Ruthenium(II) complexes of hybrid 8-hydroxyquinoline-thiosemicarbazone ligands: Synthesis, characterization and catalytic applications

Article abstract of DOI:10.1002/aoc.3052

A series of new hexa-coordinated ruthenium(II) hydroxyquinoline- thiosemicarbazone complexes of the type [Ru(CO)(EPh₃)(B)(L)] (E = P or As; B = PPh₃, AsPh₃ or Py; L = hydroxyquinoline- thiosemicarbazone) were synthesized by reacting ruthenium precursor complexes [RuHCl(CO)(EPh₃)₂(B)] (E = P or As; B = PPh₃, AsPh₃ or Py) with hydroxyquinoline-thiosemicarbazone ligands in ethanol. The new complexes were characterized by analytical and spectroscopic (FT-IR, UV-visible, NMR (¹H, ¹³C and ³¹P) and fast atom bombardment (FAB)-mass spectrometric methods. Based on the spectral results, an octahedral geometry was assigned for all the complexes. The new complexes showed good catalytic activity for the conversion of aldehydes to amides in the presence of hydroxylamine hydrochloride-sodium bicarbonate and for the oxidation of alkanes into their corresponding alcohols and ketones in the presence of m-chloroperbenzoic acid. The complexes also catalyzed the N-alkylation of benzylamine in the presence of KOtBu in alcohol medium. Copyright

Full text of DOI:10.1002/aoc.3052

Full Paper
Received: 16 May 2013
Revised: 5 August 2013
Accepted: 5 August 2013
Published online in Wiley Online Library: 24 September 2013
(wileyonlinelibrary.com) DOI 10.1002/aoc.3052
Ruthenium(II) complexes of hybrid
8-hydroxyquinoline–thiosemicarbazone
ligands: synthesis, characterization and
catalytic applications
M. Nirmala, R. Manikandan, G. Prakash and P. Viswanathamurthi*
A series of new hexa-coordinated ruthenium(II) hydroxyquinoline–thiosemicarbazone complexes of the type [Ru(CO)(EPh3)(B)
(L)] (E = P or As; B = PPh3, AsPh3 or Py; L = hydroxyquinoline–thiosemicarbazone) were synthesized by reacting ruthenium pre-
cursor complexes [RuHCl(CO)(EPh3)2(B)] (E = P or As; B = PPh3, AsPh3 or Py) with hydroxyquinoline–thiosemicarbazone ligands
in ethanol. The new complexes were characterized by analytical and spectroscopic (FT-IR, UV–visible, NMR (1H, 13C and 31P)
and fast atom bombardment (FAB)–mass spectrometric methods. Based on the spectral results, an octahedral geometry
was assigned for all the complexes. The new complexes showed good catalytic activity for the conversion of aldehydes to
amides in the presence of hydroxylamine hydrochloride–sodium bicarbonate and for the oxidation of alkanes into their cor-
responding alcohols and ketones in the presence of m-chloroperbenzoic acid. The complexes also catalyzed the N-alkylation
of benzylamine in the presence of KOtBu in alcohol medium. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: thiosemicarbazone ligands; ruthenium(II) complexes; aldehyde-to-amide conversion; alkane oxidation; N-alkylation
for the atom-economical and cost effective synthesis of amides.[10]
Aldehydes are highly desirable starting materials for amide
Introduction
In Schiff base metal complexes, the environment at the coordina-
tion center can be modified by attaching different substituents to
the ligand, which provides a useful range of steric and electronic
properties essential for the fine-tuning of structure and reactivity.[1]
Therefore, Schiff base ligands are among the most fundamental
chelating systems particularly for transition metals.[2] Among the
transition metals, ruthenium complexes have attracted much
interest mainly because of their vital applications in several organic
syntheses.[3,4] Ruthenium complexes containing carbonyl and
triphenylphosphine/arsine ligands are fascinating due to their reac-
tivity and efficiency in catalysis.[5,6] They have been used as catalyst
precursors for a variety of purposes, including hydrogenation,
oxidation, polymerization, carbon–carbon bond formation and
aldehyde-to-amide conversion.
Amides are one of the most important and prolific functional
groups, with a great importance in both research and industrial
chemistry owing to their prevalence in detergents, lubricants,
biologically active compounds and pharmaceuticals.[7] They are
routinely prepared through the union of carboxylic acids and de-
rivatives (halides, anhydrides or esters) with amines.[8] However,
these classical methods present several drawbacks, such as the
use of toxic, corrosive and/or expensive materials, highly exother-
mic reactions, low tolerance to sensitive functional groups, com-
plex reaction conditions and wasteful procedures. New, efficient
and sustainable synthetic routes for this important class of
compounds therefore need to be made available.[9] In the search
for improved methods, metal-catalyzed transformations have
emerged in the last two decades as the most promising alternative
synthesis because of their availability and non-toxic nature. In
this context, the coupling of aldehydes with hydroxylamine cat-
alyzed by metal complexes has been revealed in recent years as
an appealing tool for the generation of primary amides.[11] The
reaction involves the rearrangement of an aldoxime intermedi-
ate, generated in situ by the condensation of an aldehyde with
NH2OH, via initial dehydration to the corresponding nitrile,
followed by hydration of the C¼N bond.[10] Several catalytic
systems, based on ruthenium,[12] iron,[13] iridium,[14] palla-
dium,[15] copper,[16] zinc,[17] indium[18] and scandium[19] com-
pounds, able to promote this catalytic transformation have
already been described in the literature.[20] In a recent advance,
Williams and co-workers reported a ruthenium catalyst for this
reaction with high selectivity, excellent yield and low catalyst
loading with p-toluene sulfonic acid as an additive.[21] Further,
Crabtree et al. reported ruthenium terpyridine complexes as an
efficient catalyst to achieve high yields without any additive.[22]
In recent years, catalytic oxidation of saturated hydrocarbons
under mild conditions has become an exciting and challenging
scientific goal and has received increasing attention.[23–28]
Although iron complexes are considered to be one of the most
*
Correspondence to: P. Viswanathamurthi, Department of Chemistry, Periyar
University, Salem 636011, Tamilnadu, India. E-mail: viswanathamurthi72@
gmail.com
Department of Chemistry, Periyar university, Salem, 636011, India
Appl. Organometal. Chem. 2014, 28, 18–26
Copyright © 2013 John Wiley & Sons, Ltd.

Ru(II) complexes of hybrid 8-hydroxyquinoline–thiosemicarbazone ligands
promising catalysts for this industrially important reaction, a
variety of metal-based homogeneous catalysts for alkane oxida-
tion have been reported by replacing iron with different transi-
tion metals such as Mn, Co, Cu, Ru and Os.[29–49] Among them,
ruthenium has attracted much attention and a few Ru(II/III) com-
plexes of tripodal 4N ligands based on tris(2-pyridylmethyl)amine
(TPA)[50] and tris(benzimidazol-2-ylmethyl)amine (NTB)[51] have
been studied as catalysts for alkane hydroxylation, and the in-
volvement of a Ru(V)¼O intermediate in the catalytic cycle has
been proposed. Itoh et al. have reported Ni(II) complexes of tripo-
dal phenolate ligands, which are capable of catalyzing the oxida-
tion of cyclohexane to cyclohexanol using m-chloroperbenzoic
acid (m-CPBA) with up to 100% conversion based on the oxidant
in solvent-free conditions.[52] However, the factors determining
selectivity as well as efficiency of the catalysts and the nature of
intermediate species still remain unclear.
obtained at room temperature using tetramethylsilane as internal
standard. 31P NMR spectra of the complexes were obtained at room
temperature using o-phosphoric acid as a reference. Electrospray
ionization mass spectra were recorded by liquid chromatogra-
phy–mass spectrometry quadrupole time-of-flight Micro Analyzer
(Shimadzu) at SAIF, Panjab University, Chandigarh. Melting points
were recorded on a Technico micro heating table and are
uncorrected. The catalytic yields were determined by gas chroma-
tography–flame ionization detection (GC-FID) using an ACME
6000 series instrument with a DP-5 column of 30 m length, 0.53
mm diameter and 5.00 μm film thickness. The precursor complexes
[RuHCl(CO)(PPh3)3],[61] [RuHCl(CO)(AsPh3)3],[62] [RuHCl(CO)(Py)
(PPh3)2],[63] and ligands[64] L1 and L3 were prepared according
to literature procedures.
Preparation of 8-Hydroxyquinoline-2-Thiosemicarbazone (L2)
The N-alkylation of amines is an important transformation for
the preparation of pharmaceuticals, agrochemicals, polymers,
dyes and other fine chemicals.[53] Traditionally, the alkylation of
amines is achieved using conventional alkylating agents, such
as alkyl halides. Many alkyl halides have toxic or even mutagenic
properties and an alternative to such reagents is therefore
advantageous.[54] In recent years, a number of reports on the
hydroamination[55] and hydroamino-methylation of olefins or al-
kynes[56] for the synthesis of amines have been reported. Com-
pared to the frequently applied N-alkylations with alkyl halides
and reductive amination methods, an economically and environ-
mentally attractive method is the N-alkylation of amines using
primary and secondary alcohols. Since the early 1980s, transition
metal complexes,[57] particularly ruthenium complexes, have
been shown to be active for this transformation. Recently,
Williams, Beller and co-workers[58] were able to obtain good
yields in the N-alkylation of indoles by using 0.2–0.5 mol% of
the dimeric Shvo ruthenium catalyst (0.4–1 mol% Ru) at 110°C.
Matute also reported that small loadings of ruthenium pincer
complex (1 mol% Ru) afforded excellent results in the selective
alkylation of (hetero-)aromatic amines with alcohols.[59] With this
background, as part of our continuous interest at the design of
novel metal complexes and their catalytic applications to organic
synthesis, we herein report the synthesis and characterization of a
series of new ruthenium(II) hydroxyquinoline–thiosemicarbazone
complexes along with their catalytic activity towards the conver-
sion of aldehyde to amide in the presence of NaHCO3, alkane
oxidation in the presence of oxidizing agent m-chloroperbenzoic
acid and N-alkylation of amine with alcohols in the presence
of KOtBu.
2-Formyl-8-hydroxyquinoline[65] (0.173 g; 1 mmol) was dissolved in
dried and degassed ethanol (10 ml) and treated with
thiosemicarbazide (0.0911 g; 1 mmol) in 1:1 molar ratio. A drop of
acetic acid was added, and the reaction was subsequently refluxed
for 4 h. The light-yellow, crystalline product obtained was isolated
in good yield, washed with ether (30 ml) and dried in air. Yield
86%; yellow crystalline solid; m.p. 238°C. Anal. Found (%): C 53.52;
H 3.98; N 22.58; S 12.96. Calcd for C11H10N4OS (%): C 53.64; H 4.09;
N 22.75; S 13.02. IR (KBr, cmꢀ1): 3389 (C–OH), 3211 (N–H), 1611
(C¼N), 833 (C¼S). UV λmax (εmax) 351 (16 682), 318 (28 452), 260
(4520). 1H NMR (DMSO-d6, δ, ppm): 10.78 (s, 1Hk, N–NH–C¼S),
10.23 (s, 1He, C–OH), 3.26 (s, 2Hl, NH2), 8.77 (s, 1Hj, N¼CH), 7.38 (d,
1Ha, J = 7.4 Hz), 7.23 (d, 1Hb, J = 7.4 Hz), 6.37 (dd, 1Hg, J(Hg–Hf) = 7.2
Hz, J(Hg–Hh) = 7.2 Hz), 6.34 (dd, 1Hh, J(Hh–Hg) = 7.2 Hz, J(Hh–Hf) = 1.5
Hz), 6.03 (dd, 1Hf, J(Hf–Hg) = 7.2 Hz, J(Hf–Hh) = 1.5 Hz). 13C NMR
(DMSO-d6, δ, ppm): 176.96 (Ck, C¼S), 151.43 (Ce, C–OH), 149.93
(Cb, C–C¼N), 140.56 (Cj, CH¼N), 136.52 (Ca), 115.82 (Cd), 110.01
(Ci), 134.20 (Ch),129.31 (Cg), 126.84 (Cf),116.58 (Cc). The structure of
the ligands used in this study is given in Fig. 1.
General Procedure for Synthesis of New Ruthenium(II)
Hydroxyquinoline–Thiosemicarbazone Complexes (1–9)
All the new metal complexes were synthesized according to the
following general procedure. To a solution of [RuHCl(CO)(EPh3)2
(B)] (E = P or As; B = PPh3, AsPh3 or Py) (0.1 g; 0.1 mmol) in ethanol
(20 ml), the appropriate ligand (0.0259–0.0338 g, 0.1 mmol) was
added in 1:1 molar ratio. The mixture was heated under reflux
for 5 h in a water bath, whereby the solution turned from pale
yellow to reddish-brown. After reducing the content to half vol-
ume and standing for a day, the complex was obtained as a red-
dish-brown precipitate. It was filtered and washed several times
with ether and dried under vacuum, and the purity of the com-
plexes was checked by thin-layer chromatography.
Experimental
Materials and Methods
All the reagents used were chemically pure and AR grade. The
solvents were purified and dried according to standard proce-
dures.[60] RuCl3.3H2O was purchased from Loba Chemie Pvt Ltd.
Microanalyses of carbon, hydrogen, nitrogen and sulfur were
carried out using a Vario EL III elemental analyzer at SAIF, Cochin,
India. IR spectra were recorded as KBr pellets on a Nicolet Avatar
model spectrophotometer in the 4000–400 cmꢀ1 range. Electronic
spectra were recorded in dichloromethane using a Shimadzu
Elemental composition and properties are as follows.
a
h
i
b
c
g
f
S
l
N
H
N
R
k
N
N
d
e
j
H
OH
UV-1650 PC spectrophotometer in the 800–200 nm range. 1H, 13
C
(R= CH3 (L1), R = H (L2), R = C6H5(L3))
and 31P NMR spectra were recorded on a Jeol GSX-400 instrument
using DMSO-d6 as solvent. 1H NMR and 13C NMR spectra were
Figure 1. General structure of hydroxyquinoline–thiosemicarbazone ligands.
Appl. Organometal. Chem. 2014, 28, 18–26
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

M. Nirmala et al.
AsPh3), 2.95 (d, 3Hm, J = 4.2 Hz, CH3). 13C NMR (DMSO-d6, δ, ppm):
203.54 (Cn, CꢁO), 182.05 (Ck, C–S), 159.30 (Ce, C–O), 147.25 (Ck,
C¼N), 145.41 (Cj, N¼CH), 139.56 (Ca), 137.06 (Ch),135.85 (Cg),
132.91 (Cf), 131.85 (Cb), 130.54 (Cc), 130.21 (Cd), 129.24 (Ci), 128.01,
124.20, 121.21, 120.03 (AsPh3), 30.74 (Cm, CH3). MS (FAB),
m/z = 1001 [M+].
[Ru(CO)(PPh3)2(L1)] (1)
Yield 82%; brown solid; m.p. 125°C. Anal. Found (%): C 64.32; H
4.08; N 5.98; S 3.31. Calcd for C49H40N4O2P2SRu (%): C 64.53; H
4.42; N 6.14; S 3.52. IR (KBr, cmꢀ1): 3354 (N–H), 1981 (CꢁO),
1380 (C–O), 1574 (C¼N), 760 (C–S). UV λmax (εmax) 432 (1875),
1
366 (28 310), 310 (16 892), 261 (11 372). H NMR (DMSO-d6, δ,
ppm): 8.37 (s, 1Hj, N¼CH), 8.72 (m, 1Hl, CH3–NH–C–S), 7.23 (d,
1Ha, J = 7.4 Hz), 7.25 (d, 1Hb, J = 7.4 Hz), 7.27 (dd, 1Hg, J(Hg–Hf) = 7.2
Hz, J(Hg–Hh) = 7.2 Hz), 7.40 (dd, 1Hf, J(Hf–Hg) = 7.2 Hz, J(Hf–Hh) = 1.5
Hz), 7.43 (dd, 1Hh, J(Hh–Hg) = 7.2 Hz, J(Hh–Hf) = 1.5 Hz), 7.70–7.56
(m, 15H, PPh3), 2.75 (d, 3Hm, J = 4.2 Hz, CH3). 13C NMR (DMSO-d6,
δ, ppm): 204.26 (Cn, CꢁO), 179.85 (Ck, C–S), 158.21 (Ce, C–O),
148.32 (Ck, C¼N), 144.78 (Cj, N¼CH), 138.55 (Ca), 136.96
(Ch),137.62 (Cg), 133.55 (Cf), 133.05 (Cb), 130.12 (Cc, tert), 129.74
(Cd), 128.20 (Ci), 128.86, 128.37, 127.89 (PPh3), 30.21 (Cm, CH3).
31P NMR (CDCl3, δ, ppm): 46.79 and 48.26 (2s, 2PPh3). MS (FAB),
m/z = 911.5 [M+].
[Ru(CO)(AsPh3)2(L2)] (5)
Yield 80%; brown solid; m.p. 234°C. Anal. Found (%): C 58.15; H
3.65; N 5.42; S 3.02. Calcd for C48H38N4O2As2SRu (%): C 58.48; H
3.89; N 5.68; S 3.25. IR (KBr, cmꢀ1): 1975 (CꢁO), 1360 (C–O),
1567 (C¼N), 747 (C–S). UV λmax (εmax): 426 (5732), 358 (12 487),
1
261 (11 592), 230 (10 890). H NMR (DMSO-d6, δ, ppm): 4.65 (s,
2Hl, NH2), 8.27 (s, 1Hj, N¼CH), 7.17 (d, 1Ha, J = 7.4 Hz), 7.21 (d,
1Hb, J = 7.4 Hz), 7.26 (dd, 1Hg, J(Hg–Hf) = 7.2 Hz, J(Hg–Hh) = 7.2 Hz),
7.32 (dd, 1Hf, J(Hf–Hg) = 7.2 Hz, J(Hf–Hh) = 1.5 Hz), 7.38 (dd, 1Hh,
J(Hh–Hg) = 7.2 Hz, J(Hh–Hf) = 1.5 Hz), 7.86–7.74 (m, 15H, AsPh3).
13C NMR (DMSO-d6, δ, ppm): 203.94 (Cn, CꢁO), 182.73 (Ck, C–S),
158.59 (Ce, C–O), 149.84 (Ck, C¼N), 145.88 (Cj, N¼CH), 139.15 (Ca),
138.96 (Ch),138.22 (Cg), 134.41 (Cf), 133.28 (Cb), 132.69 (Cc), 131.90
(Cd), 130.91 (Ci), 129.78, 128.95, 125.91, 121.73, 120.10 (AsPh3).
MS (FAB), m/z = 986.2 [M+].
[Ru(CO)(PPh3)2(L2)] (2)
Yield 84%; brown solid; m.p. 278°C. Anal. Found (%): C 64.08; H
4.04; N 6.11; S 3.28. Calcd for C48H38N4O2P2SRu (%): C 64.21; H
4.27; N 6.24; S 3.57. IR (KBr, cmꢀ1): 1963 (CꢁO), 1360 (C–O),
1572 (C¼N), 759 (C–S). UV λmax (εmax) 434 (1232), 358 (22 490),
[Ru(CO)(AsPh3)2(L3)] (6)
1
261 (11 592). H NMR (DMSO-d6, δ, ppm): 3.41 (s, 2Hl, NH2), 8.28
Yield 82%; brown solid; m.p. 117°C. Anal. Found (%): C 60.89; H
3.82; N 5.07; S 2.95. Calcd for C54H42N4O2As2SRu (%): C 61.08; H
3.99; N 5.28; S 3.02. IR (KBr, cmꢀ1): 3368 (N–H), 1973 (CꢁO),
(s, 1Hj, N¼CH), 7.25 (d, 1Ha, J = 7.4 Hz), 7.28 (d, 1Hb, J = 7.4 Hz),
7.32 (dd, 1Hg, J(Hg–Hf) = 7.2 Hz, J(Hg–Hh) = 7.2 Hz), 7.42 (dd, 1Hf, J
(Hf–Hg) = 7.2 Hz, J(Hf–Hh) = 1.5 Hz), 7.48 (dd, 1Hh, J(Hh–Hg) = 7.2 Hz, J
(Hh–Hf) = 1.5 Hz), 7.72–7.65 (m, 15H, PPh3). 13C NMR (DMSO-d6, δ,
ppm): 204.87 (Cn, CꢁO), 177.56 (Ck, C–S), 156.21 (Ce, C–O),
149.23 (Ck, C¼N), 144.87 (Cj, N¼CH), 139.21 (Ca), 137.86
(Ch),136.26 (Cg), 134.11 (Cf), 131.85 (Cb), 130.74 (Cc), 129.36 (Cd),
128.54 (Ci), 128.21, 125.31, 121.85, 120.13 (PPh3). MS (FAB),
m/z = 898 [M+].
1376 (C–O), 1578 (C¼N), 737 (C–S). UV λmax (εmax): 441 (1381),
1
363 (27 890), 278 (14 821), 248 (19 243) . H NMR (DMSO-d6, δ,
ppm): 11.11 (s, 1Hl, C–NH–C–S), 8.31 (s, 1Hj, N¼CH), 7.18 (d, 1Ha,
J = 7.4 Hz), 7.24 (d, 1Hb, J = 7.4 Hz), 7.28 (dd, 1Hg, J(Hg–Hf) = 7.2
Hz, J(Hg–Hh) = 7.2 Hz), 7.33 (dd, 1Hf, J(Hf–Hg) = 7.2 Hz, J(Hf–Hh) = 1.5
Hz), 7.39 (dd, 1Hh, J(Hh–Hg) = 7.2 Hz, J(Hh–Hf) = 1.5 Hz), 7.70–7.47
(m, 15H, AsPh3). 13C NMR (DMSO-d6, δ, ppm): 203.74 (Cn, CꢁO),
182.30 (Ck, C–S), 161.2 (Ce, C–O), 149.51 (Ck, C¼N), 145.81 (Cj,
N¼CH), 138.65 (Ca), 136.89 (Ch),135.06 (Cg), 132.57 (Cf), 131.47
(Cb), 130.89 (Cc), 130.34 (Cd), 128.96 (Ci), 127.52, 126.91, 123.23,
122.05 (AsPh3).
[Ru(CO)(PPh3)2(L3)] (3)
Yield 87%; brown solid; m.p. 158°C. Anal. Found (%): C 66.24; H
4.12; N 5.62; S 3.09. Calcd for C54H42N4O2P2SRu (%): C 66.59; H
4.35; N 5.75; S 3.29. IR (KBr, cmꢀ1): 3368 (N–H), 1981 (CꢁO),
1374 (C–O), 1573 (C¼N), 759 (C–S). UV λmax (εmax) 428 (5687),
374 (22 327), 278 (10 156), 250 (9742). 1H NMR (DMSO-d6, δ,
ppm): 11.34 (s, 1Hl, C–NH–C–S), 8.39 (s, 1Hj, N¼CH), 7.18 (d, 1Ha,
J = 7.4 Hz), 7.21 (d, 1Hb, J = 7.4 Hz), 7.26 (dd, 1Hg, J(Hg–Hf) = 7.2
Hz, J(Hg–Hh) = 7.2 Hz), 7.32 (dd, 1Hf, J(Hf–Hg) = 7.2 Hz, J(Hf–Hh) = 1.5
Hz), 7.40 (dd, 1Hh, J(Hh–Hg) = 7.2 Hz, J(Hh–Hf) = 1.5 Hz), 7.71–7.49
(m, 15H, PPh3). 13C NMR (DMSO-d6, δ, ppm): 203.31 (Cn, CꢁO),
181.25 (Ck, C–S), 157.95 (Ce, C–O), 147.22 (Ck, C¼N), 145.42
(Cj, N¼CH), 137.81 (Ca), 136.78 (Ch),135.91 (Cg), 134.23 (Cf),
133.21 (Cb), 130.29 (Cc), 130.12 (Cd), 129.78 (Ci), 128.94, 128.37,
124.10 (PPh3). 31P NMR (CDCl3, δ, ppm): 47.41 and 49.19 (2s,
2PPh3). MS (FAB), m/z = 975.2 [M+].
[Ru(CO)(PPh3)(Py)(L1)] (7)
Yield 86%; brown solid; m.p. 105°C. Anal. Found (%): C 59.02; H
3.98; N 9.52; S 4.16. Calcd for C36H30N5O2PSRu (%): C 59.33; H
4.15; N 9.61; S 4.40. IR (KBr, cmꢀ1): 3357 (N–H), 1958 (CꢁO),
1390 (C–O), 1542 (C¼N), 750 (C–S). UV λmax (εmax): 450 (9971),
1
371 (24 396), 264 (12 394), 236 (18 391). H NMR (DMSO-d6, δ,
ppm): 8.21 (s, 1Hj, N–CH), 8.72 (m, 1Hl, CH3–NH–C–S), 7.19 (d,
1Ha, J = 7.4 Hz), 7.23 (d, 1Hb, J = 7.4 Hz), 7.32 (dd, 1Hg, J(Hg–Hf) = 7.2
Hz, J(Hg–Hh) = 7.2 Hz), 7.38 (dd, 1Hf, J(Hf–Hg) = 7.2 Hz, J(Hf–Hh) = 1.5
Hz), 7.41 (dd, 1Hh, J(Hh–Hg) = 7.2 Hz, J(Hh–Hf) = 1.5 Hz), 8.04–7.70
(m, 5H, Py), 7.67–7.55 (m, 15H, PPh3), 2.92 (d, 3Hm, J = 4.2 Hz,
CH3). 13C NMR (DMSO-d6, δ, ppm): 205.02 (Cn, CꢁO), 175.23 (Ck,
C–S), 162.14 (Ce, C–O), 149.45 (Ck, C¼N), 144.71 (Cj, N¼CH),
138.23 (Ca), 137.58 (Ch),137.95 (Cg), 134.53 (Cf), 133.27 (Cb),
132.36 (Cc), 131.44 (Cd), 131.20 (Ci), 128.88, 128.68, 128.25 (Py),
127.95, 127.65, 125.68 (PPh3), 29.84 (Cm, CH3). 31P NMR (CDCl3,
δ, ppm): 43.25. MS (FAB), m/z = 729.4 [M+].
[Ru(CO)(AsPh3)2(L1)] (4)
Yield 80%; brown solid; m.p. 268°C. Anal. Found (%): C 58.71; H
3.91; N 5.48; S 3.02. Calcd for C49H40N4O2As2SRu (%): C 58.56; H
4.03; N 5.60; S 3.21. IR (KBr, cmꢀ1): 3345 (N–H), 1934 (CꢁO),
1367 (C–O), 1595 (C¼N), 745 (C–S). UV λmax (εmax) 430 (5432),
359 (26 890), 270 (11 321), 241 (20 216). 1H NMR (DMSO-d6,
δ, ppm): 8.21(s, 1Hj, N¼CH), 8.79 (m, 1Hl, CH3–NH–C–S), 7.19
(d, 1Ha, J = 7.4 Hz), 7.23 (d, 1Hb, J = 7.4 Hz), 7.33 (dd, 1Hg, J(Hg–Hf) = 7.2
Hz, J(Hg–Hh) = 7.2 Hz), 7.39 (dd, 1Hf, J(Hf–Hg) = 7.2 Hz, J(Hf–Hh) = 1.5 Hz),
7.42 (dd, 1Hh, J(Hh–Hg) = 7.2 Hz, J(Hh–Hf) = 1.5 Hz), 7.87–7.64 (m, 15H,
[Ru(CO)(PPh3)(Py)(L2)] (8)
Yield 85%; brown solid; m.p. 140°C, Anal. Found (%): C 58.65; H
3.72; N 9.65; S 4.31. Calcd for C35H28N5O2PSRu (%): C 58.82; H
3.95; N 9.80; S 4.49. IR (KBr, cmꢀ1): 1938 (CꢁO), 1375 (C–O),
1553 (C¼N), 745 (C–S). UV λmax (εmax): 419 (2378), 357 (25 376),
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Appl. Organometal. Chem. 2014, 28, 18–26

Ru(II) complexes of hybrid 8-hydroxyquinoline–thiosemicarbazone ligands
290 (10 152), 257 (12 891). 1H NMR (DMSO-d6, δ, ppm): 3.75 (s, 2H,
NH2), 8.31 (s, 1Hj, N¼CH), 7.12 (d, 1Ha, J = 7.4 Hz), 7.28 (d, 1Hb,
J = 7.4 Hz), 7.30 (dd, 1Hg, J(Hg–Hf) = 7.2 Hz, J(Hg–Hh) = 7.2 Hz), 7.34
and refluxed at 110–150°C for 6 h. On completion of the reac-
tion, brine (3 ml) and CH2Cl2 (5 ml) were added. The organic
layer was separated and the aqueous layer was extracted into
CH2Cl2. The combined organic extracts were dried with mag-
nesium sulfate and concentrated. The compound was purified
by column chromatography (pentane–diethyl ether, 1:1) using
silica (200 mesh) as solid phase and characterized by compar-
(dd, 1Hf, J(Hf–Hg) = 7.2 Hz, J(Hf–Hh) = 1.5 Hz), 7.39 (dd, 1Hh, J(Hh–Hg)
=
7.2 Hz, J(Hh–Hf) = 1.5 Hz), 8.12–7.68 (m, 5H, Py), 7.55–7.44 (m, 15H,
PPh3). 13C NMR (DMSO-d6, δ, ppm): 203.41(Cn, CꢁO), 179.54 (Ck,
C–S), 154.89 (Ce, C–O), 147.47 (Ck, C¼N), 144.87 (Cj, N¼CH),
140.15 (Ca), 138.56 (Ch), 136.26 (Cg), 135.76 (Cf), 132.63 (Cb),
131.74 (Cc), 130.26 (Cd), 129.54 (Ci), 129.15, 128.36, 127.65 (Py),
126.25, 124.51, 120.78 (PPh3).
1
ing their H NMR spectra with those given in the literature.[67]
1
Yields were obtained by the H NMR integration compared to
1,3,5-trimethoxybenzene as an internal standard.
[Ru(CO)(PPh3)(Py)(L3)] (9)
Yield 80%; brown solid; m.p. 114°C. Anal. Found (%): C 62.01; H
3.85; N 8.62; S 3.95. Calcd for C41H32N5O2PSRu (%): C 62.27; H
4.08; N 8.86; S 4.05. IR (KBr, cmꢀ1): 3378 (N–H), 1916 (CꢁO),
1367 (C–O), 1541 (C¼N), 750 (C–S). UV λmax (εmax): 440 (1364),
360 (21 823), 275 (9860), 247 (19 241). 1H NMR (DMSO-d6, δ,
ppm): 11.85 (s, 1Hl, C–NH–C–S), 8.65 (s, 1Hj, N¼CH), 7.22 (d, 1Ha,
J = 7.4 Hz), 7.25 (d, 1Hb, J = 7.4 Hz), 7.27 (dd, 1Hg, J(Hg–Hf) = 7.2 Hz, J
(Hg–Hh) = 7.2 Hz), 7.34 (dd, 1Hf, J(Hf–Hg) = 7.2 Hz, J(Hf–Hh) = 1.5 Hz),
7.39 (dd, 1Hh, J(Hh–Hg) = 7.2 Hz, J(Hh–Hf) = 1.5 Hz), 8.22–7.58 (m, 5H,
Py), 7.57–7.42 (m, 15H, PPh3). 13C NMR (DMSO-d6, δ, ppm): 205.34
(Cn, CꢁO), 178.51 (Ck, C–S), 154.52 (Ce, C–O), 149.32 (Ck, C¼N),
146.45 (Cj, N¼CH), 138.91 (Ca), 137.92 (Ch),136.74 (Cg), 134.25 (Cf),
132.15 (Cb), 131.10 (Cc), 130.09 (Cd), 129.98 (Ci), 129.72, 128.36,
127.39 (Py), 126.35, 124.53, 119.18 (PPh3).
Results and Discussion
Diamagnetic, hexa-coordinated low-spin ruthenium(II) hydroxy-
quinoline–thiosemicarbazone complexes of the general formula
[Ru(CO)(EPh3)2(B)(L)] (B = PPh3, AsPh3 or Py; L = hydroxyqu-
inoline–thiosemicarbazone ligand) were synthesized in quantita-
tive yields from the reaction of [RuHCl(CO)(EPh3)2(B)] (E = P or As;
B = PPh3, AsPh3 or Py) with hydroxyquinoline–thiosemicarbazone
ligands at 1:1 molar ratio in ethanol medium (Scheme 1). In all
these reactions, it was observed that the hydroxyquinoline–
thiosemicarbazone behaved as dianionic tridentate ligand by
replacing one molecule of triphenylphosphine/arsine, one
molecule of hydride and one molecule of chloride from the
starting complexes.
The analytical data (C, H, N, S) of all the hydroxyquinoline–
thiosemicarbazone ligands and their ruthenium(II) complexes
are in good agreement with the calculated values, thus
confirming the proposed mononuclear composition for all the
complexes. Molecular ion peaks (M+) were observed with m/z
values 911.5, 898, 975.2, 1001, 986.2 and 729.4, corresponding
to complexes 1, 2, 3, 4, 5 and 7 respectively in the FAB mass
spectra of the complexes, supporting the proposed molecular
formulae (supporting information, Figs S1–S4). All the ligands
and their complexes are stable at room temperature, non-
hygroscopic, insoluble in water and soluble in common organic
General Procedure for the Ruthenium-Catalyzed
Aldehyde-to-Amide Conversion
Catalytic conversion of aldehydes to amides was carried out
using ruthenium(II) complexes as catalysts in the following proce-
dure. The reaction vessel was charged with aldehyde (1 mmol),
NH2OH.HCl (1 mmol), NaHCO3 (1 mmol) and ruthenium catalyst
(0.5 mmol), and the mixture was placed under an atmosphere
of N2. About 2 ml dry and degassed toluene was added and
the mixture was stirred for 15 min at room temperature, followed
by reflux for 24 h. On completion of the reaction, 2–3 ml metha-
nol was added to the mixture, followed by filtration through
celite to remove the catalyst and NaHCO3. The crude product
was then purified by column chromatography (MeOH/CH2Cl2,
1:1) using silica (200 mesh) as solid phase, providing the amide
in good yield. The products were characterized by comparing
their 1H NMR spectra with those given in the literature.[66]
a
h
i
b
c
g
f
S
k
l
N
H
N
R
N
N
d
e
j
H
OH
+
[RuHCl(CO)(EPh3)2(B)]
General Procedure for the Ruthenium-Catalyzed Oxidation
of Alkanes
EtOH/ reflux 5h
In a typical reaction, ruthenium(II) complex (0.35 × 10ꢀ3 mmol)
was added to a mixture of alkane (1.75 mmol) and oxidant
m-CPBA (0.35 mol) in CH2Cl2–CH3CN mixture (3:1, v/v) and stirred
at room temperature for 48 h. The reaction mixture was filtered
over a silica column, and was subjected to GC analysis. The
formation of product was determined by GC, comparing with
authentic sample.
h
a
i
g
f
b
c
j
d
N
CO
n
e
N
O
N
Ru
H
C
k
N
R
S
B
l
EPh3
m
General Procedure for the Ruthenium-Catalyzed N-Alkylation
of Amines
(E = P or As; B = PPh3, AsPh3 or Py;
R = CH3, H or C6H5)
A
mixture of amine (0.3 mmol), ruthenium(II) complex
(3 × 10ꢀ3 mmol) and KOtBu (0.12 mmol) in alcohol (0.9 mmol)
was placed in a flask under atmospheric pressure of nitrogen
Scheme 1. Synthesis of ruthenium(II) hydroxyquinoline–thiosemicarbazone
complexes.
Appl. Organometal. Chem. 2014, 28, 18–26
Copyright © 2013 John Wiley & Sons, Ltd.
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M. Nirmala et al.
solvents such as dichloromethane, chloroform, benzene, acetoni-
trile, ethanol, methanol, DMSO, etc. However, our efforts to ob-
tain single crystals of the complexes were unsuccessful.
coordination mode is azomethine nitrogen. The terminal methyl
group in ligand L1 and their complexes showed a peak around
2.90–3.10 ppm. The terminal NH2 protons in complexes 2, 5, 8 are
seen in the positions around 3.75–3.41 ppm with slight deviations
from that of the ligand spectrum, confirming the non-involvement
of this group in coordination with metal.[73] The terminal –NHl–R
proton appeared in the region 11.87–8.79 ppm in the ligands
L1, L3 and their complexes. The multiplet observed in the
7.22–8.44 ppm range has been assigned to the aromatic protons
of the hydroxyquinoline–thiosemicarbazones ligands and the
phenyl groups of triphenylphosphine/ triphenylarsine/ pyridine.
Infrared Spectroscopic Analysis
The IR spectra of free ligands were compared with those of
new complexes in order to confirm the coordination of ligands
to ruthenium metal. In the spectra of free ligand, a strong
band was observed in the region 1630–1611 cmꢀ1, which is char-
acteristic of the azomethine group, and this band has been
shifted to a lower frequency (1573–1541 cmꢀ1) in complexes,
indicating the coordination of Schiff bases with metal through
the azomethine nitrogen atom.[68] A strong band obtained around
1320–1300 cmꢀ1 in the spectra of free ligands was assigned to
phenolic C–O stretching which had been shifted to a higher
frequency (1390–1360 cmꢀ1) in the complexes, showing that the
other coordination is through the phenolic oxygen atom.[69] This
has been further confirmed by the disappearance of the broad
13C NMR Spectroscopic Analysis
The 13C NMR spectra of the complexes (supporting information,
Figs S5–S8) showed a peak in the 205.34–203.31 ppm region due
to terminal CꢁO carbon. The azomethine (>C¼N) carbon exhibited
a peak in the region 147.22–149.84 ppm. The peaks observed in
the 119.18–139.56 ppm region have been assigned to aromatic
carbons. The peak appearing in the 154.52–162.14 ppm region
can be attributed to the coordinated carbonyl carbon (C–O). A sharp
singlet appearing at 175.23–182.73 ppm was due to C–S carbon.
ν
OH band around 3388–3370 cmꢀ1 in all complexes due to binding
of the phenolic –OH group with ruthenium metal via deproton-
ation.[70] The medium-intensity band at 870 cmꢀ1 and a shoulder
at 3210–3260 cmꢀ1 was due to the presence of C¼S and NH
groups in the ligands, which disappeared in the complexes, and a
new band appeared for C–S around 750–760 cmꢀ1 due to
enolization of the ligand, followed by deprotonation prior to the
coordination of the thiolate sulfur.[71] The strong absorption band
around 1981–1938 cmꢀ1 was due to the CꢁO stretching of the
terminally coordinated carbonyl group. The complexes
containing pyridine as coordinated ligand exhibited a peak at
about 1027–1031 cmꢀ1 due to aromatic C¼N stretching.
31P NMR Spectral Analysis
31P NMR spectra were recorded for few representative complexes
(1, 3 and 7) in order to confirm the presence of triphenylphosphine
group along with heterocyclic nitrogen base and its geometry
in the ruthenium(II) hydroxyquinoline–thiosemicarbazone com-
plexes. The signals appeared at 46.79–48.26 and 47.41–49.19 ppm
in the spectra of 1,
3 and indicated that the two
triphenylphosphine ligands were cis to each other in these
complexes. However, complex 7 exhibited only one signal at
43.25 ppm, consistent with the presence of one triphenylphosphine
group. This observation indicated the retention of coordinated
pyridine in the complexes even after the coordination of
thiosemicarbazones. The 31P NMR spectral studies indicated a more
labile Ru–P bond compared to Ru–N bond in these complexes,
which is a reflection of better σ-donating ability of the nitrogen
bases compared to that of phosphorus in triphenylphosphine.[74]
Electronic Spectroscopic Analysis
All the ruthenium(II) hydroxyquinoline–thiosemicarbazone com-
plexes are diamagnetic, indicating the presence of ruthenium in
+2 oxidation state, corresponding to the electronic configuration
t62g e0g. The electronic spectra of the new complexes have been
recorded in CH2Cl2 solution in the region 800–200 nm. All the ru-
thenium(II) complexes displayed three to four bands in the region
450–240 nm. The bands in the region 450–360 nm have been
assigned to metal-to-ligand charge-transfer transitions. The other
high-intensity bands appearing around 350–240 nm were probably
due to ligand-centered transitions. The observed electronic spectral
pattern of our complexes was similar to the octahedral ruthenium
complexes of the same type reported earlier.[72]
Catalytic Aldehyde-to-Amide Conversion
The synthesized ruthenium(II) complexes were employed as a
catalyst for the conversion of aldehydes to their corresponding
amides. The effects of solvents, base and catalyst/substrate ratio
were investigated using 4-nitrobenzaldehyde as substrate. No
considerable reaction was observed in the absence of catalyst
or base. The optimized time for the reaction was 24 h and the
catalyst/substrate ratio was 1:100 (Table 1). The conversion of
4-nitobenzaldehyde to 4-nitrobenzamide was performed in dif-
ferent solvent media such as acetonitrile, benzene, chloroform,
dichloromethane, toluene and xylene. Among these, toluene
was found to be best, with high conversion (Fig. 2). It was there-
fore used as solvent for the conversion of all substrates. Similarly,
NaHCO3 was selected as base among KHCO3, CH3COONa and
Et3N (Table 2).
1H NMR Spectroscopic Analysis
The 1H NMR spectra of the ligands and corresponding ruthenium(II)
complexes were recorded in DMSO-d6 to confirm the presence
of coordinated ligand in the complexes. The signal around
10.24 ppm due to the hydrazinic proton (–NH–C¼S) of the free
hydroxyquinoline–thiosemicarbazone ligands, which disappeared
in the complexes, confirmed the thio-enolization. Spectra of free
ligands exhibited a peak in the region 9.58–9.54 ppm, which is char-
acteristic of the phenolic O–H group. The peak disappeared in the
corresponding complexes, confirming the binding of ligand with
metal ion via deprotonation of the phenolic hydroxyl group. The
azomethine proton of the ligands exhibited a signal in the region
8.26–8.10 ppm which underwent a downfield shift (8.4–8.3 ppm)
in the corresponding complexes. This indicated that the other
Using the above optimized conditions, the catalytic conversion
of aldehydes to amides in the presence of NH2OH.HCl and NaHCO3,
using all the synthesized ruthenium(II) hydroxyquinoline–
thiosemicarbazone complexes as catalysts, has also been investi-
gated (Table 3). It has been observed that the electron-withdrawing
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Ru(II) complexes of hybrid 8-hydroxyquinoline–thiosemicarbazone ligands
Table 1. Effect of catalyst:substrate (C:S) ratio on the conversion of
4- nitrobenzaldehyde to 4-nitrobenzamide using complex 1
Table 3. Catalytic aldehyde-to-amide conversion using ruthenium(II)
hydroxyquinoline–thiosemicarbazone complexes
Entry
C:S ratio
Yield (%)a
Ru(II) complex
O
O
NaHCO3/
reflux 24h
1
2
3
4
5
6
1:1000
1:500
1:400
1:300
1:200
1:100
<25
30
+
NH2OH.HCl
R
H
R
NH2
36
(R = 4-NO2Ph, Ph, 4-MePh)
45
58
Complex
1
Substrate
Product
Yield (%)a
86
aIsolated yield after column chromatography (average of two runs).
4-Nitrobenzaldehyde
Benzaldehyde
4-Nitrobenzamide
Benzamide
86
79
81
82
74
76
84
79
84
86
75
83
82
78
78
85
77
81
83
73
78
81
76
74
84
74
77
Tolualdehyde
Toluamide
2
3
4
5
6
7
8
9
4-Nitrobenzaldehyde
Benzaldehyde
4-Nitrobenzamide
Benzamide
Tolualdehyde
Toluamide
4-Nitrobenzaldehyde
Benzaldehyde
4-Nitrobenzamide
Benzamide
Tolualdehyde
Toluamide
4-Nitrobenzaldehyde
Benzaldehyde
4-Nitrobenzamide
Benzamide
Tolualdehyde
Toluamide
4-Nitrobenzaldehyde
Benzaldehyde
4-Nitrobenzamide
Benzamide
Tolualdehyde
Toluamide
4-Nitrobenzaldehyde
Benzaldehyde
4-Nitrobenzamide
Benzamide
Tolualdehyde
Toluamide
4-Nitrobenzaldehyde
Benzaldehyde
4-Nitrobenzamide
Benzamide
Tolualdehyde
Toluamide
4-Nitrobenzaldehyde
Benzaldehyde
4-Nitrobenzamide
Benzamide
Figure 2. Effect of solvents on conversion of 4-nitrobenzaldehyde to
4-nitrobenzamide using catalyst 1.
Tolualdehyde
Toluamide
4-Nitrobenzaldehyde
Benzaldehyde
4-Nitrobenzamide
Benzamide
Tolualdehyde
Toluamide
Table 2. Effect of base in the conversion of 4-nitrobenzaldehyde to
4-nitrobenzamide using complex 1
aIsolated yield after column chromatography (average of two runs).
Entry
Solvent
Base
Temp.
(°C)
Time
(h)
Yield
(%)a
transformation proceeded via the formation of a RuIV(OH)(imine)
intermediate, as proposed by Crabtree and co-workers.[66] When
compared to other ruthenium complexes,[12a] the percent yields
were higher for the new ruthenium complexes.
1
2
3
4
5
6
7
8
9
Xylene
NaHCO3
NaHCO3
NaHCO3
140
78
48
48
48
48
48
24
24
24
24
48
48b
36
76
Acetonitrile
Benzene
80
33
Dichloromethane NaHCO3
40
29
Chloroform
Toluene
Toluene
Toluene
Toluene
NaHCO3
NaHCO3
KHCO3
CH3COONa
Et3N
64
26
115
115
115
115
115
115
86
Alkane Oxidation
85
<20
<20
—
The catalytic ability of the ruthenium(II) complexes towards the
oxidation of alkanes such as cyclohexane and ethylbenzene was
explored using m-CPBA as oxidant in a dichloromethane–
acetonitrile solvent mixture (3:1, v/v) at room temperature for
48 h under nitrogen atmosphere (Table 4). In order to optimize
the reaction conditions, the effects of time (Fig. 3) and different
catalyst/substrate ratio (Fig. 4) were tested[75] using catalyst 1
as a model. Using the optimized conditions, the oxidation of al-
kanes using all the synthesized ruthenium(II) complexes were
carried out. It was observed that the oxidation of cyclohexane
proceeded to give cyclohexanol as the major product, along with
cyclohexanone and ɛ-caprolactone as minor products, where
ɛ-caprolactone is the over-oxidized product of the oxidation of
10 Toluene
11 Toluene
—
NaHCO3
—
aIsolated yield after column chromatography (average of two runs).
bReaction carried out in the absence of catalyst.
substituent (nitro) on benzaldehyde gave slightly higher yields
when compared to that of benzaldehyde. Electron-donating
substituent (methyl) on benzaldehyde gave slightly lower yields
compared with benzaldehyde. We believe that the catalytic
Appl. Organometal. Chem. 2014, 28, 18–26
Copyright © 2013 John Wiley & Sons, Ltd.
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M. Nirmala et al.
Table 4. Catalytic oxidation of alkanes using ruthenium(II)
hydroxyquinoline–thiosemicarbazone complexes
80
60
40
20
0
OH
Ru(II) complex / R.T
m- CPBA/
O
CHCl3/CH3CN
+
CyOH
CyO
Caprolactone
Complex
1
Substrate
Product
Yield (%)
Cyclohexane
Cyclohexanol
78
<11
<10
76
Cyclohexanone
ε-Caprolactone
1-Phenylethanol
Acetophenone
Cyclohexanol
Ethylbenzene
Cyclohexane
17
0
10
20
30
40
50
60
70
80
2
75
Time (hrs)
Cyclohexanone
ε-Caprolactone
1-Phenylethanol
Acetophenone
Cyclohexanol
10
<10
73
Figure 3. Effect of time on oxidation of cyclohexane catalyzed by com-
plex 1 in the presence of m-CPBA.
Ethylbenzene
Cyclohexane
16
3
78
Cyclohexanone
ε-Caprolactone
1-Phenylethanol
Acetophenone
Cyclohexanol
10
<10
69
80
Ethylbenzene
Cyclohexane
13
70
CyOH
4
77
CyO
60
Cyclohexanone
ε-Caprolactone
1-Phenylethanol
Acetophenone
Cyclohexanol
11
Caprolactone
<10
74
50
Ethylbenzene
Cyclohexane
14
40
30
20
10
0
5
74
Cyclohexanone
ε-Caprolactone
1-Phenylethanol
Acetophenone
Cyclohexanol
12
11
Ethylbenzene
Cyclohexane
68
12
6
76
Cyclohexanone
ε-Caprolactone
1-Phenylethanol
Acetophenone
Cyclohexanol
12
10
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
cyclohexane
Ethylbenzene
Cyclohexane
71
16
Figure 4. Effect of substrate concentration on oxidation of cyclohexane
catalyzed by complex 1.
7
74
Cyclohexanone
ε-Caprolactone
1-Phenylethanol
Acetophenone
Cyclohexanol
10
<10
72
Ethylbenzene
Cyclohexane
cyclohexanone by excess or unreacted m-CPBA. It is well known
that m-CPBA is a quantitative reagent for the conversion of cyclic
ketones to the corresponding lactones (Bayer–Villiger oxidation)
in the absence of any metal catalyst and so the ɛ-caprolactone
is not the metal-catalyzed product. Interestingly, only 50%
of the oxidized products were formed under air. Also, when
H2O2–t-BuOOH was used as the oxidant only trace amounts of
the oxidized products are formed, demonstrating that the ruthe-
nium(II) complexes are not effective as catalysts for these
oxidants. The most efficient conversions were found in the case
of cyclohexane with all catalysts (up to 78%), as can be seen in
Table 4. Similarly, ruthenium(II)-catalyzed oxidation of ethylben-
zene led to the formation of the corresponding alcohol and
ketone in yields of 76–68 % and 21–12 % respectively. On com-
paring the present catalytic system with that of other ruthe-
nium(II) complexes previously reported[76,77] for this conversion,
our new complexes showed better catalytic efficiency. The per-
cent yields were comparatively higher under the same reaction
conditions and media.
21
8
78
Cyclohexanone
ε-Caprolactone
1-Phenylethanol
Acetophenone
Cyclohexanol
10
<10
75
Ethylbenzene
Cyclohexane
18
9
75
Cyclohexanone
ε-Caprolactone
1-Phenylethanol
Acetophenone
Cyclohexanol
<10
<10
69
Ethylbenzene
Cyclohexane
17
Blanka
35
Cyclohexanone
ε-Caprolactone
1-Phenylethanol
Acetophenone
—
—
Ethylbenzene
27
—
aReaction carried out in the absence of catalyst.
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Appl. Organometal. Chem. 2014, 28, 18–26

Ru(II) complexes of hybrid 8-hydroxyquinoline–thiosemicarbazone ligands
Catalytic N-Alkylation of Amine
Conclusions
The ruthenium(II) hydroxyquinoline–thiosemicarbazone com-
plexes catalyzed the N-alkylation of amine with alcohols in the
presence of KOtBu. The yields were obtained by 1H NMR integra-
tion compared to the internal standard. As shown in the Table 5,
benzyl alcohols reacted smoothly to give the corresponding
products in good yields (71–86%) whereas, p-methoxybenzyl al-
cohol gave the corresponding product in moderate yield (up to
71%). Complexes containing L1 ligand is a significantly efficient
catalyst compared to the other complexes. When compared to
earlier reported systems, the catalyst loadings were very low
and the reaction conditions were very mild in the present cata-
lytic system.[78,79]
A series of new hexa-coordinated ruthenium(II) Schiff base
complexes were synthesized from hydroxyquinoline–
thiosemicarbazone-based ligands and ruthenium(II) precursor
complexes. An octahedral structure was tentatively proposed
for all the complexes based on the analytical and spectral results.
The complexes were employed as catalyst for organic reactions
such as aldehyde-to-amide conversion, alkane oxidation and
N-alkylation. The aldehyde-to-amide conversion reaction was car-
ried out in the presence of NH2OH.HCl and base. time and C:S ra-
tios were optimized for the above conversion and the effects of
different solvents and bases were also investigated. The maxi-
mum yield of amides was observed up to 86% for the substrate
4-nitrobenzaldehyde under optimized conditions. Alkane oxida-
tion was performed at room temperature in the presence
m-CPBA as oxidant and dichloromethane/acetonitrile solvent
mixture as reaction medium. The maximum yield of 78%
obtained for the oxidation of cyclohexane. N-Alkylation was car-
ried out at 110–150°C in the presence of KOtBu under nitrogen
atmosphere. N-Alkylation of benzylamine showed a maximum
yield of 86%. Overall, the new ruthenium complexes possess effi-
cient catalytic activities for the above three organic conversions.
Table 5. N-Alkylation of benzylamine catalyzed by ruthenium(II)
hydroxyquinoline–thiosemicarbazone complexes
OH
NH2
NH
Ru(II) complex
KOtBu, 100 °C, 6 h
+
Supporting information
Supporting information may be found in the online version of
this article.
R
R
(R = H or OCH3)
Acknowledgments
We are thankful to IISC, Bangalore, Department of Chemistry, MKU,
Madurai and SAIF, Cochin, for providing instrumental facilities.
Complex
1
Medium
Product
Yield
(%)a
Benzyl alcohol
p-Methoxybenzyl
alcohol
N-Phenylbenzylamine
N-Phenyl-
86
75
References
(p-methoxybenzyl)amine
N-Phenylbenzylamine
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