Di-ruthenium complexes of 1,4-diazabutadiene ligands: synthesis, characterization and utilization as catalyst precursors for the oxidative coupling of amines to imines in air

Abstract

The reaction of [Ru(PPh₃)₃Cl₂] with a group of four 1,4-diazabutadiene ligands (p-RC₆H₄NC(H)–C(H)NC₆H₄R-p; R = OCH₃, CH₃, H and Cl; abbreviated as L-R) in an equimolar ratio afforded chloro-bridged di-ruthenium complexes (1–4) of the general type [Ru₂(PPh₃)₂(L-R)₂Cl₃]Cl. The crystal structures of complexes 1 (R = OCH₃), 2 (R = CH₃) and 4 (R = Cl) were determined and the molecular structure of complex 3 (R = H) was optimized using the DFT method. Each complex showed four intense absorptions in the visible and ultraviolet regions, the origin of which was probed employing TDDFT calculations. The complexes were found to serve as efficient catalyst precursors for the oxidative and deaminative C–N coupling of a series of primary amines, producing corresponding imines in good yields. Homo-coupling of benzylamines and cross-coupling of two differently substituted benzylamines or benzylamine with anilines and other aryl/alkyl amines could be realized using this catalytic protocol with aerial oxygen as the oxidant.