Ketone transfer hydrogenation reactions catalyzed by a phosphinite ruthenium PCP complex: The X-ray crystal structure of [C6H 4-1,3-(OPPh2{Ru(η6-p-cymene)Cl 2})2]

Article abstract of DOI:10.1016/j.molcata.2005.11.041

The reaction of the potentially tridentate phosphinite PCP pincer type ligand [C₆H₄-1,3-(OPPh₂)₂] with [(η⁶-p-cymene)RuCl₂]₂ affords the bimetallic species [C₆H₄-1,3-(OPPh₂{Ru(η ⁶-p-cymene)Cl₂})₂]. Several attempts, including the change of the ruthenium starting material (e.g. [(η⁶- benzene)RuCl₂]₂) and reflux conditions to achieve the coordination of the diphosphinite [C₆H₄-1,3-(OPPh ₂)₂] in a tridentate PCP pincer fashion failed. Complex [C₆H₄-1,3-(OPPh₂{Ru(η⁶-p-cymene) Cl₂})₂] was tested in the transfer hydrogenation of ketones.

Full text of DOI:10.1016/j.molcata.2005.11.041

Journal of Molecular Catalysis A: Chemical 247 (2006) 124–129
Ketone transfer hydrogenation reactions catalyzed by
a phosphinite ruthenium PCP complex
The X-ray crystal structure of
[C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2]
∗
´
´
´
Ricardo Ceron-Camacho, Valente Gomez-Benıtez, Ronan Le Lagadec ,
∗
´
David Morales-Morales , Ruben A. Toscano
Instituto de Qu´ımica, Universidad Nacional Auto´noma de Me´xico, Cd. Universitaria, Circuito Exterior,
Coyoaca´n, 04510 Me´xico D. F., Me´xico
Received 12 October 2005; received in revised form 16 November 2005; accepted 16 November 2005
Available online 28 December 2005
Abstract
The reaction of the potentially tridentate phosphinite PCP pincer type ligand [C6H4-1,3-(OPPh2)2] with [(η6-p-cymene)RuCl2]2 affords the
bimetallic species [C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2]. Several attempts, including the change of the ruthenium starting material (e.g.
[(η6-benzene)RuCl2]2) and reflux conditions to achieve the coordination of the diphosphinite [C6H4-1,3-(OPPh2)2] in a tridentate PCP pincer
fashion failed. Complex [C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2] was tested in the transfer hydrogenation of ketones.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Ketone hydrogenation; Transfer hydrogenation; PCP pincer ligands; Phosphinite ligands; Ruthenium complexes; Catalysis; Crystal structures
1. Introduction
yields and selectivities and when P-stereogenic centers have
been included, good enantioselectivities [6].
Since its discovery, transfer hydrogenation using ruthenium
complexes as catalysts has been an increasingly useful tool in
organic synthesis [1], allowing transformations otherwise very
difficult or almost impossible to carry out. This process, has
taken higher importance due to the mild conditions generally
required, fact that has been important in the use of ruthenium
chiral complexes for enantioselective transfer hydrogenations
[2]. In addition, the chemistry of pincer type complexes has been
having a steadily increasing interest in homogeneous catalysis
[3], due the high thermal stability that these complexes exhibit
and the potential alternative mechanistic routes they follow [4].
In fact, ruthenium PCP phosphino, NCN (amino) and CNC (C:
carbene) pincer type complexes have already been employed in
ketone transfer hydrogenation reactions [5] even in its asym-
metric fashion using PCP phosphino complexes, affording good
Thus, following our continuous interest in the chemistry
of phosphinite PCP pincer ligands [7] and the applications of
cyclometalated ruthenium complexes [8], we report here our
findings in the reactivity of ruthenium starting materials with
the potentially tridentated PCP ligand [C6H4-1,3-(OPPh2)2]
and the catalytic evaluation of the dinuclear complex [C6H4-
1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2] in the catalytic transfer
hydrogenation of ketones.
2. Experimental
2.1. Materials and methods
Unless stated otherwise, all reactions were carried out under
an atmosphere of dinitrogen using conventional Schlenk glass-
ware, solvents were dried using established procedures and dis-
tilled under dinitrogen immediately prior to use. The IR spectra
were recorded on a Bruker-Tensor 27 FT-IR spectrometer as KBr
pellets. The 1H NMR spectra were recorded on a JEOL GX300
∗
Corresponding authors. Tel.: +52 555 622 4514; fax: +52 555 616 2217.
E-mail addresses: ronan@servidor.unam.mx (R. Le Lagadec),
damor@servidor.unam.mx (D. Morales-Morales).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.11.041

R. Cero´n-Camacho et al. / Journal of Molecular Catalysis A: Chemical 247 (2006) 124–129
125
Scheme 1. Synthesis of [C6H4-1,3-(OPPh2)2] with [(η6-p-cymene)RuCl2]2 (1).
spectrometer. Chemical shifts are reported in ppm down field of
2.3. Synthesis of [C6H4-1,3-(OPPh2
TMS using the solvent (CDCl3, δ = 7.27) as internal standard.
31P{1H} NMR spectra were recorded with complete proton
decoupling and are reported in ppm using 85% H3PO4 as exter-
nal standard. Positive-ion FAB mass spectra were recorded on a
JEOLJMS-SX102Amassspectrometeroperatedatanaccelerat-
ing voltage of 10 kV. Samples were desorbed from a nitrobenzyl
alcohol (NOBA) matrix using 3 KeV xenon atoms. Mass mea-
surements in FAB are performed at a resolution of 3000 using
magnetic field scans and the matrix ions as the reference mate-
rial or, alternatively, by electric field scans with the sample peak
bracketed by two (polyethylene glycol or cesium iodide) ref-
erence ions. Melting points were determined in a MEL-TEMP
capillary melting point apparatus and are reported without cor-
rection. GC–MS analyses were performed on a Agilent 6890N
GC with a 30.0 m DB-1MS capillary column coupled to an Agi-
lent 5973 Inert Mass Selective detector.
{Ru(η6-benzene)Cl2})2] (2)
The title compound was synthesized in an analogous manner
as that of complex 1 from a solution of [(η6-benzene)RuCl2]2
(128 mg, 0.21 mmol) in CH2Cl2 (10 mL) and a solution
(CH2Cl2, 10 mL) of [C6H4-1,3-(OPPh2)2] (0.21 mmol). A
microcrystalline red powder was obtained after recrystaliza-
tion from diethyl ether/CH2Cl2 (yield 106 mg, 52%). 1H NMR
(300.53 MHz, CDCl3) δ = 8.03–7.81 (m, 8H, arom), 7.36–6.93
(m, 16H, arom), 5.37 (s, 12H, benzene); 31P NMR (121.65 MHz,
CDCl3) δ = 116.86 (s). MS-FAB+ [M + H − Cl]+ = 943 (1%)
m/z, [M + H − (Cl-benzene)]+ = 864 (2%) m/z, [M + H − (2Cl-
benzene)]+ = 829 (2%) m/z, [M + H − (2Cl-2benzene)]+ = 751
(1%) m/z.
2.4. Transfer hydrogenation of ketones
The starting materials [(η6-p-cymene)RuCl2]2, [(η6-
benzene)RuCl2]2 [9] and the ligand [C6H4-1,3-(OPPh2)2] [10]
were prepared according to published procedures.
Typical procedure for the catalytic hydrogen-transfer reac-
tion: a suspension of the ruthenium complex [C6H4-1,3-
(OPPh2{Ru(η6-p-cymene)Cl2})2] (1) (11.0 mg, 0.005 mmol),
NaOH (0.5 mg, 0.012 mmol) and the corresponding ketone
(0.5 mmol) in degassed iso-propanol (2.5 mL) was refluxed for
10 h. After this time a sample of the reaction mixture is taken off,
diluted with acetone and analyzed by GC–MS, yields obtained
are related to the residual unreacted ketone.
2.2. Synthesis of [C6H4-1,3-(OPPh2
{Ru(η6-p-cymene)Cl2})2] (1)
Toasolutionof[(η6-p-cymene)RuCl2]2 (128 mg, 0.21 mmol)
in CH2Cl2 (10 mL), a solution (CH2Cl2, 10 mL) of [C6H4-1,3-
(OPPh2)2] (0.21 mmol) was added. The resulting reaction mix-
ture was allowed to proceed under stirring at room temperature
for 24 h. After this time, the solution was filtered off and the sol-
vent evaporated under vacuum, the solid residue thus obtained
is washed with diethyl ether (3 × 10 mL) and then dried under
vacuum (Scheme 1). Following recrystalization from diethyl
ether/CH2Cl2 a microcrystalline red powder was obtained (yield
127 mg, 56%), m.p. 180 ◦C (decomp.). 1H NMR (300.53 MHz,
CDCl3) δ = 7.99–7.92 (m, 8H, arom), 7.27–7.19 (m, 13H, arom),
7.00–6.94 (m, 1H, arom), 6.80–6.77 (m, 2H, arom), 5.19 (d, 4H,
2.5. Data collection and refinement for
[C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2] (1)
A crystalline red-orange prism of [C6H4-1,3-(OPPh2{Ru(η6-
p-cymene)Cl2})2] (1), grown from CDCl3 was glued to a glass
fiber. The X-ray intensity data were measured at 291 K on a
Bruker SMART APEX CCD-based X-ray diffractometer sys-
˚
tem equipped with a Mo-target X-ray tube (λ = 0.71073 A). The
detector was placed at a distance of 4.837 cm from the crystal. A
total of 1800 frames were collected with a scan width of 0.3◦ in ω
and an exposure time of 10 s/frame. The frames were integrated
with the Bruker SAINT software package using a narrow-frame
integration algorithm. The integration of the data using a mono-
clinic unit cell yielded a total of 42334 reflections to a maximum
3
3J = 6.4 Hz, C6H4), 5.08 (d, 4H, J = 6.5 Hz, C6H4), 2.43 (m,
3
2H, CH(CH3)2), 1.42 (s, 6H, CH3), 0.79 (d, 12H, J = 6.5 Hz,
CH(CH3)2); 31P NMR (121.65 MHz, CDCl3) δ = 114.78 (s).
MS-FAB+ [M + H]+ = 1091 (<1%), [M + H − Cl]+ = 1055 (1%)
m/z, [M + H-(RuCl2-η6-p-cymene)]+ = 784 (2%) m/z.

126
R. Cero´n-Camacho et al. / Journal of Molecular Catalysis A: Chemical 247 (2006) 124–129
Table 1
The numbering of the atoms is shown in Fig. 1 (ORTEP).
[13].
Summary of crystal structure data for complex [C6H4-1,3-(OPPh2{Ru(η6-p-
cymene)Cl2})2] (1)
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C51H54Cl6Ru2O2P2
1175.72
291 (2) K
3. Results and discussion
The reaction of stoichiometric amounts of [(η6-p-
cymene)RuCl2]2 and [C6H4-1,3-(OPPh2)2] affords complex
[C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2] (1) in good yield
as a red microcrystalline powder, this compound being stable to
˚
0.71073 A
Monoclinic
P2(l)/c
◦
˚
Unit cell dimensions
a = 15.9326 (10) A, α = 90
◦
˚
1
b = 18.6537 (11) A, β = 92◦.654 (2)
the open atmosphere. Analysis by H NMR reveals this com-
˚
c = 17.6065 (11) A, γ = 90
pound to be diamagnetic, exhibiting signals corresponding to
the aromatic rings for the phosphinite ligand at 6.77–7.99 ppm.
Another set of signals consisting of two doblets centered at 5.19
and 5.08 ppm are due to the presence of the aromatic protons
in the p-cymene group, this information is complemented by
the presence of signals at 2.43 and 0.79 ppm due to the CH and
CH3 of the iso-propyl groups of the p-cymene moiety. Finally, a
signal due to the presence of the methyl in the p-cymene group
is observed at 1.42 ppm. Analysis by 31P{1H} NMR exhibits
a unique signal in the spectrum, indicative of both phosphorus
being equivalent as result of the high symmetry of the complex.
The FAB+ mass spectrometry analysis of this complex exhibits
the molecular ion plus one unit [M + H]+ at 1091 m/z, other
important fragments are observed at 1055 and 784 m/z due to the
consecutive loss of a Cl− ligand and the [RuCl-(η6-p-cymene)]
fragment, respectively.
3
˚
Volume
Z
Density (calculated)
Absorption coefficient
F(0 0 0)
Crystal size
θ range for data collection
Index ranges
Reflections collected
Independent reflections
Absorption correction
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
Largest diff. peak and hole
5227.1 (6) A
4
1.494 g/cm3
0.984 mm−1
2384
0.38 mm × 0.09 mm × 0.08 mm
1.28–25.00◦
−18 ≤ h ≤ 18, −22 ≤ k ≤ 22, −20 ≤ l ≤ 20
42334
9197 [R(int) = 0.0670]
Analytical
Full-matrix least-squares on F2
9197/0/573
1.000*
R1 = 0.0585, wR2 = 0.1142**
R1 = 0.0976, wR2 = 0−.13247**
˚
0.638 and −0.570 eA
*
S = [w(Fo)2 − (Fc)2)2/(n − p)]1/2, where n: number of reflections and p: total
Crystals of [C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2] (1)
suitable for single crystal X-ray diffraction analyses (Table 1)
were obtained by slow evaporation of a saturated solution of 1 in
CDCl3. The structure determined is quite unusual exhibiting a
Ru(II) bimetallic species with the PCP phosphinite ligand acting
as a bridge between the two ruthenium centers.
The geometry around the two ruthenium centers can be
defined as slightly distorted pseudo tetrahedrons, exhibiting the
typical “piano-stool” geometry around the ruthenium, having
two chloride ligands and one diphenylphosphino group coor-
dinated and completing the coordination sphere the p-cymene
group occupying three coordination sites. To the best of our
knowledge this is the first crystallographic determination of a
bimetallic bridged structure using this sort of ligands, although
a related structure has been invoked by Balakrishna and cowork-
ers [14] for the formation of the ruthenium complex (a) where
the ligand also serves as bridge between two metal centers
(Scheme 2), however no crystallographic evidence was pre-
sented.
number of parameters.
**
R1 = |Fo − Fc|/|Fo|, wR2 = [w((Fo)2 − (Fc)2)2/w(Fo)2]1/2
.
◦
˚
2θ angle of 50.00 (0.93 A resolution), of which 9197 were inde-
pendent (Rint = 6.70%). Analysis of the data showed negligible
decay during data collection. The structure was solved by Pat-
terson method using SHELXS-97 [11] program. The remaining
atoms were located via a few cycles of least squares refinements
and difference Fourier maps, using the space group P2(l)/c, with
Z = 4. Hydrogen atoms were input at calculated positions, and
allowed to ride on the atoms to which they are attached. Ther-
mal parameters were refined for hydrogen atoms on the phenyl
˚
groups using a Ueq = 1.2 A to precedent atom. The final cycle of
refinement was carried out on all non-zero data using SHELXL-
97 [12] and anisotropic thermal parameters for all non-hydrogen
atoms. The details of the structure determination◦are given in
˚
Table 1 and selected bond lengths (A) and angles ( ) in Table 2.
Another related structure (b) has been reported by Osborn
and co-workers [15], where the phosphinite ligand PONOP
[C6H3N-2,6-(CH2OPPh2)2]behavesasbridgingligandbetween
two platinum centers, forming a large 20 membered diplatina-
macrocycle. This behavior was attributed to the length of the
arms in the ligand, being one CH2 longer that the ligand used in
this work (Scheme 3).
Mostrecently[16]asimilarstructure(c), (Scheme4)hasbeen
determined for a BINAP-based phosphinite ligand BINAPO,
where as is the case for complex 1, it is noted by the authors that
compounds bearing binaphtyl-based ligands bridging two metal
centers are very rare.
Table 2
Selected bond lengths and angles for [C6H4-1,3-(OPPh2{Ru(η6-p-
cymene)Cl2})2] (1)
Angles (◦)
˚
Bond lengths (A)
Ru(1) P(1)
Ru(1) Cl(1)
Ru(1) Cl(2)
Ru(2) P(2)
Ru(2) Cl(4)
Ru(2) Cl(3)
P(1) O(1)
2.2960 (15)
2.3956 (16)
2.4070 (16)
2.3116 (17)
2.4044 (17)
2.4128 (16)
1.625 (4)
P(1) Ru(1) Cl(1)
P(1) Ru(1) Cl(2)
Cl(1) Ru(1) Cl(2)
P(2) Ru(2) Cl(4)
P(2) Ru(2) Cl(3)
Cl(4) Ru(2) Cl(3)
O(1) P(1) Ru(1)
O(2) P(2) Ru(2)
90.65 (6)
85.03 (6)
87.48 (6)
86.62 (6)
94.69 (6)
87.75 (6)
113.79 (15)
113.63 (17)
P(2) O(2)
1.642 (4)

R. Cero´n-Camacho et al. / Journal of Molecular Catalysis A: Chemical 247 (2006) 124–129
127
Fig. 1. An ORTEP representation of the structure of [C6H4-1,3-(OPPh2)2] with [(η6-p-cymene)RuCl2]2 (1) at 50% probability showing the atom labeling scheme.
Scheme 2.
The Ru–P distances in [C6H4-1,3-(OPPh2{Ru(η6-p-
the case of the Ru–Cl distances. In all other respects the bond
distances and angles are within the expected values.
˚
cymene)Cl2})2] (1) [Ru(1) P(1) 2.2960 (15) A and Ru(2) P(2)
˚
The PCP ligands can behave as potentially tridentate pin-
cer type ligands [17], fact that has made their transition metal
complexes very valuable in organic transformations [18] and
activation of small molecules [19]. The thermal stability that
these ligands confer to their complexes, allow them to with-
stand high temperatures and thus overcome the energy bar-
riers to perform the corresponding activation. In an attempt
to obtain ruthenium complexes with the ligand [C6H4-1,3-
(OPPh2)2] behaving as pincer ligand, the starting material
[(η6-p-cymene)RuCl2]2 was changed for [(η6-benzene)RuCl2]2
[9]. However, analysis of the resulting product reveals this
compound to have a similar structure as that of [C6H4-1,3-
(OPPh2{Ru(η6-p-cymene)Cl2})2] (1). Direct reaction of the
ligand [C6H4-1,3-(OPPh2)2] with [RuCl2(PPh3)3] [20] resulted
2.3116 (17) A] are comparable to those found in the BINAPO
˚
derivative (c) (2.299 (4) A), and a similar situation occurs in
Scheme 3.

128
R. Cero´n-Camacho et al. / Journal of Molecular Catalysis A: Chemical 247 (2006) 124–129
the formation of the PCP complexes. However, in most of the
cases unworkable mixtures were obtained, which often decom-
posed readily to dark green material. In other cases, compound
1 could be isolated in lower yields than under the “mild” condi-
tions (CH2Cl2, RT) reported.
Following our continuous interest in the use of potentially
tridentated PCP pincer ligands in relevant organic transfor-
mations, we have used complex [C6H4-1,3-(OPPh2{Ru(η6-p-
cymene)Cl2})2] (1) as catalyst in the transfer hydrogenation of
ketones.
The results obtained are similar to those reported by
van Koten with the PCP pincer complex [(C6H3-2,6-
(CH2PPh2)2RuCl(PPh3)] for benzophenone and acetophenone
[5a]. Interestingly, our system is considerably faster, at least
in the case of benzophenone, since we achieve 84% yield in
only 10 h while van Koten’s system affords 98% but only after
108 h of reaction [5a]. Other ketones like the propiophenone
and 3-heptanone were also tested, however the yields under
the same reaction conditions and reaction times were only of
34% and 58%, respectively. An interesting result occurs when
the transfer hydrogenation of chalcone was attempted, afford-
Scheme 4.
in an intractable mixture of compounds that were not further
characterized. Moredrasticconditionswerealsoused. Forexam-
ple, a mixture of the ruthenium dimer, the [C6H4(OPPh2)2-1,3]
ligand (in various amounts) and KPF6 was refluxed (EtOH or
MeCN) under basic (KOH) conditions. The use of silver salts
(AgBF4 for instance) was another strategy developed to lead to
Table 3
Reduction of ketones by [C6H4-1,3-(OPPh2{Ru(η6-p-cymene)Cl2})2] (1) with PriOH/NaOH as the reducing agent
Entry
1
Substrate
Product
Yielda (%)
91
2
3
84
34
4
5
17
34
Reaction conditions: complex (1) (11.0 mg, 0.005 mmol), NaOH (0.5 mg, 0.012 mmol) and ketone (0.5 mmol) in degassed iso-propanol (2.5 mL), reflux for 10 h.
a
Yields were determined by GC–MS and related to the unreacted ketone.

R. Cero´n-Camacho et al. / Journal of Molecular Catalysis A: Chemical 247 (2006) 124–129
129
ing the hydrogenation of the alkene C C bond, without any
(c) M. Poyatos, J.A. Mata, E. Falomir, R.H. Crabtree,
Organometallics 22 (2003) 1110;
(d) W. Baratta, G. Chelucci, S. Gladiali, K. Siega, M. Toniutti, M.
Zanette, E. Sangrando, P. Rigo, Angew. Chem. Int. Ed. Engl. 44 (2005)
6214.
E
Peris,
detectable (GC–MS) amount of the alcohol as hydrogenated
product, which in fact may favor the use of this catalyst in the
regioselective hydrogenation of conjugated enones. It is note-
worthy that, extension of the reaction times up to 40 h affords in
all cases quantitative yields (Table 3).
[6] S. Medici, M. Gagliardo, S.B. Williams, P.A. Chase, S. Gladiali, M.
Lutz, A.L. Spek, G.P.M. van Klink, G. van Koten, Helv. Chim. Acta 88
(2005) 694.
It is probable, as in the case for [(C6H3-2,6-(CH2PPh2)2
RuCl(PPh3)], that in the present case hydrido-alkoxo inter-
mediates are the active species in the catalytic process [5a].
This hypothesis and other aspects of the reaction mechanism
for the transfer hydrogenation of ketones using [C6H4-1,3-
(OPPh2{Ru(η6-p-cymene)Cl2})2] (1) as catalyst are currently
under study and will be matter of a further communication.
In summary, we have synthesized and unequivocally charac-
terized a new bimetallic ruthenium complex having a phosphi-
nite PCP ligand as a bridging ligand between the two ruthenium
centers. This compound has proved to be an efficient catalyst in
the transfer hydrogenation reaction of ketones in the presence
of PriOH and NaOH.
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4. Supplementary material
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D. Adv. Synth. Catal., in press;
(j) F.E. Hahn, M.C. Jahnke, V. Gomez-Benitez, D. Morales-Morales, T.
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Morales, Inorg. Chim. Acta, in press.
Supplementary data for complex 1 have been deposited at the
Cambridge Crystallographic Data Centre. Copies of this infor-
mation are available free of charge on request from The Direc-
tor, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax:
+44 1223 336033; e-mail address: deposit@ccdc.cam.ac.uk or
url: http://www.ccdc.cam.ac.uk) quoting the deposition number
CCDC 284682.
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V.G-B would like to thank CONACYT for financial support.
We would like to thank Chem. Eng. Luis Velasco Ibarra and
´
M.Sc. Francisco Javier Perez Flores for their invaluable help
in the running of the FAB+-Mass. The support of this research
by CONACYT (40135-Q and J41206-Q) and DGAPA-UNAM
(IN114605) is gratefully acknowledged.
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