Structure, magnetism and reactivity of a {MnIII(μ-O)2MnIV}3+ core towards oxidation of phenols

Article abstract of DOI:10.1016/j.poly.2019.06.032

Two complexes [(L)₂Mn^II(OClO₃)(CH₃CN)](ClO₄)·CH₃CN (1) and [(L)₄Mn^III/IV ₂(μ-O)₂](ClO₄)₃·2CH₃CN·Et₂O (2), where L is 2-((1H-pyrazol-1-yl)methyl)-1-methyl-1H-benzimidazole, have been synthesized, structurally characterized, UV–Vis and EPR spectral properties, variable-temperature (2–300 K) magnetic susceptibility and redox behavior investigated. Structural analysis reveals that the {Mn^III(μ-O)₂Mn^IV}³⁺ core in 2 is an example of delocalized class III system. Complex 1 has S = 5/2 ground-state. The Mn^III and Mn^IV centres in 2 are antiferromagnetically coupled (J = ?192 cm^?1) and has S = 1/2 ground-state. Complex 2 exhibits characteristic 16-line EPR spectrum (120 K) centered at g ≈ 2. Reactivity of 2 towards p-X-2-tert-butylphenols (X = H, Me, OMe, ^tBu) has been investigated. The oxidation of phenols generate phenoxyl radical (2,4,6-tri-tert-butylphenol exhibits EPR signal due to radical)/radical-coupled bis-phenol product (in the case of 2,4-di-tert-butylphenol, DTBP). Kinetic experiments have allowed us to evaluate second-order rate constant values for the faster initial step (k₂ × 10^?2 M^?1 s^?1) for p-X-2-tert-butylphenols: 0.6 (X = H), 1.95 (Me), 3.0 (OMe), 1.85 (^tBu) and for the slower second step (k₂ × 10^?3 M^?1 s^?1) for p-X-2-tert-butylphenols: 0.9 (X = H), 2.5 (Me), 4.0 (OMe). Reaction between {Mn^III(μ-O)₂Mn^IV}³⁺ of 2 and phenols proceeds via hydrogen atom transfer mechanism to produce oxidation products. Initial reaction supposedly generates {Mn^III(O)(OH)Mn^III}³⁺ species, which finally ends up as (L)-coordinated Mn^II species (ESI-MS spectrum for the reaction between 2 and DTBP).

Full text of DOI:10.1016/j.poly.2019.06.032

Accepted Manuscript
III
IV 3+
Structure, magnetism and reactivity of a {Mn (μ–O) Mn } core towards ox-
2
idation of phenols
Akhilesh Kumar, Arunava Sengupta, Serhiy Demeshko, Rabindranath
Mukherjee
PII:
S0277-5387(19)30443-7
https://doi.org/10.1016/j.poly.2019.06.032
POLY 14023
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
3 February 2019
11 June 2019
Please cite this article as: A. Kumar, A. Sengupta, S. Demeshko, R. Mukherjee, Structure, magnetism and reactivity
III
IV 3+
of a {Mn (μ–O) Mn } core towards oxidation of phenols, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.
2
2
019.06.032
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Ms. Ref. No.: POLY-D-19-00126 (R1)
Graphical entry
Systematic kinetic studies have been
Structure, magnetism and reactivity of a made for the reaction between bis(–
III
IV 3+
{
Mn (–O) Mn } core towards oxidation oxo)dimanganese(III,IV) and p-X-2-tert-
2
t
of phenols
butylphenols (X = H, Me, OMe, Bu); the
oxidation of phenols generate phenoxyl
radical/radical-coupled product through
hydrogen-atom transfer reaction.
Akhilesh Kumar, Arunava Sengupta, Serhiy
Demeshko and Rabindranath Mukherjee
*
1

Ms. Ref. No.: POLY-D-19-00126 (R1)
III
IV 3+
Structure, magnetism and reactivity of a {Mn (–O) Mn } core
2
towards oxidation of phenols
a
a, b
c
Akhilesh Kumar , Arunava Sengupta , Serhiy Demeshko and
a,*
Rabindranath Mukherjee
a
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016,
India
b
Department of Chemistry, Techno India University, EM-4, Salt Lake City, Sector V,
Kolkata, West Bengal 700 091
c
ꢀ
ꢁꢂꢃꢂꢄꢂꢅꢆꢇꢈꢆ ꢀꢇꢉꢊꢋꢀꢃꢌꢆꢍꢎꢅꢏꢃꢁꢂꢉꢐꢑꢆꢒꢅꢇꢉꢊ-ꢓꢄꢊꢄꢁꢂ-ꢔꢀꢃꢕꢅꢉꢁꢃꢂꢐꢆꢒꢖttingen, Tammannstrasse
, D-37077 Göttingen, Germany
----------------------------------------------------------------------------------------------------------
4
-
ABSTRACT
Two complexes [(L) Mn (OClO )(CH CN)](ClO )•CH CN (1) and [(L) Mn 2(µ–
II
III/IV
2
3
3
4
3
4
O) ](ClO ) •2CH CN•Et O (2), where L is 2-((1H-pyrazol-1-yl)methyl)-1-methyl-1H-
2
4 3
3
2
benzimidazole, have been synthesized, structurally characterized, UV-vis and EPR
spectral properties, variable-temperature (2-300 K) magnetic susceptibility and redox
III
IV 3+
behavior investigated. Structural analysis reveals that the {Mn (µ–O) Mn } core in 2
2
is an example of delocalized class III system. Complex 1 has S = 5/2 ground-state. The
III
IV
–1
Mn and Mn centres in 2 are antiferromagnetically coupled (J = –192 cm ) and has S =
1
/2 ground-state. Complex 2 exhibits characteristic 16-line EPR spectrum (120 K)
t
centered at g  2. Reactivity of 2 towards p-X-2-tert-butylphenols (X = H, Me, OMe, Bu)
has been investigated. The oxidation of phenols generate phenoxyl radical (2,4,6-tri-tert-
butylphenol exhibits EPR signal due to radical)/radical-coupled bis-phenol product (in the
case of 2,4-di-tert-butylphenol, DTBP). Kinetic experiments have allowed us to evaluate
–
2
–1 –1
second-order rate constant values for the faster initial step (k x 10 M s ) for p-X-2-
2
2

t
tert-butylphenols: 0.6 (X = H), 1.95 (Me), 3.0 (OMe), 1.85 ( Bu) and for the slower
–
3
–1 –1
second step (k x 10 M s ) for p-X-2-tert-butylphenols: 0.9 (X = H), 2.5 (Me), 4.0
2
III
IV 3+
(
OMe). Reaction between {Mn (–O) Mn } of 2 and phenols proceeds via hydrogen
2
atom transfer mechanism to produce oxidation products. Initial reaction supposedly
III
III 3+
II
generates {Mn (O)(OH)Mn } species, which finally ends up as (L)-coordinated Mn
species (ESI-MS spectrum for the reaction between 2 and DTBP).
-----------------------------------------------------------------------------------------------------------
Keywords:
II
III,IV
Mn and Mn 2(µO)2
Crystal structures
Magnetism
Phenol oxidation
Kinetics
Hydrogen atom transfer
-----------------------------------------------------------------------------------------------------------
_
*
______
Corresponding authors. Fax: +91 512 259 7436 (R. Mukherjee).
E-mail address: rnm@iitk.ac.in (R. Mukherjee).
3

1
. Introduction
Di-/tetra-nuclear nonheme metal-oxo species are involved as key intermediates in
n+
enzymatic reactions [1-3]. In biomimetic studies, a number of high-valent [M (μ–O) ]
2
2
cores have attracted much attention for systems involving Mn [4,5], Fe [6], Co [7], Ni
7,8a] and Cu [8b,9] centres. These cores, supported by a variety of primarily N-donor
non-porphyrin ligands, have been synthesized, characterized and investigated in reactivity
[
2
+/3+/4+
III
2+
studies. In particular, [M (μ–O) ]
(M = Mn, Fe) [3-6] and [Cu (μ–O) ] [8b,9]
2 2
2
2
cores have been the subject of extensive investigations in recent times due to their
involvement or possible involvement in biological oxidation reactions. Recently,
III
attention has also been directed to particularly on the reactions of phenols with [Cu (μ–
III 2+
O) Ni ] cores [10].
2
Herein we report on the synthesis, characterization and reactivity towards phenols of
III/IV
[
[
(L) Mn
(µ–O) ](ClO ) •2CH CN•Et O (2), via chemical oxidation of isolated
2 2 4 3 3 2
4
II
(L) Mn (OClO )(CH CN)](ClO )•CH CN (1) (L = 2-((1H-pyrazol-1-yl)methyl)-1-
2
3
3
4
3
methyl-1H-benzimidazole). Both 1 and 2 have been structurally and magnetically
characterized. Absorption, ESI-MS, EPR (for 2) and redox properties have also been
investigated.
2
. Experimental
.1. General
2
All chemicals and reagents were obtained from commercial sources and used as received.
Solvents were purified/dried following standard procedures [11,12]. Tetra-n-
butylammonium perchlorate were synthesized as before [13].
4

2
.2. Syntheses
2
.2.1 Synthesis of 2-((1H-pyrazol-1-yl)methyl)-1-methyl-1H-benzimidazole (L).
The synthetic strategy comprises the following two steps.
Step 1. Synthesis of 2-(chloromethyl)-1-methyl-1H-benzimidazole [14]
N-Methylbenzne-1,2-diamine dihydrochloride (1.5 g, 7.68 mmol) and chloroacetic acid
(1.08 g, 11.52 mmol) were refluxed in 6M HCl (16 mL) for 8 h. After cooling to room
temperature and then in ice-bath, the mixture was neutralized by dropwise addition of
concentrated ammonia solution. The precipitated product was collected by filtration and
washed with cold water (5 mL). The crude product was subjected to column
chromatography on alumina (neutral) using CHCl as eluent. The volume of the solution
3
was reduced to half of its original volume, under reduced pressure. The compound was
recrystallized from n-hexane at 298 K, followed by slow evaporation in the air.
Yellowish-solid that precipitated was collected by filtration and dried in air. Yield: 1.54
g, 70%. Anal. Calc. for C H ClN (f.w. 180.63): C, 59.84; H, 5.02; N, 15.51; Found: C,
9
9
2
+
1
6
0.08; H, 4.87; N, 15.46%. ESI-MS (m/z): 181.04 (M + H ). H NMR (CDCl 400 MHz):
3,
δ 7.74 (d, J = 7.4 Hz, 1H), 7.36–7.24 (m, 4H), 4.82 (s, 2H), 3.84 (s, 3H).
Step 2. Synthesis of 2-((1H-pyrazol-1-yl)methyl)-1-methyl-1H-benzimidazole (L).
A mixture of 2-(chloromethyl)-1-methyl-1H-benzimidazole (0.5 g, 2.77 mmol) and
pyrazole (0.2 g, 2.77 mmol), aqueous NaOH solution (4.2 mL, 40%), benzene (35 mL)
and 40% aqueous tetra-n-butylammonium hydroxide (3 drops) was refluxed with stirring
for 8 h. The organic layer was then separated, washed twice with brine water, dried over
anhydrous Na SO and filtered. The solvent was evaporated under reduced pressure to
2
4
obtain a yellowish-solid. Yield: 0.425 g, 72%. Anal. Calc. for C H N (f.w. 212.25): C,
1
2
12
4
5

6
7.90; H, 5.70; N, 26.40; Found: C, 67.74; H, 5.64; N, 25.96%. ESI-MS (m/z): 213.09 (M
+
1
+
H ). H NMR (CDCl 400 MHz): δ 7.75 (dd, J = 6.2, 1.7 Hz, 1H), 7.53–7.48 (m, 2H),
3,
7
.33 – 7.25 (m, 3H), 6.26 (t, J = 2.1 Hz, 1H), 5.63 (s, 2H), 3.76 (s, 3H).
II
2
.2.2. [(L) Mn (OClO )(CH CN)](ClO )•CH CN (1)
2
3
3
4
3
II
Solid [Mn (H O) ](ClO ) (0.43 g, 0.117 mmol) was added to a stirred CH CN
2
6
4 2
3
solution (5 mL) of L (0.5 g, 0.235 mmol). The pale yellow solution was stirred for 4 h.
Volume of the solution was reduced down to ~2 mL. Diethyl ether was added to it and
kept inside a fridge. After 4h, a colorless precipitate that formed was collected by
filtration on a frit, washed with diethyl ether and dried in vacuo. Yield: 0.7 g, 78%.
Single-crystals suitable for X-ray crystallographic studies were grown by vapor-diffusion
of diethyl ether into a solution of the complex in CH CN. Anal. Calc. for
3
C H Cl MnN O (f.w. 760.46): C, 44.22; H, 3.98; N, 18.42; Found: C, 43.89; H, 3.88;
2
8
30
2
10
8
–
1
–
N, 18.34%. FT-IR (KBr pellet, selected peaks, cm ): 1146, 1120, 1086 ν(ClO ), 623
4
–
ν(ClO ).
4
III/IV
2
.2.3. [(L) Mn
(µ–O) ](ClO ) •2CH ꢍN•Eꢂ O (2)
4
2
2
4 3
3
2
To a solution of 1 (0.1 g, 0.13 mmol) in CH CN (10 mL) at 298 K, a solution of 70%
3
t
BuOOH (170 µL, 1.32 mmol) in CH CN (2 mL) was added dropwise, under magnetic
3
stirring. The colorless solution changed to green within 1 h. It was stirred for a further 2
o
h, resulting in a greenish solution. The solvent was reduced at 30 C, using a rotary
o
evaporator. Diethyl ether was added to ensure precipitation and kept in a fridge (–10 C)
6

for 3 h. The green precipitate that formed was collected by filtration, washed thoroughly
with diethyl ether and finally dried in vacuo. A green solid of the desired product was
obtained. Yield: 0.055 g, 65%. Single-crystals suitable for X-ray crystallographic studies
were grown by vapor-diffusion of diethyl ether into a solution of the complex in CH CN.
3
Anal. Calc. for C H Cl Mn N O (f.w. 1445.45): C, 46.53; H, 4.43; N, 17.44; Found:
5
6
64
3
2
18 15
–
1
C, 46.03; H, 4.09; N, 17.47%. FT-IR (KBr pellet, selected peaks, cm ): 1096 and 625
–
ν(ClO ); 750 (ν(Mn–O–Mn)); 690 (ν(Mn O )).
4
2
2
Caution! Perchlorate salts of compounds containing organic ligands are potentially
explosive; therefore, preparation and handling of these salts deserve special care!
2
.3. Physical Measurements
Elemental analyses (C, H, N) were obtained using Thermo Quest EA 1110 CHNS-O,
Italy. Spectroscopic measurements were made using the following instruments: FT-IR
–
1
(
KBr, 4000–500 cm ): Bruker Vector 22; Electronic: Agilent 8453 diode-array
spectrophotometer; HR-ESI-MS: Waters-HAB213 spectrometer; X-band EPR: Bruker
EMX 1444 spectrometer. The EPR spectra were calibrated with diphenylpicrylhydrazyl,
DPPH (g = 2.0037). Simulation of the EPR spectra was performed with WINEPR
1
SimFonia program. H NMR spectra were obtained either on JEOL JNM LA 500 (500
MHz) or JEOL JNM LA 400 (400 MHz) NMR spectrometer. Chemical shifts are
reported in ppm referenced to TMS. The kinetic experiments were carried out
spectrophotometrically, using an Agilent 8453 diode-array spectrophotometer, equipped
with 89090A temperature-controller.
7

2
.3.1. Magnetic Measurements
Variable-temperature magnetic susceptibility measurements in the solid state were
performed in the temperature range of 2–300 K with a Quantum-Design MPMS-XL-5
SQUID magnetometer, equipped with 5 T magnet (Göttingen, Germany). Solid samples
were placed in a gel capsule (for 1) or Teflon bucket (for 2) and measurements were
done at 0.5 T. Each raw data file for the measured magnetic moment was corrected for the
diamagnetic contribution of the sample holder. The molar susceptibility data were
corrected for the diamagnetic contribution.
2
.3.2. Electrochemistry
Cyclic voltammetric measurements at 298 K were performed by using CH Instruments
Electrochemical Analyzer/Workstation Model 600B Series. A standard three-electrode
cell was employed with a Beckman (M-39273) platinum-inlay working electrode, a
platinum-wire auxiliary electrode, and a saturated calomel electrode (SCE) as reference;
no correction was made for junction potential. Details of the cell configuration are as
n
+
–
described before [11,13,15]. The concentrations of solute and ( Bu N) ClO were 1.0
4
4
mM and 0.1 M, respectively.
2
.3.3. Kinetic studies
–
3
In the kinetic experiment, to a CH CN solution of 2 of fixed concentration (1.04 x 10
3
t
M), a CH CN solution of the substrates [4-X-2-tert-butylphenol; X = H, Me, OMe, Bu)
3
–
3
–3
were added incrementaly (40 x 10 to 160 x 10 M). The reactions rates (k ) of the
obs
8

pseudo-first-order reactions were obtained from the slop of the plot between log{(At–
A )/(A –A )} (A A and A are initial concentration, concentration at time t and
∞
0
∞
0,
t
∞
concentration at infinity, respectively) and t (time in sec). The second-order rate constants
–
1
(
k2) were calculated from the slopes of the plots between kobs (s ) and concentration of
the substrates.
Arrhenius-plots between the logarithm of second-order rate constants k and 1/T gave
2
‡
‡
the activation parameters (A and E ) and Eyring-plots gave the parameters (∆H and ∆S ).
a
2
.3.4. X-ray crystallography
Single-crystals of 1 and 2 of suitable dimension were used for data collection.
Diffraction intensities were collected on a Brucker SMART APEX CCD diffractometer,
with graphite-monochromated Mo-K (0.71073 Å) radiation at 100(2) K. The data were
α
collected for absorption. Data were corrected for Lorentz and polarization effects;
empirical absorption corrections (SADABS v 2.10) was applied. The structures were
solved by SHELXT, and refined with the SHELXL-2016 package incorporated into the
Olex2 1.2.8-alpha crystallographic collective package [16]. The position of the hydrogen
atoms were calculated by assuming ideal geometries, but not refined. All non-hydrogen
atoms were refined with anisotropic thermal parameters by full matrix least-square
2
procedures on F . For 2, the dimer is situated on a crystallographically imposed glide
plane. The Mn1# is generated by the symmetry element: 1/2–x, 1/2–y, z. The oxygen
atoms of the perchlorate anions are also situated on a different glide plane. The O7#, O8#,
O9# and O10# are generated by symmetry element: 3/2–x, 1/2–y, z. The diethyl ether
solvent molecule is highly distorted and as a consequence four methylene hydrogens
9

could not be located/assigned. However, these hydrogens are included in the molecular
formula of 2 (Table 1). To make the ADPs reasonable, the rigid-bond restraint DELU and
ISOR commands were used. Pertinent crystallographic parameters are summarized in
Table 1.
3
. Result and discussion
3
.1. Synthesis and characterization of the ligand and the complexes
2
-(Chloromethyl)-1-methyl-1H-benzimidazole was synthesized by following a
modified procedure [14]. 2-((1H-Pyrazol-1-yl)methyl)-1-methyl-1H-benzimidazole (L)
was synthesized in moderate yield by the reaction between 2-(chloromethyl)-1-methyl-
1
H-benzimidazole and pyrazole in benzene. The ligand was characterized by elemental
1
analysis, and H NMR (Fig. S1) and ESI-MS (Fig. S2) spectra.
3
+,4+
The synthesis of 2 is a continuation of our long-standing interest in {Mn O }
[11]
2
2
III
2
2+
II
and {Cu (µ–O) } [12] cores. Air-stable complexes [(L) Mn (OClO )(CH CN)]-
2
2
3
3
III/IV
(
ClO )•CH CN (1) and [(L) Mn
(µ–O) ](ClO ) •2CH CN•Et O (2) were synthesized
2 2 4 3 3 2
4
3
4
in good yield, following straightforward procedures. Complex 1 was prepared from the
II
reaction between Mn (ClO ) •6H O and L in CH CN. Recrystallization from CH CN
4
2
2
3
3
afforded 1. Complex 2 was prepared by chemical oxidation using tert-
butylhydrogenperoxide (70%). Recrystallization from CH CN-Et O afforded 2.
3
2
Generally, benzimidazole- and pyrazole-based ligands stabilize comparatively lower
oxidation state of metal ions. Therefore, oxidation of 1 required a strong oxidizing agent
t
such as BuOOH; H O could not bring about this oxidation. Complexes 1 and 2 are
2
2
1
0

stable under ambient conditions and are soluble in CH CN, CH COCH and DMF. The
3
3
3
molecular structures of 1 and 2 (Scheme 1) have been authenticated by single-crystal X-
ray crystallographic analysis (see below).
Scheme 1. The ligand and the complexes
3
.2. IR and electronic spectra of the complexes
IR spectra of 1 and 2 are displayed in Figs. S3 and S4, respectively. Characteristic ionic
–
1
perchlorate bands (~ 1100 and 625 cm ) are clearly observed. For 2 vibrations due to
–
1
Mn O (~690 cm ) unit [11,17] is clearly observable.
2
2
1
1

UV-vis spectrum of 1 in CH CN from 200–500 nm is shown in Fig. 1. Absorptions at
3
–
1
–1
2
4
57 ( = 13 800 M cm ), 275 (17 600), 282 (15 350) and a broad feature between 300–
00 nm are observed. These transitions are ligand-centered. The electronic spectral
III
IV 3+
feature of 2 (Fig. 1) in CH CN attests the presence of {Mn (µ–O) Mn } core [11,17].
3
2
–
1
–1
In addition to the absorptions at 675 nm ( = 325 M cm ) and 550 nm (250), shoulders
at 515 nm (330) and at 385 (1750) nm are clearly observable. Lower energy transitions
2
–
IV
are assigned as due to O  Mn (dπ*) charge-transfer [11,17].
(a)
1
2

(b)
II
Fig. 1. Electronic spectra of (a) [(L) Mn (OClO )(CH CN)](ClO )•CH CN
2
3
3
4
3
III,IV
(
1) and (b) [(L) Mn
(µ–O) ](ClO ) •2CH CN•Et O (2) in CH CN.
4
2
2
4 3
3
2
3
3
.3. Description of crystal structures
II
A perspective view of the metal coordination environment in [(L) Mn (OClO )
2
3
(
CH CN)](ClO )•CH CN (1) is shown in Fig. 2. Selected bond distance and bond angles
3 4 3
are collected in Table 2. Two benzimidazole nitrogens N1 and N5 (Mn–Nbenzim distances
.229(2) and 2.239(2) Å, respectively) and two pyrazole nitrogens N4 and N8 (Mn–Npz
distances 2.218(2) and 2.238(2) Å, respectively) from two didentate ligands, a CH CN
2
3
(
Mn–N(9)MeCN distances 2.220(2) Å) and a perchlorate oxygen O1 (Mn–OClO distances
3
II
2
.270(2) Å) provide six-coordination around high-spin Mn . The cis positions are
1
3

–
occupied by CH CN and O ClO ion. The angles between trans atoms at the metal centre
3
3
are in the range 166.27(8)–172.55(8)º. The cis angles span a wide range 83.66(8)–
00.73(8)º. Appreciable distortion from ideal octahedral geometry is apparent.
1
(a)
(b)
1
4

Fig. 2. Perspective views of the metal coordinated environment in (a)
II
[
(L) Mn (OClO )(CH CN)](ClO )•CH CN
(1)
and
(b)
2
3
3
4
3
III,IV
–
[(L) Mn
(µO) ](ClO ) •2CH CN•Et O (2). For 2, only 1.5ClO
4
2
2
4 3
3
2
4
molecules could be seen due to symmetry and solvents of crystallization are
not shown. For both the structures, only coordinated donor atoms are
labeled and hydrogen atoms are omitted for clarity.
III,IV
The crystal structure of [(L) Mn
(µO) ](ClO ) •2CH CN•Et O (2) (Fig. 2) reveals
4
2
2
4 3
3
2
that the asymmetric unit contains only half of the dimeric unit as there exits
crystallographically-imposed inversion centre. The asymmetric unit also contains three
perchlorate counteranions, and two CH CN and a diethyl ether molecule are present as
3
solvent of crystallization. Two didentate ligands L bind each Mn center and two
coordination sites of each Mn center are occupied by two µ–oxo atoms. The two
benzimidazole nitogens N1 and N5 (Mn–Nbenzim distances 2.132(4) Å and 2.094(4) Å,
respectively) are trans to each other and two pyrazole nitrogens N4 and N8 (Mn–Npyrz
distances 2.111(4) Å and 2.092(3) Å, respectively) are cis to each other. The Mn(1)–O(1)
and Mn(1)–O(2) distances are 1.822(3) and 1.824(3) Å, respectively. The angles between
trans atoms at the metal center are in the range 178.57(12)–165.59(14)º. The cis angles
III
span a wide range 82.66(13)–97.62(10)º. The coordination environment around Mn and
IV
…
Mn centers are distorted octahedral. The Mn Mn separation is (2.7237(14) Å), which is
within the range (2.588(2)–2.741(1) Å) observed for other structurally characterized bis-
III
IV
oxo Mn Mn complexes [4d]. The Mn1–O1–Mn1# and Mn1–O2–Mn1# angles are
9
6.71º and 96.56(18)º, respectively. Due to symmetry the geometry at both the Mn
III
IV 3+
centers are same. Thus, structural analysis reveals that the {Mn (µ–O) Mn } core in 2
2
is a delocalized (class III mixed-valent dimer) system [18]
1
5

3
.4 EPR Spectra
A frozen CH CN solution (120 K) EPR spectrum of 2 exhibits a 16-line signal centred
3
III
IV 3+
at g ≈ 2.00 (Fig. 3). The spectrum is characteristic of the {Mn (µ–O) Mn } core with
2
III
4
S = 1/2 ground-state, arises due to antiferromagnetic coupling of high-spin Mn (d ion)
IV
3
with a Mn (d ion) through super-exchange pathway. The spectrum has been simulated
to extract relevant parameters (Fig. 3) [19]. Similar spectra for analogues compounds
have been reported in the literature [4,5,11a,19].
III,IV
Fig. 3. EPR spectrum of [(L) Mn
(µO) ][ClO ] •2CH CN•Et O (2) in
III
4
2
2
4 3
3
2
CH CN. Upper trace: simulated EPR spectrum with parameters: A (Mn )
3
x,x
–
4
–1
III
–4
–1
III
–4
=
160 x 10 cm , A (Mn ) = 160 x 10 cm , A (Mn ) = 136 x 10
y,y
z,z
–
1
IV
–4
–1
IV
–4
–1
IV
cm ; A (Mn ) = 85 x 10 cm , A (Mn ) = 72 x 10 cm , A (Mn )
x,x
y,y
z,z
–
4
–1
=
=
71.2 x 10 cm ; S = ½. g = 2.015, g = 1.999, g = 1.913; W = 50 G, W
60 G, W = 70 G. Frequency (ν) = 9.459 GHz.
x
y
z
x
y
z
1
6

3
.5 Magnetic susceptibility measurements
Magnetic susceptibility measurements on powder samples were carried out in the
temperature range 2 –210 K and 2–295 K for 1 and 2, respectively. The value of  T at
M
3
–1
room temperature for 1 is 4.62 cm mol K and the  T values do not change on
M
lowering of temperature (Fig. 4 a), revealing S = 5/2 ground-state for 1. Satisfactory
fitting (Curie-Weiss law) gave rise to the following parameters: g = 2.07, Weiss
temperature,  = –0.12 K and temperature-independent-paramagnetism (TIP) = 350 x
–
6
3
–1
3
–1
1
0 cm mol . The value of  T at room temperature for 2 is 0.68 cm mol K, much
M
3
–1
lower than spin-only value of 4.875 cm mol K for two individual S = 2 and S = 3/2
ions, indicating very strong antiferromagnetic coupling. The  T values gradually
M
3
–1
decrease with lowering in temperature (Fig. 4 b) and reach a plateau of 0.49 cm mol K
3
–1
at 150 K, closer to spin- only value of 0.375 cm mol K for the S = 1/2 ground-state. The
experimental data were fit using Heisenberg-Dirac-Van-Vleck Hamiltonian for isotropic
exchange interaction and Zeeman splitting (eq. 1) [20]. Satisfactory fit was obtained with
–
1
–6
3
–1
the parameters: g = g = 2.25, J = –192 cm ,  = –0.06 K, TIP = 210 x10 cm mol .
1
2
The J value indicates a strong antiferromagnetic interaction between the two manganese
III
IV 3+
ions, typical for delocalized {Mn (µ–O) Mn } dimers [4d,18].
2
We note that the the g values obtained from SQUID measurement and EPR are
different. The following comments are in orer. The g value can be directly observed
(derived) from EPR measurements. However, in SQUID experiment the overall magnetic
moment is measured and the calculated susceptibility values include all experimental
1
7

errors [sample weighing, molecular weight (for example, influenced by the solvent loss)
or some minor amount of paramagnetic impurity with lower molecular weight (and
therefore higher magnetic impact]. So, such differences are quite common and in the
present case does not influence the main conclusion from the SQUID measurement of 2
(strong antiferromagnetic coupling between Mn(III) and Mn(IV) ions).
(a)
1
8

(b)
II
Fig. 4.  T vs T plots of (a) [(L) Mn (OClO )(CH CN)](ClO )•CH CN (1)
M
2
3
3
4
3
III,IV
and (b) [(L) Mn
(µO) ][ClO ] •2CH CN•Et O (2). The solid lines
4
2
2
4 3
3
2
correspond to best theoretical fit (see text).
3
.6 Electrochemistry
Redox behaviour of 1 and 2 were examined by cyclic voltammetry (CV) in CH CN
3
solution at a platinum working electrode. For 1, a quasireversible (peak-to-peak
III
II
separation, ∆E = 140 mV) oxidative response corresponding to Mn /Mn redox process
p
is observed at E1/2 = 0.71 V vs SCE (Fig. 5 a). The CV scan of 2 displays (Fig. 5 b) a
quasireversible oxidative wave at E1/2 = 1.06 V (∆E = 120 mV) and an irreversible
p
reductive response at E (cathodic peak potential) = 0.16 V vs SCE, corresponding to the
pc
1
9

IV,IV
IV/III
IV,III
III,III
redox processes Mn
2/Mn
(eq. 2) and Mn 2/Mn
(eq. 3), respectively. The
2
2
CV result of 2 is similar to those reported for closely related systems [11].
(a)
2
0

(b)
II
Fig. 5. Cyclic voltammograms of (a) [(L) Mn (OClO )(CH CN)](ClO ) •
2
3
3
4
III/IV
CH CN (1) and (b) [Mn
(µ–O) (L) ](ClO ) •2CH CN•Et O (2) in
CH CN (0.1 M TBAP) at a Pt electrode (scan rate 100 mVs ).
3
2
2
4
4 3
3
2
–
1
3
4
. Reactivity
To investigate the electrophilic reactivity of 2 towards phenols, 2,4,6-tri-tert-
butylphenol was used, as the substrate. This substrate was chosen because of its well-
known property to give a stable phenoxyl radical [21], which could be easily identified by
its EPR spectrum [10b]. Solutions of 2 and 2,4,6-tri-tert-butylphenol in CH CN were
3
mixed (1:1 mole ratio) and frozen shortly after mixing (within 1–2 min). The formation
2
1

of stable phenoxyl radical was observed due to oxidation of the substrate (Scheme 2),
based on EPR spectrum recorded after 10 min of mixing (Fig. S5). The absence of EPR
III
signal characteristic of 2 (Fig. 3) suggests the presence of Mn ions (S = 2) in the reaction
mixture.
Scheme 2. The reactions of 2 with different substituted phenols
The oxidation of phenol to generate phenoxyl radical can, in principle, occur via two
–
+
possible pathways: (i) H-atom abstraction (e and H transfer in a concerted manner) and
2
2

–
+
(
ii) proton-coupled electron-transfer (e and H transfer in a stepwise manner: electron-
transfer followed by proton-transfer (ET/PT) or PT followed by ET) [10b].
Addition of 2,4-di-tert-butylphenol to a CH CN solution of 2 (1:3 mole ratio) afforded
3
isolation (after removal of solvent and addition of diethyl ether) of the C–C coupled
1
product
, confirmed by its H
NMR (Fig. S6) and ESI-MS (Fig. S7) spectra. After 12 h, ESI-MS of such solution
II
revealed the presence of (L) Mn -containing end-product (Fig. S8). The isolated solid
2
was washed with diethyl ether and dried in vacuo. Yield 40% (with respect to complex 2).
Similar reactions were done with a series of para-substituted 2-tert-butylphenol
t
substrates (p-X-2-tert-butylphenols: X = H, Me, OMe, Bu). The yields of the products
t
are 89% (X = H), 91% (X = Me), 88% (X = OMe) and 90% (X = Bu).
In order to gain insight into the mechanism of phenol oxidation, kinetic studies were
t
performed on p-X-2-tert-butylphenols (X = H, Me, OMe, Bu) in CH CN at 303 K.
3
Kinetic experiments (large excess of substrates) were performed by monitoring
changes in the absorption of 2 at 675 nm. In each case a steady decrease of 675 nm band
was monitored and good pseudo-first-order kinetics was observed. The profile of initial
studies revealed that the reaction is a two-component decay process. The initial reaction
is faster (up to ~200 sec), followed by second slower-step (after ~200 sec) for all the
substrates. A representative case for 2,4-di-tert-butylphenol is shown in Fig. 6. Plots of
these first-order rate constants (k ) against the concentration of the phenolic substrates
obs
for the faster as well as for the slower reaction steps gave good straight lines, revealing a
first-order dependence of the rates on the concentration of phenolic substrates. The case
2
3

(fast process) for 2,4-di-tert-butylphenol is shown in Fig. 7. Kinetic results for all the
substrates are in Fig. S9–S15.
(a)
(b)
2
4

Fig. 6. Reaction of 2 with 2,4-di-tert-butyl-phenol (DTBP) in CH CN at 303
3
K. (a) UV–VIS spectral change of 2 (1.042 mM) upon addition of 40 equiv.
of DTBP. (b) The time-course of the decay of 2, monitored at 675 nm.
Fig. 7. Plot of kobs vs. [substrate] for the reaction of 2 (1.042 mM) with
DTBP.
The second-order rate constant k for all four reactions are collected in Table 3. We
2
III
IV 3+
propose that during the reaction between {Mn (O) Mn } (2) and phenols (1:1) initially
2
III
III 3+
–2
a {Mn (O)(OH)Mn } species is formed (faster process: k x 10 values 0.6 (X = H),
2
t
–1 –1
–3
1
.95 (Me), 3.0 (OMe), 1.85 ( Bu) M s ) and subsequently (slower process: k x 10
2
–
1
–1
II
2+
values 0.9 (X = H), 2.5 (Me), 4.0 (OMe) M s ) the species {(L) Mn S } is formed.
2
2
The faster step is responsible for larger part of the decay process. For the initial part of the
2
5

reactions, the kobs and k values were calculated for all the substrates. We cannot
2
comment on the nature of the ‘intermediate’, as all our attempts to trap it failed. Within
the chosen series, the fastest rate is observed for X = OMe. The electronegativity of the
para-substituents [22] influence the electrophilic reaction rates significantly. The
electrophilicity of 2 has been investigated in hydrogen-atom transfer from para-X-
t
substituted 2-tert-butylphenol derivatives (X = H, Me. OMe, Bu) (Fig. 8 a). The k values
2
of the first-component in the decay process gave a good linear Hammett correlation
+
(
log(k˝ /k˝ ) = ρσ ) for para-X-substituted 2-tert-butylphenol derivatives with ρ = –0.8 ±
X
H
p
0
.3 (Fig. 8 a). It is appropriate to mention here that for the electrophilic reactivity of
III
IV 1+
[
(L*) Mn (O) Mn } (L = dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-
2 2
acetamidate) with 4-X-2,6-di-tert-butylphenol derivatives the ρ value was reported to be –
.0 for H-atom transfer reaction [3a,5]. The closely similar ρ value (ρ = –0.8 ± 0.3) for 2
1
revealed that the H-atom abstraction of the O–H bond by 2 is the rate-determining step,
indicating that bis(μ-oxo)-dimanganese(III,IV) complex acts as an electrophile in the
hydrogen-atom transfer reaction. Similar plots are reported in the literature [22-25]. A
second informative correlation in support of H-atom abstraction is obtained from the plot
of homolytic phenolic O–H bond dissociation energies [26] vs log(k˝ /k˝ ) = log (k" ),
X
H
rel
shown in Fig. 8 b. This plot shows a linear relationship, where the relative rate decreases
with increasing O–H bond strength (in other words, the k value increases as the electron-
2
donating ability of the para-substituent increases) with a slope of –0.11 ± 0.032. For the
III
IV 1+
reported system [(L*) Mn (O) Mn } the slope obtained for the plot of log (k ) vs.
2
2
2
BDE (O–H) is –0.257 ± 0.018 [5]. As the rate variation for the present work less than
2
6

that reported the smaller value of the slope is understandable. Similar plots are reported in
the literature [5,23,24,26-28].
(a)
(b)
2
7

Fig. 8. Reactions of 2 with 2,4-di-tert-butyl-phenols in CH CN at 303 K. (a)
3
+
Hammet plot of log(k" /k" ) against  of para-substituted 2-tert-butyl-
X
H
p
phenols. (b) Plot of log(k" ) vs homolytic bond dissociation energies
rel
(
BDE ).
O–H
Using the k of the first-step, a correlation of rates with potential for the oxidation of
2
phenols has also been attempted. The potential for phenol oxidation of p-X-2-tert-
butylphenols were determined in CH CN by CV, under identical experimental conditions.
3
In each case an irreversible oxidative response was observed. The E (anodic peak
pa
potential) values (V vs. SCE) of p-X-2-tert-butylphenols are as follows: X = H (1.64), Me
t
(
1.37), OMe (1.10), Bu (1.30). The plot of (RT/F)ln(k ) vs E (theoretically should have
2
pa
been E1/2 values) of phenols is shown in Fig. 9. The slope of (RT/F)ln(k ) vs E plot is
2
pa

0.077  0.015. The k values for a H-atom transfer mechanism are expected to be
2
constant irrespective of the E values [10b,30]. Fig. 9 attests such a situation. The E
ox
1/2
value for the redox process (eq. 2) (1.06 V vs SCE in CH CN) is lower than the E
3
pa
2
8

Fig. 9. Plot of (RT/F)ln(k ) against E values of ArOH for the reactions of
2
pa
t
4
-X-2-tert-butylphenol (X = H, Me, OMe, Bu) with 2 in CH CN at 303 K.
3
t
values of the phenols (see above) [for p-X-2-tert-butylphenols (X = H, Me, OMe, Bu):
Epa = 1.10–1.64 V vs. SCE], excluding the possibility of initial rate-limiting ET process
III
IV 3+
followed by fast PT. It is also very unlikely that {Mn (–O) Mn } species is basic
2
enough to deprotonate the phenol substrates, suggesting the initial PT followed by ET is
also not feasible. Thus, all the experimental data suggest that the concerted H-atom
abstraction by 2 is the most favourable pathway. In the absence of kinetic isotope effect
(KIE) data, we are not in a position to be more definitive of the mechanism.
To get an idea whether or not a coordinative interaction of substrate prior to cleavage
of O–H bond takes place in the transition state, we determined the rate of reaction of 2
with 2,4-di-tert-butylphenol as a function of temperature. The activation parameters were
determined by Arrhenius (Fig. 10 a): A (frequency factor) = 191.88 ± 1.26 and E =
a
–
1
‡
.
–1
‡
2
7.94 ± 3.25 kJ mol and Eyring plot (Fig. 10 b): ∆H = 27.94 ± 3.48 kJ mol and ∆S =
–
1
–1
–
209.47 ± 1.35 J mol K . The values are in good agreement to that obtained by others
‡
[
25,28]. The high negative ∆S value indicates that an association of the reactive species
occurs in the transition-state. The positive enthalpy value confirm the bond forming
dimerization) reaction.
(
2
9