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

The quinoline-based pincer nickel­(II) complexes κN,κN,κN-{R2N-C6H4-(μ-N)-C9H6N}­NiX ((R2NNNQ)­NiCl: R = Me, 2a; R = Et, 2b) were synthesized by the reaction of the ligand precursors (R2NNNQ)H (R = Me, 1a; R = Et, 1b) with (DME)­NiCl2 in the presence of Et3N. Similarly, the pincer nickel­(II) derivatives (R2NNNQ)­NiX (R = Me, X = Br, 3a; R = Et, X = Br, 3b; R = Me, X = OAc, 4a) were obtained by treatment of the ligands (R2NNNQ)H with the nickel precursor (THF)2NiBr2 or Ni­(OAc)2. All of these complexes were characterized by 1H and 13C 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.