Ionic Rotors. Preparation, Structure, and Dynamic Solid-State ²D NMR Study of the 1,4-Diethynylbenzenebis(triphenylborate) Dianion

The reaction between p-(LiC2)2C6H4 (generated in situ from butyllithium and dialkynylbenzene) and 2 equiv of BPh3 affords high yields of [Li(THF)4]2[p-(Ph3BC2)2C6H4], a heretofore missing member of the isoelectronic [p-(Ph3EC2)2C6H4] (E = groups 13−15 element) series. The central phenylene linker is free to rotate in the solid state, as determined by a variable-temperature solid-state 2D spin−echo NMR spectroscopic study. At room temperature, the rate of rotation is at least 20 times faster than that of its carbon-based analogue, presumably a result of Coulombic repulsions minimizing noncovalent interactions between rotating units in the borate salt.