(Ru(II)(hedta))(-) complexes of pyrimidine, pyridazine, and pyrazinium ligands: Studies of eta(2) and fluxional behavior

摘要

The reaction of (Rd$sp{rm II}$(hedta)) $sp{-}$ (hedta$sp{3-}$ = N-hydroxyethylenediaminetriacetate) with various N-heterocycles including pyrimidine, pyrazine and pvridazine have been studied predominantly by $sp1$H/$sp{13}$C NMR and electrochemical methods. It has been shown that pyrimidine and other N-heterocycles first substitute for H$sb2$O forming N-bound complexes. However, migration to other locations which are shown to be novel $etasp2$ attachments to the pyrimidine ring at $etasp2$(1,2), $etasp2$(1,6) and $etasp2$(5,6) locations occur with changes in NMR and electrochemical properties of the ligand at 1:1 stoichiometry. Both the N-1 bound and $etasp2$ species are stereochemically rigid at 298 K. However, when two pyrimidines are coordinated (N-l)by displacement of a cis-carboxylate donor of hedta$sp{3-},$ one pyrimidine is fluxional with linkage 1,3-isomerism and the other pyrimidine remains stereochemically rigid. The process of N-1 to $etasp2$ migration is suppressed as the Ru-N $sigma$ bond is strengthened by donating ligand substituents (CH$sb3$ group) and favored by exo-ring oxo substituents in DNA nucleobases.;Pyrazine as a ligand is observed to bind initially to all three stereochemical isomers of (Ru$sp{rm II}$(hedta)) $sp{-}.$ Two isomers (cis-equatorial and trans-equatorial) are rigid complexes, whereas the cis-polar pyrazine complexes is fluxional. An equilibrium shift proceeds in ca. 2h to completion in an entropy driven manner to produce only N-bound cis-polar isomer which has the coordinated ligand undergoing 1,4-metallotropic shifts. Migration to $etasp2$-binding sites is initiated by protonation of the terminal N-4 nitrogen. The process weakens Ru-N $sigma$ bonding to the pyrazine which allows a shift to more favorable $etasp2$ locations on coordinated pyrazinium ion. The shift is again retarded by electron releasing CH$sb3$ groups.;Pyridazine as a ligand follows patterns similar to the N-bonded pyrimidines. However, the stronger $sigma$ donation of pyridazine prevents fluxional behavior or $etasp2$ migration for either mono or bis complexes. A bidentate mono complex with weakened coordination is implied by kinetic effects.;Since the $sp{13}$C NMR shifts and some $sp1$H NMR shifts which are observed for the $etasp2$(C=N) pyrimidine complexes are unusual in comparison with known $etasp2$(C=C) olefinic complexes of Ru$sp{rm II}$ and Os$sp{rm III}$ complexes in the literature, a model complex possessive of a single $etasp2$(C=N) binding site was prepared using 6-azauridine as a ligand for (Ru$sp{rm II}$(hedta)) $sp{-}$. It has been shown that the Ru(II) complexes which is produced initially with 6-azauridine forms by substitution at N-6. This N-bound isomer (complex (1)) undergoes sequential migration to the adjacent $etasp2$(5,6) position with 100% conversion (complex (2)) when the pH $le$ 2. However, above pH $approx$ 4, a second migration occurs to the deprotonated N-1 position of 6-azauridine, followed by N-1, O-4 chelation (complex (3)). By means of pH control, it was shown that the $etasp2$-bound complex (2) has $sp{13}$C and $sp1$H NMR parameters similar to the pyrimidine $etasp2$ complexes, e.g. that $sp{13}$C chemical shifts of $etasp2$ (C=N) chromophores exhibit small downfield shifts, not large (ca. 80 ppm) upfield shifts as for $etasp2$ (C=C) species.;Lastly it is shown that 2,3-dimethylpyrazine forms (Ru$sp{rm II}$(hedta)(2,3-Me$sb2$Pz)) $sp{-}$ and bridged-binuclear complex ${$(Ru$sp{rm II}$(hedta)) $sb2$(2,3-Me2Pz)) $}sp{2-}.$ In the bridged complex, $sp1$H NMR data shows that coordination the two Ru$sp{rm II}$ is asymmetric. This is attributed to steric factors and electronic factors promoting better donation to one Ru(II) center at the expense of the second Ru(II) site.