Reactivity of a Two-Coordinate Cobalt(0) Cyclic (Alkyl)(amino)carbene Complex
Jingzhen Du, Wenwei Chen, Qi Chen, Xuebing Leng, Yin‐Shan Meng, Song Gao, Liang Deng
Abstract
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.