Substrate Binding Preferences and pK_a Determinations of a Nitrile Hydratase Model Complex: Variable Solvent Coordination to [(bmmp-TASN)Fe]OTf

Authors

Martin G. O'Toole, University of Louisville
Brian Bennett, Marquette UniversityFollow
Mark S. Mashuta, University of Louisville
Craig A. Grapperhaus, University of Louisville

Document Type

Article

Language

eng

Format of Original

9 p.

Publication Date

3-2009

Publisher

American Chemical Society

Source Publication

Inorganic Chemistry

Source ISSN

0020-1669

Original Item ID

doi: 10.1021/ic802180d

Abstract

The five-coordinate iron-dithiolate complex (N,N′-4,7-bis-(2′-methyl-2′-mercatopropyl)-1-thia-4,7-diazacyclononane)iron(III), [LFe]⁺, has been isolated as the triflate salt from reaction of the previously reported LFeCl with thallium triflate. Spectroscopic characterization confirms an S = 1/2 ground state in non-coordinating solvents with room temperature μ_eff = 1.78 μ_Β and electron paramagnetic resonance (EPR) derived g-values of g₁ = 2.04, g₂ = 2.02 and g₃ = 2.01. [LFe]⁺ binds a variety of coordinating solvents resulting in six-coordinate complexes [LFe-solvent]⁺. In acetonitrile the low-spin [LFe-NCMe]⁺ (g₁ = 2.27, g₂ = 2.18, and g₃ = 1.98) is in equilibrium with [LFe]⁺ with a binding constant of K_eq = 4.7 at room temperature. Binding of H₂O, DMF, methanol, DMSO, and pyridine to [LFe]⁺ yields high-spin six-coordinate complexes with EPR spectra that display significant strain in the rhombic zero-field splitting term E/D. Addition of 1 equiv of triflic acid to the previously reported diiron species (LFe)₂O results in the formation of [(LFe)₂OH]OTf, which has been characterized by X-ray crystallography. The aqueous chemistry of [LFe]⁺ reveals three distinct species as a function of pH: [LFe-OH₂]⁺, [(LFe)₂OH]OTf, and (LFe)₂O. The pK_a values for [LFe-OH₂]⁺ and [(LFe)₂OH]OTf are 5.4 ± 0.1 and 6.52 ± 0.05, respectively.

Comments

Accepted version. Inorganic Chemistry, Vol. 48, No. 5 (March 2009): 2300-2308. DOI.

Brian Bennett was affiliated with Medical College of Wisconsin at the time of publication.

Recommended Citation

O'Toole, Martin G.; Bennett, Brian; Mashuta, Mark S.; and Grapperhaus, Craig A., "Substrate Binding Preferences and pK_a Determinations of a Nitrile Hydratase Model Complex: Variable Solvent Coordination to [(bmmp-TASN)Fe]OTf" (2009). Physics Faculty Research and Publications. 18.
https://epublications.marquette.edu/physics_fac/18