Repozytorium
Wyszukaj
Kolekcje
Inne
Disproportionation of iron(III) porphyrin π-cation radicals in the presence of sterically hindered pyridines. Spectroscopic detection of asymmetric highly oxidized intermediates.
Autorzy
Rok wydania
1996
Czasopismo
Numer woluminu
35
Strony
1136-1147
DOI
10.1021/ic950876k
Kolekcja
Język
Angielski
Typ publikacji
Artykuł
The reactivity of iron(III) tetraphenylporphyrin π-cation radical (TPP•)FeIII(ClO4)2, (1-1) iron(III) tetra-p-tolylporphyrin π-cation radical (TTP•)FeIII(ClO4)2 (1-2) and iron(III) tetramesitylporphyrin π-cation radical (TMP•)FeIII(ClO4)2 (1-3) complexes with 2,4,6-collidine, 2,3,6-collidine, 2-picoline, 2,6-di-tert-butylpyridine, and 2,6-dibromopyridine has been examined by 1H NMR spectroscopy in dichloromethane-d2 solution at low temperatures. These complexes undergo hydration processes which are essential in the generation of highly oxidized species via acid base/equilibria of coordinated water followed by disproportionation pathway, giving as sole stable products [(TPP•)FeIIIOFeIII(TPP)]+ (4-1), [(TTP•)FeIIIOFeIII(TTP)]+ (4-2), and (TMP)FeIII(OH) (6) respectively. The sterically hindered pyridines act as efficient proton scavengers. Two novel highly oxidized iron complexes have been detected by 1H NMR spectroscopy after addition of 2,4,6-collidine to (TTP•)FeIII(ClO4)2 or (TPP•)FeIII(ClO4)2 in dichloromethane-d2 solution at 202 K. New intermediates have been identified as iron porphyrin N-oxide complexes, i.e., iron(III) porphyrin N-oxide cation radical (2-n) and iron(IV) porphyrin N-oxide radical (3-n). The 1H NMR results indicate that the D4h symmetry of the parent iron(III) π-cation radical is drastically reduced upon disproportionation in the presence of proton scavengers. Both species are very unstable and were observed from 176 to 232 K. The intermediate 2-2 has a 1H NMR spectrum which demonstrates large hyperfine shifts (ppm) for the meso p-tolyl substituents (ortho 98.0, 94.8, 92.9, 91.7; meta −34.8, −38.7, −41.5, −42.3; p-CH3 −86.3, −88.0) which are consistent with presence of an N-substituted iron porphyrin radical in the product mixture. The characteristic 1H NMR spectrum of 2-2 includes six pyrrole resonances at 149.6, 118.2, 115.4, 88.3, 64.6, and 55.7 ppm at 202 K, i.e., in the positions corresponding to iron(III) high-spin porphyrins. On warming to 222 K, the pyrrole resonances broaden and then coalesce pairwaise. Such dynamic behavior is accounted for by a rearrangement mechanism which involves an inversion of the porphyrin puckering. The pattern of p-tolyl resonances revealed the cation radical electronic structure of 3-2. The p-tolyl resonances are divided in two distinct sets showing opposite direction of the isotropic shift for the same ring positions. The pyrrole resonances of 3-2 also demonstrated downfield and upfield shifts. A disproportionation mechanism of the hydrated iron porphyrin cation radicals to generate 2 and 3 has been proposed. Both intermediates react with triphenylphosphine to produce triphenylphosphine oxide and high-spin iron porphyrins. Addition of 2,4,6-collidine to (TMP•)FeIII(ClO4)2 does not produce analogs of 2 and 3 found for sterically unprotected porphyrins. It results instead in the formation of a variety of X(TMP•)FeIVO (5) complexes also accounted for by the disproportionation process.
Słowa kluczowe
Cations, Iron, Phenyls, Pyrroles, Quantum mechanics
Adres publiczny
https://doi.org/10.1021/ic950876k
Strona internetowa wydawcy
Podobne publikacje
NMR investigation of β-substituted high-spin and low-spin iron(III) tetraphenylporphyrins.
Wojaczyński Jacek, Latos-Grażyński Lechosław, Hrycyk Witold, Pacholska Ewa, Rachlewicz Krystyna, Szterenberg Ludmiła
Dendrimers with a copper(I) bis(phenanthroline) core: synthesis, electronic properties, and kinetics.
Gumienna-Kontecka Elżbieta, Rio Y., Bourgogne C., Elhabiri M., Louis R., Albrecht-Gary A.-M., Nierengarten J.-F.
Reactions of nickel(II) 2-aza--5,10,15,20-tetraphenyl-21-carbaporphyrin with methyl iodide. The first structural characterization of a paramagnetic organometallic nickel(II) complex.
Chmielewski Piotr J., Latos-Grażyński Lechosław, Głowiak Tadeusz