Pt^II−N‐Heterocyclic Carbene Complexes in Solvent‐Free Alkene Hydrosilylation

Abstract

Herein, we report the catalytic activity of a series of platinum(II) pre‐catalysts, bearing N‐heterocyclic carbene (NHC) ligands, in the alkene hydrosilylation reaction. Their structural and electronic properties are fully investigated using X‐ray diffraction analysis and nuclear magnetic resonance spectroscopy (NMR). Next, our study presents a structure‐activity relationship within this group of pre‐catalysts and gives mechanistic insights into the catalyst activation step. An exceptional catalytic performance of one of the complexes is observed, reaching a turnover number (TON) of 970 000 and a turnover frequency (TOF) of 40 417 h−1 at 1 ppm catalyst loading. Finally, an attractive solvent‐free and open‐to‐air alkene hydrosilylation protocol, featuring efficient platinum removal (reduction of residual Pt from 582 ppm to 5.8 ppm), is disclosed.

Simply accessible platinum(ii) complexes enabling alkene hydrosilylation at ppm catalyst loadings

“…The reason behind this behaviour is a large energy barrier associated with decoordination of the dvtms ligand in the Pt(0)–NHC complexes. 8,9…”

“…In our previous study on the use of Pt( ii )–NHC complexes in alkene hydrosilylation, 9 we proposed an attractive pre-catalyst, which allows the formation of the product resulting from a model reaction between 1-octene and 1,1,1,3,5,5,5-heptamethyltrisiloxane (MD′M) in quantitative isolated yield, while reaching a TON value of 970 000 at 1 ppm catalyst loading after 24 h at 80 °C. In this instance, the catalytic performance turned out to be substrate-specific and an increase of the catalyst loading was required for several reactions to reach completion.…”

“…In this instance, the catalytic performance turned out to be substrate-specific and an increase of the catalyst loading was required for several reactions to reach completion. This increase in catalyst loading did not pose a significant problem when accessing soluble small-molecule alkylsilanes, 9,12 since the use of [Pt(IPr*)(DMS)Cl 2 ] (DMS = dimethyl sulfide) as pre-catalyst allowed for an easy and efficient removal of residual Pt after the reaction was completed. However, in many industrial applications where this reaction is deployed, product output consists mainly of insoluble, polymeric hydrosilylation adducts.…”

“…1) perform as well as state-of-the-art platinum pre-catalysts, yet their use comes with a number of striking practical benefits. These easily accessible Pt( ii ) compounds have been known for almost 100 years, 13 and have been used as precursors for the synthesis of other hydrosilylation pre-catalysts, 9,11 yet, to the best of our knowledge, their catalytic activity in this reaction has never been studied. One of the biggest advantages of the title complexes is the ease of their synthesis (Scheme 1).…”

Making accessible soluble silicon-containing polynorbornenes: hydrosilylation of vinyl-addition poly(5-vinyl-2-norbornene)

“…The plausible reasons for the observed dependences are the electron-donating and steric properties of NHC-ligands. 38 Thus, the increase in the size of the heterocycle of the NHC-ligand leads to the enhancement of electron-donating properties and this should facilitate the oxidative addition of silane to Pt-centers, providing higher catalytic activity. At the same time, the increase in the size of the aryl groups or the heterocycle should lead to more efficient shielding of the Pt-center with its stabilization.…”