Synthesis, X-structure and solvent induced electronic states tuning of meso-tris(4-nitrophenyl)corrolato-copper complex

Article abstract of DOI:10.1016/j.ica.2010.07.016

Copper(II) ion insertion into the coordination core of meso-tris(p- nitrophenyl)corrole (1), in air showed the formation of meso-tris(p-nitrophenyl) corrolatoCu(III) (2), with the generation of superoxide radical as the side product of the reaction. Singl

Full text of DOI:10.1016/j.ica.2010.07.016

Inorganica Chimica Acta 363 (2010) 4313–4318
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Inorganica Chimica Acta
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Synthesis, X-structure and solvent induced electronic states tuning of
meso-tris(4-nitrophenyl)corrolato-copper complex
*
Dibyendu Bhattacharya, Pinky Singh, Sabyasachi Sarkar
Department of Chemistry, Indian Institute of Technology-Kanpur, Kanpur 208 016, Uttar Pradesh, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 15 July 2010
Copper(II) ion insertion into the coordination core of meso-tris(p-nitrophenyl)corrole (1), in air showed
the formation of meso-tris(p-nitrophenyl)corrolatoCu(III) (2), with the generation of superoxide radical
as the side product of the reaction. Single-crystal X-ray diffraction analysis revealed that 2 exhibits weak
Dedicated to Prof. Achim Muller
p–p
stacking interaction. Among the two electronic states, diamagnetic Cu(III)–corrole3À exits in DCM
solution and in contrast is paramagnetic Cu(II)–corrole2ꢀÀ cationic radical in DMF solution. This difference
in electronic behavior in solution is due to the small energy gap between HOMO and LUMO that is readily
influenced by solvent interaction.
Keywords:
Aerial oxidation
Copper(II)
Copper(III)
Corrole
Ó 2010 Elsevier B.V. All rights reserved.
Superoxide radical
X-ray
1. Introduction
Furthermore, the electronic ground states of Cu meso-triarylcor-
roles (aryl = –C6F5, –CF3, –OMe, –H, –CH3) are sensitive to periphe-
A large number of transition-metal complexes supported by a
wide variety of non-innocent ligands (e.g. porphyrins, pterins, fla-
vins, quinines, dithiolenes, or phenoxyl based systems) have so far
been evolved to evaluate the new aspects in synthetic chemistry
and with special relevance to catalysis [1–5]. For macrocyclic,
non-innocent corrole ligands, a great deal of efforts have been
made to provide profound insights into the oxidation state of the
metal centers [6–9]. High-valent metal ions have been reported
to be present in copper, iron, chromium, cobalt, and manganese
corroles [8]. It was thought that corrole ligand in these complexes
has a non-innocent character, which has stimulated renewed inter-
est in this field [6,10–12]. Corroles as free base are less stable than
their porphyrin analogs in solution. Depending on the type and po-
sition of the substituents, corroles can readily transform to various
products [13].
Copper is often used as the metal of choice for the synthesis and
functionalization of metalocorroles, due to its easy insertion, the
absence of an axial ligand and its diamagnetic ground state [14–
18]. The Cu-metalation of corrole was first reported in 1965 [14].
Generally in basic medium and at room temperature several cop-
per(II) compounds have been used for such metalation [19,20].
Cu–corroles are often oversimplified as Cu(III)-species with ther-
mally accessible paramagnetic triplet excited state [15,16,21–25].
ral substituents and recently it has been reported that such Cu–
corroles are inherently saddled as because of the Cu(d)–corrole(
p
)
orbital interactions and thus electronic structure may be described
as Cu(II)–corrole2ꢀÀ cationic radical [26]. However, electronic nat-
ure of copper b-octaethylcorrole has been nicely scrutinized which
indicates that ground state of the copper–corrole is best described
as copper in 2+ physical oxidation state [27]. This was further ver-
ified with comparative DFT/XRD studies with analogs 10-oxacor-
role copper(II) complex. The structure and electronic nature of
the meso-tris(4-nitrophenyl)corrolato-copper complex has not
yet been reported in the literature.
To understand the paradox of metal insertion inside the coordi-
nation core of corrole and its electronic structure in solution, we
embarked on a study on the metalation of meso-tris(4-nitro-
phenyl)corrole and it’s related properties. Herein, we report on
the mechanistic details of a simple synthetic methodology of a
new meso-tris(4-nitrophenyl)corrolatoCu(III) corrole and also
show the influence of a solvent to tune the relative ground state
energy between the diamagnetic Cu(III)–corrole complex and para-
magnetic, Cu(II)–corrole2ꢀÀ cationic radical state.
2. Experimental
2.1. Materials and analytical methods
Solvents, pyrrole and aldehyde were obtained from S.D. Fine
Chemicals Ltd., India and all solvents were dried and distilled
* Corresponding author. Tel.: +91 512 2597386; fax: +91 512 2597265.
E-mail address: abya@iitk.ac.in (S. Sarkar).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.016

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D. Bhattacharya et al. / Inorganica Chimica Acta 363 (2010) 4313–4318
by standard methods prior to use [28]. meso-Tris(4-nitro-
phenyl)corrole (1) was prepared under condition developed by
Paolesse et al. [29]. 1H NMR spectrum of 1 and its microanalyt-
ical data were in good agreement with the reported data. Elec-
tronic absorption spectral measurements were recorded by USB
2000 (Ocean Optics Inc.) UV–Vis spectrophotometer equipped
with fiber optics. X-ray diffraction data for 2 were collected on
a Bruker-AXS smart APEX CCDC diffractometer at 100 K. Infrared
spectra were recorded on a Bruker Vertex 70, FT-IR spectropho-
tometer as pressed KBr disks. Elemental analyses for carbon,
hydrogen and nitrogen were carried out with a Perkin–Elmer
2400 microanalyzer. Cyclic voltammetric measurements were
performed with a BASi Epsilon-EC bioanalytical systems Inc. Cyc-
lic voltammogram was recorded with glassy carbon electrode as
working electrode, Pt wire as auxiliary electrode and Ag/AgCl
electrode as reference electrode. All electrochemical experiments
were done under argon atmosphere with 0.1 M Bu4NClO4 as sup-
porting electrolyte at 298 K. Potentials are referenced against
internal ferrocene (Fc) and are reported relative to the Ag/AgCl
electrode (E1/2(Fc+/Fc) = 0.49 V versus Ag/AgCl electrode). The
scan rate of CV was 100 mV/s. Magnetic susceptibility was mea-
sured at 0.5 T as powder packed in gelatin capsules using a
Quantum Design MPMSR2 SQUID susceptometer by heating the
sample from 298 to 450 K. X-band EPR measurements were car-
ried out on frozen solutions at 120 K on a Bruker EMX spectrom-
eter. For the time dependent repetitive scan, the solution was
thawed to bring it at room temperature in air and then cooled
at 120 K to record the spectrum. EPR spectrometer setting:
microwave frequency: 9.8 GHz, modulation amplitude: 10 G,
modulation frequency: 100 kHz, microwave power: 0.21 mW.
2.2. Synthesis of meso-tris(4-nitrophenyl)corrolato-copper(III) (2)
meso-Tris(p-nitrophenyl)corrole (1) (40 mg, 0.06 mmol) and
Cu(acac)2 (0.48 mmol) were taken in 6 mL dichloromethane
and triethylamine (0.20 mmol) was added into it. Color of the
solution was finally changed from green to red-brown. The
solution was further stirred at room temperature for 15 min
in the presence of air and finally vacuum dried. The residue
was then dissolved in dichloromethane and it was subjected
to flash column chromatography using silica gel (100–200
mesh) and the eluted was evaporated under reduced pressure.
Single crystal suitable for X-ray analysis of this compound
was grown in a 1:1 mixture of dichloromethane and hexane.
Yield = 45 mg, 80%. Anal. Calc. for C56H92N6Cu(III): C, 73.69; H,
10.15; N, 9.20. Found: C, 73.39; H, 10.55; N, 9.41%. Vis. kmax/
(e
/MÀ1 cmÀ1) in DCM: 424 (2.6 Â 104), 553 (1.8 Â 104), 660
nm
(8.5 Â 104).
2.3. Crystal structure determination of 2
Crystals of 2 suitable for a diffraction study were prepared by
slow diffusion of hexane into their dichloromethane solutions,
mounted on glass fiber and transferred to a Bruker Smart CCD dif-
fractometer. Details of the structure determinations are given in
Table 1. Measurement was performed using graphite-monochro-
matized Mo Ka radiation (k = 0.71073 Å). The unit cell parameters
and crystal-orientation matrices were determined by least-squares
refinements of all reflections. The intensity data were corrected for
Lorentz and polarization effects and an empirical absorption cor-
rection were also employed using the SAINT program [30]. Data were
collected by applying the condition I > 2r(I). Intensity data were
collected at 100(2) K within the limits 0.95° < h < 28.30° for 2.
The structure was solved by direct methods and followed by suc-
cessive Fourier and difference Fourier syntheses. Full-matrix
least-squares refinements on F2 were carried out using SHELXL97
with anisotropic displacement parameters for all non-hydrogen
atoms. Hydrogen atoms were constrained to ride on the respective
carbon or nitrogen atoms with isotropic displacement parameters
equal to 1.2 times the equivalent isotropic displacement of their
Table 1
Summary of X-ray crystallographic data for 2.
Compound
2
Empirical formula
Formula weight
T (K)
C
37H20Cu1N7O6
722.15
100 (2)
k (Å)
0.71069
Monoclinic
P2/c
15.611(5)
16.462(5)
31.626(5)
90
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a
(°)
b (°)
109.493
90
7662(4)
c
(°)
V (A3)
Z
8
Dcalc (mg cmÀ3
)
1.252
Crystal color, shape
Crystal size (mm)
F(0 0 0)
Dark green, needle
0.2 Â 0.1 Â 0.08
2944
Absorption coefficient (mmÀ1
)
0.621
h Range (°) for data collection
Index ranges
2.01–28.41
À20 6 h 6 20,
À22 6 k 6 17,
À26 6 l 6 42
50 161
Reflections collected
Unique reflections
18 984
Independent reflections (Rint
Completeness to hmax (%)
Data/restraints/parameters
)
0.0992
98.5
18 984/0/836
0.958
0.1019, 0.2359
0.2421, 0.2516
0.84 and À0.93
0.0
Goodness-of-fit (GOF) on F2
b
R1,a wR2 [I > 2
r
(I)]
b
R1,a wR2 (all data)
Largest difference in peak and hole (e AÀ3
Flack parameter
)
Fig. 1. Time dependent changes in the EPR spectral line shape (X-band, 120 K)
monitored in dichloromethane with 1% triethylamine containing a mixture of
Cu(acac)2 and corrole 1 (1:1) in air.
P
P
a
R1
=
||Fo| À |Fc||/ |Fo|.
P
P
wR2 = { [w(F2o À F2c )2]/ [w(F2o)2]}1/2
.
b

D. Bhattacharya et al. / Inorganica Chimica Acta 363 (2010) 4313–4318
4315
NO2
NO2
N
Cu(acac)2/O2/3 Et3N
DCM, RT
N
N
NH
NH
N
N
-
[Et3NH]+[O2
Et3NHacac
]
NO2
Cu
NO2
HN
NO2
NO2
Scheme 1. Reaction involved in the course of copper insertion inside the corrole moiety.
parent atom in the all cases. Complex neutral atom scattering fac-
tors were used throughout for all cases. All calculations were car-
ried out using SHELXS97 [31], SHELXL97 [32], PLATON99 [33], and
ORTEP-3 [34] programs. The figures are generated by using the DIA-
MOND 3.1e program.
The structure of 2 suffered from disordered and unidentified
solvent (crystallized from DCM–Hexane) in the lattice, which was
not included in the refinement but was taken care of by the
SQUEEZE procedure (from PLATON). The volume occupied by the sol-
vent was 1079.5 Å3 and the number of electrons per unit cell de-
duced by SQUEEZE was 65.5.
4. Results and discussion
4.1. Synthesis of copper complex (2)
To understand the metalation of corrole, the synthesis of [meso-
tris(p-nitrophenyl)corrolato]copper(III) (2) was carried out by the
reaction of Cu(acac)2 with the meso-tris(p-nitrophenyl)corrole
[T(p-NO2-P)Corr] (1) in dichloromethane in the presence of a few
drops of triethylamine used as proton abstractor instead of carry-
ing out the reaction in neat pyridine [14]. The metal insertion in-
side the coordination core of 1 was followed by monitoring the
progress of the reaction by EPR spectroscopy (Fig. 1).
The diminution of the EPR signal of Cu(II) in Cu(acac)2 concom-
itant with the appearance of EPR signal due to O2 radical [41]
3. Computational details
ÁÀ
demonstrated the participation of the redox reaction between
Cu(II) and aerial oxygen (Fig. 1). The dominant green color of 1 in
the starting reaction mixture with Cu(acac)2 changed to red-brown
within minutes and 15 min was enough for the disappearance of
the EPR signal arising from Cu(acac)2. At the expense of this signal,
a new EPR signal which started to appear within minutes became
fully developed by this time. On standing for another 20 min, this
signal disappeared. The identity of this EPR signal as O2 radical
was revealed by the nitroblue tetrazolium assay [42,43] and the
reaction that followed is shown in Scheme 1.
All calculations were performed with the GAUSSIAN 03, revision
B.04, package [35] Molecular orbitals were visualized using ‘‘Gauss
View 3.0”. Gas-phase and single-point calculation was performed
based on density functional theory (DFT). The method used was
Becke’s three-parameter, hybrid exchange functional [36,37], the
non-local correlation was provided by the Lee, Yang, and Parr
expression (B3LYP), and by the Vosko, Wilk, and Nusair (1980) cor-
relation functional (III) for local (B3LYP). The 6-31g(d,p) basis set
[38] was used for H, C, and N atoms; the LANL2DZ [39] basis set
and LANL2 pseudopotentials of Hay and Wadt [40] were used for
Cu atoms. The geometry of 2 was obtained from its crystal
structure.
À
It is known in the metalation of corrole by copper(II) in air, that
the isolated complex is found to be in copper(III) state [15–23].
Fig. 2. Molecular structure of Cu[T(p-NO2-P)C] (2). ORTEP representation with thermal ellipsoids at 50% probability (H atoms omitted for clarity).

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The corrolato macrocycle form a saddle conformation with pyrrolic
subunit tilted alternatively up and below the mean N4 plane. The
single crystal of 2 contains three crystallographically independent
copper–corrole molecules with non-identical structural parame-
ters (Fig. 3). As depicted in Fig. 2 these molecules interact with
one another to form a p–p stacking. The intermolecular Cu–Cu dis-
tances between two successive planes in 2 are 4.478 Å and 4.113 Å,
respectively, and mean distance between ring planes is 3.43 Å and
4.05 Å, respectively.
4.3. Electronic absorption spectroscopy
The electronic absorption spectra of the copper–corroles are
exceptionally sensitive to peripheral substituents [13,17,18,26].
Substitution with electron-withdrawing group causes red-shifted
optical spectra. Thus, Cu[T(p-H-P)C] shows absorption at 413 nm,
but Cu[(CF3)8T(p-XP)C] complex shows absorption at 462 nm in
DCM [7]. Compound 2 shows Soret band at 424 nm in DCM which
is consistent with the expected red-shift compared to Cu[T(p-H-
P)C] and blue-shift compared to Cu[(CF3)8T(p-XP)C]. Interestingly,
2 in DMF solution shows different electronic spectrum. Although
Soret band appeared at the same position that found in DCM but
there is a new peak at 649 nm appeared in DMF. This is probably
due to the different electronic states in two different solvents. In
DCM, compound 2 exists as a Cu(III)–corrole3À, however in DMF
it exists as Cu(II)–corrole2ꢀÀ. This is further confirmed by the solu-
tion magnetic moment measurement, EPR, cyclic voltammetry and
DFT studies as discussed below. Compound 2 displayed a broad
absorption around 850 nm in DCM and also in DMF with subtle
variation (Fig. 4). The origin of this broad absorption which is
reproducible is not clear and requires further study.
4.4. Electron spin resonance and solution magnetic moment
Compound 2 is EPR inactive in the solid state and diamagnetic
in the temperature range from 273 K to 450 K, suggesting its d8
electronic configuration in a square planar Cu(III) form [6,16,25].
In dichloromethane solution, it does not show any EPR signal.
However, in DMF medium, it showed a broad EPR signal that
may be related to Cu(II) state along with a free radical signal
(g = 2.003). The narrow line width without any hyperfine structure
Fig. 3. Cu–N bond length and N–Cu–N angles of three molecules present inside the
lattice of Cu(III)[T(p-NO2-P)C] 2.
Though, the oxidizing agent in such reaction is aerial oxygen, its
fate after the oxidation of Cu(II) to Cu(III) has not been identified.
ÁÀ
Thus the spontaneous generation of O2 radical ion in the metala-
tion of corroles by Cu(II) ion as a metal ion source is a rare event
that is documented here.
4.2. Solid state structures of 2 by X-ray crystallography
ORTEP diagram of 2 are shown in Fig. 2 and the crystallographic
data is provided in Table 1. Single-crystal X-ray structure analysis
of 2 revealed that copper has been incorporated into the corrole
core with no indication for the proton bound to the central nitro-
gen supporting the +3 oxidation state of the central metal atom.
Fig. 4. Comparison of electronic spectra of 2. 2 Â 10À4 (M) dichloromethane
solution (black), DMF solution (pink). Inset indicates the EPR spectra of 2 in DMF
solution, flat cell, RT. (For interpretation of the references in color in this figure
legend, the reader is referred to the web version of this article.)

D. Bhattacharya et al. / Inorganica Chimica Acta 363 (2010) 4313–4318
4317
of 2 measured by Evans method showed
leff = 2.4 BM, a value
intermediate between one and two unpaired electrons (see Supple-
mentary material).
4.5. Electrochemistry
The cyclic voltammetric (CV) trace of the CuIII/CuII couple of 2
showed reversible value at E1/2 = À0.04 V (Fig. 5). Reversibility of
this low potential E1/2 was checked by the anodic peak Ipa versus
m1/2 plot indicating the (scan rate 100–1000 mV/s) linear nature.
The separation between anodic and cathodic peak current is close
to one of the peak-to-peak separation observed for Fc/Fc+ couple in
the same condition further substantiating the process as an one
electron redox event. As corrole macrocycle is ‘non-innocent’ in
nature that was reflected from the oxidation of the ring at a poten-
tial E1/2 = +0.92 V and two electron reversible reduction of –NO2
group present periphery of the ligand in 2.
Further inside into the metal center redox potential was ob-
tained by comparing the potential with other reported copper–cor-
role as shown in Table S1 in Supplementary material.
Reduction potential of our system was found to be more posi-
tive in comparison to the other system which reflects the effect
of the electron-withdrawing effect of the –NO2 group. Thus the re-
dox potential of the pentafluorophenyl corrole is À0.51 V but for –
NO2 substituted corrole such value appeared at À0.045 V. So, meso
substituents in metalocorroles exert a strong influence on the half
wave potential for one electron metal center reduction and this po-
tential is shifted from À0.55 V to À0.045 V, when the meso substit-
uents vary from p-OCH3 phenyl to p-NO2 phenyl group [17]. Thus,
Fig. 5. Cyclic voltammogram of 2 in DCM (1 mM) in 0.1 M TBAPF6 with scan rate,
100 mV/s vs. Ag/AgCl.
of the free radical signal indicated its identity as carbon centered
radical (Fig. 4, inset). In DMF solution, the magnetic moment value
Fig. 6. Frontier molecular orbitals determined at the DFT B3LYP/LanL2DZ level for unsubstituted Cu(III)–corrole.

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it is easier for the [{(p-NO2-P)C}CuIII] to be reduced compared to
that of [{T(p-OCH3-P)C}CuIII] analogs.
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The chemistry associated with Cu metal insertion inside the
coordination core of corrole has been successfully investigated to
show that the air present in the reaction mixture oxidized the
Cu(II) ion to Cu(III) with the generation of O2ÁÀ. The X-ray structure
of this new copper–corrole showed saddle conformation with
three crystallographically independent copper–corrole molecules
interacting through weak
p–p stacking with non-identical struc-
tural parameters. Solvent induced changes in the electronic states
were confirmed by UV–Vis, EPR, solution magnetic moment and
cyclic voltammetric measurements. This difference in electronic
behavior in different solvents is due to the small energy gap be-
tween LUMO and HOMO. The propensity of other first row transi-
tion-metal–corrole complexes with different solvent is under
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D.B. acknowledges IIT Kanpur for a Senior Teaching Fellowship,
P.S. for a Senior Research Fellowship from the CSIR, New Delhi and
S.S. thanks the DST, New Delhi for funding the project.
Appendix A. Supplementary material
CCDC 771021 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2010.07.016.
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