C-13 NMR confirmation that [Ru-II(hedta)(6-azauridine)](-) has eta(2)(5,6)and not sigma(C-5) coordination

The proton-coupled C-13 NMR spectrum of the second kinetic isomer of [Ru-II (hedta)(6-azauridine)](-) reveals a doublet for C-5 after migration of the [Ru-II(hedta)](-) moiety from the original N-6 site of 6-azauridine coordin ation. The presence of the H-5 proton is indicated by the doublet pattern c entered at 137 ppm (J(C-H) = 204 Hz). This rules out the possible formation of a sigma-bonded, sigma(C-5), species for the second isomer, and confirms the prior assignment of the second kinetic isomer as an eta(2)(5,6)-coordi nated species [Y. Chen, R.E. Shepherd, Inorg. Chim. Acta 277 (1998) 46]. Th e small magnitude of the downfield C-13 NMR shifts for the eta(2)-bound 6-a zauridine N-heterocycle with [Ru-II(hedta)](-) (0.00 ppm for C-5, -3.0 ppm for C-4, and -3.7 ppm for C-2) is in concert with the former observations o f very small or near-zero C-13 NMR shifts (+1.0 to -9 ppm) for eta(2)-coord inated pyrimidines. This contrasts with the large +40 to +80 ppm upheld shi fts which are characteristic of eta(2) complexes of Ru-II and Os-II of C=C olefinic ligands. (C) 1999 Elsevier Science S.A. All rights reserved.