Dihydrogen Activation by Lithium‐ and Sodium‐Aluminyls

High-Level Coupled-Cluster Study on Substituent Effects in H2 Activation by Low-Valent Aluminyl Anions

The synthesis of novel aluminyl anion complexes has been well exploited in recent years. Moreover, the elucidation of the structure and reactivity of these complexes opens the path toward a new understanding of low-valent aluminum complexes and their chemistry. This work computationally treats the substituent effect on aluminyl anions to discover suitable alternatives for H₂ activation at a high level of theory utilizing coupled-cluster techniques extrapolated to the complete basis set. The results reveal that the simplest AlH₂^- system is the most reactive toward the activation of H₂, but due to the low steric demand, severe difficulty in the stabilization of this system makes its use nonviable. However, the results indicate that, in principle, aluminyl systems with -C, -CN, -NC, and -N chelating centers would be the best choices of ligand toward the activation of molecular hydrogen by taking care of suitable steric demand to prevent dimerization of the catalysts. Furthermore, computations show that monosubstitution (besides -H) in aluminyl anions is preferred over disubstitution. So our predictions show that bidentate ligands may yield less reactive aluminyl anions to activate H₂ than monodentate ones.

Heavy Alkali Metal Manganate Complexes: Synthesis, Structures and Solvent-Induced Dissociation Effects

Rare examples of heavier alkali metal manganates [{(AM)Mn(CH₂ SiMe₃ )(N^'Ar )₂ }_∞ ] (AM=K, Rb, or Cs) [N^'Ar =N(SiMe₃ )(Dipp), where Dipp=2,6-iPr₂ -C₆ H₃ ] have been synthesised with the Rb and Cs examples crystallographically characterised. These heaviest manganates crystallise as polymeric zig-zag chains propagated by AM⋅⋅⋅π-arene interactions. Key to their preparation is to avoid Lewis base donor solvents. In contrast, using multidentate nitrogen donors encourages ligand scrambling leading to redistribution of these bimetallic manganate compounds into their corresponding homometallic species as witnessed for the complete Li - Cs series. Adding to the few known crystallographically characterised unsolvated and solvated rubidium and caesium s-block metal amides, six new derivatives ([{AM(N^'Ar )}_∞ ], [{AM(N^'Ar )⋅TMEDA}_∞ ], and [{AM(N^'Ar )⋅PMDETA}_∞ ] where AM=Rb or Cs) have been structurally authenticated. Utilising monodentate diethyl ether as a donor, it was also possible to isolate and crystallographically characterise sodium manganate [(Et₂ O)₂ Na(ⁿ Bu)Mn[(N^'Ar )₂ ], a monomeric, dinuclear structure prevented from aggregating by two blocking ether ligands bound to sodium.

Potassium Aluminyl Promoted Carbonylation of Ethene

The potassium aluminyl [K{Al(NON^Dipp)}]₂ ([NON^Dipp]²⁻=[O{SiMe₂NDipp}₂]²⁻, Dipp=2,6‐iPr₂C₆H₃) activates ethene towards carbonylation with CO under mild conditions. An isolated bis‐aluminacyclopropane compound reacted with CO via carbonylation of an Al−C bond, followed by an intramolecular hydrogen shift to form K₂[Al(NON^Dipp)(μ‐CH₂CH=CO‐1κ²C^1,3‐2κO)Al(NON^Dipp)Et]. Restricting the chemistry to a mono‐aluminium system allowed isolation of [Al(NON^Dipp)(CH₂CH₂CO‐κ²C^1,3)]⁻, which undergoes thermal isomerisation to form the [Al(NON^Dipp)(CH₂CH=CHO‐κ²C,O)]⁻ anion. DFT calculations highlight the stabilising influence of incorporated benzene at multiple steps in the reaction pathways.

Assessing Alkali-Metal Effects in the Structures and Reactivity of Mixed-Ligand Alkyl/Alkoxide Alkali-Metal Magnesiates

Advancing understanding of using alkali-metal alkoxides as additives to organomagnesium reagents in Mg-Br exchange reactions, a homologous series of mixed-ligand alkyl/alkoxide alkali-metal magnesiates [MMg(CH2SiMe3)2(dmem)]2[dmem= 2-{[2-(dimethylamino)ethyl]methylamino} ethoxide; M= Li, 1; Na, 2; (THF)K, 3] has been prepared. Structural and spectroscopic studies have established the constitutions of these heteroleptic/heterometallic species, which are retained in arene solution. Evaluation of their reactivity towards 2-bromoanisole has uncovered a marked alkali-metal effect with potassium magnesiate 3 being the most efficient of the three ate reagents. Studies probing the constitution of the exchange product from this reaction suggest that the putative [KMgAr2(dmem)]2 (Ar= o-OMe-C6H4) intermediate undergoes redistribution into its single metal components [KAr]n and [MgAr(dmem)]2 (5). This process can be circumvented by using a different potassium alkoxide containing an aliphatic chain such as KOR' (R'= 2-ethylhexyl) which undergoes co-complexation with Mg(CH2SiMe3) to give [KMg(CH2SiMe3)2(OR')]2 (7). This ate, in turn, reacts quantitatively with 2-bromoanisole furnishing [KMgAr2(OR')]2 (9) which is stable in solution as a bimetallic compound. Collectively this work highlights the complexity of these alkali-metal mediated Mg-Br exchange reactions, where each reaction component can have a profound effect not only on the success of the reaction; but also the stability of the final metalated intermediates prior to their electrophilic interception.