Electrosteric Stabilization of End-on Cupric Superoxide Complexes

The use of oriented internal electrostatic fields in homogenous systems is rapidly gaining attention as a nonconventional design principle for imparting unusual properties and/or reactivity profiles on metal complexes and catalysts. Herein, a series of η1 cupric superoxide complexes bound by tris(phosphinimine) ligands (X-PhMe2P3tren) are reported. Across this series of complexes, the identity of the substituent at the 4 position of a phosphinimine phenyl group was modulated (X = NMe2, H, CF3). The X-PhMe2P3tren ligands impart systematic adjustments to the electronic and secondary coordination sphere electrostatic properties of the copper complexes while maintaining a consistent steric profile in the vicinity of the O2 binding pockets. Key differences in the thermal stabilities and proton-coupled electron transfer (PCET) kinetics were observed among the η1 cupric superoxide complexes, which are discussed in the context of both intra- and inter-molecular electrostatic interactions. Notably, the cupric superoxide complex that was most resistant to decomposition – [(CF3 PhMe2P3tren)CuII(O2•1–)]+ – displays markedly improved thermal stability compared to the Me3P3tren-bound cupric superoxide complex and can be observed as high as room temperature on a multi-minute timescale. Detailed kinetic and computational analyses suggest that the improved thermal stability in this context results from an electrosteric effect, in which the accumulation of cationic charge in the secondary coordination sphere slows bimolecular decomposition.