Towards true carbaporphyrinoids: Synthesis of 21-carba-23-thiaporphyrin

Article abstract of DOI:10.1002/anie.201310999

In the search for porphyrinoids with a built-in cyclopentadienyl moiety (true carbaporphyrins), a rational synthesis of carbathiaporphyrin, the synthons, has been elaborated. The donors (C,N,S,N) in the porphyrinic core of carbathiaporphyrinoids are potentially of fundamental importance for generating organometallic complexes, as exemplified through formation of the palladium(II) complex.

Full text of DOI:10.1002/anie.201310999

Angewandte
Chemie
DOI: 10.1002/anie.201310999
Carbaporphyrinoids
Towards True Carbaporphyrinoids: Synthesis of 21-Carba-23-
thiaporphyrin**
´
Anna Berlicka, Paweł Dutka, Ludmiła Szterenberg, and Lechosław Latos-Graz˙ynski*
Abstract: In the search for porphyrinoids with a built-in
cyclopentadienyl moiety (true carbaporphyrins), a rational
synthesis of carbathiaporphyrin, the synthons, has been
elaborated. The donors (C,N,S,N) in the porphyrinic core of
carbathiaporphyrinoids are potentially of fundamental impor-
tance for generating organometallic complexes, as exemplified
through formation of the palladium(II) complex.
Scheme 1. N-Confused porphyrin 1 and true carbaporphyrins 2.
T
he formal permutation of a pyrroline nitrogen atom
and a b-methine pyrrolic group of meso-tetraarylpor-
phyrin resulted in the creation of 2-aza-21-carba-
5,10,15,20-tetraarylporphyrin (N-confused porphyrin;
1), which has fundamentally changed electronic and
coordination properties compared to the parental
macrocycle.[1,2] The synthesis of this molecule was
achieved 20 years ago, although early suggestions
about the structure of this peculiar porphyrin isomer
were reported by Aronoff and Calvin in 1943[3] and
Pauling in 1944.[4] The initial isolation of the N-
confused porphyrin prompted intensive synthetic stud-
ies to generate a class of carbaporphyrinoids.[5–10]
Scheme 2. True carbaporphyrins reported to date.
The replacement of a single pyrrole unit in a por-
phyrin by a cyclopentadiene moiety seems at first to be
a compelling strategy for creating true carbaporphyrins,
which can be regarded as fundamental extensions of the
classical porphyrinic motif.[11] In a retrospective approach,[12]
this replacement corresponds to a substitution of an imine
nitrogen atom of a pyrrole ring by a trigonally hybridized
methine unit or of an amine NH group of a pyrrole by
a tetrahedral methylene group to afford a tautomeric couple
(2-1 and 2-2; Scheme 1), which is a characteristic of all
monocarbaporphyrinoids (monocarbaheteroporphyrinoids)
including N-confused porphyrin 1.
Originally the incorporation of a b-substituted cyclo-
pentadiene moiety into a porphyrin frame to yield an
aromatic carbaporphyrin 3 (Scheme 2), which is b-substituted
at the pyrrole rings and mono- or disubstituted at the
cyclopentadiene ring, was achieved by Berlin by a [3+1]
condensation,[11] and subsequently clarified by Lash and
Hayes.[13] To date the coordination chemistry of 3 has not
been reported.
Recently we reported on 5,10,15,20-tetraaryl-21-carba-
porphyrin—the first example of a true carbaporphyrin
directly related to 2-2—which was firmly stabilized through
palladium(II) coordination (4-Pd).[14] Thus, the remarkable,
facile palladium(II)-mediated contraction of p-phenylene to
cyclopentadiene, embedded in palladium(II) p-benzipor-
phyrin, produced palladium(II) complexes of 21-carbapor-
phyrins (4-Pd). The analogous gold(III)-promoted contrac-
tion of the p-phenylene group of p-benziporphyrin afforded
the first representative of a 21-carbaporphyrin complex (5-
Au) encompassing the frame of 2-1.[15] Evidently the con-
traction route to provide complexes of true carbaporphyrins is
far from being general and hitherto is, by our present
understanding, limited to certain transition metals. Within
this context, a different direction was probed to generate
a free-base meso-substituted 21-carbaporphyrin, which would
be potentially amenable to the insertion of a variety of metal
cations. Inspired by the emerging chemistry of tetraaryl-21-
carbaporphyrins, masked however by palladium(II) or gold-
(III) coordination, we have devoted our synthetic effort to the
construction of a class of true carbaporphyrins or carba-
heteroporphyrins related to 2-1 and 2-2.
´
[*] Dr. A. Berlicka, P. Dutka, Dr. L. Szterenberg, Prof. L. Latos-Graz˙ynski
Department of Chemistry, University of Wrocław
ul. 14 F. Joliot-Curie, 50-383 Wrocław (Poland)
E-mail: lechoslaw.latos-grazynski@chem.uni.wroc.pl
Homepage: http://llg.chem.uni.wroc.pl/
[**] Financial support from the National Science Centre (grant 2012/04/
A/ST5/00593) is kindly acknowledged. DFT calculations were
Consequently, we report on the synthesis and character-
ization of meso-substituted true carbathiaporphyrinoid 10,15-
dimesityl-21-carba-23-thiaporphyrin (6) and its reduced con-
gener 7. These carbaporphyrinoids can be formally con-
´
carried out at the Supercomputer Centers of Poznan and Wrocław.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310999.
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structed by replacement of a pyrrolidene fragment of 21-
thiaporphyrin with a cyclopentadienylidene (6) or cyclo-
pentenylidene (7) moiety with preservation of the general
pattern of the parent skeleton.
A key step in the synthesis of carbathiaporphyrinoids 6
and 7 is the construction of suitable synthons for introducing
the cyclopentadiene ring into a porphyrin-like frame. We have
taken advantage of the synthetic method of Seo et al. to
produce a mixture of isomeric carba analogues of tripyrrane
8a and 8b, used previously as a suitable ligand to form ansa-
cyclopentadienylpyrrolyltitanium complexes.[16]
The synthesis involves the one-pot reaction of 2,5-
bis(mesitylhydroxymethyl)thiophene (9) with 8 in a 3%
solution of ethanol in chloroform, catalyzed by BF3·Et2O,
followed by oxidation with p-chloranil (tetrachloro-1,4-ben-
zoquinone; Scheme 3). The procedure follows the [3+1]
N(22) atoms of preformed 6 are covalently linked to a 4,5-
dichloro-3,6-dioxocyclohexa-1,4-diene-1,2-diyl unit, was also
identified as a macrocyclic side product. Evidently 11 was
formed from 6 by substitution of two chloro substituents of p-
chloranil by C(21) and N(22).
At this stage we realized that the proper oxidation
conditions are particularly important. Eventually, the oxida-
tion of carbathiachlorin 7 with one equivalent of DDQ (2,3-
dichloro-5,6-dicyano-1,4-benzoquinone) afforded 21-carba-
23-thiaporphyrin 6 (yield of oxidation 25%; Scheme 4). The
progress of the conversion was monitored by 1H NMR
spectroscopy. An excess of DDQ or longer reaction time
resulted in decomposition of 7.
Scheme 4. Oxidation of 7: [D]chloroform, DDQ (1 equiv) dissolved in
[D6]benzene, RT, 20 min.
The molecular structure of 7 reveals the bond-length
pattern expected for aromatic carbaporphyrinoids and
heteroporphyrinoids (Figure 1).[5,17–22] The bonds of the cyclo-
pentene moiety not involved in the macrocyclic delocalization
À
pathway are in the range typical for alkyl C C bonds (1.515–
1.537 ꢀ).
Carbathiaporphyrinoids 6 and 7 give intense orange
solutions in chlorinated solvents. The electronic absorption
spectrum of 6 demonstrates a Soret band at 417 nm accom-
Scheme 3. Synthesis of 7. Reaction conditions: a) BF3·Et2O (1 equiv),
3% EtOH in CHCl3, RT, 1 h; b) triethylamine (1.2 equiv), p-chloranil
(6 equiv), RT, 1 h. Mes=mesityl=2,4,6-trimethylphenyl.
method previously utilized for the synthesis of heteropor-
phyrinoids and carbaporphyrinoids. Presumably two isomeric
porphyrinogens 10-1 and 10-2 are formed on condensation
that are differentiated by the orientation of the cyclopenta-
diene unit. After oxidation and chroma-
tographic work up, the chlorin-like
derivative 7 with the built-in cyclopen-
tenyl moiety was isolated (yield 5.2%)
instead of the targeted 21-carba-23-thia-
porphyrin 6 or isomeric 6-1.
The internally bridged 21-carba-23-
thiaporphyrin 11, where the C(21) and
Figure 1. Molecular structure of carbathiachlorin 7: a) perspective view
and b) side view (mesityl groups and selected H atoms are omitted for
clarity). The vibrational ellipsoids represent 50% probability. Selected
bond lengths [ꢀ]: C1-C2 1.515(3), C2-C3 1.536(3), C3-C4 1.524(3),
C1-C21 1.403(3), C4-C21 1.404(3).
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Figure 2. The electronic spectra of 6 (solid line), 7 (dotted line), 6-Pd
(short dashed line), and 7-Pd (long dashed line) in dichloromethane.
panied by a set of four Q bands (Figure 2, solid line). The
spectra can be related to the UV/Vis absorption spectrum of
aromatic 21-thiaporphyrin, which is considered here to be an
appropriate reference.[23] The electronic spectrum of carba-
thiachlorin 7 (Figure 2, dotted line) displays a split Soret band
at 415 and 441 nm, and Q bands at 525, 621, and 682 nm,
which are typical for aromatic, chlorine-like porphyri-
noids.[24–26]
1
The H NMR spectra of carbaporphyrinoids 6, 7, and 11
displayed resonances at positions consistent with an aromatic
structure. All the macrocycles retain their macrocyclic
aromaticity with an 18-p-electron delocalization pathway
and show the basic features of aromatic carbaporphyrinoids
(Figures 3 and 4).[5,18]
The 1H NMR spectra of 21-carba-23-thiaporphyrin 6
(Figure 3a) and 21-carba-23-thiachlorin 7 (Figure 4a) mea-
sured at 300 K reflect the effective Cs symmetry, and show
similar patterns, with remarkable deshielding of the meso-
hydrogen atoms (6, 9.89 ppm; 7, 9.49 ppm) and shielding of
the internally located H(21) protons (6, À4.79 ppm; 7
À5.16 ppm). The cyclopentadienyl protons H(2) and H(3) of
6 give a doublet at 8.21 ppm. The NH protons are observed as
highly broadened signals at À2.95 ppm for 6 and À3.70 ppm
for 7. The intensity ratio of the NH to H(21) signals is 1:1 for
both 6 and 7, which is consistent with the molecular structures.
The alkyl hydrogen atoms H(2) and H(3) of the cyclopentene
moiety of 7 result in a singlet in the region typical for
dihydroporphyrins (4.91 ppm).[24,26] Significantly, the 13C
chemical shift determined for C(2) and C(3) of 7 (d =
37.1 ppm) reflects their tetrahedral geometry.
21-Carba-23-thiapophyrin 6 and 21-carba-23-thiachlorin 7
tautomerize through a rapid exchange of the NH proton
between two structurally equivalent nitrogen atoms (N(22)
and N(23)). Significant spectral changes have been detected
in variable-temperature 1H NMR studies on 6 and 7 in
[D8]toluene. As the temperature gradually decreases, the
skeleton resonances broaden. Eventually, at low temperature
(180–200 K), the exchange process between the two principal
tautomers is sufficiently slow on the 1H NMR time scale and
double the number of resonances is observed (Figures 3b and
4b for 6 and 7, respectively). The scalar coupling detected
between the inner NH and b protons of one pyrrole moiety
unambiguously confirms the structure of 6 and 7, with the NH
proton specifically localized on a single pyrrole ring.
Figure 3. 1H NMR spectra of a) 6 (300 K, [D]chloroform), b) 6 (190 K,
[D8]toluene), c) 6-Pd (300 K, [D]chloroform), and d) 11 (300 K,
[D2]dichloromethane). Atom assignments correspond to the systematic
position numbering or denote proton groups such as o-, m-, p-
positions of the meso-mesityl groups.
In the presence of TFA, 6 and 7 readily generate green
solutions of the cations, as confirmed by 1H NMR (see
Figures S1 and S2 in the Supporting Information) and UV/Vis
spectroscopy (see Figures S7 and S8 in the Supporting
Information).
The identity of 11 was unambiguously confirmed by high-
resolution mass spectrometry (m/z = 735.1611 for [M+H]+).
1
The H NMR spectrum of 11 shows it has a lower symmetry
compared to 6; however, the resonances are in the range
found for other carbaporphyrins (Figure 3d). Furthermore,
six additional signals from the benzoquinone ring were
identified in the 13C NMR spectrum of 11, two of them (d =
168.6 and 165.7 ppm) were assigned to carbonyl groups by
=
analogy to the chemical shift of the C O group in p-chloranil
(d = 168.2 ppm).
Treatment of 6 and 7 with palladium(II) acetate in toluene
resulted in the formation of palladium(II) carbathiaporphyrin
6-Pd and palladium(II) carbathiachlorin 7-Pd (Scheme 5),
which was reflected by substantial changes in the UV/Vis
electronic absorption spectra (Figure 2) and confirmed by
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Figure 5. DFT-optimized structure of 6-Pd: a) perspective view and
b) side view (mesityl groups at the 10- and 15-positions are omitted
for clarity). Selected bond lengths [ꢀ]: Pd-C(21) 1.96, Pd-N(22) 2.09,
Pd-N(24) 2.09, Pd-S(23) 2.32. The angle between the plane of the
thiophene ring and the plane of four Cmeso atoms is 30.08.
Consequently, the thiophene ring is tilted with respect to
the macrocyclic plane to allow the pyramidal side-on coordi-
nation of the palladium(II) ion, in a similar manner as
detected for the reduced palladium(II) 21-thiaporphyrin[27] or
Figure 4. 1H NMR spectra of a) 7 (300 K, [D2]dichloromethane), b) 7
(190 K, [D8]toluene) and c) 7-Pd (300 K, [D2]dichloromethane). Atom
assignments correspond to the systematic position numbering or
denote proton groups such as o-, m-, p-positions of the meso-mesityl
groups.
1
palladium(II) thiaethyneporphyrin.[28] The H NMR spectra
of complexes 6-Pd and 7-Pd (Figures 3c and 4c, respectively)
reveal similar spectroscopic patterns as those observed for the
free ligands. In the 13C NMR spectra, the inner C(21) atoms
give resonances (6-Pd, 138.6 ppm; 7-Pd, 146.8 ppm) that are
significantly shifted downfield compared to those of the
monocations (6-H+, 113.6 ppm; 7-H+, 129.9 ppm), which is
consistent with engagement of these carbon atoms in the
coordination of palladium(II) and formation of organome-
tallic complexes.
Several isomers of dihydro-10,15-dimesityl-21-carba-23-
thiaporphyrin, including these which preserve the cyclo-
pentadiene moiety, were subjected to DFT studies (see
Scheme S5 and Table S3 in the Supporting Information).
The isomers important for discussion alongside the exper-
imental data are shown in Figure 6 and are ordered according
to their total calculated energies (B3LYP/6-31G**). Evi-
dently, the unique aromatic isomer 7 has the lowest energy,
which accounts for the preference of 7 in solution and in the
solid state. Nucleus-independent chemical shift (NICS) values
of the central 16-membered ring (center ring) (Figure 6) were
Scheme 5. Palladium(II) complexes of carbathiaporphyrin (6-Pd) and
carbathiachlorin (7-Pd).
high-resolution mass spectrometry (6-Pd, m/z = 666.1382 for
[M]+; 7-Pd, m/z = 668.1477 for [M]+). Carbathiaporphyrin 6
and carbathiachlorin 7 distort to accommodate the palladiu-
m(II) cation, as shown in the DFT model of 6-Pd and 7-Pd
(Figure 5 and see Figure S11 in the Supporting Information,
respectively).
Figure 6. Geometries, calculated energies (in kcalmolÀ1), and NICS values (in ppm) of chosen dihydro-10,15-dimesityl-21-carba-23-thiaporphyrin
isomers obtained by DFT optimization.
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´
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9130.
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In conclusion we have presented a rational route to the
synthesis of free-base 21-carba-23-thiaporphyrin, based on
carba analogues of tripyrrane as synthons, which allow the
incorporation of an unsubstituted cyclopentadiene moiety
into the heteroporphyrin-like structure. This molecule and its
chlorine-like derivative act as aromatic macrocyclic ligands,
as shown by its reaction with palladium(II) ions, thus opening
up alternatives to explore the coordination chemistry of true
carbaporphyrins and true carbaheteroporphyrins. One can
expect the true carbaporphyrins to prompt developments in
carbaporphyrinoid and organometallic chemistry, building on
the specific reactivity of the cyclopentadienyl moiety.
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Received: December 19, 2013
Published online: March 3, 2014
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[20] L. Latos-Graz˙ynski, J. Lisowski, L. Szterenberg, M. M. Olm-
Keywords: carbaporphyrinoids · chlorin · NMR spectroscopy ·
palladium · porphyrinoids
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cam.ac.uk/data_request/cif.
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