Subpyriporphyrin - A [14]triphyrin(1.1.1) homologue with an embedded pyridine moiety

Article abstract of DOI:10.1002/anie.200600589

Short and to the point: A three-centered hydrogen bond is present in 6,16-dimesityl-11-phenylsubpyriporphyrin, a non-aromatic free-base triphyrin(1.1.1)-related molecule. X-ray crystal structural analysis shows this compound contains the shortest known N.

Full text of DOI:10.1002/anie.200600589

Communications
Porphyrinoids
DOI: 10.1002/anie.200600589
Subpyriporphyrin—A [14]Triphyrin(1.1.1)
Homologue with an Embedded Pyridine
Moiety**
´
´
Radomir Mysliborski, Lechosław Latos-Graz˙ynski,*
Ludmiła Szterenberg, and Tadeusz Lis
Triphyrins are porphyrin analogues which contain only three
pyrrole or pyrrole-related rings linked through meso-sp2
carbon atoms, as found in [18]triphyrin(6.1.1) (vacatapor-
phyrin),
[15]triphyrin(1.1.3),
dithiaethyneporphyrin,
[18]triphyrin(2.2.2), and [18]heterotriphyrin(2.2.2).[1]
Subphthalocyanine,[2] subporphyrazine,[3] and tribenzo-
subporphine,[4] which have been isolated and characterized
only as their boron complexes (2–4, respectively), can be
directly related to the hypothetical [14]triphyrin(1.1.1) (1;
Scheme 1).Extensive studies on the 14 p-electron system of
Scheme 1. [14]Triphyrin(1.1.1) (1) and the boron complexes of sub-
phthalocyanine 2, subporphyrazine 3, and tribenzosubporphine 4.
boron subphthalocyanines revealed interesting properties for
application in the areas of dyes, nonlinear optics, and photonic
devices.[2b] Thus, it remains a challenge to produce contracted
porphyrins that display triphyrin(1.1.1)-like molecular frame-
´
´
[*] R. Mysliborski, Prof. Dr. L. Latos-Graz˙ynski, Dr. L. Szterenberg,
Prof. Dr. T. Lis
Department of Chemistry
University of Wrocław
14 F. Joliot-Curie St., 50-383 Wrocław (Poland)
Fax: (+48)71-328-2348
E-mail: llg@wchuwr.chem.uni.wroc.pl
[**] Financial support from the Ministry of Scientific Research and
Information Technology (Grant 3T09A16228) is gratefully
acknowledged. Quantum mechanical calculations were preformed
´
at the Supercomputer Centers in Wrocław and Poznan.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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works that are potentially amenable to diverse structural
modifications and applications.[5]
Here we report on the synthesis and characterization of
subpyriporphyrin 5 (Scheme 2).Subpyriporphyrin ((Sub-
PyP)H) is a novel type of contracted porphyrin—a homo-
logue of [14]triphyrin(1.1.1) (1) with a pyridine moiety
Scheme 2. Subpyriporphyrin 5 with the systematicnumbering scheme.
Figure 1. Molecular structure of 5 (30% thermal ellipsoids) at 80 K.
The solvent molecule and hydrogen atoms (except for the directly
detected NH group) are omitted for clarity (perspective view; top and
right: side views, phenyl groups are omitted for clarity). The dihedral
angles between the N3 plane and the planes of the rings are: pyridine
12.58; pyrrole-N(18) 20.78; pyrrole-N(19) 25.58. Selected distances:
N(17)···N(18) 2.519(2) Š; N(17)···N(19) 2.534(2) Š; N(18)···N(19)
2.370(2) Š.[7]
embedded in the macrocyclic framework.It introduces the
unique structural pattern of a free-base triphyrin, as distin-
guished by the location of three nitrogen atoms at the corners
of an approximately equilateral triangle, and thus clearly
approaches the limit of porphyrin contraction defined by 1.
The coordinating properties of 5 are exemplified by the
formation of an organoboron complex.
The synthetic strategy (Scheme 3) incorporates some
features used for the synthesis of 3-azabenziporphyrin.[6] In
particular, the introduction of bulky substituents at the ortho
dipyrrane subunit are conjugated and have similar length
patterns in both pyrrolic subunits, while the bond lengths in
the six-membered ring are pyridine-like.The determined
N(18)···N(19) distance of 2.370(2) Š is the shortest N···HN
hydrogen bond reported.[8] The N(17)···N(18) distance
2.534(2) Š also lies within the range for a very strong
hydrogen-bonding interaction,[9] and thus the NH hydrogen
atom of 5 is involved in three-centered (N(17), N(18), N(19))
hydrogen bonding.
Density functional theory (DFT) has been applied to
model the molecular and electronic structure of two feasible
tautomers 5 and 5’ of subpyriporphyrin (Scheme 4), and
Scheme 3. Synthesis of subpyriporphyrin 5: a) MsCl/TEA, b) pyrrole,
c) EtMgBr, d) PhCOCl, e) NaBH4, f) BF3 or TFA/MeCN, g) TEA, DDQ
(5 equiv). Ms=methanesulfonyl, TEA=triethylamine, TFA=trifluoro-
acetic acid, DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
Scheme 4. Tautomers 5 and 5’ of subpyriporphyrin where p-delocaliza-
tion pathways (6e and 14e, respectively) are marked with thick lines.
positions of the meso-aryl groups in 6, adjacent to the
incorporated pyridine ring, increased the stability of 7 toward
acidolysis during the condensation, thus eliminating a scram-
bling effect.Monobenzoylation of 7 yielded 8.Subsequent
reduction of 8 gave the alcohol derivative which undergoes an
intramolecular condensation to produce subpyriporphyrin 5.
Compound 5 was obtained as a green solid in 6% yield after
oxidation and chromatographic workup.
shows the preference for the protonation of a single pyrrole
nitrogen atom over that of the pyridine ring (4.66 kcalmolꢀ1).
Significantly, the calculated N···N distances (N(17)···N(18)
2.518 Š; N(17)···N(19) 2.585 Š, N(18)···N(19) 2.397 Š)
closely resemble those determined by the X-ray data.
Application of the the theory of atoms in molecules
(AIM)[10] allows chemically relevant information to be
derived from the topology of the electron density in 5.
Figure 2 shows the bonding critical points (BCPs) and ring
X-ray crystal structure analysis showed that macrocycle 5
ꢀ
is slightly ruffled (Figure 1).The C C bonds within the
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Figure 2. Molecular graph of the central part of 5 illustrating the
existence of hydrogen bonding from AIM formalism. A) Position of
bond critical points (BCPs) and ring critical points (RCPs). B) Electron-
density distribution in 5. The black solid curve shows bond paths of
the hydrogen bond.
critical points (RCPs) in the central part of the structure of 5.
Most importantly, the bond critical points indicate a hydro-
gen-bonding pathway between H(18) and the N(17), N(18),
and N(19) nitrogen atoms.The values of the parameters
determined for the N(17)H, N(18)H, and N(19)H bond
critical points (see the Supporting Information) agree with
the presence of three-centred hydrogen bonds.[10b,c]
The electronic spectrum of 5 (Figure 3) shows two bands
at 346 and 730 nm (very broad) that resemble the spectro-
scopic features of non-aromatic benziporphyrin[11] and 3-
azabenziporphyrin.[6] The 1H NMR spectrum of 5 shows
Figure 4. 1H NMR spectra (downfield regions) of: A) 5 (203 K, CD2Cl2),
B) 9 (298 K, CDCl3), C) 10-s and 10-a (298 K, CD2Cl2). Resonance
assignments (obtained from COSY and NOESY maps) follow the
numbering given in Scheme 2. The resonances of 10-a and their scalar
connectivity marked by arrows are linked by dotted lines. o-Ph’, m-Ph’,
and p-Ph’ denote resonances of the s-phenyl ring. The domed
structure of 9 is reflected by the substantial differentiation of the m-
mesityl and o-CH3 resonances (B, inset). The diasterotopic splitting of
the CH2 resonances of 2-ethoxide is shown (C, inset). S=solvent.
phyrin,[13] subpyriporphyrin 5 tautomerizes extremely fast
between two equivalent tautomers even at 183 K: (N(17),
N(18)H, N(1)) and (N(17), N(18), N(19)H).The rapid
exchange of the inner proton involves two pyrrole nitrogen
atoms but excludes the pyridine one.The extreme downfield
shift of the N(18)H resonance (d = 18.07 ppm; CD2Cl2, 203 K)
results from the strong hydrogen-bonding interaction.
Two stages of protonation in [D2]dichloromethane have
been demonstrated.The addition of the first proton resulted
solely
in
the
formation
of
the
symmetrical
Figure 3. The electronic spectra of 5 (solid black line), 5-H2 (black
dotted line), 9 (solid gray line), and 10 (dashed gray line).
(N(17),N(18)H,N(19)H) tautomer 5-H as confirmed by the
(N)H–b-H scalar coupling.The significant upfield shift of the
NH resonance (d = 12 ppm) readily correlates with the
profound structural changes accompanying the breaking of
the internal hydrogen bond.[8d] The DFT calculations dem-
onstrated the significant folding of 5-H, as reflected by the
significant increase in the optimized N(18)···N(19) distance
(2.611 Š) by the tilting of the pyrrole rings in opposite
directions (see the Supporting Information).Further titration
results in protonation of the pyridine nitrogen atom to form 5-
H2.The 5-H2 species remains in fast exchange both with 5-H
and the species formed by coordination of TFA by 5-H2 and
resonances at chemical shifts that are consistent with a non-
aromatic structure (Figure 4A).The macrocyclic aromatic
ring current is practically absent for 5, as clearly illustrated by
the positions of the pyrrole and pyridine resonances, assigned
by 2D experiments (d ppm: H(8/13) 7.07, H(9/12) 7.35, H(2/4)
7.70, H(3) 8.00; CD2Cl2, 203 K).A single NH resonance
shows simultaneous scalar coupling to four pyrrolic hydrogen
[12]
atoms.Thus, similar to porphycene
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accounts for the smooth changes in the chemical shifts during
the course of the titration.
Compound 5 reacts with PhBCl2 to yield green s-phenyl
boron(iii) subpyriporphyrin [(SubPyP)BPh]+ 9, wherein the
macrocycle acts as a monoanionic tridentate ligand.The
electronic spectrum of 9 is not very different from that of 5,
except for the presence of two Q-like bands (ca.500 nm).
The coordination of the boron(iii) ion results in the
1
pyrrole and pyridine H NMR resonances of 9 being mark-
edly downfield-shifted with respect to those of 5 (Dd ppm:
H(3) 1.55, H(2/4) 1.35, pyrr 0.72, pyrr, 0.70), thus revealing a
significant contribution by the ring current effect.The most
notable feature is the marked upfield shift of the axially
coordinated s-phenyl group (Ho 4.87, Hm 6.52, Hp 6.07),
although less pronounced than that for typical diamagnetic
metalloporphyrins with s-phenyl groups,[14,15] but stronger
than
that
for
s-phenylboron
subphthalocyanine
((SubPh)BPh).[3,15] This observation confirms the aromatiza-
tion arising from coordination and reflects the structural
changes that allow delocalization of 14 p electrons as shown
in 9’ (Scheme 5).
Figure 5. The DFT optimized structure of 9 (top: perspective view;
bottom: side view).
Addition of acid to 10 recovers 9.The UV/Vis spectrum
confirms the aromaticity of 10, as evident by the large
extinction coefficients (Figure 3).
In conclusion we have presented a rational route for the
synthesis of 6,16-dimesityl-11-phenylsubpyriporphyrin—the
first free-base triphyrin(1.1.1)-related molecule—which
revealed the shortest N···N distance between two nitrogen
atoms involved in hydrogen bonding.The molecule acts as a
ligand, as shown in its reaction with boron, thus opening up
options to explore the coordination chemistry of subpyripor-
phyrin.The new system can potentially be linked into
multielement porphyrin arrays by using the perimeter adjust-
ments developed for tetraarylporphyrins, thus introducing a
trigonal structural motif instead of a tetragonal one.
Scheme 5. Transformations of boron subpyriporphyrin: a) CH3CH2Oꢀ/
CH3CH2OH, b) H+.
Experimental Section
As demonstrated by DFT calculations, the coordination
results in remarkable changes in the geometry that allows
formation of a domed structure with the boron atom
displaced from the N3 plane by 0.581 Š (Figure 5).
The perimeter reactivity of 9 is essential when considering
the perspective functionalization of subpyriporphyrins
(Scheme 5).Thus addition of ethoxide to 9 afforded two
aromatic steroisomers 10-a and 10-s as readily seen by two
sets of resonances in the 1H NMR spectrum.
As shown by a NOESY experiment and integration, a syn
addition with respect to the s-aryl group is unexpectedly
preferred (1:4.5 molar ratio). The complex multiplets (10-s:
d = 3.31 and 2.94 ppm; 10-a: d = 2.85 and 2.76 ppm) of the
ethoxymethylene group reflects the diastereotopic effect as
the stereocenter is created at the tetrahedrally hybridized
C(2) atom (13C NMR shift of 10-s C(2) d = 67.6 ppm).
5: NaBH4 (155 mg, 40 equiv) was added in several portions to a
solution of 8 (57.5 mg, 0.1 mmol) in THF/EtOH under N2.The
mixture was stirred for 3 h, and then CH2Cl2 (10 mL) was added.The
organic layer was washed 3 times with dilute NaOH and water, dried
with Na2SO4, evaporated, and dried in a vacuum for 1 h.The residue
was dissolved in dry CH3CN (200 mL) under N2, and then TFA
(200 mL, 2.6 mmol) or Et2O·BF3 (25.5 mL, 0.2 mmol) added. The
solution was then stirred for 4 h in the absence of light.The
condensation was quenched by addition of excess TEA.DDQ
(113.5 mg, 5 equiv) was then added and the reaction mixture was
stirred for 1 h before the solvent was evaporated to dryness.The
residue was dissolved in CH2Cl2 and purified by chromatography on
grade II basic alumina.The first green fraction, containing 5, was
eluted with CH2Cl2.Recrystallization from CH 2Cl2/CH3OH produced
3.3 mg of 5 (yield 6%). 1H NMR (500 MHz, CDCl3, 213 K): d = 17.82
3
(s, 1H, H(18)), 8.00, 7.76 (AB2, (1 + 2)H, JH,H = 7.95 Hz, H(3), H(2/
3
4)), 7.73, 7.53, 7.39 (A2M2X, (2+2+1)H, Ph), 7.41 (AB, 2 H, JH,H
=
4.62 Hz, H(9/13)), 7.18 (AB, 2H, 3JH,H = 4.62 Hz, H(8/14)), 7.05 (brs,
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4H, m-Mes), 2.41 (s, 6H, 4-Me), 1.99 ppm (s, 12H, 2/6-Me); 13C NMR
in idealized positions using the riding model constraints.The
CH2Cl2 was disordered into two positions.CCDC-297978 and
-297979 contain the supplementary crystallographic data for this
paper.These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
(125.7 MHz, CDCl3, 213 K): d = 157.9, 149.8, 149.3, 139.3, 137.9,
137.7, 135.3, 133.7, 131.2, 130.7, 130.0, 128.7, 127.9, 126.1, 125.8, 123.2,
111.7, 108.2, 22.0, 21.5 ppm; UV/Vis (CH2Cl2) lmax (loge) = 346 (4.40),
730 nm (very broad, 3.71). HRMS (ESI): m/z: 558.29054 (m/z calcd
for [C40H35N3+H]+: 558.29038).
9: Phenylboron dichloride (8 mL, 10 equiv) was added to a
solution of 5 (3.3 mg, 0.006 mmol) in dry toluene (under N2).The
mixture was stirred for 1.5 h. Excess AgBF4 was added and the
solvent was removed under vacuum.Green 9 was dissolved in CH2Cl2,
filtrated, and purified by recrystallization from CH2Cl2/n-hexane.
Yield 4.2 mg (95%). 1H NMR (500 MHz, CDCl3, 298 K): d = 9.33,
9.08 (AB2, (2+1)H, 3JH,H = 8.08 Hz, H(3) + H(2/4)), 8.07, 7.85 (AB,
[8] a) E.Vogel, M.Kocher, H.Schmickler, J.Lex,
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Ogawa, J. Am. Chem. Soc. 2000, 122, 5748; e) I.Saltsman, I.
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(2+2)H, JH,H = 5.01 Hz, H(9/13), H(8/14)), 7.92, 7.74, 7.67 (A2M2X,
´
(2+2+1)H, Ph), 7.16 (s, 2H, 5-Mes), 7.13 (s, 2H, 3-Mes), 6.07, 6.52,
4.87 (AM2X2, (1+2+2)H, Ph’), 2.47 (s, 6H, 4-Me), 1.79 (s, 6H, 2-Me),
1.69 ppm (s, 6H, 6-Me); 13C NMR (125.7 MHz, CDCl3, 298 K): d =
150.4, 146.0, 140.6, 140.3, 139.6, 138.8, 133.4, 132.6, 132.0, 131.3, 129.9,
129.8, 129.6, 129.5, 129.4, 129.0, 128.7, 128.0, 127.4, 117.2, 21.7, 21.5,
21.0 ppm. UV/Vis lmax (loge) = 358 (4.46), 485 (3.54), 523 (3.43),
687 nm (3.85). HRMS (ESI) m/z: 644.32356 (m/z calcd for
[C46H39N3B]+: 644.32316).
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was previously detected in the case of oxoindole (2.448 Š),[8c] N-
fused porphyrin (2.459 Š),[8d] isocorrole (2.544 Š),[8b] and por-
phycene (2.63 Š)[8a] as reflected by the N···N distances given in
parentheses.
[10] a) R.F.W. Bader, Atoms in Molecules—a Quantum Theory,
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Molecules, An Introduction, Pearson Education, Harlow, 2000;
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9925.
Received: February 14, 2006
Published online: April 26, 2006
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[12] U.Langer, C.Hoelger, B.Wehrle, L.Latanowicz, E.Vogel, H-.
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Keywords: hydrogen bonds · macrocycles · porphyrinoids ·
pyriporphyrins
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[15] [(tpp)Ge(Ph)Et],[14a] Ho 0.42; Hm 4.85, Hp 5.34 ppm;
[(tpp)AlPh],[14b] Ho 2.48; Hm 5.58, Hp 5.84 ppm), [(SubPh)BPh][3]
(Ho 5.66; Hm 6.53, Hp 6.67).
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b) M.S.Rodríguez-Morgade, S.Esperanza, T.Torres, J.Barberµ,
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[7] Data collection: the measurement was performed on an
Xcalibur PX k-geometry diffractometer using MoKa radiation
(l = 0.71073 Š), T= 100 and 80 K for the same crystal. 5·CH2Cl2:
crystals were prepared by the slow diffusion of CH3OH into a
solution of 5 in CH2Cl2 to yield a dark-green crystal of
C40H35N3·CH2Cl2, size 0.40 ” 0.12 ” 0.07 mm3, monoclinic, space
group P21/c, a = 12.737(3), b = 11.687(3), c = 22.894(4) Š, b =
3
102.94(3)8, V= 3321.4(13) Š for T= 80 K, Z = 4.total no.of
reflections collected: 50231; no.of independent reflections: 9735
(of which 4774 > 2s(I)) were included in the refinement for 442
parameters; no absorption correction was applied.The structure
was solved by using direct methods with SHELXS-97 and
refined against j F2 j using SHELXL-97 (G.M. Sheldrick,
University of Göttingen, Germany, 1997), final R1/wR2 indices
(for I > 2s(I)): 0.0459/0.1130; max./min. residual electron den-
sity: + 0.25/ꢀ0.30 eŠꢀ3; H atoms except H(18) (NH) were fixed
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