Coordination-Induced Bond Weakening and Electrocatalytic Proton-Coupled Electron Transfer of a Ruthenium Verdazyl Complex.

Coordination of the leucoverdazyl ligand 2,4-diisopropyl-6-(pyridin-2-yl)-1,4-dihydro-1,2,4,5-tetrazin-3(2 H )-one VdH to Ru significantly weakens the ligand's N-H bond. Electrochemical measurements show that the metalated leucoverdazyl Ru( VdH )(acetylacetonate) ₂ RuVdH has a lower p K _a (-5 units), BDFE (-7 kcal/mol), and hydricity (-22 kcal/mol) than the free ligand. DFT calculations suggest that the increased acidity is in part attributable to stabilization of the conjugate base Vd ^- . When free, Vd ^- distorts to avoid an 8πe ^- antiaromatic state, but it remains planar when bound to Ru. Proton-coupled electron transfer (PCET) behavior is observed for both the free and metalated leucoverdazyls. PCET equilibrium between the Vd radical and TEMPOH affords a VdH BDFE that is in good agreement with that obtained from electrochemical methods. RuVd exhibits electrocatalytic PCET donor behavior. Under acidic conditions, it reduces the persistent trityl radical ·CAr ₃ (Ar = p - tert -butylphenyl) to the corresponding triarylmethane HCAr ₃ via net 1e ^- /1H ⁺ transfer from RuVdH .