Palladium(II) Complexes of the First Pincer (Se,N,Se) Ligand, 2,6-Bis((phenylseleno)methyl)pyridine (L): Solvent-Dependent Formation of [PdCl(L)]Cl and Na[PdCl(L)][PdCl4] and High Catalytic Activity for the Heck Reaction

The reaction of PhSe- with 2,6-bis(chloromethyl)pyridine has resulted in the first pincer (Se,N,Se) ligand, 2,6-bis((phenylseleno)methyl)pyridine (L), which has formed two types of complexes, [PdCl(L)]Cl (1) and Na[PdCl(L)][PdCl4](2), when reacted with Na-2[PdCl4], depending upon the solvent. The complex 2 is formed in methanolic medium and 1 in an acetone-water (2: 1) mixture. L, 1, and 2 have been characterized by proton, carbon-13, and selenium-77 NMR spectra and single-crystal X-ray crystallography. The Pd-Se bond length is between 2.3891 (1) and 2.4313(7) angstrom due to the trans influence of two Se donor atoms on each other and the rigidity of L. The noncovalent interactions (hydrogen bonding) present in the crystal of 1 make a ring structure comprising its six molecules. In the case of complex 2, a dimeric unit is formed due to such interactions. The geometry around Pd is nearly square planar for both the complexes. 1 and 2 are both efficient as catalysts for the Heck coupling (C-C) reaction, as the yield and TON values are up to 97% and 97 000, respectively. For complex 2 yields and TON values are higher in comparison to those obtained with 1.