Importance of C-H?π interactions in stabilizing the: Syn / anti arrangement of pendant alkoxy side arms in two manganese(IV) Schiff base complexes: Exploration of catechol oxidase and phenoxazinone synthase like actIVities

Article abstract of DOI:10.1039/c7nj00766c

Two new octahedral manganese(iv) complexes [Mn(HL¹)(L¹)(NCS)]_n (1) and [Mn(HL²)₂(OH)Cl] (2) [where H₂L¹ = 1-(5-hydroxy-3-oxapentyliminomethyl)-3-ethoxyphenol and H₂L² = 1-(5-hydroxy-3-oxapentyliminomethyl)-4-chlorophenol] have been prepared and characterized. Unconventional C-H?π_(SCN) interactions play a key role in stabilizing the unexpected syn arrangement of the ligand pending arms in 1, whereas conventional C-H?π_(phe) interactions favour the anti arrangements in 2. The energetic features of these interactions have been studied by DFT calculations and characterized using Bader's theory of atoms in molecules in both complexes. Both complexes exhibit catechol oxidase and phenoxazinone synthase like activities.

Full text of DOI:10.1039/c7nj00766c

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The importance of unconventional C–H···π(SCN) interactions in stabilizing the
unexpected syn/anti arrangement of pendant alkoxy side arms in two
manganese(IV) Schiff base complexes: Exploration of catechol oxidase and
phenoxazinone synthase like activities
Nandita Sarkar a, Klaus Harms b, Antonio Frontera c,*and Shouvik Chattopadhyay a,*
a Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700 032, India, Eꢀ
mail: shouvik.chem@gmail.com
bFachbereich Chemie, PhilippsꢀUniversität Marburg, HansꢀMeerweinꢀStrasse, Dꢀ35032
Marburg, Germany.
c Departament de Química, Universitat de les Illes Balears, Crta de Valldemossa km 7.5, 07122
Palma de Mallorca (Baleares), Spain, Eꢀmail:toni.frontera@uib.es.
Abstract
Two new octahedral manganese(IV) complexes [Mn(L1)2(NCS)]n (1) and
[Mn(L2)2(OH)Cl] (2) [where HL1= 1ꢀ(5ꢀhydroxyꢀ3ꢀoxapentyliminomethyl)ꢀ3ꢀethoxyphenol and
HL2 = 1ꢀ(5ꢀhydroxyꢀ3ꢀoxapentyliminomethyl)ꢀ4ꢀchlorophenol] have been prepared and
characterized. Unconventional C–H···π(SCN) interactions plays a key role in stabilizing the
unexpected syn arrangement of the ligand pending arms in 1, whereas conventional C–H···π(phe)
interactions favours the anti arrangements in 2. The energetic features of these interactions have
been studied by DFT calculations in and characterized using the Bader’s theory of atoms in
molecules in both complexes. Both complexes exhibit catechol oxidase and phenoxazinone
synthase like activities.
1

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Keywords: Manganese(IV); Schiff base; CꢀH···π(SCN) interaction, DFT study; catalytic activity
Introduction
M
anganeseꢀbased coordination compounds have received considerable interest in the
fields of supramolecular chemistry and crystal engineering not only for their remarkable
magnetic properties,1 but also f or their rich biochemistry2 and versatile catalytic
activities.3Supramolecular interactions based on πꢀsystems are recognized in crystal engineering
since long, and it traditionally involve phenyl rings of which the role of CꢀHꢁꢁꢁπ interactions are
most explored in stabilizing crystal structures and are routinely evaluated in the analysis of
packing patterns in various structures.4 CꢀHꢁꢁꢁπ interactions involving the πꢀsystem of a chelate
ring have also been defined and described.5 CꢀHꢁꢁꢁπ interaction involving the πꢀsystem of NCS is
also ‘unconventional’ and is not usually taken into account by scientists working in crystal
engineering, despite the fact that the situation is analogous to an aromatic πꢀsystem owing to the
delocalization of the πꢀelectron density over the NCS moiety. Works available in the literature
also mention to some extent the existence of CꢀHꢁꢁꢁπ (NCS) interactions,6 however their
utilization is not very common.
The chemistry of the higher oxidation states of manganese, such as manganese (IV), is
not well explored compared to that of its lower oxidation states and represents a rich area for
studying the biological chemistry of manganese. Manganese is an essential component of
photosynthetic oxygen evolution, most likely participating as the oxidant of water. A tetra
nuclear manganese(IV) cluster is found in the oxygen evolving complex (OEC) of photo system
II (PS II).7 Manganese(IV) also catalyses the reduction of hydrogen peroxide to water as in the
pseudocatalase isolated from Lactobacillus plantarum.8
2

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In the present work, we have shown the importance of unconventional C–H···π
interactions which involves the pseudohalide SCN ligand in the solid state of one complex
stabilizing the unexpected syn arrangement of the ligand pending arms and conventional C–
H···π(phe) interactions favouring the anti arrangements in the solid state of other. The energetic
features of these interactions have been studied by means of DFT calculations and characterized
using the Bader’s theory of atoms in molecules. Besides this, catalytic efficiency (i.e catecholase
and phenoxazinone synthase like activity) of each complex has been explored. Catechol oxidase
(a wellꢀknown Type 3 copper protein) is basically a copperꢀcontaining polyphenol oxidase that
catalyses the aerial oxidation of oꢀdiphenols (catechols) to the corresponding oꢀquinones. There
has been a lot of effort to synthesize model complexes and investigate their catecholase activity
for the development of new and efficient catalysts for oxidation reactions. On the other hand, one
of the important metalloenzymes that utilize molecular dioxygen for biological oxidation process
is phenoxazinone synthase,9 an oligomeric multicopper oxidase found naturally in the bacterium
Streptomyces antibioticus, and catalyzes the oxidative coupling of a wide variety of substituted
oꢀaminophenols to the phenoxazinone chromophore.
Experimental section
All chemicals were of reagent grade and used as purchased from SigmaꢀAldrich without
further purification.
Caution!!! Although no problems were encountered in this work, metal complexes
containing organic ligands in the presence of perchlorate and azides are potentially explosive.
Only a small amount of the material should be prepared and it should be handled with care.
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Preparations
Preparation of ligand, 1ꢀ(5ꢀhydroxyꢀ3ꢀoxapentyliminomethyl)ꢀ3ꢀethoxyphenol (HL1)
The Schiff base ligand HL1 was prepared by refluxing 3ꢀethoxysalicylaldehyde (152 mg,
1 mmol) with 2ꢀ(2ꢀaminoethoxy)ethanol (0.1 ml, 1 mmol) in methanol solution (10 ml) for ca.
1h. The ligand was not purified and used directly for the preparation of complex 1.
Preparation of ligand, 1ꢀ(5ꢀhydroxyꢀ3ꢀoxapentyliminomethyl)ꢀ4ꢀchlorophenol (HL2)
It was prepared in a similar method to that of HL1, except that 5ꢀchlorosalicylaldehyde
(0.156 g, 1 mmol) was used instead of 3ꢀethoxysalicylaldehyde. It was not purified and used
directly for the preparation of the complex 2.
Preparation of [Mn(L1)2(NCS)]n (1)
A methanol solution (5 ml) of manganese(II) perchlorate hexahydrate (370 mg, 1 mmol)
was added to the methanol solution of HL1. The resulting mixture was refluxed for ca. 1 h
followed by the addition of methanol solution of Sodium thiocyanate (0.080 g, 1 mmol). It was
then cooled and kept for slow evaporation in open atmosphere. Single crystals, suitable for Xꢀray
diffraction, were obtained after few days.
Yield: 524 mg (85%). Anal. Calc. for C27H35MnN3O8S (FW 616.62): C, 47.17; H, 5.13;
N, 6.11%. Found: C, 47.1; H, 5.1; N, 6.2%. IR (KBr, cmꢀ1): 2926 (νCH), 2078 (νNCS), 1614 (νC=N).
UVꢀVis, λmax (nm) (εmax(L molꢀ1 cmꢀ1)) (CH3CN): 655 (290), 414 (3380), 296 (15400), 220
(37400). Magnetic moment = 3.91ꢀꢁ. Melting point =172°C
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Preparation of [Mn(L2)2(OH)Cl] (2)
A methanol solution (5 ml) of manganese(II) chloride tetrahydrate (197 mg, 1 mmol) was
added to the methanol solution of HL2. The resulting mixture was refluxed for ca. 1 h to get a
dark brown solution. It was then cooled and kept for slow evaporation in open atmosphere.
Single crystals, suitable for Xꢀray diffraction, were obtained after few days.
Yield: 460 mg (80%). Anal. Calc. for C22H27Cl3MnN2O7(FW 592.74): C, 44.58; H, 4.59;
N, 4.73%. Found: C, 44.5; H, 4.8; N, 4.9%. IR (KBr, cmꢀ1): 2855 (νCH), 1623 (νC=N). UVꢀ
Vis, λmax (nm) (εmax(L molꢀ1 cmꢀ1)) (CH3CN): 645 (397), 408 (3730), 328 (12330), 223 (36880).
Magnetic moment = 4.01ꢀꢁ. Melting point =149°C
Physical measurement:
Elemental analyses (carbon, hydrogen and nitrogen) were performed using a PerkinElmer
240C elemental analyzer. IR spectra in KBr (4500ꢀ500 cmꢀ1) were recorded with a PerkinElmer
Spectrum Two spectrophotometer. Electronic spectra (800ꢀ200 nm) were recorded on a
PerkinElmer Lambda 35 UVꢀvisible spectrophotometer.Molar conductivity measurements were
performed on a WENSAR microprocessor conductivity/temperature meter. The magnetic
susceptibility measurement was done with an EG & PAR vibrating sample magnetometer
(Model 155) at room temperature and diamagnetic corrections were performed using Pascal’s
constants.10 Electroꢀspray ionization mass spectra were recorded with Waters QTOF Micro
YA263.
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Xꢀray crystallography
Suitable single crystals of complexes 1 and 2 were used for data collection using a
‘Bruker D8 QUEST area detector’ diffractometer equipped with graphiteꢀmonochromated Mo
Kα radiation (λ= 0.71073 Å) at 110 K. Data were corrected for absorption effects using multiꢀ
scan reflections. Nonꢀhydrogen atoms were refined with anisotropic thermal parameters.
Hydrogen atoms attached with oxygen atoms of solvent molecules were located by difference
Fourier maps and were kept at fixed positions. Other hydrogen atoms were placed in their
geometrically idealised positions and constrained to ride on the atoms to which they were
attached. The programs used were APEX3,11 SAINT V8.37A,12 XT V2014/1,13SHELXLꢀ
2016/6,14 DIAMOND,15 ShelXle.16 Crystallographic data and refinements details of both
complexes are given in Table 1. Important bond lengths and angles are listed in Tables 2 and 3
respectively.
Theoretical methods
The calculations of the noncovalent interactions were carried out using the Gaussianꢀ0917
and the M06ꢀ2X/6ꢀ31+G* level of theory. This basis set does not include ECPs (effective core
potential) for the core electrons of manganese, therefore all electrons of manganese are described
using Gaussian functions. 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.18 The interaction energies were computed by calculating the
difference between the energies of isolated monomers and their assembly. Bader’s "Atoms in
molecules" theory has been used to study the interactions discussed herein by means of the
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AIMall calculation package.19 The MEPS calculations have been performed by means of the
SPARTAN software.20
Kinetic studies
Kinetic measurements for (a) the aerobic oxidation of model substrate 3,5ꢀditertꢀbutyl
catechol (3,5ꢀDTBC) and (b) phenoxazinone synthase like activity, catalyzed by 1–2 were
monitored by UV–vis spectroscopy. The kinetic study for catechol oxidase mimicking activity
was carried out at λmax ~ 409 nm which corresponds to the absorption maximum of 3,5ꢀditertꢀ
butyl quinone (3,5ꢀDTBQ). Similarly phenoxazinone synthase like activity was monitored by the
increase of the product 2ꢀaminophenoxazineꢀ3ꢀone at λmax ~ 433 nm for both complexes. All
experiments were carried out in a standard quartz cuvette (lightꢀpath length of 1 cm) in
acetonitrile at 25°C under aerobic conditions. In both cases, 10ꢀ5 M solution of 1–2 in acetonitrile
was added to solutions with varying concentrations (10ꢀ4 M to 10ꢀ3 M) of substrate (3,5ꢀDTBC/ oꢀ
aminophenol) in acetonitrile. In the above cases complex: substrate value ≥ 1:10 to maintain
pseudoꢀfirst order conditions to determine the dependence of rate on the substrate concentration
and various kinetic parameters. The rate of a reaction was derived from the initial rate method.
Results and Discussion
Synthesis
Schiff base ligands, HL1 and HL2, were prepared by 1:1 condensation of 2ꢀ(2ꢀ
aminoethoxy)ethanol with 3ꢀethoxysalicylaldehyde and 5ꢀchlorosalicylaldehyde, respectively.
The methanol solution of HL1 was reacted with manganese(II) perchlorate hexahydrate followed
by the addition of sodium thiocyanate to prepare the polymeric complex, [Mn(L1)2(NCS)]n (1).
For the preparation of mononuclear complex, [Mn(L2)2(OH)Cl] (2), Schiff base ligand HL2 was
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made to react with manganese(II) perchlorate hexahydrate. In both the above cases this
deprotonation is favoured by the hydroxide ions produced by the reduction of O2 during the areal
oxidation of manganese(II) to manganese(IV). Formation of both complexes is shown in scheme
1.
Scheme 1: Synthesis of complexes 1 and 2.
Structure description
Complex of [Mn(L1)2(NCS)]n (1)
Complex 1 crystallizes in the monoclinic space group P21/n. A perspective view of
complex 1 with the selective atom numbering scheme is shown in Figure 1. The asymmetric unit
ꢀ
consists of a manganese(IV) centre with two deprotonated Schiff base (L1) ligands and a
coordinated thiocyanate. The manganese(IV) center is in a sixꢀcoordinate octahedral
environment, being bonded to two imine nitrogen atoms, N(8) and N(8'), and two phenoxo
ꢀ
oxygen atoms, O(1) and O(1'), from two deprotonated Schiff base ligands (L1) . Remaining two
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coordination sites are bonded by a thiocyanate nitrogen atom, N(1N), and an oxygen atom,
ꢀ
O(14)a, of (L1) from a symmetry related molecule (a = 1/2ꢀx, ꢀ1/2+y, 3/2ꢀz), creating an 1D
zigzag polymer. The Mn(1)ꢀO(1), Mn(1)ꢀO(1'), Mn(1)ꢀN(8) and Mn(1)ꢀN(8') bond lengths
{1.856(2), 1.8556(2), 2.046(2) and 2.051(2) Å} are comparable and close to the manganeseꢀ
oxygen/nitrogen
bond
lengths
of
this
type
manganese(IV)
system.21
The
manganese(IV)···manganese(IV) distance in the complex core is 8.895(7) Å.
The pendent alkoxy side arms of ligands in complex 1 shows stable CꢀH···π interactions
with the thiocyanate moiety. The geometric features (bond lengths and bond angles) of Cꢀ
H···π(SCN) interactions are given in Table 4. These interactions stabilize the syn arrangement of
complex 1 which is explained thoroughly in ‘DFT study in noncovalent interaction’ section. Due
to having syn structure, complex 1 attend a twoꢀdimensional hydrogen bonded self assembly.
The hydrogen atom, H(14B), attached with O(14C), participate in hydrogen bonding interaction
with a symmetry related (b =1+x, y, z) oxygen atom, O(14)b (Figure 2). The detail geometric
features of hydrogen bonding interactions are given in Table 5.
Complex of [Mn(L2)2(OH)Cl] (2)
ꢂ
Complex 2 crystallizes in the triclinic space group P1. A perspective view of complex 2
with the selective atom numbering scheme is shown in Figure 3. Structure determination reveals
that the complex consists of a discrete mononuclear unit [Mn(L2)2(OH)Cl]. Manganese(IV)
center is again in a sixꢀcoordinate octahedral environment, being bonded to two imine nitrogen
atoms, N(8) and N(8)c, and two phenoxo oxygen atoms, O(1) and O(1)c, (c =1ꢀx,1ꢀy,ꢀz) from two
deprotonated Schiff base ligands (L2)ꢀ. Other vacant sites are coordinated by two oxygen atoms,
O(1H) and O(1H)c, of two OH groups. There is also an additional peak in the electron density
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map close to the O(1H) atom at ca. 0.44(9) Å. This was interpreted as chlorine atom. These two
sites [O(1H) and Cl(1H)] were refined with occupancy factors of x and 1ꢀx, with x refining to
0.5(2).
In complex 2, there is no such extra stability (CꢀH···π(SCN) interactions) as like complex 1
which probably causes the anti arrangement. Detail discussion has been given in the ‘DFT study
of noncovalent interaction’ section. Due to this anti arrangement only 1D hydrogen bonded
structure is formed (Figure 4). The hydrogen atom, H(1H)c, attached with oxygen atom, O(1H)c,
forms hydrogen bonding interaction with the oxygen atom, O(14A)d, of a symmetry related
molecule (d = ꢀx,1ꢀy,1ꢀz). The details of hydrogen bonding interactions are given in Table 5.
The aromatic ring shows πꢁꢁꢁπ stacking interaction with a symmetry related (ꢀ1ꢀx,2ꢀy,ꢀz)
aromatic ring [C(1)ꢀC(2)ꢀC(3)ꢀC(4)ꢀC(5)ꢀC(6)] of a neighbouring molecule and creates a
supramolecular 1D structure (Figure 5). Details of geometric features of π···π stacking
interactions are given in Table 6.
DFT study of noncovalent interactions
The theoretical study has been divided into two parts. In the first part it has been analyzed
that the ethoxy ethanol arms are disposed in a syn conformation in 1 and, conversely, in an anti
conformation in 2. The syn arrangement in 1 facilitates the generation of a 1D polymeric
structure, as further described below. In the second part theoretical study has been used to
analyze this issue focusing on the formation of unconventional C–H···π(SCN) and conventional
C–H···π(phe) interactions in 1 and 2, respectively.
Figure 6a represents a fragment of the polymeric 1D chain observed in the solid state Xꢀ
ray structure of 1. It can be observed that the presence of the axially coordinated linear
pseudohalide likely plays a relevant role stabilizing the syn arrangement of the ethoxy ethanol
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arms of the ligand. This stabilization is due to the formation of unconventional and short
CH3···π(SCN) interactions(back dashed lines), where the πꢀsystem of the pseudohalide is acting as
a Lewis base. Figure 6b shows the MEP surface of compound a model of 1, where the arms have
been eliminated. Otherwise, it is not possible to compute the MEP value at the SCN ligand
plotted onto the van der Waals surface due to its contacts with the arms. It can be observed that
the MEP values at the SCN ligand are large and negative (most negative value is observed at the
N atom, –43 kcal/mol).One monomer retrieved from the infinite 1D chain (see Figure 6c) has
been used to establish the intramolecular C–H···π(SCN) interactions. This theoretical model has
been simplified by reducing the size of the chain opposite to the axially coordinated
pseudohalide. Moreover, the manganese coordinated to the end of one of the ethoxy ethanol
arms, has been replaced by a proton. Since the interaction is intramolecular, the strength of the
multiple C–H···π(SCN) interactions between the syn conformation and an hypothetical
arrangement (rotamer) where the arms are coplanar to the equatorial plane, have been estimated
roughly (Figure 6d). The energetic difference between these is very large (20.4 kcal/mol) as a
consequence of the strongly negative MEP values at the pseudohalide and confirming the
relevant role of this unconventional interaction in the formation of the 1D polymer in the solid
state, since it strongly stabilizes the syn conformation. At this point, it should be mentioned that
we have used simplified models to estimate the noncovalent interactions. Other factors like steric
effects, crystal packing and long range van der Waals interactions may also influence the binding
energies and the final solid state architecture of the compounds studied herein.
The Bader’s theory of “atoms in molecules” (AIM) has been used to characterize the C–
H···π(SCN) noncovalent interaction in complex 1. A bond critical point (CP) and a bond path
connecting two atoms is an unambiguous evidence of interaction.22 The AIM distributions of
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critical points and bond paths computed for the monomeric model of compound 1 is shown in
Figure 6e. The distribution confirms the existence of the C–H···π(SCN) interactions described
above since it shows the presence of four bond CPs (red sphere) and a bond paths connecting
four H atoms of the chains to the SCN moiety (Figure 6e). Several rings CPs (yellow spheres)
are also generated due to the formation of supramolecular rings.
Complex 2 also forms infinite 1D supramolecular assemblies in its solid state architecture
where conventional C–H···π(phe) in combination with hydrogen bonding interactions are
established (dashed lines, Figure 7a). The MEP value over the phenyl ring is negative (–4
kcal/mol), thus appropriate for interacting with positive regions. The interaction energy of one
dimer retrieved from the infinite 1D chain have been computed (Figure 7b) where two
symmetrically equivalent C–H···π interactions are established. The OH and Cl ligands occupy
both axial positions with about 50% of relative occupation. For the theoretical study one possible
ꢂ
combination preserving the P1symmetry of the crystal structure have been selected (see Figure
7b). It should be mentioned that the observed distance for the C–H···π(phe) interaction is very
short (2.58 Å). Moreover, two symmetrically equivalent hydrogen bonding interactions between
the coordinated HO– ligands and the hydroxyl group of the ethoxy ethanol arms are also formed.
This combination of interactions exhibits a binding energy of ꢂE1 = –13.2 kcal/mol). In an effort
to roughly evaluate the contribution of the two C–H···π(phe) interactions, a theoretical model have
used where two HO– ligands have been replaced by hydride ligands (see small arrows in Figure
7c) in such a way that the hydrogen bonding interactions are not established. As a result, the
binding energy is reduced to ꢂE2 = –8.6 kcal/mol and the difference (–4.6 kcal/mol) is a rough
estimation of the hydrogen bonding interactions. Since the hydroxyl groups of the ethoxy ethanol
arms (highlighted in yellow, Figure 7c) also contribute to the formation of the dimer, another
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theoretical model have used where these OH groups have been replaced by hydrogen atoms
(model not shown in Figure 7). As a result the interaction energy is further reduced to ꢂE2 = –5.6
kcal/mol that is the contribution of both C–H···π(phe) interactions.
Finally, the Bader’s theory of “atoms in molecules” (AIM) has been used to characterize
the noncovalent interactions in the dimer of 2. The AIM distribution of critical points and bond
paths is shown in Figure 8. Each hydrogen bonding interaction is characterized by a bond CPs
(red sphere) and a bond path connecting the oxygen donor atom to the hydrogen atom.
Moreover, each C–H···π(phe) interaction is characterized by a bond CPs (red sphere) and a bond
path connecting the hydrogen atom to tone carbon atom of the ring, thus confirming the
interactions. Finally, it can be observed that the OH group also participate in the formation of
ancillary C–H···O interactions, since a bond CP and bond path connect the oxygen atom of the
hydroxyl group to a hydrogen atom of the alkyloxy chain.
IR and electronic spectra
The ν
C=N stretching vibrations of imine functionalities are observed around 12620 cmꢀ1
region in the IR spectra of both complexes.23 Sharp band at 2078 cmꢀ1 in the IR spectrum of 1 is
indicative of the presence of Nꢀcoordinated thiocyanate.4c, 24 The bands in the range of 2926ꢀ
2855 cmꢀ1 due to alkyl CꢀH bond stretching vibrations are customarily noticed in the IR spectra
of both complexes.25 Broad bands around 3400 cmꢀ1 in the IR spectra of both complexes may be
attributed to the presence of hydrogen bonded OH groups.26 IR spectra of both complexes are
shown in Figures S1ꢀS2, Electronic Supporting information.
The UVꢀVis absorption spectra of both complexes were recorded in CH3CN. Intense
absorption bands in the region of 220ꢀ223 nm are assigned to intraꢀligand π→π* transitions.
Bands around ~300 nm are due to n→π* transitions. Absorption bands around 410 nm may be
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attributed to the ligand to metal charge transfer (LMCT) transitions. Absorption bands around 50
nm region are due to spin allowed d–d transitions. 21a, 21d UVꢀvisible spectra of both complexes
are given in Figures S3ꢀS4.
Molar conductivity and magnetic property
The molar conductance values of both complexes were measured in acetonitrile solution
at the concentration 10ꢀ3M at 32°C. The values are 40.2 and 18.52 S cm2 molꢀ1 for complexes 1
and 2 respectively. The low values indicate the nonꢀelectrolytic nature of the complexes.
The formulation of both 1 and 2 as manganese(IV) complexes is supported by the
room temperature solid state magnetic moments close to 3.91 − 4.01ꢀꢁ. This value is expected
formagnetically nonꢀcoupled manganese(IV) complexes having three unpaired electrons (S =
3/2).21a
PXRD
The experimental PXRD patterns of the bulk products are in good agreement with the
simulated XRD patterns from single crystal Xꢀray diffraction results, indicating consistency of
the bulk samples (Figures are in Electronic Supporting information, S5). The simulated patterns
of the complexes were calculated from the single crystal structural data (Cif files) using the
CCDC Mercury software.
Hirshfeld Surface analysis
Hirshfeld surfaces and 2Dꢀfingerprint plots provide contribution of different
intermolecular interactions present in crystal structure. Crystal Explorer 3.0 program was used to
determine the Hirshfeld surfaces and the associated 2Dꢀfingerprint plots.27Two parameters, di
(the distance to the nearest nucleus internal to the surface) and de (the distance from the point to
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the nearest nucleus external to the surface) are used to define the normalized contact distance
(dnorm) as shown below:
Where rivdw and revdw were the van der Waals radii of the atoms.
Hirshfeld surfaces of both complexes, mapped over dnorm (range of ꢀ0.1 to 1.5 Å), shape
index and curvedness, are shown in Figure S6. Transparent surfaces are used so that molecular
moieties (around which the surfaces are calculated) could be visualised clearly. Red spots on the
dnorm surface indicate the interaction between hydrogen atoms. OꢁꢁꢁH,CꢁꢁꢁH and ClꢁꢁꢁH contacts
are also observed in the Hirshfeld surfaces as smaller visible spots of light colour indicating
weaker and longer contact. Distinct spikes in 2D fingerprint plot (Figure S7) indicate
intermolecular interactions, e.g. CꢁꢁꢁH/HꢁꢁꢁC or OꢁꢁꢁH/HꢁꢁꢁO interactions etc. The proportions of
CꢁꢁꢁH/HꢁꢁꢁC interactions comprise 16.2% and 14.4% of the Hirshfeld surfaces for 1 and 2,
respectively. Similarly, the proportions of OꢁꢁꢁH/HꢁꢁꢁO interactions comprise 11% and 6.6% of
the Hirshfeld surfaces for 1 and 2 respectively. Contribution of Cl···H/H···Cl interactions in 2 is
21.8%.
Catalytic activity and kinetic study
Oxidation of 3,5ꢀdiꢀtertꢀbutyl catechol
This kinetic experiment was performed with both complexes (10ꢀ5 M) and substrate 3,5ꢀ
DTBC varying the concentration from 10ꢀ4 M to 10ꢀ3 M in acetonitrile at 25°C under aerobic
conditions. Both complexes show moderate to high catalytic activities toward the aerobic
oxidation of the model substrate (3,5ꢀDTBC) to the corresponding oꢀquinone (3,5ꢀDTBQ) as sole
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oxidation product at ~409 nm. The complete conversion was achieved after 2 h and 5 min at
room temperature. Spectral changes of both complexes on adding 3,5ꢀDTBC (i.e. catechol
oxidase like activity) in CH3CN are shown in Figure 9.
Oxidation of oꢀaminophenol
The catalytic activity of both complexes for the areal oxidation of oꢀaminophenol to 2ꢀ
aminophenoxazineꢀ3ꢀone was verified with various concentration of substrate (from 10ꢀ4 M to
10ꢀ3 M) in presence of 10ꢀ5 M catalyst. 2ꢀaminophenoxazineꢀ3ꢀone shows characteristic
absorption at ~433 nm. Therefore, the catalytic conversation of oꢀaminophenol to 2ꢀ
aminophenoxazineꢀ3ꢀone in open atmosphere is confirmed by the steady increase of peak
intensity at ~433 nm. Spectral changes of both complexes on adding oꢀaminophenol (i.e.
phenoxazinone synthase mimicking activity) in CH3CN are shown in Figure 10.
Rationalization of kinetic parameters
Well known Michaelis–Menten and Lineweaver–Burk Equations (given below) are used in
general to rationalize the mechanism of aboveꢀmentioned catalytic reactions.
ꢃ
[ꢇ]
ꢄꢅꢆ
V=
ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ (1)
ꢈ ꢊ[ꢇ]
ꢉ
ꢋ
ꢃ
ꢈ
ꢋ
ꢋ
ꢉ
=
+
ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ (2)
ꢃ
ꢄꢅꢆ
[ꢇ]
ꢃ
ꢄꢅꢆ
In equation (1), V indicates initial rate; [S] indicates substrate concentration. KM is the
Michaelis–Menten constant for the catalyst and is expressed as, KM = (k2 + k3)/k1. Vmax indicates
maximum initial rate for a fixed concentration of catalyst in presence of large amount of
substrate. In equation (2), Vmax, KM and Kcat indicate maximum reaction velocity, Michaelis
constant and turnover number, respectively. Values of all kinetic parameters are given in Tables
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7 and 8. As can be observed for each complex, initially the rate increases linearly with the
increase of respective oxidation product (3,5ꢀDTBQ or 2ꢀaminophenoxazinoneꢀ3one)
concentration, but deviates gradually from linearity and finally tends toward a saturation curve
(Figure 11). In other words, it is a first order kinetics at lower concentration which gradually
differs from unity at higher concentration. The initial first order rate constants, V (min−1), are
obtained directly from the slope of the plot of log[A∞/(A∞α−At)] versus time. This is a straight
line passing through origin. The kinetic parameters for the catalyzed reactions (Vmax, kM and kcat)
have been determined from the linear plots of 1/V versus 1/[S] (Lineweaver−Burk plot) as per
Michaelis−Menten approach of enzymatic kinetics. LineweaverꢀBurk plots of complexes 1 and 2
for both catalytic activities are shown in Figure 12. Comparison on catechol oxidase and
phenoxazinone synthase mimicking activity of complexes 1 and 2 with some other
manganese(IV) complexes28 are shown in Tables 7 and 8 respectively.
It is to be noted here that catechol oxidase and phenoxazinone synthase like activities of
two other manganese(IV) Schiff base complexes were already explored by us.28d In that work, a
potential tridentate Schiff base was used to prepare manganese(IV) complexes, but in one
complex, the potential tridentate ligand behaved as a bidentate ligand with a pendant donor arm.
That complex was found to be catalytically more active. It was therefore concluded that more
catalytic activity is ‘probably due to its more open structure, which is a manifestation of the
presence of a potential tridentate ligand in its bidentate form, which could easily be replaced by
the substrates initiating catalysis’.28d This observation lead us to prepare some other
manganese(IV) complexes with potential tridentate Schiff bases which may behave as bidentate
ligand with pendant side arms (i.e. with more open structures) and to explore their catechol
oxidase and phenoxazinone synthase mimicking activities.
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In the present work, Schiff bases are apparently tridentate, but in their complexes, they
are behaving as bidentate ones, keeping pendant side arms, thereby producing more open
structures. Both complexes are found to be more active towards phenoxazinone synthase
mimicking activity, as expected. However, their activity towards the oxidation of 3,5ꢀDTBC is
less compared to the previous complexes, reported elsewhere.28d Therefore, more complexes are
needed to be prepared and their catalytic activities are to be investigated before reaching to any
concrete conclusion.
Exploration of plausible mechanisms for catechol oxidase and phenoxazinone synthase like
activities
The electrospray ionization mass (ESIꢀMS positive) spectral characterization of 1:50
mixture of the complex (1 or 2) with the substrate (3,5ꢀDTBC or oꢀaminophenol) were carried
out in acetonitrile solution to get the inside nature of complexes in presence of substrates. In
presence of 3,5ꢀDTBC, complex 1 exhibits two important peaks at m/z = 243.13 and 463.27
corresponding to quinone sodium aggregates [3,5–DTBQꢀNa]+ and [(3,5–DTBQ)2ꢀNa]+, which
suggest the completion of cycle of catechol oxidation process and formation of the final product.
Peaks at m/z = 254.13 and 276.11 indicate the presence of [L1ꢀH]+ and [L1ꢀNa]+ ions,
respectively (Figure S8) which supports the mechanistic pathway. The probable catalytic
pathway of catechol oxidation is shown in Scheme 2.
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Scheme 2: Probable mechanistic pathway showing the formation of 3,5ꢀDTBQ.
In presence of oꢀaminophenol, complex 1 shows peak at m/z = 254.13 for [L1ꢀH]+ .
Another peak at m/z = 276.11 is due to [(Phenoxazinone)(CH3CN)Na]+ which confirms the
completion of phenoxazinone synthase activity (Figure S9) and supports the following
mechanism. Probable mechanistic pathway of phenoxazinone synthase is shown in Scheme 3.
Scheme 3: Probable mechanistic pathway showing the formation of phenoxazinone
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Conclusion
In summary, two new manganese(IV) complexes, [Mn(L1)2(NCS)]n (1)
and[Mn(L2)2(OH)Cl] (2), have been synthesized and Xꢀray characterized. The importance of
unconventional C–H···π interactions involving the pseudohalide thiocyanate ligand in the solid
state of complex 1has been described that likely plays a key role in the polymer formation since
it stabilizes the unexpected syn arrangement of the ligand pending arms. Due to presence of
conventional C–H···π(phe) interactions, complex 2 shows anti arrangement. The energetic feature
of these interactions has been studied and characterized by means of DFT calculations using the
Bader’s theory of atoms in complexes 1 and 2.Both complexes found to show catalytic property
towards the oxidation of 3,5ꢀDTBC and oꢀaminophenol. The catalytic efficiency of the
complexes was assessed following conventional Michaelis–Menten enzymatic kinetics.
Acknowledgments
N.S. thanks the UGC, India, for awarding a Junior Research Fellowship (Sr. No. 2061410290,
Ref. No.: 22/06/2014(i)EUꢀV, dated 7.08.2015).
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Notes and References
CCDC 1529065ꢀ1529066 contain the supplementary crystallographic data for 1 and 2
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Table 1: Crystal data and refinement details of complexes 1 and 2:
Complex
Formula
Formula Weight
Crystal System
Space group
a(Å)
1
C27H35MnN3O8S
616.58
2
C22H27Cl3MnN2O7
592.74
Monoclinic
P21/n
Triclinic
ꢂ
P1
10.7379(5)
14.9326(7)
18.4749(9)
90
6.9737(5)
10.1021(7)
10.3328(7)
66.047(2)
73.622(2)
73.682(2)
626.64(8)
1
b(Å)
c(Å)
α (°)
β(°)
105.857(2)
90
γ(°)
V(Å3)
2849.6(2)
4
Z
d
(calc) [g/cm3]
µ [ mmꢀ1 ]
F(000)
1.437
1.571
0.590
0.892
1292
305
Total Reflections
Unique Reflections
Observed data[I > 2 σ(I)]
R(int)
95272
12830
8730
4311
7682
4091
0.033
0.018
R1, wR2(all data)
R1, wR2([I > 2 σ(I)]
0.0771,0.1404
0.0678,0.1364
0.0580, 0.1363
0.0556, 0.1355
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Table 2: Bond lengths (Å) of complexes 1 and 2:
Complex
Mn(1)ꢀO(1)
Mn(1)ꢀO(1')
Mn(1)ꢀN(1N)
Mn(1)ꢀN(8)
Mn(1)ꢀN(8')
Mn(1)ꢀO(14)a
Mn(1)ꢀO(1H)
Mn(1)ꢀCl(1H)c
1
2
1.856(2)
1.8556(2)
2.211(2)
2.046(2)
2.051(2)
2.334(1)
_
1.874(2)
_
_
2.038(2)
_
_
2.216(9)
2.625(7)
_
Symmetry transformations: a 1/2ꢀx, ꢀ1/2+y, 3/2ꢀz; c= 1ꢀx, 1ꢀy, ꢀz
=
Table 3: Selected bond angles (°) of complexes 1 and 2:
Complex
1
2
O(1)ꢀMn(1)ꢀO(1')
O(1)ꢀMn(1)ꢀN(1N)
O(1)ꢀMn(1)ꢀN(8)
O(1)ꢀMn(1)ꢀN(8')
O(1)ꢀMn(1)ꢀO(14)a
O(1')ꢀMn(1)ꢀN(1N)
O(1')ꢀMn(1)ꢀN(8)
O(1')ꢀMn(1)ꢀN(8')
O(1')ꢀMn(1)ꢀO(14)a
175.42(1)
91.91(1)
91.14(1)
89.70(9)
84.7(3)
_
_
89.92(8)
_
_
_
_
_
_
92.66(9)
88.88(9)
89.84(8)
90.8(3)
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N(1N)ꢀMn(1)ꢀN(8)
N(1N)ꢀMn(1)ꢀN(8')
O(14)aꢀMn(1)ꢀN(1N)
N(8)ꢀMn(1)ꢀN(8')
94.67(9)
_
90.89(9)
_
176.5(3)
_
174.35(9)
_
O(14)aꢀMn(1)ꢀN(8)
O(14)aꢀMn(1)ꢀN(8')
O(1)ꢀMn(1)ꢀO1(H)
O(1)ꢀMn(1)ꢀO(1H)c
O(1)ꢀMn(1)ꢀN(8)c
O(1H)ꢀMn(1)ꢀN(8)
O(1H)cꢀMn(1)ꢀN(8)
Cl(1H)ꢀMn(1)ꢀO(1)
Cl(1H)ꢀMn(1)ꢀN(8)
Cl(1H)ꢀMn(1)ꢀO(1)c
Cl(1H)ꢀMn(1)ꢀN(8)c
86.2(3)
_
88.3(3)
_
_
_
_
_
_
_
_
_
_
91.2(3)
88.8(3)
90.08(8)
89.7(2)
90.3(2)
91.16(9)
85.73(9)
88.84(9)
94.27(9)
Symmetry transformation: a = 1/2ꢀx,ꢀ1/2+y,3/2ꢀz for 1; c=1ꢀx,1ꢀy,ꢀz for 2.
Table 4: Geometric features (distances in Å and angles in °) of the C–H···π(SCN) interactions
obtained for complex 1.
C–H···π(SCN)
H··· π(SCN) (Å)
2.630
X–H··· π(SCN) (°) X··· π(SCN) (Å)
C(12')ꢀH(12U)ꢁꢁꢁC(2N)
C(12')ꢀH(12U)ꢁꢁꢁN(1N)
C(12)ꢀH(12B)ꢁꢁꢁC(2N)
C(12)ꢀH(12B)ꢁꢁꢁN(1N)
C(10)ꢀH(10A)ꢁꢁꢁN(1N)
165.48
167.46
120.35
122.84
145.58
3.597(4)
3.650(4)
3.303(6)
3.442(6)
3.450(4)
2.677
2.690
2.803
2.587
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DOI: 10.1039/C7NJ00766C
Table 5: Hydrogen bond distances (Å) and angles (◦) of complexes 1 and 2:
Complex
∠Dꢀ
H···A
165
D–H···A
DꢀH
H···A
D···A
O(14C)ꢀH(14B)ꢁꢁꢁO(14)b
O(1H)cꢀH(1H)cꢁꢁꢁO(14A)d
0.84
0.73
1.89
2.09
2.716
2.82
1
2
173
Symmetry transformations: b = 1+x,y,z; c=1ꢀx,1ꢀy,ꢀz; d = ꢀx,1ꢀy,1ꢀz. D = donor; H = hydrogen; A
= acceptor.
Table 6: Geometric features (distances in Å and angles in °) of π···π stacking interactions
obtained for complex 2.
Cg(Ring I)···Cg(Ring J)
Cg···Cg(Å)
α (°)
Cg(I)···Perp
(Å)
Cg(J)···Perp
(Å)
Cg(1)···Cg(1)e
3.845(2)
0
3.469(1)
3.470(1)
Symmetry transformation: e = 1ꢀx,2ꢀy,ꢀz.
α = Dihedral angle between ring I and ring J. Cg(I)···Perp = Perpendicular distance of Cg(I) on
ring J. Cg(J)···Perp = Perpendicular distance of Cg(J) on ring I. Cg(1) = Centre of gravity of the
ring [C(1)ꢀC(2)ꢀC(3)ꢀC(4)ꢀC(5)ꢀC(6)] for complex 2
29