Rates of Water Exchange on the [Fe4(OH)2(hpdta)2(H2O)4]0 Molecule and Its Implications for Geochemistry
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- 作者：Adele F. Panasci ; C. Andr茅 Ohlin ; Stephen J. Harley ; William H. Casey
- 刊名：Inorganic Chemistry
- 出版年：2012
- 出版时间：June 18, 2012
- 年：2012
- 卷：51
- 期：12
- 页码：6731-6738
- 全文大小：374K
- 年卷期：v.51,no.12(June 18, 2012)
- ISSN：1520-510X
文摘
The ammonium salt of [Fe4O(OH)(hpdta)2(H2O)4]鈭?/sup> is soluble and makes a monospecific solution of [Fe4(OH)2(hpdta)2(H2O)4]0(aq) in acidic solutions (hpdta = 2-hydroxypropane-1,3-diamino-N,N,N鈥?N鈥?tetraacetate). This tetramer is a diprotic acid with pKa1 estimated at 5.7 卤 0.2 and pKa2 = 8.8(5) 卤 0.2. In the pH region below pKa1, the molecule is stable in solution and 17O NMR line widths can be interpreted using the Swift鈥揅onnick equations to acquire rates of ligand substitution at the four isolated bound water sites. Averaging five measurements at pH < 5, where contribution from the less-reactive conjugate base are minimal, we estimate: kex298 = 8.1 (卤2.6) 脳 105 s鈥?, 螖H = 46 (卤4.6) kJ mol鈥?, 螖S = 22 (卤18) J mol鈥? K鈥?, and 螖V = +1.85 (卤0.2) cm3 mol鈥? for waters bound to the fully protonated, neutral molecule. Regressing the experimental rate coefficients versus 1/[H+] to account for the small pH variation in rate yields a similar value of kex298 = 8.3 (卤0.8) 脳 105 s鈥?. These rates are 104 times faster than those of the [Fe(OH2)6]3+ ion (kex298 = 1.6 脳 102 s鈥?) but are about an order of magnitude slower than other studied aminocarboxylate complexes, although these complexes have seven-coordinated Fe(III), not six as in the [Fe4(OH)2(hpdta)2(H2O)4]0(aq) molecule. As pH approaches pKa1, the rates decrease and a compensatory relation is evident between the experimental 螖H and 螖S values. Such variation cannot be caused by enthalpy from the deprotonation reaction and is not well understood. A correlation between FeIII鈥揙H2 bond lengths and the logarithm of kex298 is geochemically important because it could be used to estimate rate coefficients for geochemical materials for which only DFT calculations are possible. This molecule is the only neutral, oxo-bridged Fe(III) multimer for which rate data are available.
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