MIGRATION OF [RU-II(HEDTA)](-) AMONG 3 BINDING-SITES ON 6-AZAURIDINE

[Ru (hedta) (H2O)](-) 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 H-1 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 eta(2) coordination with T-1/2 similar t o 2.5 h. The eta(2) form has a characteristic H-1' H-1 NMR resonance a t 5.45 ppm and a very highly stabilized Ru-II center having a Ru-II/II I electrochemical wave at 1.33 V. if the pH or pD is above 6.0, the et a(2) complex further migrates to the N-3 position of 6-azauridine whic h 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 ur idine. 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 eta(2) form. Lf the synthetic sequ ence is carried out with pD below 3.0, the eta(2) form can be prepared in nearly quantitative yield. Under these conditions the C-13 NMR spe ctra reveal carbon resonances for the pyrimidine ring which are shifte d 0.0 ppm compared to the free ligand for the eta(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 eta(2) attachments at C = N units do not show t he 40 to 80 ppm upheld shifts observed for C = C units of olefins. (C) 1998 Elsevier Science S.A. All rights reserved.