The doubly oxidized, antiaromatic tetraphenylporphyrin complex [Li(TPP)][BF4]

Article abstract of DOI:10.1021/ol060772l

Oxidation of dilithium tetraphenylporphyrin, Li₂(TPP), yields the doubly oxidized porphyrin complex [Li(TPP)][BF₄]. Free TPP appears to be unstable. The crystal structure of [Li(TPP)][BF₄] reveals that the 16-atom C-N inner ring of the porphyrin consists of alternating single and double bonds. NMR spectroscopy and nucleus-independent chemical shift (NICS) calculations, in addition to the distortion from planarity observed in the crystal structure, indicate that the 16 π-electron inner C-N ring of the porphyrin is antiaromatic.

Full text of DOI:10.1021/ol060772l

ORGANIC
LETTERS
2006
Vol. 8, No. 11
2401-2404
The Doubly Oxidized, Antiaromatic
Tetraphenylporphyrin Complex
[Li(TPP)][BF4]
Julie A. Cissell,‡ Thomas P. Vaid,*,‡ and Glenn P. A. Yap§
Center for Materials InnoVation and Department of Chemistry,
Washington UniVersity in St. Louis, St. Louis, Missouri 63130, and
Department of Chemistry and Biochemistry, UniVersity of Delaware,
Newark, Delaware 19716
Vaid@wustl.edu
Received March 30, 2006
ABSTRACT
Oxidation of dilithium tetraphenylporphyrin, Li2(TPP), yields the doubly oxidized porphyrin complex [Li(TPP)][BF4]. Free TPP appears to be
unstable. The crystal structure of [Li(TPP)][BF4] reveals that the 16-atom C N inner ring of the porphyrin consists of alternating single and
double bonds. NMR spectroscopy and nucleus-independent chemical shift (NICS) calculations, in addition to the distortion from planarity
observed in the crystal structure, indicate that the 16 -electron inner C N ring of the porphyrin is antiaromatic.
−
π
−
In the vast majority of known porphyrin complexes the
oxidation state of the porphyrin ring system is 2-. We
recently reported the first crystallographic characterization
of a porphyrin complex with a ring system in the 3-
oxidation state, the aluminum tetraphenylporphyrin radical,
Al(TPP)(THF)2.1 In another recent communication we de-
scribed the antiaromatic Si(TPP)(THF)2, a porphyrin complex
with a ring system in the 4- oxidation state.2 The one
reported example of a neutral porphyrin complex with a ring
system in the 1- oxidation state is Li(TPP),3 and there are
several examples of porphyrin radical cations with ring
systems in the 1- oxidation state.4 The existence of stable
porphyrin complexes with ring systems in the 4-, 3-, 2-,
and 1- oxidation states led us to explore the possibility of
isolating a porphyrin with a ring system in the zero oxidation
state. We were encouraged by the fact that many porphyrin
complexes can be reversibly oxidized by two electrons in
cyclic voltammetry experiments.5 Our interest has been in
neutrally charged, oxidized or reduced molecules as isos-
tructural dopants for molecular semiconductors,6 and in the
case of tetraphenylporphyrin the doubly oxidized, neutral
species would be simply “TPP”, which contains no central
metal atom or hydrogens.
Li(TPP) was synthesized by the oxidation of Li2(TPP) with
ferrocenium hexafluorophosphate.3 Cyclic voltammetry of
Li(TPP) in CH2Cl2 showed that it is reversibly oxidized at
‡ Washington University in St. Louis.
§ University of Delaware.
(1) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. Inorg. Chem. 2006, 45,
2367-2369.
(4) Vangberg, T.; Lie, R.; Ghosh, A. J. Am. Chem. Soc. 2002, 124, 8122-
8130 and references therein.
(5) Kadish, K. M.; Smith, K. M.; Guilard, R., Eds. The Porphyrin
Handbook; Academic Press: San Diego, CA, 2000; Vol. 9.
(6) Vaid, T. P.; Lytton-Jean, A. K.; Barnes, B. C. Chem. Mater. 2003,
15, 4292-4299.
(2) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. J. Am. Chem. Soc. 2005,
127, 12212-12213.
(3) Gebauer, A.; Dawson, D. Y.; Arnold, J. J. Chem. Soc., Dalton Trans.
2000, 111-112.
10.1021/ol060772l CCC: $33.50
© 2006 American Chemical Society
Published on Web 05/06/2006

+0.12 V vs ferrocene0/+, indicating that oxidation of the ring
system to the zero oxidation state is possible but requires an
oxidizing agent stronger than ferrocenium. Oxidation of
tion of the structure of [Li(TPP)][BF4] in crystals of
[Li(TPP)][BF4]‚CH2Cl2 is given in Figure 1. The lithium
-
Li(TPP) with thianthrenium tetrafluoroborate (Thn+BF4 ) in
CH2Cl2 did not produce free TPP, but instead yielded a
complex that spectroscopic evidence indicated was [Li(TPP)]-
[BF4]. In [Li(TPP)][BF4] the porphyrin ring is in the desired
zero oxidation state, but TPP is acting as a neutral com-
plexing ligand for Li+. We attempted to remove the Li+
+
cation by treating [Li(TPP)][BF4] with NBu4 Cl- in CH2Cl2,
but no free TPP could be isolated. The cation K+ is
significantly larger than Li+, so while Li typically sits in
the N4 plane in porphyrin7 and phthalocyanine8 complexes,
K+ sits far outside the N4 plane.7,9 We therefore thought that
oxidation of K2(TPP) might yield free TPP. Oxidation of
Figure 1. Solid-state structure of [Li(TPP)][BF4].
-
K2(TPP) with 2 equiv of Thn+BF4 in CH2Cl2 yielded a
product mixture that contained some doubly oxidized por-
1
phyrin species (as indicated by H NMR), but TPP could
cation is displaced toward the BF4- anion by 0.440 Å from
the mean plane of the four nitrogen atoms. The ring system
is moderately distorted from planarity by both a ruffle and
a saddle distortion.12 OETPP exhibits the same combination
of ruffle and saddle distortions,10 but its deviation from
planarity is significantly larger than that of [Li(TPP)][BF4].
Figure 2 depicts the valence-bond structure that best
represents the bonding in [Li(TPP)][BF4], with each bond
not be isolated from this mixture. Similar reactions in 1,2-
dichlorobenzene, fluorobenzene, and acetonitrile did not yield
TPP. We concluded that TPP is probably not stable as a free
molecule but is stabilized when coordinated to Li+.
When we had reached this point in our study, the first
isolation of doubly oxidized porphyrins, the oxidized forms
of octaethyltetraphenylporphyrin (OETPP) and octaisobu-
tyltetraphenylporphyrin were reported.10 OETPP was syn-
thesized unintentionally by the reaction of Li2(OETPP) and
SOCl2. (We therefore carried out the reaction of Li2(TPP)
and SOCl2, but it did not yield TPP.) It appears that the steric
protection of the eight alkyl groups on the â-carbons of these
oxidized porphyrins stabilizes them enough for isolation,
although OETPP reportedly slowly decomposes in solution
at room temperature.
Because it was not possible to isolate free TPP, we
continued our study on its stabilized form in [Li(TPP)][BF4].
[Li(TPP)][BF4] was most efficiently synthesized by the
11
-
reaction of Li2(TPP)(OEt2)2 with 2 equiv of Thn+BF4 in
CH2Cl2. The orange-brown solution was filtered to remove
1 equiv of LiBF4. The product was crystallized by the
addition of toluene and isolated by filtration. After remaining
under vacuum for 1 h, orange-brown [Li(TPP)][BF4]‚1.5-
(toluene) was collected in 84% yield. The compound is stable
at room temperature in the solid state and in CH2Cl2 solution
in the absence of water, with which it reacts slowly.
[Li(TPP)][BF4] does not react with O2. It slowly decomposes
in CH3CN to yield the Li(TPP) radical.
Figure 2. Valence-bond structure of Li(TPP)+ with bond distances
(Å) from the crystal structure of [Li(TPP)][BF4] (black) and
calculated (B3LYP/6-31G*) for Li(porphine)+ (red). C-C distances
are outside of the ring, and C-N distances are inside the ring. The
letters “a” and “b” indicate where NICS calculations were
performed. The blue outline indicates the 16 π-electron system.
Crystals suitable for X-ray diffraction were grown by
heating a solution of [Li(TPP)][BF4] in CH2Cl2/heptane and
slowly cooling to room temperature. An ORTEP representa-
labeled with its appropriate bond length from the crystal
structure. Also shown in Figure 2 are the bond lengths
calculated for Li(porphine)+ [Li(TPP)+ with the phenyl
groups replaced by hydrogen atoms] in a density functional
theory (DFT) calculation (Gaussian 03,13 B3LYP/6-31G*).
The calculated structure for Li(porphine)+ is perfectly planar,
(7) Arnold, J.; Dawson, D. Y.; Hoffman, C. G. J. Am. Chem. Soc. 1993,
115, 2707-2713.
(8) Sugimoto, H.; Mori, M.; Masuda, H.; Taga, T. Chem. Commun. 1986,
962-963.
(9) Ziolo, R. F.; Guenther, W. H. H.; Troup, J. M. J. Am. Chem. Soc.
1981, 103, 4629-4630.
(10) Yamamoto, Y.; Yamamoto, A.; Furuta, S.; 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.
(11) Brand, H.; Capriotti, J. A.; Arnold, J. Inorg. Chem. 1994, 33, 4334-
4337.
(12) For nonplanar porphyrin nomenclature, see: Shelnutt, J. A.; Song,
X.-Z.; Ma, J.-G.; Jia, S.-L.; Jentzen, W.; Medforth, C. J. Chem. Soc. ReV.
1998, 27, 31-42.
(13) Frisch, M. J., et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT,
2004.
2402
Org. Lett., Vol. 8, No. 11, 2006

in constrast to the observed crystal structure of Li(TPP)+.
Nevertheless, the calculated bond lengths agree quite well
with those observed in the crystal structure. The C-C and
C-N bond lengths clearly correspond to those expected from
the most reasonable valence-bond depiction of TPP, and
neutral TPP is simply acting as a donor ligand to the Li+
cation. The doubly oxidized porphyrin OETPP has similar
C-C and C-N bond lengths throughout its porphine core.10
The aromaticity of a normal-valent porphyrin is generally
understood to be a consequence of an 18 π-electron circuit
that is present in its ring system. The doubly oxidized
Li(TPP)+ would therefore be expected to have an antiaro-
matic 16 π-electron circuit, and exactly such a circuit is
evident in Li(TPP)+sit is highlighted in blue in Figure 2.
Evidence from 1H NMR and calculated nucleus-independent
chemical shifts (NICS)14 both agree with the picture of an
antiaromatic 16-membered C-N inner ring with four ap-
pended ethene groups. The 1H NMR spectrum of [Li(TPP)]-
[BF4] in CD2Cl2 contains a singlet due to the Câ protons
(see Figure 2 for atom-type labels) at 5.94 ppm (a chemical
shift typical of alkene protons), far upfield from the typical
resonance of about 9 ppm for the Câ protons of an aromatic
porphyrin. NICS values were calculated (Gaussian 03,
B3LYP/6-31G*) for Li(porphine)+ at the centroid of a
pyrrole ring (labeled “a” in Figure 2) and at the centroid of
two C-N bonds (labeled “b” in Figure 2). The NICS value
at “a” is -0.6 and that at “b” is +36.5, indicating a strong
paratropic ring current along the outlined 16-atom circuit
with the appended ethene groups excluded. Paratropic ring
currents are characteristic of antiaromatic systems,15 as
opposed to the diatropic ring currents of aromatic molecules.
An optimized structure of neutral “porphine” (TPP with the
phenyl groups replaced by hydrogen atoms) was calculated
(B3LYP/6-31G*), and in contrast to the planar structure of
Li(porphine)+ it is moderately saddled and slightly ruffled.
NICS values calculated at the center of the ring system of
porphine and at positions “a” and “b” (analogous to those
in Figure 2) were +13.5, -4.8, and +20.2 ppm, respectively.
The structure of porphine was minimized with an enforced
planar (C4h) geometry, and the calculated NICS values at
the center, “a”, and “b” were +20.0, -6.8, and +30.4,
respectively. All of those values are consistent with a
paratropic ring current in the central 16-atom C-N circuit
of porphine. The lower NICS value at “b” in the saddled
geometry of porphine (relative to either planar porphine or
Li(porphine)+) indicates that Li+ increases the paratropic ring
current, at least in part, by maintaining planarity in the ring
system. NICS calculations in the vicinity of a bare Li+ cation
show that it has negligible influence on NICS values.
Many porhyrin radical cations, in which the oxidation state
of the ring system is 1-, undergo a distortion that is often
described as a bond-length alternation along the 16-
membered C-N inner ring.4 The distortion is the same as
that observed in Li(TPP)+, except that the magnitude of the
distortions in the radical cations is smaller than that in
Li(TPP)+, and is at most half of that observed in Li(TPP)+.
A normal-valent porphyrin has nearly degenerate a1u and a2u
HOMOs, and the bond-length changes in the radical cations
are due to a pseudo-Jahn-Teller distortion caused by the
unequal occupancy of the (former) a1u and a2u molecular
orbitals.4 The bond-length alternation in Li(TPP)+ could be
considered to be a consequence of a similar pseudo-Jahn-
Teller distortion, but of a magnitude twice as large as that
in the porphyrin radical cations because two electrons have
been removed instead of one. The calculated HOMO of C4h
Li(porphine)+ (Figure 3) has the same au symmetry as the
Figure 3. The HOMO of Li(porphine)+ (left) and N-CR-Cmeso
CR and CR-Cmeso-CR-N dihedral angles (right).
-
SOMO of the porphyrin radical cations.4 The image of the
HOMO of Li(porphine)+ in Figure 3 is in the same
orientation as the valence-bond structure of Figure 2, and
the alternating bonding and antibonding π-interactions around
the 16-membered inner ring further clarify the cause of the
bond-length alternation. An alternative explanation of the
bond-length alternation in Li(TPP)+ is simply that it adopts
the valence-bond structure of Figure 2. The situation is quite
similar to that in the singly reduced Al(TPP)1 and the doubly
reduced Si(TPP),2 which both have alternating lengthened
and shortened C-C bonds along the 20-atom periphery of
their porphine core. The bond-length changes in Al(TPP)
are due to a Jahn-Teller distortion caused by the single
electron in one of its formerly degenerate eg LUMOs. The
distortion in Si(TPP) is approximately twice as large, and
can be viewed either as a larger Jahn-Teller distortion
(caused by two electrons instead of one) or simply as Si(TPP)
adopting a new valence-bond structure.
The deviations from planarity in the porphine core of
Li(TPP)+ can be understood in relation to its au HOMO, and
those deviations support the assertion that the 16-atom inner
ring is antiaromatic. For each pyrrole ring, the five atoms of
its ring and the two Cmeso to which it is attached are very
nearly coplanar. However, the combined ruffling and sad-
dling of Li(TPP)+ lead to the N-CR-Cmeso-CR and CR-
Cmeso-CR-N dihedral angles indicated in Figure 3. The
average dihedral angle across a C-C double bond (where
there is a π-bonding interaction in the au HOMO) is 1.5°
and that across a C-C single bond (where there is a
π-antibonding interaction in the au HOMO) is 24.4°. In a
normal conjugated system such as 1,3-butadiene, which is
neither aromatic nor antiaromatic, the lowest energy con-
formation is planar s-trans,16 which maximizes the interaction
(14) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer,
P. v. R. Chem. ReV. 2005, 105, 3842-3888.
(15) Pople, J. A.; Untch, K. G. J. Am. Chem. Soc. 1966, 88, 4811-
4815.
Org. Lett., Vol. 8, No. 11, 2006
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of the π-orbitals across the single bond. In contrast, Li(TPP)+
is clearly distorting from planarity to minimize the π-anti-
bonding interactions of the HOMO across the C-C single
bonds, indicating that the 16-atom inner ring is antiaromatic,
rather than simply conjugated. It is unlikely that the Li+ is
contributing to the distortion from planarity because the ionic
radius of Li+ is appropriate for a planar porphyrin rings
both aromatic Li(porphyrin)- anions7 and the Li(TPP)
radical3 are planar. In fact, the Li+ of Li(TPP)+ may be
limiting the porphyrin’s distortion from planarity and sta-
bilizing it in that way.
The valence-bond picture of Li(TPP)+ indicates that it has
two inequivalent sets of CR, two inequivalent sets of Câ, and
two inequivalent sets of hydrogens bound to the Câ atoms.
The calculated [B3LYP/6-31G*, Li(porphine)+ model] chemi-
cal shift difference between those two types of CR is 14.3
ppm, that between the two types of Câ is 7.32 ppm, and that
between the two types of Câ hydrogens is 0.35 ppm.
However, room temperature 1H and 13C NMR spectra contain
only one resonance for each of those types of nuclei. The
averaging of the NMR resonances is due to a very fast
automerization that interconverts the double and single bonds
in the 16-atom C-N inner ring of Li(TPP)+. Even when a
solution of [Li(TPP)][BF4] in a 3:1 mixture of CD2Cl2/CDCl3
was cooled to -100 °C the resonances were still averaged.
This rapid interconversion at -100 °C is perhaps not
surprising in light of the fact that cyclobutadiene undergoes
a similar automerization quickly at 25 K.17 Nevertheless, the
interconversion is clearly stopped in crystals of [Li(TPP)]-
[BF4]‚CH2Cl2 at 120 K, the temperature at which the crystal
structure was determined.
of the absorption bands. The red shift in distorted normal-
valent porphyrins is due mainly to a rise in the HOMO
energy caused by the distortion.18 In the doubly oxidized
porphyrins, distortions from planarity (of the type observed
for the three known compounds) will cause a lowering of
the HOMO energy as it decreases the π-antibonding interac-
tions of the HOMO (see Figure 3). Hence the more-distorted
OETPP has a UV-vis spectrum that is blue-shifted relative
to Li(TPP)+.
[Li(TPP)][BF4] is an oxidizing agent and undergoes a
variety of chemical reactions in which TPP oxidizes a metal
and the LiBF4 is replaced by the oxidized metal ion. The
yields are essentially quantitative and the reaction rates
appear to be limited mainly by the surface area of the solid
metals available for reaction. For example, the reaction of
[Li(TPP)][BF4] and copper powder in THF at 22 °C to form
CuTPP is complete in an hour, while the reaction with finer
Zn dust to form ZnTPP is complete in 10 min. Similarly,
the reaction of [Li(TPP)][BF4] and Mg turnings in THF
yields MgTPP after a few hours. The reaction of [Li(TPP)]-
[BF4] with SnCl2 yields Sn(TPP)Cl2, oxidizing the Sn(II) to
Sn(IV). Finally, the reaction of [Li(TPP)][BF4] and hydro-
quinone yields H2TPP and benzoquinone.
In summary, we have isolated the doubly oxidized
porphyrin complex [Li(TPP)][BF4], which is TPP stabilized
by coordination to LiBF4. It was not possible to isolate free
TPP, but analogous oxidized porphyrins with eight alkyl
substituents on the â-carbons are stable enough to be
isolated.10 The best description of the electronic structure of
Li(TPP)+ is an antiaromatic 16 π-electron core with four
appended ethene groups. [Li(TPP)][BF4] acts as an oxidizing
agent in reactions with elemental metals and compounds to
yield metalloporphyrins.
The UV-vis spectrum of [Li(TPP)][BF4] in CH2Cl2
consists of two high-absorptivity bands at 332 and 394 nm
and a broad, lower intensity shoulder that extends to about
575 nm (see the Supporting Information). The spectrum is
similar to that of OETPP, with the two major absorptions of
OETPP blue-shifted to 275 and 339 nm.10 The blue shift of
the OETPP absorptions is due to its greater distortion from
planarity (the average of its dihedral angles of the type shown
in Figure 3 are 10.7° and 54.0°). Interestingly, that is the
opposite of the effect observed in normal-valent aromatic
porphyrins, where distortions from planarity cause a red shift
Acknowledgment. We thank the NSF (CHE 0133068)
for support, Brian Barnes for assistance with DFT calcula-
tions, and Andre´ d’Avignon for help in acquiring low-
temperature NMR spectra.
Supporting Information Available: Experimental pro-
cedures of all synthesized compounds, including their
characterization and crystalligraphic data in CIF format. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(16) De Mare, G. R.; Panchenko, Y. N.; Vander Auwera, J. J. Phys.
Chem. A 1997, 101, 3998-4004.
(17) Orendt, A. M.; Arnold, B. R.; Radziszewski, J. G.; Facelli, J. C.;
Malsch, K. D.; Strub, H.; Grant, D. M.; Michl, J. J. Am. Chem. Soc. 1988,
110, 2648-2650.
OL060772L
(18) Parusel, A. B. J.; Wondimagegn, T.; Ghosh, A. J. Am. Chem. Soc.
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