Formation of homoleptic tetracarbene versus m-Chelating dicarbene complexes of nickel(ii) and applications in kumada-Corriu couplings

Abstract

The formation of mono- versus bis(chelate) Ni11 complexes bearing N-heterocyclic dicarbene ligands can be controlled by the flexibility of the ligand bridge. A short methylene spacer exclusively gives rise to a dicationic bis(chelate) complex [Ni(MeCCmeth)2]Br2 (1), whereas a more flexible propylene spacer affords a neutral monochelate complex [NiBr2(MeCCprop)] (2). Complex 2 was found to autoionize very slowly to the corresponding dicationic bis(chelate) over ca. 45 d in [D6]dmso. The formation of the bis-versus mono-chelate complex can be attributed to the different stabilities of the resulting metallacycles. The catalytic activity of mono-chelate 2 was tested in the Kumada-Corriu coupling of aryl halides with arylmagnesium reagents at ambient temperature. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009.