Diphenylmethane oxidation catalyzed by mononuclear Mn(III) complexes

Article abstract of DOI:10.1002/jccs.201190008

Catalytic property of a series of manganese(III) complexes with tetradentate phenol-and pyridinebased ligands on the oxidation of the C-H bond of diphenylmethane in the presence of hydrogen peroxide as an oxidant at ambient conditions were investigated. In this process, diphenylmethane was oxidized to the corresponding keton and alcohol. The catalytic selectivity of the complexes was evaluated.

Full text of DOI:10.1002/jccs.201190008

470
Journal of the Chinese Chemical Society, 2011, 58, 470-473
Diphenylmethane Oxidation Catalyzed by Mononuclear Mn(III) Complexes
Hamid Golchoubian* and Nesa Ghasemi
Department of Physical-Inorganic Chemistry, University of Mazandaran,
Babolsar, Postal Code 47416-95447, Iran
and
Department of Chemistry, Mohammad Reza Hariri Science Foundation Babol,
Postal Code 47146-38474, Iran
Received January 23, 2011; Accepted April 25, 2011; Published Online April 29, 2011
Catalytic property of a series of manganese(III) complexes with tetradentate phenol- and pyridine-
based ligands on the oxidation of the C-H bond of diphenylmethane in the presence of hydrogen peroxide
as an oxidant at ambient conditions were investigated. In this process, diphenylmethane was oxidized to
the corresponding keton and alcohol. The catalytic selectivity of the complexes was evaluated.
Keywords: Diphenylmethane; Oxidation; Hydrogen peroxide; Catalyst; Tetradentate chelate; Se-
lective oxidation; Mn(III) complex.
INTRODUCTION
The catalytic oxidation of the C-H bonds under mild
stances. As a result, if the oxidation reaction can be devel-
oped to function by hydrogen peroxide, it will be of practi-
cal value and contribute to the area of green chemistry.
In continuation of our work on the selective oxidation
of hydrocarbons37-42 recently we developed a catalytic sys-
tem, Mn(III)-salen/H2O2 that can be oxidized alkanes to the
corresponding alcohols and ketons efficiently under mild
conditions.43 This promising system inspired us to investi-
gate, whether an enhance in catalyst activity can be estab-
lished by utilizing Mn(III) complexes of other tetradentate
ligands. Herein, the synthesis and catalytic properties of
several manganese(III) complexes depicted in Scheme I
are reported.
conditions is an important reaction with respect to syn-
thetic and industrial chemistry,1,2 however the catalytic oxi-
dation of the C-H bonds of alkanes is extremely difficult
because of the lack of the reactivity of alkanes. Therefore,
the catalytic oxidation of the C-H bonds remains a chal-
lenging target. A few examples of hydroxylation of alkanes
have already been reported,3,4 however, much of the cur-
rent work in this area focuses on the use of transition-metal
catalyzed processes.5-17 Recent studied have demon-
strated that manganese complexes in combination with
various oxidizing reagents catalyze a range of oxidation
reactions, oxygenation of saturated18-23 and aromatic hy-
drocarbons.24-27
Scheme I Mn(III) complexes used in this study
Although many studies performed on the oxidation of
hydrocarbons by using Mn(III) complexes28-35 and various
peroxide substances as oxidants, there are few which used
H2O2 as a source of oxygen in the C-H oxidation of al-
kanes.36
In recent decades, hydrogen peroxide has provoked
significant interest in synthetic community and demon-
strated to be a promising oxidant in organic synthesis since
the oxidizing agent is economic, environmentally benign
and powerful. In this regard, considerable attempts have
been dedicated to expanding organic oxidation by hydro-
gen peroxide with various intrinsic advantages over reac-
tions in conventional oxidants such as other peroxide sub-

Diphenylmethane Oxidation by Mn(III) Complexes
J. Chin. Chem. Soc., Vol. 58, No. 4, 2011 471
Table 1. Benzyl oxidation of diphenylmethane with 30% H2O2.a
RESULTS AND DISCUSSION
In pervious report,43 we demonstrated that MnL1 was
an efficient catalyst in oxidation of alkanes to the corre-
sponding carbonyl and alcoholic counterparts. It was then
decided to modify the ligand L1 to investigate the activity
of the resultant complexes toward the oxidation of diphen-
ylmethane. In this study, for comparison purpose, the reac-
tion conditions were chosen as the optimum conditions
used for MnL1. As a result, a series of manganese(III) com-
plexes with tetradentate (N2O2 and N4) ligands were pre-
pared and the catalytic activities of the complexes MnL1-
MnL9 were tested in the oxidation of diphenylmethane. The
reactions were carried out in acetonitrile as a solvent and
H2O2 as an oxidizing agent. The large excess of hydrogen
peroxide required was as a result of its decomposition in
the presence of the complexes. The formations of diphen-
ylmethanol and benzophenone products were determined
by standard methods and the results are collected in Table
1. It can be seen that the ligand system has a significant role
on the catalytic properties of the complexes. It also demon-
strate that the most conversion of diphenylmethane per-
tains to MnL5 (89%) and the lowest belongs to MnL7
(19%). It seems that the presence of a 5-membered iminic
chelate plays an important role in catalytic reactivity
(MnL1, MnL5 and MnL6). In terms of selectivity, MnL1 and
MnL3 demonstrated solely the formation of benzophenone.
However, MnL4 and MnL7 exhibited a significant selectiv-
ity toward diphenylmethanol.
PhCH2Ph ® Ph2C=O + Ph2CH2OH
Alcohol
selectivityb
Keton selectivityb
Catalyst
Conversionb
(Yield %)
(Yield %)
L1
L2
L3
L4
L5
L6
L7
L8
L9
69
25
36
26
89
70
19
48
47
100 (69)
79 (17)
100 (36)
15 (7)
0 (-)
21 (8)
0 (-)
85 (18)
28 (52)
27 (52)
88 (8)
49 (23)
64 (32)
72 (37)
73 (18)
12 (11)
51 (25)
36 (15)
a 0.06 mol of catalyst complex, 2 mmol diphenylmethane and 20
mmol H2O2 (30%) in 2 mL of CH3CN was stirred in 25 oC after 4
h. b Determined as isolated yield.
In summary, the results indicate that all the com-
plexes can be used as catalyst for oxidation of diphenyl-
methane as a model for oxidation of hydrocarbons using
hydrogen peroxide with diverse reactivities. Also it was
shown that if alcohol is the goal, then the use of MnL4 and
MnL7 are preferred, and if carbonyl product is the target of
the reaction MnL1 and MnL3 are much desired catalysts.
Although the exact mechanism of this transformation is
still unclear, the catalytic system probably involves the for-
mation of an intermediate oxo-Mn(V) complex (Scheme
II). Moreover, this catalytic oxidation system is clean, safe
and operationally simple.
The spectral changes observed (as shown typically in
Fig. 1) upon addition of H2O2 to the solution of the catalyst
complex MnIIIL1 in acetonitrile, is attributed to the forma-
tion of a reactive LMnV-oxo intermediate. The spectral fea-
tures noted earlier for Mn(V)-oxo complexes are compara-
ble to those observed in the present case.44-46
Proposed catalytic cycle for oxidation of
diphenylmethane
Scheme II
O
Ph-C-Ph
H2O2
LMnIII
2H2O
H2O
OH
LMnv=O
Ph-C-Ph
H
LMnv=O
Fig. 1 Spectral changes for the formation of complex
MnL7 that associated with the formation of
oxo-magnesium(V) species: (A) Absorption
spectrum of LMnIII in acetonitrile; (B) spectrum
of the resultant solution obtained after addition
of H2O2 at room temperature.
PhCH2Ph
LMnIII
H2O2

472 J. Chin. Chem. Soc., Vol. 58, No. 4, 2011
Golchoubian and Ghasemi
EXPERIMENTAL
N, 5.55%. Found: C, 48.01; H, 3.20; N, 5.70%. FT-IR
(KBr): n (cm-1), 3550 (b) n(O–H str.), 1625 (s) n(C=N str.),
1275 (m) n(phenolic C-O str.) and 1100 (s), 625 (m)
n(ClO4). Lm (in CH3CN) = 133 W-1mol-1cm2.
[MnL6(H2O)]ClO4. Yield: 62% as a dark brown solid.
Anal. Calc. for C14H18N4O6ClMn: C, 39.22; H, 4.23; N,
13.07%. Found: C, 39.51; H, 3.94; N, 13.49%. FT-IR
(KBr): n (cm-1), 3500 (b) n(O–H str.), 1643 (s) n(C=N str.),
1110 (s), 630 (m) n(ClO4). Lm (in CH3CN) = 141
W-1mol-1cm2 .
[MnL7(H2O)2]ClO4. Yield: 56% as a green solid.
Anal. Calc. for C14H20N4O6ClMn: C, 39.04; H, 4.68; N,
13.01%. Found: C, 38.88; H, 4.56; N, 12.95%. FT-IR
(KBr): n (cm-1), 3500 (b) n(O–H str.), 3430 (s) n(N–H str.),
1100 (s), 626 (m) n(ClO4). Lm (in CH3CN) = 121 W-1mol-1
cm2 .
2-Hydroxybenzaldehyde, 2-pyridinecarbaldehyde,
ethylenediamine, propylenediamine, o-diaminobenzene,
sodium borohydride and diphenylmethane were purchased
from Merck; Manganese(II) acetate tetrahydrate and aceto-
phenone (99%) were purchased from Aldrich. All other
chemicals and solvents are obtained from Fluka and used
without further purification. The isolated carbonyl and al-
cohol compounds are commercially available and were
identified through comparison of their GLC (retention
time), IR, and NMR (with literature known data) with those
of authentic samples.
Caution! Perchlorate is potentially explosive and
though we have not encountered any problem, neverthe-
less, they should be handled with care.
Conductance measurements were made at 25 °C with
a Jenway 400 conductance meter on 1.00 × 10-3 M samples
in acetonitrile. Infrared spectra (potassium bromide disk)
were recorded using a Bruker FT-IR instrument. The elec-
tronic absorption spectra were measured using a Braic2100
model UV-Vis spectrophotometer. The elemental analyses
were performed on a LECO 600 CHN elemental analyzer.
The manganese(III) complexes of L2–L9 were pre-
pared by a similar procedure used for [MnL1]ClO4.43
[MnL2(H2O)2]ClO4. Yield: 66% as a crusty dark
brown solid. Anal. Calc. for C18H26N2O8ClMn: C, 44.23;
H, 5.36; N, 5.37%. Found: C, 43.94; H, 5.56; N, 5.49%.
FT-IR (KBr): n (cm-1), 3534 (b) n(O–H str.), 3490 (s)
n(N–H str.), 1625 (b) n(O–H str.), 1275, 1286 (m) n(pheno-
lic C-O str.), 1100 (s), 620 (m) n(ClO4). Lm (in CH3CN) =
153 W-1mol-1cm2 (standard value for 1:1 electrolyte type in
CH3CN is in the range of 120-150 W-1mol-1cm2).
[MnL3(H2O)2]ClO4. Yield: 62% as a dark brown
solid. Anal. Calc. for C17H20N2O8ClMn: C, 43.37; H, 4.28;
N, 5.95%. Found: C, 42.92; H, 4.44; N, 6.19%. FT-IR
(KBr): n (cm-1), 3527 (b) n(O–H str.), 1638 (s) n(C=N str.),
1270, 1282 (m) n(phenolic C-O str.), 1105 (s), 630 (m)
n(ClO4). Lm (in CH3CN) = 137 W-1mol-1cm2.
[MnL4(H2O)2]ClO4×CH3OH. Yield: 55% as a brown
solid. Anal. Calc. for C18H26N2O9ClMn: C, 42.83; H, 5.19;
N, 5.55%. Found: C, 42.50; H, 5.61; N, 5.69%. FT-IR
(KBr): n (cm-1), 3535 (b) n(O–H str.), 3474 (s) n(N–H str.),
1620 (b) n(O–H str.), 1273 (m) n(phenolic C-O str.) and
1100 (s), 630 (m) n(ClO4). Lm (in CH3CN) = 143 W-1mol-1
cm2.
[MnL8(H2O)2]ClO4×CH3CN. Yield: 66% as a dark
green solid. Anal. Calc. for C17H23N5O6ClMn: C, 42.21; H,
4.79; N, 14.48%. Found: C, 42.56; H, 4.30; N, 14.78%.
FT-IR (KBr): n (cm-1), 3540 (b) n(O–H str.), 2253 (s)
n(CºN str.), 1645 (s) n(C=N str.), 1100 (s), 630 (m)
n(ClO4). Lm (in CH3CN) = 138 W-1mol-1cm2.
[MnL9(H2O)2]ClO4. Yield: 53% as a green solid.
Anal. Calc. for C15H22N4O6ClMn: C, 40.51; H, 4.99; N,
12.60%. Found: C, 40.88; H, 4.61; N, 12.45%. FT-IR
(KBr): n (cm-1), 3500 (b) n(O–H str.), 3408 (s) n(N–H str.),
1100 (s), 626 (m) n(ClO4). Lm (in CH3CN) = 129 W-1mol-1
cm2.
General procedure for oxidation of diphenylmethane
0.06 mmol of the catalyst complex, 2 mmol diphenyl-
methane and 20 mmol hydrogen peroxide (30%) in 2 mL of
acetonitrile was stirred at the ambient temperature for 4 h.
The progress of the reaction was followed by TLC (solvent,
n-hexane : ethyl acetate 9:2 v/v), After the completion of
the reaction 50 µL dimethyl sulfide was added to the result-
ing mixture. The aqueous acetonitrile solution was then re-
moved on a rotary evaporator and the residue was treated
with 15 mL diethyl ether. The organic solution was washed
with saturated brine. Drying (Na2SO4) and evaporation of
the solvent on a rotary evaporator afforded a residue which
was passed through a short pad of silica gel or a silica gel
plate using hexane and ethyl acetate (9:2 v/v) as eluent to
result the analytical pure products.
ACKNOWLEDGEMENTS
[MnL5(H2O)2]ClO4×CH3OH. Yield: 70% as a black
We are grateful for the financial support of Mazandaran
University and Mohammad Reza Hariri Foundation of the
solid. Anal. Calc. for C20H18N2O8ClMn: C, 47.59; H, 3.59;

Diphenylmethane Oxidation by Mn(III) Complexes
Islamic Republic of Iran.
J. Chin. Chem. Soc., Vol. 58, No. 4, 2011 473
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