Add time:08/24/2019         Source:sciencedirect.com

A series of Fe(CN)5L3 complexes, where L = 4-, 3-hydroxypyridines and 4-, 3-methoxypyridines, was prepared in aqueous solution by the reaction of Fe(CN)5OH24 with excess ligand. A metal-to-ligand charge-transfer transition was observed for the complexes at 414 (4-hydroxypyridine), 363 (3-hydroxypyridine), 336 (4-methoxypyridine) and 368 nm (3-methoxypyridine). The corresponding Fe(III) complexes als display a ligand-to-metal charge-transfer band at 565 (4-hydroxypyridine), 503 (3-hydroxypyridine), 440 (4-methoxypyridine) and 530 nm (3-methoxypyridine). Cyclic voltammetry of the complexes under study has shown that the oxidation is a one-electron reversible process with E12 values of 0.40 (4-hydroxypyridine), 0.45 (3-hydroxypyridine), 0.43 (4-methoxypyridine) and 0.45 V (3-methoxypyridine) versus NHE at 25°C, μ = 0.10 M (LiClO4) and pH = 5 (acetate). The rate constants of formation and dissociation of the Fe(CN)5L3 complexes were measured and the k1 and Kd values (25°C, μ = 0.1 M (LiClO4), pH = 5) are 0.273 M−1 s−1 and 1.08 × 10−4 si−1 (4-hydroxypyridine), 171 M−1 s−1 and 2.28× 10−4 s−1 (3-hydroxypyridine), 375 M−1 s−1 and 2.05 × 10−4 s−1 (4-methoxypyridine), 421 M−1 s−1 and 2.37 × 10−4 s−1 (3-methoxypyridine). The k1 value of the 4-hydroxypyridine complex is three orders of magnitude smaller than that of the other complexes under investigation. The difference may arise from the favorable tautomeric equilibrium of the 4-hydroxypyridine ligand to its keto-form rather than via a different reaction mechanism. By comparing the k1 results of complexes of 4-, 3-methoxypyridines and 4-, 3-hydroxypyridines, the equilibrium constants of tautomerization of hydroxypyridines towards their keto-isomers can be estimated, and the values are 1310 and 1.4 for 4- and 3-hydroxypyridines, respectively.

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