Boosting Low‐Valent Aluminum(I) Reactivity with a Potassium Reagent

Abstract

The reagent RK [R=CH(SiMe3)2 or N(SiMe3)2] was expected to react with the low‐valent (DIPPBDI)Al (DIPPBDI=HC[C(Me)N(DIPP)]2, DIPP=2,6‐iPr‐phenyl) to give [(DIPPBDI)AlR]−K+. However, deprotonation of the Me group in the ligand backbone was observed and [H2C=C(N‐DIPP)−C(H)=C(Me)−N−DIPP]Al−K+ (1) crystallized as a bright‐yellow product (73 %). Like most anionic AlI complexes, 1 forms a dimer in which formally negatively charged Al centers are bridged by K+ ions, showing strong K+⋅⋅⋅DIPP interactions. The rather short Al–K bonds [3.499(1)–3.588(1) Å] indicate tight bonding of the dimer. According to DOSY NMR analysis, 1 is dimeric in C6H6 and monomeric in THF, but slowly reacts with both solvents. In reaction with C6H6, two C−H bond activations are observed and a product with a para‐phenylene moiety was exclusively isolated. DFT calculations confirm that the Al center in 1 is more reactive than that in (DIPPBDI)Al. Calculations show that both AlI and K+ work in concert and determines the reactivity of 1.

Contrasting the Mechanism of H2Activation by Monomeric and Potassium‐Stabilized Dimeric AlIComplexes: Do Potassium Atoms Exert any Cooperative Effect?

“…Harder modeled the reaction highlighting the role of potassium cations through favorable noncovalent interactions. [32] Fernandez et al computed the CÀ H activation in toluene using the naked version of complex IV and its potassium stabilized version. [29] The authors presented quantitative evidence of a synergy effect of the potassium atoms through the whole reaction path by attractive dispersion interactions, producing a decrease of 3.9 kcal mol À 1 in the rate-limiting step encompassing the S N Ar mechanism.…”

“…Moreover, K + atoms play an essential role in the stabilization of the Meisenheimer intermediate, reinforcing the idea of including the cations in the calculation. [29,32] On the other hand, Yamashita et al reported the first example of a potassium-dialkylaluminium salt. Although the prepared system demonstrated its strong basicity and electrophilic character by reacting with benzene, MeOTf, benzyl chloride, and C 6 F 6 , prominent substrate⋯interactions were observed for C 6 H 6 and benzyl chloride (C 6 F 6 to a minor extend).…”

The Aluminyl Anion: A New Generation of Aluminium Nucleophile

“…[27] Subsequently,t wo further potassium aluminyl complexes featuring bidentate diamido ligands have been reported. [28] Thefirst of these,K 2 [Al( Si NON)] 2 (7), was reported by Coles and co-workers in 2019, [29] and more recently K 2 [Al(NCCN)] 2 (8)w as reported by Hill, McMullin and co-workers in 2020 ( Figure 3). [30] Both of these complexes were synthesised using as imilar synthetic route to 6,t hat is,b yt he two-electron reduction of the corresponding aluminium(III) iodide precursor with potassium.…”

Attempted reduction of a carbazolyl-diiodoalane

“…Later, other examples without O-donor moieties or even N-substituents were reported by Yamashita, Kinjo, Coles, Harder and Hill. [9][10][11][12][13] Braunschweig and coworkers utilised a bulky cyclopentadienyl derivative to stabilise the monomeric Al(I) compound E. 14 The quest for a genuine monocoordinated Al(I) compound was still to be tackled. The group of Power was the first to succeed when they employed the bulkiest terphenyl substituent available and prepared the alanediyl F. 15,16 Scheme 1.…”