Systematic electrochemical synthesis of reduced forms of the alpha-[S2Mo18O62](4-) anion

As proton concentration increases, the first two reversible (1 e(-))-reduction processes of the anion alpha-[S2Mo18O62](4-) 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 x 10(8), and 10(2) 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 (I 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.