Electron transfer. 140. Reactions of riboflavin with metal center reductants

Riboflavin (I) is reduced in separable steps by indium(I), vanadium(II), europium(II), and titanium(III) in 0.02-1.0 M H+, yielding first the radical ion, II (lambdamax = 495 nm), and then the dihydro compound, III. The initial reduction with InI yields 2 equiv of the radical, but kinetic profiles exhibit no irregularity due to intervention of In(II), indicating that participation by the dipositive state is much more rapid than the In(I) reaction. Predominant paths involve the protonated form of the flavin, RbH+, and that of the radical, RbH2.+. Formation of the radical with excess V(II) and Ti(III) (but not with In(I)) is strongly autocatalytic, reflecting rapid comproportionation involving the flavin and the dihydro compound. The V(II) and Ti(III) rates for both steps greatly exceed the substitution-controlled limits for these states and therefore pertain to outer-sphere processes. The very high ratio kEu/kv for the first step, however, points to an inner-sphere reduction by the lanthanide cation. A kinetic inversion is observed for In(I) (kRbH.+ > kRbH2.+), implying a bridged reduction path for the initial step with this center as well.