New conformations and binding modes in halogen-bonded and ionic complexes of 2,3,5,6-tetra(2′-pyridyl)pyrazine

The enormous potential of 2,3,5,6-tetra(2′-pyridyl)pyrazine (tppz) as a versatile multisite component for the design and construction of complex structures is demonstrated by the preparation of several new complexes that possess previously unobserved binding modes and conformations. tppz forms halogen-bonded complexes with diiodine and organoiodides and forms ionic compounds in which tppz is either di- or tetraprotonated. We have prepared two new halogen-bonded complexes of tppz with diiodine and with tetraiodoethylene (TIE). Both possess extended chain structures of alternating donors and acceptors, with the tppz donors in the diiodine complex linked by two neutral -I ₂·I ₂·I ₂- chains. Two new polyiodide complexes of tppz have also been prepared, one of which has di- and the other tetraprotonated cations. A bromide salt of the tetraprotonated tppz cation has also been prepared and found to be isomorphous with the known chloride salt. In the diiodine complex, tppz·6I ₂, the conformation of tppz is similar to that found in the metastable, tetragonal polymorph of the free donor, and obeying Ostwald's rule of stages, decomposes through solid-state loss of I ₂ to give exclusively this polymorph. In the TIE complex, tppz·TIE, has a previously unreported conformation, but decomposes to the thermodynamically stable monoclinic polymorph upon loss of TIE. The diprotonated cation, [tppz(H) ₂] ²⁺, which forms two intramolecular N-H⋯N hydrogen bonds and crystallizes with (I ₂·I ₃) ^- anions, has a twisted conformation as opposed to the bowed conformation found for this cation with tetraphenylborate counterions. Tetraprotonated tppz hydrogen bonds to two iodide anions to form [tppz(H ₂I) ₂] ²⁺ cations, similar to the bromide and chloride salts, but with triiodide anions rather than hydrated halides as in the lighter derivatives.