Base-Free and Acceptorless Dehydrogenation of Alcohols Catalyzed by an Iridium Complex Stabilized by a N, N, N-Osmaligand

Article abstract of DOI:10.1021/acs.organomet.8b00380

The preparation of a N,N,N-osmaligand, its coordination to iridium to afford an efficient catalyst precursor, and the catalytic activity of the latter in dehydrogenation reactions of hydrogen carriers based on alcohols are reported. Complex OsH₂Cl₂(PⁱPr₃)₂ (1) reacts with 3-(2-pyridyl)pyrazol to give the osmium(II) complex 2H, which contains an acidic hydrogen atom. Deprotonation of the latter by the bridging methoxy groups of the dimer [Ir(μ-OMe)(n⁴-COD)]₂ (COD = 1,5-cyclooctadiene) leads to Ir(2)( n ⁴-COD) (3), where osmaligand 2 has a free-nitrogen atom. Iridium complex 3 catalyzes the dehydrogenation of secondary and primary alcohols to ketones and aldehydes or esters, respectively, and the dehydrogenation of diols to lactones. Cyclooctatriene is detected during the catalysis by GC-MS, suggesting that the true catalyst of the reactions is a dihydride IrH₂(2)-species with osmaligand 2 acting as N,N,N-pincer. The presence of a phenyl group in the substrates favors the catalytic processes. The dehydrogenative homocoupling of primary alcohols to esters appears to take place via the transitory formation of hemiacetals.

Full text of DOI:10.1021/acs.organomet.8b00380

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Base-Free and Acceptorless Dehydrogenation of Alcohols Catalyzed
by an Iridium Complex Stabilized by a N,N,N‑Osmaligand
́
Roberto G. Alabau, Miguel A. Esteruelas,* Antonio Martínez, Montserrat Olivan,
̃
and Enrique Onate
́
́
́
́
Departamento de Química Inorganica − Instituto de Síntesis Química y Catalisis Homogenea (ISQCH) − Centro de Innovacion en
Química Avanzada (ORFEO−CINQA), Universidad de Zaragoza − CSIC, 50009 Zaragoza, Spain
S
* Supporting Information
ABSTRACT: The preparation of a N,N,N-osmaligand, its
coordination to iridium to afford an efficient catalyst precursor,
and the catalytic activity of the latter in dehydrogenation reac-
tions of hydrogen carriers based on alcohols are reported. Com-
plex OsH2Cl2(PiPr3)2 (1) reacts with 3-(2-pyridyl)pyrazol to
give the osmium(II) complex 2H, which contains an acidic
hydrogen atom. Deprotonation of the latter by the bridging
methoxy groups of the dimer [Ir(μ-OMe)(η4-COD)]2 (COD =
1,5-cyclooctadiene) leads to Ir(2)(η4-COD) (3), where osmaligand
2 has a free-nitrogen atom. Iridium complex 3 catalyzes the
dehydrogenation of secondary and primary alcohols to ketones
and aldehydes or esters, respectively, and the dehydrogenation of diols to lactones. Cyclooctatriene is detected during the
catalysis by GC-MS, suggesting that the true catalyst of the reactions is a dihydride IrH2(2)-species with osmaligand 2 acting as
N,N,N-pincer. The presence of a phenyl group in the substrates favors the catalytic processes. The dehydrogenative
homocoupling of primary alcohols to esters appears to take place via the transitory formation of hemiacetals.
Acceptorless alcohol dehydrogenation is generally an endo-
thermic reaction at room temperature. Nevertheless, the equi-
librium can be driven by removal the generated molecular
hydrogen. High temperatures and strong basic media are often
employed to promote the catalysis.8 The base is in principle able
to deprotonate the alcohol. The resulting alkoxide then binds to
the metal to generate a hydride by β-hydrogen elimination.
Some ligands are described as noninnocent, giving rise to bifunc-
tional complexes which can be engaged in deprotonation-
protonation steps, and avoid the use of additional bases.9
The basic center of the ligands can reside in an atom bonded to
the metal center (first coordination sphere) or in a remote
position. Recent studies of Dub, Gordon, and co-workers10 and
those of Gusev11 call into question the noninnocent character
of these ligands and therefore the bifunctional role of the
catalysts. Their results reveal that the ligands are involved in the
catalysis via the stabilization of some transition states through
hydrogen bonding interactions and not via a reversible proton
transfer.
INTRODUCTION
■
Metalloligands are coordination complexes, including organo-
metallic species, bearing binding groups which can coordinate to
other metal center.1 The presence of the metallic core in the
metalloligand allows a wide variety of dispositions for its donor
atoms. This enlarges the range of geometries that can be stabi-
lized around of the new metal center. Furthermore, as a con-
sequence of the presence of empty and full d orbitals in the
metal, the metalloligands generally possess variable electronic
character, which is transmitted through the donor groups.2 This
property has given rise to significant enhancements in catalytic
activity of some metal centers in notable reactions.3 Ferrocene
compounds containing donor substituents are most probably
the best known ligands of this class. The ferrocene moiety
provides specific geometries and opens the door to the control of
reactivity at the additional metal center due to the possibility of
switching the redox state of the iron center.4
Metalloligands are having a notable influence in modern
organic synthesis by stabilizing a wide range of homogeneous
catalysts for cross-coupling, hydrogenation, allylic substitution,
hydroformylation, and cyclopropanation, among other reactions
of relevance.5 However, the metalloligand strategy has been
scarcely applied to the acceptorless alcohol dehydrogenation,6
which is an oxidant-free atom-economical approach for the
transformation of these molecules to carbonyl compounds and
has great interest in connection with the hydrogen production
from biomass and the storage-transport of hydrogen in organic
liquids.7
π-Conjugated azolate ligands are particularly efficient in the
transmission of information between distant metal centers.3,5b,12
Taking into account this background, we have designed a stron-
gly basic tridentate N-donor metalloligand bearing an osmium-
(II) core, based on the 3-(2-pyridyl)pyrazolate moiety, in the
search of a novel class of catalysts for the acceptorless and
Received: June 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00380
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
base-free alcohol dehydrogenation. In this paper, we report the
preparation of a novel osmaligand and its coordination to iridium,
which generates an efficient catalyst for the acceptorless and
base-free dehydrogenation of secondary and primary alcohols as
well as diols.
RESULST AND DISCUSSION
■
Preparation of the Osmaligand and Its Coordination
to Iridium: Synthesis of the Catalytic Precursor. Complex
OsH2Cl2(PiPr2)2 (1) has been one of the cornerstones in the
development of the modern osmium chemistry due to its ability
to act as starting compound in the preparation of a wide variety
of complexes.13 In the light of these precedents, we decided to
use it in order to prepare our osmaligand. Treatment of toluene
solutions of 1 with 3.3 equiv of 3-(2-pyridyl)pyrazol (3-py-pzH),
in the presence of (piperidinomethyl)polystyrene, for 5.5 h
under reflux lead to 2H, which was isolated as a dark green solid
in 52% yield according to
Figure 1. Molecular diagram of complex 3 (50% probability ellipsoids).
All hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (deg): Os−P(1) = 2.326(3), Os−N(2) = 2.132(8),
Os−N(4) = 2.048(9), Os−N(6) = 2.083(8), Os−N(7) = 2.075(9),
Os−N(9) = 2.157(8), Ir−N(1) = 2.131(8), Ir−N(5) = 2.079(9),
C(25)−C(26) = 1.426(15), C(29)−C(30) = 1.392(15); P(1)−Os−
N(9) = 174.3(2), N(4)−Os−N(6) = 77.9(3), N(7)−Os−N(9) =
77.4(3), N(1)−Ir−N(5) = 89.5(3), N(1)−Ir−M(2) = 175.0, N(5)−
Ir−M(1) = 173.3, M(1)−Ir−M(2) = 86.4.
chelate, chelate-bridge, and bridge. The phosphine ligand is
disposed trans to the pyridyl group of the chelate ligand (P(1)−
Os-N(9) = 174.3(2)°), whereas the pz-ring is located trans to
the pz-ring of the chelate-bridge ligand (N(7)−Os-N(4) =
166.5(3)°). The pyridyl group of the latter lies trans to the
bridging ligand (N(6)−Os−N(2) = 164.1(3)°). This geometry
disposes three nitrogen atoms with free electron pairs, the
pyrazolic N(1) and N(5) and pyridinic N(3), which can act as a
pincer. Nevertheless, the metalloligand only coordinates the
pyrazolic N(1) and N(5) atoms to the Ir(η4-COD) moiety
(N(1)−Ir−N(5) = 89.5(3)°) to form a square-planar iridium(I)
species, in agreement with the high stability of the d8-com-
pounds of this geometry. If M(1) is the midpoint of the coor-
dinated C(25)−C(26) double bond and M(2) is the midpoint
of the related C(29)−C(30) bond, then the N(1)−Ir−M(2)
and N(5)−Ir−M(1) angles are 175.0 and 173.3°, respectively.
The 1,5-cyclooctadiene ligand takes its customary “tub” confor-
mation. The coordinated bonds have lengths of 1.426(15)
(C(25)−C(26)) and 1.392(15) (C(29)−C(30)) Å, which are
longer than the C−C double bonds in the free diene (1.34 Å),15
in accordance with the usual Chatt−Dewar−Duncanson model.
In agreement with the lack of symmetry of the molecule, the
1H NMR spectrum in benzene-d6 at room temperature and in
toluene-d8 at −50 °C displays four olefinic C(sp2)−H reso-
nances. Two of them appear together at about 3.4 ppm, while
the other two resonances are observed at about 4.7 and 2.8 ppm.
Consistently, the 13C{1H} NMR spectrum in fluorobenzene at
−30 °C shows four singlets at 69.0, 66.8, 66.2, and 57.6 ppm for
the C(sp2) carbon atoms of the diene. The separation between
the iridium atom and the core of the metalloligand is long,
3.9317(6) Å.
The most noticeable spectroscopic features of this species in
benzene-d6 at room temperature are a broad signal at 17.60 ppm
1
in the H NMR spectrum, corresponding to the NH-nitrogen
atom, and a singlet at −2.2 ppm, in the 31P{1H} NMR spectrum,
due to the coordinated phosphine. In contrast to 1, the
d2-hexahydride complex OsH6(PiPr3)2 reacts with 3-py-pzH to
give the osmium(IV)-trihydride derivative OsH3{κ2-N,N-(3-py-
pz)}(PiPr3)2.14
Complex 2H can be deprotonated by the bridging methoxy
groups of the dimer [Ir(μ-OMe)(η4-COD)]2 (COD = 1,5-
cyclooctadiene). The deprotonation produces the coordination
of two free nitrogen atoms of the metalloligand to iridium. Thus,
the addition of 0.5 equiv of this dimer to acetone solutions of 2H
affords heterobimetallic derivative 3, which was isolated as a
dark brown solid in 78% yield according to
Dehydrogenation of Secondary and Primary Alcohols
and Diols. Complex 3 is a catalyst precursor for the base-free
and acceptorless dehydrogenation of the above-mentioned
hydrogen donors. As expected, the rate of the reaction increases
as the precursor concentration increases (see Figure S1), reach-
ing an optimal equilibrium between conversion and time
required when the catalyst loading was 5% mol. Thus, the
Complex 3 was characterized by X-ray diffraction analysis.
Figure 1 shows a view of the molecule. The structure proves the
formation of the osmaligand and its coordination to the Ir(η4-
COD) moiety. The metalloligand can be rationalized as an octa-
hedral osmium(II) complex containing a triisopropylphosphine
ligand and three different 3-(2-pyridyl)pyrazolate groups:
B
DOI: 10.1021/acs.organomet.8b00380
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
reactions were performed in toluene at 100 °C, using a substrate
concentration of 0.19 M and 5% mol iridium complex.
Table 1 collects the studied secondary alcohols and the yields
of their dehydrogenation reactions to the corresponding
ketones, after 18 h:
metalloligand 2 is more efficient than the 3-(2-pyridyl)-
pyrazolate ligand from a catalytic point of view. Chloride and
methyl substituents at the para position have little influence in
the amount of formed ketone (runs 2 and 3), while methoxy
clearly favors the dehydrogenation. Jones and co-workers have
conversely observed that in the presence of nickel(II) precursors
supported by tris(3,5-dimethylpyrazolyl)borate and 2-hydrox-
yquinolines para-substituted 1-phenylethanol alcohols bearing
electron-donating groups react much more slowly than those
with electron-withdrawing substituents.17 In the presence of 3,
the substituent of the phenyl group influences the dehydrogen-
ation when is close to the alcohol function, decreasing the
amount of formed ketone. The same trend has been observed for
the dehydrogenations of methyl-substituted 1-phenylethanol
Table 1. Acceptorless and Base-Free Dehydrogenation of
a
Secondary Alcohols Catalyzed by 3
alcohols catalyzed by the osmium precursor OsH3{κ2-Npy,Nimine
-
(BMPI)}(PiPr3)2 (BMPI = 1,3-bis(6′-methyl-2′-pyridylimino)-
isoindolinate).18 The increase of the steric hindrance in the
vicinity of the OH group certainly hinders the dehydrogenation.
Thus, diphenylmethanol (run 7) is dehydrogenated with the
same efficiency (61%) as 1-(o-tolyl)ethanol (63%; run 6), i.e.,
worse than 1-phenylethanol (78%). For the trihydride-Os-
BMPI precursor, however, the adverse impact resulting from the
increase of the steric hindrance in the proximity of the functional
group is compensated for the higher stability of benzophenone
with regard to acetophenone. Thus, in contrast to 3, it oxidizes
diphenylmethanol better than 1-phenylethanol (77% versus
60%).18 Alcohols with two alkyl groups, such as 2-octanol and
cyclohexylethanol (runs 8 and 9) are dehydrogenated worse
than diphenylmethanol. 2-Octanone and cyclohexyl methyl
ketone are obtained in 40 and 50% yield, respectively. Complex
3 also dehydrogenates functionalized alcohols. 1-(4-Pyridyl)-
ethanol (run 10), 1-(3-pyridyl)ethanol (run 11), and 1-(2-
furyl)ethanol (run 12) are transformed into the corresponding
ketones and molecular hydrogen in 31, 45, and 56% yield,
respectively. The increase in the amount of produced hydrogen
as the heteroatom of the heterocycle approaches to the alco-
hol function agrees well with that observed for the iron pre-
cursor Fe(η5-C5H5)Cl(CO)2 in the presence of NaH; while
2-pyridylmethanol derivatives are efficiently dehydrogenated,
4-pyridylmethanol and 3-pyridylmethanol do not undergo
oxidation under the same conditions.8e
Dehydrogenation of primary alcohols is more challenging
than the dehydrogenation of secondary alcohols, since the alde-
hydes resulting from the former can generate inactive carbonyl
complexes.19 Alternatively to aldehydes, these alcohols form
esters as a consequence of a dehydrogenative homocoupling.20
Complex 3 promotes both processes: the dehydrogenation to
aldehydes and the dehydrogenative homocoupling to yield
esters, shown in eqs 4 and 5, respectively:
a
Conditions: Complex 3 (0.0093 mmol); substrate (0.19 mmol);
b
toluene (1 mL); heated at 100 °C for 18 h. Conversions were calcu-
lated from the relative peak area integrations of the reactant and
product in the GC spectra.
1-Phenylethanol (run 1) and related alcohols with a substituent
at the phenyl groups (runs 2−6) are efficiently dehydrogenated
into the ketones with yields between 63 and 95%. Under the
same conditions, the iridium complex Ir{κ2-N,N-[3-py-pz]}
(η4-COD)16 affords 15% of acetophenone, demonstrating that
The reactions were performed under the same conditions as
those used for promoting the dehydrogenation of secondary
alcohols. Table 2 collects the studied alcohols, the percentage of
alcohol dehydrogenated after 18 h, the formed oxidized pro-
ducts, and the percentages of alcohol transformed into each one
C
DOI: 10.1021/acs.organomet.8b00380
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 2. Acceptorless and Base-Free Dehydrogenation of Primary Alcohols Catalyzed by 3
a
b
Conditions: Complex 3 (0.0093 mmol); substrate (0.19 mmol); toluene (1 mL); heated at 100 °C for 18 h. Calculated by 1H NMR
spectroscopy.
of them, i.e., the percentage of molecular hydrogen generated
according to eqs 4 and 5. Benzyl alcohol (run 1) and related
alcohols with a substituent at the phenyl group (runs 2−7) are
efficiently dehydrogenated in 86−97%. The products resulting
from the oxidation show a marked dependence upon the sub-
stituents. Electron-donating groups at para position favor the
dehydrogenation according to eq 4 and therefore the formation
of aldehydes. However, electron-withdrawing substituents
benefit the generation of esters, i.e., the dehydrogenation
through eq 5. There is a decreasing linear relationship between
the percentage of alcohol transformed into aldehyde according
to eq 4 and the Hammett σp values of the substituent,21 which is
consistent with the increasing linear relationship between the
percentage of alcohol transformed into ester according to eq 4
and the above-mentioned values (Figure 2).
The position of the substituent mainly influences the amount
of generated molecular hydrogen (runs 5−7), decreasing as the
substituent approaches to the alcohol function, i.e, p ≈ m > o
(runs 5 ≈ 6 > 7). Cyclohexylmethanol (run 8) and 1-octanol
(run 9) are also dehydrogenated, although in lower extension
(26 and 45%, respectively). In these cases, the corresponding
ester is the main oxidation product. In contrast, 2-furanylmethanol
(run 10) and 3-pyridylmethanol (run 11) are dehydrogenated in
42 and 36%, respectively, mainly generating the aldehyde.
Complex 3 also dehydrogenates diols to lactones. The pres-
ence of a phenyl group in the substrate favors the process as has
D
DOI: 10.1021/acs.organomet.8b00380
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 1. Proposed Catalytic Cycle for the Dehydrogenation
of Secondary Alcohols
■
Figure 2. Percentages of alcohol transformed into aldehyde (green
)
●
and ester (red ) as a function of the Hammett σp values for the
dehydrogenation of p-substituted benzyl alcohols.
been shown for secondary and primary alcohols in Tables 1 and 2.
Thus, 1,2-phenylenedimethanol gives 1-isobenzofuranone and
molecular hydrogen in quantitative yield after 18 h, while 1,5-
pentanediol affords tetrahydro-2H-pyran-2-one in 35% yield
under the same conditions and the same time, shown in eqs 6
and 7, respectively:
coordinated hydrogen molecule should lead to hydride-alkoxide C.
Then, this intermediate could undergo a β-hydrogen elimination
reaction of the alkoxide group to afford the corresponding
ketone or aldehyde and to regenerate A.
Esters, as alternative products to aldehydes in the dehydro-
genation of primary alcohols, seem to be a result of the transitory
formation of hemiacetals. These compounds can be generated
by means of the reaction of intermediate B with a molecule of
aldehyde previously formed, according the following:
This class of reaction is of particular interest, since function-
alized lactones are ubiquitous frameworks in a variety of bio-
logically active natural products. Several catalysts have been
developed for the reaction in the presence of hydrogen
acceptors.20a,22 Catalysts working in the presence of bases are
also known,23 while efficient systems under base-free and accep-
torless conditions are more rare.20b,24
Cyclooctatriene is detected during the catalysis by GC-MS.
This suggests the formation of a dihydride-iridium(III) species
stabilized by a pincer-N,N,N-osmaligand. Pincer complexes
active in acceptorless dehydrogenation of alcohols are known.25
This (N,N,N)IrH2 species (A) should be the result of the
dehydrogenation of the coordinated diene of 3 and of the
coordination of the free nitrogen atom to the iridium center
(Scheme 1) and can be viewed as a N,N,N-counterpart of the
known dihydride (PCP)IrH2 complexes, which promote the
acceptorless dehydrogenation of alkanes.26 Complex A would
promote the dehydrogenation of the alcohols through the cycle
shown in Scheme 1. The addition of the O−H bond of the
alcohols to one of the Ir−H bonds of A should give hydride-
dihydrogen intermediate B containing an alkoxy ligand. The
addition could involve only one alcohol molecule or a dimer
formed via O−H···O hydrogen bond. Previous relevant DFT
mechanistic studies have revealed that proton shuttle type
transition states, involving a molecule of alcohol as a proton
bridge, significantly decrease the activation energy of the O−H
rupture.27 Once complex B is formed, the dissociation of the
The carbon atom of the carbonyl group of the aldehyde removes
the coordinated alkoxide group, whereas the oxygen atom
abstracts the proton of the dihydrogen ligand. In agreement with
this, it should be mentioned that consistent manner with Figure 2
E
DOI: 10.1021/acs.organomet.8b00380
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Reaction of 2H with [Ir(μ-OMe)(η4-COD)]2: Preparation of 3. A
solution of 2H (140 mg, 0.18 mmol) in acetone (5 mL) was treated
with [Ir(μ-OMe)(η4-COD)]2 (59 mg, 0.09 mmol), and the resulting
mixture was stirred at room temperature for 18 h. After this time, the
dark brown solution was evaporated to dryness to afford a dark-brown
residue. Methanol was added to afford a dark-brown solid, which was
washed with methanol (2 × 1 mL) and dried in vacuo. Yield: 150 mg
(78%). Anal. Calcd for C41H51IrN9OsP: C, 45.46; H, 4.74; N, 11.63.
Found: C, 45.24; H, 4.43; N, 11.92. HRMS (electrospray, m/z) calcd
for C41H51IrN9OsP [M]+: 1085.3248. Found: 1085.3276. IR (cm−1):
ν(CN), ν(CC) 1603 (m), 1586 (m). 1H NMR (300 MHz, C6D6,
the natural bond orbital charge (b3lyp(GD3)6-311G**) on the
carbon atom of the carbonyl group of the aldehyde increases as
the Hammett σp value also increases (see Table S1) and that the
protic character of the coordinated hydrogen molecule is a well-
known property of these ligands.28 Once the hemiacetal and
A are formed, the latter can dehydrogenate to the ester as to any
other alcohol.
CONCLUDING REMARKS
■
This study has revealed that the reaction of the known complex
OsH2Cl2(PiPr3)2 with 3-(2-pyridyl)pyrazol gives rise to an
osmium(II) compound, which can act as a N,N-bidentate or
N,N,N-pincer osmaligand by deprotonation. Thus, its coordi-
nation to iridium has allowed us to stabilize an efficient catalyst
precursor for base-free and acceptorless dehydrogenation of
secondary and primary alcohols and diols, which leads to
ketones, aldehydes or esters, and lactones, respectively. The
presence of a phenyl group in the substrates favors the reactions.
The products of the dehydrogenation of benzyl alcohols bearing
a substituent at the phenyl group shows a marked dependence
upon the Hammett σp value of the substituent, which is con-
sistent with the transitory formation of hemiacetals in the
dehydrogenative homocoupling to generate esters.
298 K): δ 8.73 (d, JH−H = 7.8, 1H, py), 8.52 (m, 1H, pz), 8.32 (d, JH−H
=
1.9, 1H, pz), 7.90 (d, JH−H = 6, 1H, py), 7.75 (d, JH−H = 2.7, 1H, pz),
7.34 (td, JH−H = 6.0, JH−H = 1.9, 1H, py), 7.05 (d, JH−H = 2.1, 1H, pz),
6.85−6.63 (m, 5H, 2H pz and 3H py), 6.56 (d, JH−H = 1.8, 1H, py), 6.49
(d, JH−H = 6, 1H, py), 6.40−6.26 (m, 2H, py), 5.85−5.73 (m, 2H, py),
4.67 (br, 1H, CH COD), 3.71 (m, 3H, PCH(CH3)2), 3.5 (br, 2H, 
CH COD), 2.80 (br, 1H, CH COD), 2.28 (m, 3H, CH2 COD), 1.95
(br, 2H, CH2 COD), 1.46 (m, 3H, CH2 COD), 1.30 (dd, JH−H = 7.2,
J
H−P = 12.0, 9H, PCH(CH3)2), 1.21 (dd, JH−H = 7.2, JH−P = 11.7, 9H,
1
PCH(CH3)2). H NMR (300 MHz, C7D8, 223 K, olefinic protons):
δ 4.7 (br, 1H, CH), 3.4 (br, 2H, CH), 2.8 (br, 1H, CH).
13C{1H} NMR (75.47 MHz, C6D6, 298 K): δ 160.9 (s, C), 157.2 (s, C),
156.9 (s, C), 154.7 (s, CH py), 154.6 (s, C), 153.0 (s, C), 150.3 (s, C),
149.5, 149.0, 146.8, 140.1, 136.0, 135.1, 134.7, 130.4, 122.8, 120.9,
120.4, 118.1, 118.1, 116.5, 108.5, 105.3, 104.7 (all s, CH py and pz),
32.9 (s, CH2, COD), 29.4 (s, CH2, COD), 27.5 (d, JC−P = 23.4,
PCH(CH3)2), 20.9, 20.1 (both s, PCH(CH3)2). 31P{1H} NMR (121.49
MHz, C6D6, 298 K): δ −4.0.
In conclusion, a novel osmaligand, which allows to stabilize
catalysts for base-free and acceptorless dehydrogenation of
alcohols and diols has been discovered.
General Procedure for the Catalytic Dehydrogenations.
A solution of complex 3 (0.0093 mmol) and the corresponding alcohol
(0.19 mmol) in toluene (1 mL) was placed in a Schlenk flask equipped
with a condenser and an oil bubbler. The reaction mixture was heated at
100 °C for 18 h. After this time the solution was cooled to room
temperature, and the extent of the reaction was determined by different
methods depending on the nature of the alcohol. Reactions were run in
duplicate.
For secondary alcohols, the extent of the conversion to ketones was
determined by CG on an Agilent Technologies 6890N gas chromato-
graph with a flame ionization detector, using a HP-Innowax column
(25 m × 0.2 mm, with 0.4 μm film thickness). The oven conditions used
are as follows: 80 °C (hold 5 min) to 200 °C at 15 °C/min (hold 7 min),
except for diphenylmethanol (150 °C (hold 5 min) to 240 °C at
15 °C/min (hold 13 min)). The identity of the ketones was confirmed
by comparison of their retention times with those of pure products, as
well as by GC-MS analyses. In the case of primary alcohols and diols,
the crude solution was evaporated under reduced pressure to obtain an
oil. Then, 0.187 mmol (20 μL) of 1,1,2,2-tetrachloroethane was added
as an internal standard, and the mixture was dissolved in CDCl3 and
analyzed by 1H NMR spectroscopy. The formed aldehydes, esters, and
lactones were characterized by 1H NMR spectroscopy.
EXPERIMENTAL SECTION
■
General Information. The reactions were carried out under
moisture- and oxygen-free atmosphere using dried solvents and
Schlenk-tube techniques. Instrumental methods, X-ray information,
and DFT computational details are given in the Supporting
Information. In the NMR spectra, chemical shifts (expressed in parts
per million) are referenced to residual solvent peaks (1H, 13C{1H}) or
external H3PO4 (31P) and coupling constants (J) are given in hertz.
29
30
OsH2Cl2(PiPr3)2 and [Ir(μ-OMe)(η4-COD)]2 were prepared by
previously published methods.
Reaction of OsCl2H2(PiPr3)2 (1) with 3-(2-Pyridyl)pyrazol:
Preparation of 2H. A solution of 1 (200 mg, 0.343 mmol) in toluene
(10 mL) was treated with 3-(2-pyridyl)pyrazol (164.2 mg, 1.131 mmol)
and (piperidinomethyl)polystyrene (323 mg, 1.131 mmol). The
resulting mixture was refluxed for 5.5 h, resulting in a dark green
solution. After cooling at room temperature, the solution was filtered
evaporated to dryness, affording a dark green residue. Further puri-
fication was carried out by silica gel (230−400 mesh, eluted previously
with a 20% solution of NEt3 in pentane) column chromatography using
a 1:1 mixture of pentane and CH2Cl2, followed by precipitation in
pentane, giving a dark green solid. Yield: 140 mg (52%). Anal. Calcd for
C33H40N9OsP: C, 50.56; H, 5.14; N, 16.08. Found: C, 50.30; H, 4.86;
N, 16.37. HRMS (electrospray, m/z) calcd for C33H39N9NaOsP [M −
ASSOCIATED CONTENT
* Supporting Information
■
S
1
H + Na]+: 807.2574. Found: 807.2576. H NMR (300 MHz, C6D6,
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00380.
298 K): δ 17.60 (br, 1H, N−H), 8.47 (d, JH−H = 4.1, 1H, py), 8.41
(d, JH−H = 2.1, 1H, pz), 8.35 (d, JH−H = 2.1, 1H, pz), 7.97 (d, JH−H = 5.7,
1H, py), 7.13 (d, JH−H = 1.8, 1H, pz), 7.00−6.89 (m, 5H, 1H pz and 4H
py), 6.83 (d, JH−H = 1.8, 1H, pz), 6.72 (d, JH−H = 5.6, 1H, py), 6.44
(dd, JH−H = 7.7, JH−H = 0.8, 1H, py), 6.37 (t, JH−H = 7.7, 1H, py), 6.29
(t, JH−H = 7.7, 1H, py), 6.17 (d, JH−H = 2.1, 1H, pz), 5.80 (m, 1H, py),
5.61 (1H, JH−H = 6.5, 1H, py), 2.83 (m, 3H, PCH(CH3)2), 1.11 (dd,
General information, crystallographic data, computa-
tional details, and NMR spectra (PDF)
Cartesian coordinates of the optimized structures (XYZ)
Accession Codes
J
H−H = 6.9, JH−P = 7.5, 9H, PCH(CH3)2), 1.07 (dd, JH−H = 7.2, JH−P =
CCDC 1845698 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 Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
7.5, 9H, PCH(CH3)2). 13C{1H} NMR (75.47 MHz, C6D6, 298 K):
δ 161.8 (s, C), 158.2 (s, C), 154.4 (s, CH py), 151.6 (s, C), 150.6 (s, C),
150.3 (s, CH py), 148.3 (s, C), 147.4, 144.5, 144.0 (s, C), 140.3, 137.8,
136.3, 135.0, 132.3, 122.2, 120.2, 119.9, 119.5, 117.3, 109.9, 104.3
(all s, CH py and pz), 26.5 (d, JC−P = 24.1, PCH(CH3)2), 20.0
(s, PCH(CH3)2). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ −2.2.
F
DOI: 10.1021/acs.organomet.8b00380
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Catalyzed Hydrogenation and Dehydrogenation Reactions. Acc. Chem.
Res. 2015, 48, 363−379.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: maester@unizar.es.
ORCID
Roberto G. Alabau: 0000-0001-7168-8859
Miguel A. Esteruelas: 0000-0002-4829-7590
(7) (a) Friedrich, A.; Schneider, S. Acceptorless Dehydrogenation of
Alcohols: Perspectives for Synthesis and H2 Storage. ChemCatChem
2009, 1, 72−73. (b) Johnson, T. C.; Morris, D. J.; Wills, M. Hydrogen
generation from formic acid and alcohols using homogeneous catalysts.
Chem. Soc. Rev. 2010, 39, 81−88. (c) Gunanathan, C.; Milstein, D.
Applications of acceptorless dehydrogenation and related trans-
formations in chemical synthesis. Science 2013, 341, 1229712.
́
Montserrat Olivan: 0000-0003-0381-0917
ate: 0000-0003-2094-719X
(d) Trincado, M.; Banerjee, D.; Grutzmacher, J. Molecular catalysts
̈
Enrique On
̃
for hydrogen production from alcohols. Energy Environ. Sci. 2014, 7,
2464−2503. (e) Nielsen, M. Hydrogen Production by Homogeneous
Catalysis: Alcohol Acceptorless Dehydrogenation. In Hydrogen
Production and Remediation of Carbon and Pollutants; Lichtfouse, E.,
Schwarzbauer, J., Robert, D., Eds.; Springer: Switzerland, 2015;
Chapter 1, pp 1−60.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the MINECO of Spain (Projects
CTQ2017-82935-P and Red de Excelencia Consolider
(8) (a) Zhang, J.; Gandelman, M.; Shimon, L. J. W.; Rozenberg, H.;
Milstein, D. Electron-Rich, Bulky Ruthenium PNP-Type Complexes.
Acceptorless Catalytic Alcohol Dehydrogenation. Organometallics
2004, 23, 4026−4033. (b) Zhang, J.; Leitus, G.; Ben-David, Y.;
Milstein, D. Facile Conversion of Alcohols into Esters and Dihydrogen
Catalyzed by New Ruthenium Complexes. J. Am. Chem. Soc. 2005, 127,
10840−10841. (c) Musa, S.; Shaposhnikov, I.; Cohen, S.; Gelman, D.
Ligand−Metal Cooperation in PCP Pincer Complexes: Rational
Design and Catalytic Activity in Acceptorless Dehydrogenation of
Alcohols. Angew. Chem., Int. Ed. 2011, 50, 3533−3537. (d) Nielsen, M.;
Kammer, A.; Cozzula, D.; Junge, H.; Gladiali, S.; Beller, M. Efficient
Hydrogen Production from Alcohols under Mild Reaction Conditions.
Angew. Chem., Int. Ed. 2011, 50, 9593−9597. (e) Kamitani, M.; Ito, M.;
Itazaki, M.; Nakazawa, H. Effective dehydrogenation of 2-pyridylme-
thanol derivatives catalyzed by an iron complex. Chem. Commun. 2014,
50, 7941−7944. (f) Song, H.; Kang, B.; Hong, S. H. Fe-Catalyzed
Acceptorless Dehydrogenation of Secondary Benzylic Alcohols. ACS
Catal. 2014, 4, 2889−2895. (g) Sengupta, D.; Bhattacharjee, R.;
Pramanick, R.; Rath, S. P.; Chowdhury, N. S.; Datta, A.; Goswami, S.
Exclusively Ligand-Mediated Catalytic Dehydrogenation of Alcohols.
Inorg. Chem. 2016, 55, 9602−9610. (h) Dutta, I.; Sarbajna, A.; Pandey,
P.; Rahaman, S. M. W.; Singh, K.; Bera, J. K. Acceptorless
Dehydrogenation of Alcohols on a Diruthenium(II,II) Platform.
Organometallics 2016, 35, 1505−1513. (i) Toyomura, K.; Fujita, K.
Synthesis of Coordinatively Unsaturated Iridium Complexes Having
Functional 8-Quinolinolato Ligands: New Catalysts for Dehydrogen-
ative Oxidation of Alcohols in Aqueous Media. Chem. Lett. 2017, 46,
808−810. (j) Wang, Z.; Pan, B.; Liu, Q.; Yue, E.; Solan, G. A.; Ma, Y.;
Sun, W.-H. Efficient acceptorless dehydrogenation of secondary
alcohols to ketones mediated by a PNN-Ru(II) catalyst. Catal. Sci.
Technol. 2017, 7, 1654−1661. (k) Wang, Q.; Chai, H.; Yu, Z. Dimeric
Ruthenium(II)-NNN Complex Catalysts Bearing a Pyrazolyl-Pyridy-
lamino-Pyridine Ligand for Transfer Hydrogenation of Ketones and
Acceptorless Dehydrogenation of Alcohols. Organometallics 2017, 36,
3638−3644. (l) Sarbajna, A.; Dutta, I.; Daw, P.; Dinda, S.; Rahaman, S.
M. W.; Sarkar, A.; Bera, J. K. Catalytic Conversion of Alcohols to
Carboxylic Acid Salts and Hydrogen with Alkaline Water. ACS Catal.
2017, 7, 2786−2790. (m) Huang, R.; Yang, Y.; Wang, D.-S.; Zhang, L.;
Wang, D. Where does Au coordinate to N-(2-pyridiyl)benzotriazole:
gold-catalyzed chemoselective dehydrogenation and borrowing hydro-
gen reactions. Org. Chem. Front. 2018, 5, 203−209.
́
́
CTQ2016-81797-REDC), the Diputacion General de Aragon
(E06_17R), FEDER, and the European Social Fund is
acknowledged.
REFERENCES
■
(1) (a) Kumar, G.; Gupta, R. Molecularly designed architectures − the
metalloligand way. Chem. Soc. Rev. 2013, 42, 9403−9453. (b) Liu, J.;
Chen, L.; Cui, H.; Zhang, J.; Zhang, L.; Su, C.-Y. Applications of metal−
organic frameworks in heterogeneous supramolecular catalysis. Chem.
Soc. Rev. 2014, 43, 6011−6061. (c) Gil-Rubio, J.; Vicente, J. The
Coordination and Supramolecular Chemistry of Gold Metalloligands.
Chem. - Eur. J. 2018, 24, 32−46.
(2) Bosnich, B. Cooperative Bimetallic Redox Reactivity. Inorg. Chem.
1999, 38, 2554−2562.
(3) (a) Esteruelas, M. A.; Garcia, M. P.; Lopez, A. M.; Oro, L. A.
́
Indirect cooperative effects leading to synergism in bimetallic
homogeneous catalysts containing azolates as bridging ligands.
Organometallics 1991, 10, 127−133. (b) Esteruelas, M. A.; Garcia, M.
́
P.; Lopez, A. M.; Oro, L. A. Hydrogen-transfer catalytic synergism in
binuclear complexes containing 2,2′-biimidazolate as a bridging ligand.
Organometallics 1992, 11, 702−705. (c) Hetterscheid, D. G. H.;
Chikkali, S. H.; de Bruin, B.; Reek, J. N. H. Binuclear Cooperative
Catalysts for the Hydrogenation and Hydroformylation of Olefins.
ChemCatChem 2013, 5, 2785−2793.
(4) See, for example, (a) Colacot, T. J. A Concise Update on the
Applications of Chiral Ferrocenyl Phosphines in Homogeneous
Catalysis Leading to Organic Synthesis. Chem. Rev. 2003, 103, 3101−
3118. (b) Barbaro, P.; Bianchini, C.; Giambastiani, G.; Parisel, S. L.
Progress in stereoselective catalysis by metal complexes with chiral
ferrocenyl phosphines. Coord. Chem. Rev. 2004, 248, 2131−2150.
(c) Fihri, A.; Meunier, P.; Hierso, J.-C. Performances of symmetrical
achiral ferrocenylphosphine ligands in palladium-catalyzed cross-
coupling reactions: A review of syntheses, catalytic applications and
structural properties. Coord. Chem. Rev. 2007, 251, 2017−2055.
(d) Toma, S.; Csizmadiova, J.; Meciarova, M.; Sebesta, R. Ferrocene
phosphane-heteroatom/carbon bidentate ligands in asymmetric
catalysis. Dalton Trans. 2014, 43, 16557−16579. (e) Stepnicka, P.
Coordination and catalytic chemistry of phosphinoferrocene carbox-
amides. Coord. Chem. Rev. 2017, 353, 223−246.
(5) See, for example, (a) Atkinson, R. C. J.; Gibson, V. C.; Long, N. J.
The syntheses and catalytic applications of unsymmetrical ferrocene
ligands. Chem. Soc. Rev. 2004, 33, 313−328. (b) Buchwalter, P.; Rose,
J.; Braunstein, P. Multimetallic Catalysis Based on Heterometallic
Complexes and Clusters. Chem. Rev. 2015, 115, 28−126.
(6) (a) 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. (b) Chelucci, G.; Baldino, S.; Baratta, W. Ruthenium and
osmium complexes containing 2-(aminomethyl)pyridine (Ampy)-
based ligands in catalysis. Coord. Chem. Rev. 2015, 300, 29−85.
(c) Chelucci, G.; Baldino, S.; Baratta, W. Recent Advances in Osmium-
̌
̌
́
̌
́
̌
̌
̌
(9) (a) Kawahara, R.; Fujita, K.; Yamaguchi, R. Dehydrogenative
Oxidation of Alcohols in Aqueous Media Using Water-Soluble and
Reusable Cp*Ir Catalysts Bearing a Functional Bipyridine Ligand. J.
Am. Chem. Soc. 2012, 134, 3643−3646. (b) Kawahara, R.; Fujita, K.;
Yamaguchi, R. Cooperative Catalysis by Iridium Complexes with a
Bipyridonate Ligand: Versatile Dehydrogenative Oxidation of Alcohols
and Reversible Dehydrogenation−Hydrogenation between 2-Propanol
and Acetone. Angew. Chem., Int. Ed. 2012, 51, 12790−12794.
́
̈
(c) Chakraborty, S.; Lagaditis, P. O.; Forster, M.; Bielinski, E. A.;
Hazari, N.; Holthausen, M. C.; Jones, W. D.; Schneider, S. Well-
Defined Iron Catalysts for the Acceptorless Reversible Dehydrogen-
ation-Hydrogenation of Alcohols and Ketones. ACS Catal. 2014, 4,
G
DOI: 10.1021/acs.organomet.8b00380
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3994−4003. (d) Zeng, G.; Sakaki, S.; Fujita, K.; Sano, H.; Yamaguchi,
R. Efficient Catalyst for Acceptorless Alcohol Dehydrogenation:
Interplay of Theoretical and Experimental Studies. ACS Catal. 2014,
4, 1010−1020. (e) Bonitatibus, P. J., Jr.; Chakraborty, S.; Doherty, M.
D.; Siclovan, O.; Jones, W. D.; Soloveichik, G. L. Reversible catalytic
dehydrogenation of alcohols for energy storage. Proc. Natl. Acad. Sci. U.
Amines and for the Dehydrogenation of Cyclic Amines. Organometallics
2017, 36, 2996−3004.
́
(14) Alabau, R. G.; Esteruelas, M. A.; Olivan, M.; Onate, E.
̃
Preparation of Phosphorescent Osmium(IV) Complexes with N,N′,C
and C,N,C′-Pincer Ligands. Organometallics 2017, 36, 1848−1859.
(15) Churchill, M. W.; Bezman, S. A. X-Ray Crystallographic Studies
on Fluxional Pentacoordinate Transition Metal Complexes. III.
(Cycloocta-1,5-diene)[1,3-bis(diphenylphosphino)propane]-
methyliridium(I). Inorg. Chem. 1973, 12, 531−536.
S. A. 2015, 112, 1687−1692. (f) Valencia, M.; Muller-Bunz, H.;
̈
Gossage, R. A.; Albrecht, M. Enhanced product selectivity promoted by
remote metal coordination in acceptor-free alcohol dehydrogenation
catalysis. Chem. Commun. 2016, 52, 3344−3347. (g) Mazloomi, Z.;
(16) Martínez, A. P.; Fabra, M. J.; García, M. P.; Lahoz, F. J.; Oro, L.
A.; Teat, S. J. Mononuclear rhodium and iridium compounds with
pyridyl-pyrazole ligands. Inorg. Chim. Acta 2005, 358, 1635−1644.
(17) Chakraborty, S.; Piszel, P. E.; Brennessel, W. W.; Jones, W. D. A
Single Nickel Catalyst for the Acceptorless Dehydrogenation of
Alcohols and Hydrogenation of Carbonyl Compounds. Organometallics
2015, 34, 5203−5206.
̀
́
Pretorius, R.; Pamies, O.; Albrecht, M.; Dieguez, M. Triazolylidene
Iridium Complexes for Highly Efficient and Versatile Transfer
Hydrogenation of CO, CN, and CC Bonds and for Accept-
orless Alcohol Oxidation. Inorg. Chem. 2017, 56, 11282−11298.
(h) Fujita, K.; Tamura, R.; Tanaka, Y.; Yoshida, M.; Onoda, M.;
Yamaguchi, R. Dehydrogenative Oxidation of Alcohols in Aqueous
Media Catalyzed by a Water-Soluble Dicationic Iridium Complex
Bearing a Functional N−Heterocyclic Carbene Ligand without Using
́
(18) Buil, M. L.; Esteruelas, M. A.; Gay, M. P.; Gomez-Gallego, M.;
Nicasio, A. I.; Onate, E.; Santiago, A.; Sierra, M. A. Osmium Catalysts
̃
for Acceptorless and Base-Free Dehydrogenation of Alcohols and
Amines: Unusual Coordination Modes of a BPI Anion. Organometallics
2018, 37, 603−617.
́
Base. ACS Catal. 2017, 7, 7226−7230. (i) Vivancos, A.; Petronilho, A.;
Cardoso, J.; Muller-Bunz, H.; Albrecht, M. Unveiling the role of
ancillary ligands in acceptorless benzyl alcohol dehydrogenation and
etherification mediated by mesoionic carbene iridium complexes.
̈
́
́
(19) See, for example, (a) Esteruelas, M. A.; Hernandez, Y. A.; Lopez,
́
A. M.; Olivan, M.; Rubio, L. Reactions of a Dihydride-Osmium(IV)
́
Dalton Trans. 2018, 47, 74−82. (j) Gonzalez Miera, G.; Martínez-
Complex with Aldehydes: Influence of the Substituent at the Carbonyl
Castro, E.; Martín-Matute, B. Acceptorless Alcohol Dehydrogenation:
OH vs NH Effect in Bifunctional NHC−Ir(III) Complexes. Organo-
metallics 2018, 37, 636−644.
́
Group. Organometallics 2008, 27, 799−802. (b) Alos, J.; Esteruelas, M.
́
A.; Olivan, M.; Onate, E.; Puylaert, P. C-H Bond Activation Reactions
̃
in Ketones and Aldehydes Promoted by POP-Pincer Osmium and
Ruthenium Complexes. Organometallics 2015, 34, 4908−4921.
(10) (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) 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.
(20) (a) Murahashi, S.-I.; Naota, T.; Ito, K.; Maeda, Y.; Taki, H.
Ruthenium-Catalyzed Oxidative Transformation of Alcohols and
Aldehydes to Esters and Lactones. J. Org. Chem. 1987, 52, 4319−
4327. (b) Zhang, J.; Balaraman, E.; Leitus, G.; Milstein, D. Electron-
Rich PNP- and PNN-Type Ruthenium(II) Hydrido Borohydride
Pincer Complexes. Synthesis, Structure, and Catalytic Dehydrogen-
ation of Alcohols and Hydrogenation of Esters. Organometallics 2011,
30, 5716−5724. (c) Langer, R.; Fuchs, I.; Vogt, M.; Balaraman, E.;
Diskin-Posner, Y.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D.
Stepwise Metal-Ligand Cooperation by a Reversible Aromatization/
Deconjugation Sequence in Ruthenium Complexes with a Tetradentate
Phenanthroline-Based Ligand. Chem. - Eur. J. 2013, 19, 3407−3414.
(11) (a) Gusev, D. G. Dehydrogenative Coupling of Ethanol and Ester
Hydrogenation Catalyzed by Pincer-Type YNP Complexes. ACS Catal.
2016, 6, 6967−6981. (b) Morris, S. A.; Gusev, D. G. Rethinking the
Claisen−Tishchenko Reaction. Angew. Chem., Int. Ed. 2017, 56, 6228−
6231. (c) Gusev, D. G. Rethinking the Dehydrogenative Amide
Synthesis. ACS Catal. 2017, 7, 6656−6662.
́
(d) Nguyen, D. H.; Trivelli, X.; Capet, F.; Swesi, Y.; Favre-Reguillon,
A.; Vanoye, L.; Dumeignil, F.; Gauvin, R. M. Deeper Mechanistic
Insight into Ru Pincer-Mediated Acceptorless Dehydrogenative
Coupling of Alcohols: Exchanges, Intermediates, and Deactivation
Species. ACS Catal. 2018, 8, 4719−4734.
́
(12) (a) Esteruelas, M. A.; Lahoz, F. J.; Lopez, A. M.; Onate, E.; Oro,
̃
L. A.; Ruiz, N.; Sola, E.; Tolosa, J. I. Quantum Mechanical Exchange
Coupling in Trihydridoosmium Complexes Containing Azole Ligands.
Inorg. Chem. 1996, 35, 7811−7817. (b) Mayboroda, A.; Comba, P.;
Pritzkow, H.; Rheinwald, G.; Lang, H.; van Koten, G. Heterotrinuclear
Complexes of the Platinum Group Metals with 2,2-Biimidazole as a
Bridging Ligand. Eur. J. Inorg. Chem. 2003, 2003, 1703−1710.
(21) σp values taken from March, J. Advanced Organic Chemistry, 3rd
ed.; Wiley Interscience, 1985.
(22) See, for example, (a) Lin, Y.; Zhu, X.; Zhou, Y. A convenient
lactonization of diols to γ- and δ-lactones catalysed by transition metal
polyhydrides. J. Organomet. Chem. 1992, 429, 269−274. (b) Suzuki, T.;
Morita, K.; Tsuchida, M.; Hiroi, K. Mild and Chemoselective Synthesis
of Lactones from Diols Using a Novel Metal-Ligand Bifunctional
Catalyst. Org. Lett. 2002, 4, 2361−2363.
(23) (a) Ito, M.; Osaku, A.; Shiibashi, A.; Ikariya, T. An Efficient
Oxidative Lactonization of 1,4-Diols Catalyzed by Cp*Ru(PN)
Complexes. Org. Lett. 2007, 9, 1821−1824. (b) Dutta, I.; Sarbajna,
A.; Pandey, P.; Rahaman, S. M. W.; Singh, K.; Bera, J. K. Acceptorless
Dehydrogenation of Alcohols on a Diruthenium(II,II) Platform.
Organometallics 2016, 35, 1505−1513.
(24) Zhao, J.; Hartwig, J. F. Acceptorless, Neat, Ruthenium-Catalyzed
Dehydrogenative Cyclization of Diols to Lactones. Organometallics
2005, 24, 2441−2446.
(25) See, for example, (a) Nielsen, M.; Kammer, A.; Cozzula, D.;
Junge, H.; Gladiali, S.; Beller, M. Efficient Hydrogen Production from
Alcohols under Mild Reaction Conditions. Angew. Chem., Int. Ed. 2011,
50, 9593−9597. (b) Tseng, K.-N. T.; Kampf, J. W.; Szymczak, N. K.
Mechanism of N,N,N−Amide Ruthenium(II) Hydride Mediated
Acceptorless Alcohol Dehydrogenation: Inner-Sphere β-H Elimination
versus Outer-Sphere Bifunctional Metal-Ligand Cooperativity. ACS
́
(13) See, for example, (a) Esteruelas, M. A.; Fernandez-Alvarez, F. J.;
́
Olivan, M.; On
Formation Promoted by Osmium: 2-Vinylpyridine-Acetylene Cou-
plings. J. Am. Chem. Soc. 2006, 128, 4596−4597. (b) Bolano, T.;
Esteruelas, M. A.; Onate, E. Osmium-carbon multiple bonds:
̃
ate, E. C-H Bond Activation and Subsequent C-C Bond
̃
̃
Reduction and C-C coupling reactions. J. Organomet. Chem. 2011,
696, 3911−3923. (c) Collado, A.; Esteruelas, M. A.; Gulías, M.;
Mascarenas, J. L.; Onate, E. Reactions of an Osmium(IV) Complex
̃ ̃
with Allenedienes: Coordination and Intramolecular Cycloadditions.
Organometallics 2012, 31, 4450−4458. (d) Esteruelas, M. A.;
́
́
Fernandez, I.; Lopez, A. M.; Mora, M.; Onate, E. Preparation,
̃
Structure, Bonding, and Preliminary Reactivity of a Six-Coordinate d4
Osmium−Boryl Complex. Organometallics 2012, 31, 4646−4649.
(e) Casanova, N.; Esteruelas, M. A.; Gulías, M.; Larramona, C.;
Mascarenas, J. L.; Onate, E. Amide-Directed Formation of Five-
̃ ̃
Coordinate Osmium Alkylidenes from Alkynes. Organometallics 2016,
́
́
35, 91−99. (f) Esteruelas, M. A.; Lezaun, V.; Martínez, A.; Olivan, M.;
Onate, E. Osmium Hydride Acetylacetonate Complexes and Their
Application in Acceptorless Dehydrogenative Coupling of Alcohols and
̃
H
DOI: 10.1021/acs.organomet.8b00380
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Catal. 2015, 5, 5468−5485. (c) de Boer, S. Y.; Korstanje, T. J.; La Rooij,
S. R.; Kox, R.; Reek, J. N. H.; van der Vlugt, J. I. Ruthenium PNN(O)
Complexes: Cooperative Reactivity and Application as Catalysts for
Acceptorless Dehydrogenative Coupling Reactions. Organometallics
2017, 36, 1541−1549. (d) Wang, Q.; Chai, H.; Yu, Z. Acceptorless
Dehydrogenation of N−Heterocycles and Secondary Alcohols by
Ru(II)-NNC Complexes Bearing a Pyrazoyl-indolylpyridine Ligand.
Organometallics 2018, 37, 584−591.
(26) (a) Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S.
Dehydrogenation and Related Reactions Catalyzed by Iridium Pincer
Complexes. Chem. Rev. 2011, 111, 1761−1779. (b) Haibach, M. C.;
Kundu, S.; Brookhart, M.; Goldman, A. S. Alkane Metathesis by
Tandem Alkane-Dehydrogenation Olefin-Metathesis Catalysis and
Related Chemistry. Acc. Chem. Res. 2012, 45, 947−958. (c) Esteruelas,
́
́
M. A.; Lopez, A. M.; Olivan, M. Polyhydrides of Platinum Group
Metals: Nonclassical Interactions and σ−Bond Activation Reactions.
Chem. Rev. 2016, 116, 8770−8847. (d) Kumar, A.; Bhatti, T. M.;
Goldman, A. S. Dehydrogenation of Alkanes and Aliphatic Groups by
Pincer-Ligated Metal Complexes. Chem. Rev. 2017, 117, 12357−12384.
(27) (a) Qu, S.; Dang, Y.; Song, C.; Wen, M.; Huang, K.-W.; Wang, Z.-
X. Catalytic Mechanisms of Direct Pyrrole Synthesis via Dehydrogen-
ative Coupling Mediated by PNP-Ir or PNN-Ru Pincer Complexes:
Crucial Role of Proton-Transfer Shuttles in the PNP-Ir System. J. Am.
Chem. Soc. 2014, 136, 4974−4991. (b) Hou, C.; Zhang, Z.; Zhao, C.;
Ke, Z. DFT Study of Acceptorless Alcohol Dehydrogenation Mediated
by Ruthenium Pincer Complexes: Ligand Tautomerization Governing
Metal Ligand Cooperation. Inorg. Chem. 2016, 55, 6539−6551.
(28) (a) Esteruelas, M. A.; Oro, L. A. Dihydrogen Complexes as
Homogeneous Reduction Catalysts. Chem. Rev. 1998, 98, 577−588.
(b) Morris, R. H. Brønsted-Lowry Acid Strength of Metal Hydride and
Dihydrogen Complexes. Chem. Rev. 2016, 116, 8588−8654.
(29) Aracama, M.; Esteruelas, M. A.; Lahoz, F. J.; Lopez, J. A.; Meyer,
U.; Oro, L. A.; Werner, H. Synthesis, Reactivity, Molecular Structure,
and Catalytic Activity of the Novel Dichlorodihydridoosmium(IV)
Complexes OsH2Cl2(PR3)2 (PR3 = P-i-Pr3, PMe-t-Bu2). Inorg. Chem.
1991, 30, 288−293.
́
(30) Uson, R.; Oro, L. A.; Cabeza, J. A.; Bryndza, H. E.; Stepro, M. P.
Dinuclear Methoxy, Cyclooctadiene, and Barrelene Complexes of
Rhodium(I) and Iridium(I). Inorg. Synth 2007, 23, 126−130.
I
DOI: 10.1021/acs.organomet.8b00380
Organometallics XXXX, XXX, XXX−XXX