Strain-induced substitutional lability in a Ru(II) complex of a hypodentate polypyridine ligand

The ruthenium(II) complex of heptadentate N,N,N′,N′-tetrakis(2- pyridylmethyl)-2,6-bis(aminomethyl)pyridine (tpap) was isolated as the hexafluorophosphate complex Ru(tpap)(PF₆)₂. The crystal structure has been determined for the triflate salt Ru(tpap)(CF ₃SO₃)₂·2H₂O, which crystallizes in the monoclinic space group P2₁/n. The structure was refined to a final R value of 0.0549 for 5894 observed reflections. The heptadentate ligand coordinates with six nitrogens, i.e. with two tertiary nitrogens and four pyridine nitrogens, one of the pyridines remaining un-coordinated. The resulting structure is significantly distorted from octahedral geometry with an equatorial N_sp3-Ru-N_pyridine angle of 120°. The consequence of the above steric strain is a labilization of the system and fluxional behaviour involving exchange between equatorially coordinated and non-coordinated pyridines has been observed by ¹H NMR for Ru(tpap)(PF₆)₂ in d₆-acetone solution. The activation parameters of ΔG^#298 = 53 kJ mol^-1, ΔH^# = 56 ± 1 kJ mol^-1 and ΔS ^# = -10 ± 3 J mol^-1 K^-1 were determined on the basis of NMR experiments. In addition electronic structure calculations applying density functional theory (DFT) have been performed in order to identify a transition state and to estimate the activation barrier. On the basis of NMR and DFT results the mechanism of isoexchange involving a hepta-coordinated intermediate has been proposed.