Tailoring Catalyst Activity and Substrate Scope via Modular Design of Adaptive Tridentate Ligands for Alkyne Metathesis

The synthetic utility of alkyne metathesis remains underdeveloped compared to olefin metathesis, its mechanistic analogue. Advancement in tridentate molybdenum alkylidyne catalysts led to an expanded substrate scope and enhanced stability. However, the systematic tuning of their steric and electronic properties has not yet been realized. Here, we report a library of adaptive tridentate ligands with a tris(biphenyl)methane backbone. A total of 35 ligands were prepared using a modular synthetic strategy, all of which were active in metathesis upon ligand exchange with a precatalyst cat-II, and 12 of them demonstrated activity surpassing that of the fastest monodentate and tridentate ligands in literature. Density functional theory (DFT) calculations suggested that their reactivity originated from the highly adaptive ligand backbone, which could accommodate different coordination geometries with minimal strain. A systematic survey established that steric factors outweigh electronic properties, and the highest reaction rates were achieved for ligands with medium-sized meta-substituents or bulky para-substituents. The catalysts demonstrated moderate stability and retained full activity after 2 h of open vial storage in air. Anthraquinone-bearing L-CP21 features a rather broad substrate scope, promoting the metathesis of challenging substrates bearing cyano, nitro, aldehyde, aniline, pyridine, and pyrimidine groups with excellent isolated yields.