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.