Synthesis, structure, characterisation and preliminary CO2 reactivity of a family of heptazine-based Re(I) tricarbonyl complexes.

Heptazine derivatives are well known for their attractive properties in applications such as organic light-emitting diodes (OLEDs) and flame-retardant materials. However, studies in this field have largely focused on fully symmetrically substituted heptazines. Thus, the coordination chemistry studies for those derivatives have been limited to multimetallic complexes, coordination polymers, and metal-organic frameworks (MOFs). Herein, the design and synthesis of three unsymmetrical heptazine-based ligands have enabled the preparation of the first monometallic heptazine complexes, 1-3, through coordination to Re(I) tricarbonyl precursors. Single-crystal X-ray diffraction analyses confirmed the coordination of the rhenium centres at the targeted binding sites. The optical and electrochemical properties of the complexes were found to be strongly dependent on the ligand structure, in agreement with theoretical calculations. The study reveals a ligand-centered first reduction process. As an initial assessment of the reactivity of this new molecular platform, the electrocatalytic reduction of CO2 was investigated. Under the conditions employed, complex 1NHPy exhibits the formation of CO as the major reduction product (FE = 39.4%), albeit with modest catalytic performance. These preliminary results demonstrate the potential of unsymmetrical heptazine derivatives as versatile ligands for the development of new molecular catalysts.