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CH Activation versus Yttrium–Methyl Cation Formation from [Y(AlMe 4 ) 3 ] Induced by Cyclic Polynitrogen Bases: Solvent and Substituent‐Size Effects
Author(s) -
Bojer Daniel,
Venugopal Ajay,
Mix Andreas,
Neumann Beate,
Stammler HansGeorg,
Mitzel Norbert W.
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201003317
Subject(s) - chemistry , cationic polymerization , steric effects , substituent , alkyl , medicinal chemistry , toluene , solvent , nuclear magnetic resonance spectroscopy , crystallography , stereochemistry , polymer chemistry , organic chemistry
Abstract The reaction of 1,3,5‐triisopropyl‐1,3,5‐triazacyclohexane (TiPTAC) with [Y(AlMe 4 ) 3 ] resulted in the formation of [(TiPTAC)Y(Me 3 AlCH 2 AlMe 3 )(μ‐MeAlMe 3 )] by CH 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) 2 YMe 2 ][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 CH activation was observed in reactions of [Y(AlMe 4 ) 3 ] with 1,3,5‐trimethyl‐1,3,5‐triazacyclohexane (TMTAC) and TiPTAC. In THF molecular dimethyl cations, such as [(TCyTAC) 2 YMe 2 ][AlMe 4 ], [(TMTAC) 2 YMe 2 ][AlMe 4 ] and [(TiPTAC) 2 YMe 2 ][AlMe 4 ], could be synthesised by addition of the triazacyclohexane at a later stage. The THF‐solvated complex [YMe 2 (thf) 5 ][AlMe 4 ] 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.