Synthesis, reactions, and electronic properties of 16 π-electron octaisobutyltetraphenylporphyrin

Article abstract of DOI:10.1021/ja102817a

The reaction of the doubly oxidized β-octaisobutyl-meso- tetraphenylporphyrin (OiBTPP, 4), which has a 16 π-electronic structure at the porphyrin core, with a variety of metal reagents was investigated. The reaction of 4 with SnCl₂ followed by

Full text of DOI:10.1021/ja102817a

Published on Web 08/23/2010
Synthesis, Reactions, and Electronic Properties of 16
π-Electron Octaisobutyltetraphenylporphyrin
Yohsuke Yamamoto,*,† Yusuke Hirata,† Megumi Kodama,† Torahiko Yamaguchi,†
Shiro Matsukawa,†,O Kin-ya Akiba,‡ Daisuke Hashizume,§ Fujiko Iwasaki,⊥
Atsuya Muranaka,| Masanobu Uchiyama,|,∞ Ping Chen,# Karl M. Kadish,# and
Nagao Kobayashi3
Department of Chemistry, Graduate School of Science, Hiroshima UniVersity, 1-3-1
Kagamiyama, Higashi-Hiroshima 739-8526, Japan, AdVanced Research Center for Science and
Engineering, Waseda UniVersity, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan, Molecular
Characterization Team and AdVanced Elements Chemistry Laboratory, Institute of Physics and
Chemistry Research, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan, X-ray Laboratory,
Rigaku Corporation, 3-9-12, Matsubaracho, Akishima, Tokyo 195-8666, Japan, Department of
Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, and Department of Chemistry,
Graduate School of Science, Tohoku UniVersity, Aoba-ku, Miyagi 980-8578, Japan
Received April 11, 2010; E-mail: yyama@sci.hiroshima-u.ac.jp
Abstract: The reaction of the doubly oxidized ꢀ-octaisobutyl-meso-tetraphenylporphyrin (OiBTPP, 4), which
has a 16 π-electronic structure at the porphyrin core, with a variety of metal reagents was investigated.
The reaction of 4 with SnCl2 followed by ethanolysis afforded an 18 π-electron tin complex, (OiBTPP)-
Sn(OEt)2 (5), in a redox manner. No reactions were observed using zerovalent metals (Zn, Cu, and Pd).
However, the reaction of 16π [(OiBTPP)Li]+[BF4]- (6), which was easily derived from 4, with Zn, Cu, and
Pd2(dba)3 gave the corresponding 18π metalloporphyrins (OiBTPP)M (7, M ) Zn(EtOH); 8, M ) Cu; and
9, M ) Pd). One-electron oxidation of the copper complex 8 by AgSbF6 afforded a 17 π-electron cation
radical complex, [(OiBTPP)Cu]•+[SbF6]- (10). The UV-visible and electron spin resonance spectra of 10
were quite similar to those of previously reported ꢀ-octaethyl-meso-tetraphenylporphyrin (OETPP)
derivatives, [(OETPP)Cu]•+X- (X ) ClO4, I). In contrast to the reaction of 6 with Zn to give the 18π complex
7, the reaction of 4 with divalent ZnCl2 enabled us to isolate a new 16π porphyrin-zinc(II) complex,
[(OiBTPP)Zn(Cl)]+[ZnCl3]- (11), in 92% yield. The solid-state structures of 5 and 7-11 were unambiguously
determined by single-crystal X-ray crystallography. The porphyrin cores of 10 (17π) and 11 (16π) are much
more distorted than those of the 18π derivatives 5 and 7-9. Furthermore, the bond distances of 10 and 11
clearly showed the presence of bond alternation in contrast to aromatic 18π species 8 and 7, respectively.
Nucleus-independent chemical shift calculations of 4 and some metalated porphyrins indicated that the
highly distorted 16π porphyrins are essentially nonaromatic, with only weak antiaromaticity. Magnetic circular
dichroism studies in conjunction with ZINDO/S calculations assisted in identifying the electronic transitions
of the UV-vis spectra of key porphyrins. Electrochemical and thin-layer UV-vis spectroelectrochemical
experiments on 4 (16π) and 11 (16π) indicated that both compounds can be electroreduced to give the
18π species, with the 16π/18π transition being reversible in the case of [(OiBTPP)Zn(Cl)]+[ZnCl3]- (11).
Introduction
derivatives, especially metalloporphyrins, have been carried out,
and through these endeavors unique structural, photochemical, and
Porphyrins, macrocycles having four pyrrole rings connected
with four methine carbons, have drawn much attention through
the years due to their close relevance to biological activities
involving heme proteins.1 Thus, a number of studies on porphyrin
electrochemical characters have been revealed.2
Ordinary porphyrins have an aromatic 18 π-electron conju-
gated system delocalized over 24 core atoms, satisfying Hu¨ckel’s
(4n + 2) π-electron rule. As for 4n π-electron porphyrins, they
could in principle be obtained by the oxidation or reduction of
the 18π system (Scheme 1). However, due to their unstable
† Hiroshima University.
‡ Waseda University.
§ Molecular Characterization Team, RIKEN.
| Advanced Elements Chemistry Lab, RIKEN.
⊥ Rigaku Corp.
(1) (a) Mansuy, D.; Battioni, P. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: Burlington,
MA, 2000, Vol. 4, pp 1-15. (b) Poulos, T. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: Burlington, MA, 2000, Vol. 4, pp 189-218. (c) Ogoshi, H.;
Mizutani, T.; Hayashi, T.; Kuroda, Y. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press:
Burlington, MA, 2000, Vol. 6, pp 279-340.
# University of Houston.
3 Tohoku University.
O Current address: Department of Chemistry, Faculty of Science, Toho
University, Funabashi 274-8510, Japan.
∞ Current address: Graduate School of Pharmaceutical Sciences, The
University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
9
10.1021/ja102817a  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 12627–12638 12627

A R T I C L E S
Yamamoto et al.
Scheme 1. Redox Transformation among 16, 18, and 20 π-Electron Porphyrin Cores
Scheme 2. Synthesis of 16 π-Electron OETPP (2) and OiBTPP (4)
expected to be much more stable than a 16π state. On the other
hand, if a porphyrin core were distorted to some extent, the
energy difference between the 18π and 16π states would be
expected to become smaller due to destabilization of the 18π
state. This rationalization implies that the unexpected stability
of the 16π state in 2 and 4 arose from the considerable distortion
of the porphyrin core caused by the steric bulk of the 12
peripheral substituents. The significance of the deformation of
the conjugated core was also supported by the fact that the more
distorted OiBTPP (4) was more stable than OETPP (2). The
bond lengths of the porphyrin skeletons in 2 and 4 clearly
showed the presence of bond alternation. As shown in Figure
1, there was a unequivocal difference in bond lengths between
corresponding adjacent bonds (1.31-1.37 and 1.42-1.49 Å)
for 16π OETPP (2), suggesting that it was nonaromatic. Another
distinct difference between the 16 and 18 π-electron states was
observed in the UV-vis spectra. The UV-vis absorptions for
18π (OETPP)H2 (1) appeared at 446, 548, and 588 nm, whereas
those for 16π OETPP (2) were highly blue-shifted to 275 and
339 nm and had smaller extinction coefficients. In addition, no
absorptions in the Q-band region were observed for the latter.
These experimental results established 2 and 4 as the first fully
characterized 16π porphyrins.
electronic structure, the isolation of such “nonaromatic” or
“antiaromatic” species is not an easy task, and the pursuit of
such species remains a challenge.2 Thus, for the reduced species
(20 π-electron state), N,N′,N′′,N′′′-tetramethylisophlorin,3a ꢀ-tet-
rakis(trifluoromethyl)-meso-tetraphenylporphyrin,3b and tetra-
phenylporphyrin-silicon3c and -germanium complexes3d are
the only examples that have been isolated and structurally
determined. Oxidized porphyrins (16 π-electron state) are even
rarer, and only two examples have been reported so far, by us4
and by Vaid’s group,5 to the best of our knowledge (vide infra).
During the course of our studies to synthesize highly distorted
porphyrin-main group element complexes,6 we fortuitously
found that reaction of the dilithiated salt of ꢀ-octaethyl-meso-
tetraphenylporphyrin ((OETPP)H2) or ꢀ-octaisobutyl-meso-
tetraphenylporphyrin ((OiBTPP)H2) with SOCl2 gives doubly
oxidized octaalkyltetraphenylporphyrins (OETPP or OiBTPP)
with a 16 π-electron core (Scheme 2).4 X-ray analysis revealed
that the 16π porphyrins 2 and 4 were more distorted than the
corresponding 18π analogues 1 and 3, respectively. If a
porphyrin core were planar, an aromatic 18π state would be
After our initial report, Vaid and co-workers disclosed their
isolation of [(TPP)Li]+[BF4]- (TPP ) meso-tetraphenylporphy-
rin) bearing a 16π porphyrin skeleton by the reaction of
dilithiated tetraphenylporphyrin (Li2(TPP)) with thianthrenium
tetrafluoroborate (Thn+BF4-).5 This is the only report on the
isolation of a 16π porphyrin-metal complex to date. They
attempted to remove the lithium cation, but metal-free 16π TPP
could not be isolated. Therefore, they concluded that the 16π
TPP was probably not stable as a free molecule and required
stabilization by coordination to the lithium cation. This is in
clear contrast to our sterically distorted 2 and 4. Nonetheless,
the UV-vis spectrum of [(TPP)Li]+[BF4]- (λmax ) 332 and
394 nm) was similar to those of 2 and 4, giving rise to a general
trend that the UV-vis spectra of 16π porphyrins show absorp-
tions that are blue-shifted compared with those of their 18π
counterparts.
(2) (a) Sanders, J. K. M.; Bampos, N.; Clyde-Watson, Z.; Darling, S. L.;
Hawley, J. C.; Kim, H.-J.; Mak, C. C.; Webb, S. J. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: Burlington, MA, 2000, Vol. 3, pp 1-48. (b) Kadish, K. M.;
Van Caemelbecke, E. Royal, G. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: Burlington,
MA, 2000, Vol. 8, pp 1-97. (c) Brothers, P. J. AdV. Organomet. Chem.
2001, 48, 289–342. (d) Hoard, J. L. In Porphyrins and Metallopor-
phyrins; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; p 317.
(3) (a) Vogel, E.; Grigat, I.; Ko¨cher, M.; Lex, J. Angew. Chem., Int. Ed.
Engl. 1989, 28, 1655–1657. (b) Liu, C.; Shen, D.-M.; Chen, Q.-Y.
J. Am. Chem. Soc. 2007, 129, 5814–5815. (c) Cissell, J. A.; Vaid,
T. P.; Rheingold, A. L. J. Am. Chem. Soc. 2005, 127, 12212–12213.
(d) Cissell, J. A.; Vaid, T. P.; Yap, G. P. A. J. Am. Chem. Soc. 2007,
129, 7841–7847.
(4) Yamamoto, Y.; Yamamoto, A.; Furuta, S.-y.; Horie, M.; Kodama, M.;
Sato, W.; Akiba, K.-y.; Tsuzuki, S.; Uchimaru, T.; Hashizume, D.;
Iwasaki, F. J. Am. Chem. Soc. 2005, 127, 14540–14541.
(5) Cissell, J. A.; Vaid, T. P.; Yap, G. P. A. Org. Lett. 2006, 8, 2401–
2404.
(6) (a) Akiba, K.-y.; Nadano, R.; Satoh, W.; Yamamoto, Y.; Nagase, S.;
Ou, Z.; Tan, X.; Kadish, K. M. Inorg. Chem. 2001, 40, 5553–5567.
(b) Yamamoto, Y.; Akiba, K.-y. J. Organomet. Chem. 2000, 611, 200–
209. (c) Yamamoto, A.; Satoh, W.; Yamamoto, Y.; Akiba, K.-y. Chem.
Commun. 1999, 147–148. (d) Yamamoto, Y.; Nadano, R.; Itagaki,
M.; Akiba, K.-y. J. Am. Chem. Soc. 1995, 117, 8287–8288.
Figure 1. Bond alternation in 16 π-electron OETPP (2).
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16 π-Electron Octaisobutyltetraphenylporphyrin
A R T I C L E S
Scheme 3. Improved Synthesis of 16 π-Electron OiBTPP (4) Using
SbCl5
Scheme 4. Reactions of 16 π-Electron OiBTPP (4) and
[(OiBTPP)Li]+[BF4]- (6) with Low-Valent Metal Reagents
Since the vast majority of known porphyrins are metallopor-
phyrins, it would be important to reveal the complexation
chemistry of 16π porphyrins toward metals. Thus, we decided
to examine reactions with metals of varying oxidation levels
using OiBTPP (4), which is more stable and easier to handle
than OETPP (2). In this article, we report the details of studies,
which culminated in the isolation and structural determination
of a 17 π-electron cation radical complex, [(OiBTPP)Cu]•+
[SbF6]- (10), and the first 16 π-electron transition metal
complex, [(OiBTPP)Zn(Cl)]+[ZnCl3]- (11). Magnetic circular
dichroism (MCD) measurements, spectroelectrochemical experi-
ments, and nucleus-independent chemical shift (NICS) calcula-
tions were found to be beneficial for elucidation of the electronic
properties of the novel porphyrins.
ordinary 18 π-electron porphyrin-metal complexes. Therefore,
it was concluded that the reaction of 16π OiBTPP (4) with
SnIICl2 had proceeded in a redox manner; i.e., 4 was reduced
to the 18 π-electron state, while the divalent tin atom was
oxidized to the tetravalent state.
Results and Discussion
Synthesis. First, we needed to improve the final step in the
synthesis of 4, i.e., oxidation of 18π (OiBTPP)H2 (3), because
the yield of the reaction of (OiBTPP)Li2 with SOCl2 as the
oxidant was low (15%) and poorly reproducible (see Scheme
2).4 Thus, we examined the direct oxidation of 3 with several
oxidants (MnO2, NOBF4, and SbCl5) and found that the reaction
using SbCl5 proceeded cleanly to give 4 in moderate yield (48%)
with high reproducibility (Scheme 3). (OETPP)H2 (1) was also
oxidized to give 2 utilizing this method, although the yield was
not improved (26%). However, the present method was found
to be ineffective for sterically uncrowded planar porphyrins,
TPP and ꢀ-octaethylporphyrin (OEP). The treatment of (TPP)H2
with SbCl5 resulted in its complete recovery. In the reaction of
(OEP)H2, a color change was observed after addition of SbCl5,
but the formation of 16π OEP could not be confirmed. These
results suggest the effectiveness of using distorted 18π porphy-
rins for obtaining metal-free 16π porphyrins.
The introduction of metals into 4 using low-valent metal
reagents was first examined. Two electronic states could be
envisaged for the products of the reaction of 4 with M(n) (where
n is the oxidation number). One was the 16π (OiBTPP)M(n)
state, where 16π OiBTPP (4) just coordinates with the metal
with no change in the oxidation states. The other was the 18π
(OiBTPP)M(n + 2) state, which would form upon reduction of
4 with the metal reagent. The redox potential of metals are
generally low, and thus, from a thermodynamic viewpoint,
products resulting from the latter would be expected. However,
if the barrier to electron transfer were large enough, a product
from the former might arise. We decided to take our chances
on this latter possibility. The reaction of 4 with SnIICl2 afforded
a tin complex, presumably (OiBTPP)SnCl2, which transformed
to (OiBTPP)Sn(OEt)2 (5) upon recrystallization from CH2Cl2/
EtOH (Scheme 4). The UV-vis spectrum of 5 showed an
intense Soret-band absorption at 465 nm, together with weak
Q-band absorptions at 605 and 657 nm, which is common for
We next attempted reactions of 4 with zerovalent metals (Mg,
Zn, Cu, and Pd2(dba)3). However, only complex mixtures were
obtained, including unidentified 18π and 16π complexes as
implicated by UV-vis measurements. Vaid and co-workers
disclosed in their study of 16π [(TPP)Li]+[BF4]- that their 16π
complex was capable of reacting with Mg, Zn, and Cu to give
the corresponding 18π (TPP)MII (M ) Mg, Zn and Cu)
complexes, respectively.5 Thus, we examined this methodology
for the synthesis of (OiBTPP)MII (M ) Zn, Cu, and Pd). The
reaction of 4 with LiBF4 in toluene gave [(OiBTPP)Li]+[BF4]-
(6) quantitatively. Although we could not isolate 6 because of
its low stability in air, the formation of 6 could be confirmed
by UV-vis spectroscopy (λmax ) 365 nm) and 7Li NMR (δ )
1.29 ppm). The reaction of lithium complex 6 with Zn, Cu,
and Pd2(dba)3 afforded 18π porphyrin-metal complexes
(OiBTPP)Zn(EtOH) (7), (OiBTPP)Cu (8), and (OiBTPP)Pd (9),
respectively (Scheme 4). This result indicates that cationic
lithium complex 6 is more favorable for reduction by zerovalent
metals than metal-free 4. The UV-vis spectra of 7-9 in CH2Cl2
were similar to that of 5, showing an intense Soret-band
absorption (λmax ) 476 nm for 7, 436 nm for 8, and 440 nm for
9) and weak broad Q-band absorptions (λmax ) 690 nm for 7,
575 nm for 8, and 553 and 589 nm for 9), which again were
indicative of the formation of 18 π-electron metalloporphyrins.
The oxidation reaction of copper complex 8 with AgSbF6
resulted in the quantitative formation of the 17 π-electron cation
radical complex [(OiBTPP)Cu]•+[SbF6]- (10) (Scheme 5). The
UV-vis spectrum of 10 showed absorptions at 306, 421 (sh),
454, and 623 nm (Figure 2a), and the compound was found to
be silent to electron spin resonance (ESR) measurements. These
features were quite similar to those reported previously for
structurally highly related 17 π-electron cation radical complexes
[(OETPP)Cu]•+X- (X ) ClO4, I).7b The lack of an ESR signal
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A R T I C L E S
Yamamoto et al.
Scheme 5. Oxidation of (OiBTPP)Cu (8) by AgSbF6
Scheme 6. Reaction of 4 with ZnCl2 to Give 16 π-Electron
[(OiBTPP)Zn(Cl)]+[ZnCl3]- (11)
absorptions (340 and 386 nm) of 11 being considerably blue-
shifted and having smaller extinction coefficients compared with
the Soret-band absorption of 7 (476 nm). In addition, no
absorptions were observed in the Q-band region for 11. This
difference is quite similar to that between metal-free analogues
3 and 4. The absorptions of 11 are more red-shifted than those
of metal-free 16π OiBTPP (4) (274 and 331 nm) by ca. 60 nm
but are comparable to those of [(TPP)Li]+[BF4]- (332 and 394
nm).5
Crystal Structure. Single crystals for 5 and 7-11 grown from
appropriate solvents (ethanol/CH2Cl2 for 5, 7-9; CH2Cl2/hexane
for 10; and 1,2-dichloroethane/hexane for 11) were subjected
to X-ray crystallographic analysis. These crystals contained
solvent molecules in the crystal lattice (5, ethanol; 7-9, CH2Cl2;
10, water;8 11, 1,2-dichloroethane). Figures S1, S2, S4-S6 (see
Supporting Information), and 3 show the ORTEP drawings of
5 and 7-11, respectively, and selected structural parameters
are summarized in Table 1.
was consistent with the general propensity for coupling between
paramagnetic metals such as Cu(II) and nonplanar radical
porphyrin ligands to be antiferromagnetic due to effective
overlap of the d-orbitals of the metal with the radical π orbitals.7
Since the reactions of low-valent and zerovalent metals
described in Scheme 4 proceeded in a redox manner to furnish
18π porphyrins, if the reaction proceeded at all, we next
examined high-valent metal reagents to avoid reduction of the
porphyrin core. No reactions were observed when NiCl2, MgCl2,
and PdCl2 were used. However, the reaction of 4 with 2 equiv
of ZnCl2 gave 16π zinc complex [(OiBTPP)Zn(Cl)]+[ZnCl3]-
(11), which was isolated in 92% yield as a brown crystalline
solid (Scheme 6). Complex 11 was not very stable in solution
but was relatively stable in the solid state. Decomposition leads
to what we believe to be products of hydrolysis. However, we
have not yet been able to identify them.
The UV-vis spectra of the two zinc complexes 7 and 11
(Figure 2b) were significantly different, with the two major
The porphyrin cores of the OiBTPP-metal derivatives are
nonplanar. Figure 4 shows the deviation of each atom from the
mean plane defined by the 24 core atoms for 5 and 7-11. The
core of 5 is distorted only at the ꢀ-positions, showing a saddle-
type distortion. For 7-11, not only the ꢀ-positions but also the
meso-positions deviate from the mean plane; therefore, these
metalloporphyrins take on essentially saddle-type conformations,
Figure 3. ORTEP drawing of [(OiBTPP)Zn(Cl)]+[ZnCl3]- (11) with the
thermal ellipsoids shown at the 50% probability level. All hydrogen atoms
and solvated molecules are omitted for clarity. Selected bond lengths (Å)
and angles (°): Zn1-N1, 2.125(4); Zn1-N2, 2.117(4); Zn1-N3, 2.119(4);
Zn1-N4, 2.127(4); Zn1-Cl1, 2.2239(15); N1-Zn1-N2, 83.03(16); N1-
Zn1-N3, 142.92(16); N1-Zn1-N4, 84.61(16); N1-Zn1-Cl1, 107.68(12);
N2-Zn1-N3, 86.35(16); N2-Zn1-N4, 146.38(17); N2-Zn1-Cl1, 107.82(12);
N3-Zn1-N4, 84.94(16), N3-Zn1-Cl1, 109.40(13); N4-Zn1-Cl1,
105.71(12).
Figure 2. UV-vis spectra of copper and zinc complexes. (a) 8 (c ) 12.1
µM; broken line) and 10 (c ) 15.3 µM; solid line). (b) 7 (c ) 12.2 µM;
broken line) and 11 (c ) 22.4 µM; solid line) in CH2Cl2. The small
absorption at 480 nm (marked with a cross) is due to a small amount of a
hydrolyzed (18π) species.
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16 π-Electron Octaisobutyltetraphenylporphyrin
A R T I C L E S
Table 1. Selected Structural Parameters for 5 and 7-11
compound
M-Na (Å)
N· · · Nb (Å)
M· · ·∆4Nc (Å)
ad (deg)
be (deg)
RMSf
ref
(OiBTPP)Sn(OEt)2 (5)
(OiBTPP)Zn(EtOH) (7)
(OiBTPP)Cu (8)
2.1025(16)
2.055(3)
1.964(2)
2.006(4)
1.967(3)
2.122(4)
4.204(2)
4.052(5)
3.920(4)
4.008(8)
3.933(5)
4.043(6)
4.268(3)
0.000
0.338
0.010
0.005
0.001
0.643
161.5
148.1
160.0
159.0
147.3
129.5
161.5
167.2
160.1
159.1
147.4
149.2
0.502
0.641
0.690
0.671
0.821
0.888
0.921
0.669
0.700
0.838
0.836
(OiBTPP)Pd (9)
[(OiBTPP)Cu]•+[SbF6]- (10)
[(OiBTPP)Zn(Cl)]+[ZnCl3]- (11)
OiBTPP (4)
4
(OETPP)Zn(MeOH)
(OETPP)Cu
2.063
1.977
1.974
0.240
0.004
0.092
11
7b
7b
4
[(OETPP)Cu]•+[ClO4]-
OETPP (2)
4.227(3)
a Average value of the four M-N lengths. Standard deviations are in parentheses. b Average value of the N1 · · ·N3 and N2 · · ·N4 distances. Standard
deviations are in parentheses. c Distance between M and the mean plane consisting of the four nitrogen atoms. d Average value of a1 and a2. e Average
value of b1 and b2. f Root-mean-square of the sum of the squares of the deviation of each atom from the mean plane defined by the 24 core atoms.
with some contribution from ruffle-type distortions. This should
be due to considerable crowding at the periphery. Although there
is no consistent trend in the orientations of the two isobutyl
groups of the pyrrole rings (up-up or up-down), it could be
that the isobutyl groups are arrayed to minimize steric repulsions.
The degree of overall distortion of the porphyrin core can be
evaluated by the root-mean-square of the deviation of each atom
from the mean plane (rms value). Among the 18π complexes,
the rms value of tin complex 5 is smaller (0.502) than those of
the others (0.641 for 7, 0.690 for 8, and 0.671 for 9), indicating
that the size of the central metal affects the degree of distortion
of the porphyrin core. This agrees with the trend observed for
OETPP-metal complexes; i.e., larger metals tend to induce
more-planar porphyrin cores.9 As a result, the transannular
N· · ·N distance of 5 (4.20 Å) is longer than those of the others
(7, 4.05 Å; 8, 3.92 Å; 9, 4.01 Å). The rms value for the 17π
copper complex 10 (0.821) is much greater than that of the
corresponding 18π complex 8 (0.690), as observed for OETPP
derivatives ((OETPP)Cu, 0.700; [(OETPP)Cu]•+[ClO4]-,
0.838).7b The situation is the same with the two zinc complexes,
18π 7 (0.641) and 16π 11 (0.888), as observed for metal-free
OETPPs (1, 0.712; 2, 0.836). These findings establish a
structural trend that the lack of aromaticity enhances distortion
in the porphyrin core.
coordinate (for 7, M ) Zn(EtOH); 11, M ) Zn(Cl)), and four-
coordinate (for 8, M ) Cu; 9, M ) Pd). The central metals for
5 (Sn), 8 (Cu), and 9 (Pd) are placed essentially in the mean
plane defined by the four nitrogen atoms, as shown by the near-
zero distance between the central metal atom and the mean plane
(M· · ·∆4N). The M· · ·∆4N distance for the 17π copper
complex 10 is 0.001 Å, whereas that for [(OETPP)Cu]•+[ClO4]-
is reported to be 0.092 Å.7b This difference is notable,
considering the fact that the only difference in the porphyrin
ligand is the identity of peripheral alkyl groups (Et and i-Bu).
The counteranion ClO4- of the latter is located near the Cu atom
(the closest Cu · · ·O contact is 2.45 Å), and this proximity is
probably the reason for the deviation of the Cu atom toward
the axial ClO4-. In contrast, for 10, the anionic counterpart
(SbF6-) is separated from the Cu atom (the closest Cu · · ·F
distance is 5.3 Å), and two solvated water molecules occupy
each side of the Cu atom (Cu · · ·O distances are 3.2-3.7 Å).
The difference between the two zinc complexes 7 and 11 is
remarkable. The M · · ·∆4N distance of 18π complex 7 (0.338
Å) is slightly larger than that of 18π (OETPP)Zn(MeOH) (0.240
Å),10 whereas that of 16π complex 11 (0.643 Å) is considerably
larger than those of the former two porphyrins. Thus, the
coordination of the pyrrole moieties to the metals is skewed
due to the distorted porphyrin cores. For the 18 π-electron
metalloporphyrins with the “in-plane” metals (5, 8, and 9), the
angles consisting of metal, nitrogen, and the centroid of the
pyrrole ring (a and b in Table 1) are ca. 160°, which deviates
from the ideal 180° for coplanarity. The more distorted 17π
copper complex 10 (rms ) 0.821) shows smaller angles (a )
147.3°, b ) 147.4°) than the 18π metalloporphyrins. It is natural
that the “out-of-plane” 18π zinc complex 7 shows different
The central metals (M) in the metalloporphyrins can be
regarded as pseudo-octahedral (for 5, M ) Sn(OEt)2), five-
(7) (a) Erler, B. S.; Scholz, W. F.; Lee, Y. J.; Scheidt, W. R.; Reed, C. A.
J. Am. Chem. Soc. 1987, 109, 2644–2652. (b) Renner, M. W.; Barkigia,
K. M.; Zhang, Y.; Medforth, C. J.; Smith, K. M.; Fajer, J. J. Am.
Chem. Soc. 1994, 116, 8582–8592.
(8) Crystallizations were performed using undistilled solvents under air.
(9) Sparks, L. D.; Medforth, C. J.; Park, M.-S.; Chamberlain, J. R.;
Ondrias, M. R.; Senge, M. O.; Smith, K. M.; Shelnutt, J. A. J. Am.
Chem. Soc. 1993, 115, 581–592.
(10) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner,
M. W.; Smith, K. M. J. Am. Chem. Soc. 1990, 112, 8851–8857.
9
J. AM. CHEM. SOC. VOL. 132, NO. 36, 2010 12631

A R T I C L E S
Yamamoto et al.
Figure 5. Bond lengths for the porphyrin cores of the copper (8 and 10)
and zinc (7 and 11) complexes.
16π 11 is larger than that of 17π 10. The differences in bond
lengths of 11 are comparable to those of metal-free 16π
porphyrins (2, ∆(N-CR) ) 0.114 Å, ∆(Cm-CR) ) 0.118 Å; 4,
∆(N-CR) ) 0.114 Å, ∆(Cm-CR) ) 0.122 Å) and those of
[(TPP)Li][BF4] (∆(N-CR) ) 0.092 Å, ∆(Cm-CR) ) 0.105 Å).
By carrying out density functional theory (DFT) calculations,
Ghosh et al. have recently identified the reason for bond
alternations observed for certain π-cation radicals to be pseudo-
Jahn-Teller distortion.11 Through structural optimizations of
model zinc-porphine complexes with 16-18 core π-electrons,
they have also predicted that the degree of bond alternation of
the dicationic zinc complex ([(P)Zn]2+) with 16 π-electrons at
the core (∆(N-CR) ) 0.0635 Å, ∆(Cm-CR) ) 0.0784 Å) would
be larger than that of the cation radical ([(P)Zn]•+) with 17
π-electrons (∆(N-CR) ) 0.0270 Å, ∆(Cm-CR) ) 0.0336 Å)
(Table 2). The trends in the bond lengths of our oxidized
metalloporphyrins 10 and 11 are in good agreement with these
calculations.
Figure 4. Deviation of each atom from the mean plane (in units of 0.01
Å) in the molecular structure of 5 and 7-11 along with rms values (see
text).
angles (a ) 148.1°, b ) 167.2°). The most distorted complex,
11 (rms ) 0.888), shows the smallest a angles (average 129.5°:
a1 ) 128.6°, a2 ) 130.3°). This is due to the large M · · ·∆4N
distance (0.643 Å) as well as the large distortion of the core, as
shown in Figure S7 (Supporting Information).
Comparison of the structurally similar [(OiBTPP)Cu]•+
[SbF6]- (10) and [(OETPP)Cu]•+[ClO4]- shows that the extent
of bond alternation is larger in the former than in the latter (10,
∆(N-CR) ) 0.066 Å, ∆(Cm-CR) ) 0.065 Å; [(OETPP)-
Cu]•+[ClO4]-, ∆(N-CR) ) 0.023 Å, ∆(Cm-CR) ) 0.028 Å),
even though the overall distortions of the porphyrin cores are
practically the same (rms ) 0.821 and 0.838). This difference
is reflected in the deviation of the meso carbon atoms from the
mean plane defined by the core atoms (Figure S8, Supporting
Information). The average deviation of the meso carbon atoms
of 10 (0.050 Å) is much larger than that of [(OETPP)Cu]•+
[ClO4]- (0.019 Å). It is interesting that the bonding schemes
for 17π radicals are sensitive to small changes in the porphyrin
ligand (Et and i-Bu in the periphery), whereas little difference
in the degree of bond alternation is observed among structurally
Figure 5 shows the bond lengths of the porphyrin cores of
the oxidized species (10 and 11) and those of the corresponding
parent 18π porphyrins (8 and 7). Characteristic structural
parameters are listed in Table 2, together with those of related
compounds. In contrast with aromatic 7 and 8, unambiguous
bond alternations were observed between adjacent C-N bonds
(N-CR1 and N-CR2) and between adjacent C-C bonds
(Cm-CR1 and Cm-CR2) in the porphyrin cores of the oxidized
species 10 and 11. The average differences between the longer
and shorter bond distances are 0.066 (∆(N-CR)) and 0.065 Å
(∆(Cm-CR)) in 17π copper complex 10, whereas those of
16π zinc complex 11 are 0.101 (∆(N-CR)) and 0.114 Å
(∆(Cm-CR)), indicating that the degree of bond alternation of
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12632 J. AM. CHEM. SOC. VOL. 132, NO. 36, 2010

16 π-Electron Octaisobutyltetraphenylporphyrin
A R T I C L E S
Table 2. Structural Parameters of the Porphyrin Cores for 7, 8, 10, and 11a
average deviation (Å)
average bond distance (Å)b
compound
no. of π electrons
Cm
CR1
CR2
N-CR1
N-CR2
Cm-CR1
Cm-CR2
ref
(OiBTPP)Zn(EtOH) (7)
(OiBTPP)Cu (8)
18
18
17
16
16
17
16
16
18
17
16
0.033
0.042
0.050
0.062
0.052
0.019
0.060
0.017
0.016
0.020
0.016
0.020
0.038
0.010
0.055
0.066
0.072
0.075
0.067
0.059
0.064
1.368(6)
1.380(4)
1.341(5)
1.312(6)
1.310(3)
1.372(6)
1.306(3)
1.316(3)
1.3735
1.372(6)
1.401(4)
1.407(5)
1.413(6)
1.424(3)
1.395(4)
1.420(3)
1.408(3)
1.3734
1.411(7)
1.405(4)
1.443(6)
1.472(7)
1.483(4)
1.421(6)
1.483(4)
1.470(3)
1.3977
1.403(6)
1.407(4)
1.388(6)
1.358(7)
1.361(4)
1.393(3)
1.365(4)
1.365(3)
1.3978
[(OiBTPP)Cu]•+[SbF6]- (10)
[(OiBTPP)Zn(Cl)]+[ZnCl3]- (11)
OiBTPP (4)
4
7b
4
[(OETPP)Cu]•+[ClO4]-
OETPP (2)
[(TPP)Li][BF4]
5
(P)Znc
11
11
11
c
[(P)Zn]•+
1.3579
1.3395
1.3849
1.4030
1.4187
1.4455
1.3851
1.3671
c
[(P)Zn]2+
a Cm, meso-carbon atom; CR1, CR2; R carbon atoms of the pyrrole ring. b Standard deviations are in parentheses. c Optimized by DFT calculations
carried out under C4h symmetry constraint. P ) porphine.
Table 3. NICS Values (ppm, B3LYP/6-31G*) for Porphyrins and
determined 16π species, regardless of the identity of the
Their Metal Complexes
porphyrin or whether metals are involved.
point ac
point bd
point ce
point de
Harmonic oscillator model of aromaticity (HOMA) values,12
derived from experimentally determined bond lengths, have been
utilized to determine the degree of aromaticity of cyclic
conjugated systems, with values close to 1 corresponding to
high aromaticity and those close to 0 corresponding to low
aromaticity. Calculations based upon this criterion for 17π cation
radical complex [(OiBTPP)Cu]•+[SbF6]- (10) give an average
value of 0.451 (0.501 and 0.402), whereas that for the parent
(OiBTPP)Cu (8) is 0.664 (0.678 and 0.649). As for the 16π
dication complex [(OiBTPP)Zn(Cl)]+[ZnCl3]- (11), the value
is 0.328, whereas the parent (OiBTPP)Zn(EtOH) (7) has an
average value of 0.603 (0.600 and 0.607). Thus, the two
electron-deficient complexes 10 and 11 can be considered to
be of low aromaticity, especially 11.
The crystals of both (OiBTPP)Cu (8) and (OiBTPP)Pd (9)
contain three dichloromethane molecules per porphyrin. These
molecules are located close to the porphyrin core, with the
distances between the pyrrole nitrogen atom and the carbon atom
of dichloromethane being 3.3-3.5 Å, and the hydrogen atoms
face the core. There might be C-H· · ·π interactions, as observed
for the OETPP · · ·CH2Cl2 complex,4 or it could be that the
dichloromethane molecules are simply trapped in the cavity
formed by the highly distorted (OiBTPP)M fragments.
compound
(OiBTPP)H2 (3)a
OiBTPP (4)a
-11.57
+0.53
-12.42
+0.93
-15.25
-15.48
+13.20
-13.45
+2.40
-14.86
+2.83
-1.22
+1.09
-0.61
+0.41
-1.97
-2.93
-4.79
-5.03
+0.42
-8.28
-4.00
-5.09
-5.86
-7.97
-10.61
-9.81
+1.00
-10.50
-0.24
-12.73
-13.25
-4.79
-5.68
-0.50
-8.28
-3.98
-5.15
-6.22
-7.98
-10.62
(OETPP)H2 (1)a
OETPP (2)a
a
PorH2
-18.78
-19.26
+19.79
-14.02
+4.38
-18.00
+35.09
-15.38
-13.09
-19.82
-3.49
b
PorH2
Porb
(OiBTPP)Zn (7)a
[(OiBTPP)Zn(Cl)]+ (11)a
PorZnb
b
[PorZnCl]+
(OiBTPP)Cu (8)a
[(OiBTPP)Cu]+ (10)a
PorCub
b
[PorCu]+
a X-ray geometry. OETPP, 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-
tetraphenylporphyrin;4 PorH2 free-base porphine.15 b Optimized
)
structure calculated at the level of B3LYP/6-31G*. Por, porphine.
c Value at the center of the inner 16-membered ring. d Average of the
values at the midpoint of two C-N bonds. e Average of the values at
the center of two pyrrole rings.
classified as aromatic, while antiaromatic systems have positive
values. Indeed, the NICS values at the ring center of aromatic
porphyrins have been calculated to be negative.14
As shown in Table 3 and Figure 6, NICS values were
calculated for the X-ray geometries of the present porphyrins
at the center of the inner 16-membered ring (point a), the
midpoint of two C-N bonds (point b), and the centers of two
inequivalent pyrrole rings (points c and d). We have also
calculated NICS indices at these points for the optimized
structures of free-base and metal porphines (porphyrins without
peripheral substituents) to probe the effect of the peripheral
substitution on the NICS values. Unfortunately, full structural
Nucleus-Independent Chemical Shift Calculations. The elec-
tronic structures of the porphyrins with the 16 and 18 π-electron
systems were studied by nucleus-independent chemical shift
calculations (NICS). Schleyer et al. proposed NICS as a
computational measure of aromaticity that is related to experi-
mental magnetic criteria.13 NICS is defined as the negative
shielding value computed at a ring center or some other points
of interest in the system. Rings with negative NICS values are
(11) Vangberg, T.; Lie, R.; Ghosh, A. J. Am. Chem. Soc. 2002, 124, 8122–
8130.
(13) Schleyer, P. v. R.; Meaerker, C.; Dransfeld, A.; Jiao, H.; Hommes,
N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317–6318.
(14) Cyran˜ski, M. K.; Krygowski, T. M.; Wisiorowski, M.; Hommes,
N. J. R. v. E.; Schleyer, P. v. R. Angew. Chem., Int. Ed. 1998, 37,
177–180.
(12) Krygowski, T. M. J. Chem. Inf. Comput. Sci. 1993, 33, 70–78. For
aromatic porphyrin systems, the value may change depending on how
the 18 bonds are selected. Therefore, the two values are shown in
parentheses with the average of the two outside of it.
9
J. AM. CHEM. SOC. VOL. 132, NO. 36, 2010 12633

A R T I C L E S
Yamamoto et al.
Figure 6. NICS values (ppm) calculated for free-base and Zn porphyrins
(B3LYP/6-31G*).
optimization of the actual complexes with multiple peripheral
substituents could not be carried out. The reliability of our
analysis could be confirmed by the good agreement between
NICS values for the experimentally reported15 and theoretically
optimized PorH2. As would be expected, the DFT-optimized
porphines adopt a more planar structure than the corresponding
X-ray geometries (see Figure S9, Supporting Information).
In the case of 18π free-base porphyrin ((OiBTPP)H2, 3), the
NICS values at points a and b were calculated to be -11.57
and -13.45, respectively, which are comparable with those
reported for aromatic porphyrins.14 On the other hand, OiBTPP
(4) exhibits markedly different NICS signatures. The values at
points a and b are positive but small in magnitude (+0.53 at
point a, +2.40 at point b), indicating the ring current to be
weakly paratropic. Similar positive NICS values were calculated
at points a and b for the X-ray geometry of OETPP (2). The
NICS values of +13.20 and +19.79 at points a and b for the
16π porphine (Por) indicate the ring current to be strongly
paratropic (see Table 3). Thus, the highly antiaromatic nature
of Por implied by the NICS values compared with those of
OiBTPP may be a consequence of the planarity of the porphyrin
skeleton in Por. The NICS values at points c and d for OiBTPP
were also different from those for Por.
Figure 7. (a) MCD and electronic absorption spectra of (OiBTPP)H2 (3,
left) and OiBTPP (4, right) measured in CH2Cl2. Bands marked with
asterisks are due to trace amounts of products of decomposition. (b)
Calculated absorption spectra of the free-base porphyrins obtained using
the ZINDO/S method.
and LanL2DZ) were used for the zinc atom instead of the 6-31G*
basis set. Thus, the effect of the basis set on the NICS values should
not be important (see Table S2, Supporting Information).
We have also calculated NICS values for the two forms (17
and 18π) of copper porphyrins. The NICS value at point b for
[(OiBTPP)Cu]•+ (10) remains negative upon oxidation, implying
that the singly oxidized form of the Cu complex more or less
has aromatic character.
Magnetic Circular Dichroism Spectra and MO Analysis.
MCD spectroscopy has been one of the most powerful tech-
niques for analyzing the electronic excited states of aromatic
porphyrins and related macrocyclic systems derived from an
18 π-electron core.16 Very recently, a subgroup of the authors
demonstrated that the electronic absorption spectra of expanded
porphyrins with a [4n] π-electron system can also be character-
ized using MCD techniques.17 Here we report the first MCD
spectra for the 16 π-electron porphyrins and rationalize the
spectral changes upon oxidation on the basis of ZINDO/S
calculations and molecular orbital analyses.
The oxidized form of the zinc complex, [(OiBTPP)Zn(Cl)]+
(11), has a positive NICS value at point b (+4.38). The NICS
calculation for [PorZn]+ indicates a more positive value at point
b (+35.09), which is almost the same as that for [LiPor]+
(+36.5) reported by Vaid et al.5 These results confirm that the
electronic structure of [(OiBTPP)Zn(Cl)]+ can be described as
a nonaromatic 16 π-electron system with only weak paratropic
ring current. The difference between the NICS signs at points c
(+0.42) and d (-0.50) is likely due to distortion of planarity, as
observed for the X-ray structure. The calculated NICS values were
essentially the same, even when other basis sets (6-311G*, SVP,
Figure 7a shows the UV-vis and MCD spectra of the two
forms (16 and 18π) of free-base porphyrin (3 and 4) in CH2Cl2.
(16) (a) Mack, J.; Stillman, M. J.; Kobayashi, N. Coord. Chem. ReV. 2007,
251, 429–453. (b) Kobayashi, N.; Nakai, K. Chem. Commun. 2007,
4077–4092.
(17) (a) Sankar, J.; et al. J. Am. Chem. Soc. 2008, 130, 13568–13579. (b)
Muranaka, A.; Matsushita, O.; Yoshida, K.; Mori, S.; Suzuki, M.;
Furuyama, T.; Uchiyama, M.; Osuka, A.; Kobayashi, N. Chem.sEur.
J. 2009, 15, 3744–3751.
(15) Saltsman, I.; Goldberg, I.; Balasz, Y.; Gross, Z. Tetrahedron Lett. 2007,
48, 239–244.
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12634 J. AM. CHEM. SOC. VOL. 132, NO. 36, 2010

16 π-Electron Octaisobutyltetraphenylporphyrin
A R T I C L E S
Table 4. Selected Results of ZINDO/S Calculations for
Low-Energy π-π* Transitions of 18 and 16 π-Electron Porphyrins
compound
λ (nm)
f
composition (weight, %)a
(OiBTPP)H2 (3)
907
0.023
HfL (39.9),
H-1fL+1 (24.7),
HfL+1 (15.7),
H-1fL (14.4)
H-1fL (33.5),
HfL+1 (31.8),
HfL (20.1),
788
0.014
H-1fL+1 (10.4)
H-1fL+1 (42.6),
HfL (22.8),
H-3fL (14.7)
HfL+1 (40.9),
H-1fL (39.2)
463
452
1.601
2.312
OiBTPP (4)
485
358
0.000
1.570
HfL [s-fs+]b (81.3)
HfL+1 [s-fl-]b (56.2),
H-1fL [h+fs+]b (18.7)
HfL+2 [s-fl+]b (56.4),
H-2fL [h-fs+]b (18.7)
358
1.570
a H ) HOMO; L ) LUMO. b Perimeter labels.
A derivative-shaped MCD signal with a negative/positive sign
sequence, with increasing energy, was observed for the Q00 band
of (OiBTPP)H2 (3). This type of MCD signal is called an
apparent Faraday A term and arises from an electronic transition
to nearly degenerate excited states.18 The spectral appearance
of 3 is similar to that of [(OEP)H4]2+, which adopts a saddle
geometry. Although large signals derived from traces of 3 were
observed in the visible region for OiBTPP (4), a distinctive
derivative-shaped MCD signal with a negative/positive sign
sequence for 4 could be detected in the UV region. Because 4
has S4 symmetry in the X-ray structure, the MCD signal should
be assigned to a positive Faraday A term, which arises from
transitions involving degenerate excited states.
Figure 8. Energy levels of key molecular orbitals and their contour plots
for (OiBTPP)H2 (3, left) and OiBTPP (4, right). The values were obtained
from the ZINDO/S calculations for the X-ray geometries. Perimeter labels
are given in parentheses.
model.20 Because the HOMOfLUMO (s-fs+) transition is of
intrashell nature in the perimeter model, the transition is
magnetic-dipole allowed, but the absorption and MCD intensities
are predicted to be zero.
The experimental and calculated spectra of the zinc complexes
are given in Figure S10 (Supporting Information). The oxidized
zinc complex (11) exhibits two distinct absorption bands in the
UV region and a broad shoulder in the visible region. The MCD
signals corresponding to the two UV absorption bands have
negative and positive signs, so these signals are assigned to
coupled Faraday B terms, which arise from magnetically induced
mixing of nondegenerate excited states. The calculated spectral
features for both zinc complexes reproduced the experimental
spectra well. The split UV transitions for 11 can be derived
from the loss of orbital degeneracy, because the insertion of
zinc lowers the four-fold symmetry (Figure S11, Supporting
Information). The nodal patterns for key frontier molecular
orbitals of the zinc complexes are quite similar to those of the
free-base porphyrins.
Transition energies and oscillator strengths for the X-ray
geometries of these porphyrins were calculated using the
ZINDO/S method (Table 4). As can be seen from Figure 7b,
the profiles of the calculated absorption spectra of both
porphyrins are in close agreement with the observed spectral
pattern. A doubly degenerate UV transition (358 nm) was
deduced for OiBTPP (4), which is in accordance with the
observed MCD A term. The calculations also predict that a
dipole-forbidden HOMOfLUMO transition is located at 485
nm. The observed absorption shoulder that extends to about 800
nm is therefore associated with this HOMOfLUMO transition.
This type of forbidden transition is characteristic of cyclic [4n]
π-electron systems.17b,19
Figure 8 shows the energy levels of key frontier π-orbitals
and their contour plots for the neutral and oxidized porphyrins.
It is evident that the nodal patterns of the frontier orbitals of
OiBTPP (4) are derived from those of (OiBTPP)H2 (3). The
four orbitals play an essential role in the low-energy electronic
transitions of 3 and 4 (see Table 4). In the case of the 16π
porphyrin, two additional occupied orbitals also contribute to
the 358 nm transition, and these six orbitals can be labeled as
h-, h+, s-, s+, l-, and l+ according to the 4n-electron perimeter
Electrochemistry and Spectroelectrochemistry. The redox
properties of (OiBTPP)H2 (3) and OiBTPP (4) were examined
(18) Mack, J.; Asano, Y.; Kobayashi, N.; Stillman, M. J. J. Am. Chem.
Soc. 2005, 127, 17697–17711.
(19) (a) Fleischhauer, J.; Ho¨weler, U.; Spanget-Larsen, J.; Raabe, G.; Michl,
J. J. Phys. Chem. A 2004, 108, 3225–3234. (b) Fleischhauer, J.; Raabe,
G.; Klingensmitj, K. A.; Ho¨weler, U.; Chatterjee, R. K.; Hafner, K.;
Vogel, E.; Michl, J. Int. J. Quantum Chem. 2005, 102, 925–939.
(20) (a) Fleischhauer, J.; Ho¨weler, U.; Michl, J. J. Chem. Soc., Perkin Trans.
2 1998, 1101–1117. (b) Fleischhauer, J.; Ho¨weler, U.; Michl, J.
Spectrochim. Acta 1999, 55A, 585–606. (c) Fleischhauer, J.; Ho¨weler,
U.; Michl, J. J. Phys. Chem. A 2000, 104, 7762–7775.
9
J. AM. CHEM. SOC. VOL. 132, NO. 36, 2010 12635

A R T I C L E S
Yamamoto et al.
Figure 9. (a) Cyclic voltammograms of OiBTPP (4) and (OiBTPP)H2 (3) in CH2Cl2 containing 0.1 M TBAP at a scan rate of 0.1 V/s. (b) Thin-layer
UV-visible spectral changes during controlled potential reduction at (1) -1.20 V and (2) -1.60 V in CH2Cl2 containing 0.1 M TBAP.
Table 5. Half-Wave (E1/2) and Peak (Ep) Potentials (V vs SEC) in
The difference in potentials between the two porphyrin ring-
centered reductions of (OEP)H2 is 0.46 V, compared to 0.52 V
CH2Cl2 Containing 0.1 M TBAP
compound
oxidation
reduction
∆ (V)c
for (OiBTPP)H2 (3). A similar separation of 0.50 V is also seen
between the second and third reductions of OiBTPP (4). This
compound undergoes an irreversible reduction at Ep ) -1.02
V prior to two additional processes at Ep) -1.49 and -1.99 V
in CH2Cl2. The latter potentials are virtually identical to the Ep
values for the two reductions of 3 (see Table 5), suggesting an
a
a
a
a
a
OiBTPP (4)
0.50
0.52
1.66
-1.02 -1.49 -1.99
a
a
a
a
(OiBTPP)H2 (3)
1.27 0.86 0.45
1.40 0.89
-1.47 -1.99
b
-1.44 -1.90 0.46
(OEP)H2
a Peak potential at a scan rate of 0.1 V/s. b Data taken from ref 22.
c Potential difference between the last two processes.
(21) (a) Kadish, K. M. Prog. Inorg. Chem. 1986, 34, 435–605. (b) Kadish,
K. M.; Caemelbecke, V.; E.; Royal, G. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San
Diego, CA, 2000; Vol. 8, pp 1-97. (c) Kadish, K. M.; Royal, G.;
Caemelbecke, V. E.; Gueletti, E. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego,
CA, 2000; Vol. 9, pp 222-413.
in CH2Cl2 containing 0.1 M TBAP. Examples of cyclic
voltammograms in CH2Cl2 are shown in Figure 9a, while a
summary of redox potentials is given in Table 5, which includes
data for the related (OEP)H2 derivative. This latter compound
exhibits reversible one-electron oxidations and reductions in
CH2Cl2, leading to a π-cation radical (17π), dication (16π),
π-anion radical (19π), and dianion (20π), respectively.21
(22) Stolzenberg, A. M.; Spreer, L. O.; Holm, R. H. J. Am. Chem. Soc.
1980, 102, 364–370.
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16 π-Electron Octaisobutyltetraphenylporphyrin
A R T I C L E S
Figure 10. (a) Cyclic voltammograms of [(OiBTPP)Zn(Cl)]+[ZnCl3]- (11) in PhCN containing 0.1 M TBAP. (b) UV-visible spectral changes during
controlled potential reduction at the indicated potentials.
electrochemical conversion of 4 to 3 under the given solution
conditions. The reduction at -1.02 V is irreversible and involves
an overall two-electron, two-proton addition, where the protons
come from the solvent or supporting electrolyte.
on longer time scales (see Figure S12, Supporting Information).
This is also demonstrated by a reduced peak current for the
process at -1.02 V whose shape is consistent with an EC-type
mechanism (electron transfer followed by a chemical reaction)
where one electron is transferred in the initial step.
More definitive evidence for the electrochemically initiated
conversion of OiBTPP (4) to (OiBTPP)H2 (3) is given by the
thin-layer spectroelectrochemical data. A CH2Cl2 solution of 3
exhibits absorption bands at 357, 464, 526, and 610 nm (Figure
9b, top right), and exactly the same spectral pattern is observed
for the product formed after controlled potential reduction of 4
at -1.20 V in the thin-layer cell (Figure 9b, top left). Further
controlled potential reduction of the electroreduced 4 solution
as well as (OiBTPP)H2 (3) at -1.60 V gave in both cases
identical spectral products, assigned to the porphyrin π-anion
radical of 3. These spectra are characterized by a broad band at
441-445 nm, and the spectral transitions during reduction are
shown in the bottom of Figure 9b. The conversion of 4 to 3
does not occur in a single two-electron, two-proton step but
rather involves one or more chemical reactions following an
initial electron transfer, after which the second electron is added
The electrochemical oxidation of free-base porphyrins is often
complicated in nonaqueous media due to the presence of coupled
chemical reactions.21b,c,23 This is also the case for (OiBTPP)H2
(3), which undergoes three irreversible oxidations, as shown in
Figure 9a. Attempts were made to electrochemically convert 3
to 4 by oxidation in CH2Cl2, but this did not occur, leading
instead to a mixture of protonated products as was reported for
(OEP)H2 and (TPP)H2.23
The electrochemistry of [(OiBTPP)Zn(Cl)]+[ZnCl3]- (11) was
also investigated, and the spectra of the oxidation and reduction
products were elucidated by thin-layer UV-vis spectroelectro-
chemistry. These studies were carried out in PhCN, where a
(23) Inisan, C.; Saillard, J.-Y.; Guilard, R.; Tabardc, A.; Mest, Y. L. New
J. Chem. 1998, 823–830.
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J. AM. CHEM. SOC. VOL. 132, NO. 36, 2010 12637

A R T I C L E S
Yamamoto et al.
well-defined reversible reduction to the 18π porphyrin is
observed at E1/2 ) 0.40 V vs SCE. The cyclic voltammogram
of 11 in PhCN, 0.1 M TBAP is shown in Figure 10a, and the
spectral changes obtained during application of selected applied
potentials between +1.4 and -1.8 V are illustrated in Figure
10b.
(Zn, Cu, Pd2(dba)3), no reaction took place. However, the redox
reaction of 4 with SnCl2 followed by ethanolysis afforded 18π
(OiBTPP)Sn(OEt)2 (5). The 18π metal complexes (OiBTPP)M
(M ) Zn(EtOH)(7), Cu (8), and Pd (9)) were successfully
synthesized by the reaction of 16π lithium complex
[(OiBTPP)Li]+[BF4]- (6) with Zn, Cu, and Pd2(dba)3, respec-
tively. One-electron oxidation of copper complex 8 by AgSbF6
afforded a 17 π-electron cation radical complex, [(OiBTPP)-
Cu]•+[SbF6]- (10). The reaction of 4 with ZnCl2 afforded a novel
16π zinc complex, [(OiBTPP)Zn(Cl)]+[ZnCl3]- (11). The
identities of the metalloporphyrins were confirmed by single-
crystal X-ray analysis and UV-vis spectroscopy. X-ray analysis
of the oxidized species 10 and 11 revealed the presence of
distinctive bond alternations, suggestive of the nonaromatic
nature of 10 and 11. The degree of bond alternation in 11 was
undoubtedly greater than that in 10. The essentially nonaromatic
and only weakly antiaromatic nature of the 16π porphyrins was
confirmed by NICS calculations. The combination of MCD
studies and MO (ZINDO/S) analyses unambiguously explained
the characteristic features of the UV-vis spectra of the 16π
porphyrins. Electrochemical studies of OiBTPP (16π 4) and
[(OiBTPP)Zn(Cl)]+[ZnCl3]- (16π 11) indicated that 4 could be
reduced to its 18π porphyrin form in CH2Cl2 and 11 was capable
of undergoing the same transition in CH2Cl2 or PhCN. The 16π/
18π transition of 11 was reversible in PhCN, and further
reduction of the 18π compound occurred at E1/2 values quite
similar to those for the reduction of (OEP)Zn to its π-anion
radical form in the same two solvents. The UV-vis spectrum
of electroreduced 11 was virtually identical to that of (OiBT-
TP)Zn(EtOH) 7, thus confirming the assignment of the 16π-
to-18π electrochemical conversion. The examination of other
porphyrin systems for the preparation of other novel 16π
complexes is currently in progress.
[(OiBTPP)Zn(Cl)]+[ZnCl3]- (11) initially exhibits absorptions
of 341 and 389 nm associated with the pure 16π compound as
well as an additional band at 473 nm which was earlier assigned
as a small amount of reduced 18π species (see Figures 2 and
10b). The 18π porphyrin was first electrochemically removed
from solution by oxidation at 1.40 V, and this was accompanied
by a disappearance of the bands at 473 and 687 nm, indicating
conversion to a pure oxidation state of the compound (see Figure
10b, top left). The applied potential was then set at 0.0 V, which
led to a rapid loss of the bands at 341 and 389 nm and the
appearance of an 18π porphyrin-like spectrum having an intense
Soret band at 473 nm and two Q-bands at 612 and 687 nm.
These spectral changes are shown in the top right of Figure
10b, and the final spectrum after reduction is virtually identical
to the spectrum of compound 7 illustrated in Figure 2b. Further
reduction of 11 at -0.60 and then -1.80 V led to a marked
decrease in the Soret band intensity, suggesting formation of a
porphyrin π-anion radical.24
To summarize the electrochemical studies, [(OiBTPP)-
Zn(Cl)]+[ZnCl3]- (11) undergoes two major redox processes
in PhCN. The first, at 0.40 V, involves conversion of the 16π
compound to its 18π porphyrin form, most likely (OiBTTP)Zn,
and the second, at -1.58 V, involves the expected conversion
of the porphyrin to its π-anion radical form. The reduction
potentials for porphyrins with OETPP and related macrocycles
are quite similar to those of the OEP derivatives with the same
center metal ion,20b,c and this is also case for 11, whose Ep value
of -1.58 V compares well with the E1/2 ) -1.64 V for reduction
of (OEP)Zn under the same solution conditions. A similar
reduction of 11 to its 18π porphyrin form occurs in CH2Cl2,
with the product of the reduction being characterized by a Soret
band at 470 nm and two Q-bands at 595 and 681 nm. A further
reduction of the compound to give the Zn(II) porphyrin π-anion
radical occurs at E1/2 ) -1.64 V in CH2Cl2.
Acknowledgment. We are grateful to the RIKEN Super
Combined Cluster (RSCC) for computational resources. This work
was supported by two Grants-in-Aid for Scientific Research on
Priority Areas (Nos. 14340199, 17350021) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan, and
the Robert A. Welch Foundation (K.M.K., Grant E-680).
Conclusion
Supporting Information Available: Complete ref 17a, ex-
perimental details, calculated structures of some porphyrins, and
CIF files for 3, 5, and 7-11. This material is available free of
charge via the Internet at http://pubs.acs.org.
We have investigated the reaction of 16π OiBTPP (4) with
a variety of metal reagents. Using zerovalent metal reagents
(24) (a) Perrin, M. H.; Gouterman, M.; Perrin, C. L. J. Chem. Phys. 1969,
50, 4137–4151. (b) Fuhrhop, J. H. Struct. Bonding (Berlin) 1974, 18,
1–67. (c) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138–163.
JA102817A
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