Inclusion of Ln(III) in the Complexes of Co(II) with a Mannich Base Ligand: Development of Atmospheric CO2 Fixation and Enhancement of Catalytic Oxidase Activities

Article abstract of DOI:10.1021/acs.inorgchem.9b00121

The reaction of the Mannich base ligand (H₂L = N,N′-dimethyl-N,N′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)ethylenediamine) with Co(OAc)₂·4H₂O and Ln(III) nitrate salts (Ln = Gd, Tb, and Dy) under basic conditions afforded three carbonato-bridged isostructural tetranuclear heterometallic Co(II)-Ln(III) complexes (Co₂Gd₂L₂(μ₄-CO₃)₂(NO₃)₂] (1), [Co₂Tb₂L₂(μ₄-CO₃)₂(NO₃)₂] (2), and [Co₂Dy₂L₂(μ₄-CO₃)₂(NO₃)₂] (3)) by atmospheric CO₂ fixation. In all structures, two dinuclear [(Co^IIL)Ln^III(NO₃)] units are linked via two μ₄-carbonato groups to form the tetranuclear Co^II₂Ln^III₂ core. The geometry around two penta-coordinated Co(II) ions is distorted square pyramid, and that around two nona-coordinated Ln(III) ions is intermediate between spherical tricapped trigonal prism and spherical capped square antiprism in all complexes. The complexes (1-3) showed catecholase-like and phenoxazinone-synthase-like catalytic activities. The k_cat values calculated for the catecholase-like reaction were 254.5, 272.4, and 291.3 h^-1, and for the phenoxazinone-synthase-like reaction they were 2930.6, 2965.2, and 2998.5 h^-1 for complexes 1-3, respectively. The probable pathways for these two oxidase reactions have been proposed by the analyses of mass spectral data. For all of the compounds, the variable temperature magnetic susceptibility and isothermal magnetization data were investigated. The complexes exhibited overall ferromagnetic behavior, which was evident from the isothermal magnetization curves. AC magnetic susceptibility measurements revealed the slow relaxation of magnetization in complexes 2 and 3 but very negligible in 1. The activation energy barriers (U_eff) for the slow relaxation process were evaluated and found to be 1.99, 2.79, and 8.98 K for 1, 2, and 3, respectively.

Full text of DOI:10.1021/acs.inorgchem.9b00121

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Inclusion of Ln(III) in the Complexes of Co(II) with a Mannich Base
Ligand: Development of Atmospheric CO2 Fixation and
Enhancement of Catalytic Oxidase Activities
Avijit Das,† Soumyabrata Goswami,‡ Rupam Sen,§ and Ashutosh Ghosh*,†
†Department of Chemistry, University College of Science, University of Calcutta, 92, A. P. C. Road, Kolkata 700009, West Bengal,
India
‡Department of Chemistry, Amity Institute of Applied Sciences (AIAS), Amity University Kolkata, Major Arterial Road, Action Area
II, Rajarhat, Newtown, Kolkata 700156, West Bengal, India
§Department of Chemistry, Adamas University, Barasat-Barrackpore Road, Barasat 700126, West Bengal, India
S
* Supporting Information
ABSTRACT: The reaction of the Mannich base ligand (H2L = N,N′-
dimethyl-N,N′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)-
ethylenediamine) with Co(OAc)2·4H2O and Ln(III) nitrate salts (Ln =
Gd, Tb, and Dy) under basic conditions afforded three carbonato-bridged
isostructural tetranuclear heterometallic Co(II)−Ln(III) complexes
(Co2Gd2L2(μ4-CO3)2(NO3)2] (1), [Co2Tb2L2(μ4-CO3)2(NO3)2] (2),
and [Co2Dy2L2(μ4-CO3)2(NO3)2] (3)) by atmospheric CO2 fixation. In
all structures, two dinuclear [(CoIIL)LnIII(NO3)] units are linked via two
μ4-carbonato groups to form the tetranuclear CoII LnIII core. The
2
2
geometry around two penta-coordinated Co(II) ions is distorted square
pyramid, and that around two nona-coordinated Ln(III) ions is
intermediate between “spherical tricapped trigonal prism” and “spherical
capped square antiprism” in all complexes. The complexes (1−3) showed
catecholase-like and phenoxazinone-synthase-like catalytic activities. The kcat values calculated for the catecholase-like reaction
were 254.5, 272.4, and 291.3 h−1, and for the phenoxazinone-synthase-like reaction they were 2930.6, 2965.2, and 2998.5 h−1
for complexes 1−3, respectively. The probable pathways for these two oxidase reactions have been proposed by the analyses of
mass spectral data. For all of the compounds, the variable temperature magnetic susceptibility and isothermal magnetization
data were investigated. The complexes exhibited overall ferromagnetic behavior, which was evident from the isothermal
magnetization curves. AC magnetic susceptibility measurements revealed the slow relaxation of magnetization in complexes 2
and 3 but very negligible in 1. The activation energy barriers (Ueff) for the slow relaxation process were evaluated and found to
be 1.99, 2.79, and 8.98 K for 1, 2, and 3, respectively.
activities of heterometallic M(II)−Mn(II) (M = Cu and Ni)
complexes can be very high compared with those of the
homometallic complexes under suitable conditions.10,11 Various
combinations of transition-metal ions have been used to
investigate different catalytic reactions,10,11,14,15 but complexes
of heterometallic Tr−Ln complexes have rarely been used for
such purposes.16 On the contrary, the fixation of CO2 by metal
complexes has drawn a great amount of attention as such a
reaction may have a useful role in eliminating CO2 from the air.
In most cases, alkaline conditions facilitate the absorption of
CO2, transforming it to carbonate, which acts as bridging ligand
between the metal centers.17,18 Such fixation of CO2 in a neutral
medium can also occur, although it is very rare.19 Even if the role
of a metal ion in such a fixation process is not very clear, homo-
or heterometallic complexes containing Ln(III) seem to be more
INTRODUCTION
■
Studies on heterometallic multinuclear complexes of various
types of ligands are of increasing attention to the chemists,
mainly due to their many functions in molecular magnetism and
catalysis.1−9 The presence of a second metal ion can drastically
modulate the catalytic activities and magnetic properties of the
metal complexes.10,11 It is now well known that polynuclear 3d−
4f complexes with various types of ligands have the potential to
be used as nanoscale magnetic materials because they show very
prominent single-molecule magnet properties below a critical
temperature.12,13 Therefore, the synthesis of new 3d−4f
complexes and the study of their magnetic properties are very
active areas in the present day research. Besides the magnetic
properties, the exploration of the catalytic activity of
heterometallic complexes is also one of the foremost areas of
research because the multimetallic cooperative effects are found
to enhance dramatically the efficiency of various types of
catalysts. For example, recently, we have shown that the catalytic
Received: January 14, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00121
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Table 1. Summary of X-ray Crystallographic Data for Complexes 1−3
parameters
1
2
3
composition
fw
C46H60Co2Gd2N6O20
1449.36
C46H60Co2Tb2N6O20
1452.72
C46H60Co2Dy2N6O20
1459.86
cryst syst
space group
a/Å
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
ρcalc/g cm−3
temp/K
λ(Mo Kα)/Å
Z
F(000) (μ mm−1
2θmax/deg
triclinic
triclinic
triclinic
P1
̅
P1
̅
P1
̅
10.831(4)
12.061(4)
12.130(5)
77.594(19)
81.60(2)
64.777(15)
1397.4(9)
1.722
298
0.71073
1
720, 3.002
54.91
10.778(3)
12.031(3)
12.124(3)
77.487(14)
81.246(14)
64.917(9)
1386.8(6)
1.740
298
0.71073
1
722, 3.184
55.12
10.7911(19)
12.054(2)
12.132(2)
77.367(7)
81.365(8)
64.862(6)
1391.1(4)
1.743
298
0.71073
1
724, 3.318
54.95
)
reflns collected, unique
Rint, GOF on F2
18958, 6280
0.058, 1.03
349
0.0361, 0.0874
1.124, −1.281
19494, 6330
0.063, 1.08
349
0.0432, 0.0943
2.629, −1.287
19461, 6304
0.050, 1.12
349
0.0410, 0.0964
2.601, −1.036
no. of parameters
R1(Fo), wR2(Fo) (all data)
largest diff. peak, deepest hole (eÅ−3
a
b
)
a
b
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2(Fo ) = [∑[w(Fo − Fc2)2/∑wFo ]1/2
.
2
2
4
prone to taking up CO2 from the atmosphere to form carbonate-
bridged polynuclear complexes compared with the transition-
metal analogues.20,21
incorporation of Ln(III) ions into a transition-metal complex
can enhance the catalytic oxidase activities.
EXPERIMENTAL SECTION
Recently we used a flexidentate Mannich base ligand (H2L)
for the synthesis of complexes with Zn(II), Ni(II), Cd(II), and
Co(II) and observed that the complexes can be di-, tri-, or
tetranuclear depending on the metal ion and studied their
magnetic properties, catalytic activities, or photophysical
properties.22−24 Homometallic (Ni(II), Cu(II)) and hetero-
metallic (3d metal(II)−lanthanide(III), metal = Ni, Co, Zn, and
lanthanide = Gd, Tb, Dy) complexes of this ligand have also
been synthesized by other groups, and their magnetic properties
have been investigated.25−28
Herein we report the synthesis, crystal structure, magnetic
properties, and catalytic oxidase activities of heterometallic
Co(II)−Ln(III) complexes obtained by the reaction of this
Mannich base (H2L) with Co(OAc)2·4H2O in the presence of
lanthanide(III) nitrate (Ln = Gd(III), Tb(III), and Dy(III))
under basic conditions. The characterization by ultraviolet (UV)
spectroscopy, infrared (IR) spectroscopy, mass spectrometry,
and single-crystal X-ray crystallography shows that the resulting
species are carbonato-bridged tetranuclear complexes
(Co2Gd2L2(μ4-CO3)2(NO3)2] (1), [Co2Tb2L2(μ4-
CO3)2(NO3)2] (2), and [Co2Dy2L2(μ4-CO3)2(NO3)2] (3))
formed by atmospheric CO2 fixation. All three Co(II)−Ln(III)
complexes (1−3) exhibit much higher biomimetic catechol- and
phenoxazinone-synthase-like activities under aerial conditions
compared with tetranuclar homometallic Co(II) complexes of
this ligand, recently reported by us.24 Mass spectrometry has
been carried out to obtain an idea about the probable pathways
for these catalytic reactions. Complexes 2 and 3 demonstrate
slow relaxation of magnetization reorientation, which was not
observed in the reported Co(II) complexes. To the best of our
knowledge, examples of complexes of this ligand formed by
fixing of the atmospheric CO2 are unprecedented until now.
Moreover, we have shown here for the first time that
■
Starting materials were procured as previously reported.24
Synthesis of Ligand H2L. The ligand, H2L, was synthesized as
previously reported22−24 and was characterized by electrospray
ionization mass spectrometry (ESI−MS) (Figure S1) and proton
nuclear magnetic resonance (1H NMR) (Figure S2).
Synthesis of the Complex [Co2Gd2L2(μ4-CO3)2(NO3)2] (1). Ten
mL of methanolic solution of Co(OAc)2·4H2O (1 mmol, 0.249 g) was
added to a 20 mL methanol solution of ligand, H2L (1 mmol, 0.388 g),
in a beaker. The mixture was stirred for 1 h, and NEt3 (3 mmol, 0.420
mL) was added to the solution. Then, Gd(NO3)3·6H2O (0.450 g, 1
mmol) salt was added to this solution and stirred for a further 2 h. The
solution was filtered, and the filtrate was kept in a desiccator for slow
evaporation of solvent. After a few days, well-shaped pink-colored
crystals started to appear. They were collected and washed with
methanol.
Yield, 0.464 g (64%). Anal. calcd for C46H60Co2Gd2N6O20
(1449.36). Calculated C, 38.12; H, 4.17; N, 5.80; Found C, 38.3; H,
−
4.4; N, 5.6. IR: ν(CO32−) = 1466 cm−1, ν(NO3 ) = 1384 cm−1, ν(C−
O/phenolate) = 1254 cm−1, ν(C−N) = 1149 cm−1 (Figure S3). UV/
vis: λmax = 289 nm. ESI−MS (positive-ion mode) calcd m/z (2+)
663.06, found 663.07 [Co(L)Gd(CO32−)]+ (Figure S4).
Synthesis of Complex [Co2Tb2L2(μ4-CO3)2(NO3)2] (2) and
Complex [Co2Dy2L2(μ4-CO3)2(NO3)2] (3). Complexes 2 and 3 were
synthesized following a similar procedure as that for complex 1, except
that lanthanide salts, Tb(NO3)3·5H2O (0.435 g, 1 mmol) and
Dy(NO3)3·xH2O (0.348 g, 1 mmol), were used instead of Gd-
(NO3)3·6H2O, respectively.
Complex 2. Yield, 0.479 g (66%). Anal. calcd for
C46H60Co2Tb2N6O20 (1452.72). Calculated C, 38.03; H, 4.16; N,
5.79; Found C, 38.2; H, 3.9; N, 5.6. IR: ν(CO32−) = 1465 cm−1,
−
ν(NO3 ) = 1384 cm−1, ν(C−O/phenolate) = 1254 cm−1, ν(C−N) =
1149 cm−1 (Figure S5). UV/vis: λmax = 290 nm. ESI−MS (positive-ion
mode) calcd m/z (2+) 664.07, found 664.08[Co(L)Tb(CO32−)]+
(Figure S6).
Complex 3. Yield, 0.504 g (69%). Anal. calcd for
C46H60Co2Dy2N6O20 (1459.86). Calculated C, 37.85; H, 4.14; N,
B
DOI: 10.1021/acs.inorgchem.9b00121
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5.76; Found C, 37.7; H, 4.3; N, 5.6. IR: ν(CO32−) = 1466 cm−1,
crystalline solid products were obtained in the absence of
triethylamine, and when triethylamine was added to the
solution, atmospheric CO2 was fixed, resulting in the
carbonato-bridged well-crystalline tetranuclear complexes.
Infrared and UV−vis Spectroscopy. The complexes
exhibit IR absorption bands for carbonate. The asymmetric
stretching vibrations for carbonate are observed at 1465 cm−1.17
A strong absorption band at 1384 cm−1 indicates the presence of
nitrate groups. The IR spectra also showed a characteristic band
at 1255 cm−1 assignable to the presence of a phenolic C−O
stretching vibration of the Mannich base.23
The electronic spectra of the complexes 1−3 were recorded in
DMF/MeOH solution at room temperature. An absorption
bands at 289, 290, and 291 nm was observed for complexes 1−3,
respectively.24 This band was assigned to a ligand-to-metal
charge-transfer transition.
−
ν(NO3 ) = 1384 cm−1, ν(C−O/phenolate) = 1256 cm−1, ν(C−N) =
1150 cm−1 (Figure S7). UV/vis: λmax = 291 nm. ESI−MS (positive-ion
mode) calcd m/z (2+) 668.07, found 668.08 [Co(L)Dy(CO32−)]+
(Figure S8).
Physical Measurements. Elemental analyses, IR spectra, UV−vis
spectra, mass spectra, and DC magnetic susceptibilities of complexes
(1−3) have been performed as previously reported.24 The measured
susceptibilities were corrected for the diamagnetism of the samples
using Pascal’s tables.29 AC magnetic susceptibilities of 1−3 were
measured down to 1.9 K.
Crystallographic Data Collection and Refinement. Crystallo-
graphic data of single crystals of complexes (1−3) have been collected
as previously reported.24 The absorption corrections were carried out
using the SADABS program.30,31 All calculations were performed using
SHELXS 2016/6,32 SHELXL 2016/6,33,34 and PLATON 99,35 and
solvents were squeezed using Olex2.36 Crystallographic data for the
complexes are given in Table 1.
Catalytic Oxidation of 3,5-DTBC and o-Aminophenol.
Catechol-oxidase- and phenoxazinone-synthase-like activities of the
complexes in DMF/methanol solution have been studied as previously
reported.10,24 The H2O2 that was produced during catalytic reactions
was estimated iodometrically.11
Description of Crystal Structures. Single-crystal X-ray
diffraction studies reveal that all three complexes are
isostructural, centrosymmetric carbonato-bridged tetranuclear
[CoII LnIII2] species (Ln = Gd, Tb, and Dy for 1, 2, 3,
2
respectively) formed by the deprotonated Mannich base ligand
L2− and bridging carbonate ion. All of them are isomorphous
RESULTS AND DISCUSSION
■
and crystallize in triclinic space group P1
(Z = 1).
̅
with the same Z value
Syntheses. The Mannich base ligand (H2L) was prepared by
putting N,N′-dimethylethylenediamine, 2-methoxy-4-methyl-
phenol, and formaldehyde under reflux, as described in the
literature. It was characterized by mass and NMR spectra and
used for the preparation of heterometallic Co(II)−Ln(III)
complexes. The carbonato-bridged tetranuclear Co(II)−Ln-
(III) complexes (1−3) resulted from the mixture of H2L,
Co(OAc)2·4H2O, Ln(NO3)3·6H2O (Ln = Gd, Tb, and Dy for 1,
2, 3, respectively), and triethylamine in 1:1:1:3 molar ratios in
methanol solution in an ambient atmosphere by self-assembly.
Block-shaped single crystals were obtained by the slow
evaporation of the solution with yields of 65−70%. A particularly
interesting point of this reaction is the fixation of atmospheric
CO2 as a carbonate that binds the metal ions in the complexes
through a rare bis(monodentate)−bidentate bridging mode of
μ4-carbonate. The synthetic procedure of complexes 1−3 is
shown in Scheme 1. It is to be noted that previously we
A full structural description of 3 is presented here as a
representative complex and in some cases compared with the
structures of 1 and 2. An ORTEP view of the molecular structure
with atom labeling of complex 3 is shown in Figure 1a, and those
of 1 and 2 are given in Figures S9 and S10, respectively. In the
centrosymmetric structure of the complex, two dinuclear
[CoII(L)LnIII(NO3)] units are connected via two μ4-carbonato
bridges to form a tetranuclear structure. The asymmetric unit of
complex 3 consists of one Co(II) ion, one deprotonated
Mannich base ligand, one Dy(III) ion, one nitrate ion, and one
carbonate ion.
For all three complexes, the Co1 metal center is penta-
coordinated, being equatorially coordinated by two nitrogen
atoms (N1 and N2) and two oxygen atoms (O1 and O3) from
the deprotonated Mannich base ligand. The fifth coordination
site is occupied by the O8 atom from the carbonato group. The
geometry is best described as square pyramid, in which the metal
center Co1 deviates from the mean basal plane passing through
O1, O3, N1, and N2 atoms by 0.42(1) Å toward the O8 atom.
These values are 0.41(2) and 0.41(1) Å for complexes 1 and 2,
respectively. In the complexes, around the Co1 metal center, the
cis angles vary from 76.39(2) to 93.97(2)°, and the trans angles
lie within the range 144.53(1)−156.94(1)°, confirming
considerable distortion in the square pyramidal structure. The
Addison parameter37 (τ = (α − β)/60, where α and β are the two
largest L−M−L angles of the coordination spheres) was
determined to be 0.22 for complex 3 (0.20 and 0.21 for
complexes 1 and 2, respectively). The τ value (τ = 0 for a
perfectly square pyramidal and τ = 1 for a perfectly trigonal
bipyramidal geometry) also indicates that the environment
around the Co(II) center in all of the complexes is distorted
square pyramid. The shape of the molecule has been checked by
using the SHAPE 2.1 program38 (detailed geometric analyses for
the Co(II) ions are described in Table S1), and it is found that
Co(II) centers adopt “spherical square pyramidal” geometry
(Figure 1b) with C4v symmetry and the continuous shape
measure (CShM) value is 0.748 for 3. These values are 0.730
and 0.773 for 1 and 2, respectively (Figures S11a and S12a).
Scheme 1. Syntheses of Complexes 1−3
synthesized the complexes of Zn(II), Cd(II), Ni(II), and Co(II)
of this ligand.22−24 Co(II) and Cd(II) resulted in tetranuclear
complexes, whereas dinuclear and trinuclear complexes were
produced with Zn(II) and Ni(II), respectively. The addition of
triethylamine had no effect on the synthesis, composition, and
structures of these complexes. However, in the present case, no
C
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Figure 1. (a) ORTEP view of complex 3 with 30% ellipsoid probability. (b,c) Polyhedral view of the coordination environments around the Co and Dy
centers, respectively. Color code: O, red; N, blue; Co, pink; Dy, violet.
In all three complexes (1−3), Ln(III) metal ion is nona-
coordinated with two phenoxido oxygen atoms, two methoxido
oxygen atoms, three oxygen atoms of two carbonato groups, and
two oxygen atoms of one nitrato group. The carbonato group
bridges the Co(II) metal ion and the corresponding Ln(III)
metal ion of the dinuclear unit via the κ1O,O′ mode and further
chelates to the another Ln(III) of the adjacent dinuclear unit
with κ2O,O′ fashion, respectively. In addition to that, one
bidentate axial nitrato group is chelated to the Ln(III) metal ion
via κ1O,O′ fashion. The coordination geometry around the
Ln(III) ions was also ascertained by the SHAPE 2.1 program,
which revealed that for all three complexes, the structures are
intermediate between “spherical tricapped trigonal prism” (D3h
symmetry) and “spherical capped square antiprism” geometry
(C4v symmetry, Figure 1c) because the obtained CShM values
for all of the complexes for D3h symmetry as well as C4v symmetry
are almost the same (detailed geometric analyses for the Ln(III)
ions are described in Table S2). The Ln−O bond distances for
the three Ln ion with similar types of oxygen atoms (i.e.,
phenoxido/methoxido/carbonato/nitrato) are comparable. For
each complex, the Ln−O distances with the phenoxido groups
are the shortest, and the Ln−O distances with methoxido groups
are the longest. The distances between Co1···Ln1 are 3.361(1),
3.346(1), and 3.354(1) Å, and Ln1···Ln2 are 4.075(1),
4.054(1), and 4.078(1) Å for complexes 1−3, respectively.
The bridging angles Co(II)−O−Ln(III) vary from 100.01(1) to
100.49(2)° in all complexes. The bond distances and bond
angles relevant to the coordination to the metal centers are listed
in Table S3.
It is worth noting that three tetranuclear Co(II) complexes,
two dinuclear, one trinuclear, and one tetranuclear Ni(II)
complex, five trinuclear Cu(II) complexes, three dinuclear
Zn(II) complexes, and three tetranuclear Cd(II) complexes of
this flexidentate Mannich base ligand are reported.22−24 Besides
these homonuclear complexes, there are reports of seven
heterometallic Tr(II)−Ln(III) (Tr = transition metal) com-
plexes, all of which are dinuclear.25,26 In addition, two Ni(II)−
Na(I) complexes (one hexanuclear and one tetranuclear) have
also been characterized structurally.28 The nuclearity of the
complexes seems to depend on the metal ion; tetranuclear
complexes are generally formed when the coordination number
of the metal ion is six or higher. However, all of the M(II)−
Ln(III) complexes are dinuclear, although the coordination
number of Ln is more than six. This reason lies in the fact that in
these complexes, four of the coordination sites of the Ln are
blocked by DBM ligand25,26 (DBM = 1,3-diphenylpropane-1,3-
dione), reducing the available coordination number of Ln to less
than six.
Structural analyses of the reported tetranuclear complexes
reveal that in all of them the metal/deprotonated ligand (L2−)
ratio is 2:1, and two triply bridging groups (e.g., hydroxido,
methoxido, chlorido, azido, and acetato) are situated at the
opposite side of the plane passing through the four metal ions.
The present complexes are exceptional in that there is no μ3-
bridging group; instead, two μ4-bridging carbonate groups hold
the metal ions together. In these complexes, coordination sites of
Ln are not blocked, and hence to satisfy the higher coordination
number of Ln, μ4-bridging carbonate seems to be preferred to
any of the μ3-bridging groups. This also explains the affinity for
CO2 and its fixation during the formation of the heterometallic
Co(II)−Ln(III) complexes.
A Cambridge Structural Database (CSD) search for
carbonato-bridged Co(II)−Ln(III) complexes reveals that
there are seven such structures, and the nuclearity of the
complexes spans from 6 to 52. The present complexes are thus
the first examples of carbonato-bridged complexes of this
Mannich base ligand and are also the first examples of
tetranuclear Co(II)−Ln(III) complexes with any kind of ligand.
Catecholase Activity Study and Kinetics. The catecho-
lase-like activity of complexes 1−3 was investigated as previously
reported by us.39−41 The oxidation reaction of 3,5-di-tert-
butylcatechol to its corresponding o-quinone (3,5-DTBQ) is
shown in Scheme 2. The variation in absorption with time for
the oxidation of 3,5-DTBC by complex 3 is depicted in Figure 2.
Similar plots for 1 and 2 are shown in Figures S13 and S14,
respectively.
The kinetic studies have been carried out to estimate the
catalytic efficiency of all complexes (1−3), as previously
reported.39−41 The observed rate versus [substrate conc.] and
the Lineweaver−Burk (inset) plots for complex 3 are shown in
Figure 3. Similar plots for complexes 1 and 2 are given in the
Figures S15 and S16. The kinetic parameters for the complexes
D
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After the addition of 3,5-DTBC into the DMF/methanol
solution of 3, a drastic change in the overall mass spectrum is
noticed (Figures S17−S21). The base peak is observed at m/z =
1049.39 (calcd 1049.38), which can be assigned to [(CoIIL)-
DyIII(3,5-DTBSQ)2]+. The peak is very significant because it
indicates the formation of a semiquinonate of 3,5-DTBC. Along
with this, two other peaks are observed at m/z = 741.24 (calcd
741.25) and m/z 927.19 (calcd 927.18), assignable to the
species [(CoIIL)DyIIICO3(DMF)]+ and [(CoIIL)DyIIICO3(3,5-
DTBC)+K]+, respectively. The latter peak clearly indicates the
formation of a complex−substrate intermediate. Another
important peak is observed at m/z = 1072.36 (calcd 1072.35)
attributable to [(CoIIIL)DyIII(3,5-DTBC)2+Na]+.
Scheme 2. Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ in
DMF/Methanol
Mechanistic Insight. The probable mechanistic studies
with model copper complexes suggest that there are two possible
mechanisms for the oxidation of 3,5-DTBCH2 to its quinine
form (3,5-DTBQ). One process proceeds through the formation
of copper(I) semiquinonate (an organic radical intermediate)
with hydrogen peroxide as the byproduct, whereas other
mechanism involves a dicopper(II) catecholate intermediate
with the reduction of O2 molecule to water. This catalytic
reaction with various homometallic (Mn(III), Co(III), Ni(II),
Zn(II), etc.) and heterometallic (Ni(II)−Mn(II), Cu(II)−
Mn(II), etc.) complexes is found to proceed through the
formation of semiquinonate species.
Figure 2. Increase in the quinone band at 402 nm after the addition of
100 equiv of 3,5-DTBC to 10−4 M DMF/methanol solution of complex
3. The spectra were recorded at 5 min time intervals.
On the basis of ESI−MS, a probable mechanism is proposed
for catechol oxidation (Scheme 3). In the DMF/methanol
solution, the tetranuclear complex [CoII2DyIII2L2(μ4-
CO3)2(NO3)2] (3) produces doubly charged species
[CoII DyIII2L2(μ4-CO3)2]2+ (A) by releasing two nitrate ions,
2
which then undergoes dissociation into two dinuclear species
[(CoIIL)DyIIICO3(DMF)]+ (B) on coordination by DMF.
When 3,5-DTBCH2 is added to the solution of complex 3, it is
deprotonated and coordinated to B via the phenolic O atom by
replacing the DMF solvent molecule, and thus intermediate (C)
[(CoIIL)DyIIICO3(3,5-DTBCH)] is formed. Then, the other
phenolic O atom of 3,5-DTBC coordinates to the Ln atom to
produce the D [(CoIIL)DyIIICO3(3,5-DTBC)] intermediate.
Another doubly deprotonated 3,5-DTBC subsequently replaces
the carbonate ion of D to generate E [(CoIIIL)DyIII(3,5-
DTBC)2]. Here one Co(II) metal ion is oxidized to Co(III).
After that, intermediate E is converted into its semiquinonate
radical forms (F and G) [(CoIIL)DyIII(3,5-DTBSQ)2] with the
reduction of Co(III) to Co(II). Finally, the semiquinone
molecules (3,5-DTBSQ) are oxidized to quinone (3,5-DTBQ)
and eliminated with the reduction of oxygen to hydrogen
peroxide.
Figure 3. Plot of the rate versus substrate concentration for complex 3.
Inset shows the corresponding Lineweaver−Burk plot.
To get information about the mechanism of the catalytic
reaction for these present heterometallic Co(II)−Ln(III)
complexes, we observed that H2O2 is produced when all
complexes (1−3) are mixed with 3,5-DTBC. The amount of
formed H2O2 was estimated iodometrically10,11(Figures S22 and
S23), and it has been found that about 84, 89, and 92% H2O2 was
generated for complexes 1−3, respectively, after 1 h of oxidation
of 3,5-DTBC to 3,5-DTBQ.
It is also important to mention that previously reported
homometallic cobalt complexes of this polytopic Mannich base
ligand do not show any catecholase-like activity (Figure S24),
whereas the present heterometallic Co(II)−Ln(III) complexes
are moderately active toward the catechol oxidation. Various
factors may affect the catalytic activity of the complexes;
however, in the present case, the generated catalytically active
dinuclear species seems to be the most important. In this
are listed in Table S4. The turnover numbers (kcat) are calculated
to be 254.5, 272.4, and 291.3 h−1 for complexes 1−3,
respectively.
ESI−MS Study. To draw the mechanistic pathways of
catecholase-like activity, we have recorded ESI−MS spectra of
complexes 1−3 and a 1:10 mixture (v/v) solution of the
complex 3 and 3,5-DTBC in DMF/methanol solution within 10
min of mixing. In the spectra of complexes, base peaks are
observed at m/z = 663.07 (calcd 663.06), m/z = 664.08 (calcd
664.07), and m/z = 668.07 (calcd 668.06) for complexes 1−3 (z
= 2), respectively, which can be assigned to doubly charged
[(CoIIL)LnIIICO3]2+ species (where Ln = Gd, Tb, and Dy for
1−3). In addition, two low-intensity peaks at m/z = 446.17
(calcd 446.16) and 468.14 (calcd 468.15) are observed for the
[(CoIIL)+H]+ and [(CoIIL)+Na]+ species, respectively.
E
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Scheme 3. Proposed Mechanism for the Catalytic Oxidation of 3,5-DTBC to 3,5-DTBQ by Complex 3
Scheme 4. Catalytic Oxidation of o-Aminophenol (OAP) to Phenoxazinone (APX) in DMF/Methanol
dinuclear species, the oxophilic and coordinatively unsaturated
Ln ion binds easily with the substrate (catechol) through the
phenolic oxygen, and the oxidation state of cobalt shuttles
between +2 and +3 to do the job of the redox reaction. Hence,
the cooperative activity of these two metal ions is considered to
be responsible for the catecholase-like activity of these
heterometallic complexes.
Phenoxazinone-Synthase-Like Activity. The phenoxazi-
none-synthase-like activity for complexes 1−3 has also been
studied. The detailed method was previously described.24,40,41
The o-aminophenol (OAP) is oxidized and dimerized to
phenoxazinone (APX) by the catalyst shown in Scheme 4.
The variation in absorption with time for the oxidation of OAP
with the addition of complex 3 is depicted in Figure 4, and these
kinds of plots for complexes 1 and 2 are given in Figures S25 and
S26.
The kinetic studies have been carried out to estimate the
extent of the catalytic efficiency of all complexes (1−3), as
previously reported.24,40−44 The observed rate versus [substrate
conc.] plot and the Lineweaver−Burk plots (inset) for complex
3 are depicted in Figure 5. Similar plots for complexes 1 and 2
are given in Figures S27 and S28.
Analyzing the experimental data, we obtained the kinetic
parameters for complexes 1−3, which are summarized in Table
F
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species, [(CoL)+Na]+, that appears at m/z = 468.15 (calcd
468.14). Along with this base peak, another two peaks are
observed at m/z = 883.03 (calcd 883.04) and m/z = 1032.62
(calcd 1032.61), which indicate the presence of species
[(CoIIL)DyIIICO3(OAP)(DMF)(CH3OH)+H+]+ and
[(CoIIIL)DyIIICO3(OAP)2(DMF)2+H+]+, respectively, indicat-
ing the coordination of OAP to the complex. Another peak is
found at m/z 741.25 (calcd 741.26) due to the formation of
[(CoIIIL)DyIIICO3(DMF)]+.
Mechanistic Insight. The mechanistic pathways for the
oxidation of o-aminophenol by a few homometallic and
heterometallic complexes containing transition-metal ions are
reported in literature. Until now there, there has been no report
of any 3d−4f heterometallic catalyst. The study of the
mechanism of the catalysis using these three complexes 1−3
reveals that this reaction proceeds via an o-aminophenolate
radical intermediate with the change of oxidation number of
cobalt between +2 and +3. In this process, no H2O2 is produced;
oxygen is reduced to H2O.
On the basis of ESI−MS, we propose a probable mechanism
(Scheme 5) for this catalysis. Like the catecholase activity, the
tetranuclear complex [Co2Dy2L2(μ4-CO3)2(NO3)2] (3) (spe-
cies A) in the DMF/methanol solution produce the catalytically
active dinuclear [(CoIIIL)DyIIICO3(DMF)] species (B). After
the addition of o-aminophenol to the solution of complex 3, an
OAP coordinates to species B, producing the intermediate
[(CoIIL)DyIIICO3(OAP)(DMF)(CH3OH)] (C). Another o-
aminophenol molecule is then attached to the C species to form
D [(CoIIIL)DyIIICO3(OAP)2(DMF)2]. In this step, Co(II) is
oxidized to Co(III) by oxygen, which is reduced to water. The
nitrogen atom of OAP is then coordinated to the Dy(III) metal
ion donating its lone pair, and species E is produced. Finally,
species F is generated by the formation of o-aminophenolate,
and the active catalyst (B) is regenerated with the release of H2O
and phenoxazinone (APX).
Figure 4. Increase in the APX band at 432 nm after the addition of 100
equiv of OAP to a 5 × 10−5 M DMF/methanol solution of complex 3.
The spectra were recorded at 5 min time intervals.
Magnetic Properties. The variable-temperature direct-
current magnetic susceptibility data of complexes 1−3 were
measured on powdered microcrystalline samples within the 2−
300 K temperature range under an applied magnetic field of 0.1
T and are presented in the form of χMT versus T plot (χM is
molar magnetic susceptibility) in Figure 6a. At 300 K, the χMT
values for complexes 1, 2, and 3 are equal to 19.65, 27.97, and
32.44 cm3 mol−1 K, respectively, which are close to the estimated
values 19.50 cm3 mol−1 K (for 1, with two uncoupled Co(II), S =
3/2, g = 2 and two Gd(III), S = 7/2, L = 0, 8S7/2, g = 1.99), 27.40
cm3 mol−1 K (for 2, with two uncoupled Co(II), S = 3/2, g = 2
and two Tb(III), S = 3, L = 3, 7F6, g = 3/2), and 32.10 cm3 mol−1
K (for 3, with two uncoupled Co(II), S = 3/2, g = 2 and two
Dy(III), S = 5/2, L = 5, 6H15/2, g = 1.33). The agreement of the
experimental and calculated χMT values at 300 K confirms the
penta-coordinate geometry around the Co(II) ions in all of the
complexes that is responsible for quenching of the orbital
contribution due to symmetry reduction compared with the
octahedral geometry.45 For complex 1, the χMT plot decreases
(Figure 6a) very gradually from 19.65 cm3 mol−1K at 300 K to
18.89 cm3 mol−1 K at 16 K, below which there is an abrupt
increase in the χMT value up to 28.71 cm3 mol−1 K at 2 K. For
complex 2, where the Gd(III) ions are replaced by Tb(III) ions,
the χMT plot shows a slow increase from 27.97 cm3 mol−1 K at
300 K to 32.19 cm3 mol−1 K at 18 K and a sharp rise to a value of
47.48 cm3 mol−1 K at 2 K. This overall behavior is due to an
intramolecular ferromagnetic interaction among the Co(II) and
Tb(III) ions, which is weak at the high-temperature zone and
Figure 5. Plot of the rate versus substrate concentration for complex 3.
Inset shows the corresponding Lineweaver−Burk plot.
S5. The kcat values for complexes 1−3 are found to be 2930.6,
2965.2, and 2998.5 h−1, respectively. Until now, the phenox-
azinone-synthase-like catalytic activity of this type of hetero-
metallic 3d−4f tetranuclear complex has not been investigated.
However, when compared with the kcat values of similar
tetranuclear Co(II) complexes of this ligand (500.41−511.20
h−1), the turnover numbers of these complexes are found to be
considerably higher.
ESI−MS Study. To get a better understanding of the
catalyst−substrate intermediate of phenoxazinone-synthase-like
activity during the catalytic reaction and to establish the
probable mechanism, we have recorded ESI−MS spectra for
complexes 1−3 and a 1:10 mixture of the complex 3 and OAP
within 5 min of mixing in DMF/methanol solvent.
Here we have focused only on the spectra of complex 3 and
the substrate mixture. In the spectrum of complex 3, a base peak
is observed at m/z = 668.07 (calcd 668.06) assignable to doubly
charged [(CoIIL)DyIIICO3]2+ species. After the addition of OAP
to the solution of complex 3, a drastic change is observed in the
overall mass spectrum (Figures S29 and S30). In the spectrum of
the solution of complex 3 and OAP, the base peak is the sodiated
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Scheme 5. Proposed Mechanism for Catalytic Oxidation of OAP by Complex 3
Figure 6. (a) Plot of χMT versus T for complexes 1 (Co2Gd2), 2 (Co2Tb2), and 3 (Co2Dy2) in the temperature range of 2−300 K. (b) Magnetization
(M (NμB) vs H) plots at 2 K for complexes 1 (Co2Gd2), 2 (Co2Tb2), and 3 (Co2Dy2) in the field range of 0−5 T.
strong in the low-temperature regime. Finally, the introduction
of Dy(III) ions instead of Gd(III) and Tb(III) ions gives a new
complex 3, for which upon cooling from room temperature, the
χMT increases to 45.71 cm3 mol−1 K at 5 K, followed by a steep
fall to 36.52 cm3 mol−1 K at 2 K. This increase in the magnetic
susceptibilities from 300 to 5 K suggests an overall intra-
H
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Figure 7. Temperature dependence of χM′ signals (a) and χM″ signals (b) for complex 2 under 1000 Oe DC field.
Figure 8. Temperature dependence of χM′ signals (a) and χM″ signals (b) for complex 3 under 1000 Oe DC field.
Figure 9. ln (χ″/χ′) versus T−1 plot for 1−3. The red lines are the best fit to the Debye equation.
molecular ferromagnetic interaction among the paramagnetic
ions, whereas the decrease in the χMT plot recorded at very low
temperatures below 2 K may be attributed to the zero-field
splitting (ZFS) effect or weak intermolecular interactions.46
The isothermal magnetization data (M (NμB)) recorded in
the field range of 0−5 T at 2 K for 1−3 (Figure 6b) show that the
magnetization curve rises sharply at low fields and achieves
maximum values of 20.72 NμB for 1, 24.23 NμB for 2, and 26.27
NμB for 3 at 5 T. These highest values obtained at 5 T
approximate to the values expected for two isolated Co(II) ions
and two lanthanide ions (M for 1 = 19.93 NμB, M for 2 = 24.00
NμB, and M for 3 = 26.00 NμB). The nature of the plots suggests
the presence of magnetic anisotropy or considerable crystal-field
effects in the complexes.47,48 Therefore, the dynamics of
magnetization for all of the complexes were investigated by
alternating-current (AC) magnetic measurements with respect
to temperature and frequency at 3.5 Oe AC field and in zero and
higher direct-current (DC) fields.
At zero DC field, the AC magnetic susceptibility plots (χM′
and χM″ vs T) show both temperature- and frequency-
dependent signals, which indicate the occurrence of slow
relaxation of magnetization behavior and a distinctive feature of
single-molecule magnets (SMMs).49,50 However, the in-phase
signals (χM′ vs T) are very weak in comparison with the out-of-
phase (χM″ vs T) signals. Although there is a prominent χM″
upsurge in the out-of-phase plots, no peaks appeared above 2 K
under zero DC field for any of the complexes, which is mainly
due to the fast quantum tunnelling of magnetization
(QTM).51−54 To reduce the possible QTM and to evaluate
the SMM behavior for the present complexes, χM′ and χM″ plots
were investigated further with a DC bias field of 1000 Oe.
Figures 7 and 8 and Figure S31 show the results of the AC
susceptibility measurements (χM′ and χM″ vs T) for 1−3 at 1000
Oe DC field. It is observed that for complexes 1 and 2, χM′ versus
T remained almost unchanged, but for complex 3, the plot shows
better frequency dependency at the lower temperature region.
On the contrary, in χM″ versus T plots, no distinct peaks were
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observed for any of the complexes, even in the presence of this
higher DC field. The above observation suggests that not
quantum tunnelling but a small energy barrier is responsible for
the absence of maxima in complexes 1−3.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00121.
■
S
To evaluate the effective energy barrier (Ueff) and relaxation
time (τ0) for the magnetization reversal phenomenon in SMMs,
fitting of χM″ versus T data is generally done according to the
Arrhenius equation, but this requires the occurrence of maxima
in the χM″ versus T plots. Because in complexes 1−3, χM″ versus
T does not exhibit peak maxima in the measured temperature
range, an estimate of Ueff and τ0, by assuming a single relaxation
process, can be obtained by using the Debye equation, ln(χM″/
χM′) = ln(ωτ0) + Ueff/kBT (ω = angular frequency). The best
linear fit according to the Debye equation (Figure 9) yields Ueff =
UV, IR, mass spectra, table of bond parameters of
complexes 1−3, catalytic activity parameters and kcat
values of complexes, as well as the magnetic plots of
complexes 1−3 (PDF)
Accession Codes
CCDC 1890698−1890700 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
1.99 K, τ0 = 6.97 × 10−6 s for 1, Ueff = 2.79 K, τ0 = 2.16 × 10−5
s
for 2, and Ueff = 8.98 K, τ0 = 3.67 × 10−6 s for 3. Further evidence
of the relaxation processes occurring in the complexes is
obtained from the Cole−Cole or the Argand plot, that is, the
plot of χM″ versus χM′, and this is shown in Figure S32.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ghosh_59@yahoo.com.
ORCID
Soumyabrata Goswami: 0000-0002-9646-1439
Ashutosh Ghosh: 0000-0003-2026-7565
Notes
■
CONCLUSIONS
■
Co(II)−Ln(III) complexes of the multidentate Mannich base
ligand, H2L = N,N′-dimethyl-N,N′-bis(2-hydroxy-3-methoxy-5-
methylbenzyl)ethylenediamine, were synthesized by reacting it
with Co(OAc)2·4H2O and lanthanide(III) nitrate (Ln =
Gd(III), Tb(III), and Dy(III)) under basic conditions. These
resulting complexes are tetranuclear, as are several reported
complexes of this ligand with other transition-metal ions, but
instead of the two μ3 anionic bridging groups, which is a
common feature of such tetranuclear species, here two μ4-
carbonato bridges are present, formed by the atmospheric CO2
fixation. The higher coordination number of the lanthanide ions
seems to be responsible for the “carbonatophilicity” of the
reaction. Incidentally, these are the first carbonato-bridged
tetranuclear Co(II)−Ln(III) complexes, although several Co-
(II)−Ln(III) complexes of other nuclearities are known. The
introduction of the Ln(III) ion has brought substantial changes
in the catalytic oxidase properties of the complexes compared
with the homometallic tetranuclear Co(II) complexes of this
ligand. All three present complexes show moderate catecholase-
like activity and quite high phenoxazinone-synthase-like activity,
although their homometallic Co(II) analogues did not exhibit
catecholase-like activity and their phenoxazinone-synthase-like
activity was considerably lower. The mass spectral analyses
suggest that the dinuclear [(CoIIL)DyIIICO3]2+ ion that is
formed by the dissociation of the tetranuclear complex is the
catalytically active species in the present cases. The higher
oxophilicity of Ln(III) ion facilitates the binding of the substrate
to it, while the cobalt ion shuttles between +2 and +3 states to do
the job of oxidation with the help of molecular oxygen. In other
words, the improved catalytic oxidase activities of the present
complexes seem to be due to the heterometallic cooperative
activity in the active species. DC magnetic susceptibility studies
reveal ferromagnetic coupling between Co(II) and Ln(III) for
all three complexes. Isothermal magnetization measurements
provided a signature of anisotropy in the complexes that
provoked the AC susceptibility measurements, which confirmed
the slow relaxation of magnetization behavior in the complexes.
Thus the results of this work demonstrate the development of
heterometallic (3d−4f) coordination complexes with interesting
structural features, catalytic behaviors, and magnetic properties
and should incite further study in this research field.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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. R.S.
thanks DST-SERB for the early carrier research grant (sanction
no. ECR/2016/001572). We are grateful to CRNN, University
of Calcutta, Kolkata, India for the facility of magnetic
measurement.
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