Bioorganometallic chemistry Part 12. Reaction of [Cp~*Rh(H_2O)_3](OTf)_2 with nicotinamide adenine dinucleotide in water: synthesis, structure, and a pH-dependent ~1H-NMR and voltammetric study of the cyclic trimer product, [Cp~*Rh(#mu#-#eta#~1(N1):#

摘要

The reaction of [Cp~*Rh(H_2O)_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(#mu#-#eta#~1(N1):#eta#~2(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(#mu#-Cl_2)Cl_2] 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. Published by Elsevier Science S.A.