Phosphines on Colloidal Nickel Nanocatalysts to Lower the Onset Temperature of Terminal Alkynes Hydrogenation

Abstract

Catalysis by colloidal suspensions of metal nanoparticles (NPs) is becoming more and more attractive as it may combine mild reaction conditions with the use of organic ligands as surface modifiers. While most examples focus on increasing the selectivity of the catalytic process, there is also an opportunity to use the ligands to boost the conversion and trigger the onset of activity at a temperature where NPs alone would not be active. This may be achieved because the ligands modify the electronic and steric environments at the surface active site. In this work, we show that phosphines with adequate steric hindrance (e.g., P n Bu 3 and P i Bu 3 ) lower the onset temperature for phenylacetylene hydrogenation by nickel NPs under 7 bar of H 2 , by ca 10 to 20 °C depending on the NP diameter. This result is not expected to have practical repercussions for the reaction at stake, as it is already well-mastered; yet, it is of conceptual value because the hydrogenation may have been driven by the frustrated Lewis pair (FLP) between the Lewis basic phosphine and the Lewis acid nickel surface, forming a so-called "NanoFLP". We investigated this proposal using 10 phosphines and used a stereoelectronic map to rationalize their ability to boost the conversion, showing that moderately hindered and fairly donating phosphines are most adequate. Moreover, we demonstrated that less than 2 phosphines per Ni surface atom are enough for the effect to arise. We showed that other terminal alkynes, like 1-octyne, can be hydrogenated with this method. Lastly, a comparison of conversions for 5 para-substituted phenylacetylenes was used to discuss the effects of electronic donation and steric hindrance at the surface active site.