Synthesis and characterization of a directly linked N-confused porphyrin dimer

Article abstract of DOI:10.1002/anie.200461361

There's nothing confusing about the dimer (see picture) that forms from N-confused porphyrin! The dimer was obtained as a directly β-β-linked structure through a simple acid-catalyzed condensation reaction of the monomeric macrocycles, and the electronic

Full text of DOI:10.1002/anie.200461361

Angewandte
Chemie
with each other. Distance, geometry, and orientation have
been recognized as important factors for the control of this
communication. Among arrays of covalently linked porphyrin
units, systems in which the aromatic subunits are directly
connected to each other by a covalent bond are particularly
important. Such a linkage enables interaction between the
subunits while the intrinsic structure of the porphyrin is
preserved. Considerable progress has been reported recently
in the synthesis of oligoporphyrins with direct b–b, meso–
meso, and meso–b links[5] to yield products that range from
dimeric species to linear structures, which contain up to 128
porphyrin subunits.[6]
The field of porphyrin arrays can be further extended by
the construction of arrays of covalently linked porphyrin
analogues, particularly porphyrin-like macrocycles that also
have carbon atoms in the coordination core. The N-confused
porphyrin 1[7] is an isomer of regular porphyrin which
preserves the fundamental porphyrinoid skeleton. In multi-
molecular assemblies, it offers newmodes of linking subunits
and different coordination properties of the resultant oligom-
ers.[8] Herein we report the synthesis of the first directly b–b-
linked dimer of N-confused porphyrin. This newmolecule
was obtained in an acid-catalyzed process, which did not
require prior activation of the macrocyclic ring. The active
site was provided here by a single a-carbon atom of the N-
confused pyrrole. 3,3’-bis(meso-tetraaryl-2-aza-21-carbapor-
phyrin) (2) was obtained in a one-step, one-pot synthesis
starting from the N-confused porphyrin 1 (Scheme 1). The
coupling process was carried out in the presence of air, which
seems to be a sufficiently strong oxidant for the dehydrogen-
ation step that is expected to followthe formation of the bond
between the two porphyrin subunits. An analogous reaction,
which leads to a 3-(2’-pyrrole)-substituted N-confused por-
phyrin, was observed previously in the reaction of 1 with
pyrrole in DMF.[9] Although the reaction proceeds in the
presence of various acids, the best results were obtained when
hydrogen bromide was added as a catalyst to a solution of 1 in
Porphyrins
Synthesis and Characterization of a Directly
Linked N-Confused Porphyrin Dimer**
Piotr J. Chmielewski*
Oligomeric porphyrin arrays[1] are present in molecular
devices that are designed for energy- and electron-transfer
processes,[2] multielectron redox catalysis,[3] and in organic
magnetic materials.[4] For all such applications, it is important
that the individual molecules within the array communicate
[*] Prof. P. J. Chmielewski
Department of Chemistry
University of Wrocław
F. Joliot-Curie Street 14
50383 Wrocław (Poland)
Fax: (+4871)32-823-48
E-mail: pjc@wchuwr.chem.uni.wroc.pl
´
[**] The author thanks Dr. M. Ste˛pien for his help in the X-ray crystal
structure analysis and for helpful discussions. This work was
supported by the State Committee for Scientific Research of Poland
(Grant No. 309A15515)
Scheme 1. Synthesis of the dimer 2 and its nickel complex 3: a) HBr, toluene/
dichloromethane, 20 h; b) Ni(CH3COO)2, toluene/DMF, 1 h. Ar=p-tolyl,
DMF=N,N-dimethylformamide.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 5655 –5658
DOI: 10.1002/anie.200461361
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5655

Communications
toluene/dichloromethane. After heating for 20 h at reflux, the
factors imposed by the meso-substituted aryl groups that flank
the N-confused pyrrole in both subunits. The lack of rota-
tional freedom around the 3,3’-axis leads to an axial chirality
of the molecule (see Supporting Information).[5i] The
observed cisoidal conformation of the bipyrrolic system is
essential for the coordination properties of this potential
chelating site.
conversion of the starting material was ꢀ 80%, and the
product 2 was obtained with an overall yield of 50%.[10]
Analysis of the 1H and 13C NMR spectroscopic data
revealed that the aromaticity of the subunits is preserved in
the bis(porphyrinoid) and that the 3,3’-bridging carbon atoms
are sp2-hybridized. Owing to the symmetry of the dimer, only
one set of resonances were detected in the NMR spectrum of
2. Nevertheless, an upfield shift of the protons that belong to
one of the meso-substituted aryl groups as well as the
through-space interaction of these protons with NH protons
located within the porphyrin crevice were observed in the
NOESYand ROESY spectra. These features indicate that the
subunits are close to one another. It is particularly evident in
the case of the p-tolyl derivative 2 for which one of the methyl
signals (5-(p-tolyl)) is located at d ꢀ À0.7 ppm (CDCl3,
298 K); that is, it is shifted more than 3 ppm upfield with
respect to the other methyl signals because of the strong
influence of the aromatic-ring current of the porphyrin (see
Supporting Information).
Formation of the bis(nickel(ii)) complex 3 (Scheme 1)
required a higher reaction temperature than necessary for the
insertion of nickel(ii) into the monomeric N-confused por-
phyrin 1.[7a] Some of the structural and spectral features
typical of nickel(ii) complexes with N-confused porphyrins—
that is, deprotonation of the internal carbon, protonation of
the external nitrogen, as well as reduction of the aromatic
character in comparison to the free base—were observed for
1
3. However, the H NMR spectrum of 3 also shows features
typical of the N-confused bis(porphyrin) system 2.[8,12] In
particular, protons of the 5-aryl substituent are strongly
shifted upfield owing to a close contact with an aromatic-ring
current of the neighboring porphyrinic ring. Analysis of the
NOESY spectrum of 3 (see Supporting Information) revealed
a mutual orientation of the subunits similar to that observed
for 2.
The crystal structure of 2 (Figure 1) confirms the integrity
of the dimer. The dihedral angle between the mean planes
defined by the C··N··N··N coordination cores of the subunits is
The interaction between the aromatic systems in 2 was
reflected by a strong bathochromic shift of the Soret and
Q bands in the electronic absorption spectra with respect to
the monomeric species 1 (Figure 2a) and a splitting of the
Figure 2. Absorbance spectra of solutions in CH2Cl2 of a) 2 (solid line)
and b) 3 (solid line). The dashed lines (a) in a) and b) represent
the spectra of the monomer 1 and its nickel(ii) complex, respectively.
Figure 1. Crystal structure of 2. Hydrogen atoms are omitted for clarity.
Thermal ellipsoids are drawn at the 50% probability level.
Soret band (DE = 736 cmÀ1). The optical properties of 3 are
affected even more by the interaction of the subunits. The
UV/Vis spectrum of the complex consists of a split Soret-type
band and strongly broadened and red-shifted Q bands
(Figure 2b).
208. The plane defined by the N-confused pyrrole is tilted by
338 from the average plane in each macrocyclic subunit. The
dihedral angle between the mean planes of the N-confused
À
pyrroles is 618. The distance of the unique bridging C3 C3’
bond is 1.468(3) Š, which is similar to the distances observed
in a zinc complex of a b–b-linked regular heptaalkylporphyrin
(1.46 Š),[5a] a meso–meso-linked bis(porphyrin) free base
(1.51 Š),[5b] or a meso–meso, b–b double-linked bis(porphyr-
inatonickel(ii)) dimer (1.45 Š),[5j] and it is much shorter than
the sp3–sp3 distance reported for a b–b-linked bis(chlorin)
(1.61 Š).[11] Molecular modeling studies of 2 showthat the
rotation around the bridging bond is restricted owing to steric
Electrochemical measurements (solution in CH2Cl2, tetra-
n-butylammonium perchlorate as a supporting electrolyte,
potentials referenced to the ferrocene/ferrocenium couple)
also reveal the mutual influence of the subunits. The first
oxidation potential of 2 was anodically shifted with respect to
that of 1 (about 65 mV) and was split into two waves (240 mV
and 340 mV). This separation is similar to that observed for
the meso–meso-linked dimers of regular porphyrin
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Angewandte
Chemie
(150 mV).[5h] For 3, two reversible one-electron couples were
observed for the oxidation (140 and 218 mV) and the
reduction (À1220 and À1315 mV) processes (see Supporting
information). A significant anodic shift was observed for the
reduction processes with respect to that of a monomeric
nickel(ii) complex of 1 (À1556 mV)[13] which corresponds to a
> 300 mV decrease in the HOMO–LUMO gap[14] of the
dimer 3.
In summary, compound 2 constitutes the first example of a
directly linked N-confused bis(porphyrin). Its surprisingly
facile synthesis makes 2 a potentially attractive building block
of novel molecular assemblies that contain covalently and
coordinatively linked subunits with peripheral nitrogen
donor-atom sites.
Centre, 12, Union Road, Cambridge CB21EZ, UK; fax: (+ 44)1223-
336-033; or deposit@ccdc.cam.ac.uk).
3: A solution of 2 (30 mg, 0.022 mmol) in toluene (30 mL) was
mixed with a solution of nickel(ii) acetate tetrahydrate (50 mg) in
DMF (5 mL), and the mixture was heated at reflux under nitrogen for
1 h. The solvents were then removed, and the residue was purified by
flash chromatography through silica gel. The first green band eluted
with CH2Cl2 was collected. Recrystallization from hexane solution
(carried out under N2) yielded the desired complex 3 (20 mg, 63%).
1H NMR: (500 MHz, CD2Cl2, 233 K, TMS): d = 9.58 (s, 1H, 2-NH),
7.88 (d, 3JHH = 4.8 Hz, 1H; pyrrole), 7.79 (d, 3JHH = 5.0 Hz, 1H;
3
4
3
pyrrole), 7.75 (dd, JHH = 7.6 Hz, JHH = 1.8 Hz, 1H), 7.73 (d, JHH
=
5.3 Hz, 1H; pyrrole), 7.72 (dd, 3JHH = 7.6 Hz, 4JHH = 1.4 Hz, 1H), 7.65
(d, 3JHH = 5.0 Hz, 1H; pyrrole), 7.62 (dd, 3JHH = 7.7 Hz, 4JHH = 1.3 Hz,
1H), 7.56 (dd, 3JHH = 7.6 Hz, 4JHH = 1.4 Hz, 1H), 7.55 (dd, JHH
=
3
4
3
7.8 Hz, JHH = 1.6 Hz, 1H), 7.51 (d, JHH = 5.3 Hz, 1H; pyrrole), 7.50
(dd, 3JHH = 7.6 Hz, 4JHH = 1.4 Hz, 1H), 7.48 (d, 3JHH = 5.3 Hz, 1H;
pyrrole), 7.47 (dd, 3JHH = 7.8 Hz, 4JHH = 1.6 Hz, 1H), 7.41 (dd, 3JHH
=
7.5 Hz, 4JHH = 1.4 Hz, 1H), 7.35 (m, 2H), 7.33 (m, 2H), 7.29 (dd,
3JHH = 8.0 Hz, 4JHH = 1.1 Hz, 1H), 7.11 (d, 3JHH = 7.3 Hz), 6.55 (d,
3JHH = 7.1 Hz), 5.96 (d, 3JHH = 7.3 Hz), 2.52 (s, 3H), 2.49 (s, 3H), 2.43
(s, 3H), 1.33 (s, 3H); UV/Vis (CH2Cl2): lmax (loge) = 243 (sh), 368
(4.84), 431 (4.86), 483 (4.79), 625 (4.49), 860 nm (sh). ESI-MS: m/z:
1452.5 ([M+1]+); elemental analysis: calcd for C96H68N8Ni2: C 79.55,
H 4.70, N 7.73; found: C 79.35, H 4.90, N 7.54%.
Experimental Section
2: HBr (40% solution in acetic acid, 50 mL, 0.25 mmol) was added in
one portion to a solution of 1 (100 mg, 0.15 mmol) in toluene (30 mL).
A further aliquot of dichloromethane (5 mL) was added to enhance
the solubility of the generated porphyrin dication. The reaction
mixture was heated at reflux for 20 h, then the solvents were removed.
The residue was dissolved in benzene and was purified by chroma-
tography through basic Al2O3 (activity III). Decomposition products
and unconverted starting material 1 (recovered 21 mg) were eluted
with benzene, whereas the desired product 2 was eluted with CH2Cl2
Received: July 19, 2004
Keywords: dimerization · nickel · nitrogen heterocycles ·
and then recrystallized from dichloromethane/ethanol (49 mg, 49%).
.
1H NMR: (500 MHz, CDCl3, 233 K, TMS): d = 8.92 (dd, JHH
=
3
porphyrinoids · supramolecular chemistry
4
3
4.4 Hz, JHH = 1.3 Hz, 1H; pyrrole), 8.84 (d, JHH = 7.5 Hz, 1H) 8.61
3
3
(d, JHH = 4.5 Hz, 1H; pyrrole), 8.56 (d, JHH = 4.5 Hz, 1H; pyrrole),
3
4
3
8.51 (dd, JHH = 4.4 Hz, JHH = 1.0 Hz, 1H; pyrrole), 8.39 (dd, JHH
=
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=
4
1.0 Hz, 1H; pyrrole), 8.01 (dd, 3JHH = 7.4 Hz, 4JHH = 1.3 Hz, 1H),
7.83 (d, 3JHH = 8.1 Hz, 1H), 7.81 (dd, 3JHH = 8.0 Hz, 4JHH = 1.4 Hz,
1H), 7.76 (d, 3JHH = 7.6 Hz, 1H), 7.64 (m, 2H), 7.56 (m, 2H), 7.46 (d,
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3
4
3
3
(d, JHH = 7.5 Hz, 1H), 4.64 (d, JHH = 7.0 Hz, 1H), 2.80 (s, 3H), 2.71
(s, 3H), 2.69 (d, 3JHH = 7.6 Hz, 1H), 2.65 (s, 3H), À0.76 (s, 3H), À2.40
(s, 1H, NH), À2.64 (b, 1H, NH), À5.25 (b, 1H, 21-CH); UV/Vis
(CH2Cl2): lmax (loge) = 257 (4.74), 290 (4.76), 348 (sh), 399 (sh), 442
(sh), 459 (5.47), 565 (4.60), 595 (sh), 703 (sh), 758 nm (4.40); ESI-MS:
m/z: 1340.5 [MH+]; elemental analysis: calcd for C96H74N8C2H5OH: C
84.97, H 5.78, N 8.09; found: C 84.74, H 5.65, N 8.30%.
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Crystals of suitable quality for X-ray analysis were obtained by
the slowdiffusion of ethanol into a solution of 2 in dichloromethane.
Crystal data for 2: C96H74N8·2.25EtOH·0.75CH2Cl2, MW = 1491.86,
T= 100 K, CuKa radiation, monoclinic, space group P21/n, a =
15.829(3), b = 18.772(4), c = 26.673(5) Š, b = 92.16(3) V=
7920(3) Š3, Z = 4, 1calcd = 1.251 MgmÀ3
1.039 mmÀ1
,
l = 1.54178 Š, m =
,
F(000) = 3124; Oxford Diffraction KM4 Xcalibur2
diffractometer with KM4CCD Sapphire detector; 3.65 ꢁ q ꢁ 73.138,
15495 collected reflections of which 11701 were independent with I >
2s(I), 1097 parameters, R1(F) = 0.0822, wR2(F2) = 0.2606, S = 1.116,
largest difference peak and hole 0.474 and À0.602 eŠÀ3. All non-
hydrogen atoms were refined with anisotropic displacement param-
eters, except for those of the disordered solvents; hydrogen atoms
were included in the geometry of the molecules and were refined
isotropically. The extensive solvent disorder was modeled with a
number of isotropic C, O, and Cl atoms with free refining occupancies
to finally establish the presence of nine molecules of ethanol and
three molecules of dichloromethane per unit cell. CCDC-245032
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic Data
Angew. Chem. Int. Ed. 2004, 43, 5655 –5658
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5657

Communications
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5658
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 5655 –5658