Metal Atom Lability in Polynuclear Complexes

The asymmetric oxidation product [(L-Ph)-Fe-3(mu-Cl)](2) [(LH6)-L-Ph = MeC(CH2NHPh-o-NHPh)(3)], where each trinudear core is comprised of an oxidized diiron unit [Fe-2](5+) and an isolated trigonal pyramidal ferrous site, reacts with MCl2 salts to afford heptanuclear bridged structures of the type (L-Ph)(2)Fe6M(mu-Cl)(4)(thf)(2), where M = Fe or Co. Zero-field, Fe-57 Mossbauer 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 [(L-Ph)Fe-3(mu-Cl)](2) with CoCl2 (2 or 5 equivalents) followed by precipitation via addition of acetonitrile afforded trinudear products where one or two irons, respectively, can be substituted within the trinuclear core. Metal atom substitution was verified by H-1 NMR, Fe-57 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 ([M-2](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 S = 1 unit ([FeCo](5+)) or S = 1/2 ([Co-2](5+)) that is weakly antiferromagnetically coupled to the high spin (S = 2) ferrous site. The kinetic pathway for metal substitution was probed via reaction of [(L-Ph)Fe-3(mu-Cl)](2) with isotopically enriched (FeCl2)-Fe-57(thf)(2), the results of which suggest rapid equilibration of Fe-57 into both the M2+ site and oxidized diiron site, achieving a 1:1 mixture.