Heterodimetallic alkaline earth metal amides: Synthesis, structure, and solvent-induced charge separation of homoleptic calcium-magnesium hexamethyldisilazide

The heterodimetallic compound [(Me3Si)(2)NMg{mu-N(SiMe3)(2)}(2)CaN(SiMe3)(2)], 3, can be prepared as single-component crystals through manipulation of the equilibrium established on mixing the homometallic derivatives Mg[N(SiMe3)(2)](2), 1, and Ca[N(SiMe3)(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(mu(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 [(Me3Si)(2)NCa center dot(pyr)(n)](+)[Mg{N(SiMe3)(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%.