Mixed substituted porphyrins: Structural and electrochemical redox properties

Article abstract of DOI:10.1021/ic052035b

To examine the influence of mixed substituents on the structural, electrochemical redox behavior of porphyrins, two new classes of β-pyrrole mixed substituted free-base tetraphenylporphyrins H₂(TPP(Ph) ₄X₄) (X = CH₃

Full text of DOI:10.1021/ic052035b

Inorg. Chem. 2006, 45, 4136−4149
Mixed Substituted Porphyrins: Structural and Electrochemical Redox
Properties
P. Bhyrappa,* M. Sankar, and B. Varghese
Department of Chemistry, Indian Institute of TechnologysMadras, Chennai-600 036, India
Received November 27, 2005
To examine the influence of mixed substituents on the structural, electrochemical redox behavior of porphyrins,
two new classes of -pyrrole mixed substituted free-base tetraphenylporphyrins H2(TPP(Ph)4X4) (X CH3, H, Br,
Cl, CN) and H2(TPP(CH3)4X4) (X H, Ph, Br, CN) and their metal (M Ni(II), Cu(II), and Zn(II)) complexes have
been synthesized effectively using the modified Suzuki cross-coupling reactions. Optical absorption spectra of
these porphyrins showed significant red-shift with the variation of X in H2(TPPR4X4), and they induce a 20 30 nm
shift in the B band and a 25 100 nm shift in the longest wavelength band [Qx(0,0)] relative to the corresponding
H2TPPR4 (R CH3, Ph) derivatives. Crystal structure of a highly sterically crowded Cu(TPP(Ph)4(CH3)4) 2CHCl3
complex shows a combination of ruffling and saddling of the porphyrin core while the Zn(TPP(Ph)4Br4(CH3OH))
CH3OH structure exhibits predominantly saddling of the macrocycle. Further, the six-coordinated Ni(TPP(Ph)4(CN)4-
(Py)2) 2(Py) structure shows nearly planar geometry of the porphyrin ring with the expansion of the core.
Electrochemical redox behavior of the MTPPR4X4 compounds exhibit dramatic cathodic shift in first ring oxidation
potentials (300 500 mV) while the reduction potentials are marginally cathodic in contrast to their corresponding
MTPPX4 (X Br, CN) derivatives. The redox potentials were analyzed using Hammett plots, and the highest
occupied molecular orbital lowest unoccupied molecular orbital (HOMO LUMO) gap decreases with an increase
in the Hammett parameter of the substituents. Electronic absorption spectral bands of H2TPPR4X4 are unique that
their energy lies intermediate to their corresponding data for the H2(TPPX8) (X CH3, Ph, Br, Cl) derivatives. The
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−
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dramatic variation in redox potentials and large red-shift in the absorption bands in mixed substituted porphyrins
have been explained on the basis of the nonplanarity of the macrocycle and substituent effects.
Introduction
formations of the porphyrin macrocycle arise from repulsive
interactions among the peripheral substituents. The introduc-
tion of halogens at the peripheral positions of the high-valent
metalloporphyrins provides robust catalysts for the oxidative
transformation of organic substrates in the presence of strong
oxygen donors to enhance cytochrome P450 like activity.10-16
The robust nature of these perhaloporphyrin catalysts has
Over the past 2 decades, considerable progress has been
made on the synthesis of highly substituted porphyrins and
their use as model compounds of nonplanar conformations
of metalloporphyrinoids in nature.1-9 Such nonplanar con-
* To whom correspondence should be addressed. E-mail:
bhyrappa@iitm.ac.in. Fax: 091-44-2257-0509.
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H.-K.; Smith, K. M.; Fajer, J. J. Am. Chem. Soc. 1991, 113, 6891.
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4136 Inorganic Chemistry, Vol. 45, No. 10, 2006
10.1021/ic052035b CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/22/2006

Mixed Substituted Porphyrins
been attributed to the stabilization of the highest occupied
π-molecular orbitals induced by the presence of halogen
substituents.16,17 Furthermore, these highly substituted por-
phyrins exhibit unique physicochemical properties.1,18
Of the synthetic porphyrin analogues, meso-tetraphen-
ylporphyrin (TPP) is the most widely explored system
because of its ease of synthesis and facile functionalization.
M(TPP) complexes with similar â-octa-substituents (alkyl
or phenyl)2-8,18-21 and halogens15,18,22-37 at the â-pyrrole and/
or meso-phenyl positions have been reported in the literature.
Electrochemical redox properties of M(TPP) complexes
bearing fewer â-pyrrole substituents have been examined by
various groups.38-43 Kadish et al.44-46 and others8,24,25,27,47
Figure 1. Molecular structure of mixed substituted porphyrins and their
metal complexes.
have reported electrochemical redox properties of a variety
of M(TPP) complexes bearing similar â-octa-substituents.
Previous reports have shown that the number of â-substit-
uents and the nonplanarity of the macrocycle influence the
redox properties of the π-system.17,48 Syntheses and proper-
ties of M(TPP) complexes with mixed antipodal â-octa-
substituents have been largely unexamined.49-51 In an effort
to elucidate the role of mixed substituents at the antipodal
â-pyrrole positions, we have synthesized two new classes
of M(TPPR4X4) complexes (Figure 1) and explored their
physicochemical properties. These porphyrins display inter-
esting structural and electrochemical redox properties.
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Experimental Section
Materials. 5,10,15,20-Tetraphenylporphyrin, H2(TPP), its metal
[Ni(II), Cu(II), Zn(II)] complexes,52,53 and 2,3,12,13-tetrabromo-
5,10,15,20-tetraphenylporphyrin,54 H2TPPBr4, were synthesized
using the literature methods.
All the solvents procured were of analytical grade and purified
prior to use.55 N-Bromosuccinimide (NBS) was recrystallized from
hot water and dried at 60 °C under vacuum (2.0 mbar) for 12 h
before use. N-Chlorosuccinimide (NCS) obtained from Aldrich was
recrystallized from benzene and dried at 60 °C under vacuum
(2.0 mbar) for 10 h. Calcium hydride, CDCl3, benzene-d6, super-
base (2,8,9-triisobutyl-2,5,8,9-tetraaza-1-phosphabicyclo[3.3.3]-
undecane), phenylboronic acid, methylboronic acid, tetrakis(tri-
phenylphosphine)palladium(0), and cuprous cyanide were purchased
from Sigma-Aldrich (India) and used as received. Metal acetates,
copper(II) acetate monohydrate, zinc(II) acetate dihydrate, and
nickel(II) acetate tetrahydrate were procured from Sigma-Aldrich
(India) and used without further purification. Silica gel (60-120
mesh) and alumina (neutral and basic) for column chromatography
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1259.
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Int. Ed. Engl. 1994, 33, 348.
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I. J. Chem. Soc., Dalton Trans. 1998, 4187.
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5495.
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Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic Press: New York, 2000; Vol. 9, p 59 and references therein.
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E.; Bianco, P.; Antonio, A.; Tagliatesta, P. Inorg. Chem. 1994, 33,
5169.
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Boschi, T.; Tagliatesta, P.; Kadish, K. M. Inorg. Chem. 1993, 32, 4042.
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E.; Kisters, M.; Medforth, C. J.; Smith, K. M. Inorg. Chem. 1993, 32,
4177.
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44, 4849.
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Chemicals; Elsevier: New York, 2003.
(47) Takeda, M.; Sato, M. Chem. Lett. 1995, 939.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4137

Bhyrappa et al.
procured from Acmes (India) were used as such. Precoated thin-
layer silica gel chromatography and aluminum plates were pur-
chased from E. Merck (Germany) and used as received. Tetrabu-
tylammonium hexafluorophosphate, TBAPF6, procured from Fluka
(Switzerland) was recrystallized thrice from absolute ethanol and
stored in a vacuum desiccator.
Fourier map. Refinement of disordered atoms were carried out with
their relative occupancies as least-squares variables. However, the
sum of the occupancies of disordered pairs was constrained to be
1.0. Complete bond lengths, bond angles, and isotropic thermal
parameters for all the three crystal structures examined in this work
are given in the Supporting Information.
I. Synthesis of Mixed Substituted Tetraphenylporphyrin,
M(TPP(Ph)4X4) Derivatives. (a) 2,3,12,13-Tetraphenyl-5,10,15,-
20-tetraphenylporphyrin (H2(TPP(Ph)4)) and Its Metal Com-
plexes. 2,3,12,13-Tetraphenyl-5,10,15,20-tetraphenylporphyrin (H2-
(TPP(Ph)4)) was synthesized using a modified Suzuki cross-coupling
reaction,57,58 and its metal [Ni(II), Cu(II), and Zn(II)] complexes
were prepared by conventional procedures.53,25
Instrumentation and Methods. Electronic absorption spectra
of porphyrins and their metal complexes were performed on a
computer interfaced JASCO V-550 model UV-visible spectro-
photometer using a pair of quartz cells of 10 mm path length in
1
CH2Cl2 at 298 K. H NMR spectra of porphyrins were recorded
on a Bruker Avance 400 MHz FT-NMR spectrometer in CDCl3
using tetramethylsilane as the internal reference. Mass spectral
measurements of the samples were carried out using electrospray
ionization (ESI) mass spectrometer model Micromass Q-TOF Micro
using 10% formic acid in methanol as the solvent medium. Low-
resolution fast atom bombardment mass spectra (FAB-MS) of the
samples were obtained on a JEOL SX 102/DA-6000 model using
m-nitrobenzyl alcohol as the matrix. In some cases, a matrix assisted
laser desorption time-of-flight (MALDI-TOF) spectrum on a
Voyager DE-PRO model mass spectrometer was used by employing
R-cyano-4-hydroxycinnamic acid as the matrix. Elemental analysis
of the samples was performed on a Perkin-Elmer CHNO/S analyzer
model 2400 series. Cyclic voltammetric measurements of the
porphyrins were taken on a CH instruments model 660B or
Bioanalytical Systems model BAS-100A. The electrochemical cell
consists of a three-electrode cell assembly: a platinum button as a
working electrode, Ag/AgCl as a reference electrode, and platinum
wire as the counter electrode. The concentrations of all the
porphyrins employed were 0.5-1.0 mM, and 0.1 M tetrabutylam-
monium hexafluorophosphate, TBAPF6, was used as the supporting
electrolyte. All cyclic voltammetric measurements were performed
in triply distilled CH2Cl2 from CaH2 and stored over 4 Å molecular
sieves.
The two-way stoppered round-bottom flask (250 mL) containing
freshly distilled toluene from metallic sodium (170 mL) was charged
with H2(TPPBr4) (0.93 g, 1.0 mmol), tetrakis(triphenylphosphine)-
palladium(0) (20 mol %, 0.232 g), 2,8,9-triisobutyl-2,5,8,9-tetraaza-
1-phosphabicyclo[3.3.3]undecane (0.070 mL, 0.20 mmol), anhy-
drous K2CO3 (3.32 g, 24.0 mmol), and phenylboronic acid (1.45
g, 12.0 mmol) and degassed by the freeze-pump-thaw method
(three cycles). Then the suspension was heated and stirred between
95 and 105 °C under N2 atmosphere for 2 days. After completion
of the reaction, toluene was removed by rotary evaporation under
reduced pressure. The crude product was redissolved in CHCl3 and
washed with saturated aqueous NaHCO3 solution (100 mL)
followed by 30% aqueous NaCl solution (100 mL). Then, the
organic layer was dried over anhydrous Na2SO4. The concentrated
solution of the crude product was chromatographed on a silica
column using CHCl3 as the eluent. Trace amounts of the hydro-
genated porphyrin product were eluted first followed by the desired
H2(TPP(Ph)4) as the major product. The product was recrystallized
from CHCl3/CH3OH (1:3, v/v) solvent mixture and dried under
vacuum (3 mbar) at 100 °C for 5 h. The yield of the product was
1
found to be 0.78 g (85%). H2(TPP(Ph)4) H NMR in CDCl3 δ
(ppm): 8.30 (s, 4H, â-pyrrole-H), 7.76 (d, 8H, meso-o-phenyl-H),
7.16 (m, 12H, meso-m- and p-phenyl-H), 6.81 (m, 20H, â-pyrrole-
phenyl-H), -2.13 (s, 2H, imino-H). FAB-MS (m/z): 919 (calcd,
919.14).
Crystal Structures. Mixed substituted porphyrins Cu(TPP(Ph)4-
(CH3)4) and Zn(TPP(Ph)4Br4) were crystallized by direct diffusion
of CH3OH to a saturated porphyrin solution in CHCl3 over a period
of 7 days. Crystals of the NiTPP(Ph)4(CN)4 complex were obtained
by slow diffusion of hexane to the saturated porphyrin solution in
pyridine. X-ray diffraction data were collected on Enraf-Nonius
CAD-4 diffractometer at 293 K. Cell refinement was done with
the CAD4 system software, data reduction with XCAD4 (wingx).
Twenty-five reflections, collected through a search routine from
different zones of the crystal, were indexed using a method of short
vectors followed by least-squares refinement. Intensity data from
one quadrant of the reciprocal space were collected. The systematic
absences clearly revealed the space groups. The intensity data were
corrected for background, Lorentz, polarization, and decay correc-
tions. Empirical absorption corrections using ψ-scan data were used
for absorption. A total of I > 2σ(I) reflections were taken for
structure solution. The SIR92 (WINGX) program was used to solve
the structure by direct methods. Successive Fourier synthesis was
employed to complete the structures. The structure was refined with
a full matrix least-squares technique on F2 using SHELXL97
software (Sheldrik, 1997).56 Non-hydrogen atoms were refined with
anisotropic thermal parameters. The positions of hydrogen atoms
were assigned geometrically and were given riding model refine-
ment. In the case of the ZnTPP(Ph)4Br4 structure, the bromine atoms
and the coordinated methanol were found to have some positional
disorder. The disordered positions were located from a difference
The metal [Ni(II), Cu(II), and Zn(II)] complexes of H2(TPP-
(Ph)4) were prepared almost in excellent (80-90%) yields using
conventional procedures.25 Ni(TPP(Ph)4) 1H NMR in CDCl3: 8.11
(s, 4H, â-pyrrole-H), 7.37 (d, 8H, meso-o-phenyl-H), 7.07 (t, 4H,
meso-p-phenyl-H), 6.97 (t, 8H, meso-m-phenyl-H), 6.76 (m, 20H,
â-pyrrole-phenyl-H). FAB-MS (m/z): 975 (calcd, 975.81). Cu(TPP-
(Ph)4) FAB-MS (m/z): 980 (calcd, 980.67). Zn(TPP(Ph)4) 1H NMR
in CDCl3: 8.42 (s, 4H, â-pyrrole-H), 7.69 (d, 8H, meso-o-phenyl-
H), 7.15 (t, 4H, meso-p-phenyl-H), 7.08 (t, 8H, meso-m-phenyl-
H), 6.88 (d, 8H, â-pyrrole-o-phenyl-H), 6.75 (m, 12H, â-pyrrole-
m- and p-phenyl-H). FAB-MS (m/z): 982 (calcd, 982.5).
(b) Preparation of 2,3,12,13-Tetrabromo-7,8,17,18-tetra-
phenyl-5,10,15,20-tetraphenylporphyrin (H2(TPP(Ph)4Br4)) and
Its Metal Complexes. The mixed substitution at the â-pyrrole
positions of the H2(TPP) was effected by the bromination of Ni-
(TPP(Ph)4) with N-bromosuccinimide in refluxing CHCl3 to produce
the corresponding Ni(TPP(Ph)4Br4) complex. Its free-base porphyrin
(H2(TPP(Ph)4Br4)) was obtained by acid demetalation reaction of
the NiTPP(Ph)4Br4 complex. Other M(TPP(Ph)4Br4) [M ) Cu(II)
(57) (a) Chan, K. S.; Zhou, X.; Lou, B.-S.; Mak, T. C. W. J. Chem. Soc.,
Chem. Commun. 1994, 271. (b) Chan, C.-S.; Tse, K.-S.; Chan, K. S.
J. Org. Chem. 1996, 59, 6084. (c) Chan, K. S.; Zhou, X.; Au, M. T.;
Tam, C. Y. Tetrahedron 1995, 51, 3129.
(56) Sheldrick, G. M. SHELXL97, Program for the Refinement of Crystal
Structures; University of Go¨ettingen: Go¨ettingen, Germany, 1997.
(58) Urgaonkar, S.; Nagarajan, M.; Verkade, J. G. Tetrahedron Lett. 2002,
43, 8921.
4138 Inorganic Chemistry, Vol. 45, No. 10, 2006

Mixed Substituted Porphyrins
and Zn(II)] complexes were prepared using their appropriate metal-
(II) acetates as metal carriers as described in the literature.25,53
Ni(TPP(Ph)4) (0.38 g, 0.39 mmol) was dissolved in CHCl3 (50
mL). To this solution, freshly recrystallized NBS (0.416 g, 2.34
mmol) was added. The reaction mixture was refluxed for 4 h and
cooled to room temperature, and the CHCl3 was removed by rotary
evaporation. The residue thus obtained was washed with methanol
(2 × 10 mL) to remove any soluble succinimide and degraded
porphyrin impurities. Then the greenish-purple colored product was
chromatographed on a silica column using CHCl3 as the eluent.
The first fraction was collected, and chloroform was removed to
obtain the Ni(TPP(Ph)4Br4) complex. This was dried for 6 h under
vacuum (3 mbar) at 100 °C, and the yield of Ni(TPP(Ph)4Br4) was
2H, imino-H). ESI-MS (m/z): 1057 (calcd, 1056.92). Zn(TPP-
(Ph)4Cl4) 1H NMR in CDCl3: 7.67 (d, 8H, meso-o-phenyl-H), 7.15
(m, 12H, meso-m- and p-phenyl-H), 6.69 (m, 20H, â-pyrrole phenyl-
H). MALDI-TOF-MS (m/z): 1119.10 (calcd, 1120.28). Cu(TPP-
(Ph)4Cl4): MALDI-TOF-MS (m/z): 1117.12 (calcd, 1118.54).
(d) 2,3,12,13-Tetramethyl-7,8,17,18-tetraphenyl-5,10,15,20-
tetraphenylporphyrin (H2(TPP(Ph)4(CH3)4)) and Its Metal
Complexes. To a 250 mL two necked round-bottom flask contain-
ing H2TPP(Ph)4Br4 (0.106 g, 0.085 mmol) in toluene (15 mL), Pd-
(PPh3)4 (20 mol %, 0.02 g), anhydrous K2CO3 (0.381 g, 2.70 mmol),
and methylboronic acid (0.083 g, 1.38 mmol) in dry THF (5 mL)
were added. The reaction mixture was degassed by the freeze-
pump-thaw method (three cycles), and it was stirred and main-
tained between 85 and 95 °C under N2 atmosphere for 3 days. After
completion of reaction, the solvent was evaporated to dryness. The
crude product was redissolved in CHCl3 and washed with saturated
aqueous NaHCO3 (25 mL) solution followed by saturated aqueous
NaCl (30 mL) solution, and the organic layer was dried over
anhydrous Na2SO4. The solution was concentrated and loaded on
a silica gel column, and trace amounts of hydrogenated porphyrin
product were eluted first with CHCl3. Then the required product
H2(TPP(Ph)4(CH3)4) was eluted as a second fraction using 4%
acetone in CHCl3. The product was recrystallized from a CHCl3/
CH3OH (4:1, v/v) mixture and dried under vacuum (2 mbar) at 90
°C for a period of 6 h. The yield of the product was 0.055 g (65%).
The metal complexes of the H2(TPP(Ph)4(CH3)4) derivative were
prepared using conventional procedures as described above. In all
these reactions, the yields were 85-90%. H2(TPP(Ph)4(CH3)4) 1H
NMR in CDCl3: 7.92 (d, 8H, meso-o-phenyl-H), 7.56 (t, 4H, meso-
p-phenyl-H), 7.46 (t, 8H, meso-m-phenyl-H), 6.74 (m, 20H,
â-pyrrole-phenyl-H), 1.82 (s, 12H, â-pyrrole-methyl-H), -1.65 (bs,
2H, imino-H). ESI-MS (m/z): 974.83 (calcd, 975.25). Anal. Calcd
for C72H54N4: C, 88.67; H, 5.58; N, 5.74%. Found: C, 88.37; H,
5.49; N, 5.57%. Ni(TPP(Ph)4(CH3)4) 1H NMR in CDCl3: 7.48 (d,
8H, meso-o-phenyl-H), 7.10 (t, 4H, meso-p-phenyl-H), 7.02 (t, 8H,
meso-m-phenyl-H), 6.68 (m, 20H, â-pyrrole-phenyl-H), 1.58 (s,
12H, â-pyrrole-methyl-H). ESI-MS (m/z): 1030.68 (calcd, 1031.92).
Cu(TPP(Ph)4(CH3)4) MALDI-TOF-MS (m/z): 1036.09 (calcd,
1
0.46 g (90%). Ni(TPP(Ph)4Br4) H NMR in benzene-d6: 7.50 (d,
8H, meso-o-phenyl-H), 7.06 (t, 4H, meso-p-phenyl-H), 6.99 (t, 8H,
meso-m-phenyl-H), 6.76 (d, 8H, â-pyrrole-o-phenyl-H), 6.64 (m,
12H, â-pyrrole-m- and p-phenyl-H). FAB-MS (m/z): 1291 (calcd,
1291.4). Anal. Calcd for C68H40N4Br4Ni: C, 63.25; H, 3.12; N,
4.34%. Found: C, 63.25; H, 3.12; N, 4.32%.
Preparation of H2(TPP(Ph)4Br4). Ni(TPP(Ph)4Br4) (0.30 g, 0.23
mmol) was dissolved in a minimum amount of chloroform. To this,
concentrated H2SO4 (2.0 mL) was added, and the solution was
stirred for 45 min at 0 °C. At the end of this period, distilled water
(100 mL) was added dropwise to the reaction mixture with stirring.
The organic layer was separated and washed with water (2 × 50
mL) followed by neutralization using aqueous ammonia (25%)
solution (20 mL). Then the organic layer was dried over anhydrous
Na2SO4 and concentrated to a small volume. This was separated
by silica gel chromatography using CHCl3 as the eluent, and the
yield of the product was found to be 0.26 g (90%).
The metal complexes of the H2(TPP(Ph)4Br4) derivative were
prepared using the reported methods,25 and the yields of the products
were 85-95%. H2(TPP(Ph)4Br4) 1H NMR in CDCl3: 7.85 (d, 8H,
meso-o-phenyl-H), 7.23 (m, 12H, meso-m- and p-phenyl-H), 6.75
(m, 20H, â-pyrrole-phenyl-H), -1.79 (bs, 2H, imino-H). FAB-MS
(m/z): 1235 (calcd, 1234.72). Cu(TPP(Ph)4Br4) FAB-MS (m/z):
1297 (calcd, 1296.25). Zn(TPP(Ph)4Br4) 1H NMR in CDCl3: 7.86
(d, 8H, meso-o-phenyl-H), 7.25 (m, 12H, meso-m- and p-phenyl-
H), 6.77 (m, 20H, â-pyrrole phenyl-H). FAB-MS (m/z): 1297
(calcd, 1298.09).
1
1036.78). Zn(TPP(Ph)4(CH3)4) H NMR in CDCl3: 7.74 (d, 8H,
meso-o-phenyl-H), 7.15 (m, 12H, meso-m- and p-phenyl-H), 6.68
(m, 20H, â-pyrrole-phenyl-H), 1.72 (s, 12H, â-pyrrole-methyl-H).
LD-MS (m/z): 1038.20 (calcd, 1038.61).
(c) 2,3,12,13-Tetrachloro-7,8,17,18-tetraphenyl-5,10,15,20-tet-
raphenylporphyrin (H2(TPP(Ph4)Cl4)) and Its Metal Complexes.
Ni(TPP(Ph)4) (0.18 g, 0.184 mM) was dissolved in 1,1,2,2-
tetrachloroethane (30 mL). To this, freshly recrystallized N-
chlorosuccinimide (0.197 g, 1.47 mM) in TCE (20 mL) was added,
and the resulting solution was refluxed for 3 h under argon
atmosphere. After completion of the reaction, the solvent was
removed by rotary evaporation. The residue thus obtained was
washed with methanol to remove any soluble succinimide impuri-
ties. Then the crude product Ni(TPP(Ph4)Cl4) was chromatographed
on a silica gel column using chloroform as eluent. The yield of the
(e) 2,3,12,13-Tetracyano-7,8,17,18-tetraphenyl-5,10,15,20-tet-
raphenylporphyrin (H2(TPP(Ph)4(CN)4 )) and Its Metal Com-
plexes. H2(TPP(Ph)4(CN)4) was obtained by the direct cyanation
of Ni(TPP(Ph)4Br4) complex by using a variation of the reported
procedure59 followed by an acid demetalation reaction. The typical
procedure is as follows: Ni(TPP(Ph)4Br4) (0.151 g, 0.117 mmol)
was dissolved in dry pyridine (30 mL). To this, CuCN (0.42 g,
4.69 mmol) was added in the solid form. Then the reaction mixture
was stirred and refluxed for a period of 48 h under argon
atmosphere. At the end of this period, the solvent was rotary
evaporated under reduced pressure. Further, the resultant residue
was redissolved in a minimum amount of CHCl3, and it was loaded
onto a silica gel column using CHCl3 as the eluent. The first fraction
[Ni(TPP(Ph)4(CN)3)] was found to be 0.025 g (20%). The desired
product [Ni(TPP(Ph4)(CN)4)] was eluted as the second fraction
1
product was 0.19 g (92%). Ni(TPP(Ph)4Cl4) H NMR in CDCl3:
7.42 (d, 8H, meso-o-phenyl-H), 7.18 (t, 4H, meso-p-phenyl-H), 7.04
(t, 8H, meso-m-phenyl-H), 6.78 (d, 8H, â-pyrrole-o-phenyl-H), 6.69
(m, 12H, â-pyrrole-m- and p-phenyl-H). MALDI-TOF-MS (m/z):
1113.05 (calcd, 1113.59). Anal. Calcd for C68H40N4Cl4Ni: C, 73.34;
H, 3.62; N, 5.03%. Found: C, 73.34; H, 3.50; N, 5.30%.
1
H2(TPP(Ph)4Cl4) and its metal [Cu(II) and Zn(II)] complexes
were prepared as described above for the synthesis of M(TPP-
(Ph)4Br4) [M ) 2H, Cu(II), and Zn(II)] derivatives. The yields of
(0.070 g, 56%). Ni(TPP(Ph4)(CN)3) H NMR in CDCl3: 8.64 (s,
1H, â-pyrrole-H), 7.47-7.39 (m, 8H, meso-o-phenyl-H), 7.34-
1
the products were almost quantitative. H2(TPP(Ph)4Cl4) H NMR
(59) (a) Callot, H. J.; Giraudeau, A.; Gross, M. J. Chem. Soc., Perkin Trans.
2 1975, 1321. (b) Callot, H. J. Tetrahedron Lett. 1973, 4987. (c) Callot,
H. J. Bull. Soc. Chim. Fr. 1974, 1492.
in CDCl3: 7.93 (d, 8H, meso-o-phenyl-H), 7.31 (m, 12H, meso-m-
and p-phenyl-H), 6.89 (m, 20H, â-pyrrole-phenyl-H), -1.80 (bs,
Inorganic Chemistry, Vol. 45, No. 10, 2006 4139

Bhyrappa et al.
7.00 (m, 12H, meso-p- and m-phenyl-H), 6.98-6.66 (m, 20H,
â-pyrrole-phenyl-H). Electronic absorption spectrum in CH2Cl2,
Br4). The other metal complexes were prepared as described earlier.
H2(TPP(CH3)4Br4) 1H NMR in CDCl3: 8.26 (d, 8H, meso-o-phenyl-
H), 7.78 (m, 12H, meso-m- and p-phenyl-H), 1.95 (s, 12H, â-pyrrole
methyl-H), -2.25 (bs, 2H, imino-H). MALDI-TOF-MS (m/z):
987.25 (calcd, 986.44). Cu(TPP(CH3)4Br4 MALDI-TOF-MS
λmax, nm (log ꢀ): 329 (4.34), 451 (5.17), 537 (3.74), 576 (sh), 635
(4.31). MALDI-TOF-MS (m/z): 1051.9 (calcd, 1050.84). Ni(TPP-
1
(Ph)4(CN)4) H NMR in CDCl3: 7.39 (d, 8H, meso-o-phenyl-H),
1
7.30 (t, 4H, meso-p-phenyl-H), 7.10 (t, 8H, meso-m-phenyl-H), 6.87
(t, 4H, â-pyrrole-p-phenyl-H), 6.78 (t, 8H, â-pyrrole-m-phenyl-H),
6.68 (d, 8H, â-pyrrole-o-phenyl-H). MALDI-TOF-MS (m/z): 1074.18
(calcd, 1075.85). Anal. Calcd for C72H40N8Ni: C, 80.38; H, 3.75;
N, 10.42%. Found: C, 80.10; H, 3.85; N, 10.14%.
(m/z): 1048.39 (calcd, 1047.97). Zn(TPP(CH3)4Br4) H NMR in
CDCl3: 8.14 (d, 8H, meso-o-phenyl-H), 7.74 (m, 12H, meso-m-
and p-phenyl-H), 1.51 (s, 12H, â-pyrrole-methyl-H). MALDI-TOF-
MS (m/z): 1053.0 (calcd, 1049.80).
(c) 2,3,12,13-Tetracyano-7,8,17,18-tetramethyl-5,10,15,20-tet-
raphenylporphyrin (H2TPP(CH3)4(CN)4) and Its Metal Com-
plexes. H2(TPP(CH3)4(CN)4) was prepared as described in the
synthesis of the Ni(TPP(Ph)4(CN)4) complex. The first fraction,
Ni(TPP(CH3)4(CN)3), was found to be 15%. The product Ni(TPP-
(CH3)4(CN)4 was eluted as a second fraction in 40% yield. Ni-
(TPP(CH3)4(CN)4) 1H NMR in CDCl3: 7.89 (d, 8H, meso-o-phenyl-
H), 7.84 (t, 4H, meso-p-phenyl-H), 7.73 (t, 8H, meso-m-phenyl-
H), 1.94 (s, 12H, â-pyrrole-methyl-H). MALDI-TOF-MS (m/z):
826.86 (calcd, 827.57). Anal. Calcd for C52H32N8Ni: C, 75.47; H,
3.90; N, 13.54%. Found: C, 75.35; H, 3.77; N, 13.78%. Ni(TPP-
The free-base porphyrin was prepared by acid demetalation
reaction as described above, and the yield of the product was 85%.
H2(TPP(Ph)4(CN)4) 1H NMR in CDCl3: 7.84 (d, 8H, meso-o-
phenyl-H), 7.51 (t, 4H, meso-p-phenyl-H), 7.35 (t, 8H, meso-m-
phenyl-H), 6.89 (m, 20H, â-pyrrole-phenyl-H), -1.48 (s, 2H, imino-
H). MALDI-TOF-MS (m/z): 1019.22 (calcd, 1019.18). Metal
complexes (Cu(II) and Zn(II)) of H2(TPP(Ph)4(CN)4) were prepared
as described above. Yields are almost quantitative (85-95%). Cu-
(TPP(Ph)4(CN)4) MALDI-TOF-MS (m/z): 1079.71 (calcd, 1080.71).
Zn(TPP(Ph)4(CN)4) 1H NMR in CDCl3: 7.63 (d, 8H, meso-o-
phenyl-H), 7.37 (t, 4H, meso-p-phenyl-H), 7.10 (t, 8H, meso-m-
phenyl-H), 6.75 (m, 20H, â-pyrrole-phenyl-H). LD-MS (m/z):
1081.73 (calcd, 1082.54).
1
(CH3)4(CN)3) H NMR in CDCl3: 8.86 (s, H, â-pyrrole-H), 7.91-
(m, 8H, meso-o-phenyl-H), 7.89-7.65 (m, 12H, meso-p- and
m-phenyl-H), 2.00-1.92 (m, 12H, â-pyrrole-methyl-H). UV-
visible in CH2Cl2, λmax, nm (log ꢀ): 329 (4.16), 443 (5.05), 533
(3.60), 573 (sh), 633 (4.31). MALDI-TOF-MS (m/z): 802.26 (calcd,
802.56).
II. Synthesis of Mixed Substituted Tetraphenylporphyrins,
M(TPP(CH3)4X4) Derivatives. (a) 2,3,12,13-Tetramethyl-5,10,-
15,20-tetraphenylporphyrin (H2(TPP(CH3)4)) and Its Metal
Complexes. The free-base porphyrin H2(TPP(CH3)4) was prepared
using the reported procedure57 or modified Suzuki cross-coupling
reaction of H2(TPPBr4) with methylboronic acid as described for
the synthesis of the H2(TPP(Ph)4) derivative. The product was
separated on a basic alumina column using CHCl3 as the eluent.
The product was recrystallized from a CHCl3/CH3OH (1:5, v/v)
solvent mixture and dried under vacuum (3 mbar) at 90 °C over a
period of 6 h. The yield of the product was found to be 90%. H2-
The free-base porphyrin was prepared by the demetalation of
the Ni(TPP(CH3)4(CN)4) complex with sulfuric acid. The yield of
1
the free-base porphyrin was 85%. H2(TPP(CH3)4(CN)4) H NMR
in CDCl3: 8.16 (d, 8H, meso-o-phenyl-H), 7.97 (t, 4H, meso-p-
phenyl-H), 7.85 (t, 8H, meso-m-phenyl-H), 2.21 (s, 12H, â-pyrrole-
methyl-H), -2.02 (s, 2H, imino-H). MALDI-TOF-MS (m/z):
771.31 (calcd, 770.90). M(TPP(CH3)4(CN)4) (M ) Cu(II) and Zn-
(II)) complexes were prepared using conventional procedures.7
Yields were almost quantitative (85-95%). Cu(TPP(CH3)4(CN)4)
MALDI-TOF-MS (m/z): 833.55 (calcd, 832.43). Zn(TPP(CH3)4-
1
(TPP(CH3)4) H NMR in CDCl3: 8.46 (s, 4H, â-pyrrole-H), 8.10
(d, 8H, meso-o-phenyl-H), 7.73 (m, 12H, meso-m- and p-phenyl-
H), 2.45 (s, 12H, â-pyrrole-methyl-H), -2.76 (s, 2H, imino-H).
ESI-MS (m/z): 671.51 (calcd, 670.86). The MTPP(CH3)4 (M )
Zn(II) and Cu(II)) complexes were prepared using a variation of
literature methods.25
1
(CN)4) H NMR in CDCl3: 8.06 (d, 8H, meso-o-phenyl-H), 7.91
(t, 4H, meso-p-phenyl-H), 7.78 (t, 8H, meso-m-phenyl-H), 2.09 (s,
12H, â-pyrrole-methyl-H). MALDI-TOF-MS (m/z): 835.68 (calcd,
834.26).
The Ni(TPP(CH3)4) complex was prepared using the reported
1
procedure53 and its yield was 90%. Ni(TPP(CH3)4) H NMR in
Results and Discussion
CDCl3: 8.49 (s, 4H, â-pyrrole-H), 7.86 (d, 8H, meso-o-phenyl-
H), 7.62 (m, 12H, meso-m- and p-phenyl-H), 2.16 (s, 12H,
â-pyrrole-methyl-H). MALDI-TOF-MS (m/z): 727.41 (calcd, 727.53).
Cu(TPP(CH3)4) MALDI-TOF-MS (m/z): 732.56 (calcd, 732.39).
Zn(TPP(CH3)4) 1H NMR in CDCl3: 8.66 (s, 4H, â-pyrrole-H), 8.08
(d, 8H, meso-o-phenyl-H), 7.70 (m, 12H, meso-m- and p-phenyl-
H), 2.37 (s, 12H, â-pyrrole-methyl-H). MALDI-TOF-MS (m/z):
734.08 (calcd, 734.22).
(b) 2,3,12,13-Tetrabromo-7,8,17,18-tetramethyl-5,10,15,20-
tetraphenylporphyrin (H2(TPP(CH3)4Br4)) and Its Metal Com-
plexes. The title porphyrin was obtained by bromination of the
Ni(TPP(CH3)4) complex followed by an acid demetalation reaction
as given in the synthesis of the H2(TPP(Ph)4Br4) derivative. The
yield of the product Ni(TPP(CH3)4Br4) was 75%. Ni(TPP(CH3)4-
Br4) 1H NMR in CDCl3: 7.95 (d, 8H, meso-o-phenyl-H), 7.67 (m,
12H, meso-m- and p-phenyl-H), 1.79 (s, 12H, â-pyrrole-methyl-
H). MALDI-TOF-MS (m/z): 1043.81 (calcd, 1043.11). Anal. Calcd
for C48H32N4Br4Ni: C, 55.27; H, 3.09; N, 5.37%. Found: C, 55.19;
H, 3.55; N, 5.18%.
Synthesis and Characterization. A series of antipodal
â-pyrrole mixed substituted TPPs were synthesized using H2-
(TPPBr4) as the precursor. The synthetic route for the
preparation of various antipodal substituted porphyrins is
shown in Scheme 1. The choice of the brominated porphyrin
for the preparation of mixed substituted porphyrins is due
to the ease of synthesis and to its ability to provide entry to
other groups at the other antipodal â-pyrrole positions.
Recent reports show that Suzuki cross-coupling reactions for
sterically hindered monobromoarenes using Pd(OAc)2 as the
catalyst with superbase in the presence of Cs2CO3 proceed
in shorter time scales (<12 h).58 This procedure was not
feasible to convert the H2(TPPBr4) to H2(TPP(Ph4)) even
under a longer reaction time (3 days) and yielded a mixture
of products. H2(TPP(Ph)4) was synthesized in good yields
using a modified Suzuki cross-coupling reaction of H2-
(TPPBr4) with phenylboronic acid in the presence of Pd(0)
catalyst using superbase and K2CO3 in a toluene/THF
medium over a period of 2 days. The observed spectral data
The demetalation of the porphyrin Ni(TPPBr4(CH3)4) was
performed as described above with 85% yield of H2(TPP(CH3)4-
4140 Inorganic Chemistry, Vol. 45, No. 10, 2006

Mixed Substituted Porphyrins
Scheme 1. Synthetic Route for Various Mixed Substituted Porphyrins
of these compounds are consistent with the reported values.57
Earlier reports show that the Suzuki cross-coupling reaction
with brominated porphyrins requires a longer reaction time
(>4 days).57 Halogenation (bromination or chlorination) of
Ni(TPP(Ph)4) with the N-halosuccinimide reagent produced
the desired Ni(TPP(Ph)4X4) derivatives in 90% yield. The
use of Ni(TPP(Ph)4) was essential due to the considerable
degradation of H2(TPP(Ph)4) or Zn(TPP(Ph)4) derivatives in
the presence of NXS (X ) Br, Cl) in refluxing chloroform/
1,1,2,2-tetrachloroethane. Further, the bromination of M(T-
PP(Ph)4) [M ) Cu(II), Ni(II)] was also carried out with liquid
bromine and produced lower yields (40-50%) of the
corresponding M(TPP(Ph)4Br4) derivatives.
The direct method was also examined as a way to
synthesize M(TPP(Ph)4(CH3)4 (M ) 2H, Ni(II)) derivatives
by the bromination of M(TPP(CH3)4) (M ) 2H, Ni(II)) with
NBS followed by the Suzuki cross-coupling reaction. This
resulted in lower yields of the M(TPP(CH3)4Br4) product.
Hence, the synthesis of the H2(TPP(Ph)4(CH3)4) derivative
was carried out using H2(TPP(Ph)4 as the precursor. Ni-
(TPPR4(CN)4) complexes were prepared in 40-55% yields
by refluxing a Ni(TPPR4Br4) complex with an excess of
CuCN in pyridine. The free-base mixed substituted porphyrin
H2(TPP(Ph)4(CH3)4) derivative was prepared by the direct
Suzuki cross-coupling reaction of H2(TPP(Ph)4Br4) with
methylboronic acid using Pd(0) catalyst. Other metal (Cu-
Figure 2. Electronic absorption spectra of H2(TPP(Ph)4) (thin line) and
H2(TPP(Ph)4(CN)4) (heavy line) in CH2Cl2 at 298 K. Q bands are magnified
3 times.
(II) and Zn(II)) complexes were prepared by demetalation
of Ni(TPPR4(CN)4) with concentrated H2SO4. The desired
metal insertion into the free-base porphyrins was performed
using the reported procedures. Similar strategy was employed
for the synthesis of MTPP(CH3)4X4 compounds. All the
synthesized compounds were characterized by electronic
1
absorption, H NMR, and mass spectral techniques.
Representative absorption spectra of H2(TPP(Ph)4(X)4) (X
) H, CN) derivatives are shown in Figure 2. Table 1 lists
the electronic absorption spectral data for the M(TPP(Ph)4X4)
Inorganic Chemistry, Vol. 45, No. 10, 2006 4141

Bhyrappa et al.
Table 1. Optical Absorption Spectral Dataa of MTPP(Ph)4X4
Derivatives in CH2Cl2 at 298 K
porphyrin
B band (s), nm
434 (5.45)
Q band (s), nm
H2TPP(Ph)4
529 (4.29), 565 (sh),
599 (3.87), 677 (3.65)
555 (4.05), 607 (3.82),
704 (3.98)
574 (3.93), 630 (3.95),
734 (3.96)
567 (3.90), 621 (3.97),
717 (4.03)
585 (3.97), 651 (3.99),
778 (4.16)
H2TPP(Ph)4(CH3)4
H2TPP(Ph)4Br4
H2TPP(Ph)4Cl4
368 (4.39),
457 (5.18)
377 (4.36),
470 (5.19)
376 (4.39),
463 (5.23)
464 (5.14)
H2TPP(Ph)4(CN)4
NiTPP(Ph)4
NiTPP(Ph)4(CH3)4
NiTPP(Ph)4Br4
431 (5.27)
439 (5.11)
340 (4.22),
449 (5.15)
331 (4.39),
444 (5.33)
344 (4.36),
457 (5.16)
428 (5.37)
345 (4.30),
458 (sh)
548 (4.18), 585 (sh)
558 (3.94), 607 (4.07)
564 (4.00), 611 (3.84)
Figure 3. Optical absorption spectra of Cu(TPP(Ph)4) (thin line) and Cu-
(TPP(Ph)4(CN)4 (heavy line) in CH2Cl2 at 298 K.
NiTPP(Ph)4Cl4
560 (4.19), 606 (4.00)
NiTPP(Ph)4(CN)4
545 (3.82), 592 (sh),
654 (4.48)
554 (4.18), 593 (sh)
570 (4.12), 610 (4.02)
Qx(0,0), of the free-base mixed substituted porphyrins shows
an interesting trend in red-shift and aligns in the following
order: H2(TPP(Ph)4(CH3)4) < H2(TPP(Ph)4Cl4) < H2(TPPR4-
Br4) < H2(TPPR4(CN)4). Notably, of the mixed substituted
porphyrins, H2(TPP(Ph)4(CN)4) shows a dramatic red-shift
of 17-44 nm in the B band and 34-123 nm in the Qx(0,0)
band relative to the corresponding bands of the â-tetrasub-
stituted H2(TPPX4) (X ) CH3, Ph, Br, CN) derivatives.42b,57,59
The red-shift in the sterically crowded porphyrins has been
ascribed to the nonplanarity of the porphyrin ring in
conjunction with the inductive or resonance interaction of
the substituents that are in direct conjugation with the
porphyrin π-system.
CuTPP(Ph)4
CuTPP(Ph)4(CH3)4
435 (5.19)
357 (4.25),
466 (sh)
441 (5.07),
354 (4.29),
459 (sh)
440 (5.15),
456 (5.10)
431 (5.26)
353 (4.35),
452 (5.15)
365 (4.37),
465 (5.23)
364 (4.45),
456 (5.31)
466 (5.11)
CuTPP(Ph)4Br4
CuTPP(Ph)4Cl4
575 (4.05), 615 (sh)
574 (4.11), 612 (sh)
CuTPP(Ph)4(CN)4
ZnTPP(Ph)4
ZnTPP(Ph)4(CH3)4
607 (sh), 662 (4.42)
559 (4.00), 601 (sh)
582 (4.02), 619 (3.83)
ZnTPP(Ph)4Br4
ZnTPP(Ph)4Cl4
ZnTPP(Ph)4(CN)4
598 (4.04), 648 (4.00)
587 (4.10), 634 (3.93)
626 (sh), 683 (4.33)
Optical absorption spectra of a representative Cu(TPP-
(Ph)4X4) (X ) H, CN) are shown in Figure 3. The
introduction of four more substituents on the other antipodal
â-pyrrole positions of M(TPP(Ph)4) showed an interesting
trend in their spectral features. Generally, metal complexes
M(TPPR4X4) exhibit a B band and two Q bands with the
extent of red-shift of the B band being about 10-25 nm and
20-50 nm in Q(0,0) transitions relative to their correspond-
ing M(TPPX4) (X ) CH3, CN, Br). Interestingly, M(TPPR4-
(CN)4) complexes showed considerable gain in the intensity
of the longest wavelength Q(0,0) band relative to the Q(1,0)
band (Figure 3), and this is possibly due to stabilization of
b1 (a1 ) relative to b2 (a2 ) and thereby further lifting of
a Error in λmax ) (1.0 nm; values in parentheses refer to log ꢀ (ꢀ in
(dm3/mol)/cm); error in ꢀ ) (7%.
compounds. H2(TPP(CH3)4X4) (X ) H, Br, CN) and H2(TPP-
(Ph)4X4) (X ) H, Br, Cl, CH3, CN) derivatives showed a
Soret, B band, and three visible Q bands when compared to
a B band and four Q bands of H2(TPP). H2(TPP(Ph)4)
exhibited red-shifts in the B band (20 nm) and in Qx(0,0)
(∼30 nm) bands relative to H2(TPP). This is possibly due
to the â-pyrrole phenyl inductive interaction with the
π-system rather than nonplanarity of the porphyrin macro-
cycle since it shows planar geometry.57a However, H2(TPP-
(CH3)4) shows marginal red-shift in the absorption bands
relative to H2(TPP). The M(TPPR4) compounds (R ) CH3
or Ph; M ) Ni(II), Cu(II), and Zn(II)) exhibited a B band
and two Q bands with molar extinction coefficients similar
to those of their corresponding M(TPP) compounds.7,60
The increase in size and number of â-pyrrole substituents
induces greater steric crowding around the peripheral posi-
tions of the porphyrin; one would anticipate an increase in
nonplanarity of the macrocycle that would lead to red-shift
of the absorption bands.61-63 The longest wavelength band,
u
u
degeneracy of the highest occupied molecular orbitals
(HOMOs).62 As anticipated (Table 1), the bands are more
blue-shifted with the increase in electronegativity of the metal
ion.64,65 Generally, M(TPP(CH3)4X4) derivatives show blue-
shifted absorption spectral features relative to M(TPP(Ph)4X4)
derivatives. This is possibly due to the variable steric
crowding that induces conformational differences between
the macrocycles as well as the substituent effects. A
pronounced red-shift in the absorption spectral features of
the mixed substituted cyano and bromo porphyrins is due to
both conjugative effect and nonplanar distortion of the
porphyrin ring. Crystal structure studies on Cu(TPP(Ph)4-
(60) Meot-Ner, M.; Adler, A. D. J. Am. Chem. Soc. 1975, 97, 5107.
(61) Barkigia, K. M.; Chantranupong, L.; Smith, K. M.; Fajer, J. J. Am.
Chem. Soc. 1988, 110, 7566.
(62) Haddad, R. E.; Gazeau, S.; Pecaut, J.; Marchon, J.-C.; Medforth, C.
J.; Shelnutt, J. A. J. Am. Chem. Soc. 2005, 125, 1253 and references
therein.
(63) Parusel, A. B. J.; Tebikie, W.; Ghosh, A. J. Am. Chem. Soc. 2000,
122, 6371.
(64) Gouterman, M. J. Chem. Phys. 1959, 30, 1139.
(65) Ortiz, V.; Shelnutt, J. A. J. Phys. Chem. 1985, 89, 4733.
4142 Inorganic Chemistry, Vol. 45, No. 10, 2006

Mixed Substituted Porphyrins
Table 2. Crystal Structure Data of Mixed Substituted
Metalloporphyrins Cu(TPP(Ph)4(CH3)4)‚2(CHCl3) (1),
Zn(TPP(Ph)4Br4(CH3OH))‚CH3OH (2), and
Ni(TPP(Ph)4(CN)4(Py)2)‚2(Py) (3)
1
2
3
empirical
formula
fw
cryst syst
space group
a (Å)
C74H54Cl6N4Cu C70H48Br4N4O2Zn C92H60N12Ni
1275.45
monoclinic
C2/c
1380.15
monoclinic
P21/c
1392.23
monoclinic
C2/c
17.255(3)
20.397(2)
18.1490(15)
106.030(10)
6139.2(13)
4
13.848(6)
13.835(6)
31.659(14)
97.49(4)
6014(5)
4
19.125(1)
18.531(1)
21.8020
114.35(6)
7038.8
4
b (Å)
c (Å)
â (deg)
vol (Å3)
Z
D
calcd (mg/m3)
1.380
1.504
1.314
wavelength (Å)
T (K)
1.541 80
293
0.710 69
293
1.541 80
293
no. of total reflns 5585
10566
6399
no. of
independent
reflns
R1a
wR2b
4949
4108
5206
0.0542
0.1575
0.0754
0.2144
0.0504
0.1460
a R1 ) ∑||Fo|| - |Fc||/∑|Fo|; Fo > 4σ(Fo). b wR2 ) [∑(Fo - Fc )2)/
2
2
∑w(Fo )2]1/2
.
2
(CH3)4) and Zn(TPP(Ph)4Br4) show considerable nonplanar
distortion of the porphyrin ring. The theoretical studies
reported earlier predicted that the nonplanar porphyrins
exhibit red-shift of the absorption bands.62,63
meso-Tetraphenylporphyrin, H2(TPP), exhibits character-
istic chemical shifts arising from the â-pyrrole and meso-
1
phenyl proton resonances.66 The H NMR spectra of free-
base mixed substituted porphyrins (H2(TPP(Ph)4X4) (X )
CH3, Cl, Br, CN) exhibit resonances arising from meso- and
â-pyrrole phenyls, â-methyl and imino hydrogens. The
chemical shifts of the meso-phenyl protons of H2(TPP-
(Ph)4X4) (X ) CH3, Cl, Br, H) are comparable to those
observed for the corresponding H2(TPP(Ph)4) derivative. The
core imino-protons of H2(TPP(Ph)4X4) (X ) Cl, Br, CH3)
(-1.65 ppm) are downfield-shifted relative to H2(TPP(Ph)4)
(-2.13 ppm).57c Further, the H2(TPP(Ph)4(CN)4) derivative
shows imino-proton resonance at -2.20 ppm. The downfield
shift of the resonances is possibly due to the nonplanar
conformation and/or substituent effects. Similar trends in
chemical shifts of imino-protons were observed for H2(TPP-
(CH3)4X4) derivatives. Nonplanarity of the macrocycle causes
the imino-protons to resonate at downfield relative to that
in planar porphyrins.67 The 1H NMR spectra of M(TPPR4X4)
(M ) Zn(II) and Ni(II)) complexes are devoid of imino-
protons, and their other proton resonances are marginally
upfielded (0.1-0.3 ppm) relative to those of the H2(TPP-
(Ph)4X4) derivative. The integrated intensity of the pro-
ton resonances of these mixed substituted porphyrins
(M(TPPR4X4)) is consistent with the proposed structures.
Figure 4. ORTEP diagrams showing (a) top and (b) side views of Cu-
(TPP(Ph)4(CH3)4)‚2CHCl3. The solvates and the hydrogens are not shown
for clarity, and in the side view, the phenyl groups are not shown for
simplicity. Displacement of porphyrin-core atoms in Å from the mean plane
(esd’s 0.003 Å) is shown in part c.
listed in Table 2. ORTEP views (top and side) of the Cu-
(TPP(Ph)4(CH3)4 and Zn(TPP(Ph)4Br4) complexes are shown
in Figures 4 and 5, respectively. The selected average bond
lengths and bond angles of Cu(TPP(Ph)4(CH3)4)‚2(CHCl3)
and Zn(TPP(Ph)4Br4)(CH3OH)‚CH3OH structures are com-
pared with the reported Zn(TPP(Ph)8) structure to examine
the role of mixed substitution on the stereochemical features
(Table 3). The Cu-N and Zn-N bond distances observed
in these complexes are similar to those reported for their
corresponding M-N bond distances.68 Nonequivalent dis-
tances in the M-N and M-N′ of the M-(N)4 core in these
mixed substituted porphyrins are possibly due to electronic
and steric strain induced by the peripheral substituents.50,69
The extent of steric crowding arising from the methyl (van
der Waals radius (VDW), 2.0 Å) group is anticipated to be
higher than that of bromo group (VDW, 1.95 Å).70 The more
Crystal Structure Discussion. Crystallographic data of
the Cu(TPP(Ph)4(CH3)4) and Zn(TPP(Ph)4Br4) complexes are
(68) Scheidt, W. R.; Lee, Y. J. Struct. Bonding (Berlin) 1987, 64, 1.
(69) Scheidt, W. R. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 3,
p 49.
(66) Scheer, H.; Katz, J. J. In Porphyrins and Metalloporphyrins; Smith,
K. M., Ed.; Elsevier: Amsterdam, The Netherlands, 1975; p 399.
(67) Smirnov, V. V.; Woller, E. K.; DiMagno, S. G. Inorg. Chem. 1998,
37, 4971.
(70) Cotton F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 3rd ed.;
Interscience Publishers: New York, 1972.
(71) Barkigia, K. M.; Nurco, D. J.; Renner, M. W.; Melamed, D.; Smith,
K. M.; Fajer, J. J. Phys. Chem. B 1998, 102, 322.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4143

Bhyrappa et al.
Table 3. Selected Bond Lengths and Geometric Parameters of
Substituted Metalloporphyrins
Cu(TPP(Ph)4(CH3))4 Zn(TPP(Ph)4Br4)) Zn(TPP(Ph)8)a
Bond Length (Å)
1.962(2)
M-N
2.001(7)
2.102(7)
2.044(8)
1.401(10)
1.407(10)
1.426(11)
1.505(12)
1.375(11)
1.294(11)
1.326(12)
1.449(11)
1.07
2.037(4)
M-N′
1.946(2)
M-O
N-CR
1.376(3)
1.373(3)
1.454(4)
1.452(4)
1.371(4)
1.363(4)
1.410(4)
1.404(4)
0.90
1.376(3)
1.456(4)
1.371(6)
1.412(11)
0.96
N′-CR
CR-Câ
CR′-Câ′
Câ-Câ
Câ′-Câ′
CR-Cm
CR′-Cm
∆Câb (Å)
Bond Angle (deg)
124.4(1) 126.9(5)
M-N-CR
124.4(2)
M-N′-CR
N-CR-Cm
N′-CR′-Cm
N-CR-Câ
N′-CR′-Câ′
Câ-CR-Cm
Câ′-CR′-Cm
CR-Cm-CR′
CR′-Cm-CR
CR-Câ-Câ
CR′-Câ′-Câ′
125.1(2)
122.2(2)
122.9(2)
109.1(2)
108.9(2)
128.1(2)
127.7(2)
122.5(2)
122.1(2)
106.9(2)
107.1(2)
119.6(6)
120.8(8)
123.8(8)
111.7(8)
102.8(7)
127.4(9)
132.7(9)
124.6(8)
124.6(8)
106.7(8)
110.5(8)
123.2(3)
109.2(3)
127.5(3)
124.5(3)
107.0(2)
Figure 5. ORTEP diagrams showing (a) top and (b) side views of the
Zn(TPP(Ph)4Br4(CH3OH))‚CH3OH complex. The lattice solvates and
hydrogens are omitted for clarity. Displacement of porphyrin-core atoms
from the mean plane in Å (esd’s 0.008 Å) is shown in part c.
donor effect of the methyl group enhances the electron
density on the pyrrole nitrogens than phenyl groups, and
hence in the Cu(TPP(Ph)4(CH3)4) complex, the Cu-N′ pair
shows a shorter distance than the Cu-N distance (Table 3).
Similarly, the Zn(TPP(Ph)4Br4)(CH3OH)‚(CH3OH) complex
shows longer Zn-N′ bond lengths for pyrroles with bromo
groups and shorter M-N bond distances for pyrroles bearing
phenyl rings. Earlier reports on Zn(TPPBr4(CH3OH))‚DMF
and Ni(TPPBr4(CN)4) structures showed longer M-N (for
the pyrroles with more electron withdrawing substituents)
distances due to decreased electron density on the imino-
nitrogens.42a,50
The nonplanarity of the porphyrin core is induced by the
steric repulsion among the peripheral substituents, which
enforces the relief of the strain through bond lengths and
angles. For the mixed substituted porphyrin structures, one
would anticipate difference in crowding along the antipodal
positions because of the variable shape and size of the
â-pyrrole substituents. Nonplanar conformation arises by the
tilting of the pyrroles to prevent repulsive interactions among
the substituents, and this results in an increase of the Câ-
CR-Cm angles with the concomitant decrease in the N-CR-
Cm and M-N-CR angles. The Cu(TPP(Ph)4(CH3)4) and
Zn(TPP(Ph)4Br4) exhibited about a 2-7° change in these
a Data from ref 71. b Average deviation of â-pyrrole carbon from the
24-atom mean plane.
angles, and this change is comparable to that of the saddle
shaped Zn(TPP(Ph)8) structure (Table 3). The methyl group
of the axially coordinated methanol in Zn(TPP(Ph)4Br4)
shows some positional disorder. It is noteworthy that the
Zn(II) ion is about 0.4033 Å above the mean plane, higher
than that reported for the Zn(TPPBr4(CH3OH)) complex
(0.28 Å),42 and it is 0.07 Å in the case of the four-coordinated
Zn(TPP(Ph)8) complex.71 Another reported five-coordinated
Zn(TPP(Ph)8(CH3CN)) structure shows a 0.233 Å displace-
ment of Zn(II) ion from the mean plane with moderate
saddling of the core (∆Câ ) (0.43 Å).72 Steric and electronic
factors are responsible for the extent of displacement of
the zinc(II) ion from the mean plane of the 24-atom core.
The observed distortions in these porphyrins are larger than
those reported for partially substituted Zn(TPPBr4) com-
plexes.42,54 Moreover, the N1-M-N3 and N2-M-N4
angles are deviated from 180° indicating the nonplanar
(72) Harada, R.; Kojima, T. Acta Crystallogr. 2002, E60, 1097.
4144 Inorganic Chemistry, Vol. 45, No. 10, 2006

Mixed Substituted Porphyrins
conformation of the M-(N)4 core. Each pyrrole ring is
alternately displaced up and down from the mean plane
formed by the 24-atom core. The magnitude of displace-
ments for the â-pyrrole carbons bearing mixed substit-
uents from the mean plane is unequal along the antipodal
directions. The phenyl groups are tilted toward the macro-
cycle plane to minimize the repulsive interaction between
the substituents. The phenyl and bromo groups at the
â-pyrrole positions are titled up and down from the porphyrin
mean plane to minimize steric crowding between the
substituents. The average dihedral angles formed by the
meso-phenyl (53-56°) are lower in contrast to that formed
by the â-pyrrole phenyl groups (65-70°) with the porphyrin
plane for Zn(TPP(Ph)4Br4) and Cu(TPP(Ph)4(CH3)4) struc-
tures. The average bond lengths of the Cm-meso-Cph and
Câ-Câ(Ph) in Zn(TPP(Ph)4Br4) and Cu(TPP(Ph)4(CH3)4) are
found to be 1.48-1.50 Å, indicating no significant conjuga-
tion of the phenyl rings with the porphyrin macrocycle
although the phenyl groups are tilted toward the porphyrin
plane. The extent of observed macrocyclic distortions is
similar to that of fully â-octaalkylated-MTPPs,73 â-octaphe-
nylated-MTPPs71,74 and Zn(TPPBr8(butyronitrile)2)75 struc-
tures. In the case of Zn(TPP(Ph)4Br4(CH3OH))‚CH3OH, the
lattice solvate is weakly hydrogen bonded to the coordinated
methanol with the shortest O‚‚‚O distance of 2.908 Å.76
The extent of mean plane deviation of the atoms in Cu-
(TPP(Ph)4(CH3)4) and Zn(TPP(Ph)4Br4) is shown in Figures
4c and 5c, respectively. The 24-atom core of the Cu(TPP-
(Ph)4(CH3)4) complex (Figure 4c) shows a combination of
saddle with severe ruffled conformation, whereas ZnTPP-
(Ph)4Br4 has more saddling of the core with very gentle
ruffled conformation (Figure 5c). As anticipated for saddle
conformations, the dihedral angles for the meso-phenyl rings
are smaller than those observed for more planar macro-
cycles.68 The size and number of â-pyrrole substituents of
MTPP induce varying degrees of nonplanarity to the por-
phyrin macrocycle. It is known in the literature that the
partially brominated porphyrins, Zn(TPPBr4(CH3OH)),42
ZnTPPBr4(tetrahydrofuran),54 and tetrabromo-tetramesityl-
porphyrins,48 exhibit moderate distortion of the porphyrin
core. The â-octahalogenotetraarylporphyrins, octabromotet-
raphenylporphinato zinc(II) dibutyronitrile,75 octabromotet-
rakis(pentafluorophenyl)porphinato zinc(II) hydrate,77 octa-
bromotetrakis(2,6-dibromo-3,5-dimethoxyphenyl)porphin-
ato copper(II) methanol,29 octabromotetraphenylporphinato
nickel,78 copper(II) and nickel(II) complexes of octabromo-
tetrakispentafluorophenylporphyrins,79 octachloro-meso-tet-
rakis(pentafluorophenyl)porphinato zinc(II),80 octafluorotetra-
phenylporphinato zinc(II),30,31 and octafluorotetrakis(pen-
tafluorophenyl)porphinato zinc(II),81 showed nonplanar dis-
tortion of the porphyrin core. The nonplanar distortion of
the mixed substituted Zn(II) and Cu(II) porphyrins has been
largely due to the steric crowding of the peripheral groups
to minimize the repulsive interactions among the substituents.
The closest distance between the porphyrin units is more
than 3.40 Å indicating the nonplanar conformation in these
systems is influenced by the repulsive interactions among
the peripheral groups. This also suggests the π-π interac-
tions between the phenyl rings of the porphyrin macrocycles
in the lattice are negligible.
The crystal structure of the six-coordinated complex Ni-
(TPP(Ph)4(CN)4(Py)2)‚2Py was examined to determine the
role of cyano and phenyl groups at the â-pyrrole positions
on the stereochemistry of the coordinated nickel(II) porphyrin
complex. The crystallographic data of the complex are shown
in Table 2. Its molecular structure is shown along with the
numbering scheme in Figure 6a. Figure 6b,c shows the side
on view and the perpendicular displacement of all the skeletal
atoms from the 24-atom core mean plane, respectively. A
comparison of Ni(TPP(Ph)4(CN)4)(Py)2) is made with the
reported Ni(TPPBr4(CN)4)(Py)2) structure (Table 4) to
examine the role of mixed â-pyrrole (phenyl and cyano vs
bromo and cyano) groups on the structural changes. The
geometry around the Ni(II) center, axial Ni-Npyridine and
equatorial Ni-N bond lengths of the structure (Table 4),
seems to suggest that the Ni(II) is in the typical S ) 1 spin
system rather than in the four-coordinate Ni(II) porphyrins
with shorter Ni-N distances of Ni(II) (S ) 0) systems.82
As reported previously, the four-coordinate low-spin Ni(II)
porphyrins82 tend to show shorter Ni-N bond lengths than
the six-coordinate high-spin50 Ni(II) porphyrin complexes.
The six-coordinate structure of the Ni(TPP(Ph)4(CN)4(py)2)‚
2Py complex is possibly due to the electron deficiency of
the Ni(II) center induced by the electron deficiency of the
porphyrin macrocycle. Interestingly, the Ni-N (for the
pyrrole with CN groups) distance seems to be shorter than
the Ni-N′ distance (for the pyrrole with phenyl groups)
(Table 4). A similar trend in Ni-N bond distances was
reported for the Ni(TPPBr4(CN)4(Py)2)‚2.5(C2H4Cl2) struc-
ture.50 The expansion of the porphyrin core in the Ni(TPP-
(Ph)4(CN)4(Py)2)‚2Py complex is reflected from the bond
lengths (N-CR and CR-Cm) and angles (CR-N-CR, N-CR-
Cm, and N-CR-Câ), and it is similar to that found in the
(79) Henling, L. M.; Schaefer, W. P.; Hodge, J. A.; Hughes, M. E.; Gray,
H. B. Acta Crystallogr. 1993, C49, 1743.
(73) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner, M.
W.; Smith, K. M. J. Am. Chem. Soc. 1990, 112, 8851.
(74) Nurco, D. J.; Medforth, C. J.; Forsyth, T. P.; Olmstead, M. M.; Smith,
K. M. J. Am. Chem. Soc. 1996, 118, 10918.
(75) Bhyrappa, P.; Krishnan, V.; Nethaji, M. J. Chem. Soc., Dalton Trans.
1993, 1901.
(76) Bhyrappa, P.; Wilson, S. R.; Suslick, K. S. J. Am. Chem. Soc. 1997,
119, 8492.
(77) Marsh, R. E.; Schaeffer, W. P.; Hodge, J. A.; Hughes, M. E.; Gray,
H. B.; Lyons, J. E.; Ellis, P., Jr. Acta Crystallogr. 1993, C49, 1339.
(78) Mandon, D.; Ochsenbein, P.; Fischer, J.; Weiss, R.; Jayaraj, K.; Austin,
R. N.; Gold, A.; White, P. S.; Brigaud, O.; Battioni, P.; Mansuy, D.
Inorg. Chem. 1992, 31, 2044.
(80) Birnbaum, E. R.; Hodge, J. A.; Grinstaff, M. W.; Schaefer, W. P.;
Henling, L.; Labinger, J. A.; Bercaw, J. E.; Gray, H. B. Inorg. Chem.
1995, 34, 3625.
(81) Smirnov, V. V.; Woller, E. K.; Tatman, D.; DiMagno, S. G. Inorg.
Chem. 2001, 40, 2614.
(82) (a) Scheidt, W. R. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1978; Vol. 3, p 463. (b) Barkigia, C. M.; Renner,
M. W.; Furenlid, L. R.; Medforth, C. J.; Smith, K. M.; Fajer, J. J.
Am. Chem. Soc. 1993, 115, 3627. (c) Hobbs, J. D.; Majumder, S. A.;
Luo, L.; Sickelsmith, G. A.; Auirke, J. M. E.; Medforth, C. J.; Smith,
K. M.; Shelnutt, J. A. J. Am. Chem. Soc. 1994, 116, 3261. (d)Veyarath,
M.; Ramassuel, R.; Marchon, J. C.; Turovska-Tyrk, I.; Scheidt, W.
R. New J. Chem. 1995, 19, 1199.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4145

Bhyrappa et al.
Table 4. Selected Bond Lengths and Geometric Parameters of
Ni(TPP(Ph)4(CN)4(Py)2)‚2(Py) and Ni(TPPBr4(CN)4(Py)2)‚2.5(C2H4Cl2)
Structures
Ni(TPP(Ph)4(CN)4(Py)2)
Ni(TPPBr4(CN)4(Py)2)a
Bond Length (Å)
2.051(2)
2.089(2)
2.216(3)
1.369(3)
1.375(3)
1.438(1)
1.462(3)
1.371(3)
1.356(3)
1.409(3)
1.402(3)
0.0438
Ni-N
2.040(2)
2.073(2)
2.153(3)
1.367(3)
1.375(3)
1.441(3)
1.463(3)
1.371(4)
1.341(4)
1.405(8)
Ni-N′
Ni-Npyridine
N-CR
N′-CR′
CR-Câ
CR′-Câ′
Câ-Câ
Câ′-Câ′
CR-Cm
CR′-Cm
∆Câb (Å)
0.625
Bond Angles (deg)
125.7(1)
126.3(2)
127.0(2)
125.0(2)
108.6(2)
109.3(2)
124.4(2)
125.7(2)
125.4(2)
107.2(2)
107.1(2)
108.4(2)
107.3(2)
Ni-N-CR
124.9(3)
125.1(3)
126.0(2)
124.6(2)
108.6(2)
108.1(2)
125.3(4)
127.2(4)
124.6(2)
107.1(2)
107.6(2)
108.4(2)
108.2(2)
Ni-N′-CR′
N-CR-Cm
N′-CR′-Cm
N-CR-Câ
N′-CR′-Câ′
Câ-CR-Cm
Câ′-CR′-Cm
CR-Cm-CR′
CR-Câ-Câ
CR′-Câ′-Câ′
CR-N-CR
CR′-N′-CR′
Figure 6. ORTEP diagrams showing (a) top and (b) side views of the
Ni(TPP(Ph)4(CN)4(Py)2)‚2Py complex. The lattice solvates and hydrogens
are omitted for clarity. In the side view, the phenyl groups are not shown
for simplicity. Displacements of the porphyrin-core atoms from the mean
plane in Å (esd’s 0.0017) are shown in part c.
six-coordinated Ni(TPPBr4(CN)4(Py)2)‚2.5(C2H4Cl2) complex
(Table 4). Another interesting feature of the complex is that
the Ni-Npyridine bond distance (2.216(3) Å) in Ni(TPP)(Ph)4-
(CN)4(Py)2)‚2Py is longer than that reported for the mixed
substituted Ni(TPPBr4(CN)4(Py)2)‚2.5(C2H4Cl2) complex
(2.153(3) Å) and shorter than that found in the Ni(TPP(CN)4-
(Py)2) complex (2.242(2) Å).83 The two axially ligated
pyridine rings are almost planar, and their planes are
perpendicular to each other. Furthermore, the axial (Ni-
a Data from ref 50. b Average deviation of â-pyrrole carbon atom from
the 24-atom mean plane.
pyridines is probably due to favorable back-donation from
the nickel dπ(dxz, dyz) to the π* orbitals of the pyridines and
also reduces the nonbonding interactions between the axial
pyridines with the porphyrin ring.
Surprisingly, the mean plane displacement of core atoms
in the Ni(TPP(Ph)4(CN)4(Py)2)‚2Py complex (Figure 6c)
shows very minimal wavelike conformation in contrast to
ruffling combined with the moderate saddling of the por-
phyrin macrocycle reported for the Ni((TPP)Br4(CN)4(Py)2‚
2.5(C2H4Cl2) complex. This is reflected from the mean plane
displacement of â-pyrrole carbons (Table 4) and meso-
Npyridine av distance is longer than the equatorial (Ni-N)av
)
distance, but it is similar to that reported for Ni(TPPBr4-
(CN)4(Py)2)‚2.5C2H4Cl2 structure (Table 4). In the case of
the Ni(TPP(Ph)4(CN)4(Py)2)‚2Py structure, the two axially
coordinated pyridines are in an almost mutually perpendicular
orientation, and the axial pyridine mean planes make dihedral
angles68 close to 45° (42.2 and 49°) with the closest
equatorial Ni-N axis. Similar features were reported previ-
ously for Ni(TPPBr4(CN)4(Py)2‚2.5(C2H4Cl2)50 and other iron
porphyrins.84 The staggered orientation of the axially ligated
(84) (a) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.;
Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935. (b) Grinstaff, M.
W.; Hill, M. G.; Birnbaum, E. R.; Schaefer, W. P.; Labinger, J. A.;
Gray, H. B. Inorg. Chem. 1995, 34, 4896. (c) Safo, M. K.; Walker, F.
A.; Raitsimring, A. M.; Walters, W. P.; Dolata, D. P.; Debrunner, P.
G.; Scheidt, W. R. J. Am. Chem. Soc. 1994, 116, 7760. (d) Safo, M.
K.; Gupta, G. P.; Watson, C. T.; Simonis, U.; Walker, F. A.; Scheidt,
W. R. J. Am. Chem. Soc. 1992, 114, 7066.
(83) Duval, H.; Bulach, V.; Fischer, J.; Weiss, R. Acta Crystallogr. 1997,
C53, 1027.
4146 Inorganic Chemistry, Vol. 45, No. 10, 2006

Mixed Substituted Porphyrins
Figure 7. Cyclic voltammograms of CuTPP(Ph)4X4 (X ) CH3, Br, CN)
complexes in CH2Cl2 containing 0.1 M TBAPF6 with a scan rate of 0.1
V/s at 298 K.
carbons (Figure 6c). Further, the phenyl groups are oriented
perpendicular to the mean plane of the porphyrin ring with
a dihedral angle of 81.2° and 80.3° for the meso-phenyl and
â-pyrrole phenyl groups, respectively. The nearly planar
conformation of the porphyrin macrocycle in Ni(TPP(Ph)4-
(CN)4(Py)2)‚2Py is probably due to the decreased steric
crowding at the antipodal positions compared to that of the
Ni(TPPBr4(CN)4)‚2.5(C2H4Cl2) complex rather than any
intermolecular interactions since the closest intermolecular
contact in NiTPP(Ph)4(CN)4(Py)2‚2Py is greater than 3.394
Å.
Electrochemistry. To probe the influence of mixed
â-pyrrole substitution on the π-electronic properties of the
MTPPR4X4 compounds (M ) 2H, Ni(II), Cu(II), and Zn-
(II)), the electrochemical redox behavior of these porphyrins
was examined by cyclic voltammetric studies. Figure 7 shows
the cyclic voltammograms of CuTPP(Ph)4X4 bearing varying
â-pyrrole substituents. The effect of core metal ion on the
redox potentials of the representative porphyrins MTPP-
(Ph)4Cl4 (M ) Ni(II), Cu(II) and Zn(II)) is shown in Figure
8. Table 5 lists the electrochemical data for the MTPP(Ph)4X4
complexes. Under similar conditions, the corresponding
MTPP compounds were also examined and the data is
listed in Table 5. The data analysis of free-base and metal
complexes of the mixed substituted porphyrins MTPPR4X4
(X ) H, CH3, Br, Cl, CN) series indicates the following
observations: (1) The mixed substituted porphyrins show
two successive one-electron electrochemical oxidations and
reductions. (2) Notably, the first ring oxidation potentials
span a range from 0.38 to 1.36 V while reduction potentials
vary from -0.28 to -1.5 V. (3) For the Hammett equation
E1/2 ) 4σF, the plot of the first ring oxidation and reduction
versus the Hammett parameter85 (σp) of the substituents (X)
in MTPPR4X4 was examined to delineate the role of X on
the potentials of MTPPR4, and they show more linear trend
in reduction than in oxidation (Figure 9 and Table 6). (4)
The reaction constant (F) for the oxidation and reduction is
Figure 8. Cyclic voltammograms of MTPP(Ph)4Cl4 complexes in CH2-
Cl2 containing 0.1 M TBAPF6 with a scan rate of 0.10 V/s at 298 K.
Table 5. Half-Wave Electrochemical Redox Dataa (vs Ag/AgCl) of
M(TPP(Ph)4X4) Derivatives in CH2Cl2 Containing 0.1 M TBAPF6 at
298 K
oxidation (V)
reduction (V)
II
porphyrin
H2(TPP)
I
II
∆E1/2d (V)
I
1.00
0.83
0.59
0.82
0.86
1.13
1.02
0.91
0.72
1.02
1.05
1.36b
0.97
0.75
0.46
0.77
0.78
1.16
0.84
0.74
0.38
0.72
0.74
1.02
1.34
0.95
0.84
0.93
0.98
1.38
1.32
1.19
1.03
1.17
1.17
2.23
2.02
1.79
1.70
1.77
1.41
2.30
2.23
2.12
2.05
2.09
1.78
2.27
2.08
1.82
1.80
1.81
1.54
2.20
2.13
1.88
1.78
1.80
1.48
-1.23 -1.54
-1.19
H2(TPP(Ph)4)
H2(TPP(Ph)4(CH3)4)
H2(TPP(Ph)4Br4)
H2(TPP(Ph)4Cl4)
H2(TPP(Ph)4(CN)4)
Ni(TPP)
-1.20
-0.88 -1.25
-0.91 -1.03
-0.28 -0.56
-1.28 -1.72
-1.32 -1.71
-1.40 -1.74
-1.03 -1.32
-1.04 -1.42
-0.42 -0.86
-1.30 -1.70
-1.33 -1.57
-1.36 -1.70
-1.03 -1.32
-1.03 -1.38
-0.38 -0.80
-1.36 -1.77
-1.39 -1.79
-1.50 -1.67c
-1.06 -1.27
-1.06 -1.27
-0.46 -0.82
Ni(TPP(Ph)4)
Ni(TPP(Ph)4(CH3)4)
Ni(TPP(Ph)4Br4)
Ni(TPP(Ph)4Cl4)
Ni(TPP(Ph)4(CN)4)
Cu(TPP)
1.35
1.26
1.04
1.29
1.29
1.38
1.15
0.88
0.66
0.90
0.88
1.35
Cu(TPP(Ph)4)
Cu(TPP(Ph)4(CH3)4)
Cu(TPP(Ph)4Br4)
Cu(TPP(Ph)4Cl4)
Cu(TPP(Ph)4(CN)4)
Zn(TPP)
Zn(TPP(Ph)4)
Zn(TPP(Ph)4(CH3)4)
Zn(TPP(Ph)4Br4)
Zn(TPP(Ph)4Cl4)
Zn(TPP(Ph)4(CN)4)
a Error in redox potentials ) (10 mV; scan rate ) 0.1 V/s. b Two
I
electron oxidation. c Irreversible. d Ioxidation - reduction.
in the range 0.15-0.22 and 0.29-0.38 V, respectively. (5)
The first ring redox potentials are cathodic (0.03-0.10 mV)
for the MTPP(CH3)4X4 series relative to the MTPP(Ph)4X4
series. (6) The difference in the first ring redox potential
(HOMO-LUMO gap) is narrower (Table 5) with the
increase in the Hammett parameter of the substituents.
Interestingly, the first oxidation potentials of MTPPR4X4
are more cathodically shifted (150-500 mV) while reduction
potentials are marginally cathodic (∼50 mV) relative to their
corresponding MTPPX4 (X ) CN, Br) derivatives.38-40,42 The
reactivity constant, F, for the MTPP(Ph)4X4 is comparable
(85) The Hammett parameter (σp) of the substituents (X) was taken from
the following: Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991,
91, 165.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4147

Bhyrappa et al.
Table 7. Comparison of B Band and Q Band Transitions and
HOMO-LUMO Gap (∆E1/2) of Various Free-Base Porphyrins in
CH2Cl2 at 298 K
b
B
(nm)
Qx(0,0)
(nm)
Qx(0,0)
(eV)
∆E1/2
(V)
porphyrin
ref
(TPP(Ph)4(CH3)4)
(TPP(Ph)4Cl4)
(TPP(Ph)4Br4)
(TPP(Ph)4(CN)4)
(TPP(Ph)4)
(TPP(CH3)4Br4)
(TPP(CH3)4(CN)4)
(TPP(CH3)4)
(TPP(CH3)8)
(TPP(Ph)8)
457
463
470
464
434
461
457
420
447
468
452
469
414
704
717
734
778
677
722
764
641
691
724
720
743
646
1.76
1.73
1.69
1.59
1.83
1.72
1.62
1.93
1.79
1.71
1.72
1.67
1.92
1.79
1.77
1.70
1.41
2.02
1.76
1.47
2.06
c
c
c
1.64
2.23
a
a
a
a
a
a
a
42, a
19
89
90
25
7
(TPPCl8)
(TPPBr8)
(TPP)
I
I
a This work. b ∆E1/2 ) oxidation - reduction. c Data not available.
complexes showed considerable nonplanar conformation of
the porphyrin ring. The ease of oxidation in mixed substituted
porphyrins relative to the corresponding M(TPPX4) com-
plexes has been ascribed to the electronic effects of the
substituents and nonplanar distortion of the macrocycle;
however, the reduction potentials are predominantly depend-
ent on the nature of the substituents.
Figure 9. Plot of first ring redox potentials versus the Hammett parameter
(σp) of the substituents (X) for H2TPP(Ph)4(X)4 derivatives.
Table 6. Summary of Hammett Plots with Reaction Constants (F) and
Correlation Coefficients (r2) of First Ring Redox Potentials for Various
Mixed Substituted Porphyrinsa
first oxidation
first reduction
porphyrin
F (V)
r2
F (V)
r2
A comparison of optical absorption spectral data and
electrochemical redox potentials of the mixed substituted
free-base porphyrins was made with the literature data of
the porphyrins with similar â-pyrrole substituents, H2(TPP-
(X8)) compounds (X ) Br, Cl, Ph, CH3), and showed an
interesting trend in their properties (Table 7). The Qx(0,0)
transitions and ∆E1/2 of H2(TPPR4X4) compounds are
intermediate in energy when compared to the parent ana-
logues of similar â-pyrrole substituted-H2TPP compounds.
Qualitatively, the ∆E1/2 from the redox potentials correlates
fairly well with the HOMO-LUMO gap calculated from
the longest wavelength band in the electronic spectra of these
derivatives.
H2(TPP(Ph)4X4)
Ni(TPP(Ph)4X4)
Cu(TPP(Ph)4X4)
Zn(TPP(Ph)4X4)
H2(TPP(CH3)4X4)
Ni(TPP(CH3)4X4)
Cu(TPP(CH3)4X4)
Zn(TPP(CH3)4X4)
0.150
0.190
0.190
0.170
0.150
0.220
0.220
0.170
0.906
0.987
0.926
0.842
0.797
0.903
0.863
0.717
0.290
0.300
0.310
0.320
0.330
0.350
0.340
0.375
0.950
0.975
0.956
0.988
0.993
0.998
0.999
0.967
a MTPPR4X4: R) Ph, X ) CH3, H, Br, Cl, CN; R ) CH3, X ) Ph, H,
Br, CN.
to that of the partially â-pyrrole substituted-TPP series,39 and
it is considerably higher than that reported for the â-octahalo-
MT(4-X-phenyl)P series86 or MT(4-X-phenyl)P series.87 The
fairly broader range of F values for the first ring redox
potentials of mixed substituted porphyrins is possibly due
to the effect of the core metal ion and the nature of the mixed
substituted porphyrin macrocycle. As reported earlier, the
oxidation potentials are largely influenced by the substituent
effect and nonplanarity of the porphyrin macrocycle while
the reduction potentials are independent of structural
change.17,48,88 The planar structure57a of H2(TPP(Ph)4) is
anticipated to show variable degrees of nonplanarity with
an increase in size or shape of the substituent X in
M(TPPR4X4) derivatives. The crystal structures of Cu(TPP-
(Ph)4(CH3)4)‚2CHCl3 and Zn(TPP(Ph)4Br4(CH3OH))‚CH3OH
Conclusions
In this article, two new families of mixed â-pyrrole
substituted tetraphenylporphyrins have been reported. The
mixed substituted free-base porphyrins and their metal
complexes exhibited dramatic shift of the longest wavelength
band relative to their corresponding H2(TPPR4) (R ) CH3,
Ph) or H2(TPPX4) (X ) H, Br, CN) derivatives. This has
been attributed to the nature of the substituents and/or
nonplanarity of the porphyrin ring. Crystal structures of Cu-
(TPP(Ph)4(CH3)4) and Zn(TPP(Ph)4Br4) complexes showed
nonplanar structure of the porphyrin ring and have been
interpreted in terms of increased steric crowding; however,
the nearly planar geometry of the porphyrin ring of a six-
coordinated Ni(TPP(Ph)4(CN)4(Py)2)‚2(Py) complex is prob-
ably due to the decreased steric repulsive interactions among
the peripheral substituents. Electrochemical redox potentials
of these porphyrins showed dramatic variation and span a
wide range in first ring oxidation (0.38-1.36 V) and in
reduction (-0.28 to -1.5 V) potentials. The redox potentials
were analyzed by Hammett plots, and it was found that the
(86) Ghosh, A.; Halvorsen, I.; Nilsen, H. J.; Steene, E.; Wondimagegn,
T.; Lie, R.; Van Caemelbecke, E.; Guo, N.; Ou, Z.; Kadish, K. M. J.
Phys. Chem. B 2001, 105, 8120.
(87) (a) Walker, F. A.; Beroiz, D.; Kadish, K. M. J. Am. Chem. Soc. 1976,
98, 3484. (b) Kadish, K. M.; Morrison, M. M. J. Am. Chem. Soc.
1976, 98, 3326.
(88) Takeuchi, T.; Gray, H. B.; Goddard, W. A., III. J. Am. Chem. Soc.
1994, 116, 9730.
(89) Medforth, C. J.; Senge, M. O.; Smith, K. M.; Sparks, L. D.; Shelnutt,
J. A. J. Am. Chem. Soc. 1992, 114, 9859.
(90) Wijesekara, T.; Dupre, D.; Cader, M. S. R.; Dolphin, D. Bull. Soc.
Chim. Fr. 1996, 133, 765.
4148 Inorganic Chemistry, Vol. 45, No. 10, 2006

Mixed Substituted Porphyrins
extent of HOMO-LUMO gap decreases with increase in
the Hammett parameter of the substituents. The similarity
in reduction rather than in oxidation potentials of mixed
substituted porphyrins M(TPPR4X4) (R ) Ph, CH3; X ) Br,
CN) relative to the corresponding M(TPPX4) has been
interpreted in terms of the greater effect of substituent X on
the LUMO levels and nonplanarity of the porphyrin mac-
rocycle and/or substituent effects on the HOMO levels of
the porphyrin π-system.
and in part by the Defense Research Development Organi-
sation (DRDO), Government of India (to P.B.).
Supporting Information Available: Tables of optical absorp-
tion and electrochemical redox data of MTPP(CH3)4X4 deriva-
tives; bond lengths, bond angles, and anisotropic thermal param-
eters for Cu(TPP(Ph)4(CH3)4)‚2(CHCl3), Zn(TPP(Ph)4Br4-
(CH3OH))‚(CH3OH), and Ni(TPP(Ph)4(CN)4(Py)2)‚2(Py) com-
plexes and their crystallographic information file in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This work was supported by the funds
from the Council of Scientific and Industrial Research (CSIR)
IC052035B
Inorganic Chemistry, Vol. 45, No. 10, 2006 4149