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

The tris-palladium tripodal N-heterocyclic carbene (NHC) complexes (timteb tBu ){Pd(ICy)I 2 } 3 ( 4a ), (timteb tBu ){Pd(PPh 3 )I 2 } 3 ( 5a ), and (timteb dipp ){Pd(PPh 3 )I 2 } 3 ( 5b ) (timteb tBu = 1,3,5-{tris( tert -butylimidazol-2-ylidene)methyl}-2,4,6-triethylbenzene, 3a; ICy = 1,3-dicyclohexylimidazol-2-ylidene; timteb dipp = 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 = PF 6, BF 4 ) with 3a or 3b produced the tris-copper(I) bridged complexes [(timteb R )Cu 3 (μ 3 -O)]X (R = t Bu, X = PF 6, 6a; R = dipp, X = BF 4, 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.