Redox chemistry of aquatitanium(II), Ti2+(aq)

The growth of industrial interest, since 1960, in organic derivatives of titanium(II) (such as alkoxides and cyclopentadienyls) stands in contrast to the elusive nature of this state as its salts in polar media. The preparation, in 2003, of triflates of this ion in HF–CF3SO3H made solutions of this cation conveniently available, although such preparations contained Ti(IV) as well. The standard potential for the Ti(II,III) couple remains in doubt. Rates of reduction, using Ti(II), of quinones and complexes of Co(III), Ru(III) and Fe(III) are often very similar to rates for reductions by Ti(III). A number of Ti(II) reductions are catalyzed by Ti(IV). Depending on the oxidant and the medium, such catalysis may involve: (a) the rapid formation of a 1:1 Ti(II)–Ti(IV) complex (Kf = 400 M−1 at 22 °C); (b) slow comproportionation, yielding Ti(II) (Ti(II) + Ti(IV) → 2Ti(III); or (c) slow conversion of Ti(II)(aq) to a more reactive coordinatively unsaturated transient. Trapping experiments with [Co(NH3)5Br]2+ demonstrate that the oxidations of Ti(II) using Cr(VI) or Cr(IV) proceed via le− transactions.

► Reductions by dipositive titanium, Ti(II). ► Solutions of Ti(II) in triflic acid were prepared from titanium wire. ► Rates of reduction with Ti(II) of quinones and high valent metal centers each resemble rates with Ti(III). ► A number of reductions by Ti(II) are catalyzed by Ti(IV). ► Oxidations of Ti(II), using Cr(VI) or Cr(IV), proceed via single unit transactions.