Rhenium(I) tricarbonyl halide complexes of pyrazolyl-bipyridine ligands. Part 1. Nuclear magnetic resonance studies of solution fluxionality

Abstract

Pentacarbonylhalogenorhenium(I) complexes reacted with 6-(pyrazol-1-yl)-2,2′-bipyridine (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 (ΔG‡) for this process were in the range 66–76 kJ mol^–1.