The redox properties of ferrocenyl-substituted aryl azines

Sauro, Vittorio A; Workentin, Mark S
March 2002
Canadian Journal of Chemistry;Mar2002, Vol. 80 Issue 3, p250
Academic Journal
A series of ferrocenyl substituted azines (1-Fc/Ar, where Ar = 4-NO[sub 2] C[sub 6] H[sub 4] , 4-CNC[sub 6] H[sub 4] , 4-OCH[sub 3] C[sub 6] H[sub 4] , C[sub 5] H[sub 4] N, ferrocene, anthracene, and pyrene) were investigated by electrochemical and photochemical techniques. All the 1-Fc/Ar exhibited oxidation waves within 60 mV of each other, consistent with the expected oxidation of the ferrocene moiety. The reduction properties of 1-Fc/Ar is governed by the nature of the Ar substituent. The standard reduction potentials suggest that ferrocene has comparable electron donating abilities as a 4-methoxyphenyl and 4-dimethyl aminophenyl group. The anthracenyl azines exhibit one-electron reversible reduction followed by dimerization of the radical anion resulting in dimerization rate constants (k[sub d] ) between 5.1 × 10[sup 4] and 1.5 × 10[sup 5] M[sup –1] s[sup –1] . 1-Fc/Anth and related azines undergo photochemical E/Z isomerization of the C=N bonds to produce E/Z and Z/Z isomers from the thermodynamically most stable E/E form. Fluorescence at 77 K, was observed for these compounds only after long- wavelength irradiation to produce a mixture of E/E, E/Z, and Z/Z isomers. Fluorescence quantum yields of 0.042, 0.090, and 0.176 were determined for 2-Anth/H, 2-Anth/Anth, and 1-Fc/Anth, respectively. The electrochemical, photochemical, and X-ray data suggest that the azine unit is a conjugation "limiter" and may be a general characteristic of azine molecules.Key words: azines, electrochemistry, cyclic voltammetry, reduction, electron transfer.Faisant appel à des techniques électrochimiques ainsi que photochimiques, on a étudié une série d'azines de ferrocényles substitués (1-Fc/Ar dans lesquels Ar = 4-NO[sub 2] C[sub 6] H[sub 4] , 4-CNC[sub 6] H[sub 4] , 4-OCH[sub 3] C[sub 6] H[sub 4] , C[sub 5] H[sub 4] N, ferrocène, anthracène et pyrène). Tous les 1-Fc/Ar présentent des vagues d'oxydation à moins de 60 mV les uns des autres, ce à quoi on peut s'attendre pour l'oxydation de la partie ferrocène. Les propriétés de réduction des 1-Fc/Ar sont définies par la nature du substituant Ar. Les potentiels de réduction standards suggèrent que les propriétés du ferrocène en ce qui a trait à ses possibilités de donner des électrons sont semblables à celles des groupes 4-méthoxyphényle et 4-diméthylaminophényle. Les azines dérivés de l'anthracène donnent lieu à une réduction réversible à un électron suivie d'une dimérisation de l'anion radical avec des constantes de vitesse de dimérisation (k[sub d] ) qui vont de 5,1 × 10[sup 4] à 1,5 × 10[sup 5] M[sup –1] s[sup –1] . La 1-Fc/Anth et les azines apparentées subissent des isomérisations photochimiques E/Z des liaisons C=N qui conduisent à des mélanges d'isomères E/Z et Z/Z à partir de la forme E/E, thermodynamiquement la plus stable. Ces composés ne donnent lieu à de la fluorescence à 77 K qu'après avoir été soumis à une irradiation à grande longueur d'onde qui permet de produire un mélange des isomères E/E, E/Z et Z/Z. On a déterminé des rendements quantiques de fluorescence de 0,042, 0,090 et 0,176 respectivement pour les 2-Anth/H, 2-Anth/Anth et 1-Fc/Anth. Les données électrochimiques, photochimiques ainsi que celles relatives à la diffraction des rayons X suggèrent que l'unité azine « limite » la conjugaison et que cette propriété est peut-être une caractéristique générale pour les molécules d'azine.Mots clés : azines, électrochimie, voltampérométrie cyclique, réduction, transfert d'électron.[Traduit par la Rédaction]


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