Prato Reaction of M₃N@I_h‑C₈₀ (M = Sc, Lu, Y, Gd) with Reversible Isomerization

The 1,3-dipolar cycloaddition of an azomethine ylide (Prato reaction) with M₃N@I_h-C₈₀ (denoted as M₃N@C₈₀; M = Sc, Lu, Y, Gd) was carried out to obtain fulleropyrrolidinebis­(carboxylic acid) derivatives as scaffolds for the preparation of various functionalized M₃N@C₈₀ materials. The formation of two monoadduct isomers (the [6,6]- and [5,6]-adducts) were detected by HPLC and identified by NMR and vis/NIR spectroscopies. In each Prato reaction with M₃N@C₈₀, the initial addition gave a [6,6]-adduct of the I_h-C₈₀ cage, and subsequently, a [5,6]-adduct was obtained by complete or partial thermal isomerization via a rearrangement reaction. The reaction rate of the latter thermal conversion of the adducts was dependent on the size of the metal cluster inside C₈₀, and interestingly, in the reactions of Y₃N@C₈₀ and Gd₃N@C₈₀, this conversion was found to be reversible for the first time. Detailed kinetic studies provided the enthalpy and entropy barriers for the reactions of the adducts of Lu₃N@C₈₀, Y₃N@C₈₀, and Gd₃N@C₈₀. The utility of the obtained Prato adducts was confirmed by preparation of a highly water-soluble Gd₃N@C₈₀ derivative.