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

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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)L 1 RuCl] Ru1, [( p -cymene)L 2 RuCl] Ru2, and [( p -cymene)L 3 RuCl]OTf Ru3 [HL 1 = 2-(2,6-dibenzhydryl-4-methylphenyliminomethyl)phenol, HL 2 = 2-(2,6-dibenzhydryl-4-methylphenyliminomethyl-5-pentan-3-yl)phenol, L 3 = 2,6-dibenzhydryl- N -(isoquinolin-1-ylmethylene)-4-methylaniline] were synthesized by the reaction of precursor complex [( p -cymene)RuCl 2 ] 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 1 H and 13 C{ 1 H} NMR. The ligands (HL 2 and L 3 ) 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 1 H and 13 C{ 1 H} NMR analysis.