Synthesis and structural characterization of 2,3,7,8,12,13,17,18- octakis(propyl), N, N, N′, N′-tetramethylaminoporphyrazines and 2,3,9,10,16,17,23,24-octa substituted phthalocyanine and the kinetic studies of their Co(II) and Cu(II) metalated complexes

Article abstract of DOI:10.1016/j.dyepig.2014.05.013

Three tetrapyrrole macrocyclic compounds 2,3,7,8,12,13,17,18- octakis(propyl)porphyrazine, N, N, N′, N′-tetramethylamino porphyrazine hybrid and 2,3,9,10,16,17,23,24-octa substituted phthalocyanine were synthesized and characterized using elemental analys

Full text of DOI:10.1016/j.dyepig.2014.05.013

Accepted Manuscript
Synthesis and structural characterization of 2,3,7,8,12,13,17,18-octakis(propyl), N,
N, N’, N’-tetramethylaminoporphyrazines and 2,3,9,10,16,17,23,24-octa substituted
phthalocyanine and the kinetic studies of their Co(II) and Cu(II) metalated complexes
David Isabirye , Fanyana Mtunzi , Temitayo Aiyelabola
PII:
S0143-7208(14)00193-4
10.1016/j.dyepig.2014.05.013
DYPI 4385
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 11 March 2014
Revised Date: 29 April 2014
Accepted Date: 9 May 2014
Please cite this article as: Isabirye D, Mtunzi F, Aiyelabola T, Synthesis and structural characterization
of 2,3,7,8,12,13,17,18-octakis(propyl), N, N, N’, N’-tetramethylaminoporphyrazines and
2,3,9,10,16,17,23,24-octa substituted phthalocyanine and the kinetic studies of their Co(II) and Cu(II)
metalated complexes, Dyes and Pigments (2014), doi: 10.1016/j.dyepig.2014.05.013.
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ACCEPTED MANUSCRIPT
•
Three tetrapyrrole macrocyclic compounds 2,3,7,8,12,13,17,18-octakis(propyl)porphyrazine 1a, N, N, N’,
N’-tetramethylamino porphyrazine hybrid 2a and 2,3,9,10,16,17,23,24-octa substituted phthalocyanine 3a
were synthesized and characterized using elemental analysis, FTIR, H, C NMR and UV-Vis spectroscopic
techniques.
1
13
•
•
Kinetics of their metalation with Co(II) and Cu(II) and redox reaction of the complexes was studied and
reported for the first time.
It was suggested that for the incorporation of the metal ion, deformation of the ring was essential for
effective coordination; and this was a function of the metal ion size and peripheral functionalities of the
ligands.
•
•
The spectroelectrochemical studies for the redox reactions indicated metal based oxidation for the reductants
and ring based reduction for the oxidants. An outer sphere electron tunneling mechanism was proposed for
the redox reaction.
It further showed that the redox activity of the metal ions was dependent on the availability of their low
energy d-orbitals. It was therefore inferred that the nature of the metal ion and the peripheral substituents of
these tetrapyrrole macrocyclic compounds and their complexes had a significant effect on their reactivity.
RO
R
R
OR
N
O
O
O
O
OR
N
N
N
M
N
N
N
M
N
N
N
N
M
N
RO
N
N
N
N
N
N
OR
N
N
N
N
N
N
OR
RO
O
O
OR
R=
(1a)
(2a)
R=N(Me)
2
(3a)

ACCEPTED MANUSCRIPT
Synthesis and structural characterization of 2,3,7,8,12,13,17,18-octakis(propyl), N, N, N’,
N’-tetramethylaminoporphyrazines and 2,3,9,10,16,17,23,24-octa substituted
phthalocyanine and the kinetic studies of their Co(II) and Cu(II) metalated complexes
a*
b
a
David Isabirye , Fanyana Mtunzi and Temitayo Aiyelabola
a
Department of Chemistry, North–West University, Private Bag X 2046. Mmabatho 2735, South Africa.
Department of Chemistry, Vaal University of Technology, Private Bag X021, Vanderbijlpark 1900, South Africa.
b
Abstract
Three tetrapyrrole macrocyclic compounds 2,3,7,8,12,13,17,18-octakis(propyl)porphyrazine, N, N, N’, N’-
tetramethylamino porphyrazine hybrid and 2,3,9,10,16,17,23,24-octa substituted phthalocyanine were synthesized
1
13
and characterized using elemental analysis, FTIR, H, C NMR and UV-vis spectroscopic techniques. Kinetics of
their metalation with Cu(II) and Co(II) and redox reaction of the complexes was studied and are reported here for the
first time. It is suggested that deformation of the ring, which is a function of their peripheral functionalities, is
essential for effective coordination of the ligands. The kinetic studies indicated metal based oxidation and ring based
reduction for the reductants and oxidant respectively. The redox activity of the metal ions is dependent on the
availability of their low energy level d-orbitals for charge transfer. It was therefore inferred that the nature of the
metal ion and the peripheral substituents of these ligands and their complexes had significant effect on their
reactivity.
Keywords: Porphyrazine hybrid, porphyrazine, phthalocyanine, metalation kinetics, redox reaction
*
Corresponding author. Tel: +27 18 3892308; Fax,+27 18 3892052; E.mail address: David.Isabirye@nwu.ac.za
1

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1
.0
Introduction
The synthesis, properties and application of porphyrazines (Pzs) and metaloporphyrazines (MPzs) have received
increasing attention in recent years [1]. This is due to their varied potential in technological applications such as
sensors and photocatalyst for energy conversion processes [2, 3]. They have also found usage as photosensitizers in
photodynamic therapy and as biomedical imaging agents [1]. This has been encouraged partly because of their
effective absorbance in the blue region of the visible spectrum, low cost of synthesis and lack of toxicity [4, 5].
Tetrapyrrole macrocycles porphyrins (Ps), porphyrazines (Pzs) and phthalocyanines (Pcs) are isoelectronic. They can
thus be viewed as variants of each other; hence their reactivity may therefore be compared. Unlike the other two
macrocycles however porphyrazines have been less studied [1, 6-8].
Peripheral functionalization of Pzs has resulted in the significant modulation of their physical and chemical properties
[
1, 6, 8]. This functionalization can be either symmetrical or unsymmetrical. Unsymmetrical peripheral
functionalization includes the porphyrazine hybrid which combines the electronic character and extended π electron
system of porphyrazines and phthalocyanines [6, 9-11]. Another way in modifying the properties of these Pzs is the
use of various transition metal ions incorporated in their inner core [1]. As a consequence, an investigation of the
relationship involving their chemical structure, the kinetics of formation and redox reactions of MPzs is important.
This is because it may provide useful understanding in designing synthetic pathways to new compounds and also
provide information on the mechanism of the reactions. Furthermore, such studies would also yield information
essential for understanding their implementation in many technological areas such as electrophotography,
photovoltaic cells, fuel cells and electrochromic displays [8].
In contrast to the metaloporphyrins (MPs) the mechanism of formation for MPcs and MPzs is yet to be exhaustively
studied. Focus has been on their synthesis, characterization and electrochemistry [6, 8]. In this regard the aim of the
present study was to synthesize symmetrical porphyrazine 1a, porphyrazine hybrid 2a and phthalocyanine 3a
derivatives and study the kinetics of incorporation of copper(II) and cobalt(II) ions into the central cavity of the
synthesized ligands to give 1b, 2b, 3b, 1c, 2c and 3c respectively. In addition, study the redox reaction of their
complexes, and then propose the mechanism for both reactions.
2
.0
.1
Material and methods
General
2
All solvents used were purified according to standard literature techniques and the reactions were done under
nitrogen atmosphere. Flash column chromatography was carried out on Merck Kieselgel 60 (230-400 mesh) under
2

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nitrogen pressure. Thinlayer chromatography was performed using Merck 60 F254 silica gel sheets. NMR spectra were
recorded on a Varian Gemini 300 MHz spectrometer in CDCl . Electronic spectra were measured on a Varian Cary
3
5
0 ultraviolet-visible spectrophotometer, using a matched pair of 1 cm path length quartz cuvettes, measurements
were made from 300 nm to 800 nm. Infra-red spectra was recorded on a Nicolet 410 impact fourier transform infrared
-1
-1
spectrophotometer using nujol or potassium bromide cells, in the range 4000 cm and 500 cm . Compounds 4-6 and
8
-10 were prepared according to published procedures, [9, 12].
2
.2
Synthesis of the compounds
2
.2.1
Synthesis of 2,3,7,8,12,13,17,18-octakis(propyl)porphyrazinato magnesium(II) 1d.
Compound 1d was prepared according to established method (Figure 1) [6]. Butanol (20 ml), magnesium (0.077 g,
.17 mmol) and iodine crystals were refluxed for 24 hours after which 2,3-dipropylmaleonitrile, 7 (0.300 g, 0.831
3
mmol) was added to the suspension of magnesium butoxide and the reaction mixture refluxed for the 24 hours away
from direct light. The brownish purple solution obtained was evaporated and the residue was re-dissolved in ethyl
acetate. It was then purified using flash column chromatography with ethyl acetate: dichloromethane (1:10) solvent
-1
system. Yield = 92 mg, 0.136 mol; %Yield = 4.3; R = 0.61, ethyl acetate: dichloromethane (1:10). IR ⱱ (cm ): 3395
f
1
(
C-H) aromatic, 2928-2851 (CH , CH ), 1643 (C=C), 1379 (C-H), 470 (N-Mg). Uv-vis λ (nm): 345, 550, 600. H
2
3
max
13
NMR (300 MHz, CDCl ) δ1.35 (t, 24H, J=7.5Hz), 2.52 (hp, 16H, J=7.4Hz), 4.06(t, 16H, J=7.5Hz); C NMR (300
3
MHz, CDCl ) δ 15.20, 26.22, 28.84, 30.08, 30.59, 122.78, 123.17, 123.50, 123.83, 134.81, 135.17, 135.50, 135.82,
3
1
2
43.92, 149.47, 149.83, 150.19, 158.82. Anal Calcd: C, 68.05; H, 7.31; N, 19.30. Found: C, 68.93; H, 7.28; N, 20.00.
.2.2 Synthesis of 2,3,7,8,12,13,17,18-octakis(propyl)porphyrazine 1a
Removal of magnesium from 1d (0.58 g) was accomplished by heating it at reflux in a CHCl /TFA (20:1) solution
3
(
200 ml), which was made basic by the addition of NaOH. The organic layer was separated, washed once with 10%
aqueous NaOH (50 ml), anhydrous Na SO and filtered. 1a was adsorbed onto a silica gel, column chromatographed
2
4
and eluted with CH Cl /Hexane (1:1). Slow evaporation of the solvent gave a microcrystalline solid, which was
2
2
recrystallized by slow diffusion of benzene into a CHCl solution of the free base Yield = 0.48g, %Yield = 0.827, R
3
f
-1
=
0.413 (Hexane: ethyl acetate, 10:1). IR ⱱ (cm ): 3021 (N-H), 2928-2851 (CH , CH ), 1209 (C=C), 723 (C-H). Uv-
2 3
1
vis λmax (nm): 339, 558, 626. H NMR (CDCl ) δ = -2.11 (s, 2H, NH), 1.86 (t, 24H, J = 7.7 Hz, CH ), 3.98 (q, 16H, J
3
3
=
7.7 Hz, CH CH CH ). Anal Calcd: C, 71.74; H, 7.97; N, 20.29. Found: C, 71.68; H, 8.01; N, 20.28.
2 2 3
3

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2
.2.3
Synthesis of N, N, N’, N’-tetramethylamino porphyrazine hybrid 2a
The synthesis of 2a was achieved by a modification of previously reported method by Forsyth et al. (Figure 2) [9].
Butanol (30 ml) and magnesium (0.210 g, 8.64 mmol) were refluxed for 24 hours. This was followed by addition of
bis(dimethylamino) maleonitrile, 11 (0.270 g, 1.642 mmol) in butanol (5 ml) and 4.7-(isopropyloxy)-1,3-
iminoisoindoline, 12 (1.230 g, 4.701 mmol) in butanol (5 ml) in suspension of magnesium butoxide. The reaction
mixture was refluxed for the 24 hours away from direct light. After which the brownish purple solution obtained was
evaporated under reduced pressure and the residue was re-dissolved in dichloromethane (60 ml) followed by addition
of trifluoroacetic acid (20 ml). The resultant product was vigorously stirred for 2 hours in an ice-bath. After 2 hours
the reaction mixture was poured into ice cold concentrated ammonium hydroxide (50 ml) and stirred for 30 minutes.
After this, the aqueous and organic layers were separated. The aqueous layer was further extracted with
dichloromethane (3 x 150 ml) and the two organic layers were mixed and dried with anhydrous sodium sulphate. The
sodium sulphate was filtered off and the solvent evaporated under reduced pressure. The sample mixture was purified
by flash column chromatography packed with silica gel using ethyl acetate: dichloromethane: toluene (1:8:6) solvent
system as eluent. Three components were separated as pure fractions. The bulk sample was re-separated using a
mixture of ethyl acetate: dichloromethane: toluene (0.5: 1: 4) as eluent on flash silica. The major product 2a was
-1
separated as pure. Yield = 97 mg, 0.108 mmol, %Yield = 6.6. IR ⱱ (cm ): 3508 (N-H), 1593 (C=C), 1265 (C-H),
1
1
040 (C-H) Uv-vis λmax (nm): 325, 556,742. H NMR (300MHz, CDCl ) δ: (s, 6H), 4.95 (m, 1H), 5.10 (m, 1H), 5.17
3
13
(
m, 1H), 7.37 (d, 2H, J=9Hz), 7.46 (d, 2H, J=8.7Hz); C (300MHz, CDCl ) δ 22.71, 22.78, 30.97, 44,57, 70.77,
3
7
1
5.24, 75.53, 114.50, 122.58, 124.26, 125.06, 128.54, 128.77, 130.84, 131.02, 135.05, 140.73, 140.80, 148.63,
50.48, 150.97. Anal Calcd: C, 64.49; H, 7.29 N, 6.47. Found: C, 65.63; H, 7.88; N, 6.73.
2
1
.2.4
Synthesis of 2,3,9,10,16,17,23,24-octa substituted phthalocyanine 3a
-3
,2-Diisocyano-4-phenoxy benzene, 13 (0.300 g, 1.36 x 10 mol) was refluxed in 1-pentanol, 14 (14 ml). The
resulting product 15 was refluxed further in the presence of 1 mol 1,8-Diazabicyclo[5.4.0]-undec-7-ene (0.21 g, 1.36
-3
x 10 mol) for 16 hours (Figure 3). Methanol (20 ml) was there after added and the precipitate 3a filtered, soxhlet
extracted with methanol and acetone and dried at 60˚ C. Yield = 0.294g, %Yield = 43 R =0.23 (ethyl acetate: hexane,
f
-
1
3
6
4
:8). IR ⱱ (cm ): 3620 (N-H), 2848 (C-H) aromatic, 1685 (C=C), 1163 (C-H), 743 (C-H). Uv-vis λ (nm): 337,
max
1
06, 640, 661, 702. H NMR (300MHz, CDCl ) δ -2.01 (s, 2H, NH), 3.66 (m, 4H, CH), 6.99 (m, 4H, ArH), 7.23 (m,
3
13
H, ArH), 7.50 (m, 4H, ArH) 8.04 (m, 4H, ArH), 8.99 (m, 4H, ArH), 9.23 (m, 4H, ArH), C NMR (300MHz,
4

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CDCl ) δ 71.01, 76.57, 76.99, 77.41, 117.45, 119.69, 119.92, 126.95, 127.48, 127.63, 128.53, 128.85, 128.94, 134.43,
3
1
35.20. Anal Calcd: C, 76.21; H, 4.19; N, 9.03. Found: C, 76.21; H, 4.18; N, 9.11.
2
.2.5
Synthesis of the complexes
The synthesis of the complexes of 1a, 2a and 3b was carried out by an adaptation of established method as described
by Forsyth et al. [9]. 0.01M solution of appropriate metal salts of cobalt(II) acetate and copper(II) acetate was
dissolved in ethanol and added to 0.015 M solution of 1a, 2a and 3b in dichloromethane with stirring until a uniform
solution was achieved. The mixture was heated for 2 h under nitrogen controlled environment. Immediate precipitate
was obtained for majority of the complexes. The solvents were removed using a rotary evaporator and the products
obtained filtered, washed and dried in vacuo at 60 °C.
2
.3
Kinetics studies
2
.3.1
Metalation
The reactions were monitored spectrophotometrically by a Varian Cary 50 ultraviolet-visible spectrophotometer with
a thermostated cell compartment maintained at 25 °C ± 0.1. Metalation of the ligands was carried out in 1 cm cells by
mixing solutions of the ligands and that of the corresponding metal ion solution, copper acetate or cobalt acetate, pre-
equilibrated at the reaction temperature. The Uv-vis absorption spectrum was scanned from 300 to 800 nm and
readings were taken at constant wavelength. The absorption spectra changed as a function of time with distinct
isobestic points until completion of the reaction. The kinetics was run under pseudo-first order conditions with the
metal ion concentration at a large excess over that of the corresponding free base ligand.
2
.3.2
Redox
Solutions of 1c, 2c and 3c in dichloromethane were allowed to equilibrate at 25 ° C under nitrogen. Solutions of 1b in
glacial acetic acid under nitrogen was also prepared and allowed to equilibrate at 25 °C. The solutions were mixed
and quickly transferred to a 1 cm quartz cell in a thermostated compartment of the Cary 50 spectrophotometer. The
Uv-vis absorption spectrum from 300 to 800 nm was scanned and readings were taken. The absorption spectra
changed as a function of time with distinct isobestic points until completion of the reaction.
5

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3
3
.0
.1
Results and Discussion
Characterization of compounds
1
13
The structure of the compounds was confirmed using elemental analysis, FTIR, H, C NMR and UV-vis
spectroscopic techniques. The elemental analysis results obtained are in close agreement with the calculated values.
Spectral assignment of the compounds was achieved by the comparison of the spectra of the compounds with those of
their starting material and that of similar compounds.
3
.1.1
2,3,7,8,12,13,17,18-octakis(propyl)porphyrazine 1a
1
The H NMR spectrum of 1a displayed a broad peak at -2.11 ppm diagnostic of the N-H protons in its inner core.
Similar peaks have been reported to be indicative of the shielding of the pyrrolenic nitrogen atoms of porphyrazines
[
13, 14]. Consequently confirming the formation of the macrocycle and hence the demetalation of 1d. Evidence of
the cyclotetramerization of the dinitrile was given by the absence of the C≡N band in this spectrum. This was
however observed at 3.00 ppm in the spectrum of 7, dipropylmalenitrile, one of the starting material [15]. A triplet
observed at 1.86 ppm was assigned to the methyl protons of the propyl substituents. The triplet at 3.98 ppm
corresponds to protons attached to carbon of the propyl substituent linked to the pyrrole rings CH CH CH . The
2
2
3
-1
infrared spectrum of 1a on the other hand showed a weak band at 3021 cm indicative of the two protonated nitrogen
atoms in the inner core of the porphyrazine ring, suggesting 1a was metal free. This was further corroborated by the
-1
absence of the N-Mg stretching frequency in the spectrum of 1a observed at 470 cm for 1d [16]. The most revealing
data for a tetrapyrrole macrocyclic system is given by its UV-vis spectra in solution [1]. Metal containing
porphyrazines have D4h symmetry consequently the Q band appears as one peak in the visible spectrum. The
symmetry of the metal-free macrocycle is however reduced to D2h and the Q band splits into two distinct bands. The
electronic spectrum of 1d exhibited a Q-band at λmax = 600 nm, which was however split into Q [λ = 558 nm] and
x
max
Q [λ = 626 nm] in 1a spectrum. This indicated that the Q-band shifted from D4h in 1d to D2h symmetry in 1a [13,
y
max
1
3
4].
.1.2
N, N, N’, N’-tetramethylamino porphyrazine hybrid 2a
1
The H NMR of 2a is in close agreement with that obtained for similar compounds in literature [9]. The spectrum
exhibited a singlet at 1.23 ppm, which was assigned to the protonated pyrrolic nitrogen in the core of the molecule,
typical of tetrapyrrole macrocycles with electron donating substituents [17]. This is suggestive of the demetalation of
2
a. The signal at 1.52 ppm a doublet of doublets and another doublet at 1.70 ppm were ascribed to protons of the
methyl substituents attached to the carbons linked to the phenoxy group. This corresponds to the trans and cis-
6

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methyl substituents respectively. A singlet at 3.7 ppm was due to the protons attached to nitrogen of the substituents
[
N(Me) ]. The signal at 5.15 ppm, a multiplet was attributed to the protons of the carbon attached directly to the
2
phenoxy oxygen. A singlet at 7.37 ppm was ascribed to the proton of the methine carbon on the phenoxy ring [1, 9,
-1
1
5]. The infrared spectrum of 2a showed characteristic N-H stretching frequency observed at 3508 cm . This
-1
supports the formation of the macrocylce. Isoindole and C-H bending were observed at 1040 and 1265 cm . Pyrrole
-1
-1
-1
breathing was observed at 1137-1198 cm . Aromatic C=C and C-H peaks were observed at 1593 cm and 3329 cm
respectively [15, 16, 18, 19]. Formation of the macrocycle was further confirmed by its electronic spectrum which
showed an intense B (soret) band at λmax= 325 nm and a Q-band that was split into Q [λ = 556 nm] and Q [λ
=
max
x
max
y
*
7
42 nm]. This was assigned to π→π transitions of the macrocycle. The shoulder at λmax= 650 nm was assigned to
*
n→π transition from the meso nitrogen of the macrocycle [9, 13].
3
.1.3
2,3,9,10,16,17,23,24-octa substituted phthalocyanine 3a
1
The H NMR spectrum exhibited a signal with chemical shift -2.016 ppm characteristic of the pyrrolenic protons in
the core [13]. The signals at 7.6-6.8 ppm were assigned to the benzyl protons. On the other hand its infrared spectrum
-1
exhibited a band at 3620 cm assigned to the N-H stretching vibration [15]. The absence of the C≡N stretching band
in the spectrum corroborated this further. It has been reported that the isoindole and pyrrole stretching frequencies
-1
-1
serve as markers for phthalocyanine formation. This was observed at 1378-1465 cm and 1321 cm respectively [18-
-1
2
0]. Consequently they serve as further evidence for the formation of the macrocycle. Bands at 1091cm and 1163
-1
cm were assigned to pyrrole and isoindole (with some small contribution from C-H in-plane bending) breathing
-1
-
frequencies [18]. Aromatic C=C and ortho-substituted benzene stretch was observed at 1582 cm and 1685-1700 cm
1
.
respectively [15]. The ultraviolet-visible spectrum gave characteristic B-band at λmax= 337 nm. In the visible region
two intense absorptions of comparable intensity were observed. These are due to the splitting of Q-band into Qx
[
λmax= 666 nm] and Q [λmax=703 nm]. This therefore confirmed the D2h symmetry of 3a. The transitions are assigned
y
*
to π→π transitions [1, 21, 22].
3
.1.4
Complexes
The characterizations of the complexes were carried out using elemental analysis, IR and Uv-vis spectrophotometric
techniques and are presented in Tables 1 and 2. The spectral assignments for the complexes were achieved by
comparing their vibrational frequencies with that of the free ligand. Absence of certain bands and shifts in others
indicated coordination. Electronic spectra for some of the complexes and their ligands are given in Figures 4 and 5.
7

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3
.2
Preliminary repetitive scanning of the ultraviolet spectral region during the incorporation of Co(II) and Cu(II) in 1a,
a and 3a gave well defined isobestic points (Table 3) for over two half-lives for the individual reactions. Typical
Kinetic studies of metalation
2
spectra changes for these reactions are presented in Figure 6. The plots of 1/[A] versus time were linear, suggesting
second order kinetics (Figure 7).
A comparative analysis showed that the rate of incorporation of the metal ions into the central cavity of the ligands is
a function of the metal ion and peripheral functionalities of the ligands. Generally the cobalt(II) complexes were more
readily formed than the corresponding copper(II) complexes. This fact is more evident in the coordination of 2a and
3
a, for which there was significant difference in the metalation rate for both metal ions. An explanation based on
difference in size could be used here [23], since cobalt(II) ion is larger than Cu(II) ion. According to Hambright and
Chock [24], these macrocycles deform causing their nucleus to rearrange to provide a suitable configuration for
metalation. It is therefore suggested that as a result of the bigger size of Co(II) ion there is less strain on the
pyrrolenic nitrogen of the macrocycles on coordination. On the other hand the nucleus would have to bend, stretching
itself on coordination to Cu(II) ion as a result of its size. This creates strain on the nucleus. There is however, no
significant difference in metalation rate for both 1b and 1c. It is proposed that the highly symmetrical nature and
flexibility of the peripheral functionality induces a form of flexibility on the cavity of the ligand. Although the rate of
incorporation of Co(II) ion was still slightly higher (Table 4). Further lending credit to the size argument.
The rate of metalation, between the ligands was of the order 2a> 1a >3a for both metal ions. The difference in the
reactivity can be interpreted by considering the electronic and steric effect of the peripheral functionalities of the
ligands. The macrocycle with the greater basicity reacts with the metal ions faster. Such similar correlation was
observed by Worthinton et al. and Inamo et al. for porphyrins [25, 26]. This is because the outer sphere association
between the deformed macrocycles and the metal(II) ions prior to coordination is considered to be driven by an
electrostatic attraction between the local negative charge on the pyrrolenic nitrogen and the positive charge on the
metal ion [26, 27]. Further difference in the rates of complexation of the ligands is attributable to the steric hindrance
brought about by the relative rigidity of the various peripheral substituents of the ligands [28]. This is reflected in 2a
with unsymmetrical peripheral functionalities giving a deformed central cavity. Although the rigid benzo group
induces some rigidity on the porphyrazine ring the dimethylamino substituent is more flexible [29]. This flexibility
creates a form of allowance for the deformation of the ring. Consequently it compels the porphyrazine ring system to
be more favourably distorted for complexation. This is in agreement with that reported for porphyrins with
unsymmetrical central nucleus [23]. It is suggested in the case of 1a that as a result of its more flexible
8

ACCEPTED MANUSCRIPT
functionalization, its core is deformed to a great extent for it to effectively coordinate with the metal ions in the
course of complex formation [26]. This and the symmetrical nature of the peripheral functionality, support the fact
that the rate of insertion is similar for both metal ions. In the 3a complex, the peripheral functionalities induce a form
of rigidity on the macrocycle inhibiting its deformation. Consequently the central cavity is more defined. The
electrostatic approach between the metal ion and the deformed 1a ring is thus restricted by this steric effect [26].
Hence slow rate of insertion. This is supported by the fact that at higher temperature the rate for insertion for the
Co(II) ion increased. It is suggested that increasing the temperature enabled the ligand to rearrange itself, attain a
form of better flexibility enabling it to deform in such a way for easy complexation of the metal ions [29]. This is
corroborated by Hambright and Chock [24], who proposed that slow rates for similar macrocycles are accounted for
in part to the unfavourable deformation equilibria [24]. This is further supported by the high activation entropy for the
reaction (Table4).
3
.3
Mechanism of metal ion incorporation.
All the reactions studied in this research were over 95% complete and the observed rate constants were obtained from
plots of 1/[A] against time, which gave a straight line where A = absorbance at time t. The following scheme
t
t
represents the possible pathways for the incorporation of the metal ions into the central cavity of 1a
Figure 8. Proposed reaction pathway for the metalation of 1a
Pathway (1) is proposed to be the removal of the central magnesium(II) metal ion from the macrocycle, followed by a
fast pick up of the metal(II) ion by the resulting free base (Figure 8). Pathway (2) involves the direct exchange of
positions between the Cu(II) metal ion and magnesium in 1d without forming the free base first (Figure 8). This
reaction pathway involves the formation of the heterodinuclear metaloporphyrazine complex intermediate. Thus it
has two different metal ions bound simultaneously to the ligand on the opposite sides [23, 30].
Process for reaction system being
2
1
d + Cu
Cu1d
(3.1)
(3.2)
Cu1d
1c + Mg
9

ACCEPTED MANUSCRIPT
According to the activation energy requirements however, pathway (1) is the more plausible mechanism. The
activation energy requirement for the metalation of 1a is lower in pathway (1) (Table 4.). Indicating that magnesium
has to be removed first before the incorporation of another metal ion
From section 3.2 above the deformation of the macrocycle is required for the metal ion to effectively interact with the
nitrogen atom of the pyrrole group. It is proposed that in line with most mechanisms the macrocycles form an outer
sphere complex with the metal species [24]. From previous studies the rate determining step for the metalation of
these macrocycles is the dissociation of a coordinated solvent molecule from the metal ion in the outer-sphere
associated complex [23, 24]. Based on these, the rate law for the reactions may be giving by the product of
equilibrium constant for the deformation of the rings K , equilibrium constant for the outer sphere complex formation
D
KOS and the rate constant for the dissociation of the coordinating solvent molecules in the inner sphere of the metal(II)
ion ki.
K
D
HL
HL
(3.3)
(3.4)
(3.5)
K
OS
2
+
2+
M(ace)6 +HL
M(ace)6 HL
k
I
2
+
M(ace)6 HL
ML
Where:
L=
1a, 2a, and 3a
Co(II) and Cu(II)
equilibrium constant for the deformation of the rings
M=
K =
D
K = equilibrium constant for the outer sphere complex formation
OS
ki=
rate constant for the dissociation of the coordinating solvent molecules in the inner sphere of the metal(II)
ion
3
.4
Kinetics of redox reaction
During the redox reaction involving Cu(II)L (L= 1a, 2a and 3a) and cobalt(II) complex of 1a, preliminary repetitive
scanning of the ultraviolet-visible spectral region gave well defined isobestic points for more than two half-lives for
the individual reactions (Table 3) and (Figures 9-11). This signifies that a single reduced species was formed during
the reactions [1, 4, 8]. The plots of 1/[A] versus time were linear, which suggested that the reactions followed second
order kinetics.
1
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Tetrapyrrole macrocyclic complexes are capable of successive four ring reduction and two ring oxidation steps.
Consequently they may act as oxidizing and or reducing agents [7, 8]. From previous reports their redox activity may
be metal or ligand based [1, 2, 4, 8]. This activity is dependent on the availability of low energy orbital for charge
transfer [2]. In this study a spectrophotometric method was used to study the kinetics of redox reaction of 1c, 2c and
3
c with 1b. The spectral changes observed during the redox reaction of 1c and 1b (Figure 9) showed the Q-band
increased in intensity with a hypsochromic shift from 581 to 627 nm and a new band was observed at 509 nm.
Previous studies have reported that these are characteristic features for the formation of the π-anion radical for
tetrapyrrole macrocyclic complexes with redox inactive central metals [1, 2, 8, 31]. Further evidence for the ring
reduction was given by the weak broad band attributable to the d-d transition; B →E indicative of Jahn Teller
1
g
g
2
+
distorted Cu which shifted hypsochromically from 665 to 700 nm with increase in intensity [32]. The redox process
resulted in clear isobestic points obtained at 581 and 822 nm in the spectrum. On the other hand the spectroscopic
changes recorded during the redox reaction involving 2c and 1b (Figure 10) showed a shift in the Q-band from 556
to 590 nm with decrease in intensity. Two new bands were recorded at 560 and 627 nm. Similar spectroscopic
changes have been reported by Tuncer et al. [1] as characteristic of porphyrazine ring reduction. The broad band
2
+
ascribable to a tetragonally distorted Cu B →E transition [32] further suggests a ligand based reduction. The
1g
g
redox process however resulted in clear isobestic points at 580 and 620 nm. Figure 11 represents the spectral changes
which occurred during the reduction of 3c. The Q band red shifted from 667 to 590 nm with retention of intensity.
Two new bands were observed at 558 and 628 nm. According to Donzello et al. [31] these are suggestive of the
formation of a π-anion radical of a tetrapyrrole macrocycle. The broad band characteristic of a tetragonal distorted
2
+
Cu B →E shifted from 700 to 673 nm. Clear isobestic points were observed at 577 and 620 nm in the spectrum
1g
g
[
32]. It is therefore proposed that the Co(II) ion reduced the Cu(II) complexes to give a π-anion with the Co(II) ion
oxidized to Co(III). This is supported by previous studies that showed that first oxidation takes place on the metal ion
for Co(II) complexes of these macrocycles. According to these studies donor solvents strongly favour trivalent cobalt
formation by coordinating in axial positions to form six-coordinate L Co(III)L species. In the absence of donors
2
molecules oxidation to Co(III) is inhibited and ring oxidation occurs first [2, 7, 8, 33]. This is suggestive of the
availability of a low energy d-orbital electron for charge transfer in the octahedral geometry of Co(II) as opposed to
*
the square planar geometry. According to Lever et al. [2] this electron lies between the HOMO (π) and LUMO (π )
orbitals of the ligands [2, 8]. The broad bands ascribed to Jahn Teller distorted Cu(II) ion, further confirmed the fact
that reduction for the Cu(II) complexes was ligand based and not metal based.
1
1

ACCEPTED MANUSCRIPT
The trend for the rate of the reduction followed the order 2c > 1c >3c (Table 4). It is suggested that the reduction rate
is influenced by the ring substituents. This is reflected more in the 2c complex with electron rich substituents. The
ease of reduction of this compound as compared to the other complexes may be attributed to the formation an
unstable species necessitating the subsequent diffusion of charge. As a consequence it is suggested that the ring
reduction is succeeded by another reaction, possibly the diffusion of charge [7, 8, 34].
Generally the rate of reaction was relatively slow compared to similar reactions [35]. Although such reactions are
fast, for acyclic compounds, the size of the macrocycle affected the rate. For the reaction to take place the reactants
have to rearrange before electron transfer could take place to satisfy the Franck Condon requirements. This stipulates
that the oxidant and the reductant must reorganised themselves before the act of electron transfer in a way that
ensures that the energies of the oxidant and reductant in their transition state are identical. Tetrapyrrole macrocycles
are relatively large and as such take time to rearrange. This inevitably slows the reaction. Therefore it can be implied
that the rate of the reaction is also dependent on the rate of rearrangement of the macrocycles (Table 4). From the
foregoing therefore, an outer sphere mechanism is proposed for the reactions. This is because the coordination sphere
of both reductants and oxidants remain the same after the reactions. There is also no bridging group common to the
coordination sphere of both; consequently the reactions followed an electron tunnelling mechanism.
4
.0
Conclusion
The study has shown the significance of peripheral substituents and metal size in tuning the reactivity of tetrapyrrole
macrocyclic compounds. It also underscores the effect of the deformation of the central cavity for effective
metalation. It also highlighted the need to obtain the free base before the incorporation of desired metal ion into the
central cavity of these macrocycles. The study further showed that the rate of redox reaction of the complexes is a
function of their peripheral functionalities.
Acknowledgements
The authors thank Prof BDG Williams, formerly of the University of Johannesburg, for the use of his laboratory,
meaningful suggestions and discussions and the North-West University Material Science Innovation and Modelling
(
MaSIM) research focus area for funding.
1
2

ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
RO
R
R
OR
N
OR
O
O
O
O
N
N
N
N
M
N
N
M
N
RO
N
N
N
M
N
N
N
N
N
N
N
OR
N
N
N
N
OR
N
N
O
O
RO
OR
R=
R=N(Me)2
(1a)
(2a)
(3a)
1
1
1
1
a
,
2a
,
,
3a; M=2H
3b; M=Co(II)
3c; M=Cu(II)
b, 2b
c,
2c,
d
; M=Mg(II)
Br2/AcOH/0.1 M LiBr
R
Br CuCN / DMF
R CN
NC CN
-
2+
MeCN
BuOH,BuO /Mg
R R
5 OC, 2-5h
N
N
2
30 OC 18 h
N
M
N
1
39 h, hv
O
(
4)
Br
R
NC R
CHCI /TFA 120 C
3
R
R
N
N
(
5)
(6)
(
7)
N
N
(
1a)
Figure 1. Reaction scheme for the formation of 1a
1
6

ACCEPTED MANUSCRIPT
O
NH
NH
OH
OH
O
O
0
Na , NH
(g)
OH
OH
Br
K
2
CO
3
3
(Me) N
N(Me)2
CN
CN
2
+
DMF,
60
NC
CN
NH
O
HOCH
2
CH OH
2
O
C
+
(11)
40-150 O
C
(12)
1
i. Mg(OBu) , BuOH
(8)
(9)
2
R
R
ii.TFA
(
10)
O
O
O
O
N
N
N
N M N
N
N
N
O
O
(2a)
R=N(Me)2
Figure 2. Reaction scheme for the formation of 2a
1
7

ACCEPTED MANUSCRIPT
RO
OR
N
OR
CN
CN
i. I-C H11OH
5
N
N
M
N
NaH
THF, 0 O
+
RO
O
CN
CN
ON2
C
N
N
OH
ii. DBU, reflux
OR
(13)
(14)
N
N
OR
RO
(
15)
OR
R=
(
3a)
Figure 3. Reaction scheme for the formation of 3a
1
8

ACCEPTED MANUSCRIPT
Figure 4. Uv-vis spectra for 1a, 1b and 1c
1
9

ACCEPTED MANUSCRIPT
Figure 5. Uv-vis spectrum of 1d
2
0

ACCEPTED MANUSCRIPT
Figure 6. Spectral changes during the reaction
2
1

ACCEPTED MANUSCRIPT
Graph of 1/[A] vs time
1.08
1.06
1.04
1.02
1/[A]
1
0.98
0.96
0.94
0
5
10
15
20
25
30
35
time (min)
Figure 7. A typical kinetic plot for the metalation of 1a free base with copper(II) ion at 25 °C
2
2

ACCEPTED MANUSCRIPT
Figure 8. Proposed reaction pathway for the metalation of 1a
2
3

ACCEPTED MANUSCRIPT
Figure 9. Spectral changes during the reduction of 1c by 1b at 25 °C
Figure 10. Spectral changes during the reduction of 2c by 1b at 25 °C.
2
4

ACCEPTED MANUSCRIPT
o
Figure 11. Spectral changes during the reduction of 3c by 1b at 25 C at λ- 588 nm
2
5

ACCEPTED MANUSCRIPT
-1
Table1. Infra-red stretching frequencies of the ligands and their complexes (cm )
Compound (N-H)
(CH , CH )
(C-H)
(C=C)
1209
1593
1685
1644
1640
(C-H)
(C-O)
(C-C)
2
3
1
2
3
1
1
2
2
3
3
a
a
a
b
c
3021
3508
3620
2928-2851
1464
2848
3051
3055
2997
3001
2999
2986
2824-2897
2925-2854
b
c
b
c
1112
1660
1586
1579
1013
2770-2897
2856-2746
2
6

ACCEPTED MANUSCRIPT
Table2. UV-Vis spectra bands (nm) and elemental analysis of the ligands and their complexes
Compound
B
Qx
Qy
d-d
C,
H,
N
(CALCULATED)
1
2
3
1
1
2
2
3
3
a
a
a
b
c
b
c
b
c
339
325
337
340
336
322
320
330
332
558
556
661
589
581
557
556
669
667
626
742
703
71.68 (71.74); 8.01 (7.97); 20.28 (20.29)
65.63 (64.49); 7.88 (7.29); 6.73 (6.47)
76.21 (76.45); 4.18 (4.19); 9.11 (9.03)
629 65.00 (65.03); 6.86 (6.90); 18.32 (18.39)
665 64.50 (64.54); 6.81 (6.85); 18.00 (18.25)
743 61.27 (60, 74); 6.33 (6.59); 15.02 (15.39)
745 61.11 (60.40); 6.54 (6.56); 14.98 (15.32)
700 72.88 (72.98); 3.80 (3.84); 8.48 (8.62
699 72.68 (72.72); 3.86 (3.83); 8.45 (8.59)
2
7

ACCEPTED MANUSCRIPT
Table3. Isobestic points for kinetic reactions (nm)
a
b
Compound
Isosbestic points
605, 566
Isosbestic points
581, 622
1
2
3
1
2
3
1
c
c
c
b
b
b
d
720
580, 620
690
577, 620
560, 620
780, 820, 840
590, 715
540, 580, 624
Isosbestic points obtained from the spectra for metalation
a
=
b
=
Isosbestic points obtained from the spectra for redox reactions
2
8