Two-State Hydrogen Atom Transfer Reactivity of Unsymmetric [Cu 2 (O)(NO)] 2+ Complexes.

We report the temperature-dependent spin switching of dicopper oxo nitrosyl [Cu 2 (O)(NO)] 2+ complexes and their influence on hydrogen atom transfer (HAT) reactivity. Electron paramagnetic resonance (EPR) and Evans method analysis suggest that [Cu 2 (O)(NO)] 2+ complexes transition from the S = 1/2 to the S = 3/2 state around ca. 202 K. At low temperatures (198 K) where S = 3/2 dominates, a strong correlation between the rate of HAT ( k HAT ) and the population of the S = 1/2 state was identified ( R 2 = 0.988), suggesting that the HAT by [Cu 2 (O)(NO)] 2+ complexes proceeds by the S = 1/2 isomer. Installation of functional groups that introduce an unsymmetric secondary coordination environment accelerates the HAT rates through perturbation of the spin equilibria. Given the often unsymmetric coordination sphere of bimetallic active sites in natural proteins, we anticipate that similar strategies could be employed by metalloenzymes to control HAT reactions.