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

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), FeLCl2 (1), and a homoleptic
octahedral iron complex, mer-[Fe­(L)2]­ClO4 [2]­ClO4, have been synthesized from
L and FeCl2 or hydrated Fe­(ClO4)2, respectively, in boiling methanol. Determination of the X-ray crystallographic
structure together with magnetic data (≈ 5.06 μB) and Mössbauer 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 dN–N > 1.30 Å), Mössbauer data, and magnetic
susceptibility measurements (≈ 1.65 μB) 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]­ClO4. 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]­ClO4 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.