Temperature- and pressure-dependent kinetico-mechanistic studies on the formation of mixed-valence {(tetraamine)Co^IIINCFe^II(CN)₅}⁻ units

Abstract

The reduction of Co^III in the tetraamine-encapsulating ligand complex [Co^III{(μ-ET)(Me₂)cyclen}(H₂O)₂]³⁺ by [Fe^II(CN)₆]⁴⁻ has been studied kinetico-mechanistically at different pH, temperatures, and pressures. The process agrees with the expected outer-sphere redox mechanism, with the value of the encounter-complex equilibrium constant large enough to allow for kinetic determination of the first-order electron transfer reaction rate constant. The value of the encounter-complex equilibrium constant, K_pre-eq, is not only dependent on the charge of the redox partners, but also on the establishment of an important network of hydrogen bonds. These can also explain the differences obtained in the activation volumes determined for the diaqua and bis-hydroxo complexes. Neither the leaching of Co^II nor the presence of [Fe^III(CN)₆]³⁻ is observed in the final reaction medium, which indicates that a fast sequence involving substitution on the transient Co^II complex followed by a fast inner-sphere electron transfer takes place. This sort of mechanism has already been established for encapsulating pentaamine ligand complexes, but this is the first example of such a sequential reaction occurring on a tetradentate ligand complex. Preliminary UV–Vis and electrochemical characterization experiments have been conducted on the final reaction mixtures, suggesting the formation of a stable cyanide-bridged Co^III/Fe^II mixed-valence complex of the same type reported in the literature for encapsulating {Co^III(N)₅} skeletons.

Graphical abstract

The kinetics of formation of a new cyanido-bridged mixed-valence Co^III/Fe^II complex has been studied at different temperatures and pressures.

Keywords:

• Redox
• Mechanisms
• Mixed-valence
• Kinetics

Acknowledgements

Financial support from the Spanish Ministerio de Ciencia e Innovación (project CTQ2012-37821-C02-01) is acknowledged. V.B. also acknowledges Campus Hungary Program of Balassi Institute (TÁMOP 4.2.4.B/2-11/1-2012-0001).

Disclosure statement

No potential conflict of interest was reported by the authors.