General methods and materials. All manipulations were carried out using standard Schlenk line and glove box techniques under dry argon. Solvents were passed through columns containing alumina or were dried by refluxing over K, and were stored over K mirrors or 4 Å molecular sieves (THF) and were degassed before use. For NMR spectroscopy, d6-benzene and d8-THF were dried by refluxing over K and degassed by three freeze-pump-thaw cycles, and vacuum-transferred before use.