Porphyrins acting as external and internal ligands: Preparation of conjugated trimetallic dimeri porphyrins

Article abstract of DOI:10.1039/b008740h

A porphyrin bearing a peripheral enaminoketone function allows the preparation of conjugated dimeric porphyrins linked through metal ions.

Full text of DOI:10.1039/b008740h

Porphyrins acting as external and internal ligands: preparation of conjugated
trimetallic dimeric porphyrins
Sébastien Richeter, Christophe Jeandon, Romain Ruppert* and Henry J. Callot*
Faculté de Chimie, 1 rue Blaise Pascal, 67008 Strasbourg, France. E-mail: callot@chimie.u-strasbg.fr
Received (in Cambridge, UK) 30th October 2000, Accepted 20th November 2000
First published as an Advance Article on the web
A porphyrin bearing a peripheral enaminoketone function
allows the preparation of conjugated dimeric porphyrins
linked through metal ions.
composition H2N–X (X = OH, NHTs, OSO3H) gave in
variable yield enaminoketone 2, the highest yield (90%) being
observed with hydroxylamine O-sulfonic acid–AcOH–NaOAc
in refluxing CH2Cl2. Enaminoketone 2a showed a UV–VIS
similar to the starting material (lmax = 451, 568, 614 and 709
nm). The b-N–H NMR signals could be detected at d 9.0 (N–H
involved in a hydrogen bond with the carbonyl group) and d
5.48 (free N–H, shielded by a vicinal phenyl ring).
Enaminoketone 2 presents remarkable coordinating proper-
ties: in addition to the internal 4N tetradentate macrocyclic
coordination site, the enaminoketone group is a potential N+O
chelating external site (an analog of the acetylacetonate ligand)
that might couple porphyrins to form homo- or hetero-
polymetallic complexes. One would predict that the coordina-
tion at the enaminoketone site should be favoured over the
introduction of metal ions in the core of the porphyrin, but one
also expects the stability of the internal complex to be of orders
of magnitude higher for the same metal ion; in particular with
first row transition metals the porphyrin–metal–porphyrin
connection is expected to be of low stability. Accordingly,
heterometallation may be performed along two routes by using
as an intermediate either an internal complex or the porphyrin
base dimer (Scheme 1).
Metallation of 2a (R = Ph) with nickel acetylacetonate (one
or more equiv.) rapidly gave a highly insoluble product
containing one metal and two porphyrins formulated as 3a (M1
= Ni; M2 = M3 = H2). Treatment with acetic acid, which is
unable to remove nickel from the 4N site of a porphyrin,
quantitatively regenerated the starting material. However, on
prolonged reaction in refluxing toluene with a 1:1 porphyrin/
nickel ratio, metallation of the porphyrin occurred and gave
mononuclear Ni–2a, while excess reagent gave trinuclear 3a
(M1 = M2 = M3 = Ni). The formation of Ni–2a is best
explained by the low stability of the nickel bridge—it did not
survive chromatographic conditions—and the slow formation
of the most stable metallation product. The same trinuclear
compound 3a (M1 = M2 = M3 = Ni) was formed when Ni–2a
was treated with an excess of nickel acetylacetonate. Similarly,
several trimetallic bisporphyrins were prepared (Ni/Cu/Ni; Ni/
VNO/Ni; Ni/Pd/Ni).
Multiporphyrin assemblies are involved in several biological
processes such as photosynthesis or electron transfer. Mimick-
ing those assemblies and their activity has been a challenge for
chemists and numerous examples of multiporphyrin com-
pounds have been described in recent years.1–22 The connection
between porphyrin nuclei may involve C–C bonds, hydrogen
bonds, direct metal–metal or m-X bonds, catenation of porphy-
rin ansa-derivatives, coordination of appropriate peripheral
substituents to metal centers, etc. More recently, after the
molecular wires described earlier by Crossley and Burn,8
several groups are pursuing efforts to obtain conjugated
oligoporphyrinic molecules directly linked through one, two or
even three covalent bonds.10–13 This covalent approach suffers
some drawbacks: synthetic and separation problems, iterative
reactions to obtain larger molecules, variations of the linking
bonds limited to C–C or C–N.
The use of coordination bonds15–21 has the advantage of
offering an additional choice: each metal center may present its
own geometry and coordination number depending on the
element and its oxidation level, in addition to the variations due
to the nature of the internal coordination sites of the porphyr-
ins.
Here we describe a substituted porphyrin acting both as an
internal (with the four pyrrole nitrogens) and as an external
(with an enaminoketone moiety) ligand. The external coordina-
tion site is coplanar and fully conjugated to the porphyrin
aromatic system.22 In contrast to most systems assembled with
metal ions, where little or no interaction is observed in the
ground state between the porphyrins, the porphyrin–porphyrin
conjugation in our dimers linked by metal ions will be
illustrated by their electronic spectra and electrochemical
behaviour.
meso-Tetraarylporphyrins bearing a keto group located on an
additional ring attached to an aryl substituent and a pyrrolic b-
carbon have been known since 1980 but their reactivity has been
little investigated.23–26 In the course of a search for extended
There remained the question of the relative arrangement of
the porphyrins around the metal, since the dimers may have a
center or a plane of symmetry. COSY and NOESY experiments
Scheme 1 Metallation of enaminoketone 2 (squares): vertical arrows =
metallation of internal site of porphyrin, slow and irreversible under the
reaction conditions; horizontal arrows = metallation of external site (fast
and possibly reversible reactions).
porphyrin chromophores we reacted ketone 1 with nitrogen
nucleophiles. Under mild acid catalysis, reagents of the general
DOI: 10.1039/b008740h
Chem. Commun., 2001, 91–92
This journal is © The Royal Society of Chemistry 2001
91

removal of the first two electrons from the dimer has led to a
disruption of the conjugation between the two rings. The dimer
(already oxidised by two electrons) now behaves like two
independant nickel monomers. All waves are quasi-reversible
(DEp = 60–90 mV), showing that dimer 3a (M1–3 = Ni)
remains intact during this overall four-electron oxidation–
reduction process.
In conclusion, we confirmed that efficient porphyrin–
porphyrin interaction could be achieved via metal coordination.
The synthesis of bis-enaminoketoporphyrins where two ex-
ternal coordination sites are located in divergent directions is in
progress. These compounds should allow the preparation of
more highly conjugated oligomers.
We thank R. Graff for the NMR experiments.
confirmed the structure and the dimeric nature of 3. Additional
information was obtained when we turned to the more soluble
meso-tri(3,5-di-tert-butyl)phenylporphyrin series. Heating
equimolecular amounts of palladium(II) acetate and enaminoke-
tone 2b in refluxing toluene for 3 h led almost exclusively to 3b
(M1 = Pd; M2 = M3 = H2). Again, NMR experiments
(ROESY) confirmed the relative position of the porphyrins: HH
ROESY cross signals were observed between two protons of the
cyclised phenyl ring and the para proton and the tert-butyl
protons of a meso-aryl group belonging to the other porphyrin.
This compound survived chromatographic purification and is a
good starting material for heterometallic dimer preparation.
Indeed, metallation with zinc acetate gave the mono- and bis-
metallated 3b (M1 = Pd; M2 = Zn; M3 = H2 or Zn).
Notes and references
1 J.-C. Chambron, V. Heitz and J.-P. Sauvage, in The Porphyrin
Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic
Press, San Diego, 2000, vol. 6, p. 1; J. K. M. Sanders, vol. 3, p. 347; S.
Fukuzumi, vol. 8, p. 115; D. Gust and T. A. Moore, vol. 8, p. 153; J.-M.
Barbe and R. Guilard, vol. 3, p. 211; J.-H. Chou, M. E. Kosal, H. S.
Nalwa, N. A. Rakow and K. S. Suslick, vol. 6, p. 43.
2 A. Osuka, N. Mataga and T. Okada, Pure Appl. Chem., 1997, 69,
797.
3 F. Li, S. I. Yang, Y. Ciringh, J. Seth, C. H. Martin III, D. L. Singh, D.
Kim, R. R. Birge, D. F. Bocian, D. Holten and J. S. Lindsey, J. Am.
Chem. Soc., 1998, 120, 10 001.
4 A. K. Burrell and D. L. Officer, Synlett, 1998, 1297.
5 D. P. Arnold, Synlett, 1999, 296.
6 M. G. H. Vicente, L. Jaquinod and K. M. Smith, Chem. Commun., 1999,
1771.
7 H. L. Anderson, Chem. Commun., 1999, 2323.
8 M. J. Crossley and P. L. Burn, J. Chem. Soc., Chem. Commun., 1987, 39;
M. J. Crossley and P. L. Burn, J. Chem. Soc., Chem. Commun., 1991,
1569.
9 W. J. Belcher, A. K. Burrell, W. M. Campbell, D. L. Officer, D. C. W.
Reid and K. Y. Wild, Tetrahedron, 1999, 55, 2401.
10 L. Jaquinod, O. Siri, R. G. Khoury and K. M. Smith, Chem. Commun.,
1998, 1261.
11 R. G. Khoury, L. Jaquinod and K. M. Smith, Chem. Commun., 1997,
1057; N. Aratani, A. Osuka, Y. H. Kim, D. H. Jeong and D. Kim,
Angew. Chem., Int. Ed. Engl., 2000, 39, 1458.
12 A. Tsuda, A. Nakano, H. Furuta, H. Yamochi and A. Osuka, Angew.
Chem., Int. Ed. Engl., 2000, 39, 558; K.-i. Sugiura, T. Matsumoto, S.
Ohkouchi, Y. Naitoh, T. Kawai, Y. Takai, K. Ushiroda and Y. Sakata,
Chem. Commun., 1999, 1957; I. M. Blake, L. H. Rees, T. D. W. Claridge
and H. L. Anderson, Angew. Chem., Int. Ed. Engl., 2000, 39, 1818.
13 A. Tsuda, H. Furuta and A. Osuka, Angew. Chem., Int. Ed., 2000, 39,
2549.
14 R. Beavington and P. L. Burn, J. Chem. Soc., Perkin Trans. 1, 2000,
605.
15 T. Imamura and K. Fukushima, Coord. Chem. Rev., 2000, 198, 133.
16 J. Wojaczynski and L. Latos-Grazynski, Coord. Chem. Rev., 2000, 204,
113.
These new porphyrin dimers all showed similar electronic
spectra and an electrochemical behaviour indicative of the
conjugation introduced through the connecting metal between
the two aromatic rings. For example, the lowest energy band of
3a (M1–3 = Ni) shifts to 700 nm (e = 33000 dm3 mol21 cm21
)
in comparison to the band observed at 649 nm (e = 18500 dm3
mol21 cm21) for Ni–2a (Fig. 1). The HOMO–LUMO gap
estimated from these data is reduced to 1.77 eV in the dimer 3a
(M1–3 = Ni) compared to 1.91 eV for the monomer Ni–2a
(already a very low value compared to the gap reported for
nickel porphyrins, E ≈ 2.3 eV). The corresponding bases such
as 3b (M1 = Pd; M2 = M3 = H2) display bands at even longer
wavelengths (730 nm, e = 39000 dm3 mol21 cm21), which can
be compared with that of 2b (711 nm, e = 8500 dm3 mol21
cm21).
The monomer Ni–2a showed two one-electron oxidations at
0.46 and 0.83 V vs. Fc/Fc+. In trimetallic 3a (M1–3 = Ni), the
first oxidation is split into two one-electron oxidation waves
(one electron per porphyrin) at 0.32 and 0.50 V, indicative of a
strong interaction between the two porphyrins. The removal of
the first electron therefore occurs at a potential 0.14 V lower
than for the monomer (a value equal to the difference between
the gaps taken from the electronic spectra). A two-electron
oxidation (or rather twice a one-electron oxidation of each
porphyrin ring) is further observed at 0.68 V. It seems that the
17 A. Harriman, F. Odobel and J.-P. Sauvage, J. Am. Chem. Soc., 1995,
117, 9461.
18 M. J. Crossley, P. L. Burn, S. J. Langford and J. K. Prashar, J. Chem.
Soc., Chem. Commun., 1995, 1921.
19 T. A. Vannelli and T. B. Karpishin, Inorg. Chem., 1999, 38, 2246.
20 I. M. Dixon, J.-P. Collin, J.-P. Sauvage, F. Barigelletti and L. Flamigni,
Angew. Chem., Int. Ed., 2000, 39, 1292.
21 Y. Diskin-Posner, S. Dahal and I. Goldberg, Angew. Chem., Int. Ed.,
2000, 39, 1288.
22 A porphyrazine allowing the coordination of metal centers in the
macrocyclic plane was recently described: N. Bellec, A. G. Montalban,
D. B. G. Williams, A. S. Cook, M. E. Anderson, X. Feng, A. G. M.
Barrett and B. M. Hoffman, J. Org. Chem., 2000, 65, 1774.
23 K. Henrick, P. G. Owston, R. Peters, P. A. Tasker and A. Dell, Inorg.
Chim. Acta, 1980, 45, 161.
24 H. J. Callot, E. Schaeffer, R. Cromer and F. Metz, Tetrahedron, 1990,
46, 5253.
25 L. Barloy, D. Dolphin, D. Dupré and T. P. Wijesekera, J. Org. Chem.,
1994, 59, 7976.
26 Y. V. Ishkov and Z. I. Zhilina, Zh. Org. Khim., 1995, 31, 136.
Fig. 1 UV–VIS spectrum of 3a (M1–3 = Ni) and Ni–2a.
92
Chem. Commun., 2001, 91–92