Perbrominated 2-nitrotetraphenylporphyrins: Electrochemical and axial ligation properties

Article abstract of DOI:10.1039/b003605f

A new series of perbrominated 2-nitro-5,10,15,20-tetraphenylporphyrins, H₂TPPBr_n(NO₂) (n = 6 and 7) and their metal (Cu^II and Zn^II) complexes have been synthesised and characterised. The presence of mixed electron withdrawing (bromo- and nitro-) substituents at the β-pyrrole positions induces interesting electrochemical and axial ligation properties. Perbrominated nitroporphyrins exhibit two successive one electron redox potentials with a positive shift of >100 mV relative to their corresponding octabromotetraphenylporphyrin (MTPPBr₈) complexes. Axial ligation of various nitrogenous bases to ZnTPPBr_n(NO₂) complexes showed facile ligand binding with >50% enhancement in the equilibrium constants, K_eq, relative to ZnTPPBr₈. Surprisingly, MTPPBr_n(NO₂) complexes show similar axial ligation and electrochemical redox behaviour. The anodic shift in electrochemical redox potentials and enhanced K_eq of MTPPBr_n(NO₂) complexes have been attributed to the increased electron deficiency of the porphyrin π-system.

Full text of DOI:10.1039/b003605f

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Perbrominated 2-nitrotetraphenylporphyrins: electrochemical and
axial ligation properties
Puttaiah Bhyrappa* and Bhavana Purushothaman
2
Department of Chemistry, Indian Institute of Technology-Madras, Chennai-600 036, India
Received (in Cambridge, UK) 5th May 2000, Accepted 15th November 2000
First published as an Advance Article on the web 20th December 2000
A new series of perbrominated 2-nitro-5,10,15,20-tetraphenylporphyrins, H TPPBr (NO ) (n = 6 and 7) and their
2
n
2
II
II
metal (Cu and Zn ) complexes have been synthesised and characterised. The presence of mixed electron with-
drawing (bromo- and nitro-) substituents at the β-pyrrole positions induces interesting electrochemical and axial
ligation properties. Perbrominated nitroporphyrins exhibit two successive one electron redox potentials with a
positive shift of >100 mV relative to their corresponding octabromotetraphenylporphyrin (MTPPBr ) complexes.
8
Axial ligation of various nitrogenous bases to ZnTPPBr (NO ) complexes showed facile ligand binding with >50%
n
2
enhancement in the equilibrium constants, K , relative to ZnTPPBr . Surprisingly, MTPPBr (NO ) complexes show
eq
8
n
2
similar axial ligation and electrochemical redox behaviour. The anodic shift in electrochemical redox potentials
and enhanced Keq of MTPPBr (NO ) complexes have been attributed to the increased electron deficiency of the
n
2
porphyrin π-system.
Over the past decade, perhaloporphyrins have aroused con-
siderable interest due to their unusual physical and chemical
properties. High-valent metalloperhaloporphyrins serve as cata-
1
lysts for the oxidative transformation of organic substrates.
The stability of metalloporphyrin catalysts has been attributed
1e
to both electronic and steric factors. There are two classes of
metalloperhaloporphyin catalysts; (a) those with electron with-
2,3
drawing substituents only on the meso-phenyl positions and
b) those with electron withdrawing substituents on both the
(
4,5
β-pyrrole and meso-phenyl positions. These increase the
electron deficiency of the porphyrin macrocycle, and the latter
type, in particular, have shown dramatic increases in catalytic
activity. The increased stability of the catalyst towards oxida-
tive degradation is due to the stabilisation of the HOMO of
the porphyrin macrocycle by the halogens at the porphyrin
4,5
4a,b,1d
periphery.
Perhaloporphyrins exhibit remarkable electronic, electro-
chemical redox and stereochemical properties, which make
these porphyrins unique. Furthermore, β-perbromoporphyrins
Fig. 1 Molecular structure of perbrominated 2-nitrotetraphenyl-
porphyrins.
6
have been employed as precursors in the synthesis of a variety
of other substituted porphyrins, incorporating the desired
donor–acceptor substituents using Suzuki cross-coupling
bromo-5,10,15,20-tetraphenylporphyrin, H
TPPBr (NO ) and
6 2
2
7
reactions. These highly functionalised porphyrins, are other-
2-nitro-3,7,8,12,13,17,18-heptabromo-5,10,15,20-tetraphenyl-
II
II
wise synthetically inaccessible. A large number of reports can
be found in the literature on dodeca-substituted porphyrins
with similar substituents at the pyrrole carbons.
porphyrin, H TPPBr (NO ) and their Zn and Cu complexes.
2 7 2
Results and discussion
The porphyrins with mixed substituents at the β-pyrrole
positions of the meso-tetraphenylporphyrin, MTPPBr (NO )
Synthesis of porphyrins with mixed substituents is of increas-
ing interest, due partly to the differences in their chemical
reactivities and also in order to generate unusually substituted
n
2
8
porphyrins. A variety of porphyrins with mixed (alkyl and
(n = 6 and 7) (Fig. 1) were synthesised by direct bromination
bromo) groups at the β-pyrrole positions have been reported
by Senge and co-workers and they exhibit interesting stereo-
of the complex CuTPP(NO ) followed by acid demetallation.
2
9
In addition to the desired products [CuTPPBr (NO ) (n = 6
n
2
chemical features. The degree of non-planarity of the porphyrin
and 7)], the formation of CuTPPBr was also observed. This is
8
8,11
ring influences the physico-chemical properties of the por-
possibly due to nucleophillic substitution
of the nitro group
10
phyrin π-system. Porphyrins with mixed substituents have
by the bromo group. With increasing bromine concentration,
been largely unexplored. To our knowledge, MTPPBr (NO )
there is an increase in the yield of CuTPPBr relative to the
7
2
8
is the first dodeca-substituted asymmetric porphyrin with
electron withdrawing hetero substituents at the β-pyrrole
positions. We report here the synthesis, electrochemical redox
and axial ligation properties of a series of perbrominated
CuTPPBr (NO ) complexes. When the ratio of liquid Br to Cu-
n
2
2
TPP(NO ) was 100:1, the product distribution was exclusively
2
of CuTPPBr (55%) and CuTPPBr (NO ) (45%) complexes.
8
7
2
Under our optimised conditions, the CuTPPBr (NO ) (n = 6, 7)
n
2
2
-nitrotetraphenylporphyrins,
2-nitro-7,8,12,13,17,18-hexa-
complexes were generated in good yields.
2
38
J. Chem. Soc., Perkin Trans. 2, 2001, 238–242
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b003605f

View Article Online
a
Table 1 UV–Visible absorption spectral data of MTPPBr (NO ) complexes in toluene at 25 ЊC
n
2
Porphyrin
B (Bands)
Q (Bands)
H TPPBr
368(4.30), 471(5.21)
373(4.31), 474(5.13)
269(4.32), 469(5.17)
368(4.43), 472(5.11)
370(4.31), 475(4.98)
365(4.37), 470(5.05)
355(4.50), 472(5.37)
354(4.43), 474(5.34)
352(4.31), 471(5.26)
570(3.88), 623(4.05), 740(3.76)
573(3.80), 629(3.93), 747(3.75)
570(3.85), 623(4.06), 738(3.75)
585(4.21), 628(3.84)
589(4.03), 634(3.78)
587(4.12), 633(3.87)
603(4.04), 658(3.99)
607(3.97), 664(4.03)
603(3.89), 657(3.91)
2
8
H TPPBr (NO )
2
7
2
H TPPBr (NO )
2
6
2
CuTPPBr8
CuTPPBr (NO )
CuTPPBr (NO )
ZnTPPBr8
ZnTPPBr (NO )
7
2
6
2
7
2
ZnTPPBr (NO )
6
2
a
3
Ϫ1
Ϫ1
λ/nm. The values in parentheses refer to log (ε/dm mol cm ) values.
Fig. 3 Cyclic voltammograms of ZnTPPBr (NO ) in CH Cl with
7
2
2
2
Ϫ1
TBAHFP as the supporting electrolyte at a scan rate of 100 mV s
.
1
II
The H NMR spectra of MTPPBr (NO ) (M = 2H and Zn )
6
2
complexes exhibit proton resonances arising from the β-pyrrole
and meso-phenyl protons, while those of MTPPBr (NO )
7
2
complexes showed no resonances due to β-pyrrole protons.
The integrated intensities are in accordance with the proposed
structures. The presence of a singlet at 8.70 ppm for the
Fig. 2 Electronic absorption spectra of ZnTPPBr (heavy line) and
8
ZnTPPBr (NO ) (thin line) in toluene.
MTPPBr (NO ) complexes indicates that the pyrrole ring
6 2
7
2
with the nitro group is free of bromine substitution. For the
1
The synthesised complexes were characterised by H NMR,
MTPPBr (NO ) complexes, the ortho-phenyl protons show
7
2
UV–Visible and mass spectroscopic methods. Optical absorp-
tion spectra of the Zn -perbrominated porphyrins are shown
an asymmetric multiplet, in contrast to their corresponding
II
MTPPBr complexes, indicating the lowered symmetry of the
8
in Fig. 2. Electronic absorption spectral data of the porphyrin
complexes are given in Table 1. MTPPBr (NO ) complexes
MTPPBr (NO ) complexes.
7
2
A wide variety of perhaloporphyrins have been examined in
n
2
12a
6a,b,j,12
exhibit similar spectral features to MTPPBr8 complexes.
Interestingly, the UV–Visible absorption spectra of MTPP-
non-aqueous media.
The electrochemical redox properties
of a series of perbrominated porphyrins have been examined
II
II
Br (NO ) (M = 2H, Zn and Cu ) complexes show red-shifted
in CH Cl2 using tetrabutylammonium hexafluorophosphate
7
2
2
B and Q bands relative to their corresponding MTPPBr or
(TBAHFP) as the supporting electrolyte. The cyclic voltammo-
gram of a representative ZnTPPBr (NO ) complex is shown
8
MTPPBr (NO ) complexes. The Q(0,0) bands in ZnTPPBr -
6
2
n
7
2
(
NO ) complexes show an increase in intensity relative to their
in Fig. 3. For comparison, MTPPBr8 complexes were also
examined under similar conditions. Table 2 lists the electro-
chemical redox data of various asymmetric MTPPBr (NO )
2
Q(1,0) transitions in toluene. According to the four-orbital-
13
model, the highest occupied molecular orbitals, a1u and a2u,
bear electron density at the β-pyrrole carbons and at the meso-
carbons and imino nitrogens, respectively. The increase
in intensity of Q(0,0)/Q(1,0) in ZnTPPBr (NO ) is possibly due
n
2
complexes. The observed redox potentials for the MTPPBr
8
12
complexes are in accordance with the reported values. The
II
MTPPBr (NO ) (M = Cu and 2H) complexes exhibited two
n
2
n
2
to a further lifting of the degeneracy of the a1u and a2u
orbitals by stabilisation of the a1u relative to the a2u orbital,
successive reduction and two oxidation potentials while the
ZnTPPBr (NO ) complexes showed two oxidation and three
n
2
in contrast to that of ZnTPPBr . The relative intensity Q(0,0)/
reduction potentials. The observed behaviour of the ZnTPP-
8
Q(1,0) for the perbrominated porphyrins follows the order
ZnTPPBr (NO ) > ZnTPPBr (NO ) > ZnTPPBr . Synthesised
Br (NO ) (n = 6 and 7) complexes is quite similar to that of the
n
2
12a
ZnTPPBr complex. Interestingly, the porphyrins with mixed
7
2
6
2
8
8
perbrominated 2-nitrotetraphenylporphyrins were found to be
substituents showed >100 mV anodic shift in redox potential
relative to their corresponding MTPPBr8 complexes. This
can be attributed to the increased electron deficiency of the
non-fluorescent due to the heavy atom effect of the bromine
12a
substituents.
J. Chem. Soc., Perkin Trans. 2, 2001, 238–242
239

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a
Table 2 Half-wave redox potentials (mV) of MTPPBr (NO ) com-
n
2
plexes in CH Cl with 0.1 M TBAHFP at 25 ЊC
2
2
Oxidation
II
Reduction
I
Porphyrin
I
II
III
b
H TPPBr8
960
—
Ϫ680b
Ϫ580
Ϫ650
Ϫ865
Ϫ720
Ϫ725b
Ϫ935b
Ϫ740b
Ϫ750
Ϫ1070
Ϫ950
Ϫ955
Ϫ1090
Ϫ925
Ϫ970
Ϫ1085
Ϫ945
Ϫ960
—
—
—
—
—
—
Ϫ1335
Ϫ1180
Ϫ1215
2
H TPPBr (NO )
1050
1055
910
1040
1060
880
1310
1290
1510
1545
—
1140
1240
1220
2
7
2
b
H TPPBr (NO )
2
6
2
CuTPPBr8
CuTPPBr (NO )
7
2
CuTPPBr (NO )
6
2
ZnTPPBr8
ZnTPPBr (NO )
955
955
7
2
ZnTPPBr (NO )
6
2
a
b
vs. saturated calomel electrode. Irreversible potentials.
Fig. 5 A plot is given for log Keq of ZnTPPBr (NO )/base against
7
2
the corresponding pK values of the bases. The designated numbers for
a
the bases are as given in Table 3.
K , relative to the ZnTPPBr complex. The ZnTPPBr (NO )
eq
8
7
2
and ZnTPPBr (NO ) complexes show quite similar K values.
6
2
eq
The plot of log Keq of ZnTPPBr (NO ) against the pK
7
2
a
values for the various bases is shown in Fig. 5. The log
Keq values follow a fairly good linear relationship with the
pKa values. Interestingly, the additional bromo group in
ZnTPPBr (NO ) has no significant influence over the K value.
7
2
eq
In summary, a new class of asymmetric β-perbrominated
-nitrotetraphenylporphyrins with mixed substituents have
Fig. 4 Titration data is shown for the ligation of pyridine to the
2
ZnTPPBr (NO ) complex in toluene. The main plot shows the
7
2
been synthesised and characterised. MTPPBr (NO ) complexes
electronic spectral change in the Soret region of the ZnTPPBr (NO )
n
2
7
2
complex on increasing addition of pyridine. The inset shows the
plot of pyridine concentration, [pyridine] vs. [pyridine] divided by the
change in absorbance at 474 nm.
showed anodic shifts in electrochemical redox potentials and
enhanced Keq values relative to MTPPBr complexes. This is
8
interpreted in terms of an enhanced electron deficiency of
the MTPPBr (NO ) complexes due to the introduction of a
nitro group that is in direct conjugation with the porphyrin
π-system.
n
2
porphyrin macrocycle of the MTPPBr (NO ) complexes.
n
2
II
II
For the MTPPBr (NO ) (M = Cu , Zn ) complexes, with an
n
2
increase in electronegativity of the core metal ion a positive
shift in redox potential was observed, as anticipated. It is of
interest to note that the MTPPBr (NO ) (n = 6, 7) complexes
Experimental
n
2
showed similar redox potentials.
Ligation of nitrogenous bases to ZnTPPBr (NO ) com-
The majority of the solvents employed in this study were of
reagent grade and were distilled before use. Toluene (Ranbaxy
n
2
plexes was performed to delineate the effect of mixed bromo-
labs, India) was refluxed and distilled from sodium. CH Cl2
2
II
and nitro-substituents on the basicity of the Zn centre.
(
Qualigens, India) was refluxed and distilled over CaH . Liquid
2
Different bases were employed to probe the effect of basicity
on the Keq values. Optical absorption spectra of the ZnTPP-
bromine, purchased from E. Merck, was used as received. Metal
carriers, Zn(OAc) ؒ2H O and Cu(OAc) ؒH O were purchased
2
2
2
2
Br (NO ) complex on successive additions of pyridine in
7
2
from Ranbaxy labs (India) and used without further purifi-
cation. Silica gel (60–120) was purchased from Acmes (India)
and used as received. The nitrogenous bases, pyridine, 3-
hydroxypyridine, imidazole and piperidine were procured from
Fluka (Switzerland) and used as received. TBAHFP was
procured from Fluka (Switzerland) and used without further
purification.
II
toluene are shown in Fig. 4. On ligation of bases to Zn -
perbrominated porphyrins, the ZnTPPBr (NO ) complexes
n
2
show a red-shifted Soret and the visible bands, as in the case
14
of ZnTPP. The stoichiometry of the complexation between
II
the Zn porphyrin and the ligand (L) were analysed using the
15
Hill method and in all cases the stoichiometry of the ligand
binding to porphyrin was found to be one-to-one complexation
[eqn. (1)].
Syntheses
K
eq
MTPPBr (NO ) complexes. 2-Nitro-5,10,15,20-tetraphenyl-
n
2
[
ZnTPPBr (NO )] ϩ [L]
[ZnTPPBr (NO )(L)] (1)
n 2
n
2
porphinatocopper(), CuTPP(NO ) was synthesised using a
2
18
reported procedure. Perbrominated H TPPBr (NO ) (n = 6, 7)
2
n
2
For comparison with the representative plot for the ligation
complexes were synthesised by bromination of CuTPP(NO )
with liquid bromine followed by demetallation. A 250 cm con-
2
16
3
of pyridine with ZnTPPBr (NO ) (shown in Fig. 4), the
7
2
ligation of bases with ZnTPPBr was also performed under
ical flask, equipped with a magnetic stir bar, was charged with
8
17
similar conditions; these data are presented in Table 3. It can
be seen that the ZnTPPBr (NO ) complexes have >50% higher
CuTPP(NO ) (0.33 g, 0.44 mmol) in CHCl (70 ml) at room
2
3
temperature. To this solution, liquid Br (4.36 g, 26 mmol)
n
2
2
2
40
J. Chem. Soc., Perkin Trans. 2, 2001, 238–242

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a
II
Table 3 Equilibrium constants (K ) for the ligation of various nitrogenous bases and DMSO with Zn -perbrominated porphyrins in toluene at
eq
2
5 ЊC
Ϫ3
Ϫ1
1
0 K /M
eq
b
Base
ZnTPPBr8
ZnTPPBr (NO )
ZnTPPBr (NO )
pKa
7
2
6
2
1
2
3
4
5
6
7
8
9
. Piperidine
1003.77
192.79
135.17
182.08
138.71
113.30
29.57
1598.52
320.23
181.32
261.22
186.39
169.85
45.89
1598.86
301.04
158.87
263.27
187.44
184.72
47.58
11.22
10.66
9.70
8.33
7.33
6.95
5.23
4.86
—
. Cyclohexylamine
. 4-Dimethylaminopyridine
. Morpholine
. N-Methylimidazole
. Imidazole
. Pyridine
. 3-Hydroxypyridine
. DMSO
39.02
9.06
64.21
14.22
62.02
14.15
a
b
Error ± 6%. Values are taken from ref. 19.
II
in CHCl (30 ml) was added dropwise over a period of 20 min.
After complete addition of bromine, the reaction mixture was
stirred for 4 h at room temperature. At the end of 4 h, pyridine
0.06 mmol) in CHCl3 (25 ml). Cu acetate monohydrate
(0.050 g, 0.22 mmol) in 3 ml of methanol was added and
the solution was stirred at room temperature for 20 min. The
solvent was then removed under reduced pressure and the crude
material purified by silica gel column chromatography using
3
(
4.0 ml) in CHCl (40 ml) was added dropwise over a period of
3
3
0 min. Then the reaction mixture was allowed to stir for a
further 6 h and then treated with 100 ml of aqueous sodium
metabisulfite solution (25%) followed by 150 ml of aqueous
HCl (20%). The organic layer was separated and dried over
anhydrous Na SO . The crude product mixture was con-
CHCl as the eluant. The yield of the porphyrin was almost
quantitative.
3
2
4
Spectra
centrated to a minimum volume and purified by column
chromatography on silica gel with CHCl3 as the eluant.
The yield of the product was found to be 0.38 g (65%). The
The synthesised free base perbrominated porphyrins and their
metal complexes were characterised by H NMR, UV–Visible,
elemental and mass spectroscopic techniques. ZnTPPBr8:
1
separation of the individual components CuTPPBr , and
8
δH (400 MHz, CDCl , TMS) 8.10 (m, 8H, aromatic H ), 7.56
3
o
CuTPPBr (NO ) (n = 6, 7) was unsuccessful on silica or alumina
n
2
ϩ
(
1
m, 12H, aromatic H ). FAB mass spectrum (m/z): 1308 (M ,
m,p
columns due to the poor solubility of the product mixture in
low polarity solvents. However, the separation the of mixture
ϩ
00%), 1230 (M Ϫ Br, 8%). The observed data are similar to
those reported for ZnTPPBr .
12a
II
8
was effected by preparation of the Zn complexes.
The acid demetallation of perbrominated porphyrins was
II
II
MTPPBr (NO ) (M ؍ 2H, Zn ) complexes. H TPPBr -
6 2 2 6
2 H 3
performed using conc. H SO . In a typical reaction, the Cu
2
4
(
8
2
NO ): δ (400 MHz, CDCl , TMS) 8.71 (s, 1H, β-H), 8.21 (m,
H, aromatic H ), 7.70 (m, 12H, aromatic Hm,p), Ϫ1.50 (br s,
H, NH) (Found: C, 46.35; H, 2.16; N, 6.10. C H N O Br
requires C, 46.86; H, 2.06; N, 6.21%). ZnTPPBr (NO ): δ (400
MHz, CDCl , TMS) 8.65 (s, 1H, β-H), 8.10 (m, 8H, aromatic
H ), 7.50 (m, 12H, aromatic H ). FAB mass spectrum (m/z):
197.0 [(M ϩ H) , 100%], 1116.9 [(M ϩ H) Ϫ Br, 12%].
complex (0.10 g, 0.08 mmol) was dissolved in CHCl (40 ml),
3
o
and to this conc. H SO (3 ml) was added. The reaction mixture
2
4
44
23
5
2
6
was stirred for a period of 30 min, then washed with distilled
water (2 × 100 ml) and the non-aqueous layer was neutralised
6
2
H
3
with saturated aqueous NaHCO solution. The organic solu-
3
o
m,p
tion was dried over anhydrous Na SO and the solvent was
2
4
ϩ
ϩ
1
removed under reduced pressure using a rotary evaporator.
Yield of the free base product was found to be 0.085 g (90%).
II
II
MTPPBr (NO ) (M ؍ 2H, Zn ) complexes. H TPPBr -
The Zn insertion into the free base perbrominated porphyrins
7
2
2
7
II
(NO ): δ (400 MHz, CDCl , TMS) 8.18 (m, 8H, aromatic H ),
was carried out using the procedure employed for Cu insertion
2
H
3
o
7
.75 (m, 12H, aromatic Hm,p), Ϫ1.45 (br s, 2H, NH) (Found:
as described below and the yield of the product was almost
II
C, 43.40; H, 1.91; N, 5.90. C H N O Br requires C, 43.60; H,
quantitative. The individual Zn complexes of H TPPBr and
44 22
5
2
7
2
8
ϩ
1
1
(
.83; N, 5.78%). FAB mass spectrum (m/z): 1213.6 [(M ϩ H) ,
00%]. ZnTPPBr (NO ): δ (400 MHz, CDCl , TMS) 8.12
m, 8H, aromatic H ), 7.73 (m, 12H, aromatic H ). FAB mass
o m,p
ϩ ϩ
spectrum (m/z): 1275.0 [(M ϩ H) , 100], 1196.9 [(M ϩ H) Ϫ
Br, 11%].
H TPPBr (NO ) (n = 6,7) were separated on a silica gel column
2
n
2
using toluene as the eluant. The ZnTPPBr complex was eluted
7
2
H
3
8
first followed by the ZnTPPBr (NO ) and ZnTPPBr (NO )
7
2
6
2
complexes. Yields of the individual fractions were found to
be ZnTPPBr8 (40%), ZnTPPBr (NO ), (30%) and ZnTPP-
7
2
Br (NO ) (30%).
6
2
Instrumentation
H TPPBr (NO ) complexes. These complexes were prepared
2
n
2
Electronic absorption spectra of free base porphyrins and their
metal derivatives were recorded on a JASCO V-550 model UV–
II
by demetallation of their Zn complexes using conc. HCl.
In a typical demetallation reaction, ZnTPPBr (NO ) (0.060 g,
7
2
Visible absorption spectrophotometer using 1 cm path length
1
0
.05 mmol) was dissolved in 20 ml of CHCl . To this, 3 ml
H NMR spectra of porphyrins were
3
quartz cells in toluene.
of conc. HCl (33%) was added with stirring for 15 min. the
reaction mixture was then washed with distilled water (3 ×
0 ml). The non-aqueous layer was neutralised with aqueous
recorded on a 400 MHz FT-NMR spectrometer in CDCl with
3
tetramethylsilane as the internal standard. Cyclic voltammetric
measurements of free base perbrominated porphyrins and
their metal complexes were performed on a BAS-100 Bio-
analytical system provided with a monitor and XY recorder.
The electrochemical cell consists of a three-electrode cell
assembly; a platinum-button-working electrode, a saturated
calomel reference, SCE, and a platinum wire as the auxiliary
electrode. The reference electrode, SCE was separated from
the bulk solution by a glassy diaphragm connected to a bridge
5
NaHCO (15%) solution and dried over anhydrous Na SO .
3
2
4
The yield of the porphyrin was found to be 0.055 g (90%).
CuTPPBr (NO ) complexes. The metal ion insertion was
n
2
II
performed in a CHCl –CH OH solvent mixture using Cu
3
3
acetate monohydrate as metal carrier. A round-bottomed
3
flask (100 cm ) was charged with H TPPBr (NO ) (0.075 g,
2
7
2
J. Chem. Soc., Perkin Trans. 2, 2001, 238–242
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filled with the supporting electrolyte. The concentrations of
the porphyrins employed were ~1 mM. All measurements were
T. K. Chandrashekar, Struct. Bonding (Berlin), 1995, 82, 105;
f ) E. R. Birnbaum, J. A. Hodge, W. P. Schaefer, L. Henling,
J. A. Labinger, J. E. Bercaw and H. B. Gray, Inorg. Chem., 1995,
4, 3625; (g) M. W. Grinstaff, M. G. Hill, E. R. Birnbaum,
W. P. Schaefer, J. A. Labinger and H. B. Gray, Inorg. Chem., 1995,
4, 4896; (h) T. P. Wijesekara, D. Dupre, M. S. R. Cader and D.
Dolphin, Bull. Chim. Soc. Fr., 1996, 133, 765; (i) M. S. Chorghade,
D. Dolphin, D. Dupre, D. R. Hill, E. C. Lee and T. P. Wijesekara,
Synthesis, 1996, 1320; (j) K. M. Kadish, J. Li, E. V. Camelbecke,
Z. Ou, N. Guo, M. Autret, F. D’Souza and P. Tagliatesta, Inorg.
Chem., 1997, 36, 6292.
(
performed at 25 ЊC in CH Cl2 under N2 atmosphere using
2
3
0
.1 M TBAHFP as the supporting electrolyte.
The ligation of various nitrogenous bases with the Zn
II
3
perbrominated porphyrins was performed at 25 ЊC in toluene
using a thermostat cell holder and the equilibrium constants,
1
6b
K , were evaluated using reported procedure.
Keq were
eq
16b
analysed at four different wavelengths and are found to be
similar within the experimental error (6%). For a given base,
three Keq values were evaluated and average of three Keq values
has been taken for data analysis.
7
K. S. Chan, X. Zhou, B.-S. Lou and T. C. W. Mak, J. Chem. Soc.,
Chem. Commun., 1994, 271; K. S. Chan, X. Zhou, M. T. Au and
C. Y. Tam, Tetrahedron, 1995, 51, 3129; X. Zhou, Z. Y. Zhou,
T. C. W. Mak and K. S. Chan, J. Chem. Soc., Perkin Trans. 1, 1994,
2
519; S. G. DiMagno, V. S.-Y. Lin and M. J. Therein, J. Am. Chem.
Acknowledgements
Soc., 1993, 115, 2513; S. G. DiMagno, V. S.-Y. Lin and M. J. Therein,
J. Org. Chem., 1993, 58, 5983; S. G. DiMagno and M. J. Therein,
Science, 1994, 264, 1105.
L. Jaquinod, C. Gros, R. G. Khoury and K. M. Smith, Chem.
Commun., 1998, 2581; K. M. Shea, L. Jaquinoid, R. G. Khoury
and K. M. Smith, Chem. Commun., 1998, 759; C. M. Muzzi,
C. J. Medforth, K. M. Smith, S.-L. Jia and J. A. Shelnutt, Chem.
Commun., 2000, 131.
We are thankful to the Department of Science and Technology,
DST (SP/S1/F09/98) and Council of Scientific and Industrial
Research, CSIR (01(1550)/98/EMR-II), India for financial
support. We thank Professor V. Krishnan (IISc, Bangalore) for
providing the facilities to carry out electrochemical studies.
8
9
M. O. Senge, V. Gerstung, K. Ruhlandt-Senge, S. Runge and
I. Lehmann, J. Chem. Soc., Dalton Trans., 1998, 4187.
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J. Chem. Soc., Perkin Trans. 2, 2001, 238–242