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

Abstract

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 Na2[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) Å 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