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

Abstract

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 E_1/2 = −0.58 and −1.53 V (vs. the saturated calomel electrode (SCE)), which correspond to the Rh^III/Rh^I 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 Rh^III–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.