Bioorganometallic chemistry: Part 12. Reaction of [Cp*Rh(H₂O)₃](OTf)₂ with nicotinamide adenine dinucleotide in water

The reaction of [Cp*Rh(H₂O)₃](OTf)₂ (1) with nicotine adenine dinucleotide (NAD⁺, 2), an important co-factor in enzymatic reactions, was studied utilizing ¹H-NMR spectroscopy, electrospray ionization mass spectroscopy (ESI/MS), cyclic voltammetry (CV), and isolation techniques, as a function of pH. The product was formulated from the above-mentioned spectroscopic data as the well-known Cp*Rh cyclic trimer structure, [Cp*Rh(μ-η¹(N1):η²(N6,N7)-9-(5′-ribose pyrophosphate-5″-ribose-1″-nicotinamide)adeninato]₃(OTf)₃, 3, which forms via a self-assembly mechanism as the pH is increased from 3 to 6 (¹H-NMR). We also compared 3 with the putative one reported that formed via reaction with [(Cp*Rh)₂(μ-Cl₂)Cl₂] and was tentatively assigned the formula, [Cp*Rh(NAD)Cl](Cl). In fact, both Cp*Rh synthons provide the same cyclic trimer product at pH 6, while a presumed mixture of [Cp*Rh(NAD)] and Cp*Rh aqua intermediates (at least eight Cp*Rh ¹H-NMR signals are evident) were formed at pH 3.0. A full analysis of the CV data reveals that some Cp*Rh aqua complexes are electroactive at potentials around −1.2 V versus Ag ∣ AgCl, but probably not the cyclic trimer, complex 3. Unfortunately, we were not able to utilize complex 3 in an intramolecular, regioselective reduction reaction, with sodium formate as the hydride source, to provide the corresponding biologically active 1,4-dihydro derivative.