SCHEDULED MAINTENANCE
Strain-induced substitutional lability in a Ru(ii) complex of a hypodentate polypyridine ligand
Abstract
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)(CF3SO3)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 Nsp3–Ru–Npyridine 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.
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