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

Abstract

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 pK 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.

Synthesis and Reactivity of Iron-Oxocyclohexadienyl Complexes toward Proton-Coupled Electron Transfer

“…Proton-coupled electron transfer (PCET), where proton and electron move concertedly (also called concerted PCET), plays an important role in reductive transformations because PCET decreases activation barrier. , In PCET, reagents with small bond dissociation free energies (BDFEs) have been employed as PCET donors to make new X–H bonds with large BDFEs. Coordination-induced bond weakening decreases the strength of ligand Y–H bonds upon coordination to low valent metal centers, generating potent H atom donors in PCET reactions. − A combination of SmI 2 and H 2 O was widely used as a PCET reagent in organic reactions. − Catalysts that worked as formal H atom donors to promote PCET steps in electrochemical reduction reactions were also reported by Peters, − Mougel, Waymouth, and co-workers.…”

Improving co-electrocatalytic carbon dioxide reduction by optimizing the relative potentials of the redox mediator and catalyst

“…17 While examples of RMs used with catalysts for the reduction of CO 2 are currently limited, 18–23 a few additional examples exist for the electrocatalytic conversion of a variety of small molecules. 23–35…”