Synthesis and characterization of some new half-sandwich ruthenium(II) complexes with bidentate N,N′-ligands and their application in alcohol oxidation

Article abstract of DOI:10.1016/j.ica.2016.11.014

A series of eight new (η⁶-arene)ruthenium(II) complexes were prepared by the reaction of pyridyl-imine ligands and the ruthenium(II) precursors of the general formula [(η⁶-arene)Ru(μ-Cl)Cl]₂, where arene = p-cymene (1) and C₆H₆(2), to form the complexes [(η⁶-arene)RuCl(C₅H₄N-2-CH[dbnd]N-Ar)]PF₆(where Ar = 2,4,6-trimethylphenyl (a), 2,4-dimethylphenyl (b), 2-methoxyphenyl (c), 2,6-diisopropylphenyl (d))_.These complexes were characterized using 1H NMR, 13C NMR, 31P NMR, IR, UV–Vis, HRMS, and TGA. The molecular structures for the complexes 1a_,1d, 2a and 2d were determined by single crystal crystallography, revealing a pseudo-octahedral piano stool geometry. In this arrangement, the ruthenium metal is coordinated to the arene ligand at the apex of the stool with one chloride and the N,N-ligand as the base. These complexes were applied as catalysts in the oxidation of cyclic, aliphatic and aromatic alcohols with NaIO₄as oxidant and the complexes showed good conversions and yields to the corresponding carbonyl products.

Full text of DOI:10.1016/j.ica.2016.11.014

Accepted Manuscript
Research paper
Synthesis and characterization of some new half–sandwich ruthenium(II) com-
plexes with bidentate N,N’-ligands and their application in alcohol oxidation.
Joel M. Gichumbi, Holger B. Friedrich, Bernard Omondi
PII:
S0020-1693(16)30931-8
http://dx.doi.org/10.1016/j.ica.2016.11.014
ICA 17354
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
19 August 2016
11 November 2016
22 November 2016
Please cite this article as: J.M. Gichumbi, H.B. Friedrich, B. Omondi, Synthesis and characterization of some new
half–sandwich ruthenium(II) complexes with bidentate N,N’-ligands and their application in alcohol oxidation.,
Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.11.014
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1
Synthesis and characterization of some new half–sandwich ruthenium(II) complexes with
bidentate N,N’-ligands and their application in alcohol oxidation.
Joel M. Gichumbi, Holger B. Friedrich*, Bernard Omondi
1
School of Chemistry, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South
Africa
*Corresponding author. Tel.: +27 312603107; fax: +27 312603091. E-mail address:
friedric@ukzn.ac.za (H.B. Friedrich).
Abstract
A series of eight new (η6-arene)ruthenium(II) complexes were prepared by the reaction pyridyl-
imine ligands and the ruthenium(II) precursors of the general formula [(η6-arene)Ru(µ-Cl)Cl]2,
where arene = p-cymene (1) and C6H6 (2) to form the complexes [(η6-arene)RuCl(C5H4N-2-
CH=N-Ar)]PF6 (where Ar = 2,4,6-trimethylphenyl (a), 2,4-dimethylphenyl (b), 2-
1
methoxyphenyl (c), 2,6-diisopropylphenyl (d)). These complexes were characterized using H
NMR, 13C NMR, 31P NMR, IR, UV-Vis, HRMS, and TGA. The molecular structures for the
complexes 1a, 1d, 2a and 2d were determined by single crystal crystallography, revealing a
pseudo-octahedral piano stool geometry. In this arrangement, the ruthenium metal is coordinated
to the arene ligand at the apex of the stool with one chloride and the N, N-ligand as the base.
These complexes were applied as catalysts in the oxidation of cyclic, aliphatic and aromatic
alcohols with NaIO4 as oxidant and the complexes showed good conversions and yields to the
corresponding carbonyl products.
1.0 Introduction
The oxidation of alcohols to carbonyl compounds, such as aldehydes, ketones and carboxylic
acids, is an important reaction in organic synthesis [1]. Alcohol oxidation reactions are also
important in industrial applications [2]. Many transition metals have been applied for this
purpose and ruthenium compounds have been found to be very attractive for this transformation
[3-5]. Ruthenium based oxidation catalysis have shown good utility for selective oxygenation in
both homogeneous and heterogeneous catalysis [6]. This may be attributed to the ability of
ruthenium to assume a wide range of coordination geometries and oxidation states which offers
unique opportunities in application to catalysis [7]. Among the ruthenium complexes,
1

2
ruthenium(II) arene complexes have been found to be versatile intermediates in organic synthesis
because the arene ligand can be easily functionalized.
The aromatic π-ligand stabilizes and protects the metal center, which prevents rapid oxidation of
ruthenium(II) [8]. The ruthenium(II) arene complexes display a three-legged piano stool
structure, in which the metal centre is in octahedral geometry and the arene ligand occupies three
coordination sites. This structural feature creates the possibility of introducing different types of
ligands into the molecule [9].
Arene ruthenium complexes have been previously employed in the oxidation of alcohols. Singh
and co-workers reported the synthesis of half-sandwich complexes of the type [(η6-
arene)RuCl(L)]PF6 (where arene = p-cymene, benzene; L = N-(2-(aryl chalcogeno) ethyl)
morpholines, where the chalcogen can be sulphur, selenium, or tellurium) [10]. They then
applied these complexes in the oxidation of primary and secondary alcohols using a variety of
oxidants [10]. Ruthenium(II) complexes with tridentate (S, N, E, where E = S, Se or Te) type
ligands also have been reported as catalysts for the oxidation of alcohols with a variety of
oxidants [11]. Raja and Ramesh reported the application of cationic organoruthenium(II)
complexes with bidentate thiocarboxamide ligands and reported high yields in the presence of 4-
methylmorpholine-N-oxide (NMO) as a co-oxidant [3]. Kumar and coworkers have also reported
on the synthesis of a series of conformationally rigid half-sandwich complexes with naphthylazo
phenolato complexes. They used the complexes to catalyze the oxidation of a range of alcohols
to their corresponding carbonyl compounds [12].
There has been great intrest in biphasic catalysis because it allows for utilization of water as a
co-solvent [13]. Water is the most desirable solvent in catalysis because it is cheap, abundant,
non-flammable and nontoxic [13]. Herein we report the synthesis and characterization of new
complexes of the type [(η6-arene)RuCl(C5H4N-2-CH=N-Ar)]PF6 (where arene = p-cymene (1)
and C6H6 (2) and R = 2,4,6-trimethyl phenyl (a), 2,4-dimethyl phenyl (b), 2-methoxy phenyl (c)
and 2,6-diisopropyl phenyl (d)). The complexes were then applied as catalysts in biphasic
alcohol oxidation.
2.0 Experimental
2.1 Materials and Methods
Standard Schlenk techniques were used for all reactions. Nitrogen gas (AFROX) was dried over
phosphorous pentoxide. Reagent grade Et2O (Merck) was distilled over sodium /benzophenone
2

3
and stored over molecular sieves, ethanol (Merck) was dried over Mg/I2, acetonitrile (Merck)
was dried over P2O5 and stored over molecular sieves. Ruthenium trichloride hydrate (DLD),
anilines (Aldrich), pyridine-2-carbaldehyde (Aldrich) and ammonium hexafluorophosphate
(Aldrich) were used as supplied. Melting points were recorded on an Ernst Leitz Wetzlar hot-
stage microscope. Elemental analyses were obtained using a Thermoscientific Flash 2000
CHNS/O analyser. Infrared spectra were obtained using a Perkin Elmer spectrum 100 FTIR
spectrophotometer between 400 and 4000 cm-1 in the solid state. Electronic spectra were
recorded in acetonitrile with a Perkin-Elmer LAMBDA 35 UV-visible spectrophotometer. Mass
spectra were recorded on a Waters Micromass LCT Premier TOF-Ms instrument using direct
infusion and ESI operated in the positive mode. The mobile phase was acetonitrile (100%) and
10 µL of the sample was injected at 0.3 ml/min flow rate. NMR spectra were recorded on a
Bruker Topspin 400 MHz spectrometer. Thermogravimetric analyses (TGA) were performed on
a SDTQ600 TGA-DSC under a nitrogen atmosphere. The deuterated solvent DMSO-d6 (Aldrich)
was used as supplied The precursors [(η6-arene)Ru(µ-Cl)Cl]2, where arene = C6H6 or p-cymene,
were prepared following reported methods [13]. The pyridyl-imine ligands were synthesized
according to reported procedures [14-16].
2.2 General procedure for the preparation of the complexes
A mixture of [(η6-p-cymene)Ru(µ-Cl)Cl]2 (0.163 mmol), ligand (0.33 mmol) and NH4PF6 (0.41
mmol) in dry methanol (15 ml) was stirred at room temperature for 3 hours. The mixture was
then placed in an ice bath for 2 h to facilitate precipitation and the precipitate was filtered off,
washed with diethyl ether and dried in vacuo.
1a
Orange powder. Yield 80 %. m.p. 188 °C (decomp). 1H NMR (400 MHz, DMSO-d6). δ 9.62 (d, JHH
= 5.52 Hz, 1H, Py); δ 8.83 (s, 1H, CH=N); δ 8.30 (d, JHH = 7.44 Hz, 1H, Py); δ 8.21 (d, JHH = 7.5
Hz, 1H, Py); δ 7.92 (t, JHHH = 4.4 Hz, 1H, Py); δ 7.15 (s, 1H, Ar); δ 7.09 (s, 1H, Ar); δ 5.93 (d,
JHH = 6.36 Hz, 1H, p-cymene); δ 5.62 (d, JHH = 6.36 Hz, 1H, Ar, p-cymene); δ 5.58 (d, JHH = 6.12
Hz, 1H, Ar, p-cymene); δ 5.31 (d, JHH = 6.06 Hz, 1H, Ar, p-cymene); δ 2.67 (sep, 1H, CH(Me)2);
δ 2.35 (s, 3H, CH3); δ 2.28 ( s, 3H, CH3); δ 2.15 (s, 3H, CH3); δ 2.09 (s, 3H, p-Me(cymene)); δ
1.05 (t, JHHH = 5.0 Hz, 6H, CH-Me2 (cymene)). 13C NMR (400 MHz, DMSO-d6). δ173.18
(CH=N); δ 155.97 (Py); δ 154.04 (Py); 148.56 (Py); δ 140.03 (Py); δ 137.31 (Py); δ 130.9 (Ar); δ
3

4
129.81 (Ar); δ 129.22 (Ar); δ 128.29 (Ar); δ 106.33 (Ar); δ 100.40 (Ar); δ 87.18 (Ar, p-cymene);
δ 86.70; (Ar, p-cymene); δ 85.72 (Ar, p-cymene); δ 84.55 (Ar, p-cymene); δ 30.57 (CH); δ 22.03
(CH3); δ 21.58 (CH3); δ 20.42 (CH3); δ 19.57 (CH3); δ 18.04 (CH3); δ 17.92 (CH3). 31P NMR
(400 MHz, DMSO-d6).-131.0 to -157.4 (sep. PF6). IR (solid state): ν(-CH=N) 1607.7 cm-1, ν(P-
F) 832.9 cm-1. Anal. Calcd for [C25H30ClN2Ru]PF6 C, 46.72; H, 4.72; N, 4.38. Found: C, 46.18;
H, 4.45; N, 4.52. MS (ESI, M/Z): 495.11 [M+]
1b
Orange powder. Yield 80 %. m.p. 188 °C (decomp). 1H NMR (400 MHz, DMSO-d6). δ 9.60 (s, JHH
= 5.4 Hz, 1H, Py); δ 8.83 (s, 1H, CH=N); δ 8.32-8.23 (m, 2H, Py); δ 7.91-7.80 (m, 1H, Py); δ
7.59 (d, JHH = 8.08 Hz, 1H, Ar), δ 7.30 (s, 1H, Ar); δ 7.24 (d, JHH = 8.16 Hz, 1H, Ar); δ 6.03 (d,
JHH = 6.28 Hz, 1H, p-cymene); δ 5.70 (m, 2H, Ar, p-cymene); δ 5.30 (d, JHH = 6.04 Hz, 1H, Ar,
p-cymene); δ 2.59 (sep, 1H, CH(Me)2); δ 2.38 (s, 3H, CH3); δ 2.35 (s, 3H, CH3); δ 2.09 (s, 3H,
13
CH3); δ 1.02 (d, JHH = 6.88 Hz, 3H, p-Me(cymene)); δ 0.95 (d, JHH = 6.88 Hz, 3H, CH3). C
NMR (400 MHz, DMSO-d6). δ 170.77 (CH=N), δ 156.42 (Py), δ 154.89 (Py); δ 140.48 (Py); δ
138.70 (Py); δ 132.47 (Py); δ 130.46 (Ar); δ 129.57 (Ar); δ 127.71(Ar); δ 123.69 (Ar); δ 106.06
(Ar); δ 102.22 (Ar); δ 87.49 (Ar, p-cymene); δ 87.00 (Ar, p-cymene); δ 86.56 (Ar, p-cymene); δ
84.89 (Ar, p-cymene); δ 30.95 (CH); δ 22.5 (CH3); δ 21.6 (CH3); δ 21.12 (CH3); δ 18.58 (CH3);
δ 18.08 (CH3). 31P NMR (400 MHz, DMSO-d6).-131.0 to -157.4(sep. PF6). IR (solid state): ν(-
CH=N) 1608.1 cm-1 ν(P-F) 841.6 cm-1. Anal. Calcd for [C24H28ClN2Ru]PF6 C, 46.05; H, 4.51; N,
4.48. Found: C, 46.2; H, 4.52; N, 4.20. MS (ESI, M/Z): 481.10 [M+]
1c
Orange powder. Yield 70%. m.p. 189 °C (decomp). 1HNMR (400 MHz, DMSO-d6). δ 9.60 (d, JHH
5.4 Hz, 1H, Py); δ 8.89 (s, 1H, CH=N); δ 8.29 (m, 2H, Py); δ 7.92-7.90 (m, 1H, Py); δ 7.52-7.48
(m, 2H, Ar), δ 7.40 (d, JHH = 8.32 Hz, 1H, Ar); δ 7.18 (t, JHHH = 7.56 Hz, 1H, Ar); δ 6.08 (d, JHH
=
=
6.24 Hz, 1H, p-cymene); δ 5.71 (d, JHH = 6.2 Hz, 1H, p-cymene); δ 5.55 (d, JHH = 6.12 Hz, 1H,
Ar, p-cymene); δ 5.34 (d, JHH = 6.08 Hz, 1H, Ar, p-cymene); δ 3.94 (s, 3H, -OCH3); δ 2.41 (sep,
1H, CH(Me)2); δ 2.13 (s, 3H, p-Me(cymene)); δ 0.97 (q, 6H, CH-Me2, p-cymene). 13CNMR (400
MHz, DMSO-d6). δ 171.1 (CH=N); δ 156.07 (Py); δ 154.26 (Py); δ 148.95 (Py); δ 140.32 (Py); δ
130.22 (Py); δ 129.09 (Ar); δ 124.50 (Ar); δ 120.64 (Ar); δ 112.93 (Ar); δ 104.01 (Ar); δ 103.61
4

5
(Ar); δ 87.01 (Ar, p-cymene); δ 86.22 (Ar, p-cymene)); δ 85.04; (Ar, p-cymene); δ 84.7 (Ar, p-
31
cymene); δ 56.23 (-OCH3); δ 30.45 (CH); δ 21.55 (CH3); δ 21.35 (CH3); δ 18.29 (CH3). P
NMR (400 MHz, DMSO-d6).-131.0 to -157.4 (sep. PF6). IR (KBr, cm-1) ν(-CH=N) 1610.6 cm-1
ν(P-F) 831.9 cm-1. Anal. calcd for [C23H26ClN2ORu]PF6 C, 43.99; H, 4.17; N, 4.46. Found: C,
43.56; H, 3.94; N, 4.58.MS (ESI, M/Z): 483.08 for [M+]
1d
Orange powder. Yield 80 %. m.p. 188 °C (decomp). 1H NMR (400 MHz, DMSO-d6). δ 9.68 (d, JHH
= 5.44 Hz, 1H, Py); δ 9.00 (s, 1H, CH=N); δ 8.35-8.28 (m, 2H, Py); δ 7.97-7.94 (m, 1H, Py); δ
7.51-7.48 (m, 2H, Ar); δ 7.41-7.39 (m, 1H, Ar); δ 6.03 (d, JHH = 6.32Hz, 1H, Ar, p-cymene); δ
5.65 (d, JHH = 6.28 Hz, 1H, Ar, p-cymene); δ 5.49 (d, JHH = 6.12 Hz, 1H, Ar, p-cymene); δ 5.34
(d, JHH = 6.08 Hz, 1H, Ar, p-cymene); δ 3.74 (sep, 1H, CH(Me)2); δ 2.63 (sep, 2H, CH(Me)2); δ
2.12 (s, 3H, CH3); δ 1.45 (d, JHH = 6.76 Hz, 3H, CH3); δ 1.26 (d, JHH = 6.76 Hz , 3H, CH3); δ 1.14
(d, JHH = 6.64 Hz, 3H, CH3); δ 1.05 (d, JHH = 6.88 Hz, 6H, (CH3)2); δ 0.838 (d, JHH = 6.68 Hz,
13
3H, CH3). C NMR (400 MHz, DMSO-d6). δ 173.31 (CH=N); δ 156.25 (Py); δ 153.87 (Py); δ
148.12 (Py); δ 141.14 (Py); δ 140.41 (Py); δ 139.54 (Ar); δ 129.85 (Ar); δ 129.48 (Ar); δ 124.43
(Ar); δ 105.51 (Ar); δ 101.51 (Ar); δ 86.8 (Ar, p-cymene); δ 85.95 (Ar, p-cymene); δ 84.7 (Ar, p-
cymene); δ 30.3 (CH); δ 27.6 (CH); δ 27.1 (CH); δ 26.5 (CH3); δ 25.7(CH3); δ 23.06 (CH3); δ
22.19 (CH3); δ 21.57 (CH3); δ 21.25 (CH3); δ 17.88 (CH3). 31P NMR (400 MHz, DMSO-d6).-
131.0 to -157.4 (sep. PF6). IR (KBr, cm-1) ν(-CH=N)1615.9 cm-1, ν(P-F) 829.3 cm-1. Anal. Calcd
for [C28H36ClN2Ru]PF6, C, 49.30; H, 5.32; N, 4.11. Found: C, 48.90; H, 4.99; N, 3.92. MS (ESI,
M/Z): 537.16 [M+]
2a
Yellow powder. Yield 80 %. m.p. 213 °C(decomp). 1H NMR (400 MHz, DMSO-d6): δ 9.68 (d, JHH
= 5.0 Hz, 1H, Py); δ 8.84 (s, 1H, CH=N); δ 8.33 (t, JHHH = 7.28 Hz, 1H,Py); δ 8.21 (d, JHH = 7. 32
Hz, 1H, Py); δ 7.93 (t, JHHH = 5.72 Hz, 1H, Py); δ 7.15 (s, 1H, Ar); δ 7.10 (s, 1H, Ar); δ 5.88 (s,
13
6H, C6H6); δ 2.36 (s, 3H, CH3); δ 2.33 (s, 3H, CH3); δ 2.14 (s, 3H, CH3). C NMR (400 MHz,
DMSO-d6): δ 173.61 (CH=N); δ 156.34 (py); δ 153.99 (py); δ 148.91 (py); δ 140.19 (py); δ
137.44 (py); δ 130.93 (Ar); δ 130.07 (Ar); δ 129.84 (Ar); δ 129.12 (Ar); δ 129.03 (Ar); δ 128.44
(Ar); δ 87.28 (C6H6); δ 20.47 (CH3); δ 19.69 (CH3); δ18.1 (CH3). 31P NMR (400 MHz, DMSO-
5

6
d6).-131.0 to -157.4 (sep. PF6). IR (KBr, cm-1) ν(-CH=N) 1608.1 cm-1 ν(P-F) 828.7 cm-1. Anal.
Calc. for [C21H22ClN2Ru]PF6 C, 43.20; H, 3.80; N, 4.80. Found C, 43.09; H, 3.87; N, 4.80. MS
(ESI, M/Z): 439.05 [C21H22ClN2Ru]+
2b
Yellow powder. Yield 70 %. m.p. 183 °C (decomp).1H NMR (400 MHz, DMSO-d6). δ 9.68 (d, JHH
= 5.46 Hz, 1H, Py); δ 8.83 (s, 1H, CH=N); δ 8.33 (d, JHH = 7.44 Hz, 1H, Py); δ 8.26 (d, JHH
=
7.44 Hz, 1H, Py); δ 7.91(q, 1H, Py); δ 7.61 (d, JHH = 7.92 Hz, 1H, Ar); δ 7.31 (s, 1H, Ar); δ 7.24
13
(d, JHH = 8.1 Hz, 1H, Ar); δ 5.90 (s, C6H6); δ 2.40 (s, 3H); δ 2.37 (s, 3H). C NMR (400MHz,
DMSO-d6). δ 171.18 (CH=N); δ 156.74 (Py); δ 154.91 (Py); δ 140.55 (Py); δ138.60 (Py); δ
132.39 (Py); δ 130.47 (Ar); δ 129.32 (Ar); δ 127.78 (Ar); δ 123.33 (Ar); δ 87.79 (Ar); δ
21.13(CH3). 31P NMR (400 MHz, DMSO-d6).-131.0 to -157.4 (sep. PF6). IR (solid state): ν(-
CH=N) 1609.5 cm-1 ν(P-F) 830.9 cm-1. MS (ESI, M/Z): 425.04 [M+]. Anal. Calc. for
[C20H20ClN2Ru]PF6 C, 40.86; H, 3.77; N, 4.77. Found C, 41.30; H, 3.74; N, 4.74. MS (ESI,
M/Z): 439.05 [C19H18ClN2ORu]+.
2c
Orange powder. Yield 70 %. m.p. 180 °C(decomp). 1H NMR (400 MHz, DMSO-d6). δ 9.68 (d, JHH
= 5.4 Hz, 1H, py); δ 8.87 (s, 1H, CH=N); δ 8.30 (d, JHH = 7.44 Hz, 2H, py); δ 8.28 (q, 1H, py); δ
7.91-7.88 (m, 1H, Ar); δ 7.68-7.66 (m, 1H, Ar); δ 7.47-7.36 (m, 1H, Ar); δ 7.16-7.12 (m, 1H,
Ar); δ 5.90 (6H, C6H6); δ 3.94 (s, 3H, -OCH3). 13C NMR (400 MHz, DMSO-d6). δ 171.49
(CH=N); δ 156.40 (Py); δ 154.29 (Py); δ 149.09 (Py); δ 140.50 (Py); δ 140.14 (Py); δ 130.16
(Ar); δ 130.08 (Ar); δ 128.94 (Ar); δ 124.22 (Ar); δ 120.66 (Ar), δ 113.05 (Ar); δ 87.25 (C6H6);
31
δ 56.34 (-OCH3). P NMR (400 MHz, DMSO-d6).-131.0 to -157.4 (sep. PF6). IR (solid state):
ν(-CH=N) 1618.4 cm-1 ν(P-F) 837.2 cm-1. Anal. Calc. for [C19H18ClN2ORu]PF6 C, 39.91; H,
3.10; N, 4.90. Found C, 40.22; H, 2.51; N, 4.70. MS (ESI, M/Z): 427.01 [C19H18ClN2ORu]+.
2d
1
Orange powder. Yield 80 %. m.p. 213 °C. H NMR (400 MHz, DMSO-d6). δ 9.72 (d, JHH = 5.4
Hz, 1H, py); δ 8.99 (s, 1H, CH=N); 8.32-8.25 (m, 2H, Py); δ 7.95-7.92 (m, 1H, Py); δ 7.51-7.41
(m, 3H, Ar); δ 5.87 (s, C6H6); δ 3.71 (sep, 1H, CH(CH3)2), δ 2.58 (sep, 1H, CH(CH3)2); δ 1.38
6

7
(d, JHH = 6.76 Hz, 3H, CH3); 1.26 (d, JHH = 6.68 Hz, 3H,CH3); δ 1.11 (d, JHH = 6.72 Hz, 3H,
CH3); δ 0.92 (d, JHH = 6.72 Hz, 3H, CH3).13C NMR (400 MHz, DMSO- d6). δ 173.13 (CH=N); δ
156.47 (Py); δ 153.86 (Py); δ 148.26 (Py); δ141.12 (Py); δ 140.43 (Py); δ 139.52 (Ar); δ 129.92
(Ar); δ 129.15 (Ar); δ 129.10 (Ar); δ 124.67 (Ar); δ 124.35 (Ar); δ 87.09 (C6H6); δ 27.45 (CH); δ
27.23 (CH); δ 26.34 (CH3), δ 25.16 (CH3); δ 22.88 (CH3); δ 22.39 (CH3). 31P NMR (400 MHz,
DMSO-d6).-131.0 to -157.4 (sep. PF6). IR (KBr, cm-1) ν(-CH=N)1616.0 cm-1 ν(P-F) 827.7 cm-1.
Anal. Calc. for [C24H28ClN2Ru]PF6 C, 46.05; H, 4.51; N, 4.86. Found C, 46.06; H, 4.50; N, 4.43.
MS (ESI, M/Z): 481.09 [C24H28ClN2Ru]+.
2.3 X-ray crystal structure determination
Crystals of compounds 1c, 1d, 2a.H2O and 2d suitable for x-ray diffraction studies were grown
using the solvent diffusion method. Thus dry acetone solutions of the compounds were layered
with a fourfold excess of hexane and then allowed to stand undisturbed at room temperature in
the dark for 2 days. Crystals of the compounds 1c, 1d, 2a.H2O and 2d were chosen, glued onto
the tip of glass fibres and mounted in a stream of cold nitrogen at 173(2) K. They were then
centred in the x-ray beam using a video camera. The crystal evaluation and data collection were
performed on a Bruker Smart APEXII diffractometer. The diffractometer to crystal distance was
4.00 cm. The initial cell matrix was obtained from three series of scans at different starting
angles. Each series consisted of 12 frames collected at intervals of 0.5 ˚in a 6˚ range with an
exposure time of about 10 s per frame. The reflections were successfully indexed by an
automated indexing routine built into the APEX II programme suite [17]. The data collection
method involved ω scans of width 0.5˚. The final cell constants were calculated from a set of
6460 strong reflections from the actual data collection. Data reduction was carried out using the
programme SAINT+ [17] and the structure solved by direct methods using SHELXS [18] and
refined by SHELXL [17]. All H atoms were refined isotropically. The absorption correction was
based on fitting a function to the empirical transmission surface as sampled by multiple
equivalent measurements [17]. Disorder was found in hexafluorophosphate molecules in
compounds 1d and 2d and those were refined using PART instructions with the major
component of the disorder being 68 and 65% for 1d and 2d respectively. All atoms in the anion
in 1d were refined with ISOR. Disorder was also found in an arene ring of one of the two
molecules in 2d which was also modelled using PART instructions in the INS file. The atoms of
7

8
the ring were refined with ISOR instruction. Crystal data and structure refinement information
for compounds 1c, 1d, 2a.H2O and 2d are summarized in Table1. Packing diagrams are shown
in the supplementary section.
Table 1.Summary of the crystal data of 1c, 1d, 2a.H2O and 2d
Compound
1c
C23H26ClN2ORu
PF6
1d
2a.H2O
2d
Formula
C28H36ClN2Ru
PF6
C21H22ClN2RuOPF C24H25ClN2RuPF6
6
Formula weight
627.95
682.08
Monoclinic
173(2)
0.71073
P21/c
599.89
Triclinic
173(2)
625.95
Crystal system
T, K
Monoclinic
173(2)
orthorhombic
173(2)
λ, Å
0.71073
P21/c
0.71073
0.71073
ꢀ
P1
Space group
a, Å
Pna21
18.3865(13)
10.3533(7)
27.1288(17)
90
17.2751(9)
10.0882(5)
20.4604(9)
90
8.7213(5)
11.3449(6)
12.4760(7)
104.4890(10)
104.2320(10)
98.4360(10)
1129.59(11)
2
17.6534(11)
b, Å
11.8893(8)
c, Å
23.7635(16)
α, °
90
β, °
98.7150(10)
90
125.304(3)
90
90
γ, °
90
V, Å
5104.6(6)
8
2910.0(3)
4
4987.6(6)
Z
8
ρcalcd, Mg/m3
µ, mm-1
F(000)
Cryst size, mm3
1.634
1.557
1.764
1.659
0.845
0.745
0.950
0.861
2528
1392
600
2504
0.230 x 0.130 x 0.240x
0.230x 0.140
0.120
x
0.130x 0.180 x 0.160 x
0.080
0.110
0.180
Theta range for 1.519 to 28.479
data collection, °
1.444 to 28.000
1.764 to 28.479
1.714 to 28.320
Index ranges
-24≤h≤24,
-13≤k≤13,
-22≤h≤22,
-13≤k≤13,
-11≤h≤11,
-15≤k≤15,
-19≤h≤23,
15≤k≤15,
8

9
-36≤l≤36
-23≤l≤27
-16≤l≤16
-31≤l≤31
Reflections
collected
76252
61681
28474
71556
Independent
reflections
12844 [R(int) =
0.0595]
6969 [R(int) =
0.0272]
5692 [R(int) =
0.0441]
12397 [R(int) =
0.0577]
Completeness to 100
99.8
100
100
θ = 25.242°, %
Absorption
corrections
Semi-empirical
Semi-empirical
from equivalents
0.875 and 0.836
Semi-empirical
from equivalents
0.892 and 0.875
Semi-empirical
from equivalents
0.933 and 0.856
from equivalents
Max.
trans
&
min. 0.911 and 0.823
Refinement
method
Full matrix least
squares on F2
Full matrix least
squares on F2
6969 / 0 / 349
Full matrix least
squares on F2
Full matrix least
squares on F2
Data / restraints / 12854 / 0 / 639
parameters
5692 / 72 / 356
12397 / 1 / 629
Goodness-of-fit
on F2
1.013
1.101
1.049
1.022
Final R indices
0.0355, 0.0742
0.0717, 0.1639
0.0753, 0.1662
0.0311, 0.0707
0.0396, 0.0742
0.0393, 0.0900
0.0555, 0.0978
R
indices (all 0.0570, 0.0827
data)
Largest diff. peak 0.670 & -0.416
& hole, e. Å-3
2.531 &-2.757
0.468 & -0.538
0.926 & -0.557
2.4 Alcohol oxidation reactions
The catalytic reactions were performed in Schlenk tubes in an oil bath maintained at 80 ˚C. All
products were analyzed using a Perkin Elmer Auto system gas chromatograph fitted with a flame
ionization detector (FID) set at 290 ˚C and a Varian capillary column (25 mm x 0.15 mm x 2
µm). The injector temperature was set at 240 ˚C. The substrate to catalyst ratio was 1:200.
Biphenyl was used as an internal standard. The identity of the products was assessed by
9

10
comparing their retention times with commercially available (Aldrich) standards, supported by
GC-MS analysis.
2.4.1 General procedure for catalytic oxidation of alcohol
Alcohol (0.5 mmol), the catalyst (0.5 mol %) and internal standard (0.5 mmol) were dissolved in
a 1:1 mixture of water and tert-butanol (6 ml). NaIO4 (3 equivalents) was then added in one
portion and the solutions were vigorously stirred in an oil bath with the temperature maintained
at 80 ˚C. The resultant solution was sampled at intervals, whereby an aliquot was removed and 1
µL was injected and analyzed by GC. Control experiments in the absence of either the catalyst or
NaIO4 were performed under identical conditions.
3.0 Results and discussion
3.1 Syntheses and characterization of arene ruthenium complexes
The bimetallic arene ruthenium precursors [(η6-arene)Ru(µ-Cl)Cl]2 reacted in methanol with the
N, Nʹ-bidentate ligands in a 1:2 ratio to give the new mononuclear cationic complexes [(η6-
arene)RuCl(C5H4N-2-CH=N-Ar)]PF6 (where Ar = 2,4,6-trimethyl phenyl (a), 2,4-dimethyl
phenyl (b), 2-methoxy phenyl (c), 2,6-diisopropyl phenyl (d)) (Scheme1), which were isolated as
their hexafluorophosphate salts. The complexes were isolated as yellow or orange air stable
solids, which were not hygroscopic, in high yields. They were soluble in acetonitrile, acetone,
DMSO, DCM and insoluble in hexane and diethyl ether.
10

11
Scheme 1: Synthesis of [(η6-arene)RuCl(C5H4N-2-CH=N-R)]PF6 (x = 2, 4, 6- trimethyl (a); 2,4-
dimethyl (b); 2-methoxy (c); 2, 6-diisopropyl (d)).
Binding of the ligands to the arene ruthenium precursor was confirmed by comparing the imine
1
proton and carbon signals of the free and coordinated ligands in their NMR spectra. The H
NMR spectra of the complexes 1a - 1d and 2a - 2d display a singlet signal for the imine proton
at 8.83 -8.90 ppm and this signal showed an up-frequency shift when compared to the imine
proton of the free ligand, which occurred at 8.21 - 8.84 ppm (Table 2). This shift is due to
deshielding of the imine nitrogen upon coordination and this observation has also been seen for
1
similar ruthenium(II) bidentate complexes [19]. In addition, the H NMR spectra of the
complexes 1a - 1d suggest a loss of the two-fold symmetry of the p-cymene arene once the N, N-
bidentate ligands are coordinated to the metal. The evidence for the symmetry loss is the
appearance of a set of four doublets in the region of 5.30-6.08 ppm for the protons of the p-
13
cymene rings in the complexes 1a to 1d. In addition the C NMR spectra of the imine protons
also showed a shift in the -C=N carbon peak from the region 159.5 - 163.5 ppm for the free
ligands to the region 170.8 - 173.6 ppm for the complexes (Table 2), supporting the coordination
of these ligands to the ruthenium through the imine nitrogen.
13
Table 2. 1H NMR, C NMR and IR shifts for the imine functionalities of compounds 1a-d and
2a-d
Complex 1H NMR
13C NMR
Imine functionality (C=N) cm-1
11

12
Ligand Complex Ligand
Complex
173.2
Ligand
1639
1628
1631
1632
1639
1628
1631
1632
Complex
1607
8.70
8.84
8.56
8.21
8.70
8.84
8.56
8.21
8.83
8.83
8.89
8.90
8.84
8.83
8.87
8.90
163.5
159.5
161.4
163.2
163.5
159.5
161.4
163.2
1a
1b
1c
1d
2a
2b
2c
2d
1608
170.8
1610
171.1
1616
173.31
173.61
171.2
1608
1609
1618
171.5
1616
173.2
The infrared spectra for the mononuclear complexes 1a - 1d and 2a - 2d further confirmed the
binding of the ligands to the complexes. The complexes showed shifts of the bands assigned to
(C=N)
stretching frequencies, which suggests coordination of the ruthenium to the imine
imine
nitrogen. The (C=N) imine stretching frequency shifted from around 1628-1639 cm-1 to the lower
frequency of 1607-1618 cm-1 for the complexes (Table 2). The shift in the (C=N)imine may be
attributed to the electron withdrawing effects from the coordinated metal, which weakens the
(C=N)
bond and thus pushes the stretching frequency to a lower frequency [19]. The
imine
complexes 1a - 1d and 2a - 2d also exhibit a strong band at around 836-845 cm-1 due to the
stretching υ(P-F) mode of the counter ion of these complexes. In addition the presence of the
-
PF6 counter ion was established by 31P NMR and all complexes showed a septet in the region of
-131.0 to -157.4 [20,21].
High resolution mass spectrometry revealed the peaks corresponding to the expected molecular
[M+] ions with the characteristic Ru isotopic pattern. Elemental analyses further confirm the
composition of these compounds. The data obtained for these complexes thus support the
proposed structures.
UV-Vis spectra of the complexes 1a - 1d and 2a - 2d were acquired in acetonitrile and the
spectra further confirm the formation of the complex (Figure 1). The low spin d6 configuration of
these complexes supplies filled orbitals of correct symmetry at the Ru(II) centers which can
interact with the low lying π* orbital of the ligands. Hence, intense bands in the range 216 - 222
nm and 282 - 312 nm can be assigned to the π-π*/n-π* transitions. The complexes also exhibit
12

13
low intensity bands in the region 400 - 423 nm assignable to the metal-to-ligand charge transfer
(MLCT) (dπ-π*) transition. This is in agreement with observations on similar complexes [19].
1
0.9
0.8
0.7
0.6
0.5
1a
1b
1c
1d
0.4
2a
0.3
2b
0.2
2c
0.1
2d
0
200
250
300
350
400
450
500
550
600
Wavelength(nm)
Figure 1: UV-Vis Spectral data for 1a - 1d and 2a - 2d
The differential thermograms of the complexes 1a - 1d and 2a - 2d were obtained between 30˚C
and 600˚C under a nitrogen atmosphere at a ramp rate of 10˚C per minute. The thermograms of
the compounds presented two main weight mass-loss steps during the decomposition of the
compounds. The first step occurs between 190 and 310 ˚C and the second step occurs between
310 and 440 ˚C (Figure 2). The first weight loss in all cases was approximately 30 % which may
be attributed to the loss of the bidentate N, N- ligand. The second decomposition step of
approximately 20% may be attributed to the loss of the arene ring.
13

14
95
85
75
65
55
45
35
1a
1b
1c
1d
2a
2b
2c
2d
20
120
220
320
420
Temperature (°C)
Figure 2: TGA curves for 1a-1d and 2a- 2d
3.2 Crystal and molecular structures
The crystal structure of the complexes 1c, 1d, 2a.H2O and 2d are shown in Figures 3, 4, 5 and 6
respectively. Selected inter-atomic distances and angles are shown in Table 3. All four
compounds have the cationic species [(η6-arene)RuCl(C5H4N-2-CH=N-Ar)]+ and the anionic
species PF6¯ . However, compounds 1c and 2d have two of each in their asymmetric units, 1d has
one of each and 2a.H2O has one of each and one third of a water molecule in its asymmetric
unit. In all compounds, the ruthenium centre is coordinated to the N,Nʹ-bidentate ligand through
the N atoms of the pyridine ring and the imine, to a Cl atom, and also to the arene ring, resulting
in a pseudo octahedral geometry, typically referred to as piano-stool geometry. In this
arrangement the ligands through the two N atoms and the Cl atom form the base of the stool and
the arene ring the apex of the stool. The ligands in the cationic species in all four compounds
have two sets of planes defined by the pyridine ring including the imine N atom and the
substituted phenyl ring. The two planes are twisted away from each other, seemingly driven by
the steric effects of the substituents on the phenyl ring. The torsion angles C—Nimine—CPh—CPh
are larger in 1c and 2a.H2O, which have OMe and Me substituents and smaller in 1d and 2d,
which have iPr as substituents (Table 4). It is not clear if the benzene and the p-cymene rings
play any role in the torsion angle mentioned above, but as pointed out by Ruzziconi et al., the
cymene moiety tends to sterically be as far as possible from the Cl atom such that it’s isopropyl
14

15
substituent is on the opposite side of the Cl ligand [22,23]. In their compound, the isopropyl
substituent was on the same side as the Cl ligand. This phenomenon, however, has also been
observed in some related compounds [21,24-26].
The Ru—N bond distances of the compounds lie between 2.059(2) (2) Å and 2.108(4) Å. These
values are comparable to those reported for other arene ruthenium complexes with N,N’ donor
ligands [21,22,24-26] and within the reported values of half-sandwich complexes of [(η6-p-
cymene)RuCl2]2 with nitrogen ligands, which range from 2.060(5) Å to 2.156(2) Å [19,22,26].
The N—Ru—N bond angles range from 76.45(9) to 77.03(19) ° and the N-Ru-Cl bond angles
range from 80.31(11) to 88.91(6) °. These values are close to those reported for related
compounds [11,19,21,22,25].
Figure 3. The molecular structure of 1c showing the atom numbering scheme of one of the two
molecules in the asymmetric unit. Displacement ellipsoids are drawn at the 50% probability level
and the H atoms along with two molecules have been omitted for clarity purposes.
15

16
Figure 4. The molecular structure of 1d showing the atomic numbering scheme. Displacement
ellipsoids are drawn at the 50% probability level and the C atoms are shown as small spheres
Figure 5. The molecular structure of 2a.H2O showing the atomic numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level and the C atoms are shown as
small spheres
16

17
Figure 6. The molecular structure of 2d showing the atom numbering scheme of one of the two
molecules in the asymmetric unit. Displacement ellipsoids are drawn at the 50% probability level
and the H atoms along with two molecules have been omitted for clarity purposes.
17

18
Table 3: Selected bond lengths (Å) and angles (deg).
1c
1d
2a.H2O
2d
Bond distance
Ru—NPy
2.077(2)
2.074(2)
2.059(2)
2.065(2)
2.3921(6)
2.3707(7)
2.079(5)
2.108(4)
2.3823(13)
2.0811(19)
2.0851(18)
2.3985(6)
2.099(5)
2.103(5)
2.099(5)
2.101(5)
2.3906(15)
2.3116(16)
Ru--NImine
Ru—Cl
Bond angles
NPy—Ru—NImine
76.78(8)
76.47(9)
86.65(6)
84.34(6)
83.26(6)
84.38(6)
76.70(17)
80.31(13)
89.22(11)
94.30
76.88(7)
81.94(5)
88.91(5)
118.32
77.16(19)
77.00(18)
80.44(13)
83.22(13
87.40(14)
87.02(14)
110.82
NPy—Ru—Cl
NImine—Ru—Cl
C—Nimine—CPh—CPh 128.00
124.18
100.09
3.3 Oxidation catalysis
3.3.1 Optimization
The synthesized complexes were investigated as catalysts in the oxidation of alcohols to ketones
or aldehydes. The reaction parameters such as catalyst loading, temperature, choice of oxidant,
solvent and oxidant to substrate ratio were varied to establish optimum reaction conditions. The
oxidation of benzyl alcohol to benzaldehyde was chosen as the model reaction for optimization
using complex 1a. First, the temperature was varied from 25°C to 60°C and 80°C. Best yield and
conversion and shortest reaction time was found at 80°C. A mixture of water and solvents in a
ratio of 1:1 were investigated. The three solvents investigated were acetonitrile, toluene and tert-
butanol. tert-Butanol was found to be the best solvent for the reactions, likely because of the
better solvation of the catalyst and substrate in this solvent. Four oxidants: tert-
butylhydroperoxide, H2O2, N-morpholine oxide and NaIO4 were tested. NaIO4 gave the best
18

19
yields and conversions within a short time. The oxidant to substrate ratio was investigated and
the best ratio was found to be three equivalents, below which the reaction took a longer time to
be completed and above which there was no change in conversion and yield. The catalyst loading
was also investigated and 0.25 mol%, 0.5 mol% and 1 mol% of the catalyst were considered. The
best conversion was obtained at 0.5 mol %, below which the reaction took a longer time to reach
completion, while a higher loading showed no change in conversion and yield. In addition, blank
studies were carried out. Using the catalyst without the oxidant showed no conversion and the
oxidant without the catalyst showed about 3 % conversion within the time limit of the catalytic
runs.
3.3.2 Catalytic results
Having established the best reaction conditions as 0.5 mol% catalyst loading, a substrate:NaIO4
ratio of 1:3 at 80 °C in tert-butanol, the catalysts were screened for the oxidation of benzyl
alcohol. All the catalysts, 1a to 1d and 2a to 2d showed high conversion and yields within half
an hour to 1 hour of reaction as shown in Table 4, with catalysts 1b showing the highest TON
and TOF. It was observed that reactions with the catalysts with the p-cymene ring reached
completion within half an hour compared to those with the benzene ring which took one hour.
This may be attributed to the cymene fragment being better at stabilizing the high oxidation state
of the proposed Ru(VI) oxo intermediate than the benzene fragment and therefore making the
reaction faster.
Table 4. Oxidation of Benzyl alcohol results for catalysts 1a-d and 2a-d
Catalyst
1a
Conversion
Yield
95
Time (h)
TONa
191
199
189
196
190
199
189
199
TOFb (h-1)
98
100
99
0.5
0.5
0.5
0.5
1
382
99
398
1b
90
378
1c
100
100
99
96
392
1d
91
190
2a
99
1
199
2b
100
100
90
1
189
2c
99
1
199
2d
aTON: mol of product/mol of catalyst
bTOF: mol of product/(mol. of catalyst x time)
19

20
Having established the optimum conditions for the reaction, the study was extended to other
alcohols aimed at establishing its scope and limitations (Table 5). For the extended study catalyst
1b was used, since it gave the highest catalytic TON of the respective series. To investigate the
steric effect on the substrate oxidation, the oxidation of 2-methyl and 4-methyl cyclohexanol was
investigated. In the oxidation of cyclohexanol to cyclohexanone it was observed that introducing
an electron donating group to the 2-position of cyclohexanone extended to the time required to
reach maximum conversion to 2 hours. However, a higher conversion in a shorter time was
obtained when the same electron donating group was in the 4-position. This may be attributed to
the steric interference that resulted in limited access to the reactive site in the 2-substituted
substrate.
In addition, to investigate the possible effects of the steric interactions on the oxidation of
straight chain alcohols, 3-pentanol and 2-pentanol were considered. 2-Pentanol was oxidized to
2-pentanone in a shorter reaction time than 3-pentanol to 3-pentanone. This may be attributed to
the lower steric hindrance, which makes the alcohol functional group more accessible in 2-
pentanol as compared to 3-pentanone.
In general, the cyclic aliphatic alcohols gave better conversions than their straight chain
counterparts, which may be attributed to the accessibility of the C-OH bond in the relatively rigid
cyclic compounds, as compared to the more sterically hindered reactive site in the straight chain
alcohols that led to longer reaction times and diminished conversion of the alcohol.
When these complexes are compared to other ruthenium-arene complexes reported by Singh and
co-workers, the complexes had lower TONs and TOFs [10], in addition the complexes reported
gave comparable yields to those over the ruthenium complexes reported by Raja and Ramesh,
who reported high yields in the presence of 4-methylmorpholine-N-oxide (NMO) as a co-oxidant
[3]. The complexes showed lower TONs and TOFs when compared to the conformationally rigid
half-sandwich complexes with naphthylazo phenolato complexes reported by Kumar and
coworkers [12].
20

21
Table 5. Oxidation of alcohols results for catalysts
Entry
1
Substrate
Product
Conversion
Yield
Time (h) TONa
TOFb (h-1)
cyclohexanol
96
95
1
190
170
192
190
O
O
2-methyl
89
97
86
96
1
170
384
2
3
cyclohexanol
O
4-methyl
0.5
cyclohexanol
3-pentanol
2-pentanol
98
97
97
99
2
194
194
199
97
4
5
6
O
O
98
1
194
398
4-methyl
100
0.5
O
H
benzyl alcohol
aTON: mol of product/mol of catalyst
bTOF: mol of product/(mol. of catalyst x time)
When 0.5 mol% of the ruthenium precursor [(η6-p-cymene)Ru(µ-Cl)Cl]2 was used as the catalyst
in the oxidation of the benzyl alcohol, the reaction took 5 h to reach completion with a
benzaldehyde yield of 92%. It thus was observed that this reaction was slow despite having twice
the concentration of ruthenium. When the ruthenium precursor and the ligand b were mixed in
the ratio of 1:2 and used in situ for the oxidation of benzyl alcohol, the reaction took three hours.
This suggests that the ligand plays a role in this oxidation system and, also, that the formed
complex is important.
Once 100% substrate conversion was achieved over catalyst 1b, studies were done to investigate
whether the used catalyst remaining in the reaction mixture was catalytically active. The reaction
mixture was able to catalyze the oxidation of fresh benzyl alcohol to give 100 % conversion and
21

22
more than 90 % yield to benzaldehyde. There was an increase in benzoic acid concentration and
this may be attributed to the catalytic intermediate being more active than the catalyst. Three
further additions of benzyl alcohol were done and a steady decrease in conversion and
benzaldehyde yield from 90% for the first cycle to 73% for the fourth cycle was observed. This
may be due to a gradual decomposition or deactivation of the catalyst. Also, the steady increase
of the product concentration in the mixture may contribute to the reduced activity. In addition,
when the catalyst 1b was reacted with NaIO4 and the isolated product used in the oxidation of
benzyl alcohol, the reaction took one hour to reach completion.
Earlier work on elucidating the mechanism of catalytic oxidation using ruthenium complexes
reported ruthenium-oxo species as the reactive intermediates [11,19,27,28]. It was suggested that
the high valent ruthenium-oxo species are formed by free radicals which are generated by
heterolytic cleavage of the oxidants [29]. In an attempt to elucidate the mechanism of this
reaction, NaIO4 (oxidant) was added to the tert-butanol solution of 1a, as a model catalyst. A
new shoulder at 360 nm appears in its UV-Vis spectrum, which is an indication for the presence
of Ru(VI)=O species [27].
In addition, when the solvent from a mixture of NaIO4 with complex 1a was evaporated, the IR
spectrum of the remaining residue was found to exhibit strong bands at 782, 764 and 700 cm-1 ,
which further supports the formation of Ru(VI)=O species [27,30,31] responsible for the
catalytic oxidation. The ESI-MS spectra of the residue from the experiment carried out using 1a
and NaIO4 was obtained. One major species observed containing the Ru fragment showed a peak
at 403 (maximum of the isotopic cluster), which may be attributed to [(p-
cymene)RuCl(MeCN)2.(H2O)]+. This may be due to the existence in solution of a ruthenium
species containing coordinated cymene, but also implies that there is cleavage of the N, N-
bidentate ligand. In addition the ESI-MS spectrum of the used catalyst 2d was obtained and the
Ru fragment showed peaks at 413 (maximum isotopic cluster), which may be attributed to
[(C6H6)RuCl(MeCN)4.(H2O)2]+. This implies the existence in solution of a ruthenium species
coordinated to a benzene ring, but again the N, N-bidentate ligand appears to be labile. One
could speculate that the arene ring decoordinates during the reaction and re-attaches at the end,
however, the catalytic data shows a clear influence of the two different arene ligands.
22

23
Using the above mentioned observations and available literature, we believe a mechanism that
involves a Ru(VI) oxo species can be proposed (Scheme 2). The catalysts react with the terminal
oxidant NaIO4 to form a Ru(VI)-cis dioxo intermediate [31] as the active species, while the
terminal oxidant is converted from NaIO4 to NaIO3.
Scheme 2. Proposed catalytic scheme for the oxidation of alcohols by the complexes.
23

24
4.0 Conclusion
Eight new mononuclear complexes with the general formula [(η6-arene)RuCl(C5H4N-2-CH=N-
Ar)]PF6 , where arene = p-cymene (1) and C6H6 (2); Ar = 2, 4, 6-trimethyl phenyl (a), 2, 4-
dimethyl phenyl (b), 2-methoxy phenyl (c), 2, 6-diisopropyl phenyl (d) have been successfully
synthesized, isolated and characterized. The structures of four of these compounds were
determined by single crystal crystallography. All the complexes showed pseudo octahedral three
legged piano stool geometry. The compounds were found to be reactive in alcohol oxidation to
give aldehydes or ketones in good yields and short reaction times. The reactions of the p-
cymene complexes were clearly faster than those of the benzene complexes. A mechanism
involving Ru(VI)=O complexes has been proposed from IR, UV-Vis and ESI data.
Supplementary material
CCDC 1469560-1469563 contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax:
+44 1223 336033).
Acknowledgement
We wish to extend our sincere thanks to the NRF, THRIP (Grant no. TP 1208035643) and
UKZN (URF) for financial support. Joel M. Gichumbi thanks Prof. E. N. Njoka for his support.
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Graphical Abstract
Research Synopsis
New ruthenium (II) pyridine-imine complexes have been prepared and characterized.
They show good catalytic activity in the oxidation of alcohols to carbonyl products using
NaIO4 in a H2O/tert-butanol mixture.
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Research Synopsis
• New [(η6-arene)RuCl(L)]PF6 (L = N,N’ bidentate ligands) complexes prepared
• These complexes were characterized using 1H & 13C NMR, IR, UV-Vis, HRMS, and
TGA
• Four crystal structures showing pseudo-octahedral piano stool geometry presented
• The compounds are good catalytics for alcohol oxidation to carbonyl products
.
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