Crown Ether Supported Alkali Metal Phosphides: Synthesis, Structures and Bonding.

Herein we present the synthesis and full characterization of the previously unpresented lithium diphenyl phosphides Li(15-crown-5)PPh2 (1Li) and Li(12-crown-4)PPh2 (2Li) as well as the heavy homologues Rb(18-crown-6)PPh2 (1Rb) and Cs(18-crown-6)PPh2 (1Cs). We thus complete the series of crown ether coordinated alkali metal diphenyl phosphides and reveal that the structural diversity increases upon descending the group. Including the previously reported Na and K derivatives, we performed a detailed study of the title compounds in solution that strongly indicated all of them are monomers with an alkali metal phosphide AM-P interaction. We further investigated the influence of the phosphorus-bound substituents on the bond between the Cs(18-crown-6) and the phosphide fragment. For this purpose, we synthesized and characterized two new cesium phosphides, alkyl-aryl Cs(18-crown-6)PtBuPh (4Cs) and bis-alkyl Cs(18-crown-6)PtBu2 (5Cs). We observed here that the heavy alkali metals Rb and Cs favor coordination via π-arene interactions over the P donor dative interactions. Finally, we used state-of-the-art quantum chemical methods to analyze the bonding situation in the title series of compounds.