Room temperature polymerization of norbornene with a hydride-bridged dinuclear ruthenium complex system

Abstract

A two-component system, [Ru(p-cymene)Cl2]2 (C1)/BH3•NMe3, allows for the catalytic ring-opening metathesis polymerization (ROMP) of norbornene at 60°C. A hydride-bridged complex isolated from the two-component system, [RuCl(p-cymene)]2(μ-H)(μ-Cl) (C2), is also active in the ROMP reaction, albeit at a reduced rate. Abstraction of one chloride ligand from C2, by the reaction with 1equiv of either AgPF6 or AgSbF6, leads to the generation of either [{Ru(p-cymene)}2(μ-H)(μ-Cl)2]PF6 (C3) or [{Ru(p-cymene)}2(μ-H)(μ-Cl)2]SbF6 (C4) and further reduction of ROMP activity. Importantly, a reaction of C2 with 2equiv each of AgSbF6 and BH3•NMe3 provides the ability to achieve room temperature ROMP of norbornene. Various hydride-bridged dinuclear ruthenium complexes have been observed in the room temperature system based on 1H NMR analysis. An independent synthesis of a complex mixture containing [{Ru(p-cymene)}2(μ-H)3]PF6 (C5) and a single-component complex [{Ru(η6-C6Me6)}2(μ-H)3]PF6 (C6) rules out the catalytic capacity of this tri-μ-hydrido species. A di-μ-hydrido complex, [{Ru(p-cymene)}2(μ-H)2(μ-Cl)]PF6 (C7), is therefore postulated to act as the ROMP reaction center at room temperature.