Regioselective Synthesis and Characterization of Tris- and Tetra-Prato Adducts of MN@C (M = Y, Gd).

The tris- and tetra-adducts of MN@C metallofullerenes were synthesized and characterized for the first time. The 1,3-dipolar cycloaddition (Prato reaction) of YN@-C and GdN@-C with an excess of -ethylglycine and formaldehyde provided tris- and tetra-fulleropyrrolidine adducts in a regioselective manner. Purification by HPLC and analyses of the isolated peaks by NMR, MS, and vis-NIR spectra revealed that the major products were four tris- and one tetra-isomers for both YN@-C and GdN@-C. Considering the large number of possible isomers (e.g., at least 1140 isomers for the tris-adduct), the limited number of isomers obtained indicated that the reactions proceeded with high regioselectivity. NMR analyses of the YN@-C adducts found that the tris-adducts were all-[6,6]- or [6,6][6,6][5,6]-isomers and that some showed mutual isomerization or remained intact at room temperature. The tetra-adduct obtained as a major product was all-[6,6] and stable. For the structural elucidation of GdN@-C tris- and tetra-adducts, density functional theory (DFT) calculations were performed to estimate the relative stabilities of tris- and tetra-adducts formed upon Prato functionalization of the most pyramidalized regions of the fullerene structure. The most stable structures corresponded to additions on the most pyramidalized (i.e., strained) bonds. Taking together the experimental vis-NIR spectra, NMR assignments, and the computed relative DFT stabilities of the potential tris- and tetra-adducts, the structures of the isolated adducts were elucidated. Electron resonance (ESR) measurements measurements of pristine, bis-, and tris-adducts of GdN@C suggested that the rotation of the endohedral metal cluster slowed upon increase of the addition numbers to C cage, which is favored for accommodating the Gd atoms of the relatively large GdN cluster inner space at the sp addition sites. This is presumably related to the high regioselectivity in the Prato addition reaction driven by the strain release of the GdN@C fullerene structure.