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 [Feu(apRen)(2)]X-2 (R = H, Me; X- = ClO4-, BPh4-, PF6-) and [Fe-II(hapRen)(2)]X (R = Me, Et; X- = ClO4-, BPh4-). Single-crystal X-ray analyses of [Fe-II(apRen)(2)] (ClO)(2) (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the Fe-II-N bond distances (1.896-2.041 angstrom) pointing to the (1)A(1) (d(n)(6)) 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 [Fe-III(hapMen)(2)]ClO4, determined at 100 K, demonstrated stabilization of the ferric state; the compression of the coordinate bonds at the metal center is in accord with the T-2(2) (d(pi)(5)) ground state. Magnetic susceptibility measurements along with EPR and Mossbauer spectroscopic techniques have shown that the iron(III) complexes are spin-crossover (SCO) materials. The spin transition within the [(FeN4O2)-N-III](+) chromophore was modulated with alkyl substituents to afford two-step and one-step (6)A(1) <-> T-2(2) transformations in [Fe-III(hapMen)(2)]ClO4 and [Fe-III(hapMen)(2)]ClO4, respectively. Previously, none of the X-salRen- and X-sal(2)trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron(II) and SCO iron(III) complexes display intense d(pi) -> p(pi)* and p(pi) -> d(pi) CT visible absorptions, respectively, which account for the spectacular color differences. All the complexes are redox-active; as expected, the one-electron oxidative process in the divalent compounds occurs at higher redox potentials than does the reverse process in the trivalent compounds. The cyclic voltammograms of the latter compounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H(2)salen-type quadridentate ketimine H2hapen complexed with an equivalent amount of iron(III) to afford the mu-oxo-monobridged dinuclear complex [{Fe-III(hapen)}(2)(mu-O)] exhibiting a distorted square-pyramidal geometry at the metal centers and considerable antiferromagnetic coupling of spins (J approximate to -99 cm(-1)).