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 Hydricity of Transition Metal Hydrides

Transition metal hydrides play a critical role in stoichiometric and catalytic transformations. Knowledge of free energies for cleaving metal hydride bonds enables the prediction of chemical reactivity, such as for the bond-forming and bond-breaking events that occur in a catalytic reaction. Thermodynamic hydricity is the free energy required to cleave an M-H bond to generate a hydride ion (H(-)). Three primary methods have been developed for hydricity determination: the hydride transfer method establishes hydride transfer equilibrium with a hydride donor/acceptor pair of known hydricity, the H2 heterolysis method involves measuring the equilibrium of heterolytic cleavage of H2 in the presence of a base, and the potential-pKa method considers stepwise transfer of a proton and two electrons to give a net hydride transfer. Using these methods, over 100 thermodynamic hydricity values for transition metal hydrides have been determined in acetonitrile or water. In acetonitrile, the hydricity of metal hydrides spans a range of more than 50 kcal/mol. Methods for using hydricity values to predict chemical reactivity are also discussed, including organic transformations, the reduction of CO2, and the production and oxidation of hydrogen.

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.