Enantiopure Calcium Iminophosphonamide Complexes : Synthesis, Photoluminescence, and Catalysis

The synthesis of calcium complexes ligated by three different chiral iminophosphonamide ligands, L-H (L=$[Ph_{2}P{N(R)CH(CH_{3})Ph}_{2}$]), L′-H (L′=[$Ph_{2}$P{NDipp}{$N(R)CH(CH_{3})Ph$}]), ($Dipp=2,6-^{i}Pr_{2}C_{6}H_{3}$), and L′′-H (L′′=[$Ph_{2}P{N(R)CH(CH_{3}naph}_{2}$]), (naph=naphthyl) is presented. The resulting structures [$L_{2}Ca$], [$L′_{2}Ca$], and [$L′′_{2}Ca$] represent the first examples of enantiopure homoleptic calcium complexes based on this type of ligands. The calcium complexes show blue–green photoluminescence (PL) in the solid state, which is especially bright at low temperatures. Whereas the emission of [$L′′_{2}Ca$] is assigned to the fluorescence of naphthyl groups, the PL of [$L_{2}Ca$] and [$L′_{2}Ca$] is contributed by long-lived phosphorescence and thermally activated delayed fluorescence (TADF), with a strong variation of the PL lifetimes over the temperature range of 5–295 K. Furthermore, an excellent catalytic activity was found for these complexes in hydroboration of ketones at room temperature, although no enantioselectivity was achieved.