Substituent effects on the electrochemical oxidation ofN,N?,N?-triphenyl-1,3,5-triaminobenzenes

Correlation analysis of the oxidation potentials of a series of N,N′,N″-triphenyl-1,3,5-triaminobenzenes (TPABs) substituted at the para positions of the outer phenyl rings shows a linear free energy relationship with resonance-enhanced substituent parameters (σ+). Reaction parameters (ρ+) for oxidation of TPABs were found to be −1.53, −1.45 and −1.34 (per substituent) in methylene chloride, acetonitrile and propylene carbonate respectively. The resonance enhancement and small magnitude of the ρ+ values are related to a significant but weak delocalization of charge onto the outer phenyl rings in the molecular orbitals of radical cations resulting from the oxidation of TPABs. Data on the oxidation of p-substituted triphenylamines were treated similarly and gave a ρ+ value of −3.27 (per substituent) in acetonitrile, greater than that for TPABs owing to a more significant delocalization of charge onto the phenyl rings in the molecular orbitals of the corresponding radical cations. To demonstrate their predictive value, these linear free energy correlations were used to estimate the oxidation potentials of similarly substituted N,N,N′,N′,N″,N″-hexaphenyl-1,3,5-triaminobenzenes, which are of interest as building blocks for molecular magnetic materials.