Synthesis and electrochemical characterisation of new tantalum (V) alkythio phthalocyanines

Article abstract of DOI:10.1016/j.ica.2010.05.003

The synthesis and electrochemical characterisation of octa-pentylthio (4a) and octa-octylthio (4b) - phthalocyaninato tantalum (III) hydroxide are hereby reported. These TaPc complexes absorb in the near infrared region (～800 nm in dichloromethane). They

Full text of DOI:10.1016/j.ica.2010.05.003

Inorganica Chimica Acta 363 (2010) 3662–3669
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and electrochemical characterisation of new tantalum (V)
alkythio phthalocyanines
*
Vongani Chauke, Tebello Nyokong
Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and electrochemical characterisation of octa-pentylthio (4a) and octa-octylthio (4b) –
phthalocyaninato tantalum (III) hydroxide are hereby reported. These TaPc complexes absorb in the near
infrared region (ꢀ800 nm in dichloromethane). They show good solubility in most common solvents
especially non-viscous solvents such as dichloromethane and chloroform. NMR, mass and infrared spec-
troscopy and elemental analysis confirmed the structures and purity of the synthesised complexes. The
cyclic voltammograms (CVs) showed reversible reduction couples and irreversible oxidation peaks. The
latter exhibited adsorption behavior. The reduction processes were observed at ꢁ0.74 and ꢁ1.13 V (ver-
sus Ag|AgCl) for 4a, and ꢁ0.67, ꢁ1.02 and ꢁ1.48 V (versus Ag|AgCl) for 4b. Spectroelectrochemistry con-
firmed one metal reduction, with the rest of the redox processes being centered on the phthalocyanine
ring.
Received 16 November 2009
Received in revised form 7 April 2010
Accepted 5 May 2010
Available online 12 May 2010
Keywords:
Tantalum phthalocyanine
Cyclic voltammetry
Spectroelectrochemistry
Adsorption
Ó 2010 Elsevier B.V. All rights reserved.
Alkyl thio
1. Introduction
to Fe–N of 1.93 Å in FePc) [22,23], while the phthalocyanine cavity
has an N–N distance of 3.96 Å [22], making it difficult for Ta to fit
Metallophthlocyanines (MPcs) have attracted a lot of attention
for many years since their discovery in the early 1900s. Their
remarkable properties that include flexibility, chemical and ther-
mal stabilities, semiconductivity and photoconductivity [1,2] have
been of great interest in research. Traditionally, phthalocyanines
(Pcs) have been used as dyes and pigments. Currently, the focus
of research on MPc complexes is directed to applications in mate-
rial science [3–5]. These include Pcs as liquid crystals [6–8],
Langmuir–Blodgetts films [9–11], electrochemical sensors [12–14]
and fuel cells [15]. Phthalocyanines with sulfur substituents
generally absorb in the near infrared (NIR) region [16]. Pcs absorb-
ing in the NIR region to match the 780 and 830 nm semiconductor
lasers are used for optical data storage (ODS) whilst for security
applications, Pcs employed cover the 700–1000 nm region
[16–18]. Thiol substituted MPcs may be used to modify gold
electrode surfaces for sensor applications [19–21], where there is
a spontaneous formation of SAMs due to the strong gold–sulfur
interactions. Thus the synthesis of thio derivatized Pc complexes
is of importance for many practical applications.
into the Pc cavity.
This results in highly unsymmetrical MPcs that are somewhat
unstable. The synthesis and X-ray crystal structure of an unsubsti-
tuted TaPc have been reported before [23], and the electrochemical
behavior of unsubstituted TaPc has been reported by our group
[24]. Ring substituted TaPc derivatives are unknown and are re-
ported here for the first time.
This work reports on the syntheses of non peripherally alkyl-
thiol substituted TaPc complexes. MPcs with electron donating
substituents, in particular sulfur substituted MPcs often show
interesting electrochemistry that involves the central metal, the
Pc ring and the substituents on the Pc ring. This has therefore moti-
vated the study in this work.
2. Experimental
2.1. Materials
Acetone, tantalum (V) butoxide, dimethylsulfoxide (DMSO), 1-
pentanethiol, 1-octanethiol, tantalum butoxide, 1,8-diazabicy-
clo{5.4.0}-undec-7-ene (DBU) and 1-pentanol were purchased
from Sigma–Aldrich. Potassium carbonate, ammonium ferrous sul-
fate, deuterated chloroform (CDCl3), 2,3-dicyanohydroquinone (1),
p-toluenesulfonylchloride, tetrahydrofuran (THF), toluene, chloro-
form and dichloromethane (DCM) were procured from Merck. Tet-
rabutylammonium tetrafluoroborate (TBABF4) was purchased from
Sigma–Aldrich.
Even though MPc complexes containing transition metals have
been extensively studied, very little is known about tantalum
phthalocyanines due to synthetic challenges. The difficulty arises
from the fact that Ta–N distances in TaPc are ꢀ2.17 Å (compared
* Corresponding author. Tel.: +27 46 603 8260; fax: +27 46 622 5109.
E-mail address: T.nyokong@ru.ac.za (T. Nyokong).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.05.003

V. Chauke, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3662–3669
3663
2.2. Equipment
3084, 2951, 2930, 2864, 2378(S–C), 2225(C„N), 1444, 1283,
1202, 1181, 1173, 1145, 877, 847, 827, 725, 547, 447.
1H NMR (CDCl3): d, ppm 7.49 (2-H, s, Ar–H), 2.99–3.02 (4-H, t, –
CH2), 1.63–1.71 (4-H, m, –CH2), 1.37–1.46 (4-H, m, –CH2), 1.28–
1.36 (4-H, m, –CH2), 0.88–0.91 (6-H, t, –CH3).
Column chromatography was performed on silica gel 60 (0.04–
0.063 mm) and preparative thin layer chromatography was per-
formed on silica gel 60 P F254. Ground state electronic absorption
spectra were performed on a Varian Cary 500 UV–Vis–NIR spectro-
photometer, infrared spectra (KBr pellets) on Perkin–Elmer Spec-
trum 2000 FT-IR Spectrometer and 1H nuclear magnetic
resonance signals on a Bruker EMX 400 NMR spectrometer. Ele-
mental analysis was performed at Rhodes University using a Var-
io-Elementar Microcube ELIII. MALDI-TOF mass spectrometry was
carried out at the University of Stellenbosch using an ABI Voyager
DE-STR MALDI-TOF instrument.
2.4.3. 3,6-Di(octylthio)-4,5-dicyanobenzene (3b), Scheme 1
Synthesis and purification of 3b were similar to that of com-
pound 3a, except 1-octanethiol was used instead of 1-pentane
thiol. The amounts of reagents employed were: 1-octanethiol
(2.39 g, 16.3 mmol), 3,6-bis(40-methylphenylsulfonyloxy) phthalo-
nitrile (2) (4.30 g, 9.18 mmol), ground anhydrous potassium car-
bonate (5.07 g, 36.7 mmol). Yield: 1.79 g, (58.1%). IR [(KBr)mmax
/
cmꢁ1]: 2920, 2850, 2388(S–C), 2225(C„N), 2023, 1637, 1466,
1422, 1204, 1143, 1032.
2.3. Electrochemical methods
1H NMR (CDCl3): d, ppm 7.40 (2-H, s, Ar–H), 2.98–3.10 (4-H, t, –
CH2), 1.62–1.75 (4-H, m, –CH2), 1.61–1.50 (4-H-broad m, –CH2),
1.40–1.52 (4-H, m, –CH2), 1.21–1.38 (18-H, m, –CH2CH2CH3).
Cyclic (CV) and square wave (SWV) voltammetry experiments
were performed using Autolab potentiostat PGSTAT 302 (Eco Che-
mie, Utrecht, The Netherlands) driven by the General Purpose Elec-
trochemical System data processing software (GPES, software
version 4.9, Eco Chemie), using a conventional three-electrode sys-
tem. A glassy carbon electrode (GCE, 3.0 mm diameter) was used as
the working electrode. Silver–silver chloride (Ag|AgCl) and plati-
num wire were used as pseudo-reference and counter electrodes,
respectively. Electrochemical experiments were performed in dry
DCM containing TBABF4 as the supporting electrolyte. Prior to
scans, the working electrode was polished with alumina paste on
a Buehler felt pad. This was followed by washing with de-ionised
water. Spectroelectrochemical data was recorded using an opti-
cally transparent thin-layer electrochemical (OTTLE) cell which
2.4.4. 1,4,8,11,15,18,22,25-Octapentylthiophthalocyaninato tantalum
(V) butoxide (4a, (OH)3TaOPTPc), Scheme 1
3,6-Dipentylthiophthalonitrile (3a) (0.8 g, 1.20 mmol) in 1-
pentanol (7.0 ml) was refluxed under a nitrogen atmosphere and
tantalum (V) butoxide (0.138 g, 0.34 mmol) was added. After the
addition of DBU (0.30 ml, 0.86 mmol), the reaction was continued
for 6 h. The mixture was cooled and column chromatography over
silica was done with CHCl3 as eluent. Yield: 0.25 g (42%). UV–Vis
(DCM): kmax (nm) (log
e) 276(6.36) 348(5.62) 722(5.44) 814
(5.81). IR [(KBr)
m
max/cmꢁ1]: 3290 (OH), 2922, 2852, 2350, 1561,
1459, 1361, 1310, 1279, 1156, 109, 932, 908(Ta–O), 750 (C–S–C).
1H NMR (400 MHz, CDCl3): d, ppm 7.65 (8H, s, Ar–H), 3.20 (16H,
broad s, S-CH2), 1.89 (16H, quintuplet, CH2), 1.61 (16H, quintuplet,
CH2), 1.50 (16H, sextuplet, CH2), 1.0 (24H, t, CH3). Anal. Calc.
was connected to
voltammograph.
a
Bioanalytical System (BAS) CV 27
2.4. Synthesis
C72H99N8S8O3Ta: C, 55.36; H, 6.38; N, 7.17. Found: C, 55.45; H,
7.21; N, 7.92%. MALDI-TOF MS m/z: Calcd: 1561.52 amu. Found:
(MꢁTa) 1330 amu.
A procedure similar to that reported in literature [25,26] was
employed for the synthesis of phthalonitriles (2 and 3) with slight
alterations as follows.
2.4.5. 1,4,8,11,15,18,22,25-Octaoctylthiophthalocyaninato tantalum
(V) butoxide (4b, (OH)3TaOOTPc), Scheme 1
2.4.1. 3,6-Bis(40-methylphenylsulfonyloxy) phthalonitrile (2),
Scheme 1
Synthesis and purification of 4b were similar to that of com-
pound 4a, except 3b was employed instead of 3a. The amounts
of reagents employed were: 3,6-dioctylthiophthalonitrile (3b)
(0.81 g, 1.20 mmol, 1-pentanol (ꢀ8 ml), tantalum (V) butoxide
(0.139 g, 0.34 mmol), DBU (0.30 ml, 0.86 mmol). Yield 0.31 (48%).
p-Toluenesulfonyl chloride (10.32 g, 27 mmol) was added to a
mixture of 2,3-dicyanohydroquinone (1) (4.04 g, 12.5 mmol) and
potassium carbonate (13.8 g, 50 mmol) in acetone (15 ml). The
mixture was refluxed for 2 h. Thin layer chromatography (TLC)
was performed to determine the consumption of 2,3-dicyanohy-
droquinone. The mixture was cooled to room temperature, poured
to water (40 ml) and stirred for 1 h in water. The light brown prod-
uct was filtered and oven dried to give 2. Yield: 9.51 g (79%) IR
UV–Vis (DCM): kmax (nm) (log
717 (4.41), 814 (5.76). IR [(KBr)
e
) 416(3.82) 662(4.00) 609(3.73)
m
max/cmꢁ1]: 3419 (OH), 2955,
2920, 2850, 2538, 2400, 1637, 1563, 1432, 1368, 1312, 1281,
1223, 1181, 1142, 1091, 1031, 910, (Ta–O), 867, 787, 751 (C–S–
C), 720. 1H NMR (400 MHz, CDCl3): d, ppm 7.51 (8H, s, Ar–H), 3.2
(16H, broad s, S-CH2), 1.90 (32H, quintuplet, CH2), 1.79 (32H, quin-
tuplet, CH2), 1.05 (56H, sextuplet, CH2CH2CH3). Anal. Calc.
[(KBr)
m
max/cmꢁ1]: 3432, 3239, 3085, 2243, 2226 (CN), 1504,
1449, 1315, 1279, 1204, 1174, 1142, 1021, 1004, 979, 934, 847,
749, 694, 638, 614.
C96H147N8S8O3Ta: C, 60.72; H, 7.80; N, 5.90. Found: C, 60.92; H,
8.41; N, 6.42%. MALDI-TOF MS m/z: Calcd: 1898.16 amu. Found:
(MꢁTa) 1666.5 amu.
2.4.2. 3,6-Di(pentylthio)-4,5-dicyanobenzene (3a), Scheme 1
1-Pentanethiol (2.39 g, 22.9 mmol) was dissolved in DMSO un-
der a nitrogen atmosphere and 3,6-bis(40-methylphenylsulfonyl-
oxy) phthalonitrile (2) (4.30 g, 9.18 mmol) was added. The
mixture was stirred for 15 min and finely ground anhydrous potas-
sium carbonate (5.07 g, 36.7 mmol) was added in portions for 2 h
while stirring. The mixture was stirred under a nitrogen atmo-
sphere for a further 12 h. Water was added and the aqueous phase
extracted using chloroform (3 ꢂ 50 ml). The extracts were further
treated with 5% sodium carbonate solution (2 ꢂ 250 ml). The solu-
tion was further treated with water (2 ꢂ 250 ml) and the solvent
was evaporated off using a rotavapor. The product 3a was recrys-
3. Results and discussion
3.1. Synthesis and characterisation
Scheme 1 gives the synthesis pathways for the TaPc complexes
discussed in this work. 3,6-Bis(40-methylphenylsulfonyloxy) pht-
halonitrile (2), was used to prepare 3,6-disubstituted phthalonitri-
le derivatives (3a and 3b), through base-catalysed nucleophilic
aromatic displacement. The reactions were carried out in DMSO
at room temperature and gave yields of 71.7% for 3a and 58.1%
tallised from ethanol. Yield: 2.19 g (71.7%). IR [(KBr)m
max/cmꢁ1]:

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V. Chauke, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3662–3669
OH
SR
OTos
K2 CO3
CN
CN
CN
CN
CN
CN
CH3 C6 H4 SO2 Cl
RSH, DMSO
Reflux, Acetone
2hrs
K2CO3, stir, 12 hrs
OH
SR
OTos
(3)
(1)
(2)
(a)
R =
Pentanol, Ta(OCH2CH2CH2CH3)5,
DBU
(b)
SR
SR
SR
N
SR
SR
N
N
M
N
N
N
N
N
SR
SR
SR
(4)
Scheme 1. Synthesis of tantalum [M = (OH)3TaV for 4a and 4b] non peripherally substituted phthalocyanine. Complexes 5a (M = OVIV [30]) 6a (M = OTiIV [29]) and 7a
(M = AcMnIII [29]). Ac = acetate.
for 3b. The syntheses of TaPc complexes [(OH)3TaOPTPc, 4a and
(OH)3TaOOTPc, 4b] were achieved by treatment of substituted
3,6-dicyanobenzenes 3a and 3b with anhydrous tantalum (V)
butoxide in 1-pentanol in the presence of DBU (Scheme 1). DBU
acts as a nucleophilic base which allows the reaction to proceed
under mild conditions while preventing the formation of side
products typical of reactions that use strong bases [27]. Tantalum
is a large metal that does not fit perfectly in a phthalocyanine core
center. It therefore needs high energy to insert it into the phthalo-
cyanine ring. Thus, a solvent with a high boiling point, 1-pentanol,
was used to achieve this purpose. Column and preparative thin
layer chromatography with silica gel were employed to obtain
the pure products from the reaction mixtures. The synthesised
complexes [(OH)3TaOPTPc, 4a and (OH)3TaOOTPc, 4b] show good
solubility in organic solvents such as dichloromethane, chloroform
and chloronaphthalene. The tantalum derivatives were character-
ized by UV–Vis, IR, mass and NMR spectroscopies. The analyses
are consistent with the predicted structures as shown in the exper-
imental section. After conversion into TaPc complexes, the charac-
teristic C„N stretch at 2225 cmꢁ1 of 1,2-dicyanobenzenes 3a and
3b disappeared, indicative of metallophthalocyanine formation.
The formation of (OH)3TaPc (containing OH instead of butoxy
Fig. 1. 1H NMR spectrum of complex 4a.

V. Chauke, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3662–3669
3665
Table 1
groups) could be a result of the purification process. The presence
OH was proved by infrared spectroscopy. C–S–C vibrations were
observed at ꢀ750 cmꢁ1 [28].
UV–Vis spectra (Q bands only) of MPc derivatives (4a, 4b, 5, 6 and 7) derivatives in
various solvents.
The substituted TaPc derivatives were found to be pure by 1H
NMR with all the substituents and ring protons observed in their
respective regions, Fig. 1. This figure shows a clean 1H NMR spec-
trum confirming the purity of the complex. The pentylthio substi-
tuted complex 4a showed the resonances belonging to ring protons
as a singlet at 7.65 ppm. Similarly, the octylthio substituted com-
plex 4b showed the resonances belonging to ring protons as a sin-
glet at 7.51 ppm. Mass spectral studies confirmed the loss of Ta
since as already stated Ta is a large metal and it does not fit prop-
erly into the cavity of a phthalocyanine core. Ta ion is vulnerable to
displacement if exposed to harsh conditions such as during ionisa-
tion in mass spectrometry hence the observed mass. However, ele-
mental analyses unequivocally confirmed the formation of the
complexes.
Complex
Solvent
Q band (nm)
Ref.
4a
Toluene
THF
DCM
1-CNP
CHCl3
795
804
808
810
809
This work
4b
Toluene
THF
DCM
1-CNP
CHCl3
794
800
812
815
809
This work
TaPc
CHCl3
697
25
29
5a (M = OVIV
)
Toluene
THF
DCM
830
834
850
841
CHCl3
The ground state electronic absorption spectra [(OH)3TaOPTPc,
4a and (OH)3TaOOTPc, 4b] show sharp Q bands (Fig. 2a and b), typ-
ical of unaggregated MPc complexes Beer’s law was obeyed for
6a (M = OTiIV
7a (M = AcMnIII
)
DCM
DCM
774
908
28
28
a
)
concentrations ranging from 1.85 ꢂ 10ꢁ7 to 2.28 ꢂ 10ꢁ5 mol dmꢁ3
.
a
Ac = acetate.
The Q bands were observed at 808 and 812 nm in DCM for
[(OH)3TaOPTPc and (OH)3TaOOTPc], respectively.
The Q bands were relatively red-shifted when compared to
unsubstituted TaPc [24] (Q band at 697 nm chloroform), Table 1.
This is typical of MPcs with substituents on the non-peripheral po-
sition and also characteristic of MPc containing sulfur substituents.
Considering the same solvent (CHCl3) and the same substituent,
MnPc derivative (7a) [29] shows the most red shifted Q band, Table
1 (Fig. 3), followed by OVPc derivative [30]. Fig. 3 shows that the
red shift is related to some extend on the ionic radii of the central
metal ion, with tantalum being an exception.
The Q band is more red shifted in chlorinated solvents: CHCl3, 1-
CNP and DCM. It has been reported before that there is more red
(a)
Fig. 3. Relationship between the ionic radii of central metal ions and the red-shift of
Q bands (Q band values ion chloroform).
shifting in chlorinated solvents [31]. The reported [32,33] trend
based on the solvents’ refractive index is not observed in Table 1.
3.2. Voltammetric and spectroelectrochemical characterisation
The cyclic [CV(a)] and square wave [SWV(b)] voltammetries for
TaPc complexes 4a and 4b are shown in Figs. 4 and 5, respectively.
These were performed in de-aerated DCM with TBABF4 as the sup-
porting electrolyte. The oxidation of these complexes was expected
to occur at the ring and this was confirmed by spectroelectrochem-
istry as discussed below. For unsubstituted TaPc complex no oxida-
tion peaks were observed [24] due to the fact that TaV has a high
oxidation state, which makes ring oxidation more difficult. The
electro-rich alkylthio substituents in this work contributed to the
ease of oxidation of the ring, since these substituents enhance elec-
tron density on the ring thus making it easy for oxidation to occur.
Hence oxidation peaks are observed.
(b)
(i)
(iv)
(ii)
(v)
(iii)
The cyclic and square wave voltammograms of complex 4a,
Fig. 4, show two reduction processes labelled III and IV at half-
wave (E1/2) potentials of ꢁ0.74 and ꢁ1.13 V versus Ag|AgCl, respec-
tively. The ratio of anodic to cathodic current (Ipa/Ipc) was near
unity for the two couples. The cathodic and anodic peak separa-
350
450
550
650
750
850
Wavelength nm
tions (DE) of 70 mV (similar to that of the ferrocene standard) for
couples, III and IV, suggests reversibility due to fast electron trans-
fer process. For all the reduction couples, the plots of peak currents
Fig. 2. Ground state electronic absorption spectra of TaPc complexes 4a (a) and 4b
(b) in (i) CHCl3, (ii) chloronaphthalene, (iii) DCM, (iv) THF, (v) toluene.
ꢀConcentrations = 1 ꢂ 10ꢁ5 M.

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V. Chauke, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3662–3669
(a)
(b)
1 μA
(I)
(II)
(III)
(ΙV)
-1.5
-1
-0.5
0
0.5
1
1.5
E/V (vs. Ag|AgCl)
(c)
1 μA
1 μA
-1.5
-1
-0.5
0
0
0.2 0.4 0.6 0.8
E/V (vs. Ag|AgCl)
1
1.2
E/V (vs. Ag|AgCl)
Fig. 4. Square wave (a), cyclic (b) voltammograms for complex 4a in DCM containing 0.1 M TBABF4. (c) Cyclic voltammgrams observed when recording from 0 V to negative
or positive potentials. Scan rate = 100 mV/s.
(ip) versus the square root of scan rate (
m
1/2) were linear suggesting
Three reduction processes labelled III, IV and V were observed
at E1/2 of ꢁ0.67, ꢁ1.02 and ꢁ1.48 V versus Ag|AgCl, respectively,
for complex 4b, Fig. 5. The anodic and cathodic peak separations
that the processes were diffusion controlled. Compared to other
octa pentyl-thio alpha substituted MPc complexes in Table 2, 4a
(Ta) and 6a (Ti) are more difficult to reduce. The first reduction
in all of the complexes containing the pentylthio substituent at
the non-peripheral position of the Pc ring (4a to 7a) occurs at the
central metal. The second reduction occurs at the ring in all com-
plexes except for OTiPc derivative (6a).
The oxidation of 4a consists of irreversible processes with sharp
peaks due to adsorption processes, Fig. 4. Process II has a broad
pre-peak followed by a sharp peak typical of adsorption [34,35].
It is known [34] that the adsorption of a product of the electrode
reaction results in the formation of a sharp peak preceded by a
more regular peak, the latter being due to the diffusion of the elec-
troactive species from solution to the electrode. On return scan, a
sharp peak appeared for process II, Fig. 4b. Smaller mainly irrevers-
ible oxidation peaks are observed at larger oxidation potentials
(process I). The irreversibility of oxidation processes and the
adsorption behavior was also reported for OTiPc complex 6a [30].
The SWV shows a very broad oxidation feature. Table 2 shows that
in general, irreversible oxidations are observed for octapentylthiol
derivatives. The irreversibility of alkyl or arylthio substituted MPc
complexes in general is well documented [36,37]. There was no
change in the shape oxidation processes when the potential was
scanned from 0 V to negative or positive potentials, Fig. 4c.
(DE) for couples III was 60 mV, suggesting reversibility. The reduc-
tion currents for III and IV are larger than for V, suggesting the pos-
sibility of different number of electrons transferred. However,
chronocoulometric analysis did not show significant differences
in the number of electrons involved. The lower currents for V could
be a result of the fact that this couple is at high negative potentials
and hence near the limits of the electrode/electrolyte system,
which may affect this peak. The effects of the limits of the system
are clearer on the SWV.
Comparing the reduction potentials of 4b with those of 4a,
shows that the former has potentials at less negative values, result-
ing in the shift of couple V to less negative values, hence to be lo-
cated within the potential window in Fig. 5.
In order to further determine the nature of the redox processes
for complex 4a and 4b, spectroelectrochemistry was performed in
an optically transparent thin-layer electrochemical (OTTLE) cell.
Fig. 6a shows the observed spectral changes for complex 4a at
potentials more negative of process III (ꢁ0.8 V versus Ag|AgCl) in
DCM containing TBABF4 as a supporting electrolyte. Upon reduc-
tion, the spectral changes observed include a shift of the Q band
from 808 to 770 nm, with a slight decrease in the intensity of the
Q band. There was also an emergence of new peaks at ꢀ450 nm

V. Chauke, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3662–3669
3667
0.8
(a)
0.6
0.4
0.2
0
300
400
500
600
700
800
900
Wavelength nm
(b)
Fig. 5. Square wave (i) and cyclic (ii) voltammograms for complex 4b in DCM
containing 0.1 M TBABF4. Scan rate = 100 mV/s.
(c)
and a slight change of colour of the solution in the OTTLE cell from
purple to blue. Diffuse isosbestic points were observed suggesting
the presence of more than one species in solution. The lack of clear
isosbestic points could be a result of change of axial ligands during
the reduction process. In related Ta porphyrin derivatives, axial li-
gand exchange reactions readily occur in slightly acid media [38].
The spectral changes observed in Fig. 6 consist of a shift in the Q
band to shorter wavelengths, without the drastic decrease in inten-
sity. These spectral changes are typical of redox processes occur-
ring at the central metal [22], whereby the Q band shifts without
a drastic decrease in intensity. The n value calculated for reduction
of 4a at potentials of process III processes was approximately ꢀ1
using Eq. (1):
Fig. 6. Observed UV–Vis spectral changes for complex 4a at potentials (a) ꢁ0.8 V
(process III), (b) ꢁ1.2 V (process IV) and (c) 1.1 V (process II), in DCM with 0.1 M
TBABF4. Solid red line; before and dotted red line; after application of the
appropriate potential. The last scan in (a) is the same as the first scan in (b). (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
Q ¼ nFCV
ð1Þ
where n is the number of electrons involved; F, Faraday’s constant;
C, solution concentration and V, volume of the solution.
From the discussion above, process III can be associated with
the reduction TaVPcꢁ2 to TaIVPcꢁ2. Further reduction at potentials
more negative of process IV (ꢁ1.2 V versus Ag|AgCl), Fig. 6b, re-
sulted in a decrease in the intensity (without shifting in position)
of the Q band and the appearance of new peaks between 500 and
650 nm together with diffuse isosbestic points near 665 and
Table 2
Electrochemical data (half-wave potential, E , unless otherwise stated) for MPc derivatives (4a, 4b, 5, 6 and 7) in DCM containing TBABF4. Potentials vs. Ag|AgCl.
½
a
b
Redox process (E
)
4a
4b
5a (M = OVIV
ꢁ0.58
)
6a (M = OTiIV)b
7a (M = AcMnIII
ꢁ0.46
)
½
Reduction 1
ꢁ 0.74
ꢁ0.67
ꢁ0.73
TaVPcꢁ2/TaIVPcꢁ2
TaVPcꢁ2/TaIVPcꢁ2
VIVPcꢁ2/VIIIPcꢁ2
(TiIV Pcꢁ2/TiIIIPcꢁ2
)
(MnIII Pcꢁ2/MnIIPcꢁ2
)
Reduction 2
Reduction 3
ꢁ1.13
ꢁ1.02
ꢁ0.93
ꢁ1.09
ꢁ1.24
TaIVPcꢁ2/TaIVPcꢁ3
TaIVPcꢁ2/TaIVPcꢁ3
VIIIPcꢁ2/VIIIPcꢁ3
TiIIIPcꢁ2/TiIIPcꢁ2
(MnIIPcꢁ2/MnIIPcꢁ3
)
ꢁ1.48
ꢁ1.18
TaIVPcꢁ3/TaIVPcꢁ4
VIIIPcꢁ3/VIIIPcꢁ4
Oxidation processes
0.37, 0.57 (EP)
0.47, 0.59, 0.99 (EP)
0.59
0.54, 0.36, 0.95
0.47 (MnIV
VIV Pcꢁ2/VIVPcꢁ1
0.97
Pcꢁ2/MnIIIPcꢁ2
0.75
)
VIV Pcꢁ1/VIVPc0
(MnIVPcꢁ1/MnIV Pcꢁ2
)
a
Data from Ref. [29].
Data from Ref. [30], Ac = acetate.
b

3668
V. Chauke, T. Nyokong / Inorganica Chimica Acta 363 (2010) 3662–3669
490 nm. These changes are typical of ring reduction processed [39],
i.e. TaIVPcꢁ2 is reduced to TaIVPcꢁ3. The regeneration (to ꢀ72%) of
the initial species was achieved, confirming the reversibility of
the couples. Application of the potentials more positive of process
II, resulted in spectral changes shown in Fig. 6c consisting of a de-
crease in the Q band with no new peaks forms indicative of the
degradation of the complex confirming irreversibility of the oxida-
tion processes. However, the oxidation peaks are attributed to the
Pc ring and the substituents, since no oxidation is expected on the
central metal.
Oxidation is expected to occur only on the ring for these
complexes.
Based on the electrochemical observations described above the
assignments for processes I–IV for complex 4a are as follows:
TaVPcꢁ1 ! TaVPc0 þ eꢁ
TaVPcꢁ2 ! TaVPcꢁ1 þ eꢁ
TaVPcꢁ2 þ eꢁ ! TaIVPcꢁ2
TaIVPcꢁ2 þ eꢁ ! TaIVPcꢁ3
ðProcess IÞ
ðProcess IIÞ
ðProcess IIIÞ
ðProcess IVÞ
Similarly, spectroelectrochemical experiments were performed
for the further investigation of the redox processes observed for
complex 4b. Fig. 7a shows the observed spectral changes for com-
plex 4b, at potentials more negative of process III (ꢁ0.7 V versus
Ag|AgCl). The changes consisted of a shift of the Q band from
814 to 765 nm, with an increase in the intensity, the emergence
of new peaks at ꢀ460 nm and a change of colour of the solution
in the OTTLE cell from purple to blue were observed. Again isos-
bestic points were diffuse, due to possible axial ligation as dis-
cussed above for 4a. The observed changes are typical of redox
processes occurring at the central metal [22] as discussed for 4a.
Therefore, process III can be attributed to the reduction TaVPcꢁ2
to TaIVPcꢁ2. Further reduction at potentials of process IV (ꢁ1.1 V
versus Ag|AgCl), Fig. 7b, resulted in a decrease in the intensity of
the Q band and the appearance of new absorptions in the
600 nm region, together with clear isosbectic points at 670 nm.
These spectral changes are typical of ring reduction processed
[39], i.e. TaIVPcꢁ2 is reduced to TaIVPcꢁ3. Again, the regeneration
of the initial species of ꢀ69% was achieved, confirming some
reversibility of the couples. Process IV is attributed to further ring
reduction.
Similarly, processes for complex 4b are assigned as follows:
TaVPcꢁ2 ! TaVPcꢁ0 þ eꢁ
TaVPcꢁ2 ! TaVPcꢁ1 þ eꢁ
TaVPcꢁ2 þ eꢁ ! TaIVPcꢁ2
TaIVPcꢁ2 þ eꢁ ! TaIVPcꢁ3
TaIVPcꢁ2 þ eꢁ ! TaIVPcꢁ4
ðProcess IÞ
ðProcess IIÞ
ðProcess IIIÞ
ðProcess IVÞ
ðProcess VÞ
The multi-electron transfer process which was observed for the
unsubstituted TaPc is not observed in this work [24] for complexes
4a and 4b, probably due to differences in geometry.
4. Conclusion
The synthesis of alkylthio substituted TaPc derivatives was suc-
cessfully carried out. These complexes were relatively red shifted,
absorbing at wavelengths over 800 nm in CHCl3, due to the pres-
ence of alkylthio substituents at the non-peripheral positions.
The reduction couples for both complexes 4a and 4b were revers-
ible, however, oxidation processes are irreversible in most cases.
Spectroelectrochemistry confirmed one metal reduction, with the
rest of the redox processes being centered on the phthalocyanine
ring.
Acknowledgements
This work was supported by the Department of Science and
Technology (DST) and National Research Foundation (NRF), South
Africa through DST/NRF South African Research Chairs Initiative
for Professor of Medicinal Chemistry and Nanotechnology as well
as Rhodes University.
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