Maigut J, Meier R, Zahl A, van Eldik R (2008)
Publication Type: Journal article, Original article
Publication year: 2008
Original Authors: Maigut J., Meier R., Zahl A., Van Eldik R.
Publisher: American Chemical Society
Book Volume: 130
Pages Range: 14556-14569
Journal Issue: 44
DOI: 10.1021/ja802842q
Paramagnetic effects on the relaxation rate and shift difference of the 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(edta)(H O)] (CN 7, hexacoordinated edta) was reinvestigated and complemented by variable pressure NMR data. The results revealed a rapid water exchange reaction for the [Mn(edta)(HO)] complex with a rate constant of k = (4.1 ± 0.4) × 10 s at 298.2 K and ambient pressure. The activation parameters ΔH, ΔS, and ΔV‡ are 36.6 ± 0.8 kJ mol, +43 ± 3 J K mol, and +3.4 ± 0.2 cm mol , which are in line with a dissociatively activated interchange (I) 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(tmdta)(HO)] are k = (5.5 ± 0.5) × 10 s at 298.2 K, ΔH = 43 ± 3 kJ mol, ΔS = ± 30 ± 13 J K mol, and ΔV = +15.7 ± 1.5 cm mol , and those for [Mn(tmdta)(HO)] are k = (1.3 ± 0.1) × 10 s at 298.2 K, ΔH = 37.2 ± 0.8 kJ mol, ΔS = +35 ± 3 J K mol, and ΔV = +8.7 ± 0.6 cm mol. The water containing species, [Fe (tmdta)(HO)] with a fraction of 0.2, is in equilibrium with the water-free hexa-coordinate form, [Fe(tmdta)] . The kinetic parameters for [Fe(tmdta)(H O)] are k = (1.9 ± 0.8) × 10 s at 298.2 K, ΔH = 42 ± 3 kJ mol, ΔS = +36 ± 10 J K mol, and ΔV = +7.2 ± 2.7 cm mol. 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(edta′)(HO)] and [Ni (tmdta′)(HO)] 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)] and the aqua-pentadentate forms [M(L′)(HO)] of the Ni-edta complex, which was studied in detail by variable temperature and pressure UV-vis experiments. For [Ni(edta′)(HO)] (CN 6, pentacoordinated edta) a water substitution rate constant of (2.6 ± 0.2) × 10 s at 298.2 K and ambient pressure was measured, and the activation parameters ΔH, ‡S‡, and ΔV were found to be 34 ± 1 kJ mol, -27 ± 2 J K mol, and +1.8 ± 0.1 cm mol, respectively. For [Ni (tmdta′)(HO)], we found k = (6.4 ± 1.4) × 10 s at 298.2 K, ΔH = 22 ± 4 kJ mol, and ΔS = -59 ± 5 J K mol. The process is referred to as a water substitution instead of a water exchange reaction, since these observations refer to the intramolecular displacement of coordinated water by the carboxylate moiety in a ring-closure reaction. © 2008 American Chemical Society.
APA:
Maigut, J., Meier, R., Zahl, A., & van Eldik, R. (2008). Triggering water exchange mechanisms via chelate architecture. Shielding of transition metal centers by aminopolycarboxylate spectator ligands. Journal of the American Chemical Society, 130(44), 14556-14569. https://doi.org/10.1021/ja802842q
MLA:
Maigut, Joachim, et al. "Triggering water exchange mechanisms via chelate architecture. Shielding of transition metal centers by aminopolycarboxylate spectator ligands." Journal of the American Chemical Society 130.44 (2008): 14556-14569.
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