Stereochemically non-rigid transition metal complexes of 2,6-bis(phenyliminomethyl)pyridine and derivatives. Crystal structure of fac-[ReBr(CO)(3)(PMTFA)] (PMTFA=2,6-pyridylene-bis-(methylene-2-trifluoromethylaniline)

The Schiff bases 2,6-bis(phenyliminomethyl)pyridine and its derivatives (L) form bidentate chelate complexes with transition metals Re-I, Pt-IV and Pd -II of general formulation fac-[ReBr(Co)(3)L], fac-[PtXMe3L] (X = Cl, Br or I), and cis-[Pd(p-CF3C6F4)(2)L]. In solution these complexes are stereoche mically non-rigid, undergoing 1,4-metallotropic shifts and restricted rotat ions of the substituted aromatic rings attached to the metal-coordinated im ino nitrogen. Rates and activation energies of these internal motions were measured by one-dimensional NMR bandshape analysis and two-dimensional EXSY NMR experiments over a range of solution temperatures. Free energy values, DeltaG(double dagger) (298.15 K), for the fluxional shifts were in the ran ge 76-97 kJ mol(-1) with the Pd-II complexes having the lowest energies. An X-ray crystal structure of fac-[ReBr(CO)(3)PMTFA] (PMTFA = 2,6-pyridylene- bis(methylene-2-trifluoroaniline)) showed the pendant imine function to be in an E-conformation with its nitrogen trans to the pyridyl nitrogen. (C) 2 001 Elsevier Science BN. All rights reserved.