Nuclear magnetic resonance studies of molecular dynamics at below-ambient temperatures and crystal structures

Abstract

The octahedral complexes fac-[ReX(CO)₃L][L = 6-(pyrazol-1-yl)-2,2′-bipyridine (pbipy), 6-(3,5-dimethylpyrazol-1-yl)-2,2′-bipyridine (dmpbipy) or 6-(4-methylpyrazol-1-yl)-2,2′-bipyridine (mpbipy); X = Cl, Br or I] have been studied by low-temperature NMR spectroscopy and in the first two cases by X-ray crystallography. The pendant pyrazol-1-yl ring is shown to be oriented considerably out of the plane of the adjacent co-ordinated pyridyl ring of the ligand. The crystal structure of [ReBr(CO)₃(pbipy)] reveals this angle of orientation to be 36.3 and 48.1° in two crystallographically independent molecules. In solution, restricted rotation of the pendant ring occurs leading to distinct pairs of rotamers at low temperatures. Rotation barriers (ΔG^‡) for the pbipy and mpbipy complexes are in the range 39–51 kJ mol^–1 and the two rotamers have near-equal solution abundances. Methyl substitution at the 3 and 5 positions of the pyrazol-1-yl ring as in the dmpbipy complexes has a major effect on the orientation of the unco-ordinated pendant ring in the solid state as evidenced by the crystal structure of [ReBr(CO)₃(dmpbipy)]. This in turn affects the relative rotamer populations in solution, with one rotamer being very dominant, and the barrier to pendant-ring rotation increases somewhat.