Development of catalysts for alkyne metathesis: Synthesis of functionalised molybdenum benzylidyne complexes and related species

Abstract

The present dissertation focused on the variation of the ligand environment of molybdenum benzylidyne complexes as potential alkyne metathesis initiators. In order to improve the robustness of these complexes, several strategies were explored. In the first place, functionalisation of the benzylidyne unit (CAr) with ether groups resulted in para-methoxybenzylidyne and trimethoxybenzylidyne derivatives [Mo(CAr)(OC(CF3)2Me)3] (Ar = 4-MeO-C6H4, 2,4,6-(MeO)3C6H2). These complexes formed 1D-polymeric chains in the solid state promoted by intermolecular contacts between the methoxy group at the para-position and the metal centre of the adjacent molecule. In the second place, a series of complexes of the type [Mo(CMes)(L)(OC(CF3)2Me)3] (Mes = 2,4,6-trimethylphenyl, L = acetonitrile, 2,4,6-trimethoxybenzonitrile, acetone, pyridine, 2,2'-bipyridine, trimethylphosphane, and 1,3,4,5-tetramethylimidazoline carbene) was synthesised by addition of coordinating ligands (L) to [Mo(CMes)(OC(CF3)2Me)3]. Stability tests revealed no observable degradation of the 2,2'-bipyridine and the carbene adducts upon exposure to air. Furthermore, selected alkynes (such as 1-heptyne, 3-hexyne, phenylacetylene, tolan, and ynediamines) were reacted stoichiometrically with [Mo(CMes)(OC(CF3)2Me)3] in an attempt to isolate intermediate metallacyclic species. Only the reaction with ynediamines yielded unusual paramagnetic molybdenacyclobutadiene complexes; otherwise, alkylidyne cross-metathesis products were generated, e.g. [Mo(CPh)(OC(CF3)2Me)3]. The catalytic activity in alkyne metathesis was determined for all investigated alkylidyne complexes in terms of TON and TOF values.