Acetocatechol functionalized viologen as polyfunctional material that responds to anion, cation and reductant in aqueous and organic solvents

Abstract

Both viologen and catechol have been studied extensively. However, the stability of catechol-Fe(III) without additional oxidant is still not well understood. In this paper, we introduced acetocatechol into viologen to investigate its interactions with anion, cation, and reductant, as well as the stability of its Fe(III) complexes. This acetocatechol functionalized viologen, 1,1′-bis(2-(3,4-dihydroxyphenyl)-2-oxoethyl)- [4,4′-bipyridine]-1,1′-diium chloride (H6V·Cl2) exists in central symmetric ketone cation form in solid state. Viologen cation increased the acidity of the aceto group and deprotonated the enolic proton to form monodeprotonated enolic H5V+ in the presence of anion/base, which had the deepest color in organic solvents. The absorbance maximum of H5V+ increased with the decrease of solvent polarity. It also interacted with B4O72− and MoO42− by forming catechol ester in DMSO solution. The catechol moiety can coordinate to metal ion, especially Fe(III), in both aqueous and DMSO solution. In particular, it coordinated to Fe(III) much more readily in aqueous solution than in DMSO. Green monocatecholato Fe(III) and red-brown bis-catecholato Fe(III) complex also formed in aqueous solution. The monocatecholato Fe(III) complex first-order dimerized in aqueous solution at room temperature but underwent second-order decomposition to Fe(II) complex at 60 °C. The biscatecholato Fe(III) complex also transferred to other Fe(III) complexes at first- and second-order at room-temperature and 60 °C respectively. The t1/2 varied from several hours at room-temperature and several minutes at 60 °C at 10−4 M concentration. The interactions of Fe(III) in DMSO is much more complex than that of acetocatecholate without viologen. Fe(III) can also be reduced to free viologen radicals in the presence of sodium Na2S2O4, but not N2H4. In conclusion, this polyfunctional compound responds to anion via aceto and catechol, metal ion via catechol hydroxyl, while reductant via viologen. Keywords: Viologen, Catechol, Anion sensor, Fe(III) complex, Stability