Stable Discrete Pt1(0) in Crown Ether with Ultra-High Hydrosilylation Activity

Obtaining reduced discrete metal atoms that are stable in liquid solvents by in-situ reduction of an ionic metal precursor has been a challenge until recently. A liquid surfactant polydimethylsiloxane-polyethylene glycol (PDMS-PEG) enabled the synthesis of stable discrete platinum atoms (Pt-1) by reducing Pt(IV) and Pt(II) salts. Here we report the successful preparation of discrete mononuclear platinum atoms (Pt-1) in a crown ether, [15]crown-5, as a structurally much simpler solvent, and the prepared Pt-1@[15]crown-5 was demonstrated for ultra-high catalytic activity and selectivity in hydrosilylation reactions. A combination of spectroscopic characterizations proves the reduced Pt species is Pt-1(0) with partially positive charge. Pt-195 NMR and DFT calculation indicate the Pt-1(0) is stabilized by the pseudo octahedral structure of ([15]crown-5)PtCl2-H2+ involving two adjacent oxygens from the crown ether ring, although the oxygens in the crown ether ring have been known to host and stabilize certain metal cations. The Pt-1@[15]crown-5 shows ultrahigh activity (TOF of 8.3x10(8) h(-1)) with excellent terminal adducts selectivity in catalytic olefin hydrosilylation. This catalyst was found to be highly stable under hydrosilylation conditions. For examples, the turnover number (TON) exceeded 1.0x10(9) for hydrosilylation between 1-octene and (Me3SiO)(2)MeSiH without showing sign of deactivation; the TON exceeded 2.0x10(8) while the catalyst remained active for a catalytically more demanding reaction between styrene and (Me3SiO)(2)MeSiH.