RHENIUM(I) TRICARBONYL HALIDE-COMPLEXES OF PYRAZOLYL-BIPYRIDINE LIGANDS .1. NUCLEAR-MAGNETIC-RESONANCE STUDIES OF SOLUTION FLUXIONALITY

Pentacarbonylhalogenbrhenium(l) complexes reacted with 6-(pyrazol-1-yl)-2.2'-bipyribine-(pbipy) to form stable octahedral complexes of type fac-[ReX(CO)(3)(pbipy)] (X = Cl, Br or I) in which pbipy acts as a bidentate chelate ligand. Mono- and di-methyl derivatives of this ligand formed similar series of complexes. In all cases chemically distinct pairs of complexes involving both bipyridyl and pyridyl/pyrazolyl coordination were formed but could not be separately isolated. The 'bipyridyl' co-ordinated complexes predominate (> 80%) in solution. These exchange with the pyridyl/pyrazolyl co-ordination species by a 1,4-metallotropic shift, the rates and mechanism of which were followed by variable-temperature NMR spectroscopy. Energy barriers (Delta G double dagger) for this process were in the range 66-76 kJ mol(-1).