Mechanistic aspects of the chemistry of mononuclear CrIII complexes with pendant-arm macrocyclic ligands and formation of discrete CrIII/FeII and CrIII/FeII/CoIIIcyano-bridged mixed valence compounds

Abstract

The kinetics and mechanism of the redox reaction between [Fe^II(CN)₆]⁴⁻ and the macrocyclic ligand complex [CrClL₁₅]²⁺ (L₁₅ = 6-methyl-1,4,8,12-tetraazacyclopentadecane-6-amine) has been studied at different pH values. In acidic solution, the expected redox process occurs with no formation of any of the possible Cr^III/Fe^II mixed valence complexes, as those seen for the Co^III species of the same family, due to the enhanced lability of the Cr^II species formed on Fe^II to Fe^III oxidation. In alkaline conditions, the formation of the complex [Cr(L₁₅)(OH)₂]⁺ takes place as an initial step that precedes a simple substitution process producing the expected cyano-bridged Cr^III/Fe^II complex. In this species the potentially pentadentate ligand, L₁₅, has a tetradentate coordination mode with a protonated exocyclic primary amine group and the redox potential is shifted to more negative values, thus disfavouring a redox driven reaction; the equivalent complex [CrCl(HL₁₄)(H₂O)]³⁺ (L₁₄ = 6-methyl-1,4,8,11-tetraazacyclotetradecane-6-amine) has been prepared by the same method and characterized by X-ray crystallography. The final [Fe^II(CN)₆]⁴⁻ substituted complex, [{(HL₁₅)(OH)Cr^IIINC}Fe^II(CN)₅]⁻ shows pK_a values of 3.8 and 7.4, as expected for the aqua and amino ligands, respectively. Its characterization indicated its Class II mixed valence character with a very intense MMCT band at 350 nm showing a much larger extinction coefficient than that observed for the Co^III complexes of the same family. This fact is in good agreement with the much larger Cr^III–Fe^II (t_2g–t_2g) coupling through cyanide bridging ligands expected for these complexes. The fully mixed metal/valence/ligand trimetallic complex [{(HL₁₅)(OH)Cr^IIINC}{L₁₃Co^IIINC}Fe^II(CN)₄]²⁺ has been prepared following the same procedures and the results are comparable. The final complex has the same Class II mixed valence character and its electronic spectrum shows the characteristics of both the Fe^II-to-Cr^III and Fe^II-to-Co^III CT bands. The study allows the application of the redox and/or substitutionally driven methodology, already described and kinetically designed, for the preparation of discrete mixed valence complexes of inert centres, so avoiding the unpredictable nature of other approaches seen in the past.