Outer-sphere 2 e^-/2 H⁺ transfer reactions of ruthenium(II)-amine and ruthenium(IV)-amido complexes

A diverse set of 2 e^-/2 H⁺ reactions are described that interconvert [Ru^II(bpy)(en*)₂]²⁺ and [Ru^IV(bpy)(en-H*)₂]²⁺ (bpy=2,2'-bipyridine, en*=H₂NCMe₂CMe₂NH₂, en*-H=H₂NCMe₂CMe₂NH^-), forming or cleaving different O-H, N-H, S-H, and C-H bonds. The reactions involve quinones, hydrazines, thiols, and 1,3-cyclohexadiene. These proton-coupled electron transfer reactions occur without substrate binding to the ruthenium center, but instead with precursor complex formation by hydrogen bonding. The free energies of the reactions vary over more than 90 kcal mol^-1, but the rates are more dependent on the type of X-H bond involved than the associated ΔG . There is a kinetic preference for substrates that have the transferring hydrogen atoms in close proximity, such as ortho-tetrachlorobenzoquinone over its para-isomer and 1,3-cyclohexadiene over its 1,4-isomer, perhaps hinting at the potential for concerted 2 e^-/2 H⁺ transfers. (copyright 2017 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim)