O,N-bidentate ruthenium azo complexes as catalysts for olefin isomerization reactions

Article abstract of DOI:10.1002/ejic.200900926

A series of ruthenium complexes [(η⁶-p-cymene) RuCl(L)] [ligands L incorporate an azo group (1-5) or an imino group (67)] havo boon synthesized and studied as olefin isomerization catalyst with allylbenzene and 1-octene as model substrates. Temperature and catalyst/substrate mol ratio have been taken into account as parameters to optimize the isomerization reaction conditions. By using ¹H, ¹³C NMR, FT-JR and microanalysis, the new complexes have been characterized and the molecular structure of complex 4 has been determined by crystal structure determination.

Full text of DOI:10.1002/ejic.200900926

FULL PAPER
DOI: 10.1002/ejic.200900926
O,N-Bidentate Ruthenium Azo Complexes as Catalysts for Olefin
Isomerization Reactions
Fu Ding,[a,b] Yaguang Sun,[b] and Francis Verpoort*[a]
Keywords: Isomerization / Olefins / Ruthenium
A series of ruthenium complexes [(η6-p-cymene)RuCl(L)] [li-
gands L incorporate an azo group (1–5) or an imino group (6–
7)] have been synthesized and studied as olefin isomerization
catalyst with allylbenzene and 1-octene as model substrates.
Temperature and catalyst/substrate mol ratio have been
taken into account as parameters to optimize the isomeriza-
tion reaction conditions. By using 1H, 13C NMR, FT-IR and
microanalysis, the new complexes have been characterized
and the molecular structure of complex 4 has been deter-
mined by crystal structure determination.
or metallacycles.[16] Moreover, the azo group (–N=N–) can
stabilize ruthenium in lower oxidation states due to its
strong π-acidic character, while naphtholate/phenolate oxy-
gen, being a hard base, stabilizes the higher oxidation states
of the metal ion.[15]
Introduction
Olefin isomerization with transition-metal catalysts is
well established in organic chemistry in terms of both aca-
demic curiosities and industrial interests.[1] Today, the avail-
able catalysts have proven their value in many conditions,
some of them are even frequently used, for example, the
Wilkinson catalyst [(PPh3)3RhCl] is employed in the isom-
erization of allylic ethers.[2] However, there is still a high
request for cheaper, readily available and highly selective
transition metal systems. Furthermore, systems providing
especially mild and efficient conditions are required ensur-
ing the C=C bond is only isomerized to a certain position.
During recent years the chemistry of half-sandwich (η6-
arene)ruthenium(II) complexes has been the subject of in-
tense research in the field of organometallic chemistry.[3–6]
Except for isomerization, the catalytic properties of these
complexes have been investigated in various fields ranging
from hydrogen transfer[7–11] to ring-closing metathe-
sis,[4,12–14] etc.
The extension of the catalytic activities towards isomer-
ization depends on the metal itself and its structure. Tuning
of the ligand environment with the aim to develop catalysts
for the isomerization is of our concern. Herein, as an ad-
dition of previous investigations on the ruthenium Schiff
base chemistry,[15] analogous ligands e.g. Sudan compounds
are selected as ligands to investigate.
The interest for these ligands (Scheme 1) is mainly based
on three reasons. Firstly, because of the π-acidic nature of
the azo function, the ligands are able to form M–C bonds
Scheme 1. Structure of the azo and imino ligands.
Secondly, the naphthol-based ligands are not only poten-
tial ligands that can coordinate with the metal by oxygen
and nitrogen in a bidentate mode, but they are also better
electron-donating agents and provide more steric crowding
compared to the phenyl group. Naphthol based azo com-
pounds have not been used very often as ligands, although
recent work has gone some way to redress the balance. Un-
til now, only three reports could be found in respect to these
ligands in combination with Ru(II)[17] and Ru(III).[18,19]
Two reports describe the ligands as mono-anionic O,N-bi-
dentate donor ligands, while the third report describes the
ligand as a di-anionic C,O,N-tridentate donor ligand.
[a] Department of Inorganic and Physical Chemistry, Division of
Organometallics and Catalysis, Ghent University,
Krijgslaan 281-S3, 9000 Ghent, Belgium
Fax: +32-9-264-49-83
E-mail: francis.verpoort@ugent.be
[b] Laboratory of Coordination Chemistry, Shenyang Institute of
Chemical Technology,
Shenyang 110142, P. R. China
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O,N-Bidentate Ruthenium Azo Complexes
Thirdly, we expect that some of the selected ligands can bit many sharp and strong vibrations within the region of
act as a tridentate donor capable of generating an extra vac- 500–1700 cm–1. To confirm the ligand coordination IR
ancy on the metal center during the catalytic reaction.
spectra of the ligands were analyzed and compared with the
As reported earlier in the literature, azonaphthol and spectra of the complexes. The infrared spectra of the ligands
azophenyl complexes have been extensively used as catalysts show bands around 1446–1476 cm–1 and 1266–1278 cm–1
for transfer hydrogenation of ketones.[17,20] Among the dif- corresponding to the azo group ν–N=N– and the aromatic
ferent metals catalyzing this reaction, ruthenium-based sys- νC–O stretch vibrations respectively. In the coordinated
tems are found to be effective.[21–23] However, in many cases, compounds, the azo vibration, ν–N=N–, exposes peaks at
double bond isomerization occurs as an undesired side reac- lower frequency 1364–1380 cm–1. This supports the as-
tion with the result that two reactions can be performed by sumption that the coordination of the nitrogen atom can
the same catalyst.[24–26]
reduce the electron density in the azo frequency due to the
In this study we focused on the synthesis, characteriza- (d-π) RuIIǞπ*(L) back bonding effect.[27] Meanwhile, in all
tion and isomerization performance of half-sandwich ruthe- the compounds, a shift to higher frequency occurs for the
nium azo and imino complexes. To the best of our knowl- aromatic νC–O stretch confirming the coordination of the
edge, this is the first time these compounds are studied as aromatic oxygen.[19]
isomerization catalysts.
X-ray Crystallography
Results and Discussion
In order to determine the coordination mode of the li-
gands in the ruthenium complexes and the stereochemistry
Synthesis
of the complexes, the crystallization of the complex [(η6-p-
cymene)RuCl(L4)] (4) was performed by slow evaporation
A series of ruthenium(II) complexes of the general for-
mula [(η6-p-cymene)RuCl(Ln)] (n = 1–7) incorporating an
azo group (1–5) or an imino group (6–7) have been synthe-
sized conveniently in the following way. To a solution of
[(η6-p-cymene)RuCl2]2 in THF, the appropriate thallium
salt (TlLn) was added and stirred for 6–8 h at room tem-
perature. Thallium chloride was removed via filtration. Af-
ter purification, the novel ruthenium complexes were ob-
tained as red-brown to dark-brown solids with 90–98%
yield.
It follows that all ligands act as bidentate ligands, al-
though some ligands, especially ligand 4 have been assumed
to act as a tridentate N,N,O donor ligand; this was not the
case here. All the complexes are found to be air-stable in
both solid and liquid states at room temperature and are
non-hygroscopic.
of a MeOH/hexane solution at 0–4 °C. It follows from X-
ray analysis that one MeOH molecule (solvent) is enclosed
in the [(η6-p-cymene)RuCl(L4)]·CH3OH crystal. The X-ray
analysis results and refinements are given in Tables 1 and
2. The complex essentially has an octahedral coordination
geometry, as shown in Figure 1. The molecular structure
shows that the O1,N2 donor in the arylazo-naphtholate li-
gand adopts a bidentate chelating mode and the p-cymene
carbons occupy one face of the octahedron. The Ru–C-
(arene) distances vary considerably from 2.182–2.212 Å.
Compared with earlier reported work, the distances of Ru1–
O1, Ru1–N2 and Ru1–C are within the normal range.[17]
A
six-membered ring planar with the naphtholate ligand is
generated by the two carbon atoms from the naphtholate
Table 1. Crystal data and structure refinement for [(η6-p-cymene)-
RuCl(L4)]·CH3OH.
Characterization
Molecular formula
Molecular weight
C35H37ClN4O2Ru
682.21
For ruthenium compounds, the methyl signal (singlet) of
the p-cymene ligand appears in the range of 2.18–2.96 ppm
and the isopropyl protons of the p-cymene ligand give rise
to signals (two doublets) in the range of 1.05–1.97 ppm.
The isopropyl CH signal appears as a septet in the range of
2.17–3.90 ppm. The signals of p-cymene ring show as either
four doublets (4 H) or two doublets (2 H) and a singlet (2
H) in the range of 3.75–5.51 ppm. In 13C NMR spectra, the
p-cymene resonances are assignable to four distinctive ran-
ges of 17.63–19.08, 21.52–23.67, 29.71–31.15 and 77.94–
101.80 ppm. The other spectral features are as expected. In
complexes 1–7, a similar peak in the range of 161.76–
165.36 ppm corresponds to the phenolato/naphtholato C
atoms.
Crystal size [mm]
Color
0.18ϫ0.16ϫ0.10
brown
Crystal system
Space group
monoclinic
P2(1)/n
a [Å]
b [Å]
c [Å]
β
V
Z
15.845(3)
14.337(3)
17.406(4)
116.27(3)
3545.8(12)
4
Dc
F(000)
1.278
1408
Absorption correction Tmax/Tmin
Goodness-of-fit on F2
Absorption coefficient [mm–1]
Largest diff. peak and hole [eÅ–3]
Data/restraints/parameters
R1, wR2[IϾ2σ(I)]
R1, wR2 (all data)
0.9469/ 0.9073
1.139
0.551
0.667/–0.629
6189/326/452
0.0993, 0.2325
0.1324, 0.2548
The IR bands from the azo group of the metal complexes
[(η6-p-cymene)RuCl(Ln)] (n = 1–5) are most useful to deter-
mine the coordination mode. For instance, all of them exhi-
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F. Ding, Y. Sun, F. Verpoort
FULL PAPER
group, the two nitrogen atoms from the coordinated diazo
To optimize the reaction conditions, the different cata-
group, the oxygen from the phenolic group and ruthenium. lyst/substrate mol ratios and temperature were chosen as
The dihedral angle of the naphthalene plane and the ben- parameters and the results are summarized in Table 3
zene plane C21–C26 in the arylazo-naphtholate ligand is (Scheme 2) and Table 4 (Scheme 3).
64.0°. Comparing the crystal data with the one reported by
Table 3. Isomerization of 1-octene by [(η6-p-cymene)RuCl(Ln)] (n
Rakesh, most of the bond angles of the crystal [(η6-p-
=
1–5). Conversions (substrate) and yields (products) were
cymene)RuCl(L4)]·CH3OH were larger due to the increased
steric congestion around the Ru center.
measured after 12 h.
Cat. T [°C] C/S[a] % Conv. % Yield % Yield
% Yield
1-octene 2-octene 3-octene
4-octene[b]
Table 2. Selected bond lengths [Å] and angles [°] for complex 4.
Bond lengths [Å]
trans
cis
1
2
3
4
5
60
80
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
a
b
90.6
87.9
87.0
75.6
80.4
68.8
68.8
59.8
92.2
78.7
86.9
84.2
37.6
36.0
92.2
78.8
87.1
84.2
44.6
27.8
82.2
73.7
74.1
56.9
32.1
0.8
14.9
51.0
10.5
14.0
10.0
10.0
42.4
59.7
8.3
9.5
7.3
6.7
5.9
3.8
8.2
9.4
7.2
6.6
8.3
3.4
8.8
10.4
10.8
6.9
4.7
0.1
0.2
0.2
0.2
0.1
49.8
28.2
55.4
51.0
11.3
46.0
17.4
0.1
51.1
59.0
55.6
55.5
27.3
28.1
52.4
59.1
55.7
55.5
31.6
21.0
51.2
51.7
51.1
42.7
24.0
0.2
0
25.8
7.5
17.6
8.9
57.0
10.3
9.0
0
25.6
7.0
19.5
17.9
3.9
3.6
24.5
7.0
19.6
17.9
4.4
Ru(1)–O(1)
Ru(1)–C(4)
Ru(1)–C(7)
Ru(1)–C(3)
Ru(1)–Cl(1)
2.038(4)
2.181(5)
2.191(6)
2.205(5)
2.4095(15)
Ru(1)–N(2)
Ru(1)–C(6)
Ru(1)–C(5)
Ru(1)–C(2)
O(1)–C(11)
2.051(5)
2.183(5)
2.202(6)
2.211(6)
1.308(6)
1.2
3.5
1.6
2.1
2.5
0
100
60
Bond angles [°]
0
O(1)–Ru(1)–N(2) 88.9(2)
O(1)–Ru(1)–C(4) 88.39(19)
O(1)–Ru(1)–C(6) 122.2(2)
N(2)–Ru(1)–C(4) 153.8(2)
N(2)–Ru(1)–C(6) 92.98(19)
N(2)–Ru(1)–C(7) 95.7(2)
N(2)–Ru(1)–C(3) 159.8(3)
O(1)–Ru(1)–Cl(1) 85.16(11)
C(4)–Ru(1)–C(6)
N(2)–Ru(1)–C(5)
N(2)–Ru(1)–C(2)
66.8(2)
116.3(2)
121.8(3)
80
6.9
3.2
4.5
4.2
0.6
0.5
7.1
3.2
4.6
4.2
0.3
0.51
4.52
1.67
1.99
1.32
0.93
0.07
0.06
0.09
0.06
0.04
100
60
N(2)–Ru(1)–Cl(1) 84.16(15)
C0–Ru(1)–Cl(1)
C0–Ru(1)–O(1)
C0–Ru(1)–N(2)
128.33(19)
125.62(12)
129.97(13)
80
100
60
2.9
80
16.8
10.1
10.2
6.0
2.4
0.4
0.5
0.5
0.5
0.4
100
60
80
1.4
1.3
1.2
0.7
0.6
0.5
0.5
0.2
100
[a] Catalyst/Substrate (C/S) ratio: a = 1:1000, b = 1:2000. [b] GC
results, only trans/cis-4-octene can be specified; the trans/cis iso-
mers of 2- and 3-octene were not separated.
Figure 1. Molecular structure of complex 4, the solvent molecule
MeOH has been omitted for clarity (30% thermal probability ellip-
soids).
Isomerization Activity
Allylbenzene is used as a model substrate for the isomer-
ization applying all kinds of catalysts, such as the transition
metals Ti, Zr,[28] Co,[29] Rh,[30,31] Ir,[32] Au,[33] and Ru,[34–37]
or Mg,[38] or even a non-metal compound like P.[39] While
1-octene is also extensively used as a model substrate,[58]
these two compounds are selected as representatives alkenes
to evaluate the performance of the complexes [(η6-p-
cymene)RuCl(Ln)] (n = 1–7).
Scheme 2. Isomerization of 1-octene.
From Tables 3 and 4 it follows that changing the C/S
To investigate the catalytic activity, all ligands and pre- ratio from 1:1000 to 1:2000, the reaction still proceeds
cursor complexes have been screened under identical reac- smoothly accompanied by a moderate drop in conversion.
tion conditions and no isomerization activity could be ob- Compared with the raise of the C/S ratio, the decrease of
served. Compound 6 and 7 are also found to be totally inac- conversion is much less than half in most cases. Higher cat-
tive for the isomerization of both substrates.
alyst concentrations favored the formation of the cis-alkene;
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O,N-Bidentate Ruthenium Azo Complexes
Table 4. Isomerization of allylbenzene by [(η6-p-cymene)RuCl(Ln)]
(n = 1–5); conversions were measured after 12 h.
It is known from the literature that azonaphthol/azo-
phenyl-Ru complexes are used as catalysts for transfer hy-
drogenation of carbonyl compounds with 2-propanol as hy-
drogen source. In this classical pathway, the ruthenium
complexes dehydrogenate the alcohol and deliver the hy-
drides to a ketone or an α,β-unsaturated ketone.[41,42] How-
ever, the coordination of the substrate to the hydride ruthe-
nium metal intermediate is not suitable here.
Catalyst
C/S
% Conversion
ratio[a] 60 °C
80 °C
100 °C
[(η6-p-Cymene)RuCl(L1)]
[(η6-p-Cymene)RuCl(L2)]
[(η6-p-Cymene)RuCl(L3)]
[(η6-p-Cymene)RuCl(L4)]
[(η6-p-Cymene)RuCl(L5)]
a
b
a
b
a
b
a
b
a
b
89.2
73.0
27.2
25.4
10.8
10.1
34.1
27.6
6.5
95.6
93.4
90.9
85.9
90.8
85.9
81.1
66.5
5.7
98.4
96.3
86.3
70.2
86.2
70.1
76.9
31.5
3.4
Moreover, studies by other teams dealing with metathesis
reactions mediated by ruthenium-arene complexes have
shown that the release of the arene ligand is crucial and is
responsible for the generation of the catalytic active spe-
cies.[43–45] Nonetheless, in this study, a careful NMR moni-
toring of the isomerization revealed no ligation of the cy-
2.3
1.6
0.4
[a] Catalyst/substrate molar ratios: a = 1:1000, b = 1:2000.
1
mene ligand (Figure 2). Comparing the H NMR spectra
of the cymene region of the catalyst before and during reac-
tion (Figure 2), it was observed that the four doublet peaks
of the cymene ring remain during the catalytic reaction. The
shift of the peaks can be understood as a result of the trans-
formation of coordination from η6 to η4. Therefore, it is
plausible to state that the proposed mechanism depicted in
Scheme 4 is responsible for the formation of the catalytic
Scheme 3. Isomerization of allylbenzene.
as the catalyst concentration was lowered, more trans-iso- active species during the isomerization reaction.
mer was formed. Comparing the different catalysts it fol-
lows that the variation in selectivity is related to the bulki-
ness of the aniline part of the azonaphthol ligand. This sig-
nifies that less steric hindering in the ortho positions gener-
ates a higher cis ratio.
Catalysts 2–4 show higher activities when temperature is
enhanced from 60–80 °C. Increasing the temperature to
100 °C diminished the final conversion. From these results,
it is clear that a temperature of 80 °C is the best compro-
mise for catalysts 2–4.
With 1-octene as substrate, the higher the temperature,
the lower the substrate conversion becomes for catalyst 1
and 5. This phenomenon can be related to the thermal sta-
bility of the catalysts. While using allylbenzene, the behavior
of catalyst 1 is consistent with the temperature, however, a
contrasting behavior of catalyst 5 is observed.
If only the conversion of 1-octene is taken into account,
the catalytic activity of catalysts 1–3 is similar (yield 92–
90%) without dimerization or polymerization. Considering
the selectivity, catalysts 1–4 all catalyze 1-octene to n-octene
in yields higher than 50% by controlling the C:S ratio, tem-
perature and reaction time. For the catalyst 2, using a C:S
= 1:2000, preferentially 2-octene was generated and the
highest product selectivity was reached.
Comparison of the catalysts reveals that catalyst 1 has a
better performance for allylbenzene, implicating an excel-
lent yield (more than 98%) and a high selectivity (trans-β-
methylstyrene exclusively). Contrary to the results from late
transition metal complexes in which the isomerized prod-
ucts are in equilibrium, this interesting result – no cis-β-
methylstyrene – indicates the lack of cis/trans-isomerization
of β-methylstyrene.[40] The conversion obtained by using
catalyst 5 was much lower in comparison to the other cata-
1
lysts indicating that the structure of naphthol plays a sig-
nificant role in isomerization.
Figure 2. H NMR spectrum of the cymene region of the catalyst
before reaction (upper) and during reaction (bottom).
Eur. J. Inorg. Chem. 2010, 1536–1543
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F. Ding, Y. Sun, F. Verpoort
FULL PAPER
Scheme 4. Proposed mechanism for the isomerization using catalysts 1–5.
The proposed pathway for this reaction is the decoordi- tronic environment of the ruthenium. So, the prospective of
nation of the nitrogen generating a vacant coordination site these catalysts in the field of isomerization can be further
followed by the substrate coordination to the metal center. improved by fine-tuning of the ligand environment. The
Subsequently, following the less common π-allyl mecha- isomerization activity of complex 1 for allylbenzene is supe-
nism, an oxidative addition of an activated allylic C–H rior to most of the reported systems.[32,35,51–54]
bond to the metal yields a π-allyl metal hydride and gener-
In general, the results of the present exploration suggest
a promising application of a new family of organorutheni-
ates the desired isomerization product.[46–50]
Furthermore, results obtained from experiments in which um(II) complexes containing an azonaphthol/azophenyl
the influence of moisture and air sensitivity was studied group. Furthermore, the fact that these catalysts have been
proved that the isomerization using catalyst 1–5 is not affec- reported for transfer hydrogenation and exhibiting good
ted and so, the isomerization can be executed using solvents isomerization activities (this work) allows them to combine
form the bottle and in open air.
these two methodologies to some interesting properties by
using new substrate combinations. Further studies concern-
ing these points are currently under investigation.
Conclusions
In conclusion, a new class of ruthenium-based catalysts
useful for isomerization has been synthesized. The X-ray
crystal structure of the complex 4 reveals an octahedral en-
vironment around ruthenium. In contrast to what is ob-
served for most other described catalysts, a noteworthy ad-
vantage of these catalysts is their inertness toward air and
moisture preventing a meticulous pretreatment of solvents
and substrates. This benefit makes the set-up of the experi-
ment and the monitoring of the reaction progress very con-
venient.
The ruthenium complexes have been tested on their
isomerization activity and their different behavior has been
explained. Catalysts 1–4 are highly active for the isomeriza-
tion without any dimerization or oligomerization. The ob-
tained results imply that the naphthol moiety plays a signifi-
cant role in the isomerization suggesting that the catalytic
Experimental Section
General: Unless otherwise stated, all reactions were carried out un-
der a dry argon atmosphere following conventional Schlenk tech-
niques. All solvents were distilled from the appropriate drying
agents and deoxygenated prior to use. 1H and 13C NMR spectra
were recorded on a Varian 300 spectrometer in CDCl3 (δ ppm).
The NMR spectroscopic data suggest a 1:1 molar ratio of the p-
cymene and the O,N ligand in 1–7. Elemental analysis was per-
formed using a Perkin–Elmer 2400 CHNS/O analyzer. The Sudan
ligands were purchased from Aldrich and used as such; [(p-
[55]
cymene)RuCl2]2 and the ligands 1-(2,4,6-trimethylphenylazo)-2-
naphthol,[56] (5-chloro-2-hydroxybenzenyl)(p-methylphenylazo)[57]
were prepared according to the literature. All other chemicals used
were of analytical grade without further purification.
Conversions, yields and selectivities of the isomerization reactions
were obtained via capillary GC using a Finnigan Trace GC Ultra
activity for 1-octene strongly depends on the steric and elec- with an Ultra Fast Column Module (PH-5 5% diphenyl/95% di-
1540 Eur. J. Inorg. Chem. 2010, 1536–1543
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O,N-Bidentate Ruthenium Azo Complexes
methyl poly-siloxane capillary, helium carrier gas, 1 mL/min) col-
umn (10 mϫ0.10 mm, 0.40 µm) and a FID detector. The tempera-
ture program starts at 50 °C increasing with 20 °C/min until the
end temperature (255 °C) is reached.
(ring of p-cymene), 121.95, 123.46, 124.49, 124.79, 127.19, 127.64,
127.71, 127.77, 128.28, 130.11, 134.89, 137.26, 153.12 (C–L1), 162.29
(C next to O) ppm.
[(η6-p-Cymene)RuCl(L2)] (2): (Scheme 5) Yield 93%, red-brown
powder. C28H29ClN2ORu (546.07): calcd. C 61.58, H 5.36, N 5.13;
Isomerization Procedure: Pretreatment of 1-octene was necessary
before screening; it was passed through a column (20 cmϫ1.5 cm)
of neutral alumina (Acros, 50–200 µm) containing 15 g of alumina
per 100 mL of 1-octene, collected in a Schlenk flask and degassed.
In an empty 15 mL reaction vessel, an appropriate quantity of the
catalyst under investigation was transferred together with toluene
as solvent followed by the substrates. The vessel was immersed in
an oil bath and allowed to equilibrate to the desired temperature
before timing. Before GC-analysis, the reaction mixture was puri-
fied over a silica filter in order to remove the catalyst. Distilled,
degassed hexane was used as solvent to prepare the GC samples
and 1-dodecane was taken as internal standard.
found C 62.01, H 5.81, N 5.34. IR: ν = ν
1372.15 cm–1,
˜
–N=N–
ν–C–O–Ru 1310.99 cm–1. H NMR: δ = 1.12, 1.24 (2d, JHH = 7 Hz,
2ϫ 3 H, 2 CH3, isopropyl of p-cymene), 2.16 (s, 3 H, CH3–phenyl),
2.33 (s, 3 H, CH3–phenyl), 2.44 (s, 3 H, CH3, p-cymene), 2.64 (sp,
1 H, CH, isopropyl), 4.31 (s, 1 H, ring H of p-cymene), 5.02 (s, 1
H, ring H of p-cymene), 5.38, 5.45 (2d, JHH = 21 Hz, 2ϫ 1 H, ring
H of p-cymene), 7.13–7.21 (m, 4 H, ring H), 7.32 (d, JHH = 6.9 Hz,
1 H, naphthol ring H), 7.51 (d, JHH = 7.8 Hz, 1 H, naphthol ring
1
H), 7.64 (d, JHH = 9.3 Hz, 1 H, naphthol ring H), 7.93 (d, JHH
=
9 Hz, 1 H, naphthol ring H), 8.13 (d, JHH = 8.4 Hz, 1 H, naphthol
ring H) ppm. 13C NMR: δ = 18.94 (CH, isopropyl), 22.15, 22.45 (2
CH3, isopropyl), 27.23 (CH3–phenyl), 30.63 (Me, p-cymene), 31.51
(CH3–phenyl), 77.94, 78.71, 80.51, 81.30 (ring of p-cymene),
121.92, 123.43, 124.31, 125.66, 127.79, 128.67, 128.91, 129.67,
133.48, 134.89, 153.67 (C–L2), 162.24 (C next to O) ppm.
General Procedure for the Preparation of the Thallium Salts: To a
solution of the ligands 1–7 in dry THF (15 mL) was added drop-
wise a solution of thallium ethoxide in THF (5 mL) at room tem-
perature. Immediately after the addition, a yellow solid formed and
the reaction mixture was stirred for 2 h. Filtration of the solid un-
der an Argon atmosphere gave the thallium salts in quantitative
yields. The salts were used without further purification.
[(η6-p-Cymene)RuCl(L3)] (3): (Scheme 5) Yield 95%, red-brown
powder. C29H31ClN2ORu (560.10): calcd. C 62.18, H 5.59, N 5.00;
found C 61.77, H 5.16, N 4.77. IR: ν = ν
1376.87 cm–1, ν–C–
˜
–N=N–
1308.22 cm–1. H NMR: δ = 1.26, 1.35 (2d, JHH = 7 Hz, 2ϫ
1
O–Ru
General Procedure for the Preparation of [(η6-p-Cymene)RuCl(Ln)] (1–
7): To a solution of [(η6-p-cymene)RuCl2]2 (0.10 g, 0.16 mmol) in
THF (5 mL) was added a solution of the corresponding appropriate
thallium salt in THF (5 mL) and stirred for 6–8 h at room tempera-
ture. Thallium chloride was removed via filtration. After evaporation
of the solvent, the residue was dissolved in a minimal amount of
toluene and cooled to 0 °C. Hexane was used to precipitate the de-
sired compound. The product was filtered, washed with hexane and
dried in vacuo. The novel ruthenium complexes were obtained as red-
brown to dark-brown solids; yields were between 90–98%.
3 H, 2 CH3, isopropyl of p-cymene), 2.03, 2.06 (s, 2ϫ 3 H, 2 CH3–
phenyl), 2.28 (s, 3 H, CH3–phenyl), 2.60 (s, 3 H, CH3, p-cymene),
2.75 (sp, 1 H, CH, isopropyl), 4.46, 4.91 (s, 2ϫ 1 H, ring H of p-
cymene), 5.33, 5.38 (d, JHH = 7 Hz, 2ϫ 1 H, ring H of p-cymene),
6.96–7.17 (m, 5 H, ring H), 7.49 (d, JHH = 9 Hz, naphthol ring H),
7.58 (d, JHH = 7 Hz, naphthol ring H), 7.95 (d, JHH = 8 Hz, naph-
thol ring H) ppm. 13C NMR: δ = 17.68, 18.02 (2 CH3–phenyl),
19.08 (CH, isopropyl), 20.98 (CH3–phenyl), 22.09, 22.53 (2 CH3,
isopropyl), 30.92 (Me, p-cymene), 79.40, 83.67, 87.31, 91.33 (ring of
p-cymene), 121.91, 123.37, 124.14, 127.54, 127.69, 128.46, 129.47,
130.48, 130.88, 136.84, 154.70 (C–L3), 162.23 (C next to O) ppm.
[(η6-p-Cymene)RuCl(L1)] (1): (Scheme 5) Although this complex has
been reported by Rakesh et al.,[17] a modified procedure was used to [(η6-p-Cymene)RuCl(L4)] (4): (Scheme 5) Yield 93%, red-brown
get high yield: 96% of a red-brown powder. C26H25ClN2ORu
powder. C34H33ClN4ORu (650.19): calcd. C 62.80, H 5.13, N 8.62;
(518.02): calcd. C 60.28, H 4.87, N 5.41; found C 59.95, H 4.47, N found C 62.57, H 5.24, N 8.23. IR: ν = ν
1370.93 cm–1,
˜
–N=N–
5.13. IR: ν = ν–N=N– 1371.63 , ν–C–O–Ru 1312.31 cm–1. 1H NMR: δ = ν–C–O–Ru 1294.59 cm–1. 1H NMR: δ = 1.13, 1.26 (2 d, JHH = 6.6 Hz,
˜
1.05, 1.12 (2d, JHH = 7 Hz, 2ϫ 3 H, 2 CH3, isopropyl of p-cymene, 2ϫ 3 H, 2 CH3, isopropyl of p-cymene), 2.20, 2.49 (s, 2ϫ 3 H, 2
), 2.25 (s, 3 H, CH3, p-cymene), 2.56 (sp, JHH = 7 Hz, 1 H, CH, CH3–phenyl), 2.96 (s, 3 H, CH3, p-cymene), 3.90 (sp, 1 H, CH,
isopropyl), 4.59, 4.96 (2d, JHH = 6 Hz, 2ϫ 1 H, ring H of p-cymene),
5.40 (s, 2 H, ring H of p-cymene), 7.14–7.56 (m, 7 H, ring H), 7.67 (d, JHH = 5.7 Hz, 2ϫ 1 H, ring H of p-cymene), 7.20–7.40 (m, 8
(d, JHH = 9 Hz, 1 H, naphthol ring H), 7.90 (d, JHH = 7 Hz, 2 H, H, ring H), 7.53 (d, JHH = 6 Hz, naphthol ring H), 7.56 (d, JHH
isopropyl), 4.43, 5.01 (s, 2ϫ 1 H, ring H of p-cymene), 5.42, 5.51
=
naphthol ring H), 8.26 (d, JHH = 8 Hz, 1 H, naphthol ring H) ppm. 9 Hz, naphthol ring H), 7.93 (d, JHH = 6.6 Hz, naphthol ring H),
13C NMR: δ = 18.79 (CH, isopropyl), 21.89, 22.72 (2 CH3, isopro- 8.14 (d, JHH = 6.4 Hz, naphthol ring H), 8.25 (d, JHH = 7 Hz,
pyl), 30.50 (Me, p-cymene), 83.41, 83.72, 85.83, 88.08, 101.14, 101.80 naphthol ring H) ppm. 13C NMR: δ = 17.63 (CH, isopropyl), 18.32,
Scheme 5. Synthesis procedure for catalysts 1–4.
Eur. J. Inorg. Chem. 2010, 1536–1543
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1541

F. Ding, Y. Sun, F. Verpoort
FULL PAPER
Scheme 6. Synthesis procedure for catalyst 5.
Scheme 7. Synthesis procedure for catalysts 6 and 7.
18.43 (2 CH3–phenyl), 21.52, 22.89 (2 CH3, isopropyl), 30.60 (Me, 22.99 (2 CH3, isopropyl), 30.60 (Me, p-cymene), 66.54 (–CH–
p-cymene), 79.68, 80.60, 83.29, 86.71, 87.69, 89.09 (ring of p- N=N), 80.52, 81.31, 83.21, 88.09 (ring of p-cymene), 114.02,
cymene), 115.31, 121.09, 121.64, 121.95, 122.75, 123.58, 123.78,
124.31, 124.58, 125.20, 126.51, 127.19, 127.85, 128.917, 130.40,
131.23, 131.39, 134.69, 136.744, 137.76, 151.26 (C–L5), 161.77 (C
next to O) ppm.
118.05, 122.34, 129.37, 129.82, 134.738, 135.14, 136.23, 164.16,
165.33 (C next to O) ppm.
Procedure for the Preparation of HL3: (Scheme 1) 7.3 g (54 mmol)
of 2,4,6-trimethyl aniline is dissolved in a solution containing
16 mL of concd. hydrochloric acid and 16 mL of water. By addition
of a solution of 4.0 g sodium nitrite in 20 mL water in small por-
tions and keeping the solution below 10 °C diazotizing occurs. The
temperature is kept below 10 °C until the colour of potassium io-
dide–starch paper dipped into the solution immediately turns blue.
A cooled solution (5 °C) of 7.8 g β-naphthol in 45 mL of 10% so-
dium hydroxide solution is vigorously stirred and added very slowly
[(η6-p-Cymene)RuCl(L5)] (5): (Scheme 6) Yield 90%, red-brown
powder. C22H23ClN2O2Ru (483.96): calcd. C 54.60, H 4.80, N 5.79;
found C 55.02, H 4.99, N 5.44. IR: ν = ν
1364.14 cm–1,
˜
–N=N–
ν–C–O–Ru 1278.24 cm–1. 1H NMR: δ = 1.24, 1.97 (2d, JHH = 4.5 Hz,
2ϫ 3 H, 2 CH3, isopropyl of p-cymene), 2.18 (s, 3 H, CH3, p-
cymene), 2.32 (sp, 1 H, CH, isopropyl), 4.60 (s, 2 H, ring H of p-
cymene), 4.99, 5.20 (d, JHH = 6 Hz, 2ϫ 1 H, ring H of p-cymene),
6.91, 7.00 (d, JHH = 9 Hz, 2ϫ 1 H, ring H), 7.12 (s, 1 H, ring H), to the cold diazonium salt solution. A red color appears and red
7.23–7.42 (m, 5 H, ring H), 9.79 (s, H of hydroxy) ppm. 13C NMR:
δ = 18.01 (CH, isopropyl), 22.85, 23.67 (2 CH3, isopropyl), 31.15
(Me, p-cymene), 79.88, 81.91, 83.30, 84.34, 97.18, 98.56 (ring of
p-cymene), 113.54, 121.46, 123.57, 127.72, 135.07, 140.01, 146.94,
152.26, 156.36, 165.36, (C next to O) ppm.
crystals of 1-(2,4,6-trimethylphenylazo)-2-naphthol (Scheme 1) pre-
cipitate. The filtered product was recrystallized from glacial acetic
acid, yielding 26% of HL3. C19H18N2O (290.36): calcd. C 78.58, H
6.26, N 9.65; found C 78.20, H 5.95, N 9.31. 1H NMR: δ = 2.35
(s, 3 H, CH3), 2.601 (s, 6 H, 2 CH3), 6.92 (s, 1 H, naphthol ring
H), 6.95 (s, 1 H, naphthol ring H), 7.00 (s, 2 H, phenyl ring H),
7.47 (t, J- = 7.5 Hz, 1 H, naphthol ring H), 7.63 (t, J- = 7.2 Hz, 1
H, naphthol ring H), 7.24 (d, J = 6.6 Hz, 1 H, naphthol ring H),
7.82 (d, J = 9.6 Hz, 1 H, naphthol ring H), 8.41 (d, J = 6.6 Hz, 1
H, hydroxy) ppm.
[(η6-p-Cymene)RuCl(L6)] (6): (Scheme 7) Yield 92%, red-brown
powder. C24H27ClN2ORu (496.01): calcd. C 58.12, H 5.49, N 5.65;
found C 58.38, H 5.48, N 5.46. IR: ν = ν
1377.76 cm–1,
˜
–N=N–
ν–C–O–Ru 1278.21 cm–1. H NMR: δ = 1.13, 1.22 (d, JHH = 1.2 Hz,
2ϫ 3 H, 2 CH3, isopropyl of p-cymene), 2.28 (s, 3 H, CH3, p-
cymene), 2.37 (s, 3 H, CH3–phenyl), 2.77 (sp, 1 H, CH, isopropyl),
1
Procedure for the Preparation of HL6: Triethylamine (4 equiv.
4.96, 5.17 (s, 2ϫ 1 H, ring H of p-cymene), 5.28, 5.45 (d, JHH
=
25.2 mmol) is added to a 50 mL methanol solution of the appropriate
6 Hz, 2ϫ 1 H, ring H of p-cymene), 6.48–7.85 (m, 8 H, ring H) tolylhydrazine hydrochloride (1 equiv., 6.3 mmol) resulting in a bright
ppm. 13C NMR: δ = 17.48 (CH3–phenyl), 18.62 (CH, isopropyl), yellow solution followed by a dropwise addition of a 10 mL methanol
22.64, 23.40 (2 CH3, isopropyl), 29.71 (Me, p-cymene), 65.45
(–CH–N=N), 81.52, 82.56, 85.74, 86.17 (ring of p-cymene), 113.09,
115.34, 121.28, 123.79, 128.23, 131.30, 132.43, 135.28, 160.85,
161.76 (C next to O) ppm.
solution of salicylaldehyde (1 equiv., 6.3 mmol). An orange-yellow
solution is obtained after 5–6 h stirring at room temperature. There-
after the solvent was evaporated till precipitation occurred.
(Scheme 1). Filtering the solid and recrystallization using ethyl ace-
tate/hexane (1:8) yielded 38% of HL6. C14H14N2O (226.28): calcd. C
74.30, H 6.25, N 12.38; found C 74.48, H 6.52, N 12.01. H NMR:
δ = 2.29 (s, 3 H, CH3), 6.86–7.25 (aromatic ring, 4 H, m), 7.40 (m, 3
H, NH), 7.82 (s, 1 H, CH), 10.91 (s, 1 H, OH) ppm.
[(η6-p-Cymene)RuCl(L7)] (7): (Scheme 7) Yield 91%, red-brown
1
powder. C24H26Cl2N2ORu (530.46): calcd. C 54.34, H 4.95, N 5.28;
found C 54.73, H 4.59, N 5.49. IR: ν = ν
1380.96 cm–1,
˜
–N=N–
ν–C–O–Ru 1293.15 cm–1. 1H NMR: δ = 1.25,1.32 (d, JHH = 6 Hz,
2ϫ 3 H, 2 CH3, isopropyl of p-cymene), 1.86 (s, 3 H, CH3, p-
cymene), 2.17 (sp, 1 H, CH, isopropyl), 2.36 (s, 3 H, CH3–phenyl),
3.75 (s, 2ϫ 1 H, ring H of p-cymene), 4.77, 5.23 (d, JHH = 3.6 Hz,
2ϫ 1 H, ring H of p-cymene), 7.18–7.36 (m, 9 H, ring H) ppm.
13C NMR: δ = 18.42 (CH3–phenyl), 18.94 (CH, isopropyl), 21.65,
Procedure for the Preparation of HL7: (Scheme 1) The same pro-
cedure was used as for HL6 except that 5-chlorosalicylaldehyde was
applied instead of salicylaldehyde. The yield after recrystallization
was 39 %. C14H13ClN2O (260.72): calcd. C 64.49, H 5.04, N 10.75;
1
found C 64.11, H 4.97, N 10.31. H NMR: δ = 2.30 (s, 3 H), 6.905
1542
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 1536–1543

O,N-Bidentate Ruthenium Azo Complexes
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Published Online: February 22, 2010
Eur. J. Inorg. Chem. 2010, 1536–1543
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1543