meso-Substituted expanded porphyrins: New and stable hexaphyrins

Article abstract of DOI:10.1039/a808952c

New hexapyrrolic macrocycles have been synthesized and characterized by UV-VIS, mass and NMR spectroscopy and X-ray crystallography.

Full text of DOI:10.1039/a808952c

meso-Substituted expanded porphyrins: new and stable hexaphyrins
Maria G. P. M. S. Neves,a Rosália M. Martins,a Augusto C. Tomé,a Armando J. D. Silvestre,a Artur M. S.
Silva,a Vitor Félix,a Michael G. B. Drewb and José A. S. Cavaleiro*a
a Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal. E-mail: jcavaleiro@dq.ua.pt
b Department of Chemistry, University of Reading, Whiteknights, Reading, UK RG6 6AD
Received (in Liverpool, UK) 16th November 1998, Accepted 8th January 1999
New hexapyrrolic macrocycles have been synthesized and
characterized by UV–VIS, mass and NMR spectroscopy and
X-ray crystallography.
esses: by reacting the blue compound with DDQ in CHCl3 the
violet one is obtained and reduction of the violet compound with
TsNHNH2 gives the blue one.
Surprisingly, the 1H NMR spectra of these compounds show
that they have aromatic features: for the major (violet)
compound, signals at d 22.43, 21.98, 9.11 and 9.49 are
observed. The analysis of this spectrum, together with the
HETCOR (1H/13C) spectrum, revealed that the singlet (4H) at d
22.43 correlates with the carbon resonance at d 122.9. The very
broad singlet at d 21.98 shows no correlation with carbons and
was, thus, attributed to NH protons. This signal became more
narrow when the proton spectrum was registered at 273.1 K.
Based on these spectra, and on the 19F NMR spectrum (vide
infra), the structure 2 is proposed for this macrocycle. For the
blue compound the hydrogenated structure 3 is considered,
based mainly on its mass and NMR spectra and its dehydroge-
In recent years there has been considerable research directed
towards the synthesis and study of systems involving expanded
porphyrin macrocycles. Such systems are finding applications
in the fields of medicine, neutral substrate binding and anion
recognition.1
Recent studies have shown that inverted2 and expanded3
porphyrins can be formed during the Rothemund synthesis of
porphyrins. Very recently the synthesis of an interesting
expanded porphyrin, the hexaphenylhexaphyrin 1, has been
reported.4 The poor stability of that compound hampered its
spectroscopic characterisation.
1
nation to 2. The H NMR spectrum of the blue compound is
Ph
Ph
significatively different from that of compound 2: signals at d
2.55 (4H, s), 4.40 (br s), 7.63 (4H, d, J 4.7) and 7.73 (4H, d, J
4.7) are observed; there are no signals at negative d values. The
signal at d 4.40 is due to the NH protons since it disappears after
shaking with D2O. From the HETCOR (1H/13C) experiment we
can observe a correlation between the singlet at d 2.55 and the
resonance at d 119.4 while the doublets at d 7.63 and 7.73
correlate, respectively, with the carbon resonances at d 130.2
and 129.2. From the NMR spectra of the blue product it is
obvious that there are no sp3 carbons in this reduced
compound.
The fact that both compounds show the same ion at m/z 1462
can be explained by a fast hydrogenation of 2 to 3 during the
acquisition of the mass spectra.† This was confirmed by the use
of deuterated nitrobenzyl alcohol as matrix: 2 gives a signal at
m/z 1466, corresponding to the addition of ‘2D’ plus the
exchange of the two NH by ND; 3 gives a set of signals at m/z
1463–1466 corresponding to the successive exchange of the
four NH by ND.
N
NH
N
N
N
Ph
Ph
C6F5 (a)
C6F5 (b)
H
N
N
B
A
F
C
N
NH
N
Ph
Ph
1
(f) C6F5
C6F5 (c)
HN
D
C6F5
C6F5
E
N
N
C6F5 (e)
C6F5 (d)
HN
NH
2
C6F5
C6F5
NH
HN
N
C6F5
C6F5
3
Here we report the synthesis and characterisation by
spectroscopic techniques of stable meso-hexa(pentafluoro-
phenyl)hexaphyrins 2 and 3; the structure considered for 2 was
also confirmed by X-ray crystallography. These new com-
pounds were obtained during the Rothemund synthesis of meso-
tetra(pentafluorophenyl)porphyrin under modified experimen-
tal conditions:5 addition of pentafluorobenzaldehyde to a
refluxing mixture of glacial AcOH and PhNO2, followed by the
dropwise addition of pyrrole. The final mixture was refluxed for
45 min, the solvents were removed by distillation, and the
residue was then purified by silica gel column chromatography.
The first fraction to be collected contained the expected
porphyrin (15%), and a more polar bluish fraction was then
eluted (ca. 1%). Thin layer chromatographic (TLC) analysis of
this fraction revealed the presence of two main compounds with
blue and violet colours: lmax(CH2Cl2)/nm (log e) 591 (4.95),
762 (3.82) and 568 (5.64), 712 (4.67), respectively (Fig. 1).
Both compounds show, in their mass spectra (LSIMS), the
same molecular ions at m/z 1462, which correspond to
molecules formed from six pyrroles and six pentafluoro-
benzaldehyde units. The fact that both compounds show the
same molecular ions was an unexpected observation, since they
can be interconverted by hydrogenation–dehydrogenation proc-
For compound 2, the resonance at d 22.43 corresponds to the
‘inner’, strongly shielded, four b-protons of the two inverted
pyrrole rings (B and E). Since the NH protons resonate at d
21.98 we can say that they are ‘inside’ the macrocycle and not
in the inverted pyrrole rings. The ‘outer’ b-pyrrolic protons
appear as two doublets (4H each) at d 9.11 and 9.44; they
correlate, in the HETCOR (1H/13C) spectrum, with the carbon
resonances at d 132.5 and 135.0, respectively. This last
observation confirms that each pyrrole ring (A, C, D and F) has
Fig. 1 Electronic spectra of compounds (a) 2 and (b) 3.
Chem. Commun., 1999, 385–386
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non-equivalent b-pyrrolic carbons and protons. The resonances
of all a-pyrrolic carbons (d 156.3 for rings B, E and 149.5, 149.7
for the other rings) were determined by the connectivities found
in the HMBC spectrum. The resonances of the meso carbons
appear at d 106.2 and 117.4, whereas those of the penta-
fluorophenyl rings appear as multiplets with low intensity (due
to both relaxation processes and coupling with the fluorine
atoms).
have some double or single bond character: 1.36(1)–1.32(1) Å
for the Cb–Cb bonds and 1.42(1)–1.46(1) Å for the Ca–Cb
bonds. The six pentafluorophenyl rings are tilted by 47.3(1),
43.3(1), 51.0(1), 68.4(1), 78.8(1) and 81.4(1)° relative to the
least-square plane defined by the atoms of the macrocyclic core.
Consequently, the average bond length between the meso
carbons and pentafluorophenyl rings of 1.50(1) Å indicates an
absence of a p interaction between those rings and the
macrocyclic core.
This set of structural features is consistent with a 26p electron
aromatic current ring extended over the nitrogens, the a-
pyrrolic carbons and the meso carbons of the macrocyclic
ring.
The nonplanarity of the macrocycle is due to its large
dimensions [N(39)···N(45) 2.93(1), N(18)···N(24) 2.82(1),
N(24)···N(39) 7.58(1) and N(18)···N(45) 7.60(1) Å] and the
bulk of the meso-pentafluorophenyl rings. In order to minimise
the steric interactions, the macrocycle looses the planarity of its
core, leading an average deviation of 0.536(8) Å of the atoms
belonging to the macrocyclic core from the least-squares plane
defined by their atomic positions.
From the 19F NMR spectrum of 2 (Table 1), by comparing the
relative signal integrals, we can conclude that there are two
types of pentafluorophenyl rings in a 2: 1 ratio. This result is
also compatible with the proposed structure: the two meso-
pentafluorophenyl rings (c) and (f) are in a different environ-
ment than the other four phenyl rings.
The acidic character of the NH protons and the stability of
meso-hexa(pentafluorophenyl)hexaphyrin 2 was demonstrated
by the following experiments: (i) addition of several drops of a
solution of NaOD (in D2O) to a CDCl3 solution of this
compound, followed by a gentle shaking, led to the appearance
of two signals (d 22.43 and 25.91) in the negative region of the
1H NMR spectrum; (ii) after a more vigorous shaking, only the
resonance at d 25.91 was observed for the b-H of rings B and
E. This strong shielding of the ‘inner’ protons suggests the
conversion of the macrocycle into the corresponding dianion;
(iii) the neutralization of this solution allowed us to recover the
meso-hexa(pentafluorophenyl)hexaphyrin 2.
We thank FCT, Lisbon, for funding the Research Unit No.
62/94 and Mr Pedro Domingues (University of Aveiro) for the
mass spectra.
All the NMR results discussed here support the proposed
structure 2 for this new 26p electron macrocycle. The final
proof for the structure of this compound was obtained by X-ray
single crystal diffraction.
The X-ray structure of 2‡ is presented in Fig. 2. The
macrocycle displays a nonplanar conformation [see Fig. 2(b)]
with four exo pyrrolic rings (A, C, D and F) and two endo rings
(B and E). p-Electron delocalization in the macrocyclic core is
apparent from the observed range of bond lengths: 1.33(1)–
1.39(1) Å for the N–Ca bonds and 1.39(1)–1.42(1) Å for the
Ca-Cmeso bonds. On the other hand, the range found for the
C–C bond lengths in the pyrrole rings suggest that these bonds
Notes and references
† A similar hydrogenation process was observed during the acquisition of
the mass spectrum of the meso-tetra(pentafluorophenyl)porphyrin.
‡ Crystal data for 2: C66H14F30N6, M = 1460.83; monoclinic, space group
P21/c, a = 10.911(10), b = 27.242(23), c = 19.466(21) Å, b = 90.31(1)°,
V = 5786(10) Å3, Z = 4, Dc = 1.677 g cm23, m = 0.168 mm21. A violet
needle-like crystal was mounted in a Lindmann capillary under saturated
atmosphere of the mother-liquor. The X-ray data were collected with
graphite monochromated Mo-Ka radiation (l = 0.71073 Å) on a Mar-
research image plate system at Reading University. The crystal was
positioned at 75 mm from the plate. An exposure time of 5 min was used per
2º frame collected. Data analysis was performed with the XDS program (ref.
6). Intensities were not corrected for absorption effects. 12030 reflections
collected in the range 2.0 < q < 26.1 (hkl range indices: 0 @ h @ 11, 233
@ k @ 33, 219 @ l @ 19) were merged in the Laue symmetry group 2/m to
7246 unique reflections with a Rint = 0.0454. The structure was solved by
direct methods and refined by full-matrix least-squares methods on F2 using
the SHELX-97 package (ref. 7) Anisotropic parameters were used for all
non-hydrogen atoms. The hydrogen atoms were included in the refinement
in geometric positions with a Uiso = 1.2Ueq of the parent nitrogen atom. In
agreement with 1H NMR results, two geometric arrangements for two N–H
protons were considered: one proton at N(39) and one at N(18) or
alternatively one proton at N(24) and the other at N(45). The first
arrangement was considered as correct since it gave a slightly lower R value
(0.0789) than the second (0.0795). The final refinement of 920 parameters
converged to R and wR values of 0.0785 and 0.2094 and GOF = 0.862 for
the data with I > 2s(I). The final R and wR values for all hkl data were
0.2304 and 0.2750, respectively. In the last difference Fourier map the
Table 1 19F NMR chemical shifts (ppm, from TFA) observed in the
spectrum of the meso-hexa(pentafluorophenyl)hexaphyrin 2
(a), (b), (d) and (e)
phenyl rings
(c) and (f) phenyl rings
o-F
m-F
p-F
2136.19 (4F)
2162.22 (4F)
2151.94 (2F)
2135.63 (2F)
2159.61 (2F)
2149.08 (1F)
residual electronic density was in the range of 20.341 to 0.281 e Å23
Molecular diagrams were made with ZORTEP (ref. 8). CCDC 182/1139.
Crystal data are available in CIF format from the RSC web site, see:
http://www.rsc.org/suppdata/cc/1999/385/
.
1 A. Jasat and D. Dolphin, Chem. Rev., 1997, 97, 2267; B. Franck and A.
Nonn, Angew. Chem., Int. Ed. Engl., 1995, 34, 1795.
2 P. J. Chmielewski, L. Latos-Grazynski, K. Rachlewicz and T. Glowiak,
Angew. Chem., Int. Ed. Engl., 1994, 33, 779; H. Furuta, T. Asano and T.
Ogawa, J. Am. Chem. Soc., 1994, 116, 767.
3 P. J. Chmielewski, L. Latos-Grazynski and K. Rachlewicz, Chem. Eur. J.,
1995, 1, 68.
4 C. Bruckner, E. D. Sternberg, R. W. Boyle and D. Dolphin, Chem.
Commun., 1997, 1689.
5 A. M. d’A. Rocha Gonsalves, J. M. T. Varejão and M. M. Pereira,
J. Heterocycl. Chem., 1985, 1228.
6 W. Kabsch, J. Appl. Crystallogr., 1988, 21, 916.
7 G. M Sheldrick, SHELX-97, University of Göttingen, 1997.
8 L. Zsolnai, ZORTEP, University of Heidelberg, 1994.
Fig. 2 (a) Top and (b) side views of 2 (thermal ellipsoids at the 40%
probability level). The side view shows the nonplanarity of the macrocyclic
core. In both views only the labelling scheme for nitrogen atoms is
presented, and in (b) the phenyl rings are omitted for clarity.
Communication 8/08952C
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Chem. Commun., 1999, 385–386