On the Reliability of C-H⋯O Interactions in Crystal Engineering: Synthesis and Structure of Two Hydrogen Bonded Phosphonium Bis(aryloxide) Salts

Two related phosphonium aryloxides, [Ph₃PCH₃]⁺₂[CH₂(C ₆H₂-3,5-tert-butyl-4-O)₂]^2-, 1a, and [Ph₃-PC₂H₅]⁺₂[CH ₂(C₆H₂-3,5-tert-butyl-4-O)₂] ^2-, 1b have been synthesized by protonation of nonstabilized phosphorus ylides with 4,4′-methylenebis(2,6-di-tert-butylphenol) LH₂. The crystal structures of 1a and 1b have been determined by single-crystal X-ray diffraction, and that of 1b has also been determined by low-temperature (20 K) neutron diffraction. Both crystallize as CH₃CN solvates, and both exhibit polymeric supramolecular structures via extensive C-H⋯O hydrogen bonding. The predominant structural motif is chelation of the aryloxide oxygen atom of the anion by phosphonium alkyl and aryl C-H groups, such as has been previously observed for phosphonium monoaryloxide salts. The use of this motif as a supramolecular synthon in crystal engineering is explored. In the case of 1a, in addition to the expected intermolecular C-H⋯O hydrogen bonding pattern, C-H⋯π interactions between the anion and this solvent are observed. In the structure of 1b, C-H⋯π interactions between the cation and anion lead to an unpredicted supramolecular structure. The possible significance of this feature is discussed the context of the general use of weak hydrogen bonds in crystal engineering.