EPR studies associated with the electrochemical reduction of C₆₀ and supramolecular complexes of C₆₀ in toluene-acetonitrile solvent mixtures

Electron paramagnetic resonance (EPR) studies at 295 K of the radical anion of buckminsterfullerene (C₆₀^.-) generated electrochemically by bulk-controlled potential electrolysis of neutral C₆₀ in mixed toluene-acetonitrile solvents (5-20% v/v acetonitrile with 0.1-0.2 M n-Hex₄NPF₆ or n-Bu₄NPF₆ as the supporting electrolyte) have shown that at least three different forms of C₆₀^.- exist in solution. The radical anions have different g values and can thus be detected by EPR spectroscopy in the form of single line signals with different line widths. The familiar species with the broad line width signal (ΔH_pp = 6 mT) was present in a high percentage (ca. >95%) along with two species with narrow line width (ΔH_pp = 0.16 and 0.07 mT) signals that were present between 1 and 5% of the total concentration of C₆₀^.-. The concentrations of the species with narrow line width EPR signals increased with increasing time so that over a 20-h period their EPR signal incensity approximately doubled. The purity of the toluene was found to be very important in determining the number of species detected with sharp EPR signals. Although two species were always detected with sharp EPR signals, when lower purity toluene (<99%) was used, the number of species with narrow line width signals increased to >2. The increase in the sharp EPR signals with increasing time is not inconsistent with a slow reaction between C₆₀^.- and low levels of impurities in the solvent to form lower symmetry paramagnetic species, the narrow line width signals being very similar to those reported for substituted fullerene radicals. EPR and cyclic voltammetry experiments were conducted on C₆₀ in the presence of several complexing agents, cyclotriveratrylene (CTV), the symmetrical tris-allyl substituted analogue of CTV [CTV(allyl)₃], and p-benzylcalix[5]arene. The intensity of the narrow and broad line width EPR signals decreased when C₆₀^.- was electrochemically generated in the presence of the complexing agents, indicating that the host-guest π-eπ interactions were sufficiently strong to alter the EPR signals of C₆₀^.-. Cyclic voltammetry experiments performed on C₆₀ in the presence of the completing agents showed that the first four reduction processes of C₆₀ split into two new processes upon complexation, with the time allowed for the host-guest reaction being critical in determining the voltammetric behavior.