Tuning the olefin epoxidation by manganese(iii) complexes of bisphenolate ligands: Effect of Lewis basicity of ligands on reactivity

Article abstract of DOI:10.1039/c3dt51766g

A new family of manganese(iii) complexes of the type [Mn(L)Cl], where H₂L is 1,4-bis(2-hydroxy-benzyl)-1,4-diazepane (H₂(L1)), 1,4-bis(2-hydroxy-4-methylbenzyl)-1,4-diazepane (H₂(L2)), 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)-1,4-diazepane (H₂(L3)) and 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane (H₂(L4)), has been isolated and studied as a catalyst for epoxidation reaction. Complexes 1-4 have been characterized using elemental analysis, electronic spectral and electrochemical methods and ESI-MS. The single crystal X-ray structures of 1 and 3 contain the MnN₂O₂Cl chromophore with a novel square pyramidal coordination geometry (τ: 1, 0.11; 3, 0.00). All the complexes possess a distorted square pyramidal coordination geometry in solution, as revealed by the characteristic bands observed in the electronic spectra. A time dependent density functional theory (TD-DFT) calculation has been performed to assist in the assignment of the electronic absorption spectral bands of the complexes. The Mn(iii)/Mn(ii) redox potentials (E_1/2) of 1-4 fall within the narrow range of 0.279-0.320 V. The catalytic ability of the complexes towards olefin epoxidation has been investigated using PhIO as the oxygen source at room temperature under an N₂ atmosphere. Addition of N-methylimidazole to the reaction mixture leads to an increase in the epoxide yield. A correlation between the Lewis acidity of the Mn(iii) center as tuned by the substituents on the phenolate ligand, and the epoxide yield and product selectivity has been observed. The present complexes act as better chemoselective catalysts for epoxidation of cyclohexene and styrene rather than cyclooctene.

Full text of DOI:10.1039/c3dt51766g

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DOI: 10.1039/C3DT51766G
Manuscript Type: Article
Submitted to: Dalton Trans.(Revised 1)
Manuscript ID: DTꢀARTꢀ07ꢀ2013ꢀ051766
Tuning the Olefin Epoxidation by Manganese(III) Complexes of Bisphenolate Ligands:
Role of Lewis Basicity of Ligands on Reactivity
Muniyandi Sankaralingam,a and Mallayan Palaniandavar*a,b
aSchool of Chemistry, Bharathidasan University, Tiruchirappalli - 620024, Tamil Nadu, India.
bDepartment of Chemistry, Central University of Tamil Nadu, Thiruvarur - 610004,Tamil Nadu,
India.
*To whom correspondence should be addressed. Email: palanim51@yahoo.com; palaniandavarm@gmail.com
1

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DOI: 10.1039/C3DT51766G
Abstract
A new family of manganese(III) complexes of the type [Mn(L)Cl], where H2L is 1,4ꢀbis(2ꢀ
hydroxyꢀbenzyl)ꢀ1,4ꢀdiazepane (H2(L1)), 1,4ꢀbis(2ꢀhydroxyꢀ4ꢀmethylbenzyl)ꢀ1,4ꢀdiazepane
(H2(L2)), 1,4ꢀbis(2ꢀhydroxyꢀ3,5ꢀdimethylbenzyl)ꢀ1,4ꢀdiazepane (H2(L3)) and 1,4ꢀbis(2ꢀhydroxyꢀ3,5ꢀ
diꢀtertꢀbutylbenzyl)ꢀ1,4ꢀdiazepane (H2(L4)), has been isolated and studied as catalyst for epoxidation
reaction. The complexes 1 - 4 have been characterized using elemental analysis, electronic spectral
and electrochemical methods and ESIꢀMS. The single crystal Xꢀray structures of 1 and 3 contain
MnN2O2Cl chromophore with a novel square pyramidal coordination geometry (τ: 1, 0.11; 3, 0.00).
All the complexes possess a distorted square pyramidal coordination geometry in solution, as
revealed by the characteristic bands observed in the electronic spectra. A time dependent density
functional theory (TDꢀDFT) calculation has been performed to assist in the assignment of the
electronic absorption spectral bands of the complexes. The Mn(III)/Mn(II) redox potentials (E1/2) of
1 - 4 fall within the narrow range of 0.279 ꢀ 0.320 V. The catalytic ability of the complexes towards
olefin epoxidation has been investigated using PhIO as the oxygen source at room temperature under
N2 atmosphere. Addition of Nꢀmethylimidazole to the reaction mixture leads to an increase in the
epoxide yield. A correlation between the Lewis acidity of the Mn(III) center as tuned by the
substituents on the phenolate ligand, and the epoxide yield and product selectivity has been observed.
The present complexes act as better chemoselective catalysts for epoxidation of cyclohexene and
styrene rather than cyclooctene.
2

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Introduction
Manganese plays a very important role in metalloenzymes such as catalase, superoxide
dismutase and photosystem II of green plants1ꢀ4 and extensive studies have been made on Mn(III)
complexes of Schiff base type ligands because they behave as functional models for the
metalloenzymes.5ꢀ10 Manganese(III)ꢀsalen (H2salen = N,N’ꢀethylenebis(salicylaldimine)) complexes
have been shown11 to be highly effective, chemoselective and stereoselective catalysts for the
epoxidation of alkenes and several such complexes with MnN2O2 chromophore have been studied as
epoxidation catalysts.5ꢀ10 Mn(III)ꢀsalen complexes with chiral centers in the diimine bridge have
been shown to act as highly enantioselective epoxidation catalysts (enantiomeric excess, ee 90%;
yield exceeding 80%, Scheme 1) when iodosobenzene is used as oxidant.9,12 Also, Mn(III)
complexes of numerous salen type ligands have been prepared and the ligand stereoelectronic
properties on the catalytic epoxidation of Mn(III) center have been investigated.13
Ph
Ph
H
H
N
O
N
N
O
N
O
Mn
Mn
Cl
O
Cl
B
A
Scheme 1. Jacobsen catalysts used for epoxidation reaction
The use of H2O2 as the oxygen source for Mnꢀsalen catalysed epoxidations has been limited
due to participation of free radicals in the reaction, which leads to nonꢀselective epoxidation of
alkenes (low ee) and susceptibility of H2O2 towards metalꢀcatalysed dismutation reaction.14 Very
recently, Stack and coworkers have identified that [Mn(bpy)2(OTf)2] (bpy = 2,2’ꢀbipyridine, OTf =
trifluoromethanesulfonate) is the most active epoxidation catalyst using peracetic acid as oxidant.15
Costas et al. have found that chiral Mn(II) complexes epoxidize a wide range of olefins with good
yields and moderate selectivity (15ꢀ46% ee) under mild experimental conditions in the presence of
peracetic acid.16 PhIO has been used as a single oxygen atom transfer reagent to generate reactive
highꢀvalent Mnꢀoxo intermediates analogous to those occurring in enzymatic reactions.17 When
NaOCl and PhIO are used as oxidizing agents, Mn(II/III) complexes show highly selective
epoxidation reactions and they are considered to oxidise Mn(III)ꢀsalen complexes directly to the
corresponding Mnꢀoxo species.11,18 Nam et al. have isolated a few Mn(II) complexes of linear
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tetradentate 4N heterocyclic ligands and studied them as catalysts for alkene epoxidation using PhIO
and peracetic acid as oxidants.19 Also, the study of mechanism of epoxidation by manganese
complexes has evoked8,20ꢀ27 a great deal of interest because of its relevance to a wide range of
important synthetic and biological pathways. Very recently, it has been shown that the epoxidation
products obtained and the reaction mechanism depend on several factors like olefin, nature of the
catalyst, structure of the Schiff base ligand and counterꢀionꢀ, oxygen source and added base.12,24b,25
Although significant progress has been made in this field, the effect of nonꢀheme ligands on the
oxidizing ability of the highꢀvalent metalꢀoxo intermediate has not been clearly understood.28 It may
be noted that the widely studied manganese(III)ꢀSchiff base ligand complexes are attractive as they
are synthetically easily accessible but they suffer from their inherent susceptibility towards
hydrolysis and dimerisation.13d,29 Also, the search for efficient, robust and recyclable catalysts
remains an important task in homogeneous catalysis.
All the above observations prompted us to design new manganese complexes and study them
as epoxidation catalysts. Very recently, we have isolated30 a few Mn(II) complexes of linear
tetradentate 4N heterocyclic ligands with diazacycloalkane backbone and studied the effect of ligand
backbone size on the catalytic activity of the complexes towards epoxidation of unfunctionalized
olefins using PhIO as oxidant. In this work Mn(III) complexes of a new family of linear tetradentate
bis(phenolate) ligands such as 1,4ꢀbis(2ꢀhydroxybenzyl)ꢀ1,4ꢀdiazepane H2(L1), 1,4ꢀbis(2ꢀhydroxyꢀ4ꢀ
methylꢀbenzyl)ꢀ1,4ꢀdiazepane H2(L2), 1,4ꢀbis(2ꢀhydroxyꢀ3,5ꢀdimethylbenzyl)ꢀ1,4ꢀdiazepane H2(L3)
and 1,4ꢀbis(2ꢀhydroxyꢀ3,5ꢀdiꢀtertꢀbutylbenzyl)ꢀ1,4ꢀdiazepane H2(L4) (Scheme 2) has been isolated
and studied as epoxidation catalysts using PhIO as oxygen source. The Mn(III) complexes
[Mn(L1)Cl] 1 and [Mn(L3)Cl] 3 exhibit a novel square pyramidal coordination geometry for the
MnN2O2Cl chromophore and the similarity of the chromophore to that of the wellꢀknown Mn(III)ꢀ
salen complexes encouraged us to study the effect of diazacycloalkane ligand backbone on the
catalytic activity of the complexes towards epoxidation of unfunctionalized olefins. The present
Mn(III) complexes catalyze the epoxidation of alkylꢀ and arylꢀsubstituted olefins with better
selectivities when PhIO is used as the oxidant (cyclooctene, 100%; cyclohexene 10ꢀ62%; styrene,
60ꢀ82%). The phenolate ring substituents in the complexes dictate the Lewis acidity of Mn(III)
center, and hence the catalytic activity of the complexes towards olefin epoxidation.
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N
N
N
N
R
OH HO
R
R
OH HO
R
R
R
R = -Me
R = -CMe3 H2(L4)
H2(L3)
R = -H
R = -Me
H2(L1)
H2(L2)
Scheme 2. Structures of Linear Bis(phenolate) Ligands.
Experimental Section
Materials
Homopiperazine, 2ꢀhydroxyꢀ5ꢀmethylbenzaldehyde, 2ꢀhydroxyꢀ5ꢀmethylbenzaldehyde, 2,4ꢀ
diꢀtertꢀbutylphenol, 3,5ꢀdiꢀtertꢀ2,4ꢀdimethylphenol, sodium cyanotrihydroborate, cyclooctene,
styrene, cyclohexene, iodobenzene diacetate (Aldrich) and manganese(II) chloride (anhydrous),
manganese(II) acetate, sodium sulfate (Merck, India) were used as received, unless noted otherwise.
Acetonitrile, dichloromethane, chloroform (Merck, India) were used as received. The supporting
electrolyte tetraꢀNꢀbutylammonium perchlorate (NBu4ClO4, G. F. Smith, USA) was recrystallized
twice from aqueous ethanol.
Synthesis of Ligands
The present ligands were synthesized according to known procedures.31ꢀ34
1,4-Bis(2-hydroxybenzyl)-1,4-diazepane (H2(L1))
To a solution of 2ꢀhydroxybenzaldehyde (2.44 g, 20.0 mmol) in methanol (100 mL) were
added homopiperazine (1.0 g, 10.0 mmol) and a small amount of acetic acid. Sodium
cyanotrihydroborate (1.26 g, 20.0 mmol) in methanol (5 mL) was added drop wise to the resulting
solution with stirring. After the solution was stirred for 3 days, it was acidified by adding conc. HCl
and then evaporated almost to dryness under reduced pressure. The residue was dissolved in
saturated aqueous solution (50 mL) of Na2CO3 and extracted withCHCl3 (3 × 50 mL). The combined
extracts were dried over anhydrous Na2SO4 and filtered. On slow evaporation of the filtrate a white
crystalline solid was obtained. Yield: 1.36 g (44%). It was purified by recrystallization from CH2Cl2.
1
mp: 108–110 °C. H NMR (400 MHz, CDCl3): d 7.14–7.26 (m, 2H), 6.74–6.97 (m, 8H), 3.79 (s,
4H), 2.83 (t, 4H), 2.79 (s, 4H), 1.93 (pentet, 1H). 13C NMR (400 MHz, C6D6): δ 26.06, 54.40, 54.21,
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61.18 (CH2), 115.96, 123.09, 128.90, 129.62, 131.17, 155.67 (ArC). Anal. Calcd. for C19H24N2O2: C,
73.05; H, 7.74; N, 8.97. Found C, 73.25; H, 7.48; N, 8.62.
1,4-Bis(2-hydroxy-4-methylbenzyl)-1,4-diazepane (H2(L2))
To a solution of 2ꢀhydroxyꢀ5ꢀmethylbenzaldehyde (1.36 g, 10.0 mmol) in methanol (50 mL)
were added homopiperazine (0.50 g, 5.0 mmol) and small amount of acetic acid. Sodium
cyanotrihydroborate (0.63 g, 10.0 mmol) in methanol (5 mL) was added drop wise to the resulting
solution with stirring. After the solution was stirred for 3 days, it was acidified by adding con. HCl
and then evaporated almost to dryness under a reduced pressure. The residue is dissolved in saturated
aqueous Na2CO3 solution (50 mL) and extracted with CHCl3 (3 × 50 mL). The combined extracts
were dried over anhydrous Na2SO4 and filtered. Slow evaporation of the filtrate gives a white
crystalline ligand H2(L2). Yield: 0.84 g (49%). The ligand was further purified by recrystallization
from CH2Cl2. mp: 110ꢀ112 °C. 1H NMR (400 MHz, CDCl3): δ 1.27 (s, 6H), 1.42 (s, 4H), 1.95
(pentet, 2H), 2.77 (t, 4 H), 3.77 (s, 4H), 6.82 (d, 2H), 7.21 (d, 2H), 7.21 (s, 2H). 13C NMR (360 MHz,
C6D6): δ 20.91 (CH3), 27.06, 53.50, 54.81, 62.28 (CH2), 116.86, 122.29, 128.20, 129.74, 130.07,
156.97 (ArC). Anal. Calcd. for C21H28N2O2: C, 74.08; H, 8.29; N, 8.23. Found C, 74.26; H, 8.47; N,
8.42.
1,4-Bis(2-hydroxy-3,5-dimethylbenzyl)-1,4-diazepane (H2(L3))
A solution of 2,4ꢀdimethylphenol (3.66 g, 30.0 mmol), homopiperazine (1.50 g, 15.0 mmol)
and 37% aqueous formaldehyde (3.5 mL, 42.0 mmol) in methanol (10 mL) was stirred and refluxed
for 24 h. The mixture was cooled and the product obtained was filtered off and washed with iceꢀcold
methanol to give a colorless product. Yield: 4.20 g (76%). The product was purified by
recrystallization from CH2Cl2. mp: 98ꢀ100 °C. 1H NMR (400 MHz, CDCl3): δ 1.89 (pentet, 2H), 2.17
13
(s, 12H), 2.74 (s, 4H), 2.79 (t, 4H), 3.70 (s, 4H), 6.83 (s, 2H), 6.58 (s, 2H), 7.23 (s, 2H, OH). C
NMR (360 MHz, CDCl3): δ 15.56, 20.34 (CH3), 26.46, 53.37, 54.36, 61.72 (CH2), 120.63, 124.60,
126.61, 127.61, 130.62, 153.51 (ArC). Anal. Calcd. for C23H32N2O2: C, 74.96; H, 8.75; N, 7.60.
Found C, 74.65; H, 8.57; N, 7.72.
1,4-Bis(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane (H2(L4))
A solution of 2,4ꢀdiꢀtertꢀbutylphenol (5.0 g, 24.2 mmol), homopiperazine (1.21 g, 12.1
mmol) and 37% aqueous formaldehyde (2.5 mL, 33.6 mmol) in methanol (10 mL) was stirred and
refluxed for 24 h. The mixture was cooled and the product was filtered off and washed with iceꢀcold
methanol to give a colorless product. Yield: 4.71 g (72%). The product was purified by
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1
recrystallization from CH2Cl2. mp: 173ꢀ175 °C. H NMR (400 MHz, CDCl3): δ 1.27 (s, 18H), 1.42
(s, 18H), 1.94 (pentet, 2H), 2.77 (s, 4 H), 2.82 (t, 4H), 3.77 (s, 4H), 6.82 (s,), 7.21 (s, 2H), 7.26 (s,
2H, OH). 13C NMR (400 MHz, CDCl3): δ 26.76, 29.61, 31.70 (CH3), 34.15, 34.86, 53.07, 54.45
(CH2), 62.45 (tertꢀC), 121.22, 123.01, 123.44, 135.66, 140.59, 154.29 (ArꢀC). Anal. Calcd. for
C35H56N2O2: C, 78.31; H, 10.51; N, 5.22. Found C, 78.28; H, 10.43; N, 5.32.
Synthesis of Manganese(III) Complexes
[Mn(L1)Cl] 1.
The complex 1 was prepared by adding of a solution of MnCl2ꢁ4H2O (0.162 g, 1.0 mmol) in
methanol (5 mL) to an acetonitrile solution (10 mL) of an equivalent amount of the ligand H2(L1)
(0.312 g, 1.0 mmol) and then triethylamine (0.20 g, 280 ꢂL, 2.0 mmol) in methanol (2 mL) was
added. The solution was stirred for an hour. The blue colored precipitate obtained was filtered off,
washed with small amounts of cold methanol and dried in vacuo over P4O10. Yield: 0.29 g (72%).
Anal. Calcd. for C19H22N2O2MnCl: C, 56.94; H, 5.53; N, 6.99. Found C, 56.86; H, 5.47; N, 6.83.
ESIꢀMS, m/z, 365.24 [(M−Cl)]. The complex was crystallized from acetonitrile to give a bright blue
crystals at room temperature with in 24 hrs it is suitable for Xꢀdiffraction were obtained.
[Mn(L2)Cl] 2.
The complex 2 was prepared as blue colored product by using the procedure employed for
preparing complex 1. Yield: 0.26 g (60.7%). Anal. Calcd. for C21H26N2O2MnCl: C, 58.82; H, 6.11;
N, 6.53. Found C, 58.75; H, 6.18; N, 6.62. ESIꢀMS, m/z, 393.18 [(M−Cl)].
[Mn(L3)Cl] 3.
The complex 3 was prepared as blue colored product by using the procedure employed for
preparing complex 1 Yield: 0.33 g (73.3%). Anal. Calcd. for C23H30N2O2MnCl: C, 60.46; H, 6.62; N,
6.13. Found C, 60.35; H, 6.72; N, 6.3. ESIꢀMS, m/z, 421.20 [(M−Cl)]. The complex was crystallized
from acetonitrile to give a dark blue crystal within one day upon slow evaporation at room
temperature, that bright blue crystals suitable for Xꢀdiffraction were obtained.
[Mn(L4)Cl] 4.
The complex 4 was prepared as a dark brown colored crystalline precipitate by using the
procedure employed for preparing complex 1. Yield: 0.49 g (78.5%). Anal. Calcd. for
C35H54N2O2MnCl: C, 67.24; H, 8.71; N, 4.48. Found C, 67.16; H, 8.62; N, 4.50. ESIꢀMS, m/z,
589.14 [(M−Cl)].
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Physical Measurements
1
Elemental analyses were performed on a Perkin Elmer Series II CHNS/O Analyzer 2400. H
NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. The electronic spectra were
recorded on an Agilent diode arrayꢀ8453 spectrophotometer. ESIꢀMass spectrometry was performed
on a Thermo Finnigan LCQ 6000 Advantage Max instrument. Cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) were performed using a three electrode cell configuration. A
platinum sphere, a platinum plate and Ag(s)/Ag+ were used as working, auxiliary and reference
electrodes respectively. The supporting electrolyte used was NBu4ClO4 (TBAP). The temperature of the
electrochemical cell was maintained at 25.0 ± 0.2 °C by a cryocirculator (HAAKE D8 G). By bubbling
research grade nitrogen the solutions were deoxygenated and an atmosphere of nitrogen was maintained
over the solutions during measurements. The E1/2 values were observed under identical conditions for
various scan rates. The instruments utilized included an EG & G PAR 273 Potentiostat/Galvanostat and
PentiumꢀIV computer along with EG & G M270 software to carry out the experiments and to acquire
the data. The product analyses were performed using a HP 6890 GC series Gas Chromatograph
equipped with a FID detector and a HPꢀ5 capillary column (30 m × 0.32 mm × 2.5 ꢀm) and GCꢀMS
analysis was performed on a Agilent instrument equipped with 7890A GC system 5975C MSD using
a HPꢀ5 capillary column with the previously reported30 temperature program.
Single Crystal X-Ray Data Collection and Structure Solution
The single crystals of 1 and 3 of suitable size was selected from the mother liquor and
immersed in paraffin oil, then mounted on the tip of a glass fiber and cemented using epoxy resin.
Intensity data for the crystal were collected using MoKα (λ = 0.71073 Å) radiation on a Bruker
SMART APEX II diffractometer equipped with a CCD area detector at 273 K. The crystallographic
data are collected in Table 1. The SMART35 program was used for collecting frames of data,
indexing reflection, and determination of lattice parameters; SAINT35 program for integration of the
intensity of reflections and scaling; SADABS36 program for absorption correction, and the
SHELXTL37 program for space group and structure determination, and leastꢀsquares refinements on
F2. The structure was solved by heavy atom method. Other nonꢀhydrogen atoms were located in
successive difference Fourier syntheses. The final refinement was performed by fullꢀmatrix leastꢀ
squares analysis. Hydrogen atoms attached to the ligand moiety were located from the difference
Fourier map. The crystallographic data and details of data collection for 1and 3 are given in Table 1.
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Reactivity Studies
The catalytic activity of all the manganese(III) complexes towards cyclooctene, cyclohexene
and styrene was examined. The reaction maintained is solid PhIO (20 equivalence per Mn) was
added to the reaction solution containing complex (10 mM) and substrate (1.5 M) in acetonitrile for
two hours under nitrogen atmosphere through schlenk line and the additive Nꢀmethylimidazole
equivalent to catalyst. The oxygenated products were identified by using GCꢀMS, quantified by
using GC HewlettꢀPacked HP 6890 series gas chromatograph equipped with an FID detector and a
HPꢀ5 capillary column (30 m, 0.32 mm i.d) with the following temperature program: initial
temperature, 80 °C, 5 degree minꢀ1; final temperature 250 °C; detector temperature 250 °C. Product
identification analysis was done on a Agilent GCꢀMS instrument equipped with 7890A GC system
5975C MSD using a HPꢀ5 capillary column and with the same temperature program noted above.
The averages of three measurements are reported.
Computational details
The ground state geometries of the two complex molecules 1 and 3 were optimized by using
density functional theory (DFT) based method with Becke’s threeꢀparameter functional and the Lee–
Yang–Parr functional (B3LYP) with LANL2DZ basis set for the Manganese (Mn) atom and 6ꢀ31G*
for the rest of the atoms. Frequency calculations were also executed at the same level of theory. The
optimizations and the vibrational data confirmed that the structures were true minima on the potential
energy surface because there were no imaginary frequencies. On the basis of optimized ground state
structure, the absorption spectra of all the compounds were computed by using time dependant
density functional theory (TDꢀDFT) approach with (B3LYP/6ꢀ31G*). As solvent effects are known
to play a crucial role in predicting the absorption spectra, the same was incorporated in the TDꢀDFT
calculations within the PCM framework. The Swizard program has been employed to evaluate the
contribution of singly excited state configurations to each electronic transition.38 All calculations
were performed using Gaussian 09 program package.39
Results and Discussion
Synthesis and Characterization of Ligands and Complexes
The tetradentate ligands H2(L1)ꢀH2(L4) were synthesized according to procedures31ꢀ33 already
reported in the literature, which involve Mannich base condensation, reductive amination and simple
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substitution reactions, with suitable modifications. The ligand H2(L1) was synthesized by the
reductive amination of 2ꢀhydroxybenzaldehyde using sodium cyanotrihydroborate as the reducing
agent.31,32 The ligand H2(L2) was synthesized by the reductive amination of 2ꢀhydroxyꢀ5ꢀ
methylbenzaldehyde using sodium cyanotrihydroborate as the reducing agent.31,34 The ligands
H2(L3) and H2(L4) were synthesized33,34 in good yields by the reaction of the Mannich base formed
in situ by ortho and para substituted phenols using excess of 37% aqueous formaldehyde. The two
phenolate [pKa (BH+): phenol, 9.95] arms with electronꢀreleasing methyl and tertꢀbutyl groups in the
ligands are expected to coordinate very strongly to Mn(III) center. The methyl and tertꢀbutyl
substituents have been introduced not only to tune the spectral and electrochemical properties but
also to effect a variation in reactivity of the model complexes by providing steric hindrance to
oxidant attack. The unsubstituted and 2,4ꢀdisubstituted phenolate moieties have been shown28 to
favor the formation of monomeric fiveꢀcoordinate complexes but with varying structural, spectral
and electrochemical properties. The reaction of MnCl2ꢁ4H2O with bisꢀphenolate ligands in the
presence of two equivalents of Et3N results in immediate formation of the Mn(III) complexes. The
complexes 1 ꢀ 4 are formulated as [Mn(L)Cl], which is supported by the Xꢀray crystal structures of 1
and 3 (cf. below). Attempts to grow single crystals of 2 and 4 suitable for Xꢀray diffraction were not
fruitful. The fiveꢀcoordinate complexes containing only one coordinated chloride ion may expand the
coordination sphere to accommodate the alkene substrates. It is interesting to note that the
replacement of both the pyridylmethyl moieties in the octahedral complex30 [Mn(L)Cl2] by 2ꢀ
hydroxybenzyl moieties gives the fiveꢀcoordinate Mn(III) complex [Mn(L1)Cl]. The ability of the
phenolate ligands to stabilize the higher Mn(III) oxidation state reveals that these ligands would
strongly facilitate the formation of highꢀvalent intermediate manganese(V)ꢀoxo species proposed to
be involved in epoxidation reactions.
Description of X-ray Crystal Structures of [Mn(L1)Cl] 1 and [Mn(L3)Cl] 3
The molecular structures of 1 and 3, which look like a butterfly, are depicted in Figures 1
and 2 respectively, together with the atom numbering scheme. The selected bond lengths and bond
angles are collected in Table 2. The compound 1 crystallizes in the monoclinic crystal system with
space group P2(1)/c. The coordination environment around Mn(III) in 1 is described as distorted
square pyramidal with the trigonality index40 τ of 0.11 [τ = (βꢀα)/60, where β = Oꢀ(1)ꢀMn(1)ꢀN(2) =
161.46° and α = O(2)ꢀMn(1)ꢀN(1) = 154.28°; for perfect square pyramidal and trigonal bipyramidal
10

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geometries the τ values are zero and unity respectively]. The two phenolate oxygens and two tertiary
amine nitrogens of the tetradentate ligand are bonded to Mn(III) at the four corners of the basal plane
of the square pyramid and the chloride ion occupies the apical site. The chloride ion interacts with
Mn(III) (MnꢀCl, 2.3946 Å) weaker than the phenolate (MnꢀO1, 1.8558; MnꢀO2, 1.8671 Å) and
amine nitrogen donors (MnꢀN1, 2.0452; MnꢀN2, 2.0389 Å) as expected. The compound 3
crystallizes in the orthorhombic crystal system with the space group Cmc21. The dianion of the
tetradentate ligand H2(L3) is coordinated to Mn(III) in a symmetric fashion leading to both the MnꢀO
(1.858 Å) and both the MnꢀN (2.052 Å) bond distances being the same. Like 1, the coordination
environment around Mn(III) in 3 is perfectly square pyramidal without any distortion towards
trigonal bipyramidal (τ, 0.00; where β = O(1)ꢀMn(1)ꢀN(1a) = 159.47° and α = O(1a)ꢀMn(1)ꢀN(1) =
159.47°), and the MnꢀO, MnꢀN and MnꢀCl bond distances being almost similar to those in 1. The
MnꢀOphenolate bond length in 3 is almost the same as those in 1; however, the MnꢀNamine bond in 3 is
longer than those in 1, and the MnꢀCl bond in 3 is shorter than those in 1. The incorporation of two
electronꢀreleasing methyl groups at 3,5 positions on the phenolate ring is expected to make the MnꢀO
bond shorter than those in 1; however, the methyl group in the 3 position would sterically hinder the
coordination of the phenolate oxygen. This results in rendering the MnꢀO bond distance in 3 almost
the same as those in 1. The bond angles OꢀMnꢀN (91.64(7)° ꢀ 93.48(6)°), OꢀMnꢀO (89.33(7)° ꢀ
92.64(9)°), OꢀMnꢀCl (99.54(5)° ꢀ 107.40(5)°), NꢀMnꢀCl (97.34(4)° ꢀ 97.95(7)°) and OꢀMnꢀN
(154.28(6)° ꢀ 161.46(6)°) in both 1 and 3 deviate from the ideal octahedral angles of 90° and 180°
respectively revealing the presence of significant distortion in the Mn(III) coordination geometry.
The mode of coordination of H2(L3) in 3 is similar to that previously observed41 for the
complex [Mn(L)Cl], where L is the Schiff base (1R,2R)ꢀ(ꢀ)ꢀbis(3,5ꢀdiꢀtertꢀbutylsalicylidine)ꢀ1,2ꢀ
cyclohexanediamine with N2O2 donor set. The MnꢀNamine bonds in the present complexes (1, 2.0389 ꢀ
2.0452; 3, 2.052 Å) are longer than the MnꢀNimine bonds (1.973, 1.994 Å), which is due to sp3 and sp2
hybridizations respectively of amine and imine nitrogen donors, whereas the MnꢀOphenolate bond
length (1.8558 ꢀ 1.8671 Å), which is very close to those (1.858 ꢀ 1.870 Å) observed in the previously
reported Mn(III) complexes.41 Also, the structures of 1 and 3 are similar to that of [Mn(acen)Cl]
complex,42 where acen is the dianion of the tetradentate ligand N,N’ꢀethyleneꢀ
bis(acetylacetoneimine). The MnꢀO bonds in the present complexes (1, 1.8558, 1.8671 Å; 3, 1.858
Å) are shorter than that (1.901 Å) in [Mn(acen)Cl], which is expected of the higher basicity of
phenolate oxygen than the enolate oxygen (pKa, BH+: phenol, 9.95; acetylacetone, 8.95) of
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H2(acen).42 The chloride ion in 1 and 3 interact less strongly (MnꢀCl, 1, 2.3946; 3, 2.3885 Å) than in
[Mn(acen)Cl] (MnꢀCl, 2.360ꢀ2.385 Å),41,42 which is due to the stronger binding of the tetradentate
phenolate ligands. The mode of coordination of the tetradentate ligand H2(L3) in 3 is similar to that
in the analogous iron(III) complex [Fe(L3)Cl].34 The MnꢀOphenolate bond length in 3 is (1.858 Å) very
close to that of FeꢀOphenolate bond length (1.855 Å) but the MnꢀNamine bond (2.052 Å) in 3 is shorter
than the FeꢀNamine bond (2.202 Å) in the latter. As a consequence, the MnꢀCl bond (2.3885 Å) is
longer than the FeꢀCl bond (2.2470 Å). This suggests that the smaller Mn(III) prefers to bind to
nitrogen donor than chloride ion donor and that the crystal field stabilization energy (CFSE) of
Mn(III) contributes to the stronger MnꢀNamine bond.
Electronic Absorption Spectral studies
The electronic spectral data of the Mn(III) complexes are summarized in Table 3 and the
typical electronic absorption spectrum of 1 is shown in Figure 3. Upon varying the substituents on
the ligand no significant changes are observed in the spectra of the complexes, which is expected.
The very intense and high energy band observed in the range 200 ꢀ 285 nm is assignable to ligandꢀ
based transitions. The intense band around 365 nm is assigned to the dxz,yz → π* charge transfer
transition while the intense band with split feature (522, 556 nm) is assigned to the dx ꢀy
2
2
→ π*
charge transfer transition. On this intense low energy LMCT band is possibly superimposed the
5
5
ligand field transition B2 → E, which is expected42 to show a low intensity split feature in the
present distorted square pyramidal coordination geometry. The lowꢀenergy 5B2 → 5A1 and 5B2 → 5B1
ligand field bands expected42 are not observed. A DFT study has been performed to confirm the
above assignment of the electronic absorption bands observed. The computed optimized ground state
geometries of 1 and 3 (Figure 4) are in good agreement with the experimentally determined crystal
structures. The absorption spectral bands calculated for these structures are in close agreement with
the experimentally observed ones in acetonitrile solvent. The absorption peak at 281 nm for complex
1 originates from HOMOꢀ9 → LUMO transition, and the LMCT nature of this band is evident from
the orbital pictures (Figure 5) of the HOMOꢀ9 electron density delocalized over the ligand part and
the LUMO localized on the metal. All the other peaks are similarly assigned (Table 4). Thus the
absorption bands at 309 and 372 nm originate from the Hꢀ7 → L and Hꢀ1 → L+1 transitions
respectively and both of them are assigned as MLCT transitions. The band at 496 nm is assigned as
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Hꢀ2 → L+2 MLCT transition. Similar assignments are made for all the bands observed for 3 (Table
4).
When PhIO was added to a solution containing the Mn(III) complexes 1 - 4 in acetonitrile the
original color was intensified, indicating formation of oxoꢀmetallic intermediate species. However,
attempts to detect the highꢀvalent manganeseꢀoxo intermediate using a lowꢀtemperature UVꢀVis
spectrophotometer cell were unsuccessful, revealing that the intermediate generated during the
reaction of [Mn(L)Cl] with PhIO has low stability.
Electrochemical Studies
The electrochemical features of the Mn(III) complexes 1ꢀ4 were investigated in acetonitrile
solution by employing cyclic (CV) and differential pulse voltammetry (DPV) on a stationary
platinumꢀsphere electrode. All the present complexes display one electron Mn(III)/Mn(II) redox
process, which is irreversible (Figure 6 and S1). The E1/2 values (279 ꢀ 320 mV, Table 3) obtained
from DPV measurements fall in the range observed for similar type of Mn(III) complexes.43 No wellꢀ
defined electrochemical responses corresponding to Mn(III)/Mn(IV) redox couple are observable.
The E1/2 values of the Mn(III)/Mn(II) redox couple follow the trend 1 > 2 > 3 > 4, reflecting a
decrease in Lewis acidity of the Mn(III) center along this series. On incorporating a methyl group on
both the phenolate rings in 1 to obtain 2, and two methyl groups at 3,5 positions of both the
phenolate rings in 1 to get 3 and on replacing the two methyl groups on both the phenolate rings in 3
by two tertꢀbutyl groups to get 4 the negative charge built on the metal center increases causing a
shift in redox potential to less positive values. It is worth noting that the negative charge built on
Mn(III) center would tend to facilitate the formation and stabilization of the highꢀvalent metalꢀoxo
intermediate involved in the olefin epoxidation reactions (cf. below).
Catalytic Properties of Manganese(III) Complexes
The catalytic activity of all the complexes towards olefin epoxidation using PhIO was
investigated (Scheme 3). Typical reaction conditions employed in these reactions involve 1
equivalent of catalyst, 20 equivalents of PhIO (oxygen source) and 150 equivalents of substrate
(olefin) and decane (internal standard) at room temperature in acetonitrile. The reaction mixture was
stirred for 2 h at room temperature under nitrogen atmosphere and then passed through a silica
column. The resulting solution was directly analyzed by GC and GCꢀMS. The yields of products
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were determined by comparison of peak area with those of known authentic samples. All the
complexes show moderate yield and good selectivity under the above mentioned conditions. It is
remarkable that all the complexes show significant catalytic activity within 2 h (Tables 5 and 6) in
comparison to MnCl2ꢁ4H2O, which is a poor catalyst under the same experimental conditions.
Catalysts
PhIO
1- 4
O
A
O
O
Catalysts
PhIO
1- 4
B
A
OH
B
O
Catalysts
PhIO
O
1- 4
A
C
Scheme 3. Epoxidation of alkenes by manganese(III) complexes
The encouraging yields for the epoxidation of cyclooctene catalyzed by the present complexes
prompted us to improve the yield by optimizing the reaction conditions in terms of solvent and
different additives. The influence of solvent on epoxidation was explored by carrying out the
reactions in different solvent media (methanol, dichloromethane and acetonitrile) and the highest
activity44 (results are not shown here) was achieved in acetonitrile among the solvents used as
reported in the literature.44 Our present complexes catalyze the epoxidation with the yields of
products lower than those for previously reported MnIIIꢀ(salen) complexes;13 however, the selectivity
is enhanced for epoxidation of cyclohexene and styrene.
Cyclooctene oxidation
The epoxidation of cisꢀcyclooctene catalyzed by [Mn(L)Cl] complexes proceeds with
moderate conversion (12.1ꢀ28.6%; Table 5) but with a high selectivity (100%) for cisꢀepoxide. The
yield of cisꢀcyclooctene oxide depends upon the catalyst and follows the trend 1 > 2 > 3 > 4, which
represents a decrease in epoxide yield with decrease in Lewis acidity of the Mn(III) center along the
series, as reflected in the MnIII/MnII redox potential (cf. above). Upon adding one equiv. of Nꢀ
methylimidazole (NꢀMeIm) as additive the product conversion increases from 12.1ꢀ28.6 to 18.0ꢀ
35.1% (Table 6). Interestingly, the product yield follows the trend 1 > 2 > 3 > 4, which is the same as
that observed in the absence of added base. At low concentrations the coordination of monodentate
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NꢀMeIm increases the electron density on Mn(III) center leading to destabilize the reactive highꢀ
valent manganeseꢀoxo species (L)MnV=O, which has been proposed45 as the reactive intermediate
for oxygen transfer reactions, and enhances the yield. At higher concentrations of the added base the
product yield decreases because the additive competes for the coordination geometry around the
metal center, and discourages the binding of the oxidant to Mn(III) center. Similar effect of added Nꢀ
MeIm has been observed30 for olefin epoxidation catalysed by Mn(III)ꢀsalen45 and
metalloporphyrins.46
To illustrate the above observations we propose a twoꢀway reaction mechanism for the
present catalytic reactions: In one mechanism, the manganeseꢀoxo intermediate, formed by the
interaction of the catalyst with PhIO (Scheme 4A), attacks the olefinic double bond in a concerted
pathway, which is similar to the one involving [(salen)MnV=O] intermediate proposed20dꢀe,21c for the
epoxidation reactions using PhIO. Such an intermediate [(salen)(Cl)MnV=O] species has been
characterized directly by mass spectrometry26 and indirectly by NMR spectroscopy.27 The lowest
Lewis acidity of 4, conferred by the electronꢀreleasing tertꢀbutyl substituents on the phenolate, would
tend to stabilize the (L)MnV=O intermediate species and decrease its reactivity and hence the amount
of epoxidation product. Similar observations of decrease in epoxide yield upon incorporating
electronꢀreleasing substituents on the diimine back bone of MnIII(salen) complex has been made.11 In
the second mechanism, the intermediate species [(L)MnꢀOꢀI+ꢀPh], which is similar to that proposed
for the epoxidation reactions using Mn(III)ꢀsalen complexes and iodoarenes,47 attacks cyclooctene
directly to form the carbo cation [(L)MnꢀOꢀC8H14]+, which collapses following the inꢀcage electronic
rearrangement to give the epoxide product and regenerates the original complex (Scheme 4B). We
have already invoked a similar type of reactive intermediate to illustrate the mechanism of the
epoxidation reaction.30 Also, we have elegantly shown48 that the intermediate peroxido species
[Cl(ntb)RuꢀOꢀOꢀtBu]+, where ntb = tris(benzimidazolꢀ2ꢀylmethyl)amine, detected by ESIꢀMS,
attacks the olefin directly to form the epoxide, without invoking the formation and involvement of
highꢀvalent [(ntb)RuIV=O] species. It may be noted that the adduct formation between the olefin and
the Mn catalystꢀPhIO adduct is facilitated by the most Lewis acidic Mn(III) complex 1 leading to the
highest epoxide yield observed. Thus, as one moves from 1 to 4 the Lewis acidity of the Mn(III)
center decreases, due to incorporation of electronꢀreleasing methyl and tertꢀbutyl groups as
substituents on the phenolate ring, resulting in the decrease in yield of the epoxide product, which
supports our above proposal.
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O
(N2O2)MnIII
+
ΙΙΙ
_
I
+
Ph
(N2O2)Mn
O
O
I
Ph
I
A
Ph
ꢀPhI
O
V
III
V
(N2O2)Mn
O
Mn(N2O2)
+
O
Mn(N2O2)
MnIII(N2O2)
O
O
MnIII(N2O2)
-PhI
(N2O2)MnIII
B
O
+
I+
Ph
+
Scheme 4. Proposed reaction mechanism for the epoxidation using Mn(III) complexes and PhIO
The epoxide yield observed for 1 (28.6%) is lower than that (35.0%) for
[Mn(BPMEN)(CF3SO3)2],
where
BPMEN
is
N,N’ꢀdimethylꢀbis(2ꢀpyridylmethyl)ꢀ1,2ꢀ
diaminoethane, with no diazacycloalkane backbone.19 This illustrates that the bulky diazapane
backbone in 1 sterically hinders the extent of interaction of the olefin with the catalystꢀPhIO adduct.
Also, the replacement of both the pyridylmethyl moieties in [Mn(L5)Cl2],30 where L5 is bis(2ꢀ
pyridylmethyl)ꢀ1,4ꢀdiazepane, by two phenolate donors to give 1 leads to decrease the yield (28.6%)
of cyclooctene oxide from 33.1 to 28.6% but with the selectivity remaining constant as 100%. The
strongly coordinating phenolate moieties would facilitate the formation of the Mn(V)ꢀoxo
intermediate and increase its stability and hence decrease its reactivity leading to a lower epoxidation
yield. Also, the possibility of adduct formation between the oxidant PhIO and the octahedral
[Mn(L5)Cl2] complex with two replaceable chloride ions (MnIIꢀCl, 2.456 Å) is higher than that
between PhIO and the fiveꢀcoordinate [Mn(L1)Cl] complex with only one easily replaceable chloride
ion (MnIIIꢀCl, 2.3946 Å). In this regard it is to be noted that the bulky substituents present on the
diimine bridges of achiral Mn(III)ꢀsalen complexes enhance11 the catalytic activity of the complexes
towards epoxidation while electronꢀreleasing substituents like tertꢀbutyl group on the phenolate ring
of the complexes decrease the catalytic activity by stabilizing the reactive [(salen)(X)MnV=O]
intermediate and making it less reactive. Also, a recent computational study49 has concluded that the
sideꢀon approach of the olefin towards the reactive [(salen)(X)MnV=O] intermediate is favorable
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more from the salicylaldehyde side rather than from the imine portion of the complex catalyst.
Similarly, the bulky diazapane backbone in 1 may be effective in forcing the Mn=O bond in the
reactive intermediate to bend away from it and facilitating the approach of the olefin substrate
leading to enhance the catalyst efficiency. However, the epoxide yield (28.6%) for the complex 1 is
lower than those (30 – 59%) for the previously reported [Mn(salen)](PF6) complexes,11 which may
be due to the bulky diazapane bridge as discussed above. Thus the catalytic activity of the metal
complex is controlled and tuned by the ligand electronic as well as steric factors imposed on the
metal center. It would be interesting to study the use of the present catalysts for epoxidation of chiral
alkenes, which would throw light on the reaction mechanism.
Cyclohexene oxidation
The epoxidation of cyclohexene has been examined in acetonitrile solvent at room
temperature using 1-4 as catalysts and PhIO as oxygen source under nitrogen atmosphere. The major
product observed is cyclohexene oxide (2.4ꢀ17.1%, Table 5) the yield of which follows the order 1 >
2 > 3 > 4. In addition, the allylic oxidation products 2ꢀcyclohexenꢀ1ꢀol and 2ꢀcyclohexenꢀ1ꢀone are
also obtained and the latter is the further oxidized product of the former involving an oxidative
dehydrogenation pathway50 catalysed by the complexes. The variation in yield of cyclohexene
epoxidation is illustrated by invoking a mechanism, which involves the highꢀvalent (L)MnV=O
intermediate, which is similar to the reactive intermediate50 (4N)RuIV=O, and [(L)MnꢀOꢀI+ꢀPh]
species proposed above for the cyclooctene epoxidation (Scheme 4A and 4B). The observation of
increased allylic oxidation products and decreased epoxide formation, is because of facile abstraction
of allylic proton by the intermediate species (L)MnV=O or by the species [(L)MnꢀOꢀI+ꢀPh] followed
by inꢀcage electronꢀtransfer to form the allylic alcohol product (Scheme 5). Similar observations
have been made previously for Ru,50 Mn30,51 and Fe complexes.52 The lower selectivity observed for
1 - 4 is due to the preferential attack of the allylic CꢀH bond rather than the olefinic double bond by
(L)MnV=O and [(L)MnꢀOꢀI+ꢀPh] species. Control experiments for cyclohexene oxidation without the
catalyst gave a total yield of only <6% under aerobic conditions, which is higher than the total yield
of < 2% obtained under anaerobic (N2) conditions for the same reaction, revealing the contribution of
autoꢀoxidation of cyclohexene in the presence of oxygen, occurring through a free radical chain
mechanism.53
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MnIII(N2O2)
H
+
OH
H
O
MnIII(N2O2)
O
(N2O2)MnIII
-PhI
+
I+
Ph
Scheme 5. Proposed reaction mechanism for allylic oxidation of cyclohexene by using Mn(III)
complexes and PhIO
In the presence of NꢀMeIm the product conversion increases (Table 6) and the yield of cyclohexene
oxide follows the trend 1 > 2 > 3 > 4, which is the same as that observed for cyclooctene. Also, the
amount of allylic oxidation product increases with decrease in selectivity on moving from 1 to 4.
Upon incorporating electronꢀreleasing methyl and tertꢀbutyl groups on the phenolate ring the
electron density around the metal center increases (cf. above), leading to an increase in stability of
the reactive highꢀvalent manganeseꢀoxo intermediate and/or decrease in the extent of interaction of
the substrate with the catalystꢀPhIO adduct, as for the epoxidation of cyclooctene, and hence the
decrease in both the yield and selectivity of epoxide product on moving from 1 to 4. This illustrates
that the yield of epoxide product is strongly influenced by the Lewis acidity of Mn(III) center, as
determined by the substituents on the phenolate ring of the linear tetradentate ligand.
Interestingly, the product conversion and selectivity observed (27.4, 62.0%) for 1 is lower
than those (66.0, 68.0%) for the corresponding complex [Mn(BPMEN)(CF3SO3)2] with no
diazacycloalkane ligand backbone.19 This reveals that the sterically hindering diazapane ligand
backbone in 1 would decrease the extent of interaction of cyclohexene with the catalystꢀPhIO adduct,
leading to a decrease in the epoxide yield. In addition, the latter complex contains two labile sites,
which increases the extent of interaction of the substrate with the catalystꢀPhIO adduct. Also, the
replacement of both the pyridylmethyl moieties in [Mn(L5)Cl2]30 by two phenolate donors to give 1
leads to a small decrease in yield of cyclohexene oxide from 18.5 to 17.1% but a large increase in
selectivity from 18.0 to 62.0%, in spite of the availability of only one Mn site for binding of the
oxidant. A similar trend (yield, 28.0 to 23.0% and selectivity 46.5 to 69.0%) has been observed in the
presence of added Nꢀmethylimidazole. This means that incorporation of sterically hindering
diazapane ligand backbone makes 1 a better chemoselective catalyst for epoxidation of cyclohexene
rather than cyclooctene.
Styrene oxidation
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The reaction of styrene with PhIO in the presence of 1 – 4 has been also investigated under
the same reaction conditions used for cyclohexene. In addition to major amounts of styrene oxide
(14.9ꢀ36.2%; Table 5), small amount of benzaldehyde (7.9ꢀ11.9%) is also obtained as a side product.
The yield of styrene oxide follows the trend 1 > 2 > 3 > 4, which is similar to that observed for
epoxidation of cyclooctene and cyclohexene. Also, the yield obtained (19.8ꢀ41.3%) in the presence
of NꢀMeIm is higher than that obtained in its absence (Table 6) and significantly varies as 1 > 2 > 3
> 4, which is the same as that observed in its absence. This variation in yield is explained on the
basis of variation in stability of the reactive highꢀvalent metalꢀoxo intermediate and/or variation in
extent of interaction between the olefin and catalystꢀPhIO adduct, followed by inꢀcage electronic
rearrangement in the carbo cation, as illustrated above for the catalytic epoxidation of cyclooctene
and cyclohexene. The catalystꢀoxidant adduct proposed could deliver the oxygen atom to the
substrate and bypass the metalꢀoxo intermediate if the formation of the metalꢀoxo species from the
oxidantꢀcatalyst adduct is relatively slow compared to the oxygen atom transfer to the substrate
(Scheme 6A), which is similar to the mechanism proposed by Collman et al.47 The amount of
benzaldehyde detected under N2 atmosphere is lower than that in the presence of oxygen supporting
its formation through further oxidation of the carbo cation directly by oxygen or through C=C
cleavage (Scheme 6B).48,54 The negative charge built on the Mn(III) center facilitates the formation
and stabilization of highꢀvalent manganeseꢀoxo intermediate, which has been invoked as the key
intermediate responsible for olefin oxidation by both the metalloenzymes55 and their model
complexes,56 and the interaction of the substrate with the catalystꢀPhIO adduct, followed by the inꢀ
cage electronic rearrangement in the carbo cation. The complexes with electronꢀreleasing methyl and
tertꢀbutyl substituents on the phenolate donor are expected to stabilize the highꢀvalent metalꢀoxo
species to a larger extent leading to the formation of lower amount of the epoxide. This reveals that
the Lewis acidity of the Mn(III) center, as determined by the ligand donor environment, plays a vital
role in dictating the product yield.
Upon replacement of both the pyridylmethyl moieties in [Mn(L5)Cl2]30 by two phenolate
donors to give 1, the epoxide yield decreases from 46.8 to 36.2% but the selectivity increases from
77.1 to 82.0%. In the presence of NꢀMeIm base also the same trend is observed with the yield
decreasing slightly from 46.5 to 41.3% and the selectivity increasing from 76.9 to 88.0%. This means
that the incorporation of sterically hindering diazapane ligand backbone as in 1 makes the complex a
chemoselective catalyst, better for epoxidation of styrene than for cyclooctene.
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H
H
Ph
O
H
H
Ph
MΙnΙΙ(N2O2)
MnIII(N2O2)
+
-PhI
(N2O2)MVn
ΙΙΙ
n
O
O
O
O )M
(N2
2
A
B
+
I
H
H
Ph
MnIII(N2O2)
Ph
H
H
H
O
Ph
MnIII(N2O2)
O
H
H
Ph
-PhI
MnIII(N2O2)
+
O
I+
Ph
H
+
H
H
H
C=
C bo
nd
clea
vage
O
MnIII(N2O2)
+
Ph
H
Scheme 6. Proposed reaction mechanisms for epoxidation of styrene
Also, the catalytic activity of the complex 1 (epoxide, 36.2; selectivity 82.0%) is lower than that
(epoxide, 71.2; selectivity, 91.4%) of the previously reported13i [Mn(salen)Cl] complex suggesting
that the ring strain of the diazapane system sterically discourages the adduct formation between the
substrate and the catalystꢀPhIO adduct and also the formation of the reactive Mnꢀoxo intermediate as
well (cf. above). Very recently, Majumder et al., reported43 that when Mn(III)ꢀmodified Schiff base
complexes are used as the catalyst in the presence of PhIO as oxidant in acetonitrile solvent the
styrene epoxide yield and selectivity fall in the ranges of 65.0ꢀ87.0% and 81.0ꢀ92.0% respectively.
Conclusions
A new family of Mn(III) complexes of linear bis(phenolate) ligands with sterically
demanding diazacycloalkane backbone has been isolated and characterized. The single crystal Xꢀray
structures of two of the complexes contain the MnN2O2Cl chromophore with a novel square
pyramidal coordination geometry. The catalytic activity of the complexes towards olefin epoxidation
using PhIO as oxidant increases upon adding Nꢀmethylimidazole as base. The Lewis acidity of the
Mn(III) center, as revealed by the redox potential of Mn(III)/Mn(II) couple, depends upon the
substituents on the phenolate rings. The epoxide yield decreases upon decreasing the Lewis acidity
of the Mn(III) center by incorporating electronꢀreleasing methyl and tertꢀbutyl substituents on the
20

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phenolate rings and the unsubstituted complex has a strong potential to be developed into a selective
catalyst for epoxidation of cyclohexene and styrene rather than cyclooctene.
Supplementary information
The crystal structures 1 and 3 have been deposited at the Cambridge Crystallographic Data
Centre and the allotted deposition numbers are CCDC 946645 and 946644 respectively. The data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223ꢀ336ꢀ033; or eꢀmail: deposit@ccdc.cam.ac.uk.
Acknowledgments
We sincerely thank the Council of Scientific and Industrial Research, New Delhi, for Senior
Research Fellowship to M.S. This work was also supported by IndoꢀFrench Centre (IFCPAR)
[Scheme No. IFC/A/4109ꢀ1/2009/993]. Professor M. Palaniandavar is a recipient of DST Ramanna
Fellowship [Scheme No. SR/S1/RFICꢀ01/2007 and SR/S1/RFICꢀ01/2010]. We thank Department of
Science and Technology (FIST program), New Delhi for the use of the Xꢀray diffractometer in
School of Chemistry, Bharathidsan University. We thank Dr. Balachandran Unni Nair, Central
Leather Research Institute, Chennai for providing the ESIꢀMS facility.
References
1. (a) Y. Kono and I. Fridovich, J. Biol. Chem. 1983, 258, 6015. (b) W. F. Beyer Jr. and I.
Fridovich, Biochemistry, 1985, 24, 6460.
2. V. K.Yachandra, K. Sauer and M. P. Klein, Chem. Rev. 1996, 96, 2927.
3. A.W. Rutherford and A. Boussac, Science, 2004, 303, 1782.
4. R. H. Holm, P. Kennepohl and E. I. Solomon, Chem. Rev. 1996, 96, 2239.
5. E. J. Larson and V. L. Pecoraro, in: V.L. Pecoraro (Ed.), Manganese Redox Enzymes, VCH
Publisher, Inc., New York, 1992, p. 1.
6. K. A. Jørgensen, Chem. Rev. 1989, 89, 431.
7. M. J. Gunter and P. Turner, Coord. Chem. Rev. 1991, 108, 115.
8. E. N. Jacobsen, in: I. Ojima (Ed.), Catalytic Asymmetric Synthesis, VCH, Weinheim,1993, p.159.
21

Dalton Transactions
Page 22 of 36
View Article Online
DOI: 10.1039/C3DT51766G
9. (a) T. Katsuki, Coord. Chem. Rev. 1995, 140, 189. (b) T. Katsuki, J. Mol. Catal. A: Chem.1996,
113, 87.
10. T. Katsuki, in: I. Ojima (Ed.), Catalytic Asymmetric Synthesis, 2nd ed., Wiley, New York, 2000,
p. 287.
11. K. Srinivasan, P. Michaud and J. K. Kochi, J. Am. Chem. Soc. 1986, 108, 2309
12. E. N. Jacobsen and M. H. Wu, in Comphrehensive Asymmetric Catalysts, eds. Pfaltz, A.;
E. N. Jacobsen, and Yamamoto, vol.2, SpringerꢀVerlag, Berlin, 1999, p. 649
13. (a) W. Zhang, J. L. Leobach, S. R. Wilson, and E. N. Jacobsen, J. Am. Chem. Soc. 1990, 112,
2801. (b) E. N. Jacobsen, W. Zhang and M. L. Guler. J. Am. Chem. Soc. 1991, 113, 6703. (c)
E. N. Jacobsen, W. Zhang, A. R. Muci, J. R. Ecker and L. Deng, J. Am. Chem. Soc. 1991, 113,
7063. (d) W. Zhang and E. N. Jacobsen, J. Org. Chem. 1991, 56, 2296. (e) M. Paulcki, P. J.
Pospisil, W. Zhang and E. N. Jacobsen, J. Am. Chem. Soc. 1994, 116, 9333. (f) S. Chang, J. M.
Galvin and E. N. Jacobsen, J. Am. Chem. Soc. 1994, 116, 6937. (g) B. D. Brandes and E. N.
Jacobsen, J. Org. Chem. 1994, 59, 4378. (h) J. F. Larrow and E. N. Jacobsen, J. Org. Chem.
1994, 59, 1939. (i) A. R. Silva, C. Freire and B. D Castro, New. J. Chem. 2004, 28, 253.
14. L. Cavallo, and H. Jacobsen, Eur. J. Inorg. Chem. 2003, 892.
15. (a) A. Murphy, G. Dubois and T. D. P. Stack, J. Am. Chem. Soc. 2003, 125, 5250. (b A. Murphy,
A. Pace, and T. D. P. Stack, Org. Lett. 2004, 6, 3119. (c) A. Murphy and T. D. P. Stack, J. Mol.
Cat. A: Chem. 2006, 251, 78.
16. (a) L. Gomez, I. GarciaꢀBosch, A. Company, X. Sala, X. Fontrodona, X. Ribas and M. Costas,
Dalton Trans. 2007, 5539. (b) I. GarciaꢀBosch, A. Company, X. Fontrodona, X. Ribas and M.
Costas, Org. Lett. 2008, 10, 2095.
17. (a) J. T. Groves and T. E. Nemo, J. Am. Chem. Soc. 1983, 105, 6243. (b) T. G. Traylor, W.ꢀP.
Fann, D. Bandycpdhyay, J. Am. Chem. Soc. 1989, 111, 8009. (c) D. Ostovic and T. C. Bruice, J.
Am. Chem. Soc. 1989, 111, 6511. (d) C. L. Hill and Schardt, J. Am. Chem. Soc. 1980, 102, 6374.
(e) J. T. Groves, W. J. Kruper, Jr. And R. C. Haushalter, J. Am. Chem. Soc. 1980, 102, 6375. (f)
R. B. Hopkins and A. D. Hamilton, J. Chem. Soc., Chem. Commun. 1987, 171. (g) C.ꢀM. Che,
W.ꢀH. Hung and C.ꢀK. Pcmn. J. Chem. Soc., Chem. Commun. 1987, 173. (h) C.ꢀM. Che and W.ꢀ
K. Cheng, J. Chem. Soc., Chem. Commun. 1986, 1443. (i) C.ꢀM. Che and V. W.ꢀW. Yam, J. Am.
Chem. Soc. 1987, 109, 1262. (j) J. D. Kcmla and J. K. Kochi, J. Org. Chem. 1987, 52, 4545. (k)
J. D. Koola and J. K. Kochi, Inorg. Chem. 1987, 26, 908. (l) J. F. Kinneary, J. S. Albert and C. J.
Burrows, J. Am. Chem. Soc. 1988, 110, 6124. (m) M. Fontecave and D. Mansuy, J. Chem. Soc.,
Chem. Commun. 1984, 879. (n) N. Murugesan and S. M. Hecht, J. Am. Chem. Soc. 1985, 107,
493. (o) C. L. Hill and R. B. Brown, Jr. J. Am. Chem. Soc. 1986, 108, 536.
18. E. G. Samsel, K. Srinivasan and J. K. Kochi, J. Am. Chem. Soc. 1985, 107, 7606.
19. K. Nehru, S. J. Kim, I. Y. Kim, M. S. Seo, Y. Kim, S. J. Kim, J. Kim and W. Nam, Chem.
Commun. 2007, 4623.
22

Page 23 of 36
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT51766G
20. (a) W.ꢀY. Hsieh and V. L. Pecoraro, Inorg. Chem. Acta 2002, 341, 113. (b) T. J. Collins and S.
W. GordonꢀWylie, J. Am. Chem. Soc. 1989, 111, 4511. (c) T. J. Collins, R. D. Powell, C.
Slebodnick, E. S. Uffelman, J. Am. Chem. Soc. 1990, 112, 899. (d) F. M. Macdonnell, N. L. P.
Fackler, C. Stern and T. V. O’Halloran, J. Am. Chem. Soc. 1994, 116, 7431. (e) N. S. Finney, P.
J. Pospisil, S. Chang, M. Palucki, R. G. Konsler, K. B. Hansen, E. N. Jacobsen, Angew Chem.
Int.Ed. Engl. 1997, 36, 1720. (f) V. L. Pecoraro, M. J. Baldwin, M. T. Coudle, W.ꢀY. Hsieh and
N. A. Law, Pure Appl. Chem.1998, 70, 925. (g) W. Adam, K. J. Roschmann, C. R. SahaꢀMoller
and D. Seebach, J. Am. Chem. Soc. 2002, 124, 5068.
21. (a) P. Battioni, J. P. Renaud, J. F. Bartoli, M. ReinaꢀArilles, M. Fort and D. Mansuy, J. Am.
Chem. Soc. 1988, 110, 8462. (b) P. E. M. Siegbahn and R. H. Crabtree, J. Am. Chem. Soc. 1999,
121, 117. (c) B. Meunier, E. Guilmet, M. De Carvalho and R. Poilblanc, J. Am.Chem. Soc. 2000,
122, 2675. (d) D. A. Plattner, D. Feichtinger, J. ElꢀBaraoui, O. Weist, J. Mass Spectrom. 2000,
195/196, 351.
22. (a) B. Mandimutsira, B. Ramdhanie, R. C. Todd, H. Wang, A. A. Zareba, R. S. Czernuszewicz,
D. P. Goldberg, J. Am. Chem. Soc. 2002, 124, 15170. (b) H. Wang, B. Mandimutsira, R. C.
Todd, B. Ramdhanie, J. P. Fox, D. P. Goldberg, J. Am. Chem. Soc. 2004, 126, 18.
23. (a) J. T. Groves and M. K. Stern, J. Am. Chem. Soc. 1988, 110, 8628. (b) R. Arasasingham,
G. He, T. C. Bruice, J. Am. Chem. Soc. 1993, 115, 7985.
24. (a) C.T. Dalton, K. M. Ryan, V. M. Wall, C. Bousquet and D. G. Gilheany, Top. Catal.1998, 5,
75. (b) W. Adam, K. J. Roschmann, C. R. SahaꢀMöller and D. Seebach, J. Am. Chem. Soc. 2002,
124, 5068.
25. K. A. Campbell, M. R. Lashley, J. K.Wyatt, M. H. Nant and R. D. Britt, J. Am. Chem. Soc. 2001,
123, 5710.
26. D. Feichtinger and D. A. Plattner, Angew. Chem. Int. Ed. 1997, 36, 1718.
27. K. P. Bryliakov, D. E. Babushin and E. P. Talsi, J. Mol. Catal. A: Chem. 2000, 158, 19.
28. Z. Li, C.ꢀG. Xia and M. Ji, Appl. Catal. A: General 2003, 252, 17.
29. D. L. Canali and D. C. S. Sherrington, Chem. Soc. Rev., 1999, 28, 85.
30. N. Saravanan and M. Palaniandavar, Inorg. Chim. Acta. 2012, 385,100.
31. M. Velusamy, M. Palaniandavar, R. Srinivasa Gopalan and G. U. Kulkarni, Inorg. Chem. 2003,
42, 8283.
32. R. Mayilmurugan, E. Suresh, H. S. Evans and M. Palaniandavar, Dalton Trans. 2009, 26, 5101.
33. E. Y. Tshuva, I. Goldberg, M. Kol and Z. Goldschmidt, Organometallics, 2001, 20, 3017.
23

Dalton Transactions
Page 24 of 36
View Article Online
DOI: 10.1039/C3DT51766G
34. R. Mayilmurugan, M. Sankaralingam, E. Suresh and M. Palaniandavar, M. Dalton Trans. 2010,
39, 9611.
35. SMART & SAINT Software Reference manuals, version 5.0; Bruker AXS Inc.: Madison, WI,
1998.
36. Sheldrick, G. M. SADABS software for empirical absorption correction; University of Göttingen:
Germany, 2000.
37. SHELXTL Reference Manual, version 5.1; Bruker AXS Inc.: Madison, WI, 1998.
38. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G.
Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.
Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K.
N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C.
Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B.
Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J.
Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski,
G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B.
Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009.
Gaussian 09 Revision A.02, Gaussian Inc. Wallingford CT 2009.
39. S. I. Gorelsky, SWizard program (version 4.6), University of Ottawa, Canada, 2007.
40. A. W. Addition, T. N. Rao, J. Reddjik, J. Rijn and G. C. Verschoor, J. Chem. Soc. Dalton
Trans. 1984, 1349.
41. (a) P. J. Pospisil, D. H. Carsten and E. N. Jacobsen. Chem. Eur. J. 1996, 2, 974. (b) J. W. Yoon,
TꢀS. Yoon, S. W. Lee and W. Shin, Acta Cryst. 1999, C55, 1766.
42. (a) L. J. Boucher, V. W. Day, Inorg. Chem. 1977, 16, 1360. (b) A. Westphal, A. Klinkebiel,
HꢀM. Berends, H. Broda, P. Kurz and F. Tuczek, Inorg. Chem. 2013, 52, 2372. (c) R. S.
Downing and F. L. Urbach, J. Am. Chem. Soc., 1969, 91, 5977.
43. S. Majumder, S. Hazra, S. Dutta, P. Biswas and S. Mohanta, Polyhedron. 2009, 28, 2473.
44. M. Sono, M. P. Roach, E. D. Coulter and J. H. Dawson, Chem. Rev. 1996, 96, 2841.
45. T. Yamada, K. Imagawa, K. Nagata and T. Mukaiyama, Chem. Lett. 1992, 2231.
46. (a) P. Pietikäinen, Tetrahedron Lett. 1994, 35, 941. (b) P. Battioni, J. P. Renaud, J. F. Bartoli, M.
ReinaꢀArtiles, M. Fort and D. Mansuy, J. Am. Chem. Soc. 1988, 110, 8462.
24

Page 25 of 36
Dalton Transactions
View Article Online
DOI: 10.1039/C3DT51766G
47. (a) J. P. Collman, A. S. Chien, T. A. Eberspacher and J. I. Brauman, J. Am. Chem. Soc. 2000,
122, 11098. (b) J. P. Collman, L. Zeng and R. A. Decreau, Chem. Commun. 2003, 2974. (c) J. P.
Collman, L. Zeng and J. I. Brauman, Inorg. Chem. 2004, 43, 2672.
48. M. Murali, R. Mayilmurugan and M. Palaniandavar, Eur. J. Inorg. Chem. 2009, 22, 3238.
49. J. ElꢀBahraoui, O. Wiest, D. Feichtinger and D. A. Plattner, Angew. Chem., Int. Ed., 2001, 40,
2073.
50. (a) L. K. Stultz, M. H. V. Huynh, R. A. Binstead, M. Curry and T. J. Meyer, J. Am. Chem. Soc.
2000, 122, 5984. (b) K. Jitsukawa, Y. Oka, S. Yamaguchi and H. Masuda, Inorg. Chem. 2004,
43, 8119.
51. D. D. Agarwal, R. P. Bhatnagar, R. Jain, and S. Srivastava, J. Chem. Soc. Perkin Trans. 1990, 2,
989.
52. A. J. Appleton, S. Evans and J. R. L. Smith, J. Chem. Soc. Perkin Trans. 1996, 2, 281.
53. (a) F. R. Mayo, Acc. Chem. Res. 1968, 1,193. (b) D. R. Paulson, R. Ullman, R. B. Sloan, J.
Chem. Soc. Chem. Commun. 1974, 186.
54. (a) W.ꢀH. Fung, W.ꢀY. Yu and C.ꢀM. Che, J. Org. Chem. 1998, 63, 7715. (b) C.ꢀJ. Liu, W.ꢀY.
Yu, C.ꢀM. Che and C.ꢀH. Yeung, J. Org. Chem. 1999, 64, 7365.
55. (a) W. Adam, K. J. Roschmann, C. R. SahaꢀMöllera and D. Seebach, J. Am. Chem. Soc. 2002,
124, 5068. (b) C. Mantel, A. Hassan, J. Pecaut, A. Deronzier, M. N. Collomb and C.ꢀD. Toia, J.
Am. Chem. Soc. 2003, 125, 12337.
56. (a) T. Linker, Angew. Chem. Int. Ed. 1997, 36, 2060. (b) M. Palucki, N. S. Finney, P. J. Pospisil,
M. L. Guler, T. Ishida and E. N. Jacobsen, J. Am. Chem. Soc. 1998, 120, 948.
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N1
N2
Mn1
O1
O2
Cl2
Figure 1. ORTEP diagram of complex [Mn(L1)Cl] 1 showing 50% probability thermal ellipsoids
and the labeling scheme for selected atoms. All hydrogen atoms are omitted for clarity.
N1i
N1
Mn1
O1i
O1
Cl1
Figure 2. ORTEP diagram of complex [Mn(L3)Cl] 3 showing 50% probability thermal ellipsoids
and the labeling scheme for selected atoms. All hydrogen atoms are omitted for clarity.
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1
0.8
0.6
0.4
0.2
0
300
500
700
900
1100
Wavelength (nm)
Figure 3. Electronic absorption spectrum of [Mn(L1)Cl] 1 (2.0 × 10ꢀ4 M) in acetonitrile solution at
25.0 oC
1
3
Figure 4. Computed geometries of the complexes 1 and 3.
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1
HOMO
HOMO-1
HOMO-2
HOMO-3
HOMO-4
HOMO-5
HOMO-6
HOMO-7
HOMO-9
LUMO
HOMO
LUMO
LUMO+1
HOMO-1
LUMO+1
LUMO+2
HOMO-2
LUMO+2
LUMO+3
HOMO-3
LUMO+3
LUMO+4
HOMO-4
LUMO+4
LUMO+5
HOMO-5
LUMO+5
LUMO+6
HOMO-6
LUMO+6
LUMO+7
HOMO-7
LUMO+7
LUMO+8
HOMO-9
LUMO+8
3
Figure 5. Frontier Molecular Orbital (FMO) of the complexes 1 and 3.
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40
30
20
10
0
ꢀ10
ꢀ20
ꢀ30
550
450
350
250
150
50
ꢀ50
E1/2 (V)
Figure 6. Cyclic voltammogram (CV) and differential pulse voltammogram (DPV) of 1 mM
complex [Mn(L1)Cl] in a acetonitrile solvent mixture at 25.0 °C. Ag(s)/Ag+ was used a reference
electrode. Supporting electrolyte: 0.1 M TBAP, Scan rate: 50 mV sꢀ1 for CV and 5 mV sꢀ1 for DPV.
29