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 (1^Li) and Li(12-crown-4)PPh₂ (2^Li) as well as the heavy homologues Rb(18-crown-6)PPh₂ (1^Rb) and Cs(18-crown-6)PPh₂ (1^Cs). 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)P^tBuPh (4^Cs) and bis-alkyl Cs(18-crown-6)P^tBu₂ (5^Cs). 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.