The structural systematics of protonation of some important nitrogen-base ligands. V. Some univalent anion salts of mono- and bis(2-picolyl)amine

Abstract

A variety of salts derivative of bis(2-picolyl)amine, 'dipic' = [{(C5NH4)(CH2)}2NH] in various stages of protonation have been structurally characterized, showing a considerable diversity of hydrogen-bonding modes and interactions. For the triprotonated species (protonating hydrogen atoms on all three nitrogen atoms) the arrays are haphazard ([picH3]X3, X = Cl(·H2O), I(·H2O), NO3, tfs (= trifluoromethanesulfonate)(·H2O). For the diprotonated species, diverse forms are also found: in [dipicH2]Br2, the central nitrogen atom and one of the peripheral are protonated, but in the remainder, both peripheral nitrogen atoms are protonated, leading to a propensity to chelating interactions with an anion (as in the mixed anion salts CL- · I3- and I-·I3-, where I3- does not interact) or a water molecule (in the I·2H2O salt) or anion (in the tfs salt) oxygen atom; in the nitrate salt, the ligand is twisted so that each pyridinium component interacts with an independent nitrate. By contrast, in the singly protonated species, [dipicH]X, the central nitrogen atom is protonated in all cases (X = Br, I, ClO4, thf). The tfs- salt is remarkable, containing a pair of cations with self-interactions. In [picH]X only the aliphatic nitrogen is protonated (X = I-, NO3-, ClO4, tfa- (= trifluoroacetate)). A single example of a diprotonated species [picH2]Cl2 has also been defined.