Unearthing a Well-Defined Highly Active Bimetallic W/Ti Precatalyst Anchored on a Single Silica Surface for Metathesis of Propane
2017-02-10T00:00:00Z (GMT) by
Two compatible organometallic complexes, W(Me)<sub>6</sub> (<b>1</b>) and TiNp<sub>4</sub> (<b>2</b>), were successively anchored on a highly dehydroxylated single silica support (SiO<sub>2−700</sub>) to synthesize the well-defined bimetallic precatalyst [(Si–O−)W(Me)<sub>5</sub>(Si–O−)Ti(Np)<sub>3</sub>] (<b>4</b>). Precatalyst <b>4</b> was characterized at the molecular level using advanced surface organometallic chemistry (SOMC) characterization techniques. The strong autocorrelation observed between methyl of W and Ti in <sup>1</sup>H–<sup>1</sup>H multiple-quantum NMR spectra demonstrates that W and Ti species are in close proximity to each other. The bimetallic precatalyst <b>4</b>, with a turnover number (TON) of 9784, proved to be significantly more efficient than the silica-supported monometallic catalyst [(Si–O−)W(Me)<sub>5</sub>] (<b>3</b>), with a TON of 98, for propane metathesis at 150 °C in a flow reactor. The dramatic improvement in the activity signifies the cooperativity between Ti and W and indicates that the key step of alkane metathesis (C–H bond activation followed by β-H elimination) occurs on Ti, followed by olefin metathesis, which occurs on W. We have demonstrated the influence and importance of proximity of Ti to W for achieving such a significantly high activity. This is the first report demonstrating the considerably high activity (TON = 9784) in propane metathesis at moderate temperature (150 °C) using a well-defined bimetallic system prepared via the SOMC approach.