Synthesis of a low-valent Al4+ cluster cation salt

Abstract

Low-valent aluminium compounds are among the most reactive and widely researched maingroup compounds. Since the isolation of [(AlCp*)4] in 1991 as the first stable, molecular Al I compound, a variety of highly reactive neutral or anionic low-valent aluminium complexes were developed. In particular, the strongly basic aluminyl anions allowed for nucleophilic activation of a large variety of small molecules and formation of elusive transition-metal complexes. By contrast, an accessible cationic, low-valent aluminium compound combining the nucleophilicity of low-valent compounds with the electrophilicity of aluminium is hitherto unknown. Here, weUnexpectedly, the complex ion forms a dimer in the solid state and in concentrated solutions.Addition of Lewis bases results in monomerization and coordination to the unique formal Al + atom giving [(L)xAl(AlCp*)3] + salts with L = hexaphenylcarbodiphosporane (cdp; x = 1), tetramethylethylenediamine (tmeda; x = 1) and 4-dimethylamino-pyridine (dmap; x = 3).Depending on the donor strength of the ligand added, the Al + -AlCp* bonds in the [(L)xAl(AlCp*)3] + cluster cations can be finely tuned between very strong (L = nothing) to very weak and approaching isolated [Al(L)3] + ions (L = dmap). We anticipate our easily accessible low-valent aluminium cation salts to be the starting point for investigation and potential application of this unusual compound class. In particular, the ambiphilic reactivity of the cationic, low-valent compounds will be studied. Moreover, knowledge gained from the stabilization of the reported complex salts is expected to facilitate the isolation and application of novel cationic, low-valent Al complexes.

[Fp(CH4)]+, [η5-CpRu(CO)2(CH4)]+, and [η5-CpOs(CO)2(CH4)]+: A Complete Set of Group 8 Metal–Methane Complexes

“…In this study and in our previous investigations, we have highlighted the importance of using the weakly coordinating solvent 1,1,1,3,3,3-hexafluoropropane (HFP) and the extremely weakly coordinating Krossing’s perfluoro tert- butoxy aluminate anion ([Al­(OC­(CF 3 ) 3 ) 4 ] − ), which has been effectively used to stabilize highly reactive cations. Neither the HFP solvent or the Krossing’s anion will “outcompete” alkanes as incoming ligands. ,− ( Safety note: HFP boils at ca . 0 °C.…”

From mixed group 13 cations [M(AlCp*)3]+ (M = Ga/In/Tl) to an Al4+ cluster

“…At rst, Me 3 TACN reacts rapidly with [Ga(AlCp* + clusters reported previously. 32 Hence, these exchange reactions represent a highly promising route towards isolation of novel cationic low-valent aluminium complexes.…”

Synthesis, Structure, and Reactivity of a Superbulky Low-Valent β-Diketiminate Al(I) Complex

“…Attempts to isolate a coordination complex with an Al(I)− Al(III) donor bond by reaction of ( DIPeP BDI)Al with AlBr 3 gave the Al bromide complex ( DIPeP BDI)AlBr 2 (3) and led to the reduction of Al(III) to Al(0) (Scheme 2). We presume this reaction proceeds by initial formation of the intermediate donor−acceptor complex, ( DIPeP BDI)Al−AlBr 3 , which after Br transfer gives ( DIPeP BDI)AlBr 2 (3) and AlBr, the latter disproportionating to Al(0) and AlBr 3 . 1 Complex ( DIPeP BDI)-AlBr 2 (3) was structurally characterized by X-ray diffraction (Figure S33).…”

“…2 Schnockel's pentamethylcyclopentadienyl (Cp*) complex crystallized as the tetramer (Cp*Al) 4 (I, Scheme 1). This cluster, which was recently also isolated in a cationic form Cp 3 Al 4 + , 3 is in equilibrium with monomeric Cp*Al in solution, indicating that the Al−Al bonds are weak. 4 The Roesky group found that sterically hindered bidentate β-diketiminate (BDI) ligands stabilized the monomeric state and reported the crystal structure of (BDI)Al (II); BDI = HC[C(Me)N(DIPP)] 2 and DIPP = 2,6-diisopropylphenyl).…”