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

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. A half‐sandwich nickel NHC‐picolyl complex effectively catalyzed the hydrosilylation of aldehydes and ketones. Studies by DLS, STEM, XPS, ICP‐AES and elemental analyses showed evidence for the involvement of NHC‐stabilized nickel nanoparticles when potassium t‐butoxide is used as an activator.