Photoelectron Spectroscopy
of Palladium(I) Dimers with Bridging Allyl Ligands
- Publication date
- Publisher
Abstract
The dianionic Pd<sup>I</sup> dimers [TBA]<sub>2</sub>[(TPPMS)<sub>2</sub>Pd<sub>2</sub>(μ-C<sub>3</sub>H<sub>5</sub>)<sub>2</sub>] (<b>1</b>) [TBA = tetrabutylammonium, TPPMS
= PPh<sub>2</sub>(3-C<sub>6</sub>H<sub>4</sub>SO<sub>3</sub>)<sup>−</sup>] and [TBA]<sub>2</sub>[(TPPMS)<sub>2</sub>Pd<sub>2</sub>(μ-C<sub>3</sub>H<sub>5</sub>)(μ-Cl)] (<b>2</b>), containing two bridging allyl ligands and one bridging allyl ligand
and one bridging chloride ligand, respectively, were synthesized.
The electronic structures of these complexes were investigated by
combining electrospray mass spectrometry with gas phase photodetachment
photoelectron spectroscopy. The major difference between the photoelectron
spectra of the anions of <b>1</b> and <b>2</b> is the
presence of a low-energy detachment band with an adiabatic electron
detachment energy of 2.44(6) eV in <b>1</b>, which is not present
in <b>2</b>. The latter has a much higher adiabatic electron
detachment energy of 3.24(6) eV. Density functional theory calculations
suggest that this band is present in <b>1</b> due to electron
detachment from the out-of-phase combination of the π<sub>2</sub> orbitals, which are localized on the terminal carbon atoms of the
bridging allyl ligands. In <b>2</b>, the Pd centers stabilize
the single π<sub>2</sub> orbital of the bridging allyl ligand,
and it is lowered in energy. The presence of the high-energy out-of-phase
combination of the π<sub>2</sub> allyl orbitals makes <b>1</b> a better nucleophile, which explains why species with two
bridging allyl ligands react with CO<sub>2</sub> in an analogous fashion
to momoneric Pd η<sup>1</sup>-allyls, whereas species with one
bridging allyl and one bridging chloride ligand are unreactive
