No abstract

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 intensity 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-benzyl-calix[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−π interactions were sufficiently strong to alter the EPR signals of C60 •-. Cyclic voltammetry experiments performed on C60 in the presence of the complexing 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.

An unusually stable five-coordinate monomeric divalent rhodium complex, [RhII(H)(CO)(PPh3)3]+, is produced by bulk oxidative electrolysis or chemical oxidation of RhI(H)(CO)(PPh3)3 in dichloromethane. Consequently, odd as well as even electronic configurations are available in this well-known catalytic system. The EPR and electronic spectra of electrogenerated paramagnetic 17-electron cation [RhII(H)(CO)(PPh3)3]+ have been obtained at low temperatures as has the EPR spectrum of the deuterated analogue. Computer simulation of the EPR spectra of the hydride and deuteride complexes reveals three g-values and anisotropic coupling constants for hydrogen, phosphorus, and rhodium. One of the phosphorus coupling constants is very large (A 1 = 175.0 G; A 2 = 176.0 G; A 3 = 230.0 G). This may be accounted for if [RhII(H)(CO)(PPh3)3]+ has the square pyramidal structure, and substantial mixing of the singly occupied metal orbital and the apical phosphorus s-orbital are considered. NMR measurements on mixtures of RhI(H)(CO)(PPh3)3 and [RhII(H)(CO)(PPh3)]+ are consistent with a very fast electron self-exchange reaction and the heterogeneous charge-transfer rate constant for the [RhII/I(H)(CO)(PPh3)3]+/0 redox couple also is very fast. One electron electrochemical oxidation of [RhII(H)(CO)(PPh)3]+ to [RhIII(H)(CO)(PPh3)3]2+ is followed by a very fast reductive elimination reaction (loss of proton) which generates [RhI(CO)(PPh3)3]+.

Segregated composite electrodes mimic microelectrode ensembles. In this preliminary report, the use of a rotating gold-polychlorotriRuoroethylene (or Kel-F, a 3M Company polymer) composite electrode in combination with pulsed amperometric detection (PAD) is described for the detection of glucose. Comparisons are made with results obtained at a solid gold disk electrode. The composite electrode exhibits a higher signal and a lower background than does the solid gold electrode. In terms of current density, the enhancement of the signal above the background is over 3-fold. similar to that observed with segregated graphite composite electrodes used in a constant potential mode. Little or no glucose signal is observed at either the solid gold or the gold composite elcctrode when employed in the constant potential mode. In the PAD mode, the signal is stable for periods in excess of an hour.