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 Na2PdCl4, resulting in [Pd-2(L1-H)(2)Cl-2] (1)/[PdL2Cl(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. Li, 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 (angstrom) are Pd-C = 2.001(9), Pd-N = 2.048(8), and Pd-S = 2.259(2) angstrom. The Pd-Se bond length in complex 2 is 2.3924(15)-2.3991(15) A. 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 Li 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).