Treatment of a Methylene‐Bridged Dialuminium Compound with Lithium Alkanides and Alkynides ‐ Single vs. Twofold Terminal Coordination

Abstract

We have investigated the coordination of alkanide and alkynide anions to the coordinatively unsaturated aluminium atoms of the methylene-bridged dialuminium compound R 2 Al-CH 2 -AlR 2 [1, R = CH(SiMe 3 ) 2 ]. Treatment of 1 with the corresponding lithium derivatives in the presence of a small excess of TMEN (TMEN = tetramethylethylenediamine) yielded mono-adducts [M] + [R 2 Al-CH 2 -AlR 2 R'] -[2a, M = Li(TMEN) 2 , R' = Me; 2b, M = Li(TMEN) 2 , R' = n-Bu; 3a, M = Li(TMEN) 2 , R' = CϵC-SiMe 3 ; 3b, M = Li(TMEN) 2 , R' = CϵC-t-Bu; 3d, M = Li(DME) 3 , R' = CϵC-Ph; 3e, M = Li(TMEN) 2 , R' = CϵC-PPh 2 )] and bis-adducts [Li(TMEN) 2 ] + [LiCH 2 (AlR 2 R') 2 ] -[4a, R' = CϵC-CH 2 -NEt 2 ; 4b, R' = CϵC-t-Bu]. In the solid state the

The Methylene-bridged Dialuminium Compound R2Al-CH2-AlR2 [R=CH(SiMe3)2] as an Effective Chelating Lewis Acid – Adducts with Halides, Perchlorate and Isopropylamide

“…The reduced strain in the larger ring results in a much larger Al-C-Al angle of 120.18 (9) • . The Al-C bond lengths of 196.4(2) and 201.8(2) pm are similar to those of [R 2 Al-CH 2 -AlR 2 (µ-H)] − [17], but are slightly shorter than those of 3b and 3d or the comparable bonds to the four-coordinate aluminium atoms in the alkyl adducts [19]. This is attributed to the lower ring strain in the six-membered ring.…”

“…2 and 4). The compounds are similar to the hydrido-complex [R 2 Al-CH 2 -AlR 2 (µ-H)] − [17], but differ from a series of alkyl or alkynyl adducts [R 2 Al-CH 2 -AlR 2 R'] − (R' = CH 2 -t-Bu [18], Me [19], n-Bu [19], C≡C-SiMe 3 [19], C≡C-Ph [19], C≡C-PPh 2 [19]) in which the Lewis base coordinates in a terminal fashion to only one of the aluminium atoms. The larger tendency of Br − or NH(i-Pr) − to coordinate in a bridging fashion may be attributed to the energetically favoured formation of two 2e-2c bonds as compared to a 3c-2e bond required in the hypothetical derivatives containing bridging alkyl groups.…”

“…Owing to the electronic influence of the aryl groups the signals of the CH 2 protons of 1b are shifted considerably to lower field relative to those of [19] or of the compounds 3a, 3b and 3d (Table 1).…”

“…The use of sterically less demanding ligands resulted in dismutation reactions with the formation of the mononuclear trialkylaluminium derivatives and a mixture of unknown secondary products [13]. The potential of the above dialuminium compounds to react as chelating Lewis acids in the presence of an auxiliary base (e. g. TMEN) was demonstrated for 1a by the synthesis and characterisation of a range of mono-adducts [Li(base) n ][1a(X)] (X = NO 2 [14], NO 3 [14], N 3 [15], CH 3 CO 2 [15], Me 2 PCHPMe 2 [16]) and [Li(base) n ][1a(R')] (R' = H [17], CH 2 -t-Bu [18], Me [19], n-Bu [19], C≡C-SiMe 3 [19], C≡C-t-Bu [19], C≡C-Ph [19], C≡C-PPh 2 [19]). In the solid state the anions X − are chelated by the two aluminium atoms via two of their heteroatoms thereby forming five-or six-membered heterocycles, while in most cases the R' anions are bound in a terminal fashion to only one aluminium atom.…”

“…Only the H − anion was chelated by the two aluminium atoms in solution and in the solid state. In the case of R' = C≡C-CH 2 -NEt 2 [19] [20], was obtained in which each aluminium atom is bound to an alkynide substituent. The solid-state structure revealed a unique molecular arrangement with one of the two lithium cations being sandwiched between two alkynide groups and showing close Li-C contacts to the ethynyl α-C atoms and the bridging CH 2 group.…”