Synthesis and Structure of Neutral and Cationic Aluminum Complexes Incorporating Bis(oxazolinato) Ligands

Abstract

Treatment of the bis(oxazolines) 1,1-bis[4,4-dimethyl-1,3-oxazolin-2-yl]ethane (1a, {BOX-Me2}H) and 1,1-bis[(4S)-4-isopropyl-1,3-oxazolin-2-yl]ethane (1b, {BOX-(S)-iPr}H) with nBuLi, followed by the addition of 1 equiv of ClAlMe2, affords the corresponding dimethyl Al complexes {BOX-Me2}AlMe2 (2a) and {BOX-(S)-iPr}AlMe2 (2b) in reasonable yields. The dichloro Al complex {BOX-Me2}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-Me2}AlMe2 (2a) and {BOX-(S)-iPr}AlMe2 (2b) react in C6D5Br with B(C6F5)3 to yield the quantitative formation of cations {BOX-Me2}AlMe+ (4a+) and {BOX-(S)-iPr}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-Me2}Al(Me)(L)+ (5a+, L = THF; 6a+, L = NMe2Ph) and {BOX-(S)-iPr}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-Me2}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.