Di-ruthenium complexes having diphosphines and carbonyls: Formation, structure, and catalytic hydrogenation of alkynes

Article abstract of DOI:10.1016/j.jorganchem.2017.01.027

Reaction of three selected diphos ligands, viz. 1,2-bis(diphenylphosphino)ethane (L¹), 1,3-bis(diphenylphosphino)propane (L²) and 1,4-bis(diphenylphosphino)butane (L³), with [{Ru(CO)₂Cl₂}_n] has afforded di-ruthenium complexes of type [Ru₂(L)₃(CO)₂Cl₄], (1–3). Crystal structure of complex 1 has been determined, and molecular structures of complexes 2 and 3 have been optimized through DFT method. Formation of the unexpected di-ruthenium complexes has been probed through DFT calculations. In dichloromethane solution all the complexes show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on the complexes shows an irreversible oxidation within 0.79–1.53 V vs SCE, and an irreversible reduction within ?1.20 to ?1.33 V vs SCE. The di-ruthenium complexes efficiently catalyze hydrogenation of alkynes to the corresponding alkenes.

Full text of DOI:10.1016/j.jorganchem.2017.01.027

Journal of Organometallic Chemistry 834 (2017) 47e57
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Di-ruthenium complexes having diphosphines and carbonyls:
Formation, structure, and catalytic hydrogenation of alkynes
Aparajita Mukherjee, Piyali Paul, Samaresh Bhattacharya*
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Reaction of three selected diphos ligands, viz. 1,2-bis(diphenylphosphino)ethane (L1), 1,3-
bis(diphenylphosphino)propane (L2) and 1,4-bis(diphenylphosphino)butane (L3), with [{Ru(CO)2Cl2}n]
has afforded di-ruthenium complexes of type [Ru2(L)3(CO)2Cl4], (1e3). Crystal structure of complex 1 has
been determined, and molecular structures of complexes 2 and 3 have been optimized through DFT
method. Formation of the unexpected di-ruthenium complexes has been probed through DFT calcula-
tions. In dichloromethane solution all the complexes show intense absorptions in the visible and ul-
traviolet regions. Cyclic voltammetry on the complexes shows an irreversible oxidation within 0.79
e1.53 V vs SCE, and an irreversible reduction within ꢀ1.20 to ꢀ1.33 V vs SCE. The di-ruthenium com-
plexes efficiently catalyze hydrogenation of alkynes to the corresponding alkenes.
Article history:
Received 15 November 2016
Received in revised form
28 December 2016
Accepted 30 January 2017
Available online 12 February 2017
Keywords:
Diphos and carbonyl ligands
Formation of di-ruthenium complexes
Catalytic hydrogenation of alkyne to alkene
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
bidentate ligands without dissociation of any pre-coordinated li-
gands [17,26e30]. Understandably, the primary aim was to obtain
There has been considerable current attention to the chemistry of
mixed-ligand ruthenium carbonyl complexes largely because of
their versatile catalytic applications [1e16]. Reactivity of such com-
plexes is dictated by the carbonyl and the ancillary ligands. Thus,
binding of ligands of selected types to the metal center in the
Ru(CO)n fragment is of significant importance for induction of
desired properties in such mixed-ligand complexes. For the present
study, which has originated from our interest in the chemistry of
mixed-ligand ruthenium carbonyl complexes [17e21], we chose
three diphos ligands (L1 e L3) with difference in separation between
the two phosphorus donor sites. These phosphines were selected as
ancillary ligands based on the fact that coordination of ruthenium by
phosphines, in combination with the carbonyls, has often been
found to be particularly useful for augmenting the catalytic activities
of the resulting complexes [1e3,5e10,15]. The chosen diphos ligands
are known to bind to a metal center usually as bidentate ligands
forming stable chelate rings (I), but they are also known to serve as
bridging ligands, where the two phosphorus sites remain linked
with two metal centers (II) [22e25]. As the ruthenium starting
material we selected [{Ru(CO)2Cl2}n], firstly because it contains a
Ru(CO)2 fragment in it, and secondly, and more importantly, because
of its demonstrated ability to readily incorporate neutral chelating
mixed carbonyl-phosphine-Ru complexes of type [Ru(L)(CO)2Cl2] (L
representing the diphos ligand) and explore their catalytic utility.
Herein we describe our observations on the interaction of the
selected diphos ligands with [{Ru(CO)2Cl2}n], with special reference
to formation of new complexes, their structure and, their catalytic
application in hydrogenation of alkynes.
(CH2)n
(CH2)n
P
P
P
P
n = 2 L1
n = 3 L2
n = 4 L3
M
I
M
P
(CH2)n
P
M
II
* Corresponding author.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
http://dx.doi.org/10.1016/j.jorganchem.2017.01.027
0022-328X/© 2017 Elsevier B.V. All rights reserved.

48
A. Mukherjee et al. / Journal of Organometallic Chemistry 834 (2017) 47e57
Table 1
2. Results and discussion
Selected bond lengths (Å) and bond angels (ꢁ) for complex 1.
2.1. Synthesis and characterization
Bond lengths (Å)
Ru1-Cl1
Ru1- Cl2
Ru1-P1
Ru1-P2
Ru1-P3
Ru1-C1
2.4221(11)
2.4001(11)
2.3580(10)
2.4937(11)
2.3980(9)
1.882(5)
C1-O1
P1-C2
C2-C3
P2-C3
1.076(6)
1.810(4)
1.524(6)
1.844(4)
As outlined in the introduction, the primary goal of this study
was to explore the interaction of the selected diphos ligands (L1
e
L3) with [{Ru(CO)2Cl2}n] with an expectation of getting mixed-
ligand complexes of type [Ru(L)(CO)2Cl2]. Accordingly a reaction
of [{Ru(CO)2Cl2}n] was first carried out with L1 in equimolar ratio,
where the mole calculation of the ruthenium precursor was done
based on the Ru(CO)2Cl2 unit. The reaction proceeded smoothly in
refluxing methanol to afford a yellow complex in rather poor yield.
The yield improved on increasing Ru:L1 ratio to 1:1.5, which is
described in the experimental section. Characterization on the
complex (vide infra) showed it to have a significantly different
composition, viz. [Ru2(L1)3(CO)2Cl4], (1), than what was expected.
The low yield is attributable to unavailability of adequate quantity
of the diphos ligand (L1), required with respect to the Ru-starting
material, for the formation of this di-ruthenium complex. This
was further demonstrated by carrying out a similar reaction with
1.5 equivalents of L1, which afforded the same complex 1 in much
better yield. Similar results were observed for reactions of
[{Ru(CO)2Cl2}n] with the other two diphos ligands, viz. L2 and L3,
both of which yielded di-ruthenium complexes of the same type,
viz. [Ru2(L2)3(CO)2Cl4], (2) and [Ru2(L3)3(CO)2Cl4], (3), respectively.
Preliminary characterizations (microanalysis, mass, NMR and IR) of
the isolated products agree well with their compositions. Mass
spectrum of each di-ruthenium complex (general formula
[Ru2(L)3(CO)2Cl4]) shows three characteristic peaks, one corre-
sponding to the [Ru2(L)3(CO)2Cl3]þ ion, and the other two peaks
indicate that, under the mass spectral condition, the di-ruthenium
complexes (1, 2 and 3) have undergone rapid fragmentation into
two parts (1a, 1b; 2a, 2b; and 3a, 3b) having general composition
Ru(L)2(CO)Cl2 and Ru(L)(CO)Cl2, respectively. In order to identify
the bridging ligand in these di-ruthenium complexes, as well as to
find out coordination mode of the diphos ligands in them, structure
of complex 1 was determined by X-ray crystallography. The
Bond angles (ꢁ)
Cl1-Ru1-Cl2
P1-Ru1-P3
P2-Ru1-C1
175.75(4)
176.73(4)
169.63(14)
P1-Ru1-P2
Ru1-C1-O1
82.95(4)
177.4(5)
structure is shown in Fig. 1, and selected bond parameters are listed
in Table 1. The structure shows that out of the three diphos ligands
present in the complex molecule, two are coordinated, separately,
to the two ruthenium centers forming five-membered chelate rings
(I, n ¼ 2, M ¼ Ru), while the third diphos ligand is coordinated to
both the ruthenium centers in a bridging mode (II, n ¼ 2, M ¼ Ru).
Two chlorides and a carbonyl are coordinated to each ruthenium
center. The chelated diphos ligand, the carbonyl and the phos-
phorus of the bridging diphos ligand constitute an equatorial plane
around each ruthenium, and the two chlorides occupy mutually
trans positions. The Ru-P bond, that is trans to the coordinated
carbonyl, is found to be significantly longer than the other two Ru-P
bonds. The remaining bond distances around ruthenium are all
found to be quite usual [17e21]. The other two di-ruthenium
complexes (2 and 3), which were synthesized similarly and show
similar properties (vide infra), are assumed to have similar struc-
tures as complex 1. As crystals of complexes 2 and 3 could not be
grown, their expected structures were optimized with the help of
Density Functional Theory (DFT) method [31]. The optimized
structures are shown in Fig. 2 and Fig. 3, and some computed bond
parameters are given in Table S1 (Supplementary material). For a
proper comparison of bond parameters in the DFT-optimized
Fig. 1. Crystal structure of complex 1.

A. Mukherjee et al. / Journal of Organometallic Chemistry 834 (2017) 47e57
49
Fig. 2. DFT-optimized structure of complex 2.
structures, structure of complex 1 was also optimized by DFT
method, which is shown in Fig. S1 (Supplementary material) and
some computed bond parameters are included in Table S1. The DFT-
optimized structure of complex 1 and the associated bond pa-
rameters are found to compare well with its crystal structure and
bond parameters obtained thereby. In the other two complexes, 2
and 3, the core structure around each ruthenium center is found to
be qualitatively similar to that in complex 1. The computed bond
parameters of the DFT-optimized structures of all the three com-
plexes are found to be very much comparable (Table S1). However,
the Ru-Ru distance is found to increase expectedly with increasing
number of CH2 fragment in the bridging diphos ligand. Similarly, in
the Ru(P-P) chelate, P-P depicting the chelated diphos ligand, size
of the chelate ring and the P-Ru-P bond angle are also found to
Fig. 3. DFT-optimized structure of complex 3.

50
A. Mukherjee et al. / Journal of Organometallic Chemistry 834 (2017) 47e57
2 Ru(CO)2Cl2
= L1, L2 or L3
P
P
3
P
P
Cl
Cl
P
P
P
OC
CO
Ru
Ru
Cl
OC
CO
P
P
P
Cl
IM1 - IM3
- 2 CO
Cl
Cl
P
P
OC
P
Ru
Ru
Cl
P
CO
P
P
Cl
1 - 3
Scheme 1. Probable steps behind formation of the di-ruthenium complexes 1 - 3.
increase with increasing number of CH2 fragment between the two
phosphorus donors.
L1 were optimized by DFT, as were the remaining species depicted
in Scheme 1. Stable geometries, having only positive eigenvalues,
were found for each species, and this allowed us to determine the
thermodynamics for the observed transformation that affords
complex 1. The optimized structures and ground-state energy
ordering for the pertinent species are depicted in Fig. 4. The
Ru(CO)2Cl2 unit (A) has a tetrahedral geometry and the uncoordi-
nated 1,2-bis(diphenylphosphino)ethane (B) has the unfolded
chain structure. The reaction of A and B gives the di-ruthenium
intermediate species (C), in which both terminal and bridging
diphos ligands are having similar unfolded geometry as the unco-
ordinated diphos ligand (B). Formation of C is exergonic and the
reaction lies 111.2 kcal/mol below the reagents. Species C is con-
verted to the final di-ruthenium complex 1 (D) via elimination of
CO (E). The C / D þ 2 E reaction is also exergonic and lies 2.5 kcal/
mol below the intermediate C. The net thermodynamics are
computed at ꢀ113.7 kcal/mol and are in agreement with the easily
prepared and readily isolable nature of the di-ruthenium
complexes.
Formation of the di-ruthenium complexes of type [Ru2(L)3(-
CO)2Cl4], instead of the expected mixed-ligand complexes of type
[Ru(L)(CO)2Cl2], even from reactions between equimolar amounts
of the diphos ligand and [{Ru(CO)2Cl2}n], has been quite intriguing.
In view of the fact that the three chosen diphos ligands remain in an
unfolded chain form in their native states [32e34], we have
invoked a set of plausible sequences behind formation of the di-
ruthenium complexes, which are illustrated in Scheme 1.
Herein, we speculate that the diphos ligands initially bind to the
coordinatively unsaturated metal center in the Ru(CO)2Cl2 unit,
retaining their native unfolded geometry, forming a di-ruthenium
species as intermediate (IM1 e IM3). In the next step, the two ter-
minal diphos ligands fold around the metal centers and thereby the
so far uncoordinated phosphorus atoms bind to the metal centers,
via dissociation of a carbonyl from each ruthenium, forming two
Ru(P-P) chelate rings, and thus affording the di-ruthenium com-
plexes 1e3.
The thermodynamics for the formation of complex 1 was
investigated computationally by DFT. The reactive Ru(CO)2Cl2
fragment in the starting ruthenium complex, and the diphos ligand
We also considered an alternative mechanism behind formation
of the di-ruthenium complexes, which was also believed to proceed
in a step-wise fashion, as illustrated in Scheme S1 (Supplementary

A. Mukherjee et al. / Journal of Organometallic Chemistry 834 (2017) 47e57
51
Fig. 4. DFT-optimized structures and ground-state energy ordering for the reaction of Ru(CO)2Cl2 (A) and 1,2-bis(diphenylphosphino)ethane (B) to give the di-ruthenium complex
[Ru2(L1)3(CO)2Cl4] (D) and CO (E), via formation of [Ru2(L1)3(CO)4Cl4] (C). The free energy values (in kcal/mol) are referenced relative to the reagents (2Aþ3B). The optimized
structure for E is not shown.
material). In the first step, the diphos ligand binds, in the chelating
mode, to the coordinatively unsaturated ruthenium center in the
Ru(CO)2Cl2 unit, yielding a complex of type [Ru(L)(CO)2Cl2] as an
intermediate (X1 e X3). In the next step, one of the two carbonyls in
this intermediate undergoes facile displacement by a phosphorus
at one end of a second diphos ligand, while the phosphorus at the
other end of the same diphos ligand links itself to a second
ruthenium center via similar displacement of a carbonyl, and
thereby affords the di-ruthenium complexes. The thermodynamics
for this alternative mechanism was also investigated by DFT for
complex 1, and all the species involved in its formation, as depicted
in Scheme S1, were optimized. The optimized structures and
ground-state energy ordering for all the relevant species are shown
in Fig. S2 (Supplementary material). Initial reaction of Ru(CO)2Cl2
(A) and 1,2-bis(diphenylphosphino)ethane (B) gives the interme-
diate (C′), in which the diphos ligand is chelated to ruthenium.
Formation of C′ is exergonic and the reaction lies 62.9 kcal/mol
below the reagents. Species C′ further reacts with the diphos ligand
(B) and is converted to the di-ruthenium complex 1 (D) via elimi-
nation of CO (E). The C′ þ ½ B / ½ D þ E step is found to be
endergonic by 6.0 kcal/mol, and hence it is thermodynamically
unfavorable. Thus we are inclined to believe that formation of the
di-ruthenium complexes proceeds through the two steps shown in
Scheme 1, where both the steps are thermodynamically favorable.
three distinct signals, each as a triplet due to coupling with the two
proximal methylene protons, consistent with the nonequivalent
nature of the three phosphorous nuclei in each half of the di-
ruthenium complex. Infrared spectra of the complexes show
many bands of varying intensities within 4000 - 450 cmꢀ1. While a
detailed spectral assignment has not been attempted due to the
complex nature of the spectra, a strong n(CO) stretch could be easily
identified within 1960e1965 cmꢀ1. The remaining bands within
489e1638 cmꢀ1 are attributable to the coordinated diphos ligands
[35]. Besides, two distinct bands near 2920 and 3054 cmꢀ1 are due
to methylinic C-H and phenyl C-H stretches respectively.
The di-ruthenium complexes were found to be readily soluble in
dichloromethane and chloroform, producing light yellow solutions.
Electronic spectra of the complexes were recorded in dichloro-
methane solutions, and the spectral data are given in Table 2. Each
complex shows an absorption of moderate intensity around
370 nm, and few intense absorptions at shorter wavelengths. In
order to gain some idea about the nature of the lowest energy
absorption, we examined the character of the HOMO and LUMO of
these complexes. Contour plots of these two orbitals for complex 1
are shown in Fig. 5, and those for the other two complexes are
presented in Fig. S3 and Fig. S4 (Supplementary material).
Composition of the HOMO and LUMO of all the complexes is given
in Table 3. In complex 1 the HOMO is found to be delocalized
2.2. Spectral studies
Table 2
Electronic spectral and cyclic voltammetric data.
Magnetic susceptibility measurements show that all the three
di-ruthenium complexes are diamagnetic, which corresponds to
the þ2 oxidation state of ruthenium (low-spin d6, S ¼ 0) in them. 1H
NMR spectra of the complexes show broad signals within
6.5e8.2 ppm due to overlapping phenyl-proton signals arising from
twelve phenyl rings from three coordinated diphos ligands in each
complex molecule. The methylene-proton signals, arising from six,
nine or twelve -CH2- fragments in the three complexes, also appear
as overlapping signals within 2.2e3.0 ppm, and hence no clear P-H
coupling was visible. 31P NMR spectrum of each complex shows
Complex Electronic spectral dataa
lmax, nm (ε, Mꢀ1 cmꢀ1
Cyclic voltammetric datab
E/V vs. SCE
)
1
2
3
371c (700), 269 (39500)
0.79d, ꢀ1.33e
370c (900), 271c (50400), 249c (62500) 1.16d, ꢀ1.20e
371c (1300), 272c (38500), 250c (60000) 1.53d, ꢀ1.26e
a
b
c
In dichloromethane.
Solvent, dichloromethane; supporting electrolyte, TBHP; scan rate, 50 mVsꢀ1
.
Shoulder.
Epa value.
Epc value.
d
e

52
A. Mukherjee et al. / Journal of Organometallic Chemistry 834 (2017) 47e57
Table 3
Composition of selected molecular orbitals of the complexes.
Complex
Contributing fragments
% Contribution of
fragments to
HOMO
LUMO
1
Ru(L1)(CO)Cl2
Ru1
35.3
13.6
0.0
1.3
2.7
33.7
13.4
0.0
13.3
10.3
0.0
10.8
26.7
14.0
13.6
0.0
Cl1 þ Cl2
CO
L1
bridging L1
Ru (L1)(CO)Cl2
Ru2
Cl3 þ Cl4
CO
L1
0.0
11.3
2
Ru(L2)(CO)Cl2
Ru1
46.6
32.9
0.0
9.8
10.7
0.0
0.0
0.0
0.0
1.4
0.0
0.0
0.0
24.1
26.3
18.5
0.0
Cl1 þ Cl2
CO
L2
bridging L2
Ru(L2)(CO)Cl2
Ru2
Cl1 þ Cl2
CO
L2
29.7
3
Ru(L3)(CO)Cl2
Ru1
45.7
36.1
0.0
8.0
10.2
0.0
0.0
0.0
0.0
21.3
19.6
0.7
10.2
26.4
11.0
10.5
0.0
Cl1 þ Cl2
CO
L3
bridging L3
Ru(L3)(CO)Cl2
Ru2
Cl1 þ Cl2
CO
L3
0.3
environments, indicates that there is no observable communication
between the two metal centers.
2.4. Catalysis
As already mentioned in the introduction that ruthenium
complexes having carbonyl and phosphine in the coordination
sphere are finding wide application as catalysts in a variety of
organic transformations, we were keen to find catalytic utility of
the present group of complexes. And encouraged by some recent
studies on the utilization of such complexes as catalyst for the
hydrogenation of alkynes to alkenes [36e43], we planned to
explore similar reactions using our di-ruthenium complexes as
catalyst. We began our study by examining the hydrogenation of
phenylacetylene using gaseous hydrogen at 1 atm and complex 1 as
catalyst to furnish the corresponding hydrogenated product, viz.
styrene. Table 4 provides information on the impact of various re-
action parameters on the efficiency of this process. After extensive
optimization, we found that 2.0 mol% catalyst, toluene as solvent,
110ꢁ C reaction temperature, and 10 h reaction time, furnish the
desired alkene in good (65%) yield (entry 1), and no alkane is
produced in the reaction. As expected, in the absence of the
ruthenium catalyst, no hydrogenation occurs (entry 2). Decrease in
the catalyst loading, relative to the optimum value, results in sig-
nificant decrease in the yield (entry 3), while increase in the cata-
lyst loading leads to only insignificant improvement in the yield
(entry 4). Upon reducing the reaction time, the yield is found to
decrease significantly (entry 5). However, upon increasing the re-
action time, to twice the optimum value, results only in slight in-
crease in the yield (entry 6). Lowering the reaction temperature,
relative to the optimum value, causes decrease in the yield (entry
7). Among the solvents tried, toluene turned out to be the most
suitable. For example, no hydrogenation is observed to take place
Fig. 5. Contour plots of the HOMO and LUMO of complex 1.
primarily (69%) over the two metal centers, with much less (27%)
contribution from the coordinated chlorides, while the LUMO is
distributed over the two metal centers (~27%), the chlorides (~24%),
the two chelated diphos ligands (~22%) and the bridging diphos
ligand (~27%). The lowest energy absorption in complex 1 is hence
assignable to a transition from a filled orbital having mixed
(metal þ chloride) character to a vacant orbital also having mixed
(metal þ chloride þ diphos) character. In complexes 2 and 3,
though distribution of HOMO and LUMO over the different frag-
ments is different than that in complex 1 (Table 3; Fig. S3 and
Fig. S4), the overall nature of the lowest energy transition is found
to be qualitatively similar to that in complex 1.
2.3. Electrochemical properties
Redox activity of the di-ruthenium complexes was examined in
dichloromethane solution (0.1 M TBHP) by cyclic voltammetry.
Voltammetric data are presented in Table 2. Each complex shows an
irreversible oxidation within 0.7e1.6 V and an irreversible reduc-
tion within ꢀ1.2 to ꢀ1.4 V (all potentials are referenced to saturated
calomel electrode (SCE)), which are tentatively assigned to Ru(II)-
Ru(III) oxidation and reduction of the diphos ligand. Appearance
of a single oxidative response for these di-ruthenium complexes,
where the two metal centers are having identical coordination

A. Mukherjee et al. / Journal of Organometallic Chemistry 834 (2017) 47e57
53
Table 4
Optimization of experimental parameters.a
H
H
H
H2
H
catalyst
Entry
Solvent
Catalyst
Mole % of catalyst
Temperature,ꢁC
Time, h
Yeildb, %
1
2
3
4
5
6
7
8
toluene
toluene
toluene
toluene
toluene
toluene
toluene
chloroform
acetonitrile
toluene
toluene
1
e
1
1
1
1
1
1
1
2
3
2.0
e
110
110
110
110
110
110
70
60
80
110
110
10
10
10
10
8
20
10
10
10
10
10
65
NOc
44
1.5
3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
71
47
69
45
NOc
NOc
67
9
10
11
64
a
Reaction conditions: phenylacetylene (0.5 mmol), H2 gas (1.0 atm), solvent (5.0 mL).
Determined by GCMS.
Not observed.
b
c
upon changing the solvent to chloroform or acetonitrile (entries 8
and 9). Under the optimized condition, the other two di-ruthenium
complexes also show similar catalytic efficiency as complex 1
(entries 10 and 11).
carbonyl generating another 16e species (iv), and the catalytic cycle
starts therefrom. The possibility of dissociation of CO first followed
by that of the Ru-P bond, however, cannot be ruled out. Activation
of molecular hydrogen takes place, initially via
s-interaction (v),
The scope of the reaction is shown in Table 5. As all the di-
ruthenium complexes have shown comparable catalytic effi-
ciency, only the results obtained with complex 1 as the catalyst are
highlighted here. Using the optimized the reaction conditions, hy-
drogenation reactions have been performed on a series of ten
different alkynes. Using phenylacetylene and substituted phenyl-
acetylenes as substrates (S1 e S7), the corresponding alkenes (P1 e
P7) were obtained in good (64e72%) yields. Both electron-
withdrawing and electron-donating substituents were used, but
the yields remained rather indifferent to the nature of the sub-
stituents. While alkynes with both R1 s H and R2 s H, viz. 1-
phenyl-1-propyne, diphenylacetylene and bis(4-pyridyl)acetylene,
were used as substrates (S8 e S10), the corresponding alkenes were
obtained in both cis (P8a e P10a) and trans (P8b e P10b) forms.
However, yield of the cis-alkene (6e10%) was always found to be
much less than that of the trans-alkene (52e63%), which is usually
observed as well as expected from steric consideration. All the
product alkenes were isolated and characterized by 1H NMR
studies. Separation of cis and trans isomers of the last three alkenes
(P8 e P10) has not been possible, however, NMR spectra recorded on
the mixture of two isomers clearly indicated the presence of trans-
isomer in considerable excess over the cis-isomer. It is noteworthy
that further reduction of alkene to alkane has not been encountered
in any of these attempted hydrogenation reactions. The catalytic
efficiency of the present group of complexes is found to be com-
parable to that of some other ruthenium complexes reported
recently [37,39,40,42].
eventually generating a dihydrido species (vi). Hydride transfer
generates the Ru-C bonded species (vii). Reductive elimination of
the alkene takes place, producing a ruthenium(II) species (viii)
having P2Cl2 coordination sphere, and such four-coordinated
ruthenium(II) species are well precedent in the literature
[44e47]. This coordinatively unsaturated species immediately
takes up an alkyne to regenerate the first member (iv) of the cat-
alytic cycle, and thus it continues.
3. Conclusions
The present study shows that the diphos ligands (L1 e L3), which
remain in the unfolded chain form in their native state, readily
interact with the metal center in the Ru(CO)2Cl2 fragment, provided
by [{Ru(CO)2Cl2}n], to afford di-ruthenium complexes of type
[Ru2(L)3(CO)2Cl4] (1e3), where the diphos ligands display both
chelating, as well as bridging, mode of binding. The di-ruthenium
complexes can efficiently catalyze hydrogenation of alkynes, un-
der relatively mild condition, to afford the corresponding alkenes.
Dissociability of the Ru-CO and Ru-P bonds seems to play key roles
behind the observed catalysis.
4. Experimental
4.1. Materials
The observed catalytic hydrogenation of alkynes is believed to
follow sequences, illustrated in Scheme 2, that are essentially
drawn from those proposed earlier [36,38,42]. In the di-ruthenium
catalyst-precursor, each ruthenium has identical P3Cl2(CO) coor-
dination environment, which is depicted as (i). Dissociation of a Ru-
P bond, which may be from the chelated diphos or from the
bridging diphos ligand, is believed to take place initially, producing
a 16e species (ii), and thus enabling the alkyne to bind to the metal
Ruthenium trichloride was purchased from Arora Matthey,
Kolkata, India. The diphos ligands (L1 e L3) were procured from
Aldrich. [{Ru(CO)2Cl2}n] was prepared by following a reported
procedure [48]. Tetrabutylammonium hexaflurophosphate (TBHP),
obtained from Aldrich, and AR grade acetonitrile, procured from
Merck, India, were used for electrochemical work. All other
chemicals and solvents were reagent grade commercial materials
and were used as received.
center in a p-fashion (iii). Which, in turn, triggers dissociation of a

54
A. Mukherjee et al. / Journal of Organometallic Chemistry 834 (2017) 47e57
Table 5
Hydrogenation of alkynes.a,b,c,d
H
H
H
R2
H
H2
R1
R2
+
catalystb
R1
R2
R1
(Sn)
(Pna)
(Pnb)
Product alkenes (Yieldc,d)
H
H
H
H
H
H
H
H
H
H
H3C
H
H
H3CO
H3C
(P2, 71% [62%])
H3CO
(P3, 68% [57%])
(P4, 67% [59%])
(P1, 65% [56%])
H
H
H
H
H
H
H
H
H
F
Cl
(P6, 69% [61%])
Br
(P7, 72% [60%])
(P5, 64% [55%])
H
H
H
H
CH3
H
H
H
H
CH3
(P9b, 62% [51%])
(P8a, 10% [7%])
(P8b, 52% [44%])
(P9a, 6% [4%])
N
H
H
H
H
N
N
N
(P10b, 63% [50%])
(P10a, 8% [5%])
a
Reaction conditions: alkyne (0.5 mmol), H2 gas (1.0 atm), toluene (5.0 mL), reaction temperature (110 ꢁC), reaction time (10 h).
Catalyst: complex 1 (2.0 mol%).
b
c
Determined by GCMS, for R2 ¼ H the product is indicated by Pn only.
d
Isolated yields are given within square brackets. For P8a e P10b isolated yields is calculated based on the ratio intensity of 1H NMR signal of the two isomers.
4.2. Physical measurements
630 spectrophotometer. Magnetic susceptibilities were measured
using a Sherwood MK-1 balance. Electrochemical measurements
were made using a CH Instruments model 600A electrochemical
analyzer. A platinum disc working electrode, a platinum wire
auxiliary electrode and an aqueous saturated calomel reference
electrode (SCE) were used in the cyclic voltammetry experiments.
All electrochemical experiments were performed under a dini-
trogen atmosphere. All electrochemical data were collected at
298 K and are uncorrected for junction potentials. Optimization of
Microanalyses (C, H, N) were performed using a Heraeus Carlo
Erba 1108 elemental analyzer. Mass spectra were recorded with a
Micromass LCT electrospray (Qtof Micro YA263) mass spectrom-
eter. NMR spectra were recorded in CDCl3 solution on a Bruker
Avance DPX 300 NMR spectrometer. IR spectra were obtained on a
Perkin Elmer Spectrum Two IR spectrometer with samples pre-
pared as KBr pellets. Electronic spectra were recorded on a JASCO V-

A. Mukherjee et al. / Journal of Organometallic Chemistry 834 (2017) 47e57
55
P
Ru CO
(i)
Ru CO
(ii)
R
R
R
R
Ru CO
(iii)
- CO
R
R
Ru
(iv)
R
H2
R
R
R
H
Ru
Ru
(v)
H
(viii)
H
R
R
H
H
R
R
R
R
H
Ru
(vii)
H
Ru
(vi)
H
Scheme 2. Probable mechanism for the observed hydrogenation of alkynes. In the pre-catalyst (i) only one ruthenium coordination environment is indicated, where P represents
the coordinated phosphorus of a diphos ligand, and besides CO, no other coordinated ligands are shown.
ground-state structure, energy calculations, and molecular orbital
calculations were carried out by density functional theory (DFT)
method using the Gaussian 09 (B3LYP/GEN) package [31]. GC-MS
analyses were performed using a Perkin Elmer CLARUS 680
instrument.
settle by cooling the solution to room temperature. Then it was
collected by filtration, washed thoroughly with diethyl ether and
dried in air. Yield: 78%. Anal. Calcd. for C80H72O2P6Cl4Ru2: C, 60.21;
H, 4.52. Found: C, 60.72; H, 4.49%. MS (ESI), positive mode: [1 - Cl]þ,
1558; [1a - Cl]þ, 960; [1b - Cl]þ, 562. 1H NMR [49]: 2.34e2.80 (12H)
*; 6.97e7.60 (60H)*. 31P NMR: 52.34 (t, JP-H ¼ 36.3); 50.36 (t, JP-
¼ 39.6); 24.45 (t, JP-H ¼ 39.7). IR: 489, 512, 521, 562, 694, 740, 836,
H
4.3. Syntheses of complexes
878, 999, 1027, 1097, 1189, 1312, 1433, 1484, 1571, 1633, 1965, 2917
and 3054 cmꢀ1
.
4.3.1. [Ru2(L1)3(CO)2Cl4], (1)
To a solution of [{Ru(CO)2Cl2}n] (50 mg, 0.22 mmol) in methanol
(50 mL) 1,2-bis(diphenylphosphino)ethane (140 mg, 0.35 mmol)
was added. The mixture was refluxed for 4 h, whereby complex 1
started precipitating as a light yellow solid, which was allowed to
4.3.2. [Ru2(L2)3(CO)2Cl4], (2)
To a solution of [{Ru(CO)2Cl2}n] (50 mg, 0.22 mmol) in methanol
(50 mL) 1,3-bis(diphenylphosphino)propane (140 mg, 0.34 mmol)

56
A. Mukherjee et al. / Journal of Organometallic Chemistry 834 (2017) 47e57
Table 6
flask was charged with the phenylacetylene (0.5 mmol) and a
Crystallographic data for complex 1.
known mol percent of the catalyst dissolved in toluene (5 mL). H2-
gas was passed through the solution continuously at 1 atm pres-
sure. The flask was placed in a preheated oil bath at the required
temperature. After the specified time, the flask was removed from
the oil bath and the resultant solution was filtered through tight-
packed slurry of silica (100e200 mesh) in hexane, and the filtrate
was analyzed by GC-MS.
Empirical formula
C80H72O4P4Cl2Ru2$2(C3H6O)
Formula weight
Crystal system
Space group
a (Å)
1711.29
monoclinic
P21/n
12.8704(4)
11.2419(3)
27.5589(8)
91.833(2)
3985.4(2)
2
b (Å)
c (Å)
b
(ꢁ)
4.5.1. 1H NMR of the product alkenes [49]
V (Å3)
Z
P1: 5.27 (d, 1H, J ¼ 10.9); 5.78 (d, 1H, J ¼ 17.6); 6.75 (d of d, 1H,
J ¼ 17.5, 10.9); 7.25e7.46 (5H)*. P2: 2.28 (s, 3H); 5.13 (d, 1H, J ¼ 9.7);
5.67 (d, 1H, J ¼ 17.0); 6.63 (d of d, 1H, J ¼ 16.3, 9.9); 7.06e7.33 (4H)*.
P3: 3.72 (s, 3H); 5.06 (d, 1H, J ¼ 11.0); 5.60 (d, 1H, J ¼ 17.8); 6.68 (d of
d, 1H, J ¼ 17.2, 10.5); 6.80e7.21 (4H)*. P4: 2.37 (s, 3H); 3.78 (s, 3H);
5.34 (d, 1H, J ¼ 10.3); 5.44 (d, 1H, J ¼ 16.9); 6.49e6.55 (2H)*; 6.90 (d
of d, 1H, J ¼ 16.2, 9.9); 7.21 (d, 1H, J ¼ 7.5). P5: 5.14 (d, 1H, J ¼ 9.6);
5.62 (d, 1H, J ¼ 16.4); 6.59 (d of d, 1H, J ¼ 16.7, 9.7); 6.86e7.23 (4H)*.
P6: 5.21 (d, 1H, J ¼ 9.8); 5.70 (d, 1H, J ¼ 16.5); 6.66 (d of d, 1H,
J ¼ 16.8, 9.9); 7.18e7.42 (4H)*. P7: 5.19 (d, 1H, J ¼ 9.7); 5.68 (d, 1H,
F (000)
1756
crystal size (mm)
T (K)
0.14 ꢂ 0.16 ꢂ 0.24
155
m
(mmꢀ1
)
0.684
65033
0.068
9170
0.0489
0.1462
0.96
Collected reflections
Rint
Independent reflections
R1a
wR2b
GOFc
a
b
c
R1 ¼ Sjj Foj -j Fcjj/Sj Foj.
J ¼ 16.5); 6.63 (d of d, 1H, J ¼ 16.8, 9.8); 7.12e7.40 (4H)*. P8a þ P8b
:
wR2 ¼ [S{w(F2o-F2c)2}/S{w(F2o)}]1/2
.
1.73 (s)#; 1.81 (s); 5.95 (d, J ¼ 10.2)#; 6.08 (d, J ¼ 14.9); 6.22 (d,
J ¼ 10.9)#; 6.34 (d, J ¼ 16.6); 6.91e7.39*. P9a þ P9b: 6.66 (s)#; 7.02
(s); 7.09e7.51*. P10a þ P10b: 6.94 (s)#; 7.17 (s); 7.05 (d, J ¼ 6.5)#; 7.32
(d, J ¼ 6.2); 8.23 (d, J ¼ 7.8)#; 8.59 (d, J ¼ 7.8).
GOF ¼ [S(w(F2o-Fc2)2)/(M-N)]1/2, where M is the number of reflections and N is
the number of parameters refined.
was added. The mixture was refluxed for 4 h, whereby a brown
solution was obtained. The solvent was evaporated and the solid
mass, thus obtained, was subjected to purification by thin-layer
chromatography on a silica plate. With 1:2 acetonitrile-benzene
as the eluant, a yellow band separated, which was extracted with
acetonitrile. Evaporation of the acetonitrile extract gave complex 2
as a yellow solid. Yield: 44.5%. Anal. Calcd. for C83H78O2P6Cl4Ru2: C,
60.87; H, 4.77. Found: C, 60.35; H, 4.79%. MS (ESI), positive mode: [2
- Cl]þ, 1600; [2a - Cl]þ, 989; [2b - Cl]þ, 577. 1H NMR: 2.30e2.98
(18H)*; 6.66e8.14 (60H)*. 31P NMR: 20.51 (t, JP-H ¼ 37.1); 17.48 (t, JP-
Acknowledgments
The authors thank the reviewers for their constructive com-
ments, which have been very helpful in preparing the revised
manuscript. Financial assistance received from the Department of
Science and Technology, Government of West Bengal, Kolkata
[Sanction No. 746(Sanc.)/ST/P/S&T/2G-4/2013)], University Grants
Commission, New Delhi [Sanction No. F.19-122/2014(BSR)] and
Council of Scientific and Industrial Research, New Delhi [Grant No.
01(2788)/14/EMR-II] is gratefully acknowledged. The authors thank
Dr. Saurabh Das (Department of Chemistry, Jadavpur University) for
his help in recording the electronic spectra.
¼ 36.0); 8.60 (t, JP-H ¼ 38.8). IR: 500, 512, 518, 579, 698, 744, 838,
H
860, 999, 1027, 1094, 1189, 1315, 1434, 1483, 1638, 1961, 2924 and
3054 cmꢀ1
.
Appendix A. Supplementary data
4.3.3. [Ru2(L3)3(CO)2Cl4], (3)
This complex was synthesized by following the same procedure
used for the synthesis of complex 1, using 1,4-
bis(diphenylphosphino)butane instead of 1,2-bis(diphenylphos-
phino)ethane. Yield: 81%. Anal. Calcd. for C86H84O2P6Cl4Ru2: C,
61.49; H, 5.01. Found: C, 61.44; H, 4.97%. MS (ESI), positive mode: [3
- Cl]þ, 1642; [3a - Cl]þ, 1017; [3b - Cl]þ, 591. 1H NMR: 2.27e2.92
(24H)*, 6.56e7.81 (60H)*. 31P NMR: 26.24 (t, JP-H ¼ 36.5); 25.61 (t,
JP-H ¼ 37.4); 18.82 (t, JP-H ¼ 39.2). IR: 503, 514, 529, 576, 588, 696,
740, 833, 857, 1001, 1028, 1096, 1185, 1359, 1433, 1484, 1572, 1587,
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2017.01.027.
CCDC 1489485 contains the supplementary crystallographic
data for this paper. DFT-optimized structure of complex 1 (Fig. S1),
selected bond lengths (Å) and bond angels (ꢁ) for the DFT-
optimized structures of complexes 1, 2 and 3 (Table S1), probable
alternative steps behind formation of the di-ruthenium complexes
1 - 3 (Scheme S1), DFT-optimized structures and ground-state en-
ergy ordering for the species shown in Scheme S1 (Fig. S2), and
contour plots of the HOMO and LUMO of complexes 2 and 3 (Fig. S3
and Fig. S4) are available as supporting information.
1618, 1961, 2922 and 3052 cmꢀ1
.
4.4. X-ray crystallography
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refinement were done using the SHELXS-97 and SHELXL-97 pack-
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