FLUXIONAL RHENIUM(I) TRICARBONYL HALIDE-COMPLEXES OF PYRAZOLYLPYRIDINE LIGANDS - A DETAILED NUCLEAR-MAGNETIC-RESONANCE INVESTIGATION

2,6-Bis(pyrazol-1-yl)pyridine (bppy) reacted with pentacarbonylhalogenorhenium(I) complexes under mild conditions to form stable octahedral complexes of type fac-[ReX(CO)3(bppy)] (X = Cl, Br or 1) in which bppy acts as a bidentate chelate ligand. In solution these tricarbonyl complexes are fluxional with bppy oscillating between equivalent bidentate bonding modes by a twist mechanism involving the breaking/making of two Re-N bonds. 2,6-Bis(3,5-dimethylpyrazol-1-yl)pyridine (tmbppy) and 2-(3,5-dimethylpyrazol-1-yl)-6-(pyrazol-1-yl)pyridine (dmbppy) formed analogous fluxional complexes. The 1,4-Re-N metallotropic shifts of [ReX(CO)3(dmbppy)] interconvert chemically distinct complexes with different solution populations. Under more severe reaction conditions the complex cis-[ReBr-(CO)2(tmbppy)] was formed in which the ligand adopts its more usual terdentate behaviour. Rates and activation energies of these 'tick-tock' twist fluxions have been investigated by one- and two-dimensional NMR methods. Activation energies are dependent on the relative donor strengths of the N atoms, with magnitudes of AG(double dagger) (298.15 K) being 55-56 (bppy), almost-equal-to 69 (dmbppy) and 70-77 kJ mol-1 (tmbppy).