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

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.