Air-stable rhenium complex catalyses alkyne metathesis

Researchers in Hong Kong have developed a rhenium catalyst for the selective metathesis of alkynes. It provides advantages over traditional catalysts for this purpose by being stable in the presence of air and moisture, and compatible with protic functional groups.

Alkyne metathesis plays a crucial role in synthetic chemistry by forming compounds with carbon–carbon triple bonds. ‘Researchers normally use high-valent d⁰ molybdenum and tungsten [compounds] to catalyse alkyne metathesis. However, these catalysts usually require strict air-free conditions and are sensitive to moisture,’ explains Guochen Jia from The Hong Kong University of Science and Technology.

Now, Jia and co-workers have shown that an air-stable rhenium (V) aqua alkylidyne complex can mediate alkyne metathesis at room temperature.

To make the catalyst, the team started with a readily available rhenium phosphine complex and reacted it with a ruthenium chloride complex in the presence of toluene and heat, to form an intermediate rhenium–ruthenium compound. Attempts to isolate this compound were unsuccessful, so the team reacted the compound with trace amounts of water at room temperature, and this yielded an aqua complex, which serves as a pre-catalyst in their system. The pre-catalyst works by first dissociating its water ligand to create a coordinatively unsaturated rhenium species, allowing an alkyne to coordinate to the metal centre and begin the alkyne metathesis.

Commenting on the work, Alex Bissember, a catalysis expert at the University of Tasmania, Australia, says ‘their functional group compatibility is really nice… and although some [compounds] required heating, it’s still a very impressive scope’.

Jia says that ‘while rhenium is indeed scarce, its demand in industry is relatively low compared to other noble metals … as a result, its market price is even lower than ruthenium and only slightly higher than silver.’ This point is reinforced by Bissember, who says ‘rhenium, in pricing terms, compares very favourably to other precious metals’.

Next Jia and co-workers will focus on two main areas. Firstly, they will conduct a detailed mechanistic study of how the rhenium aqua alkylidyne catalyst works. And secondly, they will explore catalytic terminal alkyne metathesis because Jia explains that ‘while our catalyst is effective with internal alkynes, it is less efficient for terminal alkyne metathesis’.

‘Even if broad industrial use remains limited, our work could inspire other researchers to explore middle to late transition metal alkylidynes, including more abundant metals such as manganese,’ concludes Jia.