Hydrogen-bonding, π-stacking and Cl--anion-π interactions of linear bipyridinium cations with phosphate, chloride and [CoCl4]2- anions

A series of protonated bipyridinium compounds has been synthesized and their crystal packing is discussed in terms of N-H center dot center dot center dot X contacts (X = O, Cl) together with pi-pi and Cl-center dot center dot center dot pi interactions of the positively charged aromatic nitrogen heterocycles. The bipyridinium-anion combinations are diprotonated 4,4'-bipyridin-1,1'-ium bis(dihydrogenphosphate) dihydrate (3), 4,4'-bipyridin-1,1'- ium bis(1,1'-binaphthalene-2,2'-diyl phosphate)center dot 2.5H(2)O (4), 6,6'-diisoquinolin-2,2'-ium dichloride dihydrate (5), 2,2'-bi-1,6-naphthyridin-6,6'-ium dichloride dihydrate (6) and triprotonated 2,2'-bi-1,6-naphthyridin-1,6,6'-ium-tetrachlorocobaltate(II)-chloride (7). In all structures N+-H hydrogen bonding occurs only to the anion and not to water of crystallization. The packing in 3-6 can be rationalized by a separation of the hydrophobic C-H backbone of the bipyridines from the hydrophilic N-H, phosphate or chloride and water part. N+-H center dot center dot center dot Cl-/O hydrogen bonding and pi-pi-stacking are identified as the major packing interactions in 3-6. In 7 the pi-stacking is replaced by remarkably short Cl--anion-pi interactions.