Nuclearity: Versus oxidation state in the catalytic efficiency of MnII/III azo Schiff base complexes: Computational study on supramolecular interactions and phenoxazinone synthase-like activity

Article abstract of DOI:10.1039/c7nj02280h

A novel mononuclear Mn(iii) complex, [Mn^III(L¹)Cl(H₂O)]·H₂O (1), and a tetranuclear Zn(ii)-Mn(ii) complex, [{Zn^II₂(L²)₂Cl₂}Mn^II₂(μ_1,1-N₃)₂(H₂O)₂]·2H₂O (2), have been synthesized involving azo Schiff base ligands, viz. H₂L¹ = (E)-6,6′-((1E,1′E)-(ethane-1,2-diylbis(azanylylidene))bis(methanylylidene))bis(2-methoxy-4-((E)-p-tolyldiazenyl)phenol) and H₂L² = (E)-6,6′-((1E,1′E)-((2,2-dimethylpropane-1,3-diyl)bis(azanylylidene))bis(methanylylidene))bis(2-methoxy-4-((E)-p-tolyldiazenyl)phenol), respectively. The solid-state structures were determined by single crystal X-ray crystallography. In complex 1, the Mn(iii) centre adopts slightly distorted octahedral geometry, while in complex 2 the Mn(ii) centre residing in the outer core of the ligand adopts a distorted pentagonal bipyramidal geometry. In complex 1, the chloride ion simply acts as a terminal coligand, while in complex 2, azide ions bind the metal centres in an end-on bridging fashion to produce a tetranuclear complex. The phenoxazinone synthase-like activity of both complexes has been examined and a detailed investigation of the structure-property correlation has been performed. Whereas the mononuclear complex 1 exhibits significant phenoxazinone activity, complex 2 is almost inactive, although in both complexes labile sites are available at manganese centres for substrate binding. The present work therefore highlights the importance of higher oxidation states of manganese over nuclearity for the development of better in vitro catalysts. In addition, extensive efforts have been made to visualize and quantify all supramolecular interactions present in 1 and 2.

Full text of DOI:10.1039/c7nj02280h

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DOI: 10.1039/C7NJ02280H
Nuclearity versus oxidation state in catalytic efficiency of MnII/III azo
Schiff base complexes: Computational study on supramolecular
interactions and phenoxazinone synthase like activity
Saikat Banerjee,a Paula Brandão,b Antonio Bauzá,c Antonio Frontera,*c Miquel Barceló-
Oliverc, Anangamohan Panja,d and Amrita Saha,*a
a Department of Chemistry, Jadavpur University, Kolkata- 700032, India.
E-mail: amritasahachemju@gmail.com; asaha@chemistry.jdvu.ac.in; Tel. +91ꢀ33ꢀ2457294.
b Departamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
c Departament de Química, Universitat de les Illes Balears, Crta. De Valldemossa km 7.5,07122
Palma (Baleares), Spain.
E-mail: toni.frontera@uib.es.
d Postgraduate Department of Chemistry, Panskura Banamali College, Panskura RS, WB
721152, India.
Abstract
A novel mononuclear Mn(III) complex, [MnIII(L1)Cl(H2O)].H2O (1), and a tetranuclear Zn(II)ꢀ
Mn(II) complex, [{ZnII (L2)2Cl2}MnII (µ1,1ꢀN3)2(H2O)2].2H2O (2), have been synthesized
2
2
involving azo Schiff base ligands viz. H2L1
=
(E)ꢀ6,6'ꢀ((1E,1'E)ꢀ(ethaneꢀ1,2ꢀ
diylbis(azanylylidene))bis(methanylylidene))bis(2ꢀmethoxyꢀ4ꢀ((E)ꢀpꢀtolyldiazenyl)phenol) and
H2L2=(E)ꢀ6,6'ꢀ((1E,1'E)ꢀ((2,2ꢀdimethylpropaneꢀ1,3ꢀ
diyl)bis(azanylylidene))bis(methanylylidene))bis(2ꢀmethoxyꢀ4ꢀ((E)ꢀpꢀtolyldiazenyl)phenol),
respectively. The solidꢀstate structures were determined by single crystal Xꢀray crystallography.
In complex 1, the Mn(III) centre adopts an almost perfect octahedral geometry, while in complex
2 the Mn(II) centre residing in outer core of the ligand adopts a distorted pentagonal bipyramidal
geometry. In complex 1, chloride ion simply acts as a terminal ligand. While, in complex 2 azide
ions bind the metal centres in an endꢀon bridging fashion to produce a tetranuclear complex.
Phenoxazinone synthase like activity of both complexes has been examined and a detail
structureꢀproperty correlation has been performed. Whereas, the mononuclear complex 1 exhibits
significant phenoxazinone activity, complex 2 is almost inactive, although in both complexes
1

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labile sites are available at manganese centres for substrate binding. The present work therefore
highlights the importance of higher oxidation states of manganese over nuclearity for the
development of better in vitro catalysts. Besides, an extensive effort has been made to visualize
and quantify all supramolecular interaction present in 1 and 2.
Introduction
Scientists have been engaged to develop costꢀefficient chemicals at a large scale to solve
different needs of civilized society like fuels, polymers, energy, materials, agrochemicals and
many other industrial products. Enhancement in the field of pharmaceutical and medical
applications also needs synthesis of new organic molecules with unique molecular
architectures.1–14 Therefore, the design and synthesis of different catalysts are challenging tasks
in the view point of developing new industrial technologies and subsequently facilitating the
advancement of science in several key directions. Exploration of mechanistic studies at each
stage help to uncover the intriguing nature of catalytic transformations, which further helps to
develop more efficient and selective catalytic systems. Selective oxidation of organic molecules
plays a crucial role in many biological and industrial processes. Nature has developed several
metalloenzymes to fulfil such transformations.15–23 Metalloenzyme catalysed oxidation reactions
are highly regioꢀ and stereoꢀselective, and function under mild and ‘green’ conditions. Among
them, the enzymes which efficiently activate molecular dioxygen at ambient condition have
received a great deal of attention in the past decades.24–26 Subsequently, the metal complexes,
which are able to mimic the function of oxidase (oxygenase) enzymes, are very useful choice for
the development of new, more efficient, bioꢀinspired and environment friendly catalysts for in
vitro oxidation processes. Amongst various oxidase enzymes, phenoxazinone synthase (PHS) is
of great interest mainly because of its wide clinical application.27 Phenoxazinone synthase (PHS)
is an enzyme which contains five copper atoms. Actinomycin D is a potent antineoplastic agent
that contains phenoxazinone chromophore.27 The formation of the phenoxazinone chromophore
in Actinomycin D is catalysed by PHS. Actinomycin D is an aromatic heterocyclic natural
product where 2ꢀaminophenoxazinone chromophore is linked to two cyclic pentapeptides. This is
clinically used for treatment of choriocarcinoma, Wilms tumours, rhabdomyosarcoma, and
Kaposi's sarcoma.28 Phenoxazinone synthase was discovered in 1962. The detail mechanistic
study of PHS was first done by Tadhg P. Begley and coꢀworkers.28 They concluded in their work
2

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that there are four possible mechanistic pathways for oxidation of oꢀaminophenol (OAPH) to
phenoxazinone via aiminequinone intermediate. In 1991, Simándi et al. established that kinetics
of OAPH oxidation in presence of a cobalt(II) complex follows MichaelisꢀMenten type kinetics.
In 2008, Hassanein et al. showed that OAPH oxidation follows first order kinetics with respect to
OAPH involving dioxygen species where the catalyst involved was cobalt(II)
phthalocyaninetetrasodiumsulfonate.29 Till date, several research groups have reported different
transition metal complexes which mimics PHS enzyme.30 However, there are limited studies
dealing with structureꢀproperty correlation in PHS mimicking complexes specially for Mn(II/III)
systems. Coordination compounds derived from Schiff base ligands have provided a large
number of synthetic models mainly because of synthetic simplicity.29 Here, we have used azo
appended oꢀvanillin aiming to bring extra stability in the solidꢀstate through different nonꢀ
covalent interactions such as π–π stacking interactions involving appended azoꢀaromatic part.
Previous studies have studied the ability of azoꢀaromatic compounds to interact with a
variety of systems. For instance, several coordination polymers with different architectures have
been designed and synthesized using azobenzenetetracarboxylic acid as ligand. Remarkably, the
azoꢀligand play an important role in adjusting the structural diversity.31a Moreover, it has been
recently reported that a novel azo ligand [2,2′ꢀ(1,3ꢀphenylenebis(diazeneꢀ2,1ꢀdiyl))bis(4ꢀ
chlorophenol)] and its mononuclear Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and
Cd(II) complexes present interesting antimicrobial activity.31b Similarly, the docking of new
organic/inorganic hybrid materials composed of chiral salenꢀtype Schiff base Mn(II), Co(II),
Ni(II), Cu(II), and Zn(II) complexes having azobenzene groups and laccase has been recently
demonstrated.31c In addition, it has been shown that chromium complexes with two positional
isomers of 2ꢀ[(Nꢀarylamino)phenylazo]pyridine are able to stabilize the metal ion in two
different oxidation states. Finally, combined theoretical and experimental studies on azoligands
are also available in the literature.31d For example Jana et al.31e have reported the synthesis and
Xꢀray characterization of four ruthenium–PPh3 complexes with a donor azoꢀligand, 1ꢀmethylꢀ2ꢀ
{(oꢀthiomethyl)phenylazo}imidazole in combination with DFT and TDDFT calculations that
were used to characterize the systems.
In this work, we have reported two complexes involving azo Schiff base ligands. The
Mn(III) mononuclear complex (1) exhibits strong phenoxazinone synthase like activity, whereas
Zn(II)ꢀMn(II) tetranuclear complex (2) is found to be feebly active or rather almost inactive. So,
3

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here we have focused on factors governing the reactivity of the complexes. A detail study has
been made for structureꢀproperty correlation to justify the reactivity trend of the complexes.
Besides, supramolecular interactions of the complexes have been computed using
TURBOMOLE software. The energetic features of the noncovalent interactions (Hꢀbonding and
π–π stacking) that govern the supramolecular assemblies observed in the solid state have been
studied by means of DFT calculations and MEP analysis.
Experimental section
Materials and synthesis
All reagent or analytical grade chemicals and solvents were purchased from commercial
sources and used without further purification.
Synthesis of (E)-2-hydroxy-3-methoxy-5-(p-tolyldiazenyl)benzaldehyde
The azo aldehyde [(E)ꢀ2ꢀhydroxyꢀ3ꢀmethoxyꢀ5ꢀ(pꢀtolyldiazenyl)benzaldehyde] has been
synthesized using standard procedure.32 Briefly, 4 mL conc. HCl was added to 15 mL water and
kept in an ice bath keeping the temperature 0 ºC. To it, pꢀtoluidine (4.0 mmol, 0.430 g) was
added. To this mixture an aqueous solution of sodium nitrite (6.0 mmol, 0.414 g) was added drop
wise over a time period of 30 mins. The mixture was stirred for 1 h at 0 ºC and then the resultant
solution was added to the alkaline oꢀvanillin (4.8 mmol, 0.730 g) which was kept in another ice
bath. The mixture was further stirred for 1 h at 0 ºC. The solution was neutralised using dilute
HCl and pH was maintained around 7.0. The mixture was extracted with dichloromethane and
evaporated to result red coloured aldehyde.
Synthesis
of
Schiff
base
ligand
[H2L1
=
(E)-6,6'-((1E,1'E)-(ethane-1,2-
diylbis(azanylylidene))bis(methanylylidene))bis(2-methoxy-4-((E)-p-tolyldiazenyl)phenol)]
A mixture of (E)ꢀ2ꢀhydroxyꢀ3ꢀmethoxyꢀ5ꢀ(pꢀtolyldiazenyl)benzaldehyde (2.0 mmol) and
1,2ꢀdiaminoethane (1.0 mmol, 60.1 mg) was heated to reflux for 4 h in methanolꢀ
dichloromethane solvent mixture (1:1, v/v). The resulting deep red coloured Schiff base ligand
(H2L1) was used for further complexation reaction.
IR (cmꢀ1, KBr): υ(C=N) 1610 m; υ(N=N) 1400 s.
1H NMR (CDCl3, 300 MHz) δ ppm: 2.41 (ArꢀCH3) (s, 3H), 3.96 (ꢀOCH3) (s, 3H), 4.02 (ꢀCH2)
(s, 2H), 7.26ꢀ7.29 (ArꢀH) (d, 9Hz, 2 H), 7.53 (ArꢀH) (s, 2 H), 7.74ꢀ7.7.77 (ArꢀH) (d, 9Hz, 2 H),
8.39 (ꢀCH=N) (s, 1H), 14.19 (ꢀOH) (bs, 1H).
4

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Synthesis of Schiff base ligand [H2L2 = (E)-6,6'-((1E,1'E)-((2,2-dimethylpropane-1,3-
diyl)bis(azanylylidene))bis(methanylylidene))bis(2-methoxy-4-((E)-p-tolyldiazenyl)phenol)]
A mixture of (E)ꢀ2ꢀhydroxyꢀ3ꢀmethoxyꢀ5ꢀ(pꢀtolyldiazenyl)benzaldehyde (2.0 mmol) and
2,2ꢀdimethylꢀ1,3ꢀdiaminopropane (1.0 mmol, 0.102 g) was heated to reflux for 4 h in methanolꢀ
dichloromethane solvent mixture (1:1, v/v). The resulting deep red coloured Schiff base ligand
(H2L2) was used for further complexation reaction.
IR (cmꢀ1, KBr): υ(C=N) 1616 m;υ(N=N) 1390 s.
1H NMR (CDCl3, 300 MHz) δ ppm: 1.15 (ꢀCH3) (s, 3H), 2.42 (ArꢀCH3) (s, 3H), 4.00 (ꢀOCH3) (s,
3H), 3.40 (ꢀCH2) (s, 2H), 7.27 (ArꢀH) (s, 2 H), 7.57 (ArꢀH) (s, 2 H), 7.75ꢀ7.8 (ArꢀH) (m, 2 H),
8.36 (ꢀCH=N) (s, 1H).
Preparation of [MnIII(L1)Cl(H2O)].H2O(1)
A 10 mL methanolic solution of manganese(II) chloride tetrahydrate (1.0 mmol, 0.198 g)
was added drop wise to CH2Cl2/methanol (1:3, v/v) solution of H2L1 (1.0 mmol) followed by
addition of triethylamine (2.0 mmol, ~0.28 mL), and the resultant reaction mixture was heated to
reflux for about 1 h. The solution was then cooled and filtered. Deep brown coloured block
shaped crystals resulted from the slow evaporation of methanolic solution of the complex at
room temperature. Yield: 0.613 g (89%). Anal. Calc. for C32H34ClMnN6O6: C 55.78%; H 4.97%;
N 12.20%. Found: C 55.65%; H 4.77%; N 12.08%. IR (cmꢀ1, KBr): υ(C=N) 1625 m;υ(N=N)
1425 s; ν(CꢀH) 730 s. UVꢀVis, λmax (nm), (ε (dm3molꢀ1cmꢀ1)) in MeOH: 227 (88920), 294
(35877), 391 (16047).
Preparation of[{ZnII (L2)2Cl2}MnII2(µ1,1-N3)2(H2O)2].2H2O (2)
2
A 5 mL methanolic solution of zinc(II) chloride (1.0 mmol, 0.136 g) was added drop wise
to CH2Cl2/methanol (1:3, v/v) solution of H2L2 (1.0 mmol) followed by addition of triethylamine
(2.0 mmol, ~0.28 mL), and the resulting solution was stirred at room temperature for ca. 2 h. To
the resulting mixture, 5 mL methanolic solution of manganese(II) chloride tetrahydrate (1.0
mmol, 0.198 g) was added drop wise followed by gradual addition of aqueous methanolic
solution (~1.5 mL) of sodium azide (1.00 mmol, 0.065 g). The reaction mixture was then heated
to reflux for about 4 h. The solution was then cooled and filtered. Deep bronze coloured plate
shaped crystals resulted from the slow evaporation of solution at room temperature. Yield: 1.576
g (92%). Anal. Calc. for C70H84Cl2Mn2N18O14Zn2: C 49.08%; H 4.94%; N 14.72%. Found: C
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49.01%; H 4.85%; N 14.65%. IR (cmꢀ1, KBr): υ(N3) 2070 s; υ(C=N) 1622 s; υ(N=N) 1463s; ν(Cꢀ
H) 768 s. UVꢀVis, λmax (nm), (ε
(dm3molꢀ1cmꢀ1)) in MeOH: 285 (40112), 381 (69492).
Physical measurements
Elemental analysis for C, H and N was carried out using a Perkin–Elmer 240C elemental
analyser. Infrared spectra (400–4000 cm–1) were recorded from KBr pellets on a Nicolet Magna
IR 750 seriesꢀII FTIR spectrophotometer. Absorption spectra were measured using a Cary 60
spectrophotometer (Agilent) with a 1ꢀcmꢀpathꢀlength quartz cell. Electron spray ionization mass
(ESIꢀMS positive) spectra were recorded on a MICROMASS QꢀTOF mass spectrometer.
1
Measurements of HNMR spectra were conducted using a Bruker 300 spectrometer in CDCl3.
The Xꢀband EPR spectra were recorded using a JEOL JESꢀFA 200 instrument at liquid nitrogen
temperature (77 K). Cyclic voltammetric experiment was performed using a PCꢀcontrolled PAR
model 273A electrochemical system under nitrogen atmosphere using a Ag/AgCl reference
electrode, with a Pt disk working electrode and a Pt wire auxiliary electrode in acetonitrile
containing supporting electrolyte, 0.1 M Bu4NClO4.
X-ray crystallography
Single crystal Xꢀray data of complexes 1 and 2 were collected on a Bruker SMART APEXꢀ
II CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at room
temperature. Data processing, structure solution, and refinement were performed using Bruker
ApexꢀII suite program. All available reflections in 2θmax range were harvested and corrected for
Lorentz and polarization effects with Bruker SAINT plus.33 Reflections were then corrected for
absorption, interꢀframe scaling, and other systematic errors with SADABS.34 The structures were
solved by the direct methods and refined by means of full matrix leastꢀsquare technique based on
F2 with SHELXꢀ2017/1 software package.35 All the nonꢀhydrogen atoms were refined with
anisotropic thermal parameters. All the C–H hydrogen atoms were inserted at geometrically
idealized positions with Uiso = 1/2Ueq to those they are attached. Crystal data and details of data
collection and refinement for 1 and 2 are summarized in Table 1. The four pꢀtolylmethanimine
groups present in the asymmetric unit in 2 have been split over two complementary different
positions in order to model the E/Z conformations that are possible conformations for the imino
group. To do that, DFIX, DANG and FLAT restraints have been used to keep the molecule
integrity during the refinement. The occupancies of the disordered parts were allowed to freely
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refine until a constant number was obtained and then the values were changed to fixed ones in
order to simplify the final refinement.
Table1 Crystal parameters and selected refinement details for compounds 1 and 2
Compound
compound_1
compound_2
CCDC
CCDC 1478366
C32H34ClMnN6O6
1.447
CCDC 1562304
C70H84Cl2Mn2N18O14Zn2
1.439
Formula
Dcalc./ g cmꢀ3
m/mmꢀ1
0.556
1.051
Formula Weight 689.04
1713.07
bronze
plate
0.30×0.16×0.06
150(2)
Monoclinic
.
Colour
brown
block
0.38×0.20×0.05
150(2)
Monoclinic
.
Shape
Size/mm3
T/K
Crystal System
Flack Parameter
Hooft Parameter
.
.
Space Group
P21/c
P2/c
a/Å
b/Å
20.2205(15)
12.0071(8)
13.1850(10)
90
98.761(3)
90
8.3750(2)
14.7703(4)
31.9870(9)
90
92.0290(10)
90
c/Å
a/°
b/°
g/°
V/Å3
3163.8(4)
4
3954.35(18)
2
Z
Z'
1
0.5
Wavelength/Å
Radiation type
0.71073
MoKa
1.979
26.467
17653
0.71073
MoKa
1.274
25.708
45338
Qmin/°
°
Qmax
/
Measured Refl.
Independent Refl. 6454
Reflections Used 3895
7512
5586
Rint
0.0462
430
2
1.210
ꢀ0.451
1.041
0.2284
0.1815
0.1248
0.0676
0.0347
610
287
1.315
ꢀ0.872
Parameters
Restraints
Largest Peak
Deepest Hole
GooF
wR2 (all data)
wR2
R1 (all data)
R1
1.055
0.1995
0.1815
0.0830
0.0613
7

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Catalytic oxidation of o-aminophenol
Phenoxazinone synthase like activity of the complexes were examined by the reaction of 1.0×
10ꢀ5 M dioxygenꢀsaturated methanolic solutions of the complexes with 10ꢀ2 M solution of oꢀ
aminophenol (OAPH) at 25 ºC. Upon formation of the phenoxazinone chromophore, the
successive increase in absorbance band at ca. 433 nm was monitored spectrophotometrically. To
evaluate the rate dependency of the reaction on OAPH concentration and to evaluate various
kinetic parameters like Vmax, KM, Kcat, 1.0× 10–5 (M) solution of the complexes were mixed with
various concentration of substrate maintaining minimum 10 folds excess to that of catalyst to
retain the pseudoꢀfirstꢀorder condition. Rate of a reaction was evaluated from the initial rate
method, and the average initial rate over three independent measurements was recorded.
Theoretical methods
The calculations of the noncovalent interactions were carried out using the TURBOMOLE
version 7.036 using the M06ꢀ2X/def2ꢀTZVP level of theory. To evaluate the interactions in the
solid state, we have used the crystallographic coordinates. This procedure and level of theory
have been successfully used to evaluate similar interactions.37 The interaction energies were
computed by calculating the difference between the energies of isolated monomers and their
assembly. The interaction energies were corrected for the Basis Set Superposition Error (BSSE)
using the counterpoise method.38
Results and Discussion
Syntheses and spectral characterization of ligands (H2L1 and H2L2) and complexes 1 and 2
Initially, azoaldehyde has been prepared by diazoꢀcoupling reaction between pꢀtoludine and oꢀ
vanillin.32 Then Schiff bases have been synthesized through condensation of diamines with that
azoaldehyde in a 1:2 molar ratio in reagent grade methanolꢀdichloromethane solvent mixture
(Scheme 1). Azoaldehyde and Schiff base ligands have been initially characterized by CHN
1
analysis and ESI Mass spectroscopy. In HNMR spectrum of H2L1 (CDCl3), imine proton
appeared at 8.39 ppm. Aromatic, aliphatic and methoxy protons appeared at 7.26–7.77 ppm, 4.02
ppm, and 3.96 ppm, respectively. The aromatic–OH appeared at 14.19 as a broad singlet.
1
Hydrogen atoms of methyl group of toluidine ring appeared at 2.41 ppm (Fig. S1). In HNMR
spectrum of H2L2 (CDCl3), imine proton appeared at 8.36 ppm. Aromatic, aliphatic –CH2, –CH3
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and methoxy protons appeared at 7.27–7.78 ppm, 3.40 ppm, 1.15 ppm, and 4.00 ppm,
respectively (Fig. S2). Methyl protons of toluidine appeared at 2.42 ppm. The bands appearing at
1610 cmꢀ1 and 1616 cmꢀ1, respectively, are the characteristic stretching frequency of –CH=N
bonds of both of the schiff base ligands. Whereas bands appearing at 1400 cmꢀ1 and 1390 cmꢀ1,
respectively, are the characteristic stretching frequency of azo bonds (N=N) of both of the schiff
base ligands (Figs. S3 and S4). For complexes 1 and 2, the bands appeared in the IR spectra
around 1600 and 1620 cmꢀ1, respectively, are assigned to –CH=N stretching frequency. Whereas,
the characteristic azo N=N stretching frequency of complexes 1 and 2 are obtained around 1445
and 1460 cm–1, respectively. The characteristic µꢀ1,1 azido stretching of complex 2 has been
obtained at around 2070 cm–1 (Figs. S5 and S6). All the characteristic stretching frequencies of
FTIR data are very much comparable with that of literature values.39 UVꢀVis spectra of
complexes 1 and 2 have been recorded in methanol solvent at room temperature. Complex 1
shows absorption bands around 391, 294 and 227 nm attributed to nꢀπ* transitions and πꢀπ*
transitions, respectively. Whereas, complex 2 shows absorption bands around 380 nm and 285
nm attributed to nꢀπ* transitions and πꢀπ* transitions, respectively.
Scheme 1. The route to the syntheses of ligand H2L1 and H2L2.
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Description of the crystal structure of 1
The Xꢀray data reveal that complex 1 crystallized in the monoclinic P21/c space group.
Crystal structure of complex 1 is depicted in Fig. 1. Selected bond distances and bond angles are
listed in Table 2. The Mn(III) complex is mononuclear and the metal centre is hexaꢀcoordinated,
where secondary valence has been satisfied by two imineꢀN atoms and two phenoxidoꢀO atoms
of the tetradentate Schiff base ligand, one chloride ion and one water molecule, respectively.
There is another molecule of water present in the crystal system which was contained by nonꢀ
dried solvent (methanol), used for crystallization purpose. The triꢀpositive charges of Mn(III) ion
have been satisfied by the dianionic tetra dentate ligand and one chloride ion. The geometry
around the manganese centre is best described as a slightly distorted octahedron.
Table 2. Selected bond lengths (Å) and bond angles (°) for complex 1at 150 K.
MnꢀN1
1.980(4) N1ꢀMnꢀN2 82.75(15)
1.975(4) N1ꢀMnꢀO1 91.49(14)
1.889(3) N2ꢀMnꢀO2 91.49(15)
1.891(3) O3ꢀMnꢀCl 170.06(10)
2.5407(15)
MnꢀN2
MnꢀO1
MnꢀO2
MnꢀCl
N3ꢀN4
1.260(6)
N5ꢀN6
1.229(7)
MnꢀO3
2.277(4)
Cl–O100 3.317(6)
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Fig. 1. Ortep view of complex 1. Atoms are shown as 30% thermal ellipsoids. H atoms are
omitted for clarity.
In the said octahedron environment, the equatorial sites are occupied by iminoꢀN atoms
and phenoxidoꢀO atoms, respectively, and the axial sites are occupied by chloride ion and water
molecule. It has been found that manganese centre deviates from the mean basal plane by 0.095
Å. The Mn–Nimino and Mn–Ophenoxido bond distances of the Schiff base are found to be 1.980(4) Å
(Mn–N1), 1.975(4) Å (Mn–N2), 1.889(3) Å (Mn–O1), and 1.891(3) Å (Mn–O2), which are
comparable with that of previously reported literature values.40 The MnIII–Cl and MnIII–OH2
axial band distances are 2.5407(15) Å and 2.277(4) Å, respectively, that are typical for the
elongated Jahn Teller distortion around the Mn(III) ion (d4 system), while azo N=N bond
distance is found to be ~ 1.2 Å. The bite angles of the complex are 82.75(15)º (N1–Mn–N2),
91.49(14)º (O1–Mn–N1) and 91.49(15)º (O2–Mn–N2) respectively. The value of transoid angle
is 170.06(10)º (O3–Mn–Cl). The Cl atom is tilted toward the O atom of the second sphere water
molecule [Cl···O distance 3.317(6) Å].
It is curious that a water ligand is placed at the first coordination sphere, instead of the
most abundant MeOH molecule that is used as solvent. In fact, this was the case for a related
pyridoxalꢀderived Schiff base Mn(III) complexes reported by Naskar et al.40b displaying transꢀ
oriented Cl/MeOH ligands in a similar environment. Probably equilibrium of both complexes
exists in the mother solution and, for some reason, only the water coordinated complex
crystallizes displacing the equilibrium.
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Description of the crystal structures of 2
Single crystals of complex 2 were obtained from slow evaporation of its solvent mixture of
dichloromethaneꢀmethanol. It crystallized with the space group in the monoclinic unit cell.
P2/c
Crystal structure of complex 2 is depicted in Fig. 2. Selected bond distances and bond angles are
listed in Table 3. Complex 2 is a centrosymmetric tetranuclear molecule, where the asymmetric
unit contains [Zn(L2)(Cl)Mn(H2O)(µ1,1ꢀN3)] moiety which through symmetry operation
generates the whole molecule. Asymmetric units are connected by two symmetry related µ1,1
azido bridges resulting in a formation of this tetranuclear molecule. In the asymmetric unit, the
inner core of the azo bicompartmental Schiff base ligand is occupied by zinc(II) ion and outer
core is occupied by Mn(II) ion. The Zn(II) ion placed in the inner core of the ligand is pentaꢀ
coordinated with two phenoxidoꢀO atoms, two imineꢀN atoms and one chloride ion. The
geometry index (structural or Addison parameter) (τ)41 for Zn(II) centre is found to be 0.046
which is very close to zero and the environment around the Zn(II) centre suggests to be almost
PͯQ
square pyramidal. Here, τ =
, α and β being the two greatest valence angles of the
ͪͤ
coordination centre. The equatorial sites of the pentaꢀcoordinated Zn(II) ion are occupied by two
iminoꢀN atoms and two phenoxidoꢀO atoms. The Zn–Nimino and Zn–Ophenoxido bond distances are
2.093(5) Å (Zn1ꢀN3), 2.048(6) Å (Zn1ꢀN25), 2.018(4) Å (Zn1ꢀO7) and 2.069(3) Å (Zn1ꢀO29).
The apically coordinated Zn–Cl bond distance is 2.257(2) Å. The Zn(II) ion deviates from mean
N2O2 plane by 0.503 Å. The Mn(II) ion placed in the outer core is connected with the Zn(II) ion
through two phenoxido bridges. The secondary valence of Mn(II) centre is satisfied by two
bridging phenoxidoꢀO atoms, two methoxyꢀO atoms, two bridging (µ1,1) azido ions and one
water molecule. The Mn(II) centre is heptaꢀcoordinated and adopts a distorted pentagonal
bipyramidal geometry. In the coordination sphere it is equatorially coordinated with two
phenoxidoꢀO atoms (O7, O29), two methoxyꢀO atoms (O9, O31) and a µ1,1 bridged azido
nitrogen atom (N46). Whereas, oxygen atom of a water molecule (O1W) and a symmetry related
µ1,1 bridged azido nitrogen atom (N46A) are axially coordinated with the metal centre. It has
been observed that the Mn(II) is slightly above (0.018 Å) the pentagonal plane. The value of
transoid angle is 169.45(15)° (O1W–Mn1–N46A). The MnII–Ophenoxido, MnII–Omethoxy, MnII–Nazido
and MnII–Owater bond distances are found to be 2.166(3)Å (Mn1ꢀO7), 2.160(3)Å (Mn1ꢀO29),
2.452(4)Å (Mn1ꢀO9), 2.499(3)Å (Mn1ꢀO31), 2.200(4)Å (Mn1ꢀN46) and 2.157(4)Å (Mn1ꢀ
O1W), respectively. The Zn(II)···Mn(II) and Mn(II)….Mn(II) separations are 3.381 Å and 3.495
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Å respectively. All the bond distances around Zn(II) and Mn(II) centres are found to be
comparable with that of previously reported literature values.42 The Mn1ꢀ(µ1,1)N1ꢀMn1A angle is
found to be 104.06(14)º. Two water molecules are present as solvents of crystallization in the
given crystal system. The azo N=N bond distances are found to be 1.06(1) Å (N14ꢀN15) and
1.02(1) Å (N36ꢀN37). The pictorial representation of coordination environment around the metal
centre is given in Fig. S7. The value of bite angles around Zn(II) and Mn(II) centres are 94.8(2)º
(N3–Zn1–N25), 88.27(17)º (N3–Zn1–O7), 88.03(19)º (N25–Zn1–O29), 66.88.(13)º (O7–Mn1–
O9), 70.40(13)º (O7–Mn1–O29) and 67.17(12)º (O29–Mn1–O31), respectively.
Fig. 2. Ortep view of complex 2. Atoms are shown as 30% thermal ellipsoids. H atoms are
omitted for clarity.[ Symmetry operation: A = ꢀx,ꢀy,ꢀz].
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Table 3. Selected bond lengths (Å) and bond angles (°) for complex 2 at 150 K.
Zn1ꢀN3
2.093(5)
2.048(6)
2.018(4)
Mn1ꢀMn1
N25ꢀZn1ꢀN3
O7ꢀZn1ꢀN3
3.495
Zn1ꢀN25
Zn1ꢀO7
94.8(2)
88.27(17)
88.03(19)
66.88(13)
70.40(13)
67.17(12)
Zn1ꢀO29
Zn1ꢀCl1
N14ꢀN15
N36ꢀN37
Mn1ꢀO7
Mn1ꢀO9
2.069(3) O29ꢀZn1ꢀN25
2.257(2)
1.06(1)
1.02(1)
O7ꢀMn1ꢀO9
O7ꢀMn1ꢀO29
O29ꢀMn1ꢀO31
2.166(3) O1WꢀMn1ꢀN46 169.45(15)
2.452(4) Mn1ꢀN46ꢀMn1A 104.01(2)
Mn1ꢀO29 2.160(3) N46ꢀN47ꢀN48
Mn1ꢀO1W 2.157(4)
179.8(7)
Mn1ꢀN46 2.200(4)
Mn1ꢀO31 2.499(3)
Computational Study
The theoretical study is devoted to analyse some noncovalent interactions that are present in the
crystal packing of Mn(III/II) complexes 1 and 2 focusing our attention to the role of the
coordinated water ligand influencing the crystal packing of 1. As a first approximation we
computed the Molecular Electrostatic Potential (MEP) surface of complex 1 in order to analyse
its electron donor/acceptor properties. It can be observed that the most positive region
corresponds to the Hꢀatoms of coordinated water molecule due to the enhanced acidity of these
protons as a consequence of the metal coordination. Moreover, the most negative region
corresponds to the phenoxide O atoms. In fact the four O atoms (two from the phenoxide groups
and two from methoxy groups) converge to the same point, providing an electron rich region.
Moreover, the coordinated chlorido ligand also presents a spherical region of negative potential
(Fig. 3).
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Fig.3. MEP surface of complex 1. Energies at selected points of the surfaces are given in
kcal/mol.
In Fig. 4a we show partial view of the crystal packing (2Dꢀsupramolecular chain) of complex 1
where it can be observed the presence of selfꢀassembled dimers. We have analysed these dimers
in detail (Fig. 4b) and coordinated water molecule establishes four Hꢀbonding interaction with
the oxygen atoms of the Schiff base ligand, in sharp agreement with the MEP analysis shown in
Fig. 3. We have evaluated the binding energy of this dimer, which is very large and negative
(ꢁE1 = –75.1 kcal/mol) as a consequence of the formation of eight Hꢀbonding interactions and
confirms the relevance of these interactions in the crystal packing of complex 1. Moreover, this
selfꢀassembled dimer also present two different types of πꢀstacking interactions, one between the
uncoordinated rings (4.08 Å) and the other between the coordinated rings (3.61 Å) of the ligand.
In order to evaluate the contribution of the Hꢀbonds, we have computed a theoretical model
where we have eliminated the coordinated water molecules. As a result, the interaction energy is
reduced to ꢁE2 = –37.4 kcal/mol and the contribution of the Hꢀbonds can be estimated as the
difference, ꢁE1 –ꢁE2 = – 27.7 kcal/mol. In an effort to evaluate the πꢀstacking interactions
between the uncoordinated rings, we have computed an additional model where the rings have
been eliminated. Thus, the energy is reduced to ꢁE3 = –22.3 kcal/mol and consequently a rough
estimation of the contribution of each stacking interaction is 1/2(ꢁE3 – ꢁE2) = –7.6 kcal/mol
(Fig. 4cꢀd).
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Fig.4.(a) Xꢀray fragment of 1.(bꢀd) theoretical models used to evaluate the noncovalent
interactions. Hꢀatoms omitted for clarity apart those involved in the Hꢀbonds. Distances in Å.
In complex 2 we have evaluated the energy of the self–assembled 1D supramolecular chains
observed in the solid state which are governed by the formation of symmetrically equivalent O–
H···N H–bonds (see Fig.5a). We have computed a dimer extracted from this polymeric chain
(Fig.5b) and a close examination reveals the existence of unconventional C–H···Cl Hꢀbonding
interactions that also participate in the assembly mechanism (see red dashed lines). We have
computed the binding energy of this dimer, which is large and negative (ꢁE4 = –26.2 kcal/mol)
as a consequence of the formation of two strong O–H···N(azide) interactions and four weaker C–
H···Cl Hꢀbonding interactions. In an effort to evaluate the contribution of the ancillary C–H···Cl
Hꢀbonding interactions, we have used a theoretical model where four methyl groups have been
replaced by H atoms (see small arrows in Fig.5c). Consequently, the interaction energy is
reduced to ꢁE5 = –22.0 kcal/mol that corresponds to the contribution of the strong O–H···N
hydrogen bonds. Moreover, the additional contribution of the rest of unconventional Hꢀbonds
can be estimated as the difference, ꢁE4 – ꢁE5 = – 4.2 kcal/mol.
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Fig.5. (a) Xꢀray fragment of 1, Hꢀatoms omitted for clarity apart those involved in the Hꢀbonds.
(b,c) Theoretical models used to evaluate the noncovalent interactions. Distances in Å.
Electrochemical study
It is well known that the redox flexibility between the different oxidation states of the transition
metal at the active site of a metalloenzyme plays an anchoring role in catalysing the oxidation of
various biologically important organic molecules. Therefore, it is important to check the
electrochemical behaviour of the synthetic compounds as that result could disclose the potential
of the synthetic analogues for behaving as redox catalysts. Cyclic voltammogram of complex 1 is
depicted in Fig. 6 in which potentials are referenced to the standard Ag/AgCl electrode. An
electrochemically irreversible reduction wave was obtained at ꢀ0.101 V versus Ag/AgCl
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electrode, and this can be assigned to the reduction of manganese(III) to manganese(II).43 The
value of reduction potential (ꢀ0.101 V) is found to be very interesting as that suggests that the
compound can be capable to oxidase various organic compounds. Furthermore, the reduced
manganese(II) species can easily be reꢀoxidized by the molecular dioxygen. Therefore, the
electrochemical study discloses that compound 1 could be an efficient redox catalyst for the
aerobic oxidation of various organic compounds. One of such catalytic reaction (phenoxazinone
synthase like activity) has been performed in details (vide infra).
Fig. 6. Cyclic voltammogram of complex 1 in acetonitrile containing TBAP (0.1 M) as a
supporting electrolyte at a scan rate of 100 mV/s. Potentials were referenced to the Ag/AgCl
couple.
Phenoxazinone synthase like activity
Catalytic oxidation of oꢀaminophenol (OAPH) was spectrophotometrically studied in dioxygenꢀ
saturated methanol solution at 25 ºC. Firstly, 1×10–5 M methanolic solutions of the complexes
were subjected to react with a 1×10–2 M solution of OAPH under aerobic condition. Upon
successive catalytic oxidation of oꢀaminophenol (OAPH) the characteristic absorbance band of
phenoxazinone chromophore at ca. 433 nm gradually increases, which confirms the catalytic
activity of the complexes by resulting oxidation of OAPH to 2ꢀaminophenoxazinꢀ3ꢀone in
aerobic condition. The timeꢀresolved spectral profiles are shown in Figs. 7 and 8 for 1 and 2,
respectively. A blank experiment has been performed in absence of the each of the complex
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(catalyst) under identical condition, which does not result any significant escalation in the band
intensity at 433 nm. The outcome of the experiment is that the only complex 1 is catalytically
efficient to oxidize OAPH to 2ꢀaminophenoxazinꢀ3ꢀone under aerobic condition. Complex 2 is
either feebly active or almost inactive towards OAPH. So, further kinetic study of complex 2 is
not required due to its inefficiency to oxidize OAPH to 2ꢀaminophenoxazinꢀ3ꢀone. To have
insight view of the reaction kinetics of the complex 1, 1×10–5 M methanolic solutions of the
complex were subjected to react with various concentration of substrate maintaining at least 10
folds excess of catalyst used to retain pseudo first order condition. All the kinetic studies have
been performed under aerobic condition and at 25 ºC temperature maintained using a thermostat.
For a particular kinetic study using a specific substrate ratio to that of the complex, time scan at
the maximum band (433 nm) of 2ꢀ aminophenoxazinꢀ3ꢀone was carried out for a period of 20
min, and initial rate was determined by linear regression from the slope of the absorbance versus
time; each experiment was performed thrice and average values were noted. The initial reaction
rates versus concentrations of the substrate plot shows rate saturation kinetics as depicted in Fig.
9. These results suggest that, in the catalytic cycle of OAPH oxidation, the complex initially
forms adduct with that of the substrate followed by release of oxidized product in an irreversible
manner which is the rate determining step (r.d.s.). Since the rate of the reaction depends of
substrate concentration only, the following kinetic measurement can be fitted to Michaelis–
Menten model. The kinetic parameters viz. Vmax, KM, and Kcat can be determined by linearization
of Michaelis–Menten equation which gives double reciprocal Lineweaver–Burk plot. The
observed and simulated initial rate versus substrate concentration plot and the Lineweaver–Burk
plot for the complexes are shown in Figs. 9 and 10, respectively. Michaelis binding constant
(KM) and Vmax were calculated to be (6.96 ± 0.19) × 10–3 M and 9.79×10–8 M Secꢀ1, respectively.
The turnover number (Kcat) value is calculated by dividing the Vmax by the concentration of the
complex used, and is found to be 35.24 h–1. Moreover, linearity has been observed for initial rate
of catalysis which confirms pseudoꢀfirst order kinetic of the complex 1.
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Fig. 7. UV–Vis spectral scans showing the increase in phenoxazinone chromophore band at 433
nm after the addition of oꢀaminophenol (10ꢀ2 M) to a solution of complex 1(1×10ꢀ5 M) in
methanol at 250C. The spectra were recorded for the period of 2 h.
Fig. 8. UV–Vis spectral scans showing the increase in phenoxazinone chromophore band at 433
nm after the addition of oꢀaminophenol (10ꢀ2 M) to a solution of complex 2 (1×10ꢀ5 M) in
methanol at 250C. The spectra were recorded for the period of 2 h.
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Fig. 9. Initial rate verses substrate concentration plot for the oxidation of oꢀaminophenol
catalyzed by complex 1 in dioxygenꢀsaturated methanol. Symbols and solid lines represent the
experimental and simulated profiles, respectively.
Fig. 10. Lineweaver–Burk plots for the oxidation of oꢀaminophenol catalysed by complex 1 in
aerobic condition. Symbols and solid lines represent the experimental and simulated profiles,
respectively.
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ESI mass spectral study
Mass spectrometry is a useful technique that could provide significant information regarding the
important intermediates of a chemical reaction from which the most possible mechanistic
pathway of a catalytic reaction can be framed. As compound 1 is only found active catalyst for
the aerobic oxidation of oꢀaminophenol, the ESI (positive) mass spectral studies of compound 1
itself and in the presence of excess of the substrate have been performed in methanol. The mass
spectrum of complex 1 (Fig. S8) consists of the base peak at m/z = 617.0635 which can be
assigned as the [Mn(L1)]+ species (calculated m/z = 617.17). The second most abundant peak at
m/z = 338.2629 is a fragmented molecular ion peak of azoaldehyde together with two water
molecules and one methanol molecule (calculated m/z = 338.15). When mass spectrum was
recorded in presence of excess substrate, the base peak was found at m/z = 215.0203, which is
quite interesting as the isotropic distribution patterns matched well with the protonated
intermediate IIꢀB (calculated m/z = 215.08) as shown in scheme 2. Again peak at m/z =
213.0203 is a product related peak as that agrees with the protonated species of 2ꢀ
aminophenoxazinꢀ3ꢀone (calculated m/z = 213.06). The basal peak at m/z = 617.0635 in the
mass spectrum of the compound itself become a minor component when the mass is ionized in
presence of the substrate. Moreover, a minor peak at m/z = 726.1697 related to the complexꢀ
substrate aggregate of formula [Mn(L1) + OAPH]+ is also deserved to be special mentioned as
that suggests binding of the substrate to the metal centre during the catalytic oxidation of the
substrate, which is consistent with the rate saturation kinetics (Fig.11).
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Fig. 11. ESI mass spectrum of complex 1 in presence of excess oꢀaminophenol; Inset showing
expanded mass spectrum at selected region.
Plausible mechanistic pathway and comparative study of catalytic efficiency
Both the rate saturation kinetics and the mass spectral study reveal that the catalytic cycle starts
with the formation of a stable complexꢀsubstrate aggregate by the replacement of the labile
coordinated water or chloride ion at first coordination sphere. It is well known that when the
higher oxidation of the transition metal canter is involved in the oxidation reaction, it generally
oxidises the substrate at the rate determining step and subsequently reduces to the lower
oxidation state.44 In order to check such possibility in our case, we have performed EPR
spectroscopic studies. Interestingly, when EPR spectrum was recorded of the complex in
presence of excess substrate, we observed sixꢀline EPR spectrum (Fig. 12), which indicates the
formation of manganese(II) species in presence of the substrate. The complex itself is found EPR
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silent which is characteristic for the manganese(III) complexes. These observations likely
suggest that at the first step OAPH forms adduct with the complex 1, which thereafter through
redox transformation generates OAP radical in the rate determining step. Now, the OAP radical
might be converted into oꢀbenzoquinone monoamine (BQMI) in several ways including selfꢀ
disproportionation reaction. Finally, 2ꢀaminophenoxazinꢀ3ꢀone is produced through several
oxidative dehydrogenation processes involving OAPH, O2, and BQMI as shown in Scheme 2.
Remarkably, intermediate IIꢀB, shown in Scheme 2, was eventually identified in the mass
spectrum of the complex in presence of OAPH, thereby validating the said scheme.
UVꢀVis Spectrophotometric study of the catalytic oxidation of OAPH reveals that
complex 1 is highly active whereas complex 2 is feebly active or almost inactive. So, here, a
structureꢀproperty correlation is highly demanded. Catalytic efficiency of a complex depends
upon a number of factors viz. oxidation state, nuclearity, binding site, flexibility of ligand system
etc. Like 1, a labile site (coordinated water molecule) is also available at the manganese centre
in 2 for substrate binding, and furthermore there are two adjacent manganese(II) centres present
in the molecule, still it is found almost unreactive. It can be visualized that when lower oxidation
state of the transition metal participates in the catalytic oxidation of the substrate by molecular
oxygen, generally, both the substrate binding and dioxygen activation are expected to be required
at the rate determining step. In complex 2, each manganese(II) centre has only one labile site, so
either of the two metal centres binds the substrate or molecular dioxygen. Therefore, one can
expect that this cooperation may help the catalytic oxidation of substrate by molecular oxygen.
In reality, we have found that complex 2 is almost unreactive. Careful inspection of the crystal
structure of 2 reveals that two labile water molecules are coordinated to the manganese(II)
centres in opposite directions, and consequently two metalꢀbound reactants dioxygen and OAPH
are displayed far away from each other, that rendering the system to have any effect of such
cooperation for the substrate oxidation. This result therefore suggests that the proper
stereochemical arrangements of the vacant or labile positions in the high nuclearity transition
metal complexes are extremely important for the cooperative catalytic activity when lower
oxidation state metal ions are involved in aerobic oxidation.
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Scheme 2. Probable mechanistic pathway for the formation of the phenoxazinone chromophore
catalysed by complex 1.
Fig. 12. Xꢀband EPR spectrum of complex 1 after addition of ~50 eq. of oꢀaminophenol at 77 K.
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Conclusion
We have synthesized and Xꢀray crystallographically characterized two new Mn(III/II) complexes
1 and 2. The azo appended Schiff ligands have been developed for synthesizing metal complexes
to introduce π–π stacking interactions in the solid state for bringing extra stability in the
molecules. The energetic features of the noncovalent interactions that govern the supramolecular
assemblies observed in the solid state interactions have been studied by means of DFT
calculations and MEP analysis, which disclose rich hydrogen bonding and π–π stacking
interactions in 1. Interestingly, the azo additive in oꢀvanillin part is involved in fairly strong π–π
stacking in the solid state; therefore, such azoꢀaromatic group incorporation in the systems could
bring interesting noncovalent interactions in the system for the stability. Regarding the catalytic
oxidation of oꢀaminophenol by molecular oxygen, the compound 1 is found to be an active
catalyst, while compound 2 is almost inactive, although in both cases labile positions are
available at manganese centres for substrate/dioxygen binding. The structure–reactivity
correlation study reveals that the stereochemical positions of labile sites in the manganese(II)
center in 2 are not suitable enough for the cooperative catalytic oxidation of the substrate by
molecular oxygen. These results therefore dictate that the proper stereochemical arrangements of
the vacant or labile positions in the high nuclear transition metal complexes with lower oxidation
state of the metal are extremely important for the cooperative catalytic activity. Such a
stereochemical requisite seems not to be at all required for the metal complexes with higher
oxidation state, therefore easing to develop efficient in vitro catalysts.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgment
A.S. gratefully acknowledges the financial support of this work by the DST, India (Sanction No.
SB/FT/CSꢀ102/2014, datedꢀ 18.07.2015). A. F. gratefully acknowledges the financial support of
this work by the DGICYT of Spain (projects CTQ2014ꢀ57393ꢀC2ꢀ1ꢀP and CONSOLIDER
INGENIO 2010 CSD2010ꢀ00065, FEDER funds). A. F. thanks the CTI (UIB) for free allocation
of computer time.
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Appendix A. Supplementary data
CCDC 1478366 and 1562304 contain the supplementary crystallographic data for complexes 1
and 2, respectively. These 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
email:deposit@ccdc.cam.ac.uk.
Notes & References
aDepartment of Chemistry, Jadavpur University, Kolkata- 700032, India.
E-mail: amritasahachemju@gmail.com; asaha@chemistry.jdvu.ac.in; Tel. +91ꢀ33ꢀ2457294.
bDepartamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
cDepartament de Química, Universitat de les IllesBalears, Crta. De Valldemossa km 7.5, 07122
Palma (Baleares), Spain.
E-mail:toni.frontera@uib.es.
1 A. de Meijere, S. Bräse and M. Oestreich (Eds.), Metal-Catalyzed Cross-CouplingReactions
and More, WileyꢀVCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2014.
2 K. Grela (Ed.), Olefin Metathesis, John Wiley & Sons Inc, Hoboken, NJ, USA,2014.
3 Z. ꢀJ. Shi, Homogeneous Catalysis for Unreactive Bond Activation, John Wiley & Sons Inc,
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