Stabilization of an Electron-Unsaturated Pd(I)–Pd(I) Unit by Double Hemichelation

Abstract

The competition between conventional coordination and hemichelation of a Pd(II) center of three different palladacycles was probed by reacting the tricarbonyl(η6-3-phenylprop-1-enyl)chromium anion with μ-chloro-bridged palladacycles. Structural X-ray diffraction analysis indicates that the main products of the reaction are conventional (η3-allyl)Pd(II) complexes. The latter display significant dynamic behavior in solution, as suggested by 1H NMR spectroscopy. According to DFT calculations this dynamic behavior can be related to conformational equilibria and to possible chemical exchanges leading to Pd(II) hemichelates. The (η3-allyl)Pd(II) complexes convert readily to homoleptic bis-chelates of Pd(II) and to a bis-hemichelate of a Pd(I)–Pd(I) unit via a reductive disproportionation reaction. The latter Pd(I)–Pd(I) complex bears structural features very similar to those of electron-saturated bis(μ-allyl)-bridged Pd(I) complexes in the literature: the Pd–Pd interaction is only weakly covalent and is dominated by noncovalent attractive interactions, as revealed by NCI analyses. The incipient covalent interaction of the Pd(I) centers with CO ligands of the Cr(CO)3 moieties is too weak to significantly hinder the motion of the metalcarbonyl rotor in solution, which leaves each Pd center formally unsaturated with a 14-valence-electron count. DFT investigations sustained by QTAIM, NCI, ELF, and ETS-NOCV analyses suggest the predominance of noncovalent attractive forces in the stabilization of the bis(μ-allyl)-bridged Pd(I)–Pd(I) complex.