The serendipitous discovery of the unusual acetone activation by the cobalt complex of a modified porphyrin (Scheme 1) occurred while trying to insert a cobalt ion into azuliporphyrin 1 in a different than previously solvent to form a known 4coordinate regular complex (with CNNN coordination). Instead of seeing a well-known low-spin Co II pattern in the 1 H NMR spectrum, we received a previously unobserved high-spin (HS) complex with unusually shielded signals at as far as -101.4 ppm (at 300 K). Isolation of this species (2) allowed for a detailed investigation. The 1 H NMR spectrum shown in Fig. presents a symmetric pattern coming from protons of a parent ligand with preserved Cs symmetry. However, the total number of signals suggests an additional fragment is attached. Typically, for the HS Co II porphyrin complex, pyrrole protons are strongly deshielded due to contact through bonds with the paramagnetic centre (37-47 ppm region). Previous studies show that the azulene protons, despite their significant distance from the cobalt(II) centre, are highly and alternately shifted as the spin density is effectively delocalised over the seven-membered ring through a conjugated π system. Thus, signals at -21.9, 29.5, and -25.6 ppm represent the azulene moiety. The phenyl rings remain largely within the diamagnetic window with a maximum shift of 17.1 ppm. Two signals at -15 and -101 ppm were unidentified. High-resolution mass spectrometry identified that the additional fragment consists of C2H5CO, which can be recognised as a monodeprotonated acetone fragment. The highly negative chemical shifts may indicate a significant dipolar interaction with the paramagnetic centre rather than a pure contact shift. Indeed, after the X-ray diffraction analysis of a single crystal of 2, we can confirm not only the anticipated Co-assisted activation of an acetone molecule but also its transfer onto the inner C(21) atom (Fig. ). The Co II centre is not directly bound to C(21) anymore. However, it is still in close contact (2.599(6) Å), and the acetone moiety attachment forces the azulene plane to deflect from the meso plane by a significant 53.38 (10)°. It all results in close contact between the metal centre and the substituent, especially with the CH2 fragment directly linked with the inner C(21) atom. This close alignment (Co•••C 3.059(8) Å) is responsible for the protons' strong shielding in the 1 H NMR spectrum. The cobalt(II) ion is supplemented with a chloride ligand to preserve the charge neutrality of the complex, as azuliporphyrin is monoanionic in this case. Lash et al. showed the activation of an acetone molecule by rhodium(III) azuliporphyrin, yet with no transfer of the small molecule fragment on the organic ligand part. As cobalt(II) forms more labile organometallic bonds, it only facilitates ketone α-arylation by activating both fragments. A plausible mechanism (Scheme 2) could consist of a few steps. Starting from docking a cobalt ion in a coordination cavity of the macrocycle. Then, cobalt's ligand exchange with the acetone activation facilitated by the metal presence, perhaps first through O-coordination. Next, the carbaporphyrin's internal C-H activation, reductive elimination of the organic fragments, and reoxidation of the metal centre with recoordination of the chloride. The process can be pretty efficient, with a 57% yield of the isolated product and 21-oxoazuliporphyrin as the main byproduct (S4; see SI for more details on mechanistic studies). We decided to test the generality of this transformation using another readily available carbaporphyrin, i.e., N-confused porphyrin (Scheme 3). The reaction mixture presents a very nicely defined 1 H NMR spectrum with the same features as observed for 2 (Fig. ). The difference is the macrocycle's (a)symmetry, leading to doubling the signal count. Another very characteristic feature is that the CH2 protons from the substituent became diastereotopic, resulting in two signals at ca. -105 and -120 ppm. The problem occurred while purifying the complex. After preparative TLC, we produced little to no product 4, showing that it was less stable for NCP 3 than for azuliporphyrin 1 despite the substitution product being obtained. It may be related to a higher degree of freedom in choosing the tautomeric state of the ligand and hence favouring other coordination motifs with inner C(21) involved in organometallic bonding, which could weaken the C(21)-C(acetone) bond. This, in turn, could destabilise the substitution product while exposed to water, oxygen, etc. We tried crystallising the crude mixture to get single crystals of complex 4. The obtained structure yielded answers to the stability issue. In crystallised molecule 6 (Fig. ), the inner carbon atom has been oxygenated with the removal of the acetone moiety (Scheme 3). The obtained structure is, in fact, monocationic (outer N(2) protonation), with half of the [Co2Cl6] 2-complex anion present in the asymmetric unit. A very noticeable characteristic of the molecule is a short C(21)=O bond, equal to 1.292(4) Å. This reflects the fragment's significant double bond character, which is in line with the observations for all other oxygenated carbaporphyrinoids. It also correlates with theoretical calculations presented by original discoverers of 21oxo-N-confused porphyrin. The X-ray structure obtained at that time possessed an elongated and distorted C-O bond (∠PlaneC4N•••VectorC(21)-O(21) 18.6(2)° vs. 7.3(2)° in the dication) yet was obtained for a neutral molecule and was heavily disordered due to typical pseudosymmetry of the NCP motif in crystals. We also carried out theoretical calculations, further supporting the higher stability of the 21-oxo tautomer (0 vs 13.4 kcal/mol for an enol tautomer; Scheme S2). A ca. 3% crystal 6 disorder indicates that the Co centre was present in such a modified cavity before the demetallation of the oxygenated macrocycle (in the form of complex 5, Scheme 3, Fig. ). It suggests the most plausible pathway to this peculiar transformation, i.e., cobalt binding of a dioxygen molecule, which we identified experimentally for azuliporphyrin Co II complexes. Next is the oxygenation of the inner carbon atom and removing the acetone fragment (probably also oxidised). In the end, demetallation of similar cobalt(II) 21oxoazuliporphyrin can occur in mild conditions, e.g., with any presence of acid, and this was the case here as well. We speculated that locking the NCP tautomer with two protons in the cavity could produce a more stable product. Thus, we used N(2)-methylated NCP to react with cobalt(II) chloride and acetone (7, 8, Scheme 3). This time, we could isolate the product with a good yield of 68%. 8 presents the same NMR features as 4 with an additional deshielded signal from N(2)-CH3 (Fig. ).