Preparation of (ortho-halogeno)-η1-pyridylnickel(II) complexes, precursors for the generation of pyridyne-nickel(0) complexes

Abstract

Monomeric η1-(3-chloro)-2-pyridyl complexes of nickel(II), NiCl(3-ClC5H3N-2) L2 [L2 = 2PEt3 (1), dcpe (2); dcpe = 1,2-bis(dicyclohexylphosphino)ethane, (C6H11)2 PCH2CH2P(C6H11)2], have been prepared by the reaction of 2,3-dichloropyridine with Ni(COD)2 in the presence of PEt3 or with Ni(η2-C2H4) (dcpe), respectively. Reaction of 2,3-dichloropyridine with NiCl2(PPh3)2 in the presence of zinc gave the dimeric (C,N)-bridged (3-chloro)-2-pyridyl complex [NiCl(μ-3-ClC5H3N-2)(PPh3)]2 (4), which re-forms 1 on addition of PEt3. A similar reaction between NiBr2(PPh3)2 and 3,4-dibromopyridine gave a mixture of the isomers NiBr(3-BrC5H3N-4)(PPh3)2 (5) and NiBr(4-BrC5H3N-3)(PPh3)2 (6), from which the PPh3 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 31P NMR spectra. The structure of 4 has been solved by heavy atom methods and refined by least squares methods.