The solvent dependence of -RuIII(NH3) association with macrocyclic ether species has been assessed by following intervalence charge-transfer absorbance energy changes in (bpy)2ClRuII-pz-RuIII(NH3)54+. Depending on the identity of the solvent, -RuIII(NH3)5·idicyclohexano-24-crown-8 association constants can vary by as much as 4 orders of magnitude, with the largest constants found in the solvents of lowest Lewis basicity. The solvent modulation effects are interpreted in terms of a second-sphere substitution process where association necessarily involves both an energy gain (ether oxygen/ligated ammonia hydrogen-bond formation) and an energy loss (molecular solvent/ligated ammonia hydrogen-bond destruction). The magnitude of the energy loss term is expected to increase as the Lewis basicity of the solvent increases, resulting in decreased crown association strength, as observed experimentally. In low-basicity solvents, crown association with -RuIII(NH3)5 is strongly favored over association with -RuII(NH3)5-consistent with the known enhanced Lewis acidity for the ammine hydrogens in the higher metal oxidation state. A quasi-thermodynamic analysis of crown-induced intervalence energy shifts indicates that the preference for the higher oxidation state diminishes as the solvent basicity increases and is apparently lost when the solvent and the macrocycle have equivalent basicities. Careful consideration of solvatochromic effects associated with intervalence absorption offers a means for evaluating the extent of encapsulation of -RuIII(NH3)5 by macrocyclic species in solution. The pentaammine guest is roughly one-third encapsulated by dicyclohexano-24-crown-8 and two-thirds encapsulated by dibenzo-42-crown-14, with intermediate degrees of encapsulation by dibenzo-30-crown-10 and dibenzo-36-crown-12. Finally, under some circumstances (high concentrations of dicyclohexano-24-crown-8 in low-basicity solvents) we observe 2:1 crown:metal-complex association stoichiometries.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry