Catalytic Synthesis of Bi- and Teraryls in Aqueous Medium with Palladium(II) Complexes of 2-(Pyridin

Eur. J. Inorg. Chem. 2015, 520–526

Suzuki–Miyaura coupling reactions of phenylboronic and 1,4-phenylenediboronic acid with ArBr to form bi- and teraryls have been efficiently catalyzed by the air- and moisture-insensitive complexes [PdCl2L1] [1, L1 = 2-(pyridine-2-ylmethyl)sulfanylbenzoic acid; 0.05–0.5 mol-%] and [Pd(L2-H)2] (2, L2-H = 2-pyridin-2-yl-benzo[b]thiophen-3-ol; 0.01–0.5 mol-%). The complexes were formed by the reaction of [Pd(CH3CN)2Cl2] with L1, and the catalysis proceeds in water for 1. The loading of 0.1–0.5 mol-% of Pd is very promising for the coupling reactions to form teraryls. The COOH group imparts solubility to 1 in water. Ligand L2 is formed by the unprecedented cyclization of L1 in the course of the complexation reaction. The yield of 2 increases with the reaction time (5 h: 70 %; 24 h: 85 %). The two complexes and L1 have been characterized by NMR spectroscopy (1H and 13C{1H}). The single-crystal structures of 1 and 2 have been solved, and the Pd–S, Pd–N, and Pd–O bond lengths are 2.269(2), 1.999(6)–2.057(2), and 1.9787(17) Å, respectively. Nanoparticles (NPs) with a narrow size distribution (ca. 3.0–5.5 nm for 80–85 % of the particles) formed at the start of these reactions and appear to be important for the catalytic coupling. Poisoning experiments and a two-phase test have shown that the catalysis is largely homogeneous and involves [Pd0–PdII] processes. The bond lengths and angles calculated by DFT are consistent with the experimental ones.