Datensatz

Zusammenfassung

Molybdenum alkylidyne complexes with a trisilanolate podand ligand framework (“canopy catalysts”) are the arguably most selective catalysts for alkyne metathesis known to date. Among them, complex 1a endowed with a fence of lateral methyl substituents on the silicon linkers is most reactive, although fairly high loadings are required in certain applications. It is now shown that this catalyst decomposes readily via a bimolecular pathway, which engages the Mo≡CR entities in a stoichiometric triple bond metathesis event to furnish RC≡CR and the corresponding dinuclear complex 8 with a Mo≡Mo core. In addition to the regular analytical techniques, ⁹⁵Mo NMR was used to confirm this unusual outcome. This rapid degradation mechanism is largely avoided by increasing the size of the peripheral substituents on silicon, without unduly compromising the activity of the resulting complexes. When chemically challenged, however, canopy catalysts can open the apparently somewhat strained tripodal ligand cages; this reorganization leads to the formation of cyclo-tetrameric arrays composed of four metal alkylidyne units linked together via one silanol arm of the ligand backbone. The analogous tungsten alkylidyne complex 6 endowed with a tripodal tris-alkoxide (rather than siloxide) ligand framework is even more susceptible to such a controlled and reversible cyclo-oligomerization. The structures of the resulting giant macrocyclic ensembles were established by single crystal X-ray diffraction.