Volume Changes on the Conversion from Quinquedentate EDTA Complexes of Co(III) and Cr(III) into Sexidentate Complexes

Yoshitani, Kouzou

doi:10.1246/bcsj.67.2115

Cited by 10 publications

(4 citation statements)

References 25 publications

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“…From the observed pressure dependence of the isomeric equilibrium constant K (Figure and Table S7-1, Supporting Information) a reaction volume of −7.9 ± 0.2 cm 3 mol −1 for equilibrium (7) was calculated. A similar equilibrium for [Co III (edta)] − was reported to be accompanied by a volume change of −9.5 ± 0.4 cm 3 mol −1 . The strong preference for the octahedral coordination geometry is manifested by the observation of the above-mentioned coordination equilibrium between hexadentate ⇌ aqua-pentadentate species with a ring-opened carboxylate group in solution, and thus the significant pressure dependence of the isomeric equilibrium needs to be considered for any mechanistic interpretation that is based on variable pressure experiments.…”

Section: Resultsmentioning

confidence: 72%

Triggering Water Exchange Mechanisms via Chelate Architecture. Shielding of Transition Metal Centers by Aminopolycarboxylate Spectator Ligands

et al. 2008

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Paramagnetic effects on the relaxation rate and shift difference of the (17)O nucleus of bulk water enable the study of water exchange mechanisms on transition metal complexes by variable temperature and variable pressure NMR. The water exchange kinetics of [Mn(II)(edta)(H2O)](2-) (CN 7, hexacoordinated edta) was reinvestigated and complemented by variable pressure NMR data. The results revealed a rapid water exchange reaction for the [Mn(II)(edta)(H2O)](2-) complex with a rate constant of k(ex) = (4.1 +/- 0.4) x 10(8) s(-1) at 298.2 K and ambient pressure. The activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) are 36.6 +/- 0.8 kJ mol(-1), +43 +/- 3 J K(-1) mol(-1), and +3.4 +/- 0.2 cm(3) mol(-1), which are in line with a dissociatively activated interchange (I(d)) mechanism. To analyze the structural influence of the chelate, the investigation was complemented by studies on complexes of the edta-related tmdta (trimethylenediaminetetraacetate) chelate. The kinetic parameters for [Fe(II)(tmdta)(H2O)](2-) are k(ex) = (5.5 +/- 0.5) x 10(6) s(-1) at 298.2 K, DeltaH(double dagger) = 43 +/- 3 kJ mol(-1), DeltaS(double dagger) = +30 +/- 13 J K(-1) mol(-1), and DeltaV(double dagger) = +15.7 +/- 1.5 cm(3) mol(-1), and those for [Mn(II)(tmdta)(H2O)](2-) are k(ex) = (1.3 +/- 0.1) x 10(8) s(-1) at 298.2 K, DeltaH(double dagger) = 37.2 +/- 0.8 kJ mol(-1), DeltaS(double dagger) = +35 +/- 3 J K(-1) mol(-1), and DeltaV(double dagger) = +8.7 +/- 0.6 cm(3) mol(-1). The water containing species, [Fe(III)(tmdta)(H2O)](-) with a fraction of 0.2, is in equilibrium with the water-free hexa-coordinate form, [Fe(III)(tmdta)](-). The kinetic parameters for [Fe(III)(tmdta)(H2O)](-) are k(ex) = (1.9 +/- 0.8) x 10(7) s(-1) at 298.2 K, DeltaH(double dagger) = 42 +/- 3 kJ mol(-1), DeltaS(double dagger) = +36 +/- 10 J K(-1) mol(-1), and DeltaV(double dagger) = +7.2 +/- 2.7 cm(3) mol(-1). The data for the mentioned tmdta complexes indicate a dissociatively activated exchange mechanism in all cases with a clear relationship between the sterical hindrance that arises from the ligand architecture and mechanistic details of the exchange process for seven-coordinate complexes. The unexpected kinetic and mechanistic behavior of [Ni(II)(edta')(H2O)](2-) and [Ni(II)(tmdta')(H2O)](2-) is accounted for in terms of the different coordination number due to the strong preference for an octahedral coordination environment and thus a coordination equilibrium between the water-free, hexadentate [M(L)](n+) and the aqua-pentadentate forms [M(L')(H2O)](n+) of the Ni(II)-edta complex, which was studied in detail by variable temperature and pressure UV-vis experiments. For [Ni(II)(edta')(H2O)](2-) (CN 6, pentacoordinated edta) a water substitution rate constant of (2.6 +/- 0.2) x 10(5) s(-1) at 298.2 K and ambient pressure was measured, and the activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) were found to be 34 +/- 1 kJ mol(-1), -27 +/- 2 J K(-1) mol(-1), and +1.8 +/- 0.1 cm(3) ...

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Section: Resultsmentioning

confidence: 72%

Triggering Water Exchange Mechanisms via Chelate Architecture. Shielding of Transition Metal Centers by Aminopolycarboxylate Spectator Ligands

et al. 2008

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“…It was originally believed that EDTA was in pentadentate coordination with Cr III over a wide pH range, with an aquo or hydroxo ligand occupying the sixth coordination position. , The p K a of 1.8 would then correspond to protonation of the uncoordinated carboxylate group of EDTA, and the p K a at 7.39 would result from deprotonation of the coordinated aquo ligand. In recent years, 2 H NMR, circular dichroism, Raman spectroscopy, and volume change measurements have provided evidence that EDTA acts as a hexadentate chelating agent between pH 1.8 and 7.39. Therefore, protonation at pH values below pH 1.8 must involve exit of a carboxylate Lewis base group of EDTA and entry of a water molecule: The deprotonation reaction defined by the second p K a again involves carboxylate group exit, followed by entry of hydroxide ion: …”

Section: Resultsmentioning

confidence: 99%

“…12,38 The pK a of 1.8 would then correspond to protonation of the uncoordinated carboxylate group of EDTA, and the pK a at 7.39 would result from deprotonation of the coordinated aquo ligand. In recent years, 2 H NMR, 39 circular dichroism, 40 Raman spectroscopy, 41 and volume change measurements 42 have provided evidence that EDTA acts as a hexadentate chelating agent between pH 1.8 and 7.39. Therefore, protonation at pH values below pH 1.8 must involve exit of a carboxylate Lewis base group of EDTA and entry of a water molecule:…”

Section: Resultsmentioning

confidence: 99%

Speciation of Chromium(III) and Cobalt(III) (Amino)carboxylate Complexes Using Capillary Electrophoresis

Carbonaro

Stone

2004

Anal. Chem.

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Capillary electrophoresis (CE) can very efficiently resolve different dissolved metal ion species as long as rates of ligand exchange are slow relative to time scales required for electromigration. Here, we detail the separation of several CrIII and CoIII complexes with the multidentate chelating agents iminodiacetic acid, nitrilotriacetic acid, trans-1,2-cyclohexanediaminetetracetic acid, N-(2-hydroxyethyl)ethylenediaminetriacetic acid, trimethylenediaminetetraacetic acid, and ethylenediaminetetraacetic acid. Successes in speciating some NiII and CoII complexes are also reported. For CrIII and CoIII, subtle differences in metal ion-chelating agent stereochemistry, chelating agent denticity, and number of bridging ligands are discernible due to the high resolving power of CE. New peaks and heightened baselines were encountered when a pH buffer with strong complexing properties (orthophosphate) was employed in the background electrolyte. For this reason, we recommend using pH buffers with very weak or negligible complex properties (e.g., MES and MOPS).

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“…The lability of [Cr(salen)(OH 2 ) 2 ] + and related Schiff-base complexes 7 has been attributed to a destabilizing distortion of the ground state, but this distortion has been described as slight and may facilitate the departure of the leaving group by transient coordination of the carboxyl group of the pendant arm. − The picture is complicated, however, by the likelihood − that edta is predominantly sexidentate in its complex with Cr III in the pH range 3.5−6.5, i.e., that the conjugate base of [Cr(Hedta)OH 2 ], which we will designate for the time being as “[Cr(edta)OH 2 ] - ”, is actually present mainly as [Cr(edta)] - .…”

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confidence: 99%

Volume Profile for Substitution in Labile Chromium(III) Complexes: Reactions of Aqueous [Cr(Hedta)OH₂] and [Cr(edta)]^- with Thiocyanate Ion

1996

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A procedure is given for correcting optical absorbance measurements made at variable pressure with a le Noble-Schlott ("pillbox") cell for the inner sleeve wall thickness. With this technique, the molar volume change for the acid ionization of aqueous [Cr(Hedta)OH(2)] was found to be +5.1 +/- 0.6 cm(3) mol(-)(1) (0-200 MPa, 25.0 degrees C, ionic strength 1.0 mol L(-)(1) HClO(4)/NaClO(4)), an anomalous positive value which implies a change from quinquedentate to predominantly sexidentate edta and expulsion of the coordinated water on ionization. For thiocyanate substitution into labile [Cr(Hedta)OH(2)], high pressure stopped-flow measurements gave the volume of activation as -7.8 +/- 0.9 cm(3) mol(-)(1) and the volume of reaction as +3 +/- 2 cm(3) mol(-)(1), while for the reaction of [Cr(edta)](-) with NCS(-) the activation volume is -13.6 +/- 0.6 cm(3) mol(-)(1) (same conditions). These and other data support the notion that the anomalous substitutional lability of Cr(III)(edta) complexes relative to typical Cr(III) species is due to activation by transient chelation of the pendant arm of quinquedentate edta.

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Volume Changes on the Conversion from Quinquedentate EDTA Complexes of Co(III) and Cr(III) into Sexidentate Complexes

Cited by 10 publications

References 25 publications

Triggering Water Exchange Mechanisms via Chelate Architecture. Shielding of Transition Metal Centers by Aminopolycarboxylate Spectator Ligands

Triggering Water Exchange Mechanisms via Chelate Architecture. Shielding of Transition Metal Centers by Aminopolycarboxylate Spectator Ligands

Speciation of Chromium(III) and Cobalt(III) (Amino)carboxylate Complexes Using Capillary Electrophoresis

Volume Profile for Substitution in Labile Chromium(III) Complexes: Reactions of Aqueous [Cr(Hedta)OH₂] and [Cr(edta)]^- with Thiocyanate Ion

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Volume Changes on the Conversion from Quinquedentate EDTA Complexes of Co(III) and Cr(III) into Sexidentate Complexes

Cited by 10 publications

References 25 publications

Triggering Water Exchange Mechanisms via Chelate Architecture. Shielding of Transition Metal Centers by Aminopolycarboxylate Spectator Ligands

Triggering Water Exchange Mechanisms via Chelate Architecture. Shielding of Transition Metal Centers by Aminopolycarboxylate Spectator Ligands

Speciation of Chromium(III) and Cobalt(III) (Amino)carboxylate Complexes Using Capillary Electrophoresis

Volume Profile for Substitution in Labile Chromium(III) Complexes: Reactions of Aqueous [Cr(Hedta)OH2] and [Cr(edta)]- with Thiocyanate Ion

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Volume Profile for Substitution in Labile Chromium(III) Complexes: Reactions of Aqueous [Cr(Hedta)OH₂] and [Cr(edta)]^- with Thiocyanate Ion