Ionic association effects upon optical electron transfer energetics

Studies in water with (CN)5FeII-BPE-FeIII(CN)5 5-

Robert L. Blackbourn, Yuhua Dong, L. Andrew Lyon, Joseph T Hupp

Research output: Contribution to journalArticle

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Abstract

The energetics of optical electron transfer (ET) in (NC)5FeIII-bis(pyridyl)ethylene -FeII(CN)55- (1), as monitored by intervalence absorption spectroscopy, display a remarkable sensitivity to added "inert" electrolyte. With small amounts of added NaCl, CaCl2, or LaCl3, the optical ET (or metal-to-metal charge transfer) energy (EMMCT) increases significantly. With further additions, however, it gradually decreases, ultimately approaching (for LaCl3) the value found in the absence of added electrolyte. The unusual intervalence energy effects are interpreted in terms of stepwise ionic association: unsymmetrical species of the type (NC)5FeIII-BPE-FeII(CN)5 5-·Mn+ absorb at higher energies than symmetrical species (i.e., 1 or Mn+·1·Mn+), because of the existence of a net unfavorable thermodynamic driving force. Support for the interpretation comes from (a) site-specific probes which show that metal cations preferentially associate with the {-FeII(CN)5}3- portion of 1 and (b) calculations of EMMCT (based on experimentally determined ionic association constants and localized energy shifts) which qualitatively reproduce the observed changes in intervalence energetics. Similar intervalence energy behavior is observed for (NH3)5RuIII-4,4′-bpy-Ru II(NH3)55+ with added Na2SO4. This behavior is interpreted in terms of stepwise association of SO42- with the {(NH3)5RuIII-}3+ and {-RuII(NH3)5}2+ sites, respectively, of the polycationic mixed-valence ion. Finally, we speculatively suggest that ion-pairing-induced symmetry reduction also accounts for a most unusual literature observation: the apparent dependence of EMMCT for (NH3)5RuIII-dithiaspirane-Ru II(NH3)55+ on the identity and formal potential of the oxidant used to prepare the ion from the 4+ form.

Original languageEnglish
Pages (from-to)4446-4452
Number of pages7
JournalInorganic Chemistry
Volume33
Issue number20
Publication statusPublished - 1994

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electron transfer
Metals
Association reactions
Ions
Electrolytes
Electrons
Water
water
Absorption spectroscopy
Oxidants
energy
Cations
Charge transfer
electrolytes
metals
ions
Thermodynamics
absorption spectroscopy
ethylene
charge transfer

ASJC Scopus subject areas

  • Inorganic Chemistry

Cite this

Ionic association effects upon optical electron transfer energetics : Studies in water with (CN)5FeII-BPE-FeIII(CN)5 5-. / Blackbourn, Robert L.; Dong, Yuhua; Lyon, L. Andrew; Hupp, Joseph T.

In: Inorganic Chemistry, Vol. 33, No. 20, 1994, p. 4446-4452.

Research output: Contribution to journalArticle

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title = "Ionic association effects upon optical electron transfer energetics: Studies in water with (CN)5FeII-BPE-FeIII(CN)5 5-",
abstract = "The energetics of optical electron transfer (ET) in (NC)5FeIII-bis(pyridyl)ethylene -FeII(CN)55- (1), as monitored by intervalence absorption spectroscopy, display a remarkable sensitivity to added {"}inert{"} electrolyte. With small amounts of added NaCl, CaCl2, or LaCl3, the optical ET (or metal-to-metal charge transfer) energy (EMMCT) increases significantly. With further additions, however, it gradually decreases, ultimately approaching (for LaCl3) the value found in the absence of added electrolyte. The unusual intervalence energy effects are interpreted in terms of stepwise ionic association: unsymmetrical species of the type (NC)5FeIII-BPE-FeII(CN)5 5-·Mn+ absorb at higher energies than symmetrical species (i.e., 1 or Mn+·1·Mn+), because of the existence of a net unfavorable thermodynamic driving force. Support for the interpretation comes from (a) site-specific probes which show that metal cations preferentially associate with the {-FeII(CN)5}3- portion of 1 and (b) calculations of EMMCT (based on experimentally determined ionic association constants and localized energy shifts) which qualitatively reproduce the observed changes in intervalence energetics. Similar intervalence energy behavior is observed for (NH3)5RuIII-4,4′-bpy-Ru II(NH3)55+ with added Na2SO4. This behavior is interpreted in terms of stepwise association of SO42- with the {(NH3)5RuIII-}3+ and {-RuII(NH3)5}2+ sites, respectively, of the polycationic mixed-valence ion. Finally, we speculatively suggest that ion-pairing-induced symmetry reduction also accounts for a most unusual literature observation: the apparent dependence of EMMCT for (NH3)5RuIII-dithiaspirane-Ru II(NH3)55+ on the identity and formal potential of the oxidant used to prepare the ion from the 4+ form.",
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N2 - The energetics of optical electron transfer (ET) in (NC)5FeIII-bis(pyridyl)ethylene -FeII(CN)55- (1), as monitored by intervalence absorption spectroscopy, display a remarkable sensitivity to added "inert" electrolyte. With small amounts of added NaCl, CaCl2, or LaCl3, the optical ET (or metal-to-metal charge transfer) energy (EMMCT) increases significantly. With further additions, however, it gradually decreases, ultimately approaching (for LaCl3) the value found in the absence of added electrolyte. The unusual intervalence energy effects are interpreted in terms of stepwise ionic association: unsymmetrical species of the type (NC)5FeIII-BPE-FeII(CN)5 5-·Mn+ absorb at higher energies than symmetrical species (i.e., 1 or Mn+·1·Mn+), because of the existence of a net unfavorable thermodynamic driving force. Support for the interpretation comes from (a) site-specific probes which show that metal cations preferentially associate with the {-FeII(CN)5}3- portion of 1 and (b) calculations of EMMCT (based on experimentally determined ionic association constants and localized energy shifts) which qualitatively reproduce the observed changes in intervalence energetics. Similar intervalence energy behavior is observed for (NH3)5RuIII-4,4′-bpy-Ru II(NH3)55+ with added Na2SO4. This behavior is interpreted in terms of stepwise association of SO42- with the {(NH3)5RuIII-}3+ and {-RuII(NH3)5}2+ sites, respectively, of the polycationic mixed-valence ion. Finally, we speculatively suggest that ion-pairing-induced symmetry reduction also accounts for a most unusual literature observation: the apparent dependence of EMMCT for (NH3)5RuIII-dithiaspirane-Ru II(NH3)55+ on the identity and formal potential of the oxidant used to prepare the ion from the 4+ form.

AB - The energetics of optical electron transfer (ET) in (NC)5FeIII-bis(pyridyl)ethylene -FeII(CN)55- (1), as monitored by intervalence absorption spectroscopy, display a remarkable sensitivity to added "inert" electrolyte. With small amounts of added NaCl, CaCl2, or LaCl3, the optical ET (or metal-to-metal charge transfer) energy (EMMCT) increases significantly. With further additions, however, it gradually decreases, ultimately approaching (for LaCl3) the value found in the absence of added electrolyte. The unusual intervalence energy effects are interpreted in terms of stepwise ionic association: unsymmetrical species of the type (NC)5FeIII-BPE-FeII(CN)5 5-·Mn+ absorb at higher energies than symmetrical species (i.e., 1 or Mn+·1·Mn+), because of the existence of a net unfavorable thermodynamic driving force. Support for the interpretation comes from (a) site-specific probes which show that metal cations preferentially associate with the {-FeII(CN)5}3- portion of 1 and (b) calculations of EMMCT (based on experimentally determined ionic association constants and localized energy shifts) which qualitatively reproduce the observed changes in intervalence energetics. Similar intervalence energy behavior is observed for (NH3)5RuIII-4,4′-bpy-Ru II(NH3)55+ with added Na2SO4. This behavior is interpreted in terms of stepwise association of SO42- with the {(NH3)5RuIII-}3+ and {-RuII(NH3)5}2+ sites, respectively, of the polycationic mixed-valence ion. Finally, we speculatively suggest that ion-pairing-induced symmetry reduction also accounts for a most unusual literature observation: the apparent dependence of EMMCT for (NH3)5RuIII-dithiaspirane-Ru II(NH3)55+ on the identity and formal potential of the oxidant used to prepare the ion from the 4+ form.

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