title: 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 P2(1)/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 degrees. 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 DeltaG(not equal to 298) = 53 kJ mol(-1), DeltaH(not equal) = 56 +/- 1 kJ mol(-1) and DeltaS(not equal) = -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.
authors.affiliation: Stockholms Universitet, Organisk Kemi, Arrheniuslaboratoriet, 106 91 Stockholm, Sweden.
authors.forename: Björn
authors.initials: B
authors.lastname: Akermark
authors.forename: Jens
authors.initials: J
authors.lastname: Bjernemose
authors.forename: Anna
authors.initials: A
authors.lastname: Börje
authors.forename: Piotr J
authors.initials: PJ
authors.lastname: Chmielewski
authors.forename: Hauke
authors.initials: H
authors.lastname: Paulsen
authors.forename: Ole
authors.initials: O
authors.lastname: Simonsen
authors.forename: Paul C
authors.initials: PC
authors.lastname: Stein
authors.forename: Hans
authors.initials: H
authors.lastname: Toftlund
authors.forename: Juliusz A
authors.initials: JA
authors.lastname: Wolny