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Preparation, characterization, and aquation kinetics of pyridine N-oxide complexes of chromium(III)

A series of (H2O)5Cr(X-pyO)3+ ions (pyO = pyridine N-oxide, X = H, 3-CH3, 4-CH3, 4-OCH3, 4-NO2) were prepared by the reduction of the corresponding pyridine N-oxide adducts of diperoxochromium(VI) species with acidic ferrous perchlorate. The (H2O)5Cr(X-pyO)3+ complexes undergo aquation to yield Cr(H2O)6 3+ and X-pyO according to the rate law k_obs = k_o+k_– 1[H+]– 1. The values of the rate constants extrapolated to 298 K at 1.0 M ionic strength are: k0 = 2.80×10– 6 s– 1, k– 1 =1.86×10– 8 Ms– 1 (X = 4-NO2) ; 7.80×10– 8, 6.27×10– 10 (H) ; 4.80×10– 8, 3.20×10– 10 (3-CH3) ; 3.05×10– 8, 1.60×10– 10 (4-CH3) ; and 2.37×10– 9, 4.76×10– 11 (4-OCH3). The reaction of the 4-OCH3 complex exhibits two additional terms in the rate law, k1[H+]+k– 2[H+]– 2. The binding of 4-OCH3– pyO to chromium is suggested to take place through the methoxy group.

Pyridine oxide; Chromium; Kinetics; Mechanism; Aquation

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