Experimental Evidence for Zerovalent Pd(NHC) as a Competent Catalyst in C–N Cross-Coupling (NHC = DiMeIHept^Cl)

Use of the branched N-heterocyclic carbene (NHC) ligand 1,3-bis(2,6-bis(3-methyl-1-(2-methylpropyl)butyl)phenyl)-4,5-dichloro-1,3-dihydro-2<i>H</i>-imidazole-2-ylidene (DiMeIHept^Cl) facilitated the stabilization of several relevant intermediates for Pd(NHC)-catalyzed C-N cross-coupling reactions. Complexes [Pd(DiMeIHept^Cl)]₂(μ-N₂), [Pd(DiMeIHept^Cl)]₂(μ-η²-1,2-η²-4,5-C₆H₆), and Pd(DiMeIHept^Cl)(pyridine), representing zerovalent Pd(NHC) bearing labile ligands, were isolated and structurally characterized, along with divalent PdCl(Ph)(DiMeIHept^Cl) and PdCl(Ph)(DiMeIHept^Cl)(<i>n</i>-propylamine). The former is a 14-electron Pd complex, which is stable under air and chromatography on silica gel or neutral alumina. One possible reason for this exceptional stability is the numerous dispersion interactions between the NHC alkyl chains and the Pd-Ph group. Detailed investigations of catalyst activation and oxidative addition confirmed that "Pd(NHC)" is formed from many known Pd(II)(NHC) precatalysts and provided activation rates for these different precatalysts.