Doubly N-confused hexaphyrin: A novel aromatic expanded porphyrin that complexes bis-metals in the core

Article abstract of DOI:10.1021/ja029018v

Meso-hexakis(pentafluorophenyl)-substituted doubly N-confused hexaphyrins and their metal complexes were synthesized for the first time, and the structures were elucidated by X-ray single-crystal analyses. The free base form of oxidized hexaphyrin (5) had two preorganized N₃O pockets in the macrocyclic core, where a hydrogen-bonding network was formed to keep the molecule planar (the mean plane deviation is 0.054 A). The formation of a planar bis-Cu(II) complex was confirmed by UV/vis, magnetic susceptibility measurements, and X-ray crystallography. The bis-Ni(II) complex (7), on the other hand, was distored from planarity, but it changed to the planar structure upon solvent (acetonitrile) coordination as judged by the observation of a sharp Soret-like band in absorption spectra and the X-ray structures of the complexes. Copyright

Full text of DOI:10.1021/ja029018v

Published on Web 01/01/2003
Doubly N-Confused Hexaphyrin: A Novel Aromatic Expanded Porphyrin that
Complexes Bis-metals in the Core
Alagar Srinivasan,† Tomoya Ishizuka,† Atsuhiro Osuka,† and Hiroyuki Furuta*,†,§,|
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan,
Department of Applied Chemistry, Graduate School of Engineering, Kyushu UniVersity,
Fukuoka 812-8581, Japan, and PRESTO, Japan Science and Technology Corporation, Japan
Received October 19, 2002 ; E-mail: hfuruta@cstf.kyushu-u.ac.jp
Chart 1. Schematic Drawings of Bis-metal Complexes of Various
Porphyrins are ubiquitous aromatic ligands that can chelate a
variety of metals using interior four pyrrolic nitrogens (N4) in a
square-planar arrangement. Due to the limited core size, porphyrins
usually serve as mononuclear ligands when they complex metals
in the plane.1,2 Thus, for bis-metal in-plane-coordination, another
class of porphyrinoids having more than four pyrrolic units in the
core is contemplated.3 Among such, so-called expanded porphyrins,
hexapyrrolic porphyrinoids, hexaphyrins, are promising candidates
because some kinds of hexaphyrins were reported to take a planar
conformation.4,5 For example, Sessler and co-workers have shown
that [24]hexaphyrin-(1.0.0.1.0.0), “amethyrin”, could coordinate two
Cu(II) ions with inward-pointing six pyrrolic nitrogens and two
bridging chlorides in the cavity.4 A more spacious hexaphyrin-
(1.1.1.1.1.1) system of meso-aryl type, on the other hand, has been
shown to have a pseudorectangular framework,5 but the formation
of bis-metal in-plane-complex seems difficult because it ac-
companies the activation of inner CH of the N2C2 macrocyclic core.6
If one or more interior carbon atoms is replaced by nitrogen, the
bis-metal coordination in a modified N3C or N4 core would become
more facile. Such an idea promoted us to introduce two confused
pyrrolic units into normal hexaphyrin framework to produce a
doubly N-confused hexaphyrin (N2CH).7 Herein, we present the
syntheses and spectral and structural characterization of doubly
N-confused meso-hexaarylhexaphyrin (2 and 3), the amide deriva-
tives (4 and 5), and their bis-metal complexes (6 and 7). To the
best of our knowledge, these are the first examples of doubly
N-confused hexaphyrins and square-planar coordinated in-plane bis-
metal complexes of aromatic porphyrinoid.
Hexaphyrins
On varying the oxidizing agent from p-chloranil to 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) in the condensation reaction,
two different products 4 and 5 were obtained as greenish solids in
7 and 10% yield, respectively (Scheme 1b). FABMS of 4 and 5
showed the peaks at m/z 1494 and 1492, respectively, indicating
the presence of N2CH moiety as well as two oxo groups. In fact,
the presence of -CdO was confirmed by the IR band at 1716 and
1683 cm-1 for 4 and 5, respectively. The difference in the frequency
(33 cm-1) suggested the strong hydrogen-bonding interaction in 5.
The nonaromatic [28π] characters of 4 were reflected from1H NMR
signals in the range from 14.00 to 5.75 ppm, and the electronic
absorption spectrum, where five broad bands were displayed from
325 to 755 nm. On the other hand, compound 5 [26π] showed the
aromatic features and the inner hydrogen bondings as judged by
1H NMR spectrum, in which the outer â-CH and NH signals of
the confused ring and the NH of normal pyrroles resonated at 10.74,
-0.73, and -0.24 ppm, respectively. The aromatic planar structure
of 5 was also suggested from the absorption spectrum, displaying
the intense Soret-like band at 566 nm.
When N-confused tripyrrane (1) was treated with pentafluo-
robenzaldehyde in the presence of p-toluenesulfonic acid (p-TSA),
followed by oxidation with p-chloranil, meso-hexakis(pentafluo-
rophenyl)-substituted doubly N-confused hexaphyrins, 2 and 3, were
obtained as brown solids in 7 and 3% yields, respectively (Scheme
1a). FABMS of 2 and 3 showed the molecular ion peaks at m/z
1462 and 1460, suggesting the formation of the reduced and
oxidized N2CH. The 1H NMR spectrum of 2 in CDCl3 at -50 °C
showed two different NH signals at 11.71 and 10.76 ppm and
confused pyrrolic R-CH and â-CH signals at 7.03 and 6.40 ppm,
respectively, which proved that the compound was nonaromatic
[28π]. Supporting these, the electronic absorption spectra displayed
three broad bands in the region from 380 to 590 nm. In contrast,
the aromatic feature [26π] of compound 3 was shown in a strong
Soret-like band at 591 nm and four Q-type bands from 650 to 1100
nm.8 However, 3 was quite unstable and gradually oxidized into
amide 5 in CH2Cl2 solution at room temperature.
The explicit structural details of 5 were revealed by the single-
crystal X-ray analysis. Consistent with the above observations, two
confused pyrrole rings in 5 were inverted and two oxo groups
attached at the inner R-carbons to afford the amide groups. The
two carbonyl groups were forming the intramolecular hydrogen
bondings with encountered amide NH (O(1)- - -N(5): 2.755 Å) and
adjacent pyrrole NH (O(1)- - -N(3): 2.609 Å), keeping the molecule
planar. The mean deviation was only 0.054 Å to the mean plane,
consisting of a core of 36 atoms (excluding the oxygen atoms). As
a result of oxygenation of N2CH, two N3C compartments of 2 and
3 changed to two N3O cores in 4 and 5 (Figure 1a,b).
The metal coordination chemistry was performed on stable 5 by
using Cu(II) and Ni(II) salts. When it was treated with anhydrous
Cu(II) acetate, in CH2Cl2 solution, blue-greenish solid 6 was
obtained in quantitative yield (Scheme 1c). The complexation of
bis-Cu(II) metal 6, was confirmed by FABMS peak at m/z 1614.
The electronic absorption spectrum showed the Soret-like band at
616 nm, which was 50 nm red-shifted as compared to the free base
5. The complex was paramagnetic, and the magnetic susceptibility
measurements proved that the Cu ions were present in the
† Kyoto University.
§ Kyushu university.
| PRESTO, JST.
9
878
J. AM. CHEM. SOC. 2003, 125, 878-879
10.1021/ja029018v CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
on center Ni(II) metals, resulting in a structural change to the planar
complex, was further confirmed by the X-ray structure of 8.8
The doubly N-confused hexaphyrins (N2CH) complexes de-
scribed here are rather unique in that two adjacent metals are located
in a large conjugated π-electron pool. By combining the bistability
of both the 26π and 28π systems and the redox couples of the
coordinated metals, N2CH complexes would provide attractive bis-
metal systems for the study of optoelectronic and optomagnetic
materials. Furthermore, the conformational flexibility by axial
coordination of exterior substrates observed in 7 suggests the
advantage of the N2CH complexes for the development of various
catalytic systems. Efforts to prepare homo- and hetero-bis-metal
complexes using a variety of metals are currently underway.
Figure 1. Molecular structures of 5 and 7 (a, c: top view, and b, d: side
view). Pentafluorophenyl groups were omitted for clarity in (b) and (d).
Dotted lines indicate hydrogen-bonding interactions.
Acknowledgment. We thank Professor Y. Uwatoko at Tokyo
University for SQUID measurements of 6. A.S. thanks JSPS, Japan
for Postdoctoral Research Fellowship.
paramagnetic d8 form (µeff ) 1.73 µB, at 77 K). The preliminary
X-ray analysis of 6 confirmed the bis-metal coordination and the
planarity of the molecule.8 In contrast, the bis-Ni(II) complex 7
was synthesized in a refluxing solution of 5 in toluene by using
Supporting Information Available: Synthetic details and absorp-
tion spectra (2-8), crystallographic data of 5 and 7, crystal figures of
6 and 8 (PDF/CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
1
Ni(II) acetylacetonate in 95% yield (Scheme 1d). The H NMR
References
spectrum of 7 (FABMS: m/z ) 1606) showed signals from 9.50
to 8.70 ppm, suggesting the compound was diamagnetic and both
the nickel oxidation states were +2. The electronic absorption
spectrum displayed the broad bands from 360 to 1210 nm indi-
cating that the complex was distorted from planarity. The single-
crystal X-ray analysis of 7 confirmed that each Ni metal was
coordinated by three nitrogens and an oxygen atom in a square-
planar fashion at the distance of 4.689(5) Å. The molecule was
completely distorted from the mean macrocyclic plane, and the
mean deviation of core 36 atoms was 0.6632 Å (Figure 1c,d).
Interestingly, when the absorption spectrum of 7 was measured
in CH3CN, the broad bands in 7 became sharper to give a more
porphyrin-like spectrum, suggesting the large structural change
from distorted to planar due to the solvent coordination. Such a
change was also reflected in the 1H NMR spectrum where
paramagnetic downfield shifting (∆δ ) 5-6 ppm) was observed,
probably due to Ni(II) hexacoordination. The solvent coordination
(1) Scheidt, W. R. The Porphyrin Handbook; Kadish, K. M., Smith, K. M.,
Guilard, R., Eds.; Academic Press: New York, 2000, Vol. 3, Chapter 16.
(2) Recently, bis-boron complex in the porphyrin core was reported. Belcher,
W. J.; Breede, M.; Brothers, P. J.; Rickard, C. E. F. Angew. Chem., Int.
Ed. 1998, 37, 1112-1114.
(3) Sessler, J. L.; Gebauer, A.; Weghorn, S. J. The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New
York, 2000; Vol. 2, Chapter 9.
(4) Weghorn, S. J.; Sessler, J. L.; Lynch, V.; Baumann, T. F.; Sibert, J. W.
Inorg. Chem. 1996, 35, 1089-1090.
(5) (a) Neves, M. G. P. M. S.; Martins, R. M.; Tome, A. C.; Silvestre, A. J.
D.; Silva, A. M. S.; Felix, V.; Drew, M. G. B.; Cavaleiro, J. A. S. Chem.
Commun. 1999, 385-386. (b) Shin, J.-Y.; Furuta, H.; Yoza, K.; Igarashi,
S.; Osuka, A. J. Am. Chem. Soc. 2001, 123, 7190-7191.
(6) With meso-free dodecaalkylhexaphyrin system, formation of bis-Zn(II)
complex was reported. Charriere, R.; Jenny, T. A.; Rexhausen, H.;
Gossauer, A. Heterocycles 1993, 36, 1561-1575.
(7) (a) Furuta, H.; Maeda, H.; Osuka, A. Chem. Commun. 2002, 1795-1804.
(b) Pushpan, S. K.; Srinivasan, A.; Anand, V. G.; Venkatraman, S.;
Chandrashekar, T. K.; Joshi, B. S.; Roy, R.; Furuta, H. J. Am. Chem.
Soc. 2001, 123, 5138-5139.
(8) See Supporting Information.
JA029018V
9
J. AM. CHEM. SOC. VOL. 125, NO. 4, 2003 879