New Conformations and Binding Modes in Halogen-Bonded and Ionic Complexes of 2,3,5,6-Tetra(2‘-pyridyl)pyrazine

Abstract

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 -I2·I2·I2- 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·6I2, 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 I2 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 (I2·I3)- 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(H2I)2]2+ cations, similar to the bromide and chloride salts, but with triiodide anions rather than hydrated halides as in the lighter derivatives.

Alternative Motifs for Halogen Bonding

“…Another substance class used for XB‐mediated crystal engineering includes unsaturated halogenated (usually iodinated) compounds. For example, tetraiodoethylene (TIE) has been used as a multi‐directional halogen‐bonding donor, which can be seen in the structure of the 1:1 complex (DARZOM)41 with 2,3,5,6‐tetrakis(2′‐pyridyl)pyrazine (tppz) in which TIE forms four (i.e., two pairs of symmetry independent) halogen bonds with lengths of 2.99 and 3.10 Å (15 and 12 % reduction, Figure 6).…”

“… The 1:1 complex formed between TIE and tppz with halogen bond lengths of 2.99 and 3.10 Å (15 and 12 % reduction) between the nitrogen and iodine atoms (DARZOM) 41…”

Three Novel Metal Supramolecular Polymers Directed by 1,2-bis(pyridinium)ethane Cation: Syntheses and Crystal Structures

“…The results showed that organic-inorganic hybrids could offer an important opportunity to combine useful properties from organic compounds and inorganic materials. [15,16] Further work is in progress to extend this facile method to other metal azides, to evaluate the influences of cation structure modifications and different anions on the resulting supramolecular structures, as well to elucidate the mechanism of inorganic polymeric framework formation to further probe their structure/properties relationship. [17] Supplementary Materials CCDC-623643, 623647, 623648 contains the supplementary Crystallographic data for 1-3.…”

Halogen Bonding with Dihalogens and Interhalogens

“…Most N · I 2 complexes, and all N · Br 2 , N · IBr, and N · ICl complexes are simple adducts, mode A. Exceptions for the diiodine complexes include: bridging mode (B) observed for diazines, such as pyrazine [86], tetramethylpyrazine [86], phenazine, and quinoxaline [87], and for 9-chloroacridine [89] and the 1 : 1 complex of diiodine with hexamethylenetetramine [144]; and amphoteric bridging mode (BA) observed for 2,2 -bipyridine [85], acridine [89], 9-chloroacridine [89], and 2,3,5,6-tetra-2 -pyridylpyrazine [91]. The occurrence of both B and BA complexes with 9-chloroacridine, and of B and A complexes and an ionic salt [145] for tetramethylenetetramine, highlights the obvious difficulty in predicting which mode will be favored for a given donor-acceptor pair.…”