Effective catalytic oxidation of alcohols and alkenes with monomeric versus dimeric manganese(II) catalysts and t-BuOOH

Article abstract of DOI:10.1002/aoc.3495

Two new Mn(II) complexes, [Mn(C₆H₅COO)(H₂O)(phen)₂](ClO₄)(CH₃OH) (1) and [Mn₂(μ-C₆H₅COO)₂(bipy)₄]?2(ClO₄) (2) (phen = 1,10-phenanthroline; bipy = 2,2′-bipyridine), were synthesized and characterized using UV–visible and infrared spectroscopies and single-crystal X-ray diffraction analyses. Complexes 1 and 2 have six-coordinate octahedral geometry around the Mn(II) centre. Complex 1 is a monomer and consists of a deprotonated monodentate benzoate ligand together with two neutral bidentate amine ligands (phen) and a water molecule. Complex 2 has a dinuclear structure in which two Mn(II) ions share two carboxylate groups, adopting a two-atom bridging mode, and two chelated bipy ligands. Both complexes catalyse the oxidation of alcohols and alkenes in a homogeneous catalytic system consisting of the Mn(II) complex and tert-butyl hydroperoxide (TBHP) in acetonitrile. The system yields good to quantitative conversions of various alkenes and alcohols, such as styrene, ethylbenzene and cyclohexene to their corresponding ketones, and primary alcohols and 1-octanol, 1-heptanol, cyclohexanol, benzyl alcohols and cinnamyl alcohol to their corresponding aldehydes and carboxylic acids. Complexes 1 and 2 exhibit very high activity in the oxidation of cyclohexene to cyclohexanone (ca 80% selectivity) as the main product (ca 94% conversion in 1 h) and of cinnamyl alcohol to cinnamaldehyde (ca 64% selectivity) as the main product (ca 100% conversion in 0.5 h) with TBHP at 70°C in acetonitrile. In addition, optimum reaction conditions were also determined for benzyl alcohol with complexes 1 and 2 and TBHP.

Full text of DOI:10.1002/aoc.3495

Full paper
Received: 31 December 2015
Revised: 22 February 2016
Accepted: 22 March 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3495
Effective catalytic oxidation of alcohols and
alkenes with monomeric versus dimeric
manganese(II) catalysts and t-BuOOH
Ibrahim Kani* and Serkan Bolat
Two new Mn(II) complexes, [Mn(C6H5COO)(H2O)(phen)2](ClO4)(CH3OH) (1) and [Mn2(μ-C6H5COO)2(bipy)4]Á2(ClO4) (2) (phen = 1,10-
phenanthroline; bipy = 2,2′-bipyridine), were synthesized and characterized using UV–visible and infrared spectroscopies and
single-crystal X-ray diffraction analyses. Complexes 1 and 2 have six-coordinate octahedral geometry around the Mn(II) centre.
Complex 1 is a monomer and consists of a deprotonated monodentate benzoate ligand together with two neutral bidentate
amine ligands (phen) and a water molecule. Complex 2 has a dinuclear structure in which two Mn(II) ions share two carboxylate
groups, adopting a two-atom bridging mode, and two chelated bipy ligands. Both complexes catalyse the oxidation of alcohols
and alkenes in a homogeneous catalytic system consisting of the Mn(II) complex and tert-butyl hydroperoxide (TBHP) in acetoni-
trile. The system yields good to quantitative conversions of various alkenes and alcohols, such as styrene, ethylbenzene and cy-
clohexene to their corresponding ketones, and primary alcohols and 1-octanol, 1-heptanol, cyclohexanol, benzyl alcohols and
cinnamyl alcohol to their corresponding aldehydes and carboxylic acids. Complexes 1 and 2 exhibit very high activity in the oxi-
dation of cyclohexene to cyclohexanone (ca 80% selectivity) as the main product (ca 94% conversion in 1 h) and of cinnamyl al-
cohol to cinnamaldehyde (ca 64% selectivity) as the main product (ca 100% conversion in 0.5 h) with TBHP at 70°C in acetonitrile.
In addition, optimum reaction conditions were also determined for benzyl alcohol with complexes 1 and 2 and TBHP. Copyright ©
2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site
Keywords: manganese; complex; benzoic acid; catalysis; oxidation
to obtain new Mn(II) complexes with mixed benzoate and nitroge-
nous base ligands as oxidation catalysts for various alcohols and al-
Introduction
Selective liquid-phase catalytic oxidation, in particular the oxidation
of alkenes to their corresponding diols or epoxides or of alcohols to
aldehydes, ketones or carboxylic acids, is an important reaction
usually used in the industrial synthesis of organics.[1] The resultant
carbonyl compounds are used to manufacture a wide range of fine
chemicals and pharmaceuticals. Traditionally, strong oxidants such
kenes. We used benzoic acid and bidentate amines (2,2′-bipyridine
and 1,10-phenanthroline) as terminal ligands to complete the man-
ganese coordination. We synthesized two new complexes formu-
lated as mononuclear [Mn(C6H5COO)(H2O)(phen)2](ClO4)(CH3OH)
(1) and binuclear [Mn2(μ-C6H5COO)2(bipy)4]Á2(ClO4) (2) (phen
=1,10-phenanthroline; bipy =2,2′-bipyridine). The complexes were
characterized using X-ray crystallography and successfully applied
as catalysts in the oxidation of various alcohols and alkenes with
TBHP in a variety of solvents.
[2]
as KMnO4, CrO3, peracetic acid, m-chlorobenzoic acid and HNO3
are used in stoichiometric amounts as the oxidizing agent for
liquid-phase oxidation reactions, which might result in serious pol-
lution and have potential risks. In the last few years, much attention
has been paid to catalytic oxidation reactions in the presence of
various environmentally friendly oxidants, such as hydrogen perox-
ide, tert-butyl hydroperoxide (TBHP) and oxygen.[3] Hydrogen per-
oxide is a very attractive oxidant for industrial applications
because it is inexpensive and mild and water is the only by-product,
which is easily dealt with after reactions.[4]
The oxidation of alcohols catalysed by inorganic–organic hybrid
compounds is of current interest, and various effective catalysts
have been reported, such as those based on molybdenum
and tungsten,[5] ruthenium,[6] cobalt,[7] manganese,[8] iron[9] and
titanium.[10] However, because most metal-containing catalysts
are expensive and may cause environmental pollution, poisonous
reagents are required as solvents for use in time-consuming proce-
dures, making the development of a more efficient and environ-
mentally friendly catalytic system an urgent matter.
Experimental
Materials, instrumentation, physical measurements
All chemicals were purchased from Aldrich and used as received.
Fourier transform infrared (FT-IR) spectra were recorded with a
Jasco FT/IR-300 E spectrophotometer using the KBr pellet tech-
nique in the range 400–4000 cmÀ1. Electronic spectra were
recorded with a Shimadzu UV-2450 spectrophotometer.
*
Correspondence to: Ibrahim Kani, Department of Chemistry, Faculty of Science,
Anadolu University, Eskisehir 26210, Turkey. E-mail: ibrahimkani@anadolu.edu.tr
As a continuation of our recent oxidation studies of various
homogeneous catalysts, the aim of the study reported here was
Department of Chemistry, Faculty of Science, Anadolu University, Eskisehir, 26210,
Turkey
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

I. Kani and S. Bolat
Synthesis of complexes
(30 m × 0.32 mm × 0.25 μm) and a flame ionization detector using
ultrapure nitrogen as the carrier gas (at a rate of 1.0 ml minÀ1). Each
sample was injected twice. Both the injector and detector temper-
atures were 250 °C, and the column temperature was between 100
and 180 °C. The products were confirmed using GC–MS. All catalytic
experiments were performed at least twice.
Synthesis of [Mn(C6H5COO)(H2O)(phen)2](ClO4)(CH3OH) (1)
A solution of manganese(II) perchlorate (Mn(ClO4)2Á6(H2O)) and
H2O (311.8 mg, 1.23 mmol) in methanol (10 ml) was added to a neu-
tralized solution of benzoic acid (150 mg, 1.23 mmol) with NaOH
(2.5 ml, 0.489 M). The reaction mixture was stirred for 1 h at room
temperature, and then a solution of phen (221.6 mg, 1.23 mmol)
in methanol (2 ml) was added to the mixture. The final solution
was refluxed for 5 h. The mixture was filtered over Celite, and the
solvent was evaporated to dryness. The crude product was washed
with diethyl ether and then dried under vacuum. The yellow
product was recrystallized from methanol by ether diffusion. Yield
276.6 mg, 65.5%; m.p. 330 °C. Anal. Found (%): N, 8.64; C, 58.05; H,
3.25. Calcd for C32H27ClMnN4O8 (%): N, 8.17; C, 56.16; H, 3.96. Signif-
icant IR bands (KBr pellet, cmÀ1): 3050 (vs), 1990 (s), 1581 (vs), 1396
(s), 1307 (s), 1083 (vs), 825(s), 694 (s). The complex is soluble in polar
solvents and slightly soluble in CH2Cl2 and CHCl3.
Results and discussion
Synthesis of complexes 1 and 2
The synthesis of the complexes is shown in Scheme 1. X-ray
analysis and spectral data confirm the assigned composition
of the complexes. The electronic spectra of the complexes
were recorded in methanol. The crystallographic data and cor-
responding selected bond lengths and angles of 1 and 2 are
given in Tables 1, 2.
The crystal structure of complex 1 shows a mononuclear cation
with benzoic acid and phen ligands, one perchlorate and a metha-
nol molecule. The molecular structure of 1 is shown in Fig. 1. Within
the complex cation, the Mn atom is coordinated by four N atoms
from two bidentate chelating phen ligands, a carboxylate oxygen
atom from the benzoic acid ligand, and an oxygen atom from a wa-
ter molecule to complete a significantly distorted MnN4O2 octahe-
dral environment with (Mn–N) in the range 2.262(2)–2.278(2) Å
and (Mn–O) of 2.108(1) and 2.162(2) Å. A perchlorate anion and a
methanol molecule make up the outside of the complex
cation. The phen ligands are almost planar (torsion angle
N1C1C9N2 = À0.7(2)° and N3C13C21N4 = 1.3(2)°), and the mean
plane angle between ligands is 74.56°. The cisoid angles range from
73.1(5)° to 103.4(5)°, whereas the transoid angles are 160.9(5)° (O1–
Mn1–N2), 161.2(5)° (N1–Mn1–N4) and 163.2(6)° (N3–Mn1–O8).
An intramolecular hydrogen bond is found between the coordi-
nated carboxylic acid group of benzoic acid moiety and the coordi-
nated water with an O2–O8 distance of 2.612(2) Å. Moreover,
intermolecular hydrogen bonds (O8–O7 = 2.724(2) Å, O7–
O6 = 2.748(3) Å) exist in the uncoordinated methanol, perchlorate
anion and coordinated water molecule.
Synthesis of [Mn2(μ-C6H5COO)2(bipy)4]Á2(ClO4) (2)
The procedure for the synthesis of complex 2 was similar to that of
complex 1. Yield 56.7%; light yellow colour; m.p. 260 °C. Anal. Found
(%): N, 9.31; C, 55.33; H, 3.52. Calcd for C54H42Cl2Mn2N8O12 (%): N,
9.53; C, 55.16; H, 3.60. Significant IR bands (KBr, cmÀ1): 3085 (s),
1997 (s), 1612 (s), 1550 8w), 1461 (s), 1103 (vs), 821 (s), 605 (s). The
complex is soluble in dimethylformamide (DMF), MeOH, EtOH,
(CH3)2CO and MeCN and slightly soluble in CH2Cl2.
X-ray crystallography
Diffraction data for the complexes were collected using a Bruker
AXS APEX CCD diffractometer equipped with a rotation anode at
296(2) K using graphite-monochromated Mo Kα radiation at
λ = 0.71073 Å. Diffraction data were collected over the full sphere
and were corrected for absorption. Unit cell dimensions were deter-
mined by least-squares refinement of the complete data set, and an
empirical absorption correction was made based on psi-scans. The
data reduction was performed with the Bruker SMART[11] program
package. Structure solutions were found with the SHELXS-97[12]
package using direct methods and refined with SHELXL-97[13]
against F2 using first isotropic and later anisotropic thermal param-
eters for all non-hydrogen atoms. Hydrogen atoms were added to
the structural model at calculated positions. Geometric calculations
were performed with PLATON.[14]
General procedure for catalytic oxidation experiments
Notably, only harvested crystals of the complexes were used for the
catalytic activity studies to exclude the effects of impurities. Cata-
lytic oxidation reactions were carried out in 50 ml two-neck flasks
equipped with a reflux water condenser. The whole assembly was
placed in a temperature-controlled oil bath. The reaction mixture
was heated at 70 °C for 6 h. The general procedure for
alcohol/alkene oxidation was as follows. A solution of substrate
and catalyst was purified with nitrogen via bubbling to remove
the dissolved oxygen in the solvent. A solution of oxidant (2 ml,
70%) in solvent (10 ml) was added to a stirred mixture of substrate
(0.2 ml) and catalyst Mn(II) (ca 0.5–1%) at 70 °C. The homogeneous
mixture was oxidized on a magnetic stirring apparatus at a stirring
speed of 1200 rpm. Samples of the products (0.1 ml) were
collected at certain time intervals and analysed using a Thermo
Finnigan Trace GC with an HP-5 quartz capillary column
Scheme 1. Synthesis of complexes 1 and 2.
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Appl. Organometal. Chem. (2016)

Effective catalytic oxidation of alcohols and alkenes with Mn(II) complexes
Table 1. Crystal data and structure refinement parameters of com-
plexes 1 and 2
1
2
Empirical formula
MA (g molÀ1
C
32H27ClMnN4O8
C54H42Cl2Mn2N8O12
1175.74
)
685.97
Refinement method
Full-matrix least-squares on F2
Temperature (K)
Wavelength (Å)
Colour
106(2)
0.71073
Yellow
100(2)
0.71073
Yellow
Crystal system
Space group
a (Å)
Monoclinic
P121/c1
11.6192(3)
16.0932(6)
16.9741(6)
90
Triclinic
P-1
9.0291(3)
11.6096(4)
13.3884(4)
65.727(2)
84.269(2)
87.897(2)
1272.95(7)
1
b (Å)
c (Å)
α (°)
β (°)
γ (°)
108.8220(10)
90
Volume, V (Å3)
Z
3004.26(17)
4
Density (mg mÀ3
)
1.517
1.534
Absorption coefficient
(mmÀ1
0.589
0.675
)
θ range for data
collection (°)
1.85 to 28.45
1.68 to 28.39
Goodness-of-fit on F2
R indices (all data)
0.999
1.060
R1 = 0.0491,
wR2 = 0.0984
0.787 and À
0.456 e AÀ3
R1 = 0.0372,
wR2 = 0.0917
R1 = 0.0784,
wR2 = 0.2030
2.515 and À
0.854 e AÀ3
R1 = 0.0699,
wR2 = 0.1947
Figure 1. Molecular structure of 1.
Largest diff. Peak
and hole
The crystal structure of complex 2 consists of a dinuclear cation,
[Mn2(μ-C6H5COO)2(bipy)4]2+, and two perchlorate anions, as shown
in Fig. 2. It has a centre of symmetry. The carboxylate group of the
benzoic acid ligand acts as the mono-bidentate bridging ligand,
and bridges two Mn atoms in a syn-anti fashion to form a dimer.
The Mn–O bond length is similar (2.136(3) Å) and comparable
to that of related compounds ([Mn2(OAc)2(4-aba)2(2,2′-bpy)2][15]
(average Mn–O: 2.109(8) Å), [Mn(2,2′-bipy)2(2,6-dhba)2][16] (average
Final R indices
[I > 2σ(I)]
Table 2. Selected bond lengths (Å) and angles (°)
Complex 1
Mn(1)–O(8)
2.162(15)
2.107(12)
2.277(15)
86.51(5)
102.84(6)
88.91(6)
163.19(6)
91.29(6)
89.89(5)
160.93(5)
90.35(5)
Mn(1)–N(2)
Mn(1)–N(3)
2.269(16)
2.261(16)
2.268(15)
103.45(5)
73.10(5)
93.66(5)
161.21(5)
99.10(6)
95.14(5)
73.38(5)
Mn(1)–O(1)
Mn(1)–N(1)
Mn(1)–N(4)
O(8)–Mn(1)–O(1)
O(8)–Mn(1)–N(1)
O(8)–Mn(1)–N(2)
O(8)–Mn(1)–N(3)
O(8)–Mn(1)–N(4)
O(1)–Mn(1)–N(1)
O(1)–Mn(1)–N(2)
O(1)–Mn(1)–N(4)
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–N(3)
N(1)–Mn(1)–N(4)
N(2)–Mn(1)–N(3)
N(2)–Mn(1)–N(4)
N(3)–Mn(1)–N(4)
O(1)–Mn(1)–N(3)
Complex 2
Mn(1)–O(2i)
2.127(4)
2.136(3)
Mn(1)–N(1)
Mn(1)–N(2)
2.261(4)
2.280(4)
Mn(1)–O(1)
Mn(1)–N(4)
2.253(4)
Mn(1)–N(3)
2.284(4)
O(2i)–Mn(1)–O(1)
O(2i)–Mn(1)–N(4)
O(1)–Mn(1)–N(4)
O(2i)–Mn(1)–N(1)
O(1)–Mn(1)–N(1)
N(4)–Mn(1)–N(1)
O(2i)–Mn(1)–N(2)
O(1)–Mn(1)–N(2)
103.92(14)
92.48(15)
88.61(14)
98.01(15)
92.57(14)
168.83(15)
168.25(15)
84.04(14)
N(4)–Mn(1)–N(2)
N(1)–Mn(1)–N(2)
O(2i)–Mn(1)–N(3)
O(1)–Mn(1)–N(3)
N(4)–Mn(1)–N(3)
N(1)–Mn(1)–N(3)
N(2)–Mn(1)–N(3)
96.38(15)
72.71(15)
80.83(14)
161.21(14)
72.92(15)
104.88(15)
94.40(14)
Figure 2. Molecular structure of 2 ((i) 2 À x, 1 À y, 2 À z).
Appl. Organometal. Chem. (2016)
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I. Kani and S. Bolat
Table 3. Oxidation of various alkenes with homogeneous complex 1/TBHP system
Substrate
Products
Conv. (%)
Time (h)
1.5
Total conv. (%)
94.3
TON
251
Solvent
Cyclohexene
Cyclohexene oxide
2-Cyclohexene-1-one
2-Cyclohexene-1-ol
1,2-Cyclohexanediol
Styrene oxide
4.6
76.0
2.5
Acetonitrile
11.2
1.0b
25.9
11.7
1.5
Styrene
6
6
40.1
107
DMF
Acetophenone
2-Phenylethanol
Benzaldehyde
Ethylbenzene
Acetophenone
10.0
10.0a
Acetonitrile
a3 ml addition of oxidant after 1 h of reaction increased the conversion to 44.3%, and a further increase in the oxidant (2 ml) increased the conversion to
69.1% after 24 h of reaction.
bAfter 24 h of reaction, total conversion increased to 52.4% (styrene oxide 1.2%, acetophenone 42.7%, 2-phenylethanol 6.4%, benzaldehyde 2.1%).
Reaction conditions: 1.46 × 10À2 mol of oxidant, 7.3 × 10À3 mmol of catalyst (0.8 mol% vs. substrate), substrate/catalyst =266, 70 °C, TON = number of
moles of product per mole of catalyst.
Mn–O: 2.112(2) Å) and [Mn(4-hba)(H2O)(1,10-phen)2](4-hba)
(H2O)[17] (average Mn–O: 2.10(6) Å)). Each manganese ion is further
coordinated by two bipy ligands (average Mn–N: 2.269(2) Å), lead-
ing to a distorted octahedral geometry around the metal ion with
average Mn–O distances that are much longer than the Mn–N dis-
tances (2.524 and 2.269 Å, respectively). All distances agree with
those reported for analogous compounds.[15–17] The Mn–O bond
lengths range from 2.436(2) to 2.691(19) Å, with an average value
of 2.524 Å. The Mn–Mn distance (4.465(8) Å) is significantly longer
than those found in [Mn2(OAc)2(4-aba)2-(2,2′-bipy)2] (4.081 Å) and
[Mn3(4-aba)6]n (3.452(2) Å). The oxygen atoms of the carboxylate
group and perchlorate anions form intermolecular C–HO hydrogen
bonds with the –CH groups of coordinated bipy molecules. The
compound is further stabilized by π–π aromatic stacking interac-
tions between the rings of ligands.
carboxylate ligands coordinated in a bidentate bridging mode
(μ1,3), whereas higher values are expected for compound 1 with a
monodentate coordination, although hydrogen bonds may decrease
this Δ value. Broad bands at ca 1100 cmÀ1 and a moderate intensity
band at 623 cmÀ1 in the spectra of compounds 1 and 2 are assigned
to the perchlorate anions. The phen ligand produces characteristic
bands at ca 1516, 1428, 863, 848 and 727 cmÀ1, and the bipy ligand
produces characteristic bands at ca 1500, 1474 and 1439 cmÀ1
.
The UV–visible spectroscopic data are very similar for both com-
plexes, indicating that each of the Mn(II) complexes behaves as a
high-spin octahedral d5 system (Fig. 10 (SP6)). The absorption bands
in the UV region at ca 235, ca 270 and ca 300 nm are assigned to
ligand-centred π → π* or n → π* transitions in the bipy and phen li-
gands, respectively. Due to the d5 configuration of the Mn2+ ion, no
absorptions are expected in the visible region of the spectrum.
Spectroscopic study of complexes
Catalytic studies
The FT-IR spectra of the compounds contain two strong bands at ca
1563 and ca 1394 cmÀ1 due to the asymmetric and symmetric vibra-
tions of the COO groups (Fig. 9 (SP5)). The values of Δ = νa(COO) À νs
(COO), ca 200 cmÀ1 for compounds 1 and 2, are indicative of
Complexes 1 and 2 are highly soluble in CH3CN. The complexes
produce a light-brown-coloured solution in solvent after dissolu-
tion. The colour is intensified upon addition of the terminal oxi-
dants. When the substrates are added, the colour begins to fade,
Table 4. Oxidation of various alkenes with homogeneous complex 2/TBHP system
Substrate
Products
Conv. (%)
Time (h)
1
Total conv. (%)
92.9
TON
241
Solvent
Cyclohexene
Cyclohexene oxide
2-Cyclohexene-1-one
2-Cyclohexene-1-ol
1,2-cyclohexanediol
Styrene oxide
3.8
75.1
2.8
Acetonitrile
11.8
1.2b
18.4
3.9
Styrene
6
6
25.3
11.0
78
32
DMF
Acetophenone
2-Phenylethanol
Benzaldehyde
Acetophenonea
1.3
Ethylbenzene
11.0
Acetonitrile
a3 ml addition of oxidant after 1 h of reaction increased the conversion to 61.9%, and a further increase in the oxidant (2 ml) increased the conversion to
84.1% after 24 h of reaction.
bAfter 24 h of reaction, total conversion increased to 29.6% (acetophenone 24.2%, 2-phenylethanol 2.7%, others not changed).
Reaction conditions: 1.46 × 10À2 mol of oxidant, 4.3 × 10À3 mmol of catalyst (0.45 mol% vs. substrate), substrate/catalyst =266, 70 °C, TON = number of
moles of product per mole of catalyst.
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Effective catalytic oxidation of alcohols and alkenes with Mn(II) complexes
and, after complete consumption of the substrates, the intensity of
the colour turns back nearly to the original intensity, indicating cat-
alyst regeneration. Mn(ClO4)2 was also examined as a catalyst for
oxidation reactions: no reaction occurred. Furthermore, we exam-
ined the Lewis acid co-catalyst used by others as an additive, i.e.
ascorbic acid and oxalate buffer, trichloroacetic acid and salicylic
acid, and the results are in contrast to those of previous reports:
the additives are not effective in the epoxidation of alkenes
and alcohols.[18] Shul’pin and co-workers reported a dinuclear
Mn(IV) complex that efficiently catalysed various oxidation reac-
tions with H2O2 or t-BuOOH oxidants.[19] The system requires a
carboxylic acid co-catalyst, preferably oxalic acid, to control the
solubility and oxidation state of the Mn species. Our catalytic sys-
tem contains carboxylic acid as a coordinating ligand in the
complex structure, which is probably the reason for the ineffec-
tiveness of the co-catalyst. Moreover, the results of control
experiments reveal that the presence of catalyst and oxidant is
essential for the oxidation.
Mono- (1) and homodinuclear (2) manganese complexes were
tested as catalysts for the oxidation of alcohols (cinnamyl alcohol,
cyclohexanol, 1-heptanol, 1-octanol and benzyl alcohol) to the re-
spective aldehydes and carboxylic acids and of alkenes (cyclohex-
ene, styrene and ethylbenzene) to the corresponding ketones and
alcohols using TBHP as an oxidizing agent. Typically, we dissolved
5 mmol of a substrate in 15 ml of acetonitrile with a catalytic
amount (7.3 × 10À3 mol and/or 4.3 × 10À3 mol) of the Mn(II) com-
plexes. The solution was heated to 70 °C and treated with 2 ml
(9.7 × 10À3 mmol) of the TBHP (70%, aqueous) oxidant.
Oxidation of alkenes
Figure 3. Comparison of complexes (a) 1 and (b) 2 in alkene oxidation.
The olefins (cyclohexene, styrene and ethylbenzene) are practically
active without adding any additives. The system has highest activity
with electron-rich alkenes, i.e. cyclohexene, and relatively little ac-
tivity with the electron-deficient alkenes styrene and ethylbenzene.
Tables 3, 4 and Fig. 3 present the maximum conversion and time re-
quired to obtain the maximum yield for cyclohexene, styrene and
ethylbenzene epoxidation using TBHP as the terminal oxidant in
the presence of two solvents, acetonitrile and DMF (only for sty-
rene). From the obtained data, complexes 1 (94.3%, after 1.5 h)
and 2 (92.9% after 1 h) clearly exhibit the highest efficiency in the
reaction of cyclohexene in acetonitrile (Tables 3, 4).
Complexes 1 and 2 show the highest activity for the oxidation of
cyclohexene in acetonitrile, affording cyclohexanone as a major
product with the formation of small amounts of cylohexenol,
2-cyclohexene-1-one, cyclohexene oxide and 1,2-cyclohexanediol
(due to allylic oxidation) (Tables 3, 4). Cyclohexene has two frag-
ments accessible to attack by the catalytically active species (the
double bond and relatively weak C–H bonds). Comparison of the
rates of epoxidation and C–H bond oxygenation could give valu-
able information on the nature of the oxidizing species. The very
low yield of the epoxidation product indicates that the catalytic
activity of our system proceeds through C–H bond activation. The
oxidation of cyclohexene to the allylic ketone and the very low
epoxide formation strongly imply that the system involves a radical
oxidant via one-electron oxidation. Complex 1 oxidizes cyclohex-
ene to the corresponding epoxide (4.6%), 2-cyclohexen-1-one
(76.0%), 2-cyclohexen-1-ol (2.5%) and 1,2-cyclohexanediol (11.2%)
after 1.5 h of reaction. Complex 2 exhibits very rapid oxidation
under the same conditions (reaction almost completed in 1 h) with
92.9% conversion: cyclohexene oxide (3.8%), 2-cyclohexen-1-one
(75.1%), 2-cyclohexen-1-ol (2.8%) and 1,2-cyclohexanediol (11.8%).
Figure 4. Comparison of complexes (a) 1 and (b) 2 in alcohol oxidation.
Appl. Organometal. Chem. (2016)
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I. Kani and S. Bolat
Table 5. Oxidation of various alcohols with homogeneous complex 1/TBHP system
Substrate
Products
Product distribution (%)
Time (h)
0.5
Total conv. (%)
TON
261
Cinnamyl alcohol
Cinnamaldehyde
30.0
32.8
98.2
Benzaldehyde
Styrene
13.5
3-Phenylglycidol
21.9
Cyclohexanol
Heptanol
Cyclohexanone
43.0
6
6
6
6
43.0a
114
17
,
Heptanal, Heptanoic acid
Octanal, Heptanal, Heptanoic acid
Benzaldehyde, Benzoic acid
4.9;1.3
1.8;4;1;2.7
53.4;3.63
6.2b c
1-Octanol
8.6d
92
,
Benzyl alcohol
57e f
152
a2 ml addition of oxidant after 1 h of reaction increased the conversion to 75.4%.
b2 ml addition of oxidant after 1 h of reaction increased the conversion to 18.7%: heptanal (14.6%), heptanoic acid (4.1%).
cAfter 24 h of reaction, total conversion increased to 36.1% (heptanal 24.1%, heptanoic acid 12.0%).
d2 ml addition of oxidant after 1 h of reaction increased the conversion to 34.7%: heptanal (22.4%), heptanoic acid (6.8%), octanal (0.8%), octanoic acid
(4.7%).
e2 ml addition of oxidant after 1 h of reaction increased the conversion to 76.8%: benzaldehyde (47.3%), benzoic acid (29.5%).
fAfter 24 h of reaction, total conversion increased to 89.8% (benzaldehyde 46.5%, benzoic acid 43.3%).
Reaction conditions: 1.46 × 10À2 mol of oxidant, 7.29 × 10À3 mmol of catalyst (0.8 mol% vs. substrate), substrate/catalyst =266, 70 °C, solvent =
acetonitrile, TON = number of moles of product per mole of catalyst.
The oxidation of styrene was carried out in DMF because styrene
polymerizes in acetonitrile, and was catalysed by complexes 1 and
2 using TBHP as the oxidant to yield the following products: styrene
oxide, benzaldehyde, 2-phenylethanol and acetophenone (Tables 3,
, 4). The efficiency of the mononuclear catalyst (40.1%) in styrene
epoxidation is better than that of the binuclear catalyst (25.3%)
under the same conditions. For both complexes, the main product
is acetophenone (65% selectivity with 1 and 62% selectivity with 2)
in 6 h. Further increasing the reaction time to 24 h and the ratio of
oxidant from 1:4 to 1:16 increases the total conversion of styrene to
69.1 and 29.5% for 1 and 2, respectively.
Ethylbenzene exhibits a low conversion for both complexes (ca
10%) with TBHP in acetonitrile at 70 °C. However, increasing the
TBHP concentration in the reaction mixture considerably increases
the conversion of ethylbenzene due to an increased consumption
of oxidant. The conversion of ethylbenzene to acetophenone with
100% selectivity increases with TBHP, varying from ca 10.0% (at
an ethylbenzene-to-TBHP mole ratio of 1:6) to 44.3% (for 1) and
61.9% (for 2) (at an ethylbenzene-to-TBHP mole ratio of 1:16) and
69.1% (for 1) and 84.1% (for 2) (at an ethylbenzene-to-TBHP mole
ratio of 1:30) without changing the selectivity. This excellent selec-
tivity in the oxidation of ethylbenzene to acetophenone is very
remarkable.
Oxidation of alcohols
Complexes 1 and 2 are effective in the oxidation of primary alco-
hols. A comparison of the time-dependent conversion using 1
and 2 is shown in Fig. 4 and Tables 5, 6. Among the studied alco-
hols, cinnamyl alcohol is the most reactive and results in 100%
conversion in 0.5 h (turnover number (TON) of ca 266) with both
complexes. Notably, primary aliphatic alcohols are inevitably oxi-
dized to form their secondary oxidation products, carboxylic acids,
with low to moderate conversion using a range of reaction condi-
tions, whereas secondary alcohols (cyclohexanol) are oxidized to
only the corresponding ketones (cyclohexanone) (43.0% for
Table 6. Oxidation of various alcohols with homogeneous complex 2/TBHP system
Substrate
Products
Conv. (%)
Time (h)
0.5
Total conv. (%)
100
TON
266
Cinnamyl alcohol
Cinnamyl aldehyde
30.9
Benzaldehyde
45.9
8.8
Styrene
Benzoic acid
7.1
3-Phenylglycidol
Cyclohexanone
9.9
Cyclohexanol
Heptanol
48.1
14.9
6
24
6
48.1a
50.3b
11.3c
44.6d
128
17
Heptanal
Octanalc, Heptanal, Heptanoic acid
1-Octanol
1.3;6.8;3.2
39.2;5.40
92
Benzyl alcohol
Benzaldehyde, Benzoic acid
6
152
a2 ml addition of oxidant after 1 h of reaction increased the conversion to 79.5%.
b2 ml addition of oxidant after 1 h of reaction increased the conversion to 50.3%.
c2 ml addition of oxidant after 1 h of reaction increased the conversion to 53.0% in 24 h of reaction (octanal 1.0%, heptanal 38.0%, heptanoic acid
14.0%).
d2 ml addition of oxidant after 1 h of reaction increased the conversion to 70.6% in 24 h of reaction (benzaldehyde 45.8%, benzoic acid 24.8%).
Reaction conditions: 1.46 × 10À2 mol of oxidant, 4.25 × 10À3 mmol of catalyst (0.45 mol% vs. substrate), substrate/catalyst =266, 70 °C, solvent =
acetonitrile, TON = number of moles of product per mole of catalyst.
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Effective catalytic oxidation of alcohols and alkenes with Mn(II) complexes
complex 1 and 48.1% for complex 2 in acetonitrile in 6 h of reac-
tion) without C–C chain cleavage. This is in agreement with reports
that aliphatic alcohols are less reactive than benzylic substrates.
1-Octanol, a primary aliphatic alcohol, is generally difficult to ox-
idize to its carboxylic acid in lower conversion of 8.6% for 1 and
11.3% for 2 in 6 h. Heptanol is oxidized in also very low yield
(6.2% for 1 and 14.9% for 2 in 6 h) with both complexes (Tables 5,
6). However, adding the oxidant in small portions (at an alcohol-
to-TBHP mole ratio of 1:6 to 1:10 in 6 h of reaction) over time in a
single experiment improves the conversion of alcohols: 43.0%
cyclohexanol to 75.4% cyclohexanone; heptanol to the correspond-
ing aldehyde and acid (total conversion of 6.2 to 18.7%), to
heptaldehyde (4.9 to 14.6%) and to heptanoic acid (1.3 to 4.1%);
1-octanol to the corresponding aldehyde and acid (total conversion
of 8.6 to 34.7%), heptanal (4.1 to 22.4%) and heptanoic acid (2.7 to
6.8%); and benzyl alcohol to the corresponding aldehyde and acid
(total conversion of 57.0 to 76.8), benzaldehyde (53.4 to 47.3%) and
benzoic acid (3.6 to 29.5%) (Table 5, for complex 1). After 24 h reac-
tion, the total conversion increases up to 89.8% for benzyl alcohol
oxidation.
quite different solvents indicates that coordination of the organic
solvent to the manganese complex is essential to the catalytic cycle.
We also assessed the activity of 1 and 2 in the oxidation of benzyl
alcohol with three different oxidants: hydrogen peroxide, TBHP and
molecular oxygen. The results are given in Table 8. The oxidation of
benzyl alcohol with O2 (10 bar) as an oxidant was carried out at
70 °C in a stainless steel autoclave using the same amounts of
CH3CN, TBHP and complex 1 as in the general procedure. During
the reaction, the samples were taken with a special sampling appa-
ratus and analysed by GC after filtration and dilution. The com-
plexes exhibit a unique activity in alcohol oxidation with only
TBHP (TONs of up to 152) and do not show activity with H2O2 or
molecular oxygen, in agreement with the unproductive decompo-
sition of H2O2 with Mn(II) complex under the applied reaction con-
ditions (70 °C).
Furthermore, the effect of the amount of TBHP on the
oxidation of benzyl alcohol as a function of time is illustrated in
Fig. 6 (SP2), and the results are summarized in Table 9. Four differ-
ent oxidant/substrate molar ratios of 4, 7, 11 and 15 were
considered, while maintaining fixed amounts of benzyl alcohol
(7.3 × 10À3 mol) and catalyst (7.29 × 10À6 mol) in 20 ml of CH3CN
at 70 °C. A maximum of 51.9% conversion of benzyl alcohol is ob-
tained at an oxidant/substrate molar ratio of 4 (TON =138). This con-
version improves to 70.6% at an oxidant/substrate molar ratio of 11.
Further increasing the ratio to 15 expedites the conversion to com-
pletion within 6 h, but the overall conversion is not affected very
much (76.8%, TON =204), which may have been due to the dilution
of the reaction mixture by the greater presence of water molecules
in the TBHP solution. It is therefore clear that the TBHP-to-benzyl al-
cohol molar ratio of 15 is optimal for obtaining the maximum ben-
zyl alcohol conversion of 76.8% in 6 h of reaction time.
The same effect is observed for the oxidation of cyclohexanol to
cyclohexanone catalysed by complex 2 without changing the 100%
selectivity: a conversion of 48.1 to 79.5% (at a cyclohexanol-to-TBHP
mole ratio of 1:12 and 1:20) after 6 h of reaction. In addition, 4.9%
heptanol was converted to 50.3%, and 11.3% 1-octanol was con-
verted to 53.0% after 24 h of reaction.
Benzyl alcohol oxidation
To determine the suitable reaction conditions, the oxidation of ben-
zyl alcohol was selected as a model reaction. The conversion of ben-
zyl alcohol is nearly quantitative, with benzaldehyde as the major
product along with benzoic acid as the minor product. First, the
choice of solvent was optimized in the model reaction. Different
solvents such as acetone, acetonitrile, dichloromethane and tetra-
hydrofuran were investigated, and the highest yield of benzalde-
hyde is obtained with acetonitrile for complex 1 (89.8% after
24 h) and acetone for complex 2 (100% after 24 h) (Table 7 and
Fig. 5 (SP1)). The catalytic activity of complex 1 decreases in the or-
der of the relative dielectric constants: acetonitrile (37.5) > acetone
(20.7) > dichloromethane (8.9) > tetrahydrofuran (7.6). In general,
there is good correlation between the solvent polarity and the ben-
zyl alcohol conversion for complex 1 but not for complex 2 in ace-
tone. The highest conversion in acetonitrile is possibly caused by its
high dielectric constant. The fact that activity is observed in several,
Table 8. Oxidant effect on benzyl alcohol oxidation with complexes 1
and 2
Oxidant
Substrate/ Time Total conv.
TON
TOF (hÀ1
)
catalyst
(h)
(%)
1
2
1
2
1
2
t-BuOOH
H2O2
O2
266
266
266
6
6
6
57 44.6 152 119 25 20
0
0
0
0
0
0
0
0
0
0
0
0
Reaction conditions: 70 °C, solvent = acetonitrile, 1.46 × 10À2 mol of
TBHP.
Table 7. Solvent effect on benzyl alcohol oxidation with complexes 1 and 2
Oxidant
Dielectric
T (K)
Time (h)
Total conv. (%)
TON
TOF (hÀ1
)
constant (ε)
1
2
1
2
1
2
Acetonitrile
37.5
20.7
8.93
7.58
70
70
70
70
6
24
6
57
44.6
70.6
57.7
152
119
25
20
26
10
3
89.8
37.2
79.6
17.6
64.2
8.2
Acetone
99
47
22
154
61
17
8
24
6
100
Dichloromethane
Tetrahydrofuran
23
24
6
62.5
6.1
16
4
24
17.4
27.2
Reaction conditions: 1.46 × 10À2 mol of oxidant, substrate/catalyst =266, solvent = acetonitrile, TON = number of moles of product per mole of catalyst,
TOF = number of moles of product/(number of mole of catalyst × time).
Appl. Organometal. Chem. (2016)
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I. Kani and S. Bolat
Table 9. Effect of oxidant addition on benzyl alcohol oxidation with complex 1
Oxidant/substrate
Substrate/catalyst
Time (h)
Product distribution
Total conv. (%)
TON
TOF (hÀ1
)
4
266
266
266
266
6
6
6
6
47.0% benzaldehyde; 4.9% benzoic acid
53.4% benzaldehyde; 3.6% benzoic acid
52.4% benzaldehyde; 18.2% benzoic acid
47.3% benzaldehyde; 29.5% benzoic acid
51.9
57.0
70.6
76.8
138
152
188
204
23
25
31
34
7
11
15
Reaction conditions: 70 °C, solvent = acetonitrile.
acetonitrile with both complexes. In addition, with the optimized
conditions for benzyl alcohol oxidation with complex 1, the maxi-
mum conversion was obtained at 97% (substrate/catalyst =66,
TBHP =1.46 × 10À2 mol, at 70 °C). Moreover, the present catalytic
oxidation system has the following distinct characteristics: (a)
requires only acetonitrile as a solvent without any additives; (b)
employs a very small amount of catalyst; (c) produces a high TBHP
efficiency; and (d) is safe, clean, inexpensive and operationally
simple.
Table 10. Effect of substrate/catalyst ratio on benzyl alcohol oxidation
with complex 1
Oxidant
Substrate/ T (°C) Time (h)
catalyst
Total
conv. (%)
TON
TOF
(hÀ1
)
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
66
166
266
366
70
70
70
70
6
6
6
6
97
64
158
152
166
11
26
25
28
95.1
57
45.4
Reaction conditions: 70 °C, solvent = acetonitrile, benzyl alcohol
=10 mmol, TBHP =14.6 × 10À3 mol.
Acknowledgements
The authors are indebted to Anadolu University and the
Medicinal Plants and Medicine Research Centre of Anadolu
University, Eskişehir, Turkey, for the use of the X-ray diffractometer.
This work was supported by the Scientific Research Fund of
TUBITAK project no. 113Z303. This work was also supported by
the EU COST Action CM1402 “Crystallize”.
The effect of the amount of 1 on the oxidation of benzyl alcohol
was studied considering four different substrate/catalyst molar ra-
tios of 66, 166, 266 and 366, while maintaining the amounts of ben-
zyl alcohol (1.04 g, 10 mmol) and TBHP (14.6 × 10À3 mol, 2 ml) in
20 ml of CH3CN, and the reaction was carried out at 70 °C. As shown
in Fig. 7 (SP3) and Table 10, a maximum conversion of 45.4% is
achieved with a substrate/catalyst ratio of 366, whereas a
substrate/catalyst ratio of 166 gives a maximum conversion of
95.1% in 6 h of reaction time. A further increase in the catalyst
amount (substrate/catalyst ratio of 66) results in little increase in
conversion to 97.0%.
The effect of temperature on benzyl alcohol oxidation with 1 and
2 was investigated with substrate/catalyst ratio = 266 at various
temperatures over the range 50–80 °C (Fig. 8 (SP4)). The conversion
rate is rapid, up to ca 30% conversion in the first minute of reaction,
after which the reaction proceeds slowly with both catalysts at all
temperatures. The reaction at 60 °C is slow and yields only 41 and
36.7% benzaldehyde from benzyl alcohol after 6 h with 1 and 2, re-
spectively. However, the conversion increases markedly at a MeCN
reflux temperature of 80 °C to yield 67.7% benzaldehyde with 1, but
the conversion with 2 does not show significant enhancement
(47.5%).
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