Novel bis(azole) pincer palladium complexes: synthesis, structures and applications in Mizoroki-Heck reactions

Two novel NCN-pincer complex precursors bearing frameworks of 2,6-bis(oxazol-4-yl)benzene (A) and 2-(thiazol-4-yl)-6-(oxazol-4-yl)benzene (B) were synthesized. Palladations of A and B afforded two new bis(azole) pincer complexes, [(A-κ(3)NCN)PdBr] (1) and [(B-κ(3)NCN)PdBr] (2). Both complexes were fully characterized by NMR, MS, DSC-TGA and single-crystal X-ray diffraction analysis. Complex 1 crystallizes in a noncentrosymmetric orthorhombic space group Cmc2(1) (No. 36, Z=4). Complex 2 crystallizes in a centrosymmetric monoclinic space group P2(1)/n (No. 14, Z=4). Despite the similarity in their chemical formulas, the structures of the two complexes are subtly different: they are built up of two-dimensional supramolecular layers with identical topology, but stacked in different sequences, i.e., the layers in complex 1 are stacked in an AAAA-type fashion, while those in complex 2 are stacked in an alternating AA(-1)AA(-1) sequence (A denotes a layer; A(-1) stands for A's inversion symmetry equivalent). In addition, the complexes showed good catalytic activity toward Mizoroki-Heck reactions.