Synthesis and structure of neutral and cationic aluminum complexes incorporating bis(oxazolinato) ligands

Treatment of the bis(oxazolines) 1,1-bis [4,4-dimethyl-1,3-oxazolin-2-yl]ethane (1a, {BOX-Me-2}H) and 1,1-bis [(4S)-4-isopropyl- 1,3-oxazolin-2-yl]ethane (1b, {BOX-(S)-Pr-i}H) with (n)- BuLi, followed by the addition of 1 equiv of CLAIMe(2), affords the corresponding dimethyl Al complexes {BOX-Me-2}AlMe2 (2a) and {BOX-(S)-Pr-i}AlMe2 (2b) in reasonable yields. The dichloro Al complex {BOX-Me-2}AlCl2 (3a) was synthesized in a similar way using 1 equiv of AlCl3. Compounds 2a,b and 3a are monomeric four-coordinate mono [bis(oxazolinato)aluminum] complexes, on the basis of X-ray analyses for 2b and 3a and NMR data for 2a,b and 3a. The dimethylaluminum complexes {BOX-Me-2}AlMe2 (2a) and {BOX-(S)-Pr-i}AlMe2 (2b) react in C6D5Br with B(C6F5)(3) to yield the quantitative formation of cations {BOX-Me-2}AlMe+ (4a(+)) and {BOX-(S)-Pr-i}AlMe+ (4b(+)), respectively, as fully dissociated MeB(C6F5)(3)(-)salts. Cations 4a,b(+), which are most likely either base-free three-coordinate cationic species or four-coordinate cationic Al-C6D5Br adducts, are unstable at room temperature in C6D5Br and decompose to unidentified species. When these ionization reactions are performed in the presence of a Lewis base L (L = THF, NMe2Ph), corresponding four-coordinate Al-L Lewis base adducts are cleanly generated, {BOX-Me-2}Al(Me)(L)(+) (5a(+), L = THF; 6a(+), L = NMe2Ph) and tBOX-(S)-Pr-i}Al(Me)(L)(+) (5b(+), L = THF; 6b(+), L = NMe2Ph), as determined by solution studies and, in the case of 6a+, by X-ray analysis. In contrast, the reaction of {BOX-Me-2}AlMe2 (2a) with [Ph3C][B(C6F5)(4)] yields the bis(imine) Al cation 7a(+), as determined by NMR and X-ray analysis. The formation of 7a+ most likely proceeds via a hydride abstraction by Ph3C+ at the Me group located at the back of the bis(oxazolinato) ligand in 2a.