Heterodimetallic Alkaline Earth Metal Amides: Synthesis, Structure, and Solvent-Induced Charge Separation of Homoleptic Calcium−Magnesium Hexamethyldisilazide

The heterodimetallic compound [(Me 3 Si) 2 NMg{μ-N(SiMe 3 ) 2 } 2 CaN(SiMe 3 ) 2 ], 3, can be prepared as single-component crystals through manipulation of the equilibrium established on mixing the homometallic derivatives Mg[N(SiMe 3 ) 2 ] 2, 1, and Ca[N(SiMe 3 ) 2 ] 2, 2, in toluene/hexane solution. Specifically, doping solutions with a 30% molar excess of 1 is required in order to avoid cocrystallization with 2 . Complex 3 has been characterized by single-crystal X-ray diffraction and multinuclear NMR spectroscopy and is found to adopt a heterodimetallic Mg(μ 2 -N) 2 Ca ring structure in the solid state and in arene solutions. DFT computational studies confirm the experimentally observed variations in the metrical parameters between the homo- and heterodimetallic complexes. The calculations also indicate that the reaction to form 3 is close to being thermoneutral. Addition of pyridine to 3 results in asymmetric cleavage of the mixed-metal ring structure, in turn forming a kinetically stable, charge-separated “ate” species of the type [(Me 3 Si) 2 NCa·(pyr) n ] + [Mg{N(SiMe 3 ) 2 } 3 ] -, 4 . The three bases 1, 2, and 4 react with propiophenone in pyridine media at ambient temperature to give high Z stereoselectivities of ≥98%.