Enantiopure Calcium Iminophosphonamide Complexes: Synthesis, Photoluminescence, and Catalysis

Abstract

The synthesis of calcium complexes ligated by three different chiral iminophosphonamide ligands, L‐H (L=[Ph2P{N(R)CH(CH3)Ph}2]), L′‐H (L′=[Ph2P{NDipp}{N(R)CH(CH3)Ph}]), (Dipp=2,6‐iPr2C6H3), and L′′‐H (L′′=[Ph2P{N(R)CH(CH3)naph}2]), (naph=naphthyl) is presented. The resulting structures [L2Ca], [L′2Ca], and [L′′2Ca] 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′′2Ca] is assigned to the fluorescence of naphthyl groups, the PL of [L2Ca] and [L′2Ca] 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.

Thermally Activated Delayed Fluorescence and Phosphorescence Quenching in Iminophosphonamide Copper and Zinc Complexes

“… [11] The monometallic zinc complexes 5 and 6 coordinating two iminophosphonamide ligands are analogous to the calcium complexes [Ca{( R )‐PEPIA} 2 ]. [14] In view of the bright phosphorescence and thermally activated delayed fluorescence (TADF) observed for the alkali metal and calcium derivatives,[ 11 , 14 ] a comparison with PL properties of the copper and zinc complexes 1–7 appears particularly interesting. Figures 3 and 4 show PL excitation (PLE) and emission spectra of the solid (polycrystalline) samples of 1–4 and 5–7 , respectively, at selected temperatures between 5 and 295 K, together with the emission intensity as a function of temperature.…”

“…Similar spectra were observed for the alkali metal and calcium complexes, indicating related structures. [ 11 , 14 ] The low‐temperature emission of the latter and dimeric complexes 1–3 and 5 shares the same origin: it is phosphorescence with a lifetime from tens of microseconds to a few milliseconds. The phosphorescence is a major relaxation channel at low temperatures, referring, for instance, to the quantum efficiency, Φ PL , of ≈100 % for 1 and 5 at T =5 K, as estimated from the PL temperature dependences and Φ PL values measured at room temperature using an integrating sphere (see the supporting information).…”

“…Similarly moderate delayed fluorescence efficiencies were found for the related lithium, cesium and calcium compounds. [ 11 , 14 ] Remarkably, this trend towards lower fluorescence quantum efficiency becomes dramatic in the copper complexes 2 and 3 , which emit practically no fluorescence at all. On the other hand, as mentioned above, they show the TADF‐characteristic temperature dependence of the PL (phosphorescence) decay time, correlating with a strong, thermally activated decrease of the emission intensity.…”

“…Similar observations were made for the related calcium complexes. [14] These findings encouraged us to investigate coordination of the iminophosphonamine ligands with other metals, including transition metals. Herein we report on the synthesis of copper(I) and zinc(II) iminophosphonamide complexes.…”

Cover Feature: Thermally Activated Delayed Fluorescence and Phosphorescence Quenching in Iminophosphonamide Copper and Zinc Complexes (Chem. Eur. J. 61/2021)

“…These values compare well to those determined for the alkali metal and calcium complexes. [11,14] However, delayed fluorescence in 1 and 5 has a relatively low quantum efficiency (9 and 3 %, respectively, at 295 K). The S 1 state in these compounds apparently decays to the ground state mostly nonradiatively.…”