Rare azido and hydroxido bridged tetranuclear Co(ii) complexes of a polynucleating Mannich base ligand with a defect dicubane core: structures, magnetism and phenoxazinone synthase like activity

Article abstract of DOI:10.1039/c8nj04750b

Three novel tetranuclear cobalt(ii) complexes [Co₄L₂(μ_1,1,1-N₃)₂(N₃)₂] (1), [Co₄L₂(μ₃-OH)₂(NCS)₂]·4CH₃CN (2) and [Co₄L₂(μ₃-OH)₂(NCSe)₂]·2CH₃CN (3) have been synthesized on reacting a Mannich base ligand, N,N′-dimethyl-N,N′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)ethylenediamine (H₂L), with cobalt(ii) acetate tetrahydrate in the presence of azide, thiocyanate and selenocynate salts, respectively. Single crystal X-ray structural analyses showed that these discrete tetranuclear Co(ii) complexes possess defect dicubane cores with μ_1,1,1-N₃ (for 1) and μ₃-OH bridges (for 2 and 3). Magnetic susceptibility measurements showed prominent intramolecular ferromagnetic interactions for 1 (J₁/k_B = ?19 K, J₂/k_B = 38 K) and dominant antiferromagnetic interactions for 2 and 3 (J₁/k_B = 4.7 K, J₂/k_B = ?34 K for 2 and J₁/k_B = 4.4 K, J₂/k_B = ?12 K for 3). All three complexes (1-3) exhibited phenoxazinone synthase-like catalytic activity towards the aerobic oxidation of o-aminophenol. The turn over numbers (K_cat) for the aerobic oxidation of o-aminophenol were calculated to be 500.4, 508.9 and 511.2 h^?1 for complexes 1-3, respectively. The mechanism of phenoxazinone synthase-like catalytic activity has also been proposed for these tetranuclear Co(ii) catalysts with the help of mass spectral analysis.

Full text of DOI:10.1039/c8nj04750b

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Rare azido and hydroxido bridged tetranuclear
Co(II) complexes of a polynucleating Mannich
base ligand with a defect dicubane core:
structures, magnetism and phenoxazinone
synthase like activity†
Cite this: New J. Chem., 2018,
42, 19377
b
a
Avijit Das,a Soumyabrata Goswami
and Ashutosh Ghosh
*
Three novel tetranuclear cobalt(II) complexes [Co4L2(m1,1,1-N3)2(N3)2] (1), [Co4L2(m3-OH)2(NCS)2]ꢀ4CH3CN
(2) and [Co4L2(m3-OH)2(NCSe)2]ꢀ2CH3CN (3) have been synthesized on reacting a Mannich base ligand,
N,N0-dimethyl-N,N0-bis(2-hydroxy-3-methoxy-5-methylbenzyl)ethylenediamine (H2L), with cobalt(II) acetate
tetrahydrate in the presence of azide, thiocyanate and selenocynate salts, respectively. Single crystal X-ray
structural analyses showed that these discrete tetranuclear Co(II) complexes possess defect dicubane cores
with m1,1,1-N3 (for 1) and m3-OH bridges (for 2 and 3). Magnetic susceptibility measurements showed
prominent intramolecular ferromagnetic interactions for 1 (J1/kB = ꢁ19 K, J2/kB = 38 K) and dominant
antiferromagnetic interactions for 2 and 3 (J1/kB = 4.7 K, J2/kB = ꢁ34 K for 2 and J1/kB = 4.4 K, J2/kB
=
ꢁ12 K for 3). All three complexes (1–3) exhibited phenoxazinone synthase-like catalytic activity towards
the aerobic oxidation of o-aminophenol. The turn over numbers (Kcat) for the aerobic oxidation of
o-aminophenol were calculated to be 500.4, 508.9 and 511.2 hꢁ1 for complexes 1–3, respectively. The
mechanism of phenoxazinone synthase-like catalytic activity has also been proposed for these tetranuclear
Co(II) catalysts with the help of mass spectral analysis.
Received 18th September 2018,
Accepted 29th October 2018
DOI: 10.1039/c8nj04750b
rsc.li/njc
discrete homo- or heterometallic polynuclear complexes of
different nuclearities with several metal ions.6a,9,10 For example,
Introduction
The design and synthesis of discrete polynuclear metal complexes the complexes of Zn(II) and M(II)–Ln(III) are dinuclear, those of
have attracted the attention of chemists mainly because of their Cu(II) are trinuclear whereas complexes of Cd(II) and Ni(II) are
interesting magnetic properties, efficient catalytic activities, tetranuclear. The coordination mode of this flexidentate ligand
sensing ability, luminescent behaviours etc.1–6 Various factors is found to be hexa-, hepta- or octadentate in the complexes
such as coordination geometry and hardness/softness of the depending upon the coordinative requirement of the metal ions.
metal ions, synthetic conditions (i.e., solvent, temperature, pH In the literature, there are quite a few complexes of cobalt ion
etc.), ligand : metal ratios, flexibility, denticity, and the nature of with N,O donor Schiff/Mannich base ligands11 but to date, no
the donor atoms of the ligands are to be taken into account complex of Co(II) or Co(III) of this polynucleating ligand is
to obtain the desired polynuclear structures.7,8 The polytopic reported. However, various other N,O donor ligands are known
Mannich base ligand, [H2L = N,N0-dimethyl-N,N0-bis(2-hydroxy-3- to produce discrete polynuclear complexes with cobalt ions.12
methoxy-5-methylbenzyl)ethylenediamine] is known to produce These complexes consist of the cobalt ion in +3 or +2 or mixed
valence (+3/+2) states, depending upon the nature of the ligand
and methods of syntheses. Among these complexes, tetranuclear
complexes with a Co4O4 core are relatively rare. The tetranuclear
a Department of Chemistry, University College of Science, University of Calcutta, 92,
A. P. C. Road, Kolkata 700009, India. E-mail: ghosh_59@yahoo.com
b Department of Chemistry, Amity Institute of Applied Sciences (AIAS), Amity
University Kolkata, Major Arterial Road, Action Area II, Rajarhat, Newtown,
Kolkata 700156, India
structures can exist in different types of cores but the most
common ones are the cubic cores which include cubane,
incomplete cubane, dicubane and incomplete or defective
dicubane.12c–14 The literature shows that the reported Co4O4
cores of N,O donor ligands can possess any of these structures
and in these species also cobalt ions can be in mixed valence
(+3/+2) or in single valence (+2 or +3) states.12c,15,16
† Electronic supplementary information (ESI) available: UV, IR, mass spectra,
table of bond parameters of complexes 1–3, catalytic activity parameter and Kcat
values of the complexes as well as the magnetization plot of complexes 1–3. CCDC
1863132–1863134 for complexes 1–3 respectively. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c8nj04750b
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The magnetic properties of Co(II) complexes are of interest of material should be prepared, and it should be handled
due to a considerable orbital contribution to the effective with care.
magnetic moment arising from the intrinsic orbital angular
momentum in the octahedral ground state of high-spin Co(II), Synthesis of ligand H2L
the spin–orbit coupling and generation of interesting aniso-
The Mannich base ligand H2L was prepared by following
tropic systems through its d7 configuration.17 As a result,
standard method.6a,9,10 10 mmol of N,N0-dimethylethylene-
mono- or polynuclear Co(II) complexes are a popular choice
for in-depth study of magnetic interactions via short bridges
diamine (1.1 mL) was stirred with 20 mmol of formaldehyde
(0.6 mL) in 30 mL methanol for 30 min. Then, 20 mmol of
2-methoxy-4-methylphenol (2.5 mL) was added to it and the
and also for the magnetic materials showing SMM or SIM
behaviour.18 Moreover, cobalt complexes are known to mimic
resulting solution was refluxed for about 6 h. On cooling
various biological processes.19 One such process is phenoxazinone
the resulting solution, a white coloured solid separated out.
synthase (PHS) like activity. PHS is a pentanuclear copper
The solid product was filtered and washed with methanol.
containing enzyme which is found naturally in the bacterium
Yield 2.486 g (64%). Anal. calc. for C22H32N2O4 (388.5),
Streptomyces antibiotus and medicinally used for the treatment
calculated C, 68.01; H, 8.30; N, 7.21; found C, 68.15; H, 8.21;
of different types of tumors. It is well known that PHS catalyzes
N, 7.32. IR: n(O–H) = 3436 cmꢁ1, n(C–O) = 1239 cmꢁ1. UV/Vis:
the oxidative coupling of 2-aminophenol to the phenoxazinone
lmax = 285 nm. ESI-MS (positive ion mode, CH3OH) calculated.
chromophore at the final step for the biosynthesis of actino-
mycin D.3a,12a The cobalt complexes have also been found to be a
good catalyst for this process and to date, the PHS activities of a
few mononuclear cobalt(II)/cobalt(III) complexes, three dinuclear
cobalt(III) complexes and one mixed valence trinuclear cobalt(II)/(III)
complex are reported.3a,12a,19,20
m/z 389.32, found 389.31 (100%, [H2L + H]+) (Fig. S1, ESI†).
1H NMR (300 MHz, DMSO-d6, ppm): d 6.63 (s, 2H), d 6.49
(s, 2H), d 3.69 (s, 6H), d 3.53 (s, 6H), d 2.53 (s, 4H), d 2.14 (s, 6H),
d 2.12 (s, 6H) (Fig. S2, ESI†).
Synthesis of the complex, [Co4L2(l1,1,1-N3)2(N3)2] (1)
In this paper, we report the syntheses, crystal structures,
magnetic properties and phenoxazinone synthase-like catalytic
activities of three cobalt(II) complexes, [Co4L2(m1,1,1-N3)2(N3)2] (1),
[Co4L2(m3-OH)2(NCS)2]ꢀ4CH3CN (2) and [Co4L2(m3-OH)2(NCSe)2]ꢀ
2CH3CN (3) derived from the Mannich base ligand, [H2L =
N,N0-dimethyl-N,N0-bis(2-hydroxy-3-methoxy-5-methylbenzyl)ethyl-
enediamine]. Single crystal X-ray diffraction reveals that all
three complexes possess a similar defect dicubane tetranuclear
structure. In complex 1, two rare m1,1,1-N3 bridges and in
complexes 2 or 3, two m3-OH bridges hold the metal ions
together. The magnetic properties of these complexes have also
been explored in detail. Complex 1 shows prominent intra-
molecular ferromagnetic interactions whereas 2 and 3 show
dominant antiferromagnetic interactions which have been
explained considering the bridging angles between the Co(II)
centres. All three complexes (1–3) exhibit quite high biomimetic
phenoxazinone synthase-like activity under aerobic atmosphere.
Mass spectral analyses have been performed to have an idea
about the possible intermediates of this catalytic reaction and
An acetonitrile solution of cobalt acetate tetrahydrate (2 mmol,
0.498 g) was added to a 25 mL acetonitrile solution of ligand
H2L (1 mmol, 0.388 g) in a beaker. Then 5 mL of 1 : 1 aceto-
nitrile/water (v/v) solution of sodium azide (2 mmol, 0.130 g)
was added dropwise to the reaction mixture. The resulting
solution was stirred for 2 h at room temperature. The blue
colour-solution was filtered and kept in the refrigerator for
crystallization. Block-shaped crystals suitable for structural
studies separated out from the solution after a few days.
Yield 0.423
g (73%). Anal. calc. for C44H60Co4N16O8
(1176.80). Calculated C, 44.91; H, 5.14; N, 19.04; found C,
44.83; H, 5.05; N, 19.12. IR: n(C–H) = 2863 cmꢁ1, n(N3 ) =
ꢁ
2064 cmꢁ1, n(C–O/phenolate) = 1256 cmꢁ1, n(C–N) = 1158 cmꢁ1
(Fig. S3, ESI†). UV/Vis: lmax = 244 and 295 nm. ESI-MS (positive
ion mode, CH3OH) cald m/z 446.16, found 446.17 [CoL + H]+,
cald. m/z 468.14, found 468.15 [CoL + Na]+ (Fig. S6, ESI†).
Synthesis of the complex [Co4L2(l3-OH)2(NCS)2]ꢀ4CH3CN (2)
subsequently to develop a probable mechanism. To the best of our Cobalt acetate tetrahydrate (2 mmol, 0.498 g), dissolved in
knowledge, these are the first tetranuclear cobalt(II) complexes of 20 mL of acetonitrile, was added to acetonitrile solution
which phenoxazinone synthase like activity is explored.
(20 mL) of H2L (1 mmol, 0.388 g) with constant stirring, and
then 5 mL of 1 : 1 (v/v) H2O–MeCN solution of NH4SCN (0.076 g,
1 mmol) was added drop wise to the solution at room
temperature. The resulting solution was stirred for a further
3 h. The blue colour needle shaped single crystals suitable for
X-ray diffraction were obtained after few days by slow evapora-
Experimental
Starting materials
2-Methoxy-4-methylphenol, N,N0-dimethylethylenediamine and tion of this solution in an open atmosphere. Crystals were
formaldehyde were purchased from Alfa Aesar, India and were filtered from the solution and washed with hexane.
of reagent grade. They were used without further purification.
Yield 0.463 g (70%). Anal. calc. for C54H74Co4N10O10S2
The other reagents and solvents were of commercially available (1323.07). Calculated C, 49.02; H, 5.64; N, 10.59; found C,
49.18; H, 5.70; N, 10.66. IR: n(C–H) = 2860 cmꢁ1, n(N3 ) =
ꢁ
reagent quality, unless otherwise stated.
Caution! Azide salts of metal complexes derived from 2071 cmꢁ1, n(C–O/phenolate) = 1258 cmꢁ1, n(C–N) = 1156 cmꢁ1
organic ligands are potentially explosive. Only a little amount (Fig. S4, ESI†). UV/Vis: lmax = 247 and 297 nm. ESI-MS
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(positive ion mode, CH3OH) cald m/z 446.16, found 446.17 [CoL throughout the temperature range 2–300 K. The susceptibility
+ H]+, cald. m/z 468.14, found 468.15 [CoL + Na]+ (Fig. S7, ESI†).
data were corrected by Pascal’s diamagnetic contributions.21a
Isothermal magnetization measurements were performed up to
5 Tesla magnetic fields. Using the PHI program,21b the best
fitting results were obtained over the whole temperature range
(2–300 K).
Synthesis of the complex [Co4L2(l3-OH)2(NCSe)2]ꢀ2CH3CN (3)
This complex was prepared following a procedure similar to
that for the synthesis of complex 2. The only difference was that
here KSeCN (0.144 g, 1 mmol) was used instead of NH4SCN.
Here also, needle-shaped single crystals of X-ray quality
appeared in the beaker after 4–5 days on the slow evaporation
of the solvent. The crystals were filtered from the solution,
washed with hexane and air-dried.
Crystallographic data collection and refinement
Suitable single crystals of complexes 1–3 were mounted on glass
fibres without any protection. Intensity data were collected on
a Bruker-AXS SMART APEX II CCD diffractometer equipped with
a monochromatized Mo-Ka (k = 0.71073 Å) radiation source at
298(2) K. The crystals were positioned 60 mm from the CCD, and
frames (360) were measured with a counting time of 5 s. No crystal
decay was observed during the data collection. The structures
were solved by the direct method refining on F2 by a full-matrix
least-squares procedure.22 Non-hydrogen atoms were refined with
independent anisotropic displacement parameters. Hydrogen
atoms were placed in idealized positions and their displacement
parameters were fixed to be 1.2 times larger than those of the
attached non-hydrogen atom. Absorption corrections were carried
out using the SADABS program.23 All calculations were carried out
using SHELXS 97,24a SHELXL 97,24b,c and PLATON 99.25 For
complex 1, disorder treatment was carried out and the solvent
was masked by using Olex2.26 Data collection, structure refine-
ment parameters and crystallographic data for the complexes are
given in Table 1.
Yield 0.454 g (68%). Anal. calc. for C50H68Co4N8O10Se2
(1334.76). Calculated C, 44.99; H, 5.13; N, 8.39; found C,
ꢁ
44.82; H, 5.01; N, 8.46. IR: n(C–H) = 2861 cmꢁ1, n(N3 ) =
2070 cmꢁ1, n(CꢁO/phenolate) = 1258 cmꢁ1, n(C–N) = 1156 cmꢁ1
(Fig. S5, ESI†). UV/Vis: lmax = 246 and 298 nm. ESI-MS (positive ion
mode, CH3OH) cald m/z 446.16, found 446.17 [CoL + H]+, cald. m/z
468.14, found 468.15 [CoL + Na]+ (Fig. S8, ESI†).
Physical measurements
Elemental analysis (carbon, hydrogen and nitrogen) was performed
using a Perkin-Elmer 2400 series II CHN analyzer. IR spectra in
KBr (4000–400 cmꢁ1) were collected using a Perkin-Elmer RXI
FT-IR spectrophotometer. Electronic spectra in methanol solvent
(650–300 nm) were recorded in a Hitachi U-3501 spectrophoto-
meter. The electrospray ionization mass spectrometry (ESI-MS
positive) spectra were recorded using a Xevo G2-S QTof (Waters)
mass spectrometer, equipped with a Z-spray interface spectrometer.
Catalytic oxidation of o-aminophenol
Temperature-dependent molar magnetic susceptibility for a In order to study the phenoxazinone synthase-like activity of
powdered sample of 1–3 was measured with a superconducting complexes 1–3 as a suitable catalyst towards the oxidation of
quantum interference device vibrating sample magnetometer o-aminophenol (OAP), a 10ꢁ4 M solution of each complex in
(SQUID-VSM, Quantum Design) with an applied field 0.1 T methanol solution was treated with 100 equiv. of OAP under
Table 1 Summary of X-ray crystallographic data for compounds 1–3
Parameters
1
2
3
Composition
Formula wt.
Crystal system
Space group
a/Å
b/Å
c/Å
C
44H60Co4N16 O8
C54H74Co4N10O10S2
1323.07
Monoclinic
P2(1)/c
10.494(3)
13.481(3)
22.380(5)
90
C50H68Co4N8O10Se2
1334.76
Monoclinic
P2(1)/c
10.8554(6)
19.3089(10)
13.0765(7)
90
1176.80
Trigonal
R3
32.2276(7)
32.2276(7)
13.8386(3)
90
%
a/deg
b/deg
90
120
12447.4(7)
1.413
100.166(10)
90
3116.4(13)
1.410
90.839(3)
90
2740.6(3)
1.617
g/deg
V/Å3
r
calc/g cmꢁ3
Temp/K
l (Mo Ka)/Å
298
0.71073
9
5472, 1.241
52.866
298
0.71073
2
1376, 1.174
49.84
298
0.71073
2
1360, 2.580
50.23
Z
F(000), m mmꢁ1
2ymax/[1]
Reflections collected, unique
68 327, 5670
0.023, 1.07
340
61 521, 5428
0.093, 0.99
373
0.0431, 0.1416
0.448, ꢁ0.422
66 748, 4865
0.100, 1.04
345
0.0439, 0.1477
0.761, ꢁ0.759
Rint, GOF on F2
No. of parameters
R1(F0),a wR2(F0)b (all data)
Largest diff. peak, deepest hole, e Åꢁ3
0.0435, 0.1406
0.930, ꢁ0.269
P
P
P
P
a
b
2
R1
=
||Fo| ꢁ |Fc||/ |Fo|. wR2 (Fo2) = [ [w(Fo ꢁ Fc2)2/ wFo
]
4 1/2].
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aerobic conditions at room temperature. The reactions were SeCNꢁ ions respectively.27a,b,d A moderate band at 1256 cmꢁ1
followed spectrophotometrically by monitoring the increase in indicates the presence of a phenolic C–O bond.27b
the absorbance maxima of the phenoxazinone band at 433 nm
as a function of time (time scan).
The electronic spectra of complexes 1–3 were recorded
in methanol (Fig. S9, ESI†). Complexes 1–3 show two sharp
absorption maxima near 244, 247, 246 nm and 295, 297,
298 nm respectively, attributed to ligand-to-metal charge trans-
fer transitions.27c,d
Results and discussion
Syntheses
Description of crystal structures
The condensation of N,N0-dimethylethylenediamine with for-
maldehyde and 2-methoxy-4-methylphenol in 1 : 2 : 2 molar
ratios in methanol resulted in Mannich base ligand H2L.6a,9,10
The ligand has been characterized by Mass and NMR spectro-
scopy. The reaction of cobalt acetate tetrahydrate salt with this
ligand (H2L) in 2 : 1 ratios in the presence of azide/thiocyanate/
selenocyanate in acetonitrile solvent produces complexes 1–3.
In complex 1, two m1,1,1-N3 bridges and in complexes 2 and 3,
two m3-hydroxido bridges complete the tetranuclear defect
dicubane structure. In the tetranuclear structure of the com-
plexes, two Co(II) metal centers are hexa-coordinated and
another two Co(II) are penta-coordinated. The formation of
1–3 is shown in Scheme 1. All tetranuclear cobalt complexes
are characterized by elemental analysis, IR, UV-Vis, and Mass
spectrometry. The structures of all complexes are confirmed by
single crystal X-ray diffraction.
ORTEP representations of the three complexes (1–3) are shown
in Fig. 1–3, respectively. Selected bond lengths and bond angles
for the structures are given in Table S1, ESI.† Complex 1
%
crystallizes in trigonal space group R3 whereas complexes 2
and 3 crystallize in monoclinic space group P2(1)/c. Although
there is a difference in symmetry in the crystallographic cell
settings, the molecular geometries of the three complexes are
very similar containing four Co(II) ions arranged in such a
manner as to form a centrosymmetric defect dicubane tetra-
nuclear core. The only difference among these three structures is
that two m1,1,1-N3 bridges in complex 1 but two m3–O (from–OH gr.)
bridges in complexes 2 and 3 complete the defect-dicubane core.
The asymmetric unit of complex 1 contains two Co(II) ions,
one Mannich base ligand, and two azide ions (one in m1,1,1-N3
bridging mode and another in a terminal mode). The four Co(II)
ions which occupy the four corners of a defective double
cubane are coplanar. In this complex, one of the Co atoms
Co(1) possesses a six-coordinated distorted octahedral geometry.
The equatorial plane around Co(1) is formed by O(1), N(2) and
O(3) atoms from the deprotonated Mannich base ligand L2ꢁ and
N6 atom from the m1,1,1 bridging azide group. The axial positions
are occupied by N(1) and N(6a) (a = 5/3 ꢁ x, 4/3 ꢁ y, 4/3 ꢁ z) from
the ligand L2ꢁ and another bridging azide group, respectively.
The Co(1) atom is displaced from the mean plane passing through
the equatorially coordinated atoms by 0.071(4) Å towards the N1
Infrared (IR) and UV-Vis spectroscopy
Besides elemental analyses, all the complexes were charac-
terized by FT-IR spectroscopy and important infrared (IR) bands
of them are given in the Experimental section. The spectra
of complexes 1–3 show strong and sharp peaks at 2064, 2071
and 2070 cmꢁ1 for the stretching vibrations of N3ꢁ, SCNꢁ and
Scheme 1 Syntheses of complexes 1–3.
Fig. 1 ORTEP view of complex 1 with 30% ellipsoid probability.
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bipyramid). It is to be noted that the O(4a) atom of a methoxy
group is semi-coordinated to the Co(2) centre at a distance of
2.526(1).
One of the two triply bridging azide groups is situated above
and the other below (ca. 1.145 Å) the plane of the four Co(II)
ions (Fig. S10a, ESI†). The distances between Co(1)ꢀ ꢀ ꢀCo(2) and
Co(1)ꢀ ꢀ ꢀCo(2a) are 3.268(1) Å and 3.244(1) Å, respectively. The
Co–O bond lengths lie in the range from 1.979(1) Å to 2.226(2) Å
whereas the Cu–N bond lengths are in the ranges from 1.973(2) Å
to 2.227(3) Å. The Co–O (phenolate O atoms) distances (1.979 Å)
are relatively shorter than the distances (2.226 Å) of Co–O (meth-
oxy O atoms). The cis angles around Co(1) vary from 78.25(12)1 to
101.49(2)1 whereas the cis angles around the Co(2) metal centre
vary in a wider range from 76.51(1)1 to 137.87(11)1. The bridging
angles Co(1)–O(1)–Co(2) and Co(1)–N(6)–Co(2) are 108.50(10)1 and
93.87(13)1, respectively.
Complexes 2 and 3 possess very similar tetranuclear struc-
tures to that of complex 1. Here also in the centrosymmetric
core of the complexes, two Co(II) metal ions have a hexa-
coordinated distorted octahedral geometry and the other two
Co(II) possess a penta-coordinated distorted square pyramidal
environment. In these complexes, the asymmetric unit contains
two Co(II) ions, one deprotonated Mannich base ligand, one
triply bridged (m3) hydroxide group and one terminal N-bonded
thiocyanate anion (SCNꢁ) for 2 or selenocyanate anion (SeCNꢁ)
for 3. There are also non coordinated acetonitrile solvent
molecules in their asymmetric units (two for 2 and one for 3).
In complexes 2 and 3, the Co(1) centre is hexa-coordinated
being equatorially bonded by four donor atoms [O(1), O(3),
N(2)] from the deprotonated Mannich base ligand and O(5)a
from a triply bridging OH group. The axial sites are coordinated
by the N(1) atom from the ligand and O(5) from other bridging
OH groups. A penta-coordinated Co(2) centre is equatorially
surrounded by O(1), O(4a), and O(3a) from the ligand and O(5)
from a m3 bridging OH group. The axial position is occupied
by N(3) of a terminal SCNꢁ anion (SeCNꢁ for 3). The Addison
parameters (t) for Co(2) are 0.203 and 0.215 in complexes 2 and 3,
respectively confirming distorted square pyramidal geometry. The
O atom of each OH group coordinates three cobalt centres and
completes the defect double cubane core. One triply bridging OH
group is situated above and another is below the plane of the four
Co(II) ions at the distances of 1.033 Å and 1.035 Å for complexes
2 and 3 respectively from the plane (Fig. S10b and c, ESI†). The
Co–O (phenolate O atoms) distances are relatively shorter than the
distances of Co–O (methoxy O atoms). The average distances
between two Co(II)ꢀꢀꢀCo(II) metal centres are 3.187 Å and 3.190 Å
Fig. 2 ORTEP view of complex 2 with 30% ellipsoid probability. Non
coordinated solvent molecules have been omitted for clarity.
Fig. 3 ORTEP view of complex 3 with 30% ellipsoid probability. Non
coordinated solvent molecules have been omitted for clarity.
atom. The other cobalt atom Co(2) in the asymmetric unit is for complexes 2 and 3, respectively.
penta-coordinated. The basal plane around the Co(2) centre is
It is to be mentioned here that some homo- and hetero-
formed by three oxygen atoms [O(1), O(2) and O(3a)] from the metallic polynuclear complexes of this multidentate Mannich
Mannich base ligand and one nitrogen atom N(6a) from a m1,1,1 base ligand are known.6a,9,10 Among the homometallic com-
bridging azide group. The axial position is coordinated by a N(3) plexes, five are of Cu(II), four are of Ni(II), three are of Zn(II) and
atom from a terminal azide group. The Addison parameter28 three are of Cd(II). The nuclearity of these complexes depends
(trigonality index, t = (a ꢁ b)/60, where a and b are the two largest upon the metal ion e.g. all five complexes of Cu(II) are tri-
L–M–L angles of the coordination sphere) for Co(2) is 0.279 nuclear, all three of Zn(II) are dinuclear, all three of Cd(II) are
confirming the distorted square pyramidal character (t = 0 for tetranuclear and the complexes of Ni(II) are di- (two), tri- (one)
a perfect square pyramid and a t = 1 for a perfect trigonal and tetranuclear (one). Besides these, seven heterometallic
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M(II)–Ln(III) (M = Co, Ni, Zn, and Ln = Tb, Dy, Gd) complexes are 1–3. In the case of 1, upon cooling from 300 K, wT increases very
reported and all of these complexes are dinuclear. There is also slowly (w = molar magnetic susceptibility per tetramer) from a
one tetranuclear and one hexanuclear Ni(II)–Na(I) complex. The value of 11.4 cm3 molꢁ1 K at 300 K, to a value of 12.4 cm3 molꢁ1
tetranuclear complexes are formed when the coordination K at 20 K and then increases sharply to reach a value of
number of a metal ion is six or more and the metal : deproto- 14.1 cm3 molꢁ1 K at 7.4 K (Fig. 4a). This is followed by a
nated ligand (L2ꢁ) ratio is 2 : 1 and all of them possess defect sudden sharp decrease to a value of 11.8 cm3 molꢁ1 K at 2 K.
dicubane structures. Therefore, the formation of a tetranuclear The room temperature wT value of 11.4 cm3 molꢁ1 K is higher
dicubane structure of the present complexes is not surprising than the spin-only value (7.5 cm3 molꢁ1 K) for four magnetically
as the usual coordination number of Co(II) is six. It is inter- isolated high-spin Co(II) ions in octahedral environments. This
esting to note that two triply bridging groups, one above and is due to orbital contributions typical for the 4T1g ground states
other one below the plane of the four metal ions are an integral of octahedral high-spin Co(II) ions.17b,29,30 Analyses of the 1/w
part of these tetranuclear structures. These bridging groups are vs. T data using the Curie–Weiss law (Fig. S11, ESI†) gave Curie
chlorido, azido and acetato for two, two and one structure, constant (C) = 2.85 cm3 molꢁ1 K (per Co(II) ion) and Weiss
respectively. For the preparation of the present complexes, temperature (y) = 2.72 K. The positive y value is probably due to
although very similar reaction conditions have been used, the dominating ferromagnetic interactions in the tetra nuclear
bridging group is azido in 1 but hydroxido in 2 and 3. The cluster.
higher tendency of an azide group to act as a m1,1,1 bridge than
The intramolecular ferromagnetic interactions are evident
SCN or SeCN seems to be responsible for this difference.
from the initial gradual increase and then sharp increase of wT
A CSD search for tetranuclear defect dicubane cobalt between 300 K and 7.4 K. The final decrease below 7.4 K could
complexes containing any kind of N,O donor ligand reveals be the result of antiferromagnetic intermolecular interactions
that only nine such structures are reported.12c,15,16 All of them and an anisotropic contribution (D) which is observed for
contain triply bridged alkoxido or phenoxido or hydroxide similar dicubane-like Co(II) clusters.17a,31 The 4T1g ground state
groups. Among these complexes, six are mixed valence Co(II)/ in an octahedral field (S = 3/2) is split into a set of three levels by
Co(III), only one is single valence Co(III) and in the remaining spin–orbit coupling, leading to an effective S = 1/2 ground state
two, the cobalt ion is in a +2 oxidation state. Therefore, complex with the possibility of a relatively large g value.32 This usual
1 is the first example of a triply azido bridged tetranuclear behaviour of Co(II) ions does not allow us to accurately quantify
defect dicubane Co(II) complex and complexes 2 and 3 are the the magnitude of the magnetic interactions, so a qualitative
first triply hydroxido bridged tetranuclear defect dicubane approach has been tried to simulate the wT vs. T curve using a
structures with this ligand for any metal ion. The difference simplified scheme of only isotropic intra-cluster magnetic inter-
in bridging system in 1 from that in 2 or 3 has a pronounced actions (Fig. 4b) and an anisotropic term (D). The following spin
effect on their magnetic properties (see below).
Hamiltonian (1) has been used to reproduce the experimental data:
"
#
"
#
X
X
H ¼ ꢁ2J1
SiS2 þ SiS4 ꢁ 2J2S1S3 þ D
Si2
(1)
Magnetic properties
i¼1;3
i¼1ꢁ4
Magnetic susceptibility measurements in the temperature
range of 2–300 K at 0.1 T and variable field magnetization
The best fitting parameters obtained using this simplified
measurements were performed on polycrystalline samples of model are J1/kB = ꢁ19 K, J2/kB = 38 K, D/kB = ꢁ0.242 K and
Fig. 4 (a) Plot of wT vs. T for complex 1 in the temperature range of 2–300 K; the red line indicates fitting according to eqn (1) in the text; (b) schematic
illustration of the tetranuclear Co(II) (dicubane) core in 1, blue dash lines denote the spin topology and J represents exchange pathways through different
bridges; color codes: pink – Co, gray – C, blue – N, red – O.
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g = 2.39, where, J1 and J2 represent the antiferromagnetic and the data was obtained that gave the fitting parameters as
ferromagnetic interactions respectively. The observed ferro- J1/kB = 4.7 K, J2/kB = ꢁ34 K, D/kB = ꢁ0.588 K and g = 2.6. Here,
magnetic exchange interaction can be justified on the basis J1 and J2 represent ferromagnetic and antiferromagnetic inter-
of the azide bridging in the end-on mode33 while the antiferro- actions respectively.
magnetic interactions may be due to combination of oxygen
bridging and end-on azido modes of bridging.
The isothermal magnetization curve (M vs. H plot, Fig. 7a)
shows a value of 9.9 N mB at 2 K and 5 T, lower than the
Plots of isothermal magnetization (M) vs. field (H) (Fig. 5a) saturation value for four S = 3/2 systems (Msat = 15.6 N mB for
show a quick increase of M (N mB) by increasing the external g = 2.6). This behaviour justifies the antiferromagnetic inter-
field, at the lower field region and arriving at a maximum value actions in the cluster as well as the presence of magnetic
of 6.81 N mB at 2 K and 5 T. The highest value obtained at 2 K is anisotropy due to the Co(II) ions. The anisotropic behaviour
well below the theoretical saturation value for four S = 3/2 in the complex is also confirmed by the non-superimposition of
systems (Msat = 14.34 N mB for g = 2.3). These observations along the M/N mB vs. H/T curves (Fig. 7b).
with the feature of nonsuperimposition of the isothermal
For 3, the wT vs. T plot (Fig. 8a) shows a wT value of
magnetization plots in the M/N mB vs. H/T plots (Fig. 5b) 10.9 cm3 molꢁ1 K at 300 K. With the lowering of temperature,
indicate the presence of significant magnetic anisotropy in wT remains nearly constant up to 36 K. Below this temperature
the system. No satisfactory fits were possible for the M vs. it experiences a decrease up to 9.7 cm3 molꢁ1 K followed by a
H plots. This could be due to the different orientations of the sharp increase up to 12.2 cm3 molꢁ1 K at 2.7 K and finally a
local axis of anisotropy for each Co(II) ion.34
steep fall to a value of 10.5 cm3 molꢁ1 K at 2 K. This complicated
The magnetic behavior of complex 2 in the form of a wT vs. behavior suggests weak antiferromagnetic interactions accom-
T plot (Fig. 6a) shows a room temperature wT value of panied by ferromagnetic interactions at a low temperature
12.8 cm3 molꢁ1 K, which is higher than the spin-only value region in the tetranuclear cluster. The temperature dependence
(7.5 cm3 molꢁ1 K) for four isolated high-spin Co(II) ions in of 1/w was fitted according to the Curie–Weiss law, affording
octahedral environments. As mentioned before, this is due to Curie constant (C) = 2.72 cm3 molꢁ1 K (per Co(II) ion) and Weiss
the orbital contribution for the 4T1g ground states of octahedral temperature (y) = ꢁ0.059 K (Fig. S13, ESI†). The low negative
high-spin Co(II) ions. Between 300 and 12 K the wT product Weiss temperature value indicates weak antiferromagnetic
slowly decreases from 12.8 cm3 molꢁ1 K to 10.1 cm3 molꢁ1 K. interactions in the tetranuclear cluster. To ascertain the intra-
At 12 K the wT product reaches a minimum and then increases cluster interactions, qualitative fitting of the experimental
up to 10.5 cm3 molꢁ1 K at 3.5 K. Further lowering of the tem- wT vs. T data was carried out using eqn (1), similar to complexes
perature leads to a final decrease of wT down to 9.5 cm3 molꢁ1 K 1 and 2, taking into consideration only isotropic intra-cluster
at 2 K. Fitting of the 1/w vs. T data according to the Curie–Weiss magnetic interactions (Fig. 8b) and the anisotropic term (D).
law (Fig. S12, ESI†) gave Curie constant (C) = 3.25 cm3 molꢁ1 K The best fitting of the data gave the fitting parameters as J1/kB =
(per Co(II) ion) and Weiss temperature (y) = ꢁ3.82 K. The 4.4 K, J2/kB = ꢁ12 K, D/kB = ꢁ2.842 K and g = 2.4. Here, J1 and J2
negative Weiss temperature value indicates antiferromagnetic represents ferromagnetic and antiferromagnetic interactions
interactions in the tetranuclear cluster. A qualitative fitting of respectively.
the experimental wT vs. T data have been attempted using
From the field-dependent isothermal magnetization data
eqn (1) considering only isotropic intra-cluster magnetic inter- (Fig. 9a) the highest value of 11.95 N mB is obtained at 2 K
actions (Fig. 6b) and the anisotropic term (D). Satisfactory fit of and 5 T, which is lower than the theoretical saturation value for
Fig. 5 (a) Magnetization (M/N mB vs. H) plots at different low temperatures (2, 3 and 5 K); (b) M/N mB vs. H/T plots at the indicated temperatures (2, 3 and 5 K).
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Fig. 6 (a) Plot of wT vs. T for complex 2; the red line indicates fitting according to eqn (1) in the text; (b) schematic representation of a tetranuclear Co(II)
(dicubane) core in 2, blue dash lines denote the spin topology and J represents exchange pathways through different bridges; color codes: pink – Co,
gray – C, blue – N, red – O, lemon yellow – S.
Fig. 7 (a) Magnetization (M/N mB vs. H) plots at different low temperatures (2, 3 and 5 K); (b) M/N mB vs. H/T plots at the indicated temperatures (2, 3 and 5 K).
four S = 3/2 systems (Msat = 14.41 N mB for g = 2.4). M vs. H/T data and N bridging. However, in 2 and 3, only the Co(II)–O–Co(II)
(Fig. 9b) at different fields show an increase of the magnetiza- bridges are observed [for 2, Co(II)–O–Co(II) angles ranging from
tion at lower fields and also non-superimposition of the curves. 97.8–103.51 and for 3, Co(II)–O–Co(II) angles ranging from
These features of the magnetization data indicate the presence 96.8–104.51], which mainly results in an antiferromagnetic type
of magnetic anisotropy and/or low-lying excited states in the of interaction and a very weak ferromagnetic behavior at the
complex. Structural and magnetic parameters for complexes low temperature region.
1–3 are listed in Table S2, ESI.†
Phenoxazinone synthase like activity
Magneto-structural correlations
The phenoxazinone synthase-like activity for complexes 1–3 has
The magnetic behavior of complex 1 is observed to be distinctly been studied by the catalytic oxidation of o-aminophenol (OAP)
different from that observed for 2 and 3. This can be attributed to aminophenoxazinone (APX) as reported previously.35 The
to the different bridging ligands (azido and hydroxido) and OAP is easily oxidized and dimerized to form APX according to
dissimilar Co(II)–X–Co(II) (X = N, O) bridging angles in the reaction shown in Scheme 2. APX shows a broad absorption
the complexes. In 1, both Co(II)–O–Co(II) and Co(II)–N–Co(II) peak in the range of 420–435 nm in methanol solvent. Before
bridging are observed [Co(II)–O–Co(II) angles ranging from going into the details of kinetics studies, we examined the
107.1–108.41 and Co(II)–N–Co(II) angles ranging from 93.7– catalytic activity of all three complexes for the oxidation of
100.91], the latter mostly responsible for the ferromagnetic type o-aminophenol as a substrate in methanol at room temperature.
of interaction.33 The antiferromagnetic interaction at the lower
For this purpose, 10ꢁ4 M methanol solution of the complexes
temperature regime is probably due the combined effect of O 1–3 was mixed with 10ꢁ2 M (100 equivalents) of substrate OAP
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Fig. 8 (a) Plot of wT vs. T for complex 3; the red line indicates fitting according to eqn (1) in the text; (b) schematic representation of the tetranuclear Co(II)
(dicubane) core in 3, blue dash lines denote the spin topology and J represents exchange pathways through different bridges; color codes: pink – Co,
gray – C, blue – N, red – O, chrome yellow – Se.
Fig. 9 (a) Magnetization (M/N mB vs. H) plots at different low temperatures (2, 3 and 5 K); (b) M/N mB vs. H/T plots at the indicated temperatures
(2, 3 and 5 K).
temperature. In the presence of substrate (OAP), the change in
spectral behaviour of complex 1 is shown in Fig. 10 and similar
plots for 2 and 3 are shown in the supporting information
(Fig. S14 and S15, ESI†).
Kinetic studies were performed to evaluate the extent of the
catalytic efficiency of all the complexes. The rate constant for
Scheme 2 Catalytic oxidation of o-aminophenol (OAP) to phenoxazinone
(APX) in methanol.
the oxidation reaction was determined from the log[Aa/(Aa
ꢁ
At)] vs. time plot. In all cases, a first-order kinetic dependence
under aerobic conditions at room temperature. After addition of was observed at low concentrations of substrate (OAP), whereas
OAP into the complexes, the progress of the oxidation was saturation kinetics was noticed at higher substrate concentra-
followed by recording the UV-Vis spectra of the reaction mixture tions. On the basis of the MichaelisꢁMenten approach of
at 5 min time interval. The gradual increase of an absorption enzymatic kinetics, the observed reaction rates vs. substrate
band around 433 nm was observed in the UV-Vis spectra for all concentration data were analyzed to obtain the Lineweaver–
the complexes.35 Beside this, a blank experiment without catalyst Burk (double reciprocal) plot as well as the values of kinetic
under identical conditions was performed where it does not show parameters KM, Vmax and Kcat. Both the curves of the observed
any significant growth of the band at 433 nm. These spectral rate vs. [substrate] and the Lineweaver–Burk plot for complex 1
behaviours indicate that all the complexes 1–3 show phenoxazi- are shown in Fig. 11. Similar plots for 2 and 3 are given in the
none synthase-like activity under aerobic conditions at room ESI† (Fig. S16 and S17).
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and a 1 : 10 mixture (v/v) of the complex and OAP within 20 min
of mixing in methanol.
From the spectra of all the complexes 1–3, a common peak is
observed at m/z = 446.17 (calcd 446.16) which can be assigned
to protonated mononuclear species, [(CoL) + H]+. After the
addition of o-aminophenol into the solution of all complexes
1–3, a drastic change is found in the overall mass spectrum
(Fig. S18–S20, ESI†). We focused here only on the spectrum of
the complex 1–substrate mixture. In the spectra of the mixture
of the complex and o-aminophenol, the base peak is observed
at m/z = 612.14 (calcd 612.15) which can be assigned to the
species [(CoL)(H2O)(OAP) + K]+. In addition, two peaks are
observed at m/z 468.15 (calcd 468.14) and m/z 557.62 (calc.
557.63) which indicate the presence of species [(CoL) + Na]+ and
[(CoL)(OAP) + H]+, respectively. Along with these, another
low intensity peak is observed at m/z = 610.11 (calcd 610.10)
which seems to be due to the presence of methanol and
o-aminophenol coordinated with mononuclear species in the
mixture [(CoL)(CH3OH)(OAP) + H]+. The appearance of two
other peaks at m/z 587.10 (calc. 587.11) and m/z 572.61 (calc.
572.60) may be assigned to the presence of protonated species
[(CoL)(OAP)(O2) + H]+ and protonated hydroxyl Co(III) species
[(CoL)(OAPꢂ)(OH) + H]+ which are the key intermediates of this
catalytic reaction. The peaks corresponding to the oxidized
product (APX) and its intermediate are also found at m/z
213.07 (calc. 213.06) and m/z 215.23 (calc. 215.22), respectively.
We have also checked that H2O2 is not formed as a by-product
during this oxidative dimerization reaction. A probable mecha-
nism for the catalytic oxidation of OAP by complex 1 is shown
in Scheme 3 (below).
Fig. 10 Increase of the APX band at 433 nm after the addition of
100 equiv. of OAP to a methanol solution with complex 1. The spectra
were recorded at 5 min intervals.
Mechanistic insight. The mechanism of catalytic oxidation
of o-aminophenol by some homometallic copper(II), nickel(II),
cobalt(II/III), and manganese(II/III) complexes is reported in the
literature, where o-aminophenol is converted to phenoxazinone.
Most of the reported cobalt(II/III) complexes used as a catalyst for
this catalytic reaction are mononuclear or dinuclear complexes.
Herein, we have shown similar catalytic activity by three tetra nuclear
cobalt(II) complexes for the first time. The investigation on mecha-
nism of this catalysis using these complexes (1–3) reveals that the
catalysis occurs via a radical mechanism with the formation of an
o-aminophenolate intermediate with the oxidation of Co(II) to Co(III).
Fig. 11 Plot of the rate vs. substrate concentration for complex 1. Inset
shows the corresponding Lineweaver–Burk plot.
Analysing the experimental data we obtained the kinetic During this catalytic reaction phenoxazinone is formed as a final
parameters for complexes 1–3 which are summarized in Table S3, product and oxygen is reduced to H2O as a by-product.
ESI.† The turnover number (Kcat value) is obtained by dividing the
On the basis of ESI-mass spectra, we propose a possible
Vmax by the concentration of the complex used, and is observed to catalytic pathway (Scheme 3) for this catalytic conversion
be 500.4, 508.9 and 511.2 hꢁ1 for complexes 1–3, respectively. The reaction. We assume that our cobalt(II) complexes catalyse the
Kcat values of complexes 1–3 are compared with other reported oxidation of o-aminophenol through a process as proposed by
36
´
cobalt complexes in Table S4, ESI.† Until now, phenoxazinone Simandi et al. The tretranuclear complex [Co4L2(m1,1,1-N3)2(N3)2]
synthase-like catalytic activity of tetranuclear cobalt complex has (1) dissociates in solution to produce catalytically active mono-
not been examined.
nuclear [(Co(II)L)] (A) species. It is further solvated with methanol
and produced [(Co(II)L)(CH3OH)2] (B). When o-aminophenol is
added to the methanolic solution of complex 1, it coordinates to
ESI-mass spectrometric study
In order to establish the catalytic pathway of phenoxazinone B via a N atom of its amine group by replacing one of the methanol
synthase-like activity and the complex-substrate intermediate molecules and thus intermediate [(Co(II)L)(CH3OH)(OAP)] (C) is
during the oxidative dimerization reaction of o-aminophenol formed. Then the oxygen molecule is bonded with species (C)
(OAP), we have recorded ESI-MS spectra of all the complexes replacing the methanol to result in superoxocobalt(II) complex
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Scheme 3 Proposed mechanism for catalytic oxidation of OAP by complex 1.
[(Co(II)L)(OAP)(O2)] (D). The intermediate species (E) is formed by reacted with cobalt(II) acetate tetra hydrate in the presence of
exchanging the proton of OAP with coordinated dioxygen giving an azide, thiocyanate or selenocynate when three isostructural tetra-
OAPꢂ free radical intermediate and further the peroxo linkage is nuclear Co(II) complexes with a defect dicubane core were formed.
dissociated to produce hydroxyl species (F) [(Co(III)L)(OAP)(OH)].
Considering the coordination number of a cobalt ion, formation
Finally (G) [(Co(III)L)(OAP)(OH2)] is formed by abstracting one of such a structure is not unexpected but the existence of all four
proton from the solution. The active catalyst (A) in a catalytic cycle cobalt ions in a +2 oxidation state is a bit surprising as commonly
is regenerated with the release of H2O and OAPꢂ from the species this kind of Schiff base produces complexes in which cobalt is
(G). On the other hand, being unstable, the OAPꢂ is oxidized to mixed valence (+2/+3) or in a +3 state. The two triply bridging
2-benzoquinone monoimine (BQMI). The intermediate (BQMI) atoms/groups which are an essential feature for this structure
undergoes oxidative coupling with another molecule of OAP to are azide for complex 1 and hydroxide for complexes 2 and 3.
give final product phenoxazinone (APX) and oxygen is reduced to a This difference is attributable to the greater ability of azide to
water molecule as a by-product. During this whole catalytic cycle be involved in a m1,1,1-bridging mode than thiocyanate or a
1.5O2 is reduced and 3H2O is formed as a by-product.
selenocyanate ion. The magnetic properties of the complexes
1–3 are also interesting. Depending on the nature of the bridging
co-ligands and also the angles, prominent magnetic behaviours,
ferromagnetic and antiferromagnetic are manifested. The aniso-
tropic natures of the complexes are also evident from the
Conclusions
In order to study the complex formation of the versatile polytopic magnetization plots. All three complexes show phenoxazinone
Mannich base ligand [H2L = N,N0-dimethyl-N,N0-bis(2-hydroxy-3- synthase-like catalytic activity. The catalytic efficiency of these com-
methoxy-5-methylbenzyl)ethylenediamine] with cobalt salts, it was plexes cannot be compared with that of any other tetranuclear
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248, 1717–2128; (c) M. Suzuki, H. Furutachi and H. Okawa,
Coord. Chem. Rev., 2000, 200-202, 105–129.
cobalt complexes as such study is not yet documented but is higher
than most of the previously reported cobalt complexes of other
nuclearities.
8 (a) C. T. Lyons and T. D. P. Stack, Coord. Chem. Rev., 2013,
257, 528–540; (b) P. Bhowmik, H. P. Nayek, M. Corbella,
N. Aliaga-Alcalde and S. Chattopadhyay, Dalton Trans., 2011,
40, 7916–7926; (c) K. Liua, W. Shia and P. Cheng, Coord.
Chem. Rev., 2015, 289–290, 74–122.
9 (a) Q. W. Xie, S. Q. Wu, W. B. Shi, C. M. Liu, A. L. Cuia and
H. Z. Kou, Dalton Trans., 2014, 43, 11309–11316; (b) Q.-W.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
A. G. and A. D. thank the University Grant Commission (UGC),
New Delhi for funding the CAS-V, Department of Chemistry,
University of Calcutta and for the Senior Research Fellowship
[Sanction No. UGC/1253/JR.Fellow(Sc)], respectively. We are
grateful to the Sophisticated Analytical Instrument Facility
(SAIF), Dept. of Chemistry, IIEST, Shibpur, Howrah, for providing
the single crystal X-ray diffractometer facility as well as CRNN,
University of Calcutta, Kolkata, India, for the facility of magnetic
measurements.
10 (a) L. Botana, J. Ruiz, J. M. Seco, A. J. Mota, A. R. Dieguez,
R. Sillanpaa and E. Colacio, Dalton Trans., 2011, 40,
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Dieguez, J. M. Seco, I. Oyarzabala and E. Colacio, Dalton
Trans., 2014, 43, 13509–13524.
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S. Chattopadhya, Dalton Trans., 2018, 47, 331–347;
(b) K. Ghosh, M. G. B. Drew and S. Chattopadhyay, Inorg.
Chim. Acta, 2018, 482, 23–33; (c) D. Mondal, M. C. Majee,
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S. Kundu, M. Mortel, G. Abbas, A. Endo, M. M. Khusniyarov
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