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Making Base-Assisted C–H Bond Activation by Cp*Co(III) Effective: A Noncovalent Interaction-Inclusive Theoretical Insight and Experimental Validation

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posted on 08.07.2020 by Fule Wu, Christophe Deraedt, Yann Cornaton, Julia Contreras-Garcia, Mélanie Boucher, Lydia Karmazin, Corinne Bailly, Jean-Pierre Djukic
The base-assisted cyclometalation of 2-phenylpyridine (2-phpyH) by Cp*Co­(III) was holistically addressed both theoretically and experimentally. Combined DFT and DLPNO-CCSD­(T) methods assisted by QTAIM-based noncovalent interactions plots (NCI plots), interacting quantum atoms (IQA), and local energy decomposition (LED) analyses have been used for a comparative study of the CMD-promoted cyclocobaltation and the parent cycloiridation of the 2-phpyH. Results suggest a remarkable contribution of noncovalent interactions, especially local electrostatic interactions, in the evolution of the reactive site giving a rationale for the optimization of cyclocobaltation. The theoretically predicted benefits of using the acetamidate anion as a base is rationalized and verified experimentally. Cobaltacycle [Cp*Co­(2-phpy-κC,κN)­I] was efficiently synthesized from the air-stable [Cp*CoI2]2 and 2-phpyH, in the presence of LiNHAc as base in 83% yield, whereas with anhydrous NaOAc as base only a 12% yield was achieved under similar conditions. By applying the [NHAc]-promoted cyclometalation various cobaltacycles were synthesized and analytically characterized, and their structures were resolved by X-ray crystallization analysis, confirming the importance of the acetamidate in the base-assisted cyclometalation. Experimental kinetic isotope effect (KIE) studies validated by Bigeleisen equation based KIE computations confirm that the formation of the agostic transient is indeed the kinetic determining step of the CMD mechanism in dichloromethane. Application of the [Cp*CoI2]2/LiNHAc mixture to the catalysis of the condensation of 1,2-diphenyacetylene to various aromatics reveals the coexistence of two mechanisms, i.e. CMD and electrophilic C–H activation.

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