Olivier
Siri
and
Pierre
Braunstein
*
Laboratoire de Chimie de Coordination, UMR CNRS 7513, Université Louis Pasteur, 4 rue Blaise Pascal, F-67070, Strasbourg Cédex, France. E-mail: braunst@chimie.u-strasbg.fr
First published on 7th January 2002
The first 12π-electron zwitterionic structure in quinonoid chemistry is described with the N,N,O,O -molecule 6 in which the positive charge is π-delocalized between the nitrogen atoms and the negative charge between the oxygen atoms; depending on the crystallization solvent, a 1D-tape-like H-bonded network can be generated in the solid-state.
Although compound 1 is known to be readily oxidized in 2,4 smooth reaction of 1·4HCl with RC(O)Cl (R = t-Bu) in wet CH3CN and excess NEt3 did not afford the p-benzoquinone derivative 3. Instead, the new compound 4 was obtained (25% yield) together with known 5 (72% yield) which results from direct N-substitution of 15 (Scheme 1).
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Scheme 1 (i): t-BuC(O)Cl (4 equiv.)/MeCN/H2O; (ii): LiAlH4/THF/[Ar] followed by CH2Cl2 extraction/[Air]. |
The 1H and 13C NMR data of 4 (see ESI)† were consistent with the presence in solution of either a mirror plane or an inversion center, symmetry elements compatible with isomeric structures in which the two oxygen functions would be in mutual meta or para position, respectively. An X-ray diffraction study established the mutual meta relationship between the N- and O-substituents,6 and the down,up,up,up-type orientation of the carbonyl functions which implies a lower symmetry than in solution. We are currently investigating the reaction pathways leading to 4.
Reduction of 4 with LiAlH4, followed by work-up in air, afforded 6 as a purple solid in 70% yield. An X-ray diffraction study on single crystals of 6·2H2O, obtained from acetone/H2O, showed a planar molecule, except for the t-Bu groups which are both on the same side of the plane (Fig. 1).6 The C(2)–C(3) distance of 1.517(5) Å corresponds to a single bond and indicates the lack of conjugation between the two halves of the ligand. In contrast, the other C–C distances within the ring, the C–O and C(3)–N(1) and C(5)–N(2) bond distances show a remarkable bond equalization. Delocalization of the 12π-electron system appears therefore confined within two independent subunits of the molecule: the negative charge being delocalized between the oxygen atoms and the positive charge between the nitrogen atoms. Formation of 6 could be explained by reduction of the amido and ester groups of 4 leading to an intermediate A (not isolated but related to known molecules7) which would be readily oxidized by air during work-up to give B (Scheme 1). Proton migration from oxygen onto the more basic nitrogen atoms then results in the zwitterionic benzoquinonemonoimine structure 6. In contrast to the UV–VIS. absorption spectrum of substituted benzenes like 4 and 5, that of 6 in CH2Cl2 is characterized by two strong absorptions at 350 nm (log ε = 4.49) and 343 nm (log ε = 4.45) which correspond to the intraquinone charge transfers.8 The situation in 6 represents a novel electronic pattern for N,N,O,O-systems since previously known quadrupol-merocyanine-type structures 7 and 8 display electronic delocalization exclusively between nitrogen and oxygen atoms with no occurrence of a zwitterionic structure.2
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Fig. 1 Top (A) and side (B) views of the structure of 6·2H2O in the crystal. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and angles (°): C1–C2 1.379(5), C1–C6 1.399(5), C2–O1 1.265(4), C2–C3 1.516(5), C3–C4 1.393(5), C3–N1 1.323(5), C4–C5 1.389(5), C5–C6 1.523(5), C5–N2 1.320(5), C6–O2 1.254(4); C2–C1–C6 122.0(4), C1–C2–C3 118.7(3), C3–C2–O1 115.5(4), C5–C6–O2 116.2(3), C2–C3–N1 113.4(3), C6–C5–N2 113.5(3). |
In the latter cases, theoretical studies have been performed in relation to the coupling effects which occur in polymethine dyes.9 The molecular symmetry deduced from the 1H and 13C NMR spectra (see ESI†) is clearly explained by the zwitterionic structure of 6·2H2O and not by a tautomeric equilibrium as suggested in related systems.3,10 In the solid state, intermolecular interactions develop between water molecules of solvation and O(1), O(2) and the N(2)H proton. In contrast, crystals of 6 grown from an aprotic solvent such as chlorobenzene showed a ‘tête-bêche’ arrangement of the zwitterion,6 leading to an H-bonded tape-like supramolecular array (Fig. 2). The six-membered rings of two consecutive molecules are almost orthogonal to each other. A change in the orientation of the t-Bu groups is observed, which are now situated on opposite sides with respect to the molecular plane. Bond distances in 6 are very similar to those in 6·2H2O.
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Fig. 2 Views of the supramolecular array generated by 6 in the solid-state: (A) view of the tape running in the projection plane; (B) side view with the tape orthogonal to the projection plane. Colour coding : oxygen, red; nitrogen, blue; hydrogen, green. |
The synthesis of the unprecedented zwitterion 6 results from a new strategy for the preparation of substituted aminobenzoquinones. This molecule displays a remarkable delocalization of its 12π-electron system and represents a promising building block in coordination and supramolecular chemistry.11
This work was supported by the CNRS and the Ministère de la Recherche (Paris). Thanks are due to Ms N. Kyritsakis, Dr A. DeCian and Prof. R. Welter for the X-ray structure determinations.
Footnote |
† Electronic supplementary information (ESI) available: spectroscopic data for 4 and 6 and an ORTEP view of the structure of 4. See http://www.rsc.org/suppdata/cc/b1/b107828n/. |
This journal is © The Royal Society of Chemistry 2002 |