A thermally robust ruthenium phosphonium alkylidene catalyst - the effect of more bulky N-heterocyclic carbene ligands on catalyst performance in olefin metathesis reactions

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 (H2IDEP) and 2,6-diisopropyl, L = 1,3-bis(2, 6-diisopropylphenyl) imidazolin-2-ylidene (H2ID-i-PP)) were synthesized and characterized. The H2ID-i-PP supported complex was found to exhibit excellent thermal stabilities relative to the parent N-mesityl (N-Mes) complexes as well as the H2IDEP 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 H2ID-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.