Stereochemically-rigid and fluxional pyrimidines within the bis-[Ru(hedta)(pym)2]- complex

The formation of the bis-pyrimidine complex [Ru-II(hedta)(pym)(2)](-) occurs by sequential additions of pym to [Ru-II(hedta)(H2O)](-) (hedta(3-)=N-hydroxyethylethylenediaminetriacetate; pym=pyrimidine). The second step is rate limited by the dissociation of an in-plane carboxylato donor of the hedta(3-) ligand; k = 1.57 X 10(-4) s(-1) at 22 degrees C. E-1/2 values for the Ru-II/III waves in 0.10 M NaCl, T=22 degrees C are 0.18 V for [Ru-II(hedta)(pym)](-) and 0.54 V for [Ru-II(hedta)(pym)(2)](-). The latter is similar to the bidentate 2,2'-bipyridine complex, [Ru(hedta)/(bpy)](-), with E-1/2=0.48 V, illustrative of the influence of two-coordinated N-heterocyclic pi-acceptor ligands. H-1 NMR spectra of the [Ru-II(hedta)(pym)(2)](-) complex reveal a differentiation in the coordination behavior of the two bound pyrimidines. One pyrimidine is stereochemically-rigid, exhibiting resonances at 9.25 (H2), 8.82 (H6), 8.68 (H4) and 7.45 (H5) ppm, respectively. The second pyrimidine is fluxional with rapid migration of the Ru-II center between N-1 and N-3 sites. This motion interchanges the H4 and H6 protons which resonate at 8.82 ppm; the H2 resonance appears at 9.15 ppm and H5 at 7.61 ppm. The weaker bonding exhibited by the fluxional pyrimidine also allows for rapid exchange of the fluxional pyrimidine with any free pyrimidine in the solution. The difference in the two pyrimidines is consistent with assisted displacement of the fluxional pyrimidine, promoted by associative attack by the pendant carboxylate arm which is displaced in forming the his complex. The stereochemically-rigid pyrimidine is nearer the -CH2CH2OH (alcohol) arm of hedta(3-) which does not associate strongly and provide an assisted ligand exchange path. Acidification of the [Ru-II(hedta)(pym)(2)](-) complex to pD congruent to 1.0 results in formation of eta(2)(1,2)-[Ru(hedta)(pym)](-) as the major product in 1.0 Da, rather than the previously established (Y. Chen, F.-T. Lin and R.E. Shepherd, Inorg. Chem., 36 (1997) 818) 2:43:22:33 distribution for N-1:eta(2)(1,2):eta(2)(5,6):eta(2)(1,6) isomers which is generated spontaneously from the initially N-1-bound [Ru(hedta)(pym)](-) 1:1 complex after 14 days. (C) 1998 Elsevier Science S.A.