Dinuclear mixed-valence CoIIICoII complexes derived from a macrocyclic ligand: Unique example of a CoIIICoII complex showing catecholase activity

Article abstract of DOI:10.1039/c2dt32629a

The work in this paper presents the syntheses, characterization, catecholase activity, and electrospray ionization mass spectroscopic (ESI-MS positive) study of three mixed-valence dinuclear Co^IIICo^II complexes of composition [Co^IIICo^IIL(N₃) ₃]·CH₃CN (1), [Co^IIICo ^IIL(OCN)₃]·CH₃CN (2), and [Co ^IIICo^IIL(μ-CH₃COO)₂](ClO ₄) (3), derived from a tetraimino diphenolate macrocyclic ligand H₂L, obtained on [2 + 2] condensation of 4-ethyl-2,6-diformylphenol and 2,2′-dimethyl-1,3-diaminopropane. While 1 and 2 are diphenoxo-bridged, 3 is a heterobridged bis(μ-phenoxo)bis(μ-acetate) system. Utilizing 3,5-di-tert-butyl catechol (3,5-DTBCH₂) as the substrate, the catecholase activity of all the three complexes has been checked in methanol/acetonitrile/N,N-dimethyl formamide. While 2 and 3 are inactive, complex 1 shows catecholase activity with turnover numbers of 482.16 h ^-1 and 45.38 h^-1 in acetonitrile and methanol, respectively. Electrospray ionization mass (ESI-MS positive) spectra of complexes 1-3 have been recorded in acetonitrile solutions and the positive ions have been well characterized. The ESI-MS positive spectrum of complex 1 in the presence of 3,5-DTBCH₂ has also been recorded and, interestingly, two positive ions [Co^IIICo^IIL(N₃) ₂(3,5-DTBCH^-)H]⁺ and [Co^IICo ^IIL(μ-3,5-DTBCH^2-)Na]⁺ have been identified. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt32629a

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PAPER
III
II
Dinuclear mixed-valence Co Co complexes derived
from a macrocyclic ligand: unique example of a Co Co
III
II
Cite this: DOI: 10.1039/c2dt32629a
complex showing catecholase activity†
a
a
b
a
Samit Majumder,* Suraj Mondal, Pascale Lemoine and Sasankasekhar Mohanta*
The work in this paper presents the syntheses, characterization, catecholase activity, and electrospray ion-
III
II
ization mass spectroscopic (ESI-MS positive) study of three mixed-valence dinuclear Co Co complexes of
III
II
III
II
III
II
composition [Co Co L(N
3
)
3
]·CH
3
CN (1), [Co Co L(OCN)
3
]·CH
3
CN (2), and [Co Co L(μ-CH
L, obtained on [2 + 2] condensation of
-ethyl-2,6-diformylphenol and 2,2’-dimethyl-1,3-diaminopropane. While 1 and 2 are diphenoxo-
3 2 4
COO) ](ClO )
(
3), derived from a tetraimino diphenolate macrocyclic ligand H
2
4
bridged, 3 is a heterobridged bis(μ-phenoxo)bis(μ-acetate) system. Utilizing 3,5-di-tert-butyl catechol
3,5-DTBCH ) as the substrate, the catecholase activity of all the three complexes has been checked in
(
2
methanol/acetonitrile/N,N-dimethyl formamide. While 2 and 3 are inactive, complex 1 shows catecholase
activity with turnover numbers of 482.16 h− and 45.38 h−1 in acetonitrile and methanol, respectively.
Electrospray ionization mass (ESI-MS positive) spectra of complexes 1–3 have been recorded in aceto-
nitrile solutions and the positive ions have been well characterized. The ESI-MS positive spectrum of
1
Received 3rd November 2012,
Accepted 22nd December 2012
DOI: 10.1039/c2dt32629a
2
complex 1 in the presence of 3,5-DTBCH has also been recorded and, interestingly, two positive ions
III
II
−
+
II
II
2−
+
www.rsc.org/dalton
3 2
[Co Co L(N ) (3,5-DTBCH )H] and [Co Co L(μ-3,5-DTBCH )Na] have been identified.
coordinated to three histidine nitrogens and adopts a trigonal
pyramidal environment with one nitrogen in the apical site.
Introduction
Mixed-valence chemistry has a long and rich history that is The structure determination of catecholase oxidase has
characterized by a strong interplay of experimental, theoretical encouraged an extensive investigation on model compounds to
1
–4
6a–c,7b–f,8–14
and computational studies.
The results offer enhanced understand the structure–property relationship.
As
chemical insight into metal–ligand electron-transfer situations the structure contains a dicopper(II) moiety, several dicopper(II)
and suggest that mixed-valence materials may eventually be complexes derived from nitrogen containing dinucleating
5
exploited in molecular electronics and molecular computing.
ligands have been mainly employed for this purpose. While
Catechol oxidase is an enzyme with the type-3 active site the metallo-enzyme contains hydroxo-bridged dicopper(II),
that catalyzes the oxidation of a wide range of o-diphenols activity has been observed for dicopper(II) systems having
6
a–c,11
6a–c,7a,d,8a,9,10
(
2
catechols) to the corresponding o-quinones coupled with 2e/ hydroxo
or various other bridging moieties.
O, in a process known as catecholase Again, although the enzyme contains a dicopper(II) moiety, a
The generated o-quinones are autopolymerized few monocopper(II) complexes and even a few copper(II) clus-
+
2 2
H reduction of O to H
6
–15
activity.
1
1
producing a brown polyphenolic pigment, i.e. melanin, a ters and polymers have been found as active catalysts. The
process that is considered to protect damaged tissues against catecholase activity of complexes of the ions of other metals
1
6–18
pathogens or insects. The crystal structure of the met form of such as Mn, Fe, and Co has also been observed.
All these
the enzyme reveals that the active center consists of a hydroxo- indicate that the exploration of the possibility of catalyzing
bridged dicopper(II) center in which each copper(II) center is catecholase activity by new types of species in terms of any of
the following deserves importance: ligand environment, metal
ion, metal ion combination, oxidation states of the metal ions,
a
Department of Chemistry, University of Calcutta, 92 A. P. C. Road, Kolkata 700 009,
India. E-mail: sm_cu_chem@yahoo.co.in, samitmaj@gmail.com
nuclearity of the complexes. Moreover, straightforward struc-
ture–property correlations are still lacking, indicating further
that the problem of modelling the catecholase activity remains
still open.
b
Université Paris Descartes, Laboratoire de Cristallographie et RMN biologiques
UMR 8015, Faculté de Pharmacie, 4 avenue de l’Observatoire, 75006 Paris, France
†
Electronic supplementary information (ESI) available: Tables S1–S4 and
Catecholase activity studies of complexes of metal ions,
other than copper(II), are very much less and therefore
Fig. S1–S4. CCDC 908081–908083 for 1–3 respectively. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c2dt32629a
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M
spectrometer. The molar conductivity (Λ ) of a 1 mM solution
in DMF was measured at 25 °C with a Systronics conductivity
bridge.
Syntheses
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II
Synthesis of [Co Co L(N
nitrile solution (25 ml) of [H
dropwise added successively an acetonitrile solution (15 ml) of
Co(ClO ·6H O (0.732 g, 2 mmol) and an acetonitrile solution
5 ml) of triethylamine (0.404 g, 4 mmol). After 1 h, an
3
)
3
]·CH
L](ClO
3
CN (1). To a stirred aceto-
4
4 2
)
(0.689 g, 1 mmol) were
4
)
2
2
(
aqueous solution (10 ml) of NaN (0.260 g, 4 mmol) was added
3
dropwise to the solution with stirring. Immediately, a reddish
precipitate started to appear. After stirring for an additional
2
h, the mixture was heated to get almost a clear solution
Scheme 1 Chemical structure of H
2
L.
which was filtered to remove the suspended particles and the
filtrate was kept at ambient temperature for slow evaporation.
After a few days, a red crystalline compound containing diffrac-
tion quality crystals that deposited were collected by filtration,
washed with an acetonitrile–water mixture and air dried. Yield:
determination of the structure–activity relationship has not
been possible. Regarding cobalt complexes showing the
activity, there are a few but neither their single crystal X-ray
structures are known nor their turnover numbers are deter-
(
0.53 g, 70%). Anal. calcd for C H N O Co : C, 49.81; H,
32 41 14 2 2
5
.36; N, 25.41%. Found C, 49.77; H, 5.34; N, 25.46%. IR (KBr
1
8
mined. Again, although a few dinuclear or oligonuclear com-
−1
3
pellets, cm ): ν(N ), 2055 s; ν(CvN), 1637. UV-vis (DMF) λmax
III
II
19,20
pounds containing both Co and Co are known,
the
−
1
−1
(
nm) [ε in M cm ]: 380 (18 600), 581 (332), 1065 (16).
possibility of catecholase activity of those mixed-valence
systems has not been checked. With the aim to explore cate-
cholase activity by cobalt complexes, we have isolated three
III
II
[
3 3
Co Co L(OCN) ]·CH CN (2). This compound was prepared
following the similar procedure as described above for 1,
except using sodium cyanate instead of sodium azide. Yield:
III
II
III
II
mixed-valence dinuclear Co Co
complexes [Co Co L-
]·CH CN (2), and [Co Co L-
3 3
(
0.57 g, 74%). Anal. calcd for C35
41 8 5 2
H N O Co : C, 54.48; H,
III
II
III
II
(
(
N
3
)
3
]·CH
3
CN (1), [Co Co L(OCN)
5
.36; N, 14.52%. Found C, 54.44; H, 5.42; N, 14.57%. IR (KBr
μ-CH COO)
3
2
](ClO ) (3), derived from a tetraimino diphenolate
4
−
1
pellets, cm ): ν(OCN), 2211 s; ν(CvN), 1638. UV-vis (DMF)
macrocyclic ligand H L (Scheme 1), obtained on [2 + 2] con-
2
−1
−1
λ
max (nm) [ε in M cm ]: 376 (11 600), 581 (330), 1093 (10).
densation of 4-ethyl-2,6-diformylphenol and 2,2′-dimethyl-1,3-
III
II
[
Co Co L(μ-CH
solution (30 ml) of 4-ethyl-2,6-diformylphenol (0.356 g,
mmol) were added successively a methanol solution (10 ml)
of Co(ClO )·6H O (0.366 g, 1 mmol) and Co(OAc) ·4H
0.249 g, 1 mmol) and a methanol solution (20 ml) of 2,2′-
3 2 4
COO) ](ClO ) (3). To a boiling methanol
2
1
diaminopropane. Herein, we report the syntheses, character-
ization, catecholase activity, and electrospray ionization mass
spectroscopic (ESI-MS positive) study of these three complexes.
2
4
2
2
2
O
(
dimethyl-1,3-diaminopropane (0.208 g, 2 mmol). The resulting
red solution was refluxed for one hour, after which it was con-
centrated to ca. 15 ml on a rotary evaporator. Then after fil-
tration to remove any suspended particles, the filtrate was
allowed to evaporate slowly at ambient temperature. After a few
days, a red crystalline compound containing diffraction quality
single crystals that deposited were collected by filtration,
washed with cold methanol and air dried. Yield: (0.62 g, 75%).
Experimental section
Caution! Azide and perchlorate complexes of metal ions are poten-
tially explosive. Only a small amount of material should be
prepared, and it should be handled with caution.
Materials and physical measurements
All the reagents and solvents were purchased from commercial Anal. calcd for C H N O ClCo : C, 49.68; H, 5.40; N, 6.81%.
3
4
44
4
10
2
−
1
sources and used as received. 4-Ethyl-2,6-diformylphenol was Found C, 49.71; H, 5.37; N, 6.83%. IR (KBr pellets, cm ):
3 4
prepared according to the procedure reported for the synthesis ν(CvN), 1636; ν(CH COO), 1583 vs, 1441 s; ν(ClO ), 1091 s,
of 4-ethyl-2,6-diformylphenol. Elemental (C, H, and N) ana- 623 w. UV-vis (DMF) λmax (nm) [ε in M cm−1]: 377 (11 200),
2
2
−1
lyses were performed on a Perkin-Elmer 2400 II analyzer. IR 578 (329), 956 (10).
spectra were recorded in the region 400–4000 cm− on a
1
X-ray crystallography. The crystallographic data of the com-
Bruker-Optics Alpha-T spectrophotometer with samples as KBr pounds 1–3 are summarized in Table 1. Intensity data of 1 and
disks. Electronic spectra were obtained by using a Shimadzu 2 were collected at 100 K and 296 K respectively, on a Bruker-
UV-3600 spectrophotometer. The electrospray ionization mass APEX II SMART CCD diffractometer using graphite-monochro-
spectra were recorded on a Micromass Qtof YA 263 mass mated Mo-Kα radiation (λ = 0.71073 Å) whereas intensity data
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Table 1 Crystallographic data for 1–3
Compound
1
2
3
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C
32
H
41
N
14
O Co
2 2
C
35
H
41
N
O
8 5
Co
2
C
34 44 4 2
H N O10ClCo
771.64
Monoclinic
C2/c
32.855(12)
12.339(4)
19.200(7)
90.00
771.63
Triclinic
Pˉ1
822.07
Monoclinic
P2/c
15.792(4)
11.677(6)
21.775(4)
90.00
9.631(4)
12.255(5)
16.296(7)
96.958(5)
α (°)
β (°)
113.975(5)
90.00
7112(4)
8
101.226(5)
104.979(5)
1792.4(14)
2
104.040(10)
90.00
3895(2)
4
γ (°)
V (Å )
3
Z
−3
D (calculated, g cm
)
1.441
1.430
1.402
λ (Mo-K
μ (mm
α
) (Å)
)
0.71073
0.985
0.71073
0.978
0.71073
0.978
−
1
T (K)
F (000)
100(2)
3208
296(2)
802
293(2)
1708
2
θ range for data collection (°)
2.72–50.88
−39 ≤ h ≤ 39
2.60–51.00
4.12–60.04
−22 ≤ h ≤ 22
0 ≤ k ≤ 16
−30 ≤ l ≤ 30
22 630
Index ranges
−11 ≤ h ≤ 11
−14 ≤ k ≤ 14
−19 ≤ l ≤ 19
12 975
−
14 ≤ k ≤ 14
23 ≤ l ≤ 23
−
No. of measured reflections
34 359
No. of independent reflections
6533
0.0931
6530
0.0238
11 352
0.1166
R
int
No. of refined parameters
458
4421
1.016
0.0520, 0.1282
0.0878, 0.1478
0.982, −0.647
489
4891
1.022
0.0770, 0.2088
0.1014, 0.2300
2.305, −1.604
470
4726
0.980
0.0502, 0.1228
0.1652, 0.1534
0.486, −0.391
No. of observed reflections, I ≥ 2σ(I)
2
Goodness-of-fit on F , S
a
a
b
b
R
R
1
, wR
, wR
2
[I ≥ 2σ(I)]
[all data]
1
2
−
3
Max., min. electron density (e Å
)
a
b
2
o
2 2
2 2 1/2
R
1
= [∑||F
o
| − |F
c
||/∑|F
o
|]. wR
2
= [∑w(F
− F
c
) /∑w(F
o
) ]
.
of 3 were collected on an Enraf-Nonius CAD4 diffractometer better quality crystal. During the development of structure 3
using graphite-monochromated Mo-Kα radiation (λ eq values of C8, O7 and O8 were found to be higher (>0.15).
.71073 Å) at 293 K. Data processing and absorption correction However it was only possible to split O7 over two sites, O7A
=
U
0
2
3a
of 1 and 2 were done with the packages SAINT
SADABS
and (Ueq = 0.167; occupancy = 0.50) and O7B (Ueq = 0.174; occu-
2
3b
while a CAD4 Express Enraf-Nonius programs pancy = 0.50). No appreciable density was found around C8
package and XCAD4 were used for data collection and and O8. It may be noted that thermal agitation of the methyl
reduction for 3. The structures were solved by direct and carbon atom (as C8) and the perchlorate oxygen atom (as O8)
Fourier methods and refined by full-matrix least-squares based is not specific. All the hydrogen atoms in 1–3 were placed at
2
23c
23d
on F using SHELXTL and SHELXL-97
packages. During geometrical positions with fixed thermal parameters. Except
the development of the structures of 2 it became apparent that for O4A, all the nonhydrogen atoms were refined anisotropi-
a few atoms were each disordered over two sites. These dis- cally, while O4A and all the hydrogen atoms were refined iso-
ordered atoms are Co2, N6, C32 and O4. This was allowed on tropically. The final least-squares refinements (R ) based on
1
refining freely and the final linked occupancy parameters for I > 2σ(I) converged to 0.0520, 0.0770 and 0.0502 for 1, 2 and
these disordered atoms are 0.85 and 0.15 for Co2, 0.85 and 3, respectively.
0
.15 for N6, and 0.80 and 0.20 for C32 and O4. While refining
anisotropically, the Ueq values of O4 (occupancy 0.80) and O4A
occupancy 0.20) were 0.217 and 0.360, both are larger than
(
Results and discussion
Syntheses, characterization and proposed composition in
solution
usual in spite of splitting over two sites. As Ueq (anisotropic) of
O4A was appreciably large, 0.36, this site was refined isotropi-
cally resulting in Ueq (isotropic) = 0.21. The Ueq value of C8 is
III
II
also high, 0.262. However, no electron density around this The macrocyclic mixed-valence Co Co azide/cyanate com-
III
II
III
II
atom was found and therefore splitting it over two disordered plexes
[Co Co L(N ) ]·CH CN
(1)
and
[Co Co L-
3
3
3
sites was not possible. Again, the largest residual electron (OCN) ]·CH CN (2) are readily obtained in high yield from the
3
3
density peak was 2.305 which is related to the heavy element reaction of the perchlorate salt of diprotonated macrocyclic
Co1 in the structure. All these problems (larger Ueq or larger ligand [H L](ClO ) , cobalt(II) perchlorate hexahydrate, triethyl-
4
4 2
residual density peak) in the structure of 2 are related to poor amine and NaN (for 1) or NaNCO (for 2) in a 1 : 2 : 4 : 4 ratio.
3
2
4
III
II
quality of the crystal; it has not been possible to isolate a On the other hand, the mixed-valence bis(μ-acetate) Co Co
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compound [Co Co L(μ-CH
Dalton Transactions
III
II
3
2 4
COO) ](ClO ) (3) was obtained in
the template syntheses in which 4-ethyl-2,6-diformylphenol
and 2,2′-dimethyl-1,3-diaminopropane (2 : 2) were allowed to
condense in the presence of 1 equivalent of Co(ClO
4 2 2
) ·6H O
and Co(OAc) ·4H O (1 : 1). The elemental analyses clearly indi-
2
2
III
II
cate a composition having Co and Co metal ions in each of
1
–3.
The FT-IR spectra reveal the presence of CvN moieties in
−
1
−1
1
(
–3 (1636–1638 cm ), azide in 1 (2055 cm ), cyanate in 2
2221 cm− ), perchlorate in 3 (1091 and 622 cm ), and acetate
1
−1
−
1
−1
in 3 (1583 and 1441 cm ). The difference in energy, 142 cm
,
between the antisymmetric and symmetric carboxylate stretch-
ing frequencies in 3 is in line with the bidentate bridging
2
5
mode.
The electronic spectra of the complexes were studied in
DMF at ambient temperature. The complexes 1–3 show an
intense band at 380 nm (for 1), 376 nm (for 2), and 377 nm
(
1
for 3) with an extinction coefficient (ε) in the range
1 200–18 600 M− cm that can be assigned to the π→π* tran-
1
−1
sition associated with the azomethine group. All these three
−
1
−1
complexes exhibit one low-intensity (ε = 16 M cm for 1 and
−
1
−1
−1
1
1
3
3
0 M cm for 2 and 3) near-IR band (at 1065 cm for 1,
−
1
−1
093 cm for 2, and 956 cm for 3) and a stronger band (ε =
32 M− cm−1 for 1, 330 M cm−1 for 2, and 329 M cm for
1
−1
−1
−1
) at 581 nm, which are characteristic d–d bands for, respect-
ively, high-spin Co and low-spin Co centers in valence-loca-
lized Co Co systems. Thus, the spectral feature indicates
that valences in 1–3 are localized.
II
III
III
II
Fig. 1 Crystal structure of [Co Co L(N
3
)
3
]·CH
3
CN (1). All the hydrogen atoms
III
II
26
and the solvated acetonitrile molecule are omitted for clarity.
The molar conductance values at 298 K for 1–3 in different
solvents are summarized in Table S1.† The nonelectrolytic
nature of compounds 1 and 2 in the solid state is retained in
solution as evidenced by the low molar conductivity values
2.9–40 ohm− cm mol l). The 1 : 1 electrolytic nature of
1
−1
−1
(
complexes 3 in the solid state is retained in solution as evi-
−
1
−1
denced by their molar conductance values, 75 ohm cm
−
1
−1
−1
−1
27
mol l in DMF and 158 ohm cm mol l in acetonitrile.
III
II
Description of the structures of [Co Co L(N
3
)
3
]·CH
3
CN (1),
III
II
III
II
[
(
Co Co L(OCN) ]·CH CN (2) and [Co Co L(μ-CH COO) ]-
3
3
3
2
ClO ) (3)
4
The structures of compounds 1, 2, and 3 are shown in Fig. 1–3,
respectively. The selected bond lengths and angles of 1, 2, and
3
are listed in Tables S2–S4 (ESI†), while selected bond lengths
and angles and some other structural parameters of the three
complexes are compared in Table 2. The structures show that
all the three compounds are dicobalt systems derived from tet-
2
−
raiminodiphenolate macrocyclic ligand L . While 1 and 2 are
diphenoxo-bridged, 3 is a heterobridged bis(μ-phenoxo)bis-
(
L
μ-acetate) system. In addition to the dinegative organic ligand,
2
−
, the presence of three mononegative moieties (three azides
in 1, three cyanates in 2 and two acetates and one perchlorate
in 3) in the composition of all the three complexes indicates
III
II
that all of 1–3 are Co Co systems. As described below, the
relative bond lengths involving the two metal centers indicate
that the Co1 center is in the +III state while the Co2 center is
in the +II state. Each of the two N(imine) O(phenoxo)
III
II
3 3
Fig. 2 Crystal structure of [Co Co L(OCN) ]·CH CN (2). All the hydrogen atoms
and the solvated acetonitrile molecule are omitted for clarity. Of the two disor-
dered positions of one cobalt, nitrogen, carbon and oxygen atom of the coordi-
nated cyanate molecule, the one with high occupancy is shown here.
2
2
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compartments of L − satisfy four coordination positions and atoms of two μ1,3-acetate ligands. From the least-squares N
2
O
2
2
provide the basal plane for each of the two metal ions in 1–3. basal plane, while the displacement of the hexacoordinated
III
In 1 and 2, the Co center (Co1) is hexacoordinated and metal ions in 1–3 is small (<0.16), the pentacoordinated metal
adopts a distorted octahedral environment in which the two centers (Co2) in 1 and 2 are shifted significantly (by 0.55 and
axial positions are occupied by the nitrogen atoms of two 0.60, respectively) towards the apical azide or cyanate nitrogen
II
azide (in 1) or two cyanate (in 2) ligands, while the Co center atom. As compared in Table 2, the cobalt–O(phenoxo) and
(Co2) is pentacoordinated and adopts a distorted square pyra- cobalt–N(imine) bond distances in all the three complexes and
midal environment in which the axial position is occupied by the cobalt–O(acetate) bond distances in 3 involving Co1 are
the nitrogen atom of an azide (in 1) or a cyanate (in 2) ligand. significantly shorter than the corresponding bond lengths
III
II
III
On the other hand, both the Co (Co1) and Co (Co2) centers involving Co2, indicating that Co1 is the Co center while Co2
II
in 3 are hexacoordinated and adopt a distorted octahedral geo- is the Co center. On the other hand, the cobalt(III)–N(azide)
metry in which the axial positions are occupied by the oxygen and cobalt(II)–N(azide) distances in 1 are almost identical, as
are the cobalt(III)–N(cyanate) and cobalt(II)–N(cyanate) dis-
tances in 2, which is due to the shifting of the cobalt(II) center
towards the apical atom. In all of 1–3, the cisoid angles for the
III
Co centers (overall range for 1–3 = 80.38(16)–95.4(2)°) are less
II
deviated than for the Co centers (overall range for 1–3 =
III
II
7
4.97(15)–122.4(2)°). Transoid angles for the Co and Co
centers are more significantly different; the overall range for
III
the Co centers in 1–3 is 171.82(19)–178.7(3)°, the range for the
II
Co center in 3 is 151.86(9)–169.60(10)°, and the overall range
II
for the Co centers in 1 and 2 is 139.91(9)–150.01(12)°. The
metal⋯metal separation (3.064 Å in 1, 3.067 Å in 2, 2.856 Å in
3) and the Co–O(phenoxo)–Co bridge angles (99.74(12)° and
100.82(12)° in 1, 99.36(17)° and 101.20(17)° in 2, 92.77(8)° and
92.87(8)° in 3) are comparable with each other and also with
previously reported similar macrocyclic dicopper(II) systems.
The macrocyclic ligand in 1–3 adopts a puckered configuration
as evidenced from the dihedral angle between the two aro-
matic rings, 69.96°, 58.04° and 65.70°, respectively. We have
2
8
also performed the bond valence sum (BVS) calculations to
2
0a–c
assign the oxidation states of the two cobalt centers.
The
BVS values of 3.72/4.36/3.44 and 2.31/2.34/2.40 for the Co1 and
Co2 centers, respectively, in complexes 1–3 corroborate the
assignment on the basis of bond lengths. However, it is not
III
clear why the Co (Co1) center in 1 and 2 has such a high BVS
IV
value (3.72/4.36) but there is no evidence to suggest it as a Co
III
II
center. It may be mentioned as well that such a high value of
the Co center in mixed-valence systems is not unknown.
Fig. 3 Crystal structure of [Co Co L(μ-CH
3 2 4
COO) ](ClO ) (3). All the hydrogen
III
20c
atoms and the perchlorate ion are omitted for clarity.
Table 2 Selected bond lengths and angles and some other structural parameters of the three complexes 1–3
Metal
Compound center
Co–
Cisoid
Transoid
angles
M–O
(phenoxo)–M M⋯M
phenoxo Co–imine Co–azide Co–cyanate Co–acetate angles
d
N,O
d
M
III
II
1
2
3
Co1 (Co
)
1.929(3), 1.911(3), 1.947(4),
.933(3) 1.915(3) 1.969(3)
—
—
—
—
81.41(11)– 172.89(13)– 0.008 0.004 99.74(12),
3.064
3.067
2.856
1
95.27(14) 178.30(15)
100.82(12)
Co2 (Co ) 2.042(3), 2.039(4), 1.978(4)
75.39(10)– 145.53(12)– 0.036 0.551
113.05(15) 150.01(12)
2.076(3) 2.059(3)
III
II
Co1 (Co
)
1.930(4), 1.910(5),
.945(4) 1.919(5)
—
—
1.700(12),
1.945(6)
1.950(7)
—
—
80.38(16)– 171.82(19)– 0.022 0.011 99.36(17),
95.4(2) 178.7(3) 101.20(17)
74.97(15)– 139.91(19)– 0.093 0.604
122.4(2) 149.34(19)
1
Co2 (Co ) 2.036(4), 2.055(5),
2.074(4) 2.068(5)
III
II
Co1 (Co
)
1.922(2), 1.931(3),
.923(2) 1.934(3)
—
—
—
—
1.909(2),
1.933(2)
2.100(3),
2.279(2)
87.25(9)– 176.18(10)– 0.006 0.001 92.77(8),
1
94.72(12) 177.85(10)
77.82(9)– 151.86(9)–
107.39(12) 169.60(10)
92.87(8)
Co2 (Co ) 2.018(2), 2.016(3),
0.007 0.156
2.020(2) 2.017(3)
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Dalton Transactions
Catecholase activity
3
2
,5-Di-tert-butylcatechol (3,5-DTBCH ) is usually used to study
the catecholase activity for the model complexes because of
the ease of oxidation of 3,5-DTBCH2 to its corresponding
quinone, 3,5-di-tert-butylquinone (3,5-DTBQ), the latter of
which has a characteristic transition at ca. 400 nm. To check
the ability of complexes 1–3 to behave as a catalyst for catecho-
−
4
lase activity, a 0.25 × 10 (M) solution of a complex was
treated with a 100-fold concentrated solution of 3,5-DTBCH
2
and the course of the reaction was followed by recording UV-
vis spectra of the mixture under aerobic condition times up to
3
0 or 120 min. Experiments for complex 1 were done in three
solvents, methanol, acetonitrile, and DMF, while those for 2
and 3 were done in acetonitrile and DMF because the latter
two compounds are practically insoluble in methanol.
Fig. 5 Initial rates versus substrate concentration for the 3,5-DTBCH
2
→3,5-
III
II
DTBQ oxidation reaction catalyzed by complex [Co Co L(N ) ]·CH CN (1) in
3
3
3
acetonitrile. The inset shows the Lineweaver–Burk plot. Symbols and solid lines
represent the observed and simulated profiles, respectively.
Spectral changes of complex 1 in acetonitrile are shown in
Fig. 4. Because of the addition of 3,5-DTBH
2
, the band of 1 at
3
80 nm becomes slightly red shifted to 400 nm with a gradual
increase of intensity, indicating more and more formation of
the quinone, 3,5-DTBQ. Clearly, complex 1 in acetonitrile
shows catecholase activity. Similar spectral changes take place
for complex 1 in methanol (Fig. S1, ESI†). On the other hand,
generation of the quinone band does not take place for
complex 1 in DMF and also for 2 and 3 in acetonitrile and
DMF, which clearly indicates that complexes 2 and 3 are not
catalysts of catecholase activity and complex 1 is not a catalyst
in DMF. Kinetic studies of the catecholase activity of complex
the quinone band was carried out for a period of 30 minutes
in acetonitrile and 120 minutes in methanol. It may be noted
here that blank experiments in both for acetonitrile and
methanol without the catalyst do not show formation of the
quinone up to 6 hours. The rate constant for a particular
complex–substrate mixture was determined from the optical
density versus time plot by the initial rate method. The rate
constant versus concentration of substrate data were then ana-
lyzed on the basis of the Michaelis–Menten approach of enzy-
matic kinetics to get the Lineweaver–Burk plot as well as the
1
have been performed to understand the extent of its
−
4
efficiency. For this purpose, a 0.25 × 10 (M) solution of a
complex was treated with the substrate solution having con-
centration ranging between 10-fold and 100-fold greater than
that of the complex. The experiments were done at a constant
temperature, 25 °C, under aerobic condition. For a particular
complex–substrate mixture, a time scan at the maximum of
M
values of the parameters Vmax, K , and Kcat. The observed and
simulated rate constant versus [substrate] plot and the Line-
weaver–Burk plot for complex 1 in acetonitrile are shown in
Fig. 5 while similar plots for complex 1 in methanol are shown
in Fig. S2 (ESI†). The kinetic parameters are listed in Table 3.
−1
The turnover number (Kcat) for complex 1 is 482.16 h in
acetonitrile and 45.38 h− in methanol. These values lie in the
1
range of the K values of the di- or oligonuclear copper(II)
cat
7
–15
systems
behaving as functional models of catechol oxidase.
III
II
Therefore the Co Co compound 1 can also be considered as
a functional model of catechol oxidase.
Electrospray ionization mass spectral study
The electrospray ionization mass spectra (ESI-MS positive) of
acetonitrile solutions of compounds 1–3 were recorded and
are shown in Fig. 6, S3 and S4 (ESI†), respectively. Compound
1
exhibits three peaks at m/z = 688 (52%, line to line separation
1.0), 753 (31%, line to line separation 1.0), and 302 (12%, line
to line separation 0.5). The peaks at m/z = 688 and 753 are
III
II
+
III
assignable to [Co Co L(N
3
)
2
]
(C30
H
38
N
10
O
2
Co
2
) and [Co -
II
+
Co L(N ) Na] (C H N O NaCo ), respectively, while the low
3
3
30 38 13
2
2
intensity peak at 302 arises due to the reduced species
II
II 2+
[
Co Co L] (C30
H
38
N
4
O
2
Co
2
). Compound 2 exhibits only one
peak at m/z = 688 (64%, line to line separation 1.0), which is
Fig. 4 The spectral profile showing the increase of the quinone band at
III
II
+
assignable to [Co Co L(OCN)
2
] (C32
H
38
N
6
O
4
Co
2
). Compound
4
00 nm after the addition of 100-fold of 3,5-DTBCH
2
to a solution containing
III
II
−4
3 exhibits three peaks at m/z = 663 (11%, line to line separation
complex [Co Co L(N ) ]·CH CN (1) (0.25 × 10 M) in MeCN. The spectra were
3 3 3
recorded after each 5 min.
1.0), 331 (18%, line to line separation 0.5), and 302 (58%, line
Dalton Trans.
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III
II
Table 3 Kinetic parameters for the complex [Co Co L(N
3
)
3
]·CH
3
CN (1)
max (M min−
1
)
Std. error
K
M
(M)
Std. error
K )
cat (h−1
Complex
Solvent
V
2.009 × 10−
1.891 × 10−
4
5.15 × 10−5
4.168 × 10
0.003011
0.001576
0.001227
0.0007015
482.16
45.38
1
MeCN
MeOH
5
−6
+
quinone–sodium aggregates [(3,5-DTBQ)Na] (C H O Na)
1
4
20 2
+
2 40 4
and [(3,5-DTBQ) Na] (C28H O Na). The remaining peaks at
m/z = 889 and 910 are quite interesting because the peak posi-
tion and matching of the isotopic distribution of the observed
and the simulated patterns clearly indicate that these peaks
II
arise due to 1 : 1 complex–substrate aggregates [Co
2
L(μ-3,5-
3 2
2
−
+
III
II
DTBC )Na] (C H N O NaCo ) and [Co Co L(N ) (3,5-
4
4
58
7
4
2
2
−
+
DTBCH )H] (C44
H
60
N
10
O
4
Co
2
). It is logical to consider the
−
III
II
monoanionic 3,5-DTBCH species in [Co Co
2
L(N
3
)
2
(3,5-
III
II
−
+
Fig. 6 Electrospray mass spectrum (ESI-MS positive) of [Co Co L(N
3
)
3
]·CH
3
CN DTBCH )H] as coordinated in monodentate mode through
(
1) in acetonitrile showing observed and simulated isotopic distribution
the phenolate oxygen atom to the cobalt(III) center and the di-
patterns.
2−
II
2−
+
anionic 3,5-DTBC species in [Co L(μ-3,5-DTBC )Na] as a
2
bridging ligand through the two phenolate oxygen atoms to
the two cobalt(II) centers.
Comparison with the previous related studies: significant
aspects
To understand why a metal compound is an active catalyst
while another closely related compound is not, determination
of single crystal X-ray structures of the compounds is very much
important. Again, determination of turnover number (Kcat) of
the catalysts is important to understand their relative efficiency.
In addition, a sufficient number of active catalysts having
closely similar composition/structure is required in order to
frame a structure–property or structure–efficiency correlation.
Such criteria are fulfilled for several series of dicopper(II) com-
pounds as the active catalyst of catechol oxidase activity and
therefore a few structure–efficiency correlations have been pro-
posed in which efficiency is the function of metal⋯metal dis-
tance, coordination geometry around the metal center, nature
of the exogenous bridging ligand, flexibility of the primary
Fig. 7 Electrospray mass spectrum (ESI-MS positive) of a 1 : 100 mixture of
III
II
[
Co Co L(N
5 minutes of mixing. Both the observed and simulated isotopic distribution
patterns are shown.
3 3 3 2
) ]·CH CN (1) and 3,5-DTBCH in acetonitrile, recorded after
1
6
,7,12,13
to line separation 0.5). The peaks at m/z = 331 are assignable ligand etc.
However, the literature survey reveals that the
), while the problem is rather complicated because a correlation is appli-
remaining two peaks at 302 and 663 arise due to the two cable for only a set of a few compounds; straightforward corre-
III
II
2+
to [Co Co L(μ-CH
3
COO)]
41 4 4 2
(C32H N O Co
II
II 2+
II
II
reduced species [Co Co L] (C30
(
H
38
N
4
O
2
Co
2
) and [Co Co L- lation even in dicopper(II) compounds is still lacking. On the
μ-CH COO)] (C32H N O Co ), respectively. As shown in other hand, studies of catechol oxidase activity by complexes of
+
3 41 4 4 2
Fig. 6, S3 and S4 (ESI†), the isotopic distributions of the metal ions, other than copper(II), are very much less and there-
observed and the simulated spectral patterns are in excellent fore no correlation has yet been proposed.
agreement with each other, indicating right assignment of the
positive ions.
Regarding complexes of metal ions of cobalt as an active
catalyst for catechol oxidase activity, a few examples are
To get an insight into the nature of possible complex– known. To the best of our knowledge, these include one mono-
II 18a,b
II
II 18c
substrate intermediates, ESI-MS positive spectra of
1
a
nuclear Co ,
four dinuclear Co Co , and two dinuclear
III
III
18c
: 100 mixture of complex 1 and 3,5-DTBCH2 were recorded Co Co compounds.
However, single crystal X-ray struc-
after 15 minutes of mixing in acetonitrile. The observed and tures of none of these have been determined. Moreover,
simulated patterns are presented in Fig. 7. In this case, four kinetic studies to determine Kcat values of none of these have
peaks are observed at m/z = 243 (99%, line to line separation been carried out. So, compound 1 in the present investigation
1
.0), 463 (12%, line to line separation 1), 889 (21%, line to line is the sole example of a cobalt containing catalyst of catecho-
separation 1.0), and 910 (26%, line to line separation 1.0). The lase activity for which both the single crystal X-ray structure
peaks at m/z = 243 and 463 correspond, respectively, to the and Kcat values have been determined.
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As already mentioned, while [Co Co L(N
Dalton Transactions
III
II
3
)
3
]·CH
3
CN (1) is a
Conclusions
III
II
III
II
catalyst, [Co Co L(OCN) ]·CH CN (2) and [Co Co L(μ-OAc) ]-
3
3
2
(ClO
4
) (3) are inactive regarding catecholase activity. Although The present study demonstrates first structurally characterized
catecholase activity is a property in solution, the activity or mixed-valence dinuclear cobalt complexes to check their cate-
III
II
inactivity as well as the relative extent of activity are usually chol oxidase activity. Interestingly, [Co Co L(N ) ]·CH CN (1)
explained in terms of solid state structures.
3
3
3
6
a,b,7,8,11,12
More- is the unique example of (i) a structurally characterized
over, as mentioned above, the electrolytic nature in solution complex showing catecholase activity, (ii) a cobalt complex for
and in solid state of 1–3 are the same, indicating that the brid- which turnover number has been determined, and (iii) a
III
II
ging core along with the ligand environment of 1–3 in solid mixed valence Co Co complex showing catecholase activity.
state and in solution are most probably the same. Therefore, it In the ESI-MS positive spectra, two complex–substrate aggre-
is logical to address activity/inactivity of 1–3 in terms of their gates have been identified which contain, respectively, a mono-
crystal structures. The inactivity of 3 is probably due to its dentate catechol moiety and a bidentate bridging catechol
triple bridged composite structure, which prevents the attack moiety and thus these species mimic, interestingly, the pro-
of the catechol moiety on the metal center(s). On the other posed intermediates in the mechanism of in vivo cycles of the
hand, as 1 and 2 have similar types of structures, it looks sur- native dicopper enzyme. Due to the availability of structure
prising, at least apparently, why 1 is active but 2 is inactive. and activity/Kcat data in cobalt systems here only, the probable
However, while the two cobalt(III)–N(azide) distances in 1 are reason for activity/inactivity of the three title compounds has
almost similar, 1.947(4) and 1.969(3) Å, the two cobalt(III)– been explained. The exploration of both structure and activity/
III
II
N(cyanate) distances in 2 are significantly different, 1.700(12)
Kcat of more cobalt systems in general and Co Co complexes
and 1.945(6) Å. The more tightly bound cyanate of cobalt(III) in particular will be important for further enlightenment of
and the coordinated cyanate of cobalt(II) lie at the same side, the intimate reason for activity/inactivity and framing struc-
both of which are to be eliminated for providing space of the ture–activity correlations, which are to date lacking in systems
dideprotonated, dinegative, and bridging catechol moiety. But, having metal ions other than copper(II).
elimination of the tightly bound cyanate is rather difficult,
which is probably the reason for inactivity of complex 2.
Whatever the aftermath, coordination of the catechol
moiety to the metal center is an essential requirement for a
Acknowledgements
complex to show catecholase activity. In the two proposed Financial support from the Government of India through the
mechanisms regarding the in vivo cycle, 1 : 1 adduct formation Department of Science and Technology (Project No. SR/S1/
between a dicopper(II) core and the substrate has been men- IC-42/2011) and the Council for Scientific and Industrial
tioned. While monodentate asymmetric coordination of the Research (CSIR Fellowship to S. Mondal) is gratefully acknowl-
substrate has been proposed in one mechanism (Krebs’s edged. Crystallography of 1 and 2 was performed at the
6
f,g
mechanism), simultaneous coordination of the substrate to DST-FIST, India-funded Single Crystal Diffractometer Facility
both copper centers in the dinucleating bridging fashion is at the Department of Chemistry, University of Calcutta. P. L.
6
d
suggested in the second (Solomon’s mechanism). Although thanks Université Paris Descartes, Laboratoire de Cristallogra-
II
II
III
II
complex 1 is not a Cu Cu but a Co Co system, two 1 : 1 phie et RMN biologiques UMR 8015, Faculté de Pharmacie for
III
II
−
+
complex substrate aggregates [Co Co L(N
3
)
2
(3,5-DTBCH )H]
single crystal X-ray data collection of 3.
II
2−
+
and [Co 2L(μ-3,5-DTBC )Na] having a monodentate (as in
one mechanism) catechol moiety in the first aggregate and a
bridging bidentate (as in the second mechanism) catechol
moiety in the second aggregate have been identified in the
ESI-MS positive spectrum of the present investigation. We
should keep in mind that the complex–substrate aggregate
identified in the ESI-MS spectrum may not be a real intermedi-
ate in the catalytic cycle because such aggregates may be
formed in situ because of the relative stability when compared
to the other cationic species in the time scale of the ESI-MS
spectrum. Similar should be the case for the complex–substrate
aggregates identified in the solid state, where the solid state
effects may enforce the stability. Nonetheless, the aggregates
identified here in the ESI-MS positive spectrum deserve impor-
tance because they indicate clear evidence of the coordination
and bridging ability of the substrate to the complexes. More-
over, the species mimic the species proposed in two mechan-
isms at least in terms of stoichiometry (1 : 1) and coordination
mode (monodentate in one and bridging in another).
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