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(2H)-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 pKa (−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 ·CAr3 (Ar = p-tert-butylphenyl) to the corresponding triarylmethane HCAr3 via net 1e–/1H+ transfer from RuVdH.