Electron transfer. 93. Further reactions of transition-metal-center oxidants with vitamin B12s (Cob(I)alamin)

Vitamin B/sub 12s/ (cob(I)alamin) reduces europium(III), titanium(IV) (TiO(C/sub 2/O/sub 4/)/sub 2//sup 2/minus//), and uranium(VI) in aqueous solution. These oxidants undergo one-electron changes, leading in each case to the cobalt product cob(II)alamin (B/sub 12r/). The reduction of Eu/sup 3+/, which is inhibited by TES buffer, but not by glycine, is outer sphere. Its limiting specific rate (1 /times/ 10/sup 2/ M/sup /minus/1/ s/sup /minus/1/), incorporated in the Marcus treatment, yields a B/sub 12s/,B/sub 12r/ self-exchange rate of 10/sup 4.8/plus minus/0.5/ M/sup /minus/1/ s/sup /minus/1/. Reductions of TiO(C/sub 2/O/sub 4/)/sub 2//sup 2/minus// are accelerated by H/sub +/ and by acetic acid. Kinetic patterns suggest three competing reaction paths involving varying degrees of protonation of the Ti(IV) center or its association with acetic acid. The very rapid reduction of U(VI) (k = 4 /times/ 10/sup 6/ M/sup /minus/1/ s/sup /minus/1/) yields U(V) in several buffering media, even when B/sub 12s/ is taken in excess. The much slower conversion of U(V) to U(IV), although thermodynamically favored, appears to be retarded by the extensive reorganization of the coordination sphere of oxo-bound U(V) that must accompany its acceptance of an additional electron. The observed specific rate for the B/sub 12s/-U(VI) reaction is in reasonable agreement, inmore » the framework of the Marcus formalism, with reported values of the formal potential and the self-exchange rate for U(V,VI). 37 references, 4 tables.« less