Reduction of tris(2,2′-bipyridyl) and tris(1,10-phenanthroline) complexes of iron(III) and osmium(III) by hydroxide ion

Abstract

Reaction(i)(L = 1,10-phenanthroline. 2,2′-bipyridyl, and a series of methyl-substituted derivatives of these 4 [M(LL)₃]³⁺+ 4 OH^–→ 4 [M(LL)₃]²⁺+ O₂+ 2 H₂O (i) ligands; M = Fe and Os) has been studied in aqueous (1M) NaCl–NaOH from 6·5 to 35 °C. The rate law throughout is that reported earlier for the unsubstituted iron complexes, i.e., d[Fe(LL)₃³⁺]/dt=k[Fe(LL)₃³⁺][OH^–]. Phenanthroline complexes react faster than bipyridyl complexes, Fe^III complexes faster than the analogous Os^IIIcomplexes. A linear free-energy relation between the rates and standard free-energy changes of the metal couples, analogous to that known for other reducing agents, holds only for each metal and each ligand type. It is confirmed that the enthalpies of activation are less than those calculated for the standard enthalpy change of reaction (ii). From a consideration of the stopped-flow, potentiometric, and n.m.r. data, together with those from pulse-radiolysis [M(LL)₃]³⁺+ OH^–→[M(LL)₃]²⁺+˙OH (ii) and chemiluminescence studies for these and similar reactions, we conclude that the first-formed product is a highly reactive precursor complex and formulate this as [M(LL)₂(R˙–OH)]²⁺ where R˙–OH is probably the pseudo-base of a radical formed by addition of an electron to a ligand which approaches the structure of a quaternary-nitrogen cation.