Discrete Cyanide‐Bridged Mixed‐Valence Co/Fe Complexes: Outer‐Sphere Redox Behaviour

Abstract

The outer-sphere redox behaviour of a series of [L n Co IIINCFe II (CN) 5 ] − (L n = n-membered pentadentate aza-macrocycle) complexes have been studied as a function of pH and oxidising agent. All the dinuclear complexes show a double protonation process at pH ഠ 2 that produces a shift in their UV/Vis spectra. Oxidation of the different non-protonated and diprotonated complexes has been carried out with peroxodisulfate, and of the non-protonated complexes also with trisoxalatocobaltate ( oxidants, has been carried out as a function of pH, temperature, and pressure. As a whole, the values found for the activation volumes, entropies, and enthalpies are in the follow-

Tuning the metal-to-metal charge transfer energy of cyano-bridged dinuclear complexes

“…The pH was then lowered to ca. 5 with HCl to afford the more reactive aqua complex (orange) and an equimolar amount of K 4 [Ru(CN) 6 ] (1 mmol), dissolved in a minimum amount of water, was added to the solution. The pH of the mixture was monitored and kept above 6 at all times to avoid the precipitation of an orange powder (typically at pH 5), which, if formed, could be redissolved on addition of base.…”

“…[1][2][3][4] As an example, the complex anion trans-[L 14 CoNCFe(CN) 5 ] − (Chart 1) and its analogues fall into the Robin and Day 5 Class II group of mixed valence compounds, where weak to moderate electronic coupling between the metal centres is present and electronic transitions from the individual Co III and Fe II chromophores are apparent. and acceptor (Co III/II ) redox potentials i.e.…”

Outer‐Sphere Redox Reactions Leading to the Formation of Discrete CoIII/FeII Pyrazine‐Bridged Mixed‐Valence Compounds

“…[1,2,11,17,25] Furthermore, comparison of the data with that available in the literature for equivalent [Fe II (CN) 6 ] 4-reductions of the same complexes (Table 3) indicates a definitive shift toward much lower values of ∆S ‡ for reductions with [Fe II (CN) 5 (pz)] 3-. In the same respect, the values determined for K OS are also much larger, thus indicating a more important outer-sphere interaction between the two redox counterparts before electron transfer.…”

“…[6][7][8] In circumstances in which the final Co II complex is sufficiently kinetically stable (i.e., due to its encapsulation by a macrocyclic ligand), a substitution process by the formed [Fe III -(CN) 6 ] 3-species (Oxd 2 in Scheme 1) has been observed, [9,10] especially for cases in which the outer-sphere complex-formation equilibrium constant is much larger than that expected from simple Fuoss equation electrostatic considerations. [8,11] These systems are a clear example of the importance of nonelectrostatic interactions in outer-sphere complex formation; they appear even between species with high charges of the same sign [12] that are able to tune the reactivity of {Oxd 1 ;Red 2 } species. [13][14][15] We have been involved in the kinetic and mechanistic study of the outer-sphere redox processes between [Fe II -(CN) 6 ] 4-(Scheme 1, Red 2 ) and Co III or Cr III complexes of pentadentate macrocyclic (N) 5 ligands (Scheme 1, Oxd 1 ).…”

“…The compounds include species with acceptor metal ions in different environments and protonation states. [11,17,19,20] All complexes show Robin and Day Class II character and tuning of the metal-to-metal charge transfer (MMCT) band has been achieved. [21] Scheme 2 summarizes some examples of these compounds.…”