The Structural Systematics of Protonation of Some Important Nitrogen‐base Ligands. I Some Univalent Anion Salts of Doubly Protonated 2, 2′:6′, 2″‐Terpyridyl

Abstract

Abstract A number of salts of 2,2′:6′,2″ ‐terpyridyl (‘tpy’) with univalent anions (halides : X = Cl, Br, I; oxyanions of increasing basicity: ClO 4 , NO 3 , ‘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 − ·H 2 O 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 = NO 3 , 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)].