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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 the 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 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. 

The synthesis, structure, and reactivity of a series of cyclopentadienone and hydroxycyclopentadienyl 4,4’-dimethylbipyridine (dmbpy) iridium complexes (C5Tol2Ph2O)(dmbpy)IrCl 1, [(C5Tol2Ph2OH)(dmbpy)IrCl][OTf] 2 (C5Tol2Ph2O)(dmbpy)IrH 3, and [(C5Tol2Ph2OH)(dmbpy)IrH][OTf] 4 are described. The Ir(I) complexes 1 and 3 are active catalyst precursors for transfer hydrogenation of aldehydes, ketones, and N-heterocycles with HCO2H/Et3N under mild conditions. Model studies implicate the cationic iridium hydride, [(C5Tol2Ph2OH)(dmbpy)IrH][OTf] 4 as a key intermediate, as 4 reacts readily with acetone to generate isopropanol. Selectivity over hydrogenation of alkenes is enhanced compared to other Shvo-type catalysts, and only modest C=C hydrogenation observed when adjacent to polarizing functional groups. Catalytic hydrogenation likely proceeds by a metal-ligand bifunctional mechanism similar to related cyclopentadienone complexes