Synthesis and characterization of low-coordinate divalent aryl transition-metal halide analogues of grignard reagents: Precursors for reduction to metal-metal-bonded complexes

The synthesis and characterization of the series of divalent first-row aryl transition-metal(II) halide compounds [Cr(mu;-Cl)Ar′]₂ (1) and (Li(OEt₂)Ar′Ml₂]₂ (M = Mn (2), Fe (3), and Co (4); Ar′ = C₆H₃-2,6-(C₆H ₃-2,6-ⁱPr₂)₂) are described. 1-4 were prepared by the addition of one equiv of Ar′Li to the respective transition-metal dihalides. They were characterized by UV-vis spectroscopy, magnetic measurements, and by X-ray crystallography. In dimeric 1, each chromium center has quasi-four-coordinate, square-planar geometry, in which the metal is terminally bound to a terphenyl ligand through the ipso carbon of the central ring and to two bridging chloride ligands. There is a further interaction between chromium and an ipso carbon from one of the flanking -C ₆H₃-2,6ⁱPr₂ rings. In contrast, for the iodo derivatives 2-4, Lil is not eliminated upon addition of LiAr′ to Ml₂. Instead, the diethyl ether solvated adducts, [Li(OEt ₂)Ar′MI₂]₂ (M = Mn (2), Fe (3), or Co (4)) were isolated. These possess a distorted cubane Li₂M ₂l₄ core, in which the lithiums are bound to an ether and the transition metals are bound to a terphenyl group. Magnetic measurements between 2 and 300 K reveal the expected weak antiferromagnetic exchange coupling in each of the complexes.