Manipulation of Solvent Donor Effects to Overcome the Calcium Schlenk Equilibrium

Synthetic access toward well-defined, monomeric s-block metal complexes is mired with chemical challenges, primarily attributed to the formation of homoleptic complexes promoted by the Schlenk equilibrium. Ligand redistribution is significantly pronounced for the heavier s-block metals, such as calcium, which form bischelate complexes readily through traditional synthetic routes such as transmetalation, amination, and ligand exchange. Mechanistic investigation of each of these routes with phenoxyimine (ONN) ligands was explored to ascertain the fundamental parameters promote bischelate formation. Donor effects from coordinated solvent proved to be deleterious, and in an effort to circumvent bischelation, a new calcium bisamide, {Ca[N(SiMe₃)₂]₂(diox)₂}_∞, was synthesized and characterized as a coordination polymer with a unique square planar geometry, rarely seen for group 2 complexes. Amination with {Ca[N(SiMe₃)₂]₂(diox)₂}_∞ was found to significantly shift the Schlenk equilibrium to favor heteroleptic species, allowing for the characterization of new phenoxyimine calcium-amido complexes: [ONN1Ca–N(SiMe₃)₂]₂(diox) and [ONN3Ca–N(SiMe₃)₂(diox)]_∞. Subsequent studies showed that solvent exchange from the isolated dioxane complexes with THF notably shortened the stability of the complex in solution. Although steric parameters have previously been regarded as the key to the stabilization of heteroleptic calcium complexes, it is equally important to consider the donor ability of coordinated solvent ligands to achieve longer-lived heavy s-block complexes.