Solution fluxionality of some pyridine-2,6-dialdehydes, -diketones and -diesters when acting as bidentate ligands in rhenium(I) and platinum(IV) complexes. Crystal structure of [ReBr(CO)(3)L] (L = methylethyldipicolinate)

2,6-Disubstituted pyridines, where the substituents are aldehyde, ketone or ester functions, form bidentate chelate complexes with the transition meta l moieties fac-(ReX)-X-I(CO)(3) (X = halogen). 2-Substituted pyridines, whe re the substituents are aldehyde or ester functions, form similar types of complexes with the isoelectronic transition metal moieties fac-(ReX)-X-I(CO ), and (PtXMe3)-X-IV. The fac-(ReX)-X-I(CO)(3) complexes of the 2,6-disubst ituted pyridine ligands were shown by H-1 NMR spectra to undergo metallotro pic shifts whereby the Re coordination switches between adjacent ON pairs o f the ONO ligand donor set. Rates and activation energies of these fluxiona l shifts were measured by dynamic NMR bandshape analysis. Magnitudes of Del taG(double dagger) (298.15 K) were in the range 59-64 kJ mol(-1) for the di ketone and diester ligands. The dialdehyde ligand, 2,6-pyridinedicarboxalde hyde, formed an appreciably less-stable Re-I complex that was highly fluxio nal and showed a tendency to dissociation at ambient solution temperatures. The unsymmetrical diester ligand, methylethyldipicolinate, formed two dist inct Re-I complex species in solution, in the approximate abundance ratio o f 2:1, the more abundant structure involving coordination to the carbonyl o f the ethyl ester function. This particular complex forms exclusively in th e solid state and an X-ray crystal structure of [ReBr(CO)(3)L] (L = methyle thyldipicolinate) is reported. (C) 2001 Elsevier Science Ltd. All rights re served.