Characterization of [Pd(pap)(NHC)I2]n (n = +1, 0, −1; pap = phenylazopyridine; NHC = N-heterocyclic carbene): Unusual coordination mode of pap and non-innocent behavior of NHC

The compounds [Pd(pap)(NHC)I2], 1 and 2 (pap = Phenylazopyridine, NHC = 1,3-di-n-butyl-benzimidazolin-2-ylidene, 1 and NHC = 1-n-butyl-3-isopropyl-benzimidazolin-2-ylidene, 2) were synthesized from the respective 1,3-disubstituted benzimidazolium salts and [Pd(pap)] precursor in the presence of a base. Crystallographic characterization of 1 shows an unusual coordination mode of pap, with the ligand coordinating to the Pd(II) center only through the pyridine nitrogen atom in an η1-N fashion. Both the nitrogen atoms of the azo group remain uncoordinated to the metal center. Cyclic voltammetry of 1 and 2 shows two oxidation and two reduction steps. A combination of UV–Vis and EPR spectroelectrochemistry is used to show that the reduction steps are pap based, proving its known non-innocent character. Investigation of the first oxidation step points to an electron removal from a predominantly NHC based orbital, thus showing the potential non-innocent nature of these ligands. These results can provide new mechanistic insights into the use of Pd–NHC catalysts for cross-coupling reactions, where oxidative addition is one of the deciding steps for catalysis.