Transition Metal Acetylide Rearrangement and Coupling Induced by Coordinative Unsaturation

The reaction of {Ar′Fe(μ-Br)}2 (Ar′ = C6H3-2,6-(C6H3-2,6-iPr2)2) with LiCCPh afforded the unusual 1,3-butadiene-1,4-diyl Fe(I)-coupled derivative Fe2{Ar′CC(Ph)-C(Ph)CAr′} (1), whereas the reaction of {Ar′Fe(μ-Br)}2 with LiCCtBu yielded the monomeric Fe(II) “ate” complex Ar′Fe(CCtBu)2{Li(THF)2} (2). Complexes 1 and 2 were characterized by X-ray crystallography, NMR, and UV−vis spectroscopy and magnetic measurements. In 1 the dimeric structure is a result of Ar′ group transfer to the iron-bound carbon of the acetylide ligand and subsequent dimerization via coupling of the phenyl-substituted carbons. The irons are antiferromagnetically coupled, and the iron−iron separation is 2.5559(3) Ǻ. In 2 the high-spin iron atom has distorted trigonal-planar coordination with a THF-complexed lithium ion associated with the Ar′Fe(CCtBu)2 anion via interactions with the tBu-substituted alkyne carbons.