Interaction of nitric oxide with cobalt(II) tetrasulfophthalocyanine

Article abstract of DOI:10.1016/S0277-5387(99)00373-3

The interaction of nitric oxide (NO) with cobalt(II) tetrasulfophthalocyanine [Co(II)TSPc]^4-) has been studied. Coordination of NO is accompanied by electron transfer from the central metal in [Co(II)TSPc]^4-, the resulting complex being represented as [(NO^-)Co(III)TSPc]^4-. The rate constant for the formation of this species is k(f)-= 142 ± 7 dm³ mol^-1 s^-1 and an equilibrium constant of 3.0 ± 0.5 x 10⁵ dm³ mol^-1 was obtained. When adsorbed to a glassy carbon electrode, [Co (II) TSPc]^4- catalyses the oxidation and reduction of NO, with a detection limit of the order of 10^-9 mol dm^-3. Ammonia and hydroxylamine are some of the reduction products obtained for the reduction of NO on [Co(II)TSPc]^4- -modified glassy carbon electrodes. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00373-3

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 229–234
Interaction of nitric oxide with cobalt(II) tetrasulfophthalocyanine
*
Sibulelo Vilakazi, Tebello Nyokong
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
Received 18 August 1999; accepted 30 November 1999
Abstract
The interaction of nitric oxide (NO) with cobalt(II) tetrasulfophthalocyanine [Co(II)TSPc]4y) has been studied. Coordination of NO is
accompanied by electron transfer from the central metal in [Co(II)TSPc]4y, the resulting complex being represented as
[(NOy)Co(III)TSPc]4y. The rate constant for the formation of this species is kfs142"7 dm3 moly1 sy1 and an equilibrium constant of
3.0"0.5=105 dm3 moly1 was obtained. When adsorbed to a glassy carbon electrode, [Co(II)TSPc]4y catalyses the oxidation and reduction
of NO, with a detection limit of the order of 10y9 mol dmy3. Ammonia and hydroxylamine are some of the reduction products obtained for
the reduction of NO on [Co(II)TSPc]4y-modified glassy carbon electrodes.
q2000 Elsevier Science Ltd All rights reserved.
Keywords: Cobalt; Electrocatalysis; Nitric oxide; Oxidation; Phthalocyanine; Reduction
1. Introduction
phyrins is not well understood. For example, the Ni(III)/
Ni(II) couple has been implicated in NO determinationusing
electrodes modified with nickel(II) porphyrin complexes
[19,20], but other researchers have reportedthattheNi(III)/
Ni(II) couple plays no role in the electrocatalytic oxidation
of NO [25].
Interest in the chemistry of nitric oxide (NO) has inten-
sified in the last few years due to its importance in biological
systems including neurotransmission, vasculardilation,cyto-
toxicity, immune response and regulation of blood pressure
[1,2]. Excess concentrations of NO have been implicated as
a major factor in a number of conditions, such assepticshock,
and Parkinson’s and Alzheimer’s diseases [3,4]. There has
been a surge of interest in the nature of coordination of NO
to metalloporphyrins [4–13]. NO interacts with hemoglobin,
myoglobin and cytochrome P450 forming NO compounds
[14]. NO has a short half-life (depending on the concentra-
tion of oxygen) and has high reactivity with oxygen and
hydrogen peroxide [1]. Current methods of detecting NO
are indirect, relying on the measurement of secondaryspecies
such as nitrite and nitrate after removal from biological sys-
tems. Measurement of NO using bioassay, chemilumines-
cence and UV–Vis spectroscopy do not allow for in vivo
detection, and require chemical pretreatment of the sample.
Thus there is a growing interest in electrochemical sensors
for analysis of NO in cells [15–18], based on its electroca-
talytic oxidation. Nickel(II) and iron(II) porphyrin com-
plexes and unmetallated porphyrin have been employed in
modifying electrodes for the detection of NO [19–27]. The
mechanism for the catalytic oxidation of NO by metallopor-
Metallophthalocyanine (MPc) complexes have been used
successfully as electrocatalysts for many reactions [28–36].
There are only a few reports on the interaction between NO
and MPc complexes [37–42]. The use of MPc complexes
for the electrocatalytic reduction or oxidation of NO still
needs to be explored. It has recently been suggested that
[CoTSPc]4y bound to immobilized imidazole-modified sil-
ica gel can be used as a water-soluble NO sorbent that does
not dimerize, and hence can be used for the removal of NO
from flue gas streams [37]. The present study explores the
electrocatalytic oxidation and reduction of NO on glassy car-
bon electrodes modified with cobalt(II) tetrasulfophthalo-
cyanine ([CoTSPc]4y; Fig. 1). We also report on the
kinetics and equilibria for the interaction between NO and
[CoTSPc]4y
.
2. Experimental
2.1. Materials
Cobalt(II) tetrasulfophthalocyanine(Na4[Co(II)TSPc])
was synthesized and purified according to the method of
* Corresponding author. Fax: q27-46-6225109; e-mail: t.nyokong@
ru.ac.za
0277-5387/00/$ - see front matter q2000 Elsevier Science Ltd All rights reserved.
PII S0277-5387(99)00373-3
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S. Vilakazi, T. Nyokong / Polyhedron 19 (2000) 229–234
platinum plate of area 2.2 cm2 as counter electrode and an
AgNAgCl (3 mol dmy3 KCl) reference electrode. The GCE
modified by electrodeposition was used as a working elec-
trode for bulk electrolysis. A nitrogen atmosphere was main-
tained for all electrochemistry experiments. Concentrations
of NO were of the order of 10y4 to 10y3 mol dmy3 for
electrocatalytic studies. Yields of ammonia were determined
by the Nessler method [45] and by monitoring the absorb-
ance at 400 nm. Hydroxylamine was determined spectro-
scopically using the specific test described in the literature
[9].
2.3. Kinetics and equilibria
Kinetic and equilibrium studies were run at 21.0"0.58C
and monitored with a Cary 1E UV–Vis spectrophotometer.
We monitoredtheappearanceofthespectraduetotheproduct
formed following the addition of NO to solutions of [Co-
TSPc]4y in water. Typically, a known volume of the aqueous
solution of [CoTSPc]4y was added to a cell of 1 cm path-
length and deaerated using nitrogen. Then a known volume
of a deaerated aqueous solution of NO was added to the cell
and the spectra monitored with time. Bubbling of nitrogen
into the cell was continued while adding the NO solution.
The cell was fitted with a stopper to avoid oxygen affecting
the reaction. Concentrations of [CoTSPc]4y were kept con-
stant at 5.0=10y6 mol dmy3, whereas NO concentrations
were varied from 1.4=10y5 to 5.6=10y5 mol dmy3. Con-
centrations of [CoTSPc]4y in water were determined using
the published extinction coefficient of 5.8=104 moly1 dm3
cmy1 [46]. Since the concentration of the NO was larger
than that of [CoTSPc]4y by a factor of at least 10, pseudo-
first-order conditions were assumed for kinetic studies. Solid
CoTSPc-NO was prepared by bubbling NO into aqueous
solutions of [CoTSPc]4y and allowing the solvent to
evaporate.
Fig. 1. Molecular structure of cobalt(II) tetrasulfophthalocyanines.
Weberand Busch [43]. NO waspreparedbyaddingsaturated
sodium nitrite dropwise to a 0.2 mol dmy3 solution of H2SO4
under nitrogen atmosphere. The NO produced was passed
through a column containing solid KOH to remove higher
oxides and then bubbled into nitrogen-saturated deionized
water. NO solutions in water were prepared freshly before
use and kept in glass flasks fitted with a rubber septum. NO
concentration in water was determined using the method pro-
posed by Ascenzi et al. [38] or calculated using the reported
solubility of 2.2=10y3 mol dmy3 at 258C and1.00atm[38].
Tetraethylammonium perchlorate (TEAP) was recrystalli-
zed from ethanol before use as an electrolyte. Dimethylfor-
mamine (DMF) was freshly distilled.
2.2. Electrochemical methods
Electrochemical data were collected with the Bio-
Analytical Systems (BAS) model CV-50W voltammetric
analyser. For cyclic voltammetry, a glassy carbon electrode
(GCE; 3.0 mm diameter) was used as a working electrode
and a platinum wire as an auxiliary electrode. The reference
electrode was the AgNAgCl (3 mol dmy3 KCl). Concentra-
tions of Na4[Co(II)TSPc] in water were in the 10y4 to 10y3
mol dmy3 range for cyclic voltammetry. Phosphate buffers
pH 4 and 7 were employed for electrochemical experiments.
In some experiments sodium sulfate (0.05 mol dmy3) was
employed as an electrolyte. For non-aqueous solutionsTEAP
was employed as an electrolyte.
IR spectra were recorded using a Perkin–Elmer Spectrum
2000 IR spectrophotometer.
3. Results and discussion
3.1. Interactions with NO
For electrocatalytic reactions, a glassy carbon electrode
modified with Na4[Co(II)TSPc] (CoTSPc-GCE) was
employed as a working electrode. The glassy carbon was
modified by anodic electrodeposition from a 10y3 mol dmy3
solution of [CoTSPc]4y in DMF containing 0.1 mol dmy3
TEAP as an electrolyte, as reported before [44]. The depo-
sition was carried out by scanning repetitively at 100 mV sy1
between y0.2 and 1.0 V versus AgNAgCl (non-aqueous).
Prior to coating, the GCE was polished with alumina on a
Buehler felt pad, followed by soaking in dilute nitric acid and
rinsing in water, acetone and the solvent of interest. For bulk
electrolysis, a two-compartment cell was employed, with a
The electronic absorption spectral properties of [Co-
TSPc]4y are well known. Like other tetrasulfonated phthal-
ocyanines, this species forms aggregates in aqueous solution.
The electronic absorption spectrum is explained in terms of
the monomer/dimerequilibrium. Thelowerenergybandnear
670 nm has been attributed to the monomeric species and the
higher energy band near 620 to the dimeric species [46]. On
coordination of molecules such as nitrite and histidine, the
peak due to the monomer increases in intensity and shifts to
longer wavelengths, whereas the peak due to the dimeric
species decreases in intensity [47,48]. Fig. 2 shows spectral
changes that were observed when NO was added to solutions
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231
porphine [Co(2-TMpyP)(NO)] was observed at 1722
cmy1 [9] and for CoPc(NO) at 1680 cmy1 [42]. IR spectra
of the solid formed by reacting [Co(II)TSPc]4y with NO
showed a new band at 1730 cmy1, in the range for coordi-
nated NO. This value is higher than those reported for NO
coordinated to cobalt porphyrins. Generally oxidized cobalt
porphyrin complexes show n(NO) values at higher frequen-
cies than neutral complexes [12]. Thus the higher n(NO)
valuesforthe(NO)CoTSPccomplexcouldbeaconsequence
of the oxidation of [Co(II)TSPc]4y to [Co(III)TSPc]3y
following coordination of NO. As mentioned above, oxida-
tion of [Ni(II)TSPc]4y to [Ni(III)TSPc]3y following
coordination of NO has been observed. The IR spectra of the
Ni(III)TSPc(NOy) complex showed n(NO) at 1660 cmy1
[39], much lower than the value reported here for the
Co(III)TsPc(NO) complex. Vibrational frequencies of NO
greater than 1700 cmy1 have been associated with linear
geometry of M–N_O and those of lower frequency to a bent
geometry [39]. The observation of n(NO) at 1730 cmy1
suggests a bent geometry for the Co–NO bond in the Co-
Fig. 2. Electronic absorption spectral changes observed on addition of NO
(3.5=10y5 mol dmy3)toaqueoussolutionsof[Co(II)TSPc]4y.(a)Spec-
tra immediately after addition of NO; (b) spectra 30 min after addition of
NO.
of [Co(II)TSPc]4y. The monomeric peak of [CoTSPc]4y
was observed at 659 nm before addition of NO (Fig. 2a).
The spectrum is typical for [CoTSPc]4y in water, at low
electrolyte concentrations; the peakduetothemonomerdom-
inates under these conditions [46]. On addition of NO to
[Co(II)TSPc]4y, a shift in the peak due to the monomer,
from 659 to 667 nm, was observed. The intensity of the peak
at 667 nm increased isosbestically with time (Fig. 2b). The
shift in spectrum is associated with the coordination of
NO to the [CoTSPc]4y species. A shift and increase in the
monomeric peak of the [CoTSPc]4y species on coordination
of histidine and nitrite have been reported [47,48]. An
increase and shifting of the monomer peak of [CoTSPc]4y
was also observed upon formation of an adduct between this
species and oxygen. Oxygen was carefully excluded from the
solutions containing NO and [CoTSPc]4y and no spectral
changes were observed for [CoTSPc]4y in the absence of
NO. Thus the spectral changes shown in Fig. 2 are associated
with the formation of an adduct between NO and
y
TSPc(NO) complex. Oxidation of NO to NO2 is likely
following its coordination to CoTSPc. NO2y stretching fre-
quencies were observed at 1401, 1306 and 830 cmy1 in
Co(III)2-TMpyP(NO). We did not observe new IR bands
in this region for the CoTSpc(NO) complex.
The coordination of NO to [Co(II)TSPc]4y can be rep-
resented by Eq. (1):
kf
[Co(II)TSPc]4yqNO|[(NOy)Co(III)TSPc]4y (1)
kr
The number of NO molecules coordinated to the
[Co(II)TSPc]4y species was determined using the standard
equation [37,42]:
AeqyA0
log
slogKqnlog[NO]
(2)
≥A yA ¥
`
eq
[CoTSPc]4y
.
X-ray photoelectron spectroscopy studies have indicated
oxidation of the central metal in FePc and CoPc when the
complexes are exposed to NO [41]. Electron transfer from
Ni(II) to NO has also been implicated during coordination
of NO to [Ni(II)TSPc]4y, resulting in the formation of the
[(NOy)Ni(III)TSPc]4y species [39]. Oxidation of
Co(II)Pc to a Co(III)Pc species was alsoimplicatedoninter-
action of NO with CoPc [42]. We suggest that oxidation of
the central metal also occurs on coordination of NO to the
[CoTSPc]4y species, forming a Co(III)TSPc-NO complex.
Oxidation of [CoTSPc]4y usingchemicaloxidants(e.g.bro-
mine) resulted in the spectrum similar to that shown in Fig.
2b. The lack of an isosbestic point with the spectra of the
starting complex in Fig. 2 is most likely a result of the two
processes, coordination of NO to [CoTSPc]4y and the sub-
sequent electron transfer between the two molecules, occur-
ring simultaneously.
where Aeq is the equilibrium absorbance at 667 nm, A0 is the
absorbance before addition of NO; A`, the absorbance after
complete formation of the [(NOy)Co(III)TSPc]4y com-
plex, was determined from the final absorbance at 667 nm.
Fig. 3a shows a plot of log[(AeqyA0)/(A`yAeq)] versus
log[NO]. A linear plot with a slope ns0.97"0.07 was
obtained, showing that only one molecule of NO is coordi-
nated. The equilibrium constant (K) was determined from
the intercept to be 3.0"0.5=105 dm3 moly1
.
Plots of the observed rate constant (kobs) versus the con-
centration of NO were linear (Fig. 3b), showing that the
coordination is first-order in NO and obeys the rate law:
kobsskf[NO]qkr
(3)
where kf is the rate constant for the formation of the
[(NOy)Co(III)TSPc]4y species according to Eq. (1) and
kr is the dissociation of this complex. Least-square analysis
of the data presented in Fig. 3b gave kfs142"7 dm3 moly1
sy1 from the slope, and krs5.4"0.8=10y4 sy1 was
In nitrosyl complexes, n(NO) ranges from 1500 to 1900
cmy1. n(NO) for cobalt tetrakis(N-methyl-2-pyridyl)-
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alone did not show any peaks. The reduction currents for NO
observed on GCE modified with [Co(II)TSPc]4y are much
higher than the currents observed for the reduction of the
same concentration of NO on an unmodified GCE. The peak
currents for the reduction of NO on GCE modified with
Table 1
Equilibrium and rate constants for coordination of NO
Complex
K (dm3 moly1
)
kf (dm3 moly1 sy1
)
Ref.
Fe(III) porphyrin 1.1=103
7.2=105
1.9=105
2.5=107
142
[14]
[14]
[14]
this work
Metmyoglobin
Hemoglobin
1.4=104
[Co(II)TSPc]4y 3.0=105
Fig. 3. (a) Plot of log[(AeqyA0)/(A`yAeq)] vs. log[NO]. (b) Plot of the
observed rate constant (kobs) vs. the concentration of NO, for the formation
of [(NO)yCo(III)TSPc]4y species.
obtained from the intercept. Using these values of rate con-
stants, the value of the equilibrium constant for NO coordi-
nation to [Co(II)TSPc]4y was estimated to be K1s
2.6=105 dm3 moly1 from the relationship K1skf/kr, which
is in excellent agreement, within experimental error, with the
value determined above from the equilibrium data.
Fig. 4. Cyclic voltammogram for the reduction of 1.0=10y5 mol dmy3 NO
on (a) unmodified GCE and (b) CoTSPc-GCE. Scan rate 100 mV sy1
.
Table 1 compares the values of the rate constant for the
coordination of NO to [Co(II)TSPc]4y with the values for
its coordination to iron(II) porphyrin, metmyoglobin and
hemoglobin. The values for the coordination of NO to the
latter three molecules is much higher than the value for the
coordination to [Co(II)TSPc]4y. A low rate constant was
also reported for the coordination of NO to manganese por-
phyrin complexes [11]. Axial ligand exchange reactions
occur more slowly for CoPc than for FePc complexes [49].
The equilibrium constant for the coordination of NO to
[Co(II)TSPc]4y was, however, much larger than for coor-
dination of NO to the Fe(II) porphyrin, metmyoglobin and
hemoglobin complexes (Table 1).
3.2. Electrocatalysis
Typical cyclic voltammograms for the reduction of NO
on an unmodified GCE and on a GCE modified with
[Co(II)TSPc]4y are shown in Fig. 4. On CoTSPc-GCE, a
catalytic peak was observed at y0.96 V versus AgNAgCl for
the reduction of NO (Fig. 4b). The voltammogram for the
reduction of NO on unmodified GCE is shown in Fig. 4a.
The cyclic voltammogram of the adsorbed CoTSPc in buffer
Fig. 5. Plot of the variation of the catalytic currents for (a) reduction and
(b) oxidation of NO on CoTSPc-GCE vs. the concentration of NO. Scan
rate 100 mV sy1
.
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233
[Co(II)TSPc]4y increased with increase in NO concentra-
tion as shown in Fig. 5a. The modified electrode showed
stability towards the reduction of NO in that the currents did
not decrease appreciably with scan number. The peak at
y0.96 V versus AgNAgCl is almost at the same potential as
that observed for reduction of NO (y0.93 V) on CoPc-
modified GCE [42]. The pH of the media plays an important
part in the catalytic reduction or oxidation of NO. The peak
for the catalytic reduction of NO in water containing sodium
sulfate was observed at y1.2 V versus AgNAgCl. When the
pH 7 buffer was employed the peak was observed at y1.1 V
versus AgNAgCl.
In conclusion, NO has been shown to coordinate to
[Co(II)TSPc]4y. The coordination is accompanied by elec-
tron transfer from the central Co(II) metal and the formation
of [(NOy)Co(III)TSPc]4y species. The rate constant for
the formation of this species is much lower than those
reported for the interaction of NO with metmyoglobin,
iron(II) porphyrin and hemoglobin. [Co(II)TSPc]4y
adsorbed on GCE catalyses both the reduction and oxidation
of NO. The lowest concentration of NO that couldbe detected
on GCE modified with [Co(II)TSPc]4y was of the order of
10y9 mol dmy3
.
The cyclic voltammograms for the oxidation of NO on an
unmodified GCE and on GCE modified with CoTSPc are
shown in Fig. 6. There is a considerable enhancement in the
oxidation currents of NO when GCE is modified with
[Co(II)TSPc]4y compared with unmodified GCE. A weak
broad peak is observed for NO on unmodified GCE near 1.1
V. On CoTSPc-GCE, the oxidation peak for NO is well
resolved and observed at 1.08 V. The enhancement in oxi-
dation currents when CoTSPc-GCE is employed shows that
the [Co(II)TSPc]4y species acts as a catalyst for the oxi-
dation of NO. The currents for the oxidation of NO on
CoTSPc-GCE increased with increase in NO concentration,
as shown in Fig. 5b. The lowest concentration of NO that
could be determined using either the oxidation or reduction
Acknowledgements
This work is supported by Rhodes University and by the
Foundation of Research Development in South Africa.
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