Electron transfer. Part 169 . Delayed reduction of mononuclear manganeses(IV) using vanadium(III)

The tris-chelate formed by biguanide, H2NC(NH)NHC(NH)NH2, and Mn(IV) is one of the rarely encountered examples of water-stable mononuclear complexes of this oxidation state. This cation is reduced in aqueous acid by both V(II) and V(III). In contrast to the kinetically straightforward bimolecular reduction by V(II) in 0.5 M HClO4 (k = 7.4 × 103 M−1 s−1 at 22 °C), reductions by excess V(III) yield profiles which are linear (zero-order in MnIV) until the last few percent reaction. Analyses of these composite curves are consistent with the sequences 2 V III ⇆ k - 1 k 1 ( V III ) 2 ∗ ( V III ) 2 ∗ + Mn IV → k 2 2 V IV + Mn II in which reaction is initiated by slow and reversible conversion of VIII to an activated dimeric species ( V III ) 2 ∗ Treatment of data in terms of this sequence yields a value of k1 1.9 × 10−3 M−1 s−1 and a ratio k2/k−1 2 × 106. This activated route for ( V III ) 2 ∗ appears to be preferred over a remarkably slow electron transfer from monomeric V(III) to the Mn(IV) center, reflecting an unusually high barriet to V(III/IV) self-exchange.