Chlorocarbonyl ruthenium(II) complexes of tripodal triphos {MeC(CH2PPh2)3}: Synthesis, characterization and catalytic applications in transfer hydrogenation of carbonyl compounds

Article abstract of DOI:10.1016/j.jorganchem.2010.01.011

The complex [Ru(CO)₂(triphos-κ²P)Cl₂] (1) underwent decarbonylation in dichloromethane solution under air over a period of about two weeks to afford the chelated monocarbonyl complex [Ru(CO)(triphos-κ³P)Cl₂] (2). The Single Crystal X-ray structure of 2 showed a slightly distorted metal centred complex. The catalytic activity of one of the complexes [Ru(CO)(triphos-κ³P)Cl₂] (2) was examined in the transfer hydrogenation of aromatic carbonyl compounds and was found to be efficient with conversion up to 100% in the presence of isopropanol/NaOH.

Full text of DOI:10.1016/j.jorganchem.2010.01.011

Journal of Organometallic Chemistry 695 (2010) 781–785
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Chlorocarbonyl ruthenium(II) complexes of tripodal triphos {MeC(CH2PPh2)3}:
Synthesis, characterization and catalytic applications in transfer hydrogenation
of carbonyl compounds
*
Bhaskar Jyoti Sarmah, Dipak Kumar Dutta
Materials Science Division, North-East Institute of Science and Technology (CSIR), Jorhat 785006, Assam, India
a r t i c l e i n f o
a b s t r a c t
The complex [Ru(CO)2(triphos-j
2P)Cl2] (1) underwent decarbonylation in dichloromethane solution
Article history:
Received 26 October 2009
Received in revised form 29 December 2009
Accepted 7 January 2010
under air over a period of about two weeks to afford the chelated monocarbonyl complex [Ru(CO)(tri-
phos-
3P)Cl2] (2). The Single Crystal X-ray structure of 2 showed a slightly distorted metal centred com-
plex. The catalytic activity of one of the complexes [Ru(CO)(triphos-
3P)Cl2] (2) was examined in the
j
j
Available online 14 January 2010
transfer hydrogenation of aromatic carbonyl compounds and was found to be efficient with conversion
up to 100% in the presence of isopropanol/NaOH.
Keywords:
Ó 2010 Elsevier B.V. All rights reserved.
Ruthenium carbonyl complexes
Tridentate ligands
Triphos complexes
Crystal structure
Transfer hydrogenation
1. Introduction
structure of 2 established by Single Crystal X-ray diffraction as well
as its catalytic transfer hydrogenation activities are also reported.
Tridentate tripodal phosphines are recognized as one of the
most important classes of ligands having widespread applications
in coordination chemistry [1–10]. Such ligands provide an impor-
tant advantage over monodentate phosphines with respect to
greater control of the coordination number, stoichiometry and ste-
reochemistry of their complexes [11]. Moreover, polyphosphine
complexes usually have two or more chelate rings which minimize
the unwanted isomers, and therefore, expected to show better cat-
alytic activities. Among these ligands, the C3-symmetric 1,1,1-
tris(diphenylphosphinomethyl)ethane(triphos){MeC(CH2PPh2)3}
and its derivatives are the most extensively investigated ones
forming a large verity of transition metal complexes which are
found to have applications in catalysis [12–22]. Bianchini and
coworkers in particular, pioneering the use of this ligand in transi-
tion metal catalysis using platinum group metals for a number of
processes including hydrogenation, hydroformylation, oxidation,
etc. [23–30]. One preliminary report for synthesis of complexes
such as [Ru(CO)2{MeC(CH2PPh2)3}Cl2}] was found in literature
[31]. As a part of our continuing research activities [19,32–39],
we report here the synthesis, detailed spectroscopic characteriza-
tion of ruthenium(II) carbonyl complexes of the type [Ru(CO)2(tri-
2. Results and discussion
The polymeric complex [Ru(CO)2Cl2]n reacts with equimolar
quantity of the triphos ligand by cleavage of the chloro bridge to
afford 1 in good yield as a yellow crystalline solid (Scheme 1).
The molecular composition of the complex is well supported by
elemental analyses data. The probable molecular structure of 1
(Fig. 1) was assigned by elemental analyses, IR, NMR and mass
spectrometry, where the tripodal triphos ligand attached in a
bidentate manner. The bidentate coordination mode of the tripodal
triphos ligand in 1 is confirmed by both the IR and 31P NMR spec-
tral data. The IR spectra of 1 show two equally intense m(CO) bands
at 2054 and 1988 cmÀ1 attributing the two terminal carbonyl
groups cis to one another. The 1H NMR spectra of 1 show character-
istics reresonance at d = 2.58–2.67 ppm (–CH2–) and d = 0.98 ppm
(–CH3–), respectively, along with their phenylic protons in the
range 6.61–8.05 ppm. The 31P NMR spectra for 1 at room temper-
ature exhibit resonances at d = 34.07 ppm for the coordinated
phosphorus atoms and d = À23.51 ppm due to the free dangling
phosphorus atom. The free ligand, {MeC(CH2PPh2)3}, has a single
resonance at d = À20.32 ppm in CDCl3 at 25 °C is close to the free
phosphorus in 1. Both 1H and 31P NMR spectra are in accordance
with stereochemical rigidity of 1 in CDCl3 solution at 25 °C.
However, attempts to develop suitable single crystal of 1 for
phos-j j
2P)Cl2](1) and [Ru(CO)(triphos- 3P)Cl2] (2). The molecular
* Corresponding author. Tel.: +91 376 2370 081; fax: +91 376 2370 011.
E-mail address: dipakkrdutta@yahoo.com (D.K. Dutta).
0022-328X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.01.011

782
B.J. Sarmah, D.K. Dutta / Journal of Organometallic Chemistry 695 (2010) 781–785
CO
Ru
CO
Ru
CO
Ru
Cl
Cl
Cl
Cl
CO
RuCl3.xH2O
Reflux /Alcohol
CO
CO
CO
P
Reflux, 3 hrs.
Methanol
P
P
P
=
P
P
P
P
P
CO
P
OC
Cl
Ru
P
Cl
1
P
Scheme 1. Synthesis of [Ru(CO)2(triphos-j
2P)Cl2] (1).
X-ray analysis from dichloromethane solution under air led to the
formation of
3-P coordinated complex [Ru(CO)(triphos- 3P)Cl2]
j
j
(2) over a period of about two weeks in relatively good yield
CO
(Scheme 2). The resulting monocarbonyl complex exhibits single
terminal m
(CO) band at 1959 cmÀ1. Such decarbonylation reaction
P
P
OC
Cl
was also observed in ruthenium complexes of the type [Ru(-
CO)2(Ph2PCH2P(S)Ph2)2Cl2] [36]. The dangling P atom in 1 under-
went chelation which is substantiated by a single 31P NMR
resonance data. In order to obtain an unambiguous characteriza-
tion of 2, an X-ray diffraction study was undertaken. The arrange-
ment of the atoms in the crystal is shown in Fig. 2. Ru(II) is situated
in the centre of a slightly distorted octahedral coordination envi-
ronment. Crystal data and structure refinement as well as some se-
lected bond lengths (Å) and bond angles (°) for the complex 2 are
Ru
Cl
P
Fig. 1. Probable structure of [Ru(CO)2(triphos-
j
2P)Cl2] (1).
CO
CO
P
P
Cl
OC
Cl
-CO
Ru
Ru
DCM/Hexane
Two weeks
Cl
P
P
P
Cl
2
1
P
P
=
P
P
P
P
P
Scheme 2. Decarbonylation reaction of 1.

B.J. Sarmah, D.K. Dutta / Journal of Organometallic Chemistry 695 (2010) 781–785
783
Table 2
Selected bond lengths [Å] and angles [°] for the complex 2.
Bond lengths
Ru(1)–C(43)
Ru(1)–P(2)
Ru(1)–P(1)
Ru(1)–P(3)
Ru(1)–Cl(4)
Ru(1)–Cl(6)
1.968(4)
2.3197(7)
2.3209(7)
2.3951(7)
2.4648(9)
2.4720(8)
Bond angles
C(43)–Ru(1)–P(2)
C(43)–Ru(1)–P(1)
P(2)–Ru(1)–P(1)
C(43)–Ru(1)–P(3)
P(2)–Ru(1)–P(3)
P(1)–Ru(1)–P(3)
C(43)–Ru(1)–Cl(4)
P(2)–Ru(1)–Cl(4)
P(1)–Ru(1)–Cl(4)
P(3)–Ru(1)–Cl(4)
C(43)–Ru(1)–Cl(6)
P(2)–Ru(1)–Cl(6)
P(1)–Ru(1)–Cl(6)
P(3)–Ru(1)–Cl(6)
Cl(4)–Ru(1)–Cl(6)
92.58(9)
97.27(9)
87.61(3)
175.92(10)
89.14(3)
86.49(3)
86.64(10)
174.00(3)
98.38(3)
91.27(3)
78.96(9)
89.89(3)
175.39(3)
97.35(3)
84.12(3)
Fig. 2. Single Crystal X-ray structure of [Ru(CO)(triphos-j
3P)Cl2] (2) with thermal
ellipsoids drawn at the 30% level. All hydrogen atoms omitted for clarity.
listed in Tables 1 and 2. The three bite angles (P–Ru–P) of triphos
ligand are close to a right angle and cap a triangular face of the
octahedron with fac stereochemistry. Two trans P–Ru–Cl bond an-
gles are in the range 174.00–175.39°. The Ru–P bond lengths in 2
clearly indicate the elongation of the Ru–P(3) bond due to the
strong trans influence of the carbonyl group. The Ru–P bond trans
to CO is longer than those trans to Cl by about 0.075 Å. Thus, one
of the three M–P bonds is more labile and may show hamilabile
behavior by ‘‘Opening–Closing” mechanism during the course of
catalytic reactions. The Ru–CO bond length (1.97 Å) falls within
the range 1.81–2.08 Å observed for second and third row transition
metal carbonyls [40,41].
Ruthenium mediated transfer hydrogenation reactions are
found to be effective catalytic systems in which hydrogen is trans-
ferred from one organic molecule to another and this made us to
carry out this type of reactions. One of the complexes [Ru(CO)(tri-
phos-j
3P)Cl2] (2) is taken as model catalyst and the catalytic activ-
ity in the transfer hydrogenation of various aldehydes and ketones
in the presence of isopropanol and NaOH as promoter has been ex-
plored, while 1 due to its instability does not show any catalytic
properties. Complex 2 catalyzes the reduction of carbonyl com-
pounds to corresponding alcohols with high yield and the results
are shown in Table 3. The ruthenium complex catalyzed transfer
hydrogenation reaction proposed to follow the classical mecha-
Table 1
Crystal data and structure refinement for the complex 2.
Table 3
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C42H39Cl2OP3Ru
824.61
296(2)
0.71073
monoclinic
Cc
Transfer hydrogenation of carbonyl compounds catalyzed by 2 using 2-propanol and
NaOHa.
Entry
Substrate
Product
Conversion (%)
85
1
Unit cell dimensions
O
O
OH
HO
a (Å)
b (Å)
c (Å)
21.8609(4)
10.3959(2)
18.5067(4)
90
117.467(2)
90
3731.80(13)
4
1.468
2
3
91
a
(°)
b (°)
Cl
Cl
c
(°)
Volume (Å3)
75
O
HO
Z
Density (calculated) (Mg/m3)
Absorption coefficient (mmÀ1
F(0 0 0)
)
0.725
1688
0.40 Â 0.30 Â 0.16
H3CO
MeO
Crystal size (mm3)
4
5
94
98
O
OH
Theta range for data collection (°) 2.95–28.24
Index ranges
À28 6 h 6 29, À13 6 k 6 13,
À24 6 l 6 24
Reflections collected
19538
Independent reflections
Completeness to theta = 28.24°
Absorption correction
Refinement method
8690 [R(int) = 0.0337]
99.1%
O
HO
H
H
None
Full-matrix least-squares on F2
8690/2/443
Data/restraints/parameters
6
100
Goodness-of-fit on F2
1.016
O
OH
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0314, wR2 = 0.0667
R1 = 0.0360, wR2 = 0.0692
À0.016(19)
Absolute structure parameter
a
Conditions: reactions were carried out at 82 °C using 5 mmol of substrate for
24 h. substrate/Ru/NaOH ratio: 100/1/24. Yield determined by GC analysis.
Largest diff. peak and hole (e ÅÀ3
)
0.661 and À0.409

784
B.J. Sarmah, D.K. Dutta / Journal of Organometallic Chemistry 695 (2010) 781–785
nism through the formation of metal hydrides as shown in Scheme
3 [42–44]. The base facilitates the formation of a ruthenium alkox-
ide by abstracting the proton of the alcohol (Scheme 3) followed by
b-elimination to yield a monohydride ruthenium species (A),
which is the active catalyst. The proposed mechanism for this pro-
3. Experimental
3.1. General methods
All the solvents used were distilled under N2 prior to use. Ele-
mental analyses were done on a Perkin–Elmer 2400 elemental ana-
lyzer. IR spectra (4000–400 cmÀ1) were recorded in KBr discs and
CHCl3 solution on a Perkin–Elmer system 2000 FTIR spectropho-
tometer. NMR spectra were recorded in CDCl3 solution on a Bruker
DPX-300 MHz spectrometer. RuCl3Á3H2O was purchased from M/S
Arrora Matthey Ltd., Kolkata. {MeC(CH2PPh2)3} (triphos) was pur-
chased from Aldrich, USA and used as received. The starting com-
plex [Ru(CO)2Cl2]n was prepared by following literature method
[47–50].
cess involves j3 2 dissociation of triphos ligand which is well re-
–j
ported in literature [17]. It is believed that during the catalytic
cycle one phosphine ligating site of triphos ligand of 2 is dissoci-
ated to create vacant coordination site for the incoming substrate.
The most labile phosphine site would be the one trans to CO. X-ray
crystal structure of 2 also clearly reveals the elongation of one of
the three Ru–P bonds due to the strong trans influence of CO.
Therefore, it is reasonable to assume that the phosphine ligating
site of triphos ligand trans to CO is dissociated affording metal-alk-
oxide intermadiate (B). The complex efficiently catalyzes the
reduction of cyclohexanone to the corresponding alcohol with
100% conversion. The conversion in case of acetophenone is 85%,
while benzophenone is converted to its corresponding alcohol with
94% yield. The presence of electron withdrawing (Cl) and electron
donating (OCH3) substituents on the substrates (entries 2 and 3,
Table 3) plays a significant role in the conversion of ketones to
alcohols, which is clearly reflected for higher conversion of 4-chlo-
roacetophenone (91%) over 4-methoxyacetophenone (75%). The
above process involves reduction i.e. gaining of electrons, and is
facilitated by electron deficient centers caused by the substituents
[45]. Interestingly, the complex also efficiently catalyzes the reduc-
tion of benzaldehyde to benzyl alcohol with 98% yield. It was ob-
served that the catalytic activity of 2 in terms of conversion is
comparable to that of [RuCl2(PPh3)3] [46], but the former takes
more time to achieve the required conversion. This is due to the
high stability of 2 which results the slow formation of ruthenium-
hydride species responsible for hydrogen-transferring process.
3.2. Synthesis of metal complexes
3.2.1. [Ru(CO)2(triphos-j
2P)Cl2] (1)
[Ru(CO)2Cl2]n (40 mg, 0.175 mmol) was dissolved in 10 cm3
methanol and to this 0.0877 mmol of triphos {MeC(CH2PPh2)3} in
10 cm3 dichloromethane was added. The reaction mixture was re-
fluxed for about 3 h and the solvent was evaporated under vacuum
to produce a yellow compound. The compound so obtained was
washed with diethyl ether and recrystallized from dichlorometh-
ane solution.
3.2.1.1. Analytical data for the complex 1. Yield: 92%; Anal. Calc. for
C43H39Cl2O2P3Ru: C, 60.37; H, 4.50. Found: C, 60.47; H 4.57%. Se-
lected IR data: 1988, 2054
d = 0.98 ppm (3H, s), 2.58–2.67 ppm (6H, d), 6.61–8.05 ppm (30
H, m), 31P NMR data d = À23.51 ppm (1P, s), 34.07 ppm (2P, s),
Mass Spectral data: m/z+ = 853.4.
[m
(CO) cmÀ1], 1H NMR data:
3.2.2. [Ru(CO)(triphos-j
3P)Cl2] (2)
OH
Forty milligrams of the complex 1 was dissolved in dichloro-
methane the solution was kept aside for about two weeks. The
solvent was evaporated under reduced pressure to generate yel-
low-red colored compounds (2), which was washed with diethyl
ether and stored over silica gel in a desiccator.
P
P
OC
Cl
P
P
OC
Cl
P
P
+ HCl
Ru
O
Ru
Cl
NaOH
O
3.2.2.1. Analytical data for the complex 2. Yield: 65%; Anal. Calc. for
β−hydride elimination
C42H39Cl2OP3Ru: C, 61.08; H, 4.69. Found: C 61.11; H 4.73. Selected
IR data: 1959 [m
(CO) cmÀ1], 1H NMR data: d = 0.96 ppm (3H, s),
O
2.48–2.70 ppm (6H, d), 6.95–7.95 ppm (30 H, m), 31P NMR data
O
P
R1
d = 16.05 ppm (3P, s).
R2
OC
Cl
P
P
Ru
3.3. General procedure for catalytic transfer hydrogenation
H
P
Ru
O
A
P
Ru
H
Under inert atmosphere, the carbonyl compound (5 mmol), the
ruthenium catalyst precursor (0.05 mmol) and 45 mL of propan-2-
ol were introduced into a Schlenk tube fitted with a condenser and
heated at 82 °C for 15 min. Then NaOH was added (5 mL of a
0.01 M solution in propan-2-ol, 9.6 mol%), and the reaction was
monitored by gas chromatography. The corresponding alcohol
and acetone were the only products detected in all cases. The iden-
tity of the alcohols was assessed by comparison with commercially
available pure samples.
OC
Cl
P
P
OC
Cl
P
P
O
R2
R1
P
B
OC
P
P
OH
Ru
O
Cl
R2
R1
OH
R2
R1
3.4. X-ray crystallographic study
H
R1= R2 = alkyl/aryl/H
The X-ray data were collected at 296 K with Mo K
a radiation
(k = 0.71073 Å) using a Bruker Nonius SMART CCD diffractometer
equipped with graphite monochromator. The SMART software was
used for data collection and also for indexing the reflections and
determining the unit cell parameters; the collected data were inte-
Scheme 3. Proposed mechanism for the transfer hydrogenation of carbonyl
compounds by chlorocarbonyl Ru(II) complexes of tripodal phosphine ligand
{MeC(CH2PPh2)3}.

B.J. Sarmah, D.K. Dutta / Journal of Organometallic Chemistry 695 (2010) 781–785
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