Enhancing Reactivity via Structural Distortion
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- 作者：Dirk Schweitzer ; Jason Shearer ; Durrell K. Rittenberg ; Steven C. Shoner ; Jeffrey J. Ellison ; Reza Loloee ; Scott Lovell ; David Barnhart ; and Julie A. Kovacs
- 刊名：Inorganic Chemistry
- 出版年：2002
- 出版时间：June 17, 2002
- 年：2002
- 卷：41
- 期：12
- 页码：3128 - 3136
- 全文大小：138K
- 年卷期：v.41,no.12(June 17, 2002)
- ISSN：1520-510X
文摘
To examine how small structural changes influence the reactivity and magnetic properties of biologically relevantmetal complexes, the reactivity and magnetic properties of two structurally related five-coordinate Fe(III) thiolatecompounds are compared. (Et,Pr)-ligated [Fe(III)(S2Me2N3(Et,Pr))]PF6 (3) is synthesized via the abstraction of asulfur from alkyl persulfide ligated [Fe(III)(S2Me2N3(Et,Pr))-Spers]PF6 (2) using PEt3. (Et,Pr)-3 is structurally related to(Pr,Pr)-ligated [Fe(III)(S2Me2N3(Pr,Pr))]PF6 (1), a nitrile hydratase model compound previously reported by our group,except it contains one fewer methylene unit in its ligand backbone. Removal of this methylene distorts the geometry,opens a S-Fe-N angle by ~10
, alters the magnetic properties by stabilizing the S = 1/2 state relative to the S= 3/2 state, and increases reactivity. Reactivity differences between 3 and 1 were assessed by comparing thethermodynamics and kinetics of azide binding. Azide binds reversibly to both (Et,Pr)-3 and (Pr,Pr)-1 in MeOHsolutions. The ambient temperature Keq describing the equilibrium between five-coordinate 1 or 3 and azide-bound1-N3 or 3-N3 in MeOH is ~10 times larger for the (Et,Pr) system. In CH2Cl2, azide binds ~3 times faster to 3relative to 1, and in MeOH, azide dissociates 1 order of magnitude slower from 3-N3 relative to 1-N3. The increasedon rates are most likely a consequence of the decreased structural rearrangement required to convert 3 to anapproximately octahedral structure, or they reflect differences in the LUMO (vs SOMO) orbital population (i.e.,spin-state differences). Dissociation rates from both 3-N3 and 1-N3 are much faster than one would expect forlow-spin FeIII. Most likely this is due to the labilizing effect of the thiolate sulfur that is trans to azide in thesestructures.
, alters the magnetic properties by stabilizing the S = 1/2 state relative to the S= 3/2 state, and increases reactivity. Reactivity differences between 3 and 1 were assessed by comparing thethermodynamics and kinetics of azide binding. Azide binds reversibly to both (Et,Pr)-3 and (Pr,Pr)-1 in MeOHsolutions. The ambient temperature Keq describing the equilibrium between five-coordinate 1 or 3 and azide-bound1-N3 or 3-N3 in MeOH is ~10 times larger for the (Et,Pr) system. In CH2Cl2, azide binds ~3 times faster to 3relative to 1, and in MeOH, azide dissociates 1 order of magnitude slower from 3-N3 relative to 1-N3. The increasedon rates are most likely a consequence of the decreased structural rearrangement required to convert 3 to anapproximately octahedral structure, or they reflect differences in the LUMO (vs SOMO) orbital population (i.e.,spin-state differences). Dissociation rates from both 3-N3 and 1-N3 are much faster than one would expect forlow-spin FeIII. Most likely this is due to the labilizing effect of the thiolate sulfur that is trans to azide in thesestructures.