Synthesis of tetra- and pentacoordinate palladium complexes of functionalized N-heterocyclic carbenes and a comparative study of their catalytic activities

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Palladium complexes, [Pd(1-butyl-3-((7-methyl-1,8-naphthyridin-2-yl)methyl)imidazolylidene)2](PF6)2 (1), [Pd(3-methyl-1-(pyrimidin-2-yl)imidazolylidene)2(CH3CN)](PF6)2 (4), [Pd(3-benzyl-1-(pyrimidin-2-yl)imidazolylidene)2(CH3CN)](PF6)2 (5), [Pd(3-ethyl-1-(pyrimidin-2-yl)imidazolylidene)2Cl](PF6) (6), and [Pd(3-benzyl-1-(pyrimidin-2-yl)imidazolylidene)Cl2] (7), have been prepared via transmetallation of corresponding in situ generated silver-carbene complexes. These palladium complexes have been characterized by NMR spectroscopy and elemental analyses, and their structures were determined by X-ray single-crystal diffraction. Complexes 1 and 7 are tetracoordinate displaying square-planar geometry. Complexes 4-6 are pentacoordinate showing uncommon square-pyramidal geometry with acetonitrile or chloride at the equatorial position. The Suzuki and Hiyama coupling reactions of a wide range of aryl bromides and chlorides have been comparatively investigated by using tetracoordinate moncarbene, tetracoordinate dicarbene, and pentacoordinate dicarbene palladium complexes as catalysts. The tetracoordinate moncarbene palladium complex showed the highest catalytic activity for both coupling reactions.