Kinetic stability of heteroleptic (β-diketiminato) heavier alkaline-earth (Ca, Sr, Ba) amides

Abstract

The potential of the heteroleptic heavier alkaline-earth hexamethyldisilazides [{HC(C(Me)₂N-2,6-ⁱPr₂C₆H₃)₂}Ae{N(SiMe₃)₂}(THF)] (Ae = Ca, Sr, Ba) as kinetically-stable reagents for further protolytic reaction chemistry has been assessed. Only the previously reported calcium complex was found to be stable to solution dismutation and dynamic ligand exchange. The barium complex was isolated in sufficient purity to enable characterisation by an X-ray analysis. Reactions of the kinetically robust calcium complex with cyclohexylamine and tert-butylamine resulted in displacement of THF and formation of solvated structures, which could be characterised by ¹H NMR spectroscopy. Attempts to isolate these solvated complexes were unsuccessful due to redistribution to the homoleptic complex [{HC(C(Me)₂N-2,6-ⁱPr₂C₆H₃)₂}₂Ca]. In contrast, the more acidic amine [H₂NCH₂CH₂OMe] was cleanly deprotonated resulting in the isolation of the first well defined primary amido derivative of a heavier alkaline-earth element, [{HC(C(Me)₂N-2,6-ⁱPr₂C₆H₃)₂}Ca{NHCH₂CH₂OMe}]₂, which retains its dimeric constitution in solution and is stable to further intermolecular ligand exchange. Reactions of [{HC(C(Me)₂N-2,6-ⁱPr₂C₆H₃)₂}Ca{N(SiMe₃)₂}(THF)] with a variety of ortho-disubstituted anilines also resulted in immediate protonation of the hexamethyldisilazide ligand. Only the most sterically demanding 2,6-diisopropylphenyl anilide derivative possessed sufficient kinetic stability to allow isolation of the heteroleptic complex. The crystal structure of [{HC(C(Me)₂N-2,6-ⁱPr₂C₆H₃)₂}Ca{N(H)-2,6-ⁱPrC₆H₃}(THF)] was shown to exist as a mononuclear, pseudo-five-coordinate complex in which the coordinative unsaturation of the calcium centre is relieved by a Ca⋯H–C agostic-type interaction to one of the ortho isopropyl groups of the anilide ligand. This interaction is not maintained in solution however and the complex slowly redistributes to the homoleptic β-diketiminato species and ill-defined polymeric calcium anilido products.