N-1- and η²-Pyrimidine Linkage Isomers in Complexes of [Ru^II(hedta)]^-

[Ru^II(hedta)(H₂O)]^-, hedta^3- = N-hydroxyethylethylenediaminetriacetate, forms N-1-bound pyrimidine complexes via a kinetically controlled substitution at N-1 with pyrimidine (pym), 4-methylpyrimidine (4CH₃pym), and 4,6-dimethylpyrimidine (Me₂pym); k = 31 M^-1 s^-1 for pym. Subsequent to N-1 coordination, an intramolecular redistribution of Ru^II−pyrimidine linkage isomers occurs with the formation of η² attachments, reaching equilibrium with t_1/2's of 28.8 (pym), 24 (4CH₃pym), and 1 h for Me₂pym. The N-1 forms exhibit normal Ru^II/III waves at 0.14 (pym), 0.10 (4CH₃pym), and 0.16 V (Me₂pym) whereas the η² forms shift to more positive values indicative of better π-acceptor attachments:  0.50 (pym) and 0.44 V (4CH₃pym). The ratio of isomers was determined to be as follows by ¹H NMR and ¹³C NMR methods:  (pym) η²(1,2):η²(5,6):η²(1,6):N-1 of 43:22:33:2; (4CH₃pym) η²(1,2):η²(5,6):N-1 of 10:33:57; (Me₂pym) η²(1,2):η²(5,6):N-1 of 6:26:68. ¹H NMR assignments have been made for all the observed N-1, η²(1,2), η²(5,6), and η²(1,6) isomers. ¹³C NMR shifts have been identified for the major isomers of pym and 4CH₃pym and confirmed HH COSY and HC COSY methods. Several important conclusions are drawn:  (1) η² isomers of the η²(1,2) type exhibit significant downfield shifts of H2 of ca 1.20 ppm; (2) ¹³C NMR shifts of carbon centers in η²-bound diazine rings are downfield of the free ligand by up to +9 ppm and not 40−80 ppm upfield as for η²-olefinic complexes; (3) η² protons of the η²(5,6) type shift significantly less upfield than those for η²-olefin complexes (ca. 0.4−0.8 ppm vs 1.0−2.0 ppm). It was established that the formation of η²-bound pyrimidines of [Ru^II(hedta)]^- occurs concomitantly with the dissociation of a carboxylate donor of the hedta^3- ligand. ¹³C NMR spectra reveal the predicted weighted percentage of freed carboxylates based on the isomer distribution between N-1 with all three glycinato arms of hedta^3- bound to Ru^II (¹³C resonance at 188 ppm) vs η² forms with two bound glycinato groups and one free glycinato group of hedta^3- (resonating near 174 ppm).