Two red salts derived from yellow 4-cyano-1-methylpyridinium iodide: 1,1′,1″-trimethyl-4,4′,4″-(1,3,5-triazin-2,4,6-triyl)tripyridinium trisiodide and 4-cyano-1-methylpyridinium triiodide
Graphical abstract
Two red salts were obtained unexpectedly from yellow 4-cyano-1-methylpyridinium iodide. Crystal structures of 1,1′,1″-trimethyl-4,4′,4″-(1,3,5-triazin-2,4,6-triyl)tripyridinium trisiodide (m.p. 383 °C) and 4-cyano-1-methylpyridinium triiodide (m.p.118 °C) are described.
Introduction
Numerous reports of host/guest compounds, metalloprisms or cages incorporate 2,4,6-tris(4-pyridyl)-1,3,5-triazine (usually abbreviated tpt or 4-tpt) as panels in large hexavalent [1], [2], [3], [4], [5], [6] or dodecavalent [7] cations. Therrien [8] reviewed the coordination chemistry of 2,4,6-tri(pyridyl)-1,3,5-triazine ligands and Mooibroek and Gamez [9] discussed in detail all the numerous possible s-triazine supramolecular interactions. One of the crystal structures described herein contains a trivalent cation of 4-tpt methylated at the pyridine nitrogens. This new structure will contribute to our understanding of the non-covalent interactions in previously reported, as well as yet undiscovered, compounds such as the cages and prisms containing tethered triazine panels. Shedding light on the very existence of this trivalent triazinium cation may spark new ideas for this synthon.
In addition, triazine syntheses can require extreme conditions including high temperatures and pressures, high concentrations of strong base or acid, and long reaction times [8], [10], [11]. The present study describes a cyclotrimerization product formed under mild reaction conditions. Despite Kosower [12] and others having used and studied 4-cyano-1-methylpyridinium iodide and closely related compounds extensively, there are no published reports of the formation of either of the other two title compounds from that starting material.
In acidic aqueous solutions of 4-cyano-1-methylpyridinium iodide under ambient conditions in contact with insoluble heavy metal chloride solids, an electron transfer reaction takes place to form red 4-cyano-1-methylpyridinium triiodide, containing the linear triatomic anion (Compound I). In methanol, cyclotrimerization yields 1,1′,1″-trimethyl-4,4′,4″-(1,3,5-triazin-2,4,6-triyl)tripyridinium trisiodide with three monatomic anions per trivalent cation (Compound II). Neither of these compounds has been widely discussed in the chemical literature. They have potential applications in photoelectrochemistry and supramolecular design. Methods of preparation, crystal structures, supramolecular features and other characteristics of these compounds are described herein.
Section snippets
Experimental
All solvents and reagents were purchased from commercial suppliers and used without modifications. NMR spectrum was obtained with a Varian 400 MHz NMR. Melting points were obtained using a Shimadzu DSC-60 plus differential scanning calorimeter. Crystal structures were determined using a Bruker Smart apex CCD single crystal X-ray diffractometer.
Compound I
Several salts of the 4-cyano-1-methylpyridinium cation have been produced readily by salt metathesis (double displacement) between the 4-cyano-1-methylpyridinium iodide and more soluble silver salts containing the desired anion [17], [18], [19]. However, we were unable to generate 4-cyano-1-methylpyridinium chloride by this method. Instead, crystals of the triiodide salt, I, were obtained when the reaction was done in acidic aqueous solution. Similar attempts using mercury(I) chloride and
Summary
Two relatively unknown red compounds, 4-cyano-1-methylpyridinium triiodide and 1,1′,1″-trimethyl-4,4′,4″-(1,3,5-triazin-2,4,6-triyl)tripyridinium trisiodide, were synthesized serendipitously from yellow 4-cyano-1-methylpyridinium iodide. Crystal structures and melting points were determined and described. Interaction energies for cation pairs and associated anions were calculated for the triazinium compound.
Acknowledgements
The authors thank Erin Larrabee and David Olivier for DSC measurements. LVK acknowledges support from the Earl and Gertrude Vicknair professorship. JTM thanks Tulane University for support of the Tulane Crystallography Laboratory. KER gratefully acknowledges support from the NSF through the HBCU-UP program (HRD1501259) and the EPSCoR Cooperative Agreement (EPS-1003897).
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