Olefin Metathesis Catalysts Generated In Situ from Molybdenum(VI)-Oxo Complexes by Tuning Pendant Ligands.

Tailored molybdenum(VI)-oxo complexes of the form MoOCl 2 (OR) 2 (OEt 2 ) catalyse olefin metathesis upon reaction with an organosilicon reducing agent at 70 °C, in the presence of olefins. While this reactivity parallels what has recently been observed for the corresponding classical heterogeneous catalysts based on supported metal oxide under similar conditions, the well-defined nature of our starting molecular systems allows us to understand the influence of structural, spectroscopic and electronic characteristics of the catalytic precursor on the initiation and catalytic proficiency of the final species. The catalytic performances of the pre-catalysts are determined by the highly electron withdrawing (σ-donation) character of alkoxide ligands, O t Bu F9 being the best. This activity correlates with both the 95 Mo chemical shift and the reduction potential that follows the same trend: O t Bu F9 >O t Bu F6 >O t Bu F3 .