Dithiadiazuliporphyrin: Facile generation of carbaporphyrinoid cation radical and dication

Article abstract of DOI:10.1021/ja053723x

Dithiadiazuliporphyrin is a nonaromatic porphyrinoid, readily and reversibly oxidizable to its cation radical and to the unprecedented aromatic carbaporphyrinoid dication, which can be viewed as a 21,23-dicarba-22,24-dithiaporphyrin with two fused tropyli

Full text of DOI:10.1021/ja053723x

Published on Web 09/03/2005
Dithiadiazuliporphyrin: Facile Generation of Carbaporphyrinoid Cation
Radical and Dication
Natasza Sprutta, Marta SÄ widerska, and Lechosław Latos-Graz˘ yn´ski*
Department of Chemistry, UniVersity of Wrocław, 14 F. Joliot-Curie Street, Wrocław 50 383, Poland
Received June 7, 2005; E-mail: llg@wchuwr.chem.uni.wroc.pl
Carbaporphyrinoids are porphyrin analogues that possess at least
one CH unit replacing a pyrrolic nitrogen in the coordination core.
This “internal” carbon atom normally belongs to a carbo- or
heterocyclic ring substituting one of the pyrroles.1 Introduction of
an azulene moiety to the porphyrinoid framework is of particular
interest because of the unusual electronic properties of this bicyclic
system. Exploration of this concept led to the synthesis of
azuliporphyrin and its heteroanalogues, which exhibit borderline
macrocyclic aromaticity and unusual reactivity pathways.2,3
Figure 1. Molecular structure of 1. Hydrogen atoms are omitted for clarity.
Fragments pointing away from the viewer are shown in gray.
Here we report on a high-yielding synthesis of hybrid thiophene-
azulene macrocycles and their redox chemistry. Combination of
the azulene π system with the heteroporphyrin-like macrocyclic
skeleton results in a three-state redox switchable chromophore with
a potential for electrochromic (including a near-IR region)4 or
molecular conductivity applications.5 The molecular framework
presented herein is potentially amenable to diverse structural
modifications (substitutions of meso-aryls or azulene, a choice of
heteroatom).
Scheme 1. Synthesis of Porphyrinogen 1 (5,10,15,20-tetra-p-tolyl-
22,24-dithiadiazuliporphyrinogen)
Porphyrinogen 1 is obtained in a modification of the synthesis
described for tetraaryl-21,23-dithiaporphyrins.6 This method relies
on the known suitability of azulene as a substrate for Rothemund-
type condensation (Scheme 1).2,7
Compound 1 is formed as a mixture of stereoisomers in
remarkable 95% yield. One of the isomers, possessing effective
C2h symmetry, has been characterized by X-ray crystallography
(Figure 1). 1 adopts a chair-like conformation in the solid state.
The thiophene rings are coplanar, whereas the dihedral angle
between each of the azulene moieties and the C4 plane (defined by
the four meso carbons) equals 71°. Bond lengths in the azulene
and thiophene fragments of 1 are similar to those in isolated
molecules.
Scheme 2. Formation of 2 and Its Oxidized Forms 2•+ and 22+
Oxidation of 1 with varying amounts of DDQ (2,3-dichloro-5,6-
dicyano-p-benzoquinone) leads to mixtures of the porphyrin
analogue 2, its radical cation 2•+, and dication 22+. These three
species constitute a multielectron redox system with no precedence
in carbaporphyrinoid chemistry. Tetrathiaporphyrin and its oxa-
congener are the only other porphyrin analogues for which two
different oxidation states (isophlorin and dication) could be isolated.8
X-ray quality crystals of 2 were obtained directly from the reaction
mixture. In the solid state, 2 adopts a saddle conformation, with
the azulene and thiophene rings tilted in opposite directions (Figure
2). There is an appreciable effect of the conjugation on the thiophene
fragment. The bond distances within the thiophene ring are altered
in accord with the valence bond structure of 2. Thus the CR-Câ
bond lengths (1.43-1.45 Å) are longer than the Câ-Câ distances
(1.35 Å), whereas the reverse is true for porphyrinogen 1. Similarly,
the Cmeso-CR(thiophene) bonds (1.37-1.38 Å) reflect some double-
bond character, while the Cmeso-CR(azulene) distances (1.47-1.49
Å) approach the single-bond limit for C(sp2)-C(sp2). The geometry
of azulene moieties remains largely unaltered, suggesting that they
are not conjugated with the macrocycle.
The redox system comprising 2, 2•+, and 22+ was further
investigated using chemical and physical methods. Addition of an
excess of DDQ leads to exclusive formation of the dication 22+. It
is quantitatively reduced to 2 by SnCl2 in THF. Systematic titration
of 2 with Br2 followed by UV-vis spectroscopy (Figure 3) reveals
the presence of an intermediate, assigned as 2•+ on the basis of the
ESR data. The solution of 2•+ (dichloromethane/1% THF, 298 K)
exhibits a single line at g ) 2.0031, consistent with the cation
radical electronic structure. During oxidation, the color of the
solution changes from dark red (2), through purple (2•+), to navy
blue (22+).
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C O M M U N I C A T I O N S
polar solvents, such as acetonitrile (Figure 4, trace A), whereas
in chloroform the spectrum is very broad (trace B). Conversely,
poor solubility of the charged forms 2•+ and 22+ in toluene
results in the desired sharpening of the spectrum of 2 in this solvent
(trace C).
1H chemical shifts observed for 22+, in particular the peculiar
upfield position of the inner 21,23-H protons (1.48 ppm), show
that a diatropic ring current is present in the macrocycle. In fact,
22+ can be viewed as a 21,23-dicarba-22,24-dithiaporphyrin with
two fused tropylium rings. In the spectrum of 2, the 21,23-H
resonance is located at 7.02 ppm, confirming the absence of
macrocyclic aromaticity.
In conclusion, the newly designed dithiadiazuliporphyrin 2 is
the first example of a non-nitrogenous carbaporphyrinoid in general
which is easily oxidizable to its cation radical and dication. Thanks
to their hybrid structure, the new systems can be linked into
multielement conductive arrays using technologies developed for
both azulenes10 and porphyrins.11
Figure 2. Molecular structure of 2. Solvent molecules and hydrogen atoms
are omitted for clarity. Fragments pointing away from the viewer are shown
in gray. In the side view, tolyl groups are omitted.
Acknowledgment. Financial support from the Ministry of
Scientific Research and Information Technology (Grant 3 T09A
162 28) is kindly acknowledged.
Supporting Information Available: Synthetic procedures, UV-
vis and NMR data, and crystallographic data for 1 and 2. This material
is available free of charge via the Internet at http://pubs.acs.org.
Figure 3. UV-vis absorption spectra (CH2Cl2, 298 K) of 2 (solid line),
2•+ (dashed line), and 22+ (dotted line). The spectra were obtained by
titrating 2 with Br2. The inset shows a cyclic voltammogram of 2 (CH2Cl2
solution, supporting electrolyte TBAP; working electrode, glassy carbon
disk; reference electrode, Ag/AgCl; E1/2 for ferrocene +553 mV).
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1
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