Synthesis, characterization and catalytic reactivity of Mn(III) complexes with a scorpion-like bis(phenolate) ligand: Selective oxidation of primary alcohols to aldehydes

Article abstract of DOI:10.1016/j.poly.2014.01.020

Two new mononuclear complexes of manganese(III), [Mn(bp)(N ₃)(CH₃OH)]·0.5CH₃OH (1) and [Mn(bp)(NCS)_0.9(NO₃)_0.1(CH₃OH)] ·CH₃OH (2), have been synthesized, where H₂

Full text of DOI:10.1016/j.poly.2014.01.020

Accepted Manuscript
Synthesis, characterization and catalytic reactivity of Mn(III) complexes with a
Scorpion-like bis(phenolate) ligand: Selective oxidation of primary alcohols to
aldehydes
Nader Noshiranzadeh, Marzieh Emami, Rahman Bikas, Katarzyna Ślepokura,
Tadeusz Lis
PII:
S0277-5387(14)00041-2
DOI:
http://dx.doi.org/10.1016/j.poly.2014.01.020
POLY 10518
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
14 November 2013
18 January 2014
Please cite this article as: N. Noshiranzadeh, M. Emami, R. Bikas, K. Ślepokura, T. Lis, Synthesis, characterization
and catalytic reactivity of Mn(III) complexes with a Scorpion-like bis(phenolate) ligand: Selective oxidation of
primary alcohols to aldehydes, Polyhedron (2013), doi: http://dx.doi.org/10.1016/j.poly.2014.01.020
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Synthesis, characterization and catalytic reactivity of
Mn(III) complexes with a Scorpion-like bis(phenolate)
ligand: Selective oxidation of primary alcohols to
aldehydes
Nader Noshiranzadeh,a,* Marzieh Emami, Rahman Bikas,a Katarzyna Ślepokura,b
and Tadeusz Lisb
a Department of Chemistry, Faculty of Sciences, University of Zanjan 45195-313, Zanjan,
Iran
b Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14, Wroclaw 50-383, Poland
Abstract
Two new mononuclear complexes of manganese(III), [Mn(bp)(N3)(CH3OH)]·0.5CH3OH
(1) and [Mn(bp)(NCS)0.9(NO3)0.1(CH3OH)]·CH3OH (2), have been synthesized, where
H2bp is 6,6'-(pyridin-2-ylmethylazanediyl)bis(methylene)bis(2,4-di-tert-butylphenol).
The complexes were characterized by elemental analyses and spectroscopic methods, and
fully characterized by X-ray diffraction analysis. Single-crystal X-ray diffraction
structure analysis of the complexes revealed that the manganese atom is octahedrally
coordinated, where the bis(phenolate) ligand (bp2-) is N2O2-bonded to the Mn(III) center
in a tetradentate dinegative fashion. The catalytic potential of these complexes has been
tested for the oxidation of primary alcohols using H2O2 as the oxidant. The effects of
various parameters, including the molar ratio of oxidant to substrate, the temperature and
the solvent, have been studied. On comparing the catalytic activity of 1 and 2, catalyst 2
showed the most powerful activity in the oxidation of alcohols. Excellent conversions
have been obtained for the oxidation of benzyl alcohols containing electron-donating
groups at the para-position.
1

Keywords: Bis(phenolate) ligands; Mn(III) complex; Catalytic oxidation; Crystal
structure; Scorpion-like ligands, Oxidation of alcohols
1. Introduction
Due to wide application of carbonyl compounds as intermediates for many drugs,
vitamins and fragrances, both in manufacturing and scientific researches [1], the
oxidation of alcohols to their corresponding carbonyl compounds is one of the most
important and fundamental reactions in organic synthesis [2]. However, the selective
oxidation of primary alcoholic compounds to their corresponding aldehydes remains
relatively difficult because in the oxidation of primary alcohols, the corresponding
carboxylic acids can also form [3]. The selective catalytic oxidation of primary alcohols
to aldehydes has attracted a great deal of attention in the last decade and in this context a
variety of methods have been reported. Most of reported methods have to be performed
with a stoichiometric amount of the oxidants [4]. The development of newer catalysts,
methods and methodologies are gaining much attention currently due to the significance
of this reaction [5]. In terms of economical benefit and environmental impact, catalytic
oxidations processes based on H2O2 as an oxidant are extremely valuable [6]. Recently,
the catalytic oxidation of alcohols employing transition metal complexes, such as Fe, V
[7], Cu [8], Co [9], Ag [10], Pd [11], Mn [12] etc., using H2O2 have been developed.
Among these, manganese complexes have attracted much attention because of (i) the
variable structures of manganese complexes with various accessible oxidation states (e.g.
2+, 3+, 4+, 7+) [13], (ii) the wide occurrence of manganese in enzymes, plants and
2

bacteria [14] and (iii) its potential for use as an industrial catalyst [15]. In addition,
manganese based catalysts are generally known as environmentally benign catalytic
systems [16].
It is obvious that the product selectivity depends on the catalyst, oxidant, solvent
and the reaction conditions. It is well established that Mn(III) salen complexes are good
candidates for the selective oxidation of organic substrates [17]. Investigations on
MnIII(salen)-catalyzed oxidations have shown that specific steric and electronic
properties of the substituents on the salen ligands (Scheme 1) are essential in order to
achieve high enantioselectivity [18]. Previous reports note that the effect of substituents
at the ortho- and para-positions with respect to the phenolic group is more effective than
for other sites. Incorporation of bulky substituents at the ortho-positions improves
selectivity dramatically by directing the substrate approach to the vicinity of the metal
core, while the effect of para-substituents is based on the electronic nature of the
substituents (electron-donating substituents show higher enantioselectivity). The tert-
butyl and tert-Ph groups are simple but unique directing substituents for selective
oxidation [19]. There are many reports on the application of catalysts containing the tert-
butyl group as a directing group [20].
In the present work, we report the synthesis, characterization, crystals structures
and catalytic reactivity of two new manganese(III) complexes of a tetradentate ligand
including two phenolic rings bearing the bulky tert-butyl substituent at the ortho- and
para- positions with respect to the phenolic oxygen. This ligand has been prepared by a
simple one-pot reaction through the Mannich condensation of 2,4-di-tert-butylphenol,
formaldehyde and 2-(aminomethyl)pyridine. The catalytic reactivity of these complexes
3

has been investigated in the selective oxidation of primary alcohols in the presence of
H2O2 as a highly selective, atom economic and environmental oxidant. When this ligand
coordinates to the metal ion, only one side of metallic core is available for catalytic
reactions and the other sites are closed by substituents or donor atoms of the ligand.
Having no need for axial ligands, which are usually use as a co-catalyst in Mn-salen
complexes, is one of the most remarkable features of this catalytic system.
Scheme 1.
2. Experimental
All starting chemicals were commercially available reagents and were used
without further purification. IR spectra were recorded in KBr discs with a Bruker FT-IR
spectrophotometer. UV-Vis solution spectra were recorded using a thermo-spectronic
Helios Alpha spectrometer. The elemental analyses (carbon, hydrogen and nitrogen) of
the compounds were obtained from a Carlo ERBA Model EA 1108 analyzer. The
manganese percentage of the complexes was measured by a Varian spectrometer AA 220
1
equipment. H and 13C NMR spectra were measured with a Bruker Spectrospin
spectrometer at 500 and 125 MHz, respectively. The molecular mass was determined
with an HP-MS model 5973, Network Mass Selective Detector with an electron impact
70 eV and Quadru-pole analyzer. The reaction products of oxidation were determined and
analyzed by an HP Agilent 6890 gas chromatograph equipped with an HP-5 capillary
column (phenyl methyl siloxane 30 µm × 320 µm × 0.25 µm) and gas chromatograph-
4

mass spectrometry (Hewlett-Packard 5973 Series MS-HP gas chromatograph with a
mass-selective detector).
2.1. Synthesis of
butylphenol) (H2bp)
6,6'-(pyridin-2-ylmethylazanediyl)bis(methylene)bis(2,4-di-tert-
For the synthesis of the amine-bis(phenolate) ligand, H2bp, methanol (35 mL) was
added to 2-(aminomethyl)pyridine (5.407 g. 0.05 mol) and the rapidly stirred solution
was cooled to 0 °C, after which aqueous formaldehyde (37%) solution (8 mL, 0.1 mol)
was added dropwise. The mixture was stirred for 15 minutes, then 2,4-di-tert-butylphenol
(20.63 g, 0.1 mol) was added and the mixture was refluxed for 48 h. The resultant
solution was evaporated on a steam bath to 5 mL and cooled to room temperature. The
resulting white precipitate was separated and filtered off, washed with 20 mL of cooled
methanol and re-crystallized from methanol. Yield 85% (23.15 g). M.p. 188-193 °C.
Anal. Calc. for C36H52N2O2 (MW = 544.81): C, 79.36; H, 9.62; N, 5.14. Found: C, 79.38;
H, 9.61; N, 5.17%. FT-IR (KBr, cm-1): 3100 (s, br), 2949 (s, br), 2864 (s), 1597 (vs),
1571 (s), 1509 (s), 1482 (s), 1467 (s), 1435 (s), 1415 (s), 1387 (m), 1362 (m), 1293 (w),
1254 (m), 1221 (s), 1202 (s), 1164 (w), 1154 (w), 1141 (s), 1133 (s), 1120 (w), 1090 (s),
1049 (m), 1041 (w), 1004 (m), 996 (m), 985 (w), 974 (m), 956 (w), 937 (s), 875 (m), 861
(m), 821 (m), 799 (w), 760 (m), 755 (m), 725 (w), 699 (w), 685 (w), 668 (w), 648 (w),
1
625 (w). H NMR (500 MHz, DMSO-d6, 25 °C, TMS, ppm) δ: 1.23 (s, 18 H, CH3 of t-
Bu), 1.35 (s,18 H, CH3 of t-Bu), 3.93 (s, 2 H, Ar-CH2), 3.96 (s, 2 H, Ar-CH2), 4.89 (s,
2H, py-CH2), 6.81 (d, 4J = 2.40 Hz, 2H, aryl), 7.14 (d, 4J = 2.45 Hz, 2 H, aryl), 7.29 (tm,
4
3
3J = 7.50 Hz, 1 H, py), 7.52 (d, J = 7.80 Hz, 1H, py), 7.81 (td, 1 H, py), 8.49 (dd, J =
5

4
5.75 Hz, 2H, J = 1.75 Hz, 1 H, py), 9.04 (s, broad, 2 H, OH). 13C NMR (125 MHz,
DMSO-d6, 25 °C, TMS, ppm) δ: 30.2 (-CH3 of t-Bu), 32.3 (-CH3 of t-Bu), 39.5 (-CMe3
of t-Bu), 39.7 (-CMe3 of t-Bu), 50.4 (Ar-CH2), 57.3 (Ar-CH2), 82.2 (py-CH2), 116.2,
123.1, 124.0, 130.9, 134.6, 136.2, 137.4, 142.1, 147.1, 154.0, 159.2. Mass spectrum [m/z
(%)]: 546 (M+1, 65.9), 453 (M+ - C6H6N-, 80.8), 325 (M+- C15H23O-, 100), 234
(C15H23NO+, 61.7), 219 (C15H23O+, 20.2), 203 (14.9), 189 (4.2), 161 (7.4), 116 (7.4), 107
+
(C6H6N2 , 17), 93 (C6H6N+, 28.7), 57 (t-Bu+, 27.6). UV-Vis (5×10-5 M, CH3OH) λmax, nm
(ε, M-1 cm-1): 228 (39 500), 268 (30 400), 278 (30 200).
2.3. Synthesis of [Mn(bp)(N3)(CH3OH)]·0.5CH3OH (1)
The appropriate amount of the ligand H2bp (0.45 g, 1 mmol) and NaN3 (0.13 g,
2.00 mmol) were dissolved in CH3OH (30 mL) in association with Mn(NO3)2·4H2O
(0.502 g, 2.00 mmol). The solution was gently refluxed for 12 h under an aerobic
atmosphere. After cooling, the resulting solid was filtered off, washed with cooled
absolute ethanol and dried at 60 °C. Suitable single crystals of
[Mn(bp)(N3)(CH3OH)]·0.5CH3OH (1) were grown from a concentrated methanol
solution following slow evaporation at room temperature. Yield 78% (0.54 g). Anal. Calc.
for C37.5H56MnN5O3.5 (MW = 687.81): C, 65.48; H, 8.21; N, 10.18; Mn, 7.99. Found: C,
65.55; H, 8.19; N, 10.15; Mn, 8.03%. FT-IR (KBr, cm-1): 3426 (w, br), 2952 (s), 2925 (s),
2861 (w), 2055 (vs), 1604 (w), 1477 (m), 1469 (m), 1438 (m), 1412 (w), 1387 (w), 1361
(w), 1304 (w), 1262 (m), 1239 (m), 1202 (w), 1169 (w), 1019 (w), 835 (m), 763 (w), 750
(w), 606 (w), 577 (w). Mass spectrum [m/z (%)]: 688 (M+, 2.5), 648 (3.14), 545 (H2bp+,
5.5), 453 (40.94), 325 (C21H29N2O+, 100), 234 (C15H23NO+, 62.99), 203 (26.77), 187
6

+
+
(3.93), 161 (12.59), 133 (8.66), 109 (C6H6N2 , 18.11), 93 (C6H6N+, 59.8), 77 (C6H5 ,
3.93), 57 (t-Bu+, 43.3). UV-Vis (2.5×10-5 M, CH3OH) λmax, nm (ε, M-1 cm-1): 224 (71
000), 259 (52 800), 375 (13 400), 520 (7 300).
2.3. Synthesis of [Mn(bp)(NCS)0.9(NO3)0.1(CH3OH)]·CH3OH (2)
The complex [Mn(bp)(NCS)0.9(NO3)0.1(CH3OH)]·CH3OH (2) was formed by a
similar procedure to that used for complex 1, with azide replaced by potassium
thiocyanate (H2bp, 1 mmol; Mn(NO3)2·4H2O, 2.00 mmol; KSCN, 2.00 mmol). Yield
70% (0.50 g). Anal. Calc. for C38.9H58MnN3O4.3S0.9 (MW = 720.28): C, 64.86; H, 8.12; N,
5.83; Mn, 7.63. Found: C, 64.93; H, 8.10; N, 5.78; Mn, 7.69%. FT-IR (KBr, cm-1): 3431
(m, br), 2954 (s), 2923 (m), 2865 (w), 2068 (vs), 1605 (w), 1477 (w), 1466 (m), 1438
(m), 1411 (w), 1384 (s), 1362 (w), 1261 (m), 1169 (w), 1019 (w), 837 (m), 611 (w). Mass
spectrum [m/z (%)]: 720 (M+, 2.5), 598 (Mnbp+, 4.4), 545 (H2bp+, 10.8), 453 (49), 325
+
(C21H29N2O+, 100), 234 (C15H23NO+, 68.7), 203 (30.5), 161 (12.1), 106 (C6H6N2 , 13.4),
93 (C6H6N+, 45.8), 57 (t-Bu+, 25.5). UV-Vis (2.5×10-5 M, CH3OH): λmax, nm (ε, M-1 cm-
1): 230 (69 600), 256 (61 400), 375broad (31 900), 515 (5 200).
2.4. X-ray crystallography
Data collection for the X-ray structure determinations was performed on an Xcalibur R
four-circle diffractometer with a Ruby CCD detector with graphite monochromatized
MoKα radiation. The data were collected at 100(2) K using an Oxford Cryosystems
cooler. Data collection, cell refinement, data reduction, analysis and absorption
corrections were carried out with the Xcalibur R software, CrysAlisPro [21]. The
7

structures were solved by direct methods with SHELXS [22] and refined by a full-matrix
least-squares technique on F2 using SHELXL-97 [22] with anisotropic thermal
parameters for the non-H atoms (except for some positions of disordered atoms). The H
atoms were found in the difference Fourier maps, but in the final refinement cycles they
were repositioned in their calculated positions and refined using a riding model, with C–
H = 0.95-0.99 Å, and with Uiso(H) = 1.2Ueq(CH, CH2) or 1.5Ueq(CH3), except for
methanol hydroxyl H atoms, which were located in the Fourier maps, refined
isotropically with O–H distances restrained to 0.840(2) Å and then constrained to ride on
their parent atoms (AFIX 3 instruction in SHELXL-97). Ye uncoordinated methanol
molecule and one of the tert-Bu groups in 1 are disordered and refined into two sites with
s.o.f. = 0.604(8) and 0.396(8) for MeOH, and 0.922(6) and 0.078(6) for tert-Bu. Two
different ligands share the same coordination site in the complex molecule of 2: NCS-
-
[additionally disordered into two positions with s.o.f. = 0.54(2) and 0.34(2] and NO3
with s.o.f. = 0.114(4)]. Thus, the crystals of 2 are of a solid solution type – two different
complex molecules may be distinguished in the same, symmetrically equivalent position:
[Mn(bp)(NCS)(CH3OH)]
and
[Mn(bp)(NO3)(CH3OH)],
and
the
formula
[Mn(bp)(NCS)0.9(NO3)0.1(CH3OH)]·CH3OH, accepted in the final model, corresponds to
the existence of these two complexes, randomly occupying the crystallographic sites of
the unit cell in a 0.9:0.1 ratio. In the final refinement cycles, the site occupation factors of
-
the NCS- and NO3 ligands were refined, but the sum of them was restrained to unity.
Additionally, some geometrical restrains (SAME, SADI and FLAT instructions in
SHELXL-97) and constraints on the fractional coordinates and anisotropic displacement
parameters (EXYZ and EADP instructions) were applied in the refinement procedure of
8

the disordered regions in 1 and 2. The structure plots were prepared with DIAMOND
[23]. The crystal data and refinement parameters are presented in Table 1.
2.5. Experimental set up for catalytic oxidations
The liquid phase catalytic oxidations of alcohols were carried out under air
(atmospheric pressure) in a 25 mL round bottom flask equipped with a magnetic stirrer.
In a typical experiment, H2O2 was added to a flask containing the catalyst (1.45 × 10-3
mmol) and benzylalcohol (1 mmol) in a solvent (3 mL). The course of the reaction was
monitored using a gas chromatograph equipped with a capillary column and a flame
ionization detector. The oxidation products were identified by comparing their retention
1
times with those of authentic samples or alternatively by TLC, H NMR and GC–Mass
analyses. Control reactions were carried out in the absence of catalyst, under the same
conditions as the catalytic runs. No products were detected.
Table 1.
3. Results and discussion
3.1. Synthesis of the ligand and Mn complexes
The reaction of 2-(aminomethyl)pyridine with 2,4-di-tert-butylphenol in the
presence formaldehyde gave the desired tetradentate bis(phenolate) ligand, H2bp, in
excellent yield and purity. The complex [Mn(bp)(N3)(CH3OH)]·0.5CH3OH (1) was
synthesized by the reaction of H2bp/Mn(NO3)2·4H2O/NaN3 with molar ratios 1.0:2.0:2.0
in methanol. Complex 2 was synthesized in a similar manner to 1, with KSCN being used
instead of NaN3 and the molar ratios of the materials being constant. The formation of the
9

ligand H2bp has been confirmed by elemental analysis, 1H NMR (Fig. S1), mass (Fig. S2)
1
and infrared spectroscopies. In the H and 13C NMR spectra of the ligand, two CH2
groups (those connected to the phenolic rings) are not equal and appear as two
independent peaks. In the IR spectrum of the ligand (Fig. S5), the bands at 3100, 1597
and 1262 cm-1 are due to O‒H, C=NPy and C‒Ophenolic vibrations, respectively [24]. The
elemental analysis is in good accordance with the proposed structure. Complexes 1 and 2
are slowly formed by the reaction of H2bp with manganese in the presence of the
-
pseudohalide ligand N3 or SCN- as a co-ligand and methanol as the solvent. For both
complexes 1 and 2, during the reaction manganese(II) is oxidized to manganese(III) by
air. The IR spectrum of 1 was analyzed in comparison with that of the free ligand (Fig.
S5). In both complexes, the change in the positions of the peaks in comparison to the free
ligand indicates the coordination of the ligands to the metal core. In the IR spectrum of
the complexes, the broad bands at about 3430 cm-1 are due to the O‒Hmethanol vibration. In
the IR spectrum of complex 1, a strong, well resolved peak at 2055 cm-1 was obtained,
which was assigned to ν(N3). In the IR spectrum of complex 2, an SCN peak is located at
2068 cm-1.
The electronic spectra of complex 1 (purple), complex 2 (purple) and H2bp
(colorless) in methanol and acetonitrile solutions are shown in Fig. 1. The bis(phenolate)
ligand displays strong bands at 228, 268 and 278 nm. Based on their extinction
coefficients, these bands are assigned to intraligand π → π* (228 nm) and n → π* (268,
278 nm) transitions. The shifts of the bands in the complexes relative to the H2bp bands
indicate the coordination of the ligand to the Mn(III) ion. The UV-vis spectra of the
complexes in methanol and acetonitrile solutions are approximately the same, and the
10

small differences in the shapes of the bands is due to solvent effects. For the complexes,
the broad bands at higher energies of ~375 and ~520 nm are due to ligand to metal charge
transfer (LMCT) transitions. The other higher energy intense transitions at about 225 and
260 nm are due to intraligand π → π* and n → π* charge transfer transitions. In these
complexes, the d-d transitions are overlapped with the LMCT transitions and as a result
their peaks are not visible.
Fig. 1.
3.2. X-ray structures of complexes 1 and 2
The molecular structures of complexes [Mn(bp)(N3)(CH3OH)]·0.5CH3OH (1) and
[Mn(bp)(NCS)0.9(NO3)0.1(CH3OH)]·CH3OH (2), determined by X-ray analysis, are
shown in Figs. 2 and 3, respectively, and selected bond lengths and angles are tabulated
in Table 2. In the asymmetric unit of complex 1, two crystallographically independent
molecules (described as 1A and 1B) appear. An overlay of these two molecules shows
that they are almost identical (Fig. S6). In the crystals of complex 2, which are of a solid
solution type (see the Experimental Part), 10% of thiocyanate ligands are replaced by
nitrate anions from the initial metal source, and only the molecule of the composition
[Mn(bp)(NCS)(CH3OH)] will be discussed. X-ray analyses reveal that both 1 and 2 are
neutral mononuclear complexes of Mn(III). The coordination environment around the Mn
centers is best described as a distorted mer-[MnO3N3] octahedral geometry, with
pyridinyl nitrogen, tertiary nitrogen and two oxygen atoms provided by the bis(phenolate)
ligand, an oxygen atom from methanol and a nitrogen atom from the pseudohalide ligand
-
-
(N3 in 1 and SCN-/NO3 in 2).
11

In both complexes 1 and 2, two phenolate donor atoms, provided by the
bis(phenolate) ligand, are located in a trans mode towards each other and their bond
lengths are considerably shorter than the others, which can be attributed to their high σ-
donor ability and their negative charges (see Table 2). Also, two bonds (Mn-Omethanol and
Mn-Npyridine) are considerably elongated with respect to the other four, which is consistent
with the Z-out Jahn-Teller effect, found in most octahedral Mn3+ complexes with HS-d4
electronic configuration [25]. The tertiary nitrogen and two phenolate oxygen atoms,
together with nitrogen atom of the pseudohalide, form the basal plane, and the nitrogen
atom of the pyridinyl moiety of the ligand scorpion-like tail entangles the metal core and
coordinates to the axial position. The remaining axial position in both complexes is
occupied by a methanol molecule. In complex 1, the terminal azide ligand shows
asymmetric N–N distances (N3–N4/N4–N5) of 1.2004(18)/1.1506(19) Å and
1.2063(19)/1.142(2) Å for 1A and 1B, respectively, and the N3–N4–N5 angles are
177.62(15) and 176.56(17)º, which represent the typical shape of the azide ligand [26]. In
complex 2, the Mn1–N3–C37 angle of 167.2(8)° is close to a linear coordination of NCS-
, which is the usual manner for an N-coordinated thiocyanate ligand [27].
Fig. 2.
Fig. 3.
Having both coordinated and uncoordinated methanol molecules in the structures
of 1 and 2, it is not surprising that the crystal packing of these complexes is dominated by
strong and directional hydrogen bonding. The acidic H from the coordinated methanol
molecule forms a hydrogen bond with the oxygen atom of the uncoordinated methanol. In
addition, the hydrogen atom of the uncoordinated methanol is involved in other hydrogen
12

bonding (O–H···O in 1 and O–H···S in 2), and pseudo-dimeric units form by these
interactions, as shown in Fig. 4. Geometric parameters characterizing the intermolecular
interactions are collected in Table S1. In complex 1, the pyridinyl rings of neighboring
molecules from adjacent pseudo-dimeric units interact with each other via π···π stacking,
with a centroid···centroid distance of 3.596(2) Å. In addition, the inter-dimeric contacts
are assisted by C–H···π interactions between pyridinyl C atoms and phenolate rings, as
shown in Fig. S7 [28] (H···centroid = 2.46 and 2.65 Å). Similar inter-dimeric C–H···π
contacts, involving the same rings of the ligand, are also observed in the crystal structure
of complex 2 (Fig. S8).
Fig. 4.
Table 2.
3.3. Catalytic activity studies
3.3.1. Oxidation of benzyl alcohol
The catalytic activity of complexes 1 and 2 were tested in the oxidation of
alcohols. The catalytic oxidation of benzyl alcohol (scheme 2) as a representative
substrate with hydrogen peroxide was studied in the presence of complexes 1 and 2.
Aqueous hydrogen peroxide (30%) was selected as the oxidant by considering its high
selectivity, atom economy and environmentally benign properties. The results of control
experiments revealed that the presence of catalyst and oxidant (H2O2) is essential for the
oxidation. To achieve the maximum oxidation of alcohols, effective parameters for
suitable reaction conditions, such as the oxidant concentration (moles of oxidant per
moles of substrate), solvent and temperature of the reaction, were investigated. The
results of the studies are summarized in Table 3. In all these reactions, benzyl alcohol
13

gave the benzaldehyde with 100% selectivity by catalyst and there was not any other
product. The effect of oxidant concentration on the oxidation of benzyl alcohol by
complex 1 is illustrated in Fig. 5. Different oxidant/substrate molar ratios (1:1, 2:1, 3:1
and 4:1) were considered, while the ratio of benzyl alcohol (1.0 mmol) to catalyst (1.5
µmol, 0.001 g) in 3 mL of acetonitrile was constant. The conversion of benzyl alcohol
increased on increasing the amount of hydrogen peroxide in the reaction mixture. When
the H2O2/substrate molar ratio was 2:1, a maximum conversion of 42% was obtained at
60 1 °C.
Scheme 2.
In order to get the best reaction temperature, the oxidation of alcohols was studied
at 25 (room temperature), 40, 60 and 80 °C, and the results are shown in Fig. 6. At room
temperature the oxidation of benzyl alcohol was very low. Increasing the reaction
temperature from 25 to 60 °C increased the conversion of benzyl alcohol, but a further
increase of temperature to 80 °C lowered the conversion. The influence of the solvent
nature in the catalytic oxidation of benzyl alcohol was tested and the results are presented
in Table 3. Acetone, toluene, ethyl acetate, dichloromethane, chloroform, DMF, DMSO,
THF and acetonitrile were used as solvents. Of the various solvents examined,
acetonitrile was found to be the most suitable solvent for this catalytic reaction. It was
observed that the catalytic activity of the catalyst decreased with respect to acetonitrile
(dielectric constants e = 37.5 [29], donor number D = 14.1 [30]) > acetone (e = 20.7, D =
17) > toluene (e = 2.38, D = 0) > chloroform (e = 4.9, D = 0) > dichloromethane (e =
8.93, D = 0) > THF (e = 7.3, D = 20) > ethyl acetate (e = 6.02, D = 17.1) > DMF (e = 21,
14

D = 26.6) > DMSO (e = 46.7, D = 29.8). Overall, the reactivity of the catalyst in other
solvents was very much lower than in acetonitrile. This may be due to the high relative
dielectric constant and low coordination ability of this solvent. Solvents with a high
donor ability decrease the reactivity of the catalyst, and the lowest conversion in DMSO
is due to the highest coordinating ability of this solvent. High reactivity in toluene is due
to the high solubility of the catalyst in this solvent.
The catalytic activity of complex 2 was examined under the optimized conditions
for catalyst 1 (H2O2/benzyl alcohol molar ratio = 2, acetonitrile, reaction temperature 60
°C). The reactivity of catalyst 2 is better than that of catalyst 1, which may be due to the
difference in the pseudohalide co-ligand.
Fig. 5.
Fig. 6.
Table 3.
3.3.2. Oxidation of various primary alcohols
Finally, in order to explore further the oxidation potential of the these complexes,
the oxidation of various primary alcohols was performed under the same reaction
conditions that proved to be the best for benzyl alcohol, and complex 2 was used as the
catalyst due to its high reactivity in comparison with complex 1. We have studied the
effects of the substituents and nature of the aromatic rings on alcohol oxidation. The
results are shown in Table 4 and indicate this catalytic system led to the oxidation of
different primary alcohols to the corresponding aldehydes with 100% selectivity. It seems
that the efficiency of the oxidation in this catalytic system is dependent on the electronic
and steric demands of the substrate.
15

It was observed that the presence of an electron-donating (–OMe, –OH) group at
the para position of benzyl alcohol enhanced the conversion in comparison with a
molecule having no substituent on the ring (entries 1, 2 and 3). In addition, electron-
withdrawing –Cl and especially –NO2 groups on the phenyl ring decelerated the
oxidation reaction (entries 4 and 5). This behavior is in agreement with the Hammett plot
which has been studied for the oxidation of benzyl alcohols under different conditions in
detail [31]. Moreover, the influence of steric factors in this catalytic system is also
notable. The lower conversion of ortho-substituted alcohols, such as 2-Cl- and 2-NO2-
benzyl alcohol (entries 7 and 8), in comparison with the corresponding non-hindered
alcohols after similar reaction times may be related to the steric hindrance at the vicinity
of the active Mn center. It is worth noting that the oxidation of 2-OH-benzyl alcohol is
the same as 4-OH-benzyl alcohol (entries 2 and 6), and this may be due to the formation
of hydrogen bonding through the O-H phenol group at the ortho position to the alcoholic
group of the substrate, which may help the substrate to approach near to the Mn center.
The conversion of 4-pyridylmethanol and 3-pyridylmethanol is abnormally low. This
may due to the electron withdrawing nature of the pyridine ring together with the high
donor ability of these substrates, which can coordinate to the Mn center through the N-
pyridine group. Perhaps, coordination of the pyridine group to the active site of the
catalyst inactivates the catalyst. This is also in agreement with the low activity of
catalysts in solvents that have a high donor ability (DMSO, DMF). The behavior of
catalyst 1 is similar to 2, so UV-vis spectroscopy was employed to find evidence for
coordination of these substrates to the metals centers (Fig. 7). After addition of these
substrates (3- or 4-pyridylmethanol) to catalyst 1, the band at 515 nm shifted to a higher
16

energy (blue shift), which may due to the coordination of the N-pyridine group to the
Mn(III) center.
Fig. 7.
Table 4.
The interaction of H2O2 with catalyst 1 was monitored in methanol by UV-vis
spectroscopy. Fig. 8 shows the UV-vis spectra of the complex after the addition of 1, 2
and 3 drops of H2O2 to the methanol (Fig. 8a) and acetonirile (Fig. 8b) solutions of 1 (1
drop per 10 minutes). After the addition of one drop of H2O2 to a solution of 1 (3 mL, 2.5
× 10-5 mol L-1), the intensity of the bands between 200 and 270 nm increases and undergo
a blue shift, together with a change in intensity and shape of the bands in the region 300-
520 nm. All this suggests the interaction of the peroxo group with the Mn(III) center
during the catalytic reactions.
Fig. 8.
4. Conclusion
Two new manganese(III) complexes of a tetradentate bis(phenolate) ligand were
synthesized and characterized by spectroscopic and single crystal X-ray analyses. The
catalytic abilities of these complexes were investigated using the environmentally benign
and clean oxidant H2O2 for the oxidation of primary alcohols. The effects of various
parameters, including the molar ratio of oxidant to substrate, the temperature and the
solvent, have been studied and optimized conditions were obtained. These complexes are
selective catalysts for the oxidation of primary alcohols to the corresponding aldehydes.
The influence of the presence of electron-withdrawing and electron-donating groups on
17

the phenyl ring of benzyl alcohol on the reactivity of the catalysts has been investigated.
Excellent conversions were obtained for the oxidation of alcohols containing electron-
donating groups at the para-position.
Acknowledgements
We acknowledge the University of Zanjan for funding this research work.
Appendix A. Supplementary material
CCDC 935112 and 935113 contain the supplementary crystallographic data for
complexes 1 and 2, respectively. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in the online version.
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Schemes/Figures/Tables caption
Scheme 1. a) Structure of the ligand used for the most widely active metal complexes in
selective oxidation reactions; b) The structure of the ligand H2bp used in this study
Scheme 2. Catalytic oxidation of alcohols in the presence of Mn(III) complexes
Fig. 1. The electronic spectra of H2bp, complex 1 and complex 2 in methanol and
acetonitrile
Fig. 2. The molecular structure of one of two crystallographically independent complex
molecules present in 1. Displacement ellipsoids are drawn at the 50% probability level
-
Fig. 3. The molecular structure of one of two complex molecules present in 2 (NO3 and
the disorder of the NCS- ligand are omitted for clarity). Displacement ellipsoids are
drawn at the 30% probability level
21

Fig. 4. Pseudo-dimeric units observed in 1 (left) and 2 (right), formed by methanol-
mediated hydrogen bonds (dashed lines) between complex molecules. Alternative
positions of the disordered MeOH molecule in 1 are shown with different colors. Ligand
H atoms are omitted for clarity. Symmetry code is given in Table S1
Fig. 5. Effect of H2O2 concentration on the oxidation of benzyl alcohol by 1. Reaction
conditions: Catalyst, complex 1, 1.45 µmol (0.001 g); CH3CN, 3 mL; benzyl alcohol, 1
mmol; temperature, 60 1 °C
Fig. 6. Effect of the reaction temperature on the oxidation of benzyl alcohol by 1.
Reaction conditions: Catalyst, complex 1, 1.45 µmol (0.001 g); CH3CN, 3 mL; benzyl
alcohol, 1 mmol; H2O2, 2 mmol
Fig. 7. UV-vis spectra of 1 after the addition of a) 4-pyridylmethanol b) 3-
pyridylmethanol
Fig. 8. UV-vis spectra of 1 after the addition of H2O2 (a) in methanol (b) in acetonitrile
Table 1. Crystallographic data of 1 and 2
Table 2. Selected geometric parameters (Å, °) for 1 and 2
Table 3. Comparison the catalytic activities of catalysts 1 and 2 in the oxidation of
benzyl alcohol with hydrogen peroxide under different conditions
Table 4. Oxidation of various primary alcohols by complex 2 / H2O2 /CH3CN /60 °C
22

Scheme 1.

Scheme 2.

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.