A porphyrin with a C=C unit at its center

Article abstract of DOI:10.1021/ja205738z

The molecule (C=C)TTP (TTP = tetra-p-tolylporphyrin) and the triflate salt of its dication, [(C=C)TTP][OTf]₂, have been synthesized and characterized. NMR spectroscopy, nucleus-independent chemical shift calculations, and the crystal structure of (C=C)TTP indicate that (C=C)TTP is antiaromatic and (C=C)TTP²⁺ is aromatic.

Full text of DOI:10.1021/ja205738z

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pubs.acs.org/JACS
A Porphyrin with a CdC Unit at Its Center
Thomas P. Vaid*
Department of Chemistry, University of Alabama, Shelby Hall, 250 Hackberry Lane, Tuscaloosa, Alabama 35487, United States
S Supporting Information
b
Scheme 1. Synthesis of (CdC)TTP and
[(CdC)TTP][OTf]2
ABSTRACT: The molecule (CdC)TTP (TTP = tetra-p-
tolylporphyrin) and the triflate salt of its dication, [(CdC)-
TTP][OTf] , have been synthesized and characterized.
2
NMR spectroscopy, nucleus-independent chemical shift calcu-
lations, and the crystal structure of (CdC)TTP indicate that
2+
(
CdC)TTP is antiaromatic and (CdC)TTP is aromatic.
orphyrin complexes of nearly every metal in the periodic
1,2
P
table have been prepared, as have complexes of several
3,4
main-group metalloid and nonmetallic elements. The prede-
fined size of the four-nitrogen coordination sphere of the
porphyrin ring system sets a lower limit on the covalent radius
of an atom that can form a porphyrin complex in the usual
manner, with all four nitrogen atoms bonded to that central
atom. Silicon and phosphorus seem to be near this lower limit on
5
ꢀ9
10,11
the covalent radius, as many silicon
and phosphorus
porphyrin complexes are known, but in many cases the porphyrin
ring system is ruffled to accommodate these small central
12
atoms. Porphyrin complexes with even smaller atoms (specifically,
boron and carbon) bonded to nitrogen have been synthesized; in
those compounds, however, an individual boron or carbon is
bonded to only one or two nitrogen atoms of the porphyrin, and
usually there is more than one boron or carbon atom present.
Examples include porphyrins that are methylated one, two, three,
or four times at nitrogen (the last two being a cation and dication,
+
ꢀ
(ICdCI)TTP(H) Cl after workup with HCl(aq). After chroma-
tographic purification and recrystallization of (ICdCI)TTP-
+
ꢀ
(H) Cl , deprotonation with K
TTP was straightforward. Attempts at thermal or photolytic
conversion of (ICdCI)TTP to [(CdC)TTP][I] were made,
but none were successful. Reduction of (ICdCI)TTP with
2 equiv of SmI in THF gave (CdC)TTP. Oxidation of
(CdC)TTP with 2 equiv of AgOTf in THF yielded
[(CdC)TTP][OTf]
CO (aq) to give (ICdCI)-
2
3
1
3ꢀ15
respectively).
Other porphyrins with two nitrogens bridged
16
0
17ꢀ19
20,21
by CH CH , RCdCR ,
or various one-carbon moieties
2
2
2
are known. Recently, an N-confused porphyrin with a carbene
carbon bridging a porphyrin nitrogen atom and a carbon atom has
2
2
2
been synthesized. Known boron-containing porphyrins gener-
ally have two boron atoms, each bonded to two nitrogen atoms
.
2
23ꢀ29
and often with other heteroatoms (O, Cl, F) present.
Crystals of (CdC)TTP were grown by slowly cooling a hot
heptane/toluene solution. Single-crystal X-ray diffraction re-
One reported porphyrin complex contains at its center a
30
simple BꢀB unit with a boronꢀboron single bond and each
vealed the structure shown in Figure 1. There was some
disorder in the structure: in ∼11% of the molecules, the central
CdC unit is turned 90° relative to the orientation shown in
Figure 1. Only those two disordered carbon atoms were refined
(isotropically) as separate atoms with partial occupancy, and they
are not shown in Figure 1 [see the Supporting Information (SI)
for a figure that includes the two carbon atoms with 11%
occupancy]. The rest of the molecule in the minor orientation
largely overlaps with the major orientation shown, so the
structure was refined with 100% occupancy of the remaining
major-orientation atoms. The orientations of the displacement
ellipsoids of the nitrogen atoms are consistent with the presence
of a fraction of the molecules in the second orientation described.
27
boron bound to two nitrogens. The existence and stability of
that compound (in the absence of oxygen and water) suggests an
intriguing synthetic target that contains at its center a CdC unit,
whose size is similar to that of the BꢀB unit. Such a molecule
would be a completely organic (containing only C, H, and N),
conjugated, eight-fused-ring system. Herein we report the synthesis
and characterization of (CdC)TTP (TTP = tetra-p-tolylporphyrin)
and [(CdC)TTP][OTf] (OTf = trifluoromethanesulfonate).
2
The synthetic route is shown in Scheme 1. Cobalt porphyrins
0
Co(porph)] are known to react with alkynes (RCtCR ) and an
+
[
0
oxidizing agent to yield (RCdCR )porph(H) , where the
two carbon atoms of the alkene moiety are bonded to two
adjacent nitrogen atoms of the porphyrin and one other nitrogen
1
8,19
is protonated.
We found that the reaction of CoTTP
Received: June 20, 2011
with ICtCI in CH Cl with FeCl as an oxidant yielded
Published: September 12, 2011
2
2
3
r 2011 American Chemical Society
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dx.doi.org/10.1021/ja205738z
|J. Am. Chem. Soc. 2011, 133, 15838–15841

Journal of the American Chemical Society
COMMUNICATION
Figure 2. Valence-bond representation of the bonding in the porphine
core of (CdC)TTP, with bond distances from the crystal structure shown
in black and B3LYP/6-31+G(d)-calculated bond distances shown in blue.
Figure 1. Two views of the structure of (CdC)TTP as determined by
single-crystal X-ray diffraction. C atoms are shown in gray and N atoms
in blue; H atoms are not shown.
There is a fairly significant ruffling of the porphine core of
CdC)TTP, as shown in the lower image in Figure 1. The mean
Figure 3. (left) Structure of (CdC)porphine, with its 20-π-electron
circuit highlighted in blue. The letters a, b, c, and d indicate where NICS
calculations were conducted. (right) One resonance structure of
(
deviation of the four meso-carbons from the N plane is 0.84 Å. A
4
31
2+
structurally optimized [Gaussian 09, B3LYP/6-31+G(d)]
model of (CdC)porphine (in which the p-tolyl groups are
replaced by H atoms) adopted a similarly ruffled structure, with
(CdC)porphine , with its 18-π-electron circuit highlighted in blue.
reversible one-electron waves at ꢀ0.26, ꢀ0.59, ꢀ1.91, and
+
/0
+/0
a 0.86 Å mean deviation of the meso-carbons from the N plane.
ꢀ2.17 V vs ferrocene
(Fc ). The waves at ꢀ0.59 and
4
The CdC unit is fairly small for an entity at the center of a
porphyrin, even in comparison with the single metal atoms that
are usually present in porphyrin complexes, and that small size is
ꢀ0.26 V are presumably due to oxidation of (CdC)TTP to its
cation and dication, respectively, while the waves at ꢀ1.91 and
ꢀ2.17 V are due to the reduction to the anion and dianion. Most
metalloporphyrin complexes containing a redox-inactive metal
(such as Zn or Mg) similarly undergo two reversible oxidations
and two reversible reductions, with the redox occurring on the
organic ring system of the porphyrin. However, oxidation of
those metalloporphyrins to their radical cations occurs at about
12
the cause of the ruffling in (CdC)TTP.
Figure 2 shows the bond lengths from the crystal structure
along with the calculated bond lengths for (CdC)porphine.
There are alternating CꢀC single and double bonds along the
2
0-atom periphery of the porphine ring system, as indicated in
+
/0 33
the valence-bond representation of Figure 2. The average
+0.4 V vs Fc
,
while (CdC)TTP is oxidized at a potential
differences between the lengths of single and double bonds are
about 1.0 V more negative. The easy oxidation of (CdC)TTP is
unsurprising because the removal of two electrons converts it
from an antiaromatic 20-π-electron system to an aromatic 18-π-
electron system. In addition, the N CdCN core of (CdC)TTP
0
C
.050 Å for C ꢀC , 0.049 Å for C ꢀC , and 0.065 Å for
β
β
α
β
ꢀC . The structure is not in resonance with the equivalent
meso
α
representation in which the outer CꢀC single and double bonds
2
2
have been interconverted because the porphine core of (CdC)-
is the same as that of tetrakis(dimethylamino)ethene and other
34,35
TTP is antiaromatic. A similar bond-length alternation has been
organic reducing agents.
9
2+
observed in Si(TPP)(THF) (TPP = tetraphenylporphyrin),
The dication (CdC)TTP was synthesized and isolated as its
triflate salt by oxidizing (CdC)TTP with 2 equiv of AgOTf. We
2
32
27
Ge(TPP)(pyridine)2, and (BꢀB)TTP. In each of those
complexes, the central atom (or pair of atoms for the BꢀB com-
plex) is formally in the 4+ oxidation state. The remainder of the
porphyrin ring system is in the 4ꢀ oxidation state and therefore
has been reduced by two electrons relative to the usual 2ꢀ
oxidation state. A standard porphyrin is aromatic and contains an
were not able to grow crystals of [(CdC)TTP][OTf] suitable
2
for single-crystal X-ray diffraction, but density functional theory
(DFT) calculations at the B3LYP/6-31G* or higher level have
proven to give reliable predictions of the structures of (CdC)-
9
,32,36,37
TTP (see Figure 2) and other porphyrin complexes.
The
31
1
8-π-electron system, as highlighted in blue for the (CdC)-
calculated [Gaussian 09, B3LYP/6-31+G(d)] structure of
(CdC)porphine is as ruffled as that of (CdC)TTP, with
2
+
2+
porphine cation in Figure 3. When the porphyrin ring system is
reduced by two electrons, one would expect an antiaromatic
the meso-carbons displaced from the N plane by an average of
4
2
0-π-electron system to be formed, and that 20-π-electron
0.84 Å, which indicates that the ruffling of (CdC)TTP is not
primarily due to its antiaromaticity but is instead due to the
small size of the CdC unit. The calculated bond lengths for
circuit in (CdC)porphine is highlighted in blue in Figure 3.
Cyclic voltammetry of (CdC)TTP in THF with 0.10 M
2+
[NBu ][PF ] as the supporting electrolyte showed four
(CdC)porphine are shown in Figure 4. As expected for an
4
6
1
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Journal of the American Chemical Society
COMMUNICATION
2
+
2+
Figure 5. Calculated HOMO of (CdC)porphine .
Figure 4. B3LYP/6-31+G(d)-calculated structure of (CdC)porphine .
Selected bond lengths are indicated.
aromatic porphyrin, there is no alternation of CꢀC single and
double bonds of the type observed in (CdC)TTP. The point group
2+
of (CdC)porphine is C , while that of (CdC)porphine is C .
2
2v
A comparison of the NMR spectra of (CdC)TTP and
[
(CdC)TTP][OTf] provides further evidence for the antiar-
2
2+
omaticity of (CdC)TTP and aromaticity of (CdC)TTP . A
strong magnetic field induces a diatropic ring current in an
aromatic compound, leading to the familiar downfield shift of the
resonances for nuclei outside the ring and upfield shift for nuclei
inside the ring. In an antiaromatic compound, a paratropic ring
current is induced, and the effects on the chemical shifts are in the
38
1
opposite direction. The effects are clearly evident in the H NMR
resonances of the β-pyrrole protons of [(CdC)TTP][OTf]2
and (CdC)TTP. In a typical (diamagnetic and aromatic) por-
phyrin complex, the β-pyrrole protons resonate at ∼9.0 ppm. In
Figure 6. UVꢀvisꢀNIR absorption spectrum of [(CdC)TTP][OTf]
2
in CH Cl , showing λmax values of 335, 439, 512, 559, and 864 nm.
2 2
[
(CdC)TTP][OTf] , they are slightly downfield at 9.44 and
highlighted in blue in Figure 3 are considered. That 18-π-
electron model for aromatic porphyrins is also consistent with
the antiaromatic 20-π-electron circuit present in doubly reduced
2
9
.47 ppm, perhaps as a result of the positive charge on the
molecule. In contrast, the β-pyrrole protons of (CdC)TTP
resonate far upfield at 2.38 and 2.67 ppm, indicating a paratropic
9
,27,32
porphyrins such as Si(TPP)(THF)2
and the antiaromatic
13
37,41
ring current in its porphine core. All of the C resonances of
both compounds were assigned through a combination of multi-
dimensional experiments (see the SI for full assignments). Of
most interest are the two central carbon atoms. In (CdC)TTP
their chemical shift is 135.43 ppm, and in [(CdC)TTP][OTf]2
they resonate at 102.57 ppm. The positive charge on (CdC)-
16-π-electron circuit present in doubly oxidized porphyrins.
However, H €u ckel’s (4n + 2)-π-electron rule for aromaticity was
4
2
formulated for monocyclic systems, and some caution may be
warranted when applying it to polycyclic systems such as
43
porphyrins. Also, the π electrons from the N atoms are left
uncounted in the blue circuits of Figure 3, although they would
add either four or eight π electrons and leave the predicted
aromaticity or antiaromaticity unchanged. In the (CdC)-con-
taining porphyrins there are two additional π electrons that, if
counted, would reverse the predicted aromaticity/antiaromaticity of
the porphyrin. For that reason, at the start of this project it was not
obvious which oxidation state of (CdC)TTP would be aromatic,
2
+
13
TTP should shift the central C resonance to lower field
higher chemical shift), but it is instead 33 ppm upfield of the
resonance for neutral (CdC)TTP. A diatropic ring current in
(
2
+
(
CdC)TTP and a paratropic ring current in (CdC)TTP
would explain the chemical shifts.
Nucleus-independent chemical shift (NICS) calculations pro-
39
2+
vide a computational measure of ring currents. NICS values are
commonly calculated at or near a ring center, but NICS(1)
measurements (in which the calculation is done 1 Å “above” a
ring centroid) are a better reflection of the magnitude of a
and in fact, the calculated HOMO of (CdC)porphine (Figure 5)
44
looks much like the HOMO of Zn(porphine) with an additional
component on the central CdC π bond. Nevertheless, the standard
rubric of aromaticity in porphyrins due to an 18-π-electron system
40
2+
paratropic or diatropic ring current. We performed NICS(1)
seems to apply to (CdC)TTP .
2+
calculations [B3LYP/6-31+G(d)] for both (CdC)porphine and
While (CdC)TTP contains the nominal 18-π-electron circuit
of an aromatic porphyrin, its overall electronic structure is fairly
different from that of a metalloporphyrin such as ZnTTP (see the
SI for more images of MOs and for MO energy-level diagrams).
The UVꢀvisꢀNIR absorption spectrum of [(CdC)TTP]-
2
+
(
CdC)porphine at the ring centroids denoted a, b, c, and d in
Figure 3. Because the two sides of the ring (above and below) are
inequivalent, calculations were performed both 1 Å above and
1
Å below the ring system for each position, and the results were
averaged. For (CdC)porphine, the NICS(1) values at a, b, c, and
[OTf] is shown in Figure 6. It differs markedly from the spectra
2
d were 38.5, 19.2, 25.1, and 15.6, respectively, and those for
of standard porphyrins, which have an intense Soret absorption
2
+
45
(
CdC)porphine were ꢀ13.6, ꢀ13.8, ꢀ16.2, and ꢀ13.7,
at ∼420 nm and smaller Q bands between 500 and 650 nm.
respectively. Those results are consistent with a paratropic ring
The major changes in the spectrum are not due to the positive
2+
current and antiaromaticity in (CdC)porphine and a diatropic
charge on (CdC)TTP , since doubly protonated porphyrins
2+
2+
ring current and aromaticity in (CdC)porphine .
such as H TPP have a Soret band that is red-shifted by ∼20 nm
4
4
6
The standard model of porphyrins as aromatic 18-π-electron
relative to H TPP and Q bands with some changes in intensity.
2
2+
2+
systems explains the aromaticity of (CdC)TTP and the
antiaromaticity of (CdC)TTP if the π-electron circuits
Nor is the ruffling of (CdC)TTP the cause of the unusual
spectrum, as ruffling simply causes some red-shifting of both
1
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dx.doi.org/10.1021/ja205738z |J. Am. Chem. Soc. 2011, 133, 15838–15841

Journal of the American Chemical Society
COMMUNICATION
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4
7
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2
2
7
1
some resemblance to that of (ClBꢀBCl)TTP, which implies
that it is the interaction of these two-atom central units with the π
system of the porphyrin that leads to the multiple high-absorp-
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(
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1
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can be isolated as either an antiaromatic neutral molecule or an
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(
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’
ASSOCIATED CONTENT
(
Meyer, S.; Lynch, V.; Boyd, P. D. W.; Brothers, P. J.; Sessler, J. L. Chem.
Commun. 2004, 1060.
S
Supporting Information. Synthetic, spectroscopic, and
b
crystallographic details; selected NMR spectra; a CIF for
CdC)TTP and an image of its structure with disordered carbon
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(
30) Dark block, 0.22 mm ꢁ 0.22 mm ꢁ 0.24 mm; monoclinic,
’
AUTHOR INFORMATION
1
space group P2 /n; a = 15.523(2) Å, b = 14.8902(19) Å, c = 15.937(2) Å,
3
β = 93.327(4)°, V = 3677.5(9) Å ; Z = 4; 2θmax = 52.72°; 33518
Corresponding Author
tpvaid@ua.edu
reflections collected, 7467 unique (R = 0.0497); final GOF = 1.091;
int
R
2
1
= 0.0696, wR
σ(I) (refinement on F ).
31) Frisch, M. J.; et al. Gaussian 09; Gaussian, Inc.: Wallingford,
CT, 2009.
2
= 0.177, R indices based on 5210 reflections with I >
2
(
’
ACKNOWLEDGMENT
I thank Steven Kelley for collecting the single-crystal X-ray
(32) Cissell, J. A.; Vaid, T. P.; Yap, G. P. A. J. Am. Chem. Soc. 2007,
diffraction data and assisting in the structure solution, Kenneth
Belmore for collecting and interpreting the multidimensional
NMR spectra, and the Dixon research group for some assistance
with DFT calculations. This work was supported by the Uni-
versity of Alabama.
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dx.doi.org/10.1021/ja205738z |J. Am. Chem. Soc. 2011, 133, 15838–15841