4-HYDROXYPYRIDINE AND 3-HYDROXYPYRIDINE COMPLEXES OF PENTACYANOFERRATE(II,III)

A series of Fe(CN)(5)L3- complexes, where L=4-,3-hydroxypyridines and 4-,3-methoxypyridines, was prepared in aqueous solution by the reactio n of Fe(CN)(5)OH23- with excess ligand. A metal-to-ligand charge-trans fer transition was observed for the complexes at 414 (4-hydroxypyridin e), 363 (3-hydroxypyridine), 336 (4-methoxypyridine) and 368 nm (3-met hoxypyridine). The corresponding Fe(III) complexes also display a liga nd-to-metal charge-transfer band at 565 (4-hydroxypyridine) 503 (3-hyd roxypyridine), 440 (4-methoxypyridine) and 530 nm (3-methoxypyridine). Cyclic voltammetry of the complexes under study has shown that the ox idation is a one-electron reversible process with E-1/2 values of 0.40 (4-hydroxypyridine), 0.45 (3-hydroxypyridine), 0.43 (4-methoxypyridin e) and 0.45 V (3-methoxypyridine) versus NHE at 25 degrees C, mu = 0.1 0 M (LiClO4) and pH = 5 (acetate). The rate constants of formation and dissociation of the Fe(CN)(5)L3- complexes were measured and the k(f) and k(d) values (25 degrees C, mu = 0.1 M (LiClO4), pH = 5) are 0.273 M-1 s(-1) and 1.08 X 10(-3) s(-1) (4-hydroxypyridine), 171 M-1 s(-1) and 2.28 X 10(-3) s(-1) (3-hydroxypyridine), 375 M-1 s(-1) and 2.05 X 10(-3) s(-1) (4-methoxypyridine), 421 M-1 s(-1) and 2.37 X 10(-3) s(-1 ) (3-methoxypyridine). The k(f) 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 taut omeric equilibrium of the 4-hydroxypyridine ligand to its keto-form ra ther than via a different reaction mechanism. By comparing the k(f) re sults of complexes of 4-, 3-methoxypyridines and 3-, 3-hydroxypyridine s, the equilibrium constants of tautomerization of hydroxypyridines to wards their keto-isomers can be estimated, and the values are 1310 and 1.4 for 4- and 3-hydroxypyridines, respectively. (C) 1998 Elsevier Sc ience S.A.