A novel homodinuclear manganese(III) complex with N,N′- bis(salicylidene)diethylenetriamine: Synthesis, structure and catalytic activity

Article abstract of DOI:10.1016/j.ica.2014.05.005

A novel chloride bridged dinuclear manganese(III) complex was prepared and its structure established by X-ray diffraction analysis. The complex showed high selectivity in H₂O₂ promoted oxidation of cyclohexane and cyclic alkenes. Und

Full text of DOI:10.1016/j.ica.2014.05.005

Accepted Manuscript
A novel homodinuclear manganese(III) complex withN, N ’-bis(salicylidene)di-
ethylenetriamine: synthesis, structure and catalytic activity
Hassan Hosseini-Monfared, Sohaila Alavi, Peter Mayer
PII:
S0020-1693(14)00251-5
http://dx.doi.org/10.1016/j.ica.2014.05.005
ICA 15985
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
7 February 2014
4 April 2014
5 May 2014
Please cite this article as: H. Hosseini-Monfared, S. Alavi, P. Mayer, A novel homodinuclear manganese(III)
complex with N, N ’-bis(salicylidene)diethylenetriamine: synthesis, structure and catalytic activity, Inorganica
Chimica Acta (2014), doi: http://dx.doi.org/10.1016/j.ica.2014.05.005
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A novel homodinuclear manganese(III) complex with N, N′-
bis(salicylidene)diethylenetriamine: synthesis, structure and catalytic
activity
a,*
a
b
Hassan Hosseini-Monfared , Sohaila Alavi , Peter Mayer
a
Department of Chemistry, University of Zanjan 45195-313, Zanjan, I. R. Iran. Email:
monfared@znu.ac.ir
b
Fakultät für Chemie und Pharmazie, Ludwig-Maximilians-Universität, München, Butenandtstr. 5-13,
Haus D, D-81377 München, Germany. E-mail: p.mayer@lmu.de
Abstract
A novel chloride bridged dinuclear manganese(III) complex was prepared and its structure established
2 2
by X-ray diffraction analysis. The complex showed high selectivity in H O promoted oxidation of
cyclohexane and cyclic alkenes. Under optimized conditions, 72% of cyclohexane conversion with 85%
of selectivity for alcohol was achieved after 5 h. To the best of our knowledge this activity and
remarkable selectivity to cyclohexanol is the highst for a manganese catalyst.
Keywords: Homodinuclear; Manganese; Structure; Catalysis; Oxidation
1
. Introduction
1

The design and construction of supramolecular architectures with coordination bonds and hydrogen
bonds have attracted great attention in recent years [ 1 ]. Among the various families of these
supramolecular materials, the family of closed, ring-like coordination complexes such as metallacrowns
and metallacycles is an important one, owing to their potential applications in molecular recognition,
chiral recognition, catalysis and photoluminescence [ 2]. According to previous reports, the basic
strategy to design these materials is to chelate metal ions by polydentate N,O- or N,N-containing
ligands [3]. Multidentate ligands have often been observed to coordinate with more than one metal
centre to form infinitely extended structures, such as coordination polymers, polynuclear complexes or
metallacycles [4]. The use of a flexible or less-rigid spacer for the construction of a coordination
complex has an added advantage as they have many degrees of freedom and a few conformational
restraints, which can give various topologies. These types of conformational flexible ligands give
unique opportunities to construct novel structures with desirable characteristics.
Selective oxidation of hydrocarbons is of great interest in synthetic organic chemistry and chemical
industry for the conversion of petroleum-based feed stocks to more valuable chemicals such as epoxides
and alcohols [5]. From both economic and green chemistry perspectives, H
2
O
2
and O are the most
2
attractive oxidants because of their high contents of active oxygen species and co-production of only
water. Manganese derivatives containing in many cases chelating N-ligands are among the most active
catalysts in hydrocarbon oxidations [6]. Mn-salen-type complexes have been extensively investigated
for the oxidation of hydrocarbons [7]. It has been also reported that the dinuclear Mn-triazacyclononane
2 2
complex is the only one which catalyzes the oxidation of alkanes with H O in the presence of a
carboxylic acid [8]. Selective oxidation of cyclohexane catalyzed by metallic complexes has been
reported in the literature [9]; however, the product yields are usually low [9]. Oxidations of cyclohexane
that use H
potentially viable for large-scale production. Selected examples of H
10] complexes have attracted much interest, but the search for highly selective environmentally benign
alkene epoxidation and alkane C-H bond oxidation methods with aqueous H remains a challenge.
2
O
2
in conjunction with cheap, manageable and relatively non-toxic metal-based catalysts are
2
O
2
-epoxidations catalyzed by Mn
[
2
O
2
2

Herein we report on the synthesis, structure and catalytic activity of a novel dinuclear manganese(III)
complex with N,Nꢀ-bis(salicylidene)diethylenetriamine (H saldien). The ligand has often been used in
the synthesis of metalorganic complexes [11]. The catalytic activity of the complex was investigated in
the oxidation of hydrocarbons with aqueous H as the oxidant without any co-catalyst.
2
2
O
2
2
. Experimental Section
All starting chemicals were commercially available reagents and were used without further purification.
−1
The infrared spectrum was taken on a Bruker FT-IR spectrophotometer in the 4000–400 cm region,
using KBr pellets. UV/Vis solution spectra were recorded on a thermo-spectronic Helios Alpha
spectrometer using a 1 cm quartz cell. The reaction products of the oxidation were determined and
analyzed by an HP Agilent 6890 gas chromatograph equipped with a HP-1 capillary column (phenyl
methyl siloxane 30 m × 320 µm × 0.25 µm) with flame ionization detector. Single-crystal X-ray
intensity data were collected on an Oxford Diffraction Xcalibur 3 diffractometer at 173 K (MoKꢁ
radiation).
2
2
.1. Synthesis of N,N′-bis(salicylidene)diethylenetriamine (H saldien)
Following a literature procedure, the ligand H
typical procedure, to an ethanolic solution of salicylaldehyde (2 equiv.), a solution of diethylenetriamine
1 equiv.) in ethanol was added and the resulting yellow coloured solution was allowed to reflux for 3 h.
The excess of solvent was removed under vacuum and a dark yellow coloured oily product resulted.
2
saldien was prepared in high yield and purity [12]. In a
(
-
1
Yield: 85%. IR (KBr, cm ): 3433 (br, w, O–H), 3317 (w, N–H), 3059 (w), 3009 (w), 1633 (vs, C=N),
1
1
582 (s), 1497 (m), 1461 (m), 1417 (w), 1383 (w), 1337 (w), 1279 (s), 1211 (w), 1151 (w), 1118 (w),
1
045 (w), 1025 (w), 973 (w), 894 (w), 850 (w), 755 (s), 641 (w), 566 (w), 461 (w). H NMR (CDCl
3
,
3

2
50 MHz): δ = 8.213 (2H, -CH=N), 6.734-7.227 (8H, aryl), 3.456-3.756 (4H, -CH
2
-), 2.605-2.860 (4H,
, 125 MHz): δ 49.6 (2C), 59.4 (1C), 59.6 (1C), 76.9
1C), 77.4 (1C), 77.9 (1C), 116.9 (2C), 118.6 (2C), 131.4 (2C), 132.3 (2C), 160.9 (1C), 161.1 (1C),
13
-
2 3
CH -), 1.152 ppm (1H, N-H). C NMR (CDCl
(
-4
-3
-1
-1
1
2
66.1 (1C), 166.4 (1C). UV/Vis (in CH
3
OH, c = 1.0×10 mol dm , ꢂmax [nm] with ꢃ [L mol cm ]):
21 (23000), 249 (28000), 327 (9900).
2
.2. Synthesis of the homodinuclear manganese(III) complex (1)
2
A mixture of H saldien (0.04 g, 0.14 mmol), manganese(II) chloride tetrahydrate(0.06 g, 0.30 mmol)
were placed in the main arm of a branched tube (‘branched tube’ method) and methanol was added to
fill the arms. The tube was sealed and the arm containing the reagents immersed in an oil bath at 60 °C
while the other arm was kept at ambient temperature. After 24 h, the black crystals of
NH
[
{Mn(Cl)(saldien )}
2
(µ-Cl)]Cl·H
2
O·2CH
3
OH (1) suitable for X-ray analysis were deposited in the
Mn (954.53) C 47.81, H 5.28, N
.80, Mn 11.51. Found C 47.89, H 5.28, N 8.67, Mn 11.40%. IR (KBr, cm ): 3447 (w, O–H), 3319 (m,
cooler arm in yield 72% (0.045 g). Anal. Calc. for C38
H
50Cl
4
2 6 7
N O
-1
8
-4
-3
N–H), 1611 (s, C=N), 1547(s, ꢄ(NH)). UV-Vis (in CH
3
OH, c = 1.0×10 mol dm , ꢂmax [nm] with ꢃ [L
-1
-1
mol cm ]): 227 (29000), 245 (26000), 270 (15400), 296 (9200), 370 (3800), 598 (100).
2
.3. Catalytic oxidation
The liquid phase catalytic oxidations were carried out under air (atmospheric pressure) in a 25 mL
round bottom flask equipped with a condenser and immersed in a thermo stated oil bath. In a typical
2 2
experiment, H O (3 mmol) was added to the flask containing the catalyst (1.03 µmol) and alkene (1.0
mmol) in a solvent (3 mL) and n-octane/or chlorobenzene (0.1 g) as internal standard. The course of the
reaction was monitored at periodic intervals using a gas chromatograph equipped with a capillary
column and a flame ionization detector. The oxidation products were identified by comparison of their
4

retention times with those of authentic samples. The conversion was determined by the following
equation
[
(ꢀꢁ(ꢂꢃꢄꢂꢅꢆꢇꢅꢈ)/ꢀ(ꢂꢅꢇꢉꢊꢇꢆꢊ)]ꢋꢌꢍꢁꢎꢏ[(ꢀꢁ(ꢂꢃꢄꢂꢅꢆꢇꢅꢈ)/ꢀ(ꢂꢅꢇꢉꢊꢇꢆꢊ)]ꢋꢌꢐꢁꢎ
%Conversion =
[
(ꢀꢁ(ꢂꢃꢄꢂꢅꢆꢇꢅꢈ)/ꢀ(ꢂꢅꢇꢉꢊꢇꢆꢊ)]ꢋꢌꢍꢁꢎ
A = peak area in GC chromatogram
2
.4. Titrimetric analysis for the determination of chloride (Mohr procedure)
In the titration of a neutral solution of chloride ions with silver nitrate [13], a small quantity of
potassium chromate solution was added to serve as indicator. At the end point the chromate ions
combine with silver ions to form the sparingly soluble, red, silver chromate. Aqueous solution of the
3
fresh complex 0.010 M was titrated with aqueous solution of 0.1 mol/L AgNO in the presence of a few
milliliters of dilute potassium chromate solution (0.003 M). The results showed the presence of only one
equivalent chloride ions. For determination of chloride after catalysis, the mixture of reaction solution
(
complex 1 (0.02 g), cyclohexene (1.0 mmol), n-octane (0.1 g), acetonitrile (3 ml), H
temperature 80 °C) after 5 h was diluted to 25 mL with water and then titrated with the aqueous solution
of AgNO in the presence of potassium chromate. The results showed the presence of four equivalents
chloride ions.
2 2
O 3 mmol,
3
2
.5. X-ray structure determination
A suitable single crystal of 1 was carefully selected under a polarizing microscope and X-ray diffraction
data for 1 were collected at 173(2) K on a X Calibur diffractometer using mirror collimated Mo-Kα
radiation (λ = 0.71073 Å, graphite monochromator). The structure was solved by Direct Methods with
2
SIR97 [14] and refined with full-matrix least-squares techniques on F with SHELXL-97 [15]. The
crystal data and refinement parameters are presented in Table 1. N-bound H-atoms were refined freely,
5

C-bound H-atoms constrained, methanol- and water-bound H-atoms not considered; a split model has
been applied for the refinement of disordered methanol; the free chloride ion is disordered over two
sites (occupancy sum 0.5, occupancy ratio ca. 2/1) close to a water O-atom (occupancy fixed to 0.5).
The structure plots were prepared with DIAMOND [16]. The structural data has been deposited with the
Cambridge Crystallographic Data Center (CCDC-number 939700). These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data request/cif.
3
. Results and Discussion
The Schiff base ligand H
2
saldien was synthesized by the dicondensation of diethylenetriamine with 2
saldien confirmed that the reaction
equiv. of salicylaldehyde in ethanol [17]. IR and NMR spectra of H
2
of diethylenetriamine with salicylaldehyde occurred by the Schiff condensation between the C=O and
NH
the NH
2
groups. The manganese complex [[{Mn(Cl)(saldien )}
2
(µ-Cl)]Cl· H
2
O ·2 CH
3
OH (1) was
·4H O,
prepared by treating a methanolic solution of H
2
saldien with an equimolar amount of MnCl
2
2
NH
where saldien denotes the amine protonation (Scheme 1). The Mn(II) was oxidized to Mn(III) by air
2
during the reaction. In the IR spectrum, the free ligand H saldien shows stretching bands attributed to
-1
-1
phenolic C–OH, C=N, and N–H at 1279, 1633 and 3317 cm , respectively. A band at 3412 cm is
-
1
assigned to ꢅ(O-H) vibrations involving intramolecular hydrogen bonding, while a band at 1214 cm is
attributed to δ(O-H) (phenolic) [18]. The presence of ꢅ(OH) observed for H saldien as a weak band at
433 cm suggests intramolecular hydrogen bonding (O−H···N) in the ligand. For complex 1 the
2
-
1
3
−1
absence of the δ(O-H) (phenolic) band and red shift in azomethine (C=N, 22 cm ) bands compared to
2
-
the free ligand H saldien indicate coordination of (saldien) through the phenolate oxygen and
2
azomethine nitrogen atoms in complex 1. As a result of complexation, the UV–Vis spectrum in
methanol shows absorbance bands at 227 (29000), 245 (26000), 270 (15400), 296 (9200), 370 (3800),
6

5
98 (100) (Fig. 1). The complex shows an UV-Vis spectrum similar to the Schiff base ligand. Based on
*
their extinction coefficients absorbance bands can be assigned to intraligand π ꢆ π (227, 245 nm) and
*
n ꢆπ (270, 296 nm) transitions (from the azomethine (CH=N) group [19]). The spectrum of the
complex shows an intense band in the high-energy region at 370 nm which can be attributed to charge
transfer (LMCT) transition. Furthermore, complex 1 shows a ligand field d−d transition at 598 nm (the
inset in Fig. 1).
The molecular structure of the cation with numbering is depicted in Fig. 2. The compound consists of
NH
+
a dinuclear C
2
-symmetric [{Mn(Cl)(saldien )} (µ-Cl)] cation, a chloride counter ion and solvate
2
water and methanol molecules. Selected bond lengths and angles are presented in Table 2. The complex
crystallizes in the space group C2/c. The Mn(III) atoms are coupled by both doubly l-phenolato oxygens
and the imine nitrogen atoms of the Schiff base ligand and simultaneously one chloride anion in the
anti–anti bridging mode. The coordination geometry around the Mn(III) centres may be regarded as
distorted octahedral. The equatorial plane is formed by the two imine nitrogen atoms [N(1), N(2)] and
two phenolic oxygen atoms [O(1), O(2)] coming from the two ligands, N1 and O1 belong the one Schiff
base ligand but N2 and O2 belong the other ligand. [N2–Mn1–N1 = 169.98(9)º and O2–Mn1–O1 =
1
77.04(9)º]. The apical positions are occupied by a terminal chloride [Mn–Cl1 = 2.5336(9) Å] and a
bridging chloride located on the C axis within the metallacycle [Mn–Cl2 = 2.8236(6) Å, Cl1–Mn2–Cl2
170.50(2)º]. The Mn1–imine nitrogen [2.043(2) and 2.044(2) ꢇ] as well as the Mn1–phenolic oxygen
1.852(2) and 1.869(2) ꢇ] bond distances are in the range observed for similar systems [20]. In the
2
=
[
Schiff base ligand, the N3 atom is protonated (Fig. 2). The Mn-Mn distance within a cation is 5.1334(5)
ꢇ. Within the metallacyclic cation intramolecular ꢈ-stacking is observed between the aryl ring plane of
i
i
i
i
i
i
C13/C14/C15/C16/C17/C18 and C13 /C14 /C15 /C16 /C17 /C18 ring with a short centroid-centroid
distance of 3.898(3) ꢇ (Fig. 2) [21]. The planes are almost parallel to the C axis along b resulting in a
2
dihedral angle of 1.68(3)°. As a consequence of the strong ꢈ-stacking the anellated six-membered
chelate ring (involving N2and O2) is significantly more puckered than the other one involving N1 and
+
O1 (puckering amplitudes 0.378(2) ꢇ and 0.207(2) ꢇ, respectively) [22]. One of the two amine (NH
2
)
7

hydrogen atoms is involved in a bifurcated intramolecular charge-assisted [23,24] hydrogen bonds with
O2 and the bridging Cl2 as acceptors (N3···O2 2.796(3) Å, N3···Cl2 3.255(3) Å), while the other one is
bound to the disordered free chloride ion and water molecule.
NH
2 2 3
Complex [{Mn(Cl)(saldien )} (µ-Cl)]Cl·H O·2CH OH(1) is a stable chloride-bridged dinuclear
NH 2-
manganese complex in the solution and ligand (saldien ) remains coordinated to manganese centres;
however, the bridged and terminal coordinated chlorides are separated from the complex during the
catalysis. In the UV-Vis spectrum of the complex after catalysis (Fig. 1), absence of the charge transfer
band at 370 nm suggests dissociation of the chloride ligands during the catalysis (see below). This
finding was also supported by measuring the chloride ions in the solution. Titration of the fresh
-
dinuclear complex 1 solution with AgNO
3
showed the presence of only one equivalent Cl while for the
complex after catalysis four equivalents of chloride ions were titrated [13].
In order to explore the catalytic feature of 1 the catalytic oxidation of cyclooctene with hydrogen
peroxide was studied in the presence of 1. All reactions were carried out with 1 mmol of cis-
3
cyclooctene in CH CN and in the presence of complex 1 at 80 ºC. cis-Cyclooctene oxide was the sole
product. The results of control experiments revealed that the presence of catalyst (Table 3, entry 1) and
oxidant are essential for the oxidation. The catalyst showed high activity in the oxidation of cis-
cycloocetene in acetonitrile with a high conversion value (65%), affording with excellent selectivity
(
>99%) the epoxide (Table 3, entry 2). The reaction medium showed remarkable influence on the
activity of 1. No oxidation was observed in CHCl while the conversion values in methanol and ethanol
3
were 11% and 5%, respectively. The highest conversion (65%) was obtained in acetonitrile after 5 h. It
was observed that the catalytic activity of complex decreases in the order acetonitrile (relative dielectric
constants [25], (ꢃ/ꢃ
). In general, there was a good correlation between the solvent polarity (ꢃ/ꢃ
conversion. The highest conversion in acetonitrile possibly is caused by its high dielectric constant.
Decreasing the [H ]/[substrate] molar ratio from 3 to 1 and 2 decreased the conversion to 7% and
8%, respectively (Table 3, entries 2, 6,7). This finding shows partially unproductive decomposition of
0
= 37.5) > methanol (32.7) > ethanol (26.6) > chloroform (4.9) (Table 3, entries 2-
5
0
) and the cis-cyclooctene
2
O
2
1
8

hydrogen peroxide by 1. At temperatures lower than 80 °C the reaction resulted in reduction of the
activity of 1 (Table 3, entries 8, 9). Higher temperature probably helps in faster transfer of the oxygen
#
atom from the active intermediate species [1-H
2
O
2
] to the substrate and results in the higher activity.
Furthermore, the catalytic activity of catalyst 1 was examined in the oxidation of cyclohexene and
cyclohexane under optimized conditions (H /cyclooctene molar ratio = 3, acetonitrile = 3 mL,
2
O
2
reaction temperature 80 ºC) and the results are summarized in Table 4. Cyclohexene has two fragments
accessible for the attack by catalytically active species (the double bond and relatively weak C–H
bonds). The comparison of the rates of epoxidation and C–H bond oxygenation could give valuable
information on the nature of the oxidizing species. Complex 1 oxidized cyclohexene to the
corresponding epoxide with > 99% selectivity. This excellent selectivity in the oxidation of cyclohexene
is highly remarkable. In addition, the outstanding catalysis potential of 1 was also observed in the
oxidation of cyclohexane (Table 4) with a conversion of 72% resulted after 5 h leading to the products
cyclohexanol and cyclohexanone that were yielded in a ratio of 5.7.
Various homogeneous and heterogeneous mono- and dinuclear Mn-catalysts have been used for the
oxidation of cyclohexene to cyclohexene oxide [ 26 , 27 , 28, 29, 30 , 31 ] and cyclohexane to
cyclohexanol/cyclohexanone [27,32,33,26] with H
important reports on the epoxidation of various olefins, except cyclohexene, using nonaqueous H
1.2-1.4 equiv.) and acetic acid (14 equiv, to suppress H disproportionation) within 35-90 min at
room temperature by a manganese-triazacyclononane catalyst, [Mn(OTf)
dihydroxlation and epoxidation of alkenes with H by carboxylate bridged dinuclear manganese
complexes, [Mn O(RCO (tmtacn) ] [35], (a carboxylic acid as co-catalyst is necessary for inherent
catalase activity of the catalyst and for tuning of the catalyst’s selectivity toward cis-dihydroxylation
and epoxidation) and oxidation of cis-cyclooctene to epoxide and cis-diol with H and 2-10 equiv of
solid cocatalytic supports by manganese complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn)
supported on carboxylic acid-functionalized SiO [36]. However, none of the reported catalysts shows
2 2
O and t-BuOOH. There are also interesting and
2
O
2
(
2 2
O
H,Me
2
(
PyTACN)] [34], cis-
2
O
2
2
2
)
2
2
2
O
2
2
3 6 2
such a high activity or/and selectivity as complex 1. Selectivity of the reported [LMn(µ-O) MnL][PF ]
9

catalyst in cyclohexene epoxidation is comparable to 1. Furthermore, the synthesis of trimethyl-1,4,7-
triazacyclononane (L) is not as simple as H saldien and the presence of a cocatlyst to achieve high
activity is necessary by catalyst [LMn(µ-O) MnL][PF . In the oxidation of cyclohexane, selectivity of
with respect to cyclohexanol and its turnover number were higher than those of [LMn(µ-
2
3
]
6 2
1
NH
O)
3
MnL][PF
6
]
2
. In addition to the chemical simplicity and easy synthesis of [{Mn(Cl)(saldien )}
disproportionation reaction to a great extent relative to
/acetic acid/[LMn(µ-O) MnL][PF system (which needs
) [26] and [Mn(µ-terph)(H O) ([H ]/[substrate] = 10) (terph =
2
(µ-
+
Cl)] , it has also the ability to limit the H
2
O
2
most Mn(II)/Mn(III) compounds, the H
2
O
2
3
6 2
]
4
.6 to 13.9 equiv. of H
2
O
2
2
2
]
n
2 2
O
terephthalate) [37]. Despite their power as a synthetic tool and abundant use in academic research,
oxidation reactions as a whole comprise as little as 3% of the reactions used on a preparative scale in the
pharmaceutical industry [38]. Perhaps the greatest factor influencing the hesitation to employ oxidation
2 2
reactions on a large scale is the safety of these processes. Using less amount of H O improves
oxidation process safety in large scale syntheses.
The established intermediate in the catalytic oxidation by Mn complex/H
2
2
O systems is manganese
hydroperoxy intermediate, Mn–O–OH, which leads to the formation of Mn(V)-oxo active species
39,40]; the same oxygen atom is subsequently transferred to the substrate in alkane oxidation or olefin
[
epoxidation. Various other high manganese(V)-oxo species based on salen [ 41 ], corrole [ 42 ],
corrolazine [43] and other nitrogen-based ligands have been identified [44]. In the earlier studies the
ꢉ
main problem in Mn-salen catalyzed epoxidation with H
2
O
2
was the formation of HO radicals by the
homolytic cleavage of the weak O–O bond, leading to indiscriminate oxidation [45]. Addition of Lewis
bases (cocatalysts), such as imidazole, pyridine, or 2,6-lutidine, was found to favour the heterolytic
bond cleavage, leading to reactive manganese-oxo species [46]. The added Lewis bases coordinate
axially and facilitate the heterolytic cleavage of the O–O bond in the manganese hydroperoxy
intermediate and favour the formation of a catalytically active oxomanganese(V) intermediates [39],
which are presumed to be the actual oxidizing species, lead to improvement of the selectivity and
catalyst stability [ 47 ]. The oxidation reactions are commonly catalyzed by soluble manganese
1
0

complexes. However, prevention of H
2
O
2
decomposition is one of the major issues with the complexes
to H O and O by the complexes
n+
of Mn . The major problem of unproductive decomposition of H
2
O
2
2
2
[
48] may in some cases be solved by immobilization of the catalysts [49]. In solution, the mononuclear
compounds have a strong tendency to dimerize, and H
2
O
2
is readily decomposed by these complexes
NH
via Mn–O–O–Mn intermediates. Clearly, the [[{Mn(Cl)(saldien )} (µ-Cl)]Cl complex generates
2
NH
active species upon reaction with H
2 2 2
O , because [[{Mn(Cl)(saldien )} (µ-Cl)]Cl can participate in
efficient oxidation as described above. The addition of H
2
O
2
to a CH
3
CN solution of
NH
*
*
[
[{Mn(Cl)(saldien )}
2
(µ-Cl)]Cl leads to the disappearance of n ꢆ π bands and shifts in the π ꢆ π
NH
transitions of the saldien ligand (Fig. 3). These findings suggest the formation of an active oxidizing
III
III
NH
species in the first step by replacing the group Mn -Cl with Mn -OOH unit; while saldien ligands
-
remain coordinated to manganese centres the coordinated three Cl dissociate in the oxidation medium.
Measurements of the chloride ion in the solution of complex 1 and solution of the oxidation reaction
NH
medium as well as UV-Vis spectrum of the reaction mixture of [[{Mn(Cl)(saldien )} (µ-
2
Cl)]Cl/H
2
O
2
/CH CN/cyclohexene after 5 h (Fig. 1) confirmed the dissociation of the coordinated
3
chlorides during the reaction.
Furthermore, one remarkable feature of complex 1 is showing very good catalytic activity merely
with 3 equivalents of H . Probably, similar to iron(III) complexes [50], in the oxygenation of organic
substrates and catalase-like activity there are two different kinds manganese(III)-peroxide adduct in
2
O
2
1
1
2 2
solution (Scheme 2). Unproductive decomposition of H O is performed by the adduct I (µ-ꢊ :ꢊ -peroxo
dimanganese(III)) [50] or µ-hydroperoxo dimanganese(IV) which results to the formation of hydroxide
IV
bridges on the resulting Mn centers [51]. Since the decomposition rate is very low, the concentration
1
of adduct I (or µ-hydroperoxo dimanganese(IV)) must be low. The adduct II (η -hydroperoxide
dimanganese(III)) is formed through hydrogen bonding between the hydroperoxide ion and the oxygen
atom of the phenoxide group. This type of intramolecular hydrogen bond prevents unproductive
1
decomposition of H
2
O
2
. A similar η -hydroperoxide intermediate has been assumed for a dinuclear
1
1

iron(III) species stabilized through hydrogen bonding between the hydroperoxide ion and the oxygen
atom of the amide group [50].
The catalytic performance of the manganese salts MnCl
catalysts have been examined. The manganese salts in CH CN solution showed no catalytic activity in
the oxidation of alkene with 30% H since hydrogen peroxide was decomposed instantly in the first
2
and Mn(CH
3
COO)
2
as homogeneous
3
2
O
2
minutes of the reaction [52]. Since the oxidation of cyclohexene by complex 1 did not stop and
continued it suggests that complex 1 is not decomposed in the oxidation medium. The possibility of
-
involvement of dissociated Cl ions in the catalytic oxidation was excluded by repeating the oxidation of
cyclohexene under the optimized conditions and using KCl instead of complex 1 as catalyst which gave
no product. Overall, the control experiments provide evidence that the observed catalytic results are
derived from precursor of complex 1 and no leaching of the active component and/or demetallation
occurs during the catalytic process.
4
. Conclusions
A novel chlorido-bridged dinuclear Mn(III) complex was synthesized and successfully used for
selective oxidation of cyclohexene, cis-cylooctene and cyclohexane with 3.0 equiv. of H as oxidant
2
O
2
in the absence of any cocatalyst. The complex shows comparable and better activity with respect to
reported Mn(II) and Mn(III) complexes without requirement to use high equivalents of hydrogen
peroxide. To the best of our knowledge the activity and remarkable selectivity to cyclohexanol of 1 is
the highest for a manganese catalyst.
Acknowledgments
We thank the University of Zanjan and Ludwig-Maximilians-Universität for financial support.
1
2

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Scheme / Figure / Table captions
NH
2 2 2 3
Scheme 1. Synthesis of (a) H saldien and [{Mn(Cl)(saldien )} (µ-Cl)]Cl·H O·2CH OH (1).
Scheme 2. Proposed Mn-hydroperoxide intermediate in the oxidation of hydrocarbons by
NH
[
2 2 2
[{Mn(Cl)(saldien )} (µ-Cl)]Cl/H O system.
2
Fig. 1. UV/Vis spectra of complex 1, H saldien and complex 1 after 5 h catalytic oxidation of
cyclohexene in acetonitril.
1
5

NH
Fig. 2. Thermal ellipsoid plot (50% probability) of the cation of [{Mn(Cl)(saldien )}
2
(µ-
Cl)]Cl·H O·2CH OH (1); hydrogen atoms (except hydrogen atoms of the amine) have been omitted for
2
3
clarity; bond lengths and angles in Table 2; symmetry transformation i = 1–x, y,0.5–z; intramolecular
ꢈ···ꢈ interaction (dashed line).
Fig. 3. The visible spectrum of the intermediate formed upon addition of H
2
O
2
to a solution of complex
1
in acetonitrile.
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Table 2. Selected bond distances (Å) and angles (°) for 1.
a
Table 3. Effect of the catalyst and the amount of oxidant on the oxidation of cis-cyclooctene.
a
2 2
Table 4. Oxidation of various hydrocarbons by 1/H O in acetonitrile.
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1
7

Scheme 1.

Scheme 2.

Fig. 1.

Fig. 2.

Fig. 3.

Table 1. Crystal data and refinement details for 1.
Complex
Colour
1
brown
Net formula
M /g mol
r
C H Cl Mn N O ·Cl·H O·2(CH O)
954.53
3
6
40
3
2
6
4
2
4
−1
Crystal size/mm
Crystal system
Space group
Block, 0.22 × 0.08 × 0.07
Monoclinic
C2/c
a/Å
b/Å
c/Å
β/°
V/Å
Z
18.1184 (19)
13.2952 (7)
20.132 (2)
120.715 (14)
4169.2 (6)
4
3
−3
Calc. density/g cm
μ/mm
1.521 (1)
−1
−1
0.92 mm
Absorpt. correction
Refls. measured
Rint
multi-scan
11401
0.048
Mean σ(I)/I
θ range/°
Observed refls.
0.058
4.2-26.4
3337
x, y (weight. scheme)
0.0511, 6.0103
Hydrogen refinement
a mixture of independent and constrained
refinement
4232
281
Refls in refinement
Parameters
Restraints
1
R(F )
0.0459
0.1267
1.061
0.02
0.65
−0.60
obs
2
R (F )
w
Goodness-of-fit, S
Shift/errormax
Max electr. dens./e Å
−
3
−3
Min electr. dens./e Å

Table 2. Selected bond distances (Å) and angles (°) for 1.
Bond
Length (Å)
Bond
Angle (°)
i
Mn1-Cl1
Mn1–Cl2
Mn1-O1
Mn1-O2
Mn1-N1
Mn1-N2
2.5336(9)
2.8236(6)
1.852(2)
1.869(2)
2.043(2)
2.044(2)
Mn1-Cl2-Mn1
N1-Mn1-O1
N2-Mn1-O2
N1-Mn1-O2
N1-Mn1-Cl1
N2-Mn1-Cl1
O1-Mn1-Cl1
130.72(4)
90.51(9)
90.68(9)
91.40(9)
94.54(7)
95.31(7)
93.24(7)
Symmetry code i = 1–x, y, 0.5–z.

a
Table 3. Effect of the catalyst and the amount of oxidant on the oxidation of cis-cyclooctene.
H O
Conv.
Selectivity
(%)
-
Time
Temp
(°C)
80
2
2
Entry
Solvent
[
b]
(
mmol)
(%)
(h)
5
[
c]
1
2
3
4
5
6
7
8
3
3
3
3
3
1
2
3
3
0
CH CN
3
65
11
5
100
100
100
100
0
5
80
CH CN
3
5
80
MeOH
EtOH
5
80
0
5
80
CH Cl
3
7
5
80
CH CN
3
18
32
45
100
100
100
5
80
CH CN
3
5
25
CH CN
3
9
5
60
CH CN
3
a
Reaction conditions: catalyst 1.03 µmol, cis-cyclooctene 1.0 mmol, n-octane 0.1 g, acetonitrile
3
.0 mL, H O 3.0 mmol, temperature 80 °C.
2
2
b
Conversions based on the starting substrate and determined by GC.
c
In the absence of catalyst 1.

a
Table 4. Oxidation of various hydrocarbons by 1/H O in acetonitrile.
2
2
[
c]
Substrate
Product(s)
Select.
%)
Conv.
(%) .
TON
[
b]
(
1
1
00
00
65
631
543
56
72
1
5
699
8
5
a
Reaction conditions: catalyst 1 1.03 μmol, substrate 1 mmol, n-octane 0.1 g, acetonitrile 3 mL,
H O 3 mmol, temperature 80 °C and reaction time 5 h.
2
2
b
conversions based on the starting substrate and determined by GC.
c
TON: turnover number = number of moles of product formed per mole of the catalyst.

Highlights



Synthesis and structure of a new dinuclear Mn(III) complex of a tetradentate ligand
The complex shows high selectivity in the oxidation of cyclohexene and cyclohexane
The highest reported selectivity to cyclohexanol for manganese catalysts

Graphical abstract (picture)

Graphical abstract (synopsis)
Synthesis and structure of a novel dinuclear manganese(III) complex are reported. A remarkable
selectivity to the cyclohexanol has been accomplished by the manganese catalyst in the oxidation
of cyclohexane using aqueous H O as the oxidant without any cocatalyst.
2
2