Synthesis of Quinoline-Based NNN-Pincer Nickel(II) Complexes: A Robust and Improved Catalyst System for C–H Bond Alkylation of Azoles with Alkyl Halides

Abstract

The quinoline-based pincer nickel(II) complexes κ N,κ N,κ N -{R 2 N-C 6 H 4 -(μ-N)-C 9 H 6 N}NiX (( R2 NNN Q )NiCl: R = Me, 2a; R = Et, 2b ) were synthesized by the reaction of the ligand precursors ( R2 NNN Q )H (R = Me, 1a; R = Et, 1b ) with (DME)NiCl 2 in the presence of Et 3 N. Similarly, the pincer nickel(II) derivatives ( R2 NNN Q )NiX (R = Me, X = Br, 3a; R = Et, X = Br, 3b; R = Me, X = OAc, 4a ) were obtained by treatment of the ligands ( R2 NNN Q )H with the nickel precursor (THF) 2 NiBr 2 or Ni(OAc) 2 . All of these complexes were characterized by 1 H and 13 C NMR spectroscopy as well as by elemental analysis. Further, the molecular structures of 2a and 3a, b were elucidated by X-ray crystallography. Complex 2a is found to be an efficient catalyst for the direct C–H bond alkylation of substituted benzothiazoles and oxazoles with various unactivated alkyl halides containing β-hydrogens under mild reaction conditions. The catalyst 2a is very robust and was recycled and reused five times for the alkylation reaction without a decrease in its catalytic activity. Preliminary studies reveal that the catalyst 2a acts as an active catalyst and the alkylation reaction appears to operate via a radical pathway.