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

The octahedral complexes fac-CReX(CO)(3)L] [L = 6-(pyrazol-1-yl)-2,2'-bipyridine (pbipy), 6-(3,5-dimethylpyrazol-1 -yl)-2,5'-bipyridine (dmpbipy) or 6-(4-methylpyrazol-1-yl)-2,2'-bipyridine (mpbipy); X = Cl, Pr or Il have been studied by low-temperature NMR spectroscopy and in the first two cases by X-ray crystallography. The pendant pyrazol-1-yr 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 degrees 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 (Delta G(+)+) 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.