Nickel Tetracarbonyl as Starting Material for the Synthesis of NHC-stabilized Nickel(II) Allyl Complexes

Abstract

A novel, useful in situ synthesis for NHC nickel allyl halide complexes [Ni(NHC)(η3-allyl)(X)] starting from [Ni(CO)4], NHC and allyl halides is presented. The reaction of [Ni(CO)4] with (i) one equivalent of the corresponding NHC and (ii) with an excess of the corresponding allyl chloride at room temperature leads with elimination of carbon monoxide to complexes of the type [Ni(NHC)(η3-allyl)(X)]. This approach was used to synthesize the complexes [Ni(tBu2Im)(η3-H2C-C(Me)-CH2)(Cl)] (2), [Ni(iPr2ImMe)(η3-H2C-C(Me)-CH2)(Cl)] (3), [Ni(iPr2Im)(η3-H2C-C(Me)-CH2)(Cl)] (4), [Ni(iPr2Im)(η3-H2C-C(H)-C(Me)2)(Br)] (5), [Ni(Me2ImMe)(η3-H2C-C(Me)-CH2)(Cl)] (6), and [Ni(EtiPrImMe)(η3-H2C-C(Me)-CH2)(Cl)] (7). The complexes 1 to 7 were characterized using NMR and IR spectroscopy and elemental analysis, and the molecular structures are provided for 2 and 7. The allyl nickel complexes 1–7 are stereochemically non-rigid in solution due to (i) NHC rotation about the nickel-carbon bond, (ii) allyl rotation about the Ni–η3-allyl axis and (iii) π–σ–π allyl isomerization processes. The allyl halide complexes can be methylated as was demonstrated by the methylation of a number of the complexes [Ni(NHC)(η3-allyl)(X)] with methylmagnesium chloride or methyllithium, which led to isolation of the complexes [Ni(Me2Im)(η3-H2C-C(Me)-CH2)(Me)] (8), [Ni(tBu2Im)(η3-H2C-C(Me)-CH2)(Me)] (9), [Ni(iPr2ImMe)(η3-H2C-C(Me)-CH2)(Me)] (10), [Ni(iPr2Im)(η3-H2C-C(Me)-CH2)(Me)] (11), [Ni(iPr2Im)(η3-H2C-C(H)-C(Me)2)(Me)] (12), and [Ni(EtiPrImMe)(η3-H2C-C(Me)-CH2)(Me)] (13). These complexes were fully characterized including X-ray molecular structures for 10 and 11.