Synthesis and Reactivity of Pyridine N-Oxide Complexes of Cobalt(III)

The syntheses of four new pentaamminecobalt(III) complexes with pyridine N-oxide, 4-methylpyridine N-oxide, 2-cyanopyridine N-oxide, and 4-cyanopyridine N-oxide are described. The first two ligands bond through the oxygen, while the latter two ligands are bonded via the nitrile group nitrogen. Reaction kinetics have been used to investigate the reactivity of these complexes. The rate constant for aquation of the pyridine N-oxide complex at 25°C is 2.51 × 10^-6 s^-1. Chromium(II) reduction of the pyridine N-oxide complex is an inner-sphere process with a rate constant at 25°C of 0.371 M^-1 s^-1, an activation enthalpy of 3.75 ± 1.2 kcal mol^-1, and an activation entropy of -48 ± 4 cal K^-1 mol^-1. The 4-cyanopyridine N-oxide complex undergoes facile base hydrolysis of the nitrile function to a bound amide. The rate constant for this process at 25°C is 4030 ± 125 M^-1 s^-1. The 2-cyanopyridine N-oxide complex exhibits this same nitrile group conversion even in acidic solution, where water is shown to function as a competitive attacking nucleophile.