The phthalimide N-oxyl (PINO) radical was generated by the oxidation of N-hydroxyphthalimide (NHPI) with Pb(OAc)4 in acetic acid. The molar absorptivity of PINO* is 1.36 x 10(3) L mol(-1) cm(-1) at lambda(max) 382 nm. The PINO radical decomposes slowly with a second-order rate constant of 0.6 +/- 0.1 L mol(-1) s(-1) at 25 degrees C. The reactions of PINO(*) with substituted toluenes, benzaldehydes, and benzyl alcohols were investigated under an argon atmosphere. The second-order rate constants were correlated by means of a Hammett analysis. The reactions with toluenes and benzyl alcohols have better correlations with sigma+ (rho = -1.3 and -0.41), and the reaction with benzaldehydes correlates better with sigma (rho = -0.91). The kinetic isotope effect was also studied and significantly large values of k(H)/k(D) were obtained: 25.0 (p-xylene), 27.1 (toluene), 27.5 (benzaldehyde), and 16.9 (benzyl alcohol) at 25 degrees C. From the Arrhenius plot for the reactions with p-xylene and p-xylene-d(10), the difference of the activation energies, E(a)(D) - E(a)(H), was 12.6 +/- 0.8 kJ mol(-1) and the ratio of preexponential factors, A(H)/A(D), was 0.17 +/- 0.05. These findings indicate that quantum mechanical tunneling plays an important role in these reactions.

A new class of macrocyclic metal-N(4) complexes [MN(4)](n) (M = Co and Fe) were designed and synthesized based on a triangular ligand. Their unique triangular trinuclear structure provides a high density of active sites and facilitates the reduction of dioxygen via a four-electron pathway. Among them, a [CoN(4)](3)/C catalyst (20 wt %) exhibits high catalytic activity and long-time stability for the oxygen reduction reaction (ORR) in alkaline conditions, superior to the commercial Pt/C catalyst. Such structurally well-defined [MN(4)](n) complexes provide a platform for a new generation of nonprecious metal catalysts (NPMCs) for fuel cell applications.

The reactions of the phthalimide N-oxyl (PINO) radical with several hydrocarbons having different C−H bond dissociation energies (BDEs) were investigated in HOAc at 25 °C. The slope of the Evans−Polanyi plot is 0.38, which indicates that the reactions are mildly exothermic or almost thermoneutral. This finding is supported by the O−H BDE of N-hydroxyphthalimide (NHPI), 375 ± 10 kJ mol-1, obtained by means of a thermodynamic cycle. The observed kinetic isotope effects (KIEs) are related to the reaction free-energy changes, which can be explained by a Marcus-type equation. The comparison of the PINO radical reactions with analogous hydrogen atom abstraction reactions of the dibromide radical implies that HBr2• reactions are more exothermic than the PINO radical reactions. This factor is put forth to explain the different KIEs.

Kinetic data have been obtained for three distinct types of reactions of phthalimide N-oxyl radicals (PINO(.)) and N-hydroxyphthalimide (NHPI) derivatives. The first is the self-decomposition of PINO(.) which was found to follow second-order kinetics. In the self-decomposition of 4-methyl-N-hydroxyphthalimide (4-Me-NHPI), H-atom abstraction competes with self-decomposition in the presence of excess 4-Me-NHPI. The second set of reactions studied is hydrogen atom transfer from NHPI to PINO(.), e.g., PINO(.) + 4-Me-NHPI <=> NHPI + 4-Me-PINO(.). The substantial KIE, k(H)/k(D) = 11 for both forward and reverse reactions, supports the assignment of H-atom transfer rather than stepwise electron-proton transfer. These data were correlated with the Marcus cross relation for hydrogen-atom transfer, and good agreement between the experimental and the calculated rate constants was obtained. The third reaction studied is hydrogen abstraction by PINO(.) from p-xylene and toluene. The reaction becomes regularly slower as the ring substituent on PINO(.) is more electron donating. Analysis by the Hammett equation gave rho = 1.1 and 1.8 for the reactions of PINO(.) with p-xylene and toluene, respectively.