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

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.