Aluminylenes: Synthesis, Reactivity, and Catalysis

ConspectusFor many years, carbenes were regarded as fleeting intermediates, elusive to both isolation and direct observation. This perception was overturned when Bertrand isolated the first singlet carbene in 1988, followed by Arduengo's synthesis of a crystalline N-heterocyclic carbene (NHC) in 1991. This not only demonstrates that carbenes can be tamed under ambient conditions but also ushered in a new era in which such species found widespread and indispensable applications in synthetic chemistry and materials science. Aluminylenes/alanediyls (R-Al), the aluminum analogs of carbenes, feature a monovalent aluminum center in the +I oxidation state bearing a pair of nonbonding electrons and two vacant orbitals. For a long time, however, these species remained largely confined to the realm of theory or could be inferred only under extreme conditions. Although the first Al(I) compound, (Cp*Al)4, was isolated by Schnöckel in 1991 and a monomeric, dicoordinate Al(I) complex HC(CMeNDipp)2Al (Dipp = 2,6-diisopropylphenyl) was reported by Roesky in 2000, free monocoordinate aluminylenes eluded isolation until 2020-2021. In this period, Power and Tuononen realized the isolation of AriPr8-Al (AriPr8 = C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2), while we and Hinz independently disclosed a stable carbazolyl-aluminylene, N-Al (N = 3,6-di-tert-butyl-1,8-bis(3,5-di-tert-butylphenyl)carbazolyl). These discoveries collectively establish aluminylenes as an accessible class of low-valent main-group species and open new avenues for their exploration in synthetic chemistry and beyond.This Account delineates our efforts to harness aluminylenes as potent ligands and versatile synthons. We describe the synthesis and characterization, under conventional laboratory conditions, of a broad family of unique aluminum-containing compounds, including aluminylene/alumanyl-transition-metal complexes, aluminylene-Lewis acid adducts, base-stabilized aluminylenes, alumaboranes, aluminum chalcogenides, aluminadichalcogeniranes, aluminatrichalcogenetanes, aluminacyclopropenes, aluminanorbornadienes, aluminacyclopentadienes, and aluminaheptatrienes. More recently, we discovered that a free aluminylene can also operate as a highly effective catalyst for the cyclotrimerization of alkynes via a previously uncharted Al(I)/Al(III) redox cycle, thereby extending low-valent aluminum chemistry from stoichiometric small-molecule activation to bona fide redox catalysis. Collectively, the straightforward synthesis and rich reactivity of these systems showcase the remarkable capability of aluminylenes to engage in a spectrum of transformations.The methodologies, bonding concepts, and mechanistic insights from our work deepen our understanding of low-valent aluminum chemistry and provide a framework for designing next-generation main-group reagents and catalysts. We anticipate that our results will not only inspire new synthetic strategies and redox manifolds involving aluminum but also stimulate the development of functional materials in which low-valent aluminum centers are key enabling motifs. We are optimistic that aluminylenes may ultimately parallel and perhaps complement the trajectory of stable singlet carbenes, finding broad and lasting applications across diverse areas of modern chemistry and technology.