Nickel Complexes Bearing Quinoline Derived NNS Donor Ligands as Catalytic Activators for N-Alkylation of Anilines...

Herein, we have reported a new series of NNS-donor ligands coordinated Ni(II) complexes and utilized them as catalytic activator to synthesize N-alkylated amines and 1,2-disubstituted benzimidazoles. The reaction of thiophenol/4-chlorothiophenol/4-methylthiophenol/4-methoxythiophenol with 2-bromo-N-quinolin-8-yl-acetamide in presence of sodium hydroxide in ethanol at 80 °C gave [C₉H₆N-NH-C(O)-CH₂-S-Ar] [Ar=C₆H₅ (L1); C₆H₄Cl-4 (L2); C₆H₄Me-4 (L3) and C₆H₄-OMe-4 (L4)], respectively. The corresponding reaction of L1-L4 with Ni(OAc)₂ in methanol at 80 °C for 3 hours resulted in octahedral nickel complexes [(L1-H)₂Ni] (C1), [(L2-H)₂Ni] (C2), [(L3-H)₂Ni] (C3), and [(L4-H)₂Ni] (C4), respectively. All compounds have been characterized by micro and spectroscopic analysis. The molecular structure of complexes C1-C3 has also been determined by single crystal X-ray diffraction data. The utility of complexes C1-C4 were evaluated for the N-alkylation of aniline with benzyl alcohols, and for 1,2-disubstituted benzimidazoles synthesis. The obtained results indicate that complex C1 showed better catalytic activity in both N-alkylation of amines with benzyl alcohols [catalyst loading: 2.0 mol %; Yield up to 92 %], and for 1,2-disubstituted benzimidazoles derivatives [catalyst loading: 2.0 mol %; Yield up to 94 %)]. The mechanistic studies suggested that the reaction works through hydrogen borrowing from benzyl alcohol and its subsequent utilization for in situ reduction of imine. The experimentally observed catalytic reactivity patterns of complexes C1-C4 have found in good agreement with the HOMO-LUMO energy gaps obtained by DFT analysis of corresponding complexes.