Novel Ni(0)-COT Complexes, Displaying Semiaromatic Planar COT Ligands with Alternating C−C and CC Bonds^†

Reaction of (R₂PC₂H₄PR₂)Ni(C₂H₄) with COT gives the mononuclear complexes (R₂PC₂H₄PR₂)Ni(η²-C₈H₈) (R = ⁱPr 1a, ^tBu 1b). The COT ligand in 1a,b is planar with alternating C−C and CC bonds, corresponding to a formal semiaromatic [C₈H₈]^- ligand. Reactions of 1a with (ⁱPr₂PC₂H₄PⁱPr₂)Ni(η²,η²-C₆H₁₀) and of 1b with stoichiometric amounts of {(^tBu₂PC₂H₄P^tBu₂)Ni}₂(μ-C₆H₆) or lithium afford the dinuclear complexes {(ⁱPr₂PC₂H₄PⁱPr₂)Ni}₂{μ-η⁴(1,2,5,6):η⁴(3,4,7,8)-C₈H₈} (2a) and {(^tBu₂PC₂H₄P^tBu₂)Ni}₂(μ-η²:η²-C₈H₈) (2b; two isomers). The COT ligand in 2a is tub-shaped and olefinic, whereas in 2b (as in 1a,b) it is planar and semiaromatic. The products are characterized by IR, solution and solid-state NMR spectroscopy, and by X-ray structure analysis.