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Charge-Modulated Associates of Anionic Donors with Cationic π-Acceptors: Crystal Structures of Ternary Synthons Leading to Molecular Wires

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posted on 07.01.2009, 00:00 by Jianjiang Lu, Jay K. Kochi
Tetramethylpyrazine is effectively transformed via controlled N-protonation or N-methylation to afford a series of mono- and dicationic π-acceptors of particular use in the study of charge modulation during the self-assembly of various anionic donors and π-acid acceptors into ternary synthons and infinite linear chains (molecular wires). Thus, when the charges of the anionic donor (D) and the cationic acceptor (A) are unequal (either as 1:2 or as 2:1), the corresponding crystal structures of the donor/acceptor associates all contain structure motifs of discrete synthons consisting of (A···D···A) or (D···A···D) triads. On the other hand, when the anionic donor and cationic acceptor are of equal charge (either as 1:1 or as 2:2), the corresponding crystal structures all contain infinite chain arrangements: (···D···A···D···A···D···), which are the same as those we previously found in the cocrystallization of various anionic donors with neutral π-acids. Fully appreciated anion/π interactions are found in all of these solids owing to the absence of aromatic protons in the π-acids, as well as the presence of strong electrostatic anion/cation interactions. It is noteworthy in such associates that halide anions (Cl, Br, and I) show different patterns of donor/acceptor interactions. For example, in the case of the diprotonated tetramethylpyrazinium dication, the chloride donor is π-bonded over the edge of the aromatic ring, whereas bromide and iodide donors are precisely located over the aromatic ring centrosymmetrically. We hope these structural studies will provide additional stimulus for the design of new anion receptors based on the use of charged aromatic acceptors.