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)}L-2] (M = Ni, L-2 = 2PPh(3) (2a), 2PCy(3) (2b), 2PEt(3) (2c), dcpe (2d), dppe (2e), and dppb (2f); M = Pd, L-2 = 2PPh(3) (3a), 2PCy(3) (3b), and dcpe (3d)), has been prepared. Many of these complexes react readily with (KOBu)-Bu-t to form the corresponding benzyne complexes [M(eta(2)-C6H4)L-2] (M = Ni, L-2 = 2PPh(3) (4a), 2PCy(3) (4b), 2PEt(3) (4c), dcpe (4d); M = Pd, L-2 = 2PCy(3) (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 (KOBu)-Bu-t, but in this case disproportionation of the initially formed eta(2)-C6H4 complex (5d) leads to a 1:1 mixture of a novel dinuclear palladium(l) complex, [(dcpe)Pd(mu(2)-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.