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

The heterodimetallic compound [(Me3Si)2NMg{μ-N(SiMe3)2}2CaN(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(μ2-N)2Ca 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)2NCa·(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%.