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

Authors

• Musa S. Shongwe, Sultan Qaboos University
• Usama A. Al-Zaabi, Sultan Qaboos University
• Faizah Al-Mjeni, Sultan Qaboos University
• Carla S. Eribal, Western Michigan University
• Imaddin A. Al-Omari, Sultan Qaboos University
• Hussein H. Hamdeh, Wichita State University
• Dariusz Matoga, Jagiellonian University CracowFollow
• Harry Adams, University of SheffieldFollow
• Michael J. Morris, University of Sheffield
• Arnold L. Rheingold, University of California - San DiegoFollow
• Eckhard Bill, Max-Planck-Institut für Bioanorganische ChemieFollow
• David J. Sellmyer, University of Nebraska at LincolnFollow

Document Type

Article

Date of this Version

2012

Citation

Inorganic Chemistry 51:15 (2012)

Comments

The authors declare no competing financial interest.

Abstract

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 com-plexed 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 [Fe^II(apRen)₂]X₂ (R = H, Me; X– = ClO4–, BPh4–, PF6–) and [Fe^III(hapRen)₂]X (R = Me, Et; X– = ClO_4–, BPh_4–). Single-crystal X-ray analyses of [Fe^II(apRen)₂](ClO4)₂ (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the Fe^II–N bond dis-tances (1.896–2.041 Å) pointing to the ¹A₁ (dπ⁶) ground state; the existence of this spin state was cor-roborated by magnetic susceptibility measurements and Mössbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex [Fe^III(hapMen)₂]ClO₄, determined at 100 K, demonstrated sta-bilization of the ferric state; the compression of the coordinate bonds at the metal center is in accord with the 2T2 (dπ5) ground state. Magnetic susceptibility measurements along with EPR and Möss-bauer spectroscopic techniques have shown that the iron(III) complexes are spin-crossover (SCO) materials. The spin transition within the [Fe^IIIN4O2]+ chromophore was modulated with alkyl substit-uents to afford two-step and one-step ⁶A₁ ↔ ²T₂ transformations in [Fe^III(hapMen)₂]ClO₄ and [Fe^III(hapEen)₂]ClO₄, respectively. Previously, none of the X-salRen- and Xsal2trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron(II) and SCO iron(III) complexes display intense dπ → pπ* and pπ → dπ 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 com-pounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H₂salen-type quadridentate ketimine H₂hapen complexed with an equivalent amount of iron(III) to afford the μ-oxo-monobridged dinuclear complex [{Fe^III(hapen)}₂(μ-O)] exhibiting a distorted square- pyramidal geometry at the metal centers and considerable antiferromagnetic coupling of spins (J ≈ −99 cm−1).