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.