Experimental determination of the reorganization energy of the NO2+/NO2 redox couple. Comparison with theory

Abstract

The reaction between the heteropolyanion Co II W 12 O 40 6؊ , in the form of its (Bu 4 N) 6 (2) or K 2 (Bu 4 N) 4 salt (3), and NO 2 ϩ BF 4 ؊ has been studied kinetically in acetonitrile or nitromethane at 26 ЊC. In acetonitrile, the rate constants of 2 and 3 are almost identical, 0.097 and 0.073 dm 3 mol ؊1 s ؊1 , whereas in nitromethane 2 reacts >10 2 times faster, k = 13 dm 3 mol ؊1 s ؊1 (3 has not been studied in nitromethane for solubility reasons). A second inorganic species, (Ph 4 P) 2 Os IV Cl 6 , has also been similarly investigated, for which k = 1.4 × 10 3 dm 3 mol ؊1 s ؊1 in acetonitrile. The rate constants of the reaction of 2 (3) with NO 2 ϩ could be correctly estimated by the Marcus theory of outer-sphere electron transfer, using the exceptionally high value of 140 kcal mol ؊1 of the reorganization energy of the NO 2 ϩ /NO 2 homoexchange reaction which earlier had been estimated theoretically. Thus it seems that this value is supported by experimental evidence.The reversible potential of NO 2 ϩ /NO 2 in nitromethane has been determined to be 0.95 V vs. the ferricinium/ferrocene couple, thus confirming previously reported values in the vicinity of 1.0 V, but in disagreement with a higher value, 1.62 V.

Dynamics and the Regiochemistry of Nitration of Toluene

“…18,19 With less electron-rich arenes such as toluene, however, Eberson argued that the uphill electron transfer would be far too slow to account for the observed rates of reactions. 20 Notably, the reactivity of aromatics correlates well with the stability of σ -complexes on a broad scale that includes both electron-rich and electron-poor aromatics. 21 As with Olah before him, Kochi found the selectivity observed in toluene nitrations to be a sticking point.…”

“…The idea that electron transfer is involved in nitration has a long history in the literature . In reactions of highly electron-rich aromatics, Kochi and others were able to strongly support the viability of such mechanisms. , With less electron-rich arenes such as toluene, however, Eberson argued that the uphill electron transfer would be far too slow to account for the observed rates of reactions . Notably, the reactivity of aromatics correlates well with the stability of σ-complexes on a broad scale that includes both electron-rich and electron-poor aromatics .…”

Charge‐Transfer Effects on Arene Structure and Reactivity

“…(The latter underscores the caveat that the viability of ET mechanisms cannot be pre-judged [67] solely by the value of DG ET ).…”

“…While the reduction potentials of NO þ and NO 2 þ are comparable in different solvents [67], there is a large difference between NO þ and NO 2 þ in terms of the magnitudes of their reorganization energies, which differ by a factor of 2, i.e. : l NO2 ¼ 140 kcal mol À1 and l NO ¼ 69 kcal mol À1 , largely as a result of the requisite bending of ONO attendant upon the reduction of the almost linear (triatomic) cation [67]. In order to follow through with the comparison to NO þ , let us consider the corresponding ET process, viz.…”

Nitração aromática: substituição eletrofílica ou reação com transferência de elétrons?

“…Mesmo considerando-se limites inferiores para a energia de reorganização do par NO 2 + /NO 2 38 , parece estar em considerável erro. Medidas mais recentes convergem para um valor de 1,0 V, o que implicaria que a transferência de elétrons seria um processo termodinamicamente inviável, mesmo para substratos altamente ativados, tal como o naftaleno 53 .…”