Mn(III) complexes with tridentate N,N,O-ligands as catalysts for the epoxidation of alkenes

Article abstract of DOI:10.1080/00958972.2013.809425

Mn(III) complexes with tridentate Schiff bases have been prepared and applied as catalyst precursors in epoxidation of alkenes using iodosobenzene as an oxidant providing high conversions and high selectivities when cyclohexene derivatives were studied.

Full text of DOI:10.1080/00958972.2013.809425

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Mn(III) complexes with tridentate
N,N,O-ligands as catalysts for the
epoxidation of alkenes
A. Aghmiz a , N. Mostfa a b , S. Iksi a b , R. Rivas a , M.D. González
a , Y. Díaz c , F. El Guemmout b , A. El Laghdach b , R. Echarri b &
A.M. Masdeu-Bultó a
a Departament de Química Física i Inorgànica , Universitat Rovira i
Virgili , Tarragona , Spain
b Faculté des Sciences , University Abdelmalek Essaadi , Tétouan ,
Maroc
c Departament de Química Analítica i Química Orgànica ,
Universitat Rovira i Virgili , Tarragona , Spain
Accepted author version posted online: 31 May 2013.Published
online: 28 Jun 2013.
To cite this article: A. Aghmiz , N. Mostfa , S. Iksi , R. Rivas , M.D. González , Y. Díaz , F. El
Guemmout , A. El Laghdach , R. Echarri & A.M. Masdeu-Bultó (2013): Mn(III) complexes with
tridentate N,N,O-ligands as catalysts for the epoxidation of alkenes, Journal of Coordination
Chemistry, 66:14, 2567-2577
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Journal of Coordination Chemistry, 2013
http://dx.doi.org/10.1080/00958972.2013.809425
Mn(III) complexes with tridentate N,N,O-ligands as catalysts
for the epoxidation of alkenes
A. AGHMIZ*†, N. MOSTFA†‡, S. IKSI†‡, R. RIVAS†, M.D. GONZÁLEZ†, Y. DÍAZx,
F. EL GUEMMOUT‡, A. EL LAGHDACH‡, R. ECHARRI‡
A.M. MASDEU-BULTÓ*†
†Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, Tarragona,
Spain
‡Faculté des Sciences, University Abdelmalek Essaadi, Tétouan, Maroc
xDepartament de Química Analítica i Química Orgànica, Universitat Rovira i Virgili,
Tarragona, Spain
(Received 11 February 2013; in final form 6 May 2013)
Mn(III) complexes with tridentate Schiff bases have been prepared and applied as catalyst precur-
sors in epoxidation of alkenes using iodosobenzene as an oxidant providing high conversions and
high selectivities when cyclohexene derivatives were studied.
Keywords: Manganese; Tridentate N, N, O-; Epoxidation; Catalysis
1. Introduction
Epoxides are valuable and versatile intermediates in organic synthesis [1–3] and useful
starting materials for the preparation of industrial chemicals such as epoxy resins, surfac-
tants, paints, adhesives, and surface coating agents [4]. They can be efficiently obtained by
metal-catalyzed oxidation of alkenes (figure 1) using a variety of oxidants such as oxygen,
peroxides, hypochlorites, peracids, or iodosyl arenes. The catalyzed oxidations have the
advantage of generating less waste by-products than the stoichiometric processes [4].
Schiff base manganese complexes have been extensively used as catalysts for this
reaction. In particular, tetradentate salen ligands derived from salicylic aldehydes and
diamines (figure 2) have been successfully applied in manganese-catalyzed epoxidation of
unfunctionalized alkenes, including the asymmetric version [5–9]. Catalytic systems with
tridentate N,N,O-donor ligands have been less reported than salen-based ones. Unlike
salen, tridentate N,N,O-donor ligands can form species with one coordinated ligand,
M(NNO)Lx, with coordination numbers from 4 to 6, and also species with two ligands
M(NNO)2. In the first case, easily available coordination sites can be generated when L
are labile. In the case of M(NNO)2 complexes, a coordination vacancy should be generated
by displacement of one of the arms of the ligand to allow interaction with the substrate or
*Corresponding authors. Email: ali.aghmiz@urv.cat (A. Aghmiz); annamaria.masdeu@urv.cat (A.M. Masdeu-Bultó)
Ó 2013 Taylor & Francis

2568
A. Aghmiz et al.
O
[cat]
+ [O]
R
R
Figure 1. Epoxidation of alkenes.
R
R
N
N
HO
OH
R
R
R
R
Salen
Figure 2. Salen ligands.
reagent. This pendant arm could subsequently reincorporate into the coordination sphere as
an auxiliary or stabilizing ligand. In many metal-based catalytic systems, the presence of
an auxiliary ligand increases the activity. For example, the addition of small amounts of
the so-called donor axial ligands, such as pyridine, imidazole, or pyridine oxides, was
reported to increase the yield in epoxidation of alkenes using Mn-salen catalysts [5, 10].
Furthermore, the presence of axial donors in Mn(III)-salen catalytic systems influenced the
formation of the epoxide at the expense of the aldehyde by-product [11]. DFT calculations
showed that cationic six-coordinate Mn(III)-salen species had a low energetic barrier for
the formation of epoxide [12]. Six-coordinate Mn(III) complexes with 8-quinolinato biden-
tate ligands were reported to be active catalysts for epoxidation of olefins using aqueous
H2O2 as an oxidant in AcOH/NH4OAc [13]. For these systems, a mechanism through a
five-coordinate Mn-oxo species formed by dissociation of the hydroxo fragment, forming a
pendant hydroxyl group, was proposed.
In this study we have investigated the preparation of manganese(III) complexes with
tridentate ligands 1 and 2 (figure 3), containing phenol, imino/amino, and pyridine donors.
Assuming chelate coordination through the phenoxo-imino/amino-moieties, the pyridine
could act as an axial neutral ligand in a cationic six-coordinate species, with the advantage
of being closer to the central metal than the free ligand. We have studied the application
of these complexes as catalysts in the epoxidation of styrene and cyclohexene derivatives.
Dioxo molybdenum(VI) complexes with 2 are efficient in the epoxidation of alkenes using
tert-butyl hydroperoxide as the terminal oxidant [14].
N
N
NH
OH
N
OH
tBu
tBu
tBu
tBu
2
1
Figure 3. N,N,O-donors 1 and 2.

Mn(III) Complexes
2569
2. Experimental
2.1. General procedures
Styrene and cyclohexene were distilled over CaH2 and stored under nitrogen. Dihydro-
naphthalene and Z, E-stilbene were used as purchased (Aldrich). PhIO [15] and 1 and 2
were prepared according to described procedures [14]. Solvents were purified through
either distillation or a purification system, MBraun-SPS-800, and stored under nitrogen. IR
spectra (4000–400 cmꢀ1) were recorded on a Midac Grams/386 spectrometer in ATR or
KBr. MALDI-TOF measurements of 2a and 2b were performed on a Voyager-DE-STR
(Applied Biosystems, Framingham, MA) instrument equipped with a 337-nm nitrogen
laser. All spectra were acquired in the positive ion reflector mode. Alpha-cyano-4-hydroxy-
cinnamic acid was used as a matrix. The matrix was dissolved in MeOH at 10 mg mLꢀ1
.
The complex was dissolved in CH2Cl2 (50 mg Lꢀ1). The matrix and the samples were pre-
mixed in the ratio of 2 : 1 (matrix : sample) and the mixture was then deposited (1 mL) on
the stainless steel sample holder and was allowed to dry before introduction into the mass
spectrometer. Three independent measurements were made for each sample. For each spec-
trum, 100 laser shots were accumulated. Electrospray ionization mass spectra (ESI-MS)
were obtained with an Agilent Technologies mass spectrometer. Typically, a dilute solution
of the compound in DMF/methanol (1 : 99) was delivered directly to the spectrometer
source at 0.01 mL minꢀ1 with a Hamilton microsyringe controlled by a single-syringe infu-
sion pump. The nebulizer tip operated at 3000–3500 V and 250 °C and nitrogen was both
the drying and the nebulising gas. The cone voltage was 30 V. Quasi-molecular ion peaks
[M–H]ꢀ (negative ion mode) or sodiated [M + Na]+ (positive ion mode) peaks were
assigned on the basis of the m/z values. Conductivity was measured with a Crison conduc-
timeter GLP equipped with a conductivity Pt cell. UV–visible spectra were recorded on a
Shimadzu UV-1203 spectrophotometer.
2.2. Syntheses of complexes
[Mn(1)2Cl] (1a). In a round-bottomed flask under inert atmosphere, 1 (0.130 g, 0.40 mmol)
was dissolved in 20 mL of ethanol and Mn(OAc)2·4H2O (0.147 g, 0.6 mmol) was added.
The mixture was refluxed under inert atmosphere for 2 h. LiCl (0.025 g, 0.6 mmol) was
then added and the mixture was refluxed under air for 2 h. The mixture was filtered off
and the organic solvent evaporated. The remaining solid was redissolved in CH2Cl2 and
the solution was washed with aqueous sodium chloride. The organic phase was dried with
sodium sulfate and evaporated to obtain a dark brown oily solid. Yield: 0.033 g
(0.044 mmol, 22%). HR ESI-MS: Calcd for C42H52MnN4O2 [M–Cl–2H]+ m/z = 699.3471,
found m/z = 699.3477. IR m (ATR, cmꢀ1): 2959, 2869, 1646, 1616, 1465, 1437, 1392,
1362, 1252, 1202, 1170, 1092, 1016, 801. ΛM (acetonitrile, 4 ꢁ 10ꢀ4 M) = 6.1
S·cm2·Mꢀ1
.
[Mn(1)(OAc)2]2·2H2O (1b). In a round-bottomed flask, 1 (0.400 g, 1.23 mmol) and Mn
(OAc)3·4H2O (0.332 g, 1.23 mmol) were dissolved in 25 mL of acetonitrile and stirred at
60 °C for 2 h. A dark brown solid was then obtained by solvent evaporation. The solid
was redissolved in diethyl ether and filtered off. The filtrate was evaporated to obtain a
dark brown solid. Yield: 0.226 g (0.22 mol, 36%). Anal. Calcd for C50H70Mn2N4O12
(1028.98): C, 58.36; H, 6.86; N, 5.44. Found: C, 58.90; H, 6.95; N, 5.94. HR ESI-MS:
calcd for (C50H65Mn2N4O10Na)+ [MꢀH + Na]+ m/z = 1014.3359, found m/z = 1014.4573;

2570
A. Aghmiz et al.
calcd for (C44H57Mn2N4O4)+ [M-3OAc]+ m/z = 815.3140, found m/z = 815.3966. IR m
(KBr, cmꢀ1): 3418, 2959, 2867, 1753, 1663, 1588, 1570, 1469, 1433, 1392, 1250, 1202,
996, 756.
[Mn(2)2Cl] (2a). In a round-bottomed flask under inert atmosphere 2 (0.130 g, 0.4 mmol)
was dissolved in 20 mL of ethanol and Mn(OAc)2·4H2O (0.147 g, 0.6 mmol) was added.
The mixture was refluxed under inert atmosphere for 1 h. LiCl (0.025 g, 0.6 mmol) was then
added and the mixture was refluxed under air for 30 min. The mixture was evaporated to
obtain a solid, which was washed with water. A dark violet solid was obtained after vacuum
drying. Yield: 0.080 g (0.108 mmol, 54%). MALDI-TOF: calcd for (C42H58MnN4O2)+
[M–Cl–2H]+ m/z = 703.3784, found m/z = 703.6412. IR m (KBr, cmꢀ1): 3405, 3099, 2951,
1604, 1572, 1468, 1439, 1411, 1253, 1238, 956, 830, 755, 741. ΛM (acetonitrile, 4 ꢁ 10ꢀ4
M) = 42.6 S·cm2·Mꢀ1
[Mn(2)(OAc)2]x (2b). In
.
a
round-bottomed flask, 2 (0.130 g, 0.4 mmol) and
Mn(OAc)3·4H2O (0.107 g, 0.4 mmol) were dissolved in 15 mL of acetonitrile and stirred at
60 °C for 2 h. A dark brown solid was then obtained by solvent evaporation. The solid
was redissolved in dichloromethane. Addition of hexane produced a solid, which was fil-
tered off. The filtrate was evaporated to obtain a dark solid. MALDI-TOF: dimeric species
calcd for (C42H54Mn2N4O10)+ [M-2tBu] m/z = 884.2601, found m/z = 884.7000; trimeric
species calcd for (C50H68Mn3N4O10) m/z = 1088.2714, found m/z = 1088.3200.
2.3. General procedure for the epoxidation of alkenes with PhIO [5]
The catalyst (3.2 ꢁ 10ꢀ2 mmol), PhIO (0,7 g, 3.2 mmol), substrate (1.6 mmol) and 0.3 mL
of undecane as an internal standard (for styrene and cyclohexene) were placed in a purged
round-bottomed flask under nitrogen (unless otherwise specified) and 8 mL of acetonitrile
was added. A sample at 0 h was taken (for styrene and cyclohexene) and analyzed by GC.
The mixture was stirred at 25 °C. After the indicated reaction time (table 1), the mixture
was filtered through a zelite pad and analyzed by GC (styrene and cyclohexene) or by
NMR spectroscopy after solvent evaporation (1,2-dihydronaphthalene, (Z)- and (E)-stil-
bene). The identification of the products was performed by comparison of the GC data
with commercial samples, by GC-MS or by comparison with literature NMR data.
3. Results and discussion
3.1. Syntheses of complexes
Initially, the standard method of preparation of metal salen complexes starting with a Mn
(II) salt was applied to the syntheses of Mn complexes with 1 and 2. Thus, Mn
(OAc)2·4H2O was reacted with 1 or 2 in ethanol under inert atmosphere and after two or
one hours, respectively, oxidation of Mn(II) was performed with air in the presence of lith-
ium chloride in a ratio of Mn : ligand:LiCl 3 : 1 : 3 to furnish 1a and 2a (scheme 1) [16].
When 1 was used (complex 1a), the IR spectrum of the final dark solid obtained
showed two absorptions in the region of the azomethane stretching vibration m (C=N) at m
1646 and 1616 cmꢀ1 (Supplementary material). The shift of these bands with respect to
those observed in free 1 (1636 and 1590 cmꢀ1) indicates coordination of the ligand to the

Mn(III) Complexes
2571
a) 1, EtOH
b) LiCl, air
[Mn(1)2Cl]
(1a)
(2a)
Mn(OAc)2·4H2O
a) 2, EtOH
b) LiCl, air
[Mn(2)2Cl]
1, MeCN
2, MeCN
[Mn(1)(OAc)2]2·2H2O
[Mn(2)(OAc)2]x (2b)
(1b)
Mn(OAc)3·2H2O
Scheme 1. Synthesis of 1a, 1b, 2a and 2b.
metal. The increase of frequency compared to that in free ligand could be attributed to a
poor π-backbonding of this ligand [17].
Mass spectrometric analysis of 1a showed peaks at m/z corresponding to [Mn(1)2]+
(figure 4). Similar results were obtained when the ratio of Mn:ligand:LiCl used was 3 : 2 : 3.
The product resulting from the reaction between Mn(OAc)2·4H2O and 2 (2a, scheme 1) also
showed a peak in the mass spectrum at m/z 703.6412, attributed to [Mn(2)2]+ (figure 5).
Nevertheless, the values of molar conductivity ΛM in acetonitrile for 1a and 2a, 6.1 and
42.6 S·cm2·Mꢀ1, respectively, were in the range attributed to neutral species [18], indicat-
ing coordination of the chloride. Consequently, we propose the formation of the six-coordi-
nate species [Mn(L-κ3N,N,O)(L-κ2N,O)Cl] (L= 1a and 2a), in which one ligand is
tridentate κ3N,N,O and the other is bidentate κ2N,O (one of the possible stereoisomers mer
is depicted in figure 6). The six-coordinate Co(III) complex [Co(2)][ClO4] was reported
[19], but in this case the poor coordinating perchlorate was out of the coordination sphere.
The higher value of molar conductivity of 2a compared to 1a may be attributed to partial
dissociation of chloride in solution. This dissociation was also observed for Mn–Cl–salen
derivatives [20]. It was not possible to obtain good microanalysis data for 1a and 2a,
probably due to the presence of small amounts of higher nuclearity species according to
the ESI mass spectra. Reported crystal structures of related six-coordinate amino-phenola-
to-pyridine complexes with Co(III) [19] and Fe(III) [21] show that the tridentate ligands
are coordinated in the fac mode. Unfortunately, it was not possible to obtain suitable
crystals of these complexes to determine the X-ray structures.
Figure 4. ESI mass spectrum of 1a.

2572
A. Aghmiz et al.
Figure 5. MALDI TOF mass spectrum of 2a.
O
Py
AcO
Py
N
O
O
N
O
O
O
N
Mn
N
Mn
Mn
O
O
Cl
OAc
Py
Py
1a , 2a
1b
N
= 1 or 2
O
Figure 6. Possible molecular structures for 1a, 2a and 1b.
Instead, when a Mn(III) precursor was made to react with 1 at a 1 : 1 ratio [22], the
mass spectrum (figure 7) and elemental analysis of the resulting dark solid suggested the
formation of a dinuclear species [Mn(1)(OAc)2]2·2H2O (1b). The strong bands at m
1636 cmꢀ1 and 1590 cmꢀ1 in the FTIR spectrum of free 1, attributed to the C=N and C=C
(pyridine, phenol) stretching vibrations [23], shifted to 1663 and 1588 cmꢀ1, confirming
the coordination of the ligand to the metal (Supplementary material). In comparison to the
spectrum of free 1, two medium absorptions are observed in the spectrum of 1a at 1570
and 1433 cmꢀ1 that may be assigned to the asymmetrical and symmetrical stretching m of

Mn(III) Complexes
2573
Figure 7. ESI mass spectrum of 1b.
the CO in the acetate. According to literature data, the difference of 137 cmꢀ1 between
these two absorptions may indicate bridging coordination [17, 24], although exchange pro-
cesses may take place in the sample preparation [25]. In fact, the weak absorption at
1753 cm-1 may indicate protonation of acetate in the presence of water. For related dimeric
Mn(III) complexes, the presence of small quantities of D2O (less than 1%) has been
reported to produce the loss of about half of the bound acetate [23]. Nevertheless, the
number of peaks from the ligand, which appear also in this zone, did not allow determin-
ing the presence of monodentate or free acetate [24]. The IR spectrum of the free ligand
shows a broad absorption at m 3448 cmꢀ1 attributed to O-H stretch, which sharpens in the
spectrum of 1b, indicating the presence of water. A tentative proposal of molecular
structure for 1b is shown in figure 6.
When 2 was reacted with manganese(III) acetate dihydrate, the mass spectrum of the
resulting solid showed peaks at m/z values corresponding to dimeric species, but peaks
corresponding to trimeric and tetrameric species were also observed (2b, scheme 1). When
the Mn:2 ratio was increased to 1 : 2, similar mixtures of inseparable polynuclear species
were detected in the MALDI-TOF mass spectrum.
3.2. Catalytic experiments
Complexes 1a, 2a, and 1b were tested as catalysts in the epoxidation of styrene (ST),
cyclohexene (Cy), 1,2-dihydronaphthalene (DHN), and (Z)- and (E)-stilbene (Z-STB and
E-STB) as the model substrates. Initial experiments using 1b and 2a as catalysts with
sodium hypochlorite [26], sodium periodate [27], and hydrogen peroxide [28] as the
terminal oxidants for the epoxidation of styrene did not lead to the formation of the
epoxide, so iodosobenzene was selected as an oxidant. The reactions were run in aceto-
nitrile, where this reactant is more soluble than in other organic solvents [5]. This sol-
vent was also reported to provide better conversions and selectivities in other
Mn-based catalytic systems [29]. Low selectivities in the epoxide were obtained when
using styrene as a substrate, as already reported for other Mn-salen or Mn-porphyrin
catalysts, and was attributed to partial polymerization [5] or the formation of other by-
products (benzaldehyde) [29, 30]. Therefore, our study focused on other substrates such
as stilbenes, cyclohexene, and 1,2-dihydronaphthalene. The results obtained are shown
in table 1.

2574
A. Aghmiz et al.
Table 1. Epoxidation of alkenes with PhIO and 1a, 1b and 2aa.
b
c
Entry
Cat.
Substrate
% Conv
% Sepox
1
1a
2a
2a
1b
1a
2a
1b
2a
1a
2a
1b
1a
2a
2a
1b
E-STB
E-STB
E-STB
E-STB
Z-STB
Z-STB
Z-STB
Z-STB
Cy
Cy
Cy
DHN
DHN
DHN
DHN
15
31
45
23
39
34
26
18
48
52
61
53
74
72
82
100 (0/100)d
100 (0/100)d
70 (0/100)d
100 (0/100)d
89 (53/47)d
90 (47/53)d
100 (42/58)d
93 (46/54)d
73
2
3e
4
5
6
7
8f
9e
10
11
12e
13
14g
15
71
78
73
86
86
85
Notes: aReaction conditions: catalyst 0.0037 M, molar ratio substrate/catalyst = 50/1, molar ratio oxidant/sub-
strate = 2/1, solvent CH3CN V = 8 mL, t = 24 h; bdetermined by GC based on cyclohexene converted; determined
by 1H NMR for DHN, and Z/E-stilbene; cdetermined by GC respect to epoxide formed for cyclohexene; deter-
f
mined by 1H NMR for DHN, and cis/trans stilbene; dZ/E; ereaction run under air; 0.5 equivalents of 2,6-tert-
g
butylphenol; t = 48 h.
The selectivities in the epoxide obtained with (Z)- and (E)-stilbene were high
(70–100%), with 1,2-diphenylethanone as the by-product detected [31], although the con-
1
versions in epoxide measured by H NMR were modest (15–39%, entries 1, 2, 4–7, table
1). When the reaction was run in the presence of air, the activity increased (45% versus.
31%) but the selectivity in epoxide decreased and benzaldehyde and 1,2-diphenylethanone
were detected (entry 3, table 1). The reaction from E-stilbene was completely stereoselec-
tive producing only the (E)-isomer. Retention of configuration was also reported for Mn-
salen complexes [5]. As for the epoxidation of Z-stilbene, mixtures of approx. 45/55 of Z/
E epoxides were obtained. This suggests a radical mechanism in which the C–C bond rota-
tion in the expected Mn-O-C-C intermediate formed competes with ring closure to form
the final epoxide, thus leading to Z/E epoxide mixtures [5]. In fact, the addition of 0.5
equivalents of a radical scavenger (2,6-di-tert-butylphenol) produced a decrease in conver-
sion (entry 8 versus. entry 6, table 1), as observed for Mn-salen systems [5]. In the case of
1a, the steric effect of the substrate influenced the conversion, since it was lower for the
trans-alkene than for the cis- one (entry 1 versus. entry 5, table 1), as also reported for
porphyrin catalytic systems [32]. Nevertheless, no significant differences of conversion
were observed between the cis- and trans-alkenes using 2a and 1b (entries 2 and 4 versus.
entries 6 and 7, table 1).
The conversions were higher when cyclohexene and 1,2-dihydronaphthalene were epoxi-
dated using 1a, 2a, and 1b. Conversions of 48–61% and selectivities in the epoxide up to
78% were obtained in the epoxidation of cyclohexene (entries 9–11, table 1).
4-Cyclohexenone by-product was identified by GC-mass analysis. Conversions increased
up to 70–80% in the epoxidation of 1,2-dihydronaphthalene using 2a and 1b with good
selectivities towards the epoxide (entries 13 and 15, table 1). Neither the conversion nor
the selectivity increased at longer reaction times using 2a (entry 14 versus. entry 13,
table 1). Naphthalene was identified as a by-product (ca. 15% selectivity) when 1a and 2a
were used (entries 12 and 13, table 1). Using catalysts with 1 (1a and 1b) the dialdehyde
2-(3-oxopropyl)benzaldehyde, product of the C = C bond cleavage, was also identified by

Mn(III) Complexes
2575
Figure 8. UV–visible spectra (λ, nm) of 1a (4 ꢁ 10ꢀ4 M)/PhIO (molar ratio = 1/10) in acetonitrile at (a) t = 0 h,
(b) 24 h.
1H NMR (13% selectivity). Both the sub-products were also reported to be formed in the
catalytic oxidation of 1,2-dihydronaphthalene with hydrogen peroxide using Keggin-type
metal-substituted polyoxotungstates as catalysts [33].
The reactivity of 1a with PhIO was analyzed by UV-visible spectroscopy. Complex 1a
has an absorption at λ 322 nm (ɛ 7500) due to the π → π electronic transition of the ligand.
When PhIO was added to acetonitrile solution of 1a (4 ꢁ 10ꢀ4 M) at a molar ratio Mn/
PhIO, 1/2, a weak lightening of the brown color of the solution was observed. It was ana-
lyzed after 3 h and 24 h. After 3 h, the spectrum presents a decrease in the absorption at
322 nm, probably due to the formation of a species with an absorption at λ < 300 nm,
which is unaltered after 24 h (Supplementary material). Similar changes were observed
when a ratio of Mn/PhIO of 1/10 was used (figure 8). This could be indicative of the
formation of an active species although no conclusive proposal can be done.
4. Conclusions
New catalytic systems based on Mn(III)-tridentate complexes have been prepared and used
in the epoxidation of alkenes. Tridentate ligands 1 or 2 (L) formed six-coordinate
complexes [Mn(L-κ3N,N,O)(L-κ2N,O)Cl] in which the chloride is coordinated, as indicated
by the conductivity measurements. In the absence of this coordinating anion, formation of
dimeric species with bridging acetate is proposed based on microanalysis, mass spectra,
and infrared data. The lability of chloride or coordinating arms of the tridentate or biden-
tate L may account for the catalytic activity of these systems, since formation of the active
species requires generation of a coordination site. Changes in the UV–visible spectrum of
the solution of 1a in the presence of PhIO indicate that reaction occurs, although no
conclusive proposal of catalytic active species can be done. The best conversions with
these catalytic systems were obtained for epoxidation of cyclohexene and dihydronaphtha-
lene using iodosobenzene as a terminal oxidant (up to 82% conversion in 24 h, and
selectivities in the epoxide up to 86%). The formation of allylic oxidation products such as

2576
A. Aghmiz et al.
4-cyclohexenone or 1,2-diphenylethanone and the low stereoselectivity in the epoxidation
of (Z)-stilbene suggest that a radical mechanism is operating in these processes, as
proposed for Mn-salen catalytic systems [34, 35]. The presence of a radical scavenger
produced a decrease in the conversion. As for the epoxidation of cyclohexene derivatives,
the system containing the amine functionality 2a presents slightly higher conversions than
1a, containing imine. The dimeric catalysts 1b provided the best results, although the
increase in conversion using this catalyst was only 10%. In this case, there is a two-fold
concentration of Mn in solution compared to 1a or 2a, which would be the reason for the
increased activity if the active species was a monomer. Otherwise, the coordinative
saturation of 1a and 1b may account for these differences. The epoxidation of styrene and
stilbenes with these catalysts proceed at low conversions and selectivities.
Supplementary material
Supplementary Data containing IR spectra, examples of NMR spectra of catalytic experi-
ments and UV-visible spectra of 1a and PhIO are available.
Acknowledgments
We gratefully acknowledge the Generalitat de Catalunya (SGR116), the Ministerio de
Ciencia e Innovación (CTQ2010-16676 and Consolider Ingenio CSD2006-0003) and the
Ministerio de Asuntos Exteriores (Programa de Cooperación Internacional C030686-10)
for the financial support. Dr J. Castilla is acknowledged for providing some experimental
data.
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