Synthesis and solution fluxionality of rhenium(I) carbonyl complexes of 2,4,6-tris(pyrazol-1-yl)-1,3,5-triazines (L), [ReX(CO)(3)L] (X=Cl, Br or I). Characterization of [{ReX(CO)(3)}(2)L] (X=Cl or Br)

The stable octahedral complexes fac-[ReX(CO)(3)L] [X=Cl, Br or I; L=2,4,6-tris(4-methylpyrazol-1-yl)-1,3,5-triazine (tmpzt) or 2,4,6-tris(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine (tdmpzt)] have been prepared in which the substituted triazines act as bidentate chelate ligands. In solution; the complexes;are fluxional with the nitrogen ligand oscillating between two equivalent bonding modes. Rates and activation energies for the,fluxion have been investigated by dynamic NMR methods. Activation energies Delta G double dagger (298.15 K) are in the range 47-72 kJ mol(-1) and are dependent on the steric requirements of the pyrazole groups. At low temperatures changes in the H-1 NMR spectra are interpreted in terms of varying rates of rotation of one of the unco-ordinated pyrazole rings. The non-fluxional dimetallic complexes [{ReX(CO)(3)}(2)L] (X=Cl or Br; L=tdmpzt) were also isolated and structurally characterized by H-1 NMR spectroscopy.