Molybdenum and copper catalysis of reductions by titanium(II) and titanium(III)

Reductions of vanadium(IV), benzoquinone, and tri-iodide, both by titanium(III) and by titanium(II), are catalyzed by molybdenum(VI). The VO(2+)-Ti(II) reaction is catalyzed by copper(II) as well. Reactions of Ti(II) with the oxidant in excess yield Ti(IV), as do reductions by Ti(III). Reactions proceed via competing uncatalyzed and catalyzed paths, with the latter components first order in catalyst. Kinetic patterns indicate that monomeric Mo(v) is the active species, but the dimeric Mo(v) species, Mo(2)O(4)](2+), is without catalytic action. Catalytic constants pertaining to Ti(III) are remarkably similar to those for Ti(II), despite the 0.47 V difference in the standard potentials of the two reductants.