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

The potential of the heteroleptic heavier alkaline-earth hexamethyldisilazides [{HC(C(Me)(2)N-2,6-(Pr2C6H3)-Pr-i)(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 H-1 NMR spectroscopy. Attempts to isolate these solvated complexes were unsuccessful due to redistribution to the homoleptic complex [{HC(C(Me)(2)N-2,6-(Pr2C6H3)-Pr-i)(2)}(2)Ca]. 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)(2)N-2,6-(Pr2C6H3)-Pr-i)(2)}Ca{NHCH2CH2OMe}](2), which retains its dimeric constitution in solution and is stable to further intermolecular ligand exchange. Reactions of [{HC(C(Me)(2)N-2,6-(Pr2C6H3)-Pr-i)(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)(2)N-2,6-(Pr2C6H3)-Pr-i)(2)}Ca{N(H)-2,6-(PrC6H3)-Pr-i}(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 beta-diketiminato species and ill-defined polymeric calcium anilido products.