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

摘要

The five-coordinate iron-dithiolate complex (N,N'-4,7-bis-(2'-methyl-2'-mercatopropyl)-1-this-4,7-diazacyclonon_ane)iron(lll), [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_B and electron paramagnetic resonance (EPR) derived g-values of g_1 = 2.04, 9_2 = 2.02 and g_3 = 2.01. [LFe]~+ binds a variety of coordinating solvents resulting in six-coordinate complexes [LFe-solvent]~+. In acetonitrile the low-spin [LFe-NCMe]~+ (g_1 = 2.27, g_2 = 2.18, and g_3 = 1.98) is in equilibrium with [LFe]~+ with a binding constant of K_eq = 4.7 at room temperature. Binding of H_2O, 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-OH_2]~+, [(LFe)_2OH]OTf, and (LFe)_2O. The PK_a values for [LFe-OH_2]~+ and [(LFe)_2OH]OTf are 5.4 ± 0.1 and 6.52 ± 0.05, respectively.