Organo-peroxyl compounds via catalytic oxidation of a hindered phenol and aniline utilizing new manganese(II) bis benzimidazole diamide based complexes

Article abstract of DOI:10.1016/j.ica.2010.06.064

Bis benzimidazole diamide ligand-N,N′-bis(2-methylbenzimidazolyl) propanediamide [GBMA = L] has been synthesized and utilized to prepare new Mn(II) complexes of general composition [Mn(L)X₂]·nH ₂O where X is an exogenous anionic ligand(X = Cl^-, CH ₃COO^-, SCN^-). The geometry of the ligand and its Mn(II) complex have been optimized at the level of UHF, by using ZINDO/1 method. Binding energies, heat of formation and bond lengths of geometry optimized structures for the ligand and complex have been obtained. The oxidation of 2,4,6-tri-tert.-butylphenol (TTBP) and 2,4,6-tri-tert.-butylaniline (TTBA) has been investigated using these Mn(II) complexes as catalyst and TBHP as an alternate source of oxygen. The organo-peroxyl compounds have been isolated and characterized by ¹H NMR, ¹³C NMR, IR and mass data. A different product profile was obtained when H₂O₂ is used as an oxidant.

Full text of DOI:10.1016/j.ica.2010.06.064

Inorganica Chimica Acta 363 (2010) 3477–3488
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Inorganica Chimica Acta
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Organo-peroxyl compounds via catalytic oxidation of a hindered phenol and
aniline utilizing new manganese(II) bis benzimidazole diamide based complexes
*
Ruchi Bakshi, Pavan Mathur
Department of Chemistry, University of Delhi, Delhi, India
a r t i c l e i n f o
a b s t r a c t
Bis benzimidazole diamide ligand-N,N0-bis(2-methylbenzimidazolyl) propanediamide [GBMA = L] has
been synthesized and utilized to prepare new Mn(II) complexes of general composition [Mn(L)X2]ꢀnH2O
where X is an exogenous anionic ligand(X = Clꢁ, CH3COOꢁ, SCNꢁ). The geometry of the ligand and its
Mn(II) complex have been optimized at the level of UHF, by using ZINDO/1 method. Binding energies,
heat of formation and bond lengths of geometry optimized structures for the ligand and complex have
been obtained. The oxidation of 2,4,6-tri-tert.-butylphenol (TTBP) and 2,4,6-tri-tert.-butylaniline (TTBA)
has been investigated using these Mn(II) complexes as catalyst and TBHP as an alternate source of oxy-
gen. The organo-peroxyl compounds have been isolated and characterized by 1H NMR, 13C NMR, IR and
mass data. A different product profile was obtained when H2O2 is used as an oxidant.
Article history:
Received 3 July 2009
Received in revised form 28 June 2010
Accepted 30 June 2010
Available online 13 July 2010
Keywords:
Manganese(II)
Benzimidazole diamides
Hydroperoxides
Tri-tert.-butylphenol
Tri-tert.-butylaniline
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
concentration of the oxidant. Further the difference in the product
profile has been studied by using two different oxidants, while
Mononuclear Mn(II) coordination compounds have been stud-
ied due to their importance in material chemistry, catalysis and
biological chemistry and they are often used for the development
of catalysts especially for oxidation reactions [1]. The reactivity
of such Mn(II) complexes has been correlated with their electronic
properties and hence geometric structure [2–5].
keeping the catalyst fixed. The activity of Mn(II) complexes as cat-
alysts in terms of stability of higher oxidation state has also been
probed.
2. Experimental
Oxidation of phenols is a relatively facile reaction which may be
carried out with various oxidants. However, such procedures are of-
ten non-selective, giving rise to a very complex mixture of products
[6]. The catalytic oxidation of substituted phenols and anilines has
also proved to be a useful procedure for mechanistic studies [6–8].
In earlier attempts the effect of different central atoms (Co, Mn) and
various chelating ligands (schiff bases, porphyrins, oximes) on the
catalytic activity and the product distribution has been investigated
[9]. In this context, several authors have utilized hindered phenols
as substrates for oxidation reactions. Kitajima et al. [10] and Gupta
and Mukherjee [11] have reported both coupled diphenoquinones
and bisphenols using a binuclear copper complex, while Gupta
et al. [12] have reported formation of benzoquinones from oxida-
tion of 2,4,6-tri-tert.-butylphenol, utilizing Cu(II) complexes in
the presence of molecular oxygen.
2.1. Materials and methods
Glycine benzimidazole dihydrochloride was prepared following
the procedure reported by Cescon and Day [13]. Freshly distilled
solvents were employed for all synthesis work. Spectroscopic
grade solvents were employed for spectral experiments. All other
chemicals were of AR grade. 2,4,6-Tri-tert.-butylphenol (TTBP),
and 2,4,6-tri-tert.-butylaniline were procured from Sigma Aldrich.
TTBP was twice recrystallized from CH3OH:H2O mixture. Tert.-
butylhydroperoxide (TBHP) was introduced slowly to minimize
dispropotionation of the peroxide that may be catalyzed by the
manganese complexes. Elemental analyses were obtained from mi-
cro-analytical laboratory of RSIC, Chandigarh, India. Electronic
spectra were recorded on a Shimadzu UV-1601 Spectrophotometer
at the Department of Chemistry, University of Delhi, Delhi, India. IR
spectra were recorded in the solid state as KBr pellets on a Perkin–
Elmer FTIR-2000 Spectrometer (IR stretching frequencies mea-
sured to an accuracy of 1 cmꢁ1). 1H NMR and 13C NMR spectra
were recorded on a 300 MHz Bruker-Spin instrument at the
Department of Chemistry, University of Delhi and at IIT-Delhi,
The present study reports the oxidation of a hindered phenol/
aniline utilizing Mn(II) complexes as catalysts and with low
* Corresponding author.
E-mail address: pavanmat@yahoo.co.in (P. Mathur).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.06.064

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R. Bakshi, P. Mathur / Inorganica Chimica Acta 363 (2010) 3477–3488
Delhi, India. Cyclic Voltammetric measurements were carried out
on a BAS-CV 50W electrochemical analysis system. Cyclic voltam-
mograms of all the complexes were recorded in DMSO solution
with 0.1M TBAP as a supporting electrolyte. A three electrode con-
figuration composed of a Pt-disk working electrode, a Pt wire coun-
ter electrode and an Ag/AgNO3 reference electrode was used for
measurements. The reversible one-electron Fc/Fc+ couple has an
2.2.2.1. MnC19N6O2H18ꢀCl2ꢀ3H2O. Anal. Calc.: C, 42.1H, 4.4; N, 15.5.
Found: C, 42.4; H, 4.3; N, 14.7%. Yield (%): 98 mg (75%), IR (KBr pel-
lets, cmꢁ1): 3495 (
m
m
OH), 3302 (mNH amide), 3187 (mNH benzim), 1636
C–N amideII), 1454 (mC N–C@C– benzim), 742 (mc@c
benzene ring vibration), E = +718 mV.
(mC
@O amideI), 1562 (
@
½
2.2.2.2. MnC19N6O2H18ꢀ(CH3COO)2ꢀ4H2O. Anal. Calc.: C, 45.5; H, 5.2;
E
½
of 0.145 V versus an Ag/AgNO3 electrode. The electrospray mass
N, 13.8. Found: C, 44.6; H, 5.0; N, 14.6%. Yield (%): 91 mg (70%),
spectra were recorded on a MICROMASS QUATTRO II triple quadra-
pole mass spectrometer at CDRI, Lucknow, India. Solid state EPR
spectra were recorded on the X-Band on a Bruker-spectrospin at
IIT-Bombay, Mumbai, India.
IR (KBr pellets, cmꢁ1): 3410 (
m
m
OH), 3273 (
C–N amideII), 1425 (mC
m
NH amide), 3021 (mNH ben-
C– benzim),
zim), 1642 (
738 (
m
C@O amideI), 1560 (
@
@
N–C
m
@
c
benzene ring vibration), 1340 (macetate(symm)), anodic
c
potentials = +500 mV, +750 mV.
2.2.3. [Mn(L)(SCN)2]; L = GBMA
2.2. Synthesis
A concentrated solution of potassium thiocyanate in methanol
was added dropwise to a methanolic solution (5 ml) of MnCl2ꢀ4H2O
(0.3 mmol) till KCl precipitated. This was centrifuged and the cen-
trifugate was added to a methanolic solution (5 ml) of ligand
(0.3 mmol). The mixture was stirred for 1 h when a creamish-
white precipitate was obtained. This was filtered, washed with
methanol and dried under vacuum over anhydrous CaCl2. The com-
plex was analyzed for the composition MnC19N6O2H18ꢀ(SCN)2ꢀ3H2O.
Anal. Calc.: C, 42.9; H, 4.1; N, 19.1. Found: C, 42.0; H, 4.1; N,
2.2.1. N,N0-Bis(2-methylbenzimidazolyl)-propanediamide [GBMA][L]
A solution of glycine benzimidazole dihydrochloride (5.0 g,
22.8 mmol) in pyridine (10 ml) was added to a solution of malonic
acid (1.2 g, 11.4 mmol) in pyridine (5 ml). The mixture was stirred
gently for 10 min, during which a white precipitate appeared. The
reaction mixture was then heated slowly on a water bath at a
temperature of 45 °C and to it triphenylphosphite (5.9 ml,
22.8 mmol) was added dropwise over a period of 15 min. The
reaction mixture was simultaneously stirred. After addition of
P(OPh)3 was complete and the initially formed white precipitate
dissolved, the temperature of the reaction mixture was slowly
raised to 75 °C and the clear solution was stirred for 6–7 h. A
white solid resulted which was filtered off, washed with chloro-
form and neutralized. The crystalline product so obtained was
recrystallized with EtOH/H2O (1:1) mixture and analyzed for com-
position C19N6O2H18ꢀ3H2O
18.9. Yield (%): 95 mg (68%), IR (KBr pellets, cmꢁ1): 3425 (
3299 (mNH amide), 3183 (mNH benzim), 1633 (mC@O amideI), 1526 (mC–N
amideII), 1454 (mC C– benzim), 741 ( c@c benzene ring vibration),
S–C), E = +697 mV.
mOH),
@ @
N–C
m
2041 (mC@N), 849 (m
½
The presence of water molecules was determined by IR and TGA
experiments.
2.2.4. Energy optimized structures of the ligand and the complex
The geometry of the ligand and its Mn(II) complex have been
optimized at the level of UHF (Unrestricted Hartree-Fock) approach
[14,15]. The geometry optimization of the structures was per-
formed by using ZINDO/1 (Zerner Intermediate Neglect of Differen-
tial Overlap) method. The optimization was done successively and
iteratively till the desired precision and consistency was achieved
(the optimizations were obtained by the application of the Steepest
Descent Method followed by Polak-Ribierre algorithm consecu-
tively with a conversion limit of 0.01 kcal molꢁ1 and RMS gradient
of 0.01 kcal molꢁ1/A°). The calculated values of the binding energy
and the other electronic parameters are given under Section 3.
The optimized structures of the ligand and the complex L and
Mn(II)-L (penta coordinated) are given in Fig. 2a and b (Supple-
mentary material), respectively.
Anal. Calc.: C, 54.8; H, 5.8; N, 20.2. Found: C, 53.9; H, 5.0; N,
19.5%. M.p.: 230 °C, Yield: 3.9 g (42%), IR (KBr pellets): 3485
(
m
OH), 3256 (
m
NH amide), 3092 (
m
NH benzim), 1629 (mC O amideI), 1562
benzim), 735 (mc@ benzene ring
c
@
(mC–N amideII), 1444 (mC
@ @
N–C C–
vibration), kmax, nm (log
e
): 274 (4.0), 280 (3.9), 1H NMR (d6-
DMSO): 3.1 (s, 2H), 4.3 (d, 4H, J = 5.4 Hz), 6.9 (m, 4H), 7.2 (d, 2H,
J = 6.8 Hz), 7.3 (d, 2H, J = 6.4 Hz), 8.6 (t, 2H, J = 5.1 Hz), 12.06 (s,
2H) (Fig. 1 – Supplementary material).
H2
C
HN
C
O
C
O
NH
N
CH2
CH2
2.2.5. Oxidation of 2,4,6-tri-tert.-butylphenol (TTBP) and 2,4,6-tri-
tert.-butylaniline (TTBA) using Mn(II) complexes and oxidants H2O2
(hydrogen peroxide) and TBHP (tert.-butylhydroperoxide)
N
NH
HN
A solution of TTBP (381 mM)/TTBA (127 mM) in 3 ml MeOH was
prepared. A suspension of [Mn(L)Cl2] complex (12.7 mM) in MeOH
was added to the above solution followed by addition of H2O2
(381 mM for TTBP and 127 mM for TTBA). The above reaction
was repeated by substituting H2O2 with TBHP as oxidant
(381 mM for TTBP and 127 mM for TTBA). The reaction mixture
was stirred for 48 h on a water bath at a temperature of 45 °C
and was monitored continuously using TLC. 2 spots were observed
at different Rf values. After 48 h, the reaction mixture was worked
up on a short column, eluting with 20% ethyl acetate/hexane. Evap-
oration of the eluent to a small volume and keeping it overnight
gave the desired product. The product with higher Rf could be iso-
lated in sufficient yield (Table 1). Recovery and subsequent usage
of the catalyst [Mn(L)Cl2] was found to give decreased yields of
the products. The product was further characterized by 1H NMR
(Fig. 3a–d), 13C NMR (Fig. 4a–d), mass spectrometry (Fig. 5a–d)
and IR spectral studies. 13C NMR is in reference to similar products
2.2.2. [Mn(L)X2] where L = GBMA; X = Clꢁ, CH3COOꢁ
A methanolic solution (5 ml) of ligand L (0.3 mmol) was added
to the methanolic solution (5 ml) of MnCl2ꢀ4H2O (0.3 mmol). The
resulting light yellow coloured solution was stirred for 2 h. A white
product precipitated which was filtered, washed with small
amount of methanol and dried under vacuum over anhydrous
CaCl2. The compounds were analysed for the following
compositions:

R. Bakshi, P. Mathur / Inorganica Chimica Acta 363 (2010) 3477–3488
3479
Table 1
tively, with no corresponding cathodic wave being observed. This
suggests that the higher oxidation states of Mn(III) and Mn(IV)
are highly oxidising and unstable in nature, in the acetate bound
compound. This may also explain as to why multiple products
were obtained while using [Mn(L)(CH3COO)2] as catalyst for oxida-
tion of a hindered phenol and aniline.
Turnover no. and % yield of the products.
Substrate
Oxidant
Turnover
% Yield
TTBP
TTBA
TTBP
TTBA
H2O2
H2O2
TBHP
TBHP
10
11
5
49.09
56.03
23.45
47.54
10
(i) Oxidation of TTBP in presence of H2O2
reported earlier [16]. The above studies support the formation of
the products identified as ‘A’, ‘B’, ‘C’, ‘D’ as shown below.
O
OH
8
8
7
7
1
The above oxidation reaction was also carried out by using
[Mn(L)(CH3COO)2] as catalyst instead of [Mn(L)Cl2]. Upon employ-
ing [Mn(L)(CH3COO)2] complex as catalyst, it was found that the
product profile obtained was quite complex and column chroma-
tography did not yield the separation of a single product. A com-
parison of the cyclic voltammograms of the two complexes gives
a clue as to why a complex product profile was obtained. It is found
that while a quasi-reversible wave with a E1/2 of +718 mV is ob-
served for the [Mn(L)Cl2] complex, the acetate bound compound
shows two anodic waves observed at 500 and 700 mV, respec-
6
5
Mn(II) complexes
hydrogen peroxide
2
3
4
9
10
OH
[A]
R = 0.8
TTBP - 2,4,6-tri -tert.-butylphenol
1H NMR (d6-DMSO) (ppm): 1.19 (s, 9H), 1.37 (s, 18H), 6.69 (s,
1H),7.09 (s, 2H) (Fig. 3a). 13C NMR (d6-DMSO): 141.6 (C-2, C-6),
Fig. 3a. (i) 1H NMR in d6-DMSO of oxidation product of TTBP with Mn(L)Cl2 and H2O2 (ii) 1H NMR in d6-DMSO of oxidation product of TTBP with Mn(L)Cl2 and H2O2 (D2O
exchange experiment).

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Fig. 3b. 1H NMR in d6-DMSO of oxidation product of TTBA with Mn(L)Cl2 and H2O2.
1H NMR (d6-DMSO) (ppm): 0.93 (s,2H), 1.17–1.19 (m, 9H), 1.24
(s, 9H), 1.38 (s, 18H), 6.67 (s, 2H), 7.09 (s, 2H) (Fig. 3c). 13C NMR
(d6-DMSO): 188.0 (C-1), 141.2 (C-2), 138.2 (C-3), 141.0 (C-4),
121.2 (C-5), 79.0 (C-6), 34.3 (C-7), 31.5 (C-8), 81.9 (C-9), 26.9 (C-
10), 185.9 (C-10), 79.0 (C-20), 121.2 (C-30), 141.0 (C-40), 129.9 (C-
50), 147.7 (C-60), 33.8 (C-70), 30.4 (C-80), 81.9 (C-90), 25.6 (C-100)
(Fig. 4c). ESI MS+ (m/e): 474, 432, 416, 400 (Fig. 5c).
138.7 (C-3, C-5), 121.5 (C-4), 35.1 (C-7), 30.9 (C-8), 34.5 (C-9), 32
(C-10) (Fig. 4a). ESI MS+ (m/e): 278, 262, 247, 222, 221 (Fig. 5a).
IR (cmꢁ1): 3645, 1651, 1362, % yield: 55 (Yields refer to isolated
yield).
(ii) Oxidation of TTBA in presence of H2O2
IR (cmꢁ1): 1667, 1646, 1362, % yield: 42 (Yields refer to isolated
yield).
NH2
NO
8
8
7
7
1
4
2
3
6
5
Mn(II) complexes
hydrogen peroxide
(iv) Oxidation of TTBA in presence of TBHP
NH
9
NH2
10'
8
10
[B]
10
2
12'
6
1
4
7
9
Mn(II) complexes
11
TTBA - 2,4,6-tri.-tert.-butylaniline
O
O
12
3
5
Rf =0.78
OO H
C
7
8
1H NMR (d6-DMSO) (ppm): 1.22 (s, 9H), 1.38 (s, 18H), 7.06
(s,2H) (Fig. 3b). 13C NMR (d6-DMSO): 168.0 (C-1), 142.1 (C-4),
133.0 (C-2, C-6), 121.3 (C-3, C-5), 34.8 (C-7), 30.5 (C-8), 34.3 (C-
9), 32.0 (C-10) (Fig. 4b). ESI MS+ (m/e): 274, 261, 246, 230
(Fig. 5b). IR (cmꢁ1): 1620, 1363, % yield: 75 (Yields refer to isolated
yield).
[D]
TTBA - 2,4,6-tri tertbutylaniline
Rf=0.75
1H NMR (d6-DMSO) (ppm): 1.17 (s, 9H), 1.22 (s, 9H), 1.25–1.27
(m, 9H), 1.33 (s, 9H), 6.61 (s, 1H), 7.06 (s,1H) (Fig. 3d). 13C NMR (d6-
DMSO): 166.8 (C-1), 139.9 (C-2), 133.0 (C-3), 141.5 (C-4), 134.4 (C-
5), 83.3 (C-6), 34.7 (C-7), 30.5 (C-8), 32.0 (C-9), 31.3 (C-10), 31.2 (C-
100), 79.2 (C-11), 26.8 (C-12), 25.9 (C-120) (Fig. 4d). ESI MS+ (m/e):
350, 277, 220 (Fig. 5d).
(iii) Oxidation of TTBP in presence of TBHP
IR (cmꢁ1): 1651, 1374, % yield: 50 (Yields refer to isolated yield).
8
O
1
OH
10
7
OO
9
2
Mn(II) complexes
6
5
3. Results and discussion
3
4
3.1. Description of energy optimized structure
H
H
C
OOH
4'
5'
6'
TB HP
3'
2'
The binding energy for the ligand L has been found to be
7'
1'
ꢁ15044.42 kcal molꢁ1 and that of the Mn(II) complex is
9'
8'
OO
TTBP - 2,4,6-tri-tert.-butylphenol
10'
ꢁ14824.73 kcal molꢁ1
.
The heat of formation (Hf) are
O
[C ]
ꢁ10062.55 kcal molꢁ1 and ꢁ9746 kcal molꢁ1, respectively. In the
Rf=0.6
optimized structure the benzimidazole rings are found to be nearly

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3481
Fig. 3c. 1H NMR in d6-DMSO of oxidation product of TTBP with Mn(L)Cl2 and TBHP and blow ups of relevant position.
perpendicular to one another, possibly minimizing the interactions
between the rings. The bond lengths and IR bands of geometry
optimized structures were compared with the structures for a sim-
ilar diamide ligand and a five coordinated Cu(II) complex reported
earlier [17,18], and it is found that there is a close correspondence,
supporting the structures obtained after geometry optimization
(Table 2a and b – Supplementary material).
Characteristic stretching frequencies for the coordinated anions
are also observed. Bands at 2041 and 849 cmꢁ1 in the thiocyanate
complex are assigned to mC@N and mC–S stretching of the thiocyanate
group. This shows coordination of the thiocyanate group through
the N atom. Similarly, the band at 1340 cmꢁ1 in the acetate com-
plex is assigned to monodentate bound acetate.
3.3. 1H NMR Spectra
3.2. Electronic spectroscopy and IR data
1H NMR spectrum of the free ligand L shows signals for aliphatic
and aromatic protons with theoretically predicted splittings. A sig-
nal is observed at 12.0 ppm corresponding to the N–H benzimid-
azole proton. Multiplets in the range of 7.1–7.8 ppm arise due to
the benzimidazole ring protons characteristic of an AA0BB0 pattern.
The linker –CH2 group give rise to a singlet at 3.1 ppm. The –CH2
group adjacent to the benzimidazole ring gives a doublet at
4.3 ppm due to coupling to the neighbouring amide NH proton
while the –NH between C@O and –CH2 shows a triplet at
8.6 ppm due to coupling to the neighbouring –CH2 protons
[17,18] (Fig. 1 – Supplementary material).
The electronic spectra of the ligand shows two strong bands in
*
the UV region 273–284 nm. These bands are assigned to the
transition characterizing the benzimidazole group.
p–p
The free ligand L has characteristic IR bands at 1629 cmꢁ1
amideI
(m
C@O), 1562 cmꢁ1 amideII
(mC
@
amide), 1444 cmꢁ1
N
(m
C@N@C@N benzimidazole), respectively. On complexation a de-
crease in amideI and increase in amideII has been observed, which
is in accordance with the coordination of the ligand through an
amide carbonyl oxygen [18]. Thus, the manganese complexes have
amide carbonyl coordination rather than amide NH coordination.

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Fig. 3d. 1H NMR in d6-DMSO of oxidation product of TTBA with Mn(L)Cl2/TBHP and blow up of relevant region.
Fig. 4a. 13C NMR in d6-DMSO of oxidation product of TTBP with Mn(L)Cl2 and H2O2.
3.4. EPR Spectroscopy
sitions [19]. While the EPR spectra of [Mn(L)(CH3COO)2] complex
shows the presence of a strong signal in the g = 2.0 region. No other
signal is observed in the 5000 Gauss scan spectra, as would have
been expected for non-cubic Mn(II) – complexes with appreciable
zero field splitting. Such a one line spectrum devoid of any hyper-
fine structure has been interpreted as arising from a Mn(II) com-
plex with a neighbouring atom magnetic interaction [20,21]. The
parameter D has been evaluated following the method of Korteweg
and Reijen [22] (Table 4 – Supplementary material).
X-Band EPR spectra of the Mn(II) complexes were obtained in
the solid state at liquid nitrogen temperatures.
The spectra of [Mn(L)Cl2], [Mn(L)(SCN)2] complexes reveal tran-
sitions in the 5000 Gauss scan. An intense transition is centred at g
ꢂ2.0 while medium to weak transitions are seen at g ꢂ2.3, 4.0 and
4.8. This suggests axially distorted Mn(II) complexes in the above
cases, which are predicted to give five
DMs = 1 fine structure tran-

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3483
Fig. 4b. 13C NMR in d6-DMSO of oxidation product of TTBA with Mn(L)Cl2 and H2O2.
Fig. 4c. 13C NMR in d6-DMSO of oxidation product of (i) TTBP with Mn(L)Cl2 and TBHP (ii) blow ups.

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Fig. 4d. 13C NMR in d6-DMSO of oxidation product of TTBA with Mn(L)Cl2 and TBHP.
Fig. 5a. ESI mass spectra of oxidation product of TTBP + Mn(L)Cl2 + H2O2.
Fig. 5b. ESI mass spectra of oxidation product of TTBA + Mn(L)Cl2 + H2O2.
3.5. Catalytic oxidation of hindered phenol (TTBP) and aniline (TTBA)
line. IR spectra (in KBr) of products display bands in the region of
1640–1670 cmꢁ1, confirming the presence of carbonyl group in
them, and is also in keeping with a cyclohexadienone structure
[23], the only exception is compound B.
Stable oxidation products are generated during the catalytic
oxidation of 2,4,6-tri-tert.-butylphenol and 2,4,6-tri-tert.-butylani-

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3485
Fig. 5c. ESI mass spectra of oxidation product of TTBP + Mn(L)Cl2 + TBHP.
Fig. 5d. ESI mass spectra of oxidation product of TTBA + Mn(L)Cl2 + TBHP.
1H NMR spectrum of the product A in d6-DMSO (Fig. 3ai) shows
a singlet at 6.69 ppm which is assigned to the hydroxyl proton at
para position. This is confirmed by performing a D2O exchange
experiment. Addition of D2O to the sample in d6-DMSO solution
diminishes the peak at 6.69 ppm (Fig. 3aii). The peak at 7.09 ppm
is due to C@CH protons and is assigned to protons associated with
C3–C5 of the cyclohexadienone ring suggesting that the protons at
C3–C5 are chemically equivalent as has been observed for para-
cyclohexadienones [24]. The signals of the tert.-butyl protons are
observed at 1.19 and 1.37 ppm, the relative integration ratios of
the two peaks suggest that one of the tert.-butyl group is in a dif-
ferent chemical environment than the remaining two groups. In
the 13C NMR (Fig. 4a), the signal of C1 (C@O) is expected to be at
ꢂ190 ppm but cannot be seen due to its weakness. The mass spec-
trum shows the following peaks 278 [M] (30%), 262 [Mꢁ(O)] (85%),
247 [Mꢁ(O+CH3)] (80%), 222 [M + HꢁtBu] (100%), 221 [M+ꢁ(tBu)].
The 1H NMR, 13C NMR and the fragmentation pattern observed in
the mass spectra confirm the identity of product A (Scheme 1). For-
mation of such a hydroxy bound product has been proposed earlier
also [6].
Product B is obtained by oxidation of 2,4,6-tri.tert.-butylaniline
with hydrogen peroxide which leads to a nitrosobenzene deriva-
tive. This product does not show a strong band in the IR region
of 1642–1677 cmꢁ1 excluding the possibility of a carbonyl group.
The 1H NMR (Fig. 3b) shows a single peak at 7.06 ppm suggesting
that the protons at C3–C5 are equivalent [24]. The signals of the
protons of tert.-butyl groups are found at 1.22 and 1.38 ppm, the
relative integration ratios of the two peaks suggest that one of
the tert.-butyl group is in a different chemical environment than
the remaining two groups, suggesting a para substituted deriva-
tive. In the 13C NMR the C1 carbon carrying the NO absorbs at
168 ppm which is different from that of a carbon carrying the car-
bonyl group. The mass spectrum (Fig. 5b) shows the following
peaks 274 [MꢁH] (20%), 261 [MꢁMe+H] (100%), 246 [Mꢁ2Me+H]
(80%), 230 [Mꢁ3Me] (60%). The 13C NMR, base peak and other frag-
ments confirm the identity of product B, formation of such a prod-
uct has been postulated also earlier [16,9].
Product C is obtained by reaction of 2,4,6-tri-tert.-butylphenol
with tert.-butylhydroperoxide, this leads to a >C–C< coupled prod-
uct, supported by the mass spectrum that shows peaks at: 474 [M]

3486
R. Bakshi, P. Mathur / Inorganica Chimica Acta 363 (2010) 3477–3488
.
Mn(III) L (OH) + OH
Mn(II) L + HO-OH
.
OH
O
.
OH +
H2O +
.
O
O
O
.
+
Mn(III)L(OH)
.
O
Mn(II)L +
OH
Scheme 1. Oxidation of hindered phenol using H2O2 as an oxidant.
(10%), 432 [Mꢁ(3Me)+3H)] (12%), 416 [Mꢁ(CMe3+H)] (38%) and
400 [Mꢁ(OCMe3+H)] (100%), the higher peak at 498 is assigned
to [M+Na++H]; in its 1H NMR spectra (Fig. 3c) two resonances are
observed at 6.67 and 7.09 ppm assigned to the protons on C30/5
and C3/50 on two ring system and are supported by their integral
ratios. The tert.-butyl groups are observed at 1.17–1.19, 1.24 and
1.38 ppm. The integration ratios of their signals suggest that the
two tert.-butyl groups attached to peroxyl resonate at different
positions, while the remaining two tert.-butyl groups are equiva-
lent and resonate at the same position (blow up of the aliphatic re-
gion between 0.9 and 1.5 ppm has been provided).
The presence of a peak at 0.93 ppm is assigned to two hydrogen
atoms at C4/40. In the 13C NMR (Fig. 4c) two peaks are observed for
C1 and C10 at 188 and 185 ppm confirming the presence of two car-
bonyl groups; the C7/70 carrying the tri methyl groups and C8/80
are found in the range of 33–35.5 and 30–31.0 ppm, respectively.
The signals of C9/90, C20/6 that are associated with peroxyl group
are found at 82 and 79 ppm, implying that these carbons are
widely different from the carbons carrying the regular tri methyl
group. Further it strengthens the presence of two tert.-butyl per-
oxyl groups in the product C [16]. The presence of two sets of car-
bon-peaks in this molecule suggests that the two rings are not
coplanar giving rise to a unsymmetrical structure1. The above data
justifies the C–C coupled product identified as C (Scheme 2).
Product D is obtained by oxidation of 2,4,6-tri-tert.-butylaniline
with TBHP, its mass spectrum supports a cyclohexadiene type
structure with a tert.-butylperoxyl attached at the ortho position:
350
[M+H]
(100%),
277
[MꢁOCMe3+H]
(66%),
220
[Mꢁ(OCMe3+CMe3)+H] (38%). The tert.-butyl groups in 1H NMR
(Fig. 3d) are observed at 1.17, 1.22, 1.25–1.27 and 1.33 ppm imply-
ing inequivalent tert.-butyl groups (blow up of the region 1.0–
1.5 ppm has been provided, indicating the inequivalence of methyl
groups)2. While protons associated with C3/5 are observed at 6.21
and 7.09 ppm suggesting inequivalent C@CH protons. A broad
band at 10.06 ppm is observed because of presence of NH in the
product. This assignment is confirmed by performing a D2O ex-
change experiment. Addition of D2O to the sample in d6-DMSO
solution diminishes the peak at 10.06 ppm, and leaves the remain-
ing peaks unchanged. Thus confirming that the peroxyl group is at
the ortho position. In the 13C NMR (Fig. 4d), peak at 166 ppm is ob-
served for C1. Further peaks observed at 83 and 79 ppm are as-
signed to C4 and C9 carbon and are in keeping with the position
of C9/90and C2/60 found for the product C carrying the peroxyl
groups. However, the signals of C-12 and C-120 are split I contrast
to the 1H NMR signals. 13C NMR data are in keeping with an earlier
report of such a peroxy bound product [16].
3.6. Monitoring of Mn catalyst by EPR Spectroscopy during oxidation
reaction
The catalytic reaction between TTBP, catalyst Mn(L)Cl2 and oxi-
dant TBHP was monitored by time dependent EPR Spectroscopy at
T = 100 K. In the blank experiment, three prominent signals are
1
In the context of product C, it is important to note that bianthrones (bis-tricyclic
compounds with two carbonyl groups) relieve steric interactions by deviating from
coplanarity and generate folded structures. Methyl substitution at 2,20 position
permits the identification of two isomeric forms by 1H NMR studies, and a doubling of
signals have been observed; while substitution at 1,10 position leads to exclusive
formation of one isomeric form [I. Agranat, Y. Taputi, J. Org. Chem. 12 (1979) 1941]. It
is possible that compound C is a mixture of two isomers in a 1:1 ratio that may also
account for the doubling of peaks in NMR. However as we are unable to resolve the
isomeric forms (if they exist) we are proposing C as a unsymmetrical dimer.
2
In the context of product D, it is interesting to note that the signal of two tert.-
butyl groups split into two to three lines, which indicates that the tert.- butyl groups
show hindered rotation. This has also been observed with other tert.-butyl groups at
an sp3 center [L.M. Jackman, S. Sternhell, Application of Nuclear Magnetic Resonance
Spectroscopy in Organic Chemistry, second ed, vol. 5, 1969].

R. Bakshi, P. Mathur / Inorganica Chimica Acta 363 (2010) 3477–3488
3487
.
Mn(III) L (OH) + tBuO
Mn(II) L + tBuOOH
.
OH
O
.
tBuO +
tBuOH +
.
O
O
O
O
.
.
.
.
Mn(II) L + H2O + tBuOO
Mn(III) L (OH) + tBuOOH
O
O
.
O O B u t
.
+
tB u O O
O
OOBut
O
O
OOBut
OOBut
+
H
H
.
.
ButOO
O
Scheme 2. Oxidation of hindered phenol using TBHP as an oxidant.
150
100
50
observed when [Mn(II) complex] was mixed with [TTBP] at g
ꢂ2.00, g ꢂ2.96 and g ꢂ6.00 [trace F] (Fig. 6). On adding [TBHP] it
was found that all the transitions at g = 2.0, g = 2.95 and g = 6.00
undergo a diminishing in intensity and its overall area. A change
of approx. 2.5 times in the area of peak centred at g = 2.95 is ob-
served as the reaction proceeds in a catalytic manner to generate
peroxyl products [trace B and D]. The diminishing in the intensities
of band is indicative of the formation of Mn(III) complex via oxida-
tion of the starting Mn(II) catalyst.
B
D
F
0
Acknowledgements
-50
-100
We gratefully acknowledge financial support from the Defence
Research Development Organization, Directorate of Extramural Re-
search and Intellectual Property Rights, and special research grant
from University of Delhi, Delhi, India.
We are also grateful to reviewers for their highly constructive
suggestions.
0
1000
2000
3000
4000
5000
Appendix A. Supplementary material
Supplementary data (1H NMR of the ligand, geometry opti-
mized structures and tables related to IR stretching frequencies,
bond lengths along with EPR data of complexes) associated with
this article can be found, in the online version, at doi:10.1016/
j.ica.2010.06.064.
Fig. 6. X-Band EPR spectra of Mn(L)Cl2 + TTBP in methanol [F] Mn(L)Cl2 + TTBP +
TBHP (0 min) [B] Mn(L)Cl2 + TTBP + TBHP (60 min) [D] at microwave frequency
9.4 GHz and receiver gain 103.

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R. Bakshi, P. Mathur / Inorganica Chimica Acta 363 (2010) 3477–3488
[11] R. Gupta, R. Mukherjee, Tetrahedron Lett. 41 (2000) 7763.
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