Accessibility and Selective Stabilization of the Principal Spin States of Iron by Pyridyl versus Phenolic Ketimines: Modulation of the 6A1 ↔ 2T2 Ground-State Transformation of the [FeN4O2]+ Chromophore

Several potentially tridentate pyridyl and phenolic Schiff bases (apRen and HhapRen, respectively) were derived from the condensation reactions of 2-acetylpyridine (ap) and 2′-hydroxyacetophenone (Hhap), respectively, with N-R-ethylenediamine (RNHCH2CH2NH2, Ren; R = H, Me or Et) and complexed in situ with iron(II) or iron(III), as dictated by the nature of the ligand donor set, to generate the six-coordinate iron compounds [FeII(apRen)2]X2 (R = H, Me; X– = ClO4–, BPh4–, PF6–) and [FeIII(hapRen)2]X (R = Me, Et; X– = ClO4–, BPh4–). Single-crystal X-ray analyses of [FeII(apRen)2](ClO4)2 (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the FeII–N bond distances (1.896–2.041 A) pointing to the 1A1 (dπ6) ground state; the existence of this spin state was corroborated by magnetic susceptibility measurements and Mossbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex [FeIII(hapMen)2]ClO4, determined at 100 K, demonstrated stabilization of the ferric state; the...