Metal Atom Lability in Polynuclear Complexes

The asymmetric oxidation product [(^PhL)­Fe₃(μ-Cl)]₂ [^PhLH₆ = MeC­(CH₂NHPh-<i>o</i>-NHPh)₃], where each trinuclear core is comprised of an oxidized diiron unit [Fe₂]⁵⁺ and an isolated trigonal pyramidal ferrous site, reacts with MCl₂ salts to afford heptanuclear bridged structures of the type (^PhL)₂Fe₆M­(μ-Cl)₄(thf)₂, where M = Fe or Co. Zero-field, ⁵⁷Fe Mössbauer analysis revealed the Co resides within the trinuclear core subunits, not at the octahedral, halide-bridged MCl₄(thf)₂ position indicating Co migration into the trinuclear subunits has occurred. Reaction of [(^PhL)­Fe₃(μ-Cl)]₂ with CoCl₂ (2 or 5 equivalents) followed by precipitation via addition of acetonitrile afforded trinuclear products where one or two irons, respectively, can be substituted within the trinuclear core. Metal atom substitution was verified by ¹H NMR, ⁵⁷Fe Mossbauer, single crystal X-ray diffraction, X-ray fluorescence, and magnetometry analysis. Spectroscopic analysis revealed that the Co atom(s) substitute(s) into the oxidized dimetal unit ([M₂]⁵⁺), while the M²⁺ site remains iron-substituted. Magnetic data acquired for the series are consistent with this analysis revealing the oxidized dimetal unit comprises a strongly coupled <i>S</i> = 1 unit ([FeCo]⁵⁺) or <i>S</i> = 1/2 ([Co₂]⁵⁺) that is weakly antiferromagnetically coupled to the high spin (<i>S</i> = 2) ferrous site. The kinetic pathway for metal substitution was probed via reaction of [(^PhL)­Fe₃(μ-Cl)]₂ with isotopically enriched ⁵⁷FeCl₂(thf)₂, the results of which suggest rapid equilibration of ⁵⁷Fe into both the M²⁺ site and oxidized diiron site, achieving a 1:1 mixture