Differences of pH-dependent mechanisms on generation of hydride donors using Ru(II) complexes containing geometric isomers of NAD+ model ligands: NMR and radiolysis studies in aqueous solution.
ABSTRACT The pH-dependent mechanism of the reduction of the nicotinamide adenine dinucleotide (NADH) model complex [Ru(bpy)(2)(5)](2+) (5 = 3-(pyrid-2'-yl)-4-azaacridine) was compared to the mechanism of the previously studied geometric isomer [Ru(bpy)(2)(pbn)](2+) (pbn = 2-(pyrid-2'-yl)-1-azaacridine, previously referred to as 2-(pyrid-2'-yl)-benzo[b]-1,5-naphthyridine) in aqueous media. The exposure of [Ru(bpy)(2)(5)](2+) to CO(2)(*-) leads to the formation of the one-electron reduced species (k = 4.4 x 10(9) M(-1) s(-1)). At pH < 11.2, the one-electron reduced species can be protonated, k = 2.6 x 10(4) s(-1) in D(2)O. Formation of a C-C bonded dimer is observed across the pH range of 5-13 (k = 4.5 x 10(8) M(-1) s(-1)). At pH < 11, two protonated radical species react to form a stable C-C bonded dimer. At pH > 11, dimerization of two one-electron reduced species is followed by disproportionation to one equivalent starting complex [Ru(bpy)(2)(5)](2+) and one equivalent [Ru(bpy)(2)(5HH)](2+). The structural difference between [Ru(bpy)(2)(pbn)](2+) and [Ru(bpy)(2)(5)](2+) dictates the mechanism and product formation in aqueous medium. The exchange of the nitrogen and carbon atoms on the azaacridine ligands alters the accessibility of the dimerization reactive site, thereby changing the mechanism and the product formation for the reduction of the [Ru(bpy)(2)(5)](2+) compound.
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ABSTRACT: We have developed a correlation between experimental and density functional theory-derived results of the hydride-donating power, or "hydricity", of various ruthenium, rhenium, and organic hydride donors. This approach utilizes the correlation between experimental hydricity values and their corresponding calculated free-energy differences between the hydride donors and their conjugate acceptors in acetonitrile, and leads to an extrapolated value of the absolute free energy of the hydride ion without the necessity to calculate it directly. We then use this correlation to predict, from density functional theory-calculated data, hydricity values of ruthenium and rhenium complexes that incorporate the pbnHH ligand-pbnHH = 1,5-dihydro-2-(2-pyridyl)-benzo[b]-1,5-naphthyridine-to model the function of NADPH. These visible light-generated, photocatalytic complexes produced by disproportionation of a protonated-photoreduced dimer of a metal-pbn complex may be valuable for use in reducing CO(2) to fuels such as methanol. The excited-state lifetime of photoexcited [Ru(bpy)(2)(pbnHH)](2+) is found to be about 70 ns, and this excited state can be reductively quenched by triethylamine or 1,4-diazabicyclo[2.2.2]octane to produce the one-electron-reduced [Ru(bpy)(2)(pbnHH)](+) species with half-life exceeding 50 μs, thus opening the door to new opportunities for hydride-transfer reactions leading to CO(2) reduction by producing a species with much increased hydricity.Proceedings of the National Academy of Sciences 07/2012; 109(39):15657-62. · 9.74 Impact Factor
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ABSTRACT: Successful control of photo-driven NAD(+)/NADH type hydrogenation reactions in ruthenium complexes has been accomplished by using a new NAD(+) model ligand with modulated distortion of the ligand taking advantage of the substituent effect.Chemical Communications 01/2012; 48(12):1796-8. · 6.38 Impact Factor
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ABSTRACT: In biological reduction processes the dihydronicotinamides NAD(P)H often transfer hydride to an unsaturated substrate bound within an enzyme active site. In many cases, metal ions in the active site bind, polarize and thereby activate the substrate to direct attack by hydride from NAD(P)H cofactor. This review looks more widely at the metal coordination chemistry of organic donors of hydride ion - organo-hydrides - such as dihydronicotinamides, other dihydropyridines including Hantzsch's ester and dihydroacridine derivatives, those derived from five-membered heterocycles including the benzimidazolines and benzoxazolines, and all-aliphatic hydride donors such as hexadiene and hexadienyl anion derivatives. The hydride donor properties - hydricities - of organo-hydrides and how these are affected by metal ions are discussed. The coordination chemistry of organo-hydrides is critically surveyed and the use of metal-organo-hydride systems in electrochemically-, photochemically- and chemically-driven reductions of unsaturated organic and inorganic (e.g. carbon dioxide) substrates is highlighted. The sustainable electrocatalytic, photochemical or chemical regeneration of organo-hydrides such as NAD(P)H, including for driving enzyme-catalysed reactions, is summarised and opportunities for development are indicated. Finally, new prospects are identified for metal-organo-hydride systems as catalysts for organic transformations involving 'hydride-borrowing' and for sustainable multi-electron reductions of unsaturated organic and inorganic substrates directly driven by electricity or light or by renewable reductants such as formate/formic acid.Chemical Society Reviews 03/2013; · 24.89 Impact Factor