Structure and Dynamic Rearrangements of the Pt2dba3 and Pd2dba3 Complexes in Solution.

ABSTRACT

Although the tris(dibenzylideneacetone)diplatinum complex (Pt2dba3) is an important source of Pt(0) used in catalysis and materials science, its structure has not yet been fully elucidated. A thorough study of the three-dimensional structure of Pt2dba3 and its dynamic behavior in solution was carried out using NMR spectroscopy methods at a high field (600 MHz) and molecular modeling. The complex was shown to contain three dba ligands in the s-cis,s-trans, s-trans,s-cis, and s-trans,s-trans conformations, which are uniformly oriented around the Pt2 backbone. In solution, the Pt2dba3 and Pd2dba3 complexes undergo rapid dynamic rearrangements, as evidenced by the exchange between the signals of the olefin protons of various dba ligands in the EXSY NMR spectra. According to the experimental measurements, the activation energies of the rearrangements were estimated to be 19.9 ± 0.2 and 17.9 ± 0.2 kcal/mol for the platinum and palladium complexes, respectively. Three possible mechanisms for this chemical exchange process were considered within the framework of DFT calculations. According to the calculated data, M2dba3 complexes undergo fluxional isomerization involving successive rotations of the dihedral angles formed by the carbonyl group and the C═C bond. Dissociation of dba ligands does not occur within these processes.