Mechanism of Hydride Donor Generation Using a Ru(II) Complex Containing an NAD⁺ Model Ligand: Pulse and Steady-State Radiolysis Studies

Abstract

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-diazabicyclo[2.2.2]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.

Calculation of thermodynamic hydricities and the design of hydride donors for CO 2 reduction

“…Previously, we have determined that ½RuðbpyÞ 2 ðpbnHHÞ 2þ is formed by two ½RuðbpyÞ 2 ðpbnH • Þ 2þ molecules undergoing bimolecular disproportionation (8). We can therefore think of ½RuðbpyÞ 2 ðpbnHHÞ 2þ as being a doubly reduced, doubly protonated species that "pools" the two electrons and two protons carried by the two ½RuðbpyÞ 2 ðpbnH…”

“…1 were characterized by X-ray single-crystal diffraction. We further investigated the mechanistic pathways of formation of the ½RuðbpyÞ 2 ðpbnHHÞ 2þ species from ½RuðbpyÞ 2 ðpbnÞ 2þ in an aqueous medium using pulse radiolysis and 60 Co irradiation (8). Formation of the OER species as a result of reduction by a solvated electron (k ¼ 3.0 · 10 10…”

A Series of Supramolecular Complexes for Solar Energy Conversion via Water Reduction to Produce Hydrogen: An Excited State Kinetic Analysis of Ru(II),Rh(III),Ru(II) Photoinitiated Electron Collectors

“…The homobimetallic complexes [(phen) 2 Ru(BL)Ru(phen) 2 ](PF 6 ) 4 collect up to two or four electrons on the BL (π*) orbitals (phen = 1,10-phenanthroline; BL = 9,11,20,22-tetraazatetrapyrido[3,2-a:2′3′-c:3′′,2′′-1:2′′′,3′′′-n]pentacene (tatpp) or 9,11,20,22-tetraazatetrapyrido[3,2-a:2′3′-c:3′′,2′′-1:2′′′,3′′′-n]pentacene-10,21-quinone (tatpq)) [7,8]. The Ru monometallic [(bpy) 2 Ru(pbn)](PF 6 ) 2 undergoes proton-coupled two-electron reduction of the NAD/NADH + model ligand to produce [(bpy) 2 Ru(pbnHH)] 2+ (pbn = 2-(2-pyridyl)benzo[ b ]-1,5-naphthyridine, NAD = nicotinamide adenine dinucleotide) [9,10]. These early systems display ligand-centered photoinitiated electron collection, but do not perform photocatalytic reduction of H 2 O to H 2 .…”

Nickel Complex with Internal Bases as Efficient Molecular Catalyst for Photochemical H2 Production

“…Such behavior has already been described in the literature for similar ligands. [30,31] Therefore, further studies on the photocatalytic performance of the Ni complexes were performed in basic solutions.…”