Abstract:
Heterobenzene chemistry of the rare-earth metals and group 13 elements can be considered as a rather new or neglected field, likely due to the low electronegativity of the rare-earth metals and especially aluminum, and therefore enhanced reactivity. Investigations of the reactivity of heterobenzene supported rare-earth-metal complexes [(1-Me-3,5-tBu2-C5H3Al)(μ-Me)Y(2,4-dtbp)], [(1-Me-3,5-tBu2-C5H3Ga)(μ-Me)Y(2,4-dtbp)] and derivatives of those complexes toward several Lewis bases, in the form of neutral donor molecules, showed that the Lewis adduct formation of the central rare-earth-metal cations is favored over the group 13 atom in the heterobenzene moiety. The stability of the “dianionic” alumina- and gallabenzene ligands was further evidenced by pentadienyl exchange reactions with KC5Me5 and KTpMe,Me. Since the route toward the rare-earth-metal-coordinated group 13 heterobenzenes is very robust, several one-pot reactions at variable temperatures were conducted utilizing ternary mixture of [LnMe3]n, GaMe3 and K(2,4-dtbp) targeting new types of heterobenzenes. This approach led to the isolation of the highly reactive terminal methyl complex [(2,4-dtbp)2LuMe] at lower temperatures, which can incorporate two gallium atoms into the pentadienyl ligands, resulting in a dianionic and a monoanionic trigonal planar gallabenzene. Further, molar variation of the reactants K(2,4-dtbp) and GaMe3 gave rare-earth-metal incorporated heterobenzenes as well as mixed group 13 and rare-earth-metal heterobenzenes in the same complex. Slight modifications of the synthesis protocol also led to higher aggregations of these complexes of up to five lantahanid atoms, offering a vast potential for cluster synthesis. The reactivity of the rare-earth-metal coordinated group 13 heterobenzenes was extended to the Lewis acids AlMe3 and AlCl3/GaCl3. While AlMe3 was simply added to the electron-rich heterobenzene moieties, a different behavior was observed for the chlorides. In this case, the formation of thermodynamically favored rare-earth-metal chlorides triggered the abstraction of the heterobenzene ligands as monoanionic moieties, affording a group 13 complex bearing a lid-like bis-alumo/galla-cyclobutane cycle stabilized by a pentadienyl ring. Further investigations established the synthesis of aza-crown stabilized alkyl species of the earlier, bigger rare-earth metals, which were previously not accessible. Accordingly, an efficient synthesis of monomeric (Me3TACN)LnMe3(THF)(Ln = La - Nd) was developed and some properties and reactivities toward Lewis acids and alcohols were studied, resulting in the isolation of the first monomeric rare-earth-metal alkoxides for smaller alcohols. In another approach, this synthesis route was expanded toward titanium, giving access to the first titanium trimethyl compound as (Me3TACN)TiMe3 closing the gap between TiMe4 and (DMPE)2TiMe2. Furthermore, the reaction of (Me3TACN)TiMe3 with AlMe3 produced the ion pair [(Me3TACN)TiMe2(THF)][AlMe4] and exchange reactions with trimethylsilylchloride/ triflate (TMS-Cl/OTf) accomplished defined mixed methyl complexes. Moreover, the reactions with methanol and ethanol resulted in the smallest structurally characterized alcoholates for titanium so far. Finally, the isolation of halide-containing side products let us to comprehensively study the purification of methyllithium. Since commercial MeLi is contaminated with LiCl, MeLi was purified via the addition of KN(SiMe3)2 and precipitation of KCl, resulting in the MeLi/LiN(SiMe3)2/Et2O solution which was layered with n-hexane to afford pure MeLi. A 7Li- NMR study showed that the LiCl contamination is an integrated part of an oligomeric structure emerging in a single 7Li resonance. This signal can be used to calculate the amount of LiCl via 7Li NMR-spectroscopy, instead of potentiometric analysis. Aza-crown addition to MeLi resulted in monomeric (Me3TACN)LiMe, which was characterized by X-ray diffraction, as was monomeric (Me3TACN)LiCl. A DFT study revealed that the Li–C(CH3) and the Li–Cl bond show almost the same ionicity.
Heterobenzene