Structure and reactivity of aryl(bromo)nickel complexes relevant to nickel(0) complex-promoted dehalogenative polycondensation of organic dihalides

Abstract

Reactions of 4-bromobiphenyl, 1,4-dibromobenzene, and 9,10-dibromoanthracene with [Ni(cod)₂](cod = cycloocta-1,5-diene) in the presence of PEt₃ gave arylnickel complexes, tans-[NiBr(C₆H₄C₆H₅)(PEt₃)₂]1, trans-[NiBr(C₆H₄Br-p)(PEt₃)₂]2, and trans-[NiBr(C₁₄H₈Br)(PEt₃)₂]3, respectively. 4,4′-Dibromobiphenyl reacted with a [Ni(cod)₂]–PEt₃ mixture to give trans, trans-[(Et₃P)₂BrNi(C₆H₄C₆H₄)NiBr(PEt₃)₂]4. The structures of complexes 1–4 with trans configuration around square-planar nickel centres have been determined by X-ray crystallography. The NMR (¹H, ¹³C, and ³¹P) spectra of the complexes are consistent with the trans structures. Complexes 2 and trans-[NiBr(Ph)(PEt₃)₂]5 did not undergo any thermal reaction at 60 °C in dimethylformamide. Complex 2 reacted with Mel at room temperature to give a mixture of p-bromotoluene (17%) and 4,4′-dibromobiphenyl (8%), while similar reaction of 5 gave a mixture of toluene (15%) and biphenyl (78%). Reaction of complex 4 with Mel gave 4-methylbiphenyl and quaterphenyl.