Comparative study of C^N and N^C type cyclometalated ruthenium complexes with a NAD+/NADH function

Photocatalytic hydrogen evolution using a Ru(ii)-bound heteroaromatic ligand as a reactive site

A Ru^II complex, [Ru^II(tpphz)(bpy)₂]²⁺ (1) (tpphz = tetrapyridophenazine, bpy = 2,2'-bipyridine), whose tpphz ligand has a pyrazine moiety, is converted efficiently to [Ru^II(tpphz-HH)(bpy)₂]²⁺ (2) having a dihydropyrazine moiety upon photoirradiation of a water-methanol mixed solvent solution of 1 in the presence of an electron donor. In this reaction, the triplet metal-to-ligand charge-transfer excited state (³MLCT*) of 1 is firstly formed upon photoirradiation and the ³MLCT* state is reductively quenched with an electron donor to afford [Ru^II(tpphz˙^-)(bpy)₂]⁺, which is converted to 2 without the observation of detectable reduced intermediates by nano-second laser flash photolysis. The inverse kinetic isotope effect (KIE) was observed to be 0.63 in the N-H bond formation of 2 at the dihydropyrazine moiety. White-light (380-670 nm) irradiation of a solution of 1 in a protic solvent, in the presence of an electron donor under an inert atmosphere, led to photocatalytic H₂ evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its ³MLCT* state to react with a proton and aldehydes. In photocatalytic H₂ evolution, the H-H bond formation between photoexcited 2 and a proton is involved in the rate-determining step with normal KIE being 5.2 on H₂ evolving rates. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the reaction mechanism of H₂ evolution from the ground and photo-excited states of 2 were performed to have a better understanding of the photocatalytic processes.

Protonation of a Cobalt Phenylazopyridine Complex at the Ligand Yields a Proton, Hydride, and Hydrogen Atom Transfer Reagent

Protonation of the Co(I) phenylazopyridine (azpy) complex [CpCo(azpy)] 2 occurs at the azo nitrogen of the 2-phenylazopyridine ligand to generate the cationic Co(I) complex [CpCo(azpyH)]⁺ 3 with no change in oxidation state at Co. The N-H bond of 3 exhibits diverse hydrogen transfer reactivity, as studies with a variety of organic acceptors demonstrate that 3 can act as a proton, hydrogen atom, and hydride donor. The thermodynamics of all three cleavage modes for the N-H bond (i.e., proton, hydride, and hydrogen atom) were examined both experimentally and computationally. The N-H bond of 3 exhibits a p K_a of 12.1, a hydricity of Δ G°_H- = 89 kcal/mol, and a bond dissociation free energy (BDFE) of Δ G°_H• = 68 kcal/mol in CD₃CN. Hydride transfer from 3 to the trityl cation (Δ G°_H- = 99 kcal/mol) is exergonic but takes several hours to reach completion, indicating that 3 is a relatively poor hydride donor, both kinetically and thermodynamically. Hydrogen atom transfer from 3 to 2,6-di- tert-butyl-4-(4'-nitrophenyl)phenoxyl radical (^tBu₂NPArO·, Δ G°_H• = 77.8 kca/mol) occurs rapidly, illustrating the competence of 3 as a hydrogen atom donor.

Electrochemical behavior of a Rh(pentamethylcyclopentadienyl) complex bearing an NAD+/NADH-functionalized ligand

A RhCp* (Cp* = pentamethylcyclopentadienyl) complex bearing an NAD+/NADH-functionalized ligand, [RhCp*(pbn)Cl]Cl ([1]Cl, pbn = (2-(2-pyridyl)benzo[b]-1,5-naphthyridine)), was synthesized. The cyclic voltammogram of [1]Cl in CH3CN shows two reversible redox waves at E1/2 = -0.58 and -1.53 V (vs. the saturated calomel electrode (SCE)), which correspond to the RhIII/RhI and pbn/pbn˙- redox couples, respectively. The addition of acetic acid to the solution afforded the proton-coupled two-electron reduction of [1]Cl at -0.62 V, from which [RhCp*(pbnHH)Cl]+ was selectively generated, probably via a hydride transfer from a RhIII-hydride intermediate to the pbn ligand. Complex [1]Cl is stable under acidic conditions, whereas a methyl proton of the Cp* moiety dissociates under basic conditions. The resulting anionic methylene group attacks the para carbon of the free pyridine of pbn, accompanied by protonation of the nitrogen atom of the ligand. As a result, treatment of [1]Cl with a base produces selectively the cyclic complex [1CH]Cl, which bears a reduced pbn framework (pbnCH). [1CH]Cl forms 1 : 1 adducts with PhCOO-via hydrogen bonding. A similar adduct, formed by a Ru-pbnHH scaffold and RCOO- (R = CH3, C6H5), has been reported to react with CO2 to produce HCOO- under concomitant regeneration of Ru-pbn. The adduct of [1CH]Cl with PhCOO-, however, lacks such hydride-donor ability, due to a steric barrier in the molecular structure of [1CH]Cl, which hampers the hydride transfer.

New Insight into the Mechanism of NADH Model Oxidation by Metal Ions in Nonalkaline Media

For a long time, it has been controversial that the three-step (e-H⁺-e) or two-step (e-H^•) mechanism was used for the oxidation of nicotinamide adenine dinucleotide coenzyme (NADH) and its models by metal ions in nonalkaline media. The latter mechanism has been accepted by the majority of researchers. In this work, 1-benzyl-1,4-dihydronicotinamide (BNAH) and 1-phenyl-l,4-dihydronicotinamide are used as NADH models and ferrocenium (Fc⁺) metal ion as an electron acceptor. The kinetics for oxidation of the NADH models by Fc⁺ in pure acetonitrile was monitored by using UV-vis absorption and a quadratic relationship between k_obs and the concentrations of NADH models was found for the first time. The rate expression of the reactions developed according to the three-step mechanism is quite consistent with the quadratic curves. The rate constants, thermodynamic driving forces, and kinetic isotope effects of each elementary step for the reactions were estimated. All results supported the three-step mechanism. The intrinsic kinetic barriers of the proton transfer from BNAH^+• to BNAH and the hydrogen-atom transfer from BNAH^+• to BNAH^+• were estimated by using Zhu equation; the results showed that the former is 11.8 kcal/mol and the latter is larger than 24.3 kcal/mol. It is the large intrinsic kinetic barrier of the hydrogen-atom transfer that makes the reactions choose the three-step rather than two-step mechanism. Further investigation of the factors affecting the intrinsic kinetic barrier of chemical reactions indicated that the large intrinsic kinetic barrier of the hydrogen-atom transfer originated from the repulsion of positive charges between BNAH^+• and BNAH^+•. The greatest contribution of this work is the discovery of the quadratic dependence of k_obs on the concentrations of the NADH models, which is inconsistent with the conventional viewpoint of the "two-step mechanism" on the oxidation of NADH and its models by metal ions in the nonalkaline media.

Photochemical Properties and Reactivity of a Ru Compound Containing an NAD/NADH-Functionalized 1,10-Phenanthroline Ligand

An NAD/NADH-functionalized ligand, benzo[b]pyrido[3,2-f][1,7]-phenanthroline (bpp), was newly synthesized. A Ru compound containing the bpp ligand, [Ru(bpp)(bpy)2](2+), underwent 2e(-) and 2H(+) reduction, generating the NADH form of the compound, [Ru(bppHH)(bpy)2](2+), in response to visible light irradiation in CH3CN/TEA/H2O (8/1/1). The UV-vis and fluorescent spectra of both [Ru(bpp)(bpy)2](2+) and [Ru(bppHH)(bpy)2](2+) resembled the spectra of [Ru(bpy)3](2+). Both complexes exhibited strong emission, with quantum yields of 0.086 and 0.031, respectively; values that are much higher than those obtained from the NAD/NADH-functionalized complexes [Ru(pbn)(bpy)2](2+) and [Ru(pbnHH)(bpy)2](2+) (pbn = (2-(2-pyridyl)benzo[b]-1.5-naphthyridine, pbnHH = hydrogenated form of pbn). The reduction potential of the bpp ligand in [Ru(bpp)(bpy)2](2+) (-1.28 V vs SCE) is much more negative than that of the pbn ligand in [Ru(pbn)(bpy)2](2+) (-0.74 V), although the oxidation potentials of bppHH and pbnHH are essentially equal (0.95 V). These results indicate that the electrochemical oxidation of the dihydropyridine moiety in the NADH-type ligand was independent of the π system, including the Ru polypyridyl framework. [Ru(bppHH)(bpy)2](2+) allowed the photoreduction of oxygen, generating H2O2 in 92% yield based on [Ru(bppHH)(bpy)2](2+). H2O2 production took place via singlet oxygen generated by the energy transfer from excited [Ru(bppHH)(bpy)2](2+) to triplet oxygen.

[Ru(V)(NCN-Me)(bpy)([double bond, length as m-dash]O)](3+) Mediates efficient C-H bond oxidation from NADH analogs in aqueous media rather than water oxidation

[Ru(V)[double bond, length as m-dash]O](3+) and [Ru(VI)[double bond, length as m-dash]O](4+) generated from [Ru(II)(NCN-Me)(bpy)(H2O)](PF6)2 (where NCN-Me is the neutral N-methyl-3,5-di(2-pyridyl)pyridinium iodide after deprotonation of the C-H bond) play a selective role in the C-H bond oxidation of 2-(pyridin-2-yl)-9,10-dihydroacridine (PADHH) and water oxidation, respectively.

The coordination chemistry of organo-hydride donors: new prospects for efficient multi-electron reduction

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.