Robust tetrazole-linked bimetallic Ru(II) complexes as catalysts for base-free acceptor-less double dehydrogenation of primary amines to nitriles under milder conditions

Abstract

The catalytic double dehydrogenation (DDH) of primary amines is a clean and efficient method over aerobic oxidation. Further, the utilization of bimetallic catalytic system for DDH has proven to be more efficient than its monometallic counterpart. However, its large scale adoption for industrial application is limited owing to the usage of expensive and environmentally adverse catalytic conditions. In this context, the present study describes the first example of tetrazole-derived two bimetallic Ru(II)-arene complexes [(η⁶-p-cymene)₂Ru₂Cl₃(L1)] [Ru-1] and [(η⁶-benzene)₂Ru₂Cl₃(L1)] [Ru-2] (where, L1 = 4-(1H-tetrazole-5yl)benzoic acid) for acceptor-less double dehydrogenation of primary amines under oxidant and base free conditions. The results suggest that the electron rich [Ru-1] has outperformed due to its high solubility, high electron density and more charge separation as compared to [Ru-2]. The mechanistic studies reveal that electrophilic centre of [Ru-1] easily associates with substrate, whereas nucleophilic metal centre abstracts β-hydrogen of primary amine via thermodynamically more favourable six-membered transition state as compared to traditional four membered transition state in monometallic system. Further, the catalytic investigation proves that electron rich aromatic primary amines and aliphatic amines are more powerful than bidentate substrates which deactivate the catalyst suggesting the bimetallic dehydrogenation pathway for primary amines. Overall, this research opens the possibility of exploring tetrazole linked bimetallic complexes as an industry efficient solution for transition of primary amines to nitriles.