Entrapment of a Pseudo-Tetrahedral CoII Center by Thioether Sulfur Bound {Co2(μ-L)} Fragments: Synthesis, Field-Induced Single-Ion Magnetism and Catechol Oxidase Mimicking Activity

Article abstract of DOI:10.1002/asia.201901109

Simultaneous incorporation of both Co^II and Co^III ions within a new thioether S-bearing phenol-based ligand system, H₃L (2,6-bis-[{2-(2-hydroxyethylthio)ethylimino}methyl]-4-methylphenol) formed [Co₅] aggregates [Co^IICo^III ₄L₂(μ-OH)₂(μ_1,3-O₂CCH₃)₂](ClO₄)₄?H₂O (1) and [Co^IICo^III ₄L₂(μ-OH)₂(μ_1,3-O₂CC₂H₅)₂](ClO₄)₄?H₂O (2). The magnetic studies revealed axial zero-field splitting (ZFS) parameter, D/hc=?23.6 and ?24.3 cm^?1, and E/D=0.03 and 0.00, respectively for 1 and 2. Dynamic magnetic data confirmed the complexes as SIMs with U_eff/k_B=30 K (1) and 33 K (2), and τ₀=9.1×10^?8 s (1), and 4.3×10^?8 s (2). The larger atomic radius of S compared to N gave rise to less variation in the distortion of tetrahedral geometry around central Co^II centers, thus affecting the D and U_eff/k_B values. Theoretical studies also support the experimental findings and reveal the origin of the anisotropy parameters. In solutions, both 1 and 2 which produce {Co^III ₂(μ-L)} units, display solvent-dependent catechol oxidation behavior toward 3,5-di-tert-butylcatechol in air. The presence of an adjacent Co^III ion tends to assist the electron transfer from the substrate to the metal ion center, enhancing the catalytic oxidation rate.

Full text of DOI:10.1002/asia.201901109

DOI: 10.1002/asia.201901109
Full Paper
II
Entrapment of a Pseudo-Tetrahedral Co Center by Thioether
Sulfur Bound {Co (m-L)} Fragments: Synthesis, Field-Induced
2
Single-Ion Magnetism and Catechol Oxidase Mimicking Activity
[a]
[a]
[b]
[b]
[a]
Manisha Das, Dipmalya Basak, Zden eˇ k Trꢀvnꢁ cˇ ek, Jꢀn Van cˇ o, and Debashis Ray*
II
III
Abstract: Simultaneous incorporation of both Co and Co
to N gave rise to less variation in the distortion of tetrahe-
II
ions within a new thioether S-bearing phenol-based ligand
dral geometry around central Co centers, thus affecting the
system, H L (2,6-bis-[{2-(2-hydroxyethylthio)ethylimino}meth-
D and U /k values. Theoretical studies also support the ex-
3
eff
B
II
III
yl]-4-methylphenol) formed [Co ] aggregates [Co Co L (m-
perimental findings and reveal the origin of the anisotropy
5
4 2
II
III
4
OH) (m -O CCH ) ](ClO ) ·H O (1) and [Co Co L (m-OH) (m -
parameters. In solutions, both 1 and 2 which produce
2
1,3
2
3 2
4 4
2
2
2
1,3
III
O CC H ) ](ClO ) ·H O (2). The magnetic studies revealed
{Co (m-L)} units, display solvent-dependent catechol oxida-
2
2
5 2
4 4
2
2
axial zero-field splitting (ZFS) parameter, D/hc=ꢀ23.6 and
24.3 cm , and E/D=0.03 and 0.00, respectively for 1 and
. Dynamic magnetic data confirmed the complexes as SIMs
tion behavior toward 3,5-di-tert-butylcatechol in air. The
ꢀ
1
III
ꢀ
presence of an adjacent Co ion tends to assist the electron
2
transfer from the substrate to the metal ion center, enhanc-
ing the catalytic oxidation rate.
ꢀ
8
with U /k =30 K (1) and 33 K (2), and t =9.1ꢂ10 s (1),
eff
B
0
ꢀ
8
and 4.3ꢂ10 s (2). The larger atomic radius of S compared
Introduction
ly pleasing shapes and structures of the compounds which can
be interesting in the areas of molecular magnetism and mag-
[3–6]
Novel coordination aggregates of 3d metal ions can be ach-
neto-caloric materials.
ieved by suitable modulation of the bridging assistance of judi-
Room-temperature synthesis, characterization and crystalliza-
tion of 3d metal ion aggregates have received considerable at-
tention in recent years. Many of these aggregates often pos-
sess a large ground-state spin (S) and sizeable magnetic aniso-
tropy (D) giving rise to sizeable barrier for reversal of magneti-
[
1]
ciously chosen ancillary ligands. In solution these ancillary li-
gands can often result in coordination induced aggregation of
preformed fragments of selected metal ions and ligands. Play-
ing with these ligands, either in situ generated or added from
outside, in the domain of synthesis teaches us about their po-
tential for multinuclear coordination aggregates as obtained
by the competitive occupancy of available or vacant coordina-
tion sites by the primary ligand system and supporting ancil-
lary groups. Judicious selection of primary ligand and ancillary
bridges were vital to bring about the self-condensation of frag-
zation, U . Within these multimetallic aggregates either all the
eff
centers are paramagnetic or in the middle of several diamag-
netic centers one or more of them could be paramagnetic. The
SMM behavior under the second category having a single par-
amagnetic metal ion often depends on unusual coordination
geometry generating a magnetically anisotropic ground state.
Thus interesting types of molecular magnet can be conceived
on a single paramagnetic ion, known as single-ion magnets
[
2]
ments to give rise to exciting structures. In this regard
phenol-based Schiff bases showed promise to extend immedi-
ate coordination to two metal ions to generate fragments suit-
able for aggregation. Molecular aggregates based on 3d ions
are strongly related to a variety of research fields, including
bioinorganic chemistry, catalysis, structures and magnetism.
Self-assembling of ligand bound fragments can result in visual-
(SIMs) and have shown promise in recent years when the
II [7,8]
choice for the 3d ion is Co .
Most recent advancement in
II
the field of multinuclear Co -based aggregates is due to the
fact that they display slow magnetic relaxation at low tempera-
II
ture. Co -SMMs can demonstrate much larger magnetic aniso-
tropy and higher blocking temperatures than SMMs based on
ions where the zero-field splitting originates from a second
[
a] M. Das, D. Basak, Prof. Dr. D. Ray
Department of Chemistry
Indian Institute of Technology
Kharagpur 721 302 (India)
E-mail: dray@chem.iitkgp.ac.in
[9]
order spin-orbit coupling. It is known that low-coordinate,
II
II
high-spin Fe and Co complexes can record large and negative
D values for this purpose. We have recently reported a poly-
II
nuclear cobalt complex containing a single Co ion in distorted
[
b] Prof. Dr. Z. Trꢀvnꢁ cˇ ek, Dr. J. Van cˇ o
Division of Biologically Active Complexes and Molecular Magnets
Faculty of Science
III
tetrahedral geometry surrounded by diamagnetic Co centers
[
10]
II
showing SIM behavior. The trapping of Co within a con-
Palack y´ University
III
strained environment of the surrounding Co ions resulted in a
ˇ
˚
Slechtitelu 27, 783 71 Olomouc (Czech Republic)
unique distortion of the geometry from regular tetrahedra
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
II
around the Co leading to more negative D value.
https://doi.org/10.1002/asia.201901109.
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The phenol bearing ligand (H L) family with side arms on
The deprotonated alcohol arm extends its coordination and
n
n+
III
II
both the adjacent sides is known to provide {M L} (M=3d
bridges the ligand bound Co and the central Co trapping it
in a tetrahedral environment. In our present investigation we
have extended our idea by introducing a bigger thioether S
donor hence synthesising the ligand (2,6-bis-[{2-(2-hydroxye-
2
metal ions) fragments having potential for self-aggregation in
ꢀ
ꢀ
ꢀ
[2]
presence of HO /RO and RCO ions. When M=Co, we have
2
n+
shown that aggregation of preformed {Co L}
fragments
2
2ꢀ
[1]
around CO3 groups lead to a [Co ] wheel type structure. In
thylthio)ethylimino}methyl]-4-methylphenol) (H L when X=S;
5
3
solutions such fragments assemble together to provide the
final product and these reactive species can show interesting
solution properties which aid in their solution characterization.
In the native state catechol oxidase (CO) catalyze the oxidation
of ortho-diphenols by utilizing the electron transfer properties
Figure 1left). We have investigated the reactivity of H L (Fig-
3
ure 1right) with Co(ClO ) ·6H O in the presence of two differ-
4
2
2
ꢀ
ent carboxylate salts (RCO2 , R=Me, CH Me) and present the
2
synthesis of mixed-valence-mixed-geometry [Co ] aggregates
5
II
III
[Co Co L (m-OH) (m -O CCH ) ](ClO ) ·H O
(1)
and
4
2
2
1,3
2
3 2
4 4
2
[
11]
II
III
4
of copper ions. This oxygen-utilizing enzyme controls the re-
[Co Co L (m-OH) (m -O CC H ) ](ClO ) ·H O (2) having similar
2
2
1,3
2
2
5 2
4 4
2
actions involving O by modulating the reactivity of ligand-
molecular structures with X=N. It is most likely that S being
larger in size as compared to N will introduce more flexibility
in the alcohol arm in turn affecting the amount of distortion in
2
bound metal ion fragments. Functional mimetic studies were
centered on [Cu ] complexes due to the fact that CO is a Type-
2
II/III
II
II
III copper enzyme. Recently other metal ions like Mn and Ni
have also been used for the synthesis of such functional
the tetrahedral geometry around the trapped Co ion and
hence the magnitude of D. The synthesis, growth of crystals,
X-ray structure determinations, magnetic properties and cate-
chol oxidation activity of these complexes are described and
[
12–14]
mimics.
The synergism between the chosen ligand and
used metal ions provides a strong influence on the affinity for
substrate catechol. Dinuclear and other cobalt(II/III) complexes
were less extensively explored as synthetic mimics for catechol
discoursed. Recently we have shown that use of H L resulted
3
2
+
only {Cu L(OH)} based complexes without showing any kind
2
[
15–18]
oxidation activity.
of self-aggregation around nucleating anions or metal ions
[20]
The nature of amine parts on the two sides of the ligand
from the reaction medium. Catalytic catechol oxidation stud-
ies in different solvent medium showed the influence of the
ligand alcohol arm on the mechanism of substrate oxidation
(
H L) backbone is important from their cooperative effects for
n
self-aggregation and trapping of substrates while showing
functional behavior. Thus systematic variation of X groups be-
tween O, N and S in H N(CH ) X(CH ) OH (Figure 1) resulted in
under aerobic conditions. In this work we have investigated
III
the solvent dependent catalytic potency of {Co L} fragments
2
2 2
2 2
2
three different ligand types of contrasting coordination poten-
generated in solution towards oxidation of 3,5-DTBCH2.
tial to 3d metal ions. When X=O, the resulting ligand 2,6-
bis[((2-(2-hydroxyethoxy)ethyl)imino)methyl]-4-methylphenol
II
2
showed self-aggregation reactions of preformed {Co L} frag- Results and Discussion
ments for [Co ] complexes achieved from the trapping of a
5
Single Step Aggregation Reaction. Phenol-based and thioeth-
er sulfur side arm bearing ligand has been developed from the
synthesis of 2-(2-aminoethylthio)ethanol. This amine-alcohol
having thioether sulfur donor at the center was prepared from
the reaction of 2-marcaptoethanol in 30% aqueous NaOH solu-
tion and 2-chloroethylamine hydrochloride in aqueous
medium. The ligand 2,6-bis-[{2-(2-hydroxyethylthio)ethylimino}-
II
ꢀ
central Co with the help of water-derived two HO and six
ꢀ
[19]
CH COO ions. The ether O along with the alcoholic OH re-
3
mained uncoordinated and dangling. In our previous attempt
to synthesize polynuclear aggregates based on cobalt ions we
had utilized the ligand 2,6-bis((2-(2-hydroxyethylamino)ethyli-
[
10]
mino)methyl)-4-methyl-phenol with X=N. Reactions of the
ligand with Co(ClO ) ·6H O in the presence of externally added
4
2
2
methyl]-4-methylphenol (H L) was acquired from the reaction
3
NaCOOR (R=Me, CH Me) resulted in mixed valence pentanu-
2
of 2,6-diformyl-4-methylphenol and 2-(2-aminoethylthio)etha-
nol in refluxing MeOH medium. In MeCN medium reaction of
H L, Co(ClO ) ·6H O, NEt and CH COONa in 1:2.5:1:2 ratio
II
clear aggregates arising from the entrapment of a central Co
III
ion by the initially formed {Co (m-L)(m-OH/OMe)} fragments.
2
3
4 2
2
3
3
under stirring and refluxing condition resulted in a deep
brown solution from which brown block shaped crystals of 1
were obtained after 18 days in 67% yield [Eq. (1)]. Different se-
quence of addition of reactants and changes in molar ratios of
the components failed to give any other type of complex
MeCN
2
H3L þ 5CoðClO4Þ2 ꢁ 6H2O þ 2NEt3 þ 4CH3COONa þ O2 ꢀꢀ!
½
Co5L2ðm ꢀ OHÞ2ðm1; 3 ꢀ O2CCH3Þ2ꢂðClO4Þ4 ꢁ H2O þ 4NaClO4
þ2CH3COOH þ 2ðNHEt3ÞClO4 þ 29H2O
ð1Þ
In a similar fashion, use of C H COONa in MeCN in lieu of
2
5
CH COONa and same molar ratio gave a brown solution after
Figure 1. The ligand H
3
L used in present work and its observed metal bind-
3
ing mode.
45 min of room temperature stirring followed by 1 h reflux.
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Brown crystals of 2 were obtained from this solution after
Crystal Structures Descriptions
2
0 days (0.478 g, 59% yield) [Eq. (2)].
II
III
[
Co Co L (m-OH) (m -O CCH ) ](ClO ) ·H O (1). The molecular
4 2 2 1,3 2 3 2 4 4 2
MeCN
2
H3L þ 5CoðClO4Þ2 ꢁ 6H2O þ 2NEt3 þ 4C2H5COONa þ O2 ꢀꢀ!
structure of the tetracationic part of 1 is shown in Figure 2,
and important bond distances and angles are provided in
Table S1 in Supporting Information. Complex 1 crystallizes in
orthorhombic Fdd2 space group with Z=16 and the asymmet-
ric unit consists of one pentanuclear fragment [Co L (m-
½
Co5L2ðm ꢀ OHÞ2ðm1; 3 ꢀ O2CC2H5Þ2ꢂðClO4Þ4 ꢁ H2O þ 4NaClO4
þ2C2H5COOH þ 2ðNHEt3ÞClO4 þ 29H2O
ð2Þ
5
2
4
+
OH) (m -O CCH ) ] , four perchlorate anions and a lattice
2
1,3
2
3 2
Elemental analysis and single crystal X-ray deter-
water molecule.
mination provided the composition of
1
and
2
as
A remarkable mixed-valence-mixed-geometry pentanuclear
aggregate has been identified from the coordination support
of imine-thioether-alcohol arm and imine-phenolate backbone
[Co L (m-OH) (m -O CCH ) ](ClO ) ·H O and [Co L (m-OH) (m -
5
2
2
1,3
2
3 2
4 4
2
5
2
2
1,3
III
2+
O CC H ) ](ClO ) ·H O. Thus in solution, two {Co L(OH)} units
2
2
5 2
4 4
2
2
were responsible to give a tetrahedral O coordination pocket
4
3ꢀ
3ꢀ
from two L units. The [Co ] aggregate consists of two L ,
5
to trap the fifth metal ion in the aggregate as a single para-
each of them delivering two neighboring ONSO pockets to
II
magnetic Co center (Scheme 1).
III
ꢀ
bind two smaller Co ions. Trapping of in situ generated HO
III
III
FT-IR Characterization of Powdered Samples. Com-
pounds 1 and 2 were used as KBr pellets for immediate iden-
tification following synthesis. The appearance of broad peaks
ions by initially formed {Co (m-L)} units provided {Co L(OH)}
2 2
fragments. In the following step the growth of 1 may be as-
sumed to happen by the aggregation of two {Co (m-L)(m-
2
ꢀ
1
at 3412 and 3421 cm in the spectra confirmed the presence
of hydroxido group and water molecule in the crystal lattices.
The coordination from imine functions of the ligand was rec-
OH)(m-O CCH )} units, obtained from the capping of carboxyl-
2
3
ognized from the n
=
stretching frequency at 1648 and
N
C
ꢀ
1
1647 cm for 1 and 2 respectively. The asymmetric (nas(COO)
)
and symmetric (ns(COO)) stretching vibrations for bridging ace-
ꢀ
1
tate groups in 1 appeared at 1576 and 1430 cm (Figure S1 in
the Supporting Information). In case of 2 the presence of pro-
ꢀ
1
pionate bridges were detected at 1577 and 1459 cm . The Dn
ꢀ
1
ꢀ1
values (146 cm for 1 and 118 cm for 2) confirmed the car-
[21]
boxylate bridging modes as m1,3 type in both the cases. The
presence of four perchlorate (in T symmetry) ions were also
d
ꢀ
1
detected uniformly at 1090, 626 and 625 cm due to the n (T )
3
2
[
22]
(
nClO) and n (T ) (ddOClO) stretching modes, respectively.
4 2
Figure 2. POV-ray view of the cationic part 1 with partial atom numbering
Scheme. H atoms are omitted for clarity. Color code: C, black; N, blue; O,
III
II
red; S, yellow; Co , violet; Co , purple.
Scheme 1. Synthetic routes for 1 and 2.
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II
ate anions, around the fifth and central Co ion in pseudo-tetra-
hedral geometry in the final structure. Simultaneous bridging
by ligand phenoxido and ancillary hydroxido and carboxylato
groups maintained the Co···Co separation within 2.779–2.802 ꢄ
range (Table S3 in Supporting Information). The facial folding
3
ꢀ
of the imine-thioether-alcohol (NSO) arms of L was vital for
the tetrahedral disposition of four oxygen atoms around the
II
III
central Co center. The four Co centers (Co1, Co2, Co3 and
Co4) stay in distorted octahedral NSO coordination geometry
4
(Figure 3).
Figure 4. POV-ray view of the cationic part of 2 with partial atom numbering
Scheme. H atoms are omitted for clarity. Color code: C, black; N, blue; O,
III
II
red; S, yellow; Co , violet, Co , purple.
in Table S2 in Supporting Information. Complex 2 crystallize in
monoclinic P2 /c space group with Z=4. The asymmetric unit
1
4
+
of 2 consists of one [Co L (m-OH) (m -O CC H ) ] part, four
5
2
2
1,3
2
2
5 2
perchlorate anions and one lattice water molecule. Here two
III
ꢀ
{
Co (m-L)} units bridged by HO and C H COO- groups, assem-
2 2 5
II
ble around the tetrahedral Co ion to provide 2. The structure
and all other metric parameters are similar to those for com-
Figure 3. Core view of 1 showing two types of cobalt centers. Color code: C,
black; N, blue; O, red; S, yellow; Co , violet, Co , purple.
plex 1 (Figure 5). The triply-bridged situation in {Co (m-L)} units
III
II
2
in the final structure ensued a Co···Co separations of 2.785 ꢄ.
The distorted octahedral NSO coordination geometry around
4
III
III
The Co-N distances for imine coordination around the Co
Co centers are formed from coordination of four types of
centers are shorter at 1.894(10)–1.935(11) ꢄ compared to Co-S
distances for thioether coordination involving bigger sulfur
atoms in 2.198(4)–2.206(3) ꢄ range. The phenoxido bridges
oxygen atoms along with imine nitrogen and thioether sulfur
atoms. The CoꢀN and CoꢀS separations were in the ranges of
1.889(7)–1.908(7) and 2.194(2)–2.203(3) ꢄ, respectively. The
3ꢀ
III
III
III
from the L units regulate the Co -O -Co angles to 92.3(3)–
Co ꢀO bond lengths from the bridging phenoxido, hydroxido,
Ph
ꢀ
9
3.2(3)8, while the exogenous HO bridges were responsible
propanoato groups within the {Co (m-L)(m-OH)(m-O CC H )}
2
2
2
5
for Co-O -Co angles in 94.3(4)–95.3(4)8 range. The terminal al-
fragments were in 1.879(5)–1.937(5) ꢄ range. Within the flat-
hy
II
koxido groups from the NSO arms asymmetrically bridge the
tened tetrahedral cavity (Houser’s t =0.82) the Co ꢀO bonds
4
III
II
octahedral Co and tetrahedral Co centers during aggregation.
were longer at 1.989(6)–2.043(5) ꢄ. The O-Co-O angles around
this ion span from 97.2(19) to 134.3(2)8 The C-S-C angles were
found within 102.0(5)–104.4(5)8 range which is similar to that
in 1. In comparison, the C-N-C angles for the analogous com-
pound previously reported by us fall in 109.3(2)–114.5(3)8
range which deviates considerably compared to that recorded
III
As expected the Co -O separations were shorter (1.880(8)–
1
.931(8) ꢄ range) for the four types of oxygen donors com-
II
pared to that in case of the tetrahedral Co centers (Co-O sepa-
rations in 1.991(8)–2.031(8) ꢄ range). For the facial folding of
the NSO arms the C-S-C angles remain within 102.3(9)–
ꢀ
[10]
105.5(8)8 range compared to the larger C-N-C angles in
for CH CO2 bridges (113.0(6)–117.3(2)8). The very similar t4
3
1
13.0(6)–117.3(2)8 range reported for analogous complex in
value compared to 1 confirmed almost negligible effect on the
compression of the tetrahedral geometry around central Co
[
10]
II
our previous work signifying greater amount of folding in
presence of S. The O-Co-O angles around the central tetrahe-
II
dral Co ion span from 100.9(3)–133.4(3)8. The Houser’s geome-
try index t (t =[3608ꢀ(a+b)]/1418; a and b being the two
4
4
II
largest angles) for the central Co ion is 0.83 indicating at
[
23]
slightly distortion of the T geometry. For a perfect tetrahe-
d
dral environment t4 value is 1.00 while for an ideal square
planar geometry the value is 0.00.
The assignment of +3 and +2 oxidation states to octahe-
dral and tetrahedral cobalt ion centers were evident from the
bond distances and were further confirmed by BVS analysis
[
24,25]
(
Table S2 in Supporting Information).
II
III
[
Co Co L (m-OH) (m -O CC H ) ](ClO ) ·H O (2). The tetra-
4 2 2 1,3 2 2 5 2 4 4 2
cationic part of 2 within the molecular structure is depicted in
Figure 4, and important bond distances and angles are given
Figure 5. Core view of 2. Color code: C, black; N, blue; O, red; S, yellow;
Co , violet, Co , purple.
III
II
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III
due to bridging of the ligand bound Co centers by the
upon cooling the meff value decrease gradually upto ca 50 K
and then, it drops sharply to a minimum at 1.9 K, where it
adopts the value of 3.3 m . In the case of complex 2, the m
eff
ꢀ
ꢀ
C H CO groups in place of CH CO2 . In our previous investi-
2
5
2
3
gation the variation in t values were greater for analogous
4
B
[
10]
compounds. This can be substantiated from the variation of
C-X-C (X=N, S) bond angles with the larger size of S compen-
sating for any structural strains introduced by the change in
bridging carboxylate. BVS analysis confirmed a+3 oxidation
state for the octahedral cobalt ions and +2 oxidation state for
tetrahedral cobalt ions (Table S2 in Supporting Information).
Magnetic properties. Investigation of the magnetic features
of complexes 1 and 2 proceeded firstly using a PPMS device
on which the temperature (1.9–300 K) and field (0–9 T) depen-
dent magnetic data were obtained and they are shown in
Figure 6. The magnetic properties of the complexes are primar-
value at 300 K is 5.4 m which is again much higher that the
B
calculated value for isotropic S=3/2 spin. It decreases gradual-
ly upon cooling up to ca 50 K and then, it declines sharply to a
minimum at 1.9 K reaching the value of 3.9 m . The high m
B
eff
values at 300 K indicates considerable contribution of a spin-
orbit coupling to the ground state in both the complexes. Iso-
thermal magnetization experiments (Figure 6) for 1 and 2 in
the applied field range of B=0–9 T at different constant tem-
peratures T=2, 5 and 10 K reveal that the magnetization does
not show saturation suggesting the presence of magnetic ani-
sotropy. To extract the parameters affected by magnetic aniso-
tropy arising from crystal field effects (g, D), both the suscepti-
bility and magnetization data were fitted simultaneously. The
best fit parameters were calculated using the PHI program
II
ily due to the presence of central tetrahedral Co ion since oc-
III
tahedral LS-Co ions are diamagnetic. The value of effective
magnetic moment (m ) of complex 1 at 300 K is 4.5 m , higher
than the spin-only value of 3.9 m for S=3/2, g=2.0. Thereafter
eff
B
[26]
package for S=3/2 and they are as follows: g =2.29, D=
iso
B
Figure 6. Temperature dependence of the effective magnetic moment (inset: temperature dependence of the molar magnetic susceptibility) for complexes 1
and 2 (Left). The isothermal magnetizations measured in the range of B=0–9 T and at T=2, 5 and 10 K (Right).
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ꢀ
1
ꢀ1
ꢀ
23.6 cm ,
E/D=0.03,
zJ/hc=0 cm ,
cTIP =10.3ꢂ
plexes 1 and 2. The SHAPE analyses revealed that the cobalt(III)
centres in both the complexes, labelled as Co1, Co2, Co4 and
Co5, show nearly octahedral geometry (with the values of the
deviations from the minimal distortion paths from the ideal oc-
tahedral coordination polyhedron in the range from 0.354 to
0.468), while the geometry in the vicinity of the cobalt(II)
atom, labelled as Co3, can be described as slightly distorted
tetrahedral (with the values of 1.510 for complex 1, and 1.777
for complex 2, of the deviations from the minimal distortion
paths from the ideal tetrahedron). The complete results can be
found in Supporting Information (see Tables S4 and S5).
ꢀ
9
3
ꢀ1
ꢀ1
1
0
cm ·mol for 1 and g =2.32, D=ꢀ24.3 cm , E/D=0.00,
iso
ꢀ
1
ꢀ9
3
ꢀ1
zJ/hc=0.05 cm , c =26.9ꢂ10 cm ·mol for 2. The pres-
TIP
ence of temperature-independent paramagnetism (TIP) term
III
accounts for the four surrounding Co ions. The observed D
values for 1 and 2 are relatively close compared to those
ꢀ
1
ꢀ1
(
ꢀ31.31 cm and ꢀ21.88 cm ) obtained in our previous in-
vestigations with complexes having similar molecular struc-
[
10]
tures. Incorporation of thioether S donors at the side arms in
place of N donors used in our previous work, provided new
ligand system H L utilized in the present work. Due to the
3
larger atomic size of S, it is able to accommodate more strain
in geometry around itself compared to N thus in turn reducing
Due to the presence of large anisotropy suggested by the
negative D values for 1 and 2, the dynamic magnetic data for
the complexes were acquired and thus, alternate current (AC)
susceptibility measurements were performed in zero and non-
zero static magnetic field. Whilst in the zero static magnetic
field no out-of-phase susceptibility (c“) signal was observed,
II
the difference in distortion of the central Co ion from ideal tet-
rahedral geometry. This is evident from the similar t values
4
(0.83 and 0.82) calculated for 1 and 2. The greater difference in
the values of the D term observed for the complexes used in
our earlier study arose from the fact that one of them showed
greater distortion of the tetrahedral geometry around the cen-
the measurement at an external dc field B =0.1 T revealed a
dc
frequency-dependent out-of-phase signal in 1 and 2. For 1, the
single maxima was observed in the frequency range 130–
1490 Hz and in the temperature range of 2.15–4.18 K while for
2, the same appeared in the range 240–1490 Hz and 3.47–
4.30 K (Figures 7 and 8). Since the peak maxima are frequency
dependent, both the complexes can be considered as field-in-
duced single-ion magnets (SIMs). A Debye model for single re-
laxation time was utilized to fit the data in both the cases. For
II
tral Co compared to the other.
With the aim to describe the geometry around the cobalt
centres more deeply, the analysis of coordination polyhedra
[
27]
was performed using the SHAPE 2.1 software. Based on the
single-crystal X-ray data, the hexa-coordinated cobalt(III) cen-
tres (i.e. Co1, Co2, Co4, and Co5) and tetra-coodinated cobal-
t(II) centre (Co3) were analysed in the structures of com-
Figure 7. The frequency dependence of (A) in-phase c’ and (B) out-of-phase c“ molar susceptibilities for 1 at an applied external magnetic field B=0.1 T.
C) The temperature dependence of the out-of-phase c” molar susceptibility for 1 at an applied external magnetic field B=0.1 T. (D) The Arrhenius-like plot re-
(
vealing the association with the Orbach relaxation process dominant above the temperature of 3.57 K and other type of processes contributing to the relaxa-
tion proceeding below this temperature (represented by the orange points).
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Figure 8. The frequency dependence of (A) in-phase c’ and (B) out-of-phase c“ molar susceptibilities for 2 at an applied external magnetic field B=0.1 T.
(
C) The temperature dependence of the out-of-phase c” molar susceptibility for 2 at an applied external magnetic field B=0.1 T. (D) The Arrhenius-like plot re-
vealing the association with the Orbach relaxation process above the temperature of 3.45 K.
II
1
, fitting of the ln t vs. 1/T data utilizing the Arrhenius expres-
sociated with single ion Co can be formulated from these col-
lective findings.
sion reveal the dominance of Orbach relaxation process at
higher temperatures (>3.57 K) while at lower temperatures
other types of processes like Raman relaxation process also
contribute to the magnetization relaxation (Figure 7D). A simi-
lar observation was made in case of 2 and both data were
fitted in the higher temperature region to obtain the values of
relaxation time (t ) and spin reversal barrier (U /k ). The pa-
To further corroborate the results obtained from experimen-
tal measurements and to better understand the origin of the
various anisotropy parameters, we have performed CASSCF cal-
culations employing NEVPT2 correction on the X-ray crystal
structures of 1 and 2. The zero-field splitting parameters were
[29–32]
determined using the Hamiltonian [Eq. (3)]
0
eff
B
ꢀ
8
rameters obtained are as follows: t =9.1ꢂ10 s (1), and 4.3ꢂ
0
ꢀ
8
2
Z
2
X
2
Y
b
b
b
b
ð3Þ
10
s (2), and U /k =30 K (1) and 33 K (2). The values of U /
H
¼ D½S ꢀSðS þ 1Þ=3ꢂ þ EðS ꢀS Þ
eff
B
eff
ZFS
kB are very similar to that obtained for a simple complex
[
28]
[Co(PPh ) Br ] with U /k =37 K. The U /k values for 1 and
where D is the axial zero-field splitting (ZFS) parameter, E is
the rhombic ZFS parameter, and S, S , S , and S are the total
3
2
2
eff
B
eff
B
2
are close to each other as expected from their very similar D
X
Y
Z
values. Thus the amount of distortion in geometry of the cen-
spin and its x, y, and z components, respectively. The three di-
agonal components of the D tensor are D , D , and D and D
II
tral tetrahedral Co can be varied through change in donor
ZZ
YY
XX
atom of the ligand backbone which in turn affects the aniso-
is related to DZZ as D=3/2 D . Thus the overall sign of D can
ZZ
tropy parameter D and spin reversal barrier (U /k ). In our pre-
be obtained by analyzing the sign of D . The NEVPT2 method
eff
B
ZZ
ꢀ
1
vious investigation we had shown that the magnitude of
yielded a D value of ꢀ20.7 cm and E/D value of 0.14 for com-
plex 1 while for complex 2 these were found to be D=
II
single-ion D term for pseudo-mononuclear Co units can be al-
ꢀ
1
tered by trapping them within a predefined diamagnetic envi-
ꢀ24.0 cm and E/D=0.17 (Table 1). The obtained values are
close to those obtained experimentally. The Loewdin orbital-
compositions for the d-based CASSCF orbitals for complexes 1
and 2 are shown in Tables S5 and S6 in Supporting Informa-
tion. The ground state wave function shows slight mixing with
exited states as shown in Tables S7 and S8 in Supporting Infor-
[
10]
ronment. In the present study we have been able to demon-
strate that the use of donor atoms with larger atomic radii like
S in the same ligand backbone instead of N gives rise to lesser
variation in D and U /k values and vice-versa. Thus an effec-
eff
B
tive method for altering the various anisotropy parameters as-
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ic transitions (Table S6 in Supporting Information). For com-
Table 1. Experimental and computed Spin Hamiltonian Parameters for
Complexes 1 and 2.
plex 2 the contribution comes from Y !Y (55%) and Y !
1
3
2
Y (36%) transitions (Table S7 in Supporting Information). The
3
Complex
Experimental
Calculated (NEVPT2)
energy gap between the Y and Y orbitals and Y and Y
3
1
3
2
ꢀ
1
ꢀ1
D [cm
]
E/D
g
iso
D [cm
]
E/D
g
iso
orbitals are very similar in both the complexes which is expect-
1
2
ꢀ23.6
ꢀ24.3
0.03
0.00
2.29
2.32
ꢀ20.7
ꢀ24.0
0.14
0.17
2.28
2.29
ed from the similar torsional angle (q ) in complex 1 (100 and
t
1018) and complex 2 (97 and 998). Figure 10 represents the D
and g anisotropy axes of complexes 1 and 2. The structural pa-
rameter which controls the D value is the O-Co-O or torsion
[
33]
mation with the major configuration possessing about 93%
and 90% weightage for complexes 1 and 2 respectively. This
suggests that a single electron configuration cannot explain
the given ligand field state. Both the complexes are close to
D2d symmetry leading to less mixing at the ground state wave
function as compared to our previous work. Due to less varia-
angle while the E/D value is controlled by the interplanar or
[34]
dihedral angle (Table 2). Both complexes 1 and 2 have very
similar torsion angles which explains the closeness of the D
values. In our previous study, due to greater difference in the
torsion angle, the D values were also widely spaced. Thus in-
II
tion in the geometry around the central Co ion when compar-
Table 2. Structural parameters affecting the magnitude of D and E/D in
complexes 1 and 2.
ing complex 1 with 2, both of them show similar high weight-
age of the major configuration. This is in contrast to our previ-
ous study where a considerable difference in the weightage of
the major configuration was observed (41% and 62%).
Figure 9 represents the eigenvalue plots of the d-based
CASSCF orbitals for complexes 1 and 2.
ꢀ
1
Complex
Dihedral angle
, deg]
Torsional angle
[q , deg]
D [cm
]
E/D
[
q
d
t
1
2
78.05
75.71
O9-Co3-O3=101.43,
O8-Co5-O2=100.94
O8-Co5-O2=99.72,
O9-Co3-O3=97.26
ꢀ20.7
ꢀ24.0
0.14
0.17
The major contribution to the negative D parameter in com-
plex 1 comes from Y !Y (43%) and Y !Y (43%) electron-
2
3
1
3
Figure 9. Energy levels of d-based CASSCF orbitals of complex 1 (left) and complex 2 (right).
Figure 10. D and g anisotropy axes of complexes 1 (left) and 2 (right). Colour code: Co, pink; O, red; N, blue; S, yellow (C and H atoms are omitted for clari-
ty).
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troduction of larger donor atom S in place of N in the ligand
Choice of this model substrate to monitor the reaction origi-
nates from the presence of tertiary butyl groups on the cate-
backbone tends to relax the distortion of the {CoO } tetrahe-
4
dron brought about by the constrained environment produced
chol ring thereby reducing the oxidation potential while pre-
III
[36]
by the diamagnetic Co ions and effectively alters the various
venting further oxidation and ring-opening reaction.
Two
anisotropic parameters.
different solvent media MeOH and MeCN were used to com-
pare the reaction rate for the formation of 3,5-DTBQ at 401
Functional Behavior for catechol oxidation. Variety of cate-
chol molecules are found extensively in nature and used for
several functions, including neurotransmission, pigment forma-
tion, surface adhesion, protein crosslinking and formation of
beak and cuticle materials as well as microbial iron ion uptake
and 403 nm, respectively. Solutions of
1
and
2
(ꢃ1ꢂ
ꢀ
5
ꢀ1
10 molL ) were treated with 100 equiv of 3,5-DTBCH in
2
MeOH and MeCN respectively and time dependent UV-vis
spectra were recorded under aerobic conditions up to 50 min
at 5 min interval. Both the complexes show a reactivity pattern
very similar to each other (Figure 11). Control experiments
were performed using Co(ClO ) ·6H O and 100 equiv of
II/III
in siderophores. Till today laboratory synthesized Co com-
plexes have been comparatively less probed as synthetic imita-
tors for catechol oxidase like functions. The motivation to
model the enzyme active sites originates from the potential of
the aggregating fragments to act as efficient catalyst in order
to afford insight into the mechanistic pathways of the native
enzymes in the line of designing man made catalysts. Further
to this, in recent year opinions from clinical trials and epide-
miological studies support the hypothesis that estrogen con-
tributes to breast cancer and may be a pertinent agent. The
oxidative metabolites of estrogen have been implicated in
chemical carcinogenesis. Oxidation of the catechol metabolite
of estrone gives o-quinones that produce reactive oxygen spe-
cies (ROS) and damage DNA by adduct formation and oxida-
4
2
2
DTBCH , where no change was observed in absorption intensi-
2
ty even after 1 d.
Oxidation in MeOH. The change in spectral behavior upon
treatment with 3,5-DTBCH has been shown in Figures 11a and
2
11b and the oxidation reaction was followed up to 50 min
after addition of the substrate. Initially the absorption band for
the complexes appeared at ꢃ387 nm which shifted to higher
wavelength at ꢃ401 nm, with the generation of a shoulder at
309 nm after addition of the substrate 3,5-DTBCH . With time,
2
the intensity of the former peak gradually increases whereas
the shoulder peak gradually disappears with progress of the
reaction. The appearance and disappearance of the shoulder
at 309 nm can be explained by taking into consideration the
initial formation of catalyst-substrate adduct, which decompos-
[
35]
tion.
Herein we examined the ability of complexes 1 and 2 to pro-
duce catalytically active fragments in solution to oxidize 3,5-di-
[37]
tert-butylcatechol (3,5-DTBCH ) in air to the quinone form.
es with time upon generation of 3,5-DTBQ.
2
ꢀ
5
ꢀ1
Figure 11. Time dependent UV-vis spectral changes for Complexes 1 and 2 (conc. ꢃ1ꢂ10 molL ) upon addition of excess of (100 fold) 3,5-DTBCH
2
(conc.
ꢃ1ꢂ10ꢀ molL ) in MeOH (a and b) and in MeCN (c and d) at 298 K.
3
ꢀ1
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Oxidation in MeCN. The catalytic oxidizing power of 1 and
in air was studied and compared in MeCN medium to identi-
the formation of 3,5-DTBQ was monitored by UV-vis spectros-
copy by recording the change in absorption intensity with
time at 401 nm for MeOH and 396 nm for MeCN up to first
10 minutes of mixing. The reaction rates were calculated by in-
itial rate method and were analyzed by Michaelis–Menten
model of enzyme kinetics. Important kinetic parameters such
2
fy the solvent effect in the oxidation process. Upon treatment
of 1 and 2, having an absorption maximum at 380 nm, with
1
00 equivalents of DTBCH , the peaks were blue shifted to
2
ꢃ382 nm and ꢃ388 nm and the intensity increases with time
identifying both as catalytically active compounds giving reac-
tive fragments for the aerobic oxidation of DTBCH2 (Figur-
es 11c and 11d). The oxidation reaction was followed up to
as reaction rate (Vmax), the binding constant (K ) and catalytic
M
rate constant (k ) were extracted from the corresponding
cat
ꢀ1
Lineweaver–Burk plots of 1V
vs. 1/[S] (Figure 12 and
5
0 min after addition of the substrate using UV-vis spectropho-
Figure 13 insets) for all the four cases and are listed in Table 3.
The observed kinetic parameters are comparable to those
reported in the literature for species with Co···Co separations
tometry. With progress of the reaction the peaks were shifted
to higher wavelength with increase in intensity and become
stabilized at 396 nm. Similar to the MeOH case, a shoulder ap-
peared at ꢃ308 nm which gradually disappeared with the ad-
vancement of the reaction.
[38]
in our range. The turnover number (k ) is the maximum
cat
Kinetic Study for Catechol Oxidation in MeOH and MeCN.
Table 3. Kinetic parameters for the catalytic oxidation of 3,5-DTBCH
and 2 in MeOH and MeCN medium at 258C.
2
by 1
The kinetic behavior for the conversion of 3,5-DTBCH to 3,5-
2
DTBQ by 1 and 2 were carried out by monitoring the strong
quinone (3,5-DTBQ) absorption band in MeOH and MeCN as a
function of time (Figures 12 and 13). In both the two cases, a
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
Complex Solvent
V
max [Ms
]
K
M
[M]
k
cat [h
]
M
kcat/K [s m ]
ꢀ
7
ꢀ4
ꢀ4
ꢀ4
ꢀ4
1
2
1
2
MeOH
MeOH
MeCN
MeCN
7.86ꢂ10
5.95ꢂ10
1.95ꢂ10
1.41ꢂ10
4.95ꢂ10
4.24ꢂ10
1.39ꢂ10
3.02ꢂ10
283
214
70
158
140
140
47
fixed concentration (ꢃ1ꢂ10ꢀ molL ) of complex solution
5
ꢀ1
ꢀ7
ꢀ
7
was treated with varying concentration of 3,5-DTBCH (10 to
2
ꢀ7
51
100 equivalents). For all the complex-substrate combinations
2
Figure 12. Dependence of the reaction rates on the substrate concentration for the oxidation of 3,5-DTBCH catalyzed by complexes 1 and 2 in MeOH. The
Lineweaver–Burk plots (inset).
2
Figure 13. Dependence of the reaction rates on the substrate concentration for the oxidation of 3,5-DTBCH catalyzed by complexes 1 and 2 in MeCN. The
Lineweaver–Burk plots (inset).
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number of moles of the substrate that can be converted to
product per mole of catalyst in unit time. Determination of
tion). The catalytically active fragment gave rise to the inter-
mediate with the single electron oxidized substrate as {Co (m-
2
+
turnover number (k ) in the present case is important to rec-
L)(DTSQ)}
(C H Co N O S ; Calcd 705.1277) at m/z=
31 43 2 2 5 2
cat
ognize the ability of the {Co } fragments in solution to catalyze
705.4434 when 1:100 molar ratio mixture of 2 and 3,5-DTBCH2
in MeOH was used for analysis. The peaks at m/z=243.1371
2
the oxidation reaction. Spectrophotometric detection of H O2
2
+
based on the I - anion formation and its identification as triio-
and 463.2823 were assigned for {3,5-DTBQ-Na} and {(3,5-
3
+
dide molybdate complex was important to propose the course
of oxidation reaction. A comparison of kcat values of 1–2 with
previously reported cobalt complexes is given in Table S9 in
Supporting Information.
DTBQ) -Na} (Figure S8 in Supporting Information).
2
In MeCN medium exactly identical peaks were obtained at
m/z=371.1445 and 241.9867 for the different fragments ({H L-
3
+
2+
H} and {Co (m-L)} ) as mentioned earlier. The mass spectro-
2
+
Mass Spectroscopic Analysis for the Fragments in Solu-
tion. To identify the nature of the fragments present in the
two solvents used in this study and their involvement in the
catalytic process, we scrutinized corresponding mass spectral
patterns of 1 and 2 in MeOH and MeCN solutions, respectively,
in absence and presence of 3,5-DTBCH2.
scopic signature for the fragment {Co (m-L)(C H COO)(OH)}
2
2
5
(C H Co N O S ; Calcd 575.0131) appeared at m/z=575.0153
20
29
2
2
6 2
in MeCN with an additional mononuclear fragment {Co(m-
2
+
H L)(C H COO)(H O) -K}
(C H CoKN O S ; Calcd 288.0383)
20 34 2 7 2
2
2
5
2
2
at m/z=288.0119 (Figure S9 in Supporting Information).
As found in MeOH medium, the mixture of 2 and 3,5-DTBCH2
Complex 1. The HRMS data for 1 in MeOH showed peaks
in 1:100 molar ratio in MeCN medium resulted a low
+
(
Figure S3 in Supporting Information) at m/z=371.1479 corre-
intensity peak at m/z=705.1401 for {Co (m-L)(DTSQ)}
2
+
sponding to the protonated ligand {H L-H} (C H N O S ;
(C H Co N O S ; Calcd 705.1277). At m/z=649.2227, another
3
17 27
2
3
2
31 43
2
2
5 2
Calcd 371.1463). Another peak at m/z=241.9857 of high inten-
peak of comparatively higher intensity appeared due to {Co(m-
2
+
+
sity is due to the fragment {Co (m-L)}
(C H Co N O S ;
H L)(DTSQ)-H} (C H CoN O S ; Calcd 649.2175) (Figure S10
2
17 22
2
2
3
2
2
31 46
2
5 2
Calcd 241.9863) while the peaks at m/z=545.0029 and
in Supporting Information).
EPR measurements. To apprehend the involvement of Co
centers along with semiquinone based organic radical as inter-
mediate species during the catechol oxidation, the X-band EPR
spectral measurements were performed on 1 and 2 in MeOH
II
5
60.9990 of lower intensity arise from the fragments {Co (m-
2
+
L)(CH COO)-H} (C H Co N O S ; Calcd 545.0025) and {Co (m-
3
19 27
2
2
5
2
2
+
L)(CH COO)(OH)} (C H Co N O S ; Calcd 560.9975). Mixing of
3
19 27
2
2
6 2
3
,5-DTBCH and 1 in a molar ratio of 100:1 in MeOH resulted a
2
peak (Figure S4 in Supporting Information) at m/z=705.4401
and MeCN medium as well as in the presence of 3,5-DTBCH at
2
III
6
for the catalyst and partially oxidized substrate adduct {Co (m-
room temperature (Figure 14). Low-spin Co (3d ) in an octahe-
2
+
L)(DTSQ)} (C H Co N O S ; Calcd 705.1277). Whereas pres-
dral environment is diamagnetic and hence EPR inactive.
31
43
2
+
2
5 2
+
II
7
ence of {3,5-DTBQ-Na} and {(3,5-DTBQ) -Na} were diagnosed
Octahedral Co (3d ) can exist in low spin state (S=1/2) with
2
6
1
5
2
from the peaks at m/z=243.1371 and 463.2823.
t2g eg configuration and high spin state (S=3/2) with t eg
2g
II
In MeCN, 1 provided peak (Figure S5 in Supporting Informa-
tion) at m/z=371.1485 for the protonated ligand
configuration. Low-spin paramagnetic Co ions in C or D4h
4v
symmetry show EPR activity at room temperature due to the
presence of longer spin lattice relaxation time and display
small anisotropy in the g values. Whereas in the high-spin con-
(
C H N O S ; Calcd 371.1463). A peak of considerable intensity
17 27 2 3 2
2
+
at m/z=241.9867 is due to the dimeric fragment {Co (m-L)}
2
II
(
C H Co N O S ; Calcd 241.9863) while the fragment {Co (m-
figuration, the Co ions provide EPR signals only at very low
17
22
2
2
3
2
2
+
L)(CH COO)(OH)} (C H Co N O S ; Calcd 560.9975) docu-
temperatures with large anisotropy in the g-tensor due to
3
19 27
2
2
6 2
mented a low intensity peak at m/z=560.9986. An additional
peak at m/z=281.0037, of slightly lower intensity compared to
the dimeric fragment, not present in MeOH medium can be
most reasonably assigned to the monomeric fragment {Co(m-
short spin lattice relaxation time as a result of large orbital
[12]
contribution towards the ground state magnetic moment.
Absence of any signal in the spectra of 1 and 2 in MeOH and
MeCN solutions at room temperature was consistent with the
2
+
III
H L)(CH COO)(H O) -K}
(C H CoKN O S ; Calcd 281.0305).
presence of only ligand bound Co fragments. In the presence
2
3
2
2
19 32
2
7 2
In presence of 100 equivalent 3,5-DTBCH in MeCN, 1 showed
of 100 equivalents of 3,5-DTBCH , the MeOH solutions of 1 and
2
2
a low intensity peak (Figure S6 in Supporting Information) at
2 showed a sharp signal at g=2.0023 indicating at the forma-
tion of semiquinone radical during the oxidation process. The
signal was doubly split due to coupling with a single proton,
positioning the lone electron on C4 of the benzene ring. A hy-
perfine structure of fifteen lines (one or more lines were ob-
scured by strong radical signal) also appeared on both sides of
+
m/z=705.4387 for the dimeric intermediate {Co (m-L)(DTSQ)}
2
(
C H Co N O S ; Calcd 705.1277) following partial oxidation as
31 43 2 2 5 2
obtained in the case of MeOH. One more peak at m/z=
49.2134 of comparatively higher intensity was assigned
6
+
to
a
monomeric intermediate {Co(m-H L)(DTSQ)-H}
2
(
C H CoN O S ; Calcd 649.2175).
the radical signal with giso =2.0006 and A =11 gauss clearly
31
46
2
5
2
iso
II
III
Complex 2. In MeOH very similar peaks, as found for 1,
indicating at the formation of Co species by reduction of Co
during the catalytic cycle. Generally, the interaction of an
+
were obtained at m/z=371.1461 (protonated ligand {H L-H} )
3
2
+
II
59
and 241.9857 (fragment {Co (m-L)} ) for 2. Other different
peaks at m/z=559.0148 and 575.0154 were assigned for
unpaired electron on Co with a nuclear spin of I=7/2 ( Co,
I=7/2, 100% natural abundance) should give rise to an eight
line spectra. The appearance of fifteen line signal was due to
the additional interaction with an adjacent cobalt nucleus
through bridging phenoxido oxygen. The appearance of this
2
+
fragments {Co (m-L)(C H COO)-H}
(C H Co N O S ; Calcd
2
2
5
20 29
+
2
2
5 2
5
59.0182) and {Co (m-L)(C H COO)(OH)}
(C H Co N O S ;
20 29 2 2 6 2
2
2
5
Calcd 575.0131) respectively (Figure S7 in Supporting Informa-
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2
Figure 14. EPR spectra of 1 (left) and 2 (right) in MeCN and MeOH, respectively, and in the presence of 3,5-DTBCH at 298 K.
II
signal at room temperature suggests the generation of Co ion
in a low-spin state.
spectra for 1 and 2 in MeCN by taking into consideration two
59
1
II
Co nuclei, S= = (low spin Co ), g =2.0022, Aiso =11 gauss
2
iso
In MeCN, the spectra of 1 and 2 in presence of 100 equiva-
lents of 3,5-DTBCH2 showed no representative signal corre-
sponding to the formation of organic radical intermediate. This
might probably be due to the transient nature and very low
concentration of the formed radical species in this solvent
and average line width of 0.17 mT reproduced the experimen-
tal spectrum well with regard to the signal positions
(Figure 15).
Proposed Mechanism of Oxidation. Reduction of O into
2
H O is often proposed for the catalytic oxidation of catechol
2
2
II
medium. Formation of low-spin Co species was confirmed
with {Cu } fragments or discrete complex species. In the pres-
2
from the appearance of fifteen line signals with giso =2.0006
and Aiso =11 gauss at room temperature. Simulation of the
ent case recognition of H O by spectroscopic detection of
2
2
ꢀ
triiodide (I ) anion as triiodide molybdate complex at 352 nm
3
2
Figure 15. Experimental and simulated spectra for 1 (left) and 2 (right) in MeCN in the presence of 100 equivalent 3,5-DTBCH .
&
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ꢀ
1
ꢀ1
III
III
(
26 400m cm ), formed in the presence of H O from KI,
Hence, although the {Co Co (m-L)} species still catalyzes the
2
2
points to the formation of H O during the catalytic cycle (Fig-
oxidation of the substrate, the major contribution comes from
2
2
[
38]
III
ure S11 in Supporting Information). In contrast to the {Cu2}
the {Co (H L)(CH COO)(H O) } species (Scheme 2b). Subse-
2
3
2
2
II/III
based systems, the Co systems generally yield H O during
quent oxidation of DTSQ and formation of H O takes place by
2 2
2
[
39,40]
oxidation.
a process similar to that described earlier. The kinetic experi-
ments reveal that the kcat values in MeCN are much lower com-
pared to those in MeOH for both 1 and 2. This can be attribut-
ed to the predominance of the monomeric species in the cata-
lytic oxidation process in MeCN. The substrate 3,5-DTBCH pref-
erentially binds to the monomeric fragment driven by less
steric crowding and hydrogen bonding assistance from the
The HRMS and EPR analysis for 1 and 2 in absence and pres-
ence of 3,5-DTBCH in MeOH and MeCN lead to a mechanistic
2
pathway for the catalytic activity. In MeOH solution both 1 and
III
III
2
formed {Co Co (m-L)} as the predominant fragment which
serves as the active species for catalyzing the oxidation of 3,5-
DTBCH to 3,5-DTBQ. Upon addition of 3,5-DTBCH the dimeric
2
2
III
fragment possibly forms a 1:1 adduct with the substrate identi-
fied as the shoulder at 309 nm from UV-vis experiments.
Herein we propose the coordination of 3,5-DTBCH2 in its
dangling alcohol arm. The presence of a neighboring Co in
the dimeric species facilitates the electron transfer from 3,5-
DTBCH to the metal center. This is further supported by the fif-
teen line EPR spectrum suggesting considerable delocalization
III
mono-deprotonated form as 3,5-DTBCH to only one of the Co
[
20,41]
II
centers due to the generation of H O .
One electron oxida-
of the unpaired electron on the formed low-spin Co to the ad-
2
2
III
tion of the substrate accompanied by concomitant reduction
jacent Co through the bridging phenoxido oxygen. Such as-
III
II
of Co to Co results in the semiquinone radical bound inter-
sistance is not possible in case of the monomeric active spe-
cies. The fifteen line spectrum observed in MeCN for 1 and 2
arises from a minor contribution of the dimeric active species
in the oxidation process. These results demonstrate a probable
strategy for enhancing the catalytic potency of a synthetic cat-
alyst through introduction of a “spectator” metal ion center
not involved in binding the substrate molecule. The observed
slight difference in kcat values between 1 and 2 in both MeOH
and MeCN arises from the difference in concentration of the
formed active fragments in solution.
II
III
mediate species {Co Co (m-L)}(DTSQ)} (Scheme 2a). Subsequent
II
III
coordination of O to the Co center probably generates a Co -
2
superoxo species. Loss of a second electron from the bound
·ꢀ
semiquinone to the O2 lead to the smooth formation of 3,5-
III
III
DTBQ and H O regenerating the {Co Co (m-L)} active species.
2
2
Interestingly, in MeCN the additional monomeric fragments,
Co(H L)(CH COO)(H O) } and {Co(H L)(C H COO)(H O) } could
{
2
3
2
2
2
2
5
2
2
be detected for 1 and 2 respectively. While the dimeric species
Co (m-L)} still predominates in solution, the monomers have a
{
2
considerable abundance as is evident from the HRMS relative
Probable Aggregation Pathway. Structurally characteriza-
tion of the single crystals of the end products established in
this work and their disintegrations in solution visibly point to
the presence of the building fragments necessary for the ag-
gregation processes. Based on the types of the fragments
peak intensities. In the presence of 3,5-DTBCH these form 1:1
2
adduct with the substrate followed by one electron oxidation
II
giving rise to the intermediate {Co (H L)(DTSQ)} which greatly
2
II
III
predominates over the dimeric intermediate {Co Co (L)(DTSQ)}.
Scheme 2. Proposed catalytic pathway for 1 and 2 in (a) MeOH and MeCN (Minor contribution) and (b) MeCN (Major contribution). Only bonds to ligand and
substrate has been shown, remaining sites are occupied by solvent molecules to fulfill octahedral coordination geometry of the metal ion centers.
Chem. Asian J. 2019, 00, 0 – 0
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III
found in the mass spectral analysis, it is possible to propose
aggregation process from solution and during crystallization
{Co (m-L)(RCOO)(OH)} behave as a O,O donor bidentate metal-
2
II
lo-ligand to trap the central Co ion for aggregation
(Scheme 3). The identification of different fragments from
(Scheme 4).
HRMS measurements of 1 and 2 in MeCN solvent media re-
vealed the involvement of these species in solution for the
self-aggregation process during solid state isolation and crys-
tallization.
Conclusions
Two structurally unique [Co ] complexes were synthesized to
5
establish the usefulness of the chosen ligand system to bind
III
II
four Co and one Co centers. Four thioether sulfur atom
bound [Co ] complexes described in this paper represented a
5
new family of “mixed-valent-mixed-geometry” homometallic co-
ordination aggregates. Evolution of the structural networks in
1
and 2 resulted from spontaneous assembly of diamagnetic
III
III
{
Co (m-L)(RCOO)(OH)} units (obtained from initial {Co (m-L)}
2 2
II
units) through entrapment of a central Co ion which remained
in distorted pseudo-T coordination geometry. The amount of
d
distortions from ideal tetrahedral geometry was determined
from Houser’s geometry index t values of 0.83 and 0.82, en-
4
dorsing almost identical faintly compressed tetrahedral geome-
try in both cases. The ligand H L has been responsible for the
3
coordination of two types of Co ions in these unusual struc-
tures. Both complexes 1 and 2 showed a relatively high easy-
ꢀ
1
axis magnetic anisotropy (D/hc=ꢀ23.6 (1) and ꢀ27.0 cm (2))
and dynamic magnetic data confirmed that both the com-
plexes behave as field-induced single-molecule magnets with
spin reversal barrier U /k =30 K (1) and 33 K (2), and relaxa-
eff
B
ꢀ
8
ꢀ8
tion time t =9.1ꢂ10 s (1), and 4.3ꢂ10 s (2), respectively.
0
The use of larger donor atom S lead to less variation in D and
U /k values in comparison to N used in our previous study.
eff
B
Theoretical calculations support the experimental findings.
Thus an efficient methodology for varying the anisotropic pa-
II
rameters of a pseudo tetrahedral Co ion trapped within the
III
constraint environments of diamagnetic Co based units have
Scheme 3. Trapping of central cobalt ion through aggregation of dinuclear
fragments in 1 and 2.
been established through simple variation of donor atoms. In-
terestingly the disintegration of 1 and 2 in solution gave rise
to catalytically active molecular fragments suitable for the oxi-
III
III
Fragments like {Co (m-L)} and {Co (m-L)(RCOO)(OH)} in a
dation of the prototypical substrate 3,5-DTBCH with distinct
2
2
2
step wise manner were utilized to gather the available ancillary
solvent effects. Kinetic readings on the solvent-dependent oxi-
dation reaction established that both the complexes exhibited
lower kcat values in MeCN compared to MeOH. ESI-MS (+ve) of
II
bridges and the Co ion in central position from Co(-
ClO ) ·6H O used during synthesis. The obtained fragments
4
2
2
Scheme 4. Synthesis of HAET and H
3
L.
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III
13
the complexes in solution showed that dimeric {Co (m-L)} type
tached with N atom), 2.23 ppm (3H, methyl-H). C NMR (100 MHz,
2
(
CD ) SO): d=161.94 (imine C), 159.98–121.27 (Ar C), 61.45 (methyl-
fragments were the key catalytic species in MeOH while in
3 2
ene C attached to imine N), 59.64 (methylene C attached to OH
group), 34.50–32.97 (methylene C attached to S), 20.35 ppm
MeCN the predominant active species was monomeric
III
{
Co (H L)(RCOO)(H O) } (R=-CH (1), -C H (2)). A dimeric spe-
2
2
2
3
2
5
(methyl C).
cies was found to be more efficient at catalyzing the two elec-
II
III
[
Co Co L (m-OH) (m -O CCH ) ](ClO ) ·H O (1). To a stirred MeCN
4 2 2 1,3 2 3 2 4 4 2
tron oxidation of 3,5-DTBCH to 3,5-DTBQ as compared to a
2
solution (15 mL) of H L (approx. 0.37 g, 1 mmol) a MeCN solution
3
monomeric species due to the assistance provided by the adja-
(
10 mL) of Co(ClO ) ·6H O (0.913 g, 2.5 mmol) was added. After
4 2 2
III
cent Co .
1
5 min of stirring, neat NEt (0.101 g, 1 mmol) was added followed
3
by solid sodium acetate (0.164 g, 2 mmol). The final reaction mix-
ture was stirred for 45 min and refluxed for 1 h. After cooling to
room temperature the deep brown solution was filtered and kept
in air for slow evaporation. After 18 days, brown colored block
shaped single crystals of 1 were obtained. Yield: 0.535 g, (67%
Experimental Section
Materials. The chemicals used here were obtained from the follow-
ing sources: 2-marcaptoethanol, 2-chloroethylamine hydrochloride
from Alfa Aesar, cobalt(II) carbonate, sodium acetate from SD Fine-
Chem Ltd., India and NEt3 from Merck, India. Hydrated cobalt(II)
perchlorate salt was freshly prepared by treating hydrated cobalt(II)
ꢀ1
w.r.t. H L). Anal Calcd. for C H Cl Co N O S (1597.54 gmol ): C,
3
38 56
4
5
4
29 4
2
8.57; H, 3.53; N, 3.51. Found: C, 28.71; H, 3.56; N, 3.49. Selected
FT-IR bands: (KBr, vs.=very strong, br=broad, s=strong, m=
medium, w=weak): n˜ =3412 (br), 1648 (m), 1576 (m), 1430 (m),
carbonate with 1:1 aqueous HClO solution followed by crystalliza-
tion. Sodium propionate was obtained by reacting propionic acid
4
ꢀ
1
ꢀ1
ꢀ1
1
090 (vs.), 626 cm (m). UV-vis spectra [lmax, nm (e, Lmol cm )]:
(
(
MeCN solution) 620 (506), 598 (564), 556 (702), 382 (7300), 249
54.9ꢂ10 ).
(
11.1 g, 0.15 mol) with solid sodium hydroxide (6.0 g, 0.15 mol) in
3
water, followed by concentration and crystallization on a water
bath. 2,6-diformyl-4-methylphenol was prepared following a litera-
ture procedure with modification. All other chemicals and sol-
vents used in this work were reagent-grade and were used as re-
ceived without further purification.
II
III
[Co Co L (m-OH) (m1,3-O CC H ) ](ClO ) ·H O (2). Complex 2 was
4 2 2 2 2 5 2 4 4 2
[
42]
prepared following a similar procedure as described above for 1
using sodium propionate (0.192 g, 2 mmol) instead of sodium ace-
tate. Brown colored block shaped single crystals suitable for X-ray
analysis was obtained from the reaction mixture after 20 days.
Yield: 0.478 g, (59% w.r.t. H L). Anal Calcd. for C H Cl Co N O S
Caution! Metal complexes of organic ligands with perchlorate
counter anions are potentially explosive. Although we did not face
any problems with the reported compounds, it is advisable to pre-
pare the materials in small amount and should be handled with ex-
treme care.
3
40 56
4
5
4
29 4
ꢀ
1
(
3
1621.56 gmol ): C, 29.63; H, 3.48; N, 3.45. Found: C, 29.72; H,
.46; N, 3.46%. Selected FT-IR bands (KBr, s=strong, vs.=very
strong, m=medium, br=broad): n˜ =3421(br), 1647(m), 1577(m),
ꢀ
1
1
459(m), 1090(vs.), 625 cm (m). UV/Vis spectra [lmax, nm (e,
Synthetic Protocols. HAET (2-(2-aminoethylthio)ethanol). The thi-
oether incorporated amine alcohol was obtained from condensa-
tion of 2-marcaptoethanol and 2-chloroethylamine following a
ꢀ1
ꢀ1
Lmol cm )] (MeCN solution): 619 (515), 596 (578), 557 (709), 378
3
(
6300), 249 (46.9ꢂ10 ).
[
43]
Physical Measurements. Elemental analysis was performed by a
PerkinElmer model 240C elemental analyzer. A Shimadzu UV 3100
UV/Vis-NIR spectrophotometer was used for electronic spectra and
a PerkinElmer RX1 spectrometer for the FT-IR spectra. Powder X-ray
diffraction (PXRD) patterns were measured on a BRUKER AXS X-ray
diffractrometer (40 kV, 20 mA) using CuꢀKa radiation (l=
modified literature procedure.
2-marcaptoethanol (0.780 g,
1
0 mmol) was added drop wise during 30 min to a 30% freshly
prepared aq. NaOH solution (15 mL) with stirring and then heated
to 508C. Next an aq. solution of 2-chloroethylamine hydrochloride
(
1.159 g, 10 mmol) was added drop wise to the above solution
during a time period of 2 h. Finally the mixture was stirred for an-
other 2 h period at 508C to give a pale yellow solution. The result-
ing solution was concentrated under reduced pressure, and then
dissolved in absolute EtOH and filtered over glass frit to remove
the precipitate. Removal of solvent from the filtrate under reduced
1
.5418 ꢄ) within 5–508 (2q) range and a fixed-time counting of 4 s
at 258C.
X-ray crystallography. X-ray diffraction data on appropriate single
crystals of 1 and 2 were collected on a Bruker SMART APEX-II CCD
X-ray diffractometer, equipped with a graphite monochromator
and X-ray source of MoꢀKa radiation (l=0.71073 ꢄ). wꢀscan at
293 K was used giving 5 s per frame. Space group determination,
data integration and reduction were performed with XPREP and
1
pressure afforded HAET as a pale yellow liquid. H NMR (400 MHz,
(
CD ) SO): d=2.47–2.53 (4H, -CH ), 2.63 (2H, methylene CH at-
3 2 2 2
tached with N atom), 3.48 ppm (2H, methylene CH attached with
2
1
3
O atom). C NMR (100 MHz, (CD ) SO): d=34.31, 35.99 (methylene
3
2
[
44]
C attached with S), 42.18 (methylene C attached with N atom),
1.43 ppm (methylene C attached with O atom).
SAINT software.
The Structures were solved using the direct
[45]
6
method through the SHELXS-2014 and refined with full-matrix
least squares on F using the SHELXL (2014/7) program package
incorporated into WINGX system Version 2014.1. Multiscan em-
pirical absorption corrections were applied to the data using the
2
[46]
H3L (2,6-bis-[{2-(2-hydroxyethylthio)ethylimino}methyl]-4-meth-
ylphenol). At room temperature, 2-(2-aminoethylthio)ethanol
[47]
(
1.21 g, 10 mmol) was added drop wise to 4-methyl-2,6-diformyl-
[
48]
program SADABS. All non-hydrogen atoms were refined aniso-
tropically. The H atoms were introduced in calculated positions
and refined with fixed geometry and riding thermal parameters
with respect to their carrier atoms. The locations of the heaviest
atoms (Co) were determined easily, and the O, N, and C atoms
were subsequently determined from the difference Fourier maps.
Crystallographic diagrams were presented using DIAMOND soft-
ware. Information of concerned X-ray data collection and struc-
ture refinement of the compounds is summarized in Table 4 and in
Table S1 in Supporting Information. CCDC 1857612 and 1857611
contain the supplementary crystallographic data for this paper.
phenol (0.820 g, 5 mmol) in MeOH (10 mL) with stirring and finally
the whole mixture was refluxed for 2 h. Removal of solvent under
vacuum yielded a yellow oily mass, which was characterized by
FTIR and NMR spectroscopy. The oily product thus obtained was
used directly for reactions with metal ion salts without further pu-
rification. FT-IR (KBr pellet): n˜ =3368 (br), 2918 (m), 1637(s), 1600
ꢀ1
(
w), 1458 (m), 1219 (w), 1045 (w), 1010 (w), 871 (w), 772 cm (s).
[49]
1
H NMR (400 MHz, (CD ) SO): d=8.58 (2H, imine-H), 7.49 (2H, Ar-
3
2
H), 2.81–3.74 (8H, methylene CH attached with S atom), 3.52 (4H,
2
methylene CH attached with O atom), 3.74 (4H, methylene CH at-
2
2
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MeCN using 3,5-di-tert-butylcatechol (3,5-DTBCH ) as a model sub-
2
Table 4. Crystal Data and Structure Refinement Details for 1 and 2.
strate. The oxidation of 3,5-DTBCH to 3,5-di-tert-butylquinone (3,5-
2
DTBQ) was followed on a Shimadzu UV 3100 UV-vis-NIR spectro-
photometer. The reactions were monitored for the growth of qui-
none lmax at ~400 nm. Kinetic studies were performed in MeOH
and MeCN medium, where the solutions of the complexes of con-
stant concentrations (~1.0ꢂ10ꢀ m) were reacted with varying
parameters
Formula
1
2
C
38
H
56Cl
4
Co
5
N
4
O
29
S
4
C
40 4 5 4 29 4
H56Cl Co N O S
ꢀ
1
F.W. [gmol
]
1597.54
Orthorhombic
Fdd2
1621.56
Monoclinic
P2 /c
Red
0.30ꢂ0.15ꢂ0.14
23.278(5)
12.529(3)
25.635(5)
90.00
108.458(9)
90.00
7092(2)
4
1.517
1.489
3284
296(2)
84812
0.1154
14763
8858
crystal system
space group
Crystal color
Crystal size [mm ]
a [ꢄ]
5
1
amounts of 3,5-DTBCH (10 to 100 equiv) and the spectral changes
Red
2
3
were monitored with time at the lmax of quinone. The data were
treated using the initial rate method and the rate was obtained
from the slope of the absorbance versus time plot. Kinetic analyses
were executed following the Michaelis–Menten method, and im-
portant kinetic parameters were extracted from the Lineweaver–
Burk plots.
0.35ꢂ0.15ꢂ0.12
44.596(3)
50.035(3)
13.0237(8)
90
90
90
29060(3)
16
1.459
1.453
12944
297(2)
104086
0.1650
18145
11264
690
b [ꢄ]
c [ꢄ]
a [deg]
b [deg]
g [deg]
3
Detection of H O . Generation of H O from O of air during the
2 2 2 2 2
V [ꢄ ]
oxidation of 3,5-DTBCH2 was investigated by monitoring the
Z
D
-
3
ꢀ
c
[gcm ]
growth of the characteristic absorption band for triiodide (I ) ion
3
ꢀ
1
ꢀ1
ꢀ1 [54,55,56]
m [mm
F(000)
T [K]
]
(lmax =353 nm, e=26000 Lmol cm ).
The reaction mixture
was acidified with H SO (1ꢂ10 m) to pH 2 after 10 min of mixing
ꢀ3
2
4
to quench the reaction. The formed quinone was extracted with
CH Cl and the aqueous layer was treated with 10% KI solution.
Total reflns
R(int)
Unique reflns
Observed reflns
Parameters
2
2
ꢀ
+
The reaction can be represented as H O +2I +2H !2H O+I .
2
2
2
2
The I liberated combines with excess iodide ions to form triiodide
2
ꢀ
ꢀ
ꢀ
(
I ) ions following the reaction I (aq)+I !I . Addition of ammo-
671
0.0998, 0.2940
1.012
3.234, ꢀ1.745
3
2
3
nium molybdate (3%) solution in a catalytic amount accelerates
R
1
; wR
2
2
(I>2s(I))
0.0814, 0.2316
1.121
1.756, ꢀ0.959
[57]
GOF (F )
the reaction. Blank experiments were also carried out as atmos-
pheric oxygen can oxidize I ions present in solution.
ꢀ
Largest diff peak and hole
ꢀ
3
[eꢄ
]
CCDC No.
1857612
1857611
Supporting Information
2
2 1/2
R
1
2
=S(kF
o
jꢀjF
c
k)/SjF
o
j. wR
2
=[Sw(jF
o
jꢀjF
c
j) /Sw(F
o
) ]
.
w=0.75/
2
X-ray crystallographic data in CIF format, Figures S1–S11, Ta-
bles S1–S8. CCDC 1857612 and 1857611 contain the supplementa-
ry crystallographic data in CIF format for complexes 1 and 2.
(
s (F )+0.0010F
o
o
).
These data are provided free of charge by The Cambridge Crystal-
lographic Data Centre.
Acknowledgements
Magnetic measurements. The temperature-dependent (T=1.9–
3
00 K, B=0.2 T) and field-dependent (B=0–9 T, T=2, 5, 10 K) mag-
netization measurements were performed on PPMS Dynacool
Quantum design Inc., San Diego, CA, USA) using a VSM option
M.D. and D.B. are grateful to IIT Kharagpur for their research
fellowship. We are also thankful to DST, New Delhi, for provid-
ing the Single Crystal X-ray Diffractometer facility in the De-
partment of Chemistry, IIT Kharagpur under FIST program. EPR
facility by CRF, IIT Kharagpur is duly acknowledged. M.D. and
D.B. are thankful to Mr. Sudipto Khamrui for his help with EPR
measurements. Z.T. and J.V. thank the Ministry of Education,
Youth and Sports of the Czech Republic (grant No. LO1305).
(
module. Dynamic magnetic properties were studied by measuring
alternating current (AC) susceptibility on a MPMS XL-7 SQUID mag-
netometer in B =0 and 0.1 T). The magnetic data were corrected
for the diamagnetism of the constituent atoms and for the dia-
magnetism of the sample holder. The experimental data were
fitted using the PHI program package.
Computational details. All calculations were performed using the
dc
[26]
[50]
ORCA 4.0.0 program package on the X-ray crystal structures.
def2-TZVP basis set was employed on Co, N, O and S while def2- Conflict of interest
[51–53]
SVP basis set was employed on C and H.
State averaged com-
plete active space self-consistent field (SA-CASSCF) calculations
were performed on 1 and 2 using these basis sets with the active
space comprising of seven d electrons of Co in five d orbitals i.e.,
CAS(7,5). 10 quartet and 40 doublet roots were computed using
this active space in the CI (configuration interaction) step. N-elec-
tron valence perturbation theory (NEVPT2) was employed on the
CASSCF wave function to account for the dynamic correlation. The
zero-field splitting parameters (D and E) were calculated from both
second-order perturbation theory and the modern effective Hamil-
The authors declare no conflict of interest.
Keywords: catecholase activity
·
cobalt
·
magnetic
anisotropy · single-molecule magnets · thioether ligand
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Version of record online: && &&, 0000
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These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
Manisha Das, Dipmalya Basak,
Zden eˇ k Trꢀvnꢁ cˇ ek, Jꢀn Van cˇ o,
Debashis Ray*
&& – &&
Entrapment of a Pseudo-Tetrahedral
II
Co Center by Thioether Sulfur Bound
{
Co (m-L)} Fragments: Synthesis, Field-
2
Induced Single-Ion Magnetism and
Catechol Oxidase Mimicking Activity
The magnetic properties and catalytic
activity of “mixed-valent-mixed-geome-
atom S leads to less variation in D and
U /k values in comparison to N. Cata-
eff
B
try” [Co ] aggregates containing a
lytically active fragments show catechol
oxidase behavior with assistance from a
5
II
pseudo-tetrahedral Co has been dis-
III
cussed. The use of the larger donor
“spectator” Co center.
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ꢃ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!