C-H activation versus yttrium-methyl cation formation from [Y(AlMe 4)3] induced by cyclic polynitrogen bases: Solvent and substituent-size effects

Abstract

The reaction of 1,3,5-triisopropyl-1,3,5-triazacyclohexane (TiPTAC) with [Y(AlMe4)3] resulted in the formation of [(TiPTAC)Y(Me3AlCH2AlMe3)(μ-MeAlMe 3)] by C-H activation and methane extrusion. In contrast, the presence of bulkier cyclohexyl groups on the nitrogen atoms in 1,3,5-tricyclohexyl-1,3,5-triazacyclohexane (TCyTAC) led to the formation of the cationic dimethyl complex [(TCyTAC)2YMe2][AlMe 4]. The investigations reveal a dependency of the reaction mechanism on the steric bulk of the N-alkyl entity and the solvent employed. In toluene C-H activation was observed in reactions of [Y(AlMe4)3] with 1,3,5-trimethyl-1,3,5-triazacyclohexane (TMTAC) and TiPTAC. In THF molecular dimethyl cations, such as [(TCyTAC)2YMe2] [AlMe4], [(TMTAC)2YMe2][AlMe4] and [(TiPTAC)2YMe2][AlMe4], could be synthesised by addition of the triazacyclohexane at a later stage. The THF-solvated complex [YMe2(thf)5][AlMe4] could be isolated and represents an intermediate in these reactions. It shows that cationic methyl complexes of the rare-earth metals can be formed by donor-induced cleavage of the rare-earth-metal tetramethylaluminates. The compounds were characterised by single-crystal X-ray diffraction or multinuclear and variable-temperature NMR spectroscopy, as well as elemental analyses. Variable-temperature NMR spectroscopy illustrates the methyl group exchange processes between the cations and anions in solution. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.