Synthesis, characterization, DFT studies and catalytic activities of manganese(ii) complex with 1,4-bis(2,2′:6,2′′-terpyridin- 4′-yl) benzene

Article abstract of DOI:10.1039/c2dt31544k

A new di-manganese complex with "back-to-back" 1,4-bis(2,2′:6,2′′-terpyridin-4′-yl) benzene ligation has been synthesized and characterised by a variety of techniques. The back-to-back ligation presents a novel new mononuclear manganese catalytic centre that functions as a heterogeneous catalysis for the evolution of oxygen in the presence of an exogenous oxidant. We discuss the synthesis and spectroscopic characterizations of this complex and propose a mechanism for oxygen evolution activity of the compound in the presence of oxone. The di-manganese complex also shows efficient and selective catalytic oxidation of sulfides in the presence of H₂O₂. Density functional theory calculations were used to assess the structural optimization of the complex and a proposed reaction pathway with oxone. The calculations show that middle benzene ring is distorted respect to both of metallic centers, and this in turn leads to negligible resonance of electrons between two sides of complex. The calculations also indicate the unpaired electron located on oxyl-ligand emphasizes the radical mechanism of water oxidation for the system.

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PAPER
Synthesis, characterization, DFT studies and catalytic activities of
manganese(II) complex with 1,4-bis(2,2′:6,2′′-terpyridin-4′-yl) benzene†
Mohammad Mahdi Najafpour,*a,b Warwick Hillier,*c Amir Nasser Shamkhali,d Mojtaba Amini,e
Katrin Beckmann,c Zvonko Jagličić,f,g Marko Jagodič,g Peter Strauch,h Atefeh Nemati Moghaddam,a
Giangiacomo Berettai and Mojtaba Bagherzadehj
Received 1st June 2012, Accepted 26th July 2012
DOI: 10.1039/c2dt31544k
A new di-manganese complex with “back-to-back” 1,4-bis(2,2′:6,2′′-terpyridin-4′-yl) benzene ligation has
been synthesized and characterised by a variety of techniques. The back-to-back ligation presents a novel
new mononuclear manganese catalytic centre that functions as a heterogeneous catalysis for the evolution
of oxygen in the presence of an exogenous oxidant. We discuss the synthesis and spectroscopic
characterizations of this complex and propose a mechanism for oxygen evolution activity of the
compound in the presence of oxone. The di-manganese complex also shows efficient and selective
catalytic oxidation of sulfides in the presence of H2O2. Density functional theory calculations were used
to assess the structural optimization of the complex and a proposed reaction pathway with oxone. The
calculations show that middle benzene ring is distorted respect to both of metallic centers, and this in turn
leads to negligible resonance of electrons between two sides of complex. The calculations also indicate
the unpaired electron located on oxyl-ligand emphasizes the radical mechanism of water oxidation for the
system.
and generate solar fuels (most commonly H2) via electrolytic
Introduction
reactions.1 However, one significant challenge in this endeavour,
The generation of hydrogen from the decomposition of water is
an appealing solution for the future energy concerns associated
with diminishing reserves of fossil fuels and global warming
scenarios.1 A plausible strategy is to employ solar energy to
drive artificial photosynthesis as a mean to capture light energy
a prerequisite for a sustainable hydrogen economy is designing a
catalyst for anodic oxidation reaction.1 Water oxidation in Nature
is catalysed by a Mn4O5Ca complex housed in a protein environ-
ment in photosystem II (PSII) that controls the reaction coordi-
nate, proton movements and the solvent access (Fig. 1).2–4
aDepartment of Chemistry, Institute for Advanced Studies in Basic
Sciences (IASBS), Zanjan 45137-66731, Iran.
E-mail: mmnajafpour@iasbs.ac.ir; Fax: +98 241 4153232;
Tel: +98 241 4153201
bCenter of Climate Change and Global Warming, Institute for Advanced
Studies in Basic Sciences (IASBS), Iran
cResearch School of Biology, Canberra, ACT 0200, Australia.
E-mail: Warwick.Hillier@anu.edu.au; Fax: +61 26125-5894;
Tel: +61 26125-8056
dDepartment of Applied Chemistry, Faculty of Sciences, University of
Mohaghegh Ardabili, Ardabil, P. O. Box: 56199-11367, Iran
eDepartment of Chemistry, Faculty of Science, University of Maragheh,
Golshahr, P.O. Box: 55181-83111731, Maragheh, Iran
fInstitute of Mathematics, Physics and Mechanics, University of
Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
gUniversity of Ljubljana, Faculty of Civil and Geodetic Engineering,
Jamova 2, 1000 Ljubljana, Slovenia
hInstitut für Chemie Universität Potsdam, Karl-Liebknecht-Str. 24-25,
14476 Golm, Germany
iDepartment of Pharmaceutical Sciences, Pietro Pratesi, Faculty of
Pharmacy, University of Milan, 20133 Milan, Italy
jChemistry Department, Sharif University of Technology, PO Box
11155-3516, Tehran, Iran
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt31544k
Fig. 1 A image of the Mn4O5Ca cluster within the PSII complex, Mn
purple, oxo’s red, Ca green and four aqua ligands in cyan.3
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The biological system operates low over potentials and makes
use of earth abundant Mn ions.2–4 The synthetic systems thus far
synthesised include a range of manganese oxides5 or complexes
incorporating Schiff bases, bpy, terpy or other ligands that result
in active heterogeneous or homogenous catalysts for water oxi-
dation.6 Turnover numbers for oxygen evolution of many of
these complexes can be quite low.6
Experimental
Materials
All reagents and materials used for synthesis were reagent grade
and used without further purification. All solutions were pre-
pared using doubly deionized water. Oxone was standardized by
iodometric titration and found to contain 0.21 mmol KHSO5 per
100 mg. The acetate buffer was prepared by adjusting the pH of
a 0.1 M KOAc solution to 4.0 with concentrated H2SO4.
In the synthesis of manganese complexes, typically hard N/O
donor ligands such as tridentate N-donor and carboxylate ligands
have been used to obtain catalysts capable of oxygen evolution.
One notable example is a [(terpy)(H2O)MnIII(μ-O)2MnIV(terpy)-
(H2O)]3+ (terpy = 2,2′:6′2′′-terpyridine) complex which was
reported to catalytically produce oxygen with oxone
(2KHSO5·KHSO4·K2SO4) or NaOCl, as primary oxidants.7 The
terpy ligand derivatives have triggered active research fields on
syntheses, characterizations, redox and catalytic reactions related
to water oxidation by manganese complexes.6 This di-manga-
nese catalyst was argued to invoke MnIV–OH or MnVvO as the
final intermediates that poise the oxygen evolution reactions.7 In
the recent years, computational studies particularly those of
Density Functional Theory (DFT) methods have been widely
used to study the electronic structure of transition metal com-
plexes, particularly because of its reasonable precision and lower
computational cost in comparison with high level ab initio
methods.8–15 Bencze et al. used BP86 functional with general-
ized gradient approximation (GGA) for calculation of rotational
barrier of methyl group in CH3Mn(CO)5 and CH3Ru(CO)5 com-
plexes.8 Siegbahn applied B3LYP hybrid-GGA (HGGA) func-
tional to construct model complexes for water oxidation similar
to PSII with the oxygen radical mechanism.9 Budzelaar et al. uti-
lized B3LYP HGGA functional to study electron transfer from
Synthesis of complex
Synthesis of 1: 1,4-bis(2,2′:6,2′′-terpyridin-4′-yl)benzene as a
solid was added to a solution of MnCl2·4H2O dissolved in de-
ionized water. The initial molar ratios of the input species were
MnCl2·4H2O (5): ligand (1): water (500). The reaction mixture
was transferred to a Teflon-lined autoclave and placed in an oven
at 393 K for 24 h. Orange powder separated and were filtered
off, washed with 5 mL of cooled water, 5 mL EtOH and then
dried in air at 323 K (yield is 73%). Elemental analysis: Found:
C, 49.78%; H, 3.62%; N, 9.60%. The molecule
C36H32N6O4Mn2Cl4 requires C, 50.02%; H, 3.73%; N, 9.72%.
IR (KBr, cm−1): 3446, 3400, 3062, 1600, 1552, 1474, 1402,
1013, 792, 507. UV-Vis in solid state: 292 nm (ε = 2.1 × 104 L
mol−1 cm−1), 341 nm (ε = 1.8 × 104 L mol−1 cm−1); UV-Vis in
methanol solution: 200 nm (ε = 5.1 × 104 L mol−1 cm−1),
291 nm (ε = 5.5 × 104 L mol−1 cm−1) and 329 nm (ε = 1.2 ×
104 L mol−1 cm−1). MS: m/z 673 [Mn2L(OH)]+ (ESI-MS). The
complex is not soluble enough in usual solvents to crystallize
and characterize by X-ray diffraction measurement.
the metal t2g orbitals to the ligand π* orbitals for a series of com-
x+
plexes ML2
(MvMn–Zn, L = 2,6-bis(iminomethyl)pyri-
Characterization
dine).10 Hartl et al. performed DFT calculation by B3LYP
functional to prove the formation of products of stepwise electro-
chemical reduction of face-[Mn(Br)(CO)3(4,4′,5,5′-tetramethyl-
2,2′-biphosphinine)] complex.11 Lundberg et al. used B3LYP
functional to study electronic structure of [(terpy)(H2O)MnIV-
(μ-O)2MnIII(H2O)(terpy)]3+ (terpy = 2,2′:6,2′′-terpyridine) as a
water oxidizing complex catalyst with oxyl radical mechanism.12
Sproviero et al. performed QM/MM study of O2-evolving
complex of PSII by B3LYP functional.13 Yano et al. performed
(time dependent)-DFT (TD-DFT) calculations on Mn(V)-nitrido and
Mn(V)-oxo complexes as catalytically important materials in water
splitting reaction which occurs in the PSII membrane protein.14
As a new approach a manganese binucleating ‘back-to-back’
complex (1) with 1,4-bis(2,2′:6,2′′-terpyridin-4′-yl)benzene
(terpy-ph-terpy) has been synthesized and characterized by a
variety of spectroscopic techniques. Oxygen evolving activity in
presence of oxone has also been studied and DFT calculations
were used to investigate electronic structures and proposed oxyl
intermediate. The oxidation of sulfides to sulfoxides is of signifi-
cant importance in organic chemistry, both for fundamental
research and for a wide range of applications in the synthesis of
biologically and medicinally important compounds. Some
manganese compounds with N-donor ligands were used as cata-
lysts in this reaction.15 We also tested the catalytic activity of the
complex toward oxidation of sulfides to sulfoxides in the pres-
ence of H2O2.
Elemental analyses were performed on a Perkin-Elmer 2400
CHNS/O elemental analyser.
The MIR spectra of KBr pellets of compounds were recorded
on a Genesis FT-Spectrometer (ATI Mattson) in the range
between 400 and 4000 cm−1. The FT Raman spectra of KBr
pellets were recorded at room temperature between 400 to
3500 cm−1 using 1064 nm illumination in a Bruker IFS66/S and
FRA106/S FT-Raman system. Manganese atomic absorption
spectroscopy (AAS) was performed on an Analyst spectrometer
system by PerkinElmer. Prior to analysis, suspensions of 1
(1 mL, 25.0 mg metal L−1) was added to 1 mL of concentrated
nitric acid, left at room temperature for at least 24 h to ensure
that 1 was completely dissolved. The solution was then diluted
to 25.0 mL and analysed by AAS.
Oxygen signals were detected using an isotope ratio mass
spectrometer system (IRMS) and a membrane inlet.16 The exper-
iments was under conditions of 100 mM acetate/acetic acid
buffer pH 4.0 at 25.0 °C. The IRMS system (Delta V, Thermo)
simultaneously recorded m/z = 32, 34 and 36 signals. Sample
were equilibrated for 20 min to stabilize the baseline before
injection of 18O-water and again stabilised before injection of
oxone.
EPR Spectra were collected on frozen samples with an
X-band Varian E-9 spectrometer equipped with an Oxford
ESR-900 liquid helium cryostat. Typical instrument settings
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were the following: modulation amplitude) 20 G, modulation
frequency) 100 kHz, and microwave power) 2.0 mW.
condition and oxygen evolution was not observed. After deaera-
tion of Ce(IV) solution with argon, the compound as several
small particles was added, and oxygen evolution was recorded
with the oxygen meter under stirring (Fig. S1†).
The detection of the Mn-complex was made in the positive
electrospray mode using a PE-Sciex API III+ triple quadrupole
mass spectrometer (Concord, ON, Canada) with an Ion Spray
interface. A syringe pump (Syringe Infusion Pump 22, Harvard
Apparatus Inc., Cambridge, MA, USA) operated at a flow rate of
5 μL min−1 was utilized to pump the solution through a fused
silica capillary (Polymicro Technologies, Phoenix, AZ, USA)
into the ion source of the mass spectrometer. The fused silica
spray capillary entering the mass spectrometer was centred in a
stainless steel capillary auxiliary assembly delivering 40 psi of
dry nitrogen (boil off from liquid nitrogen) for pneumatically
assisted ESI-MS. The flow rate of dry nitrogen counter-current
curtain gas (heated to 63 °C) was 1.2 L min−1 over the sampling
orifice. The mass spectrometric parameters were as follows: ion
spray voltage (ISV) 4500 V, interface plate voltage (IN) 650 V,
orifice lens voltage (OR) 50 V, and AC entrance rod (R0)
30 V. Q1 Scans between m/z 100–1400 (step size 0.1), with a dwell
time of 1 ms, were recorded in the multichannel acquisition
(MCA, summation of 10 scans) mode. Furthermore, in MS/MS
mode the collision energy was set to 20 eV with a collision gas
thickness of 2.00 × 1015 molecules cm−2. Data were collected on
a Macintosh computer by the Tune2.5-FPU software. The reac-
tion products of oxidation were determined and analysed by HP
Agilent 6890 gas chromatograph equipped with a HP-5 capillary
column (phenyl methyl siloxane 30 m × 320 lm × 0.25 lm) and
flame-ionization detector.
Calculation method
All of the DFT calculations were performed using the popular
B3LYP HGGA functional with SBKJC basis set including effec-
tive core potentials (ECPs) for inner shell electrons to reduce the
computational cost of calculations.17–21 The Firefly quantum
chemistry software used to optimize structures 1 and oxidized
intermediate (3) of proposed mechanism in Scheme 1.
Some important optimized structural properties of species 1
and 3 are listed in Table S1.† Due to symmetry, the atoms left
and right of the complex are the same. Therefore, only structural
properties for one side of structures 1 and 3 are included in
Table S1.† Note that the symbols and labels used in Table 1S† is
shown in Fig. S3.† Moreover, the Mülliken charges of Mn, N,
and oxygen atoms of species 1 and 3 in along with their pure
spin [α (↑)–β (↓)] values are given in Table 2S.†
General procedure for sulfide oxidation
To a solution of sulfide (0.4 mmol), imidazole (0.5 mmol) as
axial ligand, chlorobenzene (0.4 mmol) as internal standard and
catalyst (0.055 mmol) in CH3CN (1.0 mL) was added H2O2
(0.4 mmol) as oxidant. After stirring at room temperature for
2 h, for the products analysis, the solution was subjected to ether
extraction (3 × 10.0 mL), and the extract was also concentrated
down to 0.5 mL by distillation in a rotary evaporator at room
temperature and then a sample (2 μL) was taken from the solu-
tion and analyzed by GC. To establish the identity of the pro-
ducts unequivocally, the retention times and spectral data were
compared to those of commercially available compounds. All
the reactions were run at least in duplicate.
Oxygen evolution
In a typical experiment, after the deaeration with argon, the
complex as a solid was added to a buffered solution of oxone solu-
tion (100 ml, 0.021 M in 0.1 M acetate buffer, pH 4.0). All rates
were measured at 25.0 0.1 °C using the method of initial rates.
Photochemical water oxidation experiments were conducted in
a colourless two-neck glass flask (50 ml) flask containing
25.0 ml of aqueous buffer (acetate and acetic acid, 0.5 M) with
pH held at 4.0. [Co(NH3)5Cl]Cl2 (12.5 mM), [Ru(bpy)3]-
Cl2·6H2O (1.6 mM), and the complex (2.5 mg) as a solid. One
neck of the flask was sealed with a rubber septum whilst the
other was sealed with an electrode of oxygen meter to measure
the amount of oxygen evolved during the irradiation. To check
for possible air leaks the photo reactor flask was maintained in
dark for 10 min prior to irradiation. No oxygen signals were
typically detected. The sample chamber was maintained at
25.0 °C with a circulating water bath and light was provided by a
500 W tungsten lamp. Light intensities illuminating the sample
were determined by a MS-1300A luxmeter to be 20 000 lux.
Oxygen evolution from aqueous solutions in the presence of
(NH4)2Ce(NO3)6 (Ce(IV)) was measured using an HQ40d porta-
ble dissolved oxygen meter connected to an oxygen monitor
with digital readout. The reactor was maintained at 25.0 °C in a
water bath. In a typical run, the instrument readout was calibrated
against air-saturated distilled water stirred continually with a
magnetic stirrer in the air-tight reactor. After ensuring a constant
baseline reading, the water in the reactor was replaced with
Ce(IV) solution. Without catalyst, Ce(IV) was stable in this
Results and discussion
Metal complexes by N-donor ligands have been the subject of
numerous studies, in relation to photochemical energy and elec-
tron transfer, mixed-valence complexes, catalyst and molecular
materials for electronic applications.22 Manganese complexes
with N-donor ligands have been reported as catalysts for oxi-
dation reactions.23 In the paper, we used a terpyridine-phenyl-
terpyridine and synthesized a dinuclear manganese complex. A
few complexes with this ligand have been reported.22 This and
similar ligands containing two tpy units attached back-to-back
are of particular interest because they allow control over the geo-
metrical properties of the system and, in particular, over the
M–M distance.24 We used hydrothermal method to synthesize a
manganese complex with the ligand because the ligand is not
soluble enough in water or organic solvents (<10 mg L−1).
The IR spectrum of 1 indicates a rich IR fingerprint (Fig. 2a)
with spectral features in the range 3100–2900 cm−1 (aromatic
C–H stretching vibrations), 1600–1550 cm−1 [ν(CvN) and ν(CvC)
stretches], 1470–1020 cm−1 [ν(C–C) + ν(C–N) vibrations], and
810–710 cm−1 (aromatic C–H deformation vibrations) are
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Fig. 2 (a) The FTIR spectrum of 1 (red) and reaction of 1 after 30 min
(green) and 1 day (black) with oxone. Blue arrows show peaks for sul-
phate. Samples prepared as KBr pellets pressed with 1 as a solid. (b)
The solid state UV-Vis spectra of 1 in the presence of oxone (black) and
absence of oxone (red). Arrows show the direction of absorbance
change with adding oxone solution. 1 as a solid (4 mg) was added to a
buffered solution of oxone solution (1 ml, 0.021 M in 0.1 M acetate
buffer, pH 4.0) (b).
Scheme 1 Proposed mechanisms for 32O2 and 34O2 production from
reaction of SO52− and 1.
recorded.25 A broad underlying water O–H stretching band is
centered ∼3400 cm−1. The structural characterisation of 1 by
FTIR shows a number of IR bands that are diagnostic of CN and
C–C stretches from the ligand. The compound in the presence of
oxone decomposed after 24 hrs and sharp peaks related to sul-
phate (∼1144 and 620 cm−1, νSvO) could be observed (Fig. 2a).
The electronic spectrum of 1 in solid state shows characteristic
UV absorption with a maximum at 292 nm (ε = 2.1 × 104
L mol−1 cm−1) assigned to π → π* or n → π* transitions and a
second feature at 341 nm (ε = 1.8 × 104 L mol−1 cm−1) assigned
to charge transfer (Fig. 3). The electronic spectrum for 1 in
methanol shows UV absorption at 200 nm (ε = 5.1 × 104
L mol−1 cm−1) and 291 nm (ε = 5.5 × 104 L mol−1 cm−1), which
are nominally assigned as π → π* or n → π* and 329 nm (ε =
1.2 × 104 L mol−1 cm−1) assigned as charge transfer (Fig. 2b).25
The X-band EPR spectrum of 1 in solid state at 104 K
(Fig. 3a) reveals a spectrum with a broad peak around g = 2 that
is typical for Mn(II) in an octahedral environment.
between 2 K and 300 K in a magnetic field H = 1000 with a
MPMS-XL-5 SQUID magnetometer. The data on Fig. 3c are
already corrected for the sample holder contribution and for the
temperature constant contribution due to inner shell electrons
and temperature independent paramagnetism.
The susceptibility monotonically increases with decreasing
temperature while the product χT remains constant, which
reveals paramagnetic behaviour. The data above 100 K follows
the Curie–Weiss law χ = C/T − Θ. The best fit was obtain with
C = 8.3 emu K mol−1. The obtained value of the Curie constant
is in good agreement with a theoretical value for two Mn(II) ions
with S = 5/2 per molecule.27 Below 6 K, a possible weak antifer-
romagnetic intramolecular interaction between two Mn(II) ions
could be observed (Fig. 3c).
The broad signals above and below g = 2 are due to the S =
5/2 zero-field splitting, as reported for distorted mononuclear
Mn(II) complexes.26 The X-band EPR recorded in DMSO
revealed a Mn(II) complex (Fig. 3b) with six hyperfine lines due
to the interaction of the electron spin with the nuclear spin
(55Mn, I = 5/2).26 Magnetic susceptibility was measured
Reactivity and oxygen evolution
An important goal of this work was the synthesis of hetero-
geneous water oxidising catalyst with good stability properties
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Table 1 Isotope fractions of the evolved oxygen (in %) for reactions of
the complex with 10 eq. of oxone per complex
Reaction
%H218O
13.8
32O2
34O2
36O2
1
2
74.33 (74.31)
57.45 (57.45)
25.03 (23.79)
40.31 (36.69)
0.64
(1.90)
2.24
24.2
(5.86)
we obtained a Mn(II) complex (Scheme S1†) that is stable in
water at least for 48 h. The heterogeneous catalysis (1) showed
oxygen evolution reaction in the presence of oxone at pH = 4.0.
Decomposition of oxone at this pH is negligible without cata-
lyst7 and the experimental rate law for oxygen evolution vs.
[oxone] was determined as first order. Turnover number for 1
was ∼37.1 mol of O2/mol for the complex during 35 min. The
O2 turnover number is significantly higher than turnover
numbers for Mn 2,2′-bipyridine (∼0), picolinate (∼0.1 or 5.028)
or 1,10-phenanthroline (0.25) and also manganese(III) complex
with 2,6-dipicolinate (∼0.6). The turnover is lower than that
reported (50) for bis(μ-oxo) di-Mn(terpy) complex.2,7
To determine the molecular origin of the O2, H218O isotope-
labelling experiments coupled with membrane inlet mass spec-
trometry (MIMS) were carried out.16 The experiments conducted
with oxone yielded 36O2 with labelling percentages that are half
the theoretically expected values if both O-atoms originate
entirely from water (Table 1).29 The 50% labeling of 36O2 indi-
cates that one oxygen atom in O2 originates from labeled water
and the other from oxone (Table 1).29 It is surprising that the
ratio of 36O2(theoretical)/36O2 (experimental) is similar to that of
the bis(μ-oxo) di-Mn(terpy) complex.29 The interesting finding
from the current work is that the back-to-back Mn complex 1
indicates the presence of an μ-O bridge is not a prerequisite to
evolve oxygen from manganese complexes.
The reaction of oxone with a suspension of Mn2O3 or MnO2
ion in the presence of H218O produced no 36O2.30 These obser-
vations showed that no 36O2 formed from labelled water during
the reaction of 1 and oxone originate from reactions of Mn2O3 or
MnO2 with oxone.15 In characterizing the reaction mechanism 1
with oxone both IR and Raman spectroscopy were performed to
examine if characteristic signatures of μ-O Mn bridges could be
assigned (Fig. 2a, Fig. S2†). The typical bonds for μ-O Mn
dimers appear at29a 600–800 cm−1 but in this work we were
unable to detect the characteristic modes.
We also studied the reaction with low temperature EPR spec-
troscopy and the characteristic 16-line or related signal for μ-oxo
species was not observed. In EPR spectrum of 1 after reaction
with a solution of oxone, only EPR spectrum of Mn(II) was
detected.29 Reacting with oxone, the color of 1 changed from
orange to red and the increase of ∼500 nm absorbance in the
solid state UV-Vis spectrum (Fig. 2b) was observed that likely
shows the conversion of Mn(II) to high-valent manganese
complex.29
Fig. 3 The EPR spectrum of 1 in solid state at 104 K (a) and in
solution (DMSO) at 104 K (b). The susceptibility of 1 (c).
In a result similar to reactions of other manganese complexes
with terpyridines and oxone; no MnO4− was observed. Reactions
of 1 and Ce(IV) or in an aqueous solution containing tris(2,2′-
bipyridyl)ruthenium(II) chloride and chloro pentaammine cobalt
(III) chloride in acetate buffer and in presence of light produce no
that was derived from terpy ligands. It is worth noting there are
many manganese(II) complexes with substituted terpy ligands,
but they are typically not water stable and are precipitated. Using
a novel back-to-back 1,4-bis(2,2′:6,2′′-terpyridin-4′-yl) benzene,
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Table 2 Oxidation of sulfides catalyzed by 1a
oxygen. In this system, a Mn(IV)vO or Mn(III)–*OH is proposed
as an intermediate in oxygen evolution in attributed to the obli-
gate two-electron redox chemistry of oxone. In this mechanism,
Entry
1
Substrate
Conversionb (%)
61
2−
first SO5 coordinates to 1. As the Mn(II) complex is labile, the
rate of reaction could be fast. Then, the Mn(II) complex oxidizes
to Mn(III)–*OH complex by oxone. The reaction of the oxyl
radical (*OH) with oxone produces oxygen (Scheme 1).
As observed in Table 1, a small quantity of 36O2 is being gen-
erated and this suggesting a second reaction pathway may oper-
ational at a low rate (a proposed mechanisms as shown in
Scheme 2S†).
2
3
4
59
64
68
From DFT calculations (Table 1S†) the middle phenyl ring
which connects to side of the complex is distorted respect to
both sides of complex (C20–C18–C6–C1 with 34.8° and 24.9°
dihedral angles respectively for structures 1 and 3). Therefore
two Mn atoms in complex cannot have coupled spin states due
to the negligible resonance of electrons between two sides of
complex. This result is in agreement with EPR spectra and sus-
ceptibility of complex (Fig. 3) which emphasizes S = 5/2 spin
state for each Mn atom of complex. Also, the calculated pure
spin values in Table 2S† is in good agreement with structural
optimization and EPR results, in which pure spin value of Mn
atom is 4.85 for structure (I). Another important result of DFT
calculations is exhibited by Mülliken charges and pure spin
values of Mn68 and O69 atoms in the structures 1 and 3. The
charges of Mn68 and O69 in structure 1 are 1.00 and −0.58 a.u.
respectively. These results are reasonable because O69 in struc-
ture 1 belongs to a water molecule. Whereas, those values for
structure 3 are 0.07 and −0.29 a.u., respectively for Mn68 and
O69 atoms, which implies the net charge transfer form O69 to
Mn68. These results are along with decrease of Mn68 pure spin
from 4.85 to 3.00 and increase of O69 pure spin from 0.01 to
0.10, respectively from 1 to 3. Therefore, reaction with oxone
oxidizes Mn2+ to Mn4+ and O69 (oxyl ligand) in structure 3 may
act as both radical and electrophile. Therefore, our DFT calcu-
lations propose the radical mechanism for water oxidation in our
synthesised complex which is in agreement with results of Sieg-
bahn for model complexes of photosystem (II).9 We are now
trying to detect the species present during catalysis to elucidate
and understand the mechanism in order to able to design more
robust and effective systems. These species may be important to
understand the mechanism of oxygen evolution by oxygen evol-
ving complex in Nature. It is also possible that oxo-bridges are
formed transiently during the catalytic cycle in a small fraction
of the total Mn but are unstable in the steady-state. In this con-
dition, Mn(V) or Mn(IV)–O˙ could be considered as an
intermediate.
5
72
6
7
8
CH3CH2–S–CH2CH3
CH3(CH2)2–S–(CH2)2CH3
CH3(CH2)7–S–(CH2)7CH3
47
43
30
a The molar ratios for catalyst: imidazole:substrate:H2O2 are
1 : 9.1 : 7.3 : 7.3. The reactions were performed in CH3CN (1 mL) under
air at room temperature. b The GC conversion (%) are measured relative
to the starting sulfide after 60 min and selectivity to sulfoxide for all of
substrates is 100%.
Conclusions
In summary
a new Mn(terpy-ph-terpy)Mn ‘back-to-back’
complex was synthesized and oxygen evolving activity was
demonstrated in the presence of oxone. This catalyst appears not
to produce stable μ-oxo intermediates and O–O bond formation
probably proceeds via an Mn(III)–*OH intermediate as a hetero-
geneous catalyst with has rapid rates of turnover. The bridging
nature of this catalyst has some interesting features for designing
polynuclear Mn catalysts with terpyridine ligand. The molecule
also suggests that μ-O bridge is not prerequisites to evolving
oxygen from manganese complexes. The complex also showed
efficient catalytic activity toward oxidation of sulfides in terms
of conversion and selectivity.
Acknowledgements
MMN and ANM are grateful to Institute for Advanced Studies
in Basic Sciences for financial support. The image in Fig. 1 was
made with Pymol.
Notes and references
The next investigation was to study the catalytic activity of 1
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structurally diverse substrates – arylalkyl, diaryl, dibenzyl, ben-
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oxidation to produce the corresponding sulfoxides in good yields
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