Synthesis and solid state structures of 1,3-dihalo-1,3-bis(dialkylamino)propenium salts and related compounds

1,3-dicholoro-1,3-bis(dialkylamino)propenium salts are prepared by condensing N,N,-dialkylamides with dichloromethylene dimethyliminium chloride ("Viehe’s Salt"). The products are versatile synthons and can be used to make a variety of ligand types. Here we report the syntheses of a number of new propenium salts and provide comprehensive characterization data for some which have already been described. Additionally, we have prepared a series of related 1,3-dihalo-1,3-bis(dimethylamino)propenium salts via halogen exchange reactions (F through I). Finally, all of the compounds described, including Viehe’s Salt and several hydrolysis products, have been characterized in the solid state using single-crystal X-ray diffraction. The symmetrical cations, [Me2NC(X)CHC(X)NMe2], were found to take one of two general structural forms. The first (X = H or F) is planar, presumably maximizing the delocalization within the backbone of the cation, while the second form is markedly twisted (X = Cl, Br or I). Calculations indicate that the latter develop increasingly positive regions of the electrostatic potential on the halide substituents, allowing cation/anion interactions via halogen bonding to dominate in the extended solids. The results of these studies illustrate the importance of the differing interionic interactions in a homologous series of small organic salts.