Gallium Complexes of Tetradentate Formazanate Ligands: Synthesis, Property Benchmarking, and Radiolabeling

Main group complexes of formazanate ligands, most commonly boron, have emerged as readily accessible dyes with applications throughout the materials and health sciences on account of their unique optoelectronic and redox properties and biocompatibility. Despite these advances, gallium formazanate complexes have received limited attention. In this study, we report the synthesis of a variety of six-coordinate gallium complexes supported by tetradentate formazanate ligands and benchmark their optoelectronic/redox properties with a variety of axial ligands (water, DMAP, DMSO, acetate). DFT calculations support our findings and provide insight into the labile nature of the axial ligands based on modest fluoride ion affinities calculated for the gallium formazanate core. Finally, we demonstrate the potential of tetradentate formazanate ligands as chelators for radioactive 68Ga with radiolabeling occurring at room temperature in just 15 min and set the stage for the future development of these and related compounds as positron emission tomography (PET) imaging agents and theranostics.