Ivanovic-Burmazovic I, Hamza M, van Eldik R (2002)
Publication Type: Journal article, Original article
Publication year: 2002
Original Authors: Ivanovic-Burmazovic I., Hamza M., Van Eldik R.
Publisher: American Chemical Society
Book Volume: 41
Pages Range: 5150-5161
Journal Issue: 20
DOI: 10.1021/ic0200803
A detailed mechanistic study of the substitution behavior of a 3d metal heptacoordinate complex, with a rare pentagonal-bipyramidal structure, was undertaken to resolve the solution chemistry of this system. The kinetics of the complex-formation reaction of [Fe(dapsox)(HO)]ClO (Hdapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with thiocyanate was studied as a function of thiocyanate concentration, pH, temperature, and pressure. The reaction proceeds in two steps, which are both base-catalyzed due to the formation of an aqua-hydroxo complex (pK = 5.78 ± 0.04 and pK = 9.45 ± 0.06 at 25 °C). Thiocyanate ions displace the first coordinated water molecule in a fast step, followed by a slower reaction in which the second thiocyanate ion coordinates trans to the N-bonded thiocyanate. At 25 °C and pH <4.5, only the first reaction step can be observed, and the kinetic parameters (pH 2.5: k = 2.6 ± 0.1 M s, ΔH = 62 ± 3 kJ mol, ΔS = -30 ± 10 J K mol, and ΔV = -2.5 ± 0.2 cm mol) suggest the operation of an I mechanism. In the pH range 2.5 to 5.2 this reaction step involves the participation of both the diaqua and aqua-hydroxo complexes, for which the complex-formation rate constants were found to be 2.19 ± 0.06 and 1172 ± 22 M s at 25 °C, respectively. The more labile aqua-hydroxo complex is suggested to follow an I or D substitution mechanism on the basis of the reported kinetic data. At pH ≥4.5, the second substitution step also can be monitored (pH 5.5 and 25 °C: k = 21.1 ± 0.5 M s, ΔH = 60 ± 2 kJ mol, ΔS = -19 ± 6 J K mol, and ΔV = +8.8 ± 0.3 cm mol, for which an I or D mechanism is suggested. The results are discussed in terms of known structural parameters and in comparison to relevant structural and kinetic data from the literature.
APA:
Ivanovic-Burmazovic, I., Hamza, M., & van Eldik, R. (2002). A detailed mechanistic study of the substitution behavior of an unusual seven-coordinate iron(III) complex in aqueous solution. Inorganic Chemistry, 41(20), 5150-5161. https://doi.org/10.1021/ic0200803
MLA:
Ivanovic-Burmazovic, Ivana, Mohamed Hamza, and Rudi van Eldik. "A detailed mechanistic study of the substitution behavior of an unusual seven-coordinate iron(III) complex in aqueous solution." Inorganic Chemistry 41.20 (2002): 5150-5161.
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