Hybrid orbital deformation (HOD) effect and spectral red-shift property of nonplanar porphyrin

Article abstract of DOI:10.1021/ol100434w

A series of 5,15-meso,meso-strapped nonplanar porphyrins with different degrees of ruffling distortion, as a model system, have been synthesized and characterized. The spectral red-shift of the nonplanar porphyrins was experimentally demonstrated to mainl

Full text of DOI:10.1021/ol100434w

ORGANIC
LETTERS
2010
Vol. 12, No. 8
1780-1783
Hybrid Orbital Deformation (HOD) Effect
and Spectral Red-Shift Property of
Nonplanar porphyrin
Zaichun Zhou, Chenzhong Cao,* Qiuhua Liu, and Rongqing Jiang
School of Chemistry and Chemical Engineering, Hunan UniVersity of Science and
Technology, Xiangtan 411201, China, Key Laboratory of Theoretical Chemistry
and Molecular Simulation of Ministry of Education, Hunan UniVersity of Science
and Technology, China, and Hunan ProVincial UniVersity Key Laboratory of
QSAR/QSPR, China
czcao@hnust.edu.cn
Received February 21, 2010
ABSTRACT
A series of 5,15-meso,meso-strapped nonplanar porphyrins with different degrees of ruffling distortion, as a model system, have been synthesized
and characterized. The spectral red-shift of the nonplanar porphyrins was experimentally demonstrated to mainly originate from the hybrid
orbital deformation (HOD) effect due to the distortion in the tetrapyrrole macrocycle, which confirmed previous explanations to the red-shift
phenomenon.
Nonplanar heme distortion has been recognized to be a
conserved structural feature of particular proteins, and
significant attention has been paid to exploration of the
distortion properties of porphyrins.1-3 Distorted heme can
stabilize the unligated Fe(II) oxidation state and also generate
a high-valent iron(V)-oxo complex.4 This potent oxidant
is competent for hydroxylation chemistry of inactivated
C-H bonds.5 The out-of-plane heme distortions found in
Thermoanaerobacter tengcongensis even show >2 Å devia-
tions from planarity.6 Nonplanar porphyrins also show many
specific properties, e.g., potent catalytic ability,7 tunable
coordination,8 and an exceptional spectral red-shift.9 This
distortion is energetically unfavorable, suggesting that this
distortion is crucial to the functions of heme or porphyrin.10,11
Distortions of porphyrin macrocycles are known to result
in the spectral red-shift property.10 Shelnutt12 et al. carried
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10.1021/ol100434w  2010 American Chemical Society
Published on Web 03/22/2010

out numerous computational and experimental studies to
explore the origins of the red shifts and demonstrated that
the nonplanar deformation is the primary factor for the shift,
rather than the early in-plane nuclear reorganization (IPNR).9
Schweitzer-Stenner13 et al. subsequently offered a clearer
explanation using the static normal coordinate deformations
(SNCDs) theory. Their results implied that the spectral shifts
of the tetrapyrrole macrocycle are potentially useful indica-
tors of the unique changes in the ring nonplanarity.
To clarify the relationship between the spectral shifts and
the distortions to the porphyrin rings, we have (i) developed
an efficient approach to synthesize a series of nonplanar
porphyrins 5,15-meso,meso-strapped with different degrees
of distortion as a model system (see the Supporting Informa-
tion, S2) and (ii) augmented the previous “deformation”
theories and experimentally demonstrated that the spectral
red-shift of the nonplanar porphyrins is primarily due to the
hybrid orbital deformation (HOD) effect of the distortion in
the tetrapyrrole macrocycle.
Figure 1. Strapped porphyringen (top) and strapped porphyrin
(bottom). Take the 1,6-hexanediyl strap as the example.
and characterized according to the above mentality. Strapped
materials 6-10 were obtained from the condensation of
dialdehyde compounds 11-15 with a substituted dipyr-
romethane 16 under a molar ratio of 1:224 and then
complexed with zinc(II) to afford compounds 1-5 (Scheme
1). The dialdehyde compounds 11-15 and the dipyr-
romethane 16 were prepared according to the methods of
Whitlock25 and Lindsey,26 respectively. These types of
strapped structures effectively avoid the disturbances due to
substituent effects and the exchange of conformations in
periphery crowded porphyrins that represent the most popular
distorted porphyrin samples prepared.10,27 Direct measure-
ment of the compounds was achieved using NMR and
UV-vis spectral methods and a high-resolution mass
spectrometry (HR-MS) technique (see the Supporting Infor-
mation).
Shorter alkyl straps induce a stronger degree of distortion
to the porphyrin, and the length of the straps can be flexibly
adjusted by their carbon atom number (nC: from 7 to 3). The
1H NMR spectra of compounds 1-5 indicated a distorted
handbasket-type conformation (Scheme 1). All proton signals
in the straps showed a distinct upfield shift resonating at <3.0
ppm (Figure 2). This observation is because the straps
vertically locate to one side of the large π-system planarity28
and their protons are shielded by the porphyrin π-system.29
The distortive degree of these porphyrins can be judged
by the chemical shift of straps’ protons, e.g., the chemical
shift of ether methylene proton R decrease as the alkyl straps
shortened. For compound 5, the signal R appeared at 2.8
The field of strapped-metalloporphyrin functional assembly
has seen tremendous growth, with continuous emergence of
exciting applications in selective guest inclusion,14 photo-
induced electron transfer,15 molecular device,16,17 and recep-
tor model,18 and so on. Strapped metalloporphyrins also show
good potential in the fields of catalysis19,20 and asymmetric
catalysis.21
Porphyrin skeletons are known to originate from their
corresponding porphyrinogen, a nonplanar calyx[4]pyrrole
macrocycle,22 by using the condensation of aldehyde and
pyrrole, thus allowing the meso substituents A and B (or C
and D) to approach each other (Figure 1). If the two
substituents were bridged by a shorter strap (e.g., a 1,6-
hexandiyl group or shorter) before formation of porphyrino-
gen, the resulting strapped porphyrin would exhibit a ruffling
distortion10 after its oxidation due to the competetion between
the binding of the alkyl strap and the coplanarity of the
porphyrin π-system.23
A series of 5,15-meso,meso-strapped porphyrins with
different degrees of ruffling distortion have been synthesized
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Nurco, D. J.; Schore, N. E.; Zhang, J.; Ma, J. G.; Smith, K. M.; Gazeau,
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Org. Lett., Vol. 12, No. 8, 2010
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Scheme 1. Synthesis of Strapped Free Base Porphyrins 6-10
and Their Corresponding Zinc Complexes 1-5a
Figure 2.
1H NMR signal changes of the alkyl chain in molecules
1-5 at 293 K. The symbols R, ꢀ, γ and δ, are described in Scheme
1, and the symbols # and & represent the ligands, methanol and
water, respectively.
a Note: The protons labeled 5, 7 and the alkyl straps are at one side of
porphyrin plane, and those labeled 6, 8 are at the other side of it.
ppm (Figure 2e); for compound 1, however, the signal R
shifted to -1.0 ppm (Figure 2a) and the shift scope was up
to 3.8 ppm between compound 5 and 1; that is, proton R of
the former locate at the edge of porphyrin π-system and that
of the latter is close to the center of the π-system. The shorter
the alkyl strap is, the stronger the distortive degree of
porphyrin is. This is a unique property that distinguishes it
from many previous strapped systems.30-32
Figure 3.
1H NMR signal changes of the aromatic protons in
molecule 1-5 at 293 K. The numbering is described in Scheme 1.
The structural deformation of the strapped porphyrins can
also be reflected in the chemical shift of aromatic protons
(Figure 3). In the phenyl linked to alkyl, the diagnostic signal
(1) gradually extends shield field of porphyrin when the
number of carbon atom decrease from 7 to 3; e.g., the
diagnostic signal 1 of compound 5 and 1 appears at 7.18
and 6.24 ppm, respectively, and the latter is under the
remarkable influence of the π-system. Furthermore, the H
NMR experiments also support that these porphyrins are a
ruffling structure.
with previous reports.3,9,10 For zinc porphyrins 1-5, as the
degree of distortion increased the maxima were observed to
red-shift further from 422.7 to 446.1 nm (5 f 1) at the Soret
band (Figure 4-top). For the corresponding free base
compounds 6-10, their spectra also show a similar red-shift
trend (Figure 4-bottom, 10 f 6; 424.1 to 442.8 nm).
The spectral red-shift of the distorted porphyrin is believed
to mainly originate from the HOD effect. The sp2 orbits
would be deformed following the distortion of the large π
system. The relative sp2 orbital deformation of the C and N
atoms results in an increase of the ground energy level and
a consequent spectral red-shift. As far as each sp2 orbit is
concerned; however, the change scope is noticeably weaker
because there are 30 sp2 orbits involved in porphyrin. This
implies that it is difficult to find the HOD effect in normal
nonplanar systems (see the Supporting Information).
1
The series of nonplanar porphyrins 1-10 were selected
as model compounds to investigate the HOD effect of the
distorted porphyrins. These nonplanar porphyrins showed
strongly red-shifted electronic spectra, which is in agreement
(30) Mullen, K. M.; Gunter, M. J. J. Org. Chem. 2008, 73, 3336
(31) Fantauzzi, S.; Gallo, E.; Rose, E.; Raoul, N.; Caselli, A.; Issa, S.;
Ragaini, F.; Cenini, S. Organometallics 2008, 27, 6143
(32) Miyaji, H.; Kim, H. K.; Sim, E. K.; Lee, C. K.; Cho, W. S.; Sessler,
.
.
The changes in the degrees of distortion provided an
interesting phenomenon, in which the change scope of the
J. L.; Lee, C. H. J. Am. Chem. Soc. 2005, 127, 12510
.
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Org. Lett., Vol. 12, No. 8, 2010

hybrid orbits of the central zinc atom are also found at a
relatively higher ground energy level than those in the normal
coplanar system.
Figure 5. Graphic presentation of the hybrid orbital deformation
of the central zinc(II) in the nonplanar porphyrin.
Stronger distortions lead to higher ground energy levels;
therefore, the ∆λmax gradually increases as the length of the
strap decreases (Table 1). However, the spectral change from
the central zinc atom is much stronger than that from a sp2
hybrid orbit. The relative change in the molecular ground
energy level (-R∆EE,m: up to ∼2.7 kJ·mol-1) can be
conveniently estimated according to their spectral red-shift
(see the Supporting Information).
Figure 4. Comparison of absorption spectra of 1-5 (top) and 6-10
(bottom) in a CHCl3 solution (1.0-3.0 × 10-6 M) at 293 K. Inset:
absorption spectra at the Q-band.
red-shift is different after and prior to zinc complexation
(Table 1). When nC is 7, the spectral shift (∆λmax) between
zinc porphyrin 5 and the corresponding free base porphyrin
10 is -1.37 nm, and with decreasing nC, ∆λmax gradually
increases until nC is 3; ∆λmax between 1 and 6 is up to 3.37
nm.
Figure 6. Crystal structure (left), strain behavior, and out-of-plane
character (right) of the strapped porphyrin 5.
Table 1. Relationship between the Spectral Shift (∆λmax) and
the Number of Carbon Atoms (nC) at 293 K
In this study, the HOD model was used to interpret the
spectral shifts of porphyrin and heme by influence from the
degree of distortion in the porphyrin ring. That is, the
distortion can be availably traced through a spectral shift in
nonplanar porphyrin, which makes it feasible to investigate
the biological functions of distorted heme molecules by
applying this spectral technique. Moreover, the series of
nonplanar porphyrins can be used as an ideal model system
in exploring the characteristics of heme distortion and
optimizing the biochemical functions of metalloporphyrins.
compd
1 and 6 2 and 7 3 and 8 4 and 9 5 and 10
a
nC
3
3.37
4
1.49
5
0.24
6
7
∆λmax/nmb
-0.14
-1.37
a nC is the number of carbon atoms in the alkyl straps. b ∆λmax is the
spectral shift between the zinc porphyrin and the corresponding free base.
The HOD theory can also offer a satisfactory explanation
to this phenomenon. For zinc(II) atoms of the third periodic
elements, the hybrid orbits adopt an outer-orbital sp2d hybrid
mode due to its full 3d arrangement (Figure 5) and a smaller
size than the 4-N cavity in planar porphyrin. Zincic 4 sp2d
hybrid orbits are deformed due to the deviation of the zinc
atom from the 4-N plane after distortion of the large π
system. The determination of the crystal structure of 5 reveals
that the π-system of the strapped porphyrin is distortive and
the zinc atom is out of the 4-N plane (0.375 Å deviation,
Figure 6). As a result, the electrons in the deformed sp2d
Acknowledgment. This work was supported by the Nature
Science Foundation of China (20772028).
Supporting Information Available: Explanation of the
spectral shift, experimental procedures, spectroscopic data,
and characterization of compounds and crystallographic data
of 5 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
OL100434W
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