Oxidation products of VO(T(m-NO2)PP): EPR and cyclic voltammetric studies

Article abstract of DOI:10.1016/S1386-1425(02)00333-5

The EPR study of VO(T(m-NO₂)PP)⁺ at 77 K is reported. A triplet state spectrum of VO(T(m-NO₂)PP)⁺ at 77 K is obtained. The ZFS parameters are obtained from the computer simulation. An inter-electron distance of

Full text of DOI:10.1016/S1386-1425(02)00333-5

Spectrochimica Acta Part A 59 (2003) 1549ꢁ1556
/
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Oxidation products of VO(T(m-NO2)PP):
EPR and cyclic voltammetric studies
A. Tomba Singh, A. Lemtur *
Department of Chemistry, North-Eastern Hill University, Umshing, Mawkynroh, Shillong 793 022, Meghalaya, India
Received 20 May 2002; received in revised form 30 September 2002; accepted 30 September 2002
Abstract
The EPR study of VO(T(m-NO2)PP)ꢀ at 77 K is reported. A triplet state spectrum of VO(T(m-NO2)PP)ꢀ at 77 K is
˚
0.05 A
obtained. The ZFS parameters are obtained from the computer simulation. An inter-electron distance of 3.579
/
between the two unpaired electrons is obtained. This is attributed to an a2u ground state of the porphyrin. Also, room
temperature EPR of the pre-oxidized species of VOTPP with SbCl5 are reported. The effect on the oxidation potentials
due to substitution is also reported.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Oxidation products; VO(T(m-NO2)PP); Electron paramagnetic resonance (EPR); Cyclic voltammetry; Triplet state; Inter-
electron distance
1. Introduction
as polarography [15] and cyclic voltammetry [16]
and the reports are available in the literature. The
unoxidised VOTPP possesses an unpaired electron
in the 3dxy orbital of the vanadium atom. One
electron oxidation removes an electron from the
ligand [14] generating another unpaired electron
The possible occurrence of metalloporphyrin p-
cation in nature such as in photosynthesis [1ꢁ7],
cytochrome, heme catabolism [8,9], catalase and
peroxidase [7,10] were studied, compared and
reported in the literature. Therefore, metallopor-
phyrin p-cation serve as biological model system.
EPR studies on the oxidation products of
/
giving rise to a triplet state (Sꢂ1). In a metallo-
/
porphyrin with D4h symmetry, the two highest
occupied p-molecular orbitals are very close lying
with symmetry levels a1u and a2u. The question is
in which orbital the unpaired p-electron spin
density is localized? Generally, a cation of MTPP
VOTPP were done by a number of workers [11ꢁ
14,16,17]. The redox properties of VOTPP were
studied by different electrochemical methods such
/
(Mꢂmetal atom), the unpaired p-electron is
/
assigned to an a2u ground state [17] while it is
assigned to a1u for MOEP type. However, our
experimental results indicate that the demarcation
may not be very rigid.
* Corresponding author. Tel.:
91-364-550041x54.
ꢀ91-364-550041x26; fax:
/
ꢀ/
E-mail address: lemturl@rediffmail.com (A. Lemtur).
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1386-1425(02)00333-5

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A.T. Singh, A. Lemtur / Spectrochimica Acta Part A 59 (2003) 1549ꢁ1556
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The effect of ring substitution on the oxidation
and reduction potentials of MTPP were widely
studied and reported in the literature [18ꢁ20]. So
methane. Antimonypentachloride (Sigma chemi-
cals) was used as received.
/
far, the EPR and cyclic voltammetric studies of the
oxidation products of VO(T(m-NO2)PP) are not
reported in the literature. In this paper we present
the EPR and cyclic voltammetric studies of one
electron oxidation products of VO(T(m-NO2)PP).
We also present the room temperature EPR
spectra of the oxidation products of VOTPP which
were not reported earlier.
2.2. EPR measurement
EPR measurements were done with X-band
E109 (Varian) spectrometer. A cold finger dewar
was employed for the liquid nitrogen measure-
ments. All measurements were done after deaerat-
ing with N2 gas.
2.3. Computer simulation
A computer program QCPE 134 for the simula-
tion of EPR spectra of single crystal was modified
for PC systems. Simulated spectra were obtained
using WINEPR graphics.
2. Experimental
2.1. Chemicals
TPP and VOTPP were synthesized and purified
according to the standard method reported in the
2.4. Cyclic voltammetric measurements
literature [21ꢁ23].
/
Cyclic voltammogram were obtained using po-
larographic analyzer model 174A, PARC and a
universal programmer model 175, PARC coupled
with a digigraphic 2000XY recorder. A platinum
plate electrode and a platinum wire were used as
working electrode and auxiliary electrode, respec-
tively. A saturated calomel electrode was used as
reference electrode. All measurements were carried
out after deaerating by bubbling N2 gas through
the solution in the cell.
(T(m-NO2)PP) was synthesized and purified
according to the literature [24]. Also, a good
product was obtained by taking freshly distilled
pyrrole (1.70 ml), 3.775 g of m-NO2 benzaldehyde
(Sigma chemicals) and 300 ml of reagent grade
propionic acid and refluxed for 30 min. The
reaction mixture was cooled to room temperature
and kept standing for 18 h. The tarry material was
filtered and extracted the product in chloroform
and kept in the dark at room temperature for
about 36 days. Chloroform was recovered fol-
lowed by washing the product with methanol and
hot water till the filtrate was colorless. The
product was extracted with chloroform, dried
over anhydrous Na2SO4, filtered and evaporated
to dryness. The product thus obtained was chro-
matographed using a mixture of chloroform and
benzene. The purity of the sample was checked by
TLC and visible spectra.
2.5. UVꢁ/Vis measurement
UV
650 spectrophotometer (Beckman) using a quartz
/
Vis measurements were done with DU*
ꢁ
cuvette.
3. Results and discussion
VO(T(m-NO2)PP) was prepared and purified by
the same method used for the preparation of
VOTPP as reported in the literature [23].Tetra-n-
butylammoniumperchlorate was prepared by the
reaction of sodium perchlorate with tetra-n-buty-
lammonium iodide (Aldrich chemicals) and recrys-
tallized from methanol. The purity was checked by
recording cyclic voltammogram in dichloro-
3.1. EPR studies
Firstly, we report here the room temperature
EPR spectra of VOTPP oxidation with SbCl5
which were neither observed nor reported earlier.
The oxidation of VOTPP can be represented as
follows:

A.T. Singh, A. Lemtur / Spectrochimica Acta Part A 59 (2003) 1549ꢁ
/
1556
1551
at 77 K because these reports are available in the
literature [12]. The formation of monocation is
further supplemented by IR [25,26] spectrum
which shows a new band at 1272.76 cmꢃ1. The
spectrum a (Fig. 1) show splitting giving additional
lines on both ends. Further addition of SbCl5 have
produced the spectrum b and c (Fig. 1) and
subsequently produces the spectrum d (Fig. 1)
exhibiting inversion of the intensity and linewidth
as well. The spectrum a is attributed to complex I
while spectrum b and c to complex II. The
spectrum d is attributed to complex III which is
a mixture of the complex II and the monocation.
The hyperfine coupling constant of the spectrum d
(Fig. 1) is 71 G. This spectrum seems to indicate
the presence of VOTPPꢀ having some amount of
a1u species. Linewidth and intensity inversion are
also observed at room temperature although not
The preoxidized species I and II are observable
d). The EPR signal
of the species IV, i.e. VOTPPꢀ do not vanish
completely but broadens out. This is observable at
higher modulation as a broad signal at room
temperature (Fig. 1e). We have not measured it
at room temperature (Fig. 1aꢁ
/
Fig. 1. Room temperature X-band EPR spectra of VOTPP in dichloromethane oxidized with SbCl5, (a) species I; (b) and (c) species II;
(d) species III; (e) species IV and (f) species V. Experimental conditions: modulation frequency, 100 KHz, microwave power, 2 mW,
microwave frequency, 9.33ꢁ
/
9.4 GHz, scan range, 1600 G, scan time, 8 min, spectrum; (A) modulation amplitude, 0.8ꢄ
/10 G; (B) and
(C) modulation amplitude, 1.6ꢄ
/
10 G, (D), (E) and (F) modulation amplitude, 2.0ꢄ10 G.
/

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A.T. Singh, A. Lemtur / Spectrochimica Acta Part A 59 (2003) 1549ꢁ1556
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significantly as in the case of VO (OEP) cation [27]
and VO (Meso) cation [28]. The spectrum e (Fig.
1) is attributed to VOTPPꢀ having dominant a2u
species and hence broadens out. The hyperfine
coupling constant of the dication (spectrum f in
Fig. 1) is 80 G (lesser than the unoxidized
VOTPP).
It has been observed that most of the substituted
VOTPP undergo oxidation in the similar pattern
as that of the VOTPP. Oxidation of [VO(m-
NO2TPP)]ꢀ exhibits a very broad EPR signal at
room temperature (Fig. 2a). On freezing it to 77 K
gives a triplet state spectrum (Fig. 2b). Half-field
signal characteristic of 51V nucleus is also observed
(Fig. 2d).
The spin Hamiltonian for such a system has
been discussed elsewhere [29]. Diagonalization of
the Hamiltonian was done considering the system
to be of axial symmetry while the powder spectrum
is obtained by averaging over all angular orienta-
tions using gaussian quadrature method. The
value of A and D were varied so as to match the
experimental spectrum. The spin Hamiltonian
parameters are presented in the Table 1.
Fig. 2. X-band EPR spectra of VO(T(m-NO2)PP) in dichloromethane oxidized with SbCl5 at, (a) room temperature; (b) 77 K, (c)
computer simulated spectrum and; (d) Half-field spectrum (g :4). Experimental conditions: modulation frequency, 100 KHz,
microwave power: 2 mW, scan range, 1600 G, scan time, 8 min. Spectrum (a) microwave frequency, 9.33ꢁ9.4 GHz, modulation
amplitude, 0.5ꢄ0.1 G; (b) microwave frequency, 8.89ꢁ9.1 GHz; modulation amplitude, 0.5ꢄ0.1 G; and (c) microwave frequency,
8.89ꢁ9.1 GHz, modulation amplitude, 2.0ꢄ10 G.
/
/
/
/
/
/
/

A.T. Singh, A. Lemtur / Spectrochimica Acta Part A 59 (2003) 1549ꢁ
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1556
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Fig. 2 (Continued)
The parameter D obtained from the spectrum is
and
considered as follows: The inter-electron vector is
considered parallel to the plane of the ring and
normal to the 4-fold axis of the molecule. The D
and g tensors are obtained in the same principal
axis system with the Z-axis parallel to the 4-fold
symmetry axis. Thus, the ZFS tensor is rewritten
with the inter-electron axis on the Z-axis.
ꢃDz?z?
DI ꢂDx?x? ꢂDy?y? ꢂ
2
g2
Dz?z? (in MHz)ꢂ1:298ꢄ104
R3
For an axially symmetric system the ZFS
tensors along the inter-electron axis, (Z?)(Zeeman
and zero field interaction only):
where R is expressed in angstrom and
g2 ꢂgI2 ꢀ0:5gꢀ2
Here, we consider the dipolar interaction as the
only factor contributing to the entire zero field
splitting (ZFS). This is a crude approximation only
ꢃDz?z?
D
x?x? ꢂDy?y? ꢂ
2
Table 1
Parameters of triplet state ESR spectra of some metalloporphyrin
Complex
gꢁ
gꢀ
D (cmꢃ1
)
Reference
V(IV)OTPP
(1.97ꢁ/2.0)
0.036 (a1u)
0.048 (a2u
0.04
)
[11]
[12]
[30]
[31]
V(IV)OEPꢀ
Ni complex
ZnTPP
1.973
1.970
1.98
ꢀ/
0.0004
0.031
0.031
MgTPP
VO(T(m-NO2)PP)
[32]
a
1.919
0.04
a
Present work.

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A.T. Singh, A. Lemtur / Spectrochimica Acta Part A 59 (2003) 1549ꢁ1556
/
and is not conclusive in determining the inter-
electron distance between the two unpaired elec-
neutral VO(T(m-NO2)PP) but no demetallation is
observed.
trons. Thus, inter-electron distance of 3.57590.05
/
˚
A is obtained. It is to be noted that the dipolar
interaction contribution in the case of Nickel
3.3. Cyclic voltammetric studies of VO(T(m-
NO2)PP)
isobacteriochlorins the Ni(I)ꢂ
/
N is reported [30]
to be around 1.2 MHz (0.0004 cmꢃ1) only. The D
value we obtained is in between 0.036 and 0.048
To further supplement the EPR studies cyclic
voltammetric measurements are carried out. The
voltammogram exhibiting two oxidation peaks
with 1.287 and 1.511 V as their E1/2 values are
obtained (Fig. 4). These potentials are assigned to
the two successive one electron oxidations of the
ligand. The oxidation steps were assigned accord-
ing to the following scheme:
cmꢃ1. Considering a smaller D value of 0.036
cmꢃ1 an inter-electron distance of 3.739
/
0.05 A is
˚
obtained but no appreciable change in the simu-
lated spectrum is observed. This may be due to the
unpaired p-electron density is more in a2u. This is
in agreement with the earlier report7. The non-
vanishing EPR spectrum of the p-cation at room
temperature points to a possibility of having
considerable unpaired p-electron density in a1u.
However, the shorter inter-electron distance ob-
tained is in good agreement with that of the
VOTPPꢀ.
ꢃe
ꢃe
VO(m-NO2TPP)X VO(m-NO2TPP)ꢀ
X
ꢀe
ꢀe
E1=2ꢂ1:287
V
 VO(m-NO2TPP)ꢀꢀ
E1=2ꢂ1:511
V
The oxidation potentials are shifted more posi-
tively as compared with that of the VOTPP. The
E1/2 of the first oxidation is shifted by 0.289 V
while that of the second oxidation potential by
0.288 V. The shifts are quite uniform and large.
The difference (DE) between the oxidation poten-
tial and the reduction potential of each redox
couple are found to be 0.098 and 0.109 V,
respectively. These values are rather large for
one-electron oxidation steps. It seems that these
peaks are overlapped of very close redox couples.
3.2. UVꢁVis spectra
/
In order to ascertain the ligand oxidation and
the reversibility of the oxidation, UVꢁVis mea-
/
surements are done. VO(T(m-NO2)PP) on oxida-
tion with SbCl5 shows decrease in the intensity of
the soret band while the visible band (Q-band)
broadens out (Fig. 3). It indicates that the
porphyrin ligand is oxidized. Reducing the oxi-
dized product with dimethylamine regenerate the
Fig. 3. Visible spectra of VO(T(m-NO2)PP) in dichloromethane containing 0.1M SbCl5 Neutral*
/
, Oxidation*
/
×
/
*/
×
/
, Reduction*
/
ꢄ
/
*/
ꢄ/.

A.T. Singh, A. Lemtur / Spectrochimica Acta Part A 59 (2003) 1549ꢁ
/
1556
1555
Fig. 4. Cyclic voltammogram of VO(T(m-NO2)PP) in dichloromethane containing Tetra-n-butyl ammonium perchlorate. Scan rate 50
mV sꢃ1
.
However, the ratio of the cathodic peak and the
anodic peak seems to be unity.
cherry University for helping us with his computer
programme and useful discussions.
4. Conclusion
References
The EPR spectra of the oxidation of VOTPP
exhibits a triplet state at room temperature. This
may suggest to a considerable amount of unpaired
spin density in a1u. Similar trends were observed
for VO(T(m-NO2)PP). This may be the reason for
observing large DE values in the voltammogram.
The electron withdrawing substituents in the
phenyl ring of the porphyrin does affect the
oxidation potentials of the vanadyl porphyrin
(shifted to higher positive potentials). Similar
trends were observed by some earlier workers [14].
[1] D.H. Kohl, in: H.M. Swartz, J.R. Bolton, D.C. Borg
(Eds.), Biological Applications of Electron Spin Reso-
nance, Wiley-Interscience, New York, 1972, p. 213.
[2] J.D. McElroy, G. Feher, D. Mauzerall, Biochem. Biophys.
Acta 267 (1972) 363.
[3] W.W. Passon, Biochem. Biophys. Acta. 153 (1968) 248.
[4] P.A. Loach, K. Walsh, Biochemistry 8 (1969) 1908.
[5] J.R. Bolton, R.K. Clayton, D.W. Reed, Photo Chem.
Photo Biol. 9 (1969) 209.
[6] J.T. Warden, J.R. Bolton, J. Am. Chem. Soc. 94 (1972)
4351.
[7] D. Dolphin, R.H. Felton, Acc. Chem. Res. 7 (1974) 26.
[8] C.E. Castro, J. Theor. biol. 33 (1972) 475.
[9] (a) N. Sutin, A. Forman, J. Am. Chem. Soc. 93 (1971)
5274;
Acknowledgements
(b) N. Sutin, Chem. Br. 8 (1972) 148.
[10] C.A. Reed, in: K.M. Kadish (Ed.), Electrochemical and
Spectrochemical Studies of Biological Redox Components,
Advances in Chemistry Series, vol. 201, American Chemi-
cal Society, Washington, DC, 1982, p. 333.
One of the author is thankful to UGC (SAP)
and DRS for providing some financial assistance.
The authors gratefully acknowledge Professor J.
Subramanian, Department of Chemistry, Pondi-
[11] M. Hoshino, S. Konishi, M. Imamura, S. Watanabe, Y.
Hana, Chem. Phys. Lett. 102 (1983) 259.

1556
A.T. Singh, A. Lemtur / Spectrochimica Acta Part A 59 (2003) 1549ꢁ1556
/
[12] G.E. Selyutin, A.A. Shklyaev, V.F. Anerfrienko, Dokl.
Akad. Nauk SSR 255 (2) (1980) 390.
[22] G.M. Badger, R.A. Jones, R.L. Laslett, Aust. J. Chem. 17
(1964) 1028.
[13] C.M. Newton, D.G. Davis, J. Magn. Reson. 20 (1975) 446.
[14] K.M. Kadish, D. Sazou, C. Araullo, Y.M. Liu, A. Saoiabi,
M. Ferhat, R. Guilard, Inorg. Chem. 27 (1988) 2320.
[15] (a) A. Stanienda, G.Z. Biebl, Phys. Chem. (frank furt an
Main) 52 (1967) 54;
[23] J.G. Erdman, V.G. Ramsey, N.W. Kalenda, W.E. Hansen,
J. Am. Chem. Soc. 78 (1956) 5844.
[24] A. Bettelheim, B.A. White, S.A. Bulk, R.W. Murray,
Inorg. Chem. 26 (1987) 1009.
[25] E.T. Shimomura, M.A. Phillipi, M.M. Goff, W.F. Schuls,
C.A. Reed, J. Am. Chem. Soc. 103 (1981) 6778.
[26] A.S. Hinman, B.J. Pavelick, K. McGarty, Can. J. Chem.
66 (1988) 1589.
(b) A. Stanienda, G.Z. Biebl, Phys. Chem (Leipzig) 229
(1965) 259.
[16] R.H. Felton, in: D. Dolphin (Ed.), The Porphyrins, vol. 4
(part C), Academic Press, New York, 1979.
[17] J. Fajer, D.C. Borg, A. Forman, D. Dolphin, R.H. Felton,
J. Am. Chem. Soc. 92 (1970) 3451.
[27] G.R. Luckhurst, M. Setaka, J. Subramanian, J. Mol. Phys.
32 (1976) 1299.
[28] A. Lemtur, B.K. Chakravorty, T.K. Dhar, J. Subrama-
nian, J. Phys. Chem. 88 (1984) 5603.
[18] K.M. Kadish, M.M. Morrison, Inorg. Chem 15 (1976)
930.
[29] J. Subramanian, V.P. Shedbalkar, A. Lemtur, R. Chakra-
borty, T.N. Saloi, J. Phys. Chem. 100 (1996) 4770.
[30] J. Telser, Y.-C. Fann, M.W. Renner, J. Fajer, S. Wang, H.
Zhang, R.A. Scott, B.M. Hoffman, J. Am. Chem. Soc. 119
(1997) 733.
[19] A. Giraudeau, H.J. Callot, M. Gross, Inorg. Chem. 18
(1979) 201.
[20] L.A. Bottomley, L. Olson, K.M. Kadish, in: K.M. Kadish
(Ed.), Electrochemical and Spectrochemical Studies of
Biological Redox Component, ACS Advances, Washing-
ton, 1982, p. 279.
[31] J.-I. Fujsawa, Y. ohba, S. Yamauchi, J. Phys. Chem. A 101
(1997) 434.
[21] A.D. Adler, F.R. Longo, J.D. Finarelli, J. Goldmacher, J.
Assour, L. Korshoff, J. Org. Chem. 32 (1967) 476.
[32] J.-I. Fujsawa, Y. Ohba, S. Yamauchi, J. Am. Chem. Chem.
119 (1997) 8736.