Synthesis and Reactivity of Cyclic (Alkyl)(Amino)Carbene Stabilized Nickel Carbonyl Complexes

Cyclic (alkyl)­(amino)­carbenes cAACcy (1b) and cAACmenthyl (1c) react with [Ni­(CO)4] to give the 18 VE complexes [Ni­(CO)3(cAACcy)] (2b) and [Ni­(CO)3(cAACmenthyl)] (2c). With these in hand,
the donor-strength and
the steric profile of the respective cAAC ligands were evaluated.
CAACcy and cAACmenthyl possess similar overall-donating
properties (Tolman electronic parameter (TEP) = 2046 (1b) and 2042 (1c)) as common NHCs, though they are also
known to be better π-acceptors. 3,3-Diamino-2-aryloxyacrylimidamide 3b, arising from the reaction of cAACcy (1b) with released CO molecules, was obtained as side-product
of CO substitution reactions at nickel carbonyls. In contrast to cAACmenthyl (%Vbur = 42), the sterically
less encumbered cAACcy (%Vbur = 38) undergoes a subsequent CO substitution at [Ni­(CO)3(cAACCy)] (2b) to afford the 16 VE complex
[Ni­(CO)­(cAACcy)2] (4b). Treatment
of both [Ni­(CO)3(cAACmethyl)] (2a) and [Ni­(CO)­(cAACmethyl)2] (4a) with allyl bromides led to the formation of cAAC-stabilized allyl
nickel complexes [NiBr­(η3-H2CCH–CH2)­(cAACmethyl)] (5a) and [NiBr­(η3-H2CCH–CMe2)­(cAACmethyl)]
(5b). The chloro complex [NiCl­(η3-H2CCMe–CH2)­(cAACmethyl)] (6)
was synthesized from [Ni­(COD)2] (COD = 1,5-cyclooctadiene)
by consecutive treatment with allyl chloride and cAAC. The allyl halide
complexes 5 and 6 are thermally labile and
decompose in solution already within a few hours at room temperature.
One of the decomposition products, the dinuclear nickel complex [Ni2(μ-Br)2(η3-(cAACmethyl)CH–CH­(CH3))2] (7), was crystallographically
characterized.