Manolis M. Roubelakis

Proton-coupled electron transfer kinetics for the hydrogen evolution reaction of hangman porphyrins

Cobalt hangman porphyrins catalyze the hydrogen evolution reaction (HER). The hangman group is observed to facilitate HER by mediating a proton-coupled electron transfer (PCET) reaction. The details of the PCET pathway have been determined by comparing rate constants associated with the ET and PT processes of the hangman system to those of the corresponding values measured for porphyrins that lack an internal proton relay. A rapid intramolecular proton transfer from the carboxylic acid hanging group to the reduced cobalt centre of 8.5 × 106 s−1 provides a facile pathway for the formation of Co(II)H, which leads directly to H2 generation.

Radical Reactivity of Aza[60]fullerene: Preparation of Monoadducts and Limitations

Six aza[60]fullerene monoadducts were synthesized by the thermal reaction between the azafullerene radical C(59)N* and 9-alkyl-substituted fluorenes, 9,10-dihydroanthracene, or xanthene. Unlike fluorenes, dihydroanthracene, and xanthene, the structurally related substituted diphenylmethanes, ethylbenzene, cumene, 1,2-diphenylethane, 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene, 10,11-dihydro-5H-dibenzo[a,d]cycloheptene, 9-methylanthracene, and 9-benzylanthracene do not lead to the isolation of azafullerene monoadducts. Moreover, 1,2-dichlorobenzene, the most commonly utilized solvent for azafullerene reactions, reacts slowly with the azafullerenyl radical C(59)N* affording the corresponding aza[60]fullerene monoadduct.

Solvent‐Dependent Changes in the Triazolinedione–Alkene Ene Reaction Mechanism

The influence of the solvent on the triazolinedione-alkene ene reaction mechanism has been investigated. Both inter- and intramolecular kinetic isotope effects with tetramethylethylenes and 2,2,2-(trideuterio)methyl-7-methyl-2,6-octadiene-[D3]-1,1,1 provide, for the first time, strong evidence for changes in the mechanism of the reaction on going from non-protic to polar protic solvents. In non-protic polar or apolar solvents, an aziridinium imide that equilibrates to an insignificant extent with an open intermediate (a dipolar or a polarized biradical) is formed irreversibly in the first, rate-determining step of the reaction, which is followed by fast hydrogen abstraction. On the contrary, in polar protic solvents, hydrogen abstraction is rate limiting, allowing the main dipolar intermediate to equilibrate with its open intermediate(s) as well as with the starting reagents.