A bioinspired heterogeneous catalyst based on the model of the manganese-dependent dioxygenase for selective oxidation using dioxygen

Article abstract of DOI:10.1039/c7ra00514h

A hybrid bioinspired material with manganese(ii) complexes grafted on the surface of a mesostructured porous silica is investigated. The Mn sites mimic the manganese-dependent dioxygenase (MndD), which is an enzyme that catalyses the oxidation of catechol derivatives. The metal complexes were introduced in the silica using a dinuclear complex [Mn₂L₂(Cl)₂(μ-Cl)₂] as a precursor with a clickable ligand N,N′-bis[(pyridin-2-yl)methyl]prop-2-yn-1-amine (L). Azide moieties covalently grafted on MCM-41 type mesoporous silica were utilised to anchor the manganese complex through Huisgen cycloaddition using CuBr(PPh₃)₃ as a catalyst. A second functional group-trimethylsilyl or pyridine—was grafted on the silica to bring, together with nanopore size confinement, a similar metal environment as in MndD. The mesostructure of the materials was maintained after incorporation of the Mn complex. Catalytic oxidation of 3,5-di-t-butyl-catechol (3,5-DTBC) into quinone occurred without the need of an additional base when the metal complex was confined in the porous solid. In comparision, the oxidation of 4-t-butyl-catechol (4-TBC) that always required a basic media led to a total oxidation into the ortho-quinone, contrary to the molecular analogue.

Full text of DOI:10.1039/c7ra00514h

RSC Advances
View Article Online
View Journal | View Issue
PAPER
A bioinspired heterogeneous catalyst based on the
model of the manganese-dependent dioxygenase
for selective oxidation using dioxygen†
Cite this: RSC Adv., 2017, 7, 17336
ab
ac
a
b
Jeremy Chaignon, Marie Gourguesc, Lhoussain Khrouz, Nicolas Moliner,
´
´
´
a
b
a
Laurent Bonneviot, Fabienne Fache, Isabel Castro* and Belen Albela*
´
A hybrid bioinspired material with manganese(II) complexes grafted on the surface of a mesostructured
porous silica is investigated. The Mn sites mimic the manganese-dependent dioxygenase (MndD), which
is an enzyme that catalyses the oxidation of catechol derivatives. The metal complexes were introduced
0
in the silica using a dinuclear complex [Mn
2
2
L (Cl)
2
(m-Cl)
2
] as a precursor with a clickable ligand N,N -bis
[
(pyridin-2-yl)methyl]prop-2-yn-1-amine (L). Azide moieties covalently grafted on MCM-41 type
mesoporous silica were utilised to anchor the manganese complex through Huisgen cycloaddition using
CuBr(PPh as a catalyst. A second functional group – trimethylsilyl or pyridine—was grafted on the
3 3
)
silica to bring, together with nanopore size confinement, a similar metal environment as in MndD. The
mesostructure of the materials was maintained after incorporation of the Mn complex. Catalytic
oxidation of 3,5-di-t-butyl-catechol (3,5-DTBC) into quinone occurred without the need of an additional
base when the metal complex was confined in the porous solid. In comparision, the oxidation of 4-t-
butyl-catechol (4-TBC) that always required a basic media led to a total oxidation into the ortho-
quinone, contrary to the molecular analogue.
Received 12th January 2017
Accepted 14th March 2017
DOI: 10.1039/c7ra00514h
rsc.li/rsc-advances
activation, which is useful to prevent any spontaneous reaction
Introduction
of O with organic molecules.
2
Many industrial processes involve catalytic reactions, since they
Structural and mechanistic research on metalloenzymes in
allow working under milder conditions with high selectivity and the past decades has allowed the development of numerous
short reaction time, decreasing, therefore, the cost of the nal bioinspired systems that can be used for catalysis. Homoge-
1
product. However, these enhanced processes are not a prerog- neous biomimetic complexes are oen not optimal for reac-
ative for industry, for they existed in nature long before chem- tions involving dioxygen activation. Indeed, the activation of
ists could understand them. A class of natural metalloproteins oxygen can partially oxidise the catalyst. Graing on a solid
called metalloenzymes is able to carry out extremely specic support can stabilise the catalyst and confer connement as in
ꢀ
reactions under physiological conditions (between 10 and 40 C the enzymatic pocket.
2
for most of them). In particular, several metalloenzymes called
Amongst oxygenases, a recurrent metal coordination triad
oxygenases perform the oxidation of organic compounds is observed, composed by two histidines (His) and one
by activating molecular dioxygen. Various metal ions can be carboxylic acid, coming either from glutamate (Glu) or aspar-
found in these oxygenases, mainly copper, iron and manga- tate (Asp) moieties. This widespread structural motif is called
3–9
nese.
namically favourable, the electronic conguration of the O
Although the reaction with dioxygen is thermody- the 2-His-1-carboxylate facial triad, and is found in many
6
metalloproteins. This is the case of catechol dioxygenase,
2
molecule creates
a
strong kinetic barrier that requires which catalyses the cleavage of catechol derivatives. Two kinds
of catechol dioxygenases can be found, which differ on the
III
nature of the metal ion involved at the active site: Fe allows
a
Laboratoire de Chimie UMR CNRS 5182, Ecole Normale Sup ´e rieure de Lyon,
University of Lyon, 46, All ´e e d'Italie, 69364 Lyon, Cedex-07, France. E-mail: belen.
II
II
an intradiol cleavage while Fe or Mn performs the extradiol
one. In particular, the manganese-dependent dioxygenase
Instituto de Ciencia Molecular, Universitat de Val `e ncia, C/Catedr a´ tico Jos ´e Beltr a´ n 2, (MndD) is a system that has attracted much attention since its
6
–9
albela@ens-lyon.fr
b
4
c
6980 Paterna, Val `e ncia, Spain. E-mail: isabel.castro@uv.es
7,10–12
catalytic mechanism is still under discussion.
Therefore,
Institut de Chimie et Biochimie Mol ´e culaires et Supramol ´e culaires (ICBMS) UMR
CNRS 5246, University of Lyon, Bd. du 11 Novembre 1918, 69622 Villeurbanne,
Cedex, France
the study of bioinspired systems that mimic the structural
and/or catalytic function of MndD could help to understand its
reaction mechanism.
†
Electronic supplementary information (ESI) available: TGA proles of materials,
magnetic measurements and N sorption isotherms. See DOI: 10.1039/c7ra00514h
2
17336 | RSC Adv., 2017, 7, 17336–17345
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
Molecular models that contain a manganese centre are
RSC Advances
Results and discussion
however much less frequent than those containing iron. Only
few examples in the literature report manganese(II) complexes
as mimics of the catechol dioxygenase. The rst reported
model to our knowledge was synthesised by Sakurai et al. two
II
Incorporation of the Mn complex in the mesoporous silica:
synthesis strategy to isolate the metal sites with control of
their close environment
1
3
years aer the discovery of MndD. Their report describes the The alkyne function present in the ligand L (Scheme 1) was used
use of penicillamine with manganese(II) in the oxidation of to covalently gra the manganese complex into the pores of
catechol to the corresponding muconic acid derivatives a mesoporous silica of the LUS type, which presents a 2D
29
resulting from a true extradiol dioxygenase activity. However, hexagonal structure just like the classic MCM-41 silica. The
very low conversion rates were achieved (<2%). Ramadan and LUS silica was synthesised using a ultra-fast microwave assisted
El-Mehasseb subsequently investigated the oxidative proper- hydrothermal synthesis that affords a high quality mesostruc-
30
ties of manganese(II) with tridentate pyridyl hydrazone tured porous silica. Due to the yield and selectivity advantages
1
4
ligands. They reported the transformation of catechol into o- of the copper(I)-catalysed Huisgen azide–alkyne cycloaddition
benzoquinone in presence of catalyst and O , which would (also known as “click chemistry”), this reaction was chosen for
correspond to an oxidase activity (oxidation without incorpo- the graing process.
2
26–28
We had the choice to design our
ration of oxygen atom in the substrate). Similar results were ligands either with an azide or an alkyne arm, the latter being
obtained a few years later with tetradentate ligands containing preferred for two reasons. First, the incorporation of an azide
1
5
imidazole, phenolate and/or pyridine moieties. At the same function in the ligand could have been more problematic for the
time, the manganese(II) complex of another tetradentate synthesis of the complex than the incorporation of an alkyne
0
ligand, N,N -bis(2-pyridylmethyl)-1,2-ethanediamine (bispi- arm because of the coordinating potential of the azide moiety.
cen), managed to oxidise the 4-tert-butylcatechol into its Secondly, the synthesis of the azide tethering silane was easier
1
6
quinone equivalent mixed with extradiol intermediates.
than the alkyne tether, the azidopropyltriethoxysilane precursor
Reaction conditions turned out to have an important effect on being then synthesised starting from chloropropyltriethox-
the resulting products.
ysilane upon nucleophilic substitution of the chlorine atom by
31
We have developed a bioinspired approach for design of the an azide moiety.
metal supported catalysts based on the model of metal-
The molecular stencil patterning (MSP) technique was used
1
7–21
32–34
loproteins.
This approach was recently applied using the to incorporate the complexes into the solid (Scheme 2).
This
manganese dependent dioxygenase (MndD) as a model, the strategy takes advantage of the cationic nature of the surfactant
0
+
N,N -bis[(pyridin-2-yl)methyl]prop-2-yn-1-amine (L) as a click- (cetyltrimetylammonium, CTA ) present in the mesoporous
able ligand for graing and [Mn (Cl) (m-Cl) ] as a precursor silica. The general procedure is a four step strategy: (i) the
2
L
2
2
2
1
8
metal complex (Scheme 1). This ligand has the particular template is partially removed with hydrochloric acid; due to
advantage to mimic the facial triad present in the active site of electrostatic repulsion, the remaining surfactant rearranges
the MndD and allows an efficient graing in the silica thus making some silica surface available, (ii) a rst organo-
2
2–25
support.
Indeed, the alkyne side function allows graing of silane (F1) is graed onto this freed surface, (iii) all of the
the ligand in an azido-functionalised support using “click remaining surfactant is removed, freeing silica surface once
2
6–28
chemistry”.
In addition, the two pyridine and the tertiary again, and (iv) the second function (F2) is graed, providing
amine functions present in the ligand can coordinate to the a double functionalised material. In the present work, however,
+
Mn(II) ion and thus mimic the metal environment of the a slightly different protocol was applied where CTA was
+
MndD.
exchanged with tetramethylammonium (TMA ) prior to the rst
34
In the present work we are going further in providing partial extraction (Scheme 2). This modication allowed
a comprehensive investigation of the [Mn (Cl) (m-Cl) ] gra-
ing, knowing that this dimer is in equilibrium with the mono-
2
L
2
2
2
18
mer in solution. We will also compare two different local
environments of the metal complex in the solid to test the
inuence of a pyridine side function meant here to mimic the
histidine function neighbouring the metal active site in MndD.
Both homogeneous and heterogeneous systems are tested for
the rst time on the oxidation of catechol derivatives.
Scheme
tetramethylammonium.
2
Molecular stencil patterning strategy. TMA
¼
2 2 2 2
Scheme 1 Synthesis of complex [Mn L (Cl) (m-Cl) ].
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 17336–17345 | 17337

View Article Online
RSC Advances
Paper
a better diffusion of the F1 graing function since there was reaction with L and benzyl azide has been reported and per-
37
more available pore space. If the graed functions are modelled formed under these conditions. Instead of sodium ascorbate,
38
as spheres arranged in 2D hexagonal packing on the pore methanol can also be used as reductant. Some preliminary
surface, isolating a function F2 will result in having it sur- tests between the ligand L and azidopropyltrimethoxysilane
rounded by 6 functions F1. Every F1 will thus be adjacent to 3 were performed in order to optimise the conditions for the click
32
functions F2, which corresponds to a ratio F1/F2 of 2.
reaction on the solid support. Under these conditions, coordi-
The nomenclature used for the synthesised solids is the nation of copper to the ligand L was observed in all cases, even
+
following. LUS stands for the as-made support containing CTA
when copper(I) bromide was used. A tris(triphenylphosphine)
molecules inside the pores, while the material exchanged with copper(I) bromide complex was required as catalyst to avoid this
+
TMA is denoted as LUS-TMA. Partial extraction is represented problem. This change of catalyst was inspired by organic
+
by an – Ex suffix where x is the percentage of extracted TMA . syntheses of triethoxysilanes using the microwave-assisted
+
39
For instance, a material where 15% of TMA was extracted is Huisgen copper-catalysed reaction reported in the literature.
named LUS-TMA-E15. Totally extracted materials are simply Given that triethoxysilanes are particularly sensitive to water
+
noted LUS-E. Likewise, when the remaining TMA is removed and to many chemical functions such as alcohols or carboxylic
from a LUS-TMA-Ex material, the resulting one is denoted as acids, this kind of synthesis was quite a challenge. However,
LUS-Ex. Graing reactions are symbolised by prexes. For using microwave irradiation and CuBr(PPh
instance, silica functionalised with azide (N ) and trimethylsilyl desired organosilane was obtained in a yield typical for click
TMS) groups will be named N –TMS@LUS. In the case of chemistry (above 90%).
3 3
) as catalyst, the
3
(
3
double functionalisation, the last function incorporated is
Then, several tests were performed to click the manganese(II)
placed at the beginning of the name: a material functionalised complex [Mn L (Cl) (m-Cl) ] in the azide-functionalised silica
2
2
2
2
with F1 and then with F2 will thus be called F2–F1@LUS.
3 3 3
N @LUS-E. Aer an overnight reaction with CuBr(PPh ) , the IR
In the present work two materials containing a man- spectrum of the resulting material still exhibited a band around
18
ꢁ1
ganese(II) complex were studied using [Mn
2
L
2
(Cl)
2
(m-Cl)
2
]
as 2100 cm , which is characteristic of the presence of azide
metal precursor: a new solid {Mn}-Py@LUS-E15 (S1), where moieties. Aer a one-week reaction, no absorption occurred in
pyridine (Py) functions are in the vicinity of the metal this region, proving that all the azide moieties had reacted. This
18
complexes, and the already reported {Mn}-TMS@LUS-E (S2),
long reaction time can be explained by the porous nature of the
where the manganese complexes are surrounded by hydro- material. All the reactive sites at the silica surface are not as
phobic trimethylsilyl (TMS) functions (Scheme 3). In this accessible as in solution, and diffusion inside the porous
nomenclature, the {Mn} function is the product of the reaction material becomes a non-negligible parameter. Removal of the
2 2 2 2 3
between [Mn L (Cl) (m-Cl) ] and the azide (N ) tether present in copper catalyst was performed by washing with methanol. This
both host materials. The TMS function was chosen to mimic technique was efficient enough to remove the catalyst, as
some hydrophobic residues present in the enzymatic pocket of conrmed by EDX microscopy, EPR and elemental analysis: no
the metalloprotein, whereas Py function was selected to mimic trace of copper could be found beyond the detectable threshold.
the His200 residue present in the second coordination sphere of Therefore, this technique was applied to incorporate the
the Mn active site, which is known to help in the activation of manganese complex into the host materials N –Py@LUS-E15
3
7,35,36
molecular oxygen during the catalytic cycle of MndD.
and N –TMS@LUS-E, affording solids {Mn}-Py@LUS-E15 (S1),
3
{
Mn}-TMS@LUS-E (S2), respectively.
Optimisation of the “click chemistry” graing of the Mn
complex
Characterisation of the hybrid materials
Even though click chemistry is known to be efficient, one must The porous structure of the host materials was checked using
be careful about the conditions implied in those reactions. powder X-ray diffraction (XRD) during the different steps of the
Indeed, the copper-catalysed Huisgen cycloaddition has proven synthesis. The original 2D hexagonal porous structure was
to be sensitive to coordinating molecules when the classical maintained during the functionalisation of the silica and the
copper catalyst is used. In principle, copper(II) sulphate or incorporation of the manganese complex (Fig. 1).
acetate is directly used and reduced to copper(I) in situ, most of
the time by treatment with sodium ascorbate. For instance, exchange of CTA surfactant molecules present in the as-made
Thermogravimetric analyses (TGA) indicate that aer
+
+
silica by TMA , almost 2/3 of the internal surface of the silica
+
was covered by TMA cations, which corresponds to 2.1 mmol of
+
TMA per gram of SiO
2
. Starting from this solid, two interme-
diate materials were synthesised: TMS@LUS-TMA and Py@LUS-
TMA-E15. In order to obtain a F1/F2 molar ratio of 2, TMS
functionalisation was performed without any partial extraction.
Indeed, TMS is graed to the support with only one covalent
bond and the ammonia generated by the aperture of the aza-
silane used for the functionalisation displaces a small part of
Scheme 3 Host materials for the grafting of the Mn(II) complex: N
3
–
+
TMS@LUS-E and N –Py@LUS-E15.
3
the TMA . However, in the case of pyridine, which possesses
17338 | RSC Adv., 2017, 7, 17336–17345
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
29
3
Fig. 2 Solid Si HPDEC NMR spectrum of N –TMS@LUS-E.
2
9
Integration of M and Q massifs from solid Si-NMR could
however give a precise value of the quantity of graed TMS
functions per gram of SiO (Fig. 2).
2
The two hybrid materials {Mn}-Py@LUS-E and {Mn}-
TMS@LUS-E present a manganese weight loading of about
2%, 1.7 and 2.4%, respectively (Table 1). The chloride counter-
anions present in the initial manganese complex are only
partially retained in the nal materials. The Cl/Mn molar ratio
Fig. 1 Powder XRD patterns of the support LUS-E and the materials in the graed materials (Cl/Mn ¼ 0.7 and 0.5 for {Mn}-Py@LUS-
LUS-TMA, Py@LUS-TMA-E15, N
E15 (S1).
3
–Py@LUS-E15 and {Mn}-Py@LUS- E and {Mn}-TMS@LUS-E, respectively) is approximately 25% of
the original chloride anions present in the dinuclear precursor
(
Cl/Mn ¼ 2). This diminution tends to prove that the chloride
bridges are partially lost during the graing process question-
ing about the presence of mononuclear species.
+
a triethoxysilane tether function, 15% of TMA was extracted
before the functionalisation reaction in order to favour the
graing of the function. In other words, TMS functionalisation
was performed on LUS-TMA material while pyridine function-
alisation was carried out with LUS-TMA-E15 material. The F1/F2
molar ratio was calculated from TGA, elemental analyses, and
Nitrogen sorption isotherms were performed to measure
the remaining porous volume of the nal materials (Table 1
and Fig. 3). The pore volume, measured using the t-plot
3
ꢁ1
method, was reduced from 0.86 cm g in LUS-E down to 0.2–
3
ꢁ1
0
.3 cm g in the nal materials, indicating a partial lling of
2
9
quantitative solid Si-NMR (Table 1 and Fig. S1†). Py/N and
3
TMS/N molar ratios of 1.9 and 1.8 can be deduced, and these
3
are close to the desired value of 2.0 required to insure site
isolation. Note that the TMS amount cannot be precisely
deduced neither from TGA – the mass loss is very low and
occurs in a broad temperature range – nor from elemental
+
analyses due to the presence of carbon atoms in TMA .
2
Table 1 Data deduced from N sorption isotherms, TGA, elemental
29
a
analyses and quantitative solid Si NMR
b
3
ꢁ1
c
d
e
Material
LUS-E
V
p
(cm g
)
M
n
(wt%) F1/Si F2/Si
Cl/Mn
0.86
—
—
—
—
N
N
S1
S2
3
–TMS@LUS-E 0.26
–8Py@LUS-E15 0.28
—
—
1.7
2.4
0.046 0.025
0.064 0.034
—
—
0.7
0.5
3
1
0.31
0.22
—
—
—
—
a
b
{
Mn}-Py@LUS-E15 ¼ S1; {Mn}-TMS@LUS-E ¼ S2. Porous volume
3 ꢁ1
2
deduced from sorption N
isotherms using t-plot, ꢂ0.01 cm
g .
c
d
e
Molar ratio, F1 ¼ TMS or Py. Molar ratio, F2 ¼ N
3
.
Molar ratio, Fig. 3 Nitrogen sorption isotherms at 77 K of LUS-E, N –TMS@LUS-E,
3
deduced from chemical analyses.
{
Mn}-TMS@LUS-E (S2).
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 17336–17345 | 17339

View Article Online
RSC Advances
Paper
the pores aer the incorporation of the complex. However, the graed metal complex could be explained by the bending
there was still enough pore volume to allow the substrate of the Mn–Cl–Mn angle (q) within the double-bridged
diffusion and its access to the manganese(II) centres for cata- Mn (m-Cl) diamond core with respect to that found in the
2
2
ꢀ
18
lytic applications.
dinuclear precursor (q ¼ 97.5 ). This situation agrees with the
general trend found in related double-bridged di-m-chlor-
odimanganese(II) complexes reported in the literature, which
Nature of the metal complexes inside the solid
ꢁ1
ꢀ
40–42
exhibit weak ferro- (J ¼ 0.66–1.1 cm with q ¼ 93.5–96.4 )
to
In order to determine the relative amount of mono- and
dinuclear species, magnetic and EPR measurements were
performed for both solids {Mn}-Py@LUS-E15 (S1), {Mn}-
TMS@LUS-E (S2).
The magnetic properties of S1 and S2 are shown in Fig. 4 and
M M
S2† in the form of the c T vs. T plot (c being the molar
magnetic susceptibility per Mn ion and T the absolute
temperature). They are qualitatively similar to those of the
2 2 2 2 M
dinuclear precursor [Mn L (Cl) (m-Cl) ]. c T remains constant
from room temperature down to ca. 100 K and it abruptly
ꢁ1
moderate antiferromagnetic couplings (ꢁJ ¼ 4.8–5.2 cm with
ꢀ
43,44
q ¼ 90.2–90.7 ).
Overall, these results suggest that the
molecular structure of the dinuclear precursor was likely
modied upon graing within the pore walls of the mesoporous
silica.
EPR spectra of S1 and S2 solids conrmed the presence of
both monomeric and dimeric Mn species. In both cases the
spectra were simulated assuming the monomeric ratio
a deduced from the magnetic measurements (Fig. 4 and S2†).
The shape of the simulated signals matched well that of the
experimental signals assuming one type of monomer and two
II
18
decreases upon further cooling, suggesting the occurrence of
weak antiferromagnetic interactions between the Mn ions
II
ꢁ1
types of dimers, a dimer 1 with D ¼ 0.0333 cm and a dimer 2
within the graed metal complex. The least-squares ts of the
experimental data through the appropriate expression derived
from the spin Hamiltonian for an antiferromagnetically
coupled dinuclear manganese(II) complex (H ¼ ꢁJS
assuming the presence of a variable amount of mononuclear
manganese(II) species (a), gave J ¼ ꢁ1.5 cm and g ¼ 2.0 for
both S1 and S2 with a ¼ 0.25 (S1) and 0.38 (S2) (solid lines in
Fig. 4 and S2†). The calculated a values indicate the presence of
ꢁ
1
with D ¼ 0.0733 cm . The calculated parameters for the
different species were: for S1, 62.5% of dimer 1 with g ¼ [1.990
1
.990 2.120], 12.5% of dimer 2 with g ¼ [2.000 2.000 2.150], 25%
18
1 2
S ),
of monomer with g ¼ [2.055 2.018 1.940], and for S2, 28% of
dimer 1 with g ¼ [1.990 2.000 2.130], 34% of dimer 2 with g ¼
ꢁ
1
[
1
1.990 2.100 2.200], 38% of monomer with g ¼ [2.080 2.010
.940] (see ESI† for additional parameters). The presence of two
different dimers revealed by EPR is not surprising given the
various possibilities of interaction with the functionalised
material or the triazole cycle generated during the Huisgen
cycloaddition. The Cl/Mn molar ratio deduced from the
elemental analysis for S1 and S2 (0.7 and 0.5, respectively) led us
to suppose that the solid likely contains a dimer 1 with two
chloride bridges, a dimer 2 with one chloride bridge and
a monomer without any chloride ion in the rst coordination
sphere (Scheme 4 and Fig. S2†). Considering this hypothesis,
the calculated Cl/Mn molar ratios would be 0.68 and 0.45 for S1
and S2, respectively, which are within the margin of error of
elemental analysis. In addition, the difference of g values for
dimer 2 in both solids could be explained either by the presence
of a second bridge provided by the different supports or by
a different geometry around the metal ion induced by the solid
matrix.
II
a relatively larger amount of mononuclear Mn species result-
ing from the loss of the two bridging chlorides in S2, in agree-
ment with the slightly smaller value of the Cl/Mn ratio, as
discussed above.
The calculated ꢁJ value in S1 and S2 materials are twenty

ve-fold larger than that found for the dinuclear precursor (J ¼
ꢁ1
18
ꢁ
0.059 cm ). The stronger antiferromagnetic coupling for
M
Fig. 4 Temperature dependence of c T for S1. The solid line repre- Scheme 4 Representation of dimers 1 and 2. X represents a possible
sents the best fit discuss in the text. Insert: solid-state EPR spectra of S1 second bridge for dimer 2. Dashed bonds are possible bonds created
at 120 K and its simulation (dotted line). after the click reaction.
17340 | RSC Adv., 2017, 7, 17336–17345
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Catalytic properties
We then turned our attention to a more complex substrate,
the 4-t-butyl catechol (4-TBC), for which we mainly identied
We rst tested the oxidation of 3,5-di-t-butylcatechol (3,5-
DTBC), known to be easier to study if compared to catechol, as it
leads to more stable intermediates and thus to fewer by-
products, due to the presence of 3- and 5-t-butyl blocked posi-
1
6
47,48
two products, named B and C (Scheme 6).
Without catalyst
and in the presence of base, a total conversion of 4-TBC to
a mixture of unidentied products was obtained (Table 3, entry
1), whereas without base but in presence of [Mn L (Cl) (m-Cl) ]
2 2 2 2
no reaction occurred (entry 2). With both catalyst and base,
a mixture of products B and C were isolated with a 100% total
conversion (entry 3).
Changing the nature of oxidant (air instead of oxygen) led to
a slight modication of the B/C ratio in favour of B product
45
tions. In order to appreciate the effect of the manganese
complex anchoring into the mesoporous silica, we systemati-
cally compared the heterogeneous catalyst and its homoge-
neous analogue. We mainly identied one product, named A,
which is the quinone equivalent of 3,5-DTBC (Scheme 5) and
14
conrmed the oxidase activity of our catalysts (Table 2).
(entries 3 and 4), whereas heating under reux had almost no
Without any catalyst and in the presence of a base (Table 2,
entry 1), a mixture of unidentied products and the starting
material was obtained. In the absence of base, no reaction
signicant effect on this ratio (entries 4 and 5). Reducing the
reaction time from 5 h (entry 4) to 3 h (entry 7) diminished the
amount of B in favour of C. We thus assume that there is equi-
librium between B and C depending on the reaction conditions.
Finally, using a catalytic amount of base considerably diminished
the activity of the catalyst (entries 4 and 6). Concerning the nature
of the catalyst, with either Mn(NO ) (entry 8) or MnCl (entry 9)
2
occurred, without or with MnCl as catalyst (entries 0 and 2,
respectively). In homogeneous conditions, with a catalytic
amount of base (10%), complete conversion was observed and
product A was isolated exclusively whatever the catalyst used,
i.e. MnCl or [Mn L (Cl) (m-Cl) ] (entries 3 and 4). Working
2 2 2 2 2
under reux did not change the reactivity (entry 5).
3
2
2
no reaction occurred, which proves that the ligand L is required
for the activity of the catalyst (entry 10). In the absence of base,
the heterogeneous catalysts had almost no activity in the case of
Heterogeneous catalysts led also to product A even in the
absence of an additional base (entries 6 and 7 for {Mn}-
Py@LUS-E15 and {Mn}-TMS@LUS-E, respectively). It is not
surprising since it was already observed that pyridine has
4-TBC (entries 11 and 12), whereas it showed oxidase activity
with 3,5-DTBC. In the former case, an addition of stoichiometric
amount of base was required to go to completion, leading
selectively to only one compound, product B (entry 13). Indeed
46
a negligible effect on this reaction contrary to triethylamine.
Nevertheless, our heterogeneous catalysts turned out to be as
efficient as the homogeneous counterpart in the case of 3,5-
DTBC, with the advantage of being easier to separate and
potentially recyclable.
3
,5-DTBC is easier to oxidise since the intermediates are more
49,50
stable.
were observed in the N
It is noteworthy that in all cases no notable changes
2
sorption isotherms, before and aer
catalytic tests, leading us to believe that the material do not
undergo major modications throughout the whole catalytic
process (Fig. S3†). Nonetheless it remains to study the recycla-
bility of these bio-inspired catalysts.
a
Table 2 Oxidation of the 3,5-DTBC with Mn-based catalysts
b
Entry
Catalyst
Base (%)
Conversion (%)
0
Selectivity (%)
Experimental
Chemicals
0
1
2
3
4
5
6
7
—
—
—
200
0
10
10
10
0
0
0
c
ꢃ20
MnCl
MnCl
[Mn]
[Mn]
S1
2
100
100
100
100
100
100
100
100
100
100
100
100
All chemicals were used without further purication. Solvents
were dried using molecular sieves if needed. Propargylamine,
2
d
2
-picolyl chloride hydrochloride and 3-chloropropyltrie-
thoxysilane were purchased from Sigma-Aldrich, tetrabuty-
lammonium iodide and 2,2,6,6-tetramethylpiperidine were
purchased from Acros Organics, tetramethylammonium
bromide, hexamethyldisilazane were purchased from Alfa
S2
0
a
[
Mn
2
L
2
(Cl)
2
(m-Cl)
2
] ¼ [Mn]; {Mn}-Py@LUS-E15 ¼ S1; {Mn}-TMS@LUS-
E ¼ S2. Standard reaction conditions: 50 mL MeOH, 0.25 mmol 3,5-
DTBC, 10% catalyst, O
2
, room temperature, 5 h; base ¼ 2,2,6,6-
c
b
tetramethylpiperidine.
unknown products. T ¼ 65 C.
Selectivity for product A. Presence of Aesar, 2-(4-pyridylethyl)triethoxysilane was provided by ABCR
d
ꢀ
and bromotris(triphenylphosphine)copper bromide was
purchased from Santa Cruz Biotechnology.
Synthesis of the ligand and the azide tether
51
Synthesis of the ligand L. A mixture of propargylamine
(1.50 g, 27.3 mmol) and potassium carbonate (22.8 g, 163.8 mmol)
was stirred in 140 mL of acetonitrile for 5 min. Then 2-picolyl
chloride hydrochloride (9.84 g, 60.0 mmol) dissolved in 140 mL of
acetonitrile was added. The resulting solution was stirred under
reux for 5 days. The solution was ltered and the ltrate was
evaporated under reduced pressure. The resulting brown oil was
Scheme 5 3,5-DTBC and its quinone equivalent A.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 17336–17345 | 17341

View Article Online
RSC Advances
Paper
a
Table 3 Oxidation of 4-TBC with Mn-based catalysts
Selectivity (%)
Entry
Catalyst
Base (%)
Oxidant
Conversion (%)
B
C
b
1
2
3
4
5
6
7
8
9
—
200
0
200
200
200
10
200
10
10
10
0
0
200
O
O
O
Airc
Air
Air
2
2
2
100
0
0
0
0
0
[Mn]
[Mn]
[Mn]
[Mn]
[Mn]
[Mn]
100
100
100
48
100
0
60
74
66
100
38
0
40
26
34
0
62
0
0
0
0
0
d
Air
Mn(NO
3
)
2
O
O
O
O
O
O
2
2
2
2
2
2
MnCl
[Mn]
S1
S2
S1
2
0
0
1
1
1
1
0
1
2
3
70
<5
<5
100
100
<5
<5
100
0
a
[
Mn
2
L
2
(Cl)
2
(m-Cl)
2
] ¼ [Mn]; {Mn}-Py@LUS-E15 ¼ S1; {Mn}-TMS@LUS-E ¼ S2. Standard reaction conditions: 50 mL MeOH, 0.25 mmol 4-TBC, 10%
ꢀ
b c d
catalyst, O
2
, room temperature, 5 h; base ¼ 2,2,6,6-tetramethylpiperidine. Mixture of unidentied products. T ¼ 65 C. 3 h.
silica where azide functions were previously graed using the
34
so-called molecular stencil pattering technique. A Berghof
speedwave oven was used for the synthesis of the mesoporous
18,30
silica LUS using the procedure described elsewhere.
It
provides a microwave irradiation at 2450 MHz with a power of
ꢀ
1
450 W. The experimental conditions used were: T ¼ 180 C,
Scheme 6 4-TBC and the two main oxidation products B and C.
30
ramp ¼ 1 min, plateau ¼ 9 min (total heating time ¼ 10 min).
34
Synthesis of LUS-TMA. Tetramethylammonium bromide
(
1.7 g, 11 mmol) was dissolved in 150 mL of EtOH 70% and stirred
dissolved in 100 mL of distilled water and extracted with
dichloromethane (3 times). The organic phases were gathered
ꢀ
for 10 min at 40 C. The LUS mesoporous silica (3 g) was added
and the resulting suspension was stirred for 45 min at 40 C. The
solid was ltered on B u¨ chner and washed twice with 50 mL of
EtOH 70% and twice with 50 mL of acetone. This procedure was
repeated three times to ensure a total exchange. The solid LUS-
ꢀ
and dried over sodium sulphate. The solvent was nally evapo-
1
rated to afford L in a 94% yield. H NMR (CDCl
3
, 300 MHz):
d (ppm) 8.55 (ddd, J ¼ 4.80, 1.80, 0.90 Hz, 2H), 7.64 (td, J ¼ 7.50,
1
1
2
1
.80 Hz, 2H), 7.50 (d, J ¼ 7.80 Hz, 2H), 7.15 (ddd, J ¼ 7.50, 4.80,
ꢀ
TMA obtained was nally dried in at 80 C overnight.
.20 Hz, 2H), 3.91 (s, 4H), 3.41 (d, J ¼ 2.40 Hz, 2H), 2.29 (t, J ¼
13
.40 Hz, 1H). C NMR (300 MHz, CDCl ): d (ppm) 158.76, 149.26,
3
36.47, 123.14, 122.10, 78.32, 73.62, 59.44, 42.54. IR (KBr): 3294
Synthesis of {Mn}-Py@LUS-E15 (S1)
ꢁ
1
(w), 3062 (w), 3011 (w), 2921 (w), 2839 (w), 2095 (m, C^C) cm .
31
LUS-TMA-E15. Hydrochloric acid (0.25 mL of 1 M solution)
was added to a suspension of LUS-TMA (1 g) in 200 mL of
technical EtOH and stirred at RT for 1 h. The solid was then
Synthesis of 3-azidopropyltriethoxysilane. Sodium azide
(27 g, 0.41 mol) was added portionwise to a solution of 3-
chloropropyltriethoxysilane (20 g, 83 mmol) and tetrabuty-
lammonium iodide (153 mg, 0.42 mmol) in 220 mL of buta-
none. The mixture was then stirred at reux for 5 d under
nitrogen. The solution was then ltered over celite and the
solvent was removed under reduced pressure. The resulting oil

ltered and washed twice with 100 mL of technical EtOH, before
ꢀ
being dried overnight at 80 C.
Py@LUS-TMA-E15. LUS-TMA-E15 (500 mg) was pretreated
ꢀ
ꢀ
as at 130 C under argon for 2 h, then at 130 C under vacuum
for 2 h. Aer cooling under argon, a solution of 2-(4-pyr-
idylethyl)triethoxysilane (459 mg, 1.7 mmol) dissolved in
was redissolved in 400 mL CH
with 20 mL of distilled water. The organic phase was dried over
Na SO and the solvent was removed under reduced pressure to
afford the 3-azidopropyltriethoxysilane (84 mg, 0.34 mmol) as
2 2
Cl and quickly washed twice
1
0 mL of dry toluene was added. The suspension was stirred
2
4
for 1 h, 20 mL of toluene were added, and the mixture was
stirred at 80 C during 18 h. Aer ltration and washing
twice with 20 mL of toluene, 20 mL of technical EtOH and
2
8
ꢀ
1
3
a pale yellow oil in a 80% yield. C NMR (200 MHz, CDCl3):
d (ppm) 58.16, 53.54, 22.39, 17.97, 7.33.
0 mL of acetone, the solid was nally dried overnight at
ꢀ
0 C.
Synthesis of the graed complexes
Py@LUS-E15. Hydrochloric acid (2 mL of 1 M solution) was
[
Mn
2
L
2
8
(Cl)
2
(m-Cl)
2
] was used to synthesise the hybrid mate- added to a suspension of Py@LUS-TMA-E15 (1 g) in 200 mL of
1
rials. The complex was incorporated in a LUS mesoporous technical EtOH and stirred at RT for 1 h. The solid was then
17342 | RSC Adv., 2017, 7, 17336–17345
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
ltered and washed twice with 100 mL of technical EtOH, before with Et
RSC Advances

2
O aer addition of 2 N HCl (until pH 4–5). The organic
ꢀ
being dried overnight at 80 C.
layer was dried over MgSO . Aer evaporation of Et O, the crude
4 2
1
N –Py@LUS-E15. Py@LUS-E15 (500 mg) was pretreated mixture was analysed by H NMR (300 MHz). Silica gel ash-
3
following the same procedure used in the synthesis of Py@LUS- chromatography allowed separating the reaction products (80/
TMA-E15. Aer cooling under argon, a solution of 3-azidopro- 20: petroleum ether/AcOEt).
pyltriethoxysilane (371 mg, 1.5 mmol) dissolved in 10 mL of dry
Heterogeneous catalysis. Filtration of the reaction mixture
toluene was added. The suspension was stirred for 1 h, 20 mL of allowed recovering of the catalyst for further analyses. The
ꢀ
1
toluene were added, and the mixture was stirred at 80 C during ltrate was concentrated to dryness, analysed by H NMR (300
18 h. Aer ltration and washing twice with 20 mL of toluene, MHz) and the reaction products were recovered by silica gel
2

0 mL of technical EtOH and 20 mL of acetone, the solid was ash-chromatography (80/20: petroleum ether/AcOEt).
nally dried overnight at 80 C. Elemental analysis: C: 9.78%, H:
ꢀ
1
.99%, N: 2.14%.
Mn}-Py@LUS-E15 (S1). N
red in a mixture MeOH/MeCN (60 mL/20 mL). Aer 10 min,
complex [Mn (Cl) (m-Cl) ] (176 mg, 0.5 mmol) was added and
the suspension was stirred during 1 h, before addition of
Physico-chemical characterisation
{
3
–Py@LUS-E15 (300 mg) was stir-
Infrared spectra were recorded from KBr pellets using a Mattson
3000 IRTF spectrometer. Nitrogen sorption isotherms at 77 K
2
L
2
2
2
were determined with a volume device Belsorp Max on solids
ꢀ
that were dried at 80 C under vacuum overnight. Low angle X-
CuBr(PPh
3
)
3
(93 mg, 0.10 mmol). Then the mixture was stirred
ꢀ
ray powder diffraction (XRD) experiments were carried out
using a Brucker (Siemens) D5005 diffractometer using Cu K
monochromatic radiation. Liquid NMR spectra were recorded
on a Bruker AC 200 spectrometer. Solid NMR spectra were
measured on an Advance III Bruker 500 MHz Wide Bore spec-
trometer, with a CP/MAS DVT (double resonance, variable
temperature) probe 4 mm. The delay time between pulses (pulse
width 5 ms) in HPDEC experiments was set to 500 s. Spinning
speed was set to about 5 kHz and the number of scans was 5000.
at 60 C for 6 days before ltration. The pale brown solid was
washed with 200 mL of MeOH and dried overnight at 80 C.
Elemental analysis: C: 9.86%, H: 1.84%, N: 1.68%, Mn: 1.66%,
Cl: 0.74%.
ꢀ
18
Synthesis of {Mn}-TMS@LUS-E (S2)
34
TMS@LUS-TMA. Before any reaction, the solid was pre-
treated as follows: the LUS-TMA solid (1.5 g) was stirred at
1
The solid was let to cool to room temperature under argon,
before addition of hexamethyldisilazane (2.42 mg, 15.0 mmol)
in 20 mL of dry toluene. The suspension was further stirred at
room temperature for 1 h, and 60 mL of toluene were then
added. The suspension was stirred under argon at 80 C for
1
toluene, 20 mL of technical EtOH and 20 mL of acetone. The
solid was dried overnight at 80 C to afford the TMS@LUS-TMA
ꢀ
ꢀ
30 C under argon for 2 h, then at 130 C under vacuum for 2 h.
13
29
Frequency C ¼ 125.76 MHz, Si ¼ 99.36 MHz. EPR spectra
were recorded on a Brucker Elexsys e500 X-band (9.4 GHz)
spectrometer with a 4102ST cavity (modulation: 2 G, power: 4
mW). The simulated spectra were calculated using the Easyspin
toolbox from Matlab. Variable-temperature magnetic suscepti-
bility measurements were made using a Quantum Design
MPMS-XL-5 SQUID susceptometer, using an applied eld of
ꢀ
8 h, before being ltered and washed twice with 20 mL of
1000 G.
ꢀ
silica.
Conclusions
TMS@LUS-E. Hydrochloric acid (2 mL of 1 MM solution) was
added to a suspension of TMS@LUS-TMA (1 g) in 200 mL of
technical EtOH and stirred at RT for 1 h. The solid was then
0
The N,N -bis[(pyridin-2-yl)methyl]prop-2-yn-1-amine (L) ligand
is a versatile molecule allowing both coordination to a metal ion
and graing to a solid support using click chemistry. The
functionalisation method herein reported is not restricted to
this particular complex and can be applied to any metal
complex provided that the ligand contains a free alkyne group.
The site isolation is ensured by the molecular stencil patterning
strategy that allows homogeneous graing of two different
functions in a precise ratio. The azide anchor function is sur-
rounded by pyridyl or TMS functions. Once again, the method is
not limited to these particular groups and can be extended to
other isolating functions. We have demonstrated that the use of

ltered and washed twice with 100 mL of technical EtOH, before
ꢀ
being dried overnight at 80 C.
N –TMS@LUS-E and {Mn}-TMS@LUS-E (S2). The function-
3
alisation of TMS@LUS-E with azide functions and the incor-
poration of the Mn(II) complex [Mn
TMS@LUS-E was performed as described elsewhere.
CuBr(PPh
and reaction time for the click reaction were 60 C and 6 days,
respectively.
2 2 2 2 3
L (Cl) (m-Cl) ] in N –
18
3
)
3
was used as catalyst and the reaction temperature
ꢀ
General procedure for the catalytic tests
3 3
CuBr(PPh ) as catalyst for the Huisgen cycloaddition avoids
The substrate (0.25 mmol) and the base (2,2,6,6-tetramethylpi- copper complexation to L allowing therefore a better yield for
peridine) were dissolved in MeOH (50 mL) in a single-necked the click reaction. The Mn hybrid materials here presented have
round-bottom ask. To this solution was added the catalyst been tested in the catalytic oxidation of two catechol derivatives
and the gas was passed through the reaction mixture for 5 h (3,5-DTBC and 4-TBC). Their activity has been compared with
with magnetic stirring.
that of the molecular analogue. With both catechol derivatives
Homogeneous catalysis. The resulting solution was tested, oxidation could be achieved using molecular dioxygen as
concentrated to dryness and the resulting mixture was extracted oxidant. This reaction can be compared to the natural oxidase
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 17336–17345 | 17343

View Article Online
RSC Advances
Paper
enzymes. Nevertheless, the heterogeneous catalysts were more 16 N. Shaikh, A. Panja, P. Banerjee and M. Ali, Transition Met.
advantageous as no base was necessary in the case of 3,5-DTBC Chem., 2003, 28, 871–880.
and only one product was obtained in the case of 4-TBC. 17 S. Calmettes, B. Albela, O. Hamelin, S. Menage,
Therefore, once graed in a conned and controlled environ-
ment, the metal complex does exhibit a different and more
F. Miomandre and L. Bonneviot, New J. Chem., 2008, 32,
727–737.
advantageous behaviour than the free complex. The broad 18 J. Chaignon, S. E. Stiriba, F. Lloret, C. Yuste, G. Pilet,
range of isolating functions available offers numerous options
to control the activity of any inserted complex, which is not
L. Bonneviot, B. Albela and I. Castro, Dalton Trans., 2014,
43, 9704–9713.
possible for complexes in solution. Controlling the environ- 19 S. Abry, A. Thibon, B. Albela, P. Delich `e re, F. Banse and
ment at a larger range than the sole ligand might allow tuning L. Bonneviot, New J. Chem., 2009, 33, 484–496.
the catalyst activity, in a similar fashion to the natural enzymes. 20 V. Jollet, B. Albela, K. Senechal-David, P. Jegou, E. Kolodziej,
J. Sainton, L. Bonneviot and F. Banse, Dalton Trans., 2013,
4
2, 11607–11613.
Acknowledgements
2
1 W.-J. Zhou, B. Albela, M.-Y. He and L. Bonneviot, Polyhedron,
J. C. thanks the French “Minist `e re de l'Enseignement, de la
2013, 64, 371–376.
Recherche et de la Technologie” (MERT) for a PhD fellowship, 22 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and
and the C-MIRA program of Rh ˆo ne-Alpes region in France for
J. S. Beck, Nature, 1992, 359, 710–712.
nancial support to the collaboration between the Universitat 23 D. Brunel, N. Bellocq, P. Sutra, A. Cauvel, M. Lasperas,

de Val `e ncia (Spain) and the Ecole Normale Sup ´e rieure de Lyon
P. Moreau, F. Di Renzo, A. Galarneau and F. Fajula, Coord.
Chem. Rev., 1998, 178, 1085–1108.
(
(
(
France). The Spanish work was supported by the MICINN
Spain) (Project CTQ2010-15364) and the Generalitat Valenciana 24 K. Moller and T. Bein, Chem. Mater., 1998, 10, 2950–2963.
Spain) (Projects PROMETEO/2009/108 and ISIC/2012/002). 25 D. E. De Vos, M. Dams, B. F. Sels and P. A. Jacobs, Chem. Rev.,
002, 102, 3615–3640.
2
2
6 H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int.
Ed., 2001, 40, 2004–2021.
Notes and references
1
2
B. C. Gates, Catalytic Chemistry, Wiley, 1992.
W. Kaim and B. Schwederski, Bioinorganic chemistry:
27 H. C. Kolb and K. B. Sharpless, Drug Discovery Today, 2003, 8,
1128–1137.
Inorganic Elements in the Chemistry of Life, John Wiley & 28 J. E. Moses and A. D. Moorhouse, Chem. Soc. Rev., 2007, 36,
Sons, 1st edn, 1994. 1249–1262.
E. A. Lewis and W. B. Tolman, Chem. Rev., 2004, 104, 1047– 29 L. Bonneviot, M. Morin and A. Badiei, Patent, WO 01/55
076. 031A031, 2001.
L. M. Mirica, X. Ottenwaelder and T. D. P. Stack, Chem. Rev., 30 J. Chaignon, Y. Bouizi, L. Davin, N. Calin, B. Albela and
004, 104, 1013–1045. L. Bonneviot, Green Chem., 2015, 17, 3130–3140.
E. I. Solomon, U. M. Sundaram and T. E. Machonkin, Chem. 31 M. Ortega-Munoz, J. Lopez-Jaramillo, F. Hernandez-Mateo
3
4
5
6
7
1
2
Rev., 1996, 96, 2563–2605.
and F. Santoyo-Gonzalez, Adv. Synth. Catal., 2006, 348,
P. C. A. Bruijnincx, G. van Koten and R. J. M. K. Gebbink,
Chem. Soc. Rev., 2008, 37, 2716–2744.
2410–2420.
32 S. Abry, B. Albela and L. Bonneviot, C. R. Chim., 2005, 8, 741–
J. P. Emerson, M. L. Wagner, M. F. Reynolds, L. Que,
752.
M. J. Sadowsky and L. P. Wackett, J. Biol. Inorg Chem., 33 A. Badiei, L. Bonneviot, N. Crowther and G. M. Ziarani, J.
005, 10, 751–760. Organomet. Chem., 2006, 691, 5911–5919.
T. D. H. Bugg and S. Ramaswamy, Curr. Opin. Chem. Biol., 34 K. Zhang, B. Albela, M. Y. He, Y. M. Wang and L. Bonneviot,
008, 12, 134–140.
Phys. Chem. Chem. Phys., 2009, 11, 2912–2921.
L. Que and M. F. Reynolds, Met. Ions Biol. Syst., 2000, 37, 35 M. W. Vetting, L. P. Wackett, L. Que, J. D. Lipscomb and
05–525.
D. H. Ohlendorf, J. Bacteriol., 2004, 186, 1945–1958.
0 V. Georgiev, T. Borowski, M. R. A. Blomberg and 36 V. Georgiev, T. Borowski and P. E. M. Siegbahn, J. Biol. Inorg
P. E. M. Siegbahn, J. Biol. Inorg Chem., 2008, 13, 929–940. Chem., 2006, 11, 571–585.
1 J. P. Emerson, E. G. Kovaleva, E. R. Farquhar, J. D. Lipscomb 37 S. Huang, R. J. Clark and L. Zhu, Org. Lett., 2007, 9, 4999–
2
8
9
2
5
1
1
and L. Que, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 7347–
352.
2 A. K. Whiting, Y. R. Boldt, M. P. Hendrich, L. P. Wackett and
L. Que, Biochemistry, 1996, 35, 160–170.
3 H. Sakurai, S. Shimomura and T. Yoshimura, Biochem.
Biophys. Res. Commun., 1983, 115, 618–624.
5002.
7
38 W. S. Brotherton, H. A. Michaels, J. T. Simmons, R. J. Clark,
N. S. Dalal and L. Zhu, Org. Lett., 2009, 11, 4954–4957.
39 K. Buerglova, N. Moitra, J. Hodacova, X. Cattoen and
M. W. C. Man, J. Org. Chem., 2011, 76, 7326–7333.
40 I. Romero, M. N. Collomb, A. Deronzier, A. Llobet, E. Perret,
J. Pecaut, L. Le Pape and J. M. Latour, Eur. J. Inorg. Chem.,
2001, 69–72.
1
1
1
1
4 A. Ramadan and I. M. El-Mehasseb, Transition Met. Chem.,
1998, 23, 183–189.
5 M. U. Triller, D. Pursche, W. Y. Hsieh, V. L. Pecoraro, 41 J. Z. Wu, E. Bouwman, A. M. Mills, A. L. Spek and J. Reedijk,
A. Rompel and B. Krebs, Inorg. Chem., 2003, 42, 6274–6283. Inorg. Chim. Acta, 2004, 357, 2694–2702.
17344 | RSC Adv., 2017, 7, 17336–17345
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
4
4
4
4
2 G. A. van Albada, A. Mohamadou, W. L. Driessen, R. de 46 I. C. Szigyarto, L. I. Simandi, L. Parkanyi, L. Korecz and
Gelder, S. Tanase and J. Reedijk, Polyhedron, 2004, 23,
387–2391.
3 F. H. Kohler, N. Hebendanz, G. Muller, U. Thewalt,
G. Schlosser, Inorg. Chem., 2006, 45, 7480–7487.
47 T. R. Demmin and M. M. Rogic, J. Org. Chem., 1980, 45,
4210–4214.
2
B. Kanellakopulos and R. Klenze, Organometallics, 1987, 6, 48 M. Lanfranchi, L. Prati, M. Rossi and A. Tiripicchio, J. Mol.
15–125. Catal. A: Chem., 1995, 101, 75–80.
4 B. Brauer, D. Schaarschmidt, C. Flohrer, T. Ruffer, S. Tripke, 49 J. Mukherjee and R. Mukherjee, Inorg. Chim. Acta, 2002, 337,
1
A. Hildebrandt, B. Walfort, L. Sorace and H. Lang, Inorg.
Chim. Acta, 2011, 365, 277–281.
5 K. S. Banu, T. Chattopadhyay, A. Banerjee, M. Mukherjee,
429–438.
50 J. Reim and B. Krebs, J. Chem. Soc., Dalton Trans., 1997,
3793–3804.
S. Bhattacharya, G. K. Patra, E. Zangrando and D. Das, 51 J. Rosenthal and S. J. Lippard, J. Am. Chem. Soc., 2010, 132,
Dalton Trans., 2009, 8755–8764. 5536–5537.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 17336–17345 | 17345