Molecular Co(III)/Fe(II) cyano-bridged mixed-valence compounds with high nuclearities and diversity of Co(III) coordination environments: Preparative and mechanistic aspects

The kinetico-mechanistic study of the formation of discrete tri- and tetranuclear mixed-valence cyano-bridged (Co(III))(2)/Fe(II) and (Co(III))(3)/Fe(II) complexes has been carried out from their already described parent dinuclear Co(III)/Fe(II) and mononuclear Co(III) complexes. Different Co(III) encapsulating units have been used in order to tune both the redox potential on the mono- and dinuclear complexes and the lability of the mononuclear building blocks. The importance of outer-sphere precursor complex formation involving the oppositely charged reactants has been established by experiments conducted at high ionic strength where electrostatic effects are nullified. The influence of the pH is also crucial, and this has been linked to the pH dependence of the precursor Co(III/)(II) redox potentials in terms of enabling a redox-assisted association (at low pH) between the ferricyanide analogue and the labile Co(II) partner. A new asymmetric Co(III)L/Fe(II)/Co(III)L' complex has been fully characterized, and a series of putative (Co(III)L)(2)/Fe(II)/Co(III)L' and (Co(III)L)(3)/Fe(II) forms have been spectroscopically detected. A comparison with the mechanistic reaction pathways established for the formation of the parent dinuclear Co(III)/Fe(II) complexes indicates that an important tuning of the direct substitution and redox-catalyzed mechanism is applicable. For the formation of the trinuclear complexes, only in the most favorable conditions is a redox-assisted sequence observed. The rate and activation parameters for the reactions have been determined and are indicative of an essential outer-sphere precursor formation. Similarly, for reactions where redox-assisted mechanisms are unfavorable, only direct substitution processes have been found to be applicable, with their rate coefficients and activation parameters also agreeing with the expectations, once an outer-sphere precursor complex is formed. Formation of the tetranuclear (Co(III)L)(3)/Fe(II) complexes has only been detected on decomposition of the parent trinuclear (Co(III)L)(2)/Fe(II) complexes following reduction to their Co(II) form. The overall processes seem to be based on the outer-sphere association between the respective building blocks in such a way that both unfavorable redox potentials and Lewis basicities are overcome