Dithiaethyneazuliporphyrin - A contracted heterocarbaporphyrin

Article abstract of DOI:10.1039/b605984h

Dithiaethyneazuliporphyrin, the first contracted carbaporphyrinoid, has been synthesized; the molecule contains an azulene moiety embedded in the [18]dithiacarbatriphyrin(4.1.1) macrocyclic framework. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b605984h

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Dithiaethyneazuliporphyrin - a contracted heterocarbaporphyrin{
/
Anna Berlicka, Natasza Sprutta and Lechoslaw Latos-Graz˙ynski*
´
Received (in Austin, TX, USA) 27th April 2006, Accepted 9th June 2006
First published as an Advance Article on the web 4th July 2006
DOI: 10.1039/b605984h
Dithiaethyneazuliporphyrin, the first contracted carbaporphyr-
inoid, has been synthesized; the molecule contains an azulene
moiety embedded in the [18]dithiacarbatriphyrin(4.1.1) macro-
cyclic framework.
Carbaporphyrinoids provide a unique macrocyclic platform which
is suitable to explore organometallic chemistry in a peculiar
porphyrin-like environment forcing unusual coordination geome-
try and/or oxidation states of metal ions.1 Typically an internal
carbon donor atom belongs to a carbo- or heterocycle while the
(CNNN) core becomes the denominator of the monocarbapor-
phyrinoid structure. A subsequent replacement of one of the
pyrrolic nitrogens by
a heteroatom (O, S, or Se) yields
Scheme 1 Dithiaethyneporphyrin 1 and dithiaethyneazuliporphyrin 2.
heterocarbaporphyrinoids.2 Enlargement of the macrocycle
accompanied by introduction of a carbon atom in place of a
nitrogen afforded expanded carbaporphyrinoids.3 On the other
hand, to the best of our knowledge, no contracted carbaporphyr-
inoids have been reported to date.
Dithiaethyneporphyrin 1, reported recently, introduces a unique
pattern for contracted porphyrins created by fusing the structural
motifs of 21,23-dithiaporphyrin and acetylene.4 Actually, the
molecule can be considered as a triphyrin – a contracted porphyrin
analogue. Such systems have recently gained some attention.4,5
The synthetic route elaborated for dithiaethyneporphyrin 14 opens
access to structure 2, an unprecedented contracted heterocarba-
porphyrinoid containing the (SCS) core, confined in the rather
atypical frame of [18]triphyrin(4.1.1).
Here we report on the synthesis and characterization of the first
contracted carbaporphyrinoid – dithiaethyneazuliporphyrin 2
(Scheme 1). The coordinating ability of 2 is exemplified by the
formation of its ruthenium(II) complex (4).
Scheme 2 Synthesis of dithiaethyneazuliporphyrin: (a) Et2O:BF3, RT,
1 h; (b) Et3N, DDQ, RT, 20 min.
(Fig. 2A). The azulene moiety contributes the inner CH singlet at
5.57 ppm, upfield relative to the position of azulene (7.91 ppm)
or tetra(para-tolyl)dithiadiazuliporphyrinogen (7.72 ppm)6 but
The contracted heterocarbaporphyrin 2 was obtained by a
modification of the synthesis described for 1. The synthetic
strategy (Scheme 2) resembles the [3 + 1] approach using 1,4-bis(5-
(phenylhydroxymethyl)thien-2-yl)-1,4-diphenyl-2-butyne 34 and
azulene. This method relies on the known suitability of azulene
as a substrate for Rothemund-type condensations (Scheme 2).6 2
has been isolated in 15% yield.
The electronic spectrum of 2 (Fig. 1) contains broad absorption
bands with relatively small extinction coefficients, suggesting a
reduction in the aromatic character, which remains in contrast
with the parent system 14 and dithiaporphyrin S2TPP.7
The 1H NMR spectrum of 2 shows an AB pattern (8.00,
7.91 ppm) assigned to the b protons of the thiophene rings
/
Department of Chemistry, University of Wroclaw, F. Joliot-Curie Street
/
14, Wroclaw 50 383, Poland. E-mail: llg@wchuwr.chem.uni.wroc.pl;
Fax: +48 71 32-823-48; Tel: +48 71 37-572-56
{ Electronic supplementary information (ESI) available: Experimental
details and spectral data of 2 and 4. See DOI: 10.1039/b605984h
Fig. 1 Absorption spectra of 2 (solid line) and 4 (dashed line) in CHCl3.
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Scheme 4 Coordination geometry of 4.
spectrum in comparison to 2 (Fig. 1). Dithiaethyneazuliporphyrin
acts as a monoanionic ligand forming the Ru–C(20) s-bond.
1
Accordingly, the unique H(20) resonance seen in the H NMR
Fig. 2 1H NMR spectra of 2 (A; benzene-d6, 298 K) and 4 (B; CD2Cl2,
220 K). Peak labels follow systematic position numbering or denote proton
groups: o-, m-, p-Ph – ortho, meta and para positions of the meso-phenyls.
spectrum of 2 is missing in the spectrum of 4.
In contrast to ruthenium(II) 21,23-dithiaporphyrin10
(S2TPP)RuIICl2 and ruthenium(II) dithiaethyneporphyrin4
(S2ETPP)RuII(CO)2Cl, molecule 4 preserves the Cs symmetry
with a mirror plane passing through the ruthenium, chloride and
coordinated azulene carbon as reflected by a single AB pattern
assigned to the b protons of two thiophene rings (Fig. 2B). The
downfield with respect to meso- (3.35 ppm) or b-substituted
monoazuliporphyrins (1.59 ppm).6 One can consider the 1H NMR
shifts of the internally located H(20) proton and the peripheral
thiophene resonances as a convenient spectroscopic criterion of
macrocyclic aromaticity. Thus the molecule 2 is only weakly
aromatic, reflecting the limited contribution of the 18 p-electron
delocalization pathway resulting from the input of the dipolar
canonical structures 29 and 20.
1
geometry of 4, inferred from the H NMR spectroscopic pattern
(Fig. 2B), reflects the balance between the constraints of the
macrocyclic ligand and the requirements of ruthenium(II) for
octahedral geometry (Scheme 4). The dithiaethyneazuliporphyrin
molecule 2 has to distort to accommodate the Ru(CO)2Cl moiety.
The puckering of dithiaethyneazuliporphyrin and the difference in
coordination above and below the porphyrin plane lower the
symmetry with respect to the carbaporphyrinoid plane.
Accordingly, two ortho and two meta resonances have been
identified for one set of meso aryls (8,13-Ph) at 220 K.
Two canonical structures of 29 and 20 define 18 p-electron
macrocyclic delocalization pathways (Scheme 3). Accordingly, the
electronic structure of 2 can be described as reflecting a
combination of the acetylene (LC–CMC–CL) and cumulene
(–CLCLCLC–) character of the C18–C1–C2–C3 fragment.
The 13C NMR chemical shift of carbon atoms C(1) and C(2) of
the linker at 2 equals 106.1 ppm, upfield in relation to 1 (116.8
ppm).4 Thus the acetylene character of the CspCsp moiety of 2
prevails. The detected chemical shifts of 2 are fairly typical
for acetylene-cumulene porphyrinoids8 and acetylene-cumulene
dehydroannulenes.9
In conclusion, dithiaethyneazuliporphyrin 2, the original con-
tracted carbaporphyrinoid, is a peculiar molecule combining three
structural motifs: acetylene, azulene and thiophene moieties in a
[18]triphyrin(4.1.1) frame.
We thank the Ministry of Scientific Research and Information
Technology (Grant PBZ-KBN-118/T09/5) for financial support.
Reaction of 2 with Ru3(CO)12 in chlorobenzene under reflux
results in the formation of ruthenium(II) dithiaethyneazulipor-
phyrin (S2EATPP)RuII(CO)2Cl (4). The relatively small yield
reflects the inherent instability of the maternal macrocycle,
which decomposes in the insertion conditions. Coordination of
ruthenium is reflected in the substantial changes of the electronic
Notes and references
´
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Scheme 3 Canonical structures of 2.
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3348 | Chem. Commun., 2006, 3346–3348
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