Redox Activity, Ligand Protonation, and Variable Coordination Modes of Diimino-Pyrrole Complexes of Palladium.

Ligand-based functionality is a prominent method of increasing the reactivity or stability of metal centers in coordination chemistry. Some of the most successful catalysts use ligand-based redox activity, pendant protons, or hemilability in order to specifically accelerate catalysis. Here we report the diimino-pyrrole ligand Tol,CyDIPyH (Tol,CyDIPy = 2,5-bis( N-cyclohexyl-1-( p-tolyl)methanimine)pyrrolide), which exhibits all three of these ligand properties. Metalation of Tol,CyDIPy to Pd gives the pseudo-square planar complex (Tol,CyDIPy)PdCl, which upon reduction forms a mixture of products, including a Pd(I)-Pd(I) dimer wherein Tol,CyDIPy bridges the dimeric unit. Upon addition of PMe3, the imine arms of (Tol,CyDIPy)PdCl are displaced to yield (Tol,CyDIPy)Pd(PMe3)2Cl, where the Tol,CyDIPy ligand binds in a monodentate fashion. This complex can be reduced to generate a ligand-based radical, as shown by EPR spectroscopy. Finally, (Tol,CyDIPy)PdCl also can be protonated at the imine arm, exhibiting a total of three different coordination modes across this series of complexes. Taken together, these studies show that Tol,CyDIPy exhibits notable flexibility in its coordination and redox chemistry.