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

Abstract

The proton-coupled 13C 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 coordination. The presence of the H-5 proton is indicated by the doublet pattern centered at 137 ppm ( J CH=204 Hz). This rules out the possible formation of a sigma-bonded, σ(C-5), species for the second isomer, and confirms the prior assignment of the second kinetic isomer as an η 2(5,6)-coordinated species [Y. Chen, R.E. Shepherd, Inorg. Chim. Acta 277 (1998) 46]. The small magnitude of the downfield 13C NMR shifts for the η 2-bound 6-azauridine 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 of very small or near-zero 13C NMR shifts (+1.0 to −9 ppm) for η 2-coordinated pyrimidines. This contrasts with the large +40 to +80 ppm upfield shifts which are characteristic of η 2 complexes of Ru II and Os II of CC olefinic ligands.