At scan rates above 10 V s-l, the waves complementary to those for oxidation a t 1.1 V, and the reduction a t -0.06 V, appear. By application of the method of Nicholson and Shain,6 the specific rates for the S to 0 isomerization of the metastable 3+ species and the 0 to S isomerization of the metastable reduced species were determined as (5.0 f 0.5) X 10 s-I and (2.0 f 0.2) X 10 s-l, respectively. At low scan rates, <20 mV s-l, the reduction wave at E , = -0.06 V decreases in amplitude, as is seen by comparing Figure I C with Figure 1 b. Intramolecular electron transfer can cause such a decrease, but other processes, for example, intermolecular electron transfer and loss of the electroactive species from the diffusion layer, can also contribute, so that, in the absence' of a more complete study, we can only set the upper limit, 4.1 X s-I, on the rate of intramolecular electron transfer. The intervalence band for the stable form of the mixed-valence species, [S3+/(SO)2+], measured in acetone, has a maximum at -640 nm with c -4 X 10 M-l cm-I. The shift to high energies, as compared to substitutionally symmetric species, is a reflection of the difference in the redox potentials for the two sites. There is a second much stronger absorption at 452 nm which we attribute to ligand to metal charge transfer at R u ( I I I ) .~The more usual redox couples conform to the Marcus relat i~n s h i p ,~ a necessary condition for its validity being that the distortions that bring energy matching at the two sites are nearly harmonic. Those in which the potential profile for at least one oxidation state has two minima-in our case, this is true in both oxidation states-comprise an important class which has not been investigated systematically. Devices of the kind we have described provide a means of studying electron transfer for them in the intramolecular mode. They may also provide a means of locking in charge transfer brought about by light absorption and, as such, may find application in high-density storage of memory. The excited state [Sz+/(SO)3+]* which arises on absorption of light at -640 nm is expected to be quenched rapidly, and if intramolecular electron transfer is slow enough, isomerization to [Sz+/(OS)3+] will occur.Many examples of geometrical isomerization accompanying a change in oxidation states have been reported,'O particularly by Bond and co-workers." The appearance of linkage isomerizations, which depends on a change in back-bonding capacity attending a change in oxidation state, is predictable for many yet untested systems with a reasonable degree of certainty. The field has been greatly extended in the study of molecules in which the metal ion is bound by q2 to organic ligands,I2 including aromatic molecules, and additional chromophores can easily be built into mixed-valence molecules. It needs to be acknowledged that there is precedent for the "double square" potential diagram featured in Figure 2.13 The system that we describe is to be regarded as a prototype of others which can be devised to show "molecul...

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Dipicolinate vanadium(V) complexes oxidize lignin model complexes pinacol monomethyl ether (A), 2-phenoxyethanol (B), 1-phenyl-2-phenoxyethanol (C), and 1,2-diphenyl-2-methoxyethanol (D). With substrates having C-H bonds adjacent to the alcohol moiety (B-D), the C-H bond is broken in pyridine-d(5) solvent, yielding 2-phenoxyacetaldehyde from B, 2-phenoxyacetophenone from C, and benzoin methyl ether from D. In DMSO-d(6) solvent the reaction is slower, and both C-H and C-C bond cleavage products are observed for D. The vanadium(IV) products of these reactions have been identified and characterized. Catalytic oxidation of C and D has been demonstrated using air and (dipic)V(O)O(i)Pr. For both substrates, the C-C bond between the alcohol and ether groups is broken in the catalytic oxidation. 1-Phenyl-2-phenoxyethanol is oxidized to a mixture of phenol, formic acid, benzoic acid, and 2-methoxyacetophenone. The products of oxidation of 1,2-diphenyl-2-methoxyethanol depend on the solvent; in DMSO benzaldehyde and methanol are the major products, while benzoic acid and methyl benzoate are the major products obtained in pyridine solvent. Phenyl substituents on the model complex facilitate the oxidation, with relative rates of oxidation D > C > B.

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C(60) and liquid bromine react to form C(60)Br(24), a crystalline compound isolated as a bromine solvate, C(60)Br(24)(Br(2))(x), The x-ray crystal structure defines a new pattern of addition to the carbon skeleton that imparts a rare high symmetry. The parent C(60) framework is recognizable in C(60)Br(24), but sp(3) carbons at sites of bromination distort the surface, affecting conformations of all of the hexagonal and pentagonal rings. Twenty-four bromine atoms envelop the carbon core, shielding the 18 remaining double bonds from addition. At 150 degrees to 200 degrees C there is effectively quantitative reversion of C(60)Br(24) to C(60) and Br(2).