The one-electron reduction of carbonium ions. Part 14. Effect of successive introduction of methyl substituents on the reducibility of tropylium ion in chromium(II) ion and cathodic reductions
Abstract
The second-order rate constants (k2) of one-electron reduction with CrII ion of methylated tropylium ions [C7H7 –n(CH3)n+ClO4–, n= 0–7] were determined in 10% hydrochloric acid at 25.0°C. Reduction peak potentials determined by means of triangular wave cyclic voltammetry and charge transfer energies with pyrene were compared with the rates of CrII ion reduction in terms of the effect of methyl substituents. The log k2 values decrease linearly with an increase in the number of methyl groups up to n= 4. However, the decelerating effect per extra methyl substituent becomes smaller when n is 5 or 6, and the seventh methyl substituent accelerates the reduction. The methyl substituent effect on the reduction peak potentials of the carbocations is similar to that found for the CrII ion reduction, giving rise to a linear correlation. A Marcus treatment suggests that CrII ion reduction proceeds through an outer-sphere mechanism. The charge transfer energies with pyrene are linearly correlated with reduction potentials for ions with n= 0–6, but considerable deviation is noted when n is 7. The charge transfer energies are linearly correlated with the LUMO energy levels of the methylated tropylium ions, suggesting that the unexpectedly pronounced reducibilities of hexa- and hepta-methyltropylium ions are attributable to steric factors. The deviation from linearity in the CrII ion reduction and the reduction potential are reasonably explained by assuming that non-bonded repulsive interactions among congested methyl substituents in the carbocations are relieved upon reduction to their corresponding radicals. This explanation is consistent with our previous conclusion from a 13C n.m.r. study that the methyl substituents of 1,2,3,4,5-penta-, hexa-, and hepta-methyltropylium ions are highly congested and distorted out of plane.