Electrochemistry of trans-dioxo complexes of rhenium(V) in water

Cyclic voltammetry of trans-((py)/sub 4/Re/sup V/(O)/sub 2/)/sup +/ (py is pyridine) in aqueous solutions at glassy-carbon electrodes show that oxidation states Re(VI)(d/sup 1/) to Re(II)(d/sup 5/) are accessible within the solvent limits. Similar behavior is observed for trans-((CN)/sub 4/Re/sup V/(O)/sub 2/)/sup 3 -/ and trans-((en)/sub 2/Re/sup V/(O)/sub 2/)/sup +/. Reduction of the pyridine complex to Re(II) at 0 less than or equal to pH par. delta 14 leads to H/sub 2/; k/sub obsd/(room temperature, ..mu.. = 0.1 M) = 2.4 (+/- 1.5) x 10/sup -3/s/sup -1/ at pH 1.2. At pH 6.8 or 13.0, the addition of NO/sub 2//sup -/ or SO/sub 3//sup 2 -/ suppresses water reduction at the expense of electrocatalytic reduction of NO/sub 2//sup -/ to NH/sub 3/ and N/sub 2/O and of SO/sub 3//sup 2 -/ to H/sub 2/S or HS/sup -/. Comparisons between the Re-pyridyl-based couples and the structurally and electronically related trans-((bpy)/sub 2/Os/sup VI/(O)/sub 2/)/sup +/ (bpy is 2,2'-bipyridine) couples suggest that the pattern of couples that appear and their pH dependences are determined largely by the d-electronic configurations of the components. Differences in the magnitudes of redox potentials between electronically equivalent Re and Os couples are determined by the differences in oxidation state between the two types of couples.