The reversible reduction of [S2Mo18O62]4- to [S2Mo18O62]5- and [S2Mo18O62]6- at a glassy carbon macrodisk electrode has been studied by cyclic voltammetry in acetonitrile as a function of the concentration of [(C6H13)4N]4[S2Mo18O62] in the absence and presence of [(C6H13)4N]ClO4 as the added supporting electrolyte. Consideration is given to the influence of scan rate, reference-working electrode distance, [(C6H13)4N]4[S2Mo18O62], and electrolyte concentrations. Experimental data confirm theoretical predictions that cyclic voltammetry at a macrodisk electrode is a viable technique for studies of multiply charged electroactive species without added electrolyte, provided the influence of enhanced complexities associated with effects of increased solution resistance, the mass transport contribution from migration, and convection arising from enhanced density gradients are considered. Enhanced density gradients present in the absence of added supporting electrolyte give rise to a more marked dependence of voltammograms on the angle of the electrode and hence lead to significant distortion of wave shapes at low scan rates. The summation of all these obstacles implies that quantitative evaluation of cyclic voltammograms of multiply charged species requires significantly greater care in the absence than in the presence of added supporting electrolyte.

As proton concentration increases, the first two reversible (1 e-)-reduction processes of the anion α-[S2Mo18O62]4- in 95/5 MeCN/H2O convert to an overall (2 e-)-reduction process. Half-wave potentials for reversible one-electron reduction of [S2Mo18O62]4- itself and its two one-electron-reduced forms [S2Mo18O62]5- and [HS2Mo18O64]4- were estimated by voltammetry to be 0.12, −0.13, and 0.35 V, respectively, versus the ferrocenium/ferrocene couple. Simulation of cyclic voltammograms provided estimates of association constants of 1.4, 1.6 × 108, and 102 M-1 for protonation of the respective products of the reductions, [S2Mo18O62]5-, [S2Mo18O62]6-, and [HS2Mo18O62]5-. Equilibrium constants for disproportionation of the (1 e-)-reduced species were derived. Rates of the electron transfer and protonation processes are very fast relative to the voltammetric time scale. Consideration of the equilibrium constants, plus information obtained from acid titrations monitored by steady state voltammetry, helped define conditions for the isolation of salts of the (1 e-)-, (2 e-)-, (2 e-, 1 H+)-, (2 e-, 2 H+)-, (4 e-, 2 H+)- and (4 e-, 4 H+)-reduced derivatives of the [S2Mo18O62]4- anion. Anions at the (6 e-)- and (8 e-)-reduced levels undergo spontaneous oxidation in the acid solutions and could not be isolated experimentally. The present work shows that directed synthesis of reduced species in different protonation states is possible for these complex systems if adequate voltammetric data are available.

Isolation of a soluble [NHex4]+ salt has allowed a detailed electrochemical study of the anion alpha-[SMo12O40]2- to be undertaken. Four reversible one-electron-reduction processes are observed in CH2Cl2 solution. Controlled potential electrolysis led to isolation of tetraalkylammonium salts of the one-electron-reduced anion alpha-[SMo12O40]3- and the two-electron-reduced anion alpha-[SMo12O40].4- [SMo12O40]3- is stable to disproportionation in dry solvents (Kdis = 10(-7.4). EPR and magnetic susceptibility data indicate that [SMo12O40]3- is a simple paramagnet (S = 1/2) while [SMo12O40]4- is paramagnetic with the mu eff values decreasing at low temperatures. Solutions of the two-electron-reduced species are EPR silent, but microcrystalline powders show very weak signals. The crystal structure of alpha-[NBu4]3[SMo12O40] has been determined (triclinic P1; a = 13.840(3) A; b = 15.587(4) A; c = 19.370(3) A; alpha = 94.82(2) degrees; beta = 93.10(1) degrees; gamma = 91.05(2) degrees; Z = 2). There is disorder around the C2 axis of the central SO4(2-) tetrahedron. In the presence of aqueous HClO4 (0.045 M) in thf/H2O or MeCN/H2O (98/2 v/v), [SMo12O40]2- exhibits five two-electron-reduction processes. Under these conditions, [SMo12O40]3- protonates and disproportionates into [SMo12O40]2- and the (2e-, 2H+)-reduced anion [H2SMo12O40]2-.