Electrochemical and catalytic studies of a manganese(III)complex with a tetradentate schiff-base ligand encapsulated in NaY zeolite

Article abstract of DOI:10.1002/ejic.201201517

The manganese(III) complex with a Schiff-base salen-type ligand (1,5-bis{[(1E)-(2-hydroxyphenyl)methylene]amino}-1H-imidazole-4-carbonitrile) has been encapsulated in the nanopores of a Y zeolite by using two different methodologies, the flexible ligand and in situ complex preparation methods. Cyclic voltammetry studies revealed that the neat complex undergoes reversible oxidation in dmf, which has been attributed to the Mn^II/III redox couple, whereas the two heterogeneous catalysts show different electrochemical behaviour in aqueous medium. The encapsulated and non-encapsulated homogeneous Mn^IIIsalen complexes were screened as catalysts for olefin oxidation by using tBuOOH as the oxygen source in different solvents. Under the optimized conditions, the catalysts exhibited moderate activity. Both heterogeneous catalysts catalysed the oxidation of cyclohexene with tBuOOH, primarily to give the allylic oxidation products. These catalysts were found to be reusable after the catalytic cycle, but with some loss of activity. A DFT study confirmed the distortion of the complex in the zeolite cage, the main difference being observed for the bonded chloride. Copyright

Full text of DOI:10.1002/ejic.201201517

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DOI:10.1002/ejic.201201517
Electrochemical and Catalytic Studies of a Manganese(III)
Complex with a Tetradentate Schiff-Base Ligand
Encapsulated in NaY Zeolite
Iwona Kuz´niarska-Biernacka,*[a] Otilia Rodrigues,[a]
Maria Alice Carvalho,[a] Pier Parpot,[a] Krzysztof Biernacki,[b]
Alexandre Lopes Magalhães,[b] Antonio Mauricio Fonseca,[a] and
Isabel Correia Neves[a]
Keywords: Electrochemistry / Heterogeneous catalysis / Oxidation / Zeolites / Manganese / Alkenes
The manganese(III) complex with a Schiff-base salen-type
ligand (1,5-bis{[(1E)-(2-hydroxyphenyl)methylene]amino}-
encapsulated homogeneous MnIIIsalen complexes were scre-
ened as catalysts for olefin oxidation by using tBuOOH as
the oxygen source in different solvents. Under the optimized
conditions, the catalysts exhibited moderate activity. Both
heterogeneous catalysts catalysed the oxidation of cyclohex-
ene with tBuOOH, primarily to give the allylic oxidation
products. These catalysts were found to be reusable after the
catalytic cycle, but with some loss of activity. A DFT study
confirmed the distortion of the complex in the zeolite cage,
the main difference being observed for the bonded chloride.
1H-imidazole-4-carbonitrile) has been encapsulated in the
nanopores of a Y zeolite by using two different methodolo-
gies, the flexible ligand and in situ complex preparation
methods. Cyclic voltammetry studies revealed that the neat
complex undergoes reversible oxidation in dmf, which has
been attributed to the MnII/III redox couple, whereas the two
heterogeneous catalysts show different electrochemical be-
haviour in aqueous medium. The encapsulated and non-
reaction due to their inactive catalytic character.[2,3] To
overcome these difficulties, heterogenization of the catalyst
has received considerable attention with different immobili-
zation methods having been developed, for example, 1) in-
tercalation into clays,[4] 2) encapsulation into zeolites or
mesoporous molecular sieves[5–8] and 3) electropolymeri-
zation or grafting onto polymer matrices[9,10] or silicates.[11]
It was concluded that the formation of μ-oxo dimers is lim-
ited by immobilization, which consequently increases the
turnover frequencies of the catalytic reaction.[12]
The use of zeolites for heterogenization has attracted
considerable interest due to the size of the window pore,
the accessibility of the void space, the dimensionality of the
channel system and the number of sites and types of extra
framework cations.[13] Faujasite zeolite, including zeolite X
and Y, is frequently chosen as a support to encapsulate
metal complexes because it presents large cavities or su-
percages (11.8 Å) with smaller pore openings (7.4 Å) that
are suitable for accommodating metal complexes and a
three-dimensional pore system desirable for easy reactant
accessibility and product diffusion.[14,15] Metal complexes
encapsulated into Y zeolite are interesting for applications
as biomimetic heterogeneous catalysts for the oxidation of
alkanes, alkenes and alcohols.[6,13–16]
Introduction
Alkenyl and allylic oxidation reactions are of fundamen-
tal importance in synthetic organic chemistry, and a variety
of reagents have been used for these transformations. The
selectivity of the process depends on the oxygen source and
solvent, however, the chemical properties of the ligand as
well as the support also play a significant role. Selectivity
in the allylic oxidation reaction in the presence of tert-butyl
hydroperoxide (tBuOOH) is due to the ability of the tert-
butylperoxy radical to remove a hydrogen atom from the
activated site with the lowest C–H bond dissociation en-
ergy.[1] Manganese–salen complexes are known to effec-
tively catalyse the oxidation of olefins.
For practical use, the catalyst requires good stability
against self-oxidation, which would lead to the formation
of μ-oxo dimers and peroxo-bridged compounds. Indeed,
when molecular oxygen is used as the oxygen atom donor
in homogeneous catalysis, μ-oxo dimers are systematically
formed in the medium causing a weakening of the oxidation
[a] Departamento de Química, Centro de Química, Universidade
do Minho,
Campus de Gualtar, 4170-057 Braga, Portugal
Fax: +351-253604382
E-mail: iwona@quimica.uminho.pt
Homepage: http://www.quimica.uminho.pt/
[b] REQUIMTE, Departamento de Química e Bioquímica, Facul-
dade de Ciências, Universidade do Porto,
4169-007 Porto, Portugal
Previously we have reported the redox proprieties of dif-
ferent metal complexes encapsulated in Y zeolite and
showed that the electroactivity of zeolite-encapsulated
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metal complexes is related to the sites at which the com-
plexes are incorporated.[5,17] The electroactivity is very im-
portant for understanding the stability of these hetero-
geneous catalysts in oxidation reactions.[9,18,19] As a contin-
uation of this research, we report herein the electrochemical
and catalytic properties of the manganese(III) complex
Results and Discussion
Electrochemical Properties of the Neat and Encapsulated
MnIIIsalen Complex
To gain some insights into the electron-richness of the
formed with the Schiff base-salen-type ligand 1,5-bis{[(1E)- complex studied, the electrochemical behaviour of the
(2-hydroxyphenyl)methylene]amino}-1H-imidazole-4-carbo- 1,5-bis{[(1E)-(2-hydroxyphenyl)methylene]amino}-1H-imid-
nitrile, denoted as salen, in both homogeneous and hetero- azole-4-carbonitrile ligand, H2(salen), and its manganese
geneous media. These catalysts were tested in the oxidation complex, [Mn(salen)Cl], were studied by cyclic voltammetry
of cyclohexene in the liquid phase with tBuOOH as the oxy- at different scan rates in dmf solutions containing 0.10 m
gen source. The effect of solvent on this oxidation reaction [NBu4][BF4] as the supporting electrolyte. The electrochem-
is reported and the catalytic activity of the MnIIIsalen com- ical responses for the ligand and complex between the limits
plex is compared with that in the oxidation of styrene.[20] imposed by the solvent used are shown in Figure 1, and the
DFT studies were performed to rationalize the different most relevant data for the redox processes are summarized
electrochemical and catalytic behaviour arising from the in Table 1.
different coordination sphere of the metal complex encap-
sulated in the NaY zeolite.
The ligand shows one irreversible oxidation at 0.80 V
versus SCE (Ep ), which can be attributed to the formation
a
of the radical cation of the heterocyclic moiety, without any
cathodic counterpart. At low potential, –1.50 V versus SCE
c
(Ep ), this compound exhibits one irreversible electron pro-
cess that generates a radical anion; the electron enters the
π* anti-bonding orbital of the heterocyclic ring.[21]
Cyclic voltammetry revealed that the manganese com-
plex undergoes reversible oxidation in dmf solution (Fig-
ure 1, process IIa), which has been assigned to the MnII/III
c
a
redox couple [Ip /Ip ≈ 1, ΔEp ≈ (ferrocenium/ferrocene)].
Comparison of the cyclic voltammograms obtained for the
complex and the ligand in the positive potential range
shows that after coordination it is not subject to the anodic
process experienced by the ligand on oxidation. This can be
explained by metal-to-heterocycle π* orbital back-donation
nitrogen atoms. Reduction of the complex occurs by an
irreversible process (Figure 1, process Ic) and the reduction
potential is shifted to a less cathodic value compared with
the ligand.
The zeolite-encapsulated MnIIIsalen complex was de-
posited on Carbon Toray (CT) to determine the electroac-
tivity and stability of the complex by cyclic voltamme-
try.[5,17] The voltammetric study was carried out in 0.10 m
aqueous NaCl at room temperature. Under these condi-
tions, the modified electrodes were mechanically and chemi-
cally stable. When the modified electrode was placed in the
electrolytic solution and washed, no change in the colour
of the solution was observed. The cyclic voltammograms of
NaY-CT and Mnsalen@YA/B-CT in the positive potential
range are presented in Figure 2. The cyclic voltammogram
of NaY-CT did not exhibit any redox processes in the scan
range of 0.50–1.40 V versus SCE. The cyclic voltammog-
rams obtained for the modified electrodes Mnsalen@YA-
CT and Mnsalen@YB-CT in this potential range did exhibit
redox processes. The voltammogram obtained with the
Mnsalen@YA-CT-modified electrode shows an anodic pro-
cess at 0.98 V versus SCE with a cathodic counterpart at
0.63 V versus SCE. The Mnsalen@YB-CT-modified elec-
trode (Figure 2) shows an irreversible anodic process at
1.10 V versus SCE. The results of the electrochemical stud-
ies of the Mnsalen@YA-CT- and Mnsalen@YB-CT-modi-
Figure 1. Cyclic voltammograms of H2(salen) (grey) and
[Mn(salen)Cl] (black) in dmf containing 0.10 m [NBu4][BF4] at
room temperature under Ar recorded at a scan rate of 0.10 Vs–1
and at different potentials.
Table 1. Electrochemical data for the H2(salen) ligand and
[Mn(salen)Cl] complex.[a]
v [Vs–1]
H2(salen)
Oxidation Reduction
[Mn(salen)Cl]
Oxidation
Reduction
a
c
a
c
Ep [V]
–Ep [V]
Ep [V]
ΔE [mV]
–Ep [V]
0.02
0.05
0.10
0.20
0.50
0.77
0.78
0.80
0.81
0.82
1.47
1.48
1.50
1.51
1.52
0.53
0.54
0.56
0.59
0.60
65
80
95
100
120
1.30
1.32
1.33
1.34
1.35
[a] All measurements were taken at room temperature in degassed
dmf with 0.10 m [NBu4][BF4] as the supporting electrolyte at a car-
bon working electrode. Potentials were measured with respect to
ferrocenium/ferrocene as the internal standard. Finally, the poten-
tials were converted and are reported relative to the SCE [E(fc+/fc)
= 0.48 V vs. SCE].
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fied electrodes show that their electrochemical behaviour is [acetonitrile (acn) and decane] with tBuOOH as the oxygen
different, which is due to differences in the coordination source. The results obtained are summarized in Table 2
sphere around the metal arising from the different encapsul- along with the data obtained with the homogeneous phase
ation method used.
and the blank experiments under comparable conditions.
The following reaction products were identified: cyclohex-
ene oxide (CyO) as the alkenyl oxidation product and 2-
cyclohexen-1-ol (CyOl) and 2-cyclohexen-1-one (CyOne) as
the allylic oxidation products. These products were also
formed in the presence of a copper(II)–purine complex en-
capsulated in NaY zeolite[22] and electron-deficient manga-
nese(III)–corrole[23] when tBuOOH was used as the oxygen
source. Another significant product formed in this reaction
was 1-tert-butylperoxy-2-cyclohexene (CyOx).[15] The for-
mation of the allylic oxidation products (i.e., CyOne and
CyOl) shows preferential attack of the activated C–H bond
over the C=C bond. The selectivity for allylic oxidation by
tBuOOH is due to the ability of the tert-butylperoxy radical
to remove a hydrogen atom from the activated site having
the lowest C–H bond dissociation energy.[1]
Figure 2. Cyclic voltammograms of NaY-CT (1), Mnsalen@YA-CT
(2) and Mnsalen@YB-CT (3) at a scan rate of 0.10 Vs–1.
NaY and the blank experiments were performed, and the
same cyclohexene conversion was observed (14%). In both
cases only CyOx was detected. We have characterized this
product previously[6] and it provides rather strong evidence
for the involvement of the tBuOO· radical as the reactive
intermediate. Marked improvement in substrate conversion,
when compared with encapsulated complexes, resulted from
the presence of the MnIIIsalen complex. Both hetero-
geneous catalysts show similar substrate conversions in dec-
ane compared with [Mn(salen)Cl] and higher conversions
in acn.
Study of the Catalytic Properties of the Neat and
Encapsulated MnIIIsalen Complex
The three catalysts were screened for the oxidation of
olefins performed under aerobic conditions at room
temperature. The results of the experiments performed
with the new heterogeneous catalysts (Mnsalen@YA and
Mnsalen@YB) and its homogeneous counterpart
([Mn(salen)Cl]) in the oxidation of cyclohexene and styrene
in different solvents are shown in Tables 2 and 3, respec-
tively.
Figure 3 shows representative examples of the results of
the oxidation of cyclohexene in the homogeneous phase. It
can be observed that the product distribution changed with
time. A progressive reduction in CyOl and CyOne selectiv-
ity was observed under homogeneous conditions in the apo-
lar solvent decane, whereas a different result was obtained
Oxidation of Cyclohexene
The MnIIIsalen heterogeneous catalysts were tested in the in the aprotic polar medium acetonitrile, especially for
oxidation of cyclohexene in solvents of different polarity CyOne.
Table 2. Oxidation of cyclohexene catalysed by the MnIIIsalen complex in homogeneous and heterogeneous phases.
Catalyst
Run
T [h][a] C [%][b,c]
Products
CyO[d]
CyOl[e]
η [%]
CyOne[f]
CyOx[g]
S [%]
η [%][h]
S [%][i]
S [%]
η [%]
S [%]
η [%]
NaY
[Mn(salen)Cl]
Mnsalen@YA
1
1
1
2
1
2
1
1
1
2
1
2
24
5
14[j]
48[j]
45[j]
24[j]
43[j]
19[j]
Ͻ1[l]
42[l]
71[l]
69[l]
77[l]
36[l]
0
2
5
2
3
2
0
7
8
0
5
11
9
6
10
0
15
11
38
24
52
0
2
3
1
3
6
0
7
12
4
0
5
8
6
6
35
0
17
17
6
20
26
0
9
14
3
10
3
0
17
21
24
23
3
0
14
34
23
18
27
8
100
71
50
73
64
38
0
27
43
22
27
12
10
31
12
24
17
0
41
29
34
29
10
48
48
48
48
48
5
Mnsalen@YB
NaY
[Mn(salen)Cl]
Mnsalen@YA
0
11
30
15
21
4
6
48
48
48
26
18
19
Mnsalen@YB
15
10
[a] Reaction time at which the substrate conversion becomes constant. [b] Determined by GC against an internal standard. [c] Cyclo-
hexene conversion (C) was calculated as C = {[A(cyclohexene)/A(chlorobenzene) t=0h – [A(cyclohexene)/A(chlorobenzene) t=xh} ϫ 100/[A(cyclohexene)
A(chlorobenzene) t=0h; A = chromatographic peak area. [d] CyO = cyclohexene epoxide. [e] CyOl = 2-cyclohexene-1-ol. [f] CyOne = 2-
]
]
/
]
cyclohexene-1-one. [g] CyOx = 1-tert-butylperoxy-2-cyclohexene. [h] Product yield (η) calculated as η = CϫS/100. [i] Product selectivity
(S) calculated as S = A(product) ϫ 100/[A(product) + ΣA(other reaction products)]; A = chromatographic peak area. [j] In decane. [l] In acn.
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(active site accessibility) under homogeneous and hetero-
geneous conditions.
Low catalytic activity (Ccyclohexene Ͻ 5%) was observed
in the filtrate after removal of the catalyst, which indicates
that the reaction was not fully catalysed heterogeneously
and that a small amount of the active phase is leached into
solution; this effect is less perceptible for Mnsalen@YA in
acn. The FTIR spectra of the Mnsalen@YA heterogeneous
catalyst, as a representative example, measured before the
catalytic reaction and after the second cycle show a de-
crease in band intensity in the region 1620–1100 cm–1. This
corresponds to the frequency range in which vibration
bands for the complex occur. Conversely, the bands typical
of the zeolite matrix do not show significant changes after
the catalytic reaction. These observations suggest that no
structural changes of the zeolite matrix take place during
consecutive catalytic cycles, but that some metal complex
leaching must occur under the catalytic experimental condi-
tions. Thus, the decrease in substrate conversion with reuse
of the heterogeneous catalysts might be correlated with
some leaching of the active phase.
In a previous study[15] we encapsulated other transition-
metal complexes within the NaY zeolite by using the flexi-
ble ligand method. These catalysts led to lower cyclohexene
conversions and the selectivity for CyOne was always higher
than the selectivity for CyOl, as was observed for the cata-
lysts described in this report under the same experimental
conditions. The different behaviour exhibited by the materi-
als based on transition-metal complexes with PAN[15] and
those presented in this work can be explained by consider-
ing the metal and its donor atoms. As was reported by
Figure 3. Conversion/selectivity observed for the oxidation of cy-
clohexene in the presence of [Mn(salen)Cl] in (A) decane and
(B) acn.
All the catalysts showed considerable selectivity towards Salavati–Niasari and co-workers,[26–28] the dgree of conver-
CyO in acn. A small increase in the selectivity for CyO was sion of cyclohexene decreased in the following order:
observed in the presence of decane when the reaction was MnϾCoϾCuϾNi. The results obtained with the
performed in the heterogeneous phase, nevertheless CyOne [Mn(salen)Cl]-based catalysts fits well in this series, and
and CyOx were the main products. It seems that the zeolite the substrate conversion decreases in the order
not only protects the complex but also leads to an increase MnϾZnϾCoϾCu when complexes have the same coordi-
in the epoxide yield, as has been reported for other transi- nation mode, namely N2O2. However, the copper(II)–pur-
tion-metal complexes encapsulated in the zeolite Y.[24]
ine complex (N4 coordination mode) encapsulated by the
The effect of the solvent on the efficiency of manganese- same method[22] shows higher cyclohexene conversion than
based heterogeneous catalysts in the oxidation of cyclohex- the heterogeneous MnIIIsalen complex presented in this
ene has been widely studied recently.[25] These studies work under the same experimental conditions.
showed that the formation of CyOl increases with solvent
Oxidation of Styrene
polarity. This effect was also observed in our study in the
first run, with the selectivity for CyOl being higher in acn
It has been reported[29] that for neutral complexes such
than in decane. However, when Mnsalen@YA was used as as [Mn(salen)]@NaY, leaching of the complexes into solu-
the catatlyst and in the first cycle of Mnsalen@YB, the tion may occur, especially if the catalytic reaction is per-
selectivity for CyOne was higher than CyOl independent of formed in a highly polar solvent. To overcome this problem,
the solvent and in agreement with literature data.[25,26] The before the catalytic tests, the heterogeneous catalysts were
substrate conversion strongly depends on the properties of subjected to Soxhlet extraction with a rather polar solvent,
the complex as well as on the support materials, but less on namely ethanol. To prove the Soxhlet efficiency and effect
the solvent as there is no wide-ranging relationship between of solvent on the distribution of products, the reaction was
these factors. Comparison of the conversions obtained with screened in the polar solvents acn[20] and dichloromethane
the two heterogeneous catalysts in apolar and polar solvents (dcm). Metal complexes encapsulated in the zeolite Y, in-
shows that both catalysts exhibit significantly higher ac- cluding MnIIIsalens, can promote the formation of other
tivity in polar medium. This contrasts with the results of products by the ring opening of phenyloxirane.[30–33] Under
the reaction performed in the homogeneous phase. The dif- the experimental conditions used, the product distribution
ference might arise from different structures of the complex was the same as in the homogeneous oxidation of styrene
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with benzaldehyde as the major oxidation product followed
by styrene epoxide. No other products were detected. The
oxidation of styrene was negligible in the absence of hetero-
geneous transition-metal catalysts, which confirms that the
oxidation is indeed catalytic in nature under these experi-
mental conditions. The NaY zeolite without encapsulated
metal complexes is also catalytically inactive. Therefore the
MnIIIsalen complex encapsulated in the zeolite plays a de-
termining role.
The experimental results obtained under homogeneous
conditions confirmed that the [Mn(salen)Cl] complex shows
excellent selectivity towards benzaldehyde (Bz) over the re-
action period (Figure 4 and Table 3). This is in accordance
with previous publications[32] in which only traces of phen-
ylacetaldehyde were reported when the reaction was carried
out in an homogeneous medium. Under these experimental
conditions neither polymer nor benzoic acid formation was
observed.[33] After the first 2 h of reaction, benzaldehyde
was the major product with 95 and 100% selectivity in acn
and dcm, respectively. After 5 h (acn) and 4 h (dcm), the
selectivity towards styrene oxide (SO) increased to 30 and
20% in acn and dcm, respectively. No significant change in
activity and product selectivity was observed after acquiring
a steady state at around 6 h.
Compared with its homogeneous counterpart, the
heterogeneous catalysts showed similar styrene conversion
in the first reaction cycle. Both heterogeneous catalysts
showed similar styrene epoxide selectivity in acn, although
less than that observed for the homogeneous reaction. The
Figure 4. Conversion/selectivity for the oxidation of styrene in the
presence of [Mn(salen)Cl] in (A) acn and (B) dcm.
NaY-supported catalysts after two cycles gave reduced styr- the catalyst, which indicates that the reaction was not fully
ene conversion for all catalyst systems, except Mnsalen@YB catalysed heterogeneously and that a small amount of
in acn. This has been observed with similar supported cata- leaching (the highest substrate conversion after catalyst re-
lysts.[34]
moval was less than 10%) of the active phase into solution
To determine the cause of the decrease in styrene conver- occurs. This effect was more perceptible when the reaction
sion upon catalyst reuse, the leaching test was carried out occurred in dcm. Although complex leaching was observed
and the structural integrity of the catalysts was analysed by for all the catalysts, there is no direct correlation between
FTIR spectroscopy. Low catalytic activity was observed for the decrease in styrene conversion upon reuse and the Mn
the epoxidation of styrene in the filtrate after removal of content after catalysis. This suggests that the decrease in
Table 3. Oxidation of styrene catalysed by the [Mn(salen)Cl] complex in the homogeneous and heterogeneous phases.
Catalyst
Run
T [h][a]
C [%][b,c]
Products
η [%][d]
SO
Bz
η [%][d]
S [%][e]
S [%]
[e]
NaY
[Mn(salen)Cl]
Mnsalen@YA
1
1
1
2
1
2
1
1
1
2
1
2
48
5
Ͻ48
4
4[f]
45[f])
39[f]
12[f]
40[f]
54[f]
5[g]
0
13
10
Ͻ1
10
12
0
10
5
1
0
30
26
Ͻ1
26
23
0
29
14
9
4
100
32
74
99
74
77
100
31
86
91
73
38
70
29
11
30
42
5
25
32
13
35
3
Mnsalen@YB
5
24
48
6
48
48
48
48
NaY
[Mn(salen)Cl]
Mnsalen@YA
35[g]
37[g]
14[g]
48[g]
8[g]
Mnsalen@YB
13
5
27
62
[a] Reaction time at which the substrate conversion becomes constant. [b] Determined by GC against an internal standard. [c] Styrene
conversion (C) calculated as C = {[A(styrene)/A(chlorobenzene) t=0h – [A(styrene)/A(chlorobenzene) t=xh} ϫ 100/[A(styrene)/A(chlorobenzene) t=0h; A = chro-
]
]
]
matographic peak area. [d] Product yield (η) calculated as η = CS/100. [e] Product selectivity (S) calculated as S = A(product) ϫ 100/
[A(product) + ΣA(other reaction products)]; A = chromatographic peak area. [f] In acn.[20] [g] In dcm (this work).
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styrene conversion is determined not only by complex
leaching, but also by some metal complex deactivation dur-
ing the catalytic cycles. The catalytic centres can be deacti-
vated by decomposition of the complex inside the matrix
by the oxidant[35] or by parallel reactions that involve the
formation of dimeric μ-oxo-MnIV species, which are inac-
tive towards styrene epoxidation, as observed in homogen-
eous solution.[2] The occurrence of the latter reaction is less
probable especially because of the dimensions of these mo-
lecules as well as the difficulties in complex diffusion from
the zeolite cage. The FTIR spectra for both heterogeneous
catalysts recorded after the second reuse show band broad-
ening in the 1620–1200 cm–1 region, which corresponds to
the frequency range in which bands due to the complex
occur. During the catalytic cycles, no structural changes in
the zeolite matrix took place, but some metal complex
leaching and/or decomposition must occur under the cata-
lytic experimental conditions, which supports the results
obtained upon reuse of the catalyst. It can be concluded
that the catalytic oxidation ability of the catalysts presented
in this work depends on the structure of the complex and
the polarity of the reaction medium.
DFT Studies of the Neat and Encapsulated MnIIIsalen
Complex
Figure 5. Optimized structures: (a) top and side views of the neat
complex in gas phase and (b) Mn(III)salen complex encapsulated
into the NaY zeolite.
DFT studies were performed to verify the structures of
the MnIIIsalen complex encapsulated in NaY obtained by
different methods. It is well known that Schiff-base salen-
type ligands react with different metal ions to give intensely
coloured chelate complexes. They act as tetradentate li-
gands coordinating the metal through the hydroxy oxygen
atoms and the azomethine nitrogen atoms. The complex
[Mn(salen)Cl] shows a slightly distorted square-pyramidal
geometry. The molecular structure of the neat Mn(III)salen
complex was optimized in the gas phase at the B3LYP/6-
31G(d,p) level of theory without any symmetry constraints
and the final geometry is depicted in Figure 5a. The opti-
mized structure shows a square-pyramidal configuration of
encapsulation might have some influence on the FTIR spec-
tra of the zeolite-based materials.
Table 4. Optimized values of the most relevant geometrical param-
eters of neat [Mn(salen)Cl][a] and inside the zeolite.[b]
Distance [Å]
[Mn(salen)Cl]
Mnsalen@Y
Mn–Cl
Mn–N1
Mn–N2
Mn–O1
Mn–O2
2.327
2.034
2.027
1.873
1.863
2.206
2.069
1.977
1.820
1.845
the chromophore with coordination through the two oxy- Angle [°]
gen and two nitrogen atoms in the Schiff-base ligand and
the chloride ion (Figure 5a).
N1–Mn–N2
O1–Mn–O2
N1–Mn–O1
N2–Mn–O2
N1–Mn–O2
N2–Mn–O1
N1-Mn-Cl
N2-Mn-Cl
O1-Mn-Cl
O2-Mn-Cl
87.02
92.57
87.98
89.95
157.06
156.21
95.22
96.67
105.21
106.71
79.60
90.79
85.62
The geometry of the MnIIIsalen complex in the cage of
the NaY zeolite (see Figure 5b) reveals some distortion of
the square-pyramidal geometry found in the gas phase. The
most relevant geometrical parameters are collected in
Table 4. The distances between the metal ion and the near-
est atoms are practically the same for the complex in the
gas phase and inside the zeolite cage. However, a noticeable
perturbation is observed in the Mn–Cl distance, which is
significantly shortened (more than 0.1 Å) when the complex
is encapsulated in the zeolite cage. The most notable differ-
ences are observed in the values of the bond and dihedral
angles around the metal ion. These results show that the
chlorine atom is closer to nitrogen N1 inside the zeolite
93.23
163.66
140.09
88.80
104.07
112.51
107.26
Dihedral angle [°]
N1–O1–O2-N2
–22.00
–14.81
[a] Determined at the B3LYP/6-31G(d,p) of theory. [b] Determined
at the ONIOM [B3LYP/6-31G(d,p):PM6] level of theory.
The most representative IR frequencies of the optimized
than in the neat complex, which has a more regular geome- complex are presented in Table 5. The theoretical predic-
try. These distortions in the structure of the complex upon tions of the vibrational frequencies for the neat and encap-
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purchased from Aldrich: 2-hydroxybenzaldehyde, manganese(II)
chloride tetrahydrate (MnCl2·4H2O), triethylamine, acetonitrile
(acn), dichloromethane (dcm), decane, ethanol, tert-butyl hydroper-
oxide solution, 5.0–6.0 m in decane (tBuOOH), chlorobenzene
(PhCl), cyclohexene and styrene. Nafion solution (Nafion® 117
solution) was purchased from Aldrich and Carbon Toray from
Quintech. Spectroscopic-grade potassium bromide used for the
preparation of FTIR pellets was purchased from Merck.
sulated complex show the same shifts as observed in the
experimental FTIR spectra.[20] Moreover, the band posi-
tions are in good agreement for neat and encapsulated com-
plexes.
Table 5. Representative experimental and theoretical IR frequencies
for neat [Mn(salen)Cl][a] and inside of the zeolite.[b]
ν [cm–1
CϵN C=N Arom. C=N[c] C–O
]
C–H[d]
Encapsulation of the MnIIIsalen Complex in NaY: The synthesis and
characterization of the Schiff-base ligand 1,5-bis{[(1E)-(2-hy-
droxyphenyl)methylene]amino}-1H-imidazole-4-carbonitrile, de-
noted as H2(salen), and the corresponding manganese(III) com-
plex, denoted as [Mn(salen)Cl] has been reported by us else-
where,[20] as well as the encapsulation of the MnIIIsalen complex
guest by two different methodologies.[20] The catalyst prepared by
in situ complex formation was obtained by using the general
method.[20] Briefly, a solution of the H2(salen) ligand and hydrated
MnII chloride in ethanol was added to a suspension of NaY zeolite
and then triethylamine was added. Uncomplexed ligand and the
molecules of the complex adsorbed on the external surface were
removed by Soxhlet extraction with ethanol. The extracted sample
was further washed with deionized water to remove undesired
metal ions. The new catalyst (Mnsalen@YA) was dried overnight
under reduced pressure. The manganese content in Mnsalen@YA
was 0.55%.
[Mn(salen)Cl]
Exp.[e] 2222
Theor. 2342
1602 1566
1633 1577
1585 1541
1620 1570
1485
1484
1475
1457
1384
1345
1396
1357
1282
1279
1300
1274
Mn(salen)@Y Exp.[e] 2219
Theor. 2341
[a] Determined at the B3LYP/6-31G(d,p) of theory. [b] Determined
at the ONIOM [B3LYP/6-31G(d,p):PM6] of theory. [c] In the imid-
azole ring. [d] In-plane deformation. [e] From ref.[20]
The DFT studies show that the molecular structure of
the MnIIIsalen complex encapsulated in NaY corresponds
to the catalyst obtained by in situ method (A) in which the
counter ion is chloride. However, for the catalyst obtained
by the flexible ligand method (B), the counter ion is the
oxygen from the zeolite framework. This means that chlor-
ide and oxygen complete the square-pyramidal coordina-
tion of MnIII when methods A and B, respectively, are used.
The flexible ligand immobilization method starts with cation ex-
change in the NaY zeolite by using an aqueous solution of hy-
drated MnII chloride. MnY solid was suspended in a solution of
the H2(salen) ligand in ethanol and then triethylamine was added.
The new material was Soxhlet extracted with ethanol and dcm to
Conclusions
The manganese(III) complex with the tetradentate Schiff
base derived from 1,5-bis{[(1E)-(2-hydroxyphenyl)methyl-
ene]amino}-1H-imidazole-4-carbonitrile was encapsulated
in the Y zeolite by using two different methodologies: in
situ complex preparation (A) and the flexible ligand
method (B). The two methodologies lead to a different co-
ordination sphere around the manganese. Cyclic voltamme-
try studies of the neat complex in dmf and the hetero-
geneous catalysts in aqueous medium showed different elec-
trochemical behaviour. The neat complex [Mn(salen)Cl]
shows a reversible oxidation process, which has been as-
signed to the MnII/III redox couple. The modified electrode
of the heterogeneous catalyst obtained by method A shows
a quasi-reversible process, whereas the modified electrode
of Mnsalen@YB-CT shows an irreversible anodic process.
In the catalytic studies, the heterogeneous catalysts led to
similar or higher substrate conversions than the homogen-
eous catalyst. When acn was used as solvent, both hetero-
geneous catalysts led to a higher conversion of cyclohexene
compared with styrene under the same conditions. The sub-
strate conversion depends of the encapsulation method used
and the supporting material. DFT studies confirmed some
distortion of the complex geometry during the encapsul-
ation.
remove
unreacted
ligand.
The
new
catalyst
(Mnsalen@YB) was dried overnight under reduced pressure. The
manganese content in Mnsalen@YB was 0.80%.
Methods of Characterization: The elements Si, Al, Na and Mn were
quantitatively analysed by inductively coupled plasma atomic emis-
sion spectrometry (ICP-AES) using a Philips ICP PU 7000 spec-
trometer. FTIR spectra of solid samples were obtained with a
Bomem MB104 spectrophotometer at room temperature from
powdered samples mixed with KBr to form pellets for measure-
ment. CV was performed by using a potentiostat/galvanostat
(AUTOLAB/PSTAT 12) with the low-current module ECD from
ECO-CHEMIE and the data analysis was processed by using the
General Purpose Electrochemical System software package also
from ECO-CHEMIE. Three electrode-two compartment cells
equipped with vitreous carbon disc working electrodes, a platinum
wire secondary electrode and a silver wire pseudo-reference elec-
trode were employed for cyclic voltammetric measurements. The
concentration of the free complex was 1.0 mm and 0.10 m
[NBu4][BF4] was used as the supporting electrolyte in dry dmf. Cy-
clic voltammetry was usually conducted at 0.10 Vs–1 or at different
scan rates (0.02–0.50 Vs–1) to investigate the influence of the scan
rate. Potentials were measured with respect to ferrocenium/ferro-
cene as an internal standard. Voltammetric measurements on zeo-
lite-based materials were carried out with a computerized AMEL
instrument General-purpose Potentiostat model 2051. The experi-
ments were controlled with the home-made program LAB VIEW.
All electrochemical studies were performed at room temperature
with a three-electrode assembly including a 250 mL glass cell, a
saturated calomel electrode as reference electrode, a 90% platinum
and 10% iridium counter electrode and the working electrode was
Carbon Toray (CT) or CT modified by manganese salen complexes
encapsulated in zeolite NaY. All potentials were measured and re-
Experimental Section
Materials and Reagents: NaY zeolite (CBV100, Si/Al ratio = 2.83)
in powder form was obtained from Zeolyst International. NaY
powder was calcined at 500 °C for 8 h under a dry stream of air
prior to use. All chemicals and solvents were reagent-grade and
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
ported versus the SCE reference electrode. The GC-FID chromato-
grams were obtained with a SRI 8610C chromatograph equipped
with a CP-Sil 8CB capillary column. Nitrogen was used as the car-
rier gas. The identification of reaction products was confirmed by
GC–MS (Varian 4000 Performance).
internal standard method. After each reaction cycle, the catalysts
were washed, dried and characterized. The catalytic activity of
homogeneous [Mn(salen)Cl] was also tested; the amount of neat
complex was equivalent 0.50% Mn relative to alkene.
Computational Details: All the calculations were performed in the
gas phase and the corresponding relative energies were calculated
by the one-layer, single method, and two-layer, ONIOM method,[36]
by using B3LYP[37] for the high-level calculations and PM6[38] for
the low-level calculations. The geometry of the neat [Mn(salen)Cl]
complex was characterized by a quantum mechanical method
based on DFT. The B3LYP Becke’s three-parameter exchange-cor-
relation hybrid functional with non-local correlation corrections
provided by Lee, Yang and Parr was used.[37] The double-zeta Pople
basis set 6-31G(d,p) was employed, which ensures a superior elec-
tronic description by adding polarization functions of the p-, d-
and, especially, f-type for all the hydrogen atoms and non-hydrogen
atoms and metal ions, respectively. When encapsulated in the NaY
zeolite cage, the geometry of the [Mn(salen)Cl] complex was opti-
mized by using the two-layer ONIOM [B3LYP/6-31G(d,p):PM6]
method in which the B3LYP/6-31G(d,p) method was used for the
high-level calculation for the [Mn(salen)Cl] complex and the semi-
empirical PM6 method was used for the low-level calculation for
the zeolite framework. The geometry of the NaY zeolite, faujasite-
type structure, was taken from the database of the Structure Com-
mission of the International Zeolite Association (IZA).[39] The vi-
brational frequencies of these optimized structures were predicted
by using the same basis set. No imaginary frequencies were ob-
tained, which confirms that the molecular structures were opti-
mized at stationary positions on the potential energy surface. Usu-
ally, the harmonic frequencies calculated at the B3LYP/6-31G(d,p)
level have to be scaled,[40] however, the agreement between theoreti-
cal and experimental results was so satisfactory in this work that
further scaling of the calculated frequencies seemed unnecessary.
All geometry optimizations and energy and frequency calculations
were performed by using the Gaussian 09 package of programs.[41]
Graphical representations of the optimized structures and the mo-
lecular orbitals were produced with the MOLEKEL 4.3[42] and
Gauss-View molecular visualization programs.[43]
Acknowledgments
I. K.-B. thanks the Fundação do Ministério de Ciência e Tecnolo-
gia (FCT) for a contract under the program Ciência 2007. The
authors thank the FCT and the Fundo Europeu de Desen-
volvimento Regional (FEDER)) [COMPETE-QREN-EU, grant
number PEst-C/QUI/UI0686/2011 (FCOMP-01-0124-FEDER-
022716)] for financial support to the Research Centre at the Centro
de Química, Universidade do Minho.
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Pages: 10
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FULL PAPER
Zeolite-Encapsulated Manganese
I. Kuz´niarska-Biernacka,* O. Rodrigues,
M. A. Carvalho, P. Parpot, K. Biernacki,
A. L. Magalhães, A. M. Fonseca,
Two methods for the encapsulation of a
MnIIIsalen complex in NaY zeolite lead to
different modes of coordination at the me-
tal. Cyclic voltammetry of the neat and het-
erogeneous complexes shows different elec-
trochemical behaviour. The heterogeneous
catalysts lead to similar or higher alkene
conversion than the homogeneous catalyst.
DFT confirmed distortion of the complex
in a confined space.
I. C. Neves ...................................... 1–10
Electrochemical and Catalytic Studies of a
Manganese(III)Complex with a Tetraden-
tate Schiff-Base Ligand Encapsulated in
NaY Zeolite
Keywords: Electrochemistry
/
Hetero-
geneous catalysis / Oxidation / Zeolites /
Manganese / Alkenes
Eur. J. Inorg. Chem. 0000, 0–0
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