Reactivity of a Two-Coordinate Cobalt(0) Cyclic (Alkyl)(amino)carbene Complex

Cyclic (alkyl)­(amino)­carbenes (cAACs) are the merely known ligand set being capable of stabilizing two-coordinate cobalt(0) complexes. While two complexes of the type [(R2-cAAC)2Co] [R2-cAAC = 5,5-dialkyl-3,3-dimethyl-2-(2′,6′-diisopropylphenyl)­pyrrolidine-1-ylidene, R = Me, Et] have been reported for years, little is known regarding their reactivity. In this Article, we present the investigation on the reactivity of [(Me2-cAAC)2Co] (2). Complex 2 can be synthesized in a high yield from the reaction of [(Me2-cAAC)2CoCl] (1) with MeLi. The interaction of 2 with 1 equiv of ArBr (Ar = phenyl, mesityl) or n-C8H17X (X = Br, Cl) in THF affords the corresponding cobalt­(I) complex [(Me2-cAAC)2CoX] (X = Cl, 1; Br, 3). The formation of ArH and n-C8H18 in these reactions and the relatively high half-wave potential of [(Me2-cAAC)2Co]0/1+ over that of [ArBr]0/1+ indicate an inner-sphere electron-transfer character of the C–X bond cleavage reactions mediated by 2. The two-coordinate cobalt­(I) complex [(Me2-cAAC)2Co]­[BArF4] (4) has been synthesized from the reaction of 2 with [Cp2Fe]­[BArF4]. Complex 2 reacts with the diazo compounds (p-tolyl)2CN2 and DmpCHN2 (Dmp = 2,6-dimesitylphenyl) to form [(Me2-cAAC)­Co­((η6-p-tolyl)­(p-tolyl)­CNN­(Me2-cAAC))] (5) and [(Me2-cAAC)2Co­(σ-NNCHDmp)] (6), respectively. The formation of the azine ligand in 5 indicates the reactivity of the Co–C­(carbene) bond in 2. In addition, the reaction of 2 with 4 equiv of 2,6-dimethylphenyl isocyanide (XylNC) gives the ligand-substitution product [(Me2-cAAC)­Co­(CNXyl)3] (7) and a ketenimine Me2-cAACCNXyl (8) in good yields, and the interaction of 2 with 3-hexyne yields a three-coordinate cobalt(0) complex bearing η2-alkyne ligand [(Me2-cAAC)2Co­(η2-EtCCEt)] (9). The diversified reactivity of 2 arises from the coordination–unsaturation and electron-rich (d9) nature of the cobalt center and demonstrates the promising synthetic utility of cobalt(0) cAAC complexes.