Formation of C60Adducts with Two Different Alkyl Groups via Combination of Electron Transfer and SN2 Reactions

The formation of organofullerenes of the type R 2 C 60 and R(R‘)C 60 from C 60 2 - and alkyl halides (RX or R‘X) in benzonitrile was mechanistically investigated for 15 different alkyl halides which vary in electrophilicity and electron acceptor ability. The first step in the reaction leads to RC 60 - via an electron-transfer mechanism, followed by formation of R 2 C 60 or R(R‘)C 60 via an S N 2 mechanism. Evidence of the mechanism comes from comparison of rate constants for the stepwise addition of two R groups to C 60 2 - with rate constants for the genuine electron transfer and S N 2 reactions. The formation of t -BuC 60 - and PhCH 2 C 60 - after the first R group addition was confirmed by electrospray ionization mass spectroscopy. The t -BuC 60 - derivative will not react further with excess t -BuI, but this is not the case for the less sterically hindered PhCH 2 Br, which adds to t -BuC 60 - in benzonitrile to give t -Bu(PhCH 2 )C 60 . A protonation of t -BuC 60 - with trifluoroacetic acid can also occur to give 1,4- t -Bu(H)C 60, which rearranges rapidly to yield 1,2- t -Bu(H)C 60 . Rate constants for the second alkylation of t -BuC 60 - with a variety of different alkyl halides are compared with values of genuine S N 2 reactions and indicate that the second step in the fullerene alkylation reaction proceeds via an S N 2 mechanism. The rate constants of electron transfer from C 60 2 - to RX span a range of 10 5, but are insensitive to the steric effect of the alkyl group, i.e., they depend only on the electron-acceptor ability of RX. In contrast, the S N 2 rate constants of t -BuC 60 - with RX are highly susceptible to the steric effect of the alkyl group and no reaction at all takes place between t -BuC 60 - and t -BuI. Thus, the first addition of one sterically hindered alkyl group to C 60 2 - occurs via electron transfer and cannot be followed by further addition of a second sterically hindered group (via an S N 2 reaction). This is not the case for less sterically hindered alkyl groups such as benzyl bromide which can add via an S N 2 reaction to yield C 60 adducts with two different alkyl groups.