Cuboidal Mo3S4 Clusters as a Platform for Exploring Catalysis: A Three-Center Sulfur Mechanism for Alkyne Semihydrogenation

Article abstract of DOI:10.1021/acscatal.8b02254

We report a trinuclear Mo₃S₄ diamino cluster that promotes the semihydrogenation of alkynes. Based on experimental and computational results, we propose an unprecedented mechanism in which only the three bridging sulfurs of the cluster act as the active site for this transformation. In the first step, two of these μ-S ligands react with the alkyne to form a dithiolene adduct; this process is formally analogous to the olefin adsorption on MoS₂ surfaces. Then, H₂ activation occurs in an unprecedented way that involves the third μ-S center, in cooperation with one of the dithiolene carbon atoms. Notably, this step does not imply any direct interaction between H₂ and the metal centers, and directly results in the formation of an intermediate featuring one (μ-S)-H and one C-H bond. Finally, such half-hydrogenated intermediate can either undergo a reductive elimination step that results in the Z-alkene product, or evolve into an isomerized analogue whose subsequent reductive elimination generates the E-alkene product. Interestingly, the substituents on the alkynes have a major impact on the relative barriers of these two processes, with the semihydrogenation of dimethyl acetylenedicarboxylate (dmad) resulting in the stereoselective formation of dimethyl maleate, whereas that of diphenylacetylene (dpa) leads to mixtures of Z- and E-stilbene. The results herein could have significant implications on the understanding of the catalytic properties of MoS₂-based materials.

Full text of DOI:10.1021/acscatal.8b02254

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Letter
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Cuboidal MoS clusters as a platform for exploring catalysis: A
three-center sulfur mechanism for alkyne semihydrogenation.
Andrés G. Algarra, Eva Guillamón, Juan Andrés, M. Jesús Fernández-Trujillo,
Elena Pedrajas, Jose Angel Pino-Chamorro, Rosa Llusar, and Manuel G. Basallote
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02254 • Publication Date (Web): 09 Jul 2018
Downloaded from http://pubs.acs.org on July 10, 2018
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Cuboidal Mo3S4 clusters as a platform for exploring catalysis: A
three-center sulfur mechanism for alkyne semihydrogenation
Andrés G. Algarra,*,†,# Eva Guillamón,‡,# Juan Andrés,‡ M. Jesús FernándezꢀTrujillo,† Elena Pedrajas,‡
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Jose Ángel PinoꢀChamorro,† Rosa Llusar,*,‡ and Manuel G. Basallote*,†
† Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Universidad de Cádiz, Apartado
40, Puerto Real, 11510 Cádiz, Spain
‡ Departament de Química Física i Analítica, Universitat Jaume I, Av. Sos Baynat s/n, 12071 Castelló, Spain
ABSTRACT: We report a trinuclear Mo3S4 diamino cluster that promotes the semihydrogenation of alkynes. Based on
experimental and computational results, we propose an unprecedented mechanism in which only the three bridging sulfurs of the
cluster act as the active site for this transformation. In the first step, two of these µꢀS ligands react with the alkyne to form a
dithiolene adduct, this process being formally analogous to the olefin adsorption on MoS2 surfaces. Then, H2 activation takes place
in an unprecedented way that involves the third µꢀS center in cooperation with one of the dithiolene carbon atoms. Notably, this
step does not imply any direct interaction between H2 and the metal centers, and directly results in the formation of an intermediate
featuring one (ꢀꢀS)−H and one C−H bond. Finally, such halfꢀhydrogenated intermediate can either undergo a reductive elimination
step that results in the Zꢀalkene product, or evolve into an isomerized analogue whose subsequent reductive elimination generates
the Eꢀalkene product. Interestingly, the substituents on the alkynes have a major impact on the relative barriers of these two
processes, with the semihydrogenation of dimethyl acetylenedicarboxylate (dmad) resulting in the stereoselective formation of
dimethyl maleate, whereas that of diphenylacetylene (dpa) leads to mixtures of Zꢀ and Eꢀstilbene. The results herein could have
significant implications in the understanding of the catalytic properties of MoS2ꢀbased materials.
KEYWORDS: catalysis, molybdenum disulfide, alkyne semihydrogenation, hydrogen activation, density functional theory.
Catalytic semihydrogenation of alkynes is of great practical
significance in chemistry today, and palladium heterogeneous
catalysts have proved to be the most effective in achieving this
transformation.1ꢀ2 According to the proposed mechanism,3 H2
is adsorbed dissociatively over the surface of the metal,
whereas the alkyne is adsorbed through its interaction with the
πꢀbonds. Then hydrogenation proceeds via two stepwise C−H
bond formations. Due to its wellꢀdefined stereochemistry, Zꢀ
selectivity is intrinsic to this transformation, but the halfꢀ
hydrogenated intermediate allows for alternative cis-trans
Figure 1. Topological relationship between the molecular cluster
isomerization and H−D exchange processes.4 The use of hoꢀ
mogeneous conditions in the palladium catalyzed semihydroꢀ
genation of alkynes is, in contrast, much less frequent.2,5
[1]+ and the basal planes of a MoS2 monolayer.
about the generation of Mo−H hydrides. Nevertheless, this
option cannot be ruled out due to the facile migration of adꢀ
sorbed H atoms from Mo to S in this material.8
Molybdenum sulphides, traditionally employed in industry
for the hydrodesulphurisation (HDS) of fossil fuels, have
recently emerged as lowꢀcost alternatives to platinum group
metals as hydrogenation and hydrogen evolution reaction
(HER) catalysts.6 Similarly to the semihydrogenation mechaꢀ
nism, HDS processes start with the adsorption of the Sꢀ
containing substrate and dissociation of H2 at the surface of
the catalyst. While substrate adsorption is postulated to occur
at coordinatively unsaturated sites, there is no consensus on
the H2 activation mechanism. As a result, two possibilities
have been envisioned: the heterolytic dissociation of H2 to
generate Mo−H and S−H bonds, and the homolytic dissociaꢀ
tion on disulphides resulting in two S−H groups.7 Notably,
while the formation of S−H species has been observed specꢀ
troscopically, there is no experimental evidence
The inherent difficulties to obtain mechanistic information
from heterogeneous catalytic systems have led, over the years,
to the design of molecular models capable of emulating the
reactivity of the MoS2 edge sites. Specifically, in the context
of HER catalysis, the active edge sites have been successfully
mimicked by the monoꢀ, diꢀ, and trinuclear species
[(PY5Me2)MoS2]2+,9 [Mo2(S2)6]2ꢀ,10 and [Mo3(µ3ꢀS)(µꢀ
S2)3(S2)3]2ꢀ,11 respectively. The latter compound shows evident
similarities to the incomplete cubaneꢀtype Mo3(µ3ꢀS)(µꢀS)3
clusters. However, whereas [Mo3S13]2ꢀ represents a model for
the edges, Mo3(µ3ꢀS)(µꢀS)3 clusters can be considered as modꢀ
els of the basal planes of MoS2, as illustrated in Figure 1 for
[Mo3(µ3ꢀS)(µꢀS)3Cl3(dmen)3]+ ([1]+). In addition to the strucꢀ
tural similarity, MoS2 and Mo3S4 clusters also share behavꢀ
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ioural patterns. Thus, the reaction of Mo3S4 clusters with alꢀ
kynes leads to the formation of dithiolene products by means
of a [3+2] cycloaddition reaction,12 with the process being
formally analogous to the alkyne adsorption at the MoS2
surface. Similarly, both Mo3S4 clusters13 and MoS2 structures14
catalyze relevant organic transformations such as the hydroꢀ
genation of nitroarenes.
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In this Letter, we report that the cluster [1]+ catalyzes the
semihydrogenation of alkynes via a mechanism that is signifiꢀ
cantly different to that assumed for the hydrogenation of unꢀ
saturated hydrocarbons at the surface of MoS2. In the present
mechanism: a) only the three bridging sulfur ligands within
the cluster take part in all the bond breaking and formation
events; b) H2 activation takes place without prior interaction
with the cluster through a step that directly results in the halfꢀ
hydrogenated intermediate; c) the Mo centers are not directly
involved in the process. As the whole set of transformations
occurs at the triangle defined by the three ꢁꢀS ligands, the
possibility of a similar mechanism operating for reactions with
MoS2 must be considered.
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Figure 2. ORTEP representation of [2]+ (ellipsoids at 50% probaꢀ
This study was prompted not only by previous work on the
reactivity of cuboidal Mo3S4 clusters towards alkynes,12 but
also by the high activity of [Mo3S4Cl3(dmen)3]+ ([1]+) as a
catalyst for the selective hydrogenation of nitroarenes into
anilines.13 It is well known that the [3+2] cycloaddition reacꢀ
tion between these clusters and alkynes is affected by the
nature of the ancillary ligands at the cluster as well as the
alkyne substituents.12 In the case of [1]+, the reaction with
dmad results in formation of the corresponding dithiolene
adduct, as evidenced by the appearance of a band at 887 nm
characteristic of these addition products. The process takes
place in a single step with a secondꢀorder rate constant of 0.72
bility) with the atomꢀnumbering scheme. Hydrogen atoms have
been omitted for clarity. Intermetallic distances: Mo(1)−Mo(2) =
2.653 Å, Mo(1)−Mo(3) = 2.772 Å and Mo(2)−Mo(3) = 2.778 Å,
C(15)−C(16)=1.329 Å.
the dmad precursor (1.183 Å).16 The sp2 character of these
carbon atoms is also supported by the values of bond angles
close to 120º, similarly to those found in other dinuclear and
trinuclear dithiolate derivatives, and also in agreement with a
−2 charge for this ligand.17 This implies an internal electron
transfer process induced by the interaction with dmad, which
is also reflected in the shortening of the Mo−Mo bond between
the Mo centers not involved in the [3+2] cycloaddition reacꢀ
tion from 2.759 Å in [1]+, characteristic of a Mo−Mo single
bond, to 2.653 Å in [2]+, typical of a Mo−Mo double bond.
Thus, insertion of the alkyne results in a reduction of the metal
atoms from Mo(IV)3 to Mo(III)2Mo(IV), as the dithiolene
cluster contains eight cluster skeletal electrons for the forꢀ
mation of two single and one double Mo−Mo bonds.18 The
structure of [2]+ is well reproduced by DFT calculations (see
below) and supports the existence of an internal electron transꢀ
fer also reflected in their redox properties. Thus, whereas the
cyclic voltammogram of [1]+ shows a sole reversible reduction
at ꢀ0.45 V (vs. Ag/AgCl), [2]+ undergoes one reversible reducꢀ
tion at ꢀ0.34 V and two quasiꢀreversible oxidations at 0.84 and
1.16 V, respectively (see Figure S1).
After isolation and characterization of [2]+, we investigated
its reactivity towards H2. Hydrogenation of [2]+ at 20 bar and
70 ºC for 18 h in CH3CN resulted in the stereoselective forꢀ
mation of dimethyl maleate, with no evidence of formation of
dimethyl fumarate. No conversion was observed when the
reaction temperature was lowered to 30 ºC. DFT calculations
were then performed to unveil the reaction mechanism. A
[2+2] addition of H2 across one of the C−S bonds at [2]+ was
computed initially but, as expected, the calculations resulted in
a prohibitive free energy barrier of 59.2 kcal molꢀ1 together
with a thermodynamically uphill process (ꢁGr = 34.0 kcal molꢀ
1, see Figure S5). Instead, a more feasible mechanism was
found to involve the interaction of H2 with the remaining ꢀꢀS
ligand and one of the sp2 C atoms of the dithiolate ligands (see
Figure 3). The process described by TS2 features a free energy
±
0.02 Mꢀ1sꢀ1,
a value close to those obtained for
[Mo3S4(acac)3(py)3]+ and [Mo3S4Cl3(tBuꢀbipy)3)]+.12 In conꢀ
trast, alkynes with electron donating substituents, such as dpa,
do not react with [1]+. The free energy profiles, obtained at
DFT level (see SI for computational details), point out that
both processes take place in a concerted fashion, via transition
states in which both C−S bonds are formed simultaneously
(see Figure S4). However, while the reaction between [1]+ and
dmad is slightly exergonic (ꢀ2.6 kcal molꢀ1) and occurs with an
activation barrier of 9.5 kcal molꢀ1, the reaction with dpa is
endergonic by 4.2 kcal molꢀ1 and therefore not thermodynamiꢀ
cally favoured. These results thus show the agreement beꢀ
tween experimental and computational results.
Accordingly, the reaction between [1]+ and a ten molar exꢀ
cess of dmad in acetonitrile at room temperature quantitatively
affords a stable 1:1 adduct of formula [Mo3(µ3ꢀS)(µꢀS)(µ3ꢀ
SC(CO2CH3)=C(CO2CH3)S)Cl3(dmen)3]BF4 [2]BF4, see SI for
full experimental details). [2]+ was crystallized as the
[2]2[Mo6Cl18]•3CH3OH salt and its structure determined by
single crystal Xꢀray diffraction. An ORTEP representation of
[2]+ is given in Figure 2. The structure of [2]+ confirms the
formation of two C−S bonds, with the addition of the unsatuꢀ
rated molecule resulting in the loss of the C3 symmetry of the
incomplete cuboidal Mo(IV)3 cluster precursor. While the two
Mo−(µꢀS) bonds involved in the dithiolene formation are
slightly elongated by ca. 0.06ꢀ0.13 Å on going from [1]+ to
[2]+,15 there are no significant changes in the other Moꢀligand
bond lengths. The geometry of the resulting dithiolate correꢀ
sponds to the maleate adduct (Zꢀisomer), with a C=C bond
distance (1.329 Å) far from that of
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Figure 3. Computed pathways for the activation hydrogenation of dmad (R=CO2CH3) at [2]+. Free energy values are given in kcal molꢀ1
quoted relative to [1]+ + dmad + H2. For simplicity, dmen and Cl ligands are not drawn.
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tively. From [3]+, dimethyl maleate can be released in a subseꢀ
quent reductive elimination step between the H atom at ꢀꢀSH
and Ca via TS3. Such transition state appears only 0.5 kcal
molꢀ1 above [3]+ (see Figure 3) and so, once the latter species
is formed, it will readily undergo exergonic elimination of
dimethyl maleate. Note that, in relation to heterogeneous caꢀ
talysis, [3]+ is formally analogous to the halfꢀhydrogenated
intermediate involved in processes such as the cis-trans isomꢀ
erization of alkenes,19 and indeed the computations show that
[3]+ can also rearrange into [3iso]+ through a process that
implies the cleavage of the Cb−Sb bond. This allows for the
operation of a parallel pathway that results in the thermodyꢀ
namically more stable Eꢀalkene via TS3iso. Importantly, the
free energy difference between TS3 and TS4 determines the
extent in which kinetic (dimethyl maleate) and thermodynamic
(dimethyl fumarate) products are formed. The computations in
Figure 3 show that elimination of dimethyl maleate from [3]+
is 7.8 kcal molꢀ1 more facile than its isomerization, a ꢁꢁG#
value that is in agreement with the stereoselective formation of
dimethyl maleate.
Figure 4. For TS2, schematic representation of the two main
symmetryꢀallowed orbital interactions between [2]+ and H2. For
simplicity, additional orbital contributions from the Mo atoms
were not drawn, see Figure S7.
barrier of 35.1 kcal molꢀ1 and implies the cleavage of the H−H
σ bond together with the formation of new (ꢀꢀS)−H and C−H
bonds. In fact, the concerted nature of the process is highlightꢀ
ed by the frontier molecular orbital (FMO) analysis of such
structure in Figure 4. This shows a main interaction between
the HOMO of [2]+, which features contributions from a lone
pair of the (ꢀꢀS) ligand and the π orbital of the dithiolene C=C
moiety, and the σ* orbital of H2. Furthermore, there is an
additional interaction between the LUMO of the cluster, partly
located at the π∗ orbital of the dithiolene C=C moiety, and the
σ orbital of H2. The product of this reaction, the cluster [3]+, is
19.2 kcal molꢀ1 less stable than its precursors (see Figure 3),
and a comparison of its structure with that of [2]+ shows that
the interaction with H2 results in a rearrangement of the elecꢀ
tron density within the cluster core. Thus, [3]+ can be viewed
as featuring a C−C simple bond, in agreement with its longer
bond distance (1.50 Å, cf. 1.35 Å in [2]+), whereas the differꢀ
ent Cb−Sb and Ca−Sa bond distances (1.92 and 1.77 Å, respecꢀ
tively) are in line with simple and double C−S bonds, respecꢀ
According to our observations regarding the regeneration of
[1]+ upon semihydrogenation of [2]+ and the above calculated
mechanism, the process is expected to be catalytic. Unfortuꢀ
nately, the catalytic hydrogenation of dmad requires harsh
conditions that are incompatible with the stability of the orꢀ
ganic precursor. Then, the catalytic hydrogenation of dpa,
which does not react with [1]+ in the absence of H2 to an apꢀ
preciable extent, was investigated by varying the hydrogen
pressure, temperature, amount of catalyst and reaction time
(see Table 1 and Tables SI1 and SI2 in the SI). The best results
were obtained after 65 hours at 150 ºC in the presence of 12%
catalyst, using CH3CN as the solvent and H2 at 100 bar (see
Table 1, entry 7). These conditions led to 62% conversion,
with Zꢀ and Eꢀalkenes in yields of 53% and 9%, respectively.
Thus, in this case the reduction of the unsaturated molecule is
not stereoselective (see Table 1), and an average Z:E ratio of
6.5:1 is observed.
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Table 1. Variation of the catalyst loading and reaction time in
the semihydrogenation of dpa.a
previously generated intermediate or from its isomerized anaꢀ
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logue releases the Zꢀ and Eꢀalkenes, respectively. Hence, the
stereochemistry of the alkene product depends on the relative
barriers for the isomerization and reductive elimination of
such intermediate. Remarkably, the present results clearly
show that an adequate arrangement of three S atoms allows for
the alkyne and H2 interaction required for the semihydrogenaꢀ
tion process to take place, without direct participation of the
Mo atoms in any of the reaction steps.
Entry
Catalyst loading
(mol%)
Time (h)
Conversion
(%)b
Yield (%)b
Given the structural similarity between Mo3S4 clusters and
MoS2 basal planes, which also display threeꢀsulfur triangles
linked through Mo atoms (see Figure 1), the possibility of
analogous chemical transformations at MoS2ꢀbased materials
should be considered.
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ASSOCIATED CONTENT
12
5
Detailed synthetic and experimental procedures, kinetic data,
crystal structure determination and computational details. Carteꢀ
sian coordinates of all optimized structures. This material is availꢀ
able free of charge via the Internet at http://pubs.acs.org.
8
12
AUTHOR INFORMATION
Corresponding Authors
a
Reaction conditions: dpa (0.1 mmol), H2 (100 bar), 150 ºC,
CH3CN (2 mL). b Determined by GC analysis using nꢀhexadecane
as an internal standard.
* andres.algarra@uca.es
* rosa.llusar@uji.es
* manuel.basallote@uca.es
The reaction with dpa was also modelled using DFT calcuꢀ
lations and the results for a mechanism analogous to that in
Figure 3 are shown in Figure S8. As previously anticipated,11
alkyne substituents have a large effect not only on the kinetics
and thermodynamics of the dithiolene product formation [2Ph]+
(the subindex Ph is used to refer to dpa), but also on its further
reactivity with H2. The formation of [3Ph]+ is computed to be
thermodynamically uphill by 30.7 kcal molꢀ1, with a free enerꢀ
gy barrier of 42.0 kcal molꢀ1. Despite the unfavourable therꢀ
modynamics, the process is still feasible under the experiꢀ
mental conditions and release of Zꢀstilbene from [3Ph]+ via
TS3Ph (ꢁG# = 37.0 kcal molꢀ1), or its isomerization into
[3isoPh]+ via TS4Ph (ꢁG# = 38.9 kcal molꢀ1) and subsequent
release of Eꢀstilbene, are two significantly exergonic processes
that complete the catalytic cycle. Notably, the computed ꢁꢁG#
value for these two processes is only 1.9 kcal molꢀ1, which
translates into a predicted Z:E ratio of 9.6:1. In spite of the
slight overestimation on the formation of the Zꢀisomer, it is
worth noting that the theoretical ꢁꢁG# for the obtained averꢀ
age Z:E ratio is 1.5 kcal molꢀ1, thus leading to an error of only
0.4 kcal molꢀ1 in this calculation. This is well within the limits
of the soꢀcalled chemical accuracy, and all in all gives further
support to the proposed mechanism.
ORCID
Andrés G. Algarra: 0000ꢀ0002ꢀ5062ꢀ2858
Eva Guillamón: 0000ꢀ0002ꢀ3595ꢀ5929
Juan Andrés: 0000ꢀ0003ꢀ0232ꢀ3957
M. Jesús FernándezꢀTrujillo: 0000ꢀ0003ꢀ2832ꢀ822X
Elena Pedrajas: 0000ꢀ0002ꢀ7392ꢀ8446
Jose Angel PinoꢀChamorro: 0000ꢀ0003ꢀ3513ꢀ1003
Rosa Llusar: 0000ꢀ0002ꢀ3539ꢀ7269
Manuel G. Basallote: 0000ꢀ0002ꢀ1802ꢀ8699
Author Contributions
# These authors contributed equally to this work.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
Financial support from the Spanish Ministerio de Economía y
Competitividad and FEDER funds of the EU (Grants CTQ2015ꢀ
65207ꢀP, CTQ2015ꢀ65707ꢀC2ꢀ2ꢀP, and CTQ2015ꢀ71470ꢀREDT),
Universitat Jaume I (UJIꢀA2016ꢀ05, UJIꢀB2017ꢀ44) and Generaliꢀ
tat Valenciana (PrometeoII/2014/022) is gratefully acknowledged.
ABBREVIATIONS
In summary, herein we show that Mo3S4 clusters are able to
catalyze the semihydrogenation of alkynes. Based on experiꢀ
mental and computational results, we propose an unprecedentꢀ
ed mechanism whereby only the three (ꢀꢀS) ligands of the
cluster are directly involved in all the bond breaking and forꢀ
mation events. Two of these initially interact with the spꢀ
hybridized C atoms of the alkyne resulting in the formation of
a dithiolene adduct. This species subsequently reacts with H2
in a process that results in the concerted cleavage of the H−H
σ bond together with the formation of one (ꢀꢀS)−H and one
C−H bond. Finally, a reductive elimination step from the
PY5Me2,
2,6ꢀbis(1,1ꢀbis(2ꢀpyridyl)ethyl)pyridine;
dmen,
Me2NCH2CH2NMe2; dmad, dimethyl acetylenedicarboxylate;
dpa, diphenylacetylene.
REFERENCES
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Chemistry, Surface Chemistry and Catalytic Behaviour, Elsevier
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Hydrocarbons, Springer Science + Business Media, Inc., New York,
2005; c) F. Zaera, ACS Catal. 2017, 7, 4947ꢀ4967.
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(2) Chinchilla, R.; Nájera, C. Chemicals from Alkynes with Pallaꢀ
dium Catalysts. Chem. Rev. 2014, 114, 1783ꢀ1826.
(3) Horiuti, J. Polanyi, M. Exchange reactions of hydrogen on meꢀ
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