Reductive Coupling of Bridging Diaryl Ligands in Half-Sandwich Cobalt(II) Dimers: Revisiting Triple-Decker Cobalt(I) Complexes

The reactions of [Cp*Co­(μ-Cl)]2 with various aryl lithium reagents (ArLi) were performed with a view to access bridging diaryl dicobalt­(II) complexes. Treatment of [Cp*Co­(μ-Cl)]2 with 1 equiv of ArLi provided bridging aryl Co­(II) complexes with the formula [(Cp*Co)2(μ-Cl)­(μ-Ar)] (Ar = C6H5 –, 1; p-CH3C6H4 –, 2). With 2 equiv of C6H5Li or p-CH3C6H4Li, the reaction provided triple-decker dicobalt­(I) complexes [(Cp*Co)2-μ-(η4:η4-biphenyl)] (3) and [(Cp*Co)2-μ-(η4:η4-dimethyl-biphenyl)] (4). The biphenyl ligands in 3 and 4 underwent dissociation resulting in formation of the more stable triple-decker tricobalt complexes [(Cp*Co)3-μ-(η4:η4:η4-biphenyl)] (5) and [(Cp*Co)3-μ-(η4:η4:η4-dimethyl-biphenyl)] (6), respectively. The bridging diaryl complex [(Cp*Co)2(μ-η1:η1-Ar)2] was proposed as the intermediate in the formation of triple-decker dicobalt­(I) complexes through the reductive coupling of the aryl ligand, because the diaryl cobalt­(II) complex­[(Cp*Co)2-μ-(o-CH3C6H4)2] (7) is stabilized by increased steric hindrance at the o-position of the aryl ligand.