Anion Coordination Chemistry .3. 2nd-sphere Interaction Between the Complex Anions Hexacyanoferrate(ii) and Hexacyanocobaltate(iii), With Polycharged Tetraazamacrocycles - Thermodynamic and Single-crystal X-ray Studies

The thermodynamic parameters ΔG°, ΔH° and ΔS° for the supercomplex formation between the [Fe(CN)6]4− and [Co(CN)6]3− anions and the tetraprotonated species of the tetraazamacrocycle 1,6,11,16-tetraazacycloeicosane (L2) have been determined at 25 °C by potentiometric techniques and by direct microcalorimetry. The chemical model is very simple since only the 1:1 species (H4L2)[Fe(CN)6] and (H4L2)[Co(CN)6]+ are formed. Both species have favorable enthalpic and entropic contributions to their stability: ΔH° = −1.1 kcal mol−1 and ΔS° = 13 cal K−1 mol−1 for the first complex and ΔH° = −2.56 kcal mol−1 and ΔS° = 2.3 cal K−1 mol−1 for the second complex. The thermodynamic results have been interpreted in terms of hydrogen bond formation, electrostatic interactions and desolvation effect. The electrochemical behavior of [Fe(CN)6]4− complexed by [H4L2]4+ has been studied by the cyclic voltammetry technique. The stability constant of the oxidized species (H4L2)[Fe(CN)6]+ (log kox = 2.5) has been obtained from the potentiometric stability constant of (H4L2)[Fe(CN)6] and the redox potential shift of the complexed couple (H4L2)[Fe(CN)6]/(H4L2)[Fe(CN)6]+ with respect to the uncomplexed couple [Fe(CN)6]4−/[Fe(CN)6]3−. To better investigate the influence of the electrostatic interaction the macrocyclic polyammonium receptor 1,1,4,4, 7,7,10,10-octamethyl-1,4,7,10-tetraazacyclododecane tetrakis(iodide) (L1·4I) has been synthesized and characterized. The quaternarization of all nitrogen atoms prevents the formation of hydrogen bonds. Crystals of the compound (L1)[Co(CN)6]·I·3H2O are triclinic, space group P1, with a = 12.091(3), b = 11.696(2), c = 11.484 Å, α = 94.89(4)°, β = 102.75(4)°, γ = 92.55(3)°, and Z = 2. Refinement of the atomic parameters by least-squares methods gave a final R factor of 0.061 (Rw = 0.062) for 4063 unique reflections with I> 3σ(I). The structure consists of discrete macrocyclic L14+ cations, [Co(CN)6]3−, I− anions, and water molecules. The interaction between the cation and anions is purely electrostatic and its strength is indicated by several close contacts between carbon atoms of the macrocyclic cation and cyanide nitrogens of the exacyanocobaltate(III) anion.