Spectroscopic and electrochemical characterization of solutions and films of a new redox couple: Co(II)/Co(III) N-confused porphyrin

Article abstract of DOI:10.1016/j.inoche.2004.09.020

The films of Co(II) N-confused tetraphenylporphyrin are stable in aqueous base, insensitive to air and show a relatively high oxidation potential attributed to the Co(II)/Co(III)NCTPP couple: 0.025 V vs. Ag/AgCl/KCl (satd.) in 1 M NaOH. Co(II) ion was introduced in N-confused tetraphenylporphyrin (NCTPP) as evidenced by ESR and UV-Vis spectroscopy as well as electrochemical methods. The Co(II)NCTPP complex shows a typical ESR spectrum of a high spin 3d ⁷ Co(II) ion, Soret bands at 460 and 445 nm in CH₂Cl ₂ and CH₃CN, respectively, and oxidation potential of +0.55 V vs. Ag/AgCl/KCl (satd.). Adsorbed films of Co(II)NCTPP on glassy carbon electrodes are stable and show reversible redox waves in aqueous basic solutions attributed to the Co(II)/Co(III)NCTPP couple with E_1/2 = +0.025 V at pH 14. It is also shown that NCTPP can be electropolymerized and the films readily metallate with Co(II) ions.

Full text of DOI:10.1016/j.inoche.2004.09.020

Inorganic Chemistry Communications 7 (2004) 1238–1241
www.elsevier.com/locate/inoche
Spectroscopic and electrochemical characterization of solutions
and films of a new redox couple: Co(II)/Co(III)
N-confused porphyrin
a
I. Zilbermann , E. Maimon , R. Ydgar , A.I. Shames ,
a
a
b
c
E. Korin , L. Soifer , A. Bettelheim
c
a,c,
*
a
Nuclear Research Centre – Negev, P.O. Box 9001, Beer-Sheva 84190, Israel
Physics Department, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel
b
c
Chemical Engineering Department, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel
Received 23 September 2004; accepted 30 September 2004
Available online 19 October 2004
Abstract
Co(II) ion was introduced in N-confused tetraphenylporphyrin (NCTPP) as evidenced by ESR and UV–Vis spectroscopy as well
as electrochemical methods. The Co(II)NCTPP complex shows a typical ESR spectrum of a high spin 3d7 Co(II) ion, Soret bands at
460 and 445 nm in CH2Cl2 and CH3CN, respectively, and oxidation potential of +0.55 V vs. Ag/AgCl/KCl (satd.). Adsorbed films of
Co(II)NCTPP on glassy carbon electrodes are stable and show reversible redox waves in aqueous basic solutions attributed to the
Co(II)/Co(III)NCTPP couple with E1/2 = +0.025 V at pH 14. It is also shown that NCTPP can be electropolymerized and the films
readily metallate with Co(II) ions.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: N-confused porphyrins; Co(II) complex; Spectroscopy; Redox; Electropolymerization
N-confused porphyrins (NCP) are porphyrin isomers
where one of the pyrrole rings is inverted [1,2]. These
porphyrins possess three nitrogens and one carbon atom
which display N–H and C–H tautomerism [3,4] and act
as chelating donor atoms. They can behave both as
dianionic and trianionic ligands when complexing with
metal ions [5] and have been shown to form complexes
with transition metal ions such as iron [6], manganese
[7,8], copper [9,10], nickel [11–13], silver [5] and palla-
dium [14]. The N-confused pyrrole can play different
roles depending on the metal ion and oxidation state
as well as the synthesis method [15,16]. The internal car-
bon can be bonded to a metal ion in different ways
[2,6,8,9,17] and the external nitrogen can be free
[3,5,18], proton bearing [2,3,6,8,9,14,17] or involed in
coordination [14–16,19]. The coordination of N-con-
fused porphyrins has been recently reviewed by Harvey
and Ziegler [20].
The B12 coenzymes contain a corrin ring with a low-
spin Co(III) ion in an octahedral configuration. In one
group of enzymes, the cobalt ion coordinates to the imi-
dazole side chain of a histidine residue and in another
group the cobalt ion binds to the pendant dimethylbenz-
imidazole group of the coenzyme. The second axial site
is coordinated to a methyl or 50-deoxyadenosyl groups
and these form an organometallic Co–C bond. In view
of the possible involvement of this bond in enzymatic
reactions and in the chemistry of the different oxidation
states of cobalt ion, we have explored the complexation
*
Corresponding author. Tel.: +97286461799; fax: +97286477656.
E-mail address: armandb@bgumail.bgu.ac.il (A. Bettelheim).
1387-7003/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2004.09.020

I. Zilbermann et al. / Inorganic Chemistry Communications 7 (2004) 1238–1241
1239
Scheme 1. Schematic structure of Co(II)NCTPP.
of N-confused tetraphenylporphyrin with cobalt ions
(CoNCTPP). Very recently, X-ray crystal data of
Co(III)NCTPP with H2O, triphenylphosphine and
pyridine were reported [21]. Here we present the pre-
paration 1 of the Co(II) derivative (Scheme 1) and spect-
roscopic as well as electrochemical properties of the
Co(II)/Co(III)NCTPP couple which has not yet been re-
ported in the literature. Moreover, we show for the first
time that NCTPP can be electropolymerized on elec-
trode surfaces and metal ions can be readily introduced
into these films.
Room temperature X-band EPR spectra of polycrys-
talline Co(II)NCTPP showed a poorly resolved signal at
a g value of approximately 3.3. The spectrum is typical
of a high spin (S = 3/2) 3d7 Co(II) ion, similar to that
reported for Co(II) texaphyrin [22] and unlike other
Co(II) porphyrins which generally exist as low spin
(S = 1/2) complexes [23].
Fig. 1. Optical absorption spectra in CH2Cl2 (a), CH3CN (b) and
DMF (c) of Co(II)NCTPP (A) and non-metallated NCTPP (B).
Absorbance is in arbitrary scale.
UV–Vis spectroscopy showed that the Co(II)NCTPP
complex dissolved and was stable in CH2Cl2, even when
exposed to air (Fig. 1A, curve a: Soret band at 460 nm
and Q bands at 652 and 804 nm). However, the spec-
trum after imidazole addition was similar to that ob-
tained for non-metallated NCTPP dissolved in CH2Cl2
in the absence as well as presence of imidazole (Fig.
1B, curve a: Soret band 439 nm and Q bands at 539,
582 and 727 nm). The spectrum of Co(II)NCTPP in
CH3CN showed a Soret band at 445 nm and Q bands
at 550, 600, 719 and 793 nm (Fig. 1A, curve b) and dif-
fered from that of NCTPP (Fig. 1B, curve b: Soret at
434 and Q bands at 538, 580, 645, 695 and 722 nm).
However, as in the case of addition of imidazole to
Co(II)NCTPP, the spectrum of Co(II)NCTPP in DMF
(Fig. 1A, curve c) showed similar features to those of
the non-metallated NCTPP in this solvent (Fig. 1B,
curve c: Soret at 438 and Q bands at 537, 581 and 731
Fig. 2. Cyclic voltammogram (20 mV/s) obtained at a GC electrode
for a CH2Cl2 solution containing 0.5 mM Co(II)NCTPP and 0.1 M
Bu4NClO4.
nm). These results indicate that the Co(II)-carbon bond,
unlike the Co(III)–C bond [21], is not stable in the pres-
ence of nitrogenous bases such as imidazole and DMF
which ligate the Co(II) ion and demetallate the complex.
Cyclic voltammetry of Co(II)NCTPP complex in
CH2Cl2 at a glassy carbon (GC) electrode (Fig. 2)
showed a quasi-reversible peak (DEp = 0.11 V) with
E1/2 = 0.55 V vs. Ag/AgCl/KCl (satd.) and current peaks
which increased linearly with the concentration of
Co(II)NCTPP in solution. Although this peak differs
from the oxidation peaks obtained for the non-metal-
lated porphyrin (0.65 and 1.0 V), it is not possible in this
stage to ascribe it as metal- or ligand-centered oxidation.
1
In a typical synthesis, insertion of Co(II) ions in NCTPP
(Midcentury, Posen, IL) was achieved by mixing a solution of
15 mM Co(ClO4)2 in methanol with a solution of 1 mM of NCTPP
in 10 ml CH2Cl2 and stirring for 72 h at room temperature. The vessel
was then opened to air until the solvents totally evaporated. In order to
dissolve excess of the Co(II) salt, 20 ml of water were added and the
solution was stirred for a few hours. Collection of the non-crystalline
solid product was made after decantation, drying in air overnight and
precipitation from a CH2Cl2 solution.

1240
I. Zilbermann et al. / Inorganic Chemistry Communications 7 (2004) 1238–1241
Electrochemical
experiments
for
2
adsorbed
Co(II)NCTPP films on GC electrodes were conducted
in aqueous electrolyte solutions. Fig. 3A shows cyclic
voltammograms for such a film in a NaOH solution at
various pHÕs. A well defined and reversible wave
E1/2 = 0.025 V and DEp = 0.05 V was observed at pH
14 (curve a). Since it is improbable that a radical cation
would yield a reversible wave in aqueous solution, it is
attributed to the Co(II)/Co(III)NCTPP couple. The re-
dox potential of this couple is relatively high compared
to other cobalt porphyrins which have been examined
(dissolved or as films) in basic solutions (for example,
E1/2 = ꢀ0.16 V) for Co(III/Co(II) tetrakis(o-aminophe-
nyl)porphyrin in 1 M NaOH [23]). Moreover, the wave
shifts to more anodic potentials and becomes quasi-
reversible as the pH decreases and at pH lower than
11, it is almost undetectable. The slope of E1/2 vs. log
[OHꢀ] is linear (inset of Fig. 2) with a slope of 36 mV
per decade, indicating an electrochemical process involv-
ing one electron per two hydroxyl ions:
CoðIIIÞNCTPPðOHꢀÞ þ e
2
! CoðIIÞNCTPP þ 2OHꢀ
ð1Þ
The formation constant of Co(III) NCTPP(OHꢀ)2 as
estimated from the intercept is 1.07 · 109 Mꢀ2
Fig. 3. (A) Cyclic voltammograms (5 mV/s) for adsorbed films of
Co(II)NCTPP on GC electrodes in NaOH solutions at pH: (a) 14;
(b)13; (c) 12 and (d) 11. Inset: E1/2 of Co(II)/Co(III)NCTPP vs.
log [OHꢀ]. (B) Cyclic voltammograms (20 mV/s) at pH 13 for
electropolymerized films of NCTPP before (a) and after (b) being
contacted with a 1 mM CoCl2 aqueous solution.
.
Repetitive potential scanning in the range 0 to +1.1 V
for a CH2Cl2 solution of NCTPP at a GC electrode re-
vealed growth of oxidation peaks at ꢁ+0.4, +0.6 and +1
V. Similar processes have also been observed for 1-phe-
nylpyrrole and ‘‘non-confused’’ porphyrins with N-sub-
stituted pyrroles [24] and have been attributed to the
electropolymerization of the pyrrole derivative. Electro-
polymerized films of NCTPP (poly(NCTPP)) when
transferred to a deaerated 0.1 M NaOH solution did
not show any cyclic voltammetric peaks (Fig. 3B, curve
a). However, after dipping these films overnight in an
aqueous solution of CoCl2, rinsed and transferred to
0.1 M NaOH, a redox couple with E1/2 = 0.11 V was de-
tected (Fig. 3B, curve b). This clearly indicates that
Co(II) ions have been introduced in the poly(NCTPP)
onated nitrogen. This nitrogen is less prone to oxidation
since in contrast to the N–H form, it has a pair of elec-
trons which are in resonance with the whole porphyrin p
electrons system.
To conclude, Co(II) ion was introduced in N-con-
fused tetraphenylporphyrin as evidenced by ESR and
UV–Vis. spectroscopy as well as electrochemical meth-
ods. The Co(II)NCTPP is stable in aprotic non-coordi-
nating solvents. While nitrogenous bases decompose
the Co(II)–carbon bond, films of Co(II)NCTPP are sta-
ble in aqueous base. Since the metal–carbon bond in
Co(II)NCTPP is part of the macrocyclic compound, it
differs from that found in B12 coenzymes in which it in-
volves an axial ligand. However, this bond and the tran-
sition from high spin Co(II) to low spin Co(III) seem to
be responsible for the high redox potential found for the
Co(II)/Co(III)NCTPP couple. The Co(II)NCTPP com-
plex is insensitive to air so that use of the Co(II)/Co(III)
couple to catalyze oxygen reduction can be excluded but
it can be considered for possible application in electro-
catalytic oxidation processes. It has been demonstrated
that NCTPP electropolymerizes and readily metallates
with Co(II) and probably with a variety of other metal
ions. This offers the possibility of future research related
to these films in various fields, such as electron transfer,
biomimetic catalysis and sensors.
films.
Attempts
to
directly
electropolymerize
Co(II)NCTPP were unsuccessful. This result differs
from that obtained with ‘‘non-confused’’ amino-, pyr-
role, and hydroxy-substituted tetraphenylporphyrins
which have been electropolymerized in their non-metal-
lated as well as metallated forms [24]. This is attributed
to the oxidation of Co(II) to the Co(III) state and as
suggested for the oxidation of Cu(II) to Cu(III)NCTPP
[10] as well as for Co(III)NCTPP with pyridine axial lig-
ands [21], the peripheral N–H is converted to non-prot-
2
Adsorbed films of Co(II)NCTPP on GC electrodes (Metrohm,
0.07 cm2) were prepared by placing 20 ll of a CH2Cl2 solution
containing 2 · 10ꢀ4 Co(II)NCTPP on the surface and allowing to dry
for ꢁ1/2 h.

I. Zilbermann et al. / Inorganic Chemistry Communications 7 (2004) 1238–1241
1241
[13] Z. Xiao, B.O. Patrick, D. Dolphin, Inorg. Chem. 42 (2003) 8125.
[14] H. Furuta, N. Kubo, H. Maeda, T. Ishizuka, A. Osuka, H.
Nanami, T. Ogawa, Inorg. Chem. 39 (2000) 5424.
[15] J.D. Harvey, C.J. Ziegler, Chem. Commun. (2002) 1942.
[16] C.-H. Hung, W.-C. Chen, G.-H. Lee, S.-M. Peng, Chem.
Commun. (2002) 1516.
References
[1] H. Furuta, T. Asano, T. Ogawa, J. Am. Chem. Soc. 116 (1994)
767.
[2] P.J. Chmieleweski, L. Latos-Grazynski, K. Rachlewicz, T. Glo-
wink, Angew. Chem. Int. Ed. Engl. 33 (1994) 779.
[3] H. Furuta, T. Ishizuka, A. Osuka, H. Dejima, H. Nakagawa, Y.
Ishikawa, J. Am. Chem. Soc. 123 (2001) 6207.
[4] H. Karabiyik, A.G. Gokce, M. Aygun, J. Mol. Struct. (Theo-
chem) 673 (2004) 191.
[17] G.R. Geier, D.M. Haynes, J.S. Lindsey, Org. Lett. 1 (1999) 1455.
[18] T. Ogawa, H. Furuta, M. Takahashi, A. Morino, H. Uno, J.
Organomet. Chem. 661 (2000) 551.
[19] A. Srinivasan, H. Furuta, A. Osuka, Chem. Commun. (2001)
1666.
[5] H. Furuta, T. Ogawa, Y. Uwatoka, K. Araki, Inorg. Chem. 38
(1999) 2676.
[20] J.D. Harvey, C.J. Ziegler, Coord. Chem. Rev. 247 (2003) 1.
[21] J.D. Harvey, C.J. Ziegler, Chem. Commun. (2004) 1666.
[22] S. Hannah, V. Lynch, D.M. Guldi, N. Gerasimchuk, C.L.B.
MacDonald, D. Magda, J.L. Sessler, J. Am. Chem. Soc. 124
(2002) 8416.
[6] W.–C. Chen, C.–H. Hung, Inorg. Chem. 40 (2001) 5070.
[7] J.D. Harvey, C.J. Ziegler, Chem. Commun. (2003) 2890.
[8] D.S. Bohle, W.-C. Chen, C.-H. Hung, Inorg. Chem. 41 (2002)
3334.
[23] J.K.M. Sanders, N. Bampos, Z. Clyde-Watson, S.L. Darling,
J.C. Hawley, H.–J. Kim, C.C. Mac, S.J. Webb, in: K.M. Kadish,
K.M. Smith, R. Guilard (Eds.), The Porphyrin Handbook, vol. 3,
Academic Press, San Diego, CA, 2000, pp. 1–48.
[9] P.J. Chmieleweski, L. Latos-Grazynski, I. Schmidt, Inorg. Chem.
39 (2000) 5475.
[10] H. Maeda, Y. Ishikawa, T. Matsuda, A. Osuka, H. Furuta, J.
Am. Chem. Soc. 125 (2003) 11822.
[24] A. Bettelheim, B.A. White, S.A. Raybuck, R.W. Murray, Inorg.
Chem. 26 (1987) 1009.
[11] I. Schmidt, P.J. Chmieleweski, Inorg. Chem. 42 (2003) 5579.
[12] Z. Xiao, B.O. Patrick, D. Dolphin, Chem. Commun. (2003) 1062.