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)63+ and X‐pyO according to the rate law kobs = ko + 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 M s–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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)