Enhanced Stability of a Paramagnetic Palladium Complex Promoted by Interactions with Ethynyl Substrates

Abstract

The highly reactive palladium-centered radical cluster [Pd3(dppm)3(CO)]•+ exhibits only a limited stability in solution at room temperature (about an hour). This stability can be extended significantly to several hours by adding organic substrates such as the symmetric and asymmetric alkynes Ph−C⋮C−H and MeO2C−C⋮C−CO2Me, which reversibly bind to the Pd3 triangle. The presence of the substrate inside the cavity protects the palladium centers from reacting with the “outside world”, hence enhancing the stability. Both adducts are stable as the cluster is always totally recovered. The paramagnetic complexes along with their corresponding dications were characterized by EPR, variable-temperature 31P NMR, UV−vis and MALDI-TOF spectroscopy, and electrochemistry. For the MeO2C−C⋮C−CO2Me/[Pd3(dppm)3(CO)]2+ complex, the analysis of the low-temperature 31P NMR spectra strongly suggests a major structure modification of the ligand and substrate with respect to the starting materials.