| Abstract
| - 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.
- The structural, reactivity, and magnetic properties of two structurally related five-coordinate Fe(III) thiolate compounds 1 and 3 are compared. Removal of a methylene unit from the ligand backbone of 1 subtly distorts the geometry by opening a face (shown left), alters the magnetic properties by stabilizing the S = 1/2 relative to the S = 3/2 state, and increases reactivity significantly.
|
