Tripodal N-Heterocyclic Carbene Complexes of Palladium and Copper: Syntheses, Characterization, and Catalytic Activity

The tris-palladium tripodal N-heterocyclic carbene (NHC) complexes (timtebtBu){Pd(ICy)I2}3 (4a), (timtebtBu){Pd(PPh3)I2}3 (5a), and (timtebdipp){Pd(PPh3)I2}3 (5b) (timtebtBu = 1,3,5-{tris(tert-butylimidazol-2-ylidene)methyl}-2,4,6-triethylbenzene, 3a; ICy = 1,3-dicyclohexylimidazol-2-ylidene; timtebdipp = 1,3,5-{tris({2,6-diisopropylphenyl}imidazol-2-ylidene)methyl}-2,4,6-triethylbenzene, 3b) were prepared by reaction of Pd(II) precursors with either the free carbenes or imidazolium salts. Treatment of [Cu(NCMe)4]X (X = PF6, BF4) with 3a or 3b produced the tris-copper(I) bridged complexes [(timtebR)Cu3(μ3-O)]X (R = tBu, X = PF6, 6a; R = dipp, X = BF4, 6b). Complexes 4a, 5a, and 6a were structurally characterized. The palladium complexes were tested as catalysts for Suzuki−Miyaura and Sonogashira coupling reactions and the copper species also employed for the Sonogashira reaction, as well as for the Ullmann-type arylation of imidazoles and phenols.