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Mechanism of molecular nitrogen reduction to hydrazine by bivalent chromium hydroxide

The effect of the water concentration on the rates of the hydrazine and hydrogen formation in the reactions involving Cr(II) hydroxide is studied in alkaline CH 3 OH-H 2 O and CH 3 OD-D 2 O mixtures. It was shown that the rate of nitrogen reduction to hydrazine in the deuterated solvent is higher than that in the non-deuterated solvent in the range [H 2 O] = 1.32-4.10 mol/l. The rate constant ratio k D /k H for the hydrogen evolution is higher than unity at [H 2 O] > 2.5 and lower than unity at [H 2 O] < 2.5 mol/l. In the protium system, HD is found to be formed in the presence of D 2 . A conclusion is made that the rate of the hydrazine formation is determined by the step of intramolecular transfer of the hydride atom to the coordinated nitrogen molecule. A kinetic model proposed explains adequately the influence of the water concentration on the k D /k H value for both reactions. The structure of the active center is discussed.