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

The stable octahedral complexes fac-[ReX(CO)3L][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 ΔG‡(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 1H 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}2L](X = Cl or Br; L = tdmpzt) were also isolated and structurally characterized by 1H NMR spectroscopy.