Hydrogen activation by high-valent oxo-molybdenum(vi) and -rhenium(vii) and -(v) compounds

Article abstract of DOI:10.1039/b719375k

The high-valent oxo-molybdenum(vi) and -rhenium(vii) and -(v) derivatives MoO₂Cl₂, ReCH₃O₃ (MTO) and ReIO ₂(PPh₃)₂ catalyze the selective hydrogenation of alkynes to alkenes at 80 °C under 40 atm of pressure. The reduction of sulfoxides to sulfides has also been performed by oxo-rhenium and -molybdenum complexes using hydrogen as a reducing agent. Activation of hydrogen by MoO ₂Cl₂ and MoO₂(S₂CNEt ₂)₂ was shown by means of DFT calculations to proceed by H-H addition to the Mo=O bond, followed by hydride migration to yield a water complex. The Royal Society of Chemistry.

Full text of DOI:10.1039/b719375k

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Number 13 | 7 April 2008 | Pages 1649–1796
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PAPER
Royo et al.
COMMUNICATION
Jones et al.
COMMUNICATION
Lindoy et al.
Hydrogen activation by high valent
oxo-molybdenum(vi) and
-rhenium(vii) and -(v) compounds
Pentanuclear tetra-decker
luminescent lanthanide Schiff base
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Assembly of a trinuclear metallo-
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PAPER
www.rsc.org/dalton | Dalton Transactions
Hydrogen activation by high-valent oxo-molybdenum(VI) and -rhenium(VII)
and -(V) compounds†
Patr´ıcia M. Reis,a Paulo Jorge Costa,‡b Carlos C. Roma˜o,a Jose´ A. Fernandes,a Maria Jose´ Calhorda*b and
Beatriz Royo*a
Received 17th December 2007, Accepted 8th February 2008
First published as an Advance Article on the web 27th February 2008
DOI: 10.1039/b719375k
The high-valent oxo-molybdenum(VI) and -rhenium(VII) and -(V) derivatives MoO2Cl2, ReCH3O3
(MTO) and ReIO2(PPh3)2 catalyze the selective hydrogenation of alkynes to alkenes at 80 ◦C under
40 atm of pressure. The reduction of sulfoxides to sulfides has also been performed by oxo-rhenium and
-molybdenum complexes using hydrogen as a reducing agent. Activation of hydrogen by MoO2Cl2 and
=
MoO2(S2CNEt2)2 was shown by means of DFT calculations to proceed by H–H addition to the Mo O
bond, followed by hydride migration to yield a water complex.
Herein, we report the hydrogenation of alkynes to alkenes
Introduction
with H2 using MoO2Cl2, ReIO2(PPh3)2 and ReCH3O3 (MTO)
Recently, high-valent transition metal complexes have been used
as catalysts, and the reduction of sulfoxides to sulfides using
in catalytic hydrosilylation reactions. Since the seminal report of
H2 as a reducing agent in the presence of catalytic amounts of
Toste and coworkers showing the high efficiency of ReIO2(PPh3)2
MoO2Cl2, MoO2(S2CNEt2)2, ReIO2(PPh3)2 and ReOCl3(PPh3)2.
in the hydrosilylation of carbonyl groups,1 additional related oxo-
This is the first report of high valent oxo-molybdenum and -
rhenium complexes were reported to catalyze the hydrosilylation
rhenium complexes catalyzing the hydrogenation of alkenes. How-
of carbonyls2–8 and imines.9 We have recently shown the excellent
ever, several imido molybdenum(IV) arene complexes are known
to effect the hydrogenation of alkenes.20 DFT calculations have
shown that the activation of H2 by MoO2Cl2 and MoO2(S2CNEt2)2
can take place in a [2 + 2]-type addition of the H–H bond to the
terminal oxo–metal multiple bond, followed in the latter system
by water elimination and formation of a reduced Mo(IV) species.
This pathway of H2 activation is not favored for the tetrahedral
efficiency of MoO2Cl2 in the hydrosilylation of carbonyls by means
of Si–H bond activation,10,11 and some of us have extended the
scope of the use of MoO2Cl2 to the reduction of esters,12 imines,13
amides,14 sulfoxides and pyridine N-oxides.15 In a very recent
re-examination of their original system, Toste and coworkers
provided compelling experimental evidence for the [2 + 2] addition
=
z
of a Si–H bond across a Re O bond as the initial step of the
metal oxides MO4 −(M = Mn, Ru, Os) (or their adducts) which
catalytic carbonyl hydrosilylation cycle.16 This mechanism had also
been found to be the most likely one by DFT calculations.17 In the
case of MoO2Cl2, all attempts to observe a Mo–H intermediate
preferentially undergo [3 + 2] additions of E–H bonds (E = R3C,
R3S, H).21
=
failed but an initial [2 + 2] addition of Si–H to a Mo O bond
was also predicted by DFT calculations from us18 and others.19
Although the activation of CH4 by MoO2Cl2 has been ruled out
by these studies, on both thermodynamic and kinetic grounds,
nothing is known about the capacity of this complex to activate
H2. Given the propensity of complexes like ReIO2(PPh3)2 and
MoO2Cl2 to undergo [2 + 2] type additions, we reasoned that
their reaction with H2 might produce metal hydrides exhibiting
catalytic activity in hydrogenation or other reductive reactions.
This would extend the role of high valent oxides as catalysts
for reductive processes replacing silanes by a cheaper and more
convenient reducing agent, dihydrogen.
Experimental
General considerations
All experiments involving synthesis of metal complexes and
catalysis were carried out using standard Schlenk-line techniques
under a nitrogen atmosphere. Solvents were purified by con-
ventional methods and distilled under nitrogen, prior to use.
NMR spectra were recorded on a Bruker AMX 300 MHz and
Bruker Avance III 400 MHz spectrometers, and IR spectra were
measured on a Unicam Mattson model 7000 FTIR spectrometer.
Elemental analysis were performed in our laboratories (ITQB).
Commercially available reagents were used without further purifi-
cation. Compounds MoO2Cl2,22 MoO2(S2CNEt2)2,23 ReCH3O3,24
aInstituto de Tecnologia Qu´ımica e Biolo´gica da Universidade Nova de
Lisboa, Quinta do Marqueˆs, EAN, Apt. 127, 2781-901, Oeiras, Portugal.
E-mail: broyo@itqb.unl.pt; Fax: +351 214411277; Tel: +351 214469754
ReIO2(PPh3)2,25 and ReOCl3(PPh3)2 were prepared according
26
to the literature procedures. Catalytic reactions were assayed by
gas chromatography by comparison with authentic samples. Gas
chromatography was performed on a CE Instruments Trace GC-
series 2000 gas chromatograph equipped with a DB-5 MS capillary
column (30 m) with an injector and detector temperature of 220 ◦C
for the system alkynes/alkenes and 280 ◦C for sulfoxides/sulfides.
Helium was used as the carrier gas. The method used for
bDepartamento de Qu´ımica e Bioqu´ımica, CQB, Faculdade de Cieˆncias,
Universidade de Lisboa, 1749-016, Lisboa, Portugal. E-mail: mjc@fc.ul.pt;
Fax: +351 217 500 088; Tel: +351 217 500 196
† Electronic supplementary information (ESI) available: Coordinates of
the DFT optimized compounds. See DOI: 10.1039/b719375k
‡ Current Address: Departamento de Qu´ımica, CICECO, Universidade
de Aveiro, 3810-193, Aveiro, Portugal.
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Table 1 Hydrogenation of alkynes catalyzed by MoO2Cl2, MTO and
separating 1-hexyne/1-hexene and 3-hexyne/trans- and cis-3-
hexene was: an oven temperature of 40 ◦C, with a ramp rate
of 5 ◦C/min to a final temperature of 80 ◦C for 1 min. For
separating phenylacetylene/styrene an oven temperature of 40 ◦C,
with a ramp rate of 5 ◦C/min to a final temperature of 100 ◦C
for 1 min was used. The method for separating methyl phenyl
sulfoxide/thioanisole and dibutyl sulfoxide/dibutyl sulfide was:
a
ReIO2(PPh3)2
Entry
Substrate
Catalyst
H2 (atm)
% Yieldb
1
2
3
4
5
6
7
8
9
n-BuC≡CH
PhC≡CH
MoO2Cl2
MoO2Cl2
MoO2Cl2
MoO2Cl2
MoO2Cl2
MTO
MTO
MTO
ReIO2(PPh3)2
ReIO2(PPh3)2
MTO
8
8
8
40
40
40
40
40
40
8
100
28
4
EtC≡CEt
PhC≡CH
36
◦
an oven temperature of 100 C, with a ramp rate of 10 ◦C/min
EtC≡CEt
45c
100
70
to a final temperature of 250 ◦C for 1 min. The products
were also identified by NMR spectroscopy. The reactions with
hydrogen were performed in a 100 mL AISI 316 stainless steel
autoclave.
n-BuC≡CH
PhC≡CH
EtC≡CEt
47d
100
75
n-BuC≡CH
tert-BuC≡CH
tert-BuC≡CH
10
11
8
70
a Reaction conditions: 3.5 mmol of alkyne and 0.05 mmol of catalyst at
80 ◦C in toluene for 24 h. b Yields were determined by GC chromatography.
c A mixture of cis and trans isomers, 16 : 29 respectively. d A mixture of cis
and trans isomers 8 : 39, respectively.
General procedure for hydrogenation of alkynes
The hydrogenation reaction was carried out in a stainless steel
autoclave, which was charged with 5 mol% of the corresponding
catalyst, a magnetic stirrer and flushed with nitrogen. A solution
of alkyne (3.5 mmol) in toluene (5 mL) was placed into the
autoclave which was pressurized with H2. The reaction mixture
was then stirred at 80 ◦C. After 24 h, the autoclave was cooled to
room temperature, degassed, and opened. The reaction mixture
was filtered, and the filtrate was analyzed by gas chromato-
graphy.
Results and discussion
Hydrogenation of alkynes
The catalytic activity of MoO2Cl2 (1) was tested by using 5 mol%
of catalyst and 3.5 mmol of 1-hexyne in toluene under 8 atm
of H2 pressure. After 24 h at 80 ◦C, the reaction mixture was
filtered and analyzed by gas chromatography (GC), which showed
100% conversion of 1-hexyne to 1-hexene (Table 1, entry 1). Using
the above catalytic conditions, low conversions were achieved
for other alkynes such as phenylacetylene (28% conversion to
General procedure for reduction of sulfoxides
The deoxygenation reaction was carried out in a stainless steel
autoclave, which was charged with 10 mol% of the corresponding
catalyst, a magnetic stirrer and flushed with nitrogen. A solution
of sulfoxide (1.0 mmol) in toluene (5 mL) was placed into the
autoclave which was pressurized with H2 (50 atm). The reaction
mixture was then stirred at 120 ◦C. After 20 h, the autoclave
was cooled to room temperature, degassed, and opened. The
reaction mixture was filtered and analyzed by gas chromato-
graphy.
=
PhCH CH2) and the internal alkyne 3-hexyne (4% conversion
=
to EtCH CHEt). Higher yields of alkenes were obtained by
increasing the hydrogen pressure to 40 atm (36% conversion
=
=
to PhCH CH2 and 45% conversion to EtCH CHEt). Longer
reaction times and higher temperatures (up to 120 ◦C) for the
hydrogenation reactions did not increase the conversion of alkynes
to alkenes.
With the optimal conditions in hand, we tested the hydrogena-
tion reaction of alkynes with the high-valent rhenium complexes,
ReCH3O3 (2, MTO) and ReIO2(PPh3)2 (3). Both complexes are cat-
alytically active in the hydrogenation of 1-hexyne, phenylacetylene
and 3-hexyne giving conversions of 100, 70 and 47%, respectively,
for MTO (Table 1 entries 6–8) and 100% yield of 1-hexene for
ReIO2(PPh3)2, after 24 h at 80 ◦C. Blank experiments performed for
1-hexyne in the presence of H2, confirmed that no alkene formation
was detected unless the catalyst was used.
Under catalytic conditions, we have detected the formation of
reduced rhenium species along with the hydrogenation products.
When catalyst 3 is used in the hydrogenation of t-BuC≡CH
(Table 1, entry 10), we have isolated, by cooling down the toluene
solution, yellow crystals which have been identified as ReIO(t-
BuC≡CH)2 (4). The alkyne complex 4 has been reported in
the literature by Mayer and co-workers; it has been synthesized
by treatment of ReIO2(PPh3)2 with excess of the corresponding
alkyne.36 Therefore, it seems that free triphenyl phosphine (gener-
ated by replacement of PPh3 in 3 by alkyne) could be the reducing
agent responsible for the reduction of ReIO2(PPh3)2 to ReIO(t-
BuC≡CH)2. We examined then the hydrogenation reaction cat-
alyzed by MTO (Table 1, entry 11). For this reaction, the formation
of the reduced Re(V) alkyne complex ReCH3O2(t-BuC≡CH)
DFT Calculations
All calculations were performed with the Gaussian03 software
package,27 using the B3LYP hybrid functional. It includes a
mixture of Hartree–Fock28 exchange with DFT29 exchange–
correlation, given by Becke’s three-parameter functional30 with
the Lee, Yang and Parr correlation functional, which accounts for
both local and nonlocal terms.31,32 The standard LanL2DZ basis
set with the associated ECP33 augmented with a f-polarization
function (exponent 1.043) was used for Mo,34 and a standard 6–
31G(d,p)35 basis set for the remaining elements. The geometries
were optimized without any symmetry constraints. Frequency
calculations were performed in all species at this level of theory to
confirm the nature of the stationary points. The transition-state
structures, which yielded one imaginary frequency, were relaxed
following the vibrational mode to confirm the connecting reagents.
The reported zero-point corrected electronic energies (E0) and gas-
phase Gibbs free energies (G) were also obtained at this level of
theory. Complex MoO2(S2CNMe2)2 (6) was taken as a model of
MoO2(S2CNEt2)2.
1728 | Dalton Trans., 2008, 1727–1733
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(5, MDO) was also detected. In this case, we can be sure that
H2 is responsible for the reduction of MTO, since there is
no other reducing agent in the reaction mixture (Scheme 1).
The synthesis of ReCH3O2(t-BuC≡CH) has been reported by
Herrmann and co-workers from the reaction of MTO with an
excess of alkyne using polymer-bound PPh3 as the reducing agent
(Scheme 1).37
and some of its crude oxo–thiol–Mo(VI) models.38 Indeed, the
selective reduction of alkynes to alkenes is a challenging task in
organic synthesis, largely employed in the preparation of organic
intermediates for the synthesis of several pharmaceuticals.39 The
presently available data do not shed any light upon the nature of the
true catalyst. The most obvious products of the reduction under
catalytic conditions are given in Scheme 1. However, none of them,
per se, possesses any catalytic activity. More reduced species yet to
be identified might be the actually active catalysts. The fact that
radical processes are most likely involved in the reaction makes it
more difficult to assign a mechanism at this stage.
Reduction of sulfoxides
In view of the ability of high-valent metal complexes to activate H2,
and knowing that H2 is capable to reduce this type of complexes
in a similar way to that achieved by PPh3 (see Scheme 1), we
decided to investigate the catalytic deoxygenation of sulfoxides by
using H2 as reducing agent. The oxygen atom of the sulfoxide is
then removed as H2O and no longer as OPPh3 (see Scheme 2).
Indeed, sulfoxides are readily deoxygenated by PPh3 in a reaction
catalyzed by many cis-Mo(VI)O2 derivatives,40 including MoO2X2
(X = Cl, Br)41 and also by ReOCl3(PPh3)2.42 We were most pleased
to find that MoO2Cl2 (1) catalyzes the reduction of methyl phenyl
sulfoxide with H2 (50 atm) at 120 ◦C affording the corresponding
sulfide in quantitative yields (Scheme 2, Table 2, entry 2).
Scheme 1 Reactions of ReIO2(PPh3)2 and MTO with H2 and PPh3.
In order to find out if these reduced Re-(III) and -(V) complexes
could be the catalytic active species in the hydrogenation reaction,
complexes 4 and 5 were synthesized according to the literature
procedures, and were tested in the hydrogenation reaction. A
mixture of 3.5 mmol of 3,3-dimethyl-1-butyne and 0.2 mmol
of compounds 4 and 5 in toluene were stirred under 40 atm
of hydrogen pressure at 80 ◦C for 24 h. No conversion of 3,3-
dimethyl-1-butyne to the corresponding alkene was observed.
Therefore, species 4 and 5 are by-products of the reaction and
not intermediate active species of the catalytic cycle.
In an attempt to get additional mechanistic information,
we have carried out spin trap experiments by addition of the
radical scavenger DMPO (5,5-dimethyl-4,5-dihydro-3H-pyrrole-
N-oxide) to the reaction mixture catalyzed by MTO (2). The
addition of the spin trap causes an important decrease in the
yield which becomes only 16% of its normal value under similar
catalytic conditions. These experiments suggest that radical species
are involved in the catalytic hydrogenation of alkynes.
A similar spin trap essay was carried out in the hydrogenation
of 1-hexyne catalyzed by MoO2Cl2. In this case, the addition
of radical scavengers has a drastic effect: the hydrogenation of
alkyne is completely inhibited by the presence of DMPO. It seems
that, also for this catalyst, radical species are participating in the
hydrogenation reaction (a radical mechanism has been proposed
by us in the hydrosilylation of carbonyl groups when MoO2Cl2 is
used as catalyst).11
It is well known that in alkyne/alkene mixtures, the alkyne
functionality can be preferentially reduced because of the better
bonding capabilities of the triple bond. However, in general, when
most of the alkyne has been consumed, the reduction of alkene
occurs. This does not happen by using 1, 2 and 3 as catalysts, and in
fact, we do not detect formation of hexane over long reaction times
(48 h). The observation of an incomplete hydrogenation selectively
producing alkenes is reminiscent of the behaviour of nitrogenase
Scheme 2 Reaction of sulfoxides with H2 and PPh3 in the presence of
catalyst.
A detrimental effect in the yield of methyl phenyl sulfoxide
has been observed by decreasing the temperature (25% yield at
80 ◦C, no reaction at room temperature), time (60% yield in 7 h),
Table 2 Reduction
of
sulfoxides
catalyzed
by
MoO2Cl2,
%Yieldb
a
MoO2(S2CNEt2)2, ReIO2(PPh3)2 and ReOCl3(PPh3)2
Entry
Substrate
Catalyst
Product
1
2
3
4
5
6
7
8
(n-Bu)2SO
PhMeSO
PhMeSO
PhMeSO
(n-Bu)2SO
PhMeSO
(n-Bu)2SO
PhMeSO
MoO2Cl2
MoO2Cl2
(n-Bu)2S
PhMeS
PhMeS
PhMeS
(n-Bu)2S
PhMeS
(n-Bu)2S
PhMeS
100
100
55
31
100
87
MoO2(S2CNEt2)2
MoO(S2CNEt2)2
ReIO2(PPh3)2
ReIO2(PPh3)2
ReOCl3(PPh3)2
ReOCl3(PPh3)2
100
83
a Reaction conditions: 1.0 mmol of sulfoxide, 0.1 mmol of catalyst, 50 atm
of H2 pressure at 120 ◦C in toluene for 20 h. b Yields were determined by
GC chromatography.
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the hydrogen pressure (67% yield with 20 atm of H2) and the
amount of catalyst (64% yield when 5 mol% of catalyst is used).
Blank experiments for the reduction of sulfoxides with H2 have
been performed and no significant sulfide formation was observed
under the applied conditions.
Scheme 3 Reaction of MoO2(S2CNEt2)2 with H2.
In order to know whether the active catalytic species are stable
under catalytic conditions, new charges of methyl phenyl sulfoxide
were added to the reactor when MoO2Cl2 was used as catalyst
(a 100% conversion of methyl phenyl sulfoxide to methyl phenyl
sulfide was obtained in the first catalytic cycle). After 24 h at
120 ◦C, the mixture was analyzed by GC, showing a 65% of
conversion to the corresponding sulfide. A third catalytic cycle
under similar reaction conditions gave 55% of conversion to the
sulfide. Therefore, we can conclude that the catalyst is stable
under catalytic conditions although a slight decrease in yield was
observed.
When 1 is exposed to H2 pressure under similar conditions
(50 atm and 120 ◦C) in the absence of any substrate, no reaction
takes place and the product is recovered.
The catalytic deoxygenation was also achieved by
MoO2(S2CNEt2)2 (6), although less effectively (55% yield,
Table 2, entry 3). We chose to study complex 6 as catalyst in this
reaction because of its solubility in a variety of organic solvents
and the full characterization of its reduced Mo-(IV) and -(V)
species could help us identify intermediate species responsible for
the reduction of the sulfoxides.
The reaction of MoO2(S2CNEt2)2 with H2 (50 atm) in toluene,
in the presence of molecular sieves, at 120 ◦C for 24 h, af-
forded a reddish solid which was characterized as the oxo-
bis(diethyldithiocarbamato)molybdenum(IV) (Scheme 3). When
molecular sieves are not employed, the reaction becomes rather
irreproducible most likely due to side reactions with water at
the high temperature used. Thus, the first step in the catalytic
reaction is probably the reduction of 6 by H2 (see Computational
studies) affording the Mo(IV) complex MoO(S2CNEt2)2 (7) and
water (compound 7 was shown to be catalytically active in the
reduction of sulfoxides affording a 31% yield of methyl phenyl
sulfide).
also catalyze the reduction of sulfoxides affording quantitative
conversions to the corresponding sulfides (Table 2, entries 5–8).42
Computational studies
DFT/B3LYP29 calculations (Gaussian 03)27 were performed in
order to understand the mechanism of activation of H2 by these
Mo(VI) complexes (more details in the Experimental section).
Since the reaction of MoO2(S2CNEt2)2 with dihydrogen is well
defined, the reagent and the product being well characterized
complexes 6 and 7, we started by studying this reaction. As can
be seen from the complete pathway shown in Fig. 1, the first
step consists of the activation of the H–H bond over one of the
=
Mo O bonds of 6, similar to the first step in the activation of
Si–H bonds over the same type of Mo(VI) dioxo complex.18 An
intermediate (I1) of relatively high energy, containing a hydride
and a OH ligand, is formed. The following hydrogen migration to
the oxygen of the OH group gives rise to a complex with a bound
water molecule (I2), which then leaves the coordination sphere of
molybdenum.
The initial complex 6, MoO2(S2CNEt2)2, has a highly distorted
octahedral geometry, as can be seen in the three-dimensional
representation of Fig. 2. The Mo–S and other bonds in each
dithiocarbamate ligand are the same, despite the lack of symmetry
=
constraints of the calculations. The Mo–S bond trans to Mo O
˚
˚
is much longer (2.7◦77 A) than the other (2.486 A), and the S–
◦
=
Mo–S angle (147.5 ) is narrower than the S–Mo O (156.6 ).
These structural parameters reproduce very well those of the
experimentally determined structure,43 although the longer Mo–
˚
S bond is overestimated (exptl. 2.640 A). All the others are
˚
comparable within 0.04 A.
The H2 molecule approaches the metal octahedral coordination
sphere over one SSO face. In the transition state TS6-I1, the
˚
We have extended our studies to the use of several oxo–rhenium
complexes. We found that ReIO2(PPh3)2 and ReOCl3(PPh3)2
H–H bond is very long (1.099 A), while the O–H and Mo–H
˚
bonds are partially formed (1.197 and 1.950 A). The intermediate
Fig. 1 Reaction pathway for the dihydrogen activation by complex 6: DE0 (bold) top and DG (italics) bottom, in kcal mol−1
.
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˚
Fig. 2 Relevant distances (A) and structures of the intermediates and transition states in the dihydrogen activation by complex 6.
I1 is heptacoordinate, displaying a very distorted coordination
environment. The coordination of another ligand, the hydride,
barrier of 0.6 kcal mol−1 drops when entropy effects are taken into
account and free energies are considered. In the transition state
˚
=
is reflected in the four long Mo–S bonds, while the Mo O has
TSI2–7 the Mo · · · O distance has increased significantly to 2.769 A,
become slightly shorter (Fig. 2). This intermediate has a higher
energy than the sum of the two isolated reagents (the zero of the
energy scale), the reaction being endothermic and endergonic. The
activation free energy is calculated as 62.2 kcal mol−1, a value not
unexpected considering the reaction conditions (Table 2).
while the long Mo–S bond of I2 starts to shrink back to normal.
The final stable complex 7 has a square pyramidal structure, with
four short Mo–S bonds (2.484–2.485 A) and a very low energy. It
is the most stable species in the system, and this probably explains
the low catalytic activity in the reduction of sulfoxides (Table 2).
In the X-ray structure of the complex,44 the Mo–S bonds are a bit
shorter, though the Mo O bond has the same length. It is very
asymmetric, probably owing to crystal packing effects (there are
two molecules per unit cell).
Starting from complex MoO2Cl2 (1) a similar pathway was
investigated and is sketched in Fig. 3. The first step, the activation
of H–H over Mo O leading to the first intermediate I3, has a
˚
This is the rate limiting step of the reaction. The other two steps,
a migration and a dissociation, involve much lower activation free
energies than the associative first step. The migration of the hydride
in I1 to the oxygen of the OH group to form intermediate I2 has an
activation barrier of 10.9 kcal mol−1, or 15.7 kcal mol−1 in terms of
free energy. There is a large reorganization of the geometry of the
complex, when going from I1 to I2. I2 is again a highly distorted
octahedral species, with a long Mo–O distance to the bound water
=
=
lower activation barrier (∼5 kcal mol−1) than in the pathway
shown in Fig. 1. The free energy of I3 is higher than that of I1
relative to the reagents. In the second step, however, the migration
of the hydride to the OH group, has an activation barrier of
∼25 kcal mol−1 (TSI3-I4) almost double that for the dithiocarbamate
˚
molecule (2.385 A), and two very asymmetrically coordinated
˚
dithiocarbamate ligands. One of them exhibits a 2.808 A long
=
Mo–S distance and is trans to Mo O. It is not surprising that
the complex loses the H2O molecule very easily: the activation
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Fig. 3 Reaction pathway for the activation of H2 by MoO2Cl2, showing some relevant distances: DE0 (bold) top and DG (italics) bottom, in kcal mol−1
.
complex. The water complex I4 is less stable than the reagents
(another difference from the first mechanism) by 10.6 or 16.5 kcal
mol−1, considering electronic or free energy. The structural changes
parallel those described for the first pathway. Finally, it was
impossible to find a path for water elimination from complex I4.
The triangular MoOCl2 complex is not a stable species, owing to
its high unsaturation. This finding is in accord with the fact that
1 does not react with H2 in the absence of substrates. Therefore,
other substrate dependent reaction pathways must be considered
inorder toexplainthecatalyticactivitysummarizedinTables 1and
2. For instance, it is obvious that under the catalytic conditions
of the sulfoxide deoxygenation reactions, 1 will not be present
and [MoO2Cl2(OSR2)] will be the most likely species undergoing
reduction. As shown by Arna´iz and coworkers, such a species is
readily deoxygenated by PPh3 leading to catalytic deoxygenation
of sulfoxides.41 The parallel between O abstraction by H2 and PPh3
(reaction conditions apart) leads us to expect that MoO2Cl2(OSR2)
will also be readily deoxygenated by H2 under our experimental
conditions.
tested are quite effective. The fact that MoO2Cl2 (1) possesses a
very high catalytic activity in this deoxygenation process in spite
of having an energetically unfavored water loss pathway, means
that other reaction pathways become competitive. Further work
is under way in order to extend the scope of these novel reactions
as well as the understanding of their mechanisms.
Acknowledgements
This work was supported by FCT, POCI and FEDER through
projects POCI/QUI/55586/2004 and POCI/QUI/58925/2004.
We wish to acknowledge Maria da Conceic¸a˜o Almeida and
Dr Ana Coelho for providing data from the Elemental Anal-
yses Service at ITQB. P. M. Reis and J. A. Fernandes
thank FCT for postdoctoral grants SFRH/BPD/20655/2004
and SFRH/BDP/23461/2005, respectively. P. J. C. acknowl-
edges the FCT for grants SFRH/BD/10535/2002 and
SFRH/BPD/27082/2006.
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