Chalcogen-Dependent Palladation at the Benzyl Carbon of 2,3-Bis[(phenylchalcogeno)methyl]quinoxaline: Palladium Complexes Catalyzing Suzuki–Miyaura Coupling via Palladium–Chalcogen Nanoparticles

The reaction of PhS – /PhSe – with 2,3-bis(bromomethyl)quinoxaline has resulted in 2,3-bis[(phenyl-thio/-seleno)methyl]quinoxaline ( L1 / L2 ). They react with Na 2 PdCl 4, resulting in [Pd 2 ( L1 –H) 2 Cl 2 ] ( 1 )/[Pd L2 Cl 2 ] ( 2 ). External base-free activation of the benzyl group’s C(sp 3 )–H results in palladation of L1, forming palladacycle 1, whereas L2 forms a seven-membered chelate ring with Pd(II), resulting in 2 . L1, L2, 1, and 2 have been characterized by IR, multinuclear NMR, mass spectrometry, and single-crystal X-ray diffraction. In the case of 1 bond lengths (Å) are Pd–C = 2.001(9), Pd–N = 2.048(8), and Pd–S = 2.259(2) Å. The Pd–Se bond length in complex 2 is 2.3924(15)–2.3991(15) Å. Complexes 1 / 2 were explored in the catalyzed Suzuki–Miyaura coupling reactions of several aryl bromides (including deactivated ones). The palladacycle 1 shows better catalytic efficiency than complex 2, as its lower reaction time and catalyst loading (up to 0.006 mol %) are sufficient for significantly good conversions. The catalysis appears to be occurring via in situ generated nanoparticles (size <2 nm) composed of palladium and sulfur or selenium and protected by L1 or L2, as nanoparticles after isolation also show catalytic activity. The results of a two-phase test suggest that the catalysis is cocktail type (i.e., homogeneous and heterogeneous in parts).