Ruthenium cationic species for transfer hydrogenation of aldehydes: Synthesis and catalytic properties of [(PPh3)2Ru(CH 3CN)3Cl]+[A]- {A = BPh4 or ClO4} and structure

Article abstract of DOI:10.1016/j.jorganchem.2005.08.006

The compounds, [(PPh₃)₂Ru(CH₃CN) ₃Cl] [BPh₄] (1) and [(PPh₃) ₂Ru(CH₃CN)₃Cl] [ClO₄] (2) have been synthesized from reactions of [Ru(PPh₃

Full text of DOI:10.1016/j.jorganchem.2005.08.006

Journal of Organometallic Chemistry 690 (2005) 5006–5010
www.elsevier.com/locate/jorganchem
Note
Ruthenium cationic species for transfer hydrogenation
of aldehydes: Synthesis and catalytic properties
of [(PPh3)2Ru(CH3CN)3Cl]+[A]ꢀ {A = BPh4 or ClO4} and structure
of [(PPh3)2Ru(CH3CN)3Cl]+[BPh4]ꢀ
*
Sipra Naskar, Manish Bhattacharjee
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721 302, India
Received 6 July 2005; received in revised form 1 August 2005; accepted 9 August 2005
Available online 19 September 2005
Abstract
The compounds, [(PPh3)2Ru(CH3CN)3Cl] [BPh4] (1) and [(PPh3)2Ru(CH3CN)3Cl] [ClO4] (2) have been synthesized from reactions of
[Ru(PPh3)3Cl2] with Na[BPh4] and NaClO4 Æ H2O, respectively, in acetonitrile. The compound 1 has been structurally characterized. The
geometry of this complex is that of a distorted octahedral, with cationic Ru(II) center bonded two triphenylphosphine ligands in the axial
position and three acetonitrile and one chloride ligands present in the basal positions. The ruthenium compounds are catalytically active
for the conversion of aldehydes and ketones to alcohols at 90 ꢀC using 2-propanol as the source of hydrogen in the presence of K2CO3.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: Ruthenium; Transfer hydrogenation; Catalysis
1. Introduction
tion of ketones and aldehydes have attracted attention for
the relatively benign nature of the reagents and mild reac-
tion conditions employed [6,10–13]. In contrast to conven-
tional hydrogenation using dihydrogen and metal
catalysts, which frequently require a high hydrogen pressure
[14], the transfer hydrogenation has some unique advanta-
ges in its simplicity and avoidance of cumbersome reducing
agents. However, aldehydes are difficult to reduce by trans-
fer hydrogenation catalysts [11]. It has been shown that,
ruthenium complexes containing phosphine ligands and a
iridium complex N-heterocyclic carbene are effective cata-
lysts for reduction of aldehydes [11,13]. Recently, Yamada
and Noyori has shown that, RuCl[(R,R)-YCH(C6H5)CH-
(C6H5)NH2](g6-arene) (Y = NSO2C6H4-4-CH3 or O) and
t-C4H9OK catalyzes the asymmetric transfer hydrogenation
of various benzaldehyde-1-d-derivatives with 2-propanol to
yield (R)-benzyl-1-d alcohols in 95–99% ee and with >99%
isotopic purity. Reaction of benzaldehydes with a DCO2D-
triethylamine mixture and the R,R catalyst affords the S
deuterated alcohols in 97–99% ee [15].
The structural and reactivity studies of coordinatively
unsaturated complexes have received much attention due
to their involvement as intermediates in transition metal
catalyzed organic transformation. The ability of ruthenium
to exist in verity of oxidation states as well as to assume
wide range of coordination geometry are the main reasons
behind ruthenium compounds being effective catalysts for
verity of organic reactions including ring closing metathesis
[1], C–H activation [2–4], activation of carbon–carbon mul-
tiple bonds [2], C–C bond formation [2] and Oppenauer oxi-
dation [5], and transfer hydrogenation [6]. In most of the
cases, 16 electron cationic species are predicted to be the ac-
tive catalyst. Thus, in recent years, some 16 electron cationic
stable species have been isolated and characterized structur-
ally [7–9]. Transition metal catalyzed transfer hydrogena-
*
Corresponding author. Fax: +91 03222 282252.
E-mail address: mxb@iitkgp.ac.in (M. Bhattacharjee).
0022-328X/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.08.006

S. Naskar, M. Bhattacharjee / Journal of Organometallic Chemistry 690 (2005) 5006–5010
5007
Another frequently encountered problem in transfer
hydrogenation is the concomitant formation of alkylated
products and decarbonylation [10,11]. For example,
Ru(PPh3)3Cl2 catalyzed reaction between butanol and ace-
tophenone gives 1-phenylhexan-1-ol and 1-phenylhexan-1-
one, rather than the expected direct transfer hydrogenation
product, 1-phenylethanol [10]. It has been observed that,
when primary alcohol is used as reducing agent, usually
alkylated products are obtained [16].
We were interested in synthesizing simple catalytically
active ruthenium(II) cationic compound containing phos-
phine ligands and labile N donor ligands, which can easily
afford 16 electron species in solution.
N, 4.25. 1H NMR: 2.1 ppm(s); 7.27–7.79 ppm(m). IR
(cmꢀ1): 3055, 2265, 1675, 1485, 1440, 700, 510. UV–Vis
(nm): 332 (e = 1275), 265 (e = 3865). Yield: 72%.
3. Hydrogenation of carbonyl compounds by 2-propanol
catalyzed by 1 and 2
The carbonyl compound (10 mmol), 2-propanol
(10 cm3), solid K2CO3 (10 mmol), and 1 or 2 (0.05 mmol)
was taken in a 25 ml schlenk flask. The reaction mixture
was flashed with argon and was stirred for desired time
at 90 ꢀC. The reactions were monitored by TLC. After
the completion of the reaction, the reaction mixture was
added to water (50 cm3) and the organic product was ex-
tracted (4–5 times) by diethyl ether. The combined ether
layer was washed with water (2/3 times), dried (Na2SO4)
and finally the solvent was removed under vacuum to iso-
late the product. The products were purified by column
chromatography.
Herein we report preparation, characterization, and cat-
alytic activity of [(PPh3)2Ru(CH3CN)3Cl]+[BPh4]ꢀ (1) and
[(PPh3)2Ru(CH3CN)3Cl]+[ClO4]ꢀ (2) and structure of
[(PPh3)2Ru(CH3CN)3Cl]+[BPh4]ꢀ (1).
2. Experimental
All the chemicals used were reagent grade products.
[Ru(PPh3)3Cl2] was synthesized by reported procedure
[17]. H NMR spectra were recorded on Bruker AC 200,
4. X-ray crystallographic study
1
Suitable crystals of [(PPh3)2Ru(CH3CN)3Cl][BPh4] were
grown from dilute acetonitrile solution at room temperature
over a period of 5–6 days. The single crystal data was col-
lected on Bruker-Nonius Mach3 CAD4 X-ray diffractome-
ter that uses graphite monochromated Mo Ka radiation
200 MHz NMR spectrometer in CDCl3 solutions at room
temperature. IR spectra were recorded on a Thermo Nick-
olet Model 870 FT IR spectrometer. The UV–Vis spectra
were recorded on a Shimadzu UV 600 spectrophotometer.
Cyclic voltammetry experiments were performed using a
fully automated CH Instruments electrochemical analyzer
(Model 620a) and a three-electrode set-up consisting of a
glassy carbon working electrode, a platinum wire counter
electrode, and an Ag/AgCl reference electrode. All experi-
ments were performed in dry CH3CN containing tetra-N-
butylammonium perchlorate (0.2 mol dmꢀ3) as the back-
ground electrolyte. The magnetic measurements were done
using Gouy method. The C, H, and N were estimated using
microanalyses on a Perkin–Elmer microanalyser, 2400.
˚
(k = 0.71073 A) by x-scan method. No absorption correc-
tion was used. The structure was solved by direct methods
and refined by least square methods on F2 employing WINGX
[18] package and the relevant programs (SHELX-97 [19] and
ORTEP-3 [20]) implemented therein. Non-hydrogen atoms
were refined anisotropically and hydrogen atoms on
C-atoms were fixed at calculated positions and refined
using a riding model. Crystal data: Empirical formula:
C66H59B1Cl1N3P2Ru1, MW = 1103.43, monoclinic, space
˚
˚
group P21/c, a = 15.063(5) A, b = 19.581(5) A, c =
˚
19.529(5) A, a = 90.000ꢀ, b = 95.062(5)ꢀ, c = 90.000ꢀ,
3
2.1. Synthesis of [(PPh3)2Ru(CH3CN)3Cl][BPh4] (1)
and [(PPh3)2Ru(CH3CN)3Cl][ClO4] (2)
V = 5738.4(3) A , Z = 4,
d
(calcd.) = 1.277 Mg/m3,
˚
F(000) = 2288, T = 293 K, l = 0.418 mmꢀ1, crystal size =
0.4 · 0.4 · 0.2 mm3, data collection range 1.36 6 h 6
25.00, 0 6 h 6 17, 0 6 k 6 23, ꢀ23 6 l 6 23, R1 = 0.0355,
wR2 (all data) = 0.0923.
RuCl2(PPh3)3 (0.5 g; 0.520 mmol) and NaBPh4 or Na-
ClO4 Æ H2O (0.520 mmol) were dissolved in acetonitrile
(25 ml) and the reaction solution was refluxed for about
6 h. The color of the reaction solution turned to light yel-
low. The reaction solution was then filtered and concen-
trated where upon a yellow crystalline solid separated
out. The separated solid was filtered and washed with hex-
ane (3–4 times) and dried in vacuo. The compound was fi-
nally recrystallized from acetonitrile. Yield: 75%. Anal.
Calcd. for C66H59B1Cl1N3P2Ru1 (MW = 1103.43): C,
71.78; H, 5.39; N, 3.81. Anal. Found: C, 71.11; H, 5.37;
N, 4.05. 1H NMR: 2.0 ppm(s); 7.27–7.79 ppm(m). IR
(cmꢀ1): 3050, 2270, 1670, 1479, 1434, 696, 511. UV–Vis
(nm): 330 (e = 1270), 260 (e = 3861). Yield: 75%. Anal.
Calcd. for C42H39Cl2N3O4P2Ru1 (MW = 883.24): C,
57.06; H, 4.45; N, 4.76. Anal. Found: C, 56.78; H, 4.10;
5. Results and discussion
The title compounds 1 and 2 have been prepared from a
reaction of NaBPh4 and NaClO4 Æ H2O (0.520 mmol),
respectively, with [(PPh3)3RuCl2] (0.5 g; 0.520 mmol) in
refluxing acetonitrile in high yield (ca. 75%). It was shown
a few years back that, reactions of Ru2Cl(O2CR)4 or Ru2-
Cl(O2CAr)4 with PPh3 in acetonitrile afford compounds
of the type [RuCl(MeCN)3(PPh3)2]2[Ru2Cl2(O2CR)4] or
[RuCl(MeCN)3(PPh3)2]2[Ru2Cl2(O2CAr)4], two of which
were structurally characterized [21]. It may be mentioned
that, the yields of [RuCl(MeCN)3(PPh3)2]2[Ru2Cl2(O2CR)4]
were very poor, typically in the range of 7–10%. The highest

5008
S. Naskar, M. Bhattacharjee / Journal of Organometallic Chemistry 690 (2005) 5006–5010
˚
larger (Ru1–N1 distance = 2.034(2) A, Ru1–N2 distance =
yield was obtained in the case of [RuCl(MeCN)3(PPh3)2]2-
[Ru2Cl2(O2CMe)4] which was only 37% [21]. Fogg and
James have reported the preparation of [(PPh3)2Ru-
CH3CN)3Cl]+[PF6]ꢀ from a reaction of [(PPh3)3RuCl2] with
NH4PF6 in acetonitrile in 72% yield. However, this was nei-
ther characterized structurally, nor was its reactivity ex-
plored [22].
˚
˚
2.021(2) A, and Ru1–N3 distance = 2.014(2) A) then the
other two. Similarly, \Ru1–N1–C1 and \Ru1–N3–C5 an-
gles deviate considerably form linearity (\Ru1–N1–C1 an-
gle = 168.6 (2)ꢀ and \Ru1–N3–C5 angle = 174.8),
whereas, \Ru1–N2–C3 angle is somewhat linear (\Ru1–
N2–C3 = 178.8(3)ꢀ). Thus, two of the three coordinated
acetonitrile ligands are coordinated to ruthenium center in
a highly distorted manner. Both the Ru–P distances are
The compounds 1 and 2 have been characterized by ele-
mental analyses, IR, UV–Vis, and 1H NMR spectral studies,
and also by electrochemical and magnetic measurements.
The elemental analyses agree well with the formulations
of the compounds. Cyclic voltammetry of 1 and 2 showed
that the metal center undergoes a reversible (DEP = ca.
40 mV), one-electron oxidation process with a half-wave
potential (E1/2) of 1.40 and 1.35 V, respectively, versus Ag/
AgCl. Along with the reversible peak, the compound 1 shows
an irreversible oxidative peak at 1.06 V. This is due to oxida-
tion of BPhꢀ4 anion, as confirmed by cyclic voltammetry of
NaBPh4 in acetonitrile, which also shows an irreversible
peak at similar position. The 1H NMR spectra of the com-
pounds show a singlet at d ca. 2.0 ppm due to the –CH3 pro-
tons of the bonded acetonitrile and a multiplet in the region d
ca. 7.27–7.79 ppm due to aromatic protons. The UV–Vis
spectra show two bands at ca. 330 (e = 1270), ca. 260
(e = 3861). The compounds are diamagnetic in character,
as expected, and clearly show the presence of ruthenium(II).
The single crystal X-ray study of 1 clearly establishes the
formulation of the compound. The structure of the com-
pound is shown in Fig. 1.
The cationic complex has a ruthenium(II) center bonded
to three acetonitrile and a chloride in the equatorial plane
and two triphenyl phosphines occupy the axial position in
a trans fashion as shown earlier. The BPhꢀ4 anion is found
outside the coordination sphere and does not show any
bonding interaction with ruthenium(II) center. The ruthe-
nium(II) center is found to be in a distorted octahedral envi-
ronment. All the three Ru–N distances are not found to
be equal, one of the Ru–N distance is comparatively
˚
almost equal (Ru1–P1 distance = 2.4076(9) A and Ru1–P2
˚
distance = 2.4145(9) A). The \N3–Ru1–N2 and \N2–
Ru1–N1 angles are less then 90ꢀ (88.30(9)ꢀ and 87.91(9)ꢀ,
respectively). Similarly, the P1–Ru1–P2 angle is found to
be 177.78 (2)ꢀ. The geometry and bond lengths and bond dis-
tances are similar to those reported for [RuCl(MeCN)3-
(PPh3)2]2[Ru2Cl2(O2CR)4] [21].
Transfer hydrogenation of carbonyl compounds using
2-propanol as reducing agent as well as solvent has at-
tracted a lot of attention over last few years due to the com-
paratively benign nature of the reagents [23–28]. A number
of transition metal catalysts have been used for this trans-
formation. The main focus has been on ruthenium(II) com-
plexes [23–29]. However, in most of the cases, the reactions
are effective only in the cases of ketones. Very recently, an
iridium complex containing N-heterocyclic carbene ligand
has been found to be effective in the reduction of aldehydes
[11]. Encouraged by these reports, we wanted to explore the
efficacy of the newly synthesized compound.
Accordingly, hydrogenation of various types of alde-
hydes and ketones was carried out in 2-propanol using
0.5 mol% of the newly synthesized compound, [(PPh3)2Ru-
(CH3CN)3Cl][BPh4], (1) in the presence of K2CO3. The
products were isolated in medium to high yield and charac-
terized by 1H NMR spectra (Table 1). 1H NMR spectra of
the crude products showed the signals for the alcohols and
the starting carbonyl compounds only. We could not ob-
serve formation of any decarbonylation or condensation
product. This is also apparent form the observed values
of percentage yield and conversion (Table 1).
The catalytic activity of the compound 2 has also been
explored and compared with that of 1. The results are given
in Table 2. The catalytic activity of 2 is similar to that of 1
(Table 2).
Backvall and coworkers [27] have reported that, mono-
hydrido species, [RuHCl(PPh3)3], generated from the reac-
tion of isopropanol and [RuCl2(PPh3)3], is completely
inactive in the reduction of acetone to isopropanol. However,
Cadierno et al. [26] have recently isolated cis,cis-[RuHCl-
(CN-2,6-C6H3Me2)2(dppf)] and cis,cis,cis-[RuH2(CN-2,
6-C6H3Me2)2(dppf)] [dppf = 1,10-Bis(diphenylphosphino)-
ferrocene] and found that, both the monohydrido and dihyd-
ridoruthenium compounds are active in the reduction of
acetophenone by isopropanol, however, the dihydrido species
was found to be more active. Recently, it has been reported
that, cationic complexes of the type [CpRu(P–N)(CH3CN)]+,
where (P–N) is bidentate aminophosphene ligands, are excel-
lent pre-catalysts for transfer hydrogenation for ketones and
Fig. 1. ORTEP drawing of [(PPh3)2Ru(CH3CN)3Cl][BPh4]. The H-atoms
were not shown for the sake of clarity.

S. Naskar, M. Bhattacharjee / Journal of Organometallic Chemistry 690 (2005) 5006–5010
5009
Table 1
Transfer hydrogenation of carbonyl compounds catalyzed by 1 using 2-propanol and K2CO3
a
Entry
Substrates (10 mmol)
Product
Yield (%)b
Conversion
Turnover numberc
1
2
3
4
5
6
7
8
a
Benzaldehyde
Benzyl alcohol
75
71
79
80
60
68
65
60
77
73
79
81
60
70
65
60
154
146
158
162
120
140
130
120
4-Methoxy-benzaldehyde
2-Chloro-benzaldehyde
4-Chloro-benzaldehyde
4-Nitro-benzaldehyde
Acetophenone
4-Methoxy-benzyl alcohol
2-Chloro-benzyl alcohol
4-Chloro-benzyl alcohol
4-Nitro-benzyl alcohol
1-Phenylethanol
Benzophenone
Cyclohexanone
Diphenylmethanol
Cyclohexanol
Reaction time = 4 h. Reaction temperature = 90 ꢀC. Catalyst used = 0.05 mol.
Isolated yield.
Mol of product/mol of catalyst used.
b
c
Table 2
Comparison of transfer hydrogenation of aldehydes catalyzed by 1 and 2 using 2-propanol and K2CO3
a
Entry
1
Substrates (10 mmol)
Benzaldehyde
Catalyst (0.05 mmol)
Yield (%)b
Conversion
Turnover numberc
2
1
73
75
75
77
150
154
2
4-Chloro-benzaldehyde
4-Nitro-benzaldehyde
2
1
70
80
72
81
144
162
3
2
1
62
60
63
60
126
120
a
Reaction time = 4 h. Reaction temperature = 90 ꢀC.
Isolated yield.
Mol of product/mol of catalyst used.
b
c
a monohydrido Ru(II) intermediate has been proposed as
the active species [30]. Similarly, the mono hydride complex,
[RuH(O2CH){PPh2(C6H4SO3Na)}3] was identified to be the
active species in the aqueous medium reduction of aldehydes
by corresponding catalyst, [RuCl2L2], in the presence of ex-
cess of phosphene [31,32].
+
PPh3
]
CH3CN
CH3CN
Cl
Ru
NCCH3
PPh3
Our attempts to identify the reaction intermediates by
NMR spectroscopy failed. However, based on the reported
mechanistic works [26–32], a possible reaction pathway has
been proposed in Scheme 1.
K2CO3
+
HO
O
R1
R2
O
O
PPh3
The initial step of the reaction could be replacement of
chloride by alkoxide followed by b-hydride elimination to
give acetone and ruthenium monohydrido complex, which
may be the active species. The next step is the replacement
acetonitrile ligand by the carbonyl compound followed by
the insertion of the carbonyl compound to Ru–H bond.
This step is followed by proton transfer to coordinated alk-
oxide by isopropanol with the formation of metal hydride
by b-hydride elimination to produce the active species
back. Since in 1 and 2 only one Clꢀ is present, formation
of a monohydrido species is favored as observed earlier
[30,31]. Although, a monohydrido species has been pro-
posed as the intermediate, a dihydrido species as an inter-
mediate cannot be ruled out completely. Also, it may be
noted that, albeit monohydrido intermediate has been
completely ruled out by Backvall and coworkers by isotope
labeling method, the involvement of Ru(0) species has not
been proved conclusively [31].
+
CH3CN
CH3CN
H
Ru
NCCH3
PPh3
CH3CN
PPh3
PPh3
+
+
CH3CN
CH3CN
H
O
R2
R1
O
NCCH3
Ru
Ru
CH3CN
NCCH3
PPh3
PPh3
PPh3
R1
OH
H
CH3CN
CH3CN
R2
R2
R1
+
Ru
O
OH
PPh3
Scheme 1.

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S. Naskar, M. Bhattacharjee / Journal of Organometallic Chemistry 690 (2005) 5006–5010
[5] C. Thoumazet, M. Melaimi, L. Ricard, F. Mathey, P. Le Floch,
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9020–9022.
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631.
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Organometallics 11 (1992) 1990–1993.
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In summary, simple cationic ruthenium compounds
containing labile acetonitrile ligands have been synthe-
sized. Due to the presence of the labile acetonitrile ligands
the compounds are capable of affording electronically and
coordinatively unsaturated species. The compounds are
effective catalysts for hydrogenation of aldehydes and ke-
tones in isopropanol, which acts both as solvent and as
hydrogen donor in this homogeneous hydrogen transfer
reaction. The solvent, isopropanol, is inexpensive, and
unreactive toward most organic functional groups, and
employing this method, only
a
catalytic amount
(0.5 mol%) of the ruthenium complex is necessary and
high turnover numbers as well as conversions can be
achieved.
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[16] I. Taguchi, H. Nakagawa, T. Hirabayashi, S. Sakaguchi, Y. Ishii, J.
Am. Chem. Soc. 126 (2004) 72–73.
7. Supplementary material
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(1970) 237–240.
[18] L.J. Farrugia, WINGX. An Integrated System of Windows Programs
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Diffraction Data, Version 1.63.01, University of Glasgow, Glasgow,
Scotland, 1999;
L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.
[19] G.M. Sheldrick, Program for Solution and Refinement, University of
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Organometallics 23 (2004) 4836–4845.
Crystallographic data (excluding structure factors) for
the structure(s) reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre as Sup-
plementary Publication No. CCDC-266619. Copies of the
data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax:
int. code +44 1223 336 033; deposit@ccdc.cam.ac.uk].
Acknowledgments
We thank Department of Science & Technology, Gov-
ernment of India, New Delhi for financial support and
FIST grant for single crystal X-ray facility. S.N. thanks
Council of Scientific & Industrial Research, Government
of India, New Delhi for the award of individual fellowship.
The authors thank Dr. K. Biradha, Department of Chem-
istry, IIT, Kharagpur, for his help in solving the crystal
structure of compound 1.
[27] A. Aranyos, G. Csjernyik, K.J. Szabo, J.-E. Backvall, Chem.
Commun. (1999) 351–352.
[28] Y.R. Santosh Laxmi, J.-E. Backvall, Chem. Commun. (2000) 611–
612.
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