Mn(II) complexes of different nuclearity: Synthesis, characterization and catecholase-like activity

Article abstract of DOI:10.1039/c5dt03659c

Two "end-off" compartmental ligands, 2-formyl-4-chloro-6-N-ethylmorpholine-iminomethyl-phenol (HL1) and 2-formyl-4-methyl-6-N-ethylpyrrolidine-iminomethyl-phenol (HL2) have been designed and three complexes of Mn(ii), one mono-, one di- and a polynuclear, namely Mn(L1)(SCN)₂(H₂O)] (1), [Mn₂(L1)(OAc)₂](BPh₄)] (2), and [Mn₂(L2)(OAc)₂(dca)]_n (3) have been synthesized and structurally characterized. Variable temperature magnetic studies of 2 and 3 have been performed and data analyses reveal that Mn centers are antiferromagnetic coupled with J = -9.15 cm^-1 and J = -46.89, respectively. Catecholase activity of all the complexes has been investigated using 3,5-di-tert-butyl catechol (3,5-DTBC). All are highly active and the activity order on the basis of the k_cat value is 2 > 1 > 3. In order to unveil whether the metal centered redox participation or the radical pathway is responsible for the catecholase-like activity of the complexes, detailed EPR and cyclic voltammetric (CV) studies have been performed. In addition to the six-line EPR spectrum characteristic to Mn(ii), an additional peak at g ～ 2 is observed when the EPR study is done with the mixture of 3,5-DTBC and the catalyst, suggesting the formation of an organic radical, most likely ligand centered. The CV experiment with the mixture of 3,5-DTBC and the catalyst reveals ligand centered reduction rather than reduction of Mn(ii) to Mn(i). It is thus inferred that complexes 1-3 show catecholase-like activity due to radical generation.

Full text of DOI:10.1039/c5dt03659c

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Mn(II) complexes of different nuclearity: synthesis, characterization
and catecholase-like activity
Prateeti Chakraborty,*,a, e Ishani Majumder,a Kazi Sabnam Banu,a Bipinbihari Ghosh,b
Hulya Kara,c Ennio Zangrando,*,d and Debasis Das*,a
aDepartment of Chemistry, University of Calcutta, 92, A. P. C. Road, Kolkata – 700009, India
bDepartment of Chemistry, Indian Institute of Engineering, Science and Technology, Shibpur,
Howrahꢀ711103, India
cDepartment of Physics, Faculty of Science and Art, Balikesir University, 10145 Balikesir,
Turkey
d Department of Chemical and Pharmaceutical Sciences, University of Trieste, Via L. Giorgieri
1, 34127 Trieste, Italy
e Department of Chemistry, Amity University, Newtown, Kolkata, West Bengal 700156, India
ABSTRACT
Two “endꢀoff” compartmental ligands, 2ꢀformylꢀ4ꢀchloroꢀ6ꢀNꢀethylmorpholineꢀiminomethylꢀ
phenol (HL1) and 2ꢀformylꢀ4ꢀmethylꢀ6ꢀNꢀethylpyrrolidineꢀiminomethylꢀphenol (HL2) have
been designed and three complexes of Mn(II), one Monoꢀ, one diꢀ and other polynuclear, namely
Mn(L1)(SCN)2(H2O)] (1), [Mn2(L1)(OAc)2](BPh4)] (2), and [Mn2(L2)(OAc)2(dca)]n(3) have
been synthesized and structurally characterized. Variable temperature magnetic study of 2 and 3
have been performed and data analyses reveal that Mn centers are antiferromagetic coupled with
J = ꢀ 9.15 cmꢀ1 and J = ꢀ 46.89, respectively. Catecholase activity of all the complexes have been
investigated using 3,5ꢀdiꢀtertꢀbutyl catechol (3,5ꢀDTBC). All are highly active with activity
order on the basis of kcat value is 2>1>3. In order to unveil whether metal centered redox
participation or radical pathway is responsible for the catecholaseꢀlike activity of the complexes
detailed EPR and cyclic voltametric (CV) study have been performed. In addition to six line EPR
spectrum characteristic to Mn(II) an additional peak at g~2 is observed when the EPR study is
done with the mixture of 3,5ꢀDTBC and catalyst, suggesting the formation of organic radical
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most likely ligand centered. CV experiment with the mixture of 3,5ꢀDTBC and catalyst reveals
ligand centered reduction rather than reduction of Mn(II) to Mn(I). It is thus inferred that
complexes 1ꢀ3 show catecholaseꢀlike activity due to radical generation.
Introduction
The oxidation of organic substrates by metalloenzymes, that activate molecular oxygen under
mild conditions, has received vast deal of concentration in the arena of industrial and synthetic
processes.1ꢀ4 As a result many groups tried to build up the finest model of transition metal
complexes able to mimic oxidase activity of metalloenzymes. One of the foremost enzymes that
play a key role in these oxidation reactions is catechol oxidase (CO), a lesser well known
member of the typeꢀIII copper proteins originate in plant tissues and crustaceans. This process,
known as catecholase activity, allows to catalyze the oxidation of a wide range of oꢀdiphenols
(catechols) to the corresponding oꢀquinones (Scheme 1).5ꢀ8 Recently our group has got admirable
results while mimicking catecholase activity with phenol based compartmental ligand complexes
of Ni(II),9 Cu(II)10,11 and Zn(II)12. In this manuscript we widen our work by using manganese,
one of the most important trace transition element in biological system being present in
numerous metalloenzymes of varied nuclearity. From literature survey, it is possible to recognize
that some complexes of Mn(III)13, Mn(IV)14, and a very few ones of Mn(II)15 are found to mimic
catecholaseꢀlike activity, and these derivatives were generally made up from salenꢀtype Schiff
bases, tripodal ,and pyridineꢀcontaining ligands etc, while phenol based compartmental ligands
have been very rarely used. Our earlier studies with Mn(III) discloses metalꢀcentered redox
participation as a most probable pathway for the catecholase activity of the model compounds.13b
In case of Mn(II) complexes15 although we found excellent catalytic activities but failed to
evaluate the plausible catalytic mechanism. That failure inspires us to synthesize new Mn(II)
complexes to explore that untouched arena of catecholase activity study and thereby to unveil the
most probable mechanistic pathway. In order to reach to our goal the ligands of our choice in the
present study are two phenol based compartmental ligands, namely 2ꢀformylꢀ4ꢀchloroꢀ6ꢀNꢀ
ethylmorpholineꢀiminomethylꢀphenol (HL1) and 2ꢀformylꢀ4ꢀmethylꢀ6ꢀNꢀethylpyrrolidineꢀ
iminomethylꢀphenol (HL2). We have prepared three Mn(II) complexes having different
2

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nuclearities,
i.e.
[Mn(L1)(SCN)2(H2O)]
(1),
[Mn2(L1)(OAc)2](BPh4)]
(2),
and
[Mn2(L2)(OAc)2(dca)]n (3), structurally characterized them and have studied their catecholaseꢀ
like activity in DMSO solvent with 3,5ꢀdiꢀtertꢀbutylcatechol (3,5ꢀDTBC) as substrate. EPR and
cyclic voltametric study help us to understand the origin of the catecholaseꢀlike activity of our
Mn(II) complexes. Variable temperature magnetic study on complexes 2 and 3 has also been
performed since the compounds of phenol based compartmental ligands are well known for their
stimulating magnetic properties.16ꢀ19 This examination of magnetic properties of such compounds
helps not only in further understanding the relations between magnetic coupling centers in
metalloproteins and enzymes, but also to extend the knowledge of molecular magnetism.
Scheme. 1 Reaction Pathway of the Oxidation Catalyzed by CO.
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HL1
HL2
Fig. 1 Structure of the two Synthesized Compartmental Ligands.
Results and Discussion
Synthesis, Rationalization, and Characterization of the Metal-Complexes.
Three manganese (II) complexes (1-3) were prepared by adopting the template synthesis
technique by treating a methanolic solution of corresponding manganese salts with the Schiff
base formed in situ between 2,6ꢀdiformylꢀ4ꢀRꢀphenol (where R= chloro, methyl) and the
diamines and structurally characterized by single crystal Xꢀray diffraction (vide infra). FTꢀIR
spectral study reveals that all complexes exhibit bands due to the C=N stretch in the range
1630−1648 cm−1 and skeletal vibration in the range 1545−1555 cm−1. The presence of
coordinated SCN− anion in complex 1 has been assigned by a broad band in the region of 2065
cmꢀ1. In complex 2 the OAc− anions show bands at 1483 cmꢀ1 and in complex 3 OAc− anions and
dicyanamide show bands at 1488 and 2165 cmꢀ1, respectively. Electronic spectra of all
complexes studied in DMSO medium display very similar absorption bands in the range 375ꢀ425
nm. The observed strong higher energy single band (between 375 and 425 nm) is due to
combination of both phenoxido−Mn(II) and hydroxido−Mn(II) LMCT bands. Room temperature
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(300 K) magnetic moment calculation reveals that (5.96 µB for complex 1, 8.37 µB and 8.19 µB
for complex 2 and 3 respectively) Mn(II) in all complexes is in high spin state.
Crystal Structure Descriptions
The crystal structure analysis showed the formation of a mononuclear neutral species,
[Mn(HL1)(SCN)2(H2O)], due to the protonation of one of the imine nitrogen atoms. An ORTEP
drawing is depicted in Figure 1, and a selection of coordination bond distances and angles is
reported in Table 1. The metal is chelated by ligand HL1 through the phenoxo oxygen atom, the
imineꢀnitrogen and amine donor of the morpholine ring with MnꢀO and MnꢀN bond distances of
2.1222(14), 2.2075(18), and 2.387(13)Å, respectively. The latter value refers to the coordinated
morpholine moiety at higher occupancy (see experimental section).The distorted octahedral
coordination sphere is completed by an aqua ligand and two isothiocyanate anions (MnꢀO1w =
2.3692(18), MnꢀNCS = 2.163(2) and 2.154(2) Å. The distortions in the coordination bond angles
are attributable to a strength in the HL1 chelation and the value of O1ꢀMnꢀN3 (of 156.5(2)°) is
the more deviating from ideal geometry (Table. 1). The protonated imino nitrogen N1 forms an
intramolecular Hꢀbond with the phenolato oxygen O1 (N...O distance 2.598(2) Å), and the water
molecule is engaged in weak Hꢀbonding with N4 and S2 of a symmetry related complex at
x,1/2−y, −1/2+z (O1w…N4 = 3.130(3) and O1w…S2 = 3.330(2)Å) to form a polymeric species
developed parallel axis a. The structural data of complex 1 are comparable to those detected in
the corresponding species having a pyridine replacing the morpholine ring reported by us a few
years ago.15
Table 1. Coordination bond lengths (Å) and angles (°) for complex 1.
MnꢀO(1)
MnꢀO(1w)
MnꢀN(1)
2.1222(14)
2.3692(18)
2.2075(18)
MnꢀN(3) *
MnꢀN(3b) *
MnꢀN(5)
2.387(13)
2.413(13)
2.163(2)
2.154(2)
MnꢀN(6)
O(1)ꢀMnꢀN(6)
O(1)ꢀMnꢀN(5)
N(6)ꢀMnꢀN(5)
O(1)ꢀMnꢀN(1)
N(6)ꢀMnꢀN(1)
N(5)ꢀMnꢀN(1)
89.73(7)
89.35(8)
93.29(9)
82.07(6)
163.57(8)
100.79(9)
N(5)ꢀMnꢀO(1w)
N(1)ꢀMnꢀO(1w)
O(1)ꢀMnꢀN(3)
N(6)ꢀMnꢀN(3)
N(5)ꢀMnꢀN(3)
N(1)ꢀMnꢀN(3)
5
176.52(8)
82.69(7)
156.5(2)
111.3(2)
85.2(2)
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O(1)ꢀMnꢀO(1w)
N(6)ꢀMnꢀO(1w)
91.18(7)
83.28(8)
O(1w)ꢀMnꢀN(3)
95.7(2)
* N3 and N3b represent the donor of the disordered coordinated morpholine. Only bond angles involving
the higher occupancy is reported.
Fig. 2 ORTEP drawing (ellipsoid probability at 30%) with atom labels of complex 1. (Of the
disordered coordinated morpholine only that at higher occupancy is shown).
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Fig. 3 Complexes 1 connected through Hꢀbonds by the coordinated water molecule (primed
atoms at x,1/2−y, −1/2+z).
The structural analysis revealed that compound 2 is a dinuclear cationic species and the unit cell
contains two independent complexes located on a crystallographic mirror plane, normal to the
phenolato ring plane passing in between the manganese atoms, beside a BPh4 anion. An ORTEP
view of one of the two the ꢁꢀphenoxoꢀdiꢀꢁꢀacetatoꢀdimanganese(II) complex with atom labeling
scheme of independent atoms is shown in Fig. 4. In both species the metal ions exhibit a
distorted trigonal bipyramidal coordination sphere comprising in the equatorial plane the imine
nitrogen donor N1 and acetate oxygens O2 and O3, while the phenoxidoꢀbridged oxygen O1, and
morpholine amine nitrogen N2 occupy the axial positions (N4, O5, O6 and O4, N5 are the
corresponding atoms in the second complex). The MnꢀO and MnꢀN bond distances reported in
Table 2 indicate comparable values for the two independent complexes with the exception of the
MnꢀN(amino) bond that differs by ca. 0.1 Å (2.442(3) vs 2.335(3)Å). It is worth of note the
shorter MnꢀO(acetate) bond lengths (mean value 2.176 Å with respect the MnꢀO(oxo) ones that
average to 2.039 Å. Correspondingly a difference is observed in the intermetallic distance being
of 3.3455(9) and 3.2194(8)Å, respectively in the two complexes.
Table 2. Coordination bond lengths (Å) and angles (°) for complex 2.
Mn(1)ꢀO(1)
2.197(2)
Mn(2)ꢀO(5)
2.156(2)
Mn(1)ꢀO(2)
2.030(3)
Mn(2)ꢀO(6)
2.058(3)
Mn(1)ꢀO(3)
2.020(3)
Mn(2)ꢀO(7)
2.048(3)
Mn(1)ꢀN(1)
2.117(3)
Mn(2)ꢀN(4)
2.092(3)
Mn(1)ꢀN(2)
2.442(3)
Mn(2)ꢀN(5)
2.335(3)
Mn(1)ꢀMn(1)'
3.3455(9)
115.71(15)
123.69(15)
120.46(11)
95.80(13)
93.10(11)
85.12(9)
89.64(11)
99.89(10)
77.70(10)
162.09(9)
Mn(2)ꢀMn(2)'
3.2194(8)
130.85(18)
115.63(13)
113.52(18)
92.16(12)
92.44(14)
84.96(11)
91.86(11)
95.69(13)
80.44(11)
165.19(10)
O(3)ꢀMn(1)ꢀO(2)
O(3)ꢀMn(1)ꢀN(1)
O(2)ꢀMn(1)ꢀN(1)
O(3)ꢀMn(1)ꢀO(1)
O(2)ꢀMn(1)ꢀO(1)
N(1)ꢀMn(1)ꢀO(1)
O(3)ꢀMn(1)ꢀN(2)
O(2)ꢀMn(1)ꢀN(2)
N(1)ꢀMn(1)ꢀN(2)
O(1)ꢀMn(1)ꢀN(2)
O(7)ꢀMn(2)ꢀO(6)
O(7)ꢀMn(2)ꢀN(4)
O(6)ꢀMn(2)ꢀN(4)
O(7)ꢀMn(2)ꢀO(5)
O(6)ꢀMn(2)ꢀO(5)
N(4)ꢀMn(2)ꢀO(5)
O(7)ꢀMn(2)ꢀN(5)
O(6)ꢀMn(2)ꢀN(5)
N(4)ꢀMn(2)ꢀN(5)
O(5)ꢀMn(2)ꢀN(5)
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Primed atoms at x, 1/2−y, z.
Fig. 4 ORTEP drawing (ellipsoid probability at 30%) with atom labels of one of the two
independent cationic complexes of compound 2. The complex is located on a crystallographic
symmetry plane.
By using the coordinating dicyanoamide as counteranion, the metals assume a distorted
octahedral geometry in dinuclear complex 3. Each metal is chelated by Schiff base ligand HL2
through the phenoxo oxygen atom, the imineꢀnitrogen and amine donor of the pyrrolidine ring.
In addition the coordination sphere is filled by two oxygens from different acetate anions and a
nitrogen donor from a bridging dca ligand. With respect to complex 2, the MnꢀO and MnꢀN bond
distances are here slightly longer. In particular the MnꢀN(imine) bond distances are of 2.189(9)
and 2.192(8)Å which must be compared a mean value of 2.104 Å in 2. Here the acetate anions
bridge the metals in asymmetric fashion, with a long bond length with Mn1 (2.184(8)Å) and a
shorter one with Mn2 (2.091(7)Å),and vice versa for the other anion (2.066(8)and 2.157(9)Å,
respectively). The MnꢀO(oxo) and MnꢀN(amine) bond lengths are comparable to those measured
in 2. As mentioned above the dca anions bridge metals of different complexes giving rise to a
coordination polymer running parallel to axis a. The dca CN groups coordinate the metals with
different CNꢀMn angles, of 148.4(12) and 166.3(11)°, where the latter shows an almost linear
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coordination. The intermetallic Mn1ꢀMn2 in this neutral complex, of 3.389(3) Å, is slightly
longer than the value measured in 2.
Table 3. Coordination bond lengths (Å) and angles (°) for complex 3
Mn(1)ꢀO(1)
Mn(1)ꢀO(2)
Mn(1)ꢀO(4)
Mn(1)ꢀN(1)
Mn(1)ꢀN(3)
Mn(1)ꢀN(5)
Mn(1)ꢀMn(2)
O(4)ꢀMn(1)ꢀO(1)
O(4)ꢀMn(1)ꢀO(2)
O(1)ꢀMn(1)ꢀO(2)
O(4)ꢀMn(1)ꢀN(1)
O(1)ꢀMn(1)ꢀN(1)
O(2)ꢀMn(1)ꢀN(1)
O(4)ꢀMn(1)ꢀN(5)
O(2)ꢀMn(1)ꢀN(5)
O(1)ꢀMn(1)ꢀN(5)
N(1)ꢀMn(1)ꢀN(5)
O(4)ꢀMn(1)ꢀN(3)
O(1)ꢀMn(1)ꢀN(3)
O(2)ꢀMn(1)ꢀN(3)
N(1)ꢀMn(1)ꢀN(3)
N(5)ꢀMn(1)ꢀN(3)
2.151(8)
2.184(8)
2.066(8)
2.189(9)
2.395(9)
2.256(11)
3.389(3)
106.6(3)
88.7(3)
93.1(3)
170.6(3)
82.6(3)
92.8(3)
87.8(4)
176.5(4)
88.1(3)
90.6(4)
94.8(4)
158.6(3)
87.7(3)
76.0(4)
92.4(4)
Mn(2)ꢀO(1)
Mn(2)ꢀO(3)
Mn(2)ꢀO(5)
Mn(2)ꢀN(2)
Mn(2)ꢀN(4)
Mn(2)ꢀN(7)'
2.162(7)
2.091(7)
2.157(9)
2.192(8)
2.421(9)
2.231(11)
O(3)ꢀMn(2)ꢀO(1)
O(3)ꢀMn(2)ꢀO(5)
O(5)ꢀMn(2)ꢀO(1)
O(3)ꢀMn(2)ꢀN(2)
O(5)ꢀMn(2)ꢀN(2)
O(1)ꢀMn(2)ꢀN(2)
O(3)ꢀMn(2)ꢀN(7)'
O(5)ꢀMn(2)ꢀN(7)'
O(1)ꢀMn(2)ꢀN(7)'
N(2)ꢀMn(2)ꢀN(7)'
O(3)ꢀMn(2)ꢀN(4)
O(1)ꢀMn(2)ꢀN(4)
O(5)ꢀMn(2)ꢀN(4)
N(2)ꢀMn(2)ꢀN(4)
N(7)’ꢀMn(2)ꢀN(4)
103.3(3)
90.8(3)
92.4(3)
172.5(3)
93.9(3)
82.4(3)
88.7(4)
177.0(4)
90.6(3)
86.3(4)
99.4(3)
157.0(3)
83.9(4)
75.3(3)
93.3(4)
atom N(7)' at x−1,y,z.
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Fig. 5 ORTEP drawing (ellipsoid probability at 30%) with atom labels of the crystallographic
independent unit of compound 3.
Fig. 6 Polymeric structure of compound 3 developed along axis a.
Catecholase-Like Activity:
Catechol oxidase, a typeꢀ3 copper protein can bind oxygen reversibly at room temperature and so
it can be used to oxidize catechols to respective quinones. As a model of the enzyme here we
have taken the three complexes of Mn(II). In order to mimic the functional property of the model
compounds the most widely accepted substrate is 3, 5ꢀdiꢀtertꢀbutylcatechol (3,5ꢀDTBC) since its
reduction potential for the quinoneꢀcatechol couple is low which makes it easy to be oxidized to
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the corresponding quinone, 3, 5ꢀdiꢀtertꢀbutylꢀoꢀbenzoquinone (3, 5ꢀDTBQ) and at the same time
due to presence of bulky substituents further oxidation and polymerization are stopped.
Interestingly, all of the complexes exhibit extensive catalytic competence towards the oxidation
of 3,5ꢀDTBC to 3,5ꢀDTBQ in the solvent DMSO. Moreover, the complex 2, a binuclear Mn(II)
Schiff base complex which shows a good catalytic efficiency towards the oxidation of catechols
is the first reported di nuclear Mn(II) catalyst derived from compartmental Schiff base ligand .
Before going on into detailed kinetic study we have checked the ability of the complexes to
mimic the active site of catechol oxidase by treating 1×10ꢀ4 mol dmꢀ3 solutions of the complexes
with 1×10ꢀ2 mol dmꢀ3 (100 equivalents) of 3,5ꢀDTBC under aerobic condition. The timeꢀ
dependent UV−vis spectral scan was executed in pure DMSO. Fig. 7, 8, 9 shows the spectral
change for the complexes upon addition of 100ꢀfold 3,5ꢀDTBC (1×10ꢀ2 M ) observed at interval
of 5 minutes upto 30 minutes. The kinetics of the oxidation of 3,5ꢀDTBC was determined by
monitoring the increase of the concentration of the product 3,5ꢀDTBQ and the experimental
conditions were the same as were ported earlier.5ꢀ8 All the complexes showed saturation kinetics
and a treatment based on the Michaelis−Menten model seemed to be appropriate. The binding
constant (KM), maximum velocity (Vmax), and rate constant for dissociation of substrates (i.e.,
turnover number, kcat) were calculated for the complex by using the Lineweaver−Burk graph of
1/V vs1/[S] , using the equation 1/V = {KM/Vmax}{1/[S]} + 1/Vmax,}, and the kinetic parameters
are presented in Table 4 and in Table S1. From the data presented in Table 4 it may be inferred
that the catalytic efficiency of our present Mn(II) complexes is comparable with highly active
Mn based catalysts reported till date.
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Fig. 7 Changes observed in UV−vis spectra of complex 1 in DMSO up to 30 minutes (conc. 1 ×
10−4M) upon addition of 100ꢀfold 3,5ꢀDTBC (1 × 10−2 M).
Fig. 8 Changes observed in UV−vis spectra of complex 2 in DMSO up to 30 minutes (conc. 1 ×
10−4M) upon addition of 100ꢀfold 3,5ꢀDTBC (1 × 10−2 M).
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Fig. 9 Changes observed in UV−vis spectra of complex 3 in DMSO up to 30 minutes (conc. 1 ×
10−4M) upon addition of 100ꢀfold 3,5ꢀDTBC (1 × 10−2 M).
Fig. 10 Lineweaver−Burk plots for complexes 1, 2, 3 in DMSO medium for oxidation of
Catechol.
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Table 4. First Order Rate Constants for the Oxidation of Catechols by our complexes and the
Similar Previously Reported Mn(II/III/IV), Ni(II), Cu(II) and Zn(II) Complexes using 3,5
DTBC as substrate.
Complex
Solvent Metal
Oxidation kcat
(in hꢀ1)
Ref
present State
[Mn(L1)(SCN)2(H2O)]
[Mn2(L1)(OAc)2](BPh4)],
[Mn2(L2)(OAc)2(dca)]n
DMSO Mn
DMSO Mn
DMSO Mn
(II)
(II)
(II)
607.08
853.38
255.19
This
work
This
work
This
work
[NiL1(H2O)3]I2·H2O a
[NiL1(H2O)3]Br2·H2O a
MeOH Ni
MeOH Ni
MeOH Cu
(II)
(II)
(II)
9.27
9
8.48
9
[Cu2(H2L22)(OH)(H2O)(NO3)](NO3)3
·2H2O b
32400
10
[Cu(HL14)(H2O)(NO3)]2(NO3)2 ·2H2O b
MeOH Cu
MeOH Cu
MeOH Cu
MeOH Cu
DMSO Cu
DMSO Cu
DMSO Cu
(II)
(II)
(II)
(II)
(II)
(II)
(II)
14400
10800
14400
28800
73.70
50.50
84.60
10
10
10
10
11
11
11
b
[Cu(L11)(H2O)(NO3)]2
b
[Cu2(L23)(OH)(H2O)2](NO3)2
[Cu2(L21)(N3)3] b
[Cu2(L11)(OH)(H2O)(NO3)2] c
[Cu2(L12)(OH)(H2O)(NO3)2] c
c
[Cu(L21)(H2O)(NO3)]2
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[Cu2(L13)(OH)(H2O)(NO3)2] c
[Cu2(L14)(OH)(H2O)(NO3)2] c
[Cu2(L15)(OH)(H2O)(NO3)2] c
[Cu2(L16)(OH)(H2O)(NO3)2] c
[Cu2(L17)(OH)(H2O)2(NO3)2] c
[Cu2(L19)(OH)(H2O)(NO3)2] c
[Cu2(L110)(OH)(H2O)(NO3)4] c
[Cu2(L112)(OH)(H2O)(NO3)2] c
[Zn2(H2L1)(OH)(H2O)(NO3)](NO3)3] d
[Zn2L2Cl3] d
DMSO Cu
DMSO Cu
DMSO Cu
DMSO Cu
DMSO Cu
DMSO Cu
DMSO Cu
DMSO Cu
MeOH Zn
MeOH Zn
MeOH Zn
MeOH Zn
MeCN Mn
MeCN Mn
MeCN Mn
MeOH Mn
MeOH Mn
MeOH Mn
CH2Cl2 Mn
CH2Cl2 Mn
(II)
127.20
11
11
11
11
11
11
11
11
(II)
49.60
246.80
50.70
26.90
51.80
68.30
50.10
(II)
(II)
(II)
(II)
(II)
(II)
(II)
1060.00 12
(II)
882.0
12
[Zn2L3Cl3] d
(II)
297.00
352.00
936.64
365.34
12
[Zn2(L4)2(CH3COO)2] d
(II)
12
[MnL1(OOCH)(OH2)] e
(III)
(III)
(III)
(III)
(III)
(III)
(IV)
(IV)
13a
13a
2
[MnL2(OH2)2][Mn2(L2 (NO2)3]e
[Mn2L2(NO2)2] e
[MnL2Cl·4H2O] f
[MnL3Cl·4H2O]f
[MnL4Cl·4H2O] f
[MnIV (L1• ) (L1••)] g
1432.74 13a
247.00
360.00
720.00
12.60
13b
13b
13b
14a
14a
2
A 2
g
[MnIV(L2 )(L2•)
]
2
21.30
A
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g
[MnIV(L3 )(L3•)
]
2
CH2Cl2 Mn
CH2Cl2 Mn
CH2Cl2 Mn
CH2Cl2 Mn
CH2Cl2 Mn
MeOH Mn
MeOH Mn
(IV)
(IV)
(IV)
(IV)
(IV)
(II)
55.20
14a
14a
14a
14a
14a
A
[MnIV(L4 )(L4•)2] g
14.80
34.14
16.32
15.60
A
[MnIV(L5 )(L5•)2] g
A
[MnIV(L6 )(L6•)2] g
A
[MnIV(L7 )(L7•)2] g
A
[Mn(HL)(H2O)3](NO3)2·(H2O)] h
[Mn(HL)(SCN)2(H2O)]·0.5H2O] h
2160.00 15
1440.00 15
(II)
[Mn(HL)(N(CN)2)(H2O)2](NO3)·H2O] h
MeOH Mn
(II)
720.00
15
a
b
L1=2ꢀ[(2ꢀPiperazinꢀ1ꢀylethylimino)ꢀmethyl]phenol; L22=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀmethylꢀphenolato, R = Nꢀethylpiperazine
bL14= 2ꢀformylꢀ4ꢀmethylꢀ6Rꢀiminomethylꢀphenolato, R = Nꢀethylmorpholine,
phenolato. R= Nꢀpropylmorpholine, b L23=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀmethylꢀphenolato, R= Nꢀethylpyrrolidine, b L21=2,6ꢀbis(Rꢀ
iminomethyl)ꢀ4ꢀmethylꢀphenolato;
ethylpiperidine,
iminomethylꢀphenolato, R= Nꢀethylpiperidine,
cL14=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀtꢀbutyl–phenolato, R= Nꢀethylmorpholine, L15=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀchloro–phenolato,
R=Nꢀpropylmorpholine,
iminomethyl)ꢀ4ꢀchloro–phenolato, R= Nꢀethylpyrrolidine,
ethylpyrrolidine, L110=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀchloro–phenolato, R = Nꢀethylpiperazine, L112=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀtꢀ
butyl–phenolato, R = Nꢀethylpiperazine, L1=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀmethylꢀphenolato, R = Nꢀethylpiperazine L2=2,6ꢀ
bis(Rꢀiminomethyl)ꢀ4ꢀmethylꢀphenolato,
L11=2ꢀformylꢀ4ꢀmethylꢀ6Rꢀiminomethylꢀ
b
R=Nꢀpropylmorpholine,c
L11=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀchloroꢀphenolato,
R=
Nꢀ
L12=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀtꢀbutylꢀphenolato, R= Nꢀethylpiperidine,
L21=2ꢀformylꢀ4ꢀmethylꢀ6Rꢀ
c
c
L13=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀchloro–phenolato, R = Nꢀethylmorpholine,
c
c
L16=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀtꢀbutyl–phenolato, R=Nꢀpropylmorpholine,
L17=2,6ꢀbis(Rꢀ
c
c
L19=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀtꢀbutyl–phenolato, R= Nꢀ
c
c
c
d
d
d
R
=
Nꢀethylpyridine,
L3=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀmethylꢀphenolato, R= Nꢀ
d
e
ethylpyrrolidine,
L4=2,6ꢀbis(Rꢀiminomethyl)ꢀ4ꢀmethylꢀphenolato, R= benzylamine,
L1= 2,7ꢀbis(2ꢀhydroxyphenyl)ꢀ2,6ꢀ
diazaoctaꢀ2,6ꢀdiene, L2= 1,7ꢀbis(2ꢀhydroxyphenyl)ꢀ2,6ꢀdiazaheptaꢀ1,6ꢀdiene);
L2= N,N’ꢀ1ꢀmethylethylenebis(3ꢀformylꢀ5ꢀ
e
f
methylsalicylaldimine), L3= N,N’ꢀ1,1ꢀdimethylethylenebis( 3ꢀformylꢀ5ꢀmethylsalicylaldimine), L4= N,N¢ꢀcyclohexenebis(3ꢀ
f
f
formylꢀ5ꢀmethylsalicylaldimine); L1=1,3ꢀBis(4,6ꢀdiꢀtertꢀbutylꢀ2ꢀiminophenol)benzene, L2= 2ꢀanilinoꢀ4,6ꢀdiꢀtertꢀbutylphenol,
g
g
g
g
g
L3= 2ꢀ(3,5ꢀdi tꢀbutylꢀanilino)ꢀ4,6ꢀdiꢀtertꢀbutylphenol, L4= 2ꢀ(3,5ꢀdiꢀtrifluromethaneꢀanilino)ꢀ4,6ꢀdiꢀtertꢀbutylphenol, L5 =2ꢀ
(3,5ꢀdi methylꢀanilino)ꢀ4,6ꢀdiꢀtertꢀbutylphenol, L6 =2ꢀ(3,5ꢀdi chloroꢀanilino)ꢀ4,6ꢀdiꢀtertꢀbutylphenol, L7 =2ꢀ(3,5ꢀdi methoxyꢀ
anilino)ꢀ4,6ꢀdiꢀtertꢀbutylphenol; h HL = 2,6ꢀbis{2ꢀ(Nꢀethyl)pyridineiminomethyl}ꢀ4ꢀmethylphenolato,
g
g
Mechanistic interpretation on catecholase activity exhibited by the complexes.
Earlier our group tried to explore the origin of Catecholase like activity exhibited by Mn(II) and
Mn(III) with Schiff Base ligands.13b.15 In case of Mn(III) we are able to prove that the origin of
the activity was related to metal center Redox participation, where Mn(III) underwent reduction
to Mn(II) with concomitant oxidation of catechol to quinone. However with the case of Mn(II)
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complex we failed to evaluate the plausible mechanism for the catecholase activity. The
oxidation from catechol to quinone may occured via three pathways (i) metal centered reduction,
(ii) ligand centered imine radical formation (iii) phenoxy radical formation in case of phenolic
ligands. So in this present work we tried to evaluate the correct pathway with the help of EPR
and electrochemical studies. As the three complexes are of same kind we selected the dinuclear
one (2) as a model for EPR spectroscopic analysis. For this, we have prepared a 1:100 mixture of
complex 2 and 3,5ꢀDTBC (10−1 M) in DMSO solvent and recorded the EPR spectra within 2 min
of mixing. A characteristic six line EPR spectrum (Fig. 11) corroborates the Mn (II) species
when the complex 2 is dissolved in DMSO. An additional sharp peak (g=1.99) (Fig. 12) appears
after mixing the complex with 3,5ꢀDTBC which indicates the formation of ligandꢀcentered imine
radical and that radical formation is most probably responsible for the catalytic oxidation of 3,5ꢀ
DTBC to 3,5ꢀDTBQ in presence of aerial oxygen. We have also performed the EPR experiment
at 77 K at power saturation condition to find out any signal at the halfꢀfiled but our repeated
attempts failed to distinguish any characteristic signal. However, our present EPR study implies
that when the Mn(II) complex is mixed with 3,5ꢀDTBC, the ligand part most probably the imine
bond of the coordinated ligand undergoes reduction with concomitant oxidation of the catechol
to quinone via the formation of semibenzoquinone radical, a similar observation as we found in
our earlier study with our Zn complex.12
Fig. 11 EPR spectrum of complex 2 in DMSO medium at 77 K.
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Fig. 12 EPR spectrum of a 1:100 mixture of complex 2 and 3,5ꢀDTBC (10−1 M) in DMSO at 77
K.
Interestingly, when this catalytic oxidation was performed in an inert atmosphere, no 3,5ꢀDTBQ
formation was noticed. However, upon exposure of the reaction mixture in dioxygen atmosphere
immediately 3,5ꢀDTBQ formation was observed. This observation indeed indicates that
dioxygen is one of the active participants in the catalytic cycle; it converts the semibenzoquinone
radical, formed in the first step of catalysis, to the quinone with subsequent regeneration of the
catalyst.
Electrochemical Study
All the Mn(II) complexes show an irreversible reduction at −0.72 vs Ag/AgCl, which we
tentatively assign to imine reduction of the ligand moiety. On the oxidative side the mononuclear
Mn(II) complex exhibits one irreversible oxidation at 0.89 V, which is probably due to oxidation
of phenolate to phenoxyl radical. The binuclear complexes show two distinctive oxidation peaks,
both the processes being irreversible in nature. For complex 2, the anodic peak potentials are at
0.96 and 1.12 V (Fig. S13), whereas for 3, the corresponding values are 0.51 and 1.07 V (Fig.
S15). We tentatively assign the first anodic peak to Mn(II)/Mn(III) oxidation and second peak to
oxidation of phenolate moiety of the ligand. Now we performed the cyclic voltammetic study of
the complexes in presence of 3,5ꢀDTBC. All three complexes exhibit nearly similar response and
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the thus the behavior of complex 1 in presence of 3,5ꢀDTBC is stated here. On addition of 3,5ꢀ
DTBC to a solution of complex 1 the current height of the imine reduction peak at −0.68V
decreases, while a new peak appears at −0.29V may be due to reduction of coordinated
semiquinone to catechol. (Fig. 13) On the oxidative side a very strong peak at ~0.8 V
corresponds to formation of semiquinone is observed without any signal responsible for the
oxidation of Mn(II) to Mn(III) (Fig. S15 and S16). Thus the cyclic voltammetry data confirms
that reduction of the complex during 3,5ꢀDTBC oxidation can take place only at the imine part of
the ligand and the metal center is no way involved in this process.
Fig. 13 Time dependent CV spectrum of complex 1 after addition of DTBC at the GC electrode
in DMSO medium at 100 mV s−1 scan rate as representative.
On the basis of the results obtained from EPR and cyclic voltametric studies a plausible
mechanism has been proposed in Scheme 2. With the dinuclear complexes two catechol
molecules are oxidized per cycle, whereas only one is oxidized by the monomeric Mn(II)
complex. Here the mononuclear complex 1 is presented for simplicity.
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Scheme 2. Proposed mechanism for the catalytic cycle of oxidation of 3,5ꢀDTBC by
mononuclear Mn(II) complex.
Magnetic Properties of Complex 2 and 3.
The magnetic properties of complex 2 and 3, in the form of χMT (χM is the susceptibility per
dinuclear unit) vs. T plots, are shown in Fig. 14 in a temperature range 5ꢀ300 K for 2 and 2ꢀ300
K for 3, respectively. The observed χmT values at room temperature, 8.76 emu K molꢀ1 (µeff =
8.37 µB) and 8.39 emu K molꢀ1 (µeff = 8.19 µB) for 2 and 3, respectively are comparable with the
spinꢀonly value of 8.75 emu K molꢀ1 (µeff =8.37 µB), expected for two independent highꢀspin
Mn(II) ions [(SMn, SMn) = (5/2, 5/2) assuming g = 2.0]. As the temperature is lowered, the χmT
values decrease in a monotonous manner and become 0.39 emu K molꢀ1 for 2 at 5 K and 0.31
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emu K molꢀ1 for 3 at 2 K. This result indicates the presence of an antiferromagnetic spinꢀ
exchange interaction between Mn (II) ions.
Fig. 14 Temperature variation of the magnetic susceptibilities of 2 and 3 as χT versus T plots
(The solid line represents the best fit of the experimental data based on the Heisenberg model).
Fig. 15 Local coordination environments of Mn(II) atoms and the magnetic exchange coupling
pathway for complex 2.
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For dinuclear Mn(II) complexes (S1=S2=5/2), the theoretical expression of the magnetic
susceptibility based on the Heisenberg Hamiltonian(H=−2JS1S2)is:
2N g2 ꢀB2
[
55+ 30e−10J kT +14e−18J kT + 5e−24J kT + e−28J kT
]
χ
T =
×
× (1− ρ)
k
[
11+ 9e−10J kT + 7e−18J kT + 5e−24J kT + 3e−28J kT + e−30J kT
]
(Eq.1)
N g2 ꢀB2
+
S(S +1)
ρ
+ NαT
k
In these expression,
ρ
presents the fraction of paramagnetic impurity in the sample, Nα is a
temperature independent paramagnetism and the other symbols have their usual meanings. To
determine the exchange parameters, T was fitted using Eq.1 for the range 5–300 K gives the
χ
best agreement with the experimental data for J = ꢀ 9.15 cmꢀ1, g = 2.01,
0.00089 (R2=0.99802) for complex 2.
ρ= 0.09 and Nα=
Fig. 16 Local coordination environments of Mn(II) atoms and the magnetic exchange coupling
pathway for complex 3.
The complex 3 is a oneꢀdimensional chain and its magnetic interactions should be expressed
using alternating exchange interactions, namely, Mn–J–Mn–J´–Mn–J–Mn–J´–Mn, in which J
represents the intradimer exchange interaction via the triple oxygen bridges whereas J´ is the
interdimer exchange interaction via dicyanamide bridge in the chain (Fig. 16). To take into
account the interaction between the dimer entities, the susceptibility can be corrected by the
molecular field approximation20 (Eq. (2) where zJ´ denotes the intraꢀmolecular exchange
parameter.
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χ
T
(Eq.2)
χ
MT =
2
1−
χ
(
2zJ'/ Ng2
β
)
In these expression, z is the number of nearestꢀneighbouring dimers (in this case z = 2) and J′
accounts for the presence of magnetic interactions between neighboring dimers and the other
symbols have their usual meanings. To determine the exchange parameters, χT was fitted using
Eq.2 for the range 2ꢀ300 K gives the best agreement with the experimental data for J = ꢀ
46.89cmꢀ1, J´= ꢀ 2.38, g = 2.20, = 0.025and Nα= 0.00046 (R2=0.99852) for complex 3.
ρ
The overall magnetic behavior of the complex 2 and 3 is dominated by antiferromagnetic
interaction between the spin carriers and resembles that of related compounds.16ꢀ19
Experimental Section
Physical methods and materials. 3,5 diꢀtertꢀbutyl catechol was purchased from SigmaꢀAldrich.
Reaction solutions for 3,5ꢀDTBC was prepared according to the standard sterile techniques.
Elemental analyses (carbon, hydrogen, and nitrogen) were performed using a PerkinElmer 240C
analyzer. Infrared spectra (4000−400 cm−1) were recorded at 28 °C on a Shimadzu FTIRꢀ8400S
and PerkinElmer Spectrum Express Version 1.03 using KBr pelletsas mediums. UV−visible
spectra and kinetic traces were monitored with a Shimadzu UVꢀ2450PC spectrophotometer
equipped with multiple cellꢀholders and thermostat. Magnetic susceptibility measurements over
the temperature range 2−300 K was performed at a magnetic field of 0.0750 T using a Quantum
Design SQUID MPMSXLꢀ5 magnetometer. Correction for the sample holder, as well as the
diamagnetic correction, which was estimated from the Pascal constants.20,21 EPR experiments
were performed at liquid nitrogen temperature (77 K) in methanol, using a JEOL JESꢀFA200
spectrometer at X band (9.13 GHz).
Cyclic voltammetric and DPV measurements were performed by using a CH1106A potentiostat
with glassy carbon (GC) as working electrode, Ptꢀwire as counter electrode and Ag, AgCl/sat
KCl as reference electrode. All solutions were purged with dinitrogen prior to measurements.
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Highꢀpurity Nꢀ(2ꢀaminoethyl)ꢀpyrrolidine, Nꢀ(2ꢀaminoethyl)morpholine, were purchased from
commercial sources (Fluka, Lancaster Chemical Co. Inc., Aldrich) and used as received. The
2,6ꢀdiformylꢀ4ꢀRꢀphenols (R = methyl,chloro) were prepared according to the literature
method.22 MeOH was dried over CaH2 for 2 days and then distilled under reduced pressure prior
to use. Water used in all physical measurement and experiments was MilliꢀQ grade. All other
chemicals used were of AR grade.
Synthesis of the Complexes.
The following general template synthetic route was adopted for preparing all complexes. A
methanolic solution of the manganese salt was added to the ligand solution formed in situ via
condensation of 2,6ꢀdiformylꢀ4ꢀRꢀphenol (where R= methyl, chloro) with the corresponding
amines maintaining the same molar ratio. The preparation, composition, and other
physicochemical characteristics of all complexes using the template technique are given below.
Syntheses.
[Mn(L1)(SCN)2(H2O)], (1)
To a methanolic solution (5 mL) of Nꢀ(2ꢀaminoethyl)morpholine (0.260 g, 2 mmol) a methanolic
solution (10 mL) of 4ꢀchloroꢀ2,6ꢀdiformylphenol (0.180 g,1 mmol) was added dropwise, and the
mixture was refluxed for 30 minutes. Then a methanolic solution (15 mL) of manganese chloride
(0.604 g, 2.5 mmol) was added to it, and the resulting mixture was allowed to reflux for
additional 2 h. The deep brown solution was filtered and cooled to room temperature. Then an
aqueous solution of sodium thiocyanate (0.32g, 4 mmol) was added to the reaction mixture and
stirred for about 30 minutes. The resulting solution was filtered and the filtrate was kept in a
CaCl2 dessicator. Deep brown crystals suitable for Xꢀray data collection were obtained from the
filtrate on keeping the solution overnight (yield 75%). Anal. Calcd for C22H31Mn1N6O4S2Cl : C
(44.14%); H (5.5%); N (14.04%); Found: C (44.94%); H (5.89%); N (14.1%); FTꢀIR data (KBr
pellet): I.R: ν(C=N) 1641 cmꢀ1; ν(skeletal vibration) 1533 cmꢀ1; ν(SCNꢀ) 2065 cmꢀ1; UV/vis
(MeOH): λmax(ε) = 425nm(4200 L mol−1 cm−1).
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[Mn2(L1)(OAc)2](BPh4)], (2)
To a methanolic solution (5 mL) of Nꢀ(2ꢀaminoethyl)morpholine (0.260 g, 2 mmol) a methanolic
solution (10 mL) of 4ꢀchloroꢀ2,6ꢀdiformylphenol (0.180 g,1 mmol) was added in a dropwise
manner, and the mixture was refluxed 30 min. Then a methanolic solution (15 mL) of manganese
acetate (0.604 g, 2.5 mmol) was added to it, and the resulting mixture was allowed to reflux for 2
h. The deep brown solution was filtered and cooled to room temperature. Then an aqueous
solution of tetra phenyl borate (0.88 g, 4 mmol) was added to the reaction mixture and stirred for
about 10 min. The resulting solution was filtered and the filtrate was kept in a CaCl2 dessicator.
Deep brown crystals suitable for Xꢀray data collection were obtained from the filtrate after few
days. (yield 70%).
Anal. Calcd for C48H34Mn2N4O7BCl : C (60.30%); H (3.55%); N (5.75%); Found: C (60.50%);
H (3.95%); N (5.29%); FTꢀIR data (KBr pellet): I.R: ν(C=N) 1643 cmꢀ1; ν(skeletal vibration)
1537 cmꢀ1; ν(OAcꢀ) 1483 cmꢀ1; UV/vis (MeOH): λmax(ε) = 381nm(7390 L mol−1 cm−1).
[Mn2(L2)(OAc)2(dca)]n, (3)
To a methanolic solution (5 mL) of Nꢀ(2ꢀaminoethyl)pyrrolidine (0.228 g, 2 mmol) a methanolic
solution (10 mL) of 4ꢀmethylꢀ2,6ꢀdiformylphenol (0.164 g,1 mmol) was added in a dropwise
manner, and the mixture was refluxed for 30 min. Then a methanolic solution (15 mL) of
manganese acetate (0.604 g, 2.5 mmol) was added to it, and the resulting mixture was allowed to
reflux for 2 h. The deep brown solution was filtered and cooled to room temperature. Then to the
reaction mixture an aqueous solution of sodium dicyanamide (0.284 g, 4 mmol ) was added
continuing to stir for about 30 min. The resulting solution was filtered and the filtrate was kept in
a CaCl2 dessicator. Deep brown crystals suitable for Xꢀray data collection were obtained from the
filtrate after keeping the solution overnight (yield 68%).
Anal. Calcd for C27H37Mn2N7O5 : C (49.88%); H (5.69%); N (15.08%); Found: C (49.50%); H
(5.95%); N (15.37%); FTꢀIR data (KBr pellet): I.R: ν(C=N) 1641 cmꢀ1; ν(skeletal vibration)
ꢀ
1533 cmꢀ1; ν(OAcꢀ) 1488 cmꢀ1 ν(N(CN)2 ) 2165 cmꢀ1 ; UV/vis (MeOH): λmax(ε) = 407nm(7530
L mol−1 cm−1).
25

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DOI: 10.1039/C5DT03659C
X-ray Data Collection and Structure Determination. Data collection of the structures reported
was carried out at room temperature on a on a Bruker Smart CCDdiffractometer (1ꢀ2) and on a
Nonius DIPꢀ1030H system (3), all equipped with graphiteꢀmonochromated MoꢀKα radiation
(λ=0.71073 Å). Cell refinement, indexing and scaling of the data set were carried out using
Bruker Smart Apex and Bruker Saint packages,23 Mosflm and Scala.24All the structures were
solved by direct methods and subsequent Fourier analyses25and refined by the fullꢀmatrix leastꢀ
squares method based on F2 with all observed reflections.25In compound 1the coordinated
morpholine moiety was found disordered over two positions (occupancies refined at
0.537(6)/0.463(6) with a restrainedgeometry).All the calculations were performed using the
WinGX System, Ver 1.80.05.26 Pertinent crystallographic data and refinement details are
summarized in Table 5.
Table 5. Crystallographic Data and Details of Refinements for Complexes 1-3.
1
2
3
Empirical formula
MW
C22H31ClMnN6O4S2 C48H54BClMn2N4O7 C27H37Mn2N7O5
598.04
955.09
649.52
Crystal system
Space group
Monoclinic
P 21/c
Monoclinic
P 21/m
Triclinic
P
1
a /Å
9.1058(4)
21.8162(9)
14.3490(6)
11.1317(17)
24.757(4)
17.083(3)
10.058(3)
12.348(3)
13.366(4)
78.53(2)
84.51(3)
70.55(2)
1533.3(7)
2
b /Å
c /Å
α /°
β /°
97.9080(10)
93.100(2)
γ /°
V /Å3
2823.4(2)
4
4700.9(12)
4
Z
Dcalc /g cm–3
ꢁ /mm–1
F(000)
Theta range /°
Total data
Unique data
1.407
1.349
1.407
0.748
0.648
0.870
1244
1992
676
1.71 ꢀ 25.57
32565
5273
1.19 ꢀ 26.39
32933
9482
1.56 ꢀ 23.26
11533
4339
26

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DOI: 10.1039/C5DT03659C
Rint
0.0325
4223
0.0398
6350
0.0779
1478
ReflectionsI>2σ(I)
Parameters
395
592
370
Goodnessꢀofꢀfit
1.022
1.038
0.825
0.0731
0.1723
[a]
R1
0.0331
0.0849
0.285, –0.247
0.0557
0.1675
0.981, –0.399
wR2 (I>2σ(I)) [a]
Residuals /e Å–3
0.364, –0.362
[a]
2
2 2
2 2 ½
|
| | ||
|
|
R1 = Σ Fo – Fc / Σ Fo , wR2 = [Σw (Fo – Fc ) / Σw (Fo ) ]
[b] CCDCꢀ1425706 ꢀ1425708 contain the supplementary crystallographic data for this paper.
Conclusion
Three manganese(II) complexes having different nuclearities, one monoꢀ, one diꢀ and one
dinuclear based polynuclear, have been synthesized using two “endꢀoff” compartmental Schiffꢀ
base ligands, HL1 and HL2. Protonation of one of the imine nitrogen of HL1 in presence of SCN
as coꢀligand cancel out the possibility of metalation in the second compartment of HL1 leads to
mononuclearity. In presence of acetate as coꢀligand and BPh4 as counter anion HL1 generates
dinuclear complex 2. HL2 in presence of acetate and dca as coꢀligands forms acetate bridged
dinuclear based polynuclear complex where dca bridges two dinuclear units. All three complexes
have been structurally characterized and oxidation state of each of the manganese is +2 as is
evidenced from magnetic susceptibility and EPR study. Variable temperature magnetic study
implies the presence of an antiferromagnetic spinꢀexchange interaction between Mn(II) ions with
J = ꢀ 9.15 cmꢀ1 and J = ꢀ 46.89 cmꢀ1 for complex 2 and 3, respectively. All three complexes
exhibit catecholaseꢀlike activity towards 3,5ꢀDTBC as model substrate with varying degree,
2>1>3. In order to unveil the origin of catecholase activity whether related to metal centered
redox participation or radical generation we have performed EPR and cyclic voltametric studies.
EPR investigation of the solution of 3,5ꢀDTBC and complex suggests radical formation. Cyclic
voltametric study suggests ligand centered reduction rather than the reduction of Mn(II) to
Mn(I). It is therefore concluded that catecholaseꢀlike activity of the present Mn(II) complexes is
following radical pathway as is observed in analogous Zn(II) and Ni(II) complexes.
27

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DOI: 10.1039/C5DT03659C
Acknowledgements
The authors wish to thank CSIR, New Delhi [01(2464)/11/EMRꢀII dt16ꢀ05ꢀ11 to D.D.], for
financial support and the University of Calcutta for providing the facility of the singleꢀcrystal Xꢀ
ray diffractometer from the DST FIST program and for collecting variable temperature magnetic
study from SQUIDꢀVSM in CRNN. Authors are also thankful to Prof. Shyamal Kumar
Chattopadhyay of IIEST, Shibpur for providing the facilities of electrochemical studies. I.M is
thankful to UGC [UGC/729/Jr Fellow(Sc)] for providing fellowship.
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