Introducing a New Azoaromatic Pincer Ligand. Isolation and Characterization of Redox Events in Its Ferrous Complexes

摘要

The isolation and complete characterization of a new bis-azoaromatic ligand, 2,6-bis(phenylazo)pyridine (L), are described, and its coordination to iron(II) is reported. A pseudo-trigonal-bipyramidal mixed-ligand complex of iron(II), FeLCl_2 (1), and a homoleptic octahedral iron complex, mer-[Fe(L)_2]ClO_4 [2]ClO_4, have been synthesized from L and FeCl2 or hydrated Fe(ClO_4)_2, respectively, in boiling methanol. Determination of the X-ray crystallographic structure together with magnetic data (≈ 5.06 μ_B) and Mo?ssbauer analysis of 1 established a high-spin Fe(II) complex ligated by one neutral 2,6-bis(phenylazo)pyridine ligand. The X-ray crystallographic structure (showing d_(N-N) > 1.30 ?), Mo?ssbauer data, and magnetic susceptibility measurements (≈1.65 μ_B) as well as a nearly isotropic EPR signal with only a small metal contribution at g = 1.968, on the other hand, suggest a low-spin Fe(II) complex with a one-electron-reduced radical ligand coordination in [2]ClO_4. The ligand and the metal complexes have well-behaved redox properties, with the ligand(s) functioning as the redox-active site(s) responsible for redox events. The uncoordinated ligand, L, displays a reversible one-electron wave at -1.07 V and a quasi-reversible wave at -1.39 V. The partially reduced ligand L~(?-) shows a single-line EPR spectrum at g = 2.001, signifying that L~(?-) is a free radical. While complex 1 shows a reversible reduction at -0.08 V and an irreversible cathodic response at -0.98 V, the bis-chelate [2]ClO_4 undergoes a reversible one-electron oxidation at 0.54 V and three successive reversible one-electron reductions at -0.18, -0.88, and -1.2 V, all occurring at the ligands without affecting the metal ion oxidation state. The electronic structures of the parent monocationic complex [2]~+ and its oxidized and reduced forms, generated by exhaustive electrolyses, have been characterized by using a host of spectroscopic techniques and density functional theory (DFT). It is found that the 2,6-bis(phenylazo)pyridine ligand (L) is truly redox noninnocent and is capable of coordinating transition-metal centers in its neutral ([L]~0), monoanionic monoradical ([L~?]~-), and dianionic diradical ([L~(??)]~(2-)) forms.