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 FeCl(3) to methanol solutions of L yields [LFeCl(2)]FeCl(4) 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 NaPF(6) in methanol facilitate chloride exchange resulting in a different diastereomer for the [LFeCl(2)](+) cation. X-ray analysis of [LFeCl(2)]PF(6) finds meridional coordination of the three nitrogen donors of L. Electrochemical studies of [LFeCl(2)](+) 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 [LFeCl(2)](+/0) and [LFeCl(OCH(3))](+/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(OH(2))](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.