Radicals of Free and Zinc(II)‐Coordinated α‐Azophenols

Abstract

The tridentate 2-<i>tert</i>-butyl-4-methoxy-6-(quinolin-8-ylazo)phenol ligand HL has been synthesized and structurally characterized. Its redox activity has been investigated by electrochemical measurements and DFT calculations. Oxidation of HL (irreversible process) affords primarily a hydrogen-bonded phenoxyl radical, whereas its reduction affords an iminosemiquinonate radical species. The reaction of two equivalents of HL with Zn(OAc)₂ affords the zinc complex Zn(L)₂, which has been structurally characterized. Its oxidation is easier than that of HL and is reversible. The EPR spectrum of [Zn(L)(L^·)]⁺ shows an unresolved (<i>S</i> = 1/2) signal at <i>g</i> = 2.005, while [Zn(L^·)]²⁺ exhibits spin triplet resonances (|<i>D</i>| = 0.0118 cm^–1 and <i>E/D</i> = 0). The extra delocalization of spin density on the azo group contributes to a lowering of the <i>D</i> value relative to those of Schiff base derivatives. The temperature dependence of the EPR signal and DFT calculations reveal an excited state for the spin triplet and a weak exchange coupling constant J = –2.73 cm^–1

Copper(II)‐Coordinated α‐Azophenols: Effect of the Metal‐Ion Geometry on Phenoxyl/Phenolate Oxidation Potential and Reactivity

“…In addition, a broad band appears at approximately 1000 nm in the spectra of the cations. This evolution closely matches that observed during oxidation of the zinc complex of L 1 H {[Zn II (L 1 ) 2 ]}, which produces a phenoxyl radical species,9 and this supports the assignment of the reversible oxidation wave of 1 and 2 as the phenoxyl/phenolate redox couple. Time‐dependent DFT calculations, if considering a triplet state for 1 + (see below), correctly predict a band at 904 nm (ƒ = 0.05), which is assigned to an azoquinoline→phenoxyl ligand‐to‐ligand charge transfer (LLCT) transition, and an intense band at 578 nm (ƒ = 0.123), which is assigned to a charge‐transfer (CT) transition involving the azophenoxyl moiety.…”

“…The UV/Vis spectra of the complexes are characterized by two intense absorption bands at 384 (12360 M –1 cm –1 ) and 568 nm (12220 M –1 cm –1 ) for 1 and at 373 (13920 M –1 cm –1 ) and 575 nm (12390 M –1 cm –1 ) for 2 . These bands are comparable to those of the free L 1 H ligand and its zinc complex,9 which shows that they involve predominantly ligand‐centered orbitals (see the Supporting Information for the time‐dependent DFT assignment). The powder X‐band electron paramagnetic resonance (EPR) spectra of 1 and 2 are rather uninformative as a result of line broadening, but measurements at the Q‐band frequency show slightly rhombic ( S = 1/2) spectra with resonances at g 1 = 2.193, g 2 = 2.049, and g 3 = 2.034 for 1 and at g 1 = 2.208, g 2 = 2.052, and g 3 = 2.034 for 2 (Figure 2).…”

“…The new azo ligand L 2 H [2‐ tert ‐butyl‐4‐methoxy‐6‐(2‐methylquinolin‐8‐ylazo)phenol] was prepared by using a classical method consisting in the diazotization of the appropriate 8‐aminoquinoline with NaNO 2 in concentrated hydrochloric acid and by coupling the obtained diazonium salts with 2‐ tert ‐butyl‐4‐methoxyphenol in a way similar to that used to prepare L 1 H 9. To lower the amount of byproduct, we used fresh 8‐amino‐2‐methylquinoline, which was readily synthesized by reduction of commercially available 2‐methyl‐8‐nitroquinoline with dihydrogen over Pearlman's catalyst.…”

Synthesis, crystal structure and photophysical properties of chlorido[(E)-3-hydroxy-2-methyl-6-(quinolin-8-yldiazenyl)phenolato]copper(II) monohydrate

“…The N2-N3 bond distance of 1.293 (4) A ˚is typical for the N N double bond of an azo group. The structural features of the aromatic rings and the C11-O1 single-bond length of 1.300 (4) A ˚also indicate that the ligand adopts the azo structure, rather than the hydrazone structure, which is similar to the structures observed in analogous azo-metal complexes with other metals, including Ni, Cu, and Zn (Cai et al, 2016;Kochem et al, 2011Kochem et al, , 2014.…”