Trimetallic Nitride Endohedral Metallofullerenes:  Reactivity Dictated by the Encapsulated Metal Cluster

Abstract

The first derivatives of Y(3)N@C(80) have been synthesized and fully characterized. 1,3-Dipolar cycloaddition of N-ethylazomethine ylide yielded mainly the pyrrolidine monoadduct of the icosahedral (I(h)()) symmetry cage exclusively at a [6,6] double bond. The same regioselectivity on a [6,6] double bond was observed when the endohedral compound was cyclopropanated with diethyl bromomalonate. These results are in pronounced contrast to those observed for icosahedral symmetry Sc(3)N@C(80), for which all reported derivatives add completely regioselectively to [5,6] double bonds. (1)H NMR, (13)C NMR, and HMQC spectroscopy revealed that the addition pattern on Y(3)N@C(80) resulted in a pyrrolidinofullerene derivative with unsymmetric pyrrolidine carbons and symmetric geminal protons. The cyclopropanated monoadduct exhibited symmetric ethyl groups on the malonate, consistent with regioselective addition at a [6,6] double bond. Attempts to perform the same cyclopropanation reaction on (I(h)()) Sc(3)N@C(80) failed to yield any identifiable products. These observations clearly indicate that the reactivity of trimetallic nitride endohedral metallofullerenes toward exohedral chemical functionalization is profoundly affected and effectively controlled by the nature of the endohedral metal cluster.

Chemical, Electrochemical, and Structural Properties of Endohedral Metallofullerenes

“…[53] The same year, Echegoyen and co-workers reported the first pyrrolidine adduct of Y 3 N@C 80 (I h , 13 b; Scheme 5), but the cycloaddition occurred at a [6,6] bond for this metallic nitride EMF. [54] Later, Dorn, Gibson, and co-workers reported the functionalization of M 3 N@C 80 (M = Sc, Er) with N-methylglycine and 13 C-formaldehyde, and they observed a similar addition to a [5,6] bond as observed with the Sc 3 N@C 80 (I h ) isomer. [55] In 2006, Akasaka and co-workers [56] reported the synthesis and characterization of both [5,6] and [6,6] pyrrolidine derivatives of La 2 @C 80 (14; Scheme 6), by using 3-triphenylmethyl-5-oxazolidinone (15) as the reagent.…”

“…[69] In 2005, Echegoyen and co-workers reported the first monomethanofullerene derivative 18 a of the I h isomer of Y 3 N@C 80 by cyclopropanation with 17 a and 1,8-diazobicyclo-[5.4.0]undec-7-ene (DBU; Scheme 7) that occurred selectively at a [6,6] double bond. [54] The Er 3 N@C 80 C(CO 2 C 2 H 5 ) 2 monoadduct 18 b (Scheme 7) was also prepared and characterized by HPLC and mass spectrometry. Voltammetric data suggested it was most likely a [6,6] regioisomer.…”

Dancing on a Fullerene Surface: Isomerization of Y3N@(N‐Ethylpyrrolidino‐C80) from the 6,6 to the 5,6 Regioisomer

“…[19] Interestingly, similar cycloaddition reactions on the I h Y 3 N@C 80 derivative occur exclusively at a 6,6 ring junction (pyrene-type site). [8] Isomerization of the 6,6 regioisomer of Y 3 N@(N-ethylpyrrolidino-C 80 ) to produce exclusively the 5,6 regioisomer after thermalization has been reported by Echegoyen and co-workers. [18] Figure 1 illustrates the two possible sites of addition to the I h C 80 fullerene cage, the double bonds between a five-and a six-membered ring and between two six-membered rings.…”

Retro‐Cycloaddition Reaction of Pyrrolidinofullerenes

“…12 C-and 13 C-enriched formaldehyde was a noticeable by-product of the retro-cycloaddition reaction under oxygenated conditions when 13 C-labeled Nethylpyrrolidinofullerene was employed, as shown by the 1 H NMR spectra (Figure 3) recorded before and after retrocycloaddition. Such a strategy also worked successfully and quantitatively for the isolation of I h -symmetric Y 3 N@C 80 when N-ethylpyrrolidino- [5,6]-Y 3 N@C 80 [15] was subjected to the retro-cycloaddition conditions described above. We are currently analyzing the other by-products of the reaction and investigating the roles of oxygen and light in the reaction in order to elucidate the mechanism and compare it to the retrocycloaddition of fullerene derivatives with empty cages.…”