The structural systematics of protonation of some important nitrogen-base ligands.: I -: Some univalent anion salts of doubly protonated 2, 2′:6′, 2"-terpyridyl

A number of salts of 2,2':6',2" -terpyridyl ('tpy') with univalent anions (halides : X = Cl, Br, 1; oxyanions of increasing basicity: ClO4, NO3, 'tfa' = trifluoroacetate, 'tca' = trichloroacetate), variously solvated, have been structurally characterized by single crystal X-ray studies. In all cases the tpy moieties are found to be doubly protonated [tpyH(2)](2+), the hydrogen atoms being associated with the nitrogen atoms of the peripheral rings, these together with the central nitrogen atom being directed towards a common focus, in most cases 'chelating' one of the counter-ion components in diverse ways. Thus the chloride and bromide compounds are isomorphous [(tpyH(2))X]X-+(-).H2O arrays; a second dihydrate phase is also described for the chloride, the two forms having the unchelated anion and water molecules engaged in hydrogen-bonded networks essentially independent of [(tpyH(2))X](+). The iodide is anhydrous, and of a different structural type, the anions, presumably too large for chelation, lying out of plane to either side, and linking different cations into a one-dimensional polymer; in the perchlorate, the unsolvated aggregate is now discrete [(tpyH(2))X-2], a pair of perchlorate ions disposed to either side of the tpy plane, lying each with one oxygen atom interacting with both of the two protonating hydrogen atoms. In the anhydrous X = NO3, tfa, tca arrays, the lattices are solvated by the parent acids; one oxygen atom of each anion is chelated by the [tpyH(2)](2+) as in the chlorides, the other anion, with the acid, forming an independent 'acid salt' counterion [XHX](-) in each case, retaining the additional protonic hydrogen rather than further protonating the central ring, all being of the form [(tpyH(2))X]X.HX = [(tpyH(2))X][X(HX)].