Reactivity of a fac-ReCl(α-diimine)(CO)3 complex with an NAD+ model ligand toward CO2 reduction

Metallated dihydropyridinates: prospects in hydride transfer and (electro)catalysis

Hydride transfer (HT) is a fundamental step in a wide range of reaction pathways, including those mediated by dihydropyridinates (DHP-s). Coordination of ions directly to the pyridine ring or functional groups stemming therefrom, provides a powerful approach for influencing the electronic structure and in turn HT chemistry. Much of the work in this area is inspired by the chemistry of bioinorganic systems including NADH. Coordination of metal ions to pyridines lowers the electron density in the pyridine ring and lowers the reduction potential: lower-energy reactions and enhanced selectivity are two outcomes from these modifications. Herein, we discuss approaches for the preparation of DHP-metal complexes and selected examples of their reactivity. We suggest further areas in which these metallated DHP-s could be developed and applied in synthesis and catalysis.

Rhodium catalysts with cofactor mimics for the biomimetic reduction of CN bonds

Bio-inspired reduction of CN bonds was successfully performed using rhodium catalysts containing cofactor mimics. The intramolecular cooperation between rhodium and cofactor mimics enabled the transformation with good selectivity. A plausible mechanism was also proposed.

A local proton source from carboxylic acid functionalized metal porphyrins for enhanced electrocatalytic CO2 reduction

Metal tetraphenylporphyrin modified through the introduction of propanoic acid into the phenyl groups as a local proton donor exhibits higher CO₂ electrocatalytic conversion to CO than benzoic acid.

Importance of proton-coupled electron transfer in cathodic regeneration of organic hydrides

This communication reports a combined experimental and computational study of mechanisms by which biomimetic NADH analogs can be electrochemically regenerated.

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.

Thermodynamic and kinetic hydricities of metal-free hydrides

Thermodynamic and kinetic hydricities provide useful guidelines for the design of hydride donors with desirable properties for catalytic chemical reductions.

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.

CO2 reduction with Re(i)–NHC compounds: driving selective catalysis with a silicon nanowire photoelectrode

Excellent selectivity was observed in CO₂ reduction using Re(i)–NHC catalysts on a silicon nanowire photoelectrode.

Synthesis, photophysical properties and application in organic light emitting devices of rhenium(i) carbonyls incorporating functionalized 2,2′:6′,2′′-terpyridines

Photophysics of [ReCl(CO)₃(4′-R-terpy-κ²N)].

Rhenium(i) terpyridine complexes – synthesis, photophysical properties and application in organic light emitting devices

The structural and photophysical characterization of new Re(i) complexes was reported.

Cyclopentadiene-mediated hydride transfer from rhodium complexes

Attempts to generate a proposed rhodium hydride catalytic intermediate instead resulted in isolation of (Cp*H)Rh(bpy)Cl (1), a pentamethylcyclopentadiene complex, formed by C–H bond-forming reductive elimination from the fleeting rhodium hydride.

Mechanistic insights into electrocatalytic CO2 reduction within [Ru(II)(tpy)(NN)X]n+ architectures

A series of Ru(II)-polypyridyl complexes of the design [Ru(II)(tpy)(NN)X](n+) (tpy = 2,2':6',2''-terpyridine; NN = bidentate polypyridine; X = Cl(-) or CH3CN; n = 1 or 2) have been synthesized and analyzed for their ability to function as electrocatalysts in the reduction of CO2 to CO. Varying the electron-donating/withdrawing character of the NN polypyridyl ligand has allowed for modification of electron density at the formally Ru(II) metal center. Complexes where X = Cl(-) display ligand substitution for CH3CN with differing rates of Cl(-) dissociation (k-Cl), therefore providing a degree of insight into the electron density and thus the chemical activity at the Ru(II) center. Detailed analysis of the cyclic voltammograms under argon vs. CO2 atmospheres using multiple switching potentials and scan rates ranging from ν = 25-2000 mV s(-1) has painted a picture of how monodentate ligand lability due to NN polypyridyl electron-donating character is related to electrocatalytic CO2 reduction activity of Ru(II)-polypyridyl complexes. From these studies, multiple mechanistic pathways towards generating the catalytically active [Ru(tpy(-))(NN(-))CO2](0) species are proposed and differ via the order of electrochemical and chemical processes.