Mechanism of hydride donor generation using a Ru(II) complex containing an NAD+ model ligand: pulse and steady-state radiolysis studies

The mechanistic pathways of formation of the NADH-like Ru(bpy) 2(pbnHH)] (2+) species from Ru(bpy)2(pbn)](2+) were studied in an aqueous medium. Formation of the one-electron-reduced species as a result of reduction by a solvated electron (k=3.0 x 10(10) M(-1) s(-1)) or CO2(*-) (k=4.6 x 10(9) M(-1) s(-1)) or reductive quenching of an MLCT excited state by 1,4-diazabicyclo2.2.2]octane (k=1.1 x 10(9) M(-1) s(-1)) is followed by protonation of the reduced species (p K a = 11). Dimerization (k7a=2.2 x 10(8) M(-1) s(-1)) of the singly reduced protonated species, Ru(bpy) 2(pbnH(*))](2+), followed by disproportionation of the dimer as well as the cross reaction between the singly reduced protonated and nonprotonated species (k8= 1.2 x 10(8) M(-1) s(-1)) results in the formation of the final Ru(bpy)2(pbnHH)](2+) product together with an equal amount of the starting complex, Ru(bpy)2(pbn)](2+). At 0.2 degrees C, a dimeric intermediate, most likely a pi-stacking dimer, was observed that decomposes thermally to form an equimolar mixture of Ru(bpy)2(pbnHH)](2+) and Ru(bpy)2(pbn)](2+) (pH<9). The absence of a significant kinetic isotope effect in the disproportionation reaction of Ru(bpy)2(pbnH(*))](2+) and its conjugate base (pH>9) indicates that disproportionation occurs by a stepwise pathway of electron transfer followed by proton transfer.