Preparation of benzyne complexes of group 10 metals by intramolecular Suzuki coupling of ortho-metalated phenylboronic esters: Molecular structure of the first benzyne-palladium(0) complex

A series of nickel(II) and palladium(II) aryl complexes substituted in the ortho position of the aromatic ring by a (pinacolato)boronic ester group, [MBr{o-C₆H₄B(pin)}L₂] (M = Ni, L₂ = 2PPh₃ (2a), 2PCy₃ (2b), 2PEt₃ (2c), dcpe (2d), dppe (2e), and dppb (2f); M = Pd, L₂ = 2PPh₃ (3a), 2PCy₃ (3b), and dcpe (3d)), has been prepared. Many of these complexes react readily with KO^tBu to form the corresponding benzyne complexes [M(η²-C₆H₄)L₂] (M = Ni, L₂ = 2PPh₃ (4a), 2PCY₃ (4b), 2PEt₃ (4c), dcpe (4d); M = Pd, L₂ = 2PCy₃ (5b)). This reaction can be regarded as an intramolecular version of a Suzuki cross-coupling reaction, the driving force for which may be the steric interaction between the boronic ester group and the phosphine ligands present in the precursors 2 and 3. Complex 3d also reacts with KO^tBu, but in this case disproportionation of the initially formed η²-C₆H₄ complex (5d) leads to a 1:1 mixture of a novel dinuclear palladium(I) complex, [(dcpe)Pd(μ₂-C₆H₄)Pd(dcpe)] (6), and a 2,2′-biphenyldiyl complex, [Pd(2,2′-C₆H₄C₆H₄)-(dcpe)] (7d). Complexes 2a, 3b, 3d, 4b, 5b, 6, and 7d have been structurally characterized by X-ray diffraction; complex 5b is the first example of an isolated benzyne-palladium(0) species.