SOLVENT EFFECTS ON THE EFFICACY OF RECOGNITION OF AMMINEMETAL COMPLEXES BY MACROCYCLIC ETHERS - IN-SITU PROBES OF EXTENT OF ENCAPSULATION

The solvent dependence of -RU(III)(NH3) association with macrocyclic e ther species has been assessed by following intervalence charge-transf er absorbance energy changes in (bpy)(2)ClRuII-pz-Ru-III(NH3)(5)(4+). Depending on the identity of the solvent, -Ru-III(NH3)(5)dicyclohexano -24-crown-8 association constants can vary by as much as 4 orders of m agnitude, with the largest constants found in the solvents of lowest L ewis 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-b ond formation) and an energy loss (molecular solvent/ligated ammonia h ydrogen-bond destruction). The magnitude of the energy loss term is ex pected to increase as the Lewis basicity of the solvent increases, res ulting in decreased crown association strength, as observed experiment ally. In low-basicity solvents, crown association with -RU(III)(NH3)(5 ) is strongly favored over association with -Ru-II(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 t he higher oxidation state diminishes as the solvent basicity increases and is apparently lost when the solvent and the macrocycle have equiv alent basicities. Careful consideration of solvatochromic effects asso ciated with intervalence absorption offers a means for evaluating the extent of encapsulation of -RU(III)(NH3)5 by macrocyclic species in so lution. The pentaammine guest is roughly one-third encapsulated by dic yclohexano-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 conce ntrations of dicyclohexano-24-crown-8 in low-basicity solvents) we obs erve 2:1 crown:metal-complex association stoichiometries.