Influence of solvent and counter ion on complexes of 2,5-bis(2-pyridyl)-1,3,4-oxadiazole with iron(II) and (III) studied by electrospray ionization mass spectrometry

The effect of solvent and counter ion on the complexes of 2,5-bis(2-pyridyl)-1,3,4-oxadiazole (1) with Fe+2 and Fe+3 has been studied by electrospray ionization mass spectrometry (ESUMS). As expected, upon ESI conditions the metal reduction proceeds, but it can be deduced that complexes with Fe+2 are favored over those with Fe+3. When methanol is used as solvent, the formation of complexes of stoichiometry 2:1 and 1:1 with counter ion attached (monovalent anion) is favored, for example, [I-2 + FeCl](+) ion. The use of methanol/water (1/1) as solvent favors the formation of complexes of stoichiometry 2:1 and 3:1, namely doubly charged [I-2 + Fe](+2) and [I-3 + Fe](+2) ions. The complexes containing anion of oxidative properties (ClO4-, NO3-) when the higher cone voltage is applied, yield unusual species [1n + FeOm](+) (n = 1, 2; m = 1, 2). The use of divalent counter ion (SO4-2) resulted in formation of complexes containing two iron cations, namely [1n + Fe2SO4](+2) (n = 2, 3, 4) ions. These ions can be regarded as Fe-1 complexes bridged by a sulfate anion.