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

Abstract

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.

Amperometric Urea Biosensor Using Aminated Glassy Carbon Electrode Covered with Urease Immobilized Carbon Sheet, Based on the Electrode Oxidation of Carbamic Acid

“…This amino-groupintroduced electrode is referred to as aminated GCE. It is well known that urea is eventually converted to ammonia and carbon dioxide via carbamic acid catalyzed by urease [20,21]. This result suggests that the electroactive carbamic acid is produced during the hydrolysis of urea.…”

Shape-controlled synthesis of Mn3O4 nanocrystals and their catalysis of the degradation of methylene blue

“…In this system, urea was used as the precipitant to gradually release hydroxyl ions at the appropriate reaction temperature. It is well known that decomposition of urea affords ammonia and carbon dioxide [24][25][26][27], followed by the release of OH − ions by hydrolysis of the ammonia solution. Simultaneously, the oxidation of a portion of Mn 2+ to Mn 3+ took place due to oxygen in the air, and the color of the solution changed from colorless to red-brown.…”

Ureas as new nucleophilic reagents for SNH amination and carbamoyl amination reactions in the 1,3,7-triazapyrene series

“…Ureas are normally rather inert towards alcohols, amines, and thiols: they require high temperatures, acidic or basic conditions, or metal catalysis, to undergo nucleophilic substitution reactions. [31][32][33][34] Ureas were used earlier as specific nitrogen nucleophiles in the palladium(II)-catalyzed amino carbonylation of unsaturated amines, 35 in intramolecular cyclisation of ureido acids and esters, 36 in the synthesis of a di-and triarylamines by interaction with unactivated aryl halides under palladium catalysis 37 and in Biginelli reactions. 38 In the course of this study, and taking into account the results of oxidative nucleophilic amidation of 1,3,7-triazapyrene, 28 we have tested the possibility of S N H carbamoyl amination of this heterocycle using urea as the nucleophilic reagent.…”