title: Enhancing reactivity via structural distortion.
abstract: To examine how small structural changes influence the reactivity and magnetic properties of biologically relevant metal complexes, the reactivity and magnetic properties of two structurally related five-coordinate Fe(III) thiolate compounds are compared. (Et,Pr)-ligated [Fe(III)(S(2)(Me2)N(3)(Et,Pr))]PF(6) (3) is synthesized via the abstraction of a sulfur from alkyl persulfide ligated [Fe(III)(S(2)(Me2)N(3)(Et,Pr))-S(pers)]PF(6) (2) using PEt(3). (Et,Pr)-3 is structurally related to (Pr,Pr)-ligated [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))]PF(6) (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 approximately 10 degrees, 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 the thermodynamics and kinetics of azide binding. Azide binds reversibly to both (Et,Pr)-3 and (Pr,Pr)-1 in MeOH solutions. The ambient temperature K(eq) describing the equilibrium between five-coordinate 1 or 3 and azide-bound 1-N(3) or 3-N(3) in MeOH is approximately 10 times larger for the (Et,Pr) system. In CH(2)Cl(2), azide binds approximately 3 times faster to 3 relative to 1, and in MeOH, azide dissociates 1 order of magnitude slower from 3-N(3) relative to 1-N(3). The increased on rates are most likely a consequence of the decreased structural rearrangement required to convert 3 to an approximately octahedral structure, or they reflect differences in the LUMO (vs SOMO) orbital population (i.e., spin-state differences). Dissociation rates from both 3-N(3) and 1-N(3) are much faster than one would expect for low-spin Fe(III). Most likely this is due to the labilizing effect of the thiolate sulfur that is trans to azide in these structures.
authors.affiliation: Department of Chemistry, University of Washington, Seattle, Washington 98195, USA.
authors.forename: Dirk
authors.initials: D
authors.lastname: Schweitzer
authors.forename: Jason
authors.initials: J
authors.lastname: Shearer
authors.forename: Durrell K
authors.initials: DK
authors.lastname: Rittenberg
authors.forename: Steven C
authors.initials: SC
authors.lastname: Shoner
authors.forename: Jeffrey J
authors.initials: JJ
authors.lastname: Ellison
authors.forename: Reza
authors.initials: R
authors.lastname: Loloee
authors.forename: Scott
authors.initials: S
authors.lastname: Lovell
authors.forename: David
authors.initials: D
authors.lastname: Barnhart
authors.forename: Julie A
authors.initials: JA
authors.lastname: Kovacs
mesh_terms: D000465:Algorithms; D001386:Azides / Q000737:chemistry*; D002384:Catalysis; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D005290:Ferric Compounds / Q000138:chemical synthesis* / Q000737:chemistry*; D006836:Hydro-Lyases; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D013329:Structure-Activity Relationship; D013696:Temperature
chemical_list: D001386:Azides; D005290:Ferric Compounds; D006836:Hydro-Lyases; C048831:nitrile hydratase