LAUSR

M06 density functional theory calculations reveal that arene C–H functionalization by the p-block main-group-metal complex TlIII(TFA)3 (TFA = trifluoroacetate) occurs by a C–H activation mechanism akin to transition-metal-mediated C–H activation. For benzene, toluene, and xylenes a one-step C–H activation is preferred over electron transfer or proton-coupled electron transfer. The proposed C–H activation mechanism is consistent with calculation and comparison to experiment, of arene thallation rates, regioselectivity, and H/D kinetic isotope effects. For tetramethyl- and pentamethyl-substituted arenes, electron transfer becomes a competitive pathway and thermodynamic and kinetic calculations correctly predict the experimentally reported electron transfer crossover region. These calculations show that p-block metals activate strong hydrocarbon C–H bonds through organometallic intermediates and changes in arene functional groups can result in a shift from C–H activation to electron transfer.