NMR characterization, dynamics and crystal structure of [2,2′-dipyridyl(bis-pyridine) palladium(II)]2+ and related cations

Eight PdII complexes are chemically and structurally characterized by NMR and X-ray diffraction. Their common structural feature is the planar 2,2′-bipyridyl ligand (bipy) faced by variously substituted, vertical pyridines (n-Rpy). Dissymmetric Csn-Rpy lead, in solution, to syn and anti geometric isomers whose dynamic interconvertion can be either driven by pyridine rotation or reversible dissociation. Activation energy of the rotational motion is for Pd lower than Pt in accord with a weaker double bond character of Pd–Npy bond. The energetic barrier also owns a steric contribution which becomes dominant for 2-Mepy. Methyl bumping locks rotation and loosens Pd–Npy bond, triggering an alternative isomerization pathway.

Eight palladium complexes are fully characterized and their variable temperature behavior is interpreted on the basis of NMR analyses and crystallographic data. Theoretical DFT calculations show an extended MO delocalized over the pyridyl systems which provides to Pd–N a double bond character responsible for both vertical orientation and rotational barrier. Mechanism of syn-anti isomerisation is driven by steric and electronic factors.Figure optionsDownload as PowerPoint slide