Stability of Dinuclear Phosphane Palladium(III) Complexes: A DFT Approach

Computational density functional theory studies have been carried out for the dinuclear ortho-metalated palladium­(III) compounds [Pd2{μ-(C6H4)­PPh2}2{μ-(X1-X2)}2Cl2]. These studies have shown that the electronic and steric properties of the auxiliary ligands (X1-X2 = bridging (carboxylato) or chelating (phenolato/acetylacetonato) O,O-donor ligands, bridging N,N-donor ligands (triazenido/formamidinato/pyrazolato), and bridging N,S-donor ligands) lead to systematic trends in their stability, highlighting that (a) the electronic nature of the donor atoms trans to the P has a clear trend, the replacement of hard donor atoms (O, N) by softer S donors generally reducing the stability of the compounds, (b) the geometrically flexible ligands with bulky substituents partially blocking the axial sites (formamidinato and triazenido ones) diminish the stability, except in cases where additional intramolecular interactions provide extra stabilization, and (c) the axial Cl–Pd···Pd–Cl interactions play a major role in the stability of the studied Pd­(III) complexes. The presence of a Pd···Pd bond in these compounds was verified by analyzing the UV–vis spectra simulated via TDDFT calculations. As supported by DFT calculations, palladium­(III) intermediates have been suggested in the catalytic 2-phenylation of indoles with (Ph2I)­PF6. Detailed analysis of the Pd···Pd, Pd–X, and axial Pd–Cl and Pd–C­(Ph) interactions was executed by calculating the properties of the electron density according to QTAIM methods in order to reveal the factors affecting the overall stability of the compounds.