Migration of [Ru II(hedta)] − among three binding sites on 6-azauridine

Abstract

[Ru(hedta)(H 2O)] − reacts with 6-azauridine over an essential Zn/Hg surface at N-6 of the ligand in less than 5.0 min. This species has a characteristic H-1′ ribofuranosyl 1H NMR resonance at 5.72 ppm and a related C-5H pyrimidine resonance at 4.39 ppm. This N-6 complex has a Ru II<III electrochemical wave at −0.06 V versus NHE. The N-6 bound species rearranges to C-5-N-6 η 2 coordination with T 1 2 ∼2.5 h . The η 2 form has a characteristic H-1′ 1H NMR resonance at 5.45 ppm and a very highly stabilized Ru II center having a Ru II/III electrochemical wave at 1.33 V. If the pH or pD is above 6.0, the η 2 complex further migrates to the N-3 position of 6-azauridine which has ionized to its anion at N-3. The N-3 form becomes bidentate with N-3, O-4 chelation as observed previously for [Ru II(hedta)] − for uridine. The chelated N-3, O-4 form has a Ru III/IV wave at 0.94 V which can be eliminated by making the solution acidic and forcing N-3 only coordination, or remigration to the η 2 form. If the synthetic sequence is carried out with pD below 3.0, the η 2 form can be prepared in nearly quantitative yield. Under these conditions the 3C NMR spectra reveal carbon resonances for the pyrimidine ring which are shifted 0.0 ppm compared to the free ligand for the η 2 carbon (C-5), and downfield shifts of 3.0 ppm for C-4 and 3.7 ppm for C-2 relative to 6-azauridine. Therefore η 2 attachments at C = N units do not show the 40 to 80 ppm upfield shifts observed for C = C units of olefins.