Ru(II) hydrazone catalysts for C–N bond formation via the borrowing hydrogen strategy

A series of six-coordinate Schiff base ruthenium(II) complexes [RuH(CO)(EPh3)2(L)] [E = P or As; L = 2-(benzothiazol-2-yl-hydrazonomethyl)-quinolin-8-ol (BTHQ) or 2-(quinolin-2-yl-hydrazonomethyl)-quinolin-8-ol (QHQ)] were synthesized via reflux of [RuHCl(CO)(EPh3)3] precursors with benzothiazole/quinoline-derived hydrazone ligands in MeOH-CHCl3. The complexes were comprehensively characterized by elemental analysis, IR, UV–Vis, 1H, 1³C & 31P–NMR spectroscopy, ESI-MS spectrometry, and thermogravimetric/differential thermal analysis (TGA/DTA). The combined analytical and spectroscopic data supported tentative distorted octahedral geometries around the ruthenium(II) centers, with the ligands coordinating in a bidentate N,O-chelating mode through the quinoline nitrogen and phenolate oxygen atoms. Density functional theory calculations further corroborated the proposed geometries. The complexes efficiently catalyzed the N-alkylation of various aromatic and aliphatic amines with benzyl alcohols through a borrowing-hydrogen strategy using 1 mol% catalyst and KOH in toluene at 110 °C. The corresponding N-alkylated products were obtained in yields of up to 93%, with complex 3 exhibiting the highest catalytic activity across substrates with varying steric demands. These findings demonstrate the potential of ruthenium(II) hydrazone complexes as efficient catalysts for atom-economical C–N bond formation.