Abstract
The synthesis, isolation and structural characterization of the sulfite polyoxomolybdate clusters α‐(D~3__h__~)(C~20~H~44~N)~4~{α‐[Mo~18~O~54~(SO~3~)~2~]}⋅CH~3~CN and β‐(D~3__d__~)(C~20~H~44~N)~4~{β‐[Mo~18~O~54~(SO~3~)~2~]}⋅CH~3~CN is presented. Voltammetric studies in acetonitrile (0.1 M Hx~4~NClO~4~, Hx~4~N=tetra‐n‐hexylammonium) reveal the presence of an extensive series of six one‐electron reduction processes for both isomers. Under conditions of bulk electrolysis, the initial [Mo~18~O~54~(SO~3~)~2~]^4−/5−^ and [Mo~18~O~54~(SO~3~)~2~]^5−/6−^ processes produce stable [Mo~18~O~54~(SO~3~)~2~]^5−^ and [Mo~18~O~54~(SO~3~)~2~]^6−^ species, respectively, and the same reduced species may be produced by photochemical reduction. Spectroelectrochemical data imply that retention of structural form results upon reduction, so that both α and β isomers are available at each of the 4‐, 5‐, and 6‐redox levels. However, the α isomer is the thermodynamically favored species in both the one‐ and two‐electron‐reduced states, with β→α isomerization being detected in both cases on long time scales (days). EPR spectra also imply that increasing localization of the unpaired electron occurs over the α‐ and β‐[Mo~18~O~54~(SO~3~)~2~]^5−^ frameworks as the temperature approaches 2 K where the EPR spectra show orthorhombic symmetry with different g and hyperfine values for the α and β isomers. Theoretical studies support the observation that it is easier to reduce the α cluster than the β form and also provide insight into the driving force for β→α isomerization in the reduced state. Data are compared with that obtained for the well studied α‐[Mo~18~O~54~(SO~4~)~2~)]^4−^ sulfate cluster.