Synthesis and Single-Crystal X-ray Structures of Sterically Hindered Three Half-Sandwich (η⁶-<i>p</i>-Cymene)ruthenium(II) Schiff Base Complexes: Catalytic Applications in Selective Monoalkylation of Amines and Synthesis of Amides

Transition metal complex-mediated organic synthesis is a fascinating area of mutual interest to both organic and inorganic chemists. In this work, three new bulky half-sandwich ruthenium(II) Schiff base complexes of the general formula [(p-cymene)L1RuCl] Ru1, [(p-cymene)L2RuCl] Ru2, and [(p-cymene)L3RuCl]OTf Ru3 [HL1 = 2-(2,6-dibenzhydryl-4-methylphenyliminomethyl)phenol, HL2 = 2-(2,6-dibenzhydryl-4-methylphenyliminomethyl-5-pentan-3-yl)phenol, L3= 2,6-dibenzhydryl-N-(isoquinolin-1-ylmethylene)-4-methylaniline] were synthesized by the reaction of precursor complex [(p-cymene)RuCl2]2 with the corresponding Schiff base ligands and isolated as colored crystalline solids in high yields (85–87%). All Schiff base ligands and half-sandwich ruthenium complexes were characterized by ESI-MS, FT-IR and 1H and 13C{1H} NMR. The ligands (HL2 and L3) and complexes (Ru1–Ru3) were structurally characterized by single-crystal X-ray diffraction studies. The single-crystal X-ray structures of the complexes (Ru1–Ru3) confirmed a pseudo-octahedral half-sandwich piano-stool geometry around ruthenium. The complexes were found to be efficient catalysts for selective monoalkylation of primary amines via the hydrogen borrowing strategy and synthesis of a series of amides from various aldehyde and hydroxylamine hydrochloride. Among all three complexes, the cationic complex (Ru3) exhibited the best catalytic activity than the neutral analogues Ru1 and Ru2 in both the reactions. The synthesized organic products were characterized by 1H and 13C{1H} NMR analysis.