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 (dmp bipy) 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 th e first two cases by X-ray crystallography. The pendant pyrazol-1-yr r ing is shown to be oriented considerably out of the plane of the adjac ent 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 sol ution, restricted rotation of the pendant ring occurs leading to disti nct 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 s ubstitution at the 3 and 5 positions of the pyrazol-1-yl ring as in th e 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.