Wasserlösliche Metallkomplexe und Katalysatoren, IV. 2,2′‐Bipyridin‐5‐sulfonsäure: Synthese, Reinigung, Derivate und Metallkomplexe

Water‐Soluble Metal Complexes and Catalysts, IV. — 2,2′‐Bipyridine‐5‐sulfonic Acid Synthesis, Purification, Derivatives and Metal Complexes The synthesis of 2,2′‐bipyridine‐5‐sulfonic acid (1a) is achieved by mercury(II)‐catalyzed sulfonation of 2,2′‐bipyridine in oleum (30% SO₃) in 10‐50‐g amounts. The crude product is purified by extraction of 1a as tetra(n‐butyl)ammonium salt into dichloromethane and re‐extraction of the free acid with conc, hydrobromic acid. Melting of 1a together with potassium hydroxide gives 5‐hydroxy‐2,2′‐bipyridine (2). The salts 1b–d (Na⁺, [N(n‐C₄H₉)₄]⁺, [P(C₆H₅)₄]⁺) and sulfonamides 1f–h (tert‐butyl, benzyl, 2‐pyridylmethyl) of the acid 1a are ligands of different solubility. The coordination chemistry of these N,N‐chelating ligands is studied. The compounds 4–14 of chromium, molybdenum, tungsten, manganese, rhenium, and osmium containing carbonyl or oxo ligands are prepared as examples of chelate complexes with metals in low and high oxidation states. The solubility of the complexes is mostly determined by the cations: sodium salts are usually soluble in water and/or in short‐chained alcohols. Tetra‐n‐butylammonium and tetraphenylphosphonium salts and the sulfonamides are soluble in polar organic solvents. On the other hand, the distribution of charge has an influence on solubility. Increased polarity of the anion, caused by high oxidation states of the metals, decreases the solubility of the complex in organic solvents.