Covalent attachment of a biomimetic Ru-(terpy)(bpy) complex on silica surface: Catalytic potential

Article abstract of DOI:10.1016/j.poly.2012.07.094

The Ru-containing modified silica [Ru^II(terpy)(4′Mebpy/ 4CONH(CH₂)₃SiO_3/2)Cl]⁺ _m·zSiO₂ has been prepared by covalent attachment on silica surface of biomimetic [Ru^II(terpy)(4-CO₂H- 4′-Mebpy)Cl]⁺ complex through the formation of a pseudo-peptide bond. The catalytic ability of bio-derived silica for alkene oxidation with HOO^tBu has been evaluated exhibiting significant efficiency and, in some cases, showing increased activity compared vs. the corresponding 'net' [Ru^II(terpy)(4-CO₂H-4′-Mebpy)Cl]⁺ complex. The data supported that the covalently attached ruthenium complex preserves the catalytic behaviour of the 'net' ruthenium complex indicating that the presented grafting process was successful.

Full text of DOI:10.1016/j.poly.2012.07.094

Polyhedron xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Covalent attachment of a biomimetic Ru-(terpy)(bpy) complex on silica surface:
Catalytic potential
⇑
⇑
F. Papafotiou, K. Karidi, A. Garoufis , M. Louloudi
Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece
a r t i c l e i n f o
a b s t r a c t
The Ru-containing modified silica [Ru (terpy)(4 Mebpy/4CONH(CH ) SiO )Cl]+ ꢀzSiO has been pre-
II
0
Article history:
Available online xxxx
2
3
3/2
m
0
2
II
+
2
pared by covalent attachment on silica surface of biomimetic [Ru (terpy)(4-CO H-4 -Mebpy)Cl] complex
through the formation of a pseudo-peptide bond. The catalytic ability of bio-derived silica for alkene oxi-
Dedicated to Alfred Werner on the 100th
Anniversary of his Nobel prize in Chemistry
in 1913.
t
dation with HOO Bu has been evaluated exhibiting significant efficiency and, in some cases, showing
II
0
+
2
increased activity compared vs. the corresponding ‘net’ [Ru (terpy)(4-CO H-4 -Mebpy)Cl] complex.
The data supported that the covalently attached ruthenium complex preserves the catalytic behaviour
of the ‘net’ ruthenium complex indicating that the presented grafting process was successful.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Ruthenium complex
Grafting
Biomimetic oxidation catalysis
1
. Introduction
Covalently anchoring of bio-inspired metal complexes on silica
effectively by post-grafting method [1,24,25] through anchoring of
the organic ligands present [26–28]. This covalent route to immo-
bilisation of homogeneous catalysts is believed to be the most suit-
able, since adsorption and ionic methods lead to a decrease in
catalyst stability [1].
Although ruthenium complexes with Schiff bases or polypyri-
dine ligands have been thoroughly studied, rather few reports
are available on immobilised ruthenium complexes and their
application in oxidation catalysis. As example, we refer the work
of Ram and co-workers [29–31], Reedijk and co-workers [32] and
Che and co-workers [33]. More recently, ruthenium-salophen,
and ruthenium-bipyridine complexes supported on polystyrene,
or mesoporous silica FSM have been prepared and evaluated as
surface through the side chain functionalities offers alternative
processes to develop bio-derived silica frameworks that can be uti-
lized as catalytic materials for specific transformation in fine
chemical production [1]. This synthetic strategy is grounded on
the evidence that the active sites of metalloenzymes involve metal
complexes which are efficient and selective catalysts in vivo as well
as in vitro, in oxidation reactions of organic compounds [2–5].
Among the different oxidation reactions, the epoxidation of olefins
is of major importance for organic synthesis. Nevertheless, the syn-
thesis of epoxides catalyzed by transition metal complexes is still
important on laboratory as well as on industrial scale [3,6–8].
Based on our recent work on catalytic epoxidations using manga-
nese and iron biomimetic complexes ‘net’ and covalently attached
on inorganic matrices [9–15], we were interested in using ruthe-
nium-based catalytic materials in these reactions. The potential
of ruthenium as epoxidation catalyst arises from its extensive
redox chemistry and its propensity to form high-valent oxo com-
plexes [16].
oxidation catalysts of alkenes with NaIO
[34,35].
4
and TBHP respectively
II
0
2
The biomimetic ruthenium complex, [Ru (terpy)(4-CO H-4 -
Mebpy)Cl]Cl (Scheme 1), has been designed and served as precursor
of polypyridylruthenium conjugated peptide/amino acid complexes
[36]. This class of compounds has been shown remarkable photoin-
duced nucleolytic activity cleaving the DNA phosphorodiesteric
bonds [37].
Ruthenium complexes as homogeneous catalysts offered inter-
esting results, e.g. such as high yield and chemoselectivity [16–23],
however, they become more useful if the separation process is tar-
geted to recover them at the end of reaction. Metal-based catalysts
can be covalently anchored on the surface of porous materials most
In the present study, this Ru-complex has been immobilised on
silica surface (Scheme 2). The applied method, depicted in Scheme
3, builds up the ruthenium complex on preformed silica. Thus, the
ligand 4 -Mebpy-4-COOH (4 -methyl-2,2 -bipyridine-4-carboxylic
acid) was attached onto a modified amino-propyl-silica via pseu-
0
0
0
do-peptide bond formation, then reaction with Ru(terpy)Cl
3
yielded
II
0
the supported ruthenium complex, [Ru (terpy)(4 Mebpy/4CON-
⇑
Corresponding author.
E-mail addresses: agaroufi@cc.uoi.gr (A. Garoufis), mlouloud@uoi.gr
+
H(CH
2
)
3
SiO3/2)Cl]
m
ꢀzSiO
2
. The developed Ru-based material as well
II
0
as the ruthenium complex [Ru (terpy)(4-CO
2
H-4 -Mebpy)Cl]Cl, was
(
M. Louloudi).
0
277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.094
Please cite this article in press as: F. Papafotiou et al., Polyhedron (2012), http://dx.doi.org/10.1016/j.poly.2012.07.094

2
F. Papafotiou et al. / Polyhedron xxx (2012) xxx–xxx
+
with a flame ionization detector and a Shimadzu GC-17A gas chro-
matograph coupled with a GCMS-QP5000 mass spectrometer.
N
2.2. Catalyst preparation and characterization
N
N
N
2
Ru
+
-
Cl
2.2.1. Synthesis of amino-propyl-modified silica [NH (CH )
2 2 3
SiO3/2
]
p
ꢀySiO
2
Cl
Amino-propyl-modified silica was synthesized according to [40].
N
Anal. Calc. Found: C, 4.68; N, 1.82%; organic weight loss (%) 7.9.
ꢂ1
DRIFTS-IR (cm , selected peaks of the organic functionality):
3
658:
m
(N–H); 1661: d(N–H); 1591:
m
(C–N); 1480: d(C–C). The load-
ꢂ1
ing achieved is ca. 1.3 mmol g determined by thermogravimetric
COOH
and elemental analysis.
II
0
Scheme 1. The structure of biomimetic complex [Ru (terpy)(4-CO H-4 -
2
Mebpy)Cl]Cl.
0
2
.2.2. Synthesis of [4 -Mebpy/4-CONH(CH
2
)
3
SiO3/2]
n
ꢀxSiO
2
0
The conjugation of the ligand 4 -Mebpy-4-COOH (340 mg) to
the amino-propyl-modified silica (200 mg) was achieved with the
coupling agents benzotriazol-1-yl-oxytris(pyrrolidino)phosphani-
um hexafluorophosphate (pyBOP, 852 mg) and diisopropylethyl-
amine (DIPEA, 528 ll) in N-methyl-2-pyrrolidone (NMP, 1 ml).
After 24 h reaction at room temperature, the solid material was fil-
tered and washed carefully with NMP, EtOH and MeOH. Anal. Calc.
N
N
N
N
2
Ru
+
Found: C, 5.82; N, 2.79%; organic weight loss (%) 13.8.DRIFTS-IR
ꢂ1
Cl
(cm , selected peaks of the organic functionality): 3456:
m
(N–
O
O
H); 1705:
m (C@O); 1653: m (C@C); 1522: m (C–N). Elemental and
N
Si
Si
Si
thermogravimetric analysis indicated a post-modification degree
of 0.23 mmol attached ligand/g modified support.
O
O
II
0
+
2
zSiO
.2.3. Synthesis of [Ru (terpy)(4 Mebpy/4CONH(CH
2
)
3
SiO3/2)Cl]
m
ꢀ
O
O
Si
N
H
O
2
O
0
The
ꢀxSiO
post-modified
(200 mg) was refluxed with Ru (terpy)Cl
N (60 l), in DMF/EtOH (4.5/1.5 ml v/v) for 24 h.
silica
[4 -Mebpy/4-CONH(CH
2
)
3
SiO3/
O
III
2
]
n
2
3
(110 mg), LiCl
O
O
(
15 mg) and Et
3
l
The solid product was filtered and washed thoroughly with DMF,
EtOH and MeOH. Anal.Calc. Found: C, 12.90; N, 3.67%; Ru, 2.22; or-
ꢂ1
ganic weight loss (%) 19.5. DRIFTS-IR (cm , selected peaks of the
II
0
Scheme 2. The developed bio-functionalized silica [Ru (terpy)(4 -Mebpy/4-CON-
organic ligand): 3063:
m
(N–H); 1705:
m
(C@O); 1659:
m
(C@C);
+
H(CH )
2 3
SiO3/2)Cl]
m
ꢀzSiO
2
.
ꢂ1
1
598:
m
(C–N). Metal loading: 0.22 mmol g .The further organic
ꢂ1
loading achieved is ca. 0.21 mmol g determined by thermogravi-
metric and elemental analysis. DR-UV–Vis (kmax, nm): 274:
⁄
p ? p
tested for their catalytic ability in the oxidation of various olefins
using tert-butylhydroperoxide (TBHP), as oxidant.
⁄
(
intraligand); 329, 350 and 400: d
p
(Ru) ?
p
(ligand) (MLCT); 580:
⁄
d
p
(Ru) ?
p
(ligand) (MLCT) [41].
2
. Experimental
.1. General
The ligand 4 -Mebpy-4-COOH (4 -methyl-2,2 -bipyridine-4-car-
2.3. Catalytic evaluation
2
TBHP was slowly added (within a period of 5 min) to a tert-BuOH
solution (0.9 ml) containing the catalyst and the substrate under Ar
atmosphere, at room temperature (26 °C). As an internal standard,
acetophenone or bromobenzene were used. Catalytic reactions
were started by adding the oxidant into reaction mixture. The ratio
0
0
0
III
II
boxylic acid), and the complexes Ru (terpy)Cl
3
and [Ru (terpy)
H-4 -Mebpy)Cl]Cl were synthesized according to literature
0
(
[
4-CO
2
36,38–39]. All reagents were purchased from Aldrich Chemical
of [catalyst: oxidant: substrate] was equal to [1:100:1000]
lmol.
Co. and were used as received. Silica gel K100 was purchased from
Merck and was activated at 200 °C for 12 h before use. All used
substrates were purchased from Aldrich, in their highest commer-
cial purity, stored at 5 °C and purified by passage through a column
of basic alumina prior to use. TBHP used was also purchased from
Aldrich (ꢁ5.5 M solution in decane).
We underline that in catalytic reactions taking place under air, O
2
affected both yield and selectivity of the oxidation products. Thus,
in the data shown herein all the catalytic reactions were studied un-
der strict inert Ar atmosphere.
The progress of the reaction was monitored by GC–MS, by
removing small samples of the reaction mixture. The yields re-
ported herein are based on the amount of oxidant added. Reactions
were complete within 24 h. To establish the identity of the prod-
ucts unequivocally, the retention times and spectral data were
compared to those of commercially available compounds. Blank
experiments showed that without catalyst the oxidative reactions
do not take place.
Elemental analyses (C, H, N) were obtained using a Perkin Elmer
Series II 2400 elemental analyzer. The ruthenium amount was
determined by flame atomic absorption spectroscopy on a Perkin-
Elmer AAS-700 spectrometer. Infrared spectra were recorded on a
Spectrum GX Perkin-Elmer FT-IR System. UV–Vis spectra were
recorded using a UV/VIS/NIR JASCO Spectrophotometer and a Per-
kin-Elmer Lamda 35 with a diffuse reflectance setup. Thermogravi-
metric analyses were carried out using Shimadzu DTG-60 analyser.
GC analysis was performed using an 8000 Fisons chromatograph
Control for metal leaching from the heterogeneous catalyst dur-
ing the catalytic experiments was accomplished (a) by ruthenium
analysis into the catalytic medium and (b) by the ‘filtration method’
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F. Papafotiou et al. / Polyhedron xxx (2012) xxx–xxx
3
O
O
O
O
Si
Si
Si
Si OH
Si OH
Si OH
NH2
O
O
O
O
O
O
Si
NH2
+
Si
O
O
O
O
O
O
O
O
O
O
O
pyBOP
DIPEA
NMP
OH
N
N
24h
N
N
N
N
Ru
N
Cl
N
O
O
O
O
O
N
N
N
Si
Si
Si
Si
Si
Si
N
Ru
O
O
O
Cl
Cl
O
Cl
O
O
O
Si
N
H
O
O
Si
N
H
O
O
O
O
O
O
O
O
II
0
SiO3/2)Cl]+
Scheme 3. Synthetic procedure for the bio-derived silica [Ru (terpy)(4 Mebpy/4CONH(CH
2
)
3
m
ꢀzSiO
2
.
[
10,42]. Both methods indicated that no metal-leaching occurred
Table 1
during the catalytic process.
II
0
SiO3/2)Cl]+
Alkene oxidation catalyzed by [Ru (terpy)(4 Mebpy/4CONH(CH
modified silica with TBHP.
2
)
3
m
ꢀzSiO
2
a
3
. Results and discussion
Yield (%)b
Substrate
Products
Cyclohexene
cis-Epoxide
5.0 (5.0)c
3
.1. Covalent attachment of the ruthenium complex
2
2
-Cyclohexenol
-Cyclohexenone
22.0 (12.7)
46.0 (27.0)
21.0 (32.0)
24.0 (12.0)
8.0 (14.0)
II
The synthesis of bio-derived silica [Ru (terpy)(4’-Mebpy/4-
1Methylcyclohexene
cis-Epoxide
3-Methyl-2-cyclohexen-1-ol
SiO3/2)Cl]+
CONH(CH
Scheme 3). In the first step, 3-aminopropyltriethoxysilane was
immobilized on silica surface resulting in [NH (CH
modified silica with 1.3 mmol organic content/g
2
)
3
m
ꢀzSiO
2
was performed in three steps
3-Methyl-2-cyclohexen-1-one
(
Cyclooctene
Styrene
cis-Epoxide
2-Cyclooctenone
Epoxide
Benzaldehyde
trans-Epoxide
Benzaldehyde
cis-Epoxide
70.0 (20.0)
18.0 (5.0)
a
2
2
)
3
SiO3/2
]
p
ꢀySiO
2
24.0 (30.0)
19.0 (13.0)
20.5 (50.0)
21.5 (13.0)
54.0 (16.5)
15.0 (6.6)
20.0 (25.0)
15.0 (20.0)
31.1e (32.0)
23.0 (22.0)
modified silica. Subsequently, the attachment of the ligand
4
of a pseudo-peptide bond between the carboxy-terminal of the
ligand and the amino-functional group of the modified silica. This
conjugation was achieved by the coupling agents pyBOP and DIPEA
in N-methyl-2-pyrrolidone forming the [4 -Mebpy/4-CONH(CH
SiO3/2
groups of the amino-propyl-modified silica reacted with the
ligand. Finally, the post-modified silica [4 -Mebpy/4-CONH(CH
0
Methyl-styrene
cis-Stylbene
-Mebpy-4-COOH to the modified support occurred via formation
trans-Epoxide
Benzaldehyde
1,2-Epoxides (Z- and E-)
0
Limonene
2
)
3
Limonene alcohold
f
]
n
ꢀxSiO
2
modified material. Approximately, 18% of the NH
2
-
Limoneme ketoneg
a
0
Conditions: ratio of catalyst:TBHP:substrate = 1:100:1000. Reactions were
completed within.24 h.
2
)
3
III
SiO3/2
]
n
ꢀxSiO
2
was reacted with Ru (terpy)Cl
3
by refluxing in
b
Yield based on oxidant added.
DMF/EtOH yielding the ruthenium complex supported on silica
c
II
Data given in parenthesis () were provided by homogeneous [Ru (terpy)(4-
II
0
+
0
[
Ru (terpy)(4 Mebpy/4CONH(CH
2
)
3
SiO3/2)Cl]
m
ꢀzSiO
2
. The ruthe-
CO
2
H-4 -Mebpy)Cl]Cl catalyst.
d
Limonene alcohols were found to be a mixture of 1-ol, 2-ol and 6-ol.
nium loading on the silica was found 0.22 mmol ruthenium/g
modified silica.
e
f
3
3
1.1% Yield corresponds to 8.0% for 1-ol, 5.1% for 2-ol and 18.0% for 6-ol.
2% Yield corresponds to 6% for 1-ol, 10.0% for 2-ol and 16.0% for 6-ol.
g
The only observed ketone is the 6-one.
3.2. Catalytic evaluation of the modified material
II
0
The catalytic properties of the [Ru (terpy)(4 Mebpy/4CON-
catalyst for alkene oxidation with TBHP have been evaluated. The
oxidation reactions were carried out at room temperature (26 °C)
H(CH
2
)
3
SiO3/2)Cl]+
m
ꢀzSiO
2
modified silica as heterogeneous
Please cite this article in press as: F. Papafotiou et al., Polyhedron (2012), http://dx.doi.org/10.1016/j.poly.2012.07.094

4
F. Papafotiou et al. / Polyhedron xxx (2012) xxx–xxx
.
Fig. 1. Distribution of oxidation products catalyzed by grafted on silica or ‘net’ biomimetic ruthenium complexes in the presence of TBHP See Table 1 for further details.
under inert Ar atmosphere. For comparison reasons, the catalytic
behaviour of the ‘net’ [Ru (terpy)(4-CO H-4 -Mebpy)Cl]Cl complex
2
were trans-epoxide (20.5%) and benzaldehyde (21.5%) as oxidation
cleavage adduct. In the oxidation of cis-stilbene, the major prod-
ucts were epoxides: cis-stilbene epoxide with yield 54.0% and
trans-stilbene epoxide with yield 15.0% respectively. Considerable
amounts of benzaldehyde as oxidative cleavage product (20.0%)
have been also detected.
II
0
as homogeneous catalyst, under the same experimental conditions,
has been also investigated. The catalytic results are summarized in
Table 1. Fig. 1 provides a histogram plot of the data in Table 1.
Based on Table 1, the Ru-containing modified silica was efficient
in alkene oxidations providing significant yields. More specifically,
cyclohexene and limonene oxidation provided oxidation products
with a combined yield of 73.0% and 69.1% respectively. Cyclohex-
ene undergoes mainly allylic oxidation forming 2-cyclohexene-
Comparing the ruthenium complex covalently attached on silica
II
0
+
surface [Ru (terpy)(4 Mebpy/4CONH(CH
2
)
3
SiO3/2)Cl]
m
ꢀzSiO
2
with
II
0
2
the corresponding ‘net’ complex [Ru (terpy)(4-CO H-4 -Meb-
py)Cl]Cl (see Table 1 and Fig. 1), we observe that the attached sys-
tem leads to considerable reactivity with TBHP presenting
analogous profile of oxidation products with those of the ‘net’
homogeneous system. However, in some cases, i.e., in cyclohexene,
cyclooctene and cis-stilbene oxidation, the Ru-containing modified
silica showed increased activity providing total oxidation yield
73.0%, 88.0% and 89.0% respectively vs. 44.7%, 25.0% and 48.1%
yielded by the ‘net’ homogeneous catalyst. That is, the applied
grafting process preserves the catalytic activity of the ‘net’ complex
and, in some cases increases its reactivity. Moreover, the inorganic
support does not introduce any steric hindrance preventing the
substrate access to the active metal sites.
1
-ol and 2-cyclohexene-1-one with yield 22.0% and 46.0%
respectively. However cyclohexene epoxidation was also observed,
providing low epoxide yield of 5.0%. Overall, cyclohexene was oxi-
II
0
+
dised by [Ru (terpy)(4 Mebpy/4CONH(CH
2
)
3
SiO3/2)Cl]
m
ꢀzSiO
2
pro-
viding 73.0% total oxidation yield. The major products detected
from limonene oxidation were (i) two epoxides (Z- and E-) derived
from epoxidation of the electron-rich double bond in 1,2-position,
(
ii) alcohols derived from hydroxylation in 1-, 2- and 6-positions of
limonene ring. Oxidation products from the more accessible, but
less electron-rich, double bond at 8,9-position were not observed.
Additionally, considerable amounts of the corresponding ketone
at 6-position was also formed. In summary, the total yield were
Alkene oxidations catalyzed by both attached and ‘net’ ruthe-
nium complex with TBHP present comparable selectivity and dis-
tribution of oxidation products with those catalyzed by iron-non-
(
6
a) epoxides (Z-1,2 and E-1,2) 15.0%, (b) alcohols (1-ol, 2-ol and
-ol) 31.1% and (c) ketone (only at 6-position) 23.0%. These results
provide a total oxidation yield of 69.1% achieved by ruthenium
complex grafted on silica.
In the case of methyl-cyclohexene, the detected oxidation prod-
ucts were cis-epoxide (21.0%), 3-methyl-2-cyclohexen-1-ol (24.0%)
and 3-methyl-2-cyclohexen-1-one (8.0%) with a total yield of
2 2
heme catalysts with H O [9]. In these systems, the major oxida-
tion products of alkenes were alcohols and ketones, mainly derived
by an allylic oxidation reaction; however, considerable amounts of
epoxides have been also formed. This behavior could indicate sim-
ilar mechanistic path in the catalysis as in the iron-catalyzed-oxi-
III
t
5
3.0%. cis-Cyclooctene as substrate afforded two reaction products
dations [9], i.e., the formation here of a Ru –OO Bu intermediate
IV
giving cis-cyclooctene epoxide and 2-cyclooctenone with 70.0%
and 18.0% yield respectively. Styrene oxidation provided epoxide
which undergoes homolytic O–O bond cleavage providing Ru @O
Å
t
and O Bu species [22,35].
. Conclusion
The biomimetic ruthenium complex [Ru (terpy)(4-CO
(
24.0%) and benzaldehyde (19.0%) as oxidation products formed
by direct oxidation and oxidative cleavage respectively of the
4
II
exo-cyclic double bond. Overall, styrene was oxidised by [Ru (ter-
0
+
II
py)(4 Mebpy/4CONH(CH
2
)
3
SiO3/2)Cl]
m
ꢀzSiO
2
with total oxidation
0
2
H-4 -
+
yield 43.0%. The methyl-substituted styrene, trans-b-methyl sty-
Mebpy)Cl] has been covalently attached on silica surface through
the formation of a pseudo-peptide bond providing the bio-derived
rene provided total oxidation yield 42.0%. The identified products
Please cite this article in press as: F. Papafotiou et al., Polyhedron (2012), http://dx.doi.org/10.1016/j.poly.2012.07.094

F. Papafotiou et al. / Polyhedron xxx (2012) xxx–xxx
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II
0
SiO3/2)Cl]+
silica, [Ru (terpy)(4 Mebpy/4CONH(CH
2
)
3
m
ꢀzSiO
2
. The
[
[
13] E.R. Milaeva, Catalysis Commun. 8 (2007) 2069.
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lytic data indicate that the used grafting process was appropriate
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[
II
0
+
2
ruthenium complex [Ru (terpy)(4-CO H-4 -Mebpy)Cl] . This is a
[
very interesting and useful approach for tailoring homogeneous
catalysts via their immobilisation. The chemical behaviour of
ruthenium complexes in solution and covalently attached on silica
is quite similar and we suggest that they probably function via the
formation of ruthenium-peroxo-species with a dominant radical
mechanistic path.
1
[
2
[
(
Appendix A. Supplementary data
(
[
[
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Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.07.094.
[
[
[
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Please cite this article in press as: F. Papafotiou et al., Polyhedron (2012), http://dx.doi.org/10.1016/j.poly.2012.07.094