Manganese(II) complexes of pyridyl-appended diazacyclo-alkanes: Effect of ligand backbone ring size on catalytic olefin oxidation

Article abstract of DOI:10.1016/j.ica.2012.01.009

A series of Mn(II) complexes [Mn(L)Cl₂] 1-5, where L is a tetradentate 4N ligand such as N,N-bis(2-pyridylmethyl)-1,2-diaminoethane (L1A), 1,4-bis(2-pyridylmethyl)piperazine (L2), N,N-bis(2-pyridyl-methyl) hexahydropyrimidine (L3), N,N-bis(2-pyridylmethyl)-1,4-diazepane (L4) and N,N-bis(2-pyridylmethyl)-1,5-diazocane (L5), has been isolated, characterized by using electronic and ESI-MS spectral techniques and screened for catalytic olefin oxidation with a representative set of olefins. Interestingly, when the ligand N,N-bis(2-pyridylmethyl)imidazolidine (L1) is treated with MnCl ₂·6H₂O in methanol it undergoes imidazolidine ring hydrolysis to form the complex [Mn(L1A)Cl₂] possessing a distorted octahedral coordination geometry around Mn(II). The complex [Mn(L3)(OTf) ₂(H₂O)] contains Mn(II) with a distorted pentagonal bipyramidal coordination geometry while [Mn(L4)Cl₂] contains Mn(II) with an octahedral coordination geometry. The complex [Mn(L5)Cl₂] adopts a rare trigonal prismatic coordination geometry, presumably because of steric interactions imposed by the ligand backbone. The catalytic ability of the solvent coordinated complex species [Mn(L)(ACN)₂]²⁺ show significant activity towards olefin epoxidation using iodosylbenzene (PhIO) as oxygen source and addition of N-methylimidazole to the reaction mixture increases the epoxide yield. The epoxidation of cis-cyclooctene catalyzed by the complexes proceeds with high conversion (22-65%) and selectivity (100%). The epoxide yield and product selectivity increase upon increasing the Lewis acidity of the Mn(II) center, as modified by the variation in the diazacycloalkane ligand backbone.

Full text of DOI:10.1016/j.ica.2012.01.009

Inorganica Chimica Acta 385 (2012) 100–111
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Manganese(II) complexes of pyridyl-appended diazacyclo-alkanes: Effect of
ligand backbone ring size on catalytic olefin oxidation
Natarajan Saravanan a, Mallayan Palaniandavar a,b,
⇑
a Centre for Bioinorganic Chemistry, School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, India
b Central University of Tamilnadu, Thiruvarur 610001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Mn(II) complexes [Mn(L)Cl2] 1–5, where L is a tetradentate 4N ligand such as N,N-bis(2-pyridyl-
methyl)-1,2-diaminoethane(L1A), 1,4-bis(2-pyridylmethyl)piperazine(L2),N,N-bis(2-pyridyl-methyl)hexa-
hydropyrimidine (L3), N,N-bis(2-pyridylmethyl)-1,4-diazepane (L4) and N,N-bis(2-pyridylmethyl)-1,
5-diazocane (L5), has been isolated, characterized by using electronic and ESI-MS spectral techniques and
screened for catalytic olefin oxidation with a representative set of olefins. Interestingly, when the ligand
N,N-bis(2-pyridylmethyl)imidazolidine (L1) is treated with MnCl2ꢀ6H2O in methanol it undergoes imidazoli-
dine ring hydrolysis to form the complex [Mn(L1A)Cl2] possessing a distorted octahedral coordination geom-
etry around Mn(II). The complex [Mn(L3)(OTf)2(H2O)] contains Mn(II) with a distorted pentagonal
bipyramidal coordination geometry while [Mn(L4)Cl2] contains Mn(II) with an octahedral coordination
geometry. The complex [Mn(L5)Cl2] adopts a rare trigonal prismatic coordination geometry, presumably
because of steric interactions imposed by the ligand backbone. The catalytic ability of the solvent coordinated
complex species [Mn(L)(ACN)2]2+ show significant activity towards olefin epoxidation using iodosylbenzene
(PhIO) as oxygen source and addition of N-methylimidazole to the reaction mixture increases the epoxide
yield. The epoxidation of cis-cyclooctene catalyzed by the complexes proceeds with high conversion (22–
65%) and selectivity (100%). The epoxide yield and product selectivity increase upon increasing the Lewis
acidity of the Mn(II) center, as modified by the variation in the diazacycloalkane ligand backbone.
Ó 2012 Elsevier B.V. All rights reserved.
Received 14 August 2011
Received in revised form 1 January 2012
Accepted 5 January 2012
Available online 28 January 2012
Keywords:
Manganese(II) complexes
Diazacycloalkane ligands
Epoxidation
Electrochemistry
Catalytic properties
Iodosylbenzene
1. Introduction
and Mn enzymes [5] are involved in the oxidation of alkanes, al-
kenes and related substrates [6]. As manganese is believed to be
Oxidation reactions are among the most elementary steps of all
organic transformations and popular routes to prepare epoxides,
which are valuable building blocks in synthetic organic chemistry
and materials science [1]. The wide use of oxiranes as intermedi-
ates for production of fine chemicals as well as pharmaceuticals
stimulated the development of oxidation technologies using oxy-
gen donors such as molecular oxygen, peroxides, peracids, PhIO
and its derivatives, sodium hypochlorite, etc. The development of
catalytic epoxidation agents that are rapid, selective, scalable and
inexpensive with a wide substrate scope remains an important
goal. Since metalloenzymes catalyze the oxygenation reactions
with high regio- and stereoselectivity under mild conditions, bio-
mimetic oxygenation reactions using enzyme model compounds
have attracted much attention among the communities of bioinor-
ganic and oxidation chemistry [2]. Also, it is interesting that
biological systems like cytochrome P450 [3], non-heme iron [4]
catalytically active in a variety of metalloenzymes [7], its chemis-
try has received considerable attention and manganese-based cat-
alysts have been widely used for the oxidation of olefins to
epoxides [8]. Although numerous procedures have been developed
[9], the need to understand the mechanism of manganese medi-
ated oxygenation processes demands the synthesis and study of
new and already known catalysts.
Initial attempts using porphyrin-based catalysts with H2O2 as
oxidant for alkene epoxidation were unsuccessful due to dismuta-
tion of H2O2 into H2O and O2, leading to fast depletion of the
oxidant [10]. Many manganese porphyrins, which are able to cata-
lyze the epoxidation of olefins with high efficiency using NaOCl
[11], PhIO and its derivatives [12] and m-chloroperbenzoic acid
(m-CPBA) [13] as oxidants, have been reported. Only a few non-
heme monomeric Mn(II) complexes are known to be efficient
epoxidation catalysts with H2O2 [14]. Jacobsen et al. [15] and Kat-
suki and co-workers [16] have independently studied Mn–salen
type complexes combined with special attention to asymmetric
catalysis by using PhIO as oxidant and obtained highly enantiose-
lective (89–98% enantiomeric excess) epoxidation by Mn–salen
complexes derived from 1,2-diaminocyclohexane. Stack and
⇑
Corresponding author at: Central University of Tamilnadu, Thiruvarur 610001,
India. Tel.: +91 431 2407125; fax: +91 431 2407043.
E-mail addresses:
palanim51@yahoo.com,
palaniandavarm@gmail.com
(M. Palaniandavar).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2012.01.009

N. Saravanan, M. Palaniandavar / Inorganica Chimica Acta 385 (2012) 100–111
101
co-workers have screened a series of monomeric Mn(II) complexes
H
H
of pyridine based ligands for olefin epoxidation reaction using per-
acetic acid and identified that [Mn(bpy)2(OTf)2] (bpy = 2,20-bipyri-
dine and OTf = trifluoromethanesulfonate) is the most active
catalyst [17]. Costas et al. have described chiral Mn(II) complexes
which are robust in epoxidising a wide range of olefins with good
yields but show moderate selectivity (15–46% enantiomeric ex-
cess) under mild experimental conditions [18]. Very recently,
Browne et al. showed that pyridine-2-carboxylic acid formed
in situ via decomposition of TPTN/TPEN ligands combine with Mn
salts to function as oxidation catalyst using H2O2 in acetone as sol-
vent [19]. It has been concluded that PhIO can be used as a single
oxygen atom transfer reagent in a wide variety of studies with the
purpose of generating reactive high-valent manganese–oxo inter-
mediates analogous to those occurring in enzymatic reactions
[20]. Also, Mn(II/III) complexes show highly selective epoxidation
reactions using NaOCl and PhIO, which are considered to oxidize
Mn(III)–salen complexes directly to the intermediate manga-
nese–oxo species [21]. It is interesting to note that Nam and co-
workers have also observed two different reactivities in alkane
hydroxylation when PhIO and CH3COOOH are used as oxidants
[22].
N
N
N
N
N
N
N
N
N
N
N
N
L2
L1A
L1
N
N
N
N
N
N
N
N
N
N
N
N
L5
L3
L4
Scheme 1. Ligands employed in this study.
potential of MnII/MnIII couple, strongly influence the catalytic
activity of the complexes.
2. Experimental
In oxygen atom transfer processes catalyzed by Mn complexes
of both heme and non-heme ligands, pentavalent MnIV and MnV
oxo moieties have been characterized by using various spectro-
scopic methods and X-ray crystallography and proposed as the ac-
tive intermediate for alkane oxidation, olefin epoxidation,
halogenations, electron-transfer reactions and photosynthetic
water oxidation [23–30]. High-valent metal hydroxo complex spe-
cies like peroxo manganic acid [29] [MnIVL(O)(OOH)]+ and [MnIV
(salen)(OH)] with the sterically hindered salen platform [31] have
been proposed as intermediates in catalytic oxidation reactions.
Busch et al. have crystallographically characterized a mononuclear
MnIV complex containing a pair of hydroxo ligands and an ultra ri-
gid ethylene cross-bridged macrocyclic ligand. The mechanism of
oxygen-atom transfer from such high-valent manganese–oxo
intermediates to organic substrates has stimulated great interest
as a result of its relevance to a wide range of important synthetic
and biological pathway. Although significant progress has been
made in this field, the effect of non-heme ligands on the oxidizing
ability of the high-valent metal–oxo intermediate has been poorly
understood. Also, the search for efficient, robust and recyclable cat-
alysts remains an important task in homogeneous catalysis [32].
All the above observations prompted us to isolate Mn(II) com-
plexes of a series of systematically varied tetradentate 4N ligands
such as N,N0-bis(2-pyridylmethyl)-1,2-diaminoethane (bispicen,
L1A), N,N0-bis(2-pyridylmethyl)piperazine (L2), N,N0-bis(2-pyri-
dyl-methyl)hexahydropyrimidine (L3), N,N0-bis(2-pyridylmethyl)-
1,4-diazepane (L4) and N,N0-bis(2-pyridylmethyl)-1,5-diazocane
(L5) (Scheme 1) and study the effect of diazacycloalkane backbone
of tetradentate ligands on the catalytic activity of the complexes
towards epoxidation of unfunctionalized olefins and also to under-
stand the effect of the novel ligand systems in stabilizing manga-
nese in higher oxidation states. The single crystal X-ray
structures of the complexes [Mn(L1A)Cl2] 1, [Mn(L4)Cl2] 4 and
[Mn(L5)Cl2] 5 have been determined to show that the complexes
contain two readily replaceable chloride ions in cis positions,
which are convenient for catalytic olefin epoxidation. It would be
interesting to obtain a correlation between the coordination geom-
etry and reactivity of the complexes as the linear 4N ligands are
expected to offer changes in coordination geometry around the
metal center. The present Mn(II) complexes exhibit significant cat-
alytic activity towards epoxidation of alkyl- and aryl-substituted
olefins with high yields (54–84%) by using PhIO as the oxygen
source. Also, the divergent flexibility as well as Lewis basicity of
the ligands bound to Mn(II), as understood from the redox
2.1. Materials
Pyridine-2-carboxaldehyde, ethane-1,2-diamine, propane-1,2-
diamine, sodium triacetoxy-borohydride, sodium borohydride,
piperazine, homopiperazine, 1,5-diazacyclooctane dihydrobro-
mide, 2-picolylchloride hydrochloride, iodobenzene diacetate,
cyclooctene, cyclohexene, styrene (Aldrich), paraformaldehde and
triethylamine (Merck, India) were used as received. Acetonitrile,
dichloromethane, ethylacetate, chloroform, diethylether, hexane
(Merck, India) were used as received.
2.2. Synthesis of ligands
2.2.1. General condensation procedure
A common two step procedure was followed for the syntheses
of ligands L1 and L3 and the first step in the preparation of
N,N0-bis(2-pyridylmethyl)-1,2-diaminoethane and N,N0-bis(2-pyri-
dylmethyl)-1,2-diaminopropane, which were then reacted with
paraformaldehyde in the second step to obtain L1 and L3,
respectively.
Step 1: N,N0-Bis(2-pyridylmethyl)-1,2-diaminoethane (A) and
N,N0-bis(2-pyridylmethyl)-1,2-diaminopropane (B) were prepared
as reported [33] as yellow oils, respectively with 72% and 73%
yields. 1H NMR (400 MHz, CDCl3): A: d 2.25 (s, 3H), 2.65 (s, 2H),
3.15 (s, 2H), 7.4 (m, 3H), 8.45 (d, H); B: 1.70 (quint, H), 2.20 (s,
3H), 2.50 (s, 2H), 3.60 (s, 2H), 7.5 (m, 3H), 8.45 (d, H) ppm,
respectively.
Step 2: N,N0-Bis(2-pyridylmethyl)imidazolidine (L1) and N,N0-
bis(2-pyridylmethyl)-hexahydropyrimidine (L3) were prepared as
reported [34] and isolated as dark yellow oils respectively with
76% and 79% yields. 1H NMR (400 MHz, CDCl3): L1: d (L1) 2.96 (s,
4H), 3.61 (s, 2H), 3.92 (s, 4H), 7.17 (t, J = 12.4 Hz 2H), 7.48 (d,
J = 7.6 Hz 2H), 7.67 (t, J = 17.2 Hz 2H), 8.55 (d, J = 5.6 Hz 2H). L3:
d (L3) 2.72 (s, 4H), 3.40 (s, 2H), 3.92 (s, 4H), 7.17 (t, J = 12.4 Hz,
2H), 7.48 (d, J = 7.6 Hz, 2H), 7.67 (t, J = 17.2 Hz, 2H), 8.55 (d,
J = 5.6 Hz, 2H) ppm.
2.2.1.1. Synthesis of N,N 0-bis(2-pyridylmethyl)piperazine (L2). This
was prepared by using the procedure reported [35] already. Piper-
azine (1.010 g, 11.73 mmol) was treated with picolylchloride
hydrochloride (3.943 g, 24.04 mmol) to give L2 as a pale yellow
crystalline solid, which was used for complex preparation without

102
N. Saravanan, M. Palaniandavar / Inorganica Chimica Acta 385 (2012) 100–111
further purification. Yield: 60%; 1H NMR (CDCl3, 400 MHz) d 2.57
(s, 8H), 3.67 (s, 4H), 7.15 (m, 2H), 7.39 (d, 2H), 7.64 (m, 2H), 8.55
(m, 2H) ppm.
2.2.2.4. [Mn(L4)Cl2] (4). This complex was prepared in a manner
analogous to that described for 2 using L4 instead of L2. Pale white
crystals suitable for X-ray diffraction were grown from a slow
evaporation methanolic solution of the complex. Yield: 76%; Anal.
Calc. for C17H22Cl2MnN4: C, 50.02; H, 5.43; N, 13.72. Found: C,
50.02; H, 5.31; N, 13.24%.
2.2.1.2. Synthesis of N,N 0-bis(2-pyridylmethyl)-1,4-diazepane (L4).
This was prepared by reductive amination [36]. A mixture of
homopiperazine (0.66 g, 6.22 mmol) and pyridine-2-carboxalde-
hyde (1.41 g, 13.2 mmol) was used to give L4 as a pale yellow oil,
which is used for the complex preparation as such without further
purification. Yield: 64%; 1H NMR (CDCl3, 400 MHz) 1.74 (pentet,
J = 5.9 Hz, 2H), 2.67 (s, 4H), 2.71 (t, J = 5.8 Hz, 4H), 3.72 (s, 4H),
7.03 (m, 2H), 7.37 (d, 2H), 7.54 (m, 2H), d 8.43 (m, 2H) ppm.
2.2.2.5. [Mn(L5)Cl2] (5). This complex was prepared in a manner
analogous to that described for 2 using L5 instead of L2. Colorless
crystals suitable for X-ray diffraction were grown from a slow
evaporation methanolic solution of the complex. Yield: 85%; Anal.
Calc. for C18H24Cl2MnN4: C, 51.20; H, 5.73; N, 13.27. Found: C,
51.11; H, 5.54; N, 13.12%. ESI-MS of methanolic solution of 5 shows
prominent peak at m/z values 386.2 and 410.1 (base peak) corre-
sponding to [Mn(L5)Cl]+ and [Mn(L5)Cl + Na]+, respectively.
0
2.2.1.3. Synthesis of N,N -bis(2-pyridylmethyl)-1,5-diazocane (L5).
The procedure used was similar to that used to prepare L2. A mix-
ture of 1,5-diazacyclooctane dihydrobromide (2.327 g, 8.43 mmol)
and picolylchloride hydrochloride (2.760 g, 16.83 mmol) was used
to obtain L5 as a colorless crystalline solid, which was used for
complex preparation without further purification. Yield: 68%; 1H
NMR (CDCl3, 400 MHz): d 1.65 (pentet, J = 5.9 Hz, 4H), 2.79 (t,
J = 5.9 Hz, 8H), 3.81 (s, 4H), 7.10 (m, 2H), 7.46 (m, 2H), 7.61 (m,
2H), 8.49 (m, 2H) ppm.
2.3. Physical measurements
Elemental analyses were performed on a Perkin Elmer Series II
CHNS/O analyzer 2400. The electronic spectra were recorded on an
Agilent 8453 diode array spectrophotometer. 1H NMR spectra were
recorded on a Bruker 400 MHz NMR spectrometer. GC–MS analysis
was performed on Agilent GC–MS spectrometer using a HP-5 cap-
illary column. All ligands were purified by using Teledyne Isco
Combi Flash Rf flash chromatography. ESI mass spectra of the com-
plexes were recorded with a Thermofinnigan LCQ-6000 Advantage
Max ion trap mass spectrometer equipped with an electron spray
source by using methanol as solvent. 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 NBu4-
ClO4 (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 deoxygen-
ated and maintained under nitrogen atmosphere during measure-
ments. The E1/2 values were observed under identical conditions
for various scan rates. The instruments utilized included an EG &
G PAR 273 Potentiostat/Galvanostat to carry out the experiments
and to acquire the data. The products were quantified by using
Hewlett Packard (HP) 6890 Gas Chromatograph (GC) series
equipped with a FID detector and a HP-5 capillary column
2.2.2. Synthesis of Mn(II) complexes
2.2.2.1. [Mn(L1A)Cl2] (1). To a solution of L1 (0.25 g, 1 mmol) in
methanol (10 mL) was added MnCl2ꢀ4H2O (0.20 g, 1 mmol) in
methanol (5 mL), the solution stirred well and then cooled. The
resultant brown solution was filtered and the filtrate was left to
stand at room temperature for a week and a colorless precipitate
was obtained. This was filtered off and washed with a few drops
of ethanol, recrystallized from methanol and colorless crystals suit-
able for X-ray crystallography were grown by slow vapor diffusion
of diethyl ether into a methanolic solution of the complex. Yield:
80%; Anal. Calc. for C14H18Cl2MnN4: C, 45.67; H, 4.93; N, 15.22.
Found: C, 45.43; H, 4.62; N, 15.06%.
2.2.2.2. [Mn(L2)Cl2] (2). This complex was prepared by adding
MnCl2ꢀ4H2O (0.20 g, 1 mmol) in methanol (10 mL) to a solution
of L2 (0.26 g, 1 mmol) in methanol, the solution stirred for
30 min, and then cooled. The off-white product was filtered off,
washed with cold methanol and diethylether, and then dried under
vacuum. Yield: 72%; Anal. Calc. for C16H20Cl2MnN4: C, 48.75; H,
5.11; N, 14.21. Found: C, 48.37; H, 5.10; N, 14.02%. ESI-MS of meth-
anolic solution of 2 shows prominent peak at m/z values 347.6 and
382.13 corresponding to the [Mn2(L2)2(O)(CH3OH)]2+ and
[Mn(L2)Cl + Na]+, respectively.
(30 m ꢁ 0.32 mm ꢁ 2.5
lm).
2.4. Reactivity studies
The catalytic activity of all the complexes towards cyclooctene,
cyclohexene and styrene was examined by treating a solution of
([Mn(L1A-L5)(Sol)2]2+ prepared by treating the complexes
[Mn(L1A-L5)Cl2] with two equivalents of AgClO4 dissolved in ace-
tonitrile and centrifuging the solution to remove AgCl. The oxygen-
ated products were identified by Agilent GC–MS instrument
equipped with 7890A GC system 5975C MSD using a HP-5 capillary
column and quantified by GC using Hewlett-Packed HP 6890 series
gas chromatograph equipped with an FID detector and a HP-5 cap-
illary column (30 m, 0.32 mm i.d) with the following temperature
program: initial temperature, 80 °C, 5° minꢂ1; final temperature
250 °C; detector temperature 250 °C.
2.2.2.3. [Mn(L3)(OTf)2(H2O)] (3A). The complex [Mn(L3)Cl2] 3 was
prepared by adding MnCl2ꢀ4H2O (0.20 g, 1 mmol) in methanol
(5 mL) to a solution of L3 (0.26 g, 1 mmol) in methanol (10 mL),
stirred for 30 min, and then cooled. The resulting solution was
evaporated under vacuum and the residue was dissolved in CH2Cl2.
ESI-MS of methanolic solution of 3 shows prominent peak at m/z
values 411.5 and 457.20 corresponding to [Mn(L3)(H2O)
(CH3OH)Cl]+ and [Mn(L3)(H2O)(CH3OH)2Cl]+, respectively. A solu-
tion of AgOTf (0.52 g, 2 mmol) in DCM (30 mL) was added and
the mixture stirred in dark for 2 h under a nitrogen atmosphere.
The solution was filtered, the volume reduced to approximately
5 mL under vacuum, and pentane was added to precipitate the
product. This solid was subsequently washed with pentane and
diethyl ether to give the product as a yellow orange powder. Yield:
63%. Crystals suitable for X-ray diffraction were grown from a
CH2Cl2–pentane solution. Anal. Calc. for C18H22F6MnN4O7S2: C,
33.81; H, 3.47; N, 8.76. Found: C, 33.44; H, 3.21; N, 8.32%.
2.5. Crystal data collection and structure refinement
Single crystal X-ray diffraction data for complexes 1, 3A, 4 and 5
were collected on a Bruker SMART Apex diffractometer equipped
with a CCD area detector at 100 K with Mo-Ka (k = 0.71073 Å) radi-
ation. A crystal of suitable size was immersed in paraffin oil and
mounted on the tip of a glass fiber and cemented by using epoxy

N. Saravanan, M. Palaniandavar / Inorganica Chimica Acta 385 (2012) 100–111
103
resin. The crystallographic data and experimental parameters of
the complexes 1, 3A and 5 are listed in Tables 1 and 4 in S1 respec-
tively. For the data collection SAINT [37] software program was used
for frames of data, indexing the reflections, and determination of
lattice parameters; SAINT program for integration of the intensity
of reflections and scaling. An empirical absorption correction was
applied to the collected reflections with SADABS [38]. The structure
was solved by direct methods using SHELXTL [39] and was refined
[Mn(L3)(OTf)2(H2O)] 3A, which is supported by its X-ray structure.
The divergent flexibility of the present tetradentate diaza ligands is
expected to influence the coordination geometry as well as elec-
tronic properties of the complexes and confer a systematic varia-
tion in Lewis acidity of the Mn(II) center and hence provide a
totally different communication to the substrate, oxidizing agent
and dioxygen during oxygenation. The electronic absorption spec-
tra of complexes 1–5 exhibit no Ligand Field band, which is ex-
pected of high-spin Mn(II). In frozen methanol solution all the
complexes exhibit well-resolved six-line hyperfine EPR signals cen-
tered around g = 2.0, which is consistent with high-spin (S = 5/2)
Mn(II) species [17]. Typical spectra of complexes 1 and 3 are shown
on F2 by the full-matrix least-squares technique using the SHELXL
-
97 program package [40]. All non-hydrogen atoms were refined
anisotropically till convergence was reached for the three com-
plexes. Hydrogen atoms attached to the ligand moieties are either
located from the difference Fourier map or stereochemically fixed.
The crystallographic data and details of data collection for 1, 3A
and 5 are given in Table 1.
in Figs. S1 and S2. Conductivity measurements in acetonitrile (KM
,
6–15
X
ꢂ1 cm2 molꢂ1) reveal that the complexes behave as non-
electrolytes.[41].
3. Results and discussion
3.2. Description of molecular structure of [Mn(L1A)Cl2] 1,
[Mn(L3)(OTf)2(H2O)] 3A, [Mn(L4)Cl2] 4 and [Mn(L5)Cl2] 5
3.1. Synthesis and characterization of ligands and complexes
The X-ray crystal structure of [Mn(L1A)Cl2] 1 is depicted in
Fig. 1 together with atom numbering scheme. The selected bond
distances and bond angles are collected in Table 2. The complex
molecule possesses a distorted octahedral coordination geometry
constituted by the two pyridine nitrogen atoms (N1, N4) and two
tertiary amine nitrogen atoms (N2, N3) of the ligand L1A and
two chloride ions completing the remaining cis-coordination sites
trans to the two tertiary amine nitrogens. The linear tetradentate
The ligands L1 and L3 (Scheme 1) were synthesized according to
a known two step procedure involving condensation of pyridine-2-
carboxaldehyde with the corresponding alkylamines to form a
Schiff base followed by ring closure with paraformaldehyde. The
ligands L2 and L5 were prepared by reacting two equivalents of
picolylchloride with piperazine and 1,5-diazacyclooctane, respec-
tively. The ligand L4 was prepared by reductive amination of two
equivalents of pyridine-2-carboxaldehyde with homopiperazine
using sodium triacetoxyborohydride as reducing agent. The ligands
were treated with equimolar amounts of MnCl2ꢀ4H2O in methanol
to give mononuclear Mn(II) complexes in good yields. All the com-
plexes have been formulated as [Mn(L)Cl2] on the basis of elemental
and ESI-MS analyses, which is supported by the X-ray structures of
[Mn(L4)Cl2] 4 and [Mn(L5)Cl2] 5. Interestingly, when the ligand L1
was treated with MnCl2ꢀH2O and left to stand to obtain the complex
[Mn(L1)Cl2], only [Mn(L1A)Cl2] 1 was isolated. The imidazolidine
ring of the linear tetradentate ligand L1 has undergone hydrolysis
to form L1A in the presence of Mn(II) ion (cf. below). The complex
isolated on treating [Mn(L3)Cl2] with Ag(OTf) is formulated as
ligand adopts a cis-
a mode of coordination around Mn(II) center
with the two pyridine nitrogens trans to each other (Fig. 1). The
bond angles of N(1)–Mn(1)–N(4) (162.70(18)°), N(3)–Mn(1)–Cl(1)
(159.2(14)°) and N(2)–Mn(1)–Cl(2) (158.6(15)°) deviate markedly
from the ideal octahedral bond angle of 180° indicating that the
octahedral coordination geometry is distorted. The Mn–Npy bond
(2.324(5), 2.338(6) Å) in
1 is longer than that reported for
[Mn(BPMEN)Cl2], where BPMEN is N,N0-dimethyl-N,N0-bis(2-pyri-
dylmethyl)-1,2-diaminoethane, with symmetrical Mn–N bonds
(Mn–Npy
, 2.272(2) Å) [33]. However, the Mn–Namine bond
(2.325(6), 2.322(6) Å) in the former is shorter than that in
[Mn(BPMEN)Cl2] revealing that the orientation of the nitrogen lone
Table 1
Crystallogrphic data and structure refinement of 1, 3A and 5.
1
3A
5
Empirical formula
Formula weight (g/mol)
Crystal color
Crystal system
Space group
a (Å)
C
14H18Cl2MnN4
C18H22F6MnN4O7S2
637.44
colorless
monoclinic
P21/c
19.7016(5)
8.80609(2)
15.7990(3)
90
C18H24Cl2MnN4
422.25
colorless
monoclinic
C2/c
22.6772(6)
7.8471(2)
13.8350(4)
90
368.16
colorless
monoclinic
P21/c
16.641(9)
8.115(4)
13.182(2)
90
b (Å)
c (Å)
a
(°)
b (°)
112.407(8)
90
1645.7(15)
0.66, ꢂ0.51
4
103.122(1)
90
2669.43(10)
0.31, ꢂ0.27
4
126.09(3)
90
1989.30(12)
0.86, ꢂ0.89
4
c
(°)
V (Å3)
Residual electron density(max, min)
Z
qcalc (g/cm3)
1.486
1.591
1.410
F(000)
756
1300
876
T (K)
293
293
296
Number of reflections collected
Number of unique reflections
3224
3174
43340
4947
21070
2752
Radiation [Mo K
a
] (Å)
0.71073
1771
0.078
0.71073
3864
0.0360
0.71073
2061
0.0445
0.1119
Residuals [I > 2
r(I)]
a
R1
b
wR2
0.2223
0.0995
a
R1 = [
wR2 = {[
R
(||Fo| ꢂ |Fc||)/
R|Fo|].
b
2
R
(w(Fo ꢂ Fc2)2)/
R
(wFo4)]1/2}.

104
N. Saravanan, M. Palaniandavar / Inorganica Chimica Acta 385 (2012) 100–111
the orientation of two pyridine nitrogens trans to each other and
the chloride ions cis to each other. Such a geometry is similar to
that observed in [Mn(BPMEN)Cl2], a linear 4N ligand like L4 [33],
but different from the seven-coordinate distorted pentagonal bipy-
ramidal geometry [43] of the complex ion [Mn(L4)(CH3CN)3]2+ in
which the same ligand defines the quite ruffled equatorial plane
and three CH3CN solvent molecules complete the seven-coordinate
structure. The ability of the L4 ligand to occupy the equatorial
plane is evident also in the square pyramidal complex [35]
[Cu(L4)(ClO4)]+. Interestingly, the in situ generated complex
[Fe(L4)(CH3CN)3]2+ also had been predicted [44] to contain the
ligand in the equatorial plane with the two chloride ions
coordinated trans to each other. Interestingly, in the tetrachloroca-
techolate adduct of the iron(III) complex [Fe(L4)(cat)]+ the ligand is
bound in a cis-b orientation [36]. Thus it is evident that the
sterically unencumbered ligand L4 is capable of assuming any
orientation, cis-a or cis-b, depending on the ligand donor atoms.
Interestingly, the propylene chain in 4 adopts a chair rather than
boat conformation to minimize the steric interactions caused by
the propylene and ethylene moieties of L4 ligand (Fig. S3).
Molecular model building studies of 4 reveal that the unfavorable
steric interactions occurring between the propylene chain of the
macrocyclic ligand backbone and the two chloride ions in trans
positions force the ligand to adopt the cis-a mode of coordination
geometry.
The X-ray crystal structure of [Mn(L5)Cl2] 5 is depicted in Fig. 3
together with atom numbering scheme. The selected bond dis-
tances and bond angles are collected in Table 2. The complex mol-
ecule possesses an uncommon distorted trigonal prismatic
geometry with one of the trigonal faces (N(1)–N(2)–N(1_2)) consti-
tuted by the two pyridine (N(1), N(2)) and one amine(N(1_2))
nitrogen atoms of the tetradentate ligand and the other trigonal
face (Cl(1)–Cl(1_2)–N(2_2)) opposite to it is constituted by the
remaining amine nitrogen (N(2_2) atom of the ligand and two
chloride ions (Cl(1), Cl(1_2)). The lengths of the sides of the triangle
N(1)–N(2)–N(1_2) lie in the range 2.731(2)–4.630(3) Å while those
of the triangle Cl(1)–Cl(1_2)–N(2_2) in the range 3.490(2)–
4.438(3) Å, and the angles of both the triangles fall in the range
35.35(7)–78.80(7)°. The torsion angles about the centroid of the
triangular faces deviate markedly from the ideal angle of 0° indi-
cating that the triangular faces are not exactly parallel to each
other illustrating that the prismatic coordination environment
around Mn(II) is distorted. The Mn–Namine (2.386(3) Å), Mn–Npy
(2.361(2) Å) and the Mn–Cl bonds (Mn(1)–Cl(1), 2.463(8) Å) are
equal among themselves, which is expected of the C2 axis of
symmetry and thus the respective pyridine nitrogens, amine
nitrogens and chloride ions are symmetry related (Table 2). The
Mn–Namine bond (2.386(3) Å) is longer than the Mn–Npy bond
(2.361(2) Å) due to sp3 and sp2 hybridizations respectively of the
amine and pyridine nitrogen atoms [42]. As expected [45], the
Mn–Npy, Mn–Namine and Mn–Cl bond lengths are longer than the
respective Fe–Npy (2.305(2) Å), Fe–Namine (2.334(2) Å) and Fe–Cl
(2.428(2) Å) bond lengths in the analogous iron(II) complex
[Fe(L5)Cl2] with a similar distorted prismatic geometry [46]. Also,
the chelate ring in the iron(II) complex is in a chair conformation
but that in the Mn(II) analog is a flattened chair indicating the
strong steric clash between the two propylene groups, which is
avoided in the prismatic geometry by the flattening of the chelate
ring. Upon incorporating a methylene group in between –CH2–
groups in the diazacycloalkane ligand in 4 to obtain 5 the distorted
Fig. 1. ORTEP diagram of 1 showing 40% probability thermal ellipsoids and the
labeling scheme for selected atoms. All of the hydrogen atoms are omitted for
clarity.
pair orbital towards the Mn(II) center in the latter deviates upon
incorporation of methyl groups. The X-ray crystal structure of
[Mn(L3)(OTf)2(H2O)] 3A is depicted in Fig. 2 together with atom
numbering scheme, and the selected bond distances and bond an-
gles are collected in Table 2. The Mn(II) center in 3A adopts a rare
distorted pentagonal bipyramidal coordination geometry. The cor-
ners of the distorted pentagonal plane of the coordination polyhe-
dron are occupied by three nitrogen atoms (N2, N3, N4) of the
ligand L3, and the oxygen atoms of triflate anion (O2) and water
molecule (O3). The axial positions are occupied by the N(1) nitro-
gen of L3 and the oxygen atom O(1) of triflate anion with the N(1)–
Mn(1)–O(1) bond angle being 169.53°. The Mn–Namine bond
(2.362(3), 2.411(2) Å) is longer than the Mn–Npy bond (2.290(2),
2.323(2) Å) due to sp3 and sp2 hybridizations respectively of the
amine and pyridine nitrogen atoms [42]. The equatorial triflate
anion displays a weaker interaction with the Mn(II) center than
the axial one (Mn–O2, 2.360, Mn–O1, 2.222 Å), obviously due to
steric crowding in the equatorial plane. Out of the five equatorial
bond angles the three bond angles N(2)–Mn(1)–O(2) (165.58°),
N(4)–Mn(1)–O(3) (150.02°) and N(2)–Mn(1)–N(3) (111.78°) devi-
ate significantly from the ideal value (72°) of the pentagonal bipy-
ramidal geometry, presumably because of unfavorable steric
interactions that would occur between the two tertiary amine
nitrogens leading to strong distortion in the coordination geome-
try, and a chair conformation for the propylene moieties of the dia-
zacycloalkane ligand backbone (Fig. 2).
The crystal data for the complex [Mn(L4)Cl2] 4 are presented in
Table S1 and the ORTEP diagram along with atom numbering
scheme is presented in Fig. S3. As the R value of the structure is
high, only the geometry of the complex is discussed in comparison
with those of related complexes. The complex molecule exhibits a
distorted octahedral geometry around Mn(II) and the linear tetra-
octahedral geometry in both cis-a and trans mode of coordination
becomes unstable due to unfavorable steric interactions between
the six-membered chelate ring in 5 and leads to form the uncom-
mon trigonal prismatic geometry. Molecular model building stud-
ies reveal that the steric interactions caused between the two
propylene chains and also with the central metal atom both in
dentate ligand L4 adopts a cis-a mode of coordination leading to

N. Saravanan, M. Palaniandavar / Inorganica Chimica Acta 385 (2012) 100–111
105
Table 2
Selected bond lengths (Å) and bond angles (°) for [Mn(L1A)Cl2] 1, [Mn(L3)(H2O)(CF3SO3)2] 3A and [Mn(L5)Cl2] 5.
1
3A
5
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(3)
Mn(1)–N(4)
Mn(1)–Cl(1)
Mn(1)–Cl(2)
2.324(3)
2.332(5)
2.328(6)
2.338(5)
2.449(2)
2.454(19)
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(3)
Mn(1)–N(4)
Mn(1)–O(1)
Mn(1)–O(2)
Mn(1)–O(3)
2.290(2)
2.362(3)
2.411(2)
2.323(2)
2.222(18)
2.360(19)
2.259(2)
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(2_2)
Mn(1)–N(1_2)
Mn(1)–Cl(1)
Mn(1)–Cl(1_2)
2.361(2)
2.386(3)
2.386(3)
2.361(2)
2.463(8)
2.463(8)
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–N(3)
N(1)–Mn(1)–N(4)
N(2)–Mn(1)–N(3)
N(2)–Mn(1)–N(4)
N(3)–Mn(1)–N(4)
Cl(1)–Mn(1)–Cl(2)
Cl(1)–Mn(1)–N(1)
Cl(1)–Mn(1)–N(2)
Cl(1)–Mn(1)–N(3)
Cl(2)–Mn(1)–N(4)
Cl(2)–Mn(1)–N(1)
Cl(2)–Mn(1)–N(2)
Cl(2)–Mn(1)–N(3)
Cl(2)–Mn(1)–N(4)
71.16(18)
94.85(2)
162.70(18)
76.0(2)
95.41(2)
70.77(2)
103.39(7)
98.04(15)
92.74(17)
159.20(14)
93.35(15)
92.52(12)
158.65(15)
92.23(15)
97.48(14)
O(1)–Mn(1)–O(2)
O(1)–Mn(1)–O(3)
O(1)–Mn(1)–N(1)
O(1)–Mn(1)–N(2)
O(1)–Mn(1)–N(3)
O(1)–Mn(1)–N(4)
O(2)–Mn(1)–O(3)
O(2)–Mn(1)–N(1)
O(2)–Mn(1)–N(2)
O(2)–Mn(1)–N(3)
O(2)–Mn(1)–N(4)
O(3)–Mn(1)–N(1)
O(3)–Mn(1)–N(2)
O(3)–Mn(1)–N(3)
O(3)–Mn(1)–N(4)
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–N(3)
N(1)–Mn(1)–N(4)
N(2)–Mn(1)–N(3)
N(2)–Mn(1)–N(4)
N(3)–Mn(1)–N(4)
88.10(7)
83.80(7)
169.53(7)
95.97(7)
114.53(8)
81.28(7)
81.16(8)
84.51(7)
165.58(7)
78.66(8)
123.95(7)
87.74(8)
85.53(8)
152.01(9)
150.02(9)
89.48(8)
71.25(8)
109.02(7)
111.78(8)
70.42(8)
57.52(8)
Cl(1)–Mn(1)–N(1)
Cl(1)–Mn(1)–N(2)
84.54(6)
92.06(7)
Cl(1)–Mn(1)–Cl(1_2)
Cl(1)–Mn(1)–N(1_2)
Cl(1)–Mn(1)–N(2_2)
N(1)–Mn(1)–N(2)
Cl(1_2)–Mn(1)–N(1)
N(1)–Mn(1)–N(1_2)
N(1)–Mn(1)–N(2_2)
Cl(1_2)–Mn(1)–N(2)
N(1_2)–Mn(1)–N(2)
N(2) –Mn(1)–N(2_2)
Cl(1_2)–Mn(1)–(1_2)
Cl(1_2)–Mn(1)–(2_2)
126.39(3)
85.34(7)
132.51(7)
70.25(8)
85.34(7)
57.43(8)
130.29(8)
132.51(7)
130.29(8)
75.83(9)
84.54(6)
92.06(7)
Fig. 3. ORTEP diagram of 5 showing 30% probability thermal ellipsoids and the
labeling scheme for selected atoms. All of the hydrogen atoms are omitted for
clarity.
Fig. 2. ORTEP diagram of 3A showing 30% probability thermal ellipsoids and the
labeling scheme for selected atoms. All of the hydrogen atoms are omitted for
clarity.
ene linkage as in [Mn(L4)(CH3CN)3]2+ both the Mn–Npy bond
distances increase with the Mn–Namine bond distances remaining
almost constant [43]. As expected, on replacing the ethylene in
the ligand diazacycloalkane backbone of 4 by a methylene to obtain
3 both the Mn–Namine and Mn–Npy bond distances increase leading
to enhancement in Lewis acidity of the Mn(II) center. Further, upon
chair and boat conformations in octahedral coordination geometry
forces 5 to adopt the trigonal prismatic geometry.
For all the complexes, the observed Mn(II)–ligand bond lengths
range from 2.1 to 2.5 Å, which is expected for the high-spin Mn(II)
center. Upon connecting the two methyl groups in [Mn(BPMEN)Cl2]
to form an ethylene linkage as in [Mn(L2)(CH3CN)3]2+ and a propyl-

106
N. Saravanan, M. Palaniandavar / Inorganica Chimica Acta 385 (2012) 100–111
enlarging the ethylene linkage in 4 into propylene linkage as in 5
the Mn–Npy and Mn–Namine bond distances increase resulting in a
similar enhancement in Lewis acidity of the Mn(II) center. Thus it
is evident that changes in variation of ligand diazacycloalkane
backbone leads to variation in Lewis acidity of the Mn(II) center.
This effect is reflected in the Mn(II)/Mn(III) redox potential of the
complexes (cf. below).
The Mn(II)/Mn(III) redox potential of [Mn(L1A)Cl2] 1 is less positive
than that of [Mn(BPMEN)Cl2] (0.740 V versus SCE) [33], where
BPMEN is the N,N0-dimethyl analog of L1A, illustrating that
N-methylation leads to improper orientation of the amine nitrogen
lone pair towards Mn(II) orbital. A similar observation has been
made earlier for the iron(III) complexes of 3N ligands with N-alkyl
substituents [48]. Upon incorporating a propylene bridge in be-
tween the amine nitrogens in [Mn(L1A)Cl2] 1 to obtain 4, the
Mn(II)/Mn(III) redox potential becomes more positive suggesting
a similar improper orientation of the lone pair orbitals towards
Mn(II). Also, upon replacing the ethylene bridge in 4 by a methy-
lene bridge to obtain 3 the lone pair orbitals on the amine nitrogen
atoms in the latter deviate further from exact orientation towards
Mn(II) orbitals leading to a higher Lewis acidity of Mn(II) center
and hence a higher Mn(II)/Mn(III) redox potential for 3. Similarly,
upon replacing the ethylene bridge in 4 by a propylene bridge as
in 5 the lone pair orbitals on the amine nitrogen atoms in the latter
deviate much from exact orientation towards Mn(II) orbitals lead-
ing to increased positive charge on Mn(II) center and hence the
higher Mn(II)/Mn(III) redox potential of 5. In other words, upon
increasing the size of the diazacycloalkane ring in the ligand back-
bone on going from 4 to 5, and enhancing the steric crowding
around the metal center on going from 4 to 3 the Lewis acidity
of the Mn(II) center increases leading to significant variation in
the Mn(II)/Mn(III) redox potential.
3.3. Manganese(II) promoted hydrolysis of imidazolidine ring
In solution the ligand L1 undergoes imidazolidine ring-cleavage
reaction in the presence of Mn(II) ion. Ray et al. have reported that
the binucleating ligand 2-phenyl-1,3-bis[30-aza-40-(20-hydroxy-
phenyl)-prop-4-en-10-yl]-1,3-imidazolidine bound to Mn(II) or
Fe(III) undergoes imidazolidine ring hydrolysis with the expulsion
of one mole of aldehyde [47]. We propose that coordination of
Mn(II) to L1 with the highly strained imidazolidine ring on its
backbone leads to activation of the methylene carbon flanked by
two coordinated amine nitrogen atoms, followed by nucleophilic
attack by a water molecule leading to the formation of imi-
dazolidne ring hydrolyzed complex 1. However, 3 with a methy-
lene group also flanked by two coordinated amine nitrogen
atoms is stable towards such hydrolysis. This illustrates that it is
the ethylene linker between the amine nitrogens which tends to
confer steric congestion on the coordinated L1 ligand leading to
the activation of methylene group towards hydrolysis.
3.5. Catalytic properties
3.4. Electrochemical behavior
The catalytic activity of complexes 1–5 towards olefin epoxida-
tion was investigated by using PhIO as the oxygen source. All the
reactions were performed at least thrice, and the amounts of prod-
ucts reported represent the average obtained. In these reactions
1 equiv. of the complex, 100 equiv. of PhIO, 500 equiv. of both
the olefin substrate and decane (internal standard) are typically
mixed in acetonitrile. The solution was stirred for 1 h at room tem-
perature under nitrogen atmosphere and then passed through a
silica column. The resulting solution was directly analyzed by GC
and GC–MS. The yields of products were determined by
The electrochemical features of the Mn(II) complexes, except 2,
were investigated in acetonitrile solution by employing cyclic (CV)
and differential pulse voltammetry (DPV) on a stationary plati-
num-sphere electrode. The poor solubility of 2 in acetonitrile lim-
ited the study of its electrochemical behavior. Typical cyclic and
differential pulse voltammograms of complexes are depicted in
Figs. 4 and S4–S6. The Mn(II)/Mn(III) redox potentials (E1/2
,
Table 3) follow the trend 1 < 4 < 3 < 5, reflecting the increase in
Lewis acidity of the Mn(II) center from left to right along the series.
Fig. 4. Cyclic (CV) and differential pulse voltammograms (DPV) of [Mn(L1A)Cl2] 1 in acetonitrile solution at 25 °C. Supporting electrolyte: 0.1 M TBAP. Scan rate: 50 mV sꢂ1 for
CV and 5 mV sꢂ1 for DPV.

N. Saravanan, M. Palaniandavar / Inorganica Chimica Acta 385 (2012) 100–111
107
Table 3
epoxide formation [19]. However, when control experiments were
performed for the solvated complexes at room temperature using
1 equiv. of the complex and 100 equiv. of PhIO in the absence of
the substrate in acetonitrile solvent and the products analyzed,
no picolinic acid formation was detected. So it is evident that the
pyridyl moieties of 1–5 do not undergo any oxidative degradation
under the present catalytic reaction conditions.
Electrochemical dataa of mononuclear Mn(II) complexes in acetonitrile solution at
25 0.2 °C.
Complex
Ep,a (V) Ep,c (V) d Ep (V) E1/2 (V)
Redox process
CV
DPV
[Mn(L1A)Cl2] 0.468
0.307
0.783
0.478
–
0.160
0.114
0.148
0.387 0.389 MnII ? MnIII
0.840
MnII ? MnIII
0.552 0.553 MnII ? MnIII
0.863 MnII ? MnIII
[Mn(L3)Cl2]
[Mn(L4)Cl2]
[Mn(L5)Cl2]
0.897
0.626
–
–
The epoxidation of cis-cyclooctene catalyzed by [Mn(L)
(CH3CN)2]2+ species proceeds with both conversion (21–54%) and
selectivity (100%) for cis-epoxide higher than those previously
reported [21] for the Mn(II) complexes of linear 4N ligands such
as [Mn(BPMEN)(CF3SO3)2] and [Mn(BPMCN)(CF3SO3)2], where
BPMCN is N,N0-dimethyl-N,N0-bis(2-pyridylmethyl)cyclohexane-
(1R,2R)-diaminoethane, which exhibit conversions of only 35%
and 24%, respectively. Among the present complexes, the yield of
epoxidation product cis-cycloocteneoxide follows the trend,
1 > 4 > 5 ꢃ 2 > 3. Among 1, 3, 4 and 5 with the same octahedral
coordination geometry, the complex 1 possesses the highest reac-
tivity. Upon incorporating the diazacycloalkane ring in 1 to obtain
4, and upon replacing the ethylene bridge in 4 by a propylene
bridge to obtain 5, the Lewis acidity of the Mn(II) center increases,
as evident from the increase in Mn(II)/Mn(III) redox potential along
the above series (cf. above), leading to a decrease in ease of gener-
ating the high-valent oxo intermediate, and hence the decrease in
the epoxide yield along the above series. Similarly, upon replacing
the ethylene bridge in 4 by a methylene bridge as in 3 enormous
steric congestion is caused in the diazacycloalkane backbone, the
redox potential of the latter becomes higher than the former
resulting in a lower epoxidation activity for the latter; however,
3 shows an activity lower than 5, possibly because the dia-
zacycloalkane backbone with enormous steric congestion discour-
ages the formation of the high-valent oxo species involved in
catalysis. This also illustrates the same trend in epoxidation activ-
ity of the Mn(II) complexes towards cyclohexene and styrene.
Upon adding 2 equiv. of N-methylimidazole (N-MeIm) as addi-
tive the product conversion increases from 20–54% to 23–64%,
–
a
Potential measured ( 0.002 V) vs. Ag(s)/Ag+ (0.01 M, 0.10 M TBAP); add 0.544 V
to convert to NHE.
comparison of peak area with those of known authentic samples.
All the chloride complexes show very low yield (<5%) under these
conditions.
When
the
solvent
coordinated
species
[Mn(L)(CH3CN)2]2+ generated by treating the complexes with silver
perchlorate in acetonitrile solution to remove the coordinated
chloride ions (cf. above) was used as the catalyst, the catalytic
activity observed within 1 h (Tables 4 and 5, Scheme 2) is signifi-
cantly high in comparison to MnCl2 and Mn(ClO4)2, which are poor
catalysts under the same experimental conditions. This illustrates
that the coordinated chloride ions in the complexes are difficult
to be displaced to obtain the reactive Mn(II) complex species.
The encouraging yields observed for the epoxidation of cis-cyclooc-
tene by using the solvated complexes prompted us to improve the
yield by optimizing the reaction conditions in terms of solvent,
temperature and different additives. The influence of solvent on
epoxidation was explored by carrying out the reactions in different
solvents like methanol, dichloromethane and acetonitrile, and as
expected [49], the highest activity (results are not shown here) is
achieved in acetonitrile as solvent. It has been shown that when
a mixture of the ligand L1 and Mn(ClO4)2ꢀ6H2O was reacted with
H2O2 as oxidant in the presence of a mild base like NaOAc in ace-
tone solvent at 0 °C over 16 hours the pyridyl moiety of the ligand
underwent oxidative degradation leading to the formation of picol-
inic acid, which in the presence of the Mn(II) salt catalyzes the
Table 4
Epoxidationa of cyclooctene, cyclohexene and styrene catalyzed by Mn(II) complexesc using PhIO in acetonitrile.
Complex
Cycloocteneb
-oxide
Cyclohexeneb
-oxide
Styreneb
-oxide
Selectivity (%)
-ol
4.2
-one
Selectivity (%)
Benzaldehyde
Selectivity (%)
Mn(ClO4)2ꢀ6H2O
[Mn(L1A)(ACN)2]2+
[Mn(L2)(ACN)2]2+
[Mn(L3)(ACN)2]2+
[Mn(L4)(ACN)2]2+
[Mn(L5)(ACN)2]2+
<5
54
20.6
17.4
33.1
20.5
100
100
100
100
100
100
5.3
22.5
12.0
14.7
18.5
16.5
3.6
38.1
41.0
29.5
52.1
53.3
40.5
30.3
18.0
27.7
18.0
21.6
15.6
23.0
17.0
21.5
46.8
30
12.8
12.7
11.2
16.0
13.9
18.3
54.9
64.4
60.2
57.3
77.1
62.1
13.5
12.2
8.7
17.0
14.4
a
b
c
Conditions: ratio of catalyst:PhIO:substrate = 1:100:500 in acetonitrile.
Reaction mixtures were stirred for 1 h; yields based on oxidant used.
[Mn(L)(ACN)]2+ generated by treating the complexes with silver perchlorate in acetonitrile.
Table 5
Epoxidationa of cyclooctene, cyclohexene and styene catalyzed by Mn(II) complexesc with N-methyl imidazole additive using PhIO in acetonitrile.
Complex
Cycloocteneb
-oxide
Cyclohexeneb
-oxide
Styreneb
-oxide
Selectivity (%)
-ol
-one
Selectivity (%)
Benzaldehyde
Selectivity (%)
Mn(ClO4)2.6H2O
[Mn(L1A)(ACN)2]2+
[Mn(L2)(ACN)2]2+
[Mn(L3)(ACN)2]2+
[Mn(L4)(ACN)2]2+
[Mn(L5)(ACN)2]2+
<5
100
100
100
100
100
100
17.6
33.2
16.4
18.9
28.0
18.3
10.2
8
7.9
8.4
8.0
8.5
43.9
26.2
28.4
20.1
24.2
24.6
24.5
49.2
31.1
39.8
46.5
35.6
9.0
46.2
26.4
31.3
46.5
36.4
16.3
16.0
12.1
18.6
13.9
17.9
35.3
74.2
64.9
62.7
76.9
67.0
63.9
28.3
25.7
42.7
22.4
a
b
c
Conditions: ratio of catalyst:N-methyl imidazole:PhIO: substrate = 1:2:100:500 in acetonitrile.
Reaction mixtures were stirred for 1 h; yields based on oxidant used.
[Mn(L)(ACN)]2+ generated by treating the complexes with silver perchlorate in acetonitrile.

108
N. Saravanan, M. Palaniandavar / Inorganica Chimica Acta 385 (2012) 100–111
Catalysts
PhIO
1-5
1-5
1-5
O
OH
B
O
A
A
Catalysts
PhIO
O
C
O
O
Catalysts
PhIO
A
B
Scheme 2. Epoxidation of alkenes by Mn(II) complexes.
Fig. 5. Bar chart representation of cyclohexene epoxidation catalyzed by Mn(II)
complexes (1–5) in the presence of PhIO at room temperature in acetonitrile.
and, interestingly, the product yield follows the same trend
observed in the absence of added base, 1 > 4 > 5 ꢃ 2 > 3. At high
concentrations the coordination of monodentate N-MeIm increases
the electron density on Mn(II) center leading to stabilize the high-
valent manganese-oxo species (4 N)MnIV =O, which has been pro-
posed as the active intermediates for oxygen transfer reactions.
These species have been characterized by using various spectro-
scopic methods, and the DFT calculations confirm that this species
is indeed energetically accessible and a few of these oxo–manga-
nese(IV) compounds have been isolated successfully [30]. How-
ever, attempts to spectroscopically detect the catalyst-PhIO
adduct species or the high-valent manganese–oxo intermediate
by reacting the present complex species [Mn(L)(CH3CN)2]2+ with
PhIO even at low-temperature (ꢂ40 °C) were unsuccessful reveal-
ing their low stability. At higher concentrations of the added base
the product yield decreases as the additive competes for the coor-
dination geometry around the metal center, and discourages the
binding of the oxidant to Mn(II) center. Similar N-MeIm effects
have been observed for Mn(III)–salen [50] and metalloporphyrin
[51] catalyzed olefin epoxidation. The frozen solution EPR spectra
of the reaction mixture obtained after completion of catalytic oxi-
dation reaction show signals (Fig. S7) characteristic of high-spin
Mn(II), revealing that the catalyst does not undergo any oxidative
degradation. Based upon these observations we propose a bifur-
cated reaction mechanism: In one mechanism, the manganese-
oxo intermediate, formed by the interaction of the catalyst with
PhIO (Scheme 3A), attacks the olefinic double bond in a concerted
pathway, which is similar to the one involving (salen)MnV =O
intermediate proposed [24] for the epoxidation reactions using
Mn(III)–salen complexes and PhIO. In the other mechanism, the
intermediate species [(4 N)Mn–O–I+–Ph], which is similar to the
one proposed for the epoxidation reactions using Mn(III)–salen
complexes and iodoarenes [52], attacks cyclooctene directly to
form the carbo cation [(4N)Mn–O–C8H14]+, which collapses to give
the epoxide product and regenerates the original complex (Scheme
3B). In fact, we have elegantly shown [53] that the intermediate
peroxido species [Cl(ntb)Ru–O–O–tBu]+, detected by ESI-MS, at-
tacks the olefin directly to form the epoxide, without invoking
the formation and involvement of the high-valent [(ntb)RuIV =O]
species. It may be noted that the in-cage electronic rearrangement
in the carbo cation is facilitated by a more Lewis basic ligand. Thus,
as one moves from 1 to 4 to 5 the Lewis acidity of the Mn(II) center
increases, and hence the ligand Lewis basicity decreases from L1 to
L4 to L5 (or L3) resulting in the decrease in yield of the epoxide
product, supporting our proposal. 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.
The epoxidation of cyclohexene has been also studied in aceto-
nitrile solvent at room temperature using 1–5 as catalysts and PhIO
as oxygen source under nitrogen atmosphere. In the epoxidation of
cyclohexene, the major product observed is cyclohexene oxide (E,
17–23%, Table 4, Fig. 5), apart from allylic oxidation products like
_O
I Ph
+
O
I
Ph
(4N)MnII
O
O
(4N)MΙnΙ
+
I
(4N)MnIV
O
-PhI
(4N)MnII
(4N)MnII
+
A
Ph
A
O
ΙΙΙ
(4N)Mn
MnII(4N)
O
O
O
+
MnII(4N)
B
I+
-PhI
+
A
Ph
Scheme 3. Proposed mechanisms for epoxidation of cyclooctene by using Mn(II) complexes and PhIO.

N. Saravanan, M. Palaniandavar / Inorganica Chimica Acta 385 (2012) 100–111
109
MnII(4N)
H
+
OH
H
O
MnII(4N)
O
I+
(4N)MnII
-PhI
+
B
Ph
Scheme 4. Proposed mechanism for allylic oxidation of cyclohexene by using Mn(II) complexes and PhIO.
Fig. 6. Bar chart representation of selectivity catalyzed by Mn(II) complexes in ACN
(1–5). Epoxide selectivity of cyclohexene in the presence and absence of additive
(N-methyl imidazole).
Fig. 8. Bar chart representation of selectivity catalyzed by Mn(II) complexes in ACN
(1–5). Epoxide selectivity of styrene in the presence and absence of additive (N-
methyl imidazole).
The lower selectivity observed for 1–5 is due to the preferential
attack of the allylic C–H bond rather than the olefinic double bond
by [(4N)Mn–O–I+–Ph] and (4N)MnIV =O species, which is similar to
the reactive intermediate [54] (4N)RuIV =O. Control experiments
for cyclohexene oxidation without the catalyst gave a total yield
of only <8% 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 autooxidation of cyclohex-
ene in the presence of oxygen, occurring through a free radical
chain mechanism [56].
In the presence of N-methylimidazole, the epoxide yield in-
creases with the product conversion increasing from 30% to 50%
and the selectivity (A/E) also increasing from 1.0 to 1.8 (Table 5,
Fig. 6). The yield of cyclohexene oxide follows the trend
1 > 4 > 5 ꢃ 2 > 3, which is the same as that observed for cyclooc-
tene. This illustrates that the yield of epoxide product is strongly
influenced by the electron density on Mn(II) center, as dictated
by the linear tetradentate ligand backbone. Upon varying the back-
bone from L1 to L4 to L5 (or L3) the electron density around the
metal center decreases (cf. above), leading to decrease in stability
of the high-valent manganese-oxo intermediate and/or discour-
agement in the in-cage electronic rearrangement, as for the epox-
idation of cyclooctene, and hence the decrease in both the yield
and selectivity of epoxide product from 1 to 4 to 5 (or 3).
The reaction of styrene with PhIO in the presence of 1–5 has
been also investigated under the same reaction conditions de-
scribed above for cyclooctene and cyclohexene. In addition to ma-
jor amounts of styrene oxide (17–47%; Table 4, Fig. 7), small
amount of benzaldehyde (11–18%) is also otained as a side product.
The yield of styrene oxide follows the trend 1 > 4 > 2 ꢃ 5 > 3, which
is the same as that observed above for cyclooctene and cyclohex-
ene (Scheme 5). This variation is explained based on the variation
in stability of the reactive high-valent metal–oxo intermediate
Fig. 7. Bar chart representation of styrene epoxidation catalyzed by Mn(II)
complexes (1–5) in the presence of PhIO at room temperature in acetonitrile.
2-cyclohexen-1-ol and 2-cyclohexen-1-one, which is the further
oxidized product of the former involving an oxidative dehydroge-
nation pathway [54] catalyzed by the complexes. The mechanism
of epoxidation of cyclohexene is illustrated by invoking the involve-
ment of [(4N)Mn–O–I+–Ph] and the high-valent (4 N)MnIV =O inter-
mediate proposed above for the cyclooctene epoxidation (Scheme
3A and B). The formation of increased allylic oxidation products
and decreased epoxide (A/E, 2.2–4.1), which is similar to that ob-
served previously for RuII [54], MnIII and FeIII complexes [55], is
illustrated by invoking the facile abstraction of allylic proton by
[(4N)Mn–O–I+–Ph] or the intermediate species (4 N)MnIV =O, fol-
lowed by in-cage electron-transfer to form the allylic alcohol prod-
uct (Scheme 4).

110
N. Saravanan, M. Palaniandavar / Inorganica Chimica Acta 385 (2012) 100–111
H
H
Ph
O
H
H
Ph
MΙnΙΙ(4N)
(4N)MnII
(4N)MnII
-PhI
(4N)MnIV
+
+
ΙΙ
O
O
O
A
B
O
)Mn
(4N
+
I
H
H
A
A
Ph
H
MnII(4N)
Ph
H
+
O
Ph
MnII(4N)
O
I+
Ph
H
H
Ph
-PhI
H
H
H
H
H
C=
C bond
cleavage
O
(4N)MnII
+
C
Ph
H
Scheme 5. Proposed mechanisms for epoxidation of styrene by using Mn(II) complexes and PhIO epoxide obtained (62–77%) in the presence of N-MeIm is higher than that
obtained in its absence (Table 5, Fig. 8).
and/or to the variation in ease of the in-cage electronic rearrange-
ment in the carbo cation. Such a catalyst–oxidant adduct could de-
liver the oxygen atom to the substrate and bypass the metal–oxo
intermediate if the formation of the metal-oxo species from the
oxidant–catalyst adduct is relatively slow compared to the oxygen
atom transfer to the substrate (Scheme 5B), which is similar to the
mechanism proposed by Collman et al. [52]. The amount of benz-
aldehyde detected in the N2 atmosphere is lower than that in the
presence of oxygen supporting its formation through further oxi-
dation of the carbo cation directly by oxygen or through C@C cleav-
age (Scheme 5C) [53,57]. Also, the yield of proposed mechanisms
illustrate that for all the olefins the epoxide yield increases upon
increasing the electron density on the metal center. The negative
charge built on the Mn(II) center facilitates the in-cage electronic
rearrangement in the carbo cation and also stabilises the high-va-
lent manganese-oxo intermediate, which has been invoked as the
key intermediate responsible for olefin oxidation by both the
metalloenzymes [58] and their model complexes [59]. Also, the
complexes with less positive Mn(II)/Mn(III) redox potentials are
expected to stabilize the high-valent metal–oxo species to a larger
extent leading to the formation of higher amount of the epoxide.
This reveals that the Lewis acidity of the Mn(II) center, as dictated
by the ligand donor environment, plays a vital role in dictating the
variation in product yield.
Acknowledgements
We sincerely thank the Department of Science and Technology,
New Delhi, for supporting this research (Scheme No. SR/S5/BC-05/
2006), Professor M. Palaniandavar is a recipient of DST Ramanna
Fellowship (SR/S1/RFIC-01/2010). This work was also supported
by Indo-French Centre (IFCPAR) (Scheme No. IFC/A/4109-1/2009/
993). UGC-RFMS fellowship to N.S. We thank Department of Sci-
ence and Technology (FIST Programme) for X-ray diffractometer
and UGC-SAP for instrumentation facility at School of Chemistry,
Bharathidasan University.
Appendix A. Supplementary material
Crystallographic data for 4, Tables S1, S2 and Fig. S2. CCDC
813736–813739 contain the supplementary crystallographic data
for the crystal structures of 1, 3, 5 and 4, respectively. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2012.01.009.
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