Enhancing the Air Stability of Dimolybdenum Paddlewheel Complexes: Redox Tuning through Fluorine Substituents

The optical and electrochemical properties of quadruply
bonded
dimolybdenum paddlewheel complexes (Mo2PWCs) make them
ideal candidates for incorporation into functional materials or devices,
but one of the greatest bottlenecks for this is their poor stability
toward atmospheric oxygen. By tuning the potential at which the Mo2 core is oxidized, it was possible to increase the tolerance
of Mo2PWCs to air. A series of homoleptic Mo2PWCs bearing fluorinated formamidinate ligands have been synthesized
and their electrochemical properties studied. The oxidation potential
of the complexes was tuned in a predictable fashion by controlling
the positions of the fluorine substituents on the ligands, as guided
by a Hammett relationship. Studies into the air stability of the resulting
complexes by multinuclear NMR spectroscopy show an increased tolerance
to atmospheric oxygen with increasingly electron-withdrawing ligands.
The heteroleptic complex Mo2(DFArF)3(OAc) [where DFArF = 3,5-(difluorophenyl)­formamidinate]
shows remarkable tolerance to oxygen in the solid state and in chloroform
solutions. Through the employment of easily accessible ligands, the
stability of the Mo2 core toward oxygen has been enhanced,
thereby making Mo2PWCs with electron-withdrawing ligands
more attractive candidates for the development of functional materials.