Transfer hydrogenation of acetophenone promoted by (arene)ruthenium(II) reduced schiff base complexes: An X-ray structure of [(ν6-p-cymene)RuCl (OC6H4-2-CH2 NHC6H4-p-Me)]

Article abstract of DOI:10.1016/S0277-5387(01)00894-4

Arene ruthenium(II) reduced Schiff base complexes of formulation [ν⁶-p-cymene)RuCl(Lⁿ)] (1-5, n = 1-5) were prepared by reacting [(ν⁶-p-cymene)RuCl₂]₂ with HLⁿ in the presence of sodium carbonate in CH₂Cl₂. The complexes contain an ν⁶-p-cymene group, a chloride and a bidentate chelating N,O-donor ligand. The molecular structure of [(ν⁶-p-cymene)RuCl(OC₆H₄- 2-CH₂NHC₆H₄-p-Me)] (1) has been determined by X-ray crystallography. The complexes are found to be catalytically active for transfer hydrogenation of acetophenone to (±)1-phenylethanol in the presence of KOH and isopropanol showing percent conversion of 98 with the catalysts having a bulky substituent adjacent to the O- and N-donor sites of the auxiliary ligand.

Full text of DOI:10.1016/S0277-5387(01)00894-4

Polyhedron 20 (2001) 2735–2739
www.elsevier.com/locate/poly
Transfer hydrogenation of acetophenone promoted by
(arene)ruthenium(II) reduced Schiff base complexes: an X-ray
structure of [(h6-p-cymene)RuCl(OC6H4-2-CH2NHC6H4-p-Me)]
Rakesh K. Rath, Munirathinam Nethaji, Akhil R. Chakravarty *
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
Received 3 April 2001; accepted 5 July 2001
Abstract
Arene ruthenium(II) reduced Schiff base complexes of formulation [(h6-p-cymene)RuCl(Ln)] (1–5, n=1–5) were prepared by
reacting [(h6-p-cymene)RuCl2]2 with HLn in the presence of sodium carbonate in CH2Cl2. The complexes contain an h6-p-cymene
group, a chloride and a bidentate chelating N,O-donor ligand. The molecular structure of [(h6-p-cymene)RuCl(OC6H4-2-
CH2NHC6H4-p-Me)] (1) has been determined by X-ray crystallography. The complexes are found to be catalytically active for
transfer hydrogenation of acetophenone to (9)1-phenylethanol in the presence of KOH and isopropanol showing percent
conversion of 98 with the catalysts having a bulky substituent adjacent to the O- and N-donor sites of the auxiliary ligand. © 2001
Published by Elsevier Science Ltd.
Keywords: Arene ruthenium; Crystal structure; Transfer hydrogenation; Catalysis; Reduced Schiff base
1. Introduction
structure of complex 1 has been determined by X-ray
crystallographic study.
Half-sandwich (h6-arene)ruthenium(II) complexes
are of current interest in the development of new cata-
lytic systems for organic transformation reactions and
in asymmetric induction studies [1–3]. Recent reports
have shown that ruthenium-based complexes are effec-
tive catalysts for transfer hydrogenation of ketones in
the presence of an organic hydrogen donor like 2-
propanol [4–10]. It has been observed that the auxiliary
ligand having a NH moiety plays an important role in
the catalytic process involving a hydride intermediate
species [4]. The present work stems from our interest to
study the catalytic behavior of (p-cymene)ruthenium(II)
complexes having a N,O-donor chelating, reduced
Schiff base auxiliary ligand. Herein we report the syn-
2. Experimental
2.1. Materials and methods
All reactions were carried out in dry solvent under a
dinitrogen atmosphere. Dichloromethane, n-hexane,
methanol, petroleum ether (b.p. 40–60 °C) and iso-
propanol were distilled from appropriate drying agents
and deoxygenated prior to use. Acetophenone was dis-
tilled under vacuum. The ligands (HLn) were prepared
thesis,
structure
and
properties
of
[(h6-p-
cymene)RuCl(Ln)] (n=1–5) (Scheme 1). The crystal
* Corresponding author. Tel.: +91-80-309-2533; fax: +91-80-360-
0683.
E-mail address: arc@ipc.iisc.ernet.in (A.R. Chakravarty).
Scheme 1.
0277-5387/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0277-5387(01)00894-4

2736
R.K. Rath et al. / Polyhedron 20 (2001) 2735–2739
by reducing the Schiff bases [11] using sodium borohy-
dride (Aldrich make) in methanol. The precursor com-
plex [(h6-p-cymene)RuCl2]2 was synthesized following a
literature method [12]. Elemental analysis was per-
formed on a Perkin–Elmer 2400 CHN analyzer. 1H
NMR spectra were recorded on Bruker 200 MHz and
Bruker AMX 400 MHz spectrometer using CDCl3 as a
solvent and tetramethylsilane as an internal standard.
4°52q550° were measured in a ꢀ–2q scan mode on
CAD-4 diffractometer equipped with graphite
a
,
monochromated Mo Ka radiation (u=0.7107 A). Out
of a total 3826 unique reflections collected at 20 °C,
2901 reflections with I\2|(I) were used for the refine-
ment of the structure. The intensity data were corrected
for Lorentz and polarization effects.
The structure was solved by Patterson’s heavy-atom
method, which revealed the position of ruthenium atom
in the asymmetric unit. The remaining atoms were
located in successive difference Fourier syntheses.
Refinement of the structure was made by the full-ma-
trix least-squares procedures. An empirical absorption
correction was made on obtaining the complete struc-
tural model [13]. The transmission coefficients were in
the range 0.77–0.86. All non-hydrogen atoms were
refined anisotropically. The hydrogen atom positions
were generated and assigned isotropic thermal parame-
ters riding to the atoms they were bonded. The final
refinement was converged to R=0.0350; Rw=0.0771
2.2. Preparation of [(p6-p-cymene)RuCl(Ln)] (n=1–5,
1–5)
The complexes were prepared using a general proce-
dure in which 0.33 mmol of [(h6-p-cymene)RuCl2]2 (200
mg) was reacted with 0.66 mmol of HLn in the presence
of 1.5 mmol of sodium carbonate (160 mg) in 15 cm3
CH2Cl2 under stirring condition at 25 °C for 3 h. The
mixture was filtered and the filtrate was reduced to 5
cm3 and the product was precipitated by addition of
n-hexane. The solid was isolated, washed with n-hexane
and finally dried under vacuum. Yield ranged between
85 and 90%.
with
the
weighting
scheme:
w=1/[|2(F2o)+
(0.0340P)2+2.8307P] using 253 parameters, where
P=(F2o+2F2c)/3. The maximum shift/e.s.d., goodness-
of-fit and the highest peak were 0.001, 1.063 and 0.63
2.3. Hydrogenation of acetophenone
e A−3, respectively. All calculations were carried out
,
A general procedure was followed for the catalytic
hydrogenation of acetophenone. In a typical reaction,
0.05 mmol of the catalyst (1–5) was reacted with 0.125
mmol of KOH and 5 mmol of acetophenone (0.6 cm3)
under refluxing condition in 5 cm3 isopropanol for 6 h.
The reaction mixture was cooled to ambient tempera-
ture. The catalyst was removed by addition of 15 cm3
of petroleum ether (b.p. 40–60 °C) followed by filtra-
tion and neutralization with dilute HCl. The petroleum
ether layer was separated and passed through anhy-
drous sodium sulfate. The crude mixture containing the
hydrogenated product, (9)-1-phenylethanol and the
unreacted acetophenone was obtained by distilling the
solvent. Percentage conversion was calculated by com-
paring the methyl proton signals of acetophenone (s, l
2.62 ppm) and (9)-1-phenylethanol (d, l 1.50 ppm,
using SHELX system of programs [14].
2.4.1. Crystal data for
[(p6-p-cymene)RuCl(OC6H4-2-CH2NHC6H4-p-Me)]
C24H28NOClRu: M=482.6, monoclinic, P21/c, a=
,
9.609(7), b=14.552(4), c=15.608(4) A, i=94.90(3)°,
3
,
V=2175(2) A , F(000)=988, Z=4,
Dcalc=1.47
g cm−3, v(Mo Ka)=8.58 cm−1
.
3. Results and discussion
3.1. Synthesis and structure
Complexes 1–5 are prepared in high yield from a
reaction of [(h6-p-cymene)RuCl2]2 with the anionic
N,O-donor reduced Schiff base ligands. The complexes
are moderately stable in the solid state and unstable in
solution phase. They are characterized from elemental
1
J=6.8 Hz) in the H NMR spectrum of the mixture in
CDCl3 (s, singlet; d, doublet).
1
1
2.4. Crystallographic analysis
analysis and H NMR spectral data (Table 1). The H
NMR data suggest a 1:1 ratio of the p-cymene and the
reduced Schiff base in the monomeric complexes (Fig.
1). The methyl singlet and the isopropyl methyl protons
(two doublets) of the p-cymene ligand appear in the
ranges 1.7–2.1 and 1.1–1.3 ppm, respectively. Complex
2, however, shows a single doublet at 1.24 ppm for the
isopropyl methyl protons. The isopropyl CH proton
appears as a septet in the range 2.7–2.9 ppm. Four
doublets observed in the range 3.58–5.2 ppm are
assignable to the p-cymene ring protons. The other
spectral features are characteristic of the N,O-donor
Single crystals of 1, suitable for X-ray studies, were
obtained by using a diffusion technique in which a
dichloromethane solution of the complex was layered
below n-hexane in a Schlenk tube under a dinitrogen
atmosphere. An orange rectangular shaped crystal of
dimensions 0.32×0.20×0.18 mm was mounted on a
glass fiber with epoxy cement. Unit cell dimensions
were obtained by angular settings of 25 reflections in
the range 7.5°BqB15.9°. Intensities of reflections
(05h511; 05k517; −185l518) within the range

Table 1
Analytical a and 1H NMR b data of [(h6-p-cymene)RuCl(Ln)] (n=1–5, 1–5)
3
Complex
Analysis (%)
C
1H NMR (ppm, multiplicity, Hx, JHH (Hz))
H
N
Ln
p-Cymene
1
2
3
59.6 (59.7)
5.7 (5.8)
2.8 (2.9)
2.41 (s, Me), 3.65 (d, H7A, 10.0), 4.34 (d, H7B, 10.0), 4.35 (t, NH, 1.21–1.29 (2d, CHMe2, 6.0), 2.04 (s, Me), 2.86 (sp, CHMe2, 6.8),
10.2), 6.49 (t, H4, 7.5), 6.79 (d, H5, 7.4), 6.90 (d, H2, 7.4), 7.01 (t, 4.30–5.10 (4d, four ring H, 6.0)
H3, 7.5), 7.09–7.34 (2d, H9,10,12,13, 8.8)
/
58.3 (58.5)
64.6 (64.9)
5.7 (5.9)
7.5 (7.3)
2.7 (2.7)
2.4 (2.4)
2.40 (s, Me), 3.65 (d, H7A, 11.0), 3.88 (s, OMe), 4.23 (t, NH,
10.4), 5.15 (d, H7B, 10.0), 6.44–6.79 (m, H3–5), 7.28 (2d, H9,10,12,13
8.4)
1.24 (d, CHMe2, 6.8), 2.02 (s, Me), 2.91 (sp, CHMe2, 7.0),
4.18–5.04 (4d, four ring H, 5.8)
,
2
1.19 (s, CMe3), 1.64 (s, CMe3), 3.55 (d, H7A, 10.4), 4.37 (t, NH,
10.4), 5.26 (d, H7B, 10.4), 6.64 (s, H5), 7.13 (s, H3), 7.28 (m,
1.16–1.25 (2d, CHMe2, 7.2), 2.05 (s, Me), 2.87 (sp, CHMe2, 6.0),
4.14–5.14 (4d, four ring H, 5.8)
0
H9,10,12,13
3.83 (s, OCH3), 3.70 (d, H7A, 10.0), 3.72 (t, NH, 10.2), 4.51 (d,
H7B, 10.0), 6.75 (m, H4,5), 6.91(m, H2,3), 7.1–7.3 (m, H10–13
)
2
4
5
57.6 (57.8)
62.6 (62.5)
5.5 (5.6)
5.6 (5.4)
2.6 (2.8)
2.8 (2.7)
1.19–1.23 (2d, CHMe2, 6.8), 2.07 (s, Me), 2.80 (sp, CHMe2, 7.0),
4.65–5.05 (4d, four ring H, 6.2)
)
3.44 (d, H7A, 11.0), 4.17 (t, NH, 10.0), 5.39 (d, H7B, 10.8), 6.1–6.9 1.03–1.14 (2d, CHMe2, 7.0), 1.72 (s, Me), 2.78 (sp, CHMe2, 7.0),
(m, H2–5), 7.0–8.1 (m, H9–15 3.58–4.70 (4d, four ring H, 5.8)
2
)
a Calculated values are in parentheses.
b 200 MHz (1–4); 400 MHz (5); solvent, CDCl3; s, singlet; d, doublet; t, triplet; sp, septet; m, multiplet. Hx corresponds to the carbon number Cx of the reduced Schiff base ligand as shown
in Scheme 1.

2738
R.K. Rath et al. / Polyhedron 20 (2001) 2735–2739
sites to be coordinated by a chloride and the chelating
N,O-donor reduced Schiff base ligand. The structural
features correspond well to the analogous Schiff base
ruthenium(II) complexes [16,17].
3.2. Catalytic properties
The catalytic hydrogenation of acetophenone in the
presence of 1–5 has been studied in isopropanol–KOH
medium using a mole ratio of 1:2.5:100 for the catalyst,
KOH and the ketone in 5 cm3 of isopropanol (Eq. (1)).
The percent conversion is 70 and 72 for the complexes
1 and 2, respectively. While complexes 3 and 4 show
percent conversion of 98, the same is 93 for complex 5.
The high conversion rate could be due to the presence
of bulky substituents adjacent to the donor atom of the
reduced Schiff base ligands. Complex 3 has a bulky
t-butyl group in the ortho-position of the phenolato
ring. Complexes 4 and 5 have bulky groups like OMe at
the ortho-position of the phenyl ring or the naphthyl
group attached to the amine nitrogen.
Fig. 1. A 200 MHz 1H NMR spectrum of [(h6-p-cymene)RuCl(L2)]
(2) in CDCl3 (S, solvent peaks).
(1)
Noyori and co-workers have shown that the true
catalyst in the hydrogenation reaction is formally a
16-electron intermediate species [4]. The active catalyst
precursor is, however, a hydride 18-electron species in a
kinetically controlled reaction pathway. The role of
KOH is to generate the catalyst from the chloro precur-
sor and the reaction mediates through the hydride
species. The steric bulk of the auxiliary ligand plays an
important role in this hydrogenation process. It is likely
that the NH proton of the auxiliary ligand is suffi-
ciently acidic to undergo a facile elimination of HCl in
the presence of KOH. This results in the formation of a
reactive 16-electron species in which the metal is bound
to both anionic oxygen and nitrogen atoms in addition
to the p-cymene ring (6-electron donor). The stability of
the planar geometry of this formally 16-electron neutral
species may depend on the steric constraints generated
by the bulky substituents or groups of the auxiliary
ligand in the kinetically controlled reaction process.
Fig. 2. An ORTEP view of [(h6-p-cymene)RuCl(OC6H4-2-
CH2NHC6H4-p-Me)] (1) showing the thermal ellipsoids at 50% prob-
ability level along with the atom numbering scheme.
Table 2
Selected bond lengths (A) and bond angles (°) for [(h6-p-
,
cymene)RuCl(OC6H4-2-CH2NHC6H4-p-Me)] (1) (C0, the centroid of
the h6-p-cymene ring)
Bond lengths
Ru(1)ꢀO(1)
Ru(1)ꢀN(1)
Ru(1)ꢀC(6)
Ru(1)ꢀC(7)
Ru(1)ꢀC(3)
2.050(3)
2.136(3)
2.172(4)
2.174(4)
2.181(4)
Ru(1)ꢀC(2)
Ru(1)ꢀC(4)
Ru(1)ꢀC(5)
Ru(1)ꢀCl(1)
Ru(1)ꢀC0
2.187(5)
2.195(4)
2.200(4)
2.450(1)
1.673(5)
Bond angles
O(1)ꢀRu(1)ꢀN(1)
O(1)ꢀRu(1)ꢀCl(1) 84.75(11)
N(1)ꢀRu(1)ꢀCl(1) 81.87(10)
87.02(12)
N(1)ꢀRu(1)ꢀC0
Cl(1)ꢀRu(1)ꢀC0
O(1)ꢀRu(1)ꢀC0
131.51(18)
128.99(16)
126.56(19)
reduced Schiff base ligand. The methyl and the
methoxy protons of the bidentate ligand appear as
singlets near 2.4 and 3.9 ppm, respectively.
4. Conclusions
The complex [(h6-p-cymene)RuCl(L1)] has been
structurally characterized by X-ray crystallography. An
ORTEP [15] view of the complex is shown in Fig. 2.
Selected bond distances and bond angles are given in
Table 2. The complex has an essentially octahedral
coordination geometry. The p-cymene ring carbons oc-
cupy one face of the octahedron leaving the other three
We have prepared a series of (arene)ruthenium(II)
reduced Schiff base complexes that are catalytically
active in the hydrogenation of acetophenone to (9)-1-
phenylethanol. The steric bulk of the auxiliary ligand
has profound effect on the transfer hydrogenation pro-
cess resulting high percent conversion. The complexes
3–5 having bulky substituents or groups attached to

R.K. Rath et al. / Polyhedron 20 (2001) 2735–2739
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Crystallographic data for the structural analysis have
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This work is supported by the Council of Scientific
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