Cubane-like tetranuclear Cu(II) complexes bearing a Cu4O4 core: Crystal structure, magnetic properties, DFT calculations and phenoxazinone synthase like activity

Article abstract of DOI:10.1039/c6dt03754b

In the present work, two new copper complexes 3a and 3b with a Cu₄O₄ cubane core are reported. Both complexes are obtained by means of the in situ conversion of the imine functionality of Schiff's base ligands 1a [(E)-4-chloro-2-((thiazol-2-ylimino)methyl)phenol] and 1b [(E)-4-bromo-2-((thiazol-2-ylimino)methyl)phenol] into amino alcohols 2a (4-chloro-2-(hydroxy(thiazol-2-ylamino)methyl)phenol) and 2b (4-bromo-2-(hydroxy(thiazol-2-ylamino)methyl)phenol), respectively. The ligand transformation may be metal assisted and the generated ligands show an interesting mode of coordination in which the alkoxo-O atom binds in a μ³-manner connecting simultaneously three copper centers and forming a Cu₄O₄ cubane core. The first analysis of single crystal X-ray diffraction studies reveals that both molecules possess a [4 + 2] cubane-type core, and low temperature magnetic measurements show antiferromagnetic behaviour, in agreement with DFT calculations. However, the best fit and DFT calculations point out three pairs of coupling constants, more coherent with a [2 + 2 + 2] situation, in accordance with the fine analysis of structural data. Finally, phenoxazinone synthase activity has been measured for both molecules, finding k_cat = 86.3 h^-1 for the chloride derivative copper(ii) complex in methanol, whereas the bromide derivative copper(ii) complex displays k_cat = 3.4026 × 10² h^-1 and 10.289 × 10² h^-1 in methanol and DMSO, respectively.

Full text of DOI:10.1039/c6dt03754b

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Sengupta, S. K. Chattopadhyay, A. J. Mota, A. Espinosa, E. RIVIERE, W. Lewis and S. Naskar, Dalton Trans.,
016, DOI: 10.1039/C6DT03754B.
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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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ARTICLE
Cubane-like tetranuclear Cu(II) complexes bearing a Cu O core:
4
4
crystal structure, magnetic properties, DFT calculations and
phenoxazinone synthase like activity
Received 00th January 20xx,
Accepted 00th January 20xx
a‡
a,b‡
b
c
DOI: 10.1039/x0xx00000x
Shipra Sagar, Swaraj Sengupta, Shyamal K. Chattopadhyay, Antonio J. Mota,* Arturo
d
e
f
a
Espinosa Ferao,* Eric Riviere,* William Lewis, Subhendu Naskar*
www.rsc.org/
In the present work, two new copper complexes 3a and 3b with Cu
obtained by means of the in situ conversion of the imine functionality of a Schiff's base ligand 1a [(E)-4-chloro-2-((thiazol-
-ylimino)methyl)phenol] and 1b [(E)-4-bromo-2-((thiazol-2-ylimino)methyl)phenol)] into an amino alcohol 2a (4-chloro-2-
hydroxy(thiazol-2-ylamino)methyl)phenol) and 2b (4-bromo-2-(hydroxy(thiazol-2-ylamino)methyl)phenol), respectively.
4 4
O cubane core are reported. Both complexes are
2
(
The ligand transformation may be metal assisted and the generated ligands show an interesting mode of coordination, in
3
which the alkoxo-O atom binds in a µ -manner connecting simultaneously three copper centers and forming a Cu
4
O
4
cubane core. First analysis of single crystal X-ray diffraction studies reveals that both molecules possess a [4+2] cubane-
type core, and low temperature magnetic measurements show antiferromagnetic behaviour, in agreement with DFT
calculations. However, the best fit and DFT calculations point out three pairs of coupling constants, more coherent with a
[2+2+2] situation, in accordance with fine analysis of structural data. Finally, phenoxazinone synthase activity have been
-1
measured for both molecules, finding kcat = 86.3 h for the chloride derivative copper(II) complex in methanol, whereas
2
-1
2
-1
the bromide derivative copper(II) complex displays kcat = 3.4026 x 10 h and 10.289 x 10 h in methanol and DMSO,
respectively.
4 4
The magnetochemistry of the Cu O cubane core with hydroxo,
Introduction
alkoxo and phenoxo bridges has been explored under
6
experimental and theoretical approaches. Inherent structural
High nuclearity transition metal complexes have attracted the
scientific community owing to the diversity of their structural
features and potential application in a high number of fields,
both in material and biological sciences, such as molecular
complexity in Cu
4 4
O cubanes is manifested by two different
7
classifications proposed by R. Mergehenn et. al. and Ruiz et.
6
b
al. depending on the relative values of the measured Cu-O or
Cu-Cu, respectively, in-core distances. The first proposal
1
2
magnetism, bioinorganic chemistry (as mimics for multi-
3
centred metal-containing active sites) and catalysis. Among
4 4
classified the Cu O core as Type I (four long Cu–O distances
between two dinuclear subunits) and Type II (long Cu–O
distances within each dinuclear subunit). The second one is
based on the Cu···Cu distances and distinguishes three
categories: 1) [2+4] with two short and four long distances, 2)
them, tetranuclear Cu(II) complexes with cubane core draw
4
special attention due to their magnetic properties, as well as
5
because they represent model system for metalloenzymes.
[
4+2] four short and two long and 3) [6+0] six similar Cu···Cu
a.
Department of Chemistry, Birla Institute of Technology, Mesra, Ranchi-835215,
Jharkhand, India
Depertment of Chemistry, Indian Institute of Engineering Science and Technology-
Shibpur, Howrah-711103, India
Department of Inorganic Chemistry, Faculty of Sciences, University of Granada,
Avda. Fuentenueva, s/n, 18002 Granada (Spain).
distances. With these various types of structures no simple
magneto-structural correlation is available since small changes
in the structure carry significant changes in the magnetic
behaviour.
b.
c.
d.
Polynuclear copper systems are also found in biological system
8
in the form of oxidase enzymes. This type of metalloenzymes
Department of Organic Chemistry, University of Murcia, Campus de Espinardo,
3
0100 Murcia (Spain). Fax +34 868 88 7489. E-mail: artuesp@um.es.
e.
Equipe Chimie Inorganique, Institut de Chimie Moléculaire et des Matériaux
d'Orsay, Université Paris Sud, Bâtiment 420 - Rue du Doyen Georges Poitou,91400
Orsay, France.
developed by nature can activate molecular oxygen to
undergo spin forbidden interactions with organic substrates
9
towards biochemical oxidation processes. For instance,
f.
School of Chemistry, University of Nottingham, Nottingham NG7 2RD, UK.
‡
Equally contributed
multicopper oxidases catalyze the reduction of molecular
oxygen through
a four-electron oxidation process by a
Electronic Supplementary Information (ESI) available: Crystal refinement
parameters (Table S1), selected bond lengths and angles (Table S2), change in reducing substrate. Among them, phenoxazinone synthase
spectral pattern (Figure S1), rate vs substrate concentration and Lineweaver-Burk
plots (Figure S2), χT vs T dependence (Figure S5), and Cartesian coordinates of the
theoretical calculated model structures (Table S3). See DOI: 10.1039/x0xx00000x
10
PHS) catalyses the oxidative coupling of two molecules of a
(
substituted 2-aminophenol to the phenoxazinone
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Journal Name
chromophore at the final step in the biosynthesis of the potent
antineoplastic agent actinomycin D. Unlike other oxidase
enzymes, which catalyse only one-electron oxidation
Results and discussion
Synthetic procedure.
processes, this is a six-electron oxidation process, actually a Both pro-ligands 1a,b [X: Cl (
a), Br (b)] were synthesized in a
combination of three consecutive two-electron oxidations. PHS similar way by condensation of 2-aminothiazole with the
exists as two oligomeric forms: a less active dimer and a highly corresponding salicylaldehydes (Scheme 1) in moderate yields.
active hexamer. Structure of hexameric PHS, as determined by The resulting products were purified in silicagel column with
1
1
single crystal X-ray diffraction confirms the presence of five 25-30% DCM/hexane mixture as eluent. The Schiff base pro-
Cu(II) centres in a unit cell. Like other Cu oxidases, hexameric ligands were characterized by spectroscopic techniques, one of
PHS possesses three copper binding motifs: one type 1 (blue), them (1a) being also characterized by single crystal X-ray
one type 2 (normal), and one binuclear type 3 centers. Thus diffraction. In order to get high-nuclearity copper complexes
the active site of hexameric PHS contains a tetranuclear Cu
the complexation reaction was performed with equimolecular
motif and a fifth copper atom remains bound at a loop, amounts of Cu(ClO , the corresponding pro-ligand and a base
establishing interactions between the subunits that stabilizes (Et N), used to deprotonate the phenolic hydroxyl groups. The
the hexamer and hence renders an additional structural role. reaction mixture was refluxed for 5 h and a green solid was
3,
4
4 2
)
3
12
However, there are a limited number of model systems separated. On analysing the single crystal X-ray diffraction data
screened for phenoxazinone synthase activity. Therefore, the of the collected crystals, it turned out that the Schiff’s base
synthesis and study of polynuclear Cu(II) complexes may changed in situ into the chiral amino alcohol
enable the exploration of the PHS activity unveiling a more addition to the imine bond (Scheme 1).
clear redox mechanism and thus allowing to develop
2 through water
biomimetic catalysis strategies for new oxidation processes.
The supramolecular approach to synthesize polynuclear metal
complexes is undoubtedly the most rational way. This has
1
3
been aesthetically developed by M. Fujita et. al. by the use of
several strategies in designing the ligand e.g., the donor atoms
position and orientation that forbids coordination to the same
-
-
-
3
metal or the use of suitable ancillary ligands (N . SCN , RCOO
etc.). However, the designed assembly strategy suffers from
certain limitations as it can make incorrect predictions and,
more importantly, it limits the horizons of the new molecular
architecture.
Polynuclear metal complexes with improved magnetic
properties were not obtained by designed assembly. In 1993
Scheme 1. Synthesis of the pro-ligand
situ transformation into the protonated ligand
complex (=Cu ) formation.
1
, its Cu(II)-assisted in
2
(=LH ) and
2
the first single molecule magnets [Mn12
2 2 4
O12(O CR)16 (H O) ] (R =
3
4 4
L
Me or Ph) were made by accident from reaction involving
1
4
permanganate and a Mn(III) triangle. From then on, this
approach gained popularity as it proved to be successful in
Molecular Structure of 1a, 3a and 3b.
obtaining large assemblies of metal ions in a variety of Single crystal X-ray diffraction studies were performed on 1a
,
structures. This new approach is known as serendipitous 3a and 3b. NMR and mass spectra were also coherent with the
assembly and can result in the formation of structures that crystal structure determined for the Schiff base ligand 1a, in
cannot be imagined at synthetic level. Metal assisted which the imine bond (C7-N6) distance (Figure 1) is 1.293 Å,
serendipity is reported by S. P. Perlepes et. al. in a number of which supports its double bond nature.
1
5
cases. The ligand is transformed in situ and coordinates the Both complexes, 3a (X=Cl) and 3b (X=Br), contain the hetero-
metal producing an unexpected complex. In the presence of a nuclear Cu cubane-like structural motif and crystallize in
base, a ketone is transformed into a hemiketal or gem-diol and the triclinic P-1 space group together with two DMF and two
4 4
O
1
5a-b
an alkoxo-bridged complex is formed.
situ conversion of oxime into carboxylic acid functionality is 3a·2DMSO·2H
Metal-assisted in water molecules, therefore having the molecular formulas
O and 3b·2DMSO·2H O. Molecular structures
and the atom numbering scheme around the metal centres for
In the present work we report two new copper complexes both complexes are given in Figure 1. Crystal data and
bearing a Cu
cubane core with an antiferromagnetic refinement parameters (Table S1), as well as bond lengths and
behaviour and some phenoxazinone synthase activity. Both angles around the metal centres (Table S2) are given in the ESI.
complexes are obtained by means of the in situ conversion of The Cu core consists of four alkoxo-bridged Cu(II) centres
2
2
1
5a
also known.
4 4
O
4 4
O
the imine functionality of a Schiff's base pro-ligand into an approximately arranged in a cuboid geometry of an alternating
amino alcohol, presumably with metal assisted ligand array of Cu and O atoms that occupy the corners of the cube
(Figure 2). This structure could alternatively be seen as two
transformation.
4 4
interpenetrated Cu and O tetrahedrons.
2
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1
6
coordinated system, the trigonality index (τ) is given by the
relation τ = (α - β)/60, where α and β are the two largest
angles in the pyramid. Therefore, strictly speaking, for a square
pyramid τ = 0, and for a trigonal bipyramid τ = 1. The obtained
τ values for the Cu centres (Table 1) indicate that the metal
environment can be described as square pyramidal with a little
distortion towards a trigonal bipyramidal geometry in both
cases. Slight distortion between copper centres and the plane
of the coordinated atoms around them could be attributed to
the restrictions imposed by the tridentate ligands. The basal
plane of each Cu(II) ion is coordinated by one thiazole
nitrogen, one phenoxide-oxygen and two alkoxo-oxygen,
whereas the axial position is occupied by a third alkoxo-
oxygen. Observed Cu-N bond distances in the basal planes lie
in the normal range of 1.965-1.983 Å. The equatorial Cu-O
bonds at each Cu centre are shorter (ranging 1.897-1.995 Å)
1
7
than the axial Cu-O bond (in the range 2.375-2.409 Å),
performing a final (4+1) coordination environment, which is at
the origin of the high distortion observed for these structures.
Similar trends in the axial and equatorial bond lengths in Cu O
4
4
1
8
cubane cores have also been reported earlier. The phenolic
C-O distances in the complexes are slightly shorter (Table 1)
than the ideal value of 1.35-1.37 Å, as expected for a
coordinated catechol that could feature some semiquinonoid
contribution.
O
8
Cu
3
O8
Cu3
2
.375
9
3.20
1.947
1.943
9
6.79 96.03
2.402
108.7
Cu1
O68
Cu1
O68
1
.989
93.29
1
07.66
1
.985
9
2.52
1
.979
1.982
107.77
1
.947
Cu
2
O
28
Cu2
O28
9
3.21
96.89
2
.401
1
108.96
96.05
2.381
.954
O48
Cu4
O48
Cu4
a)
b)
Figure 2. Schematic Cu-O bond lengths (a) and Cu-O-Cu bond
angles (b) of the cubane Cu core for 3a·2DMF·2H O.
4
O
4
2
Table 1. Trigonality index (τ) of the Cu atoms in the cubane
core for each complex.
3a·2DMF·2H O
2
3b·2DMF·2H O
2
Cu1
Cu2
0.003
0.019
0.026
0.022
0.010
0.053
0.030
0.010
Figure 1. Molecular structure of 1a (top), 3a (middle) and 3b
(
bottom). Anisotropic displacement parameters are a 50% Cu3
probability; hydrogen atoms and solvent molecules are Cu4
omitted for 3a and 3b for clarity.
Each Cu O cubane exhibits both chiralities for the ligand 2,
4
4
The chiral amino alcohol ligand
base pro-ligand , behaves as a bideprotonated tridentate each cubane, the ligands are arranged as RSRS making achiral
ligand with the alkoxo-O atom acting as a
bridges three Cu(II) atoms at once in the cubane structure. Furthermore, analysis of the Cu O cubane core reveals that
2, formed in situ from Schiff's two
R and two S configurations, per cubane. Therefore, around
1
3
µ
donor, since it the whole structure and, hence, the crystal.
4
4
Beyond the cubane structure, thiazol-N and phenoxide-O there are two short (Cu2-Cu3 and Cu3-Cu4 in 3a, and Cu1-Cu4
atoms collectively provide an additional NO donor and Cu3-Cu4 in 3b), two medium (Cu1-Cu2 and Cu1-Cu4 in 3a
environment for each copper ion. In both complexes, namely and Cu1-Cu2 and Cu2-Cu3 in 3b), and two long (Cu1-Cu3 and
and 3b, each Cu(II) centre is penta-coordinated showing an Cu2-Cu4 for both complexes) intermetallic distances (Table 2)
apparent (4+1) square pyramidal geometry. For a penta- which, therefore, more properly falls within a [2+2+2] Cu O4
,
3a
4
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cubane structure category, beyond the classification of Ruiz et.
Journal Name
6
al. It is noteworthy that complex 3b has interchanged Cu2- Electrochemistry of 3a and 3b.
a
Cu3 and Cu1-Cu4 from short to medium distances with respect
Cyclic voltammograms (CV) and differential pulse
to complex 3a (Table 2). Both complexes, however, present
voltammograms (DPV) were measured for complexes 3a and
roughly the same long distances Cu1-Cu3 and Cu2-Cu4.
3b at ambient temperature in DMF and DMSO medium,
respectively, using tetraethylammonium perchlorate (TEAP) as
supporting electrolyte. The measured potentials were
·+
referenced versus the ferrocenium/ferrocene (FcH /FcH)
4 4
Table 2. Non-bonding Cu···Cu distances (Å) in the C O core.
3
a·2DMSO·2H O
2
3b·2DMSO·2H O
2
Cu1···Cu2
Cu1···Cu3
Cu1···Cu4
Cu2···Cu3
Cu2···Cu4
Cu3···Cu4
3.195
3.244
3.201
3.179
3.251
3.169
3.196
3.257
3.170
3.203
3.258
3.171
redox couple. Both complexes show one quasi-reversible peak
on cathodic scan (Figure 4). Complex 3a shows this reductive
peak at -1.3 V, whereas complex 3b has the same peak at -1.0
II
I
V. This can be assigned to a Cu /Cu reduction process. Higher
inductive -I effect of Cl over Br at the para position of the -OH
group in the phenyl ring, restricts the reduction of Cu(II) to
Cu(I) in case of 3a. Scanning on the anodic side, a peak
appeared at 0.5 V and 0.7 V, respectively, for complexes 3a
and 3b, which can be attributed to the oxidation of the
phenolic hydroxyl group to a phenoxyl radical.
Electronic spectra of 3.
Electronic absorption spectra of complexes 3a and 3b are, as
expected, quite similar (Figure 3). Complex 3a shows one
intraligand transition at 313 nm, whereas complex 3b displays
two shoulders of intraligand nature at 310 and 345 nm.
Another important feature in their absorption spectra is the
phenolate to Cu(II) LMCT transition, which takes place at 406
and 410 nm for complex 3a and 3b, respectively. Furthermore,
d-d transition bands are observed, respectively, at 700 and 678
-5
a) 2.0x10
-5
1.5x10
-5
1.0x10
-6
5.0x10
nm for complexes 3a and 3b
.
0.0
-
6
-
-
-
5.0x10
1.0x10
1.5x10
-
5
-
5
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Potential Vs. Ag/AgCl
-6
b) 8.0x10
-
6
6
6
6
4
2
.0x10
-
.0x10
-
.0x10
0.0
-
6
6
6
6
5
-
-
-
-
-
2.0x10
-
4.0x10
-
6.0x10
-
8.0x10
-
1.0x10
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential Vs. Ag/AgCl
Figure 4. Cyclic voltammogram of a 1 mM solution of
complexes with 0.1 M TEAP as supporting electrolyte. a)
3
Complex 3a in DMF (working electrode: Pt; sweep rate: 100
mV/s). b) Complex 3b in DMSO (working electrode: glassy
carbon; sweep rate: 20 mV/s).
Figure 3. Electronic spectra of complex 3a (a) and complex 3b
b). The d-d bands are shown as insets.
(
4
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Oxidation of 2-aminophenol and determination of the rate of
formation of the phenoxazinone chromophore.
Table 3. Kinetic parameters for the oxidation of 2-amino-
phenol catalyzed by 3a and 3b
.
3
a
·2DMSO·2H
2
O
2
3b·2DMSO·2H O
Catalytic behaviour of complex 3a was studied in MeOH,
whereas complex 3b was screened in both MeOH and DMSO
solutions. In MeOH the absorbance was monitored at 430 nm
Solvent
MeOH
MeOH
3.40x10
1.44x10
DMSO
-
1
-4
-3
-2
-3
Vmax (M·h )
8.63x10
10.29x10
-3
-3
1
9
K (M)
18.28x10
86.3
4.16x10
1028.9
(
ɛ = 9095 M-1cm-1), whereas for DMSO was followed at 435
m
-
1
2
0
kcat (h )
340.26
nm (ɛ = 23200 M-1cm-1). Time dependent spectral profiles
for the catalytic study on complex 3a is shown in Figure S7,
whereas for complex 3b the same is displayed in Figure 5 (in
DMSO solvent) and Figure S9 (in MeOH solvent), respectively.
The same experiment performed without catalyst under
identical conditions did not show any significant growth of the
band at the respective positions. For the kinetic study the
initial rate method was followed. To determine the
dependence of rate with the substrate (2-aminophenol, OAPH)
8
7
6
5
4
3
2
1
0
-5
concentration, a solution of a fixed concentration (1 × 10 M)
of complex used as catalyst was treated with increasing
3
concentrations of the substrate (from 20 to 200 eq) in a pseudo-
first order environment under aerobic condition at 25 °C. For a
particular catalyst-substrate mixture, time scans at the highest band
position of the 2-aminophenoxazine-3-one chromophore were
performed for 25 minutes. A linear regression was followed to
determine the initial rate from the slope of the plot of the
absorbance versus time. Plots of initial rate versus concentration of
the substrate for 3a (in MeOH) are shown in Figure S8a, and those
of 3b in Figure 6a (in DMSO) and Figure S10a (in MeOH). In both
cases, a rate saturation kinetics is followed.
0.000
0.002
0.004
0.006
0.008
0.010
[
S] (M)
(
a)
3
20
00
280
60
3
2
0
.9
.8
.7
.6
.5
.4
.3
.2
.1
.0
2
-aminophenoxazene3-one
240
220
0
2
00
80
60
40
120
00
80
0
1
1
0
-3
Km value=4.16x10
M
1
-
3
Vmax value=10.29x10 M/h
0
2
-1
1
kcat turnover no. =10.289x10 h
0
3b
6
0
0
OAPH
4
0
2
0
0
0
0
100
200
300
400
500
0
-
1
1
/[S] (M )
0
350
400
450
nm)
Figure 5. Change in the spectral pattern for complex 3b after
500
550
600
(b)
λ
(
Figure 6. a) Rate vs substrate concentration plot for the
o
oxidation of 2-aminophenol in DMSO catalysed by 3b at 25 C.
addition of OAPH up to 4.5 h in DMSO.
b) Lineweaver-Burk plot for the corresponding oxidation of 2-
The dependency of the saturation rate with the concentration aminophenol.
of the substrate can be treated with a Michaelis–Menten
approach by means of a Lineweaver–Burk plot (Figure S8b for Catalytic investigations with copper based model complexes
, and Figure 6b for 3b in DMSO and Figure S10b for 3b in for the oxidation of 2-amino phenol to phenoxazinone
3a
MeOH), which enables the calculation of all the kinetic chromophore are very rare. Moreover, the catalytic turn over
1
2
parameters such as K
, Vmax and kcat (Table3). The turn-over rate has not been determined in many of such studies. In
m
number (TON) of the complex was calculated by dividing the comparison to the available literature, the molecules under
investigation in the present study show good to efficient
value of Vmax by the complex concentration.
catalytic activity for the aforementioned reaction. kcat for 3a is
-1
-
1
8
6.3 h in methanol, whereas for 3b the values are 340.26 h
-1
and 1028.9
h
in methanol and dimethyl sulphoxide
respectively. In past few years A. Panja et. al. have made
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significant contribution towards the detailed kinetic study of antiferromagnetic (AF) interactions, which was confirmed for
2
1
22
the PHS catalysis with manganese and cobalt based model 3b·2DMF·2H
2
O by the presence of a maximum at 24 K for the
systems. M. R. Maurya et. al reported a copper complex for vs T curve. The increasing value at low temperature could
the PHS activity with kcat values 29.7 and 131.5 h in solution indicate the presence of paramagnetic impurities coming from
and after anchoring onto a chloromethylated polystyrene monomeric complexes. For 3a·2DMF·2H O, the susceptibility
χ
2
3
χ
-1
2
surface, respectively. However, from the current kinetic study increases continuously upon cooling suggesting the presence
the molecules in the present study can be considered as of a higher amount of paramagnetic impurities that mask the
efficient catalysts for this purpose. The higher rate of 3b over expected decrease.
3a
may be explained on the basis of the above mentioned We first tried to fit the magnetic data of complex 3b using two
higher -I effect of -Cl over -Br at the para position of the -OH different coupling constants, and , logically corresponding
group in the phenyl ring, which restricts the metal to undergo to the 4 shortest Cu-Cu distances (1-2 , 1-4 , 2-3 , 3-4: ) and 2
reduction to Cu(I). Since the mechanism requires the reduction longest (1-3 , 2-4: ), according to the classification from Ruiz
of Cu(II) to Cu(I) in the intermediate steps, the bromo et al. The exchange pathways with the orientation of the
J
1
J
2
J
1
J
2
6
b
derivative with relative lower -I effect shows more catalytic orbitals are represented in Figure 8 (a) for J1 and (b) for J2
.
efficiency.
Therefore, the magnetic susceptibility could be fitted
according to equation (1)
:
Magnetic Susceptibility Measurements.
ꢀꢁꢂꢃ
where ꢀꢏꢐꢒ
ꢄ ꢅ1 ꢆ ꢇꢈ. ꢀꢉꢊꢋꢌꢍꢁ ꢎ ꢇ. ꢀꢏꢐꢒ
ꢑ
ꢎ ꢓꢔꢕ (1)
Variable-temperature magnetic susceptibility measurements
were carried out in the temperature range of 2-250 K in an
ꢑ
ꢖ
is the susceptibility of a S=1/2 impurity,
ρ
is the
applied magnetic field of 1 kOe on polycrystalline samples of amount of these impurity, TIP is the temperature independent
·2DMF·2H O (complex 3a) and 3b·2DMF·2H O (complex 3b
paramagnetism and χcubane results from the full diagonalization
with a SQUID magnetometer. The temperature dependence of of the matrix arising from the spin Hamiltonian (2)
the molar susceptibility and the product of the
susceptibility and the temperature (
T) for both complexes are ꢗ ꢄꢖ ꢆꢘ
displayed in Figure 7 (complex 3b) and Figure S11 (complex 3a
in the ESI).
3a
)
2
2
:
(χ)
ꢜ ꢜ ꢜ ꢜ ꢜ ꢜ ꢜ ꢜ
. ꢚꢛꢝꢊꢙ. ꢛꢝꢊꢞ ꢎ ꢛꢝꢊꢞ. ꢛꢝꢊꢟ ꢎ ꢛꢝꢊꢙ. ꢛꢝꢊꢠ ꢎ ꢛꢝꢊꢟ. ꢛꢝꢊꢠꢡ ꢆ
ꢙ
χ
ꢜ ꢜ ꢜ ꢜ
ꢘ . ꢚꢛꢝꢊꢙ. ꢛꢝꢊꢟ ꢎ ꢛꢝꢊꢞ. ꢛꢝꢊꢠꢡꢖꢖꢖꢖꢖ (2)
ꢞ
,
-
1
-1
The best fit leads to
J
1
= -26.9 cm ,
2
J = -20.0 cm , g = 2.22, TIP
-6
-4
= 0.095 with RχT = 1.5 10 and R = 8.7 10
-
=
60 10 per Cu,
22
ρ
χ
3
.
2
.5
1
0.03
1
0
0
0
.02
χ
.01
Figure 8: Relative orientation of the Cu orbitals involved in the
0
.5
0
exchange pathway in 3a and 3b
As had been done for hydroxo- and alkoxo-bridged dinuclear
0
50
100
T / K
150
200
250
2
complexes, a great effort has been made to find magneto-
4
6
c,25
structural correlations for the Cu O core.
4
4
The J1 coupling
Figure 7. Temperature dependence of
best fit) and
complex 3b = -40.6 cm , J’
χ (o experimental data,
constant corresponds to the shortest Cu-Cu distances with
three short (~1.97 Å) and one long (~2.40 Å) Cu-O distances. In
this case the overlap of the orbitals of the two coppers that are
involved in the exchange pathway appears only in one bridge
—
χ
T (o experimental data, — best fit) for
-1
-1
-1
= +5.0 cm , g =
:
J
1
1
= -16.3 cm ,
= 0.096 with R
J
2
-6
-5
= 8.9 10 and R
2
.17, TIP = 60 10 per Cu,
ρ
χ
T
χ
=
-3
3.1 10
.
(scheme 2a).
For Tercero et al., this leads to weak ferro or antiferro
3
-1
The value of the
and 1.52 for complex 3b. These values are just slightly higher
than those expected for four uncoupled S = 1/2 spins ( T = 1.5
cm mol K with g = 2.0). For both complexes, upon cooling, the
T value decreases continuously for both complexes from
room temperature to 2 K indicating the predominance of
χT at 250 K is 1.6 cm mol K for complex 3a
coupling, the sign of the coupling being correlated with the
6c
exchange pathway displaying the shortest Cu-O distances.
χ
For complex 3a, the pathway displaying the shortest Cu-O
distances is associated with a Cu-O-Cu angle of ca. 108.9°. This
value is larger than 104° which is considered as the boundary
between ferromagnetic (angle < 104°) and antiferromagnetic
3
-1
χ
6
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-1
coupling. The
agreement with this correlation.
For , none of the bridges show the possibility of an overlap of usual) in the calculations. The approach used to determine
the orbitals containing the unpaired electrons. This the exchange coupling constants for polynuclear complexes
J
1
= -26.9 cm value extracted from the fit is in containing molecular fragments by MeO and N ones by NH
3
were used (Figure 9). Solvent molecules were discarded (as
2
7
J
2
6
c,26
6b,26
configuration should lead to a weak ferromagnetic (F)
coupling instead of the antiferromagnetic (AF) coupling that is has two basal and one apical oxygen atom belonging to the
deduced from the fitting process. Cu cubane scaffold. Since the magnetic orbitals are
However, a ferromagnetic behaviour naturally arises on positioned at the basal copper plane, the apical oxygen breaks
considering a three- isotropic spin Hamiltonian, which (as the magnetic pathway, but the remaining two basal oxygen
indicated by DFT calculations, see below) describes better this atoms are connected to one apical and one basal copper atom.
Cu cubane-like system that could, indeed, be considered as Therefore, for each Cu-Cu pair, there are two basal-basal
belonging to a [2+2+2] class: with Cu1-Cu3 and Cu2-Cu4 being exchange pathways that allow transmitting the magnetic
the longest distances ( ), Cu -Cu and Cu -Cu medium interaction (Figure 9). Nevertheless, the magnetic super-
distances (
J’ ).
has been described in detail elsewhere.
Each Cu(II) cation
4 4
O
J
4
J
2
1
2
1
4
J
1
) and Cu2-Cu3 and Cu3-Cu4 shortest distances exchange is broken for Cu1-Cu3 and Cu2-Cu4 pairs that
present just two axial-basal oxygen bridges. Therefore, we
(
1
Yet, the use of spin Hamiltonian (3) leads to a better fit for the expect a F (ferromagnetic) behaviour for these two magnetic
magnetic susceptibility of complex 3b (Figure 7) with a positive pathways (represented by and respectively, see Figure 9)
= -16.3 due to the orthogonality of the interacting magnetic orbitals.
= 0.096 To calculate nJ exchange coupling constants in a polynuclear
complex, at least + 1 energy calculations on different spin
configurations must be performed, that correspond to single-
J
4
J
5
-
1
28
2 1 1
value (ferromagnetic coupling) for J : J = -40.6 cm , J’
-
1
-1
-6
cm ,
J
2
= +5.0 cm , g = 2.17, TIP = 60 10 per Cu,
ρ
i
-5
-3
with RχT = 8.9 10 and R
χ
= 3.1 10 .
n
ꢜ
ꢜ
ꢜ
ꢜ
ꢢ
ꢜ
ꢎ
ꢝꢊꢞ ꢝꢊꢟ
ꢜ
2
9b
ꢗ ꢄꢖꢆꢘ . ꢚꢛ . ꢛ
ꢎ ꢛ . ꢛ ꢈ ꢆ ꢘ ꢅꢛ . ꢛ
determinant Kohn–Sham solutions. Therefore, since strictly
both complexes are not symmetric, eight calculations for the
eight possible broken-symmetry spin states (i.e. a single
ꢙ
ꢝꢊꢙ ꢝꢊꢞ
ꢝꢊꢙ ꢝꢊꢠ
ꢙ
ꢜ
ꢜ
ꢜ
ꢜ
ꢜ
ꢎ ꢛ . ꢛꢝꢊꢠꢡꢖꢖꢖꢖꢖꢖꢖꢖ(3)
ꢝꢊꢙ ꢝꢊꢟ
ꢜ
ꢛ
. ꢛ ꢡ ꢆ ꢘ . ꢚꢛ . ꢛ
ꢝꢊꢟ ꢝꢊꢠ
ꢞ
ꢝꢊꢞ
Since the geometry is quite similar in both complexes, we used quintuplet (high spin, + + + + HS, situation), four triplets
the same three- isotropic spin Hamiltonian for complex 3a, for (medium spin, - + + + MS1, + - + + MS2, + + - + MS3 and + + + -
which the best fit (see Figure S3 in the ESI) was obtained for MS4) and three singlets (low spin, + + - - LS1, + - + - LS2 and + -
= +6.7 cm , g = 2.23, TIP = 60 10 - + LS3)) are convenient (seven calculations are mandatory
J
J
1
-
-
1
-1
-1
=
6
-36.4 cm , J’
1
= -8.0 cm ,
J
2
-5
-3
per Cu,
ρ
= 0.23 with RχT = 9 10 and R = 6.7 10 . As always including the HS configuration) to be performed in
χ
expected, the values of the magnetic coupling constants are order to obtain the values of the six possible exchange
rather similar in both complexes, also taking into account the pathways for each complex (Figure 9). The corresponding
i
uncertainty caused by the higher amount of paramagnetic values for the J constant set were obtained by a least-square
2
9a
impurity calculated for complex 3a
.
fit-process from the energy (without spin projection) found
for the above mentioned spin configurations (see SI).
As mentioned before, it should be pointed out that a [4+2]
classification could not be a good approach. Instead, it seems
more appropriate to talk about [2+2+2] cubanes with short,
medium and long distances (Figure 9), and there arise the
pairing of the coupling constant values.
Theoretical DFT calculations.
In order to confirm the magnetic behaviour found in
complexes 3a and 3b, density functional theory (DFT)
calculations were performed. For the sake of reducing the
computational cost, models (namely 3a-model and 3b-model
)
built from the corresponding crystal structures by changing O-
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Figure 9. Left to right and up to down: Distance pairing, theoretical
J
values, spin densities (for the lowest-energy broken-
symmetry state), available magnetic pathways for each Cu(II) ion, the
or Cu2-Cu4 ferromagnetic interaction, and the vs theta ( ) correlation.
J scheme used in the calculations, the axial-axial Cu1-Cu3
J
θ
Calculated coupling constants are in the order of magnitude axial-axial relationship between the metallic centres, following
and same sign of those obtained experimentally and, the order medium < short < long. Thus, distances should never
therefore, three different pairs of values were found. From be used as the key parameter to establish any magneto-
calculations, it is also possible to assign which value structural correlation. Although the analysis of the magneto-
correspond to each Cu-Cu pair (Figure 9). One could structural data for Cu
4 4
O cubane complexes has allowed the
erroneously infer that there is a correlation between understanding of the main structural factors that govern the
6d,23a
intermetallic distances and magnetic exchange coupling values magnetic exchange coupling in this type of system,
but this is not true: for example, the highest antiferromagnetic simple magneto-structural correlations could be established
AF) values correspond to medium distances in complex 3b for them. This means that it is not only a question of long and
where values are quite different. On the other hand, in short Cu-Cu distances, but other parameters apply influencing
complex 3a values are almost the same for a medium and a the magnetic behaviour towards a more complex situation.
no
(
,
J
,
J
short distance. Short distances correlate, apparently, with Thus, since they are actually angles the parameters closely
small AF values and, therefore, long distances should exhibit related to an effective orbital overlap (through the oxygen
the highest AF values in both cases. Nevertheless they actually atoms of the cubane scaffold), which in turn is related to an
correspond to ferromagnetic ones, as predicted above by the enhanced AF behaviour, maybe some correlation could be
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found between the Cu-O-Cu angles (
θ
) for the basal-basal acknowledges Birla Institute of technology-Mesra for a
values. fellowship.
Yet, Figure 9 shows a fair correlation for these two variables. In
this context, the AF coupling increases with the value.
exchange pathways and the calculated negative (AF)
J
θ
Experimental
Ferromagnetic values are out of this plot since they are not
correlated: the reason for the F behaviour encountered is not
the angle value (about 96-97°), but the absence of magnetic
exchange for Cu1-Cu3 and Cu2-Cu4 owing to the presence of
just axial pathways between these metallic centres. Thus, the
small angle values exhibited are due to the parallel disposition
of both neighbour axial coordinations (Figure 9).
Materials and Physical Measurements.
2
-Aminothiazole, 5-bromo- and 5-chloro-salicylaldehyde were
purchased from Sigma Aldrich whereas triethylamine was
procured from Rankem. All solvents were A.R. grade and used
as received for synthetic work. HPLC grade MeOH, MeCN,
DMF, DMSO and DCM were used for spectroscopic and
electrochemical studies. All the solvents were purchased from
DFT calculations also revealed that, as expected, the main spin
density is located at the metal ions since they are the magnetic
centres, the rest of spin density being delocalized towards the
directly attached O and N atoms. Since the spin density is quite
similar in both complexes, Figure 8 depicts that of complex 3b-
model as an example along with a table indicating the most
relevant atoms supporting a non-negligible spin density value
Spectrochem. Cu(ClO
published procedure.
4
3
)
2
1
was prepared according to the
Tetraethylammonium perchlorate
(TEAP), used for the electrochemical studies, was prepared
3
2
according to the literature. Caution! Perchlorate salts of
metal ions are potentially explosive, only a small amount of
the materials should be handled with care. Infrared spectra
(in electrons) for both complexes.
(IR) were recorded on model IR prestige 21, SHIMADZU
-1
Corporation (Japan) from 400 to 4000 cm in KBr pellet.
UV/VIS spectra were recorded in PERKIN-ELMER (LAMBDA-
1
50) UV/VIS/NIR spectrophotometer. H-NMR spectra of
Conclusions
7
Serendipitous assembly of four copper atoms have been
successfully achieved from the condensation of a Schiff base
ligand were recorded using a JEOL JNM-ECZ 400S/L1 400 MHz
NMR spectrometer. ESI-MS analysis data were obtained by
using JEOL JMS-700 spectrometer. Cyclic voltammogram was
recorded in CHI6003E Potentiostat in a typical three electrode
system: Pt or glassy carbon working electrode, Ag/AgCl
reference electrode and Pt wire counter electrode. The
with Cu(ClO
into an amino alcohol leads to the formation of a tetranuclear
Cu cubane-like scaffold. Single crystal X-ray diffraction
4 2
) . In situ transformation of an imine functionality
4 4
O
analysis reveals a [4+2] cubane-type structure. However a fine
analysis points towards a [2+2+2] cubane core which is
supported by the best fitting of the low temperature magnetic
susceptibility data and DFT calculations carried out on models
0
ferrocene/ferrocenium couple was observed at E (ΔEp) = 0.4 V
(
100 mV) under these experimental conditions. TEAP was used
as supporting electrolyte.
from the crystal structures.
correlation could be stablished between the calculated
and the angle (Cu-O-Cu). From the spin density it could be
A
fair magneto-structural
Magnetic measurements were carried out with a MPMS 5
Quantum Design Magnetometer with an applied field of 1 kOe.
The independence of the susceptibility value with regard to
the applied field was checked at room temperature. The
susceptibility data were corrected for the sample holder and
the diamagnetic contributions as calculated from Pascal’s
constants tables.
J
values
θ
pointed out the AF nature of the magnetic interaction in these
kinds of complexes.
Both complexes have close structural features to those of
naturally occurring multicopper oxidase phenoxiazinone
synthase. Evaluation of the catalytic activity of both complexes
have been performed and reported here through the kinetic
parameters. According to a recent review article by A.
Synthesis of the ligands.
3
0
(E)-4-Chloro-2-((thiazol-2-ylimino)methyl)phenol) (1a)
.
A
Mukherjee et. al. there is only one reported Cu
showing phenoxiazinone synthase-like activity.
4 4
O cubane
mixture of 2 aminothiazole (15.0 g, 15 mmol) and 5-chloro-
salicylaldehyde (4.69 g, 15 mmol) was dissolved in 60 mL
methanol and the reaction mixture was refluxed for 24 h
under nitrogen atmosphere. A brown coloured solid was
obtained that was then filtered under vacuum and washed
with (2mL × 3) methanol. The crude solid was purified in Silica
gel column (100–200 mesh). Pure yellow crystalline solid was
obtained by evaporating the fraction eluted by DCM/hexane
Acknowledgements
S. N. thanks CSIR, New Delhi, Govt. of India for funding this
work through CSIR Project (No. 01(2577)/12/EMR-II,
03/04/2013). S. N. also acknowledges the NMR spectral facility
established in the Chemistry Dept. BIT-Mesra through the DST
FIST Program (SR/FST/CSI-242/2012). Different analytical
services obtained from CIF, BIT-Mesra is greatly acknowledged.
A. E. F. wishes also to thank the computation centre at Servicio
de Cálculo Científico (SCC - University of Murcia) for their
technical support and the computational resources used. S. S.
1
:3 mixture. The yield was 1.434 g (40%) based on the starting
material. M.P.: 125 °C. Elemental analysis calculated for
SOCl : % C 52.05, H 2.03, N 11.28; Found % : C 50.02, H
10 7 2
C H N
1
6
2.33, N 11.88. ESI-MS: m/z 239 (M+1); H NMR (δ, ppm) (d -
-
DMSO): 7.92 (m, 5H), 9.14 (s, 1H); 12.30 (s,1H); FTIR (KBr, cm
1
)
: 3218 (w, broad, OH ), 1628 (νC=N), 2974 (νAr-C-H ) ,1515 (ν
Ar-
C=C).
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3
4
(E)-4-bromo-2-((thiazol-2-ylimino)methyl)phenol (1b). 1b was olex2.solve structure solution program using Charge Flipping
3
5
obtained in a similar way as 1a using 2-aminothiazole and 5- and refined with the SHELXL refinement package employing
bromo-salicylaldehyde. The yield was 1.889 g (45.5%) based on Least Squares minimisation. The crystal data collection and
the starting material. M.P.: 125
calculated for C10 SOBr: % C 42.40, H 2.47, N 9.89; Found %
C 41.02, H 2.82, N 10.08. ESI-MS: m/z 283 (M ); FTIR (KBr, cm Computational Details.
°
C. Elemental analysis refinement parameters are given in the ESI (Table S1).
7 2
H N
+
-
:
1
)
: 3116 (w, broad, OH), 2914 (w, broad, C-H), 1595 (s, sharp, C
1
Quantum chemical calculations were run with the
36
=
N), 1556 (m, sharp, Car–O), 1132 (s,C-N). H NMR (δ, ppm) GAUSSIAN09 suite of programs using the hybrid B3LYP
3
7
(500MHz, CDCl
3
) : 12.27 (s, Ar-OH), 9.15 (s, 1H,C=N ), 7.70-7.26 exchange-correlation functional. The triple-ζ quality basis set
(TZV) proposed by Ahlrichs and co-workers has been used for
(m, 7H,Ar-H), 6.95-6.94 (2H, thiazole).
3
all atoms. A quadratic convergence method has been
8
3
9
employed in the self-consistent-field process.
Synthesis of metal complex [Cu (L) ] (3).
4
4
3a. To a methanolic solution (40 mL) of ligand 1a (0.952 g, 4
mmol), Et N (0.404 g, 4 mmol) was added on stirring followed
3
Notes and references
by 5 mL methanolic solution of Cu(ClO ) .6H O (1.476 g, 4
4
2
2
1
a) T. Mallah and A. Marvilliers, in Magnetism: Molecules to
Materials II; Wiley-VCH Verlag GmbH & Co., Weinheim,
Germany, 2001; pp 189−226; b) D. Gaꢀeschi and R. Sessoli, J.
Magn. Magn. Mater., 2004, 1030, 272−276; c) G. Aromí and
E. K. Brechin, Struct. Bonding (Berlin), 2006, 122, 1−68.
mmol). Colour of the solution changed immediately from
yellow to dark green. A green precipitate separated out slowly.
The reaction mixture was allowed to reflux for 5 h. The green
solid was filtered under vacuum and washed with cold
methanol (2 mL × 3). Slow evaporation of a DMSO solution of
the complex afforded X-ray quality single crystal in a couple of
days. Crude yield: 60.0% (0.737 g). Elemental analysis
calculated for C H N O S Cl Cu : % C 36.07, H 3.03, N 7.65;
2
3
R. H. Holm, P. Kennepohl and E. I. Solomon, Chem. Rev.,
1
996, 96, 2239−2314.
a) R. Wegner, M. Gottschaldt, H. Görls, E. Jäger and D.
Klemm, Chem. Eur. J., 2001, , 2143–2157; b) M. M. Dıaz-
7
4
4
44
8
12
6
4
4
Requejo and P. J. Pérez, Chem. Rev., 2008, 108, 3379–3394;
c) E. Safaei, A. Wojtczak, E. Bill and H. Hamidi, Polyhedron,
-1
Found % : C 36.49, H 3.44, N 7.19. Selected IR bands (cm )
Cu L ): 3170 (νN-H), 2952 (Ar-νC-H), 1628 (νC=N), 1508 (νAr-C=C).
2
010, 29, 2769–2775; d) E. Safaei, M. M. Kabir, A. Wojtczak,
Z. Jaglicic, A. Kozakiewicz and Y. I. Lee, Inorg. Chim. Acta,
011, 366, 275–282; e) A. M. Kirillov, M. V. Kirillova and A. J.
(
4
4
-
1
-1
UV/vis (DMF): λmax/nm (ε/M cm ) : 274 (10946), 313 ( 7415),
sh
2
404 ( 3820), 700 (125).
L. Pombeiro, Coord. Chem. Rev., 2012, 256, 2741–2759; f) S.
Löw, J. Becker, C. Würtele, A. Miska, C. Kleeberg, U. Behrens,
O. Walter and S. Schindler, Chem. Eur. J., 2013, 19, 5342–
3b. The procedure is the same than that of 3a. Yield: 0.795 g
(56.5%). Elemental analysis calculated for C H N O S Br Cu :
44 44 8 12 6 4 4
5
351.
%
C 32.16, H 2.70, N 6.82; Found % : C 32.52, H 2.52, N 7.12.IR
-1`
4
a) V. H. Crawford, H. W. Richardson, J. R. Wasson, D. J.
Hodgson and W. E. Hatfield, Inorg. Chem., 1976, 15, 2107; b)
J. Sletten, A. Sørensen, M. Julve and Y. Journaux, Inorg.
Chem., 1990, 29, 5054; c) E. Ruiz, P. Alemany, S. Alvarez and
J. Cano, J. Am. Chem. Soc., 1997, 119, 1297; d) J. K.
Eberhardt, T. Glaser, R.-D. Hoffmann, R. Fröhlich and E.-U.
Würthwein, Eur. J. Inorg. Chem., 2005, 1175; e) Y.-S. Xie, J.
Ni, F.-K. Zheng, Y. Cui, Q.-G. Wang, S. W. Ng and W.-H. Zhu,
(KBr,ʋ/cm ): 3093 (νN-H), 2951 (Ar-νC-H), 1604 (νC=N), λmax/nm
-1
-1
/M cm ): 258 (49795), 310 (15582), 345 (6573), 410 (3830),
(ε
6
78 (301).
Procedure for the oxidation of 2-amino phenol and determination
of the rate of formation of phenoxazinone chromophore.
-
Cryst. Growth Des., 2009, 9, 118; f) R. Papadakis, E. Rivière,
In this kinetic investigation 2 mL of a methanol solution (1 × 10
5
M) of complex 3 was prepared in a quartz cell and treated
M. Giorgi, H. Jamet, P. Rousselot-Pailley, M. Réglier, A. J.
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-3
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.5 (for complex 3b). The concentration of the
phenoxazinone chromophore was calculated on the basis of its
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h
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X-ray quality single crystals of complexes
3
were obtained by
7
8
evaporation of DMSO solutions. For both complexes a suitable
crystal was selected and mounted in Fomblin film on a
micromount, and data were collected on a GV1000, Atlas
diffractometer and on a SuperNova, Dual, Cu at zero, Atlas
9
diffractometer, for 3a·2DMSO·2H
2
O and 3b·2DMSO·2H
2
O,
3
. Using Olex2, the structure was solved with the
3
respectively
1
0 | J. Name., 2012, 00, 1-3
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ARTICLE
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O are Cu1 2.403 Å, Cu2 2.401 Å, Cu3 2.375 Å,
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DOI: 10.1039/C6DT03754B
Two tetranuclear copper complexes with Cu O cubane core exhibiting phenoxazinone synthase like
4
4
activity are reported. Low temperature magnetic susceptibility measurements display antiferromagnetic
interaction which has been explained theoretically by DFT calculation.

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DOI: 10.1039/C6DT03754B
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