Fate of Cobaltacycles in Cp*Co-Mediated C–H Bond Functionalization Catalysis: Cobaltacycles May Collapse upon Oxidation via Co(IV) Species

Recent
reports have identified Cp*Co-based complexes to be 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*CoIII 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%. A low-temperature EPR analysis of
a mixture
of the cobaltacycle with the tritylium cation reveals the signatures
of trityl and Co­(IV)-centered 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(μ-I)3}+. DFT investigations
of relevant noncovalent interactions using QTAIM-based NCI plots and
intrinsic bond strength indexes 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.