FLUXIONAL RHENIUM(I) TRICARBONYL HALIDE-COMPLEXES OF SUBSTITUTED PYRAZOLYLPYRIDINE LIGANDS - NUCLEAR-MAGNETIC-RESONANCE STUDIES OF 1,4-METALLOTROPIC SHIFTS

The octahedral complexes fac-[ReX(CO)(3)L][X = Cl, Br or I; L = 2,6-bi s-(4-methylpyrazol-1-yl)pyridine (bmppy), 2(pyrazol-1-yl)-6-(4-methylp yrazol-1-yl)pyridine (pmppy) and pyrazol-1-yl)-6-(3,5-dimethylpyrazol- 1-yl)pyridine (mpdmppy)] are stable octahedral complexes in which the substituted pyridines are acting as bidentate chelate ligands. In solu tion the bmppy complexes are fluxional with the nitrogen ligand oscill ating between two equivalent bidentate bonding modes, whereas for the unsymmetrical ligands pmppy or mpdmppy the fluxionality interconverts two chemically distinct species with differing solution populations. R ates and activation energies of these fluxions have been investigated by NMR methods. Activation energies are dependent on the relative dono r strengths of the nitrogen atoms and the steric requirements of the l igands with magnitudes of Delta G(double dagger) (298 K) being approxi mate to 55 (bmppy), approximate to 58 (pmppy) and approximate to 71 kJ mol(-1) (mpdmppy).