Synthesis and Characterization of Heterobimetallic Alkaline Earth Metal Amides

The synthesis and characterization of heterobimetallic alkaline-earth metal amides was attempted. This is an entirely new class of compounds and they were targeted with the purpose of evaluating their bond character aggregation behavior in solution and in the solid state. The reaction between the appropriate homometallic bis(hexamethyldisilazide) precursors in non-donating solvents yielded the products: [CaMg{N(SiMe3)2}4] (1), [SrMg{N(SiMe3)2}4] (2), [SrCa{N(SiMe3)2}4] (3), and [Ba2{N(SiMe3)2}3'¢toluene]+[Mg{N(SiMe3)2}3]- (4). The synthesis of crystalline Ba/Ca and Ba/Sr heterobimetallic complexes was attempted however they do not mix in solution. Compounds 1-4 were successfully characterized by single crystal X-ray diffraction. Structures 1, 2, and 3 are all heterodimers with a four membered ring core consisting of both metals connected by bridging amide groups. Each metal is bonded to a single amide group yielding a coordination number of three. The charge separated complex 4 consists of a magnesium trisamide anion and a dimeric barium cation with one metal site solvated by a π-bound toluene molecule. Analysis of the metrical parameters in 1-3 indicates that the more Lewis acidic metal pulls the electron density of the bridging amide ligands towards it, causing a Lauren T. Wendell change in the bonding when compared to the homodimer structures. This change in the bonding causes interesting solution behavior, where charge separated species are often seen in donating solvents. The most extreme case is complex 4 where charge separation is also observed in the solid state. Complexes 1, 2, and 3 all undergo a dynamic equilibrium between the heterobimetallic structures and the homometallic precursors in arene solvents. In contrast, Complex 4 maintains charge separation. The change in the bonding and solution behavior compared to the homodimers suggests that these compounds will have interesting reactivity with organic substrates.