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)

Abstract

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) 3L], fac-[PtXMe 3L] (X=Cl, Br or I), and cis-[Pd( p-CF 3C 6F 4) 2L]. In solution these complexes are stereochemically non-rigid, undergoing 1,4-metallotropic shifts and restricted rotations of the substituted aromatic rings attached to the metal-coordinated imino 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, Δ G ‡ (298.15 K), for the fluxional shifts were in the range 76–97 kJ mol −1 with the Pd II complexes having the lowest energies. An X-ray crystal structure of fac-[ReBr(CO) 3PMTFA] (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.