Seven-Membered Cyclic Diamidoalumanyls of Heavier Alkali Metals: Structures and C–H Activation of Arenes

Abstract

Like the previously reported potassium-based system, rubidium and cesium reduction of [{SiN Dipp }AlI] ({SiN Dipp } = {CH 2 SiMe 2 NDipp} 2 ) with the heavier alkali metals [M = Rb and Cs] provides dimeric group 1 alumanyl derivatives, [{SiN Dipp }AlM] 2 . In contrast, similar treatment with sodium results in over-reduction and incorporation of a formal equivalent of [{SiN Dipp }Na 2 ] into the resultant sodium alumanyl species. The dimeric K, Rb, and Cs compounds display a variable efficacy toward the C–H oxidative addition of arene C–H bonds at elevated temperatures (Cs > Rb > K, 110 °C) to yield (hydrido)(organo)aluminate species. Consistent with the synthetic experimental observations, computational (DFT) assessment of the benzene C–H activation indicates that rate-determining attack of the Al(I) nucleophile within the dimeric species is facilitated by π-engagement of the arene with the electrophilic M + cation, which becomes increasingly favorable as group 1 is descended.

Application of Bis(amido)alkyl Magnesiates toward the Synthesis of Molecular Rubidium and Cesium Hydrido-magnesiates

“…The alkali metal chemistry of this ligand is quite well-established, and crucially to this pursuit, the infinite supramolecular structure of unsolvated CsN­(SiMe 3 )­(Dipp) propagates in the solid state via multihapto Cs–C 6 (π) interactions, while its anionic nitrogen is naked in the sense that it does not form a bond to cesium, suggesting that it is potentially well-suited to our needs described above. Furthermore, the Dipp entity is a common feature of a wide variety of subvalent aluminum dimers which appear stitched together by heavy alkali metal cations engaging intramolecularly with N -bound Dipp groups at the periphery of the discrete molecules . Such interactions tend to be favored over similar interactions in reactions with external aromatic solvent molecules (for example, benzene, toluene, and so forth), which allow these organometallic species to propagate into extended supramolecular structures, thus enhancing the opportunities of us accessing discrete molecules.…”

“…Furthermore, the Dipp entity is a common feature of a wide variety of subvalent aluminum dimers which appear stitched together by heavy alkali metal cations engaging intramolecularly with N -bound Dipp groups at the periphery of the discrete molecules. 10 Such interactions tend to be favored over similar interactions in reactions with external aromatic solvent molecules (for example, benzene, toluene, and so forth), which allow these organometallic species to propagate into extended supramolecular structures, thus enhancing the opportunities of us accessing discrete molecules. We, thus, targeted heteroleptic alkyl/bis-amido alkali metal magnesiates [(AM)MgN′ 2 R; AM = Rb, Cs; R = alkyl group] as potential precursors to our desired amide-hydride complexes via a ligand exchange process.…”