Metallaaromaticity Reimagined: Metallaaromatic Cobalt Macrocycles Through Metal-Ligand Coordination Chemistry

Aromaticity is a fundamental concept in organic chemistry that has gradually been extended to many other areas in the chemical sciences, including organometallic chemistry. By replacing a CH unit in an aromatic hydrocarbon (e.g., benzene) with an organometallic fragment, novel metallaaromatic compounds with intriguing physicochemical properties can be synthesized. Yet, despite intense research efforts, the synthesis of their macrocyclic congeners remains challenging, mainly due to a lack of straightforward synthetic strategies. Here we report the synthesis of a carbene analog of the well-known PyBOX pincer ligand, termed CarBOX, that upon coordination to a low-valent metal center (e.g., Co or Rh) results in the formation of a bespoke macrocyclic metallaaromatic complex. 1H NMR analysis of the resulting cobalt complexes reveals a downfield chemical shift (Δδ ≈ 8.5 ppm) of a diagnostic methyl resonance, indicative of strong diatropic ring currents. The electronic properties of the resulting pincer complexes were further confirmed by density functional theory (DFT), including nucleus independent chemical shift (NICS), isomer stabilization energy (ISE), and the magnetically induced origin-independent electron current density, all of which support the metallaaromatic nature of the cobalt complexes. Overall, these findings establish a generalizable platform for accessing metallaaromaticity through coordination chemistry, expanding the design principles of aromatic systems to include through-metal π-delocalization opening up new avenues for electronic materials design and function.