Weak Interactions in the Crystal Structures of Tetraacetylenes Derived from Pentaerythrityl Tetraphenyl Ether

Tetrakis(4-ethynylphenyl)methane (7) is known to crystallize as an interpenetrated diamondoid network held together
in part by weak C⋮CH···C⋮C interactions that define characteristic cyclic quartets of ethynyl groups. To test the ability of this
motif to direct the crystallization of more flexible molecules, we synthesized a series of tetraacetylenes derived from pentaerythrityl
tetraphenyl ether, and we solved their structures by X-ray diffraction. Pentaerythrityl tetrakis(4-ethynylphenyl) ether (4) and the
more rigid analogue 7 crystallize isostructurally, even though their cores are significantly different in size and flexibility. This
observation shows that C⋮CH···C⋮C interactions may have value in engineering molecular crystals. However, widely different
structures are obtained by crystallizing pentaerythrityl tetrakis(3-ethynylphenyl) ether (5) and pentaerythrityl tetrakis(4‘-ethynyl-1,1‘-biphenyl-4-yl) ether (6), despite their close similarity to tetraacetylene 4. The unpredictably varied behavior of tetraacetylenes
4−7 highlights the difficulty of engineering crystals built from molecules that are flexible and unable to form dominant directional
interactions.