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(C6H2-3,5-tert-butyl-4-O)2]2-, 1a, and [Ph3PC2H5]+2[CH2(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.