Non-innocent electrolyte effects on bimolecular pseudo-self-exchange reactions of ruthenium ammine complexes: Evidence for electron-transfer catalysis in H-bonded ternary assemblies
“…If present, an interaction between the seven-coordinate iron complex in both its oxidation states (Fe III and Fe II ) and the ions in its substrate binding sites would result in a shift of the [Fe(dapsox)(CH 3 CN) 2 ] 0/+ couple redox potential upon increase of the salt concentration . For the electrochemical guest binding titrations, the equilibria can be represented by a redox square (Scheme ), and the interaction can be quantified by determining the apparent guest binding constant according to eq 1, 6a,, where Δ E is a shift in the host redox potential as a function of guest concentration and K II and K III correspond to the 1:1 host/guest complex in the divalent and trivalent forms of the receptor, respectively 2 …”
Section: Resultsmentioning
confidence: 99%
“…Different from the electrochemical measurements, where the effects of host−guest interactions on the redox active iron center can be observed, 7 Li NMR titrations can demonstrate the complexation effect on the Li + cation itself. The NMR chemical shift titration has been widely used as a method for the determination of binding constants. 6a, The easily observed and sensitive 7 Li isotope has nuclear spin I = 3/2 and high abundance (95%), making it an attractive nucleus for NMR titrations . In the case of host−guest complexation reactions, either the concentration of guest ( 7 Li nucleus) or of the host is varied and the resulting change in the chemical shift is followed.…”
Section: Resultsmentioning
confidence: 99%
“…Because of the good solubility, tetraalkylammonium and some lithium salts are suitable as supporting electrolytes in aprotic solvents like DMSO and CH 3 CN. However, added salts can exert strong effects on the rates of reactions between charged species in solution, affecting solvation of inorganic complexes . The interaction via hydrogen bonds should also be taken into consideration, especially in aprotic solvents where these interactions are more prominent .…”
Interactions between the seven-coordinate tweezerlike [Fe(dapsox)(H2O)2]ClO4 complex (H2dapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with different lithium salts (LiOTf, LiClO4, LiBF4, and LiPF6) in CH3CN have been investigated by electrochemical, spectrophotometric, 7Li and 19F NMR, kinetic, and DFT methods. It has been demonstrated that this complex acts as ditopic receptor, showing spectral and electrochemical ion-pair-sensing capability for different lithium salts. In general, the apparent binding constants for lithium salts increase in the order LiOTf < LiClO4 < LiBF4. From the electrochemical measurements, the apparent lithium salt binding constants for the Fe(III) and Fe(II) forms of the complex have been obtained, suggesting a stronger host-guest interaction with the reduced form of the complex. In the presence of LiPF6, the solution chemistry is more complex because of the hydrolysis of PF6-. The kinetics of the complexation of [Fe(dapsox)(CH3CN)2]+ by thiocyanate at -15 degrees C in acetonitrile in the presence of 0.2 M NBu4OTf shows two steps with the following rate constants and activation parameters: k(1) = 411 +/- 14 M(-1) s(-1); DeltaH(1) not equal = 9 +/- 2 kJ mol(-1); DeltaS1 not equal = -159 +/- 6 J K(-1) mol(-1); k(2) = 52 +/- 1 M(-1) s(-1); DeltaH(2) not equal = 4 +/- 1 kJ mol(-1); DeltaS(2) not equal = -195 +/- 3 J K(-1) mol(-1). The very negative DeltaS not equal values are consistent with an associative (A) mechanism. Under the same conditions but with 0.2 M LiOTf, k1Li and k2Li are 1605 +/- 51 and 106 +/- 2 M(-1) s(-1), respectively. The increased rate constants for the {[Fe(dapsox)(CH3CN)2] x LiOTf}+ adduct are in agreement with an associative mechanism. Kinetic and spectrophotometric titration measurements show stronger interaction between the lithium salt and the anion-substituted forms, [Fe(dapsox)(CH3CN)(NCS)] and [Fe(dapsox)(NCS)2]-, of the complex. These experiments demonstrate that in nonaqueous media lithium salts cannot be simply used as supporting electrolytes, since they can affect the kinetic behavior of the studied complex. DFT calculations revealed that the negatively charged alpha-oxyazine oxygen atoms are responsible for cation binding (electrostatic interactions), whereas the two terminal amide groups bind the anion via hydrogen bonding.
“…If present, an interaction between the seven-coordinate iron complex in both its oxidation states (Fe III and Fe II ) and the ions in its substrate binding sites would result in a shift of the [Fe(dapsox)(CH 3 CN) 2 ] 0/+ couple redox potential upon increase of the salt concentration . For the electrochemical guest binding titrations, the equilibria can be represented by a redox square (Scheme ), and the interaction can be quantified by determining the apparent guest binding constant according to eq 1, 6a,, where Δ E is a shift in the host redox potential as a function of guest concentration and K II and K III correspond to the 1:1 host/guest complex in the divalent and trivalent forms of the receptor, respectively 2 …”
Section: Resultsmentioning
confidence: 99%
“…Different from the electrochemical measurements, where the effects of host−guest interactions on the redox active iron center can be observed, 7 Li NMR titrations can demonstrate the complexation effect on the Li + cation itself. The NMR chemical shift titration has been widely used as a method for the determination of binding constants. 6a, The easily observed and sensitive 7 Li isotope has nuclear spin I = 3/2 and high abundance (95%), making it an attractive nucleus for NMR titrations . In the case of host−guest complexation reactions, either the concentration of guest ( 7 Li nucleus) or of the host is varied and the resulting change in the chemical shift is followed.…”
Section: Resultsmentioning
confidence: 99%
“…Because of the good solubility, tetraalkylammonium and some lithium salts are suitable as supporting electrolytes in aprotic solvents like DMSO and CH 3 CN. However, added salts can exert strong effects on the rates of reactions between charged species in solution, affecting solvation of inorganic complexes . The interaction via hydrogen bonds should also be taken into consideration, especially in aprotic solvents where these interactions are more prominent .…”
Interactions between the seven-coordinate tweezerlike [Fe(dapsox)(H2O)2]ClO4 complex (H2dapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with different lithium salts (LiOTf, LiClO4, LiBF4, and LiPF6) in CH3CN have been investigated by electrochemical, spectrophotometric, 7Li and 19F NMR, kinetic, and DFT methods. It has been demonstrated that this complex acts as ditopic receptor, showing spectral and electrochemical ion-pair-sensing capability for different lithium salts. In general, the apparent binding constants for lithium salts increase in the order LiOTf < LiClO4 < LiBF4. From the electrochemical measurements, the apparent lithium salt binding constants for the Fe(III) and Fe(II) forms of the complex have been obtained, suggesting a stronger host-guest interaction with the reduced form of the complex. In the presence of LiPF6, the solution chemistry is more complex because of the hydrolysis of PF6-. The kinetics of the complexation of [Fe(dapsox)(CH3CN)2]+ by thiocyanate at -15 degrees C in acetonitrile in the presence of 0.2 M NBu4OTf shows two steps with the following rate constants and activation parameters: k(1) = 411 +/- 14 M(-1) s(-1); DeltaH(1) not equal = 9 +/- 2 kJ mol(-1); DeltaS1 not equal = -159 +/- 6 J K(-1) mol(-1); k(2) = 52 +/- 1 M(-1) s(-1); DeltaH(2) not equal = 4 +/- 1 kJ mol(-1); DeltaS(2) not equal = -195 +/- 3 J K(-1) mol(-1). The very negative DeltaS not equal values are consistent with an associative (A) mechanism. Under the same conditions but with 0.2 M LiOTf, k1Li and k2Li are 1605 +/- 51 and 106 +/- 2 M(-1) s(-1), respectively. The increased rate constants for the {[Fe(dapsox)(CH3CN)2] x LiOTf}+ adduct are in agreement with an associative mechanism. Kinetic and spectrophotometric titration measurements show stronger interaction between the lithium salt and the anion-substituted forms, [Fe(dapsox)(CH3CN)(NCS)] and [Fe(dapsox)(NCS)2]-, of the complex. These experiments demonstrate that in nonaqueous media lithium salts cannot be simply used as supporting electrolytes, since they can affect the kinetic behavior of the studied complex. DFT calculations revealed that the negatively charged alpha-oxyazine oxygen atoms are responsible for cation binding (electrostatic interactions), whereas the two terminal amide groups bind the anion via hydrogen bonding.
“…18 The presence of non-innocent electrolytes (mucuonate and terephthalate) in studies of pseudo-self-exchange rates for [Ru(NH 3 ) 5 (py)] 2+ /[Ru(NH 3 ) 5 (fpy)] 2+ cause rate accelerations, explained by superexchange reactions in hydrogen-bonded association complexes, allowing electron transfer over distance in aqueous solution. 19 A detailed study of the effect of Cu 2+ on the promotion of the Fenton reaction 20 postulates the formation of the binuclear species [Fe-O-Cu] 4+ , which is more efficient in producing oxygen than any other intermediates formed in the reaction. Reaction of H 2 O 2 with a non-heme iron(II) complex bearing a pentadentate bispidene ligand in water has been shown to produce a stable Fe IV QO complex.…”
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