Reaction of an alkynyllithium reagent with boron trifluoride: the structure of a likely intermediate in alkynyl anion chemistry

Lithium phenylacetylide reacts with 1 equiv. of BF3 . OEt2 in thf-toluene to yield both a thf adduct of tris(phenylacetylido)borane 2, which is probably the true intermediate in RC=CLi/BF3-promoted alkynyl anion chemistry, and LiBF4, products which are considered to form via sequential LiF elimination from previously proposed acetylidoborate intermediates; complex 2 is the first structurally characterised tris(alkynyl)borane and in the solid state it forms an unusual hydrogen-bonded dimer.