Half‐Sandwich Nickel(II) NHC‐Picolyl Complexes as Catalysts for the Hydrosilylation of Carbonyl Compounds: Evidence for NHC‐Nickel Nanoparticles under Harsh Reaction Conditions

Abstract

The cationic [NiCp(Mes‐NHC‐CH2py]Br complex 2 a was prepared directly by the reaction of nickelocene with 1‐(2‐picolyl)‐3‐mesityl‐imidazolium bromide (1), and its PF6− derivative 2 b, by subsequent salt metathesis. X‐ray diffraction studies and Variable Temperature 1H NMR experiments run with 2 a and 2 b strongly suggest the bidentate coordination of the picolyl‐functionalized carbene to the nickel both in the solid state and in solution in both cases. These data suggest the absence of hemilabile behavior of the latter, even in the presence of a coordinating anion. Both complexes show similar activity in aldehyde hydrosilylation, further implying the absence of hemilability of the picolyl‐functionalized carbene, and effectively reduce a broad scope of aldehydes in the absence of additive under mild conditions. In the case of ketones, effective hydrosilylation is only observed in the presence of a catalytic amount of potassium t‐butoxide at 100 °C. Dynamic light scattering, scanning transmission electron microscopy and X‐ray photoelectron spectroscopy show evidence for the involvement of NHC‐picolyl‐Ni nanoparticles under these conditions.

Optimal Balance in the Catalyst Dynamics Enables C(2)−H Arylation of (Benz)imidazoles and (Benz)oxazoles by an In Situ‐Generated Ni/NHC System

“…High Ni contents in the collected precipitates were clearly established (Figures S87 and S88). It should be noted that the formation of Ni NPs is unsurprising because such precedents are known for reactions involving Ni(0) species in catalytic processes [14b,27] . On the one hand, metal NPs can represent a catalyst decomposition product.…”

“…[14b,27] On the one hand, metal NPs can represent a catalyst decomposition product. On the other hand, the participation of NHCligated Ni NPs in catalysis as alternative active centers [27][28] or Ni NPs as metal reservoirs [29] should not be completely excluded.…”

Oxidation of Styrene to Benzaldehyde Catalyzed by Schiff Base Functionalized Triazolylidene Ni(II) Complexes

“…Furthermore, the persistence of the absorbance at 439 nm even after the addition of the oxidant ruled out possible hydrolysis of the imine functionality or hemilability of the N-Ni bond during the catalytic cycle [ 52 ]. However, the inactivation of the catalyst beyond 6 h might be due to possible catalyst degradation as observed in similar imine-containing complex catalysts [ 53 , 54 ].…”

(NHC-olefin)-nickel(0) nanoparticles as catalysts for the (Z)-selective semi-hydrogenation of alkynes and ynamides

“…Furthermore, in a recent report, we have shown that a half-sandwich Ni( ii )-NHC-picolyl bromide complex effectively catalysed the hydrosilylation of a broad range of aldehydes without additive and of ketones in the presence of KO t Bu at 100 °C. 9 Dynamic light scattering, scanning electron microscopy in transmission mode and X-ray photoelectron spectroscopy showed evidence of the involvement of NHC-picolyl Ni particles under the catalytic conditions used for the ketones’ reductions.…”

“…7 To the best of our knowledge, there are only three reports on the use of NHC ligands to stabilise Ni NPs. 8,9 In the first two ones, such NHC-Ni NPs proved to be highly active catalysts for Kumada couplings 8a and the semi-hydrogenation of alkynes 8b and could be recycled a few times without significant loss of activity. Furthermore, in a recent report, we have shown that a half-sandwich Ni(II)-NHC-picolyl bromide complex effectively catalysed the hydrosilylation of a broad range of aldehydes without additive and of ketones in the presence of KOtBu at 100 °C.…”