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Three new ruthenium phosphonium alkylidene complexes incorporating N-heterocyclic carbene ligands with bulky N-aryl groups (2,6-diethyl, L = 1,3-bis(2,6-diethylphenyl)imidazolin-2-ylidene ([H.sub.2]IDEP) and 2,6-diisopropyl, L = 1,3-bis(2, 6-diisopropylphenyl)imidazolin-2-ylidene ([H.sub.2]ID-i-PP)) were synthesized and characterized. The [H.sub.2]ID-i-PP supported complex was found to exhibit excellent thermal stabilities relative to the parent N-mesityl (N-Mes) complexes as well as the [H.sub.2]IDEP supported complexes. All three phosphonium alkylidenes were evaluated in comparison to the N-Mes derivative and Grubbs second generation catalyst using standard olefin metathesis reactions and conditions. The complex containing the bulky [H.sub.2]ID-i-PP ligand was found to have excellent activity and longevity in comparison to the other catalysts. Although initiation rates were slow for this sterically bulky precatalyst, its superior activity led to the best overall efficiency in test reactions. Key words: olefin metathesis, ruthenium, catalysis, ligand design, decomposition. On a synthetase et caracterise trois nouveau complexes ruthenium phosphonium alkylidene dans lesquels sont integres des ligands carbenes N-heterocycliques contenant des groupements N-aryles volumineux (2,6-diethyle, L = 1,3-bis(2,6-diethylphenyl) imidazolin-2-ylidene ([H.sub.2]IDEP) et 2,6-diisopropyle, L = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene ([H.sub.2]IDiPP)). On a constate que les complexes supportes par [H.sub.2]ID-i-PP presentaient une excellente stabilite thermique comparativement aux complexes N-mesityl (N-Mes) parents, ainsi qu'aux complexes supportes par [H.sub.2]IDEP. On a evalue les trois phosphonium-alkylidenes comparativement au derive N-Mes et au catalyseur de Grubbs de deuxieme generation en utilisant des reactions et des conditions classiques de metathese des olefines. L'activite et la longevite du complexe contenant le volumineux ligand [H.sub.2]ID-i-PP se sont averees excellentes comparativement aux autres catalyseurs. Meme si les vitesses d'initiation de ce precatalyseur steriquement volumineux etaient faibles, son activite superieure a donne lieu a la meilleure efficacite globale dans les reactions de test. [Traduit par la Redaction] Mots-cles: metathese des olefines, ruthenium, catalyse, conception de ligands, decomposition., Introduction Ruthenium-based olefin metathesis catalysts that combine high activity (turn over frequency) with high thermal stability (turn over number) are of interest for the continued development and application of this [...] more...