Unprecedented structural variations in trinuclear mixed valence Co(ii/iii) complexes: Theoretical studies, pnicogen bonding interactions and catecholase-like activities

Article abstract of DOI:10.1039/c4dt03446e

Three new mixed valence trinuclear Co(ii/iii) compounds cis-[Co₃L₂(MeOH)₂(N₃)₂(μ_1,1-N₃)₂] (1), trans-[Co₃L₂(H₂O)₂(N₃)₂(μ_1,1-N₃)₂]·(H₂O)₂ (2) and [Co₃L^R₂(N₃)₃(μ_1,3-N₃)] (3) have been synthesized by reacting a di-Schiff base ligand (H₂L) or its reduced form [H₂L^R] (where H₂L = N,N′-bis(salicylidene)-1,3-propanediamine and H₂L^R = N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine) with cobalt perchlorate hexahydrate and sodium azide. All three products have been characterized by IR, UV-Vis and EPR spectroscopies, ESI-MS, elemental, powder and single crystal X-ray diffraction analyses. Complex 1 is an angular trinuclear species in which two terminal octahedral Co(iii)N₂O₄ centers coordinate to the central octahedral cobalt(ii) ion through μ₂-phenoxido oxygen and μ_1,1-azido nitrogen atoms along with two mutually cis-oxygen atoms of methanol molecules. On the other hand, in linear trinuclear complex 2, in addition to the μ₂-phenoxido and μ_1,1-azido bridges with terminal octahedral Co(iii) centres, the central Co(ii) is bonded with two mutually trans-oxygen atoms of water molecules. Thus the cis-trans configuration of the central Co(ii) is solvent dependent. In complex 3, the two terminal octahedral Co(iii)N₂O₄ centers coordinate to the central penta-coordinated Co(ii) ion through double phenoxido bridges along with the nitrogen atom of a terminal azido ligand. In addition, the two terminal Co(iii) are connected through a μ_1,3-azido bridge that participates in pnicogen bonding interactions (intermolecular N-N interaction) as an acceptor. Both the cis and trans isomeric forms of 1 and 2 have been optimized using density functional theory (DFT) calculations and it is found that the cis configuration is energetically more favorable than the trans one. However, the trans configuration of 2 is stabilized by the hydrogen bonding network involving a water dimer. The pnicogen bonding interactions have been demonstrated using MEP surfaces and CSD search which support the counter intuitive electron acceptor ability of the μ_1,3-azido ligand. Complexes 1-3 exhibit catecholase-like activities in the aerial oxidation of 3,5-di-tert-butylcatechol to the corresponding o-quinone. Kinetic data analyses of this oxidation reaction in acetonitrile reveal that the catecholase-like activity follows the order: 1 (k_cat = 142 h^-1) > 3 (k_cat = 99 h^-1) > 2 (k_cat = 85 h^-1). Mechanistic investigations of the catalytic behaviors by X-band EPR spectroscopy and estimation of hydrogen peroxide formation indicate that the oxidation reaction proceeds through the reduction of Co(iii) to Co(ii). This journal is

Full text of DOI:10.1039/c4dt03446e

View Article Online
View Journal
Dalton
Transactions
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. Hazari, L. K.
Das, R. M. Kadam, A. Bauza, A. Frontera and A. Ghosh, Dalton Trans., 2015, DOI: 10.1039/C4DT03446E.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/dalton

Page 1 of 15
Dalton Transactions
View Article Online
DOI: 10.1039/C4DT03446E
Graphical Abstract
Unprecedented structural variations in trinuclear mixed valence Co(II/III)
complexes derived from a Schiff base and its reduced form: Theoretical
studies, pnicogen bonding interactions and catecholase-like activities
Alokesh Hazari, Lakshmi Kanta Das, Ramakant M. Kadam, Antonio Bauzá, Antonio
Frontera* and Ashutosh Ghosh*
Solvent induced cis and trans forms of a mixed valence trinuclear Co(II/III) complex with a
salen type Schiff base ligand are isolated. The reduction of the Schiff base results in the
formation of an unprecedented cyclic trinuclear mixed valence Co(II/III) complex in which a
µ1,3-N3 coordinated azido ligand participates in pnicogen bonding interactions established by
DFT calculations. The catecholase like activities in the aerial oxidation of 3,5-di-tert-
butylcatechol to the corresponding o-quinone of all three complexes have been examined.

Dalton Transactions
Page 2 of 15
Dynamic Article Links►
Journal Name
View Article Online
DOI: 10.1039/C4DT03446E
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/xxxxxx
ARTICLE TYPE
Unprecedented structural variations in trinuclear mixed valence Co(II/III)
complexes derived from a Schiff base and its reduced form: Theoretical studies,
pnicogen bonding interactions and catecholaseꢀlike activities
5
Alokesh Hazari,a Lakshmi Kanta Das,a Ramakant M. Kadam,b Antonio Bauzá,c Antonio
Frontera*c and Ashutosh Ghosh*a
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Three new mixed valence trinuclear Co(II/III) compounds cisꢀ[Co3L2(MeOH)2(N3)2(ꢀ1,1ꢀN3)2] (1), transꢀ[Co3L2(H2O)2(N3)2(ꢀ1,1
ꢀ
10 N3)2]⋅(H2O)2 (2) and [Co3LR (N3)3(ꢀ1,3ꢀN3)] (3) have been synthesized by reacting a diꢀSchiff base ligand (H2L) or its reduced form
2
[H2LR] (where H2L = N,N′ꢀbis(salicylidene)ꢀ1,3ꢀpropanediamine and H2LR = N,N′ꢀbis(2ꢀ hydroxybenzyl)ꢀ1,3ꢀpropanediamine) with
cobalt perchlorate hexahydrate and sodium azide. All three products have been characterized by IR, UVꢀVis and EPR spectroscopies,
ESIꢀMS, elemental, powder and single crystal Xꢀray diffraction analyses. Complex 1 is an angular trinuclear species in which two
terminal octahedral Co(III)N2O4 centers coordinate to the central octahedral cobalt(II) ion through µ2ꢀphenoxido oxygen and µ1,1ꢀazido
15 nitrogen atoms along with two mutually cisꢀoxygen atoms of methanol molecules. On the other hand, in linear trinuclear complex 2, in
addition to the µ2ꢀphenoxido and µ1,1ꢀazido bridges with terminal octahedral Co(III) centres, the central Co(II) is bonded with two
mutually transꢀoxygen atoms of water molecules. Thus the cisꢀtrans configuration of the central Co(II) is solvent dependent. In complex
3, the two terminal octahedral Co(III)N2O4 centers coordinate to the central pentaꢀcoordinated Co(II) ion through double phenoxido
bridges along with nitrogen atom of a terminal azido ligand. In addition, the two terminal Co(III) are connected through a µ1,3ꢀazido
20 bridge that participates in pnicogen bonding interactions (intermoluclar N–N interaction) as acceptors. Both the cis and trans isomeric
forms of 1 and 2 have been optimized using density functional theory (DFT) calculations and it is found that the cis configuration is
energetically more favorable than the trans one. However, trans configuration of 2 is stabilized by the hydrogen bonding network
involving a water dimer. The pnicogen bonding interactions have been demonstrated using MEP surfaces and CSD search which support
the counter intuitive electron acceptor ability of the µ1,3ꢀazido ligand. Complexes 1−3 exhibit catecholaseꢀlike activities in the aerial
25 oxidation of 3,5ꢀdiꢀtertꢀbutylcatechol to the corresponding oꢀquinone. Kinetic data analyses of this oxidation reaction in acetonitrile
reveal that the catecholaseꢀlike activity follows the order: 1 (kcat = 142 h−1) >3(kcat = 99 h−1) >2 (kcat = 85 h−1). Mechanistic investigations
of the catalytic behaviors by Xꢀband EPR spectroscopy and estimation of hydrogen peroxide formation indicate that the oxidation
reaction proceeds through the reduction of Co(III) to Co(II).
Recently, it has been shown that the use of reduced Schiff base
ligands may bring about fascinating variations in the structure of
the complexes mainly due to fact that (i) the reduction of the rigid
30 Introduction
Cobalt complexes derived from Schiff bases have attracted an
extensive interest in the area of coordination chemistry due to
their intriguing structural features and various applications 55 azomethine (–CH=N–) fragment to less constrained –CH2–NH–
covering catalysis in organic synthesis, photochemistry,
35 electrochemistry and important biological applications.1–4 The
salenꢀtype diꢀSchiff base ligands on reaction with Co(II) in an
aerobic condition are known to afford mixed valence triꢀnuclear
moiety make them more flexible compared to the Schiff bases16
and (ii) the introduction of a Hꢀatom on the nitrogen not only has
considerable effect on the crystal packing but it can even stabilize
rather unusual isomers.17
complexes.5 The main structural feature of these complexes is a 60 In this present investigation, our aim is to synthesize mixed
central Co(II) ion linked to two terminal Co(III) ions through
40 double phenoxido bridges along with an additional anionic bridge
between the terminal and central cobalt atoms, which is usually a
carboxylate group6 except three examples where this bridge is
valence cobalt complexes using a salenꢀtype diꢀSchiff base
ligand and its corresponding reduced analogue along with azide
as coligand in order to study their structural variations and
catecholaseꢀlike activities.18 Catecholaseꢀlike activity of some
chloride, azido, or sulphito.7–9 It is interesting to mention that in 65 model coordination complexes has been a topic of recent interest
most of such complexes the two anionic bridges are mutually cis
45 excepting one in which two mutually transꢀacetate ions link the
terminal Co(III) to central Co(II).10 This feature is somewhat
extraordinary as the similar homoꢀ and heteroꢀtrinuclear
for the development of new bioinspired catalysts.19 Catechol
oxidase is a copperꢀcontaining typeꢀIII activeꢀsite protein that
catalyzes the oxidation reaction of a wide range of oꢀdiphenols
(catechols) to corresponding oꢀquinones through a process known
complexes of divalent metal ions (Mn(II), Co(II), Ni(II),Cu(II), 70 as catecholase activity. From the reports of distinguished research
Zn(II)) are abundant with various anionic coligands (e.g.
50 carboxylate, azide, cyanate, etc) and usually these bridges are
mutually trans to each other.11−15
groups, the ability of dicopper complexes to oxidize phenols and
catechols is well established.20 From these extensive studies, it is
concluded that various structural factors such as metal−metal
distances, electrochemical properties of the complexes,
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 |1

Page 3 of 15
Dalton Transactions
View Article Online
DOI: 10.1039/C4DT03446E
exogenous bridging ligands, the ligand structure, and the pH can
vacuum to result in a viscous liquid, followed by addition of a
mixture of petroleum ether and chloroform. The extracted pure
yellow solid mass of ligand was dissolved in methanol or ethanol
and used for complexation. The ligand was characterized by
70 elemental analyses and 1H NMR study.
Ligand (H2L). Anal. Calcd for C17H18N2O2: C 72.32, H 6.43, N
9.92; Found: C 72.48, H 6.69, N 10.05%. 1HꢀNMR (CDCl3,
300 MHz) (Fig. S1) in ppm: h = 2.08ꢀ2.17 (m, 2H, CH2); g =
3.746ꢀ3.704 (t, 2H, CH2); b,c,d and e = 6.863ꢀ7.348 (aromatic H);
75 a = 8.380 (S, 1H,CH=Nꢀ)
influence the catecholase activity.19−21 Furthermore, the
mechanistic studies for the oxidation of catecholic substrate to
their respective quinone reveals that the reaction proceeds
through the formation of either a dicopper(II) catecholate
intermediate or an organic radical intermediate such as copper(I)
semiquinonate.22 Recent investigations have also shown that
some manganese(II/III), nickel(II), zinc(II) and cobalt(II/III)
species can also mediate such catechol oxidation.23−26 It becomes
5
10 evident that in the case of Ni(II) and Mn(III) metalꢀcentered
redox participation is most probably responsible for the
catecholase activity of these model compounds whereas in case
of redox innocent Zn(II) this activity can be promoted through the
generation of ligandꢀcentered radical. However, to the best of our
15 knowledge, no mechanistic studies have yet been done on the
catecholaseꢀlike activity of cobalt complexes.
Synthesis of the reduced Schiff base ligand, N,N'ꢀbis(2ꢀ
hydroxybenzyl)ꢀ1,3ꢀpropanediamine (H2LR)
The diꢀSchiff base ligand (H2L) was synthesized as stated above.
Then 20 mL (5 mmol) of this prepared methanolic ligand solution
80 (H2L) was cooled to 0°C, and solid sodium borohydride (570 mg,
15 mmol) was added to this methanolic solution with stirring.
After completion of addition, the resulting solution was acidified
with concentrated HCl (10 mL) and then evaporated to dryness.29
The reduced Schiff base ligand H2LR was extracted from the solid
85 mass with methanol. The ligand was purified in same procedure
as described above for H2L. The purified ligand was dissolved in
methanol and used for complexation.
In this report, we have synthesized and structurally characterized
two
mixed
valence
trinucler
complexes
and
cisꢀ
transꢀ
[Co3L2(MeOH)2(N3)2(ꢀ1,1ꢀN3)2]
(1)
20 [Co3L2(H2O)2(N3)2(ꢀ1,1ꢀN3)2]⋅(H2O)2 (2) by reacting a diꢀSchiff
base ligand (H2L) (where H2L = N,N′ꢀbis(salicylidene)ꢀ1,3ꢀ
propanediamine), cobalt perchlorate hexahydrate and sodium
azide in two different solvents, methanol and ethanol,
respectively. Two solvent molecules (methanol or water) which
25 coordinate to the central Co(II) are cis in 1 but trans in 2. Another
Ligand (H2LR). Anal. Calcd for C17H22N2O2: C 71.30, H 7.74, N
9.78; Found: C 71.48, H 7.69, N 9.65%. 1HꢀNMR (CDCl3,
90 300 MHz) (Fig. S2) in ppm: h = 1.68ꢀ1.82 (m, 2H, ꢀCH2); k =
2.08 (s, H, ꢀNH) g = 2.71ꢀ3.79 (m, 2H, ꢀCH2); a = 3.99 (s, 2H, ꢀ
CH2) b,c,d and e = 6.75ꢀ7.19 (aromatic H).
complex, [Co3LR (N3)3(ꢀ1,3ꢀN3)] (3) has been synthesized with
2
the reduced form of this Schiff base ligand, N,N′ꢀbis(2ꢀ
hydroxybenzyl)ꢀ1,3ꢀpropanediamine (H2LR). The unique feature
of 3 is a ꢀ1,3ꢀN3 bridge between two terminal Co(III) and its
30 participation in pnicogen bonding interactions as acceptor.
Theoretical study has been performed using DFT calculations at
the BP86ꢀD3/def2ꢀTZVP level of theory, to analyze (i) the energy
difference between the cisꢀtrans isomers, (ii) how the solvent
molecule influences the stability of the isomers, (iii) the probable
35 explanation for the isolation of both the isomers in the solid state,
and (iv) some nonꢀcovalent interactions including pnicogen
bonding interactions27 observed in the crystal packing of complex
3. To the best of our knowledge, the cisꢁtrans configurational
changes in mixed valentCo(III)–Co(II)–Co(III) as in 1 and 2, and
Synthesis of the complex cisꢀ[Co3L2(MeOH)2(N3)2(µ1,1ꢀN3)2]
(1)
95 The yellow colored methanolic solution (20 mL) of the diꢀSchiff
base (H2L) ligand (2 mmol) was taken in a beaker. To this
methanolic solution, a water solution (1 mL) of Co(ClO4)2·6H2O
(1.098 g, 3 mmol) followed by an aqueous solution (1 mL) of
sodium azide (0.260 g, 4mmol) was added. The mixture was
100 stirred for 1 h and then filtered. The filtrate was allowed to stand
overnight when needle shaped dark brown Xꢀray quality single
crystals of 1 appeared at the bottom of the beaker. The crystals
were washed with a methanol−water mixture and dried in a
desiccator containing anhydrous CaCl2 and then characterized by
105 elemental analysis, spectroscopic methods, and Xꢀray diffraction.
40 the presence of pnicogen interaction in azide in
3 are
Complex 1: Yield: 0.834 g, 86%.
Anal. Calc. for
unprecedented. All three complexes show catecholaseꢀlike
activity. The EPR spectral investigation reveals that the process is
most likely associated with the reduction of cobalt(III) to
cobalt(II), as proposed for the copperꢀbased models of this
45 biocatalyst.
C36H40Co3N16O6: C 44.59, H 4.16, N 23.11; found: C 44.72, H
4.35, N 22.95%; UV/vis: [λmax in nm (εmax in M−1 cm−1)] (MeCN)
= 618(379), 377(21661), and 276(17532) and λmax in nm (solid,
110 reflectance) = 617 and 378. IR (KBr) in cm−1: ν(C=N) 1621,
ν(N3) 2061 and 2010. HRMS (m/z, ESI+): found for [CoL(Na)]+
= 362.0504 (calc. 362.0441), [Co2L2(CH3CN)Na]+ = 743.1162
(calc. 743.1284), [Co3L2(N3)3]+ = 863.0811 (calc. 863.0696).
Experimental
Starting Materials
115 Synthesis of the complex, transꢀ[Co3L2(H2O)2(N3)2(µ1,1
N3)2]⋅(H2O)2 (2)
Complex 2 was prepared by mixing the same components in the
same stoichiometric ratio as for 1 but using ethanol instead of
methanol as solvent. In this case, square shaped blackish brown
120 Xꢀray quality single crystals appeared at the bottom of the vessel
on keeping the filtrate at open atmosphere.
ꢀ
50 The
salicylaldehyde,
1,3ꢀpropanediamine
and
sodium
borohydride were purchased from Lancaster and were of reagent
grade. They were used without further purification.
Caution! Azide salts and Perchlorate salts of metal complexes
with organic ligands are potentially explosive. Only a small
55 amount of material should be prepared and it should be handled
with care.
Complex 2: Yield: 0.694 g, 71%. Anal. Calc. for
C34H40Co3N16O8: C 41.77, H 4.12, N 22.92; found: C 41.54, H
4.32, N 23.06%; UV/vis: [λmax in nm (εmax in M−1 cm−1)] (MeCN)
125 = 619(512), 398(26274), and 272(17537) and λmaxin nm (solid,
reflectance) = 620 and 399. IR (KBr) in cm−1: ν(C=N) 1621,
ν(N3) 2062 and 2010, HRMS (m/z, ESI+): found for [CoL(Na)]+
= 362.0431 (calc. 362.0441), [Co2L2 (CH3CN) Na]+ = 743.1058
(calc. 742.1284), [Co3L2(N3)3]+ = 863.0678 (calc. 863.0696).
130
Synthesis of the Schiff Base Ligand, N,N′ꢀbis(salicylidene)ꢀ
1,3ꢀpropanediamine (H2L)
60 The Schiff base ligand was synthesized by a reported method.28
5
mmol of 1,3ꢀpropanediamine (0.42 mL) was mixed with 10 mmol
of the salicylaldehyde (1.04 mL) in methanol or ethanol (20 mL).
The resulting solution was refluxed for ca. 2 h and allowed to
cool. The resulting yellow solution of the ligand was purified by
65 removing methanol completely from the resultant solution under
2
|Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Dalton Transactions
Page 4 of 15
View Article Online
DOI: 10.1039/C4DT03446E
Synthesis of the complex [Co3LR(N3)3(µ1,3ꢀN3)] (3)
crystallographic data for the three complexes are given in Table
Complex 3 was prepared using reduced Schiff base ligand with 65 S1.
the same molar ratios of reactant as for 1 and 2. To the colourless
methanolic solution (20 mL) of the reduced Schiff base ligand
(H2LR), a water solution (1 mL) of Co(ClO4)2·6H2O (1.098 g, 3
mmol) followed by an aqueous solution (1 mL) of sodium azide
Theoretical methods
The energies of all complexes included in this study were
computed at the BP86ꢀD3/def2ꢀTZVP level of theory. The
5
(0.260 g, 4 mmol) were added. The mixture was stirred for 1 h 70 geometries have been fully optimized for the energetic analysis of
and then filtered. The filtrate was allowed to stand overnight
when rhombic shaped deep brown Xꢀray quality single crystals of
the cis/trans isomerism. Instead, we have used the
crystallographic coordinates for the theoretical analysis of the
nonꢀcovalent interactions observed in the solid state. The
calculations have been performed by using the program
10 complex 3 appeared at the bottom of the beaker.
Complex 3: Yield: 0.840 g, 92%.
Anal. Calc. for
C34H40Co3N16O4: C 44.70, H 4.41, N 24.53; found: C 44.83, H 75 TURBOMOLE version 6.5.37 For the calculations we have used
4.31, N 24.65%; UV/vis: [λmax in nm (εmax in M−1 cm−1)] (MeCN)
= 624(167), 363(16010), and 254(16492) and λmax in nm (solid,
15 reflectance) = 621 and 365. IR (KBr) in cm−1: ν(NꢀH) 3235, ν(N3)
the BP86 functional with the latest available correction for
dispersion (D3).38 The Bader’s "Atoms in molecules" theory has
been used to study the interactions discussed herein by means of
the AIM all calculation package.39
2051 and 2027, HRMS (m/z, ESI+): found for [CoLR(Na)]+
=
366.0747 (calc. 366.0754) and [Co3LR (N3)4Na]+ = 936.1290 80 This level of theory has been shown useful and reliable to study
2
(calc. 936.1312).
noncovalent interactions like those analyzed herein.28,40
Moreover, the optimized complexes 1 and 2 exhibit Co–N and
Co–O metal ligand distances that are similar to the experimental
ones. For instance, in complex 1 the experimental distances range
20 Physical Measurements
Elemental analyses (C, H, and N) were performed using a Perkinꢀ
Elmer 2400 series II CHN analyzer. IR spectra in KBr pellets 85 from 1.90 to 2.12 Å and the theoretical ones range from 1.98 to
(4000−400 cm−1) were recorded using a Perkinꢀ Elmer RXI FTꢀ
IR spectrophotometer. Electronic spectra in acetonitrile (800−200
25 nm) and solid state (800−300 nm) were recorded in a Hitachi Uꢀ
2.21 Å.
Catalytic Oxidation of 3,5ꢀDTBC
The catecholase activity of 1−3, were studied using 3,5ꢀdiꢀtertꢀ
butylcatechol (3,5ꢀDTBC) as substrate in acetonitrile solution
1
3501 spectrophotometer. The HꢀNMR spectra at 300 MHz were
recorded in CDCl3 on a Bruker DRX 300 spectrometer. The 90 under aerobic conditions at room temperature. The reactions were
electrospray ionization mass spectrometry (ESIꢀMS positive)
spectra were recorded with a Micromass Qtof YA 105 mass
30 spectrometer. EPR spectra was recorded on a Bruker EMM 1843
spectrometer operating at Xꢀband frequency (9.80 GHz), having
followed spectrophotometrically by monitoring the increase in
the maximum absorbance of the quinone band at 400 nm as a
function of time (time scan). To detect the formation of hydrogen
peroxide during the catalytic reaction we followed the iodometric
100 kHz frequency modulation and a phase sensitive detection to 95 method as reported earlier.40
obtain a first derivative signal. Di phenyl picrylhydrazyl (DPPH)
was used for calibration of gꢀvalues of paramagnetic species. The
35 epr parameters for different samples have been precisely
determined from the calculated spectra, as obtained from Bruker
SIMFONIA program based on perturbation theory R.T. Weber,
(WINꢀEPR SIMFONIA manual, 1995).30
Results and Discussion
Syntheses of the Complexes
Crystallographic data collection and refinement
40 Well formed single crystal of each complex was mounted on a
BrukerꢀAXS SMART APEX II diffractometer equipped with a
graphite monochromator and Mo Kα (λ = 0.71073 Å) radiation.
The crystals were positioned 60 mm from the CCD, and frames
(360) were measured with a counting time of 5 s. The structures
45 were solved using the Patterson method through the SHELXS 97
program. Non hydrogen atoms were refined with independent
anisotropic displacement parameters, while difference Fourier
synthesis and leastꢀsquares refinement showed the positions of
any remaining nonꢀhydrogen atoms. The nonꢀhydrogen atoms
50 were refined with anisotropic thermal parameters. The hydrogen
atoms bound to carbon atoms were included in geometric
positions and given thermal parameters equivalent to 1.2 (or 1.5
for methyl groups) times those of the atom to which they were
attached. Hydrogen atoms that bonded to N or O were located in
55 a difference Fourier map and refined with distance constraints.
Successful convergence was indicated by the maximum
shift/error of 0.001 for the last cycle of the leastꢀsquares
refinement. Owing to the intrinsic nature of the crystal 1, the
reflection data were weak and thus R factors are high. Absorption
60 corrections were carried out using the SADABS program,31 while
all calculations were made via SHELXS 97,32 SHELXL 97,33
PLATON 99,34 ORTEPꢀ32,35 and WINGX system verꢀ1.64.36
Data collection, structure refinement parameters, and
100
Scheme 1 Formation of the complexes 1−3.
The Schiffꢀbase ligand (H2L) and its reduced analouge (H2LR)
were synthesized using the reported procedures.28,29 The Schiffꢀ
base ligand (H2L) on reaction with cobalt perchlorate hexahydrate
105 and sodium azide in 2:3:4 molar ratios resulted two different
trinuclear mixed valence Co(II/III) complexes, cisꢀ
[Co3L2(MeOH)2(N3)2(ꢀ1,1ꢀN3)2]
(1)
and
transꢀ
[Co3L2(H2O)2(N3)2(ꢀ1,1ꢀN3)2]⋅(H2O)2 (2) depending upon the
solvent used. Complex 1 separated as a needle shaped dark brown
110 single crystals from methanolic filtrate whereas complex 2
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 |3

Page 5 of 15
Dalton Transactions
View Article Online
DOI: 10.1039/C4DT03446E
separated as square shaped blackish brown single crystals from
are slightly greater than Co(II)–O bond distances (Table 1) like
ethanolic filtrate. On the other hand, when the reduced Schiffꢀ 65 the terminal cobalt atoms. The central Co−N/O bond distances
base ligand, cobalt perchlorate hexahydrate and sodium azide
were mixed in same molar ratio as for 1 and 2 in MeOH then
rhombic shaped deep brown Xꢀray quality single crystals of 3
was isolated from the reaction mixture (Scheme 1). It may also be
are significantly greater than that of the terminal cobalt centers as
the central cobalt atom is in +2 oxidation states whereas terminals
atoms are in +3 oxidation states. The range of cis angles
[73.4(4)–92.5(4)°] and trans angles [157.6(4)°–168.7(4)°] around
5
noted that no complex other than complex 3 was produced when 70 Co(II) indicate considerable deviation from the ideal values. The
the same reaction was carried out in ethanol medium.
two methanol molecules in this structure are mutually cis to each
other as usual to other mixed valence cobalt complexes reported
previously.5ꢀ9 The Co(3A)⋅⋅⋅Co(1A) and Co(2A)⋅⋅⋅Co(1A)
distances are 3.169 and 3.169 Å, respectively.
10 IR and UV−Vis Spectra of the complexes
Besides elemental analysis, all three complexes were initially
characterized by the IR spectra. A strong and sharp band due to
the azomethine υ(C=N) group of the Schiff base appears at 1621
cm−1 for both of the complexes 1 and 2 (Fig. S3−S4). On the
15 other hand, a moderately strong and sharp peak at 3235 cm−1 (due
to a N−H stretching vibration) and absence of any peak at around
1620 cm−1, for complex 3 (Fig. S5) indicate that the imine group
of the Schiff base is reduced.41 In addition, the presence of azido
ligands in all three complexes is confirmed by the appearance of
20 strong and sharp peaks at 2061, 2062 and 2051 cm−1 along with
shoulders at 2010, 2010 and 2027 cm−1 in the spectra of 1−3
respectively (Fig. S3−S5). The splitting of the band is indicative
of the presence of two different coordinated azide ions42 in
agreement with their crystal structures.
25 The electronic spectra of these compounds were recorded in
acetonitrile solutions and in solid state. The complexes show a
broad absorption band in the visible region at 618, 619, and 624
nm in acetonitrile and 617, 620, and 621 nm in the solid state for
1−3 respectively, attributed to d−d transitions of Co(II) ions.
30 Besides these bands, an absorption band in the range 363−399 nm
were observed in solution as well as solid state, assignable to
ligandꢀtoꢀmetal charge transfer transitions for all three
complexes. Moreover, absorption bands at 276, 272 and 254 nm
in acetonitrile were observed for 1−3 respectively (Fig. S6−S7),
35 assignable to intraꢀligand charge transfer transitions.
75
80
Fig. 1 The structure of 1 with ellipsoids at 50% probability. The structure
contains two similar discrete neutral trinuclear units (1A and 1B) of same
formula. One unit (1A) is shown here. Hydrogen atoms have been omitted
for clarity.
The structure of 2 is shown in Fig. 2 together with the atomic
numbering scheme. Dimensions in the metal coordination sphere
are given in Table 2. The centrosymmetric linear trinuclear unit
of
this
complex
having
molecular
formula
85 [Co3L2(H2O)2(N3)2(ꢀ1,1ꢀN3)2]⋅(H2O)2, contains two mutually trans
water molecule unlike 1. Like 1, the basal plane of the terminal
Co(III) centers are formed by two phenoxido oxygen atoms and
two imine nitrogen atoms of the chelated dianionic tetradentate
Schiffꢀbase ligand [L2ꢀ] and the axial positions are occupied by
90 one nitrogen atom of µ1,1ꢀbridging azido and one nitrogen from
terminal azido ligand. Here also, the terminal Co atom deviate
from the mean plane of the coordinated atoms and this deviation
is smaller than that in complex 1 (Table S2). The Co–O bond
distances are shorter than Co–N bond distances in the basal plane
95 and the axial Co−N bond distances are longer than the basal
bonds (Table 2) like 1. The bridging and terminal of azido ligands
in this structure are also nearly linear (Table S3). The ranges of
trans [173.0(5)–177.4(6)°] and cis [83.9(5)–96.5(6)º] angles
(Table 3) indicate similar deviation from ideal octahedral
100 geometry as in 1.
Description of the structures (1−3)
A partially labeled diagram of complex 1 is presented in Fig. 1,
and selected bond parameters are summarized in Table 1. The
molecular structure of 1 contains two discrete neutral trinuclear
40 units
(1A
and
1B)
of
same
formula,
cisꢀ
[Co3L2(MeOH)2(N3)2(ꢀ1,1ꢀN3)2] with slight differences in bond
parameters. Thus, we describe only the structure of 1A. In the
structure, three cobalt atoms are in bent arrangement with
∠Co(3A)−Co(1A)−Co(2A) angle of 130.4°. Each of the cobalt
45 atoms has a sixꢀcoordinated distorted octahedral geometry. The
basal plane of the two terminal Co(III) centers are formed by two
ꢀ
2ꢀphenoxido O atoms and two imine N atoms of the chelated
dianionic tetradentate Schiffꢀbase ligand [L2ꢀ] and the axial
positions are occupied by one nitrogen atom of 1,1ꢀbridging
ꢀ
50 azido and one nitrogen from terminal azido ligand. The basally
coordinated atoms and the Co atom deviate only slightly from the
mean plane of the coordinated atoms (Table S2). The basal Co–O
bond distances in each of the terminal cobalts are shorter than
basal Co–N bond distances and the axial Co−N bond distances
55 are longer than the basal bonds (Table 1). Both the bridging and
terminal of azido ligands are nearly linear (Table S3). The ranges
of trans [170.9(5)–176.7(4)°] and cis [80.2(4)–96.8(4)°] angles
(Table 2) indicate slight deviation from ideal octahedral
geometry.
Fig. 2 The structure of 2 with ellipsoids at 50% probability. Symmetry
element a = 2ꢀx,ꢀy,ꢀz. Hydrogen atoms have been omitted for clarity.
60 The central Co(II) atom is bonded to two phenoxido oxygen
atoms of deprotonated tetradentate Schiff base ligands, two
The central cobalt atom Co(1), which sits on the crystallographic
105 inversion center has a sixꢀcoordinated distorted octahedral
nitrogen atoms of
ꢀ1,1ꢀbridging azide ions and two oxygen atoms
geometry. The basal plane of this atom is constructed by two ꢀ2ꢀ
phenoxido oxygen atoms of deprotonated tetradentate Schiff base
of methanol molecules. Here also, the Co(II)–N bond distances
ligands and two nitrogen atoms of µ1,1ꢀbridging azide ions. Two
4
|Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Dalton Transactions
Page 6 of 15
View Article Online
DOI: 10.1039/C4DT03446E
transaxial positions are occupied by two water molecules. The 55 remaining one is azido bridged,8 angular trinuclear species in
which central Co(II) is connected to terminal Co(III) via single
phenoxido bridges and two mutually cisꢀoxygen atoms of water
molecules are bonded with Co(II). Thus this structure is very
similar to that of compound 1 of present study. It is to be noted
60 that in eight of these complexes, the two anionic bridges that link
the terminal Co(III) to central Co(II) are mutually cis and only in
one10 these bridges are mutually trans. Therefore, the structure of
complex 2 of the present work, in which the two water molecules
are trans coordinated to the central Co(II) is rather unusual.
65 Moreover, complexes 1 and 2 present the first example of the
formation of “cisꢀtrans” configuration induced by the solvent
molecule for this type of trinuclear complexes. Complex 3 is the
first example of mixed valence Co(II/III) complexes with reduced
salen type diꢀSchiff base ligands and possesses a unique
70 structural feature among these mixedꢀvalence complexes. That is,
two terminal Co(III) are joined by a µ1,3ꢀazido bridge with a
pentaꢀcoordinated central Co(II). CSD search shows that there are
few reports where µ1,3ꢀazido bridge connects the first metal ions
with the third one in the fragments of polynuclear (>3 metal
75 centers) complexes but only one hit is there of a trinuclear
complex containing Cu(II) that has such interaction.44
central Co−N/O bond distances are significantly greater (Table 2)
than that of the terminal cobalt center as expected. Though all the
trans angles of Co(1) are 180º (as the molecule is
centrosymmetric), the range of cis angles [87.9(5)–104.3(5)°]
deviates considerably from their ideal value (90°). The metalꢀ
metal distance (Co(2)⋅⋅⋅Co(1)) in this case is slightly smaller than
that in complex 1 (3.132 Å).
5
The structure of complex
3 having molecular formula
10 [Co3LR (N3)3(ꢀ1,3ꢀN3)] (3) is shown in Fig. 3. Selected bond
2
lengths and angles are listed in Table 3. In this structure, three
cobalt
atoms
are
in
a
bent
arrangement
with
∠Co(3)−Co(1)−Co(2) angle of 104.2º. The terminal cobalt(III)
atoms present a sixꢀcoordinated distorted octahedral geometry
15 like other two structures described above. The basal plane of the
two terminal Co(III) centers are formed by two phenoxido
oxygen atoms and two imine nitrogen atoms of the chelated
dianionic tetradentate reduce Schiffꢀbase ligand (LR)2ꢀ and the
axial positions are occupied by one nitrogen atom of ꢀ1,3ꢀbridging
20 azido and one nitrogen from terminal azido ligand. The terminal
Co atoms deviate only slightly from the mean plane of the
coordinated atoms and these deviations are the smaller than 1 but
greater than 2 (Table S2). The basal Co–O bond distances in each
of the terminal cobalt atoms are shorter than basal Co–N bond
25 distances and the axial Co−N bond distances are larger than the
basal bonds (Table 3). Both the bridging and terminal of azido
ligands are nearly linear (Table S3) like other two structures. The
ranges of cis [78.5(2)–96.4(3)º for Co(2) and 87.2(2)–96.0(2)º for
Co(3)] and the trans angles [170.9(2)–175.1(2)º for Co(2) and
30 171.2(2)–175.1(2)º for Co(3)] indicate slight deviation from ideal
octahedral geometry.
Theoretical study
The three trinuclear Co(III)−Co(II)−Co(III) compounds that have
80 been Xꢀray characterized and are represented in Figs. 1ꢀ3. As
aforementioned, two different compounds are obtained depending
on the solvent used. That is, compound 1 (obtained from
methanol) presents the two coordinated methanol molecules in cis
position of Co(II) whilst compound 2 (obtained from ethanol)
85 exhibits two trans coordinated water molecules. Moreover, when
the Schiff base ligand is reduced, a totally different structure 3 is
obtained.
Unlike other two structures, here the central cobalt atom is pentaꢀ
coordinated by four phenoxido oxygen atoms of two reduced
Schiff base ligands, and a terminally coordinating azido ligand.
35 The distortions of the coordination geometry from the square
pyramid to the trigonal bipyramid have been calculated by the
Addison parameter (τ).43The value of τ is defined as the
difference between the two largest donor metalꢀdonor angles
divided by 60. The τ is 0 for the ideal square pyramid and 1 for
40 the trigonal bipyramid. The τ value of Co(1) is 0.46, indicating
that the geometry is intermediate between the two ideal
geometries. The metal−metal distances (Co(3)⋅⋅⋅Co(1) 3.122 Å
and Co(2)⋅⋅⋅Co(1) 3.121 Å) are similar to those of 2.
In order to give an explanation to the aforementioned
experimental results we have fully optimized the complex 1 in cis
90 conformation starting from the crystallographic coordinates of 1
and in trans starting from the crystallographic coordinates of 2
and replacing the coordinated water molecules by methanol
moieties. The resulting optimized geometries are shown in Fig. 4
where we also indicate the relative energies. It can be observed
95 that for compound 1 the cis conformation is 10.5 kcal/mol more
stable than the trans in agreement with experiment. Using the
same methodology we have optimized the cis and trans isomers
for compound 2 and, similarly to compound 1, the cis isomer is
also considerably more favorable than the trans isomer (18.0
100 kcal/mol). Examining the intramolecular interactions, it can be
observed that both isomers exhibit two strong (O–H···O)
hydrogen bonds involving the hydrogen atoms of the coordinated
water molecules and the phenolic oxygen atom. Moreover the cis
isomers also present aromatic interactions involving the arene
105 rings (highlighted in Fig. 4) and CH2 groups of the ligands. These
interactions further stabilize the cis isomer. This result does not
agree with the experimental Xꢀray structure obtained for
compound 2 (trans isomer). A likely explanation is that
intermolecular interactions in the solid state stabilize the trans
110 isomer in compound 2. A further examination of compound 2
reveals the crucial role of the uncoordinated water molecules that
form a hydrogen bonding network connecting the coordinated
water molecules with the endꢀon coordinated azide ligands (see
Fig. 5). This hydrogen bonding network cannot be formed in the
115 cis isomer due to the relative position of the coordinated water
molecules and the azide ligands. Moreover, it is not possible in
compound 1, where methanol instead of water is coordinated to
the metal center.
45 Fig. 3 The structure of 3 with ellipsoids at 50% probability. Hydrogen
atoms have been omitted for clarity.
A CSD search for trinuclear Co(III)–Co(II)–Co(III) complexes
containing deprotonated tetradentate Schiff base ligand revels
50 that only nine such structures have been reported so far.5,6ꢀ9,10
Among them, eight complexes are linear in which terminal
Co(III) and central Co(II) are linked through acetato (six
structures)5,6,10 or sulfite9 (one structure) or chlorido7 (one
structure) bridges in addition to double phenoxido bridges. The
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 |5

Page 7 of 15
Dalton Transactions
View Article Online
DOI: 10.1039/C4DT03446E
several C–H···N interactions (Table S4) that also contribute to
the total binding energy (see red dashed lines in Fig. 7).
45
Fig. 6 Molecular Electrostatic Potential surface of compound 3 in two
orientations. Energies in kcal/mol.
Fig. 4 BP86ꢀD3/def2ꢀTZVP optimized geometries of the cis/trans isomers
of compounds 1 and 2. Relative energies are also indicated. Distances in
Å.
5
We have computed the interaction energy of compound 2 with
the water molecules since this should compensate the 18 kcal/mol
difference between the cis and trans isomers of compound 2. The
reaction used to compute this energy (denoted as ꢁEw) is shown
10 in Fig. 5 and it can be observed that ꢁEw clearly compensates the
cisꢀtrans energetic difference (18.0 kcal/mol) providing
a
plausible explanation to the different behavior of both compounds
in the solid state.
50
55
Fig. 7 Left: partial view of the crystal packing of compound 3 with
indication of the N···N interaction. The complementary MEP surfaces are
also indicated in the same orientation. Hydrogen atoms have been omitted
for clarity.
Finally, we have performed a search in the Cambridge Structural
Database (CSD) in order to know if pnicogen bonding
interactions are common in
is wellꢀknown that the CSD is a convenient and reliable tool for
60 analyzing geometrical parameters.45 Interestingly, we have found
20 Xꢀray structures where the 1,3–N3 ligand coordinated to
ꢀ1,3–N3 coordinated azide ligands. It
15 Fig. 5 Hydrogen bonding network observed in the solid state of
compound 2 and the associated interaction energy
ꢀ
transition metals participates in pnicogen π–hole interactions as
acceptor (π stands for the πꢀsystem of azide). Some examples are
shown in Fig. 8 illustrating the importance of this interaction in
65 the solid state. In some examples the electron donor is a neutral
moiety (CAPVOF46 and MILROP47) and in others it is an anion
(LIJCUC48 and MILROP49). The lp/anion–πꢀhole interaction
distances range 3.01–3.28 Å, in agreement with previous
theoretical and experimental studies on similar interactions
70 involving the nitro/nitrate group.50 As far as our knowledge
extends, the description of such interaction in azide is
unprecedented.
In the second part of the theoretical study we analyze the dual
behavior of the azide ligand since it is able to act as a Hꢀbond
20 acceptor (electron rich moiety) when it is endꢀon coordinated or
acting as a ꢀ1,1 bridging ligand and as electron acceptor (middle
nitrogen atom) when it is acting as ꢀ1,3–N3 bridging ligand. In the
description of the experimental structures (vide supra) and in Fig.
5 hydrogen bonding interactions involving the azide ligand are
25 described. In Fig. 6 we show Molecular electrostatic potential
surface (MEPS) of compound 3 that presents a ꢀ1,3–N3 bridging
ligand in the structure together with several endꢀon coordinated
azido ligands. Interestingly, it exhibits two well defined regions,
one with positive potential (blue region) close to the
ꢀ1,3–N3
30 bridging ligand and the other with negative potential close to the
endꢀon azide ligand. Therefore, azideꢀazide (pnicogen bonding)
interaction can be expected involving two nitrogen atoms, one
acting as donor and the other one as acceptor.
In sharp agreement with the MEP surface analysis, a pnicogen
35 bonding27 interaction is observed in the crystal packing of
compound 3 (see Fig. 7). This arrangement maximizes the
electrostatic interaction, as can be observed by the high degree of
complementarity of the MEP surfaces. Therefore, the
ꢀ1,3–N3
coordination mode of the azido ligand drastically changes their
40 electron donor by electron acceptor ability using the πꢀhole of the
central nitrogen atom. The interaction energy of this dimer is
large and negative not only due to pnicogen bonding but also
Fig. 8 Partial views of several Xꢀray structures retrieved from the CSD.
75 Distances in Å. Hydrogen atoms have been omitted for clarity.
6
|Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Dalton Transactions
Page 8 of 15
View Article Online
DOI: 10.1039/C4DT03446E
Catechol Oxidase Studies and Kinetics.
values obtained for complexes 1−3 are significantly higher than
60 those reported for diꢀcobalt(II) compounds containing “endꢀoff”
compartmental Schiff base ligand53 (Table 5) and considerably
lower than those reported for dinuclear mixedꢀvalence
Co(III)−Co(II) complexes derived from a macrocyclic ligand.54
The catecholaseꢀlike activity of three mixed valence Co(II/III)
complexes were determined by the catalytic oxidation of 3,5ꢀ
DTBC. This substrate is most widely used because of its low
redox potential which facilitates oxidation to quinone, and its
bulky substituents, which prevent further oxidation reactions such
as ring opening.51 Moreover, the oxidation product, 3,5ꢀdiꢀtertꢀ
butylquinone (3,5ꢀDTBQ), is highly stable and a characteristic
absorption band maxima appeared at around 400 nm (ε = 1900
5
10 M−1cm−1) in pure acetonitrile solvent system. Before going into
the detail of kinetics studies, we examined the catalytic activity of
complexes 1−3 for oxidation of 3,5ꢀdiꢀtertꢀbutylcatechol (3,5ꢀ
DTBC) to corresponding oꢀquinone (3,5ꢀDTBQ) in acetonitrile
solvent according to the reaction shown in Scheme 2.
OH
OH
O
O
Catalyst (1ꢀ3)
O2, CH3CN
65
Fig. 9 Increase of the quinone band at around 400 nm after the addition of
100 equiv of 3,5ꢀDTBC to an acetonitrile solution of complex 1. The
spectra were recorded at 5 min intervals.
15
Scheme 2 Catalytic oxidation of 3,5ꢀDTBC to 3,5ꢀDTBQ in acetonitrile
solvent.
For this purpose, 10−4 (M) acetonitrile solutions of these three
20 complexes were treated with 100 equivalent of 3,5ꢀDTBC at
room temperature under aerobic conditions. After mixing of the
3,5ꢀDTBC into the complexes (1−3), the progress of the reaction
was followed by recording the UVꢀVis spectra of the mixtures at
5 min time’s interval. The gradual increase of an absorption band
25 at around 400 nm corresponds to 3,5ꢀDTBQ52 was observed for
complexes 1−3. The variation of the spectral behavior of complex
1 in the presence of 3,5ꢀDTBC is shown in Fig. 9. Similar plots
for 2 and 3 are contained in the Supporting Information (Fig. S8
and S9).
70
30
Fig. 10 Plot of the rate vs substrate concentration for complex 1. Inset
The kinetics of oxidation of 3,5ꢀDTBC to 3,5ꢀDTBQ by the
complexes were determined by the method of initial rates by
monitoring the growth of the quinone band at 400 nm as a
function of time. The rate constant for a particular complexꢀ
35 substrate mixture was determined from the log[Aα/(Aα − At)] vs
time plot. The oxidation rates and various kinetic parameters of
shows the corresponding Lineweaver−Burk plot.
To get
a better understanding of the complex−substrate
intermediate and a mechanistic inference of catecholase activity
75 during the oxidation reaction, we have recorded ESIꢀMS spectra
of 1−3, and a 1:10 mixture of the complexes to and 3,5ꢀ DTBC
within 10 min of mixing in acetonitrile solvent (Fig. S12−S17,
Supporting Information). The spectrum of complexes 1 and 2
shows similar pattern. Both complexes show the base peak at m/z
80 = 362.0 (100%), (calcd 362.0441) which can be assigned to the
mononuclear species [CoLNa]+. In addition, peaks due to the
dinuclear [Co2L2(CH3CN)Na]+ and trinuclear [Co3L2(N3)3]+
species at m/z = 743.1 and 863.0 respectively are also observed
for both complexes. On the other hand, complex 3 displays a base
85 peak at m/z = 366.0 (calcd 366.0754), which can be assigned to
corresponding mononuclear species [CoLRNa]+ similar to 1 and
2. Besides, the peak at m/z = 936.1 corresponding to Naꢀ
containing molecular ion of trinuclear species [Co3(LR)2(N3)4Na]+
appears in this spectrum. After the addition of 3,5ꢀDTBC to the
90 solutions of complexes 1−3, drastic changes are observed in all
three spectra. In case of 1 and 2, in addition to the quinone
sodium aggregate [3,5ꢀDTBQꢀNa]+ at 243.1 (calcd 243.1415), the
base peak at m/z = 921.2 (calcd 921.2449) corresponding to
[Co2L2(3,5ꢀDTBC)Na]+, and other peaks at m/z = 582.2 (calcd
95 582.1983), 1141.4 (calcd 1141.3912) and 701.1 (calcd 701.0985)
corresponding to [CoL(3,5ꢀDTBC)Na]+, [Co2L2(3,5ꢀDTBC)2Na]+
and [Co2L2Na]+ respectively are generated. On the other hand, the
the substrate concentration were determined by using 10−4
M
solutions of 1−3 with different concentrations of 3,5ꢀDTBC under
aerobic conditions. In each cases, a firstꢀorder kinetic was
40 observed at low concentrations of 3,5ꢀDTBC, whereas higher
concentrations resulted in saturation kinetics. The observed rates
versus concentration of substrate data were then analyzed on the
basis of the Michaelis−Menten approach of enzymatic kinetics to
get the Lineweaver−Burk (double reciprocal) plot and values of
45 kinetic parameters Vmax, KM, and Kcat. Both the curves of
observed rate vs [substrate] and Lineweaver−Burk plot for
complex 1 are shown in Fig. 10. Similar plots for 2 and 3 are
given in the Supporting Information (Fig. S10 and S11). The
kinetic parameters for all the cases are listed in Table 4. The kcat
50 values can be calculated by dividing the Vmax values by the
concentration of the corresponding complexes (Table 4). The
kcat(in h−1) values are 142, 85 and 99 for 1−3, respectively. Thus,
the catecholaseꢀlike activity follows the order: complex 1>
complex 3> complex 2. These values lie in the range of the kcat
55 values of the diꢀcopper(II) systems behaving as functional models
of catechol oxidase. Thus, these three mixed valence
Co(III)−Co(II) compounds can also be considered as a functional
model of catechol oxidase. It is important to mention that, the kcat
spectrum of
3
exhibits
a base peak at m/z = 586.2
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 |
7

Page 9 of 15
Dalton Transactions
View Article Online
DOI: 10.1039/C4DT03446E
(calcd586.2218) corresponding to [CoLR(3,5ꢀDTBC)Na]+ species.
g׀׀≈ ge and g⊥= ge+ (λ/ꢁ)
The peaks at m/z = 243.1 and 929.3 can be assigned to quinone
sodium aggregate [3,5ꢀDTBQꢀNa]+ and [Co2LR (3,5ꢀDTBC)Na]+
respectively. These results reveal the formation of the
catalyst−substrate as intermediates which take part in substrate
activation during the oxidation of 3,5ꢀDTBC to 3,5ꢀDTBQ.
wherege = 2.0023, λ is the spin orbit parameter and ꢁ is the
energy difference between the dxy, yzand dz . In such cases, epr
2
2
65 signal is observed at room temperature due to sufficiently longer
spin lattice relaxation time and they exhibit small anisotropy in g
tensor (g⊥~ 2.2 > g׀׀ = 2.02ꢀ2.01). EPR parameters for low spin
reported Co complexes are given in Table 6.58 In high spin state
5
The mechanistic studies with copperꢀbased model complexes are
well established and it appears that there are two possible
mechanisms for the oxidation of 3,5ꢀdiꢀtertꢀbutylcatechol to their
t25ge2
configuration (
g ; S=3/2), Co(II) can give epr signals with
10 respective quinone. One proceeds through the formation of a 70 large anisotropy in g tensor (g1=5.0ꢀ6.8, g2=3.0ꢀ4.0 and g3=2.5ꢀ
dicopper(II) catecholate intermediate where the dicopper(II)
species stoichiometrically oxidizes the catecholic substrate to one
molecule of quinone and itself reduces to a dicopper(I) species.
Then, the dicopper(I) species reacts with an oxygen molecule to
1.85) and at very low temperatures. This is mainly due to short
spin lattice relaxation time arising due to large orbital
contribution of the unpaired electrons towards the magnetic
moment of the ground state of Co(II) ion in octahedral field.
15 generate a peroxo dicopper(II) adduct, which then oxidizes a 75 Therefore, in order to understand the redox participation of the
second molecule of the substrate to quinone; water is formed as a
byproduct through this four electron reduction process.55 The
second mechanism involves in the formation of an organic radical
intermediate such as copper(I) semiquinonate.56 Its subsequent
20 reaction with dioxygen may result in the two electron reduction
of dioxygen, leading to the reoxidation of the copper(I) ion and
release of the quinone molecule, with hydrogen peroxide as a
byproduct.57
In contrast, up to now there has been only limited study of
25 plausible mechanisms of catechol oxidation reaction catalyzed by
nickel(II), manganese(II/III) and zinc(II) complexes. However, it
has been shown that the participation of metal centre in the
catechol to quinone oxidation process is essential. Thus, this
oxidation reaction catalyzed by either Ni(II) or Mn(III)
30 complexes proceeds through the formation of semiquinonate
intermediate species similar to Cu(II) complexes. Recently, we
have studied the reaction mechanism of this oxidation reaction
catalyzed by Ni(II)ꢀoxime complexes24 and shown that the
reaction proceeds through the formation of Ni(III) species unlike
35 the dicopper complexes where the oxidation states of Cu(II)
shuttle between +I and +II. Moreover, Das et.al proposed that
conversion of 3,5ꢀDTBC to 3,5ꢀDTBQ can be promoted by
redoxꢀinnocent Zn(II) ion via radical pathway.25 In this case,
coordinated ligand undergoes reduction with concomitant
40 oxidation of the catechol to form semibenzoquinone.
In order to get a preliminary idea about the mechanism of the
catalytic reaction for these mixed valence Co(II)−Co(III)
complexes, we were interested to know whether any H2O2 was
formed and, if so, to find its amount. The estimation of H2O2
45 clearly shows that, after approximately 1 h of oxidation 91%,
79% and 82% H2O2 are generated with respect to the formation
of 3,5ꢀDTBQ for 1−3 respectively. The results indicate that
nearly equimolar amount of hydrogen peroxide is formed with
respect to 3,5ꢀDTBQ. Therefore, keeping analogy to the
50 mechanism of the oxidation by using the dicopper system, one
might assume that the reaction proceeds through the formation of
Co(II)ꢀsemiquinonate radical intermediate.
metal centers and to detect the organic radical, if any, produced as
intermediate species, we have recorded the Xꢀband EPR spectra
of these three trinuclear Co(III)−Co(II)−Co(III) complexes, as
well as the reaction mixtures in acetonitrile solutions at different
80 temperatures and time intervals. The observed features of EPR
spectra of all complexes in 10−3 M acetonitrile solutions at room
temperature were similar. The spectra consisted of an isotropic
signal at g ca 2.0045 having hyperfine structure comprised of
eight lines of almost of equal intensities (Aiso= 10.57 G). This is
85 shown in Fig. 11(a) (top). The hyperfine structure is due to
interaction of unpaired electron with Co nuclei having nuclear
spin I=7/2 (59Co, I=7/2, 100% natural abundance). The existence
of EPR signal at room temperature suggested presence of Co2+ in
low spin state. The EPR spectra of frozen complexes in
90 acetonitrile recorded at 100 K, showed two sets of eight lines,
characteristic of the 59Cohyperfine interaction and corresponding
to the parallel and perpendicular directions of the symmetry axis
with respect to the external magnetic field. The spectra were
nearly axially symmetric with g׀׀ = 2.0018, A׀׀ = 19.65, g⊥
95 =2.0059, A⊥=7.0 G. The expected ground state for the
2 1
Co2+complexes can be expressed as (dxz,yz)4 (dxy)2(dz ) . The
observed epr parameters were consistent with the unpaired
2
electron residing in the dz orbital. The typical EPR spectrum of
complex 1 in acetonitrile is shown in fig. 11(a) (bottom).
4.00E+008
3.00E+008
BLACK ꢀ COMPLEX + CATECHOL
Co COMPLEX 1
RED ꢀ COMPLEXIN ACETONITRILE
BLACKꢀEXPERIMENTAL
2.00E+008
REDꢀSIMULATED
8000000
1.00E+008
0.00E+000
-1.00E+008
6000000
4000000
-2.00E+008
2000000
-3.00E+008
300 K
-4.00E+008
0
3300
3350
GAUSS
3400
3450
-2000000
-4000000
-6000000
-8000000
2.00E+009
1.50E+009
1.00E+009
5.00E+008
0.00E+000
-5.00E+008
-1.00E+009
-1.50E+009
BLACK ꢀ COMPLEX+ CATECHOL
RED ꢀ COMPLEX INACETONITRILE
Co COMPLEX 3
Aiso
giso
3400
3450
3500
GAUSS
3550
3600
BLACKꢀEXPERIMENTAL
REDꢀSIMULATED
8.00E+007
6.00E+007
4.00E+007
2.00E+007
0.00E+000
-2.00E+007
-4.00E+007
-6.00E+007
-8.00E+007
-2.00E+009
2.00E+009
A
g
3300
3350
GAUSS
3400
3450
1.50E+009
1.00E+009 Co COMPLEX 2
5.00E+008
BLACK ꢀ COMPLEX + CATECHOL
RED ꢀ COMPLEX IN ACETONITRILE
100 K
The valence states of cobalt (Co(II), Co(III)) and its spin state
(Co(II) can exist in low spin S=1/2; or high spin, S=3/2 states) in
55 complexes can be easily distinguished by EPR spectroscopy. In
0.00E+000
-5.00E+008
g
-1.00E+009
A
-1.50E+009
t26ge0
3300
3350
GAUSS
3400
3450
-2.00E+009
3300
case of Co(III) (3d6), the
g configuration is diamagnetic and
3350
GAUSS
3400
3450
100
do not give any epr signal. In low spin state configuration Co(II)
(a)
(b)
t26ge1
Fig. 11 (a) The typical EPR spectrum of complex 1 in acetonitrile
solution. The spectra in top and bottom were recorded at room
temperature and 100K respectively. (b) The EPR spectra of different
(3d7) (
g ; S=1/2), complexes with distorted octahedral or
square planar geometries (C4v or D4h symmetry) having unpaired
60 electron density localized in dz2 orbital, the principal values of g
tensor is given by following equation
105 complexes with 3,5ꢀDTBC in acetonitrile solutions recorded at room
temperature.
8
|Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Dalton Transactions
Page 10 of 15
View Article Online
DOI: 10.1039/C4DT03446E
Conclusion
The EPR spectra of 10−3 (M) each complexes with a 10−1 (M) 35 In the present report, we have shown for the first time that the
3,5ꢀDTBC in acetonitrile solutions recorded at room temperature
were similar to that of the complexes in 10−3 M acetonitrile
without addition of 3,5ꢀDTBC (no change in g and A value for
Co2+). However, initially there was a drastic increase in the
trinuclear mixed valence Co(II/III) complexes with a salen type
Schiff base ligand can be isolated both in cis and trans
configurations depending upon the solvent used for synthesis.
The reduction of the Schiff base results in the formation of an
5
intensity of the EPR signal upon addition of solution of 3,5ꢀ 40 unprecedented cyclic trinuclear mixed valence Co(II/III) complex
DTBC in acetonitrile which gradually increased further (Fig. 12).
The increase in the integrated intensity followed the trend
in which a µ1,3−N3 coordinated azido ligand participates in
pnicogen bonding interactions as acceptor. From the DFT
calculations, we have demonstrated that the cis isomers are
energetically more stable and the trans isomer (observed
45 experimentally in 2) is stabilized by a hydrogen bonding network
involving a water dimer that cannot be formed in compound 1,
which compensates the energy of less favored trans coordination.
By means of MEP surfaces and CSD search we have
demonstrated hitherto unnoticed counterintuitive electron
50 acceptor ability of the µ1,3–N3 coordinated azide ligand. All three
complexes exhibit good catecholaseꢀlike activity in the aerial
oxidation of 3,5ꢀdiꢀtertꢀbutylcatechol to the corresponding oꢀ
quinone. Mechanistic investigations of the catalytic behaviors of
these mixed valence Co(II/III) complexes by electrospray
55 ionization mass (ESIꢀMS positive), estimation of hydrogen
peroxide formation and EPR spectroscopy indicate that, similarly
to the copper(II) analogues, the participation of metal centers
through a cobalt(III)/cobalt(II) redox process is responsible for
the oxidation of catecholic substrate to quinine.
10 complex 1> complex 3> complex 2 (Fig. 12). The catecholaseꢀ
like activity also followed similar trend complex 1> complex 3>
complex 2 which is in accordance with the data obtained from
optical absorption studies. This confirmed that reduction of some
Co3+ sites to Co2+occurred in all cases. However, we did not
15 observe any signal corresponding to the formation of the
semiquinonate radical intermediate presumably due to its
transient nature.
COMPLEX 1
COMPLEX 3
COMPLEX 2
60
50
40
30
20
60
Acknowledgement
A.H. and L.K.D. are thankful to CSIR, India, for awarding Junior
Research Fellowship and Senior Research Fellowship
respectively [Sanction No. 09/028 (0889)/2012ꢀEMRꢀI dated
65 13/12/2012 for A.H. and 09/028 (0805)/2010ꢀEMRꢀI for L.K.D.].
We thank Prof. M. G. B. Drew, School of Chemistry, The
University of Reading, U.K. for the refinement of the structure of
complexes 1ꢀ3. Crystallography was performed at the DSTꢀFIST,
India, funded Single Crystal Diffractometer Facility at the
70 Department of Chemistry, University of Calcutta. The authors
also thank the Department of Science and Technology (DST),
New Delhi, India, for financial support (SR/S1/IC/0034/2012).
AB and AF thank the DGICYT of Spain (projects CTQ2011ꢀ
27512/BQU and CONSOLIDER INGENIO 2010 CSD2010ꢀ
75 00065, FEDER funds) and the Direcció General de
RecercaiInnovaciódel Govern Balear (project 23/2011, FEDER
funds) for financial support.
-10
0
10 20 30 40 50 60 70 80
TIME AFTER MIXING CATACHOLE (Sec)
Fig. 12 The plots of EPR intensity vs the time after mixing the 3,5ꢀDTBC
20 to the different complexes (1−3).
OH
ꢀ
H+ + N3
HO
N
N
tBu
tBu
CoIII
O
O
N
N
III
Oꢀ
Co
O
O
HO
tBu
O
N3
O
tBu
tBu
tBu
+
H2O2
O2
Notes
N
N
aDepartment of Chemistry, University College of Science,
80 University of Calcutta, 92, A.P.C. Road, Kolkataꢁ700 009, India.
Eꢁmail: ghosh_59@yahoo.com
II
Co
O
O
H+
Oꢀ
2H+ + N3
ꢀ
bRadiochemistry Division, Bhabha Atomic Research Centre,
Trombay, Mumbai 400085, India
.
O
tBu
tBu
cDepartment of Chemistry, Universitat de les IllesBalears, Crta
85 de Valldemossa km 7.5, 07122 Palma de Mallorca, Baleares,
Spain. Eꢁmail: toni.frontera@uib.es
Scheme 3 Proposed mechanism for the catalytic cycle of the oxidation of
3,5ꢀdiꢀtertꢀbutylcatechol by complex 1. Complexes 2 and 3 show similar
mechanism.
Electronic supplementary information (ESI) available: The
electronic supplementary information file contains Fig. S1–
S17.CCDC 1033263ꢁ1033265 for 1−3 respectively.
25 We therefore assume that the present compounds 1−3 catalyzes
the oxidation of 3,5ꢀDTBC to 3,5ꢀDTBQ through the reduction of
Co(III) to Co(II) where an organic radical intermediate such as
Co(II)ꢀsemiquinonate species may be formed. The catalytic cycle
is completed by the reaction of Co(II) species with dioxygen,
30 leading to the reoxidation of the Co(II) to Co(III) ion and the
reduction of dioxygen. Consequently, the quinone molecule as
product and hydrogen peroxide as a byproduct are released. The
probable reaction mechanism has been shown in Scheme 3.
90
References
1 (a) V. I. Maleev, M. North, V. A. Larionov, I. V. Fedyanin, T.
F. Savelyeva, M. A. Moscalenko, A. F. Smolyakov and Y. N.
Belokon, Synth.Catal., 2014, 356, 1803–1810; (b) S. Shit, D.
95 Saha, D. Saha, T. N. G. Row and C. Rizzoli, Inorg. Chim. Acta,
2014, 415, 103–110; (c) Y. N. Belokon, V. I. Maleev, M. North,
V. A. Larionov, T. F. Savel’Yeva, A. Nijland and Y. V.
Nelyubina, ACS Catalysis, 2013, 3, 1951–1955; (d) V. I. Maleev,
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 |9

Page 11 of 15
Dalton Transactions
View Article Online
DOI: 10.1039/C4DT03446E
M. North, V. A. Larionov, I. V. Fedyanin, T. F. Savel’Yeva, M. 65 15 (a) L. K. Das, S.ꢀW. Park, S. J. Cho and A. Ghosh, Dalton
A. Moscalenko, A. F. Smolyakov and Y. N. Belokon,
AdvancedSynthesis and Catalysis, 2014, 356, 1803–1810; (e) E.
Vinck, E. Carter, D. M. Murphy and D. S. Van, Inorg. Chem.,
2012, 51, 8014–8024; (f) T. Kurahashi and H. Fujii, Inorg.Chem.,
Trans., 2012, 41, 11009–11017.
16 A. Biswas, M. G. B.Drew, C. J. GómezꢀGarcía and A. Ghosh,
Inorg. Chem., 2010, 49, 8155–8163.
5
17 A. Hazari, L. K. Das, A. Bauzá, A. Frontera and A. Ghosh,
2013, 52, 3908–3919; (g) M. Lashanizadegan and Z. Zareian, 70 Dalton Trans., 2014, 43, 8007–8015.
Catal. Lett.,2011, 141, 1698–1702.
18 A. Rompel, H. Fischer, D. Meiwes, K. BüldtꢀKarentzopoulos,
2 (a) M. Hoshino, R. Konishi, N. Tezuka, I. Ueno and H. Seki,
Journal of Physical Chemistry, 1996, 100, 13569–13574; (b) H.
R. Dillinger, F. Tuczek, H. Witzel and B. Krebs, J. Biol. Inorg.
Chem.,1999, 4, 56–63.
10 Kunkely and A. Vogle, Journal of Photochemistry and
19 I. A. Koval, P. Gamez, C. Belle, K. Selmeczi and J. Reedijk,
Photobiology A: Chemistry, 2001, 138, 51–54; (c) Y. Ohashi, 75 Chem. Soc. Rev., 2006, 35, 814−840.
Bulletin of the Chemical Society of Japan,1997, 70, 1319–1324.
3 (a) M. Volpe, H. Hartnett, J. W. Leeland, K. Wills, M.
Ogunshun, B. J. Duncombe, C. Wilson and J. B. Love, Inorg.
15 Chem., 2009, 48, 5195–5207; (b) T. Honda, T. Kojima and S.
20 (a) B. Sreenivasulu, M. Vetrichelvan, F. Zhao, S. Gao and J. J.
Vittal, Eur. J. Inorg. Chem., 2005, 4635−4645; (b) N. A. Rey, A.
Neves, A. J. Bortoluzzi, C. T. Pich and H. Terenzi, Inorg. Chem.,
2007, 46, 348−350.
Fukuzumi, J. Am. Chem. Soc., 2012, 134, 4196–4206; (c) A. 80 21 V. K. Bhardwaj, N. AliagaꢀAlcalde, M. Corbella and G.
Kamath, N. V. Kulkarni, P. P. Netalkar and V. K. Revankar,
SpectrochimicaActa ꢁ Part A: Molecular and Biomolecular
Spectroscopy, 2011, 79, 1418–1424; (d) S. Durmus, A. Atahan
20 and M. Zengin, SpectrochimicaActa ꢁ Part A: Molecular and
Biomolecular Spectroscopy, 2011, 84, 1–5.
4 (a) D. C. Ware, B. D. Palmer, W. R. Wilson and W. A. Denny,
J. Med. Chem.,1993, 36, 1839–1846; (b) P. K. Mascharak,
Coord. Chem. Rev., 2002, 225, 201–214; (c) O. A. ElꢀGammal,
25 E.ꢀR. G. Abu and S. F. Ahmed, SpectrochimicaActa Part A:
Hundal, Inorg. Chim.Acta, 2010, 363, 97−106.
22 P. Chakraborty, J. Adhikary, B. Ghosh, R. Sanyal, S. K.
Chattopadhyay, A. Bauzá, A. Frontera, E. Zangrando and D. Das,
Inorg. Chem., 2014, 53, 8257−8269.
85 23 P. Seth, L. K. Das, M. G. B. Drew and A. Ghosh, Eur. J.
Inorg. Chem., 2012, 2232–2242.
24 L. K. Das, A. Biswas, J. S. Kinyon, N. S. Dalal, H. Zhou and
A. Ghosh, Inorg. Chem., 2014, 53, 434−445.
25 R. Sanyal, S. K. Dash, S. Das, S. Chattopadhyay, S. Roy and
Molecular and Biomolecular Spectroscopy, 2015, 135, 227–240; 90 D. Das, J. Biol.Inorg.Chem., 2014, DOI 10.1007/s00775ꢀ014ꢀ
(d) M. Salehi, M. Amirnasr, S. Meghdadi, K. Mereiter, H. R.
Bijanzadeh and A. Khaleghian, Polyhedron, 2014, 81, 90–97; (e)
F.ꢀL. Mao and C.ꢀH.Dai, Synthesis and Reactivity in Inorganic,
30 MetalꢁOrganic, and NanoꢁMetal Chemistry, 2014, 44, 1059–
1148ꢀz.
26 P. Wang, G. P. A. Yap and C. G. Riordan, Chem. Commun.,
2014, 50, 5871−5873.
27 (a) M. R. Sundberg, R. Uggla, C. Viñas, F. Teixidor, S.
1063; (f) K. Das, A. Datta, P.ꢀH.Liu, J.ꢀH.Huang, C.ꢀL.Hsu, W.ꢀ 95 Paavola and R. Kivekäs, Inorg. Chem. Commun., 2007, 10, 713–
T. Chang, B. Machura and C. Sinha, Polyhedron, 2014, 71, 85–
90.
5 (a) S. Chattopadhyay, M. G. B. Drew and A. Ghosh, Eur. J.
35 Inorg. Chem., 2008, 1693–1701; (b) S. Banerjee, J.ꢀT.Chen and
C.ꢀZ. Lu, Polyhedron, 2007, 26, 686–694.
716; (b) S. Scheiner, Acc. Chem. Res., 2013, 46, 280–288; (c) A.
Bauzá, R. Ramis and A. Frontera, J. Phys. Chem. A, 2014, 118,
2827–2834. (d) A. Bauzá, T. J. Mooibroek and A. Frontera,
ChemCommun, 2014, DOI: 10.1039/C4CC09132A
100 28 L. K. Das, R. M. Kadam, A. Bauzá, A. Frontera and A. Ghosh,
Inorg. Chem., 2012, 51, 12407−12418.
6
(a) G. E. Assey, R. J. Butcher and Y. Gultneh,
ActaCrystallogr.,Sect.E: Struct. Rep. Online, 2011, 67, m303–
m304; (b) S. Chattopadhyay, G. Bocelli, A. Musatti and A.
40 Ghosh, Inorg. Chem. Comm., 2006, 9, 1053–1057.
29 M. K. Taylor, J. Reglinski, L. E. A. Berlouis and A. R.
Kennedy, Inorg. Chim.Acta, 2006, 359, 2455−2464.
30 R. T. Weber, WINꢀEPR SIMFONIA manual, 1995.
7 L. Mechi, P. Siega, R. Dreos, E. Zangrando and L. Randaccio, 105 31 SAINT, version 6.02; SADABS, version 2.03, Bruker AXS,
Eur. J. Inorg. Chem., 2009, 2629–2638.
Inc., Madison, WI, 2002.
8 A. Ray, G. M. Rosair, R. Kadam and S. Mitra, Polyhedron,
2009, 28, 796–806.
45 9 C. Fukuhara, E. Asato, T. Shimoji, K. Katsura, M. Mori, K.
32 G. M. Sheldrick, Program for Structure Solution, 1997.
33 G. M. Sheldrick, Program for Crystal Structure Refinement,
1997.
Matsumoto and S. Ooi, J. Chem. Soc., Dalton Trans., 1987, 110 34 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7−13.
1305–1311.
35 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837−838.
36 L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
37 R. Ahlrichs, M. Bär, M. Hacer, H. Horn and C. Kömel, Chem.
Phys. Lett., 1989, 162, 165–169.
10 X. He, C.ꢀZ. Lu and C.ꢀD. Wu, J. Coord. Chem., 2006, 59,
977–984.
50 11 (a) P. Seth, S. Ghosh, A. Figuerola and A. Ghosh, Dalton
Trans., 2014, 43, 990–998; (b) C. Chen, D. Huang, X. Zhang, F. 115 38 S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J.Chem. Phys.,
Chen, H. Zhu, Q. Liu, C. Zhang and L. Sun, Inorg. Chem., 2003,
42, 3540–3548.
2010, 132, 154104–19.
39 AIMAll (Version 13.05.06), Todd A. Keith, TK Gristmill
Software, Overland Park KS, USA, 2013.
12 (a) D.ꢀH. Shi, Z.ꢀL.You, C. Xu, Q. Zhang and H.ꢀL. Zhu,
55 Inorg. Chem. Comm., 2007, 10, 404–406; (b) A. Gerli, K. S.
Hagen and L. G. Marzilli, Inorg. Chem.,1991, 30, 4613–4616.
13 (a) P. Mukherjee, M. G. B. Drew, C. J. GómezꢀGarcía and A.
Ghosh, Inorg. Chem., 2009, 48, 4817–4827; (b) R. Biswas, S.
Mukherjee, P. Kar and A. Ghosh, Inorg.Chem., 2012, 51,
60 8150−8160.
40. (a) M. Mitra, P. Manna, A. Bauzá, P. Ballester, S. Kumar
120 Seth, S. R. Choudhury, A. Frontera and S. Mukhopadhyay, J.
Phys. Chem. B, DOI: 10.1021/jp510075m; (b) S. Saha, A.
Sasmal, C. R. Choudhury, G. Pilet, A. Bauzá, A. Frontera, S.
Chakraborty and S. Mitra, Inorg. Chim. Acta, 2015, 425, 211–
220; (c) M. Mirzaei, H. EshtiaghꢀHosseini, A. Bauzá, S.
14 (a) C. Biswas, M. G. B. Drew, E. Ruiz, M. Estrader, C. Diaz 125 Zarghami, P. Ballester, J. T. Maguee and A. Frontera,
and A. Ghosh, Dalton Trans., 2010, 39, 7474–7484; (b) B.
Sarkar, M. G. B. Drew, M. Estrader, C. Diaz and A. Ghosh,
Polyhedron, 2008, 27, 2625–2633.
CrystEngComm, 2014, 16, 6149−6158 (d) M. Mirzaei, H.
EshtiaghꢀHosseini, Z. Karrabi, K. Molčanov, E. Eydizadeh, J.
T.Mague, A. Bauzá and A. Frontera, CrystEngComm, 2014, 16,
5352−5363
10|Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Dalton Transactions
Page 12 of 15
View Article Online
DOI: 10.1039/C4DT03446E
(e) A. Neves, L. M. Rossi, A. J. Bortoluzzi, B. Szpoganicz, C.
Wiezbicki, E. Schwingel, W. Haase and S. Ostrovsky, Inorg.
Chem., 2002, 41, 1788−1794.
41 A. Biswas, L. K. Das, M. G. B. Drew, G. Aromí, P. Gamez
and A. Ghosh, Inorg. Chem., 2012, 51, 7993−8001.
42 S. Biswas and A. Ghosh, Journal of Molecular Structure,
2012, 1019, 32–36.
5
43 A. W. Addison, T. N. Rao, J. Reedijk, J. V. Rijn and G. C.
Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349−1356.
10 44 S. Mukhopadhyay, D. Mandal, P. B. Chatterjee, C.
Desplanches, J.ꢀP. Sutter, R. J. Butcher and M. Chaudhury, Inorg.
Chem., 2004, 43, 8501–8509.
45 A. Nangia, K. Biradha and G. R. Desiraju, J. Chem. Soc.,
Perkin Trans.,1996, 2, 943−953.
15 46 J. M. DominguezꢀVera, J. SuarezꢀVarela, I. B. Maimoun and
E. Colacio, Eur. J. Inorg. Chem., 2005, 1907−1912.
47 S. Demeshko, G. Leibeling, S. Dechert, S. Fuchs, T. Pruschke
and F. Meyer, ChemPhysChem, 2007, 8, 405−417.
48 G. A. McLachlan, G. D. Fallon, R. L. Martin, B. Moubaraki,
20 K. S. Murray and L. Spiccia, Inorg. Chem., 1994, 33, 4663−4668.
49 M. Monfort, I. Resino, M. S. E. Fallah, J. Ribas, X. Solans,
M. FontꢀBardia and H. StoeckliꢀEvans, Chem. Eur. J., 2001, 7,
280−287.
50 (a) G. SánchezꢀSanz, C. Trujillo, M. Solimannejad, I. Alkorta
25 and J. Elguero, Phys. Chem. Chem. Phys., 2013, 15, 14310–
14318; (b) J. E. del Bene, I. Alkorta and J. Elguero, J. Phys.
Chem. A, 2013, 117, 6893–6903.
51 J. Mukherjee and R. Mukherjee, Inorg. Chim.Acta, 2002, 337,
429−438.
30 52 F. Zippel, F. Ahlers, R. Werner, W. Haase, H.ꢀF.Nolting and
B. Krebs, Inorg. Chem., 1996, 35, 3409−3419.
53 A. Banerjee, A. Guha, J. Adhikary, A. Khan, K. Manna, S.
Dey, E. Zangrando and D. Das, Polyhedron, 2013, 60, 102–109.
54 (a) S. Majumder, S. Mondal, P. Lemoine and S. Mohanta,
35 Dalton Trans., 2013, 42, 4561–4569; (b) R. Modak, Y. Sikdar, S.
Mandal and S. Goswami, Inorg. Chem. Comm., 2013, 37, 193–
196.
55 E. Monzani, L. Quinti, A. Perotti, L. Casella, M. Gulotti, L.
Randaccio, S. Geremia, G. Nardin, P. Faleschini and G. Tabbi,
40 Inorg. Chem., 1998, 37, 553−562.
56 (a) M. R. MendozaꢀQuijano, G. FerrerꢀSueta, M. Floresꢀ
Álamo, N. AliagaꢀAlcalde, V. GómezꢀVidales, V. M. Ugaldeꢀ
Saldívara and L. Gasque, Dalton Trans., 2012, 41, 4985−4997;
(b) J. Ackermann, F. Meyer, E. Kaifer and H. Pritzkow, Chem.
45 Eur. J., 2002, 8, 247−258; (c) M. Kodera, T. Kawata, K. Kano,
Y. Tachi, S. Itoh and S. Kojo, Bull. Chem. Soc. Jpn., 2003, 76,
1957−1964; (d) I. A. Koval, K. Selmeczi, C. Belle, C. Philouze,
E. SaintꢀAman, I. Gautierꢀ Luneau, A. M. Schuitema, M. V.
Vliet, P. Gamez, O. Roubeau, M. Luken, B. Krebs, M. Lutz, A.
50 L. Spek, J.ꢀL. Pierre and J. Reedijk, Chem. Eur. J., 2006, 12,
6138−6150.
57 (a) R. A. Peralta, A. J. Bortoluzzi, B. Szpoganicz, T. A. S.
Brandaoa, E. E. Castellano, M. B. de Oliveira, P. C. Severino, H.
Terenzi and A. Neves, J. Phys. Org. Chem., 2010, 23,
55 1000−1013; (b) J. Balla, T. Kiss and R. F. Jameson, Inorg.
Chem., 1992, 31, 58−62; (c) K. Selmeczi, M. Reglier, M. Giorgi
and G. Speier, Coord. Chem. Rev., 2003, 245, 191−201.
58 (a) E. F. Vansant and J. H. Lunsford, Chem. Comm., 1972,
830−832; (b)R. F. Howe and J. H. Lunsford, J. Am. Chem. Soc.,
60 1975, 97, 5156–5159; (c) S. Fljlwara, T. Watanabe and H.
Tadano, J. Coord. Chem., 1971, 1, 195−206; (d) M. Baumgarten,
C. J. Winscom and W. Lubitz, Applied Magnetic Resonance,
2001, 20, 35−70; (e) Y. Nishida, A. Sumita and S. Kida, Bull.
Chem. Soc. Japan, 1977, 50, 759−760; (f) B. K. Santra and G. K.
65 Lahiri, J. Chem. Soc., Dalton Trans., 1998, 139–145.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 |11

Page 13 of 15
Dalton Transactions
View Article Online
DOI: 10.1039/C4DT03446E
Tables (1-6)
Unprecedented structural variations in trinuclear mixed valence Co(II/III)
complexes derived from a Schiff base and its reduced form: Theoretical
studies, pnicogen bonding interactions and catecholase-like activities
Alokesh Hazari, Lakshmi Kanta Das, Ramakant M. Kadam, Antonio Bauzá, Antonio
Frontera* and Ashutosh Ghosh*
Table 1 Bond distances (Å) and angles (°) for complexes 1
1A
1B
1A
1B
Co(2)–O(10)
Co(2)–O(30)
Co(2)–N(18)
Co(2)–N(22)
Co(2)–N(1)
Co(2)–N(7)
Co(3)–O(31)
Co(3)–O(51)
Co(3)–N(39)
Co(3)–N(43)
Co(2)–O(10)
Co(3)–N(4)
Co(3)–N(10)
Co(1)–O(30)
Co(1)–O(51)
Co(1)–N(10)
1.908(1)
1.934(1)
1.928(1)
1.940(1)
1.991(1)
1.941(1)
1.885(1)
1.908(1)
1.916(1)
1.923(1)
1.908(1)
1.950(1)
1.987(1)
2.115(1)
2.115(1)
2.125(1)
2.109(1)
2.091(1)
2.090(1)
87.1(4)
1.899(1)
1.928(1)
1.925(1)
1.933(1)
1.984(1)
1.937(1)
1.901(1)
1.916(1)
1.904(1)
1.945(1)
1.899(1)
1.944(1)
1.994(1)
2.112(1)
2.103(1)
2.119(1)
2.095(1)
2.075(1)
2.093(1)
86.9(4)
O(10)–Co(2)– N(7)
O(10)–Co(2)– N(1)
O(30)–Co(2)– N(1)
O(30)–Co(2)– N(7)
N(22)–Co(2)– O(10)
N(18)–Co(2)– O(30)
N(7)–Co(2)– N(1)
O(31)–Co(3)– O(51)
N(39)–Co(3)– N(43)
O(51)–Co(3)– N(43)
O(31)–Co(3)– N(39)
N(4)–Co(3)– N(43)
N(39)–Co(3)– N(4)
O(31)–Co(3)–N(4)
O(51)–Co(3)–N(4)
O(31)–Co(3)–N(43)
O(51)–Co(3)–N(39)
N(4)–Co(3)– N(10)
O(51)–Co(1)–N(10)
O(30)–Co(1)–N(10)
O(30)–Co(1)–N(1)
O(30)–Co(1)–O(1)
N(1)–Co(1)–O(1)
89.2(4)
87.7(4)
81.0(4)
90.8(4)
174.0(4)
176.7(4)
171.3(5)
87.1(4)
94.7(4)
87.9(4)
90.7(4)
88.8(5)
91.8(5)
87.8(5)
91.7(4)
173.8(4)
175.7(4)
172.4(5)
73.4(4)
92.5(4)
74.2(4)
81.8(4)
90.7(5)
111.8(4)
157.6(4)
168.7(4)
87.3(4)
89.0(5)
87.5(4)
80.2(4)
91.2(4)
174.8(4)
175.7(5)
170.9(5)
86.8(4)
95.0(5)
88.1(4)
90.3(4)
89.9(5)
93.1(4)
87.6(5)
90.7(4)
174.3(4)
175.1(4)
171.1(4)
73.9(3)
92.8(4)
73.6(4)
82.2(4)
91.1(4)
111.6(4)
157.2(4)
168.4(4)
86.6(4)
Co(1)–N(1)
Co(1)–O(1)
Co(1)–O(2)
O(10)–Co(2)– O(30)
N(18)–Co(2)– N(22)
O(30)–Co(2)– N(22)
O(10)–Co(2)– N(18)
N(22)–Co(2)– N(1)
N(22)–Co(2)– N(7)
N(18)–Co(2)– N(1)
N(18)–Co(2)– N(7)
95.3(5)
87.1(4)
90.6(4)
93.0(4)
89.2(5)
96.6(4)
91.5(4)
95.1(4)
88.1(4)
90.0(4)
93.1(4)
89.7(5)
96.8(4)
91.6(5)
O(30)–Co(1)–O(51)
N(10)–Co(1)–N(1)
O(30)–Co(1)–O(2)
O(1)–Co(1)–O(2)
1

Dalton Transactions
Page 14 of 15
View Article Online
DOI: 10.1039/C4DT03446E
Table 2 Bond distances (Å) and angles (°) for complexes 2
Co(2)–O(10)
Co(2)–O(30)
Co(2)–N(18)
Co(2)–N(22)
Co(2)–N(1)
Co(2)–N(7)
Co(1)–O(30)
Co(1)–N(1)
1.905(2) O(30)–Co(2)– N(22)
1.915(2) O(10)–Co(2)– N(18)
1.935(2) N(22)–Co(2)– N(1)
1.945(2) N(22)–Co(2)– N(7)
2.000(1) N(18)–Co(2)– N(1)
1.943(2) N(18)–Co(2)– N(7)
2.061(2) O(10)–Co(2)– N(7)
2.203(2) N(18)–Co(2)– N(22)
2.064(2) O(10)–Co(2)– N(1)
88.9(6)
90.4(6)
90.7(6)
87.8(7)
93.9(6)
88.4(6)
93.0(6)
96.5(6)
88.2(6)
83.9(5)
93.9(6)
N(22)–Co(2)– O(10)
N(18)–Co(2)– O(30)
N(7)–Co(2)– N(1)
O(30)–Co(1)–N(1)
O(30)–Co(1)–O(1)
N(1)–Co(1)–O(1)
O(30)–Co(1)–O(30)a
N(1)–Co(1)–N(1)a
O(1)–Co(1)–O(1)a
173.0(5)
174.3(6)
177.4(6)
104.3(5)
93.9(5)
87.9(5)
180.0
180.0
180.0
Co(1)–O(1)
O(10)–Co(2)–O(30)
N(18)–Co(2)– N(22)
Symmetry element a = 2ꢀx,ꢀy,ꢀz for 2.
84.2(5)
96.5(6)
O(30)–Co(2)– N(1)
O(30)–Co(2)– N(7)
Table 3 Bond distances (Å) and angles (°) for complexes 3
Co(2)–O(10)
1.925(5)
O(30)–Co(2)– N(22)
92.8(2)
N(39)–Co(3)– N(3)
90.4(2)
Co(2)–O(30)
Co(2)–N(18)
Co(2)–N(22)
Co(2)–N(1)
1.928(5)
1.992(6)
1.979(6) N(22)–Co(2)– N(7)
1.987(6) N(18)–Co(2)– N(1)
1.943(6) N(18)–Co(2)– N(7)
1.922(5) O(10)–Co(2)– N(7)
O(10)–Co(2)– N(18)
N(22)–Co(2)– N(1)
92.4(2)
86.9(3)
89.0(3)
90.6(2)
N(39)–Co(3)– N(4)
O(31)–Co(3)– N(3)
O(31)–Co(3)–N(4)
O(51)–Co(3)–N(3)
87.4(2)
90.3(2)
94.2(2)
90.7(2)
Co(2)–N(7)
Co(3)–O(31)
87.1(2)
93.8(2)
O(51)–Co(3)–N(4)
O(31)–Co(3)–N(43)
92.1(2)
171.2(2)
Co(3)–O(51)
Co(3)–N(39)
1.922(5) O(10)–Co(2)– N(1)
1.991(6) O(30)–Co(2)– N(1)
1.973(6) O(30)–Co(2)– N(7)
1.980(6) N(22)–Co(2)– O(10)
1.941(6) N(18)–Co(2)– O(30)
2.000(5) N(7)–Co(2)– N(1)
2.140(5) O(31)–Co(3)– O(51)
1.997(5) N(39)–Co(3)– N(43)
2.140(5) O(51)–Co(3)– N(43)
1.977(7) O(31)–Co(3)– N(39)
90.7(2)
90.5(2)
92.5(2)
O(51)–Co(3)–N(39)
N(3)–Co(3)–N(4)
O(31)–Co(1)–O(51)
170.7(2)
175.1(2)
71.7(2)
Co(3)–N(43)
Co(3)–N(3)
Co(3)–N(4)
Co(1)–O(10)
170.9(2) O(30)–Co(1)–O(10)
170.8(2) O(51)–Co(1)–N(10)
175.1(2) O(31)–Co(1)–N(10)
72.0(2)
105.7(3)
120.8(3)
121.0(3)
105.9(3)
91.7(2)
Co(1)–O(30)
Co(1)–O(31)
78.3(2)
96.0(2)
93.3(2)
92.5(2)
87.2(2)
88.6(2)
O(10)–Co(1)–N(10)
O(30)–Co(1)–N(10)
O(30)–Co(1)–O(31)
O(10)–Co(1)–O(51)
O(10)–Co(1)–O(31)
O(30)–Co(1)–O(51)
Co(1)–O(51)
Co(1)–N(10)
91.9(2)
O(10)–Co(2)– O(30)
N(18)–Co(2)– N(22)
78.5(2)
96.4(3)
N(3)–Co(3)– N(43)
N(4)–Co(3)– N(43)
118.2(2)
148.4(2)
Table 4 Kinetic Parameters for the Oxidation of 3,5ꢀDTBC Catalyzed by Complexes 1, 2 and 3
Complexes
Vmax (M min−1)
11.8 X 10ꢀ5
7.17 X 10ꢀ5
8.29 X 10ꢀ5
Std. error
6.79 X 10ꢀ6
0.60 X 10ꢀ6
3.87 X 10ꢀ6
KM (M)
Std. error
5.89 X 10ꢀ5
0.81 X 10ꢀ5
4.78 X 10ꢀ5
Kcat (h−1)
142
1.89 X 10ꢀ3
1.44 X 10ꢀ3
1.65 X 10ꢀ3
1
2
3
85
99
2

Page 15 of 15
Dalton Transactions
View Article Online
DOI: 10.1039/C4DT03446E
Table 5 kcat values for the oxidation of 3,5ꢀDTBC to 3,5ꢀ DTBQ catalyzed by complexes 1−3
and other reported cobalt complexes
Complexes
Kcat(hꢀ1) in CH3CN Kcat(hꢀ1) in CH3OH
References
[CoIIICoIIL1(N3)3]· 0.5CH3CN· 0.27H2O
[CoIIICoIIL1(N3)3]·CH3CN
[Co2(L2H)(H2O)2(CH3CO2)2](CH3CO2)2
[Co2(L3)(H2O)2(CH3CO2)2](CH3CO2)
[Co2(L4)(H2O)2(CH3CO2)2](CH3CO2)
Complex 1
114.24
482.16
Not done
Not done
Not done
142
Not done
45.38
447
45.9
42.9
Not done
Not done
Not done
54(a)
54(a)
53
53
53
Present work
Present work
Present work
Complex 2
Complex 3
85
99
Where, H2L1 = The [2 + 2] condensation product of 2,6ꢀdiformylꢀ4ꢀmethylphenol and 2,2ꢀdimethylꢀ1,3ꢀ
diaminopropane, H2L2 = 2,6ꢀbis(Nꢀethylpiperazine ꢀiminomethyl)ꢀ4ꢀmethylꢀphenol, H2L3 = 2,6ꢀbis(2ꢀethylpyridineꢀ
iminomethyl)ꢀ4ꢀmethylꢀphenol and H2L4 = 2,6ꢀbis(Nꢀethylpiperidineꢀiminomethyl)ꢀ4ꢀmethylꢀphenol.
Table 6 EPR parameter of low spin cobalt complexes (S=1/2) in different geometries (distorted
octahedral, square pyramid or trigonal bipyramid and square planar coordination). Hyperfine
coupling constant is given in Gauss
Matrix
g1
g2
g3
A1
68
89
12.5
12
12.2
108
ꢀꢀꢀ
A2
72
32
12
12
12.2
78
ꢀꢀꢀ
ꢀꢀꢀ
A3
72
32
17.8
21
17.2
78
165
6.78
7.0
References
58(a)
[Co(CH3NC)6]2+
[Co(CH3NC)5]2+
Co(NH,),O,ꢀY
Co(CH,NH,),O,ꢀY
Co(NH3)5O2
2.008
2.003
2.002
1.999
1.995
2.024
2.000
2.003
2.087
2.163
2.010
2.010
1.995
2.479
2.000
2.003
2.087
2.163
2.084
2.075
2.981
2.479
3.39
58(a)
58(b)
58(b)
58(c)
58(d)
58(e)
58(f)
[Co(N)4]2+
Co(N2S2)2+
[CoN6]2+/[CoN4P2]2+
2.009
ꢀꢀꢀ
Present work
Co2+ complexes 1,2 & 3
2.0018
2.0059
2.0059
19.65
7.0
Estimated errors for g value, ±0.002; for hyperfine splitting, ±0.5 G, complexes were frozen in acetonitrile solution
3