Transition Metal Ion Initiated Chain Reactions between tert-Butyl Hydroperoxide and Rhodium Hydrides. Rapid Hydrogen Atom Abstraction from Rhodium Hydrides by Methyl Radicals.
In the absence of catalysts, there is no reaction between t-BuOOH and macrocyclic rhodium hydrides LRhH(2+) (L(1) = [14]aneN(4), L(2) = Me(6)-[14]aneN(4)). The addition of Fe(aq)(2+) to such solutions results in rapid and catalytic oxidation of rhodium hydrides (LRhH(2+) + (CH(3))(3)COOH + H(+) --> LRh(3+) + CH(4) + (CH(3))(2)CO). This reaction occurs by a chain mechanism featuring methyl radicals and LRh(2+) as reactive intermediates. The rates are independent of the absolute concentrations of iron but vary with the ratio of the two oxidation states, -d[LRhH(2+)]/dt = k(obs)[t-BuOOH](2)[Fe(2+)]/[Fe(3+)]. The rate constants k(obs) are directly proportional to [H(+)]. In 0.10 M aqueous HClO(4) at 25 degrees C, the values are 80.8 M(-)(1) s(-)(1) (L(1)RhH(2+)) and 7.0 M(-)(1) s(-)(1) (L(2)RhH(2+)). A macrocyclic cobalt(II) complex, L(2)Co(2+), is also an effective chain initiator. The reaction occurs according to the rate law -d[L(1)RhH(2+)]/dt = 254[t-BuOOH][L(1)RhH(2+)], independent of [H(+)]. This reaction is not catalytic in L(2)Co(2+), which is slowly consumed in the initiation and termination steps. Methyl radicals react rapidly with L(1)RhH(2+) (k = 1.0 x 10(9) M(-)(1) s(-)(1)), L(2)RhH(2+) (1.4 x 10(8)), and L(1)Rh(2+) (8.6 x 10(8)).
Bakac
Andreja
A
Ames Laboratory, Iowa State University, Ames, Iowa 50011.