Synthesis and electrochemical studies of a covalently linked cobalt(II) phthalocyanine-cobalt(II) porphyrin conjugate

Article abstract of DOI:10.1039/b418611g

The synthesis of a cobalt phthalocyanine-cobalt porphyrin heteropentamer (cobalt(II) phthalocyaninetetrakis(cobalt(II) tetrakis(5-phenoxy-10,15,20-triphenylporphyrin))), (CoPc-(CoTPP)₄), containing four units of cobalt tetraphenylporphyrin linked to a central cobalt phthalocyanine macrocycle via ether linkages is reported. Cyclic voltammetry and spectroelectrochemistry were employed to characterize the complexes. Cyclic voltammetry and square wave voltammetry revealed nine processes. Spectroelectrochemistry of the pentamer confirmed that reduction occurs on the individual components in an alternating manner; the first reduction occurring on the CoTPP moieties, the second on the CoPc moiety, the third on the CoTPP moieties and so on. Oxidation occurred first on the CoPc moiety of the pentamer. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b418611g

View Article Online / Journal Homepage / Table of Contents for this issue
F U L L P A P E R
Synthesis and electrochemical studies of a covalently linked cobalt(II)
phthalocyanine–cobalt(II) porphyrin conjugate†
Zhixin Zhao, Kenneth I. Ozoemena, David M. Maree and Tebello Nyokong*
Department of Chemistry, Rhodes University, Grahamstown, 6140, South Africa.
E-mail: t.nyokong@ru.ac.za; Fax: + 27 46 6225109; Tel: + 27 46 6038260
Received 10th December 2004, Accepted 23rd February 2005
First published as an Advance Article on the web 7th March 2005
The synthesis of a cobalt phthalocyanine–cobalt porphyrin heteropentamer (cobalt(II) phthalocyanine–
tetrakis(cobalt(II) tetrakis(5-phenoxy-10,15,20-triphenylporphyrin))), (CoPc–(CoTPP)4), containing four units of
cobalt tetraphenylporphyrin linked to a central cobalt phthalocyanine macrocycle via ether linkages is reported.
Cyclic voltammetry and spectroelectrochemistry were employed to characterize the complexes. Cyclic voltammetry
and square wave voltammetry revealed nine processes. Spectroelectrochemistry of the pentamer confirmed that
reduction occurs on the individual components in an alternating manner; the first reduction occurring on the CoTPP
moieties, the second on the CoPc moiety, the third on the CoTPP moieties and so on. Oxidation occurred first on the
CoPc moiety of the pentamer.
Introduction
Porphyrins and phthalocyanines represent two related classes
of very versatile macrocyclic organic molecules. Conjugates of
porphyrins and phthalocyanines have attracted some attention
but generally very little is known about them. The first synthesis
of a porphyrin–phthalocyanine conjugate, a zinc porphyrin–zinc
phthalocyanine dimer, was reported about two decades ago by
Gaspardet al.1 with the aim of investigating some photochemical
properties. Peripherally linked dimers as well as sandwich type
dimers and trimers have been reported and have apparent
applications as electrochromic materials, molecular and liquid
crystals, semiconductors, gas sensors, nonlinear optical mate-
rials, molecular magnets and as materials in ionoelectronics.2–8
Metalloporphyrins are widely studied as biomimetic models for
several biological redox processes,9 and have been extensively
used in chemical and electrochemical analyses according to
a recent review.10 Metallophthalocyanines are well recognised
as efficient electrocatalysts.11–13 Multiporphyrin–phthalocyanine
arrays can serve as useful candidates for applications such
as in light harvesting and molecular photonics, and catalysis.
Porphyrin and phthalocyanine complexes of cobalt(II), in par-
ticular, have been intensively studied and found to be excellent
electrocatalysts, for example, in the electrocatalytic oxidation
of several industrially and biologically important molecules
such as thiols,13–17 nitric oxide,18 as well as for the reduction
of oxygen19–22 for potential application in fuel cell technology. A
possible synergistic means by which the full catalytic activities
of cobalt porphyrin and phthalocyanine complexes may be fully
harnessed for use in biomimetic reactions or in the design for
multi-electron redox catalysts/mediators for biomedical and
electrochemical sensing is, for example, by smart synthesis
of covalently linked cobalt porphyrin–cobalt phthalocyanine
molecules. In this work, we present a synthetic strategy for the
preparation of a cobalt phthalocyanine–cobalt tetraphenylpor-
phyrin heteropentamer complex in which four units of cobalt
tetraphenylporphyrin are covalently linked to a central cobalt
phthalocyanine macrocycle (CoPc–(CoTPP)4) (Fig. 1). Since
certain potential applications (e.g., in the design of an efficient
catalyst) of this molecule will require an understanding of the
nature of its redox processes, this work also investigates the
Fig. 1 The molecular structure of a cobalt phthalocyanine–cobalt
porphyrin CoPc–(CoTPP)4 pentamer.
electrochemical and spectroelectrochemical properties of CoPc–
(CoTPP)4.
Experimental
Equipment
UV-vis spectra were recorded on a Varian 500 UV-Visible/NIR
spectrophotometer. IR (KBr pellets) were recorded on a Perkin-
Elmer Spectrum 200 FTIR spectrometer. 1H NMR spectra were
recorded using a Bruker EMX 400 NMR spectrometer. MALDI
TOF spectra were recorded with Perseptive Biosystems Voyager
DE-PRO Biospectrometry Workstation and possessing Delayed
Extraction Technology at the University of Cape Town.
Electrochemical data were obtained under purified nitrogen
gas with BioAnalytical System (BAS) 100 B/W Electrochemical
Workstation. Cyclic voltammograms (CVs) and Osteryoung
square wave voltammograms (OSWV) were collected using
a conventional three-electrode set-up with a glassy carbon
electrode (GCE, 3.00 mm diameter) as a working electrode,
platinum wire as a counter electrode and Ag|AgCl wire as a
† Electronic supplementary information (ESI) available: Fig. S1: 1H
NMR of CoCNOTPP in DMSO-d6. Fig. S2: Mass spectrum of
CoCNOTPP. Fig. S3. Mass spectrum of CoPc–(CoTPP)4 pentamer. See
http://www.rsc.org/suppdata/dt/b4/b418611g/
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 1 – 1 2 4 8
1 2 4 1
©

View Article Online
pseudo-reference electrode. The optimised parameters for the
OSWV were: step potential 4 mV; square-wave amplitude 25 mV
at a frequency of 15 Hz. For voltammetry studies, concen-
trations of the porphyrin, phthalocyanine and the pophyrin–
phthalocyanine pentamer complexes were maintained at ca.
10−3 M in dry dimethyl sulfoxide (DMSO) containing TBAP
(tetrabutylammonium perchlorate) as supporting electrolyte.
Spectroelectrochemical data were obtained with a home-made
optically transparent thin-layer electrochemical (OTTLE) cell,
with a similar design to that previously reported by Krejcˇik
et al.23 The working and counter electrodes of the cell were
platinum grits while a piece of silver wire served as a pseudo-
reference electrode. The OTTLE cell was connected to a BAS
CV 27 voltammograph. All experiments were performed at 25
1 ◦C.
Synthesis
{5-[4-(3,4-Dicyanophenoxy)phenyl]-10,15,20-triphenylporphy-
rinato}cobalt(II) (CoCNOTPP, 3, Scheme 1). A mixture
of H2CNOTPP (2) (0.12 g, 0.16 mmol) and hydrous cobalt
acetate (0.186 g, 0.75 mmol) in DMF (10 ml) was heated at
70 ◦C for 2 h with stirring under nitrogen atmosphere. After
cooling to room temperature, the mixture was added to 40 ml
dichloromethane and washed with 100 ml water three times to
remove excess cobalt acetate, acetic acid and DMF. Column
chromatography on silica gel with dichloromethane as eluent
gave one band. Removal of dichloromethane by evaporation
gave 0.112 g (86.5%) of a purple–red solid. The purple-red crude
product was recrystallized from dichloromethane using absolute
methanol. IR [(KBr) mmax/cm−1]: 2231 (CN), 1247 (C–O–C);
UV-Vis [DMSO, kmax/nm (loge)]: 415 (5.26), 528 (4.06); 1H
NMR (400 MHz, CDCl3), d 15.82 (br s, 8H, pyrrole-H), 13.09
(br s, 8H, phenyl H), 9.56–9.92 (d, 11H, phenyl-H), 8.91 (s, 1H,
phthalonitrile H), 8.80 (d, 1H, phthalonitrile H), 8.51 (d, 1H,
phthalonitrile H); MALDI-TOF-MS m/z: calc. 813.75; found
812.98 [M]+.
Materials
4-Nitrophthalonitrile,24 5-hydroxy-10,15,20-triphenylporphy-
rin25 and 5-[4-(3,4-dicyanophenoxy)phenyl]-10,15,20-triphenyl-
porphyrin (H2CNOTPP, 2)1 were synthesized as in literature.
4-Hydroxybenzaldehyde, benzaldehyde, pyrrole, 1,8-diazabi-
cyclo[5,4,0]undec-7-ene (DBU), n-octanol, sodium methoxide,
potassium carbonate, hydrous cobalt acetate, tetraphenyl-
porphyrin (H2TPP), cobalt phthalocyanine (CoPc) were
purchased from Sigma–Aldrich and used without further
purification. DMSO was dried over alumina before use. TBAP
used as supporting electrolyte was purified by recrystallization
from ethanol. All other solvents were dried by standard methods
prior to use. Silica gel 60 (0.04–0.063 mm) for chromatography
was purchased from Merck.
Tetrakis[tetrakis(5-phenoxy-10,15,20-triphenylporphyrinato)-
cobalt(II)]–(phthalocyaninato)cobalt(II) (CoPc–(CoTPP)4, 5).
A mixture of CoCNOTPP (3, 20.3 mg, 0.025 mmol) and
sodium methoxide (5 mg) was added to distilled methanol
(5 ml). Anhydrous ammonia gas was bubbled through the
stirred suspension for 1 h. The suspension was then refluxed
for 6 h with continued addition of ammonia gas, affording
complex 4 as an intermediate which was not isolated. The
methanol was removed, then hydrous cobalt acetate (12.45 mg,
0.05 mmol), DBU (2 drops) and n-octanol (5 ml) were added.
The mixture was heated at 160 ◦C for 6 h with stirring under a
Scheme 1 Synthetic route for the pentamer CoPc–(CoTPP)4.
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 1 – 1 2 4 8
1 2 4 2

View Article Online
nitrogen atmosphere. The reaction was monitored with UV-vis
spectroscopy. After cooling to room temperature, the blue–
purple solution was poured into 20 ml dichloromethane and
washed three times with 100 ml water. The dichloromethane
layer was collected, evaporated and the solid applied to a
silica gel column. A series of purple bands were eluted by
dichloromethane. The desired compound (blue band) was
eluted using DMF. Removal of DMF by water afforded 2.8 mg
(13.5%) of a green solid (complex 5). IR [(KBr) mmax/cm−1]: 1234
(C–O–C); UV-Vis [DMSO, kmax/nm (loge)]: 331 (4.74), 419
1
(5.15), 540 (4.19), 594 (4.16), 659 (4.62); H NMR (400 MHz,
DMSO-d6), d 9.18 (br s, 8H, phthalocyanine H), d 8.96 (br s,
4H, phthalocyanine H), 8.23 (br s, 32H, pyrrole H), 7.84
(br s, 32H, phenyl-2,6H), 7.68 (br s, 44H, phenyl-3,4,5H);
MALDI-TOF-MS m/z: calc. 3314.37; found 3318.37 [M + 4]+.
Results and discussion
Fig. 2 UV-vis spectrum of ∼2 × 10−5 M solutions of (a) CoTPP, (b)
Synthesis and characterization
CoCNOTPP, (c) CoPc and (d) the pentamer CoPc–(CoTPP)4 in DMSO.
The synthetic route to the compounds presented in this work is
shown in Scheme 1. The compounds are abbreviated as indicated
in the Experimental section. H2CNOTPP (2) was synthesized as
in literature1 and in our hands we obtained a slightly higher yield
of 72%. This compound was metallated using cobalt acetate
to give CoCNOTPP, which was then characterized by IR (CN
stretch at 2231 cm−1 and aromatic ether at 1247 cm−1), UV-
vis spectroscopy as well as 1H NMR spectroscopy. The proton
resonances are broad due to the presence of the paramagnetic
(Fig. 2(d)) is a combination of the respective spectrum of CoTPP
(Fig. 2(a)) and CoPc (Fig. 2(c)). Fig. 2(d) shows a slight red shift
for both the B and Q bands of CoTPP (from 409 and 523 nm
for CoTPP alone (Fig. 2a) to 419 and 540 nm for CoTPP in
CoPc–(CoTPP)4). The Q band of CoPc (from 654 nm for CoPc
alone to 659 nm for CoPc in CoPc–CoTPP) is also found to
be shifted. The observation of shifting of the CoTPP bands in
the pentamer to longer wavelength compared to CoTPP alone
suggests that CoPc exerts some electron-withdrawing effects
on the CoTPP. CoTPP, on the other hand, confers electron-
donating effects on the CoPc since electron-donating peripheral
substituents on the MPc complexes gives rise to red shifts in
the Q absorption wavelengths.29–32 The Q and B band positions
for the CoTPP moieties in CoPc–(CoTPP)4 are typical for
CoIITPP species in Fig. 2. However this spectrum was observed
when the CoPc–(CoTPP)4 complex was freshly prepared. With
time following preparation of the solid complex, the spectrum
changed to that shown in Fig. 3(b). The spectrum (Fig. 3(b))
confirms that the CoTPP moieties of CoPc–(CoTPP)4 oxidize to
CoIIITPP with time, and the B band shifts to longer wavelengths.
It is well known33 that the B band of CoIIITPP is more red-shifted
than that of CoIITPP.
1
cobalt.26 Another remarkable characteristic of the H NMR
spectrum of this compound is the extremely deshielded pyrrole
protons which is witnessed by their resonance at 15.82 ppm.
1
The H NMR resonances for CoPc–(CoTPP)4 were generally
broad due to the complex mixture of isomers formed, typical
of a tetrasubstituted phthalocyanine species, in addition to the
presence of paramagnetic Co central metal. Useful information
is still obtained despite the broad peaks as the number of protons
in the molecule could be accurately accounted for. Further
proof of this structure being correct is obtained from the mass
spectrum of both the precursor CoCNOTPP and the product
CoPc–(CoTPP)4. The mass spectrum of CoCNOTPP had a
molecular ion peak at m/z 812.98 [M]+ and a molecular ion peak
was observed for CoPc–(CoTPP)4 at m/z 3318.37 [M + 4]+.
CoPc–(CoTPP)4 could only be synthesized in satisfactory
yields by conversion of the phthalonitrile into a diiminoisoin-
doline precursor (4) then synthesising the phthalocyanine. This
procedure gave a 13% yield of CoPc–(CoTPP)4. The product is
difficult to elute out on a silica column and it is necessary to
use a polar solvent such as dimethylformamide (DMF) to elute
the blue phthalocyanine band. After the DMF was removed by
addition of water, the product was separated and found not to be
soluble in chlorinated solvents such as dichloromethane, it does
however dissolve well in DMF or DMSO. The UV-vis spectrum
confirms the formation of a phthalocyanine ring by the appear-
ance of a phthalocyanine Q-band at 659 nm (Fig. 2). This figure
shows the electronic absorption spectra of CoTPP (Fig. 2(a)),
CoCNOTPP (Fig. 2(b)), CoPc (Fig. 2(c)) and the covalently
linked compound CoPc–(CoTPP)4 in DMSO (Fig. 2(d)). The
spectra are typical of these complexes in solution. The presence
of a phthalonitrile substituent at a ring position of the CoTPP
in CoCNOTPP resulted in a slight shift of the high (B, from 409
to 417 nm) and low (Q, from 523 to 528 nm) energy bands of the
CoTPP. This shift to longer wavelengths are usually observed
for CoTPP to which electron-withdrawing substituents have
been attached,27,28 thus indicating that phthalonitrile confers
some electron-withdrawing effect on the CoTPP. As observed for
the zinc phthalocyanine–porphyrin dimer,1 the UV-vis spectrum
of a metallophthalocyanine–metalloporphyrin conjugate should
represent the spectrum of each of the component MPc and
MP species. Thus, the UV-vis spectrum of the CoPc–(CoTPP)4
Fig. 3 UV-vis spectra of ∼1 × 10−5 M solutions of CoPc–(CoTPP)4
in DMSO: (a) freshly prepared and (b) of the solid complex four weeks
after preparation.
Voltammetric and spectroelectrochemical characterisation
The redox properties of CoPc–(CoTPP)4 (5) were studied
using cyclic voltammetry (CV) and Osteryoung square
wave voltammetry (OSWV) and compared with those of its
component molecules, H2TPP, CoTPP and CoPc, and of the
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 1 – 1 2 4 8
1 2 4 3

View Article Online
Fig. 4 Cyclic voltammograms of (a) H2TPP, (b) CoTPP, (c) H2CNOTPP and (d) CoPc (inset: OSWV) on a glassy carbon electrode in DMSO
solution containing 0.1 M TBAP. Scan rate = 100 mV s−1
.
parent molecules H2CNOTPP (2) and CoCNOTPP (3), using
similar experimental conditions. Fig. 4 shows typical CVs
for (a) H2TPP, (b) CoTPP, (c) H2CNOTPP and (d) CoPc in
DMSO containing 0.1 M TBAP. Figs. 5 and 6 present the
cyclic voltammograms for the precursor CoCNOTPP and the
pentamer CoPc–(CoTPP)4, respectively. The values for the half-
wave potential (E1/2) are summarised in Table 1. The neutral
forms of the porphyrin and phthalocyanine exist as a dianion
designated as P−2 or Pc−2 for porphyrin or phthalocyanine,
respectively.34–36 Oxidation at the porphyrin and phthalocyanine
rings occurs by successive loss of one or two electrons from
Fig. 6 Cyclic voltammograms of CoPc–(CoTPPc)4 pentamer on a
glassy carbon electrode in DMSO solution containing 0.1 M TBAP. Scan
rate = 100 mV s−1; inset: OSWV.
the highest occupied molecular orbital (HOMO) resulting
in the formation of [P−1]•+ or [Pc−1]•+ and [P0]•2+ or [Pc0]•2+
cation radicals. Reduction, on the hand, occurs by successive
gain of electrons by the lowest unoccupied molecular orbital
(LUMO) of the porphyrin and phthalocyanine complexes
resulting in the formation of P(−2− or Pc(−2− species (where
n = number of electrons). The cyclic voltammogram for H2TPP
shown in Fig. 4(a) is well known and consists of two reversible
one-electron reductions. Under our experimental conditions no
oxidation couples were observed for this molecule. The cyclic
voltammogram for CoTPP is also very well known, and the
first reduction and oxidation occur on the central metal. For
H2CNOTPP (Fig. 4(c)), only reduction processes were observed.
n)
n)
Fig. 5 Cyclic voltammograms of CoCNOTPP on a glassy carbon
electrode in DMSO solution containing 0.1 M TBAP. Scan rate =
100 mV s−1
.
1 2 4 4
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 1 – 1 2 4 8

View Article Online
Table 1 Half-wave potentials (E1/2/V) of porphyrin and phthalocyanine precursor complexes for CoPc–(CoTPP)4 in dry DMSO containing 0.1 M
TBAP
Oxidation processes (E1/2/V)
Reduction processes (E1/2/V)
Macrocycle
I (P−1/P−2
)
II (CoIII/CoII)
III (CoII/CoI)
IV (P−2/P−3
)
V (P−3/P−4
)
VI (P−4/P−5
)
H2TPP
H2CNOTPP
CoTPP
CoCNOTPP
CoPc
−0.77
−0.74
−1.68
−1.30
−1.20
−1.20
−1.15
−1.29
1.47
1.06
1.11 (1st)
1.30 (2nd)
0.35
0.44
0.37
−0.64
−0.55
−0.31
−1.62
∼−1.70
Couples IV and V for H2CNOTPP (Table 1) are similar
to those of H2TPP and are assigned to ring-based processes
given by eqns. (1) and (2). These couples are well-recognised and
are characteristic of ring-based electrode processes for metal-
free porphyrin complexes.37 A third reduction process (VI) was
observed in H2CNOTPP even though it was not observed for
H2TPP. Although it is rare to observe more than two ring-
based reduction processes in porphyrin complexes, Campbell
et al.38 observed three reduction processes for a nickel porphyrin
complex with electron-withdrawing formyl substituents.
[CoIII(CNOTPP−2)]•+ ꢀ [CoIII(CNOTPP(−1)]2+ + e−: I
CoII(CNOTPP−2) ꢀ [CoIII(CNOTPP−2)]•+ + e−: II
CoII(CNOTPP−2) + e− ꢀ [CoI(CNOTPP−2)]−: III
[CoI(CNOTPP−2)]− + e− ꢀ [CoI(CNOTPP−3)]2−: IV
[CoI(CNOTPP−3)]2− + e− ꢀ [CoI(CNOTPP−4)]3−: V
(4)
(5)
(6)
(7)
(8)
H2CNOTPP−2 + e− ꢀ [H2CNOTPP−3]−: IV
[H2CNOTPP−3]− + e− ꢀ [H2CNOTPP−4]2−: V
[H2CNOTPP−4]2− + e− ꢀ [H2CNOTPP−5]3−: VI
(1)
(2)
(3)
Electron-withdrawing substituents on the peripheral positions
of phthalocyanine or porphyrin rings lead to a decrease on the
average electron density of the total conjugated phthalocyanine
or porphyrin system thereby leading to easier reduction and
more difficult oxidation.16,27–29,44 As observed above for H2TPP
(Fig. 4(a)) and H2CNOTPP (Fig. 4(c)), the oxo-phthalonitrile
confers electron-withdrawing properties on the metallated por-
phyrin complexes. Comparing the E1/2 value of CoTPP with that
of CoCNOTPP, shows that the latter is easier to reduce.
Couples II and III for CoPc (Fig. 4(d)) are assigned to
metal-centered oxidation (CoIII/CoII) and reduction (CoII/CoI),
respectively, in comparison with the literature.36 The remainder
of the couples are ring-based, see Table 1. The observation
of weak currents for the CoIII/CoII process is a common
phenomenon for CoPc complexes.36 The inset of Fig. 4(d) is
the more sensitive OSWV depicting this CoIII/CoII process and
other ill-defined redox couples.
The pentamer, CoPc–(CoTPP)4 (Fig. 6), showed nine identifi-
able redox processes (I–VIII), characteristic of a multi-electron
redox species. The voltammograms were recorded after the
oxidation of the CoII to CoIII in CoTPP moieties, as judged
by the spectra. As stated above, the CoTPP moieties of the
solid oxidized with time to form CoIIITPP. The processes in
Fig. 6 are easier to observe on the OSWV (inset). All the
processes showed quasi-reversible (with large DEp, ≥ 100 mV)
to irreversible behaviour. The lack of total reversibility in some
of the redox couples is most likely due to overlapping of the
individual parent monomers that make up the pentamer. The
peak currents increased linearly with the square root of the
scan rates, for scan rates ranging from 25 to 800 mV s−1,
suggesting diffusion-controlled reactions. Repetitive scanning
(20 scans) of the DMSO solution of CoPc–(CoTPP)4 containing
TBAP did not show any change in the positions of the peaks
except for the process VII that shifted considerably towards
negative potential (by 0.2 V) and then broadened leading to
the loss of process VIII. However, using a freshly cleaned
working electrode surface, a scan similar to the first one in
Fig. 6 was reproduced. Aromatic systems physisorb onto glassy
carbon surfaces, indicating that this change in peak shapes
on continuous scans may be attributed to the adsorption and
formation of different products on the electrode surface.
However, to check if any of the reduction processes observed
in our work might be due to the phthalonitrile substituent,
a CV of phthalonitrile alone was recorded. An irreversible
anodic peak at ca. −0.80 V was obtained for the phthalonitrile
in DMSO containing TBAP. There is no irreversible wave in
this region in Fig. 4(c) suggesting that the phthalonitrile peak
is not observed when the molecule is bound to porphyrin.
Thus all the three reduction processes (IV–VI) are due to
H2CNOTPP. The result also indicates that the phthalonitrile
substituent exerts an electron-withdrawing effect on the redox
behaviour of the porphyrin molecule, which is apparent in
the slight shifts of the reduction potentials to more positive
(easier to reduce) potentials as compared to the H2TPP, thus
corroborating the spectroscopic observation. Couple VI was
separated from couple V by 0.14 V. The separation between ring-
based redox process is affected by the nature of electrolyte, with
smaller separations being observed when TBAP is employed
compared to TBAPF6.38 Process (VI) is represented by eqn.
(3). The reduction processes for H2CNOTPP were reversible
to quasi-reversible with anodic to cathodic peak currents being
near unity.
Displacement of the two protons in the inner porphyrin or
phthalocyanine core with a metal ion usually leads to a shift
of the first ring-based reduction potential to a more negative
value as a result of the p-back donation of the filled dp of
the metal ion into the empty porphyrin or phthalocyanine p*
orbitals.36,39 Thus, as expected, the introduction of cobalt ion
(i.e., the formation of CoCNOTPP) resulted in the shift of the
ring processes to more negative values (compare H2CNOTPP
and CoCNOTPP, Table 1). Comparison with the well-known
assignments for CoTPP complexes,28,33,35,37,40–43 nicely corrobo-
rated with the E1/2 values for CoTPP obtained in this work
(Table 1). The first oxidation (E1/2 = 0.44, II) and reduction
(E1/2 = −0.55 V, III) waves for CoCNOTPP, Fig. 5, are assigned
to CoIII/CoII and CoII/CoI processes, respectively. The second
oxidation for CoCNOTPP (labelled I) and the remainder of the
reduction couples (IV and V) are due to the ring processes.
Eqns. (4)–(8) describe redox processes I–V for CoCNOTPP,
respectively.
The redox processes (I–VIII) of CoPc–(CoTPP)4 should
involve both the central cobalt ion and the p-ring systems of the
porphyrin and the phthalocyanine subunits. Their assignments
(as detailed in Table 2) were based on spectroelectrochemistry
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 1 – 1 2 4 8
1 2 4 5

View Article Online
Table 2 Half-wave potentials (E1/2/V) or peak potential (EP/V), and redox assignments for the electrogenerated species from CoPc–(CoTPP)4 in
dry DMSO containing 0.1 M TBAP
Redox processa
E1/2 (or Ep)/V
Assignment
I
1.10
0.72
CoIIIPc−1(TPP−1CoIII)4/CoIIIPc−2(TPP−1CoIII
CoIIIPc−2(TPP−1CoIII)4/CoIIIPc−2(TPP−2CoIII
)
)
4
4
IIꢀ
II
0.40
CoIIIPc−2(TPP−2CoIII)4/CoIIPc−2(TPP−2CoIII
CoIIPc−2(TPP−2CoIII)4/CoIIPc−2(TPP−2CoII)4
CoIIPc−2(TPP−2CoII)4/CoIPc−2(TPP−2CoII)4
CoIPc−2(TPP−2CoII)4/CoIPc−2(TPP−2CoI)4
CoIPc−2(TPP−2CoI)4/CoIPc−3(TPP−2CoI)4
CoIPc−3(TPP−2CoI)4/CoIPc−3(TPP−3CoI)4
CoIPc−3(TPP−3CoI)4/CoIPc−4(TPP−3CoI)4
)
4
III
IV
V
VI
VII
VIII
−0.30
−0.52
−0.73
−1.13
−1.36
−1.70
a See Fig. 6 for the processes.
and on an established43 strategy of comparing the E1/2 values
of the parent monomeric (CoTPP and CoPc) units (Table 1).
Spectroelectrochemistry has successfully been employed to
provide an unequivocal identification of the nature of the
redox species formed for mono- and hetero-nuclear cobalt
phthalocyanine28,40,45–48 and porphyrin28,40 complexes, hence the
technique was used to confirm our cyclic voltammetry assign-
ments.
Thin layer UV-vis spectra of the CoTPP moieties of CoPc–
(CoTPP)4 in DMSO containing TBAP, before application of
potential shows the presence of CoIIITPP (at 430 nm), with small
amounts of CoIITPP at shorter wavelengths, Fig. 7(a)(i). Fig. 7(a)
shows UV-vis spectral changes observed during controlled
potential reduction of CoPc–(CoTPP)4 in DMSO solution
containing TBAP in OTTLE cell, at potential more negative
(ca. −0.5 V) than process III in Fig. 6. The B band of the
CoTPP moieties of the pentamer shifted from 430 nm of the
CoIIITPP species to 415 nm due to CoIITPP. Note the slight
difference in the B band maximum in Fig. 2(d) compared to
Fig. 7(a) probably due to the presence of the electrolyte for the
spectrum recorded for spectroelectrochemistry. Thus process III
is due CoIIPc−2(TPP−2CoIII)4/CoIIPc−2(TPP−2CoII)4, Table 2. The
reduction occurred with clear isosbestic points at 355, 427, 457,
437 and 700 nm. It can be seen in Fig. 7(a) that during the
Fig. 7 Typical UV-vis spectral changes observed using OTTLE cell during reduction processes of CoPc–(CoTPP)4. The spectra were recorded
continuosly during the electrolysis: (a) −0.5 V before (i) and after (ii) electrolysis; (b) −0.75 V: (i) same as (ii) in (a); (ii) after electrolysis; (c) −1.0 V:
(i) same as (ii) in (b), (ii) after electrolysis; (d) −1.40 V: (i) same as (ii) in (c), (II) after electrolysis.
1 2 4 6
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 1 – 1 2 4 8

View Article Online
controlled electrolysis at potentials of process III, the Q band of
the CoPc moiety (at 659 nm) decreased in intensity without any
shift in wavelength. Thus no reduction occurred on the CoPc
moiety of the pentamer, only some possible degradation.
After the spectroelectrochemical processes shown in Fig. 7(d),
the potential was changed to more positive values, to check the
reversibilities of the reduction processes. Fig. 8(a) shows that
on application of −0.3 V, following changes in Fig. 7(d), the
reduction processes on the CoTPP moiety of the pentamer could
be reversed with the disappearance in the peaks due to CoITPP
and the regeneration first of the CoIITPP Soret band at 415 nm,
together with the decrease of the Pc−3 peaks in the 480–600 nm
region (Fig. 8(a)). The peak due to CoIPc increased in intensity.
Increasing the potential to 0.4 V (Fig. 8(b)) following the
spectral changes shown in Fig. 8(a), resulted in the oxidation of
CoIPc and regeneration of the CoIIPc moiety of the pentamer.
There is also a shift of the Soret band of the CoTPP moieties of
the pentamer towards longer wavelengths, as CoIITPP is oxidized
back to CoIIITPP in the pentamer. The complete regeneration of
the CoIIITPP is only observed when a more positive potential is
Fig. 7(b) shows spectroscopic changes when the potential was
switched to potentials more negative (−0.75 V) than process
IV, following the last spectral change shown in Fig. 7(a).
A dramatic shift of the CoPc band at 659 nm to 702 nm
was observed, accompanied with formation of a new band
474 nm. These spectral changes are typical of the reduction
of CoIIPc to CoIPc,45–48 which consists of the red shifting of
the Q band and the formation of a relatively strong band
in the 450 nm region. The intense band in this region is
associated with the metal-to-ligand charge transfer (MLCT):
CoIPc−2 [d
]→p*(1b1u)(Pc(−2)).45–48 The B band of the
xz,yx
CoIITPP moieties of the pentamer increased in intensity without
any shifting in Fig. 7(b). The spectral changes occurred with
isosbestic points at 683 and 542 nm. Thus at potentials of
process IV, no significant changes occurred on the CoTPP
moieties of the pentamer and this process is assigned to
CoIIPc−2(TPP−2CoII)4/CoIPc−2(TPP−2CoII)4 (Table 2).
Increasing the reduction potential to values more negative
(−1.0 V) of process V, resulted in spectral changes shown in
Fig. 7(c). No changes were observed on the CoPc moiety of
the pentamer. The spectral changes observed on the CoTPP
moieties of the pentamer are typical of CoITPP, with a split
Soret band.33 The formation of the new peak at 358 nm
is similar to that observed by Zheng et al.33 in the 300–
350 nm region for the mononuclear CoTPP and which was
attributed to the CoITPP. Thus, process V is assigned to
CoIPc−2(TPP−2CoII)4/CoIPc−2(TPP−2CoI)4. In Fig. 7(b) the peak
at 358 nm began to be formed but complete spectral changes to
the CoITPP species were only observed in Fig. 7(c).
When the reduction potential was increased to a more negative
value (−1.40 V) (Fig. 7(d)) than process VI, the bands due to
CoIPc (703 nm) decreased in intensity and new bands were
formed between 480 and 600 nm, in addition to the MLCT
band. The bands in the 500–600 nm region are typical48 of
ring reduction in MPc complexes and the formation of Pc−3
species. The bands due to the CoITPP remained stable, hence
confirming that at potentials of process VI, CoTPP moieties of
the pentamer are not involved. Thus, process VI is assigned to
CoIPc−2(TPP−2CoI)4/CoIPc−3(TPP−2CoI)4, Table 2. There were
no further spectral changes when the potential was increased to
more negative values. The remaining reduction processes (VII
and VIII) are all ring-based and are assigned in comparison
with reduction potentials of the precursors and monomer com-
ponents (Table 1). Process VII is at potentials for ring reduction
(to P−3) in the CoCNOTPP precursor (−1.30 V). Thus, this pro-
cess is assigned to CoIPc−3(TPP−2CoI)4/CoIPc−3(TPP−3CoI)4,
and the last reduction process is at the potential for
the second ring reduction in CoPc, hence is assigned to
CoIPc−3(TPP−2CoI)4/CoIPc−4(TPP−3CoI)4. Table 3 summarizes
the spectral data of the complexes.
Table 3 Spectral data for the starting CoPc(TPPCo)4 and its oxidation
or reduction products. Data in DMSO containing TBAP. Only Q bands
(and other identifying bands) are shown for the CoPc moiety. Where
possible, both Q and Soret (B) bands are shown for the CoTPP moieties
k/nm
Complex
CoTPP
CoPc
CoIIPc−2(TPP−2CoIII
)
430 (B), 543 (Q)
415 (B), 532 (Q)
415 (B), 532 (Q)
425 and 358 (split B)
425 and 358 (split B)
430 (B), 543 (Q)
659 (Q)
659 (Q)
703 (Q), 474
703 (Q), 474
482, 533
670
4
CoIIPc−2(TPP−2CoII)4
CoIPc−2(TPP−2CoII)4
CoIPc−2(TPP−2CoI)4
CoIPc−3(TPP−2CoI)4
Fig. 8 Typical UV-vis spectral changes observed using OTTLE cell
during re-oxidation or oxidation of CoPc–(CoTPP)4 The spectra were
recorded continuosly during the electrolysis; (a) −0.3 V: (i) same as (ii)
in Fig. 7(d); (ii) after electrolysis; (b) 0.4 V: (i) same as (ii) in (a); (ii) after
electrolysis; (c) 1.0 V: (i) same as (ii) in (b), (ii) after electrolysis.
CoIIIPc−2(TPP−2CoIII
)
4
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 1 – 1 2 4 8
1 2 4 7

View Article Online
applied (1.0 V), Fig. 8(c). Application of this potential resulted
in the shift of the Q band of the CoIIPc moiety of the pentamer
from 659 nm (of CoIIPc) to 670 nm. The spectral changes
are typical of the oxidation of CoIIPc to CoIIIPc. Thus the
first oxidation of the pentamer occurs at the CoPc moiety.
Hence we assign the first oxidation process (II) in Fig. 6, to
CoIIIPc−2(TPP−2CoIII)4/CoIIPc−2(TPP−2CoIII)4 (Table 2). No fur-
ther spectral changes were observed on increasing the potential.
Processes IIꢀ and I are due to ring based processes and are
assigned to CoIIIPc−2(TPP−1CoIII)4/CoIIIPc−2(TPP−2CoIII)4 and
CoIIIPc−1(TPP−1CoIII)4/CoIIPc−2(TPP−1CoIII)4, respectively. The
latter is assigned based on similarities between its redox process
and that of the parent CoPc (Fig. 4(d)).
13 N. Phougat, P. Vasudevan and S. A. K. Shukla, J. Power Sources,
1993, 46, 61.
14 J. Zagal, M. A. Gulppi, C. A. Caro and G. I. Ca´rdenas-Jiro´n,
Electrochem. Commun., 1999, 1, 389.
15 S. Griveau, M. Gulppi, F. Bedioui and J. H. Zagal, Solid State Ionics,
2004, 169, 59.
16 S. Maree and T. Nyokong, J. Electroanal. Chem., 2000, 492, 120.
17 K. I. Ozoemena, T. Nyokong and P. Westbroek, Electroanalysis, 2003,
15, 1762.
18 T. Nyokong and S. Vilakazi, Talanta, 2003, 61, 27.
19 J. P. Collman, M. S. Ennis, D. A. Offord, L. L. Chng and J. H. Griffin,
Inorg. Chem., 1996, 35, 1751.
20 F. D’Souza, Y. Y. Hsieh, H. Wickman and W. Kutner, Chem.
Commun., 1997, 1191.
21 F. D’Souza, Y. Y. Hsieh and G. R. Deviprasad, Chem. Commun.,
1998, 1027.
In conclusion, we have shown in this work that the newly
synthesized pentamer (CoPc–(CoTPP)4) has electronic absorp-
tion spectral behaviour, which closely resembles that of the
parent (CoPc and CoTPP) complexes. The complex showed
complicated cyclic voltammetry data, which was interpreted
using spectroelectrochemistry. Using the latter technique, the
voltammograms were assigned to metal and ring-based pro-
cesses on both the CoPc and the CoTPP moieties of the
pentamer. The processes seemed to alternate between the CoPc
and CoTPP species in that the first reduction was on the CoTPP
moieties, the second reduction on the CoPc moiety, the third on
CoTPP and so on. Each of the redox processes showed distinct
spectral changes. This work has thus presented a molecule which
absorbs in both the visible and UV region and whose reduction
or oxidation gives new spectra, with potential for applications
where a wide range of absorption characteristics and/or multi-
electron transfer reactions are required.
22 Y.-H. Tse, P. Janda, H. Lam, J. Zhang, W. Pietro and A. B. P. Lever,
J. Porphyrins Phthalocyanines, 1997, 1, 3.
23 M. Krejcˇik M, M. Daneˇk and F. Hartl, J. Electroanal. Chem., 1991,
317, 179.
24 J. G. Young and W. J. Onyebuagu, J. Org. Chem., 1990, 55, 2155.
25 M. J. Crossley, L. G. King and S. M. Pyke, Tetrahedron, 1987, 43,
4569.
26 G. N. La Mar and F. A. Walker, J. Am. Chem. Soc., 1973, 95,
1790.
27 C.-L. Lin, M.-Y. Fang and S.-H. Cheng, J. Electroanal. Chem., 2002,
531, 155.
28 F. D’Souza, A. Villard, E. V. Caemelbecke, M. Franzen, T. Boschi,
P. Tagliatesta and K. M. Kadish, Inorg. Chem., 1993, 32, 4042.
29 J. Metz, O. Schneider and M. Hanack, Inorg. Chem., 1984, 23, 1065.
30 D. Dini, M. Barthel and M. Hanack, Eur. J. Org. Chem, 2001, 3759.
31 K. Ozoemena and T. Nyokong, J. Chem. Soc., Dalton Trans., 2002,
1806.
32 A. Ogunsipe, J.-Y Chen and T. Nyokong, New J. Chem., 2004, 28,
822.
33 G. D. Zheng, Y. Yan, S. Gao, S. L. Tong, D. Gao and K. J. Zhen,
Electrochim. Acta, 1996, 41, 177.
34 J. F. Myers, R. G. W. Canham and A. B. P. Lever, Inorg. Chem., 1975,
14, 461.
35 K. M. Kadish, X. Q. Lin and B. C. Han, Inorg. Chem, 1987, 26, 4161.
36 A. B. P. Lever, E. R. Milaeva and G. Speier, in Phthalocyanines:
Properties and Applications, ed. A. B. P. Lever and C. C. Leznoff,
VCH Publishers, New York, 1993, vol. 3, ch. 1, pp. 1–69.
37 F. A. Walker, D. Beroiz and K. M. Kadish, J. Am. Chem. Soc., 1976,
98, 3484.
Acknowledgements
This work was supported by Rhodes University and the
National Research Foundation (NRF) in South Africa. K. I. O.
thanks Andrew Mellon Foundation for sponsorship as an
Accelerated Development fellow/lecturer.
References
38 C. J. Campbell, J. F. Rusling and C. Bru¨ckner, J. Am. Chem. Soc.,
2000, 122, 6679.
1 S. Gaspard, C. Giannotti, P. Maillard, C. Schaeffer and T. H. Tran-
Thi, Chem. Commun., 1986, 16, 1239.
2 M. O. Liu and A. T. Hu, J. Organomet. Chem, 2004, 689, 2450.
3 M. Lachkar, A. De Cian, J. Fischer and R. Weiss, New J. Chem, 1988,
16, 729.
4 J. Jiang, K. Kasuga and D. P. Arnold, in Supramolecular Photosen-
sitive and Electroactive Materials, ed. H. S. Nalwa, Academic Press,
New York, 2001, p. 113.
¨
¨
39 A. R. Ozkaya, A. G. Gu¨rek, A. Gu¨l and O. Bekarog˘u, Polyhedron,
1997, 16, 1877.
40 A. Wolberg and J. Manassen, J. Am. Chem. Soc., 1970, 2982.
41 C. Araullo-McAdams and K. M. Kadish, Inorg. Chem., 1990, 29,
2749.
42 A. D. Kini, J. Washington, C. P. Kubiak and B. H. Morimoto, Inorg.
Chem., 1996, 35, 6904.
5 D. Chabach, M. Lachkar, A. De Cian, J. Fischer and R. Weiss,
New J. Chem, 1992, 16, 431.
6 D. Chabach, A. De Cian, J. Fischer, R. Weiss and M. M. Bibout,
Angew. Chem., Int. Ed., 1996, 35, 898.
43 M. Kadish, N. Guo, E. V. Caemelbecke, R. Paolesse, D. Monti and
P. Tagliatesta, J. Porphyrins Phthalocyanines, 1998, 2, 439.
44 A. G. Hysop, M. Orphanide, U. Javed and E. G. Megehee, Inorg.
Chim. Acta, 2003, 355, 272.
7 D. Chabach, M. Tahiri, A. De Cian, J. Fischer, R. Weiss and M. M.
Bibout, J. Am. Chem. Soc., 1995, 117, 8548.
8 T. H. Tran-Thi, J. F. Lipskier, M. Simoes and S. Palacin, Thin Solid
Films, 1992, 210–211, 150.
9 L. R. Milgrom, The Colours of Life: An Introduction to the Chemistry
of Poprhyrins and Related Compounds, Oxford Univ. Press, Oxford,
1997.
45 W. A. Nevin, M. R. Hempetead, W. Liu, C. C. Leznoff and A. B. P.
Lever, Inorg. Chem., 1987, 26, 570.
46 W. A. Nevin, W. Liu, S. Greenberg, M. R. Hempstead, S. M.
Marcuccio, M. Melnik, C. C. Leznoff and A. B. P. Lever, Inorg.
Chem., 1987, 26, 891.
47 M. J. Stilman and T. Nyokong, in Phthalocyanines: Properties and
Applications, ed. A. B. P. Lever and C. C. Leznoff, VCH Publishers,
New York, 1989, vol. 1, ch. 3, pp. 133–289.
48 M. J. Stilman, in Phthalocyanines: Properties and Applications, ed.
A. B. P. Lever and C. C. Leznoff, VCH Publishers, New York, 1993,
Vol. 3, Ch. 5, pp. 227–296.
10 M. Biesaga, K. Pyrzynska and M. Trojanowicz, Talanta, 2000, 51,
209.
11 J. Obirai and T. Nyokong, J. Electroanal. Chem., 2004, 573, 77.
12 J. H. Zagal, Coord. Chem. Rev., 1992, 119, 89.
1 2 4 8
D a l t o n T r a n s . , 2 0 0 5 , 1 2 4 1 – 1 2 4 8