Transfer hydrogenation of ketones and catalytic oxidation of alcohols with half-sandwich complexes of ruthenium(II) designed using benzene and tridentate (S, N, E) type ligands (E = S, Se, Te)

Article abstract of DOI:10.1021/om100807b

The complexes of composition fac-[(η⁶-C₆H ₆)Ru(L)][PF₆][X] (1-6; X = PF₆ or Cl), formed by reacting 2-MeSC₆H₄CH=NCH₂CH ₂E-C₆H₄-4-R (L1-L3) and 2-MeSC ₆H₄CH₂-NHCH₂CH₂E-C ₆H₄-4-R (L4-L6) (where E = S or Se, R = H; E = Te, R = OMe) with [{(η⁶-C₆H₆)RuCl(μ-Cl)} ₂] and NH₄PF₆, have been characterized by ¹H, ¹³C{¹H}, ⁷⁷Se{¹H}, and ¹²⁵Te{¹H} NMR spectroscopy and X-ray crystallography. The Ru-Se and Ru-Te bond lengths are in the ranges 2.4837(14)-2.4848(14) and 2.6234(6)-2.6333(7) A, respectively. Complexes 1-6 have been found to be efficient catalysts for catalytic oxidation of alcohols with N-methylmorpholine-N-oxide, tBuOOH, NaOCl, and NaIO₄ and transfer hydrogenation reaction of ketones with 2-propanol. The TON values are up to 9.9 × 10⁴ and 9.8 × 10⁴ for two catalytic processes, respectively. The oxidation probably involves the formation of intermediate species having Ru(IV)=O. Complexes 1-3 are as efficient as 4-6 for transfer hydrogenation of ketones. In transfer hydrogenation, the mechanism does not appear to be dependent on the availability of hydrogen on nitrogen and probably involves Ru-H bond formation. The catalytic efficiency for both processes follows the order Te > Se > S, which may be due to the presence of a MeO group on Te.

Full text of DOI:10.1021/om100807b

Organometallics 2010, 29, 6433–6442 6433
DOI: 10.1021/om100807b
Transfer Hydrogenation of Ketones and Catalytic Oxidation of Alcohols
with Half-Sandwich Complexes of Ruthenium(II) Designed Using
Benzene and Tridentate (S, N, E) Type Ligands (E = S, Se, Te)
Pradhumn Singh and Ajai K. Singh*
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India
Received August 18, 2010
The complexes of composition fac-[(η6-C6H6)Ru(L)][PF6][X] (1-6;X=PF6 or Cl), formed by reacting
2-MeSC6H4CHdNCH2CH2E-C6H4-4-R (L1-L3) and 2-MeSC6H4CH2-NHCH2CH2E-C6H4-4-R
(L4-L6) (where E = S or Se, R = H; E = Te, R = OMe) with [{(η6-C6H6)RuCl(μ-Cl)}2] and NH4PF6,
have been characterized by 1H, 13C{1H}, 77Se{1H}, and 125Te{1H} NMR spectroscopy and X-ray
crystallography. The Ru-Se and Ru-Te bond lengths are in the ranges 2.4837(14)-2.4848(14) and
˚
2.6234(6)-2.6333(7) A, respectively. Complexes 1-6 have been found to be efficient catalysts for catalytic
oxidation of alcohols with N-methylmorpholine-N-oxide, tBuOOH, NaOCl, and NaIO4 and transfer
hydrogenation reaction of ketones with 2-propanol. The TON values are up to 9.9 ꢀ 104 and 9.8 ꢀ 104 for
two catalytic processes, respectively. The oxidation probably involves the formation of intermediate
species having Ru(IV)dO. Complexes 1-3 are as efficient as 4-6 for transfer hydrogenation of ketones.
In transfer hydrogenation, the mechanism does not appear to be dependent on the availability of hydrogen
on nitrogen and probably involves Ru-H bond formation. The catalytic efficiency for both processes
follows the order Te > Se > S, which may be due to the presence of a MeO group on Te.
Introduction
ketones with η6-arene-ruthenium(II) complexes containing
bis(pyrazolyl)methane7 or bis(imidazolin-2-imine)8 ligands
has been found to be efficient. The complex is a 16-electron
species in the case of bis(imidazolin-2-imine). The ternary
system consisting of [{(η6-benzene)RuCl(μ-Cl)}2], N-tosyl-
ethylenediamine or ethanolamine, and KOH (1:1:2 molar ratio)
catalyzes the transfer hydrogenation of ketones9 including
the asymmetric one. This has been shown to occur via
a nonclassical metal-ligand bifunctional mechanism.10,11
Half-sandwich complexes of Ru(II) have been found to be
good catalysts for the oxidation of alcohols too. [η5-Cp*Ru-
(μ-Cl)3RuCl(PPh3)2] has been reported to be efficient for
catalytic oxidation of secondary alcohols with 2-butanone.12
Nonactivated secondary alcohols are oxidized to ketones by
MnO2 in the presence of a catalytic system consisting of
[{(η6-p-cymene)RuCl(μ-Cl)}2] and 2,6-di-tert-butylbenzo-
quinone.13 [{(η6-p-cymene)RuCl(μ-Cl)}2] with bis(diphenyl-
phosphino)butane catalyzes the oxidation of alcohols to
amides.14 Half-sandwich complexes of Ru have been used
Half-sandwich complexes of Ru(II) are of current interest
due to their diverse catalytic activities. The transfer hydro-
genation of ketones is one of them.1 The species of type [(η6-
arene)RuCl(N,N)]þ have been found suitable for transfer
hydrogenation of ketones even in aqueous medium.2 Ogo3
4
and Suss-Fink have reported transfer hydrogenation reac-
€
tions of ketones with sodium formate or formic acid, cata-
lyzed in aqueous solution by [(η6-arene)Ru(bipy)(OH2)]2þ
.
The base-free transfer hydrogenation of ketones is reported5
with [(η6-benzene/p-cymene)RuCl(N,N)][BPh4] ((N,N) =
2-hydroxyphenylbis(pyrazol-1-yl)methane or 2-hydroxy-
phenylbis(3,5-dimethylpyrazol-1-yl)methane). Ruthenium-
arene-sulfonated diamine complexes being soluble in water
catalyze transfer hydrogenation of R-aryl ketones in aque-
ous medium.6 The catalytic transfer hydrogenation of
*Corresponding author. E-mail: aksingh@chemistry.iitd.ac.in, ajai57@
hotmail.com.
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2010 American Chemical Society
Published on Web 11/16/2010
pubs.acs.org/Organometallics

6434 Organometallics, Vol. 29, No. 23, 2010
Singh and Singh
in a biomimetic coupled catalytic system for alcohol oxida-
tion.15 A variety of other catalytic reactions of half-sandwich
compounds of ruthenium are also known. Hydrogenation of
benzene using clusters having (η6-arene)Ru units16 was re-
cells by replacement of the coordinated O,O donor with a S,
O donor.32 The promising cytotoxic effects of water-soluble
dinuclear (η6-arene)Ru(II) complexes in human cancer cells
increase33 with an increase in spacer length between the
metal centers. The interaction of [(η6-p-cymene)RuCl2(pta)]
(pta = phosphatriazaadamantane), reported to be an effec-
tive anticancer and antimetastatic agent, with biological
nucleophiles, important with respect to its mechanism of
action, has been studied.34 The anticancer activity of organ-
ometallic (η6-arene)ruthenium(II) complexes coordinated to
maltol-derived ligands, against human tumor cell lines has
been noticed.35 The water-soluble (η6-arene)ruthenium(II)
complexes containing pyridinethiolato ligands show cyto-
toxicity toward ovarian cancer cells.36 In vitro studies have
revealed that (3,5,6-bicyclophosphite-R-d-glucofuranoside)-
(η6-p-cymene)dihalogenidoruthenium(II) is the most cyto-
toxic compound for human cancer cell lines.37 It was
therefore thought worthwhile to understand the chemistry of
half-sandwich Ru(II) compounds that have tridentate organo-
chalcogen ligands, as they are scantly studied and may be
efficient catalysts. The biological activity of such resulting
species may also be of interest due to the presence of Se or S
in the molecule. Thus ligands L1-L6 (Scheme 1) and their
complexes having a (η6-C6H6)Ru(II) unit have been synthe-
sized. They are explored for catalytic oxidation of alcohols with
N-methylmorpholine-N-oxide (NMO), tert-butylhydroper-
oxide (tBuOOH), sodium oxychloride (NaOCl), and sodium
periodate (NaIO4) and for transfer hydrogenation of ketones.
The results of these investigations constitute this paper.
€
ported by Suss-Fink et al. Arene-ruthenium-salicyloxazolines
complexes were found to be suitable as asymmetric cat-
alysts for Diels-Alder reactions.17 The compounds [(η6-
arene)RudCdCdCR2(L)(Cl)][PF6] (L = PCy3, PiPr3), are
reported as excellent catalyst precursors for ring-closing
olefin metathesis.18 The atom transfer radical polymeriza-
tion of methyl methacrylate has been catalyzed with [(η6-
p-cymene)RuCl2(PCy3)].19 [{(η6-p-Cymene)RuCl(μ-Cl)}2],
the pyrimidinium or benzimidazolium salt, and Cs2CO3
constitute a catalytic system that selectively promotes the
diarylation of 2-pyridylbenzene with aryl bromides.20 Acetate-
assisted C-H activation of 2-substituted pyridines with
[{(η6-p-cymene)RuCl(μ-Cl)}2] has been reported.21 A variety
of neutral ruthenium-carbene complexes, [RuCl2(carbene)-
(arene)], have been used in the catalytic synthesis of furans.22
Kharasch additions are catalyzed with [(η6-p-cymene)RuCl2
(PAr3)].23 The exceptional efficacy of [(η6-p-cymene)RuCl2-
(PR3)] complexes as a catalyst precursor for the ring-opening
metathesis polymerization of low-strain cyclo-olefins24 is
reported. Chiral cationic (η6-arene)(pyridylamino)ruthenium(II)
complexes act as enantioselective catalysts for Diels-Alder reac-
tions with good exo:endo selectivity.25
Several half-sandwich Ru(II) compounds show promising
anticancer activity.26-30 Consequently related chemistry has
also gotten attention. Thiolate ligand oxygenation is believed
to activate cytotoxic [(η6-arene)Ru(en)(SR)]þ complexes
toward DNA binding.31 (η6-Arene)Ru(II) complexes with
pyrone-derived ligands are rendered active against cancer
Experimental Section
Physical Measurement. A Perkin-Elmer 2400 Series II C, H, N
analyzer was used for elemental analysis. The 1H, 13C{1H},
77Se{1H}, and 125Te{1H}NMR spectra were recorded on a
Bruker Spectrospin DPX-300 NMR spectrometer at 300.13,
75.47, 57.24, and 94.69 MHz, respectively. IR spectra in the
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range 4000-400 cm-1 were recorded on a Nicolet Protege 460
2002, 41, 99.
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ꢀ
FT-IR spectrometer as KBr pellets. The UV-vis spectra were
recorded on a Lambda Bio-20 (Perkin-Elmer, USA; model 330).
The conductivity measurements were carried out in CH3CN
(concentration ca. 1 mM) using an ORION conductivity meter,
model 162. Single-crystal diffraction studies were carried out
with a Bruker AXS SMART Apex CCD diffractometer using
Mo KR (0.71073 A) radiation at 298(2) K. The software
SADABS was used for absorption correction (if needed) and
SHELXTL for space group, structure determination, and refine-
ments.38,39 All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were included in idealized positions with isotropic
thermal parameters set at 1.2 times that of the carbon atom to which
they are attached. The least-squares refinement cycles on F2 were
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Article
Organometallics, Vol. 29, No. 23, 2010 6435
Scheme 1. Synthesis of L1-L6 and Their Ru(II) Complexes, 1-6
performed until the converged model. The highly distorted sol-
vent molecule in the crystal of complex 4 was omitted using the
SQUEEZE algorithm. The resulting new data set after this omission
was generated, and the structure was refined to convergence. The
molecular structures of complexes were drawn using ORTEP-3 for
Windows, version 2.02.40 The catalytic conversions were determined
with a NUCON Engineers (New Delhi, India) gas chromato-
graph (with FID detecter), model 5765, equipped with an Alltech
(EcTM-1) column of 30 m length, 0.25 mm diameter and having a
liquid film of 0.25 μm thickness. The cyclic voltammetric studies
were performed on a BAS CV 50 W instrument at University of
Delhi (Department of Chemistry), India. A three-electrode config-
uration composed of a Pt disk working electrode (3.1 mm2 area), a
Pt wire counter electrode, and an Ag/AgCl reference electrode was
used for the measurements. Ferrocene was used as an internal
standard (E1/2 =0.500 V vs Ag/AgCl), and all the potentials are
expressed with reference to Ag/AgCl. Melting points determined in
an open capillary are reported as such.
The solvents were dried and distilled before use by standard
procedures.46
General Procedure for Synthesis of Ligands L1, L2, and L3.
2-Methylthiobenzeldehyde (0.76 g, 5 mmol) dissolved in 15 cm3
of dry CH3OH stirred at room temperature for 0.5 h was mixed
with a solution of 2-(phenylsulfanyl)ethylamine (0.77 g, 5 mmol),
2-(phenylseleno)ethylamine (1.00 g, 5 mmol), or 2-(4-methoxy-
phenyltelluro)ethylamine (1.40 g, 5 mmol) (respectively for L1, L2,
and L3) made in 10 cm3 of dry CH3OH with constant stirring. The
mixture was further stirred at room temperature for 24 h. The
solvent was evaporated on a rotary evaporator to obtain ligands
L1, L2, or L3 as an oil.
L1: Yellow oil. Yield: 1.26 g, 90%. 1H NMR (CDCl3, 25 °C vs
TMS; δ, ppm): 2.47 (s, 3H, SCH3), 3.29 (t, 3J = 6.9 Hz, 2H, H5),
3.88 (t, 3J = 6.9 Hz, 2H, H6), 7.18-7.42 (m, 8H, H1-3, H11-13),
7.81 (d, 3J = 7.8 Hz, 1H, H10), 8.72 (s, 1H, H7). 13C{1H} NMR
(CDCl3, 25 °C vs TMS; δ, ppm): 16.8 (SCH3), 34.5 (C5),
60.6 (C6), 125.5-134.0 (C1-4, C10-13), 136.0 (C8), 139.3 (C9),
160.6 (C7). IR (KBr, cm-1): 3046 (m; νC-H (aromatic)), 2929
(s; νC-H (aliphatic)), 1642, 1586 (s; νCdN), 1232 (m; νC-N), 751
(m; νC-H (aromatic)).
Chemicals and Reagents. [(η6-C6H6)RuCl(μ-Cl)]2, 2-(phenyl-
sulfanyl)ethylamine, 2-(phenylseleno)ethylamine, 2-(4-methoxy-
phenyltelluro)ethylamine, and bis(4-methoxyphenyl) ditelluride
were synthesized by reported methods.41-45 Thiophenol, diphenyl-
diselenide, 2-chloroethylamine hydrochloride, 2-methylthiobenzal-
dehyde, NH4PF6, KOH, and all the alcohols and ketones were
procured from Sigma-Aldrich (USA) and used as received.
L2: Yellow oil. Yield: 1.48 g, 90%. 1H NMR (CDCl3, 25 °C vs
TMS; δ, ppm): 2.46 (s, 3H, SCH3), 3.25 (t, 3J = 6.9 Hz, 2H, H5),
3.95 (t, 3J = 6.9 Hz, 2H, H6), 7.16-7.39 (m, 6H, H1-2, H11-13),
7.52-7.56 (m, 2H, H3), 7.80 (d, 3J = 7.8 Hz, 1H, H10), 8.72 (s,
1H, H7). 13C{1H} NMR (CDCl3, 25 °C vs TMS; δ, ppm): 16.9
(SCH3), 28.5 (C5), 61.5 (C6), 125.5-132.8 (C1-4, C10-13), 134.1
(C8), 139.3 (C9), 160.3 (C7). 77Se{1H} NMR (CDCl3, 25 °C vs
Me2Se; δ, ppm): 278.3. IR (KBr, cm-1): 3059 (m; νC-H (aromatic)),
2921 (s; νC-H (aliphatic)), 1637, 1585 (s; νCdN), 1197 (m; νC-N),
746 (m; νC-H (aromatic)).
(40) ORTEP-3 for Windows. Farrugia, L. J. J. Appl. Crystallogr.
1997, 30, 565.
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2001, 66, 5590.
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1995, 494, 199.
L3: Red oil. Yield: 1.86 g, 90%. 1H NMR (CDCl3, 25 °C vs
TMS; δ, ppm): 2.47 (s, 3H, SCH3), 3.15 (t, 3J = 7.2 Hz, 2H, H5),
3.79 (s, 3H, OCH3), 4.04 (t, 3J = 7.2 Hz, 2H, H6), 6.75 (d, 3J =
8.7 Hz, 2H, H2), 7.19-7.37 (m, 3H, H11-13), 7.71 (d, 3J =
8.7 Hz, 2H, H3), 7.79 (d, 3J = 7.8 Hz, 1H, H10), 8.71 (s, 1H, H7).
13C{1H} NMR (CDCl3, 25 °C vs TMS; δ, ppm): 10.2 (C5),
16.9 (SCH3), 55.1 (OCH3), 62.8 (C6), 100.6(C4), 115.1 (C2),
125.5-130.7 (C10-13), 134.1 (C8), 139.3 (C9), 141.0 (C3), 159.5
(44) Singh, A. K.; Srivastava, V. Phosphorus Sulfur Silicon 1990, 47,
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Longman Group U K Ltd., 1989.

6436 Organometallics, Vol. 29, No. 23, 2010
Singh and Singh
(C1), 159.7 (C7). 125Te{1H} NMR (CDCl3, 25 °C vs Me2Te; δ,
ppm): 438.3. IR (KBr, cm-1): 3052 (m; νC-H (aromatic)), 2930
(s; νC-H (aliphatic)), 1639, 1588 (s; νCdN), 1209 (m; νC-N), 747
(m; νC-H (aromatic)).
173.8 (C7). IR (KBr, cm-1): 3068 (m; νC-H (aromatic)), 2922 (s;
ν
C-H (aliphatic)), 1615 (s; νCdN), 1136 (m; νC-N), 842 (s; νP-F), 745
(m; νC-H (aromatic)).
[3 CH3CN]: Yield: 0.15 g, 85%. Anal. Calcd for C23H25-
3
General Procedure for Synthesis of Ligands L4, L5, and L6.
They were synthesized from L1, L2, and L3, respectively. L1
(0.58 g, 2 mmol), L2 (0.67 g, 2 mmol), or L3 (0.82 g, 2 mmol) was
dissolved in 20 cm3 of dry ethanol, and the solution cooled in an
ice bath for 0.5 h. NaBH4 (0.18 g, 5 mmol) was added to it in
small lots with stirring within 15 min. The reaction mixture was
further stirred at room temperature for 10 h. On a rotary
evaporator solvent was removed, resulting in a semisolid, which
was redissolved in diethyl ether (30 cm3) and stirred for 10 min.
Distilled water (20 cm3) was added, and the mixture further
stirred until both phases became clear. The diethyl ether layer
was separated, washed with distilled water (2 ꢀ 50 cm3), and
dried over anhydrous sodium sulfate. Its solvent was evaporated
off on a rotary evaporator to obtain ligands L4, L5, and L6 as
an oil.
NORuSTe [PF6]2: C, 31.32; H, 2.86; N, 1.59. Found: C, 31.36;
3
H, 2.89; N, 1.58. Mp: 256 °C.Mol. cond. (ΛM):244.5Scm2 mol-1
.
1H NMR (CD3CN, 25 °C vs TMS; δ, ppm): 3.15 (s, 3H, SCH3),
3.31-3.43 (m, 2H, H5), 3.93 (s, 3H, OCH3), 4.27-4.38 (m, 2H,
H6), 5.67 (s, 6H, Ru-ArH), 6.98 (d, 3J = 8.7 Hz, 2H, H2),
7.21-7.39 (m, 3H, H11-13), 7.84 (d, 3J = 8.7 Hz, 2H, H3), 8.67
3
(d, J = 7.5 Hz, 1H, H10), 8.74 (s, 1H, H7). 13C{1H} NMR
(CD3CN, 25 °C vs TMS; δ, ppm): 16.4 (C5), 22.4 (SCH3), 56.2
(OCH3), 75.5 (C6), 77.2 (Ru-ArC), 116.4 (C4), 117.8 (C2),
129.2-133.1 (C10-13), 135.8 (C8), 140.3 (C9), 162.0 (C3), 163.2
(C1), 173.4 (C7). 125Te{1H} NMR (CD3CN, 25 °C vs Me2Te;
δ, ppm): 692.0. IR (KBr, cm-1): 3048 (m; νC-H (aromatic)), 2926
(s; νC-H (aliphatic)), 1633, 1585 (s; νCdN), 1209 (m; νC-N), 842
(s; νP-F), 746 (m; νC-H (aromatic)).
[4 H2O]: Yield: 0.10 g, 80%. Anal. Calcd for C22H25NRuS2
3
3
L4: Red oil. Yield: 0.51 g, 88%. 1H NMR (CDCl3, 25 °C vs
TMS; δ, ppm): 1.84 (s, 1H, NH), 2.47 (s, 3H, SCH3), 2.86 (t,
3J = 6.6 Hz, 2H, H5), 3.10 (t, 3J = 6.6 Hz, 2H, H6), 3.86 (s, 2H,
H7), 7.09-7.13 (s, 1H, H1), 7.14-7.35 (m, 8H, H2-3, H10-13).
13C{1H} NMR (CDCl3, 25 °C vs TMS; δ, ppm): 15.7 (SCH3),
34.2 (C5), 47.5 (C6), 51.3 (C7), 124.9-129.8 (C1-4, C11-13), 135.7
(C10), 137.4 (C8), 137.6 (C9). IR (KBr, cm-1): 3054 (m;
[PF6][Cl]: C, 40.71; H, 3.88; N, 2.16. Found: C, 40.72; H, 3.88; N,
.
2.14. Mp: 254 °C. Mol. cond. (ΛM): 255.1 S cm2 mol-1 1H
NMR (CD3CN, 25 °C vs TMS; δ, ppm): 2.19 (s, 1H, NH), 3.01
(s, 3H, SCH3), 3.75-4.22 (m, 2H, H5), 4.41-4.81 (m, 2H, H6),
5.29 (s, 2H, H7), 5.73 (s, 6H, Ru-ArH), 7.42-7.80 (m, 7H, H1-2,
H10-13), 7.85-7.93 (m, 2H, H3). 13C{1H} NMR (CD3CN, 25 °C
vs TMS; δ, ppm): 24.4 (SCH3), 38.9 (C5), 59.1(C6), 61.2 (C7), 87.0
(Ru-ArC), 128.3-132.5 (C1-4, C10-13), 139.2 (C8), 141.6 (C9). IR
(KBr, cm-1): 3052 (m; νC-H (aromatic)), 2925 (s; νC-H (aliphatic)),
1188 (m; νC-N), 844 (s; νP-F), 747 (m; νC-H (aromatic)).
ν
C-H (aromatic)), 2930 (s; νC-H (aliphatic)), 1191 (m; νC-N), 746
(m; νC-H (aromatic)).
L5: Red oil. Yield: 0.59 g, 88%. 1H NMR (CDCl3, 25 °C vs
TMS; δ, ppm): 1.90 (s, 1H, NH), 2.47 (s, 3H, SCH3), 2.89 (t,
3J = 6.6 Hz, 2H, H5), 3.07 (t, 3J = 6.6 Hz, 2H, H6), 3.85 (s, 2H,
H7), 7.08-7.13 (s, 1H, H1), 7.20-7.50 (m, 8H, H2-3, H10-13).
13C{1H} NMR (CDCl3, 25 °C vs TMS; δ, ppm): 15.7 (SCH3),
28.5 (C5), 48.2 (C6), 51.1 (C7), 124.8-129.6 (C1-4, C11-13), 132.9
(C10), 137.3 (C8), 137.6 (C9). 77Se{1H} NMR (CDCl3, 25 °C vs
Me2Se; δ, ppm): 265.0. IR (KBr, cm-1): 3052 (m; νC-H (aromatic)),
2927 (s; νC-H (aliphatic)), 1188 (m; νC-N), 744 (m; νC-H (aromatic)).
L6: Red oil. Yield: 0.74 g, 90%. 1H NMR (CDCl3, 25 °C vs
TMS; δ, ppm): 1.90 (s, 1H, NH), 2.47 (s, 3H, SCH3), 2.93-3.03
(m, 4H, H5-6), 3.79 (s, 3H, OCH3), 3.85 (s, 2H, H7), 6.72 (d, 3J =
8.7 Hz, 2H, H2), 7.10-7.28 (m, 4H, H10-13), 7.65 (d, 3J =
8.7 Hz, 2H, H3). 13C{1H} NMR (CDCl3, 25 °C vs TMS; δ, ppm):
10.5 (C5), 15.7 (SCH3), 49.6 (C6), 51.0 (C7), 55.1 (OCH3), 100.4
(C4), 115.1 (C2), 124.8-128.9 (C10-13), 137.3 (C8), 137.7 (C9),
141.0 (C3), 159.7 (C1). 125Te{1H} NMR (CDCl3, 25 °C vs Me2Te;
δ, ppm): 414.3. IR (KBr, cm-1): 3049 (m; νC-H (aromatic)), 2923 (s;
5: Yield: 0.14 g, 87%. Anal. Calcd for C22H25NRuSSe.[PF6]2:
C, 31.32; H, 2.86; N, 1.59. Found: C, 31.36; H, 2.89; N, 1.58. Mp:
252 °C. Mol. cond. (ΛM): 253.1 S cm2 mol-1 1H NMR
.
(CD3CN, 25 °C vs TMS; δ, ppm): 2.22 (s, 1H, NH), 3.12 (s,
3H, SCH3), 3.34-3.93 (m, 2H, H5), 4.26-4.51 (m, 2H, H6), 5.17
(s, 2H, H7), 5.72 (s, 6H, Ru-ArH), 7.30-7.68 (m, 7H, H1-2,
H10-13), 7.82-7.92 (m, 2H, H3). 13C{1H} NMR (CD3CN, 25 °C
vs TMS; δ, ppm): 26.0 (SCH3), 33.8 (C5), 57.2(C6), 61.5 (C7), 87.1
(Ru-ArC), 128.1-132.6 (C1-4, C10-13), 135.3 (C8), 136.5 (C9).
77Se{1H} NMR (CD3CN, 25 °C vs Me2Se; δ, ppm): 338.3. IR
(KBr, cm-1): 3054 (m; νC-H (aromatic)), 2923 (s; νC-H (aliphatic)),
1185 (m; νC-N), 842 (s; νP-F), 746 (m; νC-H (aromatic)).
[6 CH3CN]: Yield: 0.14 g, 87%. Anal. Calcd for C23H27-
3
NORuSTe [PF6]2: C, 31.25; H, 3.08; N, 1.58. Found: C, 31.26; H,
3
3.08; N, 1.61. Mp: 250 °C. Mol. cond. (ΛM): 245.5 S cm2 mol-1
.
1H NMR (CD3CN, 25 °C vs TMS; δ, ppm): 2.19 (s, 1H, NH), 3.03
(s, 3H, SCH3), 3.17-3.60 (m, 2H, H5), 3.86 (s, 3H, OCH3),
3.90-4.12 (m, 2H, H6), 5.10 (s, 2H, H7), 5.77 (s, 6H, Ru-ArH),
7.00 (d, 3J = 8.5 Hz, 2H, H2), 7.30-7.82 (m, 4H, H10-13), 7.99 (d,
3J = 8.5 Hz, 2H, H3). 13C{1H} NMR (CD3CN, 25 °C vs TMS; δ,
ppm): 16.2 (C5), 26.3 (SCH3), 56.2 (OCH3), 56.4 (C6), 59.6 (C7),
87.1 (Ru-ArC), 116.9 (C4), 117.5 (C2), 127.6-133.5 (C10-13), 136.5
(C8), 138.3 (C9), 162.6 (C3), 163.2 (C1). 125Te{1H} NMR (CD3CN,
25 °C vs Me2Te; δ, ppm): 672.6. IR (KBr, cm-1): 3047 (m;
νC-H (aliphatic)), 1184 (m; νC-N), 745 (m; νC-H (aromatic)).
General Method of Synthesis of Half-Sandwich Ru Complexes
1 and 3-6. The solid [(η6-C6H6)RuCl(μ-Cl)]2 (0.05 g, 0.1 mmol)
was added to the solution of L (0.2 mmol) (L = L1, L3 to L6)
made in CH3OH (15 cm3). The mixture was stirred for 1 h in the
case of Schiff bases and 15 h in the case of reduced Schiff bases at
room temperature. The resulting yellow solution was filtered,
and the volume of the filtrate was reduced (∼7 cm3) with a rotary
evaporator. It was mixed with solid NH4PF6 (0.032 g, 0.2
mmol), and the resulting yellow or orange microcrystalline solid
(1 and 3 to 6) was filtered, washed with 10 cm3 of cold (∼5 °C)
CH3OH, and dried in vacuo. Single crystals of the complex
(1 and 3 to 6) suitable for X-ray diffraction were obtained by
diffusion of diethyl ether into its solution (1 cm3) made in a
mixture of CH3OH and CH3CN (v/v, 1:4).
ν
ν
C-H (aromatic)), 2920 (s; νC-H (aliphatic)), 1180 (m; νC-N), 843 (s;
P-F), 746 (m; νC-H (aromatic)).
2. This was synthesized by a procedure reported earlier by
us.47 IR spectrum and 1H and 13C{1H} NMR spectra of 2
given in the Supporting Information are consistent with an
earlier report. 77Se{1H} NMR (CD3CN, 25 °C vs Me2Se; δ, ppm):
381.5.
Catalytic Oxidation of Alcohols with NMO. A typical reaction
carried out for catalytic oxidations of primary alcohols to
corresponding aldehydes and secondary ones to ketones with
N-methylmorpholine-N-oxide and complexes 1-6 is as follows.
A solution of a complex among 1-6 (0.001 mol %) in 20 cm3 of
CH2Cl2 was mixed with neat alcohol substrate (1 mmol) and
solid NMO (3 mmol). The mixture was refluxed for 2 h with
1: Yield: 0.12 g, 76%. Anal. Calcd for C22H23NRuS2 [PF6]2:
3
C, 34.93; H, 3.06; N, 1.85. Found: C, 34.96; H, 3.03; N, 1.87. Mp:
252 °C. Mol. cond. (ΛM): 251.9 S cm2 mol-1. 1H NMR (CD3CN,
25 °C vs TMS; δ, ppm): 3.22 (s, 3H, SCH3), 3.57-4.23 (m,
2H, H5), 4.36-4.77 (m, 2H, H6), 5.63 (s, 6H, Ru-ArH),
7.40-7.84 (m, 6H, H1-2, H11-13), 8.01 (m, 2H, H3), 8.47 (m,
1H, H10), 8.92 (m, 1H, H7). 13C{1H} NMR (CD3CN, 25 °C vs
TMS; δ, ppm): 22.1 (SCH3), 35.8 (C5), 72.8 (C6), 79.7 (Ru-ArC),
129.6-132.4 (C1-3, C10-13), 136.2 (C4), 136.6 (C8), 172.9 (C9),
(47) Singh, P.; Das, D.; Singh, M.; Singh, A. K. Inorg. Chem.
Commun. 2010, 13, 223.

Article
Organometallics, Vol. 29, No. 23, 2010 6437
catalyst 1-3 and 3 h with catalyst 4-6. Thereafter solvent was
evaporated off using a rotary evaporator. The residue having an
oxidized product was extracted with 20 cm3 of petroleum ether
(60-80 °C). The complex-catalyst undissolved in petroleum
ether was recovered almost quantitatively for the next catalytic
cycle. The oxidized product present in petroleum ether was
analyzed by GC.
Catalytic Oxidation of Alcohols with tBuOOH. A mixture of a
complex among 1-6 (0.001 mol %) dissolved in 20 cm3 of
CH2Cl2 and neat alcohol substrate (1 mmol) was made.
tBuOOH (4 mmol) was added to the mixture with a dropping
funnel over 0.5 h, and the resulting mixture was stirred for 2 h
with catalyst 1-3 and 3 h with catalyst 4-6 at room tempera-
ture. The solvent from the reaction mixture was mostly evapo-
rated off with a rotary evaporator, resulting in a semisolid,
containing the complex-catalyst and the oxidized alcohol. It
was extracted with 20 cm3 of petroleum ether (60-80 °C). The
extract containing oxidized product was analyzed by GC, and
the residue containing complex-catalyst in nearly quantitative
amount was preserved for the next catalytic cycle.
Figure 1. ORTEP diagram of the cation of 1 with 30% prob-
ability ellipsoids. Hydrogen atoms and PF6 are omitted for
clarity. Bond lengths (A): Ru(1)-S(1) 2.3921(19), Ru(1)-S(2)
2.3465(19), Ru(1)-N(1) 2.073(6), Ru(1)-C 2.197(7)-2.224(7).
Bond angles (deg): S(1)-Ru(1)-S(2) 91.56(7), N(1)-Ru(1)-
S(1) 79.72(16), N(1)-Ru(1)-S(2) 86.21(17).
˚
Catalytic Oxidation of Alcohols with NaOCl and NaIO4. The
solution of a ruthenium complex among 1-6 (0.001 mol %)
made in 10 cm3 of CH2Cl2 was added to a 5 cm3 solution of
NaHCO3-Na2CO3 (1.0 M, pH = 9.5) buffer. A few drops of
aqueous NaOCl/NaIO4 (0.7 M/1.0 M) were added at 0 °C, and
the mixture was stirred vigorously until the organic phase
became yellow or orange in color, leaving the aqueous phase
colorless. The solution of alcohol substrate (1 mmol) made in
5 cm3 of CH2Cl2 was added in one lot to the reaction mixture
with stirring. The aqueous solution of 0.7 M NaOCl (5.7 cm3)/
1.0 M NaIO4 (4 cm3) was added to the reaction mixture drop-
wise with a dropping funnel over a period of 1 h. The resulting
reaction mixture was stirred for 2 h with catalyst 1-3 and 3 h
with catalyst 4-6 at room temperature, maintaining its pH at
∼9.5 and shaken thereafter with 30 cm3 of CH2Cl2. The organic
layer was separated, and its solvent was mostly evaporated
off with a rotary evaporator, resulting in a semisolid, which
was extracted with petroleum ether (60-80 °C) (20 cm3). The
complex-catalyst left as a solid residue was recovered almost
quantitatively for other catalytic cycle. The resulting products
extracted into petroleum ether were analyzed by GC.
General Procedure for Catalytic Transfer Hydrogenation Re-
action. A solution of the ketone (1 mmol), KOH (0.2 cm3 of a
0.2 M solution in 2-propanol), and the catalyst complexes 1-6
(0.001 mol %) was heated under reflux (80 °C) in 10 cm3 of
2-propanol for 5 h with catalysts 1-3 and 7 h with catalysts 4-6.
2-Propanol was removed under vacuum, and an aliquot of the
remaining product was extracted with diethyl ether, filtered
through a short column of silica gel. The column was washed
with diethyl ether. The filtrate and washings of the column were
mixed and evaporated on a rotary evaporator and analyzed by
GC. The final conversions are reported as an average of two
runs of each catalytic reaction.
common organic solvents, while complexes showed good
solubility in CH3OH, CH3CN, CH2Cl2, and CHCl3. In diethyl
and petroleum ether complexes 1-6 were almost insoluble.
However, solutions of complexes in DMSO showed signs of
decomposition within a few hours.
NMR Spectra. The 1H and 13C{1H} NMR spectra of
L1-L6 and their complexes 1-6 were in agreement with
their molecular structures. The 77Se{1H} and 125Te{1H}
NMR spectra of ligands L2, L3, L5, and L6 and their
corresponding complexes 2, 3, 5, and 6 are given in the
Supporting Information (Figures S1 to S8). The signals in
the 77Se{1H} NMR spectra of 2 and 5 appear shifted to
higher frequencies by 103.2 and 73.3 ppm with respect to
those of free L2 and L5, respectively, as Se is coordinated to
the ruthenium center. Similarly in the 125Te{1H} NMR
spectra of 3 and 6 the signals appear at frequencies higher
by 253.7 and 258.3 ppm relative to those of free L3 and L6,
respectively, which are coordinated to ruthenium through
1
Te. In the H and 13C{1H} NMR spectra of 1-6 signals of
protons and carbon atoms generally appear at higher frequency
relative to those of free ligands which coordinate with ruthe-
nium in a tridentate mode. However, the magnitude of the shift
to higher frequency is high for C5 to C7 (up to 12.2 ppm in
13C{1H} NMR) and protons attached to them (up to 1.43 ppm
in 1H NMR). The signals of SCH3 in the 13C{1H} and 1HNMR
spectra of 1-6 also appear at higher frequency (up to 10.6 and
1
0.75 ppm, respectively). The signals of NH in the H NMR
spectra of 4-6 shift to higher frequencies (up to 1.35 ppm) with
respect to those of free L4-L6.
Crystal Structures. The crystal structures of 1-6 have been
solved. The results of 2 have been reported earlier,47 and
those of the remaining five complexes are described here. The
ORTEP diagrams of 1 and 3-6 are given in Figures 1 to 5
with selected bond distances and angles. In the cations of
these five complexes, there is a pseudo-octahedral half-
sandwich “piano-stool” disposition of donor atoms around
ruthenium. The benzene ring occupies one face of octahe-
dron, and a tridentae ligand among L1 and L3-L6 the
opposite one. In the cation of 1 the Ru-S(1) bond distance,
Results and Discussions
Scheme 1 summarizes the syntheses of L1-L6 and their
ruthenium(II) complexes 1-6. The ligands L1, L4, and L5
have been synthesized for the first time. L2 has been prepared
by a synthetic procedure reported earlier.47 Similarly L3 and
L6 have been synthesized in better yield than reported
earlier48 using longer reaction time (24 h) in the case of L3
and a different workup procedure for L6 (diethyl ether-
water). The molar conductance values in acetonitrile (see
Experimental Section) indicate a 1:2 electrolyte nature of all
complexes 1-6. The solubility of each ligand was good in
˚
2.3921(19) A, is greater than its Ru-S(2) distance,
˚
2.3465(19) A. The Ru-S(1) and Ru-S(2) bond lengths of
the cation of 4 are not much different. The Ru-S bond
lengths of cations of 1 and 3-6 are also within the range
˚
2.3548(15)-2.4156(9) A, in which the Ru-S bond distances
(48) Kumar, P. R.; Upreti, S.; Singh, A. K. Inorg. Chim. Acta 2008,
361, 1426.
of [(η6-C6H6)RuCl(N-{2-(phenylthio)ethyl}pyrrolidine)]þ,

6438 Organometallics, Vol. 29, No. 23, 2010
Singh and Singh
Figure 2. ORTEP diagram of cation of 3 with 30% probability
ellipsoids. Hydrogen atoms, CH3CN, and PF6 are omitted for
clarity. Bond lengths (A): Ru(1)-Te(1) 2.6234(6), Ru(1)-S(1)
2.3484(15), Ru(1)-N(1) 2.077(4), Ru(1)-C 2.183(8)-2.205(7).
Bond angles (deg): Te(1)-Ru(1)-S(1) 91.01(4), N(1)-Ru(1)-
Te(1) 80.72(12), N(1)-Ru(1)-S(1) 86.74(12).
Figure 4. ORTEP diagram of cation of 5 with 50% probability
ellipsoids. Hydrogen atoms (except NH) and PF6 are omitted
for clarity. Bond lengths (A): Ru(1)-Se(1) 2.4848(8), Ru(1)-
S(1) 2.3742(17), Ru(1)-N(1) 2.169(5), Ru(1)-C 2.217(7)-
2.233(7). Bond angles (deg): Se(1)-Ru(1)-S(1) 91.37(5), N(1)-
Ru(1)-Se(1) 84.05(15), N(1)-Ru(1)-S(1) 85.25(16).
˚
˚
Figure 3. ORTEP diagram of cation of 4 with 30% probability
ellipsoids. Hydrogen atoms (except NH), H2O, Cl, and PF6 are
˚
omitted for clarity. Bond lengths (A): Ru(1)-S(1) 2.374(3),
Figure 5. ORTEP diagram of cation of 6 with 30% probability
ellipsoids. Hydrogen atoms (except NH), CH3CN, and PF6 are
omitted for clarity. Bond lengths (A): Ru(1)-Te(1) 2.6333(7),
Ru(1)-S(1) 2.3853(17), Ru(1)-N(1) 2.175(5), Ru(1)-C 2.204(6)-
2.226(7). Bond angles (deg): Te(1)-Ru(1)-S(1) 90.0(2), N(1)-
Ru(1)-Te(1) 84.41(14), N(1)-Ru(1)-S(1) 85.30(14).
Ru(1)-S(2) 2.379(2), Ru(1)-N(1) 2.135(5), Ru(1)-C 2.146(15)-
2.218(9). Bond angles (deg): S(1)-Ru(1)-S(2) 90.75(10), N(1)-
Ru(1)-S(1) 83.0(2), N(1)-Ru(1)-S(2) 86.51(18).
˚
[(η6-C6H6)RuCl(N-{2-(phenylthio)ethyl}morpholine)]þ, [Cp*-
Ru(PMe3)2(SC6F4H)], and [Cp*Ru(NO)(SC6F4H)2] have been
reported.49-51 The Ru-Se bond length of the cation of 5
The Ru-Se bond distances of [(η5-C5Me5)Ru(μ2-SeR)3(η5-
˚
C5Me5)Ru]Cl (R=tolyl) (2.446(4)-2.466(4) A)54 are shorter
(2.4848(14) A) is consistent with the Ru-Se bond length of
˚
the cation of 2 (2.4837(14) A)47 and the value reported for half-
˚
than those of cations of 2 and 5, because RSe- is expected to be
bonded strongly in comparison to selenated Schiff bases and
their reduced analogues. In [Ru(MeCp)(PPh3)]2(μ-Se2)2(Otf)2,
sandwich complexes of Ru(II) with N-{2-(phenylseleno)ethyl}-
49
˚
pyrrolidine (2.4770(5)-2.480(11) A). TheRu-Se bond lengths
˚
55
of 2 and 5 fall within the range 2.4756(10)-2.5240(9) A, reported
˚
Ru-Se bond distances are 2.518(1) and 2.556(1) A, some-
what longer than those of cations of 2 and 5.
for such bond lengths in clusters [Ru3(μ3-Se)(CO)7(μ3-CO)-
(μ-dppm)] and [Ru3(μ3-Se)(μ3-S)(CO)7(μ-dppm)].52 For the
Ru(IV) complex [RuCp*{η2-Se2P(iPr)2{η2-SeP(iPr)2}][PF6] the
Ru-Se bond lengths51 are reported in the range 2.538(2)-
˚
The Ru-Te bond length (2.6234(6) A) of the cation of 3 is
shorter than that of the cation of 6 (2.6333(7) A). Both the
˚
values are longer than earlier reported50 values, 2.6143(7) A
˚
˚
for [(η6-C6H6)Ru(II)Cl(N-{2-(4-methoxyphenyltelluro)-
2.590(2) A, longer than those of cations of 2 and 5 due to steric
6
ethyl}morpholine)]þ and 2.619(8) A for [(η -p-cymene)RuCl2-
crowding. The Ru-Se bond distance in [RuCp(CO)(CtCPh)-
˚
(μ-Se)ZrCp2], 2.494(1) A,53 is closer to that of cations of 2 and 5.
˚
(2-(4-ethoxyphenyltelluromethyl)tetrahydro-2H-pyran)],56 but
6
˚
consistent with the value 2.6371(4) A reported for [(η -p-
cymene)RuCl(H2NCH2CH2TeC6H4-4-OMe)Cl H2O].57 The
(49) Singh, P.; Singh, M.; Singh, A. K. J. Organomet. Chem. 2009,
694, 3872.
(50) Singh, P.; Singh, A. K. Eur. J. Inorg. Chem. 2010, 4187.
(51) Zhang, Q.-F.; Cheung, F. K. M.; Wong, W;-Y.; Williams, I. D.;
Leung, W.-H. Organometallics 2001, 20, 3777.
(52) Ahmed, S. J.; Hyder, M. I.; Kabir, S. E.; Miah, M. A.; Deeming,
A. J.; Nordlander, E. J. Organomet. Chem. 2006, 691, 309.
(53) Sunada, Y.; Hayashi, Y.; Kawaguchi, H.; Tatsumi, K. Inorg.
Chem. 2001, 40, 7072.
3
present Ru-Te distances are shorter than the reported values
(55) Amarasekera, J.; Houser, E. J.; Rauchfuss, T. B.; Stern, C. L.
Inorg. Chem. 1992, 31, 1614.
(56) Singh, A. K.; Kadarkaraisamy, M.; Misra, M.; Sooriyakumar,
J.; Drake, J. E.; Hursthouse, M. B.; Light, M. E.; Jasinski, J. P. Inorg.
Chim. Acta 2001, 320, 133.
(54) Matsuzaka, H.; Ogino, T.; Nishio, M.; Hidai, M.; Nishibayashi,
(57) Kumar, P. R.; Singh, A. K.; Butcher, R. J.; Sharma, P.; Toscano,
Y.; Uemura, S. J. Chem. Soc., Chem. Commun. 1994, 2, 223.
R. A. Eur. J. Inorg. Chem. 2004, 1107.

Article
Organometallics, Vol. 29, No. 23, 2010 6439
Scheme 2. Catalytic Oxidation of Alcohols
pH 9-10 because of the earlier reports71,72 that suggest that
in this pH range conversions are optimum. A series of blank
experiments were carried out under identical conditions
(see Experimental Section), which suggest that neither
ruthenium(II) complexes nor oxidants (NMO, tBuOOH,
NaOCl, or NaIO4) alone result in catalytic oxidation to any
significant level.
The cyclic voltammetric experiments performed at 298 K
in CH3CN (0.01 M nBu4ClO4 as supporting electrolyte) on
1-6 at a scan rate of 100 mV s-1 (anodic sweep) reveal a one-
electron irreversible oxidation with E1/2 values between 0.557
and 0.689 V (vs Ag/AgCl) (see Table S4 and Figures S9 to
S14 in the Supporting Information for details), which are not
extreme, implying that both species Ru(II) and Ru(III) are
equally stabilized by the same set of ligands. The E1/2 in such
a range has been reported to be favorable for a catalytic
oxidation process earlier;73,74 of course no one-to-one rela-
tionship has been unequivocally established. The TON values
are given in Table 1 for oxidation (catalyzed by 1-6) of
various alcohols by four oxidants, viz., NMO, tert-butyl
hydroperoxide (tBuOOH), sodium oxychloride (NaOCl),
and sodium periodate (NaIO4). 3 appears to be the most
efficient among all six Ru species with all four oxidants. The
ruthenium complexes of chalcogenated Schiff base ligands
show better catalytic efficiency than their reduced counter-
parts (Table 1). The azomethine group, >CdN, is a better
donor than NH, which in turn facilitates the formation of
Ru(IV)dO containing intermediate species, which is believed
to carry out oxidation of substrates.The catalytic efficiency
varies with chalcogen ligands in the order of Te > Se > S,
which is also the order of “softness” of these donor sites.
However, further increase in softness of Te due to a strong
electron-donating methoxy group cannot be ignored and may
be responsible for generating this order. The softer ligand
makes easier the formation of Ru(IV)dO species, which are
believed to be intermediates in the oxidation process on the basis
of several observations mentioned below. Undoubtedly deter-
mining the mechanism of these oxidative transformations un-
ambiguously is not straightforward, as intermediate species are
of low stability. On the basis of earlier work71,72,75-82 and some
observations made by us, the mechanism of catalytic oxidation
appears to involve Ru(IV)dO species, which appear to be
formed by free radicals (generated by heterolytic cleavage of
oxidants). The addition of AIBN [a free radical initiator, azo-
bis(isobutyronitrile)] to the benzyl alcohol oxidation in the pres-
ence 3 (most efficient catalyst) with any one of the four oxidants
6
˚
of 2.6528(9) A for [(η -p-cymene)RuCl2bis{2-(2-thienyl)ethyl}
telluride],58 2.651(5) A for [(η -p-cymene)RuCl2{bis(1,3-dioxan-
6
˚
2-yl)}ethyl telluride}],59 and 2.6559(9) A for [(η -p-cymene)Ru-
6
ꢀ
˚
Cl2{N-[2-(4-methoxyphenyltelluro)ethyl]phthalimide}],60 in
which hybrid organotellurium ligands bind in a monodentate
mode via Te with (η6-p-cymene)Ru(II).
˚
The Ru-N bond lengths (sum of covalent radii ca. 1.95 A)
of cations of 1 and 3-6 are between 2.073(6) and 2.175(5) A.
They are shorter than the values of the half-sandwich com-
plex of Ru(II) with N-{2-(phenylseleno)ethyl}pyrrolidine
49
(2.190(3)-2.201(5) A). The Ru-N bond distances of 5
and 6 are longer in comparison to the values reported
˚
˚
48,52
˚
recently (2.0511(17)-2.163(10) A).
In half-sandwich
complexes of (η6-p-cymene)RuCl61-65 with several nitrogen
ligands, Ru-N bond lengths have been reported generally
between 2.060(5) and 2.156(2) A, which are less than the
present values. The Ru-C bond lengths (2.146(15)-2.233(7)
˚
ꢀ
˚
A) and C-Ru-C bond angles of cations of 1 and 3-6 are in
the normal range.47-49,52,56,58-60,66-70 The solvent mole-
cules CH3CN and MeOH respectively in the crystal struc-
tures of 3 and 4 were found distorted. In the case of 4, using
the SQUEEZE algorithm, the solvent molecule (MeOH) is
removed. In the case of 3, due to disorder, the false impres-
sion of two methyl groups appears in the structure. However
in the program ORTEP-3 for Windows, version 2.02,
CH3CN appears as a linear molecule (Supporting Informa-
tion Figure S15).
Catalytic Applications. Complexes 1-6 have been used
for catalytic oxidation of primary and secondary alcohols.
Maximum conversions were reached in 2 h with catalysts
1-3 and in 3 h with catalysts 4-6 (Scheme 2). The oxidation
of alcohols with NaOCl and NaIO4 has been carried out at
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~
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1991, 21.

6440 Organometallics, Vol. 29, No. 23, 2010
Singh and Singh
Table 1. Catalytic Oxidation of Primary and Secondary Alcohols Using Complexes 1-6 As Catalysta
a Reaction time 2 h for 1-3 and 3 h for 4-6. Oxidant: a = NMO; b = tBuOOH; c = NaIO4; d = NaOCl.
Scheme 3. Catalytic Transfer Hydrogenation Reaction of
Ketone
under the same reaction conditions resulted in enhanced con-
version (85-96%). Further addition of AIBN in the same
oxidation reaction in the absence of 3 did not result in any
oxidation. In the presence of benzoquinone [a free radical
inhibitor] the conversion was in trace amounts. These observa-
tions are consistent with those made by Drago and co-workers in
h
the oxidation of alkanes82 with H2O2 and OCl. On adding NMO,
tBuOOH, NaIO4, or NaOCl to the solutions of complexes 2 and
5, the signals in their 77Se{1H} NMR spectra shift to higher
frequencies by g485 ppm. The signals in the 125Te{1H} NMR
spectra of 3 and 6 shift to higher frequencies by g305 ppm in the
presence of any of the present four oxidants. The signals in the
77Se{1H}NMR spectra of L2 and L5 and in the 125Te{1H}NMR
spectra of L3 and L6 remain unshifted on addition of any of the
four oxidants. Therefore, ruthenium is most probably oxidized
to Ru(IV)dO. On addition of any one of these oxidants to a
dichloromethane solution of 1-6, a new shoulder at 390-395
nm appears in their UV-visible spectrum, which is believed79-84
to be due to Ru(IV)dO, species reported to be responsible for
transfer of the oxygen to alcohol substrates, resulting in their
catalytic oxidation. The solvent from a mixture of any of these
four oxidants with a complex among 1-6 was evaporated off,
and the IR spectra of the remaining residue was found to exhibit
a very strong band at 845-848 cm-1 (νP-F at 842-844 cm-1 is
of medium intensity only), which further supports the formation
of Ru(IV)dO species75,80,83-86 responsible for catalytic oxida-
tion of alcohols. The 18O-labeling experiments further supported
the formation of Ru(IV)dO species. tBuOOH (70% aqueous
solution) was mixed with excess H218O (∼95%) and kept for 5
days. Thereafter the mixture was used as an oxidant for alcohols
(Table 1) using complexes 2, 3, 5, and6as catalysts. IR spectra of
the intermediate isolated by the procedure mentioned earlier
were found to exhibit an additional band at 820-826 cm-1 due
to the formation87 of Ru(IV)d18O. It is red-shifted 22-25 cm-1
with respect to that of unlabeled Ru(IV)dO species. Moreover,
the catalytic activities of Ru(IV)dO species for oxidation re-
ported earlier78-82,88 further strengthens our proposition. The
short lifetime of Ru(IV)dO transient species restricted us from
1
acquiring evidence of mass spectra. The H NMR spectra of
ruthenium complexes recorded after adding oxidants become
broad, indicating the formation of paramagnetic species as
intermediates. On the basis of these observations and literature
knowledge a detailed mechanism is difficult to assign, but a gross
one is mentioned in the Supporting Information (Figure S16).
The advantages of 1-6 in comparison to recently reported good
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17.

Article
Organometallics, Vol. 29, No. 23, 2010 6441
Table 2. Catalytic Hydrogen Transfer Reaction of Ketones Using Complexes 1-6 As Catalysta
a Reaction time 5 h for 1-3 and 7 h for 4-6.
Ru-based catalytic species77,80,83,89-93 for oxidation of alcohols
are (i) high efficiency/yield, as they are needed in less quantity,
(ii) short reaction time, and (iii) flexibility regarding oxidant.
The half-sandwich Ru(II) complexes containing chalcogenated
pyrrolidine, morpholine, and benzotriazole known to catalyze
the oxidation of alcohols efficiently49,50,94 (TON up to 9.8 ꢀ 104)
are comparable with the present complexes (TON up to
9.9 ꢀ 104).
high efficiency was exhibited in the reduction of ketones to
their corresponding alcohols with 2-propanol as hydrogen
donor in the presence of KOH, which is reported to be the
best inorganic base for such reactions.96
The catalytic efficiency varies with chalcogen ligands in
the order of Te > Se > S, which may be tailored by a strong
electron-donating methoxy group present on tellurium. The
complex catalysts 3 and 6 are the most efficient catalyst
among all. The complexes of Schiff bases and their reduced
analogues do not differ significantly in catalytic efficiency.
This implies that a nonclassical metal-ligand bifunctional
mechanism10,11 for which hydrogen on nitrogen is essential is
not operative in our case. On monitoring the transfer hydro-
genation reactions catalyzed with 2, 5, 3, and 6 by 77Se{1H}
and 125Te{1H} NMR spectroscopy, it is observed that the
signals in the spectra shift to higher frequency (20-25 ppm),
indicating that probably the Ru-S(methyl) bond is cleaved97
or weakened very significantly to make a coordination site
on ruthenium available so that formation of an intermediate
having a Ru-H bond takes place. The transfer hydrogena-
Transfer hydrogenation reaction in which hydrogen is
transferred from one organic molecule to another is of great
importance in organic synthesis since one can avoid the use
of molecular hydrogen.95 Transfer hydrogenation reactions
of ketones (Scheme 3) were explored at 80 °C using catalysts
1-6 (0.001 mol %). The most efficient conversions are found
in the case of acetophenone with all catalysts 1-6 (up to
98%), while in the case of aliphatic secondary ketones the
conversions were up to 90%. The details of percent conver-
sions and TONs are given in Table 2. The products were
identified by GC after recovering the catalyst and doing
required workup. The conversions reported are averages of
two runs in the case of all catalytic reactions (Table 2). The
1
tion reaction catalyzed with 3 and 6 was monitored by H
NMR. After 1 h a broad singlet was noticed around -10 to
-11 ppm. These signals are characteristic of hydrides and
indicate the formation of Ru-H.98 As Ru complexes of
Schiff bases and their reduced derivatives are equally efficient
as catalysts, the transfer of hydrogen to ketone via a clas-
sical mechanism appears to be most plausible (For details
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6442 Organometallics, Vol. 29, No. 23, 2010
Singh and Singh
see Supporting Information Figure S17). It is interesting
to compare recently reported transfer hydrogenation reac-
tions of ketones catalyzed with ruthenium species with
those of 1-6. Ru(II) complexes of 2-(aminomethyl)pyridine-
phosphine99 were found among the most efficient catalysts for
transfer hydrogenation (TOF up to 105 h-1 and conversion up
to 97%) in 2-propanol using NaOH as base. The percent
conversion values for 1-6 are also in the same order. The yield
of catalytic transfer hydrogenation of ketones with [(η6-
arene)RuCl(N,N)]BF4 (arene = benzene, p-cymene; N,N =
bis(pyrazolyl)methane)7 has been reported to be up to 97%,
but the amount of catalyst used in the presence of refluxing
2-propanol and KOH is almost double in comparison to those
of 1-6. The Ru(II) complexes of (N,N,N) unsymmetrical
pincer ligand 2-(benzoimidazol-2-yl)-6-(pyrazol-1-yl)-
pyridine100 are needed in 0.05 mol % (50 times more than
the amount of 1-6 used) in refluxing 2-propanol to obtain
conversions on the order similar to those of 1-6. The
NHC-carbene-based101 (η6-arene)Ru(II) complexes have
been found to reduce ketones up to 97% in refluxing
2-propanol but only when the amount of catalyst used is
100 times more than the required amounts of 1-6. Recently
the [(η6-arene)Ru(L)Cl][PF6] complex (L = dipyridyl-
amine) was reported2 to catalyze the transfer hydrogena-
tion reaction of ketones in the presence of water, but a high
amount of catalyst (5 mol %) is required, resulting in a TON
of only up to 100. These comparisons indicate the promising
performance of 1-6 as transfer hydrogenation catalysts. In
comparison to Ru(II) complexes of bidentate ligands,2,7
complexes 1-6 of Ru(II) containing tridentate ligands are
more efficient catalysts for the transfer hydrogenation reac-
tion of ketones, as the amount of catalyst used and reaction
time are lower.
(L1-L3) and 2-MeSC6H4CH2-NHCH2CH2E-C6H4-4-R
(L4-L6) (where E = S or Se, R = H; E = Te, R = OMe),
synthesized and structurally characterized by single-crystal
X-ray diffraction, are very efficient catalysts for the oxida-
tion of primary and secondary alcohols (TON up to 9.9 ꢀ 104)
and transfer hydrogenation reactions of ketones (TON up
to 9.8 ꢀ 104). The oxidation appears to occur through
Ru(IV)dO-containing species and hydrogen transfer through a
classical mechanism involving Ru-H formation. The short
reaction time and flexibility regarding the oxidant are among
the advantages of 1-6 as oxidation catalysts. The catalytic
efficiency appears to vary with chalcogen ligands in the order
of Te > Se > S. However, tellurium ligands have a strong
electron-donating methoxy group, which may also be respon-
sible for tailoring this order.
Acknowledgment. The authors thank the Council of
Scientific and Industrial Research (CSIR) New Delhi
(India) for project no. 01(2113)07/EMR-II, the Depart-
ment of Science and Technology (India) for project
no. SR/S1/IC-23/2006, and partial financial assistance
given to establish the single-crystal X-ray diffraction
facility at IIT Delhi, New Delhi (India), under its FIST
program. P.S. thanks the University Grants Commis-
sion (India) for the award of a Junior/Senior Research
Fellowship.
Supporting Information Available: Crystal data and refine-
ments table and detailed bond lengths and angles for complexes
1 and 3-6. 77Se{1H} NMR spectra of ligands L2 and L5 and
complexes 2 and 5. 125Te{1H} NMR spectra of ligands L3 and
L6 and complexes 3 and 6. 1H and 77Se{1H} NMR spectra of 2.
Cyclic voltammetric values table and figures of 1-6. ORTEP
diagram of complex 3 with solvated CH3CN. Proposed mecha-
nism for oxidation and transfer hydrogenation reactions. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Conclusion
The complexes fac-[(η6-C6H6)Ru(L)][PF6]X (1-6; X =
PF6 or Cl) of 2-MeSC6H4CHdN-CH2CH2E-C6H4-4-R
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