Reductions by aquatitanium(ii)

Abstract

Solutions of titanium(II), prepared by dissolving titanium wire in mixtures of hydrofluoric and triflic acids, reduce quinones, nitrosodisulfonate anion, and complexes of cobalt(III). When the oxidant is taken in excess, these reactions yield Ti(IV), whereas with excess reductant, the principal product is Ti(III). These reactions are compared with those by Ti(III). Despite differences in rate laws, it is clear that rate ratios for the two reductants (k_Ti^II/k_Ti^III) fall well below 10⁴, the minimum selectivity corresponding to estimated differences in formal potentials, and in some instances, Ti(II), the stronger reductant, reacts more slowly. For both Ti(III) and Ti(II), reductions within the series [Co(NH₃)₅X]²⁺ (where X = F, Cl, Br, and I), the fluoro complex reacts much more rapidly than its congeners, and the bromo and iodo complexes are slowest, an order similar to that for Eu²⁺ reductions, but opposite to that for Cr(II) and Cu(I). The [Co(NH₃)₅Br]²⁺ reaction with excess Ti(II) proceeds at rates very nearly independent of [oxidant] during the first 80–90% reaction, implying that initiation occurs via unimolecular conversion of Ti(II) to an activated cationic reducing species, in the same manner as the earlier described reduction of I₃⁻ by Ge(II) in aqueous HCl.