Osmium Hydride Acetylacetonate Complexes and Their Application in Acceptorless Dehydrogenative Coupling of Alcohols and Amines and for the Dehydrogenation of Cyclic Amines

Article abstract of DOI:10.1021/acs.organomet.7b00521

The preparation of new osmium hydride complexes, starting from OsH₆(PⁱPr₃)₂ (1) and OsH₂Cl₂(PⁱPr₃)₂ (2), and their catalytic activity in acceptorless dehydrogenative coupling of alcohols and amines and in dehydrogenation of cyclic amines are reported. Complex 1 reacts with acetylacetone (Hacac) to give the classical trihydride OsH₃(acac)(PⁱPr₃)₂ (3). The protonation of 3 with triflic acid (HOTf) produces the release of H₂ and the formation of the unsaturated osmium(IV) dihydride [OsH₂(acac)(PⁱPr₃)₂]OTf (4), which is also prepared by starting from 2 via the intermediate OsH₂Cl(acac)(PⁱPr₃)₂ (5). Treatment of an acetylacetone solution of 5 with KOH affords Os(acac)₂(PⁱPr₃)₂ (6). In the presence of 5 mol % of KOH, complexes 3-6 promote the coupling of benzyl alcohol and aniline to give N-benzylideneaniline and H₂. Under the same conditions, complex 3 catalyzes a wide range of analogous couplings to afford a variety of imines, including aliphatic imines, with yields between 90 and 40% after 1-48 h. Complex 3 also catalyzes the dehydrogenation of cyclic amines. According to the amount of H₂ released by each 1 g of employed substrate, the amines have been classified into three classes: poor (1,2,3,4-tetrahydroquinaldine, 2-methylindoline, and 2,6-dimethylpiperidine), moderate (1,2,3,4-tetrahydroquinoline and 6-methyl-1,2,3,4-tetrahydroquinoline), and good hydrogen donors (1,2,3,4-tetrahydroisoquinoline).

Full text of DOI:10.1021/acs.organomet.7b00521

Article
pubs.acs.org/Organometallics
Osmium Hydride Acetylacetonate Complexes and Their Application
in Acceptorless Dehydrogenative Coupling of Alcohols and Amines
and for the Dehydrogenation of Cyclic Amines
Miguel A. Esteruelas,* Virginia Lezaun, 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 new osmium hydride complexes,
starting from OsH6(PiPr3)2 (1) and OsH2Cl2(PiPr3)2 (2), and their
catalytic activity in acceptorless dehydrogenative coupling of alcohols
and amines and in dehydrogenation of cyclic amines are reported.
Complex 1 reacts with acetylacetone (Hacac) to give the classical
trihydride OsH3(acac)(PiPr3)2 (3). The protonation of 3 with triflic acid
(HOTf) produces the release of H2 and the formation of the
unsaturated osmium(IV) dihydride [OsH2(acac)(PiPr3)2]OTf (4),
which is also prepared by starting from 2 via the intermediate
OsH2Cl(acac)(PiPr3)2 (5). Treatment of an acetylacetone solution of 5 with KOH affords Os(acac)2(PiPr3)2 (6). In the presence
of 5 mol % of KOH, complexes 3−6 promote the coupling of benzyl alcohol and aniline to give N-benzylideneaniline and H2.
Under the same conditions, complex 3 catalyzes a wide range of analogous couplings to afford a variety of imines, including
aliphatic imines, with yields between 90 and 40% after 1−48 h. Complex 3 also catalyzes the dehydrogenation of cyclic amines.
According to the amount of H2 released by each 1 g of employed substrate, the amines have been classified into three classes:
poor (1,2,3,4-tetrahydroquinaldine, 2-methylindoline, and 2,6-dimethylpiperidine), moderate (1,2,3,4-tetrahydroquinoline and 6-
methyl-1,2,3,4-tetrahydroquinoline), and good hydrogen donors (1,2,3,4-tetrahydroisoquinoline).
iron,14 cobalt,15 and iridium16 have been also reported.
Recently, ruthenium-catalyzed coupling of nitriles with alcohols
to give imines has been independently reported by Nikonov17
and Hong.18
INTRODUCTION
■
One of the goals of the chemists of this century is to make
chemical synthesis environmentally friendly. In this respect,
improvement in the efficiency of atom-economical method-
ologies is challenging.1 Among the processes of this class, those
yielding useful byproducts from an energy point of view are of
special interest, in particular acceptorless dehydrogenation
reactions and related transformations,2 which afford molecular
hydrogen, the most efficient and clean energy carrier known.3
Imines, which are intermediates in the synthesis of several
biologically active N-heterocyclic compounds, show a rich
reactivity.4 As a consequence, they have widespread applica-
tions in the laboratory and in the industry.5 Traditional
methods for their synthesis, relatively benign from an
environmental point of view, are the condensation of a
carbonyl compound with an amine,6 the aerobic metal-
catalyzed self-condensation of amines,7 the aerobic metal-
mediated coupling of alcohols and amines,8 and the hydro-
amination of alkynes.9 In 2010, Milstein and co-workers
reported that PNN-pincer ruthenium complexes can promote
the synthesis of imines, with liberation of molecular hydrogen,
by means of the coupling of alcohols and amines under argon.10
We a year later proved that the osmium tetrahydride OsH4{κ3-
One of the barriers to adopting the hydrogen technology is
the lack of an adequate method for its storage.19 In this context,
organic liquids are a promising alternative, since they can be
reversibly dehydrogenated.20 Initial efforts to develop this class
of carriers were focused on cycloalkanes. However, their large
enthalpy of dehydrogenation is a drawback for practical
applications. The introduction of a heteroatom into the ring
system significantly decreases the enthalpy of dehydrogen-
ation.21 This has led to some groups exploring the metal-
promoted dehydrogenation of cyclic amines. As a consequence,
iron,22 ruthenium,23 cobalt,24 iridium,25 nickel,26 palladium,27
and copper28 catalysts have been developed in order to perform
this reaction under acceptorless conditions.
The ability of polyhydrides of platinum-group metals to
activate σ bonds allows them to be starting compounds for
preparing new complexes and to interact with several fields,
including organic synthetic chemistry and the storage of
regenerative energy.29 Thus, several ruthenium,30 rhodium,31
and iridium polyhydride32 complexes take part in a wide range
of metal-promoted organic synthetic processes and in the
P O P - [ d b f ( P i P r 2 ) 2 ] } ( d b f ( P i P r 2 ) 2
= 4 , 6 - b i s -
(diisopropylphosphino)dibenzofuran) is a promising alternative
to the ruthenium catalysts.11 The reaction has awakened a great
interest,12 and catalysts of other metals including manganese,13
Received: July 11, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00521
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
kinetically controlled dehydrocoupling of amine−boranes.33 In
contrast to ruthenium, rhodium, and iridium, osmium
polyhydrides have received scarce attention and only a few
compounds of this class have proved to be useful in catalysis. In
opposition to OsH4{κ3-POP-[dbf(PiPr2)2]}, the PNP-pincer
tetrahydride OsH4{κ3-PNP-[NH(CH2CH2PiPr2)2]} couples
primary alcohols and primary amines to afford secondary
amines; it also promotes the dehydrogenative coupling of
primary alcohols to symmetrical esters and hydrogen transfer
from 2-propanol to ketones.34 The cis hydride dihydrogen
complex [OsH(η2-H2){κ4-P,P,P,P-[P(CH2CH2PPh2)2]}]BPh4
catalyzes hydrogen transfer from 2-propanol to α,β-unsaturated
ketones to yield saturated ketones via isomerization of the
initially produced allylic alcohol.35 Saturated ketones by direct
reduction of the C−C double bond of α,β-unsaturated ketones
with molecular hydrogen are formed in the presence of the
carbonyl hydride dihydrogen catalysts OsHCl(η2-H2)(CO)-
(PR3)2 (PR3 = PiPr3, PtBu2Me).36 The equilibrium mixture of
the dihydride species OsH2(CO)(PiPr3)2 and the dihydride
dihydrogen complex OsH2(η2-H2)(CO)(PiPr3)2 catalyzes the
dehydrogenation of ammonia−borane,37 hydrogen transfer
from 2-propanol to ketones, α,β-unsaturated ketones,38 and
terminal alkynes,39 and the dimerization of terminal alkynes to
butatrienes.40 In contrast to the triisopropylphosphine system,
the POP-xanthene tetrahydride OsH4{κ3-POP-[xant(PiPr2)2]}
(xant(PiPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)-
xanthene) affords enynes.41
Figure 1. ORTEP diagram of complex 3 (50% probability ellipsoids).
Hydrogen atoms (except the hydrides) are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Os−P(1) = 2.3402(6), Os−P(2)
= 2.3386(6), Os−O(1) = 2.1560(16), Os−O(2) = 2.1612(16),
H(01)−H(02) = 1.59(3), H(02)−H(03) = 1.58(3); P(1)−Os−P(2)
= 172.557(19), O(1)−Os−O(2) = 83.17(6).
angle of 83.17(6)°. The classical trihydride nature of the
complex was confirmed by the DFT-optimized structure
(B3LYP(GD3)//SDD/6-31G**) and the hydride resonances
in the 1H NMR spectrum. The DFT calculations yield
separations between the hydrides of 1.590 and 1.603 Å,
which agree well with that obtained from the X-ray diffraction
analysis. The 1H NMR spectrum, in toluene-d8, is temperature-
In the search for new useful osmium polyhydrides in
catalysis, we have explored the reactions of OsH6(PiPr3)2 (1)
and OsH2Cl2(PiPr3)2 (2) with acetylacetone (Hacac). In this
paper we report the preparation of new osmium polyhydrides
containing the acetylacetonate (acac) ligand and their ability to
promote both types of reactions: the coupling of alcohols and
amines to selectively yield imines with hydrogen release and the
dehydrogenation of cyclic amines.
dependent. At room temperature, it displays a triplet (2JH−P
=
12.2 Hz) at −12.83 ppm for the hydride ligands, indicating that
the central hydride is involved in a thermally activated
intramolecular exchange process (ΔG* = 12 kcal mol−1) with
the hydrides of the corners. Thus, between 243 and 233 K
decoalescence occurs and at temperatures lower than 223 K an
A2BX2 (X = 31P) spin system is observed with δA −12.51, δB
−13.14, and JAB = 17 Hz. For these resonances 300 MHz
T1(min) values of 83 3 ms (δA) and 53 3 ms (δB) were
obtained at 213 K, which fit with a hydrogen−hydrogen
RESULTS AND DISCUSSION
■
Preparation and Characterization of the Catalyst
Precursors. The hydride ligands of the d2 hexahydride
complex 1 have proved to be basic enough to promote the
deprotonation of polar C−H,42 N−H,43 and O−H44 σ-bonds.
In agreement with this, treatment of toluene solutions of 1 with
2 equiv of Hacac, under reflux, for 1 h leads to the trihydride
derivative OsH3(acac)(PiPr3)2 (3) and H2 (Scheme 1).
separation of 1.63 Å.45 In contrast to the H NMR spectrum,
1
the 31P{1H} NMR spectrum is temperature invariant. Between
283 and 183 K, it shows a singlet at 36.6 ppm.
Scheme 1. Reaction of OsH6(PiPr3)2 with Acetylacetone
Complex 3 is also a Brønsted base. At least one of the
hydride ligands is responsible for this character. Thus, the
addition of 1.0 equiv of triflic acid (HOTf) to dichloromethane
solutions of 3 produces the release of H2, as a result of the
protonation of the basic hydride, and the formation of the six-
coordinate dihydride [OsH2(acac)(PiPr3)2]OTf (4), which was
isolated as a pale yellow solid in 87% yield. The saline character
of this compound is strongly supported by its molar
conductivity of 134 Ω−1 cm2 mol−1, at 25 °C, in acetone.
The 1H NMR spectrum, in dichloromethane-d2, at room
temperature is the expected one for a 16-valence-electron
osmium(IV) OsH2X2(PiPr3)2 species with a structure in the
solid state that can be described as a C2 square antiprism with
two missing vertexes.43c,46 Thus, it contains at 1.28 ppm the
characteristic doublet of doublets (3JH−H = 7.2 Hz, 2JH−P = 14.5
Hz) due to the PiPr3 methyl groups, whereas the OsH2 unit
gives rise to a triplet (2JH−P = 34.7 Hz) at −19.67 ppm. The
31P{1H} NMR spectrum shows a singlet at 37.1 ppm.
Complex 3 was isolated as a yellow solid in 83% yield and
characterized by X-ray diffraction analysis. Figure 1 gives a view
of the molecule. The coordination geometry around the
osmium atom can be described as a distorted pentagonal
bipyramid with axial phosphines (P(1)−Os−P(2) =
172.557(19)°). The metal coordination sphere is completed
by the hydride ligands, separated by 1.59(3) Å (H(01)−
H(02)) and 1.58(3) Å (H(02)−H(03)), and the oxygen atoms
of the acac group, which acts with an O(1)−Os−O(2) bite
B
DOI: 10.1021/acs.organomet.7b00521
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Complex 4 can be also prepared by starting from 2, by means
of the reaction sequence summarized in Scheme 2. Treatment
Scheme 2. Reaction of OsH2Cl2(PiPr3)2 with Acetylacetone
Figure 2. ORTEP diagram of complex 6 (50% probability ellipsoids).
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Os−P(1) = 2.3174(6), Os−P(2) = 2.3213(6), Os−O(1)
= 2.0669(16), Os−O(2) = 2.1038(16), Os−O(3) = 2.0706(16), Os−
O(4) = 2.0940(16); P(1)−Os−P(2) = 104.85(2), O(1)−Os−O(2) =
90.54(6), O(3)−Os−O(4) = 90.50(7).
of an acetylacetonate solution of 2 with 1.0 equiv of KOH, at
room temperature, for 5 h produces the substitution of one of
the chloride ligands by an acac group and the formation of the
seven-coordinate derivative OsH2Cl(acac)(PiPr3)2 (5), which
was isolated as a yellow solid in 64% yield. Addition of 1.0 equiv
of AgOTf dissolved in some drops of acetone to a
dichloromethane solution of 5 gives rise to the extraction of
prepared by displacement of the L ligands of Ru(acac)2L2 (L
= SbiPr3, C8H14) by PiPr3, in benzene or tetrahydrofuran, at
room temperature. When the reaction was performed at −25
°C, in tetrahydrofuran, the trans isomer was isolated. The latter
rapidly isomerizes to cis-Ru(acac)2(PiPr3)2 at room temper-
ature.
Catalytic Formation of Imines with Release of
Molecular Hydrogen. In the presence of KOH, complexes
3−6 are efficient catalyst precursors for the coupling of benzyl
alcohol and aniline, under argon, which yields N-benzylidenea-
niline with liberation of molecular hydrogen and water. The
coupling was performed in toluene under reflux, using alcohol
and amine concentrations of 1.63 M and precursor/substrate
and KOH/substrate molar ratios of 1/100 and 1/20,
respectively. The water generated during the process was
removed from the reaction medium by using a Dean−Stark
receiver filled with toluene. Under these conditions, the
trihydride complex 3 is the most active precursor (Table 1),
since it promotes the formation of the imine in 82% yield, after
1 h of reaction, while complexes 4−6 reach 73−66% of
1
the chloride anion to produce 4 in 68% yield. The H and
31P{1H} NMR spectra of 5, in benzene-d6, at room temperature
resembles those of complexes OsH2(κ2-O2CCH3)X(PiPr3)2 (X
= Cl, OC(O)CH3).47 Thus, the H NMR spectrum displays a
1
2
doublet of doublets (3JH−H = 7.2 Hz, JH−P = 12.4 Hz) at 1.24
ppm for the PiPr3 methyl groups, whereas the hydride ligands
give rise to a triplet (2JH−P = 35.5 Hz) at −12.77 ppm. The
31P{1H} NMR spectrum shows a singlet at 24.1 ppm. On the
basis of an X-ray diffraction analysis of OsH2(κ2-O2CCH3){κ1-
OC(O)CH3}(PiPr3)2, the coordination polyhedron around the
osmium atom of these compounds has been described as
halfway between a capped trigonal prism and a piano-stool
geometry. If one takes as a base the structure of 2, 4, and
related six-coordinate osmium(IV) dihydrides,46 the coordina-
tion polyhedron could be rationalized as a square antiprism
with a missing vertex.
Complex 5 has a greater tendency than 2 to release H2. In
contrast to the latter, the treatment of acetylacetone solutions
of the former with 1.0 equiv of KOH produces the reductive
elimination of H2 and the formation of the six-coordinate
osmium(II) derivative cis-Os(acac)2(PiPr3)2 (6), which was
isolated as a red solid in low yield (∼20%) and characterized by
X-ray diffraction analysis. Figure 2 shows a view of the
molecule. The geometry around the osmium atom can be
rationalized as a very distorted octahedron with cis phosphine
ligands. The distortion is mainly due to the arrangement of the
phosphine ligands, which experience a large steric hindrance
due to their large cone angle (160°).48 As a consequence, the
P−Os−P angle (104.85(2)°) strongly deviates from the ideal
value of 90°.49 The 31P{1H} NMR spectrum, in benzene-d6, at
room temperature shows a singlet at −12.2 ppm, as expected
for equivalent phosphines.
Table 1. Screening of Catalysts for the Formation of N-
Benzylideneaniline with Release of Molecular Hydrogen
a
a
Catalyst (0.0163 mmol), benzylic alcohol (1.63 mmol), aniline (1.63
We note that the related complex cis-Ru(acac)2(PiPr3)2 has
been previously reported by the groups of Werner and Bennett
and characterized by X-ray diffraction analysis.50 It was
mmol), KOH (0.081 mmol), and toluene (1 mL) were heated at 110
b
°C in a Schlenk tube under an argon atmosphere for 1 h. Yields were
1
determined by H NMR spectroscopy.
C
DOI: 10.1021/acs.organomet.7b00521
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 2. Synthesis of Imines from Alcohols and Amines Catalyzed by OsH3(acac)(PiPr3)2
a
Complex 3 (0.0163 mmol), alcohol (1.63 mmol), amine (1.63 mmol), KOH (0.081 mmol), and toluene (1 mL) were heated at 110 °C in a
b
1
Schlenk tube under an argon atmosphere. Yields were determined by H NMR spectroscopy.
D
DOI: 10.1021/acs.organomet.7b00521
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
product. Furthermore, between 14% and 24% of N-phenyl-
benzylamine is obtained as a consequence of the reduction of
N-benzylideneaniline. The lowest amine/imine ratio is also
observed when the dihydride 3 is used. Therefore, this complex
is the most active and is the most selective precursor.
benzyl alcohol, with these alcohols the reactions involving
aliphatic amines, in particular dodecylamine (runs 12 and 15),
are faster than those with anilines.
The results collected in Table 2 reaffirm that osmium is
certainly a real alternative to ruthenium for the synthesis of
imines from alcohols and amines with liberation of molecular
hydrogen. The POP tetrahydride complex OsH4{κ3-POP-
[dbf(PiPr2)2]}11 has been shown to be significantly more
efficient than the reported ruthenium systems.12 The efficiency
of 3 as a catalyst precursor for this reaction is slightly lower
than that of OsH4{κ3-POP-[dbf(PiPr2)2]} in terms of activity
and selectivity. However, the simplicity of the ligands of 3,
which are commercially available, and the speed and high yield
of the procedure discovered for its preparation are enough facts
to compensate for the lower efficiency and promote the use of
this precursor as a recommendable alternative.
Catalytic Dehydrogenation of Cyclic Amines. Complex
3 also promotes the dehydrogenation of cyclic amines, such as
1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydroquinaldine, 6-
methyl-1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydroisoquino-
line, 2-methylindoline, and 2,6-dimethylpiperidine, to the
corresponding aromatic compounds (eq 1). The reactions
Complex 3 promotes the formation of a variety of imines,
including aliphatic imines that are inherently more challenging
due to their instability. Yields of imine between 90% and 40%
are obtained after 1−48 h depending upon the nature of both
the alcohol and amine (Table 2). In addition to the reduced
product, aldehyde (1−2%) or ketone (16−23%) is also
observed during the reactions. Its presence is consistent with
a two-step procedure for the formation of the imines, which
involves the catalytic dehydrogenation of the alcohol, to afford
the aldehyde or ketone and molecular hydrogen, and the
subsequent condensation of the amine with the generated
carbonyl intermediate (Scheme 3). In contrast, the carbonyl
intermediate was not detected during the ruthenium-mediated
amide synthesis by coupling of alcohol and amines.51
Scheme 3. Two-Step Procedure for the Formation of Imines
were performed under argon, in p-xylene, at 140 °C, using a
heterocycle concentration of 0.16 M and a precursor/substrate
molar ratio of 1/10. Under these conditions, the aromatic
heterocycles were formed with yields between 100% and 29%,
after 48 h, depending upon the position of the nitrogen atom at
the heterocycle and the presence of methyl substituents
adjacent to the heteroatom (Table 3). The trend of the cyclic
amine to undergo dehydrogenation increases as the space
around the nitrogen atom also increases: i.e., in the sequence
1,2,3,4-tetrahydroquinaldine < 1,2,3,4-tetrahydroquinoline <
1,2,3,4-tetrahydroisoquinoline (runs 2 < 1 < 4). This suggests
that the coordination of the nitrogen atom to the metal center
of the catalyst is the key step for the dehydrogenation.
The studied amines can be classified into three classes,
according to the amount of molecular hydrogen released by
each 1 g of heterocycle used: poor, moderate, and good
hydrogen donors. 1,2,3,4-Tetrahydroquinaldine (Table 3, run
2), 2-methylindoline (run 5), and 2,6-dimethylpiperidine (run
6) are poor hydrogen donors. A structural characteristic of
these substrates is the presence of two carbons bonded to the
carbons adjacent to the heteroatom. They release between 0.59
× 10−2 and 0.77 × 10−2 mol g−1. 1,2,3,4-Tetrahydroquinoline
(run 1) and 6-methyl-1,2,3,4-tetrahydroquinoline (run 3) are
moderate hydrogen donors. In contrast to the poor hydrogen
donor heterocycles, in these substrates only one of the adjacent
carbons to the heteroatom has two bonded carbons. They
afford about 1.2 × 10−2 mol g−1. 1,2,3,4-Tetrahydroisoquinoline
(run 4), with both carbons adjacent to the heteroatom bonded
to one carbon, is a good hydrogen donor. After 48 h, it releases
all its capacity, 1.50 × 10−2 mol g−1.
A chloride substituent at the para position of benzyl alcohol
decreases the rate of formation of the imine. While benzyl
alcohol affords 82% of N-benzylideneaniline after 1 h (Table 2,
run 1), p-chlorobenzyl alcohol needs 24 h to give 73% of N-(4-
chlorobenzylidene)aniline (run 2). The primary or secondary
nature of the alcohol has a marked influence on the rate and
selectivity of the reactions. In contrast to benzyl alcohol, 1-
phenylethanol (run 3) and diphenylmethanol (run 4) afford
significant amounts of the carbonyl intermediate (16% and
23%, respectively), whereas the yields of the respective imines,
26 and 48%, are moderate. Benzyl alcohol couples with methyl-
substituted anilines and aliphatic amines, including benzyl-
amine, cyclohexylamine, and dodecylamine. The methyl
substituent of anilines has a marked influence on the reaction
(runs 1, 5, and 6). Its presence decreases the rate. The effect is
enhanced as the methyl group approaches the nitrogen atom.
While N-benzylidene-p-methylaniline is obtained in 80% yield
after 3 h, the formation of a similar amount of N-benzylidene-o-
methylaniline requires about 12 h. The presence of the methyl
group at the anilines also has some influence on the selectivity
of the reaction, decreasing the formation of reduced product in
the sequence aniline > p-methylaniline > o-methylaniline.
Aliphatic amines also work well; benzylamine (run 7),
cyclohexylamine (run 8), and dodecylamine (run 9) lead to
the corresponding imines in 92−70% yield after 8−12 h. The
selectivity of the reactions increases as the bulkiness of the alkyl
group also increases. Thus, the formation of the reduced
product decreases in the sequence dodecylamine > benzyl-
amine > cyclohexylamine. Cyclohexylmethanol (runs 10−12)
and n-butanol (runs 13−15) react more slowly than benzyl
alcohol. Thus, long times, 24−48 h, are necessary to obtain the
corresponding imines in moderate to high yields. In contrast to
Complex 3 is the first catalytic precursor of osmium for the
dehydrogenation of cyclic amines. Although this type of
reaction is very sensitive to the experimental conditions and
therefore a comparison with other metal systems should be
carefully done, the results collected in Table 3 show that it can
E
DOI: 10.1021/acs.organomet.7b00521
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
naldine, 1-methylindoline, and 2,6-dimethylpiperidine), mod-
erate (1,2,3,4-tetrahydroquinoline and 6-methyl-1,2,3,4-tetrahy-
droquinoline), and good hydrogen donors (1,2,3,4-tetrahy-
droisoquinoline).
Table 3. Osmium-Catalyzed Acceptorless Dehydrogenation
of Cyclic Amines
a
In conclusion, a new osmium polyhydride, which is an
efficient catalyst precursor for the formation of imines by
acceptorless dehydrogenative coupling of alcohols and amines
and for the dehydrogenation of cyclic amines to aromatic
heterocycles, has been discovered.
EXPERIMENTAL SECTION
■
General Information. All reactions were carried out with rigorous
exclusion of air using Schlenk-tube techniques. Solvents (except
acetylacetone, which was dried and distilled under argon) were
obtained oxygen- and water-free from an MBraun solvent purification
1
apparatus. H, 31P{1H}, and 13C{1H} NMR spectra were recorded on
Bruker 300 ARX, Bruker Avance 300 MHz, and Bruker Avance 400
MHz instruments. Chemical shifts (expressed in parts per million) are
referenced to residual solvent peaks (1H, 13C{1H}), external 85%
H3PO4 (31P{1H}), or external CFCl3 (19F{1H}). Coupling constants J
and N are given in hertz. Attenuated total reflection infrared spectra
(ATR-IR) of solid samples were run on a PerkinElmer Spectrum 100
FT-IR spectrometer. C and H analyses were carried out in a
PerkinElmer 2400 CHNS/O analyzer. High-resolution electrospray
mass spectra were acquired using a MicroTOF-Q hybrid quadrupole
time-of-flight spectrometer (Bruker Daltonics, Bremen, Germany).
Conductivities were measured in ca. 5 × 10−4 M acetone solutions of
complexes 4 and 5 using a Philips PW 9501/01 conductimeter.
OsH6(PiPr3)2 (1) and OsH2Cl2(PiPr3)2 (2) were prepared by the
published methods.46a The formed imines were characterized by
a
Conditions: 3 (0.0163 mmol); N-heterocyclic substrate (0.163
b
mmol); p-xylene (1 mL) heated at 140 °C for 48 h. Conversions
were calculated from the relative peak area integrations of the reactant
and product in the GC spectra.
1
comparison of their H NMR spectra with those reported previously
be a promising alternative to the classical catalysts,22−28 in
particular to those of ruthenium with which molecular
hydrogen releases between 0.16 × 10−2 and 1.50 × 10−2 mol
g−1 have been achieved.23
(N-benzylideneaniline,11 N-(4-chlorobenzylidene)aniline,52 N-(1-
phenylethylidene)aniline,53 N-phenylbenzophenone imine,54 N-benzy-
lidene-p-methylaniline,11 N-benzylidene-o-methylaniline,11,55 N-benzy-
lidenebenzylamine,11,13,15 N-benzylidenecyclohexylamine,11,13,55 N-
benzylidenedodecylamine,11 N-cyclohexylmethylene-p-methylani-
line,11 N-cyclohexylmethylenecyclohexylamine,11 N-cyclohexylmethy-
lenedodecylamine,11 N-butylidene-p-methylaniline,56 N-butylidenecy-
clohexylamine,11 and N-butylidendodecylamine11).
CONCLUSIONS
■
This study has revealed that the hydride ligands of the d2
hexahydride OsH6(PiPr3)2 are basic enough to deprotonate
acetylacetone and to generate the classical trihydride
OsH3(acac)(PiPr3)2. This complex is useful to promote
reactions of interest from an environmental point of view,
which lead to imines and aromatic amines.
Reaction of OsH6(PiPr3)2 (1) with Acetylacetone: Preparation
of OsH3(acac)(PiPr3)2 (3). Acetylacetone (80 μL, 0.78 mmol) was
added to a solution of 1 (200 mg, 0.39 mmol) in toluene (10 mL) and
heated under reflux for 1 h, and the mixture changed from colorless to
bright yellow. The resulting solution was filtrated trough Celite and
dried in vacuo, affording a yellow residue. Methanol (3 mL) was added
to give a yellow solid, which was washed with further portions of
methanol (2 × 1 mL) and dried in vacuo. Yield: 197 mg (83%). Anal.
Calcd for C23H52O2OsP2: C, 45.08; H, 8.55. Found: C, 44.76; H, 8.48.
HRMS (electrospray, m/z): calcd for C23H51O2OsP2 [M − H]+,
613.2974; found, 613.2972. IR (cm−1): ν(Os−H) 2137, 2101 (m);
The formation of imines, including aliphatic imines, takes
place by means of the catalytic dehydrogenation of alcohols and
subsequent stoichiometric condensation of the resulting
aldehyde or ketone with a primary amine. The procedure
allows the preparation of a variety of products with yields
between 90% and 40% after 1−48 h, depending upon the
nature of both alcohol and amine. The catalytic efficiency of
OsH3(acac)(PiPr3)2 is slightly lower than that of the very active
tetrahydride OsH4{κ3-POP-[dbf(PiPr2)2]}. However, the for-
mer is a recommendable alternative to the latter, given the
simplicity of its ligands and the speed and high yield of the
procedure discovered for its preparation.
The aromatic amines have been generated by dehydrogen-
ation of cyclic amines, the trihydride OsH3(acac)(PiPr3)2 being
the first catalytic precursor of osmium for this process. The
trend of the substrates to undergo dehydrogenation increases as
the space around the nitrogen atom also increases, which
suggests that the coordination of the nitrogen atom to the
metal center of the catalyst is the key step for the
dehydrogenation. According to the amount of molecular
hydrogen released by each 1 g of employed amine, the studied
heterocycles can be classified into poor (1,2,3,4-tetrahydroqui-
1
ν(CO) 1588, 1514 (s). H NMR (400 MHz, C6D6, 298 K): δ 5.18
(s, 1H, CH acac), 2.01 (m, 6H, PCH(CH3)2), 1.71 (s, 6H, CH3 acac),
1.25 (dvt, 3JH−H = 6.5, N = 12.6, 36H, PCH(CH3)2), −12.83 (t, 2JH−P
= 12.2, 3H, OsH). 1H{31P} NMR (400 MHz, toluene-d8, 193 K, high-
field region): δ −12.51 (d, 2JH−H = 17.0, 2H, OsH), −13.14 (t, 2JH−H
=
17.0, 1H, OsH). 13C{1H} NMR (100.63 MHz, C6D6, 298 K): δ 182.6
(s, CO acac), 101.4 (s, CH acac), 28.2 (s, CH3 acac), 27.3 (vt, N =
24.0, PCH(CH3)2), 20.6 (s, PCH(CH3)2). 31P{1H} NMR (121.4
MHz, C6D6, 298 K): δ 36.6 (s). T1(min) (ms, OsH, 300 MHz,
toluene-d8, 213 K): 83 3 (−12.54 ppm), 53 3 (−13.14 ppm).
Reaction of OsH3(acac)(PiPr3)2 (3) with HOTf: Preparation of
[OsH2(acac)(PiPr3)2]OTf (4). A solution of 3 (150 mg, 0.244 mmol)
in dichloromethane (10 mL) was treated with HOTf (22 μL, 0.244
mmol) and stirred for 20 min at room temperature. The solution
changed from bright yellow to orange. The resulting solution was
filtered through Celite and concentrated to ca. 0.5 mL. Diethyl ether
(5 mL) was added to afford a pale yellow solid, which was washed with
further portions of diethyl ether (3 × 2 mL) and dried in vacuo. Yield:
162 mg (87%). Anal. Calcd for C24H51F3O5OsP2S: C, 37.88; H, 6.76;
F
DOI: 10.1021/acs.organomet.7b00521
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
S, 4.21. Found: C, 37.49; H, 6.89; S, 4.01. HRMS (electrospray, m/z):
calcd for C23H51O2OsP2 [M]+, 613.2974; found, 613.2989. IR (cm−1):
ν(Os−H) 2219 (w); ν(CO) 1574, 1528 (s), ν(C−F) 1297, ν(S−
mmol), potassium hydroxide (0.081 mmol), and the corresponding
substrates (1.63 mmol) in toluene (1 mL) was placed in a Schlenk
flask equipped with a Dean−Stark apparatus and a condenser. The
mixture was stirred at reflux temperature in an open system for the
specified time (Table 2). After the mixture was cooled to room
temperature, it was evaporated under reduced pressure to obtain an
oil. Then, 0.815 mmol (52 μL) of dichloromethane was added as an
internal standard. The mixture was dissolved in CDCl3 and was
transferred to a 2 mL volumetric flask. Afterward, 0.5 mL of this
1
O) 1229, 1154, 1023 (vs). H NMR (300 MHz, CD2Cl2, 298 K): δ
6.22 (s, 1H, CH acac), 2.32 (m, 12H, PCH(CH3)2 + CH3 acac), 1.28
2
2
(dd, 3JH−H = 7.2, JH−P = 14.5, 36H, PCH(CH3)2), −19.67 (t, JH−P
=
34.7, 2H, OsH). 13C{1H} NMR (75.4 MHz, CD2Cl2, 298 K): δ 189.8
(s, CO acac), 121.5 (q, 1JC−F = 320.6, CF3SO3), 103.7 (s, CH acac),
1
28.6 (d, JC−P = 34.5, PCH(CH3)2), 27.9 (s, CH3 acac), 19.5 (s,
1
PCH(CH3)2). 31P{1H} NMR (121.4 MHz, CD2Cl2, 298 K): δ 37.1
(s). 19F{1H} (282.3 MHz, CD2Cl2, 298 K): δ −78.8 (s). T1(min) (ms,
solution was analyzed by H NMR spectroscopy. The formed imines
1
were characterized by H NMR spectroscopy.
General Procedure for the OsH3(acac)(PiPr3)2 (3) Catalyzed
Dehydrogenation Reactions of Cyclic Amines. A solution of
complex 3 (0.0163 mmol) and the corresponding substrate (0.163
mmol) in p-xylene (1 mL) was placed in a Schlenk flask equipped with
a condenser. The mixture was stirred at 140 °C for 48 h. After this
time the solution was cooled to room temperature, and the progress of
the reaction was monitored by GC on a Hewlett-Packard 5890 series
II gas chromatograph with a flame ionization detector, using a 100%
cross-linked methyl silicone gum column (30 m × 0.32 mm, with 0.25
μm film thickness). The oven conditions used are as follows: 80 °C
(hold 1 min) to 220 °C at 10 °C/min (hold 2 min).
OsH, 300 MHz, CD2Cl2, 193 K): 211
6 (−19.67 ppm). ΛM
(acetone): 134 Ω−1 cm2 mol−1.
Reaction of OsH2Cl2(PiPr3)2 (2) with Acetylacetone: Prepara-
tion of OsH2Cl(acac)(PiPr3)2 (5). A solution of 2 (100 mg, 0.17
mmol) in acetylacetone (5 mL) was treated with 1 equiv of KOH (9.6
mg, 0.17 mmol) and stirred for 5 h at room temperature. The mixture
changed quickly from brown to yellow. The resulting solution was
filtered through Celite and evaporated to dryness. Pentane (4 mL) was
added to afford a pale yellow solid, which was washed with further
portions of pentane (2 × 1 mL) and dried in vacuo. Yield: 78 mg
(64%). Anal. Calcd for C23H51ClO2OsP2: C, 42.68; H, 7.94. Found: C,
42.25; H, 7.89. HRMS (electrospray, m/z): calcd for C23H50ClO2OsP2
[M − H]+, 647.2574; found, 647.2619. IR (cm−1): ν(Os−H) 2139,
2116 (m); ν(CO) 1587, 1526 (s). 1H NMR (300 MHz, C6D6, 298
K): δ 5.42 (s, 1H, CH acac), 2.36 (m, 6H, PCH(CH3)2), 1.82 (s, 6H,
CH3 acac), 1.24 (dd, JH−H = 7.2, JH−P = 12.4, 36H, PCH(CH3)2),
−12.77 (t, JH−H = 35.5, 2H, OsH). 13C{1H} NMR (100.63 MHz,
Structural Analysis of Complexes 3 and 6. The .cif files of 3
and 6 have been deposited with the Cambridge Crystallographic Data
Center (Nos. CCDC 1547645 (3) and 1547646 (6)). X-ray data were
collected for the complexes on a Bruker Smart APEX CCD DUO (3)
or Bruker Smart APEX CCD (6) diffractometer equipped with a
normal-focus, 2.4 kW sealed tube source (Mo radiation, λ = 0.71073
Å) operating at 50 kV and 30 mA. Data were collected over the
complete sphere. Each frame exposure time was 10 s (covering 0.3° in
ω). Data were corrected for absorption by using a multiscan method
applied with the SADABS program.57 The structures were solved by
Patterson or direct methods and refined by full-matrix least squares on
F2 with SHELXL 2016/6,58 including isotropic and subsequently
anisotropic displacement parameters. The hydrogens not bonded to
the osmium atoms were observed in the least-squares Fourier maps or
calculated and refined freely or refined using a restricted riding model.
The hydrogens bonded to metal atoms were observed in the last cycles
of refinement but refined too close to metals; therefore, a restricted
refinement model was used for all of them (d(Os−H) = 1.59(1) Å).
Crystal data for 3: C23H52O2OsP2, mol wt 612.78, yellow, irregular
C6D6, 298 K): δ 183.2 (s, CO), 100.9 (s, CH acac), 27.3 (d, 1JC−P
=
31.1, PCH(CH3)2), 27.1 (s, CH3 acac), 19.2 (s, PCH(CH3)2). 31P{1H}
NMR (121.4 MHz, C6D6, 298 K): δ 24.1 (s). T1(min) (ms, OsH, 300
MHz, toluene-d8, 223 K): 161 8 (−12.77 ppm). ΛM (acetone): 1
Ω−1 cm2 mol−1.
Reaction of OsH2Cl(acac)(PiPr3)2 (5) with AgOTF: Preparation
of [OsH2(acac)(PiPr3)2]OTf (4). AgOTf (40 mg, 0.154 mmol)
dissolved in several drops of acetone was added to a CH2Cl2 solution
(8 mL) of 5 (100 mg, 0.154 mmol). After it was stirred for 1 h in the
dark at room temperature, the resulting suspension was filtered
through Celite to remove the silver salts. The solution thus resulting
was concentrated to ca. 0.5 mL, and diethyl ether (5 mL) was added to
afford a pale yellow solid, which was washed with further portions of
diethyl ether (2 × 2 mL) and dried in vacuo. Yield: 80 mg (68%).
Reaction of OsH2Cl(acac)(PiPr3)2 (5) with Acetylacetone:
Preparation of Os(acac)2(PiPr3)2 (6). A solution of 5 (100 mg, 0.15
mmol) in acetylacetone (5 mL) was treated with 1 equiv of KOH (8.7
mg, 0.15 mmol) and stirred for 4 h at room temperature, and the
mixture changed from yellow to dark red. The resulting solution was
filtered through Celite and evaporated to dryness. Methanol was added
to afford a dark red solid, which was washed with further portions of
methanol and dried in vacuo. Yield: 22 mg (20%). Anal. Calcd for
C28H56O4OsP2: C, 47.44; H, 7.96. Found: C, 47.22; H, 7.77. HRMS
(electrospray, m/z): calcd for C28H56O4OsP2 [M]+, 710.3264; found,
block (0.190 × 0.161 × 0.122 mm), triclinic, space group P1, a =
̅
8.9802(10) Å, b = 11.8374(13) Å, c = 14.3966(16) Å, α =
73.9350(10)°, β = 87.117(2)°, γ = 68.2470(10)°, V = 1363.6(3) Å3,
Z = 2, Z′ = 1, Dcalc = 1.492 g cm−3, F(000) = 624, T = 100(2) K, μ =
4.808 mm−1; 15084 measured reflections (2θ = 3−58°, ω scans 0.3°),
6894 unique reflections (Rint = 0.0222); minimum/maximum
transmission factors 0.723/0.862. Final agreement factors were R1 =
0.0197 (6520 observed reflections, I > 2σ(I)) and wR2 = 0.0481, with
6894/3/276 data/restraints/parameters and GOF = 1.029. The largest
peak and hole were 1.383 (close to the osmium atom) and −0.820 e/
Å3.
1
710.3292. IR (cm−1): ν(CO) 1559, 1509 (s). H NMR (300 MHz,
Crystal data for 6. C28H56O4OsP2, mol wt 708.86, red, irregular
block (0.174 × 0.126 × 0.113 mm), monoclinic, space group P21/n, a
= 10.6019(4) Å, b = 19.7066(7) Å, c = 15.3908(6) Å, β = 98.7460(4)°,
V = 3178.2(2) Å3, Z = 4, Z′ = 1, Dcalc = 1.481 g cm−3, F(000) = 1448,
T = 100(2) K, μ = 4.142 mm−1; 26297 measured reflections (2θ = 3−
58°, ω scans 0.3°), 7557 unique reflections (Rint = 0.0218); minimum/
maximum transmission factors 0.723/0.863. Final agreement factors
were R1 = 0.0215 (7019 observed reflections, I > 2σ(I)) and wR2 =
0.0523, with 7557/0/332 data/restraints/parameters and GOF =
1.034. The largest peak and hole were 1.713 (close to the osmium
atom) and −0.908 e/Å3.
C6D6, 298 K): δ 5.28 (s, 2H, CH acac), 2.53 (m, 6H, PCH(CH3)2),
1.67 (s, 6H, CH3 acac), 1.66 (s, 6H, CH3 acac), 1.37 (dd, 3JH−H = 7.1,
3
2
2JH−P = 4.5, 18H, PCH(CH3)2), 1.28 (dd, JH−H = 7.1, JH−P = 4.5,
18H, PCH(CH3)2). 13C{1H} NMR (75.42 MHz, C6D6, 298 K): δ
183.5 (s, CO), 178.3 (s, CO), 102.4 (s, CH acac), 30.1 (d, 1JC−P
= 27.1, PCH(CH3)2), 28.0, 27.7 (both s, CH3 acac), 20.6, 20.1 (both s,
PCH(CH3)2). 31P{1H} NMR (121.4 MHz, C6D6, 298 K): δ −12.2 (s).
Catalyst Optimization. A solution of the chosen catalyst (0.0163
mmol), potassium hydroxide (0.081 mmol), aniline (1.63 mmol), and
benzyl alcohol (1.63 mmol) in toluene (1 mL) was placed in a Schlenk
flask equipped with a Dean−Stark apparatus and a condenser. The
mixture was stirred at the reflux temperature in an open system for 1 h.
After the mixture was cooled to room temperature, it was evaporated
ASSOCIATED CONTENT
■
S
* Supporting Information
1
under reduced pressure to obtain an oil, which was analyzed by H
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00521.
NMR spectroscopy.
General Procedure for the OsH3(acac)(PiPr3)2 (3) Catalyzed
N-alkylation of Amines with Alcohols. A solution of 3 (0.0163
G
DOI: 10.1021/acs.organomet.7b00521
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1H, 13C{1H} APT, 31P{1H}, and 19F{1H} NMR spectra
of complexes 3−6, H NMR spectra of the reaction
mixtures of the acceptorless dehydrogenative coupling of
alcohols and amines, and computational details (PDF)
Cartesian coordinates of computed 3 (XYZ)
(7) See for example: (a) Schumperli, M. T.; Hammond, C.;
̈
1
Hermans, I. ACS Catal. 2012, 2, 1108−1117. (b) Largeron, M. Eur. J.
Org. Chem. 2013, 2013, 5225−5235. (c) Ray, R.; Chandra, S.; Yadav,
V.; Mondal, P.; Maiti, D.; Lahiri, G. K. Chem. Commun. 2017, 53,
4006−4009.
(8) Chen, B.; Wang, L.; Gao, S. ACS Catal. 2015, 5, 5851−5876.
(9) Muller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M.
̈
Accession Codes
Chem. Rev. 2008, 108, 3795−3892.
CCDC 1547645−1547646 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing 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.
(10) Gnanaprakasam, B.; Zhang, J.; Milstein, D. Angew. Chem., Int.
Ed. 2010, 49, 1468−1471.
(11) Esteruelas, M. A.; Honczek, N.; Olivan
M. Organometallics 2011, 30, 2468−2471.
́
, M.; Onate, E.; Valencia,
̃
(12) (a) Zeng, G.; Li, S. Inorg. Chem. 2011, 50, 10572−10580.
(b) Maggi, A.; Madsen, R. Organometallics 2012, 31, 451−455. (c) Li,
H.; Wang, X.; Wen, M.; Wang, Z.-X. Eur. J. Inorg. Chem. 2012, 2012,
5011−5020. (d) Rigoli, J. W.; Moyer, S. A.; Pearce, S. D.; Schomaker,
J. M. Org. Biomol. Chem. 2012, 10, 1746−1749. (e) Nakajima, Y.;
Okamoto, Y.; Chang, Y.-H.; Ozawa, F. Organometallics 2013, 32,
2918−2925. (f) Musa, S.; Fronton, S.; Vaccaro, L.; Gelman, D.
Organometallics 2013, 32, 3069−3073. (g) Srimani, D.; Ben-David, Y.;
Milstein, D. Angew. Chem., Int. Ed. 2013, 52, 4012−4015. (h) Saha, B.;
Rahaman, S. M. W.; Daw, P.; Sengupta, G.; Bera, J. K. Chem. - Eur. J.
2014, 20, 6542−6551. (i) Oldenhuis, N. J.; Dong, V. M.; Guan, Z.
Tetrahedron 2014, 70, 4213−4218.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.A.E.: maester@unizar.es.
ORCID
■
Miguel A. Esteruelas: 0000-0002-4829-7590
Montserrat Olivan: 0000-0003-0381-0917
́
Enrique Onate: 0000-0003-2094-719X
̃
Notes
(13) Mukherjee, A.; Nerush, A.; Leitus, G.; Shimon, L. J. W.; Ben
David, Y.; Espinosa Jalapa, N. A.; Milstein, D. J. Am. Chem. Soc. 2016,
138, 4298−4301.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the MINECO of Spain (Projects
CTQ2014-52799-P and CTQ2016-81797-REDC), the Diputa-
cion General de Aragon (E-35), FEDER, and the European
́ ́
Social Fund is acknowledged.
(14) Jaiswal, G.; Landge, V. G.; Jagadeesan, D.; Balaraman, E. Green
Chem. 2016, 18, 3232−3238.
■
(15) Zhang, G.; Hanson, S. K. Org. Lett. 2013, 15, 650−653.
(16) Xu, C.; Goh, L. Y.; Pullarkat, S. A. Organometallics 2011, 30,
6499−6502.
(17) Lee, S.-H.; Nikonov, G. I. ChemCatChem 2015, 7, 107−113.
(18) Kim, D.; Kang, B.; Hong, S. H. Org. Chem. Front. 2016, 3, 475−
479.
REFERENCES
■
(1) (a) Trost, B. M. Science 1991, 254, 1471−1477. (b) Trost, B. M.
Angew. Chem., Int. Ed. Engl. 1995, 34, 259−281. (c) Trost, B. M. Acc.
Chem. Res. 2002, 35, 695−705.
(19) Eberle, U.; Felderhoff, M.; Schuth, F. Angew. Chem., Int. Ed.
̈
2009, 48, 6608−6630.
(20) (a) Trincado, M.; Banerjee, D.; Grutzmacher, H. Energy Environ.
̈
(2) (a) Friedrich, A.; Schneider, S. ChemCatChem 2009, 1, 72−73.
(b) Johnson, T. C.; Morris, D. J.; Wills, M. Chem. Soc. Rev. 2010, 39,
81−88. (c) Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A.
S. Chem. Rev. 2011, 111, 1761−1779. (d) Gunanathan, C.; Milstein, D.
Science 2013, 341, 1229712. (e) Werkmeister, S.; Neumann, J.; Junge,
K.; Beller, M. Chem. - Eur. J. 2015, 21, 12226−12250. (f) Nielsen, M.
Hydrogen Production by Homogeneous Catalysis: Alcohol Accept-
orless Dehydrogenation. In Hydrogen Production and Remediation of
Carbon and Pollutants; Lichtfouse, E., Schwarzbauer, J., Robert, D.,
Eds.; Springer: Berlin, 2015; pp 1−60. (g) Ventura-Espinosa, D.; Mata,
J. A. Eur. J. Inorg. Chem. 2016, 2016, 2667−2675.
Sci. 2014, 7, 2464−2503. (b) Giustra, Z. X.; Ishibashi, J. S. A.; Liu, S.-
Y. Coord. Chem. Rev. 2016, 314, 134−181. (c) Preuster, P.; Papp, C.;
Wasserscheid, P. Acc. Chem. Res. 2017, 50, 74−85.
(21) Crabtree, R. H. Energy Environ. Sci. 2008, 1, 134−138.
(22) (a) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am.
Chem. Soc. 2014, 136, 8564−8567. (b) Zhou, W.; Taboonpong, P.;
Aboo, A. H.; Zhang, L.; Jiang, J.; Xiao, J. Synlett 2016, 27, 1806−1809.
(23) (a) Tsuji, Y.; Kotachi, S.; Huh, K.-T.; Watanabe, Y. J. Org. Chem.
1990, 55, 580−584. (b) Muthaiah, S.; Hong, S. H. Adv. Synth. Catal.
2012, 354, 3045−3053. (c) Tseng, K.-N. T.; Rizzi, A. M.; Szymczak,
N. K. J. Am. Chem. Soc. 2013, 135, 16352−16355. (d) Stubbs, J. M.;
Hazlehurst, R. J.; Boyle, P. D.; Blacquiere, J. M. Organometallics 2017,
36, 1692−1698.
(3) The Hydrogen Economy: Opportunities, Costs, Barriers and
R&D Needs; National Academic Press: Washington, DC, 2004.
(4) See for example: (a) Murahashi, S. I. Angew. Chem., Int. Ed. Engl.
1995, 34, 2443−2465. (b) Murahashi, S. I.; Imada, I. in Transition
Metals for Organic Synthesis, 2nd ed.; Beller, M., Bolm, C., Eds.; Wiley-
VCH: Weinheim, Germany, 2004; Vol. 2, pp 497−507. (c) Martin, S.
F. Pure Appl. Chem. 2009, 81, 195−204. (d) Gawronski, J.; Wascinska,
N.; Gajewy, J. Chem. Rev. 2008, 108, 5227−5252.
(24) Xu, R.; Chakraborty, S.; Yuan, H.; Jones, W. D. ACS Catal.
2015, 5, 6350−6354.
(25) (a) Wang, Z.; Tonks, I.; Belli, J.; Jensen, C. M. J. Organomet.
Chem. 2009, 694, 2854−2857. (b) Yamaguchi, R.; Ikeda, C.;
Takahashi, Y.; Fujita, K. J. Am. Chem. Soc. 2009, 131, 8410−8412.
(c) Li, H.; Jiang, J.; Lu, G.; Huang, F.; Wang, Z.-X. Organometallics
2011, 30, 3131−3141. (d) Zhang, X.-B.; Xi, Z. Phys. Chem. Chem. Phys.
2011, 13, 3997−4004. (e) Wu, J.; Talwar, D.; Johnston, S.; Yan, M.;
Xiao, J. Angew. Chem., Int. Ed. 2013, 52, 6983−6987. (f) Fujita, K.;
Tanaka, Y.; Kobayashi, M.; Yamaguchi, R. J. Am. Chem. Soc. 2014, 136,
4829−4832. (g) Manas, M. G.; Sharninghausen, L. S.; Lin, E.;
Crabtree, R. H. J. Organomet. Chem. 2015, 792, 184−189. (h) Talwar,
D.; Gonzalez-de-Castro, A.; Li, H. Y.; Xiao, J. Angew. Chem., Int. Ed.
2015, 54, 5223−5227. (i) Wang, C.; Xiao, J. Chem. Commun. 2017, 53,
3399−3411.
(5) See for example: (a) Kobayashi, S.; Ishitani, H. Chem. Rev. 1999,
99, 1069−1094. (b) Adams, J. P. J. Chem. Soc., Perkin Trans. 1 2000,
125−139. (c) Kobayashi, S.; Mori, Y.; Fossey, J. S.; Salter, M. M.
Chem. Rev. 2011, 111, 2626−2704. (d) Yus, M.; Gon
C.; Foubelo, F. Chem. Rev. 2011, 111, 7774−7854. (e) Yus, M.;
Gonzalez-Gomez, J. C.; Foubelo, F. Chem. Rev. 2013, 113, 5595−5698.
́ ́
zalez-Gomez, J.
́
́
(6) See for example: (a) Liu, G.; Cogan, D. A.; Owens, T. D.; Tang,
T. P.; Ellman, J. A. J. Org. Chem. 1999, 64, 1278−1284. (b) Naeimi,
H.; Salimi, F.; Rabiei, K. J. Mol. Catal. A: Chem. 2006, 260, 100−104.
(c) Reeves, J. T.; Visco, M. D.; Marsini, M. A.; Grinberg, N.; Busacca,
C. A.; Mattson, A. E.; Senanayake, C. H. Org. Lett. 2015, 17, 2442−
2445.
(26) Luca, O. R.; Huang, D. L.; Takase, M. K.; Crabtree, R. H. New J.
Chem. 2013, 37, 3402−3405.
H
DOI: 10.1021/acs.organomet.7b00521
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(27) Hara, T.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K.
Tetrahedron Lett. 2003, 44, 6207−6210.
(44) (a) Esteruelas, M. A.; Lahoz, F. J.; Lopez, J. A.; Oro, L. A.;
Schlunken, C.; Valero, C.; Werner, H. Organometallics 1992, 11,
̈
2034−2043. (b) Barrio, P.; Esteruelas, M. A.; Onate, E. Organo-
(28) (a) Mikami, Y.; Ebata, K.; Mitsudome, T.; Mizugaki, T.;
Jitsukawa, K.; Kaneda, K. Heterocycles 2010, 82, 1371−1377.
(b) Damodara, D.; Arundhathi, R.; Likhar, P. R. Adv. Synth. Catal.
2014, 356, 189−198.
̃
metallics 2004, 23, 1340−1348. (c) Eguillor, B.; Esteruelas, M. A.;
́
García-Raboso, J.; Olivan, M.; Onate, E.; Pastor, I. M.; Penafiel, I.; Yus,
̃ ̃
M. Organometallics 2011, 30, 1658−1667. (d) Esteruelas, M. A.;
García-Raboso, J.; Olivan, M. Organometallics 2011, 30, 3844−3852.
́
́ ́
(29) Esteruelas, M. A.; Lopez, A. M.; Olivan, M. Chem. Rev. 2016,
(45) The total relaxation rate for a H(n) hydride ligand (R(n) = 1/
T1(min)H(n)) is the addition of the relaxation rate due to the hydride
dipole−dipole interaction (at 300 MHz RH‑H = 129.8/r(HH)6) and that
due to all other relaxation contributors (R*). See: Jessop, P. G.;
Morris, R. H. Coord. Chem. Rev. 1992, 121, 155−284. Thus, in this
case, one can obtain the separation between the hydride ligands by
using the following equation system: RHt = R* + RHt−Hc; RHc = R* +
116, 8770−8847.
(30) (a) Borowski, A. F.; Sabo-Etienne, S.; Donnadieu, B.; Chaudret,
B. Organometallics 2003, 22, 4803−4809. (b) Prechtl, M. H. G.;
Holscher, M.; Ben-David, Y.; Theyssen, N.; Loschen, R.; Milstein, D.;
̈
Leitner, W. Angew. Chem., Int. Ed. 2007, 46, 2269−2272. (c) Caballero,
A.; Sabo-Etienne, S. Organometallics 2007, 26, 1191−1195. (d) Reg-
uillo, R.; Grellier, M.; Vautravers, N.; Vendier, L.; Sabo-Etienne, S. J.
Am. Chem. Soc. 2010, 132, 7854−7855.
2RHt−Hc
.
(46) (a) Aracama, M.; Esteruelas, M. A.; Lahoz, F. J.; Lop
́
ez, J. A.;
(31) Esteruelas, M. A.; Oro, L. A. Chem. Rev. 1998, 98, 577−588.
(32) See for example: (a) Lu, X. Y.; Lin, Y. Q. G.; Ma, D. W. Pure
Appl. Chem. 1988, 60, 1299−1306. (b) Ma, D.; Lu, X. Tetrahedron
Lett. 1989, 30, 2109−2112. (c) Lin, Y.; Zhu, X.; Zhou, Y. A. J.
Organomet. Chem. 1992, 429, 269−274. (d) Park, S.; Brookhart, M. J.
Am. Chem. Soc. 2012, 134, 640−653.
Meyer, U.; Oro, L. A.; Werner, H. Inorg. Chem. 1991, 30, 288−293.
(b) Atencio, R.; Esteruelas, M. A.; Lahoz, F. J.; Oro, L. A.; Ruiz, N.
Inorg. Chem. 1995, 34, 1004−1006. (c) Kuhlman, R.; Streib, W. E.;
Huffman, J. C.; Caulton, K. G. J. Am. Chem. Soc. 1996, 118, 6934−
6945. (d) Wolf, J.; Stuer, W.; Grunwald, C.; Gevert, O.; Laubender,
̈
M.; Werner, H. Eur. J. Inorg. Chem. 1998, 1998, 1827−1834.
(33) Rossin, A.; Peruzzini, M. Chem. Rev. 2016, 116, 8848−8872.
(34) Bertoli, M.; Choualeb, A.; Gusev, D. G.; Lough, A. J.; Major, Q.;
Moore, B. Dalton Trans. 2011, 40, 8941−8949.
(e) Bhattacharya, S.; Gupta, P.; Basuli, F.; Pierpont, C. G. Inorg.
Chem. 2002, 41, 5810−5816. (f) Baya, M.; Esteruelas, M. A.; Onate, E.
̃
Organometallics 2011, 30, 4404−4408.
(35) Bianchini, C.; Farnetti, E.; Graziani, M.; Peruzzini, M.; Polo, A.
Organometallics 1993, 12, 3753−3761.
́
(47) Crochet, P.; Esteruelas, M. A.; Lopez, A. M.; Martínez, M.-P.;
Olivan
(48) Tolman, C. A. Chem. Rev. 1977, 77, 313−348.
(49) Buil, M. L.; Esteruelas, M. A.; García-Yebra, C.; Gutier
Puebla, E.; Olivan
́
, M.; Onate, E.; Ruiz, N. Organometallics 1998, 17, 4500−4509.
̃
(36) Esteruelas, M. A.; Oro, L. A.; Valero, C. Organometallics 1992,
11, 3362−3369.
́
rez-
(37) Esteruelas, M. A.; Lop
2015, 5, 187−191.
́
ez, A. M.; Mora, M.; Onate, E. ACS Catal.
̃
́
, M. Organometallics 2000, 19, 2184−2193.
(50) (a) Grunwald, C.; Laubender, M.; Wolf, J.; Werner, H. J. Chem.
̈
(38) (a) Esteruelas, M. A.; Sola, E.; Oro, L. A.; Werner, H.; Meyer, U.
J. Mol. Catal. 1988, 45, 1−5. (b) Esteruelas, M. A.; Sola, E.; Oro, L. A.;
Werner, H.; Meyer, U. J. Mol. Catal. 1989, 53, 43−52. (c) Esteruelas,
M. A.; Valero, C.; Oro, L. A.; Meyer, U.; Werner, H. Inorg. Chem.
1991, 30, 1159−1160.
Soc., Dalton Trans. 1998, 833−839. (b) Bennett, M. A.; Byrnes, M. J.;
Chung, G.; Edwards, A. J.; Willis, A. C. Inorg. Chim. Acta 2005, 358,
1692−1708.
(51) (a) Dam, J. H.; Osztrovszky, G.; Nordstrøm, L. U.; Madsen, R.
Chem. - Eur. J. 2010, 16, 6820−6827. (b) Kang, B.; Fu, Z.; Hong, S. H.
J. Am. Chem. Soc. 2013, 135, 11704−11707.
(52) Bennett, J. S.; Charles, K. L.; Miner, M. R.; Heuberger, C. F.;
Spina, E. J.; Bartels, M. F.; Foreman, T. Green Chem. 2009, 11, 166−
168.
(39) (a) Werner, H.; Meyer, U.; Esteruelas, M. A.; Sola, E.; Oro, L. A.
J. Organomet. Chem. 1989, 366, 187−196. (b) Espuelas, J.; Esteruelas,
M. A.; Lahoz, F. J.; Oro, L. A.; Valero, C. Organometallics 1993, 12,
663−670.
(40) Esteruelas, M. A.; Herrero, J.; Lop
Organometallics 2001, 20, 3202−3205.
(41) Alos, J.; Bolano, T.; Esteruelas, M. A.; Olivan
Valencia, M. Inorg. Chem. 2013, 52, 6199−6213.
(42) (a) Baya, M.; Eguillor, B.; Esteruelas, M. A.; Olivan
E. Organometallics 2007, 26, 6556−6563. (b) Eguillor, B.; Esteruelas,
M. A.; Olivan, M.; Puerta, M. Organometallics 2008, 27, 445−450.
(c) Alabau, R. G.; Eguillor, B.; Esler, J.; Esteruelas, M. A.; Olivan, M.;
Onate, E.; Tsai, J.-Y.; Xia, C. Organometallics 2014, 33, 5582−5596.
́ ́
ez, A. M.; Olivan, M.
(53) Shirai, S.; Nara, H.; Kayaki, Y.; Ikariya, T. Organometallics 2009,
28, 802−809.
́
́
, M.; Onate, E.;
̃
̃
(54) Liu, S.; Yu, Y.; Liebeskind, L. S. Org. Lett. 2007, 9, 1947−1950.
(55) Jiang, L.; Jin, L.; Tian, H.; Yuan, X.; Yu, X.; Xu, Q. Chem.
Commun. 2011, 47, 10833−10835.
́
, M.; Onate,
̃
(56) Mastalir, M.; Glatz; Gorgas, N.; Stoger, B.; Pittenauer, E.;
̈
́
Allmaier, G.; Veiros, L. F.; Kirchner, K. Chem. - Eur. J. 2016, 22,
́
12316−12320.
(57) Blessing, R. H. Acta Crystallogr., Sect. A: Found. Crystallogr. 1995,
51, 33−38. SADABS: Area-detector absorption correction; Bruker-AXS,
Madison, WI, 1996.
̃
́
(d) Bolano, T.; Esteruelas, M. A.; Fernandez, I.; Onate, E.; Palacios,
̃
̃
A.; Tsai, J.-Y.; Xia, C. Organometallics 2015, 34, 778−789. (e) Bolano,
T.; Esteruelas, M. A.; Gay, M. P.; Onate, E.; Pastor, I. M.; Yus, M.
Organometallics 2015, 34, 3902−3908. (f) Alabau, R. G.; Esteruelas, M.
́
A.; Olivan, M.; Onate, E.; Palacios, A. U.; Tsai, J.-Y.; Xia, C.
̃
̃
(58) SHELXL 2016/6: Sheldrick, G. M. Acta Crystallogr., Sect. A:
Found. Crystallogr. 2008, 64, 112−122.
̃
Organometallics 2016, 35, 3981−3995. (g) Eguillor, B.; Esteruelas, M.
A.; Lezaun, V.; Olivan, M.; Onate, E.; Tsai, J.-Y.; Xia, C. Chem. - Eur. J.
2016, 22, 9106−9110.
(43) (a) Esteruelas, M. A.; Lahoz, F. J.; Lop
L. A.; Ruiz, N.; Sola, E.; Tolosa, J. I. Inorg. Chem. 1996, 35, 7811−
7817. (b) Castillo, A.; Barea, G.; Esteruelas, M. A.; Lahoz, F. J.; Lledos
́
́
̃
́
ez, A. M.; Onate, E.; Oro,
̃
́
,
A.; Maseras, F.; Modrego, J.; Onate, E.; Oro, L. A.; Ruiz, N.; Sola, E.
̃
Inorg. Chem. 1999, 38, 1814−1824. (c) Esteruelas, M. A.; García-
́
Raboso, J.; Olivan, M.; Onate, E. Inorg. Chem. 2012, 51, 5975−5984.
̃
(d) Esteruelas, M. A.; García-Raboso, J.; Olivan
51, 9522−9528. (e) Casarrubios, L.; Esteruelas, M. A.; Larramona, C.;
Muntaner, J. G.; Olivan, M.; Onate, E.; Sierra, M. A. Organometallics
2014, 33, 1820−1833. (f) Eguillor, B.; Esteruelas, M. A.; Fernan
Gomez-Gallego, M.; Lledos, A.; Martín-Ortiz, M.; Olivan, M.; Onate,
E.; Sierra, M. A. Organometallics 2015, 34, 1898−1910.
́
, M. Inorg. Chem. 2012,
́
̃
́
dez, I.;
́
́
́
̃
I
DOI: 10.1021/acs.organomet.7b00521
Organometallics XXXX, XXX, XXX−XXX