PREPARATION OF (ORTHO-HALOGENO)-ETA(1)-PYRIDYLNICKEL(II) COMPLEXES, PRECURSORS FOR THE GENERATION OF PYRIDYNE-NICKEL(0) COMPLEXES

Monomeric eta(1)-(3-chloro)-2-pyridyl complexes of nickel(II), NiCl(3-ClC5H3N-2) L(2) [L(2) = 2PEt(3) (1), dcpe (2); dcpe = 1,2-bis(dicyclohexylphosphino)ethane, (C6H11)(2) PCH2CH2P(C(6)H(1)1)(2)], have been prepared by the reaction of 2,3-dichloropyridine with Ni(COD)(2) in the presence of PEt(3) or with Ni(eta(2)-C2H4)(dcpe), respectively. Reaction of 2,3-dichloropyridine with NiCl2(PPh(3))(2) in the presence of zinc gave the dimeric (C,N)-bridged (3-chloro)-2-pyridyl complex [NiCl (mu-3-ClC5H3N-2) (PPh(3))](2) (4), which re-forms 1 on addition of PEt(3). A similar reaction between NiBr2(PPh(3))(2) and 3,4-dibromopyridine gave a mixture of the isomers NiBr(3-BrC5H3N-4) (PPh(3))(2) (5) and NiBr(4-BrC5H3N-3) (PPh(3))(2) (6), from which the PPh(3) ligands could easily be replaced by dcpe to give NiBr(3-BrC5H3N-4)(dcpe) (7) and NiBr(4-BrC5H3N-3) (dcpe) (8). Alkali metal reduction of 2, and of the mixture of 7 and 8, gave unstable species that could not be isolated, which are thought to be nickel(0) complexes of 2,3- and 3,4-pyridyne, respectively, on the basis of their P-31 NMR spectra. The structure of 4 has been solved by heavy atom methods and refined by least squares methods.