Reversible Chelate Ring Opening of a Sterically Crowded Palladium Bis(acyclic diaminocarbene) Complex

Reaction of cis-dichlorobis(p-trifluoromethylphenylisocyanide)palladium(II) with hydrazobenzene afforded a complex with a monodentate aminohydrazino carbene ligand and an unreacted arylisocyanide. Upon heating, this complex partially converted into a chelating bis(acyclic diaminocarbene) complex. Both the monocarbene and bis(carbene) complexes were isolated and characterized by X-ray crystallography. In solution, an equilibrium between the monocarbene and bis(carbene) complexes was observed, with equilibrium constants of 2.56(15) in CD3CN and 10.0(5) in THF-d8 at 25 °C. A van’t Hoff plot provided thermodynamic parameters of ΔH° = −1.7(3) kcal mol−1 and ΔS° = −3.7(9) eu for the monocarbene to bis(carbene) conversion in CD3CN. The instability of the bis(carbene) complex toward chelate ring opening was rationalized on the basis of steric strain apparent in the X-ray structure. The crowded syn,syn arrangement of aryl groups on the bis(carbene) backbone results in widening of the carbene NCN angles to reduce aryl−aryl steric interactions. Thermodynamic calculations on density functional theory models of the carbene complexes were in reasonable agreement with experimental data, and they provided an estimate of 16 kcal mol−1 for the maximum amount of ligand strain that can be accommodated in forming a stable bis(carbene) ligand via diamine addition to cis isocyanides. The chelating bis(carbene) is thermodynamically favored despite an unfavorable reaction entropy and a substantial amount of ligand strain.