Five coordination modes of 4-aminopyrimidine with N-hydroxy-ethylethylenediaminetriacetatoruthenate(II)

Summary The complex Ru^II(hedta)(4NH₂pym)]⁻, hedta³⁻ = N-hydroxyethylethylenediaminetriacetate, 4NH₂pym = 4-aminopyrimidine, exists at pH 7 as five different coordination isomers, which are most readily distinguished by their electrochemical waves in comparison with the 2-aminopyridine (2NH₂py) complex. The 2NH₂py complex exhibits N(1) (pyridine bound), exo-NH₂ (amine bound) and N(1), NH₂-chelated species. The 4NH₂pym complex forms N(1), exo-amine and N(3), NH₂-chelated isomers analogues to the 2NH₂py species, but also engages in η² (olefin bound) coordination of the dearomatized 4NH₂pym ring in C(5)–C(6), and another η² type of complex involving electron density between N(1) and N(3) of the ring (η³ form). N(1), η² and η³ isomers have also been detected for unsubstituted pyrimidine (pym), 4-methylprimidine (4CH₃pym) and 2-aminopyrimidine (2NH₂pym). Electrochemical waves (V versus NHE) for the five isomers are assigned as follows: (Ru^II/III) exo-NH₂ (0.06 V), N(1) (0.29 V), η² (0.49 V); (Ru^II/III) η³ (0.76 V); N(3), NH₂-chelated (1.09 V).