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

Riboflavin (I) is reduced in separable steps by indium(I), vanadium(II), eu ropium(II), and titanium(III) in 0.02-1.0 M H+, yielding first the radical ion, II (lambda(max) = 495 nm), and then the dihydro compound, III. The ini tial reduction with In-I yields 2 equiv of the radical, but kinetic profile s exhibit no irregularity due to intervention of Inn, indicating that parti cipation by the dipositive state is much more rapid than the In-I reaction. Predominant paths involve the protonated form of the flavin, RbH+, and tha t 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 comprop ortionation 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 precesses. The very high ratio k(En)/k(V) for the first step, however, points to an inner-spher e reduction by the lanthanide cation. A kinetic inversion is observed for I n-I (k(RbH).+ > k(RbH2).+), implying a bridged reduction path for the initi al step with this center as well.