Synthesis and redox properties of triarylmethane dyecation salts of anions [M6O19]2− (M = Mo, W)

Abstract

Four salts have been isolated combining the triarylmethane dye cations pararosaniline (PR⁺) and crystal violet (CV⁺) with the hexametalates [M₆O₁₉]²⁻ (M = Mo, W). A new hexatungstic acid H₂[W₆O₁₉]·4dma (dma = dimethylacetamide) was isolated and is a useful synthon for hexatungstate salts. Single-crystal X-ray diffraction confirmed the presence of PR⁺ and [Mo₆O₁₉]²⁻ ions in [PR]₂[Mo₆O₁₉]·6dmf (dmf = dimethylformamide). A number of charge-assisted hydrogen bonds N–H⋯O exist between the cation –NH₂ functions and the anion oxygen atoms. Comparative cyclic voltammetry of salts [A]Cl (A = PR, CV), [Bu₄N]₂[M₆O₁₉]²⁻ and A₂[M₆O₁₉] was established in MeCN and Me₂SO solutions and of solids in contact with the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [emim][tfsa]. In the molecular solvents, the reversible potential for the process [Mo₆O₁₉]^2−/3− is less negative than the first reduction processes of the dye cations. In contrast, that for [W₆O₁₉]^2−/3− is more negative. Spectro-electrochemistry and bulk electrolysis experiments reveal significantly different pathways in the two cases. In contrast, in the [emim][tfsa] medium, a positive shift in reduction potential of at least 400 mV is seen for the anion processes but relatively little change for the dye cation processes. This means that initial reduction of the anions always precedes that of the dyes, providing significant simplification of the complex voltammetric data. Chemically modified electrodes can be used in the ionic liquid because of slow dissolution kinetics. However, reduced anion salts dissolve rapidly, allowing dissolved phase electrochemistry to be examined. The electrochemistries of the oxidized salts A₂[M₆O₁₉] are essentially those of the individual ions, although low level interaction of A⁺ with reduced anions [M₆O₁₉]^3−,4− is evident. The work establishes protocols for synthesis and handling of intensely absorbing and relatively insoluble salts which can now be applied to systems containing more complex polyoxometalate anions.