Pyriporphyrin - A porphyrin homologue containing a built-in pyridine moiety

Article abstract of DOI:10.1002/ejoc.200600364

Pyriporphyrin 1 (6,11,16,21-tetraaryl-22-aza-m-benziporphyrin), the simplest homologue of 5,10,15,20-tetraarylporphyrin can be constructed by replacement of one of the pyrrole rings of 5,10,15,20-tetraarylporphyrin with a pyridine moiety, linked to the ma

Full text of DOI:10.1002/ejoc.200600364

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200600364
Pyriporphyrin – A Porphyrin Homologue Containing A Built-in Pyridine
Moiety
Radomir Mys´liborski,[a] Lechosław Latos-Grazyn´ski,*[a] and Ludmiła Szterenberg[a]
˙
Keywords: Core-modified porphyrin / (Porphyrin)iron / Porphyrinoid / Pyriporphyrin
Pyriporphyrin 1 (6,11,16,21-tetraaryl-22-aza-m-benziporphy- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
rin), the simplest homologue of 5,10,15,20-tetraarylporphyrin
can be constructed by replacement of one of the pyrrole rings
Germany, 2006)
of 5,10,15,20-tetraarylporphyrin with
a pyridine moiety,
linked to the macrocycle through α,αЈ-carbon atoms.
In spite of the extensive effort aimed to control porphyrin
properties involving core modifications,[1] a synthesis of
true pyriporphyrin 1 has remained a continuing challenge.
Recently reported carbaporphyrinoid 6,11,16,21-tetraaryl-
3-aza-m-benziporphyrin (N-confused pyriporphyrin) 2 was
the very first pyriporphyrin containing an unperturbed pyr-
idine ring, though in the N-confused macrocyclic frame.[2]
Previously, the significant effort was devoted to synthesize
porphyrin-related macrocyles containing pyridine instead of
pyrrole yielding, eventually, phlorin- or chlorin-like deriva-
tives.[3] Correspondingly, the incorporation of 3-hydroxy-
pyridine into the porphyrinic framework led to 2-oxypyri-
porphyrin containing a pyridone moiety.[4] Contrary to reg-
ular pyriporphyrin 1 the contracted {subpyriporphyrin – a
[14]triphyrin(1.1.1) homologue with an embedded pyridine
moiety}[5] and expanded [12-hydroxypyrisapphyrin,[6a] di-
pyrihexaphyrin(1.0.0.1.0.0),[6b,6c] torand – tetrapyriocta-
phyrin(0.0.0.0.0.0.0.0)[6d]] counterparts containing pyridine
were already reported. The relevant case of a phthalo-
cyanine modification can be exemplified by hemiporphyraz-
ines, non-aromatic cross-conjugated macrocycles, contain-
ing two pyridine and two pyrrole units bound through aza
bridges.[7] At a porphyrinogen oxidation state the homolo-
gation of pyrrole to pyridine within the meso-octaethylcalix-
[4](pyridine)n(pyrrole)4–n skeleton was achieved.[8]
Scheme 1. Pyriporphyrin 1 and N-confused pyriporphyrin 2.
The synthetic strategy (Scheme 2) applies some features
used for the synthesis of 3-azabenziporphyrin[2] and sub-
pyriporphyrin.[5] In particular, introduction of bulky sub-
stituents at the ortho positions of the meso-aryl groups at 4,
adjacent to the incorporated pyridine ring, increased the
stability of 2,6-bis[(2-pyrrolyl)(mesityl)methyl]pyridine to-
ward acidolysis during the condensation which eliminates a
scrambling effect. Dibenzoylation of 4 yielded 5. Sub-
sequently, 5 was reduced to the dialcohol derivative, which
was used in the [3+1] condensation. After oxidation and
chromatographic workup, the green pyriporphyrin 1 was
obtained in 5.5% yield.
The electronic spectrum of 1 (Figure 1) resembles those
of non-aromatic benziporphyrin[9] or 3-azabenzipor-
1
phyrin.[2] The H NMR spectrum of 1 presents the reso-
Here we report on the synthesis and characterization of
pyriporphyrin 1 (Scheme 1). The molecule reveals one of
the simplest imaginable frames for a modified porphyrin,
which involves one pyrrole ring expansion to give a pyridine
ring embedded into the tetraarylporphyrin structure.
nances at positions consistent with its non-aromatic struc-
ture (Figure 2, trace B).
Thus, the macrocyclic aromatic ring current is practically
absent for 1, as clearly illustrated by positions of pyrrole
and pyridine resonances, assigned by 2D experiments {[D2]-
dichloromethane, 203 K: δ = 7.56 (3-H), 7.28 (2/4-H), 7.04
(13/14-H), 7.00 (8/19-H), 6.62 (9/18-H) ppm}. Scalar coup-
ling, detected between pyrrole hydrogen atoms N-24-H
([D2]dichloromethane, 203 K: δ = 9.22 ppm) and 13/14-H,
is consistent with the prevalence of the symmetrical tauto-
mer 1 (N-22,N-23,N-24-H,N-25) (Scheme 3).
[a] Department of Chemistry, University of Wrocław,
14 F. Joliot-Curie St., Wrocław 50-383, Poland
Fax: +48-713-282-348
E-mail: llg@wchuwr.chem.uni.wroc.pl
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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Pyriporphyrin – A Porphyrin Homologue Containing A Built-in Pyridine Moiety
SHORT COMMUNICATION
Scheme 2. Synthesis of pyriporphyrin 1: a) MsCl/TEA, b) pyrrole,
c) EtMgBr, d) PhCOCl, e) NaBH4, f) TFA/MeCN, g) TEA, DDQ.
1
Figure 2. H NMR spectra (downfield regions presented) of 6 (A),
1 (B), 1-H (C), and 1-H2 (D). Assignments follow the numbering
given in Scheme 2. The remarkable relocation of pyridine reso-
nances of 1 due to protonation (1-H) or complexation (6) marked
with curved arrows.
Figure 1. Electronic spectra: trace A: 1 – solid black line, 1-H –
black dashed line, 1-H2 – black dotted line; trace B: spectra of 6 –
solid line, 7-Cl2 – dashed line.
(2/4-H), 8.20 (3-H) ppm; pyrrole: δ = 7.68, 7.59, 7.55; 6.10
(2×NH)] reflects the macrocyclic aromaticity related to 1-
HЈ-A (Figure 2, trace C). The further titration produces the
different tautomeric species of 1-H2. The introduction of
Two stages of protonation of 1 have been demonstrated the second proton causes a severe distortion of the macro-
by 1H NMR and electronic spectroscopy (Figure 2, cycle from planarity as demonstrated by differentiation of
Scheme 3). The addition of the first proton resulted in for- originally equivalent mesityl resonances in the 1H NMR
mation of two symmetrical trans tautomers (N-22-H,N- spectrum of 1-H2.
23,N-24-H,N-25) 1-HЈ and (N-22,N-23-H,N-24,N-25-H) 1-
The density functional theory (DFT) has been applied
HЈЈ, which remain in fast exchange even at 203 K ([D2]- to model the molecular and electronic structure of three
dichloromethane). To account for spectroscopic character- pyriporphyrin tautomers: 1 (N-22,N-23,N-24-H,N-25), 1Ј
istics of 1-HЈ, it is necessary to include the canonical form (N-22,N-23-H,N-24,N-25), 1ЈЈ (N-22-H,N-23,N-24,N-25).
1-HЈ-A in the description of 1-H (Scheme 3). It defines a The total energies, calculated using the B3LYP/6-31G** ap-
single 18e-macrocyclic π-delocalization pathway (1-HЈ-A), proach [0 (1), 2.5 (1Ј), 17.9 (1ЈЈ) kcal/mol] demonstrate
which may coexist with the [6]annulene aromaticity of the some preference for the central pyrrole protonation. The
pyridine ring (1-HЈ-B). Thus, the marked upfield relocation total energies calculated for (N-22-H,N-23,N-24-H,N-25) 1-
of 1-H macrocyclic resonances with respect to 1 [δ = 8.31 HЈ and (N-22,N-23-H,N-24,N-25-H) 1-HЈЈ reveal an energy
Eur. J. Org. Chem. 2006, 3064–3068
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3065

R. Mys´liborski, L. Latos-Grazyn´ski, L. Szterenberg
˙
SHORT COMMUNICATION
Figure 3. Molecular structure of 6 (top: perspective view, bottom:
side view with phenyl groups omitted for clarity). The thermal ellip-
soids represent 30% probability.[10]
Insertion of iron into 1 has been achieved by the addition
of iron(II) chloride hydrate to a chloroform/acetonitrile
solution of the ligand in the presence of air. The reaction
yields a six-coordinate complex 7-Cl2 which is stable as a
solid and in solution (Scheme 4). The UV/Vis spectrum of
7 markedly differs from that of 6 (Figure 1).
Scheme 3. The first protonation step of 1.
difference of 5.4 kcal/mol pointing to the larger stability of
1-HЈЈ. Contrary to 1 the protonation of the pyridine nitro-
gen atom seems to be acceptable for the monocationic form.
Compound 1 reacts smoothly with ZnCl2 in dichloro-
methane to yield chloro(6,21-dimesityl-11,16-diphenylpyri-
porphyrin)zinc(II) (6), wherein the macrocycle acts as a
monoanionic ligand. The coordination is confirmed by the
Scheme 4. (Pyriporphyrin)iron(III) complex 7-Cl2.
1
substantial changes detected in the UV/Vis and H NMR
spectra (Figures 1 and 2). The molecular structure of 6 has
1H NMR spectroscopy was shown to be a definitive
been determined by X-ray investigations (Figure 3). The co- method for detecting and characterizing (porphyrin)iron
ordinating environment of the zinc(II) ion resembles a dis- complexes[14] at different coordination/oxidation states.
1
torted square pyramid with the nitrogen atoms occupying Here, the H NMR spectroscopic data for 7-X2 (X = Cl,
equatorial positions and the chloro ligand lying at the CN) have been analyzed. Compound 7-X2 appears to have
unique apex. The zinc(II) ion is displaced by 0.512 Å from effective C2v geometry in solution, consistent with six-coor-
the N4 plane (Figure 3), resembling the structure of a chlo- dinate iron(III) with one of the mirror planes passing
ro(N-methylated porphyrin)zinc(II) complex.[11] The unique through the iron ion, the chloro ligand, and the pyridine
Zn–N(22) distance equals 2.353(3) Å and is significantly nitrogen atom. The three pyrrole resonances of 7-Cl2 are in
longer than the other three Zn–N distances [Zn–N(23) the δ = 90–60 ppm region (298 K) which is typical for high-
2.053(4), Zn–N(24) 2.108(4), Zn–N25, 2.049(4) Å]. Actu- spin (tetraarylporphyrin)iron(III) complexes (Figure 4,
ally, the Zn–N(22) bond is longer than any of the Zn–N trace A).[14a,14c,14d] The single resonance detected for the
bonds determined for axially coordinated pyridine in (por- meta positions is diagnostic of six-coordination.[14c] Ad-
phyrin)zinc(II) complexes which are typically found in the dition of potassium cyanide in [D4]MeOH to a [D8]toluene
2.1–2.2 Å range[12] and the Zn–N bond lengths of the equa- solution of 7-Cl2 resulted in its conversion to the neutral
torial pyridine rings of (tetrahydroxyhemiporphyrazine)- six-coordinate complex 7-(CN)2 (Figure 4, trace B), which
zinc(II) [2.265(1) and 2.276(1) Å].[13] All Zn–N(pyrrolic) presents the patterns of pyrrole and meso-aryl resonances
bond lengths are comparable to the Zn–N distances in (por- assigned to the (dxzdyz)4(dxy)1 less-common low-spin elec-
phyrin)zinc(II) complexes.[12]
tronic ground state.[14a,14e]
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Pyriporphyrin – A Porphyrin Homologue Containing A Built-in Pyridine Moiety
SHORT COMMUNICATION
3
6.96 (s, 4 H, m-Mes), 6.94 (s, 2 H, 13/14-H), 6.66 [AB, J(H,H) =
4.77 Hz, 2 H, 9/18-H], 2.37 (s, 6 H, 4-Me), 2.05 (s, 12 H, 2/6-
Me) ppm. 13C NMR (CDCl3, 298 K): δ = 171.6, 157.8, 155.5,
148.9, 139.8, 139.5, 138.8, 138.5, 137.9, 137.6, 134.9, 131.6, 130.3,
128.3, 127.9, 127.5, 124.8, 115.3, 21.3, 21.2 ppm. UV/Vis (CH2Cl2):
λmax (logε) = 324 (4.46), 411 (4.61), 685 (4.00) nm. HRMS (ESI):
calcd. for [C51H42N4 +H]+ 711.34822; found 711.35166.
6: 1H NMR (CDCl3, 213 K): δ = 8.61, 8.34 [AB2, 3J(H,H) =
7.88 Hz, 2 + 1 H, 2/4- + 3-H], 7.85, 7.61, 7.56–7.52 (AAЈM2X, 10
3
H, Ph), 7.47, 7.40 [AB, J(H,H) = 5.05 Hz, 2 + 2 H, 8/19- + 9/18-
H], 7.12/6.98 [s, 2 + 2 H, m-Mes], 2.44 (s, 6 H, 4-Me), 2.20/1.49 (s,
6 + 6 H, 2/6-Me) ppm. 13C NMR (CDCl3, 213 K): δ = 165.7, 161.5,
154.0, 150.0, 139.8, 138.9, 138.8, 138.3, 138.2, 137.4, 135.9, 135.3,
133.7, 133.4, 131.2, 129.9, 129.1, 128.3, 127.8, 127.7, 127.6, 127.4,
118.6, 22.0, 21.5, 20.9 ppm. UV/Vis (CH2Cl2): λmax (logε) = 278
(4.30), 335 (4.46), 409 (4.57), 460 (4.41), 492 (4.28), 545 (3,42), 587
(3.52), 638 (3.09), 848 (4.27) nm. HRMS (ESI): calcd. for
[C51H41N4Zn]+ 773.2623; found 773.2932.
7-Cl2: A solution of 1 and FeIICl2·6H2O (excess) in dichlorometh-
ane/acetonitrile was stirred in the presence of air. After 15 min, the
insertion was complete as determined by electron spectroscopy. The
mixture was concentrated to dryness, the residue dissolved in
dichloromethane and the mixture filtered. After crystallization
from dichloromethane/n-hexane, 7 was obtained.
1
Figure 4. H NMR spectra (298 K, [D8]toluene) of (A) 7-Cl2, (B)
7-CN2. The pyrrole and meta resonances of 7-Cl2 are labeled pyrr
and m, respectively. Other assignments follow the systematic label-
ing.
7-Cl2: UV/Vis (CH2Cl2): λmax (logε) = 321 (4.28), 372 (4.26), 673
(3.52), 877 (3.31) nm. MS (ESI): calcd. for [C51H41N4FeCl]+ 801.2;
found 801.0.
In conclusion, the long awaited pyriporphyrin, the sim-
plest homologue of porphyrin with a supplementary carbon
atom located in a pyrrolic β–β bond, has been synthesized.
Supporting Information (see footnote on the first page of this arti-
In light of the extensive search for suitable porphyrins and cle): Synthetic protocols and computational details.
metalloporphyrins to act as biomimetic models and cata-
lysts, pyriporphyrin 1 provides a remarkable potential. Pyri-
porphyrin creates nearly a porphyrin-like coordinating envi-
Acknowledgments
ronment easily matching the ionic radii of a large variety of
metal ions with a feasible coordination to four trigonally
formation Technology (Grant 3 T09A 162 28) is kindly acknowl-
hybridized nitrogen atoms albeit – importantly – in the
Financial support from the Ministry of Scientific Research and In-
edged. Quantum mechanical calculations were preformed at the
Supercomputer Centers in Wrocław and Poznan´.
monoanionic core instead of a dianionic one.
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(0.3 mmol) and pyrrole (1 equiv.), catalyzed by TFA (28 equiv.) in
CD3CN, was performed under nitrogen overnight. After alkaliza-
tion (TEA) and oxidation (DDQ, 5 equiv.), the solvent was re-
moved and the oily residue was dissolved in dichloromethane and
chromatographed (grade-II basic alumina, 100 g). n-Heptane
(50 mL) was added and the solvent was evaporated. Compound
1 was extracted from the solid using n-hexane. The solvent was
evaporated and the residue was dissolved in dichloromethane. Sub-
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mixture was stirred vigorously (30 min), washed with water, dried
and filtered. The product was chromatographed (silica gel). The
yellow fraction (eluted with 1–2% acetone in dichloromethane) was
collected. Recrystallization (dichloromethane/n-hexane) yielded
13.3 mg of 6 (5.5%). Next, 6 was quantitatively demetallated to 1
by using concentrated HCl. The procedure involved washing of the
dichloromethane solution of 6 with concentrated HCl (3×), water
(2×), diluted K2CO3 and finally drying with Na2SO4.
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1: H NMR (CDCl3, 298 K): δ = 9.38 (v. br. s, 1 H, 24-H), 7.54,
7.33 [AB2, 3J(H,H) = 7.79 Hz, 1- + 2-H, 3- + 2/4-H], 7.51–7.39
(A2M2X, 10 H, Ph), 7.03 [AB, 3J(H,H) = 4.77 Hz, 2 H, 8/19-H],
Eur. J. Org. Chem. 2006, 3064–3068
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3067

R. Mys´liborski, L. Latos-Grazyn´ski, L. Szterenberg
˙
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1022.
C51H41ClN4Zn·CH2Cl2, size 0.02×0.15×0.15 mm, triclinic, [14] a) F. A. Walker, in: The Porphyrin Handbook, vol. 5 (Eds.:
¯
space group P1, a = 11.161(5), b = 12.762(6), c = 17.186(9) Å,
K. M. Kadish, K. M. Smith, R. Guilard), Academic Press, San
α = 73.12(4), β = 77.94(4), γ = 69.41(4)°, V = 2177.2(2) Å3, Z
= 2, total no. of reflections collected: 20939; no. of independent
reflections: 8220 [of which 4556 Ͼ 2σ(I)] were included in the
refinement of 558 parameters; an absorption correction was
not applied. The structure was solved by using direct methods
with SHELXS-97 and refined against |F2| using SHELXL-97
(G. M. Sheldrick, University of Göttingen, Germany, 1997),
final R1/wR2 indices [for I Ͼ 2σ(I)]: 0.078/0.189; max./min.
residual electron density: +1.02/–1.00 e·Å–3; H atoms were
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Received: April 26, 2006
Published Online: May 18, 2006
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Eur. J. Org. Chem. 2006, 3064–3068