Synthesis and characterization of piano-stool ruthenium complexes with N,N′-pyridine imine bidentate ligands and their application in styrene oxidation

Article abstract of DOI:10.1016/j.jorganchem.2016.02.015

The new complexes [(η⁶-arene)RuCl(C₅H₄N-2-CH=N-R)]PF₆ (arene = C₆H₆ with R = 4- flourophenyl (1), 4-chlorophenyl (2), 4-bromophenyl (3), 4 -iodophenyl (4), 2, 5-dichlorophenyl (5) or p-cymene with R = 4-flourophenyl (6), 4-chlorophenyl (7), 4-bromophenyl (8), 4- iodophenyl (9), 2, 5 - dichlorophenyl (10)) have been synthesized by reacting the ruthenium arene precursors [(η⁶-arene)Ru(μ-Cl)Cl]₂, with the N,N′-bidentate ligands in a 1:2 ratio. Full characterization of all complexes was accomplished using ¹H and ¹³C NMR, elemental analyses, UV-Vis spectroscopy, thermal analysis, IR spectroscopy and single crystal x-ray structures for compounds 7 and 10. The single crystal structures confirmed coordination of the ligand to the ruthenium(II) centre. The Ru(II) centre has a pseudo-octahedral three legged piano stool geometry in which the arene ring occupies the apex of the stool and the ruthenium is coordinated to the N,N′-bidentate ligand and a chloride ligand at the base of the stool. Two polymorphs of 7 were identified. The synthesized Ru(II) complexes were tested as catalysts for the oxidation of styrene to benzaldehyde using NaIO₄ as a co-oxidant. All complexes were active catalysts for styrene oxidation and they gave high yields of benzaldehyde as the major product.

Full text of DOI:10.1016/j.jorganchem.2016.02.015

Accepted Manuscript
Synthesis and characterization of piano-stool ruthenium complexes with N, Nʹ-
pyridine imine bidentate ligands and their application in styrene oxidation
Joel M. Gichumbi, Holger B. Friedrich, Bernard Omondi
PII:
S0022-328X(16)30048-1
10.1016/j.jorganchem.2016.02.015
JOM 19399
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 28 October 2015
Revised Date: 5 February 2016
Accepted Date: 11 February 2016
Please cite this article as: J.M. Gichumbi, H.B. Friedrich, B. Omondi, Synthesis and characterization
of piano-stool ruthenium complexes with N, Nʹ-pyridine imine bidentate ligands and their application in
styrene oxidation, Journal of Organometallic Chemistry (2016), doi: 10.1016/j.jorganchem.2016.02.015.
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ACCEPTED MANUSCRIPT
Graphical Abstract

ACCEPTED MANUSCRIPT
Synthesis and characterization of piano-stool ruthenium complexes with N, Nʹ-pyridine
imine bidentate ligands and their application in styrene oxidation.
*
Joel M. Gichumbi, Holger B. Friedrich , Bernard Omondi.
*
School of Chemistry, University of KwaZulu-Natal, Private Bag X54001, Durban 4001, South
Africa.
*
Corresponding author. Tel.: +27 312603107; fax: +27 312603091. E-mail address:
friedric@ukzn.ac.za (H.B. Friedrich).
Abstract
6
The new complexes [(η -arene)RuCl(C
5
H
4
N-2-CH=N-R)]PF
6
(arene = C
6
H
6
with R = 4-
flourophenyl (1), 4-chlorophenyl (2), 4-bromophenyl (3), 4 -iodophenyl (4), 2, 5-dichlorophenyl
5) or p-cymene with R = 4-flourophenyl (6), 4-chlorophenyl (7), 4-bromophenyl (8), 4-
iodophenyl (9), 2, 5 - dichlorophenyl (10)) have been synthesized by reacting the ruthenium
(
6
2
arene precursors [(η -arene)Ru(ꢀ-Cl)Cl] , with the N, Nʹ-bidentate ligands in a 1:2 ratio. Full
1
13
characterization of all complexes was accomplished using H and C NMR, elemental analyses,
UV-Vis spectroscopy, thermal analysis, IR spectroscopy and single crystal x-ray structures for
compounds 7 and 10. The single crystal structures confirmed coordination of the ligand to the
ruthenium(II) centre. The Ru(II) center has a pseudo-octahedral three legged piano stool
geometry in which the arene ring occupies the apex of the stool and the ruthenium is coordinated
to the N, N’-bidentate ligand and a chloride ligand at the base of the stool. Two polymorphs of 7
were identified. The synthesized Ru(II) complexes were tested as catalysts for the oxidation of
styrene to benzaldehyde using NaIO as a co-oxidant. All complexes were active catalysts for
4
styrene oxidation and they gave high yields of benzaldehyde as the major product.
Key words: Ruthenium arene, N, N-bidentate ligands, styrene oxidation, benzaldehyde,
mechanism
1
. Introduction
The oxidation of olefins is an important reaction to fragment large compounds and to introduce
the oxygen functionality into their backbone. This transformation has played an extensive and
important role in the production of fine and pharmaceutical grade chemicals [1]. Traditionally,
ozonolysis has been used as a general technique to obtain aldehydes or carboxylic acids.
However, major safety concerns and inconvenience associated with ozone generation has
prompted researchers to develop catalytic metal-based systems for application in olefin oxidation
[2].
Amongst the metals that have potential application in catalytic selective oxidation, ruthenium
takes a special position owing to its versatility [3]. The utilization of ruthenium in olefin
1

ACCEPTED MANUSCRIPT
oxidation has been investigated widely [4]. However, there are very few reports on the use of
half-sandwich ruthenium complexes containing nitrogen ligands in olefin oxidation [5]. The half-
sandwich arene ruthenium(II) complexes with nitrogen-donor ligands have received a lot of
attention from researchers due to their potential applications in photochemical devices [6],
catalysis [7-13] and applications as cytotoxic agents for anticancer therapy [14-20]. This
versatility of arene ruthenium(II) complexes in catalysis may be attributed to the existence of
three labile coordinative sites and a rigid arene ring occupying the other three coordinate sites
[21].
The nitrogenous donor ligands are very attractive when coordinated to ruthenium complexes
because the steric and electronic properties around the metal centre can be easily changed with
these ligands [22]. One type of nitrogen-donor ligand is the bidentate pyridine-imine Schiff base.
The reactions of some pyridine-imine Schiff bases with ruthenium have been reported [23-25].
These complexes exhibited good activities as cytotoxic agents in human cancer cell lines and in
aqueous phase hydroformylation [8,26].
To the best of our knowledge, no research on the influence of altering the arene ring on the
styrene oxidation reaction has been undertaken. This prompted us to investigate the reactivity of
6
[
(η -arene)Ru(ꢀ-Cl)Cl]
2
6 6
[arene = C H or p-cymene] with pyridine-imines with electron
6
withdrawing halogen atoms as substituents in the para-position. The formation of [(η -
arene)RuCl(C N-2-CH=N-R)]PF complexes (where R = 4-flourophenyl, 4-chlorophenyl, 4-
bromophenyl, 4-iodophenyl, 2, 5-dichlorophenyl and arene = C H , p-cymene) are reported. All
5
H
4
6
6
6
the new compounds were fully characterized by various analytical techniques and the structures
of two complexes have been confirmed using single crystal x-ray crystallography. These
complexes have been employed in styrene oxidation using NaIO as the oxidant and excellent
4
conversions and yields towards benzaldehyde were achieved. In addition, steric and electronic
effects on the catalytic activity and selectivity of the complexes in the styrene oxidation reactions
were explored.
2
. Experimental
All manipulations were carried out under an inert atmosphere (UHP nitrogen) using Schlenk
techniques. Analytical grade acetonitrile (Merck) was dried over phosphorous (V) oxide, reagent
grade diethyl ether (Merck) was distilled from sodium/benzophenone and stored over molecular
sieves and ethanol was dried over Mg/I . The chemical reagents: 1, 4-cyclohexadiene, α-
2
phellandrene, 2-pyridinecarboxaldehyde, 4-chloroaniline, 4-flouroaniline, 4-bromoaniline, 4-
iodoaniline and 2, 5-dichloroaniline were purchased from Sigma-Aldrich and utilized as
received. Melting points were measured on an Ernest Leitz Wetzlar hot stage microscope.
Elemental analyses were performed on Thermal-Scientific Flash 2000 CHNS/O analyzer.
Infrared spectra were recorded using an ATR Perkin Elmer Spectrum 100 spectrophotometer
-1
between 4000-400 cm in the solid state. Mass spectra were recorded via a Waters Micromass
LCT Premier TOF-MS analyzer by direct infusion and ESI in the positive mode. Acetonitrile
2

ACCEPTED MANUSCRIPT
(100%) was used as a mobile phase and 10 ꢀL of the sample injected at 0.3 ml/min flow rate.
Thermogravimetric analysis (TGA) was performed on a SDTQ600 TGA-DSC in a nitrogen
1
13
atmosphere. H and C NMR spectra were recorded on Brucker Top Spin 400 and 600 MHz
6
spectrometers using deuterated DMSO-d from Sigma-Aldrich. The precursors of [(η -
6
arene)Ru(ꢀ-Cl)Cl] , where arene = C H or p-cymene, were prepared following reported
2
6
6
methods [27]. The pyridine-imine ligands were synthesized according to reported procedures
28].
[
6
2
.1 General procedure for the synthesis of [(η -arene)RuCl(C H N-2-CH=N-R)]PF , 1-5
5 4 6
The complexes were prepared using a modified method from Gomez et al. [29]. The precursor
6
complex [(η -arene)Ru(ꢀ-Cl)Cl]
2
(0.20 mmol) was dissolved in acetonitrile (10 ml) and added to
the ligands a-e (C H N-2-CH=N-R]PF (R = 4-flourophenyl (a), 4-chlorophenyl (b), 4-
5
4
6
bromophenyl (c), 4-iodophenyl (d), 2,5-dichlorophenyl (e)), in slight excess (0.43 mmol),
dissolved in acetonitrile. The mixture was stirred for 3 h and the solvent evaporated giving a
yellow oil that was then treated with NH PF (0.43 mmol) in 10 ml ethanol. The mixture was
4 6
stirred in an ice bath maintained at zero degrees for two hours resulting in orange or yellow
solids, which were isolated by gravity filtration, washed with diethyl ether and dried in vacuo.
The yields and characterization data for each salt are given below.
1
[C H ClN F Ru]PF
18 15 2 1 6
1
Orange powder. Yield 70%. m.p. 210.2 °C
H NMR (600 MHz, DMSO-d ): δ 9.66 (d,
6
(
decomp.).
1
6
2
4
JHH = 5.22 Hz, 1H, H ), δ 8.90 (s, 1H, CH=N, H ), δ 8.30 (d, J = 5.0 Hz, 2H, H , H ), δ 8.26
m, 3H, H , H , H ), δ 7.88 (m, 2H, H , H ), δ 5.96 (s, 6H, C H ). C NMR (400 MHz,
HH
3
8
12
9
11
13
(
6 6
6
1
5
4
3
DMSO- d ): δ 168.83 (C ); δ 161.92 (C ); δ 156.84 (C ); δ 155.11 (C ); δ 148.72 (C ); δ 140.61
6
2
7
8
12
9
11
(
C ); δ 130.54 (C ); δ 129.33 (C , C ); δ 125.25 (C ); δ 116.93 (C ); δ 87.87 (C H ). IR (KBr,
6 6
-
1
cm ): 1615.6 ν (C=N), 824.5 ν(P-F). Anal. Calcd. for [C H ClN F Ru]PF C, 38.62; H, 2.70;
N, 5.00. Found: C, 38.70; H, 2.64; N 4.85. MS (ESI, M/Z): 414.9950 [C H ClN F Ru]
1
8
15
2
1
6
+
1
8
15
2 1
2
2 2 6
[C18H15Cl N Ru]PF
1
Yellow powder, Yield 74%. m.p. 204.2 °C (decomp.). H NMR (600 MHz, DMSO- d
6
): δ 9.67 (d,
1
6
2
4
J
= 5.40 Hz, 1H, H ), δ 8.92 (s, 1H, H ), δ 8.34 (m, 2H, H , H ), δ 7.90 (m, J = 5.22 Hz, 1H,
H ), δ 7.84 (t, 2H, H , H ), δ 7.72 (t, JHHH = 5.84 Hz, 2H, H , H ), δ 5.97 (s, 6H, C
NMR (400 MHz, DMSO- d ): δ 169.09 (C ); δ 156.80 (C ); δ 155.0 (C ); δ 150.9 (C ); δ 140.60
C ); δ 134.5 (C ); δ 130.7 (C ); δ 129.99 (C , C ); δ 129.39 (C ); δ 124.81 (C ); δ 87.81
HH
3
HH
8
12
9
11
13
6
H
6
). C
6
1
5
4
6
3
2
7
8
12
9
11
(
(
-1
C H ). IR (KBr, cm ): 1614.8 ν(C=N), 824.2 ν(P-F). Anal. Calcd. for [C H Cl N Ru]PF C,
6 6 18 15 2 2 6
3
7.52; H, 2.62; N, 4.86. Found: C, 37.70; H, 2.70; N 4.86. MS (ESI, M/Z): 430.9656
+
[C H Cl N Ru]
18 15 2 2
3

ACCEPTED MANUSCRIPT
3
2 6
[C18H15ClBrN Ru]PF
1
Yellow powder, yield 80%, m.p. 200.2 °C (decomp.). H NMR (600 MHz, DMSO- d
6
): δ 9.66 (d,
1
6
2
4
J
= 5.28 Hz, 1H, H ), δ 8.91(s, 1H, H ), δ 8.32 (m, 2H, H , H ), δ 7.89 (t, J
H ), δ 7.83 (d, JHH = 8.46 Hz , 2H, H , H ), δ 7.76 (d, JHH = 8.64 Hz, 2H, H , H ), δ 5.97 (s, 6H,
C H ). C NMR (400 MHz, DMSO- d ): δ 169.09 (C ); δ 156.06 (C ); δ 151.39 (C ); δ 140.60
C ); δ 132.92 (C ); δ 130.69 (C ); δ 129.40 (C ); δ 125.08 (C , C ); δ 123.06 (C , C ); δ 87.28
= 8.24 Hz 1H,
HH
3
HHH
9 11
8
12
1
3
6
1
5
6
6
6
4
3
2
7
8
12
9
11
(
(
-1
C H ). IR (KBr, cm ): 1613.9 ν(C=N), 823 ν(P-F). Anal. Calcd. for [C H ClBrN Ru]PF : C,
6 6 18 15 2 6
3
4.88; H 2.44; N, 4.51. Found: C, 35.38, H 3.07; N 4.49. MS (ESI, M/Z): 477.9146
+
[C H ClBrN Ru]
1
8
15
2
4
[C18
H15ClIN
2
Ru]PF
6
1
Orange powder, yield 82%, m.p: 206.8 °C
. H NMR (400 MHz, DMSO- d ): δ 9.66 (d
(
decomp.)
6
1
6
2
4
J
= 5.40 Hz, 1H, H ), δ 8.90 (s, 1H, H ), δ 8.32 (m, 2H, H , H ), δ 7.99 (d, J = 8.36 Hz, 2H,
HH
H , H ), δ 7.89 (m, 1H, H ), δ 7.61 (d, J = 8.4 Hz , 2H, H , H ), δ 5.96 (s, 6H, C H ). C
NMR (400 MHz, DMSO- d ): δ 168.31 (C ); δ 156.28 (C ); δ 154.55 (C ); δ 151.29 (C ); δ
40.07 (C ); δ 138.22 (C ); δ 130.13 (C ); δ 128.84 (C , C ); δ 124.53 (C ); δ 96.07 (C ); δ
HH
3
8
12
9
11
13
HH
6
6
6
1
5
4
6
3
2
7
8
12
9
11
1
8
-1
7.28 (C H ). IR (KBr, cm ): 1614.3 ν(C=N), 822.3 ν(P-F). Anal. Calcd. for
6
6
[
5
C
18
H
2 6
15ClIN Ru]PF : C, 32.30; H, 2.26; N, 4.20. Found: C, 31.80, H 2.18; 4.12. MS (ESI, M/Z):
+
22.8999 [C H ClIN Ru]
18 15 2
5
[C H Cl N Ru]PF
18 14 3 2 6
1
6
Brown powder, yield 76%, m.p. 224.7 °C (decomp.). H NMR (400 MHz , DMSO- d ): δ 9.64 (d,
1
6
2
4
3
JHH =5.40 Hz, 1H, H ), δ 9.06 (s, 1H, H ), δ 8.37 (s, 2H, H , H ), δ 7.96 (s, 1H, H ), δ 7.88 (d,
12
9
11
J
HH = 8.4 Hz, 1H, H ), δ 7.80 (d, JHH = 2.16 Hz, 1H, H ), δ 7.69 (m, 1H, H ), δ 6.03 (s, 6H,
13
6
1
5
C H ). C NMR (400 MHz, DMSO- d ): δ 174.61 (C ); δ 154.02 (C ); δ 150.14 (C ); δ 140.88
C ); δ 132.96 (C ); δ 132.66 (C ); δ 131.80 (C ); δ 130.67 (C ); δ130.27 (C ); δ 124.2 (C , C )
δ 87.83 (C H ). IR (KBr, cm ): 1620.6 ν(C=N), 821.1 ν(P-F). Anal. Calcd. for
6
6
6
4
3
2
7
9
10
8
12
(
-1
6
6
[
C
18
H
14Cl
3
N
2
Ru]PF
6
: C, 35.60; H, 2.31; N, 4.59. Found: C, 36.06; H, 1.86; N, 4.12. MS (ESI,
+
M/Z): 466.9251 [C H Cl N Ru]
1
8
14
3
2
6
2
1
.2 General procedure for the synthesis of [(η - p-cymene)RuCl(C H N-2-CH=N-R)]PF 6-
5 4 6
0
6
To a suspension of [(η -p-cymene)Ru(ꢀ-Cl)]Cl] (0.163 mmol) in acetonitrile (10 ml) the ligand
2
a-e in slight excess (0.33 mmol) was added. The reaction mixture was stirred for four hours.
The solvent was then evaporated and the solid residue treated with NH PF (0.35 mmol) in 10 ml
4
6
of EtOH. The mixture was then stirred in an ice bath for two hours and the solid formed was
4

ACCEPTED MANUSCRIPT
collected by filtration, washed with diethyl ether and dried in vacuo. The yield and
characterization data are given below.
6
[C H ClN FRu]PF
22 23 2 6
1
6
Orange powder, yield 82%, m.p. 140.5 °C (decomp.). H NMR (400 MHz, DMSO-d ): δ 9.58 (d,
1
6
2
4
3
8
J
=5.4 Hz, 1H, H ); 8.91 (s, 1H, H ); 8.34 (m, 1H, H ); 8.29 (m, 1H,H ); 7.91 (m, 3H, H , H ,
HH
1
2
9
11
H ); 7.50 (t, 2H, H , H ), 6.09 (d, JHH = 6.78 Hz, 1H, p-CyAr ); 5.78 (d, JHH = 6.42 Hz, 1H, p-
Cy ); 5.70(d, J = 6.0 Hz, 1H, p-Cy ); 5.60 (d, J = 6.48 Hz 1H, p-Cy ), 2.57 (m, 1H, CH
Ar
HH
Ar
HH
Ar
13
(
(
1
8
(
Me) ); 2.17 (s, 3H, p-Cy ); 1.01 (m, 6H, p-Cy ). C NMR (400 MHz, DMSO-d ). δ 168.06
2 Me Me 6
6
1
10
5
4
3
2
C ), δ 159.97 (C ), δ 154.40 (C ); δ 148.12 (C ); δ 139.94 (C ); δ 130.07 (C ); δ 128.93 (C ); δ
24.78 (C ); δ 124.69 (C ); δ 116.52 (C ) ; δ 105.13 (C ); δ 103.64 (C ); δ 86.6 (p-Cy ); δ
6.04 (p-Cy ); δ 84.9 (p-Cy )); δ 84.79 (p-Cy )) ; δ 30.4 (p-Cy ); δ 21.7 (p-CyMe); δ 21.5
p-CyMe); δ 18.2 (p-CyMe). IR (solid state): 1610.0 cm ν(C=N); 829.9 cm ν(P-F). Anal. Calcd.
FRu]PF : C, 40.57; H, 3.56; N, 4.30. Found: C, 40.60; H, 3.79; N, 4.51. MS
ESI, M/Z): 471.0580 [C H ClN FRu]
7
8
12
9
11
Ar
Ar
Ar
Ar
iPro
-1
-1
for [C22
(
H23ClN
2
6
+
2
2
23
2
7
2 6
[C19H20ClN Ru]PF
1
6
Brown powder, yield 85%, m.p. 161.0 °C (decomp.). H NMR (400 MHz, DMSO-d ). δ 9.61 (d,
1
6
2
4
3
J
= 5.52 Hz, 1H, H ); 8.91(s, 1H, H ); 8.34 (m, 2H, H ,H ); 7.91 (m, 1H, H ); 7.86 (m, 2H,
HH
8
12
9
11
H ,H ), 7.72 (m, 2H, H ,H ), 6.11 (d, JHH = 6.18 Hz, 1H, p-CyAr); 5.80 (d, JHH = 6.18 Hz, 1H,
p-Cy ); 5.73 (d, J = 6.12 Hz, 1H, p-Cy ); 5.61(d, J = 6.18 Hz, 1H, p-Cy ); 2.57 (m, 1H,
CH (Me) ); 2.18 (s, 3H, p-CyMe); 1.01 (m, 6H, p-CyMe). C NMR (400 MHz, DMSO-d ). δ
2 6
68.46 (C ), δ 156.02 (C ), δ 154.37 (C ); δ 150.41 (C ); δ 139.95 (C ); δ 134.05 (C ); δ 130.05
C ) ; δ 129.55 (C ) ; δ 129.03 (C ); δ 124.33(C ) ; δ 105.17 (C ) ; δ 103.82 (C ); δ 86.68 (p-
Ar
HH
Ar
HH
Ar
1
3
6
1
10
5
4
3
1
(
2
7
8
12
9
11
Cy ); δ 86.00 (p-Cy ); δ 84.80 (p-Cy ); δ 84.73 (p-Cy ), δ 30.46 (p-Cy ); δ 21.72 (p-Cy );
Ar
Ar
Ar
Ar
iPro
Me
-1
-1
δ 21.57 (p-CyMe); δ 18.28 (p-CyMe). IR (solid state): 1615.3 cm
ν(C=N), 817.9 cm ν (P-F).
Anal. Calcd. for [C H ClN Ru]PF : C, 40.63; H, 3.87; N, 4.31. Found: C, 40.40; H, 3.79 H; N
1
9
20
2
6
+
4
.37. MS (ESI, M/Z): 487.0284 [C19
Ru]PF
H
2
20ClN Ru]
8
[C22
H23Cl
2
N
2
6
1
Yellow powder, yield 82%, m.p. 184.7 °C
. H NMR (400 MHz, DMSO-d ). δ 9.58 (d,
(
decomp.)
6
1
6
2
4
3
J
HH = 5.4 Hz, 1H, H ); 8.92 (s, 1H, H ); 8.33 (m, 2H, H ,H ); 8.02 (d, JHH = 8.34 Hz , 2H, H ,
8
12
9
11
H ); 7.90 (s, 1H, H ); 7.62 (d, J = 8.64 Hz 2H, H , H ), 6.10 (d, J = 6.18 Hz, 1H, (p-CyAr)
HH
HH
)
; 5.78 (d, JHH = 6.18 Hz, 1H, (p-Cy )); 5.71 (d, J = 6.18 Hz 1H, (p-Cy )); 5.59 (d, J = 6.24
Ar HH Ar HH
1
3
Hz, 1H, (p-Cy )), 2.56 (m, 1H, CH (Me) ); 2.17 (s, 3H, (p-Cy )); 1.01 (m, 6H, (p-Cy )). C
NMR (400 MHz, DMSO-d ). δ 168.41 (C ), δ 156.02 (C ), δ 154.03 (C ); δ 150.81 (C ); δ
1
Ar
2
Ar
Ar
6
1
10
5
6
4
3
2
7
8
12
39.97 (C ); 132.48 (C ); δ 130.26 (C ); δ 129.04 (C ); δ 124.58 (C ); δ 122.67 (C ); δ 105.13
9
11
(C ) ; 103.90 (C ); δ 86.72 (p-Cy ); δ 86.01 (p-Cy ); δ 84.79 (p-Cy ); δ 84.71 (p-Cy ); δ
Ar Ar Ar Ar
3
0.50 (p-CyiPro); δ 21.73 (p-Cy ); δ 21.60 (p-Cy ); δ 18.3 (p-Cy ). IR (solid state): 1613.9
Me Me Me
5

ACCEPTED MANUSCRIPT
-
1
-1
cm ν(C=N) 818.1 cm ν (P-F). Anal. Calcd. for [C22
4
H
2 2 6
23Cl N Ru]PF : C, 39.04; H, 3.43; N,
+
.14. Found. C, 39.19; H, 3.46; N 3.85. MS (ESI, M/Z): 530.9761 [C H BrClN Ru]
22 23 2
9
[C H ClIN Ru]PF
22 23 2 6
1
Orange powder, yield: 80%, m.p. 204.5°C (decomp.) . H NMR (400 MHz, DMSO-d
6
).
1
6
2
4
δ 9.59 (d, J = 5.46 Hz, 1H, H ); 8.93 (s, 1H, H ); 8.33 (m, 2H, H ,H ); 7.92 (m,
H, H ); 7.87(d, JHH = 8.40 Hz, 2H, H , H ); 7.78(d, JHH = 8.48 Hz, 2H, H , H );
.11 (d, JHH = 6.36 Hz,1H, (p-Cy )); 5.79 (d, J = 6.42 Hz, 1H, (p-Cy )); 5.72
HH
1
2
3
8
9
11
1
6
Ar
HH
Ar
(
d, JHH = 6.3 Hz, 1H, (p-Cy )), 5.60 (d, J = 6.12 Hz, 1H, (p-Cy ) ); 2.57 (m, 1H,
Ar HH Ar
1
3
CH (Me) ); 2.18 (s, 3H, (p-Cy )); 1.01 (m, 6H, (p-Cy )). C NMR δ (400 MHz,
DMSO-d ). δ 168.18 (C ), δ 156.01 (C ), δ 154.03 (C ); δ 151.26 (C ); 139.95 (C );
1
(
2
Me
Me
6
1
10
5
4
6
3
2
7
8
12
38.28 (C ); δ 130.23 (C ); δ 129.00 (C ); δ 124.55 (C ); δ 105.07 (C ); δ 103.90
9
11
C ) ; δ 96.20 (C ); δ 86.70 (p-Cy ); δ 86.04 (p-Cy ); δ 84.70 (p-Cy ); δ 84.67
Ar Ar Ar
(
p-CyAr); δ 30.47 (p-CyiPro); δ 21.73 (p-CyMe); δ 21.57 (p-CyMe); δ 18.30 (p-CyMe).
-
1
-1
IR (solid state): 1614.9cm
ν(C=N), 815.8cm
ν(P-F). Anal. Calcd. for
[
3
C
22
H
23ClIN
2
Ru]PF
6
: C, 34.80; H, 3.58; N, 3.69. Found: C, 34.65; H, 3.38 H; N,
+
.45. MS. (ESI, M/Z): 578.96 [C H ClIN Ru]
2
2
23
2
1
0 [C H Cl N Ru]PF
22 22 3 2 6
1
Orange, yield 78%, m.p. 210.4°C
. H NMR (400 MH , DMSO-d ). δ 9.67 (d, J = 5.4
(
decomp.)
Z
2
6
HH
1
6
4
3
Hz, 1H, H ); 9.08 (s, 1H, H ); 8.37 (d, JHH = 7.28 Hz, 2H, H , H ); 7.99 (m, 1H, H ); 7.88 (m,
H, H , H ), 7.68 (m, 1H, H ); 6.07 (d, J = 6.28 Hz, 1H, (p-Cy )); 5.83 (d, J = 6.32 Hz, 1H
HH Ar HH
9
10
12
2
(p-CyAr)); 5.80 (d, JHH = 6.2 Hz, 1H, (p-CyAr)), 5.50 (d, JHH = 6.12 Hz, 1H, (p-CyAr)), 2.64 (m,
1
6
H, CH (Me) ); 2.09 (s, 3H, (p-Cy )); 1.028 (d, J = 6.88 Hz, 3H, (p-Cy ); 0. 94 (d, J
=
HH
2
Me
HH
Me
1
3
6
1
.88 Hz, 3H, (p-CyMe). C NMR (400 MH
Z
, DMSO-d
6
). δ 173.64 (C ), δ 156.37 (C ), δ 153.55
1
1
5
4
3
2
7
9
(C ); δ 149.41 (C ); δ 140.33 (C ); 132.37 (C ); δ 132.22 (C ); δ 130.18 (C ); δ 130.05 (C ); δ
1
0
8
12
1
3
24.11 (C ); δ 106.38 (C ) ; 101.63 (C ); δ 87.04 (p-CyAr); δ 85.83(p-CyAr); δ 83.9 (p-CyAr); δ
0.47 (p-Cy ); δ 21.74 (p-Cy ); δ 21.20 (p-Cy ); δ 18.07 (p-Cy ). IR (solid state): 1612.1
iPro
Me
Me
Me
-1
-1
cm ν(C=N); 828.6 cm ν (P-F). Anal. Calcd. for [C H Cl N Ru]PF : C, 39.53; H, 3.33; N,
4
2
2
22
3
2
6
+
.20. Found. C, 39.1; H, 3.64; N, 4.19 MS (ESI, M/Z): 520.9882 [C H Cl N Ru]
22 22 3 2
2
.3 X-ray crystallography
Crystals for 7(I) were grown by the slow evaporation method. Crystals of compounds 7(II) and
0 suitable for x-ray diffraction studies were grown by the solvent diffusion method, where
1
solutions of the compounds in dry acetone were layered with a fourfold excess of hexane and
allowed to stand undisturbed in the dark at ambient temperature for 2 days. Crystals of the salts
7
(I), 7(II) and 10e were selected and glued onto the tip of glass fibres. The crystals were then
mounted in a stream of cold nitrogen at 100(1) K and centred in the x-ray beam by using a video
camera. The crystal evaluation and data collection were performed on a Bruker Smart APEXII
diffractometer to crystal distance of 4.00 cm. The initial cell matrix was obtained from three
6

ACCEPTED MANUSCRIPT
series of scans at different starting angles. Each series consisted of 12 frames collected at
intervals of 0.5 ˚ in a 6˚ range with the 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 [30] . The final cell constants were calculated from a set of 6460 strong reflections from
the actual data collection. The data collection method involved ω scans of width 0.5˚. Data
+
reduction was carried out using the programme SAINT [30] . The structure was solved by
direct methods using SHELXS [31] and refined by SHELXL [30]. Non-H atoms were positioned
geometrically and allowed to ride on their respective parent atoms. 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 [30]. Crystal data and
structure refinement information for compounds 7(I), 7(II) and 10e are summarized in Table 1.
Table 1. Summary of the crystal data of 7(I), 7(II) and 7e
Compound
7(I)
7e
7
(II)
Formula
C H Cl F N PRu
C H Cl F N OPRu
C H Cl F N PRu
2
2
23
2
6
2
25 29
2
6
2
22 22
3
6
2
Formula weight
Crystal system
Space group
632.27
690.44
666.80
Orthorhombic
Pbcn
Triclinic
P
Monoclinic
P2 /n
1
a, Å
b, Å
c, Å
α°
16.221(3)
8.967(2)
15.0341(4)
10.3961(3)
16.9722(3)
89.086(2)
75.128(2)
79.328(2)
2471.74(12)
4
20.5555(5)
12.585(3)
12.867(3)
90
14.3898(18)
90
β°
90
87.935(3)
74.251(6)
1351.4(6)
2
γ°
90
3
Cell volume, Å
4798.03(18)
8
Z
3
Dcalcd, Mg/m
1.751
1.697
1.792
T, K
173(2)
173(2)
3.235
173(2)
-1
µ, mm
2.534
2.534
Wavelength, Å
F(000)
0.71073
0.71073
696
0.71073
1328
2528
3
Cryst size, mm
θmin, θmax,°
0.12 x 0.16 x 0.6
9.638 to 99.568
140029
0.257 x 0.188 x 0.158
1.637 to 26.878
4543
0.180 x 0.140 x 0.097
1.553 to 28.473
55748
No. of reflns. collected
No of indep. Reflns.
Completeness to theta
Absorbed correction
5627[R(int)]= 0.0809]
100%
4409[R(int)] = 0.0261]
100%
6212[R(int)]= 0.0180]
100%
Semi-empirical from
Semi-empirical from
Semi-empirical from
7

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equivalents
equivalents
1.039
equivalents
1.065
2
Goodness-of-fit on F
1.063
Final R indices
0.116, 0.0809
0.1292, 0.0727
1.30 & -1.05
0.0434, 0.1235
0.0457, 0.1266
1.055 & -2.064
0.0196, 0.0486
0.0234, 0.0510
0.607 & -0.448
R indices (all data)
Largest diff.peak & hole, Å
-3
2
.4 Styrene oxidation reaction
All catalytic reactions were performed in a Schlenk tube in an oil bath maintained at 60 ˚C. All
products were analyzed using a Perkin Elmer Auto system gas chromatograph fitted with a flame
ionization detector (FID) set at 290 ˚C. A Varian wax capillary column (25 mm x 0.15 mm x 2
ꢀm) was utilized with the injector temperature set at 50 ˚C. The substrate to catalyst ratio was
set at 1:100. Benzophenone was used as an internal standard. The identity of the products was
assessed by comparing their retention times with commercially available (Sigma Aldrich)
standards, supported by GC-MS analysis.
2
.4.1 General procedure for catalytic oxidation of styrene
Styrene (0.5 mmol), the catalyst (1 mol %) and internal standard (0.5 mmol) were dissolved in a
:1 mixture of water and tert-butanol (6 ml). NaIO (4 equivalents) was then added as one
1
4
portion and the two immiscible solutions were vigorously stirred, in an oil bath, with the
temperature maintained at 60 ˚C. The resultant solution was sampled at intervals, whereby an
aliquot was removed hourly and 1 ꢀL was injected and analyzed by GC. Control experiments in
the absence of either the catalyst or NaIO were performed under identical conditions.
4
3
3
.0 Results and Discussion
.1 Synthesis and characterization of cationic iminopyridyl Ru(II)-arene complexes.
6
The reactions of the bimetallic arene precursors [(η -arene)Ru(ꢀ-Cl)Cl]
2
with the N,N-bidentate
6
ligands in dry acetonitrile at ambient temperatures give the new mononuclear complexes [(η -
+
arene)RuCl(C
flourophenyl, 2, 5-dichlorophenyl), which were isolated as their hexafluorophosphate salts
Scheme 1). The complexes formed are air-stable and non-hygroscopic. The complexes are
5 4
H N-2-CH=N-R)] (R = 4-chlorophenyl, 4-bromophenyl, 4-iodophenyl, 4-
(
soluble in polar solvents such as acetone, acetonitrile, DMSO and DMF, but are insoluble in non-
polar solvents such as hexane and diethyl ether.
8

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6
Scheme 1: Synthesis of [(η -arene)RuCl(C H N-2-CH=N-R) PF (arene = C H (1-5) or p-cymene
5
4
6
6
6
(6-10)
1
13
The formation of complexes 1 to 5 was confirmed by H and C NMR by monitoring the imine
proton and carbon signals in the ligand and the complex. The imine proton (γ (CH=N)) shifts
downfield to the region δ = 8.90 to 9.10 from the region of δ 8.57 to 8.59 of the free ligand
(
Table 2). This shift can be attributed to the deshielding of the imine proton as the nitrogen
13
atoms donate their lone pair to the metal [32]. In addition, C NMR spectra also supported the
formation of the complexes. The imine carbon shifted from the region δ 160.1 to 164.2 for the
free ligands to the region of 168.3 to 169.1 for the complexes, and this observation was in a good
6
agreement with related compounds reported [33]. For the [η -C
6
H
6
)RuCl(L)]PF
6
complexes, the
6 6
C H singlet of these complexes shifted downfield on coordination, when compared to the
precursor complex.
1
13
Table 2. H NMR and C NMR shifts for the imine functionalities for compounds 1a-e and 2a-e
1
13
Complex
H NMR
C NMR
Ligand
8.57
8.58
8.59
8.56
8.56
8.57
8.58
8.59
8.56
8.56
Complex
8.90
8.94
8.91
8.90
9.10
8.90
8.91
8.92
Ligand
161.4
160.6
160.1
162.1
164.1
161.4
160.6
160.1
162.2
164.2
Complex
168.3
169.1
169.1
168.8
174.6
168.4
169.5
168.4
168.6
173.6
1
2
3
4
5
6
7
8
9
8.89
9.08
1
0
9

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I
The formation of the p-cymene complexes 6 to 10 was also confirmed by H NMR. The binding
of the ligands to the para-cymene moiety was confirmed by the observation that the resonances
of the benzene ring protons of the p-cymene resolved to distinct peaks due to desymmetrization
of the arene ring upon coordination of the Schiff base at the ruthenium centre. This observation
is in good agreement with those of other workers for related compounds [32]. The formation of
the complex was also confirmed by the downfield shift of the imine proton γ(CH=N) to δ = 8.89
-
8.91 from δ = 8.57-8.59 for the free ligands’ imine peaks. This shift can be attributed to the
deshielding of the amine proton as nitrogen donates its lone pair to the metal (Table 2).
1
3
6
6
Furthermore, the C NMR spectra for the [(η -p-cymene)RuCl(L)]PF complexes supported the
formation of the complexes due to the downfield shift of the imine carbon from δ = 160.1-164.2
for the free pyridine-imine ligands to δ = 168.4 -173.6 for the complexes. This shift can also
also be attributed to the deshielding of the imine carbon, which is in agreement with observations
reported by other workers for related complexes [32] (Table 2).
The IR spectra also support the formation of the complexes. All the complexes show a strong
-1
absorption band in the range 1613-1620 cm , which can be assigned to the symmetrical
vibration of the γ(C=N) bond. The position of the γ(C=N) bond shifted to lower wavenumbers
-1
-1
(
1610-1620 cm ) when compared to those of the free pyridine-imine ligand (1622-1627 cm )
indicating that the ligands are coordinated to the arene moiety (Table 3). This is consistent with
the increase in the electron density on the ruthenium(II) centre caused by the coordination of the
C=N- group, which resulted in increasing the back bonding to the nitrogen and hence a lower
γ(C=N) stretching vibration. Similar trends have been well documented for related compounds
-
1
[
33] . A very strong peak is observed for all complexes in the region 818-830 cm and another
-1
-
strong peak in the region 555-556 cm which is attributed to the PF6 counter ion. This
-
assignment for PF
6
is in agreement with reports by other workers [34,35]
Table 3. Infrared shifts of the imine functionalities for compounds 3a-e and 4a-e
Complex
Ligand
Complex
(C=N)pyridine
1614.4
1614.8
1613.9
1615.6
1620.6
1614.9
1615.3
(C=N)pyridine
1
2
3
4
5
6
7
8
9
1624.5
1623.3
1622.7
1627.1
1625.3
1624.5
1623.3
1622.7
1627.1
1625.3
1613.9
1610.1
1612.7
1
0
10

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High resolution mass spectra were also obtained to confirm the formation of the mononuclear
-
complexes. They all lose the counter anion PF to give the base peak of the N, Nʹ-bidentate
6
6
+
5 4
complex [(η -arene)RuCl(C H N-2-CH=N-R)] . The characteristic multiple peaks associated
with stable isotopes of ruthenium are also observed in each fragment.
The Uv-vis data of the complexes 1 to 5 and 6 to 10 (Figs. 1 and 2) and their respective ligands
were obtained in acetonitrile. Absorption bands were observed in the regions 230 nm -249 and
2
82 nm-327 nm for the ligands which can be attributed to n-π* and π-π* transitions
respectively. These bands underwent a bathochromic shift to 255 - 273 nm and 311-316 nm in
the complexes. In addition the complexes presented bands in the regions 410-423 nm which
could be assigned to the metal to ligand (dπ-π*) charge transfer (MLCT) transition from the
filled 4d orbital to the empty π* orbital. These bands were not observed in the ligand, UV-vis
thus further confirming the formation of the complexes. This assignment is in agreement with
those of other workers for N, N’ bidentate ligands [36].
Figure 1: UV-Vis spectra of the complexes 1 to 5
11

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Figure 2: UV-Vis spectra of the complexes 6 to 10
The TGA data was obtained in the range of 25˚C to 700 ˚C (Figures 3 and 4). The thermograms
of the compounds presented two main mass loss steps during the decomposition of the
compounds. The first step occurred between 170 and 330 ˚C and the second step occurred
between 340 and 510˚C. The first decomposition in all cases reflected a weight loss of
approximately 30%, which may be attributed to the loss of the pyridine-imine ligand. The second
decomposition step of approximately 20% may be attributed to the loss of the arene ring.
The DSC results for the complexes 1-10 show two endothermic peaks at 180-290 ˚C and 310-
4
50 ˚C. The first peak indicates the compounds undergoes a phase transition on melting and the
second peak indicates possible phase transition during which the compound is completely
decomposed. This has been observed by other researchers for arene ruthenium compounds [37].
12

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Figure 3: TGA trace of 1-5
Figure 4: TGA trace of 6-10
13

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3
.2 Crystal and molecular structures
Compounds 7 and 10 crystalize as red block crystals. Single crystal X-ray analysis of these
compounds revealed that 7 crystallizes without a solvent molecule, 7(I) and as an acetone solvate
7
(II). The molecular structures of the complexes are provided in Figs. 5, 6 and 7, while Table 4
gives a comparison of important bond distances and angles. Whereas 7(I) crystallizes in the
6
+
orthorhombic Pbcn space group with a molecule of the cation [(η -p-cymene)RuL)] and two
-
half molecules of the anion PF in the asymmetric unit, 7(II) crystallizes in the triclinic
P space group with a molecule of acetone in addition to the cation [(η -p-cymene)RuL)] and
6
6
+
-
two half molecules of the counter anion PF
6
. Compound 10 on the other hand has the cation and
one molecule of the anion in the asymmetric unit. In all three cationic species the substituted N-
pyridin-2-ylmethylene) aniline ligand is coordinated to the Ru(II) center through the N atom of
(
the pyridine and the N atom of the imine bond in a bidentate manner resulting in a five member
metallacycle. The geometry around the Ru(II) center is distorted pseudo-octahedral in which the
two N atoms and the Cl atoms form the base, while the p-cymene rings form the apex of a
“three-legged piano stool” structure [38]. All three cations feature two distinct molecular planes
described by the pyridine ring including the imine N, and the para-substituted phenyl ring. The
two planes have a dihedral angle of 50.9(1) in 7(I), 52.2(1) in 7(II) and 55.7(1)° in 10
presumably to reduce steric hindrance caused by the cymene ligand, which is in agreement with
reports on related compounds [39]. The Ru-N bond lengths of the three complexes lie between
2
.066(3) Å and 2.083(3) Å and are comparable to those reported for similar arene ruthenium
complexes with N, N’ donor ligands [39,40]. The Ru-N bond lengths also fall within the
6
2
reported values of half-sandwich complexes of [(η -arene)Ru(ꢀ-Cl)Cl] with nitrogen ligands
which ranged from 2.060(5) Å to 2.156(2) Å [40]. The N-Ru-N bond angles range from 76.62(5)
to 76.78(12) and the N-Ru-Cl bond angles range from 84.46(8) to 85.39(8) ˚. These values are
close to those reported for related compounds [40,41].
14

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Table 4: Selected bond lengths (Å) and angles (deg)
7
(I)
7(II)
10e
Bond Lengths
Ru(1)-N(1)
Ru(1)-N(2)
Ru(1)-Cl(1)
N(1)-C(5)
Bond Lengths
Ru(1)-N(1)
Ru(1)-N(2)
Ru(1)-Cl(1)
N(1)-C(15)
N(1)-C(11)
Bond angles
Bond Lengths
2.103(3)
2.066(3)
2.083(3)
2.3641(10)
1.351(4)
1.330(3)
Ru(1)-N(1)
Ru(1)-N(2)
Ru(1)-Cl(3)
N(2)-C(17)
N(2)-C(16)
2.0799(12)
2.094(3)
2.4039(11)
1.364(5)
1.435 (5)
2.0687(12)
2.3910(4)
1.4310(18)
1.2900(19)
N(1)-C(7)
Bond angles
Bond angles
N(1)-Ru(1)-N(2)
76.70(12)
85.56(9)
85.77(9)
N(1)-Ru(1)-N(2)
N(1)-Ru(1)-Cl(1)
N(2)-Ru(1)-Cl(1)
76.78(12)
85.46(8)
85.39(8)
N(2)-Ru(1)-N(1)
N(1)-Ru(1)-Cl(3)
N(2)-Ru(1)-Cl(3)
76.62(5)
84.52(3)
85.06(3)
N(1)-Ru(1)-Cl(1)
N(2)-Ru(1)-Cl(1)
Figure 5. The molecular structure of 7(I) showing atomic numbering scheme. Displacement
ellipsoids are drawn at the 50% probability level. The PF counter ion has been omitted for
6
clarity.
15

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Figure 6. The molecular structure of 7(II) showing the atomic numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level.
Figure 7. The molecular structure of 10 showing the atomic numbering scheme. Displacement
ellipsoids are drawn at the 50% probability level
16

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3
.3 Styrene oxidation
.3.1 Optimization
3
The effects of the solvent, oxidant, catalyst loading and temperature were investigated in
preliminary optimization studies of the half-sandwich ruthenium compound 7 in the catalytic
oxidation of styrene. Initially the effect of temperature was investigated, at room temperature,
4
0 and 60 ˚C. The best conversion of styrene and yield to benzaldehyde was at 60 ˚C and thus
this temperature was chosen for the further studies. Next, the effect of the solvent was
investigated using acetonitrile, toluene, anisole and tert-butanol. The optimum ratio of organic
solvent to water was found to be 1:1. In addition, tert-butanol was found to be the best solvent
giving the highest conversion of styrene and highest selectivity to benzaldehyde.
In addition, three different oxidants were investigated, namely tert-butyl hydroperoxide, oxone
and NaIO . NaIO gave the best conversion and selectivity towards benzaldehyde and thus was
4
4
selected for subsequent reactions. It was found that the best conversion was obtained with 4
equivalents of the oxidant. On increasing the amount of the oxidant beyond 4 equivalents the
same conversion of styrene and yield to benzaldehyde was obtained, and on decreasing the ratio
a drop in conversion was found. The effect of catalyst loading was also explored by varying the
loading from 0.625 mol% to 2.5 mol%. The best catalytic results were achieved when 1 mol%
of catalyst was added to the reaction media. A lower amount of added catalysts showed a drop in
conversion, while catalyst loading above 1 mol % did not change the conversion and yields.
The best conditions for maximum conversion of styrene to benzaldehyde thus were found to be a
1
4
:1 tert-butanol: water ratio; 60 ˚C, 1 mol % catalyst loading and a co-oxidant equivalence of
%. A series of blank experiments were carried out under identical conditions, which showed
that the ruthenium(II) complexes did not give conversions without NaIO . The oxidant NaIO
4
4
alone gave low conversion without the presence of the catalyst. The final conversions are
reported as an average of three runs for each catalytic reaction.
3
.3.2 Catalytic results discussion
Previous research on styrene oxidation has demonstrated that oxidation at the side chain can lead
to various reaction products, depending on the catalyst and the reaction conditions. Two major
reactions take place, namely oxidative C=C cleavage into benzaldehyde and epoxidation [1,42].
Under the present catalysis conditions the major oxidation product was benzaldehyde. This may
be attributed to direct oxidative cleavage of C=C of styrene or the fast conversion of styrene
oxide to benzaldehyde. The latter route was confirmed by the oxidation of styrene oxide under
the same catalytic conditions which showed complete conversion to benzaldehyde.
17

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Complexes 1-5 and 6-10 were then tested for the catalytic oxidation of styrene. Highest
conversions were reached after three hours with catalysts 1-4, one hour with catalysts 5 and 10
(Figs. 8 and 9). For catalysts 6-9 the highest conversions were obtained after 2 hours (Table 6).
6
Figure 8. Effect of Substituents on [(η -arene)RuCl(C
5
H
4
N-2-CH=N-R)]PF
6
(arene = C
6
H
6
(1-
5
)) on styrene oxidation.
6
Figure 9. Effect of Substituents on [(η -arene)RuCl(C H N-2-CH=N-R)]PF (arene = p-cymene
5
4
6
(6-10)) on styrene oxidation.
The increase in the byproduct benzoic acid with increase in reaction time implied that it was
formed by further reaction of benzaldehyde and the formation of benzaldehyde and phenyl
18

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acetaldehyde might proceeded via parallel pathways. The product specific turn over numbers
(TON) and turn over frequencies are shown in Table 6.
Table 6. Oxidation of styrene results for catalysts 1-5 and 6-10
a
b
Catalyst
Major product Time
TON
78
76
84
76
87
76
88
76
TOF
26
25
28
25
87
38
44
41
38
69
1
2
3
4
5
6
7
8
9
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
3
3
3
3
1
2
2
2
2
1
81
69
1
a
0
TON: mol of product/mol of catalyst
b
TOF: mol of product/(mol. of catalyst x time)
On comparing the effect of changing the arene ring on styrene oxidation, the complexes with p-
cymene ligands perform better in terms of time taken to complete the reaction. There was no
remarkable effect on changing the ligand substitution, except with the 2, 5- substituted ligand
and this could be attributed to the strong electron withdrawing nature of this ligand which may
stabilize the intermediate oxo species.
Having established the optimum conditions for the reaction, the study was extended to other
olefins aimed at establishing its scope and limitations. For the extended study catalyst 5 was
used, since it gave the highest catalytic TON for the respective series. The catalyst showed high
conversions for styrene with both electron withdrawing (Cl) group and electron donating groups
(Me, OMe), giving the corresponding the aldehyde in very good yields. The catalyst was also
able to cleave the bond in trans-stilbene to give benzaldehyde and was active for straight chain
alkenes, with 1-octene giving 1-heptanal (Table 7).
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Table 7: Oxidative cleavage of olefins by 5 with NaIO4
Entry Substrate
1
Product
Conversion Yield
Time TON TOF
4-methyl benzaldehyde
100
92
0.5
93
186
2
4-methoxy benzaldehyde
98
90
2
86
86
O
3
Benzaldehyde
100
93
1
93
93
4
4-chrolo benzaldehyde
1-heptanal
100
99
94
91
1
1
94
92
94
92
Cl
5
All the catalyst systems displayed good activity and selectivity to the cleaved product.
Catalyst’’ 5 was recycled three cycles with styrene as the substrate without significant loss of
“
activity. A 5% increase in benzoic acid yield was observed and this could be attributed to the
species formed after the first catalytic cycle being significantly more active than the original
catalyst and thus leading to some over oxidation of the benzaldehyde formed. This implies that
the ruthenium(II) arene complex is not the catalyst itself, but a catalyst precursor which forms a
more catalytically active intermediate which is not deactivated at the end of the catalytic
reaction.
When the catalysts in this study are compared with other reported ruthenium catalysts, they were
found to perform better than the Ru-N-heterocyclic carbene compounds reported by Poyatos and
coworkers [43], who reported lower conversions 23-58% and longer reaction times (24 hours)
for styrene and stilbene derivatives. The catalysts in this study were also better when compared
2
+
to [Ru(dmp) (H O) ] (dmp= 2,9-dimethyl phenathroline) which was found to require longer
2
2
2
reaction times (up to eight hours) [44]. In addition, these catalyst were also better than a
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reported RuCl
3
/NaIO
4
system by Yang and Zhang, which required high catalyst loading (3.5
mol %) to afford moderate yields of oxidative cleaved products [45]. However, catalysts 1-5 and
-10 were found to be less reactive than the system reported by Daw and coworkers who used
Ru(COD)(L )Br ], where L is a fused naphthyridine–based N-heterocyclic carbon ligand, in an
6
[
1
2
1
EtOAC/CH CN/H O solvent mixture at room temperature [2]. When these catalysts are
3
2
5
compared to η -cyclopentadienyl dicarbonyl ruthenium(II) amine complexes [46], the reported
catalysts required a lower catalyst loading of 0.5 mol% but also longer reaction times.
On the basis of earlier work [41,46-50] and some observations made in this work, it is suggested
that the selective aldehyde formation involves a high valent Ru-oxo species as an intermediate.
The mechanism of catalytic oxidation seems to involve Ru(VI)=O species, which formed by free
radicals generated by heterolytic cleavage of the oxidants [51]. On the addition of NaIO4
(oxidant) to the tert-butanol solution of 6, 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 [49]. This
value is close to the values reported by other researchers for Ru(VI)=O species and the
Ru(VI)=O is the functionality reported to be responsible for transfer of the oxygen. Furthermore,
when the solvent from a mixture of NaIO with complex 6 was evaporated, the IR spectrum of
4
-1
the remaining residue was found to exhibit strong bands at 813, 764 and 718 cm which could
be assigned to the symmetric and asymmetric stretching of the Ru=O bonds of the Ru(VI) di-oxo
species, which further supports the formation of Ru(VI)=O species responsible for the catalytic
1
oxidation [47]. The H NMR spectrum of the residue showed three peaks in the region δ 6.10 to
5
.31 ppm assignable to the Ru(cymene) arene ring. This suggests that the ruthenium moiety
containing the coordinated cymene remains after the catalytic run. This observation has been
reported also for related compounds [5,46]
Furthermore, the ESI-MS spectra of the residue from the experiment carried out using 6 was
obtained. One major species observed containing the Ru fragment showed a peak at 398
(
maximum of the isotopic cluster), which may be attributed to [(p-
+
cymene)RuCl(O )(MeCN)(H O) ] . This result suggests that there is in existence in solution a
2
2
3
ruthenium species containing coordinated cymene, but also that there is cleavage of the N, N-
bidentate ligand. On the basis of the mentioned observations and available literature, a
mechanism that involves a Ru(VI) oxo species can be proposed (Scheme 2). The catalysts reacts
with the terminal oxidant NaIO to form a Ru(VI)-cis dioxo intermediate. Then, electrophilic
4
attack on the C=C bond by the high-valent dioxo-intermediate affords a Ru(IV) cyclo adduct via
a concerted [3+2] cycloaddition reaction.
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Scheme 2: Proposed mechanism for styrene oxidation using compounds 1-10.
Conclusion
The mononuclear complexes 1 to 5 and 6 to 10 have been successfully synthesized, isolated and
characterized. The structures of two of these compounds have been determined by single crystal
crystallography. Compound 7 was found to crystallize as two polymorphs, triclinic and
orthorhombic, depending on the method of crystal growth. All the complexes showed the pseudo
octahedral three legged piano stool geometry. The compounds were found to be very reactive in
styrene oxidation with good benzaldehyde selectivity. The major products obtained were
benzaldehyde (70-88%) and benzoic acid (5-16%). The oxidative cleavage of other substrates 4-
methyl styrene, 4-methoxy styrene, stilbene, 4-chloro styrene and 1-octene gave the
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corresponding aldehydes. The proposed mechanism involves a Ru(VI) oxo species as supported
1
by the UV, HNMR, ESI-MS and IR spectra.
Supplementary material
CCDC 1432508, 1432426, 1432344 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
1
EZ, 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. JG thanks Prof. E. N. Njoka for his support.
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Synthesis and characterization of piano-stool ruthenium complexes with N, Nʹ-pyridine
imine bidentate ligands and their application in styrene oxidation.
Research Highlights
•
•
•
•
New arene ruthenium(II) pyridine-imine complexes have been prepared
Their crystal structure show the pseudo-octahedral piano stool geometry
The complexes were applied in the oxidation of olefins in a H
2
O/t-butanol mixture
IR and UV-vis studies suggest the mechanism involves a Ru(IV) oxo species