Kinetics and Mechanism of Urea Hydrolysis Catalyzed by Palladium(II) Complexes

Four palladium(II) aqua complexes catalyze hydrolytic decomposition of urea into carbon dioxide and ammonia. The initial rates of carbon dioxide formation at 313 K and pH 3.3 fall in the range 6.7 x 10(-)(5) to 1.6 x 10(-)(4) M min(-)(1), depending on the catalyst. The pseudo-first-order rate constant for the formation of carbon dioxide is 1.7 x 10(-)(3) min(-)(1) in the presence of 0.30 M cis-[Pd(en)(H(2)O)(2)](2+) as the catalyst at 313 K and pH 3.3. This reaction is ca. 1 x 10(5) times faster than the uncatalyzed decomposition of urea. The reaction catalyzed by cis-[Pd(en)(H(2)O)(2)](2+) is monitored by (13)C and (15)N NMR spectroscopic methods. The following steps in the mechanism of this reaction are studied quantitatively: binding of urea to the catalyst, formation of carbamic acid (H(2)NCOOH) coordinated to palladium(II) via the nitrogen atom, and conversion of this intermediate into carbon dioxide and ammonia. These products are formed also by another pathway that does not involve carbamic acid. Kinetic effects of added acid and inhibition of the reaction by addition of thiourea and of bases are interpreted quantitatively. Ammonia inhibits the decomposition. When, however, this product is sequestered by metal cations, the reaction becomes relatively fast and catalytic turnover is achieved. The most effective of these sequestering agents is the silver(I) cation. Although the simple palladium(II) complexes are very different from the enzyme urease, which contains nickel(II) ions, the decomposition of urea catalyzed by both kinds of agents involves carbamic acid as the intermediate. Kinetic and mechanistic studies with metal complexes contribute to the understanding of the enzymatic mechanism.