Synthesis, Structure, and Solution Properties of [(mim-TASN)FeCl₂]⁺ and Its μ-Oxo Derivative

A series of iron(III) complexes based on the tetradentate ligand 4-((1-methyl-1H-imidazol-2-yl)methyl)-1-thia-4,7-diazacyclononane (L) has been synthesized, and their solution properties investigated. Addition of FeCl₃ to methanol solutions of L yields [LFeCl₂]FeCl₄ as a dark red solid. X-ray crystallographic analysis reveals a pseudo-octahedral environment around iron(III) with the three nitrogen donors of L coordinated facially. Ion exchange reactions with NaPF₆ in methanol facilitate chloride exchange resulting in a different diastereomer for the [LFeCl₂]⁺ cation. X-ray analysis of [LFeCl₂]PF₆ finds meridional coordination of the three nitrogen donors of L. Electrochemical studies of [LFeCl₂]⁺ in acetonitrile display a single Fe(III)/(II) reduction potential at −280 mV versus ferrocenium/ferrocene. In methanol, a broad cathodic wave is observed because of partial exchange of one chloride for methoxide with half-potentials of −170 mV and −440 mV for [LFeCl₂]^+/0 and [LFeCl(OCH₃)]^+/0, respectively. The equilibrium constants for chloride exchange are 7 × 10⁻⁴ M⁻¹ for Fe(III) and 2 × 10⁻⁸ M⁻¹ for Fe(II). In aqueous solutions chloride exchange yields three accessible complexes as a function of pH. Strongly acidic conditions yield the aqua complex [LFeCl(OH₂)]²⁺ with a measured pK_a of 3.8 ± 0.1. Under mildly acidic conditions, the μ-OH complex [(LFeCl)₂(OH)]³⁺ with a pK_a of 6.1 ± 0.3 is obtained. The μ-oxo complex [(LFeCl)₂(O)]²⁺ is favored under basic conditions. The diiron Fe(III)/Fe(III) complexes [(LFeCl)₂(OH)]³⁺ and [(LFeCl)₂(O)]²⁺ can be reduced by one electron to the mixed valence Fe(III)/Fe(II) derivatives at −170 mV and −390 mV, respectively. From pH dependent voltammetric studies, the pK_a of the mixed valent μ-OH complex [(LFeCl)₂(OH)]²⁺ is calculated at 10.3.