Supramolecular Recognition of Quaternary Phosphonium Cations

The modes of supramolecular recognition of quaternary phosphonium cations mediated by 1,1'-bi-2-naphthol (BINOL) are identified and characterized. In contrast to our previous work on ammonium cations, the recognition of the quaternary phosphonium cations via the formation of a PR4+center dot Br-center dot BINOL ternary complex was found to be mediated by a hydrogen bond from an alpha-carbon center of the phosphonium cation, encapsulation within a continuous hydrogen bond network between the halide-BINOL network, or a combination of these effects working in tandem. The solid state structures of these ternary complexes were analyzed by X-ray crystallography, aided by Hirshfeld surface analysis, to confirm the presence of characteristic intermolecular interactions for the identified modes. In all cases, the quaternary phosphonium cation acts as a hydrogen bond donor (HBD) in these supramolecular interactions, and thus this is the key to the recognition process with BINOL. The characterization of such mechanisms offers insight into the supramolecular and crystal engineering communities in the future design of agents capable of the supramolecular recognition of phosphonium cations and their abstraction from the solution phase.