Porphyrin-LEGO: Synthesis of a hexafullereno-diporphyrin using porphyrins programmed for [4+2]-cycloaddition

Article abstract of DOI:10.1039/c2cc31218b

A hexafullereno-diporphyrinoid was obtained in a sequence of cycloaddition steps using porphyrins programmed for [4+2]-cycloaddition reactions and C ₆₀-fullerene. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc31218b

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COMMUNICATION
Porphyrin-LEGOs: synthesis of a hexafullereno-diporphyrin using
porphyrins programmed for [4+2]-cycloadditionwz
Srinivas Banala,ya Roland G. Huber,bc Thomas Mu
¨
and Bernhard Krautler*
¨
ller,ac Martin Fechtel,a Klaus R. Liedlbc
ac
Received 17th February 2012, Accepted 13th March 2012
DOI: 10.1039/c2cc31218b
A hexafullereno-diporphyrinoid was obtained in a sequence of
cycloaddition steps using porphyrins programmed for [4+2]-
cycloaddition reactions and C60-fullerene.
of 2 exhibited two maxima at 422 and 444 nm of intense Soret
bands, consistent with two de-conjugated and only weakly
interacting porphyrin chromophores (see ESIz, Fig. S1A).
Diporphyrinoid 2 was fluorescent and showed emission bands
at 680 and 740 nm (see ESIz, Fig. S1B). The presence of an intact
Porphyrins are a rich resource of metal-chelating macrocyclic
chromophore systems1–3 and have obtained special attention as
bioinspired catalysts,4 as artificial mimics for ‘‘photo-reaction
centers’’,5–7 and as structured building blocks in multi-porphyrin
arrays.8,9 We have designed b,b0-tetra-sulfoleno-porphyrin 1 as a
versatile, reactive building block for the construction of covalent
porphyrinoid assemblies via [4+2]-cycloaddition reactions.10
The Zn-complex Zn-1 allowed the direct assembly e.g. of
Zn-porphyrins carrying up to four fullerene units11 or of
‘blackened’ quinonoporphyrins.12 Here we describe a short
sequence of [4+2]-cycloaddition and cheletropic SO2-extrusion
reactions that allowed the synthesis of a hexafullereno-diporphyrin
in a molecular LEGOs approach (see Scheme 1).
The reactive porphyrin-b,b0-diene 3, on one hand, and the
quinono-porphyrin 412,13 and C60-fullerene,14 on the other, were
used as complementary building blocks in Diels–Alder reactions.
However, the synthesis of dimer 2 further depended upon
suppression of the efficient dimerization of porphyrin-b,b0-dienes
(such as 3) by [4+2]-cycloaddition to chlorin–porphyrin-type
spiro-dimers.15,16 In the event, the unwanted dimerization was
inhibited by effective immobilization of 3 that was generated by
thermolysis of 1 adsorbed onto silica gel.
1
2,3-dihydroquinone-bridge in 2 was deduced from the H- and
13C-NMR data of 2, among which multiplets at 2.76, 3.31 and
3.54 ppm (1H-NMR) were due to H-atoms of the saturated
carbon-centres at the two newly formed (C–C)-bonds.
Incorporation of two nickel-ions by 2 occurred readily at 85 1C
under air and using a standard protocol,17 and was accompanied
by oxidation, giving the quinone-bridged di-Ni-di-porphyrin Ni2-5
in 78% yield. The new conjugated chromophore system of the
non-fluorescent brown Ni2-5 gave a characteristically different
UV/Vis-spectrum with a group of three broad absorption bands
at 425, 451 and 482 nm and two absorption bands at 559 and
597 nm (see Fig. 1). The constitutional formula of Ni2-5 was
deduced from its FAB-MS and NMR-spectra. The former showed
a group of ions centred at m/z = 2932, as calculated for its
pseudo-molecular ion M+H+ (C174H197N8Ni2O14S6), and a
prominent cluster (as base peak) at m/z = 2549 of the fragment
C174H197N8O2Ni2, corresponding to loss of all six SO2 groups. The
NMR-spectra were consistent with a dimeric compound with four
symmetry equivalent sets of atoms and an effective D2h-symmetry.
The 1H-NMR signal at 8.08 ppm of the (four equivalent) aromatic
H-atoms of the anthraquinone-bridge coupled directly with a
signal at 124.6 ppm of aromatic carbons, as well as with three
further carbons (at 130, 142.9 and 180.4 ppm), establishing the
constitution of the anthraquinone moiety.
[4+2]-Cycloaddition of the dienophilic quinono-porphyrin
412,13 and the porphyrin-b,b0-diene 3 in solution provided the
2,3-dihydroquinone-bridged porphyrin-dimer 2 (isolated in about
16% yield, based on the conversion of 1). The UV/Vis-spectrum
Thermolysis of a deoxygenated solution of Ni2-5 and of an
about 80 fold excess of C60-fullerene in 1,2-dichlorobenzene at
140 1C for 23 h furnished a reaction mixture, from which the
non-polar green hexafullereno-di-Ni-di-porphyrin Ni2-6 was
isolated in about 80% yield after size-exclusion chromatography
(for details see ESIz). The UV/vis-spectrum of the hexafullereno-
diporphyrin Ni2-6 (in toluene) showed broad porphyrin bands,
three of which clustered near 470 nm, as well as two maxima at
570 and 617 nm, i.e. all bands were shifted by about 20 nm when
compared to those in the spectrum of Ni2-5 (see Fig. 1). In
addition a broad absorption band at about 325 nm was due to
the six fullerene addends. The MALDI-TOF mass spectrum of
Ni2-6 showed a pseudo-molecular ion as a cluster of weak signals
around m/z = 6873 (corresponding to C534H197N8Ni2O2),
a Institute of Organic Chemistry, University of Innsbruck, Innrain 80/82,
A-6020 Innsbruck, Austria. E-mail: bernhard.kraeutler@uibk.ac.at;
Fax: +43-512-507-57799; Tel: +43-512-507-57700
b Institute of General, Inorganic and Theoretical Chemistry,
University of Innsbruck, Innrain 80/82, A-6020 Innsbruck, Austria
c CMBI – Center for Molecular Biosciences Innsbruck, University of
Innsbruck, Innrain 80/82, A-6020 Innsbruck, Austria
w This article is part of the ChemComm ‘Porphyrins and Phthalocya-
nines’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
and computational procedures, analytical and spectroscopic data and
their detailed interpretation. See DOI: 10.1039/c2cc31218b
y Present address: Technische Universitat Berlin, Institut fur Chemie,
¨
¨
FG Organische Chemie, Strasse des 17. Juni 124/TC 2, D-10623
Berlin, Germany.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4359–4361 4359

Fig. 1 UV-Vis absorption spectra of Ni2-5 (4.78 ꢀ 10ꢁ6 M in
CH2Cl2), and of Ni2-6 (1.51 ꢀ 10ꢁ5 M in toluene, 1 mm cell).
diastereotopic protons at each of the six methylene groups
attached to the fullerene moieties (as AB-systems with geminal
splitting), indicating a non-planar diporphyrin core. Substitution
of sulfolene groups by fullerenes also affected the chemical shifts
of the directly bound atoms, as observed earlier in monomeric
fullereno-porphyrins.11,13 Furthermore, two sets of signals in the
1H-NMR-spectrum of Ni2-6 (in CS2/CD2Cl2, at 10 1C) were
observed in a ratio of about 2 : 1 (see ESIz, Fig. S3), suggesting
the existence of two structures of the di-Ni-di-porphyrin Ni2-6 in
a 2 : 1 ratio. The two sets of nearly completely assigned signals
(see ESIz, Tables S2 and S3) suggested the existence of two
conformers that both displayed effective mirror symmetry. Weak
cross peaks among the two sets of signals (NOE-analysis at
25 1C) indicated chemical exchange from slow (conformational)
inter-conversion at this temperature. The H-atoms of individual
di-tert-butyl-phenyl groups showed strong intra-group NOEs,
allowing for their assignment to a particular aromatic moiety (see
ESIz, Fig. S7). NOE-correlations of these H-atoms with the
methylene protons provided a network of further interactions
and the individual assignment of most signals to the two
suggested conformers. The slow inter-conversion of the two
hypothetical conformers helped to correlate the signals further,
supporting the deduced basic geometric and dynamic properties.
The highly fullerene-loaded di-porphyrin Ni2-6 was thus
indicated to contain two non-planar porphyrin moieties, the
mutual attachment of which appeared to generate two (slowly
inter-converting) conformers.
Scheme 1 Outline of the synthesis of the hexafullereno-diporphyrin
Ni2-6 using porphyrin 1 as starting component in a porphyrin LEGOs
approach (Ar = 3,5-di-tert-butyl-phenyl).
a base-peak centered at m/z = 2548 (C174H196N8O2Ni2, from
loss of six C60 units), and a series of meta-stable ions from loss
of one to five C60 moieties (see ESIz, Fig. S4 and S5).
The 1H-NMR spectra of Ni2-6 were considerably more
complex than those of Ni2-5. They showed the signals of
c
4360 Chem. Commun., 2012, 48, 4359–4361
This journal is The Royal Society of Chemistry 2012

Covalent assemblies of porphyrin(oid)s and C60 are excellent core
units of artificial photosystems.20,21 As delineated here and else-
where,13,22 the [4+2]-methodology and suitably ‘programmed’
porphyrinoid systems may provide versatile routes to various
covalent arrays of porphyrinoids and redox- and photo-active units.
The hexafullereno-diporphyrinoid Ni2-6 may represent a first
example of a transition metal complex of a regularly structured
multi-modular molecular assembly of two porphyrins and six
fullerenes. Ni2-6 represents a reversibly chargeable reservoir of a
large number of electrons. Ni2-6 also features an anthraquinone
moiety as a link between its two porphyrinoid moieties. Redox-
control, as shown elsewhere with related bridges,23,24 is expected to
modify the conjugation, and thus the electronic communication
between the two porphyrin moieties. Photo-physical and electro-
chemical investigations, as well as further structural and theoretical
studies are planned or are underway, to explore the properties of
Ni2-6 and of related complexes.
We thank Paul Sintic for exploratory studies, and are
grateful to the Austrian Science Foundation (FWF, project
P-17437) and to the Tyrolean Science Foundation (TWF, UNI
404/22) for financial support (B.K.).
Notes and references
1 The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and
R. Guilard, Academic Press, San Diego, 2000, vol. 1–10.
2 The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and
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3 Handbook of Porphyrin Science, ed. K. M. Kadish, K. M. Smith
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Fig. 2 Calculated structural models of two conformers of the fullerene-
loaded di-porphyrin Ni2-6. (top) A flexible C2-symmetric ‘C-shaped’
syn-conformer. (bottom) A C2h-symmetric ‘S-shaped’ anti-conformer.
Structural models of Ni2-6 were calculated starting from a crystal
structure of the (non-planar) trisfullerenoporphyrin 718 and using
the OPLS-AA force field19 as implemented in the MOE 2011.10
modelling package (see ESIz for details). The calculated models
suggest the existence of flexible C2-symmetric ‘C-shaped’ syn- and
C2h-symmetric ‘S-shaped’ anti-conformers (as displayed in Fig. 2)
of the fullerene-loaded di-porphyrin Ni2-6, helping to rationalize the
NMR-spectroscopic data. Indeed, the chemical shifts of corres-
ponding H-atoms in the two isomeric forms of Ni2-6 had similar
patterns (see ESIz, Tables S2 and S3). The shift differences (between
1H-NMR-spectra of Ni2-5 and Ni2-6) would be due to conforma-
tional differences of the porphyrinoid skeleton and its attached 3,5-
di-tert-butyl-phenyl moieties, and the associated changes of the
mutual positions of closely situated ‘inner’ atoms and groups.
The regular structure and robustness of porphyrins, as well as
their redox- and photo-activities, have generated interest in their
use as molecular components for the purpose of the bottom-up
assembly of porphyrin-based nanoscale objects.5–9 Some of this
work was inspired by modular construction kits, e.g. by the
popular LEGOs toys. Osuka and coworkers reported an organo-
metallic strategy to stitch together remarkable covalent assemblies
of up to eight porphyrin moieties from porphyrin ‘Lego blocks’.9
The porphyrin LEGOs approach described here used the capacity
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from the ‘programmed’ porphyrin precursor 1) to form covalent
assemblies by the (thermally reversible) Diels–Alder reactions. It
gave, so far, access to Ni2-6, a regularly structured, specific covalent
assembly of a diporphyrin core and of six appended fullerene units.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4359–4361 4361