Synthesis and structure of salts of a sterically shielded, lipophilic, C-2-symmetric, fluxional aluminate

Multi-gram amounts of halogen-free lipophilic aluminate salts have been prepared, featuring two sterically demanding chelating ligands derived from a methane-2,2'-bisphenolate. The ligand is prepared by condensation of two equivalents of 2,4-di-tert-butylphenol with acetone induced by boron fluoride etherate. The C-2-symmetry of this "almebate" anion implies helical chirality. Substantial steric shielding of the aluminate core results in high stability towards aqueous bases. Almebate salts hydrolyze in aqueous acetic acid at room temperature within days. Tetraphenylphosphonium and tetrabutylammonium salts have been prepared by salt metatheses. Sodium almebate can be activated thermally, generating electrophilic sodium(I) with a low coordination number. It eliminates chloride ligands from an N-heterocyclic carbene gold chloride complex, proven by a single-crystal X-ray diffraction study. The barrier of molecular racemization by a degenerate rearrangement of the intrinsically chiral almebate anion in acetone has been determined to be Delta G(double dagger) 53.6 +/- 2 kJ mol(-1) from four H-1 NMR coalescence phenomena (500 MHz; 258 K, 273 K, 283 K, 290 K).