The base-assisted cyclometallation 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 rational 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-C,N)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 12% yield were achieved under similar conditions. By applying the [NHAc] -promoted cyclometallation various cobaltacycles were synthesized, analytically characterized and their structures resolved by X-ray crystallization analysis, confirming the importance of the acetamidate in the base-assisted cyclometallation. 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. PhD stipend. YC thanks the Centre de Calcul de l'Université de Strasbourg for providing access to the HPC facilities (project g2019a126c).

Recent reports have identified Cp*Co-based complexes as powerful catalysts for aromatic C-H bond activation under oxidative conditions. However, little is known about the speciation of Cp*Co species during catalysis. We now show that key intermediates, Cp*Co(III) metallacycles derived from 2-phenylpyridine (phpy-H), react swiftly in solution with one-electron oxidants to irreversibly collapse by a cyclocondensation of the organic ligands to afford cationic alkaloids in yields of >70 %. Low temperature EPR analysis of a mixture of cobaltacycle with the tritylium cation reveals the signatures of trityl and Co(IV)-centred radicals. Electrochemical analyses show that the oxidation of these cobaltacycles is irreversible and gives rise to several products in various amounts, among which the most salient ones are a cationic alkaloid resulting from the cyclocondensation of the phpy and Cp* ligands, and the dimeric cation {[Cp*Co] 2 (-I) 3 } + . DFT investigations of relevant noncovalent interactions using QTAIMbased NCI plots and Intrinsic Bond Strength Index suggest a ligand-dependent predisposition by "NCI-coding" for the Co(IV)templated cyclocondensation, the computed reaction network energy profile for which supports the key roles of a short lived Co(IV) metallacycle and of a range of triplet state organocobalt intermediates. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures. Full experimental procedures and details, voltammograms, EPR, Mass and NMR spectra, energies and Cartesian coordinates, high resolution NCI figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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