ETA-2 COORDINATION OF RUII(HEDTA)- AT THE C-5-C-6 BONDS OF CYTIDINE AND URIDINE

Run(hedta)L− complexes (hedta3− = N(hydroxyethyl)ethylenediaminetriacetate) were prepared for a series of ligands related to the uridine and cytidine nucleosides. The ligands studied included L = uracil, undine (U), 1-methyluracil, 1,3-dimethyluracil (1,3-DMU), 3-methyluridine, thymidine (T), cytosine, cytidine (C), 3-methylcytidine, pyrimidine, pyrazine, and pyridine. The RuII(hedta)L− complexes were characterized by 1H and 13C NMR spectroscopy, cyclic voltammetry, and differential-pulse polarography. A novel coordination mode for RuII(hedta)− at the C-5-C-6 olefinic bonds of the uridine- and cytidine-related bases is observed in addition to coordination at the more normal binding site of N-3 (and N-1) in the absence of methyl- or ribose-blocking groups at N-1 or N-3. η2 coordination at the C-5–C-6 bond is absent for the sterically hindered T nucleobase. When both the N-1 and N-3 positions of pyrimidines are blocked by CH3 or a ribose unit (1,3-DMU, 3-methyluridine, 3-methylcytidine) only the C-5–C-6 coordination mode is observed; two stereochemical isomers are detected by 1H and 13C NMR spectroscopy for the 3-methyluridine and 3-methylcytidine olefinically bound complexes in ratios of 3:2 and 1:1, respectively. All other olefinically bound complexes of the uridine/cytidine-related series are present as one stereoisomer. The η2-coordinated RuII(hedta)− complex of 1,3-DMU has a formation constant of 2.0 × 103 M−1 (µ = 0.10, 7 = 22 °C). The C-5–C-6 coordination is characterized by a RuIII/II couple of ca. 0.62 V, upfield 13C shifts of the C-5 and C-6 ring carbons (38–50 ppm), and an upfield 1H shift of the C-5 H proton (0.66–1.38 ppm), similar to the case for RuII(hedta)L− complexes of 1,3-butadiene, styrene, and other simple olefins. Pyrimidine and pyridine bases that have an α-hydroxyl or amino group are observed to form a metal chelate, with ruthenium binding between N-3 and the deprotonated exo donor group; chelation stabilizes RuIV and RuIII complexes of these ligands. An additional RuIV/III electrochemical wave at ca. 1.00 V vs NHE is observed for the chelated forms of uridine/cytidine/thymidine-related bases at higher pH; this form is abundant above pH 9. © 1990, American Chemical Society. All rights reserved.