Catalysis
Page 8 of 36
1
2
3
4
Table 1. Kinetic data for the dehydrogenation of formic acid promoted by 3a
5
6
7
8
9
T (K)
102[3] (M)
[H2CO2]0 (M)
102d[H2]/dt b
(M·min-1)
k (min-1)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
298
298
298
298
298
298
298
298
298
303
308
313
318
0.61
1.23
1.86
2.48
3.06
1.23
1.23
1.23
1.23
1.24
1.24
1.23
1.23
0.53
0.53
0.53
0.53
0.53
0.42
0.64
0.74
0.95
0.53
0.53
0.53
0.53
0.057
0.123
0.155
0.228
0.278
0.118
0.124
0.128
0.128
0.216
0.370
0.503
0.914
0.094
0.100
0.083
0.092
0.091
0.096
0.101
0.104
0.104
0.174
0.298
0.409
0.743
a Reactions were quantitative yielding TON between 17 and 87. b Calculated at 20% conversion.
A rate law for the dehydrogenation of formic acid promoted by 3 is
a
b
[
]
d H2 /dt = 푘[H2CO2]0[ퟑ]
(3)
The rate dependence on formic acid concentration was determined at 298 K, for a constant
concentration of 3 of 1.23·10-2 M, by measuring initial rates with variable initial concentrations
of formic acid ([H2CO2]0 in Table 1) from 0.42 to 0.95 M. Under these conditions, the rate is
independent of [H2CO2]0 within the experimental error (Figure 2), in agreement with a = 0 in eq
3. The rate dependence on the catalyst precursor 3 was also determined at 298 K, for a fixed
ACS Paragon Plus Environment
8

Page 9 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
initial concentration of formic acid of 0.53 M, by varying the concentration of 3 from 0.61·10-2
to 3.06·10-2 M. Now, the plot of ln(d[H2]/dt) versus ln[3] affords a straight line of slope 0.96 ±
0.17 (Figure 3), according to a first order in osmium concentration (b = 1 in eq 3). Therefore, the
rate law is:
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
[
]
d H2 /dt = k[ퟑ]
(4)
-2
[
]
Figure 2. Plot of ln(d H2 /dt) vs ln[H2CO2]0 in toluene at 298 K, for a constant [3] of 1.23·10
M.
[
]
Figure 3. Plot of ln(d H2 /dt) vs ln[3] in toluene at 298 K, for a constant [H2CO2]0 of 0.53 M.
A plot of d[H2]/dt versus [3] (Figure 4) provides a value of 9.0 ± 1.0·10-2 min-1 for k at 298 K.
The activation parameters obtained from the Eyring analysis (Figure 5) are ΔH‡ = 18 ± 3
kcal·mol-1 and ΔS‡ = -3 ± 8 cal·mol-1K-1, which yield a ΔG‡ value of 18 ± 5 kcal·mol-1 at 298 K
ACS Paragon Plus Environment
9

ACS Catalysis
Page 10 of 36
1
2
3
4
in agreement with the values reported for the dehydrogenation of formic acid catalyzed by other
5
6
homogeneous systems (17 – 26 kcal·mol-1).5a, 7d, 9f, 37
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. Plot of d[H2]/dt vs [3] using 0.53 M of H2CO2 in toluene at 298 K.
Figure 5. Eyring plot for the dehydrogenation of formic acid (0.53 M) promoted by 3 (1.23·10-2
M) in toluene.
Stoichometric Reactions. In order to have access to the key intermediates of the catalysis, we
studied the stoichiometric reaction of 3 with formic acid in addition to the stages of CO2 and H2
formation.
The addition of 1.0 equiv of formic acid to a toluene-d8 solution of 3 contained in a NMR tube
quantitatively and instantly leads to the formate derivative OsH3{κ1-O-(HCO2)}{xant(PiPr2)2}
ACS Paragon Plus Environment
10

Page 11 of 36
ACS Catalysis
1
2
3
4
(4), as a consequence of the protonation of the hydroxo ligand of 3 and the subsequent
5
6
displacement of the generated H2O by a formate anion (Scheme 2).
7
8
9
Scheme 2. Reaction of 3 with H2CO2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The presence of a formate group in the osmium coordination sphere is supported by 13C{1H} and
1H NMR spectra of the new species at 253 K. The 13C{1H} NMR spectrum shows at 167.8 ppm
a singlet due to the HCO2-carbon atom, whereas the 1H NMR spectrum contains a singlet at 8.91
ppm corresponding to the HCO2-hydrogen atom. Like in 2 and 3, the hydrides of 4 experience
position exchange thermally activated. Thus, they display a broad signal at -12.55 ppm. At about
213 K, a first decoalescence occurs to afford two signals at -12.00 and -13.32 ppm. At 193 K, the
expected second decoalescence can be intuited from the very broad form of the resonance at -
31
11.93 ppm. In the P{1H} NMR spectrum, the diphosphine gives rise to a singlet at 51.2 ppm.
These spectroscopic features are consistent with the stereochemistry proposed for 4 in Scheme 2,
which was confirmed by the X-ray diffraction analysis of a single crystal obtained from the
toluene-d8 solution (Figure 6). The Os{xant(PiPr2)2} skeleton is T-shaped with the metal center
situated in the common vertex and P1-Os-P2, P1-Os-O3, and P2-Os-O3 angles of 159.84(7)º,
83.08(12)º, and 82.63(12)º, respectively. The geometry around the osmium atom can be
described as a distorted pentagonal bipyramid with axial PiPr2 groups and the oxygen atoms of
the diphosphine and formate in the perpendicular plane, mutually cisoid disposed, along with the
hydrides.
ACS Paragon Plus Environment
11

ln
([ퟒ]/[ퟒ]0) = 푘stt
(5)
ACS Catalysis
Page 12 of 36
1
2
3
4
5
6
7
8
9
H1
P1
O1
C1
O2
Os
O3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
H01
H02
H03
P2
Figure 6. Molecular drawing of 4 with 50% probability ellipsoids. Hydrogen atoms (except
hydrides and H1) are omitted. Selected distances (Å) and angles (deg): Os-O3 = 2.231(4), Os-O1
= 2.137(6), Os-P1 = 2.2915(19), Os-P2 = 2.2848 (18); P1-Os-P2 = 159.84(7), P1-Os-O3 =
83.08(12), P2-Os-O3 = 82.63(12), O1-Os-O3 = 76.1(3)
Complex 4 is unstable and releases CO2 to afford OsH4{xant(PiPr2)2} (5), according to Scheme
3. The transformation was monitored by 31P{1H} NMR spectroscopy as a function of time
between 303 and 323 K. Figure 7 shows the transformation at 323 K. The decrease of 4 is an
exponential function of time, in agreement with a first-order process, which fits to the expression
where [4]0 is the initial concentration of 4 and [4] is the concentration at time t. The values for
the stoichiometric rate constant kst are collected in Table 2. The activation parameters calculated
by means of the Eyring analysis (Figure 8) are ΔH‡ = 20 ± 3 kcal·mol-1 and ΔS‡ = 0 ± 8 cal·mol-
1K-1. They affords a ΔG‡ value of 20 ± 5 kcal·mol-1 at 298 K, which compares well with that
obtained for the catalytic process.
Scheme 3. Decarboxylation of complex 4
ACS Paragon Plus Environment
12

Page 13 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 7. 31P{1H} spectra (161.98 MHz, in toluene-d8) for the transformation of 4 into 5 at 323
K.
ACS Paragon Plus Environment
13

ACS Catalysis
Page 14 of 36
1
2
3
4
5
6
7
Figure 8. Eyring plot for the decarboxylation of 4 (6.4·10-2 M) in toluene-d8.
Table 2. Rate Constants kst for the Decarboxylation of 4
8
9
T (K)
303
308
313
318
323
kst (min-1)
0.048
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0.072
0.138
0.192
0.396
Tetrahydride complex 5 reacts with FA to form molecular hydrogen and to regenerate the
formate derivative 4 (Scheme 4). At 243 K, the addition of 1.0 equiv of formic acid to a toluene-
d8 solution of 5 rapidly gives 4 and H2 in quantitative yield. According to previous studies, the
reaction initially implies the protonation of 4 to afford an OsH5-cation A. DFT-Calculations
revealed that this cation has two tautomers with similar energies: the trihydride compressed
dihydride Aa and the hydride-compressed dihydride-dihydrogen Ab.30c The dissociation of the
coordinated H2 ligand from the latter and the coordination of the formate anion to the resulting
unsaturated species B should lead to 4.
Scheme 4. Reaction of 5 with H2CO2
ACS Paragon Plus Environment
14

Page 15 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
The reactions summarized in Schemes 2-4, fully characterize the catalysis. The trihydride-
hydroxo complex 3 is the precursor of the catalyst and, under catalytic conditions, is
quantitatively transformed into the true catalyst, the formate derivative 4 (Scheme 2), which
releases CO2 to form the tetrahydride 5 (Scheme 3). The latter reacts with FA to produce
molecular hydrogen and regenerate 4 in a three step process: protonation, H2 dissociation, and
formate coordination (Scheme 4). Because the catalytic FA dehydrogenation promoted by 3 has
the same activation parameters as the stoichiometric release of CO2 from 4, the CO2 formation is
the rate determining step of the catalysis, while the molecular hydrogen formation is fast.
Mechanism of the CO2 Formation Stage. As previously mentioned the CO2 formation stage
can occur through three different pathways: outer-sphere, β-hydride elimination, and hydride
abstraction. An outer-sphere pathway must be excluded in this case because the system does not
bear any function which could allow such catalysis class. β-Hydride elimination reactions have
been proposed in cycles initiated by half-sandwich iridium(III) derivatives;9f, 9i, ruthenium
complexes bearing pincer PNP ligands;8e iron species stabilized by PPP groups;7c and catalysts
generated in situ from Fe3(CO)12, 2,2’:6’2’’-terpyridine or 1,10-phenanthroline, and
triphenylphosphine working under visible light irradiation,7b among others. A distinctive fact of
this mechanism is that the β-hydride elimination step occurs on unsaturated species. Hydride
abstraction has been mainly proposed in reactions catalyzed by aluminum bis(imino)pyridine
compounds,14 ruthenium tetraphosphine derivatives,8f and iron complexes bearing PNP pincer
ligands.7d, 7f In contrast to β-hydride elimination, the hydride migration takes place in two-steps,
on a saturated intermediate, which involve the slippage of the metal center from the coordinated
oxygen into the free hydrogen and the subsequent CO2 release. In order to discern between β-
hydride elimination and hydride abstraction, DFT calculations (B3LYP(GD3)//6-31G**/SDD(f))
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment
15

ACS Catalysis
Page 16 of 36
1
2
3
4
5
6
7
8
9
were carried out. The changes in ΔG‡ were calculated in toluene at 298 K and 1 atmosphere.
Although the reaction is slightly endergonic, the equilibrium is driven by removal of the gas.
Figure 9 summarizes the energy profile for the β-hydride elimination pathway. Because the
formate complex 4 is saturated, the dissociation of the oxygen atom of the pincer is necessary in
order to generate a coordination vacancy at the metal center, which allows the migration of the
hydrogen atom of the formate group. The dissociation has an activation energy of 16.3
kcal·mol-1, which is lower than the experimental values obtained for the activation energies of
the catalysis and for the stoichiometric decarboxylation of 4, and leads to a 16 e- trihydride-
hydroxo-osmium (IV) intermediate C. The latter is 13.9 kcal·mol-1 less stable than 4. The
migration leads to the Os (η2-O=CO)-derivative D. Its barrier of 24.1 kcal·mol-1 with regard to 4
lies at the experimental upper limit. Intermediate D is 4.4 kcal·mol-1 less stable than C. The
dissociation of the coordinated CO2 molecule occurs in two steps, involving the sequential
cleavage of the Os-C and Os-O bonds. Initially, intermediate D frees the carbon atom to afford
E, which subsequently undergoes the Os-O rupture. The barrier for the first step of 26.8
kcal·mol-1, with regard to 4, is out of the experimental range and suggests that the β-hydride
elimination is also a non-operating pathway. The release of the CO2 molecule leads to F, which
is a structural isomer of 5.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment
16

Page 17 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 9. Energy profile (ΔG in kcal·mol-1) for the CO2 formation via β-hydride elimination.
The hydride abstraction takes place via intermediate G (Figure 10), which can be described as an
Os{κ1-H-(HCO2)}-species and lies 12.1 kcal·mol-1 above 4. Intermediate G is generated by
means of the slippage of the metal center from the coordinated oxygen into the free hydrogen,
following a formate O-C-H path. The process has an activation energy of 22.5 kcal·mol-1, which
compares well with the experimental values obtained for the catalysis and for the stoichiometric
release of the CO2 molecule from 4. The agreement between the three values strongly supports a
hydride abstraction mechanism for the CO2 formation stage and reveals that the slippage of the
metal center from the oxygen to the hydrogen through the formate O-C-H path is the rate
determining step of the CO2 formation stage and therefore of the catalysis. According to this, the
release of CO2 from G is a barrierless process.
ACS Paragon Plus Environment
17

ACS Catalysis
Page 18 of 36
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 10. Energy profile (ΔG in kcal·mol-1) for the CO2 formation via hydride abstraction.
CONCLUDING REMARKS
Replacement of the chloride ligand of the trihydride OsH3Cl{xant(PiPr2)2} by a hydroxo group
affords the trihydride-hydroxo-osmium(IV) derivative OsH3(OH){xant(PiPr2)2} which efficiently
promotes the FA dehydrogenation to CO2 and H2.
The dehydrogenation takes place through the catalytic cycle shown in Scheme 5. Under catalytic
conditions the trihydride-hydroxo complex reacts with FA to form the compound OsH3{κ1-O-
(HCO2)}{xant(PiPr2)2}, which has been isolated and fully characterized. DFT calculations
suggest that the slippage of the metal center from the oxygen to the hydrogen through the
formate O-C-H path affords an OsH3{κ1-H-(HCO2)}{xant(PiPr2)2} intermediate, which releases
CO2 to give the previously described tetrahydride OsH4{xant(PiPr2)2}. The latter undergoes
protonation with formic acid. According to previous DFT calculations, the resulting OsH5-cation
exists as an equilibrium mixture of two tautomers: the trihydride-compressed dihydride
[OsH3(H···H){xant(PiPr2)2}]+
and
the
hydride-compressed
dihydride-dihydrogen
ACS Paragon Plus Environment
18

Page 19 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
[OsH(H···H)(η2-H2){xant(PiPr2)2}]+. The dissociation of the coordinated H2 molecule from the
latter affords the unsaturated cation [OsH3{xant(PiPr2)2}]+, which coordinates the formate anion
to regenerate the trihydride-formate and to close the cycle. The slippage of the metal center from
the oxygen to the hydrogen through the formate O-C-H path is the rate-determining step of the
catalysis.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 5. Catalytic Cycle for the Formic Acid Dehydrogenation Promoted by 3
In conclusion, the first osmium catalyst for the FA dehydrogenation to CO2 and H2 has been
discovered and the mechanism of the catalysis has been established, including the full
characterization of the key intermediates and the elucidation of the rate-determining step.
ACS Paragon Plus Environment
19

ACS Catalysis
Page 20 of 36
1
2
3
4
EXPERIMENTAL SECTION
5
6
7
8
9
Preparation and characterization of complexes 2, 3 and 4, instrumental methods, NMR spectra,
X-ray diffraction analysis information and DFT computational details are included in the
Supporting Information.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
General Methods for Formic Acid Dehydrogenation Studies. A solution of
OsH3(OH){xant(PiPr2)2} (3) in 2.5 mL of toluene was syringed through a septum into a 25 mL
flask immersed into a thermostatted bath, which was connected to a gas burette provided with a
vaseline oil reservoir. Once the system had equilibrated to atmospheric pressure, formic acid was
syringed and the solution was shaken (500 rpm). The reaction was followed by measuring the
volume of the formed gas (H2 + CO2) with the time.
NMR Spectroscopic Studies of the Decarboxylation of OsH3{κ1-O-(HCO2)}{xant(PiPr2)2}
31
(4). The decarboxylation of complex 4 was followed by P{1H} NMR spectroscopy, in the
temperature range T = 303 to 323 K. To a screw-top NMR tube containing a solution of
OsH3(OH){xant(PiPr2)2} (3) (20.8 mg, 0.032 mmol) in toluene-d8 (0.5 mL), and a capillary tube
with a solution of PPh3 in toluene-d8 as internal standard, was added H2CO2 (1.2 µL, 0.032
mmol) via syringe. After 5 min, NMR spectra were recorded at the desired temperature, to show
the formation of OsH4{xant(PiPr2)2} (5) as a function of the time. The parameters of the 31P{1H}
NMR experiment were modified to allow the integration of the signals: pulse program (zgig), d1
≥ 5T1 (d1 = 20 s). The kst value was obtained for each temperature from a plot Ln[4] / Ln[4]0 vs
time (eq 5).
ASSOCIATED CONTENT
Supporting Information
ACS Paragon Plus Environment
20

Page 21 of 36
ACS Catalysis
1
2
3
4
The Suporting Information is available free of charge.
5
6
7
8
Preparation and characterization of complexes 2, 3 and 4, instrumental methods, NMR spectra,
X-ray diffraction analysis information and DFT computational details (PDF)
Cartesian coordinates of the calculated structures (XYZ)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.A.E.: maester@unizar.es.
ORCID
Miguel A. Esteruelas: 0000-0002-4829-7590
Cristina García-Yebra: 0000-0002-5545-5112
Jaime Martín: 0000-0003-0909-3509
Enrique Oꢀate: 0000-0003-2094-719X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank MINECO of Spain (Projects CTQ2017-82935-P and Red de Excelencia Consolider
CTQ2016-81797-REDC), Diputación General de Aragón (E06_17R), FEDER, and the European
Social Fund for financial support.
REFERENCES
(1) (a) Marbán, G.; Valdés-Solís, T. Towards the Hydrogen Economy? Int. J. Hydrogen. Energ.
2007, 32, 1625-1637. (b) Nikolaidis, P.; Poullikkas, A. A Comparative Overview of Hydrogen
Production Processes. Renew. Sust. Energ. Rev. 2017, 67, 597-611.
ACS Paragon Plus Environment
21

ACS Catalysis
Page 22 of 36
1
2
3
4
5
6
7
8
9
(2) (a) Schlapbach, L.; Züttel, A. Hydrogen-Storage Materials for Mobile Applications. Nature
2001, 414, 353-358. (b) Yadav, M.; Xu, Q. Liquid-Phase Chemical Hydrogen Storage Materials.
Energy Environ. Sci. 2012, 5, 9698-9725. (c) Dalebrook, A. F.; Gan, W. J.; Grasemann, M.;
Moret, S.; Laurenczy, G. Hydrogen Storage: Beyond Conventional Methods. Chem. Commun.
2013, 49, 8735-8751. (d) Niaz, S.; Manzoor, T.; Pandith, A. H. Hydrogen Storage: Materials,
Methods and Perspectives. Renew. Sust. Energ. Rev. 2015, 50, 457-469. (e) He, T.; Pachfule, P.;
Wu, H.; Xu, Q.; Chen, P. Hydrogen Carriers. Nat. Rev. Mater. 2016, 1, 16059.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(3) Gianotti, E.; Taillades-Jacquin, M.; Rozière, J.; Jones, D. J. High-Purity Hydrogen
Generation via Dehydrogenation of Organic Carriers: A Review on the Catalytic Process. ACS
Catal. 2018, 8, 4660-4680.
(4) (a) Grasemann, M.; Laurenczy, G. Formic Acid as a Hydrogen Source – Recent
Developments and Future Trends. Energy Environ. Sci. 2012, 5, 8171-8181. (b) Kawanami, H.;
Himeda, Y.; Laurenczy, G. Formic Acid as a Hydrogen Carrier for Fuel Cells Toward a
Sustainable Energy System. Adv. Inorg. Chem. 2017, 70, 395-427. (c) Eppinger, J.; Huang, K.
W. Formic Acid as a Hydrogen Energy Carrier. ACS Energy Lett. 2017, 2, 188-195.
(5) (a) 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, 1526-1530. (b) Wang, W. H.; Himeda, Y.; Muckerman,
J. T.; Manbeck, G. F.; Fujita, E. CO2 Hydrogenation to Formate and Methanol as an Alternative
to Photo- and Electrochemical CO2 Reduction. Chem. Rev. 2015, 115, 12936-12973. (c)
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,
ACS Paragon Plus Environment
22

Page 23 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
1049-1058. (d) Artz, J.; Müller, T. E.; Thenert, K.; Kleinekorte, J.; Meys, R.; Sternberg, A.;
Bardow, A.; Leitner, W. Sustainable Conversion of Carbon Dioxide: An Integrated Review of
Catalysis and Life Cycle Assessment. Chem. Rev. 2018, 118, 434-504.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(6) (a) Singh, A. K.; Singh, S.; Kumar, A. Hydrogen Energy Future with Formic Acid: a
Renewable Chemical Hydrogen Storage System. Catal. Sci. Technol. 2016, 6, 12-40. (b)
Mellmann, D.; Sponholz, P.; Junge, H.; Beller, M. Formic Acid as a Hydrogen Storage Material
– Development of Homogeneous Catalysts for Selective Hydrogen Release. Chem. Soc. Rev.
2016, 45, 3954-3988. (c) Sordakis, K.; Tang, C. H.; 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, 372-433. (d) Wang, X.; Meng, Q. L.; Gao, L. Q.; Jin, Z.;
Ge, J. J.; Liu, C. P.; Xing, W. Recent Progress in Hydrogen Production from Formic Acid
Decomposition. Int. J. Hydrogen. Energ. 2018, 43, 7055-7071.
(7) See for example: (a) Boddien, A.; Gärtner, F.; Jackstell, R.; Junge, H.; Spannenberg, A.;
Baumann, W.; Ludwig, R.; Beller, M. ortho-Metalation of Iron(0) Tribenzylphosphine
Complexes: Homogeneous Catalysts for the Generation of Hydrogen from Formic Acid. Angew.
Chem., Int. Ed. 2010, 49, 8993-8996. (b) Boddien, A.; Loges, B.; Gärtner, F.; Torborg, C.;
Fumino, K.; Junge, H.; Ludwig, R.; Beller, M. Iron-Catalyzed Hydrogen Production from
Formic Acid. J. Am. Chem. Soc. 2010, 132, 8924-8934. (c) Boddien, A.; Mellmann, D.; Gärtner,
F.; Jackstell, R.; Junge, H.; Dyson, P. J.; Laurenczy, G.; Ludwig, R.; Beller, M. Efficient
Dehydrogenation of Formic Acid Using an Iron Catalyst. Science 2011, 333, 1733-1736. (d)
Zell, T.; Butschke, B.; Ben-David, Y.; Milstein, D. Efficient Hydrogen Liberation from Formic
Acid Catalyzed by a Well-Defined Iron Pincer Complex under Mild Conditions. Chem. - Eur. J.
2013, 19, 8068-8072. (e) Zell, T.; Milstein, D. Hydrogenation and Dehydrogenation Iron Pincer
ACS Paragon Plus Environment
23

ACS Catalysis
Page 24 of 36
1
2
3
4
5
6
7
8
9
Catalysts Capable of Metal Ligand Cooperation by Aromatization/Dearomatization. Acc. Chem.
Res. 2015, 48, 1979-1994. (f) 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, 3344-3349.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(8) See for example: (a) Gan, W. J.; Snelders, D. J. M.; Dyson, P. J.; Laurenczy, G.
Ruthenium(II)-Catalyzed Hydrogen Generation from Formic Acid using Cationic,
Ammoniomethyl-Substituted Triarylphosphine Ligands. ChemCatChem 2013, 5, 1126-1132. (b)
Mellone, I.; Peruzzini, M.; Rosi, L.; Mellmann, D.; Junge, H.; Beller, M.; Gonsalvi, L. Formic
Acid Dehydrogenation Catalysed by Ruthenium Complexes Bearing the Tripodal Ligands
Triphos and NP3. Dalton Trans. 2013, 42, 2495-2501. (c) Guerriero, A.; Bricout, H.; Sordakis,
K.; Peruzzini, M.; Monflier, E.; Hapiot, F.; Laurenczy, G.; Gonsalvi, L. Hydrogen Production by
Selective Dehydrogenation of HCOOH Catalyzed by Ru-Biaryl Sulfonated Phosphines in
Aqueous Solution. ACS Catal. 2014, 4, 3002-3012. (d) Czaun, M.; Goeppert, A.;
Kothandaraman, J.; May, R. B.; Haiges, R.; Prakash, G. K. S.; Olah, G. A. Formic Acid As a
Hydrogen Storage Medium: Ruthenium-Catalyzed Generation of Hydrogen from Formic Acid in
Emulsions. ACS Catal. 2014, 4, 311-320. (e) Pan, Y. P.; Pan, C. L.; Zhang, Y. F.; Li, H. F.; Min,
S. X.; Guo, X. M.; Zheng, B.; Chen, H. L.; Anders, A.; Lai, Z. P.; Zheng, J. R.; Huang, K. W.
Selective Hydrogen Generation from Formic Acid with Well-Defined Complexes of Ruthenium
and Phosphorus-Nitrogen PN3-Pincer Ligand. Chem. - Asian J. 2016, 11, 1357-1360. (f)
Mellone, I.; Bertini, F.; Peruzzini, M.; Gonsalvi, L. An Active, Stable and Recyclable Ru(II)
Tetraphosphine-Based Catalytic System for Hydrogen Production by Selective Formic Acid
Dehydrogenation. Catal. Sci. Technol. 2016, 6, 6504-6512. (g) Anderson, N. H.; Boncella, J. M.;
Tondreau, A. M. Reactivity of Silanes with (tBuPONOP)Ruthenium Dichloride: Facile Synthesis
ACS Paragon Plus Environment
24

Page 25 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
of Chloro-Silyl Ruthenium Compounds and Formic Acid Decomposition. Chem. - Eur. J. 2017,
23, 13617-13622. (h) Piola, L.; Fernández-Salas, J. A.; Nahra, F.; Poater, A.; Cavallo, L.; Nolan,
S. P. Ruthenium-Catalysed Decomposition of Formic Acid: Fuel Cell and Catalytic Applications.
Mol. Catal. 2017, 440, 184-189. (i) Guan, C.; Zhang, D. D.; Pan, Y. P.; Iguchi, M.; Ajitha, M. J.;
Hu, J. S.; Li, H. F.; Yao, C. G.; Huang, M. H.; Ming, S. X.; Zheng, J. R.; Himeda, Y.;
Kawanami, H.; Huang, K. W. Dehydrogenation of Formic Acid Catalyzed by a Ruthenium
Complex with an N,N'-Diimine Ligand. Inorg. Chem. 2017, 56, 438-445.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(9) See for example: (a) Celaje, J. J. A.; Lu, Z. Y.; 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. (b) Papp, G.; Ölveti, G.; Horváth, H.; Kathó, A.; Joó, F. Highly Efficient
Dehydrogenation of Formic Acid in Aqueous Solution Catalysed by an Easily Available Water-
Soluble Iridium(III) Dihydride. Dalton Trans. 2016, 45, 14516-14519. (c) Iguchi, M.; Himeda,
Y.; Manaka, Y.; Kawanami, H. Development of an Iridium-Based Catalyst for High-Pressure
Evolution of Hydrogen from Formic Acid. ChemSusChem 2016, 9, 2749-2753. (d) Li, J. J.; Li, J.
H.; Zhang, D. J.; Liu, C. B. DFT Study on the Mechanism of Formic Acid Decomposition by a
Well-Defined Bifunctional Cyclometalated Iridium(III) Catalyst: Self-Assisted Concerted
Dehydrogenation via Long-Range Intermolecular Hydrogen Migration. ACS Catal. 2016, 6,
4746-4754. (e) Czaun, M.; Kothandaraman, J.; Goeppert, A.; Yang, B.; Greenberg, S.; May, R.
B.; Olah, G. A.; Prakash, G. K. S. Iridium-Catalyzed Continuous Hydrogen Generation from
Formic Acid and Its Subsequent Utilization in a Fuel Cell: Toward a Carbon Neutral Chemical
Energy Storage. ACS Catal. 2016, 6, 7475-7484. (f) Iguchi, M.; Zhong, H.; Himeda, Y.;
Kawanami, H. Kinetic Studies on Formic Acid Dehydrogenation Catalyzed by an Iridium
Complex towards Insights into the Catalytic Mechanism of High-Pressure Hydrogen Gas
ACS Paragon Plus Environment
25

ACS Catalysis
Page 26 of 36
1
2
3
4
5
6
7
8
9
Production. Chem. - Eur. J. 2017, 23, 17017-17021. (g) Wang, L.; Onishi, N.; Murata, K.;
Hirose, T.; Muckerman, J. T.; Fujita, E.; Himeda, Y. Efficient Hydrogen Storage and Production
Using a Catalyst with an Imidazoline-Based, Proton-Responsive Ligand. ChemSusChem 2017,
10, 1071-1075. (h) Cohen, S.; Borin, V.; Schapiro, I.; Musa, S.; De-Botton, S.; Belkova, N. V.;
Gelman, D. Ir(III)-PC(sp3)P Bifunctional Catalysts for Production of H2 by Dehydrogenation of
Formic Acid: Experimental and Theoretical Study. ACS Catal. 2017, 7, 8139-8146. (i) Iguchi,
M.; Zhong, H.; Himeda, Y.; Kawanami, H. Effect of the ortho-Hydroxyl Groups on a Bipyridine
Ligand of Iridium Complexes for the High-Pressure Gas Generation from the Catalytic
Decomposition of Formic Acid. Chem. - Eur. J. 2017, 23, 17788-17793. (j) Lu, S. M.; Wang, Z.
J.; Wang, J. J.; Li, J.; Li, C. Hydrogen Generation from Formic Acid Decomposition on a Highly
Efficient Iridium Catalyst Bearing a Diaminoglyoxime Ligand. Green Chem. 2018, 20, 1835-
1840.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(10) Neary, M. C.; Parkin, G. Dehydrogenation, Disproportionation and Transfer Hydrogenation
Reactions of Formic Acid Catalyzed by Molybdenum Hydride Compounds. Chem. Sci. 2015, 6,
1859-1865.
(11) Fink, C.; Laurenczy, G. CO2 as a Hydrogen Vector – Transition Metal Diamine Catalysts
for Selective HCOOH Dehydrogenation. Dalton Trans. 2017, 46, 1670-1676.
(12) Enthaler, S.; Brück, A.; Kammer, A.; Junge, H.; Irran, E.; Gülak, S. Exploring the
Reactivity of Nickel Pincer Complexes in the Decomposition of Formic Acid to CO2/H2 and the
Hydrogenation of NaHCO3 to HCOONa. ChemCatChem 2015, 7, 65-69.
ACS Paragon Plus Environment
26

Page 27 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
(13) Correa, A.; Cascella, M.; Scotti, N.; Zaccheria, F.; Ravasio, N.; Psaro, R. Mechanistic
Insights into Formic Acid Dehydrogenation Promoted by Cu-Amino Based Systems. Inorg.
Chim. Acta. 2018, 470, 290-294.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(14) Myers, T. W.; Berben, L. A. Aluminium-Ligand Cooperation Promotes Selective
Dehydrogenation of Formic Acid to H2 and CO2. Chem. Sci. 2014, 5, 2771-2777.
(15) Iglesias, M.; Oro, L. A. Mechanistic Considerations on Homogeneously Catalyzed Formic
Acid Dehydrogenation. Eur. J. Inorg. Chem. 2018, 2125-2138.
(16) (a) Dub, P. A.; Henson, N. J.; Martin, R. L.; Gordon, J. C. Unravelling the Mechanism of
the Asymmetric Hydrogenation of Acetophenone by [RuX2(diphosphine)(1,2-diamine)]
Catalysts. J. Am. Chem. Soc. 2014, 136, 3505-3521. (b) Dub, P. A.; Gordon, J. C. The
Mechanism of Enantioselective Ketone Reduction with Noyori and Noyori-Ikariya Bifunctional
Catalysts. Dalton Trans. 2016, 45, 6756-6781. (c) Gusev, D. G. Dehydrogenative Coupling of
Ethanol and Ester Hydrogenation Catalyzed by Pincer-Type YNP Complexes. ACS Catal. 2016,
6, 6967-6981. (d) Morris, S. A.; Gusev, D. G. Rethinking the Claisen-Tishchenko Reaction.
Angew. Chem., Int. Ed. 2017, 56, 6228-6231. (e) Dub, P. A.; Gordon, J. C. Metal Ligand
Bifunctional Catalysis: The "Accepted" Mechanism, the Issue of Concertedness, and the
Function of the Ligand in Catalytic Cycles Involving Hydrogen Atoms. ACS Catal. 2017, 7,
6635-6655. (f) Gusev, D. G. Rethinking the Dehydrogenative Amide Synthesis. ACS Catal.
2017, 7, 6656-6662.
(17) (a) Kolb, H. C.; VanNieuwenhze, M. S.; Sharpless, K. B. Catalytic Asymmetric
Dihydroxylation. Chem. Rev. 1994, 94, 2483-2547. (b) Döbler, C.; Mehltretter, G. M.;
Sundermeier, U.; Beller, M. Osmium-Catalyzed Dihydroxylation of Olefins Using Dioxygen or
ACS Paragon Plus Environment
27

ACS Catalysis
Page 28 of 36
1
2
3
4
5
6
7
8
9
Air as the Terminal Oxidant. J. Am. Chem. Soc. 2000, 122, 10289-10297. (c) Döbler, C.;
Mehltretter, G. M.; Sundermeier, U.; Beller, M. Dihydroxylation of Olefins Using Air as the
Terminal Oxidant. J. Organomet. Chem. 2001, 621, 70-76. (d) Heravi, M. M.; Zadsirjan, V.;
Esfandyari, M.; Lashaki, T. B. Applications of Sharpless Asymmetric Dihydroxylation in the
Total Synthesis of Natural Products. Tetrahedron: Asymmetry 2017, 28, 987-1043.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(18) See for example: (a) Sánchez-Delgado, R. A.; Rosales, M.; Esteruelas, M. A.; Oro, L. A.
Homogeneous Catalysis by Osmium Complexes - a Review. J. Mol. Catal. A: Chem. 1995, 96,
231-243. (b) Esteruelas, M. A.; Herrero, J.; López, A. M.; Oliván, M. Alkyne-Coupling
Reactions Catalyzed by OsHCl(CO)(PiPr3)2 in the Presence of Diethylamine. Organometallics
2001, 20, 3202-3205. (c) Castarlenas, R.; Esteruelas, M. A.; Oñate, E. N-Heterocyclic Carbene-
Osmium Complexes for Olefin Metathesis Reactions. Organometallics 2005, 24, 4343-4346. (d)
Esteruelas, M. A.; García-Yebra, C.; Oliván, M.; Oñate, E.; Valencia, M. Osmium-Catalyzed
Allylic Alkylation. Organometallics 2008, 27, 4892-4902. (e) Batuecas, M.; Esteruelas, M. A.;
García-Yebra, C.; Oñate, E. Redox Isomerization of Allylic Alcohols Catalyzed by Osmium and
Ruthenium Complexes Containing a Cyclopentadienyl Ligand with a Pendant Amine or
Phosphoramidite Group: X-ray Structure of an η3-1-Hydroxyallyl-Metal-Hydride Intermediate.
Organometallics 2010, 29, 2166-2175. (f) Varela-Fernández, A.; García-Yebra, C.; Varela, J. A.;
Esteruelas, M. A.; Saá, C. Osmium-Catalyzed 7-endo Heterocyclization of Aromatic Alkynols
into Benzoxepines. Angew. Chem., Int. Ed. 2010, 49, 4278-4281. (g) Wu, L. P.; Liu, Q.;
Spannenberg, A.; Jackstell, R.; Beller, M. Highly Regioselective Osmium-Catalyzed
Hydroformylation. Chem. Commun. 2015, 51, 3080-3082. (h) Álvarez-Pérez, A.; González-
Rodríguez, C.; García-Yebra, C.; Varela, J. A.; Oñate, E.; Esteruelas, M. A.; Saá, C. Catalytic
Cyclization of o-Alkynyl Phenethylamines via Osmacyclopropene Intermediates: Direct Access
ACS Paragon Plus Environment
28

Page 29 of 36
ACS Catalysis
1
2
3
4
5
6
7
8
9
to Dopaminergic 3-Benzazepines. Angew. Chem., Int. Ed. 2015, 54, 13357-13361. (i) Batuecas,
M.; Castro-Rodrigo, R.; Esteruelas, M. A.; García-Yebra, C.; López, A. M.; Oñate, E. Aromatic
Osmacyclopropenefuran Bicycles and Their Relevance for the Metal-Mediated Hydration of
Functionalized Allenes. Angew. Chem., Int. Ed. 2016, 55, 13749-13753. (j) González-Fernández,
R.; Crochet, P.; Cadierno, V.; Menéndez, M. I.; López, R. Phosphinous Acid-Assisted Hydration
of Nitriles: Understanding the Controversial Reactivity of Osmium and Ruthenium Catalysts.
Chem. - Eur. J. 2017, 23, 15210-15221.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(19) See for example: (a) Esteruelas, M. A.; Oro, L. A.; Valero, C. Hydrogenation of
Benzylideneacetone Catalyzed by OsHCl(CO)(PR3)2 (PR3 = PiPr3, PMe-tBu2) - New Roles of
Dihydrogen Complexes in Homogeneous Catalytic-Hydrogenation. Organometallics 1992, 11,
3362-3369. (b) Barrio, P.; Esteruelas, M. A.; Oñate, E. Reactions of a Hexahydride-Osmium
Complex with Aldehydes: Double C-Hα Activation-Decarbonylation and Single C-Hα
Activation-Hydroxylation Tandem Processes and Catalytic Tishchenko Reactions.
Organometallics 2004, 23, 1340-1348. (c) Esteruelas, M. A.; Honczek, N.; Oliván, M.; Oñate,
E.; Valencia, M. Direct Access to POP-Type Osmium(II) and Osmium(IV) Complexes: Osmium
a Promising Alternative to Ruthenium for the Synthesis of Imines from Alcohols and Amines.
Organometallics 2011, 30, 2468-2471. (d) Baratta, W.; Bossi, G.; Putignano, E.; Rigo, P. Pincer
and Diamine Ru and Os Diphosphane Complexes as Efficient Catalysts for the Dehydrogenation
of Alcohols to Ketones. Chem. - Eur. J. 2011, 17, 3474-3481. (e) Bertoli, M.; Choualeb, A.;
Lough, A. J.; Moore, B.; Spasyuk, D.; Gusev, D. G. Osmium and Ruthenium Catalysts for
Dehydrogenation of Alcohols. Organometallics 2011, 30, 3479-3482. (f) Spasyuk, D.; Gusev, D.
G. Acceptorless Dehydrogenative Coupling of Ethanol and Hydrogenation of Esters and Imines.
Organometallics 2012, 31, 5239-5242. (g) Spasyuk, D.; Vicent, C.; Gusev, D. G.
ACS Paragon Plus Environment
29