Solvent effects on the efficacy of recognition of amminemetal complexes by macrocyclic ethers: In situ probes of extent of encapsulation

Xiao Lian Zhang, Christopher R. Kankei, Joseph T Hupp

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Abstract

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·dicyclohexano-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 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 circumstance (high concentrations of dicyclohexano-24-crown-8 in low-basicity solvents) we observe 2:1 crown:metal-complex association stoichiometries.

Original languageEnglish
Pages (from-to)4738-4743
Number of pages6
JournalInorganic Chemistry
Volume33
Issue number21
Publication statusPublished - 1994

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Ethers
Encapsulation
ethers
Alkalinity
probes
Association reactions
Ammonia
Ether
ammonia
Energy dissipation
Hydrogen bonds
energy dissipation
hydrogen bonds
ammines
Oxidation
oxidation
Coordination Complexes
Acidity
acidity
Stoichiometry

ASJC Scopus subject areas

  • Inorganic Chemistry

Cite this

Solvent effects on the efficacy of recognition of amminemetal complexes by macrocyclic ethers : In situ probes of extent of encapsulation. / Zhang, Xiao Lian; Kankei, Christopher R.; Hupp, Joseph T.

In: Inorganic Chemistry, Vol. 33, No. 21, 1994, p. 4738-4743.

Research output: Contribution to journalArticle

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abstract = "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·dicyclohexano-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 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 circumstance (high concentrations of dicyclohexano-24-crown-8 in low-basicity solvents) we observe 2:1 crown:metal-complex association stoichiometries.",
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